Patent Grant
8639073
The present invention relates to the field of coupling optical fibers to planar waveguides, and more particularly concerns optical coupling assemblies for the coupling of light between an optical fiber and a planar waveguide.
As telecommunication systems evolve from 10 Gb/s transmission rate to 40 Gb/s and now 100 Gb/s, more sophisticated modulation scheme are developed. For example, phase modulation is increasingly used rather than simpler amplitude modulation. At 100 Gb/s, coherent communication involving polarization multiplexing further adds to the required complexity at the transmitter and receiver sides. In particular, more optical processing is required and calls for photonic integrated solutions.
The more complex hardware required at the transmitter and receiver sides also mean additional physical space. The integration on both optical and electronic devices thus implies considerable amount of space in new systems.
For at least these reasons, there is an immediate need to reduce the size of the optical functions that are implemented into optical systems. To this end, using optical integrated circuits can be an excellent approach for reducing by orders of magnitude the size of optical devices. Yet, efficient coupling of optical fiber inputs and outputs to a waveguide substrate remains a challenge when using integrated optics.
Connecting optical fibers to lightwave circuits may be achieved by various techniques. When the cross-sectional area of the fiber core is larger in size than that of the waveguide, the use of diffraction grating couplers is one of the most power efficient ways for coupling an optical signal. In a typical configuration, a diffraction grating is positioned on the surface of the waveguide and the signal enters the diffraction grating at a nearly normal angle from the surface.
The angle of incidence on the diffraction grating needs to be close to perpendicular, but a small incidence angle is preferred to avoid the strong back reflection due to the second order of diffraction of the grating. An angle of between 5° and 20° from the vertical (i.e. the normal to the surface of the waveguide) is typical, but diffraction grating designs can be made to accommodate angles of incidence of up to 30°.
Attachment of the fiber with a butt coupling at a normal angle from the waveguide surface is not very practical for integration into systems, since it requires considerable additional space. For examples, the waveguide may include opto-electronic functions (e.g. photodiodes, variable optical attenuators, lasers, and the like) whose integration is normally made in the same plane as that of the circuit board on which the integrated photonic chip is typically mounted. Coupling at a normal angle thus implies that the optical fiber would exit perpendicularly to the supporting circuit board, which would be impractical in a dense integration scheme.
One approach known in the art has been proposed in U.S. Pat. No. 7,162,124 the contents of which are incorporated herein by reference, and consists in cutting the end of the fiber at 45° or less with respect to the fiber core in order for light propagating therein to be reflected either by total internal reflection or from a coated mirror deposited on the cleaved surface defined by the cut angle. This geometry allows the fiber to be parallel to the waveguide and reduces significantly the size of the assembly.
One drawback of the angled tip reflection is that the output light diverges when exiting the core of the fiber and goes through its cladding. The diverging angle causes power losses that are detrimental in high efficiency telecom applications. In order to overcome this problem, it has been proposed in U.S. Pat. No. 7,162,124 to reduce the fiber cladding thickness while maintaining the fiber parallel to the substrate. While resolving the power efficiency problem in theory, this method proves to be very difficult to implement efficiently in production. This is mainly due to the difficulty of precisely polishing the fiber core to an exact length along a significant portion of the fiber, since standard polishing techniques and equipment cannot be used.
There therefore remains a need for an improved optical coupling between an optical fiber and a waveguide that alleviates at least some of the above-mentioned drawbacks.
According to an aspect of the present invention, there is provided an optical coupling assembly. The optical coupling assembly includes:
One of ordinary skill in the art will understand that in the optical coupling assembly according to this aspect of the invention, the light-guiding fiber core of the optical fiber is generally parallel to the light-guiding path of the planar waveguide, so that the orientation of the reflection interface with respect to the waveguide coupling element is determined by a cut angle of the angled tip. In a preferred embodiment, this cut angle is greater than about 45 degrees, which advantageously allows the angled tip to be more easily manufactured than in prior art designs. Indeed, a steeper cut angle reduces polishing difficulties since the resulting angled tip is less fragile and prone to chipping. As will be understood by one skilled in the art, chipping of the angled tip of the optical fiber may cause signal loss since the output signal generally exits in the vicinity of the fiber end.
