Hirohito Yamada

Lightwave propagation through a 120° sharply bent single-line-defect photonic crystal waveguide

We have demonstrated 1.55 μm wavelength lightwave propagation through a 120° sharply bent waveguide formed in a triangular-lattice two-dimensional photonic crystal (2D PC). Such propagation has not previously been experimentally confirmed. The photonic crystal was fabricated in a silicon-on-insulator (SOI) wafer with the top silicon layer of the wafer used as a core layer. A 877-μm-long single-line-defect waveguide was formed in the PC with a sharp 120° bend near the middle of the waveguide. A tapered-hemispherical-end fiber was coupled to the input end of the waveguide for the light input, and the output from the other end of the waveguide was directly observed by scanning its near-field profile with another tapered-hemispherical-end fiber.

Enhanced spontaneous emission from GaAS quantum wells in monolithic microcavities

Enhanced spontaneous emission has been observed with wavelength‐sized monolithic Fabry–Perot cavities containing GaAs quantum wells. With an on‐resonance cavity structure, the photoluminescence intensity increases in the cavity axis direction, and the spontaneous emission lifetime is experimentally found to decrease.

Compact 1 × N thermo-optic switches based on silicon photonic wire waveguides

Using silicon photonic wire waveguides, we constructed compact 1 x 1, 1 x 2, and 1 x 4 Mach-Zehnder interferometer type optical switches on a silicon-on-insulator substrate and demonstrated their switching operations through the thermo-optic effect. These switches were smaller than 140 x 65, 85 x 30, and 190 x 75 mum, respectively. At a 1550-nm wavelength, we obtained an extinction ratio larger than 30 dB, a switching power as low as 90 mW, and a switching response time of less than 100 mus. Furthermore, switching operations were successfully demonstrated for the 1 x 4 switch.

Nonlinear-Optic Silicon-Nanowire Waveguides

Using a 4-mm-long compact silicon-nanowire waveguide, we demonstrated nonlinear-optic effects such as the spectral broadening of optical short pulses due to self-phase modulation and nonlinear transmittance due to two-photon absorption. At a 12 W input power level, we observed a 1.5-π nonlinear phase shift and a strong saturation of optical output power in a sample. We also estimated the third-order nonlinear coefficient n2 and the two-photon absorption coefficient β, and compared them with those previously reported.

1.5-μm-wavelength light guiding in waveguides in square-lattice-of-rod photonic crystal slab

We have experimentally demonstrated the guiding of light at a 1.5 μm wavelength in straight and 90°-bent line-defect waveguides in two-dimensional square-lattice-of-rods photonic crystal slabs. The light was guided by being confined in a row of the rods that were thinner than the surrounding ones. A new structural design to greatly facilitate their fabrication process without degrading the guiding property was used. The propagation loss measured for a 1.8-mm-straight waveguide was 4.8 dB/mm, which is small enough to allow us to proceed to integrated optical circuit application.

Light propagation in a photonic-crystal-slab line-defect waveguide

The guided and leaky modes of a photonic-crystal-slab line-defect waveguide were investigated by means of the direct three-dimensional (3-D) finite-difference time-domain (FDTD) simulation. The simulation predicted quasi-guided modes with long lifetimes in the cladding-mode continuum, while quasi-3-D analyses using projected band diagrams revealed only the existence of guided modes outside that continuum. It is shown that the direct 3-D analysis is necessary to account for measured transmission data for the photonic-crystal-slab line-defect waveguide with a finite length.

Precise wavelength control for DFB laser diodes by novel corrugation delineation method

Precise wavelength control of a multiple-wavelength DFB InGaAsP strained MQW laser-diode (LD) array was achieved using weighted-dose allocation variable-pitch EB-lithography (WAVE) and highly uniform MOVPE. Multiple-wavelength 1.3 /spl mu/m /spl lambda//4-shifted DFB LD arrays with wavelength spacing of 2.0 nm were successfully demonstrated. The standard deviation of the wavelength was as low as 0.37 nm over 2-in wafers.

Systematic design of antireflection coating for semi-infinite one-dimensional photonic crystals using Bloch wave expansion

We present a systematic method for designing a perfect antireflection coating (ARC) for a semi-infinite one-dimensional (1D) photonic crystal (PC) with an arbitrary unit cell. We use Bloch wave expansion and time reversal symmetry, which leads exactly to analytic formulas of structural parameters for the ARC and renormalized Fresnel coefficients of the PC. Surface immittance (admittance and impedance) matching plays an essential role in designing the ARCs of 1D PCs, which is shown together with a practical example.

A compact optical sampling measurement system using mode-locked laser-diode modules

A compact optical sampling measurement system with a temporal resolution of 2 ps has been developed. External-cavity mode-locked laser-diode modules, which directly generate coherent 2-ps optical pulses, were used as the optical sampling pulse sources. Real-time measurement of the recovery dynamics in semiconductor saturable absorber devices and 160-Gbit/s pseudorandom return-to-zero optical signals with this system has been demonstrated.

Optical-confinement-factor dependencies of the K factor, differential gain, and nonlinear gain coefficient for 1.55 mu m InGaAs/InGaAsP MQW and strained-MQW lasers

Optical-confinement-factor Gamma dependencies of the K factor, differential gain, dg/dN, and nonlinear gain coefficient epsilon , for 1.55 mu m InGaAs/InGaAsP multiple-quantum-well (MQW) and compressively strained MQW lasers, were investigated experimentally. For both MQW and strained-MQW lasers, when Gamma is increased, the K factor is reduced, dg/dN is increased, but epsilon is almost constant. These results indicate that the Gamma dependence of the K factor mainly results from a change in dg/dN, and does not result from a change in epsilon . For the strained MQW lasers, the K factor, dg/dN, and epsilon are, respectively, half as large, twice as large, and the same as those for the MQW lasers, when both types of lasers have the same Gamma (=0.05). This suggests that the strained MQW lasers with a large Gamma have a small K factor and thus are preferable for achieving large modulation bandwidths.<<ETX>>