Optical device for controlling light from an external light source

Abstract

An optical device (1) for controlling light from an external light source (2), comprising: at least a first and a second waveguide (3, 4) for guiding a light beam emitted from the external light source (2); and at least a first beam shaping structure (5) having a first light emitting area (6) for emitting the light beam, and a second beam shaping structure (7) having a second light emitting area (6) for emitting the light beam, wherein the first waveguide (3) guides the light beam emitted from the external light source (2) to the first beam shaping structure (5), and the second waveguide (4) guides the light beam emitted from the external light source (2) to the second beam shaping structure (7); wherein the first and second beam shaping structures (5, 7) are both configured such that the uniformity of the radiation density profile of the light beam emitted from the corresponding light emitting area (6, 8) of the corresponding beam shaping structure (5, 7) is higher than the uniformity of the radiation density profile of the light beam guided by the corresponding waveguide (3, 4) to the corresponding beam shaping structure (5, 7). A backlight unit (22) comprising such an optical device (1); and an optical system, in particular a holographic display, comprising such a backlight unit (22) and a spatial light modulator (17).

Description

Technical Field

[0001] The present invention relates to an optical device for controlling light from an external light source, comprising at least first and second waveguides for guiding a light beam emitted from the external light source, and at least a first beam shaping structure having a first light emitting area for emitting the light beam and a second beam shaping structure having a second light emitting area for emitting the light beam, wherein the first waveguide guides the light beam emitted from the external light source to the first beam shaping structure and the second waveguide guides the light beam emitted from the external light source to the second beam shaping structure. Background Art

[0002] Optical devices are known which distribute coherent light over a larger area. This is necessary, for example, for backlight units of holographic displays. Holographic displays are able to reproduce the light field emitted by objects essentially faithfully and are therefore visually indistinguishable. They have substantial advantages over, for example, (auto)stereoscopic displays, which provide one image per eye and thus introduce a sense of three-dimensionality, but suffer from a fundamental problem: since the brain perceives a three-dimensional (3D) scene, the eye should be able to focus on any part of it, but the display can only show one focal plane of the scenery. This mismatch leads to visual fatigue and nausea, which is a major problem and one of the factors hindering the widespread adoption of 3D displays. Holographic displays avoid this problem and create images that look completely natural to the eye.

[0003] The task of dynamically creating the wavefront is performed by a spatial light modulator (SLM), which can alternate the relative phase (and amplitude) between pixels between 0 and 2π, thereby creating an arbitrary wavefront. Both transmissive SLMs, which work similarly to liquid crystal displays, and reflective SLMs, i.e., liquid crystal on silicon (LCOS) and / or digital light processing (DLP), have been used to create holographic images. Both SLMs and LCOS are compact and suitable for use in, for example, head-mounted holographic goggles.

[0004] However, a key challenge is how to provide a coherent plane wave to the SLM in a compact and reliable manner. While it is relatively easy to expand a laser beam to a large diameter using lenses in a laboratory setting, this approach is neither practical nor economical for commercial devices, e.g. due to their large size. Interference-based backlight units can be implemented in a fairly compact and even transparent manner, and various types have been demonstrated / proposed. Interference-based devices are very sensitive to temperature and humidity changes and cannot compensate for these effects because the phase of the emitted beam is not well defined or cannot be well determined under such conditions, which poses a challenge for commercial applications. This is a problem especially for consumer hardware because the standard temperature range is 0°C to 70°C. The frequently used holographic volume gratings are also susceptible to temperature and humidity changes, resulting in changes in the emitted wavefront, which degrades the holographic image quality, and cannot fully compensate for the degradation of the holographic image quality for the same reasons. In addition, the efficiency of holographic gratings remains a major challenge.

[0005] Well-known is the backlight unit for conventional display, namely produces incoherent backlight. For example, US 2013 / 0308339A1 discloses a light valve device with a transparent plate (slab). Light emitted from a (coherent or incoherent) light source is diffused in the plate, which acts as a single waveguide, and the light is reflected on its surface. The transparent plate includes a stepped rear surface, which is used to emit light from the front surface of the transparent plate. However, the light leaving the front surface at one point may take many different light paths within the plate, so the emitted light cannot be coherent, and its phase is not well defined. In addition, the light will not be emitted uniformly on the front surface. Therefore, the device is not suitable as a backlight unit for, for example, a holographic display.

[0006] On the other hand, US 9,395,690 B2 shows a holographic display with an illumination device, an amplification unit and a light modulator. The illumination device comprises at least one light source and a light collimation unit. The light collimation unit collimates the light of the at least one light source. Cascaded fiber switches in the light collimation unit allow the lenses in the collimating lens array to be illuminated. The collimating lens array may comprise cylindrical lenses. However, this does not allow a compact design. The subsequent use of a transmissive volume grating further reduces the clarity of the phase at emission and, associated with this, also reduces the stability to changes in environmental conditions (e.g. ambient temperature). Therefore, it cannot provide a stable homogeneity of coherent light in certain areas.

[0007] US 2009 / 180282 A1 shows an LCD backlight unit in which light propagates in a panel and is extracted from the panel by a plurality of light extraction features. Therefore, the light takes an arbitrary path, no coherent light is provided, and the uniformity of the light extracted by each light extraction feature is not increased.

[0008] US 2010 / 259804 A1 shows a lighting unit with a waveguide and output coupling elements. However, the output coupling elements only emit light, and the uniformity of the light emitted from each output coupling element is not increased.

[0009] Therefore, no known optical devices are suitable for compact, in particular flat, reliable holographic displays. However, other applications also require optical devices that distribute coherent light over a larger area, such as structured light sensing (3D sensing) or LIDAR. In particular, the efficiency of the optical device is also a very important aspect for these applications and, among other points, cannot yet be fully achieved with current optical devices. Summary of the invention

[0010] An object of the present invention is to solve or alleviate at least some of the problems of the prior art and to provide an optical device that allows uniform illumination of an area with coherent light in a compact manner by controlling light from an external light source. In particular, in a compact manner it is meant that the optical device should allow such illumination of the area while having a reduced dimension perpendicular to the area compared to a solution that essentially covers the same area using a single beam shaping structure (e.g. a single set of lenses).

