To meet the growing demand for data created by 5G, the Internet of Things, and cloud and streaming video, many data centers are upgrading their networks to 400-Gbit/s optical links using photonic integrated circuit (PIC) technology. Compared with electronic integrated circuits, PICs have several advantages, including lower power consumption, much higher frequency coverage, and greater efficiency. Furthermore, in combination with III-V materials, silicon substrates are being used to fabricate hybrid PICs that generate less heat, provide greater electronic mobility at room temperature, and allow faster transfer of electrical currents through silicon semiconductors.
A photonic integrated circuit (PIC) or integrated optical circuit is a device that integrates multiple (at least two) photonic functions and as such is similar to an electronic integrated circuit. The major difference between the two is that a photonic integrated circuit provides functions for information signals imposed on optical wavelengths typically in the visible spectrum or near infrared 850 nm-1650 nm. The integration of photonic components such as lasers, optical amplifiers modulators,MUX/DEMUX components and photodiodes on chip would enable them to be used in optical signal processing, optical communication, biophotonics, and sensing applications.
Examples of new enabling optical technology for 5G transport include self-tunable, multivendor interoperable, bidirectional dense wavelength division multiplexers (DWDMs). These devices increase bandwidth over existing fiber networks by combining and transmitting multiple signals simultaneously at various wavelengths on the same fiber. Also important are bidirectional, “gray” point-to-point devices for connectivity between antennas over very short distances (<2 km); 400G coherent optical interfaces for short distances (<20 km); and high-speed (50G to 200G) direct-detection optical interfaces.
As with datacom, integrated photonics is playing a crucial role in enabling this new generation of low-cost, high-speed, tunable, and small-form-factor technology. An example of a company developing PIC-based devices for 5G is PICadvanced. The company currently provides a range of pluggable NG-PON2 (next-generation passive optical network 2) transceivers that supports a data rate of 10 Gbit/s. These devices come with a standard interface for small-cell base stations and can directly feed common routers with PON ports.
For these reasons, PICs have shown significant benefits for data centers and cloud computing by enabling simpler, more reliable, and higher-bandwidth communications. An example of a company that is providing PIC-based transceivers for data centers is HG Genuine, which offers a range of pluggable transceivers from 100G to 800G. However, although several new PIC-based processes have recently been brought to the data communications market, several challenges remain for the transceivers’ full implementation.
Material Platform Technologies
Manufacturers currently fabricate PICs on a variety of waveguide platforms, including InP, silicon nitride (SiN), silicon carbide, and lithium niobate, as well as glass and various polymers. Because each material has its advantages and disadvantages in the specific applications in which it is used, different PIC foundries have come to prefer particular waveguide materials
The base materials used in photonic integrated circuits mainly include silicon and silicon dioxide, lithium niobate (LiNbO3), gallium arsenide (GaAs), and indium phosphide (InP).
Silicon / Silicon Dioxide
Silicon and silicon dioxide is the basic material used in the production of electronic integrated circuits. Its price is low and it has a stable performance. The processing technology is simple and mature, and the yield is high, which is very suitable for large-scale production. But silicon-based materials have three fatal flaws when applied in photonic integrated circuits:
● Its laser emission efficiency is very low, so the silicon-based lasers are very difficult to produce.
● Silicon-based materials cannot detect the light of 1310nm and 1550nm wavelengths, which are exactly the bands used for optical communications;
● Due to the limitations of silicon-based materials, electro-optic modulation cannot be implemented.
Lithium Niobate Crystals
Many research institutions such as Intel have been studying silicon-based active optical devices and strive to make breakthroughs. Silicon-based materials are mainly used in passive photonic integrated circuits like the array waveguide grating (AWG), and the hybrid integrated large-scale photonic integrated circuits with silicon-based materials has made some progress.
Lithium niobate crystals are mainly used to make high-performance electro-optic modulators with high modulation bandwidth and good modulation linearity. However, the lithium niobate crystal cannot be applied for lasing, nor can it be used as a photodetector. At the same time, its processing technology is very complicated. so for large-scale photonic integrated circuits, lithium niobate crystal has no practical application value.
