Reference Cavity

The optical setups for these experiments are very similar to the setups described in the previous chapters. The pentacene study used a pinhole to reduce the excitation volume. The setup consisted of a home-buflt pressure cell and an external home-built reference cavity, to account for the slow frequency drifts of the commercial dye laser. In the terrylene study a magnet controlled lens, immersed in liquid helium, focused the laser light to a tiny spot and an ellipsoidal mirror collected the fluorescence. 

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Similarly, when arrays of these test resonant cavities loaded with functionalized wicks for various toxins are phase-locked to the reference cavity as shown in Fig. 15.5 they can act as detectors for targeting various toxins as well as their precursors through an array of suitably tuned cavities with specifically functionalized nanotubes. By using the approach as demonstrated in this document, it has been shown that the apparatus can be used to successfully detect low levels of toxin vapors associated with the drag Methamphetamine, in a laboratory-controlled environment. Some of the results of this study are highly sensitive in nature and are not reported in this document. These results can be obtained by other avenues. 

Fig. 15.5 Arrays of cavities phase-locked to the reference cavity and loaded with functionalized wicks to specifically determine the toxin and its precursors...

Figure 12. Gas dilatometer designed by Farris (26) has an aluminum body with two cavities, a test cavity, and a pressure reference cavity. The specimen is bonded to special tabs and is fitted into upper and lower sample holders. The upper jaw is attached to a load cell, and a compensating rod enters the cavity during extension...

Fig. 12.2 Recorder trace of Rb 52s1/2 —> 242s1/2 signal at pressures of 0.29,1.00,1.45, and 1.96 Torr of argon, with different detector sensitivities, as a function of laser detuning v. The sharp spikes are superimposed signals from a 250 MHz reference cavity (from ref. 11).

To provide the 243 nm radiation needed for the 1S-2S transition, a laser at 486 nm is stabilized to a reference cavity that reduces the linewidth to less than 1 kHz. The frequency is doubled in a BBO-crystal to produce 243 nm radiation. 

Ultrahigh resolution spectroscopy requires ultrastable lasers. Fortunately, there has been major progress in this areas. A laser locked to an external reference cavity [47] has yielded a resolution of a few parts in 1015 for an Hg+ ion in a trap [48]. 

It was found that the normal method of scanning the frequency doubling laser by rotating the tipping Brewster plate in the reference cavity was insufficiently smooth over the small ranges required a satisfactory alternative was found to be to shift electronically the reference point used for locking the reference cavity. Further refinements to the lasers were unnecessary, because their performance did not limit the accuracy of the measurements. The calibration procedure was responsible for most of the uncertainty, and was the least satisfactory aspect of the experiment as we now discuss. 

Frequency stabilisation and scanning is accomplished by use of a confocal cavity of free spectral range matched to the dye laser repetition rate. Phase modulated sidebands are put on to the mode spectrum of the mode-locked pulse train and used to lock the laser to the reference cavity. The frequency modulation technique is also used to lock the ultra-violet enhancement cavity to the mode-locked pulse train. 

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. 

Oxytraps, 8—Molecular sieve trap, 9—Line to sample cavity, 10—Line to reference cavity, 11—N2 purge streams through plastic shrouds, 12—Draft shield, 13—Aluminum block. 

Curve (a) departs from zero some distance below the 50°C inversion temperature and readies its peak some 20°C above the inversion temperature. In curve b), the sample material surface temperature would correspond to the temperature of the metal block in which the sample and reference cavities are located. This curve starts deviating from the baseline at the inversion temperature, which, if this temperature could be accurately determined, would have useful significance on such a curve. If the differential temperature is plotted against the temperature at the center of the sample material, as shown in curve (c), the peak maximum temperature would be equal to the inversion temperature. 

Sweep gas outlet Central electrode Black thermocouple station Sample cavities for dielectric measurements Sample anti reference cavities tor DTA Holes for support ocs... 

Fig. 9.91 Exciting the hydrogen 15-25 transition with two counterpropagating photons in a standing-wave field at 243 nm. This radiation is obtained from a dye laser frequency doubled in a BBO crystal and stabilized to a reference cavity. While scanning the hydrogen resonance the frequency of this laser is measured with a frequency comb to be 2 466 061413 187 074 (34) Hz for the hy-perfine centroid [1327]...

The bonding process can be readily performed in vacuum, allowing hermetic sealing or zero pressure reference cavity to be formed (or sealing with special gas mixtures). 

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