The optical readout for AURIGA
In the effort of improving the sensitivity and widen the bandwidth of the detector, the AURIGA group is carrying out a R&D project to develop a new readout chain, based on laser techniques. The sensitivity and the useful bandwidth (see below) are expected to improve by more than one order of magnitude with respect to present day figures. This would have the double effect of increasing the chances of signal detection and of allowing a better reconstruction of the signal properties (namely, arrival time and frequency components).
The optical readout makes use of a high-Q 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 high-reflectivity 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.
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 measurement 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.
Therefore the laser beam emitted by the source has to be 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 needs also to be guaranteed; it is now not possible to follow the cavity with the laser frequency, as this is already locked to the transducer cavity. Therefore it is now the reference cavity length that has to be corrected: this is done by means of piezoelectric actuators in a feedback-loop that is active only at low frequency (i.e. up to frequencies much lower that the typical detection frequency).
The measurement 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: the reference cavity displacement noise needs to be as low as 5x10-20m/sqrt[Hz] around 900 Hz. For a 20 cm long cavity this is equivalent to a fractional frequency noise of 3x10-19/sqrt[Hz], better than what ever achieved!
Delivery of the laser beam to the transducer cavity is accomplished by a single-mode polarization maintaining optical fiber.
Main figures of the optical readout are listed in the table below; for the explanation of technical expressions please refer to the glossary.
Main noise sources are: the thermal noise, the laser power noise (that acts as back-action noise force in the sense that it determines a noise in the radiation pressure on the transducer cavity mirrors), the seismic/mechanical noise and the electronic noise (essentially the photon counting error at the photodiodes).
The detector sensitivity to be expected with the above parameters is shown in the picture below. The vertical axis is the usual Shh. To better appreciate the detector improvement expected with the optical readout, this curve is to be compared to the present experimental AURIGA sensitivity.