Field of the invention

[0001] The present invention is generally related to apparatus and methods for detecting a two-photon absorption event.

Background of the invention

[0002] Near and short-wavelength Infrared imaging is important for several applications. It can be used for 3D imaging with active illumination, for spectroscopy, for inspection tasks and for measurements, detection and classification of specific molecules, substances or concentrations. In many systems, an active illumination is used with either infrared light emitting diodes (LEDs) or infrared lasers. Lasers provide coherent light and can be pulsed at very high pulse rates.

[0003] Near-infrared lasers have become much more widespread now thanks to 3D imaging applications. Vertical Cavity Surface Emitting Lasers or VCSELs are a new class of lasers that has become popular thanks to their use in 3D face recognition and 3D imaging systems. In most systems, the lasers are emitting light in the near-infrared wavelength band, which is commonly defined as the wavelength band between 700 and 1100 nm. In this wavelength band, silicon-based photodetectors are light sensitive. A single photon at this wavelength has enough energy to excite an electron from the valence band to the conduction band in a silicon-based detector, since the bandgap energy of Silicon is approx. 1.12 eV. Popular wavelengths are 940 nm and 850 nm, because the solar spectrum has a dip at these wavelengths. At these wavelengths, the undesired signal from background light emitted from the sun is lower while the silicon sensitivity is still reasonably good. A lot of research has been done on improving silicon sensitivity at these wavelengths whilst keeping the cross-talk between pixels at a reasonable level. For longer wavelengths, above 1050 nm, it is commonly proposed to use other detector materials than silicon with lower bandgap energy. For example, Indium-Gallium- Arsenide or InGaAs detectors are used. These detectors use an alloy of InAs and GaAs. The bandgap can be adapted by the composition of the alloy. Standard InGaAs has a wavelength cut-off at 1.68 pm, but this can be extended to be sensitive to for example 1.9 pm. For telecommunications using optical fibres, 1.55 pm and 1.3 pm are wavelengths used in detectors, which are often also InGaAs based. Other materials have been proposed too. Platina-Silicide (PtSi), Mercure-cadmium-telluride (MCT) and other detectors have been studied and used. Recently, quantum dot films and organic detectors have been proposed sensitive in part of the short-wavelength infrared (SWIR) band. All these detectors use more exotic materials, which are complex to manufacture and where integration with readout circuits in silicon based technologies (such as CMOS, Complementary Metal Oxide Semiconductors) is difficult. This makes the availability of such detectors problematic and not easily scalable for volume markets.

[0004] One important application of infrared detectors is three-dimensional imaging. This is important in fields such as self-driving vehicles, for imaging the environment of a vehicle. Other use cases for 3D imaging are machine vision, e.g. for inspection, 3D scanning, robot navigation or security; facial recognition with enhanced security (e.g. to distinguish some 3D features from the face). Several techniques are used, most of which employ active illumination.

A LIDAR (light detection and ranging) method can be used for longer range 3D imaging (several 10m to 200-300m), in which light is used to illuminate a scene. The light reflects from objects in the environment and a single-photon avalanche photodetector (SPAD) array is used to detect the scattered light. The electronics connected to the SPAD detect the arrival of the photon and measure the travel time between the photon emission (by the laser) and the arrival of this photon. This travel time is proportional to the distance of the object that reflected the light. This is called 'direct Time-of-flight' (or dTOF). SPAD detectors tend to be based upon silicon detectors, and illumination is then done with lasers emitting light at 940 nm or 850 nm. Scanning LIDARs emit a small laser beam or slit and scan the scene by moving the projected beam. Flash LIDARs illuminate the entire scene simultaneously and detect in each pixel the arrival time. LIDARs operating with SPADs using InGaAs detectors in the SWIR wavelength band have also been proposed.

For shorter distances, indirect Time-of-Flight (or iTOF) pixels can be used. The pixels contain a photodiode and some partition electronics. The partition electrons generated by incoming photons can fall into one of two bins: a first bin collects electrons arriving before a certain time, and a second bin collects electrons arriving after this time. The ratio between the amount of electrons in each bin is proportional to the distance travelled by the light detected at each pixel.

3D imaging at closer distance can also be done by stereovision. The signal of two cameras is detected, and by triangulation, knowledge of the position of the cameras, and finding corresponding items in both images, the depth can be estimated. This is similar to how humans and many animals estimate depth. One problem with stereovision is that the scene may be uniform (like a uniformly painted wall), in which case no correspondence can be found. To solve this, a light pattern (like a raster, a pattern of regular or irregular dots, or similar) can be projected, usually with infrared light (in order not to disturb the object). Such systems are called 'active stereovision' systems due to the presence of active illumination. A similar technique is "structured light projection" where a pattern is projected and imaged by a single camera. The projected pattern (usually a set of pseudo-random dots) is detected by the camera, and the correspondence between each point in the image and the projected pattern is used, together with the position information of the projector and camera, to estimate depth. Again, the projected pattern is usually in the near-infrared range.

