US20190293883A1 - Method And System For Implementing High-Speed Electrical Interfaces Between Semiconductor Dies In Optical Communication Systems - Google Patents
Method And System For Implementing High-Speed Electrical Interfaces Between Semiconductor Dies In Optical Communication Systems Download PDFInfo
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- US20190293883A1 US20190293883A1 US16/424,136 US201916424136A US2019293883A1 US 20190293883 A1 US20190293883 A1 US 20190293883A1 US 201916424136 A US201916424136 A US 201916424136A US 2019293883 A1 US2019293883 A1 US 2019293883A1
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/42—Coupling light guides with opto-electronic elements
- G02B6/4201—Packages, e.g. shape, construction, internal or external details
- G02B6/4274—Electrical aspects
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/42—Coupling light guides with opto-electronic elements
- G02B6/4201—Packages, e.g. shape, construction, internal or external details
- G02B6/4286—Optical modules with optical power monitoring
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- Certain embodiments of the invention relate to signal processing. More specifically, certain embodiments of the invention relate to a method and system for implementing high-speed electrical interfaces between semiconductor dies in optical communication systems.
- Optical communication systems have been widely adopted for applications ranging from internet backbone, local area networks, data centers, supercomputing, to high-definition video. Due to superior bandwidth and low loss, optical fibers are the medium of choice for transporting high-speed binary data.
- a system and/or method for implementing high-speed electrical interfaces between semiconductor dies in optical communication systems substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims.
- FIG. 1 is a block diagram of a photonically enabled CMOS optical communication system, in accordance with an embodiment of the invention.
- FIG. 2 is a block diagram of split receiver and transmitter electrical interfaces, in accordance with an embodiment of the invention.
- FIG. 3 is a block diagram of a flip-chip-type photonically-enabled CMOS optical receiver system, in accordance with an embodiment of the invention.
- FIG. 4 is a block diagram illustrating a controlled-impedance split-transmitter interface, in accordance with an embodiment of the invention.
- FIG. 5 is a block diagram illustrating a low-impedance split-transmitter cascode interface, in accordance with an embodiment of the invention.
- Certain aspects of the invention may be found in a method and system for implementing high-speed electrical interfaces between semiconductor dies in optical communication systems.
- Exemplary aspects of the invention may comprise communicating electrical signals between an electronics die and an optoelectronics die via coupling pads on the electronics die and the optoelectronics die in an optical communication system.
- the coupling pads may be located at low impedance points in one or more transmit paths and/or one or more receive paths in the optical communication system.
- the electrical signals may be communicated via one or more current-mode interfaces that may comprise a cascode amplifier stage split between source terminals of transistors on the electronics die and drain terminals of transistors on the optoelectronics die.
- the electrical signals may be communicated via one or more controlled-impedance interfaces that may comprise one or more transmission line drivers on a first die and one or more transmission lines on a second die of the optical communication system.
- the electrical signals may be communicated via one or more capacitively-coupled interfaces that may comprise capacitors formed by one or more of the contact pads on the electronics die and one or more of the contact pads on the optoelectronics die.
- the electrical signals may be communicated via one or more inductively-coupled interfaces.
- the coupling pads may be connected via one or more of: wire bonds, metal pillars, solder balls, or conductive resin.
- the electronics die and the optoelectronics die may comprise CMOS dies.
- the electronics die and the optoelectronics die may be coupled in a flip-chip configuration.
- FIG. 1 is a block diagram of a photonically-enabled CMOS optical communication system, in accordance with an embodiment of the invention.
- optoelectronic devices comprising optical modulators 105 A- 105 D, photodiodes 111 A- 111 D, monitor photodiodes 113 A- 113 H, and optical devices comprising taps 103 A- 103 K, optical terminations 115 A- 115 D, and grating couplers 117 A- 117 H.
- electrical devices and circuits comprising amplifiers 107 A- 107 D, analog and digital control circuits 109 , and control sections 112 A- 112 D.
- the amplifiers 107 A- 107 D may comprise transimpedance and limiting amplifiers (TIA/LAs), for example.
- TIA/LAs transimpedance and limiting amplifiers
- Optical signals are communicated between optical and optoelectronic devices via optical waveguides fabricated in the chip or chips comprising the devices.
- the optical modulators 105 A- 105 D comprise Mach-Zehnder or ring modulators, for example, and enable the modulation of the CW laser input signal.
- the optical modulators 105 A- 105 D comprise high-speed and low-speed phase modulation sections and are controlled by the control sections 112 A- 112 D.
- the high-speed phase modulation section of the optical modulators 105 A- 105 D may modulate a CW light source signal with a data signal.
- the low-speed phase modulation section of the optical modulators 105 A- 105 D may compensate for slowly varying phase factors such as those induced by mismatch between the waveguides, waveguide temperature, or waveguide stress and is referred to as the passive phase, or the passive biasing of the MZI.
- the phase modulators may have a dual role: to compensate for the passive biasing of the MZI and to apply the additional phase modulation used to modulate the light intensity at the output port of the MZI according to a data stream.
- the former phase tuning and the latter phase modulation may be applied by separate, specialized devices, since the former is a low speed, slowly varying contribution, while the latter is typically a high speed signal. These devices are then respectively referred to as the LSPM and the HSPM.
- LSPM thermal phase modulators
- PINPM forward biased PIN junctions
- An example of an HSPM is a reversed biased PIN junction, where the index of refraction is also modulated via the carrier density, but which allows much faster operation, albeit at a lower phase modulation efficiency per waveguide length.
- the outputs of the modulators may be optically coupled via waveguides to the grating couplers 117 E- 117 H.
- the taps 103 D- 103 K comprise four-port optical couplers, for example, and are utilized to sample the optical signals generated by the optical modulators 105 A- 105 D, with the sampled signals being measured by the monitor photodiodes 113 A- 113 H.
- the unused branches of the taps 103 D- 103 K are terminated by optical terminations 115 A- 115 D to avoid back reflections of unwanted signals.
- the grating couplers 117 A- 117 H comprise optical gratings that enable coupling of light into and out of the chip or chips comprising the optical gratings 117 A- 117 H.
- the grating couplers 117 A- 117 D may be utilized to couple light received from optical fibers into a photonically-enabled CMOS chip, and the grating couplers 117 E- 117 H may be utilized to couple light from the photonically-enabled CMOS chip into optical fibers.
- the grating couplers 117 A- 117 H may comprise single polarization grating couplers (SPGC) and/or polarization splitting grating couplers (PSGC). In instances where a PSGC is utilized, two input, or output, waveguides may be utilized.
- SPGC single polarization grating couplers
- PSGC polarization splitting grating couplers
- the optical fibers may be epoxied, for example, to the photonically-enabled CMOS chip, and may be aligned at an angle from normal to the surface of the chip to optimize coupling efficiency.
- the optical fibers may comprise single-mode fiber (SMF) and/or polarization-maintaining fiber (PMF).
- optical signals may be communicated directly into the surface of the photonically-enabled CMOS chip without optical fibers by directing a light source on an optical coupling device in the chip, such as the light source interface 135 and/or the optical fiber interface 139 . This may be accomplished with directed laser sources and/or optical sources on another chip flip-chip bonded to the photonically-enabled CMOS chip.
