A Bidirectional Electronic Counter for Use in Optical Interferometry*

Abstract

A bidirectional electronic counter has been developed for both additive and subtractive counting of electrical impulses in either binary or decimal notation. Using two coherent syncopated sine-wave input signals, such as may be derived from a Twyman-Green interferometer with a stepped mirror and two phototubes, direction sensing is obtained by pulse forming and selecting circuits which feed pulses to either the “add” or “subtract” channel, depending on the phase relation between the two input signals.In the binary counting circuit, the add and subtract channels each operate separate coincidence amplifiers which select the proper phase for transferring the trigger pulses from one binary stage to the next. In the decimal counting circuit, which consists of a binary stage followed by a reversible ring-of-five, additional coincidence amplifiers determine the direction in which the trigger pulses transfer the count around the ring. Counting rates in excess of 150 kc have been achieved with both circuits and higher rates are possible with proper design.Although the bidirectional counting circuit was designed primarily for the purpose of eliminating the tedium of counting interference fringes visually, it can also be used for differential counting with two separate (incoherent) pulse inputs. A review of contemporary developments in this field is given, along with a discussion of applications of the circuit in interferometry and in analog-to-digital conversion devices.