CONTINUOUS-WAVE, ALL-SOLID-STATE, YELLOW LASER SOURCE AT 588 nm (2006)

We report continuous-wave powers up to 704 mW and quasi-cw powers up to 1574 mW (50% duty cycle) at 588 nm from a diode-pumped Nd:GdVO4 laser with intracavity Raman-shifting (in KGd(WO4)2) and intracavity frequencydoubling (in LBO), with diode-to-588 nm conversion efficiencies of up to 7%. We also present a rate-equation model of the laser which is used to outline the key design issues for cw intracavity doubled Raman lasers, in particular the roles played by thermal loading in the Nd:GdVO4 laser crystal, resonator mode sizes in the gain and nonlinear media and passive resonator losses. Introduction There is significant current interest in continuous-wave (cw) solid-state yellow laser sources for a diverse range of applications including ophthalmology, biomedicine and visual displays, and a variety of approaches have been taken to develop promising devices. These include an extra-cavity, frequency-doubled diode-pumped Yb fibre laser generating 40 mW at 575 nm with a diode-yellow conversion efficiency of 7% [1], a 3 W, 589 nm source based on extracavity frequency-doubling of the 1179 nm output of a Raman fibre laser pumped by a diode-pumped Yb fibre laser (estimated overall optical efficiency ~3%) [2], and a 0.75 W, 593.5 nm laser source based on sum frequency mixing of the 1064 and 1342 nm lines of two Nd:YVO4 lasers with a diode-yellow conversion efficiency ~11% [3]. Our approach to developing yellow sources uses diode-end-pumped Nd lasers incorporating intracavity Raman-shifting and second-harmonicgeneration in crystalline materials; we believe this approach could yield practical, cost-effective devices at low-medium powers (up to ~1 W) due to the small number of components, small physical size, and the low complexity to assemble. Crystalline Raman lasers, based on a variety of Nd-doped pump sources, have significantly increased the number of devices that can access the spectral region between 1.1 and 1.5 μm and also to the yellow-orange spectral region through frequency doubling of the Stokes output (or by direct pumping the Raman material with frequency-doubled Nd lasers [4]). A wide variety of experimental configurations have been employed to achieve Raman conversion, including intracavity Raman generation, external resonator based lasers and Raman generators (primarily used with ultrashort pulse pump lasers). A recent review [5] provides an overview of the field. Prior to 2004, all reported crystalline Raman lasers had been pulsed devices, and it was believed that the high peak powers required to reach threshold for SRS necessitated a Q-switched fundamental laser as the pump source for SRS. The first continuous-wave (cw) crystalline Raman laser was reported by Grabitchikov, using an external Raman resonator pumped by an Ar laser at 514 nm [6] Three subsequent papers [7,8,9] reported cw Raman laser action using intracavity resonator configurations, and for the first time diode-pumped laser systems. In this paper, we report a 704 mW cw intracavity frequency-doubled Raman laser which operates in the yellow at 588 nm. To the best of our knowledge this is the first demonstration of an all solid-state cw yellow laser based on crystalline media. We also outline the key design considerations required for efficient cw intracavity Raman laser action, and use numerical modelling to illustrate the interplay and trade-offs between the various design parameters. Laser Design The Raman laser configuration is illustrated in Figure 1. The resonator was bounded by a pair of flat mirrors having high transmission at the diode (808 nm) and yellow (588 nm) wavelengths, and high reflectivity at fundamental (1063 nm) and Stokes (1176 nm) wavelengths. Two sets of mirrors were used in the experiments. Mirror set A, for operation at the first Stokes wavelength, and also for the yellow, had coatings with 85%T at 808 nm, 0.09%T at 1063 nm, 0.4%T at 1176 nm and 93%T at 588 nm). Mirror set B, used to obtain the highest yellow powers, had coatings with 96%T at 808 nm, 0.006%T at 1063 nm, 0.004%T at 1176 nm and 95%T at 588 nm). Resonator stability was achieved by way of the strong positive thermal lens in the laser gain medium (the thermal lens in the Nd:GdVO4 laser crystal for a incident pump power of 21 W had focal length estimated from measurements of resonator stability to be +66 mm). Cavity length was kept to a minimum, and was ~45 mm for operation at 1176 nm (no LBO in cavity) and ~62 mm for operation at 588 nm. Australian Institute of Physics 17th National Congress 2006 – Brisbane, 3-8 December 2006 RiverPhys Paper No. WC016 2 The pump source was a 30 W fiber-coupled 808 nm diode laser (φ=400 μm, NA ~ 0.22), imaged with unity magnification onto an AR-coated (1064-1200 nm) a-cut 0.3at. % Nd:GdVO4 crystal (3x3x10 mm). Raman shifting was obtained using a KGd(WO4)2 (KGW) crystal with dimensions of 5x5x25 mm, AR-coated for the near-infrared and cut and oriented for propagation along the Np axis with the plane of polarization parallel to Nn. KGW was selected for its superior thermal properties, good Raman gain coefficient and high damage threshold [10]. Second harmonic generation (SHG) of the 1176 nm Stokes line was obtained using a temperature controlled (~45 °C), 3x3x10 mm non-critically phase matched (NCPM, θ=90°, φ=0°) LBO crystal coated AR at 1064-1200 nm. CW Laser Operation at 1176 nm (mirror set A) Threshold for lasing at the fundamental occurred for 0.7 W power from the laser diode, and we obtained Raman threshold for 6.6 W of incident pump power. Figure 2 shows the Stokes output as a function of pump power, together with the residual fundamental which becomes strongly depleted above threshold for SHG. Above threshold the 1176 nm firstStokes Raman power increased linearly with pump power, reaching 1563 mW from the output mirror for a maximum pump power of 20.4 W, limited by the onset of coating damage. Note also that the transmission of the input mirror was equal to that of the output mirror (0.4%T at 1176nm), thus approximately 1.5 W Raman power was lost from the input mirror. The low output coupler transmission at 1176 nm (0.4%) in comparison with the other resonator losses (estimated to be at least 1%) substantially limited the Raman laser output that could be obtained; clearly optimization of output coupling in relation to other losses would increase the output powers. CW Laser Operation at 588 nm (mirror set B) Nonlinear output coupling through frequency-doubling of the Stokes optical field is particularly well suited to extracting the Stokes optical field efficiently. Low thresholds are possible due to the high-Q cavity at both the fundamental and Stokes wavelengths), while at higher circulating fundamental and Stokes powers, losses are dominated by the nonlinear conversion to the visible, which is coupled from the resonator through a dichroic end-mirror. The cw yellow output power at 588 nm as a function of diode-pump power incident on the laser crystal in shown in Figure 3 (solid squares). At 15.5 W pump power the cw visible output was stable at 704 mW, with amplitude noise of approximately 12%. At higher pump powers, the visible output decreased and became unstable as the resonator approached the stability limit. We note a similar yellow power would have been lost either through the input mirror or would have been absorbed in the Nd:GdVO4 laser crystal (α588nm>2cm ). 2 4 6 8 10 12 14 16 18 20 22 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 O ut pu t p ow er ( W ) Incident pump power ( W ) Residual 1063nm Raman 1176nm 2 4 6 8 10 12 14 16 18 20 22 24 26 0 200 400 600 80