Photonic integrated circuit comprising a numerical aperture controller for speckle-reduced projection and method therefor

Abstract

The present invention relates to a photonic integrated circuit, comprising a substrate, a waveguide arranged in the substrate and comprising an end facet at a free edge of the substrate for projecting light and a numerical aperture (NA) controller arranged in the substrate in the vicinity of the end facet, and configured to vary in time the numerical aperture of the waveguide at the end facet. In particular, by varying the NA in time, the formation of speckle noise upon projecting light is mitigated.

Description

[0001]

[0002] The following description relates to a photonic integrated circuit (PIC) comprising a numerical aperture (NA) controller, for the purpose of speckle reduction in coherent light projection applications and a method therefor.

[0003] Augmented reality (AR) is the integration of computer-generated visual elements, sounds and other stimuli with the real-world environment of the user in real time. In this rapidly developing field of technology, small and efficient red, green, and blue (RGB) lasers are key to the development of AR projections applications, such as AR glasses.

[0004] Challenges that AR projection applications are faced with include miniaturization, integration, and energy efficiency. Additionally, the fabrication method needs to be ready for mass volumes for AR projections applications to be commercially successful.

[0005] A challenge that laser projection applications are faced with includes the reduction of speckle noise. Speckle noise is the phenomenon of quality degradation of projected light due to interference of diffuse reflections from a projection surface. This phenomenon is particularly prominent in coherent light projection applications as the coherent illumination wave is highly sensitive to small variations in phase changes. This speckle issue is also relevant in metrology, microscopy and machine vision.

[0006] Typical projection surfaces are generally extremely rough when compared to the size of the wavelength used. This applies for instance in case of visible light used in AR projection applications but is also applicable to non-visible wavelengths for other applications, like metrology. The projection surface can consequently be interpreted as an array of scatterers. Scattered light self-interferes either constructively or destructively depending on the relative phases of each scattered waveform, resulting in a pattern of bright and dark dots in the image, known as speckle. Speckle is then usually quantified by means of speckle contrast.

[0007] Speckle contrast reduction typically involves creating a plurality of independent speckle patterns and having them average out on the retina of the observer or the detector.

[0008] 2. Description of Related Art

[0009] Speckle contrast reduction, from a scientific perspective, can be achieved by means of angle diversity, polarization diversity or wavelength diversity. In an angle diverse reduction method, the projection surface is illuminated from different angles. In a polarization diverse reduction method, different polarization states of the projected light are used. In a wavelength diverse reduction method, laser sources are used which differ in wavelength by a small amount.

[0010] Although reduction methods are known from a scientific point of view, a challenge remains concerning the implementation of such speckle reduction methods into practical technical applications. In particular, this is a challenge for AR applications, which are also faced with the aforementioned challenges of miniaturization, integration and energy efficiency.

[0011] One method known from the art involves projecting the laser onto a vibrating screen, see for example CN108873589A. The vibration decorrelates the generated speckle pattern. The downside of this method is that it requires mechanical parts that induce a controlled movement in a projection screen. This is difficult to achieve and also undesirable in many applications, such as for example AR glasses.

[0012] There is therefore a need for a speckle reduction method to be integrated into photonic integrated circuits for free space projection applications.

[0013] SUMMARY

[0014] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0015] The present invention relates to a photonic integrated circuit for free-space light projection, comprising a substrate, a waveguide arranged in the substrate and comprising an end facet at a free edge of the substrate for projecting light and a numerical aperture (NA) controller arranged in the substrate in the vicinity of the end facet, and configured to vary over time the numerical aperture of the waveguide at the end facet.

[0016] In other words, the aperture of the projected light cone into free-space can be varied over time, dispersing interferences and thus reducing speckle formation. In this way, a photonic integrated circuit (PIC) for free space projection applications with reduced speckle formation can be achieved. The integration of a speckle reduction solution at the free edge of the substrate further allows miniaturization rendering the PIC particularly suitable for any free space applications, but more particularly for AR head mounted devices like glasses or headsets.

[0017] According to a preferred embodiment, the NA controller is configured to vary over time the halfangle of the maximum cone of light that can exit the waveguide at the end facet by varying the refractive index of the substrate around the end facet. In this way, the numerical aperture can be controlled on the PIC without any external additional elements, improving integration and miniaturization.

[0018] According to a preferred embodiment, the NA controller varies the numerical aperture according to a control scheme configured to mitigate the formation of speckle noise upon projecting light. Preferably the control scheme is periodic. In this way, interferences can be dispersed and averaged over a period. Alternatively, the control scheme may be random to randomly disperse interferences.

