Jipeng Lin

Continuous-wave Watt-level Nd:YLF/KGW Raman laser operating at near-IR, yellow and lime-green wavelengths.

A Nd:YLF/KGW Raman laser has been investigated in this work. We have demonstrated CW output powers at six different wavelengths, 1147 nm (0.70 W), 1163 nm (0.95 W), 549 nm (0.65 W), 552 nm (1.90 W), 573 nm (0.60 W) and 581 nm (1.10 W), with higher peak powers achieved under quasi-CW operation. Raman conversion of the 1053 nm fundamental emission is reported for the first time, enabling two new wavelengths in crystalline Raman lasers, 549 nm and 552 nm. The weak thermal lensing associated with Nd:YLF has enabled to achieve good beam quality, M(2) ≤ 2.0, and stable operation in relatively long cavities.

Spectral broadening in continuous-wave intracavity Raman lasers

Spectral broadening of the fundamental field in intracavity Raman lasers is investigated. The mechanism for the spectral broadening is discussed and the effect is compared in two lasers using Raman crystals with different Raman linewidths. The impact of the spectral broadening on the effective Raman gain is analyzed, and the use of etalons to limit the fundamental spectral width is explored. It was found that an improvement in output power could be obtained by using etalons to limit the fundamental spectrum to a single narrow peak.

Highly efficient picosecond diamond Raman laser at 1240 and 1485 nm

We present a highly efficient picosecond diamond Raman laser synchronously-pumped by a 4.8 W mode-locked laser at 1064 nm. A ring cavity was adopted for efficient operation. With a low-Q cavity for first-Stokes 1240 nm, we have achieved 2.75 W output power at 1240 nm with 59% overall conversion efficiency. The slope efficiency tended towards 76% far above the SRS threshold, approaching the SRS quantum limit for diamond. A high-Q first-Stokes cavity was employed for second-Stokes 1485 nm generation through the combined processes of four-wave mixing and single-pass stimulated Raman scattering. Up to 1.0 W of second-stokes at 1485 nm was obtained, corresponding to 21% overall conversion efficiency. The minimum output pulse duration was compressed relative to the 15 ps pump, producing pulses as short as 9 ps for 1240 nm and 6 ps for 1485 nm respectively.

Cascaded self-Raman lasers based on 382 cm-1 shift in Nd:GdVO4

We report quasi-continuous-wave, cascaded Nd:GdVO(4) self-Raman lasers based on a secondary Raman transition at 382 cm(-1) for which the Raman gain was estimated to be 0.7 cm/GW. Laser output was obtained in the near-infrared at 1108 nm, 1156 nm and 1227 nm. By incorporating intracavity sum-frequency generation (SFG) or second-harmonic generation (SHG), high power output at four discrete visible wavelengths could be selected, specifically 3.4 W at 542 nm, 2.8 W at 554 nm, 1.4 W at 566 nm and 0.8 W at 577 nm, with corresponding diode-to-visible optical conversion efficiencies of 11.7%, 9.7%, 4.8% and 2.7% respectively.

Near-infrared and orange-red emission from a continuous-wave, second-Stokes self-Raman Nd:GdVO4 laser.

Stimulated-Raman-scattering (SRS) is a cascading nonlinear optical process, enabling the generation of traditional hard-to-reach wavelengths in the near-infrared, visible and UV spectral range [1]. To generate high order Stokes emission in the CW regime, at reasonable threshold and efficiency, high circulating power densities are required (typically 10–100 MW/cm2), and so the resonator is usually designed with high-Q for both fundamental and lower order Stokes wavelengths. The first CW crystalline second-Stokes Raman laser was reported by Grabtchikov et al. in 2009 [2]. They used a Raman resonator externally pumped at 514 nm by an Ar+ laser, for which second-Stokes emission occurred with a threshold of 3.67 W, and a maximum output power of 21 mW at 576 nm was achieved.

Efficient diamond Raman laser generating 65 fs pulses

We report a synchronously-pumped femtosecond diamond Raman laser operating at 890 nm with a slope efficiency of 32%. Pumped using a mode-locked Ti:Sapphire laser at 796 nm with a pulse duration of 194 fs, the bandwidth of the Stokes output was broadened to enable subsequent pulse compression to 65 fs using a prism-pair. Modelling results provide an understanding of the physical mechanisms involved in the Raman conversion of femtosecond pulses, supporting an in-depth characterization of these ultrashort pulsed lasers.

Investigation of blue emission from Raman-active crystals: Its origin and impact on laser performance

The origin and consequences to laser performance of blue emission observed in some Raman crystals has been studied in detail, leading us to attribute the origin of the phenomenon to fluorescence from Tm3+(1G4) impurity ions which are excited via sequential upconversion. For the specific case of a Nd:YLF/KGW Raman laser, we show that the blue fluorescence has modest but significant impacts on laser performance and thermal loading. If the blue fluorescence was eliminated, then laser efficiency could be increased by 15% and thermal loading in the KGW crystal reduced by 17%.

Study of relaxation oscillations in continuous-wave intracavity Raman lasers

We study the relaxation oscillations in a continuous-wave intracavity Raman laser both theoretically and experimentally. Analytic expressions for the relaxation oscillation frequency are derived from the rate-equations and are validated by experiments. We show that some important experimental parameters such as the effective Raman gain coefficient and intracavity Stokes loss can be determined simply by measuring the relaxation oscillation frequency versus pump power.

Birefringence and piezo-Raman analysis of single crystal CVD diamond and effects on Raman laser performance

Defect-induced stress has been mapped in optical-grade synthetic diamond (chemical vapor deposition grown, low nitrogen, low birefringence) using Metripol polarimetry, Mueller polarimetry, and Raman microscopy. Large circular retardance was observed in the 8 mm long ⟨110⟩ cut crystal with values up to 28° for some paths along the major axis. Metripol-determined values for linear birefringence magnitude and fast-axis direction in such regions have significant error. Stress-induced shifts in Raman frequency were observed up to 0.7  cm−1, which we deduce result from uniaxial and biaxial stresses up to 0.86 GPa. We also elucidate the effect of stress on diamond Raman laser performance. For high cavity Q Raman lasers, the direction of the linear birefringence axis is found to be a primary factor determining the laser threshold and the input–output polarization characteristics.

Continuous-wave VECSEL Raman laser with tunable lime-yellow-orange output

We report a compact CW KGW Raman laser with intracavity nonlinear mixing, pumped by the intracavity field of a VECSEL. By temperature tuning an intracavity LBO crystal, we obtained two separate tunable emissions bands, namely 548.5 - 566 nm for sum-frequency-generation (SFG) of the fundamental and Stokes wavelengths, and 577.5 - 596 nm for second-harmonic-generation (SHG) of the Stokes wavelength. The maximum output powers for SFG and SHG were 0.8 W @ 560 nm and 0.52 W @ 592.5 nm, with corresponding diode-to-visible optical conversion efficiencies of 4.2% and 2.9%. These preliminary results show strong potential for expanding the spectral coverage of VECSEL lasers.