Optical transmitter

Abstract

A transmitter (200) is an optical transmitter comprising: an optical modulator (13); a driver integrated circuit (driver IC) (12) which supplies a modulated electrical signal for the optical modulator; a first wiring substrate (22) which connects the optical modulator and the driver IC and which is mounted face down using flip-chip mounting; a first Peltier element (17) which controls the temperature of the optical modulator; and a second Peltier element (16) which controls the temperature of the driver IC.

Description

Technical Field

[0001] The present invention 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] In order to cope with the rapid increase in traffic in communication networks, digital coherent optical transmission combining 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 integrating an optical receiver and an optical transmitter is used. In the optical transceiver device of a system 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 [Patent Document 2] Patent No. 6770478 [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] [Problems that the invention aims to solve]

[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, the present invention provides a novel configuration and implementation form of an optical transmitter that suppresses the temperature dependence of the optical transmitter including the driver IC, exhibits excellent high speed, and enables stable operation regardless of ambient temperature. [Means for solving the problem]

[0011] One aspect of the present invention is an optical transmitter, comprising an optical modulator, a driver integrated circuit (driver IC) for supplying a modulation electrical signal to the optical modulator, a first wiring board face-down mounted by flip-chip mounting for connecting the optical modulator and the driver IC, a first Peltier element for controlling the temperature of the optical modulator, and a second Peltier element for controlling the temperature of the driver IC.

Advantages of the Invention

[0012] According to the present invention, it is possible to realize a novel configuration and mounting form of an optical transmitter that suppresses the temperature dependence of an optical transmitter including a driver IC, is excellent in high speed, and can operate stably regardless of the environmental temperature.

Brief Description of the Drawings

[0013] [Figure 1] Side cross-sectional view showing a conventional HB-CDM mounting form [Figure 2] Side cross-sectional view of the mounting form of the optical transmitter according to the HB-CDM of the present invention (Embodiment 1) [Figure 3] Top view of a modified mounting form of the optical transmitter of the present invention. [Figure 4] Side cross-sectional view of the mounting form of the optical transmitter according to the HB-CDM of the present invention (Embodiment 2)​​​​​​​​​​​​​​​​​​​​ In the drawings, parts having the same function may be denoted by the same reference numerals, and their description may be omitted.

[0015] [Embodiment 1] The present invention presents a new configuration for improving the temperature dependence of the high-frequency characteristics of an optical transmitter in which an optical modulator and its driver IC are integrally packaged, and a mounting 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 mounting forms of the driver IC, the optical modulator chip, and the spatial optical component adapted to the new usage form of the TEC are also proposed. Various mounting forms of the driver IC, the optical modulator chip, and the spatial optical component adapted to the new usage form of the TEC are also proposed.

[0016] 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 an element formed in a plate shape, heat absorption occurs on one side and heat dissipation occurs on the other side. Since heat absorption and heat dissipation are switched by reversing the direction of the current, local and accurate temperature control of ICs and electronic components is possible. In the following description, for 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.

[0017] Hereinafter, the problem of the temperature dependence of the high-frequency characteristics in the optical transmitter will be first explained using an optical modulator in the form of the conventional HB-CDM as an example. Thereafter, a novel configuration for improving the temperature dependence of the high-frequency characteristics by the optical transmitter of the present invention will be described together with various mounting forms.

[0018] Figure 1 is a side cross-sectional view showing the implementation configuration of a conventional HB-CDM optical transmitter. The optical transmitter 100 houses a driver IC 102, an optical modulator chip 103, and spatial optical components such as lenses 112 and 113 inside a package housing 101 made of ceramic, metal, or a combination thereof, in accordance with the specifications of the HB-CDM. More specifically, the optical modulator chip 103 is mounted on the bottom surface inside the housing 101 via a subcarrier 104 on top of a Peltier element 105. The right end of the optical modulator chip 103 in the diagram is the output end face for modulated light, and the lenses 112 and 113 for optically coupling the modulated light with the optical fiber 114 are also mounted on the subcarrier.

[0019] Adjacent to the optical modulator chip 103, the driver IC 102 is mounted on a metal block or ceramic material 106. Furthermore, the left-hand wall of the package housing 101 in the diagram includes a wiring board base 107 and a package wall 108, which together with the package housing 101 separate the external space from the internal space of the optical transmitter. The optical transmitter 100 can also be configured so that the entire package is hermetically sealed.

