Patent Description:
A photonic integrated circuit (PIC) is suitable for mass production and can achieve a significant economy of scale. Silicon-based PICs may benefit from well-developed fabrication experience, technology, and scalability already for silicon-based microelectronics industry. A PIC may comprise two or more optical components integrated on a same substrate, for example a semiconductor wafer. The optic components may perform different or same functions (e.g., splitter, combiner, coupler, interferometer, modulator, filter, isolator, delay line, among others) to build an optical system. Optic waveguides may be utilized to link the optic components to conduct optical signals between them. The components may be connected in parallel or in series.

Non-guided stray light in radiation mode may be generated by the individual components or by waveguides. One example of the stray light source may be a Y-junction combiner, such as is implemented at the output end of an integrated Mach-Zehnder interferometer used in an optical modulator, or for a Sagnac interferometer used in a fiber optic gyroscope. When light modes in the two branch waveguides of the Y-junction are in opposite phase, the combined light is not in a fundamental mode and so is not guided in the common base waveguide. Instead, the light is coupled into a radiative mode, which causes the light to scatter throughout the photonic integrated circuit. The non-guided stray light may stay in the same plane of the optical system or may be reflected and refracted back to the same plane at interfaces. Another example of a stray light source may be radiation modes generated at bended waveguides. This type of stay light source may occur in a filter comprising micro-ring waveguide resonators, or a polarizer comprising cascade of bending waveguides. Light may become non-guided and radiated into the substrate either in the operation polarization mode or in the orthogonal polarization mode due to the bending radiations. The stray light may be recoupled by the later circuit or the neighboring components. When the recoupled light joins with the desired optic signal, an erroneous signal may be generated.

The optic system comprising a PIC may be constructed for applications that require stable output polarization state. These applications may include optic interferometric modulators, optic interferometric sensors, wavelength-division multiplexing, and coherent communication. Optic birefringence may be deliberately introduced into the components and waveguides. The waveguides and components may conduct and process light of a single linear polarization. To generate and maintain the light in a signal linear polarization state, a polarizer may be included into a PIC. One example of an optic interferometer may be a hybrid fiber optic gyroscope comprising a PIC and optic fiber coil. Another example of an optic interferometer may be an optic coherence tomographic system. Erroneous signal may be produced due to a formation of parasitic interferences. Light in radiation mode that is recoupled into the circuit may coherently interfere with the desired signal. The recoupled light that is originally in the orthogonal polarization mode may be cross coupled into the operation polarization mode and results in interference type of errors.

The publication <CIT> discloses an optical waveguide device capable of highly efficiently guiding unnecessary light outside a substrate or outside an entire optical waveguide even when optical waveguides are integrated. The optical waveguide device comprises on a substrate, an optical waveguide formed by thermally diffusing a material with a high refractive index, and the optical waveguide includes a main waveguide for propagating signal light, and unnecessary-light waveguides for removing unnecessary light from the main waveguide. The unnecessary-light waveguides are configured to remove high-order mode light propagating the main waveguide from the main waveguide as the unnecessary light and to guide it outside the substrate or outside the entire optical waveguide, and the unnecessary-light waveguides are parted by the main waveguide on both sides thereof at an intersection part where the unnecessary-light waveguides and the main waveguide intersect.

The embodiments described herein are directed to a photonic integrated circuit (PIC) that mitigates detrimental effects of stray light generated by one or more components of the PIC. The PIC comprises 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 are 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.

A hybrid photonic integrated circuit (HPIC) may be constructed to expand the functionality of an integrated optic system. The HPIC may be constructed by bonding together several integrated chips. These integrated chips may be optic, electrooptic, or optoelectronic chips, or may be optic chips made of different materials, or optic chips fabricated with two or more different processes. Electronic chips may also be attached to optoelectronic or electrooptic chips and electrically coupled by, for example, solder bumps or wire-bonds. Optic fibers may be attached to the optic, electrooptic, or optoelectronic chips. The methods of the attachment may be direct-coupling between two polished facets, using an optic lens for mode conversion, shaping a micro-lens at a fiber tip, or fabricating a surface grating coupler. Non-guided stray light may be produced in a HPIC. Stray light may be generated at the transition area where waveguides on the two bonded chips been connected.

