TECHNICAL FIELD

[0001] The present disclosure relates to an emitter array, and in particular to an emitter array for a LIDAR system.

BACKGROUND

[0002] Conventional integrated optical phased arrays launch and receive beams of light at a variety of controllable angles for various applications, including free-space communications, holography, and light detection and ranging (LIDAR). A LIDAR sensor is an optical remote sensor that measure the distance to a target, by irradiating the target with light, using pulses or a modulated signal from a laser, and measuring the time it takes the light to travel to and from the target to a receiver in the LIDAR sensor. When, the reflected pulses or modulated signals are detected, the time of flight of the pulses or modulated signals correspond to the distance to the sensed target. LIDAR sensors are important components in autonomous vehicles, drone navigation systems, and robot interaction, but is currently costly and relatively large.

[0003] Conventional methods to achieve large aperture on-chip non-mechanical beam steering, such as phased-arrays may have one or more of the following problems: 1) high power consumption, 2) limited to one-dimensional steering, 3) sophisticated beamforming algorithms, and 4) strict requirement for fabrication process uniformity.

[0004] To overcome some of the aforementioned problems a one-dimensional or a two- dimensional array of point emitters are arranged on a chip. When the point emitters are placed on the focal plane of a lens system, each individual point emitter will point to a specific free space angle depending on the position of the point emitter relative to the longitudinal central axis of the lens system, as in WO 2020/0506307, entitled Beam Steering and Receiving Method Based on an Optical Switch Array, published March 19, 2020, which is incorporated herein by reference. However, the point emitters that can be fabricated in commercially available silicon photonics foundries are typically grating couplers, which may have one or more of the following problems: 1) inefficient emission, 2) non-uniformity of fabrication process, 3) strong wavelength dependence, and 4) inability to implement a low loss monostatic system leveraging the polarization of light.

SUMMARY

[0005] Accordingly, the present disclosure relates to an optical emitter device comprising:

[0006] a plurality of point emitters arranged in an array comprising a plurality of rows of point emitters and a plurality of columns of point emitters, each of the plurality of point emitters comprising: one of a plurality of end-fire tapers configured to emit a respective beam of light in a respective transmission direction; and

[0007] a plurality of reflectors for redirecting the respective beams of light substantially perpendicular to the respective transmission direction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:

[0009] Figure 1 is a side view of an optical emitter device in accordance with an embodiment of the present disclosure;

[0010] Figure 2 is a plan view of an emitter array of the device of Fig. 1 with the turning substrate removed;

[0011] Figure 3A is a plan view of a section of the emitter array of Fig. 2 with the turning substrate removed;

[0012] Figure 3B is an end view of the section of the emitter array of Fig. 3 A including the turning substrate;

[0013] Figure 3C is a cross-sectional view of the section of the emitter array of Fig. 3A including the turning substrate; [0014] Figure 3D is a cross-sectional view of the section of the emitter array of Fig. 3 A with an alternative example turning reflector and including the turning substrate;

[0015] Figure 4A is a plan view of a section of an alternative embodiment of the emitter array of Fig. 2 with the turning substrate removed;

[0016] Figure 4B is an end view of the section of the emitter array of Fig. 4A including the turning substrate;

[0017] Figure 4C is a cross-sectional view of the section of the emitter array of Fig. 4A including the turning substrate;

[0018] Figure 4D is a cross-sectional view of the section of the emitter array of Fig. 4A with an alternative example turning reflector and including the turning substrate;

[0019] Figure 5 is a cross-sectional view of a point emitter of the emitter array of Fig. 2 with the tuning substrate;

[0020] Figure 6 is a top view of the point emitter of Fig. 5;

[0021] Figure 7 is a cross-sectional view of an alternative embodiment of a point emitter of the emitter array of Fig. 2;

[0022] Figure 8 is a top view of the point emitter of Fig. 7;

[0023] Figures 9A is a side view of an example embodiment of a turning substrate for the optical emitter device of Fig. 1;

[0024] Figures 9B is a top view of the turning substrate of Fig. 9A; and

[0025] Figures 9C is a bottom view of the turning substrate of Fig. 9A.

