Optical transceiver

Abstract

A configuration of a smooth high-frequency connection between a DSP and an optical module in an optical transceiver is presented and an optical transceiver capable of operating at ultra-high speed is provided. An optical transceiver according to the present disclosure comprises: at least one optical module mounted on a PCB; the DSP is mounted on the PCB; the FPC is connected with the DSP and the optical module or the PCB and the optical module; and a differential high-frequency line configured such that the p-side line and the n-side line form a differential pair, and formed such that a signal propagates inside the DSP substrate of the DSP, inside the optical module, and between the DSP and the optical module, in which the differential high-frequency line disposed between the DSP and the optical module has at least one first bent portion.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an optical transceiver for optical communication. More specifically, the present disclosure relates to a mounting form of an optical transceiver. BACKGROUND

[0002] In order to cope with the increasing demand for communication traffic, a high-speed optical modulator and an optical receiver compatible with an advanced optical modulation system are required. In the optical modulator and the optical receiver in an early 100G digital coherent communication system, each component is packaged and the parts are mounted on a printed circuit board (PCB). For example, in the case of an optical modulator, a driver IC and an optical modulator chip are packaged separately, and the packaged IC and the chip are mounted on a PCB. In the case of an optical receiver, a trans-impedance amplifier (TIA) and an optical light-receiving chip are packaged separately and mounted as separate parts on a PCB.

[0003] In a digital coherent communication system of more than 400G in which the speed is further increased, the optical modulator and the optical receiver also need to have a wide band in order to be able to handle a signal of 40GHz or more, and need to reduce high-frequency loss and need to reduce the size. In the case of an optical modulator, a driver IC and an optical modulator chip are integrally mounted in one package as an optical module. In addition, in the case of an optical receiver, a TIA and an optical light-receiving chip are integrally mounted in a single optical module. In addition, for a signal form for input and output baseband signals, a design from a single-ended form to a differential form has been popularized as one of means for achieving high speed, small size, and low power consumption. Currently, development of devices for realizing 800Gbps and 1Tbps (128GBd operation) is in progress.

[0004] The Optical Internetworking Forum (OIF) standardized the name of a high-bandwidth coherent drive modulator (HB-CDM) as an optical transmitter in which a driver IC and an optical modulator are mounted in an integrated package. In Non-Patent Literature 1, a physical configuration, an interface specification, and the like are defined as various types of modules. In addition, on the receiving side, a TIA and an optical light receiver are mounted in an integrated package, and such an integrated package is also called a high-bandwidth intra-differential coherent receiver (HB-ICR). An optical transceiver (optical transmission / reception device) includes a transmitting side (Tx) optical module and a receiving side (Rx) optical module.

[0005] Initially, surface mount technology (SMT) packages, which offer excellent mountability, were used in the aforementioned optical modules. In SMT-packaged optical modules, since the optical module is mounted on the PCB of the optical transceiver, vias (VIAs) are essential for transmitting high-frequency electrical signals into the package. However, degradation of high-frequency characteristics is unavoidable in VIAs, making the VIA structure unsuitable for further broadband implementation. Furthermore, in SMT packages, the high-frequency transmission characteristics of electrical signals are degraded at the connection between the lead pins and the ceramic package due to electromagnetic field mode mismatch or impedance mismatch (Non-Patent Document 2). Therefore, a package configuration using flexible printed circuits (FPCs) was re-standardized (Non-Patent Document 1).

[0006] When achieving speed increases for the entire optical transceiver, in addition to increasing the speed of individual optical modules, optimized design of the digital signal processor (DSP) is also crucial. In early optical transceivers, the DSP, transmitter module, and receiver module were housed in separate packages and mounted in different locations on the PCB. Therefore, speed improvements were limited by propagation losses generated within the separate packages or on the PCB. As one solution, a "co-package structure" was investigated, in which the optical modulator chip, optical receiver chip, and DSP are mounted close to each other in a single package. This structure is ideal for significantly reducing losses in high-frequency electrical signals; however, the DSP, being the largest heat source, is placed near the periphery of the optical modulator or receiver. When using optical modulators requiring temperature control (e.g., InP modulators that excel in speed), the optical modulator chip needs to be mounted on a Peltier element. Peltier elements pose risks of increased power consumption and thermal runaway, and it is difficult to mount them close to the periphery of the DSP, which would cause heat inflow, within a single package. Co-package structures present various challenges during installation. Therefore, another configuration of the optical transceiver has been proposed (Patent Document 1).

[0007] Figure 1 This is a side cross-sectional view showing a configuration of an optical transceiver 800 suitable for high-speed operation according to conventional technology. A side cross-section (zx plane) of the optical transceiver 800 is shown, taken along a line passing through a transmitting module 807 mounted on a substrate surface (xy plane) of a PCB 801. The optical transceiver 800 includes a DSP 802, a transmitting module 807, and a receiving module (not shown). In the DSP 802, a DSP chip 805 is mounted on a DSP substrate 804 using a ball grid array (BGA). The entire DSP 802 is further mounted on the PCB 801 via a BGA 803. The transmitting module 807 includes a driver IC, an optical modulator chip, etc. (not shown) in a package having a platform, and includes an optical fiber 808.

[0008] In optical transceiver 800, high-frequency (RF) signals are connected between DSP 802 and optical module 807 via FPC 806. By using FPC 806 as the RF interface, RF signal lines formed on the upper surface of DSP substrate 804 and RF signal lines on the platform of the optical module are directly connected. In early optical transceivers including SMT-packaged optical modules, signal paths were configured through the VIA of DSP substrate 804, the RF signal line pattern of PCB 801, and the VIA of the optical module 807 package. Compared to early optical transceivers utilizing SMT-packaged... Figure 1 The optical transceiver 800 is configured to minimize high-frequency losses. Furthermore, the option of optical modulators is not limited to optical transceivers with a co-package structure, and installation is more feasible. High-frequency losses are also significantly suppressed compared to earlier optical transceivers using SMT packages.

