Detector wavelength calibration

Abstract

A method for calibrating the driving parameters of an optical component within its operating wavelength range. The method includes placing a material layer in an optical path, the material layer being substantially planar and substantially transparent, and having a thickness on the order of wavelengths within the range; operating the component to change the driving parameters while detecting light transmitted through the material layer to obtain driving parameter and light intensity data. The obtained data is then compared with previously derived characterization data for the material layer to calibrate the driving parameters.

Description

Technical Field

[0001] This disclosure relates to detector wavelength calibration, and specifically, though not necessarily, to spectrometer wavelength calibration. Background Technology

[0002] An optical spectrometer is an instrument used to measure the properties of light in a specific part of the electromagnetic spectrum. The variable being measured is sometimes the intensity of the light, where the independent variable is the wavelength of the light. Some spectrometers measure the spectral region in or near the visible part of the electromagnetic spectrum, but some spectrometers can also measure other wavelengths, such as the infrared (IR) or ultraviolet (UV) parts of the spectrum.

[0003] In a reflectance spectrometer, the spectrometer measures the proportion of light reflected from a surface as a function of wavelength. Reflectance measurements can be used to determine, for example, the color of a sample, or to examine differences between objects for classification or quality control. Reflectance measurements can also be used to identify specific materials because irradiating different materials will result in different reflectance profiles.

[0004] In some cases, spectrometers are manufactured as small, compact modules that house the necessary optoelectronic components (e.g., a light source and optical sensors) within a housing beneath a cover glass. Light generated by the light source is emitted from the module toward the sample under test. The light reflected from the sample is detected by the sensors.

[0005] The manufacturing process of spectrometer modules can lead to variations in the manufacturing, tolerances, and variability of multiple components of the system. These variations can result in unexpected changes from one module to the next; for example, the reflectance spectrum of a given sample and a given spectrometer module may show wavelength shifts compared to the reflectance spectrum of the same sample and different spectrometer modules. Therefore, it is typically necessary to perform individual module calibration before they leave the factory or, perhaps at a later stage, before using them to measure the reflectance of unknown samples.

[0006] Optical spectrometers can employ silicon microelectromechanical systems (MEMS) technology, and particularly MEMS using tunable Fabry-Perot interferometers (FPIs). These devices are referred to herein as “spectral sensors.” MEMS-based FPIs typically comprise a vertically integrated structure consisting of two mirrors separated by an air gap. Wavelength tuning is achieved by applying a voltage between the two mirrors, resulting in an electrostatic force that pulls the mirrors closer together. Calibration of such a spectrometer may involve directing narrowband light onto the spectrometer’s detector and varying the control voltage to determine when the detector output is at its maximum. The crosstalk and dark noise configuration values ​​for calibration can then be determined. This data is then used to measure the system response against a reference material with a known reflectivity response to determine a complete calibration record. The complete calibration record is stored in a memory coupled to the spectrometer. In some cases, these operations must be repeated to calibrate the sensor over a specific temperature range, such as when using a climate chamber.

[0007] Calibration can be performed using, for example, selective light sources (such as sunlight, Hg-Ar, etc.), gas discharge emission lamps, and lasers. Alternatively, broadband light sources can be used in combination with selective filters (such as monochromators, Fabry-Perot filters, laser frequency combs, and gas absorption cells). The purpose of these arrangements is to provide the system with a well-defined wavelength and to correct the spectrometer's response based on the provided offset.

[0008] Calibration of spectral sensors is a significant portion of the cost of sensor production. Specifically, calibrating MEMS-based spectrometer sensors requires measuring various spectra at all operating points and recording a lookup table of drive voltages and transmission wavelengths in memory. While this is not a critical issue in the case of small-batch production of essentially laboratory instruments, it does pose a significant obstacle to the large-scale production of spectral sensors as now desired, for example, in situations where such sensors are being greatly miniaturized for deployment in consumer electronics devices such as smartphones.

