WO2016024992A1

WO2016024992A1 - Ir array sensor incorporating per-pixel lock-in amplifier capability

Info

Publication number: WO2016024992A1
Application number: PCT/US2014/051251
Authority: WO
Original assignee: Pandata Research Llc
Priority date: 2014-08-15
Filing date: 2014-08-15
Publication date: 2016-02-18

Abstract

An optical measurement system (100) produces light modulated at one or more modulation frequencies, directs the modulated light from within an internal reflection element (110) onto an interface (114) between the internal reflection element (110) and a sample (150), directs light reflected from the interface (114) onto a multi-pixel detector (140), produces electronic signals (142) corresponding to the detector pixels, analyzes the electronic signals at the modulation frequencies and/or at harmonics thereof, and extracts values of reflectivity of the sample at the interface. One or more lock-in amplifiers (160) perform the signal analysis. In some examples, one lock-in amplifier can be used for each detector pixel. Modulating the light at one or more modulation frequencies, and using the lock-in amplifiers to extract the reflectivity values can improve resistance to noise. In some examples, the reflectivity values are used in an intermediate stage during optical characterization of the sample.

Description

IR ARRAY SENSOR INCORPORATING PER-PIXEL

LOCK-IN AMPLIFIER CAPABILITY

[0001] 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 lock-in amplifier to improve signal detection.

BACKGROUND

[0002] 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.

[0003] 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.

[0004] 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, 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.

SUMMARY OF THE DISCLOSURE

[0005] 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 (e.g., prism) 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 may be detected by a first detector. The system can include at least two repositionable mirrors in an optical path between the collimated source and the internal reflection element. The system tilts or translates the repositionable mirrors, in order to produce a desired propagation direction for the incident beam. In some examples, the repositionable mirror redirects the collimated source, so that the position of the light on the second detector does not vary as the mirrors are repositioned. [0006] In one embodiment, a system includes a first optical power detector. The system may be configured to direct light from a collimated source provided at multiple incident angles into the internal reflection element, and thereby applies the light to the sample using the internal reflection element. The internal reflection element may be configured to have an index of refraction higher than that of the sample to be analyzed, thereby providing a mechanism for the light to strike the sample at incident angles that can be greater than the critical angle. The array detector may be configured to detect light reflected from the sample as it exits the internal reflection element. The array detector detects light power at each pixel, and sends the power information to a lock-in amplifier module. The lock-in amplifier module includes a lock-in amplifier corresponding to each pixel, where each lock-in amplifier may be in a closed loop with the modulated light source. By tuning the lock-in amplifier module to the frequency and phase of the amplitude-modulated light source, the lock-in amplifier module allows the array detector to extract the modulated light signal in a noisy environment. The modulation frequencies are below the time constant of the detector pixels.

[0007] Lock-in amplifiers can be built using analog components, or implemented digitally by taking samples of the source signal and performing the lock-in function digitally. A detector array of pixels may be used to locate a laser beam, and to measure the change in intensity (e.g., absorption) caused by the chemicals of interest. A conventional scanning array of N pixels can be considered equivalent to N signals sampled at the array- scanning rate. For example, in video applications, the array- scanning rate may be 50 Hz or 60 Hz. To perform a digital lock-in operation for each pixel, the modulation frequency of the light source should be below time constant for the detector pixels.

[0008] To improve robust operation of a lock-in amplifier, the lock-in filter function may need to capture many frames at slow scanning frequencies. For example, for a 60 Hz array- scanning rate, frames may be captured over a few seconds. Lock-in amplifier operation may also be improved by modulating at a low frequency. Noise levels may be close to noise levels of a basic DC measurement. In an embodiment, the lock- in amplifier may use a modulation and lock- in frequency between 10 kHz and 1 MHz, which may reduce noise levels while simplifying the electronic design. Ideally, the modulation should be high enough to overcome low-frequency 1/f noise, but lower than a cutoff frequency of the detector response and its associated electronics.

