Optical transmitter

Abstract

The present invention aligns the optical axes of a chip and a spatial optical system with each other and improves a radio frequency band. Provided is an optical transmitter in which a chip, to which a radio frequency signal is applied, is mounted on a sub-carrier on which radio frequency wiring is formed. This optical transmitter includes a career on which an optical component of a spatial optical system, which shares an optical axis with the chip, and the subcarrier are mounted, and a ground block that is inserted between the career and subcarrier and electrically connects the career to the subcarrier.

Description

Technical Field

[0001] The present invention relates to an optical transmitter that mounts a chip to which a high-frequency signal is applied and an optical component of a spatial optical system.

Background Art

[0002] As light sources for optical transmitters applied to next-generation ultra-high-speed optical networks, a directly modulated laser (DML) and an electro-absorption modulator integrated laser (EML) are known. The DML modulates the optical output by directly modulating the current injected into the semiconductor laser (see, for example, Non-Patent Document 1). The EML modulates the continuous (CW) light output from the semiconductor laser (LD) with an EA modulator. The EML has an advantage that a large extinction ratio can be obtained and the LD and the EA modulator can be individually optimized as compared with the DML. However, since the LD and the EA modulator are integrated on one chip (hereinafter referred to as an EML chip), the structure is complicated and the manufacturing process is also complicated.

[0003] Fig. 1 shows the structure of a conventional EML subassembly. Fig. 1(a) is a top view of a part of the EML subassembly, and Fig. 1(b) is a cross-sectional view along the high-frequency wiring. The EML subassembly 10 mounts an EML chip 12 on a sub-carrier 11 on which high-frequency wiring is integrated. Although not shown, the sub-carrier 11 is mounted with a PD for monitoring the optical signal intensity, a drive circuit for the LD, an RF circuit for driving and controlling the EA modulator, and the like. The EML chip 12 integrates a distributed feedback (DFB) laser and an EA modulator, and drive electrodes 12a and modulation electrodes 12b are formed on the upper surface of the chip, respectively. The high-frequency wiring is a coplanar line 13 in which grounds 13b and 13c are arranged on both side surfaces of the signal line 13a. The RF circuit for driving and controlling the EA modulator supplies a high-frequency signal to the modulation electrode 12b via the coplanar line 13 and the bonding wire 14.

[0004] FIG. 2 shows the structure of a conventional lens-mounted assembly. FIG. 2(a) is a top view of a part of the lens-mounted assembly, and FIG. 2(b) is a cross-sectional view along the high-frequency wiring. The lens-mounted assembly mounts the EML sub-assembly 10 shown in FIG. 1 and a lens holder 23 to which a lens 22 is fixed on a carrier 21. The output from the EA modulator integrated in the EML chip 12 is emitted to the outside through the lens 22. Therefore, in order to align the optical axes of the EA modulator and the lens, it is necessary to align the height from the upper surface of the carrier 21 to the center of the waveguide 12c of the EA modulator and the height from the upper surface of the carrier 21 to the center of the lens 22. Conventionally, since it is difficult to adjust by the height (thickness) of the EML chip 12, the height (thickness) of the sub-carrier 11 has been adjusted.

[0005] However, when the thickness of the sub-carrier 11 is increased, there is a problem that the high-frequency signal resonates with the substrate inside the sub-carrier 11, resulting in deterioration of the bandwidth as an EML. On the other hand, it is also conceivable to use a material with a low dielectric constant as the substrate of the sub-carrier 11 so as not to generate substrate resonance. However, it is desirable to use a material with the same coefficient of thermal expansion for the EML chip 12 and the sub-carrier 11 so that no stress is applied to the EML chip 12. When an InP substrate is used as the EML chip 12, it is necessary to use aluminum nitride as the material of the sub-carrier 11, and there is a problem that a material with a low dielectric constant cannot be selected.

