Patent Application
20230258871
In order to reduce size, weight, and power in telecommunication systems, optical sensing systems, position-navigation-timing (PNT) systems, and quantum computing systems, it is desirable to shift from benchtop optical systems and fiber optical systems to on-chip photonic integrated circuits. However, coupling light from a fiber to a chip-based waveguide introduces loss due to mode-mismatches and scattering at the fiber-air-chip interfaces.
Mode matching can be achieved by optimizing the design of the chip-based waveguide at the edge of the chip. Scattering at the fiber-chip interface can be reduced by using index matching fluids or curable resins. However, index matching fluids are typically a non-permanent solution, and curable resins can suffer from poor environmental stability as well as coefficient of thermal expansion (CTE) mismatches with the fiber and chip. Additionally, both index matching fluids and curable resins can degrade and cause absorption under high optical powers.
Methods for fiber-to-chip coupling are described herein. In one method, a photonic integrated circuit (PIC) is provided that includes a substrate, a cladding layer on the substrate, and at least one waveguide embedded in the cladding layer, wherein the at least one waveguide has a waveguide interface. An optical fiber is positioned adjacent to the PIC, wherein the optical fiber has a fiber interface, and the fiber interface is aligned with the waveguide interface. A flowable inorganic oxide in liquid form is added to an area between the fiber interface and the waveguide interface. Thereafter, heat is applied to the area between the fiber interface and the waveguide interface for a period of time to cure the inorganic oxide, such that the optical fiber is coupled to the PIC. The cured inorganic oxide has a refractive index that substantially matches the refractive indices of the cladding layer and the optical fiber.
Features of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings. Understanding that the drawings depict only typical embodiments and are not therefore to be considered limiting in scope, the invention will be described with additional specificity and detail through the use of the accompanying drawings, in which:
In the following detailed description, embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be utilized without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense.
Methods for fiber-to-chip coupling using flowable index matching materials are described herein. The methods can use a flowable oxide liquid as an index matching material when cured, for coupling optical fibers to photonic integrated circuits or chips.
In one embodiment, the flowable oxide liquid can be an inorganic oxide liquid that produces a silicon dioxide (SiO2) matching material when cured. Since claddings for both optical fibers and chips are commonly made of SiO2, an SiO2 index matching material offers near-perfect index matching, environmental stability, high power handling, and low coefficient of thermal expansion (CTE) mismatch in the fiber-matching material-chip interface. This makes SiO2 index matching materials a desirable choice for applications that need high stability and/or operation in harsh environments.
The present approach can enable permanent environmentally stable low loss coupling of light from fibers to optical chips, which is important for various systems which have tight optical power budgets. The present methods provide a low cost way of minimizing power loss at a place where high loss is common, enabling system level power budgets to be met. Thus, the present methods can aid in producing various systems that require lower power, due to lower power loss from the fiber-to-chip coupling techniques described herein.
In one implementation method, a photonic integrated circuit or chip with an SiO2 cladding is fabricated using standard microfabrication processes. The chip is diced and edge polished at the position of the coupling waveguides. A light guiding SiO2 based fiber is aligned to the edge of the chip, and the light intensity or power at an output waveguide is maximized. Once optimal alignment is achieved, a drop of flowable oxide in liquid form is placed between the interfaces of the fiber and the chip. After the drop is placed, the fiber and chip are exposed to high heat until the flowable oxide is cured and becomes SiO2. Alternatively, the heat can only be applied locally to the flowable oxide and surrounding regions.
Alternatively, the drop of flowable oxide can be placed and the fiber can be further optimized while the flowable oxide is still in liquid form as the optimal point could shift. Once cured, the interfaces between the fiber and chip should be index matched and the cured SiO2 will be stable under harsh conditions.
Further details related to the present methods are described as follows and with reference to the drawings.