[0005] Three-dimensional imaging methods often use near or short wavelength infrared light as, amongst other reasons, it is invisible to humans. In particular a wavelength of 940nm or 850nm is preferred in some applications as the solar radiation spectrum on Earth has a dip at these wavelengths due to absorption of sunlight by water in the atmosphere. These applications tend to use a vertical cavity surface-emitting laser (VCSEL) as the light source, and a high output power is desirable as the scattered laser light is in competition with background light from the scene. However, too high output powers as may be required to provide a good signal-to-noise ratio, can be at levels that are damaging to the eyes of humans and animals. This limits the use of such systems at high background light levels, as the emitted laser light is limited to the maximum allowed for eye safety. Laser light emitted above 1400 nm is much less damaging for the eye and emitted light power can be increased considerably, but silicon-based detectors cannot be used in this range.

[0006] Another issue with such imaging is the signal generated by background light. For example, the paper "A 64x64-pixel digital silicon photomultiplier direct ToF sensor with 100M photons/s/pixel background rejection and imaging/altimeter mode with 0.14% precision up to 6km for spacecraft navigation and landing" (2016 IEEE International Solid-State Circuits Conference (ISSCC), Feb. 2016) describes an image sensor which implements a pixel based on a digital silicon- photomultiplier (dSiPM), i.e., a set of SPADs providing on a single output a stream of short pulses corresponding to each avalanche, coupled to a smart triggering logic for concurrent-event detection, allowing uniquely correlated photons' time-stamping and counting. Triggering of more than one detector in the same time window is an indication that the signal came from the light source at 532nm, which releases many photons at the same time, as opposed to the background, where photons arrive uncorrelated in time.

[0007] This method loses spatial resolution as it relies on correlation of pixels at different spatial positions. In addition, much electronic circuitry is required in order to detect coincident triggering. This results in large area for the circuitry, large pixels and low spatial resolution of the detector. Furthermore, it increases power consumption considerably.

[0008] In some imaging applications, the transmission of light through a substance needs to be measured. A two-dimensional image can be required, e.g. when detecting small molecules inside a fluid that streams in front of the image sensor. The molecule or species to be detected may have a strong absorption peak at a specific wavelength, while the fluid in which this species is embedded may have a weak absorption peak at that same wavelength. The scene can be illuminated by a laser, such as a VCSEL and an image sensor can be used, together with a lens, to image the scene. Dark spots are be seen in the image where the species were present. Illumination at multiple wavelengths, using tuneable lasers or a set of VCSELs tuned for different wavelengths, can be used to measure absorption at different wavelengths. The latter creates a set of 2D images showing the absorption energy spectrum. It is a form of 2D spectroscopy. Most of the interesting information can be found in the SWIR wavelength band, between 1050 nm and 2000 nm. Today, expensive InGaAs image sensors have to be used. Due to the low bandgap of these detectors, they are sensitive in this SWIR band. However, due to the low bandgap, a lot of charges are generated inside the photodetectors thermally, just by thermal energy, when operating at room temperature. To avoid this, the detectors have to be cooled by an active cooling circuit. This not only increases the power consumption, but also makes the camera very bulky and expensive.

[0009] In “Characterisation of Two-Photon Absorption Detectors for Applications in High-

Speed Optical Systems" (Reza Salem, Master's thesis, University of Maryland, 2003) the process of two-photon absorption is described in optical detectors comprising a silicon avalanche photodiode. Avalanche photodiodes have a high internal current gain due to impact ionization when a high reverse bias voltage is applied. The voltage at which this effect due to impact ionization starts to occur, is called the avalanche breakdown voltage. However, because of their principle of operation of avalanche detectors they cannot detect the arrival of another photon during an avalanche event and shortly afterwards, when the diode is quenched. These detectors only detect the timestamp of the generation of the electron, and the light intensity information is lost.

[0010] Patent document US2009/180099 relates to a distance measuring system comprising a single photodetector (not an array). Photodetectors with an avalanche photodiode are discussed. [0011] The paper “Monostatic all-fiber scanning LADAR System" (J. Leach et al., Applied

Optics, vol.54, no.33, Nov.2015, p.9752-9757) relates to a compact scanning LADAR system comprising a position-sensing detector and an avalanche photodiode receiver.

[0012] Hence, there is a need for an apparatus to detect two-photon absorption events wherein the above-mentioned drawbacks and limitations are avoided or overcome.

Summary of the invention

[0013] It is an object of embodiments of the present invention to provide for an apparatus and method for measuring two-photon absorption that has high spatial resolution and high tolerance to background light. [0014] The above objective is accomplished by the solution according to the present invention.

