WO2015156779A1

WO2015156779A1 - Optical measurement system having a beamsplitter element

Info

Publication number: WO2015156779A1
Application number: PCT/US2014/033328
Authority: WO
Original assignee: Pandata Research Llc
Priority date: 2014-04-08
Filing date: 2014-04-08
Publication date: 2015-10-15

Abstract

A system (100) directs light along an optical path, with a variable propagation direction, from a collimated source (122) into an internal reflection element (110), onto an interface (114) between the internal reflection element (110) and a sample (170) to be measured, out of the internal reflection element (110), and onto a first detector (150). The system (100) further includes at least two repositionable mirrors (124,126) in the optical path between the collimated source (122) and the internal reflection element (110). One of the repositionable mirrors is configured as a beamsplitter (124) that separates light from the collimated source (122) into a reflected portion (PI) and a transmitted portion (P2). The reflected portion (PI) is directed toward the internal reflection element (110). The transmitted portion (P2) is directed onto a second detector (128), which provides an electrical signal proportional to the amount of optical power produced by the collimated source (122). A ratio of electrical signals produced by the first (150) and second (128) detectors can determine a reflectivity of the sample (170).

Description

OPTICAL MEASUREMENT SYSTEM HAVING A BEAMSPLITTER

ELEMENT

The present disclosure relates generally to a system for analyzing a sample using light directed at the sample by an internal reflection element in proximity to the sample, and in particular to such a system using a beamsplitter element in an optical path of the system.

BACKGROUND

Many optical techniques are known for characterizing a sample, several of which involve launching a beam of light at the sample under a particular set of conditions, and measuring light reflected from the sample. Some techniques tailor the set of conditions toward measuring a particular structure. For instance, ellipsometry makes use of polarization state to perform measurements, and is particularly useful for measuring the refractive indices and the layer thicknesses for thin film structures. As another example, interferometry makes use of coherent light interference to perform measurements, and is particularly useful for measuring the physical profile of the surface of a sample. As still another example, spectroscopy makes use of multiple wavelengths to perform measurements, and is particularly useful in determining a presence and/or a concentration of a particular constituent in a sample.

Some measurement techniques measure a reflectivity of a sample as a function of incident angle, for a range of incident angles. In some of these techniques, the incident light is directed from within an internal reflection element, such as a prism or a partial sphere, onto an interface between the internal reflection element and the sample. For internal reflection elements having a refractive index greater than that of the sample, the range of incident angles can include the critical angle, as well as incident angles less than or greater than the critical angle.

For some techniques, measurements of reflectivity at or near the critical angle may provide particular sensitivity to a sample constituent that is below the surface of the sample. For instance, in techniques that use attenuated total reflectance (ATR), such as ATR spectroscopy, light is directed onto the sample at one or more incident angles greater than the critical angle. In ATR, nearly 100% of the light is reflected. In ATR, the decrease from 100% to the actual reflectivity corresponds to a small amount of light that enters the sample as an evanescent wave and is absorbed by the sample. ATR measurement techniques typically use this decrease in reflectivity from 100% to determine a presence and/or a concentration of a particular constituent in the sample, often with an intermediate calculation of a complex refractive index of the sample and a specified relationship between refractive index and presence and/or

concentration.

Measurements of reflectivity from the sample are particularly sensitive to power fluctuations from the light source. Accordingly, there exists a need for a measurement technique that can monitor the output power of the light source, and can reduce or eliminate the effects of power fluctuations from the light source from measurements of reflectivity from the sample.

