Photonic integrated circuit comprising a multi-level waveguide

Abstract

The present invention relates to a photonic integrated circuit, comprising a waveguide for guiding light received from a light source; an end facet at an end of the waveguide for outputting the received light into free space; wherein the waveguide is a multi-level waveguide comprising a first level and a second level layered on the first level and a third level layered on the second level; wherein each level comprises a region of higher refractive index surrounded by a region of lower refractive index.

Description

[0001] PHOTONIC INTEGRATED CIRCUIT COMPRISING A MULTI-LEVEL WAVEGUIDE

[0002] BACKGROUND

[0003] 1. Field

[0004] The following description relates to photonic integrated circuits (PICs), comprising a waveguide for guiding light received from a light source and an end facet for outputting the received light. 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.

[0005] 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.

[0006] A challenge that laser projection applications are faced with includes outputting light from a waveguide with a substantially dispersionless numerical aperture (NA). That means that the halfangle of the maximum cone of light with which the light is projected is substantially the same for different colors. This challenge is particularly prevalent in visible light projection applications, as the difference in used wavelengths may be relatively large.

[0007] A further challenge that laser projection applications are faced with includes outputting light from a waveguide in a substantially round optical mode field. The roundness of the optical mode field is commonly related to the degree of symmetry of the waveguide used.

[0008] In PICs for projection applications, light for projection enters into free space at an end facet of a waveguide. Obtaining a high resolution then requires that both abovementioned challenges are overcome. Commonly, in practice, symmetric waveguides such as square waveguides are used to obtain substantially round optical modes at the cost of retaining significant dispersion in the NA.

[0009] 2. Description of Related Art

[0010] A (surface) waveguide is a light-guiding element, much like an optical fiber, which may be formed on the surface of a rigid substrate. Waveguides are used for many applications including telecommunications, chemical sensing, and force sensing.

[0011] A (surface) waveguide is characterized as having a central region or "core" and a surrounding "cladding." An optical signal travels through a (surface) waveguide as an optical mode propagating through the core. The optical signal is substantially confined to the core by the cladding. The guiding property of a (surface) waveguide arises from a difference in the refractive index, n, between the core and the cladding. For a (surface) waveguide, the refractive index of the cladding is typically lower than the refractive index of the core.

[0012] Surface waveguides can be fabricated in various forms including slab waveguides, ridge waveguides, and stripe waveguides. A slab waveguide comprises a planar thin film optical core sandwiched between two planar thin film claddings. The cladding above and below the core confine the propagating optical mode vertically, but not laterally.

[0013] A ridge waveguide is similar to a slab waveguide, but also includes a protruding ridge of material through which an optical mode propagates. The structure of the ridge substantially confines the mode both vertically and laterally, except where the ridge meets the slab. It is possible that a mode can exist in the slab area outside the ridge portion.

[0014] A stripe waveguide is essentially a ridge waveguide in which the slab portion has been etched away. The optical mode is confined to the stripe since there is no core material anywhere else. A stripe waveguide may have a square cross section, such that its optical mode field is substantially round.

[0015] Square waveguides inherently suffer from a large refractive index contrast. A first problems then arises in fabrication, where growing a thick layer of a higher refractive index material may cause the layer to fracture as a result of stress at the interface between the high refractive index material and the lower refractive index material. A second problem is that the NA of these symmetric waveguides is known to be wavelength dependent. In projection applications such as AR, in which the use of different colors is desirable, wavelength dependence of the NA may pose a significant problem.

[0016] A method for manufacturing surface waveguides is known from EP 1 704 432 Bl, Lionix B.V, proprietor of the abovementioned patent, herein sets out a method for fabricating different types of surface waveguides. In particular, the so-called critical thickness during thin-film growth is discussed, at which the induced strain exceeds the fracture strain, such that the thin-film fractures. The patent discloses how such fracturing can be prevented during fabrication. Possible fields of applications are briefly mentioned as telecommunications, chemical sensing, and force sensing. Although a fabrication method for surface waveguides that prevents fracturing is thus known from the art, EP 1 704 432 Bl remains silent on applying these surface waveguides into the field of light projection applications. The related art also remains silent on the problem of NA dispersion, which is especially prevalent in this field. Namely, the range of (visible) wavelengths used in light projection applications is typically much broader (wavelength difference between blue and red may range up to 300 nm) than the range wavelengths used in many other applications, such as telecommunication applications (in which the range is typically between 50 and 100 nm). The related art also remains silent on obtaining a substantially round optical mode field. A waveguide that intends to overcome the problems of the state-of-the-art should thus need to be configured such that it outputs the light guided by the waveguide with a substantially constant NA over a wide range of wavelengths while maintaining a substantially round optical mode field in order to be applicable into high resolution light projection applications. Additionally, the waveguide should have such a configuration that it may be manufactured without fracturing due to stress. It is the goal of the invention to improve the state-of-the-art in view of at least one of the abovementioned problems.

[0017] SUMMARY

[0018] 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.

[0019] The present invention relates to a photonic integrated circuit, comprising a waveguide for guiding light received from a light source; an end facet at an end of the waveguide for outputting the received light into free space; wherein the waveguide is a multi-level waveguide comprising a first level and a second level layered on the first level; wherein each level comprises a region of higher refractive index surrounded by a region of lower refractive index. Higher refractive index and lower refractive index, throughout this description, should be interpreted as being relative to each other, without implying a specific magnitude in absolute terms.

[0020] The invention lies in the realization that a multi-level waveguide with two levels allows setting both an optical mode field and a numerical aperture of the waveguide (i.e., at the end facet of the waveguide), furthermore substantially independently of each other.

[0021] By optical mode field is meant the intensity distribution (also called profile) of the light guided in the waveguide at a cross section of said waveguide. 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. Here the numerical aperture is defined at the end facet, by the local effective refractive index of the waveguide at the end facet. By effective refractive index of the waveguide is meant an index indicative of all (combined) refractive indexes of regions (typically core and cladding) of a cross section of the waveguide.

[0022] By layering multiple regions of different refractive indexes, an effective refractive index of the waveguide can be set, defining in turn the numerical aperture into free space of the waveguide. Combining a plurality of regions having different refractive indexes to obtain a given effective refractive index offers further a better dispersion behavior than using a prior art waveguide with the same given effective refractive index. Indeed regions of low refractive indexes have typically a lower dispersion behavior than regions of high refractive indexes. A multi-level waveguide allows thus to improve the dispersion behavior of the waveguide, by reducing the influence of wavelength on numerical aperture.

[0023] In addition, by layering multiple regions of different refractive indexes, an optical mode field for the waveguide can be set, derived from the (combined) intensity distributions of each level. The presence of high and low refractive index regions in each level offers thus degrees of freedom to set both an optical mode field and a numerical aperture of the waveguide independently of each other. It is further noted that each level should comprise a region of higher refractive index and a region of lower refractive index, yet the lower index region of a level need not necessarily surround the whole of the higher index region of that level but may only partially surround the higher index region of its level.

[0024] According to a preferred embodiment, the multi-level waveguide may further comprise a third level, layered on the second level, and comprising a region of higher refractive index surrounded by a region of lower refractive index. It is noted that in further embodiments, the multi-level waveguide may comprise one or more further levels. In this way, the multi-level waveguide is provided with additional degrees of freedom in the design of the optical mode field and numerical aperture of the multi-level waveguide.

