What on earth is silicon photonics?

Silicon photonics is a popular concept in the field of optical communication. Many technology giants, including NVIDIA, Intel, and Cisco, are actively promoting silicon photonics. The industry generally believes that silicon photonics will be the future of optical communication.

So, what exactly is silicon photonics? Why do we need to develop it? And how does it work?

In this article today, let's find out.

What is Silicon Photonics

Before introducing silicon photonics, let's first look at a traditional optical communication infrastructure model:

This model should be relatively easy to understand. Two network devices each have their own optical modules. An optical module is an "optical - electrical converter" that can convert electrical signals into optical signals and vice versa. Between the optical modules is an optical fiber, which transmits optical signals.

After the optical signal reaches the device, it is converted into an electrical signal by the optical module, and then sent to the switching chip through the electrical channel inside the device for data processing.

SerDes is a key part of this electrical channel. It is an abbreviation for SERializer/DESerializer. We can understand it as a "serial - parallel converter + channel", as shown in the following figure:

As we all know, optical communication has high speed, low energy consumption, low cost, and better anti - interference ability, far superior to electrical communication using copper media.

If we want to improve the performance of the entire communication system, we should: Change all data transmission channels to optical channels.

There are two implementation ideas:

1. Place the optical module as close to the switching chip as possible to shorten the distance of the electrical channel:

In fact, SerDes has always been a communication bottleneck. In the past, when the bandwidth of communication devices was not high, SerDes was barely sufficient.

Now, with the surging wave of AI, the network interfaces of computing clusters often require bandwidths of 400G, 800G, or even 1.6T. This poses a huge challenge to the electrical channel.

In fact, the electrical channel is struggling. Electrical communication has large losses. If the distance of the SerDes channel is a little longer, the signal will attenuate significantly, and the speed will drop sharply.

2. Let's think one step further. Since we want to place the optical module as close to the switching chip as possible, can we simply integrate the optical module and the switching chip into "one chip"?

Yes! This technology of "co - packaging" the network switching chip and the optical engine (optical module) is the very popular CPO (Co - packaged optics) technology in the field of optical communication today.

Behind the CPO technology, the technical concept of "integrating multiple optical devices on a silicon - based substrate" is silicon - based optoelectronics, also known as "silicon photonics".

More simply:

The CPU and GPU in computers, as well as the SoC in mobile phones, are basically semiconductor chips based on silicon materials, which are integrated circuits.

Silicon photonics combines silicon semiconductor technology with optical communication technology, manufacturing and integrating optical devices on silicon wafers to realize the transmission and processing of optical signals, becoming a "integrated optical circuit".

Architecture and Principle of Silicon Photonic Optical Modules

Next, let's take a look at the technical details of silicon photonics by comparing silicon photonic optical modules with traditional optical modules.

The main function of an optical module is to emit and receive light. A traditional optical module contains multiple components, including active devices such as lasers (light sources), modulators, and detectors, as well as passive devices such as lenses, alignment components, and fiber end - faces.

When manufacturing a traditional optical module, these devices need to be manufactured separately first and then assembled into a complete optical module. This process can be called "discrete device packaging".

A traditional optical module contains both electrical chips and optical chips.

Some electrical chips provide supporting functions for optical chips, such as LD (laser driver), TIA (transimpedance amplifier), and CDR (clock and data recovery circuit). Some are responsible for adjusting the power of electrical signals, such as MA (main amplifier). In addition, there are also complex digital signal processing (DSP) chips.

Optical chips are mainly responsible for the conversion of optical and electrical signals, such as laser chips and detector chips.

Electrical chips are mainly based on silicon - based materials. Optical chips are mainly based on III - V group semiconductor materials, such as InP (indium phosphide)/GaAs (gallium arsenide).

Here is an explanation. There are three main types of semiconductor materials, including single - element semiconductor materials, III - V group compound semiconductor materials, and wide - bandgap semiconductors.

The III - V group compounds InP (indium phosphide) and GaAs (gallium arsenide) belong to the second - generation semiconductors, which have the advantages of high frequency, good high - and low - temperature performance, strong radiation resistance, and high photoelectric conversion efficiency, so they are very suitable as the substrate materials for optical chips.

There are many types of lasers. Different types use different semiconductor materials. You can refer to the following table:

Now let's look at silicon photonic optical modules.

Silicon photonic optical modules use CMOS manufacturing processes (the same processes used to manufacture electrical chips, such as photolithography, etching, and deposition) to directly manufacture modulators, detectors, and passive optical devices on silicon - based (Si) materials. Their integration level is significantly higher than that of traditional optical modules.

Internal structure of a silicon photonic optical module (Source: Intel)

Zooming in:

Source: Intel

In terms of function, silicon photonic optical modules are actually similar to traditional optical modules. They all have the following architecture:

Source: "Research on 400G FR4 Silicon Photonic Transceiver Module" (Song Zeguo et al.)

The only difference is that silicon photonic optical modules integrate all devices, making them more compact:

Architecture of a 400G silicon photonic optical module (Source: Intel)

Source: imec

The following is a schematic diagram of the packaging structure of a silicon photonic optical module:

Next, let's take a look at the specific implementation of each part one by one.

Laser

For an optical module, emitting light is the first step. And light emission mainly depends on the laser.

Interestingly, silicon photonic technology has no problem with other devices, but the laser is its biggest shortcoming.

Silicon is an indirect - bandgap semiconductor, and its inherent property is not suitable for light emission (the efficiency of photon emission when electrons and holes recombine is low). Therefore, when manufacturing silicon photonic optical modules, lasers are usually not directly fabricated on silicon chips. Instead, III - V group semiconductor materials such as InP and GaAs in traditional optical devices are used to make lasers, which are then "externally attached" to the silicon - based chips. The attachment methods include heterogeneous integration and epitaxial growth (monolithic integration).

Currently, the industry tends to use CW (Continuous Wave) laser chips as external light sources. This type of laser has a stable working state, can emit continuous laser light, and has the advantages of good coherence, high reliability, tunable wavelength, and long service life.

Modulator

After having light, we also need to modulate it so that it can represent more "0s and 1s". After modulation, the bandwidth of the optical signal is increased, enabling it to support higher speeds.

In silicon - based electro - optical modulators, the most widely used modulation mechanism is the plasma dispersion effect: by applying a voltage, the carrier concentration in the silicon material is changed, thereby changing the refractive index and absorption coefficient, and then controlling the intensity or phase of the optical signal.

Schematic diagram of silicon photonic devices

Common modulator schemes based on the plasma dispersion effect include the Mach - Zehnder Modulator (MZM) and the Micro - ring Resonator (MRR) modulator.

Mach - Zehnder modulator and micro - ring modulator

Mach - Zehnder modulator and micro - ring modulator

The micro - ring resonator is a closed - loop optical waveguide structure made of curved waveguides. Its resonant wavelength is related to the manufacturing material, structural characteristics, whether charge is injected, or the temperature is changed. Currently, the micro