Optoelectronic transmitter with phased array antenna and integrated control device

Abstract

The present invention relates to a photoelectron emitter with a phased array antenna on a photonic chip, comprising a power splitter (3), an array of phase shifters (5) and elementary emitters (6), and an integrated control device. The integrated control device includes an interference focusing lens (8) whose entrance (8.1) and exit (8.3) faces are curved and define a free propagation region (8.2) with a uniform refractive index. An input waveguide (7) is connected to the entrance face (8.1) perpendicular to the entrance face (8.1) and has an effective refractive index for the guided modes configured so that the optical paths of the input waveguide (7) are identical to each other.

Description

[Technical field]

[0001] The field of the invention is that of on-chip photonic circuits, and more particularly that of phased array antenna type photoelectron emitters fabricated on a photonic chip, the invention being particularly applicable in the field of LIDAR (Light Detection and Ranging). [Background technology]

[0002] Photoelectron emitters with phased array antennas (or OPA (Optical Phased Array)) are optoelectronic devices that emit monochromatic light beams directionally into free space. They are particularly applied in the field of Laser Ranging and Estimation (LIDAR), but also in the fields of free-space optical communications, holographic screens and medical imaging.

[0003] Fig. 1A shows the working principle of such a photoelectron emitter 1 diagrammatically. A laser source 2 outputs an optical signal that is distributed by a power splitter 3 in N arms 4 of the photoelectron emitter 1. Each arm 4 comprises a phase shifter 5 and an elementary emitter 6, also called an optical antenna. Each elementary emitter 6 outputs an optical signal in free space, for example by diffraction, which are then combined by interference to form an optical beam. This optical beam has a far-field emission pattern that is determined, among other things, by the relative phase Δφ applied by the phase shifter 5 to the optical signals propagating in the arms 4.

[0004] Such an optoelectronic emitter 1 can be manufactured using integrated photonics, i.e. its various optical components (waveguides, power splitters, elementary emitters, etc.) are manufactured on and from a photonic chip.

[0005] For this purpose, FIG. 1B shows, in a schematic and partial representation, an example of a photoelectron emitter 1 fabricated on a photonic chip and including an integrated control device, as described in the article Fully integrated hybrid silicon two dimensional beam scanner by Hulme et al. in Opt. Express 23 (5), 5861-5874 (2015).

[0006] This photoelectron emitter 1 comprises a control device consisting of a laser source 2, a power splitter 3, a phase shifter 5 and an array of elementary emitters 6, as well as a gradient index lens 8A coupled to the output end of the elementary emitters 6 at the entrance face 8A.1 and an array of photodetectors 10 at the exit face 8A.2. This control device allows the relative phase Δφ actually existing between the optical signals propagating in the arm 4 to be determined. For this purpose, the non-diffracted part of these optical signals is transmitted by the elementary emitters 6 to the lens 8A by the input waveguide 7. The lens 8A provides an image of the far-field optical emission emitted by the photoelectron emitter and then detected by the photodetector 10. A control module 11 allows the optical far-field information to be obtained (in particular the angle Φ of the directivity of the optical beam relative to the Y axis in the XY plane) and allows the photoelectron emitter 1 to be calibrated.

[0007] However, gradient index lenses are of the photonic crystal type and are formed by multiple holes of different subwavelength diameters formed in the silicon thin film of an SOI substrate. The holes are arranged so that the effective refractive index of the optical field has a parabolic profile along the width of the lens. However, these holes, which form the photonic crystal of the lens, are particularly difficult to manufacture, and any manufacturing defects are likely to significantly affect the performance of such photoelectron emitters. Summary of the Invention [Problem to be solved by the invention]

[0008] The object of the present invention is to at least partially overcome the drawbacks of the prior art, and more specifically to propose an optoelectronic emitter with a phased array antenna on a photonic chip, in which the influence of the manufacturing method on its performance capabilities is reduced or eliminated. [Means for solving the problem]

[0009] To this end, the subject of the present invention is a photoelectron emitter with a phased array antenna on a photonic chip, comprising: - a power splitter intended to be coupled at the input to a laser source and at its output to N waveguides, N>1, forming the arms of a photoelectron emitter; - an array of phase shifters and elementary emitters arranged in the arms, the elementary emitters being laterally aligned and extending parallel to one another; - an integrated controller including an input waveguide coupled to the elementary emitter; a focusing lens including an entrance face and an exit face coupled to the input waveguide; an output waveguide coupled to the exit face; a photodiode coupled to the output waveguide; and a control module configured to control the phase shifter as a function of a signal received by the photodiode.

[0010] In accordance with the invention, the focusing lens is an interference lens whose entrance and exit faces are curved and define a free propagation region having a uniform refractive index, and whose exit face extends along the focal line of the focusing lens.

[0011] The input waveguide is connected to a plane of incidence perpendicular to the input waveguide.

[0012] Finally, the effective refractive index of the guided modes in the so-called compensation section of the input waveguide is configured such that the optical paths of the input waveguide are identical to each other.

[0013] Some preferred but non-limiting aspects of this photoelectron emitter are as follows:

[0014] Each of the input waveguides may have a change in at least one lateral dimension in the compensation section, said change in lateral dimension being predetermined for each input waveguide such that the optical paths of the input waveguides are identical to one another.

