[0001] The present invention is associated with the operation concept and the design of active imaging sensors that are based on the generation of broadband light from an incoherent light source, the use of an optical phased array (OP A) for the emission of this light to the surrounding environment, the collection of the light that is back- reflected from reflectors that reside inside this environment, and the spectral analysis of the back-reflected light with the help of an optical spectrum analyzing unit. Imaging systems that are relevant to the present invention include the Light Detection and Ranging (LiDAR) systems, as well as any other system, where the surrounding environment or an object under investigation is scanned by a light beam with the purpose to create 1-dimensional (ID) or two-dimensional (2D) reconstructions of it. Compared to the use of monochromatic light from a laser source, the use of broadband light as input to the OPA enables the simultaneous emission of different spectral components towards different directions, eliminating the need for the execution of an ultra-fast scanning process. The identification of the direction that corresponds to a reflecting object in the surrounding environment or to a specific point on the object under investigation is based on the separation of the spectral components in the detection part of the active imaging sensor by means of an optical spectrum analyzing unit. In compact implementations, this unit can have the form of photonic integrated arrayed waveguide gratings (AWGs), the form of a charge-coupled device (CCD) with a linear variable filter (LVF) in front of it or any other suitable form. More specifically, the present invention is related to the operation concept and the design of an imaging sensor using a linear (ID) OPA with wavelength-independent optical antenna elements (AEs), the operation concept and the design of an imaging sensor using a plane (2D) OPA with wavelength-independent optical AEs, and the operation concept and the design of an imaging sensor using a linear (ID) OPA with wavelength-dependent optical AEs.

[0002] Typically, the active imaging sensors inside the systems that are relevant to the present invention are based today on the use of a laser source and the scanning of the surrounding environment or the object under investigation by the laser beam with the help of moving mirrors. The use of OP As, and more specifically the use of photonic integrated OP As has been introduced as an alternative way to control the direction of the emitted laser beam, aiming at replacing the moving mirrors and making the entire sensor more compact and robust. The optical AEs that are employed in those OP As are classified into two main categories: those with radiation pattern that is practically independent from the operation wavelength within a certain wavelength band (referred to hereafter as wavelength independent AEs), and those with radiation pattern that has strong wavelength dependence (referred to hereafter as wavelength dependent AEs). Examples of the first category are the edge-emitting waveguides and the waveguide integrated mirrors, whereas examples of the second category are the grating couplers. Four are in turn the most common types of OP As that make use of these AEs. The first one refers to the linear OP As with wavelength independent AEs that can facilitate the execution of a ID beam scanning process. The scanning in this case is based on the assignment of appropriate phase relations between the AEs of the OPA by means of phase shifters. The second type refers to the plane OP As with wavelength independent AEs that can facilitate the execution of a 2D beam scanning process. The scanning is based again on the assignment of proper phase relations between the AEs of the OPA by means of phase shifters. The third type refers to the linear OP As with wavelength dependent AEs, which can support the execution of a 2D beam scanning process. The scanning along the axis of the OPA is based on the assignment of proper phase relations between the AEs in the same way as in the OP As of the first type, whereas the scanning along the perpendicular axis is based on the use of a tunable laser source (TLS) and the wavelength dependence of the radiation pattern. Finally, a fourth type refers to the plane OP As with wavelength dependent AEs that can support a 2D beam scanning process. The scanning in this case is not based on the wavelength dependence of the AEs. It involves instead a single operation wavelength, and is based again on the assignment of proper phase relations between the AE by means of phase shifters.

[0003] In all those OP As, the speed requirements with respect to the reconfiguration of the phase shifters and the wavelength sweeping of the TLS depend on the target scanning rate and the target number of scanning points in each imaging direction. Taking only as example a ID scanning case with target field of view (FOV) equal to 40° and target resolution ® equal to 0.1°, we end up with a scanning process that involves 401 scanning points, which can be taken for simplicity equal to 400. For a target scanning rate of 100 Hz, the required rate for the reconfiguration of the phase shifters is thus equal to 40 kHz. Extension of this example to a 2D scanning case with the same FOV and the same resolution in the second imaging direction has as a result a scanning process with 160,000 points. If this process should be carried out using a 2D OPA with wavelength independent or wavelength dependent AEs, and the target scanning rate is still 100 Hz, the required rate for the reconfiguration of the phase shifters is 16 MHz. Finally, if the 2D scanning process should be carried out using a TLS and a ID OPA with wavelength dependent AEs, the required rate for the transition of the TLS from an emission wavelength to the next one is 40 kHz. The required rate for the reconfiguration of the phase shifters remains however ultra-high (16 MHz).

[0004] Efforts to reduce the number of the reconfigurable elements and the overall complexity of the scanning process in the case of the linear OP As with wavelength dependent AEs have recently resulted in designs with a fixed differential length AL between the AEs. By virtue of this differential length, the proper phase relations between the AEs that control the scanning process along the OPA axis are not assigned via the reconfiguration of the phase shifters, but rather via the selection of the emission wavelength of the TLS. With reference to the target scanning rate and the target number of scanning points in our 2D scanning example, the rate for the transition of the TLS from a wavelength to the next one in the wavelength sweeping process should be 16 MHz. Moreover, the wavelength step in this case should be extremely precise and short, making the use of a high-end TLS necessary. Such a TLS tends to be complex and bulky, and thus impractical for the realization of active imaging sensors with high compactness and low development cost.