Although gallium arsenide materials can be used for active photoelectric devices, the device can only work in the wavelength range of 850nm because of the intrinsic bandgap of gallium arsenide. Therefore, active optical devices with gallium arsenide materials is only applicable to the local area network. And for long-distance and large-capacity WDM(wavelength division multiplexing) transmission systems, its application is greatly restricted.
The most commercially utilized material platform for photonic integrated circuits is indium phosphide (InP), which allows for the integration of various optically active and passive functions on the same chip. Initial examples of photonic integrated circuits were simple 2 section distributed Bragg reflector (DBR) lasers, consisting of two independently controlled device sections – a gain section and a DBR mirror section.
Consequently, all modern monolithic tunable lasers, widely tunable lasers, externally modulated lasers and transmitters, integrated receivers, etc. are examples of photonic integrated circuits. Current state-of-the-art devices integrate hundreds of functions onto single chip. Pioneering work in this arena was performed at Bell Laboratories. Most notable academic centers of excellence of photonic integrated circuits in InP are the University of California at Santa Barbara, USA, and the Eindhoven University of Technology in the Netherlands.
Indium phosphide material can simultaneously integrate active and passive optical devices, and ensure the operating waveband is of 1310nm and 1550nm that are widely used in optical communications. Meanwhile, standardized semiconductor production processes can be used to achieve mass production to save costs.
The use of indium phosphide materials can simultaneously realize laser emission, detection, optical amplification and electro-optic modulation, as well as wavelength multiplexing/demultiplexing, optical switching, and dispersion compensation, making it possible for the production of large-scale monolithic indium phosphide photonic integrated circuits.
Future Technology Directions
Whether in technology or in the market, photonic integrated circuits have made breakthrough progress in recent years. However, there is still a huge gap between the photonic integrated circuit and electronic integrated circuits in terms of integration, performance, and cost.
It can be predicted that the future development direction of PIC will mainly focus on the following aspects:
The Use of Indium Phosphide Materials
As the substrate, indium phosphide has its own shortcomings. It is a rare material, so that the cost of InP PIC products is relatively high. And at the same time, using indium phosphide as the base material is not convenient for large-scale integration of the existing silicon-based materials, which won’t contribute to the large-scale integration of photonic devices and electronic devices in the future.
Companies are working hard to make indium phosphide the only material that can achieve commercial large-scale photonic integrated circuits. It can be predicted from the situation that silicon-based photonics is difficult to make a breakthrough in the next few years. Therefore, it is wise to use indium phosphide material as the substrate for large-scale PIC development.However, the use of indium phosphide also faces many technical problems. How to improve integration and chip performance, and how to further simplify the process and reduce costs are still in need of continued research.
Fraunhofer Heinrich Hertz Institute (Fraunhofer HHI), a major European organization involved in photonics integration, has a foundry dedicated to the fabrication of InP PICs for both research and industrial use. For Fraunhofer HHI, the advantages of InP over SiN and other platforms include the following:
• Complex structures containing lasers, amplifiers, detectors, gratings, and waveguides can be integrated in a monolithic process.
• InP is a more developed technology; therefore, engineers are more experienced with it than with other platforms.
• With PICs, InP (a relatively expensive semiconductor material) can be used very efficiently with active and passive optical functions. In addition, the integration effort of the hybrid systems — which is otherwise required when integrating different functional components — is eliminated.
• Because established and detailed design kits are available, people with limited knowledge of photonics can design their own PICs after only a couple weeks’ training.
Ultra-compact integrated photonic device using tungsten diselenide—with a photonic waveguide reported in March 2022
Photonic integrated circuits employ optical isolators that allow photons to travel in only one direction, which prevents light from re-entering the system and destabilizing it. But guiding light in one direction often requires large magnets, making these circuits difficult to create on a small scale.
