**3. Occupational therapy models in forensic rehabilitation**

The use of occupational therapy models in forensic rehabilitation focus on client-centered, holistic and occupation-focused practice with the approach of clinical reasoning based on individual preferences and needs. Normally, individuals engage in occupations which they prefer or want throughout their life; however, in secure environment situations, this ability of the individual can be limited or can be restricted by the individual's mental health/disorder/ learning disability, their perceived and their actual risks to themselves or others and institutional regulations, policies or legal restrictions. Individuals who need forensic rehabilitation face some participation limitations to all or a combination of activities and this can cause occupational deprivation additionally to the sense of hopelessness and poor mental health [6, 11, 15]. Moreover, community life skills and performing daily living activities and interaction with the environment of the individual can be limited. Therefore, group or individual occupational therapy programs often target basic living skills, self-care, vocational skills, adaptive coping strategies, creative arts and anger or stress management. The general aim of occupational therapy is to enable individual to experience occupational enrichment and achieve optimal occupational functioning. Occupational enrichment in forensic settings can be considered as both the goal and process of occupational therapy interventions, so evidence-based practice is very important [15].

Occupational therapy guideline recommendations show that Model of Human Occupation (MOHO) and its associated assessments are the most used occupational therapy model in forensic occupational therapy. The model was developed in the 1980s by Professor Gary Kielhofner and has had some revisions and collaborations until now. MOHO supports that human occupation is motivated, patterned and performed. Humans are conceptualized as three interrelated components: volition, habituation and performance capacity [16]. Also, environmental considerations are very important to increase the occupational participation of the individual having forensic occupational therapy and rehabilitation [15].

According to the Model of Human Occupation, performance capacity is related to an individual's adaptive interaction with the environment, and the ability to do activities provided by physical and mental components and the associated subjective experience. Occupationally restricted individuals have problems with occupational performance skills for independence in daily living. Moreover, performance skills may not be acquired or learned during transition from child to adult. Occupational therapists in secure environments have a major role to play in helping patients to develop, maintain or acquire new skills for successful community reintegration or transition to less secure settings, for example in the area of vocational rehabilitation. Occupational therapists can guide individuals to identify possible vocational areas

Occupational Therapy in Forensic Settings http://dx.doi.org/10.5772/intechopen.79366 59

Occupational therapy interventions should help the individual to identify prevocational needs and sometimes individuals have not been or will not have a productive activity like working again and therefore the therapists need to help them to establish different prosocial, productive and meaningful occupations to increase health, wellbeing, occupational performance and general quality of life. For these reasons, it is important for occupational therapists to measure and follow the progress of the individual with outcome measures during daily

Social and physical isolation of the individuals can cause problems to access their own complex occupational and environmental worlds. Being cut from own life can cause limitations over occupational choices and experience, so environmental changes and supports during activity performance can provide opportunities, constraints and demands to the individual. The literature supports that individuals in secure environments spend much of their time in passive leisure, personal care and rest occupations and occupational therapists are one of the core elements in increasing activity participation of the individual and develop occupations of their choice [15, 16]. Also, occupational therapists can facilitate the exploration of new or

As it is given earlier, there is a great model need to understand the volition, habituation, performance skills, physical and social environments in which an individual's occupation takes place. MOHO assists the understanding of occupation(s) and problems of occupation that occur in terms of volition, habituation, performance capacity and environmental context. This system-based model includes well-designed assessments, observational, self-report and interview schedules. One of the advantages of this model is that because of its extensive use in mental health settings; a forensic version of an assessment tool 'Occupational Circumstances Assessment and Interview and Rating Scale' was designed. But, the literature also supports that this tool is not the only one for the use of occupational therapists, and occupational therapists may find any particular model, or standardized assessment/outcome measures to support their interventions [4, 15, 16, 21, 22].The literature supports that the use of occupational therapy models in forensic mental health may increase evidence-based practice and help the professionals to show the effect of occupational therapy. As Model of Human Occupation is seen to be the most used occupational therapy model, models including environmental and individual issues such as psychological issues, desires, wants, activity performance and satisfaction from the activity performance can help the occupational therapist to plan more

unknown occupations to increase positive life experiences of the individual.

such as study/education, voluntary or paid employment [16, 20, 21].

living activities [17, 18, 21].

Volition presents the individual's motivation for occupation and relates to individual's motivation to participate to occupations combined with their self-belief and capacity to succeed which means personal causation. Motivation and personal causation can be affected by the individual's mental health (such as depression, schizophrenia, personality disorder) or their perception of the reason and need for their admission. Therefore, the literature supports that it is important to establish the individual's own goals to ensure treatment readiness. Some individuals can have complex occupational histories, which are situated within social tensions related to their antisocial or criminal occupations present lack of motivation to engage and participate activities they want and these individuals can view the environment as a barrier to participation in usual activities which can impact the individual's mental health and well-being negatively. It is supported that volitional problems are likely to be highly relevant in the secure setting which can cause decrease in personal causation, difficulty in identifying or having unrealistic goals and an inability to and meaning or interest in activities [15, 16].

Occupational therapists have the skills and expertise to assess and engage people in those occupations which are meaningful and motivating. This requires a careful understanding and appreciation of what underlies the motivation and creating occupational opportunities like self-care, productivity and recreational activities. Occupational therapists also help people to identify and achieve their own hopes and aspirations such as vocational rehabilitation and work skills [15, 16].

Habituation refers the individual's roles and behavior patterns consistent with his/her lifestyle. It presents automatically and effectively doing routine tasks related to their environment. Roles of the individual are responsibilities of the individual associated with personal identity, occupations and activities of daily life and extraordinary occupations. Individual with criminal lifestyles can have problems on participating prosocial roles with their daily routines and occupations [15, 16].

Occupational therapy interventions in secure environments help individual to participate in prosocial roles and occupations in an effort to live within society without resorting to previous criminal or new antisocial behaviors. The imposed legal and security restrictions in secure environments can mean that patients are unable to participate in their habitual or chosen occupations; this may be because such occupations are antisocial, or due to lack of resources, facilities or particular environments being available in secure settings. Often patients benefit from the structure, stability and consistency of admission [19].

For occupational therapy interventions age, ethnicity and culture, finding the 'right' occupations that are culturally relevant, risk-managed and appropriate to the 'typical' forensic population can be challenging. The literature supports that not only redesigning lifestyle but also technological advances have an impact on the range of occupations that occupational therapist is able to offer to extend the inclusion level of the individual such as contemporary videogames and Nintendo® WiiTM additionally to participating actual sports and recreation activities [16].

According to the Model of Human Occupation, performance capacity is related to an individual's adaptive interaction with the environment, and the ability to do activities provided by physical and mental components and the associated subjective experience. Occupationally restricted individuals have problems with occupational performance skills for independence in daily living. Moreover, performance skills may not be acquired or learned during transition from child to adult. Occupational therapists in secure environments have a major role to play in helping patients to develop, maintain or acquire new skills for successful community reintegration or transition to less secure settings, for example in the area of vocational rehabilitation. Occupational therapists can guide individuals to identify possible vocational areas such as study/education, voluntary or paid employment [16, 20, 21].

environmental considerations are very important to increase the occupational participation

Volition presents the individual's motivation for occupation and relates to individual's motivation to participate to occupations combined with their self-belief and capacity to succeed which means personal causation. Motivation and personal causation can be affected by the individual's mental health (such as depression, schizophrenia, personality disorder) or their perception of the reason and need for their admission. Therefore, the literature supports that it is important to establish the individual's own goals to ensure treatment readiness. Some individuals can have complex occupational histories, which are situated within social tensions related to their antisocial or criminal occupations present lack of motivation to engage and participate activities they want and these individuals can view the environment as a barrier to participation in usual activities which can impact the individual's mental health and well-being negatively. It is supported that volitional problems are likely to be highly relevant in the secure setting which can cause decrease in personal causation, difficulty in identifying or having unrealistic goals and an inability to and meaning or interest in activities [15, 16].

Occupational therapists have the skills and expertise to assess and engage people in those occupations which are meaningful and motivating. This requires a careful understanding and appreciation of what underlies the motivation and creating occupational opportunities like self-care, productivity and recreational activities. Occupational therapists also help people to identify and achieve their own hopes and aspirations such as vocational rehabilitation and

Habituation refers the individual's roles and behavior patterns consistent with his/her lifestyle. It presents automatically and effectively doing routine tasks related to their environment. Roles of the individual are responsibilities of the individual associated with personal identity, occupations and activities of daily life and extraordinary occupations. Individual with criminal lifestyles can have problems on participating prosocial roles with their daily

Occupational therapy interventions in secure environments help individual to participate in prosocial roles and occupations in an effort to live within society without resorting to previous criminal or new antisocial behaviors. The imposed legal and security restrictions in secure environments can mean that patients are unable to participate in their habitual or chosen occupations; this may be because such occupations are antisocial, or due to lack of resources, facilities or particular environments being available in secure settings. Often patients benefit

For occupational therapy interventions age, ethnicity and culture, finding the 'right' occupations that are culturally relevant, risk-managed and appropriate to the 'typical' forensic population can be challenging. The literature supports that not only redesigning lifestyle but also technological advances have an impact on the range of occupations that occupational therapist is able to offer to extend the inclusion level of the individual such as contemporary videogames and Nintendo® WiiTM additionally to participating actual sports and recreation

work skills [15, 16].

activities [16].

routines and occupations [15, 16].

from the structure, stability and consistency of admission [19].

of the individual having forensic occupational therapy and rehabilitation [15].

