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A Photonic Integrated Circuit (PIC) is conceptually very similar to an electronic IC. While the latter integrates many transistors, capacitors and resistors, a PIC integrates multiple optical components such as lasers, modulators, detectors, attenuators, multiplexers/de-multiplexers and optical amplifiers. Large-scale PICs, like their electronic counterparts, extend the scope of integration so that upwards of dozens or more distinct optical components are integrated into a single device.
Passive and Active PICs
Photonic integrated circuits can be further characterized into passive and active PICs. Passive PICs, typically built using silica materials, integrate functions such as filters, wavelength multiplexers, couplers, and photonic switches. They don't generate or amplify light, but are "light in, light out." Active PICs, by contrast, integrate optoelectronic functions such as lasers, modulators, PIN detectors, and amplifiers. They may include passive devices as well. Active PICs, which can be used to convert between electronic signals and optical signals, are built using compound semiconductor materials such as Indium Phosphide (InP) so that they may generate, amplify, or detect light.
Infinera uses both active and passive PICs in its products. Each of our line cards includes two active PICs, one to convert 100Gb/s digital signals into DWDM bandwidth, and another to receive that optical bandwidth and convert it back to digital information. Our ILS2 line system uses passive PICs to provides wavelength filtering and multiplexing into 25GHz channel spacing.
Hybrid vs. Monolithic Integration
As in electronics, photonic integration can include both hybrid and monolithic integration. In a hybrid PIC, multiple single-function optical devices are assembled into a single package, sometimes with associated electronic ICs, and inter-connected to each other by electronic and/or optical couplings internal to the package. Many integrated photonic devices available today utilize hybrid integration to consolidate packaging.
However, the assembly of hybrid integrated components can be highly complex, as many discrete devices must be interconnected internal to the package with sub-micron tolerances required for aligning optical components. Adding to the packaging challenge is the fact that different materials may require different packaging designs due to differences in optical, mechanical and thermal characteristics. For example, if two materials have different coefficients of expansion, they can become misaligned at different operating temperatures and require different thermo-electric coolers, thus compounding packaging complexity and cost. In practice, this has limited hybrid PICs to integrating at most three to four optical components into a common package.
In contrast, monolithic integration consolidates many devices and/or functions into a single photonic material. As in electronic ICs, the fabrication of monolithic PICs involves building devices into a common substrate so that all photonic couplings occur within the substrate and all functions are consolidated into a single, physically unique device.
The term photonic integration has also sometimes been incorrectly applied to sub-system modules, for example the 300-pin 10Gb/s transponder multi-source agreement (MSA) modules. This is misleading as such modules are actually comprised of individually packaged single-function components connected by external fiber couplings and electronic traces. Thus the only "integration" actually achieved is the incorporation of all devices into one module, and this is generally not considered true integration.
As can be seen in Table 1 below, monolithic integration provides the greatest level of benefits, including significant packaging consolidation, testing simplification, reduction in fiber couplings, improved reliability and maximum possible reduction in space and power consumption per device.
|Module Integration||Packaging Integration (Hybrid Integration)||Monolithic Integration|
|Description||Integrate discrete devices and packages into a common module||Integrate multiple discrete optical and/or electrical devices into a single package||Integrate multiple devices and/or functions into a single optical "chip" and package|
|Combine electronic IC and photonic functions||++++||+++||Difficult in practice|
|Integrate different optical materials||++++||+++||++|
|Integrate different optical functions||++++||++++||++++|
|Consolidation of electrical connections||+||++||+++|
|Consolidation of optical connections||+||+||++++|
|Fiber coupling consolidation||+||++||++++|
|Power consumption savings||+||++||++++|
Table 1: Different levels of photonic integration offer differing level of benefits depending on the degree ofintegration achieved.