Yb-doped fiber laser and amplifiers are strong contenders as combinable sources used in power-scalable spectral beam combined and coherent beam combined High Energy Laser (HEL) systems. Power levels well beyond 300kW is anticipated for HEL systems for DoD applications. Pumping these fiber amplifiers at 976 nm has become an imperative. Furthermore, technology demonstrations of HELs have resorted to slow turn-on time. This needs to change for fieldable HEL systems. The need for an “instant-on” of HEL beam on demand cannot be over emphasized. Pumping a fiber laser/amplifier on this strong absorption peak also leads to a reduction in cost due to the shorter fiber and higher threshold power for nonlinear effects such as Stimulated Raman Scattering (SRS) and Stimulated Brillouin Scattering (SBS). But the absorption band of Yb-doped fiber is very narrow (<5 nm) and very sharp drop-off occurs around the peak near 976 nm. nLIGHT has developed high-efficiency, OSL technology by incorporating into the semiconductor laser chip, a wavelength-selective element which forces the laser to operate only at the desired wavelength over all operating current (output power) and a wide range of operating temperature. Although wavelength-stabilization in semiconductor laser pumps have been done in the past using external Volume Bragg Gratings (VBGs) they are not as efficient for HEL application. Furthermore, VBG adds cost, mass and volume to the pump package. We have demonstrated nearly penalty-free wavelength-stabilized high power, high efficiency chips. These chips can be readily introduced into a fiber-coupled package without needing any modification to the opto-mechanical design of the package – a drop-in replacement to the current pump packages. We have packaged them in a low SWaP fiber-coupled packages to produce ⪆550W with 55% at 25C and ⪆530W with ⪆52% at 50C using wavelength-locked diodes with a narrow spectral bandwidth ⪅0.4 nm at FWHM. The center wavelength shifts at 0.065 nm/K. The full power-in-the-band (within ±2nm of Yb-absorption peak) can be achieved in millisecond time scale enabling instant-on capability for HEL systems. Wavelength-stabilized pumps will be an imperative in fielded HEL systems because these pumps will enable high efficiency, high fiber amplifier channel power, low SWAP, low cost and “instant on” over a wide operating temperature range.
Combinable high average power fiber amplifiers with diffraction-limited beam that is suitable for High Energy Laser (HEL) applications is currently primarily limited in output power by TMI and SBS. SBS is better understood and numerous methods have been shown to be effective for scaling power to several kilowatt levels. Currently, TMI limits output power to ~3-kW for approximately 20-µm core conventional step-index dual-clad fiber amplifiers. We report on the use of low-quantum defect pumping scheme to increase the TMI threshold. We have demonstrated 5kW and further power-scaling is underway and the latest results will be presented.
Tm-doped fiber amplifiers are good sources for high energy laser, DIRCM, LIDAR, remote sensing and spectroscopy. The reported Tm-doped amplifiers have efficiencies between 55% to 65%. The diode lasers that are commercially available at 793 nm produce about 35% ex-fiber efficiency. Therefore, the power conversion efficiency of Tm-doped amplifiers is only in the range of 20% - 23% which is less than half that of Yb-doped amplifiers. One straightforward way to improve the e-o power conversion efficiency of Tm-doped fiber amplifiers is to boost the efficiency of 793 nm diode pump from 35% to >55%. This will enable >36% efficient Tm-doped fiber amplifiers. With improvement in epitaxial design, we have demonstrated 64% at 15.5 W producing fiber-coupled package with 400 W in 225-μm/0.22 NA and 54% ex-fiber efficiency in packages with <0.63 kg/kW and <0.49 cc/W specific mass and specific volume, respectively. Further improvement is underway and we will update the latest results.
We report development of 85µm core Yb-doped and Ge-doped chirally-coupled-core (CCC) fibers, and their integration via fusion-splicing into an all-fiber optical amplifier system. This system, consisting of a CCC fiber amplifier and a 6+1 fusion-spliced signal-pump-combiner with a passive CCC fiber feed-through produces robust single mode output (diffraction-limited) in a counter-pumped configuration with passive-fiber leads as short as ~30cm. The Yb-doped 85µm core CCC fiber amplifiers had produced ~10mJ energy pulses at close to ~100W of average power. This achieved performance and monolithic all-fiber integration are required for compact and robust coherently-combined laser array drivers of laser plasma accelerators.
A better understanding of stress effects that affect the bending losses in active and passive optical fibers allows us to improve fiber system designs and helps to optimize refractive index profiles in high power, large mode area laser fibers. Bending an optical fiber affects the light in a fiber core by two different phenomena. First, the curved shape of the waveguide changes the light propagation. The second phenomenon is the refractive index change caused by the mechanical stress in a bent optical fiber. The refractive index changes due to bending stresses are estimated by the elasto-optic and stress optic models. The light propagation in a curved waveguide can be modelled applying the electromagnetic wave theory together with the conformally transformed refractive index profiles that include the stress effects. The modelled refractive index profiles that include the bending stress induced index changes are compared to the refractive index profiles that were measured from actual bent optical fibers. We tested if this comparison would allow us to estimate C2 values in stress optic model. Measured bend loss values are compared to the bend loss values simulated with the modelled refractive index profiles.
