Defocused spherical mirror Fabry—Pérot etalons, in which the mirror separation is slightly less than the common radius of curvature, produce a multiple-beam fringe pattern of concentric rings, with quasi-linear spectral dispersion over an appreciable annular region corresponding to two free spectral ranges. The characteristics of these interferograms are discussed in relation to their many advantages for pulsed laser spectroscopy. These advantages include: (i) accuracy of frequency difference measurement; (ii) high illumination of the detector with moderate energy density in the laser beam; (iii) ease of alinement and permanent adjustment of the mirrors leading to the attainment in practice of a very high instrumental finesse (N R values of up to 90 have been achieved); (iv) measurement of degree of spatial coherence of laser beam; (v) ease of matching the interferogram to the spatial resolution of the detector. A simple optical path relation determines the positions of the fringes and the location of the quasilinear dispersion region. The interfering wavefronts, formed by multiple reflexion, have been numerically computed and summed to provide information on the finesse, fringe profiles, contrast and optimum conditions of use of this new, very high resolving power (107 to 108) quasi-linear spectrographic disperser. Constructional details are described and optical design criteria are discussed, together with the various experimental arrangements for employing the instrument. Comparison is made with the equivalent confocal and plane Fabry—Pérot etalons and methods of simultaneously measuring
Fiber cavity ring-down using an optical time-domain reflectometer
