Method and system for measuring optical properties of an object - Patents.com

Abstract

The present invention relates to a method and system for measuring optical properties of an object, in particular for measuring the birefringence of the object, the method comprising the steps of directing a uniformly polarized measurement beam at the object, automatically rotating the polarization state of the measurement beam with a hologram, detecting, for each polarization state of the measurement beam, a polarization characteristic of the measurement beam that does not interact with the object and a polarization characteristic of the measurement beam after interacting with the object, and determining a measurement of the birefringence of the object using, for each polarization state, the detected polarization characteristic of the measurement beam that does not interact with the object and the detected polarization characteristic of the measurement beam after interacting with the object.

Description

[Technical field]

[0001] The present invention relates to a method and system for measuring the optical properties of an object, in particular for measuring the birefringence of an object. [Background technology]

[0002] A variety of instruments and techniques are used to determine the polarization properties of materials. Early polariscopes used reflections off transparent materials to generate polarized light that was then applied to the sample as a probe and analyzed by another polarizer of the same type but positioned orthogonally. Any incoming light was blocked unless its polarization changed. In this way, by observing the analyzer, it was possible to see if the sample caused any change in the polarization of the light and where this change occurred. These early devices were improved with the discovery of the polarization properties of other materials such as tourmaline and agate and quickly found industrial application.

[0003] The invention of instruments such as the Nicol prism, Wollaston prism, and Glan-Thompson prism improved the quality of observations by providing efficient, high-quality sources of polarized light and polarization filtering. The creation of lasers and phase retarders such as half-wave plates allowed greater control over the polarization of light, allowing more precise measurements. The high coherence of lasers made them the perfect tool for precision measurement techniques such as phase-shifting interferometry.

[0004] The original design of the polarizer was similar, but the discovery of effects such as the photoelastic effect and liquid crystal birefringence improved both the accuracy and controllability of the measurement process.

[0005] Polariscopes have historically been used in optical metrology, the sugar industry, and chemical analysis. Recent applications include biology, crystallography, and local stress measurements in materials. Their importance lies not only in the measurement of birefringence, but also in the spatial distribution of birefringence and the principal axes of optical rotation.

[0006] Birefringence has historically been known to be caused by mechanical stress, thermal excitation, Kerr medium excitation and the Faraday effect. One of the effects of birefringence on light is a change in the state of polarization. When the birefringence is circular (between right-handed and left-handed circularly polarized light), the observed effect is often called optical activity and / or optical rotation. The effect observed in light is a rotation of the state of polarization around the axis of the Poincaré sphere, where the axis of rotation is the same as the axis that has birefringence. In materials analysis, circular birefringence is related to chirality in media such as crystallite twisting in crystals, fibers, thin films and banded spherulites.

[0007] Some conventional methodologies for measuring birefringence utilize phase shifting techniques implemented by mechanical measurement systems. However, these methodologies and / or systems are often time consuming to implement and require a high degree of precision and calibration in their setup to achieve accurate measurements of birefringence. In this regard, one of the objectives of the present invention is to provide a means for measuring at least the birefringence of a material in a setup that is compact, fast, and requires little calibration and little requirement for mechanical moving parts. Summary of the Invention

[0008] According to a first aspect of the present invention there is provided a method for measuring an optical property of an object, the method comprising the steps of: directing a uniformly polarized measurement beam at the object; rotating the polarization state of the measurement beam with the hologram; detecting, for each polarization state of the measurement beam, a polarization characteristic of the measurement beam not interacting with the object and a polarization characteristic of the measurement beam after interacting with the object; For each polarization state, determining a measurement of at least one optical property of the object using the detected polarization properties of the measurement beam that does not interact with the object and the detected polarization properties of the measurement beam after interacting with the object.

[0009] The method may include generating a uniformly polarized beam.

[0010] The measurement beam that does not interact with the object may also be referred to as a reference beam. The reference beam may be identical to the measurement beam or may be a separate beam derived from the measurement beam. In any case, the reference beam, and in particular its polarization properties, may substantially correspond / be equal / identical to the polarization properties of the measurement beam before interaction with the object. In other words, detecting the polarization properties of the reference beam substantially corresponds to detecting the polarization properties of the measurement beam that does not interact with the object.

[0011] The method may include deriving and / or obtaining a reference beam from the measurement beam. In a preferred embodiment, the method may include splitting the uniformly polarized beam into two identical beams, one of which is the measurement beam directed towards the object and the other of which is the reference beam.

[0012] The measurement beam may be a first measurement beam, and after rotation of the polarization state, the measurement beam may be another measurement beam.

[0013] The method may include rotating the polarization state of the measurement beam a number of times through a corresponding number of holograms.

[0014] The method may include automatically rotating the polarization state of the measurement beam multiple times with a hologram, the rotation may be digitally controlled as described herein.

[0015] The multiple times may be 30 or more times.

[0016] The method may include continuously rotating the state of polarization of the measurement beam.

[0017] In another definition, a method for measuring an optical property of an object is provided, comprising the steps of: a) generating an initial uniformly polarized measurement beam having an initial polarization state; b) directing an initial measurement beam towards the object; c) detecting a polarization characteristic of an initial measurement beam that does not interact with the object and a polarization characteristic of the initial measurement beam after interacting with the object; d) generating, by the hologram, a uniformly polarized subsequent / second measurement beam having a subsequent polarization state, the subsequent polarization state being rotated by the hologram from the initial polarization state of the measurement beam and / or any preceding polarization state; e) directing a subsequent measurement beam towards the object; f) detecting a polarization characteristic of a subsequent measurement beam that does not interact with the object and a polarization characteristic of a subsequent measurement beam after interacting with the object; g) repeating steps d) through f) a predetermined number of times; and h) determining a measurement of at least one optical property of the object using the detected polarization characteristics of the initial measurement beam and the subsequent / second measurement beam that do not interact with the object and the detected polarization characteristics of the initial measurement beam and the subsequent / second measurement beam after interacting with the object.