SUMMARY OF THE DISCLOSURE

One embodiment of the present subject matter relates to a system for analyzing a sample using light directed at the sample by an internal reflection element in proximity to the sample. The system directs an incident beam, with a variable propagation direction, from a collimated source into the internal reflection element at a first location on the internal reflection element. The incident beam forms an internal beam inside the internal reflection element. The internal reflection element directs the internal beam having a variable incident angle onto the sample. The incident angle varies with the propagation direction of the incident beam. The internal reflection element has an index of refraction greater than that of the sample to be analyzed, so that the internal beam strikes the sample at incident angles that can be greater than the critical angle. The internal beam reflects at the interface of the internal reflection element and the sample, and then a portion of the internal beam exits the internal reflection element at a second location on the internal reflection element, and is detected by a first detector. The system further includes at least two repositionable mirrors in an optical path between the collimated source and the internal reflection element. The system tilts and/or translates the repositionable mirrors, in order to produce a desired propagation direction for the incident beam. At least one of the repositionable mirrors is configured as a beamsplitter that separates light from the collimated source into two portions, which include a reflected portion and a transmitted portion. One of the two portions is directed to form the incident beam. The other of the two portions is directed onto a second detector, which provides an electrical signal proportional to the amount of optical power produced by the collimated source. The system may use a ratio of the electrical signals produced by the first and second detectors to determine a reflectivity of the sample. In some examples, the second detector receives light transmitted through the repositionable mirror that is directly adjacent to the collimated source, so that the position of the light on the second detector does not vary as the mirrors are repositioned.

In one embodiment, a system includes a first optical power detector. The system also includes a light source configured to produce a beam of light. The beam of light propagates along a repositionable optical path. The optical path extends between the light source and the first optical power detector. The system also includes an internal reflection element disposed in the optical path adjacent the first optical power detector. The internal reflection element is configured to direct the optical path onto a measurement face of the internal reflection element from within the internal reflection element. The optical path strikes the measurement face at a variable incident angle. The measurement face is configured to contact a sample. The optical path includes a reflection at the measurement face. The reflection is formed at an interface between the internal reflection element and the sample. The system also includes a plurality of repositionable mirrors spaced apart in the optical path between the light source and the internal reflection element. The optical path includes reflections from the plurality of repositionable mirrors. The plurality of repositionable mirrors are configured to reposition the optical path so as to vary the incident angle of the optical path at the measurement face of the internal reflection element. One of the plurality of repositionable mirrors is a beamsplitter. The beamsplitter reflects a first portion of incident light. The first portion is directed along the optical path. The beamsplitter transmits a second portion of incident light. The second portion is directed out of the optical path and onto a second optical power detector.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of an example of an optical measurement system having an example of a beamsplitter element.

FIG. 2 is a schematic drawing of another example optical measurement system having an example of a beamsplitter element.

FIG. 3 is a schematic drawing of an example of a beamsplitter.

FIG. 4 is a flowchart of an example of a measurement process.

FIG. 5 is a schematic drawing of an example of an optical measurement system having an example of a multi-pixel detector.

DETAILED DESCRIPTION FIG. 1 is a schematic drawing of an example of an optical measurement system 100. The measurement system 100 includes a repositionable optical path (P) extending from a light source 122 to an optical power detector 150. The optical path (P), and the various optical elements within the optical path (P), are described in greater detail below.

A light source 122 produces a light beam that propagates along the optical path (P). The light beam is typically collimated, but may alternatively be converging or diverging. In some examples, the light beam may include a single wavelength or a relatively narrow band of wavelengths. In other examples, the light beam may include two wavelengths, or two relatively narrow bands of wavelengths. In still other examples, the light beam may include more than two wavelengths, or more than two relatively narrow bands of wavelengths.

Examples of suitable light sources 122 can include a semiconductor laser, a semiconductor laser with a collimating lens or a collimating mirror, a light emitting diode (LED), an LED with a collimating lens or a collimating mirror, a broadband source, a broadband source with a collimating lens or a collimating mirror, a broadband source with a spectral filter, a broadband source with a spectral filter and a collimating lens or a collimating mirror, a quantum cascade laser, a quantum cascade laser with a collimating lens or a collimating mirror, an amplified spontaneous emission source, and an amplified spontaneous emission source with a collimating lens or a collimating mirror. Suitable light sources 122 can optionally include two or more light-producing elements that emit light at different wavelengths. The light at different wavelengths can be combined onto the same optical path (P) by suitable wavelength- sensitive filters that transmit one wavelength or band of wavelengths and reflect another wavelength or band of wavelengths. A driving electrical signal 142 controls the light source 122, and may control switching between or among different wavelengths in the light source 122.

