We have demonstrated a pulsed 1064 nm PM Yb:fiber laser system incorporating a seed source with a tunable pulse repetition rate and pulse duration and a multistage fiber amplifier, ending in a large core (>650 μm2 mode field area), tapered fiber amplifier. The amplifier chain is all-fiber, with the exception of the final amplifier’s pump combiner, allowing robust, compact packaging. The air-cooled laser system is rated for >60 W of average power and beam quality of M2 < 1.3 at repetition rates below 100 kHz to 10’s of MHz, with pulses discretely tunable over a range spanning 50 ps to greater than 1.5 ns. Maximum pulse energies, limited by the onset of self phase modulation and stimulated Raman scattering, are greater than 12.5 μJ at 50 ps and 375 μJ at 1.5 ns , corresponding to >250 kW peak power across the pulse tuning range. We present frequency conversion to 532 nm with efficiency greater than 70% and conversion to UV via frequency tripling, with initial feasibility experiments showing >30% UV conversion efficiency. Application results of the laser in scribing, thin film removal and micro-machining will be discussed.
We demonstrate a robust, compact, low-cost, pulsed, linearly polarized, 1064 nm, Yb:fiber laser system capable of generating ~100 kW peak power pulses and >17 W average power at repetition rates of 80 – 285 kHz. The system employs a configurable microchip seed laser that provides nanosecond (~1.0 – 1.5 ns) pulse durations. The seed pulses are amplified in an all-fiber, polarization maintaining, large mode area (LMA) fiber amplifier optimized for high peak power operation. The LMA Yb:fiber amplifier enables near diffraction limited beam quality at 100 kW peak power. The seed laser, fiber amplifier, and beam delivery optics are packaged into an air-cooled laser head of 152×330×87 mm3 with pump power provided from a separate air-cooled laser controller. Due to the high peak power, high beam quality, spectral purity, and linearly polarized nature of the output beam, the laser is readily frequency doubled to 532 nm. Average 532 nm powers up to 7 W and peak powers exceeding 40 kW have been demonstrated. Potential for scaling to higher peak and average powers in both the green and infrared (IR) will be discussed. This laser system has been field tested and demonstrated in numerous materials processing applications in both the IR and green, including scribing and marking. We discuss recent results that demonstrate success in processing a diverse array of representative industrial samples.
We report on the development of ultra-high brightness laser diode modules at nLIGHT Photonics. This paper
demonstrates a laser diode module capable of coupling over 100W at 976 nm into a 105 μm, 0.15 NA fiber
with fiber coupling efficiency greater than 85%. The high brightness module has an optical excitation under
0.13 NA, is virtually free of cladding modes, and has been wavelength stabilized with the use of volume
holographic gratings for narrow-band operation. Utilizing nLIGHT's Pearl product architecture, these
modules are based on hard soldered single emitters packaged into a compact and passively-cooled package.
These modules are designed to be compatible with high power 7:1 fused fiber combiners, enabling over
500W power coupled into a 220 μm, 0.22 NA fiber. These modules address the need in the market for high
brightness and wavelength stabilized diode lasers for pumping fiber lasers and solid-state laser systems.
Er:YAG solid state lasers offer an "eye-safe" alternative to traditional Nd:YAG lasers for use in military and industrial
applications such as range-finding, illumination, flash/scanning LADAR, and materials processing. These laser systems
are largely based on diode pumped solid state lasers that are subsequently (and inefficiently) frequency-converted using
optical parametric oscillators. Direct diode pumping of Er:YAG around 1.5 μm offers the potential for greatly increased
system efficiency, reduced system complexity/cost, and further power scalability. Such applications have been driving
the development of high-power diode lasers around these wavelengths. For end-pumped rod and fiber applications
requiring high brightness, nLIGHT has developed a flexible package format, based on scalable arrays of single-emitter
diode lasers and efficiently coupled into a 400 μm core fiber. In this format, a rated power of 25 W is reported for
modules operating at 1.47 μm, with a peak electrical to optical conversion efficiency of 38%. In centimeter-bar on
copper micro-channel cooler format, maximum continuous wave power in excess of 100 W at room temperature and
conversion efficiency of 50% at 6C are reported. Copper heat sink conductively-cooled bars show a peak electrical-to-optical
efficiency of 43% with 40 W of maximum continuous wave output power. Also reviewed are recent reliability
results at 1907-nm.
KEYWORDS: Semiconductor lasers, Diodes, Fiber Bragg gratings, Reliability, Optical design, Solid state lasers, High power lasers, Epitaxy, Nd:YAG lasers, Control systems
We report on recent progress in the control of optical modes toward the improvement of commercial high-performance
diode laser modules. Control of the transverse mode has allowed scaling of the optical mode volume, increasing the
peak output power of diode laser emitters by a factor of two. Commercially-available single emitter diodes operating at
885 nm now exhibit >25 W peak (12 W rated) at >60% conversion efficiency. In microchannel-cooled bar format, these
lasers operate >120 W at 62% conversion efficiency. Designs of similar performance operating at 976 nm have shown
>37,000 equivalent device hours with no failures. Advances in the control of lateral modes have enabled unprecedented
brightness scaling in a fiber-coupled package format. Leveraging scalable arrays of single emitters, the conductively-cooled
nLIGHT PearlTM package now delivers >80 W peak (50 W rated) at >53% conversion efficiency measured from
a 200-μm core fiber output and >45 W peak (35 W rated) at >52% conversion efficiency measured from a 100-μm fiber
output. nLIGHT has also expanded its product portfolio to include wavelength locking by means of external volume
Bragg gratings. By controlling the longitudinal modes of the laser, this technique is demonstrated to produce a narrow,
temperature-stabilized spectrum, with minimal performance degradation relative to similar free-running lasers.