- the photodiodes 111 A- 111 D may convert optical signals received from the grating couplers 117 A- 117 D into electrical signals that are communicated to the amplifiers 107 A- 107 D for processing.
- the photodiodes 111 A- 111 D may comprise high-speed heterojunction photodiodes, for example, and may comprise germanium (Ge) in the collector and base regions for absorption in the 1.55 ⁇ m optical wavelength range, and may be integrated on a CMOS silicon-on-insulator (SOI) wafer.
- the photodiodes 111 A- 111 D may instead comprise phototransistors.
- the analog and digital control circuits 109 may control gain levels or other parameters in the operation of the amplifiers 107 A- 107 D, which may then communicate electrical signals to other circuitry, such as in a deep sub-micron CMOS chip with for further digital signal processing, for example.
- the plurality of optical, optoelectronic, and electronic devices in the photonically-enabled CMOS optical communication system 100 may be integrated in different chips depending on the speed and other device requirements. For example, complex, high-speed digital processing may be performed on electronic devices in a deep sub-micron CMOS chip that is communicatively coupled to a photonically-enabled CMOS chip comprising optical, electrical, and optoelectronic devices.
- the control sections 112 A- 112 D comprise electronic circuitry that enable modulation of the CW laser signal received from the splitters 103 A- 103 C.
- the optical modulators 105 A- 105 D require high-speed electrical signals to modulate the refractive index in respective branches of a Mach-Zehnder interferometer (MZI), for example.
- MZI Mach-Zehnder interferometer
- the control sections 112 A- 112 D may include sink and/or source driver electronics that may enable a bidirectional link utilizing a single laser.
- the chips comprising the photonically-enabled CMOS optical communication system 100 may comprise semiconductor-on-insulator (SOI), bulk silicon, silicon-on-sapphire, indium phosphide, indium-gallium-nitride, indium-gallium-arsenide, gallium arsenide, or an organic polymer, for example.
- SOI semiconductor-on-insulator
- bulk silicon silicon-on-sapphire
- indium phosphide indium-gallium-nitride
- indium-gallium-arsenide indium-gallium-arsenide
- gallium arsenide gallium arsenide
- organic polymer for example.
- III-V semiconductors excel in light generation and detection
- SiGe HBT technologies work very well in high-speed analog and mixed signal applications
- deep submicron CMOS provides speed and density necessary for implementing complex digital functions and signal processing on VLSI scale, with a wide variety of existing digital cores readily available for integration.
- Overlaps between these technologies exist in some areas, but in many cases it may be more efficient to choose the right technology for each function.
- This approach may enable systems to be assembled from multiple semiconductor dies. Electrical interfaces between these dies may create performance bottlenecks or lead to inefficiencies, such as reduced transmission speed, increased timing jitter, or additional power consumption.
- high-speed electronic devices and circuits may be integrated on a CMOS VLSI chip and optical, optoelectronic, and electronic devices may be integrated on a second photonically-enabled CMOS chip.
- the chips may be configured adjacent to each other or may be configured face-to-face in a flip-chip configuration.
- Coupling techniques may comprise wire bonds, solder balls, metal pillars, conductive resin, and/or proximity interconnects such as capacitively-coupled interfaces or inductively-coupled interfaces.
- Capacitively-coupled interfaces may comprise metal pads on adjacent chips comprising capacitors for AC-coupling of signals, whereas inductively-coupled interfaces may comprise inductor coils on adjacent chips with signals being communicated between chips via inductive coupling between the coils.
- the photonically-enabled CMOS optical communication system 100 may be operable to transmit and/or receive and process optical signals.
- the grating couplers 117 A- 117 D may be operable to receive optical signals from optical fibers coupled to a photonically-enabled CMOS chip, and may convert the optical mode of the fiber into the much smaller mode of a Si waveguide fabricated on the photonically-enabled CMOS SOI wafer.
- the grating couplers 117 A- 117 D may comprise a single-polarization or a polarization-splitting type: in the first case, only a specific polarization is coupled to a single Si waveguide, while in the second case, two orthogonal polarizations are split into two output waveguides.
- optical, electrical, and optoelectronic devices may be integrated on chips that may be optimized for a particular device.
- optical and optoelectronic, and some electronic devices may be integrated on a photonically-enabled CMOS chip whereas high-speed electronic devices may be integrated on an adjacent deep sub-micron CMOS chip.
- the coupling of the two chips may then be configured to minimize effects from parasitic impedances.
- the interconnect in a current-mode interface, the interconnect may be configured between transistors in a cascode configuration with one pair of transistors on one chip an open-drain configuration and another pair of transistors on a second chip in the cascode configuration.
- configurable impedance transmission lines may be utilized on a photonically-enabled CMOS chip to match impedance with low input impedance of Tx amplifiers in a deep sub-micron CMOS chip, which also mitigates the effects of parasitic impedances resulting from the interconnect between chips.
- FIG. 2 is a block diagram of split receiver and transmitter electrical interfaces, in accordance with an embodiment of the invention.
- a photonically-enabled CMOS optical communication system 200 comprising a CMOS very-large-scale-integration (VLSI) die 201 and a CMOS optoelectronics die 203 .
- the Tx path in the lower half of the dies comprises a controlled-impedance interface, whereas the Rx path in the upper half of the dies comprises a current-mode interface.
- VLSI very-large-scale-integration
- the CMOS optoelectronics die 203 comprises passive optical structures, such as grating couplers 225 A and 225 B, waveguides, splitters, and combiners.
- the CMOS optoelectronics die 203 also comprises active optoelectronic elements such as the photodetector 221 for the purpose of detecting optical signals in receivers and optical phase modulators.
- the phase modulators may be configured to function as Mach-Zehnder interferometers, such as the MZI 213 , for the purpose of modulating optical signals in transmitters.
- the optoelectronics die also comprises a common-source pair comprising transistors M CS1 and M CS2 , resistors R PD1 , R PD2 , R T1 , R T2 and RB, capacitors C 1 and C 2 , and other integrated circuit elements.
- the invention is not limited to the number of optical, optoelectronic, and electronic devices shown, as it comprises representative sections of the chips.
- the CMOS optoelectronics die 203 also comprises transmission lines 217 , a light source 215 , unit drivers 227 , coupling pads 219 , and a bias voltage VB.
- the CMOS VLSI die 201 comprises digital cores 205 A and 205 B, receiver (Rx) gain stages 207 , transmitter (Tx) gain stages 209 , CMOS transistors M C1 and M C2 , capacitor C C1 , bias resistors R V1 and R V2 , coupling pads 219 , and a bias voltage V C .
- the Rx gain stages 207 and the Tx gain stages 209 may comprise suitable gain control elements for communicating signals to and from the digital cores 205 A and 205 B.
- the Tx gain stages 209 may comprise transmission line drivers for communicating electrical signals from the CMOS VLSI die 201 and the transmission lines 217 on the optoelectronics die 203 .
- the digital cores 205 A and 205 B may comprise high-speed digital processing cores, for example, that may be operable to generate signals to be transmitted and/or process received signals.
- the transistors M C1 and M C2 may comprise a cascode configuration, such that the impedance seen at the coupling pads 219 may be low. In this manner, the interface between the CMOS VLSI die 201 and the CMOS optoelectronics die 203 may be less sensitive to parasitic impedances.
