Detecting light reflected from dbrs using tdm and WDM

Abstract

A measurement system for detecting a wavelength range of light reflected from at least one of a plurality of Distributed Bragg Reflectors, DBRs, arranged at known initial locations in an optical waveguide. A light emitter is configured for emitting light into the optical waveguide at a selectable wavelength range, wherein the wavelength range of the emitted light is selected such that the emitted light is reflected from a specific responsive DBR of the plurality of DBRs. A detector unit comprises a modulator, a mapper and a detector. The modulator is configured for to either pass the reflected light from the specific DBR or to stop the reflected light. The mapper is configured for mapping the selected wavelength range onto a specific wavelength subrange spanning at most a part of the total wavelength range of the light emitter. The detector is configured for detecting the specific wavelength subrange. A controller is coupled to the light emitter to determine a point in time when the light emitter has begun emitting the light and is configured for synchronizing a time of activation of the detector based on the point in time and on an estimated time of flight of the emitted light, taking into account a specific known initial location of the specific DBR.

Description

[0001] Detecting light reflected from DBRs using TDM and WDM

[0002] TECHNICAL FIELD

[0003] The present disclosure generally relates to detecting light reflected from Distributed Bragg Reflector (DBR) sensors. Particular embodiments relate to a measurement system and a method.

[0004] BACKGROUND

[0005] Fiber Bragg Gratings (FBGs) are a type of Distributed Bragg Reflectors (DBRs), i.e. a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristic (such as height) of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide. A DBR, in particular an FBG, can be constructed in a short segment of optical waveguide, such as an optical fiber, to reflect a particular wavelength range of light and to transmit all others (i.e. the DBR is tuned to or is responsive to that particular wavelength range). This is achieved by creating the aforementioned periodic variation in the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror and reflects light. DBRs can be used as sensors (DBR sensors) in an optical waveguide, by emitting light into the waveguide towards those DBRs and analysing the light reflected from the DBR sensors. A DBR can thus be used as a sensor by the fact that its reflected wavelength is a direct result of DBR material changes from the environment (change in index, geometric variation), such that observing a shift in wavelength response by means of illumination and detection of reflectance or transmission constitutes a sensor measurement of the DBR’s environment. This process of detection, analysis or monitoring is often called DBR sensing or interrogation and is a way of measuring. Typical applications of DBR sensing may include temperature analysis (as the length and dielectric constant of the optical waveguide increases with temperature) and strain analysis of physical bodies when the waveguide is attached to the body to act as an optical strain gauge, e.g. for structural health monitoring, although other types of applications may also be considered. A wide range of measuring approaches for DBR sensors exists. Two main approaches of interrogation can be categorized by the following:

[0006] Scanning method: Scanning a tuneable narrow-linewidth laser across the DBR reflection band to determine its spectrum over time (sometimes also expressed as a centre-of-gravity).

[0007] Spectrometric method: Applying a broadband source to the DBR(s) and determining the wavelength on a detector by means of spectrometry.

[0008] Most DBR-based measurement systems have the ability to multiplex the number of point sensors on a single or multiple waveguide paths. One of the multiplexing methods is to allocate to each DBR a different wavelength range. The DBRs function as sensors that can be uniquely monitored on a single optical fiber strand by designating distinct wavelength ranges of operation for each DBR sensor. However, the extent of this multiplexing method is limited to the availability of wavelength ranges in the measurement system, determined by both the light source spectral range (i.e. the total range of wavelength ranges that the light source can output) and the light detector properties, such as its operating wavelength span (i.e. the range of wavelength ranges that the light detector can detect) and its spectral resolution (i.e. the minimal wavelength spacing that can be resolved by the detector).

[0009] The use of time of flight for selecting one particular DBR sensor to be interrogated among a plurality of DBR sensors has in the past been introduced. As is well-known, the time of flight up to and back from that one particular DBR sensor to be interrogated can be taken into account such that only the reflected signal from that one particular DBR sensor is detected, and signals reflected from other DBR sensors can be dropped or ignored or compensated. Of course, this requires that the time of flight, and therefore, given a known propagation speed of the light being emitted, the distance up to such particular DBR sensors is known.

[0010] In a more recent advancement, the plurality of DBR sensors can be subdivided in divisions of DBR sensors. In this context, a division of DBR sensors is defined by a plurality of consecutive DBR sensors in an optical waveguide wherein each DBR sensor is responsive to a particular, different wavelength range. For example, a division may include 5 or 10 or 20 or any other number of DBR sensors, wherein each individual DBR sensor is responsive to one particular wavelength range (for example a wavelength range of around 1400-1450 nm or a wavelength range of around 1450- 1500 nm or a wavelength range of around 1500-1550 nm) in a total wavelength range. Of course, more fine-grained ranges than ranges of about 50 nm may be considered, and / or of course other total wavelength ranges than in this example the total wavelength range of 1400-1550 nm may be considered.

[0011] It has been found advantageous to use an identical number of DBR sensors in each division, such that the entire band of wavelength ranges to which the individual DBR sensors of a particular division are responsive, is shared by all divisions. In other words, in a schematic example, there may be 3 divisions, wherein each division may number 4 DBR sensors denoted as ‘A’, ‘B’, ‘C’, and ‘D’, wherein A may be responsive to a wavelength range of around 1400-1450 nm, B to a wavelength range of around 1450-1500 nm, C to a wavelength range of around 1500-1550 nm, and D to a wavelength range of around 1550-1600 nm. In this schematic example, the individual DBR sensors could for example be arranged in the following order: [ABCD ABCD ABCD ABCD], Each “cluster” of ABCD represents one division, and it is clear from this schematic example that each division can use the same overall wavelength band, namely1400-1450 nm, 1450-1500 nm, 1500-1550 nm, and 1550-1600 nm. By carefully timing detection of reflected light, this method allows to single out a particular DBR sensor at a particular distance, due to the known propagation speed (and thus the propagation time) of the light (to select a particular division, e.g. the second division ABCD) and the specific wavelength range to which that particular DBR sensor is responsive (to select a particular DBR sensor within the division, e.g. the DBR sensor C within that second division ABCD).

[0012] Of course the skilled person will understand that different divisions may instead have different numbers of DBR sensors and may use different bands of wavelength ranges, e.g. [ABCD BC ABCD BCDEF], etc.