One of ordinary skill in the art of waveguide design will also recognize the advantage of having the flexibility of reversing the waveguide propagation direction with respect to the fiber propagation direction. In particular, this flexibility generally eases the constraints imposed on the design of the optical coupling assembly. As a result, lightwave circuits including one or more optical coupling assemblies as above may support denser waveguide integration schemes. Furthermore, path crossovers on waveguides of such circuits, which may lead to the degradation of optical signals, may be reduced.
According to another aspect of the present invention, there is provided an optical coupling assembly. The optical coupling assembly includes:
Advantageously, one of ordinary skill in the art will understand that in the optical coupling assembly according to this aspect of the invention, the waveguide coupling element may couple light in the light-guiding path of the planar waveguide along a waveguide propagation direction that is either substantially the same or opposite to a fiber propagation direction.
In one embodiment, the fiber cladding includes a tapered fiber cladding portion that defines at least part of the tapered peripheral portion. In another embodiment, the optical fiber component further includes a fiber support (e.g. an optical ferrule or a v-groove support) extending along the fiber cladding proximate the angled tip thereof, the fiber support including a tapered support portion that defines at least part of the tapered peripheral portion.
In one embodiment, the orientation of the reflection interface with respect to the waveguide coupling element is determined by a combination of the cut angle of the angled tip and the tapering angle of the tapered peripheral portion. Preferably, these two angles are jointly selected so that light reflected by the reflection interface toward the waveguide coupling element is incident on waveguide coupling element at an incidence angle of between 5 and 20 degrees on either side of a normal to a plane of the planar waveguide.
The tapering angle may be designed in a given embodiment of the optical coupling assembly to accommodate the geometry or configuration of the waveguide coupling element used with a particular planar waveguide. Furthermore, depending on the intended application of the optical coupling assembly, the tapering angle may also be designed in order to minimize or maximize the contact surface between the optical fiber component and the planar waveguide.
Other features and advantages of the present invention will be better understood upon a reading of the preferred embodiments thereof, with reference to the appended drawings.
The present invention generally relates to optical coupling assemblies for coupling light from an optical fiber into a planar waveguide.
Embodiments of the present invention may be particularly useful in any applications where it is necessary to couple light propagating in an optical fiber into a planar waveguide, or vice versa. In particular, although embodiments described herein may refer to the coupling of light from an optical fiber into a planar waveguide, it will be understood that the optical coupling assembly may also be employed in order to couple light from a planar waveguide into an optical fiber. Optical coupling assemblies according to embodiments of the present invention may be part of photonic integrated circuits used for various applications including, without being limited to, telecommunications, instrumentation, signal processing, and optical sensors.
In this embodiment, the optical coupling assembly 20 first includes a planar waveguide 22 having a light-guiding path 24. In the present description, the term “planar waveguide” is understood to refer to an optical waveguide provided in a substantially integrated configuration, such as in a planar light circuit, and including a light guiding path supported by a suitable substrate. While the term “planar” suggests a flat structure, the term “planar waveguide” is also meant to encompass structures made of a plurality of flat layers. Likewise, the term “light-guiding path” is understood to refer herein to a plane or an axis of a planar waveguide along which light can be guided.
It will be understood by one of ordinary skill in the art that the planar waveguide 22 according to embodiments of the present invention may be embodied by any appropriate planar waveguide including, but not limited to, a slab waveguide, a strip waveguide, a ridge waveguide and a rib waveguide. In various embodiments, the planar waveguide 22 may include a plurality of layers stacked on a substrate, one of these of layers defining the light-guiding path 24. For example, in the embodiment illustrated in
Examples of SOI waveguides may be found, for instance, in U.S. Pat. No. 7,065,272 and U.S. Pat. No. 7,184,625 the contents of which are incorporated herein by reference. It is however understood that this structure is presented by way of example only and that different planar waveguide structures may be considered without departing from the scope of the present invention. Likewise, the optical coupling assembly 20 according to embodiments of the present invention is not limited to SOI technology but may be based on various kinds of materials such as indium phosphide and gallium arsenide.
