Optical circuit

Abstract

Disclosed is a new form of installation of a capacitor for cutting off a bias voltage in an optical circuit comprising an optical chip that includes an optical modulator, a wiring board that supplies a high-frequency electric signal, and a connecting board that connects these elements. The wiring board and an optical modulation light source chip are mounted on a subcarrier. The wiring board and the optical modulation light source chip are connected by the connecting board, and a high-frequency electric signal is inputted from outside of the optical circuit to a modulation input electrode. The connecting board is provided with a termination resistor on a first surface that is connected to two boards by gold bumps. One electrode of the termination resistor is connected to the modulation-input-electrode-side end of a transmission line. The other electrode of the termination resistor is connected to a high-impedance line. The high-impedance line is formed continuously from a side surface of the connecting board to a second surface that is the reverse side of the first surface, the high-impedance line being connected to the capacitor.

Description

Technical Field

[0001] The present invention relates to an apparatus used in a network. Specifically, it relates to an optical circuit applicable to high-speed Ethernet or the like.

Background Art

[0002] In recent years, in order to meet the strong bandwidth demand accompanying the spread of mobile and cloud services, the study of high-speed and large-capacity networks has been active. Wireless communication has entered the 5G era, and the transmission speed of Ethernet, which is widely used, has already been commercialized at 400 Gbps, and Beyond400G Ethernet is also under study. In optical circuits such as optical transceiver modules for optical fiber transmission, performance improvement, miniaturization, and cost reduction are required.

[0003] In the Ethernet standard, a miniaturized transceiver (optical transceiver) has been standardized, and an optical circuit including an optical modulator has become an important device. In order to adapt an optical transmitter to high-speed transmission, flip-chip mounting including an optical chip and a high-frequency signal substrate has been proposed as a mounting structure suitable for high-speed operation.

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In an optical circuit including an optical modulator that uses a conventional flip-chip implementation, the implementation of capacitors required for applying a bias voltage has been a problem in terms of miniaturization and reliability of the optical circuit.

Means for Solving the Problems

[0006] One embodiment of the present invention includes a wiring board that receives an external high-frequency electrical signal, a light source chip that includes an optical modulator, a sub-carrier on which the wiring board and the light source chip are mounted, and a first surface having a transmission path connecting between a signal line of the wiring board and a modulation input terminal of the light source chip, and a connection board having a termination resistor at one end on the modulation input side of the transmission path, and the connection board further includes a capacitor connected to ground in series with the termination resistor.

Effects of the Invention

[0007] Provided is an optical circuit including an optical modulator suitable for high-speed operation and miniaturized and highly reliable.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0009] The optical circuit of the present disclosure includes an optical chip including an optical modulator, a wiring board that supplies a high-frequency electrical signal, and a connection substrate that connects between them, and provides a novel implementation form of a capacitor for blocking a bias voltage. In the following description, first, problems in the implementation of the capacitor in an optical circuit according to the structure of a flip-chip implementation of the prior art will be described. Next, a novel configuration for capacitor implementation in the optical circuit including the optical modulator of the present disclosure will be described.

[0010] FIG. 1 is a diagram showing the configuration of an optical circuit of the prior art including an optical modulation light source chip. The optical circuit 1 in FIG. 1 is a component in a module form that can be mounted on a transceiver standardized in Ethernet, and includes an optical modulation light source chip 40 that is an electro-absorption modulator integrated laser (EML). On the sub-carrier 10, a wiring board 20 and an optical modulation light source chip 40 are mounted, and the wiring board 20 and the optical modulation light source chip 40 are connected by a connection substrate 30.

[0011] In FIG. 1, (a) shows an overall top view (x-y plane) of the optical circuit 1, (c) shows a cross-sectional view (x-z plane) of the optical circuit 1 cut along the C-C' line, and (b) shows the back surface (x-y plane) of the connection substrate 30 mounted on the optical circuit 1. (a) in FIG. 1 shows only four gold bumps between the connection substrate 30 and the outer shape (two-dot chain line) of the connection substrate 30 to show the connection form with the optical modulation light source chip 40. It should be noted that in the actual optical circuit 1, only the substrate surface or the ground surface is visible in the top view of (a) in FIG. 1 depending on the form of the transmission line.

