Hideki Yokoi

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.

Demonstration of an optical isolator with a semiconductor guiding layer that was obtained by use of a nonreciprocal phase shift

We present the experimental study of an optical isolator with a semiconductor guiding layer that was obtained by use of a nonreciprocal phase shift. The isolator is equipped with an optical interferometer composed of tapered couplers, nonreciprocal phase shifters, and a reciprocal phase shifter. The nonreciprocal phase shifter was constructed by wafer direct bonding between the semiconductor guiding layer and the magneto-optic cladding layer. The isolator, designed for the 1.55-mum wavelength, was fabricated to investigate the characteristics of each component. By applying an external magnetic field to the nonreciprocal phase shifter, we achieved an isolation ratio of approximately 4.9 dB in the interferometric isolator.

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.

Demonstration of an optical isolator by use of a nonreciprocal phase shift

The experimental study of an optical isolator by use of a nonreciprocal phase shift is demonstrated. The isolator has an optical interferometer composed of tapered couplers, nonreciprocal phase shifters, and a reciprocal phase shifter. The isolator, designed for a 1.55-microm wavelength, was fabricated to investigate the characteristics of each component. The branching and coupling characteristics of the tapered coupler were measured. The nonreciprocal and reciprocal phase shifts were also evaluated. By applying an external magnetic field to the interferometer, we confirmed the nonreciprocal phase shift in the interferometric isolator.

Ultralow-jitter passive timing stabilization of a mode-locked Er-doped fiber laser by injection of an optical pulse train

The pulse timing of a mode-locked Er-doped fiber laser was stabilized to a reference pulse train from a Cr:forsterite mode-locked laser by all-optical passive synchronization scheme. The reference pulses were injected into a ring cavity of the fiber laser by using a 1.3-1.5 mum wavelength-division multiplexer. The spectral shift induced by cross-phase modulation between copropagating two-color pulses realizes self-synchronization due to intracavity group-delay dispersion. The rms integration of timing jitter between the fiber laser pulse and the reference pulse was 3.7 fs in a Fourier frequency range from 1 Hz to 100 kHz.

Analysis of GaInAsP Surfaces by Contact-Angle Measurement for Wafer Direct Bonding with Garnet Crystals

GaInAsP surfaces with various treatments were analyzed by contact-angle measurement for wafer direct bonding between GaInAsP and garnet crystals. The contact angle of a water droplet was measured to estimate the hydrophilicity of the wafer surfaces. The most hydrophilic surface was obtained after an O2 plasma activation process. Direct bonding was successfully achieved between GaInAsP activated by O2 plasma and garnet crystals.

Improved Heat Treatment for Wafer Direct Bonding between Semiconductors and Magnetic Garnets

The optical propagation loss of rib waveguides fabricated on magnetic garnet films increased upon annealing in H2 ambient during wafer direct bonding. The heat treatment in wafer direct bonding between InP and Gd3Ga5O12 was investigated with the aim of circumventing the loss increase. The bonding was achieved by heat treatment in H2 ambient at temperatures of 330° C or in N2 ambient.

Calculation of nonreciprocal phase shift in magnetooptic waveguide with Si guiding layer

A nonreciprocal phase shift in a magnetooptic waveguide with a Si guiding layer is discussed. The magnetooptic waveguide was installed in an optical isolator employing the nonreciprocal phase shift. The magnetooptic waveguide had a magnetic garnet/Si/SiO2 structure, which was realized by wafer bonding technique. The nonreciprocal phase shift in the magnetooptic waveguide with the Si guiding layer was calculated at a wavelength of 1.55 µm. A required thickness of the magnetic garnet cladding layer was obtained by calculating the nonreciprocal phase shift. An optical isolator employing the nonreciprocal phase shift, which could be operated under unidirectional magnetic field, was proposed by contriving the layer structures in the interferometer.

Interferometric optical isolator employing a nonreciprocal phase shift operated in a unidirectional magnetic field

An interferometric optical isolator that employs a nonreciprocal phase shift was studied. The optical isolator consisted of an interferometer with distinct layer structures. A traveling light wave underwent distinct nonreciprocal phase shifts such that the optical isolator could be operated in a unidirectional magnetic field. The optical isolator, in which the waveguide had a HfO2 cladding layer in one of the arms, was designed at a wavelength of 1.55 microm. The propagation distance of the nonreciprocal phase shifter required for the isolators operation was less than 1.5 mm. The devices total length was less than 2 mm. An optical isolator with distinct layer structures was fabricated and evaluated. An isolation ratio of approximately 9.9 dB was obtained in the unidirectional magnetic field.

Direct Bonding between InP Substrate and Magnetooptic Waveguides

Wafer direct bonding was demonstrated between InP and magnetooptic materials. The effects of chemical treatment of two wafers before placing them into contact were investigated in order to enhance the durability of the bonded wafers through the various device fabrication processes. Phosphoric acid etching at a low temperature proved to be the most suitable treatment for the bonding between InP and gadolinium gallium garnet Gd3Ga5O12. In the case of this treatment, the bonding was studied between a planar InP substrate and magnetooptic waveguides on the garnet films.