High-speed optical transmitter and receiver

Abstract

An optical transmission / reception device 6 comprises: pads (613, 614) formed on the surface of a DSP package substrate (610); an optical modulation module (625); a package (630) for housing the optical modulation module (625); and pads (623, 624) formed on the surface of the package (630). The optical transmission / reception device 6 is configured such that the DSP package substrate (610) and the package (630) are directly connected to each other by means of the pads (613, 614) and the pads (623, 624).

Description

[Technical Field]

[0001] The present disclosure relates to high-speed optical transceivers. [Background technology]

[0002] Digital signal processing technologies, including digital coherence, have been introduced into optical fiber communication systems, establishing backbone network transmission technology of 100 Gbps per wavelength, and currently reaching practical speeds of 400 to 600 Gbps per wavelength.

[0003] FIG. 1(a) is a top view illustrating a known 100G digital coherent system, and FIG. 1(b) is a cross-sectional view taken along the arrows Ib and Ib in FIG. 1(a). The cross-sectional views in FIG. 1(b), 2(b), 3(b), 4, and 5 are intended to illustrate the layout of components, and therefore do not show the internal configuration of the components. Each component (an integrated circuit (IC), a photo integrated circuit (IC)) shown in FIG. 1(a) and FIG. 1(b) is individually packaged, and is mounted, for example, on a printed circuit board (PCB) 100. FIG. 1(a) and FIG. 1(b) show an example of a known 100G digital coherent system. In a known 100G digital coherent system, a DSP (Digital Signal Processing) package substrate 110 is mounted on a PCB board substrate 100, and the DSP package substrate 110 is electrically connected to the PCB board substrate 100 by a BGA (Ball Grid Array) 101. A DSP-ASIC (Application Specific Integrated Circuit) 111 chip is mounted on the DSP package substrate 110.

[0004] The electrical input and output of the DSP package substrate 110 is connected to a driver / TIA 130 via surface-mounted lead pins 102 by printed wiring on the PCB board substrate 100, and is then connected to an optical modulation module / optical receiving module (hereinafter also referred to as an optical modulation (optical receiving) module) 120. If 120 is an optical modulation module, 130 corresponds to a driver, and if 120 is an optical receiving module, 130 corresponds to a TIA. The optical modulation (optical receiving) module 120 receives a modulated electrical signal, performs optical modulation, and outputs modulated light to an optical fiber 140. It also converts the signal light received from the optical fiber 140 into an electrical signal and sends it to the DSP package substrate 110, where the DSP-ASIC 111 processes the received signal.

[0005] Systems exceeding 400G require broadband analog components (e.g., modulation bandwidths of 40 GHz or greater), necessitating further reductions in high-frequency loss and miniaturization. Figures 2(a) and 2(b) show a known 400G digital coherent system designed to meet these requirements. Figure 2(a) is a top view, and Figure 2(b) is a cross-sectional view along the arrows IIb and IIb in Figure 2(a). The 400G digital coherent system shown in Figures 2(a) and 2(b) is configured by mounting a DSP package substrate 210 equipped with a DSP-ASIC 211 and an integrated optical modulation (optical receiver) module 225, in which a driver / TIA 130 and an optical modulation (optical receiver) module 120 are integrated, on a PCB board substrate 200. Reference numeral 240 denotes an optical fiber, which transmits and receives light. In this way, the configuration in which the RF driver and optical modulator are integrated into a single package on the transmitting side (Coherent Driver Modulator: CDM) and the configuration in which the transimpedance amplifier TIA and optical receiver PD are integrated into a single package on the receiving side (Integrated Coherent Receiver: ICR) are collectively referred to as the CDM configuration.

[0006] 3(a) and 3(b) show a known 400G digital coherent system for suppressing degradation of high-frequency characteristics due to package mounting, where 3(a) is a top view and 3(b) is a cross-sectional view taken along the arrows IIIb and IIIb in 3(a). The 400G digital coherent system shown in 3(a) and 3(b) includes a DSP package substrate 310 on a PCB board 300, and all high-frequency analog ICs (DSP-ASIC 311, driver / TIA 330, and integrally mounted optical modulation (optical receiver) module 325) are mounted on the DSP package substrate 310 (DSP co-package mounting). An optical fiber 340 is connected to the integrally mounted optical modulation (optical receiver) module 325. In addition, in such a configuration, since the DSP-ASIC 311, which generates heat in the watt range, and the optical transceiver device are placed in close proximity on the same DSP package substrate 310, it is preferable to select an optical transceiver device that has small characteristic fluctuations (small temperature dependency) in response to temperature changes and increases.

