Optical Module

Abstract

Provided is an optical module that causes no degradation of high frequency characteristic to occur. An optical module according to the present disclosure comprises: a printed circuit board (PCB); a DSP that is mounted on the PCB; a high frequency package that is mounted on the PCB and that includes an optical transmission element, an optical modulation element or an optical reception element; and an FPC that electrically connects the signal pad of the PCB to the signal pad of the high frequency package and that electrically connects the ground pad of the PCB to the ground pad of the high frequency package. The optical module has a structure in which: the thicknesses of portions of the board where the ground pads of the PCB and of the high frequency package are arranged are thinner than the thickness of a portion of the board where each of the signal pads of the PCB and of the high frequency package is arranged; and the thickness of the ground pad of the FPC is thicker than the thickness of the signal pad of the FPC.

Description

[Technical Field]

[0001] The present disclosure relates to an optical module including an optical transmitter, an optical modulator, and an optical receiver. [Background technology]

[0002] In recent years, the demand for communication traffic has increased, resulting in a demand for high-speed optical modules compatible with advanced optical modulation methods. In the field of optical communications, the introduction of digital signal processing technologies, including digital coherence, into optical fiber communication systems has established backbone network transmission technologies of 100 Gbps per wavelength (equivalent to 32 GBd operation), and this has led to rapid increases in the speed of optical communications. Currently, advances in digital signal processing technology have brought high-speed optical communications systems of 400-600 Gbps per wavelength (equivalent to 64 GBd operation) to a practical level.

[0003] In early 100G digital coherent systems, each component (e.g., a driver IC and optical modulator chip in the case of a modulator, and a transimpedance amplifier (TIA) and optical receiver chip in the case of a receiver) was individually packaged and then mounted on a printed circuit board (PCB). However, as optical communication speeds increased (e.g., in systems exceeding 400G), wider bandwidth (specifically, modulation bandwidth of 40 GHz or more) was required for optical modules, which necessitated the reduction of high-frequency loss. Accordingly, it became necessary to integrate the driver IC and light source chip in an optical transmitter, the driver IC and optical modulator chip in a modulator, and the TIA and optical receiver chip in a receiver into the same package.

[0004] Furthermore, since the realization of such systems exceeding 400G, designs based on differential operation have become commonplace, rather than single-ended operation, in order to achieve higher speeds, smaller size, and lower power consumption. Currently, with the further expansion of communication traffic demand, 800Gbps and 1Tbps (12 Discussions have begun on systems that will achieve even higher speeds, such as 8GBd operation, and technologies that will broaden the bandwidth and reduce high-frequency loss are required.

[0005] Until now, surface mount technology (hereinafter referred to as SMT) has generally been used for high-frequency packages in optical modules for digital coherence (e.g., HB-CDM (High Bandwidth-Coherent Driver Modulator) and HB-ICR (High Bandwidth-Intradyne Coherent Receiver)). However, high-frequency packages with an SMT structure require vias and lead pins to transmit high frequencies, which are known to cause degradation of high-frequency characteristics. For this reason, a new package structure that does not cause degradation of high-frequency characteristics is required. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2015-146515 A Summary of the Invention

[0007] The present disclosure has been made in view of the above-mentioned problems, and an object of the present disclosure is to provide an optical module having a new package structure that does not cause degradation of high-frequency characteristics.

[0008] In response to the above-described problems, the present disclosure provides an optical module comprising: a PCB; a DSP mounted on the PCB; a high-frequency package mounted on the PCB and including an optical transmitting element, an optical modulation element, or an optical receiving element; and an FPC that electrically connects the signal pads of the PCB to the signal pads of the high-frequency package and electrically connects the ground pads of the PCB to the ground pads of the high-frequency package, wherein the thickness of the substrate at the locations where the PCB and the ground pads of the high-frequency package are arranged is thinner than the thickness of the substrate at the locations where the signal pads of the PCB and the high-frequency package are arranged, and the thickness of the ground pads of the FPC is thicker than the thickness of the signal pads of the FPC. [Brief explanation of the drawings]