[0009] The light sources currently used for calibration (some of which are already labeled above) are not well-suited for use in large-scale production processes. Furthermore, they are not well-suited for providing low-cost and easy-to-use calibration solutions, such as those needed when end-users expect to calibrate or recalibrate the spectrometer of their devices in typical commercial or home environments. For example, consider a smartphone device incorporating a spectrometer that can be used by the device to detect the properties of materials presented to it (e.g., fabrics, plastics, metals, etc.). The smartphone owner may need to perform initial calibration of the spectrometer after unpacking the device, or may need to perform recalibration to account for offsets since factory calibration. Such users cannot be expected to utilize laser light sources or gas absorption cells. Summary of the Invention

[0010] According to a first aspect of the invention, a method is provided for calibrating the driving parameters of an optical component within its operating wavelength range. The method includes placing a material layer in an optical path, the material layer being substantially planar and substantially transparent, and having a thickness on the order of wavelengths within the range; operating the component to change the driving parameters while detecting light transmitted through the material layer to obtain driving parameter and light intensity data. The obtained data is then compared with previously derived characterization data for the material layer to calibrate the driving parameters.

[0011] The comparison step may include aligning one or more features of the acquired data with one or more features of the characterizing data in order to calibrate the driving parameters.

[0012] The thickness of the material layer can be between 1 and 20 times the maximum wavelength of the operating wavelength range of the optical component, preferably between 1 and 10 times. The material can be plastic.

[0013] The step of calibrating the drive parameters may include creating a lookup table that maps the drive parameters to wavelengths.

[0014] The optical component may be a spectrometer, wherein the material layer is inserted between the light source and the spectrometer. The method includes operating the spectrometer to change the driving parameters while detecting light transmitted through the material layer to obtain driving parameter and light intensity data. The method may include placing a reflector in the optical path to reflect light from the light source through the material layer to the spectrometer. The method may also include obtaining a system response by operating the spectrometer in the absence of the material layer, and using the response to correct detected light or for a calibration process. The step of obtaining a system response may include detecting light from the light source or from another light source.

[0015] The optical components may be tunable light sources, and the material layer may be disposed between the spectrometer and the light source.

[0016] According to a second aspect of the invention, an apparatus is provided comprising an optical component operable within an operating wavelength range by means of driving parameters. The apparatus includes a material layer movably disposed in the optical path, being substantially planar and substantially transparent, and having a thickness on the order of wavelengths within the said range. The apparatus is configured to operate the component to change the driving parameters while detecting light transmitted through the material layer to obtain driving parameter and light intensity data, and to compare the obtained data with previously derived characterization data for the material layer to calibrate the driving parameters.

[0017] The optical component may be a spectrometer, wherein the material layer is slidably disposed in front of the spectrometer. The device may include a light source and may be provided in combination with a light reflector for redirecting light from the light source to the spectrometer. Attached Figure Description

[0018] Figure 1 The transmittance of the thin-layer material is shown;

[0019] Figure 2 A system including a spectrometer for calibrating a light source and a light source is shown;

[0020] Figure 3 This is a flowchart illustrating a method for calibrating a spectrometer;

[0021] Figure 4 and Figure 5 A system for calibrating a spectrometer using a reflective target is schematically illustrated; and

[0022] Figure 6 A system for calibrating a spectrometer using a reflectance target and taking into account the system response is illustrated schematically. Detailed Implementation

[0023] The introduction of mass-produced spectrometers (such as those used in consumer products) has created a demand for low-cost and easy-to-use spectrometer calibration processes. Ideally, the cost of the process should be a fraction of the cost of the spectrometer and equipment, and should be operable by even inexperienced users. The processes and systems described herein are designed to meet this need.

[0024] According to at least some embodiments, a process is proposed herein that utilizes at least a white light source and a thin, substantially transparent material layer. The light source may be, for example, sunlight or commonly used internal lighting. The thin material layer may be plastic, glass, silicon, etc., and is preferably of a type whose optical properties do not change significantly over time (e.g., as a result of oxidation), or at least not significantly during the expected lifespan of the spectrometer to be calibrated or the equipment using the spectrometer.

[0025] The thickness of the thin layer is chosen to correspond to the wavelengths within the operating range of the spectrometer to be calibrated. For example, a thickness of 10 μm might be suitable if the spectrometer has an operating wavelength range of 1.3 μm to 2.15 μm. This will induce multiple reflections of light between the upper and lower reflective surfaces of the layer, which in turn will induce constructive and destructive interference of light within the layer. As a result, the light exiting the layer will contain intensity peaks at wavelengths that are integer multiples of the layer thickness.