- the coupling between the CMOS VLSI die 201 and the optoelectronics die 203 is not limited to wire bonds, as illustrated by the wire bonds 223 in FIG. 2 . Accordingly, other techniques may be utilized, such as solder balls, metal pillars, conductive resin, and/or proximity interconnects, which may comprise capacitively-coupled interfaces or inductively-coupled interfaces, for example.
- Capacitively-coupled interfaces may comprise the coupling pads 219 on adjacent chips comprising capacitors for AC-coupling of signals, whereas inductively-coupled interfaces may comprise inductor coils on adjacent chips with signals being communicated between chips via inductive coupling between the coils.
- the optical receiver interface may utilize transistors available on the CMOS optoelectronics die 203 .
- the pair of NMOS transistors M CS1 and M CS2 may be in a common-source configuration with the drain terminals coupled to the coupling pads 219 .
- the CMOS transistors M CS1 and M CS2 may communicate modulated currents to another pair of NMOS transistors M C1 and M C2 in cascode configuration on the CMOS VLSI die 201 .
- the input impedance looking into source terminals of these cascode devices may be very low, and they may accept the currents transmitted by the common-source transistor pair without creating large voltage signals.
- Low impedance may desensitize this type of interface to parasitic inductance, capacitance, and resistance associated with pads, bond wires, or other interconnect elements.
- the transmitted currents may then be converted to a voltage signal on the CMOS VLSI die 201 by a pair of resistors R V1 and R V2 connected to drain terminals of the cascode transistors M C1 and M C2 .
- the voltage signal may be further amplified by subsequent voltage gain stages, the Rx gain stages 207 , until it reaches an amplitude suitable for use by the digital cores 205 A.
- the bias voltage VB may be operable to bias the common-source pair comprising M CS1 and M CS2 and the resistors R PD1 and R PD2 may be operable to bias the photodiode 221 .
- An electrical signal may be generated by the photodiode 221 in response to an input light signal. The generated signal may be AC-coupled via the capacitors C 1 and C 2 to the gate terminals of the transistors M CS1 and M CS2 for communication to the CMOS VLSI die 201 via the cascode circuit comprising M C1 and M C2 .
- the transmission lines 217 may comprise tunable impedances for matching to prior gain stages, such as the Tx gain stages 209 , and may also provide a low impedance for reduced sensitivity to parasitic impedances in the coupling between the CMOS VLSI die 201 and the optoelectronics die 203 .
- the signals received from the CMOS VLSI die 201 may be communicated to the unit drivers 227 , which may be operable to modulate CW optical signals in the MZI 213 received from the light source 215 .
- FIG. 3 is a block diagram of a flip-chip-type photonically-enabled CMOS optical receiver system, in accordance with an embodiment of the invention.
- a photonically-enabled CMOS optical communication system 300 comprising a CMOS VLSI die 301 , an optoelectronics die 303 , and metal pillars 323 .
- the CMOS VLSI die 301 comprises a plurality of digital cores shown as the digital cores 305 A and 305 B, transistors M 2A and M 2B , resistors R 2A and R 2B , inductors L P1 and L P2 , and coupling pads 319 .
- control voltage V casc supply voltage V DD
- output voltages V on and V op are also shown.
- the optoelectronics die 303 comprises a photodiode 321 , photodiode bias resistors R D1 and R D2 , coupling capacitors C 1A and C 1B , bias resistors R B1 and R B2 , a current source I B , and transistors M 1A and M 1B .
- the transistors M 1A , M 2A , M 1B , and M 2B comprise a differential cascode circuit for amplifying a signal generated by the photodiode 321 .
- the metal pillars 323 may provide electrical interconnection as well as structural support between the CMOS VLSI die 301 and the optoelectronics die 303 .
- the die may be configured in a face-to-face die attachment via metal pillars 323 instead of side-by-side dies connected with bond wires, for example, but the principle of operation and the advantages of low-impedance interfaces are the same.
- the transistors M 1A and M 1B on the optoelectronics die 303 comprise a differential pair whose drains may be coupled to the metal pillars 323 that may communicate differential currents to the CMOS VLSI die 323 .
- the cascode transistors M 2A and M 2B may receive the communicated currents that may be converted to a differential voltage signal via the resistors R 2A and R 2B and bandwidth-enhancing inductors L P1 and L P2 .
- the optoelectronics die 303 comprises passive and active circuit elements to provide bias voltage for the integrated photodetector, to convert photocurrent to a voltage signal, and to couple the photo-generated signal to the gates of transistors M 1A and M 1B .
- FIG. 4 is a block diagram illustrating a controlled-impedance split-transmitter interface, in accordance with an embodiment of the invention. Referring to FIG. 4 , there is shown a photonically-enabled CMOS optical communication system 400 CMOS VLSI die 401 , an optoelectronics die 403 , and wire bonds 423 .
- the CMOS optical communication system 400 comprises transmission lines 417 as a break point between the CMOS VLSI die 401 and the optoelectronics die 403 , which may thus comprise a controlled-impedance interface.
- the CMOS VLSI die may contain digital cores, such as the digital cores 205 A, 205 B, 305 A, and 305 B, described with respect to FIGS. 2 and 3 , where the transmitted signal originates, as well as analog signal conditioning circuitry such as transmission line drivers.
- the CMOS VLSI die 401 also comprises Tx gain stages 409 , a current source I B , transistors M T1 and M T2 , and coupling pads 419 .
- the optoelectronics die 403 comprises transmission lines 417 , which may provide proper electrical to optical velocity matching for MZI operation.
- the optoelectronics die 403 also comprises distributed amplifier stages implemented with transistors, as illustrated schematically by the unit drivers 427 . These distributed amplifiers may provide large signals to optical phase modulators in the MZI.
- the optoelectronics die 403 also comprises sense resistors R S1 and R S2 , termination resistors R T1 and R T2 , a capacitor C R , and a common-mode feedback circuit comprising a feedback gain stage AFB, a feedback control transistor M FB , and a replica reference inverter 425 .
- the common-mode feedback circuit may be operable to convert limited-swing current-mode logic (CML) signals to full-swing CMOS signals by matching the common mode voltage on the transmission lines 417 to the transition threshold of the unit drivers 427 .
- CML limited-swing current-mode logic
- signals for transmission may be generated in one or more digital cores in the CMOS VLSI die 401 and communicated to the gate terminals of the transistors M T1 and M T2 comprising a differential pair.
- the signal amplified by the differential pair may be communicated to the optoelectronics die 403 via the coupling pads 419 , which may be coupled between die using the wire bonds 423 .
- the coupling between the CMOS VLSI die 401 and the optoelectronics die 403 is not limited to wire bonds, as illustrated by the wire bonds 423 in FIG. 4 . Accordingly, other techniques may be utilized, such as solder balls, metal pillars, conductive resin, and/or proximity interconnects, which may comprise capacitively-coupled interfaces or inductively-coupled interfaces, for example.
- Capacitively-coupled interfaces may comprise the coupling pads 419 on adjacent chips comprising capacitors for AC-coupling of signals, whereas inductively-coupled interfaces may comprise inductor coils on adjacent chips with signals being communicated between chips via inductive coupling between the coils.