[0009] However, in Figure 1 In the conventional optical transceiver 800, the specific connection configuration between FPC 806 and DSP 802, the structure of FPC 806, and the connection configuration between FPC 806 and optical module 807 are not yet clear. The configuration of the DSP and DSP substrate is also unclear. Furthermore, in... Figure 1 In the transmitter module 807, the heat-generating components within the package (e.g., the driver IC) are thermally connected to the platform side and the PCB side, and the heat dissipation direction is downward (-z direction) relative to the optical transceiver 800. On the other hand, since the DSP chip 805 is located above the DSP 802, the heat dissipation direction is upward (+z direction) relative to the optical transceiver. This division of heat dissipation into two directions is not preferable for simplifying the heat dissipation structure.

[0010] Reference List

[0011] Patent documents

[0012] Patent Document 1: WO 2021 / 171599 A

[0013] Non-patent literature

[0014] Non-Patent Document 1: OIF, Implementation Agreement for the High Bandwidth Coherent Driver Modulator (HB-CDM), [Online], July 15, 2021, [Searched on January 20, 2023], Internet<URL:https: / / www.oiforum.com / wp-content / uploads / OIF-HB-CDM-02.0.pdf>

[0015] Non-patent literature 2: J. Ozaki et al., “Over-85-GHz-Bandwidth InP-Based CoherentDriver Modulator Capable of 1-Tb / s / λ-Class Operation”, Journal of Lightwave Technology, vol.41, No.11, pp.3290-3296, June 1, 2023, doi:10.1109 / JLT.2023.3236962. Summary of the Invention

[0016] The objective of this disclosure is to present a configuration of smooth high-frequency connection between a DSP and an optical module in an optical transceiver, and to provide an optical transceiver capable of operating at ultra-high speed.

[0017] To achieve the above objectives, this disclosure provides an optical transceiver, comprising: at least one optical module mounted on a printed circuit board (PCB); a digital signal processor (DSP) mounted on the PCB; a flexible printed circuit board (FPC) connecting the DSP and the optical module or the PCB and the optical module; and a differential high-frequency line configured such that p-side lines and n-side lines form a differential pair, and configured to allow signals to propagate within the DSP substrate of the DSP, within the optical module, and between the DSP and the optical module, wherein the DSP includes a DSP chip mounted on the DSP substrate, the FPC includes a first connection pad on a first surface soldered to a pad on the platform surface of the optical module, and a second connection pad on a second surface soldered to a pad on the upper surface of the DSP substrate, the DSP chip being connected to the DSP chip via a via (VIA) through signal lines in the inner layer of a multilayer wiring substrate, and the differential high-frequency line disposed between the DSP and the optical module having at least one first bent portion. Attached Figure Description

[0018] Figure 1 This is a side cross-sectional view showing the configuration of an optical transceiver 800 adapted for high speed according to conventional technology.

[0019] Figure 2 These are top and cross-sectional views showing the outline of the configuration of the optical transceiver 100 according to this disclosure. Figure 2 (a) is a top view of the substrate surface (xy plane) of PCB 101, and Figure 2 (b) is a diagram of the side section (xz plane) taken along IIb-IIb passing through Tx module 400.

[0020] Figure 3 Is with Figure 2Enlarged view of the part corresponding to FPC 500-1 in (a).

[0021] Figure 4 The simulation results for FPC500-1 are shown, including the differential high-frequency line 501 in which the phase difference adjustment curve 502 is not formed. Figure 4 (a) shows the model used for simulation. Figure 4 (b) shows the simulation results for the group delay, and Figure 4 (c) shows the calculated results of the in-phase characteristics.

[0022] Figure 5 The simulation results for FPC 500-1 are shown, including the differential high-frequency line 501 in which a phase difference adjustment bend 502 with a total length of approximately 400 μm is formed. Figure 5 (a) shows the model used for simulation. Figure 5 (b) shows the simulation results for the group delay, and Figure 5 (c) shows the calculated results of the in-phase characteristics.

[0023] Figure 6 The simulation results for FPC 500-1 are shown, including the differential high-frequency line 501 in which a phase difference adjustment bend 502 with a total length of approximately 450 μm is formed. Figure 6 (a) shows the model used for simulation. Figure 6 (b) shows the simulation results for the group delay, and Figure 6 (c) shows the calculated results of the in-phase characteristics.

[0024] Figure 7 It is shown in a similar Figure 2 A graph showing the results of comparing the presence and absence of in-phase characteristics in an FPC 500-1 with two curved sections, based on the phase difference adjustment bend 502. Figure 7 (a) shows a simulation model in which the phase difference adjustment bend 502 is not formed in the differential high-frequency line 501. Figure 7 (b) shows a simulation model of a phase difference adjustment curve 502 formed in each of p-side line 501a and n-side line 501b. Figure 7 (c) shows the results of the in-phase characteristics when the phase difference adjustment bend 502 is not formed in the differential high-frequency line 501, and Figure 7 (d) shows the results of the in-phase characteristics when a phase difference adjustment curve 502 is formed in each of the p-side line 501a and n-side line 501b.

[0025] Figure 8 This is a diagram showing a summary of the configuration of the optical transceiver 600 according to this disclosure.Figure 8 (a) shows a top view, and Figure 8 (b) shows a cross-sectional view taken along line VIIIb-VIIIb.

[0026] Figure 9 This is a diagram showing a summary of the configuration of the optical transceiver 700 according to this disclosure. Figure 9 (a) shows a top view, and Figure 9 (b) shows a cross-sectional view taken along line IXb-IXb.

[0027] Figure 10 This is a diagram showing the cross-sectional structure of the Tx module of the optical transceiver disclosed herein. Detailed Implementation

[0028] In the following, various embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The same or similar reference numerals denote the same or similar components, and repeated descriptions may be omitted. Numerical values ​​are for illustrative purposes only and are not intended to limit the scope of the present disclosure. Furthermore, the following description is merely illustrative, and some configurations may be omitted, modified, or implemented with additional configurations without departing from the spirit of the embodiments of the present disclosure. Additionally, the accompanying drawings illustrate examples of embodiments of the present disclosure and are not intended to be limiting. For example, in an optical transceiver according to the present disclosure, the number of channels may be greater than or less than the indicated number.