[0013] Architectures for both of the above-described main interrogation approaches with this time-domain multiplexing (TDM) method are known. The main principle regards the pulsation of light being emitted into the optical waveguide and a means of selection / suppression of the reflected light, e.g. by a pulsed return-path modulator functionality or synchronising the detection to the time of flight of the returning light. In the first approach (i.e. by a pulsed return-path modulator functionality), by synchronising the pulse generator and the modulator, the field of view can be selected and the reflected light can be detected by a slow detector, whose integration time is much longer than the pulse period.

[0014] The above-described TDM method typically imposes certain constraints on the sensor architecture, particularly regarding the geometric placement flexibility of the sensors. The main obstacle in this regard relates to the prevention of sensor crosstalk. In other words, the freedom to place DBRs wherever desired in the optical waveguide is limited. For a given block form pulse in the light source and the amplifier, the response strength in the optical waveguide segment of interrogation, also called the field of view, is the convolution of the pulse block form with itself, a triangular response curve as a function of response within the selected optical waveguide segment. In principle, the length (i.e. the number of DBR sensors or groups of DBR sensors) of each DBR division can be as long as the pulse width, but in practice, the DBRs located at the edge of the field of view, will suffer from low reflection power due to the convolution mechanism. Increasing the pulse width to more than one division length can solve this problem, but the extended length will generate crosstalk from other DBR divisions. To prevent sensor crosstalk, the DBRs responsive to the same wavelength range need to be placed sufficiently far apart, such that their responses do not overlap each other. This implies that in practice there are limitations to the DBRs placement and that therefore so-called “dead zones” are needed, i.e. optical waveguide segments without DBRs.

[0015] Besides the extended pulse width, there is another cause of crosstalk called the shadowing effect, wherein each reflecting DBR also transmits the remaining spectrum (i.e. its input minus its reflection spectrum) towards the more distal sensors. Regardless of TDM, this reduces the amount of light reaching the more distal sensors if they would be designed in the same wavelength window. This can be solved by arranging the DBR wavelength ranges such that DBRs from different divisions use slightly different wavelength ranges. Now the DBRs from different divisions are not direct copies of each other, but have a small offset wavelength range shift from one division to another. The shadowing effect can be minimized in this way and the power spectral density (PSD) of the light source can be more efficiently used.

[0016] SUMMARY

[0017] It is therefore an aim of embodiments according to the present invention to overcome one or more of the above-described problems. In particular, it is an aim of at least some embodiments to increase the freedom of placement of DBRs wherever desired in the optical waveguide.

[0018] Accordingly, there is provided in a first aspect according to the present disclosure a measurement system for detecting a wavelength range of light reflected from at least one of a plurality of Distributed Bragg Reflectors, DBRs, arranged at known initial locations in an optical waveguide, the plurality of DBRs comprising at least a first DBR responsive to a first wavelength range, a second DBR responsive to the first wavelength range, and a third DBR responsive to a different wavelength range than the first wavelength range. The measurement system comprises a light emitter, a detector unit, and a controller. The light emitter is configured for being coupled to the optical waveguide and configured for, when coupled to the optical waveguide, emitting light into the optical waveguide at a selectable wavelength range, wherein the wavelength range of the emitted light is selected such that the emitted light is reflected from a specific DBR of the plurality of DBRs, the specific DBR being responsive to the selected wavelength range of the emitted light. The detector unit comprises a modulator, a mapper and a detector. The detector unit is configured for being coupled to the optical waveguide. The modulator is configured for being operable at any one time in either a pass mode of operation to pass a signal representing the reflected light from the specific DBR or in a stop mode of operation to stop a signal representing the reflected light from the specific DBR. The mapper is configured for mapping the selected wavelength range onto a specific wavelength subrange spanning at most a part of the total wavelength range of the light emitter. The detector is configured for detecting the specific wavelength subrange. The controller is coupled to the light emitter to determine a point in time when the light emitter has begun emitting the light. The controller is further configured for synchronizing a time of activation of a particular mode of operation of the detector based on the point in time when the light emitter has begun emitting the light and based on an estimated time of flight of the emitted light based on the selected wavelength range of the emitted light, taking into account a specific known initial location of the specific DBR, wherein the time of flight relates to the propagation time of the emitted light from the light emitter to the specific DBR and the propagation time of the reflected light from the specific DBR to the detector.

[0019] The skilled person will appreciate that the difference between the first DBR and the second DBR may relate to enabling TDM (Time Division Multiplexing), i.e. using the same wavelength and distinguishing by means of spatial selectivity, whereas the difference between these two DBRs and the third DBR may relate to enabling WDM (Wavelength Division Multiplexing), i.e. distinguishing by means of non-overlapping wavelength operating and detection ranges.

[0020] The controller is thus configured to apply a form of time-domain multiplexing, TDM, by synchronizing the modulator of the detector unit with the light emitter. The detector unit can then detect the wavelength spectrum of the reflected light at the selected wavelength range, thus from only the subset of DBRs that is responsive to the selected wavelength range of the emitted light. In this context, a wavelength range may refer to any range of wavelengths within the optical spectrum, and the optical spectrum (also known as emission spectrum) refers to how the optical energy or power is distributed over different wavelengths. A wavelength range may include all wavelengths within a wide range or a narrow range, and may even include just one wavelength. Also, the modulator may be synchronised with the light emitter, by the controller, and thus its operation may take into account an estimated time of flight of light emitted to and reflected from a specific DBR of that subset of responsive DBRs, such that only the light reflected from that specific DBR is detected and not the light reflected from other DBRs of that subset of responsive DBRs than the specific DBR. The detector does not detect the light emitted from those other DBRs, although those other DBRs are also responsive to the same wavelength range and thus in principle also reflect the part of the emitted light that reaches them, because those other DBRs are characterised by another estimated time of flight because they are located at a different (known) initial location. As the modulator, and thus the detector unit, is carefully synchronised based on the estimated time of flight, and thus is able to block reflected light from other DBRs, it is possible to avoid (or ignore) detecting reflected light from those other DBRs. This requires that the DBRs are arranged at known initial locations in the optical waveguide. This knowledge may have been for example obtained by adhering to strictly standardised specifications, and / or by calibrating (i.e. scanning the optical waveguide) beforehand to determine the time of flight (and thus the location) of each DBR.

