WO2022167786A1

WO2022167786A1 - Methods of optical detection and optical detection apparatus

Info

Publication number: WO2022167786A1
Application number: PCT/GB2022/050267
Authority: WO
Inventors: Gilberto Brambilla; Ali MASOUDI
Original assignee: University Of Southampton
Priority date: 2021-02-03
Filing date: 2022-02-02
Publication date: 2022-08-11
Also published as: GB202101476D0

Abstract

A method of optical detection comprises arranging a optical transmission channel to propagate photons as a single channel of information; and detecting an optical output from the optical transmission channel, the optical output comprising the photons carrying the single channel of information only, using an optical detector comprising a plurality of single photon detector elements arranged in a spatial array, each single photon detector element producing an output, the outputs of all the plurality of single photon detector elements being combined into a detector output for the optical detector.

Description

TITLE OF THE INVENTION

METHODS OF OPTICAL DETECTION AND OPTICAL DETECTION APPARATUS

BACKGROUND OF THE INVENTION

The present invention relates to methods of optical detection and optical detection apparatus.

Sensors based on optical fibres and optical waveguides are available for sensing, detecting, monitoring and measuring a variety of parameters. Some examples are configured to provide a discrete output, in that measurement is obtained for a single point or location only. However, the longitudinal extent of waveguides and especially optical fibres enables distributed measurements, in which the profile of a parameter over a distance can be determined from a single interrogation of the sensor. Other arrangements that propagate light include communication systems in which a detector decodes photon sequences that have been intentionally formatted or coded to carry or represent data, and computing systems where each photon constitutes a fundamental calculation unit. These include quantum systems in which entanglement between photons makes the detection of individual photons extremely important.

As an example of distributed optical sensors, the optical fibre distributed temperature sensor (DTS) is of particular interest. The DTS is attractively simple because rather than requiring dedicated temperature detectors installed at predetermined locations in order to obtain a temperature profile, it uses the actual fibre as the sensing element so that no additional detectors or transducers are needed. The DTS makes use of an inherent property of the fibre material, which scatters light propagating in the fibre from scattering centres in the material. Some of the scattering, known as Raman scattering, is temperature-dependent, varying according to the temperature of the fibre. To make a temperature measurement, a light pulse, referred to as a probe light is launched into the fibre which has been deployed in a region where the temperature is of interest, and the Raman scattered light that travels back along the fibre is detected. Using time to resolve the detected scattering reveals the location of each scattering event along the fibre, and the amount of detected light depends on the temperature at that location. Hence temperature data can be extracted from the detected light. Hence, a temperature map or profile showing the variation of temperature with length along the fibre is obtained.

Raman scattering is very weak, however, with only a tiny proportion of the energy of the original pulse being returned for detection in the DTS. A range of techniques are used to manage this issue. These include the use of long probe pulses, in the order of 10 m, together with pulse coding. This optical time domain reflectometry (OTDR)-based approach is widely used in industry and commercialised systems, and offers a relatively long sensing range (the obtained temperature profile spans a large distance, typically tens of kilometres), but has limited spatial resolution (owing to the long probe pulses) so is not suitable for some applications. Very high spatial resolution is offered by the alternative of optical frequency domain reflectometry (OFDR), but such systems have a very short sensing range of a few hundred metres only. A different approach is the use of a single photon detector to collect the Raman scattering. This can achieve high spatial resolution and enables detection of very low light levels, but is extremely slow at obtaining a full profile because the temperature data is collected one photon at a time. Hours can be required to map the temperature over a desired measurement range.

These various difficulties can also apply to other optical applications in which the amount of light available for detection is very low.

Consequently, improved approaches for optical detection are of interest.

SUMMARY OF THE INVENTION

Aspects and embodiments are set out in the appended claims.

According to a first aspect of certain embodiments described herein, there is provided a method of optical detection comprising: arranging a optical transmission channel to propagate photons as a single channel of information; and detecting an optical output from the optical transmission channel, the optical output comprising the photons carrying the single channel of information only, using an optical detector comprising a plurality of single photon detector elements arranged in a spatial array, each single photon detector element producing an output, the outputs of all the plurality of single photon detector elements being combined into a detector output for the optical detector.

