A division-of-amplitude photopolarimeter (DOAP) is described that employs a diffraction grating in the conventional spectrometer orientation with the grating grooves normal to the plane of incidence. Four coplanar diffracted orders are used for polarimetric analysis to determine all four Stokes parameters of incident light simultaneously and virtually instantaneously (with the speed being determined solely by the photodetectors and their associated electronics); a fifth order is used for alignment by autocollimation or by use of a positionsensing quadrant detector. To sensitize the instrument for the +45° and -45° azimuths of incident linearly polarized light and for the handedness of incident circular polarization (i.e., for the third and fourth Stokes parameters), we insert two linear polarizers in two diffracted orders with their transmission axes inclined at appropriate angles with respect to the plane of incidence. The calibration and testing of an instrument of this type that uses an Al-coated 600-groove/mm holographic grating at 632.8-nm wavelength are reported as an example.

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, "Achieving a given reflectance for unpolarized light by controlling the incidence angle and the thickness of a transparent thin film on an absorbing substrate: application to energy equipartition in the four-detector photopolarimeter," Appl. Opt. 31, 935-942 (1992) Achieving a given reflectance for unpolarized light by controlling the incidence angle and the thickness of a transparent thin film on an absorbing substrate: application to energy equipartition in the four-detector photopolarimeter R. M. A. Azzam and Kurt A. GiardinaAt a given wavelength X we determine all possible solution pairs (+, ) of the incidence angle + and the thickness t of a transparent thin film on an absorbing substrate that achieve a given unpolarized light reflectance Mu. The trajectory of the point that represents a solution pair in the , + plane depends on the optical properties of the film and substrate and on whether R. is greater than or less than the normal-incidence reflectance N of the bare substrate. When R. > No, the specified reflectance is achieved over a limited range of +. At the least possible incidence angle, the film thickness is = / 8 th wave. As an application we consider SiO, films on Si detectors that produce R. = 0.75, 0.6667, and 0.50 atA = 337 and 633 nm. If the first three detectors of the four-detector photopolarimeter (FDP) are coated to have these reflectance levels, with the reflectance diminishing in the direction of propagation of the light beam, and the last detector is antireflection coated (e.g., with a quarter-wave SiN 4 layer), equipartition of energy among the four detectors is accomplished for incident unpolarized light. Such a condition is desirable in the operation of the FDP. The ellipsometric parameters of the coated surfaces and the FDP instrument matrix are also calculated.

A new division-of-amplitude photopolarimeter1 with no moving parts or modulators is described that employs the conical diffraction of light from a grating which is set for oblique incidence with the grating lines inclined with respect to the plane of incidence, Fig. 1. The grating G splits the incident light beam, whose state of polarization is to be measured, into four or more beams that are intercepted by linear photodetectors Dm (m = 0, 1, 2, …). This produces four or more signals im that represent multiple projections of the unknown Stokes vector S. S is reconstructed from its projections by matrix inversion. We have built a prototype photopolarimeter of this type using an Al-coated, 600-grooves/mm, holographic grating set at 65° angle of incidence with the grooves oriented at 45° with respect to the plane of incidence, and a 633-nm He-Ne laser as the light source. This novel photopolarimeter has been calibrated and tested, with and without the linear polarizers Pm in front of the photodetectors, using the same procedures recently established for the four-detector photopolarimeter2,3 . This polarization-analysis scheme has several advantages and degrees of flexibility. In particular, it can be adapted using detector arrays so as to perform time-resolved spectropolarimetry.