[0015] According to a first aspect of the present invention there is provided apparatus for detecting a two-photon absorption event. The apparatus comprises a pulsed laser light source having an emission spectrum with a linewidth centred at a first wavelength U; a detector comprising a two- dimensional array of n x m pixels, with m and n > 2, wherein each pixel comprises a photodiode having a bandgap energy E _g and operable below the avalanche breakdown voltage of the photodiode, and filtering means disposed between the pulsed laser light source and the detector, said filtering means comprising a first filter configured to substantially block light from the first pulsed laser light source with a wavelength less than or equal to hc/E _g . The first wavelength Li is less than 2hc/E _g and is greater than hc/E _g . The apparatus also comprises synchronization means for enabling synchronization between the pulsed laser light source and the detector.

[0016] It is an advantage of embodiments of the present invention that the proposed invention can be used in systems using lasers emitting light in the short-wavelength band, typically between 1050 nm and 2100 nm, and does not require detector materials with lower bandgap which can suffer from thermal noise and increased leakage current.

[0017] It is an advantage of embodiments of the present invention that the apparatus can be operated at voltages below the avalanche breakdown voltage. In this way the need for quenching that comes along with an avalanche event is avoided. The quenching requires a certain amount of time, during which the arrival of another photon cannot be detected. Further the need for performing additional process steps to generate a high voltage is avoided.

[0018] It is an advantage of embodiments of the present invention that an improved spatial resolution can be provided for distance measurements using the apparatus.

[0019] It is an advantage of embodiments of the present invention that two-photon absorption can be detected with a reduced sensitivity to background light.

[0020] The filtering means may comprise a second filter being a background light filter configured to substantially block light having a wavelength which is greater than a background light upper threshold wavelength.

[0021] The background light upper threshold wavelength may be at least 1050 nm.

[0022] The at least one light source may comprise a further coherent light source having an emission spectrum with a linewidth centred at a second wavelength L , wherein the second wavelength L is less than 2hc/E _g and is greater than hc/E _g , and wherein the synchronisation means is configured to synchronise the further light source and the detector. [0023] The filtering means may comprise a third filter configured to substantially block light from the pulsed laser light source and light with a wavelength less than or equal to hc/E _g .

[0024] The first filter may be disposed over at least a first pixel and the second or third filter may be disposed over the at least a second pixel which is different to the first pixel, without spatial overlap of the first filter with the second or the third filter.

[0025] In some embodiments the apparatus may comprise a lens disposed between the at least one light source and the detector, wherein the detector is located at the focal plane of the lens. [0026] At least one optical bandpass filter may substantially block light with a wavelength less than 1060 nm and greater than 1550 nm.

[0027] In an advantageous embodiment the laser is a vertical cavity surface-emitting laser.

[0028] The first wavelength Li and the second wavelength L may be between 1300nm and

1550nm.

[0029] In one embodiment the detector is a silicon photodetector.

[0030] In embodiments the at least one coherent light source may be a laser configured to operate in a pulsed mode.

[0031] In another aspect the present invention relates to the use of the apparatus according to the first aspect for performing a time-of-flight measurement.

[0032] According to a further aspect of the present invention there is provided a method of determining a distance between a light source and a target point. The method comprises the steps of providing an apparatus according to the first aspect; causing the at least one coherent light source to emit coherent light towards the target point at a first time ti; recording a time t at which the coherent light is received by the detector; and determining a distance between the at least one light source and the target point based on the difference between time t and time ti.

[0033] The invention is not limited to 3D applications. It is also useful for imaging in the short-wavelength infrared band where active laser-based illumination is used. The current invention allows capturing these images with silicon-based image sensors. Since the bandgap of silicon is much larger, thermal generation is much lower (and the same as in traditional cameras) and these devices can be operated without cooling.

[0034] For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

[0035] The above and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Brief description of the drawings

[0036] The invention will now be described further, by way of example, with reference to the accompanying drawings, wherein like reference numerals refer to like elements in the various figures. [0037] Fig. la illustrates a schematic view of an apparatus according to embodiments of the present invention. Fig. lb illustrates an array of pixels comprised in a detector of an apparatus according to embodiments of the present invention.

[0038] Fig.2 shows a not-to-scale representation of the relationship between wavelength Li and the bandgap energy E _g .

[0039] Fig.3a illustrates a first manner in which apparatus according to embodiments of the present invention can be arranged relative to a region of interest. Fig.3b illustrates a second manner in which apparatus according to embodiments of the present invention can be arranged relative to a region of interest. Fig.3c illustrates a third manner in which apparatus according to embodiments of the present invention can be arranged, for example for a 3D time-of-flight measurement.

[0040] Fig.4 illustrates a schematic view of a first modified apparatus according to embodiments of the present invention comprising a second bandpass filter.

[0041] Fig.5 is a schematic view of a second modified apparatus according to embodiments of the present invention comprising a second light source.

[0042] Fig.6 provides a schematic view of an apparatus according to embodiments of the present invention comprising a lens.