[0009] In some embodiments, combining a lock-in amplifier with laser signal detection at a pixel array may allow the pixel array to be tuned to a beam of light that may be amplitude modulated at a very specific frequency. In an embodiment, the signal used to modulate the light source may be used by the control electronics for each pixel, where the control electronics also induce phase changes to compensate for time-of-flight delays. In such an embodiment, the lock-in computation occurs in parallel for all pixels. In some embodiments, conventional video scanning logic may be used to read, manipulate, and display the output.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the following detailed description of example embodiments of the invention, reference is made to the accompanying drawings which form a part hereof, and which is shown by way of illustration only, specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

[0011] FIG. 1 is a schematic diagram of an example of an optical measurement system having an array detector and a lock-in amplifier.

[0012] FIG. 2 is a schematic diagram of another example of an optical measurement system having an array detector and a lock-in amplifier.

[0013] FIG. 3 is a flowchart of an example of a measurement method.

[0014] FIG. 4 is a flowchart of an example of a lock-in amplifier.

DETAILED DESCRIPTION

[0015] FIG. 1 is a schematic diagram of an example of an optical measurement system 100 having an array detector and a lock- in amplifier. The measurement system 100 includes a repositionable optical path (P) extending from a light source 122 to an optical power array detector 140. The optical path (P), and the various optical elements within the optical path (P), are described in greater detail below. The measurement system 100 shown in FIG. 1 is but one example; other suitable measurement system 100 can also be used.

[0016] 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 tunable light source having a selectable peak wavelength or center wavelength, 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 132 controls the light source 122, and may control switching between or among different wavelengths in the light source 122.

[0017] A first repositionable mirror 124 may be 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 may have a relatively high reflectivity, preferably as close to 100% as may be practical. Preferably, the reflectivity may be as close to uniform as is practical over the surface of the first repositionable mirror 124. In addition, the reflectivity may be as close to uniform as is practical over a range of incident angles at the first repositionable mirror 124, the range including a nominal condition and including extreme positions for the optical path (P). The first repositionable mirror 124 may be configured to change position in response to a driving electrical signal 134. The change in position can include translation in one or two dimensions, or rotation along one or two axes. In the example shown in FIG. 1, the first repositionable mirror 124 may be 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 may be perpendicular to the plane of the page of FIG. 1, and may be disposed at the center of the first repositionable mirror 124. The pivot axis may alternatively be located at any suitable location.

[0018] The first repositionable mirror 124 may be configured to reflect the incident light from the light source 122. The light may be directed along the optical path (P) to downstream optical elements in the optical path (P). As the first repositionable mirror 124 may be repositioned, the light and the optical path (P) are also repositioned.

[0019] A second repositionable mirror 126 may be disposed in the optical path (P) and receives the light of light from the first repositionable mirror 124. The second repositionable mirror 126 may be also configured to change position in response to a driving electrical signal 136, 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 may be as close to uniform as is practical over the surface of the second repositionable mirror 126. In addition, the reflectivity may be 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).

[0020] The light source 122, the first repositionable mirror 124, and the optical power detector 128 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.

[0021] 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.

[0022] 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.

[0023] 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 SN₁₁₄. 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.

[0024] During operation, a sample 150 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 150. 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 150 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 150 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 less than 100%, and is frequently 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 150, for the condition known as total internal reflection), or slightly less than 100% (for an absorbing sample 150 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 SN₁₁₄) 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.

[0025] 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.

[0026] An optical power array detector 140 may be disposed in the optical path (P) and receives the external beam from the exiting face of the internal reflection element 110. The optical power array detector 140 includes an array of optical power detecting elements. The power detected by the optical power detecting elements produces an electrical signal 142 that relates to, or is proportional to, the amount of optical power in the external beam. In an alternate configuration, the detector 140 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 array detector 140. For this alternate configuration, the detector 140 senses the optical power in the internal beam and an external beam is not produced.

