NL2035679B1

NL2035679B1 - PIC device and method for resonant sensing in a waveguide

Info

Publication number: NL2035679B1
Application number: NL2035679A
Authority: NL
Inventors: Maria Van Leest Matheus; Benjamin Haverdings Michael; Cui Moxi; Johannes Docter Vincent
Original assignee: Photonfirst Ip B V
Priority date: 2023-08-25
Filing date: 2023-08-25
Publication date: 2025-03-10
Also published as: WO2025048636A1

Abstract

A method of operating a Photonic Integrated Circuit, PIC, device; the PIC device comprising an emission element and a gain element, wherein the emission element and the gain element are coupled to a waveguide, such as an optical fiber, and wherein the emission element and the gain element are embodied by a single element or are embodied by two distinct elements; the method comprising, on the PIC device, the following steps: in step A, activating the emission element, in order to emit broadband light towards a target optical reflector in the waveguide, such that the light is reflected from the target optical reflector to the gain element; and in step C, activating the gain element, in order to resonantly amplify the light into the waveguide in a temporally incoherent manner, wherein step C lags step A based on a time-of-flight of the light emitted in step A, the time-of-flight being defined as a propagation time of light propagating from the emission element to the target optical reflector in the waveguide and back to the gain element.

Description

PIC device and method for resonant sensing in a waveguide

TECHNICAL FIELD

The present disclosure generally relates to Photonic Integrated Circuit, PIC, devices.

Particular embodiments relate to a method of operating a PIC device, and to a PIC device.

BACKGROUND

Waveguide sensing, or in particular fiber sensing, is a field that involves the use of optical fibers as sensing elements to detect and measure physical parameters such as temperature, strain, pressure, and chemical composition. It relies on the unique properties of optical fibers, such as their small size, flexibility, immunity to electromagnetic interference, and ability to transmit light over long distances.

The basic principle behind fiber sensing is the modulation of light signals traveling through the fiber in response to the physical parameter being measured. This modulation can be achieved through various mechanisms, such as changes in the fiber's refractive index, length, or scattering properties.

One of the most widely used techniques in fiber sensing is based on the phenomenon of fiber Bragg gratings (FBGs). FBGs are a type of optical reflector. In particular, FBGs are periodic variations in the refractive index of the fiber core that act as wavelength-selective reflectors. When light is transmitted through an FBG, it interacts with the grating structure, causing certain wavelengths of light to be reflected back. The reflected wavelengths depend on the physical parameter being measured, such as strain or temperature. By analysing the reflected light, it is possible to determine the value of the measured parameter. Other methods of creating a wavelength specific reflector by means of creating periodic modulation in a waveguide by geometric modulation of the core size (DBR grating, in case of integrated waveguide structures) or the formation of optical cavities (Fabry-Perot cavity)

Fiber sensing finds applications in various industries, including structural health monitoring, oil and gas, aerospace, environmental monitoring, and biomedical sensing. It offers several advantages over traditional (electronic) sensing technologies, including distributed sensing capability, immunity to electromagnetic interference, high sensitivity, withstanding harsh environments and the ability to cover large areas or long distances.

In summary, fiber sensing is a rapidly growing field that leverages the unique properties of optical fibers for accurate and reliable measurement of physical parameters. With ongoing research and technological advancements, fiber sensing is expected to find even broader applications and contribute to the advancement of various industries.

SUMMARY

Accordingly, there is provided in a first aspect of the present disclosure a method according to claim 1, i.e. a method of operating a Photonic Integrated Circuit, PIC, device; the PIC device comprising an emission element and a gain element, wherein the emission element and the gain element are coupled to a waveguide, such as an optical fiber, and wherein the emission element and the gain element are embodied by a single element or are embodied by two distinct elements; the method comprising, on the PIC device, the following steps: - obtaining a set of values relating to distances from the gain element to one or more optical reflectors in the waveguide; and - in step A, activating the emission element, in order to emit broadband light towards a target optical reflector among the one or more optical reflectors in the waveguide, such that the light is reflected from the target optical reflector to the gain element in a matter; and - in step C, activating the gain element, in order to resonantly amplify the light into the waveguide in a temporally incoherent manner, wherein step C lags step A based on a time-of-flight of the light emitted in step A, on a condition that the time-of-flight, corresponding with a distance to the target optical reflector, is greater than a coherence time of light reflected from the target optical reflector to the gain element; the time-of-flight being defined as a propagation time of light propagating from the emission element to the target optical reflector in the waveguide and back to the gain element.

Various embodiments of the method according to the present disclosure allow to operate a PIC device not only for sensing, but also as (part of) a tuneable light source, and/or as (part of) a data communication device, in particular a data communication device arranged to communicate with a specific node in an optical network, due to the ability to single out one or more specific optical reflectors. It is noted that application as (part of) a tuneable light source is based on the insight that these embodiments provide a form of pulse filtering, rather than a form of mode selection as would be the case for tuneable lasers, for instance.

In this context, the expression “to resonantly amplify light” may be interpreted to refer to a process wherein light signals become stronger by stimulated emission in order to resonate, i.e. become stronger and/or more pronounced, at one or more particular pulse repetition frequencies {known as eigenfrequencies). The light signals may be made to amplify in this constructive manner by a precise control of their relative timing, to ensure that they not only co-exist on a waveguide but moreover strengthen each other.

In this context, the resonance may be interpreted as follows: light is first emitted from the emission element to the target optical reflector and then back to the gain element, constituting a first pass; then, light is amplified at the gain element in such a manner and using such a timing, that it is emitted into the waveguide in an increased state, constituting a second pass; subsequently, optionally, the second step may be repeated in order to even further amplify the light reflected from the target optical reflector, so as to keep on increasing the state of the light.

The skilled person will appreciate that there may be various ways of producing the resonant amplification of the light, by activating the gain element.

In a first and preferred example, which corresponds with claims 8 and 22, a back reflector, i.e. an at least partially reflective element, may be provided at a rear side of the gain element. The rear side of the gain element is defined as the side of the gain element that is not coupled with the waveguide, that is, the side that is facing away from the coupling with the waveguide, such that light leaving the waveguide passes (at least) twice via the gain element before returning to the waveguide, namely a first pass from the coupling with the waveguide to the back reflector, and then a second pass from the back reflector to the coupling with the waveguide. Given that the method ensures that the gain element's activation respects a particular timing after the light has been emitted from the emission element (which may be the same single, i.e. integral, element as the gain element, in which case that same single element has both an emission function and a gain function), based on a particular time-of-flight, the reflected light may be amplified when emitted into the waveguide, which establishes the resonance together with light that is newly emitted into the waveguide, in particular with light resulting from stimulated emission of the gain element.

In a second example, a more elaborate setup may be considered, wherein instead of the above-described back reflector, a light guiding mechanism may be used to guide the light in a continuous manner out of the gain element and back into the gain element. In such a setup, it is preferable to ensure that the length of the light guiding mechanism is short enough to fit on the PIC device, in order to minimize time, in order to maximally benefit from the advantage that the gain element may then still be controlled via single continuous pulses instead of via multiple discrete pulses for each pass.

An advantage of various embodiments of the method according to the present disclosure is that, due to the method steps being performed on the PIC device, using elements comprised by the PIC device, the scales in terms of distance and time are small (e.g. on the order of less than a millimetre, and less than a nanosecond), regardless of whether the emission element and the gain element are embodied by a single element or are embodied by two distinct elements. This makes it possible to control the gain element with a relatively simple controller, as no provisions have to be made for long travel times of light, in comparison to an approach where the scale would be larger than that of a PIC device.

In other words, an advantage of various embodiments according to the present disclosure is that operating the PIC device, i.e. working in an on-chip setting, provides the situation that there is such a small distance between the gain element on the one 5 hand and whatever element is further used to help create the resonance on the other hand, that this distance can be considered negligible. This may help to reduce timing requirements demanded of electronic control pulses to control these elements. In contrast, if one were to scale up the distance to a macro scale (i.e. larger than PIC- size) which could no longer be considered to represent a negligible distance, this might no longer allow fine-grained fiber sensing due to the widened field of view.

In yet other words, the micro scale of the PIC may help to allow use of a much more simple pulse pattern (because only one single repetition rate is needed), and may help to better integrate any additionally desired interrogator optics on the PIC device, because the device is smaller in form and more readily self-contained than macro scale devices.

The realization of the elements of various embodiments according to the present disclosure, on all optical elements with or without the waveguide with reflectors, can be implemented on a PIC having a modest footprint, based on readily available building blocks. The solution therefore provides a strong economic advantage, from the perspective of scalability of the (fully automated} manufacturing technology behind

PICs. Simplification on the control side of the emission/gain activation also supports economic aspects on the electronic part of the solution.

The resonance also provides an efficiency advantage, as emission gain energy is specifically targeted to the reflector element of interest, resulting in high signal to noise ratio for a modest activation energy. This is an essential development for low-power remote sensing solutions.

Additionally, it is an advantage of various embodiment according to the present disclosure that an element that already has to be present for an application of sensing (e.g. fiber sensing), namely the target optical reflector, can be put to even better use by utilizing it to generate the resonance. Or, from the inverted point of view, an element that has to be present to generate the resonance can be put to even better use by utilizing it as a sensor.

