The current method for measuring an optical frequency relative to the primary time standard, the cesium beam standard at ~9.2 GHz, utilizes a complex frequency synthesis chain involving harmonics of laser and klystron sources. The method has been extended recently to the visible region [1], to the 633 nm He-Ne laser locked to a molecular iodine line, with an impressive accuracy of 1.6 parts in 1010. With the new definition of the meter, the distance traversed by light in vacuum during the fraction 1/299 792 458 of a second, the speed of light is now fixed and both time and length measurements can be realized with the same accuracy as an optical frequency measurement. In view of the complexity of optical frequency synthesis, these developments set the stage for originating complementary techniques for stabilizing and measuring laser frequencies which are more convenient.
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