[0025] According to a preferred embodiment, a thickness of at least one of the regions of higher refractive index may be dimensioned in relation to a wavelength of the received light, to weakly guide the received light. A substantially small thickness in relation to a wavelength of the received light may ensure that at least one optical mode guided in said at least one of the regions of higher refractive index is a weakly guided mode. Typically, when the size of a region of higher refractive index (typically core) is larger than the wavelength of the guided wave, the optical mode field is confined (almost) completely to the region of higher refractive index. In a certain range of variations, upon decreasing the size of the region of higher refractive index, the size of the optical mode field typically decreases as well. However, if the size of the region of higher refractive index becomes smaller than the wavelength of the guided wave, then the optical mode field enters a new regime in which the optical mode field is typically no longer confined to the region of higher refractive index but extends significantly beyond the region of higher refractive index. In this regime, the optical mode may be referred to as being weakly guided. Entering the weakly guided regime may thus be recognized by an increase of the size of the optical mode field in this regime upon decreasing the size of the region of higher refractive index. For a specific thickness, the thickness may be small enough for a relatively long wavelength to be weakly guided, whereas the same thickness may not be small enough for a relatively short wavelength to be weakly guided. The preferred thickness of a region of higher refractive index mentioned here may be based on a thickness at which optical modes of visible light may be weakly guided. Preferably, for visible light (in particular for wavelengths in the range of 420nm (blue) to 660nm (red), eventually with a wavelength around 800 to 850nm) , the thickness of at least one of the regions of higher refractive index may be between 5 to 40 nm, preferably between 10 to 25 nm. This range may apply in particular for a wave guide of trisilicon tetranitride (SisN^ embedded in silicon dioxide (SiOz). A skilled person in the art would without inventive step come with a suitable thickness range for other wavelengths and / or materials.

[0026] Although arranged for a photonic integrated circuit having an end facet for outputting light into the free space, photonic integrated circuits could be arranged for other purposes in general. The principle of the photonic integrated circuit described here is therefore not limited to projection into free-space insofar as the concept of a multi-level waveguide as disclosed here may also be declined accordingly for other types of photonic integrated circuits. Depending of the purpose of the photonic integrated circuit, the guided light need not necessarily be weakly guided if the size of the mode field need not be tuned (unlike in free-space projection applications where the numerical aperture is desired to be tuned). According to a preferred embodiment, at least a first region of higher refractive index and a second region of higher refractive index from a pair of adjacent levels may be arranged substantially close together to act as a single waveguide. The multi-level waveguide of the invention is supposed to act as a single waveguide. In case the distance between adjacent regions of higher refractive index would be excessively large, the individual levels would act as multiple individual waveguides. However, in case the distance between adjacent regions of higher refractive index is small, the optical modes of the individual regions of higher refractive index may hybridize and form a single optical mode.

[0027] According to a preferred embodiment, a distance between at least two regions of higher refractive index of adjacent levels may be dimensioned in relation to at least a size of at least one of the individual mode fields of said adjacent levels (4) to act as a single waveguide. A sufficiently small distance between two adjacent regions of higher refractive index may ensure that the multi-level waveguide acts as a single one. In this context, sufficiently small may mean that the regions are substantially closer together than a size of an optical mode field of the weakly guide mode of an individual region of higher refractive index. Provided that the optical mode field of a weakly guided mode may extend beyond the region of higher refractive index, it may then overlap with the optical mode field of a weakly guide mode of an adjacent region of higher refractive index. Preferably, for visible light, a distance between the first region and the second region of higher refractive index may be in the range of 50nm to 500 nm, preferably in the range of 50 to 400 nm, more preferably in the range of 100 to 400nm. For visible light, the distance between the first region and the second region of higher refractive index may be in the order of ten times the thickness of first and / or second regions of higher refractive index. This range may apply in particular for a wave guide of trisilicon tetranitride (ShNi) embedded in silicon dioxide (SiOz). A skilled person in the art would without inventive step come with a suitable distance range for other wavelengths and / or materials.

[0028] According to a preferred embodiment, the multi-level waveguide may be configured to output the received light with a substantially constant numerical aperture (NA) over a wide range of wavelengths, said range of wavelengths preferably including the visible light range. As mentioned before, the structure of the multi-level waveguide allows for tuning of the effective refractive index of a cross section of the multi-level waveguide. Obtaining an effective refractive index from a plurality of contrasted regions (i.e. having different refractive indexes) may in addition reduce the dispersion behavior of the NA. Consequently, a double( / multi)-level waveguide may emit light with a more constant NA over a range of wavelengths when compared to a single-level waveguide. Additionally, a triple-level waveguide may emit light with an even more constant NA over a range of wavelengths when compared to the double-level waveguide.

[0029] According to a preferred embodiment, the multi-level waveguide may be configured to output the received light with an NA varying less then + / - 50 %, preferably less than + / - 10%, more preferably less than + / - 5%, for light in the visible light range. By contrast typical single stripe priori art solutions exhibit a variation of typically + 200 % of the numerical aperture. The currently claimed solution reduces thus greatly the dispersion.

[0030] According to a preferred embodiment, the NA may be lower than 0,2, more preferably lower than 0, 1 for light in the visible light range. In this way, the NA is sufficiently constant for the multilevel waveguide to be used for visible light projection applications. The NA is then furthermore sufficiently small, such that the projected light is sufficiently collimated, which is another desirable effect for visible light projection applications. In this case, less advanced external optical elements may be required for, for example, focussing the light.

[0031] According to a preferred embodiment, the NA may be based on an effective refractive index of a cross section of the multi-level waveguide at or near the end facet. As mentioned before, tuning of the effective refractive index of a cross section of the multi-level waveguide may affect the dispersion behaviour of the NA. The influence of the effective refractive index of a cross section of the multi-level waveguide on the NA will be most pronounced in case said cross section is located at or near the end facet, as this is where the light will exit the multi-level waveguide.

[0032] According to a preferred embodiment, the effective refractive index of a cross section of the multilevel waveguide may be based on any one or more of the following: a number of levels of the multi-level waveguide; a distance between at least a first region of higher refractive index and a second region of higher refractive index from at least a pair of adjacent regions of higher refractive index; a shape of a cross section of at least one of the regions of higher refractive index (in particular the aspect ratio of the cross section, more in particular the width thereof corresponding to the thickness of the region in question); an area of a cross section of at least one of the regions of higher refractive index; the respective materials of the regions of higher refractive index and the regions of lower refractive index.

[0033] The above enumeration provides a list of design parameters which may affect the effective refractive index of a cross section of the multi-level waveguide, and therefore may affect the NA behaviour of the multi-level waveguide. These parameters may be tuned in order to obtain an optimal design for the multi-level waveguide that are appropriate for the requirements of the specific application. In particular, when considering a fixed height of a minimum cross-sectional area containing all regions of higher refractive index (referred later to as effective thickness), the width of a cross section of a region of higher refractive index- corresponding to the thickness of the region in question- may affect particularly the numerical aperture.

[0034] According to a preferred embodiment, the multi-level waveguide may be configured (dimensioned) such that an aspect ratio of the optical mode field of an optical mode guided by the multi-level waveguide may have a value between 0,5 and 1,5, preferably between 0,8 and 1,2, more preferably between 0,9 and 1,1. Preferably the optical mode field of the multi-level waveguide may be substantially round. A symmetrical optical mode field is desirable for the application of visible light projection. Namely, symmetrical mode fields may be beneficial for obtaining a high resolution upon projection. It also removes the need for external optical components like lenses and beam shapers. The degree of symmetry of an optical mode field may be defined by the aspect ratio of the optical mode field. For an elliptical optical mode field, this may be defined as the ratio of the semi-major axis and the semi-minor axis. If the value of this ratio approaches 1, then the size of the semi-major axis approaches the size of the semi-minor axis such that the elliptical optical mode field approaches being a circular optical mode field.