[0027] The detector 140 may detect light power at each pixel, convert the detected power level to a corresponding electrical signal, and direct the corresponding electrical signal to a lock-in amplifier module 160. The lock-in amplifier module 160 may include a lock- in amplifier corresponding to each pixel. In some embodiments, each lock-in amplifier may include a multiplying amplifier, a rectifier, and a filter. Each lock-in amplifier may be in a closed loop with the computer 130, in which the computer 130 provides control signals 138 in response to the output of each lock-in amplifier. The electrical signal 138 that controls the lock-in amplifier module 160 may provide information about the modulation of the light source 122. The electrical signal 138 information may be used to tune the lock- in amplifier module 160 to the frequency and phase of the amplitude-modulated light source 122, which allows the lock-in amplifier module 160 to extract the modulated light signal in a noisy environment. In some embodiments, conventional video scanning logic may be used to read, manipulate, and display the output. The lock-in amplifiers can be analog, digital, or a combination of analog and digital. In some examples, the lock-in function can be provided by digital processing.

[0028] In some examples, in addition to receiving the electrical signal 138 produced by the lock- in amplifier module 160 and generating the electrical signal 138 that controls the lock- in amplifier module 160, the computer 130 may generate the electrical signal 132 that controls the light source 122 and the electrical signals 134, 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 150 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.

[0029] 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.

[0030] 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.

[0031] In some examples, as a beam propagates along the optical path (P), including reflection off various elements and refraction through various surfaces, the beam' s polarization state can change. For instance, a plane of polarization can rotate, or a phase shift can change between polarization directions, which can lead to an ellipticity. Such changes in polarization state are well understood by one of ordinary skill in the art.

[0032] FIG. 2 is a schematic diagram of another example of an optical measurement system 200 having an array detector and a lock-in amplifier. FIG. 2 depicts a cross-sectional representation of an example internal reflection element 210, which may be in contact with a sample 150, in combination with other system components. FIG. 2 depicts a perspective view of an internal reflection element 210, with a highlighted optical path (P) within the internal reflection element 210. The internal reflection element 210 for use with the systems and methods described herein has a cross-section, taken in the plane of the page in FIG. 2, which includes four sides arranged as a trapezoid. A sample 150 may be placed in contact with measurement face 214. A second of the four sides may be a detector face 218, which may be disposed opposite the measurement face 214, and may be generally parallel to the measurement face 214 to within typical manufacturing tolerances. A longitudinal axis of the internal reflection element 210, denoted as A, may be parallel to both the measurement face 214 and the detector face 218. The remaining two faces of the prism are disposed at opposite longitudinal ends of the prism, and include an incident face 212 and an absorbing face 216. In the example geometry of FIG 2, both the incident face 212 and the absorbing face 216 are angled, with respect to the measurement face 214 and the detector face 218, with the angling occurring within the plane of the page of FIG. 2. In alternate configurations, a cross- section of the internal reflection element 210 may have more than four sides.

[0033] The measurement face 214, the detector face 218, the incident face 212, and the absorbing face 216 have surface normals denoted as SN214, SN21₈, SN212, and SN21₆, respectively. In the example of FIG. 2, the surface normals SN214, SN218, SN212, and SN216 are coplanar, and are disposed within the plane of the page of FIG. 2. In FIG. 2, the plane of the page may be an incident plane that includes the longitudinal axis (A) and may be perpendicular to the measurement face 214. In other examples, one or more of the surface normals extend out of the plane of the page of FIG. 2.

[0034] The measurement face 214 may be capable of sustaining attenuated total reflectance with the sample material. During the measurement, when the sample 150 may be in contact with the measurement face 214, light strikes the measurement face 214 at a particular incident angle determined by the geometry of the internal reflection element 210 and the entrance angle of the incident beam.

