Photonic Integrated Circuits
Photonic integrated circuits (also called planar lightwave circuits = PLC or integrated optoelectronic devices) are devices on which several or even many optical (and often also electronic) components are integrated. The technology of such devices is called integrated optics. Photonic integrated circuits are usually fabricated with a wafer-scale technology (involving lithography) on substrates (often called chips) of silicon, silica, or a nonlinear crystal material such as lithium niobate (LiNbO3). The substrate material already determines a number of features and limitations of the technology:
- Silica-on-silicon integrated optics builds on silicon wafers, for which many aspects of the powerful microelectronics technology can be used. Silica waveguides allow the realization of couplers, filters (e.g. for multiplexers and demultiplexers in wavelength division multiplexing technology), power splitters and combiners, and even active elements with optical gain. They can also be connected to optical fibers. A natural solution for coupling multiple waveguides is to use fiber arrays.
- An area of strong current interest is silicon photonics, where photonic functions are implemented directly on silicon chips.
- An already commercialized photonic integrated circuits technology is based on indium phosphide (InP); it is used mainly in optical fiber communications.
- The relatively new silicon nitride (Si3N4) platform is suitable for photonic devices operating in the 1-μm spectral region or even at shorter wavelengths. Low-loss waveguides with small bend radius can be made, also various types of photonic components such as couplers, filters, arrayed waveguide gratings and others. The high nonlinear coefficient gives the potential for nonlinear signal processing.
- Waveguides can be fabricated on silica glass (fused silica) e.g. with lithographic techniques involving chemical processing or indiffusion of dopants, or with laser micromachining. The latter techniques can be used for fabricating waveguides far below the surface (embedded waveguides), so that three-dimensional circuit designs become possible. Amplifiers and lasers can be made by using rare-earth-doped glasses.
- Lithium niobate (LiNbO3) as a nonlinear crystal material is suitable for devices performing nonlinear functions, for example electro-optic modulators or acousto-optic transducers. Waveguides can be fabricated on lithium niobate substrates e.g. via proton exchange or by indiffusion of titanium, in any case controlled by a lithographic method. Doping with rare earth ions makes possible amplifiers and lasers. The birefringence of this material creates opportunities for polarization control, which may then be used e.g. for filtering purposes. On the other hand, the birefringence makes it more difficult to obtain polarization-independent devices, as are often required for optical fiber communications.
Photonic integrated circuits can either host large arrays of identical components, or contain complex circuit configurations. However, for various reasons the complexity achievable is not nearly as high as for electronic integrated circuits. Their main application is in the area of optical fiber communications, particularly in fiber-optic networks, but they can also be used for, e.g., optical sensors and in metrology.
An important distinction is that between devices with smaller or larger mode areas:
- Some waveguides (e.g. made in silicon-on-insulator technology) exhibit strong confinement, leading to small effective mode areas and allowing for tight bends without excessive bend losses. They are therefore potentially suitable for chips with a very high level of integration. However, such devices are essentially always polarization dependent, having a strong built-it birefringence. Polarization-insensitive designs would be possible in principle, but would introduce unrealistic fabrication tolerances.
- Other waveguides exhibit much weaker guidance and can be made in polarization-insensitive form. However, such waveguides do not allow tight bends and thus prevent a high level of integration.
Photonic integrated circuits can find applications in different area; some examples:
- Optical fiber communications and free-space optical communications can utilize circuits for signal generation, detection, regeneration and other processing.
- Optical metrology e.g. in the form of LIDAR and fiber-optic sensors can profit from integrated circuits where even delicate devices such as interferometers can be realized in a compact and stable manner. Optical frequency metrology can utilize highly compact frequency comb sources as optical frequency synthesizers .
- Terahertz imaging usually involves photonic elements for generating and detecting terahertz waves and processing the signals.
- Quantum cryptography and quantum computing are another field of application.
Very often, photonic integrated circuits are specially designed and fabricated for a specific application. They may then be called ASPIC = application-specific photonic integrated circuits.
Fabrication in Foundries
In microelectronics, the model of foundries has been widely accepted. This means a separation between a company designing and later selling an integrated circuit for a specific purpose and another company (the foundry) for fabricating it. Only the foundry needs to have the complex machinery and detailed know-how for fabrication.
The same model is also suitable for photonic integrated circuits. Again, a complex technology is required, which is mastered by some foundries, and their customers can focus on designing circuits for specific purposes and bringing those into the markets. A suitable interface needs to be developed, where the foundry exactly describes its capabilities and receives the designs to be fabricated. For the designs, certain elements (e.g. for realizing certain device functions like waveguides, couplers, resonators, modulators, photodetectors etc.) may already be predefined and can be appropriately connected by the circuits designer. The foundry may fabricate bare chips or possibly also offer packaging solutions.
A foundry may support different technology platforms, such as indium phosphide (InP), gallium arsenide (GaAs), silicon on insulator (SOI), silica on silicon, etc. Note that these can substantially differ in technical details of the required machinery. It can also be important to over the combination with other technologies such as micro-electronics and micro-electromechanical systems (MEMS).
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