[0007] Fig. 4 is a longitudinal cross-sectional view showing a digital coherent system using a low-loss FPC (Flexible Printed Circuits) as a high-frequency interface of an optical module in a known CDM-implemented system. The digital coherent system shown in Fig. 4 has a DSP package substrate 410 connected to a PCB board substrate 400 via a BGA 401, and a DSP-ASIC 411 mounted on the DSP package substrate 410. The DSP package substrate 410 is connected to an integrally mounted optical modulation (light receiving) module 425 via an FPC 450. Input light and output light of the integrally mounted optical modulation (light receiving) module are transmitted through an optical fiber 440.

[0008] Furthermore, semiconductor-based optical modulators have attracted attention as optical transceiver device materials, replacing conventional lithium niobate (LN) optical modulators in terms of miniaturization and cost reduction. Compound semiconductors, such as InP, are primarily used for higher-speed modulation. For systems where miniaturization and cost reduction are important, research and development of Si-based optical devices is underway. Semiconductor optical modulators also have material-specific advantages and disadvantages. For example, InP optical modulators require temperature control during modulation to control the band-edge absorption effect. On the other hand, while Si modulators have the advantage of not requiring temperature control, their smaller electro-optic effect compared to other materials necessitates a longer electro-optic interaction length, which can result in increased high-frequency loss. Therefore, further increases in speed (broader bandwidth) are a significant challenge.

[0009] To further increase the speed of the conventional digital coherent systems shown in Figures 1(a) to 3(b), it is important not only to increase the speed of ICs (e.g., Si-CMOS) and PICs (e.g., circuits including optical modulators, photodetectors, etc.), but also to increase the speed of the packages and high-frequency wiring (reducing RF loss) and reduce the loss (reducing reflection) of the electrical connections between each component. From this perspective, the multi-chip copackaging configuration shown in Figures 2(a) to 3(b) is more advantageous for achieving high speeds than the configurations shown in Figures 1(a) and 1(b). For these reasons, a highly integrated DSP copackaging configuration is being considered for Si-based optical modulators, which have low temperature dependency. On the other hand, for InP-based optical modulators, which have high temperature dependency, a configuration in which only the high-frequency amplifier (driver IC) is packaged separately from the DSP, which generates a large amount of heat, is often adopted (e.g., CDM). The optical modulators are generally mounted on a thermoelectric cooler (TEC) to maintain a constant temperature. A technology for suppressing the deterioration of transmission characteristics due to high frequency loss in an internal high frequency line of a digital coherent optical receiving device is described in, for example, Patent Document 1. Patent Document 2 describes a package substrate and lightA high-speed optical transceiver that connects a module with a flexible substrate and transmits and receives light at high speed is described. Non-Patent Document 1 discloses a wideband CDM that operates at rates of 64 GBd, 96 GBd, 128 Gbd or more. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-146515 [Patent Document 2] WO2021 / 171599A1 [Non-patent literature]

[0011] [Non-Patent Document 1] Richard JRB Ward and two others, “Implementation Agreement for High Bandwidth Coherent Driver Modulator (HB-CDM)” [online], July 15, 2021. [Retrieved June 24, 2022], Internet: https: / / www.oiforum.com / wp-content / uploads / OIF-HB-CDM-02.0.pdf Summary of the Invention

[0012] Known mounting forms of semiconductor optical modulators can be broadly divided into the CDM form shown in Figure 2 (also called an ICR on the receiver side, or an IC-TROSA (Integrated Coherent Transmitter and Receiver Optical Sub-Assembly) when the transmitter and receiver are integrated into a package) and the DSP co-package form shown in Figure 3. To further increase the overall speed of the optical transmitter (receiver), it is necessary to increase the speed of each IC and PIC, as well as the wiring connecting them and the package mounting, all of which must be made faster (wider bandwidth). However, each of the two known mounting forms mentioned above has the following issues that hinder wideband operation.

[0013] (Issues with CDM format) For example, a high-speed analog electrical signal output from a digital-to-analog converter (DAC) in a DSP-ASIC is transmitted from the ASIC to the DSP package substrate, then to the PCB board, and finally to the optical modulation module, where it is converted into an optical signal. For electrical interfaces, for example, surface mount SMT (Surface Mount Technology), FPC (Flexible Printed Circuits), and flexible printed wiring boards are used. In this case, the electrical signal must be transmitted across multiple, different high-frequency circuit boards, and the longer the electrical wiring, the greater the electrical loss.

[0014] Furthermore, solder balls with diameters of 100 to several hundred microns are used for connections between boards, especially in BGA (ball grid array) connections between DSP package boards and PCB boards. When the propagating electrical signals reach high frequencies above 50 GHz, electrical reflections due to impedance mismatch at the solder ball connections significantly degrade the high-frequency characteristics. While this degradation of high-frequency characteristics was not a major issue in conventional 400G systems (64 GBaud rate, required bandwidth of approximately 40 GHz), it poses a significant barrier to the realization of next-generation 800G and 1T systems (required bandwidths >50 GHz). Therefore, even when using optical modulation modules equipped with InP modulation elements with a modulation bandwidth of 50 GHz or more, it has been difficult to ensure the overall bandwidth characteristics of the optical transmitter (receiver).

