Optical sensor based on multi-channel interferometer

Abstract

An optical sensor includes a substrate, a Fabry-Perot interferometer, and first and second photodetectors. The Fabry-Perot interferometer includes first and second reflectors and is mounted on the substrate such that light is transmitted through the interferometer to the substrate. The first and second photodetectors are configured to detect light transmitted through the etalon and the substrate. The first photodetector is sensitive to a first wavelength range, and the second photodetector is sensitive to a second wavelength range, wherein the first and second wavelength ranges each correspond to a different mode of the interferometer.

Description

Technical Field

[0001] The present invention relates to optical components. In particular, the present invention relates to a wavelength-discriminating optical sensor combining an interferometer and a photodetector. Background Art

[0002] Miniaturized wavelength-differentiating optical sensors are usually constructed by an optical interferometer mounted on a substrate and a detector located below the substrate. Figure 1A In the detector shown, interferometer 101 is a Fabry-Perot interferometer (also known as an etalon) that includes a top mirror 102, a bottom mirror 103, and a MEMS (micro-electromechanical system) element 104 configured to control the spacing between the top and bottom mirrors. The interferometer is mounted on a substrate 105, and light transmitted by both the interferometer and the substrate is picked up by a detector 106. The wavelength range 111 of the detector is shown in FIG1b and contains a minimum transmission peak 112 and a maximum transmission peak 113 of the interferometer in a particular optical mode (i.e., peaks corresponding to the maximum and minimum gaps between the mirrors controlled by the MEMS).

[0003] Additional optical components (e.g., lenses or optical filters) may be present to control the light entering the interferometer, or to control the light transmitted through the substrate. For example, a lens may be used to capture more light, or an optical filter may be used to filter out unwanted light (e.g., higher-order optical modes of the interferometer).

[0004] "Optical" and "light" are used herein to refer to visible light and adjacent regions of the spectrum - namely, infrared and ultraviolet light. Summary of the Invention

[0005] Detectors such as those described in the background are sensitive to a single mode of the interferometer (i.e., a single set of overtones of the received light). However, in many spectrometric applications, particularly in the near-infrared region, the behavior of these overtones is highly anharmonic, i.e., they do not behave in a simple way, which complicates the identification of molecules from their spectra.

[0006] This problem could be mitigated if there were independent detections of light in different overtones, which could then be compared to more accurately determine the cause of the detected wavelength (i.e., the underlying structure of the molecule).

[0007] While spectroscopy is presented as a specific example use case, optical sensors capable of detecting multiple optical modes of an etalon may be useful in other applications, including color sensors, spectral sensors, or dual-frequency proximity or time-of-flight sensors.

[0008] Therefore, the present invention provides an optical sensor. The optical sensor includes a substrate, a Fabry-Perot interferometer, and first and second photodetectors. The Fabry-Perot interferometer includes first and second reflectors and is mounted on the substrate such that light is transmitted through the interferometer to the substrate. The first and second photodetectors are configured to detect light transmitted through the etalon and the substrate. The first photodetector is sensitive to a first wavelength range, and the second photodetector is sensitive to a second wavelength range, wherein the first and second wavelength ranges each correspond to a different mode of the interferometer.

[0009] To provide different wavelength ranges, the first photodetector may include a photodiode formed of a first material, and the second photodetector may include a photodiode formed of a second material different from the first material. In this case, the first material may be transparent to the second wavelength range, and the second photodetector is positioned so that light transmitted by the interferometer passes through the first photodetector before reaching the second photodetector.

[0010] As an alternative to providing different wavelength ranges, the first photodetector can include an optical filter that blocks at least the second wavelength range and transmits the first wavelength range, and the second photodetector can include an optical filter that blocks at least the first wavelength range and transmits the second wavelength range. In this case, the first and second photodetectors can be provided on the same die.