- the transmission lines 417 may provide tunable impedances for matching to prior gain stages, such as the differential pair M T1 and M T2 , and may also provide a low impedance for reduced sensitivity to parasitic impedances in the coupling between the CMOS VLSI die 401 and the optoelectronics die 403 .
- the signals received from the CMOS VLSI die 401 may be communicated to the unit drivers 427 , which may be operable to modulate optical signals in a MZI modulator, such as the MZI 213 , described with respect to FIG. 2 .
- FIG. 5 is a block diagram illustrating a low-impedance split-transmitter cascode interface, in accordance with an embodiment of the invention.
- a photonically-enabled CMOS optical communication system 500 comprising a CMOS VLSI die 501 , an optoelectronics die 503 , and wire bonds 523 .
- the CMOS optical communication system 500 comprises transmission lines 517 on the optoelectronics die 503 as in the controlled-impedance interface shown in FIG. 4 , but also comprises a break point between the CMOS VLSI die 501 and the optoelectronics die 503 utilizing a splitting of the interface circuits in a differential cascode amplifier stage comprising the transistors M T1 -M T4 , as in the current-mode die interface, described with respect to FIGS. 2 and 3 . This further lowers interface impedance compared to a directly-driven transmission line, resulting in a lower voltage swing and even less sensitivity to interconnect inductance and capacitance.
- the interface comprises an open-drain differential driver on the CMOS VLSI die 501 and cascode-configured transistors on the optoelectronics die 503 .
- the open-drain differential driver comprises a pair of NMOS transistors M T1 /M T2 and the cascode transistors comprise the transistors M T3 /M T4 with the drain terminals of M T1 /M T2 and the source terminals of M T3 /M T4 connected to the coupling pads 519 for launching differential current signals into terminated transmission lines 517 on the optoelectronics die 503 .
- the optoelectronics die 503 also comprises a common-mode feedback loop, which matches the common-mode voltage on the transmission lines 517 to the switching threshold of the distributed amplifier stages comprising the unit drivers 527 .
- the CMOS VLSI die 501 may contain digital cores, such as the digital cores 205 A, 205 B, 305 A, and 305 B, described with respect to FIGS. 2 and 3 , where the transmitted signal originates, as well as analog signal conditioning circuitry such as transmission line drivers.
- the CMOS VLSI die 501 also comprises Tx gain stages 509 , a current source I B , transistors M T1 and M T2 , and coupling pads 519 .
- the optoelectronics die 503 comprises cascode transistors M T3 and M T4 , capacitors Cc, and transmission lines 517 , which may provide proper electrical to optical velocity matching for MZI operation.
- the optoelectronics die 503 also comprises distributed amplifier stages implemented with transistors, as illustrated schematically by the unit drivers 527 . These distributed amplifiers may provide large signals to optical phase modulators in the MZI.
- the optoelectronics die 503 also comprises sense resistors R S1 and R S2 , termination resistors R T1 and R T2 , a capacitor C R , and a common-mode feedback circuit comprising a feedback gain stage AFB, a feedback control transistor M FB , and a replica reference inverter 525 .
- the common-mode feedback circuit may be operable to convert limited-swing current-mode logic (CML) signals to full-swing CMOS signals.
- transistors on the optoelectronics die 503 may be used to implement analog and control functions associated with the optoelectronic devices. For example, this may include low-noise voltage regulators for integrated photodetectors and calibration or real-time control loops for the MZIs to compensate for the effect of process and temperature variations. Retaining these functions on the optoelectronics die 503 reduces the total number of interconnects between the two dies and keeps sensitive analog and control nets local and isolated from possible sources of crosstalk. Digital state machines and low-speed serial communication interfaces for digital control and monitoring can also be implemented on the optoelectronics die 503 as needed.
- signals for transmission may be generated in one or more digital cores in the CMOS VLSI die 501 and communicated to the gate terminals of the transistors M T1 and M T2 comprising a differential pair.
- the signal amplified by the differential pair may be communicated to the optoelectronics die 503 via the coupling pads 519 , which may be coupled between die using the wire bonds 523 .
- the coupling between the CMOS VLSI die 501 and the optoelectronics die 503 is not limited to wire bonds, as illustrated by the wire bonds 523 in FIG. 5 . Accordingly, other techniques may be utilized, such as solder balls, metal pillars, conductive resin, and/or proximity interconnects, which may comprise capacitively-coupled interfaces or inductively-coupled interfaces, for example.
- Capacitively-coupled interfaces may comprise the coupling pads 519 on adjacent chips comprising capacitors for AC-coupling of signals, whereas inductively-coupled interfaces may comprise inductor coils on adjacent chips with signals being communicated between chips via inductive coupling between the coils.
- the transmission lines 517 may provide tunable impedances for matching to prior gain stages, such as the differential pair M T1 and M T2 .
- the low impedance at the source terminals of the cascode transistors M T3 and M T4 provide reduced sensitivity to parasitic impedances in the coupling between the CMOS VLSI die 501 and the optoelectronics die 503 .
- the signals received from the CMOS VLSI die 501 may be communicated to the unit drivers 527 , which may be operable to modulate optical signals in a MZI modulator, such as the MZI 213 , described with respect to FIG. 2 .
- a method and system are disclosed for communicating electrical signals between an electronics die 201 , 301 , 401 , and 501 and an optoelectronics die 203 , 303 , 403 , and 503 via coupling pads 219 , 319 , 419 , and 519 on the electronics die 201 , 301 , 401 , and 501 and the optoelectronics die 203 , 303 , 403 , and 503 in an optical communication system 200 , 300 , 400 , and 500 .
- the coupling pads 219 , 319 , 419 , and 519 may be located in low impedance points in one or more transmit paths and/or one or more receive paths in the photonically-enabled CMOS optical communication system 200 , 300 , 400 , and 500 .
- the electrical signals may be communicated via one or more current-mode interfaces that may comprise a cascode amplifier stage split between source terminals of transistors M C1 , M C2 , M T1 , M T2 , M 2A , and M 2B on the electronics die 201 , 301 , 401 , and 501 and drain terminals of transistors M CS1 , M CS2 , M T3 , M T4 , M 1A , and M 1B on the optoelectronics die 203 , 303 , 403 , and 503 .
- a cascode amplifier stage split between source terminals of transistors M C1 , M C2 , M T1 , M T2 , M 2A , and M 2B on the electronics die 201 , 301 , 401 , and 501 and drain terminals of transistors M CS1 , M CS2 , M T3 , M T4 , M 1A , and M 1B on the optoelect
- the electrical signals may be communicated via one or more controlled-impedance interfaces that may comprise one or more transmission line drivers 209 and 409 on a first die and one or more transmission lines 217 , 417 , and 517 on a second die of the optical communication system 200 , 300 , 400 , and 500 .
- the electrical signals may be communicated via one or more capacitively-coupled interfaces that may comprise capacitors formed by one or more of the contact pads 219 , 319 , 419 , and 519 on the electronics die 201 , 301 , 401 , and 501 and one or more of the contact pads on the optoelectronics die 203 , 303 , 403 , and 503 .
- the electrical signals may be communicated via one or more inductively-coupled interfaces.
