Silicon Photonics: Opening the Door to High-Speed Optical Communications
Research Team Led by Professor Hao-Chung Kuo
Department of Photonics, National Yang Ming Chiao Tung University
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I. Introduction |
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Figure 1. Fields of Application for Silicon Photonics |
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II. An Overview of Silicon Photonics |
Why use silicon instead of another material for optical communication transmission? Figure 2 is a chart comparing silicon and other materials used in optical communication transmission. Take, for example, the III-IV material InP. Its advantage is that it allows the light source to be directly integrated, eliminating the need for additional external light source coupling and packaging. However, the disadvantage is that the current process for this material is just below 4 inches. The yield is poor and the cost is high. The advantage of using glass is that the manufacturing cost is low and the material is easy to obtain. The disadvantage is that the process technology is still under development and therefore unstable. Finally, there is Polymer. Its advantages are low costs and a fast process, but it has poor reliability, its quality is difficult to control, deformation happens easily, and it is easily impacted by high-frequency transmissions.
 Figure 2. A Comparison of Optical Communication Waveguide Materials |
Silicon photonics refers to the use of silicon as a base material for the manufacturing of optical components to achieve the transmission, modulation, detection and processing of optical signals. In contrast to traditional electrical signals, optical signals can transmit data at higher speeds while reducing power consumption, making silicon photonics a key player in high-performance computing and high-speed networks.
The core advantages of silicon photonics technology include:
- High-Speed Transmission: The bandwidth of optical signals far exceeds that of electrical signals. It can reach and even exceed hundreds of GHzs.
- Low Energy Consumption: Optical signal transmission has a lower energy loss, which means a lower heat generation and lower power consumption than electrical transmissions.
- High Integration: Silicon photonics technology is compatible with existing CMOS processes, enabling single-chip integration of electronic and photonic components.
- Scalability: It can be mass produced using existing semiconductor processes, thus reducing production costs.
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III. Key Technologies of Silicon Photonics |
The core components of silicon photonics systems include active components (such as lasers, modulators and detectors) and passive components (such as waveguides, couplers and beam-splitters). It is the waveguide component that is responsible for the transmission and manipulation of optical signals. These core technologies are as follows:
- Active Components:
- Laser Light Source: This is typically a high-powered (>40mW) Distributed Feedback Laser with wavelengths in the O-band and C-band used as a side-emitting light source.
- Light Detector: A Ge-doped optical detector, usually located at the receiving end Rx, receives optical signals from the output end Tx and converts it into an optical current which is then processed into a signal by an electronic chip such as a TIA.
- Optical Modulator: By controlling the RF power supply, the modulator can be changed to produce a switching effect with the light. When the speed is increased, a high-frequency digital signal is generated.
- Passive Components:
- Mux/Demux: This is usually used for wavelength division, multiplexing and optical mixing in for medium and long-distance transmission. It is used mainly for processing multi-mode light of different wavelengths.
- Coupling I/O: Optical coupling, which introduces external light into silicon photonic components, is divided into Edge coupling, Grating coupling, and V-groove coupling.
- Optical Filter: Optical filters can be used in medical testing.
- Interferometer/Switch: Modulates the phase of the light and adjusts its direction
- Splitter/Combiner: Used mainly for splitting single-mode optical signals into multiple outputs or multi-channel optical integration, optical power distribution and direction adjustment
- Polarization Diversity: Polarization control; TE (transverse electric field) and TM (transverse magnetic field) modes have different propagation characteristics. This can lead to some devices being suitable only for specific polarization states. For example, MZI modulators and optocouplers mainly support the TE mode, so the TM mode may be unable to transmit effectively.
- Waveguide: The main path taken by light within a silicon photonic chip, known as the optical path, functions like the circuit within an IC.
