A novel technique for obtaining enhanced control of pulsing parameters in a laser is described and implemented for the first time in a bounce geometry laser. The method uses a secondary laser cavity to control the gain in a -switched primary laser cavity and has enabled clean single-pulse -switched operation to be obtained across a repetition rate range of 1–800 kHz, where previously laser breakthrough had occurred below 150 kHz. Control of the pulse energy from the -switched laser is also demonstrated at a fixed repetition rate of 100 kHz by this technique.
© 2014 Optical Society of America
Pulsed lasers are useful for a wide range of applications, often where high peak powers are advantageous, or even necessary, for some processes [1–3]. An important example of this is in industrial manufacturing, where lasers find their single biggest commercial application, and the global market for industrial laser systems grows yearly . In this sector, pulsed lasers provide precise material removal or material interaction (material processing) and are used for cutting, drilling, micromachining, and marking [5–7]. Here the pulsing parameters of the laser such as pulse repetition rate, pulse duration, and pulse energy are vital as they influence the laser-material interaction . As the parameter requirements for processes differ, the usefulness of a laser for industrial manufacturing applications is enhanced if these can be varied, ideally on a pulse-to-pulse basis.
In this Letter, a novel pulse-control technique is presented, which uses a secondary laser cavity to control the gain in a -switched primary laser cavity. This is shown to enhance the performance and flexibility of a -switched laser, adding significant value to the pulsed source. A problem in -switched lasers, which the technique addresses, is the difficulty in operating well across a wide range of repetition rates. The issue arises because the high gains necessary to achieve high repetition rates () are often problematic at low repetition rates () where they can result in unwanted laser breakthrough before -switching. As shown in this Letter, a secondary cavity can be used to extract excess gain at low repetition rates, enabling clean, single-pulse operation across a wide repetition-rate range with no subsequent adjustment to the system required. Another highly useful ability of the technique demonstrated in this work is to control the -switched pulse energy at a given repetition rate through adjustment of the secondary cavity parameters. Furthermore, the presence of the secondary cavity is beneficial in itself, as it results in a more uniform extraction of gain across repetition rates and leads to more consistent thermal loading and hence spatial quality of the laser output.
Figure 1 is a schematic representation of one possible configuration for implementing the technique. In this setup, two laser cavities are constructed around a single gain medium. The first resonator, designated as the primary cavity, is shown in red. This incorporates the gain medium and a loss modulation element to induce switching. The resonator is formed by a highly reflective back mirror (HR1) and a partially reflective mirror for output coupling of the laser radiation (OC1). The second resonator, designated as the secondary cavity, is shown in blue. This utilizes the same gain medium as the primary cavity to achieve laser action and is again formed by a highly reflective back mirror (HR2) and a partially reflective mirror for output coupling (OC2).
The function of the secondary cavity is to clamp the primary cavity gain at a level that the loss modulation element can suppress. This concept can be described with reference to Fig. 2, which shows the temporal evolution of the primary cavity inversion for high (blue lines) and low (red/black lines) repetition rate -switching . The black dashed lines marked “high loss” and “low loss” represent the two loss levels, which the loss modulator switches between to achieve -switching of the primary cavity.
During the “high loss” interval, inversion accumulates in the gain medium, and the threshold inversion for laser action, , must not be exceeded. When the losses are reduced to the “low loss” level, the new threshold inversion for laser action, , must be surpassed for the stored energy to be released as a -switched pulse. To obtain high pulse repetition rates (), high gains are necessary to ensure is exceeded and this condition is fulfilled. However, this can introduce difficulties at low repetition rates where the time between pulses is extended, and the gain can build to uncontrollable levels, exceeding and resulting in unwanted laser breakthrough. The pulse-control technique uses a secondary cavity to prevent this and can be explained as follows. When the secondary cavity reaches its laser threshold, , it undergoes a transient period of relaxation oscillation before the secondary cavity laser mode saturates the gain down to its threshold value. At this point, a steady-state operation is achieved, and the inversion is clamped at the secondary cavity threshold level. As the primary and secondary laser cavity modes overlap in the gain medium, this means that the primary cavity gain is similarly limited to this threshold. Therefore if then the -switch hold-off cannot be exceeded. The secondary cavity laser threshold can be set through adjustment of parameters, which determine the losses in its cavity. These include, but are not limited to, reflectivity of the cavity mirrors, the path of the cavity mode through the gain medium and the alignment of the cavity mirrors.
The novel pulse-control technique was implemented in an actively -switched laser (the experimental setup is shown in Fig. 3). The arrangement utilizes the bounce geometry in which the laser mode takes a grazing incidence total internal reflection (TIR) off the pump face of the laser crystal, providing access to the region of highest inversion as well as spatial averaging of gain and thermal nonuniformities [10,11]. This geometry has been shown to be effective in obtaining the high gains necessary for high-repetition-rate switching, making it appropriate for use here to beyond 1 MHz [12,13]. The laser crystal is a slab measuring . This is side pumped by an 808 nm diode laser, which is focused to a horizontal line on the face using a vertical cylindrical lens () with . The primary cavity is formed by a high reflectivity (HR1) back mirror and an output coupler (OC1). An adjustable aperture vertical slit in the back arm of the laser cavity enforces operation and active switching is obtained using an acousto-optic modulator (AOM) for loss modulation. The internal bounce (TIR) angle is with respect to the pump face and the laser output is denoted P1. The secondary cavity is formed by a high reflectivity (HR2) back mirror and an output coupler (OC2). The laser mode of the secondary cavity overlaps the gain region of the primary cavity but has a larger internal bounce (TIR) angle . Both the primary and secondary cavity modes propagate through two common intracavity VCLs (), which ensure good vertical overlap between them and the pump region.
