OPTOELECTRONIC EMISSION WITH PHASE-CONTROLLED GROUP ANTENNA WITH ENLARGED LASER SOURCE

Description

TECHNICAL FIELD

[0001] The field of the invention is that of photonic circuits on chip, and more precisely that of optoelectronic transmitters, of the phased array antenna type, produced on at least one photonic chip. The invention finds application in particular in the field of LIDAR ( Light Detection and Ranging, in English). STATE OF THE PRIOR ART

[0002] Optoelectronic phased array antenna (OPA) transmitters Optical Phased Array, in English) are optoelectronic devices that emit a monochromatic light beam in a directional manner into free space. They are used in particular in the field of laser detection and distance estimation (LIDAR), but also in the field of free-space optical communications, holographic displays and medical imaging.

[0003] There Figure 1Aschematically illustrates the operating principle of such an optoelectronic transmitter 1. A laser source 3 emits an optical signal which is distributed by a power divider 4 in N arms 5 of the optoelectronic transmitter 1. Each arm 5 comprises a phase shifter 6 and an elementary transmitter 7 also called an optical antenna. Each elementary transmitter 7 emits an optical signal in free space, for example by diffraction, the optical signals then combining by interference to form a light beam. This has a far-field emission pattern determined in particular by the relative phase Δφ applied by the phase shifters to the optical signals propagating in the arms.

[0004] Such optoelectronic transmitters can be produced using integrated photonics, meaning that its various optical components (waveguides, power dividers, elementary transmitters, etc.) are produced on and from the same photonic chip.

[0005] In this respect, the Figure 1B schematically and partially illustrates an example of such an optoelectronic transmitter 1 described in the article by Hulme et al entitled Fully integrated hybrid silicon two dimensional beam scanner, Opt. Express 23 (5), 5861-5874 (2015). This optoelectronic transmitter 1 comprises a laser source 3, here of type III-V, and is produced on the same photonic chip. It therefore comprises a semiconductor laser source 3, the power divider 4, the phase shifters 6 and the elementary emitters 7. In this example, the laser source 3 is produced by transferring, onto the photonic chip (of type SOI), a III-V material followed by structuring it to form the gain medium.

[0006] There is, however, a need for such an optoelectronic transmitter comprising a high-power laser source. Indeed, particularly in the context of estimating a distance (LIDAR application), the optical power of the signal backscattered by the scene is only a small fraction of that of the emitted optical signal. The use of a high-power laser source may then be necessary to detect distant objects. However, the generation of a light beam of high optical power in a small-sized waveguide of the optical cavity of the laser source would lead to saturation of the gain, or even to deterioration of the gain medium. Furthermore, the use of a laser source with a very large optical cavity would lead to generating a light beam of high power, but of poor quality since the waveguide in the optical cavity would then no longer be single-mode.

[0007] Furthermore, document US2018 / 0217472 A1 and the article by Guo et al. entitled InP Photonic Integrated Circuit with on-chip Tunable Laser Source for 2D Optical Beam Steering, Optical Fiber Communication Conference / National Fiber Optic Engineers Conference 2013, OSA Technical Digest (online) (Optical Society of America, 2013), paper OTh3I.7, describe examples of such an optoelectronic transmitter comprising a laser chip coupled to a photonic chip. STATEMENT OF THE INVENTION

[0008] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose an optoelectronic transmitter with phased array antenna on a photonic chip, comprising a laser source capable of emitting a high-power, good-quality optical signal, the laser source then being optically coupled efficiently to the array of elementary transmitters.

[0009] For this, the subject of the invention is an optoelectronic transmitter with phased array antenna, comprising a photonic chip, comprising N waveguides forming arms of the optoelectronic transmitter, with N>1; and a plurality of phase shifters and elementary transmitters arranged in the arms.

[0010] According to the invention, the optoelectronic transmitter comprises a laser chip, distinct from the photonic chip and assembled thereto in a coplanar manner, and comprising a flared laser source. The flared laser source is formed of a single-mode cross section and a flared section in a main plane, extending along an optical axis Δ and ending with an emission surface of an optical signal. In addition, the flared laser source is configured to emit the optical signal, a wavefront of which is circular in the main plane and centered at a position zl located in the flared section on the optical axis Δ.

