A significant drop in the FTS Zeeman split servo laser had been noted over the past few months. Careful attention to cleaning of the optical surfaces inside the FTS vacuum tank and realignment failed to recover the signal. In fact, the signal continued to decline to the point that the instrument was becoming noticeably less stable. Tuning the electronics, likewise, didn't regain the lost signal. The FTS was beginning to suffer from poorly registered coadded scans. The data were beginning to suffer.
The servo laser is housed in a "pressure bubble" inside the FTS vacuum tank, which is kept at a pressure of one atmosphere for cooling by an air compressor. The laser bubble was inspected and found to be badly contaminated by an oily film. The compressor, it was found, had a plugged drain valve and a large amount of water and oil condensed in its tank. It was this oil that had found its way to the FTS servo laser. The plug was removed and the oily water drained out of the system.
The laser head, an HP 5500C, was opened and the intracavity optics were cleaned. This cleaning recovered about a factor of two in signal; however, it was still somewhere around a factor of two below what the FTS records show it should have been. The next step was to remove and clean the non-exposed optics. This included the laser's beam-expander/collimator and a set of 1/4 and 1/2 waveplates. The waveplates create the orthogonal linear polarization that the FTS is aligned to and so is only disturbed with good cause. The beam-expander and waveplates were removed and cleaned along with the exit window of the laser tube by "drag-wiping" with a lens tissue and alcohol.
The cleaned optics were reinstalled and aligned and the polarization axes were tuned. Surprisingly, almost a factor of three increase in servo signal was found! The combination of careful cleaning, aligning, and tuning has left the FTS with the highest servo signal ever recorded! Mike Dulick ran the instrument on 18th and 19th July and reported that the servo system was behaving stably.
Claude Plymate