- the coupling pads 219 , 319 , 419 , and 519 may be connected via one or more of: wire bonds 223 , 423 , and 523 , metal pillars 319 , solder balls, or conductive resin.
- the electronics die 201 , 301 , 401 , and 501 and the optoelectronics die 203 , 303 , 403 , and 503 may be coupled in a flip-chip configuration.
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Abstract
Description
- This application is a continuation of U.S. application Ser. No. 16/110,587 filed on Aug. 23, 2018, which is a continuation of U.S. application Ser. No. 15/716,103 filed on Sep. 26, 2017, now U.S. Pat. No. 10,061,094, which is a continuation of U.S. patent application Ser. No. 13/033,439 filed on Feb. 23, 2011, now U.S. Pat. No. 9,772,460, which makes reference to and claims priority to U.S. Provisional Application Ser. No. 61/338,690 filed on Feb. 23, 2010, each of which is hereby incorporated herein by reference in its entirety.
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- Certain embodiments of the invention relate to signal processing. More specifically, certain embodiments of the invention relate to a method and system for implementing high-speed electrical interfaces between semiconductor dies in optical communication systems.
- As data networks scale to meet ever-increasing bandwidth requirements, the shortcomings of copper data channels are becoming apparent. Signal attenuation and crosstalk due to radiated electromagnetic energy are the main impediments encountered by designers of such systems. They can be mitigated to some extent with equalization, coding, and shielding, but these techniques require considerable power, complexity, and cable bulk penalties while offering only modest improvements in reach and very limited scalability. Free of such channel limitations, optical communication has been recognized as the successor to copper links.
- Optical communication systems have been widely adopted for applications ranging from internet backbone, local area networks, data centers, supercomputing, to high-definition video. Due to superior bandwidth and low loss, optical fibers are the medium of choice for transporting high-speed binary data.
- Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.
- A system and/or method for implementing high-speed electrical interfaces between semiconductor dies in optical communication systems, substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims.
- Various advantages, aspects and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.
-
FIG. 1 is a block diagram of a photonically enabled CMOS optical communication system, in accordance with an embodiment of the invention. -
FIG. 2 is a block diagram of split receiver and transmitter electrical interfaces, in accordance with an embodiment of the invention. -
FIG. 3 is a block diagram of a flip-chip-type photonically-enabled CMOS optical receiver system, in accordance with an embodiment of the invention. -
FIG. 4 . is a block diagram illustrating a controlled-impedance split-transmitter interface, in accordance with an embodiment of the invention. -
FIG. 5 . is a block diagram illustrating a low-impedance split-transmitter cascode interface, in accordance with an embodiment of the invention. - Certain aspects of the invention may be found in a method and system for implementing high-speed electrical interfaces between semiconductor dies in optical communication systems. Exemplary aspects of the invention may comprise communicating electrical signals between an electronics die and an optoelectronics die via coupling pads on the electronics die and the optoelectronics die in an optical communication system. The coupling pads may be located at low impedance points in one or more transmit paths and/or one or more receive paths in the optical communication system. The electrical signals may be communicated via one or more current-mode interfaces that may comprise a cascode amplifier stage split between source terminals of transistors on the electronics die and drain terminals of transistors on the optoelectronics die. The electrical signals may be communicated via one or more controlled-impedance interfaces that may comprise one or more transmission line drivers on a first die and one or more transmission lines on a second die of the optical communication system. The electrical signals may be communicated via one or more capacitively-coupled interfaces that may comprise capacitors formed by one or more of the contact pads on the electronics die and one or more of the contact pads on the optoelectronics die. The electrical signals may be communicated via one or more inductively-coupled interfaces. The coupling pads may be connected via one or more of: wire bonds, metal pillars, solder balls, or conductive resin. The electronics die and the optoelectronics die may comprise CMOS dies. The electronics die and the optoelectronics die may be coupled in a flip-chip configuration.
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FIG. 1 is a block diagram of a photonically-enabled CMOS optical communication system, in accordance with an embodiment of the invention. Referring toFIG. 1 , there is shown optoelectronic devices comprisingoptical modulators 105A-105D,photodiodes 111A-111D,monitor photodiodes 113A-113H, and opticaldevices comprising taps 103A-103K,optical terminations 115A-115D, andgrating couplers 117A-117H. There are also shown electrical devices and circuits comprising amplifiers 107A-107D, analog and digital control circuits 109, andcontrol sections 112A-112D. The amplifiers 107A-107D may comprise transimpedance and limiting amplifiers (TIA/LAs), for example. Optical signals are communicated between optical and optoelectronic devices via optical waveguides fabricated in the chip or chips comprising the devices. - The
optical modulators 105A-105D comprise Mach-Zehnder or ring modulators, for example, and enable the modulation of the CW laser input signal. Theoptical modulators 105A-105D comprise high-speed and low-speed phase modulation sections and are controlled by thecontrol sections 112A-112D. The high-speed phase modulation section of theoptical modulators 105A-105D may modulate a CW light source signal with a data signal. The low-speed phase modulation section of theoptical modulators 105A-105D may compensate for slowly varying phase factors such as those induced by mismatch between the waveguides, waveguide temperature, or waveguide stress and is referred to as the passive phase, or the passive biasing of the MZI. - The phase modulators may have a dual role: to compensate for the passive biasing of the MZI and to apply the additional phase modulation used to modulate the light intensity at the output port of the MZI according to a data stream. The former phase tuning and the latter phase modulation may be applied by separate, specialized devices, since the former is a low speed, slowly varying contribution, while the latter is typically a high speed signal. These devices are then respectively referred to as the LSPM and the HSPM. Examples for LSPM are thermal phase modulators (TPM), where a waveguide portion is locally heated up to modify the index of refraction of its constituting materials, or forward biased PIN junctions (PINPM) where current injection into the PIN junction modifies the carrier density, and thus the index of refraction of the semiconductor material. An example of an HSPM is a reversed biased PIN junction, where the index of refraction is also modulated via the carrier density, but which allows much faster operation, albeit at a lower phase modulation efficiency per waveguide length.
- The outputs of the modulators may be optically coupled via waveguides to the grating couplers 117E-117H. The
taps 103D-103K comprise four-port optical couplers, for example, and are utilized to sample the optical signals generated by theoptical modulators 105A-105D, with the sampled signals being measured by themonitor photodiodes 113A-113H. The unused branches of thetaps 103D-103K are terminated byoptical terminations 115A-115D to avoid back reflections of unwanted signals. - The
grating couplers 117A-117H comprise optical gratings that enable coupling of light into and out of the chip or chips comprising theoptical gratings 117A-117H. Thegrating couplers 117A-117D may be utilized to couple light received from optical fibers into a photonically-enabled CMOS chip, and the grating couplers 117E-117H may be utilized to couple light from the photonically-enabled CMOS chip into optical fibers. Thegrating couplers 117A-117H may comprise single polarization grating couplers (SPGC) and/or polarization splitting grating couplers (PSGC). In instances where a PSGC is utilized, two input, or output, waveguides may be utilized. - The optical fibers may be epoxied, for example, to the photonically-enabled CMOS chip, and may be aligned at an angle from normal to the surface of the chip to optimize coupling efficiency. In an embodiment of the invention, the optical fibers may comprise single-mode fiber (SMF) and/or polarization-maintaining fiber (PMF).
