Optical cavity stability

The absolute frequency position of the two-photon transition is measured by comparing the infrared dye laser wavelength with an I - stabilized He-Ne reference laser at 633 nm (see Fig.2). The hey of the wavelength comparison is a nonconfocal etalon Fabry-Perot cavity (indicated as FPE in Fig.2) kept under a vacuum better than 10-6 mbar. This optical cavity is built with two silver-coated mirrors, one flat and the other spherical (R = 60 cm), in optical adhesion to a zerodur rod. Its finesse is 60 at 633 nm and 100 at 778 nm. An auxiliary He-Ne laser as well as the dye laser are mode-matched and locked to this Fabry-Perot cavity. Simultaneously the beat frequency between the auxiliary and etalon He-Ne lasers is measured by a frequency counter. 

Acetylene and methane can be detected in remote locations using diode lasers coupled with optical fibres Environmental measurements of acetylene are possible based on the overtone transition at 2 789 nm, using a stabilized AlGaAs diode laser, operating in an external optical cavity configuration. The detection limit is 0.2ppm/km for acetylene... 

An alternative approach for tunable singlemode operation is to build an external cavity. In this case, a FP laser is placed into an optical cavity, and the wavelength is tuned by using a dispersive element, such as a grating. The wavelength can be tuned continuously over many wave-numbers (20-50 cm" ) by rotating the grating. The linewidth depends on the construction of the external cavity and its stability and is usually less than 1 MHz. 

Finally we discuss the frequency stabilization of gas lasers and show how the techniques of saturated absorption and frequency stabilization have been combined in a recent measurement of the velocity of light which attained a hundred-fold increase in precision. However, since this chapter is very largely concerned with the frequency distribution of the laser output, we first discuss in more detail the frequencies of the modes of the passive optical cavity. 

The idea of stabilizing the laser frequency 37-39) and making it as far as possible independent of cavity parameters, has been realized by many authors in different ways (see for instance the review article by Basov on optical frequency standards 339). 

Laser instabilities were experimentally investigated in many kinds of lasers (see an overview of early papers [14]), but the first experimental observation of the optical chaos was performed by Arecchi et al. [30] in 1982. They used a stabilized CO2 laser with modulated cavity loss = y(l + a cos fit) and by changing the frequency of modulation 2, they found a few period doubling oscillations of the output intensity, both numerically and experimentally. 

To sweep the dye laser its beam is split and the secondary beam is driven into an acousto-optic device. The frequency-shifted beam is reflected back into the acousto-optic crystal so that one of the emerging beams is shifted twice. This beam then enters a reference Fabry-Perot cavity (indicated as FPR in Fig. 2) of very high finesse, whose length is locked to an I2 - stabilized... 

The rms linewidth of the dye laser has so far been reduced to 300 Hz relative to a reference cavity with the help of an intracavity ADP phase modulator and a fast servo system which compensates for small rapid optical path fluctuations in the liquid dye jet [24]. A perhaps even more elegant alternative is the external laser frequency stabilizer [25] which compensates for phase and frequency noise after the light has left the laser cavity. J. Hall and coworkers [26] have recently reduced the linewidth of a commercial ring dye laser to sub-Hz levels with such a device. 

---