[0019] According to a preferred embodiment, the NA controller is configured for varying over time one or more physical parameters modifying the refractive index of at least part of the substrate in the vicinity of the end facet. In this way, an indirect control of the refractive index of at least part of the substrate in the vicinity of the end facet is possible, using the physical properties of at least part of the substrate in the vicinity of the facet, allowing a precise control of the local (i.e. at the end facet) effective refractive index of the waveguide. By effective refractive index of the waveguide is typically meant an index indicative of the refractive indexes of core and cladding. By modifying the refractive index of at least part of the substrate in the vicinity of the end facet, the transverse refractive index profiles of the fiber at the end facet may be modified, impacting in turn the numerical aperture into free-space at the end facet. Preferably the one or more physical parameters modifying the refractive index of at least part of the substrate in the vicinity of the end facet are any one or more of the following: temperature, stress, electric field. Yet any other parameter able to change the refractive index locally may be considered by a skilled person without inventive step. Temperature, stress and electric field allow a proportional control of the refractive index while offering different time constants. Temperature control may be used for slow variations while stress and electric field may be used for fast(er) variations.

[0020] According to a preferred embodiment, the NA controller comprises one or more thermo-optic controllers arranged in or on the substrate in the vicinity of the end facet for varying over time a temperature of at least part of the substrate in the vicinity the end facet. In other words, the influence of temperature on the optical properties of the substrate can be used to vary the refractive index over time around the end facet, modulating thus the numerical aperture at the free-edge to mitigate in fine speckle formation. Preferably the one or more thermo-optic controllers comprise one or more resistive elements receiving a controlled electrical input to modulate the temperature dissipated by said one or more resistive elements in at least part of the substrate in the vicinity the end facet. In this way, the temperature can be controlled through an electrical control. The control may be an open loop control or a closed loop control involving an sensor feedback. More preferably, at least part of the substrate in the vicinity of the end face is made of a first material having a temperature dependent refractive index. According to a preferred embodiment, the NA controller comprises a stress-optic controller arranged in or on the substrate in the vicinity of the end facet for varying over time a stress exerted on at least part of the substrate in the vicinity of the end facet. In other words, the influence of stress on the optical properties of the substrate can be used to vary the refractive index over time around the end facet, modulating thus the numerical aperture at the free-edge to mitigate in fine speckle formation. Preferably the stress-optic controller comprises one or more piezo-electric actuators receiving a controlled electrical input to modulate the stress exerted on at least part of the substrate in the vicinity of the end facet. In this way, the applied stress can be controlled through an electrical control. More preferably, at least part of the substrate in the vicinity of the end face is made of a second material having a stress dependent refractive index.

[0021] According to a preferred embodiment, the NA controller comprises an electro-optic controller arranged in or on the substrate in the vicinity of the end facet for varying over time an electric field applied to at least part of the substrate in the vicinity of the end facet. In other words, the influence of electrical field on the optical properties of the substrate can be used to vary the refractive index over time around the end facet, modulating thus the numerical aperture at the free-edge to mitigate in fine speckle formation. Preferably, the electro-optic controller comprises one or more capacitive elements receiving a controlled electrical input to modulate the electric field on at least part of the substrate in the vicinity of the end facet. In this way, the applied electrical field can be controlled through an electrical control. More preferably, at least part of the substrate in the vicinity of the end face is made of a third material having an electric field dependent refractive index.

[0022] According to a preferred embodiment, the substrate is made of any one of a ferroelectric material, a polymer or organic electro-optic material, preferably wherein the ferroelectric material is any one of lithium niobate (LiNbOs), barium titanate (BaTiCh).

[0023] According to a preferred embodiment, the projected light is coherent illumination preferably white coherent light derived from combining green, red and blue light from respective lasers. In this way, free space projection can be realized, in particular for AR glasses and / or headsets However the principle of the present invention are not limited to the choice of the above mentioned wavelength ranges / colors. A skilled would realize that other wavelength ranges / colors of light may be combined to derive white coherent light within the scope of the present invention.

[0024] According to another aspect, a method of operation of a photonic integrated circuit for free space projection is provided. The method comprises providing a photonic integrated circuit comprising a substrate, a waveguide arranged in the substrate, said waveguide comprising an end facet at a free edge of the substrate for projecting light, a numerical aperture, NA, controller arranged in or on the substrate in the vicinity of the end facet, and configured to vary over time the numerical aperture of the waveguide at the end facet; projecting light at the end facet; and varying over time the numerical aperture of the waveguide at the end facet through the NA controller. In this way, the formation of speckle may be mitigated.

[0025] According to a preferred embodiment, varying over time the numerical aperture of the waveguide at the end facet comprises varying the refractive index of the substrate around the end facet over time to vary the half-angle of the maximum cone of light that can exit the waveguide at the end facet.

