| The high power carbon dioxide laser has become a very important tool in the area of industrial materials processing. This is due primarily to the enormous optical intensities which the device can attain. Unfortunately, it has proved increasingly difficult to construct machines capable of maintaining a consistently uniform output beam at elevated power levels, for extended periods. These difficulties are often attributed to thermal distortion of the resonator optics. Because of this fact, researchers have invested considerable effort in the construction of water cooled, thermally stable optical benches, upon which to mount the resonator components. This approach is very expensive, and has provided only a partial solution to the problem.;A different approach, and the subject of this thesis, is to construct a feedback control system which actively re-positions the optical elements, so as to maintain optimum resonator alignment. Preliminary experiments demonstrated that this concept was indeed feasible. In this first study, a circular array of small thermistors was utilized as the feedback element. Indicators of mode uniformity in the vertical and horizontal directions were extracted from the array using electronic techniques. A relay type controller monitored these error measures and subsequently activated a pair of motor-micrometers fitted to one of the primary resonator mirrors. Although this thermistor-based system demonstrated the viability of the concept, the sensor array's slow response limited its performance. Subsequent experiments utilizing high speed pyroelectric detectors in place of the thermistors solved the problem.;The next series of experiments was dedicated to finding a method of simultaneously aligning both primary resonator mirrors. This was accomplished by configuring the sensor array to monitor not only beam uniformity, but also beam position. A new controller was then developed which adjusted one mirror based on the position of the output beam, while the second mirror was employed to continuously maintain mode uniformity. This "two mirror" control system has demonstrated the ability to consistently maintain optimum resonator alignment for indefinite periods, at arbitrary power levels up to 10 kW. |