Still referring to
The optical fiber 34 has an angled tip 40 at one extremity thereof, the angled tip 40 making a cut angle θ1 with respect to the light-guiding fiber core 36. The formation of the angled tip 40 of the optical fiber 34 may be achieved by polishing, cleaving, grinding, sawing, etching, focused-ion-beam (FIB) machining or laser machining one extremity of the optical fiber 34 at the selected cut angle θ1.
The optical coupling assembly 20 further includes a reflective layer 42 provided on the angled tip 40, thereby defining a reflection interface 44, which is preferably substantially flat. The reflective layer 42 may for example be embodied by a metallic coating such as a gold, silver or aluminum coating. Alternatively, in other embodiments, the reflective layer 42 may be a dielectric coating. The reflective layer 42 may be deposited on the angled tip 40 using numerous techniques including, but not limited to, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), ion beam deposition (evaporation coating), nanoscale growth, atomic layer deposition and molecular beam epitaxy.
It will be understood that the reflective layer 42 need not cover the entire reflection interface 44 as long as it extends at least over the area on which light guided in the light-guiding fiber core 36 of the optical fiber 34 impinges. Accordingly, light traveling along the light-guiding fiber core 36 will be reflected by the reflection interface 44 regardless of whether or not the angle of incidence of light thereon exceeds the critical angle predicted by Snell's law for total internal reflection at the reflection interface 44 between the angled tip 40 and the surrounding medium.
The optical coupling assembly 20 also includes a waveguide coupling element 32 for coupling light between the optical fiber 34 and the planar waveguide 22. As used herein, the term “waveguide coupling element” is understood to refer broadly to an optical element adapted for coupling light that is incident thereon from a given optical component (e.g. an optical fiber) into a waveguiding mode, and vice versa. More particularly, in the context of the coupling of light between an optical fiber and a planar waveguide, the term “waveguide coupling element” refers to an optical element capable of coupling a substantial fraction of an optical fiber mode into a waveguide mode, and vice versa.
In the embodiment illustrated in
It will be understood by one of ordinary skill in the art that, while the waveguide coupling element 32 is disposed in a region of the upper cladding 30 proximate to the core layer 28 in the embodiment of
In the context of the present description, the term “light” is understood to refer to electromagnetic radiation in any appropriate region of electromagnetic spectrum and is not limited to visible light. For example, in embodiments of the invention for use in the telecommunications industry, the term “light” is meant to encompass electromagnetic radiation with a wavelength included in a range of between about 1520 nm to 1630 nm encompassing the C-band and L-band transmission windows for telecommunication applications, and of between about 1000 nm and 2000 nm for sensing applications.
The optical fiber 34 extends along the planar waveguide 22 with the angled tip 40 positioned such that light propagating in the light-guiding fiber core 36 along a fiber propagation direction is reflected by the reflection interface 44 toward the waveguide coupling element 32.
In the embodiment illustrated in
In the embodiment of
The angle of incidence θ2 at which the light traveling along the light-guiding fiber core 36 is reflected by the reflection interface 44 toward the planar waveguide 22 depends on the cut angle θ1 and may be determined by the Snell's law which describes the behavior of light when moving between media of differing refractive indices, and by simple geometry. In the embodiment of
θ2=2×(45°−θ1)
Upon inspection of Equation (1), one of ordinary skill in the art will understand that, in practice, the angle of incidence θ2 should be different from zero in order to avoid back-reflections due to Fresnel reflection and second-order Bragg diffraction at the top surface 46 of the planar waveguide 22.
