Laser pulse control of a Q-switched Nd:YVO<inf>4</inf> bounce geometry laser using a secondary cavity

Abstract

Pulsed laser operation is desirable for a wide range of applications such as laser micromachining in industrial manufacturing and laser marking for product identification [1]. In these cases it is beneficial to have flexibility in the parameters of the laser pulse to suit the specific application. This can include the ability to achieve a wide range of pulse repetition rates, with some applications requiring variation of laser pulse rate from high rate (multi-kHz) to low rate or even an off-state in a fast timescale. To generate ultrahigh pulse rates requires Qswitched lasers with ultrahigh gain, but problems can arise if the modulation element is insufficient to prevent laser action or hold-off lasing at low repetition rates. In these cases, lasing output can occur when it is not desired [2]. In this work we present a novel method for pulse control in a high gain bounce amplifier Qswitched system by using a secondary cavity to clamp the gain and allow for clean single pulse operation from very high (800kHz) to very low (e.g.1kHz) repetition rates. A schematic of the experimental system is shown in Fig. 1(a). This set-up utilised the bounce geometry in which the laser mode takes a total internal reflection off the pump face of the laser crystal. The laser crystal was a 1.1at. % doped Nd:YVO 4 slab crystal with dimensions of 20mm x 5mm x 2mm. Side pumping was achieved via the 20mm x 2mm face using a 50W diode bar operating at 808nm. Two laser cavities, a primary cavity with output P1 and a secondary cavity with output P2, were formed using the same laser crystal. The path of the primary cavity mode (shown by the black line in Fig. 1(a)) includes an acousto-optic Q-switch (AO-QS) for pulsed operation. A secondary cavity, with a slightly larger internal bounce angle is shown in red and its threshold can be set by its cavity optics.With the secondary cavity blocked, 26W of Q-switched output at a repetition rate of 800kHz was achieved in TEM 00 mode at a pump power of 50W from the primary cavity. In this scheme it was not possible to achieve clean pulsed operation at repetition rates below ~100kHz as the Q-switch could not effectively hold off lasing. With the secondary cavity operating, the gain could be clamped at a level that the Q-switch was able to suppress. The effect of this is seen in Fig. 1(b) which shows the Q-switched pulse duration and output power (P1) from the primary cavity alongside the output power (P2) from the secondary cavity for repetition rates ranging from 1-800kHz at a pump power of 37W. At 800kHz, an output power of 15W was obtained from the primary cavity. Decreasing the repetition rate led to a decrease in P1 and a corresponding increase in P2. Although this suppression of the primary cavity resulted in lower average power and Q-switched pulse energy, it allowed clean single pulse operation to be achieved at all repetition rates shown. In addition, at repetition rates below 100kHz, the Q-switched pulse duration is almost constant at ~18ns due to the clamping of the gain. References[1] J.