Phase and Amplitude Measurements of Coherent Optical Wavefronts

A laser phase-locked loop1 consisting of two laser oscillators has been used to measure the relative phase and amplitude of the wavefront of a laser beam. A phase resolution 011 the order of one degree and an amplitude accuracy better than one dB have been obtained. This system was used to analyze the optical qualities of devices placed in the beam's path by measuring their effect 011 the wavefront. The system and techniques used along with the results obtained are described and illustrated in this paper. I I . D E S C R I P T I O N O F T H E MODIFIED P H A S E - L O C K E D LASER LOOP The phase-locked system is shown in Fig. 1. It consists of controlled and uncontrolled optical oscillators which are single-frequency heliumneon lasers operated at 6328A. Details of the oscillators' characteristics are shown in Figs. 2 and 3. The beam waists and spot sizes are defined and calculated in Appendix A. The two lasers used initially are shown in Fig. 2; however, tube replacements required a different combination of mirrors to maintain a single transverse mode, so the final measurements were made with the lasers shown in Fig. 3. Results are identified with the lasers used. Prior to combining the two beams (Fig. 1) 011 the surface of the photomultiplier by means of a mirror and a beam splitter, the output beam of 1583