[0035] According to a preferred embodiment, the aspect ratio of an optical mode field of an optical mode guided by the multi-level waveguide may be set based on any one or more of the following: a number of levels of the multi-level waveguide; a distance between at least a first region of higher refractive index and a second region of higher refractive index from a pair of adjacent regions of higher refractive index; a shape of a cross section of at least one of the regions of higher refractive index; an area of a cross section of at least one of the regions of higher refractive index, preferably a width of at least one region of higher refractive index, respective materials of the regions of higher refractive index and the regions of lower refractive index . The size and shape of the optical mode field are largely determined by the size and shape of a region comprising all regions of higher refractive index with a minimal circumference. The size and shape of such a region may be affected by parameters provided above. In particular, when considering a fixed height of a minimum cross-sectional area containing all regions of higher refractive index (referred later to as effective thickness), the length of a cross section of a region of higher refractive indexcorresponding to the width of the region in question- may affect particularly the aspect ratio of the optical mode of the multi-level waveguide.

[0036] These parameters may be tuned in order to obtain a substantially round optical mode field. For example, the width of the regions of higher refractive index may be chosen such that it equals the sum of the thicknesses of the regions of higher refractive index and the distance between adjacent regions of higher refractive index. Alternatively, the width of the regions of higher refractive index may be chosen such that it equals the distance from the bottom of the bottom region of higher refractive index to the top of the top region of higher refractive index.

[0037] According to a preferred embodiment, the multi-level waveguide may be configured to output the received light with a predetermined NA and with a predetermined optical mode field, said predetermined NA and said predetermined optical mode field being set substantially independently from each other. The effective refractive index of a cross section of the multi-level waveguide may be varied by varying a cross-sectional area (width X thickness) of at least one region of higher refractive index. Furthermore, varying a width of at least one region of higher refractive index may be used for varying an optical mode field of an optical mode guided by the multi-level waveguide. As long as the aspect ratio (width / thickness) is sufficiently high, a small change in the width would change to the cross-sectional area significantly less than an equally sized change in the thickness would. In other words, the optical mode field may be varied without substantially varying the effective refractive index of a cross section of the multi-level waveguide.

[0038] According to a preferred embodiment, the multi-level waveguide may be configured to output the light with a substantially low NA, below 0,2, preferably around and / or below 0,1, such that the outputted light may be substantially collimated. In this way, the outputted light from the multilevel waveguide may be appropriate to be applied into light projection applications, preferably visible light projection applications, because again less advanced external optical elements may be required.

[0039] According to a preferred embodiment, at least one of the regions of higher refractive index may have a width in the range of 100 nm to 300 nm, preferably 200 nm. In this way, the appropriate aspect ratio of the regions of higher refractive index may be obtained such that the predetermined NA and predetermined optical mode field may be set substantially independently from each other, while ensuring that the thickness of the regions of higher refractive index remains sufficiently small such that the multi-level waveguide may guide modes in the weakly guided regime. These dimensions may furthermore be conveniently realized during fabrication. Fabrication limitations may impose a lower limit on the realizable width.

[0040] According to a preferred embodiment, a cross section of at least one of the regions of higher refractive index may have a shape of a rectangle or a trapezoid. In this way, the aspect ratio of the regions of higher refractive index (and of the associated optical mode field) may be easily tuned, while fabrication may be conveniently realized. Additionally, a trapezoid may provide additional degrees of freedom in the design, which may allow for further finetuning of the NA and optical mode field. Different regions of higher refractive index may have either the same or different shapes. In case of an embodiment where the cross section has the shape of a trapezoid, the relevant dimensions may be the length of the first parallel side, the length of the second parallel side, and the height, i.e. the perpendicular distance between the aforementioned sides. Other shapes may be envisioned, but their use may be limited by fabrication capabilities.

[0041] According to a preferred embodiment, a cross section of at least one of the regions of higher refractive index may have a substantially high aspect ratio, preferably a width being more than four times a thickness thereof. In this way, it may be ensured that the multi-level waveguide is configured such that the predetermined NA and predetermined optical mode field may be set substantially independently from each other. A change in width may then substantially affect the optical mode field without substantially affecting the NA behaviour.

[0042] According to a preferred embodiment, an optical mode field of an optical mode guided by the multi-level waveguide may have a larger size than an optical mode field of an optical mode guided by a square waveguide with corresponding dimensions. A square waveguide with corresponding dimensions means a square waveguide with thickness equal to an effective thickness and width equal to an effective width. Generally speaking it may be assumed that the effective thickness is the distance from the lowest point of the region of higher refractive index corresponding to the bottom level to the highest point of the region of higher refractive index of the top level, and that the effective width is the distance from the leftmost point of any of the regions of higher refractive index to the rightmost point of any of the regions of higher refractive index. For a multi-level waveguide comprising aligned, equally sized rectangular regions of higher refractive index, this would correspond to the effective thickness being equal to the sum of the thicknesses and the distances between adjacent regions of higher refractive index, while the effective width would correspond to the width of a single region of higher refractive index. Furthermore, because a square waveguide is considered, the effective thickness would equal the effective width.

[0043] According to a preferred embodiment, the multi-level waveguide may be configured such that it may allow for growing a region of higher refractive index on top of a region of lower refractive index without the region of higher refractive index fracturing due to stress at the interface of the different regions. In conventional waveguides, growing a thick layer of a higher refractive index material may cause stress to accumulate at the interface between the high refractive index material and the lower refractive index material. Layering more or thicker levels onto existing levels may induce substantially large amounts of stress at the interface of regions of different materials, until the induced strain exceeds the fracture strain, such that the thin-film fractures. The multi-level waveguide may be configured such that it allows for growing a region of higher refractive index on top of a region of lower refractive index without the regio of higher refractive index fracturing due to stress at the interface of the regions of different materials. This configuration may comprise carefully chosen materials and layer thicknesses.

[0044] According to a preferred embodiment, a region of higher refractive index may comprise trisilicon tetranitride (SisN^. According to a preferred embodiment, a region of lower refractive index may comprise silicon dioxide (SiOz). The refractive indices of the above materials allow for efficient waveguiding. Additionally, stress at the interface of these materials may be limited to below the fracture strain.

[0045] According to a preferred embodiment, the outputted / received light may comprise light in the visible spectrum, preferably red, green, or blue light. According to a preferred embodiment, the photonic integrated circuit may further comprise a light source, preferably being a laser diode. Light in the visible spectrum, especially a combination of red, green and blue light, is particularly suitable for light projection applications. A laser diode may be an efficient light source for providing the visible light, which may be conveniently integrated into the photonic integrated circuit. Alternatively, the outputted / received light may comprise light outside of the visible spectrum.

[0046] According to a preferred embodiment, at least one of the levels may comprise a stripe waveguide structure. According to a preferred embodiment, at least one of the levels may comprise a ridge waveguide structure. According to a preferred embodiment, at least one of the levels may comprise a box shell waveguide structure. According to a preferred embodiment, at least two of the levels may comprise a same waveguide structure. According to a preferred embodiment, at least two of the levels may comprise a different waveguide structure. Each different waveguide structure may have its own advantages depending on the application or specific requirements. In this way, any combination of different waveguide structures is possible, allowing for the maximal level of versatility.

