Abstract: Using a side-pumped, Nd:YLF slab laser design, we have obtained a TEM₀₀-mode output power of 25.7 W with 43.5% optical efficiency and a multimode output of nearly 36 W with 46% efficiency. As an amplifier of a SESAM-mode-locked laser, the same design has allowed us to generate, with 4.5-ps pulses at a 100-MHz rate, 15.7 W of average power in the fundamental and 2.9 W of power at the fourth harmonic.

OCIS codes: 140.3480, 140.4050

We have previously reported on a diode-pumped, Nd:YLF, multi-pass slab (MPS) laser, side-pumped by two 20-W cw diode-laser bars [1]. The laser, configured as an oscillator, can generate as much as 14 W of cw, TEM₀₀ power with an M² » 1.1, for an optical efficiency of 35%. Subsequent work on this system has involved power scaling with the same design employed as an amplifier. In one case, we have generated as much as 60 W, TEM₀₀-mode cw power with an oscillator followed by four amplifiers, and amplified a single-frequency, Q-switched laser to generate 40 W of average power at a 5-kHz rate [2]. In the work reported here, we discuss operation of the MPS design with higher-power pump lasers. In addition, we describe results obtained with a MPS amplifier system driven by a SESAM-mode-locked Nd:YLF oscillator.

The MPS design configured as an oscillator is shown schematically in Figure 1. It consists of an astigmatic resonator formed by a plano-convex cylindrical high reflector, a plano-concave spherical turning mirror, a flat output coupler and the diode-pumped Nd:YLF "gain module." The latter includes the Nd:YLF crystal, the two pump diodes, multi-pass mirrors, copper cooling blocks for the crystal and diodes, and a manifold to distribute water through the blocks. For the high-power results reported here, the gain module consists of the standard 28-mm-long Nd:YLF slab used in prior work, but now pumped by a pair of opposed and offset 30-W or 40-W, collimated, 806-nm laser diode bars, both sets of which produce a pump height of about 400 m m along the vertical plane. Segmented, high-reflection coatings are used to double-pass the pump light to obtain a uniform absorption profile with >90% absorbed pump power. External, closely coupled high reflectors on the gain module are used to make multiple passes (five) through the slab, which increases the gain and improves the fundamental- mode extraction efficiency by reducing the effective aperture by a factor of three.

Fig. 2. CW output power, multimode (MM) and TEM₀₀ vs. pump power
for the MPS oscillator pumped by 20, 30 and 40-W diodes.

The 1047-nm cw output power as a function of incident pump power is shown in Figure 2. Data taken with 20-W bars is included for reference. With the resonator optimized for TEM₀₀-mode operation, (circles), we obtained, with the 30-W bars, an output power of 25.7 W at a pump power of 59.1 W, resulting in an optical efficiency of 43.5% and a slope efficiency of 50.6%. The electrical input power to the 30-W laser diodes is 140 W, resulting in an electrical efficiency of 18.4%. Using 30-W bars, we obtained a slightly higher output power of 26.8 W in multimode operation (squares) with an overall optical efficiency of 45%. Finally, with 40-W bars, we generated multimode powers as high as 35.7 W, with a slope efficiency of 52% and an optical efficiency of 46% at the highest pump power. By comparison, our best result with 20-W pump lasers is 14 W of TEM₀₀-mode output, for an optical efficiency of 35%.

Using a Coherent Modemaster, we measured the oscillator beam properties for pump powers (from 30-W bars) in the range 40-59 W. Both the horizontal and vertical beam-quality parameters were nearly constant with M_x² » 1.15 and M_y² » 1.01 over the entire range. Consistent with the constant beam quality, the measured spot beam diameters ranged from 1.24-1.33 mm in the horizontal plane and 0.61-0.62 mm in the vertical plane. The weak dependence of beam parameters is due to the low inherent thermal lensing of the YLF material and the pump geometry [3].

