Abstract
Hoping to develop a future primary standard of length to use for precision interferometric length measurement, a frequency-stabilized He-Ne laser has been made. The frequency of the 0.633μm output is locked to one of the hyperfine components of an absorption line of moleculer iodine 127I2 by the method of saturated absorption.
The laser cavity in which the absorption cell of iodine is placed, is servo-controlled by dual feedback loops; one is for proportional control by a piezoelectric transducer, and the other is for on-off control driving a specially designed fine tuning apparatus with a parallel spring system until the servo error is reduced to less than a set value.
The stability of the frequency is evaluated by Allan variance and standard deviation measurements of the beat frequency between two stabilized lasers. Results of the measurement for five hours and a half give an Allan variance of 8kHz for a 10 second averaging period and a standard deviation of 96kHz. From these data, long term stabilities of 1.2×10-11 from the Allan variance and 1.4×10-10 from the standard deviation are evaluated for the individual laser.
The relations between the temperatures of molecular iodine in the absorption cell and of the laser output, between the maximum frequency deviation of the modulated laser output and the first and third derivative signals of inverse Lamb dip produced by saturated absorption in iodine vapour are obtained. And systems for measuring the beat frequency by analogue and digital methods are described.
