Aaron McKay

Continuous-wave wavelength conversion for high-power applications using an external cavity diamond Raman laser

We demonstrate continuous-wave (cw) operation of a diamond Raman laser at 1240 nm in an external cavity configuration. The output power increased linearly with pump power with a 49.7% slope efficiency and reached 10.1 W at the maximum available pump power of 31 W. The combination of resonator design with diamond provides a novel approach to power-scalable cw wavelength and beam conversion.

Investigating diamond Raman lasers at the 100 W level using quasi-continuous-wave pumping

Quasi-cw pumping is used to investigate the high-power characteristics of cw beam conversion in diamond Raman lasers (DRLs). We show that thermal gradients establish in DRLs at approximately 50 μs for a 100 μm pump beam diameter, and thus that the steady state for cw operation can be reached within the 100-300 μs pulse duration of conventional quasi-cw pump laser technology. Using this approach, a steady-state on-time output power of 108 W was obtained from an external-cavity DRL during 250 μs pulses with 34% conversion efficiency. No thermal lens in the diamond was evident, showing excellent prospects for further power scaling.

An efficient 14.5?W diamond Raman laser at high pulse repetition rate with first (1240?nm) and second (1485?nm) Stokes output

Efficient operation of a pulsed diamond Raman laser at output powers up to 14.5 W is reported. An external-cavity Raman resonator 2.5 cm long employing a low-loss CVD diamond 9.5 mm long generated first (1240 nm) and second (1485 nm) Stokes output powers of up to 3 W and 13 W respectively when pumped with a 30 W q-switched Nd:YVO4 laser with approximately 22 ns pulses at a 36.5 kHz pulse repetition frequency. We show that this approach to efficient wavelength conversion is promising for a range of practical pulsed laser systems with high average power at first and cascaded Stokes orders in the near-infrared and including eye-safe laser output.

Tunable Terahertz Signals Using a Helicoidally Polarized Ceramic Microchip Laser

A two-frequency microchip laser based on highly doped ceramic Nd:YAG and twisted polarization modes is presented. The frequency difference between modes was tunable continuously over a frequency range of 150 GHz, from radio frequency to terahertz frequencies. This tuning is limited by the gain bandwidth of the neodymium-doped YAG laser medium. The long-term frequency stability of the resulting narrow-linewidth beat-note signal is very good. This offers a simple, yet widely tunable terahertz source with potential for portable frequency reference applications.

Modelling and optimization of continuous-wave external cavity Raman lasers

We report an analytical model describing power and efficiency of a 23 W quasi-continuous-wave diamond Raman laser. The model guides the optimization of the first Stokes output power as a function of resonator and crystal parameters. We show that, in the limit of a weak thermal lens, efficient operation requires strong focussing, low output coupling and low-absorption crystals. Efficient damage-free operation at higher pump powers is predicted to benefit greatly from increased optimum output couplings that act to limit the intracavity Stokes field.

Polarisation-mode coupling in (100)-cut Nd:YAG

We investigate mode coupling experimentally and with an anisotropic rate equation model, for orthogonally-polarized laser states in dual-mode (100)-cut Nd:YAG crystal lasers. By optimizing the weak coupling between these laser modes we can increase the self-heterodyne beat signal.

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.

Effect of gain anisotropy on low-frequency dynamics in four-level solid-state lasers

Our anisotropic rate equation model outlines the relationship between the relaxation dynamics in a four-level solid-state laser and its anisotropic gain properties. Anisotropic pump rates and stimulated emission cross-sections were included to account for specific atom orientations in the gain material. The model is compared with experimental measurements of two relaxation oscillation frequencies which are related to the anisotropic atom-laser interaction in orthogonally polarized dual-mode lasers. The model predicts that crystal orientation and pump polarization affect the laser operation characteristics, as found experimentally. The gain anisotropy influences the fast laser dynamics, as in single-mode relaxation oscillations.

A comparison of tunable, passively-stabilized two-frequency solid-state lasers for microwave generation

We are developing tunable microwave sources based on a passively stabilized diode-pumped Nd: YAG laser. The laser operates on two orthogonal modes, which may be tuned by applying a voltage to an intra-cavity electro-optic crystal or by rotating an intra-cavity waveplate. We compare and characterize these alternative cavity designs in terms of power, tunability and noise.

Single longitudinal mode diamond Raman laser in the eye-safe spectral region for water vapor detection

We report a narrowband and tunable diamond Raman laser generating eye-safe radiation suitable for water vapor detection. Frequency conversion of a tunable pump laser operating from 1063 to 1066 nm to the second order Stokes component in an external standing-wave cavity yielded 7 W of multimode output power in the wavelength range from 1483 to 1488 nm at a conversion efficiency of 21%. Stable single longitudinal mode operation was achieved over the whole tuning range at low power (0.1 W), whereas incorporation of a volume Bragg grating as an output coupler enabled much higher stable power to be attained (0.5 W). A frequency stability of 40 MHz was obtained over a minute without active cavity stabilization. It was found that mode stability is aided via seeding of the second Stokes by four-wave mixing, which leads to a doubling of the mode-hopping interval. The laser was employed for the detection of water vapor in ambient air, demonstrating its potential for remote sensing applications.