[0011] This is achieved by an optical device for controlling light from an external light source as described at the beginning, wherein the first and second beam shaping structures are each configured so that the radiation density and / or intensity profile of the light beam emitted from the corresponding light emitting area of ​​the corresponding beam shaping structure has a higher uniformity than the radiation density and / or intensity profile of the light beam guided to the corresponding beam shaping structure by the corresponding waveguide.

[0012] In this way, uniform illumination of the first and second light-emitting areas can be achieved. At the same time, since the light beam is guided by the first and second waveguides to the corresponding beam shaping structure and emerges from the corresponding light-emitting area, the light path of the light emerging from the corresponding light-emitting area and therefore its phase is well defined. In addition, from coupling into the corresponding waveguide until emission from the corresponding light-emitting area, the light from the first and second corresponding elements will not interfere with each other, and there will be no mixing between them. In other words, the present disclosure generally avoids mixing between two or more light paths used to cover the entire light-emitting area. Due to this separation of the light paths, the device is relatively insensitive to temperature changes and the resulting deformations, thereby maintaining the consistency (coherence) of each light path separately. Ideally, the phase of the light at each point in the area can be well defined or easily determined, especially depending on the temperature of the optical device or the ambient temperature. In addition, this arrangement with at least a first and a second waveguide, each of which guides light to a beam shaping structure as defined above and is associated with one of the waveguides, allows a more compact optical device compared to, for example, a single lens that expands the light beam to a large diameter.

[0013] The influence of the beam shaping structure should manipulate the beam shape to become more similar to a top-hat beam shape. Under uniformity of the radiant density profile, the beam uniformity (U η ), where η is 0.3 and lower (i.e. closer to 0), U η =U means higher beam uniformity. Therefore, the higher uniformity of the radiation density profile of the beam emitted from each light-emitting area of ​​each beam shaping structure than the radiation density profile of the beam guided to each beam shaping structure by each waveguide means that the beam uniformity of the beam emitted from the corresponding light-emitting area (when emitted from the corresponding light-emitting area) is higher than the beam uniformity of the beam guided to the corresponding beam shaping structure by the corresponding waveguide (when leaving the corresponding waveguide), that is, U η Lower and closer to 0. U η Optionally, the effect of the beam shaping structure is reduced by at least 0.05, at least 0.1 or at least 0.2. Optionally, the beam uniformity is also increased (hence U ) due to η being one of 0.1, 0.2, 0.4, 0.5, 0.6 or 0.7. η This measure is applied under the assumption that the beam provided by the external light source is a Gaussian beam.

[0014] The optical device does not necessarily form a complete backlight unit, but can, for example, be a component of a backlight unit. The optical device does not necessarily need to emit a plane wave, nor does the optical device necessarily comprise an external light source. The individual light emitting areas do not need to be physical surface areas, but can also be, for example, boundaries between different optical elements. The edges of the first and / or second light emitting areas do not need to be structurally defined, but they represent areas through which the light beam is emitted from the respective beam shaping structure. Furthermore, the light emitted from the first and / or second light emitting area does not necessarily need to emit immediately from the optical device; however, it can emit immediately from the optical device.

[0015] The optical device comprises at least two (i.e., first and second) waveguides. Preferably, the optical device comprises a number of waveguides greater than two for guiding a light beam emitted from an external light source, and a number of beam shaping structures greater than two, each of the number of beam shaping structures comprising a corresponding light emitting area for emitting a light beam, wherein each of the number of waveguides guides the light beam emitted from the external source to a corresponding one of the number of beam shaping structures, wherein each of the number of beam shaping structures is configured such that: the uniformity of the radiation density profile of the light beam emitted from the corresponding light emitting area of ​​the corresponding one of the number of beam shaping structures is higher than the uniformity of the radiation density profile of the light beam guided to the corresponding beam shaping structure by the corresponding waveguide. Therefore, the preferred embodiments of the first and second beam shaping structures and / or the first and second waveguides may also be applied to a number of waveguides and / or beam shaping structures greater than two and additional elements interacting with the first and second waveguides, and / or the first and second beam shaping structures may also interact with a number of waveguides and / or beam shaping structures greater than two in a similar manner. Preferably, the optical device comprises a number of at least ten waveguides, beam shaping structures, and corresponding associated elements.

[0016] The at least first and second waveguides may be optical waveguides and may be separate from each other, i.e. not overlapping. The first and / or second light emitting areas may be planar or flat. The first and second light emitting areas may be located in the same plane. The first and second light emitting areas may be adjacent to each other, i.e. less than 10% of the maximum dimension of the light emitting areas that are spaced apart from each other. Furthermore, the first and second light emitting areas may be continuous or substantially continuous at least in one or more directions. The first and / or second light emitting areas may, for example, have an elliptical, in particular circular or hexagonal shape. The maximum dimension of the first and / or second light emitting areas is preferably less than 1 mm. When used in a BLU of a holographic display, the light emitting areas of the optical device may, but need not, correspond to pixels of the holographic display.

[0017] In an optional embodiment, each of the first and second beam shaping structures may include an optical shaping element, and the first and second waveguides guide the beam emitted from the external light source to the corresponding optical shaping elements of the first and second beam shaping structures, and the optical shaping element is configured to manipulate the wavefront and / or radiation intensity distribution of the beam directed to the corresponding optical shaping element. Preferably, the manipulation is a bijective mapping and / or the beam is manipulated so that the rays of the beam do not intersect. If the incident beam is coherent, the manipulation should make the resulting beam coherent and / or the coherence length of the beam not reduced. Optionally, the phase profile distribution and the amplitude profile distribution are manipulated in the same manner. Further optionally, the corresponding optical shaping element is spaced apart from the corresponding light emitting area. Optionally, the first and second waveguides guide the beam to the corresponding optical shaping element so that the beam is directed to the light emitting area after acting with the corresponding optical shaping element.

[0018] The optical shaping elements of the first and / or second beam shaping structure may be optical profiler elements which are configured to increase the uniformity of the radiation intensity (or density) profile of the light beam directed to the respective optical shaping element. The optical profiler element may in particular be configured for manipulating a Gaussian beam in this way. Optionally, the optical profiler element is configured for shaping the Gaussian beam into a top-hat beam. The optical profiler element may be a diffractive optical element (DOE). In this way, it is easy to illuminate a certain area uniformly with coherent light. Furthermore, this allows a thin layout of the optical arrangement.

