P.J. Hardman

Energy-transfer upconversion and thermal lensing in high-power end-pumped Nd:YLF laser crystals

Thermal lensing in an end-pumped Nd:LiYF/sub 4/ rod, under lasing and nonlasing conditions, has been investigated. Under lasing conditions, a weak thermal lens, with dioptric power varying linearly with pump power, was observed. Under nonlasing conditions, where higher inversion densities were involved, hence relevant to Q-switched operation or operation as an amplifier, a much stronger thermal lens was measured, whose power increased nonlinearly with pump power. This difference has been attributed to the increased heat deposition due to the subsequent multiphonon decay following various interionic upconversion processes, which increase strongly under nonlasing conditions, and is further exacerbated by the unfavorable temperature dependencies of heat conductivity and the rate of change of the refractive index with temperature. A strategy for reducing upconversion and its associated thermal loading, without degrading laser performance, is discussed.

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.

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

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

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 (M 2 < 1.2)

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

We describe efficient Q-switched operation of a single-frequency Nd:YLF ring laser, end-pumped by two 20W diode-bards. The thermal problems at low Q-switching repetition rates, due to inter-ionic upconversion, and the degradation in efficiency, associated with the slow switching speed of the A-O modulator, have been alleviated, yielding ~3.5 mJ of single frequency TEM00 output with excellent beam quality (M2<1.1).

The Influence of Energy-Transfer Upconversion on Thermal Lensing in End-Pumped ND:YLF and ND:YAG Lasers

A large increase in thermal lens power in Nd:YLF (factor ~6) and Nd:YAG (factor ~2), caused by upconversion, has been experimentally measured and theoretically calculated under conditions of high excitation density (typical of Q-switched lasers) when compared to CW lasing conditions.

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

Summary form only given. 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 active stabilisation of the cavity to an external reference cavity, but at considerable increase in complexity and cost. Here we report an efficient diode-end-pumped Nd:YLF master oscillator and power amplifier (MOPA), with >15 W of reliable single-frequency output at 1.053 /spl mu/m, exploiting the weak thermal lensing behaviour on the lower gain /spl sigma/-polarisation to avoid degradation in beam quality, and utilizing a simple technique to suppress axial-mode-hopping in the master oscillator.

Mode-hop-free tuning in high-power intracavity-frequency-doubled Nd:YAG and Nd:YLF ring lasers

By simple cavity length adjustment, continuous, mode-hop-free tuning over many axial-mode spacings has been observed in efficient, intracavity-frequency-doubled, single-frequency Nd:YAG and Nd:YLF ring lasers generating multiwatt output powers at 532nm and 526.5nm

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 bars 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.