- In another exemplary embodiment, optical signals may be communicated directly into the surface of the photonically-enabled CMOS chip without optical fibers by directing a light source on an optical coupling device in the chip, such as the light source interface 135 and/or the optical fiber interface 139. This may be accomplished with directed laser sources and/or optical sources on another chip flip-chip bonded to the photonically-enabled CMOS chip.
- The
photodiodes 111A-111D may convert optical signals received from thegrating couplers 117A-117D into electrical signals that are communicated to the amplifiers 107A-107D for processing. Thephotodiodes 111A-111D may comprise high-speed heterojunction photodiodes, for example, and may comprise germanium (Ge) in the collector and base regions for absorption in the 1.55 μm optical wavelength range, and may be integrated on a CMOS silicon-on-insulator (SOI) wafer. In another embodiment of the invention, thephotodiodes 111A-111D may instead comprise phototransistors. - The analog and digital control circuits 109 may control gain levels or other parameters in the operation of the amplifiers 107A-107D, which may then communicate electrical signals to other circuitry, such as in a deep sub-micron CMOS chip with for further digital signal processing, for example. In an embodiment of the invention, the plurality of optical, optoelectronic, and electronic devices in the photonically-enabled CMOS
optical communication system 100 may be integrated in different chips depending on the speed and other device requirements. For example, complex, high-speed digital processing may be performed on electronic devices in a deep sub-micron CMOS chip that is communicatively coupled to a photonically-enabled CMOS chip comprising optical, electrical, and optoelectronic devices. - The
control sections 112A-112D comprise electronic circuitry that enable modulation of the CW laser signal received from thesplitters 103A-103C. Theoptical modulators 105A-105D require high-speed electrical signals to modulate the refractive index in respective branches of a Mach-Zehnder interferometer (MZI), for example. In an embodiment of the invention, thecontrol sections 112A-112D may include sink and/or source driver electronics that may enable a bidirectional link utilizing a single laser. - The chips comprising the photonically-enabled CMOS
optical communication system 100 may comprise semiconductor-on-insulator (SOI), bulk silicon, silicon-on-sapphire, indium phosphide, indium-gallium-nitride, indium-gallium-arsenide, gallium arsenide, or an organic polymer, for example. - As complexity and data throughput of optical communication systems increase, it is often inefficient, difficult, or even impossible to monolithically integrate all system subcomponents on a single semiconductor die. This may be caused by diversity of semiconductor technologies and materials used in constructing such systems. Some of these technologies are either highly specialized or are optimized for a fairly narrow range of applications. For example, III-V semiconductors excel in light generation and detection, SiGe HBT technologies work very well in high-speed analog and mixed signal applications, and deep submicron CMOS provides speed and density necessary for implementing complex digital functions and signal processing on VLSI scale, with a wide variety of existing digital cores readily available for integration. Overlaps between these technologies exist in some areas, but in many cases it may be more efficient to choose the right technology for each function. This approach may enable systems to be assembled from multiple semiconductor dies. Electrical interfaces between these dies may create performance bottlenecks or lead to inefficiencies, such as reduced transmission speed, increased timing jitter, or additional power consumption.
- In an embodiment of the invention, high-speed electronic devices and circuits may be integrated on a CMOS VLSI chip and optical, optoelectronic, and electronic devices may be integrated on a second photonically-enabled CMOS chip. The chips may be configured adjacent to each other or may be configured face-to-face in a flip-chip configuration. Coupling techniques may comprise wire bonds, solder balls, metal pillars, conductive resin, and/or proximity interconnects such as capacitively-coupled interfaces or inductively-coupled interfaces. Capacitively-coupled interfaces may comprise metal pads on adjacent chips comprising capacitors for AC-coupling of signals, whereas inductively-coupled interfaces may comprise inductor coils on adjacent chips with signals being communicated between chips via inductive coupling between the coils.
- In operation, the photonically-enabled CMOS
optical communication system 100 may be operable to transmit and/or receive and process optical signals. Thegrating couplers 117A-117D may be operable to receive optical signals from optical fibers coupled to a photonically-enabled CMOS chip, and may convert the optical mode of the fiber into the much smaller mode of a Si waveguide fabricated on the photonically-enabled CMOS SOI wafer. Thegrating couplers 117A-117D may comprise a single-polarization or a polarization-splitting type: in the first case, only a specific polarization is coupled to a single Si waveguide, while in the second case, two orthogonal polarizations are split into two output waveguides. - In an exemplary embodiment of the invention, optical, electrical, and optoelectronic devices may be integrated on chips that may be optimized for a particular device. For example, optical and optoelectronic, and some electronic devices may be integrated on a photonically-enabled CMOS chip whereas high-speed electronic devices may be integrated on an adjacent deep sub-micron CMOS chip. The coupling of the two chips may then be configured to minimize effects from parasitic impedances. For example, in a current-mode interface, the interconnect may be configured between transistors in a cascode configuration with one pair of transistors on one chip an open-drain configuration and another pair of transistors on a second chip in the cascode configuration.
- In addition, in a controlled-impedance interface, configurable impedance transmission lines may be utilized on a photonically-enabled CMOS chip to match impedance with low input impedance of Tx amplifiers in a deep sub-micron CMOS chip, which also mitigates the effects of parasitic impedances resulting from the interconnect between chips.
