the principle of coherent microfluids resonance can best be understood by reference to the microstructure optical fiber 10 of fig1 . as shown , fiber 10 comprises a core region 12 surrounded by an inner cladding layer 14 and an outer cladding layer 16 . as will be described in detail below , inner cladding layer 14 comprises a periodic disposition of an optically active material 18 , active material 18 being in this example disposed within a plurality of separate air channels 20 axially disposed so as to surround core region 12 and extend along the length of fiber 10 . an important factor in the tenability of the structure of the present invention is the periodic disposition of active material 18 within channels 20 , where separate “ bubbles ” or “ plugs ” of active material 18 ( see photograph associated with fig1 ) are formed to exhibit a period of λ . as will be discussed in detail below , the presence of periodically - spaced active material plugs 18 causes phase matching between the propagating fundamental mode and higher order modes . to achieve coupling between co - propagating waveguide modes , phase matching needs to satisfy the following relationship : where β fun and β high are the propagation constants of the fundamental and higher order modes , respectively , and λ is the period of the active material sections 18 , as illustrated in fig1 . in microstructured optical fiber 10 , the evanescent field of the fundamental mode overlaps with channels 20 of inner cladding 14 , as shown in fig1 and therefore passes through active material 18 which functions to provide coupling to the higher order mode . when the two modes are phase matched , optical power is exchanged between them . the amount of light transferred into the higher order mode at certain wavelengths is related to the coupling coefficient . the coupling coefficient depends on the index difference in the periodic structure and the overlap between the mode fields e fun and e high through the following : κ =∫∫ ωε 0 / 2 ( δ n ) 2 e fun { circle over ( x )} e high da as will be described below , the refractive index of one exemplary optically active material ( trifluorotoluence ) is 1 . 405 , so that the index difference between the fluid and the air in each channel 20 gives rise to an index difference δn = 0 . 405 . although the index difference is much larger than the index changes obtained in photosensitive optical fiber gratings , the coupling coefficient κ is related to the overlap of the fundamental and higher order mode . the calculated coupling coefficient κ for a conventional fiber with similar dimensions as that inner cladding of the fiber as described below is on the order of approximately 1 × 10 − 5 / μm , which is approximately the same order of magnitude as in a conventional long period grating . it is to be noted that while the exemplary embodiment discussed herein utilizes a microstructured optical fiber , the principles of the present invention are equally applicable for use with any microstructured optical waveguiding arrangement , such as a slab optical waveguide . [ 0021 ] fig2 illustrates a particular apparatus that may be used to form a periodic microstructure optical fiber , such as fiber 10 of fig1 . as shown , apparatus 30 comprises a fluid reservoir 32 filled with optically active material 18 . a motor 34 is coupled to a clamping apparatus 36 , where clamping apparatus 36 is attached ( as shown ) to an outer housing surrounding fiber 10 . the motion of motor 34 is redirected through clamping apparatus 36 to provide periodic linear dipping ( illustrates as ± y in fig2 ) of endface 22 of fiber 10 into fluid reservoir 32 . a vacuum pump 38 is attached to opposing endface 24 of fiber 10 such that as a vacuum is applied via endface 24 , fluid plugs 18 and air ( alternating ) are drawn into each channel 20 of fiber 10 . the period a of fluid plugs 18 is controlled , in this particular example , by adjusting the rotational frequency ω of motor 34 . the ratio of fluid to air can also be controlled by adjusting the displacement of fiber 10 into fluid 18 , as compared to the displacement of fiber in air during one cycle . a microstructured optical slab waveguide may be similarly clamped within such apparatus to form a periodic disposition of active material within the cladding layers of the waveguide structure . in the particular embodiment of fiber 10 as described thus far , fiber 10 comprises a set of six approximately cylindrical channels ( in this case , “ tubes ”) 20 within inner cladding layer 14 . in a fiber that comprises an 8 μm , germanium - doped core region 12 , the propagating light signal will not interact with active plugs 18 in cladding layer 14 . in order to achieve interaction between light propagating in the fiber and active plugs 18 , fiber 10 needs to be tapered and stretched , as shown in fig3 to create a tapered region 40 . within tapered region 40 , the mode field expands into cladding layer 14 and thus becomes affected by active plugs 18 . as shown in fig3 fiber 10 has been tapered in region 40 to a waist outer diameter of approximately 30 μm ( inner diameter of approximately 8 μm ) over a length of 7 cm . periodic microfluidic plugs 18 , in this example trifluorotoluence , are spaced with a period λ of approximately 460 μm . as discussed above , light propagating in tapered region 40 will spread out of core region 12 and interact with periodic plugs 18 . coherent coupling is thus achieved between the fundamental mode ( lp 01 ) and the higher order mode ( lp 02 ) by virtue of the presence of periodic plugs 18 . the generated higher order mode then propagates through the adiabatic up - tapered section 42 and transforms undisturbed into the section 44 of inner cladding layer 14 . although the higher order mode is guided in inner cladding 14 , it will be attenuated when it reaches a splice s with a section of conventional single mode fiber 50 . the resonant coupling to the higher order mode thus manifests itself in a resonant loss peak centered at a wavelength governed by the period and the propagation constants of the respective modes . in order to obtain coupling at a desired wavelength , the required period of the perturbation is given by the first above - described equation and is based on the knowledge of the effective indices of the respective core and higher order modes ; the latter can be calculated using conventional beam propagation methods . the calculated difference between the effective indices of the lp 01 and lp 02 modes ( δn eff 01 - 02 )= 0 . 