Eundeok Sim

Monolithic dual-mode distributed feedback semiconductor laser for tunable continuous-wave terahertz generation

We report on a monolithic dual-mode semiconductor laser operating in the 1550-nm range as a compact optical beat source for tunable continuous-wave (CW) terahertz (THz) generation. It consists of two distributed feedback (DFB) laser sections and one phase section between them. Each wavelength of the two modes can be independently tuned by adjusting currents in micro-heaters which are fabricated on the top of the each DFB section. The continuous tuning of the CW THz emission from Fe(+)-implanted InGaAs photomixers is successfully demonstrated using our dual-mode laser as the excitation source. The CW THz frequency is continuously tuned from 0.17 to 0.49 THz.

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.

Temperature-dependent photoluminescence of ZnSe/ZnS quantum dots fabricated under the Stranski–Krastanov mode

Self-assembled ZnSe/ZnS quantum dots (QDs) have been grown in the Stranski–Krastanov (S–K) mode using a metalorganic chemical vapor deposition technique under the atomic-layer epitaxy mode. Atomic-force-microscopy measurements on the uncapped ZnSe/ZnS QDs reveal that lens-shaped ZnSe QDs are formed after 1–2 monolayer ZnSe is deposited. The ZnSe QDs are estimated 1–2 nm in height and 25–35 nm in radius. The temperature-dependent behavior of confined carriers in the ZnSe QDs has been investigated through photoluminescence (PL) measurements. PL spectra show a substantial PL linewidth narrowing accompanied by a large redshift of the emission peak energy with increasing temperature. This unusual temperature-dependent behavior is interpreted as the dot-to-dot carrier transfer through the wetting layer, which is common to QDs grown in the S–K mode.

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

Asymmetric multiple-quantum-well laser diodes with wide and flat gain.

Asymmetric multiple-quantum-well laser diodes with wide and flat gain spectra were designed, fabricated, and analyzed. The active layer was composed of three 10-nm, one 8-nm, and two 6-nm 0.5% compressive strained wells and four 10-nm and one 5-nm 0.4% tensile strained barrier layer. Measured spectra of antireflection-coated ridge waveguide laser diodes with such quantum-well structures have shown that -1-dB spectral gain bandwidth can be as large as 90 nm.

The characterization of all-optical 3R regeneration based on InP-related semiconductor optical devices

We report characterization and experimental realization of recent InP-related semiconductor optical devices for all-optical 3R regeneration. A Mach-Zehnder interferometric wavelength converter (MZ IWC) with preamplifiers gives an extinction ratio (ER) improvement and a negative power penalty for all-optical reamplification and reshaping (2R) of pseudorandom bit sequence (PRBS) nonreturn-to-zero (NRZ) data at 10 Gb/s. In addition, we briefly describe the mechanism for injection locking of the multisection laser diode with a distributed feedback (DFB) reflector. Using this device, we demonstrate clock recovery from PRBS return-to-zero (RZ) and NRZ data. The achieved rms timing jitter is less than 1 ps for the input of 11.727 Gb/s PRBS RZ and NRZ. All-optical 3R regeneration leads to a good performance using the MZ IWC and multisection DFB LD. The combination of these functions provides all-optical 3R regeneration with NRZ to RZ conversion. For the realization of a small and compact 3R regeneration module, we propose the hybrid module packaging process, which is composed of 2R and 1R semiconductor chip in single butterfly-type module

Measurements of coupling and reflection characteristics of butt-joints in passive waveguide integrated laser diodes

Amplified spontaneous emission (ASE) spectra of passive waveguide integrated lasers have been studied to estimate the coupling efficiency and the reflectivity at the butt-joints between active and passive waveguides. A new method has been proposed for the analysis of ASE to extract the coupling efficiency and reflectivity at the butt-joints. This method was applied experimentally to estimate the coupling and reflection of the passive waveguide integrated lasers with different amounts of vertical misalignments of active and passive waveguides.

Effects of downstream modulation formats on the performance of RSOA-based WDM PON

We evaluate the effects of the downstream modulation formats (such as NRZ, inverse RZ, and Manchester) on the performance of the WDM PON implemented by using the reflective semiconductor optical amplifiers (RSOAs). The results show that, when we modulate the downstream signals in Manchester format, the performance of the upstream signal becomes insensitive to the operating condition of the RSOA.

A novel self-pulsation in laser diodes with a DFB reflector

This paper presents a novel self-pulsation in the laser diode with a DFB reflector in a compound cavity mode beating structure. This study also shows that dispersive Q-switching is a most probable origin of self-pulsation

μ-Heater integrated dual-mode multisection laser for tunable continuous-wave terahertz generation

We report on a novel micro-heater integrated dual mode multisection laser as a optical beat source for tunable continuous-wave THz generation. The beat frequency from this dual-mode multisection laser is continuously tuned from 0.10 to 0.57 THz.