[0026] Figure 1 The transmittance is shown for a planar material layer with a thickness d of 10 μm, a refractive index n of 1.5, and reflectivities R on both the upper and lower surfaces R of 9. This assumes an incident angle θ of 0 on the upper surface. The phase difference δ between each consecutive transmission pair is given by:

[0027]

[0028] The transmittance function of the plate is:

[0029]

[0030] This will produce a well-defined transmittance function with valleys and peaks. Figure 1 This can be used for wavelength calibration of spectrometers. While valleys and peaks can be used individually to calibrate a spectrometer, for example by moving values ​​in a pre-filled (lookup) calibration table installed in the device during manufacturing, all points obtained from the (transmittance) measurement can be used in the calibration process, not just the peak / valley locations, due to the known mathematical form.

[0031] Figure 2 The diagram schematically illustrates how this technique can be used. The system includes a broadband light source 1 providing electromagnetic wavelengths within the range of interest, a collimation system 2 (not strictly required by the invention itself, but necessary in this particular case), a thin-film material plate 3, and a spectrometer 4 to be calibrated. The spectrometer can be integrated into a device 5 such as a smartphone. If the spectrometer is a MEMS-based Fabry-Perot spectrometer, a precisely driven control voltage is required, which in turn establishes the wavelength of the signal to be measured. The thickness of the plate 3 is defined and selected based on the operating wavelength range of the spectrometer 4.

[0032] The plate (or a substantially similar plate) has been previously characterized such that its transmittance characteristics are known (i.e., will be obtained such as...). Figure 1 (The outline shown). The characteristics are pre-stored in the memory 7 of device 6 (possibly installed during device manufacturing or downloaded at a later stage). Once the spectrometer performs its scan, the transmittance curve is reproduced as the acquired spectrum. Since the behavior of light passing through the plate is known, the correct response is mathematically established. In other words, the correct position of the entire spectrum is known (based on the pre-stored data). The control voltage range can then be mapped to the wavelength range, for example, by a voltage-wavelength lookup table stored in memory 7. Device 6 includes a processor 8 for performing the calibration process (using, for example, code stored in memory 7). In an alternative arrangement, the spectrometer is pre-calibrated (using a thin plate), and the lookup table is calculated and stored in memory 7.

[0033] Figure 3 This is a flowchart illustrating the method, and includes:

[0034] S100. Insert a material layer between the light source and the spectrometer.

[0035] S200. Operate the spectrometer to change drive parameters, such as control voltage, within the operating range.

[0036] S300. Detects transmitted light.

[0037] S400. Access the characterization data of the material layer to identify one or more expected features at one or more wavelengths in the transmittance properties of the material layer.

[0038] S500. Identify one or more features in the driving parameters and light intensity data, and calibrate the driving parameters.

[0039] Spectrometers that can be calibrated using the above methods include sensors used for food analysis, color sensing, infrared sensing, biomedical sensors, spectral sensing, counterfeiting, cosmetic analysis, drug analysis, process control, thickness measurement, high temperature measurement, LED measurement, reaction analysis, fluorescence analysis, etc.

[0040] Although the method described above is in the context of (miniature) spectrometers, it can be used to calibrate any suitable photodetector. It can also be used to calibrate tunable light sources (e.g., UV, visible, or infrared).

[0041] The method described here is faster, cheaper, and at least as accurate as previously known methods. It requires no special light source, no reference signal, and no external materials. The achieved wavelength response is highly stable—only variations in material thickness and refractive index cause a shift. These can be extremely low. Temperature stability is also good (only the refractive index changes with temperature; dimensional changes due to thermal expansion are negligible). In cases where the spectrometer is integrated into a smartphone or other consumer device, the device can be provided with a small layer of suitable material (e.g., plastic). Calibration can be performed during the initial setup of the device. This material layer can even be integrated into the device housing or provided above the sensor with instructions to peel it off after calibration.

