Ring-laser cavity

One variation in dye laser constmction is the ring dye laser. The laser cavity is a reentrant system, so that the laser light can circulate in a closed loop. The ring stmcture provides a high degree of stabiUty and a narrow spectral width. The spectral width of a conventional dye laser on the order of 40 GH2 is narrowed to a value as small as a few MH2. Such systems offer very high resolution in spectroscopic appHcations. 

Figure 3.25. Laser resonators applicable to molecular glasses A = microdroplet, B = microdisk, C = ring laser, D = vertical cavity distributed bragg laser, E = distributed feedback laser, F = random laser.

We have developed FM lasers based on a commercial ring laser (Coherent 699-21). In this case all the intracavity etalons are removed and replaced by a lithium niobate phase modulator. This modulator can be resonantly driven at a frequency close to the cavity mode spacing. A simple theory of FM operation of a laser suggests that the modulation index is given by [12]... 

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. 

Romanini D., A. A. Kachanov and F. Stoeckel Diode laser cavity ring down spectroscopy, Chem. Phys. Lett. 270 (1997b) 538-545. 

Fig. 3. IB CRLAS apparatus, nable infrared laser radiation U coupled into the ring-down cavity. The light transmitting the output mirror is focused into an InSb detector. The resulting signal is amplified, digitized, and sent to a PC where it is fit to a firsi order exponential, which is directly related to the total cavity loss per laser pass. The PC additionally controls the scanning of the dye laser. Base line losses are determined by scanning the laser with the expansion turned off and are then subtracted from the data, yielding the absolute sample absorbance, from which the carrier concentrations are extracted." ...

FIGURE 11 Transmission functions of the filters in the tunable single frequency ring laser filter stack (a) at low resolution and (b) at 300 times higher resolution. Solid lines are for a filter element used alone, and dashed lines are for the composite stack. The curvature of the dye tuning curve is barely noticeable in (a) and the separation of the selected mode to the next adjacent cavity mode is barely visible in (b), emphasizing the enormous span of frequency over which the filter stack must work to select one mode out of 400,000. (Courtesy of Coherent, Inc., Palo Alto, CA.)... 

The analysis uses the optimum output coupling values / = to determined ejqterimentally with the tuning curves and the estimate of the single-frequency cavity chssipative losses a = 2.5% for the ring laser of Fig. 10. Differentiating Eq. (9) with respect to t to find the output minor transmission that maximizes the output power gives... 

Figure 6.13 Schematic diagram for tuneable laser cavity ring-down apparatus...

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