Elsevier

Optics & Laser Technology

Volume 39, Issue 6, September 2007, Pages 1135-1139
Optics & Laser Technology

High-average-power and high-conversion-efficiency continuous wave mode-locked Nd:YVO4 laser with a semiconductor absorber mirror

https://doi.org/10.1016/j.optlastec.2006.09.006Get rights and content

Abstract

A high-power continuous wave (cw) mode-locked Nd:YVO4 solid-state laser was demonstrated by use of a semiconductor absorber mirror (SAM). The maximum average output power was 8.1 W and the optic-to-optic conversion efficiency was about 41%. At the maximum incident pump power, the pulse width was about 8.6 ps and the repetition rate was 130 MHz. Experimental results indicated that this absorber was suitable for high power mode-locked solid-state lasers.

Introduction

Since the first cw mode-locked (CML) solid-state laser with SESAM was demonstrated the mode-locked solid-state lasers were well developed in recent years [1], [2], [3], [4], [5]. One of the important frontiers is high average power [6]. Picosecond high-power diode-pumped solid-state lasers with good beam quality are attracting growing interest because of numerous applications in medicine, material processing and nonlinear frequency conversion. The main challenges for high-power lasers were Q-switching instabilities and the SESAM damage. With single LD pumping, one group has achieved 6.2 W average output power with a slope efficiency of 35% and the other group has achieved 5.3 W average output power with a conversion efficiency of 30% [7], [8]. The thermal lens of the laser crystal and the SESAM damage were the main reason for the limiting average output power. In this experiment, we used a super SAM as the saturable absorber which had a high damage threshold and we designed the laser cavity elaborately for mitigating the effect of the thermal lens. Finally, we achieved a high average output power of 8.1 W at the maximum incident pump power in CML operation; the optic-to-optic conversion efficiency was about 41%. The nonsaturable losses of the SAM was small and the SAM was not damaged in the experiment.

Section snippets

Theoretical discussion about the thermal lens

It is worthwhile to analyze the thermal lens in the Nd:YVO4 crystal, which affects the output power of the laser and the stability of the resonator. For a laser pumped by a fiber-coupled diode the focal length of the thermal lens fth can be approximately given by [9], [10], [11] fth=πKcωp2Pph(dn/dT)11-exp(-αl).Where Kc is the thermal conductivity, ωp is the average pump size in the laser crystal, Pph is the fraction of pump power that results in heating, dn/dT is the thermo-optic coefficient, α

Experimental setup and results

The SAM was grown on GaAs substrate by metal-organic chemical-vapor deposition. The SAM consisted of 22 pairs of GaAs/AlAs quarter-wave Bragg layers with high reflectivity of 99.5% at lasing wavelength of 1064 nm and a 15-nm relaxed In0.3Ga0.7As single quantum well (embedded in the topmost layer of the Bragg stack) for achieving saturable absorption at 1064 nm. The Bragg layers and the In0.3Ga0.7As absorber were grown at temperatures of 720 and 500 °C, respectively. For getting a high damage

Conclusion

We have demonstrated a high-power diode-end-pumped cw mode-locked Nd:YVO4 laser by using a semiconductor absorber mirror as the saturable absorber. Considering the thermal lens effect, we elaborately designed the laser cavity, and obtained 8.1 W of average output power in 8.6 ps Gaussion pulses at a repetition rate of 130 MHz. The maximum optic-to-optic conversion efficiency was about 41%. The Q-switching instability was well suppressed and the cw mode-locked pulse train was stable. The low

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