A root-cause analysis of thermal power saturation in broad area diode lasers is presented. Thermal power saturation limits in largest parts the optical power in continuous wave (CW) driven diode lasers, as junction temperature increases with bias. Accordingly, gain decreases and hence the carrier density in the quantum well increases (non-pinning) to compensate the low gain and the additional photon loss. A systematic variation of epitaxial designs was used to experimentally clarify the impact of modal gain on the temperature dependence of differential internal efficiency and optical loss. To account for this effect in simulation, a rate equation model was applied to determine temperature dependent slope efficiency. Longitudinal spatial hole burning (LSHB) causes an asymmetry of the carrier density profile, leading to high non-linear carrier loss at the back facet, but also the average carrier density increases with temperature, enhancing the impact of LSHB. This enhancement effect leads to rapid degradation of internal differential efficiency with temperature and hence early power saturation. The extreme triple asymmetric vertical design is introduced, that enables devices to be realized that show minimized degradation of internal differential efficiency with temperature and therefore increased performance in CW operation.

In this letter we present new results on high power 970 nm ridge waveguide lasers based on an extreme double asymmetric epitaxial design. Due to an improved lateral-longitudinal design an M2-value below 1.1 and efficiency over 60% could be maintained at power levels slightly above 2 W. The lasers consist of a narrow mode-filter section at the back facet, a relatively broad section at the front facet for high power capability, and a tapered section in between with a very small taper-angle. We achieved the highest reported conversion efficiency for 1 W single lateral mode power to date of 67%, as well as the highest lateral (linear) brightness of 7.5 W/mm·mrad with 55% efficiency at 2.5 W. Results of first life time tests are presented which show the potential for commercial use.

Experimental studies into the beam parameter product ( BPP ) of 940–980 nm GaAs-based high-power diode lasers are presented. Such lasers exhibit broadening far field and narrowing near field with increasing bias, with BPP increasing tenfold over the diffraction limit. First, spectrally-resolved beam profile measurements of lasers with monolithically-integrated gratings are presented, showing that a reproducible series of spatially-extended optical modes makes up the optical field. Then, changes to the device construction are presented, enabling effects limiting BPP to be inferred and addressed. Process- and package-induced effects can be minimized by design, while the effects of carrier, gain and temperature profiles dominate. Self-heating within the laser stripe raises the refractive index, forming a thermal lens, and the variation in curvature of this lens with bias and device construction directly affects BPP . Temperature non-uniformity along the resonator is also shown to strongly degrade BPP . Moreover, current spreading and the resulting lateral carrier accumulation (LCA) amplify high-order, high- BPP modes, thus degrading BPP for any given thermal lens. This LCA-induced degradation is shown to be suppressed by regrown lateral current-blocking structures. Finally, a flatter thermal lens and lower BPP can be achieved using thermal engineering, via changes to the epitaxial design or device layout.

We report on the design of a photonic crystal surface emitting laser (PCSEL) with an all-semiconductor (InGaP/GaAs) photonic crystal suitable for very-large-area emission and high-power operation. Using coupled-wave theory for PCSELs we model infinite- and finite-size cavity PCSELs and show that a photonic crystal unit cell with square lattice periodicity and a rotated and stretched triangular feature is suitable for the realization of PCSELs with very large areas (1 mm<L < 3 mm for a square cavity of size L × L) while maintaining high mode discrimination between the fundamental laser mode and higher order cavity modes as well as high external efficiency. This was achieved by exploiting a single-lattice photonic crystal unit cell design that minimizes one-dimensional coupling in the photonic crystal, providing a promising alternative to double-lattice PCSELs.

The output power of modern 975 nm GaAs-based broad area diode lasers is limited by increasing carrier and photon losses at high bias. We use experiment and one-dimensional calculations on these devices to reveal that higher current densities (and hence higher local recombination rates and higher losses) arise near the front facet due to spatial hole burning and that the non-uniformity is strongly affected by laser geometry, which is more severe for longer resonators and less severe for higher front facet reflectivity. Specifically, we use devices with a segmented p-contact to directly measure the current distribution along the resonator and compare this with laser simulation. Devices with a 6 mm resonator show 29% more current at the front than back, twice as large as the 15% current non-uniformity in devices with a 3 mm resonator. In contrast, increased front facet reflectivity (20% rather than 0.8%) is shown to almost halve the current non-uniformity from 29% to 18% in devices with a 6 mm resonator and reduces power saturation. Although the magnitude of current non-uniformity in experiment and theory is broadly consistent, in experiment, an additional divergence is seen in current flow (and hence recombination rate) near the facets, and earlier power saturation occurs. We discuss the possible saturation mechanisms that are not included in the simulation.

Diode laser pumps are a critical enabling technology for inertial fusion energy (IFE), and will remain the largest contributor to facility cost, even assuming tenfold cost-reduction at high volumes. An overview of diode requirements for IFE and a technology path to achieve them are presented.