According to a second aspect of certain embodiments described herein, there is provided an optical apparatus comprising: an optical transmission channel configured to propagate photons as a single channel of information and deliver an optical output comprising the photons carrying the single channel of information only; and an optical detector arranged to detect the optical output from the optical transmission channel, the optical detector comprising a plurality of single photon detector elements arranged in a spatial array, each single photon detector element configured to produce an output, and the optical detector configured to produce a detector output comprising the combined outputs of the single photon detector elements.

These and further aspects of certain embodiments are set out in the appended independent and dependent claims. It will be appreciated that features of the dependent claims may be combined with each other and features of the independent claims in combinations other than those explicitly set out in the claims. Furthermore, the approach described herein is not restricted to specific embodiments such as set out below, but includes and contemplates any appropriate combinations of features presented herein. For example, methods of optical detection and optical detection apparatus may be provided in accordance with approaches described herein which includes any one or more of the various features described below as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which:

Figure 1 shows a schematic representation of an example distributed temperature sensing system according to the present disclosure;

Figure 2 shows a schematic representation of an example multichannel optical detector suitable for use in methods and apparatus according to the present disclosure;

Figure 3 shows a graph of multichannel optical detector output with sensing fibre length measured using the sensing system of Figure 1 ;

Figure 4 shows a graph of multichannel optical detector output with sensing fibre temperature measured using the sensing system of Figure 1 ;

Figure 5 shows a graph of multichannel optical detector output with sensing fibre length measured using a modified version of the sensing system of Figure 1 ;

Figure 6 shows a highly schematic representation of a first example of apparatus configured generally for multichannel optical detection according to the present disclosure;

Figure 7 shows a highly schematic representation of a second example of apparatus configured generally for multichannel optical detection according to the present disclosure; and

Figure 8 shows a flow chart of steps in an example method of optical detection according to the present disclosure.

DETAILED DESCRIPTION

Aspects and features of certain examples and embodiments are discussed I described herein. Some aspects and features of certain examples and embodiments may be implemented conventionally and these are not discussed I described in detail in the interests of brevity. It will thus be appreciated that aspects and features of methods and apparatus discussed herein which are not described in detail may be implemented in accordance with any conventional techniques for implementing such aspects and features.

The single photon detector is a type of optical detector configured to detect individual photons, and is hence used for the detection of light at very low intensity levels, and allows the counting of photons. In contrast with a conventional photodetector, which produces an analogue output signal which is proportional to the intensity of the incident light (photon flux), a single photon detector produces a single fixed level pulse of output signal when a photon is incident on the detector. The output is therefore effectively digital or binary, being zero when no photon is incident and one when a photon arrives at the detector. Counting these output signal pulses with suitable electronics enables the counting of incident photons. Various formats of single photon detector are available, which in some cases comprise conventional photodetector designs specially configured for the detection of individual photons. Examples include, but are not limited to, photomultipliers, Geiger counters, single photon avalanche diodes, transition edge sensors, scintillation counters and superconducting nanowire single photon detectors. Applications for single photon detectors are varied, and include quantum technologies such as communications, computing, encryption and sensing, and techniques where low light levels require detection, such as distributed sensing in which scattered or reflected light from an optical fibre is measured to determine a physical property on which the amount of scattering or reflecting depends, various imaging techniques in medicine and astronomy, and light detection and ranging.

A single photon detector is able to detect any incident photon which has an energy higher than an intrinsic detection energy threshold of the detector. A photon has to have sufficient energy to trigger the relevant physical process in the single photon detector which generates the output signal pulse. The level of the threshold will depend on the configuration of the single photon detector. Additionally, single photon detectors are characterised by other properties, including detection efficiency or counting efficiency, time resolution, dark count rate (DCR) and recovery time. This last property indicates the time taken for the single photon detector to recover its receptive state, in which it is able to detect a photon, after detection of a previous photon. During the recovery time, the single photon detector is not able to detect photons, and so is unresponsive and blind to any photons that arrive at the detector during the recovery time. Also, two photons arriving at the detector at the same time cannot be distinguished, and will be recorded as one photon only.

