UV illumination of a lithium niobate Q-switch was demonstrated as an effective means to eliminate a loss in hold-off and associated prelasing that occurs under cold temperature operation of Q-switched lasers. This degradation occurs due to the pyroelectric effect, where an accumulation of charge on crystal faces results in a reduction in the Q-switch hold-off and a spatially variable loss of the Q-switch in its high-transmission state, both resulting in lowering of the maximum Q-switched pulse energy. With UV illumination, the resulting creation of photo-generated carriers was shown to be effective in eliminating both of these effects. A Q-switched Nd:YAG laser utilizing UV-illuminated LiNbO3 was shown to operate under cold temperatures without prelasing or spatially variable loss.
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Lithium niobate (LiNbO3) is currently the preferred material for active Q-switching of military Nd:YAG lasers, and offers the advantages of a lower half-wave voltage and an electro-optic coefficient that is less sensitive to temperature compared to KD*P . However, a well known problem with lithium niobate Q-switches is the degraded performance at cold temperatures due to pyroelectric effects . The pyroelectric response to a temperature change for LiNbO3 can result in prelasing, where a spatially dependent decrease in hold-off results in lasing in advance of opening of the Q-switch shutter. Prelasing manifests itself as a reduction in energy available for the Q-switched pulse, as well as the possibility of damage to the optical components within the laser cavity.
The pyroelectric properties for lithium niobate have been well studied [3–5], and have been identified as a key mechanism for loss of contrast for LiNbO3 modulators [6,7]. For the Z-cut lithium niobate Q-switch, a variation in temperature results in an imbalance for free charges on the surface of the ± z faces. If un-compensated, i.e. open circuit conditions, these unscreened “pyrocharges” result in a large electric field across the sample, and via the electro-optic effect, impart an uncontrolled birefringence on light propagating within the cavity. As the bulk conductivity for LiNbO3 is negligible at temperatures below about 60 °C, pyrocharges accumulated on the surface of lithium niobate can remain uncompensated with a time constant on the order of days .
With military Q-switched lasers, the preferred method for compensating pyrocharges on the endfaces of the LiNbO3 Q-switch is via ionization using a radioactive alpha particle emitter, a technique that has proven quite effective in bleeding static charge from a non-conductive surface [8,9]. The energetic release of alpha particles results in the ionization of air into pairs of positive and negative ions in the vicinity of the charged surface. With an increase or decrease in temperature, the surface pyrocharges may be positive or negative depending on the sign of temperature change, and therefore the appropriate ion from the ionized charged pair is available to compensate the surface. Although it is effective and inexpensive, the radioactive material is subject to regulation that can be burdensome to manufacturers.
It is the objective of this research to explore alternative ways of compensating pyrocharges on the end face of a lithium niobate Q-switch. To this end, we have investigated the role of UV illumination as a simple, low cost technique to increase the bulk conductivity. The role of UV in holography applications (optical damage effect in lithium niobate) suggests that conductivity can be significantly enhanced in the presence of above band-gap light. Studies of this optical damage effect have identified a bulk photovoltaic property, where photo-generated carriers are swept along the ferroelectric polar axis [10–13]. UV illumination has also been found to control photo-refractive effects in LiNbO3 via an increase in photo-conduction . For our application, UV illumination can provide the necessary increase in charge carriers; and in the presence of a strong pyroelectric field, these carriers can be effectively swept in such a direction to eliminate this field. With sufficient UV illumination, the unscreened surface pyrocharges can be effectively compensated, allowing the Nd:YAG laser to be operated without prelasing or degradation in efficiency over a large temperature range and in the presence of rapid changes in the temperature.
2.1 Extinction studies on LiNbO3 Q-switches
A series of extinction measurements was performed on LiNbO3 Q-switch crystals using a crossed polarization technique arranged according to Fig. 1 . The output of a linearly polarized 1064nm CW laser was used as the probe beam, which was expanded to provide uniform illumination over the entire LiNbO3 crystal. A 9x9x25mm LiNbO3 Q-switch (Crystal Technology) and half wave plate were inserted between crossed polarizers, and placed into a temperature test chamber. After passing through the output polarizer, the beam was apertured to only pass the central 5mm diameter circular area, representing the size of the Nd:YAG gain medium. The passed light was collected onto a silicon detector for power measurement. To record the beam intensity distribution, a small percentage of the passed probed beam was deflected into a SWIR camera (Indigo Systems – Alpha NIR).
The maximum extinction ratio was measured by rotation of the half wave plate. For baseline conditions, with the half wave plate adjusted for maximum and minimum transmission, transmitted powers were 10.5mW and 25µW respectively, corresponding to an extinction ratio of 420x.
UV illumination was provided by two UV LEDs operating at 365nm (Nichia NCSU033A), placed in close proximity to the crystal, and flood-illuminating the sample from the side. The UV array provides a uniform illumination of the LiNbO3, with a manufacture’s specified intensity distribution of over 90% relative intensity at a 30 degree radiation angle. The diodes were operated to a maximum current of 500mA, corresponding to a rated output power of 250mW per diode. A thermocouple was attached to the LiNbO3 to monitor its temperature.
2.2 Evaluation of a LiNbO3 Q-switched Nd:YAG laser
The LiNbO3 Q-switched Nd:YAG laser with UV illumination was evaluated over temperature. The cavity components were mounted to a common rail (Fig. 2 ), with the output coupler (R=50% at 1064nm) mounted within a PZT controlled 2-axis gimbal. Mirror angle optimization was performed over temperature to eliminate the effect of cavity misalignment. The entire laser was placed into the temperature control chamber, with the Q-switched pulse exiting through a port window. Temperature cycling was performed to evaluate the performance of the laser in the presence and absence of UV illumination. The maximum rate for temperature change for the laser measurements was 2°C/min.
