Air Force

Optical isolator/coupler

Simple design, high levels of optical isolation, low forward loss, all in a small footprint

Photonics

Commercially available isolator

An optical isolator allows the transmission of light in only one direction. It is typically used to prevent unwanted feedback into an optical oscillator, such as a laser cavity. Isolators are important components for photonic systems and for protection of communications and sensor systems. Due to the strong push to develop high-performance, low-power photonic integrated circuits (PIC), there is a pressing need for footprint reduction and silicon integration in optical isolators. These desired isolator characteristics must also be combined with optimal isolation performance and propagation loss in order to provide the performance demanded by system architects.

Common isolators based on the Mach-Zehnder Interferometer design are relatively complicated with four waveguides and an ongoing need to properly balance the beam spitting. They also take up considerable space on a board and are prone to losses. To address these shortcomings, Air Force scientists and engineers have developed a non-reciprocal coupler/isolator using just two waveguides. One waveguide has a cladding magnetized transversely to its propagation direction. The second waveguide has a non-magnetic cladding with a refractive index matching that of the magnetic cladding of the first waveguide. This configuration leads to a much more simple design, with fewer avenues for loss during the process of transporting light.

Simulations show improved performance compared with other chip scale optical isolator designs. An isolation ratio of 43 dB was calculated with 4 dB of forward propagation loss due to a mismatch between forward and backward coupling lengths. This mismatch loss can be significantly reduced by proper adjustment of an inter-waveguide coupling constant. Absorption losses into the garnet material are estimated at 2.5 dB and the isolation ratio stays above 30 dB over a 60 nm bandwidth. The high isolation ratio is due to the fact that a nonreciprocal action has been engineered into one of the waveguide ridges via a magneto-optic cladding layer, while the other ridge is cladded with a reciprocal material which has a matching refractive index. This leads to complete transfer of power between the two ridges in one direction and almost zero power transfer in the opposite direction. Thinner silicon films yield smaller footprints, estimated at less than 1 mm in length.

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