Long Period Fiber Grating Near Turn Around Point: Suitable Design for Bio-Sensing

Abstract

We report our experimental study on the add-layer sensitivity of LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,11</sub> cladding mode (CM) of Long Period Fiber Grating (LPFG) near turn around point (TAP). The CM response was tuned by controlled modification of the cladding diameter and deposition of overlay layers of polymer material. In this process an add-layer sensitivity of 2.308 nm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">WL</sub> /nm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> has been achieved. It is interesting to observe that add-layer sensitivity obtained in this process is close to the sensitivity of the same CM operated near the mode transition (MT). The advantage is that, the sensor designed as per the proposed methodology will not be plagued by the basic limitations of operating a CM near MT, i.e. loss of contrast of the resonant band and limited dynamic range. In our experiment, the dynamic range has found to be ~100 nm where the contrast remains greater than -10 dB throughout the range of measurement which is practically impossible to get for any CM operating at MT. The experimental result was supported by theoretical calculations. Also the SRI sensitivity of the sensor was found out to be 6400 nm/ SRIU.