Photonic integrated circuits (PICs) integrate multiple optical and optoelectronic functions in a single device, fabricated on a semiconductor substrate. These functions include both passive routing elements, such as waveguides, splitters, couplers and resonators; and active elements, such as tuners, modulators and detectors, as well as lasers. Some PICs incorporating lasers that are currently known in the art are fully implemented on III-V substrate, resulting in low-yield and high cost. Other PICs are made on silicon substrates using CMOS-compatible processes and are then integrated with a separately-fabricated laser (III-V) and isolator components, usually on an optical interposer. This latter approach allows utilization of the large-scale production, low cost and high yield that Si-photonics inherit from CMOS processing, but suffers from difficult, costly assembly based on “pick and place” tools. Recently, however, a number of foundries have developed the capability of integrating III-V laser dies on SOI (silicon on insulator) wafers, together with silicon-based waveguides and other optical and electro-optical components.
Optical isolators are commonly used in laser-based optical systems to prevent unwanted feedback into the laser cavity caused by back reflections from other system components. Components providing optical isolation are necessarily non-reciprocal to ensure that the laser can be protected from backwards-propagating optical fields while allowing the passage of forward-direction fields. The most common type of optical isolator is based on a Faraday rotator, which uses the magneto-optic effect to rotate the polarization of incident light. Optical isolators of this sort are generally referred to as Faraday isolators. Typically, the operating parameters of the Faraday rotator (choice of magneto-optic material, length, and magnetic field strength) are chosen to rotate the polarization by ±45° depending upon the direction of travel. Light enters the Faraday rotator through an input polarizer and exits through an output polarizer, which is rotated by 45° relative to the input. Any backward-propagating light that passes back through the output polarizer (in the opposite direction) will have its polarization counter-rotated by 45° by the Faraday rotator and will thus be blocked by the input polarizer. Faraday rotation can also be applied to unpolarized light, typically utilizing birefringent crystals to spatially separate polarization components.