Beam dividers

Beam dividers, or beamsplitters, which divide reflected and transmitted waves nearly equally, are essential components in Michelson interferometers, but are also used in other instruments. Since they are based on thin film technology we discuss such dividers in this section. The solution for a two-interface (three-layer) stack, Eq. (5.6.15), is squared to derive the reflected intensity. After a straightforward, although tedious, calculation, one finds 

The solutions for the TE and TM waves are obtained by substituting the values for g as defined by Eqs. (5.6.5) and (5.6.7), respectively. With the assumption that the relative permeability of all layers is unity, the g values for both polarizations become 

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 

One may ask what would be the optimum refractive index of a monolayer to be used as a beamsplitter To answer this question we solve Eq. (5.6.21) for different values of i for the maximum reflectivity, cos Anvd n cosfix) = — 1. Again, both TE and TM values according to Eq. (5.6.22) have been used (Fig. 5.6.5). The optimum value of i for 30° and the TE wave is just above 2 and for the TM wave about 2.8. Overall i 2.4 would be the best compromise for optimum conditions at the peak. However, it is desirable to use a larger n i if a broader wavenumber range is to be covered. For example, with = 3 one obtains double maxima in the Art curves, as shown in Fig. 5.6.6. A transparent material of refractive index 3 would be an attractive beam divider over a substantial wavenumber range unfortunately 

In the middle and near infrared it is common to constmct beamsplitters by vacuum deposition of a thin film on transparent substrates. To analyze such a case we consider a 0.5/rm thick germanium ( = 4) layer on a potassium bromide (n = 1.5) substrate. Again, Eqs. (5.6.21) and (5.6.22) are solved for a beamsplitter at 45° the result is shown in Fig. 5.6.7. The substrate cut-off below 400 cm is omitted. Such a beamsplitter may be used between 400 and 2100 cm it shows an excellent efficiency (high 4r t) for the TM wave between 700 and 1800 cm, but considerably lower values for the TE wave over the same range. Overall, the average efficiency for unpolarized radiation is about 0.83 near 1300 cm but as high as 0.92 near 600 and 1900 cm Again, better performance may be obtained with additional layers. 

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In order to achieve better performance for the Michelson interferometer, metal meshes and wire grids have also been used as beam dividers. An especially interesting solution of this problem is the polarizing interferometer as developed by... 

FIGURE 11 Optical interconnects (a) beam divider (3 dB if x=50), (b) beam deflector, (c) blazed grating, (d) array generator, (e) waveguide interconnect, and (f) substrate interconnect. 

The BS has the function of transmitting half of the incident infrared beam and reflecting the remaining half. The beams divided by BS are directed into the two arms of the interferometer, reflected back by Mj and M2, and join again at BS. Then both of them travel toward the detector (Detector). 

The spin of the electron was measured experimentally by Stem and Gerlach. A beam of silver atoms was passed through a strong inhomogeneous magnetic field. It was found that the beam divided sharply into two beams. One beam was deflected as though each atom were a... 

Beam splitter (optics) An optical filter or reflector that reflects some of the incident radiation and transmits the rest. Also called a Beam divider. 

The importance of these materials goes beyond that of the design of broad-band transmission filters. The very same substances are also used in the constmction of interference filters, prisms, Fabry-Perot etalons, beam dividers, dichroic mirrors, and sometimes as windows to seal parts of an instrument while permitting radiation to pass. 

In Subsection 5.6.a we review the theory of multilayer thin films. In Subsection 5.6.b this theory is applied to the design of antireflection coatings for infrared windows, lenses, and other components. The same theory is used again in Section 5.6.C to find suitable beam dividers of the free-standing, self-supporting type as well as of the type requiring a transparent substrate. Subsection 5.6.d deals with interference filters and Fabry-Perot interferometers. 

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