[0015] 4, in a known CDM mounting system, in an example where a low-loss FPC 450 is used as a high-frequency interface for an optical module, the FPC is connected from optical module terraces at different heights to the PCB board substrate 400. With this configuration, the low-loss FPC 450 needs to be strongly bent for mounting, which raises concerns about fluctuations in high-frequency characteristics (changes in characteristic impedance) due to bending and increased electrical loss due to longer wiring.

[0016] FIG. 5 is a vertical cross-sectional view of a digital coherent system in which a DSP package substrate 410 and an integrally mounted optical modulation module 425 are directly and flatly connected using an FPC 550 in order to solve the above problem.

[0017] (Issues with DSP co-packaging) A widely known method for solving the above problems is the DSP co-package mounting configuration shown in Figure 3. As shown in Figure 3, this mounting configuration mounts not only a DSP-ASIC 311 but also a driver (TIA) 330 and an optical modulator (optical receiver) PIC325 on a DSP package substrate 310, and supplies high-frequency electrical signals to the optical modulator via the shortest possible wiring without passing through solder balls or the like. However, as currently used optical modulators are mainly Si-based modulators with little temperature dependency, as mentioned above, improving the characteristics of the optical modulation element itself is a major challenge for further speed increases (wider bandwidth).

[0018] In general, there is a trade-off between the bandwidth and modulation efficiency (corresponding to the driving voltage Vπ, modulated output optical intensity, etc.) of an optical modulator. Therefore, simply prioritizing bandwidth expansion in a design can actually lead to a degradation of the SNR (signal-to-noise ratio) of the modulated light, resulting in a degradation of signal quality. Furthermore, if a compound semiconductor optical amplifier such as an SOA is installed in addition to the Si modulator to compensate for the degradation of SNR, issues arise, such as temperature control of the amplifier itself and increased costs and power consumption due to the increased number of mounted components. Furthermore, when an InP modulator is co-packaged with a DSP instead of a Si modulator, it is necessary to change the composition of the InP modulator core (to reduce the band-edge absorption of the material). However, this reduces the modulation efficiency of the InP modulator itself (decreasing the quantum confined Stark effect: QCSE), which leads to a degradation of SNR.

[0019] The present disclosure has been made in view of the above points, and relates to a high-speed optical transceiver that shortens the length of wiring connecting a digital signal processing circuit and a module including an optical element, and that is fast and has little signal loss.

[0020] In order to achieve the above object, a high-speed optical transceiver according to one embodiment of the present disclosure is a high-speed optical transceiver including: a digital signal processing circuit; a first electrode formed on a first package substrate of the digital signal processing circuit; an optical element; a second package accommodating the optical element; and a second electrode formed on a surface of the second package, wherein the first package substrate and the second package are directly connected by the first electrode and the second electrode. The first package substrate and the second package are supported by a same support substrate, and the heat dissipation surfaces of the first package substrate and the second package are arranged on the opposite side of the support substrate. .

[0021] According to the above-described embodiment, electrodes are formed directly on the package of the digital signal processing circuit and the package of the module including the optical element, and by connecting them directly, the length of the signal wiring connecting the two can be minimized, thereby enabling faster signals and reduced loss. [Brief explanation of the drawings]

[0022] [Figure 1]FIG. 1(a) is a top view illustrating a known 100G digital coherent system, and FIG. 1(b) is a cross-sectional view taken along the arrows Ib, Ib in FIG. 1(a). [Figure 2] FIG. 2(a) is a top view illustrating a known 400G digital coherent system, and FIG. 2(b) is a cross-sectional view taken along the arrows IIb and IIb in FIG. 2(a). [Figure 3] FIG. 3(a) is a top view illustrating another known 400G digital coherent system, and FIG. 3(b) is a cross-sectional view taken along the arrows IIIb and IIIb in FIG. 3(a). [Figure 4] FIG. 1 is a longitudinal cross-sectional view showing a digital coherent system using a low-loss FPC as a high-frequency interface of an optical module. [Figure 5] FIG. 1 is a vertical cross-sectional view showing a digital coherent system in which a DSP package substrate and an integrally mounted optical modulation module are directly and flatly connected using an FPC. [Figure 6] 1 is a longitudinal cross-sectional view illustrating an optical transceiver according to an embodiment of the present disclosure. [Figure 7] FIG. 7 is a diagram for explaining the height or thickness of each part of the configuration shown in FIG. 6. [Figure 8] Figure 8(a) is a plan view illustrating the pads formed on the DSP package substrate, Figure 8(b) is a plan view illustrating the pads formed on the optical modulation module, Figure 8(c) is an enlarged view of the pads shown in Figures 8(a) and 8(b), and Figure 8(d) is a diagram illustrating the heating pads formed on the back side of the surface shown in Figure 8(b). [Figure 9] FIG. 8 is a vertical cross-sectional view of the optical modulation module shown in FIGS. 6 and 7. [Figure 10] 10 is a diagram showing a state in which the optical modulation module shown in FIG. 9 is mounted on a DSP package substrate. DETAILED DESCRIPTION OF THE INVENTION

