Mylar® beamsplitter

The beamsplitter of the polymer-film type used in the far-infrared region is a tightly spread polyester film with high flatness this is often commonly referred to as a Mylar beamsplitter (Mylar is a EHiPont trade name for a poly(ethylene terephthalate) film). The wavenumber region covered by such a beamsplitter is determined by the film thickness the peak of efficient interference shifts toward lower wavenumbers with increasing film thickness. A thin plate of silicon may also be used as the beamsplitter of the solid-film type in the far-infrared region. 

A pellicle beamsplitter is a high tensile strength elastic membrane, which is stretched like a drumhead over a flat frame. The membrane has a high refractive index and its absorption is negligible. The most common membrane material is Mylar. An ideal beamsplitter divides the incoming intensity into two equal parts. This is, however, achieved only in a definite wavenumber region, due to the interference... 

There have been several important advances in the interferometer optics for far infrared F.T. spectrometers. In the far infrared there is no satisfactory solid material than can be used for beamsplitter. The beamsplitter material in common use is a stretched film of Mylar of appropriate thickness so that constructive interference between front and back surface provides adequate reflection. Out of necessity such beamsplitters are highly wevelength dependent and as a consequence an extensive wavelength region requires the use of several different beamsplitters which must be interchanged. 

With these preparations the one-layer beamsplitter can be treated ( o = 2 = 1 til = n i). In the far infrared a self-supporting layer, such as a sheet of mylar stretched over a plane circular surface, is widely used. The intensity reflected from a 10 /xm thick sheet of refractive index 1.85 (mylar) is shown in Fig. 5.6.4 for angles of incidence of 30° and 45°. The same flgure gives the product 4rt = 4r(l — r) for all conditions. Eor an ideal beamsplitter this product would be unity. Eor the TE... 

Fig. 5.6.4 Reflectivity (solid curve) and beamsplitter efficiency (dashed curve) of a 10 /xm thick sheet of Mylar operated at 45° and 30°. Both planes of polarization are shown for both cases.

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