[0020] The modulated electrical signal supplied from an external digital signal processor (DSP) is delivered to the optical modulator chip 103 via the wiring layer 109 of the wiring board base 107 and the driver IC 102. Gold wires 110, 111, etc., connect the wiring layer 109 and the driver IC 102, and the driver IC 102 and the optical modulator chip 103, respectively. In the case of polarization multiplexed IQ optical modulation, the modulated electrical signal includes an I channel and a Q channel for both X polarization and Y polarization. If one channel is supplied as an electrical signal in differential signal format, at least eight signal lines and a GND line are required for one optical modulator, but the modulated signal format is not limited to this. The optical transmitter 100 shown in Figure 1 can be mounted on a common device board together with an ICR package or DSP that integrates the receiving side TIA and photodetector, as shown in Patent Document 1, to constitute an optical transceiver.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] Degradation of electrical signal characteristics at high frequencies due to ambient temperature causes waveform distortion of 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.

[0025] As the demand for wider bandwidth in modulated electrical signals increases, 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 invention 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.

[0026] Figure 2 is a side cross-sectional view showing the mounting configuration of the HB-CDM optical transmitter 200 of the present invention. Similar to the conventional configuration shown in Figure 1, the optical transmitter 200 of the present invention has an InP optical modulator chip 13 and its driver IC 12 and other components integrated inside a package housing 11 that follows 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 usage of the TEC, i.e., the Peltier element, which controls the temperature. Unlike the usage of the Peltier element in Figure 1, the driver IC 12 is also mounted on a second Peltier element 16. The second Peltier element 16 of the driver IC 12 is separate and independent from the first Peltier element 17 that controls the temperature of the optical modulator chip 13, and the optical transmitter 200 has two Peltier elements.

[0027] On the first Peltier element 17, a light modulator chip 13 and lenses 23 and 24 are mounted via a subcarrier 14.

[0028] The subcarrier 14 serves as a base for fixing and holding the optical modulator chip and spatial optical components.

[0029] As for the material of the subcarrier 14, it is desirable that it has good thermal conductivity because it will mount the optical modulator chip 13, which is the target of temperature control. As mentioned above, considering both the ease of handling DC wiring and thermal conductivity, a substrate made of a dielectric rather than a metal is desirable, and a ceramic substrate such as an AlN substrate is preferred. In particular, 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 first Peltier element 17 be made of AlN.

[0030] Furthermore, the subcarrier 14 may be a metal block, and if a metal block is used, it is preferable to use CuW or the like, which has excellent heat dissipation properties.

[0031] Although Figure 2 depicts a single-layer configuration, if the carrier 14 is constructed from a dielectric substrate, it may be multilayered. Multilayering allows for flexible element and wiring layouts utilizing multilayer wiring, especially when there are many DC connections to the optical modulator or when cross-wiring is required to change the order of terminals. Furthermore, when using a dielectric substrate, it becomes possible to form positioning markers for mounting spatial optical components using metal patterns.

[0032] Furthermore, if wiring for extracting the DC wiring of the optical modulator chip is essential on the carrier 14, the carrier 14 may be made of a dielectric substrate only (single-layer or multi-layer is acceptable), or it may be made of both a metal block and a dielectric substrate. When the carrier 14 is made of both a metal block and a dielectric substrate, a dielectric substrate for forming wiring should be provided on at least a portion of the upper surface of the carrier 14.

[0033] More specifically, if markings and wiring are not required, it is possible to use only a metal block, or to mount an AlN substrate on top of the metal block. The AlN substrate can be the same size as the metal block, and smaller versions can be installed next to chips, etc.

[0034] The driver IC 12, whose temperature is controlled independently of the optical modulator chip 13, is also preferably mounted on the second Peltier element 16 via a retaining member 15 in order to align its height with the RF terrace, which is the upper surface of the optical modulator chip 13 and the wiring board base 18. A metal block or ceramic substrate can be used as the retaining member 15. Considering thermal conductivity, for example, if DC wiring is not required for the driver IC 12, a metal block such as a CuW block can be used, and if DC wiring for the driver is required, a ceramic substrate such as an AlN substrate can be used. If an AlN substrate is used and the number of wires to the driver IC is large and complex, a multilayer substrate can be used, similar to the subcarrier 14 of the optical modulator chip mentioned above.

[0035] 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.

