A heterodyne interferometer operating in white light

Abstract

Heterodyne interferometry is a well known technique commonly used to measure phase difference in two beam interferometers operating with narrow-band light [l]. Unfortunately, it is much more difficult to use heterodyne methods with white light because of the need to shift the frequency of each wavelength component of the beam in one arm by the same amount. However, interferometers operating in white light are useful because of their ability to distinguish zero path difference. We present a heterodyne interferometer operating in white light where the desired frequency shift is produced using polarisation effects after the interferometer. The configuration used allows a reference beam to be created outside the interferometer which is independent of the interferometer phase and allows displacement measurements to be made.The white light phase that is measured is the phase of the temporal-coherence function of the light source modified by the system spectral response and dispersion in the interferometer. White light phase is not necessarily linear with path difference [2] and we use the interferometer to measure the variation of the white light phase with path difference where the path difference is found using the same system operating with laser light. The interferometer could easily be modified to make surface profile measurements and when combined with measurements of the fringe envelope can be used to measure the complex temporal-coherence function.As shown in Figure 1 white light from a dispersive, polarising Michelson-type interferometer is passed through a U4Fresnel rhomb then through a rotating polariser and is detected using a photodiode. Light from a laser source propagates the same path as the white light beam and is measured at detector at separate detector. Light from a second polarised laser passes directly through the rotating polariser and is used as a reference signal with a phase independent of the interferometer path difference. The intensity of all three beams vary sinusoidally at twice the angular speed of the rotating polariser and their phase relative to the reference depends on the interferometer path difference. Note that the phase of the laser beam is used as a measure of path difference as there is a linear relationship between them. laser white reference ro,a(ing light . light laser beam polariser 1s.w nhnce (rad) Figure 1 Diagram of system for measuring white light phase variation.h h is approximately equal to the mean wavelength. This is limited by the electronic noise in the phase meter used and could potentially he improved. Independent phase measurements could be made at a rate of about 200s , The laser phase and white light phase was measured over a range of path differences (about 7.5pm) and the deviation of the white light phase from a straight line can be seen in Figure 2.