In this paper, we show results of further brightness improvement and power-scaling enabled by both the rise in chip brightness/power and the increase in number of chips coupled into a given numerical aperture. We report a new chip technology using new extra Reduced-mode (x-REM) diode design providing a record ~363 W output from a 2×12 nLIGHT element® in 105 μm diameter fiber. There is also an increasing demand for low size, weight and power-consumption (SWaP) fiber-coupled diodes for compact High Energy Laser (HEL) systems for defense and industrial applications. Using thirty single emitters that were geometricallyand polarization-combined, we have demonstrated 600 watts and 62% efficiency at in 225 μm/0.22 NA fiber resulting in specific mass and volume of 0.44 kg/kW and of 0.5 cm3/W respectively. Furthermore, we have increased the number of chips to forty and increased the output power to 1kW and 52% in the same fiber diameter and numerical aperture. This results in a fiber-coupled package with specific mass and volume of <0.18 kg/kW and <0.27 cm3/W, respectively.
It is well documented that increases in pump module power enables higher power DPSS or CW fiber lasers, but it is important to recognize that increasing the efficiency by which the DPSS or CW fiber laser is pumped drives down both system complexity and cost. Additionally due to the narrow absorption band of the common laser mediums like Ytterbium and Neodymium, it is advantageous to maximize the spectral overlap between the emission of the pump module and the absorption band of the host medium; one way to accomplish this is by the use of Volume Bragg Gratings (VBGs) to both narrow and stabilize (meaning to minimize change with current and/or temperature) the emission of the diode pump module. To this end, we report on the continued progress by nLIGHT to develop and deliver the highest efficiency wavelength-stabilized, diode-laser pumps using single-emitter technology at ~885nm for neodymium DPSS pumping, and 969/976 nm for ytterbium laser pumping. The basis for these improvements is the ensuring the epitaxial structure of the laser diode is optimized not only for efficiency and power but is also properly optimized to minimize the amount of spectral shift with current. Due to the proprietary nature of our epitaxial structures, we are unable to provide exact details. However, throughout this paper, we will abstractly discuss the improvements made to our epitaxy, and how those changes directly affect, and improve upon the module level performance with VBGs, and provide COS and module level results for our element® packages with VBGs to support these claims, with key examples being: at 969/976 nm a 2×6 module with 140 W into 105 μm – 0.16 beam NA, and a 969/976 nm 400 W 2×12 into 200 μm – 0.16 beam NA, along with 888 nm diode module, in a 2×12 layout outputting a maximum of 370 W with 52 % electro-optical efficiency when coupled into 200 μm – 0.18 beam NA
In this paper, we show results of further brightness improvement and power-scaling enabled by both the rise in chip brightness/power and the increase in number of chips coupled into a given numerical aperture. We report a new chip technology using x-REM design providing a record ~340 W output from a 2×12 nLIGHT element® in 105 μm diameter fiber. These diodes will allow next generation of fiber-coupled product capable of >250W output power from 105 μm/0.15 NA beam at 915 nm. There is also an increasing demand for low SWaP fiber-coupled diodes for enabling compact high energy laser systems for defense applications. We have demonstrated 600 watts and 60% efficiency at 15C in 220 μm/0.22 NA fiber resulting in specific mass and volume of 0.44 kg/kW and of 0.5 cm3/W respectively.
Multi-kilowatt, continuous-wave fiber lasers continue to drive the need for higher power, higher brightness, and more efficient diode laser pump modules. It is well documented that increases in pump module power either enable higher power CW fiber lasers or minimize complexity of the multi-stage fiber combiners for a given power. Additionally, increasing pump module brightness positively impacts the SRS threshold of a given multi-kilowatt CW fiber lasers architecture. We report on the continued progress by nLIGHT to develop and deliver the highest brightness diode-laser pumps using single-emitter technology at 976 nm for Ytterbium fiber laser pumping. Building upon our prior developments that have enabled higher emitter counts in the element® packages, nLIGHT is releasing two new 976 nm module configurations: a 2×7 module with 155 W into 105 μm – 0.15 beam NA, and a 375 W 2×12 into 200 μm – 0.16 beam NA. Additionally, we have demonstrated high efficiency designs utilizing a new chip on submount (COS) architecture: with a 430 W 2×12 into 200 μm – 0.16 beam NA and 57% electro-optical efficiency, and an alternative 2×15 design resulting in 600 W at 57 % electro-optical efficiency at 23 A when coupled into 200 μm – 0.18 beam NA.
