Watt-level green random laser at 532 nm by SHG of a Yb-doped fiber laser

We have developed a Watt-level random laser at 532 nm. The laser is based on a 1064 nm random distributed ytterbium-gain assisted fiber laser seed with a 0.35 nm line-width 900mW polarized output power. A study for the optimal length of the random distributed mirror was carried out. An ytterbium-doped fiber master oscillator power amplifier architecture is used to amplify the random seeder laser without additional spectral broadening up to 20 W. By using a periodically poled lithium niobate (PPLN) crystal in a single pass configuration we generate in excess of 1 W random laser at 532 nm by second harmonic generation with an efficiency of 9 %. The green random laser exhibits an instability 70 dB, 0.1 nm linewidth and excellent beam quality. Random distributed fiber lasers (RDFLs) based on distributed Rayleigh scattering, have been thoroughly investigated due to their high performances and unique features [1]. Where traditional laser schemes are based in resonant cavities for feedback generation, RDFLs use the Rayleigh scattering of a long fiber as distributed mirror, generating a modeless-behavior laser [2,3]. The research in this field has led to the generation of ultra-high power RDFLs from hundreds of Watts [4] to kWs [5,6], narrower linewidth RDFLs up to sub-gigahertz [7], polarized output RDFLs [8-10], tunable RDFLs [11-13] and pulsed generation [14,15]. Gain in RDFLs can be generated from Raman scattering [2,16], by rare earth-doped fibers [12,13] or a hybrid of both [17]. However, to date random lasers in the visible based on second harmonic generation (SHG) of RDFLs have been only reported in [18] with the generation of 110 mw at 654nm in a magnesium periodically poled lithium niobate (MgPPLN) crystal. In this letter, we report for the first time, to the best of our knowledge, a Watt-level visible random laser at 532 nm based on SHG of a RDFL, with a polarized output power in excess of 1W, instability 70 dB and excellent beam quality. The RDFL is based on a half open-cavity setup assisted by a 3m-long Yb-doped double-clad fiber as gain medium (see Fig. 1). The core and clad radii of the fiber are respectively 10 and 130 m, and its clad absorption is 4.6 dB/m at 976 nm. The Yb-fiber is forward-pumped through a multi-mode (MM) combiner with a 9W multimode laser diode (LD) at 976nm. Forward pumping has been previously reported as more efficient in Yb-gain assisted RDFL [19]. The wavelength selection is carried out by a high-reflective (99.85%) fiber Bragg grating (FBG) centered at 1064.39 nm and with a 0.57 nm bandwidth. The distributed mirror is based on single-mode fiber (SMF28). Although this fiber operates in multimode regime at 1064 nm, the splice with the 1060 fiber at the output-isolator filters the high order modes. Moreover, the SMF core radius (∼9 m) is comparable to the Yb-fiber one, reducing the losses in the splice. A pump power stripper was used before the SMF fiber in order to remove the residual pump. Fig. 1. Schematic of the 532nm random laser. (RDFL: Random Distributed Fiber Laser, MO: Master Oscillator, PA: Power Amplifier, SHG: Second Harmonic Generation, PM ISO: Polarization Maintaining Isolator) To optimize the length of the distributed mirror, we carried out measurements for different SMF lengths from 1.5Km to 3Km. Figure 2 shows the spectra and the output power before the isolator for the different SMF lengths. As expected, Raman scattering becomes significant for longer fibers, starting to be critical for lengths over 3Km. Moreover, the high attenuation of SMF28 at 1064 nm (∼1.5 dB/Km) reduce the efficiency, making shorter fibers more attractive. However, the key-point for the distributed mirror length selection was determined by the random laser behavior. RDFL dynamics are less investigated in rare-earth doped-fiber gain assisted systems than in the based on Raman gain. In order to contribute to the understanding of this class of laser, we carried out a consistent study of the RDF