[0026] According to a preferred embodiment, varying over time the numerical aperture of the waveguide at the end facet comprises varying the numerical aperture according to a control scheme configured to mitigate the formation of speckle noise upon projecting light.

[0027] According to a preferred embodiment, the control scheme is periodic.

[0028] According to a preferred embodiment, varying over time the numerical aperture of the waveguide at the end facet comprises varying over time one or more physical parameters modifying the refractive index of at least part of the substrate in the vicinity of the end facet. Preferably, the one or more physical parameters modifying the refractive index of at least part of the substrate in the vicinity of the end facet are any one or more of the following: temperature, stress, electric field, magnetic field. Yet the list is not exhaustive and a skilled person would envisage depending on circumstances other parameters suitable for modifying the effective refractive index of the waveguide. For semiconductor materials, carrier injection may for example be envisaged as parameter.

[0029] According to a preferred embodiment, varying over time the numerical aperture of the waveguide at the end facet comprises varying over time a temperature of at least part of the substrate in the vicinity of the end facet, preferably using an NA controller comprising one or more thermo-optic controllers arranged in or on the substrate in the vicinity of the end facet. Preferably, varying over time a temperature of at least part of the substrate in the vicinity of the end facet comprises controlling the temperature variation based on a received electrical input, wherein preferably the one or more thermo-optic controllers comprise one or more resistive elements receiving a controlled electrical input to modulate the temperature dissipated by said one or more resistive elements in at least part of the substrate in the vicinity of the end facet.

[0030] According to a preferred embodiment, varying over time the numerical aperture of the waveguide at the end facet comprises varying over time a stress exerted on at least part of the substrate in the vicinity of the end facet, preferably using an NA controller comprising one or more stress-optic controllers arranged in or around the substrate in the vicinity of the end facet. Preferably, varying over time a stress exerted on at least part of the substrate in the vicinity of the end facet comprises controlling the stress variation based on a received electrical input, wherein preferably the one or more stress-optic controllers comprise one or more piezo-electric actuators receiving a controlled electrical input to modulate the stress exerted on at least part of the substrate in the vicinity of the end facet.

[0031] According to a preferred embodiment, varying over time the numerical aperture of the waveguide at the end facet comprises varying over time an electric field of at least part of the substrate in the vicinity of the end facet, preferably using an NA controller comprising one or more electro-optic controllers arranged in or on the substrate in the vicinity of the end facet. Preferably, varying over time an electric field of at least part of the substrate in the vicinity of the end facet comprises controlling the electrical field variation based on a received electrical input, wherein preferably the one or more electro-optic controllers comprise one or more capacitive elements receiving a controlled electrical input to modulate the electric field on at least part of the substrate in the vicinity of the end facet.

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1A illustrates a schematic side representation of an integrated photonic circuit for free space projection according to the prior art while FIG.l.B illustrates a projected light beam according to FIG. 1A.

[0034] FIG. 2 illustrates a schematic side representation of an integrated photonic circuit for free space projection according to an embodiment.

[0035] FIG. 3 is a schematic side representation illustrating the principle of operation of FIG.2 over time using wo exemplary light beams at two different times.

[0036] FIG. 4 illustrates a schematic representation of possible embodiments of an NA controller.

[0037] FIG. 5A illustrates a schematic side representation of an integrated photonic circuit according to another embodiment and detailing further an NA controller according to said embodiment, while FIG. 5B illustrates a schematic front representation of the circuit of FIG.5A.

[0038] FIG. 6A illustrates a schematic side representation of an integrated photonic circuit according to another embodiment and detailing further an NA controller according to said embodiment, while FIG. 6B illustrates a schematic front representation of the circuit of FIG. 6A.

[0039] FIG. 7 illustrates a method for operating an integrated photonic circuit for free space projection according to an embodiment.

[0040] Throughout the drawings and the detailed description, unless otherwise described or provided, the same or like drawing reference numerals will be understood to refer to the same or like elements, features, and structures. The drawings may not be to scale, and the relative size, proportions, and depiction of elements in the drawings may be exaggerated for clarity, illustration, and convenience. DETAILED DESCRIPTION

[0041] FIG. 1A illustrates a schematic side representation of an integrated photonic circuit 1000 for free space projection according to the prior art while FIG.l.B illustrates a projected light beam according to FIG. 1A. By free-space projection is referred to optical applications for coupling a light beam from a photonic chip to a projection surface outside of said photonic chip through free- space travel of the light beam. Typical prior art photonic integrated circuits meant for free-space light projection, such as illustrated circuit 1000, comprise a substrate 10, a light source 1 (typically a laser diode) arranged on the substrate and a waveguide 20 arranged in the substrate 10. By substrate is meant a semiconductor material layer having typically predetermined refractive index properties rendering it suitable for photonic applications. The substrate 10 may be arranged as a layer onto a wafer (not represented).

