Fabry scanning

Fig. 15. Simplified diagram of scanning Fabry - Perot interferometer77 showing piezoelectric ceramic tube (P), multilayer dielectric mirrors (Mj, M2), micrometer adjustment for parallelism (X, Y). Mirror spacing (d) is adjustable...

Yeh and Keeler 244) extended the method of laser-scattering spectroscopy to probe systems undergoing rapid chemical reactions. They observed the spectral line broadening in light from a singlemode He-Ne laser scattered from multicomponent solutions, as a function of time. The experiment employed a pressure-scanned Fabry-Perrot interferometer and photon counting techniques. 

Although the focus of this chapter is tropospheric HO measurements, it is worthwhile to mention techniques that have proven useful in the laboratory or in other regions of the atmosphere. As a small molecule in the gas phase, HO has a much-studied and well-understood discrete absorption spectrum in the near UV (29), shown in Figure 1, that lends itself to a variety of absorption and fluorescence techniques. The total atmospheric HO column density has been measured (30-32) from absorption of solar UV radiation, observed with a high-resolution scanning Fabry-Perot spectrometer. Long-path measurements of stratospheric HO from its thermal emission spectra in the far infrared have been reported (33-35). Long absorption paths in the atmospheric boundary layer have been used for HO detection from its UV absorption (36-42). 

If we have a two-level sample we expect to see a series of resonances separated by half the inverse of the repetition rate of the laser as the carrier frequency is scanned. If a second transition is within the bandwidth of the laser then this too will give rise to a series of resonances. The resulting spectrum is rather like that obtained from a Fabry-Perot interferometer with overlapping orders. However, in the mode-locked case the modes are precisely equally spaced in frequency. 

Fig. 2. A typical pulsed, tunable dye laser, of the Hansch design, in an oscillator— amplifier configuration. Suitable pump lasers are Nj, excimer and Nd YAG-harmonics. The broken line represents a pressure chamber which is required to scan the wavelength if an air spaced Fabry—Perot etalon is used. With solid etalons, tuning is achieved by tilting the etalon and the pressure chamber is not required.

It should be noted further that an increase in resolution is easily achieved in this case by increasing the maximum path difference and the scanning time. The power flux is not influenced by an increase of Smax- However, there will be an increase in noise, as we shall see later. An increase in resolution means for a grating instrument a reduction of slit width and hence, a reduction of the power flux, which is proportional to the square of the slit width [see Eq. (5.12)]. It also seems worth mentioning that the Jacquinot or throughput advantage exists not only in the Michelson interferometer but also in other instruments, e.g. a Fabry-Perot interferometer. 

In the above FLN spectra the observed linewidth is limited by the spectral resolution of 0.3 cm-1 of the monochromator used for recording the spectra. However, at 1.5 K, the homogeneous linewidth, Thom) is generally much smaller than this. It can in principle be resolved using a scanning Fabry-Perrot interferometer. Provided the linewidth of the laser is sufficiently small... 

An etalon, reduced to its bare essentials, consists of two partially transmissive reflecting surfaces separated by a distance L. When a beam of monochromatic radiation is input to the etalon [see Demtroder (1996) pp. 140-152 for a discussion of Fabry-Perot etalons], and the frequency of this radiation is scanned,... 

Figure 2.5b Wideband frequency sweep of a Fabry-Perot cavity with a spatial filter in place. Note how the non-TEMooq modes are suppressed. In this scan the mode number q ranges from 111 to 114...

V.G. Cooper, B.K. Gupta, A.D. May Digitally pressure scanned Fabry-Perot interferometer for studying weak spectral lines. Appl. Opt. 11, 2265 (1972)... 

Figure 10.11 Apparatus for measurement of two-photon absorption profiles in Na vapor using counterpropagating circularly polarized beams. PMT denotes photomultiplier tube. The dye laser is wavelength-scanned by rotating an intracavity Fabry-Perot etalon. Profile (a) was obtained by two-photon absorption from one linearly polarized beam. Profile (b) shows the Doppler-free f=1 - 1 and 2-+2 hyperfine peaks, obtained using counterpropagating circularly polarized beams. Used with permission from M. D. Levenson and N. Bloembergen, Phys. Rev. Lett. 32, 645 (1974) Note that this work arrived in a dead heat with that of Biraben et al., Fig. 10.10.

Fig.6.29. Examples of a Fabry-Perot recording obtained by pressure scanning. The hyperfine structure of Mn (1=5/2) in a J = 7/2 <- 5/2 transition at 5395 A has been recorded with an instrumental free spectral range of 15 GHz [6.69]... 

Sensory systems Cornea verticillata has been studied in 22 patients with Fabry disease and in 11 patients taking amiodarone, comparing the corneal microstructure in both types [34 ]. Confocal laser-scanning... 

Falke K, Buttner A, Schittkowski M, Stacks O, Kraak R, Zhivov A, Rolfs A, Guthoff R. The microstructure of cornea verticillata in Fabry disease and amiodarone-induced keratopathy a confocal laser-scanning microscopy study. Graefes Arch Chn Exp Ophthalmol 2009 247 523-34. 

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