Chul Wook Lee

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 tunable dual-wavelength Er 3+ -doped fiber laser for tunable continuous-wave terahertz radiation

We propose a widely tunable dual-wavelength Erbium-doped fiber laser that uses two micro-heater-integrated Fabry-Perot laser diodes (FP-LDs) and two fiber Bragg gratings (FBGs) for tunable continuous-wave (CW) terahertz (THz) radiation. Each wavelength can be independently tuned by using an FP-LD and an FBG. The wavelength fine tuning is achieved by simultaneously applying current to the micro-heater on the FP-LD and strain to the FBG. The side-mode suppression ratio is more than 35 dB for both wavelengths. The wavelength spacing of the dual wavelength can be continuously tuned from 3.2 nm to 9.6 nm. Continuous frequency tuning of the CW THz radiation is also successfully achieved using an InGaAs-based photomixer with our dual-wavelength fiber laser as the optical beat source. The emitted CW THz radiation is continuously tuned from 0.3 to 0.8 THz.

Room-temperature operation of InP-based InAs quantum dot laser

A ridge waveguide quantum dot (QD) laser with a stripe width of 15 /spl mu/m was fabricated by using the seven-stacked InAs QD layers based on the InAlGaAs-InAlAs material system on InP [001] substrate. Room-temperature lasing operation was observed at 1.501 /spl mu/m, which is the first observation from the InAs QDs with the InAlGaAs-InAlAs structure. The characteristic temperature of the InAs QD laser calculated from the temperature dependence of threshold current density was 135 K in the temperature range from 200 K to room temperature.

Tunable continuous-wave terahertz generation/detection with compact 1.55 μm detuned dual-mode laser diode and InGaAs based photomixer

We demonstrate a tunable continuous-wave (CW) terahertz (THz) homodyne system with a novel detuned dual-mode laser diode (DML) and low-temperature-grown (LTG) InGaAs photomixers. The optical beat source with the detuned DML showed a beat frequency tuning range of 0.26 to over 1.07 THz. Log-spiral antenna integrated LTG InGaAs photomixers are used as THz wave generators and detectors. The CW THz radiation frequency was continuously tuned to over 1 THz. Our results clearly show the feasibility of a compact and fast scanning CW THz spectrometer consisting of a fiber-coupled detuned DML and photomixers operating in the 1.55-μm range.

Manipulation of the structural and optical properties of InAs quantum dots by using various InGaAs structures

The structural and optical properties of self-assembled InAs quantum dots (QDs) with various InGaAs structures were investigated by transmission electron microscopy (TEM) and photoluminescence (PL). The emission peak position of InAs QDs covered by a 6 nm In0.15Ga0.85As layer was 1.26 μm with PL linewidth of 31 meV, which is narrower than that of QDs in a GaAs matrix. By inserting a 1 nm In0.15Ga0.85As layer below the InAs QD layer with a 6 nm In0.15Ga0.85As overgrowth layer, the emission peak position was redshifted with larger energy-level spacing between the ground states and the first excited states compared to that of QDs with an In0.15Ga0.85As overgrowth layer only. By covering the InAs QDs on a 1 nm In0.15Ga0.85As layer with an 8 nm InxGa1−xAs layer having graded In composition, the emission peak position was 1.32 μm with relatively larger energy-level spacing and narrower PL linewidth compared to QDs covered by an In0.15Ga0.85As layer. The longer emission wavelength with relatively larger energy-l...

Optical fiber-coupled InGaAs-based terahertz time–domain spectroscopy system

The successful demonstration of an optical fiber-coupled terahertz time-domain spectroscopy (THz-TDS) system is described in this study. The terahertz output power of the emitter with two optical band rejection filters was 132 nW, which is an improvement of 70% over the output power without any filters. This improvement is due to the suppression of an optical modulated signal that is reverse-generated when an alternating current bias exceeding a certain threshold is applied to the emitter. Under the optimal alignment conditions, the terahertz detector in a fiber-coupled THz-TDS system clearly measured water vapor dips in the free space.

