Patent Application
20110149282
The present invention relates generally to polarimetry and, more particularly, concerns a multiple wavelength polarimetry method and apparatus which achieve high resolution measurement, while avoiding errors due to stray magnetic fields or stray reflections
Polarimetry is the measurement and interpretation of the polarization of transverse waves, such as radio or light waves. Typically polarimetry is done on electromagnetic waves that have traveled through or have been reflected, refracted, or diffracted by some material in order to characterize that material. Polarimetry can be used to measure various optical properties of a material, including linear birefringence, circular birefringence (also known as optical rotation or optical rotary dispersion), linear dichroism, circular dichroism and scattering.
A polarimeter is the basic scientific instrument used to make these measurements. To measure these various properties, there have been many designs of polarimeters. Typical polarimeters are based on arrangements of polarizing filters, wave plates or other devices.
The light beam emanating from sample cell 4 then passes through an analyzer 5, which has a rotatable polarizer 5a driven by a motor 8 under control of a controller 9. The output of analyzer 5 is provided to a phase sensitive detector 6, which also receives a signal from generator 7, and it is therefore responsive to the received beam to produce a signal representing the difference in beam polarization direction between light emanating from analyzer 5 and light emanating from sample cell 4. Phase sensitive detector 6 produces a signal representing the rotation difference which is provided to controller 9.
More specifically, the intensity of the beam arriving at detector 6 is proportional to the cosine squared of the angle between the polarization directions of the beam exiting from cell 4 and the beam emanating from analyzer 5. An analysis of this intensity variation with time determines the direction of the minimum angle separating the analyzer polarization direction and the sample cell polarization direction. By design, that angle is maintained sufficiently small relative to the amplitude of the oscillating polarization produced by the Faraday cell 3 to permit determination of the magnitude of the minimum angle separating the analyzer beam polarization and the sample cell beam polarization. This direction and magnitude information is used in controller 9 to rotate the polarizer in analyzer 5 so as to “null” the component of rotation of the polarization induced by the sample being measured (sample cell 4). That is, motor 8 and controller 9 are in a feedback loop which adjusts the polarizer 5a in analyzer 5 so as to just compensate the polarization introduced by sample cell 4.
In practice, before use, the device of
In its simplest form, Faraday cell 3 would be a transparent rod made of a suitable material which is oriented axially to the direction of the light beam. A coil driven by signal generator 7 is wound helically over the rod. Signal generator 7 would typically provide an oscillating current of sufficient strength to produce the desired effect within the rod. As an example, a faraday cell for use with visible light could be made with a rod made of dense flint glass. The coil wound over the rod would preferably include 1000 to 3000 windings, and signal generator 7 would produce a current in the range of one ampere at a frequency in the range of 10 to 100 Hertz. The resulting rotation of the plane of polarization by the Faraday cell would have an amplitude of a few degrees over the visible wavelengths.
In polarimeters of the type just described, the resolution of angular polarization measurement that can be achieved is typically limited by imperfections in the mechanics of the rotatable analyzer. For example, bearing roughness, backlash, static friction, lubrication issues, as well as the resolution and linearity of available encoders to operate the drive motor limit the practical resolution to the 1 millidegree range.
In an effort to avoid the limitations of mechanically based polarimeters, such as the one illustrated in
The essential difference between the polarimeter of
By limiting the measurable range of angular polarization introduced by sample cell 4, the art has been able to further simplify the structure of a polarimeter. Specifically, if one is willing to accept measurable polarization which is on the order of the same, or less, of the rotation as produced by Faraday cell 3, a third type of prior art polarimeter becomes possible. This type of polarimeter it is represented by the schematic block diagram of
Fixed analyzer 5 is then set up to extinguish the mean plane of polarization when no sample is present. When a sample is present, Fourier analysis of the intensity variation of light presented to phase sensitive detector 6 determines the sign and magnitude of polarization rotation introduced by the sample to be measured. In particular, if the current in Faraday cell 3 is a pure sinusoid of a given frequency, then the sign and magnitude of the optical polarization rotation introduced in the sample is determined by the Fourier coefficients of the fundamental and harmonic of the given frequency. Unlike the first two systems, this is not a nulling arrangement. The result is determined by analysis of the system operating in an unbalanced or non-nulled state.
One limitation of polarimeters of the first type is the mechanical accuracy of the motor.
Another limitation on the performance of prior art polarimeters of the second and third types is related to stray magnetic fields from the Faraday cell permeating the sample cell area. Such fields may either interact directly with the sample to be measured by inducing additional optical rotation, or residual magnetism and the materials of the sample cell may cause the Faraday cell to exhibit nonlinearity in the relationship between coil current and optical rotation. These fields are difficult to shield, and the size constraints of a benchtop instrument typically do not allow the sample cell to be sufficiently distant, from the Faraday cell. Although mechanical polarimeters also utilize a Faraday cell, they are not affected similarly, because the angular position of the null is not a function of the exact amplitude of the oscillating polarization.
