WO2024122071A1 - Optical device and optical device positioning method - Google Patents

Abstract

The present invention comprises a wiring substrate (53) and an optically functional element (51) that has an input/output optical waveguide (511) for an optical signal and is flip-chip mounted on the wiring substrate (53), an end surface of the input/output optical waveguide (511) in the optically functional element (51) is butted and coupled with an end surface of another input/output optical waveguide (521), the wiring substrate (53) comprises aligning through-holes (531) that penetrate through the wiring substrate (53), and consistency or inconsistency between the position of the end surface of the input/output optical waveguide (511) and the position of the end surface of the other input/output optical waveguide (521) can be visually recognized via the alignment through-holes (531).

Description

Optical device and method for aligning optical device

This disclosure relates to an optical device in which an optical functional element having an optical waveguide and the optical functional element are flip-chip mounted on a substrate.

In recent years, with the spread of optical signal transmission using optical fibers, there is a demand for technology to integrate a large number of optical circuits at high density. Known examples of such optical circuits include quartz-based planar lightwave circuits (PLCs) and silicon photonics-based optical circuits (System in Package: SiPs). PLCs are waveguide-type optical devices that have low loss, high reliability, and high design freedom, and PLCs that integrate functions such as multiplexers/demultiplexers, branchers, and couplers are installed in transmission equipment at the optical communication transmission end. Although SiPs are inferior to PLCs in terms of low loss, they have high design freedom and are optical devices that can realize even smaller optical circuits due to their small optical waveguide bending radius. In addition to PLCs and SiPs, optical devices also include photodiodes (PDs) that convert optical and electrical signals, laser diodes (LDs), and optical functional elements such as optical modulators.

To further expand communication capacity, there is a demand for high-performance optoelectronic integrated devices that integrate optical waveguides such as PLCs that perform optical signal processing with optical devices such as PDs. These optical devices are made of InP-based materials and perform high-speed photoelectric conversion. PLCs and SiPs are promising platforms for such integrated optical devices, and integrated optical devices that hybridly integrate an InP optical modulator chip and a PLC chip have been proposed. Such integrated optical devices are described, for example, in Non-Patent Document 1.

In the integrated optical device described in Non-Patent Document 1, a method is adopted in which a phase modulator is integrated on an InP chip, a polarization rotator and a polarization beam combiner are integrated on a PLC, and the two chips are optically coupled via a lens. The method of using a PLC as a polarization Mux chip requires a smaller mounting area than the conventional method of constructing polarization synthesis using a spatial optical system, and optical axis alignment can be simplified by integrating it into an optical circuit. This form of optical coupling by combining an optical circuit element such as PLC and InP has advantages in terms of device miniaturization and freedom of optical circuit design. In addition, integrated devices including PDs with a waveguide structure suitable for broadbanding using InP-based materials and optical phase modulators with high-speed phase modulation functions have been developed in order to expand communication capacity. Furthermore, in recent years, there has been a demand for integrated optical devices in which optical circuit elements are directly connected to each other without using lenses in order to achieve further miniaturization.

E. Yamada et al., "112-Gb/s InP DP-QPSK modulator integrated with a silica-PLC polarization multiplexing circuit", Proc. Opt. Fiber Commun. Conf. Expo. Nat. Fiber Opt. Eng. Conf., Mar. 2012.)

Here, for example, in order to connect a PLC and an InP-based optical functional element, if we assume that the input and output optical waveguides of each are butted together and coupled, it is necessary to align and fix the optical waveguides while monitoring the intensity of the light input to one of the optical elements and passing through the connection surface. For example, in butt-connecting an optical fiber and a PLC, it is necessary to perform alignment. Alignment is performed by adjusting the end face of the glass fiber block to which the fiber is fixed and the end face of the PLC so that they are parallel, and then aligning the optical output position from the fiber with the input optical waveguide of the PLC while inputting light into the fiber, and adjusting the position so that optimal optical coupling is obtained while monitoring the output from the output optical waveguide connected to the input optical waveguide. After that, a UV-curing adhesive is filled in the adjusted position, and the adhesive is hardened in a short time by irradiating it with UV light, and the two are bonded together. If such a butt-connecting method can be used to integrate optical functional elements made of Si or InP using the optical circuit of the PLC as a platform, or to integrate optical functional elements made of InP using the SiP as an optical circuit platform, it will be possible to provide a smaller integrated optical device.

Furthermore, to increase the speed of devices, it is necessary to shorten the electrical wiring between the IC that inputs and outputs electrical signals to the optical functional element in order to transmit high-speed signals with low loss. A configuration in which the IC and optical functional element are mounted on a substrate using a flip-chip technique allows the IC and optical functional element to be connected over a short distance, and is suitable for transmitting signals with low loss. Furthermore, a small, high-speed optical device can be realized by using a SiP that integrates an optical circuit that controls polarization as an optical circuit element and connecting it to a flip-chip phase modulator using a dissimilar material.