In one embodiment, the inorganic oxide liquid can be a compound that produces silicon dioxide when cured. For example, the inorganic oxide liquid can be a hydrogen silsesquioxane (HSQ), which is a class of inorganic compounds having the chemical formula [HSiO3/2]n. In one example, the hydrogen silsesquioxane comprises a cubic cluster compound having a chemical formula H8Si8O12.
The PIC 200 can be fabricated using standard microfabrication processes. In one embodiment, PIC 200 is formed with a silicon substrate, a lower cladding layer deposited on the silicon substrate, and an SiO2 upper cladding layer deposited on an upper surface of the waveguide. A waveguide core is formed on the lower cladding layer, and embedded between the upper and lower cladding layers. The PIC 200 is then diced and edge polished at the position of an on-chip coupling waveguide.
As shown in the top view of
Once an optimal alignment is achieved, a flowable oxide liquid 310 is added to an area between waveguide interface 217 and fiber interface 222, as depicted in the top view of
Next, as shown in the top view of
The heat for curing the flowable oxide liquid can be in a temperature range of about 250° C. to about 1000° C. (or higher), and can be applied for a period of time of about 1 hour to about 24 hours. The cured inorganic oxide has a refractive index that substantially matches the refractive indices of the cladding layer on the PIC and the optical fiber.
The PIC 500 includes a substrate 510, and at least one on-chip waveguide 512. The substrate 510 can be composed of silicon, for example. The on-chip waveguide 512 includes a core layer 514 of a higher refractive index material, which is embedded in a cladding layer 516 of a lower refractive index material. The cladding layer 516 is formed on one side of substrate 510 (see
The PIC 500 can be fabricated using standard microfabrication processes. In one embodiment, PIC 500 is formed with a silicon substrate and an SiO2 cladding layer on an upper surface of the silicon substrate. A waveguide core is formed on the silicon substrate and embedded in the cladding layer. The PIC 500 is then diced and edge polished at input and output edges 520, 524.
As shown in the top view of
Once an optimal alignment is achieved, a flowable oxide liquid 610 is added to an area between input waveguide interface 518 and input fiber interface 532, as depicted in the top view of
Next, flowable oxide liquids 610, 612 are cured at a high temperature and become solid oxide materials 710, 712, as shown in the top view of
The heat for curing the flowable oxide liquid can be in a temperature range of about 250° C. to about 1000° C. (or higher), and can be applied for a period of time of about 1 hour to about 24 hours. The cured inorganic oxides have a refractive index that substantially matches the refractive indices of the cladding layer on the PIC and the optical fibers.
Example 1 includes a method comprising: providing a photonic integrated circuit (PIC) that includes a substrate, a cladding layer on the substrate, and at least one waveguide embedded in the cladding layer, wherein the at least one waveguide has a waveguide interface; positioning an optical fiber adjacent to the PIC, wherein the optical fiber has a fiber interface; aligning the fiber interface with the waveguide interface; adding a flowable inorganic oxide in liquid form to an area between the fiber interface and the waveguide interface; and applying heat to the area between the fiber interface and the waveguide interface for a period of time to cure the inorganic oxide, such that the optical fiber is coupled to the PIC, wherein the cured inorganic oxide has a refractive index that substantially matches the refractive indices of the cladding layer and the optical fiber.
Example 2 includes the method of Example 1, wherein the substrate comprises silicon, glass, quartz, sapphire, III-V materials, III-nitride materials, lithium niobate, or combinations thereof.
Example 3 includes the method of any of Examples 1-2, wherein the at least one waveguide includes a core comprising a higher refractive index material, and the cladding layer comprises a lower refractive index material.
Example 4 includes the method of Example 3, wherein: the core comprises silicon nitride, silicon, aluminum oxide, lithium niobate, III-V materials, III-nitride materials, or combinations thereof; and the cladding layer comprises silicon dioxide, aluminum oxide, or combinations thereof.
Example 5 includes the method of any of Examples 1-4, wherein the optical fiber comprises a glass based fiber.