[0015] Each input waveguide is adjusted in the compensation section to a reference value W e0 From the above, the maximum value W ec(i) The thermal insulation can have a range of up to

[0016] The photoelectron emitter may include a waveguide, called the additional waveguide, having a refractive index different from that of the input waveguide, the additional waveguide extending only within the compensation section and arranged opposite and parallel to the input waveguide to allow mode coupling with the input waveguide, each of the additional waveguides having a predetermined length such that the optical paths of the input waveguides are identical to each other.

[0017] An additional phase shifter can be coupled to the input waveguides at the correction section and can apply a predetermined relative phase shift between the guided modes in the input waveguides for each of the input waveguides such that the optical paths of the input waveguides are identical to one another.

[0018] The focusing lens can be a star coupler or an echelle grating.

[0019] The focusing lens can have a Rowland mounting or a confocal configuration.

[0020] Photonic chips can be fabricated from SOI substrates.

[0021] The photoelectron emitter may include a laser source, which may be fabricated on a surface of the photonic chip, referred to as the top surface. [Brief description of the drawings]

[0022] Further aspects, objects, advantages and features of the present invention will become more apparent on reading the following detailed description of preferred embodiments of the invention, given by way of non-limiting example and with reference to the accompanying drawings, in which: [Figure 1A] FIG. 1A, already described, is a schematic and partial view of a photoelectron emitter with a phased array antenna. [Figure 1B] FIG. 1B, already mentioned, is a schematic and partial top view of such a photoelectron emitter including an integrated control device according to an example of the prior art. [Diagram 2] FIG. 1 is a schematic and partial top view of a photoelectron emitter with a phased array antenna according to an embodiment in which the interference lens is a star coupler. [Figure 3A] FIG. 3 is a schematic and partial top view of a portion of the photoelectron emitter shown in FIG. 2 illustrating the focusing of the optical field in three different regions of the exit surface of the interference lens. [Figure 3B] FIG. 3 is a schematic and partial top view of a portion of the photoelectron emitter shown in FIG. 2 illustrating the focusing of the optical field in three different regions of the exit surface of the interference lens. [Figure 3C] FIG. 3 is a schematic and partial top view of a portion of the photoelectron emitter shown in FIG. 2 illustrating the focusing of the optical field in three different regions of the exit surface of the interference lens. [Figure 4A] FIG. 3 is a schematic and partial top view of a portion of the photoelectron emitter shown in FIG. 2, illustrating the dimensioning of the interference lens in a Rowland mounting setup. [Figure 4B] FIG. 4B is a detailed view of a region of the entrance surface of the interference lens shown in FIG. 4A. [Figure 4C] FIG. 4B is a detailed view of a region of the exit surface of the interference lens shown in FIG. 4A. [Diagram 5] FIG. 3 is a schematic and partial top view of a portion of the photoelectron emitter shown in FIG. 2, illustrating the longitudinal variation of the width of the input waveguide in the compensation section. [Figure 6] 1 is a schematic partial cross-sectional and exploded view of a portion of a photoelectron emitter according to an alternative embodiment, in which an additional waveguide is coupled to an input waveguide of a correction portion. [Figure 7] FIG. 13 is a schematic and partial cross-sectional and exploded view of a portion of a photoelectron emitter according to another alternative embodiment, in which a thermo-optic phase shifter is disposed in the input waveguide within the correction portion; [Figure 8] FIG. 13 is a schematic and partial top view of a photoelectron emitter with a phased array antenna according to another embodiment in which the interference lens is an echelle grating. [Figure 9A] 13A and 13B show top and cross-sectional views of portions of photoelectron emitters according to two other alternative embodiments in which a phase change section is disposed in the input waveguide of the correction section; [Figure 9B] 13A and 13B show top and cross-sectional views of portions of photoelectron emitters according to two other alternative embodiments in which a phase change section is disposed in the input waveguide of the correction section; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] In the remainder of the figures and description, the same reference numbers represent the same or similar elements. Additionally, the various elements are not drawn to scale to improve the clarity of the figures. Additionally, the various embodiments and alternative embodiments are not mutually exclusive and can be combined together. Unless otherwise specified, the terms "substantially", "approximately" and "to the extent of" mean to the nearest 10%, preferably to the nearest 5%. Additionally, the term "in the range between ... and ..." and equivalents mean that the limits are inclusive unless otherwise specified.

[0024] The present invention relates to a photoelectron emitter with a phased array antenna including an integrated control device. Such a photoelectron emitter is fabricated on a photonic chip. It is configured to emit a light beam, the intensity of which has a determined angular distribution around the main emission axis in the far field. This angular distribution of the far-field light beam emitted by the photoelectron emitter is called the "far-field emission pattern". As such, it differs from the near-field emission pattern of the elementary emitter (optical antenna). The far-field (or Fraunhofer region) is the region of the emission wavelength λ e(in this case, the length L along the Z axis) ee ) is greater than the ratio of the squares of the larger dimensions of the distance D, i.e., D>2L ee 2 / λ e The far-field emission pattern in this case can be adjusted by an integrated controller.

[0025] 2 is a schematic and partial view of a photoelectron emitter 1 according to an embodiment. In this example, the photoelectron emitter 1 includes a laser source 2, a power splitter 3, an array of phase shifters 5, and an elementary emitter 6. It also includes a control device connected to the phase shifters 5, which can determine or change the phase shift Δφ applied to the optical signal circulating in the N-arm 4 to improve the performance capabilities of the photoelectron emitter 1 and in particular to generate a desired emission pattern of the far-field optical beam emitted by the photoelectron emitter 1.

