Patent ID: 12199681

The foregoing and other features, aspects and advantages will be more apparent from the following detailed description, taken in conjunction with the accompanying drawings, of example embodiments, which description is by way of example only.

DETAILED DESCRIPTION

The above referenced related U.S. patent application Ser. No. 17/687,803, filed Mar. 7, 2022, entitled “Vertically Integrated Electro-Absorption Modulated Laser and Methods of Fabrication”, discloses monolithically integrated EML of some example embodiments which comprise a DFB laser and an EAM which are vertically integrated, fabricated using MGVI (Multi-Guide Vertical Integration), wherein the DFB laser and the EAM are vertically coupled by a laterally tapered vertical optical coupler.

For example,FIG.1shows a schematic longitudinal cross-sectional view100of an electro-photonic module comprising a monolithically integrated EML. The EML comprises a DFB laser and EAM, fabricated using MGVI. This method of fabrication provides for vertical integration of active and passive components formed in multiple vertically stacked waveguides, and is compatible with single epitaxial growth of III-V semiconductor, e.g. using an InP-based material system. In this example, the structure comprises a semi-insulating (SI) substrate, e.g. Fe doped InP, on which an epitaxial layer stack (which may be referred to as an epi-layer stack or epi-stack or epilayers) is grown to define layers of: a first level waveguide, labelled output waveguide; a second level waveguide, labelled EAM waveguide; and a third level waveguide labelled DFB laser waveguide, on which is defined a surface-etched-grating (SEG). The waveguides are optically coupled vertically by vertical optical couplers formed by laterally tapered portions of the respective waveguides, as illustrated schematically in the schematic plan view200of the monolithically integrated EML ofFIG.1, which is shown inFIG.2. The third level waveguide is processed to define a laser mesa and a first laterally tapered vertical optical coupler1. The SEG is etched in a top surface of the DFB laser portion of the mesa to form the DFB laser and the laterally tapered vertical optical coupler extends from the optical output of the laser over a length of the second level waveguide for vertically optically coupling of the emitted mode from the laser to the second level waveguide. In this embodiment, the EML is monolithically integrated with EAM driver circuitry and laser driver circuitry. The latter comprises bias control for operation of the laser as a CW light source, and optionally comprises other elements, e.g. for temperature sensing, power monitoring, a control loop for temperature stabilization and power adjustment. Additional semiconductor layers for the laser driver circuitry and EAM driver circuitry are provided between the SI substrate and the waveguide layers as illustrated schematically. These layers comprise layers for forming high-speed electronic circuitry for the laser driver and EAM driver circuitry. For example, the additional layers comprise InP based semiconductor layers which are structured to fabricate heterojunction bipolar transistors (HBT), which are labelled as HBT epilayers for electronic circuitry, and a spacer, which comprises one or more layers and includes one or more etch stop layers, which allow for separate processing of the optical components and the electronic circuitry. The laser driver circuitry comprises bias control for operation of the laser as a CW light source, and optionally comprises other elements, e.g. for temperature sensing, power monitoring, a control loop for temperature stabilization and power adjustment.

As shown inFIG.2, which is a schematic plan view200of the electro-photonic module comprising the EML shown inFIG.1, the multilevel optical waveguide structure for the DFB laser, EAM and output waveguide of this embodiment are formed on a first area of the SI substrate, and the electronic circuitry is formed on an adjacent second area of the SI substrate, e.g. laterally spaced from the optical components.

For EML of some embodiments, the epitaxial layer structure is compatible with a single epitaxial growth process, fabricated using III-V semiconductor materials. For example, in some embodiments, the integrated EML is fabricated using an InP-based material system, comprising selected binary, ternary and quaternary and other compositions of In, Ga, As, P, Al and Sb. For example, the SI substrate may be iron-doped InP. Optionally, fabrication may use multiple epitaxial growths.

The general principles of selecting materials and structuring the waveguide layers for vertical optical coupling using laterally tapered vertical optical couplers, i.e. appropriate selection of bandgap wavelength and refractive index, is described in, e.g. U.S. Pat. No. 7,444,055B2 to Tolstikhin, entitled “Integrated Optics Arrangements for Wavelength (de) Multiplexing in a Multi-Guide Vertical Stack”, and references cited therein.