Researchers at University of Chicago’s Pritzker School of Molecular Engineering (PME) have developed a new way to guide light in one direction on a tiny scale. By coupling light confined in a nanophotonic waveguide with an atomically thin, two-dimensional semiconductor, the researchers exploited the properties of both the light and the material to guide photons in one direction.
To create a new element for photonic circuits, High and his team interfaced a two-dimensional material—tungsten diselenide—with a photonic waveguide. The unique properties of the material’s band structure enables it to interact with light differently based on the helicity of the light’s polarization. In nanophotonic structures, where light is confined below its wavelength, circular polarization arises naturally, and the helicity is locked to the light’s propagation direction.
This means that light emitted from the tungsten diselenide will couple into the waveguide in a preferred direction. The team can also switch this biased coupling on and off by adding electrons to the system, creating a tunable emission router on a tiny micron-sized length scale.
The result – a small, tunable on-chip photonic interface – could lead to smaller photonic integrated circuits that could be more easily integrated into modern technologies, including computing systems and self-driving cars.
That small scale design and versatile fabrication method will help integrate such photonic elements into existing optoelectronic systems. An obvious application would be for on-chip lasers, which could find application in self-driving cars as part of a LIDAR navigation system (a system that uses laser pulses to measure ranges). The photonic element could be configured as an on-chip isolator, allowing compact protection of the laser system.
Ultimately, these types of photonic devices could be integrated into future optical computers, which would compute with light instead of electrons, using less energy and creating less heat. “We already use photonics to carry information throughout the country in fiber-optic networks, but advances like this could help fully control the flow of light on the nanoscale, thus realizing on-chip optical networks,” Hao said.
Continued Research on Silicon-Based Large-Scale PIC
Silicon-based materials are widely used in electronic integrated circuits. Silicon-based electronic integrated circuits have not only achieved great commercial success, but also profoundly changed the lifestyle of human beings. Since the birth of PIC technology, people have been working hard to achieve photonic integrated circuits through the mature technology and process of electronic integrated circuits. However, the luminous efficiency of silicon-based materials is very low, and the device can not detect the light of 1310nm and 1550nm. Moreover, it cannot achieve electro-optic modulation. These greatly limit the development of silicon-based PIC technology.
The research direction of silicon-based photonic integrated circuits mainly focuses on the implementation of silicon-based active optical devices through hybrid integration, and the study of achieving photonic integrated circuits with the use of CMOS technology. Companies and research institutions such as Intel and Bell Labs are all conducting research in this area, and have achieved certain progress.
Silicon photonics (SiP)
Silicon photonics (SiP) is a disruptive technology that is poised to revolutionize a number of application areas, for example, data centers, high-performance computing (HPC). The key driving force behind SiP is the ability to use CMOS like fabrication resulting in high-volume production at low cost and possible monolithic integration with CMOS electronics. This is a key enabling factor for bringing photonics to a range of technology areas where the costs of implementation using traditional photonic elements such as those used for the telecommunications industry would be prohibitive.
As the information rates required from transceivers continue to increase, more sophisticated monolithic integration techniques are being deployed both within the discrete devices themselves and also in the creation of advanced integrated transmitter and receiver circuits. Band-gap engineering through epitaxial regrowth and selective area growth allow unprecedented chip-level function and performance in an ever-decreasing footprint. Lasers are combined with modulators, amplifiers, multiplexers, detectors and hybrids in wafer-scale processes.
Silicon photonics (SiP) research can be dated back to the 1980s. However, the previous decade has witnessed an explosive growth in the field.
SiP is entering a phase of increased commercialization and manufacturing growth in datacenter and HPC communication markets, due to the cost and power efficiency that high-density integration and WDM bring. The SiP working spectral range is from 1.1 micro-m up to 2 micro-m. This spectral range is possible extended by Ge on Si up to 8 m without leaving the CMOS technology.