[0011] The first reflector may have a greater refractive index than the second reflector, and the second reflector may be positioned closer to the substrate than the first reflector. For example, the first reflector may be formed of silicon dioxide (SiO2), and the second reflector may be formed of polycrystalline silicon (poly-Si). BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1A An optical sensor is shown;

[0013] Figure 1B Shown Figure 1A The wavelength range of the sensor;

[0014] Figure 2A is a schematic diagram of an exemplary optical sensor;

[0015] Figure 2B Shown Figure 2A The wavelength range of the sensor;

[0016] Figure 3 shows the wavelength-dependent reflectivity of a first exemplary interferometer;

[0017] Figure 4shows the wavelength-dependent reflectivity of a second exemplary interferometer;

[0018] Figure 5 Example wavelengths of interest in spectroscopy are shown DETAILED DESCRIPTION

[0019] An idealized Fabry-Perot interferometer will have a transmission peak at a certain wavelength that depends only on the distance between its mirrors. The transmission peak satisfies the relationship λ = 2d / m, where d is the distance between the mirrors and m is an integer called the "interference order" or "optical mode". Figure 1A The prior art sensor shown operates in a single optical mode—that is, the sensor is configured to detect only light in a specific optical mode, and light corresponding to transmission peaks in other modes is either filtered out within the range where the detector is insensitive or is filtered out within the range where the interferometer does not transmit (e.g., due to the reflective properties of the mirror).

[0020] Operation in a single mode is sufficient for many applications. However, in some cases it would be useful to detect multiple modes simultaneously, i.e. to detect the wavelength λ m =2d / m and wavelength λ n = 2d / n, where m and n are different optical modes. For example, in spectroscopy, it is often difficult to confirm whether a species has been detected without comparing across multiple wavelengths and multiple overtones of the species' characteristic wavelength. An interferometer can be constructed in which two of the modes are aligned with different overtones of the species' characteristic wavelength, in principle allowing two measurements to be made simultaneously. This can also be used in other applications that require simultaneous monitoring of two or more different wavelengths, such as color sensors, spectral sensors, or dual-frequency versions of proximity or "time-of-flight" sensors.

[0021] Figure 2A An example of a practical construction of such a sensor is shown in FIG. The sensor comprises an interferometer 201, which comprises a top mirror 202, a bottom mirror 203, and may comprise a MEMS (micro-electromechanical system) element 204 configured to control the spacing between the top mirror and the bottom mirror. The interferometer is mounted on a substrate 205, and light transmitted by both the interferometer and the substrate is picked up by a first detector 206 and a second detector 207. Figure 2BAs shown, the first and second detectors are each sensitive to different wavelength ranges 211, 212. These wavelength ranges each correspond to a different optical mode of the interferometer, so that in operation, the first detector detects wavelengths transmitted by the first optical mode (maximum transmission peak 213 and minimum transmission peak 214 shown), and the second detector detects wavelengths transmitted by the second optical mode (maximum transmission peak 215 and minimum transmission peak 216 shown).

[0022] The materials of the first and second reflectors can be selected to ensure good transmission in the wavelength range of the first and second detectors. For example, for visible light, metal reflectors generally provide good transmission. In the near infrared spectrum, reflectors made of alternating layers of two materials, one of which has a greater refractive index than the other, will provide good transmission. The materials can be silicon compounds. For example, Figure 3 The reflectivity curve of an interferometer comprising mirrors formed from alternating layers of Si3N4 and SiO2 is shown, wherein the main usable range 301 is between 1300 and 1800 nm (corresponding to the fourth optical mode of the 400-450 nm system). Figure 4 The reflectivity curve for an interferometer comprising mirrors formed from "polysilicon" and SiO2 is shown, and the main usable range 401 is quite large - extending from about 1200 nm to over 2000 nm. In addition, Figure 3 and Figure 4 Both have a secondary usable range around 550 nm 302, 402. When these materials are used in the detectors described with reference to FIG2 , the first and second detectors may both have wavelength ranges within the primary usable range, or one may have a wavelength range within the primary usable range and the other may have a wavelength range within the secondary usable range.

[0023] Additional filters may be applied before the interferometer or between the interferometer and the detector to block light outside the wavelength range of the detector (thereby reducing interference).

[0024] In order to generate different wavelength ranges, the first detector and the second detector can be implemented as:

[0025] Two separate photodetectors, each with different filters applied to them;

[0026] Two photodetectors on the same die, each with a different filter applied to them;

[0027] Two separate photodetectors, each made of a different material, optionally with different filters applied;

[0028] Two photodetectors formed at different depths within the same wafer;

[0029] • Two photodetectors, one formed on the upper side of the wafer and one formed on the lower side of the wafer.

[0030] Equivalent configurations can be used for more than two detectors.

