Martin Reufer

Yellow AlGaInP/InGaP laser diodes achieved by pressure and temperature tuning

The emission wavelength of broad-area AlGaInP/InGaP quantum-well lasers is tuned by the application of high hydrostatic pressure and low temperature from 645 down to 575 nm, i.e., from the red through the orange to yellow spectral range. Emission powers up to 300 mW are obtained in the full tuning range. The pressure and temperature dependence of threshold currents indicates that leakage occurs into the L and X minima in the barriers.

Temperature-power dependence of catastrophic optical damage in AlGaInP laser diodes

Facet temperature changes in broad-area red-emitting high-power AlGaInP lasers are analyzed by means of micro-Raman spectroscopy. Measurements as a function of injection current demonstrate that the temperature at the laser output facet rises linearly with optical output power. Temperature profile measurements across the laser stripe show a strong correlation between near field intensity, facet temperature, and catastrophic optical damage (COD). Additionally, temperature-power analyses reveal that a critical facet temperature is needed to induce COD. The consistent results produced by complementary measurement techniques indicate that absorption of stimulated photons at the laser facet is the major source of facet heating.

The physics of catastrophic optical damage in high-power AlGaInP laser diodes

An innovative combination of concepts, namely microphotoluminescence (μPL) mapping, focused ion beam (FIB) microscopy, micro-Raman spectroscopy, and high-speed thermal imaging, was employed to reveal the physics behind catastrophic optical damage (COD), its related temperature dynamics, as well as associated defect and near-field patterns. μPL mapping showed that COD-related defects are composed of highly nonradiative complex dislocations, which start from the output facet and propagate deep inside the cavity. Moreover, FIB analysis confirmed that those dark line defects are confined to the active region, including the quantum wells and partially the waveguide. In addition, the COD dependence on temperature and power was analyzed in detail by micro-Raman spectroscopy and real-time thermal imaging. For AlGaInP lasers in the whole spectral range of 635 to 650 nm, it was revealed that absorption of stimulated photons at the laser output facet is the major source of facet heating, and that a critical facet temperature must be reached in order for COD to occur. A linear relationship between facet temperature and near-field intensity has also been established. This understanding of the semiconductor physics behind COD is a key element for further improvement in output power of AlGaInP diode lasers.

Infrared emission from the substrate of GaAs-based semiconductor lasers

We report on the origin of three additional low-energy spontaneously emitted bands in GaAs-based broad-area laser diodes. Spectrally and spatially resolved scanning optical microscopy and Fourier-transform infrared spectroscopy assign the different contributions to bandtail-related luminescence from the gain region as well as interband and deep-level-related luminescences from the GaAs substrate. The latter processes are photoexcited due to spontaneous emission from the active region followed by a cascaded photon-recycling process within the substrate.

New approaches towards the understanding of the catastrophic optical damage process in in-plane diode lasers

The microscopic processes accompanying the catastrophic optical damage process in semiconductor lasers are discussed. For 808 and 650 nm edge-emitting broad-area devices relevant parameters such as surface recombination velocities, bulk and front facet temperatures are determined and discussed. Facet temperatures vs. laser output and temperature profiles across laser stripes reveal a strong correlation to near-field intensity and degradation signatures. Furthermore, the dynamics of the fast catastrophic optical damage process is analyzed by simultaneous high-speed infrared thermal and optical imaging of the emitter stripe. The process is revealed as fast and spatially confined. It is connected with a pronounced impulsive temperature flash detected by a thermocamera.

Self-consistent modeling of edge-emitting GaInP/AlGaInP red lasers

A self-consistent laser simulator has been set up for the simulation of edge-emitting GaInP/AlGaInP red lasers. The modeling results have been compared with experiments in broad area 635 nm GaInP/AlGaInP laser diodes. The leakage of electrons and its dependence on the temperature and the p-doping level are analyzed.

Monolithically stacked high-power diode laser bars in quasi-continuous-wave operation exceeding 500 W

In this paper we report on quasi-continuous-wave (q-cw) operation of monolithically stacked laser diode bars. Monolithically stacked laser diode bars consist of more than one laser diode grown on top of each other. In between every two laser diodes a tunnel junction is included to ensure proper current injection to all lasers. In comparison to a standard laser operated at the same optical power level, the monolithic laser stack has a significantly reduced optical mirror load. Furthermore the required current is reduced drastically, which has positive consequences on both laser lifetime and diode driver costs. If one otherwise compares a monolithic integrated laser bar stack with a setup of three separate standard laser bars, the monolithic laser bar stack is characterized by very low costs per watt as well as high brilliance. By using monolithically stacked laser diode bars we were able to exceed an optical power of 500 W in q-cw mode and are moving to even higher output power levels. Typical wavelengths are in the range between 800 and 1000 nm.

