Polarization Patent - Ellipsometer for determining the polarization state of monochromatic light
ellipsometer is designed for determining the
Background of the Invention:
present invention relates to an ellipsometer, a device that senses the
inventor is aware of these publications describing such techniques. The
first is US4158506 and the second is DE3931540. The techniques disclosed
in the prior art evolved for laser sources. Similarly, the extensive use
of waveplates as well as optical detectors which are not of significant
importance for discriminating purposes. Furthermore, the Stokes parameters
acquired form these elements are not sufficient to clearly distinguish the
Summary of the Invention:
the subject invention is directed to an effective, inexpensive and precise
way to define the effect of a clean, well-conductive smooth surface on the
polarization of monochromatic light using ellipsometry techniques.
the source of light used in this device is an LED designed to give a pulse
of light when required. Previous work, assume continuous light sources or
laser beams. The first is a slow technique and unnecessary, while the
latter is of very short pulse width that accuracy and information are
compromised. Therefore, a pulsating LED with a controlled pulse width is
the optimum solution with high accuracy.
the parameters of an optical pulse involves an array of four
photodetectors. A polarizer, each with a different orientation, covers
each photodetector. This configuration is ideal for obtaining the
The electric signals acquired from the photodetectors are used in conjunction with a microprocessor to produce information about the state of polarization. These calculations involve Stokes parameters, however, further calculations and derivations (e.g. Linear polarization) were required to obtain a clear and significant discriminator on the effect/change materials inflict on the polarization state of light, particularly, the effect of subclasses of certain materials on the orientation/state of polarized light.
Description of Figures:
Side view of polarization sensor
Figure 2: Orientation of polarization filters on the four photodetectors (top view of photodetectors).
Description of the Embodiment:
operational principle of the device representing the invention is
described with reference to figure
1. Each element of the four-element photodetector array is
connected to a microprocessor, where a program is designed to acquire the
values from the four elements simultaneously. These values then undergo
signal conditioning and calculation to obtain all the parameters of the
The principle of getting the polarization parameters is described with reference to figure 2. To capture the intensities for the polarization state parameters calculations, the photodetectors are covered with polarizers oriented in the following directions:
section gives a mathematical description of the calculation of the degree
and orientation of the polarization of a beam, based on the observed
intensities. Each target exposure measures the component of the incoming
light polarized in two different orthogonal directions. The symbol Iα
is used to represent the intensity of the component polarized at an angle α
to the reference direction, then in each exposure the O
ray image records Iα
and the E
ray image records Iα+90.
Consider a beam of light in an arbitrary state of polarization that is analyzed by four different types of polarizers. The four transmitted intensities are given by I0, I90, I45, and I135 where:
α = 0 → I0 and I90
= 45 → I45
Ip and Iu
are the polarized and unpolarized intensities of the incoming light, and
θ is the angle between the plane of polarization and the reference
1852 George Stokes proposed the use of a vector which contains only four
observable quantities in order to describe the
these definitions the following can be derived,
Where, Ip is the polarized intensity and θ is the orientation of the plane of polarization.
We found there is no need to implement two more photodetectors and waveplates to acquire the value of the last Stokes parameters (V), since it can be derived from the previous equations by substituting the acquired values to obtain ε as follows:
We found that the Stokes parameters are not enough to distinguish between subclasses of materials where the differences are small, therefore, further calculations are required:
Pl is the
Pc is the circular polarization
Pe is the elliptic polarization
Pd is the degree of polarization
find clearly that the effect of the material is on the orientation of the
elliptically polarized beam, as linear polarization increases, circular
polarization decreases, and vise versa for an elliptically polarized beam.
1. 1. Optical constants depend on chemical composition and therefore are unique to every type of material. For industrial applications, Hohner research team developed a sensor using the polarization of light to distinguish between materials and their sub-categories.
2. 2. The system as defined by claim 1 herein comprises of an ellipsometer with no moving parts, consisting of an LED light source and four photodetectors, each covered by an independently oriented fixed polarizers.
system as defined by claim 2 herein comprises of four fixed photodetectors
and polarizers placed in a way that the
4. 4. The system as defined by claim 3 herein comprises of four polarizer elements having mutually orthogonal angles of transmission in one orientation ̃ 0° and 90°, while the second orientation for the other two polarizer elements having mutually orthogonal angles of transmission at ̃ 45° and -45° — refer to figure 2.
5. 5. The system as defined by claim 4 herein does not require waveplates for calculating parameters of polarization, on the contrary, software application is to compensate and mathematical computations are the substitute.
6. 6. The system as defined by claim 5 herein comprises signal conditioning and amplifiers to obtain higher accuracy.
7. 7. The system as defined by claim 1 herein comprises a microprocessor to simultaneously capture the various intensity values from the photodetectors and compute/calculate all required parameters giving results in msec.
8. 8. The system as defined by claim 7 herein comprises computation based on Malus’ law and Stokes parameters from the intensities acquired from the photodetectors. Where I0, I90, I45, and I135 are obtained: the first Stokes parameter (I) is computed as I = I0 + I90, the second Stokes parameter (Q) is computed as Q = I0 - I90, the third Stokes parameter (U) is computed as U = I45 – I135, and the last Stokes parameter (V) is computed through substitution in the above equations to obtain e and then compute V as V = sin2e.
system as defined by claim 7 and 8 herein requires further computation to
clearly distinguish the effect of materials on the
10. The system as defined by claim 1, 7, 8 and 9 herein will output a single value from claim 9 to distinguish and discriminate clearly and accurately between materials and their sub-categories.