Study & Design Of The Interferometric PCF Grating Sensor

The fiber grating is one of the most important optical passive devices and playing an importantrole in the field of optical communications and optical sensing. Recently, the appearance and rapiddevelopment of the photonic crystal fiber provide the further development of fiber gratingtechnology. Compared to the common fiber, the transmission characteristics of the photonic crystalfiber can be adjusted by the air-hole layers, duty cycle, and lattice shape, etc. Thus, the spectralcharacteristics of photonic crystal fiber grating can be controlled flexibly. Interferometric opticalfiber sensors have been widely used in many fields because of its advantages compared totraditional sensor such as simple structure, flexible design, high sensitivity.A type of interferometric sensor based on cascaded photonic crystal fiber long period gratingis studied in this paper. First, the basic theory (coupled mode theory) and calculation method(transfer matrix method) of the grating is introduced. Then the spectrum of the grating and itscascading are calculated, the analysis of the effect of the photonic crystal fiber structure isprocessed. Based on these work, the strain and index sensing with this cascade photonic crystalfiber long period grating is studied, and the structural design features of the two applications aregiven. The results show that the center wavelength and bandwidth of the transmission spectrum areaffected by the structure parameters of the photonic crystal fiber and the grating. Besides, thespectrum of the cascaded structure is affected by the photonic crystal fiber between the gratings.The sensitivity is optimized when the PCF has the duty cycle of smaller than 0.406,the air hole of4μm diameter, the cladding of 6 layers and the two LPGs are the same; pm magnitude order of thestrain sensing sensitivity and nm magnitude order of the refraction index sensing are achieved, withthe PCF length 100 times and 10 times of the grating period respectively. So that, the sensingsensitivity of strain is1.8pm /μεand the sensing sensitivity of refractive index is 800nm / RIU .