[0019] In a further optional embodiment, the optical shaping elements of the first and / or second beam shaping structure may be optical divergence elements, which are configured to increase the beam divergence of the light beam directed to the respective optical divergence element. The optical divergence element may be a mirror, such as a micromirror, in particular a curved micromirror, or a diffractive optical element. The optical divergence element widens the beam divergence angle, i.e. widens the radiation intensity distribution. This allows illuminating a larger area with a beam emitted from an external source. In another optional embodiment, each optical shaping element is both an optical profiler element and an optical divergence element.

[0020] According to yet another optional embodiment, the optical device may include an interface, wherein the first and second light emitting areas are provided by the interface. For example, each of the first and second light emitting areas may be defined by total internal reflection outside the corresponding light emitting area of ​​the light beam directed toward the interface after acting with the corresponding optical shaping element. In this way, the unwanted interference effect of the light cones overlapping at the interface can be avoided by using total internal reflection. This allows only a portion of the light beam / light cone to be selected to leave the interface, and the rest is "cut off". This "cut-off" effect can be used to select a portion of the light beam to be transmitted outside the light emitting area and then used for lighting purposes, while the other portion is reflected at the interface. This can be used to produce a uniform intensity distribution on the first and second light emitting areas or interfaces, respectively, because the central area of ​​the light beam or the central area of ​​the light cone of the light beam can be selected, thereby increasing the uniform intensity distribution. For example, the external light source can emit a Gaussian-like mode, and in order to limit the field intensity fluctuations on the interface (and / or the light emitting surface defined below) to 20%, only the central part within a radius of approximately 50% of the beam intensity of the light beam is used, while the rest is totally internally reflected. Furthermore, if the interface is arranged as the front surface of a plate with a parallel rear surface, the portion of the beam that undergoes total internal reflection will never leave the plate through the interface and leave the plate from the side of the plate. This mechanism is particularly useful if the optical shaping element is an optical divergent element. By adjusting how much the beam divergence is increased, it is possible to adjust which part of the beam will leave the interface and which part will be totally internally reflected. The part that is totally internally reflected does not belong to the beam emitted from each beam shaping structure and therefore from each light emitting area. Therefore, in this way, the uniformity of the radiation density profile of the beam emitted from each beam shaping structure can be increased by cutting off the outer part of the beam. Alternatively, the optical shaping element can guide the beam to the interface so that the beam is irradiated perpendicularly on the interface. That is, the central axis of the beam defined by the first moment of the power density distribution (as defined in ISO 13694:2018) for different z between the optical shaping element and the interface is perpendicular to the interface. This can increase uniformity and the corresponding light emitting area can be circular.

[0021] The distance of the light emitting area or the interface, respectively, from the individual optical shaping elements and the distance of the optical shaping elements from one another is given by the beam shaping technology used and whether total internal reflection is used. For example, if a Gaussian beam reflected from a curved micromirror is used in combination with total internal reflection, then for a distance of 500 μm from the optical shaping element to the interface, the distance of the optical shaping elements from one another is about 880 μm, in particular from 886 μm (if it should be configured for a blue external light source) to 877 μm (for a red external light source).

[0022] The interface may be a boundary interface between a first medium (in which the light beam propagates before being emitted from the corresponding light emitting region) and a second medium, wherein the refractive index of the first medium is higher than or equal to the refractive index of the second medium. In particular, it may be a boundary from a (glass) substrate to air, or a boundary from substrate to substrate.

[0023] It is advantageous if the first light emitting area does not overlap with the second light emitting area. In this way, a coherent light beam (with a well defined phase or optical path length, respectively) is emitted from the interface.

[0024] Optionally, the optical device comprises a light emitting surface, wherein the first light emitting area and the second light emitting area are provided by the light emitting surface, and wherein a (preferably hexagonal) honeycomb structure of an absorbing material is provided on top of the light emitting surface. For example, a first (preferably hexagonal) unit of the (preferably hexagonal) honeycomb structure may contain a first light emitting area, and a second (preferably hexagonal) unit of the (preferably hexagonal) honeycomb structure may contain a second light emitting area. The honeycomb structure and its units may also be, for example, rhombohedral or triangular. The light emitting surface may be spaced apart from the optical shaping element. Optionally, the light emitting surface is the interface. Typically, the light emitting area will have an elliptical, in particular circular, perimeter. Therefore, if the light emitting areas do not overlap, the light emitting surface may not be completely filled by the light emitting areas, so that light beams from different light paths interfere with each other. By using a honeycomb structure on top of the light emitting surface, an absorptive boundary can be formed between different light emitting areas, so that the spot shape of light beams with different light paths can be defined, thereby achieving a higher lighting fill rate of the light emitting surface, up to 100% (at the expense of additional light loss). Alternatively, the light emitting surface may be partially coated with an absorbent material to shape the light emitting area to achieve an effect similar to an elevated honeycomb structure.

[0025] The optical device may include at least one bus waveguide for guiding a light beam emitted from an external light source, and at least first and second optical couplers, in particular directional couplers (i.e. couplers arranged to distribute light from the bus waveguide to the waveguides by evanescent wave coupling), wherein the first optical coupler couples the light emitted from the external light source from the bus waveguide to the first waveguide, and the second optical coupler couples the light beam emitted from the external light source from the bus waveguide to the second waveguide. In particular, the first and / or second optical couplers include the bus waveguide and the first and second waveguides, respectively, and / or are formed (at least partially) by the bus waveguide and the first and second waveguides, respectively. For example, the optical coupler may be formed simply by bringing the first waveguide and the second waveguide, respectively, sufficiently close to the bus waveguide. This allows the light beam emitted from the external light source to be easily distributed to the first and second waveguides. The bus waveguide may also guide the light emitted from the external light source to more than two waveguides and / or more than one bus waveguide may be provided. At least one higher-level bus waveguide may also be provided to guide a light beam emitted from an external light source to at least one lower-level bus waveguide or to couple light into a lower-level bus waveguide (and optionally to one or more beam shaping structures) via an optical coupler, wherein the lower-level bus waveguide guides light to the one or more beam shaping structures.

