Mid-infrared pulse generation via Orientation Patterned Gallium Arsenide Parametric Devices

Abstract

Mid-infrared (mid-IR) pulsed lasers are finding increasing applications in the areas of industrial processing, spectroscopy, medicine, and chemistry due to the presence of characteristic vibrational absorptions of a large number of molecules in this spectral region. Optical parametric devices (OPDs), as one of the most promising solutions for mid-infrared generation, have attracted much attention. Nonlinear crystals are key elements for OPDs and the recently developed material system, orientation-patterned gallium arsenide (OP-GaAs), is deemed as an outstanding nonlinear gain element for the mid-IR spectral region. This thesis describes a systematic study of the development of mid-IR OPDs based on OP-GaAs, including optical parametric generators (OPGs), amplifiers (OPAs), oscillators (OPOs) and their variants. Several 2-µm, picosecond, thulium-doped-fibre master oscillator power amplifier (Tm:MOPA) systems, employing gain-switched laser diode seeding, are developed as the pump source for the OP-GaAs OPDs, greatly increasing their flexibility and potential for real-world applications. In a simple single-pass configuration, an OP-GaAs OPG is pumped by a high-peak-power, lowrepetition rate (1 MHz) Tm:MOPA, and provides a wide wavelength tuning range of 2550-2940 nm (signal) and 5800-8300 nm (idler) as well as a high peak power of 2.8 kW (signal) and 1.7 kW (idler). The OPG naturally generates an output with a broad spectral linewidth. In order to control and narrow the spectral linewidth, a wavelength-tunable Cr:ZnSe laser is employed as a seed laser to change the OPG into an OPA. Compared to the OPG, the OPA offers a lower pump power requirement and higher conversion efficiency. The OPA signal and idler spectral linewidth is narrowed to 0.7 cm-1 and 1.4 cm-1, respectively, and a maximum output peak power of 11.4 kW and 2.8 kW for signal and idler is obtained. However, the OPA requires an external seed laser and therefore its tuning range is mainly limited by that of the seed laser. To overcome the tuning limitation, a novel cascaded OPG-OPA is proposed and developed that combines the good features of the OPG and OPA. In the cascaded OPG-OPA, the OPG output signal spectrum is controlled and filtered and then used as the seed source for the OPA. Without using an external seed laser, a tuning range of 2552-2960 nm (signal) and 5733-8305 nm (idler) covering the entire wavelength regime that the OP-GaAs gratings provided is demonstrated, and controlled output signal and idler linewidths of 1.4 cm-1 and 9 cm-1 are achieved, respectively. The mid-IR output beam of OPGs and OPAs are typically not diffraction limited due to the lack of any active spatial control. In contrast, OPOs with cavity structures, can offer mid-IR output at high beam qualities. An idler-resonant OP-GaAs OPO, synchronously pumped by another Tm:MOPA system operating at a high repetition rate of 100 MHz, is demonstrated and delivers a neardiffraction-limited output beam for the mid-IR idler. Power scaling of the OP-GaAs OPO is then investigated, pumped by a high-average-power Tm:MOPA, and a total mid-IR maximum output power of 9.7 W is obtained with wide wavelength tunablity (2.9-3.3 μm for signal, 4.9-6.4 μm for idler). The output characteristic shows a thermal roll-over which is cured in the final work section via active water-cooling of the OP-GaAs crystal. The final work also demonstrates the flexibility allowed by the gain-switched-diode seeding of the pump laser, delivering a controllable pulse repetition-rate (100 MHz – 1 GHz) and duration (95 ps – 1.1 ns) OPO with a maximum total output power of 13.7 W at a signal and idler wavelength of 3.3 μm and 4.9 μm, respectively.

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Identifiers

ORCID for David Richardson: orcid.org/0000-0002-7751-1058

ORCID for Lin Xu: orcid.org/0000-0002-4074-3883

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