[0047] According to another aspect, the invention relates to a method for manufacturing a photonic integrated circuit, comprising forming a waveguide for guiding light received from a light source; and an end facet at an end of the waveguide for outputting the received light into free space; wherein forming the waveguide comprises forming a multi-level waveguide by forming a first level and forming a second level layered on the first level; wherein each level comprises a region of higher refractive index surrounded by a region of lower refractive index. Similar preferred embodiments as for the circuit embodiments exist as method preferred embodiments.

[0048] BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG. 1 illustrates a longitudinal cross-sectional side view of a photonic integrated circuit (PIC) according to one or more embodiments.

[0050] FIG. 2A illustrates a lateral cross-sectional view of a PIC according to the related art, comprising a single level waveguide according to the related art.

[0051] FIG. 2B-C illustrate lateral cross-sectional views of PICs according to one or more embodiments, comprising respectively a double and triple-level waveguide according to one or more embodiments.

[0052] FIG. 3A illustrates a lateral cross-sectional view of a PIC according to the related art, comprising a single level waveguide according to the related art.

[0053] FIG. 3B-C illustrates lateral cross-sectional views of PICs according to one or more embodiments, comprising respectively a double and triple-level waveguide with relevant dimensions according to one or more embodiments.

[0054] FIG. 4A illustrates a lateral cross-sectional view of a PIC according to the related art, comprising a single level waveguide according to the related art and an optical mode field corresponding thereto. FIG. 4B-C illustrate lateral cross-sectional views of PICs according to one or more embodiments, comprising respectively a double- and triple-level waveguide according to one or more embodiments and optical mode fields corresponding thereto.

[0055] FIG. 5A illustrates a lateral cross-sectional view of a PIC according to the related art, comprising a square single level waveguide according to the related art and an optical mode field corresponding thereto.

[0056] FIG. 5B illustrates a lateral cross-sectional view of a PIC according to one or more embodiments, comprising a triple-level waveguide with dimensions corresponding to that of the square waveguide of FIG. 5A according to one or more embodiments, and an optical mode field corresponding thereto.

[0057] FIG. 6 illustrates the wavelength dependence of the numerical aperture for a single level waveguide according to the related art, and a double and triple-level waveguide according to one or more embodiments.

[0058] 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.

[0059] DETAILED DESCRIPTION

[0060] FIG. 1 illustrates a longitudinal cross-sectional side view of a photonic integrated circuit (PIC) (1) according to one or more embodiments. The PIC (1) comprises a multi-level waveguide (2) for guiding light. The particular multi-level waveguide (2) of FIG. 1 comprises three levels. However, multi-level waveguides (2) of different embodiments may comprise a different number of levels, the number of levels being more than one. The multi-level waveguide (2) may be, for example but not limited to, a double-level waveguide or a triple-level waveguide.

[0061] A light source (not shown here) may be used to couple light into the multi-level waveguide (2). The light source may output light in the visible spectrum, preferably red, green, or blue light. The light source may furthermore be a laser diode. At an end facet (3) of the multi-level waveguide (2), the light is outputted into free space with a numerical aperture (NA) corresponding to a half-angle of the maximum cone of light 6.

[0062] In general, the NA may vary depending on the wavelength of the emitted light. However, the multi-level waveguide (2) may be configured to output the received light with a substantially constant NA, over a wide range of wavelengths. The NA may be said to be substantially dispersionless, preferably over a range of wavelengths including the visible spectrum.

[0063] In other words, the half-angle of the maximum cone of light for red light, dred, may be approximately equal to the half-angle of the maximum cone of light for green light, 9green, which may be approximately equal to the half-angle of the maximum cone of light for blue light, dbiue , Such that Gpe 0green blu -

[0064] The NA may be based on an effective refractive index of a cross section of the multi-level waveguide (2) at or near the end facet (3). Relevant parameters that may affect the effective refractive index of a cross section of the multi-level waveguide (2) will be discussed with respect to FIGs. 2 and 3.

[0065] FIGs. 2A-C, FIGs. 3A-C and FIGs. 4A-C may respectively describe similar embodiments. Additionally, FIG. 5B may describe a similar embodiment as that of FIG. 2C / 3C / 4C. Reference numbers referring to corresponding features may not be repeated.

[0066] FIG. 2 shows an overview of three lateral cross-sectional views of different PICs. The cross sections may be taken in a direction perpendicular to a direction of propagation of the guided light. FIG. 2A illustrates a lateral cross-sectional view of a PIC according to the related art. The figure illustrates a waveguide comprising a single level. FIG. 2A is incorporated in FIG. 2 for the purpose of comparison. FIG. 2B illustrates a lateral cross-sectional view of a PIC (1) according to one or more embodiments. The figure illustrates a multi-level waveguide (2), comprising a first level (4a) and a second level (4b) layered on the first level (4a). Each level comprises a region of higher refractive index (5a, 5b), surrounded by a region of lower refractive index (6a, 6b). The regions of higher refractive index (5 a, 5b) of the first and second level (4a, 4b) are layered above each other.

[0067] A region of higher refractive index (5a, 5b) may typically have a single refractive index and / or be made of a single material. Alternatively, a region of higher refractive index (5a, 5b) may comprise multiple refractive indices and / or multiple materials. A region of lower refractive index (6a, 6b) may typically have a single refractive index and / or be made of a single material. Alternatively, a region of lower refractive index (6a, 6b) may have multiple refractive indices and / or be made of multiple materials.

[0068] The region of higher refractive index (5a, 5b) may comprise, for example but not limited to, trisilicon tetranitride (SisN^. The region of lower refractive index (6a, 6b) may comprise, for example but not limited to, silicon dioxide (SiOz). Other suitable materials may be selected by a skilled person using common general knowledge. The choice of materials may affect the effective refractive index of a cross section of the multi-level waveguide (2).

[0069] FIG. 2C illustrates a lateral cross-sectional view of a PIC (1) according to one or more embodiments. The figure illustrates a multi-level waveguide (2), comprising a first level (4a), second level (4b) layered on the first level (4a), and a third level (4c) layered on the second level (4b). Each level comprises a region of higher refractive index (5a, 5b, 5c), surrounded by a (respective) region of lower refractive index (6a, 6b, 6c).

[0070] In an embodiment, different regions of higher refractive index (5a, 5b, 5c) may be made of different materials. In another alternative embodiment, different regions of higher refractive index (5a, 5b, 5c) may be made of the same material.

[0071] In an embodiment, different regions of lower refractive index (6a, 6b, 6c) may be made of different materials. In another alternative embodiment, different regions of lower refractive index (6) may be made of the same material.

[0072] Although the number of levels in FIG. 3 is limited to three, it should be noted that the number of levels can be extended by layering additional levels on top, in a similar fashion to how the third level (4c) is layered on the second level (4b). However, layering more or thicker levels onto existing levels may induce substantially large amounts of stress at the interface of regions of different materials. During the growth of a region of higher refractive index onto a region of lower refractive index, this stress may become excessive such that the region of higher refractive index may fracture. The multi-level waveguide (2) may be configured such that it allows for growing a region of higher refractive index on top of a region of lower refractive index without the regio of higher refractive index fracturing due to stress at the interface of the regions of different materials. It should furthermore be noted that regardless of the particular configuration and shape of the regions of higher refractive index (5a, 5b, 5c) and lower refractive index (6a, 6b, 6c) of embodiment of FIG. 2, each level (4a, 4b, 4c) may individually comprise any type of waveguide structure. At least one of the levels (4a, 4b, 4c) may comprise a stripe waveguide structure. At least one of the levels (4, 4b, 4c) may comprise a ridge waveguide structure. At least one of the levels (4, 4b, 4c) may comprise a box-shell waveguide structure. In an embodiment, at least two of the levels (4, 4b, 4c) may comprise a same waveguide structure. In an embodiment, at least two of the levels (4, 4b, 4c) may comprise a different waveguide structure.