[0042] The waveguide 20 is coupled to guide the light emitted by the light source 1 to an end facet 25 at a free edge 10a of the substrate 10. By end facet is meant a surface at an end of the waveguide where light exits the waveguide, where the surface is substantially perpendicular to the waveguide propagation direction. By free edge of the substrate is meant an exposed side surface of the substrate. In addition, outside of the photonic integrated circuit 1000 are typically arranged an (optional) optical element 50 (typically a lens) and a surface 70 onto which a projected light beam 60 is projected. The diverging light beam (or cone of light) 40 emitted at the end facet 25 is typically converged by the lens 50 to form the projected converging light beam (cone of light) 60 which is projected onto the projection surface 70. The projected light is typically coherent illumination, preferably white coherent light derived from combining green, red and blue light from respective laser diodes 1 in said colors.

[0043] Projection surfaces 70 are generally extremely rough when compared to the size of the wavelength of visible light used in such projection applications. The projection surface 70 can consequently be interpreted as an array of scatterers. Scattered light self-interferes either constructively or destructively depending on the relative phases of each scattered waveform, resulting in a pattern of bright and dark dots in the image, known as speckle. Fig. IB illustrate such bright and dark dots in the projected image on the projection surface 70. A challenge that laser projection applications are faced with includes the reduction of speckle noise. Speckle noise is the phenomenon of quality degradation of projected light due to interference of diffuse reflections from a projection surface. This phenomenon is particularly prominent in coherent light projection applications as the coherent illumination wave is highly sensitive to small variations in phase changes.

[0044] FIG. 2 illustrates a schematic side representation of an integrated photonic circuit 100 for free space projection according to an embodiment. The photonic integrated circuit (PIC) 100 for free-space light projection may be part of a large PIC 200 comprising a light source 1. Yet how the light is generated lies outside of the scope of the present application which rather focusses on the the coupling from chip towards free-space for the purpose of beam projection, typically surface normal beam projection as discussed in in FIG.1A, such that same numbers will be used for similar elements. As previously mentioned, a cone 40 of light may be emitted from an end facet 25 of a waveguide 20 at a free-end 10a of the substrate 10, in which the waveguide 20 is formed. The embodiment of Fig. 2 differs from the prior art in that it further comprises a numerical aperture, NA, controller 30 arranged in the substrate 10 in the vicinity of the end facet 25 and configured to vary over time the numerical aperture of the waveguide 20 at the end facet 25. By numerical aperture is typically meant a dimensionless number that characterizes the range of angles over which an optical system can accept or emit light. It is further noted that in embodiments with a lens, such a lens may further increase the effect of the numerical aperture variations.

[0045] As previously explained, the end facet 25 may debouch in free-space. In the vicinity of the end facet 25, the waveguide 30 may extend along an optical axis A extending substantially perpendicular to the end facet 25, i.e. substantially perpendicular to a (vertical) free-end wall of the substrate 10. The substrate 10 may be a silicon dioxide (SiOz) layer and the (not represented) wafer may then be a silicon (Si) underlayer. Yet other embodiments may be envisaged with other suitable materials and / or in which the substrate 10 and the wafer are one and the same. The waveguide 30 may be made of trisilicon tetranitride (SisN^ and may be arranged within the substrate 10. An optical waveguide, also referred to as just waveguide, in general may refer to an optical system comprising a higher refractive index material, sometimes referred to as the core, surrounded by a lower refractive index material, sometimes referred to as the cladding. Total internal reflection may confine, in one or more directions, a propagating electromagnetic wave to the region of the higher refractive index material. The higher refractive index material thus guides the wave, and by itself also may be referred to as the waveguide. This latter definition will be adhered to throughout the description. In the present embodiments, the substrate may be the lower refractive index material, while the waveguide may be the higher refractive index material. Different orientations of the higher refractive index material with respect to the lower refractive index material may lead to the formation of different types of waveguides. The used type of waveguide may be a slab waveguide or planar waveguide but is not limited thereto. Also, other types of such as ridge waveguides, rib waveguides, stripe waveguides, triplex waveguides, wire waveguides, strip-loaded waveguides, slot waveguides, buried waveguides, diffused waveguides, ARROW waveguides or SWG waveguides may be used. Additionally, the skilled person may apply any other type of currently existing or future waveguide to the technology described in the invention. The NA controller 30 is represented in the embodiment of FIG. 2 as arranged in the substrate 10, yet other alternatives can be envisaged depending on circumstances, including for instance the type of waveguide, as long as the NA controller remains able of varying the numerical aperture of the waveguide at the end facet 25 of the waveguide 20.