Although adiabatic transition may be utilized, stray light may still result due to possible differences of the dimensions and effective refractive indices of the two connected waveguides. Stray light may be produced at the connection interface of end-fired coupling between two integrated chips, or between an integrated chip and an optic fiber. Scatter light may escape the core of the waveguide due to an optic misalignment of the input optic mode, or an optic mode-field mismatching of the two connected waveguides. The non-overlapped portion of the fiber mode fields may be non-guided and scattered across the area of photonic integrated circuit. An HPIC integrated with optoelectronic chips may be more susceptible to receiving erroneous signal. Components that particularly sensitive to stray light may be light generating electrooptic components such as optic amplifiers and lasers, light receiving optoelectronic components such as photodetectors, and phase sensitive components such as interferometers. Stray light, if allowed to freely propagate within the substrate and to reflect from the substrate surfaces, may interfere with the proper operation of the optic system.

A higher degree of integration of PICs or HPICs may be implemented to reduce the size, or lower the cost, of an optic system. The high degree of integration may be achievable by reducing the size of the PICs or introducing more functional components into PICs, so a
larger number of PICs or PICs with more functions can be fitted in a same semiconductor wafer. Optic components may be placed closer to realize the function expansion and the size reduction. However, stray light generation and reception may become more notable if the optic components are more closely located. Light scattered from one component may be readily coupled to a nearby component and generate the erroneous signal in that component, degrading its optic performance.

In one aspect, the invention provides a photonic integrated circuit (PIC) according to claim <NUM>.

In another aspect, the invention provides a photonic integrated circuit (PIC) according to claim <NUM>.

In another aspect, the invention provides a method of mitigating stray light generated on a photonic integrated circuit (PIC) according to claim <NUM>.

Further preferred embodiments are defined by the dependent claims.

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.

Referring to <FIG>, a photonic integrated circuit may have a Y-junction, built on a substrate, that comprises a common base waveguide <NUM>, a splitting waveguide structure, and two branch waveguides 106a, 106b. The light propagates in one of the branch waveguides from left to right. At the splitting waveguide structure <NUM>, part of the light power may continue to be guided and to propagate 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 <NUM> and is not guided but is rather radiated out from the waveguide into the substrate. <FIG> 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 <NUM> above and below the common base waveguide <NUM>. The radiation light may be recoupled or received by 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 shields built with metal walls and doped semiconductor regions, the open mouth of an optical trap, and light absorbing films.

In the described arrangements, an integrated Y-junction may be built on a substrate with an array of collectors. The array of collectors <NUM> may be made from the same materials and fabricated with the same processes as the main circuit waveguide structure. The array of collectors <NUM> 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>. The array of collectors <NUM> may be directed as fanning out, as shown in <FIG>. 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.

Tapering the tips of the collectors enlarges the mode-field size at the waveguide tips of the collectors <NUM>, so that the collectors <NUM> 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.

A forward taper may be used to increase the mode-size when the index difference of refractions (Δn) between the core and cladding materials is small, such as Δn < <NUM>, 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 Δn larger than <NUM>. Examples of such waveguides may include a waveguide with silicon nitride core and silicon oxide cladding or a silicon-on-insulator (SOI) waveguide.

The non-guided light that is sourced at the junction <NUM> may be collected and guided by the array of the waveguide collectors <NUM>, as demonstrated by the contour plot of the light power distribution shown in <FIG>. The stray light collected by the waveguide collectors <NUM> is further guided by secondary waveguides <NUM> towards damping areas <NUM>, where the light energy may be converted into heat and dissipated thermal-conductively, as shown in <FIG>. The optic dampers <NUM> may comprise an area where the evanescent waves of each of the secondary waveguides <NUM> are exposed for a length l at the ends of the waveguides <NUM>, 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 20dB suppression of the stray light may be achievable using the collectors <NUM> of the described embodiments. As shown in <FIG> and <FIG>, approximately 12dB 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.