[0031] While the above sensors have many possible use cases, one particular use case is in spectroscopy. When detecting certain species in a spectrum, each species has a characteristic set of "overtones," which are harmonics of the fundamental emission wavelength of that species. However, the relationship between the fundamental wavelength and the overtones is not purely harmonic—several overtones may be stronger, weaker, broader, or narrower than would be expected from purely harmonic behavior. This is particularly true in Figure 5 This is shown in the example for several species (each row of the graph corresponds to a species or a group of closely related species). Thus, by simultaneously measuring at corresponding wavelengths in, for example, the first and second over-band regions, it is possible to more accurately determine which species are present in a sample.

[0032] Embodiments of the present disclosure may be used in many different applications, including spectroscopy, proximity or time-of-flight sensing, color measurement, etc., for example, in scientific settings, security, automation, food technology, and other industries.

[0033] List of reference numerals:

[0034] 101 Interferometer

[0035] 102 top reflector

[0036] 103 bottom reflector

[0037] 104MEMS components

[0038] 105 substrate

[0039] 106 detector

[0040] Wavelength range of 111 detector

[0041] 112 minimum transmission peak

[0042] 113 Maximum transmission peak

[0043] 201 interferometer

[0044] 202 top reflector

[0045] 203 bottom reflector

[0046] 204MEMS components

[0047] 205 substrate

[0048] 206 First Detector

[0049] 207 Second Detector

[0050] 211 Wavelength range of the first detector

[0051] 212 Wavelength range of the second detector

[0052] 213 Maximum transmission peak of the first mode

[0053] 214 Minimum transmission peak of the first mode

[0054] 215 Maximum transmission peak of the second mode

[0055] 216 Minimum transmission peak of the second mode

[0056] The main applicable range of 301 interferometer

[0057] 302 Secondary usable range of the interferometer

[0058] The main applicable range of 401 interferometer

[0059] 402 Secondary usable range of the interferometer

[0060] Those skilled in the art will understand that in the foregoing description and the appended claims, positional terms such as "above," "along," and "to the side" are made with reference to conceptual illustrations such as those shown in the accompanying drawings. These terms are used for ease of reference and are not intended to be limiting. Therefore, these terms should be understood to refer to an object when in the orientation shown in the accompanying drawings.

[0061] Although the present disclosure has been described in terms of the preferred embodiments described above, it should be understood that these embodiments are merely illustrative and that the claims are not limited to those embodiments. In light of this disclosure, those skilled in the art will be able to make modifications and substitutions, which are considered to fall within the scope of the appended claims. Each feature disclosed or shown in this specification may be combined in any embodiment, either alone or in any appropriate combination with any other features disclosed or shown herein.

Claims

1. An optical sensor comprising: substrate(205); a Fabry-Perot interferometer (201), comprising a first reflector (202) and a second reflector (203), and mounted on the substrate such that light is transmitted through the interferometer to reach the substrate; wherein the Fabry-Perot interferometer is configured to transmit light of different optical modes within a common optical path; a first photodetector (206) and a second photodetector (207), the first photodetector and the second photodetector being configured to detect light transmitted through the Fabry-Perot interferometer and the substrate; wherein the first photodetector is sensitive to a first wavelength range (211) and the second photodetector is sensitive to a second wavelength range (212), and wherein the first wavelength range and the second wavelength range each correspond to a different mode of the interferometer, wherein the different modes of the interferometer correspond to different optical modes of the interferometer.

2. The optical sensor according to claim 1, wherein The first photodetector includes a photodiode formed of a first material, and the second photodetector includes a photodiode formed of a second material different from the first material.

3. The optical sensor according to claim 1 or 2, wherein: The first photodetector is transparent to the second wavelength range, and the second photodetector is positioned such that light transmitted by the interferometer passes through the first photodetector before reaching the second photodetector.

4. The optical sensor according to claim 1, wherein The first photodetector and the second photodetector are formed on the same wafer such that the first photodetector is located on a side of the wafer opposite to the second photodetector, and the wafer is arranged such that light transmitted by the interferometer passes through the first photodetector before reaching the second photodetector.

5. The optical sensor according to claim 1, wherein The first photodetector includes an optical filter that blocks at least the second wavelength range and transmits the first wavelength range, and the second photodetector includes an optical filter that blocks at least the first wavelength range and transmits the second wavelength range.

6. The optical sensor according to claim 5, wherein The first photodetector and the second photodetector are disposed on the same die.

7. The optical sensor according to claim 1, wherein Each reflector is formed from alternating layers of a first material and a second material, the first material having a greater refractive index than the second material.

8. The optical sensor according to claim 7, wherein The first material is silicon dioxide SiO2, and the second material is polycrystalline silicon poly-Si.

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