[0026] In this case, for the remainder of the description, a direct Cartesian XYZ coordinate system is defined, in which the XZ plane is parallel to the plane of the photonic chip, the Z axis is oriented along the axis of the elementary emitter 6 and the Y axis is oriented towards free space where the light beam is emitted by the photoelectron emitter 1.

[0027] The photoelectron emitter 1 is, for example, integrated on a photonic chip in the context of photonic-on-silicon technology. The photonic chip, also called photonic integrated circuit (PIC), comprises a supporting substrate (not shown) from which active (modulators, diodes, etc.) and passive (waveguides, multiplexers, demultiplexers, etc.) photonic components, optically coupled to one another, can be produced. In the context of photonic-on-silicon technology, the supporting substrate and the photonic components are manufactured from silicon. The supporting substrate can be of the SOI (Silicon On Insulator) type.

[0028] The photoelectron emitter 1 emits light of wavelength λ eThe photoelectron emitter 1 includes a laser source 2 configured to output a pulsed or continuous monochromatic light signal of . The wavelength can be equal to 905 nm, for example. The laser source 2 can be tunable, in particular to change the emission angle θ that the far-field light beam emitted by the photoelectron emitter forms with respect to the Y axis in the YZ plane. The laser source 2 can be a hybrid light source formed by a gain medium produced from III / V compounds and bonded to the surface of the photonic chip. In this way, a Bragg mirror type optical reflector can be manufactured in a waveguide integrated and coupled to the gain medium. As an alternative embodiment, the photoelectron emitter 1 can be free of the laser source 2, can be remote and therefore not assembled in contact with the surface of the photonic chip. It can then be coupled to the integrated waveguide of the photonic chip, in particular by a grating coupler.

[0029] The power splitter 3 is coupled to the output of the laser source 2. It therefore has an input port and N output ports, N>1. The number N defines the number of arms 4 of the photoelectron emitter 1. In this example, the power splitter 3 is a cascade of multiple MMI (Multimode Interferometer) type splitters, although other optical components can be used.

[0030] The photoelectron emitter 1 comprises an array of N waveguides 4, preferably parallel to one another, each extending between a first end coupled to an output port of a power splitter 3 and a second end coupled to a fundamental emitter 6. Each waveguide 4 is thus adapted to receive an optical signal from the power splitter 3 and to propagate this optical signal to the fundamental emitter 6.

[0031] The photoelectron emitter 1 also includes an array of phase shifters 5 and elementary emitters 6. More specifically, the arms 4 each include at least one phase shifter 5 configured to modify the phase of the optical signal circulating in that arm, thereby generating a phase difference Δφ, i.e. a relative phase, between the optical signals circulating in adjacent arms 4. The phase shifter 5 is arranged between the power splitter 3 and the elementary emitter 6. A phase shifter 5 can be provided in each arm 4 or only in some of the arms, for example in every other arm. Furthermore, the reference arm may not include a phase shifter 5.

[0032] The phase shifter 5 can be an electrorefractive or a thermo-optical effect phase shifter. In both cases, the phase is modified by modifying the refractive index of the material forming the core of the considered waveguide 4. This change in refractive index can be obtained by varying the free carrier density in the case of an electrorefractive phase shifter, or by varying the applied temperature in the case of a thermo-optical phase shifter.

[0033] The phase shifters 5 are preferably configured to apply identical phase values ​​Δφ to the optical signals propagating in the arms 4 to obtain a predetermined non-zero angle Φ of the main emission axis relative to the Y axis in the XY plane (orthogonal to the axis of the elementary emitter 6). However, the relative phases Δφ must not be identical between the arms 4 in order to obtain different far-field patterns or to account for and compensate for any phase errors. These phase errors may be due to degradation of some components of the photoelectron emitter 1 over time, inconsistencies during the manufacturing method, non-zero tolerances of the manufacturing method, effects of the environment of the photoelectron emitter 1 (for example possible effects of packaging elements covering the elementary emitter).

[0034] The phase shifters 5 are preferably connected to a control module 11, which will be described below. Depending on control signals sent by the control module 11, the phase shifters 5 are able to generate predetermined relative phases Δφ in the optical signals circulating in the various arms 4.

[0035] The photoelectron emitter 1 is coupled to an arm 4 and thus comprises N elementary emitters 6 or optical antennas arranged downstream of a phase shifter 5. The relative phase Δφ between the optical signals output by the elementary emitters 6 determines, inter alia, the value of the angle Φ that the main emission axis of the far-field optical beam makes with respect to the Y axis in the XY plane of the photoelectron emitter 1.

[0036] The elementary emitters 6 in this case are diffraction gratings formed in the waveguides of the arms 4. They run parallel to each other along the Z axis and are aligned along the X axis, i.e. the first end is at the same position along the Z axis as the second end. They are offset from each other, preferably by λ e / 2 and 2λ e For reference, the number N of elementary emitters can range from about 10 to about 10,000.

[0037] As a result, a part of the optical signal circulating in the arm 4 is output by diffraction from the elementary emitter 6 in free space. The extracted optical signals propagate in free space and recombine by interference to form the optical beam emitted from the photoelectron emitter 1 in the far field, determining the angular distribution of the optical beam around the main emission axis and defining the far-field emission pattern of the photoelectron emitter 1.