58 Occupational Therapy - Therapeutic and Creative Use of Activity

Occupational therapy interventions should help the individual to identify prevocational needs and sometimes individuals have not been or will not have a productive activity like working again and therefore the therapists need to help them to establish different prosocial, productive and meaningful occupations to increase health, wellbeing, occupational performance and general quality of life. For these reasons, it is important for occupational therapists to measure and follow the progress of the individual with outcome measures during daily living activities [17, 18, 21].

Social and physical isolation of the individuals can cause problems to access their own complex occupational and environmental worlds. Being cut from own life can cause limitations over occupational choices and experience, so environmental changes and supports during activity performance can provide opportunities, constraints and demands to the individual. The literature supports that individuals in secure environments spend much of their time in passive leisure, personal care and rest occupations and occupational therapists are one of the core elements in increasing activity participation of the individual and develop occupations of their choice [15, 16]. Also, occupational therapists can facilitate the exploration of new or unknown occupations to increase positive life experiences of the individual.

As it is given earlier, there is a great model need to understand the volition, habituation, performance skills, physical and social environments in which an individual's occupation takes place. MOHO assists the understanding of occupation(s) and problems of occupation that occur in terms of volition, habituation, performance capacity and environmental context. This system-based model includes well-designed assessments, observational, self-report and interview schedules. One of the advantages of this model is that because of its extensive use in mental health settings; a forensic version of an assessment tool 'Occupational Circumstances Assessment and Interview and Rating Scale' was designed. But, the literature also supports that this tool is not the only one for the use of occupational therapists, and occupational therapists may find any particular model, or standardized assessment/outcome measures to support their interventions [4, 15, 16, 21, 22].The literature supports that the use of occupational therapy models in forensic mental health may increase evidence-based practice and help the professionals to show the effect of occupational therapy. As Model of Human Occupation is seen to be the most used occupational therapy model, models including environmental and individual issues such as psychological issues, desires, wants, activity performance and satisfaction from the activity performance can help the occupational therapist to plan more effective assessments and interventions. Although the literature is still limited; different occupational models such as KAWA model, creative ability model, PEOP (Person, environment, occupation, performance), the Canadian model of occupational performance and engagement and the individual placement and support model with can be effectively used with standardized assessments and outcome measures in various individuals, situations, cultures and environments in provision of occupational therapy services in a cost-effective way [6, 15, 16].

in the business association during the presentation of information, evaluation and outcome

Occupational Therapy in Forensic Settings http://dx.doi.org/10.5772/intechopen.79366 61

Occupational therapists are aware that the occupational performance of an individual is influenced by factors related to the individual as well as by the performance patterns and the environment. Roles, routines, rituals, and habits constitute performance patterns [15]. Routines and habits allow the individual to perform his/her daily activities without thinking about how to move, without trying to remember. Occupational therapists working in forensic health services care about whether the prisoner has useful habits and routines for him. It is necessary to know how individuals spend their days and which routines they create from day to guide to get new routines and habits to use the time and energy more efficiently when the living conditions change [11]. Roles are the whole of the behaviors that an individual imposes on his/her responsibility, which is imposed by the environment and culture. Rituals are symbolic behaviors that are understood by social, cultural and spiritual values that shape the occupational identity of the individual. During the evaluations, the roles of the prisoner and the importance of these roles and the determination of meaningful rituals in the individual's life provide significant benefits for the therapist's intervention plan. Changes in location and time can also cause changes in the roles and rituals of individuals. The change in the role and ritual of the individual after conviction can cause occupational alienation in the individual. In the context of a forensic health service, acquisition of the prisoner's new skills and habits, and the new roles and rituals that are well integrated with the environment make an important contribution to the occupational balance of the

Understanding the environments in which occupational performance takes place, it is important for occupational therapists to understand the underlying effects of occupational participation. The environment includes dimensions related to physical, social (including individuals in the individual's life) and policies, and at the same time creates a supportive or restrictive effect for the occupational adaptation of the individual. Situations such as an absence of freedom for the individual, individual secrecy, and the meaningful and socially acceptable occupations constitute a barrier to prisoners' participation in their environment and occupation [15, 24]. Occupational therapists should also be thoroughly evaluating the environment of individuals who are establishing an intervention plan with prisoners apply-

We have already mentioned the preferred models for forensic occupational therapy applications. MOHO, one of these theories, includes structured and unstructured assessment and information gathering tools [3, 4, 11] for collecting and evaluating information about offend-

• Occupational performance history interview (OPHI II)—A semi-structured measure of

• Assessment of communication and interaction skills (ACIS)—evaluates three subdomain individuals, including the physical dimension of communication, information exchange,

self-care and information about the individual's life history;

and relationships, in an occupational pattern or in a social group [25];

measures, and the prisoner is actively involved in this process.

individual [3, 16, 23].

ers. Some of those:

ing to the occupational therapy service.


**5. Conclusions**

Hydrogen and fuel cell technology have advanced considerably over the last fifteen years. At the global level, this area continues to face significant challenges—technical, commercial and infrastructure-related—that need to be overcome before fuel cells can realize the full potential of which they are capable. Policy makers have included hydrogen and fuel cell on the map of future energy strategies and have already taken into account the fact that fuel cells have great real potential and can successfully meet the technical, social, economic and environmental objectives in the context of the multidisciplinary concept of sustainable development.

In this paper, the review of literature and agencies' reports on specialized metrics in the domain of the hydrogen fuel cell technologies, highlights the essential considerations regarding stationary applications, as follows:

The SWOT analysis conducted in the present study highlights that the implementation of the hydrogen economy depends decisively on the following main factors: legislative framework, energy decision makers, information and interest from the end beneficiaries, potential investors, and existence of specialists in this field.

**Author Contributions:** conceptualization, R.-A.F.; methodology, R.-A.F. and E.C.; validation, E.C., M.S.R. and C.F.; formal analysis, C.N.T.; investigation, M.S.R. and E.C.; data curation, C.T.; writing—Original draft preparation, R.-A.F., M.S.R. and C.N.T.; writing—Review and editing, E.C. and C.F.; visualization, R.-A.F.; supervision, C.F.; project administration, R.-A.F. and E.C.

**Acknowledgments:** This work was supported by a grant from the Romanian Ministery of Research and Innovation, CCCDI-UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0776/No. 36 PCCDI/15.03.2018, within PNCDI III.

**Conflicts of Interest:** The authors declare no conflict of interest.


**1. Introduction**

The COVID-19 pandemic struck in 2019 had caused varying effects on different sectors and industries. It affected educational systems worldwide, resulting in an almost complete closure of higher education institutions. Students and teachers were compelled to immediately adjust and switch to online teaching and learning activities. Universities also had to allow some flexibility when it comes to conducting examinations to eliminate in-person physical interaction.

Two years after the outbreak, all educational institutions are back open. However, there is no denying that everything is going back to how it used to be. The 'new normal' has forced us to move into digital, involving a hybrid education that combined face-to-face and virtual activities. Adaptation to new technologies seems obligatory and has become a part of the daily routine for educational systems globally. We can observe that online learning is on the rise and assessments can be conducted remotely.

With regard to medical education, e-learning helps students to adjust and adapt to an online medical environment. Yet, it limits students' interpersonal contact

with patients and opportunities for clinical practice and professional development. Nevertheless, the pandemic has brought a new insight that medical teaching and learning as well as student assessment can be conducted virtually. At Universiti Putra Malaysia, conducting online assessments during the pandemic made us realise the necessity to remain maximising the use of technology. The transition from the traditional method of assessment and the paradigmatic shifts are discussed further.

### **2. Student assessment in medical education**

Assessment is the process of documenting the level of a learner's knowledge, skills, and attitude and its purpose is to make judgement and decisions about a student's learning against a certain standard or benchmark [1]. Assessment can be classified as a formative or summative assessment. Formative assessment, also known as 'assessment for learning', is an ongoing process that aims to monitor student's learning. It is usually low stake and conducted informally in class. This assessment is a powerful diagnostic tool for students to pinpoint which areas they have mastered and which areas of weakness so they can concentrate their efforts in those areas moving forward. Constructive feedback on the strengths and weaknesses of students is the cornerstone of formative assessment to shape and improve future learning. In many cases, educators modify their instructional materials and clarify contents to ensure students to achieve the expected learning outcomes. Examples of formative assessment include short quizzes during class, direct observation of procedural skills (DOPS) and mini-clinical examination (mini-CEX).

Summative assessment, on the other hand, known as 'assessment of learning', takes place at the end of a course of study which is usually high-stake. The purpose is to provide an accurate pass-or-fail decision about students and a final measure of student performance. In health professions education, summative assessment is conducted to determine whether students have met the minimum standards during progression, graduation and licensure to assure that the public is protected from incompetent practitioners. Concurrently, medical educators may obtain feedback on the appropriateness of learning outcomes and the effectiveness of learning instruction based on post-assessment analysis [2]. Examples of summative assessments include those that occur at the end of a course, semester, year or before the newly graduated doctors can begin to practise medicine professionally.