[0073] FIG. 3 shows an overview of three lateral cross-sectional views of different PICs. The cross sections may be taken in a direction perpendicular to a direction of propagation of guided light. FIG. 3A illustrates a lateral cross-sectional view of a PIC identical to that of FIG. 2A. FIG. 3A is incorporated in FIG. 3 for the purpose of comparison.

[0074] FIG. 3B illustrates a lateral cross-sectional view of a PIC (1) identical to the cross-sectional view of FIG. 2B. FIG. 3B illustrates relevant dimensions for a double-level waveguide, according to one or more embodiments. The double-level waveguide of FIG. 3B may represent a double-stripe waveguide. As noted before, however, the multi-level waveguide (2) of the invention is not limited to such a particular embodiment.

[0075] A width w is indicated corresponding to the width of the region of higher refractive index (5a) corresponding to the first level (4a). A thickness of the same region is indicated by t. In the particular embodiment of FIG. 3B, the width w and thickness t of the region of higher refractive index (5a) of the first level (4a) are equal to the width and thickness of the region of higher refractive index (5b) of the second level (4b).

[0076] Varying a thickness (t) of at least one region of higher refractive index (5a, 5b) may be used for varying an effective refractive index of a cross section of the multi-level waveguide (2). Varying a width (w) of at least one region of higher refractive index (5a, 5b) may be used for varying an optical mode field (7) of an optical mode guided by the multi-level waveguide (2) (see FIG. 4A- 4B). The optical mode field (7) represents the intensity distribution of the optical mode at or near the end facet of the multi-level waveguide (2). As long as the aspect ratio (w: t) is sufficiently high, a small change in the width would change to the cross-sectional area significantly less than an equally sized change in the thickness would. In other words, the optical mode field (7) can be varied without substantially varying the effective refractive index of a cross section of the multilevel waveguide (2). In the embodiment of FIG. 3B, a cross section of at least one of the regions of higher refractive index (5a, 5b) has a substantially high aspect ratio, preferably higher than four. It should furthermore be noted that, although the shape of the cross sections of the regions of higher refractive index of the particular embodiment of FIG. 3 is rectangular, the regions of higher refractive index may have any shape. The shape of a cross section of a region of higher refractive index may be, for example but not limited to, a rectangle or a trapezoid.

[0077] Varying the shape of a cross section of a region of higher refractive index and varying its dimensions may be used for varying the effective refractive index of a cross section of the multilevel waveguide (2). The shape of a cross section of a region of higher refractive index (5) may be set per level (4). The shape of cross sections, in particular the thickness and / or width, of regions of higher refractive index (5) from a pair of adjacent regions of higher refractive index (5) may be the same. In a different embodiment, the shapes of cross sections of regions of higher refractive index (5) from a pair of adjacent regions of higher refractive index (5) may be different.

[0078] In the embodiment of FIG. 3B, the thickness of at least one of the regions of higher refractive index (5a, 5b) may be substantially small such that the optical mode is weakly guided by the multilevel waveguide (2). Furthermore, the thickness of at least one of the regions of higher refractive index (5a, 5b) may be substantially small in relation to a wavelength of the light guided by the multi-level waveguide (2). Lastly, the thickness of at least one of the regions of higher refractive index (5) may be between 10 and 40 nm, preferably 25 nm.

[0079] The minimum width of the region of higher refractive index (5 a, 5b) may be limited by the limitations of the fabrication method. In the embodiment of FIG. 3B, at least one of the regions (preferably all regions) of higher refractive index (5 a, 5b) may have a width in the range of 100 to 300 nm, preferably 200 nm.

[0080] Referring to FIG. 3B, the distance between the two adjacent regions of higher refractive index (5a, 5b) is indicated by d. In general, if the distance d is sufficiently large, the two adjacent regions of higher refractive index will operate as independent waveguiding regions. Upon decreasing the distance d, an interaction between the two adjacent regions of higher refractive index will cause them to operate as directional couplers. Upon decreasing the distance d even further, and provided that the thickness of the individual guiding regions are such that an optical mode in the single level waveguide may be weakly guided, an interaction between the two adjacent regions of higher refractive index causes them to operate as one single waveguide. In the embodiment of FIG. 3B, the distance d may be substantially small such that the interaction between two adjacent regions of higher refractive index (5a, 5b) causes them to operate as a single waveguide (2). In the embodiment of FIG. 3B, the distance d is small in relation to a size of at least one of the individual (weakly guided) optical mode fields of said adjacent levels (4a, 4b) to form an effective optical mode field for the multi-level waveguide (2). By effective optical mode field is meant here an optical mode field created by the interaction of the individual optical mode fields of all the levels. In particular when at least one level is weakly guiding its received light, and this weakly guided mode field interacts with the one of its adjacent level, said levels act together as a single waveguide. In the embodiment of FIG. 3B, the distance d is in the range of 50 to 400 nm.

[0081] Varying a distance d between the two regions of higher refractive index (5a, 5b) may further be used for varying an optical mode field (7) of an optical mode guided by the multi-level waveguide (2). Varying a distance d between the two regions of higher refractive index (5a, 5b) may furthermore be used for varying an effective refractive index of a cross section of the multi-level waveguide (2). The distance d may be set in order to obtain a satisfactory effective refractive index of a cross section of the multi-level waveguide (2). The width w of at least one of the regions of higher refractive index (5) may then be set, without substantially affecting the effective refractive index, in order to obtain a satisfactory optical mode field (7).

[0082] FIG. 3C illustrates a lateral cross-sectional view of a PIC (1) identical to the cross-sectional view of FIG. 2C. All that has been described above relating to the embodiment of FIG. 3B, also applies to the embodiment of FIG. 3C. Additionally, the multi-level waveguide (2) of the embodiment comprises a third level (4c).

[0083] Referring to FIG. 3C, it illustrates relevant dimensions for a triple-level waveguide, according to one or more embodiments. The double-level waveguide of FIG. 3C may represent a triple-stripe waveguide. As noted before, however, the multi-level waveguide (2) of the invention is not limited to such a particular embodiment.

[0084] As a consequence of comprising three levels, the embodiment of FIG. 3C comprises two pairs of adjacent regions of higher refractive index (5a, 5b, 5c), with two corresponding distances. The distance between the adjacent regions of higher refractive index (5) of the first level (4a) and the second level (4b) is defined by d12. The distance between the adjacent regions of higher refractive index (5) of the second level (4b) and the third level (4c) is defined by d23.

[0085] The distances d12and d23are represented as the same in the particular embodiment of FIG. 3. However, these distances may be different in other embodiments, i.e. the inequality d12¥= d23may hold. This provides an additional degree of freedom in the design of a cross section of the multilevel waveguide (2), in particular with respect to varying the effective refractive index and varying the optical mode field (7). This principle may be extended to embodiments comprising additional levels (4). Varying the number of levels (4) of the multi-level waveguide (2) may also vary the effective refractive index and the optical mode field (7).

