Optical transmitter

Abstract

This optical transmitter (10) comprises a Peltier element (16), an optical modulator (13), and a driver integrated circuit (IC) (12) that supplies a modulation electrical signal for the optical modulator, wherein the optical modulator and the driver IC are mounted in a flip-chip manner while facing down toward the upper surface of the Peltier element. An optical modulator chip that includes the optical modulator, and the driver IC are mounted in a flip-chip manner on the upper surface of a subcarrier (14) disposed on the upper surface of the Peltier element. The subcarrier is formed using aluminum nitride (AlN), the optical modulator chip is formed using InP, and an underfill material (17) having a thermal conductivity of 3 W / (mk) or more is filled in a gap between the subcarrier and the optical modulator chip and the driver IC. The temperature of the Peltier element is controlled at 25°C to 50°C inclusive.

Description

【Technical Field】 【0001】 The present disclosure relates to an optical transmitter used in optical communication. More specifically, it relates to an implementation form of an optical transmitter including a semiconductor optical modulator and its driver IC. 【Background Art】 【0002】 To cope with the rapid increase in traffic in communication networks, digital coherent optical transmission that combines a coherent communication method and digital signal processing technology has been introduced into optical fiber communication systems. Starting from the establishment of the initial backbone network transmission technology of 100 Gbps per wavelength, currently, transmission at a higher speed of 400 - 600 Gbps per wavelength has been put into practical use. 【0003】 In the above-mentioned digital coherent optical transmission, an optical transceiver device in which an optical receiver and an optical transmitter are integrated is used. In optical transceiver devices of systems with a transmission capacity exceeding 400 Gbps, broadbanding of analog components such as high-frequency (RF) electrical circuits is required. For example, in an optical modulator, a modulation bandwidth of 40 GHz or more is necessary. For reducing high-frequency loss leading to broadbanding and miniaturizing the device, for example, on the transmission side, a form in which an RF driver IC and an optical modulator are mounted in an integrated package has attracted attention. This implementation form of the optical transmitter is named High-Bandwidth Coherent Driver Modulator (HB-CDM: High-Speed Driver Integrated Optical Modulator) and is also standardized by the Optical Internetworking Forum (OIF) (Non-Patent Document 1). On the receiving side of the optical transceiver device, a transimpedance amplifier (TIA) and an optical receiver are also mounted in an integrated package and are also called an Integrated Coherent Receiver (ICR). 【0004】 Turning our attention to materials for optical transceiver devices, semiconductor-based optical modulators are attracting attention as an alternative to conventional lithium niobate (LN) optical modulators, due to their miniaturization and cost-reduction potential. For faster modulation operations, compound semiconductors such as InP are primarily used. Furthermore, in systems where miniaturization and cost reduction are crucial, research and development are concentrated on Si-based optical devices. 【0005】 Even in the semiconductor optical modulators mentioned above, there are material-specific advantages and disadvantages. For example, in InP optical modulators, temperature control of the optical modulator chip is essential during operation to control the band-edge absorption effect. On the other hand, while Si optical modulators have the advantage of not requiring temperature control, they have a smaller electro-optic effect compared to other material systems. This necessitates a longer electro-optic interaction length, which can lead to increased high-frequency losses as the device length increases. There are many challenges in further increasing the speed and bandwidth of optical modulators, including implementation technologies for broadband and miniaturization. 【0006】 The operating temperature (case temperature) of an HB-CDM optical transmitter is required to be at least in the range of -5°C to 75°C. To ensure such an operating temperature, and taking power consumption into consideration, it has been common practice to mount only the optical modulator chip on a Peltier element (Patent Document 1). [Prior art documents] [Patent Documents] 【0007】 [Patent Document 1] International Publication No. 2021 / 171599 [Non-patent literature] 【0008】 [Non-Patent Document 1] OIF, Implementation Agreement for the High Bandwidth Coherent Driver Modulator (HB-CDM), [online], July 15, 2021, [Retrieved September 1, 2022], Internet<URL: https: / / www.oiforum.com / wp-content / uploads / OIF-HB-CDM-02.0.pdf> [Non-Patent Document 2] J. Ozaki et al., "500-Gb / s / λ Operation of Ultra-Low Power and Low-Temperature-Dependence InP-Based High-Bandwidth Coherent Driver Modulator," in Journal of Lightwave Technology, vol. 38, no. 18, pp. 5086-5091, 15 Sept.15, 2020, doi: 10.1109 / JLT.2020.2998466. [Overview of the project] 【0009】 However, conventional optical transmitters have suffered from the degradation of the driver IC's high-frequency characteristics at high temperatures. Specifically, when the ambient temperature is high, the driver IC's high-frequency bandwidth, peaking amount, and gain deteriorate. As optical transmitters become faster and wider bandwidth, the impact of the aforementioned degradation on signal quality can no longer be ignored. Therefore, there is a need for an optical transmitter that can maintain constant high-frequency characteristics regardless of changes in ambient temperature. 