[0026] The optical device may comprise a transparent substrate, wherein the first and second waveguides are formed in the transparent substrate. In this way, the waveguides can easily be arranged in a compact manner and with a narrow spacing between them. Furthermore, temperature sensitivity is well known and can therefore be easily compensated. For example, the first and second waveguides are formed in the transparent substrate by direct laser writing, in particular femtosecond laser writing. In this way, the transparent substrate can be very thin, for example having a thickness of less than 500 μm. (Femtosecond) laser written waveguides combine the ability to build 3D optical circuits with low propagation losses (e.g. <0.2 dB / cm), high temperature stability, ease of manufacture and integration with other (laser written) micro-optical components. Furthermore, the at least one bus waveguide may also be formed in the transparent substrate (again, for example, by direct laser writing). The light emitting surface and / or the interface may be provided by the (front) surface of the transparent substrate.

[0027] The first beam shaping structure and the second beam shaping structure may be provided at least in part by a transparent substrate, in particular, the respective optical shaping elements may be provided by a transparent substrate. The optical shaping elements, for example in the form of micromirrors, may be processed using (femtosecond) laser ablation to very high precision and almost arbitrary shapes. Subsequently, they may be polished, for example, by a (CO2) laser to improve the surface quality. For example, the optical shaping elements may be formed at or near the back side of the substrate. Specifically, the optical shaping elements may be micromirrors formed on the back side of the substrate. Furthermore, the optical shaping elements may be diffractive optical elements contacting the back side of the substrate. The back side is, for example, a surface opposite to the front surface, in particular a surface opposite to the light emitting surface and / or opposite to the interface (regardless of the optical shaping elements, such as micromirrors, that are ultimately formed). The back side may be substantially parallel to the front surface, the light emitting surface and / or the interface.

[0028] The backlight can then work as follows: (femtosecond) laser-written waveguides and marked bus waveguides embedded in the thin glass substrate deliver laser light from an external laser (which may not be part of the optical device) to the entire surface area of ​​the (transparent) substrate; optical (directional) couplers are provided to distribute the light from the bus waveguides to the waveguides and thus to various locations on the back side of the substrate where the micromirrors or DOEs are located, by evanescent coupling between the waveguides; the latter are used to create light cones, each of which illuminates multiple pixels of, for example, an SLM (which may not be part of the optical device). By using total internal reflection at the glass-air boundary, those light cones that would otherwise overlap and cause unwanted interference effects are clipped and isolated.

[0029] Light originating from an external light source (which can be located at the edge of the transparent substrate) is distributed over the front surface area using a femtosecond laser to write waveguides and beam shaping structures. These waveguides can follow any desired 3D trajectory within the substrate, and optical (directional) couplers are used to couple light between them. A minimum waveguide spacing of 20 μm is easily achieved, enabling very dense waveguide networks.

[0030] Micromirrors can be made very small (e.g. <30 μm in diameter), depending on the surface quality of the mirror structure and the free space propagation distance (in the substrate) of the light beam after leaving the corresponding waveguide. Reflection from the optical shaping element (e.g. micromirror) can occur purely due to total internal reflection at the glass-air interface or by coating the mirror structure with a reflective material (e.g. silver). The optical shaping element, in particular the micromirror, can also be manufactured by any means such as glass press molding or polymer microstructure fabrication.

[0031] The optical path lengths from the external light source to the light emitting area, especially to an external device such as an SLM, are different, so the phases from different points will be different. These phase shifts can be easily calibrated and compensated in the SLM. Due to the different optical path lengths, temperature changes will affect the different phases in different ways and in proportion to their respective optical path lengths. To reduce this effect, glasses with very low thermal-optical expansion coefficients can be used for the transparent substrate (e.g. Corning EAGLE XG, δ l =31.7·10 -7 / °C), so the phase shift can be small and can be more easily corrected using, for example, an SLM.

[0032] For commercial applications and devices, the optical device, and more specifically the device in which it may be used, such as a holographic backlight, should operate stably over a large temperature range, such as 0°C to 70°C for mobile devices or even -40°C to 125°C for the automotive industry. Temperature variations can lead to phase shifts between different pixels (e.g. different luminous areas) of the optical device and in particular to phase shifts between different pixels (e.g. different luminous areas) of the SLM illuminated by the optical device, which can degrade the image quality or even completely destroy it. Nevertheless, since the task of the SLM is to introduce a phase shift at each pixel anyway, any phase shift can be easily compensated if (a) the phase shift occurs slowly and (b) the phase shift is known. A typical SLM can update the phase value of each pixel within about 10 ms, wherein it can be assumed that temperature shifts of no more than 1 mK occur under usual conditions. Furthermore, it can be assumed, for example, that a temperature change of 0.01°K occurs in a sample of 100 mm in length. The volume expansion coefficient of borosilicate glass, for example, is 31.7·10 -7 / °K, the refractive index change rate due to temperature change is 2.2·10 -6 / ° K. At a wavelength of 450 nm, the refractive index is, for example, n0 = 1.51825. The total phase change is then calculated to be 0.5% of the phase-flip and can therefore be considered negligible.

[0033] For longer time scales and larger temperature drifts, condition (b) becomes relevant. The simplest way to combat these phase shifts is to use a Mach-Zehnder-like structure, in particular in a transparent substrate. Therefore, advantageously, the optical device comprises: at least first and second interference waveguides formed in the substrate, a first interferometer coupler for coupling a light beam from the first interference waveguide to the second interference waveguide, a second optical interferometer coupler for coupling a light beam from the first interference waveguide to the second interference waveguide, and a photodetector connected to the first interference waveguide.

[0034] Alternatively, a second optical interferometer coupler may be used to couple the light beam from the second interference waveguide to the first interference waveguide, and the photodetector may be connected to the second interference waveguide. Furthermore, alternatively, the optical device, in particular the Mach-Zehnder structure, may only include: at least first and second interference waveguides formed in a substrate, a photodetector for measuring interference of the light beam from the first interference waveguide and the light beam from the second interference waveguide.

[0035] More than one such Mach-Zehnder structure may be provided. The photodetector is, for example, a photodiode. The first and second interference waveguides may be comparable to different arms of an interferometer of the Mach-Zehnder structure and their lengths may be different. A temperature-dependent phase shift occurs between the first and second interference waveguides, which can be read out in terms of intensity fluctuations by a photodetector (e.g. at the end / side of the substrate). Multiple Mach-Zehnder structures may be constructed to improve the measurement accuracy. Preferably, the first interference waveguide is configured to receive a light beam emitted from an external light source. Any wavelength may be used, but it is most convenient to use the same wavelength that is also propagated in the other waveguides. Over a temperature range from 0°C to 70°C, the total phase drift of a waveguide of 100 mm length may be (in the example above) 366·2π. Using one or more Mach-Zehnder structures, the phase drift may be determined and may be compensated for, for example, by an SLM. The lengths of the first and second interference waveguides should be selected so that for the maximum expected temperature excursion, the resulting phase shift is no greater than 2π. Additional Mach-Zehnder structures with different lengths can be constructed to improve the measurement accuracy.

