Accuracy of measuring the laser optical frequency and velocity of light in interference experiments

Abstract

The following of the known methods of measuring the laser frequency can be extracted: the method of frequency synthesis by using a laser chain [1-3]; the method of beats by using multifrequency lasers [4-6]; the method of transferring lower frequencies (both radio and microwave) into the optical range, accomplished by means of radio frequency modulation of the laser frequency [7] or mixing the radiation of two lasers in nonlinear crystals [8-10]; and the method of direct measurement of the ratio between the laser frequencies of the optical and lower-frequency domains [ii, 12]. Measurement of the beat wavelength and of the radiation of a corresponding frequency is required to determine the speed of light in all cases. The last three methods which use interferometers to measure the optical frequencies will indeed be analyzed below. The frequency-synthesis method is not considered here since it, firstly, uses purely frequency measurements, and secondly, its possibilities are as yet limited to the infrared spectrum band (the maximum laser frequency measured is 88 THz). However, as will be seen later, its application together with interference methods permits resolution of the problem of highly accurate measurements of laser frequencies in the optical band. The first velocity measurements of light from a laser by an interference method were apparently made in 1964 [4]. Up to now, in addition to the single-minded works, a number of incorrect interference methods have been proposed and attempts to realize them have been made. Apropos the separate works, a polemic has already occurred in the press [13, 14]. Overestimation of the achievable accuracies in measuring v and C is a widespread error.