Effectiveness of a Quartz Rod as a Light Depolarizer

D. L. Portigal Sperry Rand Corporation, Univac Division, Philadelphia, Pennsylvania 19101. Received 25 November 1968.

It is often necessary to depolarize a light beam in an optical system. Translucent diffusers such as opal glass have been used for this purpose,1 but in many situations, use of such dif­fusers leads to unacceptable loss of intensity. Various depolar-

838 APPLIED OPTICS / Vol. 8, No. 4 / April 1969

Table I. Intensity Variation and Apparent Polarization Under Various Conditions λ = 435 mμ

Fig. 1. Schematic diagram of apparatus used to measure de­polarization of quartz rods.

izers involving birefringent material have also been used.2-4

F . Cerdeira et al. have reported5 the use of an ordinary quartz rod. However, these authors5 have not published results show­ing the effectiveness of such a depolarizer. By using such a rod to depolarize the output of a monochromator, we have been able to decrease the polarization by at least two orders of mag­nitude with minimal intensity loss.

The apparatus used in these measurements is shown in Fig. 1. A Hilger & Watts D-330 monochromator, with a 600 line/mm grating blazed at 300 mμ in first order is used. This has an angular aperture ƒ/5.8. The measurements were taken using 100-μ wide slits. The 1P28 detecting photomultiplier is mounted on the telescope of a Gaertner L-114 spectrometer. A diffuser was mounted in front of the photomultiplier to eliminate its polarization dependence. A quartz-iodine light source was used. The end of the quartz rod was placed at the collimator focus, and its image was focused just in front of the photomultiplier diffuser. The objective lenses on the spectrometer are 28 mm in diameter. When this full aperture was used, the angular aperture of the detector assembly was ƒ/8.9. Some measure­ments were taken with an 18-mm aperture. In that case, the angular aperture was reduced to ƒ/13.9. The quartz rods tested were commercial grade, nominally 5 mm in diameter. In Table I, we show, as a function of rod length and objective aperture, the maximum relative variation in light intensity as the polarizer is rotated as well as the apparent polarization, i.e., the difference between the intensities of horizontally and vertically polarized light, divided by the mean intensity. Measurements were taken at 435 mμ. Note that the total variation and the polarization increase as the angular aperture decreases. This is the result tha t we would expect if depolarization results from multiple internal reflection, since the number of reflections which a ray undergoes increases with the angle that it makes with the rod axis.

In Table I I , the maximum variation and apparent polarization are tabulated as a function of wavelength, with the optics aligned for minimum total variation at 435 mμ. A 0.6-m long rod and the full objective aperture were used in those measurements. A maximum variation of 0.5%, with a polarization of 0.2% was measured at 510 mμ with a 0.9-m rod. Better depolarization was obtained at 350 mμ by adjusting the optics for that wave­length and inserting a filter tha t rejects light whose wavelength is longer than 420 mμ.

The measurements of the polarization of the monochromator

Table I I . Wavelength Variation of Intensity Change and Apparent Polarization

output without the quartz rod are not as precise, since the measurements were made without a diffuser in front of the photomultiplier. Also, a low pressure mercury arc lamp, which has higher polarization than the quartz-iodine lamp, was used. However, the polarization dependence of the photomultiplier is only about 1%, and, at 400 mμ and 435 mμ, the polarization with and without a diffuser in front of the light source differed by 2 % or less. Taking these factors into account, we infer that the monochromator output polarization at these wavelengths was 19 rfc 1%. I t is somewhat larger at 510 mμ. At 365 mμ, the polarization was found to be 10% when the light from mercury lamp was depolarized, and 20% otherwise. We have also determined that the polarization of the quartz-iodine lamp was about one-third that of the mercury lamp. Taking this and the photomultiplier polarization into account we may infer that the monochromator output has a polarization of about 12% at 360 mμ.

The light losses were not explicitly measured. No appreciable variation with rod length was noted. A review of our data indicates an overall loss of about 50%. In contrast, opal glass reduced the output by three to four orders of magnitude

From the foregoing results, we see that the light polarization can be decreased by two orders of magnitude or more using a commercial grade quartz rod. This result is obtained with reasonable loss of light, over a wide wavelength range, with a low cost, readily available material.

References 1. A. C. Hardy and F . H. Perrin, The Principles of Optics

(McGraw-Hill Book Company, Inc., 1930), p . 605. 2. B. H. Billings, J. Op. Soc. Amer. 44, 66 (1951). 3. R. H. Hughes, Rev. Sci. Instrum. 31, 1156 (1960). 4. C. J. Peters, Appl. Opt. 12, 1502 (1964). 5. F . Cerdeira et al., Bull Amer. Phys. Soc. 12, 1049 (1967).

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