OPTOELECTRONIC EMITTER WITH PHASE-CONTROLLED ARRAY ANTENNA WITH OPTICAL ANTENNAS FOR EMISSION OF LIGHT RADIATION ACCORDING TO A PREDEFINED EMISSION PROFILE AND IN A PREDEFINED DIRECTION

Description

TECHNICAL FIELD

[0001] The field of the invention is that of optoelectronic transmitters of the phased array antenna type preferably produced on a photonic chip of the silicon photonic type. 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 monochromatic light radiation 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 2 emits an optical signal which is distributed by a power divider 3 in arms 4 of the optoelectronic transmitter 1. Each arm 4 comprises a phase shifter 6, and an elementary transmitter 7 also called an optical antenna. Each optical antenna 7 emits an optical signal in free space, for example by diffraction, the optical signals then combining by interference to form light radiation. This has a far-field emission pattern determined in particular by the relative phase Δφ applied by the phase shifters 6 to the optical signals propagating in the arms 4.

[0004] Such optoelectronic transmitters can be produced using integrated photonics, i.e. its various optical components (waveguides, power divider, optical antennas, etc.) are produced on and from the same photonic chip, for example in document US 10,983,273. As such, the Figure 1Bschematically 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 2, here of type III-V, and is produced on the same photonic chip. It therefore comprises a semiconductor laser source 2, the power divider 3, waveguides 5, the phase shifters 6 and the optical antennas 7 located in the arms 4. In this example, the laser source 2 is produced by transferring, onto the photonic chip (of type SOI), a III-V material followed by structuring it to form the gain medium.

[0005] Furthermore, to form a light radiation in the far field that is not very divergent, it is necessary in particular for each optical antenna to emit over a long length, for example over one to several hundred microns. Also, one solution consists of reducing the extraction rate of the optical antennas, for example by making the diffraction grating not in the waveguide but at a distance, for example above it, so that only the evanescent part of the optical mode is sensitive to the periodic structures of the diffraction grating.

[0006] In this respect, the article by Han et al. entitled Highly directonal waveguide grating antenna for optical phased array, Current Applied Physics, 18 (2018) 824-828 describes an example of an optical antenna in which the diffraction grating is located above and at a distance from a silicon waveguide, and is formed in the upper face of the cladding covering the waveguide. The values ​​of the dimensional parameters of the waveguide and the diffraction grating remain constant along the optical antenna, and are chosen in particular to limit optical losses linked to parasitic reflections and to optimize the directivity of the optical antenna.

[0007] Furthermore, the article by Wang et al. entitled Silicon nitride assisted 1x64 optical phased array based on a SOI platform, Opt. Express 29(7), 10513-10517 (2021) describes another example of an optical antenna where the diffraction grating is located above and at a distance from a silicon waveguide, and here formed from periodic pads of silicon nitride encapsulated in a layer of silicon oxide. Here too, the values ​​of the dimensional parameters of the waveguide and the diffraction grating remain constant along the optical antenna.

[0008] However, the optical mode circulating in the waveguide of the optical antenna has an optical power P(x) which decreases exponentially along the longitudinal axis as a part of it is extracted. Also, for an extraction rate α r (x) of the diffraction grating which remains constant longitudinally and equal to α 0 , the near-field emission profile S(x) of the light radiation emitted by the optical antenna also has an exponential decrease along the longitudinal axis, according to the relation S(x) ~ α 0 ×P(x), which can result in a degradation of the far-field emission pattern of the light radiation.

[0009] Also, to avoid this reduction in the effective emission length of the optical antenna, a solution could be to longitudinally modulate the extraction rate and therefore to vary the values ​​of the dimensional parameters of the diffraction grating along the longitudinal axis. However, this would lead to producing a diffraction grating with particularly small dimensions of the periodic structures, for example less than 100nm, and therefore little or not compatible with the conventional technologies usually used in manufacturing processes for example in silicon photonics. STATEMENT OF THE INVENTION

[0010] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose an optoelectronic transmitter with a phased array antenna whose far-field light radiation is not very divergent and has a predefined emission pattern, for example constant or Gaussian. For this, each optical antenna is adapted to emit light radiation according to a desired emission profile S c (x) in the near field, for example constant or Gaussian, and oriented according to a predefined emission angle θ c which is constant longitudinally, without it being necessary to produce diffraction gratings with dimensions that are too small.

[0011] For this, the subject of the invention is a method for manufacturing an optoelectronic transmitter with a phased array antenna, which comprises: a divider, intended to be coupled to a laser source; a plurality of waveguides, coupled to the divider and extending along a longitudinal axis in a main plane, forming arms of the optoelectronic transmitter; and a plurality of phase shifters and optical antennas, arranged in the arms, each optical antenna being formed from the corresponding waveguide and a diffraction grating located above and at a distance from the waveguide along a vertical axis orthogonal to the main plane.

[0012] The process involves the following steps: ∘ definition of a target emission profile S c (x) in the near field and of a target emission angle θ c of a light radiation emitted by each optical antenna, and of a structural configuration Cs ref of said optical antennas, comprising: physical parameter values ​​Pp wg of the waveguides defining optical transmission properties of an optical mode coming from the laser source; and physical parameter values ​​Pp r of the diffraction gratings defining optical diffraction properties of the optical mode; ∘ determination of a relation Λ r =f(wc ) expressing an evolution of the pitch Λ r of periodic structuring of the diffraction grating as a function of a width wc of the waveguide, so that, taking into account said structural configuration Cs ref , an emission angle θ(x) of the light radiation emitted by the optical antenna is equal to said target emission angle θ c ;∘ determination of a relation α r =g(wc ) expressing an evolution of an extraction rate α r of the diffraction grating as a function of the width wc of the waveguide, taking into account said relation Λ r =f(wc ) and said structural configuration Cs ref ; ∘ determination of a longitudinal variation wc (x) of the width wc of the waveguide and deduction of a corresponding longitudinal variation Λ r (x) of the pitch Λ r of the diffraction grating from said relation Λ r =f(wc ), so that, taking into account said relation α r =g(wc ) and said structural configuration Cs ref , a near-field emission profile S(x) of the light radiation emitted by the optical antenna is equal to said target emission profile S c (x);∘ manufacturing of the optoelectronic transmitter whose optical antennas have the reference structural configuration Cs ref , supplemented by said longitudinal variation wc (x) of the width wc of the waveguide and said longitudinal variation Λ r (x) of the pitch Λ r of the diffraction grating. ;

[0013] Some preferred but non-limiting aspects of this optoelectronic transmitter are as follows.