[0029] The optical transceiver disclosed herein presents a specific configuration for achieving a smooth high-frequency connection between a DSP and an optical module. The optical module may include an optical receiver module and an optical modulator module. The optical transceiver may include two separate optical modules, or may include a single optical module integrating the functions of both an optical receiver and an optical modulator. For the high-frequency connection, a flexible substrate (hereinafter referred to as FPC) is used. The FPC uses two surfaces to electrically connect pads on the upper surface of the DSP substrate to pads on the optical module's package. Connection pads on one surface of the FPC are connected to the DSP substrate side, and connection pads on the other surface are connected to the optical module side. The DSP chip can be mounted on the DSP substrate within the DSP, and the entire DSP substrate, except for the area of ​​the DSP substrate connected to the FPC, can be covered with a cover.

[0030] The detailed structure of the connection pads on the DSP substrate, the shape of the high-frequency wiring, the structure of the multilayer wiring substrate, and the structure of the FPC are disclosed. These are used to avoid the degradation of the transmission characteristics of high-frequency signals due to impedance mismatch, high-frequency crosstalk, etc. The detailed configuration of the optical transceiver will be described below with reference to the accompanying drawings.

[0031] In the following description, for simplicity, the transmitter-side optical module including the optical modulator and its driver IC will be referred to as a "Tx module," and the receiver-side optical module including the photodetector (PD) and TIA will be referred to as an "Rx module." A single optical module integrating the functions of both Tx and Rx modules will be referred to as a TRx module. In the following description, an optical transceiver including both Rx and Tx modules is described as an example, but both modules can be replaced with TRx modules. Furthermore, the following description will be based on the cross-section of the Tx module, but for the Rx module, whose shape is substantially symmetrical to the Tx module, the connection method using the FPC for the DSP and optical module is the same. Similar configurations of the FPC pad shapes between the DSP and Tx modules, and between the DSP and Rx modules, are also effective in high-frequency design. This also helps to suppress differences in solder mounting quality and connection strength throughout the optical module.

[0032] [Basic Configuration of Optical Transceiver]

[0033] Figure 2 The above view and cross-sectional view show an overview of the configuration of the optical transceiver 100 according to this disclosure. Figure 2 (a) is a top view of the substrate surface (xy plane) of PCB 101, and Figure 2 (b) is a diagram of the side section (xz plane) taken along IIb-IIb passing through Tx module 400. In optical transceiver 100, DSP 200, Rx module 300 and Tx module 400 are mounted on PCB 101.

[0034] In addition, various electronic and mechanical components, such as chip capacitors and resistors, are mounted on the upper and lower surfaces of PCB 101, but... Figure 1 The text is omitted. In the optical transceiver 100, considering manufacturability when each component is mounted on the PCB 101 and simplification of the heat dissipation structure of the optical transceiver 100, the heat dissipation surfaces of the DSP 200, Rx module 300, and Tx module 400 are aligned in one direction. Considering that the heat dissipation from the DSP chip 202 is the greatest in the optical transceiver 100, it is desirable that the heat dissipation direction be aligned upward (+z direction), which is suitable for the heat dissipation of the DSP chip 202.

[0035] refer to Figure 2In the cross-sectional view in (b), compared to the configuration of conventional technology, the optical module 100 is configured such that the upper and lower surfaces are interchanged when mounted on the PCB 101, and the side where the platform is located is the upper side of the optical module in the height direction (z-axis). The optical module is mounted such that the platform surface faces the upper surface of the PCB, and in the case of the Tx module 400, heat from the driver IC and Peltier elements is configured to move towards the side where the platform surface of the package is located (along the +z direction).

[0036] Figure 10 This diagram illustrates the cross-sectional structure of the Tx module in the optical transceiver of this disclosure. In the Tx module 400, a Peltier element 406, a substrate 407, and an optical modulator chip 409 are arranged in this order within a housing 417. Lenses 410 and 411 are disposed on the substrate and are optically coupled to an optical fiber 402. In this module, a driver IC 408 is formed on the surface of a substrate 415, and wiring 412 is formed on a package substrate 416. The optical modulator chip 409 is connected to wiring 412 via wirings 413 and 414. The interior and exterior of the module are divided by the housing 417, the package substrate 416, and the package sidewall 405. The platform surface of the optical module is configured by exposing a portion of the package substrate 416 from the housing. An FPC 500-1 is attached to the platform surface.

[0037] A Peltier element 406 for temperature control and a base 415 thermally connected to the driver IC, which serves as a heat-generating component, are fixed to the upper surface inside the housing 417. Heat dissipation from inside the Tx module is configured such that heat is moved from the driver IC or Peltier element to the outside along the +z direction. Alternatively, in the Rx module, a TIA is provided instead of a driver IC, and heat from inside the module is moved to the upper side of the module (+z direction).

[0038] As described above, a heat-generating component is mounted within the optical module's package to dissipate heat to the bottom surface opposite the mounting surface on the PCB. Therefore, a heat dissipation mechanism (housing) with a common heat dissipation surface (not shown) on the upper side of the optical transceiver 100 can be used to dissipate heat from both the DSP 200 and the Tx module 400, relative to heat dissipation from the DSP 200 and the Tx module 400. The optical module and DSP are positioned close to each other, and both have the same upper surface height, allowing for easy heat dissipation of both the DSP and the optical module using a single heat dissipation mechanism. In the case of a common heat dissipation mechanism, it is necessary to carefully consider preventing heat transfer from the DSP to the optical module. Alternatively, to suppress heat movement via the heat dissipation mechanism, the DSP and optical module can use independent heat dissipation mechanisms on the same plane.

[0039] DSP 200 includes a DSP substrate 201, a DSP chip 202, and a cover 203. The DSP substrate 201 includes a multilayer wiring substrate, which will be described below. The DSP chip 202 and the DSP substrate 201 are connected via a BGA 204. Similarly, the DSP substrate 201 and the PCB 101 are also connected via a BGA 205.