Kilowatt-class fiber lasers and amplifiers are becoming increasingly important building blocks for power-scaling laser systems in various architectures for directed energy applications. Currently, state-of-the-art Yb-doped fiber lasers operating near 1060 nm operate with optical-to-optical power-conversion efficiency of about 66%. State-of-the-art fiber-coupled pump diodes near 975 nm operate with about 50% electrical-to-fiber-coupled optical power conversion efficiency at 25C heatsink temperature. Therefore, the total system electrical-to-optical power conversion efficiency is about 33%. As a result, a 50-kW fiber laser will generate 75 kW of heat at the pump module and 25 kW at the fiber laser module with a total waste heat of 100 kW. It is evident that three times as much waste heat is generated at the pump module. While improving the efficiency of the diodes primarily reduces the input power requirement, increasing the operating temperature primarily reduces the size and weight for thermal management systems. We will discuss improvement in diode laser design, thermal resistance of the package as well as improvement in fiber-coupled optical-to-optical efficiency to achieve high efficiency at higher operating temperature. These factors have a far-reaching implication in terms of significantly improving the overall SWAP requirements thus enabling DEW-class fiber lasers on airborne and other platforms.
We report on the performance of a standard Yb-doped DC-LMA fiber and compare it to a similar core-size chirally-coupled core (3C®) fiber in a co-pumped fiber amplifier configuration. We used Yb-doped 20/400/0.064 DC LMA fiber for the power amplifier and achieved ~2.4 kW of signal power at 2.79 kW of absorbed pump power. However, we observed an onset of TMI at ~2.2 kW. The spectral bandwidth of this amplifier was 20 GHz and there was no sign of SBS at 2.4 kW of output power. We then used an Yb-doped 21.9/400/0.059 DC 3C fiber with a coiling diameter of ~30 cm to test the efficacy of HOM suppression in this fiber with respect to improving TMI threshold. We achieved 2.6 kW of output power (pump combiner limited) without TMI. Further power-scaling experiments are underway and we will report on the latest findings. However, it is clear from these results that 3C fiber has a better HOM suppression capability compared to 10-cm diameter coiled DC-LMA fiber. Even a 30-cm coiled 3C fiber shows no sign of TMI at 2.6 kW; while, a slightly smaller diameter and tightly coiled 10-cm diameter LMA fiber amplifier shows signs of TMI ~ 2.2 kW. We also measured Brillouin shift, gain bandwidth and gain coefficient and they were found to be ~15.3 GHz, ~83 MHz and 0.47 to 0.7 ×10-11 m/W respectively compared to reported values of 16.1 GHz, ~64 MHz and 5 ×10-11 m/W. This significantly lower Brillouin gain and slightly larger gain bandwidth leads to eight times higher SBS threshold for amplifiers using nLIGHT fiber with near single-frequency seed compared to literature values. This is a distinct advantage which will enable optimization of both the LMA and 3C fiber geometry to achieve higher TMI threshold in the future.
High-power, high-brightness diode lasers have been pursued for many applications including fiber laser pumping, materials processing, solid-state laser pumping, and consumer electronics manufacturing. In particular, ~915 nm – and ~976 nm diodes are of interest as diode pumps for the kilowatt CW fiber lasers. As a result, there have been many technical thrusts for driving the diode lasers to have both high power and high brightness to achieve high-performance and reduced manufacturing costs. This paper presents our continued progress in the development of high brightness fiber-coupled product platform, nLIGHT element®. In the past decade, the power coupled into a single 105 μm and 0.15 NA fiber has increased by over a factor of ten through improved diode laser brightness and the development of techniques for efficiently coupling multiple emitters. In this paper, we demonstrate further brightness improvement and power-scaling enabled by both the rise in chip brightness/power and the increase in number of chips coupled into a given numerical aperture. We report a new chip technology using x-REM design with brightness as high as 4.3 W/mm-mrad at a BPP of 3 mm-mrad. We also report record 315 W output from a 2×12 nLIGHT element with 105 μm diameter fiber using x-REM diodes and these diodes will allow next generation of fiber-coupled product capable of 250W output power from 105 μm/0.15 NA beam at 915 nm.
Both the fibber laser and diode-pumped solid-state laser market continue to drive advances in pump diode module brightness. We report on the continued progress by nLIGHT to develop and deliver the highest brightness diode-laser pumps using single-emitter technology. Continued advances in multimode laser diode technology [13] and fiber-coupling techniques have enabled higher emitter counts in the element packages, enabling us to demonstrate 305 W into 105 μm – 0.16 NA. This brightness improvement is achieved by leveraging our prior-reported package re-optimization, allowing an increase in the emitter count from two rows of nine emitters to two rows of twelve emitters. Leveraging the two rows off twelve emitter architecture,, product development has commenced on a 400 W into 200 μm – 00.16 NA package. Additionally, the advances in pump technology intended for CW Yb-doped fiber laser pumping has been leveraged to develop the highest brightness 793 nm pump modules for 2 μm Thulium fiber laser pumping, generating 150 W into 200 μm – 0.18 NA and 100 W into 105 μm – 0.15 NA. Lastly, renewed interest in direct diode materials processing led us to experiment with wavelength multiplexing our existing state of the art 200 W, 105 μm – 00.15 NA package into a combined output of 395 WW into 105 μm –– 0.16 NA.
