Investigation of a versatile pulsed laser source based on a diode seed and ultra-high gain bounce geometry amplifiers A. Teppitaksak,* G. M. Thomas, and M. J. Damzen Photonics Group, The Blackett Laboratory, Imperial College London, London SW7 2BW, UK *
[email protected]
Abstract: We present an investigation of a versatile pulsed laser source using a low power, gain-switched diode laser with independently variable repetition rate and pulse duration to seed an ultra-high gain Nd:YVO4 bounce geometry amplifier system at 1064nm. Small-signal gain as high as 50dB was demonstrated in a bounce geometry pre-amplifier from just 24W pumping, with good preservation of TEM00 beam quality. The single amplifier is shown to be limited by amplified spontaneous emission. Study is made of further scaling with a second power amplifier, achieving average output power of ~14W for a pulsed diode seed input of 188μW. This investigation provides some guidelines for using the bounce amplifier to obtain flexible pulse amplification of low-power seed sources to reach scientifically and commercially useful power levels. ©2015 Optical Society of America OCIS codes: (140.3280) Laser amplifiers; (140.3538) Lasers, pulsed.
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1. Introduction Pulsed laser sources are useful for industrial applications such as drilling, cutting, high precision micromachining and laser marking [1–3]. Among laser sources, diode-pumped solid-state lasers such as Nd:YAG and Nd:YVO4 are proven sources of pulsed power with good efficiency and high beam quality. Most variable pulse rate solid-state lasers are achieved using active Q-switching, producing pulse durations typically ~10–100ns, although subnanosecond pulses can be achieved from passively Q-switched microchip Nd:YVO4, Nd:GdVO4 and Nd:YAG lasers with ultra-compact cavities [4–8]. However, Q-switched laser systems are limited in pulse versatility since the pulse rate, pulse energy and pulse duration are in general not independent but all interlinked by the stored inversion and cavity dynamics. Another approach to producing pulsed laser sources is by amplifying a seed source with variable pulse parameters. In this work we consider the case where the seed is a gain-switched semiconductor laser diode. The gain-switching of laser diodes is controlled by the temporal form of the current applied to the laser, allowing flexibility and independence in pulse duration and pulse repetition rate. However, the peak power of gain-switched laser diodes is limited by optical damage, and the low duty cycle of the diode when operating with short temporal pulses can lead to very low average output power (typically sub-mW) [9,10]. To provide useful pulse energy and sufficient average output power for subsequent applications, an amplifier, or set of amplifiers, must be employed in a master oscillator power amplifier (MOPA) configuration. The amplifier system must provide extremely high amplification to produce a commercially useful output power e.g. for industrial material processing. The MOPA approach with diode seed is extensively used in high gain fibre laser systems, often using a series of fibre amplification stages [11,12]. A difficulty arises in fibre amplifiers due to high amplified spontaneous emission (ASE) and, more significantly, the onset of optical damage and fibre nonlinearity that typically limits the pulse energy and peak power achievable [13]. An alternative approach involves amplification using a bulk solid-state gain medium. However, barring recent work by Delen et al. [14], this approach usually provides limited gain. An exception to this is the ultra-high gain that can be achieved in the bounce amplifier geometry [15–22]. The bounce amplifier is a diode-side-pumped configuration, where the laser mode experiences total internal reflection (TIR) and amplification at a grazing incidence to the pump face of the crystal. By using a highly absorbing gain material with a correspondingly small absorption depth, such as Nd:YVO4 or Nd:GdVO4, the laser mode experiences a region of high inversion at the pump face, allowing efficient power extraction and extraordinarily high gain levels. High average powers of over 100W have been achieved using Nd:GdVO4 in a MOPA bounce configuration [16]. Nd:YVO4 in the bounce geometry has achieved high output powers with good beam quality in both CW and in different pulsed regimes [17–22]. In this work, we present a study of the flexibility of using the bounce geometry as an ultrahigh gain (50dB) amplifier for creation of a versatile variable repetition rate and pulse
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Received 15 Dec 2014; revised 7 Jan 2015; accepted 7 Jan 2015; published 1 May 2015 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.012328 | OPTICS EXPRESS 12329
duration system. The seed source in this study is a gain-switched semiconductor diode laser at 1064nm. Prior work has been performed on the bounce amplifier MOPA configuration using sub-ns pulses from a passively Q-switched Nd:YAG laser [20] and using a passively modelocked (picosecond) Nd:YVO4 [21] as the seed sources. In this paper, we provide a more systematic investigation of the gain and power scaling of the bounce amplifier when using a gain-switched diode seed operated at different (ns-class) pulse durations, and repetition rates up to 2MHz. We study the effects of varying the bounce angle and pump parameters using a pre-amplifier based on diode-pumped Nd:YVO4 slab for providing ultra-high gain when using μW seed powers. We find the limits of the amplifier are due to ASE, so a second power amplifier is utilised to further boost the average power levels. With 188μW seed power we obtain an output of ~14W. The flexible pulse duration and pulse repetition rates in this system allow for the possibility of optimising this type of diode-seeded bulk laser amplifier system for a wide range of processes requiring flexible pulse and high peak power format. 2. Experimental system The schematic of the investigated flexible MOPA pulse source is shown in Fig. 1. The system comprised of a pulsed laser diode seed at 1064nm, a Nd:YVO4 pre-amplifier and a Nd:YVO4 power amplifier, both in the bounce geometry.
Fig. 1. Schematic diagram of the MOPA system with gain-switched diode laser seeding a preamplifier and power amplifier, both in the bounce geometry. HCL and VCL are horizontal and vertical cylindrical lenses, respectively; OI are optical isolators and HWP are half waveplates.
2.1 Pulsed laser diode seed system The pulsed seed source used in this system was a semiconductor laser diode, connected to a pulsed electrical driving circuit for gain-switched operation. The electrical drive circuit was capable of generating pulse durations from 3.5ns to continuous wave (CW) and with adjustable pulse repetition rate, ranging from single shot up to 2MHz. The minimum pulse duration of this system was limited by the finite inductance of the laser diode and our cabling system, rather than being a fundamental limit. The laser diode was spatially single mode and had a nominal free-running wavelength of 1064nm, and CW rated power of 200mW. It was housed in a 9mm TO-can package and temperature stabilised with a thermo-electric (Peltier) device. It was collimated with an aspheric lens producing an elliptical (~3:1 ratio) spatial output but with near-diffractionlimited beam quality. The diode laser was formed of a Fabry-Perot cavity and had a spectrally multimode output. Whilst the central wavelength of the spectral band could be temperature tuned, mode hopping and discrete longitudinal mode spectrum meant that the wavelength could not be continuously tuned, nor fully matched in general to the narrow (