[0046] An insight of the present invention lies among other in that the NA controller 30 is configured to vary over time the half-angle a of the maximum cone 40 of light that can exit the waveguide 20 at the end facet 25 by varying the refractive index of the substrate 10 around the end facet 25. As explained above, typically, the substrate is made of a low(er) refractive index material and the waveguide is made of a high(er) refractive index material, such that the propagation of an electromagnetic wave is confined by total internal reflection, in one or more directions, to the region of the higher refractive index material. By modifying the refractive index of the substrate 10 around the waveguide 20 at the end facet 25, the area at the end facet 25 in which the propagating electromagnetic wave is confined can be varied, modifying thus the half-angle a of the cone 40 and hence the numerical aperture at the end facet 25. Varying the angle a of the cone of light emitted from the end facet 25 amounts to varying the angle P of the projected cone 60 onto the surface 70, leading to angle diversity and thus speckle contrast reduction. In the present case, the numerical aperture NA at the end facet 25 may be defined as (n* sin a), with n the refractive index of the free space medium (1.00 for air). The variation of the numerical aperture may be such as to reduce speckle without entirely removing it in the final projected image. The NA controller 30 may in particular vary the numerical aperture according to a control scheme configured to mitigate the formation of speckle noise upon projecting light. The control scheme may be periodic so as to average the numerical apertures over the period of the periodic control. Alternatively, speckle contrast may be further improved by introducing a control scheme varying randomly the numerical aperture.

[0047] FIG. 3 is a schematic side representation illustrating the principle of operation of FIG.2 over time using two exemplary light parcourse at two different times. The NA controller 30 may control the numerical aperture at the end facet 25 to obtain a first value NAi at a time tl, such that NAi=sinon, and obtain a first value NA2 at another time t2, such that NA2=sina2 where the angle <7.2 is larger than the angle ai. In this way, cones 41 and 42 may respectively be created with different half-cone angles C and on leading in turn to different half-cone angles P2 and Pi of light projected onto the surface 70. The speckle patterns in the images projected at times tl and t2 onto the surface 70 will then be different due to the different reflections from the projection surface 70 creating different self-interferences. By varying the numerical aperture at the end facet 25, speckle noise may thus be impacted, varied. By spreading over time different speckle patterns, a user may then experience a better image quality. Especially in the field of AR projection, even a partial speckle reduction may be appreciated by the user. The control scheme used by the NA controller 30 may be provided by a control circuit 35 sending one or more control signals 37 to the NA controller 30 physically integrated in / or the PIC 100.

[0048] FIG. 4 illustrates a schematic representation of possible embodiments of an NA controller 30. The NA controller 30 is arranged in or around the substrate 10 in the vicinity of the end facet 25 for varying over time one or more physical parameters modifying the refractive index of at least part of the substrate 10 in the vicinity of the end facet 25. The one or more physical parameter(s) in question may be called in the rest of the text “control physical parameter(s)” as they control the refractive index at the end facet 25. The one or more control physical parameters may be varied using one or more electrical control signals 37 (as illustrated in previous FG. 3). The one or more control physical parameters may be a temperature T and / or a stress P and / or an electric field E. When using temperature as control physical parameter, the NA controller 30 may comprise one or more thermo-optic controllers (also called heaters) 31 for varying over time a temperature of at least part of the substrate 10 in the vicinity of the end facet 25. The one or more thermo-optic controllers 31 may comprise one or more resistive elements R receiving a controlled electrical input to modulate the temperature T dissipated by said one or more resistive elements R in at least part of the substrate 10 in the vicinity of the end facet 25. In particular, at least part of the substrate 10 in the vicinity of the end face 25 may be made of a first material having a temperature dependent refractive index. Several options may be envisaged concerning the material(s) of the substrate 10. According to an embodiment, the substrate 10 may consist of the first material.