[0035] In example configurations in which the sample 150 may be evaluated by light at an angle within the peri-critical region, light strikes the sample at or close to the critical angle. As discussed above, the critical angle is given by the numerical value of sin^"1 (n_sampi_e / n_prism), where n_sampi_e and n_prism are the refractive indices of the sample 150 and internal reflection element 210, respectively. The precise angles used will vary as a function of the wavelengths used and the nature of the investigation. In some systems, the utilized angles of incidence may be at a desired increment within a range of up to approximately two degrees to one side, or to both sides, of the critical angle. In other applications, the range of angles may be substantially less, for example, less than a degree (or even half-degree) to one side, or to both sides, of the critical angle, and potentially in increments as small as milli-degree increments through at least a portion of that range.

[0036] Although the true value of the refractive index of the sample may not be known precisely beforehand, an expected value for the refractive index of the sample will preferably be determined, either specifically in reference to that sample, or by prior experimental evaluation of a plurality of comparable samples. The expected value may be included within a range of possible measured values. Such determination of an approximate refractive index may, if it is sufficiently accurate, narrow the range of angles of incidence to be examined. The internal reflection element 210 material may be selected to have a refractive index n_pri_Sm greater than the expected value of the refractive index of the sample n_sampie.

[0037] A series of measurements may be taken for a variety of incident angles, each of which measures the amount of light reflected at the measurement face 214. The quantity being measured may be the power reflectivity, which may be sometimes referred to as a power reflectance. The power reflectivity can vary between 0 and 1, or 0% and 100%. In most examples, the values measured will be close to 100%, with the difference between the measured value and 100% corresponding to the amount of light absorbed by the sample.

[0038] The incident face 212 of the internal reflection element 210 may be configured to receive an incident beam from upstream optical components, to allow the incident beam to refract through the incident face 212, and to allow the refracted incident beam to propagate within the internal reflection element 210 as an internal beam. The optical paths of the incident beam and the internal beam are collectively denoted as P in FIGs. 1 and 2.

[0039] In some examples, it may be desirable to reduce reflections at the incident face 212 by as much as may be practical, in order to maximize the amount of light transmitted through the incident face 212. In many examples, this high transmission and low reflection may be achieved by an anti-reflection coating on an exterior surface of the incident face 212. As will be apparent to those skilled in the art, the precise coating used will typically be dependent on the wavelengths of the light contemplated to be used, the incident angles a at which the incident light strikes the incident face 212, and in some cases the composition of internal reflection element 210. In some examples, the coating includes one or more dielectric materials. In other examples, the incident face 212 may be uncoated.

[0040] In the example of FIGs. 1 and 2, the incident face 212 may be angled so that the surface normal to the incident face, SN212, extending into the internal reflection element 210, intersects the measurement face 214. (The intersection is not shown explicitly in FIG. 2, but would occur if the surface normal to the incident face, SN212 were extended further inside the internal reflection element 210.) In general, the angling of the incident face 212 may be selected to allow upstream optical components, such as the components that produce and position an incident light beam on the incident face 212, to be disposed within a convenient location within a packaged system. For instance, the incident face 212 in the example of FIG. 2 may be angled such that an incident beam on the internal reflection element 210 may be roughly perpendicular to the

measurement face 214 (allowing for a pivoting of the incident beam during operation by a few degrees in either or both directions away from

perpendicularity to the measurement face 214). Such an example orientation may allow the optical components of the system to be packaged in a relatively compact footprint underneath the measurement face 214 (e.g., having a relatively small left-to-right extent in the view of FIG. 2). In other examples, the incident face 212 may be angled in the opposite direction; such an orientation may allow the incident beam to have an angular orientation relatively close to the longitudinal axis (A), which may be useful if it may be desired that the packaged optical system be relatively thin (e.g., having a relatively small top-to- bottom extent in the view of FIG. 2).

[0041] The detector face 218 of internal reflection element 210 reflects a fraction of the incident light back toward the measurement face 214 and transmits a fraction of the incident light to an array detector 240 that, as noted earlier herein, will be secured in fixed relation to the detector face 218 of the internal reflection element 210. The detector face 218 may also be parallel to measurement face 214.

