Method and Apparatus For Control and Suppression of Stray Light in a Photonic Integrated Circuit

Abstract

In a photonic integrate circuit (PIC) architecture, non-guided stray light that is radiated from components, junctions, discontinuous and scattering points in an integrated optic device, may be received by an integrated waveguide structure in the path of the stray radiation. The integrated waveguide structure may comprise a plurality of collectors that are configured to collect the non-guided stray light from the radiating source. Each of the collectors may comprise an integrated waveguide with a front end that is tapered to increase the mode-field size and pointed toward the stray light source, and with a back end that is connected to a secondary waveguide. The collectors are placed in the path of the stray light and aligned in the propagation direction of the stray light. The collected stray light is guided to a light energy damper through the second waveguide for converting light energy into heat.

Claims

1. A photonic integrated circuit (PIC), comprising: an integrated optic device disposed on a substrate; and an integrated optic structure disposed on the substrate around the integrated optic device, the integrated optic structure comprising: at least one stray light collector arranged to collect non-guided stray light produced by the integrated optic device; and a light damper configured to receive the non-guided stray light collected by the at least one stray light collector and to mitigate the non-guided stray light.

2. The PIC of claim 1, wherein the at least one stray light collector further comprises a waveguide having a first end and a second end, the first end disposed proximal to the integrated optic device, and the second end coupled to a secondary waveguide that conveys collected stray light to the light damper.

3. The PIC of claim 2, wherein the first end of the waveguide is tapered to increase a mode-field size.

4. The PIC of claim 2, wherein the non-guided stray light propagates along a path, and the at least one stray light collector is disposed in the path so as to be aligned with a propagation direction of the non-guided stray light and configured to facilitate reception of the non-guided stray light into the first end of the waveguide.

5. The PIC of claim 1, wherein the integrated optic structure comprises first materials, the integrated optic device comprises second materials, and the first materials are same as the second materials.

6. The PIC of claim 1, wherein the integrated optic structure and the integrated optic device are monolithically fabricated on the substrate.

7. The PIC of claim 1, wherein the integrated optic device is bonded onto the substrate.

8. The PIC of claim 7, wherein the integrated optic device is a light-emitting device.

9. The PIC of claim 1, wherein the integrated optic device comprises an integrated Y-junction.

10. The PIC of claim 1, wherein the integrated optic device comprises an integrated polarizer that is a cascade of one or more optically-coupled bended waveguides.

11. The PIC of claim 1, wherein the integrated optic device comprises an integrated polarizer, and wherein the integrated polarizer is a filter that comprises one or more micro-ring waveguide resonators.

12. The PIC of claim 1, wherein the light damper comprises light absorptive material.

13. The PIC of claim 1, wherein the light damper comprises metal material.

14. A photonic integrated circuit (PIC), comprising: an integrated optic device disposed on a substrate; a second optic device disposed on the substrate and coupled to the integrated optic device at a coupling joint; and an integrated optic structure disposed on the substrate around the coupling joint, the integrated optic structure comprising: at least one stray light collector arranged to collect non-guided stray light produced by one or more of (i) the integrated optic device, (ii) the second optic device, and (iii) the coupling joint; and a light damper configured to receive the non-guided stray light collected by the at least one stray light collector and to mitigate the non-guided stray light.

15. The PIC of claim 14, wherein the at least one stray light collector further comprises a waveguide having a first end and a second end, the first end disposed proximal to the integrated optic device, the second optic device, and the coupling joint, and the second end coupled to a secondary waveguide that conveys collected stray light to the light damper.

16. The PIC of claim 15, wherein the first end of the waveguide is tapered to increase a mode-field size.

17. The PIC of claim 15, wherein the non-guided stray light propagates along a path, and the at least one stray light collector is disposed in the path so as to be aligned with a propagation direction of the non-guided stray light and configured to facilitate reception of the non-guided stray light into the first end of the waveguide.

18. The PIC of claim 14, wherein the integrated optic structure comprises first materials, the integrated optic device comprises second materials, and the first materials are same as the second materials.

19. A method of mitigating stray light generated on a photonic integrated circuit (PIC), comprising: collecting, through at least one stray light collector, non-guided stray light produced by an integrated optic device disposed on a substrate of the PIC; and conveying, by the at least one stray light collector, the collected stray light to a light damper configured to receive the non-guided stray light collected by the at least one stray light collector and to mitigate the non-guided stray light.

20. The method of claim 19, further comprising disposing the at least one stray light collector in a path along which the non-guided stray light propagates, wherein the at least one stray light collector is aligned with a propagation direction of the non-guided stray light.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

[0020] The foregoing will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments.

[0021] FIG. 1A is a top view of a conventional Y-junction of a photonic integrated circuit.

[0022] FIG. 1B is a top view of a light intensity distribution within and nearby the conventional Y-junction shown in FIG. 1A.

