Optics: measuring and testing – By polarized light examination – With birefringent element
Reexamination Certificate
2002-05-14
2004-11-09
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With birefringent element
C356S364000, C356S369000
Reexamination Certificate
active
06816261
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to devices and methods for detecting, quantifying, indicating or otherwise responding to the polarization characteristics of an input electromagnetic signal, specifically a light input signal. The invention also relates to measurement of polarization aspects of light such as degree of polarization, extinction ratio measurements and one or two dimensional polarization mapping.
Multiple measurements are taken in conjunction with filters and/or waveplates that process light or selectively pass portions of the input signal. These measurements discriminate among light signals having distinct polarization states or polarization components, by taking measurements at different relative phase relations and/or different orientations in space or time. According to an inventive aspect, the measurements are taken using certain tunable elements, arranged to provide the necessary diversity of measurement states to resolve polarization information and optionally wavelength information. Moreover, in addition to enabling the measurements, this technique facilitates calibration that can be automated and employed without external reference signals, and allows controlled optimization of the measurement states used, to obtain the greatest accuracy of which the device is capable.
According to an inventive aspect, a novel calibration technique is provided. The calibration comprises comparison of input signals having one or more polarization attributes that are known to be related in a way that can be checked mathematically. A device transfer function containing a matrix of factors is adjusted so that the measurement sets obtained in this way, achieve values that prove true according to the known values or relationships.
In a preferred arrangement, the calibration, optimization and measurement aspects of the invention are under control of a processor that operates tunable elements in the input signal path(s) to selectively control optimization and calibration conditions. Preferably, controllable birefringence elements, optionally including narrow band wavelength bandpass filters, are tuned for selective optimization, or optimization within certain conditions or for selected wavelengths.
The input for the calibration process can be one or more arbitrary quasi-monochromatic input signals with diverse polarization components. The polarization characteristics of the signals are measured through a matrix of scaling or similar factors representing the device matrix, i.e., the factors defining the transfer function of the measurement signal path. Along one or more signal paths, at least one polarization transformation occurs. However, the at least one attribute that can be checked mathematically, as described above, remains true before and after the transformation. Thus the transformed and un-transformed signals provide distinct measurement sets that when checked should still prove true. Calibration of the device comprises adjusting the matrix of factors representing the device matrix, if necessary, so that the at least one attribute as measured is made to be true.
A plurality of different measurements are taken on signals or transformed versions of signals, of which any two or more signals or versions share at least one attribute that can be checked as true. Preferably, twelve or more polarization transformations are applied to one given input signal. Measurements are taken for each one. An iterative process is then accomplished as described herein, for homing in on a correctly calibrated matrix of coefficients or factors of the characteristic instrument matrix that accurately define the response characteristics of the polarization measurement system, i.e., the calibrated response to any arbitrary input. This technique permits calibration without the need for any calibrated reference input (although the input needs to have diverse polarization components to fully exercise the measurement signal paths).
Attributes which can be checked, and which can be the attribute(s) checked for true to test and correct calibration, might be any of various attributes and/or relationships between attributes that should remain true in a calibrated unit. Exemplary polarization transformations might comprise differential phase delay through a waveplate or reorientation or the like. Examples of attributes that are not changed by a particular transformation could include the degree of linear polarization (independent of the axis of orientation), the Stokes S
3
variable value, or other attributes as discussed herein. Whether a given attribute or relationship remains true after a polarization transformation depends on the nature of the transformation, in a known manner.
Concepts employed with respect to calibration are further employed according to the invention to optimize the measurements that are taken. This is advantageous according to the invention because different transformations as discussed above regarding calibration steps, can be made tunably and/or automatically selectable, and thus can be chosen to arrive at optimal sets of measurements capable of obtaining the greatest distinctions between measurement values obtained for light inputs of different polarization states. Such selections are used to choose the most spatially- or temporally-separated instrument states that are available and that exercise the largest available scale of measurement of the device. A theoretical explanation of the physical implications of factors in a Muller Matrix, in conjunction with the Jones reciprocal matrix, is provided herein as an aid to understanding.
2. Prior Art
The polarization state of an electromagnetic wave such as a light wave can be quantified uniquely by reference to four Stokes parameters. The four values of the Stokes parameters make up a Stokes vector.
The Stokes vector can have four values, S
0
, S
1
, S
2
, S
3
, which encode intensity as well as the distribution of the intensity among components of different relative orientation and phase. It is frequently helpful to ignore absolute intensity and to consider only polarization. For that purpose, three Stokes values, S
1
, S
2
, S
3
, are considered. Assuming that the intensity is a constant, the three Stokes values (which now encode the relative intensity as a function of orientation and phase) can be graphed to points on the surface of a sphere because they meet the mathematical definition of a sphere, S
1
2
+S
2
2
+S
3
2
=R
2
. For a nominal unit sphere, R=1. The values of S
1
, S
2
and S
3
can vary between −1.0 and +1.0. Based on the sum of the squares begin equal to one, however, if any of the Stokes values is equal to one, then the others must be zero (indicating a particular exclusive polarization state). The S
1
variable encodes between vertical and horizontal polarization orientations. The S
2
variable is associated with ±45°. The S
3
variable is associated with clockwise versus counterclockwise circular orientation (i.e., orthogonal component phase difference between ±90°).
For encoding Stokes values, the orientation of reference system used is relevant to whether the intensity falls into one of the S values or another. However, the values of a Stokes vector in one frame of reference are readily transformable to comparable values for the same light wave according to a different frame of reference.
In a polarization analysis unit, it is convenient to consider a frame of reference in which the Z axis is the propagation axis, an X-Y plane is perpendicular or normal to the Z axis, and optical elements such as polarizing filters and/or waveplates have some orientation relative to the X-Y-Z coordinate system. In a polarization analysis unit with plural optical elements (each having some orientation and position in the X-Y-Z coordinate system) and perhaps also plural optical paths, the relative position and alignment of the various elements affect the measurements that are obtained.
The polarization state of light can vary with wavelength.
Patel Jayantilal S.
Yeazell John A.
Zhuang Zhizhong
Duane Morris LLP
Optellios, Inc.
Smith Zandra V.
Valentin II Juan D
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