Highly-Efficient, Transversely-Pumped, 25-W, TEM00 Nd:YLF Slab Laser

Kevin J. Snell, Dicky Lee, Kevin F. Wall

Q-Peak Incorporated

135 South Rd., Bedford, MA 01730

e-mail: ksnell@qpeak.com

Abstract: A CW output power >25 W with an optical efficiency >43% has been achieved in TEM00 mode operation (M2<1.2) at 1047 nm in a Nd:YLF multi-pass slab laser side-pumped by two 30 W collimated laser diode bars.

©1999 Optical Society of America

OCIS Codes: (140.3580)

 

Increasingly many applications require a high-average power laser source with high peak power and brightness for efficient frequency conversion to UV and other wavelengths. In practice end-pumped systems offer high brightness and high efficiency [1,2] but scaling to high average power is difficult, whereas side-pumped lasers offer greater power capability with less complexity than end-pumped designs, at the expense of efficient fundamental-mode extraction efficiency. We present a simple, side-pumped slab laser design using Nd:YLF for low thermal lensing and high energy storage, which has high TEM00-mode extraction efficiency.

Figure 1. Schematic of the side-pumped multi-pass Nd:YLF laser.

 

The side-pumped, multi-pass slab (MPS) Nd:YLF laser is shown schematically in Figure 1 [3]. It consists of a astigmatic resonator formed by a plano-convex cylindrical high reflector, a plano-concave spherical turning mirror and a flat output coupler and a diode-pumped Nd:YLF gain module. The gain module consists of a 30 mm long Nd:YLF slab pumped by a pair of opposed and offset 30 W collimated 806 nm laser diode bars (OPC-L030-806CS/2). These bars are collimated with a 440 m m focal length aspherical lens to produce a pump spot 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 are used to make multiple passes (five) through the slab, which increase the gain and improve the fundamental mode extraction efficiency by reducing the effective aperture by a factor of three.

Figure 2. Output power as a function of pump power for the side-pumped, multipass Nd:YLF slab laser.

Figure 3. Beam diameter (closed symbols) and M2 (open symbols) as a function of pump power.

The 1047 nm output power as a function of incident pump is shown in Figure 2. In TEM00-mode operation, (circles) a CW output power of 25.7 W was obtained 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 laser diodes is 140 W resulting in an electrical efficiency of 18.4%. A slightly higher output power of 26.8 W was obtained in multimode operation (squares) with an optical efficiency of 45.3%.

Beam quality data were measured using a Coherent Modemaster and are shown in Figure 3 for pump powers from 40 W to 59 W. Both the horizontal and vertical beam quality parameters were nearly constant with Mx2 =1.15 and My2 = 1.01 over the entire range of pump power. Consistent with the constant beam quality, the measured spot sizes ranged from 1.24-1.33 mm in the horizontal plane and 0.61-0.62 mm in the vertical plane. This is due to the low inherent thermal lensing of the YLF host and the pump geometry. The laser has also been operated in the Q-switched regime using an acousto-optic modulator. At a repetition frequency of 10 kHz, the output energy was 1.8 mJ per pulse with a pulse width of 27 ns resulting in a peak power of 67 kW.

Compared with previous results using 40 W of pump [4], we have obtained a two-fold increase in output power without reaching the fracture limit. Given this result, we are now planning to scale the laser to 80 W of pump power. The TEM00 mode efficiency of this laser represents, to our knowledge, the highest yet obtained in a side-pumped design. Furthermore, it rivals many end-pumped designs, particularly when considering losses in fiber coupling. This laser is an ideal source for frequency conversion to UV and other wavelengths.
  1. W. L. Nighan, N. Hodgson, E. Cheng, D. Dudley, "Quantum-limited 35W TEM00 Nd:YVO4 laser," CLEO’99 Technical Digest, Paper CMA1, Optical Society of America, Washington D.C. (1999).
  2. L. Gonzalez, K. Kliene, L. R. Marshall, "High Brightness MOPA," CLEO’99 Technical Digest, Paper CMA2, Optical Society of America, Washington D.C. (1999).
  3. U.S. Patent #5,774,489.
  4. 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).

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