【0010】 In view of the above-mentioned problems, this disclosure provides a novel configuration and implementation form of an optical transmitter that suppresses the temperature dependence of the optical transmitter including the driver IC, offers excellent high speed, and enables stable operation regardless of ambient temperature. 【0011】 One aspect of the present disclosure is an optical transmitter comprising a Peltier element, an optical modulator, and a driver integrated circuit (IC) that supplies a modulating electrical signal for the optical modulator, wherein the optical modulator and the driver IC are flip-chip mounted face-down with respect to the top surface of the Peltier element. 【0012】 This disclosure enables the realization of a novel configuration and implementation form of an optical transmitter that suppresses the temperature dependence of the optical transmitter, including the driver IC, and is capable of high speed and stable operation regardless of ambient temperature. [Brief explanation of the drawing] 【0013】 [Figure 1] This is a side cross-sectional view of a conventional HB-CDM optical transmitter. [Figure 2] This is a side cross-sectional view of an optical transmitter using HB-CDM according to one embodiment of the present disclosure. [Figure 3] This is a side cross-sectional view of an optical transmitter using HB-CDM according to another embodiment of the present disclosure. [Figure 4] This figure illustrates the height limitations of the wire connection points and the spacing limitations between the driver IC and the optical modulator chip 103 in an optical transmitter according to one embodiment of the present disclosure. [Figure 5] This is a top view of an optical transmitter using HB-CDM according to another embodiment of the present disclosure. [Figure 6] This figure illustrates limitations on the position of pads and other parameters within the circuit plane of an optical transmitter according to one embodiment of the present disclosure. [Figure 7] This figure illustrates the density arrangement of Peltier elements in an optical transmitter according to one embodiment of the present disclosure. [Modes for carrying out the invention] 【0014】 The present disclosure presents a new configuration for improving the temperature dependence of the high-frequency characteristics of an optical transmitter in which a modulator and its driver IC are integrally packaged, and an implementation form suitable for each configuration. The configuration for improving the temperature dependence includes a new usage form of a temperature regulator (TEC: ThermoElectric Cooler) in the optical transmitter. Further, various implementation forms of the driver IC, the optical modulator chip, and the spatial optical components that are compatible with the new usage form of the TEC are also proposed. 【0015】 The TEC is also called a thermoelectric cooler and is known as a small cooling device using a Peltier junction. The TEC is composed of an n-type semiconductor, a p-type semiconductor, and a metal. When a direct current is passed through both sides of the element formed in a plate shape, heat absorption occurs on one side and heat dissipation occurs on the other side. If the direction of the current is reversed, heat absorption and heat dissipation are switched, so local and accurate temperature control of ICs and electronic components is possible. In the following description, for the sake of simplicity, the temperature regulator is called a TEC and described as a Peltier element. It is not limited to the one using the Peltier element as long as it can control the temperature of the driver IC and the optical modulator chip. 【0016】 In the following, taking an optical modulator in the form of the prior art HB-CDM as an example, the problem of the temperature dependence of the high-frequency characteristics in the optical transmitter will be first explained. Then, a novel configuration for improving the temperature dependence of the high-frequency characteristics by the optical transmitter of the present disclosure will be described together with various implementation forms. 【0017】 FIG. 1 is a side sectional view showing an implementation form of an optical transmitter according to the prior art HB-CDM. The optical transmitter 100 houses a driver IC 102, an optical modulator chip 103, lenses 112, 113 which are spatial optical components, etc. inside a package housing 101 made of ceramic, metal, etc. or a combination thereof, in accordance with the specifications of HB-CDM. More specifically, an optical modulator chip 103 is mounted on the bottom surface inside the package housing 101 via a sub-carrier 104 on a Peltier element 105. On the right end of the optical modulator chip 103 in the drawing, there is an emission end face of the modulated light, and lenses 112, 113 for optically coupling the modulated light with an optical fiber 114 are also mounted on the sub-carrier. 【0018】 Adjacent to the optical modulator chip 103, a driver IC 102 is mounted on a metal block or a ceramic material 106. Further, as the left wall surface in the drawing of the package housing 101, a wiring board base 107 and a package wall surface 108 are provided, and together with the package housing 101, they partition the external and the internal space of the optical transmitter. The optical transmitter 100 can also be configured to ensure the airtightness of the entire package. 