I’ll be doing a deep dive into a few topics centered around Marvell/Celestial AI’s Electro-Absorption (EAM) approach to scaling optical interconnects. The outline of my post is as follows:
Motivation behind Optical Interconnects
An Overview of GeSi Electro-Absorption Modulator (EAM)
Franz-Keldysh effect
Comparison with other external modulation options
🔒Why Component Level Test Vehicles are Needed to Reduce Overall Integration Risk of 3D Heterogenous Integrated Packages
🔒An Introduction to Marvell’s Photonic Link: A Fully Integrated Solution for XPU, CPO, and HBM
🔒Marvell / Celestial OMIB: Overcoming Edge Density Constraints
🔒Photonic Fabric Test Vehicle of EIC, PIC, and FAU w/ measurement results
🔒Example 3D Packaged Application: Photonic Fabric Memory Module
🔒Key Co-design Integration Challenges
🔒Thermal Management
🔒EAM Thermal Stability
🔒Package Thermal Steady State Characteristics
🔒Package Thermal Transient Characteristics
🔒Power Delivery Challenges
🔒How NVIDIA DWDM and Marvell’s EAM technologies can be powerful together
The research from this post comes from various Marvell’s papers from ECTC and their ISSCC forum presentation. I also incorporate relevant fundamental material from ISSCC short courses as well when developing the theory.
This post is aimed at more advanced understanding of some of the cross domain integration challenges that EAM is facing. Fortunately, I provide several references to several of my posts on advanced packaging, optical communications, and power architecture for all knowledge levels to understand the underlying fundamentals of Marvell’s Photonic Fabric Technology.
Before writing this, I wrote a similar deep dive on NVIDIA DWDM summarizing the challenges from both their ISSCC and ECTC papers, having attended both conferences myself. These posts come at a timely moment when NVIDIA recently announced they invested $2B into Marvell and Jensen announced at Computex that Marvell will be the next $1T company, causing the stock to soar.
I recommend you read my other NVIDIA post to put GeSi into its proper context and to read between the lines behind that stance:
Motivation behind Optical Interconnects
Scaling AI compute has many different bottlenecks: power delivery, thermal, and high speed interconnect density. Industry standard interconnects such as PCIe have limitations for the workloads AI compute demands. As a result, there is demand for high bandwidth, long reach interconnects that makes optical communications such an attractive option.
Within the optical domain, lasers themselves can be directly modulated (such as VCSELs) or light coming from lasers can be externally modulated. There are three primarily ways to externally modulate a light in the optical domain: Mach-Zehnder Interferometers (MZI), Ring, and Electro-Absorption. MZI modulates the phase of light travelling through two arms to cause destructive interference when recombined, and ring modulators act as band-stop filters for specific wavelengths of light.
I discuss these options in the following post:
GeSi Electro-Absorption modulators
EAMs operate by optical absorption of light through an electrically-controlled PIN junction at a single wavelength. A PIN Diode is a PN junction diode with an undoped intrinsic region to alter the PN diodes characteristics for the desired application, such as:
High-frequency RF switches / attenuators (Si, GaAs, InP),
High voltage tolerance by lowering the capacitance and thus increases breakdown voltage (GaN, SiC),
Photodetectors by converting optical signals to electric current through generation of Electron-Hole pairs (InGaAs, Ge)
In optical communications, EAMs device have a high bandwidth and a fairly compact footprint for CPO applications.
EAMs have a property called the Franz-Keldysh effect which is where the optical absorption in a semiconductor changes when an electric field is applied. This is shown as the black curve above shifting to the red curve when a bias voltage is applied. This allows a fixed wavelength light to be externally modulated at high speeds by quickly switching the absorption curve at that wavelength to affect how much light goes through.
As shown in the equations above, the extinction ratio can be calculated as the ratio of the exponentials of the absorption factors of the two states, and the insertion loss can be calculated as the exponential of the absorption at 0V. Higher extinction ratio and lower insertion loss is desired.
GeSi is a common material used for the intrinsic region. The GeSi is epitaxially grown on top of a flat surface that is polished with CMP. Light is routed through the GeSi which has a higher refractive index than the surrounding SiO2, thus causing total internal reflection through the device.
One common FoM is the the transmitter penalty (TP) which is a measure of the loss due to the modulation itself and combines both the IL and ER:
Here we see some example characteristics of Marvell’s EAM IL over wavelength over bias voltage and different temperatures. High temperature causes the IL to shift up and affect the absorption that each signal level sees.
This means that temperature stability and thermal management is crucial to ensuring that EAM devices can operating consistently within its intended environment. Sophisticated mixed circuit control circuitry is necessary to anticipate and correct for rapid temperature swings in the GPU and package.
While ring modulators face challenges with precise thermal control and MZI are quite large, EAMs offer a promising “middle range” alterative in high performance CPO applications. Advantages include:
Relatively small size
Good thermal stability
High speed
Low power
High optical bandwidth
However, there are a few downsides:
EAMs struggle from power handling because they absorb light, not phase shift it. There is an almost guaranteed 3dB loss from light passing through the EAM that must be accounted for in the link budget.
I believe the link budget is an extremely important concept that anyone investing in or designing optics needs to know because it affects which components in the supply chain can be successfully integrated in real systems. I dive more into link budgets here in the free section:
EAM devices are non-linear that, if uncorrected, affects the size of the individual eyes for PAM4 where the noise margin is governed by the smallest one.
To combat this, pre-distortion circuits are needed to pre-distort the four voltage levels so that the signal can be sent through the channel and recovered as linearly as possible.
EAM devices are difficult to reliably manufacture at scale. Growing the GeSi epitaxially requires a fairly flat surface and sophisticated means to control the crystal growth. High volume testing is needed to screen for KGD.
In short, GeSi offers several promising benefits for CPO, but does suffer from inherent device level challenges as well as circuit complexity challenges to control the characteristics of the PAM4 signal.
Why Component Level Test Vehicles are Needed to Reduce Overall Integration Risk of 3D Heterogenous Integrated Packages
There are several excellent research works being performed across the industry and academia on components that comprise integrated 3D packaging solutions:
Different substrates: organic, silicon, glass
Different bridge option: LSI, embedded bridge
Interconnect options: C2, C4, and Hybrid bonding
Different integration options: 2.1D, 2.3D, 2.5D 3D, 3.3D, 3.5D.
Different optical component integration and waveguide routing options in each substrate
Here are several of my other posts that give you a fundamental understanding of the key tradeoffs in each of these areas:
