D. Dawson

Mitigation of thermal lensing effect as a brightness limitation of high-power broad area diode lasers

Recent efforts to improve the reliability of high-power broad-area diode lasers operating in the 9xx nm wavelength band have yielded single emitter devices with excellent reliability to >15 W. However, in applications requiring fiber coupling, the fiber coupling power of a single emitter device is rather limited by its linear power density, and hence brightness of the device. Unfortunately, due to a rapid increase in the slow-axis divergence, a typical broad-area diode laser offers a much lower brightness increase and an earlier rollover than its output power. In this work, we show that thermal lensing (rather than carrier- or gain-induced guiding) in the slow axis is the predominant cause of beam quality degradation at increased driving current. Some of the techniques which are presented include the use of cavity length scaling and thermal path engineering. It is expected these approaches are critical to enabling continued scaling of highbrightness fiber coupled diode lasers.

High brightness fiber coupled pump modules optimized for optical efficiency and power

We report on the continued development of high performance fiber coupled laser diode modules at nLIGHT. We show that by optimizing the laser resonator design single emitter diode lasers can be tailored for high brightness or for reduced

Reliability of high power/brightness diode lasers emitting from 790 to 980 nm

/W applications. For instance, a fiber laser pump module based on 6 single emitter diode lasers couples efficiently into a 105 μm, 0.15 NA fiber with peak operating efficiency <59% and output power < 65W. These results are made possible by optimizing the diode laser slow axis brilliance and by increasing the optical to optical efficiency to 90%. We will also report on the development of tailored laser resonator that meets the power, brightness, and cost targets for industrial applications. For instance, a wider emitter has reliable performance of <18W of output power while maintaining the slow axis divergence required for coupling into a fiber with a 12 mm-mrad beam parameter product. The corresponding 50% increase in output power significantly improves the

Long-term wavelength stability of high-power laser diode bars on microchannel coolers

/W performance. These results of high brightness and high efficiency demonstrate the pump technology required for next generation solid state, fiber lasers, and materials processing applications.

Advances in high-brightness fiber-coupled laser modules for pumping multi-kW CW fiber lasers

This paper presents recent progress in the development of high power single emitter laser diodes from 790 nm to 980 nm for reliable use in industrial and pumping applications. High performance has been demonstrated on diode lasers from 790 nm to 980 nm, with corresponding peak efficiency ~65%. Reliability has been fully demonstrated on high power diode lasers of 3.8 mm laser cavity at 3 major wavelengths. We report on the correlation between photon-energy (wavelength) and device failure modes (reliability). A newly released laser design demonstrates diode lasers with 5.0 mm laser cavity at 915-980 nm and 790 nm, with efficiency that matches the values achieved with 3.8 mm cavity length. 915-980 nm single emitters with 5.0 mm laser cavity were especially designed for high power and high brightness applications and can be reliably operated at 12 W to 18 W. These pumps have been incorporated into nLIGHT’s newly developed fiber coupled pump module, elementTM. Ongoing highly accelerated diode life-tests have accumulated over 200,000 raw device hours, with extremely low failure rate observed to date. High reliability has also been demonstrated from multiple accelerated module-level lifetests.

High-brightness, fiber-coupled pump modules in fiber laser applications

Laser diode reliability depends on both power and spectral stability over time. This report examines cases in which both corrosion and ionic deposition resulted in wavelength shifts from less than 1 nm to greater than 7 nm in 60 - 100W bars on microchannel coolers. Both corrosion and deposition seemed to be exacerbated by frequent and/or lengthy periods of stagnation in the DI water system. Analytical results including SEM images of FIB cross-sections illustrate deposits of up to several microns thickness of dielectric (oxide) material, as well as voiding caused by corrosion of Ni-plating out from under Au-plating through pinhole defects. Thermal modeling confirms the effect of such features on thermal resistance, correlating to observed wavelength shifts. Actions taken to address these issues are discussed.