【0019】 The modulated electrical signal supplied from an external digital signal processor (DSP) is supplied to the optical modulator chip 103 via the wiring layer 109 of the wiring board base 107 and the driver IC 102. Between the wiring layer 109 and the driver IC 102, and between the driver IC 102 and the optical modulator chip 103, they are respectively connected by gold wire lines 110, 111, etc. The modulated electrical signal includes an I channel and a Q channel for each of the X polarization and the Y polarization in the case of the polarization multiplexed IQ optical modulation method. When one channel is supplied as an electrical signal in differential signal form, at least 8 signal wirings and further GND wiring are required for one optical modulator, but the modulation signal form is not limited to this. The optical transmitter 100 shown in FIG. 1 can be mounted on a common device substrate together with an ICR package and a DSP in which the receiving side TIA and the optical receiver are integrally integrated, as shown in Patent Document 1, to form an optical transceiver. 【0020】 Here, we again focus on the Peltier element 105 inside the optical transmitter. Temperature control is essential for the optical modulator chip 103 fabricated on an InP substrate, and the Peltier element 105 controls it to a predetermined operating temperature. As shown in Figure 1, the Peltier element 105 is sized to cover at least the entire area of ​​the optical modulator chip 103, and its position may overlap with the area of ​​spatial optical components such as lenses. On the other hand, in the conventional optical transmitter 100, temperature control of the driver IC 102 was not considered necessary, and it was fixed inside the package by a material 106 such as a metal block or ceramic. If the external temperature (ambient temperature) of the optical transmitter 100 rises, that rise becomes the operating temperature of the driver IC 102. In reality, the driver IC is also a heat source, so considering the heat generated from the driver, the operating temperature of the driver IC is estimated to be about 5 to 10°C higher than the external temperature. When the optical transceiver including the optical transmitter reaches the maximum ambient temperature of 85°C in which it is used, the temperature of the driver IC 102 itself is at least 85°C or higher. The driver IC also consumes a significant amount of power, and therefore generates heat itself. Consequently, due to the heat generated by the driver IC, the backside temperature of the driver IC exceeds the maximum ambient temperature of 85°C. 【0021】 Driver ICs exhibit temperature dependence in their amplification characteristics (high-frequency characteristics) of high-frequency electrical signals. At high temperatures, the high-frequency bandwidth tends to decrease compared to room temperature. Conversely, at low temperatures, the high-frequency bandwidth tends to increase compared to room temperature. Thus, the high-frequency characteristics of the driver IC differ between low and high temperatures. The modulation signal supplied to the driver IC undergoes various optimizations and compensations by a DSP at room temperature. However, dynamically updating such compensations in response to temperature fluctuations is a complex process and is generally not implemented. Because the driver IC continues to operate with a constant compensation state at room temperature, when the temperature changes to low or high, the compensation state of the modulation signal deviates from the optimal point. This results in fluctuations and degradation of the optical transmission characteristics and waveform quality of the optical transmitter. 【0022】 The IQ modulator in the optical modulator chip 103 is a linear modulator that preserves the amplitude and phase of the electrical signal. Fluctuations in the level and waveform quality of the modulated electrical signal directly affect the quality of the modulated output light. When the external temperature changes during the operation of the optical transmitter, the optical modulator chip itself is maintained at a constant temperature because its temperature is controlled by a Peltier element, but the operating temperature of the driver IC changes. As a result, fluctuations in the level and quality of the modulated light of the HB-CDM occur, and the transmission characteristics deteriorate and become unstable due to temporal changes in ambient temperature. 【0023】 Degradation of electrical signals at high frequencies due to ambient temperature causes waveform distortion in the modulated signal, leading to a deterioration in the modulation accuracy of the modulated output light from the optical modulator. In optical receivers receiving such degraded modulated light, this can result in a floor in the BER characteristic, leading to a decrease in the system's transmission characteristics. 【0024】 As the demand for wider bandwidth in modulated electrical signals progresses, and a modulation bandwidth of 40 GHz or higher is required, the impact of the degradation of the high-frequency characteristics of the driver IC at high temperatures, as described above, cannot be ignored. This disclosure presents a new configuration and mounting form that improves the temperature dependence of high-frequency characteristics and optical transmission characteristics in an optical transmitter in which an optical modulator and its driver IC are integrated into a single package. 