Patent ID: 12222563

DETAILED DESCRIPTION

The present disclosure generally relates to a multi-tip waveguide coupler that provides improved alignment guidance. In order to couple a laser or other optical device to a photonic integrated circuit (PIC), highly-precise mechanical alignment between the laser and waveguides formed in the photonic integrated circuit must be achieved. Currently, vertical alignment tends to be more precise due to existing process controls. However, horizontal alignment has proven to be much more difficult. It is difficult to satisfy various constraints relating to strict horizontal alignment tolerances as costs are incurred not only to establish alignment during manufacturing, but also because horizontal and vertical alignment have to be maintained after assembly. Imperfect horizontal alignment causes significant optical loss, leading to low manufacturing yields. To relax the alignment accuracy constraints with respect to the horizontal direction, tapered waveguides have been employed, where the waveguides are tapered at the edge of the chip and widen as the waveguide approaches an input interface of the circuit or chip. Alternatively, fork-type designs have also been employed.

As such, most solutions for efficiently coupling an optical element, such as a laser, to a photonic integrated circuit utilize a waveguide having a very large cross-section inside the photonic integrated circuit. The large cross-section leads to large chip sizes and increased manufacturing costs. This solution, however, is not feasible in silicon photonics, as a very small waveguide cross-section is needed to enable compact chip size and efficient electro-optic modulation. Other solutions include the use of polymers or three-dimensional printed materials to make spot-size converters. However, these converters are not compatible with existing complementary metal-oxide-semiconductor (CMOS) fabrication processes. Any solutions that are compatible with existing CMOS fabrication processes require a very large area on the photonic integrated circuit (e.g., in a millimeter size scale), which drastically increases chip size and cost.

Accordingly, in various embodiments, a photonic integrated circuit is described. The photonic integrated circuit may include an input interface, an output interface, and a waveguide array, where the waveguide array is compatible with existing photonic integrated circuit manufacturing processes. Further, the waveguide array improves overall coupling efficiency and horizontal alignment tolerance, which leads to better manufacturing yield and more favorable optical performance budgets, as will become apparent.

To this end, in various embodiments, the waveguide array of the photonic integrated circuit may include a first waveguide, a second waveguide, and a third waveguide, where the second waveguide is positioned between, and parallel to, the first waveguide and the third waveguide. As such, the second waveguide may be referred to as a centrally-located or central waveguide. The waveguide array may be integrated with a coupler of the photonic integrated circuit.

The first waveguide and the third waveguide of the waveguide array may be coupled to the input interface of the photonic integration circuit in some embodiments. Notably, the first waveguide and the third waveguide are not coupled to the output interface in various embodiments. The term “not coupled to” can refer to the first waveguide and the third waveguide not extending to or being physically connected to the output interface. However, the second waveguide may be coupled to both the input interface and the output interface. In some embodiments, the second waveguide may include a tapered body such that an output end of the second waveguide coupled to the output interface is wider than an input end of the second waveguide coupled to the input interface.

The waveguide array described herein can relax alignment tolerances required to achieve high optical coupling efficiencies without substantial optical alignment. Notably, the waveguide array relaxes requirements in horizontal alignment accuracies, where horizontal alignment includes a direction parallel to the surface of the primary photonic chip.

In the following discussion, a general description of a multi-tip coupler and its components is provided, followed by a discussion of the operation of the same.

Turning now toFIG.1, a schematic diagram of an optoelectronic system100is shown. The optoelectronic system100may include, for example, an optical element105and a photonic integrated circuit110, among other components not separately shown. The optical element105may include an optical device, such as a laser or a fiber, that provides an input beam115or other optical beam to the photonic integrated circuit110. In some embodiments, the optical element105is one of a photonic chip, or similar type of chip, separate from the photonic integrated circuit110or a chip in which the photonic integrated circuit110is incorporated. As may be appreciated, the photonic integrated circuit110may include an optoelectronic integrated circuit and, as such, may include an input interface120and an output interface125. For instance, single mode optical paths may be routed away from the photonic integrated circuit110by way of the output interface125. In some embodiments, the photonic integrated circuit110includes a laser-integrated photonic integrated circuit (LPIC).

