The drving waveform consists of a large amplitude pump signal (fp) with a smaller amplitude acoustic (fa) signal superimposed on top of it. In this case the pump signal is 30 kHz and the acoustic signal (with 1/10th the amplitude) is at 1 MHz. The resulting membrane displacement, as a function of time, is show here.
The response of a FPI to this signal for a low finesse system: water-fibre interface at ordinary reflectivity (6%) and a gold coated membrane (95%); is shown in blue. The signal is observed to clip at the far extemeties of the pump displacement. The green line shows the signal after being high-pass filtered to remove the large oscillation associated with the pump signal. The desired signal is that of the largest amplitude as this corresponds to the linear region of the FPI device.
The next curve shows the same scenario when the pump signal is reduced to 100 Hz. The fine structure of the 1 MHz can not be seen but is still present. Again the filtered response has a maximum when the system is in the linear region of the FPI transfer function.
This curve shows the response for a higher finesse system. This finesse corresponds to a fibre-water interface that has been altered to yield a reflection coefficient of 50%. The swing of the optical output is now close to the full scale. The amplitude of the received signal is about 6 times that of the lower finesse system.
Click on the following image to see a movie of the response of the system when the membrane follows a gross movement over half an optical wavelength. The peaks of interest moves are in the system. In this case the pump signal has an amplitude of 0.6 optical wavelengths and the acosutic signal is 5% of the pump signal.
Here is a case where the motion of the pump signal has an amplitude of just 40% of an optical wavelength, that is not quite large enough to swing the membrane through a complete fringe pattern. The acoustic signal is 10% of the pump signal. Note that for the region around frames 20 to 30 there is not a good response as the FPI is not properly pushed through its fringe pattern.
Last Updated August 2000
This page is maintained by robinc@bu.edu