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[18] A commercial device is now available using this technology. The MOPO 700 series from

Specu-a-Physics

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CHAPTER 6

Conclusions

The main aim of this work was to demonstrate that the high quality output obtainable

from a diode pumped solid-state laser could be used to efficiently pump an OPO when the energy available from the laser was limited. The results presented in the previous chapter demonstrate the achievement of that aim. In this chapter, each aspect of the work leading up to the achievement of the above general aim is discussed, as are ways in which improvements and further work could be implemented.

Chapter 4 described the construction of a number of pump lasers, with the end-pumped Nd: YLF laser being of sufficient energy to pump the OPO above threshold. Therefore the work on pump lasers can be considered to have achieved its aim in the sense that a laser was built which allowed operation of the OPO. The important parameters for this laser were the selection of an end pumping geometry and the host material Nd: YLF. The work with the side pumped lasers using NdiYAG as the gain medium showed that the large spatial spread of the gain associated with this geometry resulted in higher

thresholds and poorer efficiencies than were obtained with end pumping. The coupling

scheme adopted here where the output of a single diode bar is focused into a small gain

volume showed that diode bars could be used effectively in end pumping geometries. The resulting high single pass gain and good mode overlap obtained showed relative insensitivity to intracavity loss and high energy extraction was achieved, as witnessed by an output of 2 mJ in an 18 ns Q-switched pulse for a pump energy of 12 mJ.

Immediate improvement of this efficiency might be obtained by replacing the prism pair with its relatively high insertion loss (4 % per pass, which was likely due to poor quality manufacture) by a cylindrical telescope, where the small astigmatism can also be compensated for by incorporating a small power into the telescope.

In the low average power regime, it was seen in this work that YLF is a superior host to Y AG for the Neodymium ion. The main reason for the superior performance of YLF in this work was the stronger absorption of the pump light. The shorter absorption depth therefore meant that the highly diverging pump light was confined to a smaller volume in the YLF material. The higher gain per unit area resulted in a smaller threshold, and the improved confinement of the gain to the axis of the rod allowed more efficient coupling of the laser mode to the gain resulting in more efficient output. YLF would also appear to be a superior material when scaling to higher output power is required. The natural birefringence and superior thermal lensing properties result in considerably

Ch. 6 : Conclusions

reduced thermal problems in YLF. This argument only holds up to the thermal fracture limit, which is smaller in YLF than in Y AG, requiring that Y AG, or an alternative host, be used for very high average power operation. However, the low thermal fracture of YLF may be due to the growth technology of this material being less mature than that

of YAG as recent work in these labs has shown that cw powers of 18 W can be input to thin slabs of Nd:YLF without causing fracture.

Scaling of these devices to higher energies and average powers appears to be relatively straightforward. A number of schemes have been reported in the literature for coupling the output of more than one diode bar into a single gain element. Thermal fracture considerations dictate the maximum power density that can be input, restricting the

focusing of the diode light in the plane perpendicular to the diode junction. However, this has the advantage that a more circular gain region is obtained, and so anamorphic expansion of the laser mode would no longer be required.

The use of multiple gain elements in oscillators and also oscillator/amplifier set ups allows high power and energy outputs to be obtained by using the more efficient geometry of end pumping. With the price of diode laser bars decreasing due to increased demand it will not be long before stable, compact and efficient diode pumped solid-state lasers can effectively replace their lamp pumped equivalents.

The agreement between the thresholds calculated for a pulsed singly resonant OPO with the computer model, based on introducing build-up time effects to the Guha analysis, and the experimentally measured ones was seen to be as good as 50 % under the conditions investigate here. Considering the complexity of the interaction involved this agreement is encouraging. Direct comparison between this model and the established model due to Brosnan and Byer (BB) shows that when the interacting beams are focused to a reasonable degree, the model developed here with less limiting assumptions is expected to give a better prediction of performance than the BB model which is only directly applicable to the weakly focused regime. Measurement of threshold under a wider range of conditions (e.g. pump and signal focusing) and with improved determination of actual experimental parameters (e.g. crystal losses) should

give a better vindication of the modelling. In any event, the threshold curves produced from the model provide a basis for optimisation of the OPO parameters.