A typical recovery time duration may be about 50 ns or 100 ns. This limitation inhibits the use of single photon detectors for some purposes. The recovery time defines a count rate, being the maximum rate at which a single photon detector can detect photons. This in turn limits the rate at which information can be transferred or carried by photons along an optical transmission or propagation channel outputting the photons as an optical output to a single photon detector. Where the photons being transmitted or propagated comprise the response of an optical sensing or measurement arrangement, the sensitivity and resolution of the sensing is similarly limited. An example of an optical sensing system where single photon detectors are utilised is the distributed optical fibre temperature sensor (DTS). An optical fibre is deployed into or along an environment (region, area, or within an item, article or component) where the temperature and/or change of temperature are of interest. The optical fibre itself is able to act as a temperature sensor, with no need for dedicated sensors or transducers. One or more pulses of light (probe pulses) are launched into one end of the optical fibre to interrogate the optical fibre, and propagate along it. During propagation, the light undergoes the usual optical interactions with the material from which the optical fibre is fabricated, which include Raman scattering. This is a process by which a very small fraction of the photons comprised in the interrogating pulse are inelastically scattered by molecules in the optical fibre. The effect is temperature-dependent, in that the amount of Raman scattering that occurs varies with the temperature of the fibre material. The inelastic nature of the scattering means that the energy of the photons is changed. Hence, the Raman scattered photons have a different wavelength from the interrogating pulse, so they can be readily distinguished, and an amount that depends on the temperature of the fibre material. Detection of the Raman scattered photons can therefore reveal information about the fibre temperature, which reflects the temperature of the environment in which the DTS optical fibre is deployed (when other factors such as optical loss along the fibre are taken into account). For convenience, photons scattered in the back-propagation direction (back-scattering), opposite to the propagation direction of the launched probe pulse, are typically detected, since this arrangement requires access to only one end of the optical fibre for both probe pulse launch into the optical fibre and detection of Raman backscattered photons as an optical output from the optical fibre. This allows very long lengths of optical fibre to be deployed if required, enabling remote sensing. Forward Raman scattering may also be detected at the other end of the fibre, however. The use of probe light in the form of a discrete pulse of known duration together with the known velocity of light in the optical fibre means that the location of the scattering event responsible for any given detected backscatter can be deduced from the time period between probe pulse launch and backscatter detection. In this way, a map or profile of temperature distribution along the length of the optical fibre can be obtained by converting time to position and backscattered photon level to temperature. Hence, the temperature sensing is “distributed” along the fibre. The use of multiple probe pulses launched in a sequence can produce a temperature profile from a single point in time with the different pulses causing scattering at different locations along the fibre at the same time. This is useful for environments undergoing sudden temperature changes, for example.

The very low intensity of Raman backscatter (small number of photons which undergo the relevant scattering event) has led to the use of single photon detectors to detect the optical fibre output in DTS systems. While single photon detector DTS can provide a high spatial resolution along the length of the fibre, the recovery time characteristics of single photon detectors discussed above make the detection process very slow; one backscattered photon is detected per probe pulse. It can take a significant amount of time to obtain a temperature profile from a long optical fibre.