[0005] Accordingly, the present invention provides the operation concept and the design of active imaging sensors in compact form, which are based on the use of OP As, do not require the use of a TLS, and can either eliminate the need for the reconfiguration of the phase shifters in the OP As or can drastically reduce the speed requirements of such a reconfiguration. In this sense, the present invention enables the development of active imaging sensors in compact form using low-speed solutions for the implementation of the phase shifters of the OP As and low-speed electronics for their control. The operation concepts and the designs that are provided by the present invention are compatible with imaging sensors based on linear OP As with wavelength independent AEs, plane OP As with wavelength independent AEs, and linear OP As with wavelength dependent AEs. The key point of the present invention is the replacement of the laser source in an active imaging sensor by an incoherent light source. Either in the form of a fixed wavelength source or in the form of a TLS, a laser is a coherent light source that provides monochromatic light. On the contrary, an incoherent light source, either in the form of a light emitting diode (LED) or in the form of a super-luminescent diode (SLD) or in the form of a semiconductor optical amplifier (SOA) or in any other form of optical gain element, provides broadband light that can be considered to be spectrally allocated around a central wavelength and be bounded by a lower and an upper wavelength limit. Unlike the common perception that an OPA can operate only with coherent light, the present invention is based on the fact that it is also possible to operate with broadband light. This possibility is associated with the fact that each individual spectral component can retain its coherence with itself, when it is emitted by the AEs of an OPA, enabling the generation of constructive and destructive interference effects. In the simplest case of a linear OPA with wavelength independent AEs, when this broadband light is used as input, it is split by an optical power splitting unit, and its individual parts feed the AEs. If the lengths of the optical paths from that unit to the individual AEs are equal, every spectral component of the broadband light is emitted to the same direction, which is controlled by the phase relations between the AEs. If however the lengths of the optical paths from the optical power splitting unit to the individual AEs have a fixed differential length AL, the spectral components of the broadband light are emitted to different directions, covering a continuum of emission angles. The angular width of this continuum depends both on the spectral width of the broadband light and on the AL. Finally, the emission angle that corresponds to the central wavelength of the broadband light depends on the phase relations between the AEs, and can be controlled in a static or quasi-static way by the phase shifters of the OPA.

[0006] Accordingly, the present invention provides a concept for the operation of a novel active imaging sensor based on the use of a linear OPA with wavelength independent AEs, wherein the information about the angular position of a reflector is not encoded in the time variable as a result of a beam scanning process, but in the wavelength of the back-reflected light. Since all the wavelengths within the spectrum of the broadband light are simultaneously emitted by the OPA, and since each individual emission angle corresponds to a unique wavelength for a certain configuration of the phase shifters of the OPA, the extraction of the angular position of the reflector requires the analysis of the back-reflected light and the examination of its spectral components.

[0007] Accordingly, the present invention provides a concept for the design of a device that can serve as the front-end of such an active imaging sensor. In the simplest implementation case, the device can have the form of a photonic integrated circuit (PIC), comprising an input waveguide, a set of optical couplers, a set of phase shifters, a set of static optical delay lines (ODLs), and a set of wavelength independent AEs in a linear OPA. The broadband light that is required for the operation of the imaging sensor is generated by an external incoherent light source. It is coupled into the PIC, and is subsequently split by the optical couplers of the PIC into as many parts as the number of the AEs of the linear OPA. Before their emission by the AEs, the individual parts of the broadband light propagate through the phase shifters that control their phase relations, and through the ODLs that provide the differential path length AL between the AEs. In the reverse direction, the light that is back-reflected by the reflectors that reside inside the surrounding environment is coupled back into the same PIC with the help of the same AEs, which in this direction serve as in-coupling elements. The individual parts of the back-reflected light propagate in the reverse direction through the ODLs and the phase shifters, and are combined by the optical couplers, which in this direction serve as an optical power combination unit. Finally, the back-reflected light finds its way out of the PIC from the same waveguide that serves in this direction as the output waveguide, and is guided to an optical spectrum analyzing unit for spectral analysis and detection of the individual spectral components.

[0008] It is noted that the same functionality can be also obtained using two OP As with the same design as above on a single PIC. The first OPA can be used for the out- coupling (emission) of the broadband light to the surrounding environment, whereas the second one for the in-coupling of the back-reflected light in the reverse direction. The use of separate OP As eliminates the need for the use off-chip of optical components like couplers or circulators for the separation of the emission from the detection part of the imaging sensor, but increases the size of the PIC and the number of the phase shifters that need to be controlled.

[0009] In another case that corresponds to a more compact implementation of the frontend device of the same imaging sensor, the incoherent light source can be part of the PIC that comprises the optical couplers, the phase shifters, the ODLs and the wavelength independent AEs of the OPA, eliminating the need for the use of an external light source.

[0010] In another case that corresponds to an even more compact implementation of the front-end device of the same imaging sensor, the optical spectrum analyzing unit can be also part of the PIC that comprises the optical coupler, the phase shifters, the ODLs, the wavelength independent AEs of the OPA, and potentially the incoherent light source. The optical spectrum analyzing unit can have in this case the form of a single filter or the form of a fabric of filters like for example a fabric of AWGs. The individual spectral components of the back-reflected light can thus be detected at the output waveguides of this fabric using a detector array.

[0011] In another case that corresponds to an alternative implementation of the detection part of the same imaging sensor, the broadband light that is generated by an incoherent light source is emitted to the surrounding environment by a photonic integrated OPA with optical couplers, phase shifters, ODLs and wavelength independent AEs, as it has been described above. The OPA in this implementation however is not used for the detection of the back-reflected light. This light is collected instead without any beamforming asset by a lens system, which can be placed nearby with proper orientation, and is coupled into an optical fiber in order to be brought to an optical spectrum analyzing unit, either on-chip or external, for spectral analysis and detection of the individual spectral components. Alternatively, the back-reflected light is collected by a lens system, which can be placed again nearby with proper orientation, and is directly detected by a detector array such as a charge-coupled device (CCD) array. An optical linear variable filter (LVF) is placed in between the lens system and the CCD array to enable the spectral analysis of the back-reflected light.

[0012] In all cases that have been described above and correspond to implementations of an imaging sensor based on a linear OPA with wavelength independent AEs, the broadband light from the incoherent light source can be considered to be spectrally allocated around a central wavelength 0, and to be spectrally bounded by a lower wavelength limit XI and an upper wavelength limit X2. The total spectral span of the broadband light Ak| ₂ can thus be approximated as 2- 1. In the simplest operation mode, the fixed differential path length AL, which is provided by the static ODLs, is adequately selected to ensure that the continuum of emission angles has an angular width A9i2 that can match the target FOV of the OPA along the imaging direction. Moreover, the phase shifters of the OPA are adequately tuned to ensure that the differential phase shift between the AEs becomes zero when the wavelength is equal to 0, imposing that the continuum of emission angles is created in a symmetric fashion with respect to the zero angle (0°) along the imaging direction. With this operation mode, the emission of the broadband light and the detection of its reflections within the target FOV can be supported with a static operation of the phase shifters, and thus without any light scanning process. The price that has to be paid for this simplification is the need for a spectral analysis of the back-reflected light in the detection part of the imaging sensor. The number and the spacing of the spectral channels that should be used in this analysis are calculated using the FOV and the angular resolution that the imaging sensor should accommodate along the imaging direction. For a large FOV and high resolution, this number can become large.

