Using noncritically phasematched, 1-cm²-aperture, KTiOAsO₄ (KTA) crystals in an optical parametric oscillator (OPO), we have demonstrated a sustained average signal power of 33 Watts at 1534.7 nm. To our knowledge, this is the highest-average-power signal ever generated by an OPO. The pump source was a 100-Hz, Q-switched 1064-nm, Nd:YAG laser. In comparison to the similar and more common material KTiOPO₄ (KTP), idler absorption in KTA is negligible, allowing high-power operation with minimal thermally induced refractive distortion in the OPO crystal.

The KTP OPO has been extensively studied for frequency conversion of Nd-doped lasers to longer wavelengths and can be used as the basis for eye-safe lidar systems operating in the 1550-nm wavelength region [1]. One of the major advantages of OPOs based on KTP and its isomorphs is the ability to operate with non-critical phase matching (NCPM). The large acceptance angle for NCPM permits efficient OPO operation even with multi-transverse-mode pump lasers. The development of flux-growth techniques [2] has allowed fabrication of 1-cm²-aperture KTP crystals and the subsequent scale-up of KTP OPO output. A high-power, nanosecond-pulse OPO utilizing KTP was previously reported to: generate a record signal energy of 450 mJ, have an average signal power of 7.4 W, and operate with a pump-to-signal conversion efficiency of over 40% [3,4]. These previous results led us to investigate the possibility of a further increase in the average power from KTP OPOs. One possible limit to higher powers in KTP OPOs results from the absorption of the idler wave in the crystal under NCPM conditions. The isomorph KTA has similar phase-matching characteristics to KTP but greatly reduced idler absorption and thus is a candidate for operation at high output powers as well. Recent advances in the flux growth of KTA and its isomorphs [5] have allowed fabrication of crystals similar in size to KTP.

In this letter we provide a comparison between KTA and KTP for performance in high-average-power, NCPM OPOs. Next we present selected efficiency data on a ring KTA OPO using different output coupler reflectivities. We also show plots of the highest average-signal-power demonstrated by our OPO system using two separate pump lasers. Next we present experimental validation of the recently published Sellmeier coefficients [6] for KTA. Finally, we note the observation of bulk damage in some of our KTA crystals.

For our OPO experiments, we utilized 100-Hz, pulsed-diode-pumped, Q-switched, multimode Nd:YAG lasers. We employed two systems in our work, each of which consisted of an oscillator and two amplifier stages. Both had a measured output-beam M² of 5 and were capable of power outputs at the exit aperture of 100 W (Laser 1) and 130 W (Laser 2). Optics used to relay the laser outputs to the OPO (1:1 imaging) and provide isolation and variable attenuation of the pump power reduced the maximum power deliverable to the OPO by approximately 20%. The output pulsewidths (FWHM) were 22.5 and 17.5 ns for Lasers 1 and 2, respectively, and the pulse shape was approximately Gaussian with 20% modulation from mode-beating superimposed. The measured beam profiles were nominally flat-top (8-mm-diameter) with about 30% RMS deviation from the mean irradiance, with the two lasers differing somewhat in intensity profiles.

We chose the singly resonant, ring-cavity OPO design, represented in Figure 1, for a variety of practical reasons including: higher damage threshold due to reduced pump fluence, greatly reduced isolation requirements for pump laser, intrinsic ability to remove idler from cavity without specifically coating optics for this purpose (45° incidence on final cavity mirror with p-polarization for idler only), and ease of injection seeding if future applications require a narrow linewidth signal. A double pass, standing wave cavity, using only half of the crystal length used in the ring, did produce lower thresholds and equivalent peak efficiencies for the OPO signal, but the practical issues mentioned above made this option undesirable for our application.

Transmission data are shown, in Figure 2, for identically AR-coated, x-cut, KTP and KTA crystals. These data were used to calculate a linear absorption coefficient of 0.59 cm^-1 in KTP at the idler wavelength of 3297 nm expected with NCPM and a 1064-nm pump. Assuming negligible absorption in KTA for wavelengths shorter than 3500 nm [7]; the reduced transmission in our KTA sample, in the range from 3000 to 3500 nm, is due to absorption and/or reflection at the coated optical surfaces and not in the bulk crystal. Increased absorption is known to cause phasemismatch and/or thermal lensing at high average power and detailed numerical studies have been published (e.g. ref. [8]).