[0038] As is well known, the angle θ between the main emission axis and the Y axis in the YZ plane is e and the period Λ of the diffraction grating formed by the elementary emitters 6. As can be seen, the angle Φ of the main emission axis with respect to the Y axis in the XY plane depends on the value of the relative phase Δφ that the phase shifter 5 applies to the optical signal circulating in the arm 4. In general, it is required that a zero relative phase Δφ results in a zero angle Φ, i.e. the main emission axis of the far-field optical beam is parallel to the Y axis.

[0039] In this case, the extraction efficiency of the basic emitter is less than 100%, and the intensity I out is non-zero. Therefore, the intensity I outThe length L of the basic emitter is set so that ee Alternatively or additionally, a non-zero intensity I out can be obtained by adjusting the etching depth of the teeth of the lattice forming the elementary emitter and / or by adjusting the fill factor of the lattice, i.e. the ratio between the length of the teeth and the length of the recesses, over the period (formed by the teeth and the recesses) of the elementary emitter.

[0040] However, there may be cases where the relative phase value Δφ applied by the phase shifter 5 needs to be controlled or corrected. As mentioned above, this may be the case, for example, of calibrating the photoelectron emitter 1 so that zero relative phase Δφ is in fact zero angle Φ of the main emission axis, i.e. in the XY plane, parallel to the Y axis. This may also be the case of compensating for phase errors resulting from the way some components of the photoelectron emitter are manufactured or aged. This may finally be the case of correcting the value of the angle Φ precisely and in real time, so that the far-field emission pattern has the desired characteristics.

[0041] To this end, the optoelectronic emitter 1 comprises a control device formed by an interferometric focusing lens 8, a number of photodetectors 10 and a control module 11. The control device also comprises N input waveguides 7 providing coupling between the elementary emitters 6 and the interferometric lenses 8, and an output waveguide 9 providing coupling between the interferometric lenses 8 and the photodetectors 10. The control device is said to be integrated to the extent that the waveguides 7, 9, the interferometric lenses 8 and the photodetectors 10 are manufactured on or from a control chip.

[0042] N input waveguides 7 connect the second ends of the elementary emitters 6 with the entrance face 8.1 of the interference lens 8. They are identical in optical path from one input waveguide 7 to the next, so as not to degrade the phase information (Δφ value) carried by the (undiffracted) optical signal transmitted by the elementary emitters 6.

[0043] As will be explained in more detail below, each has at least two portions: a first portion 7.1, where the input waveguide 7 extends parallel to each other from the primary emitter 6, and a second portion 7.2, where the input waveguide 7 extends to the entrance face 8.1 of the interference lens 8.

[0044] In the first section 7.1, the input waveguides 7 can have the same length. In the second section 7.2, they are connected to an input port of an input plane 8.1 that is orthogonal to the plane that is tangent to it, i.e. each input waveguide 7 is connected to an input port of an input plane 8.1 that is orthogonal to the plane that tangent to it.

[0045] However, since the entrance surface 8.1 of the interference lens 8 is curved in the XZ plane, the length of each input waveguide 7 is not the same. e corresponds to the distance between a first end of the input waveguide 7 connected to the elementary emitter 6 and its other second end, which is connected to the entrance face 8.1 of the interference lens 8.

[0046] The focusing interference lens 8 is related to Fourier optics and provides at its exit face 8.1 the Fourier transform of the optical field provided at the entrance face 8.1 by the input waveguide 7 and formed by the optical signal transmitted by the elementary emitter 6. The exit face 8.3 forms the image plane of the interference lens 8, and the diffraction pattern formed at the exit face 8.3 correlates with the far-field emission pattern of the optical beam emitted by the photoelectron emitter 1.

[0047] For this purpose, the interference lens 8 is defined by an entrance surface 8.1, an exit surface 8.3, and a free propagation region (FPR) 8.2 located between the two surfaces 8.1, 8.3. Each of the entrance 8.1 and exit 8.3 surfaces is curved, with the respective radius of curvature located on the optical axis Δ of the interference lens 8 (parallel to the Z axis in this case). The interference lens 8 is planar since it extends in the XZ plane of the photonic chip. In the FPR 8.2, the optical field is contained along the Y axis orthogonal to the XZ plane of the photonic chip, but can propagate freely in the XZ plane. The FPR 8.2 differs from the GRIN lens described in the aforementioned article by Hulme et al. in 2015 in that it is defined by a uniform refractive index medium in the XZ plane. In other words, the FPR 8.2 has a refractive index at any point in this region 8.2 that is preferably the same over its entire thickness and then constant over the entire region 8.2.

[0048] Such an interference lens 8 is also called a star coupler. It focuses the optical signal received at the entrance surface 8.1 to the exit surface 8.3. The optical signals propagate in the FPR 8.2, interfere constructively and focus towards a region or point of the focal line defined by the exit surface 8.3 (image plane). Thus, the spatial distribution of the intensity of the optical field on the exit surface 8.3 correlates with the far-field emission pattern of the photoelectron emitter 1. Thus, the features of the far-field emission pattern are found on the exit surface 8.3 and the angle Φ can be deduced from the angle Φ formed in the far field by the optical beam of the photoelectron emitter with respect to the Y axis and in the XY plane.

