The POET Optical Interposer™

Wafer-level integration of electronics and photonics

The POET Optical Interposer™ utilizes a novel waveguide technology that allows the integration of electronic and photonic devices into a single multi-chip module.  By applying advanced wafer-level semiconductor manufacturing techniques and novel packaging methods, POET’s Optical Interposer eliminates costly components, assembly, alignment and testing methods employed in conventional photonics solutions.  In addition to lowering costs compared to conventional devices, POET’s Optical Interposer provides a flexible and scalable platform for a variety of photonics applications ranging from data centers to consumer products.


What is an Optical Interposer?

Interposers have been used for several years to allow electronic devices (microprocessors, memory devices, etc.) to communicate with each other via high-speed metal traces embedded in a semiconductor wafer. POET invented methods for embedding or placing photonic devices or features (lasers, detectors, waveguides, filters, etc.) within the top layer of an interposer, allowing high speed communication between electronic and photonic devices.


The POET Optical Interposer consists of layers of materials built on a standard semiconductor wafer of any size, using standard semiconductor (CMOS) processes. The bottom layers include the high-speed metal traces found in electronic interposers, while the top-most layers incorporate waveguides that guide and focus light, other passive devices (e.g., filters and gratings) and features which enable the accurate placement and integration of active devices.


The POET Optical Interposer is a platform technology.  It offers the ability to produce Opto-electronic devices of various kinds in high volumes with the efficiencies of wafer-level processing.  The low-cost integration scheme and scalability of the POET Optical Interposer brings value to any device or system that integrates electronics and photonics, including some of the highest growth areas of computing, such as Artificial Intelligence (AI), the Internet of Things (IoT), autonomous vehicles and high-speed networking for cloud service providers.


What makes the POET Optical Interposer so unique?

  • Low-loss, single mode passive waveguides with unique properties that facilitate device integration on any size silicon wafer - adaptable to a wide range of uses
  • Total CMOS compatibility for exploiting true wafer-scale assembly - driving down complexity and cost
  • An elegant  design and assembly scheme that provides maximum flexibility and scalability


The POET OPTICAL INTERPOSER PLATFORM - a novel approach to photonics device integration


Optical Interposer


Photonic components are fabricated or placed on the top layer of a silicon wafer or device produced with standard CMOS processing. High-speed communication is achieved between the electrical layers and the optical layers using the copper traces embedded in the wafer.




CMOS compatibility allows true wafer-scale fabrication, assembly, alignment and testing.  Electrical devices can either be standard photonic control devices (e.g., laser and modulator drivers) or more complex devices, such as graphics processors and network switches. The platform can also be utilized to solve key issues with conventional Silicon Photonics devices, such as laser attach and optical filter integration.


Cross-Section of Optical Interposer Architecture


The POET Optical Interposer Platform:

  • Extends the functionality of electrical interposers with optical waveguides
  • Low Loss, athermal waveguides enable integration with conventional Indium Phosphide and Silicon technologies
  • Waveguides built above top metal enables integration with IC’s
  • Passive alignment of optics using optical reference planes designed into the Optical Interposer plus advanced CMOS packaging technology for “x”, “y” and “z” alignment without active testing


Optical Engines for Transceiver Applications

What is a transceiver?

Transceivers convert digital electrical signals generated by conventional electronic devices into light waves that can be transmitted over optical fibers with higher data density, lower loss and lower cost than copper wire.  In the reverse direction, the light waves are detected and converted back to digital electrical signals that can be transmitted using copper or another transmission medium.



Transceivers used in data communications


Video on demand is the largest driver of mega data centers that are being built at an astonishing rate.  Such data centers are often referred to as the “cloud.” There are hundreds of thousands of transceivers in a mega-scale data center, connecting acres of racks holding thousands of on-line memory storage devices and servers to switches and then to distant points for distribution to users. The cost of building a data center (including the transceivers) is very large, and any technology that promises to reduce cost will catch the attention of the data center operators.  POET’s Optical Engines reduce the cost of transceivers. Under a different configuration, POET’s Optical Interposer also promises to allow a different architecture of the data center itself, by co-packaging network switching devices directly with optics, further reducing costs for data center operators.


What is an Optical Engine?

An Optical Engine (OE) includes only those parts of the transceiver module that convert the digital electrical signals into light signals, something that can only be done using certain types of compound semiconductor materials (e.g., Indium Phosphide, Gallium Arsenide). These materials are fabricated into so-called “active” devices (e.g., lasers, detectors, etc.). In addition, the OE includes waveguides that couple the light from the active devices and move it through “passive” devices (e.g., multiplexers and de-multiplexers, spot size converters) and eventually into the fiber. The reverse process of converting light signals back into electronic signals is also included in an OE.  The Optical Engine is not a complete transceiver, but it is the highest value part of the transceiver module, typically representing over 50% of the cost.



The POET Optical Engine consists of the POET Optical Interposer, lasers, photo detectors, laser monitor, waveguides, and filters integrated within the waveguides for multiplexing and de-multiplexing signals.  The green portion shows an electronic device, such as Trans-Impedance Amplifiers (TIAs) and Laser Drivers that not part of the Optical Engine.  These are placed on a PCB and wire-bonded to the Optical Engine. Depending on the customer’s requirements, the TIA, Laser Driver and other electronic devices can be mounted directly on to the Optical Interposer platform, utilizing the high-speed metal interconnects in the lower layers of the Optical Interposer platform for communication. The current design of the Optical Interposer includes both Copper (Cu) and Aluminum (Al) layers below the waveguide layer, providing excellent thermal conductivity.


