Karl Joachim Ebeling

Transverse mode selection in large-area oxide-confined vertical-cavity surface-emitting lasers using a shallow surface relief

Transverse mode selection has been introduced in a large area oxide-confined vertical-cavity surface-emitting laser (VCSEL) by etching a shallow (only 40-nm deep) surface relief. The circular relief pattern, intended for fundamental mode selection, selects low order modes, resulting in significantly reduced beam divergence (from 48/spl deg/ to 13/spl deg/ and less over the entire drive current range) and improved spectral purity (width of emission spectrum reduced from 5 to less than 0.3 nm) compared to VCSELs without surface relief. A maximum output power of 10 mW was measured.

Evaluation of the temperature stability of AlGaN/GaN heterostructure FETs

Temperature stress experiments up to 800/spl deg/C have been applied to AlGaN/GaN FETs grown by MOVPE on sapphire and their individual technological building blocks. It was found that the temperature limit is given by the irreversible degradation of the intrinsic active heterostructure material itself during operation above 600/spl deg/C. The irreversible degradation was observed for both unconnected and electrically operated devices during temperature stress.

Large-area single-mode VCSELs and the self-aligned surface relief

The effect of mode-profile specific etching of the top layer in selectively oxidized vertical-cavity surface-emitting laser (VCSEL) structures at 850-nm emission wavelength is examined. For high reproducibility, a self-aligned etching technique is demonstrated which aligns surface etch and oxide aperture by only one additional photoresist step. By optimizing layer structure and etch spot size, completely single-mode devices with aperture diameters up to 16 /spl mu/m are obtained. Maximum single-fundamental-mode output power of 3.4 mW at room temperature and over 1 mW at 0/spl deg/C is obtained with a maximum far-field angle of 5.5/spl deg/. Using parameters for etch spot height and diameter, Gaussian beam spot size and phase curvature, the measured diffracted far-field distribution is fitted well over a 20-dB intensity range. The chosen fit parameters therefore enable one to estimate the amount of phase curvature within the VCSEL for different operation currents, which cannot be obtained with available measurement methods.

Design and analysis of single-mode oxidized VCSELs for high-speed optical interconnects

Oxide-confined vertical-cavity surface-emitting laser diodes (VCSELs) are fabricated for applications in high-performance optical interconnects. Both 980-nm as well as 850-nm wavelength devices in one- and two-dimensional arrays are investigated. Noise properties of single- and multimode devices under different operation conditions are relative intensity noise of single-mode devices can be as low as -150 dB/Hz at output powers of about 1 mW and feedback levels up to -30 dB. Data rates up to 12.5 Gb/s with bit error rates below 10/sup -11/ are achieved with VCSELs showing stable single-mode emission at large-signal modulation, combined with modulation bandwidths exceeding 10 GHz. Arrays with 4/spl times/8 elements flip-chip mounted on Si CMOS driver chips ready for use in parallel data transmission systems are presented.

High-performance oxide-confined GaAs VCSELs

We present GaAs based selectively oxidized VCSELs with record high 57% wallplug efficiencies emitting in the 820-860-nm wavelength regime. Solid source molecular beam epitaxy with carbon as p-type dopant is used for crystal growth. Multimode devices show continuous-wave (CW) output powers up to 42 mW and stable operation from -80/spl deg/C up to +185/spl deg/C. Efficient single-mode output power of some milliwatts is maximized by controlling optical waveguiding that depends on the position of the 30-nm thin oxide aperture. Elliptically shaped current apertures are applied to stabilize output polarization.

Improving single-mode VCSEL performance by introducing a long monolithic cavity

We report on the improvement of several selectively oxidized vertical-cavity surface emitting laser characteristics by introducing a long monolithic cavity. The samples compared are grown with various cavity lengths using solid-source MBE. The 980 nm-regime is chosen as emission wavelength to facilitate growth by using binary GaAs cavity spacers. A record high single-transverse mode output power of 5 mW at a series resistance of 98 /spl Omega/ is obtained for a 7-/spl mu/m aperture device with a 4-/spl mu/m cavity spacer. Using an 8-/spl mu/m cavity spacer, devices up to 16-/spl mu/m aperture diameter emit 1.7 mW of single-mode power with a full-width at half-maximum far-field angle below 3.8/spl deg/.

High-power VCSELs: single devices and densely packed 2-D-arrays

We report on vertical-cavity surface-emitting lasers (VCSELs) and laser arrays providing high output powers in the 980-nm wavelength regime. Extensive investigations on size scaling behavior of single top- and bottom-emitting devices concerning fundamental electrooptical and thermal properties show limits of attainable output characteristics. Maximum experimentally achieved continuous-wave (CW) optical output powers at room temperature are 180 and 350 mW for top- and bottom-emitting VCSELs, respectively. Detailed analysis on the thermal interaction between closely spaced elements have been carried out to describe the thermally induced power limitations of two-dimensional arrays. Fabricated heat sunk bottom-emitting arrays of 23 elements and 40-/spl mu/m aperture size of individual elements show output powers of 0.56 W CW at room temperature and 0.8 W actively cooled, resulting in 0.33 kW/cm/sup 2/ and 0.47 kW/cm/sup 2/ maximum spatially averaged optical power density, respectively.

Top surface-emitting vertical-cavity laser diodes for 10-Gb/s data transmission

Intensity modulated proton-implanted top surface-emitting vertical-cavity InGaAs QW lasers (VCSELs) with a small-signal modulation bandwidth of 12 GHz butt-coupled to multimode fibers are investigated as light source for optical interconnection. At 10-Gb/s pseudorandom data rates the bit-error rate (BER) remains under 10/sup -11/ after transmission over 500 m of graded index multimode fiber. Optimum transmission behavior is achieved for linearly polarized nearly single-mode laser operation with a side-mode suppression of better than 25 dB under modulation. Spectral characterization indicates that linearly polarized single-mode light output is essential for good BER performance.

Modeling and design of proton-implanted ultralow-threshold vertical-cavity laser diodes

Detailed theoretical models are presented and used for investigating the optical, electrical, and thermal characteristics of gain-guided vertical-cavity lasers. A two-dimensional description of current flow in the proton-implanted region is achieved. Heterojunctions are taken into account for the first time, and their influence on current spreading is demonstrated. Thorough finite-element calculations of the temperature profile in the whole device indicate the formation of a thermally induced waveguide. The temperature dependence of threshold current is evaluated, and it is shown that different mode positions greatly influence the output characteristics of the laser. The models are used to analyze the behavior of three-terminal vertical-cavity lasers. Good agreement is obtained between experimental and theoretical results. Using optimized design, record low threshold currents of 650 mu A have been achieved. >

3-Gb/s data transmission with GaAs VCSELs over PCB integrated polymer waveguides

GaAs quantum-well (QW)-based vertical-cavity surface-emitting lasers (VCSELs) at 855-nm emission wavelength are investigated for intraboard polymer waveguide links. We report a 3-Gb/s pseudorandom bit sequence (PRES) nonreturn-to-zero (NRZ) data transmission over about 5-cm long printed circuit board (PCB) integrated multinode polymer waveguide arrays of two different geometries at bit-error rates (BERs) of less than 10/sup -11/.