【0025】 Figure 2 is a side cross-sectional view showing an HB-CDM optical transmitter according to one embodiment of the present disclosure. The optical transmitter 10 of this embodiment, like the conventional optical transmitter 100 shown in Figure 1, has an InP optical modulator chip 13 and its driver IC 12 and other components integrally configured inside a package housing 11 along the HB-CDM. The left wall surface of the package housing 11 in the drawing is provided with a wiring board base 18 and a package wall surface 19, and the configuration that partitions the inside and outside of the package is also the same. The difference from the conventional configuration in Figure 1 lies in the way the temperature control TEC, i.e., Peltier element, is used. Unlike the way the Peltier element is used in the optical transmitter 100 in Figure 1, the driver IC 12 is also mounted on the same Peltier element 16 as the optical modulator chip 13 and the optical mounting components, lenses 23 and 24. This makes it possible to control the temperature of the driver IC as well. 【0026】 On the Peltier element 16, a driver IC 12, an optical modulator chip 13, and lenses 23 and 24 are mounted via a subcarrier 14. The subcarrier 14 functions as a base for fixing and holding the driver IC, optical modulator chip, and spatial optical components. In addition, the subcarrier 14 has metal patterns 15 formed on it, including wiring for connecting to the DC wiring of the optical modulator chip, RF lines for connecting the driver IC and the optical modulator chip, and positioning markers for mounting the spatial optical components. 【0027】 As for the material of the subcarrier 14, it is desirable to have good thermal conductivity because it will mount the driver IC 12 and optical modulator chip 13, which are subject to temperature control. Specifically, a ceramic substrate such as an AlN substrate is preferred. Since the material constants of the AlN substrate are close to those of InP, it is compatible with the optical modulator using InP in terms of its behavior with respect to temperature changes. For the same reason and from the standpoint of material compatibility, it is also desirable that the ceramic on the upper surface of the Peltier element 16 be made of AlN. A metal with good thermal conductivity such as CuW may also be used as the material for the subcarrier 14. When a metal is used as the material for the subcarrier, a wiring board will be placed on at least a portion of the upper surface of the subcarrier as a substitute for the metal pattern 15 on which the RF lines and positioning markers described above are formed, and the RF lines and positioning markers will be formed on the wiring board. 【0028】 From the perspective of thermal dissipation in the Peltier element, it is desirable that the Peltier element 16 and the subcarrier 14 be joined by an adhesive with low thermal resistance and high thermal conductivity. Specifically, an adhesive with a thermal conductivity of 30 W / m·K It is desirable to mount the subcarrier 14 on the upper surface of the Peltier element 16 using a paste or solder with excellent thermal conductivity as described above. 【0029】 In Figure 2, subcarrier 14 is depicted as being composed of a single layer of AlN, but it can also be a multilayer AlN substrate. By using a multilayer substrate, it is possible to create a flexible element and wiring layout that makes full use of multilayer wiring when there are many DC wirings to the optical modulator or when cross-wiring is required to change the order of terminals. From an RF design perspective, using multilayer wiring makes it possible to place GNG on the back side of the wiring, which is very effective as it increases the freedom of wiring layout, such as narrowing the width of individual wiring or wiring at high density. 【0030】 The driver IC 12 and optical modulator chip 13 are flip-chip mounted face-down using pillars or bumps, or a combination of them and solder. The driver IC 12 and optical modulator chip 13 are positioned on a metal pattern 15 formed on the surface of the subcarrier 14, with the circuit side (face) of the main surface with electrode pads facing the bottom surface of the package housing 11 (i.e., with the electrode pad-formed side facing downwards). Pillars can be, for example, Au pillars or Cu pillars. 【0031】 To ensure that the driver IC 12, optical modulator chip 13, and lenses 23 and 24 are not tilted during flip-chip mounting, it is crucial to control the flatness of the mounting surface of the subcarrier 14. For example, by setting the flatness of the mounting surface of the subcarrier 14 to 0.05 mm or less, stable flip-chip mounting can be achieved. 【0032】 The gap between the face-down, flip-chip mounted driver IC 12 and optical modulator chip 13 and the subcarrier 14, which is equivalent to the height of the Au pillar or bump, is filled with an underfill material 17 that has excellent thermal conductivity. For example, 3W / m·K It is desirable to use an underfill material having the above thermal conductivity. In this way, the bonding strength of the face-down flip-chip mounted driver IC 12 and optical modulator chip 13 can be increased and temperature control can be effectively performed. It is also possible to omit the underfill material. Since the underfill material is a dielectric and has a certain dielectric constant and dielectric loss tangent, it may lead to losses in high-frequency wiring such as optical modulators. Depending on the high-frequency bandwidth and baud rate required for the optical modulator, it should be noted that it may be possible to prioritize high-frequency characteristics over temperature stability and bonding strength, and to omit the underfill material. 