[0021] Moreover, because the modulator is configured for being operable at any one time in either a pass mode of operation to pass a signal representing the reflected light from the specific DBR or in a stop mode of operation to stop a signal representing the reflected light from the specific DBR, it is operable at any one time either to let the signal representing the reflected light pass from the DBRs in the optical waveguide to the detector, or to block the signal representing the reflected light. In this way, even if the detector is slow, e.g. is of the integrating type, and thus needs a relatively long time for actually detecting the reflected light, by imposing a sufficiently fast switching behaviour on the modulator, it is possible to effectively utilize each pulse of reflected light at the selected wavelength range, in order to have the detector actually detect it. It is noted that a signal representing the reflected light may either be the reflected light itself (as this is a signal), or may be an electrical signal derived from the reflected light signal, so downstream from photodiodes doing the actual detecting of the reflected light - in that case, the pass and stop modes of operation are of course electronically implemented.

[0022] Furthermore, the mapper is configured for mapping the selected wavelength range onto a specific wavelength subrange spanning at most a part of the total wavelength range of the light emitter. In this way, it is possible to economically use a single detector that is able to multiplex over multiple wavelength ranges. In this way, the measurement system can implement a form of wavelength-division multiplexing (WDM), as the light emitter on the one hand and the mapper and the detector of the detector unit on the other hand are able to coordinate their use of the selectable wavelength range, i.e. they are able to use a single light band at a time. This allows to keep the internal components of the detector as simple as possible. In other words, the mapper may allow for the projection of specific segments of the entire spectral detection operation range onto the same detector array, and consecutive usage of the detector for any of those spectral segments at any time, thereby enabling an economic realization of both a wide spectral operation range and yet high spectral resolution in the detector (i.e. limited number of detection bins / pixels), while retaining high overall resolution and overall spectral operation span.

[0023] It is noted that the detector and the controller can be separate physical modules using e.g. a serial interface communication within the device, or they can be combined into an integral physical device having multiple logical modular or intertwined functions.

[0024] Given that there is selective amplification on the return path for a specific range of wavelengths, a specific wavelength range and thus specific DBRs can be selected, which ensures that there will be less disturbance from other wavelength ranges (i.e. from other DBRs responsive to other wavelength ranges).

[0025] In a practical implementation, at least some of, preferably all of, the DBRs may e.g. be Fiber Bragg Gratings (FBGs). Alternatively or additionally, at least some of, preferably all of, the DBRs may e.g. be Fabry-Perot etalons (FPEs).

[0026] In an embodiment, the mapper comprises any one or more of the following means for mapping the selected wavelength range of the emitted light onto the specific wavelength subrange: an Arrayed Waveguide Grating, AWG; a diffraction grating and mirroring arrangement; and a prism and mirroring arrangement.

[0027] This embodiment presents a further developed arrangement of the detector unit, in a generalised manner, in particular with respect to WDM. The fact that the detector unit comprises these means ensures that the detector unit can be used more efficiently. This may also be referred to as “cyclic behaviour” in the context of the present disclosure, in the sense that the detector unit can detect wavelength ranges in a cyclically repeating manner. This in turn further facilitates wavelength-division multiplexing. A further advantage is that the specific amplitude does not really matter. In an embodiment, the detector comprises a plurality of photodiodes, PDs, at least one PD of the plurality of PDs being configured to detect at least an overlapping part of a wavelength range detectable by another PD of the plurality of PDs.

[0028] In this way, the detector is not only simpler but also more precise. By having the PDs overlap in detectable wavelength ranges with each other, it is made possible to resolve a specific wavelength of the optical spectrum much more precisely, instead of being limited to the PDs in a specifically delineated bin. This may allow for a precision scale on the order of picometres, instead of just nanometres, because it enables a spectral footprint precision (i.e. a center-of-gravity determination in wavelength) far beyond (roughly a thousandfold) the optical resolution of the detector array. This may also mean that, per DBR, multiple PDs detect the optical spectrum, and that no other DBR contributions on those PDs should be allowed to ensure proper calculation of the wavelength of the optical spectrum.

[0029] Preferably the plurality of PDs is arranged to detect the entire wavelength range from the lowest wavelength range to the highest wavelength range detectable by the entire plurality of PDs. In other words, they are preferably arranged to leave no undetectable gap.

[0030] In an embodiment, the light emitter comprises a plurality of broadband light sources corresponding with respective wavelength ranges; and the light emitter is configured for emitting light into the optical waveguide at the selectable wavelength range by activating a light source of the plurality of broadband light sources corresponding with the selectable wavelength.

[0031] In this way, it is possible to physically limit which wavelength ranges are to be emitted, as otherwise one cannot readily discern contributions from DBR reflections originating from other wavelength ranges (and projected on a different FSR (free spectral range, i.e. the diffraction order of the mapper)) onto the same physical photodiodes.

[0032] In an embodiment, the measurement system comprises a postprocessing module configured for removing noise from the reflected light, preferably based on a previously measured spontaneous emission of the plurality of light sources, and / or preferably based on at least one heuristic configured for suppressing noise.

[0033] The background spectrum may preferably be obtained without any DBR contribution, in order to properly subtract the purely spontaneous emission spectrum from the overall signal (i.e. multiplying the background with a scaling factor, and adding the DBR spectra).

[0034] In an embodiment, the modulator comprises at least one of the following: an optical shutter configured to limit the passage of light by opening and closing an aperture; an electro-optic modulator configured for controllably allowing or suppressing or minimizing particular wavelength ranges of light; a current source modulator configured to vary the amplitude, frequency, and / or phase of a light wave; and an electrical shutter configured to limit the current detection and digitization for the generated photocurrent only for controllably allowing or suppressing contributions with the desired Time-of-Flight window. Such an electrical shutter may in other words be an electronic detector modulator for controllably allowing or suppressing the detection / integration of current / charge originating from photodetection of signals from different divisions.

[0035] In this context, the modulator may be seen as an in-line valve, activated for pass- through by providing a current, e.g. a pulse train. In other words, it may act as an absorber when it is not activated. Along with activation normally there will also be generation of light by spontaneous emission. The passthrough signal of a DBR may be maximal when the DBR reflected pulse passes through the light emitter on the way back, exactly during the timeframe that the light emitter is (re)activated in the pulse train. In other words, the time of flight between the light emitter and the targeted DBR may preferably equal the pulse repetition time of the light emitter operation, in order to increase the detection efficacy of the detector.