Prior Art Documents

Non-Patent Documents

[0006]

Non-Patent Document 1

Summary of the Invention

[0007] An object of the present invention is to provide an optical transmitter that can freely set the height from a carrier to a waveguide of a chip in order to align the optical axes of the chip and optical components of a spatial optical system, and can improve a high-frequency band.

[0008] In order to achieve such an object, an embodiment of the present invention provides an optical transmitter having a chip to which a high-frequency signal is applied mounted on a sub-carrier on which a high-frequency wiring is formed, wherein the chip shares an optical axis with an optical component of a spatial optical system, a carrier on which the sub-carrier is mounted, and a ground block inserted between the carrier and the sub-carrier and electrically connecting the carrier and the sub-carrier.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

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DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0011] [First Embodiment] FIG. 3 shows the structure of the EML sub-assembly according to the first embodiment. FIG. 3(a) is a top view of a part of the EML sub-assembly, and FIG. 3(b) is a cross-sectional view along the high-frequency wiring. The EML sub-assembly 30 mounts an EML chip 32 on a sub-carrier 31 on which high-frequency wiring is integrated. The EML chip 32 integrates a DFB laser and an EA modulator, and drive electrodes 32a and modulation electrodes 32b are formed on the upper surface of the chip, respectively. The high-frequency wiring is a coplanar line 33 in which grounds 33b and 33c are arranged on both side surfaces of the signal line 33a. The RF circuit for driving and controlling the EA modulator supplies a high-frequency signal to the modulation electrode 32b via the coplanar line 33 and the bonding wire 34.

[0012] In the first embodiment, a ground-signless coplanar line is shown, but a microstrip line having no ground on both side surfaces of the signal line may also be used.

[0013] Fig. 4 shows the structure of the lens-mounted assembly according to the first embodiment. Fig. 4(a) is a top view of a part of the lens-mounted assembly, and Fig. 4(b) is a cross-sectional view along the high-frequency wiring. The lens-mounted assembly mounts, on a carrier 41, the EML sub-assembly 30 shown in Fig. 3 and a lens holder 43 to which a lens 42, which is an optical component of the free-space optical system, is fixed. The output from the EA modulator integrated in the EML chip 32 is emitted to the outside through the lens 22. Therefore, in order to align the optical axes of the EA modulator and the lens, it is necessary to make the height from the upper surface of the carrier 41 to the center (optical axis) of the waveguide 32c of the EA modulator the same as the height from the upper surface of the carrier 41 to the center (optical axis) of the lens 42.

[0014] In the first embodiment, the EML sub-assembly 30 is mounted on the carrier 41 via a ground block 44, and the height (thickness) of the ground block 44 is adjusted to align the heights of the center of the waveguide 32c and the center of the lens 42. The thickness of the EML chip 32 is 150 μm, the thickness of the sub-carrier 31 is 150 μm, and the height from the upper surface of the carrier 41 to the center of the lens 42 is 700 μm. The waveguide 32c of the EA modulator integrated in the EML chip 32 is an embedded waveguide, but its thickness is about two digits thinner compared to the chip scale and can be regarded as being on the upper surface of the EML chip 32. Therefore, the thickness of the ground block 44 is set to 400 μm.

[0015] The sub-carrier 31 is made of aluminum nitride, a material having the same coefficient of thermal expansion as the EML chip 32. The ground block 44 is made of kovar and has its surface gold-plated. The ground block 44 electrically connects the sub-carrier 31 and the carrier 41 and can provide a low-impedance ground connection and a sufficient heat dissipation path for the EML chip 32. The sub-carrier 31 and the ground block 44 are joined using solder with a small height tolerance.

[0016] For example, in the case of an EML chip using an InP substrate, in order to suppress the resonance of high-frequency signals in the substrate, it is desirable that the thickness of the sub-carrier be 250 μm or less. Therefore, according to the first embodiment, there is no need to increase the thickness of the sub-carrier 31, the resonance of high-frequency signals in the substrate is suppressed, and the band degradation as an EML can be suppressed. In addition, since the sub-carrier 31 made of a material having the same coefficient of thermal expansion as the EML chip 32 can be used, the influence of stress on the EML chip 32 can be suppressed, and the band degradation can be suppressed.