[0018] The predetermined number of times in step g) may be at least 30. Note that each of the subsequent measurement beams has a polarization state that is rotated differently from each other and from the initial measurement beam. In this way, at least 30 different measurement beams, each having a rotated polarization state, are detected for both the measurement beam that does not interact with the object and the measurement beam after interacting with the object.

[0019] The hologram may be configured to interact with a pair of incident light beams to produce a uniformly polarized measurement beam having a predetermined polarization state. To this end, the hologram may include two holograms arranged in a superimposed manner to interact with the incident light beam to produce a subsequent uniformly polarized measurement beam having a different polarization state that is rotated from the polarization state associated with the preceding measurement beam. The hologram may include a plurality of superimposed holograms. The hologram may include two spatial diffraction gratings.

[0020] The method may include providing a hologram that interacts with an incident light beam from a light source to generate a measurement beam in a computer-controlled manner by a digitally controlled phase / polarization sensitive device, which may be a Digital Micromirror Device (DMD).

[0021] The method may include detecting polarization characteristics of the measurement beam after interference / interaction with the object to generate a polarization measurement, where the polarization measurement represents the output polarization state of the measurement beam after interference / interaction with the object.

[0022] The method may include detecting a polarization characteristic of the measurement beam that does not interfere / interact with the object to generate a polarization reference value, where the polarization reference value represents an output polarization state of the measurement beam that does not interfere / interact with the object, in other words, the polarization reference value represents the output polarization state of the measurement beam before interacting with the object.

[0023] The optical property of the object to be measured may be the birefringence of the object. Thus, the method may include evaluating the polarized measurements and the polarized reference value to determine at least one birefringence parameter representative of the birefringence of the object. The birefringence parameter may correspond to the amount of birefringence of the object and may be calculated by numerical fitting with minimized error using the polarized measurements and the polarized reference value.

[0024] The method may include detecting the polarization characteristics of the measurement beam using at least one suitable optical detection device. The method may include detecting the polarization characteristics of the reference beam using the same optical detection device used to detect the polarization characteristics of the measurement beam. However, the method may include detecting the polarization characteristics of the reference beam using a separate suitable optical detection device. In one embodiment, the optical detection device described herein may be in the form of a charge-coupled device (CCD) camera (or cameras) having a plurality of pixels. In this regard, the polarization measurements and / or the polarization reference values ​​may be associated with intensity values ​​for each pixel in an image captured by the optical detection device (or devices).

[0025] The method may include generating an initial measurement beam by an initial hologram, where the subsequent measurement beam is generated by a subsequent hologram. The method may include transforming the initial hologram into a subsequent hologram; and transforming the subsequent hologram in a digital and / or computer-controlled manner to rotate the polarization state of the measurement beam prior to interaction with the object.

[0026] The method may include providing an initial hologram and a subsequent hologram to interact with an incident light beam from a light source to generate the measurement beam.

[0027] The method may include a step of automatically providing the holograms to the incident light beam in a predetermined time interval. Thus, the measurement beam and / or the reference beam are automatically generated in a predetermined time interval in a sequential manner. To this end, the method may include a step of providing the holograms to the incident light beam in a computer-controlled manner by a digitally controlled phase / polarization sensitive device. The phase / polarization sensitive device may be a digital micromirror device (DMD). In this way, the holograms may be automatically switched relatively quickly by the DMD. In this way, the polarization of the measurement beam is rotated by the digitally controlled hologram, making it unnecessary to measure the birefringence using mechanically rotating parts. The predetermined time interval may be about 1 second. However, it is not excluded that the time interval is less than 1 second or more than 1 second.

[0028] The method may include controlling the DMD to switch between providing holograms at predetermined time intervals. Thus, the method may include switching between holograms in less than one second. In this manner, the polarization state of the measurement beam may be rotated in a relatively short period of time, since there is no need to mechanically rotate the polarization state of the measurement beam.

[0029] The method may include splitting a light beam from a light source into two paths and providing a DMD to interact with the two paths, where the two paths interact with the DMD with an angular offset relative to each other. The method may include splitting the light beam into two differently polarized beams / components, each beam traveling one of the two paths. This splitting may be done via a half-wave plate and / or a polarizing beam splitter. The angular offset between the two polarization paths may be about 1.5° before interacting with the DMD. Other angles may be used, but it is understood that a higher resolution DMD, and especially a higher resolution display of the DMD, is required for a larger angular offset. The purpose of the offset is to result in two beams propagating in directions X and Y that differ by a small angle.

[0030] Each hologram may include a hologram that interacts with one polarized beam and another hologram that interacts with another, differently polarized beam superimposed thereon, such that both polarized beams are modulated such that the first diffraction orders of both polarized beams emerge from the hologram with uniform polarization.

[0031] Those skilled in the art will understand that a reference to a "DMD" refers to a reference to a hologram provided by the DMD, and therefore, unless otherwise indicated or apparent to those skilled in the art, a reference to a DMD refers to a reference to a hologram provided by the DMD.

[0032] The measurement beams may have a Gaussian amplitude profile with an induced phase difference between them due to interaction with the holograms. The method may include providing at least 30 holograms in a manner described herein to obtain 30 measurement beams with mutually rotated polarization states.

[0033] The beams reaching the DMD may be vertically and horizontally polarized beams traveling two paths interacting with a hologram provided by the DMD to provide a uniformly polarized measurement beam. In particular, only the first diffraction orders of the vertically and horizontally polarized beams after interacting with the holograms are used as the measurement beam. The holograms may each include a hologram interacting with the vertically polarized beam and another superimposed hologram interacting with the horizontally polarized beam, such that both polarized beams are modulated such that the first diffraction orders of both polarized beams exit the hologram with uniform polarization.

[0034] The light source may be in the form of a laser, in which case the method includes expanding and collimating a beam of light from the laser before splitting it into two paths.

[0035] The method may include directing the measurement beam through a first polarizing element prior to interaction with the object. The polarizing element may be in the form of a quarter wave plate.

[0036] The method may include directing the measurement beam after interacting with the object through a second polarizing element, which may be in the form of a linear polarizer or a polarizing beam splitter.

