Experimental Studies Into the Beam Parameter Product of GaAs High-Power Diode Lasers

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.