Definition: the technology dealing with the construction of photonic integrated circuits
Integrated optics is a technology which aims at constructing so-called integrated optical devices or photonic integrated circuits or planar lightwave circuits, containing several or many optical components which are combined to fulfill some more or less complex functions. Such components can e.g. be optical filters, modulators, amplifiers, lasers and photodetectors. They can, e.g., be fabricated on the surface of some crystalline material (such as silicon, silica, or LiNbO3) and connected with waveguides.
The original inspiration of integrated optics came from the technology of electronic integrated circuits, which has shown rapid development over several decades and has led to amazing achievements, such as complex and powerful microprocessors containing many millions of transistors, specialized signal processors and computer memory chips with huge data storage capacity. Unfortunately, integrated optics has not been able to match the progress of microelectronics in terms of the complexity of possible devices. This results from a number of technical limitations:
While electronic circuits can contain extremely small wires, optical components need to be connected via waveguides, the dimensions of which usually cannot be much smaller than the wavelength, and which often cannot tolerate very sharp bends. (This limitation might be eliminated by using waveguides with very high index contrast, e.g. nanofibers or photonic bandgap waveguides.)
Optical connections, e.g. between waveguides, and couplers are significantly more critical than electrical connections.
Waveguides, device connections and passive optical components exhibit optical losses, which often need to be compensated with optical amplifiers. These are larger and more complex than electronic amplifiers based on transistors.
Some types of optical components can hardly be miniaturized.
There are different material platforms, which differ substantially in terms of capabilities and limitations, and it is not always easy to find a platform which can meet all requirements of an application. In contrast to that, the CMOS platform for microelectronics can cover most requirements of that area.
For these reasons, integrated optical circuits have not reached by far the complexity of electronic integrated circuits. However, devices of moderate complexity can still be useful for example for optical fiber communications, where they can host multiple data transmitters and/or receivers, consisting of distributed feedback lasers, optical modulators, photodiodes, and optical filters (e.g. in the form of arrayed waveguide gratings). Recently, new hope for a powerful and cost-effective integrated optical technology has arisen from developments in silicon photonics.
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