Diode Pump Power [W]
3.4 C onclusions
The cw operation proved that the design aims for the Nd:YLF slabs were reached.
o The poor thermal properties of the material could be compensated for by choosing the right dimensions of the slab, making it feasible to end-pump it with up to 20W per surface (e.g. 40W total),
o The geometry of the slab combines wavelength selection in the anisotropic material and low losses without additional AR-coatings.
o The end-pumped approach ensures high efficiency into the TEMqo mode. An all-optical slope efficiency of 32% into T E Mqohas been achieved.
The table below gives a summary of the im portant experimental results and data.
M aximum output power Multimode 12W @ 36W pum p TEMqo m2 = 1.6 6.2W @ 20W pum p m2 = 1.2 3.0W @ lo w pum p
Slope efficiencies
TEMoo 32% all optical
35.5% absorbed power Multimode 36.5% all optical
40.5% absorbed power
Optimum output coupling
10% lo w pum p level
15% 20W pump level
Intra-cavity losses -2% near threshold (i.e.: up to 2W output)
Table 3.19: Summary of the key results and experimental data under cw operation at 1047nm.
The limitations of the end-pumped approach in a monolithic slab laser, utilising focused linear diode arrays, also became evident. Thermal distortions of the pum ped surfaces introduce diffraction losses into the cavity and ultimately limit the maximum achievable TEMoo/ as well as multi-mode, output powers.
These aberrations of the thermal lens could not be compensated for by using spherical mirrors. Only the idealised parabolic part of the thermal lensing could be offset by choosing the right curvature and geometrical length of the cavity.
c w and Q-switched performance at 1047nm - Chapters
The scalability of these folded resonator geometries is therefore dependent on the pum p power density per folding point and the total num ber of folding points. In the case of the proposed two pum p module scheme, the limit was 17W at each reflection point and therefore below the available pum p powers. Introducing more folding points by scaling the slab in length would reduce this pum p power per folding point further, as the distortions, if uncorrected, add up, so that not too much might be gained. Furthermore, additional degrees of freedom would be introduced, which might render the scheme impractical in term s of alignm ent. The proposed geom etry therefore seems a good compromise in terms of output power and complexity.
The preferred mode of operation is repetitive Q-switching of the cw pum ped NdiYLF laser, as it enhances its capabilities as a pum p laser. In particular, the beam quality, which is the most critical parameter of high power end-pum ped lasers, is im proved, as well as the reliability in achieving this high quality performance.
The combination of pum p geometry, laser material and acousto-optical Q- switching are responsible for the positive characteristics of this laser system:
o Short pulses and efficient TEMoo mode operation are a consequence of the end-pumped approach,
o Efficient linearly polarised output at high pum p powers, w ithout depolarisation losses, and with good energy storage, leading to short pulses and relatively high energy levels per pulse, are the consequences of the natural birefringence and long upper state life time of the chosen NdiYLF laser material.
o Unproblem atic high repetition rate operation, which is inherent to the acousto-optical switching technique, is reflected in the good pulse to pulse stability of the Q-switched system. The flawless performance of the Q-switch is one of the foundations of the accomplished results.
Depending on the pum p power level and resonator configuration, three different modes of operation, optimising either beam quality / pulse duration
(low
pump) or beam quality / average power(20W
pump) or average power (30W pump) have been investigated. The results and im portant experimental parameters are shown in the table on the next page.c w and Q-switched performance at 1047nm - Chapter 3 M aximum output power m2 = 1.8 7.6W ©8kHz, 25ns
30W pum p power Cavity: Lgeo. = 20cm
M aximum efficiency (i.e.: Best compromise between M^ and Pout)
m2 = 1.3 5W © 6kHz, 27ns
20W pum p power Cavity: Lgeo. = 20cm
Shortest pulses M2 = 1.1 1W@ 2kHz, 12ns lo w pum p power Cavity: Lgeo. = Hem
O utput coupling T = 30%
Table 3.20: Results and experimental data at characteristic pump power levels for the Q- switching experiments.
The most restrictive limitation in the present set-up is the coating damage on the Nd:YLF slab. This restricts the safe operating peak power to 40kW per pulse (80M W /cm2 intra-cavity). onset of coating dam age is not clearly defined and this grey zone reaches up to 70 - 80kW per pulse. These levels are possible to sustain for short periods of time. This technical problem should be cured with harder coatings, possibly at the expense of slightly higher losses, which are associated with this type of coating.
Finally, the switching time of the acousto-optical Bragg-cell will set the peak power limit. This is a principal restriction, which can only be lessened by m inim ising the mode size at the location of the Bragg-cell. However, a resonator set-up approaching the optim ised configuration - close to the stability border, with the Q-switch as close as possible to the mirror on which the cavity mode will focus during its transition into the unstable region - is already employed. Therefore, a further reduction of the mode size, and thus switching time reduction, will only be marginal and is also limited by the damage threshold of the Bragg-cell (^00M W /cm 2, estimate by supplier).
Despite these restrictions under extreme modes of operation, a reliable, high average power, near diffraction lim ited, coherent light source has been achieved with the present system.
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