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.
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