A first repositionable mirror 124 is disposed in the optical path (P) after the light source 122, and receives the light beam produced by the light source 122. The first repositionable mirror 124 is configured to change position in response to a driving electrical signal 144. The change in position can include translation in one or two dimensions, and/or rotation along one or two axes. In the example shown in FIG. 1, the first repositionable mirror 124 is configured to pivot around a pivot axis. The pivot axis may be a physical element, or may be a mathematical construct that pertains to a particular mechanical structure. In the example of FIG. 1, the pivot axis is perpendicular to the plane of the page of FIG. 1, and is disposed at the center of the first repositionable mirror 124. The pivot axis may alternatively be located at any suitable location.

The first repositionable mirror 124 is configured as a beamsplitter, which reflects a first portion (PI) of the incident light from the light source 122, and transmits a second portion (P2) of the incident light from the light source 122. The first portion (PI) is directed along the optical path (P) to downstream optical elements in the optical path (P). As the first repositionable mirror 124 is repositioned, the first portion (PI) and the optical path (P) are also repositioned. The second portion (P2) is directed out of the optical path (P) onto an optical power detector 128. As the first repositionable mirror 124 is repositioned, the second portion (P2) remains essentially stationary. The optical power detector 128 receives the second portion (P2) of the light beam, and produces an electrical signal 148 proportional to the amount of optical power received from the second portion (P2) of the light beam. Advantageously, the second portion (P2) of the light beam does not move as the first repositionable mirror 124 is repositioned. The immovability of the second portion (P2) may be desirable because a relatively small optical power detector 128 may be used, which may help reduce noise in the power measurement. Suitable optical power detectors 128 can include one or more photodiodes, one or more photoconductive elements, one or more photovoltaic elements, and/or one or more pyroelectric elements.

A second repositionable mirror 126 is disposed in the optical path (P) and receives the first portion (PI) of light from the first repositionable mirror 124. The second repositionable mirror 126 is also configured to change position in response to a driving electrical signal 146, in the same manner as the first repositionable mirror 124. The second repositionable mirror 126 may have a relatively high reflectivity, preferably as close to 100% as is practical.

Preferably, the reflectivity is as close to uniform as is practical over the surface of the second repositionable mirror 126. In addition, the reflectivity is as close to uniform as is practical over a range of incident angles at the second repositionable mirror 126, the range including a nominal condition and including extreme positions for the optical path (P).

The light source 122, the first repositionable mirror 124, the optical power detector 128, and the second repositionable mirror 126 may be collectively referred to as a light production and direction module 120. The first repositionable mirror 124 and the second repositionable mirror 126 are spaced apart in the optical path (P) and may be controlled together, so that downstream, the optical path (P) may strike a particular measurement face at a single, fixed location, but may strike the single, fixed location with an incident angle that can vary over a range of incident angles.

An internal reflection element 110 is disposed in the optical path (P) and receives the output from the light production and direction module 120. In the example configuration of FIG. 1, the internal reflection element 110 is shaped as a prism, with flat sides. Other shapes for the internal reflection element 110 may also be used, as is discussed in more detail below with regard to FIG. 2.

The internal reflection element 110 has an incident face 112 disposed in the optical path (P). The incident face 112 is configured to receive the output from the light production and direction module 120 at an incident location 162. As the mirrors 124, 126 in the light production and direction module 120 are repositioned during operation, the incident location 162 may move along the incident face 112. Light from the light production and direction module 120 refracts through the incident face 112 of the internal reflection element 110 to form an internal beam inside the internal reflection element 110. The optical path (P) may bend at the incident face 112, in accordance with Snell's Law.