[0009] [Figure 1] 1A and 1B are diagrams illustrating the structures of an optical modulator or an optical receiver according to the first embodiment of the present disclosure and the prior art, where FIG. 1A shows the structure of the first embodiment of the present disclosure and FIG. 1B shows the structure of the prior art. [Figure 2] 2A, 2B, and 2C are diagrams conceptually showing cross sections of the high-frequency transmission line portions of the PCB 11, the high-frequency package 13, and the FPC 14, respectively. [Figure 3] 3A, 3B, and 3C are diagrams conceptually showing cross sections of the connection pad portions of the PCB 11, the high-frequency package 13, and the FPC 14, respectively. FIG. 3A shows a cross section of the PCB 11, FIG. 3B shows a cross section of the high-frequency package 13, and FIG. 3C shows a cross section of the FPC 14. [Figure 4] 10 is a cross-sectional view illustrating the structure of the connection pad portions of the PCB 11 and the FPC 14 after connection according to the first embodiment of the present disclosure. FIG. [Figure 5] 5A and 5B are cross-sectional views showing the structures of the connection pad portions of a PCB 51 and a high-frequency package 52 according to a second embodiment of the present disclosure, where FIG. 5A is a cross-sectional view of a PCB 51 and FIG. 5B is a cross-sectional view of a high-frequency package 52. [Figure 6]FIG. 10 is a plan view showing the structure of a PCB 60 according to a third embodiment of the present disclosure. [Figure 7] 10A and 10B illustrate the structure of a PCB 70 having ground pads 71a and 71b incorporating alignment marks 72a and 72b according to a third embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] 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. Materials and numerical values ​​are for illustrative purposes only and are not intended to limit the technical scope of the present disclosure. 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 gist of one embodiment of the present disclosure.

[0011] The optical modulator or optical receiver according to the present disclosure uses a package structure that uses a flexible printed circuit (hereinafter referred to as FPC) instead of the SMT mounting method used in conventional technology. This makes it possible to suppress the degradation of high-frequency characteristics caused by vias and lead pins, and enables compatibility with optical communication systems that are faster than conventional ones.

[0012] Furthermore, the optical modulator or optical receiver according to the present disclosure achieves high alignment by employing a fitting structure at the connection pads between the substrate (PCB) and the FPC, and between the high-frequency package in which the optical modulation element or optical receiving element is packaged and the FPC, making it possible to prevent wiring shorts due to misalignment.

[0013] Additionally, in the optical modulator or optical receiver according to the present disclosure, at least a portion of the ground plane included in the PCB laminate substrate has a cutout structure. Furthermore, in the ground pads and signal pads included in the connection pad portions of the PCB, high-frequency package, and FPC, the surfaces where the ground pads and signal pads face each other have a meander structure. This allows for greater flexibility in impedance design.

[0014] (First embodiment) Hereinafter, a first embodiment of the present disclosure will be described in detail with reference to the drawings. An optical modulator or an optical receiver according to this embodiment relates to a form in which an FPC is used as a mounting technique, whereas SMT is used in conventional techniques.

[0015] 1A and 1B are diagrams illustrating the structures of an optical modulator or an optical receiver according to a first embodiment of the present disclosure and a conventional technology, with FIG. 1A showing the structure of the first embodiment of the present disclosure and FIG. 1B showing the structure of the conventional technology. As shown in FIG. 1A, an optical modulator or an optical receiver 10 according to the first embodiment of the present disclosure includes a PCB 11 serving as a substrate, a digital signal processor (hereinafter referred to as DSP) 12 mounted on the PCB 11, high-frequency wiring 111 mounted on the PCB 11, a high-frequency package 13 including an optical modulation element or an optical reception element, and an FPC 14 electrically connecting the high-frequency wiring 111 and the high-frequency package 13. Although not shown in FIG. 1, the PCB 11, the high-frequency package 13, and the FPC 14 have transmission lines (see FIG. 2, described below) and connection pads (see FIG. 3, described below), and all connection points are joined with solder. In addition, in FIG. 1, as an example, a form in which a DSP 12 for controlling an optical module is mounted on the same PCB 11 is illustrated, but the DSP 12 does not necessarily have to be mounted on the same PCB 11.

