Mathias Ziegler

Above room temperature operation of short wavelength (λ=3.8μm) strain-compensated In0.73Ga0.27As–AlAs quantum-cascade lasers

We demonstrate the design and implementation of a broad-gain and low-threshold (Jth=860A∕cm2 at 8K) quantum-cascade laser emitting between 3.7 and 4.2μm. The active region design is based on strain-compensated In0.73Ga0.27As–AlAs on InP. Laser operation in pulsed mode is achieved up to a temperature of 330K with maximum single-facet output peak powers of 6W at 8K and 240mW at 296K. The temperature coefficient T0 is 119K.

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.

Catastrophic optical mirror damage in diode lasers monitored during single-pulse operation

Catastrophic optical mirror damage (COMD) is analyzed for 808 nm emitting diode lasers in single-pulse operation in order to separate facet degradation from subsequent degradation processes. During each pulse, nearfield and thermal images are monitored. A temporal resolution better than 7 μs is achieved. The thermal runaway process is unambiguously related to the occurrence of a “thermal flash.” A one-by-one correlation between nearfield, thermal flash, thermal runaway, and structural damage is observed. The single-pulse excitation technique allows for controlling the propagation of the structural damage into the cavity. We propose this technique for the analysis of early stages of COMD.

Physical limits of semiconductor laser operation: A time-resolved analysis of catastrophic optical damage

The early stages of catastrophic optical damage (COD) in 808 nm emitting diode lasers are mapped by simultaneously monitoring the optical emission with a 1 ns time resolution and deriving the device temperature from thermal images. COD occurs in highly localized damage regions on a 30 to 400 ns time scale which is determined by the accumulation of excess energy absorbed from the optical output. We identify regimes in which COD is avoided by the proper choice of operation parameters.

Real-time thermal imaging of catastrophic optical damage in red-emitting high-power diode lasers

The dynamics of the catastrophic optical damage process under continuous wave operation is analyzed in red-emitting high-power diode lasers by means of combined thermal and optical near-field (NF) imaging with cameras. The catastrophic process is revealed as extremely fast (Δt⩽2.3ms) and spatially confined. It is connected with a pronounced impulsive temperature change. Its coincidence with the most intense NF filament is indicative of the critical nature of thermal runaway in the catastrophic process.

Catastrophic optical damage at front and rear facets of diode lasers

Single-pulse tests of the catastrophic optical damage (COD) are performed for three batches of diode lasers with different gain-regions. The tests involve in situ inspection of front, rear, and side of the devices by a thermocamera. Devices with an Al-containing gain-region show COD at the front facet, as expected for strong facet heating via surface recombination and reabsorption of laser light. In contrast, Al-free devices with low surface recombination rates tend to fail at the rear facet, pointing to a different heating scenario. The high carrier density at the rear facet favors heating and COD via Auger recombination processes.

Surface recombination and facet heating in high-power diode lasers

Surface recombination velocities and surface temperatures at front facets of standard broad-area lasers emitting at 808nm were investigated by time-resolved two-color photoluminescence and micro-Raman spectroscopy. Surface recombination velocities in the range between <105 and 106cm∕s are determined for devices with tailored surface properties. The results clearly show that increased surface recombination velocities are accompanied by increased facet temperatures. Reabsorption of light generated in the diode lasers leads to an additional enhancement of facet heating for surfaces of minor structural quality. The methodological approach presented here paves the way for improved analytical access to diode laser facet properties.

Time-resolved analysis of catastrophic optical damage in 975 nm emitting diode lasers

Catastrophic optical damage (COD) is analyzed during single current pulse excitation of 975 nm emitting diode lasers. Power transients and thermal images are monitored during each pulse. The COD process is unambiguously related to the occurrence of a “thermal flash” of Planck’s radiation. We observe COD to ignite multiple times in subsequent pulses. Thermography allows for tracing a spatial motion of the COD site on the front facet of the devices. The time constant of power decay after the onset of COD has values from 400 to 2000 ns, i.e., an order of magnitude longer than observed for shorter-wavelength devices.

Optical and thermal characteristics of narrow-ridge quantum-cascade lasers

Quantum-cascade lasers operating at λ≈3.9μm at room temperature with narrow w≈5μm ridge widths are described. The lateral confinement due to the narrow ridge is similar to the vertical confinement and the resulting beam is circular in cross section with a single TM00 spatial mode. The beam divergence is 46° both parallel and perpendicular to the surface. The beam quality factor along the slow axis is about M2=1.6. The narrow ridges also increase the relative lateral heat dissipation from the active region, resulting in a thermal conductance per unit area of about Gth=380WK−1cm−2 for a 3mm long laser. Maximum average power is obtained with duty cycles between 10% and 30%; in spite of the very narrow ridge, the total average power with thermoelectric cooling exceeds 60mW with a peak power of 460mW. The circularly symmetric beam with very good beam quality suggests essentially zero astigmatism and indicates that these narrow-ridge quantum-cascade lasers are well suited for applications in midinfrared spectro...

Efficient data evaluation for thermographic crack detection

We present an all-purpose crack detection algorithm for flying spot thermography which is directly applicable to a thermogram sequence without the need of any additional information about the experimental setup. A single image containing distinct crack signatures is derived in two steps. Firstly, the spatial derivative is calculated for each frame of the sequence and, secondly, the resulting data set is sorted pixel wise along the time axis. The feasibility of the proposed procedure is proven by testing a piece of rail that comprises roll contact fatigue cracks and by comparing the results with magnetic particle testing.