The internal reflection element 110 has a measurement face 114 disposed in the optical path (P). The measurement face 114 is configured to receive, at a measurement location 164, the internal beam from the incident face 112. The optical path (P) strikes the measurement face 114 at an incident angle Θ, formed with respect to a surface normal SNn₄. In some examples, as the mirrors 124, 126 in the light production and direction module 120 are repositioned during operation, the measurement location 164 remains stationary on the measurement face 114 as the incident angle Θ varies.

During operation, a sample 170 under measurement is placed into contact with the measurement face 114 of the internal reflection element 110. The internal beam reflects off the measurement face 114, which during operation is an interface between the material of the internal reflection element 110 and the material of the sample 170. At this interface, there is a characteristic incident angle Θ at the measurement face 114 known as a critical angle, which is an inherent property of the materials on either side of the interface.

Mathematically, the critical angle at the measurement face 114 is given by the numerical value of sin^"1 (n_sampi_e / n_pri_Sm), where n_sampi_e is a refractive index of a sample 170 under measurement, and n_pri_sm is a refractive index of the internal reflection element 110. In many examples, the internal reflection element 110 may have a refractive index greater than that of a sample 170 under

measurement, so that the internal reflection element 110 may direct light onto the sample at incident angles that exceed the critical angle. For incident angles Θ less than the critical angle, the power reflectivity of the interface is usually significantly less than 100%. For incident angles Θ greater than the critical angle, the power reflectivity of the interface is either 100% (for a fully transparent internal reflection element 110 and a fully transparent sample 170, for the condition known as total internal reflection), or slightly less than 100% (for an absorbing sample 170 and a transparent internal reflection element 110, for the condition known as attenuated total reflectance, or ATR). The internal beam reflects off the measurement face 114 with an exiting angle (formed with respect to the surface normal SNn₄) equal to the incident angle Θ. The optical path (P) is directed from the measurement location 164 on the measurement face 114 back into the internal reflection element 110.

The internal reflection element 110 has an exiting face 116 disposed in the optical path (P). The exiting face 116 is configured to receive, at an exiting location 166, the internal beam from the measurement face 114. Light reflected from the measurement face 114 refracts through the exiting face 116 of the internal reflection element 110 to form an external beam outside the internal reflection element 110. The optical path (P) may bend at the exiting face 116, in accordance with Snell' s Law. Note that the internal beam includes the portion of the optical path (P) that extends from the incident face 112, to the measurement face 114, to the exiting face 116 of the internal reflection element 110.

An optical power detector 150 is disposed in the optical path (P) and receives the external beam from the exiting face of the internal reflection element 110. The optical power detector 150 produces an electrical signal 152 proportional to the amount of optical power in the external beam. In an alternate configuration, the detector 150 may be bonded or optically contacted to the exiting face 116, so that there is no space between the exiting face 116 and the optical power detector 150. For this alternate configuration, the detector 150 senses the optical power in the internal beam and an external beam is not produced.

A computer 130 receives the electrical signals 148, 152 produced by the detectors 128, 150, generates the electrical signal 142 that controls the light source 122, and generates the electrical signals 144, 146 that control the repositionable mirrors 124, 126. In some examples, the computer collates and processes the system measurements, such as collecting and saving a series of reflectivity measurements, processing the collected reflectivity measurements, and determining one or more optical properties of the sample 170 from a best fit of the collected reflectivity measurements; in other examples, these operations are performed externally to the computer 130. The computer 130 includes at least one processor, memory, and a machine-readable medium for holding instructions that are configured for operation with the processor and memory. The computer may also include additional hardware as needed, such as volatile and/or non- volatile memory, one or more communication ports, one or more input/output devices and ports, and so forth, to provide the control functionality as described herein. These functions may be implemented by separate processing units, as desired, and additional functions may be performed by such one or more processing units.