Medical students must acquire and demonstrate various domains of competency throughout the training. However, there is no single method of assessment that can adequately evaluate their performance across all domains. Each assessment method has its own advantages and disadvantages. Therefore, a variety of assessment methods are required to ensure that students achieve all required competencies before graduation.

More than 30 years ago, psychologist George Miller proposed a hierarchical framework for assessing clinical competence [3]. It is a valuable model showing the levels of knowledge and skills assessed in medical education. The iconic Miller's pyramid model divides between assessment of cognition and behaviour in practice (**Figure 1**). The base of the pyramid is knowledge ('knows'), followed by the application of knowledge ('knows how'). Acquiring medical knowledge is the essential precursor for clinical problem-solving. The 'knows' level can be assessed by written assessment such as multiple-choice questions (MCQs) while 'knows how' add a level of complexity to the cognitive scheme. Students need to apply their knowledge,

*Perspective Chapter: Paradigm Shift on Student Assessment Due to COVID-19 Pandemic... DOI: http://dx.doi.org/10.5772/intechopen.109555*

#### **Figure 1.** *Miller's pyramid [3].*

manipulate the information, and demonstrate an understanding of the relationship between concepts and applications [1]. Appropriate assessment methods include higher-order MCQs, essay and viva or oral exams.

The third level of the pyramid moves the method of assessment to performance assessment and represents clinical skills competency, usually assessed under a controlled environment ('shows how'). The assessments are rather simulated and standardised. Objective structured clinical communication (OSCE) is an example of assessment, in which students may demonstrate clinical skills such as communication or performing a physical examination on a simulated patient. Finally, the top of the pyramid is clinical performance, assessed by direct observation in authentic clinical settings ('does'). Examples of assessments include workplace-based assessments such as mini-CEX or DOPS where students demonstrate clinical performance with actual patients by integrating their knowledge, skills and abilities in the real-world clinical setting.

Miller's pyramid is frequently used with other taxonomy frameworks such as Bloom's revised taxonomy. Bloom's taxonomy encompasses six levels of the cognitive domain, from the lowest level, which is remembering information, up to successively more to complex higher-order levels, which is creating [4]. The taxonomy model is useful while deciding expected cognitive outcomes and constructing written assessment items.

In selecting the appropriate assessment methods in the program, the purpose of the assessment should be considered. *Is it for formative or summative purposes?* As stated above, there are different levels of clinical competence are required. *Are the different assessment methods cover all clinical competence in Miller's pyramid? Are they adequate?*

In 1996, van der Vleuten proposed a conceptual model for defining the utility of an assessment method [5]. There are five criteria of an assessment method involved in this model, namely reliability (*does it consistently measure what it is supposed to?*), validity (*does it measure what it is purported to?*), educational impact (*how does it affect teaching and learning?*), acceptability (*is it acceptable to relevant stakeholders?*) and cost (*does it practical and feasible?*). Using this model, the utility of an assessment method can be derived by conceptually multiplying all the weights of each criterion.

*Assessment utility reliability validity educ* = ×× × × *ational impact acceptability cost* (1)

It is important to note that this is not a mathematical formula, but a notional one. The weight of each criterion depends on the purpose of the assessment. For formative purposes, more weight is given to educational impact while for summative purposes, more weight is given to reliability [6].

Later, Norcini et al. published a consensus statement identifying seven criteria of a good assessment. Five of them were derived from van der Vleuten's model, while another two are equivalence (*does it produce similar results in different groups?*) and catalytic effect (*does it create, enhance, and support education?*) [7]. Therefore, it is essential to evaluate these criteria when considering the appropriate and suitable assessment methods or tools in the programme.

#### **2.1 Assessment of knowledge acquisition and application**

Written assessments are widely used in medical education to assess knowledge acquisition, comprehension of basic principles and clinical reasoning. Although these skills are positioned at the base of Miller's pyramid ('knows' and 'knows how'), they form a foundational set of skills that students need to master prior to achieving clinical competence. They are inexpensive, convenient and produce reliable scores. There are many types of written assessment commonly used for medical students which will be covered in the next section.

#### *2.1.1 Multiple choice questions (MCQs)*

This is certainly the most popular assessment method globally because of its validity, reliability and practicality. The A-type MCQs require examinees to select one best answer from several options. They are also known as single-best answer questions (SBAQs) or one-best answer (OBA) questions. The question consists of a stem, which can be a clinical or non-clinical vignette, a lead-in statement and three or more answer options.

The R-type MCQs, also called as extended matching items (EMIs) or extended matching questions (EMQs), are an extended version of the A-type format. In a set of EMIs, there is a theme of the questions, a list of options (can be from seven to 20), a lead-in statement and a minimum of two items or vignettes. All items should be relevant to the theme. For each item, examinees choose the correct answer from the list of options. Both A-type and R-type MCQs can be used to assess the theory and application of knowledge, critical thinking and problem-solving skills.

Multiple true false (MTF) questions are becoming less popular among medical schools. They are normally used to test factual recall. However, this type of assessment is able to cover more breadth of a topic, which is suitable to be used in formative assessment. Each item consists of a stem, followed by five statements related to the stem. For each statement, examinees may select either true or false. In a pen-andpaper examination, optical mark recognition sheets, better known as OMR sheets used by the examinees to mark their answers. Those sheets are analysed by an OMR machine and the scores can be obtained instantly. Certain OMR machines can also perform a concurrent evaluation of the quality of the questions based on the examinee's response and scores.

*Perspective Chapter: Paradigm Shift on Student Assessment Due to COVID-19 Pandemic... DOI: http://dx.doi.org/10.5772/intechopen.109555*

#### *2.1.2 Short answer questions (SAQs) and essay questions*

This type of assessment consists of open-ended questions which require the examinees to write either brief or long answers in SAQs or essays, respectively. They can either assess lower-order or higher-order thinking. Usually, examinees are assessed on the application of knowledge ('knows how'), and clinical reasoning. In both methods, the disadvantage is that they have to be marked manually by examiners. It can be resource intensive with a large number of examinees per cohort, particularly for essay questions. In certain cases, the answer scripts are marked by more than one examiner, based on the answer scheme. Although it can reduce the examiner's workload, this may affect the reliability of the scores with multiple examiners per question.

#### **2.2 Assessment of clinical performance**

#### *2.2.1 Objective structured clinical examination (OSCE)*

OSCE consists of several structured stations in a circuit in which an examinee moves in sequence. The number of stations and the duration for each station can vary based on the complexity of the skills being assessed [8]. It allows examinees to demonstrate a specific clinical skill in each station in a standardised medical scenario. It is usually conducted in summative assessment. OSCE is widely implemented due to its high validity and reliability to assess across different cases and skills. A large number of students can be assessed in the same way with multiple concurrent circuits. The use of standardised or simulated patients is common during OSCE so that examinees may interact with them to perform history taking, physical examination, counselling and others. There will be an examiner at each station to observe and score the examinees based on a pre-determined checklist.

#### *2.2.2 Long case and short case examinations*

The long case is a traditional clinical examination that assesses student competence at the 'shows how' level in Miller's pyramid. It requires a student to spend approximately an hour with a patient, taking history and carrying out a physical examination, unobserved. Then, the student summarises the findings to one to three examiners and answers several questions. The examiners score the student using unstructured marking criteria. Although many concerns regarding its reliability [9], the long case is still popular due to its authenticity and ability to assess clinical approach holistically. To increase the validity and reliability of long cases, several modifications have been implemented such as observing students while they interact with a patient, using a structured marking scheme and increasing the number of cases [10].

Short case, on the other hand, requires a student to spend about 5–10 minutes with a patient to examine the patient and detect signs under observation. Then, the student needs to formulate a clinical or differential diagnosis of the patient. Similar to long case, the student is scored according to unstructured marking criteria. In many medical schools, the introduction of OSCE has replaced long-case and short-case examinations, especially for high-stake examinations.

#### *2.2.3 Workplace-based assessment (WBA)*

WBA encompasses a group of assessment methods that evaluates students' performance in an actual clinical setting. Examples of WBA include mini-clinical evaluation exercise (mini-CEX), direct observation of procedural skills (DOPS) and case-based discussion (CBD). These assessment methods have high authenticity and are located at the tip of Miller's pyramid ('does'). They are usually conducted as formative assessments with the main aim to aid learning through feedback.

Mini-CEX expects a student to conduct a focused clinical skill such as history taking or physical examination with an actual patient within a short and specified time. The performance is graded using a structured evaluation form and constructive feedback is provided. This assessment occurs on multiple occasions in daily practice with different assessors and in different settings. DOPS is a variation on the mini-CEX, which focuses mainly on procedural skills. It is specifically designed to evaluate practical skills for example in surgical, medical or general practice against pre-determined criteria, followed by a face-to-face feedback session.

On the other hand, CBD is a focused discussion driven by an existing case the student has encountered. The discussion centres on what was done, why it was done and how any investigation and intervention was made. After the discussion, the assessor scores the quality of performance and provides constructive feedback.