Still referring to
One of ordinary skill in the art will also understand that, in addition to the simple geometrical considerations outlined above and mathematically expressed in Equation (1), other factors may impact the value of the angle of incidence θ2 for a given value of the cut angle θ1. In particular, Equation (1) assumes that light reflected by the reflection interface 44 travels in straight line from the reflection interface 44 to the waveguide coupling element 32, which, in the embodiment of
It also will be recognized by one of ordinary skill in the art that counter propagation of light in the planar waveguide 22 allows, for a given absolute value of θ2, to cut the optical fiber 34 at a greater cut angle θ1 when θ2 is negative, as in the embodiment shown in
According to another aspect of the invention, there is provided an optical coupling assembly including a planar waveguide and an optical fiber component having, among other features, a tapered peripheral portion. Referring now to
As will be understood from the following description, the optical coupling assemblies 20 illustrated in
Referring now specifically to
The optical coupling assembly 20 also includes an optical fiber component 48, having a light-guiding fiber core 36 surrounded by a fiber cladding 38. In the embodiment illustrated in
Referring back to
The optical coupling assembly 20 further includes a waveguide coupling element 32 (e.g. a diffraction grating) for coupling light between the optical fiber 34 and the planar waveguide 22. Again, it will be understood that, while the waveguide coupling element 28 is disposed in a region of the upper cladding proximate to the core layer 28 in the embodiment of
As opposed to the optical fiber 34 of the embodiment of
θ2=2×(45°−θ1)−θ3
It will be understood that, in this configuration, counter propagation of light in the planar waveguide 22, corresponding to the condition θ2<0, may be realized when the cut angle θ1 satisfies the inequality θ1>45°−θ3/2. Likewise, forward propagation of light in the planar waveguide 22, corresponding to the condition θ2>0, may be realized when the cut angle θ1 satisfies the inequality θ1<45°−θ3/2.
As will also be understood by one of ordinary skill in the art, in a given embodiment of the optical coupling assembly 20, the tapering angle θ3 may be designed according to the geometry or configuration of the waveguide coupling element 32 provided with a particular planar waveguide 22. Furthermore, depending on the intended application, the tapering angle θ3 may also be designed in order to minimize or maximize the contact surface between the optical fiber component 48 and the planar waveguide 22.
In some embodiments, the cut angle θ1 of the angled tip 40 and the tapering angle θ3 of the tapered peripheral portion 50 may be jointly selected so that the light impinging on the reflection interface 44 and reflected thereby toward the waveguide coupling element 32 is incident on the waveguide coupling element 32 at an incidence angle θ2 of between about 5 and 20 degrees or between about −5 and −20 degrees with respect to the normal N to a plane of the planar waveguide 22, that is, on either side of the normal N. For example, in the embodiment shown in
The waveguide coupling element 32 provided with the optical coupling assembly 20 illustrated in
The fiber cladding 38 of the optical fiber 34 preferably includes a tapered fiber cladding portion 52 defining at least part of the tapered peripheral portion 50 of the optical fiber component 48. For example, in the embodiments of
Referring back to
In the embodiment illustrated in
The optical coupling assemblies 20 of the embodiments illustrated in
Referring now to
By contrast, providing an optical fiber with a tapered peripheral portion tapering at a tapering angle toward one extremity thereof may be performed with standard off-the-shelf polishing, grinding or dicing equipment and methods, so that no additional modification or complex equipment is required. Moreover, contrary to the case of
It will be understood by one of ordinary skill in the art that the tapered peripheral portion according to embodiments of the invention need not be fashioned directly into the fiber cladding of the optical fiber, as is the case for the embodiments of
It will be understood that in other embodiments, the fiber support may be adapted to hold a plurality of optical fibers to be coupled on a given planar waveguide without departing from the scope of the present invention. In such embodiments, the fiber support may be, for example, a multiple-hole optical ferrule, a v-groove array or a rectangular block having multiple holes bored therethrough.
The optical coupling assembly 20 as exemplified in
The many features and advantages of the invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and, accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the invention.
This application is a Nonprovisional of U.S. Patent Application Ser. No. 61/509,256, filed on Jul. 19, 2011, the disclosure of which is incorporated herein by reference in its entirety.
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| Number | Date | Country | |
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| Number | Date | Country | |
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| 61509256 | Jul 2011 | US |