The internal reflection element 110 is shaped as a prism, as in FIG. 1. In the example of FIG. 1, the prism has a cross-section with three sides, and with angles of 45 degrees, 45 degrees and 90 degrees. Other prism cross-sections are contemplated, including three-sided prisms having angles different than 45 degrees, 45 degrees, and 90 degrees, four-sided prisms, five-sided prisms, six- sided prisms, and prisms having more than six sides. In some of these configurations, only three of the sides, in cross-section, are disposed in the optical path.

Although there are many possible prism configurations, in certain applications it may be advantageous to arrange the prism sides so that the incident 112 and exiting 116 faces are oriented at or near normal incidence, with respect to a nominal position of the optical path (P). One aspect of this arrangement is that it maximizes the change in incident angle Θ at the measurement face 114 for a given change in beam output angle from the light production and direction module 120. This may ease some of the optical and mechanical requirements from the repositionable mirrors 124, 126. Another aspect of this arrangement is that it simplifies design of anti-reflection coatings that may be used on the incident 112 and exiting 116 faces of the internal reflection element 110. Yet another aspect of this arrangement is that it may allow for greater insensitivity to polarization orientation at the incident 112 and exiting 116 faces of the internal reflection element 110.

FIG. 2 is a schematic drawing of another example of an optical measurement system 200. Compared with the system 100 of FIG. 1, the system 200 uses the light production and direction module 120, detector 150, and computer 130, but with a differently shaped internal reflection element 210. Unlike the prism-shaped internal reflection element 110 of Fig 1, the internal reflection element 210 includes a curved face 212 that functions as both the incident face and the exiting face of the internal reflection element 110. In some examples, the curved face 212 is a portion of a sphere, such as a half-sphere.

The internal reflection element 210 is disposed in the optical path (P) and receives the output from the light production and direction module 120. The internal reflection element 110 has a curved face 212 disposed in the optical path (P). The curved face 212 is configured to receive the output from the light production and direction module 120 at an incident location 262. As the mirrors 124, 126 in the light production and direction module 120 are repositioned during operation, the incident location 262 may move along the curved face 212. Light from the light production and direction module 120 refracts through the curved face 212 of the internal reflection element 210 to form an internal beam inside the internal reflection element 210. The optical path (P) may bend at the curved face 212, in accordance with Snell's Law. The internal reflection element 210 has a measurement face 214 disposed in the optical path (P). The measurement face 214 is configured to receive, at a measurement location 264, the internal beam from the incident face 212. The optical path (P) strikes the measurement face 214 at an incident angle Θ, formed with respect to a surface normal SN214. In some examples, as the mirrors 124, 126 in the light production and direction module 120 are repositioned during operation, the measurement location 264 remains stationary on the measurement face 214 as the incident angle Θ varies. During operation, a sample 170 under measurement is placed into contact with the measurement face 214 of the internal reflection element 210. The internal beam reflects off the measurement face 214 with an exiting angle (formed with respect to the surface normal SN214) equal to the incident angle Θ. The optical path (P) is directed from the measurement location 264 on the measurement face 214 back into the internal reflection element 210. The curved face 212 is configured to receive, at an exiting location 266, the internal beam from the measurement face 214. Light reflected from the measurement face 214 refracts through the curved face 212 of the internal reflection element 210 to form an external beam outside the internal reflection element 210. The optical path (P) may bend at the curved face 212, in accordance with Snell's Law. The curved face 212 may include an anti-reflection coating disposed thereon. Note that the internal beam includes the portion of the optical path (P) that extends from the curved face 212, to the measurement face 214, and to a different location on the curved face 212 of the internal reflection element 210.

