Abstract
This paper reviews the direct bonding technique focusing on the waveguide optical isolator application. A surface activated direct bonding technique is a powerful tool to realize a tight contact between dissimilar materials. This technique has the potential advantage that dissimilar materials are bonded at low temperature, which enables one to avoid the issue associated with the difference in thermal expansion. Using this technique, a magneto-optic garnet is successfully bonded on silicon, III-V compound semiconductors and LiNbO3. As an application of this technique, waveguide optical isolators are investigated including an interferometric waveguide optical isolator and a semileaky waveguide optical isolator. The interferometric waveguide optical isolator that uses nonreciprocal phase shift is applicable to a variety of waveguide platforms. The low refractive index of buried oxide layer in a silicon-on-insulator (SOI) waveguide enhances the magneto-optic phase shift, which contributes to the size reduction of the isolator. A semileaky waveguide optical isolator has the advantage of large fabrication-tolerance as well as a wide operation wavelength range.
Highlights
An optical isolator allows light waves to propagate only in a specified direction
We have reviewed the direct bonding technique for the waveguide optical isolator application
The surface activated direct bonding technique is developed for integrating a magneto-optic garnet onto commonly used waveguide platforms of SOI and III-V compound semiconductor wafers
Summary
An optical isolator allows light waves to propagate only in a specified direction. It plays an essential role in optical circuits by preventing light waves from propagating in the undesired direction. It is hard to grow a single-crystalline magneto-optic garnet on commonly used optical waveguide platforms made of III-V compound semiconductors, silicon and silica. In order to resolve this issue, several approaches are investigated, which include deposition and bonding techniques Another issue to be considered for integrating optical isolators is the compatibility of the device structure with the platform of other optical devices. The optical amplification is obtainable by the current injection in case of III-V compound semiconductors This is not the case for other waveguide platforms such as a silicon-based waveguide.
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