However, when connecting optical circuit elements or optical fibers to optical functional elements, in order to align the input/output optical waveguides of the optical functional element and the input/output optical waveguides of the optical circuit element, it is necessary to pre-align them to a rough position where the light intensity of both can be confirmed. In the case of the flip-chip phase modulator described above, the optical waveguide surface is on the wiring board side, so the optical waveguide is not visible and cannot be observed from above, making it difficult to pre-align the optical waveguides.

The present disclosure has been made in consideration of the above points, and aims to provide an optical device that is applicable to the integration of an optical functional element having an optical waveguide input/output structure and an optical circuit element having an optical waveguide input/output structure for inputting/outputting optical signals between the optical circuit element, and that realizes end-face optical coupling by a structure that enables simple alignment when inputting/outputting optical signals between the optical element and the optical circuit by end-face connection.

In order to achieve the above object, an optical device according to one embodiment of the present disclosure includes a wiring board, and an optical functional element having an input/output optical waveguide for an optical signal and flip-chip mounted on the wiring board, wherein an end face of the input/output optical waveguide in the optical functional element is butt-coupled to an end face of another input/output optical waveguide, and the wiring board includes an alignment through hole penetrating the wiring board,
Whether the position of the end face of the input/output optical waveguide coincides with the position of the end face of the other input/output optical waveguide can be visually confirmed via the alignment through hole.

In addition, the method for aligning an optical device according to one embodiment of the present disclosure includes a wiring board and an optical functional element having an input/output optical waveguide for an optical signal and flip-chip mounted on the wiring board, an end face of the input/output optical waveguide in the optical functional element is butt-coupled to an end face of another input/output optical waveguide, the wiring board includes a through-hole for alignment that penetrates the wiring board, and the match or mismatch between the position of the end face of the input/output optical waveguide and the position of the end face of the other input/output optical waveguide can be visually confirmed through the through-hole for alignment, and after adjusting the X-axis and Y-axis of the input/output optical waveguide and the other input/output optical waveguide while visually confirming the end face of the input/output optical waveguide and the end face of the other input/output optical waveguide through the through-hole for alignment, a light intensity profile transmitted through the input/output optical waveguide and the other input/output optical waveguide is obtained, and the peak of the light intensity profile is aligned in the Y-axis direction to perform pre-alignment.

The above embodiment is applicable to the integration of an optical functional element having an optical waveguide input/output structure and an optical circuit element having an optical waveguide input/output structure for inputting/outputting optical signals between the optical circuit element, and can provide an optical device that realizes end-face optical coupling by a structure that enables simple alignment when inputting/outputting optical signals between the optical element and the optical circuit by end-face connection.

FIG. 1 is a diagram showing a comparative example of the present disclosure. FIG. 13 is a diagram showing another comparative example of the present disclosure. 13A and 13B are diagrams showing another comparative example of the present disclosure. 13A and 13B are diagrams showing another comparative example of the present disclosure. 1A, 1B, and 1C are plan views illustrating an optical device according to a first embodiment of the present disclosure. 13A and 13B are plan views illustrating an optical device according to a second embodiment of the present disclosure. 13A and 13B are plan views illustrating an optical device according to a second embodiment of the present disclosure. FIG. 13 is a diagram showing an output light intensity profile obtained after performing pre-alignment according to the present embodiment. FIG. 13 is a diagram showing an output light intensity profile of a comparative example.

Prior to explaining the present disclosure, comparative examples of the present disclosure will be explained below using drawings. The drawings of the present disclosure are intended to explain the configuration, positional relationships of each part, action, function, and technical concept of the present disclosure, and do not limit the specific shape of the present disclosure, nor do they necessarily accurately represent the aspect ratio, etc.

[Comparative Example]
FIG. 1 is a diagram showing a comparative example of the present disclosure, and shows a state in which an optical functional element 11 and an optical circuit element 12 are connected with different materials. The optical functional element 11 shown in FIG. 1 is a phase modulation chip, which is made of InP integrating Mach-Zehnder interferometers 112 and 113 as phase modulators, and includes an optical waveguide 111 that connects the Mach-Zehnder interferometers 112 and 113. The optical circuit element 12 is a polarization control chip, which integrates a polarization rotor 122, a polarization beam combiner (Polarization Beam Combiner: PBC) 123, and an optical waveguide 121. In the following description of both the comparative example and the embodiment, the surface on which the Mach-Zehnder interferometers 112 and 113 shown in FIG. 2 are formed is referred to as the front surface of the optical functional element 11, and the surface on which the polarization rotor 122 and the polarization beam combiner 123 are mounted is referred to as the front surface of the optical circuit element 12. The surface opposite to the front surface is referred to as the back surface.