SiN material platform
SiN material platform is a versatile dielectric waveguide platform, called TriPleX and patented by LioniX International, which is based on alternating silicon nitride and silicon dioxide films. Fabrication with CMOS-compatible equipment based on low-pressure chemical vapor
deposition enables the realization of stable material compositions being a prerequisite to the control of waveguide properties and modal shape. The transparency window of both materials allows for the realization of low-loss waveguides over a wide wavelength range (400 nm–2.35 μm). Propagation losses as low as 5 × 10–4 dB/cm are achieved. PIC enabled products outperform equivalent combinations of discrete components at the functional level.
InP and SiN are perfectly complementary. A successful hybrid platform integrating InP and SiN components will offer, therefore, superior performance for both active and passive components and is a very promising approach for complex PICs which require very low propagation losses, e.g. in delay lines or high-Q filters. Ultra-low linewidth lasers by combining InP and TriPleX chips from LioniX and Fraunhofer HHI have recently been reported. The TriPleX platform will not only allow the combination of efficient InP lasers with ultra-low loss passive waveguides, but it will also enable testing of InP circuits on wafer scale by using TriPleXbased optical waveguide probes.
LIGENTEC — a manufacturer of PICs for ground and space communications, as well as for quantum, lidar, and biosensor applications — focuses on SiN. In LIGENTEC’s view, SiN has important advantages over other platforms in these specialties, due to its low propagation loss, its ability to stabilize temperature, and its usefulness for dispersion engineering and for the ability to carry multiple watts of optical power per waveguide. Moreover, SiN is already employed as a material in CMOS foundries and can be efficiently scaled to high-volume production.
Phase change material breakthrough
University of Southampton researchers have been able to demonstrate a new material family that will revolutionise optical circuits to replace parts of traditional electronic hardware. The materials that have been developed allow rapid reversible switching between two states, known as phase change, which has previously been limited to electronic circuits as standard commercially available materials suffer from large optical losses.
Scientists from the Quantum, Light and Matter group and Optoelectronics Research Centre (ORC) have designed the phase change materials to exhibit no loss of light at telecommunication wavelengths and be switched with very low power. The technology is compatible with existing silicon photonic circuits and opens the door for more advanced applications. Researchers have published their findings in Advanced Functional Materials. Lead authors Dr Matthew Delaney and Dr Ioannis Zeimpekis pinpointed the material structure and composition to enable high transparency while exhibiting low power modulation of light. They found that the new composition has 100 times less loss than the current state-of-the-art optical materials.
Their material was deposited on top of optical chips, where a short laser pulse was used to crystalize the material and change the phase of the guided light. The researchers demonstrated this property reversibly thousands of times. Importantly, the material remembers its last state without any applied signals, leading to large potential power savings. Professor Otto Muskens, Head of the Integrated Nanophotonics group, said, “This new technology will simplify and enable newly emerging applications such as solid-state LiDAR, quantum and neuromorphic computing that are currently limited by the performance of the existing materials.
“Neuromorphic and programmable photonics are set to revolutionise the industry as they offer new paradigms for data processing going far beyond existing hardware. Quantum optical circuits are on the horizon and ultralow loss components are needed to make the next step in controlling and routing quantum information.” Traditional communication electronics consume a huge proportion of their energy at the interconnection level, and their bandwidth is directly limited by the communication length. Using photons instead of electrons alleviates these shortcomings.
Phase change photonics offer much promise for the future of integrated silicon photonic circuits, with some of the world’s largest companies competing in the race for fully integrated optical solutions. However, the high absorption losses in current commercially available materials have prevented their use in larger photonic systems such as in interconnects between data servers, where the photonic technology is projected to excel. Professor Dan Hewak, ORC co-author said, “This is a significant breakthrough for optoelectronics. Our team has now demonstrated a material which bridges the gap between electronics and photonics and we expect to see further advances resulting from their discovery.”
The new phase change family has been designed as part of a series of research projects funded by the Engineering and Physical Sciences Research Council (EPSRC), including ChAMP, the Manufacturing and Application of Next Generation Chalcogenides (EP/M015130/1), Cornerstone (EP/L021129/1), The Physics and Technology of Photonic Metadevices and Metasystems (EP/M009122/1) and Nanostructured photonic metamaterials (EP/G060363/1). The new materials reside within the chalcogenide family as they combine antimony and sulfur or selenium.
