Optical transceiver

Abstract

The present invention provides an optical transceiver capable of ultrafast operation by presenting a configuration for a smooth high-frequency connection between a DSP and an optical module in the optical transceiver. An optical transceiver according to the present disclosure comprises: at least one optical module that is mounted on a PCB; a DS that is mounted on the PCB; an FPC that connects the DSP and the optical module, or the PCB and the optical module; and a differential high-frequency line that is configured from a differential pair formed by a p-side line and an n-side line, and is formed such that a signal propagates inside a DPS substrate of the DSP, inside the optical module, and between the DSP and the optical module. The differential high-frequency line disposed between the DSP and the optical module has at least one first bent part.

Description

Optical Transceiver

[0001] The present disclosure relates to optical transceivers used in optical communications, and more particularly to implementations of optical transceivers.

[0002] In order to meet the increasing demand for communication traffic, high-speed optical modulators and optical receivers compatible with advanced optical modulation methods are required. In the optical modulators and optical receivers used in early 100G digital coherent communication systems, each component was packaged and mounted on a printed circuit board (PCB). For example, in the case of an optical modulator, the driver IC and optical modulator chip were individually packaged, and the packaged ICs and chips were mounted on the PCB. In the case of an optical receiver, the transimpedance amplifier (TIA) and optical receiver chip were individually packaged and mounted on the PCB as separate components.

[0003] In digital coherent communication systems exceeding 400 Gbps, which are experiencing ever-increasing speeds, optical modulators and optical receivers are also required to have wider bandwidths to handle signals of 40 GHz and above, necessitating reduced high-frequency loss and miniaturization. In the case of optical modulators, the driver IC and optical modulator chip are now integrated into a single package as an optical module. In the case of optical receivers, the TIA and optical receiver chip are now integrated into a single optical module. Regarding the signal format for inputting and outputting baseband signals, a shift from single-ended to differential designs has become common as one means of achieving higher speeds, smaller size, and lower power consumption. Currently, device development is underway to achieve 800 Gbps and 1 Tbps (128 GBd operation).

[0004] The Optical Internetworking Forum (OIF) has standardized an optical transmitter in which a driver IC and an optical modulator are mounted in an integrated package under the name High-Bandwidth Coherent Driver Modulator (HB-CDM). Non-Patent Document 1 specifies various types of modules, including their physical configurations and interface specifications. On the receiving side, a TIA and an optical receiver are mounted in an integrated package, also known as an HB-ICR (High-Bandwidth Intradyne Coherent Receiver). An optical transceiver (optical transmitting and receiving device) includes a transmitting (Tx) optical module and a receiving (Rx) optical module.

[0005] Initially, the optical modules described above used surface-mount (SMT) packages, which offer excellent mountability. Optical modules using SMT packages are mounted on the PCB of an optical transceiver, so a via (VIA) structure is essential for transmitting high-frequency electrical signals inside the package. Because degradation of high-frequency characteristics is unavoidable with VIAs, the VIA structure is unsuitable for further broadening the bandwidth. Furthermore, SMT packages suffer from electromagnetic field mode mismatch and impedance mismatch at the connection between the lead pins and the ceramic package, resulting in degradation of the high-frequency transmission characteristics of electrical signals (see Non-Patent Document 2). Therefore, a new package configuration using flexible printed circuits (FPCs) has been standardized (see Non-Patent Document 1).

[0006] In addition to increasing the speed of individual optical modules, optimizing the design of the digital signal processor (DSP) is crucial to achieving overall high-speed optical transceivers. In early optical transceivers, the DSP, transmitter module, and receiver module were mounted in separate packages, each located at a distance on the PCB. This limited the speed of optical transceivers due to propagation losses within each package and on the PCB. One solution being explored is a "co-package structure" in which the optical modulator chip, optical receiver chip, and DSP are closely spaced within a single package. While this structure is ideal for significantly reducing high-frequency electrical signal loss, it places the DSP, the largest heat source, in close proximity to the optical modulator and receiver. When using optical modulators that require temperature control, such as InP modulators, which offer excellent speed performance, the optical modulator chip must be mounted on a Peltier element. Peltier elements have the potential for increased power consumption and thermal runaway, making it difficult to mount the Peltier element in close proximity to the DSP, which is a source of heat inflow, within a single package. The co-package structure has various problems in terms of the mounting process and realization, so another optical transceiver configuration has been proposed (Patent Document 1).

[0007] FIG. 1 is a side cross-sectional view showing the configuration of a conventional optical transceiver 800 adapted for high-speed operation. The cross-sectional view (z-x plane) of the optical transceiver 800 is shown, cut along a line passing through a transmitter module 807 mounted on the board surface (x-y plane) of a PCB 801. The optical transceiver 800 includes a DSP 802, a transmitter module 807, and a receiver module (not shown). The DSP 802 includes a DSP chip 805 mounted on a DSP board 804 using a ball grid array (BGA). The entire DSP 802 is further mounted on the PCB 801 using a BGA 803. The transmitter module 807 includes a driver IC, an optical modulator chip, and other components (not shown) in a terraced package, and is equipped with an optical fiber 808.

[0008] In the optical transceiver 800, the radio frequency (RF) signal connection between the DSP 802 and the optical module 807 is performed via an FPC 806. Using the FPC 806 as an RF interface allows direct connection between the RF signal line formed on the top surface of the DSP board 804 and the RF signal line on the terrace of the optical module. In early optical transceivers including optical modules in SMT packages, the signal path was configured via the VIA on the DSP board 804, the RF signal line pattern on the PCB 801, and the VIA on the package of the optical module 807. The optical transceiver 800 in Figure 1 minimizes high-frequency loss compared to early optical transceivers using SMT packages. Furthermore, the optical modulator options are not limited as in optical transceivers with a co-package structure, making it more feasible to implement. High-frequency loss can also be significantly reduced compared to early optical transceivers using SMT packages.