Itis noted that the light emission described here may be a spontaneous emission pulse in some embodiments, and may be broadband in various embodiments. In the context of the present disclosure, ‘broadband’ may be taken to refer to ‘spectrally broadband’.

Spontaneous emission in a semiconductor, such as in a Semiconductor Optical

Amplifier (SOA), results from random transitions of excited electrons to lower energy states. These transitions lead to the emission of photons with random phases. The emitted photons will be further amplified by stimulated emissions, leading to a broadband amplified spontaneous emission spectrum, with a low coherence time (or, equivalently, a low coherence length).

An example of the light that could be emitted by the emission element is the broadband light emitted by a SOA (Semiconductor Optical Amplifier), wherein broadband can be defined as light that contains a wide range of frequencies or wavelengths within a single optical signal. This can be achieved through the interaction of multiple frequencies within the gain medium of the SOA, resulting in the emission of photons with various energies. The resulting emitted light covers a wide spectral range, making it "broadband." Broadband light generated through stimulated emission in such a SOA tends to become temporally incoherent, after a well-defined time, namely the coherence time, which is the time over which a propagating wave may be considered coherent, meaning that its phase is, on average, predictable. The coherence time is usually defined as T = = x L ‚ where A is the central wavelength of the source,

Av and AA is the spectral width of the source in units of frequency and wavelength respectively, and c is the speed of light in vacuum.

Temporal coherence refers to the stability of the phase relationship between different parts of a wave over time. In a temporally coherent source, like a laser, the emitted photons have a predictable and stable phase relationship, leading to well-defined interference patterns. However, in the case of broadband light generated through stimulated emission in a SOA, the emitted photons obtain varying phases and frequencies due to the broad spectrum of input photons that stimulated their emission, after the coherence time. These emitted photons thus do not maintain a consistent and predictable phase relationship over time, resulting in temporal incoherence after the coherence time. This means that the phases of the emitted photons are not well- correlated with each other, and they do not exhibit stable interference patterns.

Of course, it is implicitly clear that the gain element should be situated in or should be switched on in the path of the light reflected from the target optical reflector, from the fact that the activation of the gain element during the guiding causes the resonant amplification.

In various embodiments, the activating of the gain element in step C is continuous while the light reflected from the target optical reflector is being guided into the waveguide via the gain element.

In various embodiments, the emission element and the gain element are embodied by a single optical element, preferably by a semiconductor optical amplifier, SOA, or by a superluminescent diode, SLED.

The gain element may be based on the usage of directly active (semiconductor) material for the waveguide material or by the implementation of the active function by means of element doping, such as for instance Erbium, commonly applied in silica fibers and known as an Erbium-Doped Fiber Amplifier, EDFA.

It is possible to implement the emission element using a SOA or an EDFA: 1. Semiconductor Optical Amplifier (SOA): While an SOA is primarily used as an optical amplifier, it can also operate in a gain-switched configuration to generate light pulses. In gain-switching, an electrical pulse is applied to the SOA to induce a transient change in the gain of the device, resulting in the emission of a pulsed optical signal.

However, these methods of light generation have limitations in terms of pulse width and coherence compared to dedicated light sources such as lasers.

2. Erbium-Doped Fiber Amplifier (EDFA). An EDFA is primarily used as an optical amplifier in telecommunications and other optical systems. The EDFA is pumped with a high-power laser, and it amplifies the optical signal passing through it without introducing additional noise. However, it relies on an external optical signal as its input to perform amplification.

To generate light in an even more controlled and specific manner, dedicated light sources such as laser diodes or superluminescent diodes (SLEDs or SLDs) may be used. Lasers are capable of producing coherent, narrowband, and well-controlled optical signals. SLEDs, although less coherent than lasers, can provide broader bandwidth and moderate coherence. These light sources are often integrated with amplification devices like SOAs or EDFAs to boost the power of the generated light.

In various embodiments, the method comprises: -in step B, activating a gating function between step A and step C, in order to absorb at least some, preferably all light reflected from other optical reflectors in the waveguide than the target optical reflector; wherein the gating function is preferably an inherent gating function of the gain element.

In this context, gating may be defined as absorbing most or all light and may be implemented by not biasing or reverse biasing an SOA in case an SOA is used.

It is noted that the activating of the gating function is a preferred feature of the method according to the present disclosure, but is not currently understood to be necessary in all embodiments of the method. For example, it can be envisaged to not utilize active gating, possibly via a switch, or via another form of modulator. In this example, this effectively still means it needs to pass only a part of the pulse. In another example, the complete gating function could be left out, and then the signal would be received with a small delay in time from each of the optical reflectors. This option is especially advantageous if there is a very fast detector which can obtain the time information during the return pulse. Alternatively, in case one only wants to measure one optical reflector along the waveguide, there is no need for the timing information, and it is possible to measure continuously.

In various embodiments, the method comprises: - in step D, activating a gating function after step C, in order to absorb at least some, preferably all light reflected from other optical reflectors in the waveguide than the target optical reflector; and wherein the gating function is preferably an inherent gating function of the gain element.

In various embodiments, the method comprises repeating step C at least one additional iteration, each additional iteration of step C lagging a previous iteration of step C based on the time-of-flight.

In various embodiments, if the method includes step D, the method further comprises repeating step D after each additional iteration of step C whilst ensuring that the gating function is deactivated prior to each additional iteration of step C.

In various embodiments, an at least partially reflective element is provided on the PIC device, separated from the waveguide by the gain element, and the light reflected from the target optical reflector is guided into the waveguide via the gain element using the at least partially reflective element.

In other words, the at least partially reflective element may be situated on the other side of the gain element, as seen from the waveguide.

Not only does this at least partially reflective element (also denoted as “backreflector” or “back-reflector” or “back reflector”) provide an elegant implementation for helping to create the resonance, but in some cases (e.g. when the emission element and the gating function are situated on a single component) it can also strengthen light emitted prior to the build-up of that resonance, because the initial activation of the emission element also causes light reflection from this at least partially reflective element, which also impacts even the very first light pulse.

In various further developed embodiments, step C is sufficiently long to cover: - performing at least one of, preferably both of: step C1 and step C4, and

- performing both of: step C2 and step C3; and wherein: - step C1 comprises a first amplification of light stemming from the target optical reflector and being guided through the gain element (i.e. first amplification within the gain element); - step C2 comprises a passage of light, the light resulting from step C1, from the gain element to the at least partially reflective element (i.e. passage from the gain element to the back reflector); - step C3 comprises a passage of light, the light resulting from step C2, from the at least partially reflective element to the gain element (i.e. passage from the back reflector to the gain element); and - step C4 comprises a second amplification of light, the light resulting from step C3, stemming from the at least partially reflective element and being guided through the gain element (i.e. second amplification within the gain element).

In various further developed embodiments, steps C2 and C3 are instantaneous.

It is noted that this is in particular the case if the back reflector is the back facet of the gain element, as else steps C2 and C3 would take a non-zero time duration.

In various embodiments, the method comprises: - imposing a data signal onto the light emitted in step A.

In various embodiments, the method comprises: - detecting light resulting from the resonant amplification in step C, after said light has reflected from the target optical reflector.

In various embodiments, the method comprises: - in step A, activating at least one additional emission element, in order to emit broadband light towards the target optical reflector or towards at least one other target optical reflector in the waveguide or in at least one other waveguide, such that the light is reflected from the target optical reflector or the at least one other target optical reflector to at least one additional gain element; and

- in step C, activating the at least one additional gain element, in order to resonantly amplify the light into the waveguide or into the plurality of waveguides in a temporally incoherent manner.

Advantageously, this allows to extend on the same chip into multiple channels, i.e. into multiple separate waveguides, such as multiple separate optical fibers.

In various further developed embodiments, the method comprises: - multiplexing the light resonantly amplified by the gain element and the at least one additional gain element in step C, using an Arrayed Waveguide Grating, AWG; and/or - demultiplexing the light resulting from the resonant amplification in step C, after said light has reflected from the target optical reflector, prior to the step of detecting, using an AWG; and/or - demultiplexing and multiplexing in the waveguide, in order to generate wavelength specific waveguides and path length differences therebetween.

Advantageously and surprisingly, the detector and SOA can be combined on the same

PIC. The detector may be an AWG, but it could be another type of component, as long as one can measure the wavelength with it.

An AWG can split and detect the polarization, for example in the manner described in published European patent EP3144633B1, titled “Fiber bragg grating interrogator assembly and method for the same”.

Another advantage is that multiple SOAs can be arranged on a single chip, in order to increase the number of channels one can detect.

In various embodiments, the step of obtaining a set of values relating to distances from the gain element to one or more optical reflectors in the waveguide comprises: - storing a manually predefined set of said values; or - scanning the waveguide to determine said values, and storing said values.