Additionally, the photonic integrated circuit110may include a waveguide array130. In various embodiments, the waveguide array130may include a first waveguide135, a second waveguide140, and a third waveguide145. As shown inFIG.1, the second waveguide140may be positioned between the first waveguide135and the third waveguide145. Additionally, as shown inFIG.1, the second waveguide140may be positioned near and parallel to the first waveguide135and the third waveguide145, respectively, as will be described.

The first waveguide135and the third waveguide145may be coupled to the input interface120of the photonic integration circuit110. Notably, the first waveguide135and the third waveguide145do not reach and are not physically connected to the output interface125, instead terminating prior to the second waveguide140. In some embodiments, the waveguide array130may be implemented in a coupler150embedded inside the photonic integrated circuit110, where the coupler150connects an optical signal, i.e., the input beam115, received from the optical element105to the photonic integrated circuit110. As may be appreciated, the coupler150separates the input beam115into the three or more waveguides in the waveguide array130on the photonic integrated circuit110and, as such, may be referred to as a “multi-tip” coupler150in some embodiments. Further, in some embodiments, the multi-tip coupler150may be embedded in a single chip carrier package device and, as such, the system may be described as a system-in-a-package (SiP) multi-tip coupler150.

As the first waveguide135and the third waveguide145terminate before reaching the output interface125, the first waveguide135and the third waveguide145serve to widen a mode of the coupler150, while not carrying any appreciable optical power. Additionally, the first waveguide135and the third waveguide145, acting in combination with the second waveguide140being centrally located and having a tapered body, result in a wider optical mode, which provides a better match to the input beam115emitted by the optical element105, such as a laser. This results in a larger fraction of the input beam115, such as laser light, being coupled to the photonic integrated circuit110, and also results in a wider “target”’ to which the input beam115may be horizontally aligned. Thus, the various embodiments described herein permit a significantly greater tolerance in lateral or horizontal misalignment between the optical element105(e.g., a laser) and the photonic integrated circuit110. This may result in a direct improvement in manufacturing yield.

Referring now toFIG.2andFIG.3, various perspective views of a substrate170of a photonic integrated circuit110having the waveguide array130ofFIG.1are shown in accordance with various embodiments of the present disclosure. It is important to note that the components inFIG.2andFIG.3are not necessarily to scale and, instead, serve to clearly illustrate the principles of the disclosure. The waveguide array130is shown integrated on the substrate170.

As shown inFIG.2andFIG.3, the first waveguide135and the third waveguide145do not have a length sufficient to reach or physically touch the output interface125and, instead, terminate before the second waveguide140. For instance, the first waveguide130has a first waveguide length L1, the second waveguide has a second waveguide length L2, and the third waveguide has a third waveguide length L3. In some embodiments, the first waveguide length L1may be equal or substantially similar to the third waveguide length L3. Further, in some embodiments, the second waveguide length L2is greater than the first waveguide length L1and/or the third waveguide length L3.

In some embodiments, the second waveguide length L2is approximately 30-100 μm. Further, the first waveguide length L1is at least 30 μm and the third waveguide length L3is at least 30 μm. Optically, if the first waveguide length L1and the third waveguide length L3are approximately 30 μm, an increase in the first waveguide length L1and the third waveguide length L3does not alter the output mode of the coupler150.

As shown inFIG.2andFIG.3, the second waveguide140may be coupled to both of the input interface120and the output interface125. Further, as shown inFIGS.2and3, the second waveguide140may be positioned parallel to and between the first waveguide135and a third waveguide145such that the second waveguide140is centrally located. While the first waveguide135and a third waveguide145are positioned near the second waveguide140, the first waveguide135and the third waveguide145are not integrated and do not come into contact with the second waveguide140. In other words, the distance between the first waveguide135and the second waveguide140, and the third waveguide145and the second waveguide140, is large enough such that the waveguides are substantially independent and not physically connected to each other.