DETAILED DESCRIPTION

[0026] While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art.

[0027] Long range LIDAR systems rely on efficient transmitting and receiving a highly focused or collimated beam to and from different angular directions. While lenses are typically associated with imaging, lenses may be applied to both beamforming and beam-steering. With reference to Figure 1, an optical emitter device 1 includes an emitter array 2 and a beam steering lens system 3. For beamforming, a highly collimated output beam 4 ₀ may be transmitted when a point emitter 5 n to 5nmfrorn the emitter array 2 is placed on the focal plane F of the lens system 3 (infinite conjugation). The reverse propagation is also true based on the reciprocity theorem, whereby a parallel input beam 4i shining on the lens system 3 will focus at a point spot to be captured by one of the point emitters 5uto 5nm, with a slight spread limited by lens aberration and diffraction. For beam-steering, the far-field beam angle a of the shaped, e.g. substantially collimated or focused, output beam 4 ₀ depends on the location of the point emitter 5 n to 5nm on the focal plane F relative to the longitudinal central optical axis OA of the lens system 3. The beam angle a is governed by the equation: a = arctan(d/f), where d is the distance from the center of the focal plane, i.e. the point where the optical axis OA coincides with the focal plane F, and f is the focal length of the lens system 3. Therefore, a full LIDAR system may be implemented by placing an emitter array 2 of point emitters 511 to 5nm on or near the focal plane F of the lens system 3 , then selectively switching on and off each point emitter 511 to 5nm to steer the one or more output beams 4 ₀ in the desired directions at the desired beam angles a. This method is fundamentally different than optical phased arrays as the relative optical phase between the emitters does not need to be controlled, and only one point emitter 5 n to 5nm needs to be turned on at a time. Moreover, a plurality of point emitters 5u to 5nm may be activated simultaneously for transmitting multiple output beams 4o pointing in different directions, i.e. at different beam angles an to anm.

[0028] The emitter array 2 may include: a main substrate 7 for supporting an optical waveguide structure 8, including the point emitter 5uto 5nm; and an upper turning substrate 9 for supporting beam directing and/or beam shaping elements, as hereinafter described. Ideally, the point emitters 5uto 5nmare arranged into an array of point emitters 5uto 5nm comprising a plurality (n) of rows of point emitters 5 n to 5nm, and a plurality (m) of columns of point emitters 5 n to 5nm. Typically, the point emitters in the rows of point emitters are aligned, and the point emitters in the columns of point emitters are aligned, but the rows and/or columns of point emitters may be offset. There are many ways that the point emitters 5i to 5n may be realized, including end-fire tapers, end-fire tapers with a turning mirror, single layer grating couplers, and bilayer grating couplers.

[0029] The design of the lens system 3 may be critical to the system’s performance. The lens system 3 may comprise a plurality of lens elements, if required. Most of the design of the lens system 3 is a compromise between the F-number, the field-of-view, and the aperture size. However, there may be a few design priorities: e.g. a) to have an image-plane telecentric design, where the chief rays from the point emitters 511 to 5nm are all parallel to the optical axis OA in the image space, b) reaching diffraction limit across the field-of-view, and c) the image space numerical aperture (NA) of the lens system 3 substantially matches the NA of the point emitters 5ii to 5nm. Chief rays parallel to the optical axis OA will enable the point emitters 511 to 5nm to be designed fully vertical. Minimizing the effect of lens curvature aberrations enables the smallest spread in the output beams 4 ₀ and the best possible focusing for the receiving input beams 4i. The point emitters 5n to 5nm preferably emit output beams 4 ₀ at a beam angle a that may be fully captured by the lens system 3. For example, if the NA of one or more of the point emitters 5 n to 5nm is larger than the image space NA of the lens system 3, then a portion of the light emitting from the point emitters 5 n to 5nm will not transmit through the lens system 3, therefore rendered as loss.