Moreover, in a second aspect of the present disclosure, there is provided a Photonic

Integrated Circuit, PIC, device according to claim 15; the device comprising: - a gain element configured to amplify light; - a data storage medium configured for storing a set of values relating to distances from the gain element to one or more optical reflectors in a waveguide; - an emission element configured to emit broadband light towards a target optical reflector among the one or more optical reflectors in the waveguide, such that the light is reflected from the target optical reflector to the gain element; wherein the emission element and the gain element are coupled to a waveguide, such as an optical fiber, and wherein the emission element and the gain element are embodied by a single element or are embodied by two distinct elements; and the device further comprising: - at least one controller configured to control the emission element and the gain element, on a condition that a time-of-flight, corresponding with a distance to the target optical reflector, is greater than a coherence time of light reflected from the target optical reflector to the gain element, the time-of-flight being defined as a propagation time of light propagating from the emission element to the target optical reflector in the waveguide and back to the gain element, such that the light is resonantly amplified into the waveguide in a temporally incoherent manner, by activating the gain element while guiding the reflected light via the gain element into the waveguide, after activating the emission element, based on the time-of-flight of the emitted light.

The skilled person will understand that considerations and advantages applicable to embodiments of the method according to the present disclosure may also be applicable to embodiments of the PIC device according to the present disclosure, mutatis mutandis and vice versa.

In various embodiments, the at least one controller is configured to control the gain element for activating the gain element via a continuous control pulse while the light reflected from the target optical reflector is being guided into the waveguide via the gain element.

In various embodiments, the emission element and the gain element are embodied by a single optical element, preferably by a semiconductor optical amplifier, SOA.

In various embodiments, the PIC device comprises: - a gating element configured to absorb at least some, preferably all light reflected from other optical reflectors in the waveguide than the target optical reflector; wherein the gating element is preferably integrated with the gain element; and wherein the at least one controller is configured to activate the gating element after activating and deactivating the gain element.

In various embodiments, at least two elements of the emission element, the gain element and the gating element, preferably all three of the emission element, the gain element and the gating element, are embodied by a single integrated element.

In various embodiments, the at least one controller is configured for activating the gain element at least one additional iteration, each additional iteration lagging a previous iteration based on the time-of-flight.

In various further developed embodiments, if the PIC device comprises the above- described gating element, wherein the at least one controller is configured for activating the gating element after each additional iteration of activating and deactivating the gain element whilst ensuring that the gating element is deactivated prior to each additional iteration of activating the gain element.

In various embodiments, the PIC device comprises: - an at least partially reflective element defined on the PIC device and arranged to be separated from the waveguide by the gain element, and wherein the light reflected from the target optical reflector is guided into the waveguide via the gain element using the at least partially reflective element.

In various embodiments, the at least one controller is configured for activating the gain element for a sufficiently long time duration to cover: - performing at least one of, preferably both of: step C1 and step C4, and

- performing both of: step C2 and step C3; and wherein: - step C1 comprises a first amplification of light stemming from the target optical reflector and being guided through the gain element; - step C2 comprises a passage of light, the light resulting from step C1, from the gain element to the at least partially reflective element; - step C3 comprises a passage of light, the light resulting from step C2, from the at least partially reflective element to the gain element; - step C4 comprises a second amplification of light, the light resulting from step C3, stemming from the at least partially reflective element and being guided through the gain element.

In various embodiments, the at least one controller is configured such that steps C2 and C3 are instantaneous.

In various embodiments, the PIC device comprises a data interface configured for imposing a data signal onto the light emitter by the emission element.

In various embodiments, the PIC device comprises a detector configured for detecting light resulting from the resonant amplification by the gain element, after said light has reflected from the target optical reflector.

In various embodiments, the PIC device comprises: - at least one additional emission element configured to emit broadband light towards the target optical reflector or towards at least one other target optical reflector in the waveguide or in at least one other waveguide, such that the light is reflected from the target optical reflector or the at least one other target optical reflector to at least one additional gain element; and further comprises: - the at least one additional gain element configured to amplify the light into the waveguide or into the plurality of waveguides in a temporally incoherent manner.

In various embodiments, the PIC device comprises an Arrayed Waveguide Grating,

AWG, configured for:

- multiplexing the light resonantly amplified by the gain element and the at least one additional gain element; and/or - demultiplexing the light resulting from the resonant amplification by the gain element and the at least one additional gain element, after said light has reflected from the target optical reflector; and/or - demultiplexing and multiplexing in the waveguide, in order to generate wavelength specific waveguides and path length differences therebetween. This makes it possible to place the reflector elements more closely together for specific implementations and still have overall sufficient path length difference between multiple reflector elements, beyond the minimal pulse length practically attainable.

It is noted that the above-described detector for measuring the wavelength may for example be embodied by a demultiplexer arranged for said demultiplexing, in particular in case there is just one gain element.

In various embodiments, the PIC device comprises: - a coupler configured to couple light passed through the at least one additional gain element towards a or the detector. lt is noted that such a coupler may be an optical coupler arranged between multiple gain elements and the detector, in particular if the detector is outside of the PIC’s chip.

If an AWG or a similar device is used, and if the AWG is on the PIC’s chip, it may advantageously be possible to use the AWG as the coupler.

An advantage of this embodiment is that just one single detector is required; which allows to increase density of detectors and allows to spread detection over several different wavelength ranges. Also, the controller can control multiple amplifiers at once.

A further advantage of this embodiment is that using one single detector may cause no increase to drift and thus no decrease in accuracy, may require only one calibration instead of multiple calibrations, and may require fewer components overall, which in turn is beneficial for dimensioning, cost, power consumption, weight, etc.

In various embodiments, the at least partially reflective element is provided as a waveguide interface characterized by an effective index mismatch.

In various embodiments, the at least partially reflective element is provided as a partially reflective coating on a side of the amplifier oriented away from the waveguide when in use.

In various embodiments, the at least partially reflective element is variably reflective.

Advantageously, this allows to optimize the PIC device for use with different types of optical reflectors in the waveguide having different reflectivity values. For example, a first FBG in a fiber waveguide may have a first reflectivity, whereas a second, other

FBG in the fiber waveguide may have a second, different reflectivity. If the at least partially reflective element is variably reflective, it is a matter of configuration to tune the at least partially reflective element to the reflectivity of the intended FBG.

In various embodiments, the at least partially reflective element is provided as an end facet of the PIC device.

In various embodiments, the at least partially and variably reflective element comprises a splitter having a variable splitting ratio.

In various embodiments, the PIC device comprises a waveguide coupling the at least partially reflective element with a detector configured for detecting a wavelength of light from the waveguide.

In various embodiments, the PIC device comprises a or the detector provided on the

PIC and configured for detecting a wavelength of light stemming from the waveguide; wherein the at least partially reflective element is defined between the detector and the amplifier.

The embodiments described herein are provided for illustrative purposes and should not be construed as limiting the scope of the invention. It is to be understood that the invention encompasses other embodiments and variations that are within the scope of the appended claims. The invention is not restricted to the specific configurations, arrangements, and features described herein. The invention has wide applicability and should not be limited to the specific examples provided. The embodiments disclosed are merely exemplary, and the skilled person will appreciate that various modifications and alternative designs can be made without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description, a number of exemplary embodiments will be described in more detail, to help understanding, with reference to the appended drawings, in which:

Figure 1 schematically illustrates a first embodiment of the PIC device according to the present disclosure;

Figure 2 schematically illustrates a second embodiment of the PIC device according to the present disclosure;

Figure 3 schematically illustrates a third embodiment of the PIC device according to the present disclosure;

Figure 4 schematically illustrates a fourth embodiment of the PIC device according to the present disclosure;

Figure 5 schematically illustrates selective amplification of optical pulses in a TDM scheme;

Figure 6 schematically illustrates various pulse patterns that can be used in a method embodiment according to the present disclosure;

Figure 7 schematically illustrates another embodiment of the PIC device according to the present disclosure;

Figures 8a, 8b, 8c schematically illustrate operation of a method embodiment according to the present disclosure;

Figure 9 schematically illustrates the way convolution and resonance result in selective amplification;

Figure 10 schematically illustrates the way convolution and resonance interact;

Figure 11 schematically illustrates another embodiment of the PIC device according to the present disclosure;

Figure 12 schematically illustrates another embodiment of the PIC device according to the present disclosure;

Figure 13 schematically illustrates another embodiment of the PIC device according to the present disclosure;

Figure 14 schematically illustrates another embodiment of the PIC device according to the present disclosure;

Figure 15 schematically illustrates another embodiment of the PIC device according to the present disclosure;

Figure 16 schematically illustrates another embodiment of the PIC device according to the present disclosure;

Figure 17 schematically illustrates another embodiment of the PIC device according to the present disclosure;

Figure 18 schematically illustrates another embodiment of the PIC device according to the present disclosure;

Figure 19 schematically illustrates another embodiment of the PIC device according to the present disclosure;

Figure 20 schematically illustrates another embodiment of the PIC device according to the present disclosure;

Figure 21 schematically illustrates the impact of reflectivity of the back reflector in the context of another embodiment of the PIC device according to the present disclosure; and

Figure 22 schematically illustrates a method embodiment according to the present disclosure.

It will be appreciated that similar elements in the drawings may be referred to using identical or similarly structured reference numbers. For example, most or all reference numbers ending with ‘09’ may be taken to refer to a back reflector, even if this is not explicitly stated with respect to a particular drawing, based on the description accompanying one or more other drawings.

DETAILED DESCRIPTION

A wide range of measurement setup configurations for FBG wavelength sensing exists. The two main principles of interrogation relate to: - Scanning method: Scanning a tunable narrow-linewidth laser across the FBG reflection band to obtain its central or average wavelength. Interferometric principles are used for the detection. - Spectrometric method: Applying a broadband source to the FBG(s) and obtaining the central or average wavelength on a detector by means of spectrometry.

