定义： 
具有光学功能的集成回路 

光子集成回路（也被称为平面光波回路，或者集成光电装置）是指其中存在一些光学器件（通常也存在电子器件）集成在其回路中。这一装置采用的技术为集成光学技术。光子集成回路通常制备在二氧化硅，硅或者非线性晶体材料例如铌酸锂的衬底上。衬底材料对这一技术产生一定的限制： 

1. 硅基二氧化硅集成光学建立在硅片上，会用到许多成熟的微电子技术。二氧化硅波导可以得到耦合器，滤波器（用于波分复用中的复用器和解复用器），功率分束器和合束器，甚至具有光学增益的有源器件。还可以与光纤连接。 
2. 一个的热点领域是硅光子学，直接采用硅芯片实现光子学功能。 
3. 已经实现商用的光子集成回路是基于磷化铟（InP）的技术，通常用于光纤通信中。 
4. 可以用二氧化硅玻璃（熔融二氧化硅）制备波导，采用与化学过程和掺杂物内扩散相关的光刻技术，或者采用激光微加工技术。后者可以制备远离表面的波导结构（嵌入式波导），因此可以实现三维回路的设计。放大器和激光器采用稀土掺杂玻璃得到。 
5. 铌酸锂（LiNbO3）是一种非线性晶体材料可用于具有非线性功能的装置中，例如，电光调制器或者声光换能器。在铌酸锂沉底上可以制备波导结构，例如，通过质子数交换或者钛内扩散，都是利用的光刻技术。掺杂稀土离子可以得到放大器和激光器。而该材料的双折射效应可以实现偏振控制，因此可以用于滤波等。然而，双折射导致很难得到与偏振无关的器件，而光纤通信通常需要器件与偏振态无关。 

光子集成回路不仅可以集成相同器件组成的阵列，还可以包含复杂的回路的结构。但是，由于一些原因目前能实现的复杂结构还不能像电子集成回路那么复杂。主要的应用领域是光纤通信，尤其是光纤网络，也可以应用到光学传感和测量学中。  
具有小模式面积和大模式面积器件间的重要区别： 

1. 一些波导结构（采用绝缘硅片技术）具有强的束缚，有效模式面积很小，这样在很大的弯曲时也不会有附加的弯曲损耗。因此适合应用在集成化程度高的芯片中。但是这种器件与偏振有关，因此具有很强的双折射。原则上需要进行偏振不敏感的设计，但又会引入加工公差。 
2. 其它的波导结构对光束缚弱，但是可以是偏振不敏感的。然而，这种波导不能承受强烈的弯曲，因此不能应用于很高集成度的情况。

Acronym: PIC or PLC = planar lightwave circuit

Definition: integrated circuits with optical functions

Alternative term: planar lightwave 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.

Application Areas

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 [8].
• 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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