OPTICAL DEVICE

Abstract

The application concerns an optical device including: a primary fan-out waveguide; at least one secondary fan-out waveguide; a fan-out optical coupler for coupling a light beam between the primary fan-out waveguide and the secondary fan-out waveguide; and at least one bus waveguide associated with the at least one secondary fan-out waveguide and different from each secondary fan-out waveguide; wherein a reflecting and coupling structure connecting the secondary fan-out waveguide and the bus waveguide.

Claims

1. An optical device comprising: a primary fan-out waveguide; at least one secondary fan-out waveguide-; a fan-out optical coupler for coupling a light beam between the primary fan-out waveguide and the secondary fan-out waveguide; and at least one bus waveguide associated with the at least one secondary fan-out waveguide and different from each secondary fan-out waveguide; wherein a reflecting and coupling structure connecting the secondary fan-out waveguide and the bus waveguide.

2. The optical device according to claim 1, wherein the reflecting and coupling structure is an interferometric structure for coupling a light beam traveling in the secondary fan-out waveguide into the bus waveguide or coupling a light beam traveling in the bus waveguide into the secondary fan-out waveguide such that at least a fraction of the coupled light beam travels in the opposite direction after leaving the interferometric structure.

3. The optical device according to claim 1, wherein at least a section of the bus waveguide extends alongside at least a section of the secondary fan-out waveguide, in particular substantially parallel to a section of the secondary fan-out waveguide.

4. The optical device according to claim 1, wherein the reflecting and coupling structure comprises an optical coupler for coupling a light beam between the secondary fan-out waveguide and the bus waveguide, wherein optionally substantially 50% of the light beam is coupled between the secondary fan-out waveguide and the bus waveguide on passing the optical coupler in one direction.

5. The optical device according to claim 1, wherein the reflecting and coupling structure comprises a bus reflective face for at least partially reflecting a light beam in the bus waveguide and a secondary fan-out reflective face for at least partially reflecting a light beam in the secondary fan-out waveguide.

6. The optical device according to claim 4, wherein the optical path length between the optical coupler of the reflecting and coupling structure is the same as the optical path length between the optical coupler of the reflecting and coupling structure and the secondary fan-out reflective face.

7. The optical device according to any one of the preceding claim 1, wherein the secondary fan-out waveguide and the bus waveguide are provided in a transparent substrate, optionally they are produced by femtosecond direct laser writing.

8. The optical device according to claim 1, wherein the bus reflective face and/or the secondary fan-out reflective face are provided by a facet of the transparent substrate and/or a coating of a facet of the transparent substrate.

9. The optical device according to claim 1, wherein at least one pixel waveguide, which receives a light beam from the at least one bus waveguide or directs a light beam to the at least one bus waveguide and characterized in that the pixel waveguide bends away from the bus waveguide.

10. The optical device according to claim 1, wherein a phase adjusting element for adjusting the relative optical path length of the secondary fan-out waveguide and of the bus waveguide in the reflecting and coupling structure, wherein the phase adjusting element optionally comprises a phase shifter and/or a piezo-mirror.

11. The optical device according to claim 1, wherein at least a further bus waveguide associated with the secondary fan-out waveguide and different from each secondary fan-out waveguide, wherein the reflecting and coupling structure also connects the secondary fan-out waveguide and the further bus waveguide.

12. The optical device according to claim 11, characterized in that wherein the reflecting and coupling structure comprises a tritter for coupling a light beam between the secondary fan-out waveguide, the bus waveguide and the further bus waveguide.

13. The optical device according to claim 1, wherein an additional primary fan-out waveguide; at least one additional secondary fan-out waveguide; and an additional fan-out optical coupler for coupling a light beam between the additional primary fan-out waveguide and the additional secondary fan-out waveguide.

14. The optical device according to claim 1, characterized in that the primary fan-out waveguide is coupled to a light source for receiving a light beam from the light source.

15. A backlight unit for a display, comprising an optical device according to claim 1.

Description

[0047] By way of example, the disclosure is further explained with respect to some selected embodiments shown in the drawings. However, these embodiments shall not be considered limiting for the disclosure.

[0048] FIG. 1 schematically shows a top view of a prior art optical device.

[0049] FIG. 2 schematically shows a top view of another prior art optical device.

[0050] FIG. 3 schematically shows a top view of an embodiment of an optical device according to the present disclosure.

[0051] FIG. 4 schematically shows a side view of the same embodiment of the optical device as FIG. 3.