Fabrication of an electro-photonic circuit module comprising an EML and monolithically integrated EAM driver circuitry and laser driver circuitry, using an InP based material systems and vertical integration, provides for miniaturization and a compact design with a small form factor, e.g. an EML module that is ˜1 mm long and less than a mm wide.

Beneficially, the integrated EAM driver and control circuitry comprises a high-speed electro-optical control loop for very high-speed linearization and temperature compensation, e.g. to enable advanced modulation schemes, such as PAM-4 and DP-QPSK, for analog optical data center interconnect applications.

FIG.3shows a schematic block diagram of an electro-photonic module comprising an EML of example embodiments configured for bidirectional operation in a transmitter mode20) and in a receiver mode. The electro-photonic module comprises a laser and an electro-absorption (EAM) modulator, and integrated electronic control circuitry comprising a laser driver, an EAM driver and a transimpedance amplifier (TIA). The laser and the EAM forming the EML are optically coupled, e.g. using laterally tapered vertically optical couplers, or a lateral waveguide optical coupler or butt-coupling. The EAM comprises a first part M1and a second part M2and electrical connections from the EAM driver to the first part and the second part of the EAM so that the first and second parts of the EAM can be independently biased. The control circuitry is configured to bias said electrical connections to operate the first and second parts of the EAM independently in a transmitter mode and in a receiver mode. In the transmitter mode, the laser and first and second parts of the EAM are operable as an EML to provide a modulated optical output; and in the receiver mode, the second part of the EAM is operable as photodiode receiver to receive an optical input and output a photocurrent to the TIA. This configuration saves the die area and power associated with having a separate photo-detector comprising a pin-TIA. The control circuitry has a connection for a bi-directional data bus, and comprises a mode select input for receiving a transmit/receive enable signal to switch between the transmitter mode and receiver mode.

For example,FIG.4shows a diagram to illustrate bi-directional operation of the electro-photonic module ofFIG.3in a transmitter mode and in a receiver mode. In the transmitter mode, the laser is on, to provide a cw output which is optically coupled to the modulator, and the EAM driver drives both parts M1and M2of the modulator to provide a modulated optical output. That is, the device functions as an EML. In the receiver mode, the laser is turned nearly off: the laser driver reduces the laser drive current down to close to the threshold current to reduce the light output; the EAM driver drives the first part of the modulator M1to absorb residual laser light and operates the second part of the modulator M2as a photo-diode receiver to receive optical input and provide photocurrent input to the TIA.

For example, to provide a compact design, the electro-photonic module with monolithically integrated electronics may be fabricated using an InP based semiconductor materials system, with vertical integration, using laterally tapered vertical optical couplers for optical coupling of the laser waveguide and EAM waveguide.FIG.5shows a schematic top plan view300of an electro-photonic module comprising an integrated EML of a first embodiment configured for bidirectional operation as a transmitter and a receiver. The integrated bidirectional EML comprises a laser302and an EAM modulator310which are vertically integrated using a laterally tapered vertical coupler304. In this embodiment, the integrated driver circuitry306provides for driving the EML in a transmit mode and in a receive mode. The driver circuitry306comprises on-chip laser driver circuitry, driver circuitry for the EAM modulator, and also includes transimpedance amplifier (TIA)308.

FIG.6shows a schematic top plan view400of first and second EMLs300A and300B, as shown inFIG.5, configured for bidirectional operation as a transmitter and a receiver wherein one EML is configured as a transmitter and the other EML is configured as a receiver. When one of the EMLs is configured in transmit mode, it operates as an EML as normal, with the laser turned on using the laser driver circuit to provide an appropriate laser drive current, and the EAM driver circuit operates the modulator. When one of the EMLs is configured in receive mode, the EML operates as a receiver by turning “off” the laser, e.g. by reducing the laser drive current to a threshold current Ith plus a small value e.g. +20 mA, and one half of the EAM is biased to absorb the remaining light, and the other half of the EAM operates to see only the light coming from the other end, which is detected by the TIA circuit. Thus in receive mode, the modulator and the TIA becomes the receiver. This configuration saves the die area and power associated with having a separate detector comprising a pin-TIA. During the receive mode operation, if the laser drive current is reduced below the threshold current, to entirely turn off the laser, this would introduce undue delay in turning-on the laser for transmit mode. It is therefore desirable to maintain a low drive current close to, slightly above the threshold current (Ith), e.g. enough above Ith to preserve fast switching, and use part of the EAM waveguide to absorb the remaining laser light during operation in receive mode.