-
FIG. 2 is a block diagram of split receiver and transmitter electrical interfaces, in accordance with an embodiment of the invention. Referring toFIG. 2 , there is shown a photonically-enabled CMOSoptical communication system 200 comprising a CMOS very-large-scale-integration (VLSI) die 201 and a CMOS optoelectronics die 203. The Tx path in the lower half of the dies comprises a controlled-impedance interface, whereas the Rx path in the upper half of the dies comprises a current-mode interface. - The CMOS optoelectronics die 203 comprises passive optical structures, such as
grating couplers photodetector 221 for the purpose of detecting optical signals in receivers and optical phase modulators. The phase modulators may be configured to function as Mach-Zehnder interferometers, such as theMZI 213, for the purpose of modulating optical signals in transmitters. The optoelectronics die also comprises a common-source pair comprising transistors MCS1 and MCS2, resistors RPD1, RPD2, RT1, RT2 and RB, capacitors C1 and C2, and other integrated circuit elements. The invention is not limited to the number of optical, optoelectronic, and electronic devices shown, as it comprises representative sections of the chips. The CMOS optoelectronics die 203 also comprisestransmission lines 217, alight source 215,unit drivers 227,coupling pads 219, and a bias voltage VB. - The CMOS VLSI die 201 comprises
digital cores coupling pads 219, and a bias voltage VC. The Rx gain stages 207 and the Tx gain stages 209 may comprise suitable gain control elements for communicating signals to and from thedigital cores transmission lines 217 on the optoelectronics die 203. - The
digital cores coupling pads 219 may be low. In this manner, the interface between the CMOS VLSI die 201 and the CMOS optoelectronics die 203 may be less sensitive to parasitic impedances. - The coupling between the CMOS VLSI die 201 and the optoelectronics die 203 is not limited to wire bonds, as illustrated by the
wire bonds 223 inFIG. 2 . Accordingly, other techniques may be utilized, such as solder balls, metal pillars, conductive resin, and/or proximity interconnects, which may comprise capacitively-coupled interfaces or inductively-coupled interfaces, for example. Capacitively-coupled interfaces may comprise thecoupling pads 219 on adjacent chips comprising capacitors for AC-coupling of signals, whereas inductively-coupled interfaces may comprise inductor coils on adjacent chips with signals being communicated between chips via inductive coupling between the coils. - In operation, the optical receiver interface may utilize transistors available on the CMOS optoelectronics die 203. In an exemplary embodiment, the pair of NMOS transistors MCS1 and MCS2 may be in a common-source configuration with the drain terminals coupled to the
coupling pads 219. The CMOS transistors MCS1 and MCS2 may communicate modulated currents to another pair of NMOS transistors MC1 and MC2 in cascode configuration on the CMOS VLSI die 201. The input impedance looking into source terminals of these cascode devices may be very low, and they may accept the currents transmitted by the common-source transistor pair without creating large voltage signals. Low impedance may desensitize this type of interface to parasitic inductance, capacitance, and resistance associated with pads, bond wires, or other interconnect elements. The transmitted currents may then be converted to a voltage signal on the CMOS VLSI die 201 by a pair of resistors RV1 and RV2 connected to drain terminals of the cascode transistors MC1 and MC2. The voltage signal may be further amplified by subsequent voltage gain stages, the Rx gain stages 207, until it reaches an amplitude suitable for use by thedigital cores 205A. - The bias voltage VB may be operable to bias the common-source pair comprising MCS1 and MCS2 and the resistors RPD1 and RPD2 may be operable to bias the
photodiode 221. An electrical signal may be generated by thephotodiode 221 in response to an input light signal. The generated signal may be AC-coupled via the capacitors C1 and C2 to the gate terminals of the transistors MCS1 and MCS2 for communication to the CMOS VLSI die 201 via the cascode circuit comprising MC1 and MC2. - The
transmission lines 217 may comprise tunable impedances for matching to prior gain stages, such as the Tx gain stages 209, and may also provide a low impedance for reduced sensitivity to parasitic impedances in the coupling between the CMOS VLSI die 201 and the optoelectronics die 203. The signals received from the CMOS VLSI die 201 may be communicated to theunit drivers 227, which may be operable to modulate CW optical signals in theMZI 213 received from thelight source 215. -
FIG. 3 is a block diagram of a flip-chip-type photonically-enabled CMOS optical receiver system, in accordance with an embodiment of the invention. Referring toFIG. 3 , there is shown a photonically-enabled CMOSoptical communication system 300 comprising a CMOS VLSI die 301, an optoelectronics die 303, andmetal pillars 323. The CMOS VLSI die 301 comprises a plurality of digital cores shown as thedigital cores coupling pads 319. There is also shown control voltage Vcasc, supply voltage VDD, and output voltages Von and Vop. - The optoelectronics die 303 comprises a
photodiode 321, photodiode bias resistors RD1 and RD2, coupling capacitors C1A and C1B, bias resistors RB1 and RB2, a current source IB, and transistors M1A and M1B. The transistors M1A, M2A, M1B, and M2B comprise a differential cascode circuit for amplifying a signal generated by thephotodiode 321. Themetal pillars 323 may provide electrical interconnection as well as structural support between the CMOS VLSI die 301 and the optoelectronics die 303. The die may be configured in a face-to-face die attachment viametal pillars 323 instead of side-by-side dies connected with bond wires, for example, but the principle of operation and the advantages of low-impedance interfaces are the same. - In operation, the transistors M1A and M1B on the optoelectronics die 303 comprise a differential pair whose drains may be coupled to the
metal pillars 323 that may communicate differential currents to the CMOS VLSI die 323. The cascode transistors M2A and M2B may receive the communicated currents that may be converted to a differential voltage signal via the resistors R2A and R2B and bandwidth-enhancing inductors LP1 and LP2. The optoelectronics die 303 comprises passive and active circuit elements to provide bias voltage for the integrated photodetector, to convert photocurrent to a voltage signal, and to couple the photo-generated signal to the gates of transistors M1A and M1B. -
FIG. 4 . is a block diagram illustrating a controlled-impedance split-transmitter interface, in accordance with an embodiment of the invention. Referring toFIG. 4 , there is shown a photonically-enabled CMOSoptical communication system 400 CMOS VLSI die 401, an optoelectronics die 403, andwire bonds 423. - In an exemplary embodiment, the CMOS
optical communication system 400 comprisestransmission lines 417 as a break point between the CMOS VLSI die 401 and the optoelectronics die 403, which may thus comprise a controlled-impedance interface. The resulting impedance may be relatively low, such as Z0=50Ω, thereby providing an insensitivity to parasitic inductance, capacitance, and resistance associated with pads, bond wires, or other interconnect elements. - The CMOS VLSI die may contain digital cores, such as the
digital cores FIGS. 2 and 3 , where the transmitted signal originates, as well as analog signal conditioning circuitry such as transmission line drivers. The CMOS VLSI die 401 also comprises Tx gain stages 409, a current source IB, transistors MT1 and MT2, andcoupling pads 419. - The optoelectronics die 403 comprises
transmission lines 417, which may provide proper electrical to optical velocity matching for MZI operation. The optoelectronics die 403 also comprises distributed amplifier stages implemented with transistors, as illustrated schematically by theunit drivers 427. These distributed amplifiers may provide large signals to optical phase modulators in the MZI. - The optoelectronics die 403 also comprises sense resistors RS1 and RS2, termination resistors RT1 and RT2, a capacitor CR, and a common-mode feedback circuit comprising a feedback gain stage AFB, a feedback control transistor MFB, and a
replica reference inverter 425. The common-mode feedback circuit may be operable to convert limited-swing current-mode logic (CML) signals to full-swing CMOS signals by matching the common mode voltage on thetransmission lines 417 to the transition threshold of theunit drivers 427. - In operation, signals for transmission may be generated in one or more digital cores in the CMOS VLSI die 401 and communicated to the gate terminals of the transistors MT1 and MT2 comprising a differential pair. The signal amplified by the differential pair may be communicated to the optoelectronics die 403 via the
coupling pads 419, which may be coupled between die using the wire bonds 423. - The coupling between the CMOS VLSI die 401 and the optoelectronics die 403 is not limited to wire bonds, as illustrated by the