[0013] Accordingly, the present invention provides a concept for a hybrid operation mode of an active imaging sensor based on a linear OPA with wavelength independent AEs, where the use of the broadband light is combined with the execution of a scanning process with coarse scanning step and low scanning speed. The main motivation for this hybrid operation mode is to relax the design requirements with respect to the number of the spectral channels and detection elements in the detection part of the sensor, while keeping the operation compatible with the use of low-speed phase shifter solutions and low-speed control electronics.

[0014] Moreover, the present invention provides a concept for extending the use of broadband light to the case of an active imaging sensor based on a plane OPA with wavelength independent AEs for 2D imaging of the surrounding environment. The PIC that serves in this case as the front-end device of the active imaging sensor comprises an input waveguide, a set of optical couplers for the distribution of the input light among the AEs, a set of phase shifters, a set of static ODLs and a set of AEs organized in rows and columns. The incoherent light source for the generation of the broadband light and the optical spectrum analyzing unit for the spectral analysis of the back-reflected light may be part of the same PIC or may be employed as external components. Moreover, the detection part of the sensor may be based on the same OPA as the emission part, on a twin OPA in the same PIC as the emission part or on an external setup with a lens system for the collection of the back-reflected light without any beamforming asset, as already described above for the case of the imaging sensors with linear OP As for ID imaging.

[0015] In the most practical implementation of this extension concept, the static ODLs in the OPA of the 2D imaging sensor provide a fixed differential path length AL _X between the AEs that reside in the same row of the OPA, but an equal path length between the AEs that reside in the same column of it, assuming that the rows are parallel to the x-axis and the columns are parallel to the y-axis. In the first imaging direction along the x-axis, the imaging operation can thus be accommodated by the spectral analysis of the back-reflected light and the direct mapping of each spectral component to a specific angle. By virtue of the differential path length AL _X , the input light with bandwidth Ak ₁₂ can be used in the same way as in the ID imaging sensor to create a continuum of emission angles that can entirely cover the target FOV along the x-direction (FOV _X ). A hybrid operation mode with simultaneous use of a coarse scanning step along the same direction can be also used to relax the implementation requirements in terms of number of spectral channels and detectors in the detection part of the imaging sensor. In the second imaging direction along the y-axis, the imaging operation is accommodated solely by a standard beam scanning process, using the phase shifters of the OPA for the dynamic introduction of the required phase relations between the AEs that reside in the same column of the OPA.

[0016] In a more comprehensive implementation of the same extension concept, the ODLs of the plane OPA are designed to provide both a differential path length AL _X between the AEs that reside in the same row of the OPA, and a differential path length ALy between the AEs that reside in the same column of it. The 2D imaging operation is accommodated in this case by the spectral analysis of the back-reflected light and the direct mapping of each spectral component to a specific pair of angles along the two imaging directions. By virtue of the differential length ALy along the second imaging direction, which is taken parallel to the y-axis, the input light with bandwidth Az.| ₂ can create a continuum of emission angles with an angular width A9I ₂ that can entirely cover the target FOV in this direction (FOV _y ). The number and the spectral spacing AL, of the channels that should be used for the creation of imaging points along the y-axis can be calculated using the target field of view (FOV _y ) and the target resolution (R _y ) along this axis. In the first imaging direction, which is taken parallel to the x-axis, the differential length AL _X should provide a continuum of emission angles with angular width (A0 _X ) equal to 180° for a spectral span equal to Aky. Provided that this condition can be met, multiple imaging lines that are parallel to each other and almost parallel to the x-axis can be created, resulting in a well-defined imaging grid on the x-y plane. It is noted that the actual width A0 _L of the continuum of emission angles that is used for imaging along the x-axis is smaller than A9 _X , since the target FOV in this direction (FOV _X ) is always narrower than 180°. Equivalently, the bandwidth that is exploited for the creation of each imaging line is lower than AL,. The number and the spectral spacing Ak _x of the channels that should be used for the creation of the points along each imaging line can be calculated using the target field of view (FOV _X ) and the target resolution (R _x ) along the x-axis.

[0017] Finally, the present invention provides a concept for extending the use of the broadband input light and the spectral analysis of the back-reflected light to the case of a 2D imaging sensor based on a linear OPA with wavelength dependent AEs, which most commonly have the form of grating couplers. In its typical operation mode, a linear OPA with wavelength dependent AEs is combined with a TLS that supports the execution of a wavelength sweeping process within a certain spectral range. The scanning of the laser beam along the axis of the linear OPA is enabled by the phase shifters of the OPA, whereas the scanning of the laser beam along the longitudinal axis of the grating couplers is enabled by the wavelength sweeping process itself due to the wavelength dependence of the out-coupling (emission) angle of the grating couplers. The light that is back-reflected from reflectors inside the surrounding environment is collected and detected by a single photodetection element. The replacement of the TLS by an incoherent light source, and as a consequence, the operation of the OPA with broadband light creates a continuum of emission angles along the axis of the grating couplers, which in combination with the collection and the spectral analysis of the back- reflected light can enable imaging along this direction without any wavelength sweeping process. In the other imaging direction, the imaging can still be enabled by a standard scanning process using the phase shifters of the OPA. It is noted that the PICs that can serve as the front-end of this 2D imaging sensor, and the optical setups that can implement its entire optical part are almost the same as the corresponding PICs and setups that have been described before in the case of the ID imaging sensors based on linear OP As of wavelength independent AEs. Their only differences are associated with the type of their AEs, and with the elimination of the ODLs from the PICs of the 2D imaging sensors in order to keep the scanning process along the axis of the OPA wavelength independent.

[0018] Embodiments of the invention will be described with reference to Figures 1 to 15, whereby:

Figure 1 shows a PIC with a linear OPA of wavelength independent AEs that can serve as the front-end device of an active imaging sensor for ID imaging, accommodating the emission of the broadband light and the collection of the back-reflected light.

Figure 2 shows a PIC with two identical OP As of wavelength independent AEs that can separately accommodate the emission of the broadband light and the collection of the back-reflected light.

Figure 3 shows the optical part of an active imaging sensor for ID imaging, comprising the PIC of Figure 1, and external components for the generation of the broadband light and the spectral analysis of the back-reflected light.