Detailed description of illustrative embodiments

[0043] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. [0044] Furthermore, the terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. [0045] It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

[0046] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments. [0047] Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

[0048] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0049] It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re defined herein to be restricted to include any specific characteristics of the features or aspects of the invention with which that terminology is associated. [0050] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0051] Referring to Fig. la, an apparatus 1 for detecting two-photon absorption is shown. The apparatus 1 comprises a light source 2, a detector 3, and a filter 4 between the light source 2 and the detector 3.

[0052] The light source 2 is a coherent light source, for example a laser. The light source 2 is preferably a vertical cavity surface-emitting laser (VCSEL), but may alternatively be, for example, a diode laser, a fibre laser, a dye laser, or other type of laser. The light source 2 is configured to emit coherent light with an emission spectrum having a full width at half maximum (FWHM) centred at a first wavelength Li.

[0053] The first wavelength Li is preferably outside the visible range for humans and animals, for example preferably in the near or short infrared region of the spectrum. However, in some embodiments, the first wavelength may be within the visible range for humans and animals. The first wavelength Li is preferably between 1300nm and 1550nm. However, use of other wavelengths is also possible.

[0054] Referring to Fig. lb, the detector 3 comprises a two-dimensional array of n x m pixels, each of which comprising a semiconductor photodiode. The indices n and m are both greater than or equal to 2, preferably greater than or equal to 4, more preferably greater than or equal to 16. Each photodiode has a bandgap energy E _g , and can be labelled by the indices n,m which denote the location of the pixel in the two dimensional array.

[0055] The bandgap energy is determined by the type of semiconductor. If a photon having an energy greater than or equal to the bandgap energy is incident on a photodiode, an electron-hole pair is created in the photodiode. When a sufficient number of electron-hole pairs are generated, a photocurrent is generated which is roughly proportional to the intensity of the incident light. This is known as free carrier absorption (FCA).

[0056] If the incident photons have energy less than the bandgap energy, a photocurrent can still be generated by two-photon absorption (TPA). In TPA, if two photons arrive at the photodiode within a certain time of each other and the sum of their energies is greater than or equal to the bandgap energy of the photodiode, an electron-hole pair can be created. A corresponding photocurrent can be generated which is roughly proportional to the intensity of the incident light. [0057] For FCA, as only a single photon is required to create an electron-hole pair, the measured intensity can include a significant unwanted contribution from background light, which can worsen the signal to noise ratio. In TPA, as the two photons need to have an energy within a particular range and need to arrive almost simultaneously, which is unlikely to occur with background light, imaging can be achieved which is much more tolerant to background light than FCA.

[0058] In addition, where three-dimensional time-of-flight (TOF) imaging is concerned, the spatial depth resolution is greatly improved when two-photon absorption is used. A common issue in three-dimensional TOF imaging is the corner ambiguity problem. In the area around a corner formed by two walls, or in general an edge formed by two planes, light can propagate from the light source to the detector by direct reflection from one of the walls, i.e. only one reflection takes place between the light source and the light detector. Light can also propagate from the source to the detector by reflecting first from one wall, being redirected towards the second wall, and reflected from the second wall to the detector. A traditional time-of-flight system cannot distinguish between these two cases, because the time resolution for single photon absorption is limited to a few 100 ps, and it has been shown that a practical limit is around 28 ps for red light, and longer for infrared light. In 30 ps, light travels about 9mm. This limits the spatial depth resolution of the 3D TOF imaging system at edges and corners of objects.

[0059] In order for two-photon absorption to take place, the two photons must arrive within approximately 100 fs to a few ps of each other, depending on the material properties of the detector. Light travels a distance of 30 pm in 100 fs in free space and thus this is the practical limit for depth resolution, which is more than sufficient to resolve the corner ambiguity problem.

[0060] The first wavelength Li is related to the bandgap energy E _g . The first wavelength Li is less than 2hc/E _g and is greater than hc/E _g . At wavelengths greater than hc/E _g , the probability of single photon absorption is reduced. At wavelengths less than 2hc/E _g , two photon absorption is more likely; above 2hc/E _g , the combined energy of the photons is less than the bandgap energy and so two- photon absorption does not occur.

[0061] The detector 3 is preferably a silicon photodetector. Silicon has a bandgap energy at

273K of 1.14 eV. The corresponding wavelength range within which the first wavelength Li should be chosen is 1087 nm - 2174 nm. Suitable coherent light sources include, for example, Vertical Cavity Surface Emitting Lasers (VCSELs) and edge-emitting lasers. A proof of concept of two-photon absorption in a commercial 2D CMOS image sensor was demonstrated with a 1550nm laser generating 450fs pulses of 150 pJ to 600 pJ, in which two photon absorption was observed. Other possible laser light sources are other semiconductor laser diodes, such as quantum well lasers, quantum cascade lasers, distributed Bragg reflector lasers or external cavity diode lasers. External cavity diode lasers have tuneable output wavelength, which may be advantageous in applications such as spectroscopy. In such case, several images can be acquired with different wavelengths though the same optical system.

