Abstract: We report on the operation of a high beam quality (M² < 1.2), 40-W, single-frequency, Q-switched Nd:YLF master-oscillator/power-amplifier system based upon transversely-pumped Nd:YLF gain modules.

OCIS codes: (140.3520, 140.3530, 140.3540, 140.3580)

A common method to obtain wavelength diversity of laser radiation is to use a 1-mm "pump laser" and employ nonlinear crystals to convert the radiation to shorter wavelengths (harmonic conversion) or optical parametric oscillators (OPOs) to convert to longer wavelengths. In order to efficiently convert the 1-mm radiation, it is desirable to have high peak power and high beam quality at ever-higher average powers. Nd:YLF, because of its long upper-state lifetime and low thermal lensing, is a particularly attractive material for such a pump laser.

While there have been many approaches to generating high-average-power pump sources with Nd:YAG, there are only a few notable examples with Nd:YLF. A cw, lamp-pumped Nd:YLF laser has produced as much as 80 W multimode and 40 W cw in a TEM₀₀ beam at 1.047 m m.[1] A passively cooled Nd:YLF laser has demonstrated 72 W in a multimode beam.[2] We have developed a Q-switched, Nd:YLF master oscillator/power amplifier (MOPA) system that produces 50 W of time-averaged-power (60 W cw) with high beam quality (M² < 1.3). In this paper we will report on the performance of single-frequency MOPA that produces 40 W with an M² < 1.2.

The basis for the Nd:YLF laser systems we have developed is a transversely-pumped, multi-pass slab (MPS gain module), [3, 4] with external high reflectors, shown schematically in Fig. 1. The overall design takes advantage of the low thermal lensing and long upper-state lifetime (525 ms) in Nd:YLF compared to other laser materials. Based upon this gain-module technology, we have designed and constructed a single-frequency, Nd:YLF master oscillator/power amplifier (MOPA) capable of producing 40-W of time-averaged power in 25-ns pulses at a repetition rate of 5 kHz. The system consists of a single-frequency seed laser, a two-gain-module ring laser, and three amplifier stages.

The 2.8-cm long Nd:YLF crystal is transversely pumped by a pair of 1-cm long, 20-W, cw diode-laser bars. The diode-laser bars are coupled to the Nd:YLF through a single cylindrical collimation lens attached directly to each bar package. These lenses collimate the "fast" axis (perpendicular to the 1-cm emission region) producing a Gaussian profile with a height of <500 m m. The bars are offset on opposite sides of the Nd:YLF crystal to create a uniform sheet of gain. The pump faces of the crystal are anti-reflection (AR) coated and the opposite faces are high reflectors to enable double- pass pump absorption.

Figure 1. Top view schematic of the Nd:YLF MPS gain module.

Heat is removed from the diode lasers by mounting them on water-cooled heat sinks. The diode laser wavelength is stabilized by controlling the temperature of the cooling water to ± 0.1 ° C. The diode laser wavelength is tuned off the Nd:YLF absorption peak to make the gain sheet as uniform as possible.

Efficient extraction and high gain per pass are obtained by passing the laser cavity mode five times through the gain sheet in the slab. The faces of the slab are AR coated, angled to facilitate the multi-passing, and slightly wedged to inhibit parasitic oscillation. The gain sheet is purposely kept away from the ends of the slab to minimize thermal distortion associated with end effects.[4] The unsaturated small-signal single-pass gain for a Nd:YLF MPS gain module is >4 and this high-gain design is important for generating short Q-switched pulse widths. The relatively low pump power density in the crystal minimizes upconversion loss.

Heat is removed from the top and bottom surfaces of the slab (perpendicular to the plane of Fig. 1) by means of water-cooled heat sinks, resulting in nearly one-dimension heat flow. This geometry results in a total astigmatic thermal focussing for five passes of ~100 cm and ¥ in planes parallel and perpendicular to the gain sheet, respectively. The uniaxial nature of Nd:YLF results in linearly polarized emission.