Figure 4 shows a PIC that integrates the entire optical part of Figure 3. Figure 5 shows the optical part of an active imaging sensor for ID imaging, wherein the collection of the back-reflected light is accommodated by an optical setup without beamforming asset.

Figure 6 shows the model for the optical power spectral density of the broadband light that is used in the active optical sensors of the present invention. Figure 7 shows the creation of a continuum of emission angles, when broadband light is emitted by an OPA of wavelength independent AEs with a differential path length between these AEs.

Figure 8 shows the combination of the creation of a continuum of emission angles with a scanning process with coarse scanning step for ID imaging.

Figure 9 shows a plane OPA of wavelength independent AEs, having a differential path length between the AEs that reside in the same row of the OPA but an equal path length between the AEs that reside in the same column of it.

Figure 10 shows the creation of multiple imaging lines, when the plane OPA of Figure 9 is used as the front-end device of an active imaging sensor for 2D imaging.

Figure 11 shows a plane OPA of wavelength independent AEs with differential path lengths both between the AEs that reside in the same row of the OPA and in the same column of it.

Figure 12 shows the creation of multiple imaging lines, when the plane OPA of Figure 11 is used as the front-end device of an active imaging sensor for 2D imaging.

Figure 13 presents the utilization of the optical spectrum when the plane OPA of Figure 11 is used as the front-end device of an active imaging sensor for 2D imaging.

Figure 14 shows a PIC with a linear OPA of wavelength dependent AEs that can serve as the front-end device of an active imaging sensor for 2D imaging.

Figure 15 shows a diagram that presents the creation of multiple imaging lines, when the linear OPA of Figure 14 is used as the front-end device of an active imaging sensor for 2D imaging.

[0019] Figure 1 is a schematic representation of a PIC 6 that implements a linear OPA with wavelength independent AEs 5, and serves as the front-end of an active imaging sensor for ID imaging of the surrounding environment, based on the use of broadband light and the spectral analysis of the back-reflected light. The PIC comprises an input waveguide 1 for the ingress of the broadband light, a set of optical couplers that serves as an optical power splitting unit 2, a set of phase shifters 3 for the adjustment of the phase relations between the AEs of the OP A, a set of static ODLs 4 for the introduction of a fixed differential length AL between the individual paths from the optical power splitting unit to each AE, and a set of wavelength independent AEs 5 for the out- coupling (emission) of the broadband light to the surrounding environment. In the reverse direction, the AEs are used for the in-coupling of the back-reflected light, the set of the optical couplers serves as an optical power combination unit 2, and the input waveguide serves as an output waveguide 1 for the egress of the back-reflected light from the PIC. The wavelength independent AEs 5 are represented as edge emitting waveguides on the x-z plane. With this orientation of the AEs 5, the imaging direction is parallel to the x-axis.

[0020] Figure 2 is a schematic representation of a PIC 7 that serves again as the frontend of an active imaging sensor for ID imaging of the surrounding environment, comprising two OP As with exactly the same design and orientation as the OPA in the PIC 6 of Figure 1. In the case of Figure 2, the emission and the detection part of the imaging sensor are decoupled and served by separate OP As, which are integrated however in the same PIC.

[0021] Figure 3 is a schematic representation of an optical setup that implements the optical part of an active imaging sensor for ID imaging of the surrounding environment, based on the use of broadband light and the spectral analysis of the back-reflected light. The PIC 6 from Figure 1 serves within this setup as the front-end device of the imaging sensor. The broadband light is generated by an external incoherent light source 8. An optical component 11 at the input of the PIC 6 is responsible for the separation of the emission from the detection part of the sensor. This component can have the form of an optical circulator or the form of an optical coupler with three active ports. The optical fiber 10 that is pigtailed to the first port of the optical component 11 brings the broadband light from the incoherent light source 8 to this component. The optical fiber 12 that is pigtailed to the second port forwards the broadband light to the front-end PIC 6, and brings in the reverse direction the back-reflected light from the PIC 6 to the optical component 11. Finally, the optical fiber 13 that is pigtailed to the third port of the optical component 11 forwards the back-reflected light to an optical spectrum analyzing unit 14, which is responsible for the spectral analysis of this light and the detection of its individual components. It is noted that if the optical component 11 is a simple optical coupler, an optical isolator 9 may be necessary at the output of the incoherent light source 8 to prevent all kinds of back-reflections from getting back and destabilizing this source. [0022] Figure 4 is a schematic representation of a PIC 23 that implements the entire optical setup of Figure 3 in a photonic integrated form. Compared to the components that are already integrated in the PIC 6 of Figure 1, the PIC 23 of Figure 4 integrates additionally on-chip an incoherent light source 15, an optical component 17 for the separation of the emission from the detection part of the sensor, a fabric of photonic integrated filters 19 for the spectral analysis of the back-reflected light, and an array 21 of individual detection elements 22, which are coupled to the output waveguides 20 of the filters at the last stage of the fabric. The waveguide 16 brings the broadband light from the incoherent light source to the optical component 17, whereas the waveguide 18 brings the back-reflected light from the optical component 17 to the fabric of photonic integrated filters 19. The optical component 17 that separates the emission from the detection part of the sensor can be an optical coupler or an optical circulator if the photonic integration platform supports that. If it is indeed an optical coupler, the use of an optical isolator, either in the form of an on-chip component if this can be supported by the photonic integration platform or in the form of an external component, may be necessary to protect the incoherent light source 15 from back-reflections.

[0023] Figure 5 is a schematic representation of an alternative implementation of the optical part of the same imaging sensor, wherein the linear OPA with the wavelength independent AEs 5 is used only in the emission part of the sensor. The collection of the back-reflected light is thus realized in the detection part of the sensor without any beamforming asset. In the specific example that is illustrated in Figure 5, an external incoherent light source 8 feeds the PIC 6 of Figure 1. An optical isolator 9 is used at the output of the incoherent light source 8 to protect it from back-reflections. The optical setup that serves as the front-end of the detection part is external to the PIC 6, but it remains in close proximity to it. It comprises a lens system 24 for the collection of the back-reflected light and a detector array 26 (as for example a CCD array) with a large number of detection elements 27. In between the lens system 24 and the detector array 26, a linear variable filter (LFV) 25 is used for the execution of an optical spectrum analysis of the collected light. Alternatively, the optical setup that serves as the frontend of the detection part of the sensor can comprise a similar lens system that collects the back-reflected light and couples this light into an optical fiber, which can guide it in turn to an optical spectrum analyzing unit for spectral analysis and detection of its individual spectral components. [0024] Figure 6 plots the theoretical model for the optical power spectral density of the broadband light that serves as input to the OP As of the active imaging sensors of the present invention. The basic assumption that underlies this model is that the broadband light is spectrally allocated around a central wavelength X0, and is spectrally bounded by a lower wavelength limit XI and an upper wavelength limit X2. Its spectral span AX _i2 can thus be approximated as X2-X1.