[0062] The detector 3 comprises a matrix of n x m pixels (with m, n > 1). The resolution may in some embodiments be for example at least 16 x 16 pixels but preferably more, e.g. 640 x 480 pixels or several megapixels. In embodiments of the present invention the detector pixels contain a photodiode which operate in a charge-accumulation mode during an exposure time. In other words, the photodiodes of the detector pixels are not biased in avalanche regime but operated with a low reverse voltage bias below the avalanche breakdown voltage. Such pixels are commonly used in CMOS image sensors and CCD image sensors. In most cases, the image sensors use pinned photodiodes as photodetectors. Such pinned photodiodes are buried inside the silicon and when an electron is collected by such photodiode, it is stored in the charge collecting well which is at a buried location in the silicon. The potential well is made empty at the start of the exposure time. At the end of the exposure time, the charge is transferred to a sense node for readout. This way of operation allows measuring the amount of collected electrons (which would not be possible with avalanche photodiodes). Such pixels are used for 2D image sensors, linear line scan image sensor and also for 3D indirect time-of-f light sensors. Two-photon absorption can be combined with these 2D image sensors. Though the generation of a charge due to two-photon absorption is very rare, pixels in CMOS image sensors can be read out with very low noise levels. Noise levels down to less than 0.21 e- RMS have been reported. With such low noise levels, even if TPA only generates a few electrons, these electrons can be detected. This low noise level must be compared to the noise of low-bandgap detector pixels (e.g. made with InGaAs or MCT photodiodes), which is more than 20 times higher. Obviously, for TPA a pulsed laser light source is required, because two photons need to arrive within a few 100 fs at the same photodiode location.

[0063] The apparatus 1 comprises synchronisation means 5 for enabling synchronisation between the light source 2 and the detector 3. Such synchronisation can be achieved in various ways. [0064] One form of synchronisation is related to the exposure time. The detector is composed, as mentioned hereinbefore, of an array of pixels. Each of the pixels can be configured to have a specific exposure time during which incoming light can be detected. This exposure time is synchronized to emission of light by the light source. For example, the synchronisation means 5 may comprise a control module 5i in communication with the light source 2 and with the detector 3, the control module 5i being configured to send a first trigger pulse to the light source 2 to cause the light source 2 to emit light and a second trigger pulse to the detector 3 to cause the pixels, or a subset thereof, to be exposed. The first trigger pulse and the second trigger pulse are preferably simultaneous or slightly offset such that the second trigger pulse occurs within a predetermined time of the first trigger pulse. For example, for indirect time-of-flight, the pixels of the detector are configured to switch to collect charges in a first collection bin during the first X ns after the laser trigger pulse, and collect charge in a second collection bin during the next X ns. The time shift is adjusted to fit to the distance range to be covered by the sensor. In 1 ns, light travels forth-and-back from the laser to the reflecting object and the detector over a distance of 15 cm. If the range of detection is 1.5m and the average range is half of this, then X may be 5 ns so that the charges are collected on the first charge collection bin during the first 5ns, and the second collection bin during the next 5ns. The ratio of charges in both bins is a measure in each pixel of the distance range of the object that reflected the light.

[0065] The first and second trigger pulses may have a predetermined duration such that, for the first trigger pulse, the light source 2 emits light for the entire duration of the first trigger pulse and then stops emitting light when the first trigger pulse ends (returns to an off state) and, for the second trigger pulse, the detector 3 causes the pixels (or a subset thereof) to be exposed for the entire duration of the second trigger pulse and then stops the exposure when the second trigger pulse ends (returns to an off state). Alternatively, the first and second trigger pulses may cause the start of light emission and pixel exposure, respectively, and a third and fourth trigger pulse may subsequently be provided to stop the light emission and pixel exposure, respectively.

[0066] The control module 5i may communicate with the light source 2 and the detector 3 by any means of wired or wireless communication, for example WiFi, Bluetooth, Ethernet.

[0067] The time for which the pixels (or a subset thereof) are exposed - e.g. the duration of the second trigger pulse, or the time between the second and the fourth trigger pulses - is preferably similar to the duration of time for which light is emitted by the light source - e.g. the duration of the first trigger pulse, or the time between the first and the third trigger pulses. Multiple short exposure times can be added together to obtain a larger signal in a pixel, by causing the light source 2 to emit multiple light pulses and by providing corresponding trigger pulses to the detector 3 to allow accumulation of carriers over multiple exposures. This may require additional transfer gates or switch transistors inside the pixel.

[0068] The synchronisation means 5 may comprise a control module 5i as described above along with a memory module 5 ₂ for storing inputs received from the light source 2 and/or the detector 3, and a compute module 5 ₃ for processing data stored in the memory module 5 ₂ or received directly from the light source 2 and/or the detector 3.