wire bonds 423 inFIG. 4 . Accordingly, other techniques may be utilized, such as solder balls, metal pillars, conductive resin, and/or proximity interconnects, which may comprise capacitively-coupled interfaces or inductively-coupled interfaces, for example. Capacitively-coupled interfaces may comprise thecoupling pads 419 on adjacent chips comprising capacitors for AC-coupling of signals, whereas inductively-coupled interfaces may comprise inductor coils on adjacent chips with signals being communicated between chips via inductive coupling between the coils. - The
transmission lines 417 may provide tunable impedances for matching to prior gain stages, such as the differential pair MT1 and MT2, and may also provide a low impedance for reduced sensitivity to parasitic impedances in the coupling between the CMOS VLSI die 401 and the optoelectronics die 403. The signals received from the CMOS VLSI die 401 may be communicated to theunit drivers 427, which may be operable to modulate optical signals in a MZI modulator, such as theMZI 213, described with respect toFIG. 2 . -
FIG. 5 . is a block diagram illustrating a low-impedance split-transmitter cascode interface, in accordance with an embodiment of the invention. Referring toFIG. 5 , there is shown a photonically-enabled CMOSoptical communication system 500 comprising a CMOS VLSI die 501, an optoelectronics die 503, andwire bonds 523. - In an exemplary embodiment, the CMOS
optical communication system 500 comprisestransmission lines 517 on the optoelectronics die 503 as in the controlled-impedance interface shown inFIG. 4 , but also comprises a break point between the CMOS VLSI die 501 and the optoelectronics die 503 utilizing a splitting of the interface circuits in a differential cascode amplifier stage comprising the transistors MT1-MT4, as in the current-mode die interface, described with respect toFIGS. 2 and 3 . This further lowers interface impedance compared to a directly-driven transmission line, resulting in a lower voltage swing and even less sensitivity to interconnect inductance and capacitance. - The interface comprises an open-drain differential driver on the CMOS VLSI die 501 and cascode-configured transistors on the optoelectronics die 503. The open-drain differential driver comprises a pair of NMOS transistors MT1/MT2 and the cascode transistors comprise the transistors MT3/MT4 with the drain terminals of MT1/MT2 and the source terminals of MT3/MT4 connected to the
coupling pads 519 for launching differential current signals into terminatedtransmission lines 517 on the optoelectronics die 503. The optoelectronics die 503 also comprises a common-mode feedback loop, which matches the common-mode voltage on thetransmission lines 517 to the switching threshold of the distributed amplifier stages comprising theunit drivers 527. - The CMOS VLSI die 501 may contain digital cores, such as the
digital cores FIGS. 2 and 3 , where the transmitted signal originates, as well as analog signal conditioning circuitry such as transmission line drivers. The CMOS VLSI die 501 also comprises Tx gain stages 509, a current source IB, transistors MT1 and MT2, andcoupling pads 519. - The optoelectronics die 503 comprises cascode transistors MT3 and MT4, capacitors Cc, and
transmission lines 517, which may provide proper electrical to optical velocity matching for MZI operation. The optoelectronics die 503 also comprises distributed amplifier stages implemented with transistors, as illustrated schematically by theunit drivers 527. These distributed amplifiers may provide large signals to optical phase modulators in the MZI. - The optoelectronics die 503 also comprises sense resistors RS1 and RS2, termination resistors RT1 and RT2, a capacitor CR, and a common-mode feedback circuit comprising a feedback gain stage AFB, a feedback control transistor MFB, and a
replica reference inverter 525. The common-mode feedback circuit may be operable to convert limited-swing current-mode logic (CML) signals to full-swing CMOS signals. - Additionally, transistors on the optoelectronics die 503 may be used to implement analog and control functions associated with the optoelectronic devices. For example, this may include low-noise voltage regulators for integrated photodetectors and calibration or real-time control loops for the MZIs to compensate for the effect of process and temperature variations. Retaining these functions on the optoelectronics die 503 reduces the total number of interconnects between the two dies and keeps sensitive analog and control nets local and isolated from possible sources of crosstalk. Digital state machines and low-speed serial communication interfaces for digital control and monitoring can also be implemented on the optoelectronics die 503 as needed.
- In operation, signals for transmission may be generated in one or more digital cores in the CMOS VLSI die 501 and communicated to the gate terminals of the transistors MT1 and MT2 comprising a differential pair. The signal amplified by the differential pair may be communicated to the optoelectronics die 503 via the
coupling pads 519, which may be coupled between die using the wire bonds 523. - The coupling between the CMOS VLSI die 501 and the optoelectronics die 503 is not limited to wire bonds, as illustrated by the
wire bonds 523 inFIG. 5 . Accordingly, other techniques may be utilized, such as solder balls, metal pillars, conductive resin, and/or proximity interconnects, which may comprise capacitively-coupled interfaces or inductively-coupled interfaces, for example. Capacitively-coupled interfaces may comprise thecoupling pads 519 on adjacent chips comprising capacitors for AC-coupling of signals, whereas inductively-coupled interfaces may comprise inductor coils on adjacent chips with signals being communicated between chips via inductive coupling between the coils. - The
transmission lines 517 may provide tunable impedances for matching to prior gain stages, such as the differential pair MT1 and MT2. The low impedance at the source terminals of the cascode transistors MT3 and MT4 provide reduced sensitivity to parasitic impedances in the coupling between the CMOS VLSI die 501 and the optoelectronics die 503. The signals received from the CMOS VLSI die 501 may be communicated to theunit drivers 527, which may be operable to modulate optical signals in a MZI modulator, such as theMZI 213, described with respect toFIG. 2 . - In an embodiment of the invention, a method and system are disclosed for communicating electrical signals between an electronics die 201, 301, 401, and 501 and an optoelectronics die 203, 303, 403, and 503 via
coupling pads optical communication system coupling pads optical communication system - The electrical signals may be communicated via one or more current-mode interfaces that may comprise a cascode amplifier stage split between source terminals of transistors MC1, MC2, MT1, MT2, M2A, and M2B on the electronics die 201, 301, 401, and 501 and drain terminals of transistors MCS1, MCS2, MT3, MT4, M1A, and M1B on the optoelectronics die 203, 303, 403, and 503. The electrical signals may be communicated via one or more controlled-impedance interfaces that may comprise one or more
transmission line drivers more transmission lines optical communication system contact pads coupling pads wire bonds metal pillars 319, solder balls, or conductive resin. The electronics die 201, 301, 401, and 501 and the optoelectronics die 203, 303, 403, and 503 may be coupled in a flip-chip configuration. - While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims.
Claims (27)
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US8877616B2 (en) | 2008-09-08 | 2014-11-04 | Luxtera, Inc. | Method and system for monolithic integration of photonics and electronics in CMOS processes |
US8831437B2 (en) | 2009-09-04 | 2014-09-09 | Luxtera, Inc. | Method and system for a photonic interposer |
US10425165B1 (en) * | 2008-09-11 | 2019-09-24 | Luxtera, Inc. | Method and system for a distributed optical transmitter with local domain splitting |
US9772460B2 (en) * | 2010-02-23 | 2017-09-26 | Luxtera, Inc. | Method and system for implementing high-speed interfaces between semiconductor dies in optical communication systems |
TWI502689B (en) * | 2012-03-16 | 2015-10-01 | Luxtera Inc | Method and system for a photonic interposer |
US9042684B2 (en) | 2012-09-05 | 2015-05-26 | International Business Machines Corporation | Electro-optic modulator |
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US10606004B2 (en) * | 2018-06-01 | 2020-03-31 | Intel Corporation | Distributed optoelectronic receiver |
US10678113B1 (en) * | 2019-01-23 | 2020-06-09 | Lockheed Martin Corporation | Low impedance optical modulator |
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US9772460B2 (en) | 2017-09-26 |
US10061094B2 (en) | 2018-08-28 |
US20180364430A1 (en) | 2018-12-20 |
US20110206322A1 (en) | 2011-08-25 |
US10302880B2 (en) | 2019-05-28 |
US20180017747A1 (en) | 2018-01-18 |
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