Abstract for 2009 URECA Celebration

Implementing thermal feedback control of a helium-neon laser for frequency stabilization

Measurement is an essential part of the physical sciences and frequency stabilized lasers that provide exact time and length standards are indispensable in metrology. Such lasers are used as laboratory tools and a frequency stabilized laser is employed in the detection of gravitational waves at LIGO, Laser Interferometer Gravitational Wave Observatory. In the present work, an electronic feedback circuit for stabilizing the frequency of a helium-neon (He-Ne) laser was constructed and tested. The goal was to create a frequency stabilized laser system and investigate the properties of the system. A thin-film heater attached to the laser and controlled by the feedback circuit regulates the cavity length of the laser and was the basis for stabilization.

The feedback circuit, built on the basis of proportional, integral, derivative (PID) control, is constructed of inexpensive integrated circuits. The laser is a short.tube laser that supports at most two orthogonally polarized modes resonating inside the tube. A polarizing beam splitter separates these two modes into two separate beams that hit photodiodes, creating a photocurrent. The current is converted into voltage and fed into the PID circuit. Stabilization of laser frequency output was routinely achieved. By analyzing the intensity stability of a single resonant mode, frequency stability was estimated to be better than one part in 10^8, a 300-fold improvement compared to an unstabilized laser.

Properties of the laser and stabilization system were investigated. A study was performed of the response of the laser output to constant oscillation in heater power without the feedback circuit engaged. It was found that despite decreasing the frequency of heater power oscillation from 150 mHz to 5 mHz, the phase shift of the system response remained around pi/2, indicating a growing time interval between heater power change and system response.