Linear alignment
The specifications and requirements for the linear alignment sensing scheme have been set by the alignment sensing and control group of Advanced Virgo. The mirror angles of the test masses have to be aligned with great precision to prevent degradation of the overall signal to noise ration of the gravitational wave detector.
Quadrant diodes
A high precision interferometric alignment sensing method, called wavefront sensing, is used to measure the motion of the optical propagation axis though out the interferometer. The method involves using RF modulation/demodulation methods to up convert the measurement band away from technical sensing noise and then down-convert the signals to a useful frequency band respectively. Fig. 1 shows the measurement ports used for Advanced Virgo, where ports B2, B5, B1s, and B1p contain RF (tuned circuit) quadrant photodiodes, measuring the beat frequencies originating from the beat between the laser carrier frequency and some modulated sideband frequency.
For Advanced Virgo the most crucial alignment signals, those for the arm-cavities, are derived by monitoring the DC transmitted intensity, which are located directly behind the end test masses. This is illustrated in Fig. 1 where ports B7 and B8 are used to measure the arm cavity near-field and far-field motions. The choice of using both RF and DC signals demands the development of separate, optimized, quadrant photodiodes.
Demodulation boards
This project is a continuation from the development of the Virgo+ demodulator boards, which were part of the linear alignment system. The board receive a RF input signal from a front end module, which is equipped with a quadrant photo diode. The quadrant photo diode is exposed to laser light that is modulated at specific signal frequencies, providing the alignment information of the optical components. The control system subsequently uses the demodulated signals in closed-loop feedback to keep the laser beam aligned with the cavity optical axis.
The demodulator board fits into a two connector VME crate. Only the -12 Volts and +12 Volts are used. On the front there are horizontal and vertical RF inputs and local oscillator reference inputs. The board has an I&Q detector (quadrature detector) for the horizontal signal and also for the vertical signal. Each I&Q detector has an I-output and a Q-output. The conversion gain of the board can be set at four different levels in 12dB steps. This is done by applying the appropriate voltage on the gain-input. To see in what gain setting the board is, one can read the voltage of the gain-output.