**4. Conclusions**

In this paper, several virgin and extra-virgin olive oils from Italy, mainly from the Italian central region of Tuscany, were studied to determine their pigments' content, quantified by means of two different methods of analysis based on near-ultraviolet-visible absorption spectroscopy. Extra-virgin olive oil samples produced from cultivars typical of Tuscany, such as Frantoio, Moraiolo, Leccino, and Pendolino, were investigated in terms of pigments' content. The first approach proposed by Mínguez-Mosquera et al. [11] defines two indexes, K670 and K470, related to single absorbance values at wavelengths of 670 and 470 nm, respectively. To our best knowledge, this method has never been validated, but, since it is relatively fast and simple, it has been widely used to estimate the total concentration of chlorophylls' derivatives and the total concentration of carotenoids in several works on the chemical-physical characterization of olive oils [11,13,16,32,33]. The second approach, proposed by Domenici et al. [17], is based on a mathematical deconvolution of the whole near UV-vis absorption spectrum of the oil, recorded in the bulk without any sample treatment, to obtain the concentrations of four main pigments present in olive oils: β-carotene, lutein, pheophytin A, and pheophytin B. This spectroscopic method has been validated in previous works [17–21,24] and it can be considered a robust, precise, and reproducible spectroscopic analytical method for main pigments' determination in not fresh olive oils. The main outputs of the present study, can be here summarized: 1. The calculation of the two K670 and K470 indexes according to the equations proposed by Mínguez-Mosquera et al. [11] underestimates both the content of carotenoids and the content of chlorophylls' derivatives, with respect to the deconvolution method proposed by Domenici et al. [17]; 2. A good linear correlation between the values of concentration of chlorophylls' derivatives obtained by means of the two methods was observed in a relatively large concentration range, which can be considered significant for virgin and extra-virgin olive oils; 3. In the case of the total amount of carotenoids, the linear correlation was worse, especially in the range between 1 and 3 ppm; 4. The statistical analysis of these data showed that the differences between the two methods were statistically significant; 5. The method proposed by Domenici et al. [17] is recommended for the quantification of the total amount of carotenoids and the total amount of chlorophylls' derivatives in the case of not fresh olive oils, with respect to the method proposed by Mínguez-Mosquera et al. [11].

The optimization of new spectroscopic methods and their comparison in terms of pigments' quantification, as reported in this work, are justified by the continued request of fast, cheap, and not-destructive methods to characterize olive oils and their quality. A further development is represented by the possibility to provide simple methods to be implemented in portable devices in order to check olive oils' quality and authenticity at the service of consumers and producers.

**Author Contributions:** Conceptualization, methodology and supervision: V.D.; experimental investigation and analyses of data: E.B. Both authors contributed in the writing and reviewing the final manuscript.

**Funding:** This research received no external funding. **Acknowledgments:** Authors thank Mario Cifelli for helpful discussions and Beatrice Massaza from "Agriturismo Santa Annunziata" in San Vincenzo (LI) for providing us part of the extra-virgin olive oil samples.

**Conflicts of Interest:** The authors declare no conflict of interest.


*<IP_ADDRESS> Urban class extraction*

Urban class is defined in this case as 'all manmade features including buildings, roads, and pavements in addition to vegetation covered areas such as oases, farms, parks, and farmed areas within the city boundary'. To extract urban areas, a bitmap is created to include urban and non-urban only.

## *<IP_ADDRESS> Spatial metrics*

The application of a number of spatial metrics is used to characterize the urban growth of the city. As such, the following spatial metrics are used:


**2. Photonic crystal fibers**

The authors of the paper in 1996 [3] were looking for photonic bandgap guidance in the defect core of the PCF with a core size of ~4.6 μm. The reason for the absence of photonic bandgap guidance in a solid-core PCF with air holes in the cladding only became clear a few years later. To their surprise, they observed very broadband single-mode guidance from 458–1550 nm (extended to 337-1550 nm in [4]). A conventional single-mode step-index optical fiber would become multimode at shorter wavelength. Even more surprising, the diffraction angle of the output beam was smaller at shorter wavelengths, indicating a smaller fiber numerical aperture at short wavelength.

#### **2.1. Fundamental space-filling mode of photonic crystal cladding**

In a follow-on paper [4] the initial observation of the "endlessly single-mode" nature of photonic crystal fibers was explained by the dispersive nature of the cladding, which can be viewed as a composite of two materials, i.e. silica and air. The effective refractive index of the composite cladding was identified for the first time as the effective index of the fundamental mode of the composite cladding, referred to as the *fundamental space-filling mode (FSM)*. In the extreme case of conventional optical fibers with an infinite cladding, the effective index of the fundamental space-filling mode becomes the refractive index of the cladding. In PCFs with air holes, the cladding index ncl, i.e. the effective index of the fundamental space-filling mode, is lower than the refractive index of the background glass nb due to the existence of the air holes, i.e. ncl<nb. The core index nco is the same as the index of the background glass, nco=nb. Funda‐ mental optical guidance in PCFs can, therefore, in principle, be understood in a similar way to that in conventional optical fibers (see Figure 1).

This paper [4] established the principle for understanding the basic guidance properties of PCFs. In conventional optical fibers, the refractive indexes of core and cladding are only weakly dependent on wavelength due to material dispersion. This is also expected of the core refractive index of a PCF. The refractive index of the composite photonic crystal cladding, however, behaves very differently. At the long wavelength extreme, i.e. λ→∞, the wavelength is much larger than the air holes and the fundamental space-filling mode will occupy all areas

**Figure 1.** Refractive index of photonic crystal fiber cladding.

propagation of a higher-order mode in a specially designed multimode fiber [11, 12]. It is argued that perturbations mostly have anti-symmetry in optical fibers and promote mode coupling mostly between modes of opposite symmetries. The mode spacing between a radially symmetric LP0n mode and its nearest neighbor modes with opposite symmetry is, in fact, larger for higher order modes. These higher-order modes are also more resistant to bendinduced mode compression. A special fiber design facilitates the ease of mode conversion to

In this chapter, we will give a brief introduction to the key approaches to effective mode area scaling which have shown great promise for future high power fiber lasers. Basic concepts are

The authors of the paper in 1996 [3] were looking for photonic bandgap guidance in the defect core of the PCF with a core size of ~4.6 μm. The reason for the absence of photonic bandgap guidance in a solid-core PCF with air holes in the cladding only became clear a few years later. To their surprise, they observed very broadband single-mode guidance from 458–1550 nm (extended to 337-1550 nm in [4]). A conventional single-mode step-index optical fiber would become multimode at shorter wavelength. Even more surprising, the diffraction angle of the output beam was smaller at shorter wavelengths, indicating a smaller fiber numerical aperture

In a follow-on paper [4] the initial observation of the "endlessly single-mode" nature of photonic crystal fibers was explained by the dispersive nature of the cladding, which can be viewed as a composite of two materials, i.e. silica and air. The effective refractive index of the composite cladding was identified for the first time as the effective index of the fundamental mode of the composite cladding, referred to as the *fundamental space-filling mode (FSM)*. In the extreme case of conventional optical fibers with an infinite cladding, the effective index of the fundamental space-filling mode becomes the refractive index of the cladding. In PCFs with air holes, the cladding index ncl, i.e. the effective index of the fundamental space-filling mode, is lower than the refractive index of the background glass nb due to the existence of the air holes, i.e. ncl<nb. The core index nco is the same as the index of the background glass, nco=nb. Funda‐ mental optical guidance in PCFs can, therefore, in principle, be understood in a similar way

This paper [4] established the principle for understanding the basic guidance properties of PCFs. In conventional optical fibers, the refractive indexes of core and cladding are only weakly dependent on wavelength due to material dispersion. This is also expected of the core refractive index of a PCF. The refractive index of the composite photonic crystal cladding, however, behaves very differently. At the long wavelength extreme, i.e. λ→∞, the wavelength is much larger than the air holes and the fundamental space-filling mode will occupy all areas

and from the LP0n mode using a *long period grating* (LPG).

222 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

introduced and the latest developments are also discussed.

**2.1. Fundamental space-filling mode of photonic crystal cladding**

to that in conventional optical fibers (see Figure 1).

**2. Photonic crystal fibers**

at short wavelength.

of the cladding equally. In this case, the refractive index of the FSM can be obtained by averaging the square of the refractive index of the composite photonic crystal cladding, i.e. ncl 2 =nb 2 (1-F)+nair2 F=nb 2 -F(nb 2 -nair2 ) where F is the area fraction of air; and nair is the refractive index of air. The effective NA of the PCF for λ→∞ is, therefore, F1/2(nb 2 -nair2 ) 1/2. At the short wavelength extreme, i.e. λ→0, the wavelength is much smaller than the pitch of the air holes and the fundamental space-filling mode is mostly in the region with high refractive index, i.e. the background glass. In this case, the cladding index is the refractive index of the background glass nb. The effective NA of a PCF at λ→0 trends to zero. The guidance of a PCF still gets weaker at large λ, as in a conventional optical fiber, due to the inverse wavelength dependence of the normalized frequency V. In addition, its guidance also gets weaker at short wavelengths due to the diminishing NA, which also limits the growth of V and leads to the "endlessly singlemode" nature of the PCFs with small air holes. Guidance of PCF will, therefore, diminish at both long and short wavelength ends, possessing two bend-induced cut-off edges.