3. Results and Discussion
3.1 Extinction measurements for LiNbO3 Q-switches over temperature
Figure 3 graphs the evaluation of extinction for a LiNbO3 crystal over a cooling and warming cycle. The chamber was first cooled at its maximum rate without UV illumination. After an initial temperature drop of only ~5 °C, an increase in the power leaking through the crossed polarizers was evident. This power increased with continued cooling as pyrocharges accumulated, and reached a steady value of ~400 μW as thermal equilibrium was re-established, corresponding to a decrease in extinction ratio from 420x to 26x. It should be noted that this was the average extinction over the 5mm diameter aperture. The image of the transmitted beam however showed significant spatial variability, with a spatial pattern that was observed to rapidly change (flicker) in the presence of pyroelectric fields. This rapidly changing spatial pattern resulted in several large fluctuations in the measured leakage as the overlap with the measurement aperture changed (e.g., at t=10 minutes, 23 minutes, etc.).
With the initiation of UV illumination (at ~38min), the extinction was shown to rapidly return to its room temperature level. A small amount of compensation could be expected by the ~6C increase in crystal temperature due to recombinative heating by photo-generated carriers. However, for full return of extinction to its nascent level a second mechanism must be responsible for the bleeding of excess charge from the surface, and is believed to be the sweeping of photo-generated carriers within the strong pyroelectric field. The time constant for this effect was on the scale of several minutes, as opposed to hours for the case of no UV.
With the UV on at cold temperature, within several minutes the charges on the surfaces were fully compensated. When the UV was turned off at the 58 minute mark, the crystal cooled by ~6C, resulting in accumulation of cooling pyrocharges. These pyrocharges resulted in a small increase in the leaked power. As the chamber was then warmed in the absence of UV, the generation or pyrocharges of opposite sign returned the surface to its compensated state (baseline extinction level) after about a 6°C temperature rise. Warming beyond this point then produced the accumulation of warming pyrocharges, with a subsequent loss in extinction. If the only effect of the UV illumination was heating of the crystal, then turning on of the UV during the warming cycle could be expected to provide a more rapid heating, thereby increasing the pyroelectric charge and loss in extinction. However, as can be seen from Fig. 3, turning on of the UV during the warming cycle (at 93 minutes) resulted in a rapid increase in extinction. This clearly indicates that, even during warming, UV illumination fully compensates the surface charge. This confirms that the pyroelectric charge compensating mechanism is indeed an increase in bulk conductivity via photoconduction.
As previously mentioned, a significant spatial variation in the transmitted beam was observed. This phenomenon is illustrated in Fig. 4 , which shows the transmitted intensity distribution during the cooling portion of a temperature cycle. The images highlight the spatially-dependant loss of extinction in the LiNbO3 Q-switch, with a 5 mm dashed red circle superimposed to show the detector collection aperture used to measure the leaked power (blue curve). In region (b), corresponding to the most rapid temperature change (dT/dt), the spikes in measured leaked power coincided with the rapidly changing pattern for the transmitted light distribution. Illumination with UV had a profound effect on the extinction patterns. Within about a minute of UV illumination, the spatial variation of the leaked light distribution homogenized, and over several minutes faded to its state of high extinction associated with the fully compensated surface.
3.2 Evaluation of laser performance over temperature
To validate the efficacy of UV illumination on the performance of a LiNbO3 Q-switched laser, a fully operational Q-switched laser was evaluated over a temperature cycle. Figure 5(a) shows the maximum Q-switched pulse energy that could be generated, without prelasing, as a function of temperature. Temporal characteristics of laser output pulses were observed using a fast detector, and an example of a Q-switched pulse in presence of prelasing is shown in Fig. 5(b). To eliminate the energy contribution from prelasing during the measurement, the following procedure was used: at each temperature the Q-switched pulse was monitored, and the pump pulse duration was reduced from its 300 μs maximum length until prelasing was avoided. A significant decrease in the energy of the Q-switched pulse was first observed when the temperature dropped by about 10°C from room temperature, where a drop for the Q-switch hold-off resulted in the onset of prelasing. An additional cause of decreased Q-switched energy was attributed to the fact that in the presence of pyroelectrically induced fields, the effective Q-switch transmission in its on state (high voltage applied) is degraded due to the fact that the crystal does not impart a spatially uniform quarter wave polarization retardation. This was confirmed by observing that in this condition the spatial pattern of the laser Q-switched output beam became very non-uniform, indicating spatially dependant loss in the cavity. Even where the hold-off was sufficient to prevent prelasing, a degradation in the output energy and beam uniformity occurred due to the spatially non-uniform polarization rotation in the Q-switch.
In contrast, as shown in Fig. 5a, with UV illumination the Q-switched pulse energy remained relatively constant, with the residual variation over temperature caused by changes in pump absorption due to pump diode wavelength tuning. With a fixed 300 μs pump duration during the entire temperature cycle there was no prelasing or degradation in beam uniformity.
UV illumination was shown to have high efficacy in dissipating the pyroelectric charge accumulation of the surface of a LiNbO3 Q-switch associated with temperature cycling. This result was attributed to an increase in conduction via the photo-generation of carriers, where the strong pyroelectric fields enable the sweeping of carriers to compensate the surface charge. A UV illuminated LiNbO3 Q-switched Nd:YAG laser was shown to operate with no prelasing or degradation in pulse energy, under conditions of rapid temperature cycling and cold temperatures.
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