High-energy picosecond-pulsed mid-infrared optical parametric oscillators

Abstract

Short-pulsed mid-infrared (MIR) lasers are highly attractive for applications in areas such as spectroscopy, materials processing, free-space communications, and medical treatment. Synchronously pumped optical parametric oscillators (SPOPOs), converting near-infrared pulses to the MIR, are often used to service these applications. In the application of materials processing, successful implementation of resonant infrared pulsed laser deposition of polymer films requires picosecond (ps) pulses of μJ energy. In medical applications, high peak power laser sources with output in the MIR spectral range are a highly attractive choice for high-precision surgery as they are suitable for the ablation of both soft and hard tissues with very little collateral damage. Temporal synchronization between the pump pulses and the resonant signal is required in ultrashort pulse MIR SPOPOs. This becomes a challenge when trying to generate high-energy and high-peak-power ps pulses at low repetition rates, since this dictates the need for long OPO cavities. This thesis presents the development of high-energy ps-pulsed MIR OPOs based on periodically poled Lithium Niobate (PPLN) crystals and pumped using an ytterbium-doped fibre (YDF) master oscillator power amplifier (MOPA) system. For the first method, a high-repetition-rate, tabletop-sized, MIR ps-pulsed SPOPO was realised with burst-mode operation. Under continuous mode (CM) pulsed operation, at a 1.5-GHz repetition rate and 14-W pump power, an idler (2992 nm) average power of 2.4 W (~30 W peak power) was achieved, with an idler wavelength tunability of 2260 – 3573 nm. Through the addition of an electro-optic modulator (EOM) to the MOPA pump system, acting as a time gate to suppress a variable number of pulses per 1 μs, burst-mode operation of the OPO at a 1-MHz inter-burst repetition rate was realised. By varying the burst window time with the EOM, controllable idler peak powers of up to 1.2 kW were then achieved. The next method proposed to reduce the footprint of SPOPOs was to use a fibre-feedback OPO cavity, where a long length of optical fibre is employed in the cavity, replacing the free space beam path for synchronous pumping and hence acting as an intracavity delay line. Initially a fibre-feedback OPO using a standard solid-core feedback fibre was realised. The OPO operated at 1-MHz repetition rate and generated maximum idler and signal pulse energies (peak powers) of 1.24 µJ (9.7 kW) and 3.10 µJ (17.1 kW) respectively. However, due to the high intracavity signal peak powers, strong intracavity nonlinearities were observed even in a cavity with 90 % signal loss. This significantly hinders the power-scalability of the system. To reduce the cavity nonlinearities, the solid-core feedback fibre was replaced by, for the first time ever, a novel hollow-core fibre (HCF). In this first demonstration of an HCF fibre-feedback OPO, MIR (2948 nm) pulses with a pulse energy (peak power) of up to 1.50 μJ (11.7 kW) were achieved and significant reduction in cavity nonlinearities were observed. The OPO also has signal and idler wavelength ranges of 1472.0 – 1758.2 nm and 2559.1 – 3562.7 nm. The further power-scaling of the system was shown to be limited by fibre nonlinearities within the YDF MOPA pump system due to the high peak powers. Through the reduction of fibre nonlinearities in the YDF MOPA pump system, the HCF fibre-feedback OPO was successfully power-scaled achieving a maximum signal (1600 nm) pulse energy (peak power) of 10.05 µJ (72.3 kW) nm and a maximum idler (2967 nm) pulse energy (peak power) of 5.13 µJ (36.9 kW). This represents the maximum MIR, as well as the maximum total converted pulse energy (15.18 µJ), ever achieved from a fibre laser pumped ps SPOPO. Using two different PPLN crystals of different poling periods, a wider tunable output of 1329 – 1641 nm (signal) and 2841 – 4790 nm (idler) were also achieved with the HCF fibre-feedback OPO. Using the HCF fibre-feedback OPO developed, MIR power delivery in a novel HCF fibre was conducted showing a possible real-world application. MIR power delivery of ranges 2.85 – 3.33 μm and 3.12 – 3.58 μm are achieved with two different pieces of HCF with different transmission windows. A maximum average (peak) power of 592 mW (4.9 kW) and 133mW (1.1 kW) were delivered over 5-m and 108-m lengths of HCF, respectively, at a coupling efficiency of ∼70%. This is the first hundred-meter-scale (108-m) high-power, near-diffraction-limited MIR pulse delivery using HC-ARFs demonstrated.

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Identifiers

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

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

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