[0011] Furthermore, according to the invention, the photonic chip comprises a coupler, ensuring optical coupling with the flared laser source to collect and transmit at least part of the emitted optical signal, comprising: a collection input arranged opposite the emission surface of the flared laser source, and a transmission output comprising N rectilinear waveguides coupled to the N arms of the optoelectronic transmitter and oriented so that their longitudinal axes intersect at the position zl.

[0012] Some preferred but non-limiting aspects of this optoelectronic transmitter are as follows.

[0013] Each of the N rectilinear waveguides may have an upstream end oriented towards the collection input and an opposite downstream end. The downstream ends may define the transmission output of the coupler and may be arranged laterally in an arc of a circle with a center of the radius of the arc located at the position zl.

[0014] The coupler may be a star coupler comprising: a free propagation region made of a medium of homogeneous refractive index, having an input face forming the collection input and coupled to the emission surface of the flared laser source, and an output face curved in an arc of a circle whose center of the radius of the arc is located at the position zl; and the rectilinear waveguides are output waveguides, each connected to the output face on the one hand and to the N arms on the other hand, rectilinear and oriented longitudinally in the direction of the position zl, the downstream ends of which are arranged laterally along a line forming the transmission output curved in an arc of a circle with a center of the radius of the arc located at the position zl.

[0015] The output waveguides may be tip guides, and have a width that decreases from the output face of the free-propagating region.

[0016] The rectilinear waveguides may be rectilinear tip waveguides oriented longitudinally in the direction of position zl, and arranged laterally so that their upstream ends are arranged along a line forming the collection inlet and their downstream ends are arranged along a line forming the transmission outlet, the collection inlet and transmission outlet being curved in an arc with a center of the radius of the arc located at position zl.

[0017] The photonic chip can be spaced from the laser chip along the optical axis Δ by a value less than or equal to 1µm. They can be assembled to each other by an adhesive layer.

[0018] The arms can be straight and parallel to each other, and connected to the transmission output by connecting portions of different lengths.

[0019] The photonic chip can be made from an SOI substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which: there Figure 1A , already described, is a schematic and partial view of an optoelectronic transmitter according to an example of the prior art; Figure 1B , already described, is a top view, schematic and partial, of an optoelectronic transmitter produced on the same photonic chip, according to an example of the prior art; the Figure 2A is a top view, schematic and partial, of an optoelectronic transmitter according to one embodiment, in which the coupler is a star coupler; the Figure 2Bis a top view illustrating more precisely the laser chip and the coupler of the optoelectronic transmitter of the fig.2A ; there Figure 2C is a top view illustrating more precisely the photonic chip of the optoelectronic transmitter of the fig.2A ; there figure 3 is a top view, schematic and partial, of an optoelectronic transmitter according to an alternative embodiment, in which the coupler is formed of pointed waveguides; the figure 4 is a schematic and partial top view of a structure comprising several optoelectronic emitters produced on the same laser and photonic chips. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0021] In the figures and in the remainder of the description, the same references represent identical or similar elements. In addition, the different elements are not shown to scale so as to enhance the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and may be combined with each other. Unless otherwise indicated, the terms "substantially", "approximately", "of the order of" mean to within 10%, and preferably to within 5%. Furthermore, the terms "between ... and ..." and equivalent mean that the limits are included, unless otherwise indicated.

[0022] The invention relates to an optoelectronic transmitter with a phased array antenna, comprising a flared laser source. Such an optoelectronic transmitter comprises a photonic chip on which the set of phase shifters and elementary emitters is produced, and a laser chip, separate from the photonic chip and assembled thereto in a coplanar manner, on which the flared laser source is produced.

[0023] The optoelectronic transmitter is adapted to emit a light beam whose intensity has, in the far field, a determined angular distribution around a main emission axis. This angular distribution of the light beam in the far field emitted by the optoelectronic transmitter is called the 'far field emission pattern'. It is thus distinguished from the near field emission patterns of elementary transmitters (optical antennas). The far field (or Fraunhofer zone) corresponds to a distance D greater than the ratio between the square of a large dimension of the elementary transmitter (here, the length L ee along the Z axis) and the emission wavelength λ e , in other words: D > 2L ee 2 < / λ e .

