A novel multiwavelength Raman fiber laser based on the mixed-cascaded Stokes effects of phosphosilicate fiber is proposed and demonstrated experimentally. By using stimulated Raman scattering of both P(2)O(5) and SiO(2) along 1 km phosphosilicate fiber pumped with a 1064 nm double-clad fiber laser, the mixed-cascaded Raman linear cavity is formed by a pair of fiber Bragg gratings at 1239 nm, a polarization-maintaining fiber (PMF) Sagnac loop filter, and a conventional optical loop mirror. Up to 15-wavelength stable oscillations around 1320 nm are obtained with a wavelength spacing of 0.44 nm and power nonuniformity of less than 4 dB. By changing the length of the PMF in the Sagnac loop filter from 10 to 5.5 m, the wavelength spacing is adjustable from 0.44 to 0.8 nm. The extinction ratio of the laser is more than 30 dB. Excellent stability is also observed with a peak power fluctuation of less than 0.8 dB in 1 h.

Theoretical design optimization of the first-order P-doped fiber Raman laser (FRL) by an explicit approach was investigated. The authors derived an explicit expression for the output power of the laser without using the depleted-pump approximation. The proposed solution shows excellent agreement with numerical simulation. According to the explicit solution, one can clearly know the effects of fiber length, reflectivity of output fiber Bragg grating (FBG), Raman gain and loss of the P-doped fiber and extra losses on the output power. The solution also present a criteria by which one can determine whether the depleted-pump approximation is valid or not. It is very fast and convenient to optimize the output power of the FRL using the proposed explicit solution. The optimal values of fiber length, reflectivity of output FBG and conversion efficiency are obtained under different pump power. While increasing pump power, the optimal fiber length and reflectivity of output FBG decrease and the optimal conversion efficiency increase. There exists a certain tolerance of the optimal parameters, in which the conversion efficiency decreases only slightly. The results provide us an intuitive physical understanding to the laser and are instructive to experimental design of the laser