【0033】 As mentioned earlier, the driver IC is a heat source and was not considered something that should be temperature-controlled by a Peltier element. Operating a Peltier element requires driving power, and it was not considered necessary to use extra power specifically for the heat source. However, in order to achieve wider bandwidth in optical transmitters, the inventors came up with the novel idea of ​​applying temperature control to the heat source. 【0034】 The optical transmitter 10 of this disclosure is capable of simultaneously controlling the temperature of the optical modulator chip 13 and the driver IC 12 using a single Peltier element 16. Although not explicitly shown in Figure 2, the Peltier element 16 is connected to a control current source. Regarding the specific control temperature of each part, since the modulation efficiency of the InP optical modulator decreases if the temperature is too low, it is generally desirable to use it at around 45±10℃. 【0035】 On the other hand, it is known that the driver IC 12 exhibits better high-frequency characteristics at lower temperatures than at higher temperatures. Therefore, the Peltier element 16 only needs to be controlled at any temperature between 25°C and 50°C, so as not to significantly degrade the characteristics of the InP optical modulator in the optical modulator chip 13, and to fully utilize the characteristics of the driver IC. 【0036】 To minimize variations in adhesive thickness due to temperature changes, all spatial optical components such as lenses 23 and 24 for optically coupling the modulated light with the optical fiber 25 are mounted on the Peltier element 16. This minimizes variations in optical insertion loss due to shifts in the optical axis caused by temperature changes. Spatial optical components also include fiber fixing members and polarization beam combiners (PBCs). To align the lenses 23 and 24 with the optical axis of the light output from the flip-chip mounted optical modulator chip 13 in a face-down configuration, the portion of the subcarrier 14 on which the optical components are mounted is made thin. When the diameters of the lenses 23 and 24 are small, or when a step for alignment is not required, a subcarrier 14 with a constant thickness may be used. Figure 2 shows an example in which one subcarrier 14 has two parts of different thicknesses, but the two parts may also be separated. Figure 3 shows a modified form of the optical transmitter 10, which has a subcarrier 14-1 on which the optical modulator chip 13 and driver IC 12 are mounted, and a subcarrier 14-2 on which the lenses 23 and 24 are mounted. The subcarrier 14-2 may be omitted, and the lenses 23 and 24 may be mounted directly on a TEC such as a Peltier element 16. 【0037】 Figure 2 shows an example of an HB-CDM optical transmitter 10, but similar effects can be obtained with other package configurations as long as the driver IC and optical modulator are integrated into an optical transmission module. Figure 2 also shows an example where the wiring from the DSP supplying the modulation signal to the driver IC 12 is connected on the RF terrace using a flexible printed circuit board (FPC). That is, it is connected to an FPC cable (not shown) in a metal pattern 20 formed on the wiring layer on the upper surface of the wiring board base 18 on the outside of the optical transmitter. Compared to configurations using surface mount technology (SMT), the FPC interface has superior high-frequency characteristics because it does not require RF vias (VIAs), etc. 【0038】 Next, we will describe the mounting structure that contributes to improving the high-frequency characteristics of the driver IC 12 and the quality of the high-output modulation of the optical modulator. As shown in Figure 2, in the HB-CDM type optical transmitter 10 of this embodiment, the electrode pads of the driver IC 12 and the electrode pads of the optical modulator chip 13 are connected by high-frequency wiring of the metal pattern 15 of the subcarrier 14 made of AlN substrate, using flip-chip mounting with pillars or bumps, or a combination of them and solder. On the other hand, since flip-chip mounting is difficult between the electrode pads of the driver IC 12 and the electrode pads of the metal pattern 20 on the RF terrace, they are connected by wire 21. The inductance between the driver IC 12 and the optical modulator chip 13 contributes most to the high-frequency characteristics. For example, if the wire is long, the series inductance component increases, causing the roll-off frequency in the high-frequency characteristics to shift to the lower frequency side due to LC resonance. Therefore, in order to expand the high-frequency characteristics of the driver IC and improve the quality of the high-output modulation, it is desirable for the inductance of the wire to be low. In this respect, in this embodiment, the driver IC 12 and the optical modulator chip 13 are connected by flip-chip mounting. This reduces the inductance between the driver IC 12 and the optical modulator chip 13 to about one-tenth to one-fifth of that of a wire connection like the optical transmitter 100 in Figure 1, enabling wider bandwidth. 【0039】 Although the inductance of the wire connection between the electrode pads of the driver IC 12 and the electrode pads of the metal pattern 20 on the RF terrace has a small effect on the high-frequency characteristics, it is desirable to minimize the inductance as much as possible. Therefore, specifications are provided for the height and planar directions of the wire connection portion. 