[0014] The steps of determining the relations Λ r =f(wc ) and α r =g(wc ) can be carried out for a range of width wc going from a predefined minimum value wc,out and a predefined maximum value wc,in.

[0015] The step of determining the longitudinal variation wc (x) may comprise the following operations: definition of a power function of exponent n representative of a longitudinal variation wc (x) of the width wc between the predefined maximum value wc,out and the minimum value wc,in; determination, for several values ​​n (m) of the exponent n, of a longitudinal variation wc =p (m) (x) of the width wc of the waveguide and deduction of a corresponding longitudinal variation Λ r =q (m) (x) of the pitch Λ r of the diffraction grating; and determination of a longitudinal variation α r (m) (x) of the corresponding extraction rate α r from said relation α r =g (wc ), then of a corresponding emission profile S (m) (x); then determination of an optimal value n (mopt) among the values ​​n (m) of the exponent n, for which the emission profile S (mopt) (x) has a minimal deviation from the target emission profile S c (x).

[0016] The power function can be wc (x) = W c,in + (x / L a ) n< ×(wc,in - wc,out ), where L a is the total length of the part of the optical antenna which presents the longitudinal variations of the width wc of the waveguide and the pitch Λ r of the diffraction grating.

[0017] The invention also relates to an optoelectronic transmitter with a phased array antenna, comprising: a splitter intended to be coupled to a laser source; a plurality of waveguides coupled to the splitter and extending along a longitudinal axis in a main plane, forming arms of the optoelectronic transmitter; a plurality of phase shifters and optical antennas arranged in the arms, each optical antenna being formed from the corresponding waveguide and a diffraction grating located above and at a distance from the waveguide along a vertical axis orthogonal to the main plane.

[0018] According to the invention, over at least part of the length of each optical antenna: the waveguide has a width wc =p(x) varying longitudinally according to a predefined function p; the diffraction grating has an arrangement pitch Λ r =q(x) of periodic structures varying longitudinally according to a predefined function q; the functions p and q being predefined so that a near-field emission profile S(x) of the light radiation emitted by the optical antenna is equal to a predefined target emission profile S c (x), and that a local emission angle θ(x) of the emitted light radiation is equal to a predefined target emission angle θ c, constant longitudinally.

[0019] The function p of longitudinal variation of the width wc can be a decreasing function, and the function q of longitudinal variation of the pitch Λ r can be a growing function.

[0020] Each of the periodic structures can extend across all the waveguides of the optical antennas.

[0021] Waveguides and diffraction gratings can be realized in a silicon-based photonic chip.

[0022] The periodic structures of the diffraction gratings can have a filling factor, defined as the ratio between a transverse dimension of the periodic structures along the longitudinal axis on the step Λ r , constant along the longitudinal axis of the optical antennas. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] 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 with phased array antenna according to an example of the prior art; Figure 1B , already described, is a top view, schematic and partial, of such an optoelectronic transmitter produced in integrated photonics according to an example of the prior art; Figures 2A and 2B are schematic and partial views, in longitudinal section and in top view, of an optical antenna of an optoelectronic transmitter according to one embodiment, where it comprises an apodized part having longitudinal variations in the width of the waveguide and the pitch of the diffraction grating; Figure 2C is a top view, schematic and partial, of an optical antenna of an optoelectronic transmitter according to an alternative embodiment; the figure 3 is a flowchart illustrating steps in a manufacturing process for an optoelectronic transmitter similar to that of the fig.2B ; there Figure 4Aillustrates an example of the relationship Λ r =f(wc ) expressing an evolution of the pitch Λ r of the diffraction grating as a function of the width wc of the waveguide, so that the emission angle θ(x) in the near field of the light radiation emitted by the optical antenna is equal to a target value θ c ; the Figure 4B illustrates an example of a relationship α r =f(wc ) expressing an evolution of the extraction rate α r of the diffraction grating as a function of the width wc of the waveguide, so that the emission angle θ(x) in the near field of the light radiation emitted by the optical antenna is equal to a target value θ c , and that the emission profile S(x) in the near field is equal to the target emission profile S c (x); the Figure 4C illustrates an example of an emission profile S(x) of a light radiation emitted for an optical antenna comprising an apodized part followed by a non-apodized part, and the target emission profile S c (x). DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0024] 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.

[0025] The invention relates to an optoelectronic transmitter with a phased array antenna, comprising a plurality of optical antennas each formed from a waveguide and a diffraction grating located above and at a distance from the waveguide. The optical antennas are preferably produced on a photonic chip of the silicon photonic type. The optoelectronic transmitter is adapted to emit light radiation having a predefined far-field emission pattern, for example constant or Gaussian, oriented according to a predefined emission angle, and little diverging at least in a vertical plane parallel to the optical antennas.