[0049] The control device includes M photodetectors 10 coupled to the exit face 8.3 by M output waveguides 9. The number M may be the same as or different from the number N. It, together with the radius of the lens, defines the resolution of the control device as well as the observation angular width of the far-field light beam. However, the resolution of the control device is not limited by the size of the ports of the exit face. In fact, if the beam is located between two ports, its position can be extracted exactly by calculating the ratio of the intensities of the photodiodes connected to two adjacent ports. Thus, the output waveguides 9 extend from the ports of the exit face 8.3 to the inputs of the photodetectors 10. The output ports are therefore located along the focal line of the interference lens 8.

[0050] The control module 11 is connected to the photodetector 10 and advantageously to the phase shifter 5. The photodetector and the phase shifter are controlled from the measurement signal output by the photodetector 10. The photodetector 10 thus receives the optical signal leaving a port of the exit face 8.3 and outputs a measurement signal representative of the optical intensity of the optical field received on the exit face 8.3 to the control module 11. The control module 11 comprises a computer capable of determining, based on the measurement signal, the actual phase shift value Δφ between the optical signals circulating in the arm 4 and in particular the phase shift value Δφ to be applied to the optical signals by the phase shifter 5 to correct the phase error.

[0051] Then, for example, a zero relative phase Δφ is applied, and the angle θ of the peak of the intensity of the optical field received on the exit face 8.3 of the interference lens 8 is cr It is possible to calibrate the photoelectron emitter 1 by determining the angle θ cr is the inclination angle between the optical axis Δ of the interference lens 8 and the line passing through the peak intensity on the exit face 8.3 and the entrance point Pe of the interference lens 8. Therefore, the relative phase Δφ is not zero at any angle θ cr If it is determined that a predetermined relative phase Δφ is induced, a compensation value for the relative phase Δφ can be determined. crBy determining the far-field light beam emitted by the photoelectron emitter, it is also possible to control the photoelectron emitter 1 precisely in real time. Thus, the need to use an external device for detecting and measuring the far-field light beam emitted by the photoelectron emitter is avoided, such an external device comprising a matrix photodetector arranged opposite the elementary emitter along the Y axis. Furthermore, the fact that the interference lens 8 is a lens with a uniform refractive index FPR8.2 prevents manufacturing uncertainties of this lens from significantly affecting the performance of the photoelectron emitter 1.

[0052] 3A-3C are therefore schematic and partial top views of an interference lens 8 and a photodetector 10. In this case, zero relative phase Δφ corresponds to a zero angle Φ of the main emission axis, and the peak intensity of the optical field at the exit face coincides with the optical axis Δ of the interference lens 8 (FIG. 3A). However, if the relative phase Δφ has a non-zero value Δφ1, inducing a non-zero angle Φ1 on the main emission axis, the peak intensity at the exit face will also decrease with a non-zero angle θ cr,1 Finally, if the relative phase Δφ has a non-zero value -Δφ1, inducing a non-zero angle -Φ1 of the main emission axis, the peak intensity at the exit surface will be cr,1 (Figure 3C).

[0053] The interference lens 8 can have a Rowland mounting or a confocal structure. These two variants differ fundamentally in the value of the radius of curvature of the circle that defines the exit surface 8.3. As will be seen later, it can be a star coupler, or an echelle grating, which is a special case of a star coupler.

[0054] Figure 4A is a schematic and partial top view of a portion of a photoelectron emitter according to one embodiment, where a focusing interference lens has a Rowland mounting. Figures 4B and 4C are schematic and partial top views, respectively, of the area Ze of the entrance surface 8.1 and the area Zs of the exit surface 8.3 of the interference lens 8.

[0055] In the Roland mounting, the entrance surface 8.1 has a radius of curvature R crThe exit surface 8.3 is curved with a radius of curvature R cr The sizing of the interference lens 8 depends in particular on the desired detection resolution of the light field focused on the exit face 8.3 and on the angle 2×θ around the optical axis Δ. cr,max The maximum detection aperture angle is defined as

[0056] More specifically, the optical axis Δ of the interference lens 8 in this case extends along the Z axis between the entrance point Pe of the entrance surface 8.1 and the exit point Ps of the exit surface Ps. cr The center of the Rowland circle is located at the exit point Ps and has a radius of R cr The arc of the Rowland circle of radius R defines the plane of incidence 8.1. cr The arc of a circle of .times. / 2 defines an exit surface 8.3, the centre of which lies on the optical axis .DELTA..times ...

[0057] Thus, the angle Φ of the far-field optical beam depends on the relative phase Δφ by the following relationship: Δφ=p ee / λ e ×sinΦ, where p ee is the lateral step between elementary emitters 6 along the X axis (in general, λ e / 2 and 2λ ej As an example, the emission wavelength λ e can be equal to 0.905 μm, and the lateral step p ee can be equal to about 1.6 μm.

[0058] The deflection angle θ formed by the peak intensity of the optical field focused on the exit surface 8.3 cr is sinθ cr =p ee / (λ e ×p cr,e ×β FPR ) ×sinΦ, where p cr,e is the lateral step between the ports on the entrance face 8.1, and β FPRdepends on the far-field angle Φ by the relation that Φ is the propagation constant at FPR8.2. This relation can be deduced from the article "PHASAR-Based WDM-Devices: Principles, Design and Applications, IEEE J Sel. Top." by Smit & van Dam in Quantum Electron, 2 (2), 236-250 (1996). As an example, the lateral step p cr,e can be equal to approximately 1.2 μm (in this case N=16), and the constant β FPR is approximately 12.31 μm -1 The propagation constant β FPR is 2π×n eff,FPR / λ e where n eff,FPR is the effective refractive index of the optical mode at the FPR, which can be determined by finite element simulations.

