Keisuke Shinozaki

Wavelength conversions ∼1.5 μm by difference frequency generation in periodically domain‐inverted LiNbO3 channel waveguides

Wavelength conversions ∼1.5 μm are realized by difference frequency generation in periodically domain‐inverted LiNbO3 channel waveguides for the first time. A wide wavelength conversion bandwidth, which is very important for future wavelength division multiplexing optical communication system, is demonstrated in this kind of devices.

A comparison of optical second-harmonic generation efficiency using Bessel and Gaussian beams in bulk crystals

Abstract The conversion efficiencies of optical second-harmonic generation (SHG) of light by Bessel beams and Gaussian beams in nonlinear crystals are compared. A Bessel beam, which has a quasi-nondiffractive nature, is generated by illuminating an axicon lens with a Gaussian beam. The interaction path lengths are the same for both beams, and are set to be twice the confocal parameter of the Gaussian beams. The Bessel beam is found to have greater SHG efficiency than the Gaussian beam regardless of the interaction path length and the fundamental wavelength.

Successive excited-state absorption through a multistep process in highly Er 3+ -doped fiber pumped by a 1.48-μm laser diode

The fluorescence spectra ranging from 400 to 1000 nm were investigated on highly Er(3+) -doped fiber pumped by a 1.48-microm laser diode. A strong green emission was observed. An investigation of the fluorescence intensity dependence on pump power revealed that the green emission is attributed to successive excited-state absorption through a three-step process and also that saturation of the atomic levels that contribute to the emission occurs.

Second‐harmonic generation device with integrated periodically domain‐inverted regions and distributed Bragg reflector in a LiNbO3 channel waveguide

We propose a new structure of LiNbO3 guided wave second‐harmonic generation device, in which the fundamental light beam is sufficiently confined by two distributed Bragg reflectors (DBRs) formed at input and output ports. The 40% increase in the conversion compared with a conventional device without DBR was achieved under a quasi‐phase matching condition.

Self‐quasi‐phase‐matched second‐harmonic generation in the proton‐exchanged LiNbO3 optical waveguide with periodically domain‐inverted regions

A self‐quasi‐phase matching technique for second‐harmonic generation (SHG) that uses a proton‐exchanged LiNbO3 optical waveguide with periodically domain‐inverted regions is proposed and demonstrated. The fundamental wave satisfying quasi‐phase‐matching (QPM) condition was generated by an InP/InGaAsP laser diode (LD) with antireflection coated facets which was optically connected to the LiNbO3 waveguide. As the domain‐inverted regions in the optical waveguide act as the distributed Bragg reflector (DBR), the LD is lased by the feedback waves from the DBR. We designed the period of domain‐inverted regions to satisfy the QPM conditions and the high‐reflectance conditions of the fundamental wave. We confirmed that the LD was lased at the wavelength satisfying the QPM conditions and SHG was observed simultaneously.

THREE WAVE MIXING USING A FIBER RING RESONATOR

Three wave mixing using a fiber ring resonator scheme is discussed, and experimental results are presented. In this scheme, an optical nonlinear medium is placed into the fiber ring laser cavity, and a semiconductor optical amplifier and a tunable optical band pass filter (TOBPF) are used as the gain medium and lasing wavelength selector. The phase-matching condition, which is necessary for efficient wave mixing, can easily be satisfied by tuning the TOBPF. Second-harmonic generation (SHG) and sum-frequency generation (SFG) were demonstrated using a LiNbO3 waveguide with a periodically domain inverted structure. Enhancements of SHG and SFG were achieved.

High-power operation of 830-nm AlGaAs laser diodes

We have developed a high‐power, semicylindrical‐shaped waveguide inner stripe laser diode (SCILD) using a current confinement structure. The structure is optimized for high‐power operation without catastrophic optical damage (COD) which limits the maximum available optical power. We confirm that the optical electric field shape in the LD waveguide layer obtained from our analysis coincides with the micrograph of the COD on the device facet. The SCILD is emitted in the fundamental transverse mode up to 190 mW. The full beam angles between the half‐power points were 10° and 30° in the directions parallel and perpendicular to the junction plane, respectively. The characteristic temperature of the threshold current was 152 K.

Second‐harmonic generation using a fiber ring resonator with a LiNbO3 waveguide and a semiconductor optical amplifier

Optical second‐harmonic generation (SHG) using a fiber ring resonator has been demonstrated. A LiNbO3 waveguide with a periodically domain inverted structure and a semiconductor optical amplifier were used as the nonlinear and gain media, respectively. The quasiphase‐matched condition was satisfied simply by tuning an optical bandpass filter in the resonator. SHG was enhanced by a factor of 280.

Reduced alignment accuracy requirement using focused Gaussian beams for free‐space optical interconnection

The use of a focused Gaussian beam in the space variant micro-optical interconnection system is proposed for decreasing the alignment accuracy requirement. The relationship between both longitu- dinal and lateral misalignment of the light source and the power loss due to beam shift and beam spot size mismatch at the collimating lens and the photodetector are derived theoretically using ABCD matrices, and numerical simulation results are also given. A focused Gaussian micro- optical system is introduced by changing the beam radius ratio on the two lenses of the one-to-one Gaussian micro-optical system from unity. It is shown that the focused Gaussian micro-optical system with a smaller beam radius on the second collimating lens than on the first one has improved misalignment tolerances compared with the one-to-one Gaussian micro-optical system.

Automatic quasiphase matching for second‐harmonic generation in a periodically poled LiNbO3 waveguide

A novel technique is proposed and experimentally demonstrated to achieve a quasiphase matching (QPM) condition for second‐harmonic generation (SHG) in a nonlinear optical waveguide with a periodically poled LiNbO3 waveguide. The fundamental wave satisfying the QPM conditions is automatically generated when the laser diode (LD) and the optical waveguide are optically connected. Using the proton‐exchanged LiNbO3 waveguide with a periodically poled grating, which was fabricated by Ti diffusion and the InP/InGaAsP LD with antireflection‐coated facets, it was confirmed that the LD oscillated at the wavelength satisfying the QPM condition, and SHG was observed simultaneously. Using a Ti:sapphire laser, it was also confirmed that the SHG was realized at the third order of QPM condition, as was expected.