Retardance sweep polarimeter and method

Details Retardance sweep polarimeter and method Retardance sweep polarimeter and method

2002-08-20

2004-06-01

Stafira, Michael P. (Department: 2877)

Optics: measuring and testing

By polarized light examination

With polariscopes

Reexamination Certificate

active

06744509

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to polarimeters and, more specifically, to polarimeters based on liquid crystal variable retarders for determining the state of polarization of light incident thereon.
In applications ranging from astronomy to telecommunications, it is often desired to have knowledge of the state of polarization (SOP) of light. For example, astronomical applications include the utilization of polarization information of light received at a telescope as a tool for mapping solar magnetic fields. Chemical and pharmaceutical industries exploit the effect of enantiomerically enriched chiral compounds on the state of polarization for light passed through such compounds, i.e. optical activity. The state of polarization plays a significant role in telecommunications since polarization mode dispersion and polarization-dependent loss present considerable impediments to increased optical bandwidth. Furthermore, polarimetric measurements are used in a wide array of materials characterization, such as in quantification of thin film thickness and index and as a tool for mapping internal material strain via stress-induced birefringence.
The art and science of polarimetry is vast with a history that extends well over a century, and, accordingly, various mathematical descriptions of polarized light have long been established. For example, in the Stokes vector representation, the full SOP is characterized as a four element Stokes vector {overscore (S)}, which is defined as
S
_
≡
(
S
0
S
1
S
2
S
3
)
(
1
)
where
S
0
=Total light intensity,
S
1
=Intensity difference between horizontal and vertical linearly polarized components,
S
2
=Intensity difference between ±45° linearly polarized components and
S
3
=Intensity difference between right and left circularly polarized components
Other important and often utilized polarization parameters, such as the degree of polarization (DOP), degree of linear polarization (DOLP), degree of circular polarization (DOCP), ellipticity and orientation of major axis, are directly obtainable from the Stokes vector components. For example,
DOP
=
S
1
2
+
S
2
2
-
S
3
2
S
0
(
2
)
DOLP
=
S
1
2
+
S
2
2
S
0
(
3
)
DOCP
=
S
3
S
0
(
4
)
Turning now to the drawings, wherein like components are indicated by like reference numbers throughout the various figures where possible, attention is immediately directed to
FIG. 1
, which illustrates a Poincaré sphere
10
. The Pioncaré sphere is a commonly used graphical visualization aid for the SOP. As shown in
FIG. 1
, a Poincaré sphere
10
represents a mapping of all possible SOPs onto the surface of a sphere. A north pole
12
and a south pole
14
of Poincaré sphere
10
correspond to right and left circularly polarized light, respectively. An equator
15
corresponds to linearly polarized light. Arbitrarily chosen opposing points
16
and
18
along the equator represent horizontal and vertical linear polarizations, and opposing points
20
and
22
, which define a line orthogonal to the line defined by points
16
and
18
, represent +45° and −45° linear polarizations, respectively. In terms of the Stokes vector of Eq. (1), the bottom three components of the Stokes vector define a three-dimensional vector that points from the center of the Poincaré sphere to a point on the surface of the sphere.
A Stokes polarimeter is a device for determining the SOP of light incident thereon by measuring the components of the Stokes vector of Eq. (1). In terms of the Poincaré sphere, the Stokes polarimeter determines the components of the Stokes vector by measuring the projections along the orthogonal axes of the Poincaré sphere. For example, passing the light through a horizontal linear polarizer is equivalent to measuring the projection of the Stokes vector along the horizontal axis. As another example, for measurements of circular polarization components, a quarterwave plate can be utilized to convert circular polarization components into a linear polarization, from which a linear polarizer may then be used to determine the projection. In general, multiple measurements must be made in order to obtain all four components of the Stokes vector.
Currently available polarimeter technologies use polarization optics to extract the polarization information of input light, which is received at one or more detectors and converted to electrical signals. There are mainly four types of existing polarimeters, the basic configurations of which are illustrated in
FIGS. 2A-2D
.
FIG. 2A
illustrates a manually operated polarimeter
30
including an optical assembly
32
and a detector
39
. Optical assembly
32
includes a casing
33
, which contains passive optical elements (not shown) such as a polarizing element and an optical retarder. Casing
33
includes an opening
35
for accepting an input light
37
such that input light
37
is acted upon by the polarizing element and the optical retarder within optical assembly
32
, and at least a portion of input light
37
is transmitted through optical assembly
32
to be detected by a detector
38
. During normal operation, the user of polarimeter
30
manually rotates and flips optical assembly
32
to obtain data at detector
38
. Optical assembly
32
is usually configured to have at least four measurement positions, and the data obtained at detector
38
is analyzed by a computer
39
to convert the four measurements into Stokes parameters. A device based on the design as shown in
FIG. 2A
is available from Optics for Research, for example, and such a design has been described in the literature.
1
A manually operated polarimeter such as polarimeter
30
is limited in that a relatively long time (i.e., several seconds) is required to take the full set of measurements. As a result, the calculated parameters are susceptible to inaccuracies due to power and polarization fluctuations in the input light. Also, since passive, static optical elements are used in the optical assembly, the wavelength range is limited due to the effective range of the optical elements.
Another prior art polarimeter is shown in
FIG. 2B. A
polarimeter
40
of
FIG. 2B
is a “division of aperture” or “division of amplitude” type polarimeter. Polarimeter
40
includes a beam expander
42
, a collimator
43
, a Stokes filter array
45
, which includes at least four filters
46
, and a detector array
47
, in which a plurality of detectors
48
are aranged to detect light transmitted through each of filters
46
. Input light
37
is expanded by beam expander
42
then collimated by collimator
43
to be incident on Stokes filter array
45
. Each filter
46
is configured to be preferentially sensitive to different polarizations such that at least four simultaneous measurements may be taken to obtain the complete Stokes vector. Polarimeters based on the design shown in
FIG. 2B
are commercially available from companies such as A flash Corporation, Gaertner Scientific, Santec and General Photonics. Various modifications of polarimeter
40
are disclosed in the literature, such as the “photopolarimeter” which uses non-normal illumination of four detectors arranged in a non-planar configuration.
2
Polarimeter
40
is advantageous in comparison to polarimeter
30
of
FIG. 2A
due to the high speed in which data may be acquired, limited only by the detector speed. However, polarimeter
40
is still limited in the useful wavelength range due to the wavelength-dependence of the Stokes filters, and the need for a plurality of balanced detectors adds to the total cost of the system. Also, polarimeter
40
is extremely sensitive to the angle of incidence of the input light. In order to overcome this incidence angle sensitivity, other researchers have suggested various configurations in which light propagation into the polarimeter is confined by the use of optical fibers.
3-9
Yet, the use of fibers adds to the complexity and cost of the polarimeter while further limiting useful optical bandwidth, and therefore is not desirable in many applicat

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