[0040] The cover 203 is made of a single piece of metal to cover at least the entire area of ​​the upper surface of the DSP chip 202, and the area of ​​the upper surface of the DSP substrate except for the area where FPCs 500-1 and 500-2 are connected / fixed. The cover 203 may cover the side surface of the DSP chip 202, or it may protrude from the periphery of the DSP substrate 201. Through the finely designed shape of the cover 203, the surface of the DSP substrate 201 can be kept open from the RF connection pads for connection with the FPC to the substrate end. As described above, the DSP 200 can be fabricated as a DSP module in which the DSP chip 202 and the cover 203 are mounted on the DSP substrate 201. Although the manufacturing process of the optical transceiver 100 will be described below, when the DSP 200 is fabricated as a covered DSP module, the DSP 200 can be safely mounted on the PCB 101 without damage during manufacturing, similar to the Rx module 300 and the Tx module 400.

[0041] As another form of DSP 200, DSP 200 can be fabricated with only the DSP chip 202 mounted on the DSP substrate without the cover 203. Alternatively, the DSP 200 without the cover 203, along with optical modules 300 and 400, can be connected via FPCs 500-1 and 500-2 respectively, and then the cover 203 is installed last. When installing the cover, there is a risk of accidental force acting on the FPC or the cover 203 causing a short circuit in the wiring of FPCs 500-1 and 500-2. Therefore, the pad positions, including the connection pads on the upper surface of the DSP substrate 201, and the shape of the cover 203, need to be carefully optimized. Since the DSP chip and DSP substrate are exposed until the cover is installed, extra care is required to avoid damaging the DSP chip, etc.

[0042] When the DSP 200 includes a cover 203, the area occupied by the cover 203 on the substrate surface (xy plane) is always smaller than the area occupied by the DSP substrate 201. On the other hand, the area occupied by the DSP chip 202 is included within the area occupied by the cover 203. On the substrate surface of the DSP substrate 201, the cover 203 is not provided only in areas where at least RF connection pads are provided.

[0043] During the manufacturing process of the optical transceiver 100, the cover 203 can be omitted if damage to the DSP chip 202 and BGAs 204 and 205 can be avoided. To effectively dissipate heat from the DSP chip 202 to the outside of the optical transceiver, the cover 203 can be provided only on the upper surface of the DSP chip 202. The shape of the cover 203 can be modified in various ways.

[0044] Both the Rx module 300 and the Tx module 400 are housed within a package. The two optical modules 300 and 400, along with the DSP 200, are directly connected via FPCs 500-1 and 500-2 as RF interfaces for electrical signals. Here, "direct" connection means that the connection pads on the optical module platform and the connection pads on the DSP substrate are connected only via FPCs 500-1 and 500-2. Therefore, in the optical transceiver 100, there is no high-frequency path via the VIA connecting the optical module to the PCB, the RF signal lines in the PCB, the BGA connecting the PCB and the DSP substrate, and the VIA in the DSP substrate. In the optical transceiver 100, compared to optical transceivers including SMT-type packages using conventional technology, electrical signals do not pass through the VIA or BGA, thus avoiding degradation of high-frequency characteristics. The DSP and optical module can be connected via the shortest high-frequency path, significantly reducing high-frequency losses. Furthermore, due to the reduction in the number of connection parts such as VIAs and BGAs, Figure 2 The configuration of the optical transceiver is also very effective in reducing high-frequency losses caused by impedance mismatch at discontinuous parts of the high-frequency path.

[0045] In addition, Figure 2 Although the diagram is omitted for simplicity, the differential high-frequency line 501 also includes a phase difference adjustment bend 502. The phase difference adjustment bend 502 is intentionally configured to eliminate the phase difference between p and n after the RF signal bends as it propagates through the p-side line 501a and n-side line 501b forming the differential pair (see below). Figure 3 A detailed description of the differential high-frequency line 501 and the phase difference adjustment bend 502 is given.

[0046] The DC interface of optical modules 300 and 400 can be, for example... Figure 2 DC lead pins 301 and 401 in (a). The DC interface is not limited to the lead pin structure and is similar to the RF interface with DSP 200, which can use an FPC. DC lead pins 301 and 401 are soldered onto pads (not shown) on PCB 101 so that power can be supplied from outside the optical transceiver.

[0047] Refer again Figure 2The top view of the optical transceiver 100 in (a) shows the optimal arrangement of the Rx module 300, Tx module 400, and DSP 200 to minimize the loss of the RF signal, which is a high-frequency electrical signal. These two modules and the DSP 200 are arranged symmetrically with respect to the centerline along the longitudinal direction of the PCB 101. To achieve a high-speed optical transceiver, it is important to suppress high-frequency losses in the RF signal from the DSP 200 to the Tx module and from the Rx module to the DSP 200. The length of each of the FPCs 500-1 and 500-2, which act as RF interfaces, needs to be as short as possible. Figure 2 In the optical transceiver 100, the distance (gap) between the two optical modules in the y-axis direction is preferably close to 1 mm or less in order to transmit high-speed signals exceeding 128 GBd.

[0048] To position the two optical modules close to each other, the DC lead pins need to be clustered on one side of each module and facing opposite directions. For example... Figure 2 As shown in (a), the DC lead pin 401 of the Tx module 400 points towards the outer periphery of the PCB 101 along the -y-axis direction (downward in the figure), and the DC lead pin 301 of the Rx module 300 points towards the outer periphery of the PCB 101 along the +y-axis direction (upward in the figure). This configuration follows the lead pin orientation defined in the OIF-standardized HB-CDM, as described in Non-Patent Document 1.

[0049] The packages for optical modules 300 and 400 are made of ceramic. Considering the layout design of the high-frequency signal lines and DC lines in optical modules 300 and 400, in... Figure 2 In the cross-sectional view of (b), the height from the upper surface of PCB 101 to the platform surface of optical modules 300 and 400 is expected to be about 1 to 2 mm. Similarly, the thickness of DSP substrate 201 from the angle that matches the platform surface of optical modules 300 and 400 is expected to be in the range of about 1 to 2 mm.