[0009] However, in the prior art optical transceiver 800 shown in Figure 1, the specific connection configuration between the FPC 806 and the DSP 802, the structure of the FPC 806, and the connection configuration between the FPC 806 and the optical module 807 are unclear. The configuration of the DSP and DSP board is also unclear. Furthermore, in the transmitter module 807 shown in Figure 1, heat-generating components such as the driver IC inside the package are thermally connected to the PCB side, which has a terrace, and the heat dissipation direction is downward (in the -z direction) from the optical transceiver 800. On the other hand, in the DSP 802, the DSP chip 805 is located on the upper side, so the heat dissipation direction is upward (in the +z direction) from the optical transceiver. Heat dissipation is split into two directions, which is undesirable for simplifying the heat dissipation structure.

[0010] International Publication Number WO2021 / 171599

[0011] OIF, Implementation Agreement for the High Bandwidth Coherent Driver Modulator (HB-CDM), [online], July 15, 2021, [Retrieved January 20, 2023], Internet <URL: https: / / www.oiforum.com / wp-content / uploads / OIF-HB-CDM-02.0.pdf> J. Ozaki et al., "Over-85-GHz-Bandwidth InP-Based Coherent Driver Modulator Capable of 1-Tb / s / λ-Class Operation," in Journal of Lightwave Technology, vol. 41, no. 11, pp. 3290-3296, June 1, 2023, doi: 10.1109 / JLT.2023.3236962.

[0012] An object of the present disclosure is to provide a configuration for a smooth high-frequency connection between a DSP and an optical module in an optical transceiver, and to provide an optical transceiver capable of ultra-high-speed operation.

[0013] Based on the above object, the present disclosure provides an optical transceiver including 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 (FPC) connecting between the DSP and the optical module or between the PCB and the optical module, and a differential high-frequency line in which a p-side line and an n-side line form a differential pair and which is formed so that a signal propagates inside a DPS board of the DSP, inside the optical module, and between the DSP and the optical module, The present invention provides an optical transceiver including a DSP chip mounted on a P substrate, wherein the FPC has a first connection PAD on a first surface that is solder-connected to a PAD on the terrace surface of the optical module, and a second connection PAD on a second surface that is solder-connected to a PAD on the top surface of the DSP substrate, the DSP chip being connected to the DSP chip by signal lines on an inner layer of the multilayer wiring board via vias from the PAD on the DSP substrate, and the differential high-frequency line arranged between the DSP and the optical module has at least one first bend.

[0014] 2A and 2B are side cross-sectional views showing the configuration of a conventional optical transceiver 800 adapted for higher speeds.

[0034] (a) is a top view of the PCB 101 viewed from the board surface (x-y plane), and (b) is a cross-sectional side view (x-z plane) taken along line IIb-IIb through the Tx module 400. (b) is an enlarged view of the portion corresponding to the FPC 500-1 in FIG. 2A. (c) is a diagram showing simulation results for an FPC 500-1 having differential high-frequency lines 501 without bends 502 for adjusting a phase difference. (a) shows the model used in the simulation, (b) shows the simulation results for group delay, and (c) shows the calculated in-phase characteristics. 10A and 10B are diagrams showing simulation results for an FPC 500-1 having differential high-frequency lines 501 with bends 502 for adjusting a phase difference and a total length of approximately 400 μm, where (a) shows the model used in the simulation, (b) shows the simulation results for Group Delay, and (c) shows the calculated results for in-phase characteristics. 10C are diagrams showing simulation results for an FPC 500-1 having differential high-frequency lines 501 with bends 502 for adjusting a phase difference and a total length of approximately 450 μm, where (a) shows the model used in the simulation, (b) shows the simulation results for Group Delay, and (c) shows the calculated results for in-phase characteristics. Similar to FIG. 2 , this figure shows the results of comparing the in-phase characteristics of an FPC 500-1 having two bends with and without bends 502 for adjusting the phase difference, where (a) shows a simulation model when no bends 502 for adjusting the phase difference are formed in the differential high-frequency lines 501, (b) shows a simulation model when one bend 502 for adjusting the phase difference is formed in each of the p-side line 501a and the n-side line 501b, (c) shows the results of the in-phase characteristics when no bends 502 for adjusting the phase difference are formed in the differential high-frequency lines 501, and (d) shows the results of the in-phase characteristics when one bend 502 for adjusting the phase difference is formed in each of the p-side line 501a and the n-side line 501b.8A and 8B are diagrams showing an outline of the configuration of an optical transceiver 600 according to the present disclosure, where (a) is a top view and (b) is a cross-sectional view taken along the line VIIIb-VIIIb. Also, FIG. 8A is a diagram showing an outline of the configuration of an optical transceiver 700 according to the present disclosure, where (a) is a top view and (b) is a cross-sectional view taken along the line IXb-IXb. Also, FIG. 8B is a diagram showing the cross-sectional structure of a Tx module of an optical transceiver according to the present disclosure.

[0015] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. Numerical values ​​are for illustrative purposes only and are not intended to limit the technical scope of the present disclosure. Furthermore, the following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the spirit of an embodiment of the present disclosure. Furthermore, the drawings show an example of an embodiment 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 more or less than the number shown in the drawings.

[0016] The optical transceiver disclosed herein provides 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 a single optical module integrating the functions of both the optical receiver and the optical modulator. A flexible printed circuit (FPC) is used for the high-frequency connection. Both sides of the FPC electrically connect the pads on the top surface of the DSP substrate to the pads on the optical module package. The connection pads on one side of the FPC are connected to the DSP substrate, and the connection pads on the other side are connected to the optical module. The DSP may have a DSP chip mounted on the DSP substrate, and the entire DSP may be covered by a lid except for the area of ​​the DSP substrate that is connected to the FPC.

