Wavelength-stabilized high-brightness single emitters are commonly used in fiber-coupled laser diode modules for pumping Yb-doped lasers at 976 nm, and Nd-doped ones at 808 nm. We investigate the spectral behavior of single emitters under wavelength-selective feedback from a volume Bragg (or hologram) grating (VBG) in a multi-emitter module.
By integrating a full VBG model as a multi-layer thin film structure with commercial raytracing software, we simulated wavelength locking conditions as a function of beam divergence and angular alignment tolerances. Good correlation between the simulated VBG feedback strength and experimentally measured locking ranges, in both VBG misalignment angle and laser temperature, is demonstrated.
The challenges of assembling multi-emitter modules based on beam-stacked optical architectures are specifically addressed, where the wavelength locking conditions must be achieved simultaneously with high fiber coupling efficiency for each emitter in the module. It is shown that angular misorientation between fast and slow-axis collimating optics can have a dramatic effect on the spectral and power performance of the module.
We report the development of our NEON-S wavelength-stabilized fiber laser pump module, which uses a VBG to provide wavelength-selective optical feedback in the collimated portion of the beam. Powered by our purpose-developed high-brightness single emitters, the module delivers 47 W output at 11 A from an 0.15 NA fiber and a 0.3 nm linewidth at 976 nm. Preliminary wavelength-locking results at 808 nm are also presented.
Fiber laser manufacturers demand high-brightness laser diode pumps delivering optical pump energy in both a compact
fiber core and narrow angular content. A pump delivery fiber of a 105 μm core and 0.22 numerical aperture (NA) is
typically used, where the fiber NA is under-filled to ease the launch of laser diode emission into the fiber and make the
fiber tolerant to bending. At SCD, we have developed high-brightness NEON multi-emitter fiber-coupled pump modules
that deliver 50 W output from a 105 μm, 0.15 NA fiber enabling low-NA power delivery to a customer’s fiber laser
network.
Brightness-enhanced single emitters are engineered with ultra-low divergence for compatibility with the low-NA
delivery fiber, with the latest emitters delivering 14 W with 95% of the slow-axis energy contained within an NA of
0.09. The reduced slow-axis divergence is achieved with an optimized epitaxial design, where the peak optical intensity
is reduced to both lessen filamentation within the laser cavity and reduce the power density on the output facet thus
increasing the emitter reliability.
The low mode filling of the fiber allows it to be coiled with diameters down to 70 mm at full operating power despite the
small NA and further eliminates the need for mode-stripping at fiber combiners and splices downstream from our pump
modules. 50W fiber pump products at 915, 950 and 975 nm wavelengths are presented, including a wavelengthstabilized
version at 976 nm.
Fiber laser manufacturers demand high-brightness laser diode pumps delivering optical pump energy in both a compact
fiber core and narrow angular content. A pump delivery fiber of a 105 μm core and 0.22 numerical aperture (NA) is
typically used, where the fiber NA is under-filled to ease the launch of laser diode emission into the fiber and make the
fiber tolerant to bending. At SCD, we have developed multi-emitter fiber-coupled pump modules that deliver 50 W
output from a 105 μm, 0.15 NA fiber at 915, 950 and 976 nm wavelengths enabling low-NA power delivery to a
customer’s fiber laser network.
In this work, we address the challenges of coupling and propagating high optical powers from laser diode sources in
weakly guiding step-index multimode fibers. We present simulations of light propagation inside the low-NA multimode
fiber for different launch conditions and fiber bend diameters using a ray-racing tool and demonstrate how these affect
the injection of light into cladding-bounded modes. The mode filling at launch and source NA directly limit the bend
radius at which the fiber can be coiled. Experimentally, we measure the fiber bend loss using our 50 W fiber-coupled
module and establish a critical bend diameter in agreement with our simulation results. We also employ thermal imaging
to investigate fiber heating caused by macro-bends and angled cleaving.
The low mode filling of the 0.15 NA fiber by our brightness-enhanced laser diodes allows it to be coiled with diameters
down to 70 mm at full operating power despite the low NA and further eliminates the need for mode-stripping at fiber
combiners and splices downstream from our pump modules.
Fiber-coupled laser diodes have become essential sources for fiber laser pumping and direct energy applications. Single emitters offer reliable multi-watt output power from a 100 m lateral emission aperture. By their combination and fiber coupling, pump powers up to 100 W can be achieved from a low-NA fiber pigtail. Whilst in the 9xx nm spectral range the single emitter technology is very mature with <10W output per chip, at 800nm the reliable output power from a single emitter is limited to 4 W – 5 W. Consequently, commercially available fiber coupled modules only deliver 5W – 15W at around 800nm, almost an order of magnitude down from the 9xx range pumps. To bridge this gap, we report our advancement in the brightness and reliability of 800nm single emitters. By optimizing the wafer structure, laser cavity and facet passivation process we have demonstrated QCW device operation up to 19W limited by catastrophic optical damage to the 100 μm aperture. In CW operation, the devices reach 14 W output followed by a reversible thermal rollover and a complete device shutdown at high currents, with the performance fully rebounded after cooling. We also report the beam properties of our 800nm single emitters and provide a comparative analysis with the 9xx nm single emitter family. Pump modules integrating several of these emitters with a 105 μm / 0.15 NA delivery fiber reach 35W in CW at 808 nm. We discuss the key opto-mechanical parameters that will enable further brightness scaling of multi-emitter pump modules.
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