Long-wavelength laser based on self-assembled InAs quantum dots in InAlGaAs on InP (001)

Seven stacks of self-assembled InAs quantum dots (QDs) separated by 28nm thick InAlGaAs barriers were grown on InP (001) substrate by a solid-source molecular-beam epitaxy and were investigated by cross-sectional transmission electron microscopy and photoluminescence spectroscopy. Gain guided broad-area lasers with a stripe width of 75μm were fabricated by using the seven-stacked InAs QD layers with the InAlGaAs–InAlAs material system on InP (001). The lasing operation from InAs QDs was observed up to 260K and the characteristic temperature of the uncoated QD laser calculated from the temperature dependence of threshold current density was 377K for temperatures up to 200K, and 138K above 200K. The drastic decrease in the characteristic temperature above 200K was mainly related to the thermal behavior of carriers in QDs, and possibly the thermal coupling of the QDs to the wetting layer and the waveguide region.

Effects of Asymmetric Grating Structures on Output Efficiency and Single Longitudinal Mode Operation in

We report on the design of the λ/4 -shifted DFB laser that can provide the high output efficiency as well as the stable single-mode operation. In particular, we investigate the effects of the asymmetry in the grating structure on the lasing characteristics of the λ/4-shifted DFB laser theoretically and experimentally. The steady-state and mode-stability analyses are performed for the λ/4 -shifted DFB lasers having the asymmetric phase shift (APS) and asymmetric coupling coefficient (ACC) grating structures by numerical simulations. Although the λ/4-shifted DFB laser fabricated with APS structure is found to be capable of providing high output efficiency, its lasing mode is relatively unstable due to the increased longitudinal spatial hole-burning. On the other hand, by using the ACC grating structure, we can significantly improve both the output efficiency and single-mode stability. To verify these simulation results, we fabricate both types of λ/4-shifted DFB lasers having APS and ACC grating structures. The measured data agree well with the simulated results. For example, when we fabricate the DFB laser with the ACC structure and coupling coefficient ratio of 0.55, the output power ratio between the front and rear facets of ~ 2.5 and the side-mode suppression ratio (SMSR) of >;50 can be achieved as long as the operating current is in the range of 60 mA ~200 mA.

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The formation characteristics on the vertical stacks of shape-engineered InAs∕InAlGaAs quantum dots (QDs), which were formed by the alternate growth method (AGQDs), were studied in terms of the modulation in strain field and phase separation. The threshold current of the broad-area laser diodes (LDs) with five stacks of the AGQDs (AGQD-LDs) was 4.5 times smaller than that of the LDs with seven stacks of the conventionally grown QDs (CQD-LDs). The slope efficiency for the AGQD-LDs was 0.16W∕A, which was higher than that of the CQD-LDs of 0.9W∕A. These results can be attributed to better confinement of the electron wave function in QDs.

-Shifted DFB Laser

We present a ten-channel distributed feedback laser diode array (DFB-LDA) developed for the transmission of 100-Gb/s (10 × 10 Gb/s) signals separated by an 8 nm wavelength grid at a center wavelength of 1.55 μm. For the fabrication of this type of laser array, a selective area growth (SAG) technique, electron-beam lithography, and a reverse-mesa ridge waveguide LD processing technique were adopted to offer a tailored gain spectrum to each channel, providing both accurate lasing-wavelength control and excellent single-mode yield over all channels, and reducing the fabrication cost and electrical and thermal resistances. To evaluate the operational performance of the fabricated chip systematically, we also developed a sub-assembly module containing a ten-channel λ/4-shifted DFB-LDA, ten matching resistors, flexible printed circuit board (FPCB) wiring, and a thermistor on a metal optical bench. The static and dynamic properties of all channels of the fabricated array are examined in this paper. The developed sub-assembly module shows a side-mode suppression ratio (SMSR) of > 50 dB, a modulation bandwidth of > 10 GHz, and a clear eye-opening before and after a 2-km transmission with dynamic extinction ratio of > 5 dB.