UC Santa Barbara

Integrated optical isolators are nonreciprocal optical components that allow light to pass in one direction only. They are useful in conjunction with lasers, as they block undesired reflections from entering the laser cavity, where it might destabilize the device. Optical circulators are extensions of isolators, as they reroute the backwards propagating light into another direction. Thus, they can be used to separate counterpropagating signals. Both devices have many uses in photonic integrated circuits, but are challenging to implement, due to the reciprocal nature of most semiconductor and dielectric materials.

Magnetic materials such as garnets can break the symmetry and are well suited for optical isolators and circulators. However, they are difficult to integrate with silicon, III-V, and other commonly used optical materials. Heterogeneous integration through wafer bonding can overcome this obstacle and is used successfully in this work to achieve integrated optical isolators and circulators on silicon with record performance. This is done through waveguide optimization, careful process development, and a novel idea to integrate the source of magnetic fields, an electromagnet, directly onto the chip. This not only shrinks the footprint of the devices, but also provides flexibility in design as well as wavelength tunability, which is critical if the device is to be used in a circuit.

Two flavors of the isolator and circulator are presented. One is a resonant device using a microring that can achieve up to 32dB of isolation. Slight modifications to the design can result in a microring optical circulator as well, a first to the best of our knowledge. The other device architecture is a nonresonant device using a Mach-Zehnder interferometer. While these devices have larger footprint, they can achieve optical isolation over 20dB over a wide wavelength range of 18nm. This is extremely useful in applications such as data transmission, where backwards propagating light may be spread over several nanometers.

Of course, the isolator should be paired with a laser to realize its true potential. Several design and fabrication challenges stand in the way of this, which are addressed in this work. Polarization rotators are implemented to match the operating polarization between the laser and the isolator, and fabrication is carefully tailored such that both devices can be integrated on the same chip. Preliminary results show that the laser and isolator integration can happen in the near future. Such a demonstration would open up new opportunities in photonic integrated circuits, and would be of great interest in optical communications, sensing, RF photonics, as well as new, unexplored fields.