James Roy Taylor

We review our development of wavelength tuneable, high average power, picosecond-pulse, mid-infrared sources in the three micron widow for proposed applica

Shi W, et al. Brain Tumor Res Treat. 2022 Mar;10(Suppl):S180. https://doi.org/10.14791/btrt.2022.10.F-1389

A new method of seeding X 2 optical parametric converters with X 3 fiber optical parametric sources is introduced. We demonstrate a tunable mid-infrared source operating at around 3 µm with the technique and discuss the potential of this architecture.

We report a CdSiP₂ (CSP) based seeded optical parametric generator (OPG), emitting sub-nanosecond duration, 3 MHz repetition rate, wavelength tunable mid-infrared (MIR) light at 4.2-4.6 μm. We generate up to 0.25 W at 4.2 μm with a total pump conversion efficiency of 42%. The OPG is pumped by a 1.24 μm Raman fiber amplifier system. This is the first demonstration of pumping CSP with a Raman fiber source in this region, and we show that Raman fiber sources in the near-infrared (NIR) are ideal pump sources for non-critically phasematched (NCPM) CSP devices. Pumping CSP at 1.24 μm permits the use of NCPM whilst decreasing the negative effects of both two-photon absorption and linear absorption losses, when compared to conventional 1 μm pumping. This offers a potential advantage for MIR power scaling of CSP parametric devices due to a reduced thermal load in the crystal from residual pump absorption. The OPG is seeded with a continuous-wave fiber supercontinuum source emitting radiation in the 1.7 μm region, to lower the threshold pump intensity required for efficient conversion. NCPM and temperature tuning of the crystal allow for simple wavelength tuning of the idler radiation. We report on laser damage induced by elevated crystal temperatures, which we propose is linked to the decrease in CSP bandgap energy with increasing temperature.

Extensive spectral and temporal versatility are achieved through the integration of nonlinear fibres and crystals with seeded master-oscillator power fibre amplifier configurations through diverse generation processes. Various schemes will be reviewed.

For nearly fifty years the supercontinuum source, a result of the understanding and control of the underlying physical processes, has evolved as a scientific and commercial success, providing spectral versatility well beyond the limitations of the transmission window of silica and pumped by diverse temporal formats. The relevant physical processes, characteristics and current status are reviewed with a look to future development and application.

Fiber-based lasers and master oscillator power fiber amplifier configurations are described. These allow spectral versatility coupled with pulse width and pulse repetition rate selection in compact and efficient packages. This is enhanced through the use of nonlinear optical conversion in fibers and fiber-coupled nonlinear crystals, which can be integrated to provide all-fiber pump sources for diverse application. The advantages and disadvantages of sources based upon supercontinuum generation, stimulated Raman conversion, four-wave mixing, parametric generation and difference frequency generation, allowing spectral coverage from the UV to the mid-infrared, are considered.

A fully nonlinear numerical model is used to investigate spontaneous wave generation during two-dimensional frontogenesis forced by a horizontal strain field. The model uses the idealised configuration of an infinitely long straight front and uniform potential vorticity, with a uniform imposed convergent strain across the front. Shakespeare & Taylor (J. Fluid Mech., vol. 757, 2014, pp. 817–853) formulated a generalised analytical model (ST14) for this system that extends the classical Hoskins & Bretherton (J. Atmos. Sci., vol. 29, 1972, pp. 11–37) model (HB) to large strain rates ( ). Here, we use a numerical model to simulate the fully nonlinear problem and compare the results with the predictions of the analytical model for a variety of strain rates. Even for weak strains ( ), the confinement of the secondary circulation and the spontaneous generation of waves, predicted by ST14, are shown to be important corrections to the HB solution. These inviscid predictions are also robust for an equilibrated front where strain-forced frontogenesis is balanced by diffusion. For strong strains the wavefield becomes of leading-order importance to the solution. In this case the frontal circulation is tightly confined, and the vertical velocity is an order of magnitude larger than in the HB model. The addition of a strain field that weakens with time allows the release and propagation of the spontaneously generated waves. We also consider fronts with both large vorticity and strain rate, beyond the validity of the ST14 model.