The optical readout makes use of a resonant transducer coupled and tuned to the bar, as present days AURIGA. The difference lies in the way the mechanical signal of the bar vibration is converted into an electromagnetic one: here a resonant optical cavity (a Fabry-Perot cavity) is housed between the bar and the transducer. The cavity is made out of two mirrors facing each other: one of the mirrors is fixed to the bar while the other to the transducer. The bar vibration induces a time varying relative displacement between bar and transducer, thus modulating the length of the cavity, i.e. its optical resonant frequency. The conversion of the mechanical signal is thus achieved. The resonant optical cavity housed between the bar and the transducer is named transducer cavity.
Here you will find a picture showing the full experimental scheme of the optical readout.
Here the scheme is shown in more details.
By means of a feedback loop, a Nd:YAG (wavelength=1064nm, near IR) laser source is frequency locked to the transducer cavity: this means that the frequency of the laser beam is actively kept equal to the optical resonant frequency of the transducer cavity. Thus by measuring the laser frequency one extract information about the relative displacement of the bar and the transducer, i.e. about the bar vibration.
The measureament is performed by comparison with a frequency reference, provided by a second resonant optical cavity, named reference cavity: again, this is a Fabry-Perot cavity. The laser beam emitted by the source is split in two: one beam is sent to the transducer cavity and the other to the reference cavity. The optical resonance of the reference cavity is guaranteed by a low frequency feedback loop that corrects the length of the cavity by means of piezoelectric actuators.
The measureament is performed in a frequency range around bar and transducer modes, i.e. around 900Hz. The reference cavity must be a good standard in this frequency range.