[0042] The detector face 218 may also have a thin coating thereon to facilitate the identified partial transmissivity and partial reflectivity. As will be apparent to those skilled in the art, the precise coating used will typically be dependent on the wavelengths of the light contemplated to be used, the incident angle at which the internal beam strikes the detector face 218, and in some cases the composition of internal reflection element 210. In some examples, the coating on the detector face may be made from a metal, such as gold. In other examples, the coating may include one or more dielectric materials, or may include alternating layers of relatively high and relatively low refractive index materials.

[0043] In some examples, it may be desirable to have the detector face 218 coating transmit a relatively small fraction of the incident light. Such a low transmissivity may be desirable for either of two reasons: first, the low transmissivity may reduce the effects of light that reflects off the detector surface, and may prevent such reflected light from entering back into the internal reflection element 210; and second, the low transmissivity may be beneficial for particular detectors that operate best at relatively low light levels.

[0044] In many examples, the array detector 240 will be secured directly to the internal reflection element 210. In other embodiments, for example, the internal reflection element 210 and array detector 240 might be coupled to a common structure that will support the two components in a desired, fixed relation to one another. Regardless of the specifics of how the array detector may be secured in fixed relation to the detector face, by virtue of such fixed relation the two components are integrated into an assembly in which the detector can consistently measure both the intensity of the light beam at each imprint at the detector and the position of each imprint. In many examples, the partially transmissive coating will extend along the detector face 218 at least where the beam may be contemplated to imprint on the array detector 240.

[0045] In some examples, the array detector 240 is optically bonded to detector face 218. In some examples, the array detector 240 is immersed in refractive index-matching material that has a refractive index greater than or equal to that of the sample; this condition can allow light to overcome the condition of total internal reflection and exit the internal reflection element 210 at the detector face 218, for all ranges of the incident angles at the measurement face 214. In some examples, the array detector 240 can be fabricated directly onto the internal reflection element 210.

[0046] The array detector 240 may be a commercially available model, or a straightforward modification to a commercially available unit. For instance, there are some commercially available sensors that use mercury cadmium telluride (MCT), or HgCdTe. These are cooled detectors that work in the infrared spectrum discussed herein. The sensors are available off-the-shelf in a 640 pixel by 480 pixel design, with a 12 μιη pixel-to-pixel pitch; and also in a 1000 pixel by 720 pixel design, having a 500 Hz. scanning frequency with a either a 5 or 10 μιη pixel-to-pixel pitch. It is straightforward to use the sensor technology of these MCT sensors with a custom size and shape for the array detector. While array detector 108 will beneficially be formed as a single detector, in some cases, it may be formed from multiple smaller array detectors (for example, from multiple discrete semiconductor devices) cooperatively both physically coupled and electrically configured to effectively form a single detector.

[0047] In some applications, the sensor may be made as a one-dimensional array, with each pixel in the sensor having its long dimension in the x-y plane and having its short dimension along the longitudinal z-axis. Such a 1-by-N array of pixels would be able to record the intensity and location along the longitudinal z-axis of each bounce of the internal beam off the detector face 218. Alternatively, it may be beneficial to form the sensor as a 2-by-N array, rather than a 1 -by-N array, because the split pixels may be useful for aligning the optics in the system; it may be desirable to keep the internal beam, and its multiple bounces, centered on the array detector 240 as it travels along the longitudinal extent of the internal reflection element 210. Higher pixel-count detectors may also be used.

[0048] The array detector 240 will receive a light imprint from each bounce off the detector face 218. As noted earlier herein, it may be preferable that the light imprints on array detector 240 be spatially separated. From adjacent light imprints, the location information, along with a predetermined geometry of the internal reflection element 210, may be used to provide a calculated value of incident angle off the measurement face 214. Likewise, from adjacent light imprints, the intensity information may be used to calculate a power reflectivity from the measurement face 214.

