A photonic integrated circuit (IC) is a complicated IC or chip that integrates several optical devices into a single photonic system. It consists of several photonic parts that function with the help of light, or photons. These devices are analogous to electronics ICs with several embedded optical components, such as optical lasers, optical amplifiers, detectors, de-multiplexers, multiplexers and attenuators.
In electronic ICs, an electron flux moves via electronic components including inductors, resistors, capacitors and transistors, whereas in its photonic counterpart, photons travel via optical devices such as phase shifters, lasers (analogous to transistors), waveguides (analogous to a resistor) and polarizers.
Operation of photonic ICs
Photonic ICs utilize a laser source to produce light, which is harnessed to power optical devices, just as activating a switch produces electricity to power electrical components. Integrated photonic technology solves problems common to electronic circuits such as heat generation and integration by utilizing photons rather than electrons. This takes photonic technology-based products to the next level, which is ruled by the “more than Moore” concept to enhance data transmission speed and capacity.
Typically, in a photonic IC, the signals are applied with wavelengths ranging from the visible spectrum to the infrared, primarily between 800 nm to 1700 nm. In 2005, a quantum noise problem arose when a laser light was being developed with a silicon-based electronic IC, which deferred this generation. To fix this issue, a photonic IC was used, which produced the laser in a single medium and with higher bandwidth.
Photonic IC fabrication methods
Two major categories of photonic IC synthesis are hybrid photonic fabrication and monolithic photonic fabrication.
In hybrid photonic fabrication, ICs are developed using a single platform that includes several photonic components that are utilized for the same function. This method allows for the incorporation of many optical devices.
In the monolithic photonic fabrication method, several optic devices with dissimilar functions are joined together to develop a single photonic IC. Such device manufacturing is complex as many fabrication materials must be integrated in a single substrate. In the end, several tasks can be performed on a single IC board.
Benefits of photonic ICs
Photonic ICs deliver several benefits such as higher speed, miniaturization, large integration capacity, low thermal effects and support of current processing methods that result in lower expenses, high volume manufacturing and high yield. The whole IC-based system becomes more compact and discrete with the use of optical devices and assists in delivering high performance. Photonic ICs can even be incorporated with fundamental electronic circuits and thus, can be used for developing more functions.
Although it is rare, photonic ICs are susceptible to the neutron flux effect that can deteriorate their function. However, when it comes to the hazards of electromagnetic pulse, these devices do not cause performance issues associated with electronic ICs.
Substrate materials for photonic ICs
The major substrate materials used to fabricate photonic ICs are lithium niobate, silicon and silicon dioxide, indium phosphide and gallium arsenide. Lithium niobate crystals are mostly consumed for developing electro-optic modulators with good modulation linearity, high performance and high modulation bandwidth. Nevertheless, these crystals cannot be used for lasing or as a photodetector. Moreover, crystal processing methods are also very complex, which makes it practically infeasible for large-scale photonic ICs.
Silicon and silicon dioxide materials are basic elements for developing electronic ICs as they offer stable performance and low cost. The processing methods for these materials are mature and simple, which deliver high yield, and are appropriate for big projects. In photonic ICs, silicon-related materials have three critical vulnerabilities. First, silicon-based lasers are complex to develop and their laser emission efficiency is reduced. Second, these materials cannot identify the specific light wavelengths that are used for optical communications at 1310 nm and 1550 nm. Third, because of the restrictions of silicon-based components, the implementation of electro-optic modulation is out of the question. Several academic organizations, including Intel, are seeking to make breakthroughs with silicon-based optical devices. Currently, they are being used in passive photonics ICs and hybrid large-scale photonic ICs.
Because indium phosphide integrates both active and passive optical devices, it can meet the requirements for use in communications at both 1310 nm and 1550 nm operating wavebands. In the meantime, in mass manufacturing, standardized semiconductor technologies could be able to further cut costs. These materials can concurrently provide optical amplification, detection, laser emission and electro-optic modulation services along with optical switching, dispersion compensation and wavelength multiplexing/demultiplexing. This allows photonic ICs to use indium phosphide materials for big projects.
Due to the intrinsic bandgap of gallium arsenide, photonic ICs can operate in the range of 850 nm when utilized for active photoelectric systems. Thus, active optical devices with these materials can just be used for local area network communications. It will not be feasible for large-capacity and long-distance transmission systems.
Photonic ICs leave electronic ICs behind when it comes to problems of heat generation, integration and electromagnetic pulse. The compact and discrete ICs offer several benefits such as higher speed, miniaturization, large integration capacity, low thermal effects and support of current processing methods that results in lower expenses, high volume manufacturing and high yield. This technology is still in its infancy, but research advances are expected to enhance its popularity and versatility with increases in efficiency and declines in cost.