Example 6 includes the method of any of Examples 1-5, wherein the inorganic oxide comprises a compound that produces silicon dioxide when cured.
Example 7 includes the method of any of Examples 1-6, wherein the inorganic oxide comprises a hydrogen silsesquioxane (HSQ) with a chemical formula [HSiO3/2]n.
Example 8 includes the method of Example 7, wherein the hydrogen silsesquioxane comprises a cubic cluster compound having a chemical formula H8Si8O12.
Example 9 includes the method of any of Examples 1-8, wherein the inorganic oxide is added in droplet form.
Example 10 includes the method of any of Examples 1-8, wherein the heat applied to the area between the fiber interface and the waveguide interface has a temperature of about 250° C. to about 1000° C., which is applied for a period of time of about 1 hour to about 24 hours.
Example 11 includes the method of any of Examples 1-10, wherein the heat is applied by a furnace, a heat gun, or a laser.
Example 12 includes the method of any of Examples 1-11, wherein the fiber interface is aligned with the waveguide interface such that a light intensity or power at an output waveguide on the PIC is maximized when a light beam is transmitted from the optical fiber into the PIC.
Example 13 includes the method of any of Examples 1-12, wherein: the waveguide interface is located at an input edge of the PIC; the optical fiber is an input fiber positioned adjacent to the input edge of the PIC; and the fiber interface is aligned with the waveguide interface at the input edge of the PIC such that the input fiber is edge coupled to the PIC when the inorganic oxide is cured.
Example 14 includes the method of any of Examples 1-12, wherein: the waveguide interface is located at an output edge of the PIC; the optical fiber is an output fiber positioned adjacent to the output edge of the PIC; and the fiber interface is aligned with the waveguide interface at the output edge of the PIC such that the output fiber is edge coupled to the PIC when the inorganic oxide is cured.
Example 15 includes a method for fiber-to-chip coupling, the method comprising: providing a photonic integrated circuit (PIC) that includes a substrate, a cladding layer on the substrate, and at least one waveguide embedded in the cladding layer, the PIC having an input edge and an output edge, the at least one waveguide having an input waveguide interface at the input edge and an output waveguide interface at the output edge; positioning an optical input fiber adjacent to the input edge of the PIC, wherein the optical input fiber has an input fiber interface; aligning the input fiber interface with the input waveguide interface; positioning an optical output fiber adjacent to the output edge of the PIC, wherein the optical output fiber has an output fiber interface; aligning the output fiber interface with the output waveguide interface; adding a flowable inorganic oxide in liquid form to a first area between the input fiber interface and the input waveguide interface; adding the flowable inorganic oxide in liquid form to a second area between the output fiber interface and the output waveguide interface; applying heat to the first and second areas for a period of time to cure the inorganic oxide, such that the optical input fiber and the optical output fiber are edge coupled to the PIC, wherein the cured inorganic oxide has a refractive index that substantially matches the refractive indices of the cladding layer, the optical input fiber, and the optical output fiber.
Example 16 includes the method of Example 15, wherein: the core comprises silicon nitride, silicon, aluminum oxide, lithium niobate, III-V materials, III-nitride materials, or combinations thereof; the cladding layer comprises silicon dioxide, aluminum oxide, or combinations thereof; and the input and output fibers comprises silicon dioxide based fibers.
Example 17 includes the method of any of Examples 15-16, wherein the inorganic oxide comprises a compound that produces silicon dioxide when cured.
Example 18 includes the method of any of Examples 15-17, wherein the inorganic oxide comprises a hydrogen silsesquioxane (HSQ) with a chemical formula [HSiO3/2]n.
Example 19 includes the method of any of Examples 15-18, wherein the inorganic oxide is added in droplet form.
Example 20 includes the method of any of Examples 15-19, wherein the heat is applied by a furnace, a heat gun, or a laser.
From the foregoing, it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without deviating from the scope of the disclosure. Thus, the described embodiments are to be considered in all respects only as illustrative and not restrictive. In addition, all changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.