[0025] Figure 7 presents the basic concept of the operation of a linear OPA with wavelength independent AEs using broadband light. By virtue of the fixed differential length AL between the paths from the optical power splitting unit 2 to the individual AEs 5, which is introduced by the static ODLs 4, the different spectral components of the broadband light are emitted towards different directions, creating a continuum of emission angles on the x-z plane. The emission angles 01 and 92 that correspond to the lower (XI) and the upper (X2) wavelength limits of the broadband spectrum define the angular width A9 ₁₂ of this continuum along the x-axis. Provided that the phase shifters 3 impose the same phase shift on the parts of the input light that propagate along each optical path to the individual AEs 5, the created continuum of emission angles is centered around the zero angle (0°).

[0026] Figure 8 presents the concept for the operation of the same OPA in a hybrid mode, where the use of the broadband light is combined with a scanning process with a coarse scanning step. In the example of Figure 8, the fixed differential length AL between the AEs 5 is smaller than in Figure 7. As a result, the angular width A9I ₂ of the created continuum of emission angles is approximately four times smaller than in Figure 7. If the imaging operation along the x-direction has to accommodate the same FOV as in Figure 7, the continuum of the emission angles in Figure 8 should be shifted at least three times along the x-direction via the execution of sequential scanning steps. In the starting state of the OPA, the continuum of emission angles covers the first sector from Ola to 62a. After the execution of the first scanning step, it covers the sector from 61b to 62b. After the execution of the second scanning step, it covers the sector from 61c to 62c, whereas after the execution of the third scanning step, it covers the fourth sector from Old to 62d. The scanning steps can be realized with the help of the phase shifters 3 of the OPA in the same way as in a standard scanning process with monochromatic light.

[0027] Figure 9 is a schematic representation of a plane OPA with wavelength independent AEs that can serve as the front-end of an active imaging sensor for 2D imaging of the surrounding environment. The wavelength independent AEs 5 of the OPA have in this example the form of edge emitting waveguides, and are supposed to be part of a 3-dimensional (3D) PIC with multiple waveguiding layers. They are also supposed to be organized in rows along the x-axis and columns along the y-axis, resulting in a plane OPA. The broadband light enters the OPA from the input waveguide 28, and is split by a set of vertical and horizontal optical couplers that form an optical power splitting unit 29 into as many parts as the number of the AEs 5. The individual parts propagate through the phase shifters 3 and the static ODLs 4 before they get emitted by the AEs 5. The ODLs 4 are adequately designed to provide a differential path length AL _X between the AEs 5 that reside in the same row of the OPA, but an equal path length between the AEs 5 that reside in the same column of it. As a result, the 2D imaging operation can be accommodated using a hybrid operation mode, wherein the imaging in the direction that is parallel to the x-axis is based on the emission of broadband light and the spectral analysis of the back-reflected light, and the imaging in the direction that is parallel to the y-axis is based on a standard scanning process with short scanning step with the help of the phase shifters 3 of the OPA.

[0028] Figure 10 presents a diagram that reveals the way, in which the plane OPA of Figure 9 can be used for imaging along the x- and the y-direction. The two axes of this diagram represent the two angles of an imaging point on the x-y plane along the x- and the y-direction, respectively. The differential path length AL _X between the AEs 5 that reside in the same row of the OPA is adequately selected to create a continuum of emission angles along the x-axis (A0i ₂ ), which is equal to the target FOV along this axis (FOV _X ). The entire spectrum from kl to 2, which is made available by the incoherent light source, is used for the creation of this continuum of emission angles, and thus for the extraction of each imaging line. The angular spacing between the imaging points along the x-direction is equal to the target resolution R _x . Since the optical paths between the AEs 5 that reside in the same column of the OPA are equal to each other, the imaging lines are parallel to the x-axis. The imaging operation along the y-direction is accommodated via a standard scanning process with the help of the phase shifters 3. The range and the step of this scanning process are adequately selected to match the target field of view (FOV _y ) and the target resolution (R _y ) along this imaging direction.

[0029] Figure 11 is a schematic representation of a plane OPA with wavelength independent AEs that can also serve as the front-end of an active imaging sensor for 2D imaging of the surrounding environment. Unlike the OPA of Figure 9, the static ODLs 4 in the OPA of Figure 11 do not provide only a differential path length AL _X between the AEs 5 that reside in the same row of the OP A, but also a differential path length AL _y between the AEs 5 that reside in the same column of it. In this way, they offer the possibility for 2D imaging of the surrounding environment both along the x and the y direction.

[0030] Figure 12 presents a diagram that reveals the way, in which the plane OPA of Figure 11 can be used for imaging along the x- and the y-direction. The two axes of this diagram represent again the two angles of an imaging point on the x-y plane along the x- and the y-direction, respectively. Moreover, each imaging point or equivalently each pair of angles in this diagram corresponds to a unique wavelength, which falls within the spectral span that is made available by the incoherent light source of the sensor. The differential path length ALy between the AEs 5 that reside in the same column of the OPA is selected to create a continuum of emission angles along the y-direction (A0 _y ), which is equal to the target FOV along this direction (FOV _y ). The angular spacing between the imaging points along the same direction is equal to the target resolution R _y . Moreover, the differential path length AL _X between the AEs 5 that reside in the same row of the OPA is selected to create a continuum of emission angles with a theoretical width of 180° for a spectral span that is equal to the spectral distance between the imaging points along the y-direction. This condition is necessary in order to ensure a cyclic imaging process with imaging lines that are parallel to each other and almost parallel to the x-direction. It is noted that the continuum of emission angles that is actually used for imaging along the x-direction (denoted as A0 _L ) has a width smaller than 180° and equal to the target FOV along this direction (FOV _X ). Finally, the angular spacing between the imaging points along the x-direction is equal to the target resolution R _x .

