Spectrometer, Fabry-Perot

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). 

All the components necessary to assemble a complete Fabry-Perot spectrometer are available commercially. The total cost would be below 50,000. Thus, Brillouin scattering should become a more widely used technique in polymer science. 

Spectra were recorded on a multipass Fabry-Perot spectrometer, described in detail elsewhere 11,12,22), using the 514.5 nM line of an Ar" laser. The spectrometer was operated at an optimized free spectral range in each case with a finesse of approximately 40. Spectra were recorded digitally and instrumental distortions were corrected by the method of Lindsay, Burgess, and Shepherd (23). Temperatures above 293 K were obtained with an electrically heated jacket around the sample cell and the temperature was measured with a thermocouple placed in the sample near the scattering volume. Lower temperatures were obtained with an Oxford Instruments CF-104 light-scattering cryostat. Temperature control and measurement was accurate to 0.1°C. 

Earlier work by Lee and White was repeated by Fehse et al using a double resonance crossed Fabry-Perot spectrometer with frequency modulation of the probe source. Both worked in the 26-40 GHz region for the probe frequency, with 15 GHz and 23 GHz pump radiation respectively. The work was not reported in any great depth but served to illustrate that the technique was viable according to both sets of workers, if less sensitive than conventional Stark spectrometry. 

Using a radiogenic 99.5% Os-enriched Os sample, Guthohrlein etal. [24] have measured with a Fabry-Perot spectrometer the doublet splittings of Os in three lines ... 

A Liquid Helium Cooled Mid-Infrared Imaging Fabry-Perot Spectrometer... 

Spectroscopic observation at mid-infraTed(Mnt) region is one of the most important subjects in infrared astronomy. For the ground-based observation at MIR region, it is important to reduce the thermal background. So a cooled Fabry-Perot spectrometer with high spectral resolution is a powerful instrument to observe a specific MIR line. In this proceeding we briefly describe the characteristics of our Si P material, preliminary results of the array and the design of spectrometer. 

The construction of our Fabry-Perot spectrometer is quite simple. The driving mechanism is similar to that used for FIR Fabry-Perot system developed by Okuda et al.(1986). The etalon is supported by plate spring, and... 

KF-IO3 Hz. Since conventional spectrometers (e.g., a grating spectrometer or a Fabry-Perot interferometer) are not capable of resolving such small frequency shifts, it is necessary to rely on beating techniques to obtain the information contained in the scattered spectrum. 

A schematic diagram of the spectrometer is shown in figure 10.16 its successfid operation depends critically upon the ability to achieve accurate timing for a sequence of several events. First, a short pulse of gas is produced from a pulsed-nozzle source, the gas travelling in a direction perpendicular to the axis of an evacuated Fabry Perot cavity, described later. This gas pulse lasts for about 1 ms, and the expansion in the cavity is in an essentially collision-free environment... 

The most important and unique part of a Fourier transform microwave spectrometer is the Fabry Perot cavity. A fairly complete description of the principles has been given by Balle and Flygare [14] and we here summarise the main aspects, with the aid of figure 10.19. We use the cavity built by Balle and Flygare as a typical example. It is formed by two parallel, spherical concave mirrors made from solid aluminium. The mirrors are 36 cm in diameter, have a radius of curvature (R) of 84 cm, and are situated... 

Interference filters are used in photometers and spectrometers as fixed wavelength or tunable wavelength filters. An interference filter, equivalent to a Fabry-Perot etalon. 

A laser is a radiation source which produces a very high spectral radiance in a small spectral range at a fixed wavelength. A laser combines a radiation source with spectral isolation of its radiation - two important components of a spectrometer. The word laser is an acronym which stands for light amplification by stimulated emission of radiation. The essential elements of a laser are an active medium a pumping process to produce a population inversion and a suitable geometry or optical feedback elements (Moore et al., 1993). Most lasers are essentially Fabry-Perot interferometers whose cavities contain... 

Fig. 1. 249.9-GHz FIR-ESR spectrometer. A, 9-T magnet and sweep coils B, phase-locked 250-GHz source C, 100-MHz master oscillator D, Schottky diode detector E, resonator and modulator coils F, 250-GHz quasioptical waveguide G, power supply for main coil (100 A) H, current ramp control for main magnet I, power supply for sweep coil (50 A) J, OC spectrometer controller K, lock-in amp for signal L, field modulator and lock-in reference M, Fabry-Perot tuning screw N, vapor-cooled leads for main solenoid O, vapor-cooled leads for sweep coil P, He bath level indicator Q, He transfer tube R, bath temperature thermometer S, " He blow-off valves. [From Lynch et al. (1988), by permission of the AIP.]...

We see that reducing the beam waist increases We note that is more weakly dependent on P. We will derive an expression for he field at the sample in a Fabry-Perot resonator in Section VIII after we have developed the appropriate lumped equivalent circuit for a transmission mode spectrometer. 

The dimensions of the sample are important in determining the performance of the spectrometer because the sample can extend over several wavelengths in several dimensions, at least in principle, which enhances interferometric effects within the sample. Neglecting losses in the sample for the moment, we note that if the sample is an integral number of half-wavelengths thick, it functions like a Fabry-Perot. In order to understand this, we will sketch a derivation that takes into account the index of refraction of the dielectric material and reflection from the sample-air interfaces. First, note that the optical phase difference across the sample is nkt, where n is the index of refraction and t is the thickness. The resonance condition for such a slab is given by Eq. (44) with kt replaced by nkt, namely,... 

Although all laboratories that perform high-field ESR have experimented with Fabry-Perot resonators instead of fundamental mode microwave cavities, few laboratories have as yet explored quasioptical implementations of common microwave devices such as a magic T or circulator in an FIR-ESR spectrometer (see Earle and Freed, 1995 Earle et al. 1996b Smith, 1995). Part of the problem is the unfamiliar appearance of optical circuits to spectroscopists who are only familiar with... 

The layout for a novel scheme that overcomes the limitations of a Michelson duplexer is shown in Figure 7. The most important element of the spectrometer in Fig. 7 is the polarization-transforming reflector (PTR), which functions as a quarter-wave plate in this configuration. We will defer a detailed discussion of PTRs for the moment and focus instead on its functionality. To that end, consider Fig. 8a, where we have unfolded the optical layout between the PTR and the Fabry-Perot interferometer (FPI) in order to see the evolution of the electric field polarization more clearly. 

A5. Amity, I., Fabry-Perot cavity for millimetre and sub-millimetre electron spin resonance spectrometers. Rev. Sci. Imtrum. 41, 1492-1494 (1970). 

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