Beam splitter employing birefringent optics provides either widely different or precisely equal beam ratios, it can be used with laser light source systems for interferometry of lossy media, holography, scattering measurements, and precise beam ratio applications

Brown, R. M.

English

NASA Technical Reports Server

August 1971	 Brief 71-10265 
NASA TECH BRIEF 
Ames Research Center 	
_icv 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the. Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Variable Ratio Beam Splitter for Laser Applications 
A rugged, versatile, highly efficient, variable ratio 
beam splitter employing birefringent optics can 
provide either widely different or precisely equal 
Polarizer
intensity ratio is equal to tan 2 a, where a is the 
angle between the polarization plane and the prism's 
principal axis. As the plate rotates, this intensity 
Calcite/Glass	 Polarization (w/30 rad Dev. Angle) 
Monochromatic	 H a If—Wave	 Wretringent 
Beam	 Plate	 Prism 
Figure 1. Schematic of Birefringent Beam Splitter 
beam ratios and can be used with laser light source 
systems, for interferometry of lossy media, holog-
raphy, scattering measurements, and precise beam 
ratio applications. The device is similar to one pre-
viously described (see Reference). 
A Glan-Thompson polarizer linearly polarizes 
the incoming light beam (see Fig. 1) and passes it 
through a half-wave plate which can be rotated 
about the optical axis. The plane of the emerging 
light is rotated by twice the angle, 0, between 
the incident'light polarization plane and, the plate's 
principal axis. The light enters a birefringent prism 
(a Rochon prism, in this case) which divides it 
into two orthogonally polarized components whose
ratio continuously changes. For identical beam 
polarizations, a polarization rotator or a second 
half-wave plate can be added to one beam. 
Because the entering beam must be collimated 
and monochromatic, the beam splitter is most 
useful with laser sources. If the beams are already 
polarized, the device is very efficient because its 
only inherent losses are caused by surface reflections 
and intramaterial scattering. Also, the beam splitter 
can be constructed of air-spaced or optically con-
tacted components, without reflective films, making 
it highly resistant to high power density damage and 
possibly usable with Q-switched ruby lasers. 
The device was employed in a less-than-optimum 
(continued overlcztl( 
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arrangemet (see Fig. 2) to examine its practical 
usefulness. No polarizer was used, an SiO-over-
coated aluminum mirror was placed in the path, 
and only standard precautions were taken to ensure 
optical surface cleanness. A PIN photodiode detector, 
for measuring the light intensity of each beam, 
Rochon Prism 
Half—Wave 
Plate
plate was varied between 0 and r/2 rad. At the 
larger angle, almost all light was directed to the 
deflected beam. 
With the arrangement in Figure 2, a beam ratio 
range of from 4.4 x 10to 7.4 x 10, over seven 
decades, was available. With the arrangement in 
Figure 1, the range was extended to beyond 2 x 108. 
The ellipticity introduced into the laser beam by the 
mirror (Fig. 2) caused no appreciable difference be-
tween experimental results and expected values. In 
fact, for those applications involving mirror incidence 
Detector 
Aluminum 
Mirror	 Filter
Wedge Beam 
Splitter
Discarded Light 
Figure 2. Experimental Setup 
was inserted alternately in the two beams and ad-
justed for maximum signal. Because the intensity 
of the unattenuated laser light was too strong for the 
detector, an attenuator suitable for coherent light 
beam use was needed. 
A photographic neutral density filter was tried 
and rejected because of interference produced by 
transmission beam interaction with the scattered 
light. An interference filter with a nominal trans-
mission of 0.10 at 6328A was also rejected because 
of varying transmission across the surface, ap-
parently due to substrate thickness variations. 
A thin glass prism, with an apex angle of ir/90 
rad (2°), at the minimum deviation position was 
finally chosen. A 20 A bandwidth interference filter 
in front of the detector intercepted almost all back-
ground light, while the detector sampled the high 
intensity light resulting from the double reflections 
of the system. The half-wave plate was adjusted 
to achieve maximum extinction in the deflected 
beam, and that position was marked as 0 rad. The 
output beam intensities were measured while the 
angle of the linearly polarized light leaving the
angles that are non-varying as a function of time, 
the extra phase change may be considered a small, 
constant, optical path length difference which can 
often be ignored. 
Reference: 
Champagne, E. B., Ph.D.: dissertation, A Qualita-
tive Study of Holographic Imaging, The Ohio 
State University, 1967. Available through University 
Microfilms, Ann Arbor, 1968. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP71-10265 
Patent status: 
No patent action is contemplated by NASA. 
Source: R. M. Brown
Ames Research Center
(ARC-10391) 
Brief 71-10265	 Category 03


Variable ratio beam splitter for laser applications

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