[0012] The optical modulation light source chip 40 includes a laser section using an optical waveguide structure 42 formed on an optical semiconductor, and an electro-absorption type optical modulator section. A high-frequency electrical signal, which is a modulation signal, is supplied to a modulation electrode 41 of the optical modulation light source chip 40 via a wiring board 20 and a connection substrate 30. The wiring board 20 includes signal lines 21 formed on a substrate and ground planes 22a and 22b on both sides thereof, and constitutes a transmission path. The wiring board 20 functions as a high-frequency wiring board that receives a high-frequency electrical signal from the outside and transmits it without loss.

[0013] Referring to FIG. 1(b), the connection substrate 30 includes a transmission path 31 and a ground plane 35 surrounding the periphery thereof on a surface (connection surface) connected by solder bumps. When a coplanar line is used as the transmission path, the upper surface of FIG. 1(a), i.e., the opposite side of the connection surface, is in a state where the substrate material appears as it is. When a grounded coplanar line is used as the transmission path, the opposite side of the connection surface becomes a ground plane.

[0014] One end of the transmission path 31 is connected to the signal line 21 of the wiring board 20 via a solder bump 32a. Also, the other end of the transmission path 31 is connected to the modulation input electrode 41 of the optical modulation light source chip 40 via a solder bump 32b. The modulation signal, which is a high-frequency electrical signal, is input to the modulation input electrode 41 in the direction of the arrow from the outside of the optical circuit 1 above FIG. 1(a) via the signal line 21 of the wiring board 20 and the transmission path 31 of the connection substrate 30. Also, the ground planes of the wiring board 20 and the connection substrate 30 are electrically and mechanically connected by two solder bumps on both sides of the solder bump 32a. Such a mounting form that electrically and structurally connects two different substrates 20 and 40 using the facing connection substrate 30 and bumps is known as flip-chip mounting. The structure by flip-chip mounting using the connection substrate 30 does not require wires for connection between the wiring board 20 and the optical modulation light source chip 40, which is useful for broadening the bandwidth of the optical modulation characteristics.

[0015] However, in the prior art optical circuit including the optical modulation light source chip 40 of FIG. 1, a capacitor for bias voltage cut-off that is originally required for efficient operation is omitted, resulting in a simplified circuit configuration. Referring again to FIG. 1(b), at the end of the modulation input electrode 41 side of the transmission line 31 of the connection substrate 30, a termination resistor 34 is mounted between the ground plane 35 so as to form a parallel circuit with the modulation input terminal. Usually, it is necessary to apply a bias voltage to the modulation input terminal 41 of the optical modulation light source chip 40. When a bias voltage is applied to the optical modulation light source chip 40, the bias voltage is also applied to the termination resistor 34 at the same time. Therefore, the power consumption of the optical circuit steadily increases by the amount of the direct current flowing through the termination resistor 34.

[0016] Normally, in order to suppress the direct current power consumption due to the bias voltage in an electric circuit, the bias path may be directly blocked by a capacitor for direct current cut-off. Specifically, a capacitor is mounted on the sub-carrier 10 and the termination resistor 34 and the capacitor are electrically connected by wire bonding or the like, and the current flowing through the termination resistor 34 can be blocked. However, in the optical circuit including the optical modulation light source chip, miniaturization has already advanced significantly, and the available space on the sub-carrier 10 is already limited. If a space for mounting the capacitor is secured on the sub-carrier, the size of the optical circuit as a sub-assembly will increase, which will go against the requirement for miniaturization of the optical circuit. In addition, the process of directly performing wire bonding on the connection substrate 30 for connection with the capacitor causes a problem that the connection between the connection substrate 30 and the gold bumps 32a, 32b is broken due to the physical impact by the bonding tool.