The present disclosure proposes to use a different type of optical detector. In the existing single photon detector arrangement, a single channel of optical information, comprising the Raman backscattered photons in the DTS example, is detected using a detector configured to detect a single channel only. The terms “channel” and “channel of information” are used herein to indicate that the optical signal (photons) being propagated describes a single continuously evolving stream of information or data conveyed between a first point and a second point according to a particular carrier arrangement. In the example of DTS, the channel is physically provided by the optical fibre, which propagates the optical signal in the form of Raman backscattered photons from the scattering site to the detector, the photons carrying information about the fibre temperature. The single photon detector, able to detect only one photon at a time, is suited to this and other single channel applications, but the recovery time means that not all photons will be detected. Intermittent parts of the optical signal that arrive at the detector during the recovery time will be missed. For DTS and other sensing applications, this leads to longer mapping times in order to collect enough photons to provide data about temperature (since temperature is related to the number of Raman scattered photons). For communications applications, like quantum communications, the undetected photons can produce errors since there will be gaps in the received optical signal. For quantum computing applications, long recovery times limit the achievable computational speed because every photon needs to be detected.

Accordingly, it is proposed to use an optical detector that comprises a plurality of detector elements or cells, each element being an individual single photon detector, which are arranged in a spatial array such as a square, rectangular or circular grid or matrix. Thus, a number of single channel detectors (each of the single photon detectors) are provided in a parallel arrangement. The optical detector is configured such that each of the single photon detector elements produces an output, and the outputs of all the individual single photon detector elements are added or combined together into an overall detector output for the optical detector. This provides a multichannel optical detector, comprising many detector elements each designed to detect a single channel, but according to the concept herein, the multichannel optical detector is used to detect a single channel only.

If the optical output of the single channel is incident across the optical detector as a whole (rather than being directed one single photon detector element only), the issues arising from single photon detector recovery time can be largely addressed. There are many other detector elements available to detect a next photon while any individual detector element is recovering from the previous photon. For a low intensity optical detection application, the use of plural detector elements provides redundancy such that there can always be at least one detector element in a state ready for photon detection, so that photons are not missed. The recovery time of an individual element can be considered as averaged across the whole of the optical detector, so that it can be greatly reduced. Also, the availability of multiple detector elements allows two (or more) simultaneously arriving photons to both be detected, giving a detector output at that instant which is twice the output for a single photon strike.

This parallel detection arrangement for a single channel can greatly improve the figure-of-merit (FoM) for a system in which it is used. For example, an experimental DTS system has shown a FoM of 3105, which is believed to be almost sixty times better than the highest reported value for DTS using a conventional single photon detector configuration. Moreover, this FoM value was limited by the particular experimental set-up used. Modelling and analysis suggest that an optimised system may have a FoM of 30,000 or higher. Also, the parallel detection offers general improvements, since a DTS system configured in this way can be suitable for either very long range sensing (fibre lengths of 50 km or more) with much improved temperature and spatial resolution or short range sensing offering spatial resolution on the centimetre scale with a greatly shortened response time of the order of 100 ms.

Specific examples of DTS systems configured in accordance with the proposal are described in more detail below. However, the concept is not limited in this regard, and can be applied to any optical system or apparatus requiring detection of a low light intensity output from a single optical transmission channel. The system may be a sensing system, in which the channel propagates photons arising from some phenomenon required to be sensed or detected, or a detecting or monitoring system such as an optical time domain reflectometry (OTDR) system that measures optical attenuation in an optical channel, the photons being the attenuated light, or a communication system, in which the channel propagates photons that have been intentionally formatted or coded to carry or represent data. In all cases, the channel is propagating photons that carry information being conveyed on that channel. In the DTS example, the information is the level or amount of Raman backscatter which in turn indicates the temperature of the optical fibre. The channel transmits the photons from their source or point of origin to the multichannel detector. In the DTS example, the source is the many scattering events that arise inside the sensing fibre. Conveniently, as in DTS, the channel is physically embodied as an optical fibre, but this is not essential, and an optical transmission channel may alternatively be arranged such that the photons comprising the optical signal propagate from source to detector in free space.

The multichannel optical detector may be, for example, a type of detector known as a multi-pixel photon counter (MPPC), which comprises a collection of typically hundreds of individual single photon detector cells or elements packaged together into a single unit. Although each element has a low detection bandwidth, the combination can measure high frequency and high bandwidth data from a single optical channel. A MPPC can be formed from single photon detector elements that comprise single photon avalanche photodiodes, for example. The avalanche photodiodes may be formed from indium gallium arsenide (InGaAs), for example. MMPCs may also be referred to as discrete amplification photon detectors (DAPD). These types of detector produce an output which is an electrical current which is linearly proportional to the number of photons incident on the detector, and its quantum efficiency. However, the optical detector is not limited in this regard, and single photon detectors of any type, including the examples give above, may be formed into an array for a combined output.