[0059] In general, the effective refractive index n associated with the optical mode supported by the waveguide eff 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 and the properties of the waveguide (refractive index and lateral dimensions). The effective index of an optical mode corresponds in some way to the refractive index of the waveguide as "seen" by the optical mode. It is generally included between the refractive index of the core and the refractive index of the cladding of the waveguide.

[0060] Next, the lateral step p cr,s The output port has an output lateral dimension d extending along the X-axis. cr,s , which is the maximum angle Φ max and radius R cr Depends on d cr,s =(2×R cr ×p ee ) / (λ e ×p cr,e ×β FPR )×sinΦ max Thus, as an example, the radius R cris approximately equal to 310 μm, and the maximum angle Φ max is equal to approximately 30°, the exit lateral dimension d cr,s In this case, 25 output ports are arranged with a lateral step p equal to about 1.5 μm. cr,s It is possible to arrange it in the following way.

[0061] Furthermore, the radius R cr The choice of is based on the desired resolution (p cr,s value) and the desired maximum detection aperture angle 2×θ cr,max This makes it unnecessary to manufacture a gradient index lens (GRIN lens), which is technically difficult to obtain and whose manufacturing tolerances may degrade the performance of the photoelectron emitter 1, as in the above-mentioned conventional example. In contrast, by forming the interference lens 8 with two curved surfaces 8.1, 8.3 that define an FPR 8.2 having a uniform refractive index, it is possible to simplify the manufacturing method while suppressing the effect of manufacturing tolerances on the performance of the photoelectron emitter 1.

[0062] However, the input waveguides 7 are locally connected orthogonally to the entrance face 8.1, i.e. each input waveguide 7 is connected to an exit point Ps (of radius R cr 8.2) at the entrance face 8.1 toward the center of the circle of the input waveguide 7. As a result, the input waveguide 7 has the same physical length L e Does not have L e(i) denotes the physical length of the input waveguide 7 in row i, where i ranges from 1 to N, and rows 1 and N are located at the edges of the array of input waveguides 7.

[0063] The input waveguides 7 then have a correction section Pc of the effective refractive index of the guided mode so that the optical paths are identical between the input waveguides. In general, the optical path of a waveguide is equal to the product of its length (physical) and the effective refractive index of the guided mode. This correction section Pc is preferably located in the first section 7.1 of the input waveguide 7. eff,ec(i) is the effective refractive index of the input waveguide 7 at the correction portion Pc, and n eff,e0(i)is the effective refractive index of the input waveguide 7 outside the compensation section Pc. (i) =n eff,ec(i) ×L ec +n eff,e0(i) ×(L e -L ec ) can also be obtained.

[0064] Optical path C of input waveguide 7 (i) In order to make the effective refractive index of the guided mode in each input waveguide identical, several possibilities can be implemented. The first possibility (shown in FIG. 5) is to add a length L ec(i) The first possibility involves modifying the lateral dimensions of the input waveguides over a given length. The second possibility (shown in FIG. 6) involves performing mode coupling between each input waveguide 7 and an additional waveguide 12 located on the opposite side of the Y axis and having a refractive index different from that of the input waveguide 7 to obtain a supermode locally and over a given length. The third possibility (shown in FIG. 7) involves actively modulating the effective refractive index of the guided mode in each input waveguide 7 by means of an additional thermo-optic phase shifter (heater) 14.

[0065] FIG. 5 is a schematic and partial top view of some input waveguides 7 in the compensation section Pc.

[0066] Outside the compensation section Pc, the input waveguide 7 has a thickness e e and width W e0 and have the same lateral dimension, which is the length L e(i) -L ec(i) where L e is the total length of the input waveguide of row i between the downstream end of the elementary emitter 6 and the entrance face 8.1, where L ec(i) is the length of the compensation section Pc for this same input waveguide. The length L e(i) It has been shown above that the length L varies for each input waveguide 7 to the extent that the entrance face 8.1 is curved and these input waveguides 7 emerge orthogonally thereon. ec(i)can be the same or different from one input waveguide to the next.

[0067] In the correction section Pc, the optical path C of the input waveguide 7 (i) The maximum lateral dimension of the input waveguide 7, here the width W ec(i) differs for each input waveguide 7.

[0068] More specifically, the input waveguide 7 in this case is W e0 From W ec(i) Width W to the point, and length L from the point ec(i) Thus, the input waveguide in row 1 has a gradual change in width W over the ec(1) / 2 Over the range, the value W e0 From W ec(1) Then, the linear expansion of the width in the range from ec(1) / 2 Throughout, W ec(1) From W e0 The linear shrinkage ranges from ec(i) / 2 is selected so that the width change is adiabatic, i.e., there is no optical loss. It may be equal to at least about 25 μm. ec(i) Since the effective refractive index of the guided mode varies for each input waveguide 7, the effective refractive index varies for each input waveguide 7. (i) is determined to be the same over the entire length. In other words, the longitudinal variation of the width W is adjusted as follows: TIFF0007660126000001.tif1698 where A is a non-zero constant that is the same for all input waveguides.