[0024] Furthermore, the flared laser source can be of the type described in particular in the article by Wenzel et al. entitled High-brightness diode lasers, CR Physique 4 (2003), 649-661. It comprises an active waveguide (located in the optical cavity) formed of a single-mode straight section followed by a flared section, which ends with an emission surface of the laser source. The single-mode straight section can be of the ribbon type ( ridge, in English) of small transverse dimensions so as to form a spatial filter and force the optical signal to be single transverse mode, and the flared section can ensure the amplification of the optical signal. The emission surface of the flared laser source can then have a width of the order of one to several hundred microns. Such a flared laser source can thus emit a high-power optical signal, for example of the order of several watts.

[0025] According to the invention, the photonic chip comprises a coupler ensuring the optical coupling function between the flared laser source and the N arms comprising the phase shifters and the elementary emitters, as well as the power division function.

[0026] There Figure 2A is a schematic and partial view of an optoelectronic transmitter 1 according to one embodiment, in which the laser chip 10 is optically and coplanarly coupled to the photonic chip 20.

[0027] Here and for the remainder of the description, we define a direct orthonormal XYZ reference frame, where the XZ plane is parallel to the main plane of the laser chip 10 and the photonic chip 20, and where the longitudinal axis Z is oriented along the optical axis Δ of the flared laser source 12. The X axis is called the horizontal axis, and the Y axis is called the vertical axis. The terms “upstream” and “downstream” refer to an increasing positioning along the direction of propagation of the optical signal, here along the +Z direction.

[0028] The laser chip 10 is distinct from the photonic chip 20, and comprises the flared laser source 12. The latter is adapted to emit a pulsed or continuous monochromatic optical signal, of wavelength λ e for example equal to approximately 905nm, or even 1550nm. It is said to be flared in the sense that the active waveguide 13 located in the optical cavity comprises, along an optical axis Δ, a single-mode straight section 14 followed by a flared section 15 ending in an emission surface 16. Also, the spatial distribution in an XY plane of the intensity of the emitted optical signal is elliptical and 'flattened': it has, in the near field, a small dimension along the vertical axis Y, for example of the order of a micron, and a large dimension along the horizontal axis X, for example of the order of a hundred microns, with an aspect ratio (large dimension over small dimension) of the order of 50 to 100, or even more.The flared laser source 12 can be realized in the same or similar way as the article by Delepine et al. entitled How to Launch 1 W Into Single-Mode Fiber from a Single 1.48µm Flared Resonator, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 7, No. 2, (2001), 111-123.

[0029] It comprises a support substrate 11 on which rests an active layer (not shown) comprising multiple quantum wells which extend in the XZ plane. The active layer can be framed along the vertical Y axis by confinement layers. A structured upper layer covers the active layer, in which a so-called active waveguide is formed and extends along the optical axis Δ.

[0030] This active waveguide 13 comprises a straight single-mode section 14 of ribbon type ( ridge,in English), defined laterally in the XZ plane by localized etching. This single-mode cross-section 14 has transverse dimensions such that a single guided transverse optical mode, for example the fundamental transverse mode, is supported at the emission wavelength λ e . Furthermore, the single-mode cross-section 14 has a modal filtering function insofar as the higher order transverse modes possibly excited in the flared section 15 cannot be guided in the single-mode cross-section 14.

[0031] It also comprises a flared section 15, defined in the XZ plane of the upper layer by localized doping. The optical mode is then guided there by the amplification gain. The section 15 is said to be flared insofar as it has a width in the XZ plane which increases linearly here as one moves away from the single-mode cross section 14. It may have an angle of inclination relative to the optical axis Δ of the order of a few degrees, for example between approximately 4° and 6°. The flared section 15 delimits the emission surface 16 of the flared laser source 12, which extends orthogonally to the optical axis Δ. The emission surface 16 then has a width wl , here following the horizontal axis X, of the order of one to several hundred microns.

