Highly efficient 110-W closed-cycle cryogenically cooled Nd:YAG laser operating at 946 nm.

We report a new, to the best of our knowledge, record power and efficiency for a 946 nm Nd:YAG laser, producing >110W (5TWm-2sr-1) with a slope efficiency of 80%, with respect to absorbed pump power, and an optical-to-optical efficiency of 74%, with respect to incident power. To achieve this performance, we utilized a closed-cycle acoustic Stirling cryostat to maintain the gain medium temperature at ∼80K and direct in-band pumping with a volume-Bragg-grating stabilized diode laser bar operating at 868 nm. Daily operation, including cycling of the laser crystal temperature, akin to a comparable water-cooled solid-state laser system, is demonstrated.

Characterising energy transfer upconversion in Nd-doped vanadates at elevated temperatures.

The Energy Transfer Upconversion (ETU) macroparameter is measured for Nd-doped GdVO4 and YVO4 samples at temperatures ranging from Room Temperature (RT) to 450K, by means of a simple and automated z-scan technique. Furthermore, the ground state absorption cross section into the 2H9/2 +4F5/2 energy levels is characterised for both crystals over the same range of temperatures. The 808 nm π-polarisation absorption cross section is found to decrease from (58.6 ± 0.2) pm2 to (30.9 ± 0.6) pm2 for Nd:YVO4 and (54.0 ± 0.3) pm2 to (25.7 ± 0.5) pm2 for Nd:GdVO4, from RT to 450K. Over the same range the ETU coefficient decreases from (3.2 ± 0.7) 10-16cm3/s to (1.8 ± 0.4 10-16cm3/s and (5.0 ± 0.5) 10-16cm3/s to (3.4±0.2) 10-16cm3/s for 0.6 at. % and 1 at. % Nd:YVO4 respectively, and (3.3 ± 0.5) 10-16cm3/s to (0.8 ± 0.2) 10-16cm3/s and (5.5 ± 0.5) 10-16cm3/s to (3.1 ± 0.3) 10-16cm3/s for 0.5 at.% and 1.1 at.% Nd:GdVO4.

Analytical thermal model for end-pumped solid-state lasers.

Fundamentally power-limited by thermal effects, the design challenge for end-pumped "bulk" solid-state lasers depends upon knowledge of the temperature gradients within the gain medium. We have developed analytical expressions that can be used to model the temperature distribution and thermal-lens power in end-pumped solid-state lasers. Enabled by the inclusion of a temperature-dependent thermal conductivity, applicable from cryogenic to elevated temperatures, typical pumping distributions are explored and the results compared with accepted models. Key insights are gained through these analytical expressions, such as the dependence of the peak temperature rise in function of the boundary thermal conductance to the heat sink. Our generalized expressions provide simple and time-efficient tools for parametric optimization of the heat distribution in the gain medium based upon the material and pumping constraints.

Amplification of a radially polarised beam in an Yb:YAG thin-slab.

The use of an Yb:YAG thin-slab architecture for amplification of a radially polarised beam at 1030 nm is investigated and shown to be a promising route for power scaling. The detrimental impact of the Gouy phase shift on radial polarisation purity is considered and a simple scheme for effective phase shift management to restore polarisation purity is presented. Preliminary experiments based on a double-pass amplifier configuration yielded an output beam with a high radial polarisation extinction ratio of 15 dB and no degradation in polarisation purity despite the non- axial symmetry of amplifier gain medium. At 50 W of launched pump power a small-signal gain of 7.5 dB was obtained for a 25 mW input, whilst 4.4 dB gain was obtained for a 1.45 W input. The prospects for further power scaling are discussed.

Adaptable beam profiles from a dual-cavity Nd:YAG laser.

We report a technique to tailor a laser beam profile from a donut to quasi-top-hat intensity distribution, directly from the laser, simply achieved by simultaneous excitation and control of the relative contributions of the fundamental (TEM00) and first-order Laguerre-Gaussian (LG01) transverse modes. Exploiting a dual-cavity configuration with a single Nd:YAG gain element, adaptable continuous-wave laser beam profiles from the primary cavity could be obtained by varying the diffraction loss of an acousto-optic modulator in the secondary cavity. We investigate the resultant beam profiles as a function of pump power and the AOM diffraction loss, and discuss the prospects for tunable laser beams profiles.

Cryogenically cooled 946nm Nd:YAG laser.

We present the first multi-Watt demonstration of a diode pumped cryogenically cooled neodymium-doped yttrium aluminum garnet (YAG) laser operating at 946 nm on the (4)F(3/2) → (4)I(9/2) transition. 3.8 W of continuous wave output power for 12.8 W of absorbed pump was obtained with a slope efficiency of 47%. In addition, we made an extensive characterization of the spectroscopic properties around the pump and laser wavelengths over the temperature range of 77 K to 300 K to find an increase of ~2.5 times for both the absorption and emission cross sections at the lowest temperature.

High-power Er:YAG laser with quasi-top-hat output beam.