Signal light (TE polarization (transverse electric field polarization)) is input from optical waveguide 121 and passes through Mach-Zehnder interferometers 112 and 113 to polarization rotor 122 and polarization beam combiner 123, respectively. The TE polarization input to polarization rotor 122 is converted to TM polarization (transverse magnetic field polarization) and input to polarization beam combiner 123. The TE polarization and TM polarization are output from polarization beam combiner 123 as signal light.

Furthermore, to increase the speed of the device, it is necessary not only to increase the speed of the optical functional element 11 itself, but also to shorten the wire (electrical wiring) 21 between the optical functional element 11 and the driver IC 20 that connects to the optical functional element 11 and inputs and outputs electrical signals, in order to transmit high-speed signals with low loss. Fig. 2 is a plan view showing a known structure in which a driver IC 20, optical functional element 11, and optical circuit element 12 are integrated on a substrate 10. In the structure shown in Fig. 2, the optical functional element 11 and the driver IC 20 are mounted face-up and connected by wire bonding, but the wire 21 needs to be several hundred μm long, and it is difficult to design a signal line suitable for high frequencies, which causes a problem of large losses in the band of 50 GHz or more.

In the structure shown in Figure 2, variations in the length of the wires 21 are a cause of variations in the transmission characteristics for each channel. Taking this into consideration, there is a structure in which the driver IC 20 and the optical functional element 11 are flip-chip mounted on a substrate 30 having wiring 31, as shown in Figure 3(a). The structure shown in Figure 3(a) is configured by connecting the surface of the optical functional element 11 toward the surface of the wiring substrate 30 on which the wiring 31 is formed. Figure 3(b) shows the state in which the optical functional element 11 and driver IC 20 shown in Figure 3(a) are connected to the wiring substrate 30. When viewed from above, the back surface of the optical functional element 11 is visible.

The connection is made using metal bumps 32 or solder. Therefore, the configuration shown in FIG. 3(a) can connect the optical functional element 11 to the wiring board 30 over a short distance. Furthermore, the width and pitch of the wiring 31 on the wiring board 30 can be designed to suit high frequencies, making it suitable for transmitting signals with low loss. In the example shown in FIG. 3, the optical functional element 11 is a phase modulator, and is mounted on the wiring board 30 by flip-chip mounting together with the driver IC 20, making it possible to connect the two with ideal electrical wiring. Furthermore, the configuration shown in FIG. 3(a) can realize a small, high-speed optical device by integrating optical circuit elements together.

4(a) and 4(b) are diagrams for explaining the connection of the optical circuit element 12 to the configuration shown in FIG. 3(a). FIG. 4(a) is a plan view of the configuration shown in FIG. 3(a) and the optical circuit element 12 as viewed from the front side of the wiring board 30. FIG. 3(b) is a plan view of the configuration shown in FIG. 3(a) as viewed from the back side of the wiring board 30. When connecting the optical circuit element 12 or optical fiber to the optical functional element 11, in order to align the input/output optical waveguide of the optical functional element 11 and the input/output optical waveguide of the optical circuit element 12, it is necessary to pre-align them to a rough position where the light intensity of both can be confirmed. In the case of the optical functional element 11 of the flip-chip phase modulator described above, the surface on which the input/output optical waveguide is formed faces the front side of the wiring board 30, and the input/output optical waveguide cannot be seen from the top. For this reason, the alignment shown in FIG. 4(a) and FIG. 4(b) makes it difficult to pre-align the input/output optical waveguides with each other because the input/output optical waveguides cannot be observed from the top.

To solve the above problem, it is conceivable to provide alignment marks indicating the connection positions on the back surface of the substrate for the optical functional element 11 and the optical circuit element 12, and perform pre-alignment without checking the positions of the input and output optical waveguides. However, forming alignment marks on the back surface of the substrate requires the addition of an additional process to the processing steps on the surface of the substrate, which makes the manufacturing process more complicated and increases costs.

As another configuration to solve the above problem, it is possible to design the input/output optical waveguide of the optical circuit element 12 so that it is located outside the wiring board 30. With this configuration, the input/output optical waveguide is not hidden by the wiring board 30, and pre-alignment can be performed while checking the position of the input/output optical waveguide. However, with this configuration, the chip of the optical circuit element 12 cannot be placed in the center of the wiring board 30, and the degree of freedom in design is reduced. This becomes a problem when placing chips at high density. Furthermore, when the optical functional element 11 and the optical circuit element 12 are mounted on the wiring board 30 by flip chip soldering, there is also the problem that the adhesive deteriorates due to heat caused by heating the solder when the optical functional element 11 and the optical circuit element 12 are mounted on the wiring board 30 after being connected.