[0036] As described above, the optical transmitter 200 of the present invention is equipped with two independently controlled second Peltier elements 16 and a first Peltier element 17, enabling independent temperature control of the driver IC 12 and the optical modulator chip 13. Although not explicitly shown in Figure 2, the two Peltier elements are connected to separate control current sources. 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℃.

[0037] On the other hand, it is known that the driver IC 12 exhibits better high-frequency characteristics at lower temperatures than at higher temperatures, and a lower set temperature is desirable. However, lowering the set temperature too much increases the power consumption of the Peltier element while only slightly improving the high-frequency characteristics of the driver IC. Therefore, operating the driver IC at around room temperature, for example, 30±10℃, is the most appropriate from the standpoint of balancing power consumption and high-frequency characteristics. By independently setting the optical modulator chip 13 and the driver IC 12 to different temperatures, an optical transmitter can be realized that can operate each under optimal conditions. However, if power consumption can be disregarded and transmission characteristics are prioritized, then a lower temperature is desirable, and this rule does not apply.

[0038] Therefore, the optical transmitter 200 of the present invention can be implemented as an optical transmitter comprising an optical modulator chip 13, a driver integrated circuit (driver IC 12) that supplies a modulation electrical signal for the optical modulator, a first wiring board 22 that is face-down mounted by flip-chip mounting and connects the optical modulator and the driver IC 12, a first Peltier element 17 that controls the temperature of the optical modulator, and a second Peltier element 16 that controls the temperature of the driver IC.

[0039] Components whose temperature is controlled by a Peltier element must be mounted with a conductive paste or solder that has excellent thermal conductivity of 30 W / mK or higher to ensure good heat dissipation by the Peltier element. For the purpose of controlling the module's manufacturing process temperature, the same conductive paste or solder may be used throughout, or a combination of materials with different fixed temperatures may be used.

[0040] In the optical transmitter 200 shown in Figure 2, the second Peltier element 16 and the first Peltier element 17 control the temperature of the driver IC and optical modulator chip via a common subcarrier 14. Because the driver IC and optical modulator chip are connected via the subcarrier 14, the two temperature controls cannot be performed completely independently. However, the first Peltier element 17 significantly reduces the temperature of the driver IC 12, thereby improving high-frequency characteristics. Furthermore, using a single subcarrier 14 reduces material costs and simplifies the mounting process. For example, to achieve independent control, as will be described later, it is also effective to provide thermal isolation grooves on either the upper or lower surface of the subcarrier 14, or on both the upper and lower surfaces, to achieve thermal isolation between the optical modulator and the driver IC.

[0041] To minimize variations in adhesive thickness due to temperature changes, all spatial optical components such as lenses 23 and 24 are mounted on the first Peltier element 17. This minimizes variations in optical insertion loss caused by shifts in the optical axis due to temperature changes. Spatial optical components also include fiber fixing members and polarization beam combiners (PBCs).

[0042] Figure 2 shows an HB-CDM optical transmitter 200 as an example, 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. Also in Figure 2, an example is shown in which the wiring from the DSP that supplies the modulation signal to the driver IC 12 is connected on the RF terrace by a flexible printed circuit board (FPC). That is, it is connected to an FPC cable (not shown) on the metal pattern 20 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.

[0043] Next, we will describe the mounting structure for ensuring the high-frequency characteristics of the driver IC and optical modulator. In the HB-CDM optical transmitter 200 shown in Figure 2, the electrode pads of the driver IC and the electrode pads of the RF terrace, and the electrode pads of the driver IC and the electrode pads of the optical modulator are connected by wires 21, a wiring board (first wiring board) 22, and a first pillar / bump 22a, respectively. An increase in the series inductance component causes the roll-off frequency in the high-frequency characteristics to shift to the lower frequency side due to LC resonance. Therefore, in order to suppress the degradation of the high-frequency characteristics of the driver IC and realize a wideband HB-CDM, it is important to keep the inductance of this connection low. In the optical transmitter 200, since the inductance in the connection between the driver IC and the modulator would be high, a wiring board (first wiring board) 22 is used instead of the commonly used wire. This reduces the inductance of this connection and enables wideband operation. Furthermore, as mentioned above, the inductance between the driver IC and the modulator has a very large impact on the high-frequency characteristics, but the inductance between the driver IC and the RF terrace has a smaller impact compared to the former. Therefore, the connection between the driver IC and the modulator is of highest priority, followed by the connection between the driver IC and the RF terrace.