[0049] Alternatively, the substrate 10 may comprise different materials including the first material and the first material may be arranged at least in the vicinity of the end facet 25, preferably as a layer on, under or around the waveguide 20

[0050] When using stress as control physical parameter, the NA controller 30 may comprise one or more stress-optic controllers 32 arranged in or on the substrate 10 in the vicinity of the end facet 25 for varying over time a stress exerted on at least part of the substrate 10 around the end facet 25. The one or more stress-optic controllers 32 may comprise one or more piezo-electric actuators A receiving a controlled electrical input to modulate the stress exerted on at least part of the substrate 10 in the vicinity of the end facet 25. In particular, at least part of the substrate 10 in the vicinity of the end face 25 may be made of a second material having a stress dependent refractive index. Several options may be envisaged concerning the material(s) of the substrate 10. According to an embodiment, the substrate 10 may consist of the second material. Alternatively, the substrate 10 may comprise different materials including the second material and the second material may be arranged at least in the vicinity of the end facet 25, preferably as a layer on, under or around the waveguide 20. PCT Publication WO2002 / 243827 describing further details of known thermo-optic controllers and stress-optic controllers, is hereby included by reference. It is noted that this document discloses stress-induced or thermos-induced refractive-index change for controlling a propagation characteristic like phase or speed of a light signal propagating through a waveguide. This document yet does not address numerical aperture, nor coupling to free-space.

[0051] When using electric field as control physical parameter, the NA controller 30 may comprise one or more an electro-optic controllers 33 arranged in or on the substrate 10 in the vicinity of the end facet 25 for varying over time an electric field applied to at least part of the substrate 10 around the end facet 25. The one or more electro-optic controllers 33 may comprise one or more capacitors C receiving a controlled electrical input to modulate the electric field applied to at least part of the substrate 10 in the vicinity of the end facet 25. In particular, at least part of the substrate 10 in the vicinity of the end face 25 may be made of a third material having a n electric field dependent refractive index. Several options may be envisaged concerning the material(s) of the substrate 10. According to an embodiment, the substrate 10 may consist of the third material. Alternatively, the substrate 10 may comprise different materials including the third material and the third material may be arranged at least in the vicinity of the end facet 25, preferably as a layer on, under or around the waveguide 20. The first, second and / or third material may be the same.

[0052] The substrate may be made of any one of a ferroelectric material, a polymer or organic electrooptic material, preferably wherein the ferroelectric material is any one of lithium niobate (LiNbOs), barium titanate (BaTiCh).

[0053] FIG. 5A illustrates a schematic side representation of an integrated photonic circuit 600 according to an embodiment and detailing further an NA controller according to said embodiment, while FIG.5B illustrates a schematic front representation of the circuit 600 of FIG.5A. Compared to Figure 2, the embodiment of FIG. 4a and 4b differs primarily in that the NA controller 30 is arranged on the substrate 10 and not in the substrate 10 (external arrangement). Further the NA controller 30 of FIG. 4A and 4B like in the previous embodiment remains in the vicinity of the end facet 25. The NA controller 30 may be arranged as a layer disposed on a top surface of the substrate 10. The NA controller 30 may be arranged in a portion of the substrate comprising the free end 10a of the substrate and the end facet 25. The NA controller 30 of Fig.4A may be a thermo-optic controller 31. When electric current 37 passes through the heater 31, it generates heat that propagates into the material of the substrate 10 acting as cladding above the waveguide 20, changing its temperature, and via the thermos-optic effect, inducing a refractive -index change in the material, modifying in return the numerical aperture at the end facet 25 and in the end the speckle pattern of the projected image output from the end-facet 25. The heater 31 may be a layer of platinum disposed on the top surface of the substrate 10. Although not represented, a plurality of controllers 31, 32 and / or 33 may be combined in an external arrangement. For instance, a layered thermo-optic controller 31 may be disposed on top of a layered stress-optic controller 32 arranged on the top surface of the substrate. A layered stress-optic controller 32 may comprise a piezoelectric material arranged between electrodes. When a voltage differential is applied between said electrodes, the piezoelectric material may then generate a stress in the material(s) of the substrate 10, inducing a refractive -index change in the material(s), modifying in return the numerical aperture at the end facet 25 and in the end the speckle pattern of the projected image output from the end-facet 25. A similar operation may be obtained with a layered electro-optic controller using a current fed to a capacitor to create an electric field in the material(s) of the substrate 10, inducing a refractive -index change in the material(s), modifying in return the numerical aperture at the end facet 25 and in the end the speckle pattern of the projected image output from the end-facet 25.

[0054] FIG. 6A illustrates a schematic side representation of an integrated photonic circuit 600 according to another embodiment and detailing further an NA controller according to said embodiment, while FIG.6B illustrates a schematic front representation of the circuit 600 of FIG.6A. In comparison to FIG. 5a and 5B, FIG 6A and 6B illustrate embodiments in which the NA controller 30 is nested in the substrate (nested arrangement). Further, FIG. 6B illustrates a plurality of thermo-optic controllers 31 arranged surrounding the waveguide 20. In this way, the temperature of the material surrounding the waveguide in all directions may be controlled. Alternatively, a single heater may be arranged, typically above the waveguide to create a thermal gradient. Via the control signals 37 (electrical currents) the refractive index of the material around the waveguide 20 may be spatially controlled, affecting not only the numerical aperture but also potentially the spot shape.

