Beam combining techniques for high-power high-brightness diode lasers
2015
Laser diodes are efficient and compact devices operating in a wide range of wavelengths. Boosting power by beam
combining while maintaining good beam quality has been a long-standing challenge.
We discuss various approaches for beam combining with emphasis on solutions pursued at DirectPhotonics. Our design
employs single emitter diodes as they exhibit highest brightness and excellent reliability. In a first step, after fast axis
collimation, all single emitter diodes on one subunit are stacked side-by-side by a monolithic slow-axis-collimator thus
scaling the power without enhancing the brightness.
The emissions of all diodes on a subunit are locked by a common Volume Bragg grating (VBG), resulting in a
bandwidth < 0.5nm and high wavelength stability. Second, two subunits with identical wavelength are polarization
coupled forming one wavelength channel with doubled power and brightness. Third, up to five channels are serially
spectrally combined using dichroic filters. The stabilized wavelengths enable dense spectral combining, i.e. narrow
channel spacing. This module features over 500W output power within 20nm bandwidth and a beam parameter product
better than 3.5mm*mrad x 5mm*mrad (FA x SA) allowing for a 100μm, 0.15NA delivery fiber [1].
The small bandwidth of a 500-W-module enables subsequent coarse spectral combining by thin film filters, thus further
enhancing the brightness.
This potential can only be fully utilized by automated manufacturing ensuring reproducibility and high yield. A precision
robotic system handles and aligns the individual fast axis lenses. Similar technologies are deployed for aligning the
VBGs and dichroic filters.
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