[0032] Note that a flared laser source 12 is intrinsically astigmatic, in the sense that the horizontal waist plane (i.e. in the XZ plane) is located at a different position from the vertical waist plane (i.e. in the YZ plane). In a known manner, the waist plane of a laser source, literally the neck radius plane, is located at a position where the considered wavefront (horizontal or vertical) is flat (infinite radius of curvature). As indicated in particular in the article by Delepine et al. 2001, the horizontal waist plane is located in the active waveguide 13, and more precisely in the flared section 15, at a position zl distant from the emission surface 16 by a non-zero value δ l along the optical axis Δ, while the vertical waist plane is located at the level of the emission surface 16 (δ l =0). The distance δ l (and therefore the position zl ) can be determined by a wavefront analyzer.

[0033] Also, the emitted optical signal presents, in the XZ plane which is the main plane of the laser chip 10 and that of the photonic chip 20, a circular wavefront whose center is located at the position zl. In other words, in the XZ plane, the optical signal emitted by the flared laser source 12 seems to be emitted from the position zl.

[0034] The photonic chip 20 comprises the N arms 23 of the optoelectronic transmitter 1 in which the phase shifters 24 and the elementary transmitters 25 are located. It is distinct from the laser chip 10 in the sense that these two chips comprise their own support substrate 11, 21. It further comprises an integrated coupler 30 (produced on the photonic chip 20) which ensures on the one hand the optical coupling with the laser chip 10, and on the other hand the power division of the optical signal received in the direction of the N arms 23.

[0035] The photonic chip 20, also called a photonic integrated circuit (PIC, for Photonic Integrated Circuit,in English), comprises a support substrate 21 from which active photonic components (modulators, diodes, etc.) and passive components (waveguides, multiplexers or demultiplexers, etc.) optically coupled to each other can be made. In the context of silicon photonics, the support substrate 21 and the photonic components are made from silicon. The support substrate 21 can thus be of the silicon-on-insulator (SOI) type, for Silicon On Insulator, However, many other technology platforms can be used. Typically, the use of silicon waveguides for 905nm applications is not recommended since silicon absorbs at this frequency. Examples include silicon nitride (SiN), aluminum nitride (AIN), doped silica guides, etc.

[0036] The photonic chip 20 has a first lateral face located opposite the emission surface 16 of the laser chip 10, so that the coupler 30 can collect a significant part of the optical signal emitted by the flared laser source 12. The laser chip 10 and the photonic chip 20 are assembled in a coplanar manner, in the sense that the optical signal propagates in the same XZ plane passing from the laser chip 10 to the photonic chip 20.

[0037] They are assembled to each other here by a layer 2 of an adhesive material, such as an optical glue, the refractive index of which is chosen so as not to disturb the propagation of the optical signal. The chips 10, 20 are therefore not optically coupled to each other by passive optical elements such as lenses. The refractive index of the optical glue 2 may be identical to that of the cladding material of the waveguides of the coupler 30. For example, the coupler 30 and the waveguides of the arms 23 may be made of a silicon nitride and the cladding of a silicon oxide. In addition, the spacing between the laser chip 10 and the photonic chip 20 along the longitudinal axis Z is preferably less than 1 µm to collect the maximum of the optical signal emitted by the flared laser source 12 (insofar as the signal diverges). Adhesive layer 2, however, may not be used, and the emitted optical signal may be transmitted through the air between the two chips.In any case, the optoelectronic transmitter 1 then has significant compactness, and the coupling efficiency is avoided from being impacted by relative positioning errors between the passive optical elements which could have been used, in particular positioning errors along the X axis.

[0038] The coupler 30 is adapted to collect and transmit at least part of the optical signal emitted by the flared laser source 12 in the N arms 23. Generally, the coupler 30 comprises a collection input 31 of the incident optical signal, and a transmission output 32 of the collected optical signal to which the N arms 23 are coupled.

[0039] The collection inlet 31 has transverse dimensions in the XY plane preferably having an aspect ratio at least of the same order of magnitude as that of the spatial distribution of the intensity of the emitted optical signal. More precisely, the collection inlet 31 has a height along the vertical axis Y corresponding to the thickness of a guiding layer of the photonic layer in which the coupler 30 and the waveguides of the arms 23 are made, for example of the order of a micron (from a few tens of nanometers to a few microns), and a width along the horizontal axis X of the order of one to several hundred microns. Thus, the coupler 30 is able to receive a significant portion of the emitted optical signal.