[0050] When the DSP substrate has a thickness of approximately 1 to 2 mm, more layers can be used than are required for DC and RF lines. Generally, in multilayer wiring substrates, to reduce costs, it is desirable to minimize the number of layers; therefore, a core layer can be used internally to adjust the overall thickness of the substrate. Multilayer wiring substrates including such a core layer can be called assembly substrates. The core layer acts as a separator layer, separating the RF wiring layer from the DC wiring layer in addition to adjusting the overall substrate thickness. By providing a core layer, the DC and RF wiring layers can be largely separated from each other, thus suppressing mutual interference and noise effects between wiring layers.

[0051] Considering the high-speed DSP operation described above, it is desirable to house the DSP 200 within a smaller DSP substrate 201 to suppress high-frequency signal loss and reduce cost. The two FPCs 500-1 and 500-2, and the differential high-frequency line 501 formed thereon, should be configured to converge from the optical module 300 and 400 sides toward the DSP 200 side on the centerline of the DSP 200. As will be described below, Figure 2 Both FPCs 500-1 and 500-2 shown have a curved shape within the plane (xy plane) of the substrate, which is pre-formed into a predetermined shape. Although FPCs can also be formed into straight shapes without bending, it is necessary to spread and arrange pads in a fan shape on the substrate surface of the DSP substrate 201. This increases high-frequency losses on the DSP substrate and also increases the size of the DSP substrate, which is not desirable.

[0052] As described above, since the two FPCs 500-1 and 500-2 are curved in the plane (xy plane) of the substrate, such as Figure 3 As shown, the differential high-frequency lines 501 formed thereon also have a curved shape within the plane (xy plane) of the substrate to follow this curved shape. In differential high-frequency lines 501 with this shape, the following problem exists: a phase difference appears after the curved portion between the p-side lines 501a, 501c, 501f, and 501h forming the differential pair and the n-side lines 501b, 501d, 501e, and 501g. Specifically, at the curved portion, since the line length of the inner line is shorter than that of the outer line, the signal propagates first, resulting in a phase difference between the signal propagating through the inner line and the signal propagating through the outer line. When a signal is transmitted while this phase difference is generated, this leads to degradation of high-frequency characteristics and the generation of noise. More specifically, at the bend between p-side lines 501a, 501c, 501f, and 501h and n-side lines 501b, 501d, 501e, and 501g, a phase difference shift occurs between the p-side lines 501a, 501c, 501f, and 501h and the n-side lines 501b, 501d, 501e, and 501g. When this phase difference shift occurs at the straight section following the bend, the signal is transmitted unstablely, and characteristic degradation occurs in the in-phase characteristics (Sdc21 and Scd21). Furthermore, in differential-driven optical transceivers, the dominant characteristic is typically the differential high-frequency characteristic (Sdd21 characteristic, etc.), while the in-phase characteristic represents noise. The increased noise caused by this phase shift in the differential pair also leads to degradation of the transmission characteristics. Therefore, a configuration to suppress the degradation of the in-phase characteristic is needed. Specifically, the effect of this phase difference becomes more significant when signals are transmitted at speeds exceeding 128 GBd, making it important to introduce configurations to suppress in-phase characteristic degradation.

[0053] [Configuration of FPC and Differential High-Frequency Lines]

[0054] Figure 2 Is with Figure 3 An enlarged view of the portion corresponding to FPC 500-1 in (a). Note that, although... Figure 3 The image shows FPC 500-1, but FPC 500-2 can also have a similar structure. For example... Figures 4 to 7 As shown, the differential high-frequency line 501 formed on the FPC 500-1 of the optical transceiver 100 according to this disclosure also includes a phase difference adjustment bend 502 in the line on the inner (shorter side) of the bend following the FPC 500-1. The phase difference adjustment bend 502 compensates for the shortened line length of the inner line and eliminates the phase difference generated after the bend between the p-side line 501a and the n-side line 501b, and between the p-side line 501c and the n-side line 501d. Since the propagation characteristics in a state of phase shift are undesirable, it is necessary to introduce such a phase difference compensation mechanism as soon as possible after the bend.

[0055] For example, to eliminate the impact on transmission characteristics, the in-phase characteristics (Sdc21 and Scd21) of each component (e.g., PCB 101, FPCs 500-1 and 500-2, DSP substrate 201, high-frequency packages constituting the optical module, etc.) need to be configured to be at least -30dB or lower within the desired operating frequency band (e.g., the frequency is approximately 0.6 to 0.7 times the baud rate). To achieve in-phase characteristics of -30dB or lower within the desired frequency band, a phase difference adjustment bend 502 needs to be formed at a position within half the length of the waveguide wavelength at the maximum frequency of the desired frequency band, before and after the bend of FPC 500-1. This length is referenced to the length without the phase adjustment bend 502. When this length exceeds half the waveguide wavelength, significant characteristic degradation due to phase difference occurs at the wavelength corresponding to that frequency, and in-phase characteristics of -30dB or lower cannot be achieved. Furthermore, to achieve highly favorable in-phase characteristics, it is very effective to position the location where the phase difference adjustment curve forms closer to the bend (e.g., within 1 / 4 of the length). When this position is within 1 / 4, even moving it closer (e.g., within 1 / 8) does not expect further improvement. Additionally, after eliminating the phase difference through the phase difference adjustment curve, the phase difference between the p-side line 501a and the n-side line 501b needs to be at least ±0.2 ps or less. When a phase difference exceeding this value occurs, the in-phase characteristics deteriorate significantly, and it becomes difficult to achieve the desired in-phase characteristics (-30 dB or lower) (see below for reference). Figures 4 to 7 To describe the quantitative assessment of phase difference and in-phase characteristics.

[0056] On the other hand, by forming the phase difference adjustment bend 502, the net length of the differential high-frequency line 501 itself increases, which may also produce adverse effects such as high-frequency signal loss. Therefore, from the perspective of preventing a significant impact on the differential high-frequency characteristics (Sdd21) at the wavelength of the maximum frequency, it is necessary to suppress the length of the phase difference adjustment bend 502 (the increased length due to the formation of the phase difference adjustment bend 502) to half or less of the waveguide wavelength at the maximum frequency of the desired frequency band. However, it goes without saying that from the perspective of preventing any abnormalities in the characteristics of the differential pair caused by the introduction of the phase difference adjustment bend 502, a smaller value such as 1 / 4 or less is desirable.