[0036] In an embodiment, the measurement system comprises a channel switch configured for multiplexing over multiple optical waveguides representing distinct channels, such as a fiber-optic switch, wherein the controller is configured for ensuring that the channel switch is switched to a desired channel in synchronization with the time of activation of the particular mode of operation of the modulator.

[0037] In this way, this embodiment may provide channel multiplexing.

[0038] In an embodiment, the plurality of broadband light sources comprises a plurality of Semiconductor Optical Amplifiers, SOAs.

[0039] In a further developed embodiment, the modulator and the plurality of broadband light sources may be defined by the same plurality of SOAs.

[0040] In order to operate the measurement system in a dynamic manner, the measurement system may comprise a processor.

[0041] The processor and the controller may in some embodiments be integrated into a single physical device.

[0042] The processor may allow the measurement system to not only detect the wavelength range but also to determine the centre-of-gravity wavelength thereof.

[0043] The processor may in some embodiments be housed separately.

[0044] In some embodiments, the detected wavelength range may be output to a separate analysis system.

[0045] There is also provided in a second aspect according to the present disclosure a method for detecting a wavelength range of light reflected from at least one of a plurality of Distributed Bragg Reflectors, DBRs, arranged at known initial locations in an optical waveguide, the plurality of DBRs comprising at least a first DBR responsive to a first wavelength range, a second DBR responsive to the first wavelength range, and a third DBR responsive to a different wavelength range than the first wavelength range.. The method comprises the following steps. Emitting light into the optical waveguide at a selectable wavelength range, using a light emitter, wherein the wavelength range of the emitted light is selected such that the emitted light is reflected from a specific DBR of the plurality of DBRs, the specific DBR being responsive to the selected wavelength range of the emitted light. Operating a modulator at any one time in either a pass mode of operation to pass a signal representing the reflected light from the specific DBR or in a stop mode of operation to stop a signal representing the reflected light from the specific DBR. Mapping the selected wavelength range onto a specific wavelength subrange spanning at most a part of the total wavelength range of the light emitter. Detecting the specific wavelength subrange, using a detector. Determining a point in time when the light emitter has begun emitting the light. Estimating a time of flight of the emitted light based on the selected wavelength range of the emitted light, taking into account a specific known initial location of the specific DBR, wherein the time of flight relates to the propagation time of the emitted light from the light emitter to the specific DBR and the propagation time of the reflected light from the specific DBR to the detector. Synchronizing a time of activation of a particular mode of operation of the modulator based on the point in time when the light emitter has begun emitting the light and based on the estimated time of flight.

[0046] The skilled person will understand that the considerations and advantages presented above with respect to embodiments of the measurement system may also apply analogously to embodiments of the method, mutatis mutandis.

[0047] In an embodiment, the method comprises mapping the selected wavelength range of the emitted light onto the specific wavelength subrange using any one or more of the following means: an Arrayed Waveguide Grating, AWG; a diffraction grating and mirroring arrangement; and a prism and mirroring arrangement.

[0048] In an embodiment, the method comprises activating a light source of a plurality of broadband light sources corresponding with respective wavelength ranges, the activated light source corresponding with the selectable wavelength.

[0049] In an embodiment, the method comprises removing spontaneous emission noise from the reflected light, preferably in post-processing, preferably based on a previously measured spontaneous emission of the plurality of light sources, and / or preferably based on at least one heuristic configured for suppressing noise.

[0050] In an embodiment, the method comprises multiplexing over multiple optical waveguides representing distinct channels, and switching to a desired channel in synchronisation with the time of activation of the particular mode of operation of the detector.

[0051] It will be understood that the above-described embodiments are not meant to limit the invention, whose scope is solely determined by the claims. In particular, the skilled person will appreciate that various individual or combined features selected from various embodiments within this description (above and below) may be added to other embodiments as long as there is no technical hindrance to doing so, without departing from the scope of the invention as defined by the claims.

[0052] BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The embodiments described herein will be more fully understood with the help of the detailed description below and with reference to the appended drawings, in which:

[0054] Figure 1 schematically represents an exemplary embodiment of a measurement system 100 according to the present disclosure;

[0055] Figure 2 schematically represents another exemplary embodiment of a measurement system 200 according to the present disclosure;

[0056] Figure 3 schematically represents yet another exemplary embodiment of a measurement system 300 according to the present disclosure;

[0057] Figure 4 schematically represents an example of wavelength ranges that may reach the detector of an embodiment of a measurement system according to the present disclosure; and

[0058] Figures 5 and 6 represent two exemplary embodiments 500, 600 of a detector unit of a measurement system embodiment according to the present disclosure.

[0059] DETAILED DESCRIPTION As stated above, there is provided in a first aspect according to the present disclosure a measurement system for detecting a wavelength range of light reflected from at least one of a plurality of Distributed Bragg Reflectors, DBRs, arranged at known initial locations in an optical waveguide, the plurality of DBRs comprising at least a first DBR responsive to a first wavelength range, a second DBR responsive to the first wavelength range, and a third DBR responsive to a different wavelength range than the first wavelength range. The measurement system comprises a light emitter, a detector unit, and a controller. The light emitter is configured for being coupled to the optical waveguide and configured for, when coupled to the optical waveguide, emitting light into the optical waveguide at a selectable wavelength range, wherein the wavelength range of the emitted light is selected such that the emitted light is reflected from a specific DBR of the plurality of DBRs, the specific DBR being responsive to the selected wavelength range of the emitted light. The detector unit comprises a modulator, a mapper and a detector. The detector unit is configured for being coupled to the optical waveguide. The modulator is configured for being operable at any one time in either a pass mode of operation to pass a signal representing the reflected light from the specific DBR or in a stop mode of operation to stop a signal representing the reflected light from the specific DBR. The mapper is configured for mapping the selected wavelength range onto a specific wavelength subrange spanning at most a part of the total wavelength range of the light emitter. The detector is configured for detecting the specific wavelength subrange. The controller is coupled to the light emitter to determine a point in time when the light emitter has begun emitting the light. The controller is further configured for synchronizing a time of activation of a particular mode of operation of the detector (‘particular’ in the sense that it is a particular mode from among the available modes) based on the point in time when the light emitter has begun emitting the light and based on an estimated time of flight of the emitted light based on the selected wavelength range of the emitted light, taking into account a specific known initial location of the specific DBR, wherein the time of flight relates to the propagation time of the emitted light from the light emitter to the specific DBR and the propagation time of the reflected light from the specific DBR to the detector. Consequently, various embodiments of the measurement system according to the present disclosure enable wavelength-division multiplexing (WDM) with DBRs, as well as time-division multiplexing (TDM).

