Yuya Shoji

Magneto-optical isolator with silicon waveguides fabricated by direct bonding

A magneto-optical isolator is demonstrated for use with a Si waveguide. The isolator is based on a Mach–Zehnder interferometer employing a nonreciprocal phase shift and is fabricated by bonding a magneto-optic garnet CeY2Fe5O12 (Ce:YIG) directly onto the Si waveguide. The surface-activated bonding is based on oxygen-plasma exposure in a high-vacuum chamber. The nonreciprocal phase shift is observed by applying an external magnetic field. An isolation ratio of 21dB is obtained at a wavelength of 1559nm.

Optical nonreciprocal devices with a silicon guiding layer fabricated by wafer bonding

Optical nonreciprocal devices with a silicon guiding layer fabricated by wafer bonding are proposed. The optical nonreciprocal devices are composed of a magneto-optic waveguide with a magnetic garnet/Si/SiO2 structure. Nonreciprocal characteristics are obtained by an evanescent field penetrating into the upper magnetic garnet cladding layer. Several kinds of the optical nonreciprocal device are investigated with the magneto-optic waveguide and designed at a wavelength of 1.55 microm. As a preliminary experiment, wafer bonding between Gd3Ga5O12 and Si was studied. Wafer bonding was successfully achieved with heat treatment at 220 degrees C in H2 ambient.

Magneto-optical non-reciprocal devices in silicon photonics

Abstract Silicon waveguide optical non-reciprocal devices based on the magneto-optical effect are reviewed. The non-reciprocal phase shift caused by the first-order magneto-optical effect is effective in realizing optical non-reciprocal devices in silicon waveguide platforms. In a silicon-on-insulator waveguide, the low refractive index of the buried oxide layer enhances the magneto-optical phase shift, which reduces the device footprints. A surface activated direct bonding technique was developed to integrate a magneto-optical garnet crystal on the silicon waveguides. A silicon waveguide optical isolator based on the magneto-optical phase shift was demonstrated with an optical isolation of 30 dB and insertion loss of 13 dB at a wavelength of 1548 nm. Furthermore, a four port optical circulator was demonstrated with maximum isolations of 15.3 and 9.3 dB in cross and bar ports, respectively, at a wavelength of 1531 nm.

Single-trench waveguide TE-TM mode converter

A TE-TM mode converter is proposed in a single trench GaInAsP/InP waveguide, which is fabricated by a single masking and etching process. Use of single-trench structure makes the design and the fabrication much simpler. The design of single-trench mode converter is described together with its fabrication in this article. We investigated the dependence of conversion efficiency on the waveguide width, trench depth, and trench position. Also, the wavelength dependence of mode conversion efficiency was calculated in a wavelength range between 1.5 microm to 1.58 microm. 95% TE-TM mode conversion was measured at a wavelength of 1.55 microm in a fabricated device with a 210-microm half-beat length.

Polarization-Independent Magneto-Optical Waveguide Isolator Using TM-Mode Nonreciprocal Phase Shift

A polarization-independent magneto-optical waveguide isolator using nonreciprocal phase shift for a TM mode is proposed. The isolator is a Mach-Zehnder interferometer (MZI) that is composed of transverse electric (TE)-transverse magnetic (TM) polarization converters and nonreciprocal phase shifters. The MZI induces a phase difference between the reciprocal TE and nonreciprocal TM modes that are converted to each other by the polarization converter. Since the design uses a nonreciprocal phase shift only for the TM mode, it allows fabrication of an integrated structure. This paper describes the principle of operation and its design.

Waveguide Optical Isolators for Integrated Optics

Waveguide optical isolators are investigated for an application to integrated optics. A nonreciprocal loss shift isolator has the advantage of high compatibility with optical active devices. The interferometric waveguide optical isolator that uses nonreciprocal phase shift brought about by the first-order magneto-optic effect is applicable to a variety of waveguide platforms. In a silicon-on-insulator (SOI) waveguide, the low refractive index of buried oxide layer enhances the magneto-optic phase shift, which contributes to reduce the device size of isolator. We developed the surface activated direct bonding technique to integrate a magneto-optic garnet crystal onto dissimilar crystals. The performance of SOI waveguide optical isolator was demonstrated with an optical isolation of 21 dB at a wavelength of 1559 nm.

Ultra-wideband design of waveguide magneto-optical isolator operating in 1.31μm and 1.55μm band

The design of an ultra-wideband waveguide magneto-optical isolator is described. The isolator is based on a Mach-Zehnder interferometer employing nonreciprocal phase shift. The ultra-wideband design is realized by adjusting the wavelength dependence of reciprocal phase difference to compensate for that of nonreciprocal phase difference in the backward direction. We obtained the ultra-wideband design that provides isolation > 35dB from 1.25mum to >1.65mum. This is the proposal of magneto-optical isolator that operates both in 1.31mum band and 1.55mum band.

Direct Wafer Bonding and Its Application to Waveguide Optical Isolators

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.

Wideband design of nonreciprocal phase shift magneto-optical isolators using phase adjustment in Mach-Zehnder interferometers

A wideband design is proposed for nonreciprocal phase shift magneto-optical isolators based on Mach-Zehnder interferometers. The wavelength dependence of nonreciprocal phase difference between the backward waves propagating in two interferometer arms is compensated for by that of reciprocal phase difference. This is realized by introducing an appropriate phase bias in one of interferometer arms. Two design examples are presented with a backward loss of >30 dB in the wavelength range of 1.40-1.63 microm for a magnetic garnet waveguide isolator and of 1.485-1.630 microm for a Si-wire waveguide isolator.

Optical Nonreciprocal Devices Based on Magneto-optical Phase Shift in Silicon Photonics

Silicon waveguide optical nonreciprocal devices that use the magneto-optical phase shift are reviewed. The phase shift caused by the first-order magneto-optical effect is effective in realizing optical nonreciprocal devices on semiconductor waveguide platforms. In a silicon-on-insulator waveguide, the low refractive index of the buried oxide layer contributes to the large penetration of the optical field into a magneto-optical material used as an over-cladding layer. This enhances the magneto-optical phase shift and, hence, contributes greatly to reducing the device footprint. A surface-activated direct bonding technique plays a key role in the fabrication of magneto-optical nonreciprocal devices. This technique makes it possible to use a high-quality single-crystalline magneto-optical garnet that exhibits a large first-order magneto-optical effect. An optical isolator based on the magneto-optical phase shift was demonstrated in a silicon waveguide with an optical isolation ratio as high as 30 dB and an insertion loss of 13 dB at a wavelength of λ = 1548 nm. Furthermore, a four-port optical circulator was demonstrated with maximum isolation ratios of 33.5 and 29.1 dB in the cross and bar ports, respectively, at λ = 1543 nm.