[0026] A far-field emission pattern is the angular distribution of the intensity of the far-field light radiation emitted by the optoelectronic transmitter, around a principal axis oriented along the emission angle. The far field (or Fraunhofer zone) corresponds to a distance D greater than the ratio between the square of a large dimension of the optical antenna (here, its length L tot along the longitudinal axis) and the wavelength λ of the light radiation, and more precisely: D > 2L tot 2 < / λ.

[0027] In order for the optoelectronic transmitter to emit light radiation whose far-field pattern and emission angle correspond to what is expected, the optical antennas are adapted to emit light radiation according to a near-field profile and emission angle which are equal to a predefined target profile and angle. In the remainder of the description, we will refer to a far-field emission pattern of the optoelectronic transmitter, and to an emission profile S(x) (or emission pattern) in the near field of the optical antennas, where x is the longitudinal abscissa associated with the optical antenna. Furthermore, we also note θ(x) the local emission angle of the light radiation emitted by the optical antenna with respect to a vertical axis Z.

[0028] We refer again to the Figure 1B, described briefly above, to now describe in more detail basic optical components of an optoelectronic transmitter 1 according to one embodiment.

[0029] Here and for the remainder of the description, we define a direct orthonormal XYZ reference frame, where the XY plane is parallel to the plane of the photonic chip, the X axis being oriented along the longitudinal axis of the optical antennas 7, and where the Z axis is oriented towards the free space in which the light radiation is emitted by the optoelectronic transmitter 1. The terms “lower” and “upper” relate to a distance from a support substrate 10 (cf. fig.2A ) along the +Z direction.

[0030] In this embodiment, the optoelectronic transmitter 1 is integrated on a photonic chip, for example in the context of so-called silicon photonic technology. The photonic chip, also called photonic integrated circuit (PIC, for Photonic Integrated Circuit,in English), comprises a support substrate 10 from which active photonic components (modulators, diodes, etc.) and passive components (waveguides, multiplexers or demultiplexers, etc.) optically coupled to each other can be produced. In the context of silicon photonics, the support substrate 10 and the photonic components are made from silicon. The support substrate 10, illustrated in the fig.2A , can thus be of type SOI ( Silicon On Insulator, in English). So, in this example, the waveguides 5 are made of silicon.

[0031] However, many other technological platforms can be used depending on the intended applications and the wavelength of the light radiation. Thus, waveguides 5 can be made, for example, from silicon nitride (SiN), aluminum nitride (AlN), doped silica, etc.

[0032] The optoelectronic transmitter 1 comprises a laser source 2 adapted to emit a pulsed or continuous monochromatic optical signal, of wavelength λ. The wavelength may be, for illustration purposes, equal to 1550nm. The laser source 2 may be wavelength tunable, in particular to modify the emission angle θ formed by the light radiation emitted by the optical antennas 7 with respect to the vertical axis Z in the ZX plane. The laser source 2 may be a hybrid source formed from a gain medium made from a III / V compound and bonded to the surface of the photonic chip. Optical reflectors of the Bragg mirror type may thus be made in an integrated waveguide and coupled to the gain medium. Alternatively, the photonic chip may not comprise the laser source 2, the latter then being offset and therefore not assembled on the surface of the photonic chip.It can then be coupled to an integrated waveguide of the photonic chip, in particular by a grating coupler.

[0033] A power divider 3 is coupled to the output of the laser source 2. It thus comprises an input and a plurality of outputs each coupled to a waveguide of the optoelectronic transmitter 1. The number of waveguides 5 corresponds to the number of arms 4 of the optoelectronic transmitter 1. In this example, the power divider 3 is formed of several MMI type dividers ( Multimode Interferometer, in English) arranged in cascade, but other types of optical components can be used.

[0034] The optoelectronic transmitter 1 comprises a plurality of waveguides 5 which extend between a first end coupled to one of the outputs of the power divider 3 and a second opposite end. Each waveguide 5 is therefore adapted to receive an optical signal from the power divider 3, and to allow the propagation of this optical signal to an optical antenna 7.

[0035] The optoelectronic transmitter 1 also comprises a plurality of phase shifters 6 arranged in the arms 4. More precisely, a waveguide 5 is coupled to at least one phase shifter adapted to modify the phase of the optical signal circulating in the waveguide 5 considered, and thus to generate a phase difference Δφ, or relative phase, between the optical modes circulating in the adjacent waveguides 5. The phase shifters 6 are arranged between the power divider 3 and the optical antennas 7. Each waveguide 5 may be equipped with a phase shifter, or only a part of the waveguides 5, such as for example one waveguide 5 out of two. In addition, a reference waveguide 5 may not comprise a phase shifter.

[0036] The phase shifters 6 can be electro-refractive or thermo-optical phase shifters. In both cases, the phase modification is obtained by a modification of the refractive index nc of the waveguide 5 considered. This modification of the refractive index can be obtained by modification of the free carrier density in the case of the electro-refractive phase shifter, or by modification of the applied temperature in the case of the thermo-optical phase shifter.

[0037] The phase shifters 6 are adapted to apply a predefined relative phase value Δφ to the optical modes propagating in the waveguides 5, so as to obtain a determined non-zero angle Φ of the main emission axis with respect to the vertical axis Z in the YZ plane (orthogonal to the longitudinal axis X of the optical antennas 7). However, the relative phase Δφ may not be identical between the waveguides 5, either to obtain a different far-field pattern, or to take into account and compensate for possible phase errors. These phase errors may arise from degradation over time of certain components of the optoelectronic transmitter 1, non-uniformities during the manufacturing process, non-zero tolerances of the manufacturing process, the impact of the environment of the optoelectronic transmitter 1 (e.g., possible effect of the encapsulation elements ( packaging ) covering the elementary emitters).