[0057] As a measure to prevent the phase difference adjustment bend 502 from being too long, suppressing the phase difference between p and n at the bend following the FPC 500-1 is effective. To suppress the phase difference at the bend, the signal line width in the bend can be made narrower relative to the straight portion of the differential high-frequency line 501, and the bend can be made smaller. However, if the signal line width is narrowed only in the bend, the impedance increases, and narrowing the signal line width only in the bend will lead to impedance mismatch compared to the signal line width in the straight portion where impedance matching is only performed, and the associated characteristic degradation may occur. In this case, for example, the FPC 500-1 has a GSSG configuration or a GGSSG configuration, and the impedance mismatch can be solved by shortening the distance between the G (ground) line and the S (signal) line at the portion where the signal line width is narrowed and increasing the impedance. In the case of the SS line configuration, this adjustment cannot be performed because there is no GND pattern on both sides, and this adjustment is undesirable even in a differential line configuration.

[0058] On the other hand, even if only parts of FPC 500-1 and 500-2 have advantageous characteristics, the transmission characteristics of other components, such as DSP substrate 201, PCB 101, and optical modules 300 and 400, deteriorate when their characteristics are low. Therefore, it is desirable to adjust the phase difference by introducing such a phase difference adjustment bend structure, so that the in-phase characteristics (Sdc21, Scd21) of not only the components of FPC 500-1 and 500-2, but also the components of DSP substrate 201, PCB 101, and optical modules 300 and 400 are -30dB or lower. As described above, in the case of forming a phase adjustment bend 502 for each component of the DSP substrate 201, PCB 101, and optical modules 300 and 400 (similar to forming a phase adjustment bend 502 on FPC 500-1 and 500-2), the phase adjustment bend 502 needs to be formed on the front and rear sides or within the range of the bend of the differential high-frequency line 501 (i.e., the part where there is a difference in line length between the inner and outer sides).

[0059] The phase difference between p and n (between p-side line 501a and n-side line 501b) and the configuration for eliminating the phase difference (phase difference adjustment bend 502) have been described above. However, considering the application of the optical transceiver, it is desirable that there is no phase change between channels, and for example, it is desirable that the lengths between the channels in the aforementioned FPCs 500-1 and 500-2 are equal. Furthermore, it is desirable to perform length equalization not only in FPCs 500-1 and 500-2, but also in the DSP substrate 201, PCB 101, and optical modules 300 and 400.

[0060] [Simulation Verification Results]

[0061] The following will refer to Figure 4 The simulation verification results are described to illustrate the elimination of the phase difference between p and n by forming the aforementioned phase difference adjustment bend 502. In the simulation, a polyimide resin FPC 500-1 (in which metal is formed on both surfaces of a substrate 503 having differential high-frequency lines 501) was used as the model. Furthermore, the FPC 500-1 used as the model in the simulation was designed to be compatible with the 150GBd level. In this case, for frequencies up to approximately 100GHz, the in-phase characteristics (Sdc21 and Scd21) need to be -30dB or lower.

[0062] Figure 4 The simulation results (left half of the figure) show a curved section of the FPC500-1, including the differential high-frequency line 501 in which the phase difference adjustment curve 502 is not formed, for evaluating the phase shift. Figure 4 (a) shows the model used for simulation. Figure 4(b) shows the simulation results for the group delay, and Figure 2 (c) shows the calculated results of the in-phase characteristics. In a structure like the FPC 500-1, where two bends are performed and the first and second bend angles cancel each other out (the differential high-frequency line before the first bend and the differential high-frequency line after the second bend are parallel), the phase difference completely disappears through the two bends (the bend on the input side and the bend on the output side), and is not visible in the simulation. Therefore, in order to observe the phase difference caused by the phase adjustment bend, the FPC 500-1 model is not... Figure 4 The shape shown has two bends, but it has only one bend. This allows for accurate determination of the phase difference caused by the bend. The total length of the differential high-frequency line 501 is approximately 6.5 mm on average (however, as mentioned above, the line is actually shorter on the inside). Additionally, the model used in the simulation is one where an RF signal is input from the right side of the figure and transmitted to the left side. The signal linewidth is set to approximately 100 μm, and the differential impedance is adjusted to approximately 100 Ω ± 5%. Furthermore, the model is configured to include a ground portion 504 and a via 505. The phase difference is shown when the set delay difference is at the point mentioned above.

[0063] like Figure 4 As shown in (b), when the phase difference adjustment bend 502 is not set, the phase difference between p and n (between p-side line 501a and n-side line 501b) at a frequency of 100 GHz is approximately 1 ps. Additionally, as... Figure 5 As shown in (c), at 12 GHz, Sdc21 and Scd21 are -30 dB or higher, and it can be seen that the in-phase characteristic at 100 GHz is about -12.5 dB, and the in-phase characteristic at 100 GHz does not meet the above-mentioned -30 dB or lower.

[0064] Figure 5 The simulation results for FPC 500-1 are shown, including the differential high-frequency line 501 in which a phase difference adjustment bend 502 with a total length of approximately 400 μm is formed. Figure 5 (a) shows the model used for simulation. Figure 5 (b) shows the simulation results for the group delay, and Figure 4 (c) shows the calculated results of the in-phase characteristics. Besides forming the phase difference adjustment curve 502, the simulation conditions are the same as... Figure 3 The simulation conditions are the same as in the model. Note that when the relative permittivity of polyimide is 3.3, the total length of the phase difference adjustment bend 502 (approximately 400 μm) is set to be shorter than 1 / 4 of the waveguide wavelength at 100 GHz (approximately 500 μm). In this model, as an example, multiple small bends are inserted, along with... Figure 5The difference is as follows. As mentioned above, the number of bends is not necessarily one, and multiple bends can be set. Of course, when two bends are set, the size of each bend is often smaller than that of a single bend. However, when multiple bends are generated, the bend angle becomes acute due to the smaller bend size, which may degrade the high-frequency characteristics. Therefore, it is desirable that the number of additional phase-adjustment bends is one and the bend angle is as small as possible.