【0040】 Figure 4 illustrates the height limitations of the wire connection points in the optical transmitter 10 and the spacing limitations between the driver IC and the optical modulator chip 103. In Figure 2, the vicinity of the electrodes of the RF terrace metal pattern 20, the driver IC 12, and the upper surface of the optical modulator chip 13 is shown in a height-enlarged view. The inductance between the electrodes of the RF terrace metal pattern and the electrodes of the driver IC has a smaller impact on the characteristics compared to the inductance between the electrodes of the driver IC and the electrodes of the optical modulator, but it is desirable to keep it as small as possible. As shown in Figure 4, it is desirable that the height difference between the upper surface of the RF terrace electrodes connected by the wires 21 and the upper surface of the metal pattern 15 of the subcarrier 14 be 100 μm or less. This limitation is the smallest feasible range considering variations in the thickness of the driver IC and variations in the mounting material of the subcarrier 14. For example, the height of the Peltier element 16 may be changed, the thickness of the subcarrier 14 may be changed, or the height of the RF terrace of the wiring board base 18 made of ceramic may be changed. Similarly, it is desirable that the gap between the electrode pads of the RF terrace on the wiring board base 18 and the electrode pads of the driver package (not shown) be 100 μm or less. Note that when connecting the electrodes of the metal pattern on the RF terrace and the electrodes of the driver IC by ribbon bonding or the like, a height difference between the upper surface of the RF terrace electrodes and the upper surface of the metal pattern 15 of the subcarrier 14 is not necessarily required. That is, the upper surface of the RF terrace electrodes and the upper surface of the metal pattern 15 of the subcarrier 14 may be configured to be at the same height. 【0041】 Furthermore, from the perspective of high-frequency characteristics, the longer the wiring length, the greater the wiring loss and the more the high-frequency characteristics deteriorate. It is desirable to keep the distance between the driver IC 12 and the optical modulator chip 13 close. However, if the driver IC 12 and the optical modulator chip 13 are too close, the temperature control for both the driver IC 12 and the optical modulator chip 13 by a single Peltier element 16 may not work sufficiently, and the heat from the driver IC may be transferred to the optical modulator chip, potentially causing the operation of the optical transmitter 10 to become unstable. Therefore, it is desirable to keep the distance between the driver IC 12 and the optical modulator chip 13 at least 500 μm. Keeping the distance at 500 μm or more also makes it easier to access jigs and other equipment during mounting, so it can be said to be appropriate from a manufacturing standpoint as well. However, from the perspective of high-frequency characteristics, the longer the distance between the driver IC 12 and the optical modulator chip 13, the greater the deterioration of the characteristics. Therefore, it is desirable to keep the distance between the driver IC 12 and the optical modulator chip 13 at 5 mm or less. For example, the length of the RF wiring connecting the electrode pads of the optical modulator chip 13 and the electrode pads of the driver IC 12 should preferably be between 500 μm and 5 mm. 【0042】 Figure 5 is a top view showing a modified implementation of the optical transmitter of the present invention. It corresponds to a top view of the circuit surface inside the module, obtained by cutting the package housing 11 of the optical transmitter 10 shown in Figure 2. To prevent the underfill material 17 from flowing into the high-frequency signal lines of the metal pattern 15 of the subcarrier 14, grooves 30-1 and 30-2 are formed on the top surface of the subcarrier 14, as shown by the dotted lines. The high-frequency wiring of the subcarrier 14 is configured in the region 33 shown by the dotted line between the driver IC 12 and the optical modulator chip 13. Electrode pads for RF connection are formed around the driver IC 12 and the optical modulator chip 13, respectively. By forming grooves on the top surface of the subcarrier 14, inside these surrounding electrode pads, excess underfill material from the manufacturing process is contained within the grooves. The excess underfill material can be contained within the grooves without spreading it onto the high-frequency wiring around the driver IC and the optical modulator chip. 【0043】 Figure 5 shows an example in which a linear groove 30-2 is formed on only one side of the high-frequency wiring region 33 for the driver IC 12, and a rectangular groove 30-1 is formed near the area around 4 of the optical modulator chip 13. The shape of the grooves is not limited to the configuration shown in Figure 5 and can be changed depending on the properties of the underfill material 17 and the shape of the wiring that should avoid influence on the subcarrier 14. For example, in Figure 5, the groove 30-2 on the driver IC 12 side is only on one side on the optical modulator chip 13 side, but it may also be formed in a rectangular shape at a position corresponding to the area around 4 of the driver IC. In addition to the configuration in Figure 5, a linear groove may also be added to the RF terrace side of the driver IC 12, i.e., on one side on the wiring board base 18 side. Furthermore, in Figure 5, a rectangular groove 30-1 is formed at a position corresponding to the area around 4 of the optical modulator chip 13, but grooves may also be formed only on the driver IC 12 side and the two sides corresponding to the lenses 23 and 24, which will be described next. 