This paper presents the highest average power passively Q-switched Nd:vanadate laser, to the best of our knowledge. A maximum average output power greater than 11W was demonstrated using a Nd:YVO4 bounce geometry laser operating at 1064nm in TEM00 mode, with a spatially stigmatic design. Pulse energies greater than 58µJ and peak powers in excess of 1.9kW were obtained; the maximum repetition rate recorded was 190kHz, close to the upper limit achievable with Cr4+:YAG saturable absorbers.
© 2011 OSA
Q-switching of lasers is a simple and effective way of producing high peak powers for use in a variety of applications, including materials processing. Passive Q-switching can have advantages over active Q-switching in terms of compactness and affordability, since bulky and potentially expensive electronic devices are not required. Cr4+:YAG experiences absorption at wavelengths between 900 – 1100nm and its use as a passive Q-switch for lasers operating at 1064nm has been widely documented [1–4]. However, passively Q-switched lasers are generally limited to low average output powers, typically ~1W. In a few cases, multi-Watt performance has been demonstrated  but >10W has not been achieved in Nd:vanadate systems so far, to the best of our knowledge. The bounce geometry  has shown to be a successful utilisation of a slab laser that can obtain higher average powers than end-pumped systems.
Traditional bounce geometry lasers are implemented to exploit the high gain of a-cut Nd:vanadate crystals; the laser mode takes a path of total internal reflection at the pump face, where a high inversion density forms. This has lead to extremely high gains, up to ~105 , but this is not ideal for passive Q-switching.
The use of c-cut crystals has been successful in producing high peak power passively Q-switched lasers [7–9] but they experience increased thermal lensing as compared with a-cut crystals and may not be suitable for use in the bounce geometry due to the increased absorption depth. Mixed vanadate technology has also been implemented, with high average powers [5,10], but these materials are still not widely available.
In this paper, we discuss the use of an a-cut Nd:YVO4 laser crystal in a novel design of the bounce geometry , which experiences lower gain but exhibits high circularity with low astigmatism, giving excellent spatial beam quality. By incorporating a Cr4+:YAG saturable absorber into such a cavity, we show high average power, high repetition rate passive Q-switching at 1064nm with good beam quality. The highest average power achieved with this system was 11.2W and the highest repetition rate was 190kHz. We believe these two figures of merit are the highest obtained from any passively Q-switched Nd:vanadate laser operating at 1064nm.
The schematic of Fig. 1 shows the experimental cavity design for the passively Q-switched stigmatic bounce laser. The laser crystal was an a-cut slab of 1.1 at.% doped Nd:YVO4. The cavity was formed of two plane-plane mirrors: a back mirror high reflection (HR) coated for 1064nm placed at L1 = 70mm away from the centre of the crystal, and an output coupler (OC) of partial reflectivity (R) placed at L2 = 120mm away from the centre of the crystal. A Cr4+:YAG saturable absorber crystal was placed near the back mirror. Two output couplers, one of R = 30% and the other, R = 70%, were implemented separately to assess if the output coupler reflectivity affects the Q-switching performance in any way.
The Nd:YVO4 crystal had dimensions 20 × 5 × 2mm and was contact cooled via the two 20 × 5mm faces. The two 5 × 2mm faces were anti-reflection (AR) coated for the lasing wavelength of 1064nm and angled to inhibit any parasitic oscillations. The crystal was pumped by an 808nm fast-axis collimated laser diode via the 20 × 2mm face, which was AR coated for this wavelength.
The pump radiation was directed onto the crystal using a vertical cylindrical lens (VCLD) of focal length 12.7mm, placed approximately twice its focal length away from the crystal. An experimental bounce angle of ~3° was used. The combination of small bounce angle and defocused pump radiation was implemented to equalise both the gain profile and the thermal lensing in the horizontal and vertical dimensions. This has been shown to produce highly circular spatial beam profiles, with low astigmatism . It is thought that the improved stigmatic spatial quality in combination with the lower gain of the laser will enhance the Q-switching performance compared to the standard, tightly-focused bounce geometry design .
The Cr4+:YAG crystal had dimensions 5 × 5 × 3mm and initial transmission (T) of 84%. It was placed in a copper mount, 5mm away from the back mirror. It was initially implemented in the R = 30% output coupler system and subsequently in the R = 70% output coupler system.
3. Experimental results
3.1 Output power
The power curves for both continuous-wave (CW) (with the Cr4+:YAG removed from the cavity) and Q-switched operation, for both output coupler (R = 30%; R = 70%) cavities, are shown in Fig. 2 . The maximum output power obtained during CW operation for the R = 30% output coupler cavity was 14W (Fig. 2(a), closed squares) and the maximum for the R = 70% cavity was 13.8W (Fig. 2(b) closed squares).
While experiencing a slightly lower output power, the R = 70% cavity showed a decreased threshold of 13W as compared with the R = 30% cavity threshold of 17W, as expected. The maximum output power obtained during Q-switched operation for each cavity was 11.2W (Fig. 2(a) open squares) and 11W (Fig. 2(b) open squares) for the R = 30% and R = 70% output couplers, respectively. This 11.2W is the highest average power of any passively Q-switched Nd:vanadate laser, to the best of our knowledge. The maximum fluence incident on the Cr4+:YAG during Q-switched operation was 0.65Jcm−1. During the entirety of the experiment, no damage was observed on the Cr4+:YAG.