[0038] The phase shifters 6 are preferably connected to a control module (not shown). Depending on the control signals sent by the control module, the phase shifters 6 can generate a predetermined relative phase Δφ in the optical signals circulating in the different waveguides 5. An example of such a control module is described in the article by Hulme et al entitled Fully integrated hybrid silicon two dimensional beam scanner, Opt. Express 23 (5), 5861-5874 (2015), or in document WO2021 / 130149A1.

[0039] The optoelectronic transmitter 1 comprises a plurality of optical antennas 7 arranged downstream of the phase shifters 6, at the rate of one optical antenna 7 per arm 4. The relative phase Δφ between the optical signals emitted by the optical antennas 7 determines in particular the value of the angle Φ formed by the main emission axis of the light beam in the far field with respect to the vertical axis Z in the YZ plane of the optoelectronic transmitter 1.

[0040] THE Figures 2A and 2B are partial and schematic views, in longitudinal section ( fig.2A ) and in top view ( fig.2B ), of one of the optical antennas 7 according to one embodiment. Each optical antenna 7 is formed by the waveguide 5 and a diffraction grating 8, which is located above and at a distance from the waveguide 5 along the vertical axis Z.

[0041] According to the invention, over at least part of the length of the optical antenna 7, the waveguide 5 has a longitudinal variation wc =p(x) of its width wc according to a predefined function p, and the diffraction grating 8 has a longitudinal variation Λ r =q(x) of the pitch Λ r of arrangement of the periodic structures according to a predefined function q.

[0042] The functions p and q are predefined so that the optical antenna 7 emits light radiation whose near-field emission profile S(x) is equal to a predefined target emission profile S c (x), and which is oriented according to a local emission angle θ(x) relative to the vertical axis Z (and along the +Z direction) which is equal to a predefined target value θ c which is constant longitudinally.

[0043] This area of ​​the optical antenna 7 which comprises these longitudinal variations wr (x) and Λ r (x) is a so-called apodized part of the optical antenna 7. This may comprise, upstream and / or downstream, a non-apodized part where the width wc of the waveguide 5 and the pitch Λ r of the diffraction grating 8 remain constant. Furthermore, L tot denotes the total length of the optical antenna 7, and L a the length of the apodized part.

[0044] The waveguide 5 is defined by physical parameters Pp wg which are representative of the transmission properties of the optical mode by the waveguide 5, and therefore which are also representative of the spatial distribution of the intensity of the optical mode in the transverse plane YZ. These physical parameters Pp wg are here the refractive index nc of the waveguide 5 and the refractive index ng of the cladding, and the dimensional parameters of the waveguide 5 such as the thickness ec along the vertical axis Z, and its width wc along the Y axis.

[0045] The waveguide 5 and the diffraction grating 8 are produced here in a photonic chip, here in silicon photonic technology. This is formed of a support substrate 10, here made of silicon, of a lower layer 11 of buried oxide (BOX in English, for Buried Oxide) which participates in forming the sheath of the waveguide, of the waveguide 5 made in a layer of silicon, of a spacing layer 12 made of a silicon oxide, and here of periodic pads 13 made for example of a silicon nitride encapsulated in a filling layer 14 of a silicon oxide.

[0046] Preferably, the refractive index nc of the waveguide 5 (i.e. of the core of the waveguide 5) and its thickness ec remain constant over the entire length L tot of the optical antenna 7. On the other hand, the waveguide 5 has, in the apodized part, a longitudinal variation wc (x) of its width wc between an upstream value wc,in and a downstream value wc,out. The width wc thus varies longitudinally according to a predefined function p such that wc = p (x). For example, in the case of a Gaussian target profile S c (x), the function p is a decreasing monotonic function.

[0047] This longitudinal variation wc (x) of the width wc of the waveguide 5 results in a longitudinal variation n eff (x) of the effective index n eff of the optical mode circulating in the waveguide 5, insofar as the effective index of an optical mode depends on the transverse dimensions of the waveguide 5. In general, the effective index n eff associated with an optical mode supported by a waveguide is defined as the product of the propagation constant β and λ / 2π. The propagation constant β depends on the wavelength λ of the optical mode, as well as on the properties of the waveguide (refractive index nc and ng , and transverse dimensions ec and wc ). The effective index n eff of the optical mode corresponds, in a certain way, to the refractive index of the waveguide 'seen' by the optical mode. It is usually between the refractive index nc of the waveguide and the refractive index ng of the cladding.

[0048] Thus, the longitudinal variation wc (x) of the width wc of the waveguide 5 causes a longitudinal variation n eff (x) of the effective index n eff and therefore a modification of the spatial distribution of the optical mode along the longitudinal axis X, which contributes to determining the local influence of the diffraction grating 8 on the evanescent part of the optical mode, both in terms of extraction rate and emission angle. Indeed, in this respect, let us recall that the emission angle θ(x) of the light radiation by a diffraction grating 8 can be determined by the following relation: sin θ x = n eff x + m λ Λ n c where m is the diffraction order.

[0049] It therefore appears that to keep the emission angle θ(x) equal to the target value θ c and constant along the longitudinal axis X, it is important to also define a longitudinal variation Λ r (x) of the pitch Λ r of the diffraction grating 8.

[0050] The optical antenna 7 therefore comprises a diffraction grating 8 which is not produced directly in the waveguide 5, but is located above and at a distance from that along the vertical axis Z. It is therefore spaced from it by a non-zero distance dr defined as the distance between the upper surface of the waveguide 5 and the lower surface of the periodic structures of the diffraction grating 8. Note that this distance dr can be zero. In any case, the diffraction grating 8 is not produced in the waveguide 5, for example by localized partial etching thereof.

