Laser frequency standards

This paper gives a brief review of the present status of the problem of developing optical frequency standards. The basic principles of laser-frequency stabilization based on narrow nonlinear optical resonances are presented. The basic physical factors determining the shape and the shift of narrow resonances are discussed. The design of the modern laser frequency standard, based on the use of supernarrow resonances with a relative width of 10$^{-11}$ and smaller as the reference, is studied. The currently used techniques for stabilizing the frequency of lasers for different ranges and the methods used for measuring the characteristics of the frequency stability are described. Specific achievements in the development of lasers with a narrow radiation line and high long-term frequency stability and reproducibility are presented. Special attention is devoted to the He–Ne laser. For this laser a radiation line width of less than 0.1 Hz and a long-term stability of 10$^{-14}$–10$^{-15}$ have been obtained, and the effect of different factors on the shifts of the stabilized frequency are studied in detail. Results achieved in the development of the first optical clocks in the world are presented. The progress achieved in the absolute measurement of submillimeter, IR, and visible range laser frequencies is examined. Some applications of laser frequency standards and the prospects for their further development are described briefly.