Fig. 1. Wavelength-sensitive characteristic of the MMF. (a) Normalized speckle patterns at different input wavelengths for an MMF length of 1.5 m. (b) Simulated and (d) measured spectral correlation functions versus wavelength change for different lengths of MMF. (c) Simulated and (e) measured spectral correlation bandwidths for MMFs with different lengths.

Fig. 2. Working mechanism and experimental setup of the spectrally programmable Raman fiber laser. (a) Working mechanism and (b) schematic diagram. FBG, fiber Bragg grating; WDM, wavelength division multiplexer; Combiner, signal-pump combiner; LD, laser diode; YDF, ytterbium-doped fiber; SLM, spatial light modulator; P1, polarizer; L1, L2, L3, lens; SMF, single-mode fiber; MMF, multimode fiber; OSA, optical spectrum analyzer. The inset shows the optimization process.

Fig. 3. Performance of the filter. (a) Spectra before and after optimization. The optimized wavelength is 1555.5 nm. The inset shows the filtering bandwidth as a function of MMF length. (b) Optimization process corresponding to (a). The inset shows the optimal phase pattern.

Fig. 4. Output characteristics of first-order RFL. (a) Tunable single-wavelength output. (b) Output power of Raman lasing versus 976 nm LD pump power. (c) Dual-wavelength and (d) five-wavelength output.

Fig. 5. Output characteristics of high-order RFL. (a) Wavelength tunability of the second-order RFL. The inset is the corresponding first-order Raman lasing spectrum. (b) Wavelength tunability of the third-order RFL. The inset shows the corresponding lower-order Raman lasing spectrum. (c) Evolution of wavelength switching from 1203 to 1210 nm for the second-order RFL. (d) Intensity value enhancement at the wavelength of 1210 nm corresponding to (c). (e) Output power of the high-order RFL versus 976 nm LD pump power.

Fig. 6. Output characteristics of the spectrally programmable RFL. (a) Optimized lasing spectra and spectral purity for each Stokes order. The pump powers corresponding to the first-, second-, and third-order Raman lasing spectra are 9, 15, and 20 W, respectively. (b) Wavelength tunability of second-order RFL at a fixed pump power of 9 W. (c) Spectral stability of single-wavelength lasing at 1142.5 nm. (d) Output power stability corresponding to (c). The inset shows the output power fluctuations in one optimization period.