#### **2.2. Single mode and multimode regime of photonic crystal fibers**

In conventional fibers, a mode is guided when the effective mode index neff is between the core and cladding index, i.e. nco>neff>ncl. The mode cut-off can be identified when neff=ncl. The second-order mode cuts off in a step-index fiber at V=2.405. In the first reported work on determining the single-mode regime of PCFs with a 1-cell core, i.e. one hole missing at the defect core, by Birks et al [4], a somewhat arbitrary equivalent step-index core radius equaling *pitch*Λ, i.e. center-to-center hole separation, was used to calculate the V value. Work by Saitoh [13] using a *finite element model* (FEM) to determine nFSM and the condition neff=ncl to determine the second-order mode cut-off, put the effective step-index core radius to be Λ/31/2 for a onecell core. Later work by Saitoh [14] determined the effective step-index core radius to be Λ for a 3-cell core and 21/2Λ for a 7-cell core.

The optical waveguide equation allows the scaling of all parameters measured in the length scale by the same factor. For PCF, the most convenient scaling factor is 1/Λ. The second-order mode cut-off is typically plotted as a normalized wavelength λ/Λ versus normalized hole diameter d/Λ plot. This is shown for PCFs with 1-cell, 3-cell and 7-cell core in Figure 2 [14]. For each of the curves, the area above the curve is in the single-mode regime, i.e. the wave‐ lengths above the second-order mode cut-off wavelength, and, below it, multimode regime. The "endlessly single-mode" nature of PCFs can be easily identified in Figure 2. When d/ Λ<0.424, 0.165 and 0.046 respectively for 1-cell, 3-cell and 7-cell core PCFs, the PCFs will remain in the single-mode regime for all wavelengths. It is worth noting that Figure 2 is for PCFs with infinite cladding. For PCFs with finite cladding, guidance is weaker and the second-ordermode cut-off is expected to happen at slightly shorter wavelengths. The curves in Figure 2 are expected to move downwards slightly. It needs to be noted as discussed earlier, that the PCFs are too weak to guide any light at long and short extremes of wavelength. The critical bend radius at the short wavelength edge is determined to be dependent on pitch Λ and wavelength λ such that [4]

$$\mathcal{R}\_c \ll \frac{\Lambda^3}{\lambda^2} \tag{1}$$

The critical bend radius at the short length edge increases in proportion to 1/λ2 as wavelength decreases. This relation was verified experimentally for the critical bend radius at 3dB excessive bend loss [4]. It comes directly from the dispersive nature of the photonic crystal cladding.

**Figure 2.** The second-order mode cut-off determined by neff=nFSM (dots) and V=2.405 (solid lines) using core radius ρ=Λ/31/2, Λ and 21/2Λ respectively for 1-cell, 3-cell and 7-cell cores [14].

#### **2.3. Waveguide loss of photonic crystal fibers**

PCFs are intrinsically leaky, i.e. there is always a finite waveguide loss for each mode in a PCF with a finite cladding. The waveguide loss can be found by calculating the imaginary part of the effective mode index using a numerical mode solver. When plotting the waveguide loss versus wavelength, the slope is expected to change around cut-off. This can also be used to determine the second-order-mode cut-off. This was done for a 1-cell core PCF [15]. The results are consistent with those in [13].

In conventional optical fibers, the fully enclosed core and cladding boundary ensures that waveguide loss is zero for all guided modes, i.e. those that satisfy the conditions for total internal reflection at the boundary. For the open structure of PCFs with a finite cladding, all modes are leaky with finite waveguide losses. At the extreme of an infinite number of layers, the waveguide losses are zero. The waveguide losses are lower for larger air holes and can be substantially lowered by increasing the number of air-hole layers. In practice, the waveguide loss can be made below other material and process related losses. By employing appropriate polishing, etching and dehydration processes, a PCF with a loss 0.28dB/km at 1550nm has been demonstrated [16]. For applications in fiber lasers where a length of not more than a few tens of meters is used, the waveguide loss does not present any problem if appropriate designs are used.

### **2.4. Modeling of photonic crystal fibers**

lengths above the second-order mode cut-off wavelength, and, below it, multimode regime. The "endlessly single-mode" nature of PCFs can be easily identified in Figure 2. When d/ Λ<0.424, 0.165 and 0.046 respectively for 1-cell, 3-cell and 7-cell core PCFs, the PCFs will remain in the single-mode regime for all wavelengths. It is worth noting that Figure 2 is for PCFs with infinite cladding. For PCFs with finite cladding, guidance is weaker and the second-ordermode cut-off is expected to happen at slightly shorter wavelengths. The curves in Figure 2 are expected to move downwards slightly. It needs to be noted as discussed earlier, that the PCFs are too weak to guide any light at long and short extremes of wavelength. The critical bend radius at the short wavelength edge is determined to be dependent on pitch Λ and wavelength

224 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

Rc∝ Λ3

decreases. This relation was verified experimentally for the critical bend radius at 3dB excessive bend loss [4]. It comes directly from the dispersive nature of the photonic crystal

**Figure 2.** The second-order mode cut-off determined by neff=nFSM (dots) and V=2.405 (solid lines) using core radius

PCFs are intrinsically leaky, i.e. there is always a finite waveguide loss for each mode in a PCF with a finite cladding. The waveguide loss can be found by calculating the imaginary part of the effective mode index using a numerical mode solver. When plotting the waveguide loss versus wavelength, the slope is expected to change around cut-off. This can also be used to determine the second-order-mode cut-off. This was done for a 1-cell core PCF [15]. The results

In conventional optical fibers, the fully enclosed core and cladding boundary ensures that waveguide loss is zero for all guided modes, i.e. those that satisfy the conditions for total

ρ=Λ/31/2, Λ and 21/2Λ respectively for 1-cell, 3-cell and 7-cell cores [14].

**2.3. Waveguide loss of photonic crystal fibers**

are consistent with those in [13].

The critical bend radius at the short length edge increases in proportion to 1/λ2

λ2 (1)

as wavelength

λ such that [4]

cladding.

Due to the complexity of the geometrical structures of PCFs, numerical models are typically used to find mode properties including the effective mode index, waveguide loss and effective mode area. For any numerical mode solvers, electric and magnetic fields are described by an expansion series. Eigenvalue equations are then established by requiring the fields to satisfy all boundary conditions. These equations are typically expressed as a set of linear equations, which can be solved for complex effective mode index. The more complex a waveguide design is, the larger is the number of linear equations. The waveguide dispersion can be calculated from the real part of the complex effective mode index. The waveguide loss can be obtained from the imaginary part of the complex effective mode index. Electric and magnetic fields can be calculated once the effective mode index is determined. The most common and flexible numerical mode solver is a *finite element mode solver* (FEM). This is commercially available from COMSOL (*http://www.comsol.com/*). A FEM is capable of handling practically any design. It can, however, be computationally very demanding. If only circular boundaries are involved, a Multipole mode solver is a good option [17, 18]. It is based on the decomposition of fields into circular Bessel series, which are the most accurate and efficient method for modeling circular boundaries. For non-commercial research and teaching purposes, it can be downloaded from the University of Sydney website (*http://sydney.edu.au/science/physics/cudos/research/mofsoft‐ ware.shtml*). A plane wave expansion method can also be used. This, however, assumes an infinite cladding and, therefore, cannot determine the waveguide loss.

#### **2.5. Mode area scaling with photonic crystal fibers**

The first demonstration of a large-core PCF was performed by Knight el at [5]. The 1-cell PCF with relative hole diameter d/Λ≈0.12 and a core diameter of 2ρ=22.5μm, provided robust single-mode guidance at 458nm. According to Figure 2, a 1-cell PCF with d/Λ<0.424 is expected to be single-mode over its entire wavelength range. It is, therefore not surprising that the PCF was single mode at 458nm. The critical bend radius was measured to be 50cm at 458nm and 4cm at 1.55μm [5]. This fiber is expected to have a critical bend radius of ~10cm at 1μm using equation 1.

Considering the wavelength scalability of the waveguide equation, this 1-cell PCF with 2ρ/ λ≈50 can be scaled by a factor of ~2.2 to a ~50μm core diameter to operate at 1μm. The critical bend radius for this single-mode PCF with 50μm core diameter is expected to be ~1.1m at 1μm according to equation 1! A similarly scaled 1-cell PCF with a core diameter of 30μm will have a critical bending radius of ~24cm. This is probably close to the practical limit of coiled singlemode 1-cell PCFs. Above a core diameter of ~40μm, 1-cell single-mode PCF can only be used in a straight configuration in practice. The single-mode operation of PCFs with large core diameters comes at the cost of weak guidance as a result of the diminishing effective NA. This fundamentally limits the use of single-mode PCFs with large cores in coiled configurations. If PCFs are allowed to operate in the few-moded regime, coiled PCFs with slightly larger core diameters are possible. For high average power fiber lasers with outputs exceeding kW, effective thermal management becomes increasingly critical. Long fiber lengths of many meters are required. The constraint of not being able to coil the fibers can become a major issue considering the additional space constraints.

#### **2.6. Rare-earth-doped glass for large-core photonic crystal fibers**

The diminishing NA of PCFs with large cores also has additional implications, as realized by the authors of [5]. To fabricate rare-earth-doped large-core PCFs, the core refractive index needs to be accurately controlled at levels far below the very small effective index difference between the core and cladding in order not to disturb the guidance properties of the PCFs. This requires much improved techniques for the fabrication of the active core in large-core PCFs.