[0016] 1(b), the optical modulator or optical receiver 100 according to the prior art does not include an FPC 14, but rather electrically connects the PCB 11 and the high-frequency package 13 by applying a lead pin structure 101. As described above, the optical modulator or optical receiver 10 according to the first embodiment of the present disclosure does not use mounting with the lead pin structure 101, and therefore has the characteristic that no degradation of high-frequency characteristics occurs due to vias, lead pins, etc., and can also support a band of 70 GHz or higher, which is faster than conventional bands.

[0017] The material used for the core substrate of the FPC 14 (for example, the core substrate 141 described below) can be liquid crystal polymer, fluororesin, or polyimide. In particular, when prioritizing broadband performance, it is desirable to use liquid crystal polymer or fluororesin, which have excellent properties and can suppress loss. Furthermore, the connection between the FPC 14 and PCB 11 is preferably soldered. This is because the connection in this area requires high conductivity and bonding strength, and is also compact. Additionally, because the pitch between the signal pads and ground pads is narrow, short circuits can occur with welding materials other than solder. For this reason, it is most desirable to use solder resist and take measures to prevent short circuits. The solder used for bonding can be solder with a melting point of 230°C or lower.

[0018] FIG. 2 is a conceptual diagram showing cross sections of the high-frequency transmission line portion of the PCB 11, the high-frequency package 13, and the FPC 14, with FIG. 2(a) showing a cross section of the PCB 11, FIG. 2(b) showing the high-frequency package 13, and FIG. 2(c) showing the cross section of the FPC 14. For example, in FIG. 2, the differential line configuration is ground-signal-signal-ground (GSSG), but it may also be a GSGSG configuration. Also, while FIG. 2 shows a differential operation-based structure, it may also be a single-ended drive configuration. However, as mentioned above, 2 Considering application to high speed operation such as 8 GBd operation, GSSG or GSGSG with a differential line configuration is preferable from the viewpoint of high speed operation and crosstalk.

[0019] Furthermore, if there is a conversion from GSSG to GSGSG between each component (PCB 11, high-frequency package 13, etc.), the shape of the propagation mode changes, which can result in degradation of high-frequency characteristics. Therefore, it is desirable that the PCB 11, high-frequency package 13, and FPC 14 all have the same line configuration. Hereinafter, all differential line configurations described in this specification will be GSSG as an example.

[0020] 2(a), the PCB 11 includes a laminated substrate 113 in which core substrates 111a and 111b, ground planes 111a and 111b are laminated, and ground pads 114a and 114b and signal pads 115a and 115b are arranged on the surface of the laminated substrate 113. On the other hand, as shown in FIG. 2(b), the high-frequency package 13 includes a laminated substrate 133 in which dielectric layers 131a and 131b made of ceramic or the like and ground planes 132a and 132b are laminated, and ground pads 134a and 134b and signal pads 135a and 135b are arranged on the surface of the laminated substrate 133. On the other hand, as shown in FIG. 2(c), the FPC 14 includes a laminated substrate 143 in which a core substrate 141 and a ground plane 142 are laminated, ground pads 144a and 144b and signal pads 145a and 145b are arranged on the surface of the laminated substrate 143, and a coverlay 146 that covers them. Note that, although FIG. 2 shows an example in which there are no ground vias (not shown), in reality, ground vias are periodically arranged in the propagation direction of the high-frequency signal at a pitch that is sufficiently smaller than the propagation wavelength in order to electrically connect the ground pads (e.g., ground pad 134a, ground pad 144a, etc.) to the ground planes (e.g., ground plane 132a, ground plane 142, etc.).

[0021] For simplicity, each laminated substrate is depicted with 2-4 layers in Fig. 2, but in reality, the number of layers may be more than this, and the thickness of each layer may be set as desired. In addition, Fig. 2 depicts one channel, but in actual optical modulators and receivers, there are I and Q channels, and polarization in the X and Y directions, so a total of four channels are integrated.