One of the key features of an FBG-based system is the ability to multiplex the number of point sensors that can be uniquely monitored on a single fiber strand, by designating distinct wavelength bands of operation for each sensor. But the extent of this multiplexing is limited to the available wavelength bandwidth in the measurement system. Besides that, it is common to interrogate multiple sensor fibers (also called channels) by means of fiber-optic switches and/or splitters.

One of the recent advancements for increasing the number of sensors that can be monitored with a single interrogator, is based on the use of time-of-flight for selecting which sensors are interrogated. Architectures for both main families of interrogation principles with this time-domain multiplexing (TDM) method have been published.

The main principle regards the pulsation of light being sent into the fiber and a means of selection/suppression on the return signal by either pulsed return gain/gate functionality or interferometric principles (in coherent/scanning method schemes).

In the context of the present disclosure, the term “incoherent” {or its synonym “non- coherent”) may be taken to refer to light sources or systems where there is no well- defined phase relationship between different parts of the wave, regardless of whether the lack of coherence is due to complete randomness or only partial correlation of phases. In other words, the term “incoherent” may thus be used to encompass both fully incoherent and low-coherent situations.

For broadband sources, the optical signal strength of reflected FBG signal reaching the detector is typically very low in TDM architectures, compared to continuous mode sensing schemes. Effectively the light source /amplifiers are on with a low duty cycle, and the FBGs are desired to have low reflectivity to enable more sensors in the chain without resulting in too much losses in the light path towards the most distal

Sensors.

In other words, the duty cycle to emit should be short, and it is desirable to have a low reflectivity of the FBGs in order to allow many FBGs, in order to spread the sensor over a greater sensing area.

The pulse controls, timing and the pulse shape need to be very precise and consistent to ensure the correct sensors are interrogated at good performance (e.g. noise levels or inaccuracies from crosstalk originating from other FBG contributions).

Some publications have suggested the use of cyclic architectures to boost the signal strength, but these architectures are complicated to use and control.

In other words, signal control in known approaches may be very demanding.

As described in the summary section above, various embodiments according to the present disclosure may benefit from the occurrence of a positive re-enforcing feedback loop for TDM architectures in which the light generation and amplification may preferably be based on a single optically active source, but not limited to, a semiconductor optical amplifier. (Other examples are VCSELs, EDFAs or similar waveguide integrated broadband amplifying elements.) There can be a single travel of light from spontaneous emission in the active source, travel to and FBG signals back into the active region, which can then be amplified and transmitted further towards a wavelength detector.

However, when the gain is large enough, and the reflectivity from the SOA non- negligible, a portion of amplified light can be reverted and amplified again traveling back through the active medium, and further towards the FBG for a second back and forth travel. Thus, an FBG selective resonator can be formed at the FBG wavelength of interest as selection for amplification. Considering the broadband operation and

FBG bandwidth, an incoherent resonance effect can occur. The amplification can create a competition of signals, for FBGs in tune and in wavelength.

An alternative perspective on a particular embodiment of the method according to the present disclosure, is along the lines of the following method: a method of sensing a waveguide; the method comprising, on a Photonic Integrated Circuit, PIC, device: - emitting light into a waveguide; - receiving light reflected from an optical reflector in the waveguide; - passing the received light to a partially reflective element, wherein the partially reflective element is situated on the PIC device; - passing light reflected from the partially reflective element through the amplifier to amplify said light in a resonant manner, and emitting the resonantly amplified light into the waveguide; and - receiving light reflected from the optical reflector in the waveguide; wherein the step of passing the light reflected from the partially reflective element through the amplifier to amplify said light in a resonant manner, and emitting the resonantly amplified light into the waveguide is based on a time of flight between the amplifier and the optical reflector in the waveguide.

Regardless of which perspective of formulation is chosen, various embodiments according to the present disclosure may help to offer simplification of the architecture and reduction of specification level required for the components. The reduction in specification level may also apply to the pulse definition, due to the ability of locking observed. Selectivity competition in terms of signal strength (i.e. the amount of power presented to detector stemming from target FBG w.r.t. other FBGs) of FBG signal w.r.t to other FBG signals (not selected for interrogation) occurs in the sensor on both the spectral response as well as spatially due to the pulsed operation. Hence the (current)

pulse definition and flank rise/fall time specifications are not as stringent to still enable readout of closely spaced sensors without erroneous readings.

The combination of multiplexing forms, as illustrated in a particular exemplary form in

Figure 2 (which will be described in more detail further below), may help to enable: - Increased number of low reflectivity sensors to be interrogated with respect to non- resonant TDM. - FBG precision improved w.r.t non-resonant readout, by enhancing/stimulating the

FBG response of interest w.r.t. the background spontaneous emission and from other

FBG sensors. - The system does not requires very specific or narrowly defined FBG wavelengths, freedom of operation in the range of the SOA amplification window (its FWHM) This allows multiplexing in wavelength as well for further multiplexing combinations. - The architecture allows for data transfer if desired to combine with the sensing architecture. - Less stringent requirements on SOA specifications as to output power (balanced ASE of initial spontaneous and further amplification emission). - Less stringent requirements on the driving electrical current pulse shape definition and timing as repetition rate precision, allowing cheaper driver electronic component selection. - Compatibility and integration perspective with spectrometer type detection PIC design. - Relative ease of integration of the schematics into photonic integrated circuits. - Tunability of reflector makes it possible to adjust the system to work with a wide variety of FBG reflectivity values, increasing signal dynamic range. - Detection: an integration (collecting) type of current-to-digital conversion makes the device compatible with standard DC operating interrogator, and the insensitivity to overall amplitude, makes that the interrogation method is still robust even when long divisions (thus long repetition time) are used close to the sampling rate of the detector.

From a product integration perspective, the architecture is simple and thus allows for full photonic integration on-chip of the optical functions and direct implementation as true system-in-a-package interrogator solution whilst maintaining strong interrogator specifications. The pulsed operation of the light source may also reduce heat generation and thus may help to prevent distortions in readout.

Comparing the energy consumption of a light source in DC mode, a typical duty cycle of 5% or less may be achieved. In resonant mode, the drop in overall optical power may be well compensated in the specific amplification of power generated on FBG reflected power towards the detector, exceeding that even of the DC operation. Thus, a power consumption/heat generation reduction of a factor of 20 or more can be achieved.

Further specific advantages may include: - The use of the reflector directly on the SOA chip reduced the number of components. - Ensures that there is no time delay between the back reflected pulse and the initial pulse. This means also shorted pulses can be used and still maintain the resonant mode. - Tuning the reflectivity factor can be used to obtain an equal signal level when measuring FBGs with various reflection coefficients. - No required limitation to wavelength-selective filtering when the reflector is only operating in a limited spectra bandwidth.

It is noted for some embodiments according to the present disclosure that the total gain in a single round trip can be less than 1, equal to 1, or greater than 1. This single roundtrip includes the gain of the amplifier, loss of the reflectors, and other optical losses. With a gain less than 1, the signal goes to an equilibrium after a few roundtrips.

This is a form of resonance in the context of the present disclosure. With a gain greater than 1, the signal keeps increasing with each roundtrip, in resonance, until a maximum output is approached and the gain saturates. These are two distinct cases.

Figure 1 schematically illustrates a first embodiment 100 of the PIC device according to the present disclosure. The PIC device 100 comprises a first emission element 101, denoted also as S1 to handily represent that it may for example be a

SOA or a SLED, and a second emission element 102, denoted also as Sn for similar reasons. Note that there may be more than two emission elements, which is indicated by the ‘n’ used to denote the second emission element 102 as ‘Sn’.

The first emission element 101 is coupled to a waveguide 103, and likewise the second emission element 102 is coupled to a waveguide 104. By virtue of this coupling, the respective emission elements may be configured to emit light towards a target optical reflector in the respective waveguides.

The waveguides 103 and 104 may for example be optical fibers. One or more optical reflectors 105, 106 may be arranged in the waveguides 103, 104. Handily, these optical reflectors are also denoted in the figure as F1, F2, ..., Fn, indicating that there may be any number of optical reflectors, which implies that parts 107 and 108 between F2 and Fn may optionally comprise many more optical reflectors. Moreover, the number of optical reflectors in each waveguide of the two or more waveguides 103, 104 need not be identical.

The PIC device 100 may further comprise a data storage medium (not shown) configured for storing a set of values relating to distances from the gain element to one or more optical reflectors in a waveguide.

In this particular exemplary embodiment, the first emission element 101 is also a gain element configured to amplify light, and likewise the second emission element 102 is also a gain element configured to amplify light. This can for example be done by embodying both elements by a single optical element, e.g. a SOA or a SLED, or any other suitable optical element that can provide both (or more) functions. Of course, it is possible to use individual optical elements to embody the individual elements. Moreover, it may be considered to use a single optical element for one but a combination of optical elements for another.

Itis noted that during gain operation, the amplified light from the first 101 and second 102 gain elements would be transmitted in the direction of the detector 115 via suitable waveguides 111, 112.

