Creating a Cavity-dumped Helium Neon Laser

Kevin Zheng, Martin G. Cohen, and John Noé
Laser Teaching Center, Stony Brook University

Lasers, as popularly viewed, operate in the continuous wave (cw) regime. In this regime, the laser emits a continuous beam of coherent light. Lasers can also operate in the pulsed regime, in which emission is a repetitive pulse train with high peak powers. Individual pulse durations can be on the order of nanoseconds to femtoseconds, and repetition rates can range from kilohertz or less to gigahertz or more. Pulsed lasers have many practical uses in research and industry. In research, the high peak powers attainable with pulsed lasers have enabled expanded study of nonlinear optical effects, high harmonics, and time-resolved spectroscopy. In industry, uses of pulsed lasers include laser eye surgery, metal machining, and fiber optic telecommunication. Methods of generating pulsed laser light include cavity dumping, Q-switching, modelocking, and pulsed pumping.

A conventional cw laser contains a highly reflective (HR) mirror and a partially transmissive output coupler (OC) mirror. A cavity-dumped laser contains two HR mirrors to enable high levels of circulating power to build up. When the circulating power peaks, it is extracted from the cavity as a pulse of light within several round trips. After extraction, the cavity rebuilds its circulating power. Extraction is carried out actively with a fast optical switch such as an acousto-optic modulator (AOM) or electro-optic modulator (EOM).

In this project, we develop and characterize a cavity-dumped helium neon (HeNe) laser. Our laser cavity is 1.4 m long. One HR mirror (0.45 m radius of curvature, ROC) is sealed in the back end of a 0.25 m commercial Brewster-window HeNe tube. The other HR mirror has a 1.0 m ROC and is freely mounted. Intracavity power is quite high: the entire beam is dramatically visible in a darkened room. An AOM with AR-coated faces inserted into the cavity at the Bragg angle does not stop lasing action. The 25 MHz RF signal driving the AOM is gated on when the circulating power reaches a maximum. The resulting first order diffracted beam is extracted from the cavity with a mirror. The RF signal is gated off when light extraction is complete. We monitor the circulating power and pulse train with high speed photodetectors. We are able to generate 100 ns pulses (full width at half maximum) with peak power of 2.0 mW at a rate of up to 1 MHz.

This work is supported by the Simons Foundation and the Laser Teaching Center. We also thank Samuel M. Goldwasser for providing some of the key equipment used.