[0036] Another consideration is the wavelength stability of the external light source. Vertical Cavity Surface Emitting Lasers (VCSEL) and Distributed Feedback Laser (DFB) diodes typically have a wavelength stability of 0.07nm / °C. Therefore, a wavelength variation of 0.002nm will result in a 2π phase difference between pixels 100mm apart. Therefore, the external light source is preferably temperature stabilized, as is typically done for laser diodes used for telecommunication purposes, for example. Advantageously, the external light source can be positioned at a distance from the optical device and the light beam emitted from the external light source is guided to the optical device via an optical fiber.

[0037] In an optional embodiment, the optical device comprises a beam divergence reducing structure for reducing the beam divergence of the light beam emitted from the first and second light emitting areas. Preferably, the beam divergence structure shapes the light beam emitted from the first and second light emitting areas such that the wavefront of the light beam has the shape of a (spatially confined) plane wave. In this way, the optical device can provide a coherent plane wave. The optical divergence reducing structure is preferably arranged in front of or formed by the interface, the light emitting surface and / or the front surface of the transparent substrate. The optical divergence reducing structure is preferably a DOE or a lens array, in particular a Fresnel array.

[0038] Another factor to consider is that the beam emanating from the interface, which has a smaller angle but close to the critical angle for total internal reflection, propagates outside the substrate at an angle almost parallel to the front surface. DOEs or lens-like structures can be used to generate plane waves, for example for SLMs. Such beam divergence reduction structures can be separated from the substrate by air and / or made of, for example, low refractive index materials to keep the total internal reflection principle valid.

[0039] In an advantageous embodiment, the optical device is configured to control light from at least one further external light source. For example, the optical device can be configured to control light from three external light sources, one of which can be a blue light source (wavelength 467nm), one is a green light source (532nm), and one is a red light source (630nm). The optical device can provide multi-color illumination in this way, for example for a multi-color holographic display. For example, the first and second waveguides may also be used to guide a light beam emitted from one of the at least one additional external light sources, wherein the first waveguide guides the light beam emitted from the one additional external light source to the first beam shaping structure, and the second waveguide guides the light beam emitted from the one additional external light source to the second beam shaping structure; or, alternatively, the optical device may include at least first and second additional waveguides for guiding a light beam emitted from one of the at least one additional external light sources, and at least a first additional beam shaping structure having a first additional light emitting area for emitting the light beam and a second additional beam shaping structure having a second additional light emitting area for emitting the light beam, wherein the first additional waveguide guides the light beam emitted from one of the at least one additional external light sources to the first additional beam shaping structure, and the second additional waveguide guides the light beam emitted from the one of the at least one external light sources to the second additional beam shaping structure, wherein each of the first and second additional beam shaping structures is configured to increase the uniformity of a radiation density profile of the light beam guided to the respective additional beam shaping structure when emitted from the respective additional light emitting area of ​​the respective additional beam shaping structure.

[0040] That is, the light beams emitted from the at least one further external light source, for example light beams emitted from three external light sources (where each external light source has a different color from a set of three primary colors), can be combined within the same optical device, or propagate together in the same waveguide (each beam shaping structure), or propagate in separate waveguides. In particular if the waveguides are laser written waveguides, it is easy to achieve a waveguide spacing of, for example, only 10 μm and no crosstalk, and the light beams can, for example, be reflected by the same (or different) optical shaping elements, in particular micromirrors.

[0041] The optical efficiency of an optical device depends mainly on the waveguide propagation losses, the coupling losses of the external source to the waveguide and the amount of total internal reflection. The waveguide propagation losses are, for example, <0.2 dB / cm for all wavelengths, the exact total loss depending on the waveguide layout (in the substrate). The efficiency of an external light source (especially a laser diode) coupled to a (single-mode) waveguide depends mainly on the mode matching and Fresnel reflections at the optical interface. Vertical cavity surface emitting lasers (VCSELs) have a circular mode profile that is well matched to one of the waveguides and can achieve coupling efficiencies of more than 90%. In the case of edge-emitting laser diodes, the coupling efficiency is lower due to the highly elliptical mode shape. Typically, coupling efficiencies of 50% to 80% can be achieved. The amount of light loss due to total internal reflection depends on the required uniformity and the beam shaping method used. For example, regarding a Gaussian beam profile: to achieve 80% uniformity (minimum / maximum), about 50% of the total beam intensity must be cut (i.e. discarded).

[0042] The invention also relates to a backlight unit comprising an optical device as described herein and comprising an external light source. Preferably, the external light source is a laser diode.

[0043] The invention further relates to an optical system, in particular a holographic display, comprising a spatial light modulator and a backlight unit or an optical arrangement as described herein.

[0044] In general, optical devices, backlight units and / or optical systems can be used in many applications where it is useful to generate a large coherent wavefront. One example is 3D sensing, either actively through beam steering, where an SLM is used to actively control one or more laser beams for light detection and ranging (LIDAR) applications, or passively, where a DOE is used to create structured light.

[0045] Structured light sensing or structured light 3D sensing has many promising applications in mobile devices, introduced by Apple with its FaceID authentication system. A grid of dots is projected into the environment, the deformation of which is mapped by a camera, which then generates 3D information. Future applications will increase the resolution of the dot grid and move from single-shot to continuous measurement. Both of these things are challenging in terms of accuracy and power consumption, but can be easily solved using the optical device, backlight unit or optical system described in this article. The resolution of the dot grid depends on the number of beams and their divergence angles, which can be significantly increased by using a plane wavefront and modulating it with a DOE. Energy consumption can be reduced by amplifying the signal-to-noise ratio: Due to temperature fluctuations and manufacturing variations of the laser diodes, very wide spectral filters (e.g. 25nm) must be used in current devices, which reduces the signal-to-noise ratio. Using the currently disclosed optical device, backlight unit or optical system, multiple laser diodes can be used in parallel, and only the laser diodes that closely match the much narrower filter spectrum under given conditions will be turned on. The method is applicable to any desired wavelength and can be selected at will.

