Confocal Fabry-Perot cavity

The first stage was the production of a pulsed free-jet molecular beam of helium containing 20% CO, which was then crossed with an electron beam to produce ionisation. The ions were produced close enough to the beam nozzle for cooling to occur downstream. Some 4 cm from the nozzle the beam entered a confocal Fabry-Perot cavity where it was exposed to millimetre wave radiation close to 120 GHz in frequency. Following microwave excitation, when on resonance, the beam was probed with a Nd YAG pumped dye laser beam with the frequency chosen to drive rovibronic components of the A 2 U-X 2 + band system. Figure 11.54 shows two recordings of a spin component of the lowest rotational transition the line shown in (a) is... 

N.D. Rezgui, J. Allen, J.G. Baker, J.F. Alder, Quantitative millimetre wave spectrometry. Part I design and implementation of a tracked millimetre wave confocal Fabry-Perot cavity spectrometer for gas analysis,. dwa/. Chim. Acta., 311, 99-108 (1995). 

J.G. Baker, N.D. Rezgui and J.F. Alder, Quantitative MMW Spectrometry (III) Theory of Spectral Detection and Quantitative Analysis in a MMW Confocal Fabry-Perot Cavity Spectrometer, Anal. Chim. Acta, 1996, 319, 277-290. 

This glitch is caused by interaction between the cavity and the sample resonance profiles. It may be converted into an apparent frequency shift of the cavity resonance that is directly related to the absolute absorption coefficient. The linewidth can be determined fi-om the distance between the peaks of the glitch. This phenomenon became even more marked with the Mark II confocal Fabry-Perot cavity (Section 5.3), when it could be observed as a glitch in the correction voltage applied by the servo amplifier to the piezoelectric actuator of the moveable cavity mirror. Figure 4.7 shows a spectrum for the water line obtained in this manner. 

Figure 2.4 Configurations for a Fabry-Perot optical cavity. The radii indicate the focal point of the mirrors. In (c) the hatched area indicates the beam shape in the confocal configuration (see text)...

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