The multi-channel optical detector may also be thought of a multi-pixel single photon detector, where each single photon detector element acts a separate pixel, the outputs of which are combined to create an overall output for the detector that represents the number of incident photons.

Figure 1 shows a schematic representation of an example DTS system according to the present disclosure, configured for demonstration of the concept. The DTS system 10 comprises three parts: a probe pulse generating part, a sensing fibre, and an optical detection part. The optical probe pulses required for DTS can be generated and launched in any convenient way. In the present example, which is not limiting, the pulses are generated, and carried by optical fibre to the sensing fibre. An optical source 12 comprises a laser 14 in the form of a distributed feedback diode laser 14 gain-switched with a pulse generating arrangement 16 so as to output pulses of light 18, these being the probe pulses or interrogating pulses for the sensor. Any pulse generation technique may be used as preferred, such as Q-switching, electro-optic modulation or modelocking. The laser 14 has an optical output wavelength of 1064 nm in this example (other wavelengths may be used as preferred), and pulses 18 of 1 ns duration are generated. This pulse length gives a 10 cm spatial resolution for mapping the temperature along the fibre; other pulse lengths may be used. The pulses 18 are launched into a single mode optical fibre 20 that transmits the pulses 18 to the sensing fibre 22. Multimode fibre may be used for this instead. The single mode fibre 20 conveys the pulses 18 via a series of optical components. Firstly, the pulses 18 pass through an optical isolator 24 that blocks any back-propagating light in order to protect the laser 14. Then, an ytterbium-doped fibre amplifier 26 is used to increase the optical power of the pulses. Optical fibre is inherently attenuating, so probe pulses need a sufficient power level to propagate effectively along a potentially very long sensing fibre 22. An optical bandpass filter 28 is used to remove unwanted spectral components (amplified spontaneous emission) output from the amplifier 26, and finally the pulses 18 are passed through an acousto-optic modulator 30 in order to create a probe pulse with a very high extinction ratio for launch into the sensing fibre 22.

In this example, the sensing fibre 22 of the DTS system is a multimode fibre. This is to allow the maximum detection of Raman backscattered light, which may be coupled into higher order modes than the fundamental mode of the sensing fibre and would be suppressed and lost in a single mode sensing fibre. The single mode launch fibre 20 is spliced at its end to a first end of the sensing fibre 22, at a splice 32. Hence, the probe pulses 18 at 1064 nm are launched into the sensing fibre 22, and propagate along it in a forward direction, away from the splice 32. In this example, the sensing fibre 22 has a length of around 300 m, although in practice DTS sensing fibres can be many times this length, comprising kilometres or tens of kilometres. In order to provide a thermally variable environment for the sensing fibre 22 for adequate assessment of the system, a portion of the sensing fibre 22 is arranged for exposure to an adjustable heating component 34, such as a hot plate or a water bath. The temperature of the heating component 34 can be adjusted in order to change the temperature of the sensing fibre’s environment, and hence the temperature of the sensing fibre, thereby altering the level of Raman scattering from this portion of the sensing fibre 22. In particular, the sensing fibre enters the environment of the heating component at a distance of about 240 m from the splice 32. Raman scattering of the forward propagating probe pulses 18 occurs in the sensing fibre 22 in accordance with physical principles, and a proportion of the scattered light is directed in the back-propagation direction along the sensing fibre 22, towards the splice 32. In order to detect the Raman back-scattering, a wavelength-division multiplexing optical splitter 36 is arranged near the splice end 32 of the sensing fibre. Raman scattering is wavelength-shifted from the original light, and in this example, the sensing fibre is such that the 1064 nm probe pulses produce Raman scattering at 1016 nm. The optical splitter 36 is therefore configured to couple 1016 nm light away from the splice 32 and into an output fibre 38, also multimode in order to avoid suppression of any high order Raman scattering. The output fibre 38, carrying the optical output of the single optical transmission channel embodied by the sensing fibre 22, passes the Raman scattered light through a second bandpass filter 40 to remove light at non-Raman wavelengths, and delivers the Raman scattered output light to a multi-channel optical detector 42. The optical detector 42 outputs an electrical signal 44 (voltage) indicative of the combined outputs of each of its individual single photon detector elements 43 (of which only a small number are represented in Figure 1). This detector output 44 is supplied to an amplifier 46 such as transimpedance amplifier to increase the signal level (recalling that Raman scattering is a low-level phenomenon), before reaching an analogue-to-digital converter 48. The final digital output can be provided to a processor (not shown) for processing of the detected signal. This handling of the detector output is purely an example and is not limiting.