Furthermore, there is provided at least one controller (not shown) configured to control the emission and gain elements 101, 102, on a condition that a time-of-flight, corresponding with a distance to the target optical reflector, is greater than a coherence time of light reflected from the target optical reflector to the gain element, the time-of-flight being defined as a propagation time of light propagating from the emission element to the target optical reflector in the waveguide and back to the gain element, such that light is resonantly amplified into the respective waveguides 103, 104 in a temporally incoherent manner. This resonant control may thus be based on a time-of-flight of the light emitted by the respective emission elements 101, 102. In this context, the time-of-flight may be defined to extend from the emission element 101, 102 to the target optical reflector (e.g. F2 for S1 and F2 for Sn, or e.g. F2 for S1 and F1 for Sn) in the waveguide and back to the respective gain element 101, 102 again. This light path is shown via bidirectional arrows at the waveguides 103, 104.

Even though the controller is not shown, the skilled person will understand that any suitable electronic controller can be used, if it can drive the emission and gain elements in a suitable manner.

In order to generate this resonance, the controller may be configured to impose a particular pulse pattern 113, 114 on the emission and gain elements 101, 102. Note that, given that the target optical reflector need not have the same number in each waveguide, and moreover that the waveguides may have different lengths and different arrangements of optical reflectors, the pulse patterns need not be the same.

Another element helping to generate this resonance, is the at least partially reflective back reflectors 109, 110 defined on the PIC device and arranged to be separated from the waveguides 103, 104 by the gain elements 101, 102. In other words, the back reflectors 109, 110 are at a back facet of the gain elements 101, 102 from the perspective of the waveguides 103, 104 and thus of the optical reflectors F1-Fn. This ensures that signals stemming from the optical reflectors and reflected by the back reflector pass at least once, preferably twice through the gain elements 101, 102, in order to amplify them.

Additionally, a detector 115 (handily also denoted as ‘D’) may also be provided. This detector 115 may be configured for detecting light resulting from the resonant amplification by the gain elements 101, 102, after said light has reflected from the target optical reflector. To this end, the detector 115 may be coupled to the respective gain elements 101, 102 via a suitable waveguide 111, 112, e.g. an on- chip waveguide on the PIC device (in particular if the detector is situated on the PIC device), or a direct chip-to-chip coupling, or via the air, or using a length of optical fiber. Since this light path is likely to extend from the gain element towards the detector only, unidirectional arrows indicating the path of light are shown at these pieces 111, 112.k

The architecture blocks may be separate photonic components or integrated functional blocks on a photonic integrated platform. Figure 2 schematically illustrates a second embodiment 200 of the PIC device according to the present disclosure, showing separation of modules. Figure 2 is similar to Figure 1, except as noted in the following.

In this exemplary embodiment, a splitter 218 is arranged, coupling the waveguides 203, 204 corresponding with the gain elements 201, 202 to a single further waveguide 217. In this case, resonance can be built up using only the first emission/gain element 201, meaning that a back reflector 209 needs only be arranged at that emission/gain element 201 and not at the other 202. By virtue of the splitter 216, the return path of the resonantly amplified light signal can be guided via the second emission/gain element 202, towards the detector 215.

Various known approaches use resonance for amplification, in particular on a large scale and not properly on a PIC device, but a distinct and external (to the SOA) reflector is always used in those various known approaches to enable back reflection for generating the resonance. Such solutions require a pulse driving timing sequence with two alternative repetition rates to account for both the time-of-flight between the

SOA towards the back reflector and SOA and selected sensor/range for interrogator, and to ensure amplification of signals bouncing back and forth between the back reflector and the selected FBG of interrogation. The requirements of such a pulse driving timing sequence are therefore very demanding, which is costly.

Figure 3 schematically illustrates a third embodiment 300 of the PIC device according to the present disclosure. Figure 3 is similar to Figures 1 and 2, except as noted in the following. In this exemplary embodiment of the PIC device, a data communication can be set up between a first detector 315A (also handily denoted as

DA) and a second detector 315B (also handily denoted as DB), thereby defining a data communication system comprising a data-enabled detector and the PIC device (wherein the detector may be situated on the chip of the PIC device or not). Note that this data communication can be set up in addition to or instead of the above- described classical sensing of the optical reflectors 305. Further the architecture can be used for redundancy, i.e. to have the second interrogator operate in stand-by and activate in case of failure at the first interrogator or for other reasons (for example, if needed in critical function applications requiring redundancy measures).

By having the controller (not shown) impose a desired data signal 313 onto the activation of the emission/gain element, the data signal 313 can be sent from DA to

DB.

In the status of this example, S1 may be in a master or transmitter state, while S2 may be in a continuously on (i.e. DC) state. This allows to use D2 as a data receiver.

At other times, D1 may be in an idle state to save power. Of course, D1 and D2 may reverse roles. Preferably, the initial states and state transitions may be agreed upon in a handshake protocol.

Figure 4 schematically illustrates a fourth embodiment of the PIC device according to the present disclosure. This figure shows how a gain/emission element 401 generates a broadband light signal 402 and emits it onto a waveguide 403, towards a target optical reflector 404. The light signal is reflected by various optical reflectors, resulting in a reflected signal 405. Signal 405 contains reflected signals of different wavelengths, which is the spectrum showed in the figure. Note that there may also be atime delay between the reflected signals. By imposing onto the gain element 401 a pulse train 409 that is carefully timed to correspond with the time-of-flight of the light between the gain element 401 and the target optical reflectors 404, the reflected signal 405 can be narrowed and amplified. The result of this is shown as superposition 406, which shows clear, high peaks corresponding to the amplified reflection signal 405 and an additional initial broadband emission from the gain element. This superposition 406 is transmitted via waveguide 407 to detector 408.

Figure 5 schematically illustrates selective amplification of optical pulses in a TDM scheme. Graph 501 shows a driving signal (i.e. a control signal provided by a controller) towards an emission/gain element, e.g. an SOA, ranging over time t between values on (high) and off (low) (on at time t1, off at time t2 and on again at time t3), while graph 502 shows the reflected optical signal from the target optical sensors as function of time as received by the gain element.

This may result in an output signal of the SOA as shown in graph 503, showing that only the high signal values at time t1 and t3 stemming from input 502 are transmitted because control signal 501 was on (high) only at those times.

Only the optical pulses of which the time-of-flight correspond to the repetition rate between t1 and t3 are transmitted. This can also be a higher order mode time-of flight. when the time-of-flight towards the target optical reflector (FBG) is a multiple of the time between t1 and t3.

Figure 6 schematically illustrates various pulse patterns that can be used in a method embodiment according to the present disclosure. A first pulse pattern 601 represents an example of a pulse pattern intended to build up or maintain resonance, by virtue of intermittently delivering a stable high value of ‘1’ to a gain element. The timing of the intermittent signal may of course be determined based on the distance and thus the time-of-flight between the gain element and the target optical reflector.

Since the signal is always brought to high, resonance is generated or at least maintained each time. The type of pulse pattern 601 may also be termed a *111’- mode, because it consists of consistent ‘1’ values.

In contrast, a second pulse pattern 602 represents an example of a pulse pattern intended to halt or reduce resonance, by virtue of delivering at least one (in this example one) low value of ‘0’ to the gain element. Because of this low value, the gain element will not build up the resonance, leading to a decrease, or even a cancellation, of the resonance. The type of pulse pattern 602 may also be termed a ‘110°-mode.

Figure 7 schematically illustrates another embodiment 700 of the PIC device according to the present disclosure. This figure shows the combination of time and wavelength multiplexing. A driving signal 713, 714 may be provided towards a gain element 701, e.g. by a controller (not shown). The driving signal 713 may have a particular repetition rate L corresponding to the time-of-flight towards a target optical reflector, such as a particular FBG division, in this example FBG division 703.

Driving signal 714 on the other hand may have a lower repetition rate than driving signal 713, wherein the repetition rate of driving signal 714 may correspond with a time-of-flight to another target optical reflector, for example a different FBG division, in this example FBG division 704. In other words, dependent on the repetition rate a different division in the fiber can be selected. Each division 703, 704, 706 may comprise multiple FBG sensors at different wavelengths, for example an individual

FBG sensor 705. Using an AWG 715 with N-channels 716 or another suitable spectrometer, the different wavelengths may be measured simultaneously.

Furthermore, a delay line 702 is also provided in the waveguide containing the optical reflectors 703, 704, 706. The delay line 702 helps to prevent that the times- of-flight to FBG divisions 703 and 706 are a higher order mode time-of-flight.

The delay line 702 functions as time delay such that the time-of-flight towards sensor division 706 is less than twice the time-of-flight towards FBG division 703. In order to prevent the gain element will amplify a higher order mode time-of-flight, i.e. amplify every 2nd pulse from 706, in case that time-of-flight is exactly twice the time-of-flight towards sensor division 703.

A delay line of a particular length enables the safe and geometrically unrestricted placement of FBGs within the specified measurement range of equal length. This helps to mitigate or eliminate crosstalk of higher order modes (i.e. multiple FBG reflection signals being within the n*TOF matching the pulse repetition time).

Figures 8a, 8b, 8c schematically illustrate operation of a method embodiment according to the present disclosure. These figures will be best understood with reference to Figure 4 described above, and with the following explanation.