In an example embodiment, the EML is fabricated to be 1.2 mm long and 0.6 mm wide, per channel. This is significantly shorter, e.g. 15 to 20 time shorter in length, than TFLN modulators, and also significantly shorter than any demonstrated micro-ring resonator solution. As an example, in some embodiments of an EML, the laser, EAM and driver circuitry can be integrated for an input of 250 mV in from 50Ω, and for a pin-TIA output of 50 mV into 50Ω, at 100 GB (e.g. 106 Gb/s NRZ or 212 Gb/s PAM4)

A monolithically integrated laser and EAM can be tuned without heat. For a DFB laser, e.g. having a wavelength temperature sensitivity of 0.09 nm/C and a QCSE EAM with 0.46 nm/C, the bias Vb on the EAM can be varied using a temperature sensor based on the bandgap, to control Vb for temperature compensation, e.g. as disclosed in U.S. Pat. No. 10,673,532, issued Jun. 2, 2020, entitled “Electro-absorption modulator with integrated control loop for linearization and temperature control”.

FIG.7shows a schematic top plan view500of a four-port electro-photonic integrated circuit of a third example embodiment comprising four integrated EMLs configured for bidirectional operation as transmitters and receivers. Some dimensions are shown by way of example only. For example, the four-port integrated EML may be integrated on a substrate having a length of 1.2 mm and a width of 1.2 mm, to provide an optical port-to-port spacing of 300 μm. Based on current technology, the port-to-port spacing could feasibly be reduced to 250 μm, or 200 μm, to provide a compact form factor. For example, a transmit bandwidth of >120 GHz allows for 1 Tb PAM-4 on 4 lanes, and an efficiency of 1.1 pJ/bit. Using a single laser for 4 lanes would improve the efficiency to 0.5 pJ/bit.

FIG.8shows a schematic top plan view600of a substrate of a co-packaged optical module of a fourth example embodiment, comprising an arrangement of 128 optical ports provided by InP electro-photonic chiplets and a plurality of switch core ASICs, to illustrate an example use case. In this example, 128 optical ports are provided around the periphery of a 10 mm×10 mm substrate, with a port density of 32 ports per side; e.g. 300 μm port spacing along 9.6 mm each side, and for 250 μm port spacing, along 8 mm each side. In this example, the total I/O area is 38 mm2, leaving a large central area for the ASIC switch cores to be co-packaged on the substrate. The proposed small form factor matches 125 μm diameter optical fibers. The integrated bidirectional EML electro-photonic modules can be provided as chiplets for co-packaging with silicon chiplets, e.g. switch core ASICs.

FIG.9shows a simplified circuit schematic700to illustrate conceptually an example of driving logic for bidirectional operation of the integrated EML of the first embodiment in transmitter and receiver modes. When the EML is operated in transmit mode, it operates as an EML as normal, with the laser turned “on” using the laser driver circuit to provide the appropriate laser drive current, and the EAM driver circuit operates the modulator. When the EML is operated in receive mode, the EAM is operated as a receiver by turning “off” the laser, e.g. by reducing the laser drive current to close to a threshold current Ith, e.g. Ith+˜20 mA, and one half of the EAM is biased to absorb the remaining light, and the other half of the EAM operates to see only the light coming from the other end, which is detected by the TIA circuit. The control circuitry is connected to a bidirectional data bus and comprises a mode select input for receiving a transmit/receive enable signal for switching between the transmitter mode and receiver mode.