The seed source is a proprietary, single-frequency, Nd:YLF laser. The pump source is a fiber-coupled diode laser (OPC-A001-797-FC/100) that is mounted on a thermoelectric cooler to control the pump laser wavelength. The seed laser has a passive stability of ~150 MHz/°C . To further stabilize the seed laser, the laser cavity is mounted in a water-cooled block that is maintained at a constant temperature with an accuracy of ± 0.1 ° C. Mechanical vibrations from the water cooling introduced a short jitter measured to be 250 ± 80 kHz during a 200-ms interval, however. The seed laser was tunable over 190 GHz. The laser has a cw output power of >10 mW in a TEM₀₀, circularly symmetric beam profile. Two 30-dB Faraday isolators in series are used to isolate the seed laser from the ring laser. Injection seeding is done through the ring-laser output coupler, which has a reflectivity of 50%.

We used the pulse-build-up-time-reduction (PBUTR) technique to frequency lock a longitudinal mode of the ring laser to a longitudinal mode of the seed laser.[5] The operating principal of the PBUTR technique is that the build-up time of the Q-switched pulses is shortest when a longitudinal mode of the ring laser corresponds to the seed-laser frequency. One of the ring-laser cavity mirrors is mounted on a PZT (PI S-310.10) to allow control of the cavity length and this mirror is dithered by a small fraction of a free spectral range of the ring laser about the locked position at a 2.5-kHz rate.

The buildup-time is measured by using the time difference between the Q-switch trigger pulse and the optical pulse. A photodiode monitors leakage through one of the cavity mirrors to provide the optical pulse trigger. The difference in build-up time between the locked and unlocked condition was ~40 ns. Ideally, the circuitry dithers about the minimum build-up time and the difference in build-up time between two successive pulses is zero. If the laser cavity length drifts due to environmental factors such as ambient temperature changes, the difference in build-up time between two successive pulses is nonzero and is used to generate an error signal that restores the locked condition. The PBUTR technique of seeding provides very small temporal jitter at the expense of a small (~5% of the free spectral range) frequency jitter.

Using this technique, we were able to injection seed the laser for periods as long as 30 min. before the PZT error voltage would drift to an extreme and automatically reset. Reacquisition of lock would occur within seconds. When frequency-locked, the Q-switched pulses measured with a fast photodiode and 1-GHz-bandwidth oscilloscope showed no signs of mode beating, indicating single-mode operation. Using a pulsed wavemeter (Burleigh RFP-3600E), we measured the linewidth of the injection seeded ring laser to be < 100 MHz, the limit of the instrument. The transform limit of the pulses was estimated to be ~20 MHz.

The design of the ring laser is based upon the design of our commercial linear oscillator, which is built with a single gain module. The dashed line in Fig. 2 indicates a plane about which the oscillator laser mode is nearly symmetric and each half is similar to our linear oscillator. Cylindrical and spherical lenses were used to shape the laser mode in the vertical and horizontal planes to provide good overlap of the laser mode with the gain sheet produced in the gain modules. The resulting beam shape in the gain modules matches the height of the gain sheet in the vertical direction. In the horizontal dimension, the size of the mode is a compromise between one that is as wide as possible (to efficiently extract gain) and one which does not suffer significant diffraction loss (or compromise beam quality).

The cavity was designed to try to minimize the round-trip path length to provide the shortest possible pulses. All of the cavity mirrors are plane mirrors and all of the intracavity lenses were made from fused silica. A 41-MHz, fused-silica, acousto-optic (AO) Q-switch operated at a 5-kHz repetition rate was located in the cavity at a minimum in the horizontal (in the plane of the ring) beam diameter. The Nd:YLF MPS gain modules were driven in series by a single current source, operated at the average 20-W current setting for the four diode laser bars.

Using an external high reflector to force the ring laser to operate unidirectionally, we measured the cw and time-averaged Q-switched output powers of the ring laser to be 20.2 W and 14.2 W, respectively. We determined the M² to be 1.17 and 1.12 in the horizontal and vertical dimensions, respectively using a Spiricon M²-101 beam analyzer. The pulsewidth at a 5-kHz Q-switching rate was 22 ns.

