Absolute frequency measurement of CH/sub 4/ stabilized He-Ne 3.39 mu m line

A frequency chain for measuring the absolute frequency of the CH/sub 4/-stabilized He-Ne 3.39- mu m laser line is described. The chain connects a Cs frequency standard and the stabilized laser line. The frequency chain is composed of three stages. The first stage connects a 90-GHz microwave source and the CH/sub 3/OH 70.5- mu m line; the second stage connects the 70.5- mu m line and the CO/sub 2/ 10.2- mu m line; the third stage connects the 10.2- mu m line and the high-power He-Ne laser, which is offset locked to the stabilized 3.39- mu m line. A Gunn oscillator is used for the 90-GHz frequency source. As other microwave sources, klystrons that are phase locked to a stabilized 10-GHz microwave frequency and calibrated to the Cs frequency standard are used. As frequency mixers, a Josephson diode is used at the first stage and W-Ni diodes at the second and third stages. To make reliable counter measurements, the microwave frequencies are phase-locked to the beat signals. The beat frequency between those microwave frequencies and the harmonics of the stabilized 10-GHz frequency are then measured by counters. All data from the counters are collected and analyzed by a microcomputer.<>