[0069] For example, for direct time-of-flight measurements, the distance travelled by the emitted light is calculated based on the time delay between the emission of photons by the laser and the detection of two photons through TPA by photodetector inside a pixel of the detector. In this case, the detector can also contain a time-to-digital converter, which is configured to measure the time between the detector receiving the trigger pulse and the time of the first Two-Photon Absorption (TPA) event. This time is proportional to the distance of the object from which the light was reflected to the detector. Furthermore the detector 3 may comprise means for sending data to the control module 5i and the control module 5i may comprise means for receiving data from the detector. Such means may be for example a two-way communications port, such that trigger pulses can be sent and data received at the control module through the same port, and trigger pulses can be received and data sent at the detector through the same port. Such means may alternatively be separate communications ports, for example the trigger pulses may be sent/received by different communications ports to those used for sending/receiving data.

[0070] The memory module 5 ₂ of the control module 5i is configured to store received data and the compute module is configured to retrieve data from the memory module and to process the retrieved data, for example by calculating a distance travelled based on a time measurement received from the detector 3 and a time measurement stored in the memory module 5 ₂ corresponding to a time at which the light source 2 was triggered to emit light. The detector 3 would thus comprise an internal clock configured to be synchronised with an internal clock of the control module 5i. The detector 3 does not necessarily need to send a time measurement; the detector 3 may instead send a simple trigger signal once a two photon event has been detected, and the control module 5i may comprise an internal clock against which the time at which the trigger signal is received can be referenced, in order to determine a time difference between the time at which a trigger signal is sent to the light source 2 to cause it to emit light, and a time at which a trigger signal denoting two photon absorption is received from the detector 3. In some embodiments, the detector may comprise a time- to-digital converter for measuring times between events such as receiving trigger pulses and detecting TPA events.

[0071] The detector 3 may send additional data in addition to a time measurement or a trigger signal indicating two photon absorption. For example, the detector 3 may send a pixel identifier identifying a pixel at which two photon absorption was detected. The processing module 5 ₃ can use this information for example to construct a two dimensional array of location in the pixel array and respective time delay between laser pulse and detection of two photon absorption event. The pixels of the detector may provide information on the light intensity of the background light in that pixel. This may be collected by an additional charge collection bin inside the pixel that is active when the laser is not triggered, and collects the signal in the same way as a regular image sensor used in a camera. [0072] For indirect time-of-flight measurements, each pixel can make use of at least two charge collecting bins. Charges are collected in one of these bins during a specific time window. This time window is synchronized with the time during which the laser light source emits light, for example by starting charge collection on receipt of a second trigger signal as described hereinbefore. Light received soon after emission by the light source 2 is started is collected in a first bin, and light received later is collected in another bin. The pixel may comprise a set of switches, such as transfer gates, to direct the detected carriers to the appropriate collection bin. The detector 3 may then transmit to the control module 5i a measure of the counts in each collection bin, along with a pixel identifier for the pixel concerned.

[0073] The synchronisation means 5 may comprise output means (not shown) for providing an output, such as measurements received from the detector and/or results of calculations performed on data received from the detector e.g. calculated distances. The output means may be for example an output port (not shown) for providing output to a display means such as a screen, which may be comprised in for example a computing device such as a smartphone or tablet or may be an independent screen. The output means may be a communications port (not shown) for communicating an output to a separate computing device (not shown) via wired or wireless communication.

[0074] It will be understood that other means of synchronisation are possible within the scope of the present invention.

[0075] Referring again to Fig.l, the apparatus 1 comprises a filter 4 disposed on a light path between the light source 2 and the detector 3. The function of the filter is to reduce the intensity of light from the background or environment, which could cause FCA in the detector 3 and to pass photons which can cause TPA in the detector 3.

[0076] The filter 4 is an optical long-pass filter. The filter 4 is configured to substantially block light from the light source 2 having a wavelength less than or equal to hc/E _g . Photons having an energy of less than E _g cannot give rise to single photon absorption and so by using such a filter, the intensity of light from the light source and the background which could cause single photon absorption is reduced.

[0077] By "substantially block" is meant a transmission of less than 90%, preferably less than

95%, more preferably less than 98%, still more preferably less than 99.9%, or less than 99.999%.

[0078] The filter 4 can be provided directly on or over the pixels of the detector 3, such that the filter 4 is an integral or removable part of the detector 3, or can be separate from and spaced apart from the detector 3. The detector may comprise a cover glass disposed over the pixels, the cover glass comprising the filter 4. [0079] Preferably the filter is a bandpass filter and has a bandwidth which is wider than the

FWHM of the monochromatic light emitted by the coherent light source. Preferably, the linewidth of the filter is less than 50 nm wider than the FWHM of the coherent light source.