[0052] FIG. 5 schematically shows a top view of another embodiment of an optical device according to the present disclosure.

[0053] FIG. 6 schematically shows a side view of the same embodiment of the optical device as FIG. 5.

[0054] FIG. 7 schematically shows a top view of yet another embodiment of an optical device according to the present disclosure.

[0055] FIG. 8 schematically shows a side view of the same embodiment of the optical device as FIG. 7.

[0056] FIG. 9 schematically illustrates an estimation model of optical losses due to the number of pixel waveguides.

[0057] FIG. 10 schematically illustrates considerations concerning the reflecting and coupling structure.

[0058] FIG. 11 schematically shows a side view of another embodiment of the optical device.

[0059] FIG. 1 shows a top view of a prior art optical device 101 for fanning-out light. The optical device 101 has a light source 102 that couples light to a fan-out waveguide 103. From there, light is subsequently in a cascading way coupled out to further fan-out waveguides 103. For coupling, at first the fan-out waveguides 103 must propagate in parallel to allow for the light coupling from the first to the second fan-out waveguide 103. Then the second fan-out waveguide 103 starts curving away from the first one to reach the required distance, e.g. the pixel pitch in a display application. After fan-out to the complete width of the optical device 101 has been achieved, illumination of some active area 104 can be started. However, optical losses limit the minimal bending radii of the fan-out waveguides 103. Using a bending radius which still allows for low propagation loss, e.g. 10 mm, and a target distance of 100 μm (equivalent to the intended pixel pitch) the required propagation distance in the z-direction is ca. 2 mm for one optical coupler. Thus, it is impossible to achieve a fan-out without a considerable bezel 105 on at least one side of the active area.

[0060] FIG. 2 shows a top view of an alternative prior art optical device 101 for fanning-out light. Therein, a light source 102 is oriented at 90° with regard to a direction of extension of fan-out waveguides 103 over some active area 104. However, this still requires the bezel 105 around the active area 104 to be on the order of the bending radius at two sides of the display. In particular for mobile display applications, this is undesirable.

[0061] FIG. 3 schematically shows an embodiment of an optical device 1 according to the present disclosure in a top view (y-z-plane) and FIG. 4 shows the same embodiment in a side view (x-z-plane). The optical device 1 comprises a primary fan-out waveguide 3, two secondary fan-out waveguides 4, and a fan-out coupler 5 for coupling a light beam between the primary fan-out waveguide 3 and the secondary fan-out waveguides 4. In this embodiment, the fan-out coupler 5 is a tritter, such that light can be coupled to both secondary fan-out waveguides 4 with one coupler. However, the optical device 1 can equally only comprise one secondary fan-out waveguide 4 and the fan-out optical coupler 5 does not need to be a tritter. FIG. 4 is a side view in the plane, in which the primary fan-out waveguide 3 extends. However, it will be understood, that a side view of the secondary fan-out waveguides 4 will look similar, apart from that its extension in z-direction to the left will only reach to the fan-out optical coupler 5, as can be seen in FIG. 3.

[0062] Further, the optical device 1 comprises one bus waveguide 6 associated with each of the secondary fan-out waveguides 4 and there is also provided for one bus waveguide 6 associated with the primary fan-out waveguide 3. The optical device 1 comprises a reflecting and coupling structure 7 connecting each secondary fan-out waveguide 4 and the respective bus waveguide 6. A similar reflecting and coupling structure also connects the primary fan-out waveguide 3 and the respective bus waveguide 6. As can be seen from the figure, a majority of the bus waveguide 6 extends substantially parallel to a section of the secondary fan-out waveguide 4, which section makes up more than 50% of the length of the secondary fan-out waveguide 4.

[0063] Each reflecting and coupling structure 7 is an interferometric structure 8 for coupling a light beam traveling in the secondary fan-out waveguide 4 into the respective associated bus waveguide 6 (or, in particular if the optical device 1 is used for sensing applications, coupling a light beam traveling in the bus waveguide 6 into the respective associated secondary fan-out waveguide 4) such that at least a fraction of the coupled light beam travels in the opposite direction after leaving the interferometric structure 8. The arrows next to the waveguides 4, 6 are an idealized illustration of the direction the light beams are traveling in the respective waveguide.

