Andrea Knigge

650-nm vertical-cavity surface-emitting lasers: laser properties and reliability investigations

650-nm AlGaInP-AlGaAs-based oxide-confined VCSELs are investigated in dependence on the current aperture size. VCSELs with small aperture (a=5 /spl mu/m) have a maximum continuous-wave (CW) output power of about 1 mW at room temperature. They reach higher operating temperatures (T/sub max/=55/spl deg/C), have narrower beam profiles, less transverse modes, and a higher side mode suppression compared to large aperture VCSELs (a>13 /spl mu/m). The latter devices emit a CW-output power P=3 mW at 20/spl deg/C. Reliability tests of 655-nm devices show at 20/spl deg/C an output power of P/spl ap/0.4 mW over more than 1000 h and at 40/spl deg/C P/spl ap/0.1 mW over 500 h.

Optically pumped semiconductor disk laser with graded and step indices

Summary form only given. This paper reports continuous-wave (cw) and passive mode-locked laser operation of diode-pumped semiconductor disk lasers based on gain sections with step and graded index designs, each containing 6 InGaAs quantum wells. Mode-locked laser operation is achieved applying both structures, using a V-shaped cavity including a fast saturable semiconductor absorber mirror. Results show that the graded index structure exhibits clear advantages over the step index structure in cw-laser operation with respect to the saturation of the absorption. However, for mode-locked operation, shorter pulses are generated using the step index structure.

Measurement and simulation of top- and bottom-illuminated solar-blind AlGaN metal-semiconductor-metal photodetectors with high external quantum efficiencies

A comprehensive study on top- and bottom-illuminated Al0.5Ga0.5N/AlN metal-semiconductor-metal (MSM) photodetectors having different AlGaN absorber layer thickness is presented. The measured external quantum efficiency (EQE) shows pronounced threshold and saturation behavior as a function of applied bias voltage up to 50 V reaching about 50% for 0.1 μm and 67% for 0.5 μm thick absorber layers under bottom illumination. All experimental findings are in very good accordance with two-dimensional drift-diffusion modeling results. By taking into account macroscopic polarization effects in the hexagonal metal-polar +c-plane AlGaN/AlN heterostructures, new insights into the general device functionality of AlGaN-based MSM photodetectors are obtained. The observed threshold/saturation behavior is caused by a bias-dependent extraction of photoexcited holes from the Al0.5Ga0.5N/AlN interface. While present under bottom illumination for any AlGaN layer thickness, under top illumination this mechanism influences the EQE-bias characteristics only for thin layers.

Anisotropic Responsivity of AlGaN Metal–Semiconductor–Metal Photodetectors on Epitaxial Laterally Overgrown AlN/Sapphire Templates

Al0.4Ga0.6N metal–semiconductor–metal photodetectors on epitaxial laterally overgrown (ELO) AlN/sapphire templates show anisotropic device characteristics depending on the orientation of the electrode stripes with respect to the stripe pattern onto which the underlying ELO AlN buffer layers have been grown. With electrodes perpendicular to the stripes, a quantum efficiency (QE) of ∼140 was found for 20-V bias at room-temperature. This gain is explained by carrier transport along channels with increased Ga content resulting from faceted growth at the steps of the ELO template. The resulting potential barrier is confirmed by the activation energy found for the temperature dependence of the QE. In contrast, photodetectors with electrodes running parallel to these channels do not show gain but have an enhanced QE at elevated bias voltage compared to devices on planar AlN buffer layers. This effect is attributed to different densities of threading dislocations in the absorber layer.

AlGaN Metal–Semiconductor–Metal Photodetectors on Planar and Epitaxial Laterally Overgrown AlN/Sapphire Templates for the Ultraviolet C Spectral Region

Schottky type metal–semiconductor–metal (MSM) Al0.4Ga0.6N photodetectors (PDs) for the ultraviolet C spectral region on conventional planar AlN templates are compared with epitaxial laterally overgrown (ELO) AlN templates. On planar templates solar blind MSM PDs with state-of-the-art dark current in the pA range and a power independent responsivity are obtained. PDs on ELO templates with fingers parallel to the etched stripes have properties similar to those on planar templates. PDs on ELO templates with contact fingers oriented perpendicular to the etched stripe pattern exhibit photoconductive gain leading to external quantum efficiencies of up to 77 at 30 V applied bias surpassing that of the planar grown PDs by a factor of 100. In spite of the high gain these PDs also show low dark currents, short switching times and two operating regimes with power independent responsivity.

