Fig. 1. The fiber Raman gain spectrum measurement setup of small signal method (BPF: Band-pass filter, CIR: Circulator, OSA: Optical spectrum analyzer)

Fig. 2. (a) The pump spectrum after band-pass filter; (b) The transmission spectrum of the P2-P3 passage of CIR

Fig. 3. (a) The Raman output spectra from phosphorus-doped fiber (PDF) at different signal wavelengths; (b) The Raman output spectra from germanium-doped fiber (GDF) at different signal wavelengths; (c) The calculated Raman gains of the PDF and GDF at different frequency shifts

Fig. 4. The schematic of fiber with N numbers of section

Fig. 5. (a) The transmission spectra of the two fibers; (b) The calculated Raman output power as function of Raman gain coefficients at 1070 nm

Fig. 6. The Raman output powers as functions of Raman gains at different signal wavelengths in (a) 500 m PDF and (b) 500 m GDF (red lines represent the results obtained from equation (10); green dots represent the results obtained from power balanced model)

Fig. 7. The fiber Raman gain spectrum measurement setup of spontaneous Raman emission method (BPF: Band-pass filter, OSA: Optical spectrum analyzer)

Fig. 8. (a) The backward Raman scattering spectra from the two fibers; (b) The calculated Raman gain spectra of the two fibers (scattered point data are the results obtained by the small signal amplification method, solid line data are the results obtained by the spontaneous Raman emission method)