In the Q-switched mode, the increased pump power available from the higher-power bars leads to an increase in the pulse energy as well as a reduction in the pulse width. Figure 3 plots peak power vs. pulse rate for both a 63-W pump level (from two 40-W bars) and a 40-W level (from two-20-W bars.) At the higher pump level and at pulse rates of 15-kHz and above, the peak power is 3-4X greater than that produced by 20-W bars. With 63 W of pump power, we generated an average TEM₀₀-mode power of 26 W at Q-switched pulse rates of 40 kHz and above. At lower pulse rates, we observed a loss in pulse energy associated with the finite switching time of the acousto-optic switch. The pulsewidth at the 63-W pump level increased in a nearly linear fashion with pulse rate in the 10-100-kHz range, from 27 to 193 ns.

Compared with to our previous results [1], with the higher-power pump sources we have obtained significantly higher cw and pulsed output powers and efficiencies without reaching the fracture limit of the laser crystal. The TEM₀₀-mode efficiency of this laser represents, to our knowledge, the highest yet obtained in a side-pumped design, and compares favorably with end-pumped configurations. Even higher TEM₀₀ powers will be generated as we develop appropriate resonators for use with the 40-W bars.

The MPS design also works well as an amplifier, with an unsaturated cw gain exceeding 10. We have used a two-gain-module amplifier (pumped by 20-W bars) to increase the power of a commercial SESAM-mode-locked laser (Time-Bandwidth Products, Zurich, Model GE-100, modified). The mode-locked laser generates 0.7 W of average power, in the form of 4.5-ps pulses at a 100-MHz rate.

Figure 3, below, is a schematic of our amplifier system, which uses one gain module as a double-pass preamplifier and the other as a single-pass power amplifier. Double-passing of the preamplifier is accomplished by means of a Faraday rotator. Figure 4 (next page) plots the preamplifier output as a function of pump diode power. At ~30 W of pump power, the preamplifier output power is linear with the pump power, indicating that the preamplifier is becoming saturated. Typically the output power of the double-pass preamplifier is ~6.7 W at 40 W of pump power for an overall signal gain of ~15. We further amplified the output of the preamplifier to 15.7 W in the second Nd:YLF gain module. We single-passed the input beam through this amplifier since the input intensity is on the order of the Nd:YLF saturation intensity (2 kW/cm²) and measured the output beam M² to be £ 1.1 for both the horizontal and vertical axes. The lenses following the amplifier are intended to stigmatize and circularize the output beam.

We have focused the output of the amplifier into a 2-cm-long, Type I non-critically-phase-matched LBO crystal and have generated second-harmonic powers as high as 8.8 W, for a conversion efficiency of 55 %. The second harmonic power was further focused into a Type I, critically phase-matched, 1-cm-long, CLBO crystal to generate 2.9 W of average power at 262 nm. We would expect to generate even higher mode-locked powers through the addition of more amplifier stages and/or the use of higher-power pump diodes. When saturated and pumped by 20-W bars, each amplifier stage would increase the average power by >10 W, while we would expect a >20 W increase with 30-W bars.

Fig. 4. Preamplifier output power as a function of pump power. The input signal intensity was fixed at 0.45 W

1. J. Harrison, P. F. Moulton, G. A. Scott, "13 W, M²<1.2 Nd:YLF Laser Pumped by a Pair of 20 W Diode Laser Bars," CLEO’95 Post Deadline Papers, Paper CPD20, Optical Society of America, Washington D.C. (1995).

2. K.F. Wall, M. Jaspan, A. Dergachev, A. Szpak, J.H. Flint, and P.F. Moulton, "A 40-W, single-frequency, Nd:YLF master oscillator/power amplifier system," OSA Trends in Optics and Photonics Vol. 26, Advanced Solid State Lasers, Martin M. Fejer, Hagop Injeyan and Ursula Keller, eds. (Optical Society of America, Washington, DC 1999), pp. 216-221.

3. U.S. Patent #5,774,489

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