Note that in the system 100 of FIG. 1, the optical surfaces that refract and reflect the optical path (P) are all flat. As a result, the light beam propagating through the system 100 may remain collimated along the entire optical path (P), from the light source 122, through the internal reflection element 110, and to the detector 150. In contrast, the system 200 of FIG. 2 includes a curved surface 212 in the optical path (P). Such a curved surface may change the collimation of a light beam refracting therethrough; a collimated incident beam from the light production and direction module 120 may form a converging internal beam upon refraction through the curved face 212. In most cases, it is desirable to use a collimated beam inside the internal reflection element 210. For a collimated beam, the light incident on the measurement face 214 has only a single incident angle Θ, instead of a cone of incident angles that would be present in a converging or diverging beam. As such, it may be desirable to insert a lens 280 in the optical path between the light production and direction module 120 and the internal reflection element 210. The lens 280 may have a negative amount of optical power (a "negative lens"), which may be matched to the curved face 212 of internal reflection element 210. The lens 280 receives a collimated beam from the light production and direction module 120. The light refracts through the lens 280. The refracted light transmitted through the lens is diverging. The diverging light strikes the curved face 212 of the internal reflection element 210 and forms a collimated, or nearly collimated, internal beam. Upon exiting the internal reflection element 210, through the curved face 212, the external beam becomes converging. This converging light strikes the detector 150; the effect of non-collimation at the detector is negligible.

As noted above, the repositionable mirror 124 is configured as a beamsplitter. Such a beamsplitter is typically configured as a generally planar substrate having front and back surfaces, optionally with thin film coatings on the front surface and/or the back surface. The beamsplitter is configured to couple with suitable mechanical element(s) that can perform repositioning. FIG. 3 is a schematic drawing of a beamsplitter 300, which is suitable for use as the optical element in the repositionable mirror 124. The beamsplitter 300 includes a generally planar substrate 302. The substrate 302 may optionally include a relatively small amount of wedge between its front and back surfaces, to prevent interference effects from distorting the optical power readings from the detectors 128, 150. The substrate 302 may be formed from an optical material that is transparent in the wavelength ranges subtended by the light source 122.

The beamsplitter may include an incident thin-film coating 304 on the front side of the substrate (e.g., the side that faces the light source 122). Light from the light source 122, propagating along optical path (P), strikes the incident thin- film coating 304 with incident angle Q (formed with respect to surface normal SN₃oo)- A reflected portion (PI) reflects off the incident thin-film coating 304. The optical path (P) follows the reflected portion (PI). In general, the incident thin- film coating 304 is designed to reflect a relatively high percentage of incident light at incident angle Q, such as 80%, 90%, 95%, 98%, or 99%. In general, the incident thin-film coating 304 is also designed so that as the incident angle varies in a specified range around nominal value Q, the reflectivity does not vary significantly. In some examples, the range of mirror incident angles Q includes a Brewster's angle; mathematically, the Brewster's angle occurs at an incident angle of tan^"1 (n_SUbstrate), where n_SUbstrate is the refractive index of the substrate 302. In some cases, the incident face of the beamplitter is left bare, with a reflection occurring from an interface between air and the bare substrate material.

A transmitted portion (P2) enters the substrate 302. The transmitted portion (P2) strikes an exiting thin- film coating 306 on the back surface of the substrate 302. Typically, the exiting thin-film coating 306 is an anti-reflection coating, which minimizes reflections at the back surface of the substrate. The transmitted portion (P2) exits the substrate, through the exiting thin-film coating 306, and propagates toward the detector 128.

Other configurations for the beamsplitter are also possible. For instance, the front surface of the substrate 302 may include an anti-reflection coating, while the back surface may include the beamsplitter coating. In addition, the second repositionable mirror 126 may be configured to be the beamsplitter, rather than the first repositionable mirror 124; the detector 128 may be positioned to receive the light transmitted through the second repositionable mirror 126 for this configuration.

