Single emitter based diode lasers with high brightness high power and narrow linewidth

Abstract

Multiple Single Emitter (MSE) modules allow highest power and highest brightness fiber coupled diode lasers based on standard broad area diodes. 12 single emitters, each rated at 11W, can be stacked in fast axis and yield more than 100W in a fully collimated beam with a beam quality of 7mm*mrad in both axes. Optical transfer efficiencies of >88% from diode facet to after the fiber are achieved resulting in efficient and compact fiber coupled modules. Volume Bragg Gratings (VBG) stabilize the wavelength over a tuning range of >10nm and narrow the linewidth of individual diodes to less than 2nm. The brightness is scaled by polarization multiplexing and optical stacking is deployed for larger fibers resulting in 700W delivered from a 200μm fiber, 0.2 NA. Wall plug efficiencies of 35% are achieved. The challenge of MSE fiber coupled diode lasers lies in high precision, high yield manufacturing and not so much the optical design of the device, since only collimating lenses and a focusing optic are used. However, a large number of individual components must be handled and consistently aligned with high precision. The 100W module comprises 12 single emitters and the 700W/200μm/0.2 laser comprises 120 single emitters with 85% optical fill factor. Pointing tolerances and collimation errors of all emitters cannot exceed 10% of the spot size to realize the benefits of highest brightness from single emitters compared to bars. The two major assembly processes of MSE fiber coupled diode lasers are the precision diode reflow process and the accurate 5 axis alignment of the fast axis collimation lens (FAC). The reflow process enables positioning of 12 single emitter diodes on submounts within +/-5μm on a common heatsink. Special image processing software performs automated precision alignment and fixation of the FAC with a consistent accuracy of better 0.3um and 0.12mrad. It is also deployed for automated alignment of the external VBG. Wavelength stabilization in an external resonator aims to maximize the locking range and to minimize the drop of output power as well as linewidth. Front facet reflectivity of the diode laser, reflectivity of the volume Bragg grating (VBG) and different resonator designs are investigated.