All-fiber bandwidth-tunable band-rejection filter based on a composite grating induced by CO₂ laser pulses

We propose an all-fiber band-rejection filter with a tunable bandwidth, which is realized by putting a normal long-period fiber grating in series with a rotary long-period fiber grating written in a twisted singlemode fiber by CO₂ laser pulses. Bandwidth tuning is achieved by applying torsion to the composite grating. Our experimental filter shows a bandwidth tuning of ∼-16.3 nm at a rejection level of ∼15 dB and a polarizationdependent loss lower than ∼0.9 dB. © 2009 Optical Society of America.