An empirically derived relation for internal efficiency (pump depletion) allows calculation of expected system efficiencies and optimisation of this. The parameters which need to be optimised are mode overlap (optimisation of pump and signal

focusing), pulse duration and output coupling. It would be assumed that AR coating of the crystal faces for pump and signal would allow external efficiencies to match internal

Ch. 6 : Conclusions be expected.

Scaling to higher powers seems feasible, and has been demonstrated by Marshall and co-workers at Fibertek [1]. Optical damage to the crystal coatings (no bulk damage was observed) was seen to be the limiting factor on how much power could be input to the OPO. No damage was observed when only the pump was incident, and the cause of the damage was seen to be the resonant wave (signal in this case). By using a sufficiently high output coupling (27 %), when 2.65 mJ from a Nd:YAG laser was incident on the OPO, 0.7 mJ were produced at the signal, where the reduced Q of the signal cavity avoided damage to the coatings without increasing the threshold dramatically. A high

output coupling also has the advantage that when there are intracavity losses present, the external efficiency is greatest with a large output coupling. The limits of scalability of the present device are therefore dominated by the coating damage threshold.

Increasing pump and signal spot sizes would allow more input pump power at the expense of threshold, however, pumping sufficiently above threshold would result in an increased output at signal and idler.

The use of the concentric 'zero-power' mirrors was seen to be a central component in the

operation of the OPO. The zero-power nature eased considerably the alignment difficulties that had been experienced with plano-concave mirror substrates. The small modes sizes and strong overlap produced by the curved mirrors was responsible for the low thresholds, but also seemed to limit the tuning to within the gain bandwidth as adjustment of the mirrors was unable to compensate for the perturbation of the cavity as the crystal was rotated away from normal incidence. The mirrors also had a pronounced effect on the detailed spectral properties of the oscillator due to their resonant reflector behaviour. It was seen that this characteristic of the mirrors restricted the number of signal axial modes that were above threshold, and was seen to produce single-axial mode operation on occasion. Although this was not the original intention, proper design

of the mirror coatings might provide a way of achieving single mode operation without the requirement of additional intracavity mode selection optics, or a seeding source. The use of two mirror substrates of different thicknesses, and hence different free-spectral ranges, might allow controlled selection of a single axial mode. The weak resonant

reflection (weak as one side was specified to be highly transmitting) was seen to be sufficient to have some control over the modes, suggesting that any mirror resonances would not have to be very sharp to obtain mode control, which would make overlapping

of the resonances easier and less critical.

The NCPM geometry and the cavity geometry resulted in the OPO operating as an efficient down-converter. The 1.54 jxm signal wavelength is a particularly useful wavelength as it is described as 'eyesafe' and has many applications including

Ch. 6 : Conclusions

produces it more efficiently than the usual alternative method which relies on an inefficient three level transition in Er^+ doped phosphate glass.

Wavelength diversity could be obtained, while retaining the low threshold and high efficiency operation, by the use of a tunable pump source. For example, use of a Ti:sapphire laser as the pump would allow tuning over most of the region 1-3.3 jim. Alternatively, if a higher power Nd: YLF pump laser was constructed, then critical phase matching could be employed where the higher power available from the laser would be required to offset the increase in threshold that would ensue due to the operation away from NCPM. Operation further into the IR could be achieved if other non-linear

materials were considered. The material KTA, an isomorph of KTP which is currently under development, has higher non-linear coefficients than KTP and transmits to 5 |im, which would allow access to the technologically important regime of 3-5 jim, which is inaccessible with KTP.

In summary, the work here describes an extension to the theory of pulsed OPOs brought about by a computer model which introduces time dependence to the steady-state theory of Guha et al. Use of the high quality output from an end pumped, Q-switched Nd;YLF laser allowed demonstration of a low threshold, efficient OPO based upon non-critically