Figure 2 shows a highly schematic representation of an example multi-channel optical detector such as that in the system of Figure 1. The optical detector 42 comprises a plurality of individual single photon detector cells or elements 43 arranged in an array, in this case a square grid. The array is a 17x17 grid, and therefore comprises 289 individual elements 43, but this is purely for the sake of illustration and a multi-channel detector 42 may comprise more, fewer, many more or many fewer elements 43. An array of 100 elements, arranged for example as a 10x10 grid, can be sufficient or useful. A smaller array such as nine elements as a 3x3 grid may also be relevant for some applications. At the other extreme, around a million elements may be used, configured for example as a 1000x1000 grid. Intermediate quantities such as around 50 elements, 500 elements, 1000 elements, 5000 elements, 10,000 elements or 50,000 elements, or grids of around 20x20, 50x50, 100x100 or 500x500, and values between these examples, may be used according to the application. An increased number of elements in the same overall detector size is particularly useful for increasing the detector sensitivity and reducing the overall recovery time. However, the larger the number of elements, the greater the total dark count rate since each element generates its own dark current independently of the other elements, so this may be a limiting factor for the total number of elements in some cases. It is anticipated that developments in single photon detector technology will reduce dark current, however, so this may become less of a consideration when combining large numbers of elements. Each element 43 produces its own output signal 45 (only some of which are shown in Figure 2, for clarity) and these are combined to produce a final overall output signal 44 for the optical detector 42, which is proportional to or at least indicative of the total number of photons incident on the detector 42 at that time.

Figure 3 shows a graph of experimental results obtained from the DTS system of Figure 1, for the output voltage of the optical detector 42 measured over time. Optical time domain reflectometry was performed to convert time into distance along the sensing fibre, so the graph shows the variation of detector output in millivolts for distance along the sensing fibre, for a portion of the sensing fibre between 238.5 m and 241.5 m from the splice end. A relatively abrupt increase in voltage can be clearly seen at around 240 m, being the location at which the sensing fibre transitioned from a room temperature environment to the environment of the heating component (see Figure 1).

Although the fact that a change in temperature with distance along the sensing fibre occurred can be readily appreciated from Figure 3, if absolute values of temperature are required, the output of the optical detector can be converted. In order to enable this, calibration can be performed, by detecting the amount of Raman backscatter and hence the output voltage that is produced for known values of temperature, such as by adjusting the heating component of Figure 1. The relationship between temperature and output voltage can thereby be ascertained.

Figure 4 shows a graph of voltage difference (mV) of the detector output (as compared to a baseline output at room temperature) with temperature, measured using the Figure 1 DTS system. As expected, the voltage difference increases as the temperature increases, since Raman scattering increases with temperature. The relationship is linear, also as expected from a Raman-based DTS system.

The example DTS system shown in Figure 1 included multimode fibre, as noted. This is conventional for DTS. However, as discussed, the proposed use of a multichannel optical detector to detect a single channel optical output is widely applicable for both sensing and communications applications, and may be employed with both optical fibre channels and free space channels. For some applications, single mode optical fibre will be more suitable than multimode fibre. In order to demonstrate applicability, the example DTS system of Figure 1 was modified by replacing the multimode sensing fibre with a single mode sensing fibre.