[0023] An embodiment of the present disclosure will be described below with reference to the drawings. The drawings used in this embodiment are intended to explain the configuration of the present disclosure, the components included in the configuration, the positional relationships between the components, the functions, effects, and technical concepts. Therefore, the drawings do not limit the specific shape of the present disclosure, and the drawings do not necessarily accurately depict the aspect ratio or thickness of the configuration of the present disclosure. In particular, cross-sectional views omit illustration of the internal configuration, with some exceptions.

[0024] [Connection between digital signal processing circuits and modules containing optical elements] FIG. 6 shows an optical transceiver 6 configured by connecting a DSP package substrate 610 and an optical modulation module 625. The optical transceiver 6 is a high-speed optical transceiver of this embodiment. FIG. 6 is a longitudinal cross-sectional view for explaining the optical transceiver 6 of this embodiment. The optical transceiver 6 includes a PCB board 600, a DSP package substrate 610, a DSP-ASIC 611, and an integrally mounted optical modulation module 625 (hereinafter simply referred to as an "optical modulation module"). The DSP package substrate 610 is mounted on the PCB board 600, and the DSP-ASIC 611 is mounted on the DSP package substrate 610. The length in the stacking direction from the top surface of the PCB board 600 to each component of the optical transceiver 6 is referred to as the "height." Here, the stacking direction refers to the direction in which the DSP package substrate 610 is placed (stacked) on the PCB board 600. The length in the stacking direction of each component of the optical transceiver 6 is referred to as the "thickness."

[0025] The optical transceiver 6 includes a DSP package substrate 610 including a digital signal processing circuit. Pads 613 and 614 (FIG. 8(a)) that are first electrodes are formed on the DSP package substrate 610. The optical modulation module 625 also includes an optical element and a package 630 (second package) that houses the optical element. As will be described later, this embodiment illustrates an example in which the optical modulation module 625 includes an optical modulator PIC (FIG. 9) that is an optical modulation element. However, this embodiment is not limited to this example, and the module may be an optical reception module including a light receiving element, or an optical transmission and reception module that includes both an optical modulation element and a light receiving element.

[0026] In this specification, the term "module" refers to a set of multiple elements integrated to perform a specific function, and includes both the elements constituting the set and the elements housed in a package. A module may include elements other than optical elements. As will be described later, the optical modulation module 625 houses an optical modulator PIC 727, gold wire wiring 751, high-frequency wiring 753, a TEC 760, a module wiring substrate base 770, an optical element base 780, a chip focusing lens 781, a fiber focusing lens 782, and a high-frequency amplifier IC (driver IC) 730 in a package 630 (FIG. 9).

[0027] Package 630 represents a high-frequency ceramic package used in general optical modules. Package 630 mainly includes an RF terrace portion 630b, a fiber pipe portion 630c, and a package body 630a. Package body 630a mainly houses the above-mentioned components as a unit. RF terrace portion 630b extends toward DSP package substrate 610. RF terrace 630b is made of ceramic, and has pads 623 and 624 (FIG. 8(b)) serving as second electrodes on the surface of lower surface 630bb. DSP package substrate 610 and package 630 of optical modulation module 625 are connected by directly connecting pads 613 and 614 and pads 623 and 624.

[0028] A package is a structure that includes a case portion that seals and protects electronic circuits and elements, and terminals and pads for electrically connecting the sealed circuits and elements to the outside. However, the term "package" in this specification mainly refers to the case portion.

[0029] The integrally mounted optical modulation module 625 is an optical modulation module in which a driver IC 730 (FIG. 9) described later and the optical modulation module are integrally mounted. The fiber pipe section 630c indicates the pipe section of the package 630 from which the fiber 640 extends. The ceramic RF terrace section 630b has connection pads formed thereon and is used for RF connection with the DSP package substrate 610.

[0030] FIG. 7 is a diagram illustrating the height or thickness of each component of the configuration shown in FIG. 6 , showing a state in which the DSP package substrate 610 and the optical modulation module 625 are not yet connected. In FIG. 7 , the height of the upper surface 610a of the DSP package substrate 610 is h1, the height of the lower surface 630bb of the RF terrace portion 630b is h2, the height to the bottom surface of the package 630 is h4, the thickness of the underfill material 629 filling the gap between the bottom surface of the package and the lower surface 630bb is h3, and the thickness of the RF terrace portion 630b is h5. The optical transceiver 6 has pads on both the DSP package substrate 610 and the optical modulation module 625, and by directly connecting these pads, wiring is minimized, thereby achieving high-speed operation. Because of this configuration, in this embodiment, it is preferable that the difference in height between the height h2 of the lower surface 630bb of the RF terrace portion and the height h1 of the upper surface 610a of the DSP package substrate 610 is zero or as small as possible.

