Christoph S. Harder

5.2-GHz bandwidth monolithic GaAs optoelectronic receiver

A high-speed monolithic optoelectronic receiver consisting of a photodetector, a transimpedance amplifier and a 50- Omega output buffer stage has been fabricated using an enhancement/depletion 0.35- mu m recessed-gate GaAs MESFET process. The interdigitated metal-semiconductor-metal photodetector has a dark current of 0.8 nA, a responsivity of 0.2 A/W, and a capacitance of 12 fF. The bandwidth of the receiver is 5.2 GHz with an effective transimpedance of 300 Omega into a 50- Omega load, which corresponds to a transimpedance bandwidth product of 1.5 THz- Omega .<<ETX>>

Beam divergence and waist measurements of laser diodes by near-field scanning optical microscopy

We demonstrate the use of near-field scanning optical microscopy (NSOM) for the measurement of the beam properties of single quantum well, graded index separate confinement heterojunction ridge laser diodes. Using NSOM, we measure the field intensity in the transverse plane at near field and as a function of distance from the facet. The divergence of the laser beam and the beam waists in vertical and lateral dimensions are directly measured and the astigmatism of the mode is determined. In the near field, we observe a nearly ideal Gaussian shape in the vertical dimension which is consistent with the beam divergence as measured in the far field. In the lateral dimension, the beam shape deviates from the ideal Gaussian since the mesa structure of the laser diode provides an effective step-index waveguide. The non-Gaussian structure of the mode is also observed in the beam divergence properties.

L-I characteristics of fiber Bragg grating stabilized 980-nm pump lasers

Light versus current (L-I) characteristics, of fiber Bragg grating stabilized 980-nm pump lasers, is experimentally studied and theoretically modeled. It is shown that a conventional design of such laser modules can result in sudden transitions between a coherence-collapse multimode emission, and a coherent single-mode state of operation.

Noncritically phase‐matched frequency doubling using 994 nm dye and diode laser radiation in KTiOPO4

The characteristic properties of noncritically phase‐matched frequency doubling in KTiOPO4 have been investigated using ∼1 μm radiation from a Styryl‐13 dye laser. Noncritically phase‐matched type II second‐harmonic generation was found to occur at 994.3 nm. The unusually wide angle, temperature, and wavelength tolerances observed for this nonlinear process are attractive for the design of practical blue‐green laser sources. A special strained‐layer InGaAs diode laser was fabricated to operate at 994 nm and generation of blue‐green 497 nm radiation by noncritically phase‐matched frequency doubling was demonstrated.

Spontaneous emission and gain in GaAlAs quantum well lasers

Measurements of spontaneous emission from GaAlAs single quantum well lasers over a wide range of carrier densities are discussed. From these measurements, the optical gain, refractive index, and linewidth enhancement factor are determined, all of which contribute to the overall performance of a semiconductor laser. In addition, a simple model which adequately describes the important features in the spontaneous emission (and therefore the gain) data is presented. With this model, quantitative information about the carrier density-dependent bandgap shrinkage, lifetime broadening, and the shape of the broadening functions is obtained. >

Molecular beam epitaxy of GaAs/AlGaAs quantum wells on channeled substrates

GaAs/AlGaAs quantum wells (QWs) were grown by molecular beam epitaxy on GaAs (100) substrates patterned with ridges and grooves in the [011] direction. Low‐temperature cathodoluminescence was used to measure the Al fraction and QW thickness on top of the ridges and grooves as a function of ridge and groove width. Surface diffusion during growth depletes Ga from the side facets while increasing the incorporation of Ga on the (100) sections of ridges and grooves. The QW thickness on top of a ridge grown at 710 °C increases from 72 to 95 A, and the Al fraction x decreases from x=0.33 to x=0.29 as the ridge width is narrowed from 30 to 4 μm. Graded refractive index separate confinement heterostructure lasers with nominally 70 A QWs and Al0.2Ga0.8As barriers were grown on patterned substrates at 695 and 725 °C. Lasers fabricated on the overgrown 4‐μm‐wide ridges have a 20 meV decrease in emission energy compared to laser diodes on 30 μm ridges.

Surface diffusion effects in MBE growth of QWs on channeled substrate (100) GaAs for lasers

The diffusion and desorption of Ga atoms in MBE-grown AlGaAs layers and GaAs QWs on channeled substrate planar structures have been studied by scanning electron microscopy, photoluminescence and cathodoluminescence. The (m11)A side facets of ridges and grooves act as sources for additional Ga which increases the growth rate on the (100) sections of ridges and grooves, leading to lower Al compositions and thicker quantum wells on narrow stripes. The adatom flux is increased for growth temperatures above 700°C and an As4:(Ga,Al) flux ratio below 1. Growth interruption after the QW leads to a complete desorption of Ga at the (311)A facets, leaving an embedded (100) QW.

Wavelength and threshold current of a quantum well laser in a strong magnetic field

The threshold current and the wavelength of a high‐power ridge waveguide AlGaAs graded index seperate confinement heterostructure quantum well laser have been studied in strong magnetic fields up to 20 T, to simulate the complete quantum confinement of carriers in a quantum box laser. It will be shown both experimentally and theoretically that the threshold current is increased by the application of a strong magnetic field, while its temperature sensitivity is reduced. It will further be shown that at low temperatures (T 100 K) exciton laser emission is only observed after application of a strong magnetic field, i.e., reduction of the dimensionality.

Lateral mode discrimination and self-stabilization in ridge waveguide laser diodes

Lateral mode discrimination and output stability is experimentally investigated in ridge waveguide laser diodes, having various residual guide thicknesses outside the ridge region. It is found that a critical residual thickness exists below which the lasers emit in a single mode, with a low threshold current. Above this critical value, the threshold current rises rapidly with the residual guide thickness, and the lasers oscillate simultaneously in the two lowest order lateral modes. Increasing the injected current intensity, in this regime, results in nonlinear light output-current curves, but also improves mode discrimination in favor of the fundamental mode, until single-mode operation is re-established. A simple mechanism is suggested to explain this phenomenon

Excitonic effects in gain and index in GaAlAs quantum well lasers

Spontaneous emission and gain measurements in GaAlAs single quantum well lasers are presented. The gain is derived from the spontaneous emission detected through an opening in the top metallic contact of the lasers. Excitonic effects are seen in the gain (absorption) spectra for low carrier densities, and the step‐like nature of the two‐dimensional density of states is evident. From the gain spectra, refractive index changes are derived via a Kramers–Kronig transformation, and this is used to evaluate the linewidth enhancement factor as a function of photon energy.