Kilowatt-class fiber lasers and amplifiers are becoming increasingly important building blocks for power-scaling laser systems in various different architectures for directed energy applications. Currently, state-of-the-art Yb-doped fiber lasers operating near 1060 nm operate with optical-to-optical power-conversion efficiency of about 66%. State-of-the-art fiber-coupled pump diodes near 975 nm operate with about 50% electrical-to-fiber-coupled optical power conversion efficiency at 25C heatsink temperature. Therefore, the total system electrical-to-optical power conversion efficiency is about 33%. As a result, a 50-kW fiber laser will generate 75 kW of heat at the pump module and 25 kW at the fiber laser module with a total waste heat of 100 kW. It is evident that three times as much waste heat is generated at the pump module. While improving the efficiency of the diodes primarily reduces the input power requirement, increasing the operating temperature primarily reduces the size and weight for thermal management systems. We will discuss improvement in diode laser design, thermal resistance of the package as well as improvement in fiber-coupled optical-to-optical efficiency to achieve high efficiency at higher operating temperature. All of these factors have a far-reaching implication in terms of significantly improving the overall SWAP requirements thus enabling DEW-class fiber lasers on airborne and other platforms.
There is a great interest in the development of high-power, high-efficiency, and low-cost quasicontinuous wave (QCW) diode laser bars and arrays for pumping solid state lasers. We report on the development of kW-class 88x-nm diode laser bars that are based on a bipolar cascade design, in which multiple lasers are epitaxially grown in electrical series on a single substrate and separated by low-resistance tunnel junctions with resistance as low as 8.0×10−6 Ω-cm2. QCW power of 630 W was demonstrated in a 3-mm-wide minibar with 3-mm cavity length. Peak efficiency of 61% was measured with 200 μs and 14 Hz pulses, at 10°C. Further power scaling was demonstrated in a 1-cm-wide bar with 3-mm cavity length, where a record peak power of 1.8 kW was measured at 1-kA drive current. Ongoing work for further power scaling includes development of triple-junction diode laser bars.
KEYWORDS: Diodes, Semiconductor lasers, Fiber lasers, Manufacturing, Continuous wave operation, Chemical elements, Optical components, High power lasers, Near field optics
High-power, high-brightness diode lasers from 8xx nm to 9xx nm have been pursued in many applications including fiber laser pumping, materials processing, solid-state laser pumping, and consumer electronics manufacturing. In particular, 915 nm - 976 nm diodes are of interest as diode pumps for the kilowatt CW fiber lasers. Thus, there have been many technical efforts on driving the diode lasers to have both high power and high brightness to achieve high-performance and reduced manufacturing costs. This paper presents our continued progress in the development of high brightness fiber-coupled product platform, elementTM. In the past decade, the amount of power coupled into a single 105 μm and 0.15 NA fiber has increased by over a factor of ten through improved diode laser brilliance and the development of techniques for efficiently coupling multiple emitters into a single fiber. In this paper, we demonstrate the further brightness improvement and power-scaling enabled by both the rise in chip brightness/power and the increase in number of chips coupled into a given numerical aperture. We report a new x-REM design with brightness as high as 4.3 W/mm-mrad at a BPP of 3 mm-mrad. We also report the record 272W from a 2×9 elementTM with 105 μm/0.15 NA beam using x-REM diodes and a new product introduction at 200W output power from 105 μm/0.15 NA beam at 915 nm.
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.
KEYWORDS: Diode pumped solid state lasers, High power lasers, Semiconductor lasers, Laser packaging, Laser stabilization, Diodes, High power diode lasers, Reliability, Failure analysis, Reflectivity, Fiber Bragg gratings, Fermium, Frequency modulation
There is an increasing demand for high power diode laser packages with stabilized wavelength in the range of 878 nm to 888 nm for DPSS laser pumping applications. In this paper we present nLIGHT’s most recent development of wavelength-stabilized high power, single emitter laser diode packages, elementTM , for DPSS laser pumps. We will report on how we have scaled single emitter power from 10 W per emitter with our prior generation of 200 μm wide and 3.8 mm long devices to 15 W per emitter for next generation of 5 mm cavity length device for 200 μm - 0.22 NA fiber products. The improvement in power at the chip-on-submount level results in approximately 40% increase in wavelength-stabilized power out of 200 μm fiber excited with a 0.19 NA beam, compared to the current generation elementTM products. Additionally, we will report on the improvements to wavelength-stabilization utilizing volume Bragg gratings, and chip-on-submount reliability for these new 885 nm devices, which drives the overall package reliability.