[0086] FIG. 4 shows an overview of three lateral cross-sectional views of different PICs. The cross sections may be taken in a direction perpendicular to a direction of propagation of guided light. FIG. 4A illustrates a lateral cross-sectional view of a PIC identical to that of FIG. 2A / FIG. 3A and an optical mode field corresponding thereto. FIG. 4A is incorporated in FIG. 4 for the purpose of comparison.

[0087] FIG. 4B illustrates a lateral cross-sectional view of a PIC (1) according to the embodiment of FIG 2B and 3B, comprising a double-level waveguide and an optical mode field (7) associated thereto. An optical mode field is commonly quantified by means of its mode field diameter (MFD). In circular optical mode fields, this refers to the diameter of a circle which comprises the collection of points at which the intensity has dropped to e-2times its maximum intensity, with the maximum intensity being located at the center of the circle. The optical mode field (7) of the embodiments of FIG. 4A-4C are in contrast elliptical in shape. The length of the semi-major axis of the ellipse may be defined by a. The length of the semi-minor axis of the ellipse may be defined by b. The ratio (a:b) may be referred to as aspect ratio of the optical mode field.

[0088] Analogous to the MFD in circular optical mode fields, the perimeter of an elliptical optical mode field may correspond to the collection of points at which the intensity of the optical mode has dropped to e ~2times its maximum intensity, with the maximum intensity being located at the center of the ellipse. A size of an optical mode field (7), throughout this description, may then refer to the area confined by the perimeter of the ellipse.

[0089] In the particular embodiments of FIG. 4, the aspect ratios of the individual regions of higher refractive index (5a, 5b, 5c) may be substantially high. Generally, this may cause an elliptical optical mode field with a substantially large aspect ratio, as can be seen in the optical mode field of FIG. 4A, corresponding to the related art.

[0090] In FIG. 4B, it is clearly shown that the addition of a second level (4b) to the multi-level waveguide (2) significantly benefits the roundness of the optical mode field (7). Compared to FIG4a, FIG.4B thus shows a reduction of an effective aspect ratio of the effective mode field is reduced, wherein by effective aspect ratio is meant the aspect ratio of the effective mode field of the multi-level waveguide. As defined before, by effective optical mode field is meant an optical mode field created by the combination of the individual optical mode fields of all the levels of the multi-level waveguide. The effective aspect ratio will be discussed in more detail with reference to FIG. 5. The effects of changing specific parameters on the optical mode field (7) have already been discussed with respect to FIG. 3.

[0091] FIG. 4C illustrates a lateral cross-sectional view of a PIC (1) according to the embodiment of FIG 2C and 3C, comprising a triple-level waveguide and an optical mode field (7) associated therewith. From FIG.4C derives that the addition of a third level (4c) to the multi-level waveguide (2) significantly benefits the roundness of the optical mode field (7). In the particular embodiment, the value of b may approach the value of a such that the aspect ratio of the ellipse may approach a value of 1. This may be attributed to the effective aspect ratio of the effective mode field of the multi-level waveguide becoming closer to 1.

[0092] FIG. 5A illustrates a lateral cross-sectional view of a PIC according to the related art, comprising a square single level waveguide according to the related art and an optical mode field corresponding thereto. The length of the sides of the square cross section is defined as I. The optical mode field of the optical mode that is guided by the square single level waveguide has the shape of a circle, with a radius R1. In such a prior art waveguide, R1is typically dimensioned smaller than I such that the optical mode field is confined to the interior of the guiding region (to obtain a typical strong guidance).

[0093] FIG. 5B illustrates a lateral cross-sectional view of a PIC (1) according to an embodiment according to FIG 2C, 3C, 4C, wherein the triple-level waveguide has regions of higher refractive index with a width w identical to the length I of the square waveguide of FIG. 5A.

[0094] The triple-level waveguide of that embodiment comprises three regions of higher refractive index (5a, 5b, 5c) with equal thickness t and comprises two pairs of adjacent regions of higher refractive index (5a, 5b, 5c) having equal distances d between their respective regions of higher refractive index (5a, 5b, 5c). In an embodiment, the distance d may be in the order of ten times the thickness t. The total distance from the top of the third region of higher refractive index (5c) to the bottom of the first region of higher refractive index (5a), also referred to earlier as effective thickness is then equal to 31 + 2d by construction. This effective thickness may be defined as the distance from the lowest point of the region of higher refractive index (5a) corresponding to the first level (4a) to the highest point of the region of higher refractive index (5c) of the third level (4c) while the effective width may be defined as the distance from the leftmost point of any of the regions of higher refractive index (5) to the rightmost point of any of the regions of higher refractive index (5).

[0095] FIG. 5A and 5B illustrate how for an equivalent width of higher refractive index (1 in Fig 5A, w in FIG. 5B), a multi-level guide compares to a standard square wave waveguide. Instead of having only the dimension I as single design criteria, a multi-level waveguide may offer at least three dimensions w, I and t as design criteria to tune the aspect ratio (and / or the size) of the mode field, as well as the numerical aperture. Typically for an equivalent width of higher refractive index, a multi-level waveguide enables to attain a similar size mode field having a substantially round distribution and a relatively more dispersion free numerical aperture, than a standard square wave waveguide. The numerical aperture varies less depending on the wavelength while the same desired mode field size and shape may still be obtained. The multi-level waveguide offers thus clear advantages over the prior art within the same space constraints. FIG. 6 illustrates the wavelength dependence of the numerical aperture for a single level waveguide according to the related art, and a double and triple-level waveguide according to one or more embodiments. The dispersion character of an embodiment may be characterized by comparison of the NA at specific, relevant wavelengths, or alternatively by evaluating a change in NA over a specific wavelength range. This range may be taken infinitesimally small, such that the change in NA becomes equal to the slope at that point.

[0096] In general, the NA is expected to decrease upon increasing wavelength. If we assume that the decrease of NA upon increasing wavelength is linear, then the slope of a graph representing the NA vs. wavelength characteristic may quantify the dispersion of the NA. Referring to FIG. 6, it becomes immediately apparent that for all A within the provided range, where the subscripts si, dl, and tl, refer to single level, double-level and triplelevel, respectively, and where A is the wavelength. In other words, the magnitude of the slope of the NA vs. wavelength characteristic may become increasingly small upon increasing the number of levels (4) in the multi-level waveguide (2). The parameters of the multi-level (ml) waveguide (2) may be set such that (A) | ~ 0. In a preferred embodiment, the NA is particularly constant over a range of wavelengths within the visible spectrum.

[0097] In case the NA vs. wavelength characteristic is non-linear, the multi-level waveguide (2) may be configured such that « 0, where ANA is represents the difference between the NA at a first preferred wavelength and the NA at a second preferred wavelength and A represents the difference in wavelength between the first and second preferred wavelength. In other words, the NA at a first preferred wavelength may be approximately equal to the NA at a second preferred wavelength, such that NA ( ) « NA(2). This principle may be extended to any number of preferred wavelengths. In a preferred embodiment, the multi-level waveguide (2) may be configured such that the NA of red light may be approximately equal to the NA of green light, which may be approximately equal to the NA of blue light, i.e. / VA .

[0098] This improved dispersion behavior may be attributed to the combination of the levels to obtain an effective refractive index while using relatively more material with low refractive indexes than the prior art having the same effective refractive index (typically materials with a low refractive index have a better dispersion behavior than materials with a high refractive index).