[0064] The reflecting and coupling structure 7 comprises an optical coupler 9 (labeled “bus optical coupler”) for coupling a light beam between the secondary fan-out waveguide 4 and the bus waveguide 6 on passing the bus optical coupler 9 in one direction, wherein substantially 50% of the light beam is coupled between the secondary fan-out waveguide 4 and the bus waveguide 6 on passing the bus optical coupler 9 in one direction. The length of the bus optical coupler 9, where the secondary fan-out waveguide 4 and the bus waveguide 6 are brought close to each other, can be less than 1 mm. Furthermore, the reflecting and coupling structure 7 comprises a bus reflective face 10 for at least partially reflecting a light beam in the bus waveguide 6 and a secondary fan-out reflective face 11 for at least partially reflecting a light beam in the secondary fan-out waveguide 4. The optical path length between the bus optical coupler 9 and the bus reflective face 10 is the same as the optical path length between the bus optical coupler 9 and the secondary fan-out reflective face 11. Thus, the reflecting and coupling structure 7 forms a balanced Michelson interferometric structure and/or a folded Mach-Zehnder interferometric structure, and substantially all light that enters the reflecting and coupling structure 7 in the secondary fan-out waveguide 4 leaves the reflecting and coupling structure 7 in the bus waveguide 6 in the reversed direction (due to constructive interference in the bus waveguide 6 on passing the bus optical coupler 9 for a second time and destructive interference in the secondary fan-out waveguide 4 on passing the bus optical coupler 9 for a second time).

[0065] The primary fan-out waveguide 3, the secondary fan-out waveguide 4 and the bus waveguide 6 are provided in a transparent substrate 12. They are produced by femtosecond direct laser writing, which is a method to achieve 3D trajectories of the waveguides. As can be seen from FIGS. 3 and 4, the bus waveguide 6 and the secondary fan-out waveguide 4 or the primary fan-out waveguide, respectively, are substantially extending in parallel along the z-direction and are spaced from one another in the x-direction and in the y-direction for most of their length. Thus, there is no problem with interference at their seeming point of intersection in the top view. In the vicinity of the reflecting and coupling structure 7, they are brought to the same level in the x-direction, and at the bus optical-coupler 9 they are brought close to one another also in the y-direction. The bus reflective face 10 and the secondary fan-out reflective face 11 are provided by a facet 13 of the transparent substrate 12.

[0066] The optical device 1 comprises a plurality of pixel waveguides 14, which each receive a light beam from one of the bus waveguides 6 (or, in particular when using the optical device 1 for sensing applications, direct a light beam to one of the bus waveguides 6). The pixels waveguides 14 bend away from the respective bus waveguide 6 and bend towards a top surface 15 of the substrate 12, which comprises an active area that is to be illuminated by the optical device 1. If the optical device 1 is used as a backlight unit, each pixel waveguide 14 can for example illuminate one pixel or one color subpixel e.g. of a liquid crystal display (LCD). The pixel waveguides 14 can receive the light from the respective bus waveguide 6 via an optical coupler, e.g. by bringing the pixel waveguide 14 close to the respective bus waveguide 6.

[0067] The optical device 1 comprises a light source 2, wherein a light beam emitted from the light source 2 is coupled into the primary fan-out waveguide 3, and is subsequently distributed via the secondary fan-out waveguides 4 and the bus waveguides 6 to the pixel waveguides 14. By using the reflecting and coupling structures 7 for coupling light between the secondary fan-out waveguides 4 and the bus waveguides 6, the entire extension of the substrate 12 in the y- and z-directions can be used for fanning-out, the active area can cover essentially the entire top surface 15 and there is no necessity for a bezel around the active area.

[0068] FIG. 5 schematically shows another embodiment of an optical device 1 in a top view (y-z-plane) and FIG. 6 shows the same embodiment in a side view (x-z-plane). This embodiment is in most parts identical to the embodiment shown in FIGS. 3 and 4, however, some of the pixel waveguides 14 are placed differently. In particular, there is provided for pixel waveguides 14, that are connected to the secondary fan-out waveguide 4 and to the primary fan-out waveguide 3, on both sides of the bus optical coupler 9. Thus, even the area of the coupler can be part of the active area.

