LASER locking

Ultrahigh resolution spectroscopy requires ultrastable lasers. Fortunately, there has been major progress in this areas. A laser locked to an external reference cavity [47] has yielded a resolution of a few parts in 1015 for an Hg+ ion in a trap [48]. 

Fig. 4. Results of the absolute frequency measurements shown are the absolute frequencies and hyperfine line separation of the hyperflne components ai, aio and ais of the P(54)32-0 and the hyperflne component aio of the R,(56)32—0 iodine absorption line. Numbers in brackets correspond to an independent heterodyne frequency measurement where the beat between two ILP lasers, locked to corresponding HFS components of the P(54)32-0 and the R,(56)32-0 line, were observed (for details see text)...

In 1986, R. BEAUSOLEIL and D. McINTYRE completed their thesis research at Stanford with an absolute frequency measurement of the F = 1 component of hydrogen 1S-2S [11,20], As frequency reference they employed a 486 nm cw dye laser, locked to a narrow absorption line of 1J Te2 vapor. This line was chosen near a reference line, calibrated to within 4 parts in 1010 by A. FERGUSON et al. [21]. Its second harmonic coincides very nearly with the resonance frequency of the hydrogen two-photon transition, so that the frequencies can be precisely compared by observing a radio frequency beat signal. 

Where does this spectacular development lead concerning frequency standards Traps for neutral atoms unfortunately perturb the atomic energy levels both the magnetic fields used as well as the laser light for cooling. As an example of what can be achieved, mention may be made of a tuneable dye laser, locked to the intercombination line at 657 nm of Ca atoms, either in an atomic beam or in a magneto-optic trap, has an estimated relative uncertainty of below 10 [38]. 

An example of such a chain links a 5-MHz frequency derived from a primary Cs clock to the iodine-stabilised He-Ne laser at 473 THz and is shown in figure 3. The first part of the chain (figure 3 (a)) was used to measure the frequency of a CO2 laser, locked at 29 THz to the OSO4 molecule, with an accuracy of lO" [68]. The second stage links this laser to the -stabilised He-Ne laser at 473 THz with four intermediate laser lines in the infra-red [69],... 

An examination of the above equations shows that the intermodulated DFM technique realizes an FM Differential Interferometer which produces a locking signal similar to conventional FM laser locking, but in which the optical tuning parameter A=(0Q-2tr n c/2L is replaced by the radio frequency (differential) tuning parameter 5=(0j-2tr c/2L. The DFM sideband structure creates an optical subtraction in the photodiode of the (n+l)-th cavity resonance curve from the (n-l)-th curve and thus permits accurate, low noise measurements of the cavity mode spacing. 

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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