Extinction ratio of germanium wedge-plate infrared polarizers

Narayan P. Murarka and Kalman Wilner IIT Research Institute, 10 West 35 Street, Chicago, Illi­nois 60616. Received 13 May 1981. 0003-6935/81/193275-02$00.50/0. © 1981 Optical Society of America.

Transmitting-type polarizers and analyzers for the far IR region of the optical spectrum can be constructed from plates mounted at the Brewster angle rather than Glan, Rochon, or Wollaston configurations. This constraint is caused mainly by the fact that at long wavelengths the required angle be­tween the crystal optical axis and the crystal cut exceeds the critical angle of total internal reflection.

The degree of polarization P of the transmitted light when the incident light impinges on a pile of plates mounted at the Brewster angle is given by1

where m = number of plates, n = index of refraction, and

Ip ,IS = intensities of parallel and perpendicular polar­ization components.

Equation (1) assumes no material absorption but takes into account multiple reflections inside the plates. From Eq. (1) it can be easily seen that for a given number of plates, the higher the index of refraction the larger the value of the degree of polarization for the transmitted light. When used with a high-power laser, for example, as part of an electrooptic modulator system, internal reflection in flat parallel plates may produce excessive heat causing deterioration of the de­gree of polarization that otherwise can be obtained at low-power levels.

Fig. 1. Polarizer/analyzer configuration.

To reduce the number of internal reflections, we designed two sets of three germanium Brewster angle plates for which two out of the three plates were constructed to have 0.5° wedge. The two wedged plates were mounted opposite to each other as shown in Fig. 1. The third plate was designed to have parallel surfaces. The deviation of a single plate having a 0.5° wedge is ~7° for light incident at the Brewster angle. Therefore, to obtain a beam with reasonable angular deviation, wedges were placed opposite to each other as shown in Fig. 1. It should be pointed out that for the test setup two identical sets of polarizers were constructed.

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Fig. 2. Experimental setup for low level test.

Fig. 3. Polarizer extinction ratio vs wavelength.

Figure 2 illustrates the experimental setup for the low-power level tests. It consists of a blackbody source which was a globar heated to ~1200°C. The radiation emitted by the source was then intensity modulated by a 13-Hz chopper. The light source was positioned at the focal point of an alu-minized mirror with a focal length of 26.92 cm. The colli-mated beam was then directed toward the device and refo-cused via a flat mirror and a focusing mirror to a Perkin-Elmer model 99 monochromator. The thermocouple detector out­put was fed to a Brower model 31 lock-in voltmeter synchro­nized with a chopper. This type of electronic setup enables us to experiment at very low levels of optical energy.

The first set of measurements consisted of measuring the front polarizer insertion loss which, when ignoring the ger­manium absorption, should be 50% for unpolarized light. The test results gave an average value of 51.2% for insertion loss. The monochromator spectral bandwidth resolution for these measurements was ~100° A. Inserting the analyzer into the system with its transmission axis parallel to the input polarizer introduced an additional loss of 8.3% resulting in an overall insertion loss of ~59.5%.

The extinction ratio of the polarizer/analyzer elements is defined as the ratio of light intensity when the front and back polarizers transmission axes are parallel to that when the

transmission axes are crossed perpendicular to each other. The measured extinction ratio at 3.8 μm was 898. Extinction ratio values from ~ 2 to 5.6 μm were also measured and the results are shown in Fig. 3. The figure shows some variation of the extinction ratio as a function of wavelength. This may perhaps be caused by a change in polarization of the light emitted by the globar source after its reflection from the Al-coated mirror, since it is possible that the radiation falling on the polarizer/analyzer unit may not be completely polarized randomly.

High values of the extinction ratio obtained in these ex­periments indicate the viability of a germanium wedge po­larizer configuration operating in the transmission mode.

References 1. W. G. Driscoll and W. Vaughan, Eds., Handbook of Optics

(McGraw-Hill, New York, 1978).

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