the invention provides a fiber - optic sensing system , utilizing a fiber - grating - based sensor , for a physical parameter , e . g ., a pressure or a temperature . different kinds of fiber - grating - based sensors may be used for this purpose but in - fiber gratings such as fiber bragg grating , long period grating and surface corrugated long period fiber grating are particularly suitable . due to the small size of the optical fiber and the fact that same fiber acts as the sensing element as well as the signal conducting medium , it is possible to install the sensor in a small diameter needle which is commonly used for medical diagnosis and treatment . as a result , when the fiber - optic sensing system of the invention is used for in - vivo measurement of a biological parameter , such a sensing needle can be used for different in - vivo pressure or temperature sensing applications without causing too much harm and discomfort to the subject tested .