Magnitude and Spatial Distribution of Gain in a He-Ne Discharge.
Supervisor: Dr Bob Hurst
The large ring lasers operated in the Cashmere Cavern, when compared with 'normal'
He-Ne lasers, have much larger diameter beams, simply because of diffraction and the
longer optical paths. This necessitates a larger diameter tube for the electrical
discharge that provides the laser gain medium. It is known that Ne atomic collisions
with the walls of the tube play an important role in depopulating the lower laser level,
and so the diameter affects the available laser gain and the spatial distribution of this
gain. In addition, we operate our lasers at total gas pressures several times higher than
is typical for He-Ne lasers. This affects the mean free path in the gas, further affecting
the gain distribution. We have a pressing need for more understanding of the interplay
of pressure, gas composition (He:Ne ratio) and tube diameter.
In this project, we want a student to measure directly the gain distributions in
discharges confined within tubes of a range of diameters, at a range of gas pressures,
and for different compositions. A suitable starting point is some equipment previously
used in an experimental linear laser. The old mirrors can be removed, leaving just the
discharge tube which is connectable to a vacuum and gas-filling plant. The beam from
a small He-Ne laser can be used as a probe, directly measuring its amplification in a
single pass through the discharge. The project will involve some interesting and
challenging optical measurements.
There is much scope for physical modelling of the situation, building upon earlier
theoretical work done in this department [1].
Experience to be gained:
Design, layout, execution of optical experiments
Data acquisition, Labview programming
Physical modelling
Microseismic Background
Supervisor: Dr Bob Hurst
When we look in fine detail at the rotation signals from the ring lasers in the
Cashmere cavern, we see a continuous disturbance, quasi-periodic, with period of ~5
s. It is also present on the output of our seismometers. This phenomenon is known as
the microseismic background, and it is a significant source of noise in some of our
measurements. It is widely believed to be generated by ocean waves. (If so, an
interesting problem arises immediately because the spectrum of the microseismic
signal peaks at ~0.2 Hz whereas the ocean wave spectrum peaks at ~0.1 Hz or even
lower.)
This project calls for somebody to investigate the basic phenomenology of the
microseismic background, to try and relate it to whatever drives it. In particular, is it
possible from 3-axis seismometry and 2-axis rotation measurements, to determine
what direction it arrives from, and is it possible to predict the rotation signal from the
seismic signals? If this were so, we might be able to in effect subtract the
microseismic rotation from the ring laser signals, getting cleaner estimates of other
rotation signals.
Experience to be gained:
Labview programming and data acquisition
General statistical signal processing
Interesting theoretical work on seismic propagation