[0046] Autonomous navigation of cars, drones and robots relies heavily on the use of LiDAR. Essentially, LiDAR relies on a pulsed laser beam that is scanned over a large area and measures the reflection delay (time-of-flight measurement). Although LiDAR is widely used, it is still a very expensive technology, which prevents its adoption in the mass market. The main difficulty lies in the challenge of controlling the laser beam with high accuracy over long distances in the automotive environment with large temperature changes and strong vibrations. Using currently disclosed optical devices, backlight units or optical systems (and SLMs), compact and very stable LiDAR systems can be built. The large possible aperture of the backlight / SLM allows the use of wide laser beams with very long beam divergence lengths, which increases the sensing distance. In addition, the price can be very low when mass-produced, whether it is laser writing or SLM. It is simple to design waveguides for the most commonly used wavelengths in 3D sensing (940nm and 1550nm). In addition, it is also possible to build Flash LiDARs with collimated beams and avoid active beam control. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] As an example, the present invention will be further explained with respect to some selected embodiments shown in the accompanying drawings. However, these embodiments should not be considered as limiting the present invention. The drawings show:

[0048] Figure 1 is a preferred embodiment of the optical device;

[0049] Figure 2 are details of a preferred embodiment of an optical device having a micromirror as an optical shaping element;

[0050] Figure 3 Details of another preferred embodiment of an optical device having micromirrors as optical shaping elements and DOEs as beam divergence reducing structures.

[0051] Figure 4 is a detail of another preferred embodiment of an optical device having a micromirror as an optical shaping element and a lens as a beam divergence reducing structure.

[0052] Figure 5 is a detail of another preferred embodiment of an optical device having a DOE as an optical shaping element and a second DOE as a beam divergence reducing structure;

[0053] Figure 6 is a detail of another preferred embodiment of an optical device having a DOE as an optical shaping element and a beam divergence reducing structure;

[0054] Figure 7 is a schematic top view of a preferred embodiment of a backlight unit;

[0055] Figure 8 is a schematic top view of another preferred embodiment of a backlight unit;

[0056] Fig. 9 is with Figure 8 A schematic cross-sectional view of a backlight unit in the same embodiment; and

[0057] Fig.10 A Mach-Zehnder configuration is a preferred embodiment of the optical device. DETAILED DESCRIPTION

[0058] Figure 1A preferred embodiment of an optical device 1 for controlling light from an external light source 2 (which is not part of the present embodiment) (e.g. a laser diode with a Gaussian beam shape) is shown. The optical device 1 comprises a first waveguide 3, a second waveguide 4 and one further waveguide. It may of course comprise more than one further waveguide. Furthermore, the optical device 1 comprises a first beam shaping structure 5 having a first light emitting area 6 and a second beam shaping structure 7 having a second light emitting area 8. The optical device 1 further comprises a bus waveguide 9. The bus waveguide 9 is configured for guiding a light beam emitted from the external light source and has a first optical coupler 10 and a second optical coupler 11, wherein the first optical coupler 10 couples a light beam emitted from the external light source from the bus waveguide 9 to the first waveguide 3 and the second optical coupler 11 couples a light beam emitted from the external light source from the bus waveguide 9 to the second waveguide 4. More specifically, each of the optical couplers 10, 11 redirects a portion of the light beam guided by the bus waveguide 9 to the respective first and second waveguides 3, 4. Then, the first waveguide 3 guides the light beam emitted from the external light source to the first beam shaping structure 5 , and the second waveguide 4 guides the light beam emitted from the external light source to the second beam shaping structure 7 .

[0059] The first beam shaping structure 5 and the second beam shaping structure 7 are both configured such that the uniformity of the radiant density profile of the light beam emitted from the respective light emitting area 6, 8 of the respective beam shaping structure 5, 7 is higher than the uniformity of the radiant density profile of the light beam guided to the respective beam shaping structure 5, 7 by the respective waveguide 3, 4. The first and second beam shaping structures 5, 7 each comprise a respective optical shaping element 12, and the first and second waveguides 3, 4 guide the light beam emitted from the external light source to the respective optical shaping element 12. The optical shaping element 12 is configured to manipulate the wavefront of the light beam directed to the respective optical shaping element 12. In this embodiment, the optical shaping element 12 is an optical diverging element 13, in particular a (curved) micromirror, which widens the opening angle of the light beam. Due to the widening of the beam angle, the light emitting area 6, 8 can also be closer to the respective optical shaping element 12 while providing the same total illumination area.

[0060] The optical device 1 comprises a transparent substrate 14. The first waveguide 3, the second waveguide 4 and the bus waveguide 9 are formed in the transparent substrate 14, in particular by direct laser writing. The front surface of the transparent substrate 14 provides an interface 15, which in the present case is a boundary between the substrate 14 and air. The first and second light emitting areas 6, 8 are provided by the interface 15. The first and second light emitting areas 6, 8 are each defined by a total internal reflection occurring outside the respective light emitting area 6, 8 for a light beam directed to the interface 15 after interfering with the respective optical shaping element 12. Note that in Figure 1 In FIG. 1 , only the portion of the light beam reflected from the optical shaping element 12 that passes through the interface 15 is shown, i.e., it does not undergo total internal reflection at the interface 15. In contrast, in FIG. Figures 2 to 6 The portion of the light beam that will be reflected at the interface 15 is depicted in FIG. By using the optical diverging element 13 to broaden the light beam and the subsequent total internal reflection at the interface 15, the uniformity of the radiation density profile of the light beam emitted from the individual light emitting areas 6, 8 can be increased. Alternatively, the optical shaping element 12 can of course be an optical profiler element 19 (see Figure 5 and Figure 6 ), which can increase the uniformity of the radiation density profile of the light beam emitted from the respective light emitting areas 6, 8 (in combination with or without total internal reflection at the interface 15). As is well known, the critical angle for total reflection is determined by the refractive index of the transparent substrate and - in this embodiment - the refractive index of air. The optical shaping elements 12 (or their respective widening of the light beam), their distance from the interface 15 and the critical angle for total reflection at the interface 15 (or, respectively, the related refractive index) are configured so that the first light emitting area 6 does not overlap with the second light emitting area 8.