[0037] The method may include storing in a suitable memory device the detected polarization characteristics associated with the measurement beam that does not interact with the object and the detected polarization characteristics associated with the measurement beam after interacting with the object for processing to determine a measurement value of at least one optical property of the object.

[0038] The method may include directing the measurement beam through one or more Fourier imaging systems before directing the measurement beam towards the light detection device.

[0039] According to another aspect of the present invention, there is provided a system for measuring an optical property of an object, the system including: a beam generating device including a holographic device, the beam generating device configured to generate and direct a uniformly polarized measurement beam towards the object, the holographic device configured to rotate a polarization state of the measurement beam with a hologram; a detection device configured to detect, for each state of polarization of the measurement beam, a polarization characteristic of the measurement beam not interacting with the object and a polarization characteristic of the measurement beam after interacting with the object; A control unit that determines, for each polarization state of the measurement beam, a measurement of at least one optical property of the object using the detected polarization property of the measurement beam that does not interact with the object and the detected polarization property of the measurement beam after interacting with the object.

[0040] The measurement beam may be an initial measurement beam, and after rotation of the polarization state, the measurement beam may be a subsequent measurement beam. The measurement beam that does not interact with the object may be a reference beam.

[0041] The beam generating device may be configured to generate an initial uniformly polarized measurement beam having an initial polarization state; and generate a plurality of subsequent uniformly polarized measurement beams having subsequent polarization states by a hologram provided by the holographic device, the subsequent polarization states being rotated relative to one another and / or from the initial polarization state by the hologram as referred to herein.

[0042] The detection apparatus may include a light-sensitive detection device, such as a CCD camera, configured to detect the polarization characteristics of the measurement beam before it interacts with the object and the polarization characteristics of the measurement beam after it interacts with the object.

[0043] The system may include a number of optical elements for directing the measurement beam towards the object and the detection device. In other words, the optical elements may be arranged to direct the measurement beam via the object towards the detection device. Similarly, in some embodiments, the system may include a number of optical elements for directing a measurement beam that does not interact with the object, i.e. a reference beam, towards the detection device. Optical elements as described herein may in some embodiments be part of the detection device.

[0044] The control unit may be configured to use the detected polarization characteristics of the initial measurement beam and the subsequent measurement beam after interacting with the object to determine a measurement of at least one optical property of the object.

[0045] Each hologram provided by the holographic device may include two holograms arranged in a superimposed manner to interact with an incident light beam to produce a uniformly polarized measurement beam having a polarization state that is rotated relative to one another.

[0046] The beam generating device may include a light source. The light source may be a laser light source. The holographic device may be configured to provide a hologram that interacts with an incident light beam from the light source to generate the measurement beam in a computer-controlled / electronic / digital manner. The holographic device may be a digital micromirror device (DMD).

[0047] The detector may be configured to detect polarization characteristics of the measurement beam after interaction with the object to generate respective polarization measurements, where the polarization measurements represent an output polarization state of the measurement beam after interaction with the object.

[0048] In some embodiments, the detector may be configured to detect polarization characteristics of the measurement beam that does not interact with the object to generate a polarization reference value, where the polarization reference value represents an output polarization state of the measurement beam that does not interact with the object.

[0049] It should be noted that in some embodiments, the system may include a reference detector configured to detect the polarization characteristics of the measurement beam that does not interact with the object in order to generate a polarization reference value. Accordingly, the system may include a suitable beam splitter device configured to split the measurement beam into two paths, one path being directed to intersect with the object and the detector, and the other path being directed to the reference detector. The polarization reference value should be identical to the measurement beam before interacting with the object.

[0050] The reference detector may be substantially similar to the detectors described above.

[0051] The optical property of the object to be measured may be the birefringence of the object. In this manner, the controller is configured to evaluate the polarization measurements and the reference value to determine at least one birefringence parameter representative of the birefringence of the object. The birefringence parameter may correspond to an amount of birefringence of the object, and the controller may be configured to calculate the birefringence parameter for each state of polarization using the polarization measurements and the reference value. This may be done by a numerical fitting technique with minimized error.

[0052] The CCD camera may have multiple pixels, in which case the polarization measurements and / or the reference value may be associated with an intensity value for each pixel in the image captured by the light detection device, in other words the polarization measurements may be the intensity of the measurement beam and / or the reference beam detected by the CCD camera.

[0053] The holographic device may be configured to provide different holograms to generate the initial and subsequent measurement beams, in particular the holographic device may be configured to provide a hologram that interacts with an incident light beam from the light source to generate the measurement beam.

[0054] The holographic device may be configured to automatically provide the incident light beam sequentially at predetermined time intervals. Thus, the measurement beam and / or the reference beam are automatically generated sequentially at predetermined time intervals. To this end, the method may comprise a step of computer-controlled providing of a hologram to the incident light beam by a digitally controlled phase / polarization sensitive device. The phase / polarization sensitive device may be a digital micromirror device (DMD). In this way, the hologram may be automatically switched at a relatively high speed by the DMD. In this way, the polarization of the measurement beam is rotated by the digitally controlled hologram, making it unnecessary to measure the birefringence using mechanically rotating parts. The predetermined time interval may be about 1 second. However, it is not excluded that the time interval is less than 1 second or more than 1 second.

[0055] The beam generating device may include a suitable beam splitter to split the light beam from the light source into two paths. The holographic device may be located downstream of the light source and may intersect the two paths such that both light beams travelling along the two paths intersect the holographic device. The light beams travelling along the two paths may be differently polarized beams / have two different polarization components. The beam generating device may include a half wave plate and / or a polarizing beam splitter to split the light beam from the light source into two polarized beams. The two paths may intersect at an angle at the holographic device. The angle may be about 1.5°.

[0056] Each hologram may include a hologram interacting with one polarized beam and another superimposed hologram interacting with another different polarized beam, such that both polarized beams are modulated such that the first diffraction orders of both polarized beams emerge from the hologram with uniform polarization. In other words, the hologram may be a multiplexed hologram.

