Longitudinal Modes in a Laser Resonant Cavity

A laser can only lase at longitudinal modes which are an integer multiple of half wavelength and that fit within the laser resonant cavity geometry. There are an infinite number of integer multiples of cavity length for any fix length, providing infinite number of possible lasing modes. However, a particular laser gain medium does not have gain at all frequencies, but instead there is a narrow gain profile that describes the gain as a function of longitudinal modes. The resulting laser spectrum corresponds to the set of possible modes that fit within the laser cavity geometry and that fall inside the gain profile of the laser gain medium. Therefore, the light output from a laser cavity is not perfectly monochromatic, but instead there are several modes represented. However, the spacing between the longitudinal modes is so small that for most purposes, laser light is referred as monochromatic light. Nevertheless, it is possible to measure the longitudinal modes from a laser source. Here are some pictures of the equipment that was used in one of my labs to measure these modes from a Helium-Neon Laser (pink color light at approx. 632.8 nm).


Complete Experimental Set-Up on an Optical Table


Piezoelectric Scanning Interferometer


HeNe Laser, Interferometer, Photo-detector, and Some Mirrors


Oscilloscope Reading

As seen in the Oscilloscope picture above, at least three modes were detected, which is more less the number of modes expected for this particular laser which has a typical bandwidth of 1.5 GHz.

It is important to mention that small changes in the laser resonant cavity length, even those caused by thermal expansion or acoustic effects, can result in significant changes in the longitudinal mode characteristics of a laser. Frequency-stabilized lasers actively control the laser cavity length to keep the longitudinal modes at constant frequency.

Here are some more pictures from different angles:



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