ELECTRONIC LIMITATIONS IN PHASE METERS FOR HETERODYNE INTERFEROMETRY

Limitations imposed by the phase meters used in heterodyne interferome ters are evaluated. These instruments measure the phase relationship b etween electrical signals generated by the heterodyning process, allow ing the interferometers to resolve fractions of an optical fringe. Mea surements indicate that the phase meters used in currently available h eterodyne interferometers probably limit achievable accuracy to a grea ter extent than barriers imposed by the optics. We show that a new cla ss of time interval counters offers a means of greatly improving accur acy in these instruments.