With a few simplifying assumptions we can use the idler absorption data to make a crude estimation of thermal lensing induced in a high power, 1064-nm-pumped, NCPM KTP OPO crystal that generates 25 W of signal power. The assumptions are: a cylindrical crystal with beam propagation down the length of the cylinder, the entire cylindrical surface held at a constant temperature, negligible heat loss through optical faces of the crystal, uniform heating down the length of the crystal and located on the exact center of the cylinder axis, isotropic thermal conductivity, and steady state operation. Under these conditions, we derived an analytic solution to the heat conduction equation that dictates a parabolic temperature gradient as a function of radial distance from the axis of the crystal.

For a 4-cm length of crystal (many of our experiments utilized two 2-cm-long crystals), this temperature gradient, coupled with the temperature dependent change of optical index [10] and thermal conductivity of KTP [11], leads us to predict a thermal-lens of focal length +3.5 meters. Introduced into a typical flat-mirror OPO cavity, this lens results in a significant reduction in mode volume for the lowest-order resonant mode in the cavity, increasing the risk of optical damage to cavity components. The use of alternative thermo-optic data for KTP [11] leads to the calculation of an even more powerful thermal lens. Previous calculations that assume an infinitely long slab of KTP lead to similar results [7].

Experimentally, for a KTP ring-cavity OPO using two 2 cm crystals, we observed the onset of a significant thermal lens at the modest signal power level of 8 W. When we increased the signal power level to 12 W the measured beam diameter of the resonated signal decreased to approximately one-fifth of the 0.8-cm, low-power diameter. To avoid possible optical damage, we did not increase the power of the KTP OPO beyond this level. In contrast, the beam diameter, for an optimized ring cavity using KTA, as imaged at the plane of the ring-cavity output coupler, did not have any measurable change over our entire signal power range up to 33 Watts.

In Figure 4, we present average signal power vs average pump power, for each of the two pump lasers. These data were collected for a ring cavity with four 2-cm-long KTA crystals, a 30%-reflectivity signal-wave output coupler, and an OPO physical round-trip length of 52 cm. Figure 5 shows the overall conversion efficiency of pump-to-signal average power as a function of integrated average pump irradiance incident on the OPO input mirror. Laser 1 had slightly more intensity variation in the beam than Laser 2, but both top hat beams were 0.8 cm in diameter. The performance of the OPO for data taken with Laser 2 may have suffered slightly from the presence of bulk damage sites in the OPO. The values of M² for OPO signal output beam at maximum power were 30 and 41 with Lasers 1 and 2, respectively. We measured the signal wavelength of the KTA ring cavity to be 1534.7 nm ± 0.6 nm, compared to a calculated value of 1533.7 nm based on published Sellmeier coefficients [6]. The output linewidth was 0.6 nm FWHM. We note that our calculated and measured wavelengths do not agree as well and are slightly higher than those listed in [7]. We measured a signal wavelength of 1571.6 nm ± 0.6 nm for a multi-crystal KTP ring OPO, which, within experimental error, agrees with the predicted value of 1571.1-nm [7].

With regard to bulk damage, after several months of experimentation with a set of six KTA crystals, we noticed the appearance of small, bubble-like inclusions in four, 2-cm-long crystals. The inclusions occurred in different distributions and areas for each of the crystals. The other two, 2.5-cm-long crystals, fabricated from crystal boules grown at a later date than those used to make the shorter crystals, showed no visible sign of similar inclusions. Whether the differences in bulk damage were related to particular experimental conditions in the OPO testing or differences in original defects in the source boules is a matter of continuing investigation. We are also studying the extent to which these bulk damage sites will have long term detrimental effects on the performance of the high-average-power OPOs. Ongoing improvements in KTA crystal growth may eliminate the issue of bulk damage in the future.

By direct comparison with KTP, we have experimentally validated the claim that KTA is more suitable than KTP for high-average-power, Nd-laser-pumped OPO operation in the eye-safe region near 1550 nm. The highest average-signal-power OPO, to date, has been operated and net efficiencies in excess of 30% at average power levels above 30 W have been demonstrated.

We wish to acknowledge the technical support of Gail Scott and Elias Fakhoury and the prior OPO studies of Glen Rines, Richard Schwarz, and Henry Zenzie. This work was sponsored by the Chemical & Biological Defense Command (CBDCOM), Aberdeen Proving Ground, Edgewood Area. The ongoing KTA crystal growth work is also supported by CBDCOM in cooperation with Night Vision Electronic Sensor Directorate (NVESD).

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