Ultra-Broadband Visible Random Laser Generation via Hybrid SHG and SFG Processes

Limited by stringent phase matching conditions, the realization of broad visible spectrum through nonlinear frequency up-conversion in
${{\bm{\chi }}}_2$
media remains a formidable challenge. Here, we propose that ultra-broadband visible frequency up-conversion is feasible through pumping a periodically poled lithium niobate (PPLN) crystal with a spectrally designed laser based on non-perfect phase matching (NPPM). To that end, the input spectrum was designed to deviate from perfect phase matching wavelength and has an intensity-increasing spectral profile. In simulation, a >200-nm-width frequency up-converted visible laser can be obtained with an ideal pump spectrum. In experiment, a specific random fiber laser is used to pump a PPLN, and a ∼150-nm-bandwidth (from ∼500 to 650 nm) visible random laser is obtained through hybrid second harmonic generation and sum-frequency generation. The visible random laser is also pulsed with a maximum pulsing repetition rate of ∼4.7 kHz and a pulse duration of 734 ps. The maximum output power is 1.03 mW, corresponding to a slope efficiency of 1.8‰. The ultra-broadband NPPM frequency up-conversion mechanism is verified through both simulation and experiment. To the best of our knowledge, this is the first pulsed and broadest-spectrum visible random laser achieved through nonlinear frequency conversion by a random fiber laser pumping.