[0017] Fig. 5 shows the frequency response characteristics of the lens-mounted assembly of the first embodiment. The lens-mounted assembly shown in Fig. 4 and the conventional lens-mounted assembly shown in Fig. 2 were fabricated, and their frequency response characteristics were compared. The sub-carrier 11 of the conventional lens-mounted assembly is made of aluminum nitride and has a thickness of 550 μm.

[0018] In the conventional lens-mounted assembly, the 3 dB band was about 31 GHz, but in the lens-mounted assembly of the first embodiment, the 3 dB band could be improved to 37 GHz. For example, when applied to an ultra-high-speed optical network with a baud rate of 50 Gbaud of a modulation signal, a band of 35 GHz or higher, which is about 0.7 times the baud rate, is required. The conventional lens-mounted assembly cannot meet this requirement, but the lens-mounted assembly of the first embodiment can realize an optical transmitter that meets this requirement.

[0019] According to the first embodiment, the height from the carrier to the output waveguide of the EML chip can be freely set, and the high-frequency band of the EML can be improved.

[0020] [Second Embodiment] Fig. 6 shows the structure of the lens-mounted assembly according to the second embodiment. In the first embodiment, the EML as the light source of the optical transmitter was described as an example, but in the second embodiment, the sub-assembly of the optical modulator alone will be described as an example. A Mach-Zehnder interferometer type modulator (MZM) is used as the optical modulator.

[0021] Fig. 6(a) is a top view of a part of the lens-mounted assembly, and Fig. 6(b) is a cross-sectional view along the high-frequency wiring. The MZM sub-assembly 50 mounts the MZM chip 52 on the sub-carrier 51 on which the high-frequency wiring is integrated. Although not shown, the sub-carrier 51 is mounted with a PD for monitoring the optical signal intensity, an RF circuit for driving and controlling the MZM, and the like. The MZM chip 52 has two arm waveguides as a Mach-Zehnder interferometer, and a modulation electrode 52a formed on one of the arm waveguides is formed on the upper surface of the chip. The high-frequency wiring is a coplanar line 53 in which grounds 53b and 53c are arranged on both side surfaces of the signal line 53a. The RF circuit for driving and controlling the MZM supplies a high-frequency signal to the modulation electrode 52b via the coplanar line 53 and the bonding wire 54.

[0022] In addition, also in the second embodiment, although the groundless coplanar line is shown, a microstrip line having no grounds on both side surfaces of the signal line may be used.

[0023] The lens-mounted assembly mounts the above-described MZM sub-assembly 50 and a lens holder 63 to which a lens 62, which is an optical component of the spatial optical system, is fixed on the carrier 61. The output from the output waveguide of the MZM is emitted to the outside through the lens 62. Therefore, in order to align the optical axes of the MZM and the lens, it is necessary to make the height from the upper surface of the carrier 61 to the center (optical axis) of the waveguide 52c of the MZMEA the same as the height from the upper surface of the carrier 61 to the center (optical axis) of the lens 62.

[0024] In the second embodiment, an MZM sub-assembly 50 is mounted on a carrier 61 via a ground block 64, and the height (thickness) of the ground block 64 is adjusted to align the height of the center of the waveguide 52c with the center of the lens 62. The thickness of the MZM chip 52 is 150 μm, the thickness of the sub-carrier 51 is 250 μm, and the height from the upper surface of the carrier 61 to the center of the lens 62 is 800 μm. The waveguide 52c of the MZM chip 52 is an embedded waveguide, but its thickness is about two orders of magnitude thinner compared to the chip scale and can be regarded as being on the upper surface of the MZM chip 52. Therefore, the thickness of the ground block 64 was set to 400 μm.

