Single and Multiple Wavelength DFB Fiber Lasers
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Single mode and multiple mode distributed feedback fiber lasers (DFB-FLs) are analyzed and experimentally investigated, with emphasis on properties that have importance for sensor applications. It is experimentally found that optical feedback from discrete reflectors and Rayleigh backscattering is a serious challenge for remotely pumped DFB-FL sensors. Strong and long DFB-FL gratings are therefore desired to reduce feedback sensitivity in some sensor applications. Unfortunately, higher order modes can be induced if the DFB-FL grating strength is too high. The threshold grating strength of DFB-FL higher order modes is calculated and compared with the fundamental mode thresholds as a function of gain, background loss and various types of grating errors. Higher order modes are found to be greatly enhanced by quadratic chirp. It is proposed to reduce the problem of higher order mode operation by spatially window the gain. The spatial overlap between the fundamental and higher order modes is small, and windowing the gain around the former therefore dramatically increases the threshold of the latter.
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