A multi-mode 9XXnm-wavelength laser diode was developed to optimize the divergence angle and reliable ex-facet power. Lasers diodes were assembled into a multi-emitter pump package that is fiber coupled via spatial and polarization multiplexing. The pump package has a 135μm diameter output fiber that leverages the same optical train and mechanical design qualified previously. Up to ~ 270W CW power at 22A is achieved at a case temperature ~ 30ºC. Power conversion efficiency is 60% (peak) that drops to 53% at 22A with little thermal roll over. Greater than 90% of the light is collected at < 0.12NA at 16A drive current that produces 3.0W/(mm-mr)2 radiance from the output fiber.
Hongbo Yu, Dahv A. Kliner, Kai-Hsiu Liao, Jeff Segall, Martin Muendel, James Morehead, Jane Shen, Matt Kutsuris, Johnny Luu, Justin Franke, Kelvin Nguyen, Dave Woods, Fred Vance, David Vecht, David Meng, Richard Duesterberg, Lei Xu, Jay Skidmore, Matthew Peters, Nicolas Guerin, James Guo, Jane Cheng, Jihua Du, Brad Johnson, Dongliang Yin, Allen Hsieh, Peter Cheng, Abdullah Demir, Jason Cai, Rupa Gurram, Kong-Weng Lee, Reddy Raju, Daniel Zou, Raman Srinivasan, Mandeep Saini, Laura Zavala, Victor Rossin, Erik Zucker, Hiroaki Ishiguro, Hiroshi Sako
We have demonstrated a monolithic (fully fused), 1.2-kW, Yb-doped fiber laser with near-single-mode beam quality.
This laser employs a new generation of high-brightness, fiber-coupled pump sources based on spatially multiplexed
single emitters, with each pump providing 100 W at 915 nm within 0.15 NA from a standard 105/125 μm fiber. The
fiber laser is end pumped through the high-reflector FBG using a 19:1 fused-fiber pump combiner, eliminating the need
for pump/signal combiners. The output wavelength is 1080 nm, with a linewidth of < 0.5 nm FWHM. A peak power of
1.5 kW was reached in modulated operation (1-ms pulse duration) with M2 < 1.2.
We report the development of fused-fiber pump and signal combiners. These combiners are enabling components of a ytterbium fiber-laser emitting 4 kW of 1080-nm radiation. The fiber-laser system consists of seven fiber laser modules and a 7:1 signal combiner. The laser modules are end-pumped by 90 915-nm JDSU L4 diode-lasers, yielding a nominal pump power of 900 W. The diode laser radiation is coupled into the laser fiber through a 91:1 fused-fiber pump combiner. The input fibers of this pump combiner are standard 105/125-um multimode fibers with an NA of 0.22. These fibers form a hexagonally packed fused-fiber bundle, which is tapered to match the cladding diameter of the laser fiber. Eighty-six percent of the light exiting the pump-combiner is emitted within an NA of 0.32, and all measurable power is emitted within an NA of 0.45. The typical insertion loss of the pump combiners is <1%. The high-brightness radiation of seven laser modules is combined into a single output fiber using a 7:1 fused-fiber signal combiner providing a total power of >4 kW in the single output beam. The beam parameter product of the combined output was 2.5 mm-mrad. The low insertion loss of < 2% indicates that the signal combiner is suitable to handle even higher laser powers.
We have developed a commercial 4-kW fiber laser consisting of seven, 600-W modules whose outputs are combined
with a fused-fiber combiner. The system architecture has several practical advantages, including pumping with reliable
single-emitter diodes, monolithic fused-fiber construction (no free-space beams), and end pumping using a 91:1 pump
combiner (eliminating the need for complex pump/signal combiners). Typical results at 4-kW output power are a beamparameter
product of 2.6 mm-mrad, 8-hr power stability of < 0.5% rms, central wavelength of 1080 nm, and linewidth of
1.2 nm FWHM. These lasers have been incorporated into Amada machines used for cutting metal sheet and plate and
have been used to cut aluminum, mild steel, stainless steel, brass, titanium, and copper with a thickness up to 19 mm. A
world-record cutting speed of 62 m/min has been demonstrated for 1-mm aluminum sheet metal.
How nearly circular can a double-clad fiber's pump guide be and still have good absorption in the core? D's and octagons
have good absorption but can be hard to cleave and lose pump power or brightness through splices with circular fibers.
Ray tracing cannot be trusted since the perturbations to the circle can be smaller than a transverse wavelength. Using an
algorithm well suited to near-circular boundaries, we have computed statistics of core-overlap of the lowest 10,000
modes of many pump-guide shapes. Some with 3% radial modulation have nearly as good core-overlap as an octagon.
