CHARACTERISTICS IN NEODYMIUM-DOPED FIBER AMPLIFIERS AT 1.06-MU-M

We have investigated the gain and noise figure characteristics of a Nd -doped silica single-mode fiber amplifier (NDFA) at 1.06 mu m which is applicable to various systems using a Nd:YAG laser light source at 1. 06 mu m, such as free-space laser communications, a fiber sensing syst em, and a Lidar system. A fluorescence spectrum observation of the Nd- doped fibers with various co-dopants shows that the Nd-Al co-doped fib er is suitable for realizing a high-gain amplifier for the 1.06-mu m w avelength region. The pump wavelength tolerance at around 0.81 mu m, t he gain bandwidth and the sufficient value of the Nd concentration and length product for achieving maximum small signal gain are clarified. A noise figure of almost 3 dB and small signal gain of more than 30 d B are attained by 50-mW pump power. The unique four-level system chara cteristics, even in low pump power conditions, provide low noise ampli fication in the NDFA. These gain and noise characteristics are well de scribed by a simple theoretical model. We also demonstrated high-power operation of the NDFA with four pump LD modules adopting a polarizati on-multiplexing technique. More than 100-mW signal output power is ava ilable for 1-mW signal input power at 200-mW launched pump power. Thes e features of the NDFA as a compact, polarization-independent, spatial -beam-distortion-free amplifier, will allow it to replace the solid sa te laser in various applications using a Nd:YAG laser light source at 1.06 mu m.