[0040] Furthermore, the transmission output 32 comprises N rectilinear waveguides coupled to the N arms 23 of the optoelectronic transmitter 1 and oriented so that their longitudinal axes intersect at the position zl (within the uncertainties of realization). The rectilinear waveguides can be the output waveguides 34 illustrated in the fig.2A to 2C , or be the 35-point waveguides illustrated on the fig.3 This longitudinal orientation of the N rectilinear waveguides makes it possible to optimize the collection efficiency of the emitted optical signal.

[0041] The N rectilinear waveguides each have an upstream end oriented towards the collection inlet 31 and an opposite downstream end. Preferably, the downstream ends of the rectilinear waveguides define the transmission outlet 32, and are arranged laterally (i.e. along the X axis) in an arc of a circle with a center of the radius of the arc located at the position zl, which makes it possible to optimize the transmission efficiency of the collected optical signal in the direction of the arms 23 of the optoelectronic transmitter 1.

[0042] In the example of the fig.2A to 2C, the transmission output 32 is curved in an arc of a circle with a center of the radius of the arc located substantially at the position zl. Thus, the output of the coupler 30 has a curvature which coincides with the curvature of the wavefront of the optical signal emitted by the flared laser source 12 and received by the coupler 30. Thus, as indicated previously, the transmission efficiency of the collected optical signal is optimized (in addition to the fact that the collection efficiency of the emitted optical signal is optimized), and the introduction of a phase error in the transmitted optical signal is also avoided (the phase errors can however be corrected by the phase shifters 24). Here, the slight difference between the wavefront of the optical signal emitted by the source and the wavefront of the optical signal received by the coupler 30 can be taken into account due to the difference in refractive index between the material of the active layer of the flared laser source 12 and that of the coupler 30.

[0043] In this embodiment, the coupler 30 is a star coupler. It comprises a free propagation region 33 (FPR) and output waveguides 34. The collection inlet 31 is here an input face of the FPR region 33, here orthogonal to the optical axis Δ and located opposite the flared laser source 12. It may have a longitudinal dimension along the X axis greater than that of the emission surface 16. The transmission outlet 32 ​​is here formed by the downstream ends of the output waveguides 34 and has a curvature in the form of an arc of a circle, the center of the radius of the arc of which is positioned at the position zl of the flared laser source 12. The FPR region 33 is formed by a medium of homogeneous refractive index and is delimited along the optical axis Δ by the collection inlet 31 and by an output face 33.1 to which the output waveguides 34 are coupled. The output face 33.1 is also curved in an arc of a circle whose center of the radius of the arc is also positioned at the position zl of the flared laser source 12. The coupler 30 therefore comprises N output waveguides 34, sized to optimize the collection of the emitted optical signal, then the transmission of the collected optical signal, and are preferably geometrically identical to each other. On the . fig.2A , they extend from the output face 33.1 of the FPR region 33 to the dotted arcuate line which illustrates the transmission output 32.

[0044] The N output waveguides 34 ensure the transmission of the collected optical signal towards the N waveguides of the arms 23. They are connected to the arms 23 by connection portions 22, as illustrated more precisely by Figure 2C . Here they are pointed waveguides ( type,in English) with a width in the XY plane which decreases along the direction of propagation of the optical signal in an adiabatic manner to optimize the transmission efficiency of the coupler 30. The width can thus decrease monotonically, or even linearly. As illustrated in Figure 2B, they have identical dimensions to each other, in terms of length L t and transverse dimensions wt(z). They are rectilinear and are each oriented longitudinally in the direction of the position zl of the flared laser source 12. The upstream ends of the output waveguides 34, as well as the downstream ends, are arranged laterally (in the XZ plane and along the X axis) along a circular arc whose center of the arc radius is located at the position zl . The upstream ends of the output waveguides 34 are connected to the output face 33.1 in an arc of a circle of the FPR region 33, and the downstream ends of the output waveguides 34 are located on the transmission output 32 illustrated by the arc-shaped line centered on the position zl .