There is an increasing demand for high-power, high-brightness diode lasers from 8xx nm to 9xx nm for applications such as fiber laser pumping, materials processing, solid-state laser pumping, and consumer electronics manufacturing. The kilowatt CW fiber laser pumping (915 nm - 976 nm), in particular, requires the diode lasers to have both high power and high brightness in order to achieve high-performance and reduced manufacturing costs. This paper presents continued progress in the development of high brightness fiber-coupled product platform, elementTM. Further brightness improvement and power-scaling have been enabled by both the rise in chip brightness as well as the increase in number of chips used to couple into a given numerical aperture. We have developed a new generation of high power broad area laser known as reduced-mode diode (REM-diode) which suppresses many of the higher order modes in the slow axis and reduces divergence up to two times at the same operating conditions. To date, we have achieved slow-axis brightness as high as 4.3 W/mm-mrad for devices with thermal resistance of ~2.5 C/W. As a result, we have achieved >75 watts from a 1×6 elementTMin the 9xx nm spectral range; and 177 watts of peak power from a 2×6 elementTM. We have also improved our optics for fiber-coupling which accommodates 7 emitters per polarization in the same numerical aperture. Using this configuration, we project 200 watts of peak power from a 2×7 elementTM with a reliable product at 176 W of power from 105 μm and 0.15 NA fiber. REM-diodes can also be wavelength stabilized using VBGs. The reliability of REM-diodes are equal or better than broad area lasers (BALs). We present current status on ongoing reliability assessment of chip-on-submount.
In this paper we present nLIGHT’s most recent reliability assessment of both the released and newly developed high
power, high brightness single emitter laser diodes for fiber laser pumps and material processing applications. We report
on the latest updates of lifetests performed on released 18W-rated diode lasers which have been successfully
incorporated into nLIGHT’s 210W 200μm/0.18NA elementTM pump module. A total of 371 units of 18W-rated single
emitters at 915 nm, were assessed at 22A and 2 A at a junction temperature, Tj~70ºC. Cumulatively, these devices have
accrued ~ 6.0 million equivalent device hours at module use conditions. The initial reliability analysis based on these
lifetest results support <99% module reliability for 2-year of continuous operation. Industry leading dollars-per-watt
elementTM e06, e12 and e18 packages based on these diode lasers are also presented. Two elementTM e18 packages have
been lifetested for <5400 hours with only one device failure so far. We also report on the initial lifetest of the newly
developed high brightness REM-diodes (Reduced Mode diodes) for new elementTM configuration. Preliminary highly
accelerated lifetest on ~15 W REM-diodes show very low failure rate compared to the control diode lasers under the
same conditions. The more optimized <15W REM-diodes have been lifetested for almost 4000h with no failures
observed so far. Superior performance has already been demonstrated on the initialelementTMe06, e12 and e18 packages
with these new REM designs, supporting a 25% increase in power with a minimal degradation in NA. Module level
reliability assessment is underway.
We report on continued progress in nLIGHT’s high power and high efficiency single emitter laser diodes from 915 nm to 980 nm range used for industrial and pumping applications. High performance has been demonstrated in nLIGHT’s diode laser technology in this spectral range with peak electrical-to-optical power conversion efficiency of ~65%. These diodes have been incorporated into nLIGHT’s fiber-coupled pump module, elementTM. We have reduced the slow-axis divergence of our brightest diodes by a half at the same operating power. This results primarily from suppression of higher-order lateral modes leading to lower beam-parameter-product at a given power compared to conventional broad area lasers. We have device designs that produce slow axis brightness of up to 4.3 W/mm-mrad which is 48% higher compared to our brightest broad area laser. This paper presents nLIGHT’s most recent improvement in slow-axis brightness resulting from reduced number of allowed modes in the slow-axis in a new broad area laser architecture called reduced-mode diodes (REM-diodes). We will detail the resulting power and brightness improvement along with preliminary reliability assessment of these diodes.
There is great interest in the development of high-power, high-efficiency and low cost QCW 88x-nm diode laser bars and arrays for pumping solid state lasers. We report on the development of kW 88x-nm diode laser bars that are based on a bipolar cascade design, in which multiple lasers are epitaxially grown in electrical series on a single substrate. Multiple laser junctions, each of which is based on nLight’s high performance 88x-nm epitaxial design, are separated by low resistance tunnel junctions with resistance as low as 8.0x10-6 Ω-cm2. Optimization of bar geometry and wafer fabrication processes was explored for electrical and optical performance improvement in double-junction diode lasers. A QCW power of 630 W was demonstrated in a 3-mm wide mini-bar with 3-mm cavity length. Peak efficiency of 61% was measured with 200 s and 14 Hz pulses, at a heatsink temperature of 10 °C. Further power scaling was demonstrated in a 1-cm wide bar with 3-mm cavity length, where a record high peak power of 1.77 kW was measured at 1 kA drive current. Ongoing work for further power scaling includes development of triple-junction diode laser bars and double-junction bar-stack that emits < 10kW optical power.