The first ytterbium-doped large-core PCF with 15μm core diameter was reported in 2001 at Bath University [19]. The effective mode area at 1μm was ~100μm2 . The laser operated in single mode with poor efficiency. The key advance was the use of a repeated stack-and-draw process to achieve a uniformly doped core with a refractive index close to silica. Conventional fabrication techniques for rare-earth doped silica fibers result in significant non-uniformity in refractive index across the core as well as a raised refractive index above silica. Due to the weak guidance in large-core PCFs, any index non-uniformity across the core can lead to mode distortion. It also requires the refractive index of the core to be very close to the silica back‐ ground in PCFs. The authors of [19] fabricated ytterbium-doped glass with low doping levels. The doped glass is surrounded by some undoped silica glass. The glass is stacked and drawn twice before being finally incorporated into the core of a PCF. The resulting PCF has an ytterbium-doped core which consists of 425 original doped glass sections. The dimension of each of the original doped glass sections is much smaller than the wavelength of light and the core of the PCF, therefore, appears to be homogeneous to light at the operating optical wavelength. The doped core is also heavily diluted by the addition of silica glass (90%), leading to an average refractive index close to that of silica [20]. In the same paper, it was pointed out that rare-earth-doped glass with a higher index can be stacked and drawn with undoped glass with a low index to achieve a better match to that of silica background [20].

#### **2.7. Double-clad photonic crystal fibers with high NA air-clad for pump guidance**

The concept of a composite air-glass clad with a high air-filling fraction to provide a high NA pump guide in a double-clad fiber was first proposed in 1999 at what was then then Lucent Technologies [21]. The high pump NA can enable a significant improvement in pump coupling especially from low brightness multimode diode lasers for a given pump waveguide dimen‐ sion. In conventional double-clad fibers, low-index polymer coatings are typically used to form the pump cladding. The air-clad pump isolates high pump powers from the polymer coatings, leading to potentially improved reliability.

according to equation 1! A similarly scaled 1-cell PCF with a core diameter of 30μm will have a critical bending radius of ~24cm. This is probably close to the practical limit of coiled singlemode 1-cell PCFs. Above a core diameter of ~40μm, 1-cell single-mode PCF can only be used in a straight configuration in practice. The single-mode operation of PCFs with large core diameters comes at the cost of weak guidance as a result of the diminishing effective NA. This fundamentally limits the use of single-mode PCFs with large cores in coiled configurations. If PCFs are allowed to operate in the few-moded regime, coiled PCFs with slightly larger core diameters are possible. For high average power fiber lasers with outputs exceeding kW, effective thermal management becomes increasingly critical. Long fiber lengths of many meters are required. The constraint of not being able to coil the fibers can become a major issue

The diminishing NA of PCFs with large cores also has additional implications, as realized by the authors of [5]. To fabricate rare-earth-doped large-core PCFs, the core refractive index needs to be accurately controlled at levels far below the very small effective index difference between the core and cladding in order not to disturb the guidance properties of the PCFs. This requires much improved techniques for the fabrication of the active core in large-core

The first ytterbium-doped large-core PCF with 15μm core diameter was reported in 2001 at

mode with poor efficiency. The key advance was the use of a repeated stack-and-draw process to achieve a uniformly doped core with a refractive index close to silica. Conventional fabrication techniques for rare-earth doped silica fibers result in significant non-uniformity in refractive index across the core as well as a raised refractive index above silica. Due to the weak guidance in large-core PCFs, any index non-uniformity across the core can lead to mode distortion. It also requires the refractive index of the core to be very close to the silica back‐ ground in PCFs. The authors of [19] fabricated ytterbium-doped glass with low doping levels. The doped glass is surrounded by some undoped silica glass. The glass is stacked and drawn twice before being finally incorporated into the core of a PCF. The resulting PCF has an ytterbium-doped core which consists of 425 original doped glass sections. The dimension of each of the original doped glass sections is much smaller than the wavelength of light and the core of the PCF, therefore, appears to be homogeneous to light at the operating optical wavelength. The doped core is also heavily diluted by the addition of silica glass (90%), leading to an average refractive index close to that of silica [20]. In the same paper, it was pointed out that rare-earth-doped glass with a higher index can be stacked and drawn with undoped glass

. The laser operated in single

considering the additional space constraints.

PCFs.

**2.6. Rare-earth-doped glass for large-core photonic crystal fibers**

226 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

Bath University [19]. The effective mode area at 1μm was ~100μm2

with a low index to achieve a better match to that of silica background [20].

**2.7. Double-clad photonic crystal fibers with high NA air-clad for pump guidance**

The concept of a composite air-glass clad with a high air-filling fraction to provide a high NA pump guide in a double-clad fiber was first proposed in 1999 at what was then then Lucent Technologies [21]. The high pump NA can enable a significant improvement in pump coupling The first such fiber was demonstrated in 2000, where the pump cladding was mostly air except for a single connecting element [22]. The measured pump NA over 1m was below 0.2. The fiber was passive and there was no laser demonstration in this first attempt. The first claddingpumping demonstration in an active fiber with an air-clad pump guide and a conventional core design was in 2001 at Southampton University [23]. The measured NA of the pump guide with air-cladding was 0.4-0.5. In a separate demonstration from Southampton University later in the same year, a conventional core with a very low NA together with a photonic crystal cladding was used in combination with a pump guide with air-cladding [24]. The core guidance came from a combination of the raised core index and the photonic crystal cladding. Cladding pumping with a low brightness laser diode at 915nm (100μm core with a NA=0.22) was demonstrated with a slope efficiency of 70% relative to the absorbed pump power. The measured pump NA over a short length (~10cm) was 0.3-0.4. NA over much longer fiber length as in a fiber laser is expected to be lower. The first true active double-clad photonic crystal fibers with a pump air-cladding was demonstrated in 2003 with a measured pump NA of 0.8 at 1μm [20].

The theoretical basis for the high NA of optical waveguides formed by air-cladding was established in [25]. It had been understood previously that the effective index of a glass and air composite can be obtained by the effective index of the fundamental space filling mode. An example of high NA air-clad used in double-clad fibers is shown in Figure 3 from [25]. In such air-glass composites, the glass webs can be considered as slab waveguides. The separa‐ tions between the webs are typically far enough that the webs can be considered as isolated. In this case, the effective index of the glass-air composite cladding can be well approximated by that of the fundamental slab waveguide mode. This can be easily calculated. The resulting pump NA is only dependent on the ratio of the web width w and wavelengthλ. This is plotted in Figure 3(c), showing that a small w/λ is critical for high NA.

**Figure 3.** (a) High NA air-clad from [25], (b) close-up of the high NA air-clad, and (c) NA of air-clad waveguide based on slab waveguide model plotted against the ratio of waveguide width w and wavelength λ.

### **2.8. Progress of active large-core photonic crystal fibers**

Since 2003, the group at Friedrich-Schiller-University Jena has been playing a significant role in the development large-mode-area photonic crystal fibers with pump air-clad. In their first work in collaboration with then Crystal Fibre A/S, now NKT Photonics, an ytterbium-doped PCF with effective mode area of ~350μm2 and mode field diameter (MFD) of 21μm was demonstrated [26] (see Figure 4). The 3-cell PCF had a hole diameter d=2μm and pitch Λ=11.5, giving an d/Λ=0.18. The core diameter was 28μm. A relatively small circular area of 9μm was doped in the 3-cell core. The doped area had a high ytterbium-doping level of ~0.6 at%. It was further co-doped with aluminum and fluorine to provide an index merely 2×10-4 above the silica background. The PCF with the raised index in the doped area was simulated, showing that the fiber guides the second-order mode, which is, however, close to cut-off. The fiber had a 150μm pump guide with an air clad. The webs in the air clad were ~50μm long with a thickness of ~390nm, giving w/λ=~0.4 at 976nm. The measured pump NA was 0.55, slightly below the NA=0.68 predicated by the slab model in Figure 3. The fiber had an outer glass diameter of 450μm and was coated with standard acrylic coating. Due to the high doping level, the fiber has a pump absorption of ~9.6dB/m at 976nm. An impressive slope efficiency of 78% was demonstrated with respect to the launched pump power.

Figure 4 SEM image of the air‐clad ytterbium‐doped large‐mode‐area photonic crystal fiber in [26]. **Figure 4.** SEM image of the air-clad ytterbium-doped large-mode-area photonic crystal fiber in [26].

In a subsequent paper in 2003 [27], the fiber in [26] and a new fiber with narrower pump air‐clad were studied with the FEM for temperature distributions in the fiber at various thermal loads in the core, considering both convective and radiative heat transportation by air at the fiber surface. The results show that, in the case of natural cooling, the thermal transportation is mainly limited by the heat transfer at the fiber surface. The air‐clad impedes heat flow, especially when the width of the air clad is large. A narrower pump air‐clad is advantageous, especially in actively cooled fiber lasers. In a subsequent paper in 2003 [27], the fiber in [26] and a new fiber with narrower pump airclad were studied with the FEM for temperature distributions in the fiber at various thermal loads in the core, considering both convective and radiative heat transportation by air at the fiber surface. The results show that, in the case of natural cooling, the thermal transportation is mainly limited by the heat transfer at the fiber surface. The air-clad impedes heat flow, especially when the width of the air clad is large. A narrower pump air-clad is advantageous, especially in actively cooled fiber lasers.