FIG. 4 is a flowchart of an example measurement method 400. The example method 400 may be executed using the system 100 of FIG. 1, the system 200 of FIG. 2, or by another suitable measurement system. It is assumed for this example method that more than one wavelength is used for the measurements; it will be understood that measurements may also be taken at a single wavelength. Step 402 places a sample 170 in contact with the

measurement face 114; 214 of the internal reflection element 110; 210. Step 404 selects an initial wavelength and an angular orientation for a beam produced by the light production and direction module 120. Step 406 generates a beam at the particular wavelength and angular orientation as selected at step 404. The light production and direction module 120 generates the beam. Step 408 directs the beam produced at step 406 into the internal reflection element 110; 210. Step 410 measures the power reflectivity from the sample 170. The power reflectivity is a ratio between 0% and 100%, typically close to 100%, and is proportional to the signal 152 produced by the detector 150, divided by the signal 148 produced by the detector 128. Calculation of the power reflectivity may include one or more small corrections, which can account for variation in performance of one or more optical elements as a function of incident angle. For instance, a transmissivity or a reflectivity of a particular thin film coating may vary as a function of incident angle. In addition, the reflectivity curve resulting from the plurality of power reflectivity measurements may be used to determine the presence and/or concentration of a constituent in the sample 170. Step 412 iterates the wavelength and the angular orientation of the beam, so that the wavelength and/or the angular orientation may subtend a predetermined range. Either the wavelength or the angular orientation may be iterated first, while the other quantity is held constant or is also iterated. At step 412, following an iteration, the beam is generated with the iterated quantities. Once the desired ranges of wavelengths and angular orientations have been covered, and measurements taken at each wavelength and angular orientation, the sample may be removed at 414.

FIG. 5 is a schematic drawing of another example optical measurement system 500. Elements 500-522, 526, 530-546, and 550-570 have the same structure and function as similarly numbered elements in FIG. 1. Compared with the system 100 of FIG. 1, the system 500 of FIG. 5 omits the detector 128, omits the signal 148 obtained from detector 128, increases the reflectivity of mirror 124 so that the mirror 524 has 100% or nearly 100% reflectivity, omits the first and second portions of light PI and P2, and adds a multi-pixel detector 598 that receives light reflected from the incident face 512 of the internal reflection element 510 at incident location 562.

The multi-pixel detector 598 senses the intensity of the beam before the beam interacts with the sample 570. This intensity is a scaled measure of the power produced by the light source 522. As such, the multi-pixel detector 598 provides the function of the detector 128 in FIG. 1.

In addition, as the first 524 and second 526 repositionable mirrors reposition the optical path P to achieve desired incident angles Θ on the measurement face 514 of the internal reflection element 510, the beam translates on the multi-pixel detector 598. The multi-pixel detector 598 can sense the position of the beam, and can provide the position and intensity information to the computer 530. The computer can relate the sensed position to the incident angle Θ on the measurement face 514 of the internal reflection element 510.

In some examples, the position information can be used as a monitoring signal, where the repositionable mirrors 524, 526 are run open-loop. In other examples, the position information is used as a servo signal in a feedback loop, and is used in a close-loop manner to control the repositionable mirrors 524, 526.

The resulting information obtained from the foregoing apparatus and process is used to produce information about bulk properties of the sample, such as the presence or absence of certain chemical elements, and/or concentration of certain chemical elements. The bulk properties of the sample can be used to provide bulk biological information about the sample, such as glucose concentration, alcohol concentration, and others. The description of the invention and its applications as set forth herein is illustrative and is not intended to limit the scope of the invention. Variations and modifications of the embodiments disclosed herein are possible, and practical alternatives to and equivalents of the various elements of the embodiments would be understood to those of ordinary skill in the art upon study of this patent document. These and other variations and modifications of the embodiments disclosed herein may be made without departing from the scope and spirit of the invention.

