Temporal Coherence of a Short Laser

Jan Steinbrener, Stony Brook University

The objective is to illustrate partial coherence of a HeNe laser by means of interferometry. A HeNe laser with a small cavity has a mode-spacing of approximately 1 GHz, which is also the order of the linewidth of the gain curve. Thus, it will operate only at few modes. More precisely, the length of the cavity, and hence the mode-spacing, can be chosen in such a way, that at most two modes can lase at the same time. However, the cavity length is not stable during time, since the cavity (usually glass) expands due to thermal expansion. As a consequence, the longitudinal modes "sweep" through the gain curve, which is fixed at the central frequency, causing alternating single-mode and dual-mode operation of the laser. To visualize this effect, I plan to make use of the changing coherence length, which is much longer for single-mode operation (~300 cm) than for dual-mode operation (~30 cm), by setting up an interferometer with a path length difference that can be varied from 0 - 30 cm or so. At certain distances one should then be able to see the fringes of the interference pattern periodically fade in and fade out as the laser changes from dual-mode to single-mode operation and back, respectively.

This research is part of a requirement for the Prof. Weinacht's Lasers course, PHY562.

Danielle Bourguet
Tennisailax (at) AOL.com
November 2004
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