[0022] 3A and 3B are diagrams conceptually illustrating cross sections of the connection pad portions of the PCB 11, the high-frequency package 13, and the FPC 14. FIG. 3A illustrates a cross section of the PCB 11, FIG. 3B illustrates a cross section of the high-frequency package 13, and FIG. 3C illustrates a cross section of the FPC 14. As shown in FIGS. 3A and 3B, the connection pad portions of the PCB 11 and the high-frequency package 13 have a structure in which the thicknesses of the laminated substrates 113 and 133 at the locations where the ground pads 114a, 114b and the ground pads 134a, 134b are disposed are thinner (concave) than the thicknesses of the locations where the respective signal pads (signal pads 115a, 115b and signal pads 135a, 135b) are disposed. With this structure, if the signal pads 145a, 145b of the FPC 14 are convex, the connections between the connection pad portions of the PCB 11 and the FPC 14 and the high-frequency package 13 and the FPC 14 are interlocked, enabling more accurate alignment than with conventional technology.

[0023] Alternatively, the ground pads 114a and 114b of the PCB 11, the ground pads 134a and 134b of the high-frequency package 13, and the core layer 111a where they are located may all be removed, and the outermost surfaces of these portions may become the ground planes 112a and 132a. In this case, the ground planes 112a and 132a function as ground pads.

[0024] Generally, if the pad capacitance of the connection is large, it becomes difficult to transmit high-frequency signals, so minimizing pad capacitance is important for transmitting high-speed signals. 1 2For optical communication systems transmitting high-speed signals such as 8 GBd, the width of each signal pad (the length in the direction in which the ground pads are arranged) must be at least 300 μm or less. Considering factors such as solder connectivity and manufacturing variability, a width of approximately 100-200 μm is desirable. It is particularly important to reduce the size of the pads on PCBs and high-frequency packages, which have high dielectric constants. Furthermore, from the perspective of high-frequency characteristics, and considering the suppression of crosstalk, it is desirable for the channel pitch to be 1 mm or more, and for the width of the ground pad (the length in the direction in which the signal pads are arranged) to be at least 1.5 times the width of the signal pad.

[0025] On the other hand, if the size of the pads is reduced in this way, misalignment is more likely to occur when connecting the PCB 11 and the FPC 14, and the high-frequency package 13 and the FPC 14. This increases the risk of short-circuiting the wiring patterns, so high-precision alignment is required. The optical module 10 according to the first embodiment of the present disclosure, which is equipped with the PCB 11, the high-frequency package 13, and the FPC 14, achieves high alignment by virtue of this fitting structure, while also achieving high accuracy. 2 This enables support for high-speed optical communications equivalent to 8GBd operation.

[0026] In FIG. 3, the interlocking structure is depicted as being achieved by making the ground pads 144a, b arranged on the FPC 14 thicker than the signal pads 145a, b, but the interlocking structure may also be achieved by adjusting the thickness of the solder used for joining.

[0027] FIG. 4 is a cross-sectional view illustrating the structure of the connection pad portions of the PCB 11 and the FPC 14 according to the first embodiment of the present disclosure after connection. The connection pad portions of the PCB 102 and the FPC 104 according to the first embodiment of the present disclosure are connected by solder 41. As described above, in the PCB 11 according to the first embodiment of the present disclosure, the thickness of the laminated substrate 113 is thin in the portion where the ground pads 114a and 114b are arranged. Therefore, when connected to the FPC 14, the volume of the ground pads covering the connection portions between the signal pad 115a and the signal pad 145a, and between the signal pad 115b and the signal pad 145b (the connection portion between the ground pad 114a and the ground pad 144a, and the connection portion between the ground pad 114b and the ground pad 144b) is larger than that of the conventional technology. This reduces inter-channel crosstalk and achieves better high-frequency characteristics. Although FIG. 4 shows the connection between the PCB 11 and the FPC 14 as an example, the same can be said for the connection between the high-frequency package 13 and the FPC 14.

