Microwave-Optical Frequency Chains

With fast electronic counters, frequencies up to a few gigahertz can be measured directly and compared with a calibrated frequency standard, derived from the cesium clock, which is still the primary frequency standard [1316]. For higher frequencies a heterodyne technique is used, where the unknown frequency Vx is mixed with an appropriate multiple tmvr of the reference frequency vr (m = 1,2, 3.). The integer m is chosen such that the difference frequency Av = - tmvr at the output 

This frequency mixing in suitable nonlinear mixing elements is the basis for building up a frequency chain from the Cs atomic beam frequency standard to the optical frequency of visible lasers. The optimum choice for the mixer depends on the spectral range covered by the mixed frequencies. When the output beams of two infrared lasers with known frequencies v and V2 are focused together with 

In addition, other frequency chains have been developed that start from stabilized CO2 lasers locked to the cesium frequency standard in a similar way, but then use infrared color-center lasers to bridge the gap to the l2-stabilized HeNe laser [14.155a]. 

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One way an optical standard could be provided is by harmonic multiplication of a microwave frequency standard in a synthesis chain. By use of this technique, a laser at 88 THz (3.39 pm) has been made phase coherent with a microwave oscillator. - The best optical frequency standards may be made by locking a local oscillator (laser) to an atomic or molecular resonance line. State-of-the-art accuracies are characterized by measurements on methane stabilized He-Ne lasers in which reproducibilities in the 10— -- range have... 

Starting from microwave frequencies which can be readily compared with the frequency standard, a chain of frequencies can be built up which uses, for example, the HCN laser, the H2O laser, the CO2 laser, and the He-Ne laser, and their harmonics and which has been extended up to 197 THz (1.52 ym). Recently even optical frequencies in the visible range have been directly measured by this synthesis technique. Figure 6.36 shows such a laser frequency synthesis chain as used by EVENSON et al. [6.34]. 

In this paper, we address two aspects of this general problem. First, we discuss the problem of frequency standards in the optical spectrum. (An analogue in the microwave region of the spectrum is the cesium beam frequency standard.) If one or a few of these reference frequencies can be accurately calibrated (perhaps by a frequency synthesis chain- -) then it may be possible to compare optical spectra to these standards. As an example of the precision that might be achieved, we discuss only optical standards based on stored ions. Second, we discuss the problem of frequency comparison of unknown frequencies to the standards. Here we primarily restrict discussion to generation of wideband frequency "combs". 

For a number of purposes, the accuracy obtainable by the interferometric measurement of wavelength is not adequate. The most obvious of these purposes is molecular spectroscopy of the lasing molecule itself, which was discussed in Sect. 2. When used as a local oscillator in an astronomical receiver one would also like to know the laser frequency to within a few megahertz so as to know the radial velocity of the observed objects to within a few km/s. In metrology too, where the laser might be used in a chain to link microwave measurements with those made in the optical, high precision is necessary. For such purposes heterodyne measurements, which yield the frequency directly, are to be preferred, and these are now discussed. 

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