[0055] In addition to a nested arrangement, FIG. 6A and 6B also disclose a substrate 10 comprising at least two different materials 11 and 12. A material 11 selected for its controllable refractive index properties may be arranged in the vicinity of the end facet 25. Material 11 may be called a controllable refractive index material. The controllable refractive index material may have a temperature and / or stress and / or electric field dependent refractive index. A different material 12 may be arranged in the rest of the substrate 10. In this way, the variation of the refractive index may be focused to the area made with the controllable refractive index material 11. The area in question may in particular be an area in immediate contact with the waveguide 20, typically a tubular area surrounding the end of the waveguide 20 debouching at the end facet 25 In FIG. 6B, it can be seen how the thermo-optic elements 31 are embedded within the controllable refractive index material 11. In this way, the system power efficiency may be increased as the temperature need not travel over a long distance before influencing the material acting as cladding around the waveguide 20 which refractive index should be changed to impact the numerical aperture. It is noted that the use of one or more materials is entirely unrelated to the type of arrangement, whether external or nested and that these aspects may be combined with each other without restrictions (for instance nested arrangement / one material, or external arrangement / multiple materials, etc...).

[0056] FIG. 7 illustrates a method for operating an integrated photonic circuit for free space projection according to an embodiment. The method of operation of a photonic integrated circuit for free space projection comprises a first step S10 for providing a photonic integrated circuit comprising a substrate, a waveguide arranged in the substrate, said waveguide comprising an end facet 25 at a free edge of the substrate 10 for projecting light, a numerical aperture, NA, controller 30 arranged in or on the substrate 10 in the vicinity of the end facet 25, and configured to vary over time the numerical aperture of the waveguide 20 at the end facet 25; followed by a step S20 for varying over time the numerical aperture of the waveguide at the end facet 25 using the NA controller 30. Whilst the principles of the invention have been set out above in connection with specific embodiments, it is understood that this description is merely made by way of example and not as a limitation of the scope of protection which is determined by the appended claims.

[0057] Whilst the principles of the invention have been set out above in connection with specific embodiments, it is understood that this description is merely made by way of example and not as a limitation of the scope of protection which is determined by the appended claims.

Claims

CLAIMS1. A photonic integrated circuit for free-space light projection, comprising: a substrate; a waveguide arranged in the substrate and comprising an end facet at a free edge of the substrate for projecting light; a numerical aperture, NA, controller arranged in or on the substrate in the vicinity of the end facet and configured to vary over time the numerical aperture of the waveguide at the end facet.

2. The photonic integrated circuit of claim 1, wherein the NA controller is configured to vary over time the half-angle of the maximum cone of light that can exit the waveguide at the end facet by varying the refractive index of the substrate in the vicinity of the end facet.

3. The photonic integrated circuit of any of the above claims, wherein the NA controller varies the numerical aperture according to a control scheme configured to mitigate the formation of speckle noise upon projecting light.

4. The photonic integrated circuit of the previous claim, wherein the control scheme is periodic.

5. The photonic integrated circuit of any of the above claims, wherein the NA controller is configured for varying over time one or more physical parameters modifying the refractive index of at least part of the substrate in the vicinity of the end facet.

6. The photonic integrated circuit of the previous claim, wherein the one or more physical parameters modifying the refractive index of at least part of the substrate in the vicinity of the end facet are any one or more of the following: temperature, stress, electric field.

7. The photonic integrated circuit of any of the above claims, wherein the NA controller comprises one or more thermo-optic controllers arranged in or on the substrate in the vicinity of the end facet for varying over time a temperature of at least part of the substrate in the vicinity the end facet.

8. The photonic integrated circuit of the previous claim, wherein the one or more thermo-optic controllers comprise one or more resistive elements receiving a controlled electrical input to modulate the temperature dissipated by said one or more resistive elements in at least part of the substrate in the vicinity of the end facet.

9. The photonic integrated circuit of any of the last two above claims, wherein at least part of the substrate in the vicinity of the end face is made of a first material having a temperature dependent refractive index.

10. The photonic integrated circuit of any of the above claims, wherein the NA controller comprises a stress-optic controller arranged in or on the substrate in the vicinity of the end facet for varying over time a stress exerted on at least part of the substrate in the vicinity of the end facet.

11. The photonic integrated circuit of the previous claim, wherein the stress-optic controller comprises one or more piezo-electric actuators receiving a controlled electrical input to modulate the stress exerted on at least part of the substrate in the vicinity of the end facet.