[0057] Those skilled in the art will understand that a reference to a "DMD" or "holographic device" refers to a reference to a hologram provided by the DMD or holographic device, and therefore, unless otherwise indicated or apparent to those skilled in the art, a reference to a DMD or holographic device refers to a reference to a hologram provided by the DMD or holographic device.

[0058] The measurement beams may have a Gaussian amplitude profile with an induced phase difference between the different measurement beams due to interaction with the hologram.

[0059] The beam reaching the holographic device may be a vertically polarized beam and a horizontally polarized beam that travels two paths, interacting with a hologram provided by the holographic device to result in a uniformly polarized measurement beam.

[0060] Only the first diffraction order of the differently polarized beams in the two paths after interacting with the hologram is used as the measurement beam. In particular, the hologram may comprise a hologram interacting with one polarized beam, e.g. a vertically polarized beam, and another superimposed or multiplexed hologram interacting with another differently polarized beam, e.g. a horizontally polarized beam, so that the two polarized beams are modulated to obtain the first diffraction orders of the two polarized beams as a result of the interaction with the hologram with a uniform polarization. In other words, the hologram acting on beams with different polarization states in the first and second paths allows the first diffraction orders of the incident beam to spatially overlap.

[0061] In this regard, the system may include a suitable aperture for spatially filtering the measurement beam from the holographic device.

[0062] The beam generating device may include suitable optical components to expand and collimate the light beam from the laser before splitting it into two paths.

[0063] The system may include a first polarizing element disposed downstream of the holographic device and upstream of the object, The polarizing element may be in the form of a quarter wave plate.

[0064] The system may include a second polarizing element disposed downstream of the object, which may be in the form of a linear polarizer or a polarizing beam splitter.

[0065] The system may include a suitable memory device configured to store the detected polarization characteristics associated with the measurement beam in a suitable memory device for processing to determine a measurement of at least one optical property of the object.

[0066] Non-limiting embodiments of the present invention will now be described, by way of example only, with reference to the following drawings: [Brief description of the drawings]

[0067] [Figure 1] 1 shows a highly schematic block diagram of a system for measuring the optical properties, in particular the birefringence, of an object according to one embodiment of the present invention. [Diagram 2] FIG. 1 shows a detailed diagram of an example of a system for measuring optical properties, in particular birefringence, of an object according to one embodiment of the present invention. [Diagram 3] 2 shows a detailed view of another example of a system for measuring optical properties, in particular birefringence, of an object according to an embodiment of the present invention; [Figure 4] 1 shows a highly schematic block flow diagram of a method for measuring optical properties, in particular birefringence, of an object according to one embodiment of the present invention. [Diagram 5]FIG. 13 shows how the measured intensity of an obliquely polarized projection changes as the phase difference between the resulting components changes with and without passing through a birefringent material. [Figure 6] Images showing the phase difference between the right- and left-handed circularly polarized components induced by a liquid crystal q-plate (a) and an alanine crystal (b), and images showing the phase difference between the vertical and horizontally polarized components induced by a metasurface q-plate (c) and a j-plate (d), (e) are shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0068] The following description of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognize that many changes can be made to the described embodiments while still achieving the beneficial results of the invention. It will also be apparent that some of the desired advantages of the invention can be achieved by selecting some of the features of the invention without utilizing other features. Thus, those skilled in the art will recognize that modifications and adaptations to the invention are possible and may be desirable in certain situations and are a part of the invention. Thus, the following description is provided as an illustration of the principles of the invention, not in limitation.

[0069] The phrases "for example," "e.g.," and variants thereof should be understood to describe non-limiting embodiments of the subject matter disclosed herein. Reference herein to "in one embodiment," "in another embodiment," "some embodiments," or variants thereof means that a particular feature, structure, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the subject matter disclosed herein. Thus, use of the phrases "in one embodiment," "in another embodiment," "some embodiments," or variants thereof does not necessarily refer to the same embodiment(s).

[0070] Unless otherwise indicated, some features of the subject matter described herein, which are for clarity described in the context of separate embodiments, may also be provided in combination in a single embodiment. Similarly, various features of the subject matter described herein, which are described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.

[0071] 1 and 2 of the drawings, a system for measuring the optical properties of an object, in particular the birefringence of an object S, is generally designated by the reference number 10. The system 10 may therefore be referred to as a birefringence measurement system 10. The system 10 is configured to use a hologram to rotate the polarization state of a measurement beam B for the purpose of measuring the birefringence of the object S, as described herein. The object S may be a sample of a material of interest having optical properties and whose birefringence properties are measured by the system 10. The birefringence measured by the system 10 described herein may be linear birefringence and / or circular birefringence.

[0072] The optical property of birefringence is measured with the systems and methodologies described herein, however, those skilled in the art will understand that other optical properties of an object may be determined or measured mutatis mutandis using the systems and methodologies described herein.

[0073] In any event, before proceeding to a detailed description, the following follows a theoretical explanation of the operating principles and methodology of the system 10 for measuring the birefringence of an object S described herein.

[0074] Theoretical explanation Stokes Vector:

number

number

number

number

[0075] Now consider passing this state through a linear polarizer with its transmission axis at 45° to the horizontal. In this case, we can act on this state using the Mueller matrix of the polarizer to obtain:

number

[0076] The overall intensity (i.e., Ψ' 11 =S 0 ), it should be noted that when the weightings (α=π / 2) are equal, an intensity pattern is obtained that results in a cosine curve that depends on φ, whereas when there is no phase difference between the polarization components (φ=0), the intensity pattern depends on α as follows:

number

[0077] Now, if we place a general elliptical retarder in front of the polarizer, the variables β and γ' characterize the linear retardance (phase difference between the horizontal and vertical polarization components) and optical rotation (rotation of the linear polarization due to the phase difference 2γ between the right and left circular polarization components), respectively. We obtain the output Stokes vector Ψ according to:

number

[0078] Now, considering a fixed β=π along with (α=π / 2) from before, we get:

number

[0079] On the other hand, if we consider the case where we fix γ=π / 2 and φ=0, we obtain:

number

[0080] Thus, by varying either α or φ and measuring the intensity of the 45° projection after passing through a sample exhibiting linear or circular birefringence, respectively, the linear retardation β or the angle of optical rotation γ′ can be determined through fitting the measured intensities to Equation 8 and Equation 7.