[0017] The present invention discloses detailed structures of the connection pad on the DSP board, the shape of the high-frequency wiring, the structure of the multilayer wiring board, and the FPC structure, in order to avoid 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 drawings.

[0018] For simplicity, the following description refers to the transmitting optical module containing an optical modulator and its driver IC as the "Tx module," and the receiving optical module containing a photodetector (PD) and TIA as the "Rx module." A single optical module integrating the functions of both a Tx module and an Rx module is referred to as a TRx module. The following description uses an optical transceiver containing an Rx module and a Tx module as an example, but these two modules can also be replaced with a single TRX module. While the following description focuses on the cross section of a Tx module, the connection between the DSP and optical module via FPC remains the same for an Rx module, which has a roughly symmetrical shape to the Tx module. Having similar pad shapes for the FPC between the DSP and Tx module and between the DSP and Rx module is efficient in terms of high-frequency design. It also reduces variations in solder mounting quality and connection strength throughout the optical module.

[0019] [Basic Configuration of Optical Transceiver] Figure 2 shows a top view and a cross-sectional view illustrating the overall configuration of an optical transceiver 100 according to the present disclosure. Figure 2(a) is a top view of the PCB 101, looking at the board surface (xy plane), and Figure 2(b) is a cross-sectional side view (xz plane) taken along line IIb-IIb passing through the Tx module 400. The optical transceiver 100 has a DSP 200, an Rx module 300, and a Tx module 400 mounted on the PCB 101.

[0020] Various other electrical and mechanical components, such as chip capacitors and resistors, will be mounted on the top and bottom surfaces of the PCB 101, but these are omitted from Fig. 1. In the optical transceiver 100, the heat dissipation surfaces of the DSP 200, Rx module 300, and Tx module 300 are aligned in one direction, taking into consideration manufacturability when mounting each component on the PCB 101 and simplification of the heat dissipation structure of the optical transceiver 100. Considering that the DSP chip 202 generates the most heat in the optical transceiver 100, it is desirable to align the heat dissipation direction upward (+z direction), which is appropriate for heat dissipation from the DSP chip 202.

[0021] 2B, in contrast to the configuration of the prior art, the optical transceiver 100 is mounted on the PCB 101 upside down, with the terrace side facing upward in the height direction (z-axis) of the optical module. The optical module is mounted with the terrace side facing the upper surface of the PCB, and in the case of the Tx module 400, heat is transferred from the driver IC and Peltier element toward the terrace side of the package (in the +z direction).

[0022] FIG. 10 is a diagram showing a cross-sectional structure within a Tx module in an optical transceiver according to the present disclosure. The Tx module 400 includes a Peltier element 406, a substrate 407, and an optical modulator chip 409 arranged in this order within a housing 417. Lenses 410 and 411 are provided on the substrate, which are optically coupled to an optical fiber 402. Within the module, a driver IC 408 is formed on the surface of a base 415, and wiring 412 is formed on a package base 416. The optical modulator chip 409 is connected to the wiring 412 via wires 413 and 414. The housing 417, package base 416, and package sidewall 405 separate the interior and exterior of the module. A portion of the package base 416 is exposed from the housing, forming a terrace surface of the optical module. An FPC 500-1 is connected to the terrace surface.

[0023] A Peltier element 406 for temperature control and a base 415 thermally connected to the driver IC, which is a heat-generating component, are fixed to the top surface of the inside of the housing 417. Heat is dissipated from inside the module in the +z direction, with heat moving from the driver IC and Peltier element to the outside. The Rx module also has a TIA instead of a driver IC, and heat from inside the module moves to the upper side of the module (+z direction).

[0024] As described above, the optical module is mounted with heat-generating components inside the package so that heat is dissipated toward the bottom surface, opposite the surface mounted on the PCB. Therefore, a heat dissipation mechanism (housing) with a single common heat dissipation surface (not shown) can be used above the optical transceiver 100 to dissipate heat from the DSP 200 and the Tx module 400. By arranging the optical module and DSP in close proximity and aligning the top surfaces of the DSP and optical module, a single heat dissipation mechanism can easily dissipate heat from the DSP and optical module. When using a common heat dissipation mechanism, careful consideration must be given to preventing heat from leaking from the DSP to the optical module. Furthermore, to prevent heat from leaking through the heat dissipation mechanism, separate heat dissipation mechanisms on the same surface can be used for the DSP and optical module.

[0025] The DSP 200 is composed of a DSP substrate 201, which is made up of a multilayer wiring substrate (described later), a DSP chip 202, and a lid 203. The DSP chip 202 and the DSP substrate 201 are connected by a BGA 204. Similarly, the DSP substrate 201 and the PCB 101 are connected by a BGA 205.

[0026] The lid 203 is made of a single piece of metal and covers at least the entire top surface of the DSP chip 202 and the top surface of the DSP substrate excluding the areas where the FPCs 500-1 and 500-2 are connected and fixed. The lid 203 may cover the side surfaces of the DSP chip 202 or may extend beyond the periphery of the DSP substrate 201. By carefully designing the shape of the lid 203, it is possible to leave the surface of the DSP substrate 201 open from the RF connection pad for connecting to the FPC to the edge of the substrate. In this way, the DSP 200 can be prepared as a DSP module with the DSP chip 202 and lid 203 mounted on the DSP substrate 201. The manufacturing process of the optical transceiver 100 will be described later, but if the DSP 200 is prepared in the form of a DSP module with a lid, it can be safely mounted on the PCB 101 without causing damage during the manufacturing process, just like the Rx module 300 and Tx module 400.