12. The photonic integrated circuit of any of the last two claims, wherein at least part of the substrate in the vicinity of the end face is made of a second material having a stress dependent refractive index.

13. The photonic integrated circuit of any of the above claims, wherein the NA controller comprises an electro-optic controller arranged in or on the substrate in the vicinity of the end facet for varying over time an electric field applied to at least part of the substrate in the vicinity of the end facet.

14. The photonic integrated circuit of the previous claim, wherein the electro-optic controller comprises one or more capacitive elements receiving a controlled electrical input to modulate the electric field on at least part of the substrate in the vicinity of the end facet.

15. The photonic integrated circuit of any of the last two above claims, wherein at least part of the substrate in the vicinity of the end face is made of a third material having an electric field dependent refractive index.

16. The photonic integrated circuit of any of the above claims, wherein the substrate is made of any one or more of a ferroelectric material, a polymer or organic electro-optic material, preferably wherein the ferroelectric material is any one of lithium niobate (LiNbOs), barium titanate (BaTiCh).

17. The photonic integrated circuit of any of the above claims, wherein the projected light is coherent illumination, preferably white coherent light derived from combining green, red and blue light from respective lasers.

18. A method of operation of a photonic integrated circuit for free space projection, comprising: providing a photonic integrated circuit comprising a substrate, a waveguide arranged in the substrate, said waveguide comprising an end facet at a free edge of the substrate for projecting light, a numerical aperture, NA, controller arranged in or on the substrate in the vicinity of the end facet, and configured to vary over time the numerical aperture of the waveguide at the end facet; varying over time the numerical aperture of the waveguide at the end facet using the NA controller.

19. The method of the previous claim, wherein varying over time the numerical aperture of the waveguide at the end facet comprises varying the refractive index of the substrate around the end facet over time to vary the half-angle of the maximum cone of light that can exit the waveguide at the end facet.

20. The method of any of the above method claims, wherein varying over time the numerical aperture of the waveguide at the end facet comprises varying the numerical aperture according to a control scheme configured to mitigate the formation of speckle noise upon projecting light.

21. The method of the previous claim, wherein the control scheme is periodic.

22. The method of any of the above method claims, wherein varying over time the numerical aperture of the waveguide at the end facet comprises varying over time one or more physical parameters modifying the refractive index of at least part of the substrate in the vicinity of the end facet.

23. The method of the previous claim, wherein the one or more physical parameters modifying the refractive index of at least part of the substrate in the vicinity of the end facet are any one or more of the following: temperature, stress, electric field, magnetic field.

24. The method of any of the above method claims, wherein varying over time the numerical aperture of the waveguide at the end facet comprises varying over time a temperature of at least part of the substrate in the vicinity of the end facet, preferably using an NA controller comprising one or more thermo-optic controllers arranged in or on the substrate in the vicinity of the end facet.

25. The method of the previous claim, wherein varying over time a temperature of at least part of the substrate in the vicinity of the end facet comprises controlling the temperature variation based on a received electrical input, wherein preferably the one or more thermo-optic controllers comprise one or more resistive elements receiving a controlled electrical input to modulate the temperature dissipated by said one or more resistive elements in at least part of the substrate in the vicinity of the end facet.

26. The method of any of the above claims, wherein varying over time the numerical aperture of the waveguide at the end facet comprises varying over time a stress exerted on at least part of the substrate in the vicinity of the end facet, preferably using an NA controller comprising one or more stress-optic controllers arranged in or around the substrate in the vicinity of the end facet.

27. The method of the previous claim, wherein varying over time a stress exerted on at least part of the substrate in the vicinity of the end facet comprises controlling the stress variation based on a received electrical input, wherein preferably the one or more stress-optic controllers comprise one or more piezo-electric actuators receiving a controlled electrical input to modulate the stress exerted on at least part of the substrate in the vicinity of the end facet.

28. The method of any of the above claims, wherein varying over time the numerical aperture of the waveguide at the end facet comprises varying over time an electric field of at least part of the substrate in the vicinity of the end facet, preferably using an NA controller comprising one or more electro-optic controllers arranged in or on the substrate in the vicinity of the end facet.

29. The method of the previous claim, wherein varying over time an electric field of at least part of the substrate in the vicinity of the end facet comprises controlling the electrical field variation based on a received electrical input, wherein preferably the one or more electro-optic controllers comprise one or more capacitive elements receiving a controlled electrical input to modulate the electric field on at least part of the substrate in the vicinity of the end facet.

30. A device for free space projection comprising a photonic integrated circuit according to any of the previous apparatus claims, and further comprising an optical element, preferably a lens, configured to converge the light beam emitted at the end facet of the photonic integrated circuit to form a projected converging light beam for projection onto a projection surface.

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