Nonlinear Optical Fibres for UV and Infrared Generation

Abstract

The research presented in this thesis is in three parts. First is an investigation into the generation of supercontinuum using tapered optical fibre. Second, is the creation of side-holes in hollow-core fibre, using an oxygen-butane flame, with the aim to be used for gas sensing. Finally, variables affecting the amount of power in a near infrared idler, of a four-wave mixing fibre were explored.

Supercontinuum generation is most commonly generated using photonic crystal fibre. The purpose of this research is not to provide alternative to photonic crystal fibre but to investigate some of the processes that affect supercontinuum generation that utilises soliton fission. Using a master oscillator power amplifier to deliver a 1064 nm pump, it was found that supercontinuum could be generated using tapered optical fibre with a waist length under 250 mm, even though the pulses were as long as 10 ps. The tapers were made using the flame brush technique which allows the creation of tapers with micrometer diameters. These tapers are connected at either end by standard/full size optical fibres. This presents an opportunity to investigate taper waists of many designs whilst keeping the input and output wavelength dependence of coupling independent of the taper structure, unlike in photonic crystal fibres. Investigating the limits of soliton trapping led to non-uniform designs of taper waists. The design with the broadest spectrum began with a 5 µm diameter that was connected directly to a 2.5 µm section. This allowed dispersive waves to be generated, with decent power, in the 5 µm section before the change in group velocity induced by the 2.5 µm diameter, coupled with soliton trapping, dragged the dispersive to low wavelengths. This design produced a supercontinuum, that maintained a power level above -30 dBm, over a range of 420 - 1550 nm.

Using hollow-core fibres for gas sensing is an active area of interest. Side-holes allow gases to enter the core of the fibre in order to be detected. Ultrafast lasers are the most common tool when creating these holes. Here an alternate method of using an oxygen-butane flame on a pressurised hollow-core fibre was investigated. It was found that reducing the inner diameter of the burner, delivering gases to the flame, can only reduce the size of the flame to ≈300 µm. Side-holes were successfully created on a hollow-core fibre of 125 µm diameter. However the minimum loss from a hole created this way was 3 dB which makes this method unsuitable for detection of gases over large areas.

A master oscillator power amplifier laser system was used to deliver a 1064 nm pump to a photonic crystal fibre with an phase matched idler of 1915 nm. The width of the pulses was varied between 37 - 90 ps and had their maximum peak powers kept around 2200 W. The long width of these pulses allowed a narrow bandwidth that produced an idler peak that had a full width at half maximum of ≈30 nm. Three pulse repetition rates of 40, 20 and 10 MHz were used and the maximum average power adjusted in order to keep the same peak power. Over 100 mW of idler power was generated. It was found that the relation of peak power to length of fibre needed to generate power in the idler, is complicated by confinement loss of the idler. This means that to generate the most power the fibre should be short and the peak power of the pulse large.

Date of Award	22 Jul 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Tim Birks (Supervisor) & William Wadsworth (Supervisor)