[0065] like Figure 4 As shown in (b), when a phase difference adjustment bend 502 with a total length of approximately 400 μm is formed, the phase difference at a frequency of 100 GHz has a value slightly exceeding 0.2 ps, and it can be seen that, compared with Figure 5 Compared to the case where no phase difference adjustment curve was formed, the phase difference was improved. However, as shown... Figure 6 As shown in (c), at a frequency of 100 GHz, Sdc21 and Scd21 are approximately -23 dB and do not meet the required in-phase characteristics (-30 dB or lower).

[0066] Figure 6 The simulation results for FPC 500-1 are shown, including the differential high-frequency line 501 in which a phase difference adjustment bend 502 with a total length of approximately 450 μm is formed. Figure 6 (a) shows the model used for simulation. Figure 6 (b) shows the simulation results for the group delay, and Figure 5 (c) shows the calculated results of the in-phase characteristics. Figure 4 The situation is the same as shown, except that the phase difference adjustment curve 502 is formed, the simulation conditions are the same. Figure 6 The simulation conditions are the same.

[0067] like Figure 6 As shown in (b), when a phase difference adjustment bend 502 with a total length of approximately 450 μm is formed, the phase difference at 100 GHz is approximately 0 ps, ​​and it can be seen that in other frequency bands, the phase difference can be suppressed to approximately 0.1 to 0.15 ps. Additionally, as... Figure 6 As shown in (c), when the phase difference between p and n is suppressed to ±0.2 ps or less in this manner, the Sdc21 and Scd21 at 100 GHz are approximately -30 dB, and it can be seen that sufficient in-phase characteristics can be achieved within the desired frequency band for the required baud rate. Note that, also in Figure 7 In the simulation, the length of the phase difference adjustment bend (approximately 450 μm) was adjusted to be shorter than approximately 500 μm, which corresponds to one-quarter of the guide wavelength of the FPC 500-1 at a frequency of 100 GHz. Further improvements in in-phase characteristics can be expected when the phase difference at all frequencies can be brought close to near zero.

[0068] Figure 2 It is shown in a similar Figure 7 A graph showing the results of comparing the presence and absence of in-phase characteristics in an FPC 500-1 with two curved sections, based on the phase difference adjustment bend 502. Figure 7 (a) shows a simulation model in which the phase difference adjustment bend 502 is not formed in the differential high-frequency line 501. Figure 7 (b) shows a simulation model of a phase difference adjustment curve 502 formed in each of p-side line 501a and n-side line 501b. Figure 7 (c) shows the results of the in-phase characteristics when the phase difference adjustment bend 502 is not formed in the differential high-frequency line 501, and Figure 7 (d) shows the results of the in-phase characteristics when a phase difference adjustment curve 502 is formed in each of the p-side line 501a and n-side line 501b. Figure 6 In (d), the phase difference near the curved portion and the shape / arrangement of the curved portion conform to the rules described above. For example... Figure 2 As shown in (c) and (d), when no phase difference adjustment bend 502 is formed in the differential high-frequency line 501, the in-phase characteristic of Sdc21 and Scd21 at 100GHz is approximately -22dB. However, when a phase difference adjustment bend 502 is formed in each of the p-side line 501a and n-side line 501b, the in-phase characteristic of Sdc21 and Scd21 is approximately -40dB. From this result, it can be seen that for... Figure 7 The FPC 500-1 shown with two bends forms a phase difference adjustment bend 502, which can achieve sufficient in-phase characteristics in high-speed signal transmission exceeding 128 GBd.

[0069] Note that in Figure 8 In this model, a phase difference adjustment curve 502 is formed in each of the p-side line 501a and the n-side line 501b, but there is no limit to the number of phase difference adjustment curves 502 to be formed, and any number of phase difference adjustment curves 502 can be formed according to the design.

[0070] In the model used in the above simulation, for simplicity, similar to the straight section, the signal line width of the curved section following FPC 500-1 in the differential high-frequency line 501 is constant. Furthermore, although the ground portion 504 of the phase difference adjustment bend 502 has a constant shape, to further stabilize the in-phase characteristics, it is desirable to maintain a constant distance between the signal line (differential high-frequency line 501) and the ground portion 504. Therefore, according to the phase difference adjustment bend 502, this bend can be applied only to the portion of the ground portion 504 adjacent to the signal line (differential high-frequency line 501) in which the phase difference adjustment bend 502 is formed, so that the distance between the signal line (differential high-frequency line 501) and the ground portion 504 remains constant.

[0071] [Another form of FPC and differential high-frequency line]

[0072] As mentioned above, in order to improve the high-frequency characteristics of the optical transceiver, it is desirable for FPCs 500-1 and 500-2 to be directly connected to the DSP substrate 201. However, as Figure 9 and Figure 8 As shown, they can also be connected via PCB 101.

[0073] Figure 8 This is a diagram showing a summary of the configuration of the optical transceiver 600 according to this disclosure. Figure 8 (a) shows a top view, and Figure 9 (b) shows a cross-sectional view taken along line VIIIb-VIIIb. The optical transceiver 600 has FPCs 500-1 and 500-2 configured to be relatively short and without bends, preventing characteristic degradation in FPCs 500-1 and 500-2. In this optical transceiver 600, FPCs 500-1 and 500-2 have a straight shape when viewed from above; however, due to the need for pitch conversion in the signal path connection to the DSP substrate, bends are required at the portion of the differential high-frequency line 501 drawn into the PCB 101, and characteristics may degrade with these bends. Therefore, in the optical transceiver 600, the aforementioned phase difference adjustment bend 502 needs to be formed in the portion of the differential high-frequency line 501 drawn into the PCB 101. However, generally speaking, PCBs have higher manufacturing and design costs than FPCs, and design changes are more difficult. Therefore, in PCB101, it is desirable to form only straight lines and not provide curved structures.

