1. Field of the Invention
The invention relates generally to a single spectral line laser and more particularly to a system for forcing a multi-spectral line laser to operate on a single spectral line.
2. Description of the Prior Art
The prior art has attempted to force a laser to operate on a single spectral line by making the loss of the cavity extremely sensitive to the optical frequency by use of a diffraction grating, prism, absorption cell, filter, selective coating, or birefringent material. Such prior art attempts all add additional loss to the undesired frequencies and are difficult to make selective enough to discriminate between closely-spaced spectral lines. Most of the systems of the prior art are easily damaged, are relatively expensive, are alignment sensitive, are significantly to the cavity length, and have temperature dependent passbands. Also, most such systems cannot readily discriminate between closely-spaced spectral lines.
As a specific prior art example, for an infrared carbon dioxide laser, a infraction grating has been the only practical method of selecting spectral lines, but it adds a four percent loss to the cavity for the selected line; does not totally reject adjacent lines in many of its implementations; is easily damaged; is expensive; and will change spectral lines with alignment and with only a few degrees change in temperature, as the thermal expansion of the diffraction grating substrate changes the spacing of the diffraction grooves.