【0044】 Of the linear groove 30-2 on the driver IC 12 side and the rectangular groove 30-1 on the optical modulator chip 13 side, the groove on the driver IC side also serves as a thermal isolation groove between the optical modulator chip and the driver IC. That is, a groove can be provided on the surface of the subcarrier 14 near at least one of the opposing sides of the driver IC 12 and the optical modulator chip 13. If the subcarrier 14 is made of a multilayer substrate, a groove can also be formed in region 33 because high-frequency wiring can be formed in the inner layers. By forming a groove between the driver IC 12 and the optical modulator chip 13 on at least one of the upper or lower surfaces of the subcarrier 14, this groove can also serve as a thermal isolation groove. 【0045】 It is desirable to also provide grooves on the subcarriers near the exit point of the waveguide of the optical modulator chip 13 to allow the underfill material to escape. Referring again to Figure 2, if the underfill material rises up near the tip end face on the lens side of the optical modulator chip 13, the underfill material may adhere to the exit end face, worsening the optical coupling with lenses 23 and 24. The groove on one side of the rectangular groove 30-1 on the optical modulator chip 13 side shown in Figure 45, on the lens 23 side, is also effective in avoiding such optical coupling problems. 【0046】 If the subcarrier 14 is formed by a multilayer structure, the influence of the underfill material can be avoided by configuring the high-frequency line in the inner layer of the subcarrier. Furthermore, if the high-frequency wiring is configured in the inner layer, a groove can be formed at any location on the upper surface of the subcarrier between the optical modulator chip and the driver IC. Needless to say, sufficient consideration must be given to the breakage of the inner layer wiring and its impact on characteristic impedance. On the other hand, due to the effect of the effective dielectric constant of the subcarrier, the signal line width becomes narrower in the inner layer wiring when designing high-frequency wiring with the same line impedance. In addition, it is also affected by the dielectric loss tangent of the subcarrier, so if only the loss of the high-frequency line is considered, it is desirable to have the wiring pattern on the outermost surface of the subcarrier 14. 【0047】 In the arrangement of the spatial optical components in Figure 5, lenses 23 and 24 are positioned on the opposite side of the optical modulator chip 13 from the driver IC 12. However, for example, at least one lens could be positioned above or below the optical modulator chip 13 as seen in the top view of Figure 5. Also, the PBC may be positioned on a side of the optical modulator chip 13 different from the driver IC 12. That is, the spatial optical components are mounted on a side of the optical modulator chip 13 different from the side facing the driver IC 12, and above the Peltier element 16. A groove for accommodating excess underfill material can be formed near the edge of the optical modulator chip corresponding to the spatial optical component. 【0048】 Figure 6 illustrates the limitations on pads and other components within the circuit plane of the optical transmitter, and is a view from the subcarrier 14 side. As an example, Figure 6 shows the output electrode pad 26 of the driver IC 12 and the input electrode pad 27 of the optical modulator chip 13. As shown in Figure 6, pillars and solder 30 and 31 for flip-chip connection with the metal pattern 15 formed on the surface of the subcarrier 14 are formed on the electrode pads 26 and 27, respectively. 【0049】 As shown in Figure 6, the output electrode pad 26 of the driver IC 12 is configured as a GSGSG, and the input electrode pad 27 of the optical modulator chip 13 is configured as a GSSG. Considering the efficiency of the driver IC 12, differential drive is preferable to single-ended drive, and the optical modulator chip 13 is also differential driven for connectivity with the differentially driven driver IC 12. Differential lines such as GSGSG or GSSG configurations are laid out as RF wiring in the metal pattern 15 formed on the surface of the subcarrier 14. Differential lines tend to degrade in characteristics when they include bends. Therefore, in this embodiment, as shown in Figure 6, the positions of the output electrode pad 26 of the driver IC 12 and the input electrode pad 27 of the optical modulator chip 13, as well as the channel pitch, are kept from being significantly misaligned so that the RF wiring, i.e., the differential line, is formed in a straight line. Note that in Figure 6, the differential line is shown as a dashed line. However, if it is not possible to align the electrode pads on the input side of the optical modulator chip with the electrode pads on the output side of the driver IC, the metal pattern 15 may be provided with a differential line that includes bends or tapered shapes that do not significantly degrade the characteristics of the differential line. 【0050】 The above explanation showed the case where lenses 23 and 24 are mounted as spatial optical components, but it also includes members for fixing the fiber and PBCs, etc. 