[0099] Furthermore, in an embodiment, the multi-level waveguide (2) is configured to output the light with a substantially low NA. As a consequence, the outputted light from the PIC (1) is substantially collimated, such that less advanced external optical elements are required for application purposes. In a preferred embodiment, the NA lies below 0,2, preferably around and / or below 0, 1. The relevant parameters during fabrication may be set to values as to achieve the desired effects described above.

[0100] 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.

[0101] Further embodiments of the invention are described in the following clauses:

[0102] 1. A photonic integrated circuit (1), comprising: a waveguide (2) for guiding light received from a light source; an end facet (3) at an end of the waveguide (2) for outputting the received light into free space; wherein the waveguide (2) is a multi-level waveguide (2) comprising a first level (4a) and a second level (4b) layered on the first level (4a); wherein each level (4a, 4b) comprises a region of higher refractive index (5a, 5b) surrounded by a region of lower refractive index (6a, 6b).

[0103] 2. The photonic integrated circuit (1) of clause 1, wherein the multi-level waveguide (2) further comprises a third level (4c), layered on the second level (4b), and comprising a region of higher refractive index (5c) surrounded by a region of lower refractive index (6c).

[0104] 3. The photonic integrated circuit (1) of clause 1 or 2, wherein regions of higher refractive levels are each arranged substantially thin enough to obtain individually weakly guided mode fields extending beyond said regions.

[0105] 4. The photonic integrated circuit (1) of the previous clause, wherein a thickness of at least one region of higher refractive index (5a, 5b, 5c) is dimensioned in relation to a wavelength of the received light to weakly guide the received light, wherein more preferably the thickness of the at least one of the regions of higher refractive index (5a, 5b, 5c) is between 5 to 40 nm for light in the visible spectrum.

[0106] 5. The photonic integrated circuit (1) of any of the above clauses, wherein regions of higher refractive index of adjacent levels are arranged substantially close together to act as a single waveguide.

[0107] 6. The photonic integrated circuit (1) of the previous clause, wherein a distance between at least two regions of higher refractive index of adjacent levels is dimensioned in relation to at least a size of at least one of the individual mode fields of said adjacent levels (4) to act as a single waveguide, wherein preferably the distance between at least two regions of higher refractive index (5) of adjacent levels is in the range of 50 to 400 nm for light in the visible spectrum.

[0108] 7. The photonic integrated circuit (1) of any of the previous clauses, wherein the multi-level waveguide (2) is configured to output the received light with a substantially constant numerical aperture, NA, over a wide range of wavelengths, said range of wavelengths preferably including the visible light range.

[0109] 8. The photonic integrated circuit (1) of any of the previous clauses, wherein the multi-level waveguide (2) is configured to output the received light with a numerical aperture, NA, varying less then + / - 50 %, preferably less than + / - 10%, more preferably less than + / - 5%, for light in the visible light range.

[0110] 9. The photonic integrated circuit (1) of any of the previous clauses, wherein the numerical aperture is lower than 0,2, more preferably lower than 0, 1 for light in the visible light range.

[0111] 10. The photonic integrated circuit (1) of any of the last three clauses, wherein the NA is based on an effective refractive index of a cross section of the multi-level waveguide (2) at or near the end facet (3).

[0112] 11. The photonic integrated circuit (1) of the previous clause, wherein the effective refractive index of a cross section of the multi-level waveguide (2) is set based on any one or more of the following:

[0113] - a number of levels (4) of the multi-level waveguide (2);

[0114] - a distance between at least a first region of higher refractive index (5) and a second region of higher refractive index (5) from at least a pair of adjacent regions of higher refractive index (5);

[0115] - a shape of a cross section of at least one of the regions of higher refractive index (5);

[0116] - an area of a cross section of at least one of the regions of higher refractive index (5);

[0117] - respective materials of the regions of higher refractive index (5) and the regions of lower refractive index (6).

[0118] 12. The photonic integrated circuit (1) of any of the previous clauses, wherein the multi-level waveguide (2) is configured such that an aspect ratio of the optical mode field (7) of an optical mode guided by the multi-level waveguide (2) has a value between 0,5 and 1,5; wherein preferably the optical mode field is substantially round.

[0119] 13. The photonic integrated circuit (1) of the previous clause, wherein the aspect ratio of an optical mode field (7) of an optical mode guided by the multi-level waveguide (2) is set based on any one or more of the following:

[0120] - a number of levels (4) of the multi-level waveguide (2);

[0121] - a distance between at least a first region of higher refractive index (5) and a second region of higher refractive index (5) from at least a pair of adjacent regions of higher refractive index (5);

[0122] - a shape of a cross section of at least one of the regions of higher refractive index (5);

[0123] - an area of a cross section of at least one of the regions of higher refractive index (5), preferably a width of at least one region of higher refractive index (5);

[0124] - respective materials of the regions of higher refractive index (5) and the regions of lower refractive index (6).

[0125] 14. The photonic integrated circuit (1) of any of the previous clauses, wherein the multi-level waveguide (2) is configured to output the received light with a predetermined numerical aperture, NA, and with a predetermined optical mode field (7), said predetermined NA and said predetermined optical mode field (7) being set substantially independently from each other.

[0126] 15. The photonic integrated circuit (1) of any of the previous clauses, wherein the multi-level waveguide (2) is configured to output the light with a substantially low numerical aperture, NA, below 0,2, preferably below 0,1, such that the outputted light is substantially collimated.

[0127] 16. The photonic integrated circuit (1) of any of the previous clauses, wherein at least one of the regions of higher refractive index (5) has a width in the range of 100 nm to 300 nm, preferably 200 nm.

[0128] 17. The photonic integrated circuit of any of the previous clauses, wherein a cross section of at least one of the regions of higher refractive index (5) has a shape of a rectangle or a trapezoid. 18. The photonic integrated circuit (1) of any of the previous clauses, wherein a cross section of at least one of the regions of higher refractive index (5) has a substantially high aspect ratio, preferably a width being more than four times a thickness thereof.

[0129] 19. The photonic integrated circuit of any of the previous clauses, wherein an optical mode field (7) of an optical mode guided by the multi-level waveguide (2) has a larger size than an optical mode field (7) of an optical mode guided by a square waveguide (2) with corresponding dimensions.

[0130] 20. The photonic integrated circuit (1) of any of the previous clauses, wherein the multi-level waveguide (2) is configured such that it allows for growing a region of higher refractive index (5) on top of a region of lower refractive index (6) without the region of higher refractive index (5) fracturing due to stress at the interface of the different regions.

[0131] 21. The photonic integrated circuit (1) of any of the previous clauses, wherein a region of higher refractive index (5) comprises trisilicon tetranitride (SisN^.

[0132] 22. The photonic integrated circuit (1) of any of the previous clauses, wherein a region of lower refractive index (6) comprises silicon dioxide (SiOz).

[0133] 23. The photonic integrated circuit (1) of any of the previous clauses, wherein the outputted light comprises light in the visible spectrum, preferably red, green, or blue light.

[0134] 24. The photonic integrated circuit (1) of any of the previous clauses, further comprising a light source, preferably being a laser diode.

[0135] 25. The photonic integrated circuit (1) of any of the previous clauses, wherein at least one of the levels (4) comprises a stripe waveguide structure.

[0136] 26. The photonic integrated circuit (1) of any of the previous clauses, wherein at least one of the levels (4) comprises a ridge waveguide structure.

