A method for optical triggering of a Q-switched Nd:YAG laser by direct bleaching of a Cr:YAG saturable absorber is described. This method involves the bleaching of a thin sheet of the saturable absorber from a direction orthogonal to the lasing axis using a single laser diode bar, where the Cr:YAG transmission increased from a non-bleached value of 47% to a bleached value of 63%. For steady state operation of a passively Q-switched laser (PRF=10Hz), the pulse-to-pulse timing jitter showed ~12X reduction in standard deviation, from 241 nsec for free running operation to 20 nsec with optical triggering.
©2009 Optical Society of America
Q-switched Nd:YAG lasers offer robust performance for a variety of industrial and military applications including range finding, designating, and marking . Passive Q-switching offers simplicity in laser design, compactness, low cost, and weight. However, relative to active Q-switching, passive Q-switching has several deficiencies: 1) a limited hold-off of ~6-8 dB for passive vs. >20dB for active Q-switches, which limits the buildup of gain and energy available for extraction, 2) a residual non-saturable absorption in the Cr:YAG that leads to a decrease in laser efficiency, and 3) a relatively large pulse-to-pulse timing jitter. For some applications it is this last deficiency, timing jitter, that mandates the use of the active Q-switching, despite the inherent increase in complexity and cost. This paper describes a simple and low cost technique for significantly reducing the timing jitter for a passively Q-switched laser, allowing its utilization in applications where low pulse-to-pulse jitter is required.
The timing jitter in a passively Q-switched laser can be attributed to a variety of factors including pump intensity variations, changes in pump spatial and spectral properties, spontaneous emission noise, and changes in distribution of the spatial modes for emission in the laser cavity. These changes influence threshold conditions, where lasing and subsequent Q-switching occur at slightly different times relative to the start of the pump pulse. Previously, jitter reduction using a composite pump was demonstrated [2,3]. A second technique of jitter reduction by direct bleaching of a saturable absorber was shown to produce a small reduction in pulse timing jitter . For both techniques, jitter reduction can be attributed to a rapid change induced in the laser cavity loss or gain, causing the Q-switching to occur at a precisely defined time. For the composite pump pulse, the Nd:YAG inversion was allowed to build to just below threshold, at which point a more intense pump pulse was added to rapidly increase the cavity gain. For the direct bleaching method, an optical pulse was used to bleach the saturable absorber, rapidly decreasing the cavity loss and driving the laser above threshold.
Figure 1 is a semi-empirical illustration of the concept of how jitter originates and the method of optical bleaching to minimize its magnitude. The evolution of gain and loss over the course of a single pump pulse are derived from empirical data measured on a laser used in the experiment. This laser is estimated to have a round trip cavity loss of 8.24 dB based on a Cr:YAG having a measured single pass non-saturated absorption of 3.24 dB, with an additional 1.76 dB attributed to other losses including the output coupler transmission of 30%. For illustrative purposes, two curves for the buildup of gain as a function of pump pulse duration are plotted that represent a 1% variation in the gain coefficient, and is intended to represent two sets of conditions that might be associated for the evolution of successive Q-switched pulses. In the absence of external optical bleaching, threshold for the two pulses occurs at significantly different times, separated by ∆t. The application of a bleaching pulse at a suitable level of inversion will rapidly drive the cavity loss attributed to the saturable absorber down to the lasing threshold, initiating the Q-switch at well defined times and separated by a smaller time variation, ∆t. With external bleaching, the Q-switched pulse timing is determined by the steep slope of the loss vs. time curve, rather than by the gradual slope of the gain vs. time curve of the free running laser. The reduction in the jitter can then be expected to be comparable to the ratio of the two slopes.
2.1 Bleaching of a Cr:YAG saturable absorber
A series of experiments was conducted to evaluate the bleaching behavior for the Cr:YAG saturable absorber. The approach consists of bleaching a thin sheet within the saturable absorber from a direction orthogonal to the direction of laser emission in a passively Q-switched laser cavity. The bleaching process was experimentally characterized according to Fig. 2. A laser diode driver with <200nsec rise time generated a ~920 Watt bleaching pulse from a single 1036 nm laser diode bar. The pulse was collimated in the fast divergence axis with a 300 μm rod lens that shaped the beam into a sheet with a length of ~1cm and measured width (1/e2) of 122 μm. The resultant peak intensity within the Cr:YAG was calculated to be 120 kW/cm2.
The magnitude of bleaching was characterized by passing a low power 1064nm probe beam along the same direction as the axis of the laser cavity that the crystal would later be inserted into. The probe beam diameter measured ~98 μm (1/e2) within the Cr:YAG. Half wave plates were used to control the polarization for both the bleaching and probe beams. The transmitted probe power was measured by an integrating sphere with a silicon detector. Using this technique, a measurement of the non-saturated absorption and bleaching of the absorption for a (100) cut Cr:YAG crystal, supplied by SMC Inc., was performed.