As described above, when hybrid-integrating an optical functional element such as an optical modulator with an optical circuit element such as a PLC or SiP that integrates a polarization control circuit, a flip chip is suitable for inputting and outputting high-speed signals. In this case, the challenge is to simply butt-couple the input and output optical waveguides. This disclosure has been made with a focus on this point, and aims to provide an optical device that butt-couples, in a simple manner, the input and output optical waveguides of an optical functional element and an optical circuit element that are flip-chip mounted on a wiring board, thereby achieving highly efficient optical coupling.

[First embodiment]
5(a), 5(b), and 5(c) are plan views for explaining an optical device according to a first embodiment of the present disclosure. The optical device according to this embodiment includes a wiring board 53, an optical functional element 51, an optical circuit element 52, and a driver IC 55. The optical functional element 51 and the driver IC 55 are connected by a wire 56. The optical functional element 51 may have a configuration similar to that of the phase modulation chip shown in FIG. 1, for example, and includes an input/output optical waveguide for an optical signal whose end face is located at the end of the optical circuit element 51. The optical circuit element 52 may be a polarization control chip similar to that of the optical circuit element 12 shown in FIG. 1, for example, and includes an input/output optical waveguide for an optical signal whose end face is located at the end of the optical circuit element 52. In the explanation of FIGS. 5(a) to 5(c), the side in front of the paper is referred to as "upper" and the side in the back is referred to as "lower".

FIG. 5(a) shows the back side of the surface on which the input/output optical waveguides of the optical functional element 51, the optical circuit element 52, and the IC driver 55 are formed, and shows the front side on which the wiring of the wiring board 53 is formed. FIG. 5(b) shows the state in which the configuration shown in FIG. 5(a) is rotated 180 degrees toward the front about the Z axis. FIG. 5(b) shows the back side of the wiring board 53, and the optical functional element 51, the optical circuit element 52, and the driver IC 55 arranged below the wiring board 53. The optical functional element 51 and the optical circuit element 52 are flip-chip mounted so that their front surfaces are in contact with the wiring board 53. As shown in FIG. 5(a) and FIG. 5(b), the wiring board 53 has three alignment through holes 531 penetrating the wiring board 53. FIG. 5(c) is an enlarged view of the alignment through hole 531 shown in FIG. 5(b).

The end face of the input/output optical waveguide in the optical functional element 51 is butted against and coupled to the end face of the other input/output optical waveguide. In the example shown in FIG. 5(a) and FIG. 5(b), the other input/output optical waveguide is the input/output optical waveguide of the optical circuit element 52. As shown in FIG. 5(c), the butted portion between the end face of the input/output optical waveguide of the optical functional element 51 and the end face of the input/output optical waveguide of the optical circuit element 52 is visible through the through hole for core alignment 531. Note that the "butted portion" in the first and second embodiments refers to the position where the end faces come into contact before being fixed with an adhesive or the like. That is, the butted portion C of the input/output optical waveguide 511 provided on the optical functional element 51 side and the input/output optical waveguide 521 provided on the optical circuit element 52 side can be observed from the through hole for core alignment 531. Note that "visible" here means that the state can be seen visually from above the through hole for core alignment 531 or by a camera.

As described above, the optical device of the first embodiment can confirm and pre-align the input/output optical waveguide 511 of the optical functional element 11 and the input/output optical waveguide 521 of the optical circuit element 52 from the wiring board 53 side through the alignment through hole 531. By flip-chip mounting on the wiring board 53, the surface on which the input/output optical waveguides of the optical functional element 51 are provided is covered by the wiring board 53. However, according to the first embodiment, since the alignment through hole 531 is provided corresponding to the coupling position of the input/output optical waveguides 511 and 512, the positions of the input/output optical waveguides 511 and 512 can be confirmed, and pre-alignment can be performed in the same manner as with known input/output optical waveguides that are not flip-chip mounted.

The configuration of the first embodiment allows the input/output optical waveguide 511 of the optical functional element 51 and the input/output optical waveguide 521 of the optical circuit element 52 to be aligned in advance at positions that facilitate alignment. Therefore, according to the first embodiment, it is possible to initially align the optical functional element 51 to a position close to the position that will be aligned during alignment performed while inputting signal light and monitoring the output light. The first embodiment allows for highly accurate alignment with a simple configuration and procedure, even for an optical functional element 11 flip-chip mounted on a wiring board 53.

As explained above, in the first embodiment, an optical functional element is flip-chip mounted on a wiring board, an input/output optical waveguide is provided on the optical functional element, and an alignment through-hole is provided at a position on the wiring board corresponding to the position of the input/output optical waveguide. With this configuration, the alignment of the input/output optical waveguide can be performed from the back side of the wiring board while checking the position of the input/output optical waveguide via the alignment through-hole. This is the greatest feature of the first embodiment.