Referring to <FIG>, an example of a PIC may comprise a photonic integrated circuit having a waveguide <NUM> built on a substrate <NUM> and an optic fiber <NUM>. The waveguide <NUM> may have its end <NUM> optimized to connect to the optic fiber <NUM>. The mode-field dimensions of the integrated waveguide <NUM> and the fiber <NUM> may not be matching and, therefore, non-guided stray light <NUM> may radiate out from the waveguide-fiber joint point <NUM> into the cladding layer of waveguides and substrate <NUM> of the PIC, as shown in a simulated result in <FIG>. The radiated stray light <NUM> (shown in <FIG> as a two-dimensional contour plot of the light power distribution near the waveguide-fiber joint point <NUM>) 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 reaching the later circuit or the neighboring components. A described PIC may have an array of collectors <NUM> that may be made from the same materials and fabricated using the same processes as the waveguides <NUM> of the main optic circuit. The collectors <NUM> may be arranged in such a way that the collectors <NUM> are aligned in the direction that the stray light radiates out from the joint point <NUM>. A plurality of collectors may be employed so that the collectors <NUM> may be arranged in a fanned-out configuration, as shown in <FIG>. As with the collectors <NUM> associated with the Y-junction configuration described with respect to <FIG>, the tips of the collectors <NUM> 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 <FIG>, tapering the tips of the collectors <NUM> is 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 <NUM> may then be collected by the waveguide collector array <NUM>, as shown in <FIG>. The stray light collected by the waveguide collector array <NUM> is 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 <NUM> depicted in <FIG> with respect to the Y-junction arrangement. More than 20dB suppression of the stray light may be achievable by using the collector arrays of the described embodiments.

Turning now to <FIG>, 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>, the waveguides <NUM> are in the shape of a half circle, although other curved arrangements may alternatively be used. The example waveguides <NUM> may be built on a substrate and arranged to be coupled in series, as shown in <FIG>, to form an "m" shape. The birefringence of the waveguides <NUM> may result in a higher confinement of 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 <NUM> may be optimized such that the waveguides <NUM> guide the TE polarization light with a low propagation loss, while imposing a large bending loss on the light in TM mode. The series of the half circles, therefore, cumulatively constitute a polarizer with a high propagation extinction ratio (PER).

The actual achievable PER of such an integrated polarizer may, however, be limited. At the bending waveguides <NUM>, the TM polarization-mode light may be not completely guided by the waveguide <NUM> and may be radiated into the substrate and cladding layer of the waveguides <NUM>. 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 <NUM> may be placed alongside with the curved waveguide sections <NUM> as indicated in <FIG>. The collector waveguides <NUM> 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 <FIG> and <FIG>, the tips of collectors <NUM> may be optimized in shape to improve the efficiency for receiving the coming stray light. Tapering the tips of the collectors is 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 <NUM> is guided by secondary waveguides <NUM> towards damping areas <NUM>, where absorption materials may be utilized.

Claim 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:
an array of stray light collectors (<NUM>, <NUM>, <NUM>) arranged to collect non-guided stray light produced by the integrated optic device, each stray light collector (<NUM>, <NUM>, <NUM>) further comprising a waveguide (<NUM>, <NUM>, <NUM>) that collects stray light, the waveguide (<NUM>, <NUM>, <NUM>) having a first end and a second end, the first end disposed proximal to the integrated optic device, and the second end linked to a secondary waveguide (<NUM>, <NUM>) so that the stray light is guided continuously from the stray light collector (<NUM>, <NUM>, <NUM>) to the secondary waveguide (<NUM>, <NUM>); and
a light damper configured to receive the non-guided stray light collected by the array of stray light collectors (<NUM>, <NUM>, <NUM>) and to mitigate the non-guided stray light;
wherein each secondary waveguide (<NUM>, <NUM>) continuously guides collected stray light to the light damper,
wherein the first end of each waveguide (<NUM>, <NUM>, <NUM>) is tapered to increase a mode-field size and the non-guided stray light propagates along a path, and the array of stray light collectors (<NUM>, <NUM>, <NUM>) 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 (<NUM>, <NUM>, <NUM>).