In some embodiments, a distance between the first waveguide135and the second waveguide140is approximately one micron and a distance between the second waveguide140and the third waveguide145is approximately one micron. It is understood, however, that other distances between respective ones of the waveguides in the waveguide array130can be employed, such as two microns, three microns, and so forth. In alternative embodiments, a distance between the first waveguide135and the second waveguide140is between approximately 0.5 μm to approximately 3 μm and, similarly, a distance between the second waveguide140and the third waveguide145is between approximately 0.5 μm to approximately 3 μm.

In some embodiments, the distance between the first waveguide135and the second waveguide140, and a distance between the second waveguide140and the third waveguide145, may be determined as a function of a desired wavelength. For instance, if a larger wavelength is used, the distance between the respective ones of the waveguides in the waveguide array130may be increased. In some examples, the distance between the respective ones of the waveguides in the waveguide array130is half the distance between two or three wavelengths.

However, in embodiments in which a wavelength of approximately 1.2 to 1.6 microns is employed, the distance between the first waveguide135and the second waveguide140is approximately one micron and a distance between the second waveguide140and the third waveguide145is approximately one micron.

Further, in various embodiments, the second waveguide140may include a tapered body such that an output end of the second waveguide140coupled to the output interface125is wider than an input end of the second waveguide140coupled to the input interface120. For instance, in various embodiments, the second waveguide140may include a tapered body that is tapered close to the input interface of the optical element105. The tapered body can include a progressive widening of the second waveguide140as the waveguide body approaches the output interface125of the photonic integrated circuit110.

Notably, in some embodiments and as shown inFIGS.1-3, none of the first waveguide135, the second waveguide140, and the third waveguide145include branches or forks that are commonly seen in fork-type structures and inverse taper waveguides.

The waveguide array130may be compatible with existing photonic integrated circuit manufacturing processes, as may be appreciated. For instance, the first waveguide135, the second waveguide140, and the third waveguide145may be etched or otherwise formed in a silicon material. The waveguide array130may improve an overall coupling efficiency as well as a horizontal alignment tolerance, thereby providing improved manufacturing yield and more favorable optical performance budgets.

Further, the photonic integrated circuit110may be fabricated with a silicon-on-insulator (SOI) material. More specifically, the first waveguide135, the second waveguide140, and the third waveguide145may be formed in a top silicon layer of a silicon-on-insulator material. For instance, the waveguide array130may be formed in a single silicon layer positioned on top of an silicon dioxide layer, where the silicon dioxide layer is positioned above a silicon wafer (or silicon substrate layer). The single silicon layer in which the waveguide array130is formed may be referred to as a waveguide layer.

The assembly of the photonic integrated circuit (e.g., on a chip) may be performed to allow vertical alignment of the input beam115to the photonic integrated circuit110. Even further, the use of the silicon-on-insulator material facilitates the use of an input beam103coupled to a unique single mode optical path on the photonic integrated circuit110, or if it is coupled to a multi-mode optical path on the photonic integrated circuit110.

Moving along toFIG.4, a three-dimensional perspective view of a conventional waveguide coupler400is shown. The conventional waveguide coupler400is composed of a single inverse tapered waveguide having a rectangular cross section. As such, only the width of the tip can be adjusted to optimize the structure for matching a mode. Due to this limitation of the single inverse tapered waveguide, coupling efficiency (CE) is greatly limited in many systems.

Turning now toFIG.5, a three-dimensional perspective view of the waveguide array130ofFIGS.1-3is shown in accordance with various embodiments of the present disclosure. As shown in the three-dimensional perspective view, the waveguide array130includes the first waveguide135, the second waveguide140, and the third waveguide145, where the second waveguide140is positioned between, and parallel to, the first waveguide135and the third waveguide145. Additionally, the second waveguide140is positioned close to the first waveguide135and the third waveguide145. For instance, a distance between the first waveguide135and the second waveguide140is approximately one micron, and a distance between the second waveguide140and the third waveguide145is approximately one micron. However, the distance between the first waveguide135and the second waveguide140, and the third waveguide145and the second waveguide140, is large enough such that the waveguides in the waveguide array130are independent and not physically connected to each other.