[0049] FIG. 2 shows the optical path P for the internal beam inside the internal reflection element 210. The beam enters the internal reflection element 210 at the incident face 212, then alternately reflects off the measurement face 214 and the detector face 218, then terminates at the absorbing face 216. The internal beam reflects off the measurement face 214 at locations 206, 208, and 210. The internal beam reflects off the detector face 218 at locations 202 and 204; which are representative of the location of the imprints on the array detector 240. As noted earlier herein, it may be preferable that the light imprints on array detector 240, at locations 202 and 204, be spatially separated. From adjacent light imprints, the location information, along with a predetermined geometry of the internal reflection element 210, may be used to provide a calculated value of incident angle off the measurement face 214. Likewise, from adjacent light imprints, the intensity information may be used to calculate a power reflectivity from the measurement face 214. In many examples, such as the examples of FIGs. 1 and 2, the optical path (P) remains in a single plane, including both the beam incident on the incident face 212 and the internal beam propagating inside the internal reflection element 210. Such a single plane may include the plane of incidence, which may be parallel to the longitudinal axis (A) and may be perpendicular to the measurement face 214. FIG. 2 shows three reflections off the measurement face 214 and two reflections off the detector face 218.

However, suitable geometries for the internal reflection element 210 and the beam may also include more or fewer than three reflections off the measurement face 214 or more than two reflections off the detector face 218.

[0050] The cross-section of the internal reflection element 210, taken perpendicular to the longitudinal axis (A), may be the same at each location along the longitudinal extent of the internal reflection element 210 between the incident face 212 and the absorbing face 216. The cross-section may be generally uniform, where the internal reflection element 210 may have typical manufacturing tolerances on the sizes and orientations of the various faces. The cross-section may be uniform at each point along the longitudinal axis, even if manufactured parts may vary slightly from part-to-part due to manufacturing tolerances. In some examples, such as the examples of FIGs. 1 and 2, the internal reflection element 210 may have a rectangular cross section, taken perpendicular to the longitudinal axis (A). In other examples, the internal reflection element 210 may have a cross-section that may be square, hexagonal, trapezoidal, polygonal, or another suitable shape.

[0051] As the incident angle a of the beam may be scanned over its angular range, the location at which the beam enters the internal reflection element 210 may correspond to the location where surface normal SN212 intersects the incident face 212. This location may translate across the incident face 212, provided that the beam does not become clipped upon entry into the internal reflection element 210 or during its propagation along the optical path P, and provided that the beam does not translate off the sample at the measurement face 214. The beam may remain in a single plane during propagation through the internal reflection element 210.

[0052] After reflecting alternately between the measurement face 214 and the detector face 218, the internal beam may strike the absorbing face 216 in the internal reflection element 210, where the absorbing face 216 may be at the opposite longitudinal end of the internal reflection element 210 from the incident face 212. The absorbing face 216 may absorb the internal beam completely, thereby minimizing reflections from the absorbing face 216. This high absorption may be achieved by a coating on the exterior surface of the absorbing face 216. The precise coating used may be dependent on the wavelengths of the light contemplated to be used, and in some cases, on the composition of internal reflection element 210. In some examples, the high-absorption coating may have a relatively thick metallic portion, and an anti-reflection portion disposed between the metallic portion and the internal reflection element 210 material. In these examples, the metallic portion may absorb the light, so that essentially no light transmits through the metallic portion, and the anti-reflection portion minimizes any reflections that would have otherwise arisen at the interface between the internal reflection element 210 material and the metallic material. In other examples, other suitable absorbing materials and structures may be used. In some examples, the absorbing face 216 may direct all or a portion of the internal light out of the internal reflection element 210, which can prevent internal reflections within the internal reflection element 210 from directing stray light onto the detector. In some examples, an absorber can be disposed on any or all of the faces of the prism proximate absorbing face 216.