 Figure 3. Silicon Photonics Components are Comprised of Active, Passive and Waveguide Structures (Intel) |
2.5D packaging technology occupies an area between traditional 2D packaging (chips on a planar circuit board) and 3D packaging (multilayer chip stacking). It mainly involves using silicon Interposers to place multiple chips (such as processors and memory, etc.) on the same plane and enhancing performance using Through-Silicon Via (TSV). By incorporating silicon photonic components to produce optoelectronic integrated modules will convert some of the high-speed electrical signals into optical ones, thereby reducing losses and achieving high-speed transmission.
 Figure 4. Basic optical components in 2.5D packaging systems include laser light sources, modulators, demodulators, microring resonators and optical waveguides. |
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1. Light Modulation Technology |
Light modulation is one of the cornerstones of silicon photonics technology as it determines how efficiently electrical signals are converted into optical signals. Common light modulation technologies include:
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Figure 5. In a traditional Mach-Zehnder Modulator, two waveguides are electrically modulated to produce optical output signals with different phases. |
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Figure 6. Microring/Microdisk Modulator and the Schematic Diagram of the as Seen from the Structure from the Side |
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Figure 7. Microring-Assisted MZM
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Figure 8. Schematic Diagram of an Electro-Absorption Modulator and Performance Enhancement Through the Use of Graphene Integration |
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2. Optical Modulation Mechanisms in Silicon Photonics |
Optical modulators use various physical mechanisms to change the properties of light in order to achieve high-speed data transmission. Some of the main modulation mechanisms include:
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Figure 9: Optical Modulation Mechanisms and the Related Theoretical Frameworks in Silicon Photonics |
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3. Silicon Photon Detectors |
Photodetectors (PDs) are responsible for converting optical signals into electronic ones, making them an indispensable part of silicon photonics technology. Common photodetectors include:
- Germanium/Silicon (Ge/Si) Photodetectors: These detectors utilize the light absorption properties of germanium integrated with silicon chips to achieve high-performance detection.
- Graphene Photodetectors: These detectors take advantage of graphene’s high carrier mobility and broadband absorption properties to achieve ultra high-speed detection.
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4. Silicon Photonic Waveguides and Optical Connections |
- Waveguide Technology: Silicon photonic waveguides direct optical signals to different components, improving overall system performance.
- Optical Interconnections: Silicon photonics can be applied in data centers and supercomputers to reduce transmission delays and power consumption.
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IV. Silicon Photonics Applications |
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1. High-Speed Optical Communications |
The main applications for silicon photonics technology are in high-speed optical communications, such as:
- Data Center Interconnection: Silicon photonics is used to increase the speed of data transmission between servers.
- Fiber to the Home (FTTH): Silicon photonic modules are used to improve the performance of fiber optic networks.
- Satellite and Space Communications: Silicon photonics technology can be used in optical laser communications to enhance long-distance transmission capabilities.
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2. High-Performance Computing (HPC) |
- Artificial Intelligence and Machine Learning: Silicon photonics technology can accelerate data transmission, thereby improving AI computing.
- Quantum Computing: Silicon photonics is being used in the development of quantum computing platforms.
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3. Biomedicine and Sensor Technology |
- Optical Biosensors: Silicon photonic biosensors can be used in disease diagnoses. For instance, it can be used in the detection of the COVID-19 virus.
- Environmental Monitoring: Optical sensors can be used to detect air and water pollution.
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V. The Future of Silicon Photonics |
- Improve the modulation speed and bandwidth
- Research new materials, such as silicon nitride and lithium niobate (LiNbO₃) to improve light modulation performance.
- Reduce power consumption and thermal effects
- Reduce power consumption and temperature variation by adopting the use of new technologies such as graphene and photonic crystals.
- In-Depth Integration of Photonics and Electronics
- Develop silicon photonic chips that can be more closely integrated with existing electronic components and improve overall computing performance.
- Expand Emerging Applications
- Promote further development of silicon photonics applications in fields such as biomedicine, smart sensing and quantum communications.