[0030] With reference to Figure 2, the optical emitter device 1 may also include at least one light source, preferably an array of light sources, and at least one photodetector, preferably an array of photodetectors optically coupled to corresponding point emitters 5uto 5nm in the emitter array 2. Preferably, the array of light sources and the array of light detectors comprises an array of transceivers 111 to 1 In. Each transceiver 111 to 1 In may comprise a laser, which generates at least one of the output beams 4 ₀ , and a photodetector, which detects at least one of the input beams 4i. Selectively sending and receiving light to and from the point emitters 5 n to 5nm may be provided by a switching matrix 12 between the transceivers 111 to 1 Inand the emitter array 2. Accordingly, to select a desired point emitter 5 n to 5nm, corresponding to a desired beam angle a, a controller 13 may select one of the light sources in one of the transceivers 1 li to 1 In, corresponding to one of the rows, e.g. 1 to n, of point emitters 5 n to 5nm, then select one of the point emitters 5 n to 5nm, in that row by turning on and/or off various switches 14 in the switching matrix 12 . For example, with four point emitters 5 n to 5nm in each row, m = 4, the switching matrix 12 may have a single input port optically coupled to a switch tree comprising (m-l=3) switches 14, e.g. 2x2 on-chip Mach-Zehnder interferometers (MZI), which can be selectively activated to output the output beam 4 ₀ to a desired output port. A plurality of optical waveguide cores 15 extend parallel to each other between the output ports of the switching matrix 12 to the point emitters 5i to 5n. Each of the optical waveguide cores 15 may include a curved portion, e.g. a 90° curve, at an end thereof, each curved portion with a different radius of curvature configured to align each of the point emitters 5ii to 5nm in a row. Each row of point emitters 5 n to 5nm may be aligned with the other rows forming columns of point emitters 5 n to 5nmin a n x m emitter array 2 of point emitters 5 n to 5nm. Ideally, the pitch of the point emitters 5uto 5nmin the emitter array 2 is 5 pm to 1000 pm or based on the focal length f, size L of the emitter array 2 and the angular resolution required by the LIDAR system:

[0031] Pitch = resolution/(2*arctan(L/2f))*L

[0032] Similarly, when one of the incoming beams 4i is received at the same point emitter 5i to 5n, the incoming beam 4i is transmitted in reverse via the corresponding optical waveguide core 15 to the switching matrix 12 back to the corresponding photodetector in the corresponding transceiver 111 to 1 In.

[0033] With reference to Figures 3A-4D, the point emitters 5uto 5nm may each comprise an end-fire taper 21 combined with a turning reflector 22, e.g. mirror, and an optional micro-lens 23, (See Figs 5 and 6 for further details). Unlike grating couplers, end-fire tapers 21 enable uniform broadband transmission of light with all possible polarization states. The turning reflector 22 may be disposed in a cavity or trench 24 provided in the optical waveguide structure 8 to direct the light emission from the end-fire tapers 21 to parallel with the optical axis OA of the lens system 3, e.g. vertically upwards from and perpendicular to an upper surface of the emitter array 2, which enables both a two-dimensional point emitter array 2 and a more streamlined assembly process.

[0034] A single trench 24 may be provided for a plurality of point emitters into which the ends of a plurality of the end fire tapers 21, positioned adjacent thereto, are directed. Ideally, one trench 24 is provided for an entire row, e.g. 5uto 514, of point emitters; however, one trench 24 for each point emitter, e.g. point emitter 534, or one trench 24 for a group of, e.g. 2 or 3, point emitters, e.g. point emitters 523 and 524, is also possible. Each trench 24 is configured to receive the one or more corresponding turning reflectors 22 aligned with the ends of the end fire tapers 21, and may be between 2 pm and 150 pm deep, e.g. extend past the end fire taper, or preferably to the bottom of the optical waveguide structure 8 to the main substrate 7, and/or more preferably into the main substrate 7 (shown in dashed lines).