In 2004, the bar was raised in a collaborative work between Friedrich Schiller University Jena and Crystal Fibre A/S [28]. Their 7-cell fiber has an effective area of ~1000μm2 and MFD of ~35μm (see Figure 5). Core diameter is ~40μm. Hole diameter d is 1.1μm and the pitch Λ is 12.3μm, giving a d/Λ=0.09. The fiber has a pump guide diameter of 170μm and a measured pump NA of 0.62 at 950nm. The pump absorption is ~13dB/m at 976nm. The pump air-clad is

Figure 5 Microscope image of the air‐clad ytterbium‐doped single‐mode PCF and close‐up to the 40‐μm core formed by seven missing air holes [28].

Figure 6 Microscope image of a rod‐type photonic crystal fiber and close‐up view of the inner cladding and core regions.

To further mitigate bend loss, a rod‐type PCF was developed in 2005 [29] (see Figure 6). The rod had an outer diameter of 1.7mm so that it cannot be bent. A 48cm long length was used in the demonstration. The relative hole diameter was increased to d/=0.33. This reduces bend sensitivity. This larger d/ in a 7‐cell core PCF, however, puts this fiber firmly in the multimode regime (see Figure 2). The pump guide was reduced to a hexagon with flat‐ to‐flat dimension of 117m and corner‐to‐corner dimension of 141m, similar to earlier fibers. The ytterbium

multimode regime for a 7‐cell PCF (see Figure 5). This helps to ease bend loss in the weakly guided fiber.

In 2004, the bar was raised in a collaborative work between Friedrich Schiller University Jena and Crystal Fibre A/S [28]. Their 7‐cell fiber has an effective area of ~1000m2 and MFD of ~35m (see Figure 5). Core diameter is ~40m. Hole diameter d is 1.1m and the pitch is 12.3m, giving a d/=0.09. The fiber has a pump guide diameter of 170m and a measured pump NA of 0.62 at 950nm. The pump absorption is ~13dB/m at 976nm. The pump air‐clad is much narrower for more efficient heat diffusion. It is worth noting that the fiber is in the

especially in actively cooled fiber lasers.

especially in actively cooled fiber lasers.

Figure 4 SEM image of the air‐clad ytterbium‐doped large‐mode‐area photonic crystal fiber in [26].

Figure 4 SEM image of the air‐clad ytterbium‐doped large‐mode‐area photonic crystal fiber in [26].

In a subsequent paper in 2003 [27], the fiber in [26] and a new fiber with narrower pump air‐clad were studied with the FEM for temperature distributions in the fiber at various thermal loads in the core, considering both convective and radiative heat transportation by air at the fiber surface. The results show that, in the case of

In a subsequent paper in 2003 [27], the fiber in [26] and a new fiber with narrower pump air‐clad were studied with the FEM for temperature distributions in the fiber at various thermal loads in the core, considering both convective and radiative heat transportation by air at the fiber surface. The results show that, in the case of

**2.8. Progress of active large-core photonic crystal fibers**

228 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

was demonstrated with respect to the launched pump power.

PCF with effective mode area of ~350μm2

especially in actively cooled fiber lasers.

especially in actively cooled fiber lasers.

Since 2003, the group at Friedrich-Schiller-University Jena has been playing a significant role in the development large-mode-area photonic crystal fibers with pump air-clad. In their first work in collaboration with then Crystal Fibre A/S, now NKT Photonics, an ytterbium-doped

demonstrated [26] (see Figure 4). The 3-cell PCF had a hole diameter d=2μm and pitch Λ=11.5, giving an d/Λ=0.18. The core diameter was 28μm. A relatively small circular area of 9μm was doped in the 3-cell core. The doped area had a high ytterbium-doping level of ~0.6 at%. It was further co-doped with aluminum and fluorine to provide an index merely 2×10-4 above the silica background. The PCF with the raised index in the doped area was simulated, showing that the fiber guides the second-order mode, which is, however, close to cut-off. The fiber had a 150μm pump guide with an air clad. The webs in the air clad were ~50μm long with a thickness of ~390nm, giving w/λ=~0.4 at 976nm. The measured pump NA was 0.55, slightly below the NA=0.68 predicated by the slab model in Figure 3. The fiber had an outer glass diameter of 450μm and was coated with standard acrylic coating. Due to the high doping level, the fiber has a pump absorption of ~9.6dB/m at 976nm. An impressive slope efficiency of 78%

Figure 4 SEM image of the air‐clad ytterbium‐doped large‐mode‐area photonic crystal fiber in [26].

In a subsequent paper in 2003 [27], the fiber in [26] and a new fiber with narrower pump airclad were studied with the FEM for temperature distributions in the fiber at various thermal loads in the core, considering both convective and radiative heat transportation by air at the fiber surface. The results show that, in the case of natural cooling, the thermal transportation is mainly limited by the heat transfer at the fiber surface. The air-clad impedes heat flow, especially when the width of the air clad is large. A narrower pump air-clad is advantageous,

**Figure 4.** SEM image of the air-clad ytterbium-doped large-mode-area photonic crystal fiber in [26].

Figure 5 Microscope image of the air‐clad ytterbium‐doped single‐mode PCF and close‐up to the 40‐μm core formed by seven missing air holes [28].

In 2004, the bar was raised in a collaborative work between Friedrich Schiller University Jena and Crystal Fibre A/S [28]. Their 7-cell fiber has an effective area of ~1000μm2 and MFD of ~35μm (see Figure 5). Core diameter is ~40μm. Hole diameter d is 1.1μm and the pitch Λ is 12.3μm, giving a d/Λ=0.09. The fiber has a pump guide diameter of 170μm and a measured pump NA of 0.62 at 950nm. The pump absorption is ~13dB/m at 976nm. The pump air-clad is

Figure 6 Microscope image of a rod‐type photonic crystal fiber and close‐up view of the inner cladding and core regions.

To further mitigate bend loss, a rod‐type PCF was developed in 2005 [29] (see Figure 6). The rod had an outer diameter of 1.7mm so that it cannot be bent. A 48cm long length was used in the demonstration. The relative hole diameter was increased to d/=0.33. This reduces bend sensitivity. This larger d/ in a 7‐cell core PCF, however, puts this fiber firmly in the multimode regime (see Figure 2). The pump guide was reduced to a hexagon with flat‐ to‐flat dimension of 117m and corner‐to‐corner dimension of 141m, similar to earlier fibers. The ytterbium

multimode regime for a 7‐cell PCF (see Figure 5). This helps to ease bend loss in the weakly guided fiber.

In 2004, the bar was raised in a collaborative work between Friedrich Schiller University Jena and Crystal Fibre A/S [28]. Their 7‐cell fiber has an effective area of ~1000m2 and MFD of ~35m (see Figure 5). Core diameter is ~40m. Hole diameter d is 1.1m and the pitch is 12.3m, giving a d/=0.09. The fiber has a pump guide diameter of 170m and a measured pump NA of 0.62 at 950nm. The pump absorption is ~13dB/m at 976nm. The pump air‐clad is much narrower for more efficient heat diffusion. It is worth noting that the fiber is in the

In a subsequent paper in 2003 [27], the fiber in [26] and a new fiber with narrower pump air‐clad were studied with the FEM for temperature distributions in the fiber at various thermal loads in the core, considering both convective and radiative heat transportation by air at the fiber surface. The results show that, in the case of natural cooling, the thermal transportation is mainly limited by the heat transfer at the fiber surface. The air‐clad impedes heat flow, especially when the width of the air clad is large. A narrower pump air‐clad is advantageous,

and mode field diameter (MFD) of 21μm was

40‐μm core formed by seven missing air holes [28]. In 2004, the bar was raised in a collaborative work between Friedrich Schiller University Jena and Crystal Fibre Figure 5 Microscope image of the air‐clad ytterbium‐doped single‐mode PCF and close‐up to the 40‐μm core formed by seven missing air holes [28]. **Figure 5.** Microscope image of the air-clad ytterbium-doped single-mode PCF and close-up to the 40-μm core formed by seven missing air holes [28].

much narrower for more efficient heat diffusion. It is worth noting that the fiber is in the multimode regime for a 7-cell PCF (see Figure 5). This helps to ease bend loss in the weakly guided fiber. A/S [28]. Their 7‐cell fiber has an effective area of ~1000m 2 and MFD of ~35m (see Figure 5). Core diameter is ~40m. Hole diameter d is 1.1m and the pitch is 12.3m, giving a d/=0.09. The fiber has a pump guide diameter of 170m and a measured pump NA of 0.62 at 950nm. The pump absorption is ~13dB/m at 976nm. The pump air‐clad is much narrower for more efficient heat diffusion. It is worth noting that the fiber is in the In 2004, the bar was raised in a collaborative work between Friedrich Schiller University Jena and Crystal Fibre A/S [28]. Their 7‐cell fiber has an effective area of ~1000m 2 and MFD of ~35m (see Figure 5). Core diameter is ~40m. Hole diameter d is 1.1m and the pitch is 12.3m, giving a d/=0.09. The fiber has a pump guide

multimode regime for a 7‐cell PCF (see Figure 5). This helps to ease bend loss in the weakly guided fiber.

Figure 6 Microscope image of a rod‐type photonic crystal fiber and close‐up view of the inner cladding and core regions. **Figure 6.** Microscope image of a rod-type photonic crystal fiber and close-up view of the inner cladding and core re‐ gions.