[0069] For example, the input waveguide 7 has an emission wavelength λ of 905 nm. e The refractive index of is 2, the thickness is 300 nm, and the width W e0 The optical fiber can be formed with a silicon nitride core with a diameter of 600 nm. The core is surrounded by a cladding made of silicon oxide with a refractive index equal to 1.45. In this case, the fundamental polarization guided mode TE 00 The effective refractive index n eff,e0is 1.72. However, the width W ec(i) is equal to 1 μm, the effective refractive index n eff,ec(i) is equal to 1.78.

[0070] In addition, the local width W e The change in the longitudinal direction of the effective refractive index of the guided mode, n eff,ec (i) induced change in the optical path C of the input waveguide 7. (i) This limits or isolates the degradation of the phase information associated with the guided modes, so that the angular distribution of the intensity of the optical field focused on the exit face 8.3 of the interference lens 8 actually correlates with the far-field emission pattern of the photoelectron emitter 1.

[0071] This is because the lateral step of the input waveguide is generally limited, for example, in this case, to about 1.5 μm, and in particular, the lateral step p ee (λ e / 2 and 2λ e This is particularly advantageous in that the length L for correcting the optical path deviation of the input waveguide 7 depends on the e(i) The change is insufficient.

[0072] 6 is a schematic and partial cross-sectional view of several input waveguides 7 in a correction part Pc according to an alternative embodiment of the photoelectron emitter 1. The input waveguides 7 are shown in an exploded view.

[0073] In this embodiment, the additional waveguide 12 is positioned opposite the input waveguide 7 along the Y axis to allow mode coupling between the two waveguides 7, 12. The additional waveguide 12 has a different refractive index than the input waveguide 7, preferably higher than the input waveguide. Preferably, the input waveguide 7 has a refractive index of 1.0 nm to 1.5 nm over its entire length L e(i) The upper waveguide 12 preferably also has lateral dimensions (thickness and width) which are constant over its entire length. Around the waveguides 7, 12 is provided a cladding 13, for example made of silicon oxide.

[0074] Mode coupling is understood to mean that a guided mode circulating in the input waveguide 7 spreads spatially both in the input waveguide 7 and in the upper waveguide 12, forming a supermode. In particular, the electric field components of the supermode have a spatial distribution covering the input waveguide 7 and the upper waveguide 12. The mode coupling is provided by a suitable vertical spacing of the upper waveguide 12 relative to the input waveguide 7, for example a spacing equal to tens to hundreds of nanometers.

[0075] In this way, the guided mode has an effective refractive index different from that outside the compensation section (due to mode coupling) in the compensation section Pc due to the mode coupling between the two waveguides 7 and 12. es(i) The difference between the lengths L of the waveguides 12 associated with the input waveguides 7 in row 1 changes the effective refractive index of the guided mode of each input waveguide 7. Thus, as shown in FIG. es(1) is itself L es(3) Smaller than L es(2) Thus, the guided mode circulating in the third row of waveguides 7 has an optical path equal to that of the second row of waveguides 7 and the first row of waveguides 7. Therefore, the length L es(i) is predetermined for each waveguide 7 such that the optical path of the waveguide 7 is identical from one waveguide 7 to the next over its entire length.

[0076] 7 is a schematic and partial cross-sectional view of several input waveguides 7 in a correction part Pc according to another alternative embodiment of a photoelectron emitter. The input waveguides 7 are shown in an exploded view.

[0077] In this case, the change in the effective refractive index of the guided mode for each input waveguide 7 is actively provided by several additional phase shifters 14, preferably thermo-optical phase shifters, to limit optical losses, each positioned opposite an input waveguide 7 of the correction part Pc along the Y axis.

[0078] The value of the temperature applied by each additional phase shifter 14 is the optical path C (i)is predetermined as a function of the change in effective refractive index obtained in each input waveguide 7 to equal . This makes it possible to limit or avoid degradation of the phase information carried by the guided modes in the input waveguides 7.

[0079]

[0043] Specific embodiments have been described above. Various alternative embodiments and modifications are possible without departing from the scope of the present invention. In particular, the various alternative embodiments described above are not mutually exclusive and may be combined.

[0080] The interference lens 8 can have other types of mounting, for example a confocal mounting. In this case, the interference lens 8 essentially has an exit face 8.3 located opposite the entrance face 8.1 and a radius R cr 2 in that they are defined by the same Rowland circle.

[0081] 8 is a schematic and partial top view of a photoelectron emitter 1 according to another embodiment, in which the interference lens 8 is an echelle grating. The entrance surface 8.1 and the exit surface 8.3 have a radius R cr The input 8.1 and output 8.3 regions are identically curved regions defined by the Rowland circle of . Opposite each of the input 8.1 and output 8.3 regions is a reflective surface 8.4 that defines an FPR 8.2 having a uniform refractive index. The reflective surface 8.4 includes an array of elementary reflectors 8.5 that reflect the optical signals transmitted by the input waveguide 7 towards a focal line that defines the output surface 8.3.

[0082] 9A and 9B are schematic and partial views of one of the input waveguides 7 in the correction section Pc according to two other alternative embodiments of the photoelectron emitter. In this case, the change in the effective refractive index of the guided modes of each input waveguide 7 is given by a phase change material participating in the cladding formation of the waveguide 7. The phase change material can in particular be selected from chalcogenides of the GST type, i.e. chalcogenides formed on the basis of germanium Ge, antimony Sb and tellurium Te. Reference can be made to the document "Tunable nanophotonics enabled by chalcogenide phase-change materials" by Abdollahramezani et al., Nanophotonics 2020, 9 (5), 1189-1241. The portion 15 of phase change material statically or dynamically applies a relative phase shift between the guided modes in the input waveguides 7, which are respectively predetermined for each other, such that the optical paths of the input waveguides 7 are identical to each other.