In the first embodiment, flip-chip mounting makes it easy to easily check the positions of the input and output optical waveguides covered by the wiring board. This can be used to pre-align the optical waveguide positions to positions that are easy to align. Aligning the optical waveguides to be pre-aligned in close positions is desirable because it makes it possible to reduce the time required for fine adjustments during subsequent alignment while monitoring the optical output of the signal light. As a result, butt coupling to a flip-chip mounted optical functional element can be achieved with a simple procedure.

In general, the cross-sectional structure of a PLC is formed by depositing a thin film _{of SiO 2 of} about 20 μm as an undercladding, 3 μm to 10 μm as a core, and about 20 μm as an overcladding on a substrate of Si or SiO 2. The first embodiment assumes a PLC formed on a Si substrate. In addition, in an optical circuit element of Si photonics, a few μm of SiO ₂ forming an SOI layer as an undercladding, a few hundred nm of Si as a core, and a few μm of SiO ₂ as an overcladding are deposited on a Si substrate. In addition, an optical functional element using InP as a substrate has an InP substrate as an undercladding, a few hundred nm of a compound semiconductor as a core, InP as an overcladding, and SiN or SiO ₂ as a passivation. Metal patterns as electrodes are provided on the front and back surfaces.

In the first embodiment, the optical waveguide formed in the end surface region of the substrate is assumed to be an input/output optical waveguide for inputting and outputting optical signals. The input/output optical waveguide is optically coupled to other input/output optical waveguides by the mode field at the end surface. The wiring board is used to input and output electrical signals to and from the chip. Such a wiring board may be a ceramic board made of ceramic material such as aluminum nitride or aluminum oxide, or an organic board based on epoxy or the like, on which a metal wiring pattern is provided and laminated. Conduction between each layer of the laminate is achieved by conductive vias provided by providing through holes in each board and coating or filling the surface with a conductive material.

The alignment through-hole is formed by providing a through-hole at a position on the wiring board where the part that serves as the marker for the optical functional element can be confirmed during pre-alignment for alignment. A typical through-hole via is a conductive via filled with a conductive material that does not transmit light in order to electrically connect the front and back surfaces of the board. In contrast, the alignment through-hole of the first embodiment is preferably not filled with a conductive material or filled with a light-transmitting material in order to confirm the surface of the optical functional element. The alignment through-hole is preferably located at the input/output optical waveguide position of the optical functional element. However, in the first embodiment, the alignment through-hole may be formed in the process of forming the conductive via, and used as the alignment through-hole without being filled with a conductive material, simplifying the manufacturing process.

In this way, the first embodiment is devised to provide an alignment through hole in the wiring board at the position of the input/output optical waveguide of the optical functional element flip-chip mounted on the wiring board in an optical device in which optical functional elements and optical circuit elements are hybridly integrated. Therefore, in the first embodiment, the butt optical coupling with the optical functional element or optical fiber can be performed by confirming the optical waveguide position from the wiring board side, pre-aligning to a rough alignment position, and then transitioning to alignment by optical signal. Therefore, the first embodiment makes it possible to efficiently align the butt coupling, and makes it possible to provide an optical device achieved by simple butt optical coupling.

Furthermore, according to the first embodiment, in order to obtain the above-mentioned effects, there is no need to provide a marker on the back surface of the optical functional element as a guide for alignment, nor is there any need to restrict the mounting position of the optical functional element in order to check the input/output optical waveguide. In this respect, the first embodiment is superior to the previously mentioned known techniques.

Second Embodiment
The second embodiment differs from the first embodiment in that alignment markers corresponding to both the optical functional element and the optical circuit element are provided, and alignment through holes are formed in the wiring board at positions corresponding to the alignment markers. In this configuration, the alignment markers can be observed from the alignment through holes.

FIGS. 6(a), 6(b), 7(a), and 7(b) are plan views for explaining the optical device of the second embodiment. The optical device of the second embodiment includes a wiring board 63, an optical functional element 61, and an optical circuit element 62. FIG. 6(a) is a diagram showing the state in which the optical functional element 61 and the optical circuit element 62 are mounted on the surface of the wiring board 63. The optical functional element 61 and the optical circuit element 62 are flip-chip mounted on the wiring board 63, and their surfaces are in contact with the wiring board 63. The optical circuit element 62 includes optical waveguides for inputting and outputting optical signals, and corresponding input/output optical waveguides for butt coupling. FIG. 6(b) is a diagram showing the wiring board 63 as viewed from the back side. FIG. 7(a) is a diagram showing the wiring board 63 and the optical functional element 61 and the optical circuit element 62 under the wiring board 63 as viewed from the back side of the wiring board 63. Figure 7(b) is a view from the front of an optical device in which an optical functional element 61 and an optical circuit element 62 are butt-coupled. However, the optical device shown in Figure 7(b) is a connection test device in the second embodiment, and has a configuration in which the phase modulation section, polarization rotator, and polarization beam combiner are removed from the optical functional element of the integrated optical modulation device (shown in Figure 1).