Claims

What is claimed is:

1. A system, comprising:

a first optical power detector;

a light source configured to produce a beam of light propagating along an optical path extending between the light source and the first optical power detector;

an internal reflection element disposed in the optical path adjacent the first optical power detector, the internal reflection element being configured to direct the beam of light onto a measurement face of the internal reflection element from within the internal reflection element, the beam of light striking the measurement face at a variable incident angle, the measurement face being configured to contact a sample, the optical path including a reflection at the measurement face, the reflection being formed at an interface between the internal reflection element and the sample;

a plurality of repositionable mirrors spaced apart in the optical path between the light source and the internal reflection element, the optical path including reflections from the plurality of repositionable mirrors, the plurality of repositionable mirrors being configured to reposition the optical path so as to vary the incident angle of the optical path at the measurement face of the internal reflection element;

wherein one of the plurality of repositionable mirrors is a beamsplitter, the beamsplitter reflecting a first portion of incident light, the first portion being directed along the optical path, the beamsplitter transmitting a second portion of incident light, the second portion being directed out of the optical path and onto a second optical power detector.

2. The system of claim 1, wherein the system determines a value of power reflectivity from the sample from a ratio of electrical signals produced by the first and second optical power detectors.

3. The system of claim 1 , wherein an electrical signal produced by the second optical power detector is proportional to an optical power in the beam of light produced by the light source.

4. The system of claim 1 , wherein the repositionable mirror closest in the optical path to the light source comprises the beamsplitter.

5. The system of claim 1 , wherein the internal reflection element is a prism.

6. The system of claim 1,

wherein the internal reflection element includes an incident face and an exiting face; and

wherein the incident face and the exiting face include anti-reflection coatings disposed thereon.

7. The system of claim 1 , wherein the internal reflection element is a partial sphere.

8. The system of claim 1,

wherein the internal reflection element includes a curved face; and wherein the curved face includes an anti-reflection coating disposed thereon.

9. The system of claim 7, further comprising a lens disposed in the optical path between the plurality of repositionable mirrors and the internal reflection element, the lens having a negative power matched to a curved face of the internal reflection element, so that a beam collimation in the optical path within the internal reflection element matches a beam collimation in the optical path between the lens and the plurality of repositionable mirrors.

10. The system of claim 1,

wherein the beamsplitter comprises a substrate, the substrate an incident surface facing the optical path and an exiting surface facing away from the optical path;

wherein the exiting surface of the substrate includes an anti-reflection coating disposed thereon.

11. The system of claim 10, wherein the incident surface of the substrate includes a thin film coating disposed thereon.

12. The system of claim 10, wherein the incident surface of the substrate is devoid of an optical coating.

13. The system of claim 1 , wherein the beam of light is collimated.

14. The system of claim 1 , wherein the plurality of repositionable mirrors are configured to reposition the optical path so that the optical path strikes the measurement face at a single measurement location on the measurement face, the single measurement location being invariant with respect to incident angle.

15. The system of claim 1 , wherein the plurality of repositionable mirrors are configured to reposition the optical path to subtend a range of incident angles at the measurement face, the range of incident angles including a critical angle between the sample and the internal reflection element.

16. The system of claim 1 , wherein the plurality of repositionable mirrors are configured to pivot over a range of mirror incident angles, the range of mirror incident angles including a Brewster' s angle.

17. The system of claim 1 , further comprising a processor receiving information from the first and second optical power detectors, and providing one or more signals to control positions of the plurality of repositionable mirrors to change the position of the optical path.

18. The system of claim 17, further comprising the processor controlling the light source.

19. A system, comprising:

a first optical detector and a second optical detector;

a light source configured to produce a beam of light;

a beamsplitter, a mirror, and an internal reflection element configured to direct a first portion of the beam of light to the first optical detector and a second portion of the beam of light to the second detector, the configuration further providing the first beam of light along a substantially constant optical path and the second portion of the beam of light along a variable optical path that is controllable by at least a positioning of the mirror,

wherein the internal reflection element has a planar reflective surface that can accommodate a sample for an optical measurement based on some of the second portion of the beam of light penetrating the sample when placed in proximity to the surface, the optical measurement occurring at least at the second detector, and wherein the internal reflection element has an index of refraction that is higher than the sample for the optical measurement.

20. The system of claim 19, further comprising a processor receiving information from the first and second optical detectors, and providing one or more signals to control positions of the plurality of repositionable mirrors to change the position of the optical path, the processor controlling the light source.