[0027] As another form of the DSP 200, it is also possible to prepare the DSP 200 without the lid 203, with only the DSP chip 202 mounted on the DSP substrate. The DSP 200 without the lid 203 can be connected to the optical modules 300 and 400 using FPCs 500-1 and 500-2, respectively, and then the lid 203 can be attached. When mounting the lid, there is a risk of applying inadvertent force to the FPC or causing the lid 203 to short-circuit the wiring of the FPCs 500-1 and 500-2. Therefore, it is necessary to thoroughly optimize the position of the pads in the area including the connection pads on the top surface of the DSP substrate 201 and the shape of the lid 203. Because the DSP chip and DSP substrate are exposed until just before the lid is mounted, extreme care must be taken to avoid damaging the DSP chip and other components.

[0028] When the DSP 200 includes a lid 203, the footprint of the lid 203 on the substrate surface (xy plane) is necessarily smaller than the footprint of the DSP substrate 201. On the other hand, the footprint of the DSP chip 202 is contained within the footprint of the lid 203. On the substrate surface of the DSP substrate 201, the lid 203 is absent only in the area where at least the RF connection pad is located.

[0029] If damage to the DSP chip 202 and the BGAs 204 and 205 can be avoided during the manufacturing process of the optical transceiver 100, the lid 203 may be omitted. To effectively dissipate heat from the DSP chip 202 to the outside of the optical transceiver, the lid 203 may be simplified and provided only on the top surface of the DSP chip 202. The shape of the lid 203 can be modified in various ways.

[0030] The Rx module 300 and the Tx module 400 are each housed in a package. The two optical modules 300, 400 are directly connected to the DSP 200 via FPCs 500-1, 500-2, which serve as an RF interface for electrical signals. Here, "directly connected" means that the connection pads on the terrace of the optical modules are connected to the connection pads on the DSP board only via the FPCs 500-1, 500-2. Therefore, in the optical transceiver 100, there are no high-frequency paths that pass through the vias connecting the optical modules to the PCB, the RF signal lines within the PCB, the BGAs connecting the PCB to the DSP board, or the vias within the DSP board. In contrast to optical transceivers that include conventional SMT-type packaged optical modules, the electrical signals in the optical transceiver 100 do not pass through the vias or BGAs, which can degrade high-frequency characteristics. The DSP and optical module can be connected via the shortest high-frequency path, significantly reducing high-frequency loss. Furthermore, because the number of VIA and BGA connections is reduced, the optical transceiver configuration shown in Figure 2 is also extremely effective in reducing high-frequency loss due to impedance mismatching that occurs at discontinuous parts of the high-frequency path.

[0031] 2, for simplicity, the differential high-frequency line 501 further includes a bend 502 for adjusting a phase difference. The bend 502 for adjusting a phase difference is a bend that is intentionally provided to eliminate the phase difference between the p-side line 501a and the n-side line 501b of the differential pair after bending the RF signal that propagates through each of the p-side line 501a and the n-side line 501b (a detailed description of the differential high-frequency line 501 and the bend 502 for adjusting a phase difference will be given later with reference to FIG. 3).

[0032] The direct current (DC) interface of the optical modules 300 and 400 can be, for example, DC lead pins 301 and 401 as shown in FIG. 2A. The DC interface is not limited to a lead pin structure, and an FPC may be used, similar to the RF interface with the DSP 200. The DC lead pins 301 and 401 are soldered to pads (not shown) on the PCB 101 so that power can be supplied from outside the optical transceiver.

[0033] Referring again to the top view of the optical transceiver 100 in Figure 2(a), the Rx module 300, Tx module 400, and DSP 200 are optimally positioned to minimize loss of the RF signal, a high-frequency electrical signal. The two modules and the DSP 200 are symmetrically positioned about the centerline along the longitudinal direction of the PCB 101. To achieve a high-speed optical transceiver, it is important to minimize high-frequency loss of the RF signal from the DSP 200 to the Tx module and from the Rx module to the DSP 200. The lengths of the FPCs 500-1 and 500-2, which function as RF interfaces, must each be as short as possible. In the optical transceiver 100 of Figure 2, to transmit high-speed signals exceeding 128 GBd, it is preferable to keep the distance (gap) between the two optical modules in the y-axis direction to 1 mm or less.

[0034] To place two optical modules close to each other, the DC lead pins must be grouped on one side of each module and oriented in opposite directions. As shown in Figure 2(a), the DC lead pins 401 of the Tx module 400 are oriented in the -y-axis direction (downward in the figure) toward the periphery of the PCB 101, while the DC lead pins 301 of the Rx module 300 are oriented in the +y-axis direction (upward in the figure) toward the periphery of the PCB 101. This configuration conforms to the lead pin orientation specified in the HB-CDM standardized by the Optical Fiber Forum (OIF) as shown in Non-Patent Document 1.

[0035] The packages of the optical modules 300 and 400 are made of ceramic. Considering the layout design of the high-frequency signal lines and DC lines within the optical modules 300 and 400, it is desirable that the height from the top surface of the PCB 101 to the terrace surfaces of the optical modules 300 and 400 in the cross-sectional view of Figure 2(b) be approximately 1 to 2 mm. Similarly, it is desirable that the thickness of the DSP substrate 201 be set in the range of approximately 1 to 2 mm, from the viewpoint of matching the height with the terrace surfaces of the optical modules 300 and 400.

