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The concept of the photonic integrated circuit is similar to the concept of electronic integrated circuits. However, electronic integrated circuits integrate transistors, capacitors, resistors, and other electronic devices, while PIC integrates various optical devices or optoelectronic devices, such as lasers, electro-optical modulators, photodetectors, optical attenuators, optical multiplexers/demultiplexers, and optical amplifiers.
It's also known as all-optical photonic integrated circuits. The integrated devices of this kind of PIC are all passive optical devices, such as optical filters, optical multiplexers/demultiplexers, and adjustable optical attenuators.
We also call it photoelectric photonic integrated circuits, which can integrate active optical devices such as lasers, modulators, PIN detectors, and optical amplifiers. It can also integrate passive filters such as optical filters and adjustable attenuators. Active photonic integrated circuits often integrate optical devices of different materials, so it’s more difficult to implement. Until 2004, dozens of active and passive optical devices successfully integrated on one chip, making active photonic integrated circuits available for commercial use.
Optical Filters
It usually refers to a photonic integrated circuit with less than 10 monolithic function components, such as a laser with an integrated modulator and the optical transceiver module. Small-scale PIC products have been available for widespread commercial use.
The number of monolithic function components is 10-50. In addition to the integration of several functional components, it can also be the parallel integration of multiple channels. The medium-sized photonic integrated circuits commercial products are basically the integration of passive optical devices.
The number of monolithic functional components is greater than 50, which means that each wavelength integrates several functional modules, realizing the concurrent integration of multiple wavelengths at the same time, for example. The large-scale photonic integrated circuits that have been used commercially are only the Infinera PIC products. One of the famous ones is the 10×10Gbit/s optical transmission photonic integrated circuit. It integrates 10 wavelength channels, and each wavelength channel realizes the integration of the laser, modulator, adjustable attenuator, and other devices. The large-scale photonic integrated circuits can achieve photonic integration to the greatest extent. In the long term, it is the development direction of photonic integration in the future.
Integration Level | Number of Monolithic Function Components |
Small-scale | 10 |
Medium-scale | 10-50 |
Large-scale | >50 |
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 and silicon dioxide is the basic material used in the production of electronic integrated circuits. Its price is low and it has 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 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.
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 have 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 are 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.
WDM Operating Principle
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.
As with dividing electronic integrated circuits, photonic integrated circuits can also be divided into hybrid integration photonic integrated circuits and monolithic integration photonic integrated circuits.
Hybrid integrated photonic integrated circuits refer to the integration of different optical devices with a single function in one device. Many common optical devices are implemented by using hybrid integration technology. However, as the physical characteristics (such as expansion coefficient) and packaging requirements of the materials are different, the optimal materials used for active and passive optical devices are not the same, which makes it very complicated to integrate multiple discrete components and ensure device performance, especially in the implementing of the large-scale photonic integrated circuits.
In contrast, a monolithic integration photonic integrated circuit is applied in various active and passive optical devices with one material, so there are no adaptation issues between different materials. Compared to hybrid photonic integrated circuits, Single-chip photonic integrated circuits have obvious advantages in terms of energy-saving and reliability.
From the above analysis, we can see that the key technologies of photonic integrated circuits mainly include the following aspects:
● What materials and processes are used to achieve photonic integrated circuits; ● Different optical devices often use different substrate materials. How do we integrate the optical devices of different materials, or how can we realize the functions of the all-optical device on the same substrate material; ● How to improve the large-scale production capacity of photonic integrated circuits. This will be the key to reducing the cost of photonic integrated circuits and achieving large-scale applications. |
Because of the overall upgrade of the network, telecom operators need to invest a lot of capital and manpower, and due to the continuous influx of data traffic on the network, operators have to upgrade the optical transport network(ONT) with less and less profit.
Most of the practical WDM(Wavelength Division Multiplex) transmission systems use separate components. In order to make use of the optical fiber resources to achieve long-distance transmission of 40G and 100G, 40G and 100G systems need to use a more complex modulation format. As a result, the structure of the optical transceiver is extremely complicated, and the cost is greatly increased. For example, a telecom operator with 10% of the Internet transmission capacity will need 4000 DWDM transponders every day, and the number of technicians should be increased by 200 times. How to reduce unit costs while upgrading the network has become a major problem for operators and equipment vendors. To solve this problem, photonic integrated circuits will play a vital role.
Structure of an Optical Transceiver
By integrating many optical components into a single chip, the large-scale single-chip photonic integrated circuits greatly improve the size, power consumption, and reliability of the system and greatly reducing the cost.
First, the application of a photonic integrated circuit greatly reduces the number of independent optical devices required for the transmission system, and the number of packaging times. Generally, the cost of the optical device packaging and related assembly process accounts for more than half of the entire cost. For complex optical devices, packaging and assembly costs can even be as high as 80%. Therefore, integrating dozens of optical devices into a single chip and then packaging it can greatly reduce the cost of the system.