[0061] In this embodiment, the first and second beam shaping structures 5 , 7 are thus formed by a respective optical shaping element 12 and an interface 15 , at which total reflection occurs outside the respective light emitting area 6 , 8 .

[0062] The present invention allows the optical device 1 to be very thin, and the transparent substrate 14 can be a thin plate, in which case, Figure 1 is a cross-sectional view parallel to the smaller dimension of the transparent substrate 14. The optical shaping element 12 is formed on the back side of the transparent substrate 14. The back side (excluding the indentation forming the optical shaping element 12) is substantially parallel to the interface 15. Advantageously, since the front and back surfaces of the substrate 14 are parallel, the portion of the light beam reflected from the interface 15 due to total internal reflection can leave the substrate through the back side (if it has a larger critical angle than the interface), or repeatedly reflect from the back and front surfaces of the substrate so that it will eventually be emitted from the side of the substrate 14. Therefore, it will not be emitted from the interface, and it will not interfere with the light beams emitted from the light emitting areas 6, 8. The small amount of light that is totally internally reflected from the interface 15 and then scattered at an angle below the critical angle by some other optical shaping element 12 at the interface 15 and thus emitted from the interface 15 can be ignored.

[0063] The optical device 1 further comprises a beam divergence reducing structure 16 for reducing the beam divergence of the light beams emitted from the first and second light emitting areas 6, 8. The beam divergence reducing structure 16 is a DOE which preferably transforms the light emitted from the light emitting areas 6, 8 into a (spatially confined) plane wave.

[0064] also, Figure 1 A SLM 17 is shown for modulating the phase of the emitted light on top of the optical device 1. Instead of the SLM 17, a DOE could also be used to generate a static light pattern. Figure 1 It may also be considered to show details of the optical system 18, in particular excluding external light sources.

[0065] Figures 2 to 6 Each shows details of a preferred embodiment of the optical device 1. For example, they do not show the second waveguide 4. The optical device 1 works in the same way as Figure 1 Similar to the description in the context of Figure 1 In the embodiment shown, a light beam emitted from an external light source 2 (not part of the optical device 1) is guided by a bus waveguide 9 and partially coupled to a first waveguide 3, which in turn guides the light beam to an optical shaping element 12, which is a light diverging element 13. Figures 2 to 4 In FIG. 1 , the optical shaping element 12 is a micromirror formed on the back side of the substrate 14. Figure 5 and Figure 6 In the embodiment, the optical shaping element 12 is formed as a DOE that contacts and is aligned with the back side of the substrate 14. Figure 5 and Figure 6 The optical shaping element 12 in can also be an optical profiler element 19. Figures 2 to 6 It can be seen from the figure that the optical shaping element 12 widens the angle of the light beam directed to the optical shaping element 12. In addition, it can be seen that the portion of the light beam directed by the optical shaping element 12 at the interface 15 having an incident angle greater than the critical angle is reflected at the interface 15.

[0066] Figures 3 to 6The embodiment shown in also includes a honeycomb structure 20, in which the boundaries of a cell 21 surround the first light emitting area 6. The honeycomb structure 20 is located on top of and in contact with the light emitting surface 28, which provides the first light emitting area 6. The light emitting surface 28 is provided in this embodiment by the front surface of the substrate 14, i.e. (in this embodiment) the interface 15. The honeycomb structure 20 is made of an absorptive material. Total internal reflection at the interface 15 generally only allows the light emitting areas 6 and 8 to be elliptical. In particular, in the case where the optical shaping element 12 is a micromirror, the light emitting areas 6 and 8 can be circular, for example, or can be any shape in the case where the optical shaping element 12 is a DOE. Therefore, if the light emitting areas 6 and 8 do not overlap or the light beams from different waveguides 3 and 4, respectively, cannot be mixed and therefore do not have a well-defined phase, it is impossible to emit uniformly from a larger area of ​​the interface. The honeycomb structure 20 has a specific (cell) height perpendicular to the front surface of the substrate 14. In this way, the area illuminated by each light emitting area 6 and 8 can be composed of a plurality of light emitting areas having a varying distance from the corresponding light emitting area 6 and 8 (see Figure 8 ) is shaped such that the walls of the unit 21 are shaped and therefore absorb different proportions of the light beams emitted from the boundary regions of the light emitting regions 6, 8.

[0067] Figures 3 to 6 The embodiment shown in FIG. 1 also includes a beam divergence reducing structure 16. Thus, the beam will eventually travel from the Figures 3 to 6 The optical device 1 in the emission, which is not Figure 2 The bold arrows indicate the direction of the light beams, in particular their refraction and reflection at the interface 15. Note that these arrows are for illustration purposes only and are not precise; for example, Figures 3 to 6 In FIG. 1 , they seem to have indicated a plane wave before the beam divergence reducing structure 16, but this is not the case.

[0068] Figure 7 and Figure 8 is a top view of the backlight unit 22, which includes an optical system 1 and an external light source 2. The working principle of the optical system 1 is similar to Figures 1 to 6 The light beam emitted from the external light source 2 is coupled into the higher-order bus waveguide 9, from which a part is coupled into the lower-order bus waveguide 9. The light beams from both the higher-order and lower-order bus waveguides 9 are coupled into the waveguides 3, 4 and guided to the beam shaping structures 5, 7. The light beams are guided from the optical shaping elements 12 of the beam shaping structures 5, 7 toward the front surface of the substrate 15 and emitted from the light emitting areas 6, 8. Figure 7 Compared with the embodiment, Figure 8The embodiment comprises a honeycomb structure 20. It can be seen that the cells 21 of the honeycomb structure 20 are hexagonal and each surrounds a light-emitting area 6, 8, which can increase the lighting filling factor of a larger area.

[0069] Fig. 9 yes Figure 8 An embodiment of the backlight unit 22 is along Figure 8 Schematic cross-sectional view of the plane indicated by A in FIG. 2 (perpendicular to the drawing plane). It can be seen that for this cross section along line A, the wall of the unit 21 of the honeycomb structure 20 is at a certain distance from the light-emitting areas 6 and 8.