M. Kanskar, L. Bao, J. Bai, Z. Chen, D. Dahlen, M. DeVito, W. Dong, M. Grimshaw, J. Haden, X. Guan, M. Hemenway, K. Kennedy, R. Martinsen, J. Tibbals, W. Urbanek, S. Zhang
KEYWORDS: Reliability, Semiconductor lasers, Diodes, Fiber lasers, High power lasers, Continuous wave operation, Near field, Performance modeling, Resistance, Near field optics
We report on continued progress in the development of high power and high brightness single emitter laser diodes from 790 nm to 980 nm for reliable use in industrial and pumping applications. High performance has been demonstrated in nLIGHT’s diode laser technology in this spectral range with corresponding peak electrical-to-optical power conversion efficiency of ~65%. These pumps have been incorporated into nLIGHT’s fiber-coupled pump module, elementTM. We report the latest updates on performance and reliability of chips and fiber-coupled modules. This paper also includes a new chip design with significantly narrower slow-axis divergence which enables further improved reliable power and brightness. Preliminary reliability assessment data for these devices will be presented here as well.
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 $/W decreases over the next several years.
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.
KEYWORDS: Semiconductor lasers, Reliability, Continuous wave operation, Diodes, High power lasers, Mirrors, Laser development, High power diode lasers, Laser applications
There is increasing market demand for high power reliable red lasers for display and cinema applications. Due to the fundamental material system limit at this wavelength range, red diode lasers have lower efficiency and are more temperature sensitive, compared to 790-980 nm diode lasers. In terms of reliability, red lasers are also more sensitive to catastrophic optical mirror damage (COMD) due to the higher photon energy. Thus developing higher power-reliable red lasers is very challenging. This paper will present nLIGHT’s released red products from 639 nm to 690nm, with established high performance and long-term reliability. These single emitter diode lasers can work as stand-alone singleemitter units or efficiently integrate into our compact, passively-cooled Pearl™ fiber-coupled module architectures for higher output power and improved reliability. In order to further improve power and reliability, new chip optimizations have been focused on improving epitaxial design/growth, chip configuration/processing and optical facet passivation. Initial optimization has demonstrated promising results for 639 nm diode lasers to be reliably rated at 1.5 W and 690nm diode lasers to be reliably rated at 4.0 W. Accelerated life-test has started and further design optimization are underway.
Electrically-injected vertical external cavity surface emitting laser (VECSEL) arrays are an attractive source for lowcost, high-brightness applications. Optical pumping can be used to investigate the emission properties of such devices without undergoing complex device fabrication. The design of such arrays is based on a single VECSEL chip, a 2D lens array, and a flat output coupling dichroic mirror. In this work, we report on the demonstration of an optically pumped, coherently-coupled VECSEL array. The array achieves a maximum total output power of >60 mW and lasing spectrum indicates single-mode operation. Near-field characterization reveals 37 individual lasing elements in a hexagonal array. Far-field measurements show an interference pattern which is consistent with inphase coherent coupling, with >60% of the total output power present in the on-axis central lobe. The physical origin of coherent coupling is attributed to diffractive coupling. The simplicity of the optical cavity design suggests scalability to much larger arrays, making the result of particular interest to the development of low-cost, highbrightness diode sources.
KEYWORDS: Cryogenics, Semiconductor lasers, Temperature metrology, Internal quantum efficiency, Waveguides, Diodes, High power lasers, Laser damage threshold, Ionization, Laser development
There is great interest in the development of high-power, high-efficiency InP-based broad area pump diode lasers operating in the 14xx-15xx nm band to be used for resonant-pumping of Er-doped solid state lasers. Cryogenic cooling of diode lasers can provide great benefit to performance, arising from the dramatic reduction in the threshold current and the increase in the diode’s slope efficiency. These improvements are attributed to reduction in the non-radiative losses and leakage current associated with thermionic emission of carriers from the quantum well. This is, however, at the expense of a large increase in the diode voltage, limiting the power conversion efficiency at cryogenic temperatures. In this work, we report on the development of high-power, high-efficiency diode lasers and stacked arrays operating at 15xx-nm, which are specifically designed and optimized for operation at cryogenic temperatures. We show that the diode voltage defects under cryogenic operation can be greatly reduced through reducing the energy band offsets at the hetero-interface, and through material change to reduce the dopant ionization energy, effectively mitigating carrier freeze-out at low temperatures. Optical cavity designs and band engineering optimization are also explored for low intrinsic optical loss and low carrier leakage. A peak power conversion efficiency of >74% was demonstrated at a temperature of ~100K in a 15xx-nm single emitter. Record high peak conversion efficiency of 71% and peak power of > 500 W were also demonstrated in a stacked array, under QCW pulses of 1 ms and 10 Hz.