Besides, the integrated photonic integrated circuit eliminates the fiber connection between different optical devices, thereby avoiding the need for high-precision fiber coupling and reducing the coupling cost. At the same time, each fiber coupling is a possible failure point. Under the influence of mechanical dither, temperature changes, and vibration, the fiber coupling is prone to fail. Therefore, the use of fiber coupling will reduce the reliability of the system.
For this reason, 70% of optical communication equipment failures are caused by fiber coupling failures. After adopting the photonic integrated circuit, the reliability of the system will be significantly improved. Moreover, when we upgrade an optical transmission system with a photonic integrated circuit, all-optical devices inside the photonic integrated circuits need only be upgraded once, which greatly saves the upgrade cost. Therefore, photonic integrated circuits can meet the needs of current network upgrades, and can greatly increase the transmission capacity while reducing unit costs.
The development of small and medium-scale integration technologies is relatively mature now. Common products include passive photonic integrated circuits (such as AWG, ROADM, etc.) and active photonic integrated circuits (DFB + EA laser, DFB laser array, etc.). Some optical device companies are also working on the integration of tuned lasers and Mach-Zehnder modulator products to achieve tunability on XFP transceivers.
Schematic View of a Mach-Zehnder Modulator
From the current perspective, when there is no substantial breakthrough in silicon-based photonic integrated circuits, monolithic integration with indium phosphide material is an effective solution to achieve large-scale photonic integrated circuits. However, the development of PIC technology is very slow since it’s introduced to us 40 years ago, and even large-scale single-chip photonic integrated circuits have only achieved a breakthrough in recent years. How to integrate active and passive optical devices on a single chip has always been an inherent technical problem.
The bursting of the Internet bubble in 2001 also had a great impact on the development of PIC technology. Many manufacturers chose to withdraw from the R&D of PIC technology. In 2004, PIC technology made an important breakthrough. The commercial 100Gbit/s light-emitting and the light-receiving PIC chips using indium phosphide materials were launched, integrating more than 50 optical components and 6 different functional units.
The overall design of PIC technology is not limited to pursuing the specific performance indicators of individual optical devices, but to treat these devices as a whole and strive to maximize their overall network performance. In the process of making large-scale photonic integrated circuits based on indium phosphide, manufacturers combine the advantages of indium phosphide and silicon and carefully consider the growth process of two to improve the repeatability of production and process control. Indium phosphide is a rare material, so the price of InP PIC chips is much higher than that of electronic integrated circuit chips with silicon-based materials, but it is the only material that can achieve large-scale photonic integrated circuits. In addition, manufacturers have made substantial progress in PIC chip technology of 10×40Gbit/s and 40×40Gbit/s, which will be applied in high-speed transmission equipment in the future.
Infinera is a leader in large-scale PIC technology and industry. Alcatel-Lucent Bell Labs is also conducting research on photonic integrated circuits and is committed to developing 10×10Gbit/s PIC chips for the commercial “1696” DWDM(Dense Wavelength Division Multiplexing) equipment. The chip uses a high-performance AlInAs / GaInAs APD detector, and its maximum transmission distance without amplification can reach 70km. Besides, Bell Labs is also developing a 100Gbit/s PIC chip with a new modulation format and has proposed a new design and concept to implement coherent detection. In recent years, Huawei has also invested a lot of energy in the development of the PIC field. In 2011, it launched a 100Gbit/s PID chip for commercial use.
DWDM Functional Schematic
The great success of Infinera PIC products has attracted the attention of many other equipment manufacturers and research institutions on PIC research. They're all striving to keep up with the pace of Infinera, and also committed to the study of silicon-based large-scale PIC technology.
Silicon-based materials have some natural defects in the realization of photonic integrated circuits. They can not achieve lasing by themselves, can not be used as a photodetector in the commonly used communication band, and can not realize the function of electro-optic modulation. However, due to the great success of silicon-based electronic integrated circuits, many research institutions commit to using silicon-based materials in PIC research. Intel produced the first silicon-based lasers in the world through hybrid integration. It develops a chemical processing technology that integrates an InP laser on a silicon-based substrate. The main driving factor for Intel to keep efforts to develop silicon-based photonic integrated circuits is its urgent need for high-speed photonic interconnections among chips and interconnections among motherboards. It envisages a large-scale silicon-based PIC that integrates a silicon-based laser and a high-speed electro-optic modulator with a transmission capability of 1Tbit/s. The modulation bandwidth of the modulator is 40Gbit/s and 25 wavelengths are integrated.
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:
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.
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
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.
This is the key technology in the realization of the photonic integrated circuit. The logic devices of Electronic logic devices are very simple and easy to implement, and the photonic logic devices are only simple logic, which is bulky and can only be applied in the laboratory. Though the research of integrated optical logic devices is still in the basic stage, it’s still the prerequisite for the implementation of optical logic signal processing. Only by making breakthroughs in the integration of optical logic devices can it be possible to truly implement photonic integrated circuits.
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