100 120 140 160 180 200 Quasi CW Power
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Figure 57 – Quasi-CW output power and delay between pump and signal pulses
of pulse pumped 0.5mm long Nd:YVO4 99/HR microchip laser. Different
average powers were achieved by varying the period between 50μs, 400mW peak
power diode pump pulses.
Returning to the use of a constant peak power of 400mW, the mode matching and efficiency of our microchip laser were considered as a function of average power. These characteristics were interpreted from the lasing output of the microchip laser. The efficiency was taken to be described by the peak power of the quasi-CW part of the lasing output from the microchip laser and the delay between the pump pulse and the start of the microchip gain switching (pulse build-up time) was used as a measure of the amount the gain available exceeded the losses for the lasing mode.
The results for this experiment are shown in Figure 57. The peak power of the quasi- CW level can be seen to peak around an average power of 75mW, indicating the conditions under which there is optimal coupling between the pump and lasing mode volumes. This indicates the lasing mode volume is generally less than the pumping mode volume for CW systems operating well above threshold, limiting their efficiency. The use of a 6.5mm focal length lens to image the diode pump light into the microchip laser was therefore a non-optimal choice of focussing lens. This is in
agreement with the observations in the previous chapter that longer focal length lenses (e.g. 8mm) provide higher efficiency through better mode overlap. However, a 6.5mm lens was used here to provide a smaller pump spot size within the crystal which potentially would offer a region of higher gain.
The relationship between the delay between the pump and microchip pulses and the average thermal power is similar in shape to that of the previous experiment (Figure 56), again indicating the transition between thermal and gain guiding. At high average powers, there is excellent overlap between the high gain region in the centre of the pumped volume and a small lasing beam waist within the microchip laser. This gives a high gain/loss ratio and consequently a short pulse build-up time. With decreasing thermal load, the beam waist increases to include lower gain regions and unpumped regions, leading to a longer build-up time. At very low average powers, the build-up time tends to a fixed values determined by the gain induced refractive index guiding of the lasing mode due to the fixed peak power. Although not fully considered here, the preferential amplification of the oscillating mode in the central portions of the lasing mode will also contribute to the confinement of the lasing mode.
The final experiment carried out repeated an experiment described by Longhi et al. [222]. Their theoretical modelling of gain-guiding using the Maxwell-Bloch equations hypothesised that cavity detuning in a cavity with two longitudinal modes would cause hopping between the two modes, observable by a change in the beam waist. This change in beam waist arises from one of the longitudinal modes being positively detuned from line centre and therefore anti-guided by the negative dispersion, giving a large minimum beam waist within the cavity. The converse is true for the negatively detuned second longitudinal mode, which would have a smaller beam waist. This hypothesis was experimentally verified using a 0.7mm long 94%R/HR Nd:YVO4 microchip laser operating at 1064nm pumped by a Ti:sapphire laser. The cavity detuning was achieved both with different focal length lens to give different pump spot sizes, and by displacing the microchip laser from the focal point of the lenses.
For our verification of this experiment, we used a 0.5mm long 5%R/HR Nd:YVO4 microchip laser coated for 1064nm pumped with a CW 0.5W laser diode through an 8mm collimating lens and 6.5mm focussing lens. The microchip laser was then
translated through the focus of the 6.5mm lens to simulate different pump spot sizes. The far-field divergence of the resulting output was used to calculate the minimum beam waists as described previously. The results from this experiment are shown below in Figure 58, and agree with the observations and modelling of Longhi et al.
21.2 21.3 21.4 21.5 21.6 21.7 20 25 30 35 40 45 50 55 60
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Figure 58 – Minimum beam waist as a function of the 6.5mm focal length pump lens position (relative). The beam waists were calculated from far field
divergence measurements. The 5x5x0.5mm 95%R/HR Nd:YVO4 microchip laser
was pumped with 300mW of absorbed power.
As with Longhi’s microchip laser, our microchip laser operated on two longitudinal modes, either side of the gain centre. The laser operated on a single lobed intensity profile in the far field over the experimental range (21.2-21.7mm) and on an obviously multi-mode output outside this region. No attempt was made to verify the single-lobbed output was a single transverse mode in the near field, though the calculations are consistent with this assumption. There was also evidence of increased elliptically in the output with detuning from the focal point, though again this was not quantified for this experiment.
The focal point was incident on the back surface of the microchip laser at approximately 21.44mm in Figure 58. This is characterised by a dip in both the
minimum beam waist and output power of the microchip laser. This dip is attributed to an increased heating of the rear surface by the strong absorption within the smallest pump spot size. Further characterisation of this effect could provide further insight into the relative contributions of both thermal expansion and thermal lensing to the guiding of the transverse mode.
In summary, we have carried out a range of experiments examining the conditions under which gain guiding of the transverse mode is observable. The transitional period between thermal and gain guiding has been investigated, with thermal guiding dominating in CW high average power systems and gain guiding dominating in high peak power, low average power systems. Although these experiments were carried out using Nd:YVO4 microchip lasers operating at 1064nm to take advantage of their high gain, in principle the same effects should be observable in other monolithic microchip lasers, though perhaps with weaker gain guiding.
Our results also agreed with previous work showing how gain can effect the transverse mode of a Nd:YVO4 microchip laser. This final experiment indicates a mechanism by which the Q of the cavity can be altered by cavity detuning. This ‘self Q-switching’ could arise from the different guiding properties of the two longitudinal modes providing different gain/loss ratios for each mode. Indeed we observed large inexplainable spikes from monolithic Nd:YVO4 microchip lasers which we will now consider in the context of self Q-switching through gain induced refractive index guiding and cavity detuning.