As shown in FIG. 7(b), the optical circuit element 62 has an input optical waveguide 621a and an output optical waveguide 621c for the signal light, and corresponding input/output optical waveguides 621b and 621d for butt coupling. The optical function element 61 has butt-coupled optical waveguides 611a and 611b that butt-couple with the butt-coupled input/output optical waveguides 621b and 621d. The optical function element 61 and the optical circuit element 62 are flip-chip mounted on the wiring board 63 as shown in FIG. 6(a) and FIG. 7(a). The wiring board 63 inputs and outputs electrical signals between the optical function element 61 and the optical circuit element 62.

The optical device of the second embodiment may be an optical modulation device in which an optical function element 61 and an optical circuit element 62 are coupled together. When the optical device of the second embodiment is an optical modulation device, the optical function element 61 is composed of a phase modulation optical waveguide that changes the phase of light by inputting an electrical signal from the wiring board 63 and an optical waveguide for butt coupling, and the input/output optical waveguides 611a and 611b are arranged on one end surface by a U-shaped optical waveguide arrangement. When the optical device of the second embodiment is an optical modulation device, the light input to the butt input/output optical waveguides 621b and 621d of the optical circuit element 62 is optically coupled to the side of the optical function element 61 through the butt coupling section, and then converted into an optical signal phase-modulated by the phase modulation optical waveguide, optically coupled to the optical circuit element 62 again through the butt coupling section, and output after polarization synthesis by the polarization rotator and the polarization beam combiner.

As shown in FIG. 7(b), the surface of the optical functional element 61 is provided with connection pads 613 for flip-chip mounting on the wiring board 63. As shown in FIG. 6(b), the surface of the wiring board 63 is provided with connection pads 633 that are electrically connected to the connection pads 613. As shown in FIG. 6(a), the optical functional element 61 and the optical circuit element 62 are electrically connected to the wiring board 63 via the connection pads 613, 633. Also, in the wiring board 63, four alignment through holes 632 are formed, as in the first embodiment. As in the first embodiment, the alignment through holes 632 are through holes that penetrate the wiring board 63.

A SiP chip measuring 2.5 mm in length and 2.0 mm in width can be used as a platform for the optical circuit element 62. The second embodiment uses a Si photonics chip in which a _SiO2 underclad with a thickness of 3.0 μm, a Si core with a thickness of 0.22 μm and a width of 0.5 μm, and a _SiO2 overclad with a thickness of 1.5 μm are formed on a Si substrate with a thickness of 0.625 mm.

The optical circuit element 62 inputs and outputs signal light from one long side, and connects the other long side to the optical function element 61. The connection surface is polished for connection. The optical waveguide structure from the input optical waveguide 621a to the butting input/output optical waveguide 621b and from the output optical waveguide 621c to the butting input/output optical waveguide 621d is S-shaped. Markers 625 are provided near the butting input/output optical waveguides 621b and 621d. The markers 625 serve as guides when the optical circuit element 62 is aligned with the optical function element 61.

The optical functional element 61 is an InP chip measuring 2.5 mm in length, 4.0 mm in width, and 0.25 mm in substrate thickness. The InP chip has an InP substrate as the underclad, a compound semiconductor with a width of 2.0 μm and a thickness of 0.3 μm as the core, and an InP overclad with 2.0 μm of InP deposited on it. Input/output optical waveguides 611a and 611b for butting are provided on the short side. Markers 615 are provided near the input/output optical waveguides 611a and 611b for butting. Marker 615 is paired with marker 625 of optical circuit element 62, and serves as a guide when aligning with optical circuit element 62. Markers 615 and 625 are collectively referred to as alignment markers 65.

The wiring board 63 is an aluminum nitride ceramic substrate measuring 4.0 mm in length, 8.0 mm in width, and 0.45 mm thick, on which connection pads 633 and alignment through holes 632 are formed. The connection pads 633 are gold pads. The alignment through holes 632 are through holes with a diameter of 150 μm that penetrate the substrate.

When flip-chip mounting the optical functional element 61 onto the wiring board 63, a gold bump approximately 20 μm in height is formed on the connection pad 633 of the wiring board 63 using the ball formation function of a wire bonder. The flip-chip optical functional element 61 is mounted onto the wiring board 63 by connecting the connection pads 633 and 613 via the gold bumps by thermocompression bonding.

The alignment through holes 632 of the wiring board 63 are formed at positions corresponding to the alignment markers 65 or the butt portions of the butt input/output optical waveguides of the optical function element 61 and the optical circuit element 62. Therefore, as shown in FIG. 6(a), the alignment markers 65 can be seen from the back surface of the wiring board 63 through the alignment through holes 632. Note that in the second embodiment, as in the first embodiment, the butt portions of the butt input/output optical waveguides of the optical function element 61 and the optical circuit element 62 can be seen through the alignment through holes 632.