[0031] Figure 13 presents the scheme for the utilization of the optical spectrum in an active imaging sensor, which is based on the plane OPA of Figure 11 and on the operation mode presented in Figure 12. The total spectral span that is made available by the incoherent light source spans from XI to 2. The spectral distance between the imaging points along the y-direction is AXy. Only a part of it corresponds to the spectral span AXj _y , which is used in fact for the creation of the imaging points along each imaging line in the diagram of Figure 12. The lower and the upper spectral component that correspond to the first imaging line are denoted as X _Li and ui, respectively. The corresponding spectral components for the second imaging line are denoted as _L2 and X _U2 , and so on so forth. [0032] Figure 14 is a schematic representation of a PIC 31 that implements a linear OP A with wavelength dependent AEs 30, and serves as the front-end of an active imaging sensor for 2D imaging of the surrounding environment. In this example, the wavelength dependent AEs 30 have the form of grating couplers that can diffract off- plane the light that propagates in the corresponding waveguides of the PIC 31. Compared to the PIC 6 of Figure 1, which also implements a linear OP A, the PIC 31 of Figure 14 comprises wavelength dependent instead of wavelength independent AEs 30. Moreover, it does not comprise any ODLs since the optical paths from the optical power splitting unit 2 of the OPA till the individual AEs 30 must be of equal length. With this design, the PIC 31 can support a hybrid operation mode for 2D imaging, wherein the imaging in the direction along the axis of the grating couplers (parallel to the y-axis) is based on the emission of broadband light and the spectral analysis of the back-reflected light, while the imaging along the direction of the linear OPA (parallel to the x-axis) is based on a standard scanning process with short scanning step, using the phase shifters 3 of the OPA.

[0033] Figure 15 presents a diagram that shows how the PIC 31 of Figure 14 with a linear OPA with wavelength dependent AEs can be used for imaging along the x- and the y-direction. The two axes of this diagram represent the two angles of an imaging point on the x-y plane along the x- and the y-direction, respectively. The imaging lines are parallel to the y-direction. The imaging points along each line correspond to a continuum of emission angles, which is created as a result of the wavelength dependence of the AEs 30 and the use of broadband light. The imaging along the x- direction relies on the other hand on a standard scanning process with short scanning step using the phase shifters 3 of the OPA.

[0034] In a specific embodiment of the present invention, a linear OPA with wavelength independent AEs has the form of the photonic integrated circuit (PIC) 6 of Figure 1, and is used inside the optical setup of Figure 3 as part of an active imaging sensor for ID imaging of the surrounding environment. The broadband light is generated by the incoherent light source 8, and is coupled into the input waveguide 1 of the PIC 6 via the optical component 11 that separates the emission from the detection part of the sensor. The broadband light is split on-chip by the optical power splitting unit 2 into as many parts as the number of the wavelength independent AEs 5 of the OPA. The individual parts propagate through the phase shifters 3 and the static ODLs 4 of the OPA towards the AEs 5 that have the form of edge-emitting waveguides. Without loss of generality, we use the ID imaging example that has been described in the introductory part of the present invention. In this example, the target FOV along the x- direction is 40°, the target resolution is 0.1°, and the bandwidth AX ₁₂ of the incoherent light source is 40 nm. For this bandwidth, the differential path length AL, which is provided by the ODLs 4, creates a continuum of emission angles with an angular width A9i2 equal to the target FOV (40°). The orientation of the emission of the broadband light with respect to the coordinate system is illustrated in Figure 7. The shortest wavelength of the broadband light (XI) corresponds to the angle 01, which in this example is -20°, the central wavelength of the broadband light (X0) corresponds to the angle 90, which in this example is 0°, whereas the longest wavelength of the broadband light (X2) corresponds to the angle 92, which in this example is +20°. The symmetry of the continuum of emission angles around the zero angle (0°) is controlled and ensured by the phase shifters 4. In the detection part of the setup, the back-reflected light is coupled into the PIC 6 by the same AEs 5. Its individual parts propagate through the same ODLs 4 and the same phase shifters 3, and are combined by the same set of optical couplers 2, which in this direction serves as an optical power combination unit. With reference to the optical setup of Figure 3, the output light is routed by the optical component 11, and is analyzed by the optical spectrum analyzing unit 14. Given that the total spectral span is 40 nm, that the specific span corresponds to 40°, and that the target resolution of the sensor is 0.1°, the number of the spectral channels that should be used at the optical spectrum analyzing unit 14 is 400. Using a linear approximation for the mapping between the individual wavelengths inside the 40 nm span and the individual angles inside the 40° angular sector, it is found that the spectral spacing of these channels should be 0.1 nm.

[0035] In another embodiment of the present invention, the same OPA with wavelength independent AEs has the form of the PIC 6 of Figure 1, and is used again inside the optical setup of Figure 3 as part of an active imaging sensor for ID imaging of the surrounding environment. Compared to the previous embodiment, the operation mode of the OPA is hybrid since the emission of broadband light and the spectral analysis of the back-reflected light are combined with a scanning process along the x-direction with coarse scanning step and low speed. With reference to the same imaging example as above and the representation of the emission angles in the diagram of Figure 8, it is evident that the target FOV (40°) can be covered with the use of four sequential scanning states. The ODLs 4 are designed in this case to provide a differential path length AL between the AEs 5 that creates a continuum of emission angles with an angular width A9 ₁₂ equal to 10°. Given that the target resolution remains 0.1°, the number of the active channels at the optical spectrum analyzing unit 14 should be 100 and their spectral spacing 0.4 nm. If for whatever reason, the spectral spacing of the channels should be shorter, the ODLs 4 can be redesigned to provide a differential path length AL that can cover the same angular width (10°), using a spectral span shorter than 40 nm. In either case, the reduction in the number of the channels and the corresponding reduction in the detection elements from 400 to only 100 is a significant benefit from the use of this hybrid operation mode. The scanning process in this example has a coarse step of 10°. At the first (a), second (b), third (c), and fourth (d) scanning state, the continuum of the emission angles is set by the phase shifters 3 to be centered around -15°, -5°, +5° and +15°, respectively. For a target image extraction rate of 100 Hz as in our initial imaging example, each scanning step should be realized at a rate of 400 Hz. This low reconfiguration rate is compatible with low-speed phase shifter solutions on various photonic integration platforms, and with low-speed control electronics, enabling low-cost implementations.