In situation a) (i.e. in Figure 8a), the emission/gain element 801 is driven with a high signal 813 to emit broadband light 802 into the waveguide towards the target optical reflector 803. After reflection on that target optical reflector 803, the reflected signal 804 returns to the gain element 801. Note that, due to operation of the emission/gain element 801, a copy 805 of the broadband signal 802 is sent towards the detector 815.

In situation b) (i.e. in Figure 8b), the resonance has begun to build up, which is visible in the superposition of the reflected signal 804 on the background spectrum stemming from the original broadband signal 802. Note that the amplitude of the wavelength corresponding with the target optical reflector 803 is now somewhat higher than in situation a), thus higher than the wavelengths corresponding with other optical reflectors.

In situation c) (i.e. in Figure 8c), the resonance is either further increased (by imposing a high ‘1’ with signal 813) or cancelled (by imposing a low ‘0’ with signal 813). In the first respective option, this leads to a wavelength corresponding with the target optical reflector 803 that is now significantly higher than the wavelengths corresponding with other optical reflectors, which makes detection of the target optical reflector 803 much more accurate. Note that the signal need not be limited to a repetition of three pulses, and that there can also be continuous pulses. In the second respective option, the signal is essentially cancelled (or at least significantly reduced), which is represented via a cross instead of a spectrum.

Figure 9 schematically illustrates the way convolution and resonance result in selective amplification. Starting from a square pulse 901 as a function of time, the pulse is repeatedly convoluted 902, 904, 906, resulting in the peak of the pulse becoming very more narrow 903, 905, 909. This narrowing is due to the repetition rate matching with a target optical reflector at a specific distance, because reflectors at slightly different distances will have less overlap and thus will reflect less, which is explained further below with respect to Figure 10. This effect represents a form of resonant locking.

Figure 10 schematically illustrates the way convolution and resonance interact.

Locking via the convolution effect explained above with respect to Figure 9 during square pulse amplification: a) a current pulse 1001 is sent to a gain element to amplify b) an incoming FBG reflection pulse 1004 and effectively convolve to form c) an amplified optical pulse 1006. The figure also shows which wavelengths correspond with a set of optical reflectors 1003, 1005 (denoted as FBG no. 1-7). Axes are not to scale.

As the skilled person will appreciate from the above description, various embodiments according to the present disclosure may help to achieve an enhanced signal from the optical reflector. This may be at the cost of a somewhat higher spontaneous emission arriving as noise at the detector (which is the background level that has to be corrected), but this is deemed an acceptable trade-off.

The condition for the resonant mode is determined by the single pass gain and the total reflection coefficient.

This is further explained with reference to Figure 11, which schematically illustrates another embodiment 1100 of the PIC device according to the present disclosure.

In a single round trip 1190, light experiences a reflection at back reflector 1109 (having a reflection coefficient of R1) and target optical reflector 1180 (having a reflection coefficient of R2), and also a gain in gain element 1101 (also denoted as S1) (with amplitude G). For a resonant mode, the sum of R1+R2+2*G must approach 0 dB.

When the sum is << 0 dB, there is no significant amount of power in the resonant pulses. When the sum is >> 0 dB there is a strong gain, but as long as the reflection is temporally incoherent, the spectral bandwidth of the target optical reflector can be maintained.

When the sum of R1, R2 and 2*G is slightly larger than 0 dB there will be a net gain.

This will lead to a stimulated emission of the FBG wavelengths and in the end a suppression of the total gain and ASE background spectrum, since the FBG power will start to saturate the amplifier (with the increasing FBG reflection) and both gain and

ASE will drop, i.e. will saturate, to a level where the net gain is 0 dB. Enhanced stimulated emission results in the suppression of the spontaneous emission, for a given current bias of the SOA.

STRUCTURAL LAYOUTS

For best operation, the reflection coefficient R1 should be engineered to a value inversely proportional to the FBG reflectivity, such that also low reflective FBGs can still reach a net gain in a roundtrip.

Back reflector 11109 can be engineered in different ways, specifically when a certain coefficient needs to be achieved:

Directly on the back-facet of component 1101: e Using a reflective coating. e Using a waveguide interface with a small effective index mismatch. e Via an absorber which absorbs part of the light. (Absorber could be a weakly biased SOA, for example, or another light-absorbing medium on chip.)

For the different type of reflector possibilities, various designs can be considered. A few examples are shown in the following figures. Of course, other designs than those described here may be used instead, without departing from the scope of the appended claims.

Figure 12 schematically illustrates another embodiment 1200 of the PIC device according to the present disclosure, which shows variable reflection, by means of a reflective coating 1209 on a back-facet of a SOA chip 1260. The reflector 1209 can be realized in or on the SOA element by, for instance: cleaving and coating the SOA chip (end-facet cleaving and coating of the exit waveguide), by bonding or by the introduction of a designed end pattern / taper by means of lithography (reflector MMI or other perturbation of the waveguide resulting in a controlled transmission/reflectivity)} on the (active) waveguide, or a vertical grating coupler introduced on the waveguide. Another means of creating a controlled reflector interface is by bonding the SOA to another photonic waveguide medium (hybrid or heterogeneous integration of different photonic integrated platforms).

The design of the reflector 1209 can be based on Fresnel reflection from an interface with an effective index contrast.

For example, in Figure 12, S1 1201 may be used as light source while S2 1260 may be used to partially absorb the light by changing the reverse bias voltage. In this way a variable reflector can be made on the chip 1280 itself. Another method for creating the variable reflectivity is using a splitter with variable splitting ratio.

Figure 13 schematically illustrates another embodiment of the PIC device according to the present disclosure. Light in any optical structure or waveguide will see an effective index. Any change in the geometry of the optical structure or waveguide will lead to a change in the effective index. This will induce a Fresnel reflection. By careful design this reflection can even be made polarization dependent. In this example, a waveguide width mismatch 1309 is introduced between a relatively thicker waveguide 1360 and a relatively thicker waveguide 1370. Light coming from

S1 1301 and having passed through waveguide 1370 and encounters waveguide 1360 may be at least partially reflected back into waveguide 1370.

There is also a possibility to combine the light emitter S1 1401 on the same chip 1480 as the detector 1415. In this way the reflector 1409 can be included on the same chip 1480, as is shown in Figure 14, which schematically illustrates another embodiment 1400 of the PIC device according to the present disclosure.

Figure 15 schematically illustrates another embodiment 1500 of the PIC device according to the present disclosure. When the wavelength detector 1515 and SOA 1501 are located on the same chip, this can also be extended easily to a system with multiple channels by adding multiple SOAs on the same chip 1580. The wavelength detector 1515 (in case an AWG is used) can have multiple input waveguides without changing the overall PIC device footprint of the wavelength detector and/or reflector and SOA section. This can be any detector with multiple inputs or the inputs can be combined before entering the detector.

Figure 16 schematically illustrates another embodiment 1600 of the PIC device according to the present disclosure. A variable splitter 1616 is placed in the architecture of Figure 2. An emission element S1 1601 may be used as a light emission source. In combination with target optical reflectors F1,F2,Fn 1605, a resonant signal may be built up. Using variable splitter 1616, a part of the resonant light can be tapped towards gating element S2 1602.

Figure 17 schematically illustrates another embodiment 1700 of the PIC device according to the present disclosure, using a variable reflector 1716, embodied by a splitter 1716 with a variable splitting ratio. With one connection 1703 going towards the reflector 1709, the amount of reflection can be tuned by tuning the power ratio of coupling (back and forth) at splitter 1716. Optionally (indicated by the dotted line), the optical path 1715 behind the, in this case partial, reflector 1709 may be combined with the other branch 1760 towards the detector 1715.

Figure 18 schematically illustrates another embodiment 1800 of the PIC device according to the present disclosure. The architecture is of interest to use in the redundancy and data-communication implementation of Figure 3, where the S2 element is reserved for sole activation in stand-by monitoring and/or data receival mode (D2 of Figure 3), not bothered by complex pulsed reflections being brought back into the waveguide line.

Figure 19 schematically illustrates another embodiment 1900 of the PIC device according to the present disclosure. This embodiment is similar to the embodiment illustrated in Figure 12, but with the addition that the reflector 1909 in this example may be a reflective component on chip and is not necessarily limited to a reflective coating.

Figure 20 schematically illustrates another embodiment 2000 of the PIC device according to the present disclosure. This example shows a specific variable reflector based on a MIR (Multimode Interference Reflector) 2061, and a MZI modulator to tune the ratio of power sent to the MIR. The MZI modulator, also called a MZI interferometer, comprises a 2x1 splitter 2066, a 2x2 splitter 2063, and a phase modulator 2064. Light is generated at the emission element 2001, e.g. a SOA, and propagates towards the 2x1 splitter 2066. At 2x1 splitter 2068, the light is split into two paths: one direct path 2085 towards the 2x2 splitter 2063, and another path passing through the phase modulator 2064. It is advantageous to ensure that both paths have at least approximately the same length and that the phase modulator 2084 can change the phase over a range of 21 (i.e. 360 degrees). Light from the two separate paths is then combined again using the 2x2 splitter 2063. Based on the phase of both light paths at the 2x2 splitter 2063, the light will be coupled with a certain ratio towards the MIR 2061 and the other waveguide 2082. This ratio can vary between O and 100%. At the MIR component 2061, light is reflected back towards the gain element 2001 (in this example an SOA). By tuning the phase of the phase modulator 2064, the amount of reflection back towards the SOA 2001 can be tuned. The light that is not going to the MIR 2061 is coupled into the output waveguide 2062 going to the detector (not shown).

