Facet overheating is considered a potential source for device degradation of diode lasers. We test two different concepts for the reduction of facet temperatures of high-power diode lasers by measuring the facet temperatures by means of Raman spectroscopy. For conventional high-power broad area lasers we demonstrate the reduction of the facet overheating by the introduction of current blocking layers by a factor of 3-4. For another set of devices among them quantum well and quantum-dot lasers with almost the same device design we find a reduction of the overheating by 40 to 60 percent for the dot devices. Thus we qualify two very different but promising technological approaches for increasing device reliability.
Semiconductor lasers with high beam quality and high optical output power are very attractive for a variety of applications such as optical pumping of solid-state lasers, fiber amplifiers and medical treatment. When easy and low-cost fabrication is a further requirement, devices based on tapered gain sections are the most promising candidates. Low modal gain, single quantum well InGaAs/A1GaAs devices emitting at 1040 nm were grown by molecular beam epitaxy. The lateral design consists of a tapered gain guided and a ridge-waveguide section having an overall length of mm. An output power of more than 11 W in qcw mode, lifetimes of more than 20,000 h and a record value for the beam quality factor M2of less than 1.5 up to a cw output power of 3.5 W are achieved resulting in an improved brightness of more than 255 MW/(cm2sr). In addition an external-cavity diode laser including a ridge-waveguide tapered amplifier structure is demonstrated to emit more than 2 W cw. The wavelength is tunable over a 60 nm range centered at 1020 nm. The beam quality parameter M2 remains below 1.4 for output powers of 1 W over the whole range demonstrating the nearly diffraction limited behavior.
Semiconductor lasers with high beam quality and high optical output power are very attractive for a variety of applications such as optical pumping of solid-state lasers, fiber amplifiers and medical treatment. When easy and low- cost fabrication is a further requirement, devices based on tapered gain sections are the most promising candidates. Low modal gain, single quantum well InGaAs/AlGaAs devices emitting at 940 nm were grown by molecular beam epitaxy. The lateral design consists of a tapered gain guided and a ridge-waveguide section having an overall length between 2 mm and 3 mm. Whereas the length of the tapered structure determines the high output power, the high brightness requires a ridge-waveguide structure with sufficient length. Here the length of the ridge section has been chosen to 500 micrometers . We achieved an optical output power of up to 5.3 W at room temperature in continuous wave mode. The threshold current density depends on the tapered length with values between 200 A/cm2 and 650 A/cm2. The slope efficiency is around 0.9 W/A for all devices. The wall plug efficiency reaches 44% at a current of 3 A. The beam quality factor remains nearly constant up to about 2.2 W having an M2-value of 1.3. At higher optical powers M2 increases fast. The lifetime of such devices has been extrapolated to more than 7500 h at room temperature.
To study the influence of the carrier density on the lifetime of broad area high power diodes three different epitaxial structures (double and single quantum wells, different confinement factors) were investigated. The fabricated lasers have similar emission wavelengths of 880 nm and 890 nm, the same lateral design and identical facet coatings. Because of the different vertical structures the lasers have different threshold current densities and therefore different carrier densities during operation. The experimental results show that the lifetimes depend strongly on the carrier densities. The measured catastrophical optical mirror damage (COMD) levels and the facet temperatures show the same dependence. The results achieved are explained by a theoretical model for additional heat generation at the facets in comparison to the bulk material. The calculations show a proportional relationship between the heat generation, leading to additional degradation mechanism, and the carrier density a few diffusion lengths away from the facet.
A high power semiconductor laser with a novel lateral design using angular filtering by total reflection for increased brightness is demonstrated. In this so called `Z-Laser' two inner surfaces guide the laser beam by total reflection in a Z-shaped path through the laser. Higher order laser modes with larger divergence angles are suppressed because of a smaller reflectivity. This results in a reduced far-field angle. Simulations based on a 2D steady state wave equation solved by using the Pade approximation, an 1D carrier diffusion equation and a logarithmic gain model have been performed to design the device.
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