[0017] The optical circuit of the present disclosure described below provides a novel capacitor mounting structure that suppresses the direct current power consumption in the above-described optical modulation circuit and simultaneously satisfies the requirements for miniaturization and reliability. In the optical circuit of the present disclosure, a capacitor for direct current cut-off is mounted on the connection substrate. With this configuration, it is not necessary to connect between the capacitor and the termination resistor by wire bonding or the like, and breakage of the electrical connection via the gold bumps does not occur.

[0018] FIG. 2 is a diagram showing the configuration of the optical circuit of the present disclosure including an optical modulation light source chip. The optical circuit 100 in FIG. 2 has a form of a sub-assembly that can be mounted on a substrate such as an Ethernet transceiver or an optical transmission device. In FIG. 2, (a) shows a top view (x-y plane) of the entire optical circuit 100, (c) shows a cross-sectional view (x-z plane) of the optical circuit 100 cut along the C-C' line, and (b) shows the back surface (x-y plane) of the connection substrate 50 mounted on the optical circuit. Similar to the prior art optical circuit 1 shown in FIG. 1, on the sub-carrier 10, a wiring board 20 and an optical modulation light source chip 40 are mounted in the optical circuit 100. The wiring board 20 and the optical modulation light source chip 40 are connected by the connection substrate 50, and the high-frequency electrical signal is input from the outside of the optical circuit 100 to the modulation input electrode 41, which is also the same as the optical circuit 1. Also, the mutual connection form in the transmission path (signal line) and the ground from the wiring board 20 to the light source chip 40 via the connection substrate 50 is the same as that in FIG. 1.

[0019] The difference from the optical circuit 1 of the prior art structure shown in FIG. 1 lies in the configuration of the connection substrate 50 shown in FIGS. 2(b) and 2(c). The connection substrate 50 is provided with a termination resistor 54 on the first surface connected to the two substrates 20 and 40 by gold bumps 52a, 52b, and 53. One electrode of the termination resistor 43 is connected to the end of the modulation input electrode 41 side of the transmission path 51. The other electrode of the termination resistor 54 is connected to the high-impedance line 55a. The high-impedance line 55a continues and is formed on the second surface, which is the back surface of the first surface, through the side surface of the connection substrate 50, and is connected to the capacitor 57.

[0020] FIG. 3 is a diagram for explaining in more detail the configuration of the connection substrate in the optical circuit of the present disclosure. (a) of FIG. 3 is a top view (x-y plane) of the second surface of the connection substrate 50, (b) is the first surface (connection surface) (x-y plane) of the connection substrate 50, and (c) is a side view of the short side of the connection substrate (y-z plane). Referring to FIG. 3(b), one electrode of the termination resistor 54 is connected to the end of the modulation input electrode 41 side of the transmission line 51. To the other electrode of the termination resistor 54, a high-impedance line 55a is connected, the distance to the ground planes on both sides of which is larger than that of the transmission line 51. The high-impedance line 55a extends to the high-impedance line 55b on the side surface shown in (c), and further to the high-impedance line 55c on the second surface shown in (b), and is connected to the electrode pad of the capacitor. A capacitor 57 is mounted on the second surface of the connection substrate 50, and one electrode pad is connected to the ground.

[0021] In the above-described connection substrate 50, the termination resistor 54, the high-impedance lines 55a to 55c, and the capacitor 57 are connected in series, and as an electric circuit, this series circuit is inserted between the modulation input terminal and the ground. With this configuration, when a bias voltage is applied to the optical modulation light source chip 40, the capacitor 57 in series with the termination resistor 54 can cut off the current. The DC power consumption due to the bias current flowing through the termination resistor can be suppressed. Further, since the capacitor 57 is mounted on the connection substrate 50, there is no need to connect the capacitor 57 and the termination resistor 54 by wire bonding or the like. Breakage of the electrical connection portion formed via the gold bumps does not occur due to the impact of the bonding tool.