[0051] The diffraction grating 8 is formed of periodic structures arranged along the waveguide 5 along the longitudinal axis X. Different types of structures are possible. In this example where the diffraction grating 8 is of the same type as that of the Wang 2021 article mentioned above, the periodic structures are formed of pads 13 of a high refractive index material, here a silicon nitride, separated from each other by a through notch filled with a low refractive index material (identical to that of the cladding surrounding the waveguide 5, and here made of a silicon oxide). The notches are through here but could not be. Alternatively, the diffraction grating 8 could be of the same type as that of the previously mentioned Han 2018 article, i.e. be formed of notches made in the upper face of the cladding layer of the waveguide 5.

[0052] The diffraction grating 8 is defined by physical parameters Pp r which are representative of the diffraction (and therefore of the extraction) of the evanescent part of the optical mode, namely here the spacing dr of the diffraction grating 8 with respect to the waveguide 5, the refractive indices of the structuring, namely here nr for the pads 13 made of silicon nitride and n mr for the material of the filling layer 14, the thickness er of the pads 13, the width wr of the pads 13 along the longitudinal axis X, the pitch Λ r and the filling factor ff r = wr / Λ r defined as the ratio between the width wr and the pitch Λ r .

[0053] Preferably, the refractive index nr , the thickness er and the width wr of the pads 13 remain constant over the entire length L tot of the optical antenna 7. Similarly, the notches remain through. The filling factor ff r remains constant here, but it could also have a longitudinal variation. On the other hand, the diffraction grating 8 has, in the apodized part, a longitudinal variation Λ r (x) of its pitch Λ r between an upstream value Λ r,in and a downstream value Λ r,out . The pitch Λ r thus varies longitudinally according to a predefined function q such that Λ r =q(x). For example, in the case of a Gaussian target profile S c (x), the function q is an increasing monotonic function. We can note in a discretized manner the longitudinal variation Λ r (i) of the pitch Λ r , using an index i varying from 1 to M, where M is the number of periods of the diffraction grating 8 in the apodized part.

[0054] The diffraction grating 8 is located at a distance dr from the waveguide 5 so as to weakly impact the optical mode, and here only its evanescent part, this distance dr being able to be zero or non-zero. It has an extraction rate α, sometimes also called emission force in French, and emission strength Or scattering strength in English. As stated in the article by Zhao et al. entitled Design principles of apodized grating couplers, Journal of Lightware Technology, vol. 38, no. 16, pp. 4435-4446, 2020, the extracted local optical power, which defines the emission profile S(x), depends on the local power of the optical mode P(x) (here of its evanescent part) and the extraction force α(x) by the following relation: S(x) ~ α(x)×P(x).

[0055] The optical antennas 7 are identical to each other. They all have the same reference structural configuration Cs ref , which is formed by the physical parameters Pp wg of the waveguide 5 (and therefore by the longitudinal variation wc (x)) and by the physical parameters Pp r of the diffraction grating 8 (and therefore by the longitudinal variation Λ r (x)). The optical antennas 7 are arranged laterally along the Y axis and parallel to each other. They are spaced from each other by a distance preferably between λ / 2 and 2λ. For information purposes, the number of optical antennas 7 can range from about ten to about ten thousand, so as to limit the divergence of the light radiation in the far field in the YZ plane.

[0056] Thus, the optical signals circulating in the arms 4 are progressively transmitted into free space by diffraction by the optical antennas 7, so that the light radiation emitted by each optical antenna 7 has the target emission profile S c (x) and the target emission angle θ c constant along the longitudinal axis X. The emitted light radiation propagates in free space, recombines by interference, and thus forms in the far field the light radiation emitted by the optoelectronic transmitter 1 whose angular distribution around the main emission axis is determined and defines the far-field emission pattern of the optoelectronic transmitter 1.

[0057] Thus, the optoelectronic transmitter 1 emits far-field light radiation that may be slightly divergent at least in the XZ plane, and have a predefined emission pattern, for example constant or Gaussian. This is therefore obtained by the fact that each optical antenna 7 comprises a diffraction grating 8 located above and at a distance from the waveguide 5, which makes it possible to reduce the extraction rate α r by impacting only the evanescent part of the optical mode circulating in the waveguide 5, and therefore to increase the emission length of the optical antenna 7. In addition, the local value of the extraction rate α r can be easily adjusted via the width wc of the waveguide 5, unlike the apodized diffraction gratings of the prior art where the authors only modify the dimensions of the periodic structures, as shown for example in the article by Mekis et al.entitled A Grating-Coupler-Enabled CMOS Photonics Platform, IEEE Journal of Selected Topics in Quantum Electronics, vol. 17, no. 3, pp. 597-608, May-June 2011. Finally, by longitudinally varying the width wc of the waveguide 5 and at least the pitch Λ r of the diffraction grating 8, we limit the dimensioning constraints of the periodic structures which could lead to dimensions (width wr of the pads 13 and / or the notches) which are particularly small, for example less than 100nm, and therefore little or not compatible with the conventional technologies usually used in manufacturing processes for example in silicon photonics.

[0058] There Figure 2Cis a top view of an optical antenna 7 of an optoelectronic transmitter 1 according to an alternative embodiment. In this example, the diffraction gratings 8 share the same pads 13. In other words, the pads 13 of the diffraction gratings 8 extend continuously opposite the waveguides 5 of the optical antennas 7. Furthermore, the waveguides 5 have a longitudinal variation wc (x) of a different shape from that illustrated in the fig.2B .

[0059] There figure 3 is a flowchart of a method of manufacturing an optoelectronic transmitter 1 according to one embodiment.

[0060] In a step 10, the target emission profile S c (x) in the near field of the light radiation to be emitted by each optical antenna 7 is defined, as well as the target emission angle θ c . For example, the emission profile S c (x) is Gaussian. Furthermore, the target emission angle θ c is constant for any value x of the longitudinal axis X. It is considered here that the optical mode has a wavelength λ equal for example to 1550nm.

