Optimising Signal-Pump Overlap

In the first amplifier stage, the pump beam spot size was chosen based on con- focal focussing of the beam through the crystals to achieve approximately con- stant pump size over the crystal length. For the 400µm core diameter, 0.22N A fibre used for pump delivery in the first amplifier, the corresponding pump beam M2 value was ∼170. This implied a pump waist radius of 330µm for

confocal focusing of the pump through a 10mm long Nd:YVO4 crystal. To

determine the optimum size of the signal beam in the amplifier crystals and also to test the validity of the numerical model, described in Chapter 4, an ex- periment was conducted to vary the signal size while keeping the pump size constant. The experimental layout is shown in Figure 5.9. The output from the oscillator, while operating in cw mode with the mode-locker switched off, was first passed through an isolator to eliminate feedback from the amplifier. The beam was then passed through a lens, which produced a slowly converging beam which slowly reduced in size, so that the amplifier position could be ad- justed to give access to a range of signal spot sizes. This gave beam radii in the crystals in the range 1000-200µm over a distance change of 15cm. This im- plied that the beam remained approximately constant over the crystal length to within±25µm and the relay imaging ensured that both crystals had the same signal and pump beam sizes.

Figure 5.9: Experimental layout used to determine the influence of signal-pump overlap on power and beam quality of the amplifier output. BS: 1% reflectivity beam splitter

The amplifier was pumped from both ends by the two 30W fibre coupled diode bars giving a total incident pump power of∼60W. The output from the ampli- fier was passed through a beam-splitter so that a small fraction of the power could be analysed using a Coherent Modemaster to measure the M2 _{value of}

the beam. The remaining power was simultaneously measured using a Gentec thermal power meter. The resulting power gain, as a function of signal-pump ratio, is shown in Figure 5.10. The graph also includes the predicted gain from the numerical model, in Section 4.2.5, where the saturation fluence and input fluence is replaced by the saturation intensity and input intensity in equation (4.43). The measured total input power was 4.66W and the input beam had anM2 _{value of 1.08. The total small signal gain used in the models was deter-}

mined experimentally to be 18 (12.6dB) when the overlap ratio was 1.

Figure 5.10: Power gain as a function of signal-pump overlap com- pared to theory.

Good agreement between the models and the experimental results can be seen and in this regime there is little difference between the ‘top-hat’ and Gaussian pump beam approximations. There is a slight increase in the rate at which the power gain reduces at larger values ofwl/wp, which it is thought, could be due

to losses caused by the thermal lens aberrations which are particularly severe in the wings of the pump distribution. The M2 _{value of the output beam,}

averaged over the two orthogonal directions, as a function of the signal-pump overlap is shown in figure 5.11.

In this case there is a large discrepancy between theory and experiment. The measured M2 parameter is at a minimum when the overlap ratio is approxi- mately equal to 1 and rises slowly on either side of this minimum approaching

Figure 5.11: Output M2 _{parameter as a function of signal-pump}

overlap for anM2 _{= 1.08input beam.}

2 forwl/wp = 3. The theory from Section 4.3, using a truncated Gaussian ap-

proximation for the pump beam profile, predicts no increase in M2 _{for low}

values ofwl/wp < 0.3 but a very sharp increase when the overlap ratio is in-

creased further. For wl/wp approaching and greater than 1 the predicted M2

value is much higher than the measured value which was also observed by Clarkson [14]. In this regime, the degradation in beam quality caused by ther- mal lensing is clearly much less than that predicted by the theory because the quartic phase aberration approximation is no longer valid and higher order terms become dominant. Additionally the truncated Gaussian pump beam profile used to predict the thermal aberration deviates, somewhat, from the actual pump beam profile, limiting the accuracy of the model. Another feature of the experimental results which is not predicted by this theory is an increase inM2_asw

l/wpis reduced below 1. This is not thought to be a result of the aber-

rated thermal lens since this becomes less aberrated aswl/wpis reduced, but is

instead an effect caused by re-shaping of the signal profile, due to transversely varying gain, which leads to a less Gaussian amplitude profile and therefore a reduction in beam quality. For low values ofwl/wp, the gain is higher in the

wings of the signal distribution than in the centre because the central portion of the signal causes greater gain saturation and yet the gain in the wings remains high. This could lead to flattening of the output signal profile and therefore

a reduced beam quality is likely. This effect was not fully investigated and a significant change in signal profile was not confirmed. However, the effect of the radial gain profile on the beam quality degradation is discussed further in Section 5.5.

Figure 5.12: Brightness gain (G/M2

xMy2) as a function of signal pump

overlap.

The brightness gain, calculated from equation (4.66) as a function of signal- pump overlap, is shown in Figure 5.12. As expected from the limitations of the quartic phase aberration theory, the experimental results are quite different from the theory from Section 4.4. The graph shows an optimum experimental overlap ratio∼1 which falls off more steeply at higher wl/wp values than the

power gain in Figure 5.10. This indicates that the overlap ratio is more critical for achieving high brightness gain than simply high power gain, as expected.