Wolfgang Schmid

Investigation of dark line defects induced by catastrophic optical damage in broad-area AlGaInP laser diodes

The authors present a detailed investigation of defects generated during catastrophic optical damage (COD) in high-power 650nm AlGaInP lasers using microphotoluminescence (μ-PL) mapping, focused ion beam (FIB) microscopy, and deep-etching techniques. High-resolution μ-PL images demonstrated that during COD, nonradiative dark line defects (DLDs) originate from the front mirror of the laser and propagate in several branches into the laser perpendicular to the output facet. Furthermore, FIB microscopy identified the epitaxial layers affected by COD, revealing that DLDs are confined to the active region. In addition, deep etching confirmed that these defects have a noncrystalline nature.

Optically pumped semiconductor thin-disk laser with external cavity operating at 660 nm

The optically pumped semiconductor thin-disk laser with external-cavity (OPS-TDL) is a new type of semiconductor laser structure with the capability of achieving high output power while retaining good beam quality. We demonstrate the first AlGaInP-based red light emitting OPS-TDL structure. The device has been pumped optically with an argon-laser at 514~nm. The device has an epitaxial backside mirror and a multiple quantum well active region, consisting of strained InGaP quantum wells arranged in several groups as a periodic gain structure. A peak single-mode output power of more than 200mW at 660nm has been obtained in pulsed operation. Various designs for the active layer have been investigated.

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.

Observation of deep level defects within the waveguide of red-emitting high-power diode lasers

The waveguides of 650nm emitting high-power laser diodes are analyzed regarding the presence of deep level defects by photoelectrical techniques, namely, photocurrent spectroscopy, laser beam induced current, and near-field optical beam induced current (NOBIC). Deep level configurations in pristine devices and the kinetics of defect creation during device operation are monitored and discussed. The localization of the defects within the epitaxial layer sequence is done by NOBIC. We show that light, which is confined within the laser waveguide, interacts with the deep level defects detected. This demonstrates that the presence of deep level defects directly affects the device properties.

45% quantum-efficiency light-emitting diodes with radial outcoupling taper

10We have investigated efficient light outcoupling from light- emitting diodes (LEDs) by introducing lateral tapers. The concept is based on light generation in the very central area of a circularly symmetric structure. After propagating between two highly reflecting mirrors light is outcoupled in tapered mesa region. By proper processing we achieve quantum and wallplug efficiencies of almost 30% for outcoupling via a planar surface or, respectively, 45% and 44% for encapsulated devices.

New developments on high-efficiency infrared and InGaAlP light-emitting diodes at OSRAM Opto Semiconductors

We present our latest results on developments of infrared and red light emitting diodes. Both chiptypes are based on the Thinfilm technology. For infrared the brightness has been raised by 25% with respect to former products in a package with standard silicon casting, corresponding to a brightness increase of 33% for the bare chip. In a lab package a wallplug efficiency of more than 72% at a wavelength of 850nm could be reached. For red InGaAlP LEDs we could demonstrate a light output in excess of 200lm/W and a brightness of 133lm at a typical operating current of 350mA.

Mode shaping in semiconductor broad area lasers by monolithically integrated phase structures

By broadening the stripe width of the active waveguide region, it is possible to extract high optical powers from semiconductor broad area lasers. However, a weak output beam quality, optical filamentation, and high peak power densities will result, which are invoked by the amplification of higher order modes. We show an approach to influence the optical field inside the resonator by integrating optical phase structures directly into the waveguide. Those elements offer the possibility to enlarge the active gain area for the desired fundamental laser mode, while additional diffraction losses for higher order modes are generated, thus achieving an improved beam quality. We report on considerations in designing those elements and demonstrate a first experimental realization.

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.

Efficient light-emitting diodes with radial outcoupling taper at 980- and 630-nm emission wavelength

We have investigated efficient light outcoupling from light- emitting diodes (LEDs) by introducing lateral tapers. The concept is based on light generation in the very central area of a circularly symmetric structure. After propagating between two highly reflecting mirrors light is outcoupled in a tapered mesa region. By proper processing we achieve quantum efficiencies of almost 40% for outcoupling via a planar surface or quantum and wallplug efficiencies of 52% and 48%, respectively, for encapsulated devices. Neglecting reabsorption, approximative equations yield optimum design parameters.

Control of slow axis mode behaviour with waveguide phase structures in semiconductor broad-area lasers

One of the most common methods to increase the output power of semiconductor waveguide lasers is broadening the stripe width of the active region. However, this results in higher order transverse modes to be amplified which impairs the beam quality and increases the beam divergence. By integrating optical elements into the cavity, it is possible to control the amplitude shape and the number of modes which are amplified in the laser. This paper reports on aspects to integrate phase and amplitude modifying microstructures into a semiconductor waveguide resonator by adding an additional lithographic step to the fabrication process of broad area laser diodes. The latest experimental results of such structured InGaAIP broad area lasers revealed a significant improvement of the beam quality even at a high operation current. Hence, the expansion of the stripe width of the amplifying region without degrading the beam quality is possible. The demonstrated power-current characteristics of structured laser diodes exhibit a low threshold current and a high efficiency.