[0036] If a DSP board has a thickness of about 1 to 2 mm, more layers than necessary for DC and RF lines can be used. Since minimizing the number of layers in a multilayer wiring board is typically desirable from a cost perspective, a core layer can be used internally to adjust the overall thickness of the board. Multilayer wiring boards that include such core layers are sometimes called build-up boards. In addition to adjusting the overall board thickness, the core layer also functions as a separation layer separating the RF wiring layer from the DC wiring layer. By providing a core layer, it is possible to significantly separate the DC and RF wiring layers, thereby reducing mutual interference and noise effects between the wiring layers.

[0037] Considering the need to increase the speed of DSP operation as described above, it is desirable to house the DSP 200 on a small DSP board 201 to reduce high-frequency signal loss and costs. The two FPCs 500-1 and 500-2 and the differential high-frequency transmission line 501 formed thereon should be configured to converge on the center line of the DSP 200 as they move from the optical modules 300 and 400 toward the DSP 200. As described below, the FPCs 500-1 and 500-2 shown in FIG. 2 are both pre-shaped to a predetermined shape and curved within the plane of the base material (x-y plane). While it is possible to make the FPCs straight without bending, this would require the PADs to be arranged in a fan-shaped fashion on the surface of the DSP board 201. This is undesirable because it increases high-frequency loss on the DSP board and increases the size of the DSP board.

[0038] Because the two FPCs 500-1 and 500-2 are curved in the plane (x-y plane) of the base material, the differential high-frequency line 501 formed thereon is also curved in the plane (x-y plane) of the base material to follow this, as shown in FIG. 2 . The differential high-frequency line 501 having such a shape has a problem in that a phase difference occurs between the p-side lines 501a, c, f, and h and the n-side lines 501b, d, e, and g that form the differential pair after the bend. Specifically, at the bend, the line located on the inside is shorter than the line located on the outside, causing the signal to advance, resulting in a phase difference between the signal propagating through the inside line and the signal propagating through the outside line. If signals are transmitted with this phase difference, it can lead to degradation of high-frequency characteristics and noise generation. More specifically, at the bends between the p-side lines 501a, c, f, h and the n-side lines 501b, d, e, g, a phase difference between the p-side lines 501a, c, f, h and the n-side lines 501b, d, e, g occurs. This causes signals to be transmitted with an unstable phase difference in the straight sections following the bends, resulting in degradation of the common-mode characteristics (Sdc21 and Scd21). Furthermore, while differential high-frequency characteristics (such as Sdd21 characteristics) are generally the primary focus in differential-drive optical transceivers, the common-mode characteristics represent noise. Increased noise due to such a phase shift in the differential pair also leads to degradation of transmission characteristics, so a configuration that suppresses the degradation of the common-mode characteristics is necessary. In particular, the impact of such phase differences becomes more pronounced when transmitting signals at speeds exceeding 128 GBd. Therefore, it is important to introduce a configuration that suppresses the degradation of the common-mode characteristics.

[0039] [Configuration of FPC and Differential High-Frequency Transmission Lines] FIG. 3 is an enlarged view of a portion corresponding to the FPC 500-1 in FIG. 2( a). While FIG. 3 illustrates the FPC 500-1, the FPC 500-2 may have a similar structure. As shown in FIG. 3, the differential high-frequency transmission lines 501 formed on the FPC 500-1 of the optical transceiver 100 according to the present disclosure further include a bend 502 for adjusting a phase difference on the inner (shorter) side of the bent portion that follows the FPC 500-1. This bend 502 for adjusting a phase difference compensates for the shortened line length of the inner transmission line and eliminates the phase difference that occurs after bending between the p-side transmission line 501a and the n-side transmission line 501b, and between the p-side transmission line 501c and the n-side transmission line 501d. Because propagation in a phase-shifted state is undesirable from a performance perspective, it is necessary to introduce such a phase difference compensation mechanism as early as possible immediately after bending.

[0040] For example, in order to eliminate the influence on the transmission characteristics, the in-phase characteristics (Sdc21 and Scd21) of each component (e.g., PCB 101, FPCs 500-1 and 500-2, DSP board 201, high-frequency packages constituting the optical module, etc.) must be configured to be at least -30 dB or less within the required operating frequency band (e.g., a frequency of about 0.6-0.7 times the baud rate). In order to achieve in-phase characteristics of -30 dB or less within the required frequency band, the phase difference adjustment bend 502 must be formed at a position within 1 / 2 the length of the guide wavelength at the maximum frequency of the required frequency band, from the bend that follows the FPC 500-1, based on the length when the phase adjustment bend 502 is not formed. If the length exceeds 1 / 2 the guide wavelength, significant degradation of characteristics due to phase difference occurs for the wavelength corresponding to that frequency, and it becomes impossible to achieve in-phase characteristics of -30 dB or less. Furthermore, to achieve very good in-phase characteristics, it is very effective to position the bend for phase difference adjustment closer to the bend, such as within 1 / 4 of the length. If the bend is within 1 / 4, moving it closer (for example, within 1 / 8) does not offer much improvement. Furthermore, the phase difference between the p-side line 501a and the n-side line 501b must be at least ±0.2 ps or less after the phase difference is eliminated by the bend for phase difference adjustment. A phase difference greater than this significantly degrades the in-phase characteristics, making it difficult to achieve desirable in-phase characteristics (below −30 dB). (Quantitative evaluation of the phase difference and in-phase characteristics will be described later with reference to FIGS. 4-7.)

[0041] On the other hand, forming the bend 502 for adjusting the phase difference increases the net length of the differential high-frequency line 501, which may result in adverse effects such as high-frequency signal loss. Therefore, the length of the bend 502 for adjusting the phase difference (the length increased by forming the bend 502 for adjusting the phase difference) must be kept within 1 / 2 of the guide wavelength at the maximum frequency of the required frequency band, from the perspective of not significantly affecting the differential high-frequency characteristics (Sdd21) for the wavelength of the maximum frequency. However, it goes without saying that a smaller value, such as 1 / 4 or less, is preferable from the perspective of not making any abnormalities in the characteristics of the differential pair due to the introduction of the bend 502 for adjusting the phase difference visible.