[0031] 6 shows a state in which the height of the RF terrace lower surface 630bb is equal to the height of the upper surface 610a of the DSP package substrate 610. However, it is known that a difference may occur between the heights h1 and h2 when manufacturing the optical transceiver 6 and manufacturing tolerances of the optical transceiver 6 are taken into consideration. As shown in FIG. 7, when the thickness of the main body 625a is smaller than the height h1 of the DSP package substrate 610, the allowable difference in height between the thickness of the main body 625a and the height h1 during mounting is 500 μm or less. This difference in height is a value that takes into consideration the stability of the connection with the optical modulation module 625 and actual variations.

[0032] If the height difference is 500 μm or less, an underfill agent (conductive adhesive) 629 can be filled between the bottom surface of the package 630 and the PCB board substrate 600 to fill and fix the gap between the optical modulation module 625 and the PCB board substrate 600, thereby preventing the optical modulation module 625 from floating and ensuring the long-term reliability of the connection.

[0033] Furthermore, when the mounting process is taken into consideration, the DSP package substrate 610 is mounted on the PCB board 600 before the optical modulation module 625. Therefore, if the height of 625bb becomes higher than the upper surface 610a of the DSP package substrate 610 during mounting, a difference in height will occur between the DSP package substrate 610 and the optical modulation module 625 at that point. If 625bb becomes higher than the upper surface 610a of the DSP package substrate 610 at this time, it will be impossible to connect the DSP package substrate 610 and the optical modulation module 625. For this reason, the height h3 of the lower surface 625bb needs to be equal to or less than the height h1 of the upper surface 610a of the DSP package substrate 610.

[0034] In this embodiment, the DSP-ASIC 611 and the optical modulation module 625 have heat dissipation surfaces, and both have the heat dissipation surface as the upper surface. In this embodiment, the "heat dissipation surface" or "heat dissipation side" does not refer to all surfaces or sides where heat dissipation occurs, but rather refers to the surface or side where heat dissipation occurs primarily among the surfaces or sides where heat dissipation occurs. The surface or side where heat dissipation primarily occurs may be, for example, the surface or side where heat is radiated by a heat dissipation mechanism. Examples of the heat dissipation mechanism include a Peltier element and a heat sink. In this embodiment, the heat dissipation surface can be located on the lower side. However, in this case, a heat dissipation mechanism would need to be provided on the PCB board substrate 600. This is undesirable because it increases the number of components or processes required for the optical transceiver. Furthermore, since the heat dissipation surface of the DSP package substrate 610 is located on the upper side, if the heat dissipation surface of the optical modulation module 625 were located on the lower side, the optical transceiver 6 would have heat dissipation surfaces on both the top and bottom. In this embodiment, it is desirable to form the heat dissipation surface of the optical modulation module 625 on the upper side, and to unify the entire optical transceiver 6 so that the heat dissipation surface is formed on the upper side.

[0035] [Electrode connection] Next, electrodes formed on the upper surface 610a of the DSP package substrate 610 and the lower surface 630bb of the package 630 will be described. FIGS. 8(a), 8(b), and 8(c) are diagrams for explaining such electrodes, with FIG. 8(a) showing the upper surface 610a and FIG. 8(b) showing the lower surface 630bb. That is, FIG. 8(a) is a plan view of the DSP package substrate 610 seen from above, and FIG. 8(b) is a plan view of the package 630 of the optical modulation module 625 seen from the lower surface 630bb side (from below). FIG. 8(c) is an enlarged view of the upper surface of the signal pad shown in FIG. 8(b). FIG. 8(d) is a view showing the upper surface 630bd, which is the reverse side of the lower surface 630bb shown in FIG. 8(b).

[0036] 8(a) and 8(b), the upper surface 610a and the lower surface 625bb facing the upper surface 610a of the DSP package substrate 610 are both provided with two types of electrodes (pads) of different sizes. Of the pads formed on the upper surface 610a, the larger pad 613 functions as a GND pad, and the smaller pad 614 functions as a signal pad. Similarly, of the pads formed on the lower surface 625bb, the larger pad 623 functions as a GND pad, and the smaller pad 624 functions as a signal pad.

[0037] The pads 613 and 623, and the pads 614 and 624 are arranged so as to overlap each other when the upper surface 610a and the lower surface 630bb are overlapped. In this embodiment, the upper surface 610a corresponds to the formation surface of the pads 613 and 614, and the lower surface 630bb corresponds to the formation surface of the pads 623 and 624. The formation surfaces are the surfaces of the package 630.

[0038] The examples shown in Figures 8(a) and 8(b) show a GSSG configuration with a differential line configuration. However, this embodiment is not limited to such a configuration and may also use a GSGSG configuration. Furthermore, the number of pads shown in Figures 8(a) and 8(b) is just an example, and the number of pads can be any number depending on the number of channels required.

