Fig. 1. Measured Raman gain spectrum of the utilized phosphosilicate fiber.

Fig. 2. Schematic of the high power low quantum defect Raman fiber amplifier. PF, phosphosilicate fiber; QBH, quartz block head; PM, power meter; MMF, multimode fiber; OSA, optical spectrum analyzer. Inset, refractive index profile of the phosphosilicate fiber.

Fig. 3. (a) Maximum output powers and corresponding output spectra at different fiber lengths. (b) Spectral evolution and (c) power evolution characteristics of the low QD RFA under an optimized fiber length of 150 m. ASE, amplified spontaneous emission.

Fig. 4. (a) Highest temperature increment measured at the fiber coating of each RFA as a function of output power. (b) Thermal image of the gain fiber in the low QD RFA at 815 W signal power. (c) Thermal image of the gain fiber in the conventional RFA at 813 W signal power.

Fig. 5. (a) Calculated output power evolution characteristics in the low QD RFA; the scattered points are the corresponding experimental results. (b) Calculated longitudinal power distribution of the low QD RFA at pump power of 1.6 kW.

Fig. 6. Output powers of Raman signal, residual pump, and spontaneous Raman generation at (a) different seed powers and (b) different inner-cladding diameters.

Fig. 7. (a) Longitudinal heat load distribution along the phosphosilicate fiber in the two RFAs. (b) Temperature increment distribution in the low QD RFA and (c) conventional RFA operating at the same signal power of 1 kW.

Fig. 8. Integrated heat load in the two RFAs as a function of output power with (a) 150 m and (b) 50 m Raman fiber. (The solid line and dashed line represent the integrated heat load of the low QD RFA and the conventional RFA, respectively. The red, green, and blue lines represent the total heat load, propagation-loss-induced heat load, and QD-induced heat load, respectively.)

Fig. 9. Cross section of the triple-clad phosphosilicate fiber.