[0042] One effective measure to prevent the length of the bend 502 for adjusting the phase difference from becoming excessively long is to suppress the phase difference between the pn and n-channels at the bend that follows the FPC 500-1. One way to suppress the phase difference at the bend is to narrow the signal line width at the bend relative to the straight portion of the differential high-frequency line 501, thereby bending it more narrowly. However, narrowing the signal line width only at the bend tends to increase the impedance. Narrowing the signal line width only at the bend, compared to the signal line width of the straight portion where impedance is simply matched, can result in impedance mismatching and associated degradation of characteristics. In such a case, for example, if the FPC 500-1 has a GSSG or GSGSG configuration, the impedance mismatch can be resolved by shortening the distance between the G (ground) and S (signal) lines and increasing the impedance in the portion where the signal line width is narrow. In the case of an SS line configuration, since there are no GND patterns on both sides, such adjustment is not possible, and therefore, although it is a differential line configuration, it is undesirable.

[0043] On the other hand, even if only the FPCs 500-1 and 500-2 have good characteristics, poor characteristics of other elements, such as the DSP board 201, PCB 101, and optical modules 300 and 400, will result in degraded transmission characteristics in these elements. Therefore, it is desirable to adjust the phase difference by introducing a bending structure or the like for adjusting the phase difference so that the in-phase characteristics (Sdc21, Scd21) of not only the FPCs 500-1 and 500-2 but also the DSP board 201, PCB 101, and optical modules 300 and 400 are all −30 dB or less. In this way, when forming the phase adjustment bends 502 for each element of the DSP board 201, PCB 101, and optical modules 300 and 400, just as when the phase adjustment bends 502 are formed on the FPCs 500-1 and 500-2, the phase adjustment bends 502 need to be formed before or after the bend portion of the differential high-frequency line 501 (i.e., the portion where there is a difference in line length between the inside and outside) or within the range of the bend portion.

[0044] The above explanation has been about the phase difference between pn (between the p-side line 501a and the n-side line 501b) and the configuration for eliminating it (the bend 502 for adjusting the phase difference). However, considering the use of the optical transceiver, it is desirable to eliminate phase variations between channels, and it is desirable to equalize the lengths of the channels in, for example, the above-mentioned FPCs 500-1 and 500-2. Furthermore, it is desirable to equalize the lengths not only in the FPCs 500-1 and 500-2 but also in the DSP board 201, PCB 101, and optical modules 300 and 400.

[0045] [Simulation Verification Results] The results of a simulation verifying the elimination of the phase difference between pn by forming the bends 502 for adjusting the phase difference described above are described below with reference to FIGS. 4 to 7. In the simulation, an FPC 500-1 made of polyimide resin with metal formed on both sides of a base material 503 having differential high-frequency lines 501 was used as a model. Furthermore, the FPC 500-1 used as a model in the simulation was designed to support 150 GBd-class signals. In such a case, the common-mode characteristics (Sdc21 and Scd21) are required to be −30 dB or less for frequencies up to approximately 100 GHz.

[0046] 4 shows the results of a simulation (left half of the figure) for evaluating the amount of phase shift for one bent portion of an FPC 500-1 having differential high-frequency lines 501 without a bend 502 for adjusting a phase difference. (a) shows the model used in the simulation, (b) shows the simulation results for group delay, and (c) shows the results of the calculated in-phase characteristics. In a structure like the FPC 500-1, which has two bends and in which the angles of the first and second bends cancel each other out (the differential high-frequency lines before the first bend and after the second bend are parallel), the two bends on the input side and the output side eliminate the total phase difference, making the simulation impossible to see. Therefore, in order to be able to observe the phase difference due to the bend for phase adjustment, the model of the FPC 500-1 has a shape with only one bend, rather than a shape with two bends as shown in FIG. 2. This makes it possible to accurately grasp the occurrence of a phase difference due to bending. The total length of the differential high-frequency line 501 is approximately 6.5 mm on average (however, as mentioned above, in reality the line is shorter on the inner side). The model used in the simulation is one in which an RF signal is input from the right side of the drawing and transmitted toward the left side of the drawing. The signal line width is approximately 100 μm, and is adjusted so that the differential impedance is approximately 100 Ω±5%. Furthermore, the model is configured to include a ground 504 and a through-hole 505. The difference in group delay indicates the phase difference that is the point mentioned above.

[0047] 4B, when there is no bend 502 for adjusting the phase difference, the phase difference between pn (between the p-side line 501a and the n-side line 501b) is about 1 ps at a frequency of 100 GHz. Also, as shown in FIG. 4C, Sdc21 and Scd21 are −30 dB or more at about 12 GHz, and the in-phase characteristic at a frequency of 100 GHz is about −12.5 dB, which does not satisfy the above-mentioned requirement that the in-phase characteristic at a frequency of 100 GHz be −30 dB or less.

[0048] FIG. 5 shows the simulation results for an FPC 500-1 having a differential high-frequency line 501 with a bend 502 for adjusting a phase difference, the total length of which is approximately 400 μm. (a) shows the model used in the simulation, (b) shows the simulation results for group delay, and (c) shows the calculated in-phase characteristics. The simulation conditions were the same as those in FIG. 4, except for the formation of the bend 502 for adjusting a phase difference. The total length of the bend 502 for adjusting a phase difference (approximately 400 μm) is set to be shorter than ¼ of the guide wavelength (approximately 500 μm) at a frequency of 100 GHz, assuming that the relative dielectric constant of polyimide is 3.3. In this model, as an example, unlike FIG. 3, multiple small bends are inserted. As such, the number of bends does not necessarily need to be one; multiple bends may be provided. Naturally, when two bends are provided, the size of each bend tends to be smaller than in a structure with one bend. However, if multiple bends are made, the bends will be small and the bend angle will be acute, which may cause deterioration of high-frequency characteristics. Therefore, it is preferable to limit the number of additional bends for phase adjustment to one, and to make the bend angle gentler, whenever possible.

