Dirk Lorenzen

Simultaneous quantification of strain and defects in high-power diode laser devices

Photocurrent spectroscopy is applied for an analysis of both packaging-induced strain and strain-induced defect creation in InAlGaAs/GaAs high-power diode laser arrays. Strain profiles across 50 W diode lasers processed by different packaging procedures are measured and compared to model calculations. We demonstrate that packaging-induced strain giving rise to spectral shifts of the laser transition correlates with packaging-induced defects in the waveguide that are quantified via a sub-band gap absorption band. Packaging on a Cu–diamond multilayer heat spreader appears as optimized solution simultaneously minimizing strain and defect creation.

Transient thermal properties of high-power diode laser bars

The transient thermal properties of high-power diode laser bars with active and passive cooling are analyzed experimentally with thermal imaging and through their thermal wavelength tuning behavior and modeled with the finite element method.

Direct spectroscopic measurement of mounting-induced strain in high-power optoelectronic devices

Thermally induced strain caused by device packaging is studied in high-power semiconductor lasers by a noninvasive technique. Fourier-transform photocurrent measurements with intentionally strained laser array devices for 808 nm emission reveal spectral shifts of quantum-confined optical transitions in the optical active region. These shifts by up to 7 meV serve as a measure for strain and are compared with model calculations. For a given packaging architecture, about one quarter of the mounting-induced strain is transferred to the quantum-well region of the device. Spatially resolved measurements demonstrate a lateral strain gradient in the devices.

Quantitative strain analysis in AlGaAs-based devices

We present a strategy for quantitative spectroscopic analysis of packaging-induced strain using both finite element analysis and band-structure calculations. This approach holds for a wide class of AlGaAs-based, and related, devices, among them high-power “cm-bars.” The influence on the results of particular device structure properties, such as intrinsic strain and quantum-well geometry, is analyzed. We compare theoretical results based on a unaxial stress model with photocurrent data obtained from an externally strained cm-bar, and obtain better agreement than from alternative strain models. The general approach is also applicable to the analysis of all data that refer to changes of the electronic band structure, such as absorption and photoluminescence.

Increased power of broad-area lasers (808nm/980nm) and applicability to 10-mm bars with up to 1000Watt QCW

The new packaging technology from JENOPTIK Laserdiode GmbH and the new chip technology from JENOPTIK Diode Lab GmbH increases the output power, the quality and durability of new broad area lasers. Tests with different pulse widths and duty cycles have been conducted. A maximum linear power density of 213mW/&mgr;m has been found for 808nm and 980nm laser, limited by thermal rollover. The tests were performed for duty cycles from 0.1% to 5% and pulse widths of 50&mgr;s and 100&mgr;m. Over 32W output power was reached for 150&mgr;m emitter at a 0.1 % duty cycle and 50&mgr;s pulse length. With the new diode laser technology 10mm bars with a 44% filling factor were produced. These laser bars, mounted on micro channel coolers, reached a maximum output power of 1000W. To our knowledge this is the highest power reported up to now for 980nm material with 100&mgr;s pulses and 0.1% duty cycle.

High-brightness high-power 9xx-nm diode laser bars: developments at Jenoptik Diode Lab

We report present advantages of high power 9xxnm diode laser bars for pumping of disc laser and especially for pumping fibre lasers and amplifiers. The strong demand for reduce system costs needs to have a good compromise in improved diode laser power, conversion efficiency, reliability and beam quality leading to simplified system designs. Basis of the new generation for the 9xxnm laser diode bars at JENOPTIK Diode Lab is a low loss wave guide AlGaAs - structure with low vertical far field angle of 27° (FWHM). Recently we demonstrate an output power in excess of 500W in CW operation from a diode laser bar with 50% filling factor and 3.0mm cavity length. This record was possible due to high power conversion efficiency of >68 %, optimised facet coating technology and an excellent active cooling. New results on conductive cooled high brightness laser bars of 20% filling factor with special emphasis to the needs of high efficiency fibre coupling will be presented. Lifetime tests under long pulse conditions have demonstrated a very high reliability for 120 W laser bars with 50 % filling factor and for 60 W laser bars with 20 % and 30 % filling factor.

Spatially resolved and temperature dependent thermal tuning rates of high-power diode laser arrays

Thermal tuning properties of passively cooled 808nm emitting high-power diode laser bars are analyzed. Data from standard devices packaged on Cu heat sinks and identical devices mounted on expansion-matched Cu–W heat sinks are compared. For a standard device, we find up to one-fifth of the thermal tuning rate of −(0.56±0.04)meVK−1 to be caused by pressure tuning driven by the relaxation of compressive packaging-induced stress for increasing temperatures. For devices packaged on expansion-matched heat sinks the observed tuning rate of −(0.46±0.01)meVK−1 represents almost the genuine thermal tuning rate of the semiconductor device structure. Thus this technology potentially leads to improved device properties.

Passively cooled diode lasers in the cw power range of 120 to 200W

Improvements of laser diode bar efficiency and mounting technology have boosted output powers of passively cooled diode lasers beyond the 100W cw limit. After an introduction about reliablity statements and reliability assessment, the performance increase by technology improvements is documented in current-step failure discrimination tests. Electro-optical parameters of improved diode lasers are subsequently presented in detail as well as the results of lifetime tests at different powers and in different operation modes - steady-state and repetitive/intermittent (hard pulse) cw operation.

Spectroscopic measurement of packaging induced strains in high-power laser diode arrays

Summary form only given. High-power diode lasers are important for a wide range of applications, e.g. as pump sources for solid state lasers and tools for material processing. For managing the thermal load connected with high power operation, complex device and heat sink architectures are required. In particular, the semiconductor structure must be mounted p-side down on a heat sink of high thermal conductivity. In such a geometry, mechanical strain of the active region represents a central issue, affecting both the parameters of laser emission and the lifetime of the device.

Measurement of mounting-induced strain in high-power laser diode arrays

Thermally induced strain caused by device packaging is studied in high-power semiconductor laser arrays by a novel non-invasive technique. Measurements with intentionally strained laser array devices for 808 nm emission reveal spectral shifts of quantum-confined optical transitions in the optical active region. These shifts by up to 10 meV serve as a measure for strain and are compared with model calculations. We demonstrate that different packaging techniques cause different packaging-induced strains. We also show that the packaging-induced strain portion, which gets transmitted through the solder material, differs for different packaging technologies. An intentionally strain- reduced packaging technique is shown to transmit about one quarter of the potential packaging-induced strain towards the optical active layer, whereas another packaging technique, which provides highly reliable single-chip devices is found to transmit about half of the potential amount. Spatially resolved measurements demonstrate strain gradients within the devices. Also temporal strain evolution is monitored. We show that the burn-in is accompanied by strain accumulation whereas for long-term operation strain relaxation occurs.