Sung-Bock Kim

Photonic bandedge lasers in two-dimensional square-lattice photonic crystal slabs

Square-lattice bandedge lasers are realized by room-temperature optical pumping of photonic crystal air-bridge slabs of InGaAsP quantum wells emitting at 1.5 μm. Lasing modes corresponding to the second bandedges near the X and M points are identified from their spectral positions and polarization directions. A low threshold incident pump power of less than 1 mW is achieved for the laser operating at the second bandedge near the X and M points, with only 15×15 lattice points. The measured characteristics of the bandedge lasers closely agree with the result of calculations based on the plane-wave-expansion method and the finite-difference time-domain method.

Photonic quasicrystal single-cell cavity mode

We propose and realize the photonic quasicrystal (PQC) single-cell resonator based on a InP-InGaAsP freestanding slab. A well-defined hexapole-like localized state following a C6ν symmetry is identified from the PQC single-cell resonator. In this type of hexapole mode, the electromagnetic energy is strongly concentrated on the dielectric region, in contrast to that in a triangular lattice photonic crystal. By tailoring the structural parameters, the hexapole mode shows a maximum theoretical quality factor of ∼20000. The PQC single-cell resonator lases in a single hexapole mode with a threshold of ∼0.6mW at room temperature.

Small, low-loss heterogeneous photonic bandedge laser

We have demonstrated the operation of a new type of heterogeneous photonic crystal laser, a five-lattice-constant large photonic bandedge laser assisted by a photonic bandgap, in a triangular lattice at room temperature. When the air hole radius of the surrounding photonic crystal (PC) is slightly smaller than that of the bandedge mode region, most in-plane losses of the first K point bandedge mode in the central region are suppressed and the quality factor of the mode is greatly enhanced to 50000. We identified the photonic bandgap-bandedge (PBG-BE) lasing modes through the spectral position, near-field pattern, and the state of polarization, which correspond well with the results of the three-dimensional (3D) finite-difference time-domain (FDTD) method computation. The two-dimensional (2D) feedback mechanism of the first K bandedge was verified through the Fourier analysis. Low threshold incident peak pump power of ~ 0.24 mW is achieved owing to the low optical loss of the PBG-BE mode.

Analysis and characterization of traveling-wave electrode in electroabsorption modulator for radio-on-fiber application

Comparing with a lumped electroabsorption modulator (EAM), we show the merits of a long EAM with traveling-wave electrode with high radio-frequency (RF) gain that could be used in high-frequency analog application. By terminating the RF output port with the characteristic impedance of 30 /spl Omega/, the device exhibited a large enhancement of 6 dB above 10 GHz in the electrical-to-optical response and a wide fractional bandwidth as estimated from simulation. In addition, an input impedance matching circuit of stub embedded on the device chip was found to be very effective for improving RF characteristics in the narrow band of frequency.

Multiwavelength Lasers for WDM-PON Optical Line Terminal Source by Silica Planar Lightwave Circuit Hybrid Integration

We have demonstrated a very compact eight-channel 200-GHz-spacing multiwavelength laser (MWL) module for the optical line terminal source of wavelength-division-multiplexing passive optical network. The wavelength shift of MWLs and external cavity lasers from the ITU grid was within +0.14 nm for eight channels, which was well matched to the target wavelength shift of less than plusmn0.2 nm. The oscillation characteristics of our MWL module are better than those of conventional MWLs with hybrid integration.

Monolithic integration of a multiwavelength laser array associated with asymmetric sampled grating lasers

We present a novel approach for a monolithically integrated multiwavelength laser array based on asymmetric sampled grating lasers. The asymmetric sampled grating laser combines sampled gratings of different periods with an index shifter to utilize the first-order reflection for lasing operation. With this structure, a simple fabrication procedure as well as high yield could be achieved without using complex and time-consuming e-beam lithography for multiperiod gratings. With numerical analysis based on a transfer matrix method, the effect of grating strength and mirror coating was analyzed to improve single-mode and power extraction performance. By using high-reflection/antireflection coatings on both facets, it was shown theoretically that high-power extraction efficiency as well as high single-mode yield was achieved. A four-channel laser array with 400-GHz wavelength spacing was fabricated and its operation at designed wavelengths was demonstrated. The individual laser showed a threshold current of 9-13 mA and a slope efficiency of around 0.21 W/A. A high sidemode suppression ratio over 44 dB was observed as well.

Loss-coupled distributed-feedback lasers with amplified optical feedback for optical microwave generation

Multisection semiconductor lasers for optical microwave generation have been fabricated that consist of a loss-coupled distributed feedback (LC-DFB), a phase control, and an amplifier section. High-frequency self-pulsations are generated according to the concept of a single-mode laser with short optical feedback. The effect of the optical feedback via the phase control and the amplifier section on the self-pulsation is apparently shown as a result of the superior single-mode characteristic of the LC-DFB section. Continuous frequency tuning is achieved in the range of 17-35 GHz.

Widely frequency-tunable amplified feedback lasers for 10-GHz optical pulsation

Monolithic amplified feedback semiconductor lasers are demonstrated as a new solution to 10-GHz optical pulsation, where self-pulsations are generated according to the concept of a single-mode laser with shortly delayed optical feedback. They consist of a loss-coupled distributed feedback section operating as a single-mode laser and an integrated feedback cavity including a phase control, an amplifier, and a transparency section. The pulsation frequency is continuously tunable in the range of 7-11.5 GHz with an extinction ratio above 6.5 dB, which indicates that precise control of a cavity length is not needed.

Polarization-selective resonant photonic crystal photodetector

Resonance-assisted photonic crystal (PhC) slab photodetectors are demonstrated by utilizing six 7-nm-thick InGaAsP quantum wells. In order to encourage efficient photon coupling into the slab from the vertical direction, a coupled-dipole-cavity-array PhC structure is employed. Inheriting the characteristics of the dipole mode, this resonant detector is highly polarization selective and shows a 22-nm-wide spectral width. The maximum responsivity of 0.28A∕W, which is >20 times larger than that of the identical detector without the pattern, is observed near 1.56μm.

Self-pulsation in multisection laser diodes with a DFB reflector

A novel self-pulsation regime is observed in multisection laser diodes which consist of a loss-coupled distributed-feedback (DFB) section, a phase control section, and gain sections, where 10-GHz self-pulsation due to compound cavity mode beating has been reported with the DFB section operated as a single-mode laser. When the DFB section is below threshold current, the devices give the self-pulsation in a very wide operating range. We attribute the pulsation to passive mode-locking and also confirm that this structure is applicable to 40-GHz operation