[0044] In this embodiment, the driver IC 12 and the optical modulator chip 13 are connected by a flip-chip mounting of the first wiring board 22 in a face-down configuration, and the driver IC 12 and the metal pattern 20 on the wiring board base 18 are connected by a wire 21. This makes it possible to reduce the inductance at the connection between the driver IC 12 and the optical modulator chip 13. In this way, a wideband HB-CDM can be realized.

[0045] Furthermore, from the perspective of high-frequency characteristics, in order to best utilize the characteristics of the driver IC 12, it is desirable that both the driver IC 12 and the optical modulator chip 13 have a differential line configuration. Moreover, since the characteristics of high-frequency (RF) differential lines deteriorate significantly if they have a bend, it is desirable that the high-frequency lines on the wiring board be formed in a nearly straight shape. In order to form a nearly straight shape, the PAD pitch for connecting each component should be as consistent as possible. That would be preferable.

[0046] The connection between the driver IC 12 and the optical modulator chip 13 is face-down on the first wiring board 22. The flip-chip mounting uses a first pillar / bump 22a made of Au or Cu, etc., and for stable mounting, it is desirable that the height of the top surface of the driver IC 12 and the top surface of the optical modulator chip 13 are the same, and that the bottom surface of the first wiring board 22 is mounted flat without tilting relative to the top surfaces of the driver IC 12 and the optical modulator chip 13. Solder or the like may be provided at the tip of the pillar / bump to ensure connection strength, etc.

[0047] For example, if the inclination of the main surface (bottom surface) of the first wiring board 22 relative to the main surface (top surface) of the driver IC 12 or the main surface (top surface) of the optical modulator chip 13 exceeds ±3°, then When connecting components, gaps may occur, preventing a proper connection, or the load on the connection point may become too large, potentially causing the connection to break, making it difficult to ensure reliability. Therefore, the inclination of the lower surface of the first wiring board 22 relative to the upper surface of the driver IC 12 and the upper surface of the optical modulator chip 13 in the height direction is set to within ±3°. Managing tolerances is extremely important, such as ensuring that components are not tilted during mounting, adjusting the height of each component so that the top surface of the driver IC 12 and the top of the optical modulator chip 13 are at the same height, and controlling tolerances.

[0048] Regarding the height difference between the top surface of the driver IC 12 and the top surface of the optical modulator chip 13, the size of the first pillar / bump 22a, which is generally made of Au or Cu, is typically 100 μm or less in both diameter and height. Therefore, the height difference between the top surface of the driver IC 12 and the top surface of the optical modulator chip 13 is controlled to be 100 μm or less (ideally 50 μm or less). That is preferable.

[0049] For example, when mounting the driver IC 12 and the optical modulator chip 13 on the same subcarrier, the thickness of the driver IC 12 and the optical modulator chip 13 must be the same. By making the thickness of the driver IC and the modulator chip the same, the height of the top surface of the driver IC 12 and the top surface of the optical modulator chip 13 can be made the same. Since the same carrier is used, this configuration offers the most advantages in terms of the number of components and tolerances.

[0050] However, considering the heat inflow from the driver IC to the modulator chip, from the standpoint of thermal isolation, mounting the driver IC and modulator chip on the same subcarrier in this manner is not very desirable. It is preferable to use separate carriers for the driver IC and modulator chip, as described later. Furthermore, if the same substrate is used, it is desirable to provide thermal isolation grooves as described later.

[0051] When the driver IC 12 and the optical modulator chip 13 are mounted via separate components, the height of the outermost surfaces connecting the driver IC 12 and the optical modulator chip 13 to the wiring board can be matched by controlling the thickness of the components on which the driver IC 12 and the optical modulator chip 13 are mounted. In this case, the carrier itself may be a separate component, or it may remain a single component, with adjustments such as creating steps in the carrier according to the thickness of the driver IC and the modulator chip.

[0052] From the perspective of minimizing variations in the height difference between the top surface of the driver IC 12 and the top surface of the optical modulator chip 13, it is most desirable that the optical modulator chip 13 and the driver IC 12 be mounted on the same subcarrier with the same chip thickness. For example, if the thickness of the driver IC 12 is 300 μm, then the chip thickness of the optical modulator chip 13 should also be 300 μm.