Yet another limitation on the performance of prior art polarimeters of the second and third type is related to stray, or ghosts, reflections from optical surfaces in the path. Such reflections cause a portion of the light to make multiple passes through the Faraday cell. Inasmuch as the direction of rotation in the Faraday cell depends on the direction of the light propagation relative to the magnetic field direction, each pass accumulates additional rotation of the plane of polarization, increasing the amount of rotation induced for a given current. When these reflections are caused by the surfaces of movable elements, such as the windows of removable sample cells or calibration plates, the total induced rotation for a given current becomes unpredictable. If the instrument is designed to operate at a single wavelength, the resultant error can be reduced by using anti-reflection coatings optimized for that wavelength on the critical surfaces. However, modern laboratory instruments operate over multiple wavelengths, and the compromised performance of broadband antireflection coatings is not sufficient to eliminate this source of error.
There is therefore need in the art for a method and apparatus to achieve multiple wavelength polarimetry with high measurement resolution, without suffering errors due to stray magnetic fields or stray reflections or mechanical imperfections associated with moving elements.
The present invention relates to polarimeters of the type in which light polarization rotating properties of a sample are measured by interposing the sample in the path of a light beam having base plane polarization in a plane of known orientation; along the beam path, compensating or nulling the rotation introduced by the sample; and determining the optical rotational properties of the sample based on the amount of compensation introduced to the light beam. In accordance with one aspect of the present invention, the light beam is subjected to plural compensations along its path, the compensations being of at least two different types. Preferably, one of the types of compensation is mechanical, introduced through a device in which polarization rotation is adjusted mechanically, and the second type of compensation is provided through a device in which polarization rotation is controlled electrically.
In accordance with another aspect of the invention, a first polarization rotation compensation is performed with the sample in the beam path, the sample is removed, and compensation is restored by performing a second polarization rotation compensation, the second compensation being used to determine the polarization rotation introduced by the sample. Preferably, the first compensation is of a type which will not introduce disturbances in the sample that can interfere with accuracy of rotation compensation, while the second type of compensation is of a type that may introduce such disturbances.
In a preferred embodiment, a Faraday cell in the beam path provides electrically controlled polarization rotation compensation, and a mechanically operated polarizer in the beam path provides mechanically controlled polarization rotation compensation. The mechanical compensation is performed with the sample present and compensation via the Faraday cell is performed after the sample is removed.
The foregoing brief description and further objects, features and advantages of the present invention will be understood more completely from the following detailed description of presently preferred, but nonetheless illustrative, embodiments in accordance with the present invention, with reference being had to the accompanying drawings, in which:
Phase sensitive detector 6 is responsive to the received light beam, to produce a signal, presented to a controller 9, which signal represents the difference in beam polarization direction between light emanating from analyzer 5 and light emanating from sample cell 4. As was the case in
In operation, the signal from signal generator 7 effectively passes through summing amplifier 12 to the winding of Faraday cell 3. As was the case in
Wavelength selector 13 is preferably a motorized monochrometer, or filter wheel, provided to isolate the wavelength of interest. Controller 9 acts of wavelength selector 13 to set it to the appropriate wavelength. Certain wavelengths may be totally absorbed by a sample, so it is necessary to use a different wavelength. Also the wavelength may be chosen to maximize the rotation induced by the sample.
As already explained above, the intensity of the beam arriving at the phase sensitive detector 6 is proportional to the cosine squared of the angle between the beam polarization direction of the analyzer 5 and the beam polarization direction of light emanating from sample cell 4. Fourier analysis of the intensity variation with time determines the direction and magnitude of the minimum angle separating sample cell polarization and analyzer polarization directions. This direction and magnitude information is used by controller 9 to rotate polarizer 5a of analyzer 5 or to determine a DC level to be delivered to summing amplifier 12 to null rotation of polarization induced by the sample to be measured. The adjustment required to achieve nulling is indicative of the polarization rotation induced by the sample.
It should be noted that it is only after the sample to be measured is removed from the optical path that the final adjustment is made to the Faraday cell to null the system. This eliminates the three undesirable interactions between the sample and the Faraday cell: additional rotation induced from the sample by stray magnetic fields, disturbance of the Faraday cell residual magnetism of the sample cell materials, and stray or ghost reflections from the optical surfaces of the sample cell.
Although preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the will appreciate that many additions, modifications, and substitutions are possible, without departing from the scope and spirit of the invention as defined by the accompanying claims.