High-power fiber-coupled diode lasers with superior brightness, efficiency, and reliability

High-power continuous wave (CW) fiber lasers with excellent beam quality continue to drive demand for higher brightness pump modules at 920 nm and 976 nm. Over the last decade, the brightness requirement for pumping state-of-the-art CW fiber lasers (CWFLs) has risen from approximately 0.5 W/(mm-mR)2 to ~2 W/(mm-mR)2 for today’s mutlikW CWFLs. The most advanced CWFLs demand even higher brightness pump modules in order to minimize design complexity, maximize efficiency, and maximize the stimulated Raman scattering threshold. This need has resulted in a reoptimization of the nLIGHT elementTM line to enable a commercial 200 W, 18-emitter package with a 0.15 NA beam in a 105 μm fiber, corresponding to a brightness of 3.2 W/(mm-mR)2 and a 25 % increase in power over the existing elementTM e14 at 155 W. Furthermore, we have demonstrated the further scalability of this reoptimized design with our next generation COS, resulting in a maximum of 272 W into 105 μm fiber with a brightness of 3.8 W/(mm-mR)2.

Brightness scaling of diodes lasers and fiber coupled modules at 9xx nm

High-power, high-brightness, fiber-coupled pump modules enable high-performance industrial fiber lasers with simple system architectures, multi-kW output powers, excellent beam quality, unsurpassed reliability, and low initial and operating costs. We report commercially available (element™), single-emitter-based, 9xx nm pump sources with powers up to 130 W in a 105 μm fiber and 250 W in a 200 μm fiber. This combination of high power and high brightness translates into improved fiber laser performance, e.g., simultaneously achieving high nonlinear thresholds and excellent beam quality at kW power levels. Wavelength-stabilized, 976 nm versions of these pumps are available for applications requiring minimization of the gain-fiber length (e.g., generation of high-peak-power pulses). Recent prototypes have achieved output powers up to 300 W in a 200 μm fiber. Extensive environmental and life testing at both the chip and module level under accelerated and real-world operating conditions have demonstrated extremely high reliability, with innovative designs having eliminated package-induced-failure mechanisms. Finally, we report integrated Pump Modules that provide < 1.6 kW of fiber-coupled power conveniently formatted for fiber-laser pumping or direct-diode applications; these 19” rack-mountable, 2U units combine the outputs of up to 14 elements™ using fused-fiber combiners, and they include high-efficiency diode drivers and safety sensors.

Improvement in reduced-mode (REM) diodes enable 300 W from 105-µm 0.15-NA fiber-coupled modules

Advances in high performance fiber coupled diode lasers continue to enable new applications as well as strengthen existing uses through progressive improvements in power and brightness [1]. These improvements are most notable in multi-kW direct diode systems and kW fiber laser platforms that effectively transform better beam quality into superior system performance and in DPSS (Diode pumped solid state) application striving to scale TEM00 (fundamental transverse mode) power. We report on our recent single-emitter based fiber-coupled product platform, the elementTM, that addressed these applications at 8xx/9xx nm with optical powers over 200W in a range of fiber core sizes down to 105um and 0.14NA (Numerical Aperture). The product is a culmination of numerous packaging improvements: improving wall plug efficiencies (~50% electrical-to-optical) while improving volume manufacturability, enabling lower costs, improving usable chip brightness by, < 20% over previous generation chips, and increasing the reliable output power to 15W per chip. We additionally report on current developments to extend the power of the product platform to as high as 300W. This will be realized primarily through new chip architectures projected to further increase the useable chip brightness by an additional 20 % and correspondingly scaling reliable output powers. Second order improvements are proposed in packaging enhancements that capitalize on the increased chip power and brightness as well as expand the package’s thermal capabilities. Finally, an extended performance roadmap will translate expected power advances and increasing volumes into a projection of relative

Continued advances in high brightness fiber-coupled laser modules for efficient pumping of fiber and solid-state lasers

/W decreases over the next several years.