[0028] As described above, the optical module 10 according to the first embodiment of the present disclosure electrically connects the PCB 11 and the high-frequency package 13 using the FPC 14, thereby suppressing degradation of high-frequency characteristics due to vias at the connection. In addition, unlike conventional technology, the connection is achieved by a mating structure, which provides high alignment. Therefore, by reducing the size of each pad (ground pad or signal pad), the pad capacitance can be suppressed, while the risk of wiring shorts due to misalignment or the like can be reduced.

[0029] (Second embodiment) A second embodiment of the present disclosure will be described in detail below with reference to the drawings. In this embodiment, the optical module has a structure in which at least a part of the ground plane in the laminated substrate of the PCB and the high-frequency package has a cutout structure with a hole in the center.

[0030] FIG. 5 is a cross-sectional view showing the structure of each connection pad portion of a PCB 51 and a high-frequency package 52 according to a second embodiment of the present disclosure, with FIG. 5(a) being a cross-sectional view of the PCB 51 and FIG. 5(b) being a cross-sectional view of the high-frequency package 52. As shown in FIG. 5, the basic structure of the PCB 51 and the high-frequency package 52 according to this embodiment is the same as that of the PCB 11 and the high-frequency package 13 shown in FIG. 3, except that some of the ground planes (ground plane 511a and ground plane 521a in FIG. 5) have a cutout structure in which the center of the ground plane is partially removed. This structure makes it possible to increase the impedance of differential signals while preventing the common-mode impedance value from becoming too high. Therefore, an optical module incorporating the PCB 51 and the high-frequency package 52 has the advantage of increasing the degree of freedom in impedance design.

[0031] However, the cutout area is set so as not to extend to the area directly below the ground pads (ground pads 114a, b, ground pads 134a, b) arranged on the surface layer, because if the cutout extends to the area directly below the ground pads arranged on the surface layer, the common-mode impedance will also increase, which may result in degradation of high-frequency characteristics.

[0032] 5, ground planes 511a and 521a have cutout structures as an example, but if the number of layers in laminated substrates 113 and 133 increases, the number of ground planes with cutout structures may be determined arbitrarily depending on the impedance matching condition. However, it is preferable that the ground planes arranged on the bottom layer (ground plane 111b and ground plane 132b in FIG. 5) do not have cutout structures because electromagnetic fields may leak and spread.

[0033] In this way, by forming at least a portion of the ground plane into a cutout structure with a central cutout, it is possible to improve the impedance of differential signals and, conversely, suppress the common-mode impedance. In other words, the optical module in which the PCB 51 and the high-frequency package 52 according to this embodiment are mounted is compatible with higher-speed optical communications than the prior art, while providing greater flexibility in impedance design.

[0034] (Third embodiment) Hereinafter, a third embodiment of the present disclosure will be described in detail with reference to the drawings. The optical module according to this embodiment relates to a configuration in which the ground pad and the signal pad have a meander structure, thereby increasing the degree of freedom in designing impedance.

[0035] FIG. 6 is a plan view showing the structure of a PCB 60 according to a third embodiment of the present disclosure. FIG. 6 depicts the PCB 60 as viewed from above. As shown in FIG. 6, the PCB 60 according to this embodiment includes meandering ground pads 61a, 61b and signal pads 62a, 62b on the top surface of a laminated substrate 113. The meandering structure is introduced only on the surfaces where the ground pads and signal pads face each other, and not on the end surfaces (surfaces without opposing ground or signal pads). While FIG. 6 illustrates a PCB as an example, meandering ground pads and signal pads may also be arranged on high-frequency packages and FPCs.

[0036] In the optical module including the ground pads 61a, b and signal pads 62a, b according to this embodiment, capacitance can be added to the common-mode side, thereby suppressing common-mode impedance. However, when the ground pads and signal pads are arranged in an S-to-S configuration, such as in a GS-SG configuration, where S and S are opposed to each other, a meander structure is not introduced between the SS pads. This is because introducing a meander structure between the SS pads would also add capacitance to the differential impedance, reducing the differential impedance. Furthermore, depending on the shape of the meander structure, introducing a meander structure that affects the differential signal side (main signal) could affect the high-frequency signal. For this reason, it is preferable to introduce a meander structure only between the GS pads.