How does the cost of building a POET Optical Interposer compare to more conventional approaches?

The costliest steps in the assembly of an optical engine are the placement of lasers, lenses, detectors, filters and fibers on the device.  Each time such a component is positioned, the optical alignment must be tested to ensure optimal transmission of light.  This can only be done one single placement and alignment at a time. Each time it is done and fails, production yield falls. There are several “yield points” associated with conventional approaches to transceiver design.


Conventional Approach Cost Drivers:

  • Most companies utilize this conventional approach to transceiver design
  • Consumable materials such as solder, silver paste and UV epoxies are needed
  • Active alignments of laser to waveguide is required
  • Other incidental materials like carriers, plates and bases, lenses, etc. are used
  • Expensive CAPEX investment is needed to scale production
  • And, there are many yield points to manage that can lower yield and increase cost



In contrast to Conventional approaches, the POET approach:

  • Eliminates active alignment of the laser to waveguide (huge savings)
  • Eliminates the need for many incidental materials, including consumables
  • Minimizes expensive alignment equipment for scaling production
  • Reduces number of yield points
  • Utilizes a known good OE die to increase final yield and lower cost



Wafer-level bonding and testing, passive alignment with embedded mux/de-mux, use of known good die - all reduce BOM cost, yield points, assembly and testing costs as well as required CAPEX investment.


Inside the POET Optical Interposer

What are the key components to the Optical Interposer?


POET Optical Interposer Key Components:


POET’s proprietary waveguides are the essential component to the POET Optical Interposer. Waveguides guide light through a device, from laser at one end to optical fiber at the other. In between those two ends, fabricated out of the waveguide material are “passive” devices that require no separate controls to function. They include de-multiplexers that separate light into different wavelengths and multiplexers that mix signals into a back into a single beam. In an industry first, POET has built a Coarse Wavelength Division Multiplexer (CDWM) filter using our unique low-loss material that is fully integrated into a POET Optical Interposer-based Optical Engine.
“Active” devices include lasers, photodetectors and modulators that are fabricated out of compound semiconductor materials. In POET’s case they are built from Indium Phosphide, a material that emits light when charged with an electric current.  The POET Optical Interposer uses a version of active devices that are constructed to allow them to be “flip-chipped” onto the surface of the Interposer.  Importantly, this design allows a wide variety of active devices available from many sources to be integrated onto the POET Optical Interposer platform.  This feature provides maximum flexibility for device designers to use the POET Optical Interposer for a wide variety of applications.
Mirrors built into the waveguides using POET’s proprietary waveguide material increase the flexibility of the POET Optical Interposer even further, allowing certain industry-standard devices, such as “top-entry” photodetectors to be used instead of the “side-entry” ones designed by POET. The smooth facets of the waveguides reduce optical losses while the light travels into and out of the waveguides, increasing the efficiency of the coupling of active devices on one end and the optical fibers on the other. The ability to increase the size of the light beam using integrated spot-size converters also increases coupling efficiency.


How does the POET Optical Interposer achieve chip-scale integration and wafer-scale assembly and testing?

Combined, all the features noted above and those shown below, enable the passive “pick-and-place” assembly of the POET Optical Interposer, eliminating the need for active optical alignment required by conventional approaches.



To retain its elegant design and to enable placement of devices without the costly procedure of active alignment with each device placement, lasers and other active devices are designed to be “flip-chipped” onto the surface of the POET Optical Interposer.  All passive components (mux – de-mux filters, gratings, spot-size converters, etc.) are built into the POET Optical Interposer waveguide layer. Any active device, including lasers, modulators, detectors, etc., can be coupled into the waveguide. Because loss from the waveguides is low across a wide energy spectrum, including visible and non-visible light, the POET Optical Interposer can be used to enable a wide variety of applications, providing a truly versatile platform technology.



An Optical Interposer-based Optical Engine for a 100G transceiver



From fabrication of an 8” POET Optical Interposer wafer, to the placement of active devices and fiber blocks onto the wafer using pick and place tools, to the testing of each Optical Engine, to the capping, sealing and singulation of the devices, the entire process is done at wafer scale, providing an across the board reduction in cost.


The power of wafer-scale fabrication, test and assembly – drives down cost with unlimited scalability.



A POET Optical Engine for a 400G transceiver – over 500 devices processed at the same time on a standard 8”silicon wafer.


POET Optical Engine Performance

Notwithstanding the cost reductions and scalability achieved with the POET Optical Interposer approach to building an Optical Engine, how does it measure up to other technologies with respect to performance?


Waveguide-Integrated Mux and DeMux Performance


Each of the lines of this 4-channel waveguide-integrated Multiplexer was formed from testing hundreds of individual devices.  The smoothness of the lines shows the consistency of performance, with virtually no insertion loss.



A high level of performance of a De-Multiplexer is more difficult to achieve.  Shown are the results of both Single and Multi-Mode waveguide integrated DeMux devices.



Both Single Mode and Multi-Mode 4-channel filters show minimal “insertion loss”, i.e., loss experienced from the filter itself, Pass Band and Cross talk well within specifications for overall system performance.  Desired “flat tops” are achieved for each channel, providing uniform tolerance of incoming light within an acceptable bandwidth with minimal noise.


POET’s own design of a high-speed, side-entry PIN photodetector is capable of operating at speeds suitable for 400G transceivers.



For more information about POET’s Optical Interposer Platform, please contact the Company directly.