Wavelength diversification of high-power external cavity diamond Raman lasers using intracavity harmonic generation.

We report a high power quasi-continuous-wave (QCW) 620 nm laser from an external cavity diamond Raman laser utilizing intracavity frequency doubling in lithium triborate. Output power of 30 W for durations of 0.25 ms at 15% conversion efficiency was achieved with a beam quality factor M2 = 1.1 from a free-running Nd:YAG pump laser of M2 = 1.5. The critical design parameters that affect conversion efficiency and power were analysed with the aid of an analytical model. By adaptation to other pump technologies, the diamond approach provides a novel pathway towards high brightness CW beam generation in the visible and ultraviolet regions.

Large brightness enhancement for quasi-continuous beams by diamond Raman laser conversion.

High average power lasers with high beam quality are critical for emerging applications in industry and research for defense, materials processing, and space applications. However, overcoming thermal effects in the gain medium remains the key challenge for increasing laser brightness at high powers. Here we report a means for increasing the beam brightness of high-power continuous-wave (CW) beams based on external cavity Raman lasers using diamond, a material with thermal properties far superior to any other laser material. With pump beam quality in the range M2=2.3-7.3, efficient pump-limited conversion to an M2=1.1 Stokes beam is achieved in all cases, with increases in brightness from the pump by factors as high as 12.7. The influence of pump beam quality on laser threshold and slope efficiency is analyzed. This Letter foreshadows an alternative approach for scaling the brightness of CW lasers using high-power, moderate beam quality pumps up to M2=20 or more, such as thin-disk and slab lasers and fiber lasers operating in a mode instability regime.

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.

High-gain 87 cm-1 Raman line of KYW and its impact on continuous-wave Raman laser operation.

We report a quasi-continuous-wave external cavity Raman laser based on potassium yttrium tungstate (KYW). Laser output efficiency and spectrum are severely affected by the presence of high gain Raman modes of low frequency (< 250 cm-1) that are characteristic of this crystal class. Output spectra contained frequency combs spaced by the low frequency modes but with the overall pump-to-Stokes conversion efficiency at least an order of magnitude lower than that typically obtained in other crystal Raman lasers. We elucidate the primary factors affecting laser performance by measuring the Raman gain coefficients of the low energy modes and numerically modeling the cascading dynamics. For a pump polarization aligned to the Ng crystallo-optic axis, the 87 cm-1 Raman mode has a gain coefficient of 9.2 cm/GW at 1064 nm and a dephasing time T2 = 9.6 ps, which are both notably higher than for the 765 cm-1 mode usually considered to be the prominent Raman mode of KYW. The implications for continuous-wave Raman laser design and the possible advantages for applications are discussed.

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.

SRS in the strong-focusing regime for Raman amplifiers.

The theoretical analysis of stimulated Raman scattering (SRS) in crystalline amplifiers with a tightly-focused pump geometry is presented. We predict the minimum Stokes seed power required for an efficient Raman power amplifier and verify this result experimentally. Conversion of a pump to a Stokes beam in a single-pass diamond amplifier is demonstrated using nanosecond pulses with gains of 5.8 from a 1.2-kW peak-power Stokes seed beam. The results demonstrate the possibility of amplifying and combining high-power continuous-wave lasers using current diamond Raman laser technology.

High power tungstate-crystal Raman laser operating in the strong thermal lensing regime.

We report an investigation into a double metal tungstate Raman laser when pumped at elevated average powers. Potassium gadolinium tungstate (KGW) was placed in an external cavity configured for second-Stokes output and pumped at pulse repetition rate of 38 kHz with up to 46 W of average power. For output powers above 3 W, we observe preferential excitation of Hermite-Gaussian transverse modes whose order in the X(1)(') principal direction of the thermal expansion tensor scales linearly with Raman power. We deduce that strong astigmatic thermal lensing is induced in the Raman crystal with a negative component in the X(1)(') direction. At maximum pump power, 8.3 W of output power was obtained at a conversion efficiency of 18%.

Thermal lens evolution and compensation in a high power KGW Raman laser.

The transient thermal lens in a high-average power double metal tungstate Raman laser has been investigated. An external cavity potassium gadolinium tungstate (KGW) laser designed for second-Stokes output was burst-pumped with up to 46 W of average power at a pulse repetition rate of 38 kHz. At low duty-cycle, the laser generated up to 18 W of on-time average Raman power with a conversion efficiency of 40%. At high duty cycle, efficiency is reduced and the near-field beam profile expands in the X1' crystal direction over a period of tens of milliseconds. The evolution of the spatial beam properties occurs in response to the development of a highly astigmatic thermal lens with fast-axis susceptibility of approximately -1.7 m-1 per watt of Raman output power. We show that the likely cause for astigmatism is primarily photo-elastic in origin. Beam circularization was achieved by incorporating an intracavity convex cylindrical lens.

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.

Theoretical modeling and experiments on a DBR waveguide laser fabricated by the femtosecond laser direct-write technique.

We present a model for a Yb-doped distributed Bragg reflector (DBR) waveguide laser fabricated in phosphate glass using the femtosecond laser direct-write technique. The model gives emphasis to transverse integrals to investigate the energy distribution in a homogenously doped glass, which is an important feature of femtosecond laser inscribed waveguide lasers (WGLs). The model was validated with experiments comparing a DBR WGL and a fiber laser, and then used to study the influence of distributed rare earth dopants on the performance of such lasers. Approximately 15% of the pump power was absorbed by the doped "cladding" in the femtosecond laser inscribed Yb doped WGL case with the length of 9.8 mm. Finally, we used the model to determine the parameters that optimize the laser output such as the waveguide length, output coupler reflectivity and refractive index contrast.

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.

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.

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

We investigate the polarisation-mode dynamics and Lamb's mode coupling constant for orthogonally polarised laser states in a dual-mode (100)-cut Nd:YAG laser with feedback, and compare with an anisotropic rate equation model. The anisotropic (100)-cut Nd:YAG exhibits thermally-induced depolarisation and polarisation-mode coupling dependent on the pump polarisation, crystal angle and laser polarisation directions. Here, the links between the depolarisation and polarisation-mode coupling are discussed with reference to a rate equation model which includes gain anisotropy in a quasi-isotropic laser cavity.

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.