Finding and Imaging White Light Fringes

Benno Reuter Gesellschaft fur Strahlen- and Umweltforschung mbH, Munchen-Neuherberg, 8042 Neuherberg, Germany. Received 18 September 1972.

Two methods are described that are useful in adjusting two-beam interferometers. The first method will lead to an equalization of the optical paths. By the second method the plane, localized fringes are imaged on, can easily be found.

Two-beam interferometers have often to be adjusted ex­actly for equal optical path. In this position white light fringes can be seen. Due to the short coherence length of white light the fringes are only to be seen if both of the optical paths do not differ more than about six wave­lengths.

Primak1 used a source with a narrow bandwidth to in­crease the region in which fringes can be seen and adjust­ed for maximum contrast. Then the exact position can be found by the method of Guerra2 with the aid of a pho­tocell and an oscilloscope. In spite of this expansive method a prepositioning has to be made to within a few mm of the correct setting.

Today the aligning of optical components is usually done with a small He-Ne laser as we do our interferomet­er setups. Besides this, the equalization of the optical paths can be done with this coherent source. After the mirrors are tilted to the proper position, the expanded laser beam is focused onto the mirrors. Now a nonlocal-ized circular fringe system can be observed. If one mirror is translated in the direction of an equalized setting, the

412 APPLIED OPTICS / Vol. 12, No. 2 / February 1973

fringes contract toward the center, and the angular scale of the pattern increases.3 If the optical pathlengths are equal, the field of view is uniform, and without a disper­sive element in one beam white light fringes can be ob­served. if there is a small amount of dispersion an addi­tional slight translation is necessary to produce fringes with white light illumination.

When an extended source is used in two-beam interfer-ometry the fringe system is localized and must often be imaged onto the recording plane. The same problem arises in holographic Fourier transform spectroscopy4

where fringes of equal thickness have to be imaged by a lens.

The proper method for finding exactly the image plane is the illumination of the interferometer with an extended source of strongly monochromatic radiation through a variable aperture. With the aperture the region of local­ization can be varied, and the coherence length of the monochromatic source makes it possible to visualize high contrasted fringes in the whole field of view. The method is realized by illumination of a moving diffusor by an ex­panded beam from a He-Ne laser. The fringes are visual­ized by a microscope at the position of highest contrast. By increasing the aperture the region of localization de­creases, and the exact image plane is found.

References 1. W. Primak, Appl. Opt. 3,432 (1964). 2. R. Guerra, Appl. Opt. 6,170 (1967). 3. M. Born and E. Wolf, Principles of Optics (Pergamon, New

York, 1970), p. 301. 4. G. W. Stroke and A. Funkhouser, Phys. Lett. 16, 272 (1965).