[0069] FIG. 7 schematically shows an embodiment of an optical device 1 in a top view (y-z-plane) and FIG. 8 shows the same embodiment in a side view (x-z-plane). The optical device 1 comprises three light sources 2, which each couple light into a different primary fan-out waveguide 3. Each light source 2 can be configured to emit a different wavelength, e.g. red, blue and green light. The distribution of light beams from each of the primary fan-out waveguides 3 to the respective pixels waveguides 14 (not shown in FIG. 7) works in the same way as described in the context of FIGS. 3 and 4. However, the primary fan-out waveguide 3 and the secondary fan-out waveguides 4 of each light source 2 are on different levels of depth (x-axis) in the substrate 12 for most of their length (see FIG. 8), such that they can be interleaved. In particular, the primary fan-out waveguides 3 and the secondary fan-out waveguides 4 are brought to the same level only right before the reflecting and coupling structures 7. Thus, the fan-out of each light source 2, in particular of each color, is independent from the other light sources 2, in particular the other colors. Therefore, also the reflecting and coupling structures 7 remain independent for different light sources 2 and can operate each on a single wavelength (and a different wavelength, depending on which light source they are connected to). In this way, low losses are achieved, while different colors are spread over the active area.

[0070] FIG. 9 schematically illustrates an estimation model of optical losses due to the number of pixel waveguides. When using the proposed architecture where a lot of pixel waveguides are connected to a single bus waveguides, waveguide losses in the coupling become important. Since the waveguides come very close during the evanescent coupling, small losses can occur and if a lot of them occur sequentially, exponential power loss would happen. The following will show that these losses are low enough for the present disclosure to work.

[0071] A typical pixel row consists of power carrying bus waveguides (i.e. the bus-line), and multiple couplers (i.e. the pixels), that remove/redirect part of the power carried by the bus-line towards the output facet of the substrate, in particular glass. The simplified model used for the following calculation is shown in FIG. 9.

[0072] In this arrangement, power P.sub.0 is coupled to the bus-line and propagates from left to right. The bus-line (waveguide from P.sub.0 to P.sub.bus) is coupled to a reference coupler and N couplers in series (i.e. pixels, in this case the straight diagonal parallel lines). After each coupler a portion of the power carried by the bus-line is removed. Additionally, a reference waveguide coupled to the bus-line ends at the output facet on the right (P.sub.ref). The steering of the reference waveguide towards the output facet is performed with a bending radius of 50 mm, high enough to neglect the bending losses coming from this region, and allowing a faithful estimation of the coupling C. Assuming no other power losses, the power at the output of the bus-line (P.sub.bus) is a function of the number (N) of the couplers and their coupling coefficient C. Thus, by knowing N and C, and by monitoring P.sub.ref, one can calculate P.sub.bus and estimate the losses due to the evanescent coupling mechanism.

[0073] By fabricating multiple bus-lines in the arrangement presented above, and by varying the number N of the couplers it is possible to identify and measure the presence of coupling losses, and therefore the optical transmission “t”. The reference waveguide is used to characterize the coupling coefficient of the couplers, and also verify the repeatability and stability of the fabrication process.

[0074] For this study bus-lines with N=1, 20, 40 and 80 couplers were fabricated (N=1 is a device with only the reference waveguide to evaluate the coupling coefficient, whereas the other consist of 1 reference waveguide and N−1 pixel waveguides). 12 devices for each N were made, in order to have sufficient statistics (minimum statistical validity is achievable with 4 devices for each N). The couplers-waveguides were made with an interaction distance of 7.75 μm, interaction length of 0 μm, and bends with a radius of 10 mm. Bending radii related to pixel waveguides and couplers should be in the range of 10 mm to 50 mm to maintain low optical transmission losses lower than 0.2 dB.

[0075] Specifically, for the devices with N=1 the coupling coefficient was measured to the value of C=0.0132. For N=20, t.sub.avg=0.9800±0.0278, for N=40, t.sub.avg=0.9969±0.0113 and for N=80, t.sub.avg=0.9968±0.0015. The results agree with each other, and for higher N values there is a higher statistical certainty. All results are within t=1, thus no apparent optical power losses seem to be induced due to the evanescent field coupling between the waveguides.

[0076] FIG. 10 schematically illustrates considerations concerning the reflecting and coupling structure, which should however not be considered limiting to the general concept of this disclosure. Integrated directional couplers whose output facet is metal coated have been fabricated and characterized, corresponding to FIGS. 3 and 4, but with all waveguides in the same x-plane for convenience of fabrication.

[0077] In particular, the interaction region of the coupler is positioned 1 mm from the output surface, making it insensitive to the amount of polished material (300 μm to 600 μm), while proving its compactness. The interaction region of the coupler can have a distance from the output (coated and thus reflective) surface of less than 1 mm. In the case of polishing as post-processing the output surface a thickness of less than 600 μm is removed. This post-processing step can be avoided.