[0037] In the optical module according to this embodiment having such a configuration, it is conceivable to introduce alignment marks in order to provide even higher alignment performance.

[0038] FIG. 7 is a diagram showing the structure of a PCB 70 having ground pads 71 ​​a, 71 b with alignment marks 72 a, 72 b according to a third embodiment of the present disclosure. Similar to FIG. 6, FIG. 7 shows a top view of the PCB 70. As shown in FIG. 7, the ground pads 71 ​​a, 71 b included in the PCB 70 according to this embodiment further include alignment marks 72 a, 72 b on their end surfaces. While FIG. 7 shows a PCB as an example, ground pads and signal pads with alignment marks may also be arranged on high-frequency packages and FPCs.

[0039] In the optical module including the ground pads 71a, 71b and the signal pads 73a, 73b according to this embodiment, which is configured as described above, high alignment can be achieved and wiring shorts can be prevented because the module can be mounted while being aligned based on the alignment marks 72a, 72b. However, if the implementation of alignment marks increases the implementation space, this may affect the high-frequency characteristics. For this reason, it is preferable to implement alignment marks only on the outermost ground pads.

[0040] In this embodiment, the above-mentioned alignment marks 72a, b are described as being introduced into a ground pad having a meander structure, but the same effect can be achieved even if they are introduced into a ground pad that does not have a meander structure, as described in the prior art or the first and second embodiments. [Industrial Applicability]

[0041] As described above, the optical module disclosed herein connects components such as PCBs and high-frequency packages with FPCs, thereby suppressing degradation of high-frequency characteristics due to vias and enabling higher-speed optical communications than conventional technologies. Furthermore, by reducing the thickness of the laminated substrate where the ground pads are located, a mating structure is achieved, providing high alignment and preventing short circuits in the wiring pattern due to misalignment. Additionally, by providing a cutout structure for a portion of the ground plane or by providing a meander structure on each surface where the ground pads and signal pads face each other, the degree of freedom in impedance design is increased. The optical module disclosed herein, which offers these advantages, is expected to be applied to optical communication systems with higher speeds than conventional systems, such as 800 Gbps and 1 Tbps.

Claims

1. An optical module, PCB and a DSP mounted on the PCB; a high-frequency package mounted on the PCB and including an optical transmitting element, an optical modulating element, or an optical receiving element; an FPC that electrically connects the signal pads of the PCB to the signal pads of the high-frequency package and electrically connects the ground pads of the PCB to the ground pads of the high-frequency package; Equipped with a substrate of the PCB is thinner at a portion where the ground pads of the PCB are arranged than at a portion where the signal pads of the PCB are arranged, and a substrate of the high frequency package is thinner at a portion where the ground pads of the high frequency package are arranged than at a portion where the signal pads of the PCB are arranged; The thickness of the ground pads of the FPC is greater than the thickness of the signal pads of the FPC. Optical module.

2. 2. The optical module according to claim 1, wherein the PCB, the high-frequency package, and the FPC have a differential operation-based structure in which the ground lines (G) and the signal lines (S) are arranged in the order GSSG or GSGSG.

3. The width of the signal pad in the direction in which the ground pads are arranged is 100 μm or more and 300 μm or less, the width of the ground pad in the direction in which the signal pads are arranged is 1.5 times or more the width of the signal pads; The channel pitch is 1 mm or more.

3. The optical module according to claim 1.

4. An optical module described in any one of claims 1 to 3, having a cutout structure in which at least a portion of the ground plane is cut out in the range from the center to the area directly below the ground pad.

5. 5. The optical module according to claim 1, wherein the surfaces of the signal pad and the ground pad facing each other have a meander structure.

6. The optical module according to claim 1 , wherein the outermost ground pads of the ground pads further include alignment marks.

7. 7. The optical module according to claim 1, wherein the FPC is made of a material selected from the group consisting of a liquid crystal polymer and a fluororesin.

8. An optical module described in any one of claims 1 to 7, wherein the connection pad portion is joined by solder having a melting point of 230°C or less.

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