[0045] For example, as illustrated by the Figure 2Bwhich is a top view, schematic and partial, illustrating in detail the flared laser source 12 and the coupler 30, the flared laser source 12 can emit an optical signal at a wavelength λ e equal to 905nm. The flared section 15 has a length L l of 2000µm and a maximum width wl of 150µm at the emission surface 16. The flared laser source 12 here has an astigmatism such that the distance δ l is equal to approximately 600µm. The coupler 30 here comprises 100 output waveguides 34 made in the guiding layer of a silicon nitride with a thickness of 0.3µm, and arranged at the output face of the FPR region 33 with a pitch P along the X axis equal to approximately 1.3µm. Each output waveguide 34 has a length L t of 100 µm and a maximum width wt of 1.2 µm at its upstream end. The upstream ends are spaced from each other by a spacing g equal to approximately 0.1 µm.Under these conditions, at least 90% of the emitted optical signal is collected and transmitted in the output waveguides 34.

[0046] Thus, the coupler 30 has a high collection and transmission efficiency insofar as the output waveguides 34 are rectilinear and oriented longitudinally towards the position zl and insofar as the transmission output 32 is curved in an arc of a circle having its center in the position zl, that is to say having a curvature substantially identical to that of the wavefront of the emitted optical signal. It is also avoided to introduce phase errors into the optical signals transmitted by the different output waveguides 34 (phase errors which can nevertheless be corrected by the phase shifters 24) and the transmission efficiency is optimized.Furthermore, the collection input 31 of the coupler 30 advantageously has transverse dimensions (horizontal along the X axis, and vertical along the Y axis) at least of the same order of magnitude as those of the spatial distribution in the XY plane of the intensity of the emitted optical signal, thus improving the collection efficiency. It should also be noted that such a coupler 30 (that of the . fig.2A or that of the fig.3 ) ensures an intensity distribution in the arms 23 of Gaussian shape and not an equal distribution from one arm to the other. This Gaussian distribution makes it possible to eliminate the side lobes from the far-field intensity distribution of the light beam emitted by the optoelectronic transmitter 1. Indeed, an equal distribution results in a far-field intensity distribution in the form of a cardinal sine.

[0047] The coupler 30 therefore performs the function of power division of the optical signal collected in the direction of the N arms 23. The output waveguides 34 are connected to the N arms 23 by connecting portions 22. Insofar as the N arms 23 are here preferably rectilinear and parallel to each other, the connecting portions 22 are curved (except that which is in the extension of the optical axis Δ) and have a different length from one connecting portion to the other. Note that the connecting portions 22 and the arms 23 may have the same width, equal to that of the output waveguides 34 at their downstream end. As indicated below, the phase shift introduced by the connecting portions 22 is corrected by the phase shifters 24.

[0048] The optoelectronic transmitter 1 also comprises a set of phase shifters 24 and elementary transmitters 25. More precisely, at least a portion of the arms 23 is equipped with at least one phase shifter 24 adapted to modify the phase of the optical signal circulating in the arm 23 considered, and thus to generate a phase difference Δφ, or relative phase, between the optical signals circulating in the adjacent arms 23. The phase shifters 24 are arranged between the coupler 30 and the elementary transmitters 25. Each arm 23 may be equipped with a phase shifter, or only a portion of the arms 23, such as for example one arm 23 out of two. In addition, the reference arm 23 may not comprise phase shifters 24.

[0049] The phase shifters 24 may be electro-refractive or thermo-optical phase shifters. In both cases, the phase modification is obtained by a modification of the refractive index of the material forming the core of the waveguide in question. This modification of the refractive index may be obtained by modification of the free carrier density in the case of the electro-refractive phase shifter 24, or by modification of the applied temperature in the case of the thermo-optical phase shifter 24.

[0050] The phase shifters 24 are preferably adapted to apply the same relative phase value Δφ to the optical signals propagating in the arms 23, so as to obtain a determined non-zero angle θ formed by the inclination of the main emission axis with respect to the Y axis in the XY plane (orthogonal to the axis of the elementary emitters 25). However, the relative phase Δφ may not be identical between the arms 23, either to obtain a different far-field diagram, or to take into account and compensate for possible phase errors. These phase errors may come from degradation over time of certain components of the optoelectronic emitter 1, non-uniformities during the manufacturing process, non-zero tolerances of the manufacturing process, the impact of the environment of the optoelectronic emitter 1 (e.g., possible effect of the encapsulation elements ( packaging ) covering the elementary transmitters 25).