[0023] FIG. 1C is a top view of the Y-junction of a PIC according to embodiments of the invention, with stray light collectors placed in the recommended places and directions.

[0024] FIG. 1D is a top view of a light intensity distribution within and nearby the Y-junction with stray light collectors shown in FIG. 1C.

[0025] FIG. 2 is a top view of a PIC device comprising a Y-junction with stray light collectors shown in FIG. 1C, a series of second waveguides, and light dampers.

[0026] FIGS. 3A and 3B are examples of test results of stray light suppression on a Y-junction according to embodiments of the invention.

[0027] FIG. 4A is a top view of a conventional optic component that comprises a PIC having a waveguide built on a substrate and an optic fiber.

[0028] FIG. 4B is a top view of the light intensity distribution within and nearby the conventional optic component shown in FIG. 4A;

[0029] FIG. 4C is a top view of the optic component according to embodiments of the invention, with stray light collector waveguides placed in the recommended places and directions.

[0030] FIG. 4D is a top view of the light intensity distribution within and nearby the optic component with stray light collectors shown in FIG. 4C.

[0031] FIG. 5 is a top view of a m-shaped integrated waveguide polarizer with stray light collectors, according to embodiments of the invention.

DETAILED DESCRIPTION

[0032] A description of example embodiments follows.

[0033] The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.

[0034] A photonic integrated circuit (PIC) may comprise a substrate that has an optical device integrated onto the substrate. The integrated optical device may comprise, for example, a Y-junction, a waveguide-to-optical fiber coupling, and/or a polarizer, among others. The described embodiments may be directed to apparatus configured to collect and selectively direct stray light from the optical device to a facility capable of mitigating the stray light by, for example, converting the stray light into heat, although other techniques for mitigating the stray light may alternatively be used.

[0035] Referring to FIG. 1A, a photonic integrated circuit may have a Y-junction, built on a substrate, that comprises a common base waveguide 102, a splitting waveguide structure, and two branch waveguides 106a, 106b. The light propagating in one of the branch waveguide from left to right. At the splitting waveguide structure 104, part of the light power may continue to be guided and propagating in the common base waveguide in a single mode if the waveguide is a single mode waveguide. Another part of the optic power of the input light may be in an asymmetrical mode after passing through the splitting waveguide structure 104 and is not guided but is rather radiated out from the waveguide into the substrate. FIG. 1B shows a two-dimensional contour plot of the light power distribution in the Y-junction area, including the light in asymmetric mode radiated out from the splitting waveguide structure. The non-guided stray light spreads out along angles 108 above and below the common base waveguide 102. The radiation light may be recoupled or received by the any components in the path of the radiation, which may add erroneous signal to the desired signal. It is desirable, therefore, to prevent the stray light from reaching such in-path circuits or neighboring components. Various techniques have been suggested to suppress stray light, including deep etched trenches filled with an absorbing material, light shield built with metal walls and doped semiconductor regions, the open mouth of an optical trap, and light absorbing films.

[0036] In the described embodiments, an integrated Y-junction may be built on a substrate with an array of collectors. The array of collectors 110 may be made from the same materials and fabricated with the same processes as the main circuit waveguide structure. The array of collectors 110 may be arranged such that the collectors are aligned in the direction that the stray light is radiated out from the junction, depicted in FIG. 1B. The array of collectors 110 may be directed as fanning out, as shown in FIG. 1C. The tips of the collectors that are directed to the junction area may be optimized into a shape to improve the efficiency for receiving the stray light.

[0037] Tapering the tips of the collectors may enlarge the mode-field size at the waveguide tips of the collectors 110, so that the collectors 110 operate as efficient antennas for the signal collections. The mode-field size may be enlarged by using the forward taper, which has a waveguide core increased gradually in size at the waveguide tip either in horizontal, or in vertical direction or in both directions. The mode-field size may also be enlarged by using an inverse taper, in which the waveguide core is reduced gradually in size either in the horizontal direction, or in vertical direction, or in both directions.

[0038] A forward taper may be used to increase the mode-size when the index difference of refractions (n) between the core and cladding materials is small, such as n<0.1, so the increase of waveguide core size may not readily facilitate supporting a high-order mode, which would increase the propagation loss. On the other hand, an inverse taper is often used in a waveguide that has a large index difference between the core material and the cladding material, such as n larger than 0.5. Examples of such waveguides may include a waveguide with silicon nitride core and silicon oxide cladding or a silicon-on-insulator (SOI) waveguide.