[0035] Furthermore, a single turning reflector 22 may be provided for a row of point emitters, e.g. 5ii to 5u, at which the output beams 4 ₀ (and input beams 4i) of a plurality of end fire tapers 21 is directed. Ideally, one turning reflector 22 is provided for an entire row, e.g. 511 to 514, of point emitters; however, one turning reflector 22 for each point emitter, e.g. point emitter 534, or one turning reflector 22 for a group of, e.g. 2 or 3, point emitters, e.g. point emitters 523 and 524, is also possible. Some or all of the turning reflectors 22 may be mounted on the turning substrate 9 (Figs. 3C and 4C) or mounted, e.g. deposited or etched, in the trench 24 (Figs. 3D and 4D), as in hereinafter described with reference to Figs. 9A to 9C. The turning reflector 22 width and height are about 5 pm to 100 pm, i.e. larger than the near field mode size of the end fire taper 21 divided by cos(45°).

[0036] Figure 3 A illustrates a top view of a section of the point emitter array 2 with the turning substrate 9 removed, i.e. showing one row of point emitters 5n to 514. Four point emitters are illustrated; however, additional point emitters are also within the scope of the invention. Figure 3B illustrates a cross-sectional view of the section of the emitter array 2 taken along section B-B. Figures 3C and 3D are cross-section views of the emitter array 2 with alternative turning reflectors 22, taken along section C-C, i.e. the outer optical waveguide core 15 to the fourth point emitter 514. The emitter array 2 may include the optical waveguide structure 8, comprised of one or more optical waveguide layers configured to form the optical waveguide cores 15 and the end-fire tapers 21 surrounded by cladding, i.e. a material with a lower index of refraction. The optical waveguide cores 15 and the end-fire tapers 21 may be comprised of silicon (Si) or silicon nitride (SiN), or both Si and SiN or any other suitable optical waveguide core material. The optical waveguide structure 8 may be mounted on, e.g. grown on top of, the main substrate 7 with upper and lower cladding 32 and 33 surrounding the optical waveguide cores 15 and the end-fire tapers 21. The upper and lower cladding 32 and 33 may be comprised of on oxide material, such as silicon dioxide (SiCh), e.g. 2-5 pm thick, and the main substrate 7 may be comprised of silicon, quartz or any suitable material. At least some of the end-fire tapers 21 may be 100 pm to 400 pm in length and taper down, e.g. by 25% to 75%, preferably by about one 50%, from the original width of the optical waveguide core 15, e.g. 400 nm to 500 nm wide by 200 nm to 250 nm thick, to a tip with a width of between 50 nm and 400 nm and the original thickness, e.g. 200 nm to 250 nm, although the thickness may also be tapered to less than the optical waveguide core 15, if required. Preferably, the end of the end-fire tapers 21 may be symmetrical, e.g. square (200 nm x 200 nm). At least some of the end-fire tapers 21, e.g. point emitter 5 n, may comprise reverse tapers, which expand, at least in width, from the original dimensions, e.g. width, of the optical waveguide core 15 to a wider width, e.g. 2x to lOx wider or to between 1 pm and 4 pm in width. The thickness may also expand, if required. Some of the end fire tapers 21 may be narrowing in width and some of the end fire tapers 21 may be widening in width. Some of the end fire tapers 21 may narrow more or less than other end fire tapers 21, and some of the end fire tapers may widen more or less than the other end fire tapers 21.

[0037] Upon transmission from the end of the end-fire tapers 21 the guided optical mode travelling in the feeding optical waveguide core 15 expands. The mode expansion controls both the beam divergence and the efficiency of the emission through the lens system 3. The minimum achievable NA for bare silicon end-fire tapers into the, e.g. air, around the lens system 3 is about 0.38, which is difficult for the design of the lens system 3, because portions of the output beam 4i may expand beyond the NA of the lens system 3 and be lost. Alternatively, even if the lens system 3 has sufficiently high NA, optical aberrations often present in high-NA lenses may reduce the performance of the LIDAR system. High-NA systems without aberration are often expensive to manufacture and sensitive to misalignment and environmental disturbances like shock and temperature.