A simple method for simultaneously exciting the fundamental (TEM00) transverse mode and first order Laguerre-Gaussian (LG01) donut mode in an end-pumped solid-state laser to yield a quasi-top-hat output beam is reported. This approach has been applied to an Er:YAG laser, in-band pumped by an Er,Yb fiber laser, yielding 9.6 W of continuous-wave output at 1645 nm in a top-hat-like beam with beam propagation factor (M2)<2.1 for 24 W of incident pump power at 1532 nm. The corresponding slope efficiency with respect to incident pump power was 49%. The prospects of further scaling of output power and improved overall efficiency are considered.

High power Er:YAG laser with radially-polarized Laguerre-Gaussian (LG01) mode output.

A simple method for conditioning the pump beam in an end-pumped solid-state laser to allow direct excitation of the first order Laguerre-Gaussian doughnut (LG01) mode is reported. This approach has been applied to a hybrid (fiber-laser-pumped) Er:YAG laser yielding 13.1 W of continuous-wave output at 1645 nm in a radially-polarized LG01 doughnut beam with beam propagation factor (M(2)) < 2.4 for 34 W of incident pump power at 1532 nm. The corresponding slope efficiency with respect to incident pump power was 48%. The prospects of further power scaling and improved laser performance are discussed.

Efficient in-band pumped Ho:LuLiF(4) 2 microm laser.

An efficient Ho:LuLiF(4) laser in-band pumped by a cladding-pumped Tm-doped silica fiber laser operating at 1937 nm is reported. At low-cavity output coupling, the Ho:LuLiF(4) laser yielded 5.1 W of output at a wavelength of 2066 nm for 8.0 W of absorbed pump power with a slope efficiency of 70%. At high-cavity output coupling, the lasing wavelength shifted to 2053 nm and the laser produced an output power of 5.4 W with a slope efficiency of 76%. The beam propagation factor (M(2)) was measured to be approximately 1.1 at the maximum output power confirming fundamental transverse mode (TEM(oo)) operation. The influence of resonator design on laser performance is discussed, along with prospects for further power scaling and improvement of the laser efficiency.

Influence of energy-transfer-upconversion on threshold pump power in quasi-three-level solid-state lasers.

A simple analytical expression for threshold pump power in an end-pumped quasi-three-level solid-state laser, which takes into account the influence of energy-transfer-upconversion (ETU), is derived. This expression indicates that threshold pump power can be increased dramatically by ETU, especially in low gain lasers and lasers with pronounced three-level character due to the need for high excitation densities in the upper manifold to reach threshold. The analysis has been applied to an Er:YAG laser operating at 1645 nm in-band pumped by an Er,Yb fiber laser at 1532 nm. Predicted values for threshold pump power as a function of erbium doping concentration are in very good agreement with measured values. The results indicate that very low erbium doping levels (approximately 0.25 at.% or less) are required to avoid degradation in performance due to ETU even under continuous-wave lasing conditions in Er:YAG.

Intra-cavity side-pumped Ho:YAG laser.

We present a novel, compact and power scalable Ho:YAG laser based on intracavity side-pumping by a high-power Tm:YLF slab laser. 14 W of continuous wave output power is obtained at 2.09 microm in the current experiments, with the clear prospect of reaching the 100 W regime in a power scaled version.

End-pumped, passively Q-switched Yb:YAG double-clad waveguide laser.

We present a diode-pumped, double-clad Yb:YAG waveguide laser that contains an integrated section of a Cr(4+): YAG saturable absorber for passive Q switching. Using two 4-W polarization-coupled, broad-striped diode-pumped lasers, we obtained 30-microJ pulses of 1.6-ns duration at repetition rates of as much as 77 kHz. The slope efficiency was ~50% with respect to absorbed pump power, with a maximum output average power of 2.3 W and a peak power of ~18 kW . The output beam was single lobed, with M(2) values as great as 1.5x1.3 . We also demonstrate a passively Q -switched Nd:YAG waveguide laser of similar design, operating at 1.064 microm and 946 nm.

Longitudinally diode-pumped Nd:YAG double-clad planar waveguide laser.

We report the demonstration of a near-diffraction-limited, compact, diode-end-pumped double-clad planar waveguide Nd:YAG laser. Efficient laser operation was achieved for the three dominant Nd(3+) transitions, at 1.064, 0.946, and 1.32microm , with TE polarized output powers of 1.33, 0.57, and 0.33 W for the available output couplers. The output beam from the monolithic plane-plane laser cavity had measured M(2) values of 1.0 and 1.8 perpendicular and parallel, respectively, to the plane of the waveguide.

Ion-exchanged tapered-waveguide laser in neodymium-doped BK7 glass.

We report what is to our knowledge the first operation of a planar dielectric tapered-waveguide laser. The waveguide laser is fabricated by potassium-ion exchange in Nd(3+) -doped BK7 glass and consists of a single-mode channel waveguide of a few micrometers' width followed by a linear taper up to a broad region with a width of ~180microm . A slope efficiency of 42% is found both in the tapers and in standard channel waveguides fabricated upon the same substrate, indicating that the tapers and the channels have similar internal losses; hence the low-loss nature of the tapered beam expansion. The output from either end of the tapered structure is found to be nearly diffraction limited.