[0053] Next, the configuration of the connection between the driver IC and the RF terrace will be described. As mentioned above, the effect of inductance between the driver IC and the RF terrace is smaller than the effect of inductance between the driver IC and the modulator, so Figure 2 shows a wire connection. When the connection between the driver IC 12 and the metal pattern 20 on the wiring board base 18 is made with a wire, it is desirable to keep the difference between the height of the top surface of the metal pattern 20 on the wiring board base 18 and the height of the top surface of the driver IC 12 to about 100 μm from the viewpoint of stabilizing the wire length and mounting stability. Furthermore, when considering a wire connection using the ball bonding method, from the viewpoint of minimizing the wire length, it is desirable to set the top surface of the driver IC 12 lower than the top surface of the metal pattern 20 on the wiring board base 18 and connect the wire from the driver IC 12 to the metal pattern 20 on the wiring board base 18.

[0054] Next, considering the heat inflow from the driver IC 12 to the optical modulator chip 13 and interference from jigs during mounting, it is desirable to separate the driver IC 12 and the optical modulator chip 13 by 300 μm or more. Furthermore, the first wiring board 22 described above should have a matching coefficient of linear expansion with the InP modulator. From a certain perspective, an AlN substrate might be preferable, but since AlN substrates have excellent thermal conductivity, an SiO2 substrate is preferable in terms of further suppressing heat inflow from the driver IC 12 to the optical modulator chip 13. It is desirable to use resin substrates with low thermal conductivity, such as those made of dielectric materials.

[0055] On the other hand, from the standpoint of joint strength and degradation of high-frequency characteristics, it is desirable to keep the length of the first wiring board 22 to a maximum of 2 mm or less, and the smaller the dielectric constant and dielectric loss tangent values, the higher From a frequency standpoint, it is advantageous.

[0056] The same applies to ceramic substrates other than those made of the above materials, such as alumina substrates other than AlN substrates. It is possible to obtain the effect.

[0057] Figure 3 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 housing 11 of the optical transmitter 200 shown in Figure 2. To prevent underfill material from flowing into the high-frequency signal lines of the subcarrier 14, grooves 26-1 and 26-2 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 dotted-line region 27 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 IC and chip.

[0058] Figure 3 shows an example in which a linear groove 26-2 is formed on only one side of the high-frequency wiring region 27 for the driver IC 12, and a rectangular groove 26-1 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 3 and can be changed depending on the properties of the underfill material and the shape of the wiring that should not be affected on the subcarrier. For example, in Figure 3, the groove 26-2 of the driver IC 12 is only on one side on the optical modulator chip side, but it may also be formed in a rectangular shape around 4 of the driver IC. In addition to the configuration in Figure 3, a linear groove may be added to one side of the driver IC 12 on the RF terrace side, i.e., the wiring board base 18 side. Furthermore, in Figure 5, a rectangular groove 26-1 is formed near the area around 4 of the optical modulator chip 13, but grooves may also be formed on only two sides: the driver IC side and the lens side, which will be described next.

[0059] The linear groove 26-2 on the driver IC 12 and the rectangular groove 26-1 on the optical modulator chip 13, specifically the groove on the driver IC side, also serve as thermal isolation grooves between the optical modulator chip and the driver IC. That is, grooves 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. In the optical transmitter of the present invention, where the second Peltier element 16 and the first Peltier element 17 operate via a common subcarrier 14, the aforementioned grooves improve the independence of temperature control, significantly mitigating the high-temperature state of the driver IC 12 and improving high-frequency characteristics. Furthermore, if the subcarrier 14 is composed of a multilayer substrate, high-frequency wiring can be formed in the inner layers, allowing grooves to be formed in region 27 as well. By forming grooves 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, these grooves can also serve as thermal isolation grooves.

[0060] 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 26-1 of the optical modulator chip 13 shown in Figure 3, which is on the lens 23 side, is also effective in avoiding such optical coupling problems.

[0061] 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.

[0062] In the arrangement of the spatial optical components in Figure 3, 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 4. Also, the PBC may be positioned on a different side from that of the driver IC. That is, the spatial optical components are mounted above the first Peltier element 17 on a side of the optical modulator chip that is different from the side facing the driver IC 12. A groove for accommodating excess underfill material can be formed near the side of the optical modulator chip corresponding to the spatial optical components.

[0063] [Embodiment 2] Figure 4 is a side cross-sectional view of the implementation configuration of the HB-CDM optical transmitter 300 of the present invention.