[0038] Figure 4A illustrates a top view of a section of an alternative embodiment of the point emitter array 2 with the turning substrate 9 removed, i.e. showing one row of point emitters 5 n to 514. Figure 4B illustrates a cross-sectional view of the section of the emitter array 2 taken along section B-B. Figures 4C and 4D are cross-sectional views of the emitter array 2 with alternative turning reflectors 22 taken along section C-C, i.e. the outer bi-layer optical waveguide core 15’ to the fourth point emitter 514. The emitter array 2 may include the optical waveguide structure 8 comprised of two optical waveguide layers configured to form bi-layer optical waveguide cores 15’ and bi-layer end-fire tapers 21’. Including a second layer of optical waveguide enables mode profile engineering that may also enable modification of the NA of the emitter array 2, i.e. launching light into a coupled mode that has a broader mode spread results in a smaller NA. The bi-layer optical waveguide cores 15’ and the bi-layer end-fire tapers 21’ may be comprised of two similar optical waveguide materials with similar indexes of refraction, e.g. both silicon (Si) or both silicon nitride (SiN), or of two different optical waveguide materials with different indexes of refraction, such as a first index of refraction, e.g. Si, larger than a second index of refraction, e.g. SiN, or any other suitable optical waveguide core material. The waveguide layers may be mounted on, e.g. grown on top of, the main substrate 7 with upper and lower cladding 32 and 33 surrounding the dual optical waveguide cores 15’ and end-fire tapers 21’. The upper and lower cladding 32 and 33 may be comprised of on oxide material, such as silicon dioxide (SiCh), e.g. 2 pm thick, and the main substrate 7 may be comprised of silicon or any suitable material.

[0039] Figures 5 and 6 illustrate a cross-section and a top view, respectively, of the turning reflector 22 and the optional micro-lens 23, if required, combined with the end-fire taper 21 or the dual end fire taper 21’ . The turning reflector 22 may be formed, e.g. etched, out of a separate, e.g. silicon or quartz, turning substrate 9, with an oblique wall angle, e.g. at 45° to the longitudinal axis of the end-fire taper 21 defining the transmission direction, and may be coated or configured with a reflective layer or coating 42, e.g. silver, copper, aluminum, gold, or a Bragg grating. If the turning reflector 22 has sufficiently high index of refraction n _re fiector, e.g. silicon, and the trench 24 has sufficiently low index of refraction nreflecotr, e.g. air, such that the majority of the beam 4 ₀ strikes the oblique wall at greater than the critical angle arcsin(nreflector / ntrench), the coating 42 may be omitted and the beam 4 ₀ may be reflected via total internal reflection. A flat vertical sidewall of the turning reflector 22 facing the end-fire taper 21 or 21’ may be coated with an anti -refl ection (AR) coating 43 to minimize the Fresnel reflection therefrom. Similarly, the top surface of the micro-lens 23 or the turning substrate 9 may be coated with an AR coating. The output beam 4 ₀ coming out of the end-fire tapers 21 or 21’ adjacent to the trench 24 will expand, cross an air gap, e.g. 1 pm to 10 pm, and transmit through the vertical sidewall, i.e. AR coating 43, then hit and reflect off of the oblique reflective layer or coating 42 that redirects the light path upwards substantially perpendicular to the original transmission direction in the end-fire taper 21 and the upper surface of the point emitter array 2. The emission pattern of each output beam 4 ₀ (and input beam 4i) may then be reshaped, e.g. collimated or focused, through the corresponding micro-lens 23. The goal of the micro-lens 23 is to convert the point emitter’s NA to a smaller value, e.g. less than 0.2, preferably less than 0.15 for a more practical lens design. Each micro-lens 23 may be 25 pm to 200 pm in diameter. Each turning reflector 22 may have edges with lengths between 6 pm to 90 pm. The gap and/or the trench 24 may include an index matching material between the endfire tapers 21 and the turning reflectors 22, i.e. a material with an index of refraction between the effective index of refraction of the mode in the end-fire tapers 21 and the index of refraction of the turning reflector 22, to at least reduce back reflections at the interface between the end fire taper 21 and the gap and/or the interface between the gap and the turning reflector 22.

