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.
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.
The problem of high-brightness, narrow line semiconductor lasers sources is important for different kinds of applications. The proposed solution of the problem is the use of an external cavity with volume Bragg grating for effective angular and spectral selection. High-efficient volume Bragg gratings provide complete selection directly in space of wave vectors and serve as a diaphragm in angular space. The condition of effective selection is the provision of a substantial difference in losses for a selected mode by matching angular selectivity of a Bragg grating with divergence of the selected mode. It was proposed off-axis construction of an external cavity with a transmitting volume Bragg grating as an angular selective element and a reflecting volume Bragg grating as a spectral selective feedback. In such external cavity broad area laser diodes have shown stable near-diffraction limited generation in the wide range of pumping current. For LD with 0.5% AR-coated mirror and 150 μm stripe it was achieved 1.7 W output power with divergence of 0.62° at current exceeding six thresholds. Total LD slope efficiency in the considered external cavity is less then slope efficiency of free running diodes by 3-5% only. Spectral width of such locked LD emission was narrowed down to 250 pm in the whole range of pumping current.
Interest is rapidly growing in solid-state lasers emitting from 1500-nm to 2100-nm with applications in eye-safe range finding, LIDAR, infrared countermeasures, medicine, dentistry, and others. Traditionally, these solid-state lasers have been pumped by flash lamps or more recently, by semiconductor diode lasers. In the case of the latter, the diodes of choice have been those emitting below 1-μm. The sub-micron class of semiconductor diode lasers is highly mature and has enjoyed recent rapid advances in power and efficiency. Unfortunately, the quantum defect generated when converting to the desired wavelengths results in large amounts of excess heat generation leading to costly and heavy, expensive cooling systems and performance problems related to thermal lensing. System complexity adds further cost and weight when intermediaries, such as optical parametric oscillators, are required to reach the desired longer wavelengths. Recent advances in laser diodes emitting from 1400-nm to over 1900-nm now enable the near resonant pumping of such solid state media as Er:YAG, Ho:YAG and Cr:ZnSe. Record results in the peak output power and electrical-to-optical conversion efficiency of diode lasers emitting around 1470-nm, 1700-nm and 1900-nm are presented here.
Paul Crump, Jun Wang, Steve Patterson, Damian Wise, Alex Basauri, Mark DeFranza, Sandrio Elim, Weimin Dong, Shiguo Zhang, Mike Bougher, Jason Patterson, Suhit Das, Mike Grimshaw, Jason Farmer, Mark DeVito, Rob Martinsen
Peak optical power from single 1-cm diode laser bars is advancing rapidly across all commercial wavelengths. Progress in material performance is reviewed and we show that current trends imply there is no fundamental barrier to achieving peak powers of 1-kW per 1-cm diode laser bar. For bars with such high peak powers, commercially available reliable devices would be expected to deliver ~ 300-W per bar. 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 and 24W at 1900-nm, complementing the 100-W bar previously demonstrated at 1470-nm. Peak power is seen to correlate closely peak 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 73% efficient 100-W bars on copper micro-channel) and a reduction in packaging thermal resistance by optimizing micro-channel performance (leading to < 0.2-oC/W thermal resistance).
High power GaAs-based high power diode bars produce wavelengths in the range of 780 to 980 nm and are widely used for pumping a broad range of rare earth doped solid-state lasers. As the markets for these laser systems mature, diode lasers that operate at higher power levels, greater overall efficiency, and higher reliability are in high demand. In this paper we report efficiencies of over 70% in the 9xx-nm band, continuous wave power levels over 340 Watts in the 8xx-nm band, and reliability data at or above 100 Watts. We will also review the latest advances in performance and detail the basic physics and material science required to achieve these results.
Operation of 808-nm laser diode pumps at elevated temperature is crucial to many applications. Reliable operation at high power is limited by high thermal load and low catastrophic optical mirror damage (COMD) threshold at elevated temperature range. We demonstrate high efficiency and high power operation at elevated temperatures with high COMD power. These results were achieved through device design optimization such as growth conditions, doping profile, and materials composition of the quantum-well and other layers. Electrical-to-optical efficiency as high as 62 percent was obtained through lowered threshold current and lowered series resistance and increased slope efficiency. The performance of single broad-area laser diodes scales to that of high power single bars on water-cooled copper micro-channel heatsinks or conductively-cooled CS heatsinks. No reduction in bar performance or significant spectral broadening is seen when these micro-channel coolers are assembled into 6-bar and 18-bar cw stacks for the highest power levels.
We have demonstrated record high cw and quasi-cw operation of InP-based 1.5 μm laser arrays (bars) and water-cooled stacks. High-efficiency and high-power operation were achieved through device design optimization including the multi-quantum well design, crystal growth process, doping profile, and material composition. Internal quantum efficiency, mode loss, gain parameters, and temperature sensitivity parameters are reported. Single-stripe devices produced 3 watts of cw output power and 35 percent electrical-to-optical efficiency. We demonstrated 40 watts of cw power from single bars on water-cooled copper-microchannel heatsinks. A stack of 20 bars that were collimated using fast axis microlenses achieved greater than 350 watts of cw power.
We describe the wavelength tunability and conversion efficiency of 532-nm pulse pumped optical parametric oscillators (OPOs) using periodically pulsed lithium tantalate (PPLT). The OPOs reported here used PPLT crystal lengths of 1,2 and 4 cm with signal wavelengths between 660 and 950 nm. These OPO's were pumped with a pump pulse energy of 100 (mu) J at 1 kHz yielding internal slope efficiencies of almost 90 percent and pump depletions of over 70 percent. Average power scaling experiments were also performed with a pump pulse energy of 66 (mu) J at 33 kHz yielding internal slope efficiencies of 50 percent.
We have developed an x-ray telescope that uses a new technique for focusing x-rays with grazing incidence optics. The telescope was built with spherical optics for all of its components, utilizing the high quality surfaces obtainable when polishing spherical (as opposed to aspherical) optics. We tested the prototype x-ray telescope in the 300 meter vacuum pipe at White Sands Missile Range, NM. The telescope features 2 degree graze angles with tungsten coatings, yielding a bandpass of 0.25-1.5 keV with a peak effective area of 0.8 cm2 at 0.83 keV. Results from x-ray testing at energies of 0.25 keV and 0.93 keV (C-K and Cu-L) verify 0.5 arcsecond performance at 0.93 keV. Results from modeling the x-ray telescope's response to the SUn show that the current design would be capable of recording 10 half arcsecond images of a solar active region during a 300 second NASA sounding rocket flight.
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