Sometimes we need to measure things moving to a very high accuracy. The LISA Pathfinder mission had to measure the motion of two cubes of gold-platinum alloy with nanometre precision – a length equivalent to a 100,000th of a human hair. Further, these measurements had to be made at low frequency – over thousands of seconds.
Making measurements this precisely is particularly difficult on these timescales as everything has a tendency to ‘breath’, and slowly drift. At the University of Glasgow we have successfully designed, built and demonstrated a sensor that uses a an optical fibre to make these kinds of measurements – in fact it is capable of making
measurements a hundred times more precisely at higher frequencies.
The sensor uses laser light reflected from the end of the fibre combined with light reflected from the surface being monitored. Laser light can be used as a very fine ruler as it has a very consistent wavelength – the oscillations it makes as it propagates. By, effectively, counting as additional wavelengths of the light reflected from the surface pass by the stationary light reflected from the fibre end, we can monitor the motion of the surface. The wavelength of laser light can be very small – a millionth of a metre – and we can count small fractions of a wavelength, enabling us to make very fine measurements with this fibre sensor.
The sensor that we have developed has other advantages, too. The sensor head is very small, around a centimetre by a millimetre, and doesn’t have any active parts. This means it can be easily retrofitted to systems and doesn’t disturb the system it is trying to measure by heating it up. The sensor has performed excellently in tests and we are now looking for new ways to implement it.