[0060] This may increase the number of sensors that can be interrogated. Moreover, the use of multiple sources may allow a continuous coverage in an optical waveguide (also called a sensor line) without needing dead zones in between, since the sources do not have to be operated simultaneously. With the use of 2-4 sources, the intermediate areas can be filled, enabling full flexibility on the geometric design of sensors in the sensing line. The combination of the two approaches of multiplexing may allow continuous sensor spacing but may also allow placing large amounts of DBR sensors in close proximity within each division.

[0061] In a specific embodiment, the modulator and the plurality of broadband light sources may be defined by the same plurality of SOAs. In other words, the pulse and modulation function for each group can be combined on a single semiconductor optical amplifier source (SOA), i.e. a combined architecture, or can be separated, i.e. a separated architecture. The pulse function, i.e. the function of producing light in pulsed form, is driven by an electronic pulse generator, and the actual light pulse is of course produced by the SOA. In the combined architecture, the light emitter and the amplifier may be embodied into one SOA chip. The advantage of using a single source is that fewer components are required, and that selection of a particular division of DBR sensors is made possible via the repetition frequency of the light emitter / amplifier. However, this approach requires a balanced design of spontaneous emission (ASE of the active chip) and amplification (of the return signal). The advantage of using multiple sources is that the amplifier may be inline, or may be positioned behind (i.e. after from an optical signal point of view) the circulator. The light emitter can operate at its maximal output without having to worry about the amplification ability. In the separated architecture, the selection of a division can be achieved by controlling the repetition frequency of the amplifier, and / or by controlling the time delay between the light emitter and the amplifier. Both architectures may benefit from background noise correction, by means of acquiring a non-DBR background spectrum and scaled correction.

[0062] The measurement system may comprise modules configured for channel selection, optical switch.

[0063] Wavelength ranges between adjacent groups can overlap, and certain DBRs can be interrogated with multiple sources.

[0064] The measurement system may comprise delay coils present at any position for the benefit of creating appropriate timing schemes, duty cycle, prevention of higher- order TDM mode crosstalk influences (meaning pulses arriving with twice or higher multiple of the shortest time of flight matching the repetition rate).

[0065] The measurement system may comprise band filters in place to select and cutoff bands or selectively combine groups at any position in the system.

[0066] Sources may be combined using passive combiners, e.g. WDM filters (fiberoptic or PIC-based).

[0067] The components may be integrated on an integrated photonic platform, of single or hybrid / heterogeneous format.

[0068] The modulator may be an optical or electronic shutter, an electro-optic modulation and / or current source modulation may be used for the timing of the modulating functionality (also called gain / gating functionality).

[0069] Wavelength group division multiplexing can also be implemented by blocking / discarding part of the obtained spectra, so as to effectively create two groups of operation using one set of sources. This effectively allows to ignore the part of the spectrum containing mixed DBR contributions.

[0070] An embodiment of the mapper can be realized by using an Arrayed Waveguide Grating. (AWG) based detector. The AWG as a component separates and bins any input light from an entrance waveguide into an array of waveguides with a distinct phase retardation between each waveguide (or waveguide length). Then the light from the array is recombined in a freely propagating section in which a diffraction pattern is generated. By the placement of output collecting waveguides across that interference pattern and photodetectors behind each, an AWG-based spectrometer function is created. The AWG can be designed so as to accommodate the projection of multiple diffraction orders onto the same set of output waveguides, thereby intrinsically enabling the mapping functionality. The stitching of the diffraction orders in wavelength can be designed such that no gaps occur in the continuous measurement range is realized, (i.e. all wavelengths are captured in the output waveguides). Thus the AWG may be used to create a hereby called ‘cyclic’ or ‘circular’ spectrometer.

[0071] An embodiment of mapping can be in the use case of a conventional free-space diffraction grating or prism to spatially spread the receiving spectrum and use a mirroring arrangement comprising precisely positioned mirrors to reflect spectral segments onto a single detection array, as illustrated in Figures 5 and 6.

[0072] The working principle of the mapper, thus the principle of WDM in at least some embodiments according to the present disclosure, is so-called ‘circular’. This means that the free spectral range (FSR) of the mapper (i.e. the wavelength range that one diffractive order occupies) is adjacent to other FSRs created by the number of channels and channel spacing, as illustrated for a particular embodiment of the measurement system with the help of Figure 4. A benefit of using such a circular mapper is the use of different diffractive orders of projection for detection. In this way, the detector is reusable for expanded wavelength ranges. Preferably, the measurement system and any analysis methods making use of the measurement system are designed so as to allow smooth transitioning at the FSR crossing, therefore the exact matching of group bands and mapper (e.g. AWG) FSRs is not required, but can be defined arbitrarily as an interrogation range.

[0073] Advantageously, the mapper may be designed not only for the C-band but also for other bands - in the higher and lower order modes of the mapper, e.g. the AWG. The band can be referred to as a wavelength-group. The number of wavelength-groups may basically be limited by the response characteristics of the photodiodes. Reusing the same photodiodes for an extended range of operation, may help to reduce cost of components and electronic conversion complexity. Matching the source of operation, the detected spectrum can be “unwrapped” to the correct identified wavelength band. Therefore, by selecting a specific wavelength range (e.g. by applying a different light source), the same mapper can be used to measure a different set of DBRs.

[0074] As stated above, the ‘circular’ behaviour, i.e. the ‘rotating’ or ‘cyclic’ dynamic range may mean that a wavelength shift passing beyond the end-point of one spectral range of the mapper will reappear at the beginning of the next spectral range. The cyclic feature of the mapper comes in use in the design of the ASPIC in order to allow as many DBR sensors as possible in a single optical waveguide preserving the required measurements performance.