[0137] 27. The photonic integrated circuit (1) of any of the previous clauses, wherein at least one of the levels (4) comprises a box shell waveguide structure. 28. The photonic integrated circuit (1) of any of the previous clauses, wherein at least two of the levels (4) comprise a same waveguide structure.

[0138] 29. The photonic integrated circuit (1) of any of the previous clauses, wherein at least two of the levels (4) comprise a different waveguide structure.

Claims

25CLAIMS1. A photonic integrated circuit (1) configured for free space projection, comprising: a waveguide (2) for guiding light received from a light source; an end facet (3) at an end of the waveguide (2) configured for projecting the received light into free space; wherein the waveguide (2) is a multi-level waveguide (2) consisting of a first level (4a), a second level (4b) layered on the first level (4a), and a third level (4c) layered on the second level (4b); wherein each level (4a, 4b) comprises at the end facet a region of higher refractive index (5a, 5b) surrounded by a region of lower refractive index (6a, 6b).

2. The photonic integrated circuit (1) of claim 1, wherein at the end facet the regions of higher refractive index have the same width and the same thickness.

3. The photonic integrated circuit (1) of claim 1 or 2, wherein regions of higher refractive levels are each arranged substantially thin enough to obtain individually weakly guided mode fields extending beyond said regions.

4. The photonic integrated circuit (1) of the previous claim, wherein a thickness of at least one region of higher refractive index (5a, 5b, 5c) is dimensioned in relation to a wavelength of the received light to weakly guide the received light, wherein more preferably the thickness of the at least one of the regions of higher refractive index (5a, 5b, 5c) is between 5 to 40 nm for light in the visible spectrum.

5. The photonic integrated circuit (1) of any of the above claims, wherein regions of higher refractive index of adjacent levels are arranged substantially close together to act as a single waveguide.

6. The photonic integrated circuit (1) of the previous claim, wherein a distance between at least two regions of higher refractive index of adjacent levels is dimensioned in relation to at least a size of at least one of the individual mode fields of said adjacent levels (4) to act as asingle waveguide, wherein preferably the distance between at least two regions of higher refractive index (5) of adjacent levels is in the range of 50 to 400 nm for light in the visible spectrum.

7. The photonic integrated circuit (1) of any of the previous claims, wherein at the end facet the ratio between regions of lower refractive index and regions of higher refractive index within the multi-level waveguide is such that the multi-level waveguide (2) is configured to output the received light with a substantially constant numerical aperture, NA, over a wide range of wavelengths, said range of wavelengths preferably including the visible light range.

8. The photonic integrated circuit (1) of any of the previous claims, wherein the multi-level waveguide (2) is configured to output the received light with a numerical aperture, NA, varying less then + / - 50 %, preferably less than + / - 10%, more preferably less than + / - 5%, for light in the visible light range.

9. The photonic integrated circuit (1) of any of the previous claims, wherein the numerical aperture is lower than 0,2, more preferably lower than 0, 1 for light in the visible light range.

10. The photonic integrated circuit (1) of any of the last three claims, wherein the NA is based on an effective refractive index of a cross section of the multi-level waveguide (2) at or near the end facet (3).

11. The photonic integrated circuit (1) of the previous claim, wherein the effective refractive index of a cross section of the multi-level waveguide (2) is set based on any one or more of the following:- a number of levels (4) of the multi-level waveguide (2);- a distance between at least a first region of higher refractive index (5) and a second region of higher refractive index (5) from at least a pair of adjacent regions of higher refractive index (5);- a shape of a cross section of at least one of the regions of higher refractive index (5);- an area of a cross section of at least one of the regions of higher refractive index (5);- respective materials of the regions of higher refractive index (5) and the regions of lower refractive index (6).

12. The photonic integrated circuit (1) of any of the previous claims, wherein the multi-level waveguide (2) is configured such that an aspect ratio of the optical mode field (7) of anoptical mode guided by the multi-level waveguide (2) has a value between 0,5 and 1,5; wherein preferably the optical mode field is substantially round.

13. The photonic integrated circuit (1) of the previous claim, wherein the aspect ratio of an optical mode field (7) of an optical mode guided by the multi-level waveguide (2) is set based on any one or more of the following:- a number of levels (4) of the multi-level waveguide (2);- a distance between at least a first region of higher refractive index (5) and a second region of higher refractive index (5) from at least a pair of adjacent regions of higher refractive index (5);- a shape of a cross section of at least one of the regions of higher refractive index (5);- an area of a cross section of at least one of the regions of higher refractive index (5), preferably a width of at least one region of higher refractive index (5);- respective materials of the regions of higher refractive index (5) and the regions of lower refractive index (6).

14. The photonic integrated circuit (1) of any of the previous claims, wherein the multi-level waveguide (2) is configured to output the received light with a predetermined numerical aperture, NA, and with a predetermined optical mode field (7), said predetermined NA and said predetermined optical mode field (7) being set substantially independently from each other.

15. The photonic integrated circuit (1) of any of the previous claims, wherein the multi-level waveguide (2) is configured to output the light with a substantially low numerical aperture, NA, below 0,2, preferably below 0,1, such that the outputted light is substantially collimated.

16. The photonic integrated circuit (1) of any of the previous claims, wherein at least one of the regions of higher refractive index (5) has a width in the range of 100 nm to 300 nm, preferably 200 nm.

17. The photonic integrated circuit of any of the previous claims, wherein a cross section of at least one of the regions of higher refractive index (5) has a shape of a rectangle or a trapezoid.2818. The photonic integrated circuit (1) of any of the previous claims, wherein a cross section of at least one of the regions of higher refractive index (5) has a substantially high aspect ratio, preferably a width being more than four times a thickness thereof.

19. The photonic integrated circuit of any of the previous claims, wherein at the end facet the regions of higher refractive index of the three levels are layered on each other such that an optical mode field (7) of an optical mode guided by the multi-level waveguide (2) has a larger size than an optical mode field (7) of an optical mode guided by a square waveguide (2) with corresponding dimensions.

20. The photonic integrated circuit (1) of any of the previous claims, wherein the multi-level waveguide (2) is configured such that it allows for growing a region of higher refractive index (5) on top of a region of lower refractive index (6) without the region of higher refractive index (5) fracturing due to stress at the interface of the different regions.

21. The photonic integrated circuit (1) of any of the previous claims, wherein a region of higher refractive index (5) comprises trisilicon tetranitride (SisN^.

22. The photonic integrated circuit (1) of any of the previous claims, wherein a region of lower refractive index (6) comprises silicon dioxide (SiOz).

23. The photonic integrated circuit (1) of any of the previous claims, wherein the outputted light comprises light in the visible spectrum, preferably red, green, or blue light.

24. The photonic integrated circuit (1) of any of the previous claims, further comprising a light source, preferably being a laser diode.

25. The photonic integrated circuit (1) of any of the previous claims, wherein at least one of the levels (4) comprises a stripe waveguide structure.

26. The photonic integrated circuit (1) of any of the previous claims, wherein at least one of the levels (4) comprises a ridge waveguide structure.

27. The photonic integrated circuit (1) of any of the previous claims, wherein at least one of the levels (4) comprises a box shell waveguide structure.2928. The photonic integrated circuit (1) of any of the previous claims, wherein at least two of the levels (4) comprise a same waveguide structure.

29. The photonic integrated circuit (1) of any of the previous claims, wherein at least two of the levels (4) comprise a different waveguide structure.

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