Heung Ro Choo

Low-threshold loss-coupled laser diode by new grating fabrication technique

In this letter, we introduce a novel fabrication method to make InGaAs absorptive first-order grating for loss-coupled distributed feedback laser diode (DFB-LD) using reactive ion etching. By this technique, it is possible to control duty rate of the first-order grating. A very-low-threshold current of 6 mA was obtained from our loss-coupled LD at the wavelength of 1.547 /spl mu/m, which was unusually high for a loss-coupled LD because of the excessive light absorption. In addition to the low-threshold current, it shows high SMSR and longitudinal single-mode yield as high as 50 dB and 90%, respectively.

Improvement of linewidth enhancement factor in 1.55-μm multiple-quantum-well laser diodes

The improvement of the linewidth enhancement factor in complex-coupled laser diode (CC-LD), or loss-coupled, was confirmed by measuring the spontaneous emission spectra below threshold from the sidewall of laser diodes. In addition, the serial resistance of the device was measured. The linewidth enhancement factor is improved by the presence of a light absorbing InGaAs grating for loss coupled distributed-feedback (DFB) laser diode (LD). We report the comparison of the linewidth enhancement factors of Fabry-Perot (FP) LD, conventional DFB-LD, and loss coupled DFB-LDs.

High frequency modeling for 10 Gbps DFB laser diode module packaging

In packaging the laser diode several electrical requirements must be satisfied. The conventional packaging method which consists of transmission line, matching resistor and wire bonding interconnect showed good result for the modulation bandwidth requirement of greater than 10 GHz. However the return loss requirement was not content with less than -10 dB. To analyze the effects of package parasitics on return loss, we proposed a small signal circuit model of a laser diode module. From the small signal measurement data, we obtained the small signal circuit model of the laser diode for each bias current, which contains package parasitics and intrinsic laser diode characteristics. Considering both the intrinsic laser diode characteristics and total parasitic effects of the packaged module, the overall frequency response of the laser diode module was simulated and compared to the experimental results. The simulation results on the small signal modulation bandwidth and return loss showed a good agreement with measurements. However, the return loss does not meet the requirement for 10 Gbps laser diode module. Therefore, the wire bonding effect on the return loss was analyzed. The characteristics of wire bonding inductance which affect the return loss were simulated using an interconnect analysis program. As a result, shorter wire bonding length would be the best step to reduce the package induced inductance.

Fabrication of InP-Based Reflection Type Arrayed Waveguide Grating with Metal Coated Reflection Facet

We have fabricated a reflection type arrayed waveguide grating that consists of one free propagation region and arrayed waveguides which are all strainght waveguides. Each arrayed waveguide has a reflection facet formed by lithography. All reflection facets were coated with metal for high reflection. The device is an 1×8 multi/demultiplexer with 1.6 nm channel spacing. The chip size is as small as 0.83 ×2.7 mm2. The on-chip loss is about -13 dB and the crosstalk is less than -13 dB.

Cavity length dependence of high-speed 1.55-μm multiple-quantum-well laser diode characteristics

Spontaneous emission spectra below threshold were measured from the side wall of InGaAs QW laser diodes to extract laser design parameters such as cavity length dependence of gain, linewidth enhancement factor, and serial resistance. The threshold current varies according to the change of cavity length, and thus, the lasing peak shifts and the serial resistance changes. It is interesting that the linewidth enhancement factor, however, is not deteriorated by shortening cavity length. The short cavity length would rather improve the linewidth enhancement factor mainly by shifting the lasing peak to smaller wavelength side, where the linewidth enhancement factor is inherently low.

The contraction of lattice constant and the reduction of growth rate in p-InGaAs grown by organometallic vapor phase epitaxy

We have investigated the effect of diethylzinc (DEZn) on the lattice constant and the growth rate of InGaAs. Introducing DEZn for p-type doping induces the contraction of lattice constant and the reduction of growth rate compared to undoped InGaAs. Depletion of indium is responsible for these effects. These effects are reduced at lower growth temperatures or at lower growth pressures. From the observed effects of the growth temperatures and the growth pressures on the contraction of the lattice constant, it is concluded that depletion of indium occurs in the gas phase.