To further mitigate bend loss, a rod‐type PCF was developed in 2005 [29] (see Figure 6). The rod had an outer diameter of 1.7mm so that it cannot be bent. A 48cm long length was used in the demonstration. The relative hole diameter was increased to d/=0.33. This reduces bend sensitivity. This larger d/ in a 7‐cell core PCF, however, puts this fiber firmly in the multimode regime (see Figure 2). The pump guide was reduced to a hexagon with flat‐ to‐flat dimension of 117m and corner‐to‐corner dimension of 141m, similar to earlier fibers. The ytterbium To further mitigate bend loss, a rod-type PCF was developed in 2005 [29] (see Figure 6). The rod had an outer diameter of 1.7mm so that it cannot be bent. A 48cm long length was used in the demonstration. The relative hole diameter was increased to d/Λ=0.33. This reduces bend sensitivity. This larger d/Λ in a 7-cell core PCF, however, puts this fiber firmly in the multimode regime (see Figure 2). The pump guide was reduced to a hexagon with flat-to-flat dimension of 117μm and corner-to-corner dimension of 141μm, similar to earlier fibers. The ytterbium doping level was also increased compared with earlier fibers. The increased doping level and reduced pump guide led to a high pump absorption of ~30dB/m at 976nm. The fiber was used to demonstrate power extraction of ~250W/m. Figure 6 Microscope image of a rod‐type photonic crystal fiber and close‐up view of the inner cladding and core regions. To further mitigate bend loss, a rod‐type PCF was developed in 2005 [29] (see Figure 6). The rod had an outer diameter of 1.7mm so that it cannot be bent. A 48cm long length was used in the demonstration. The relative hole diameter was increased to d/=0.33. This reduces bend sensitivity. This larger d/ in a 7‐cell core PCF, however,

The rod-type PCF was further developed with the demonstration of a 19-cell PCF with a core diameter of 60μm, effective mode area of ~2000μm2 and MFD of 75μm in 2006 [30] (see Figure puts this fiber firmly in the multimode regime (see Figure 2). The pump guide was reduced to a hexagon with flat‐ to‐flat dimension of 117m and corner‐to‐corner dimension of 141m, similar to earlier fibers. The ytterbium

multimode regime. The effective mode area was ~4500m2 and MFD was ~75m.

power of 4.5MW and pulse energy of 4.3mJ with M2

3 Leakage channel fibers

7). The fiber had a d/Λ=0.19, again firmly in the multimode regime (19-cell PCF not shown in Figure 2). The pump guide was 175μm in diameter. The pump air-clad had a web thickness of 400nm, which was 10μm long. The measured pump NA was 0.6 at 975nm, fairly close to the 0.67 NA predicted by the slab model (see Figure 3). The pump absorption is ~30dB/m at 976nm. The rod diameter is ~1.5mm. A 58cm long fiber was used to demonstrate 55W/m power extraction. In the same paper, a passive 100μm core fiber was also demonstrated. The 19-cell fiber had a d/Λ=0.2, again in the multimode regime. The effective mode area was ~4500μm2 and MFD was ~75μm. led to a high pump absorption of ~30dB/m at 976nm. The fiber was used to demonstrate power extraction of ~250W/m. The rod‐type PCF was further developed with the demonstration of a 19‐cell PCF with a core diameter of 60m, effective mode area of ~2000m2 and MFD of 75m in 2006 [30] (see Figure 7). The fiber had a d/=0.19, again firmly in the multimode regime (19‐cell PCF not shown in Figure 2). The pump guide was 175m in diameter. The pump air‐clad had a web thickness of 400nm, which was 10m long. The measured pump NA was 0.6 at 975nm, fairly close to the 0.67 NA predicted by the slab model (see Figure 3). The pump absorption is ~30dB/m at 976nm. The rod diameter is ~1.5mm. A 58cm long fiber was used to demonstrate 55W/m power extraction. In the same paper, a passive 100m core fiber was also demonstrated. The 19‐cell fiber had a d/=0.2, again in the doping level was also increased compared with earlier fibers. The increased doping level and reduced pump guide led to a high pump absorption of ~30dB/m at 976nm. The fiber was used to demonstrate power extraction of ~250W/m. The rod‐type PCF was further developed with the demonstration of a 19‐cell PCF with a core diameter of 60m, effective mode area of ~2000m2 and MFD of 75m in 2006 [30] (see Figure 7). The fiber had a d/=0.19, again firmly in the multimode regime (19‐cell PCF not shown in Figure 2). The pump guide was 175m in diameter. The pump air‐clad had a web thickness of 400nm, which was 10m long. The measured pump NA was 0.6 at

975nm, fairly close to the 0.67 NA predicted by the slab model (see Figure 3). The pump absorption is ~30dB/m at

doping level was also increased compared with earlier fibers. The increased doping level and reduced pump guide

Figure 7 Microscope‐image of the extended‐mode‐area rod‐type photonic crystal fiber, SEM‐picture of the micro‐ structured region and measured near‐field intensity profile of the 60μm core fiber [30]. **Figure 7.** Microscope-image of the extended-mode-area rod-type photonic crystal fiber, SEM-picture of the microstructured region and measured near-field intensity profile of the 60μm core fiber [30].

The demonstration of a 100m active 19‐cell PCF finally came in 2006 [6]. The pump guide had a diameter of

290m. The rod diameter was 1.5mm. A 90cm long fiber was used to amplify 1ns pulses at 9.6kHz to record peak power of 4.5MW and pulse energy of 4.3mJ with M2 =1.3. The demonstration of a 100μm active 19-cell PCF finally came in 2006 [6]. The pump guide had a diameter of 290μm. The rod diameter was 1.5mm. A 90cm long fiber was used to amplify 1ns pulses at 9.6kHz to record peak power of 4.5MW and pulse energy of 4.3mJ with M2 =1.3. Figure 7 Microscope‐image of the extended‐mode‐area rod‐type photonic crystal fiber, SEM‐picture of the micro‐ structured region and measured near‐field intensity profile of the 60μm core fiber [30]. The demonstration of a 100m active 19‐cell PCF finally came in 2006 [6]. The pump guide had a diameter of 290m. The rod diameter was 1.5mm. A 90cm long fiber was used to amplify 1ns pulses at 9.6kHz to record peak

=1.3.

with a mode area of ~700m2 was demonstrated in 2005 [31] (see left figure in Figure 8). The pitch was 12.3m with a d/ of 0.2. In the weakly guided PCFs, the polarization modes on slow and fast axis have different bend Figure 8 SEM image of the single polarization PCF with an effective area of ~700m2 [31] (left) and 2300m2 (right) [32]. **Figure 8.** SEM image of the single polarization PCF with an effective area of ~700μm2 [31] (left) and 2300μm2 (right) [32].

Polarization‐maintaining (PM) PCFs have also been developed by introducing stress elements. A PM 7‐cell PCF

losses. This enables single‐polarization operation where the polarization mode on the slow axis is still guided while polarization mode on the fast axis suffers high bend loss. Another single‐polarization 19‐cell PCF was demonstrated in 2008 with a mode area of ~2300m2 (right figure in Figure 8) [32]. The corner‐to‐corner distance of the core was 70μm. The pitch was 11m with a d/ of 0.1. 3 Leakage channel fibers A 2D micro‐structured cladding, which is made possible by the stack‐and‐draw technique developed for Polarization‐maintaining (PM) PCFs have also been developed by introducing stress elements. A PM 7‐cell PCF with a mode area of ~700m2 was demonstrated in 2005 [31] (see left figure in Figure 8). The pitch was 12.3m with a d/ of 0.2. In the weakly guided PCFs, the polarization modes on slow and fast axis have different bend losses. This enables single‐polarization operation where the polarization mode on the slow axis is still guided while polarization mode on the fast axis suffers high bend loss. Another single‐polarization 19‐cell PCF was demonstrated in 2008 with a mode area of ~2300m2 (right figure in Figure 8) [32]. The corner‐to‐corner distance of the core was 70μm. The pitch was 11m with a d/ of 0.1. Polarization-maintaining (PM) PCFs have also been developed by introducing stress elements. A PM 7-cell PCF with a mode area of ~700μm2 was demonstrated in 2005 [31] (see left figure in Figure 8). The pitch Λ was 12.3μm with a d/Λ of 0.2. In the weakly guided PCFs, the polarization modes on slow and fast axis have different bend losses. This enables singlepolarization operation where the polarization mode on the slow axis is still guided while polarization mode on the fast axis suffers high bend loss. Another single-polarization 19-cell

photonic crystal fibers, enables new designs which do not possess the closed core‐and‐clad boundaries of conventional optical fibers. When a mode is guided in a conventional optical fiber, total internal reflection everywhere around the closed core‐and‐clad boundary, traps light entirely in the core, leading to zero waveguide

A 2D micro‐structured cladding, which is made possible by the stack‐and‐draw technique developed for photonic crystal fibers, enables new designs which do not possess the closed core‐and‐clad boundaries of conventional optical fibers. When a mode is guided in a conventional optical fiber, total internal reflection everywhere around the closed core‐and‐clad boundary, traps light entirely in the core, leading to zero waveguide PCF was demonstrated in 2008 with a mode area of ~2300μm2 (right figure in Figure 8) [32]. The corner-to-corner distance of the core was 70μm. The pitch Λ was 11μm with a d/Λ of 0.1.