[0075] Related to this spectral behaviour of an mapper allows one (1) ASPIC to interrogate wavelengths in different bandwidths. By subsequently turning on one of the light sources, a specific array of DBRs is projected on the mapper output and interrogated independently from DBRs of other wavelength bands. To enable stitching of multiple diffraction orders across the entire range, spectral overlap may in some practical implementations be required at some order transitions (i.e. wavelengths being detected on both ends of the detector range). In these operating conditions (WDM range), one of the involved overlapping PD signals may be ignored in post-processing (i.e. the pixel range may be limited so as not to process DBR signals detected twice on the detector).

[0076] Figure 1 schematically represents an exemplary embodiment of a measurement system 100 according to the present disclosure. The figure shows a detector D and a plurality of SOAs, Semiconductor Optical Amplifiers, indicated SOA 1-SOA n, serving as a light emitter with n selectable wavelength ranges, wherein each particular SOA is associated with a particular wavelength range, and also serving as a modulator, as will be explained below. The figure also shows an optical waveguide 101 , in particular an optical fiber, comprising one division with a plurality of ‘n’ groups of DBRs schematically indicated with G1-Gn. Each group of DBRs is associated with a wavelength range A1-An to which it is responsive. The DBRs may for example be FBGs. The figure also shows an optional switch S1 connecting the detector D and the plurality of SOAs, and an optional switch S2 connecting the plurality of SOAs and the optical waveguide 101. In this context, the DBR sensors may be grouped in ‘n’ groups, as indicated above, in order to separate the full band of wavelength ranges to which the entire division is responsive into a number ‘n’ of smaller sub-ranges. Each group may correspond with the wavelength range that one SOA occupies, and all FBGs within one group may occupy at most one FSR of one diffractive order of the grating, in order to prevent crosstalk between diffractive orders. The skilled person will of course understand that it is not necessary to group the DBR sensors into groups, and that the DBR sensors may be arranged individually, either within one division (which is equivalent to not having any divisions at all) or in multiple divisions. The figures showing DBR groups may in fact be interpreted as showing individual DBR sensors instead of DBR groups.

[0077] Of course, the exemplary embodiment of this figure may further comprise any electronic support equipment that is necessary or useful for its functioning, e.g. an electronic controller including an electronic pulse generator to control light pulse generation of the light emitter and to control the modulator of the detector unit, etc. Such a controller is not shown in this figure. In some embodiments, such as the embodiment of Figure 1 , the light emitter and the modulator may optionally be combined into a single SOA, such that they are synchronized automatically - however, in other embodiments, they may of course be separate from each other. It will be clear to the skilled person that the appended drawings focus on showing the optical components and leave implementing the required electronics to the skilled person. The skilled person will understand that the light emitter (a light pulse generator), the modulator and the detector are all optical components (with some minor control electronics), but of course the electronic controller including an electronic pulse generator to control light pulse generation of the light emitter and to control the modulator of the detector unit, is just some electronics configured to control the opening time of the light emitter and the modulator.

[0078] In other words, this figure illustrates a measurement system 100 for detecting a wavelength range of light reflected from at least one of a plurality of Distributed Bragg Reflectors, DBRs, G1 , G2, ... , Gn, arranged at known initial locations in an optical waveguide 101 , the plurality of DBRs G1-Gn comprising at least a first DBR responsive to a first wavelength range, a second DBR responsive to the first wavelength range, and a third DBR responsive to a different wavelength range than the first wavelength range. The measurement system 100 comprises a light emitter, a detector unit, and a controller.

[0079] The light emitter SOA1-SOAn is configured for being coupled to the optical waveguide 101 (e.g. via switch S2 to allow WDM) and configured for, when coupled to the optical waveguide 101 , emitting light into the optical waveguide 101 at a selectable wavelength range, wherein the wavelength range of the emitted light is selected such that the emitted light is reflected from a specific DBR of the plurality of DBRs G1-Gn, the specific DBR being responsive to the selected wavelength range of the emitted light.

[0080] The detector unit comprises a modulator SOA1-SOAn, a mapper and a detector D. The detector unit is configured for being coupled to the optical waveguide. The modulator SOA1-SOAn is configured for being operable at any one time in either a pass mode of operation to pass the signal representing the reflected light from the specific DBR or in a stop mode of operation to stop the signal representing the reflected light from the specific DBR. The mapper is configured for mapping the selected wavelength range onto a specific wavelength subrange spanning at most a part of the total wavelength range of the light emitter. The detector D is also configured for detecting the specific wavelength subrange. In the example of this figure, the mapper is preferably integrated within the housing of detector D.

[0081] Of course, in the present example, SOAs (Semiconductor Optical Amplifiers) are used, but the skilled person will understand that this is intended as an illustrative example, and that in various other embodiments, other types of light emitters / amplifiers and / or modulators may be used instead of or in addition to SOAs.

[0082] The controller is coupled to the light emitter SOA1-SOAn to determine a point in time when the light emitter SOA1-SOAn has begun emitting the light. The controller is configured for synchronizing a time of activation of a particular mode of operation of the modulator SOA1-SOAn based on the point in time when the light emitter SOA1- SOAn has begun emitting the light and based on an estimated time of flight of the emitted light based on the selected wavelength range of the emitted light, taking into account a specific known initial location of the specific DBR, wherein the time of flight relates to the propagation time of the emitted light from the light emitter SOA1-SOAn to the specific DBR and the propagation time of the reflected light from the specific DBR to the detector D. In order to effect this synchronization in practical implementations, it is possible to alter the repetition rate, or it is possible to add a time delay (phase delay) between the emitter modulation and detection modulation.

[0083] Figure 2 schematically represents another exemplary embodiment of a measurement system 200 according to the present disclosure. The figure shows an optical waveguide 201 comprising a plurality of DBRs schematically indicated with G1-Gn. The measurement system 200 comprises an optional switch S1 and an optional switch S2, to allow wavelength-division multiplexing. The light emitters E1-En and the modulators M1-Mn are pulsed and synchronized, to allow time-division multiplexing. The light emitters E1-En as well as the modulators M1-Mn may for example be SOAs, but alternatively the light emitters could be SLED, VCSEL, etc., and the modulators could be EDFA, or a quick optical / electrical shutter. The detector D contains different diffractive orders so all bandwidth groups can be mapped onto it. The measurement system 200 further comprises circulators 202-203 to allow light transmitted by the light emitters E1-En to pass through towards the optical waveguide 201 , as well as to allow light reflected from the DBRs in the optical waveguide 201 to return towards the detector unit.