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.
nLIGHT optimized both the high-temperature (HT) and the high-efficiency (HE) epitaxial designs for efficient highpower continuous-wave (CW) operation by implementing nLIGHT’s facet passivation technology (nXLT) into our 808 nm diode laser bars. The application of a refined phenomenological model of the diode lasers allowed tailoring of the device parameters to obtain optimized bar performance. In other words, we adjusted modeling inputs such as bar layout and front facet coating reflectivity to optimize operational indicator outputs such as wall-plug efficiency and operation currents at specific power ratings. Thus, both time and cost were saved without the need of extra experimental runs. We demonstrated that both HT and HE epitaxial designs can support centimeter bar geometries with power ratings above 100 W/bar. At the standard power rating of 100 W/bar, the HE designs show advantages in both operating current and wall-plug efficiency when compared to the HT design. With the newly released HE epitaxial designs, wall-plug efficiency ~58% is achieved for a power rating of 150 W/bar.
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 $/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 $/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.
Surface-emitting distributed feedback (SE-DFB) diode lasers possess many advantages compared to edge emitters, in
terms of power, brightness and spectral characteristics. We have demonstrated 312 W and 68 W of peak power at 1-μs
pulse duration and 100-Hz repetition rate from single 97x nm SE-DFB laser chips with electrically-pumped stripe of
3.75 mm2 and 0.18 mm2 respectively, achieving a power density of 378 W/mm2. Under the same pulsing condition, we
have also demonstrated current-limited 206 W of peak power out of a 100 μm / 0.22 NA fiber with only two 2x1 chiplevel
SE-DFB laser arrays.
We present our recent efforts to improve power rating, efficiency, reliability, and cost of diode laser bars in the 88x nm
wavelength band. QCW kW-class diode laser bars are grown by metal-organic chemical vapor deposition (MOCVD),
and are cleaved, passivated, coated, and die bonded onto either standard copper CS-style heat sinks using indium solder,
or onto expansion matched CuW CS heat sinks using AuSn solder. In an effort to realize high power operation, the high
efficiency 880-nm epitaxial design has been optimized. Bars of varying fill factors, cavity lengths, and facet coating
reflectivities are explored to improve the rated electrical to optical (E-O) efficiency up to approximately 70% under low
duty cycle QCW operations. The enhanced E-O efficiency makes possible not only the passive cooling of the devices,
but also reliable operation in the kW power range. We demonstrate that the semiconductor laser bars can survive over
100 million laser shots working in QCW mode. It is expected that the development of these passively cooled, highly
efficient and highly reliable QCW kW-class diode laser bars will enable commercial applications.
We report on our progress to obtaining a 1540 nm SE-DFB laser with curved, second order grating capable of producing
extremely high peak power for eye-safe range finding applications. Higher peak power can be extracted from SE-DFB
lasers since the emission area is three orders of magnitude larger than for edge emitters. Ten-times narrower spectral
width and four-times lower thermal shift of emission wavelength allows for the use of narrower filters on the range
finder detector, providing a higher signal-to-noise ratio. Furthermore, a curved second-order diffraction grating in our
SE-DFB laser allows for a higher brightness source, which makes transmitter optics geometry more compact and
simpler.
Over the past few decades, diode laser technology development has achieved remarkable improvement in power,
reliability and efficiency. Spectral brightness,
wavelength-stabilization and spatial brightness are becoming very
important for pumping of novel solid-state gain media and fiber lasers especially for efficient and power-hungry
industrial and military applications. We will discuss the benefits of using 975 nm narrow-band curved grating Surfaceemitting
Distributed Feedback lasers for pumping fiber lasers and thin disk lasers. SE-DFB lasers with less than 0.25 nm
emission bandwidth, 0.07nm/°C thermal wavelength drift with over 50% power conversion efficiency has been achieved
with a single emitter producing 73 W of CW power. Two-dimensional arrays of these lasers have been made for power
scaling to achieve 1kW of power with less than 1nm spectral bandwidth. We will discuss the results and key advantages
of using spectrally and spatially bright diodes for pumping fiber and thin disk lasers.
We report on improvement from 50% to 70% power conversion efficiency on a 5-bar stack with 500 W of CW power at 25C coolant temperature resulting from a multi-pronged optimization approach. We also report on wavelength stabilization (0.07 nm/C) and emission bandwidth narrowing (0.3 nm at FWHM) of diode laser pump sources for precision pumping the upper transition levels of lasers that require narrow and stable pump sources such as Er/Yb co-doped or Yb:YAG lasers. These results have been achieved by integration of a Bragg grating inside a semiconductor laser cavity forming a low-loss, weak distributed feedback (DFB) laser, which results in record 53% wall-plug efficiency at 3 W CW operation and 25°C heatsink temperature from a 100-μm aperture diode laser and 45 W of wavelength-locked CW power from a 20% fill factor bar. This technique can be readily applied to diode laser structures for other strategic pump wavelengths.