[0070] Fig.10 A Mach-Zehnder type structure is shown, which can be used to determine the temperature-dependent change of the optical path length and thus the phase change. A light beam is coupled from an external light source 2 into a waveguide in a substrate 14, which waveguide can be, for example, a bus waveguide 9. From this waveguide, the light beam is coupled into a first interferometer waveguide 23 formed in the substrate 14. A portion of the light beam is then coupled into a second interferometer waveguide 24 via a first interferometer coupler 25. Subsequently, the light beam is coupled from the second interferometer waveguide 24 back into the first interferometer waveguide 23, where it interferes with the portion of the light beam that was not coupled into the second interferometer waveguide 24. Subsequently, the interference of the light beam propagating alone in the first interferometer waveguide 23 and the light beam also propagating in the second interferometer 24 is measured in a photodetector 27, which is connected to the first interferometer waveguide 23. Thermal expansion of the substrate 14 will result in a change in the interference measured by the photodetector 27 (e.g. depending on the path of the first and second interferometer waveguides 23, 24). Fig.10 A second Mach-Zehnder structure is shown, in which a second interference waveguide 24' is connected to the first interference waveguide 23' at a shorter distance than the Mach-Zehnder structure formed by the interference waveguides 23, 24. The light beam of the second Mach-Zehnder structure is then coupled into another photodetector 27'. Using more than one such Mach-Zehnder structure allows for a more sensitive temperature calibration by means of different arrangements of the interference waveguides 23, 24, 23', 24'.

Claims

1. An optical device (1) for controlling light from an external light source (2), comprising: at least a first waveguide (3) and a second waveguide (4) for guiding a light beam emitted from the external light source (2), and at least a first beam shaping structure (5) having a first light emitting area (6) for emitting a light beam, and a second beam shaping structure (7) having a second light emitting area (8) for emitting a light beam, wherein the first waveguide (3) guides the light beam emitted from the external light source (2) to the first beam shaping structure (5), and the second waveguide (4) guides the light beam emitted from the external light source (2) to the second beam shaping structure (7), It is characterized in that The first beam shaping structure (5) is configured so that the uniformity of the radiation density profile of the light beam emitted from the first light emitting area (6) corresponding to the first beam shaping structure (5) is higher than the uniformity of the radiation density profile of the light beam guided to the first beam shaping structure (5) by the first waveguide (3); the second beam shaping structure (7) is configured so that the uniformity of the radiation density profile of the light beam emitted from the second light emitting area (8) corresponding to the second beam shaping structure (7) is higher than the uniformity of the radiation density profile of the light beam guided to the second beam shaping structure (7) by the second waveguide (4), wherein the optical device (1) comprises at least one bus waveguide (9) for guiding a light beam emitted from the external light source (2), and at least a first optical coupler (10) and a second optical coupler (11), wherein the first optical coupler (10) couples the light emitted from the external light source (2) from the bus waveguide (9) to the first waveguide (3), and the second optical coupler (11) couples the light beam emitted from the external light source (2) from the bus waveguide (9) to the second waveguide (4), The at least first optical coupler (10) and the second optical coupler (11) are directional couplers configured to distribute light from the bus waveguide (9) to a respective one of the at least first waveguide (3) and the second waveguide (4) by evanescent coupling.

2. The optical device (1) according to claim 1, characterized in that The first beam shaping structure (5) and the second beam shaping structure (7) each include an optical shaping element (12), and the first waveguide (3) and the second waveguide (4) guide the light beam emitted from the external light source (2) to the corresponding optical shaping elements (12) of the first beam shaping structure (5) and the second beam shaping structure (7), and the optical shaping elements are configured to manipulate the wavefront of the light beam directed to the corresponding optical shaping elements (12).

3. The optical device (1) according to claim 2, characterized in that The optical shaping element (12) of the first beam shaping structure (5) and / or the second beam shaping structure (7) is an optical profiler element (19), and the optical profiler element (19) is configured to increase the uniformity of the radiation intensity profile of the light beam directed to the corresponding optical shaping element (19).

4. The optical device (1) according to claim 2 or 3, characterized in that The optical shaping element (12) of the first beam shaping structure (5) and / or the second beam shaping structure (7) is an optical divergence element (13), and the optical divergence element (13) is configured to increase the beam divergence of the light beam directed to the corresponding optical divergence element (13).

5. The optical device (1) according to any one of claims 2 to 4, characterized in that The optical device (1) comprises an interface (15), wherein the first light emitting area (6) and the second light emitting area (8) are provided by the interface (15) and are each defined by total internal reflection occurring outside the respective light emitting area (6, 8) for a light beam directed to the interface (15) after interacting with the respective optical shaping element (12).

6. The optical device (1) according to claim 5, characterized in that The first light-emitting area (6) and the second light-emitting area (8) do not overlap.

7. The optical device (1) according to any one of claims 1 to 6, characterized in that The optical device (1) comprises a light emitting surface (28), wherein the first light emitting area (6) and the second light emitting area (8) are provided by the light emitting surface (28), and wherein a honeycomb structure (20) of an absorbent material is arranged on top of the light emitting surface (28), wherein a first cell (21) of the honeycomb structure (20) surrounds the first light emitting area (6), and a second cell (21) of the honeycomb structure (20) surrounds the second light emitting area (8).

8. The optical device (1) according to any one of claims 1 to 7, characterized in that A transparent substrate (14), wherein the first waveguide (3) and the second waveguide (4) are formed in the transparent substrate (14).

9. The optical device (1) according to claim 8, characterized in that at least a first interference waveguide (23) and a second interference waveguide (24) formed in the substrate (14), a first interferometer coupler (25) for coupling the light beam from the first interference waveguide (23) to the second interference waveguide (24), a second optical interferometer coupler (26) for coupling the light beam from the first interference waveguide (23) to the second interference waveguide (24), and A photodetector (27) is connected to the first interference waveguide (23).

10. The optical device (1) according to any one of claims 1 to 9, characterized in that A beam divergence reducing structure (16) for reducing the beam divergence of the light beams emitted from the first light emitting area (6) and the second light emitting area (8).

11. The optical device (1) according to any one of claims 1 to 10, characterized in that It is configured to control light from at least one further external light source.

12. A backlight unit (22), characterized in that Comprising an optical device (1) according to any one of claims 1 to 11, and comprising an external light source (2).

13. The backlight unit (22) according to claim 12, characterized in that: The external light source (2) is a laser.

14. An optical system (18) comprising a spatial light modulator (17), characterized in that Comprising a backlight unit (22) according to claim 12 or 13.

15. The optical system (18) according to claim 14, characterized in that The optical system (18) is a holographic display.

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