Hybrid master oscillator power amplifier high-power narrow-linewidth nanosecond laser source at 257 nm

Abstract : We report on a high-power narrow-linewidth pulsed laser source emitting at a wavelength of 257 nm. The system is based on a master oscillator power amplifier architecture, with Yb-doped fiber preamplifiers, a Yb:YAG single crystal fiber power amplifier used to overcome the Brillouin limitation in glass fiber and nonlinear frequency conversion stages. This particularly versatile architecture allows the generation of Fourier transform-limited 15 ns pulses at 1030 nm with 22 W of average power and a diffraction-limited beam (M 2 < 1.1). At a repetition rate of 30 kHz, 106 μJ UV pulses are generated corresponding to an average power of 3.2 W. There is an increasing need for narrow linewidth, high average power ultraviolet (UV) laser sources for high-resolution spectroscopy, remote environmental sensing, laser-induced fluorescence, semiconductor inspection, and LIDAR applications [1,2]. A standard way of designing efficient sources in the UV is to frequency convert near-infrared diode-pumped solid-state lasers [3,4]. In this case, obtaining high nonlinear conversion efficiency requires a large peak power in the fundamental wave. At a fixed repetition rate and average power, this results in a trade-off between attainable linewidth, related to the pulse duration, and the final nonlinearly converted average power. Ytterbium-doped fiber amplifiers are particularly attractive to design high power sources at 1030 nm, providing compactness, efficiency, and high beam quality. In narrow linewidth regime (<100 MHz), the physical effect that limits the achievable power is stimulated Brillouin scattering (SBS). The use of large mode area (LMA) fibers and various SBS mitigating techniques has allowed scaling up of the peak power to around 1 kW in a 10 MHz bandwidth [5]. However, this peak power level is still too low for efficient frequency quadrupling to reach the wavelength of 257 nm. In this case, resonant cavities might be used to generate UV light efficiently [6], at the price of an increased complexity. Recently, single-crystal fiber (SCF) amplifiers have demonstrated a great potential to scale fiber systems beyond their limits fixed by nonlinear effects, while retaining the possibility to generate high average powers [7]. This power scaling has been recently reported in a chirped-pulse femtosecond amplifier [8], allowing the generation of 1 mJ 380 fs pulses with a moderate stretching ratio impossible to attain with glass fibers due to self-phase modulation. In this Letter, we demonstrate that Yb:YAG SCF amplifiers can be used to scale the power of SBS-limited 15– 150 ns pulse ytterbium-doped fiber sources. Subsequent single-pass nonlinear conversion in bulk crystals permits the generation of Fourier transform-limited 15 ns pulses with 3.2 W of average power, or 150 ns pulses with 430 mW of average power at a repetition rate of 30 kHz at 257 nm. The source is built following a master oscillator power amplifier (MOPA) architecture similar to [9], with an experimental setup depicted in Fig. 1. It starts with a CW single-frequency distributed-feedback (DFB) laser diode emitting 5 mW at 1030 nm with spectral linewidth below 1 MHz. First, this signal is amplified to 50 mW in a single-mode fiber preamplifier with 6 μm core diameter. A fiber-pigtailed acousto-optic modulator (AOM) is then used to carve nanosecond pulses with a variable duration of 15–150 ns at a high repetition rate of 330 kHz in order to limit the amplified spontaneous emission (ASE). This signal is amplified in two stages of single-mode 6 and 15 μm core diameter Yb-doped fiber amplifiers to an average power of 400 mW. A second AOM further decreases the repetition rate to a few tens of kilohertz before coupling to the final LMA fiber amplifier. This amplifier is made of a 2 m long polarizing double-clad 40∕200 μm Yb-doped photonic crystal fiber pumped by a 25 W
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Optics Letters, Optical Society of America, 2013, 38 (6), pp.995
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Xavier Delen, Loïc Deyra, Aurélien Benoit, Marc Hanna, François Balembois, et al.. Hybrid master oscillator power amplifier high-power narrow-linewidth nanosecond laser source at 257 nm. Optics Letters, Optical Society of America, 2013, 38 (6), pp.995. 〈hal-01303647〉

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