Using a Very Thin Glass Slide to Observe Transverse Modes:

Again in this experiment, the laser beam was projected on to a surface so as to maximize the cross sectional area of the beam. A cardboard box with sheets of white paper taped to it was used as a "projection screen" for the beam. Transverse modes were produced by using a very thin glass slide. The thin glass slide acts as an etalon with a very short cavity length. As was discussed before, the etalon serves as a frequency selector. Only light of certain frequencies will be transmitted through the slide depending on the angle it makes with the laser beam. Because only certain frequencies can pass through the etalon, only certain modes can be produced at any one time. Because the cavity length is so short for the etalon made by using the thin glass slide, instead of sharp peaks at discrete frequencies, we see a broad peak (See figures below).

The modes were "photographed" and the pictures digitized using a video camera and some computer software. These images resemble the spacial mode distribution diagrams discussed in the introduction.

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