[0081] Substantive Description It should be understood that system 10, as discussed herein, is configured to employ the above principles to determine a measure of birefringence of object S. To this end, system 10 includes a beam generating device 12 configured to generate a uniformly polarized measurement beam B and to sequentially and / or continuously rotate the measurement beam B a predetermined number of times. In this manner, multiple beams with different polarization states are generated by beam generating device 12.

[0082] As discussed herein, for one rotation of the measurement beam, the beam generating device 12 is configured to generate an initial or first measurement beam B1 and a subsequent or second measurement beam B2 having a polarization state rotated from the first measurement beam B1. This may be one iteration of the rotation of the beam B by the beam generating device 12 as discussed herein. Those skilled in the art will appreciate that the beam generating device 12 is further configured to generate one or more subsequent beams Bn (n=3 to N) or another second beam B2 (not shown), each rotated from the preceding measurement beam. The beam generating device 12 may be configured to generate multiple subsequent measurement beams Bn, where each subsequent measurement beam Bn has a polarization state rotated from the immediately preceding measurement beam Bn. The beam generating device 12 may be configured to generate multiple subsequent measurement beams Bn over a predetermined number of iterations N by rotating the polarization state of the initial or first measurement beam to a different polarization state of the subsequent or second measurement beam B2 in a manner similar to that described herein. Alternatively or additionally, the beam generating device 12 may be configured to generate multiple subsequent measurement beams Bn over a number of iterations N that allow the system 10 to determine or measure the birefringence of the object S. The number of iterations N may be a minimum of 30. It should be noted that the term "rotated" with respect to the beam B may be understood to mean "rotated state of polarization" or "rotated polarization state". Those skilled in the art will understand that these terms are used interchangeably, where applicable.

[0083] The system 10 also includes a detection device 14 downstream of the beam generating device 12 for detecting the beam B' after interacting with the object S. By "detecting" the beam B' after interacting with the object S, it should be understood that the detection device may be configured to measure the intensity of one or more beams B' after interacting with the object S.

[0084] System 10 further includes a controller 16 electrically / communicatively connected to detection device 14 and memory device 18, and configured to use the detected / received / measured intensity of beam B' from detection device 14 to determine a measurement of birefringence, for example, by fitting the measured intensity to Equation 8 and Equation 7 depending on whether the sample is linearly birefringent or circularly birefringent, respectively, as described herein.

[0085] The detection device 14 is also configured to detect the beam B that does not interact with the sample or object S as a reference beam R. The reference beam R is effectively identical / substantially similar / identical to the beam B from the device 12 before it interacts with the sample S.

[0086] The detection device 14 includes a CCD camera 15, which includes a number of pixels that respond to the measurement beam B in a conventional manner. In particular, the CCD 15 measures or detects the polarization intensity associated with the measurement beam. With reference also to FIG. 5 of the drawings, a plot of the polarization intensity of the measurement beam B measured / detected by the CCD 15 in the absence of the object S, in other words the reference beam R, is indicated by the reference numeral 70 and has a cosine profile. A plot of the polarization intensity of the measurement beam B after interaction with the sample S is indicated by the reference numeral 72. Like the plot 70, the plot 72 also has a cosine profile, but due to an induced phase difference, it is shifted from the plot 70 as shown.

[0087] The detection arrangement 14 and / or the system 10 further include an optical system / component that facilitates directing the measurement beam B through the object S to a CCD camera 15 .

[0088] With reference to FIG. 3 of the drawings, another embodiment of a system according to an embodiment is generally designated by the reference numeral 100. Those skilled in the art will appreciate that the system 100 is substantially similar to the system 10 and similar components are designated by the same reference numerals where appropriate. The system 100 comprises an additional beam splitter 102, collection optics 104 and an additional reference detector 115 in the reference optical path, which allow the system 100 to efficiently detect the measurement beam B, in other words the reference beam R, that does not interact with the object S. To this end, the beam splitter 102 splits the measurement beam into i) the measurement beam B that actually interacts / intersects with the object S and ii) the reference beam R that does not interact with the object S, as described above. In this way, the system 100 can be used for objects S that are too large for the measurement beam B to pass through the object S and enter the detector 15 and at the same time enter the detector 15 without interacting with the sample S (to provide the reference beam R). This is because the reference beam R, which corresponds to the measurement beam B that does not interact with the object, is detected separately by a detector 115 that is communicatively connected to the control unit 16 and the memory device 18 .

[0089] From the above, it can be seen that systems 10 and 100 are effective arrangements for detecting the intensity of measurement beam B after it has interacted with sample S, and the intensity of measurement beam B (referred to in this specification as reference beam R) that does not interact with the sample.

[0090] The following description will be made with reference to system 10, however, those skilled in the art will appreciate that the same discussion and explanations are applicable to system 100 as well.

[0091] The controller 16 is electrically connected to the CCD 15 and receives intensity values ​​or data / signals representative of the intensities detected or measured by the CCD 15. The controller 16 may generally be one or a combination of a microcontroller, a processor, a graphic processor, or a field programmable gate array (FPGA) operable to achieve the desired operations as described herein. The controller 16 is operable according to instructions stored in the internal or external memory device 18 to perform the operations described herein. In particular, the controller 22 is configured to receive the intensity values ​​or data indicative thereof from the CCD 15 and use it as an input to determine a birefringence measurement of the object S.

[0092] System 10 may further include an output module (not shown) in the form of a display, e.g., an LCD (liquid crystal display), a light emitting diode (LED) screen, a CRT (cathode ray tube) screen, a printer, etc., communicatively connected to controller 16 for outputting the measured birefringence. In an alternative embodiment, the output module may be in the form of an audible output, which may be, for example, a speaker used to provide an audible output of the measured birefringence.