[0049] As shown in Figure 5(b), when a bend 502 for adjusting the phase difference with a total length of approximately 400 μm is formed, the phase difference at a frequency of 100 GHz is a value slightly exceeding 0.2 ps, and it can be seen that the phase difference is improved compared to the case where no bend for adjusting the phase difference is formed as shown in Figure 4. However, as shown in Figure 5(c), Sdc21 and Scd21 at a frequency of 100 GHz are approximately -23 dB, which does not satisfy the required in-phase characteristics (-30 dB or less).

[0050] 6 shows the results of a simulation of an FPC 500-1 having differential high-frequency lines 501 with bends 502 for adjusting a phase difference, the total length of which is approximately 450 μm, where (a) shows the model used in the simulation, (b) shows the simulation results for group delay, and (c) shows the results of the calculated in-phase characteristics. As in the case shown in FIG. 5, the simulation conditions were the same as those in FIG. 4, except that bends 502 for adjusting a phase difference were formed.

[0051] As shown in Figure 6(b), when a phase difference adjustment bend 502 with a total length of approximately 450 μm is formed, the phase difference at a frequency of 100 GHz is approximately 0 ps, ​​and it can be seen that the phase difference can be suppressed to approximately 0.1 to 0.15 ps in other frequency bands. Furthermore, if the phase difference between pn is suppressed to ±0.2 ps or less, as shown in Figure 6(c), Sdc21 and Scd21 at 100 GHz are approximately -30 dB, and it can be seen that sufficient in-phase characteristics are achieved within the required frequency band for the required baud rate. Note that in the simulation of Figure 6, the length of the phase difference adjustment bend (approximately 450 μm) is adjusted to be shorter than approximately 500 μm, which corresponds to 1 / 4 the length of the guide wavelength of FPC 500-1 at a frequency of 100 GHz. If the phase difference can be made closer to zero at all frequencies, further improvement in in-phase characteristics can be expected.

[0052] 2, Fig. 7 shows the results of comparing the in-phase characteristics of an FPC 500-1 having two bends with and without bends 502 for adjusting the phase difference. (a) shows a simulation model in which the differential high-frequency lines 501 do not have bends 502 for adjusting the phase difference. (b) shows a simulation model in which one bend 502 for adjusting the phase difference is formed in each of the p-side line 501a and the n-side line 501b. (c) shows the in-phase characteristics when the differential high-frequency lines 501 do not have bends 502 for adjusting the phase difference. (d) shows the in-phase characteristics when one bend 502 for adjusting the phase difference is formed in each of the p-side line 501a and the n-side line 501b. In (d), the phase difference near the bends and the shape and arrangement of the bends comply with the rules described above. 6(c) and 6(d), when the differential high-frequency line 501 does not have the bends 502 for adjusting the phase difference, the in-phase characteristics at a frequency of 100 GHz are approximately −22 dB for both Sdc21 and Scd21, whereas when one bend 502 for adjusting the phase difference is formed in each of the p-side line 501a and the n-side line 501b, both Sdc21 and Scd21 are approximately −40 dB. From these results, it can be seen that forming the bends 502 for adjusting the phase difference in the FPC 500-1 having two bends as shown in FIG. 2 can achieve sufficient in-phase characteristics in high-speed signal transmission exceeding 128 GBd.

[0053] In addition, in FIG. 7, the model shows that one bend 502 for adjusting the phase difference is formed on each of the p-side line 501 a and the n-side line 501 b. However, there is no limit to the number of bends 502 for adjusting the phase difference that can be formed, and any number of bends may be formed depending on the design.

[0054] In the model used in the simulation described above, for simplicity, the signal line width of the bent portion that follows the FPC 500-1 in the differential high-frequency lines 501 is constant, just like the straight portion. Also, although the ground 504 of the bend 502 for adjusting the phase difference has a constant shape, it is desirable to maintain a constant distance between the signal line (differential high-frequency lines 501) and the ground 504 in order to further stabilize the in-phase characteristics. Therefore, a configuration may be adopted in which a bend is added only to the portion of the ground 504 adjacent to the signal line (differential high-frequency lines 501) on which the bend 502 for adjusting the phase difference is formed, so that the distance between the signal line (differential high-frequency lines 501) and the ground 504 is constant in accordance with the bend 502 for adjusting the phase difference.

[0055] [Another form of FPC and differential high-frequency line] As described above, in order to improve the high-frequency characteristics of the optical transceiver, it is desirable to directly connect the FPCs 500-1 and 500-2 to the DSP board 201, but it is also possible to connect them via the PCB 201, as shown in Figures 8 and 9.

[0056] 8A and 8B are diagrams illustrating an outline of the configuration of an optical transceiver 600 according to the present disclosure, with (a) a top view and (b) a cross-sectional view taken along the VIIIb-VIIIb line. The optical transceiver 600 has a configuration in which the FPCs 500-1 and 500-2 are relatively short and have no bent portions to prevent performance degradation. In this optical transceiver 600, the FPCs 500-1 and 500-2 are linear when viewed from above. However, because pitch conversion is required for connection to the DSP board in the signal path, a bend must be formed in the portion of the differential high-frequency line 501 routed within the PCB 101. This bend can potentially cause performance degradation. Therefore, in the optical transceiver 600, the bend 502 for adjusting the phase difference described above must be formed in the portion of the differential high-frequency line 501 routed within the PCB 101. However, PCBs generally have higher manufacturing and design costs than FPCs, and it is difficult to change the design, etc. Therefore, it is desirable that the PCB 201 be formed only in straight lines without providing any bent structures.

