Simple optical interferometer for measuring phase changes smaller than 10−5 Å

To measure subangstrom phase changes in the ac plasma display panel, the Jamin interferometer shown in Fig. 1 was used. The system features a double detector differential amplifier arrangement that balances out to the shot noise limit the amplitude variations from the laser. A servo system rotates a glass plate to compensate for low frequency vibrations and thermal variations. The optical flats have beam splitters that cause nearly 100% of the laser power to fall on the detectors, thus increasing SNR. A lens focuses the beam to give diffraction-limited spatial resolution in the region of the plasma cell.