The team is currently working to implement more photonic circuit components with the aim to design a neuromorphic computing photonic chip with in-memory computing capabilities. It is expected that this method will replace current technologies within the next couple of years enabling a leap forward for the technology of photonic computing.
As PIC technology development continues, novel schemes will emerge either to enable new applications by combining different technologies in a reliable way or to continue scaling the density of components for VLSI photonic systems. Research lines are targeting new technology
nodes within a period of five to ten years. For both SiP and SiN, lasers and amplifiers have to be integrated in a hybrid way.
Common Approaches for Hybrid Integration of InP PICs:
(a) Edge-coupling method through active alignment between the PICs (b) Edge-coupling approach employing flip-chip alignment
(c) Surface-coupling approach with grating couplers (d) Surface-coupling approach using 45° vertical m
By hybrid integration we under stand an approach in which different chips are coupled after processing. Hybrid integration technology has made considerable progress. It is especially favorable where the PIC requires component properties which cannot be provided by a single platform, for example optical gain and very low loss. The integration of InP active components (lasers and modulators) is on the roadmap of LioniX International, and will be implemented in the TriPleX PDK and offered in MPW services.
By heterogeneous integration we understand an approach in which an unprocessed or partially processed wafer or die of different materials, e.g. InP is bonded to a processed wafer, e.g. silicon photonics or CMOS, and further processed on wafer scale after bonding. Heterogeneous
integration of InP lasers and amplifiers on SiP circuits offers more flexibility in placement of lasers and amplifiers than hybrid integration, at the cost of added complexity in the fabrication process, because processing of both InP and SiP circuitry is now required.
Further, the coupling between the InP and the SiP layer introduces coupling losses of 1 dB or more, and the coupling structures
require significant space, of in the order of 100 μm per coupler depending on the substrate design. Heterogeneous integration of InP-Photonics on Silicon Electronics avoids optical coupling losses as only one PIC technology is used, and offers high-performance electrical connections.
The approach consists in adapting the generic InP photonic integration process such as to make it suitable for wafer scale bonding onto a (Bi) CMOS wafer in which the driver, receiver and control electronics are integrated. The bonding is achieved by means of a polymer layer that is optically and thermally insulating, and thermal, mechanical and electrical connections are made with vias through the bonding layer.
The InP membrane on Silicon (IMOS) platform aims at further reducing the footprint of photonic devices by moving towards a high contrast waveguide technology embedded in an InP membrane that can be fabricated on silicon wafers. It has seen significant progress since the
start of the technology and now offers a range of passive waveguide components such as MMIs, ring resonators, directional couplers, dielectric and metallic grating couplers, and polarization converters.
Silicon photonics technology is leading an epochmaking technology transformation of the optical network industry from the discrete-component era into an integrated-chip era that features automation and large-scale production. The influence of the technology is akin to the
impact on the electronic circuit industry caused by the transformation from the electron tube era to the transistor integrated circuit era.
Graphene, a type of two-dimensional material, is currently used on photonic components in research phase. Compared with silicon, graphene can theoretically bring higher bandwidth, low drive voltage, and smaller dimensions. Additionally, the manufacturing of graphene components is compatible with the silicon-based CMOS process. Therefore, the current silicon photonics technology and stand an approach in which different chips are coupled after processing process can be used for manufacturing graphene components.
Using graphene materials in photonic components means combining the advantages of the two types of materials, implementing next-generation beyond-silicon photonics technology. In the future, silicon photonics technology working with graphene materials can achieve various optical switching, optical routing, optical logic, optical storage, and optical signal processing functions of next generation all-optical networks. These components are superior in implementing a new optical manipulation mechanism and have the potential to change the component types and structures of existing optical networks, dramatically simplifying optical networks and implementing revolution
of the optical communications industry. Optical packet switching networks with optical memories will be possible realized.