[0053] In this example, a light source 222 produces a collimated light beam that propagates along an optical path (P). Suitable light sources can include one or more semiconductor lasers, one or more light emitting diode, a combination of semiconductor lasers and light emitting diodes, and suitable collimating optics. A plurality of repositionable mirrors 224, 226, disposed in the optical path (P) between the light source 222 and the internal reflection element 210, direct the light beam onto the internal reflection element 210. Although two mirrors are shown in the optical path (P) in FIG. 2, more than two mirrors may also be used, as needed. The light source 222 and mirrors 224, 226 may collectively be referred to as a beam emitter assembly 220. The beam produced by the beam deflection assembly 220 has variable insertion angle a, formed with respect to a surface normal SN212 from the incident face 240 of the internal reflection element 210.

[0054] An example computer 230 controls the light source 222 and the repositionable mirrors 224, 226. The processing unit 230 can also receive and process measurements from the array detector 240. The array detector 240 may consist of an array of optical power detecting elements. The power detected by the optical power detecting elements may produce an electrical signal 242 proportional to the amount of optical power in the external beam. The detector 140 may detect light power at each pixel and send the power information to a lock-in amplifier module 260. The lock-in amplifier module 260 may include a lock-in amplifier corresponding to each pixel. In some embodiments, each lock- in amplifier may include a multiplying amplifier, a rectifier, and a filter. Each lock-in amplifier may be in a closed loop 238 with the computer 230, in which the computer 230 provides control signals 238 in response to the output of each lock-in amplifier. The electrical signals 238 that control the lock-in amplifier module 260 may provide information about the modulation of the light source 222. The electrical signal 238 information may be used to tune the lock-in amplifier module 260 to the frequency and phase of the amplitude-modulated light source 222, which may allow the lock-in amplifier module 260 to extract the modulated light signal in a noisy environment. In some embodiments, conventional video scanning logic may be used to read, manipulate, and display the output.

[0055] The processing unit 230 can include one or more processors, in combination with additional hardware as needed (e.g., volatile memory, nonvolatile memory, communication ports; I O device, or I/O ports; etc.) to provide the control functionality as described herein. Hardware functionality may include processing the measurements from the array detector 240 or

incrementally incrementing the angle of beam deflection assembly 220 to vary the insertion angle a of the incident beam on internal reflection element 210.

Such processing may include performing the identified correlations to determine the presence or concentration of constituents in the sample (such as analytes in the blood in the described examples). These functions may be implemented by separate processing units, and additional functions may be performed by such one or more processing units.

[0056] FIGS. 1 and 2 show examples of optical measurement systems that direct light onto a sample at incident angles within a range of incident angles and at wavelengths within a range of wavelengths, collect light reflected from the sample, and characterize the sample based on the collected light. In both of these examples, the light can be pulsed or modulated at one or more frequencies, and then demodulated by the lock-in amplifier to determine reflectivity values from the sample. The modulation and demodulation can improve resistance to noise in the optical and electronic signals. The examples of FIGS. 1 and 2 show two particular configurations for the internal reflection element; other suitable configurations can be used as well. In some examples, a reference detector (not shown) can provide a reference value of optical power. In some examples, a first bounce inside the internal reflection element can be configured to direct a reference portion of a light beam onto the detector, which can provide the reference value of optical power. The optical measurement systems can generate values of reflectivity of the sample at a plurality of incident angles and at a plurality of wavelengths, then can analyze the generated values to determine a particular property of the sample, such as refractive index, composition, concentration level of a particular analyte, or others.

[0057] FIG. 3 is a flowchart of an example measurement method 300, according to an example embodiment. The example method 300 may be executed using the system 100 of FIG. 1 , the system 200 of FIG. 2, or by another suitable measurement system. It may be assumed for this example method that more than one wavelength may be used for the measurements; it will be understood that measurements may also be taken at a single wavelength. Step 302 places a sample 150 in contact with the measurement face 114; 214 of the internal reflection element 110; 210. Step 304 selects an initial wavelength and an angular orientation for a beam produced by the light production and direction module 120. Step 306 generates a beam at the particular wavelength and angular orientation as selected at step 304. The light production and direction module 120 generates the beam. Step 308 directs the beam produced at step 306 into the internal reflection element 110; 210. Step 310 measures the power reflectivity from the sample 150. The power reflectivity may be a ratio between 0% and 100%, typically close to 100%, and may be proportional to the signal 142 produced by the detector 140, 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 or

concentration of a constituent in the sample 150. Step 312 iterates the wavelength and the angular orientation of the beam, so that the wavelength or the angular orientation may subtend a predetermined range. Either the wavelength or the angular orientation may be iterated first, while the other quantity may be held constant or may be also iterated. At step 312, following an iteration, the beam may be 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 314.