In the second embodiment, the marker 615 formed on the optical functional element 61 side and the marker 625 formed on the optical circuit element 62 side are arranged so as to be visible from the alignment through hole 632. The optical functional element 61 and the optical circuit element 62 are pre-aligned by adjusting their positions so that the markers 615, 625 properly form the alignment marker 65. In such a process, it is preferable that the markers 615, 625 are linearly symmetrical with respect to an axis perpendicular to the opposing direction of the optical functional element 61 and the optical circuit element 62, and have the same shape. This is because it is easy to visually determine whether the shape formed by butting the markers 615, 625 together is proper.

In the second embodiment, a transparent material that transmits visible light is provided on the back surface of the wiring board 63 at a position that covers the alignment through-hole 632. Note that the "cover" here is not limited to a configuration in which the alignment through-hole 632 is covered with a transparent material, but may be a configuration in which the alignment through-hole 632 is filled with a transparent material. In the second embodiment, the transparent material is a thin-film glass substrate 635 with a thickness of 0.03 μm. With this configuration, the second embodiment can prevent thermally conductive adhesive from entering the alignment through-hole 632 even when the wiring board 63 is attached to another substrate with a thermally conductive adhesive or the like.

Next, an example of the present disclosure will be described. This example is applied to pre-alignment, in which the positions of the input and output optical waveguides are adjusted to positions where alignment by optical signals is possible in the alignment of butt-coupled optical functional elements and optical circuit elements flip-chip mounted on a wiring board. In this example, the input and output optical waveguides were observed through the alignment through holes to evaluate whether pre-alignment was possible. Note that the optical functional element of this example is a test device in which the polarization rotator and polarization beam combiner are removed from the phase modulation chip, as described in the second embodiment. The optical functional element of this example includes a signal light input and output optical waveguide and an input and output optical waveguide for butt-coupled. In this example, the optical functional element is an InP chip, and the optical circuit element is a SiP chip.

To butt-couple the SiP chip to the InP chip, the connection end faces of the SiP chip and the InP chip are observed from the back side of the wiring board through the alignment through holes, and the end faces of the input and output optical waveguides are brought close to each other. After this, the worker observes the two alignment through holes and pre-aligns them by adjusting the wiring board horizontally so that the input and output optical waveguides match at each point.

If the end faces are aligned using one of the two alignment through holes, but not the other, it is determined that the Z-axis rotation is misaligned, and the Z-axis rotation is adjusted so that they are aligned. The subsequent alignment is performed by inputting light with a wavelength of 1.55 μm from the signal light input optical waveguide of the SiP chip, propagating it through the InP chip, and monitoring the light intensity output from the signal light output optical waveguide. In this alignment, first, the output light intensity profile in the Y-axis is obtained, and the position is adjusted so that the output light intensity peaks, thereby adjusting to a rough alignment position. After that, the alignment is completed by aligning the X-axis and Y-axis so that the output light intensity is maximized, and adjusting the Z-axis rotation, and adjusting the gap between the end faces to the design position (1 μm).

Pre-alignment using the alignment marker is also performed by observing the alignment marker through the alignment through-hole, and adjusting the X-axis and Z-axis rotation so that the alignment markers of the InP chip and the SiP chip are in their designed positions. This type of alignment allows pre-alignment to be performed in the same way as pre-alignment performed by observing the end faces of the input and output optical waveguides.

The butt joint of the SiP chip and the InP chip is fixed by filling the space between the SiP chip and the InP chip with a UV-curable adhesive that is transparent in the infrared region, and then irradiating UV light to harden the adhesive. At this time, an anti-reflection film that corresponds to the refractive index of the resin to be filled is provided on the end face of the input/output optical waveguide of the InP chip. Along with such an optical device, the discloser of this embodiment separately prepared a wiring board that does not have an alignment through hole, and confirmed the difference in the complexity of alignment depending on whether or not there is an alignment through hole.

Figure 8 shows the output light intensity profile in the Y-axis direction obtained after observing the input/output optical waveguide using the alignment through-hole and performing pre-alignment according to the above procedure, and the output light intensity profile in the Y-axis direction obtained after observing the alignment marker through the alignment through-hole and performing pre-alignment. The profile shown by the solid line in the figure shows the result of alignment performed while visually observing the butt joint of the input/output optical waveguide through the alignment through-hole. The profile shown by the dashed line shows the result of alignment performed while visually observing the alignment marker through the alignment through-hole. Note that the two profiles were obtained by first performing alignment using the butt joint, then releasing the chip once, and then using the alignment marker. The horizontal axis of Figure 8 shows the scan position on the Y-axis, and the vertical axis shows the light intensity obtained at the scan position.

