Polarization Sensitivity of the RCA 6903 Photocathode Tube

Stuart A. Hoenig and Albert Cutler III Department of Aerospace and Mechanical Engineering, The University of Arizona, Tucson, Arizona. Received 14 March 1966. This work was supported by State Funds and by the Depart­ment of Astronomy under contracts from ARPA and ONR.

Measurement of the polarization ellipse of weak light sources is difficult especially in astronomy where filters must often be used to eliminate unwanted light, i.e., sunlight. I t is advantageous to have a detector that is directly sensitive to the polarization of the incident light. This is possible with certain photoelectric detec­tors: the RCA 6903 was chosen as an example.

June 1966 / Vol. 5, No. 6 / APPLIED OPTICS 1091

Fig. 1. (a) Experimental setup. (b) Rotation system.

Fig. 2. (a) Normalized photocurrents. (b) Normalized current vs polarization.

The setup of the optical system is shown in Fig. 1(a) and the results are shown in Fig. 2(a) and (b). In Fig. 2(a) the normal­ized currents Iθ=0, Iθ=90 are plotted vs the tube angle. (The subscript θ = 90° represents polarization perpendicular to the plane of incidence.) I t is clear from Fig. 2(a) that, as increases, the difference between I0 and I90 increases up to = 70°. Above

= 70° the size of the light spot on the tube was so large that the data became somewhat uncertain. For the present study, the angle = 70° was chosen to provide the highest sensitivity and best signal-to-noise ratio.

It is interesting that even at = 70° the absolute magnitude of I0 is not much less (20%) than that observed at = 0°. This is understandable if we note that the 6903 cathode is semitrans-parent and that at = 0° the light strikes the photocathode only once, while at » 0° multiple reflections occur. This effect has been exploited recently in order to increase the sensitivity of a tube of this type.1

Choosing = 70° as the optimum setting, values of I/I90 are plotted vs θ in Fig. 2(b). The curve shown in Fig. 2(b) is quite reproducible. In a series of runs where θ was varied from 0° to 90°, points were reproduced with an average scatter of 1.5% of the reading itself. The maximum scatter recorded, from one run to another, several hours later, was 3.5%.

To check on the optical alignment, the prism was generally ro­tated through 360°, thereby producing four curves similar to that in Fig. 2(b). In all cases the curves were quite symmetrical and displayed no unusual peaks or valleys.

In order to use the 6903 tube as a polarization detector, we note that over most of the range 10° < θ < 75° the change in current with angle (1/I90)(d/dθ)(I) is constant: (1/I90)(d/dθ)(I) = — 0.01. Now if we consider that a change in θ of 5° will change I/I90 by 5%, it is clear that θ can be measured to within about 5° by this method.

For use as an astronomical polarization detector, the 6903 tube would probably be rotated on a mount, as shown in Fig. 1(b). Using this mount, the tube could be rotated through approxi­mately 180° about the optical axis. The resultant photocurrent output would be a direct measure of the polarization of the inci­dent light. The system would have the advantage that no light is lost because of filters or polarizers in the optical detector system.

There is evidence that other types of optical detectors are polar­ization sensitive.2 However, in Ref. 2 no attempt was made to exploit the effect for purposes of measurement.

The assistance of Ira Clough in the construction of the tube mount system is gratefully acknowledged.

References 1. W. D. Gunter, Jr., E. F. Erickson, and G. R. Grant, Appl.

Opt. 4, 512 (1965). 2. A. Smith, Rev. Sci. Instr. 1, 433 (1936).

1092 APPLIED OPTICS / Vol. 5, No. 6 / June 1966