Many plasma diagnostic techniques use a swept-wavelength laser to obtain atomic absorption spectra from a plasma stream. Most of these measurements are dependent upon the scan width of the laser, typically measured in GHz. Since there are no instruments that measure scan width directly, the laser operator must attempt to manually adjust the scan width to specifications (within a few percent), and keep it there at all times. Results of many spectroscopic measurements, such as plasma density, are directly proportional to scan width. Two instruments are presently available to assist the operator in this scan width adjustment effort: wavemeters and Fabry-Perot confocal interferometers (F-Ps).
Wavemeters directly measure absolute wavelength by comparison with a reference light source or with a precision grating. However, only the most expensive and exotic wavemeters have sufficient resolution to measure the difference between the endpoints of a narrow scan, and even the best cannot track the wavelength as it rapidly changes.
Another way to directly measure scan width employs Fabry-Perot confocal interferometers (F-Ps). The F-P samples the laser beam at some point in the optical system. As the laser is scanned, the F-P interferometer produces a voltage pulse each time the wavelength crosses through a free-spectral resonant mode of the interferometer's cavity. The laser operator can observe the F-P pulses on an oscilloscope and attempt to visually count them, then adjust the laser until the proper number appears. A simple digital counter, synchronized with the scan, could also help to automate the counting process. With a Coherent, Inc., model 216 Fabry-Perot interferometer, a pulse occurs every 300 MHz. For example, with a scan of 3 GHz, 10 F-P intervals appear during each scan. The laser operator could then trim the scan width by manually adjusting the scan width knob on the laser's control chassis. However, the precision of this measurement is only .+-.10% (one in 10). Thus, a 10% peak error in any wavelength-dependent measurements may be expected. Also, the operator must periodically adjust the laser to compensate for drift. For this reason, an improved, automated measurement and control system for scan width using a real-time control computer and some specialized CAMAC (Computer Automated Measurement and Control, IEEE 595) modules designed specifically for this application would be desirable. The present invention presents such a system.