Figure 5 shows a graph of results obtained from the modified single mode fibre DTS system, as the variation of the output of the multichannel optical detector (in arbitrary units) with distance along the sensing fibre, again covering the portion where the sensing fibre transitioned from room temperature to the heated environment, at about 240 m. Comparison with Figure 3 shows that the overall performance is similar for both fibre types, with the increase in detector output at increased temperature being clear. The higher noise level in Figure 5 compared with Figure 3 arises because the sensing system overall was not optimised for use with single mode fibre.

Figure 6 shows a highly schematic representation of a first example of apparatus configured generally for multichannel optical detection according to the present disclosure. This may be part of a sensing, measuring or monitoring system, or a communication system, as noted. In this example, an optical transmission channel comprises an optical waveguide 50, which may be an optical fibre (solid core or hollow core) or a planar waveguide or other photonic waveguiding device, for example, which propagates or transmits photons comprised in a single channel of information. The optical waveguide may be configured for single mode or multimode optical propagation. The optical output 52 of the optical waveguide 50 is incident on a multichannel optical detector 42, where the photons are detected. The output 52 can be coupled to the optical detector 42 using any preferred optical arrangement, such as the intermediate output fibre shown in Figure 1, or free space, or free space with intervening optical elements such as lenses, mirrors or diffusers, or direct coupling. The optical detector 42 produces a detector output 44 which is passed to a further component such as a processor (again possibly via intermediate components as in Figure 1) for further handling or manipulation, such as conversion to a sensor output such as temperature or strain, or decoding/encoding in a communication system, or simple photon counting, or image processing, etc.

Figure 7 shows a highly schematic representation of a second example of apparatus configured generally for multichannel optical detection according to the present disclosure. In this example, the optical transmission channel is configured as a free space propagation path 64 to propagate photons in a information channel from a channel source 62 to the multichannel optical detector 42. The free space channel 64 may include optical components for light beam directing and shaping, such as mirrors and lenses. Figure 8 shows a flow chart of steps in an example method according to the present disclosure. In a first step S1 , an optical transmission channel is arranged or provided, for the propagation of photons as a single channel of information. In a second step S2, the optical output of the optical transmission channel, which comprises the photons carrying the information of the single channel only, is detected using a multichannel optical detector comprising an array of individual single photon detector elements as described above. In a third step S3, the output from the optical detector is obtained, where the detector output comprises the combined outputs of the individual single photon detector elements. In some cases, the detector output will itself be adequate, but in other cases the method can comprise an optional step S4 in which the detector output is processed, for example to determine information carried in the single channel, such as the temperature date obtained using a DTS system.

The concept proposed herein can bring improved optical detection, and hence can enhance the performance of systems and arrangements that utilise optical detection. Considering sensing as an example, subsurface defects and damage in composite materials may pose a significant risk to structural failure if undetected. Reliable structural health monitoring (SHM) technologies are in demand to detect the signs of damage development and avoid catastrophic failure of a structure. Optical fibres can locally map temperature and other parameters such as strain within a composite structure. Optical detection as described herein offers high speed and high resolution/sensitivity and by strategically distributing an optical sensor throughout a structure, it is possible to improve the detection of damage and reduce structural failure. In general, sensitivity and response time of an optical sensor system can be enhanced.

Considering quantum applications, the speed of a quantum communication or sensing system will be limited by the response time of an individual single photon detector. Using the massive parallelism described herein, it is possible to decrease the dead time that occurs between the detection of consecutive single photons, and increase the overall speed of a quantum system. Information transmission rates and computational times can be improved. Operation at cryogenic temperatures can decrease the dark current of each single photon detector, allowing larger arrays of single photon detectors to be assembled without compromising the total dark current.

