High Degree Of Spatial Coherence Research Articles

A Multiple-beam Interferometer Coherence Analyser

A laser coherence analyser is described which yields simultaneous temporal and spatial coherence analysis of laser beams. It is of particular application to high power pulsed laser systems, for high speed holography and non-linear optics, where the short duration of the pulse (often less than 50 nsec) excludes the use of photoelectric spectrometers, employing time variation scanning of the optical path length. The instrument consists of two high quality spherical mirrors, having dielectric multilayer coatings of reflectivity typically 0.95 to 0.97, separated by a cylindrical invar spacer of adjustable length. In use, the spacer is normally set so that the pole separation of the plates is a few hundred micrometres less than their common radius of curvature (typically 10 cm). The resulting defocusing term combines with spherical aberration to yield a multiple beam interferogram having an annular region of nearly linear dispersion. Spectral resolving powers in the range 107 to 108 are readily obtainable. When the input beam possesses a high degree of spatial coherence, the fringes of the concentric rings interference pattern are alternately of high and low intensity, the visibility of the alternation being a direct measure of the spatial coherence. This alternation effect may be eliminated by obstructing one-half of the input aperture. Any spatial, spectral or intensity variations across the laser beam are superposed on the fringe pattern, since the spherical interferometer is not translationally invariant in a parallel beam. This is in contrast to the behaviour of a plane Fabry-Pérot etalon. The experimental characteristics of the instrument, and its properties, as predicted by numerical computation, are examined both for C.W. and pulsed lasers. The effect of the finite response time is described. This is particularly important when the coherence analyser is employed in conjunction with an image tube streak camera to obtain time-resolved spectra of pulsed systems. Both time-integrated and time-resolved fringe profiles are considered for various types of laser pulse. The effect of plate defects on fringe profiles is also considered in outline. In addition to providing quasi-linear dispersion and information on coherence and other spatial properties of the input, the instrument described has the advantages of providing high illumination, and of being easy to use in practice, the plates remaining permanently in alignment.

Lasers —half a decade of development

Laser light has a high degree of spatial coherence and can therefore be beamed sharply with little divergence over large distances. It is also coherent in time in much the same way as an r.f. oscillation. In these two respects it differs from ordinary light, which has no coherence either in space or in time and, in the language of the radio engineer, is broadband noise at optical frequencies.

A Scanning Spherical Mirror Interferometer for Spectral Analysis of Laser Radiation

The results of a theoretical and experimental investigation of a scanning spherical mirror interferometer designed specifically for analysis of laser radiation are given. It is shown that the high degree of spatial coherence and monochromaticity of laser radiation makes it possible to excite individual interferometer modes selectively. A theory of single-mode excitation is presented, and a specific example treated. The mechanical construction of the instrument and the dynamics of the scanning system are described. The capability of the instrument for observation of laser mode separation, amplitudes, and frequency shifts is illustrated by specific experiments. It is shown that the instrument can easily be used in both the visible and infrared. Results of simultaneous observation of beats between laser modes with an rf spectrum analyzer and of the laser optical field with the scanning interferometer are given, and it is shown that the combined system, in certain cases, will permit determination of the relative phases of the laser modes. Experiments illustrating the use of the scanning interferometer for observation of mode competition and the effects of magnetic fields on laser output are recounted, as is also the operation of an active scanning interferometer with which resolving powers of 2.5 × 109 and finesses of 1040 were obtained. Effects of coupling between laser and interferometer are shown, and a polarization isolator for decoupling is described.