[0083] FIG. 9A shows a first embodiment as a top view and a cross-sectional view, in which the phase change material has a predefined crystalline phase during the method of manufacturing a photoelectron emitter. In this case, the waveguide 7 is placed on a substrate 13 forming a cladding. A portion 15 of thin film phase change material is deposited on and around the waveguide 7 and extends along the waveguide for a length, for example in the range of 1 μm to 100 μm. In this case, it has a uniform thickness, for example in the range of 5 nm to 100 nm. The crystalline phase of the phase change material was defined during the manufacturing method, for example by laser pulses. The effective refractive index of the guided mode therefore depends on the refractive index of the phase change material (which depends on its crystalline phase).

[0084] FIG. 9B shows a second embodiment as a top view and a cross-sectional view, in which the phase change material has a crystalline phase that can be dynamically (or statically) changed during the operation of the photoelectron emitter. In this case, the waveguide 7 is embedded in the substrate 13 forming the cladding. The heater 16 is arranged on the surface of the substrate 13 above the waveguide 7 and extends on either side of the waveguide along the X-axis. A portion 15 of the phase change material extends over the heater 16 and over the waveguide 7 for a length ranging, for example, from 1 μm to 100 μm. In this case, it has a uniform thickness, for example, ranging from 5 nm to 100 nm. Two electrodes 17 are in contact with the heater 16. Thus, when a voltage is applied between the electrodes, the heater 16 is heated and the crystalline phase of the phase change material changes. The effective refractive index of the guided mode is modified accordingly.

Claims

1. A photoelectron emitter (1) having a phased array antenna on a photonic chip, the photoelectron emitter including: o a power splitter (3) intended to be coupled at the input to a laser source (2) and at the output to N waveguides (4), N>1, forming the arms of the photoelectron emitter (1); an array of phase shifters (5) and elementary emitters (6) arranged in the arms (4), the elementary emitters (6) being laterally aligned and extending parallel to one another; o Integrated control system including: ● An input waveguide (7) coupled to the fundamental emitter (6); ● A focusing lens (8) including an input face (8.1) and an output face (8.3) coupled to an input waveguide (7); ● An output waveguide (9) coupled to the exit face (8.3); ● A photodiode (10) coupled to an output waveguide (9); ● a control module (11) configured to control the phase shifter (5) as a function of the signal received by the photodiode (10); It is characterized by: • The focusing lens (8) is an interference lens, whose entrance surface (8.1) and exit surface (8.3) are curved and define a free propagation region (8.2) with a uniform refractive index, and whose exit surface (8.3) extends along the focal line of the focusing lens (8). ● An input waveguide (7) connected to an orthogonal plane of incidence (8.1); ● The input waveguide (7) has a portion called the compensation portion (Pc) in which the effective refractive index of the guided mode is configured so that the optical paths (C(i)) of each input waveguide (7) are identical to each other.

2. Each of the input waveguides (7) has at least one lateral dimension change in the compensation portion (Pc), the lateral dimension change being proportional to the optical path (C (i) 2. The photoelectron emitter (1) according to claim 1, wherein the input waveguides (7) are predetermined to be identical to each other.

3. Each input waveguide (7) is adjusted in the compensation section (Pc) to a reference value W e0 From the above, the maximum value W ec(i) 3. A photoemitter (1) according to claim 2, characterized in that it has an adiabatic change in width up to .

4. The optical waveguide further includes a waveguide, called additional waveguide (12), having a refractive index different from that of the input waveguide (7), extending only within the correction portion (Pc) and arranged opposite and parallel to the input waveguide (7) so as to enable mode coupling with the input waveguide (7), each of the additional waveguides (12) being connected to the optical path (C) of the input waveguide (7). (i) 4. A photoelectron emitter (1) according to claim 1, wherein the first and second electrodes (11, 12) have a length that is equal to one another.

5. In the compensation section (Pc), an additional phase shifter (14) is coupled to the input waveguide (7), and the optical path (C (i) 5. A photoelectron emitter (1) according to claim 1, characterized in that for each of the input waveguides a predetermined relative phase shift is applied between the guided modes in the input waveguides (7) such that the phase shifts (Fr) of the input waveguides are identical to each other.

6. A portion (15) of phase change material located in the compensation portion (Pc) is associated with each input waveguide (7) and participates in forming its cladding, the portion (15) of phase change material being connected to the optical path (C) of the input waveguide (7). (i) 6. A photoelectron emitter (1) according to claim 1, further comprising a step of applying a static or dynamic relative phase shift between the guided modes in the respective predetermined input waveguides (7) such that the phase shifts of the guided modes in the respective input waveguides (7) are identical to each other.

7. 7. A photoelectron emitter (1) according to any one of the preceding claims, characterized in that the focusing lens (8) is a star coupler or an echelle grating.

8. 8. The photoelectron emitter (1) according to any one of claims 1 to 7, wherein the focusing lens (8) has a Rowland mounting or a confocal structure.

9. 9. A photoelectron emitter (1) according to any one of the preceding claims, characterized in that the photonic chip is manufactured from an SOI substrate.

10. A photoelectron emitter (1) according to any one of claims 1 to 9, comprising said laser source (2).

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