Overall, the proposed use of a multichannel detector to detect a single channel of photons can benefit any application that would be improved by a reduction of the measurement time or an increase in the number of measurements possible per unit time.

The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and/or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and/or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in the future.

Claims

1. A method of optical detection comprising: arranging a optical transmission channel to propagate photons as a single channel of information; and detecting an optical output from the optical transmission channel, the optical output comprising the photons carrying the single channel of information only, using an optical detector comprising a plurality of single photon detector elements arranged in a spatial array, each single photon detector element producing an output, the outputs of all the plurality of single photon detector elements being combined into a detector output for the optical detector.

2. A method according to claim 1 , wherein the optical transmission channel comprises an optical waveguide.

3. A method according to claim 2, wherein the optical waveguide comprises an optical fibre.

4. A method according to claim 2, wherein the optical waveguide comprises a planar waveguide.

5. A method according to any one of claims 2 to 4, wherein the optical waveguide is configured for single mode optical propagation.

6. A method according to any one of claims 2 to 4, wherein the optical waveguide is configured for multimode optical propagation.

7. A method according to claim 1 , wherein the optical transmission channel comprises optical propagation through free space.

8. A method according to any one of claims 1 to 7, wherein the plurality of single photon detector elements comprises at least 100 single photon detector elements.

9. A method according to any one of claims 1 to 7, wherein the plurality of single photon detector elements comprises between 10 and 1 ,000,000 single photon detector elements.

10. A method according to any one of claims 1 to 7, wherein the plurality of single photon detector elements comprises between 100 and 1000 single photon detector elements.

11. A method according to any preceding claim, wherein each of the plurality of single photon detector elements comprises a single photon avalanche photodiode.

12. A method according to any preceding claim, wherein the method further comprises processing the detector output to obtain or determine information carried by the photons in the single channel.

13. A method according to any one of claims 1 to 12, wherein the photons arise from an optical sensor and carry information relating to a parameter detectable by the optical sensor.

14. A method according to claim 13, wherein the parameter comprises temperature.

15. A method according to claim 13 or claim 14, wherein the optical transmission channel also provides the optical sensor.

16. A method according to any one of claims 1 to 12, wherein the photons carry encoded information and the single channel of information is a communication channel between a source of the photons and the optical detector.

17. A method according to claim 1 , wherein the optical transmission channel comprises an optical fibre deployed for sensing temperature in an environment, and the optical output comprises photons of Raman backscattered light arising from an interrogating pulse of light input to the optical fibre and scattered by the optical fibre at a level depending on a temperature along the optical fibre, the method further comprising: processing the detector output to determine the temperature along the optical fibre.

18. A method according to claim 17, wherein the optical fibre is embedded in a structure formed from composite material. -17-

19. An optical apparatus comprising: an optical transmission channel configured to propagate or manipulate photons as a single channel of information and deliver an optical output comprising the photons carrying the single channel of information only; and an optical detector arranged to detect the optical output from the optical transmission channel, the optical detector comprising a plurality of single photon detector elements arranged in a spatial array, each single photon detector element configured to produce an output, and the optical detector configured to produce a detector output comprising the combined outputs of the single photon detector elements.

20. An optical apparatus according to claim 19, wherein the optical transmission channel comprises an optical waveguide.

21. An optical apparatus according to claim 19, wherein the optical transmission channel comprises an optical fibre deployed or deployable into an environment where temperature is to be sensed, and the optical apparatus further comprises an optical source operable to launch interrogating pulses of light into the optical fibre to be scattered by the optical fibre as photons of Raman backscattered light at a level depending on a temperature along the fibre in order to provide the optical output.

22. An optical apparatus according to any one of claims 19 to 21 , wherein the plurality of single photon detector elements comprises between 10 and 1,000,000 single photon detector elements.

23. An optical apparatus according to any one of claims 19 to 22, wherein each of the plurality of single photon detector elements comprises a single photon avalanche photodiode.

24. An optical apparatus according to any one of claims 19 to 23, wherein the optical apparatus is operable at cryogenic temperatures.