Due to the power-dependent thermal lensing in the laser amplifier [5,11,12], the laser passes through two zones of stability, separated by a zone of unstable operation. The onset of the unstable zone can be seen in both Fig. 2(a) and (b) by the sharp decrease in power around 35W of pumping. The power recovers and as the laser enters the second zone of stability, the Q-switching threshold is reached. For the R = 30% output coupler cavity, this Q-switching threshold was 40W and for the R = 70% output coupler, this was 42W.
3.2 Spatial characteristics
The spatial profiles of the laser output in CW and Q-switched operation are shown in Fig. 3 for both output couplers. Figures 3(a) and (b) show the beam profiles using the R = 30% output coupler; (a) for CW operation with the Cr4+:YAG removed from the cavity and (b) for Q-switched operation. Figures 3(c) and (d) show the corresponding beam profiles obtained using the R = 70% output coupler.
The M2 beam quality factors for CW operation were 1.2 in both the horizontal and vertical for the R = 30% output coupler system and 1.3 in the horizontal and vertical for the R = 70% output coupler. During Q-switched operation, the beam quality M2 factor for the R = 30% cavity was measured to be 1.3 in the horizontal and 1.4 in the vertical and for the R = 70% cavity, was 1.45 in the horizontal and 1.5 in the vertical.
3.3 Temporal characteristics
Figure 4 shows the temporal output under pulsed operation. Figures 4(a) and (b) show the Q-switched pulse train and a single pulse at 48W pumping in the R = 30% output coupler cavity. Figures 4(a) and (b) show a small amount of pulse jitter and minor modulation on the temporal profile, possibly due to thermal effects in the Cr4+:YAG. At this pump power, the repetition rate was measured to be 140kHz.
Figures 4(c) and (d) show the pulse train and a single pulse obtained using the R = 70% output coupler, also at 48W pumping. They show an improvement, with less pulse jitter evident and a smoother form to the single pulse. The pulse repetition rate at this pump power was measured at 190kHz which we believe is the highest repetition rate achieved from any Cr4+:YAG passively Q-switched laser. Furthermore, this value is close to the maximum permissible for Cr4+:YAG, which has an upper state lifetime of ~4µs.
The variations of repetition rate and pulse duration with pump power are plotted in Fig. 5 . The repetition rate of the R = 30% output coupler (Fig. 5(a)) cavity varied between 120 – 160kHz with pump power. At 49W pumping and a repetition rate of 160kHz, the pulse duration was ~30ns, corresponding to a pulse energy of 70µJ and a peak power of 2.3kW. In the R = 70% output coupler cavity (Fig. 5(b)), the repetition rate varied between 145 – 190kHz; the maximum repetition rate occurred at a pump power of 47W, where the pulse duration was also ~30ns. The corresponding pulse energy was therefore 58µJ and the peak power was 1.9kW.
The complex temporal characteristics of Fig. 5 are, at least in part, accountable by the variation in mode area at the Cr4+:YAG crystal. The radial mode size at the Nd:YVO4 amplifier is ~300µm , while at the Cr4+:YAG the mode size is observed to vary from 200µm at 40W pumping to 70µm at 48W pumping, due to the thermal lensing in the Nd:YVO4 in the second stability zone (as discussed in Sect. 3.1). Most of the rapid mode size variation occurs between 40 – 44W. In the case of the R = 70% cavity (Fig. 5(b)), the repetition rate does not increase above a pump power of 46W. Beyond this pump power, the repetition rate may be limited by the upper state lifetime (~4 µs) of the Cr4+:YAG and its inability to fully recover.
3.4 Spectral characteristics
Insertion of any saturable absorbing element has been shown to stabilise single longitudinal mode (SLM) operation . The effect of the introduction of the Cr4+:YAG crystal on the spectral behavior of the laser with 30% output coupler was investigated by analysing the laser output with a Fabry-Pérot etalon of free spectral range (FSR) 3.4GHz and finesse (F) of ~50. The ring pattern produced by this was viewed on a CCD camera via imaging with an f = 300mm lens. A longitudinal mode spacing of the experimental laser cavity was ~700MHz.
With the insertion of the Cr4+:YAG, during Q-switched operation at pump powers up to approximately 48W, the laser appeared to be operating in SLM. The SLM operation was evidenced by the single ring per FSR in Fig. 6(a) , together with the smooth temporal pulse shape shown in Fig. 4(d). However, when the pump power was increased further, multi-longitudinal mode operation was evident, shown by the multi-ring pattern in Fig. 6(b). This also coincided with an increase in pulse jitter and modulation on the form of the single pulse, also evidenced in Fig. 4(b). The same trend was observed with the R = 70% output coupler, though the cavity remained operating in SLM until over 49W of pumping.
A passively Q-switched Nd:YVO4 bounce laser has been presented with two different output coupling reflectivities, using Cr4+:YAG as the saturable absorbing mechanism. For the case of the R = 30% output coupler, a maximum output power of 11.2W was achieved at a pulse repetition rate of 160kHz, corresponding to a pulse energy of 70µJ and peak power of 2.3kW. We believe this to be the highest average power passively Q-switched Nd:vanadate laser operating at 1064nm ever reported.
For the R = 70% output coupler system, a maximum output power of 11W was attained with a pulse repetition rate of 190kHz, pulse energy of 58µJ and peak power of 1.9kW. We believe this to be the highest repetition rate achieved from a Cr4+:YAG Q-switched Nd:vanadate laser at 1064nm. This 190kHz repetition rate obtained is close to the maximum obtainable repetition rate for Cr4+:YAG Q-switched lasers due to the recovery time of the saturable absorber. Additionally, no damage to the Cr4+:YAG was observed during the experiment. These results show promise for further power scaling in high repetition rate passive Q-switching, using the bounce geometry laser design.
References and links
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