Figure 21 schematically illustrates the impact of reflectivity of the back reflector in the context of another embodiment of the PIC device according to the present disclosure. This figure illustrates the resonant build-up of signals as a function of reflection, referring to the resonant build-up of signals described with reference to

Figure 11. If the reflection is high (2109A), a resonant mode will amplify the reflection from the target optical reflectors (2105A), and will suppress the background spectrum (net gain > 1). 2130A indicates the background spectrum, while 2131A indicates the resonantly amplified signal from the target optical reflector, which is high. If the reflection is low (2109B), the peaks will be amplified by a small amount (2131B), but the background (2130B) will not be fully suppressed.

Figure 22 schematically illustrates various method embodiments 2201, 2202, 2203, 2204, 2205 according to the present disclosure.

In method 2201, the method comprises, on the PIC device, the following steps: - obtaining (not shown) a set of values relating to distances from the gain element to one or more optical reflectors in the waveguide; and -in step A, activating the emission element, in order to emit broadband light towards a target optical reflector among the one or more optical reflectors in the waveguide, such that the light is reflected from the target optical reflector to the gain element; and - in step C, activating the gain element while guiding the reflected light via the gain element into the waveguide, in order to resonantly amplify the light into the waveguide in a temporally incoherent manner, wherein step C lags step A, which lagging is represented a step t, based on a time-of-flight of the light emitted in step

A, on a condition that the time-of-flight, corresponding with a distance to the target optical reflector, is greater than a coherence time of light reflected from the target optical reflector to the gain element; the time-of-flight being defined as a propagation time of light propagating from the emission element to the target optical reflector in the waveguide and back to the gain element.

In method 2202, step B is added during the lagging t. In step B, a gating function is activated, so between step A and step C, in order to absorb at least some, preferably all light reflected from other optical reflectors in the waveguide than the target optical reflector.

In method 2203, step D is added after step C. In step D, a gating function is activated, in order to absorb at least some, preferably all light reflected from other optical reflectors in the waveguide than the target optical reflector.

In method 2204, step C is repeated at least one additional iteration, represented as the flowchart returning to lagging t after completion of step C, each additional iteration of step C lagging a previous iteration of step C based on the time-of-flight. Another way of defining this, is to represent the rate at which steps C happen as a pulse rate.

Optionally, in all embodiments according to the present disclosure featuring that the emission element is distinct from the gain element, it is possible to further distinguish between the time-of-flight starting from the emission element as opposed to the time- of-flight starting from the gain element, depending on which light emission is being considered.

In method 2205, wherein only the details of step C are shown, step C is shown to be sufficiently long to cover: - performing at least one of (which is represented by the dashed steps C1 and C3), preferably both of: a step C1 and a step C4; and - performing both of: a step C2 and a step C3; and wherein: - the step C1 comprises a first amplification of light stemming from the target optical reflector and being guided through the gain element; - the step C2 comprises a passage of light, the light resulting from the step C1, from the gain element to the at least partially reflective element; - the step C3 comprises a passage of light, the light resulting from the step C2, from the at least partially reflective element to the gain element; and - the step C4 comprises a second amplification of light, the light resulting from the step

C3, stemming from the at least partially reflective element and being guided through the gain element.

It is noted that various embodiments according to the present disclosure are efficient in requiring fewer components than known approaches (for instance, a typically required circulator may not be necessary, which is a costly fiber-optic component, typically costing a few hundred euros). Moreover, no suitable PIC building block is known in the art yet, hindering full photonic integration of the architecture. Also, building a cavity may require long fibers for path length in typical applications (> 100 m), which is cumbersome. Various known approaches moreover may require the use of an extra 2x2 splitter to create a cavity configuration.

With regards to the application of various embodiments according to the present disclosure for the use of data communication, the following is noted. The resonance build up can also be interrupted by means of a pulse pattern definition. By omitting any pulse starting from the third pulse (i.e. a 110-mode) in a train pulse sequence, the resonant mode build may be diminished or even interrupted, because the first pulse sends the signal and the second pulse establishes the resonance. The 110-mode is thus a means to interrupt the occurrence of the resonance. The 111-mode in contrast continues the buildup of resonance until saturation is reached, that is, until the gain of an SOA has a saturation level above which signal cannot be amplified further.

In principle, one can use higher harmonics instead of fundamental to establish resonance (so “skip” second pulse). This may be useful to increase the energy of the signal in order to reach very remote sensors (as there may be long dead times in between pulses).

In the context of the present disclosure, an Arrayed Waveguide Grating (AWG) is a component that can be used for multiplexing and demultiplexing optical signals in wavelength-division multiplexing (WDM). It allows multiple wavelengths of light to be combined (multiplexed) or separated (demultiplexed) for transmission or reception in optical communication networks.

Functionality of an AWG:

A first function of an AWG is to separate or combine optical signals based on their wavelengths. It may achieve this by utilizing the principle of interference and diffraction: 1. Multiplexing (Combining signals):

In the multiplexing mode, an AWG may combine multiple input signals of different wavelengths onto a single output fiber. The input signals may be coupled into a set of input waveguides that act as individual channels.

Inside the AWG, the input waveguides may be arranged in a specific pattern, typically in the form of a star coupler. The light from each input waveguide may be split into multiple arms known as arrayed waveguides. These waveguides may have different path lengths, which may create a phase difference between the light waves.

The phase difference may lead to constructive interference at specific output waveguides, corresponding to the desired output wavelengths. 2. Demultiplexing (Separating signals):

In the demultiplexing mode, an AWG may separate the incoming optical signal into its constituent wavelengths. The different path lengths of the waveguides may create phase differences that may cause the individual wavelengths to separate.

The separated signals may then be directed to their respective output waveguides.

Each output waveguide may carry a specific wavelength component, which can be routed to different fibers or detectors for further processing.

The actual structure and design of an AWG can of course vary depending on the specific application and wavelength range involved. AWGs can be implemented using different materials, such as silica-based waveguides, semiconductor-based waveguides, or polymer waveguides, depending on the desired performance characteristics and fabrication processes.

It is noted that the present disclosure is not limited to utilizing AWGs, but that other spectrometer-on-chip systems or other integrated solutions may be used for implementing the above-described embodiments as well, e.g. using integrated refractive, diffractive or interference based implementations for wavelength determination and/or discrimination, such as a Mach-Zehnder interferometer scheme.

It is noted that, using AWGs, multiple wavelengths can be detected simultaneously, as can be done using a regular spectrometer, whereas interferometry typically is only suitable for one wavelength at a time.

Furthermore, in the context of the present disclosure, it is noted that the term “optical reflector” may refer to a Fiber Bragg Grating (FBG), but also to any one of the following: a Fabry-Perot etalon device, which may be a bulk optic Fabry-Perot etalon; an optical fiber Fabry-Perot etalon; an optical waveguide grating based Fabry-Perot etalon; an end of an optical fiber, which may be a mirrored end; the end of an optical fiber patch-

cord; a break within a section of optical fiber; a crystal-based reflective optical element; or a mirror element. It is also noted that, where the optical reflectors are described as (comprising) gratings, a different number of gratings may be used than the number described or shown, and the respective gratings may be at different spacing distances than those described and may be tuned to different wavelengths than those described.

As used in this application and in the claims, the singular forms “a,” “an,” and “the” include the plural forms unless the context clearly dictates otherwise. The systems, apparatus, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed systems, methods, and apparatus are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed systems, methods, and apparatus require that any one or more specific advantages be present or problems be solved. Any theories of operation are to facilitate explanation, but the disclosed systems, methods, and apparatus are not limited to such theories of operation.

Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus. Additionally, the description sometimes uses terms like “obtaining” and “outputting” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by the skilled person.

It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals may have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the examples described herein.

However, it will be understood by the skilled person that the examples described herein can be practiced without these specific details.

In other instances, methods, procedures and components have not been described in detail so as not to obscure the related relevant feature being described.

The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features.

The description is not to be considered as limiting the scope of the examples described herein.

Claims

CONCLUSIONS

1. A method of operating a Photonic Integrated Circuit (PIC) device; the PIC device comprising an emitter and a gain element, the emitter and the gain element being coupled to a waveguide, such as an optical fiber, and the emitter and the gain element being embodied by a single element or being embodied by two separate elements; the method comprising, on the PIC device, the steps of: - obtaining a set of values relating to distances from the gain element to one or more optical reflectors in the waveguide; and - in step A, activating the emitter to emit broadband light toward a target optical reflector from the one or more optical reflectors in the waveguide such that the light is reflected from the target optical reflector to the gain element; and - in step C, activating the amplifying element while guiding the reflected light through the amplifying element into the waveguide, so as to resonantly amplify the light in the waveguide in a temporally incoherent manner, wherein step C lags behind step A based on a transit time of the light emitted in step A, provided that the transit time, corresponding to a distance to the target optical reflector, is greater than a coherence time of light reflected from the target optical reflector to the amplifying element; wherein the transit time is defined as a propagation time of light propagating from the emitting element to the target optical reflector in the waveguide and back to the amplifying element.