[0040] With reference to Figures 7 and 8, to further reduce the NA of the point emitters 5 n to 5nm, a suspended optical waveguide structure 50 may be provided optically coupled to the end of some or each of the end-fire tapers 21 or 21’. The suspended optical waveguide structure 50 may be comprised of the cladding material, e.g. SiCh, now forming the optical waveguide core, surrounded by a pocket of material with a lower index of refraction, e.g. air, forming cladding. The suspended optical waveguide structure 50 may be suspended above the main substrate 7 by removing, e.g. etching, one or more of the substrate material from the main substrate 7 and/or the turning substrate 9 and/or the cladding material from the upper and lower cladding 32 and 33 beneath and/or around of the suspended optical waveguide structure 50 forming a pocket or chamber 51 around the suspended optical waveguide structure 50. Ideally, each trench 24 may be enlarged to extend underneath and/ around the suspended optical waveguide structures 50 to form the pocket or chamber 51. The turning substrate 9, as in Figure 8, may also be etched in selected areas above the suspended waveguide structure 50 forming channels 52 (Fig. 9C), such that the optical mode in the suspended optical waveguide structure 50 does not leak into either the main substrate 7 and/or the turning substrate 9. Accordingly, the NA for suspended waveguide structure 50/end-fire tapers 21 or 21’ may be reduced to less than about 0.25, preferably less than 0.2, enabling the micro-lens 23 to convert the point emitter’s NA to less than 0.20, preferably less than 0.15. The suspended optical waveguide structure 50 may extend 2 pm to 50 pm into the chamber 51 or the trench 24, whereas the end fire taper 21 or 21’ may extend somewhat into the chamber 51 or the trench 24, but less than the full length of the suspended optical waveguide structure 50. The suspended optical waveguide structure 50 may have a thickness, e.g. 6 pm to 8 pm, the same as the total optical waveguide structure 8, or may be made thinner than the optical waveguide structure 8 by the local removal of some of the upper cladding 32. The suspended optical waveguide structure 50 may have a constant width about the same as the thickness, e.g. 6 gm to 8 pm. The suspended optical waveguide structure 50 may taper, i.e. narrowing width and/or height towards the outer free end thereof (dashed lines) or may reverse taper, i.e. widening width and/or height towards the outer free end thereof. Ideally, the end-fire taper 21 is positioned in the center both vertically and horizontally of the waveguide structure 50.

[0041] Furthermore, in some or all of the aforementioned embodiments, the turning reflector 22 may include an integrated curved reflector 53 on or forming the oblique surface thereof for further reducing the NA of the point emitters 5n to 5nm. For example, a spherical, conic, or aspheric surface may be provided, e.g. etched or deposited, on the oblique surface of the turning reflector 22, e.g. with a radius of curvature of 0.1 mm to 1.0 mm. In embodiments with or without the curved reflector 53, the micro-lens 23 may not be required and may be omitted.

[0042] With reference to Figures 9A to 9C, the turning reflectors 22 and the micro-lenses 23 may be fabricated on the same turning substrate 9, whereby the plurality of turning reflectors 22 and the plurality of micro-lenses 23 may be configured on the same turning substrate 9, which may then be bonded on top of the photonics chip comprising the emitter array 2. Accordingly, the reflective layers or coatings 42, the AR coatings 43 and an AR coating over each of the microlenses 23 may be provided, e.g. coated, onto the corresponding features of the turning substrate 9 in a separate fabrication process to the fabrication of the optical waveguide structure 8. Furthermore, a plurality of the turning reflectors 22 may comprise a single monolithic structure, extending the length of the turning substrate 9 for reflecting a plurality of output beams 4 ₀ and input beams 4i from and to the point emitters, e.g. 514, 524, 534, 544, and 5n4, in a column of the emitter array 2.

[0043] The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.