Stable, continuous-wave, fiber-laser-based, ultraviolet generation in BiB<inf>3</inf>O<inf>6</inf>

Abstract

Summary form only given. Nonlinear frequency conversion has been proven to be an attractive alternative to access spectral regions that are inaccessible to conventional laser sources. Still, the high-power, continuous-wave (cw) ultraviolet (UV) generation, based on frequency conversion processes, has been challenging due to the short-wavelength transparency cut-off and low nonlinearity of the available nonlinear materials. Some of the promising birefringent nonlinear materials for UV generation are LiB<;sub>3<;/sub>O<;sub>5<;/sub> (LBO), b-BaB<;sub>2<;/sub>O<;sub>4<;/sub> (BBO), and bismuth triborate, BiB<;sub>3<;/sub>O<;sub>6<;/sub> (BIBO). The commercially available all-solid-state cw UV sources employ intracavity sum-frequency generation (SFG) schemes in LBO, which require mandatory active stabilization of the cavity and temperature phase-matching. On the other hand, single-pass schemes are simple, compact, robust, and do not require active stabilization. However, the low nonlinearity of LBO and BBO precludes them from single-pass SFG. The quasi-phase-matched (QPM) nonlinear material such as MgO:sPPLT with high nonlinearity and no spatial walk-off, also has transparency into the UV. However, 1<;sup>st<;/sup>-order QPM SFG into the UV requires a grating period of ~2 μm, which is quite challenging to fabricate. As a result 3<;sup>rd<;/sup>-order QPM, with a reduced effective nonlinear coefficient of 2.7 pm/V has been used to generate UV radiation under temperature phase-matching [1]. On the other hand, BIBO can be phase-matched at room temperature for UV generation by SFG of infrared and green, under type-I (ee→o) configuration [2]. It possesses high nonlinearity (d<;sub>eff<;/sub>~3.4 pm/V), bulk UV damage threshold (50 MW/cm<;sup>2<;/sup>) and low UV absorption coefficient (α<;sub>UV<;/sub><;0.02 cm<;sup>-1<;/sup>).Here we report, for the first time to our knowledge, the generation of cw UV generation at 354.7 nm in BIBO using SFG of a cw Yb-fiber laser as the fundamental source. The schematic of the experimental setup is shown in Fig. 1, where a 30-mm-long MgO:sPPLT (A=7.97 μm) is used for second-harmonic generation into green and a 10-mm-long BIBO (θ=146.3o, φ=90o) is used for SFG of the generated green and the fundamental into the UV.To characterize the generated UV output, we performed power scaling measurements, with the results shown in Fig. 2. We achieved as much as 68 mW of UV power at 354.7 nm for a fundamental power of 27.8 W at 1064 nm. During the measurements, as the fundamental power is increased, the phase-matching temperature of MgO:sPPLT crystal is always adjusted to generate maximum UV power. The long-term passive power stability of the generated UV output at -50 mW of output power has been recorded, and found to be better than 3.2% rms over 2 hours and 1.5% rms over 50 minutes. The inset of Fig. 2 shows frequency stability of the generated UV output, measured using a wavemeter (High finesse, WS/U-30). Under free-running conditions and in the absence of thermal isolation, the UV output exhibits a peak-to-peak frequency deviation <;437 kHz over the >2.5 hours, measured at a central wavelength of 354.7944 nm. In order to confirm the Gaussian distribution, we measured the M2 factor of the UV beam at the highest available fundamental power, resulting in Mx 2<;1.6 and My2<;1.8, indicating a TEM00 spatial mode. The UV beam exhibits an ellipticity of 66% due to the effects of spatial walkoff, but can be readily circularized using suitable cylindrical optics. Detailed discussion of experimental results, including the optimization of UV power, effects of the mixing ratio of the green and input fundamental power on UV power, and long-term performance of the system after 10 hours of operation will be presented.