Polarimeter and corresponding measuring method

Details Polarimeter and corresponding measuring method Polarimeter and corresponding measuring method

1999-10-07

2001-01-23

Font, Frank G. (Department: 2877)

Optics: measuring and testing

By polarized light examination

With polariscopes

C356S367000, C356S369000, C359S490020

Reexamination Certificate

active

06177995

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a polarimeter and a method for measuring polarisation states of a light beam.
2. Description of the Related Art
Various types of polarimeters enabling to measure all the polarisation components of light, i.e. the four components of the Stokes S vector of light, are known. Notably the article ‘Multichannel polarisation state detectors for time-resolved ellipsometry’ by R. M. A. AZZAM, Thin Solid Films, vol. 234, pp. 371-374, 1993, exhibits techniques consisting in separating a beam to be measured into several beams and in processing then these different beams in parallel. The number n of final beams is at least equal to four, so that the n intensities measured of the final beams enable to access the four states of polarisation of light, i.e. the four components I, Q, U, V of the Stokes S vector.
Thus, AZZAM has described in the article ‘Division-of-amplitude photopolarimeter (DOAP) for the simultaneous measurement of four Stokes parameters of light’, Optica Acta, Vol. 29, N°5, pp. 685-689, 1982, a division-of-amplitude polarimeter whose principle is as follows: the incident light beam is first of all separated into a reflected beam and a transmitted beam by a beam-separating plate, then into four using two Wallaston prisms.
Such polarimeters enable, thanks to information multiplexing, real-time measurement of the polarisation components of light. Moreover, they require neither modulation nor mobile portion. However, since the separating plate produces interference effects, the properties of the polarimeter highly depend on the wavelength of light. Spectroscopic applications are therefore out of the question and it is generally necessary to conduct monochromatic measurements.
Another shortcoming of this device is its high sensitivity to the angle of incidence of the beam.
SUMMARY OF THE INVENTION
The present invention relates to a division-of-amplitude type polarimeter, which can be used in a very wide spectral window.
The polarimeter according to the invention can also be little sensitive to the angle of incidence. The more so, it may enable to obtain a good level of measuring sensitivity and low propagation of relative errors onto measured intensities, hence good accuracy on the Stokes S vector.
The invention also relates to a method for measuring polarisation states of a light beam with the advantages mentioned above.
To this end, the invention relates to a polarimeter comprising:
an initial separator designed for separating an incident beam of light to be measured with a Stokes vector into a reflected beam and a transmitted beam, whereas the separator has a reflection coefficient R, a couple of ellipsometric angles in reflection (&psgr;
r
, &Dgr;
r
), a transmission coefficient T and a couple of ellipsometric angles in transmission (&psgr;
t
, &Dgr;
t
);
two final separators designed for separating respectively each of the reflected and transmitted beams into at least two final beams,
means of detection designed for measuring the intensities of the final beams, and
a processing unit linked to the means of detection, producing the Stokes vector of the light to be measured.
According to the invention, the initial separator comprises a prism that does not induce any interference effect and having properties, notably such a refraction index n
p
and a configuration that the transmitted beam undergoes at least one reflection internal to the prism according to an angle of internal reflection.
By ‘prism’ is meant a solid obtained by cutting a prismatic surface with two parallel planes, whereas the prismatic surface is by definition generated by a fixed direction straight line that moves while hugging the periphery of a plane polygon constantly. The base of the prism is delineated by this plane polygon.
Each internal reflection of the transmitted beam inside the prism generates elementary phase shift between the linear component of polarisation in the incidence plane (component p) and the linear component perpendicular to the incidence plane (component s). The sum of these elementary phase shifts is equal to the ellipsometric angle &Dgr;
t
.
According to a preferred embodiment of the polarimeter, the absence of interference effect during an internal reflection is obtained by ensuring that this reflection is total. This total reflection is produced thanks to the difference in the refraction index of the prism and of the air and thanks to the incidence of the beam transmitted onto the diopter of the prism on which internal reflection occurs.
According to another preferred embodiment of the polarimeter, the diopters of the prism on which internal reflections occur are covered with a thick absorbing layer. By ‘thick absorbing layer’ is meant a layer consisting of non-dielectric material having a thickness equal to several penetration lengths (the latter being defined as the reverse of absorption).
The final separators advantageously separate each of the reflected and transmitted beams into two final beams with different linear polarisations. The means of detection thus provide four measured intensities from which the processing unit calculates the components of the Stokes S vector. Preferably, the linear polarisations are orthogonal, whereby the final separators are Wallaston prisms. Moreover, the Wallaston prisms should be oriented by 45° with respect to the incidence plane. Other types of polarisers separating two orthogonal linear polarisations of a received beam are also quite suitable as final separators. Such elements enable to preserve wavelength independence.
The means of detection consist for instance of a set of photodetectors associated with the final beams respectively.
Unlike the division-of-amplitude polarimeter that uses a separating plate, the polarimeter according to the invention does not require any surface treatment producing interference effects and can, therefore, be very little sensitive to wavelength. Moreover, the absence of interference layer enables also to be relatively insensitive to the angle of incidence of the beam incident onto the initial separator. The influence of the angle of incidence is preferably reduced by appropriate selection of the refraction index n
p
of the prism, of its geometric properties and of the value of the angle of incidence.
The polarimeter according to the invention is also advantageously compact, which is quite important for in situ uses and need not integrate expensive optical components.
According to a first set of applications, the polarimeter is integrated to a conventional ellipsometer in order to improve the accuracy of measurements and to widen the scope of applications to depolarising media.
According to a second set of applications, the polarimeter is used as an independent component, for instance in astrophysics.
According to a third set of applications, the polarimeter is associated with a Mueller ellipsometer. It also enables to perform measurements on rough surfaces or particle systems, including in situ and in real time. Different ranges of uses include the treatment of various surfaces (steel and metals, . . . ), the detection of metallic objects (military applications, . . . ), the survey of anisotropic media (superconductors, . . . ), powder detection and characterisation (granulometry), environmental survey (aerosols, . . . ) and various medical applications.
The mathematical principles underlying the polarimeter are exposed below. They provide notably information useful for optimising the parameters of the polarimeter, in order to ensure low dependence on the angle of incidence, a good level of measuring sensitivity and/or little propagation of relative errors on the intensities when calculating the Stokes vector.
The number of measured intensities i
k
being equal to n (n≧4), the polarimeter is represented by a matrix A of dimension n×4, which verifies the relation (<<
T
>> designating the transposition operator):
(i
1
i
2
i
3
. . . i
n
)
T
=AS
In case when the number n of final beams is equal to 4

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