[0036] In other embodiments of the present invention, the same OPA as in the previous two embodiments is used in different optical setups, as for example in the setups of Figure 4 and Figure 5, and is combined with different off-chip and on-chip components for the generation of the broadband light, the collection of the back-reflected light, the spectral analysis of the back-reflected light, and the detection of its spectral components. In all these embodiments, the core idea of this invention and the basic operation concept remain the same, irrespective of whether a companion scanning process takes also place or not.

[0037] In another embodiment of the present invention, a plane OPA with wavelength independent AEs has the form of the 3D PIC of Figure 9, and is used inside an optical setup like the setup of Figure 3, as part of an active imaging sensor for 2D imaging of the surrounding environment. The broadband light is generated by an incoherent light source, and is coupled into the input waveguide 28 of the OPA by an optical component, which resides either on-chip or off-chip and separates the emission from the detection part of the sensor. The broadband light is split by the optical power splitting unit 29, which comprises a set of vertical and lateral optical couplers, into as many parts as the number of the wavelength independent AEs 5 of the OPA. The individual parts propagate through the phase shifters 3 and the ODLs 4 towards the AEs 5 of the OPA. The AEs have the same form of edge-emitting waveguides as the AEs 5 of the linear OPA in the PIC 6 of Figures 1-5 and Figures 7-8. In the reverse direction, the back- reflected light is coupled into the 3D PIC with the help of the same AEs 5. The individual parts of it pass through the same ODLs 4 and phase shifters 3, and are combined by the same set of vertical and lateral couplers 29, which in this direction serve as an optical power combination unit. The back-reflected light leaves the OPA from the output waveguide 28, passes through the optical element that separates the emission from the detection part of the sensor, and is guided to the optical spectrum analyzing unit of the setup for spectral analysis and detection of its spectral components. With reference to the coordinate system and the orientation of the plane OPA of Figure 9, the ODLs 4 are designed to provide an equal path length between the AEs 5 that reside in the same column of the OPA along the y-axis, but a differential path length AL _X between the AEs 5 that reside in the same row of the OPA along the x- axis. This design imposes in turn that the imaging along the x-direction can be enabled by the emission of broadband light from the OPA and the spectral analysis of the back- reflected light, whereas the imaging along the y-direction can be only enabled by a standard scanning process with the help of the phase shifters 3. This hybrid operation mode is presented in the diagram of Figure 10. For spectral span equal to A I ₂ , the differential path length AL _x is able to create a continuum of emission angles along the x- direction with angular width A9I ₂ equal to the target FOV along this direction (FOV _X ). Without loss of generality, we use the 2D imaging example from the introductory part of the present invention with target FOV equal to 40° and target resolution equal to 0.1° in both imaging directions. The available bandwidth from the incoherent light source is 40 nm, and is bounded by a lower wavelength limit XI and an upper wavelength limit 2. The emission angle along the x-direction that corresponds to the wavelength I is - 20°, and the emission angle along the same direction that corresponds to the wavelength X2 is +20°. The relative phase shifts between the AEs 5 that reside in the same row of the OPA are zero so as to keep the continuum of the emission angles symmetric with respect to the zero angle (0°) along the x-direction. The number of the imaging points on each imaging line along the x-direction, and thus the number of the spectral channels in the detection part of the sensor should be 400. The spectral spacing of these channels should be 0.1 nm. Given that the path lengths of the AEs 5 that reside in the same column of the OPA are equal to each other, no wavelength dependence is present along the y-direction. This implies in turn that the imaging lines can be perfectly parallel to the x-axis. The position of each imaging line along the y-direction is controlled by a standard scanning process based on the relative phase shifts between the AEs 5 that reside in the same column of the OPA. The angular distance between the first and the last imaging line along the y-direction has to be 40° so as to match the target FOV along this direction (FOV _y ). The number of the imaging lines has to be 400, and the angular step between the neighboring lines has to be equal to the target resolution R _y along this direction (0.1°). For a target imaging (frame) rate of 100 Hz as in our initial 2D imaging example, the above set of specifications imposes that the reconfiguration rate of the phase shifters 3 has to be 40 kHz. For the same frame rate, the requirements with respect to the reconfiguration rate of the phase shifters in a 2D OPA that operates with monochromatic light and beam scanning in both imaging directions is 16 MHz. Obviously, this radical reduction (by 400 times) of the required reconfiguration rate in the plane OPA of our invention can be a key factor for the development of compact and low-cost active imaging sensors for 2D imaging.

[0038] In another embodiment of the present invention, a plane OPA with wavelength independent AEs has the form of the 3D circuit of Figure 11, and is used inside an optical setup like the setup of Figure 3, as part of an active imaging sensor for 2D imaging of the surrounding environment. Compared to the OPA of the previous embodiment, the ODLs 4 of the OPA are designed to provide both a differential path length AL _X between the AEs that reside in the same row of the OPA, and a differential path length ALy between the AEs that reside in the same column of it. With the help of this additional differential path length, the OPA eliminates the need for a standard scanning process along the y-direction, and makes possible the imaging in both directions using only the emission of broadband light and the spectral analysis of the back-reflected light. The creation of imaging lines on the x-y plane is represented in the diagram of Figure 12. For a spectral span that is almost equal to A I ₂ , the differential path length ALy is adequately selected to create a continuum of emission angles along the y-direction with angular width A0 _y equal to the target FOV along this direction (FOVy). Without loss of generality, we use a different 2D imaging example with target FOV equal to 40° and target resolution equal to 1° in both directions. The available bandwidth from an incoherent source is now 80 nm, and the target frame rate is again 100 Hz. The emission angle along the y-direction that corresponds to the lower wavelength limit XI of the available bandwidth is slightly less than -20°, whereas the emission angle along the same direction that corresponds to the upper wavelength limit X2 is slightly more than +20°. The number of the imaging points that should be comprised in each column of the image along the y-direction is only 40 in this example. Using a linear approximation for the mapping between the spectral components of the broadband light and their emission angles, the spectral distance AXy between the wavelengths that correspond to the neighboring imaging points along the y-direction is calculated equal to 2 nm. For this spectral distance, the ODLs 4 of the OPA should provide a differential path length AL _X that creates a continuum of emission angles that covers the entire 180° angle along the x-direction. Provided that this condition can be met, each imaging line can cover the same FOV around the zero angle (0°) along the x- direction, and can be parallel to all other imaging lines, enabling the organization of the imaging points into well-defined columns parallel to the y-axis. It is noted that the imaging lines can be parallel to each other, but they cannot be parallel to the x-axis since there is in this embodiment a wavelength dependence of the emission angle also along the y-direction. Moreover, since the FOV along the x-direction (FOV _X ) is 40°, and thus 4.5 times smaller than the entire 180° angle, the actual spectral span AU _y , which has to be used for the creation of each imaging line, is much shorter than AXy. Using again a linear approximation, the spectral span AX _1y that creates a continuum of emission angles with 40° angular width (A0i _x ) is 4.5 times shorter than AX _y . In our example, this is almost 0.44 nm. Given the values of the FOV _X and the angular resolution R _x along the x-direction, the number of the spectral channels that have to be used for the points along each imaging line is 40. Moreover, given the value of AXi _y , it can be extracted that the spectral spacing between the neighboring channels has to be approximately 0.01 nm, which is challenging but still possible. Figure 11 presents the wavelength plan for the exploitation of the available bandwidth AX ₁₂ in the specific embodiment of the present invention. The lower wavelength limit of each imaging line is denoted as X _L i, k _L2 and so on so forth. The upper wavelength limit of each imaging line is denoted as ui, k _U2 and so on so forth. The ultimate limits of the bandwidth that is actually used in the specific embodiment correspond to the wavelengths _Li and X _Uq , where q the order of the last imaging line. It is noted that the shaded areas in this plan are not represented in scale. They correspond to spectral areas that are not used for the reconstruction of the 2D image.

