Direct Diode Pumped Raman Amplifier Based on a Multimode Graded Index Fiber

The direct pumping of a Raman number amplifer (RFA) was attempted using an array of four 25W, number pigtailed diodes at 936nm, combined via a 7 channel number beam combiner. The initial attempt was conducted using a 1.8 km, 100 um core, GRIN number with an NA of .29 and attenuation 3.6 dB/km at 936nm. While amplification was not achieved, over 200mW of conversion was shown, with 10.4W of pump power and 3.5W of seed. This corresponds to an average conversion e ciency of 2.2%. The subsequent effort utilized a 2km long, 200um core, GRIN number, with NA of .27 and attenuation of 2.8dB/km at pump wavelength. Again, amplification was not achieved, but a gain of 80mW was present, with 3.5W of seed power and 14W of pump. This corresponds to an average conversion e ciency of 0.6% and only 2.5% of the coupled seed power. The final experiment attempted to solve the problem of coupling e ciency and high Raman threshold by utilizing a 50/250 dual- clad number, with NAs of .28 and .46 for the core and inner cladding, respectively. Again, amplification was not realized, as only 100mW of gain was present, with 25W of pump and 4.2W of coupled seed power, corresponding to 36% of the coupled seed, and an average conversion efficiency of only 0.4%. Although amplification was not achieved for any of the three numbers, gain was shown possible in all, showing that directly pumping an RFA with a diode source is possible.