[0078] A difference in the length of the two arms leads to an unbalanced optical power of the device, i.e. a reduction of the power transfer from the forward-direction waveguide to the backward-direction one. For example, to convey 90% of the input power to the backward-direction waveguide, optionally the reflective facet could be perpendicular with a precision of 0.1 degrees. A mechanical polishing process may easily produce a similar angle in the output facet. To minimize this contribution, the arms of the coupler should be placed as close as possible. The distance of the arms of the coupler typically are set within a range of 15 μm to 100 μm. At the same time, the arms should not interact in order to preserve the behavior of the interferometer. To comply with these requirements, the distance between two arms of the coupler is set to 15 μm. It is worth to notice that close arms allow to shrink the length of the overall device. Supposing to have a perfectly perpendicular facet, the distance between the arms may increase up to e.g. 100 μm.

[0079] This optical circuit has been built in a 25 times 25 times 0.5 mm.sup.3 alumino-borosilicate glass (Eagle XG, Corning), through a technique known as “femtosecond-laser direct writing”. This process exploits the nonlinear interaction between focused ultra-short optical pulses and a dielectric substrate, to produce permanent modifications within the material. By tuning the irradiation conditions of the sample, this approach allows the fabrication of single-mode waveguides featuring a core of circa 3 μm and a refractive index contrast of about 5*10{circumflex over ( )}−3. The wavelength adopted for the measurements is 638 nm, at which these waveguides exhibit a mode field diameter of 4.2 μm, propagation losses of 0.1 dB/cm and bending losses of 0.45 dB/cm at a bending radius of 10 mm.

[0080] The characterization has been divided in two parts: first, the operation of the directional coupler without coating is evaluated. In this phase, light is coupled through a fiber in one mode on the input facet and the power distribution among the two output-modes is measured with a power-meter. The splitting ratio is then retrieved.

[0081] Afterward, the reflection on the output facet is enabled by means of its metallization: a gold-coated layer of thickness ˜50 nm has been sputtered on the side facet of the sample. Then, a microscope objective (NA=0.20) is used both to couple light in one input-mode and to collect the optical power propagated back and forth through the device. A balanced external beam splitter is placed in front of the objective to isolate the output power from the input beam. The Fresnel reflection on the input facet, that is overlapped to one of the two output-modes, is measured and considered in the evaluation of the device behavior.

[0082] During our tests, the relative power transferred from the input-mode to the reflected coupled one was 92.64%, with a standard deviation of 0.91% among 10 identical devices. This working principle is also proved to be independent on the arms distance and length. The complete power transfer is hindered both by the unbalancement of the directional coupler (50.24% splitting ratio with a standard deviation of 0.96%) and, more strongly, by the tilt of the output facet. The latter, randomly produced during the polishing process, leads to a difference in the path-length of the two arms and hence imperfect device performance, which can be easily avoided by using more precise—currently available—polishing/cutting techniques, such as laser cutting.

[0083] FIG. 11 shows a side view of another embodiment of the optical device 1 according to the present disclosure. This embodiment comprises a multi-layer layout. The optical device 1 functions between the light source 2 up to the bus waveguides 6 and the pixels waveguides 14 (not shown in FIG. 11) leading to the top surface 15 in substantially the same way as described in the context of the embodiment shown in FIGS. 3 and 4 (just with the x-dimension shown reversed). However, at the end of at least one of the bus waveguides 6, there is provided for another reflecting and coupling structure 7a, in particular another interferometric structure, with another bus optical coupler 9a and a reflection at another facet 13a. The other reflecting and coupling structure 7a couples light between the bus waveguide 6 and another bus waveguide or detection waveguide 16. For simplicity, FIG. 11 shows this for the bus waveguide 6 connected to the primary fan-out waveguide 3. However, this may also (or instead) be provided for at least one bus waveguide 6 connected to a respective secondary fan-out waveguide 4. The direction of light travelling in the respective waveguides is schematically illustrated by arrows. A crossing of the detection waveguide 16 with the primary fan-out waveguide 3 or, respectively, one of the secondary fan-out waveguides 4 is avoided by bypassing it in the y-dimension at around the spot 17. The detection waveguide 16 leads to a respective detector 18, in particular a photodiode, which makes it possible to analyze the light arriving there. By the bus waveguide 6 being close to the top surface 15, this embodiment is particularly useful for touch sensing applications.