[0022] A further feature of the above-described connection substrate configuration is that high-impedance lines 55a to 55c are arranged between the terminating resistor 54 and the capacitor 57. The high-impedance lines 55a to 55c are set at a large distance from the ground plane and have an impedance higher than the characteristic impedance of the transmission line 51 and the optical circuit 100. Therefore, series resonance occurs between the inductive line and the capacitor 57, thereby enabling the generation of a desired peaking in the modulation frequency response characteristics. The capacitor 57 and the high-impedance lines 55a to 55c can be three-dimensionally arranged on the connection substrate 50, contributing to the miniaturization of the optical circuit.

[0023] Therefore, the optical circuit of the present disclosure can be implemented as an optical circuit including a wiring board that receives a high-frequency electrical signal from the outside, a light source chip including an optical modulator, a sub-carrier on which the wiring board and the light source chip are mounted, and a connection substrate having a transmission line connecting between the signal line of the wiring board and the modulation input terminal of the light source chip on a first surface, and having a terminating resistor at one end on the modulation input side of the transmission line. The connection substrate further includes a capacitor connected in series to the terminating resistor and grounded.

[0024] The connection substrate 50 uses aluminum nitride having the same value as the thermal expansion coefficient of an InP substrate generally used in a modulation light source chip 40. For this reason, it has a structure in which stress caused by environmental temperature fluctuations is not applied to the chip connection portion. Hereinafter, as Example 1 of the optical circuit of the present disclosure, a specific configuration example will be shown.

Example

[0025] According to the structure of the connection substrate shown in FIGS. 2 and 3, an optical circuit of Example 1 including an optical modulation light source chip (light source chip) was fabricated. This optical circuit 100 has a connection substrate 50 shown in detail in FIG. 3 and is in the form of a sub-assembly that can be mounted on a substrate such as an Ethernet transceiver. The optical modulation light source chip 40 is an electro-absorption type optical modulator integrated laser in which an optical semiconductor modulator including an optical waveguide structure is integrated, and the electrode length of the electro-absorption type optical modulator (EA modulator) is 75 μm. The optical modulation light source chip 40 uses an InP substrate. The material of the connection substrate is aluminum nitride. The width of the transmission line 31 of the connection substrate 30 is 0.08 mm, and the distance d from the center line along the length direction of the transmission line 31 to the ground electrode 35 is 0.08 mm. The thickness T of the connection substrate 30 is set to 0.15 mm or more so as not to affect the characteristic impedance when the characteristic impedance of the transmission line 31 is 50 Ω.

[0026] The capacitance of the mounted capacitor 57 was set to 100 nF. To compare the modulation characteristics, an optical circuit according to the prior art configuration shown in FIG. 1 was also fabricated. The bias current of the laser section of the optical modulation light source chip 40 was set to 80 mA, and the bias voltage of the EA modulator was set to -1.4 V. In all the fabricated optical circuits, the configurations of the gold bumps 32a, 32b, and 33 used for the connection between the connection substrate 50, the wiring board 20, and the optical modulation light source chip 40 had a diameter of 60 μm and a height of 30 μm.

[0027] FIG. 4 is a diagram showing a comparison of the modulation frequency characteristics of the optical circuits of each configuration of Example 1 and the prior art. The modulation frequency (GHz) is plotted on the horizontal axis, and the frequency response of the modulation output characteristics is shown in dB, normalized at the level near DC. In the optical circuit with the prior art configuration, the 3 dB bandwidth was 54 GHz, whereas in the optical circuit with the configuration of Example 1, an equivalent 3 dB bandwidth of 54 GHz was obtained. In addition, in the modulation frequency characteristics of the configuration of Example 1, the peaking level can be increased by up to 1 dB compared to the case of the prior art configuration. In the conventional optical circuit, the current value of the EA bias was -39 mA and the total power consumption was 0.0546 W, whereas in the optical circuit of Example 1, the EA bias current was -11 mA and the total power consumption was 0.0154 W. Compared with the case where the capacitor of the prior art was omitted, the power consumption could be reduced to 1 / 3 or less.