2.2 Timing jitter measurements for a Nd:YAG laser.
An end pumped Nd:YAG laser Q-switched with the previously characterized SMC Cr4+:YAG saturable absorber was used as shown in Fig. 3. The laser cavity makes use of a 3mm × 3mm × 35mm long 0.7% Nd:YAG crystal with a 5mm long undoped YAG thermally fused to the pumped end. An 809 nm 12-bar laser diode stack was coupled to the Nd:YAG via a lens duct having a 5 × 10 mm input end and 3 × 3mm output end. A 70% reflective flat mirror was used as the output coupler. Optical triggering was accomplished using a single 1036 nm diode bar that was collimated in the fast axis and close coupled into the saturable absorber in the plane orthogonal to the lasing axis. A half wave plate inserted between the bleaching diode and saturable absorber ensured an appropriate polarization alignment for the maximum bleaching effect. The laser was operated at 10 Hz PRF, with energy and jitter measurements made over a duration of 2 minutes for each experiment. The arrival time, defined as the time between the start of the pump pulse and measurement of the Q-switch pulse, was measured with a sub-nsec detector. The jitter was determined by measuring the ∆t’s between successive pulses and generating a count histogram with 10 nsec bins.
3. Results and discussion
3.1 Bleaching of Cr:YAG saturable absorbers
Figure 4 illustrates bleaching of the crystal by the 1036 nm laser diode bar. In the absence of a bleaching pulse, a probe signal transmission of ~47% is measured, with rotation of the probe half wave plate showing no angular dependence. When a 15μsec long 1036nm bleaching pulse was applied, it was observed that significant bleaching was realized when the polarizations for both the bleaching beam and the probe beam lay along a common crystallographic axis, defined for these experiments as . With this alignment of the polarizations, the transmission of the saturable absorber was increased from the 47% non-saturated value to 63%. The bleaching was observed to saturate after a few microseconds, as expected from the 3.5 μsec radiative lifetime of the Cr:YAG, which causes re-population of the ground state for longer times. A 90° rotation for the probe polarization (bleaching and probe polarizations orthogonal) resulted in a much smaller 2% increase in transmission. Rotation of the polarization for the bleaching pulse to along  results in minimum bleaching, independent of the polarization of the probe.
As YAG is a cubic garnet crystal, this anisotropy would not typically be expected. However, this effect is commonly observed in Cr:YAG, and is attributed to the local environment of the Cr4+ ions having a distorted tetrahedral symmetry that generates a distinct dipole for absorption [5–7]. In the cubic garnet, symmetry operations acting on the tetrahedral site results in this dipole falling along the , , and  crystallographic axis, yielding the observed polarization effect [8,9].
3.2 Measurement of timing jitter in a Nd:YAG laser
Figure 5 plots the arrival time for the Q-switch pulse for both the case of operation as a passively Q-switched laser and for optically triggered operation with orthogonal bleaching of the saturable absorber. With the free running operation, the Q-switched pulse is observed to have a mean arrival time of 207.41 ± 1.07 μsec into the pump pulse with a measured output energy per pulse of 14.4 mJ. The application of a trigger pulse ~10 μsec before the natural event initiates the Q-switch pulse at a measured mean time for arrival of 198.10 ± 0.02 μsec. with an output energy per pulse of 13.9 mJ. The Q-switch pulse is observed to have a temporal duration of ~20 nsec for both normal and optically triggered operation. Figure 5 (b) provides a histogram count for the Q-switch arrival times over the 2 minute experimental duration and shows that drift is significantly reduced from having a standard deviation of ±1.07 μsec for passive Q-switching to a standard deviation of 0.02 μsec with optical triggering.
Figure 6 contrasts the free running pulse-to-pulse jitter against the case for operation with optical triggering. A count histogram for timing jitter was generated for each case and plotted as Fig. 7. The data was approximated as normally distributed, where a standard deviation for jitter was calculated to be ±241 nsec for the free running laser. In contrast, the application of the bleaching pulse effectively reduces the jitter to ±20 nsec, a reduction of 12x. The ±20 nsec minimum measured jitter was likely to have been limited by the accuracy of the measurement system and the inherent 20 nsec duration of the Q-switched pulses.
It is important to note that with the sheet-shaped bleaching beam, initially the laser reaches threshold only within a small volume of the active medium. We experimentally observe, however, that the laser cavity spatial modes fill the entire cavity cross-section, with depletion of the inversion taking place within the entire volume of the Nd:YAG rod. A small decrease for extracted energy is observed with the optical triggering, as expected from the fact that the Q-switched pulse is initiated a lower level of inversion relative to the non-triggered case.
The direct bleaching of a saturable absorber was found to be very effective in minimizing pulse-to-pulse timing jitter for a Q-switched Nd:YAG laser. Optical triggering was achieved using emission from a single laser diode bar incident on the Cr:YAG along a direction orthogonal to the laser axis, and shaped to bleach a thin slice of the crystal. For efficient triggering, it was found that the polarization of the bleaching pulse and polarization of the laser lay along a common crystallographic axis, where the transmission of the Cr:YAG could be increased from a non-bleached value of 47% to a bleached value of ~63%. A 12X reduction in pulse to pulse timing jitter was measured where the standard deviation was reduced from ±241 nsec for the free running operation to a value of ±20 nsec with optical triggering.
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