Richard Duesterberg

1.2-kW single-mode fiber laser based on 100-W high-brightness pump diodes

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.

Advancements in laser diode chip and packaging technologies for application in kW-class fiber laser pumping

A new 100μm aperture, 920nm laser diode chip was developed to improve fiber coupling efficiency and reliability. These chips have been assembled into single-emitter and multi-emitter packages with 105μm diameter fiber-coupled output. The single-emitter package is rated for 12W operation, while the multi-emitter package is rated at 140W. Power conversion efficiency is 50%. Over one year of accelerated active life testing has been completed along with a suite of passive, environmental qualification tests. These pumps have been integrated into 2kW, 4kW, and 6kW fiber laser engines that demonstrate excellent brightness, efficiency, and sheet metal cutting quality and speed.

High-power, high-efficiency fiber-coupled multimode laser-diode pump module (9XX nm) with high reliability

We have developed a single-emitter multi-mode laser-diode-pump platform for high efficiency, brightness and high reliability in a small form factor. This next-generation package is scalable to higher optical power and offers a low-cost solution for industrial applications, such as fiber lasers, graphic arts and medical. The pump modules employ high coupling efficiency, >90%, high power-conversion efficiency, >50%, and low thermal resistance, 2.2°C/W, in an electrically-isolated package. Output powers as high as 18W have been demonstrated, with reliable operation at 10W CW into 105μm core fiber. Qualification results are presented for 0.15NA and 0.22NA fiber designs.

Reliability of high-power multi-mode pump modules

Developers building high-power fiber lasers and diode pumped solid state lasers can receive significant benefits in thermal management and reliability by using single emitter multi-mode diodes in distributed pump architectures. This proposed distributed architecture relies on independent single emitter pump lasers and a modest level of pump redundancy. Driving the remaining diodes slightly harder componensates individual diode failures. A model of the ensemble lifetime based on module failure rates and power-scaling factors demonstrates that the distributed pump architecture requires random failure rates corresponding to better than 200,000h mean time between failure (MTBF), which meets typical industrial requirements. A high power, pigtailed, multi-mode pump module suitable for commercial applications is created through this model. Critical elements are based on telecom architectures, including the optical train and the fiber alignment. The module has a low thermal resistance of 4°C/W from the chip-on-sub-mount to the external heat sink, coupling efficiency of over 80% into 0.2 NA, and demonstrated reliable output power of over 5W cw with peak wavelengths near 915 nm. Individual pump modules are predicted to produce 5W cw output power with an MTBF of more than 400,000h. The relationship between anticipated MTBF requirements, test duration and test population is shown.

100W high-brightness multi-emitter laser pump

We report results of a spatially-multiplexed broad area laser diode platform designed for efficient pumping of fiber lasers or direct-diode systems. Optical output power in excess of 100W from a 105μm core, 0.15NA fiber is demonstrated with high coupling efficiency. The compact form factor and low thermal resistance enable tight packing densities needed for kW-class fiber laser systems. Broad area laser diodes have been optimized to reduce near- and far-field performance and prevent blooming without sacrificing other electro-optic parameters. With proper lens optimization this produces ~5% increase in coupling / wall plug efficiency for our design. In addition to performance characteristics, an update on long term reliability testing of 9XX nm broad area laser diode is provided that continues to show no wear out under high acceleration. Under nominal operating conditions of 12W ex-facet power at 25C, the diode mean time to failure (MTTF) is forecast to be ~ 480 kh.

Semiconductor Laser Power Enhancement by Control of Gain and Power Profiles

We present theoretical calculations investigating the output power limitations of GaAs-based semiconductor lasers and the experimental results showing significant improvement of output power. To understand the influence of power limitation mechanisms, semiconductor laser with standard and unfolded cavity designs is studied. Our analysis reveals that an unfolded cavity laser enables more homogeneous longitudinal gain and intracavity optical intensity with reduced levels as compared with the standard cavity. Hence, an unfolded laser has theoretically lower power penalties induced by linear and nonlinear effects. For a 5.7-mm long laser cavity with 100-μm wide aperture, the experimental results demonstrate 21-W output from standard cavity whereas the unfolded cavity design achieves 33-W at 920 nm, which is >55% enhancement of the output, confirming the prediction of the theoretical calculations. The method represents a major step toward understanding semiconductor laser power limitations and realizing higher power output by control of longitudinal gain and power profiles.

Recent progress in fiber-coupled multi-mode pump module and broad-area laser-diode performance from 800-1500 nm

We present performance improvements of fiber-coupled pump modules and broad-area lasers at 8xx nm, 9xx nm and 14xx nm wavelengths. Broad-area lasers with a 200 μm aperture at 808 nm for direct diode applications emit 11W CW and 30W pulsed. Pump modules at 830 nm for printing applications show excellent linearity, power stability of 2% and 95% of the power within 0.12 NA into a 50 μm core fiber at 1W CW. Broad-area lasers at 880 nm for pumping applications emit 18W CW with a peak wallplug efficiency of 64%. An improved design of 9xx pump modules is demonstrated with built-in feedback-protection (>30 dB at 1060 nm) that allows safe operation in multi-kW peak-power fiber lasers. Up to 3W of optical power with slope efficiency and peak wallplug efficiency of 0.64 W/A and 46%, respectively, is presented for 14xx nm broad-area lasers with a 100 μm wide aperture.

kW fiber lasers

We describe a monolithic (fully fused), 1.2-kW, Yb-doped fiber laser with single-mode beam quality. This laser employs a new generation of 100-W, high-brightness, fiber-coupled pump sources based on spatially multiplexed single emitters and a simple, end-pumped design for high efficiency, reliability, and environmental stability.

29.5W continuous wave output from 100um wide laser diode

We report modeling and experimental results that demonstrate mechanisms limiting the output power of semiconductor lasers and a method experimentally yielding a dramatic increase of the maximum continuous wave output power. Unfolded cavity is used to achieve higher power and efficiency by improving the alignment between the carrier and photon density profiles in a long cavity device. This method offers reduced longitudinal spatial hole burning (LSHB) and lower photon density inside the laser cavity; therefore, it decreases possible LSHB and non-linear effects that could limit the output power of a semiconductor laser. We have demonstrated 29.5W output from 5.7mm long and 100um wide waveguide at 9xx nm using an unfolded cavity. A semiconductor laser with an unfolded cavity allows scaling of the output power by increasing the cavity length.

High power, high brightness diode lasers for kW lasers systems

Laser diodes with high power and brightness are critical for pumping kW-class fiber lasers and direct diode laser systems. We present performance data of current generation broad area chips and new chips as well as multi-emitter fiber-coupled modules. Improved lateral beam quality at high power allows new designs to reach 15W power with lateral beam parameter product (BPP) <;4 mm-mrad. A fiber-coupled module that spatially and polarization combines multiple emitters shows a brightness of 3.4 W/(mm-mrad)2 at 185 W of power, which represents a 30% improvement over the present generation.