Student buiding a Michelson interferometer.
A Michelson interferometer. The beam splitter splits the He-Ne laser beam into two beams that travel different optical paths. After reflecting from the two mirrors, the beams rejoin at the beam splitter. The resulting interference pattern, a series of bright and dark concentric rings, can be seen on the white screen. The lens spreads out the beams and the pattern for easier viewing.
Close-up of the interference pattern formed using the He-Ne laser.
The He-Ne laser has been replaced with a sodium lamp.
Close-up of the sodium pattern. There are two closely spaced yellow wavelengths in the visible emission spectrum of sodium, one at 589.0 nm and the other at 589.6 nm. A clear interference pattern is seen only when the optical path difference between the two beams in the interferometer is such that the pattern formed by the one wavelength coincides with that of the other wavelength. By moving mirror 1 and changing the optical path, students were able to make the total pattern change from a very clear pattern to no pattern at all and back to a clear pattern. By measuring the distance that was required to do this, the students were able to calculate the difference between the two emission wavelengths. Their results confirmed a difference of 0.6 nm!



The photos of these holograms do not look nearly as good as the actual holograms. It is a bit of a challenge to get the correct exposure and distance on a digital camera to get a clear picture. The students used a red Helium-Neon laser with Kodak High Speed Holographic Film. Kodak D-19 developer and rapid fixer were used to process the film. The holograms can be viewed with any nearly monochromatic light source. We used a red He-Ne laser and a green He-Ne laser for viewing.

Here is a top view of the set-up used to make a transmission hologram. The laser (not shown) sends light along the path of the arrow and hits the beamsplitter (BS). Some light reflects from the beamsplitter, is expanded by lens L1, and is reflected by mirror M1 towards the film. The remaining light that was transmitted through the beamsplitter reflects from mirror M2, is expanded by lens L2, and is reflected by the object towards the film. The interference pattern formed between these two beams is recorded on the film. The film is then developed. When nearly monochromatic light is sent through the developed film, the hologram is seen by looking through the film in the direction of the light source.

Here is one object that was used, a ceramic swan mounted in a lens holder.

Here are the holograms of the swan viewed in red light.

A lens acting as a magnifying glass was placed in front of the swan and more holograms were made. Here are those holograms viewed in green light. It is hard to see in these photos, but the lens is indeed magnifying.

Here is another object, a watch mounted in a lens holder with the magnifying lens in front.

Here are holograms of the watch viewed in green light.



We are often told that laser light is monochromatic. This isn't strictly true. Consider this photo which shows the output of a spectrum analyzer on an oscilloscope screen. Red light from a Helium-Neon laser (wavelength of 6328 Angstroms) was sent into the analyzer. One can see three peaks in the laser output spectrum. These are longitudinal modes of the laser. Each major divsion on the screen represents 0.2 GHz of frequency. The frequency separation of the modes is approximately 0.46 GHz. The corresponding wavelength separation is 0.006 Angstroms!
(Photo by Jeff Crisp.)
Two spectra, recorded a few minutes apart, are shown. Each major divsion on the screen represents 1 GHz of frequency. Four longitudinal modes can be seen in each spectrum. Notice that the laser output is not strictly stable. The intensity of the modes changes. The laser is exhibiting longitudinal "mode hopping".
(Photo by Jeff Crisp.)



One way that the properties of an image can be modified is through spatial filtering, one example of optical processing. The imaging system is built to take advantage of the Fourier transform properties of diffraction and lens imaging. In the focal plane of the main imaging lens, filtering of the light is done. The distribution of the light in this plane is related to the Fourier transform of the light distribution in the object plane. Low 'spatial frequencies' in the focal plane occur near the center of the plane and are associated with smoother transitions in the light distribution in the object plane. High 'spatial frequencies' in the focal plane occur out from the center of the plane and are associated with abrupt transitions in the light distribution in the object plane.
Consider the three photos below. The first photo shows the unfiltered image of the object. In the second photo, a high-pass filter was used. This consisted of an opaque disk in the focal plane which blocked the lower spatial frequencies. Note how the edges of the image are enhanced since only the higher spatial frequencies are being passed. The third photo shows the image when a low-pass filter is used. This filter was a small circular aperture that blocked the higher frequencies. Note how the edges are now blurred. The information for sharp edges is blocked. (Photos by John Varriano)
Here is an example of the usefulness of a spatial filter. The image on the left shows an unaltered thumbprint image. On the right, the image using a high-pass filter. The edges of the lines are enhanced allowing for easier reading of the print.



Pattern formed by a Fresnel zone plate using the 543 nm light from a He-Ne laser. The bright spot in the pattern center is the first focal point of the plate. This pattern was produced by Orion Millaway and Steven Schmidt in their lab project involving zone plates.
(Photo by Steven Schmidt.)


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