[0025] The sub-carrier 51 is made of aluminum nitride, a material with the same coefficient of thermal expansion as the MZM chip 52. The ground block 64 uses an alumina substrate with gold vapor-deposited on the upper surface, lower surface, and side surfaces, and has a structure in which the upper and lower surfaces are electrically conductive. The ground block 64 electrically connects the sub-carrier 51 and the carrier 61 and can provide a low-impedance ground connection and a sufficient heat dissipation path for the MZM chip 52. The sub-carrier 51 and the ground block 64 are joined using solder with a small height tolerance.

[0026] For example, in the case of an MZM chip using an InP substrate, in order to suppress the resonance of the high-frequency signal with the substrate, it is desirable to make the thickness of the sub-carrier 250 μm or less. Therefore, according to the second embodiment, it is not necessary to increase the thickness of the sub-carrier 51, the resonance of the high-frequency signal with the substrate is suppressed, and the bandwidth degradation as an MZM can be suppressed. In addition, since the sub-carrier 51 made of a material with the same coefficient of thermal expansion as the MZM chip 52 can be used, the influence of stress on the MZM chip 52 can be suppressed, and the bandwidth degradation can be suppressed.

[0027] Fig. 7 shows the frequency response characteristics of the lens-mounted assembly of the second embodiment. A lens-mounted assembly shown in Fig. 6 and a conventional MZM sub-assembly without a ground block mounted thereon were fabricated in the same manner as in the first embodiment, and their frequency response characteristics were compared. The sub-carrier of the conventional lens-mounted assembly is made of aluminum nitride and has a thickness of 650 μm.

[0028] The 3dB bandwidth of the conventional lens-mounted assembly was about 30 GHz, but the 3dB bandwidth of the lens-mounted assembly of the second embodiment could be improved to 36 GHz. For example, when applied to an ultra-high-speed optical network with a modulation signal baud rate of 50 Gbaud, a bandwidth of 35 GHz or more, which is about 0.7 times the baud rate, is required. The conventional lens-mounted assembly cannot meet this requirement, but the lens-mounted assembly of the first embodiment can realize an optical transmitter that meets this requirement.

[0029] According to the second embodiment, the height from the carrier to the output waveguide of the MZM chip can be freely set, and the high-frequency band of the MZM can be improved.

[0030] In the first and second embodiments, the EML chip and the MZM chip are described as examples of the chips mounted on the carrier, but the present invention is not limited thereto. The present embodiment can be applied to an optical transmitter that mounts a chip that shares the optical axis with an optical component of a spatial optical system including a lens, such as the above-described DML chip. The ground block inserted between the carrier and the sub-carrier facilitates the optical axis alignment between the chip and the optical component of the spatial optical system and can improve the high-frequency band of the chip. In addition, a low-impedance ground connection and a sufficient heat dissipation path can be provided for the chip.

Claims

1. An optical transmitter having a chip to which a high-frequency signal is applied mounted on a sub-carrier on which a high-frequency wiring is formed, an optical component of a spatial optical system sharing an optical axis with the chip, and a carrier on which the sub-carrier is mounted, and a ground block inserted between the carrier and the sub-carrier to electrically connect the carrier and the sub-carrier. The optical transmitter is characterized by comprising the above.

2. The optical transmitter according to claim 1, characterized in that the ground block has a thickness that makes the height of the optical axis of the chip and the height of the optical axis of the optical component the same.

3. The optical transmitter according to claim 1, characterized in that the high-frequency wiring is a microstrip line or a ground-signal-ground coplanar line.

4. The optical transmitter according to claim 1, 2, or 3, characterized in that the chip is an electro-absorption modulator integrated laser, a Mach-Zehnder interferometer optical modulator, or a directly modulated laser.

5. The optical transmitter according to claim 4, characterized in that the modulation signal baud rate of the electro-absorption modulator integrated laser and the Mach-Zehnder interferometer optical modulator is 50 Gbaud or more.

6. An InP substrate is used for the chip, The optical transmitter according to any one of claims 1 to 5, characterized in that the sub-carrier is made of aluminum nitride and has a thickness of 250 μm or less.

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