Adrian Love

Double pass gain in Helium-Xenon discharges in hollow optical fibres at 3.5 μm

Gain is observed in a double pass of a Helium-Xenon gas DC discharge in a 90cm long flexible hollow core fibre. Output at 3.5μm increases with discharge current up to the maximum of 0.55mA.

Experiment Assembly and CW Measurements of the He-Xe Laser

This chapter marks the start of the experimental part of this thesis, beginning with a description of the apparatus and set-up used throughout the project before reporting on a number of important measurements made in the CW regime.

Conclusions and Future Prospects

The research presented within this thesis is experimental evidence of the first functional hollow core fibre based electrically pumped gas laser. Taking the form of an afterglow xenon laser, this experiment is primarily a proof of concept that opens the door to a new generation of compact, flexible gas discharge laser systems.

Introduction to Laser Physics

The laser (light amplification by stimulated emission of radiation), first demonstrated by Theodore Maiman in 1960 [1], has found considerable uses ranging from manufacturing, spectroscopy, telecommunications, metrology and even recreational, all thanks to its nature of producing highly monochromatic, spatially coherent light.

Hollow Core Optical Fibre Based Gas Discharge Laser Systems

The humble electrically pumped gas laser has undergone little development in its fifty year life span due to the lack of an effective method to confine light within a hollow waveguide of any appreciable length in which an electrical discharge could be contained. New technologies in the field of anti-resonant guiding hollow core fibres present an opportunity to re-invent the gas laser. A recent breakthrough in the field demonstrated that DC pumped glow discharges of a helium and xenon gas mixture could not only be sustained in such a fibre, but also exhibited signs of gain on a number of mid-IR neutral xenon laser lines [1]. The research presented in this thesis is a continuation of that project. The system was redesigned to incorporate two mirrors so that a cavity could be constructed. The previously hinted at gain on the 3.51 μm xenon line was confirmed through a series of CW measurements of the cavity, as was a polarisation of the laser due to a polarisation dependent output coupler. Further observation of the discharges revealed that they were of a pulsed nature, and that the mid-IR laser light was present in the discharge afterglow. A response to the cavity mirrors was observed in this afterglow pulse on the 3.11 and 3.36μm xenon lines in addition to the 3.51 μm line previously seen. Through fast detection a modulation of the output power due to cavity mode beating effects was detected. The high gain and narrow bandwidth of the xenon laser lines resulted in a frequency pulling effect, and the mode separation in the ‘hot’ laser cavity was measured to be lower than in the ‘cold’ cavity. It was observed through pressure optimisation experiments in helium-xenon that higher output powers could be achieved by using lower partial pressures of xenon. This was exploited with neon-xenon mixtures, where the lower ionisation potential of neon allowed a lower pressure of xenon. Discharges were also achieved in helium-neon and argon gas mixtures.

Introduction to Optical Fibres

The concept of using a glass fibre to transmit light from one place to another dates back to as early as the start of the twentieth century [1]. As technologies improved throughout the century more applications for optical fibres were developed, with the uses of modern fibres stretching across a wide range of fields including lasing, sensing, imaging, computing and telecommunications.

Introduction to Discharge Physics

The basis of an electrically pumped gas laser is the gas discharge. This chapter covers the basics of direct current (DC) discharges including how breakdown is achieved, the structure of a discharge, the key parameters involved, the particle dynamics within the discharge and methods of operating a pulsed discharge.

Electrically Pumped Noble Gas Lasers

Laser systems that use noble gases as a laser medium have attracted lots of attention due to the abundance of high gain emission lines Willett (Prog Quantum Electron 1:273, 2007 [1]) and the ease of performing stable electrical discharges.

Experiments with New Gas Mixtures

Having demonstrated a fully functional afterglow helium-xenon laser, attention was turned to the use of helium-neon. These helium-neon experiments were unsuccessful, however they did demonstrate that using neon instead of helium had the potential to improve the xenon laser.

Pulsed Measurements of the He-Xe Laser

Following some fortunate measurements made by the author it was discovered that rather than a continuous discharge, the system was in fact operating in a semi-stable pulsing regime with a strong mid-IR afterglow light pulse. This led to measurements with a more favourable signal-to-noise ratio, and so a simpler alignment.