High-power diode-bar-pumped Nd:YLF laser at 1.053-µm

Scaling diode-pumped solid-state lasers to multiwatt average power levels is an area which has attracted growing interest over recent years, stimulated by the wide commercial availability and relatively low cost of high-power cw diode-bar pump sources. Recent developments in this area have included; efficient, TEM00, end-pumped Nd:YVO4 and side-pumped Nd:YLF lasers at 1.064µm and 1.047µm respectively with cw powers in excess of 13W. So far, the scaling of diode-pumped solid-state lasers to >10W average power, whilst retaining high overall efficiency has generally been restricted to only the highest gain Nd transitions. Extension of efficient high average power operation to include other useful, but lower gain, transitions such as the 1.053µm transition in Nd:YLF, has been hindered by the inconvenient shape of the diode bar's output beam. The diode bar, with its highly elongated emitting region produces an output having M2 beam quality factors ~1 in the plane perpendicular to the array, but >1000 in the plane of the array. It is therefore difficult to focus to the small beam sizes required, particularly for low gain transitions in efficient end-pumped configurations

Efficient operation of an acousto-optically induced unidirectional and single-frequency Q-switched Nd:YLF laser

A highly-efficient Q-switched Nd:YLF ring laser, end-pumped by two 20W diode-bars is reported, yielding 3.5 mJ of single frequency TEMoo output for ~25W of incident pump power

High power diode-bar-pumped intracavity-frequency-doubled Nd:YLF ring laser

We report efficient cw operation of an intracavity-frequency-doubled Nd:YLF ring laser end-pumped by two beam-shaped 20W diode bars. A single-frequency, polarised output of 6.2W at 526.5nm (8.3W generated in the doubling crystal), was obtained in a TEMoo mode (M2 < 1.2

High power diode-bar-end-pumped Nd:YLF laser at 1.053µm

We describe efficient cw operation of a Nd:YLF laser on the 1.053µm transition, end-pumped by two beam-shaped 20W diode bars. Fundamental-transverse mode operation with output power of 11.1W for ~29.5W of incident pump power has been demonstrated. In Q-switched operation 8.4W of average power at a pulse repetition frequency of 40kHz and ~2.6mJ pulse energy at pulse repetition frequency of 1kHz were achieved

High-power diode-bar-pumped intracavity-frequency-doubled Nd:YAG and Nd:YLF ring lasers

The use of diode-pumped solid-state lasers as sources of high power visible light is an area which has attracted growing interest in recent years. This interest stems from the increased efficiencies available and compact size compared to Ar+ lasers. A further attractive feature of intracavity-frequency-doubled, single-frequency lasers, on which initial results were recently reported, is that axial-mode-hopping is suppressed. The explanation for this behaviour is based on the fact that adjacent (non-lasing) axial modes are further suppressed by an additional loss due to sum-frequency generation. This is twice the loss experienced by the lasing mode due to second harmonic generation. In a low loss resonator with efficient intracavity-frequency-doubling this extra loss can more than offset any gain advantage of adjacent modes closer to the gain peak. The net result is that continuous (mode-hop-free) tuning is possible over many axial mode spacings. Here we describe an intracavity-frequency-doubled Nd:YAG ring laser end-pumped by a 20W diode bar, and a Nd:YLF ring laser end-pumped by two 20W diode bars. In each case, a simple bow-tie cavity design was employed, with a Brewster-angled LBO crystal. In the case of Nd:YAG for a non-optimised laser mode size in the LBO crystal the laser produced ~1.4W of single-frequency output in the green at 532nm. By varying the cavity length we obtained a single-frequency, continuous (mode-hop- free) tuning range of ~40GHz corresponding to ~80 axial mode spacings. This range is consistent with predictions of a simple model accounting for the effects of nonlinear loss due to sum frequency generation. Nd:YLF offers the potential of an extended tuning range through its broader linewidth. Furthermore Nd:YLF is attractive for operation at higher powers due to its superior thermo-optical properties on the sigma-polarisation compared to Nd:YAG, providing that appropriate steps are taken to avoid thermally-induced stress-fracture. Results for this laser, end-pumped by two 20W diode-bars, include the generation of ~10.3W of single frequency 1053nm output in a TEMoo beam (M2 < 1.1), and 6.2W of green output at 526.5nm (corresponding to ~8.5W generated internally in the LBO) and a conversion efficiency of ~5% with respect to intracavity power. We have obtained a single frequency, continuous (mode-hop-free) tuning range of 72GHz corresponding to ~150 axial mode spacings. So far, the mode-hop-free tuning range has been limited by etalon effects due to imperfect AR coatings on the TGG Faraday rotator. With the elimination of these etalon effects, mode-hop-free tuning over a considerable fraction of the gain bandwidth should be achievable