Effects of heat-up procedure and grating depth on the erosion of InGaAs grating

Abstract Effects of heat-up procedure and grating depth on the erosion of InGaAs absorptive layers and the quality of strained multiple quantum wells (MQWs) were investigated using transmission electron microscopy (TEM) and double crystal X-ray diffraction (DXRD) analysis, in the fabrication of loss-coupled distributed feedback laser diodes (DFB-LDs) that have absorptive grating on the side of substrate. Decomposition of an InGaAs grating depends on the growth starting temperature and grating depth. Even before starting InP growth, a part of the InGaAs was decomposed during the heat-up process. These decomposed adatoms of indium, gallium, and arsenic seem to migrate to the concave region of the grating and then form a new material phase at the bottom of the concave region. At the initial stage of InP planarization layer growth, InP growth on the InGaAs mesa does not take place until the concave region of grating is fully filled, while a part of InGaAs is decomposed. Desorbed adatoms seemed to migrate to the upper part of the growing InP on the concave region and form another new material phase in this region. The growth technique of n-InP planarization layers over shallow gratings with the growth starting temperature of 550°C allows strained MQWs of good crystalline quality, leading to the successful fabrication of very low threshold loss-coupled DFB-LDs.

The effect of strain on the dielectric constants of strained In0.7Ga0.3AsyP1−y films

We measured the dielectric constants of strained In0.7Ga0.3AsyP1−y (y=0.2, 0.4, 0.8, 1.0) and lattice-matched 1.32 μm In1−xGaxAsyP1−y thin films grown on InP substrates by metalorganic chemical vapor deposition. Measurements were performed by phase-modulated spectroscopic ellipsometry in the range of 0.76–4.9 eV. Our data bridge the gap between literature data in the near-infrared region and those in the visible-ultraviolet region. The critical point energies of strained In0.7Ga0.3AsyP1−y were compared with unstrained counterparts and were found to be shifted in accordance with the theory, which predicts that the compositional shift is compensated. Thus, the critical point energies of strained In1−xGaxAsyP1−y thin films of arbitrary composition can be estimated accurately and, conversely, the composition of strained In1−xGaxAsyP1−y thin films can be estimated by measuring their critical point energies, as for unstrained materials.

Device performance of SI-PBH DFB-LD with undulation of SL-MQW layers

Strained layer multiple quantum well (SL-MQW) distributed feedback laser diodes (DFB-LD) have been widely used as single mode light sources for long-haul, high-speed optical telecommunication systems. Much work has been performed for thermal deformation of surface corrugations on substrates. In the growth of SL-MQW layers on first-order corrugation grating, one may observe the undulation of such layers. Such undulation deteriorates the device performance of DFB-LD. In this work, we observed that this undulation was dependent upon AsH/sub 3/ partial pressure and heat-up time before main growth, and was removed by controlling the partial pressure and the time. The influence of undulation on the device characteristics of semi-insulating planar-buried heterostructure (SI-PBH) DFB-LD is also discussed. The epilayers used in this study were grown on (100) InP substrates with a first-order corrugation grating using low-pressure metalorganic vapor phase epitaxy. The layer structure consists of compressively strained MQW (InGaAs(well)/lnGaAsP(barrier), 10 periods) as an active layer emitting at 1.55 /spl mu/m, and 1.24 /spl mu/m InGaAsP waveguide layers. The layer qualities of SL-MQW are analyzed using DXRD, TEM, and SEM. We fabricated the high speed DFB-LD with a highly resistive Fe-doped SI InP layer as a current blocking layer to minimize the parasitic capacitance.

Highly reproducible methods for fabrication of InP reflective arrayed waveguide grating demultiplexer

We have designed and fabricated a reflective type of InP-based arrayed waveguide grating demultiplexer using highly reproducible and novel methods, such as lithography, metal coating for high reflection, and lift-off, not cleaving a chip in half.