KEYWORDS: Semiconductor lasers, Diodes, Laser applications, Infrared countermeasures, Solid state lasers, High power lasers, Defense and security, Directed energy weapons, Aerospace engineering
Concepts and markets of semiconductor diode lasers are introduced in the context of Defense and
Security applications. Specific, high profile applications are reviewed and current and future diode laser
technology is discussed. Advanced diode laser technology as it pertains to Infrared Countermeasures
applications is reviewed in detail along with recent 2μm diode laser results and advanced packaging
product architectures from nLIGHT.
A conductively cooled laser diode package design with hard AuSn solder and CTE matched sub mount is presented. We discuss how this platform eliminates the failure mechanisms associated with indium solder. We present the problem of catastrophic optical mirror damage (COMD) and show that nLight's nXLTTM facet passivation technology effectively eliminates facet defect initiated COMD as a failure mechanism for both single emitter and bar format laser diodes. By combining these technologies we have developed a product that has high reliability at high powers, even at increased operation temperatures. We present early results from on-going accelerated life testing of this configuration that suggests an 808nm, 30% fill factor device will have a MTTF of more than 21khrs at 60W CW, 25°C operating conditions and a MTTF of more than 6.4khrs when operated under hard pulsed (1 second on, 1 second off) conditions.
Focused development under the DARPA SHEDs program has lead to extremely high power conversion efficiency in the
9xx-nm wavelength band, leading to bars with efficiency in excess of 74%. We review progress in advancing efficiency
and detail the route to > 85% at room temperature. The 9xx-nm wavelength band is commercially used for pumping
Ytterbium-doped solid-state crystals and fiber lasers - only one of many diode laser markets. Fortunately, the lessons
learned under SHEDs are transferable to other wavelengths. We report breakthrough efficiency results in the 8xx-nm
band, for example showing 71% power conversion efficiency from 790-nm bars at powers > 100-W for CW and QCW
packaging and testing. These wavelengths are required for pumping Neodymium-doped crystals, as used in the majority
of fielded high power Diode Pumped Solid-State Laser systems. High efficiency is delivered using low voltage SHEDs
designs, in combination with work to optimize the performance of the quantum well.
Diode lasers supply high power densities at wavelengths from 635-nm to 2000-nm, with different applications enabled
by providing this power at different wavelengths. As the range of available wavelengths broadens, many novel medical
and atmospheric applications are enabled. Traditional quantum well lasers provide high performance in the range 635-
nm to 1100-nm range for GaAs-based devices and 1280-nm to 2000-nm for InP, leaving a notable gap in the 1100 to
1280-nm range. There are many important medical and sensing applications in this range and quantum dots produced
using Stranski-Krastanow self-organized MBE growth on GaAs substrates provide an alternative high performance
solution. We present results confirming broad area quantum dot lasers can deliver high optical powers of 16-W per
emitter and high power conversion efficiency of 35% in this wavelength range. In addition, there are growing
applications for high power sources in wavelengths > 1500-nm. We present a brief review of our current performance
status in this wavelength range, both with conventional quantum wells in the 1500-nm to 2500-nm range and MOCVD
grown quantum cascade lasers for wavelengths > 4000-nm. At each wavelength, we review the designs that deliver this
performance, prospects for increased performance and the potential for further broadening the availability of novel
wavelengths for high power applications.
Peak optical power from single 1-cm diode laser bars is advancing rapidly across all commercial wavelengths. Progress
to date has allowed us to demonstrate > 400-W peak output from single 1-cm diode laser bars at emission wavelengths
from 800-nm to 980-nm. The available range of emission wavelengths has also been increased, with 90-W bars shown at
660-nm, 37W at 1910-nm and 25W at 2070-nm, complementing the 100-W bar previously demonstrated at 1470-nm.
Peak power is seen to correlate closely peak power conversion efficiency. Further advances in diode laser efficiency and
low thermal resistance packaging technology continue to drive these powers higher. The most critical improvements
have been the reduction in the diode laser operating voltage through optimization of hetero-barriers (leading to 74%
efficient 100-W bars on micro-channel at 975-nm) and a reduction in packaging thermal resistance by optimizing microchannel
performance (leading to < 0.2-°C/W thermal resistance). We have also recently extended our high efficiency
designs to shorter wavelengths, now delivering over 70% efficiency at 790-nm. Ever-increasing power levels (projected
to eventually exceed 1-kW per bar) reduce the cost in Euro per W of diode laser systems, enabling broader application in
military, industrial and medical markets. In addition, increasing availability of high powers at new wavelengths is
enabling many new applications.
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