[0058] FIG. 4 is a flowchart of an example lock-in amplifier 400, according to an example embodiment. The example lock- in amplifier 400 may combine the input signal 402 with a reference signal 404, and may measure 406 the combined signal and noise. In some embodiments, the reference signal is a carrier wave of a particular frequency, where the carrier frequency is different from the frequency of one or more noise sources. The measured data may be passed through a bandpass filter 408. In some embodiments, the bandpass filter is a narrow-band bandpass filter centered at the reference signal frequency. The filtered data may be amplified 410. The reference signal 404 may be phase- shifted 412 to compensate for any phase delays induced by the system (e.g., time-of-flight delays of the measurements). The phase-shifted reference signal and filtered and amplified measurements may be multiplied 414 in a phase- sensitive detector and integrated 416. In some embodiments, the integration time may be longer than the reference signal period. Following integration 416, the example lock-in amplifier 400 provides the recovered signal 418.

[0059] In one example of a method for optically characterizing a sample, the method modulates light at one or more modulation frequencies. The method directs the modulated light from within an internal reflection element onto an interface between the internal reflection element and the sample. The method directs light reflected from the interface onto a multi-pixel detector. The method produces electronic signals corresponding to the detector pixels. The method analyzes the electronic signals at the modulation frequencies and/or at harmonics thereof. The method extracts values of reflectivity of the sample at the interface. In some cases, one or more lock-in amplifiers analyze the electronic signals at the modulation frequencies and/or at harmonics thereof. In some cases, the one or more lock-in amplifiers analyze the electronic signals at both the modulation frequencies and at second harmonics of the respective modulation frequencies. A ratio of the second harmonic, divided by the fundamental, can reduce or eliminate the need for a reference power level, which can ease calibration requirements.

[0060] 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.

[0061] 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:

an optical array detector including a plurality of pixels;

a lock-in amplifier configured to:

receive one or more outputs from the optical array detector;

receive frequency and phase control signals; and

provide a measurement signal representative of a unit under test for each pixel in the plurality of pixels;

a light source configured to produce a beam of light; and

an internal reflection element configured to direct the beam of light to the optical array detector; and

a processor configured to:

receive the measurement signal from the lock-in amplifier;

provide one or more signals to control a position of the second mirror to change the optical path of the beam of light; and

provide frequency and phase control signals to the lock-in amplifier;

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 the optical array detector, and wherein the internal reflection element has an index of refraction that is higher than the sample for the optical measurement.

2. The system of claim 1, further comprising the processor controlling a position of the first mirror.

3. The system of claim 1, further comprising the processor controlling the frequency of the light source.

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

5. 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.

6. The system of claim 1, further comprising a lens disposed in the optical path between the second mirror 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.

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

8. The system of claim 1, wherein the first mirror and the second mirror 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.

9. The system of claim 1, wherein the first mirror and the second mirror 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.

10. 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.

11. A method for optically characterizing a sample, comprising:

modulating light at one or more modulation frequencies;

directing the modulated light from within an internal reflection element onto an interface between the internal reflection element and the sample; directing light reflected from the interface onto a multi-pixel detector; producing electronic signals corresponding to the detector pixels;

analyzing the electronic signals at the modulation frequencies and/or at harmonics thereof; and

extracting values of reflectivity of the sample at the interface.

12. The method of claim 11, wherein one or more lock- in amplifiers analyze the electronic signals at the modulation frequencies and/or at harmonics thereof.