Lamb-dip stabilization

F.R. Petersen, D.G. McDonald, F.D. Cupp, B.L. Danielson, Rotational constants of C 02 from beats between Lamb-dip stabilized laser lines. Phys. Rev. Lett. 31, 573 (1973), also in Laser Spectroscopy, ed. by R.G. Brewer, A. Mooradian (Plenum, New York, 1974), p. 555... 

In principle at least, CO isotope lasers can most conveniently serve as reference lasers in the 4.9 to 8.0 pm wavelength domain. In terms of spectral purity, sealed-off operation ) a d abundance of readily available lasing transitions CO isotope lasers are at least as good as their COi counterparts. Lamb-dip stabilization of CO lasers was also accomplished 3) nearly 15 years ago. However, the resettability of the Lamb-dip stabilization method is at least 100 times less accurate than the 4.3 pm fluorescence stabilization of CO2 lasers. The absolute accuracy of presently available CO laser transition frequencies is also only good to within about one or two MHz. If one MHz or so accuracy is not sufficient, a direct comparison of a CO reference laser line with an appropriately selected frequency doubled line-center stabilized CO2 laser transition is always possible and was so demonstrated several years ago. ... 

F.R. Petersen, D.G. McDonald, F.D. Cupp, B.L. Danielson Rotational constants of Cj 2 from beats between Lamb-dip stabilized laser lines. Phys. Rev. Lett. 

Lamb dip stabilization onto very narrow saturation peaks allows one to achieve a very good frequency stability, especially when pressure or power broadening or transit broadening can be minimized, as in the case of the 3.39 ym He-Ne laser stabilized onto a CH transition. 

The idea of using the same medium as absorber and active material has been proposed and realized by several authors 340-343) Leg and Skolnick 40) used a neon gas discharge at low current and low pressure as saturable absorber inside the cavity of a He-Ne laser oscillating at X = 6328 A. The Lamb-dip halfwidth obtained was 30 Mc/sec compared to 1500 Mc/sec for the doppler line. The disadvantage of this arrangement is that the frequency of the neon transitions depends upon pressure and current 341) in the absorption cell, and this limits the stability and reproducibility of the Lamb dip center frequency. 

The last two chapters discussed spectroscopic studies which used coincidences between laser lines and transitions in other atoms or molecules. These investigations have been performed either with lasers as external light sources, or inside the laser cavity. In the latter case coupling phenomena occur between the absorbing species and the laser emission, one example of which is the saturation effect employed in Lamb dip spectroscopy and laser frequency stabilization. This chapter will deal with spectroscopic investigations of the laser medium itself and some perceptions one may obtain from it. 

Saturated dispersion and polarization spectroscopy. The general relation between absorption and dispersion is well known. The saturation of absorption also corresponds to a change in the index of refraction of the vapour, which is called saturated dispersion and was already predicted by Lamb in his theory of gas Lasers 1161. This saturated dispersion plays a role in the frequency stabilization of Lasers on the Lamb dip 1201. Clear evidence of saturated dispersion was given by using a ring interferometer 21. But, in practice, the observation of saturated dispersion is much easier by interferences of polarized light. That introduces us to polarization spectroscopy. 

The steep zero crossing of the third derivative of narrow Lamb dips gives a good reference for accurate stabilization of the laser frequency onto an atomic or molecular transition. Either the Lamb dip in the gain profile of the laser transition or Lamb dips of absorption lines of an intracavity sample can be used. 

In the infrared spectral range. Lamb dips of a vibration-rotation transition of CH4 at 1 = 3.39 pm or of CO2 around 10 pm are commonly used for frequency stabilization of the HeNe laser at 3.39 pm or the CO2 laser. In the visible range various hyperfine components of rotational lines within the -> system of the I2 molecule are mainly chosen. The experimental setup is the same as that shown in Fig. 2.18. The laser is tuned to the wanted hfs component and then the... 

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. ... 

The accuracy of stabilizing the two lasers onto molecular transitions increases with decreasing linewidth. Therefore, the narrow Lamb dips of Doppler-broadened molecular transitions measured with saturation spectroscopy (Sect. 2.2) are well suited [921]. This was proved by Bridges and Chang [922] who stabilized two CO2 lasers onto the Lamb dips of different rotational lines within the vibrational transitions (00°1) (10°0) at 10.4 pm and (00°1) (02°0) at 9.4 pm. The superimposed beams of the two lasers were focused into a GaAs crystal, where the difference frequency was generated. 

Two lasers are stabilized onto the Lamb dips of two molecular transitions. The width Av of the Lamb dip is 10 MHz and the rms fluctuations of the two laser frequencies is = 0.5 MHz. How accurately measured is the separation vi - V2 of the two transitions, if the signal-to-noise ratio of the heterodyne signal of the two superimposed laser beams is 50 ... 

With techniques of sub-Doppler spectroscopy, even small collisional broadening effects can be investigated with high accuracy. One example is the measurement of pressure broadening and shifts of narrow Lamb dips (Sect. 2.2) of atomic and molecular transitions, which is possible with an accuracy of a few kilohertz if stable lasers are used. The most accurate measurements have been performed with stabilized HeNe lasers on the transitions at 633 nm [975] and 3.39 um [976]. When the laser frequency co is tuned across the absorption profiles of the absorbing sample inside the laser resonator, the output power of the laser Pl(co) exhibits sharp... 

Often the narrow Lamb dip at the center of the gain profile of a gas laser transition is utilized (Sect. 7.2) to stabilize the laser frequency [5.78,5,79]. However, due to collisional line shifts the frequency vq of the line center slightly depends on the pressure in the laser tube and may therefore change in time when the pressure is changing (for instance, by He diffusion out of a HeNe laser tube). 

The accuracy of stabilizing the two lasers onto molecular transitions increases with decreasing linewidth. Therefore, the narrow Lamb dips of Doppler-broadened molecular transitions measured with saturation spectroscopy (Sect. 7.2) are well suited [12.76]. This was proved by Bridges and... 

Chang [12.77] who stabilized two CO2 lasers onto the Lamb dips of different rotational lines within the vibrational transitions (00° 1) (10°0) at 10.4 xm... 

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