[0080] The optical filter can be a dichroic mirror (also called Bragg's reflector), which is made of an alternating stack of materials with high and low refractive index, tuned to a specific transmission wavelength. It can be an etalon structure, also called Fabry-Perot interference filter, which is a set of two dichroic mirrors at a fixed distance. The fixed distance determines the passband wavelength. Such etalon structures can have very narrow passband wavelengths. It can also be an absorbing filter, which absorbs the out-of-band light rather than reflecting it. Alternatively, it can be a combination of several filter principles (e.g. a Bragg reflector coated on top of an absorbing filter). An etalon structure may be most preferred because of its capability to get a narrow transmission bandwidth. The transmission bandwidth is also dependent on the angle-of-incidence of the light. At the edge of the detector, the angle-of-incidence of light may be up to 30 degrees, while in the centre the light falls perpendicular on the detector. This means that the transmission band of the filter should be chosen wide enough to transmit the light with high transmittance over the entire detector area.

[0081] Referring to Fig.2, the relationship between the filter passband, the wavelength corresponding to the bandgap energy of the detector (i.e. the wavelength of incident photons below which single photon absorption can take place), and the wavelength Li of the laser is shown schematically. The wavelength L of an optional second light source, which will be described in more detail hereinafter, is also shown.

[0082] Referring to Figs.3a and 3b, in use, the apparatus 1 is preferably arranged relative to a region of interest R such that light emitted from the light source 2 can interact with or pass through the region of interest R and subsequently be detected by the detector 3.

[0083] For example, referring in particular to Fig.3a, in a transmission imaging arrangement the apparatus 1 is arranged such that the light emitted by the light source 2 has a direct path from the light source 2 to the detector 3. The detector 3 can thus detect only those photons that propagate directly through the region R without interaction, and does not detect any scattered light.

[0084] Referring to Fig.3b, in a scattering imaging arrangement the apparatus 1 is arranged such that there is no direct path from the light source 2 to the detector 3. Light from light source 2 can only reach detector 3 by scattering occurring in region R such that the path of the light is deflected towards the detector 3. Since the apparatus 1 is configured to detect two-photon absorption, the spatial depth resolution is improved as compared with single photon detection methods. [0085] Referring to Fig.3c, a preferable arrangement of the apparatus for a 3D time-of-flight measurement is shown. The arrangement is similar to that of Fig.3b, with a lens provided between the region R and the filter. Furthermore the light source and the camera are arranged such that the direction in which light is emitted by the light source is perpendicular to the focal plane of the image sensor, being the plane which includes all pixels of the detector. Preferably the laser is located very close to the image sensor, such that from a relatively short distance the light rays emitted towards the object and returned from the object towards the detector can be considered as parallel light rays, which simplifies the distance calculation. Since light travels approximately 30 cm in 1 ns, and light travels from the light source to the region R and then to the detector, an object placed at a distance of 15 cm from each of the light source and the detector will result in a time difference between the receiving light pulse at the detector and the emission of this light pulse by the light source of Ins. [0086] Referring to Fig.4, a first modified apparatus 1' according to embodiments of the present invention comprises a second filter 6 between the light source 2 and the detector 3. The second filter 6 is configured to substantially block light having a wavelength which is greater than a background light upper threshold wavelength. The second filter can be used to allow for a first subset of pixels to be used to detect two photon absorption, and a second subset of pixels to be used to detect single photon absorption using, for example, background light or an additional light source configured to emit light having a wavelength of less than 1050 nm. This advantageously provides a compact imaging system which can detect both single photon and two photon events using a single detector and lens.

[0087] Preferably, the first and second filters 4, 6 are arranged such that they have no spatial overlap at the pixels of the detector 3, such that each filter provides filtered light only for the associated subset of pixels. For example, the first and second filters 4, 6 can be provided directly on or over subsets of pixels of the detector 3.

[0088] Referring to Fig.5, a second modified apparatus 1" according to embodiments of the present invention comprises a second light source 7. The second light source 7 has an emission spectrum with a linewidth centred at a second wavelength h. The second wavelength L is less than 2hc/E _g and is greater than hc/E _g . The second light source is connected to the synchronization means as described hereinbefore with respect to the first light source. The second wavelength L is preferably between 1300nm and 1550nm.

[0089] The modified apparatus 1' may comprise a third filter 8 configured to substantially block light from the second light source with a wavelength less than or equal to hc/E _g . The third filter 8 may be provided, for example, if the first filter is operational for light having a wavelength in a range that does not include the second wavelength L . [0090] One advantage of such an apparatus comprising more than one light source is that it can be used for two photon absorption detection in a transmission mode where the light from the light sources is directed through a region containing more than one material, each of which has a different absorption spectrum. By choosing the wavelengths Li and L depending on absorption characteristics of a material of interest, it is possible to probe more than one material using one detector and achieve the advantages of two photon absorption described hereinbefore. For example, if a first material has a single-photon absorption peak at a wavelength L'i, the first wavelength Li is preferably chosen close to the wavelength L'i.