[0051] Note here that the phase shifters 24 can therefore correct the undesirable phase shift introduced by the connection portions 22 (due to their different length from one portion to another). This correction may have been taken into account during a prior step of calibrating the optoelectronic transmitter.

[0052] The optoelectronic transmitter 1 comprises N elementary transmitters 25, or optical antennas, coupled to the arms 23, and therefore arranged downstream of the phase shifters 24. The relative phase Δφ between the optical signals emitted by the elementary transmitters 25 determines in particular the value of the angle θ formed by the main emission axis of the light beam in the far field with respect to the Y axis in the XY plane of the optoelectronic transmitter 1.

[0053] The elementary emitters 25 are here diffraction gratings formed in waveguides of the arms 23. They extend along the Z axis in a manner parallel to each other and are aligned along the X axis, in other words their first ends have the same position along the Z axis, just like their second ends. They are spaced from each other by a distance preferably between λ e / 2 and 2λ e . For information purposes, the number N of elementary emitters 25 can range from around ten to around ten thousand.

[0054] Thus, the optical signals circulating in the arms 23 are partly transmitted into free space by diffraction by the elementary emitters 25. The extracted optical signals propagate in free space, recombine by interference, and thus form in the far field the light beam emitted by the optoelectronic emitter 1 whose angular distribution around the main emission axis is determined and defines the far field emission diagram of the optoelectronic emitter 1.

[0055] The angle θ formed by the main emission axis with respect to the Y axis in the YZ plane depends, in a known manner, on the emission wavelength λ e of the flared laser source 12 and on the period Λ of the diffraction grating formed by the elementary emitters 25. As indicated previously, the angle θ of the main emission axis with respect to the Y axis in the XY plane depends on the value of the relative phase Δφ applied by the phase shifters 24 to the optical signals circulating in the arms 23. It is generally desired that a zero relative phase Δφ results in a zero angle θ, i.e. that the main emission axis of the light beam in the far field is parallel to the Y axis. This is generally determined during a prior calibration step of the optoelectronic emitter.

[0056] Note that it may be desired to connect the phase shifters 24 to a control device (not shown) adapted to calibrate and control the phase shifters as a function of the effective far-field emission pattern of the optoelectronic transmitter. Depending on the control signals sent by the control device, the phase shifters 24 can generate in the optical signals circulating in the different arms 23 a predetermined relative phase Δφ, so as to obtain the desired far-field emission pattern. The control device may be formed of an interferometric focusing lens, a plurality of photodetectors and a control module, as described in international application PCT / EP2020 / 087385 filed on 12 / 21 / 2020.

[0057] There figure 3is a schematic and partial top view of an optoelectronic transmitter 1 comprising a coupler 30 according to an alternative embodiment. In this example, the coupler 30 is formed from a network of pointed waveguides 35, which have an adiabatic variation in their width. Unlike the output waveguides 34 connected to the FPR region 33 ( fig.2A), the tip waveguides 35 here have an increase in their width along the direction of the optical signal. In addition, the coupler 30 does not have an FPR region, so that the upstream ends and the downstream ends of the tip waveguides 35 form, respectively, the collection input 31 and the transmission output 32 of the coupler 30. The tip waveguides 35 are straight and oriented longitudinally towards the position zl . The upstream ends are arranged laterally along a first arcuate line in the XZ plane centered on the position zl , and the downstream ends are also arranged laterally along a second arcuate line in the XZ plane centered on the position zl . The tip waveguides 35 therefore have the same length. As in the example of the fig.2A, the number of tip waveguides 35, their width and length, their lateral arrangement, etc. can be determined so as to optimize the coupling and transmission efficiency.

[0058] There figure 4 is a schematic and partial top view of a structure comprising several optoelectronic emitters 1 arranged laterally next to each other. Thus, the laser chip 10 comprises a plurality of flared laser sources 12 each oriented along optical axes parallel to each other, and produced on the same support substrate 11. The flared laser sources 12 are preferably identical to each other. The photonic chip 20 comprises several sets of couplers 30, phase shifters 24 and elementary emitters 25, each set being optically coupled to a flared laser source 12. These sets are produced on the same support substrate 21.

