Recommendations for further work.

The series of experiments performed in this thesis form the initial step in developing an experimental gravity wave research effort at the Physics Department of the Australian National University. As part of an effort to build increasingly complex and more realistic bench top prototype interferom eters in support of a full scale A ustralian gravity wave observatory, there are a number of significant experiments that should be performed, building on the work of this thesis.

The power recycling interferometer, studied in chapter 7, was designed and built for increasingly complex optical configurations. The interferometer is based on a triangular invar frame, to which all interferometric optical components are attached. The whole invar structure sits in a custom made, vacuum chamber (yet to be evacuated).

After the initial noise problems concerning the pow er recycling configuration (discussed in chapter 7) are overcome, it will be possible to install Fabry-Perot arms and a signal recycling mirror in the prototype interferometer.

Assuming that a finesse of approximately one hundred can be achieved for the Fabry-Perot arm cavities, the maximum shot noise limited sensitivity can be estimated based on the slope of the reflected cavity optical phase as a function of cavity length. Fritschel12 derives the result d ö /d l = 8FA , where F is the cavity finesse. Using the achieved shot noise limited sensitivity for the power recycling interferometer of ~ 5 x 10"9 radians/VHz, the Fabry -Perot arm, external modulation interferometer sensitivity can be estimated

at - 10-17 m/VHz ( \/( 8 F ) x 5 x 10'9 radians/VHz = 10'17 m/VHz). Signal recycling techniques have the potential to improve upon this sensitivity in particular regions of the signal spectrum26.

In order to achieve this sensitivity at moderate signal frequencies (~ 20kHz or less) several significant improvements will have to be implemented:

• The m echanical/electrical noise present in the current experiment must be reduced by several orders of magnitude. For the experiments reported here signal frequencies of greater than 100kHz were used. In this frequency range, mechanical/electrical noise at the mirrors was not directly observable, however as the signal frequency is pushed dow n to 20kHz and the interferometer sensitivity is im proved, mechanical mirror noise will become evident in the signal spectrum.

• The present power recycling interferometer has ~ 1 milli radian RMS phase noise in the Michelson locking system. This phase error couples laser intensity noise into the demodulated signal spectrum (see Fig. (6.2)). When Fabry-Perot cavities are introduced, the sensitivity should improve by ~ 60 (based on a Finesse of - 100). This w ill necessitate a reduction in the tolerable RMS phase error by approximately the same amount if demodulated laser intensity noise is not to dominate the signal spectrum.

In order to relax this stringent requirement, the laser should be intensity stabilised (at present free running), and so reduce the RIN in the signal bandwidth by ~ 35dB (see Fig. (3.9)).

• C om p lex in terferom eters freq u en tly rely on asym m etric interferometer arms in conjunction with frontal m odulation68 in order to generate sufficient error signals to control the interferometer. If this technique is adopted, independent frequency stabilisation of the laser becomes necessary to avoid frequency noise appearing in both the signal spectrum and the error signals.

Irrespective of whether a deliberate asymmetry is introduced into the interferometer arms, a residual asymmetry error is unavoidable. This becomes particularly significant when Fabry-Perot cavities are utilised in the interferometer arms as it is difficult to match the cavity

storage time to better than a few percent. As there are large frequency noise features on the free running laser (see Fig. (3.17)) in the desired signal range (20kHz to ~ 100kHz), frequency stabilisation will become essential in achieving shot noise limited operation.

With these improvements successfully implemented, it should be possible to trial complex servo system error signal responses by accurate manual positioning and scanning of optical components prior to design of the feedback PID. Currently, free running electrical and mechanical noise precludes this possibility on all but the most insensitive optical components (ie the local oscillator mirror).

Lock acquisition of multi-dimensional servo systems will also become much simpler if accurate manual positioning of critical optical components is possible.

Finally, the improved interferometer should provide a convenient test bed for investigating novel interferometer configurations such as resonant sideband extraction69.

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