Stabilizing a Helium-Neon Laser.     Victor Kim, Harold Metcalf and John Noé, Laser Teaching Center, University at Stony Brook   11794-3800.

Lasers have become an increasingly important component in undergraduate physics and engineering labs. Most of these labs are provided with He-Ne lasers. One advantage of the He-Ne laser is that it is very inexpensive. But this inexpensive laser often exhibits very large intensity fluctuations, which result from mode switching and persist for many hours after the laser is turned on. This becomes a very big problem when an experiment depends on laser intensity. Although rapid sampling or modulation techniques can be used to overcome these fluctuations, they are a serious hindrance to many experiments that depend on measurement of laser intensity. I encountered this problem myself when I was trying to look for the wave profile of the He-Ne laser.

It is possible to stabilize the He-Ne laser. This can be done by a optical-thermal feedback technique. Based on a study done in the Laser Teaching Center with the He-Ne by a former REU research program student, Stephanie Lim, the tube temperature, the cavity length and the output polarization are correlated to mode sweeping. As the laser is kept running, the tube length changes with time, due to thermal expansion, causing the modes to move position on the gain profile. As the mode moves out of the grain profile, another mode moves in on the other side of the profile, producing large intensity fluctuations. By providing an optical-thermal feedback, the temperature and hence the cavity length of the laser can be regulated and stabilized, stabilizing the modes, and the intensity.

In order to construct a device like this, a full understanding of electronics, laser physics, and solid state is required. My first step will be to study the behavior of the He-Ne laser in detail, specially the behavior of frequency modes with respect to the temperature of the laser. Once I study these relationships I will have a fairly complete background to set up my device.

I will construct a device that has the following characteristics. First the beam originated from the He-Ne laser will go through a beam splitter separating the beam in two with different intensities. One of those beams, the one with the lower intensity will go to another beam splitter, but this time it will separate the beam with two different polarizations. Two detectors will record each polarized intensity in volts. Once I turn the feedback on, a power supply will rise and regulate the temperature of the laser until the difference of voltage for the two detectors is equal to zero. As mentioned before, the temperature of the laser changes with respect to time, which results in the change of intensity. When the detector detects little change in the difference of intensity, it will send a signal to a power supply that will heat up or cool down the laser, according to its needs, not allowing the mode switching and the change in intensity.

Victor Kim June-2000