[0039] In another embodiment of the present invention, a linear OP A with wavelength dependent AEs has the form of the PIC 31 of Figure 14, and is used inside an optical setup that is similar to the setup of Figure 3, as part of an active imaging sensor for 2D imaging of the surrounding environment. The broadband light is generated by an incoherent light source off-chip, and is coupled into the input waveguide 1 of the PIC 31 via an optical component that separates the emission from the detection part of the sensor. The broadband light is split on-chip by the optical power splitting unit 2 into as many parts as the number of the wavelength dependent AEs 30 of the OP A. The individual parts propagate through the phase shifters 3 of the OP A towards the AEs 30, which have the form of grating couplers and deflect the broadband light off-plane. In the reverse direction, the back-reflected light is coupled into the PIC 31 by the same AEs 30. The individual parts of this light pass through the same phase shifters 3, and are combined by the same set of optical couplers 2, which in this direction serve as an optical power combination unit. The back-reflected light leaves the PIC from the output waveguide 1, passes through the optical element that separates the emission from the detection part of the sensor, and is guided to the optical spectrum analyzing unit of the setup for spectral analysis and detection of its individual spectral components. Compared to Figure 3, the coordinate system in Figure 14 has been rotated so that the 2D imaging can be carried out again on the x-y plane along the x- and the y-direction. Imaging along the y-direction is enabled by the use of the broadband input light, which in combination with the wavelength dependence of the AEs creates a continuum of emission angles along this direction. The operation is very similar to the standard operation of a linear OPA with wavelength dependent AEs, where a TLS is used for the sequential creation of different spectral components. In the case of the standard operation, the back-reflected light does not have to be analyzed since the emission angle is time-encoded as a result of the extremely demanding wavelength sweeping process of the TLS. In the case of the specific embodiment of the present invention however, the spectral analysis is necessary since the entire broadband spectrum is emitted simultaneously by each AE 30. In the other direction along the x-axis, there is not any wavelength dependence of the emission angles since there is not any differential length between the optical paths from the optical power splitting unit 2 to the individual AEs 30. Imaging along this direction can thus be enabled only via a standard scanning process using the phase shifters 3 of the OPA in the PIC 31. The creation of the imaging lines on the x-y plane is represented in the diagram of Figure 10. As illustrated in this diagram, the imaging lines are parallel to the y-axis. The upper wavelength limit X2 of the bandwidth that is made available by the incoherent light source corresponds to the first point of each line along the y-direction, whereas the lower wavelength limit XI of the same bandwidth corresponds to the last point of each line. The scanning of the broadband light along the x-direction and the creation of multiple imaging lines can be achieved through the introduction of a well-controlled differential phase shift between the AEs 30 with the help of the phase shifters 3 of the OPA. Without loss of generality, we use here an example with available bandwidth equal to 80 nm, wavelength dependence of the emission angle of the grating couplers equal to 0.2°/nm, target FOV equal to 20° and target resolution equal to 0.1° in both imaging directions. The target imaging (frame) rate is 100 Hz. Given the span of the available bandwidth and the wavelength dependence of the AEs 30, the possible FOV along the y-direction (FOV _y ) is only 16°. Taking also into account the target resolution along the same direction (R _y ), we can calculate that the number of the spectral channels that should be used in the optical spectrum analyzing unit of the sensor is 160 and their spectral spacing is 0.5 nm. The number of the scanning steps along the x-direction should be 200 given the target FOV and the target resolution along this direction. Finally, taking also into account the target frame rate (100 Hz), the required rate for the reconfiguration of the phase shifters 3 is 20 kHz.

[0040] In another set of embodiments of the present invention, the linear OP As with wavelength independent AEs that have been described in the previous embodiments as part of an active imaging sensor for ID imaging of the surrounding environment are combined with an external micro-optic system that comprises at least a lens and a moving mirror, and supports the scanning of the broadband light along a second imaging direction. By virtue of this combination, the active imaging sensor can extend its operation, accommodating 2D imaging. The imaging along the first direction is based on the use of broadband light and on the spectral analysis of the back-reflected light, whereas the imaging in the second direction is based on a standard scanning process with the help of a moving mirror.

[0041] Finally, in another set of embodiments of the present invention, the OP As and the corresponding setups that have been described in all previous embodiments as constituent parts of an active imaging sensor for ID or 2D imaging of the surrounding environment are combined with a unit for the modulation of the broadband light and the creation of pulses. These pulses can be used in turn for the encoding of the information about the distance of the reflecting objects from the sensor by means of time-of-flight (TOF) measurements as per the basic operation concept of the LiDAR systems. The unit for the modulation of the broadband light can be a broadband optical modulator or an electrical driving circuit for the driving of the incoherent light source with non- continuous injection current.