2. The method of claim 1, wherein activating the gain element in step C is continuous while the light reflected from the target optical reflector is guided through the gain element into the waveguide.

3. The method according to any of the preceding claims, wherein the emission element and the amplification element are embodied by a single optical element, preferably by a semiconductor optical amplifier, SOA, or by a superluminescent diode, SLED.

4. The method according to any one of the preceding claims, comprising: - in step B, activating a gate function between step A and step C, to absorb at least a part, preferably all of the light reflected from optical reflectors in the waveguide other than the target optical reflector; wherein the gate function is preferably an inherent gate function of the gain element.

5. The method according to any one of the preceding claims, comprising: - in step D, activating a gate function after step C, to absorb at least some, preferably all, of the light reflected from optical reflectors in the waveguide other than the target optical reflector; and wherein the gate function is preferably an inherent gate function of the gain element.

6. The method of any preceding claim, comprising repeating step C with at least one additional iteration, each additional iteration of step C lagging behind a previous iteration of step C based on runtime.

7. The method of claim 6 when dependent on claim 5, comprising repeating step D after each additional iteration of step C while ensuring that the gate function is disabled prior to each additional iteration of step C.

8. The method of any preceding claim, wherein an at least partially reflective element is provided on the PIC device separated from the waveguide by the gain element, and wherein the light reflected from the target optical reflector is guided through the gain element into the waveguide using the at least partially reflective element.

9. The method of claim 8, wherein step C is sufficiently long to include: - performing at least one, preferably both of: a step C1 and a step C4; and - performing both of: a step C2 and a step C3; and wherein:

- step C1 comprises: a first amplification of light from the target optical reflector and passed through the amplifying element; - step C2 comprises: a passage of light, the light resulting from step C1, from the amplifying element to the at least partially reflective element; - step C3 comprises: a passage of light, the light resulting from step C2, from the at least partially reflective element to the amplifying element; and - step C4 comprises: a second amplification of light, the light resulting from step C3, from the at least partially reflective element and passed through the amplifying element.

10. The method of claim 9, wherein steps C2 and C3 occur immediately.

11. The method according to any one of the preceding claims, comprising: - imposing a data signal on the light emitted in step

A.

12. The method according to any one of the preceding claims, comprising: - detecting light resulting from the resonant amplification in step C, after said light has been reflected from the target optical reflector.

13. The method according to any one of the preceding claims, comprising: - in step A, activating at least one additional emitting element, to emit broadband light towards the target optical reflector or towards at least one other target optical reflector in the waveguide or in at least one other waveguide, such that the light is reflected from the target optical reflector or the at least one other target optical reflector towards at least one additional amplifying element; and - in step C, activating the at least one additional amplifying element, to resonantly amplify the light in the waveguide or in the plurality of waveguides in a temporally incoherent manner.

14. The method of claim 13, comprising: - multiplexing the light resonantly amplified by the amplifying element and the at least one additional amplifying element in step C, using an Arrayed Waveguide Grating, AWG; and/or - demultiplexing the light resulting from the resonant amplification in step C, after said light has been reflected from the target optical reflector, prior to the detecting step, using an AWG; and/or - demultiplexing and multiplexing in the waveguide, to generate wavelength-specific waveguides and path length differences therebetween.

15. The method of any preceding claim, wherein the step of obtaining a set of values relating to distances from the gain element to one or more optical reflectors in the waveguide comprises: - storing a manually predefined set of said values; or - scanning the waveguide to determine said values, and storing said values.

16. A Photonic Integrated Circuit (PIC) device; the device comprising: - a gain element configured to amplify light; - a data storage medium configured to store a set of values relating to distances from the gain element to one or more optical reflectors in a waveguide; - an emitter configured to emit broadband light toward a target optical reflector from the one or more optical reflectors in the waveguide such that the light is reflected from the target optical reflector to the gain element; wherein the emitter and the gain element are coupled to a waveguide, such as an optical fiber, and wherein the emitter and the gain element are embodied by a single element or are embodied by two separate elements; and wherein the device further comprises:

- at least one control unit configured to control the emitter and the amplifying element, provided that a transit time, corresponding to a distance to the target optical reflector, is greater than a coherence time of light reflected from the target optical reflector to the amplifying element, the transit time being defined as a propagation time of light propagating from the emitter to the target optical reflector in the waveguide and back to the amplifying element, such that the light is resonantly amplified in the waveguide in a temporally incoherent manner, by activating the amplifying element while guiding the reflected light through the amplifying element in the waveguide, after activating the emitter, based on the transit time of the emitted light.

17. The PIC device of claim 16, wherein the at least one control unit is configured to control the gain element to activate the gain element via a continuous control pulse while guiding the light reflected from the target optical reflector into the waveguide via the gain element.

18. The PIC device according to any of claims 16 to 17, wherein the emission element and the gain element are embodied by a single optical element, preferably by a semiconductor optical amplifier, SOA, or by a superluminescent diode, SLED.

19. The PIC device of any one of claims 16 to 18, comprising: - a gate element configured to absorb at least part, preferably all, of the light reflected from optical reflectors in the waveguide other than the target optical reflector; wherein the gate element is preferably integrated with the gain element; and wherein the at least one control unit is configured to activate the gate element after activating and deactivating the gain element.

20. The PIC device of claim 19, wherein at least two elements of the emission element, the gain element and the gate element, preferably all three of the emission element, the gain element and the gate element, are embodied by a single integrated element.

21. The PIC device of any of claims 16 to 20, wherein the at least one control unit is configured to activate the gain element with at least one additional iteration, each additional iteration lagging a previous iteration based on the runtime.

22. The PIC device of claim 21 when dependent on claim 19, wherein the at least one controller is configured to activate the gate element after each additional iteration of activating and deactivating the gain element while causing the gate element to be deactivated prior to each additional iteration of activating the gain element.

23. The PIC device of any one of claims 16 to 22, comprising: - an at least partially reflective element defined on the PIC device and arranged to be separated from the waveguide by the gain element, and wherein the light reflected from the target optical reflector is guided via the gain element into the waveguide using the at least partially reflective element.

24. The PIC device of claim 23, wherein the at least one control unit is configured to activate the gain element for a sufficient period of time to include: - performing at least one, preferably both, of: a step C1 and a step C4; and - performing both of: a step C2 and a step C3; and wherein: - the step C1 comprises: a first amplification of light from the target optical reflector and passing through the gain element; - the step C2 comprises: a passage of light, the light resulting from the step C1, from the gain element to the at least partially reflective element;

- step C3 comprises: a passage of light, the light resulting from step C2, from the at least partially reflective element to the amplifying element; and - step C4 comprises: a second amplification of light, the light resulting from step C3, originating from the at least partially reflective element and passing through the amplifying element.

25. The PIC device of claim 24, wherein the at least one control unit is configured such that steps C2 and C3 occur immediately.

26. The PIC device of any one of claims 16 to 25, comprising a data interface configured to impose a data signal on the light emitted by the emitting element.

27. The PIC device of any one of claims 16 to 26, comprising a detector configured to detect light resulting from the resonant amplification by the gain element after said light has been reflected from the target optical reflector.

28. The PIC device of any one of claims 16 to 27, comprising: - at least one additional emitting element configured to emit light toward the target optical reflector or toward at least one other target optical reflector in the waveguide or in at least one other waveguide such that the light is reflected from the target optical reflector or the at least one other target optical reflector toward at least one additional amplifying element; and further comprising: - the at least one additional amplifying element configured to amplify the light in the waveguide or in the plurality of waveguides in a temporally incoherent manner.

29. The PIC device of claim 28, comprising an Arrayed Waveguide Grating, AWG, configured to:

- multiplexing the light resonantly amplified by the gain element and the at least one additional gain element; and/or - demultiplexing the light resulting from the resonant amplification by the gain element and the at least one additional gain element, after said light has been reflected from the target optical reflector; and/or - demultiplexing and multiplexing in the waveguide, to generate wavelength-specific waveguides and path length differences therebetween.

30. The PIC device of any one of claims 28 to 29, comprising: - a coupler configured to couple light passing through the at least one additional gain element to one or more of the detector.

31. The PIC device of claim 23 or any of claims 24 to 30 when dependent on claim 23, wherein the at least partially reflective element is provided as a waveguide interface characterised by an effective index mismatch.

32. The PIC device of claim 23 or any of claims 24 to 31 when dependent on claim 23, wherein the at least partially reflective element is provided as a partially reflective coating on a side of the amplifier facing away from the waveguide in use.

33. The PIC device of claim 23 or any of claims 24 to 32 when dependent on claim 23, wherein the at least partially reflective element is variably reflective.

34. The PIC device of claim 23 or any of claims 24 to 31 when dependent on claim 23, wherein the at least partially reflective element is provided as an end facet of the PIC device.

35. The PIC device of claim 33, wherein the at least partially and variably reflective element comprises a splitter having a variable splitting ratio.

36. The PIC device of any of claims 23 to 34, further comprising a waveguide coupling the at least partially reflective element to a detector configured to detect a wavelength of light from the waveguide.

37. The PIC device of any one of claims 23 to 36, further comprising a detector provided on the PIC and configured to detect a wavelength of light from the waveguide, the at least partially reflective element being defined between the detector and the amplifier.