According to Figure 8, although the initial position in the Y-axis direction is off from the peak, the pre-alignment has almost achieved alignment in the X-axis direction and Z-axis rotation direction, so the peak position of the output light intensity can be confirmed by simply acquiring the Y-axis profile once. According to this embodiment, further adjustments to the alignment and Z-axis rotation can then be made to quickly complete the alignment.

Figure 9 shows the light intensity profile for comparison with this embodiment. The horizontal axis of Figure 9 indicates the Y-axis scan position, and the vertical axis indicates the light intensity obtained at the scan position. When no alignment through-hole is used, the SiP chip and InP chip are observed from the back side and placed in their approximate positions. After this, signal light is input from the SiP chip, and the Y-axis profile is obtained while moving away from the initial position by ±2 μm on the X-axis to search for the peak position. Figure 9 shows the Y-axis profile for each movement amount from the initial X-axis position. The profile shown by the solid line in Figure 9 is 0 μm from the X-axis position, the profile shown by the dashed line is -16 μm, the profile shown by the dashed line is -18 μm, and the profile shown by the two-dot chain line is -20 μm.

As shown in Figure 9, no peak was observed when the Y axis was at 0 μm, but the peak was first confirmed when the X axis was moved repeatedly to a position -16 μm away from the initial position, and the maximum peak was obtained at -18 μm, allowing the transition to alignment. In this way, without an alignment through-hole, the light intensity profile must be acquired multiple times while moving the X axis before the peak can be confirmed, making the process complicated. In contrast, when an alignment through-hole is used, as shown in Figure 8, the alignment position on the X axis can be adjusted to the approximate position in advance, the peak can be confirmed with a single profile acquisition, and alignment can be performed with a simple procedure.

As described above, the optical functional element flip-chip mounted on the wiring board provided with the alignment through-holes of this embodiment allows the optical waveguide and markers to be observed from the wiring board side through the alignment through-holes when butt-coupled with the optical circuit element. Therefore, even when the markers cannot be confirmed from the back side due to flip-chip mounting on the wiring board, pre-alignment is possible, making it possible to provide an optical device that allows easy butt-coupling optical coupling.

10, 30: Substrate 11, 51, 61: Optical functional element 12, 52, 62: Optical circuit element 20, 55: Driver IC
Reference Signs List 21, 56 Wires 30, 53, 63 Wiring board 31 Wiring 32 Metal bump 65 Alignment markers 111, 121 Optical waveguides 112, 113 Mach-Zehnder interferometer 122 Polarization rotor 123 Polarization beam combiner 511, 512, 521 Input/output optical waveguides 531 Alignment through holes 611a, 611b, 621b, 621d Butt optical waveguide 613 Connection pads 621a Input optical waveguide 621c Output optical waveguides 615, 625 Marker 632 Alignment through holes 633 Connection pads 635 Thin-film glass substrate

Claims (6)

A wiring board;
an optical functional element having an input/output optical waveguide for an optical signal and being flip-chip mounted on the wiring substrate;
an end face of the input/output optical waveguide in the optical functional element is butt-coupled to an end face of another input/output optical waveguide;
the wiring board includes an alignment through hole penetrating the wiring board,
whether the position of the end face of the input/output optical waveguide coincides with the position of the end face of the other input/output optical waveguide can be visually confirmed through the alignment through hole;
Optical devices. The optical device according to claim 1, wherein the coupling portion between the end face of the input/output optical waveguide and the end face of the other input/output optical waveguide is visible through the alignment through hole. The optical device according to claim 1, in which an alignment marker indicating the end face of the input/output optical waveguide and an alignment marker indicating the end face of the other input/output optical waveguide are visible through the alignment through hole. The optical device according to claim 3, wherein the alignment marker is provided on an element on which the input/output optical waveguide is formed and on an element on which the other input/output optical waveguide is formed, and is composed of two markers that are linearly symmetrical with respect to an axis perpendicular to the opposing direction between the input/output waveguide and the other input/output optical waveguide and have the same shape. The optical device of claim 1, wherein the alignment through-hole is covered with a material that transmits visible light. 1. An alignment method for an optical device comprising: a wiring board; and an optical functional element having input and output optical waveguides for optical signals and flip-chip mounted on the wiring board, an end face of the input and output optical waveguide in the optical functional element is butt-coupled to an end face of another input and output optical waveguide, the wiring board has an alignment through hole penetrating the wiring board, and a match or mismatch between a position of the end face of the input and output optical waveguide and a position of the end face of the other input and output optical waveguide can be visually confirmed via the alignment through hole,
adjusting the X-axis and Y-axis of the input/output optical waveguide and the other input/output optical waveguide while visually checking the end face of the input/output optical waveguide and the end face of the other input/output optical waveguide through the alignment through hole, and then acquiring a light intensity profile transmitted through the input/output optical waveguide and the other input/output optical waveguide, and aligning the peak of the light intensity profile in the Y-axis direction, thereby performing pre-alignment.
A method for aligning optical devices.

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