Patent Application
20230304861
The present invention relates to optical components. In particular, the present invention relates to wavelength-discriminating optical sensors incorporating interferometers and photodetectors.
Miniaturised wavelength discriminating optical sensors are often constructed with an optical interferometer mounted on a substrate, and a detector located below the substrate. For example, in the detector shown in
There may be further optical components (e.g. lenses or optical filters) to control light entering the interferometer, or control light transmitted through the substrate. For example, lenses may be used to capture more light, or optical filters may be used to filter out unwanted light (e.g. higher order optical modes of the interferometer).
“Optical” and “light” are used herein to refer to both visible light and adjacent spectral regions—i.e. infra-red and ultra-violet light.
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 spectrometry applications, particularly in the near-infrared region, the behaviour of these overtones is highly aharmonic—i.e. they do not behave in a simple fashion, which complicates the identification of molecules from their spectra.
This problem can be mitigated if there is independent detection of light in different overtones, which can then be compared to more accurately determine the cause of the detected wavelengths (i.e. the underlying structure of the molecule).
While spectroscopy is presented as a particular example use case, an optical sensor capable of detecting multiple optical modes of an etalon could be useful in other applications—including color sensors, spectral sensors, or dual-frequency proximity or time of flight sensors.
As such, the present invention provides an optical sensor. The optical sensor comprises a substrate, a Fabry-Perot interferometer, and first and second photodetectors. The Fabry-Perot interferometer comprises a first mirror and a second mirror, 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, and wherein the first and second wavelength ranges each correspond to a different mode of the interferometer.
In order to provide the different wavelength ranges, the first photodetector may comprise a photodiode formed from a first material, and the second photodetector may comprise a photodiode formed from a second material which is different from the first material. In this case, the first material may be transparent to the second wavelength range, and the second photodetector may be located such that light transmitted by the interferometer passes through the first photodetector before reaching the second photodetector.
As an alternative way to provide the different wavelength ranges the first photodetector may comprise an optical filter which blocks at least the second wavelength range and transmits the first wavelength range, and the second photodetector may comprise an optical filter which blocks at least the first wavelength range and transmits the second wavelength range. In this case, the first and second photodetectors may be provided on the same die.
The first mirror may have a greater refractive index than the second mirror, and the second mirror may be located closer to the substrate than the first mirror. For example, the first mirror may be formed from silicon dioxide, SiO2, and the second mirror may be formed from poly-silicon, poly-Si.
A idealised Fabry-Perot interferometer will have transmission peaks at certain wavelengths depending only on the distance between its mirrors. The transmission peaks satisfy the relationship λ=2d/m, where d is the distance between the mirrors and m is an integer called the “order of interference” or “optical mode”. Existing sensors such as that shown in
Operation in a single mode is sufficient for many applications. However, in some instances it would be useful to detect in multiple modes simultaneously—i.e. to simultaneously detect light of wavelength λm=2d/m and of wavelength λn=2d/n, where m and n are different optical modes. For example, in spectroscopy, it is often difficult to confirm whether a certain species has been detected without comparison across multiple wavelengths, and multiple overtones of that species' characteristic wavelength. An interferometer can be constructed where two of the modes line up with different overtones of the species' characteristic wavelength, allowing both measurements to be taken simultaneously, in principle. Alternatively, this may be used in other applications requiring simultaneous monitoring of two or more different wavelengths—e.g. color sensors, spectral sensors, or dual-frequency versions of proximity or “time of flight” sensors.
An example of a practical construction of such a sensor is shown in
The materials of the first and second mirrors may be selected to ensure good transmission within the wavelength ranges of the first and second detectors. For example, for visible light, metal mirrors generally provide good transmission. In the near-infra red spectrum, mirrors made from alternating layers of two materials, where one material has a greater refractive index than the other, will provide good transmission. The materials may be silicon compounds. For example,
Further filters may be applied either before the interferometer, or between the interferometer and the detectors, to block light outside of the wavelength ranges of the detectors (thereby reducing interference).
In order to produce the different wavelength ranges, the first and second detectors may be implemented as:
Equivalent constructions may be used for more than two detectors.
While the sensor described above has many possible use cases, one particular use case is in spectroscopy. When detecting certain species in spectroscopy, each species has a characteristic set of “overtones”, i.e. harmonics of the base emission wavelength of that species. However, the relationship of the base wavelength to the overtones is not purely harmonic—several overtones may be stronger, weaker, wider, or narrower than would be expected for purely harmonic behaviour. This is shown in the example of
Embodiments of the present disclosure can be employed in many different applications including spectroscopy, proximity or time of flight sensing, color measurement, etc, for example, in scientific apparatus, security, automation, food technology, and other industries.
The skilled person will understand that in the preceding description and appended claims, positional terms such as ‘above’, ‘along’, ‘side’, etc. are made with reference to conceptual illustrations, such as those shown in the appended drawings. These terms are used for ease of reference but are not intended to be of limiting nature. These terms are therefore to be understood as referring to an object when in an orientation as shown in the accompanying drawings.
Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in any embodiments, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009904.0 | Jun 2020 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2021/050354 | 6/21/2021 | WO |
