Highly efficient Raman amplification in a polarization-preserving optical fiber

In a forward pump configuration, a Raman gain coefficient of 4.8×10−11 cm/W and a gain as high as 40 dB/W have been successfully attained with a combination of 1.34-μm pump and 1.42-μm signal pulses. A 20-dB gain at a wavelength of 1.52 μm was also obtained at a 3.7-W input. The test fiber is a 1-km-long polarization-preserving and absorption-reducing type polarization-preserving fiber which has a GeO2 highly doped core with a small core diameter of 4.5 μm. It is also shown that the 3-dB gain bandwidth is around 130–135 cm−1 and that optical power ratio of the amplified signal to the spontaneous emission is about 24 dB for the 1.42-μm 1st Stokes line.