Raman amplification in 14–15-μm spectral region in polarization-preserving optical fibers

Saturation characteristics, polarization, and wavelength dependences of the stimulated Raman gain in polarization-preserving fibers have been studied to construct an active transmission line. In a backward-pump configuration, a Raman gain coefficient of 1.1 × 10−11 cm/W and a gain as high as 25 dB have been attained with a combination of a 1.34-μm pump and a 1.42-μm signal pulse, where the pump power is 35 W and the interaction length is 100 m. A 3-dB Raman gain at 1.51 μm has also been obtained under the same conditions. It was also found that there is a nonlinear polarization mode coupling that couples the vertical axis to the pump axis. This type of nonlinear coupling seems to be large when the linear mode-coupling coefficient of the fiber is large.