Al-free active diode lasers emitting near 970 nm wavelength have been optimized for high electrical-to-optical power conversion efficiency. There are numerous key contributors such as scattering and absorption losses, band alignment, Joule heating, carrier leakage and below-threshold losses that contribute to power loss mechanisms. We report on improvement from 50% to a record-high 73% power conversion efficiency for a 1 cm bar at 10C, resulting from a multi-pronged approach that has been taken to minimize each of the loss mechanisms as to improve the overall power conversion efficiency.
Long term lifetest data is presented for Al-free active region 980 nm multimode laser diodes configured as chip-on-submount devices, as packaged fiber-coupled devices, and as multi-emitter laser bars on a microchannel cooled heatsink. Single emitter devices have been tested in chip-on-submount form. A first set of 120 of these devices were tested in a five-cell matrix at varying junction temperatures and optical output levels to obtain measured values for both random and wear-out failure model parameters. A second set of 187 packaged lasers were placed on accelerated lifetest to measure FIT data. In both cases, the devices were operated for up to 9,000. Another set of chips were packaged and tested inside a fiber-coupled, TEC cooled, 14 pin butterfly case as part of a Telcordia qualification process. These devices were operated for up to 5,000 hours with no failures and no degradation of either the chip or the package. Bar devices with a 20% fill factor were mounted on microchannel heatsinks and tested for one second on, one second off quasi CW operation for 4,000 hours. This test condition places a thermal expansion cycle stress on the devices, however once past the initial burn-in period very little degradation is seen in the output characteristics of the device.
While high-powered broad area lasers emitting between 915nm and 975nm are required for pumping Er+ and Yb+ doped dual clad fiber lasers and amplifiers, the single mode 980nm lasers are used for pumping EDFAs. We report on the performance and a systematic reliability assessment of Alfalight’s first generation Al-free multimode laser diodes with 100µm aperture and 2mm cavity length emitting between 950nm and 980nm. Data from 120 devices in five different multi-cell conditions show median life due to wear-out failure to be over 75.5 years. In addition, over 1,307,600 device-hours of accelerated lifetest data at 3A and a 70C heatsink temperature have been accumulated demonstrating 55 FIT (60% confidence level) at a 2W and 25C operation condition. We also present results from a packaged multimode diode laser with wavelength stabilized at 972nm with a spectral FWHM of 0.3nm demonstrating the capability to use such a device for pumping Er+ and Yb+ doped fibers near the more efficient 975nm portion of the absorption spectrum. Advances made in anti-resonant reflective optical waveguide (ARROW) type single mode diode lasers and the advantages over the conventional positive index guided ridge waveguide type lasers will be discussed. Single mode operation of ARROW single mode laser up to 450mW (ex-facet) was achieved. Results from the facet passivation studies showing successful implementation of non-absorbing mirror (NAM) due to quantum well intermixing using Si implantation in Al-free diode lasers will also be discussed. We have demonstrated reliable operation in excess of 5500 hours in index-guided Al-free diode lasers at a constant power of 500mW at a heatsink temperature of 25C.
Design issues relating to high-power broad-area semiconductor lasers are reviewed. Device optimization is performed using a phenomenological model. This model provides a simple means of predicting the laser performance and assessing a high-power laser design. Emphases are placed on parameters that impact the power conversion efficiency,junction temperature, optical intensity, and near field uniformity.
This paper focuses on the laser diode performance. The devices of interest operate in the spectral range of 800 nm to 915 nm. Some fabrication aspects of highly efficient laser diodes are drawn. Brief comparison of two major material systems is presented.
Realization of microelectromechanically wavelength tunable Fabry-Perot filters using high index-contrast distributed Bragg reflectors (DBRs) comprising GaAlAs/AlOx and Si/SiO2 material system are reported. Due to the broadband nature of these high index-contrast DBRs, the 3dB transmission bandwidth of the cavity resonance is narrow and stable over the tuning range. While the three different sets of GaAlAs/AlOx based filters with different number of DBRs exhibited linewidths of 0.5nm, 2.0nm and 0.47nm with tuning ranges of 59nm, 83nm, and 60nm respectively, the silicon-based filter exhibited a linewidth of 0.3nm and a tuning range of 12nm. Transmission spectra from these devices displayed varying magnitudes of higher order spatial modes were attributed to lensing effect caused by partially oxidized AlGaAs layers within the mirror layers. One of the GaAlAs filters showed a frequency response of 500 KHz at 3dB cutoff point indicating a switching time of 2 microseconds.
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