[0039] Fig. 8(c) is an example of a detailed diagram of the pad 624. The pad 624 includes a signal pad 628 shown as a rectangle, a land 626 formed in the signal pad 628, and a through-hole 627 formed in the land 626. Note that Fig. 8(c) only shows the pad 624 that serves as a signal pad, but the pad 623 that serves as a GND pad is configured in the same manner as the pad 624. For this reason, in this embodiment, illustration and description of the configuration related to the through-hole of the pad 623, etc. will be omitted.

[0040] Forming the through-holes 627 allows for the application of heat to melt the solder when connecting to the DSP package substrate 610. Here, the through-holes 627 are used as an example, but from the perspective of heat conduction, they do not necessarily need to be hollow and can be buried vias. However, buried vias do not allow the solder to flow as described below. While only one through-hole 627 is shown as an example, multiple through-holes or half-through-holes can also be used. Similarly, at least one through-hole 627 must be formed on the pad 623 (GND PAD) side to apply heat. As described above, the pads 623 and 624 are configured to be heated via the through-holes 627. However, to facilitate heating, it is effective not only to increase the number of through-holes 627 but also to provide heating pads with a width equal to or smaller than the width of the signal pad 628 on the bottom surface 630bb of the RF terrace portion 630b and the top surface 630bd opposite the bottom surface 630bb. However, since this increases the capacitance, it is desirable that the size of the heating pad (mainly in the width direction) be smaller than the size of the connection pad.

[0041] FIG. 8(d) shows a specific example of heating pads 663 and 664. The heating pads 663 and 664 are formed on the upper surface 630bd. The pad (signal pad) 624 formed on the lower surface 630bb is connected to the heating pad 664 on the upper surface 630bd through the through-hole 627 described above. The heating pad 664 has a narrower width (w1) than the pad 624 on the lower surface 630bb. Similarly, the pad (GND pad) 623 formed on the lower surface 630bb is connected to the heating pad 663 formed on the upper surface 630bd through the through-hole 627. In FIGS. 8(b) and 8(d), the widths of the pad 623 (GND pad) and the heating pad 663 are shown to be the same, but this is not necessarily the case. Furthermore, heating pads 663 and 664 on top surface 630bd are connected to high-frequency wiring 753, and transmit high-frequency signals to the inside of package 630. A cross-sectional image taken along arrows IX and IX in FIG. 8(b) is shown in FIG.

[0042] Figure 9 is a longitudinal cross-sectional view of the optical modulation module 625 of this embodiment taken along the arrows IX and IX in Figure 8(b). The longitudinal cross-sectional view shown in Figure 9 includes the pad 624 and the fiber pipe portion 630c. The package 630 includes a temperature regulator (TEC) 760 and a subcarrier (optical element base) 780 disposed on the TEC 760. An optical modulator PIC727, a chip condenser lens 781, and a fiber condenser lens 782 are disposed on the optical element base 780, and modulated light is output to the optical fiber 740. A driver IC 730 is disposed between the module wiring board base 770 and the optical modulator PIC727.

[0043] To achieve ultra-high speed operation of over 100 GBd, it is preferable that the width W1 of the signal pad 628 and the diameter of the land 626 are narrower. This is because if the width W1 and the diameter of the land 626 are wide, the capacitance of the signal pad 628 increases, causing degradation of high-frequency characteristics. Furthermore, the signal pads 614 and 624 are very small in size to improve high-frequency characteristics. Therefore, in order to ensure connection strength, it is desirable that the GND pads 613 and 623 be at least twice the width of the pads 614 and 624. Making the pads 613 and 623 at least twice the size of the pads 614 and 624 is extremely effective not only in terms of connection strength but also in terms of crosstalk.

[0044] Specifically, the width W1 must be at least 200 μm or less. However, if the widths W1 of the signal pads 628 on both the DSP package substrate 610 and the optical modulation module 625 are both very small, such as 100 μm or less, there is a risk that the signal pads may not be properly connected due to manufacturing tolerances or misalignment during assembly. Therefore, it is conceivable to reduce the size of the signal pad 628 only on the optical modulation module 625 side, which often uses a material with a higher dielectric constant, by making the width of the signal pad 628 on the DSP package substrate 610 side 100 μm and the size of the signal pad 628 on the DSP package substrate 610 side 200 μm, thereby ensuring mountability and further widening the signal bandwidth. As an example, although the signal pad 628 on the optical modulation module 625 side is small, the pad 614 (signal pad) on the DSP package substrate 601 side may also be small. However, in terms of effectiveness, due to the material constants and layer structure of commonly used packages, it is possible to achieve a greater reduction in the capacitance of the area where the pads connect by making the signal pad 628 on the optical modulation module 625 smaller.