Thermal lensing in high-power end-pumped Nd:YLF lasers

One of the major limitations of scaling diode-end-pumped solid-state lasers to high powers is introduced by thermal effects. An attractive feature of Nd:YLF has been its superior thermo-optical properties compared to other laser crystals. This is due to a decrease of refractive index with increasing temperature, creating a negative thermal lens, which partially compensates for the positive lens due to bulging of the rod end faces. Other advantages of Nd:YLF include its natural birefringence and its long fluorescence lifetime. The latter feature is of interest for high-power Q-switched operation. Problems in realising the true potential of the laser, however, have often been encountered, for underlying spectroscopic reasons as indicated, e.g., in [1]. We investigated the thermal lensing under lasing and non-lasing conditions within a diode-bar-pumped system. Under non-lasing conditions the thermal lens was measured using a Nd:YAG probe laser which double-passed the Nd:YLF rod. The resulting change in beam divergence was measured. Under lasing conditions the laser-beam waist size on the output coupler was measured. Hence, using the ABCD-matrix formalism, focal-length values for the thermal lenses were determined. The results showed a significant difference in the thermal lens under lasing and non-lasing conditions. In the former case a weak thermal lens was observed which varied linearly with pump power. Under non-lasing conditions a much stronger thermal lens was measured, whose power increased non-linearly with pump power. With 11 W of pump power incident on the crystal, a factor of 6 difference between lasing and non-lasing values of focal length was determined (Pi-polarisation, plane perpendicular to c-axis). These measurements demonstrate that significant additional heat is generated in the non-lasing case. A finite-element calculation, which considered the relevant processes including interionic upconversion, their contribution to thermal loading, as well as the temperature distribution in the Nd:YLF crystal, was performed. An experimentally observed fluorescence saturation at 1.05µm of more than 50 % under Ti:sapphire pumping was numerically reproduced, and the value of the published upconversion parameter [2] was thereby confirmed. With this information, the heat generation, spatial temperature distribution, and thermal lens under diode pumping were determined. The calculated thermal lens powers were in reasonable agreement with experimental results. Upconversion processes as well as the temperature dependencies of heat conductivity and thermo-optical parameters were found responsible for strong thermal lensing under non-lasing conditions and its non-linear behaviour with respect to absorbed pump power. Design improvement by a significant decrease of thermal lens power and spherical aberrations under Q-switched conditions can be achieved by increasing the pump-spot size, decreasing the dopant concentration and using a longer crystal, or detuning the pump wavelength from the absorption peak

Efficient Nd:YLF master-oscillator-power-amplifier with 15W single-frequency output at 1053nm

Efficient solid-state sources with high power, good beam quality and narrow-linewidth output are required for many applications. Power-scaling of diode-pumped single-frequency solid-state lasers to meet the requirements of these applications has been hindered by strong thermal effects which can degrade beam quality and efficiency, and often make the selection of a single-axial-mode difficult. A further problem is that changes in cavity length due to temperature fluctuations become more pronounced at high pump powers leading to mode-hopping. The latter problem can, in principle, be eliminated by employing frequency selective components (e.g. etalons) intracavity and by active stabilisation of the cavity length to an external reference cavity, but at considerable increase in complexity and cost, and often at the expense of a reduction in efficiency

Upconversion lifetime quenching and ground-state bleaching in Nd³⁺:LiYF₄

Since the Nd3+:LiYF4 system has some advantage over Nd3+:YAG and Nd3+:YVO4 for high-power scaling of diode-end-pumping, this system has been investigated under strong excitation. in this case using a Ti:sapphire pump. The interionic processes responsible for fluorescence saturation have been determined, due allowance being taken for the significant ground-state bleaching under these conditions. Their temperature dependence, which is relevant to scaling consideration, has been investigated theoretically, and found to be rather small over a wide temperature range. By comparing the experimental data with finite-element rate-equation calculations, the influence of interionic upconversion is determined quantitatively, and a published value of the upconversion parameter is confirmed. The spatial dependence of ground-state bleaching and quenching of the fluorescence lifetime is calculated. Analytical expressions are derived, including the influence of interionic upconversion, for the dependence of ground-state bleaching, excitation density, and storage time on pump parameters and dopant concentration