[0039] The non-guided light that is sourced at the junction 104 may be collected and guided by the array of the waveguide collectors 110, as demonstrated by the contour plot of the light power distribution shown in FIG. 1D. The stray light collected by the waveguide collectors 110 may be further guided by secondary waveguides 202 towards damping areas 204, where the light energy may be converted into heat and dissipated thermal-conductively, as shown in FIG. 2. The optic dampers 204 may comprise an area where the evanescent waves of each of the secondary waveguides 202 are exposed for a length/at the ends of the waveguides 202, and light energy absorptive material being filled in the exposed area so that the exposed waveguide sections are covered with the absorptive material. More than 20 dB suppression of the stray light may be achievable using the collectors 110 of the described embodiments. As shown in FIGS. 3A and 3B, approximately 12 dB suppression of the stray light was measured on an example ultrathin silicon nitride waveguide after placing six collectors on each side of a Y-junction.

[0040] Referring to FIG. 4A, an example embodiment of a PIC may comprise a photonic integrated circuit having a waveguide 402 built on a substrate 404 and an optic fiber 406. The waveguide 402 may have its end 408 optimized to connect to the optic fiber 406. The mode-field dimensions of the integrated waveguide 402 and the fiber 406 may not be matching and, therefore, non-guided stray light 410 may radiate out from the waveguide-fiber joint point 409 into the cladding layer of waveguides and substrate 404 of the PIC, as shown in a simulated result in FIG. 4B. The radiated stray light 410 (shown in FIG. 4B as a two-dimensional contour plot of the light power distribution near the waveguide-fiber joint point 409) may be recoupled by any components or waveguides in the path of the propagation, which may add erroneous single to the desired signal. It is desirable, therefore, to prevent the stray light from reach to the later circuit or the neighboring components. A PIC of described embodiments may have an array of collectors 420 that may be made from the same materials and fabricated using the same processes as the waveguides 402 of the main optic circuit. The collectors 420 may be arranged in such a way that the collectors 420 are aligned in the direction that the stray light radiates out from the joint point 409. A plurality of collectors may be employed so that the collectors 420 may be arranged in a fanned-out configuration, as shown in FIG. 4C. As with the collectors 110 associated with the Y-junction configuration described with respect to FIGS. 1A through 1D, the tips of the collectors 412 that are arranged to be pointing to the joint point may be optimized into a shape to improve the efficiency for receiving the radiated stray light. As was described with respect to the Y-junction configuration of FIGS. 1A through 1D, tapering the tips of the collectors 412 may be beneficial due to the enlarged mode-field size that occurs at the tips of the collectors. The mode-field size may be enlarged by using the forward taper or may be enlarged by using an inverse taper. The non-guided light generated at the joint point 409 may then be collected by the waveguide collector array 412, as shown in FIG. 4D. The stray light collected by the waveguide collector array 412 may be guided continuously by secondary waveguides, which are linking to the back ends of the collector waveguides respectively, and led towards damping areas where absorption materials may be utilized, such as the dampers 204 depicted in FIG. 2 with respect to the Y-junction arrangement. More than 20 dB suppression of the stray light may be achievable by using the collector arrays of the described embodiments.

[0041] Turning now to FIG. 5, an example embodiment of a PIC may comprise a birefringent waveguide, or a cascade of birefringent waveguides, each being curved at least to some extent. In the example embodiment of FIG. 5, the waveguides 502 are in the shape of a half circle, although other curved arrangements may alternatively be used. The example waveguides 502 may be built on a substrate and arranged to be coupled in series, as shown in FIG. 5, to form an m shape. The birefringence of the waveguides 502 may result in a higher confinement on light propagating in a transverse electric (TE) polarization mode than light propagating in a transverse magnetic (TM) mode. The radius of each of the half circle waveguides 502 may be optimized to such that the waveguides 502 guide the TE polarization light with a low propagation loss, while imposes a large bending loss to the light in TM mode. The series of the half circles, therefore, cumulatively constitute a polarizer with a high propagation extinction ratio (PER).

[0042] The actual achievable PER of such an integrated polarizer may, however, be limited. At the bending waveguides 502, the TM polarization-mode light may be not completely guided by the waveguide 502 and may be radiated into the substrate and cladding layer of the waveguides 502. The non-guided light may be recoupled back into the optic circuit, which may add light power in the TM mode of the waveguide, and effectively degrade the polarizer. A series of the collector waveguides 504 may be placed along side with the curved waveguide sections 502 as indicated in FIG. 5. The collector waveguides 504 may be aligned in the direction of the tangent lines of the curvature of the polarizer waveguide. As for the collectors described with respect to FIGS. 1C and 4C, the tips of collectors 504 may be optimized in shape to improve the efficiency for receiving the coming stray light. Tapering the tips of the collectors may utilized to enlarge the mode-field size at the tip of the collectors. The mode-field size may be enlarged by using the forward taper or may be enlarged by using an inverse taper. The stray light collected by the waveguide collectors 504 may be guided by secondary waveguides 506 towards damping areas 508, where absorption materials may be utilized.

[0043] While example embodiments have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the embodiments encompassed by the appended claims.