Fabrication of all-fiber bandpass filter using single fiber Bragg grating

Abstract

We analyze and experimentally demonstrate that both the bandwidth of the fiber Bragg grating (FBG)'s short wavelength loss and the bandwidth of the FBG's reflection increase with the increase of strain gradient along the length of the grating. Based on this phenomenon, we experimentally demonstrate a simple method of fabricating tunable all-fiber monochannel bandpass filter using a single strong FBG. A filter with a ∼3.5 nm width stopband and a ∼0.4 nm width passband is achieved in the standard single mode fiber. The strong short wavelength loss of FBG acts as a stopband on the short wavelength side, through which the incident light cannot pass because its core mode is coupled to the cladding mode. The reflection peak of FBG acts as a stopband on the long wavelength side, through which incident light cannot pass because the forward propagating core mode is coupled to backward propagating core mode. The gap between the reflection and the short wavelength loss acts as a passband, through which the light can pass with a low amount of loss. The strain gradient along the grating is used to improve the performance of the filter; it not only broadens the two stopbands but also reduces the width of the passband.