[0061] In addition, the same reference structural configuration Cs ref is defined for the optical antennas 7. This structural configuration Cs ref includes the values ​​of the physical parameters Pp wg of the waveguide 5 which define the optical properties of transmission of the optical mode by the waveguide 5, namely the refractive indices nc and ng of the waveguide 5 and the cladding, and the thickness ec of the waveguide 5, so that we have: Pp wg = {nc , ng , ec}. For example, the waveguide 5 can be made of silicon and have a refractive index nc of 3.48, and the cladding can be made of SiO 2 and have a refractive index ng of 1.45 at the wavelength λ of 1.55 µm. Furthermore, the waveguide 5 here has a constant thickness ec equal to 220 nm. The list of these physical parameters Pp wg will be completed by the longitudinal variation wc (x) of the width wc of the waveguide 5, which is determined later.

[0062] The structural configuration Cs ref also includes the values ​​of the physical parameters Pp r of the diffraction grating 8 which define the optical properties of diffraction of the optical mode by the diffraction grating 8. Thus, the physical parameters Pp r may include the refractive index nr of the high-index pads 13, here equal to 2.0 for silicon nitride, the refractive index of the filling material n mr , here equal to 1.45 for SiO 2 , the thickness er of the pads 13, for example equal to 400nm, and the spacing dr with the waveguide 5, for example equal to 200nm. The filling factor ff r is here constant along the longitudinal axis X and is here equal to 0.5. Knowing further the longitudinal variation Λ r (x) of the pitch Λ r the filling factor ff allows to determine the width wr of the plots 13.The list of these physical parameters Pp r will be completed by the longitudinal variation Λ r (x) of the pitch Λ r of the diffraction grating 8, which will be determined later.

[0063] In a step 20, a relationship Λ r =f(wc ) is determined expressing an evolution of the pitch Λ r of the diffraction grating 8 as a function of the width wc of the waveguide 5, taking into account the structural configuration Cs ref , so that the local emission angle θ is equal to the target emission angle θ c for all x along the longitudinal axis X.

[0064] To this end, we carry out several three-dimensional simulations of an optical antenna 7 by finite differences in the time domain (FDTD, for Finite Difference Time Domain,in English). In each simulation, the optical antenna 7 has the reference structural configuration Cs ref and a given pair of constant values ​​of width wc and pitch Λ r . Also, the optical antenna 7 does not have any longitudinal variation of its width wc or its pitch Λ r (it is not apodized). For each simulation, the value of the emission angle θ of the simulated optical antenna 7 is determined.

[0065] We thus obtain a relationship between the emission angle θ of the simulated optical antenna 7 as a function, for example, of the pitch Λ r for different values ​​of width wc . Then, for a value of the simulated emission angle θ equal to the target value θ c , we obtain the relationship Λ r =f(wc ) for which the optical antenna 7 emits the light radiation at the target emission angle θ c .

[0066] In this respect, the Figure 4Aillustrates an example of the evolution of the pitch Λ r of the diffraction grating 8 as a function of the width wc of the waveguide 5, for the reference structural configuration Cs ref defined previously, and for which the emission angle θ of the simulated optical antenna is equal to a target value θ c , here equal to 8°. The simulations are carried out to scan a range of values ​​of the width wc , between a minimum value here equal to 350nm and a maximum value here equal to 600nm.

[0067] During a step 30, a relationship α r =g(wc ) is determined expressing an evolution of an extraction rate α r of the diffraction grating 8 as a function of the width wc of the waveguide 5, taking into account the reference structural configuration Cs ref and the relationship Λ r =f(wc ) determined previously (therefore for which the emission angle θ(x) is equal to the target value θ c ).

[0068] To this end, several three-dimensional simulations of an optical antenna 7 are carried out here by finite differences in the time domain (FDTD). In each simulation, the optical antenna 7 has the reference structural configuration Cs ref and a constant value of width wc , and the corresponding pitch Λ r is deduced from the relation Λ r =f(wc ). Also, the optical antenna 7 does not have any longitudinal variation in its width wc or its pitch Λ r (it is not apodized), and we know that the corresponding emission angle is equal to the target value θ c . For each simulation, the value of the extraction rate α r,sim of the simulated optical antenna 7 is determined, i.e. the ratio of the value of the intensity of the diffracted optical mode to the value of the intensity of the optical mode introduced into the waveguide 5.

[0069] We thus obtain the relation α r =g(wc ) between the extraction rate α r of the simulated optical antenna 7 as a function of the width wc , for which the pitch Λ r respects the relation Λ r =f(wc ) for which the optical antenna 7 emits the radiation emitted at the target emission angle θ c .

[0070] In this respect, the Figure 4B illustrates an example of the evolution of the extraction rate α r (here 1-α r ) of the diffraction grating 8 as a function of the width wc of the waveguide 5, for the reference structural configuration Cs ref defined previously, and for which the emission angle θ of the simulated optical antenna is equal to a target value θ c equal to 8°. The simulations are carried out for the width range wc from 350nm to 600nm.

[0071] During a step 40, a longitudinal variation wc (x) of the width wc of the waveguide 5 is determined, resulting in a longitudinal variation α r (x) of the extraction rate α r for which the emission profile S(x) is equal to the target emission profile S c (x).

[0072] To do this, using a so-called theoretical approach, we start by determining the longitudinal variation α r,c (x) of the target extraction rate α r,c for which the emission profile S(x) is equal to the target emission profile S c (x). We can use the theoretical relationship identical or similar to that described in the article by Zhao 2020 mentioned above, namely: α r , c = S c x 2 ∫ x ∞ S c x dx

[0073] Then, knowing the longitudinal variation α r,c (x) of the target extraction rate α r,c , the longitudinal variation wc,c =p(x) of the width wc of the corresponding waveguide 5 is determined, from the relationship α r =g(wc ) determined previously, as well as the longitudinal variation Λ r =q(x) from the relationship Λ r =f(wc ) determined previously. However, although this 'theoretical' approach can be used within the framework of the invention, there is a risk that the longitudinal variation wc,c (x) of the width wc obtained does not entirely correspond to the technological constraints, and / or to the values ​​wc,in , wc,out of the width of the waveguide 5 upstream and downstream of the apodized part (for example here 350nm and 600nm).