New Tech Marries the Best of Photonics and Electronics on the Same Silicon Chip
Researchers from MIT, the University of California at Berkeley, and Boston University has earlier announced they’d managed to build a microprocessor that combined electronic and optical components using existing manufacturing processes. They built their device on “silicon-on-insulator” wafers, which feature a layer of silicon dioxide glass beneath the top silicon layer. This is the material typically used for silicon photonics and for some high performance electronic chips, but it is much more expensive than the bulk silicon used for most microchips.
Now the same researchers have unveiled a new technique in the journal Nature that allows for combined electronics and photonics using the same low-cost starting material and processes as conventional microchips. They devised a way to add islands of silicon dioxide to bulk silicon and then deposit a thin film of polycrystalline silicon on top. This film could then be fashioned into both photonic components over the glass islands and electronic components over the bulk silicon. Aside from making it possible to use a much cheaper starting material, the new technique’s use of standard manufacturing processes means that chipmakers interested in adding photonic components to their devices .
Making both sets of components from the same material did require a bit of compromise. With polysilicon there’s a trade-off between optical and electrical efficiency so it took a fair bit of optimizing to find the right combination of silicon types, deposition techniques, and processing temperatures and time. Writing an analysis in the same issue of Nature, Goran Mashanovich, a professor at the UK’s University of Southampton who works on silicon photonics, also notes that the microchip built by the researchers used 65-nanometer transistor processes. That technology came out in 2006 and semiconductor giants have now progressed to 10-nanometer processes, so the approach’s success is likely to depend on whether it can be further scaled down.
High performance InP photonics for large-scale micro-transfer printing
Photonic integrated circuit (PIC) technology powers the modern internet, enabling video calls, data-mining and associated cloud services. Besides that, PIC technology is also penetrating into applications of medical diagnostics, sensing and ranging, quantum signal processing and artificial intelligence. These applications are driving unprecedented levels of integration and complexity. But the circuit-level performance metrics are stretching the limits of the established PIC platforms. The three major PIC platforms, namely InP, silicon and silicon nitride (SiN), each excels in certain key metrics, but also shows critical weakness.
It is the goal of the EU H2020 project INSPIRE to combine the power of InP technology with the full capability of silicon and SiN photonic technology, creating and leveraging volume manufacturing techniques. INSPIRE will leverage micro-transfer printing technology to realize heterogeneous InP/SiN photonic integrated circuits, combining the best of both the InP and Si/SiN photonics technologies. Wafer scale coverage of InP devices on SiN circuits will be enabled, using new and innovative hybrid building blocks and printing methods. This can facilitate order-of-magnitude changes in density, performance and function of the PICs.
Photonic Integrated Circuits Market
The photonic integrated circuits market is expected to cross US$ 3,168.8 Mn by 2028, expanding at a CAGR of 18.2% during the forecast period 2020 to 2028. Superior benefits offered by photonic integrated circuits in terms of power consumption (energy efficiency), size, speed, and cost are collectively driving the photonic integrated circuits (PIC) market. In addition, escalating demand for high speed communication, especially in optical communication field have further fueled market momentum. These factors are expected to contribute towards a compounded annual growth rate (CAGR) of 18.2% during the forecast period 2020 – 2028. However, slower transition towards digitization and issues related to design and fabrication are some of the major challenges faced by the photonic IC market.
Indium Phosphide (InP) is the most preferred raw material used in photonic integrated circuits. In 2019, Indium Phosphide accounted for around 30% of the global market revenue. It is expected to remain the most preferred raw material used for photonic integration during the forecast period 2020 – 2028.
The dominance of Indium Phosphide can be attributed to its ability to integrate active as well as passive optical functions onto one single material substrate. In addition, other benefits offered in terms of cost, reliability, and energy efficiency make Indium Phosphide a preferred raw material for photonic integration. Other raw materials including silicon and silicon-on-insulator are also widely used in photonic integrated circuits on account of low cost, easy availability, and simple fabrication offered by these materials.
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