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

We analyze and experimentally demonstrate that both the bandwidth of the fi ber Bragg grating (FBG)'s short wavelength loss and the bandwidth of the FB G's reflection increase with the increase of strain gradient along the leng th of the grating. Based on this phenomenon, we experimentally demonstrate a simple method of fabricating tunable all-fiber monochannel bandpass filte r using a single strong FBG. A filter with a similar to 3.5 nm width stopba nd and a similar to 0.4 nm width passband is achieved in the standard singl e 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 be cause 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 shor t 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 imp rove the performance of the filter; it not only broadens the two stopbands but also reduces the width of the passband. (C) 2000 Elsevier Science B.V. All rights reserved.