Partitioning of the EAM into a first part that acts as a controlling EAM and a second part that acts as a modulating EAM, allows for bidirectional operation of the EAM in a transmit mode and in a receive mode. This configuration saves the die area and power associated with having a separate chip for a detector comprising a pin-TIA, e.g. as described in the above referenced patent publication US20230019783 A1 “Optical Receiver comprising monolithically integrated photodiode and transimpedance amplifier”, e.g. as illustrated schematically inFIG.15AandFIG.15B.FIG.15Ashows a schematic plan view1300of a monolithically integrated pin-TIA chip1310, comprising PIN PD1320and TIA circuitry which have a direct on chip connection, and contact areas1326.FIG.15Bshows a schematic cross-sectional view1310of the PIN PD comprising a substrate1302, HBT epilayers1304for the TIA circuitry, and PIN epilayers1306for the PIN PD.

FIG.10shows a simplified circuit schematic800to illustrate more details of the driving logic for bidirectional operation of the integrated EML of the first embodiment in transmitter and receiver mode, to illustrate components of the InP transceiver chip and CMOS bus logic to switch between transmit and receive modes of the integrated EML. The control circuit comprises an Rx switch that is connected to a mode select input for receiving a transmit/receive enable signal to switch between the transmitter mode and receiver mode.

FIG.11shows a simplified circuit schematic900to illustrate more details of the driving logic for bidirectional operation of an integrated EML of the third embodiment in transmitter and receiver modes, with four channels per laser, e.g. as illustrated schematically inFIG.7. A single laser output is split into 4 channels by a 4:1 optical splitter, to provide optical input to 4 EML, each channel coupled to an individual EML, to further optimize performance, e.g. to reduce power to <0.5 pJ/bit.

Referring toFIG.12,FIG.13andFIG.14, and U.S. Pat. No. 11,092,762 B2 “Surface Mount Packaging for Single Mode Electro-Optical Module” issued Aug. 17, 2021 (which is incorporated herein by reference), this reference discloses examples of surface-mount packaging of electro-photonic modules in which connector technology is miniaturized to allow for vertically-coupled or edge-coupled optical fibers. As an example for surface-coupled optical fibers,FIG.12shows a substrate1002, on which is mounted a surface-mount optical module1004having 4 optical ports1006, and an optical connector1010carrying four optical fiber pigtails1012. The optical connector is removably secured to the surface mount optical module1004by a latch arrangement, such as clip1008. As an example for edge coupled optical fibers,FIG.13shows a substrate1102and a surface mount optical module1104having a plurality of lateral optical ports1106; an optical connector carrying a plurality of optical fiber pigtails is inserted into the optical ports1106, which is removably secured with latches.FIG.14shows a schematic cross-sectional view of one optical fiber1200carried by an optical connector1110inserted into an optical I/O port of a surface mount optical module comprising an electro-photonic chiplet1202, for lateral coupling of the optical fiber to an optical I/O port. This I/O concept is scalable to a plurality of optical I/O ports, along one or more sides of the module, and allows for co-packaging of electro-photonic chips and electronic die on ceramic substrates designed for fully automated assembly, including optical alignment. Materials are selected to be robust enough to withstand soldering processes without damage.

In alternative embodiments, monolithically integrated electro-photonic modules (chiplets) comprising an EML and integrated control electronics for bi-directional operation in a transmitter mode and in a receiver mode may be fabricated using lateral optical coupling, instead of vertical optical coupling, of the laser and EAM waveguides. Structures comprising vertical optical coupling or lateral optical coupling may be fabricated with InP based semiconductor materials, or other semiconductor materials capable of monolithic integration of the optical components of EML and control electronics.

Monolithically integrated electro-photonic modules (chiplets) comprising transceivers of some example embodiments disclosed herein, fabricated with InP based semiconductor materials, and vertical optical integration using laterally tapered vertical optical couplers, provide for a compact design with a small form factor, and provide higher speeds, lower power (e.g. <0.5 pJ/bit), higher reliability, and lower cost, compared with some existing solutions. The electro-photonic chiplets can be co-packed with other electronics, e.g. silicon chiplets comprising switch cores. The small form factor can match 125 μm micron fiber diameters. As an example, an electro-photonic module on a substrate having a 10 mm×10 mm edge, with 128 optical ports can provide high optical performance, e.g. 30 Tb/s, leaving an area of 8 mm×8 mm for processor cores in the central area.

Although example embodiments have been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and not to be taken by way of limitation, the scope of the present invention being limited only by the appended claims