Analysis of a thermal lens in a diamond Raman laser operating at 1.1 kW output power.

We report experimental observations of thermal lens effects in a diamond Raman laser operating up to 1.1 kW output power in a quasi- steady-state regime. Measured changes in the output beam parameters as a function of output power, including beam quality factor and beam divergence after a fixed focusing lens, are compared to modelling enabling us to track the development of a thermal lens up to 16 diopters at maximum output power. Analysis shows that good agreement between model and experiment is obtained by considering the power deposition profile and the spatial overlap with the laser mode. The results clarify previous work that raised questions about thermal lens effects in the diamond gain medium and provides increased confidence in thermal models for determining the power limits for the current design.

1.2 kW quasi-steady-state diamond Raman laser pumped by an M2 = 15 beam.

An external cavity diamond Raman laser with 1.2 kW output power is demonstrated for durations 7 times longer than the thermal lens time constant. An 83% slope efficiency and a 53% optical-to-optical efficiency were obtained for conversion from a 1.06 µm pump to the 1.24 µm first Stokes. The pump had an M2 of 15, demonstrating that efficiency is maintained at the highest levels even when using exceptionally poor quality pumps. We show that a measured decrease in the output beam quality factor from M2=2.95 to M2=1.25 as power increased is evidence for thermal lens development in the diamond. The results foreshadow development of continuous-wave kilowatt-class lasers or amplifiers based on single diamond elements and pumped efficiently by lasers having poor spatial coherence such as line-narrowed diode laser arrays.

Diamond Raman laser with continuously tunable output from 3.38 to 3.80 µm.

We report a pulsed mid-infrared diamond Raman laser with output tuned from 3.38 to 3.80 µm through varying the optical parametric oscillator (OPO) pump wavelength. To our knowledge this is the longest reported wavelength from a solid-state Raman laser. We generated up to 80 µJ with good beam quality and 22% quantum conversion efficiency. Whilst the conversion process itself is efficient, approximately 40% of the generated Stokes light is lost to multiphonon absorption. By introducing a secondary pump beam at the anti-Stokes wavelength to initiate a seed at the Stokes wavelength through Raman resonant four-wave mixing, the laser threshold was reduced by approximately half, and the maximum output increased by 44% to 115 µJ.

Efficient conversion of a 1.064 µm Nd:YAG laser to the eye-safe region using a diamond Raman laser.

We report an efficient 1.485 µm external cavity diamond Raman laser operating on the 2nd Stokes shift of a 1.064 µm Nd:YAG pump laser. 1.63 W pulsed at 5 kHz is produced with a quantum conversion efficiency of 71% and excellent beam quality. Numerical modelling confirms that optimal operation is achieved with low output coupling reflectivity.

1240 nm diamond Raman laser operating near the quantum limit.

An external-cavity diamond Raman laser generating up to 2.0 W at 1240 nm from 3.3 W of 1064 nm pump power is investigated as a function of pump polarization direction. The maximum conversion efficiency was 61%, and the slope efficiency of 84% closely approaches the quantum limit of 85.8%. The lowest threshold for Raman lasing is achieved for pump polarization parallel to the <111> axis, which we show is consistent with theory.

Polarization conversion in cubic Raman crystals.

Nonlinear conversion of unpolarized beams to lower frequencies is generally inefficient in c(2) materials, as it is challenging to achieve phase-matching for input ordinary and extraordinary beams simultaneously in the normal dispersion regime. Here, we show that cubic Raman crystals having doubly and triply degenerate (E and F type) modes provide a method for efficient nonlinear frequency downconversion of an unpolarized beam and yield a linearly polarized output state. Using Mueller calculus, optimal crystal directions for such polarization conversion are determined. Using diamond, an example of an F-class Raman crystal, we have verified that such conversion is possible with near quantum-defect-limited slope efficiency and a linear polarization contrast of more than 23.9 dB.