[0042] Of course, those skilled in the art will understand that various modifications can be made to the above embodiments without departing from the scope of the invention. For example, Figure 4 An apparatus including an integrated light source 10 and a spectrometer (photodetector) 11 is shown. In this configuration, a material layer 12 is slidably disposed on the apparatus to cover and expose the spectrometer. A reflective target 13 is provided, for example as a component separate from the apparatus, for placement in front of the optical components to redirect light from the light source to the spectrometer. The reflective target 13 may be, for example, a layer of white reflective material. Although Figure 4 The material layer 12 is shown sliding out from the front of the spectrometer (e.g., in the normal operating position), but Figure 5 The material layer 12 is shown sliding in front of the spectrometer for calibration. The material layer 12 can be slid by manual operation of some electromechanical system.

[0043] Figure 6 Another alternative arrangement is shown, in which the device includes a pair of light sources 21 and 22 in addition to the spectrometer 20. Only one light source 22 has a thin material layer 23 disposed thereon in a non-slip configuration. In this device, the light source 21 is used to obtain the system response of the spectrometer 20. The response is then obtained using the second light source 22, where the material layer 23 is disposed in the optical path. The system response can be used to correct the data obtained using the second light source 22.

Claims

1. A method for calibrating the driving parameters of an optical component within its operating wavelength range, the method comprising: A material layer is placed in the optical path, the material layer being substantially planar and substantially transparent, having a thickness on the order of wavelengths in the range, and having a known transmission profile having multiple transmission maxima and multiple transmission minima over at least a portion of the operating wavelength range of the component. The component is operated to change the driving parameters, while light transmitted through the material layer is detected to obtain driving parameter and light intensity data. as well as The obtained data is compared with the characterization data previously derived for the material layer in order to calibrate the driving parameters.

2. The method according to claim 1, wherein, The comparison includes aligning one or more features of the obtained data with one or more features of the characterization data in order to calibrate the driving parameters.

3. The method according to claim 1 or 2, wherein, The thickness of the material layer is between 1 and 20 times the maximum wavelength of the operating wavelength range of the optical component.

4. The method according to claim 1 or 2, wherein, The material is plastic.

5. The method according to claim 1 or 2, wherein, The collimating optics are arranged within the optical path.

6. The method according to claim 1 or 2, wherein, Calibrling the drive parameters includes creating a lookup table that maps the drive parameters to wavelengths.

7. The method according to claim 1 or 2, wherein, The optical component is a spectrometer.

8. The method according to claim 7, wherein, The material layer is placed between the light source and the spectrometer, and the method includes operating the spectrometer to change the driving parameters while detecting light transmitted through the material layer to obtain driving parameter and light intensity data.

9. The method of claim 8, further comprising providing a reflector in the optical path to reflect light from the light source through the material layer to the spectrometer.

10. The method of claim 8, further comprising obtaining a system response by operating the spectrometer in the absence of the material layer, and using the response to correct detected light or a calibration process.

11. The method according to claim 10, wherein, The method of obtaining a system response by operating the spectrometer in the absence of the material layer, and using the response to correct detected light or for calibration processes, includes: Detect light from the light source or from another light source.

12. The method according to claim 1 or 2, wherein, The optical component is a tunable light source, and the material layer is disposed between the spectrometer and the tunable light source.

13. The method according to claim 1, wherein, The thickness of the material layer is between 1 and 10 times the maximum wavelength of the operating wavelength range of the optical component.

14. A device including an optical component capable of operating within an operating wavelength range by means of driving parameters, the device including a material layer movably disposed in an optical path, substantially planar and substantially transparent, having a thickness on the order of wavelengths within the range, and having a known transmission profile having a plurality of transmission maxima and a plurality of transmission minima over at least a portion of the operating wavelength range of the component, the device being configured to: Operate the component to change the driving parameters while detecting light transmitted through the material layer to obtain driving parameter and light intensity data; and The obtained data is compared with the characterization data previously derived for the material layer in order to calibrate the driving parameters.

15. The device according to claim 14, wherein, The optical component is a spectrometer.

16. The device according to claim 15, wherein, The material layer is slidably disposed in front of the spectrometer.

17. The device according to claim 15 or 16 further includes a light source.

18. The apparatus of claim 17, further comprising a light reflector for redirecting light from the light source to the spectrometer.

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