Monolithic passively mode-locked semiconductor quantum-well laser emitting at 1070 nm: Picosecond pulse generation and pulse train stability analysis

We study experimentally the pulsed emission of a passively mode-locked multi-section quantum-well semiconductor laser emitting picosecond short optical pulses at a fundamental repetition rate of 20 GHz and at a wavelength of 1070 nm. We identify different regions of picosecond short pulses dependent on the laser biasing conditions and relate them to the investigated mode-locking stability by means of timing jitter and amplitude jitter analysis. We obtain timing jitter values in a two-digit femtosecond range and operating regions without relative amplitude jitter at pulse widths in the order of a few picoseconds.

Femtosecond semiconductor laser system with resonator-internal dispersion adaptation

We present a femtosecond laser diode system that is capable of autonomously adjusting itself to compensate for the external dispersion in an arbitrary application. The laser system contains a spatial light modulator inside the cavity which is controlled by an evolutionary algorithm in order to allow for phase and amplitude shaping of the laser emission. The cavity-internal dispersion control is shown to be much more efficient than an external control with a pulse shaper.

Influence of Carrier Lifetime, Transit Time, and Operation Voltages on the Photoresponse of Visible-Blind AlGaN Metal--Semiconductor--Metal Photodetectors

We investigated the influence of lifetime and transit time of photogenerated carriers on the performance of visible-blind Al0.25Ga0.75N metal–semiconductor–metal photodetectors by a combination of experimental studies and numerical simulations. Good agreement between simulated and measured current–voltage (I–V) characteristics was achieved for several geometries of the interdigitated contact structure. Simulations of the external quantum efficiency (EQE) at low bias voltages showed that a long hole lifetime in the AlGaN absorption layer significantly influences the EQE due to the slow carrier transit in weak electric fields. At 1 V the EQE can be enhanced by a factor of 3 by increasing the hole lifetime from 10 ps to 1 ns. Reducing the electrode spacing from 10 to 1 µm as well as operating the device at higher voltages additionally increases the ratio between carrier lifetime and transit time, resulting in an enhancement of the EQE at a fixed carrier lifetime by one order of magnitude.

AlGaN-based metal-semiconductor-metal photodetectors with high external quantum efficiency at low operating voltage

Solar blind Al0.5Ga0.5N/AlN metal-semiconductor-metal photodetectors (MSM PDs) are characterized by means of photocurrent spectroscopy. In order to enhance the external quantum efficiency (EQE) at low bias voltages several strategies have been adopted including absorber layer thicknesses, electrode layout and metallization scheme. Analysis of experimental EQE-bias characteristics under top and bottom illumination conditions reveals (1) a correlation between EQE and electrode pair density for symmetric electrode designs and (2) a slight asymmetry of the EQE with respect to bias polarity for bottom-illuminated MSM PD consisting of electrode pairs with different electrode widths (asymmetric design) and (3) zero-bias operation for a-MSM PD consisting of electrode pairs with different metallization schemes. In addition, the combination of thin absorber layer and asymmetric electrode design leads to high EQE values under bottom illumination at very low voltages and zero-bias operation is achieved for the a-MSM detector. The zero-bias EQE of the a-MSM is further enhanced by combining the symmetric detector design with a high electrode pair density.

Highly reliable 75W InGaAs/AlGaAs laser bars with over 70% conversion efficiency

Diode lasers with a high very conversion efficiency can be obtained when carefully taken into account several partly opposed requirements for the design of the layer structure. Results are given for 940nm laser structures based on the well established InGaAs/AlGaAs material with a relatively low vertical divergence of about 45° including 95% of optical power. Laser bars were processed and mounted on passively cooled heat sinks. 73% conversion efficiency was achieved at 70W output power. 150μm stripe lasers with only 1500μm resonator length mounted on usual C-mounts have a thermal rollover of about 18W, which is a record high value for a resonator length below 2mm. Reliability tests show an excellent stability at 75W in CW and 95W in long pulse operation mode over about 10000h test time.