[0093] Those skilled in the art will appreciate that the controller 16, memory device 18, and output module need not be in close proximity to the CCDs 15, 115 in order to receive intensity-indicative signals therefrom. Although not shown, the system 10 includes associated bias and / or drive circuitry, and power supplies, for operating the electrically driven and / or controlled components of the system. Similarly, the system 10 may include suitable optical elements, such as mirrors, for steering light in the system 10 in a desired manner, as described below.

[0094] The beam generating apparatus 12 can be better seen in Figure 2, where the apparatus 12 includes a light source in the form of a laser 20, for example a 633 nm HeNe laser. The beam generating apparatus 12 further includes lenses EL22 (f = 2 mm) and CL24 (f = 200 mm), which are configured to expand and collimate the beam from the laser 20, respectively.

[0095] The beam generating device 12 further includes a half-wave plate (HWP) 26 disposed downstream of the lens 24 to control the plane of polarization, and a polarizing prism or polarization separation element 28, such as a polarizing beam splitter (PBS), Wollaston prism, or the like, and an associated mirror (M) 29, configured to split the beam received from the HWP 26 into two differently polarized beams traveling along two paths X and Y. It should be noted that in this specification, the beams traveling along paths X and Y with different polarizations may also be referred to as two polarization components. Here, a horizontally polarized beam travels along path X and a vertically polarized beam travels along path Y. The polarized beams traveling along paths X and Y may be polarized orthogonal to each other.

[0096] Beam generating device 12 advantageously includes a polarization-sensitive holographic device in the form of a digital mirror device (DMD) 30 configured to intersect paths X and Y with an angular offset of approximately 1.5° between paths X and Y. Paths X and Y are aligned such that horizontally and vertically polarized beams (with central regions of approximately constant amplitude) intersect at DMD 30. DMD 30 handles two multiplexed holograms (A and B).

[0097] DMD 30 is configured to provide a multiplexed first hologram to the intersecting beams from paths X and Y to generate a first measurement beam B1 having a first polarization state. DMD 30 is further configured to switch from providing a multiplexed first hologram to the beams from paths X and Y to providing a multiplexed second hologram to generate a second measurement beam B2 whose polarization state is rotated from the first measurement beam B1 and / or rotated with respect to each other. As alluded to above, DMD 30 may be configured to provide multiplexed holograms to the incident beam to generate another measurement beam Bn, each having a polarization state rotated from the previous measurement beam Bn-1. This may be accomplished in a computer-controlled / digital / electronic manner, where DMD 30 is controlled to handle multiple holograms, particularly the first hologram and then the second hologram, in a relatively fast manner. In other words, DMD 30 is configured to rotate the polarization of measurement beam B by handling an appropriate hologram that facilitates the aforementioned rotation of the polarization. Holograms can achieve this by interacting with the phase of the incident beam.

[0098] Those skilled in the art will appreciate that the phrase DMD 30 "handles" a hologram can be read as DMD "providing" a hologram. It is understood that DMD 30 may electronically / computer-controlledly alter the hologram it provides, which interacts with the two light beams from the light source as described above to change the polarization state of the resulting measurement beam B.

[0099] In any event, system 10 includes an aperture (A) 32 disposed downstream of DMD 30 for spatially filtering the overlapping diffraction orders of the beam from DMD 30. System 10 further includes a first 4f Fourier imaging system 34 for spatially filtering beam B with overlapping diffraction orders and imaging it onto object S.

[0100] System 10 may also include a quarter-wave plate (QWP) 36 upstream of object S and downstream of first Fourier imaging system 34, configured to convert the horizontally and vertically polarized components to right- and left-handed circular polarizations, respectively, as needed. QWP 36 may be removed depending on whether linear or circular birefringence is being measured.

[0101] The system 10 may include an obliquely oriented linear polarizer (LP) 38 positioned downstream of the object S, and a second 4f Fourier imaging system 40 for imaging the measurement beam B onto the detector 14 after it has passed through the sample.

[0102] Each multiplexed hologram provided by DMD 30 includes two superimposed or multiplexed holograms (A and B) that may be defined by the following function:

number

number

[0103] The beams generated using this setup (with or without the QWP) can be described as follows:

number

[0104] In some embodiments, the system 10 as described herein may be implemented in a suitable housing with a suitable containment for the object S.

[0105] 4 of the drawings, there is shown a flow chart generally indicated by the reference numeral 50 of a method for measuring an optical property of an object S, typically a method for measuring the birefringence of the object S, according to one embodiment of the present invention. The method 50 is typically performed by a system 10, 100 as described herein, and the following description is therefore based thereon. However, it is understood that the method 50 may also be performed by another system not described herein.

[0106] An object S, which may be a sample, is typically positioned in the path of the measurement beam B so as to intersect with the measurement beam B, between the DMD 30 and the CCD 15 as illustrated in FIG.

[0107] The method 50 is described with reference to two iterations of measurement beams B1 and B2 interacting with the sample S. However, the methodology described herein may be repeated multiple times for a minimum of 30 iterations, as described below.

[0108] The method 50 includes, at block 52, generating a uniformly polarized measurement beam, e.g., an initial or first measurement beam B1 having a first polarization state, and directing it toward the sample S. This may be accomplished by expanding and collimating the beam from laser 20 before it is split into two differently polarized beams, e.g., a horizontally polarized beam and a vertically polarized beam in paths X and Y, and directing the two differently polarized beams in paths X and Y toward the DMD 30 handling the first hologram.

[0109] The measurement beam B1 is spatially filtered by the aperture 32 and directed to the object S in block 54. This may be via the first Fourier system 34. Furthermore, depending on whether circular or linear birefringence is measured, the measurement beam B1 may be directed to the object S via a QWP 36. The QWP 36 may be introduced to change the polarization state of the beam when measuring linear birefringence, and therefore the basis being measured. The QWP 36 may be introduced before the DMD 30, but this is optional in either case.