[0091] The multiple light sources 2, 7 can allow for a first subset of pixels to be used for probing a first material and a second subset of pixels to be used for probing a second material. Preferably the first subset of pixels are synchronised with the first light source 2 and the second subset of pixels are synchronised with the second light source 7.

[0092] Preferably, the first and third filters 4, 8 are arranged such that they have no spatial overlap at the pixels of the detector 3, such that each filter provides filtered light only for the associated subset of pixels. For example, the first and third filters 4, 8 can be provided directly on or over the pixels of the detector 3, e.g. in a cover glass. The filters may be manufactured directly on the pixel by dichroic mirrors or etalon filters as described before, where the thickness of the etalon filter or the characteristics of the dichroic mirror is adjusted for each pixel to tune the pixel response to a specific wavelength. Organic colour filters, such as for example the red, green and blue transmission filters commonly used for 2D colour imaging, can also be used, if their wavelength bands suit the application and the used light sources. These types of filters directly on top of the pixels are patterned using photolithographical techniques known in the art and manufactured through semiconductor process steps on wafers, such as spin coating, etching and developing, as known in the art.

[0093] The second modified apparatus 1" may additionally comprise the second filter 6.

[0094] Referring to Fig.6, any of the apparatus 1, first modified apparatus 1', and the second modified apparatus 1" may comprise one or more lenses 10 located between the at least one light source 2, 7 and the detector 3, such that the detector is located at the focal plane of the lens. This can allow for more light to be provided to the detector, increasing the available signal, and the lens allows for 2D imaging (as regularly used in cameras). For example, the second modified apparatus 1" may comprise a first lens for focusing light emitted by the first light source and a second lens for focusing light emitted by the second light source.

[0095] The first filter and the third filter, in addition to substantially blocking light from the first and second light sources, respectively, and light which has a wavelength of less than or equal to hc/E _g , may additionally be configured to substantially block light with a wavelength greater than 1550 nm, for example if such light does not cause TPA and only contributes as unwanted background light. [0096] The first light source 2, and the second light source 7 if provided, are preferably configured to operate in a pulsed mode. In a pulsed emission mode, power consumption is less than in a continuous emission mode. Pulsed light is needed for time of flight applications, as this provides the required time information, but is not necessary for all two photon absorption detection; continuous emission can be used, for example, for probing the absorption characteristics of a material.

[0097] Typically pulsed light is generated as a sequence of a few hundreds to a few thousands of short pulses, each about 1 to 100 ns wide. The amount of pulses can be adjusted depending on the signal level received at the detector. If the objects are relatively close and/or highly reflective, more signal is received by the detector or pixel element and fewer pulses is required. If objects are relatively far away or not very reflective, more pulses are needed to detect a substantial signal in the detector or pixel element. The amount of pulses is preferably controlled so as to receive a substantial signal on most of the pixels of the detector. The time duration of each pulse can be adjusted to the range of the objects, and may vary depending on an expected range for a 3D ranging system comprising the apparatus according to embodiments of the present invention. When objects are close, more and relatively shorter pulses can help to improve the distance accuracy. For objects further away, relatively longer pulses can be used. Pulses may be emitted by the light source at regular time intervals, or the time intervals between light pulses may be modulated with some random or sinusoidal or other known pattern, which can help to reduce interference from other unrelated pulsed light sources and to reduce false echoes from reflective objects at wider distance which can cause disambiguities in the range calculation, as known in the art.

[0098] An apparatus according to embodiments of the present invention can be used to determine a distance between a light source and a target point. The method is capable of determining the distance with a precision of at least lOOpm. Such a method comprises the following steps.

Step SI : an apparatus according to embodiments of the present invention as described hereinbefore is provided

Step S2 : the at least one coherent light source is caused to emit coherent light towards the target point at a first time ti. The light is emitted starting from time ti

Step S3 : a time t at which light is received by the detector is measured. Since the apparatus is configured to measure two-photon absorption, the detection event can be assumed to be a two- photon absorption event Step S4 : a distance between the at least one light source and the target point is determined based on the difference between time t and time ti.

[0099] Various modifications of the embodiments herein described are possible within the scope of the present invention.

[0100] For example, the present invention is not limited to one or two coherent light sources. The apparatus may comprise three or more coherent light sources, which may have different emission wavelengths.

[0101] Although the detector is preferably a silicon-based detector, the present invention is not limited thereto. Other detector materials can be used provided that a coherent light source is also available which is capable of emitting light at a suitable wavelength which is less than 2hc/E _g and is greater than hc/E _g , for the bandgap energy E _g of the detector material concerned, and provided a suitable filter is available. For example, a GaAs-based detector, having a bandgap energy of 1.42 eV, can be used in conjunction with a 1200 nm diode laser. An AIAs-based detector, having a bandgap energy of 2.16 eV, could be used in conjunction with an 800 nm diode laser.

[0102] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the invention may be practiced in many ways. The invention is not limited to the disclosed embodiments.

[0103] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.