We observed that the Q-switched output power dropped to ~13 W when the external mirror was removed and the ring laser was injection seeded. It was also apparent that as the build-up time of the pulse decreased, the pulse height also decreased. Finally, we found that the AO Q-switch generated diffracted beams as well as the main beam. When the build-up time was decreased (by increasing the pump power), the amount of power in the diffracted beams increased. Ideally, no diffracted beams should be present if the optical pulse builds up after the Q-switch has fully opened. The evidence suggests that the Q-switched pulse was building up while the AO Q-switch was still opening and that this was causing a loss in power. To correct this problem, the resonator could be redesigned to generate a smaller beam diameter at the AO modulator or electro-optic Q-switching could be used.

Three amplifiers, which use gain modules identical to those used in the oscillator, form the power-amplifier portion of the system. The amplifier chain was isolated from the oscillator by a 30-dB Faraday isolator. Each Nd:YLF MPS gain module was driven by separate current sources and were operated a the average 20-W current setting for each pair of diode laser bars. All gain modules, including those in the oscillator, were water cooled using a recirculating chiller (Tec-Temp LK-10).

Relay optics were used between the ring laser and each amplifier stage to reproduce in the amplifier gain modules the elliptical beam shape that exists in the ring laser gain modules. A pair of turning mirrors between the oscillator and amplifier stages facilitated alignment.

The system was first set up without injection seeding by using a external mirror to force the ring laser to operate unidirectionally. With 14.2 W from the ring laser amplifier 1 produced 21.3 W, amplifier 2 produced 31.5 W, and amplifier 3 produced 41 W. With injection seeding,, the output from the last amplifier was 38.6 W. Using a Glan-Thompson polarizer, we measured the polarization of the laser to be >600:1. The measured FWHM pulsewidth after the final amplifier was 23 ns and the standard deviation of the pulse jitter was 1.3 ns measured over 1000 pulses. The standard deviation of the pulse amplitude was measured to be 3.4 % (also measured over 1000 pulses).

An important attribute of the Nd:YLF MPS amplifiers is their high fidelity. This results partially from the low intrinsic thermal distortion of Nd:YLF. In the vertical dimension, the gain sheet in the Nd:YLF MPS gain module has a Gaussian profile and the gain aperturing that results from this also aides in maintaining high beam quality. M² was measured to be 1.17 and 1.00 ± 5% for the horizontal and vertical dimensions, respectively, at the 40-W power level. Using a single cylindrical lens after the final amplifier the beam profiles were near Gaussian as shown in Fig. 3. The power stability measured over 1-h was ± 1.5 % and the pointing instability measured over 0.5 h was less than 1.5 m rad.

Using gain modules based upon transversely pumped, multi-pass Nd:YLF slabs, we have built a two-gain-module single-frequency ring laser Q-switched at a 5-kHz rate. Single-frequency operation was obtained by injection seeding and stabilized using the pulse-build-up-time reduction technique. A reduction in the ring laser output power was observed when seeded and this was attributed to the build up of the Q-switched pulse during the opening time of the acousto-optic Q-switch. We demonstrated amplification of the ring laser to a time-averaged power of 40-W, with an M² of < 1.2.

1. G. Cerullo, S. De Silvestri, V. Magni, "High efficiency, 40 W Nd:YLF laser with large TEM₀₀ mode," Opt. Comm., 17, 77 (1992).

Y. Hirano, T. Yanagisawa, S. Ueno, K. Kasahara, O. Uchino, T. Nagai, and C. Nagasawa, "High average power conductive-cooled diode-pumped Nd:YLF laser," in Conference on Lasers and Electro-Optics, Vol. 6, 1998 OSA Technical Digest Series (Optical Society of America, Washington DC, 1998) p. 103.

J. Harrison, P. F. Moulton, and G. A. Scott, "13-W, M² < 1.2 Nd:YLF Laser Pumped by a Pair of 20-W Diode-Laser Bars," CLEO’95 Postdeadline Paper CPD-20.

P. F. Moulton, J. Harrison, and R. J. Martinsen, "Transversely Pumped Solid State Laser," U. S. Patent #5,774,489 (1998).

L. Rahn, "Feedback stabilization of an injection-seeded Nd:YAG laser," App. Opt., 24, 940 (1985).

The authors would like to thank Fab Brennan and Al Tidd for their assistance and Mike Michailik for the design and construction of the PBUT circuitry. The contributions of Jim Harrison, Andy Finch, Jeff Manni, and Mike Francis in the development of the gain module technology is also acknowledged. This work was partially supported by Ushio, Inc

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