[0110] Next, in block 54, the method 50 includes detecting a) the polarization characteristics of the first measurement beam B1' after interacting with the object S, and b) the polarization characteristics of the first measurement beam B1 not interacting with the object, i.e., the first reference beam R1. In particular, the method 50 includes detecting and measuring the intensity of the measurement beam B1' and the first reference beam R1 using the CCD 15. The method 50 may include directing the first measurement beam B1' to the CCD 15 after interacting with the object S via a second Fourier system or a suitable lens system / collection system 40. Furthermore, the method 50 includes directing the first measurement beam B1' to the CCD 15 after interacting with the object S via a linear polarizer 38. The method 50 may include directing the first reference beam R1 to the CCD 15, or to another reference CCD 115 via a suitable lens arrangement 104 as in the case of the system 100. In the latter embodiment, the method 50 may include splitting the first measurement beam B1 using a beam splitter 102 into i) a measurement beam B1 that intersects the sample S and ii) a first reference beam R1 that does not intersect the sample S, and the intensities of the two beams are measured by the detection device 14.

[0111] The detected polarization characteristics of the first measurement beam B1' and the first reference beam R1 may be first polarization measurements and first reference values ​​in the form of a first polarization intensity and a first reference intensity detected or measured by the CCD 15. These intensities, values ​​and / or data may be stored in the memory device 18 in block 56.

[0112] The method 50 determines whether a predetermined minimum number of iterations of the measurement beam or measurement has been reached at block 58. The inventors have found that a minimum of 30 measurements are required to determine the birefringence of the sample S. In this regard, it is noted that at least 30 measurement beams and associated reference beams, each with a rotated polarization state, are generated and used in the process described herein to determine the birefringence of the sample S.

[0113] The method 50 includes rotating the polarization state of the measurement beam from the previous measurement beam generated if the number of measurement repetitions recorded by the CCD is less than 30. If the previous measurement beam is B1, the method includes rotating the polarization state of the measurement beam to generate a second measurement beam B2 and a second reference beam R2 in block 60, and includes controlling the DMD 30 to automatically switch from the first hologram to the second hologram in block 52. Here, the second hologram is configured to rotate the polarization state of the beams in paths X and Y that are incident on the DMD 30. It is therefore possible to rotate the polarization state of the measurement beam B without using mechanically moving parts, simply by changing the hologram handled by the DMD 30. For this purpose, the second hologram may have a different phase compared to the first hologram. The second reference beam R2 is generated in the same manner as the reference beam R1, as described herein. Further measurement beams Bn are similarly generated to have a polarization state rotated from the previous measurement beam Bn-1 generated. This may be repeated 30 times.

[0114] The controller 16 may be connected to communicate with other electronic components described herein to control them. For example, the controller 16 may be configured to control the DMD 30 to switch the hologram handled by the DMD 30. In this regard, the controller 16 may be configured to control the DMD 30 to automatically switch from a first hologram to a second hologram, and to automatically switch from a preceding hologram to a succeeding hologram after a predetermined period of time of about 1 second or less. This rapid switching of holograms advantageously allows for rapid rotation of the polarization state of the measurement beam B, with little or no calibration of the system.

[0115] For the first iteration, the method 50 includes detecting, in block 54, a) the polarization characteristics of the second measurement beam B2′ after interacting with the object S, and b) the polarization characteristics of the second measurement beam B2, i.e., the second reference beam R2, that does not interact with the object, in a manner similar to that described above with respect to the first measurement beam B1′ and the reference beam R1. In particular, the method 50 includes detecting and measuring, by the CCD 15, i) the second polarization intensity of the second measurement beam B2′, and ii) the second polarization intensity of the second reference beam R2. The intensities, or values ​​and / or data associated therewith, may be stored in the memory device 18 in block 56.

[0116] Thus, the methodology described above may be repeated at block 58 until a predetermined minimum number of iterations of polarization state rotation, beam formation, and measurement is at least thirty.

[0117] Once the measurement iteration count reaches 30, the method 50 includes, in block 62, using: a) the detected polarization characteristics of the first, second and subsequent measurement beams B1', B2', Bn' (not shown) after interaction with the object, i.e., the first, second and subsequent polarization intensities or values; b) the polarization characteristics of the first, second and subsequent reference beams R1, R2, Rn, i.e., the first, second and subsequent reference polarization intensities or values. In particular, the control unit 16 is configured to use the first, second and subsequent polarization intensities to determine the linear retardation β or the optical rotation angle γ' of the object S by fitting the measured intensities to the above-mentioned equations 8 and 7.

[0118] Referring also to FIG. 5 of the drawings, a plot of the reference polarized intensity detected / measured by the CCD 15 is indicated by reference numeral 70 and has a cosine profile. Similarly, a plot of the measured polarized intensity detected / measured by the CCD 15 is indicated by reference numeral 72. As can be seen from this plot, the four iterations repeated in i, ii, iii and iv correspond to the four measurement and reference beams used to generate the plot. The shift observable in the cosine-like curve 72 of the measurement beam passing through the sample S and the associated cosine-like curve 70 of the reference beam is used to determine the birefringence of the sample S. Those skilled in the art will appreciate that the shift in the cosine-like curve corresponds directly to linear birefringence and twice the circular birefringence. The above is measured in radians and can be converted to other quantities via material properties including the refractive index for the analyzed wavelength and the length traveled within the material.

[0119] A person skilled in the art will recognize that plots 70, 72 and / or their associated data, i.e., intensity values ​​of multiple measurement beams having different polarization states passing through an object and their associated reference intensity values, may be used in conjunction with an appropriate curve matching algorithm to determine the birefringence of sample S.

[0120] Referring to FIG. 6 of the drawings, the above system 10 and method 50 were used, by way of example, with the following circularly birefringent samples: a liquid crystal (LC) q-plate (q=½), which imparts an equal but opposite geometric phase to the right and left circularly polarized components, thereby inducing a phase difference β between them; an L-alanine crystal (attached to a glass slide by paratone oil), which imparts a phase difference V due to the presence of a chiral center within the molecule.

[0121] In Figure 5a) the results are illustrated showing the measured β for an LCq plate, the measurements obtained are consistent with the values ​​expected from an LCq plate with q=1 / 2 within the resolution of the CCD15.