[0084] The exemplary embodiment of Figure 2 is similar to the exemplary embodiment of Figure 1 , except in that the two functions of the SOAs have been separated, namely into a distinct light emitter (comprising a plurality of light sources E1-En) and a distinct modulator (comprising a plurality of modulating elements, such as SOAs, M1-Mn). Of course, this separation may also require that the circulators 202-203 are present.

[0085] The exemplary embodiment of Figure 2 differs from the exemplary embodiments of Figure 1 (and Figure 3) in that the plurality of DBRs is further divided in multiple divisions instead of just one division, in this example two divisions Div1 and Div2. As stated, it is an advantage of the exemplary embodiment of Figure 1 that by integrating the light emitter and the modulator into a single element (a SOA), their functions may be synchronized automatically. Moreover, the cost, complexity, and heat generation may be reduced by limiting the number of components. In contrast, the exemplary embodiment of Figure 2 has the advantage by clearly separating the two functions into separate elements, that various combinations of implementations may be used for those functions, and that, for example, the light emitter is not limited to having to be a SOA just because the modulator integrated with the light emitter is chosen to be implemented as a SOA. The separation also allows for the implementation of a modulation phase delay between pulsed emission and signal modulation (gain / gating), as a means of selecting a division for detection (where ToF changes in switching between one and the next division are matched with an equal time delay in activation).

[0086] Figure 3 schematically represents yet another exemplary embodiment of a measurement system 300 according to the present disclosure. This embodiment of the measurement system 300 comprises a switch S1 , a switch S2, and a switch S3, as well as a circulator 302.

[0087] The exemplary embodiment of Figure 3 differs from the exemplary embodiment of Figure 2 in that the number of circulators is reduced (just one circulator 302 in Figure 3 as opposed to two circulators 202-203 in Figure 2), at the cost of having to add one switch.

[0088] In the exemplary embodiments of Figure 2 and Figure 3, each of the switches S1 , S2 and S3 may be an optical switch, a passive splitter / combiner, or a passive WDM filter.

[0089] Figure 4 schematically represents an example of wavelength ranges that may reach the detector of an embodiment of a measurement system according to the present disclosure. The figure shows on the horizontal axis wavelength A and on the vertical axis optical power spectral density of the light sources. Over the wavelength axis A, multiple adjoining free spectral ranges F1-F3 may be defined, which correspond to wavelength ranges of three diffractive orders of the AWG. It can be seen that, for a particular free spectral range, for example free spectral range F2, multiple groups of the partially overlapping wavelength ranges may be involved, for example groups G2 and G3. The two groups won’t have any crosstalk because each of them is defined to cover a smaller range than the free spectral ranges of the AWG, and at most one of the SOAs is turned on at one time. The advantage of this is that all wavelength bands can be mapped onto one detector, at different FSRs, or at the same FSR, but different PD ranges.

[0090] Figures 5 and 6 show two respective embodiments of a detector unit 500, 600, wherein incoming white light 501 , 601 is passed through a diffraction grating 502 or a prism 602 configured to spread the incoming white light 501 spatially, in order to reach the detector D. In a first preferred embodiment, the measurement system comprises a mirroring arrangement comprising one mirror to guide the spread light onto the detector D. In a second preferred embodiment, the measurement system comprises a mirroring arrangement comprising at least two, more preferably exactly two, mirrors 503, 504, 603, 604 configured to guide the spread light onto the detector D, in such a way as to not invert the order of the wavelengths when reaching the detector D.

[0091] Both a diffraction grating and a prism have traditionally been used to create a spectral separation of light across a spatial projection. In a guided medium, as is the case for an on-chip AWG or fiber arrangement, this is different in the sense that multiple orders are being projected towards the same physical projection location. That is, in AWG, different diffractive orders are projected on the same reception PDs. This allows to use one detector array to detect different spectral orders.

[0092] The combination of multiplexing approaches may help to enable:

[0093] Increased number of sensors that can be interrogated.

[0094] Compatibility with spectrometer type detection PIC, which allows a modular approach in system layout, and enabling readout of many sensors, i.e. a broadband range, while not requiring more pixels in the spectrometer readout, due to the cyclic behaviour. Flexibility of the sensor layout. There are no required “dead zones”, i.e. segments without sensors, in the optical waveguide to prevent sensor crosstalk. This is beneficial for practical freedom and reduced complexity for the sensor layout and applicability.

[0095] Increased spatial density of sensors, enabled by maximizing the sensors that are available within each division. The practical constraint from TDM is the pulse definition - i.e. short, strong ns-pulses - and the associated low power levels. This architecture may e.g. enable more than 30 DBRs within one pulse width, resulting to sub-centimetre DBR spacings, or even a continuous coverage of sensors across the entire sensing fiber. In general, one is free to place the DBRs as pleased physically on the waveguide, within each division & group constraint.

[0096] An integration (i.e. collecting) type of current-to-digital conversion makes the device compatible with standard DC operating interrogators (e.g. a transimpedance amplifier (TIA) array, or a data acquisition (DAQ) card, commercially available from various companies). The insensitivity to overall amplitude ensures that the operation of the measurement system is still robust, even when long divisions and thus a long repetition time are used close to the sampling rate of the detector.

[0097] In some embodiments, the plurality of broadband light sources may comprise a plurality of semiconductor instruments other than Semiconductor Optical Amplifiers, SOAs. Examples of such light sources include SLEDs (Superluminous Light Emitting Diodes), VCSELs (Vertical Cavity Surface Emitting Lasers, which are on the narrow side for a broad band source but are quite inexpensive and are widely used in photonics general), and all other suitable types of broadband sources. These SLEDs and VCSELs may be used for their function as light sources due to spontaneous emission.

[0098] From an integration perspective, the architectures only require optical functions that can be integrated using available photonic integrated circuit building blocks. For wide range bands the use of hybrid platforms or selective growth is needed for band-gap variation. The above-described functions can therefore be integrated into a compact and flexible format. The procedure of measurement may for example be as follows:

[0099] A specific wavelength group is selected, by choosing the corresponding light source and amplifier of that wavelength range.