[0059] This structure is made possible by the compactness of the optoelectronic emitters 1. In addition, the elliptical shape of the optical signals emitted by the flared laser sources 12 makes it possible to reduce the alignment constraints of the two chips, while maintaining a particularly high coupling efficiency between the flared laser sources 12 and the couplers 30. This structure then makes it possible to perform a scanning of the scene in two dimensions, without using particularly expensive tunable laser sources. Indeed, each optoelectronic emitter 1 can be adapted to scan the scene in a different angular plane.

[0060] Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art.

Claims

1. Phased-array optoelectronic emitter (1), comprising: ∘ a photonic chip (20), comprising: N waveguides forming arms (23) of the optoelectronic emitter (1), with N>1; a plurality of phase shifters (24) and of elementary transmitters (25) placed in the arms (23); ∘ a laser chip (10), distinct from the photonic chip (20), comprising a laser source (12); ∘ characterized in that: ∘ the laser chip (10) is joined to the photonic chip (20) in a coplanar manner; ∘ the laser source (12) is a flared laser source: formed of a straight single-mode section (14) and of a section (15) that is flared in a main plane, and that extends along an optical axis Δ and ends in a surface (16) for emission of an optical signal; configured to emit the optical signal, a wavefront of which is circular in the main plane and centred on a position zl located in the flared section (15) on the optical axis Δ; ∘ the photonic chip (20) comprises a coupler (30), ensuring optical coupling to the flared laser source (12) for collecting and transmitting at least one portion of the emitted optical signal, comprising: a collection input (31) placed facing the emission surface (16) of the flared laser source (12), and a transmission output (32) comprising N rectilinear waveguides (34, 35) that are coupled to the N arms (23) of the optoelectronic emitter (1) and that are oriented so that their longitudinal axes are secant at the position zl.

2. Optoelectronic emitter (1) according to Claim 1, wherein each of the N rectilinear waveguides (34, 35) has an upstream end oriented towards the collection input (31) and an opposite downstream end, the downstream ends defining the transmission output (32) of the coupler (30) and being arranged laterally in a circular arc with a centre of the radius of the arc located at the position zl.

3. Optoelectronic emitter (1) according to Claim 2, wherein the coupler (30) is a star coupler comprising: a free propagation region (33) made of a medium of uniform refractive index, having an entrance face forming the collection input (31) and coupled to the emission surface (16) of the flared laser source (12), and an exit face (33.1) that is circularly arcuate and the centre of the radius of the arc of which is located at the position zl; the rectilinear waveguides are output waveguides (34), each connected to the exit face (33.1) on the one hand and to the N arms (23) on the other hand, the downstream ends of which are arranged laterally along a line forming the transmission output (32) that is circularly arcuate with a centre of the radius of the arc located at the position zl.

4. Optoelectronic emitter (1) according to Claim 3, wherein the output waveguides (34) are tapered waveguides, and have a width that decreases from the exit face (33.1) of the free propagation region (33).

5. Optoelectronic emitter (1) according to Claim 2, wherein the rectilinear waveguides are tapered waveguides (35) arranged laterally so that their upstream ends are placed along a line forming the collection input (31) and their downstream ends are placed along a line forming the transmission output (32), the collection input (31) and transmission output (32) being circularly arcuate with a centre of the radius of the arc located at the position zl.

6. Optoelectronic emitter (1) according to any one of Claims 1 to 5, wherein the photonic chip (20) is spaced apart from the laser chip (10) along the optical axis Δ by a value less than or equal to 1 µm.

7. Optoelectronic emitter (1) according to any one of Claims 1 to 6, wherein the laser chip (10) and the photonic chip (20) are joined to each other by an adhesive layer (2).

8. Optoelectronic emitter (1) according to any one of Claims 1 to 7, wherein the arms (23) are rectilinear and parallel to one another, and are connected to the transmission output (32) by connecting segments (22) of various lengths.

9. Optoelectronic emitter (1) according to any one of Claims 1 to 8, wherein the photonic chip (20) is made from an SOI substrate.