[0045] FIG. 10 shows the optical modulation module 6 shown in FIG. 2510 shows a state in which the optical modulation module 625 is connected to a DSP package substrate 610. The purpose of Fig. 10 is to explain the connection to the DSP package substrate 610, and the scale and aspect ratio of the optical modulation module 625 do not necessarily match those of Fig. 9. Furthermore, the optical modulation module 625 in Fig. 10 shows the main components for explaining the connection, and some parts are not shown.

[0046] As shown in Figure 10, the optical modulation module 625 shown in Figure 9 is mounted upside down. That is, the optical modulation module is mounted upside down from the state shown in Figure 9, and is heated simultaneously from the optical modulation module side by a hot bar. Mounting it in this inverted state allows the heat dissipation surface of the modulation module to be on the upper side. This makes it possible to unify the heat dissipation surfaces of the DSP package substrate 610 and the modulator, facing upward. The DSP package substrate 610 and the optical modulation module 625 are connected via their respective connection pads for high-frequency communication. Specifically, a high-frequency signal is transmitted from the DSP package substrate 610 via pads 613 and 614 to pads 623 and 624 of the optical modulation module 625, and then from the pads 623 and 624 via the through-hole 627, through heating pads 663 and 664, and then transmitted to the inside of the package 630 via high-frequency wiring 753, where the high-frequency signal propagates to the driver IC and optical modulator PIC.

[0047] The optical modulator PIC727 uses an InP-based IQ optical modulation element with excellent broadband performance. The optical modulator PIC727 uses an InP substrate and includes at least two Mach-Zehnder type optical interference waveguides.

[0048] On the input side of the optical modulator PIC727, a module wiring board base 770 and a module package wall 771 are arranged as the left wall of the package of the optical modulation module 625. The module wiring board base 770 and the module package wall 771 are made of, for example, ceramics of different thicknesses. High-frequency wiring 753 on the upper surface of the module wiring board base 770 passes between the module wiring board base 770 and the module package wall 771 and inputs a modulated electrical signal to the optical modulator PIC727 via the gold wire wiring 751. Note that for long-term stabilization of the optical lens, an inert gas such as Ar or N2 may be sealed inside the package 630 and hermetically sealed.

[0049] (Electrode connection method) Next, we will explain how to connect the pads, which are electrodes in this embodiment. Considering the small pad size and the need for multi-channel integration and module miniaturization, the pad spacing is typically very narrow, on the order of a few hundred microns. Connecting pads using conductive paste or UV-curable resin is difficult due to the high risk of short circuits caused by overflowing paste. Therefore, it is desirable to use solder, which has the characteristic of wetting and spreading only on metal surfaces. Furthermore, since there is a risk of short circuits when the pitch is narrow, it is possible to combine solder with solder resist. Using solder and solder resist in combination can suppress the solder's wetting and spreading, reducing the risk of short circuits. In this case, the solder resist is applied to cover the periphery of the pads to be connected.

[0050] The solder resist can be used on either the DSP package substrate 610 or the optical modulation module 625; it is not necessarily required to use it on the pads on both sides. However, using it on both sides is naturally more effective in terms of reducing the risk of short circuits. When using solder resist on either the DSP package substrate 610 or the optical modulation module 625, providing it on the DSP package substrate 610 is more versatile and preferable. This is because providing a solder resist on the ceramic package side is not common in the manufacturing process. Because solder is fixed by heating, the configuration of the optical modulation module 625 must take heating into consideration. In particular, considering the heat resistance of adhesives used to fix optical components and various internal components in the optical modulation module 625, the temperature inside the optical modulation module 625 must be kept below 150°C during solder heating. For this reason, the solder material used in this embodiment must be a low-melting-point solder with a melting point of 150°C or less. An example of a solder with a melting point of 150°C or less is Sn-Bi solder.

[0051] Next, the process of fixing the pads by soldering will be described. In the manufacturing process of the optical transceiver 6, the DSP package substrate 610 is mounted on the PCB board 600 before the optical modulation module 625. Therefore, the optical modulation module 625 is fixed to the PCB board 600 from above the DSP package substrate 610. For heating to connect the pads 613, 623 and the pads 614, 624, it is desirable to heat them all at once using, for example, a hot bar tool.

[0052] Heating using a hot bar tool cannot directly heat the top surface 610a or bottom surface 625bb; instead, heating is performed via heating pads or through-holes formed on the surface of the RF terrace portion 625b of the package 630 opposite 625bb. Before connection, solder is applied to at least one of the DSP package substrate and the optical modulation module package, and then both are heated simultaneously from the optical modulation module side using a hot bar. Considering the flow of solder and ease of heating, the diameter of the through-hole 627 is preferably φ100 μm or greater. Furthermore, because the through-hole is open, it is not only useful for heating, but also for pouring additional solder. In this respect, a diameter of φ100 μm or greater is an extremely effective size.

