M. Grabherr

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.

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.

Efficient single-mode oxide-confined GaAs VCSEL's emitting in the 850-nm wavelength regime

Single- and multimode vertical-cavity surface-emitting lasers (VCSELs) with three unstrained GaAs quantum wells (QWs) and emission wavelengths around 850 nm have been fabricated using molecular beam epitaxy (MBE) for crystal growth. Wet chemical etching and subsequent selective oxidation are applied for current confinement. The influence of oxide layer position on lateral index guiding is studied in detail in order to increase maximum single-mode output power. A device of 3-/spl mu/m active diameter and reduced index guiding shows maximum single-mode output power of 2.25 mW with a side-mode suppression ratio (SMSR) of more than 30 dB for high-efficiency oxidized VCSELs.

Bottom-emitting VCSEL's for high-CW optical output power

Bottom-emitting vertical-cavity surface-emitting InGaAs MQW lasers operating in the 980-nm wavelength regime have been designed for high continuous-wave optical output power. Devices of 200-/spl mu/m active diameter and optimized performance reach 350-mW maximum output power when mounted on a heat sink. 50-/spl mu/m-size lasers produce 100 mW at 25% electrical to optical power conversion efficiency. Thermal properties and size dependent basic characteristics are investigated in detail.

Improved output performance of high-power VCSELs

The intention of this paper is to report on state-of-the-art high-power vertical-cavity surface-emitting laser diodes (VCSELs), single devices as well as two-dimensional (2-D) arrays. Both approaches are studied in terms of electrooptical characteristics, beam performance, and scaling behavior. The maximum continuous wave (CW) output power at room temperature of large-area bottom-emitting devices with active diameters up to 320 /spl mu/m is as high as 0.89 W, which is to our knowledge the highest value reported for a single device. Measurements under pulsed conditions show more than 10-W optical peak output power. Also, the CW performance of 2-D arrays has been increased from 0.56 W for 23 elements to 1.55 W for 19 elements due to significantly improved heat sinking. The extracted power densities spatially averaged over the area close to the honeycomb-like array arrangement raised from 0.33 kW/cm/sup 2/ to 1.25 kW/cm/sup 2/. Lifetime measurements have proven acceptable reliability for over 10000 h at a degradation rate of less than 1% per 1000 h. The emission wavelength of bottom-emitting devices is restricted to about 900 nm or higher due to fundamental absorption in the GaAs substrate. Windowing of the substrate has been studied to allow for shorter wavelength emission.

High-power selectively oxidized vertical-cavity surface-emitting lasers

Summary form only given. Vertical-cavity surface-emitting lasers (VCSELs) emitting around 980 nm wavelength are of much interest for short distance, high-data-rate optical interconnects. Recently, VCSELs with record high conversion efficiencies of 50% and record low threshold currents of 9 /spl mu/A have been reported, employing selective oxidation of AlAs for current confinement and MOCVD growth with carbon as p-type dopant. In these small diameter devices, the maximum output power was emitted to a few milliwatts. We have fabricated MBE-grown oxidized VCSELs using beryllium as p-type dopant. Nonheatsinked 25 /spl mu/m active diameter lasers with 82% differential quantum efficiency reach maximum output powers of 47 mW and wallplug efficiencies up to 42%.Summary form only given. We have demonstrated MBE-grown oxidized InGaAs QW VCSELs combining both high output power and high conversion efficiency. Applications for these devices are foreseen in large two-dimensional arrays for optical pump sources.

Noise characteristics of 850 nm single-mode vertical cavity surface emitting lasers

We have measured the noise characteristics of single-mode oxide confined surface emitting vertical cavity laser diodes. For pumping levels of more than 3.5 times threshold current, the relative intensity noise is below −150 dB/Hz up to 5 GHz at output powers near 1 mW. For various frequencies, we observe sub-Poissonian noise.

Spatial structure of broad-area vertical-cavity regenerative amplifiers.

We investigate the spatial structure of broad-area vertical-cavity regenerative amplifiers injected with a homogeneous beam. The emerging patterns have a predominantly sixfold rotational symmetry, verifying the recent prediction of formation of hexagons. The length scale is controllable by means of detuning and follows the prediction for tilted waves.

Oxide-confined 2D VCSEL arrays for high-density inter/intra-chip interconnects

We have designed and fabricated 4 X 8 vertical-cavity surface-emitting laser (VCSEL) arrays intended to be used as transmitters in short-distance parallel optical interconnects. In order to meet the requirements of 2D, high-speed optical links, each of the 32 laser diodes is supplied with two individual top contacts. The metallization scheme allows flip-chip mounting of the array modules junction-side down on silicon complementary metal oxide semiconductor (CMOS) chips. The optical and electrical characteristics across the arrays with device pitch of 250 micrometers are quite homogeneous. Arrays with 3 micrometers , 6 micrometers and 10 micrometers active diameter lasers have been investigated. The small devices show threshold currents of 600 (mu) A, single-mode output powers as high as 3 mW and maximum wavelength deviations of only 3 nm. The driving characteristics of all arrays are fully compatible to advanced 3.3 V CMOS technology. Using these arrays, we have measured small-signal modulation bandwidths exceeding 10 GHz and transmitted pseudo random data at 8 Gbit/s channel over 500 m graded index multimode fiber. This corresponds to a data transmission rate of 256 Gbit/s per array of 1 X 2 mm2 footprint area.

High-power VCSELs: modeling and experimental characterization

Broad-area oxide confined vertical cavity surface emitting lasers are investigated theoretically and experimentally as high-power laser sources. A self-consistent laser model comprising current and temperature distribution, carrier diffusion, and a simplified optical submodel is employed to explain measured output characteristics. Top and bottom emitting devices of various diameters are designed and fabricated and the scaling laws for various laser parameters are extracted. A comparison between both emission schemes is provided. Maximum output powers of 180 mW and 350 mW obtained from both top emitters and heat-sink mounted bottom emitters of 150 micrometers and 200 micrometers active diameter, respectively, represent the state-of-the-art. Even higher output powers at improved conversion efficiencies are suggested to be obtained from densely spaced 2D arrays with properly applied heat-sinking.