[0074] Also, another advantageous, more 'pragmatic' approach can be used. We first define an equation representative of a longitudinal variation wc (x) of the width wc of the waveguide 5 between two predefined values ​​upstream wc,in and downstream wc,out. We want this equation to include few parameters to adjust (few degrees of freedom) but to cover a wide range of waveguide shapes 5 so that the corresponding emission profile is equal to the target emission profile S c (x). Preferably, we choose the power function with exponent n, expressed here in a continuous form and in a discretized form: w c x = w c , in − x L a n × w c , in − w c , out ⇔ w c i = w c , in − i M n × w c , in − w c , out where x is the abscissa which varies between x 0 =0, i.e. the beginning of the apodized part of the optical antenna 7 where the width is wc,in and L a , i.e. the end of the apodized part where the width is wc,out , and where i is the number of the period considered of the diffraction grating 8, and M the total number of periods of the apodized part, and where finally n is a free parameter to be determined.

[0075] A parametric study is then carried out by varying the parameter n here. In this example, we consider that the apodized part of the optical antenna 7 is formed of M=1000 periods, followed by a part of 1000 additional periods where the width wc and the pitch Λ r remain constant (width equal to the value wc,out ). We carry out N FDTD simulations of the optical antenna 7, where the parameter n takes a value n (m), where m is here an index varying from 1 to N associated with the simulation carried out. Thus, for each simulation of index m, we obtain a longitudinal variation wc(m) =p (m) (x) of the width wc(m) from the previous equation where the parameter n takes a value n (m). We also deduce the longitudinal variation Λ r(m) =q (m) (x) from the relation Λ r =f(wc ) determined previously.

[0076] We then determine the longitudinal variation α r(m) (x) of the extraction rate from the relation α r =g(wc ), taking into account the longitudinal variations wc(m) =p (m) (x) and Λ r(m) =q (m) (x), and we then determine the corresponding emission profile S (m) (x). We then determine a similarity parameter of the emission profile S (m) (x) with the target emission profile S c (x), in other words a parameter of deviation between these two profiles, for example from an overlap integral. We finally choose the optimal value n (mopt) of the parameter n which gives the best value of the similarity parameter, i.e. which minimizes the deviation between these two profiles.We thus obtain the optimal longitudinal variation wc =p (mopt) (x) of the width wc of the waveguide 5, as well as the corresponding longitudinal variation Λ r =q (mopt) (x) of the pitch Λ r of the diffraction grating 8, for which the corresponding emission profile S(x) is equal to the target emission profile S c (x) (in the sense of the best similarity) with an emission angle θ(x) equal to the target value θ c constant for all x. Thus, according to this 'pragmatic' approach, we are assured that the local values ​​of the width wc , associated with the local values ​​of the pitch Λ r , effectively respect the technological constraints as well as the desired values ​​wc,in , wc,out of the width of the waveguide 5 upstream and downstream of the apodized part.

[0077] Finally, during a step 50, the optoelectronic transmitter 1 is manufactured for which the optical antennas 7 all have the same structural configuration Cs ref , this being therefore completed by the longitudinal variation wc =p(x) of the width wc of the waveguide 5 and by the longitudinal variation Λ r =q(x) of the pitch of the diffraction grating 8 which have just been determined.

[0078] There Figure 4Cillustrates the emission profile S(x) of such an optical antenna 7, as well as the target emission profile S c (x). In this example, the optical antenna 7 comprises an apodized part of M=1000 periods (length L a of approximately 750µm) followed by a non-apodized part of 1000 additional periods (of a length of 850µm), with the reference structural configuration indicated previously. In the apodized part, the width wc varies between the upstream value wc,in equal to 600nm and a downstream value wc,out of 350nm. The filling factor ff r remains equal to 0.5 along the longitudinal axis X, and the parameter n is equal to 0.3. In addition, the target emission angle is here equal to 8°. In the apodized upstream part going here from x=0 to x=L a =750nm approximately, the emission profile S(x) is Gaussian and presents a good similarity with the target Gaussian profile S c (x).In the non-apodized downstream part, the emission profile shows an exponential decay, which also corresponds here with the target profile. The transition in the optical antenna 7 between the apodized part and the non-apodized part is represented by a dotted vertical line.

[0079] Thus, the manufacturing method makes it possible to produce an optoelectronic transmitter 1 in which each optical antenna 7, by the longitudinal variations of the width wc of the waveguide 5 and the pitch Λ r of the diffraction grating 8 over at least a portion of the length of the optical antenna 7, emits light radiation having the desired near-field emission profile and emission angle. The far-field emission pattern thus corresponds to the desired pattern, and is little divergent at least in the XZ plane. In addition, it avoids having to produce a diffraction grating having dimensions that are not or only slightly compatible with the constraints of the usual manufacturing processes for photonic platforms.

[0080] Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art.

[0081] Thus, the manufacturing method may also include a phase of adjusting the filling factor ff, or even determining a longitudinal variation ff(x), a phase of adjusting the length of the apodization part, or other physical parameters. As indicated previously, the emission profile may be of different types, for example Gaussian or constant. Furthermore, the diffraction grating 8 may include, as here, high-index pads 13, or even, among other things, be formed by notches made on the surface of the cladding layer.