As shown inFIG.5, the first waveguide135and the third waveguide145terminate before the second waveguide140. In other words, the first waveguide130has a first waveguide length L1, the second waveguide has a second waveguide length L2, and the third waveguide has a third waveguide length L3, where the first waveguide length L1and the third waveguide length L3are each less than second waveguide length L2.

Moving now toFIG.6, a simulated electron microscopic image depicting performance of the waveguide array130ofFIG.1is shown in accordance with various embodiments of the present disclosure. As can be seen fromFIG.6, as the first waveguide135and the third waveguide145terminate before reaching the output interface125, the first waveguide135and the third waveguide145do not carry any appreciable optical power. However, the size and position of the first waveguide135and the third waveguide145serve to widen a mode of the coupler150. More specifically, the first waveguide135and the third waveguide145, in combination with the second waveguide140being centrally located and having a tapered body, provide a wider optical mode. The wider optical mode results in a better alignment with the input beam115emitted by the optical element105, such as a laser. This results in a larger fraction of the input beam115, such as laser light, being coupled to the photonic integrated circuit110, and also results in a wider “target”’ to which the input beam115may be horizontally aligned. Thus, the various embodiments described herein permit a significantly greater tolerance in lateral or horizontal misalignment between the optical element105(e.g., a laser) and the photonic integrated circuit110. This may result in a direct improvement in manufacturing yield.

Turning now toFIGS.7and8, a first chart700and a second chart800are shown. The first chart700and the second chart800plot simulated coupling performance of a conventional taper coupler versus the multi-tip coupler150having the waveguide array130as described herein. As can be seen from the first chart700and the second chart800, compared to that of a conventional taper coupler, the multi-tip coupler150in accordance with the embodiments described herein has a higher overall coupling efficiency, a much higher coupling efficiency under severe horizontal misalignment, much lower wavelength dependence, and the same back reflection and tolerance to vertical misalignment. Each of the different line styles inFIG.7andFIG.8correspond to different wavelengths, as can be appreciated.

As such, in accordance with various embodiments described herein, a method for providing a waveguide array130may include forming a first waveguide135in a material, such as a silicon-on-insulator material; forming a second waveguide140in the material parallel to and between the first waveguide135and a third waveguide145, the second waveguide140comprising a tapered body; and forming the third waveguide145in the material, where the first waveguide135and the third waveguide145as formed each have a length less than that of the second waveguide140. The method may further include providing a photonic integrated circuit110or a chip having the photonic integrated circuit110incorporated therewith, where the waveguide array130is formed in the photonic integrated circuit110such that the photonic integrated circuit110includes an input interface120and an output interface125.

The features, structures, or characteristics described above may be combined in one or more embodiments in any suitable manner, and the features discussed in the various embodiments are interchangeable, if possible. In the following description, numerous specific details are provided in order to fully understand the embodiments of the present disclosure. However, the person skilled in the art will appreciate that the technical solution of the present disclosure may be practiced without one or more of the specific details, or other methods, components, materials, and the like may be employed. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the present disclosure.

Although the relative terms such as “on,” “below,” “upper,” and “lower” are used in the specification to describe the relative relationship of one component to another component, these terms are used in this specification for convenience only, for example, as a direction in an example shown in the drawings. It should be understood that if the device is turned upside down, the “upper” component described above will become a “lower” component. When a structure is “on” another structure, it is possible that the structure is integrally formed on another structure, or that the structure is “directly” disposed on another structure, or that the structure is “indirectly” disposed on the other structure through other structures.

In this specification, the terms such as “a,” “an,” “the,” and “said” are used to indicate the presence of one or more elements and components. The terms “comprise,” “include,” “have,” “contain,” and their variants are used to be open ended, and are meant to include additional elements, components, etc., in addition to the listed elements, components, etc. unless otherwise specified in the appended claims. The terms “first”, “second”, etc. are used only as labels, rather than a limitation for a number of the objects.

It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.