[0053] If the thickness h5 of the ceramic layer of the RF terrace portion 625b on which the pads on the package 630 side are formed is thicker than the appropriate range, it will not be possible to supply enough heat to melt the solder, and excessive heating will be required, which will result in heating the entire package 630. For this reason, it is preferable that the thickness h5 of the ceramic forming the pads be 1 mm or less. Note that the "thickness" referred to here refers to the overall thickness, regardless of whether the package 630 is single-layered or multi-layered.

[0054] Furthermore, to achieve higher speed operation, the optical transceiver of this embodiment may include not only a modulation element but also a driver IC in the same package. This configuration is also called CDM. Furthermore, for the receiver module, it is desirable to integrate a transimpedance amplifier together with the photodetector in order to achieve higher speed. This configuration is also called ICR.

[0055] Furthermore, considering high speed, it is desirable to use an InP-based optical modulator element for the optical modulation module. However, InP-based optical modulator elements require temperature control for stable operation. For this reason, in this embodiment, it is preferable to mount a Peltier element inside package 630 of optical modulation module 625. It is preferable that the heat dissipation surface of the Peltier element is on the same side as the heat dissipation surface of the driver IC. This is because if the heat dissipation surfaces of the Peltier element and the driver IC are on different sides, both the top and bottom of the optical modulation module will become heat dissipation surfaces, making it difficult to use.

[0056] Furthermore, ICs designed for high-speed operation have a risk of oscillation. In consideration of this point, in this embodiment, it is desirable to use a radio wave absorber 790 that can absorb frequency bands with a risk of oscillation, by attaching it to the lid 630d that serves as the package cover for the digital signal processing circuit, the optical modulation module, and the optical receiving module. [Explanation of symbols]

[0057] 6 Optical transmitter and receiver 600 PCB board substrate 610 DSP package board 610a top side 611 DSP-ASIC 613,623,623,624 Pad 623a signal line 625 Optical Modulation Module 626 rand 627 through hole 628 Signal Pad 629 Underfill agent (conductive adhesive) 630, 631 packages 630a package body 630b RF terrace section 630c Fiber pipe section 630d Lid 663, 664 Heating pad 727 Optical Modulator PIC 740 Optical Fiber 751 Gold Wire 753 High Frequency Wiring 760 Temperature Controller (TEC) 770 Module wiring board base 771 Module Package Wall 780 Optical element base 781 Chip Condenser Lens 782 Fiber condenser lens 790 Radio Wave Absorber

Claims

1. A high-speed optical transceiver, a digital signal processing circuit; a first electrode formed on a first package substrate of the digital signal processing circuit; an optical element; a second package that houses the optical element; a second electrode formed on a surface of the second package; the first package substrate and the second package are directly connected by the first electrode and the second electrode; A high-speed optical transceiver device, wherein the first package substrate and the second package are supported by a same support substrate, and the heat dissipation surfaces of the first package substrate and the second package are arranged on the opposite side of the support substrate.

2. 2. The high-speed optical transmitting and receiving device according to claim 1, wherein said optical element includes at least one of an optical modulation element and a light receiving element.

3. 3. The high-speed optical transceiver device according to claim 1, wherein the first electrode and the second electrode have signal pads functioning as signal lines each having a width of 200 μm or less, the width of the signal pad of the second package being smaller than the width of the signal pad of the first package substrate, and the high-speed optical transceiver device has a through-hole for heating.

4. 2. The high-speed optical transceiver according to claim 1, wherein a solder resist is provided on at least one of the first electrode and the second electrode, and the first electrode and the second electrode are connected using a low-melting-point solder having a melting point of 150°C or less.

5. 2. The high-speed optical transmitter / receiver according to claim 1, wherein said second package is a ceramic package, and the total thickness of the region of said second package where said second electrode is formed is 1 mm or less.

6. The first electrode is provided on a first forming surface of the first package substrate; the second package includes an optical element, a main body portion that houses the optical element, and a terrace portion that extends from the main body portion toward the first package substrate; 2. The high-speed optical transceiver device of claim 1, wherein the second electrode is formed on a second forming surface of the terrace portion, the thickness from the bottom surface of the main body to the second forming surface is less than or equal to the height of the first forming surface, and the difference between the height of the first forming surface and the thickness is 500 μm or less.

7. 2. The high-speed optical transceiver according to claim 1, wherein the second package is fixed to the support substrate by a conductive adhesive as an underfill agent.

8. 3. The high-speed optical transceiver according to claim 2, wherein, when the optical element includes the optical modulation element, a driver IC is integrated together with the optical modulation element, and when the optical element includes the light receiving element, a transimpedance amplifier is integrated together with the light receiving element, and a Peltier element is provided as a temperature adjustment mechanism, and the Peltier element dissipates heat on the same side as the driver IC or the transimpedance amplifier dissipates heat.

9. 2. The high-speed optical transceiver according to claim 1, wherein the first package substrate or the second package has a radio wave absorber on a lid.

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