]). Thus rather than lock the interferometer to the side of a
fringe as discussed above in
4.3, it is usual to make use of a modulation technique to operate
the interferometer close to a null in the interference pattern.
An electro-optic phase modulator placed in front of the
interferometer can be used to phase modulate the input laser
light. If the arms of the interferometer are arranged to have a
slight mismatch in length this results in a detected signal which
when demodulated, is zero with the cavity exactly on a null
fringe and changes sign on different sides of the null providing
a bipolar error signal; this can be fed back to the transducer
controlling the interferometer mirror to hold the interferometer
locked near to a null fringe.
In this situation if the mirrors are of very low optical loss, nearly all of the light supplied to the interferometer is reflected back towards the laser. In other words the laser is not properly impedance matched to the interferometer. The impedance matching can be improved by placing another mirror of correctly chosen transmission - a power recycling mirror - between the laser and the interferometer so that a resonant cavity is formed between this mirror and the rest of the interferometer; in the case of perfect impedance matching no light is reflected back towards the laser [32, 89]. There is then a power build-up inside the interferometer. This can be high enough to create the required kilowatts of laser light at the beamsplitter, starting from an input laser light of only about 10 W.
To be more precise, if the main optical power losses are those
associated with the test mass mirrors - taken to be A per
reflection - the intensity inside the whole system considered as
one large cavity is increased by a factor given by
, where the number of bounces, or light storage time, is
optimised for signals of timescale
and the other symbols have their usual meaning. Then:
|
Gravitational Wave Detection by Interferometry (Ground
and Space)
Sheila Rowan and Jim Hough http://www.livingreviews.org/lrr-2000-3 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |
