H. Wenzel

Efficient High-Power Laser Diodes

High-power broad-area diode lasers are the most efficient light sources, with 90-μm stripe GaAs-based 940-980 nm single emitters delivering > 10 W optical output at a power conversion efficiency η_E(10 W) > 65%. A review of efforts to increase η_E is presented here and we show that for well-optimized structures, the residual losses are dominated by the <i>p</i> -side waveguide and nonideal internal quantum efficiency η_i . The challenge in measuring efficiency to sufficient precision is also discussed. We show that η_E can most directly be improved using low heat sink temperature T_HS with η_E(10 W) reaching > 70% at <i>T</i>_HS = -50 °C. In contrast, increases in η_E at T_HS = 25 °C require improvements in both material quality and design, with growth studies targeting increased η_i and reduced threshold current and design studies seeking to mitigate the impact of the <i>p</i>-side waveguide. “Extreme, double asymmetric” (EDAS) designs are shown to substantially reduce <i>p</i>-side losses, at the penalty of increased threshold current. The benefit of EDAS designs is shown here using diode lasers with 30-μm stripes, (in development as high beam quality sources for material processing). Efficiency increases of ~ 10% relative to conventional designs are demonstrated at high powers.

Fundamental-Lateral Mode Stabilized High-Power Ridge-Waveguide Lasers With a Low Beam Divergence

We compare ridge-waveguide lasers with trench widths of 5 and 20 mum. The emission wavelength is around 1064 nm and the ridge width is 5 m. The maximum output power exceeds 2 W. The 5-mum trench-width device exhibits a much more stable lateral far-field. The full-width at half-maximum of the vertical far-field profile is only 15deg due to a super-large optical cavity.

High-power 980-nm DFB RW lasers with a narrow vertical far field

We compare 980-nm distributed-feedback ridge-waveguide lasers having cavity lengths of 1.5 and 3 mm. The maximum single-mode output powers are 500 and 700 mW, respectively. The full-width at half-maximum of the vertical far-field profile is only 22/spl deg/ due to a superlarge optical cavity.

5-W DBR Tapered Lasers Emitting at 1060 nm With a Narrow Spectral Linewidth and a Nearly Diffraction-Limited Beam Quality

Distributed Bragg reflector tapered lasers emitting at a wavelength of about 1060 nm were realized. The expitaxial layer structure leads to a vertical far-field angle of 15deg (full-width at half-maximum). The devices with a total length of 4 mm consist of 2-mm-long ridge waveguide and tapered sections. The input currents to both sections can be independently controlled. The laser reached 5-W output power with a narrow spectral linewidth below 40 pm (95% power) and a nearly diffraction-limited beam quality.

Combination of Low-Index Quantum Barrier and Super Large Optical Cavity Designs for Ultranarrow Vertical Far-Fields From High-Power Broad-Area Lasers

When active regions that use low refractive index quantum barriers (LIQB) are combined with super large optical cavity (SLOC) designs in GaAs-based diode lasers, high-power operation with extremely narrow vertical far-fields is observed. However, LIQB designs are found to have lower slope efficiency and increased operation voltage. Comparison of experiment and finite element device simulation shows that this is due to hole accumulation at the edge of the active region. Example devices using an 8.6-μm thick SLOC deliver 30 W at 1065 nm with vertical divergence of 15.6° (95% power).

Basic Aspects of High-Power Semiconductor Laser Simulation

The aim of this paper is to review some of the models and solution techniques used in the simulation of high-power semiconductor lasers and to address open questions. We discuss some of the peculiarities in the description of the optical field of wide-aperture lasers. As an example, the role of the substrate as a competing waveguide in GaAs-based lasers is studied. The governing equations for the investigation of modal instabilities and filamentation effects are presented and the impact of the thermal-lensing effect on the spatiotemporal behavior of the optical field is demonstrated. We reveal the factors that limit the output power at very high injection currents based on a numerical solution of the thermodynamic based drift-diffusion equations and elucidate the role of longitudinal spatial hole burning.

Narrow Linewidth DFB Lasers Emitting Near a Wavelength of 1064 nm

We report on the realization of narrow linewidth high power DFB diode lasers emitting near 1064 nm in stable longitudinal and lateral single mode. The linewidth is analyzed in dependence of the output power for lasers with cavity lengths of 1 and 2 mm by means of a heterodyne beat note technique. The minimum intrinsic linewidth is 22 kHz FWHM (full width at half maximum, at 100 μ s time scale) for an output power of 150 mW and a cavity length of 2 mm. The minimum total linewidth is mainly determined by technical noise and corresponds to 234 kHz FWHM at an output power of 70 mW. The influence of current noise on the linewidth is investigated and compared for different cavity lengths. Re-broadening at high output power is only observed for the contribution of technical noise to the linewidth. The intrinsic linewidth shows the theoretically expected 1/Pout-dependence at all power levels.

High-Power 980-nm Broad-Area Lasers Spectrally Stabilized by Surface Bragg Gratings

We report on broad-area distributed Bragg reflector (DBR) lasers with a stripe width of 90 ¿m providing up to 14-W optical power and 50% maximum conversion efficiency. Ninety-five percent of the power is included within a wavelength range of less than 1 nm. The wavelength shift between threshold and the maximum output power is less than 3.5 nm. The wavelength stabilization is achieved with a 500-¿m -long DBR containing a sixth-order Bragg grating defined by i-line wafer stepper lithography and fabricated by reactive ion etching.

Beam Properties of 980-nm Tapered Lasers With Separate Contacts: Experiments and Simulations

The beam properties of 980-nm tapered lasers with separate current drives for the ridge waveguide and tapered sections are analyzed by means of a comparison between simulations and experimental results. The simulations are performed with a new model for this type of tapered lasers, providing a good qualitative agreement with experiments. The observed improvement in the beam quality by a stronger pumping of the ridge waveguide section with respect to the tapered section is attributed to the reduction of the backward field intensity. The simulations show that this improvement, far from being a general rule, depends on the details of the device geometry.

Nearly Diffraction-Limited Tapered Lasers at 675 nm With 1-W Output Power and Conversion Efficiencies Above 30%

High brightness, highly efficient tapered lasers emitting around 675 nm have been developed. The devices have a 500- μm-long straight section and a 1500-μm -long tapered section with a flare angle of 3°. At a temperature of 25°C and a current of 1.5 A, the lasers emit an output power of nearly 1.2 W. The maximum conversion efficiency of 31% is reached at an output power of 1 W. The emitted beam is nearly diffraction-limited with a beam propagation ratio (second moments) of 2.2.