0045 , assuming that no fluid is present in the air gaps , and suggests a period of 435 μm for resonant coupling around 1 . 5 μm . it is to be noted that the effective indices are affected by the presence of plugs 18 in channels 20 by virtue of the interaction of the evanescent fields with the optically active material of plugs 18 . [ 0024 ] fig4 illustrates the transmission spectra for both a fiber with periodic microfluidic plugs ( such as plugs 18 ), as well as for a continuous fluid - filled inner cladding region . curve a illustrates the transmission over a wavelength range of 1530 nm to 1600 nm for a continuous filled fiber , showing very little difference in transmission as a function of wavelength . in contrast , curve b illustrates the transmission for a structure with a periodic plug structure , where in this case , a significant loss ( filtering effect ) is introduced at a wavelength of approximately 1565 nm . [ 0025 ] fig5 shows the experimentally measured transmission spectra associated with different dipping frequencies , as discussed above in association with fig2 . as mentioned above , the period λ of the optically active material plugs in the fiber cladding channels is determined by the dipping frequency , that is , the number of times ( n ) the fiber is dipped into a reservoir of optically active material during a time interval ( t ), as well as the velocity ( v ) in which the microfluidic plugs travel along the tube . during each time interval t , the fluid fills a certain distance determined by the velocity , which may be ( for example ) 1 cm / sec , as the fluid is being infused . thus , for a dipping frequency of 125 hz , the period λ of the fluid in the fiber will be approximately 80 μm . in a tapered fiber section ( such as section 40 shown in fig3 ), the period will increase by a predetermined factor , which is the ratio of the tapered fiber diameter to that of the un - tapered section . in the arrangement of fig3 the tapered diameter is one - fourth that of the non - tapered fiber , so that the period will increase to about 460 μm . the calculated period inferred from the above equation and that observed may be slightly different since the infusion of the fluid into the fiber is extremely sensitive to any change in the frequency of the motor , or the amount of dipping the fiber into the fluid . further , the resonance position on the transmission spectrum is very susceptible to the difference in the effective indices . in accordance with the practice of the present invention , the period λ of plugs 18 can be “ tuned ” by simultaneously heating the air in channels 20 on both sides of tapered region 40 . the heated air applies pressure on both sides of the periodic microfluidic plugs 18 , causing the air gap between plugs 18 to shrink and thus decrease the separation between adjacent plugs 18 ( decreasing the period λ ). fig6 illustrates the effect of temperature changes on a microstructured optical fiber including periodic plugs in accordance with the present invention . curve a in fig6 shows the transmission spectrum for a fiber such as fiber 10 when the end portions are maintained at 25 ° c . in this case , the transmission spectrum contains a notch at a wavelength just above 1590 nm . by heating both ends of fiber 10 to 125 ° c ., the center wavelength of the device will shift downward to a value of approximately 1583 nm , as a result of the decreased period of plugs 18 after heating . therefore , by controlling the temperature applied to both ends , wavelength tuning can easily be achieved . [ 0027 ] fig7 illustrates a set of different transmission spectra measured after directly heating plugs 18 within tapered section 40 of an exemplary fiber 10 . in this case , the optically active material &# 39 ; s refractive index exhibited a decrease with increasing temperature ( dn / dt of approximately − 10 − 4 /° c .). therefore , the coupling efficiency is reduced since the difference between the refractive indices of the fluid and air ( δn ) becomes smaller . curve a illustrates a sharp resonance at room temperature ( 25 ° c . ), which becomes weaker at higher temperatures . it is also to be noted that the background loss decreases with temperature . this is to be expected since the average loss over the entire tapered section decreases . the resonance tends to shift toward higher wavelengths as the periodic plugs are heated , since the air between the plugs tends to expand and the increases the period of the grating structure formed by the plugs . in essence , the present invention discloses periodically - spaced microfluidic plugs disposed in channels along the inner cladding layer of a microstructured optical waveguide , such as an optical fiber . coherent resonance structures can therefore be formed within such a waveguide , where the resonance condition is controlled by adjusting the period of the active material plugs ( in one example , by adjusting the “ dipping frequency ”) or heating the waveguide / fiber on either side of the tapered central region . moreover , the resonance may be attenuated by heating a fluid whose refractive index varies as a function of temperature . various and other modifications may be made to the microstructured optical fiber as discussed above , where such modifications are considered to fall within the spirit and scope of the present invention as defined by the claims hereinbelow .