In this setup, the primary cavity was constructed first, and -switching results were taken before implementation of the pulse-control technique for later comparison. Figure 4(a) shows the output power from the -switched primary cavity without the secondary cavity over a repetition rate range of 1–800 kHz and at pump power of 45.5 W. The output power remains approximately constant at between 800 and 300 kHz, before decreasing steadily to reach its minimum value of at 1 kHz. The significant average power seen at 1 kHz suggests that single-pulse -switching is not achieved here, and visualization of the temporal output confirms this.
Figure 4(b) shows temporal traces of the -switched output at two different repetition rates. The right-hand trace shows a clean, single -switched pulse obtained at 300 kHz and is representative of the temporal output from the laser at repetition rates between 150 and 800 kHz. The left-hand trace, obtained at 100 kHz, shows subsidiary pulsing and is representative of the output observed at repetition rates below 150 kHz. This demonstrates the difficulty with low repetition rate switching of high gain lasers, where insufficient -switching hold-off has resulted in laser breakthrough causing subsidiary lasing peaks and ruining the quality of the main -switched pulse. The pulse-control technique aims to improve this situation.
Results from the implementation of the pulse-control technique using a secondary cavity are shown in Fig. 5. Figure 5(a) shows the output power from the -switched primary cavity P1 (solid red squares) and the secondary cavity P2 (solid blue triangles) for repetition rates between 1 and 800 kHz. The -switching data from the primary cavity before implementation of the pulse control technique is also included for comparison (solid black squares). All results are obtained at CW pump power of 45.5 W. Using the pulse-control technique, clean single pulse -switched operation was obtained from the primary cavity across the entire repetition rate range. At high repetition rates, the primary cavity output power shows little deviation from the results obtained before implementation of the pulse-control technique and at low repetition rates, the output power approaches zero, signaling where laser breakthrough has been inhibited. The secondary cavity lases across the entire repetition rate range, with an increase at low repetition rates mirroring the decrease in P1. The total output power from both cavities () and the output power from the primary cavity before implementation of the pulse control technique (P1 (no secondary cavity) are presented in Fig. 5(b). The total output power () is greater than the primary cavity output power before secondary cavity implementation suggesting the secondary cavity accesses gain not available to the primary cavity. Additionally, the total output power () remains almost constant across the repetition rate range, signaling a near uniform extraction of the gain. A useful consequence of this is to produce laser output with a spatial profile, which remains consistent across all repetition rates. This is not always possible in pulsed systems, where the variation in gain extracted at different repetition rates leads to variation in thermal lens strength and, consequently, in the spatial profile of the beam.
Figure 6 shows the spatial intensity profile of the laser output from the primary (top) and secondary (bottom) cavities at (a) 10 kHz, (b) 150 kHz, and (c) 500 kHz.
The primary cavity output was across the repetition rate range. The secondary cavity output was multimode with the cavity mode oscillating mainly in the wings at high repetition rates (500 kHz) and filling out in the center at low repetition rates (10 kHz).
Examining the pulse energy and pulse duration of the output from the primary cavity demonstrates the gain clamping effect of the secondary cavity. Figure 7 shows the pulse energy (left axis) and pulse duration (right axis) of the primary cavity -switched output. Between 800 and 80 kHz, the pulse duration decreases from 50 to 17 ns, and, below this repetition rate, the pulse duration remains constant. This fixing of the pulse duration below 80 kHz is consistent with the expected result of the clamping of the primary cavity gain. The pulse energy mirrors the pulse duration, increasing at lower repetition rates with maximum pulse energy of 100 μJ at 1 kHz and showing evidence of pulse energy clamping below .
We can gain further insight into the dynamics of the pulse-control technique by investigating the temporal output from the primary and secondary cavities. Figure 8(a) shows two -switched pulses from the primary cavity pulse train at a repetition rate of 100 kHz (with secondary cavity implemented) in red and the output from the secondary cavity in blue. The secondary cavity reaches threshold and undergoes a period of relaxation oscillation before the primary cavity -switched pulse terminates the secondary cavity output [see Fig. 8(b), with an expanded temporal resolution].
Jitter was observed in the timing at which the primary cavity pulses were released and their peak amplitude, but quantitative measurements are yet to be undertaken. However, the extent to which the pulses suffer from jitter is expected to be affected by variations in the number and form of the secondary cavity relaxation oscillations, which precede them.
A further capability of the pulse-control technique is to enable manipulation of the pulse energy of the primary cavity -switched pulses. This was achieved experimentally by changing the secondary cavity alignment through adjustment of the back mirror vertical control at a chosen fixed repetition rate of 100 kHz.
Figure 9 shows the three pulses of varying pulse energy (91, 77, and 67 μJ) obtained through this method with the pump power kept constant at 45.5 W. This capability could prove highly beneficial in laser material processing applications where pulse energy affects qualities such as feature depth and finish.
In conclusion, this work demonstrates experimentally the first implementation of the secondary cavity method for -switch pulse control. The technique was executed in an AO -switched bounce geometry laser and enabled clean single-pulse output across the entire repetition rate range of 1–800 kHz, where previously there had been laser breakthrough at repetition rates below 150 kHz. Pulse energy control was also demonstrated, where adjustment of the secondary cavity alignment enabled variation of the pulse energies of the primary cavity at a given repetition rate.
E. A. Arbabzadah would like to thank the UK Engineering and Physical Sciences Research Council (EPSRC) for funding.
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