Frequency-offset locked laser

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

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... 

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. ... 

This controllable shift of a laser frequency V] against a reference frequency yR 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 frequency-offset locked to a stable reference laser in such a way that the difference frequency / = vl — can be controlled electronically. This technique has been described by Hall [5.98b] and is used in many laboratories. 

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. 

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. 

Fig. 1. Set-up of the PTB laser system. The Nd YAG laser is frequency stabilized onto a selected iodine absorption line using the phase modulation method. The probe beam is modulated at 2.05 MHz by an electro-optic modulator (EOM), the pumb beam is frequency shifted by an acousto-optical modulator (AOM). The driving AOM rf power is chopped in order to cancel frequency offsets introduced by the Doppler background using a lock-in detection scheme. The transmitted probe beam signal is detected by a photodiode (PD) and mixed with the EOM rf in a double balanced mixer (DBM)...

Figure 2 The stronger component of the 1S-2S two photon transition in deuterium. The signal is the normalised Lyman-a fluorescence observed as a function of the frequency difference between lasers LI and L2 (fig. 1) when LI is locked to the appropriate transition (b2) in 13°Te2. The measured offset frequency is 20 MHz greater than the true value because of the shift introduced by the acousto-optic modulator. The pressure in the deuterium cell was 270 mtorr...

Fig.13.19. First derivative of the output power of laser number 3 versus offset-lock frequency (f2"f2).

Fig. 5. Experimental setup for locking the offset frequency u>o- The femtosecond laser is located inside the shaded box. Solid lines represent optical paths, and dashed lines show electrical paths. The high-reflector mirror is mounted on a transducer to provide both tilt and translation...

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