[0028] From the modulation frequency characteristics in FIG. 4, it was confirmed that the optical circuit with the configuration of Example 1 can obtain a bandwidth almost the same as that of the prior art configuration and a larger peaking, and is effective for broadening the bandwidth of the optical modulation light source. In addition, since no capacitor is mounted on the sub-carrier, it conforms to the trend of miniaturization of the optical circuit and is also useful for suppressing power consumption. In the next Example 2, another configuration example in which a capacitor is mounted on a connection substrate is shown for an optical circuit including another light source chip including a Mach-Zehnder optical interferometric modulator (MZ modulator) instead of the EA modulator.

Example

[0029] FIG. 5 is a diagram showing the configuration of Example 2 of the optical circuit of the present disclosure including an optical modulation light source chip. The optical circuit of Example 2 has a form of a sub-assembly 200 that can be mounted on a package such as inside an Ethernet transceiver or an optical transmission device. In FIG. 5, (a) shows an overall top view (x-y plane) of the optical circuit 200, (c) shows a cross-sectional view (x-z plane) of the optical circuit 200 cut along the line C-C´, and (b) shows the back surface (x-y plane) of the connection substrate 60 on which the capacitors mounted on the optical circuit 200 are mounted. The optical circuit 200 of Example 2 uses an optical modulator chip (light source chip) 50 that includes only a Mach-Zehnder optical interferometric modulator (MZ modulator) 53 without including a light source, instead of the optical modulation light source chip 40 of Example 1.

[0030] As can be seen from the cross-sectional view of FIG. 5(c), the MZ modulator 53 configured in the optical modulator chip 50 has a two-arm waveguide structure. A modulation input electrode 52 (P-side electrode), which is an input terminal for a high-frequency electrical signal, is formed on one arm waveguide, and a P-side electrode 51 for phase adjustment is formed on the other arm waveguide.

[0031] The wiring board 20 and the optical modulator chip 50 mounted on the sub-carrier 10 are connected by the connection substrate 60, and a high-frequency electrical signal is input from the outside of the optical circuit 200 to the modulation input electrode 52. The connection form of the transmission path (signal line) from the wiring board 20, through the connection substrate 60 to the optical modulator chip 50 and the ground substrate and chip is the same as that of Example 1.

[0032] As shown in FIG. 3, in the connection substrate 50 of Example 1, the capacitor 57 was mounted on the second surface opposite to the first surface (connection surface) on which the transmission path was formed. Different from Example 1, in Example 2, the capacitor 66 of the connection substrate 60 is mounted on the first surface (connection surface) connected to the wiring board 20 and the optical modulator chip 50 by gold bumps 62a, 62b, and 63.

[0033] In the connection substrate 60, the termination resistor 64, the high-impedance line 65, and the capacitor 66 are connected in series, and as an electric circuit, this series circuit is inserted between the modulation input terminal and the ground. With such a configuration, when a bias voltage is applied to the optical modulator chip 50, the capacitor 66 in series with the termination resistor 64 can cut off the bias current.

[0034] To compare the modulation frequency characteristics of the optical circuit of Example 2 with the configuration of the prior art, an optical circuit using an optical modulator chip with the capacitor omitted as shown in FIG. 1 was created. FIG. 6 is a diagram showing the configuration of an optical circuit with a prior art configuration using an MZ modulator. In FIG. 6, (a) shows the top view (x-y plane) of the entire optical circuit 2, (c) shows the cross-sectional view (x-z plane) of the optical circuit cut along the C-C' line, and (b) shows the back surface (x-y plane) of the connection substrate 30 mounted on the optical circuit. Compared with the configuration of Example 2 in FIG. 5, in the connection substrate 30, the termination resistor 34 is directly connected to the ground 35, and a current constantly flows through the termination resistor 34 due to the bias voltage applied to the modulation input electrode 52.