[0074] Figure 9 This is a diagram showing a summary of the configuration of the optical transceiver 700 according to this disclosure. Figure 9 (a) shows a top view, and ​(b) shows a cross-sectional view taken along line IXb-IXb. In optical transceiver 700, the pitch conversion and bending formation of differential high-frequency line 501 are performed in FPCs 500-1 and 500-2, and the portion of differential high-frequency line 501 drawn into PCB 101 has a configuration that is only straight and without bending. Generally speaking, FPCs have the following characteristics: since they are less expensive and more readily available than PCBs, it is easier to change the design of differential high-frequency lines. Therefore, there is an advantage that the aforementioned phase difference adjustment bend 502 is easier to form compared to the case of optical transceiver 600. With this in mind, as with optical transceiver 100 and optical transceiver 700, it is desirable to form the pitch conversion and the bending of differential high-frequency line 501 due to the pitch conversion in FPCs 500-1 and 500-2.

[0075] Similarly, differential high-frequency lines with bends can also be formed in the DSP substrate 201 and the optical modules 300 and 400. In this case, introducing a phase difference adjustment bend 502 is also useful from the perspective of improving characteristics.

[0076] As described above, from the perspective of improving characteristics (reducing loss), the optical transceiver according to this disclosure is configured such that the differential high-frequency line has a curved portion. In the above example, a description assuming that the FPC also has a curved portion has been given, but the FPC can have any shape as long as the differential high-frequency line is configured to have a curved portion.

[0077] In addition, the configuration of differential high-frequency lines can be arbitrary, and can be, for example, SS line configuration, GSSG configuration, GGSSG configuration, SGS configuration, coplanar line, microstrip line configuration or other similar configurations.

[0078] Industrial applicability

[0079] The optical transceiver disclosed herein can be used for optical communication.

[0080] List of reference numerals

[0081] 100 optical transceivers

[0082] 200 DSP

[0083] 201 DSP substrate

[0084] 202 DSP chip

[0085] 203 Lid

[0086] 300 Rx Module (Optical Module)

[0087] 301 DC lead pins

[0088] 400 Tx module (optical module)

[0089] 401 DC lead pins

[0090] 501 Differential High Frequency Line

[0091] 501a, 501c, 501f, 501h P sideline

[0092] 501b, 501d, 501e, 501g n lateral lines

[0093] 503 substrate

[0094] 504 grounding part

[0095] 505 through hole

[0096] 600 optical transceiver

[0097] 700 optical transceiver

[0098] 800 optical transceiver

[0099] 802 DSP

[0100] 804 DSP substrate

[0101] 805 DSP chip

[0102] 807 Launch Module

[0103] 808 fiber optic cable.

Claims

1. An optical transceiver, comprising: At least one optical module is mounted on a printed circuit board (PCB). A digital signal processor (DSP) is mounted on the PCB. A flexible printed circuit board (FPC) connects the DSP and the optical module, or the PCB and the optical module. as well as Differential high-frequency lines are configured such that the p-side lines and n-side lines form a differential pair, and are configured to allow signals to propagate within the DSP substrate of the DSP, within the optical module, and between the DSP and the optical module. in, The differential high-frequency line disposed between the DSP and the optical module has at least one first bent portion.

2. The optical transceiver according to claim 1, wherein, The differential high-frequency line further includes at least one phase adjustment bend for eliminating the phase difference caused by the first curved portion, and The phase adjustment bend is formed before and after the portion of the differential high-frequency line where the line length difference occurs, or within the range of the portion where the line length difference occurs, and is configured such that: after propagation through the phase adjustment bend, the phase difference between the signals propagating through the p-side line and the n-side line is within ±0.2 ps.

3. The optical transceiver according to claim 2, wherein, The phase adjustment bend is formed at a position within half the length of the waveguide wavelength at the maximum frequency of the desired frequency band, before and after the portion where the line length difference occurs between the p-side and n-side lines of the differential high-frequency line. The length of the phase adjustment bend is within 1 / 2 of the waveguide wavelength at the maximum frequency of the required frequency band.

4. The optical transceiver according to claim 3, wherein, The FPC connects the PCB and the optical module. The differential high-frequency line is also disposed in the PCB and configured to propagate the signal between the optical module and the DSP via the differential high-frequency line disposed in the PCB. The first curved portion and the phase adjustment bend are formed in the differential high-frequency line provided in the PCB, or The first curved portion and the phase adjustment bend are formed in the differential high-frequency line provided on the FPC.

5. The optical transceiver according to claim 3, wherein, The differential high-frequency line configured between the DSP and the optical module is either a GSSG configuration or a GSGSG configuration. The line width of the first curved portion is narrower than the line width of the straight portion, and In the first curved section, the distance between the grounding portion and the differential high-frequency line serving as a signal is configured to be shorter than the distance between the grounding portion and the differential high-frequency line serving as a signal in the straight section.

6. The optical transceiver according to claim 3, wherein, The differential high-frequency line configured between the DSP and the optical module is either a GSSG configuration or a GSGSG configuration. In the first curved section, the distance between the grounding portion and the differential high-frequency line serving as the signal is constant, and The differential high-frequency lines have equal lengths between channels.

7. The optical transceiver according to claim 3, wherein, The at least one optical module: Includes: an optical transmitter module, comprising an optical modulator and driver integrated circuit; and an optical receiver module, comprising a photodetector and a transimpedance amplifier, or It is a single optical transmitter / receiver module, which includes the functions of both the optical transmitter module and the optical receiver module. The at least one optical module is mounted such that the platform surface faces the upper surface of the PCB, and a heat-generating component is mounted inside the package to dissipate heat towards a surface opposite the mounting surface on the PCB. The FPC connects the DSP and the optical module, and The phase adjustment bend is formed in the differential high-frequency line set on the FPC.

8. The optical transceiver according to claim 7, wherein, The DSP also includes: The cover covers an area that includes at least the area of ​​the DSP chip, but excludes the pads of the DSP substrate.

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