Section 5.1

Michelson Interferometer

The Michelson interferometer is a simple two-beam amplitude splitting interferometer that is useful for measuring the change in optical path of one beam relative to another.

A basic Michelson interferometer requires only a beamsplitter and a pair of mirrors. The mirrors each retroreflect one output of the beamsplitter. If the physical path lengths of the arms are not similar, it may be necessary to collimate the beam entering the interferometer or to use lenses in the arms to mode match the output beams to each other so that the interfering wavefronts are well matched producing coarse and easily observable fringes with maximum sensitivity to differential effects.

Section 5.2

Twyman-Green Interferometer

The Twyman-Green interferometer is a variation of the Michelson interferometer that is used to measure the aberrations induced by an optical element placed in one arm of the interferometer. The spatial distribution of the interference pattern reveals information about the aberrations, so it is necessary to have well matched and uniform wavefronts of large spatial extent.

A spatial filter before the interferometer provides uniform wavefronts, but care must be taken to keep the arm lengths similar and/or to mode match the retroreflected beams to the input beam so that the wavefronts from the arms match in the absence of aberration. When testing flat optics, this is most easily achieved by using a large, collimated beam. For curved optics, a lens needs to focus the light in the arm so that the wavefront curvature at the optic matches the nominal curvature of the optic under test.

Section 5.3

White-Light Michelson Interferometer

An issue in analyzing the interference patterns from a Michelson interferometer illuminated with monochromatic light, is that the fringe pattern is repeated with every optical path length difference of λ. Thus it is only possible to extract the phase difference of the beams modulo 2π.

Illuminating the interferometer with broadband illumination, however, causes the interference pattern to consist of a superposition of the (wavelength dependent) interference patterns from all of the spectral components of the illumination. With white light, it becomes possible to fully determine the phase difference of the beams from the interference pattern.

The challenge in setting up a Michelson interferometer for broad band illumination is that the short coherence length of the illumination means the arm lengths need to be very closely matched - typically to within a few microns. This is beyond the precision that can be achieved my using a ruler to position the mirrors on the table. Rather, it is necessary to scan the length of one arm while looking at the output of the interferometer for a visible interference pattern. The interference pattern, however, won’t be visible unless the mirrors are well aligned first. Typically this requires using a laser for initial alignment of the mirrors.

Section 5.4

Mach-Zehnder Interferometer

This is another two-beam interferometer, like the Michelson interferometer. It uses a separate beam splitter and beam combiner. One of the main advantages over the Michelson is the ease in which both output ports can be observed. An additional advantage is that it is easier to avoid feedback into the laser, since the beams are not retroreflected like they are in a Michelson interferometer. Alignment of the Mach-Zehnder requires one beam be directed though the final beam splitter, and then the second be steered so that it overlaps it at the plane of the beamsplitter where the beams recombine. The orientation of the beamsplitter can then be adjusted so that the output beams become parallel. Iterate this procedure if necessary until there are visible interference fringes.

Section 5.5

Fabry-Perot Cavities

A Fabry-Perot cavity can have very high resolving power, and is often used to investigate the frequency spectrum of a narrow frequency band, such as the modulation sidebands on a laser beam. Using a piezo mounted mirror, and having measured the Gaussian beam parameters from our laser, and calculated the necessary mode matching lens, we can set up a Fabry-Perot cavity. The transmitted power from the cavity will only be significant when a frequency component of the light is resonant in the cavity, thus by scanning the cavity length the frequency spectrum of the light can be mapped out over one free spectral range. Use the Fabry-Perot cavity simulator to get a feeling for how the cavity parameters affect the measurement.