Stabilizing the Wavelength of a HeNe laser with a Thermal Feedback Circuit

Matthew Whitrock, Kings Park High School; John Noe and Harold Metcalf, Laser Teaching Center, Stony Brook University.

The operating frequency (wavelength) of many lasers can be stabilized by various techniques to a precision of one part in a million, or one part in a billion or better. Stabilized lasers are important for a wide array of applications, including metrology, interferometry and holography, and surface modeling. A precisely stable frequency provides a benchmark against which other emasurements can be made.

The goal of this project is to stabilize the frequency of a red HeNe laser (nominal wavelength 632.8 nm) through the implementation of a thermal feedback system. The feedback system functions by measuring the relative intensities of the two active laser modes, producing an output which controls cavity length by varying its thermal expansion. As in any feedback system, a deviation off-center in either direction causes an output change designed to drive the system back to its central position. This method of stabilization is analogous to a driver manuevering his/her vehicle to the center of a highway lane.

In many short-cavity lasers, adjacent modes of the laser cavity are polarized orthogonally to one another. The mode spacing is inversely proportional to the cavity length, and in our laser (L=15 cm) this spacing is comparable to the 1 GHz widgh of the gain curve of the lasing medium. As the lser cavity heats up and expands the modes move ("sweep") across the cain curve, causing a periodic variation of the laser frequency. In our setup, a polarizing beamsplitter is used to separate the two active modes and direct each to a polarizer. The electronic circuit compares the output signals of the two detectors and generates an error signal which is sent to a thermofoil heater wrapped around the center of the glass laser tube. The thermofoil heats the laser cavity a few degrees above its normal operating temperature (~60 C); the small changes in heater output controlled by the circuit vary the cavity length in such a way as to equalize the intensity of the two modes.

At the present time the optical setup has been assembled and the op-amp feedback circuit is being debugged. Future research areas of interest include the construction of a Scanning Fabry-Perot Interferometer (SFPI) for resolving the individual modes of the laser and experimentation with other types of laser stabilization.

This work was supported by the Simons Foundation.