Novel Mid-IR Light Sources for Emerging Applications

Abstract

The short-wave mid-infrared, i.e., the wavelength band within 2.5–5 μm has been of keen interest in recent research due to the presence of the molecular fingerprinting region exhibiting strong absorption and resonance of bonds of organic molecules like H2O, CO2, CO and CH4 and a highly efficient water absorption maxima within this spectral range, making this wavelength window exceedingly viable for spectroscopy and material processing applications, that can be extended to medical diagnostics and surgery amongst others. Such high precision applications require the development of novel sources in the mid-IR that can deliver sufficient output power or high pulse energies, along with reasonably broad tunability and good beam quality to cover the region of interest. Additionally, it necessitates the prospect of using ns- and ps-laser sources to treat, and ablate materials with high accuracy, particularly ones containing O-H molecular bonds, usually present as contaminants in glasses, and as natural component of organic materials, and biological tissues. As tunable sources, optical parametric oscillator (OPO) can generate tailored output wavelengths by the process of nonlinear frequency conversion, with reasonable output powers in CW and pulsed mode operations. Selecting suitable nonlinear materials like MgO:PPLN, that can be quasiphasematched for nonlinear conversion, provides adequate transparency to generate mid-IR wavelengths up to 5 μm. For applications that require delivery of power with high precision and good beam quality, fibre amplifiers are excellent choices. For mid-IR, currently, fluoride-based fibres, like ZBLAN, with rare earth doping have been able to deliver sufficiently high pulse energies for several applications with Er3+ dopant capable of offering the highest output powers. From the analysis of energy levels and lifetimes of Erbium doping in ZBLAN, these fibres can also allow sufficiently broad tunability, covering the water absorption window. The PhD work presented here investigates the amplification, gain-bandwidth and wavelength tuning of a 2.1 m long, single stage, double-clad, single mode, 7 mol% Erbium-doped ZBLAN fibre amplifier, pumped by a high-power multimode diode near 980 nm, and seeded by nanosecond pulses of 5.2 ns pulse-width, derived from a broadly tunable MgO-PPLN based OPO, operating at a repetition rate of 10 kHz. At the most efficient wavelength of 2790 ± 1 nm, the highest gain of up to 20 dB with 52.7 μJ pulse energy and up to 8 kWpeak power, using 0.5 μJ seed pulses, were obtained. Over hundred nanometre continuous wavelength tuning between 2712-2818 nm was achieved at a slightly lower seed pulse energy of 0.27 μJ, and a lower pump power, recording a maximum gain of 26 dB at 2790 ± 1 nm, corresponding to 37.5 μJ peak pulse energy. The investigation also sheds light on practical limitations to power scaling and wavelength tuning, arising from self-phase modulation, self-lasing and fibre degradation due to back reflections.