[0122] The results for alanine crystals can be seen in FIG. 5b), where the crystals are arranged in the shape of a "W". 1 2 1 2 1 Those crystals of the symmetry group exhibit different chiral structures in the direction of propagation depending on the orientation of the sample, so that crystals with different orientations will show different values ​​for birefringence. Comparing the measured β, it can be seen that the first anti-diagonal crystal (left to right) has the observed chiral structure, which is reversed in handedness from the first anti-diagonal crystal.

[0123] The linearly birefringent samples used were the metasurface q-plate (l = 5) and the metasurface j-plate (l H =6,l V =5 and l H =5,l V These metasurfaces consist of subwavelength TiO posts on a SiO substrate, which use spatially varying rectangular width dimensions to impart independent phase to the horizontally and vertically polarized components of light passing through them.

[0124] The results for the metasurface q-plate are consistent with what would be expected from an element attempt to impart l=±5 OAM to the horizontally and vertically polarized components. The j-plate results clearly show a partial phase ramp corresponding to the independent OAM introduced to the horizontally and vertically polarized components.

[0125] Thus, by varying the input polarization state incident on the medium, which gives each polarization component (birefringence) a different phase, the phase-shift polarimetry technique can be used to measure the birefringence at each pixel of the image projected on the camera after projection in a fixed polarization direction. Since the analysis is performed at a fixed angle of the analyzer, the reference for the phase-shift method can be taken directly from the measurement image or a copy of it can be used by placing a beam splitter just before the sample. The measurement process can reach extremely high speeds, the speed being limited by the frame rates of the DMD and the camera, which in this case are of the order of kHz and 30 fps, respectively. The data analysis is only limited by the processing power of the hardware used.

[0126] Unlike other devices, the present disclosure uses no mechanical moving parts in the setup, making it ideal for automation. Similar setups that do not include moving parts often use birefringent materials to rotate the polarization, which makes calibration very sensitive, unlike the present disclosure where the DMD is digitally controlled and the polarization rotation is based on holographic methods. Furthermore, all optical components employed herein have been used extensively in both academia and industry, proving their possible durability.

[0127] The system and method disclosed herein provides a robust and simple setup that can measure the birefringence of materials very quickly with spatial resolution. The system can perform polarimetric measurements or function as a digitally controlled polarizer. The spatial resolution is directly related to the imaging system, allowing the method to be used on a wide range of scales.

Claims

1. 1. A method for measuring an optical property of an object, comprising: directing a uniformly polarized measurement beam at the object; rotating the polarization state of the measurement beam with a hologram; detecting, for each polarization state of the measurement beam, a polarization characteristic of the measurement beam before interacting with the object and a polarization characteristic of the measurement beam after interacting with the object; and for each polarization state, determining a measurement of at least one optical property of the object using the detected polarization properties of the measurement beam that do not interact with the object and the detected polarization properties of the measurement beam after interacting with the object.

2. A method as described in claim 1, wherein the measurement beam that does not interact with the object is a reference beam.

3. The method of claim 2, comprising the step of splitting a uniformly polarized beam into two identical beams, one of which is the measurement beam directed toward the object and the other of which is the reference beam.

4. The method of claim 1, wherein each of the holograms is configured to interact with a pair of incident light beams to produce the uniformly polarized measurement beam having a predetermined polarization state.

5. The method of claim 1, wherein each of the holograms includes at least two holograms arranged in a superimposed manner to interact with an incident light beam to produce the uniformly polarized measurement beam.

6. The method comprises: splitting a light beam from a light source into said two paths so that differently polarized beams travel along said two paths; 2. The method of claim 1, further comprising the step of: providing holograms that interact with the two paths, each of the holograms including one hologram that interacts with one polarized beam and another superimposed hologram that interacts with the other, differently polarized beam, thereby modulating both of the differently polarized beams so that first diffraction orders of both polarized beams to be used as the measurement beam exit the hologram with uniform polarization.

7. The method of claim 1, further comprising a step of guiding the measurement beam through a second polarizing element after interacting with the object.

8. A system for measuring optical properties of an object, comprising: a beam generating device including a holographic device configured to generate and direct a uniformly polarized measurement beam toward an object, the holographic device configured to rotate the polarization state of the measurement beam with a hologram; a detection device configured to detect, for each polarization state of the measurement beam, a polarization characteristic of the measurement beam before interacting with the object and a polarization characteristic of the measurement beam after interacting with the object; a control unit that, for each polarization state of the measurement beam, determines a measurement of at least one optical property of the object using the detected polarization property of the measurement beam before interacting with the object and the detected polarization property of the measurement beam after interacting with the object.

9. The system described in claim 8, wherein the beam generating device is configured to generate an initial uniformly polarized measurement beam having an initial polarization state, and to generate multiple subsequent uniformly polarized measurement beams having subsequent polarization states by a hologram provided by the holographic device, each subsequent polarization state being rotated from the immediately preceding polarization state by the hologram.

10. A system as described in claim 8 or 9, wherein each hologram provided by the holographic device includes two holograms superimposed and arranged to interact with an incident light beam to produce the uniformly polarized measurement beam having a predetermined polarization state.

11. A system as described in claim 8, wherein the beam generating device includes a suitable beam splitter that splits the light beam from the light source into two paths, and the light beam traveling along the two paths is a different polarized beam or has two different polarized components.

12. The system described in claim 11, wherein the holographic device is positioned downstream of the light source and intersects the two paths so that both of the light beams traveling along the two paths intersect the holographic device.

13. A system as described in claim 11 or 12, wherein the two paths intersect at an angle between the two paths in the holographic device.

14. The system of claim 11, wherein each hologram includes a hologram that interacts with one polarized beam and another superimposed hologram that interacts with another different polarized beam, whereby both polarized beams are modulated so that the first diffraction orders of both polarized beams emerge from the hologram with uniform polarization.

15. The system described in claim 11, wherein the beam generating device includes appropriate optical components to expand and collimate the light beam from the light source before splitting it into the two paths.

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