SELECTING A SINGLE LONGITUDINAL MODE

A laser that operates with a single longitudinal mode is called a single-frequency laser. There are two ways to force a conventional two-mirror laser to operate with a single longitudinal mode. The first is to design the laser with a short enough cavity that only a single mode can be sustained. For example, in the helium neon laser described above, a 10-cm cavity would allow only one mode to oscillate. This is not a practical approach for most gas lasers because, with the cavity short enough to suppress additional modes, there may be insufficient energy in the lasing medium to sustain any las- ing action at all, and if there is lasing, the output will be very low.
The second method is to introduce a frequency-control ele- ment, typically a low-finesse Fabry-Perot etalon, into the laser cav- ity. The free spectral range of the etalon should be several times the width of the gain curve, and the reflectivity of the surfaces should be sufficient to provide 10 percent or greater loss at fre- quencies half a longitudinal mode spacing away from the etalon peak. The etalon is mounted at a slight angle to the optical axis of the laser to prevent parasitic oscillations between the etalon surfaces and the laser cavity.
Once the mode is selected, the challenge is to optimize and main- tain its output power. Since the laser mode moves if the cavity length changes slightly, and the etalon pass band shifts if the etalon spac- ing varies slightly, it important that both be stabilized. Various mechanisms are used. Etalons can be passively stabilized by using zero-expansion spacers and thermally stabilized designs, or they can be thermally stabilized by placing the etalon in a precisely con- trolled oven. Likewise, the overall laser cavity can be passively sta- bilized, or, alternatively, the laser cavity can be actively stabilized by providing a servomechanism to control cavity length, as dis- cussed in Frequency Stabilization.