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VI. Conclusion |
Silicon photonics technology is advancing rapidly, driving innovation in optical communications, computing and sensing technologies. By continuing to improve modulation efficiency, reduce power consumption and enhance integration, silicon photonics will become one of the cornerstones of future digital infrastructures. In the times to come, advancements in manufacturing technology and material science will enable silicon photonics technology to further expand its applications, changing the way we communicate and compute. With the exponential growth in the volume of data, the demand for high-speed optical modulators in modern optical communication networks is also on the rise. The table below summarizes the performance, advantages and challenges of different types of modulators in silicon photonics technology to assist in the selection of the most suitable modulation technology according to various application requirements.
Table 1. An Overview of the Capabilities of Different Types of Silicon Photomodulators
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Parameters |
Mach-Zehnder Modulator (MZM) |
Ring Modulator |
Ring-Assisted MZM (RAMZM) |
Electro-Absorption Modulator (EAM) |
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Bandwidth (GHz) |
Max >110 GHz Standard: 30–110 GHz |
Max >77 GHz Standard: 30–77 GHz |
Max: 58.5 GHz Standard: 8.5–58.5 GHz |
Max: 89 GHz Standard: 26.8–89 GHz |
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Voltage-Length Product (Vπ·L) |
Min. : 0.003 V·cm Standard: 0.003–1.6 V·cm |
Min. : 0.52 V·cm Standard: 0.52–0.8 V·cm |
Min. : 0.025 V·cm Standard: 0.025–1.73 V·cm |
Not enough data |
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Extinction Ratio (ER, dB) |
Max: >50 dB Standard: 3.15–50 dB |
Max: 25 dB Standard: 3.5–25 dB |
Max: 30 dB Standard: 8–30 dB |
Max: 14.15 dB Standard: 3–14.15 dB |
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Insertion Loss (IL, dB) |
Range from about 1.7 dB to 18 dB, depending on length and material of device |
From less than 0.7 dB to about 14 dB, typically on the low side |
From 2 dB to 10.5 dB, depending on design complexity |
From 1.8 dB to 6.2 dB, affected by material absorption |
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Component Size |
Lengths range from 0.12 mm to 3 mm, occupies a relatively large area |
With a radius of 3.7 µm 至 15 µm, it is very compact |
With an area of about 80×60 µm², it’s on the larger side; includes ring and interferometer structures |
At about 40×0.3 µm², its structure is simple and compact |
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Data Rate (Gb/s) |
Max: 560 Gb/s Standard: 80–560 Gb/s |
Max: 330 Gb/s Standard: 128–330 Gb/s |
Max: 320 Gb/s Standard: 12–320 Gb/s |
Max: >112 Gb/s， Standard: 32–112 Gb/s |
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Advantages |
High linearity, wide bandwidth, suitable for long-distance transmission, performance can be improved via hybrid integration |
Compact size, high-speed, low power consumption suitable for high-density, integrated applications |
Achieves better extinction ratio and bandwidth, strikes a balance between insertion loss and bandwidth |
High modulation efficiency, low power consumption, and CMOS compatibility graphene integration can be used to extend the spectral range |
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Challenges |
Silicon MZM has limited bandwidth, and manufacturing process for hybrid materials is complex |
The balance between speed, power consumption and thermal stability is delicate, making it sensitive to manufacturing errors |
Design and manufacturing processes are more challenging due to requiring simultaneous integration of a ring and an interferometer |
Needs to deal with the contact resistance in graphene devices to balance speed and energy efficiency |
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Common Materials |
Silicon, Lithium Niobate (LiNbO₃), Silicon Organic Composite Materials |
SOI (Silicon Insulator), Silicon Nitride (SiN), Hybrid Materials |
InP (Indium Phosphide) Thin Films, SOI Built-In Microring Structure |
Germanium (Ge), Silicon Germanium (SiGe) Quantum Wells, Graphene-Silicon Hybrid Structures |
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Typical Applications |
Long-distance communication, data centers, high-capacity networks |
Chip interconnections, data centers, and compact modulation applications |
High-performance RF photonics and applications that require high extinction ratios and bandwidths |
Data centers, telecommunications, fiberoptic wireless communications, high-speed optical detection |
