F. G. Johnson

Dependence of the linewidth enhancement factor on the number of compressively strained quantum well in lasers

We have systematically studied the well number dependence of the linewidth enhancement factor in strained quantum-well (QW) lasers and have demonstrated experimentally that the linewidth enhancement factor can be reduced from /spl sim/9.4 to /spl sim/2.0 by increasing the number of compressively strained QWs from 2 to 8. This behavior is primarily due to an increase in the differential gain with the number of QWs.

Alignment-tolerant structures for ease of optoelectronic packaging

Expanded mode alignment tolerant optical structures will play an important role in low-cost, large-scale packaging of optoelectronic devices. In this paper, we present two expanded mode structures for operation at 1.55 micrometers . Our devices use single epitaxial growth and conventional fabrication schemes. High butt-coupling efficiencies (> 40%) to a single mode fiber with relaxed alignment tolerances were achieved. The first of our devices uses adiabatic transformation over 500 micrometers . The second device uses resonant coupling over a much shorter region of 200 micrometers . The second scheme offers an interesting possibility for monolithic integration of active-passive components. We present the design and simulation results of such an integrated device.

Detuned loading effect and high-speed modulation of fiber grating semiconductor lasers

We have studied both theoretically and experimentally the detuned-loading effect of a fiber grating semiconductor laser. This effect is observed experimentally with a fiber grating semiconductor laser constructed with a single-angled facet diode and a conically lensed grating. Operation of such a laser is demonstrated at 2.5 Gb/s and good eye diagrams are obtained.

Dependence of the lateral oxidation rate of an AlAs layer used as a current aperture in vertical-cavity surface-emitting lasers on different physical parameters

The wet oxidation kinetics of an AlAs layer used as a current aperture in selectively oxidized vertical-cavity- surface-emitting-lasers (VCSELs) is investigated in details. The process is modeled as a diffusion-reaction process. A strong dependence of the oxidation rate on the temperature, at which the wet oxidation is being carried out, is observed. The temperature dependence of the oxidation process is explained theoretically by considering equivalent reaction activation energies for the oxidation reaction. Also for oxidation over a long time interval, variation of the oxidation rate with the variation of the radius of the etched mesa of the VCSEL is observed. A theory has been developed considering the 3D diffusion of the oxidant modules is an already oxidized cylindrical AlAs region and the reaction of the diffusion of the oxidant molecules in an already oxidized cylindrical AlAs region and the reaction of the oxidant molecule at the oxidized-unoxidized AlAs interface. Relevant material parameters, that are independent of the size and geometry of the etched VCSELs, are extracted from the experimental results. Using them in the theoretical model, the rate equation of the lateral oxidation process is obtained. The theory predicts the dependence of the oxidation process on the size of the VCSEL, the AlAs layer thickness, as well as on the physical properties of the AlAs layer. The theoretical predictions have been verified by a number of experiments with reproducible results.

Passive/active resonant coupler (PARC) platform with tapered passive waveguide

There is a need for integrating various active and passive devices on a single substrate to increase the functionality of optical modules [1-4]. One of the methods is to use regrowth for creating a low-loss passive waveguide butt-coupled to the active waveguide [1]. Besides being a complex technology, issues like low-loss coupling over multiple runs is still a challenge. A second technique used for integration is selective area growth [2]. In general, this technology does not allow for freedom in the design of the various layer thickness and bandgaps of the integrated waveguides. Quantum well interdiffusion [3] has also been used for integration by altering the bandgaps of the waveguides but also suffers from a lack of freedom in waveguide design and in the selection of the proper bandgaps.

Programmable high bit rate pattern generator using a segmented semiconductor-optical amplifier

We have demonstrated that spatial gain modulation in a segmented semiconductor optical amplifier can be converted to a temporal signal. A four-segment amplifier was used to generate digital return-to-zero patterns at 40 Gbits/s , and this technique should be readily scalable to more than 100 Gbits/s .

Packaging of electrically switchable tunable tapered lasers

High power near diffraction limited external cavity semiconductor tapered lasers that use a single-angled-facet input preamplifier are demonstrated for the first time. Four electrodes (three in the preamplifier region and one in the power section) on the device were implemented to investigate the switching contrast ratio of the output laser intensity. More than 1 W CW of power was obtained with a slope efficiency of 0.7 W/A, and close to 20 dB intensity contrast ratio was obtained by switching off two and three electrodes in the input ridge section. Also, more than 50 dB side-mode suppression ratio and 60 nm tuning bandwidth were obtained. These powers were found to be emitted in a near-diffraction- limited beam.

1.55-μm mode expander using single epitaxial growth

We report on two techniques developed at the University of Maryland, College Park for fabricating expanded mode laser arrays. Both of these techniques use single epitaxial growth and conventional fabrication techniques. The first of these techniques is based on adiabatic mode transformation from a tightly confined active waveguide to a loosely confined large underlying passive waveguide over a mode transmission region 500 micrometers long. The devices butt couple to a standard single mode fiber with a coupling loss of 3.6 dB and reduced farfield divergence angles of 22 degree(s) and 9 degree(s) in the transverse and lateral directions respectively. The excess mode transformation loss is 1.3 dB. The second device is based on a novel resonant coupling scheme between two waveguides of different dimensions and refractive indices. The mode is transformed over a taper length of 200 micrometers with excess mode transformation loss of 0.6 dB. Butt coupling efficiencies of 41% (3.8 dB coupling loss) is achieved to a standard single mode fiber. The farfield divergence angles achieved are 24 degree(s) and 13 degree(s) in the transverse and lateral directions respectively.

Stable single mode hybrid DBR laser using a fiber grating and a curved semiconductor waveguide structure

Hybrid DBR lasers composed of a semiconductor Fabry-Perot laser diode and a fiber Bragg grating have been demonstrated. In our approach, we use a single-angled facet laser diode and a conically lensed grating to obtain stable single-longitudinal mode operation over more than 85 mA current range with high side mode suppression ratio and narrow linewidth.