Frequency offset locking

The expression frequency-offset locking was coined by Barger and Hall in 1969. Two lasers oscillate with frequencies whose difference Av may lie in the microwave region. One laser is stabilized to keep its frequency fixed, e.g., by locking it to a spectral line. One compares the difference frequency Ap with the frequency p of a radiofrequency oscillator. The frequency of the second laser can be scanned synchronously with a change of j rf via a ramp voltage proportional to Ay - which is obtained with a linear frequency-to-voltage converter. 

Beat frequency generation between visible lasers in the 80-GHz range using GaAs Schottky barrier diodes was published in Ref. 275. 

The development of metal-insulator-metal (MIM) point contact diodes, and of point contact and planar Schottky barrier mixers 

The combination of lasers and microwave sources also plays a very important role in metrology. The frequency of 473 THz of the iodine-stabilized HeNe laser at A = 633 nm was measured directly against the cesium standard of time with a chain of lasers and klystrons starting up from a Cs atomic clock. The authors give a total uncertainty of the 

Investigations are under way to combine a cesium beam atomic clock with laser optical pumping, replacing the A and B magnets of conventional clocks by two laser-beam-Cs-beam crossing regions. There would be no spatial selection of atoms with certain velocities as occurs with the A magnet. The preliminary results support the potential of this technique 

This controllable shift of a laser frequency vl against a reference frequency Vr can be also realized by electronic elements in the stabilization feedback circuit. This omits the Pockels cell of the previous method. A tunable laser is Trequency-offset locked to a stable reference laser in such a way that the difference frequency / = vl — vr can be controlled electronically. This technique has been described by Hall [5.104b] and is used in many laboratories. More details will be discussed in Vol. 2, Chap. 2. 

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Fig. 2.20 Schematic diagram of a frequency-offset locked laser spectrometer...

This extremely high stability can be transferred to tunable lasers by a special frequency-offset locking technique [221]. Its basic principle is illustrated in Fig. 2.20. A reference laser is frequency stabilized onto the Lamb dip of a molecular transition at coq. The output from a second, more powerful laser at the frequency (D is mixed in detector D1 with the output from the reference laser at the frequency coq- An electronic device compares the difference frequency coq — co with the frequency co of a stable but tunable RF oscillator, and controls the piezo P2 such that a>o — CO = co dt all times. The frequency co of the powerful laser is therefore always locked to the offset frequency co = coo — co which can be controlled by tuning the RF frequency co. ... 

Fig.14.51, has been built [14.123] which starts at the frequency of the hy-perfine transition in cesium at p = 9.1926317700 GHz accepted up to now as the primary frequency standard. A clystron is frequency-offset-locked (Sect.7.3.4) to the Cs clock and stabilized with the accuracy of this clock at the frequency Vq = 10.600 363 690 GHz. ITie 7th harmonic of Pq is mixed with the frequency of another clystron in a nonlinear diode, and Pj is again frequency-offset locked to the frequency Pj = 7po+Pjb where Pjg is... 

This controlled frequency shift of the laser frequency against a reference frequency can be also realized by electronic elements in the servo loop. This dispenses with the modulation by the Pockels cell of the previous method. Such a frequency-offset locking technique has been demonstrated by HALL [6.33], who locked a tunable single-mode laser with a variable frequency offset to an ultrastable He-Ne laser, stabilized onto the center of a CH line. 

The spectrum in Fig.10.26 has been obtained with such a frequency offset locked laser spectrometer. The real experimental setup is somewhat more complicated. A third laser is used to eliminate the troublesome region near zero... 

Fig.13.18. Schematic diagram of apparatus used by Barger and Hall (1969) for saturated absorption experiments in methane employing the frequency-offset locking technique. (After Hall (1973).)...

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