[0057] FIG. 9 illustrates an outline of the configuration of an optical transceiver 700 according to the present disclosure, with (a) being a top view and (b) being a cross-sectional view taken along the line IXb-IXb. In the optical transceiver 700, pitch conversion and the resulting bends in the differential high-frequency lines 501 are performed in the FPCs 500-1 and 500-2. The portions of the differential high-frequency lines 501 routed within the PCB 201 are configured as straight lines without any bends. Generally, FPCs are less expensive to prototype than PCBs and are readily available, making it easier to modify the design of the differential high-frequency lines. Therefore, an advantage of FPCs is that they are easier to form the bends 502 for adjusting the phase difference than the optical transceiver 600. Considering these points, it is desirable to perform pitch conversion and the resulting bends in the differential high-frequency lines 501 in the FPCs 500-1 and 500-2, as in the optical transceivers 100 and 700.

[0058] Similarly, differential high-frequency lines having bends may be formed in the DSP board 201 and the optical modules 300 and 400. In this case, too, from a similar perspective, it is useful to introduce bends 502 for adjusting the phase difference in terms of improving characteristics.

[0059] As described above, the optical transceiver according to the present disclosure is configured so that the differential high-frequency lines have bent portions from the viewpoint of improving characteristics (reducing loss). In the above example, the FPC has also been described as having bent portions, but the FPC may have any shape as long as the differential high-frequency lines are configured to have bent portions.

[0060] Furthermore, the differential high-frequency lines may have any configuration, such as an SS line configuration, a GSSG configuration, a GSGSG configuration, an SGS configuration, a coplanar line configuration, a microstrip line configuration, or other similar configurations.

[0061] The optical transceiver according to the present disclosure can be used for optical communications.

[0062] 100 Optical transceiver 200 DSP 201 DSP board 202 DSP chip 203 Lid 300 Rx module (optical module) 301 DC lead pin 400 Tx module (optical module) 401 DC lead pin 501 Differential high frequency line 501a, c, f, h P-side line 501b, d, e, g N-side line 503 Base material 504 Ground 505 Through hole 600 Optical transceiver 700 Optical transceiver 800 Optical transceiver 802 DSP 804 DSP board 805 DSP chip 807 Transmission module 808 Optical fiber

Claims

1. 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 (FPC) connecting between the DSP and the optical module, or between the PCB and the optical module; and a differential high-frequency line in which a p-side line and an n-side line form a differential pair and which is formed so that signals can propagate inside a DPS board of the DSP, inside the optical module, and between the DSP and the optical module, wherein the differential high-frequency line arranged between the DSP and the optical module has at least one first bent portion.

2. The optical transceiver of claim 1, wherein the differential high-frequency lines further include at least one phase adjustment bend for eliminating a phase difference caused by the first bend, the phase adjustment bend being formed before or after a portion where a difference in line length occurs between the p-side line and the n-side line of the differential high-frequency lines, or within a range of the portion where a difference in line length occurs, and the optical transceiver 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. An optical transceiver as described in claim 2, wherein the phase adjustment bend is formed at a position within a length of 1 / 2 the guide wavelength at the maximum frequency of the required frequency band before and after a portion where a difference in line length occurs between the p-side line and the n-side line of the differential high-frequency line, and the length of the phase adjustment bend is within 1 / 2 the guide wavelength at the maximum frequency of the required frequency band.

4. The optical transceiver of claim 3, wherein the FPC connects between the PCB and the optical module, the differential high-frequency lines are further arranged within the PCB and configured so that the signal propagates between the optical module and the DSP via the differential high-frequency lines arranged within the PCB, and the first bend portion and the bend for phase adjustment are formed in the differential high-frequency lines arranged within the PCB, or the first bend portion and the bend for phase adjustment are formed in the differential high-frequency lines arranged on the FPC.

5. The optical transceiver according to claim 3, wherein the differential high-frequency lines arranged between the DSP and the optical module have a GSSG configuration or a GSGSG configuration, the line width of the first bent portion is narrower than the line width of a straight portion, and the distance between the ground and the differential high-frequency lines which are signals at the first bent portion is shorter than the distance between the ground and the differential high-frequency lines which are signals at the straight portion.

6. The optical transceiver according to claim 3, wherein the differential high-frequency lines arranged between the DSP and the optical module have a GSSG configuration or a GSGSG configuration, the distance between the ground and the differential high-frequency lines which are signals is constant at the first bent portion, and the differential high-frequency lines are configured so that the lengths of the channels are equalized.

7. The optical transceiver of claim 3, wherein the at least one optical module comprises an optical transmitter module including an optical modulator and a driver integrated circuit, and an optical receiver module including a photodetector and a transimpedance amplifier, or is a single optical transmitting and receiving module including the functions of both the optical transmitter module and the optical receiver module, the at least one optical module is mounted so that the terrace surface faces the upper surface of the PCB, and heat-generating components are mounted inside the package so that heat is dissipated toward the surface opposite to the mounting surface on the PCB, the FPC connects between the DSP and the optical module, and the bend for phase adjustment is formed in the differential high-frequency line arranged on the FPC.

8. The optical transceiver according to claim 7, wherein the DSP further comprises a lid including at least an area of ​​a DSP chip and covering an area of ​​the DSP board excluding the PAD.

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