[0035] The optical modulator chips 50 in FIGS. 5 and 6 are Mach-Zehnder modulators (MZ modulators), and the electrode length of the MZ modulator is 100 μm. The optical modulator chip 50 uses an InP substrate. The capacitance of the capacitor 66 mounted in the optical circuit of Example 2 in FIG. 5 is 10 nF. The input optical power to the optical modulator chips 50 in FIGS. 5 and 6 is +8 dBm, and the bias voltage of the MZ modulator is -1.5V.

[0036] The material of the connection substrate 60 is aluminum nitride, the width of the transmission line 31 of the connection substrate 60 is 0.08 mm, and the distance d from the center line along the length direction of the transmission line 61 to the ground electrode 35 is 0.08 mm. The thickness T of the connection substrate 60 is set to 0.15 mm or more so as not to affect the characteristic impedance when the characteristic impedance of the transmission line 61 is 50 Ω.

[0037] FIG. 7 is a diagram showing a comparison of the modulation frequency characteristics of each configuration of the optical circuit in Example 2 and the prior art. The modulation frequency (GHz) is plotted on the horizontal axis, and the frequency response of the modulation output characteristics is normalized at the level near DC and shown in dB on the vertical axis. In the optical circuit 2 with the prior art configuration shown in FIG. 6, the 3 dB bandwidth was 57 GHz, whereas in the optical circuit 200 with the configuration of Example 2 shown in FIG. 5, an equivalent 3 dB bandwidth of 57 GHz was obtained. In addition, in the modulation frequency characteristics of the optical circuit 200 with the configuration of Example 2, the peaking level can be made 1 dB or more higher than in the case of the prior art configuration.

[0038] In the optical circuit 2 with the prior art configuration, the bias current value of the MZ modulator was -35 mA and the total power consumption was 0.0525 W, whereas in the optical circuit 200 of Example 2, the bias current of the MZ modulator was -5 mA and the total power consumption was 0.0075 W. Compared with the case of the optical circuit with the configuration omitting the capacitor of the prior art, the power consumption due to the bias current could be reduced to 1 / 7.

[0039] From the modulation frequency characteristics in FIG. 7, it was confirmed that the optical circuit with the configuration of Example 2 can obtain a bandwidth almost the same as that of the prior art configuration and a larger peaking, which is effective for broadening the bandwidth of the optical modulation light source. In addition, since no capacitor is mounted on the sub-carrier, it is in line with the trend of miniaturization of the optical circuit, and the power consumption can also be suppressed.

[0040] As described above, the optical circuit of the present disclosure realizes an optical circuit including an optical modulator suitable for high-speed operation, miniaturized and highly reliable.

Industrial Applicability

[0041] The present invention can be used in network devices for optical communication.

Claims

1. A wiring board for receiving a high-frequency electrical signal from the outside, A light source chip including an optical modulator, A sub-carrier on which the wiring board and the light source chip are mounted, A connection board having a transmission line connecting between a signal line of the wiring board and a modulation input terminal of the light source chip on a first surface, and having a termination resistor at one end on the modulation input side of the transmission line and comprising The connection board further includes a capacitor connected to the ground in series with the termination resistor, an optical circuit.

2. The optical circuit according to claim 1, wherein the capacitor is mounted on the same first surface as the termination resistor via a high-impedance line.

3. The optical circuit according to claim 1, wherein the capacitor is connected via a high-impedance line extending from the termination resistor through a side surface of the connection board to a second surface opposite to the first surface, and is mounted on the second surface.

4. The light source chip uses an InP substrate, The material of the connection board is aluminum nitride and the thickness is 0.15 mm or more, the optical circuit according to any one of claims 1 to 3.

5. The optical circuit according to any one of claims 1 to 4, wherein the light source chip is an electric field absorption type optical modulator or a Mach-Zehnder interference type optical modulator.

6. The optical circuit according to any one of claims 1 to 5, wherein the signal line and the transmission line, and the transmission line and the modulation input terminal are respectively connected by bumps.

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