A diode-pumped frequency-doubled Nd:YAG laser has been demonstrated which emits 7.2 W of 532-nm radiation in the
CW mode as well as 23 ns, 2.7 mJ pulses at a repetition rate of 10 kHz in the Q-switched mode. The high power in both
modes was achieved by intra-cavity second harmonic generation in lithium triborate. The nonlinear output coupling
through SHG in this laser causes a factor of 5.7 change of intra-cavity power between the CW and the Q-switched mode.
The resulting variation of the thermal lens in the laser rod makes it challenging to maintain a geometrically stable cavity
in both operation regimes, which is essential for diffraction limited beam quality. Diffraction limited beam quality with
M2 values of less than 1.1 in the CW and less than 1.2 in the Q-switched regime was achieved by a novel dual-stabilityrange
cavity-design. This design provides geometrically stable cavity configurations in both operation regimes, which
are separated by an unstable region. This cavity makes it possible to switch between the two operation regimes without
any moving components.
We report on the pulse shape of an actively Q-switched fiber laser. This master oscillator power amplifier architecture
generates pulses with multiple peaks due to its intrinsic dynamics. Modeling and experimental results provide us a
detailed understanding of the relative importance of the different time constants on the dynamics of the laser, which
allows us to define optimized design parameters that lead to smooth and controlled pulse shapes. This solution is simple
and robust; operation over a broad range of repetition rate and output power is achieved without any adjustment of the
laser settings, and the corresponding variation of the optical performances is minimal.
Optimization of nonlinear coupling and use of aggressive Q-switch clipping on the pulse falling edge has enabled high
beam quality, internally frequency-converted, Nd:YAG lasers with output powers greater than 35 W at both green and
ultraviolet wavelengths. By retaining more energy in the rod after the pulse, such lasers operate with enhanced gain
levels and achieve greater nonlinear conversion. Operation in this gain-enhancement regime has enabled lasers with
highly variable pulse-widths. We demonstrate here that gain-enhanced laser operation with variable pulse energy and/or
delay between pulses is also possible. This combination of features and flexibility available in gain-enhanced,
frequency-converted lasers allows optimization of laser processing for a wide range of candidate materials.
We demonstrate a variable pulse width, internally-frequency-converted, near-diffraction-limited Nd:YAG laser with output power up to 40 Watts at 532 nm and pulse widths electronically adjustable over a 40-300 ns range. The variable pulse width is achieved by clipping the pulse decaying edge with the Q-switch in a laser cavity optimized for post-pulse gain insensitivity. This approach makes possible frequency converted lasers with pulse width and output power substantially independent of repetition rate.
Many micromachining operations, particularly in the electronics sector, utilize pulsed solid-state UV lasers. These processes demand high levels of stability, as the yield and quality relate directly to the repeatability of each laser pulse. Critical stability issues arise with single-pulse processes (e.g. repair), situations requiring bursts of pulses (e.g. drilling), and continuous pulsing applications (e.g. cutting). To realize optimal stability specific design choices must be made, certain transient problems must be solved, and pulse energy measurements must be standardized. Solid-state UV lasers originate as infrared lasers, and nonlinear optics converts the infrared to the UV. This conversion introduces instability. Performing the conversion within the infrared laser cavity suppresses the instability, relative to performing the conversion outside of the laser cavity. We explain this phenomenon. Ideally, a versatile and stable solid-state laser can generate pulses in many formats. Thermal effects tend to prevent this versatile ideal, resulting in transient problems (unstable pulse trains), or less than optimal performance when the laser is pulsing continuously. Many methods of measuring pulse energy exist. Each method can produce surprisingly different results. We compare various techniques, discuss their limitations, and suggest an easily implemented pulse energy stability measurement.
The trend in micro-machining lasers is toward greater average power and higher repetition rate, in order to increase throughput, with pulse energy and peak power held roughly constant, as determined by the small scale of the feature. At repetition rates beyond 500 kHz, conventional high-power Q-switched Nd lasers will reach fundamental limits. We demonstrated a fiber-based oscillator-amplifier architecture which produces pulse repetition rates in the 0.5 - 5 MHz range and pulse durations in the 0.5 - 1.5 nsec range. The oscillator is a compact (35 cm3 package) passively Q-switched Nd:YVO4 laser oscillating at a single frequency. By amplifying this laser in fiber, we demonstrated 10-W average power at the two wavelengths of 914 nm and 1064 nm. At 1064-nm, Yb-doped large mode area fiber will allow scaling of average power to over 100 Watts, with peak power of tens of kW, in a diffraction-limited beam. Excellent conversion will be possible to visible and UV using the robust nonlinear material LBO. By opening up a new range of repetition rates and pulse lengths, at IR, visible and UV wavelengths, in a high power design that has the packaging and efficiency advantages of fiber, new micro-machining applications may be enabled.
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