[0100] A specific division on the sensing waveguide is activated, by synchronising the light source and the amplifier, with a specific repetition rate.

[0101] A sample is acquired on the detector unit while maintaining the source settings.

[0102] Any background signal is removed in the post-processing of the acquired sample (e.g. from the known I previously measured SOA spontaneous emission)

[0103] The correct wavelength correspondence (FSR mapping) is determined from the selected group.

Claims

CLAIMS1. A measurement system for detecting a wavelength range of light reflected from at least one of a plurality of Distributed Bragg Reflectors, DBRs, arranged at known initial locations in an optical waveguide, the plurality of DBRs comprising at least a first DBR responsive to a first wavelength range, a second DBR responsive to the first wavelength range, and a third DBR responsive to a different wavelength range than the first wavelength range; the measurement system comprising:- a light emitter configured for being coupled to the optical waveguide and configured for, when coupled to the optical waveguide, emitting light into the optical waveguide at a selectable wavelength range, wherein the wavelength range of the emitted light is selected such that the emitted light is reflected from a specific DBR of the plurality of DBRs, the specific DBR being responsive to the selected wavelength range of the emitted light;- a detector unit comprising a modulator, a mapper and a detector; wherein the detector unit is configured for being coupled to the optical waveguide; wherein the modulator is configured for being operable at any one time in either a pass mode of operation to pass a signal representing the reflected light from the specific DBR or in a stop mode of operation to stop a signal representing the reflected light from the specific DBR; wherein the mapper is configured for mapping the selected wavelength range onto a specific wavelength subrange spanning at most a part of the total wavelength range of the light emitter; and wherein the detector is configured for detecting the specific wavelength subrange; and- a controller coupled to the light emitter to determine a point in time when the light emitter has begun emitting the light; the controller being configured for synchronizing a time of activation of a particular mode of operation of the modulator based on the point in time when the light emitter has begun emitting the light and based on an estimated time of flight of the emitted light based on the selected wavelength range of the emitted light, taking into account a specific known initial location of the specific DBR, wherein the time of flight relates to the propagation time of the emitted light fromthe light emitter to the specific DBR and the propagation time of the reflected light from the specific DBR to the detector.

2. The measurement system of claim 1 , wherein the mapper comprises any one or more of the following means for mapping the selected wavelength range of the emitted light onto the specific wavelength subrange:- an Arrayed Waveguide Grating, AWG;- a diffraction grating and mirroring arrangement; and- a prism and mirroring arrangement.

3. The measurement system of any previous claim, wherein the detector comprises a plurality of photodiodes, PDs, at least one PD of the plurality of PDs being configured to detect at least an overlapping part of a wavelength range detectable by another PD of the plurality of PDs.

4. The measurement system of any previous claim, wherein the light emitter comprises a plurality of broadband light sources corresponding with respective wavelength ranges; and wherein the light emitter is configured for emitting light into the optical waveguide at the selectable wavelength range by activating a light source of the plurality of broadband light sources corresponding with the selectable wavelength.

5. The measurement system of any previous claim, comprising a postprocessing module configured for removing noise from the reflected light, preferably based on a previously measured spontaneous emission of the plurality of light sources, and / or preferably based on at least one heuristic configured for suppressing noise.

6. The measurement system of any previous claim, wherein the modulator comprises at least one of the following:- an optical shutter configured to limit the passage of light by opening and closing an aperture;- an electro-optic modulator configured for controllably allowing or suppressing or minimizing particular wavelength ranges of light;- a current source modulator configured to vary the amplitude, frequency, and / or phase of a light wave;- an electrical shutter configured to limit the current detection and digitization for the generated photocurrent only for controllably allowing or suppressing contributions with the desired Time-of-Flight window.

7. The measurement system of any previous claim, comprising a channel switch configured for multiplexing over multiple optical waveguides representing distinct channels, such as a fiber-optic switch, wherein the controller is configured for ensuring that the channel switch is switched to a desired channel in synchronization with the time of activation of the particular mode of operation of the modulator.

8. A method for detecting a wavelength range of light reflected from at least one of a plurality of Distributed Bragg Reflectors, DBRs, arranged at known initial locations in an optical waveguide, the plurality of DBRs comprising at least a first DBR responsive to a first wavelength range, a second DBR responsive to the first wavelength range, and a third DBR responsive to a different wavelength range than the first wavelength range; the method comprising:- emitting light into the optical waveguide at a selectable wavelength range, using a light emitter, wherein the wavelength range of the emitted light is selected such that the emitted light is reflected from a specific DBR of the plurality of DBRs, the specific DBR being responsive to the selected wavelength range of the emitted light;- operating a modulator at any one time in either a pass mode of operation to pass a signal representing the reflected light from the specific DBR or in a stop mode of operation to stop a signal representing the reflected light from the specific DBR;- mapping the selected wavelength range onto a specific wavelength subrange spanning at most a part of the total wavelength range of the light emitter;- detecting the specific wavelength subrange, using a detector;- determining a point in time when the light emitter has begun emitting the light;- estimating a time of flight of the emitted light based on the selected wavelength range of the emitted light, taking into account a specific known initial location of the specific DBR, wherein the time of flight relates to the propagation time of the emitted light fromthe light emitter to the specific DBR and the propagation time of the reflected light from the specific DBR to the detector; and- synchronizing a time of activation of a particular mode of operation of the modulator based on the point in time when the light emitter has begun emitting the light and based on the estimated time of flight.

9. The method of claim 8, comprising mapping the selected wavelength range of the emitted light onto the specific wavelength subrange using any one or more of the following means:- an Arrayed Waveguide Grating, AWG;- a diffraction grating and mirroring arrangement; and- a prism and mirroring arrangement.

10. The method of any one of claims 8-9, comprising activating a light source of a plurality of broadband light sources corresponding with respective wavelength ranges, the activated light source corresponding with the selectable wavelength.

11. The method of any one of claims 8-10, comprising removing noise from the reflected light, preferably based on a previously measured spontaneous emission of the plurality of light sources, and / or preferably based on at least one heuristic configured for suppressing noise.

12. The method of any one of claims 8-11 , comprising multiplexing over multiple optical waveguides representing distinct channels, and switching to a desired channel in synchronization with the time of activation of the particular mode of operation of the detector.

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