Analysis of wavefront structures of diode lasers by their spatial and current dependent evolution

For nearly Gaussian beams the characterization of changes in the beam parameters with variation of the operating conditions using a Shack-Hartmann sensor is a well-known technique. For broad-area semiconductor lasers the beam analysis based on wavefront detection is still a field of high innovation potential for quality control since the wavefront gives an additional insight to the modal composition of the laser which is strongly influenced by the processes inside the resonator. The diode lasers used for investigation are based on the material system of GaAs (λ=808nm-980nm). They show a multimodal behavior strongly affected by thermal and electrical effects inside the active medium resulting in a complex structure of the intensity and wavefront pattern. The investigations in this paper have been carried out using a so called Gaussian telescope which allows a magnification of the beam cross section as well as the investigation of the spatial evolution of the intensity and wavefront distribution in vicinity to the beam waist. Furthermore the pattern of the detected wavefront is associated with Legendre polynomials to obtain a quantitative expression of the pattern. Additionally simulation software is used to connect the modal composition of the intensity with a set of Hermite Gaussian modes. The aim of our work is to combine these two tracks of information to find a way to forecast whether the laser under test shows a stable or instable intensity distribution over the whole operation current range.

Analysis of the emission characteristics of diode lasers by their wavefront structure

In recent years wavefront measurements using a Shack-Hartmann Sensor became a fast and easy way to analyze the change of laser beam characteristics over a wide range of parameters. This method is well known for nearly Gaussian laser beams while the wavefront analysis of broadarea semiconductor lasers is still an open field of current research. Detailed analysis of the wavefront gives an additional path to get insight into the modal composition of semiconductor lasers, which has a dominant impact on the output parameters of the devices. For our investigations we utilize lasers based on the material system of GaAs emitting light in the near infrared. These types of laser emit typically more than one optical mode. The composition of these modal structures is highly affected by thermal and electric effects inside the active medium. By using a simulation software the intensity distribution at various diode currents can be associated with an assembly of Hermite Gaussian modes and thus gives insight into the basic modal structure. Additionally the change of modal composition can be recorded within the wavefront deflection. This delivers an extra track of information to the light emission. The aim of our research is to associate the wavefront with the modal structure gained by measuring the intensity distribution under changing working conditions. Furthermore we use a lens system to receive a magnified image of the beam and investigate the spatial evolution of the intensity and wavefront distribution of the laser emission along the propagating axis.

Diode laser modules based on new developments in tapered and broad area diode laser bars

In the last few years an increasing demand for high-brightness diode laser sources is observable, which is mainly driven by applications for fiber laser pumping and materials processing. A number of different approaches have been investigated in the past for the realization of such systems. In this paper we compare different concepts for high-brightness, high-power diode laser modules that are based on the new generation of tapered diode laser bars and new developments in broad area diode laser bars, respectively. One of the main advantages of tapered diode laser bars is the good beam quality in the slow-axis direction, which allows the design of high-power laser systems with a symmetric beam profile without the necessity of using sophisticated beam shaping systems. Such laser modules with multiple bars aiming for kilowatt output power can be realized with different incoherent coupling principles, including spatial multiplexing, polarization multiplexing and wavelength multiplexing. On the other hand, modules with a single or only a few tapered diode laser bars aim for very high brightness suitable for fiber coupling with fiber diameters down to 50 μm with a numerical aperture (NA) of 0.22. In this paper we present a detailed characterization of the new generation of tapered diode laser bars, including typical electro-optical data, measurements of beam quality and lifetime data. Tapered diode laser bars typically suffer from a broad spectrum which is extremely obstructive for pumping applications with small absorption bandwidths. To overcome this disadvantage we used volume bragg gratings (VBG) to improve the spectral quality of tapered diode laser bars. In addition to further improve the brightness of such diode laser systems we investigated external phaseplates to correct for smile and lens aberrations.