Optical Shop Testing

Chapter 2.2 - Beam Splitter

2.2.   BEAM-SPLITTER

The beam-splitter can take the form of glass plate as shown in Figure 2.4 or a cube. It
is made in such a way that the splitting face reflects the light in appropriate amount by
means of a partially reflective coating. If it is a plate, the other face should not reflect
any light. To avoid reflections on the second face, a multilayer antireflection coating

FIGURE 2.4. Twyman Green interferometer with beam splitter (a) at 45 and (b) at the Brewster’s angle.

can be used. However, an easier solution is to place the plate at Brewster’s angle as
shown in Figure 2.4(b) and to use a source of light with p polarization, which goes
through the interface without any reflection. Still another solution is to introduce a
wedge angle on the plate so that the unwanted reflected light escapes from the
system. The contrast of the fringes does not depend on the reflectance R of the beam
splitter; only the irradiance I on the fringe maxima is affected, since

 

where I0 is the irradiance of the incident wavefront and T is the transmittance. If there
is no absorption (nonmetallic coating), T ≈ 1 - R and there is a maximum value of I
for R = T - 1/2.

If one of the two mirrors has a higher reflectance than the other, for example if one
of the them is a highly reflective mirror and the other is still uncoated, the intensity of
one of the two interfering beams can be about 25 times higher than the other. Under
these conditions, the fringe contrast is greatly reduced. A solution is to use a beam
splitter with a different reflectivity. Another possibility is to use both mirrors without
coating.

 

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