[0064] In practice, it is often difficult to make the driver IC 12 and the optical modulator chip 13 the same thickness, and in such cases, it is preferable to control the thickness by making them separate components, as shown in Figure 4.

[0065] For example, there are cases where there is a difference in thickness, such as the driver IC 12 being 100 μm thick and the optical modulator chip 13 being 300 μm thick. In this case, for example, the optical modulator chip 13 and the driver IC 12 are mounted on the first Peltier element 17 and the second Peltier element 16, respectively, and the height of the top surface of the driver IC 12 and the top surface of the optical modulator chip 13 are controlled. For example, the driver IC 12 is mounted on a metal block 15 made of CuW or similar material, considering heat dissipation and GND stability. By setting the thickness of the metal block 15 and the subcarrier 14 on which the optical modulator chip 13 is mounted to, for example, 500 μm for the metal block 15 and 300 μm for the subcarrier 14, the height of the top surface of the driver IC 12 and the top surface of the optical modulator chip 13 can be made the same.

[0066] As shown in Figure 4, when separate subcarriers are used for the driver IC and the modulator chip (i.e., they are not mounted on the same subcarrier), the thicknesses of the second Peltier element 16 and the first Peltier element 17 do not necessarily have to be the same. For example, considering the thermal resistance of the Peltier elements, the lower the height of the Peltier element, the better the efficiency. Therefore, it is effective to set the height of the Peltier element on which the driver is mounted lower than the height of the Peltier element on which the modulator is mounted.

[0067] [Embodiment 3] Figure 5 is a side cross-sectional view of the implementation configuration of the HB-CDM optical transmitter 400 of the present invention.

[0068] If the thickness of the driver IC 12 and the optical modulator chip 13 are the same, for example, in Figure 2, a subcarrier 14 is used, but as shown in Figure 5, various DCs can be applied to the AlN substrate on the upper surface of the first Peltier element 17 and the second Peltier element 16 without using a subcarrier 14. By incorporating alignment marks for wiring and optical mounting, it is possible to reduce the number of components. Reducing the number of components leads to a reduction in thermal resistance, which is very effective from a temperature control perspective.

[0069] [Embodiment 4] Figure 6 is a side cross-sectional view of the implementation configuration of the HB-CDM optical transmitter 500 of the present invention.

[0070] If the thickness of the driver IC 12 and the optical modulator chip 13 are different, it is possible to omit the subcarrier mounted beneath the optical modulator chip 13. Furthermore, the configuration of the metal block 15, which controls the thickness of the Peltier element, is not required. In this configuration, the driver IC 12 is directly formed on the Peltier element.

[0071] [Embodiment 5] Figure 7 is a side cross-sectional view of the implementation configuration of the HB-CDM optical transmitter 600 of the present invention.

[0072] As shown in Figure 7, the connection between the driver IC 12 and the metal pattern 20 on the wiring board base 18 can also be made using a flip-chip mounting method with a second wiring board 61, instead of using wires 21.

[0073] In this case as well, due to the difference in height between the top surface of the optical modulator chip 13 and the top surface of the driver IC 12, and for similar reasons due to the inclination of the first wiring board 22, the difference in height between the top surface of the driver IC 12 and the top surface of the metal pattern 20 on the top surface of the wiring board base 18 should be 100 μm or less (ideally 50 μm or less). (Preferred) The inclination of the lower surface of the wiring board relative to the upper surface of the metal pattern 20 on the upper surface of the driver IC 12 or the upper surface of the wiring board base 18 in the height direction must be controlled to within ±3°. There is a requirement. The materials used for the first wiring board 22 and the second wiring board 61 may be the same or different. The materials used for the first bump 22a and the second bump 61a may be the same or different.

[0074] However, considering the cost, it is very effective to use the same wiring board for the first wiring board 22 and the second wiring board 61. In this case, by making the input / output pads of the driver IC 12 the same and matching the pad shape and pitch of the connection part of the metal pattern 20 on the upper surface of the optical modulator chip 13 and the wiring board base 18, it is possible to reduce costs by using the same wiring board.

[0075] However, as shown in Figure 7, when separate subcarriers are used for the driver IC and the modulator chip (i.e., they are not mounted on the same subcarrier), the thicknesses of the second Peltier element 16 and the first Peltier element 17 do not necessarily have to be the same. For example, considering the thermal resistance of the Peltier elements, the lower the height of the Peltier element, the better the efficiency. Therefore, it is effective to set the height of the Peltier element on which the driver is mounted lower than the height of the Peltier element on which the modulator is mounted.

