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Among the work done to improve laser performance by optimizing the efficiency of vertical laser structures and thermal management, the spectral quality of the emitted light is a key criterion for the usability of the device. Especially the exact adjustment of the emission wavelength, a small linewidth and a preferably small dependence of the wavelength on temperature changes are important for many applications. These requirements are met by implementing surface gratings into the devices. In the first part of the paper the theoretical tools which are used to simulate and optimize grating reflectivity are presented and discussed. 2D and 3D simulation models confirm possible high reflectivities (more than 90 %) of surface Bragg gratings, if the duty cycle is high ( 9 @ λ = 630 nm) are required. Electron beam lithography allows better design flexibility and larger etch depth tolerances, but requires a more complex etch mask preparation. With both techniques gratings up to a depth of 1.7 μm can be etched, which is sufficient to reach a high reflectivity for typical AlGaAs based laser structures. A single grating can be thermally tuned up to 7.5 nm. As an example for high power applications a 6 mm long distributed feedback broad laser with a stripe width of 30 μm as part of a spectral beam combining setup is presented. This device achieved a maximum output power of 6 W and a spectral width of 0.3 nm.

Single longitudinal mode continuous-wave operation of distributed-feedback (DFB) laser diodes based on GaN is demonstrated using laterally coupled 10th-order surface Bragg gratings. The gratings consist of V-shaped grooves alongside a 1.5 µm wide p-contact stripe fabricated by using electron-beam lithography and plasma etching. By varying the period of the Bragg grating, the lasing wavelength could be adjusted between 404.8 and 408.5 nm. The feasibility of this device concept was confirmed by mode-hop-free operation up to an optical output power of 90 mW, a low temperature sensitivity of the lasing wavelength, and a Gaussian lateral far-field distribution.

Distributed Bragg reflector tapered diode lasers (DBR-TPL) emitting at 1154 nm are ideal light sources to be implemented into medical devices and hand-held tools for treatment in dermatology and ophthalmology at 577 nm due to their high spectral radiance enabling second harmonic generation from near infrared to yellow. In this work, we present DBR-TPLs which are able to emit more than 10 W in continuous-wave operation with a narrow spectral emission at 1154 nm and a very good beam quality providing excellent spectral radiance. The investigated DBRTPLs are based on three different epitaxial structures with varying vertical far field angles of 35°, 26°, and 17°. To optimize the coupling efficiency into non-linear crystals we studied DBR-TPL with a vertical far field angle of approx. 17° based on an asymmetrical super large optical cavity epitaxial structure. At a pump current of 18 A these devices are able to emit more than 9 W at 25°C and nearly 11 W at 10°C. The spectral emission is very narrow (ΔλFWHM = 18 pm) and single mode over the entire current range. While the beam quality factor M2 according to the 1/e2-level remains 1.1, the M2 according to second order moments deteriorates when the laser is pumped with higher currents. Therefore, the power content in the central lobe increases somewhat less rapidly than the total power.

We report on the simultaneous definition and fabrication of Bragg gratings and ridge waveguides using wafer stepper lithography and reactive ion etching, respectively. Single-longitudinal mode emission from two-section ridge-waveguide distributed feedback lasers with sixth and seventh order gratings will be reported. This technology enables a cheap fabrication of wavelength-stabilized lasers and a simple variation of the parameters of the gratings on the wafer using optical lithography.

In contrast to Distributed Feedback DFB) lasers for wavelengths of about 1060nm for α-DFB lasers the fabrication of Bragg gratings using conventional wafer stepper lithography is possible, due to the necessary larger grating period. The use of a wafer stepper enables us to study especially the influence of slant angle of the fabricated gratings and stripe width on optical properties of the fabricated lasers on the same wafer independent from fluctuations of the vertical structure. Best beam quality was achieved for 15° and 13.5° tilted gratings with a small period of 594nm and 658nm respectively. For optimised structures a nearly diffraction limited beam with an output power of more than 1W, a lateral far field divergence angle of θ= 0.3°, a beam quality factor M2 = 1.1 and 3.2 at 2.1W respectively, and a line width of 5.8 pm with 28dB side mode suppression rate was achieved.