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Improved brightness using tapered diode lasers

Verbeserte Brillanz durch Diodenlaser mit Trapezstruktur
: Kelemen, M.T.; Weber, J.; Mikulla, M.; Weimann, G.

European Physical Society -EPS-, Geneva:
CLEO Europe 2003 and EQEC 2003 : Munich, Germany, June 22-27, 2003
Mulhouse: EPS, 2003 (Europhysics conference abstracts 27E)
ISBN: 2-914771-15-0
Conference on Lasers and Electro-Optics Europe (CLEO Europe) <2003, München>
European Quantum Electronics Conference (EQEC) <2003, München>
Fraunhofer IAF ()
diode laser; Diodenlaser; tapered laser; Trapezlaser; brightness; Brillanz; InGaAs/GaAs

Semiconductor lasers with high beam quality and high optical output power are very attractive for a variety of applications such as molecular spectroscopy, fiber optic communication systems and nonlinear frequency conversion for laser TV. 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 between 940 nm and 1040 nm were grown by molecular beam epitaxy. The lateral design consists of a tapered and a ridge-waveguide section having an overall length of 2.5 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 tapered section has been chosen to 2 mm leading to an emitting facet width of 200 µm.
Besides an electro-optical characterisation, the near fields and the beam quality parameters M2 have been carefully investigated. The tendency of near field profiles towards filamentation at higher output powers can be described with the help of the linewidth enhancement factor alpha(exp H). Assuming a strong dependence of alpha(exp H) on the current density, this behaviour can be simulated in good accordance with the experiment. The corrected farfields and the linewidth enhancement factor have been measured to detect the influence of the carrier density on the beam quality. By increasing the length of the ridge section or the taper section, it is possible to reduce alpha(exp H). A smaller alpha(exp H) leads to a lower tendency of filamentation and hence to a better beam quality.
By combining these two mechanisms, we achieved an optical output power of up to 4 W in continuous wave mode (cw) and 11.6 W in quasi-cw mode. The wall plug efficiency reaches 42% at a current of 4 A. The beam quality parameter M2 remains nearly diffraction limited up to about 3.5 W with values below 1.3 resulting in an improved brightness of more than 260 MW/(cm2sr).
In conclusion, the combination of a tapered structure and a ridge-waveguide structure is a promising design for applications, which depend on both high power and high brightness.