[0076] In embodiments 1 to 5 described above, the spatial optical components are described assuming lens mounting, but configurations other than lens mounting are also acceptable. In addition to the lenses 23 and 24 shown, spatial optical components also include members for fixing fibers, polarization beam combiners (PBCs), etc.

[0077] (Examples) Figure 8 illustrates the density arrangement of Peltier elements in the optical transmitter of the present invention. The Peltier element arranges numerous n-type and p-type semiconductor elements between 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.

[0078] As shown in Figure 8, the second Peltier element 16 that controls the driver IC should have the highest element density. In the first Peltier element 17 that controls the optical modulator chip, the region directly below the optical modulator can have a medium density, while the region 17-2 containing spatial optical components can have a low density.

[0079] As described in detail above, the optical transmitter of the present invention makes it possible to suppress the temperature dependence of the optical modulation output characteristics and realize a novel configuration and implementation form of an optical transmitter with excellent high-speed performance. [Industrial applicability]

[0080] This invention can be used in optical communication networks.

Claims

1. It is an optical transmitter, Optical modulator, A driver integrated circuit (driver IC) that supplies a modulated electrical signal for the optical modulator, A first wiring board, which is face-down mounted by flip-chip mounting, connects the optical modulator and the driver IC. A first Peltier element for controlling the temperature of the optical modulator, A second Peltier element for controlling the temperature of the driver IC, A subcarrier provided on the upper surface of the first Peltier element and the second Peltier element, with the driver IC and the optical modulator mounted on its upper surface, On the upper surface of the subcarrier, a first groove is provided in a position inside the electrode pads provided around the optical modulator, and along at least one side facing the driver IC, The upper surface of the subcarrier includes a second groove provided on the inner side of the electrode pads provided around the driver IC, and along at least one side facing the optical modulator, The first groove and the second groove serve both to separate the light modulator and the driver IC for heat and to block the underfill material. A spatial optical component is mounted on the first Peltier element, on the side of the optical modulator chip opposite the driver IC, and a lens, a fiber fixing member, and a polarization beam combiner (PBC), which are spatial optical components necessary to constitute the optical modulator, are mounted on the Peltier element on which the optical modulator is mounted. An optical transmitter in which the first Peltier element and the second Peltier element are configured such that the in-plane density of n-type semiconductor elements and p-type semiconductor elements is such that the second Peltier element > the mounting area of ​​the optical modulator chip of the first Peltier element > the mounting area of ​​the spatial optical component of the first Peltier element.

2. The optical transmitter according to claim 1, characterized in that the difference in height between the upper surface of the driver IC and the upper surface of the optical modulator is 100 μm or less, and the inclination of the lower surface of the first wiring board in the height direction with respect to the upper surface of the driver IC and the upper surface of the optical modulator is within ±3°.

3. The optical transmitter according to claim 1 or 2, characterized in that the distance between the optical modulator and the driver IC is 300 μm or more and 2 mm or less, and the high-frequency (RF) line on the first wiring board is formed in a substantially straight shape.

4. The temperature of the second Peltier element is set lower than the temperature of the first Peltier element, and the optical modulator is composed of InP. The first Peltier element has an upper surface made of aluminum nitride (AlN), A paste or solder layer having a thermal conductivity of 30 W / m·K or higher is provided between the first Peltier element and the chip of the optical modulator, and between the second Peltier element and the driver IC. The optical transmitter according to feature 1.

5. The temperature of the first Peltier element is set to a range of 45 ± 10°C. The optical transmitter according to claim 1, characterized in that the temperature of the second Peltier element is set in the range of 30 ± 10°C.

6. The optical modulator chip and the driver IC are mounted in a high-speed driver integrated optical modulator (HB-CDM) package. In the electrical signal paths of the input section of the package, the driver IC, and the optical modulator chip, the electrode pads are formed by a differential signal interface. The optical transmitter according to claim 1, characterized in that the difference in height between the upper surface of the RF terrace on which the high-frequency (RF) electrode pads of the input section are formed and the upper surface of the driver IC is 100 μm or less, the RF electrode pads of the RF terrace and the electrode pads of the driver IC are connected via a second wiring board, and the inclination of the lower surface of the second wiring board in the height direction with respect to the upper surface of the driver IC and the upper surface of the RF electrode pads is within ±3°.

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