Optical frequency measurement and standards

Summary form only given. The last three years have seen the rapid fruition of the long-standing dream in which we can easily and accurately measure optical frequencies. This capability has matured so powerfully that papers are beginning to appear with the words "Optical Clock" in their titles. The in-principle advantages of optical transitions regarding line-Q also are appearing in real systems, although some challenges arise from the complexity of operating simultaneously with laser atomic cooling AND optical frequency stabilization AND accurate frequency measurement. An interesting avenue for practical clocks, albeit with some reduction of accuracy-capability, is possible with gas cells. For example, using molecular iodine\´s visible transitions, performance superior to 1/spl times/10/sup 14/ at 1 s should be obtained with a variant of NICE-OHMS spectroscopy at 532 nm. Recent advances in controlling to sub-10 ppm levels the Residual Amplitude Modulation of the Electro-Optic Phase Modulator should lead to frequency repeatability at the range of a few Hz (/spl sim/1/spl times/10/sup -14/).