【0051】 Figure 7 is a diagram illustrating the density arrangement of Peltier elements in an optical transmitter according to one embodiment of the present disclosure. The Peltier element 16 arranges a large number of n-type and p-type semiconductor elements between its upper and lower metal surfaces to achieve heat transfer between both surfaces as a whole. Therefore, the arrangement density of semiconductor elements within the Peltier element can be set according to the amount of heat generated by the object whose temperature is to be controlled. Considering the amount of heat generated by each part in the optical transmitter, the driver IC generates the most heat, followed by the optical modulator chip, and then the spatial optical component. Specifically, the element density of the Peltier element is set so that the mounting area of ​​the driver IC > the mounting area of ​​the optical modulator chip > the mounting area of ​​the spatial optical component. 【0052】 As shown in Figure 7, region 16-1, which contains the Peltier elements that control the driver IC 12, should have the highest element density. Region 16-2, which contains the Peltier elements that control the optical modulator chip 13, should have a moderate element density, and region 16-3, where spatial optical components including lenses 23 and 24 are located, can have a low element density. 【0053】 As explained in detail above, this disclosure makes it possible to realize a novel configuration and implementation form of an optical transmitter that suppresses the temperature dependence of the optical modulation output characteristics and has excellent high-speed performance. [Industrial applicability] 【0054】 According to this disclosure, it is possible to provide an optical transmitter with improved temperature dependence of optical modulation output characteristics that can be used in optical communication networks. [Explanation of symbols] 【0055】 10, 100 Optical Transmitters 11, 101 Package enclosure 12, 102 Driver ICs 13, 103 Optical modulator chip 14, 14-1, 14-2, 104 subcarriers 15 Metal Patterns 16, 10⁵ Peltier element 17 Underfill material 18, 107 Wiring board base 19, 108 Package wall 20 Metal Patterns 21, 110, 111 wires 23, 24, 112, 113 lenses 25,114 optical fibers 26, 27 Electrode pads 30, 31 Pillars and Solder 32-1, 32-2 Groove 106 Member 109 Wiring layer

Claims

[Claim 1] It is an optical transmitter, Peltier element and A multilayer subcarrier structure is arranged on the upper surface of the Peltier element, An optical modulator chip including an optical modulator, A metal pattern formed on the upper surface of the subcarrier and a multilayer wiring formed using the multilayer structure, A driver integrated circuit (IC) that supplies a modulated electrical signal for the optical modulator, Equipped with, The optical modulator chip and the driver IC are flip-chip mounted face-down relative to the upper surface of the subcarrier. An optical transmitter in which the metal pattern includes wiring connecting the electrode pads of the optical modulator and the electrode pads of the driver IC, the length of the wiring being 500 μm or more and 5 mm or less, and the wiring being a straight differential line. [Claim 2] The optical transmitter according to claim 1, wherein the temperature of the Peltier element is controlled to be between 25°C and 50°C. [Claim 3] The aforementioned subcarrier is formed using aluminum nitride (AlN), The optical modulator chip is formed using InP, The flatness of the upper surface of the subcarrier is 0.05 mm or less. The optical transmitter according to claim 2, wherein the optical transmitter further comprises an underfill material having a thermal conductivity of 3 W / m·K or more, which is filled in the gap between the subcarrier, the optical modulator chip, and the driver IC. [Claim 4] The optical transmitter according to claim 3, further comprising a groove formed on the upper surface of the subcarrier near at least one of the sides of the optical modulator chip and the side of the driver IC that are opposite to each other, or a groove formed between the optical modulator chip and the driver IC on at least one of the upper or lower surfaces of the subcarrier. [Claim 5] The optical transmitter according to claim 3, further comprising an optical component associated with the optical modulator, and an optical component disposed on the upper surface of the subcarrier. [Claim 6] The optical transmitter according to claim 5, wherein the thickness of the portion of the subcarrier on which the optical components are arranged is thinner than the thickness of the portion of the subcarrier on which the optical modulator is flip-chip mounted. [Claim 7] The optical transmitter according to claim 5, wherein the Peltier element comprises an n-type semiconductor element and a p-type semiconductor element, and the density of the n-type semiconductor element and the p-type semiconductor element is configured such that the lower part of the driver IC > the lower part of the optical modulator chip > the lower part of the optical component. [Claim 8] The optical modulator chip and the driver IC are mounted in a package in the form of a high-speed driver integrated optical modulator (HB-CDM). Each of the package, the driver IC, the optical modulator, and the metal pattern has electrode pads for inputting and outputting radio frequency (RF) differential signals. The optical transmitter according to claim 1, wherein the electrode pads of the package are formed on the upper surface of the RF terrace of the package, the height difference between the upper surface of the RF terrace of the package and the upper surface of the subcarrier on which the metal pattern is formed is 100 μm or less, and the electrode pads of the RF terrace and the electrode pads of the metal pattern are wire-connected.

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