Claims

1. A process for fabricating an optical phased array transmitter (1) comprising: ∘ a splitter (3), intended to be coupled to a laser source (2); ∘ a plurality of waveguides (5), coupled to the splitter (3) and extending along a longitudinal axis in a main plane, forming arms (4) of the optoelectronic transmitter (1); ∘ a plurality of phase shifters (6) and optical antennas (7), arranged in the arms (4), each optical antenna (7) being formed by the corresponding waveguide (5) and by a diffraction grating (8) located above and at a distance from the waveguide (5) along a vertical axis orthogonal to the main plane; ∘ the process comprising the following steps: ∘ defining (10) a target near-field emission profile Sc(x) and a target emission angle θc for light radiation emitted by each optical antenna (7), and a structural configuration Csref of said optical antennas (7), comprising: values of physical parameters Ppwg of the waveguides (5) defining optical properties of transmission of an optical mode from the laser source (2); and values of physical parameters Ppr of the diffraction gratings (8) defining optical properties of diffraction of the optical mode; ∘ determining (20) a relationship Λr=f(wc) expressing a change in the pitch Λr of periodic structures of the diffraction grating (8) as a function of a width wc of the waveguide (5), such that, taking into account said structural configuration Csref, an emission angle θ(x) of the light radiation emitted by the optical antenna (7) is equal to said target emission angle θc; o determining (30) a relationship αr=g(wc) expressing a change in an extraction rate αr of the diffraction grating (8) as a function of the width wc of the waveguide (5), taking into account said relationship Λr=f(wc) and said structural configuration Csref, ∘ determining (40) a longitudinal variation wc(x) in the width wc of the waveguide (5) and deducing a corresponding longitudinal variation Λr(x) in the pitch Λr of the diffraction grating (8) based on said relationship Λr=f(wc), such that, taking into account said relationship αr=g(wc) and said structural configuration Csref, a near-field emission profile S(x) of the light radiation emitted by the optical antenna (7) is equal to said target emission profile Sc(x); ∘ fabricating (50) the optoelectronic transmitter (1) whose optical antennas (7) have the reference structural configuration Csref, supplemented by said longitudinal variation wc(x) in the width wc of the waveguide (5) and said longitudinal variation Λr(x) in the pitch Λr of the diffraction grating (8).

2. The fabrication process as claimed in claim 1, wherein the steps of determining the relationships Λr=f(wc) and αr=g(wc) are carried out for a range of widths wc ranging from a predefined minimum value wc,out and a predefined maximum value wc,in.

3. The fabrication process as claimed in claim 2, wherein the step of determining the longitudinal variation wc(x) comprises the following steps: • defining a power function with an exponent n representative of a longitudinal variation wc(x) in the width wc between the predefined maximum value wc,out and the predefined minimum value wc,in; • determining, for multiple values n(m) of the exponent n, a longitudinal variation wc=p(m)(x) in the width wc of the waveguide (5) and deducing a corresponding longitudinal variation Λr=q(m)(x) in the pitch Λr of the diffraction grating (8); and • determining a longitudinal variation αr(m)(x) in the corresponding extraction rate αr based on said relationship αr=g(wc), and then a corresponding emission profile S(m)(x); and then • determining an optimum value n(mopt) from among the values n(m) of the exponent n for which the emission profile S(mopt)(x) exhibits a minimum deviation from the target emission profile Sc(x).

4. The fabrication process as claimed in claim 3, wherein the power function is wc(x) = wc,in + (x / La)n×(wc,in - wc,out), where La is the total length of the part of the optical antenna (7) that exhibits the longitudinal variations in the width wc of the waveguide (5) and the pitch Λr of the diffraction grating (8).

5. An optical phased array transmitter (1) comprising: ∘ a splitter (3), intended to be coupled to a laser source (2); o a plurality of waveguides (5), coupled to the splitter (3) and extending along a longitudinal axis in a main plane, forming arms (4) of the optoelectronic transmitter (1); ∘ a plurality of phase shifters (6) and optical antennas (7) arranged in the arms (4), each optical antenna (7) being formed by the corresponding waveguide (5) and by a diffraction grating (8) located above and at a distance from the waveguide (5) along a vertical axis orthogonal to the main plane; ∘ characterized in that, over at least part of the length of each optical antenna (7): • the waveguide (5) has a width wc=p(x) that varies longitudinally according to a predefined function p; • the diffraction grating (8) has an arrangement pitch Λr=q(x) of periodic structures that varies longitudinally according to a predefined function q; - the functions p and q being predefined such that a near-field emission profile S(x) of the light radiation emitted by the optical antenna (7) is equal to a predefined target emission profile Sc(x), and that a local emission angle θ(x) of the emitted light radiation is equal to a predefined, longitudinally constant target emission angle θc.

6. The optoelectronic transmitter (1) as claimed in claim 5, wherein the function p regarding longitudinal variation in the width wc is a decreasing function, and the function q regarding longitudinal variation in the pitch Λr is an increasing function.

7. The optoelectronic transmitter (1) as claimed in claim 5 or 6, wherein each of the periodic structures extends facing all of the waveguides (5) of the optical antennas (7).

8. The optoelectronic transmitter (1) as claimed in any one of claims 5 to 7, wherein the waveguides (5) and the diffraction gratings (8) are produced in a silicon-based photonic chip.

9. The optoelectronic transmitter (1) as claimed in any one of claims 5 to 8, wherein the periodic structures of the diffraction gratings (8) have a vertical dimension, along the vertical axis, that is constant along the longitudinal axis of the optical antennas (7).

10. The optoelectronic transmitter (1) as claimed in any one of claims 5 to 9, wherein the periodic structures of the diffraction gratings (8) have a filling factor, defined as the ratio between a transverse dimension of the periodic structures along the longitudinal axis and the pitch Λr, that is constant along the longitudinal axis of the optical antennas (7).