Multi-color switchable visible light source generated via nonlinear frequency conversion of a random fiber laser

Nonlinear frequency conversion of random fiber lasers could provide new possibilities to realize visible and mid-infrared light with flexible wavelength and low temporal/spatial coherence. Frequency doubling of random fiber laser is reported to generate visible light with single-color output. Here, we propose a new way to generate multi-color switchable visible light source from a dual-wavelength switchable 1st-order random Raman fiber laser (RRFL) with phosphosilicate fiber. Taking advantage of the existence of the two Raman gain peaks with significant different Raman gain bandwidth at the frequency shifts of 13.2 THz (silica-related one with broad Raman gain bandwidth) and 39.9 THz (phosphorus-related one with narrow Raman gain bandwidth) in phosphosilicate fiber, a dual-wavelength switchable RRFL is developed which can emit 1120 and 1238 nm random Raman lasing individually or simultaneously with 3-watt level output power and sub-1 nm bandwidth by precisely tuning the pump wavelength to manipulate the Raman gain at two fixed Raman Stokes wavelengths. It is expected that the output power can be further increased with a shorter fiber length and more powerful pump, and the spectral bandwidth can be much narrower by adopting a narrowband point reflector in 1st-order RRFL. Based on the dual-wavelength RRFL with a flexible power ratio and a periodically poled lithium niobate (PPLN) crystal array containing three separate poled gratings with different periods, the second-harmonic generation of 1120 nm or 1238 nm random lasing and sum-frequency generation of 1120 nm and 1238 nm random lasing can be performed. As a result, the switchable output of green light at 560 nm, yellow light at 588 nm and red light at 619 nm can be realized with optical power of 22.2 mW, 16.9 mW and 18.5 mW, respectively. Our work demonstrates dual-wavelength RRFL could act as a new platform for generating visible light source with flexible color output which has potential applications in imaging, sensing and visible temporal ghost imaging.