We describe the performance and reliability of multi-bar diode stacks assembled with hard solder attachment of the laser
diode bar to the conduction-cooled package substrate. The primary stack package design is based on a modular platform
that makes use of common piece parts to incorporate anywhere from 2-7 bars, operating at peak powers of 80W/bar to
200W/bar. In assembling monolithic type diode stack packages, it is typical to use a soft solder material such as indium
for P-side bar attachment into the package. Due to its low melting point and low yield stress, indium can provide a solder
joint that transfers low stress to the laser bar. However, during CW and QCW operation, indium is prone to migration
that can cause device failure due to a number of well-known mechanisms. This shortcoming of soft-solder bar
attachment can limit the number of shots the stack delivers over its operating life. By replacing the soft solder typically
used for P-side attachment with a hard solder, it is possible to greatly reduce or eliminate certain failure modes, thereby
increasing the operating life of the part. We demonstrate lifetime of > 1E9 shots at 80 W/bar, 250 us/40 Hz pulses, and
50C package operating temperature.
We describe the performance and reliability of high power vertical diode stacks based on silicon monolithic microchannel
coolers (SiMMs) operating at >1000W/cm2 CW at 808 and 940nm. The monolithic nature of these stacks makes
them inherently robust and compact. Typical emitting dimensions for a 10-bar stack are ~8.8mm × 10mm with CW
output power up to 1.5kW. Originally developed at Lawrence Livermore National Laboratory and now actively being
developed for commercial applications at Coherent, this technology offers several advantages over current copper-based
micro-channel coolers. These devices do not require use of DI water, strict monitoring and control of the pH level,
careful control of the water velocity, or sealed cooling systems. The need for hydrostatic seals is also drastically reduced.
A typical ten bar stack requires only 2 o-ring seals, compared to 20 such seals for a similar stack using copper microchannel
cooling. Mature and readily available wet etching technology allows for cost effective batch fabrication of the
sub-mount structure while achieving repeatable high precision components based on photolithographic fabrication
processes.
Manufacturers of Nd:YAG lasers continue to demand 808 nm pump sources that deliver ever lower operating costs (measured in $/kW-hour). Responding to this demand, Coherent has developed a new generation of high power, 808 nm laser bars. These lasers are most ideal for high power QCW applications, but also perform very well in CW pumping applications. The key to the improved power for QCW bars is increase in catastrophic optical damage (COD) threshold. Through a combination of advances in epitaxial structure design and coating technology after aging COD limit for new generation of bars has been increased by 40%. This allowed us to achieve reliable QCW operation at 270W of peak power. Life test results shows that lifetime of these bars at these conditions exceed 2e9 shots. We also developed similar structure optimized for CW operations. When mounted on micro-channel water cooled packages CW bars operate reliably at an output power of 150 W. Highest power conversion efficiency (PCE) for CW bars was more
then 55% with typical PCE value >50%.
The 880 nm diode laser is emerging as the source of choice for pumping Nd:YV04 laser crystals because it offers higher pumping efficiency than 808 nm diode lasers[1]. This paper reports on recent progress in the development of high power, high reliability, 880 nm laser bars. Specifically, high performance has been achieved based on Coherent's aluminum-free active (AAA) epitaxial structures while maintaining lifetimes greater than 10,000 hours. This includes 30% fill factor, 1 cm bars on conductively cooled packages (CCP) operating at 51 W with proven manufacturability. We observed power conversion efficiency (PCE) of up to 56%. These lasers have a far field fast axis divergence of 32° (FWHM), and slow axis divergence of <7° (FWHM). Typical value of the FWHM of output spectrum is 2.5 nm. These bars were used to build fiber array packages (FAPs) operating at 45 W. We have achieved FAP PCE of 50% and numerical aperture of <0.12. Reliability of both bars and FAP was shown to exceed 10000h MTBF.
Solid-state-laser and fiber laser pumping, reprographics, medical and materials processing applications require high power, high-brightness bars and fiber-coupled arrays. Conductively cooled laser diode bars allow customers to simplify system design and reduce operational size, weight, and costs. We present results on next generation high brightness, high reliability bars and fiber-coupled arrays at 790-830 nm, 940 nm and 980 nm wavelengths. By using novel epitaxial structures, we have demonstrated highly reliable 808 nm, 30% fill-factor conductively cooled bars operating at 60W CW mode, corresponding to a linear power density (LPD) of 20 mW/μm. At 25°C, the bars have shown greater than 50% wall-plug-efficiency (WPE) when operating at 60W. Our novel approach has also reduced the fast-axis divergence FWHM from 31° to less than 24°. These bars have a 50% brightness improvement compared to our standard products with this geometry. At 980nm, we have demonstrated greater than 100W CW from 20% fill-factor conductively cooled bars, corresponding to a LPD of 50 mW/µm. At 25°C, the WPE for 976nm bars consistently peaks above 65% and remains greater than 60% at 100W. We coupled the beam output from those high-brightness bars into fiber-array-packages (“FAPs”), and we also achieved high-brightness and high-efficiency FAPs. We demonstrated 60W from a 600µm core-diameter fiber-bundle with a high WPE of 55%, and a low numerical aperture of 0.115. The brightness of such FAPs is four times higher than our standard high-power 40W FAP products at Coherent. Ongoing life test data suggests an extrapolated lifetime greater than 10,000 hours at 80W CW operating-condition based on 30%FF conductively cooled bar geometry.
Directly modulated External Cavity Laser Diode (ECLD) is an attractive alternative for extended reach source lasers for metro area telecommunications. This paper reports ECLD with transmission rate of 2.5Gbit/s and transmission distance of 650 km packaged in industry standard 14pin butterfly package. High stability package was the key in achieving Telcordia qualification. Combination of narrow linewidth and high bandwidth allows OC-48 transmission for distances comparable with the ones typical for EML. This lasers exhibit +/-30 pm wavelength stability over temperature range of -15 to +75C which makes it suitable for 100 and 50 GHz spacing applications without wavelength locker. ECLD can be operated using standard laser driver. Results using Vitesse 7940 and Analog Devises 2841 laser driver are presented. Typical extinction ratio obtained with these drivers was 10dB. Peak to peak jitter was <40ps. Dispersion penalty with these drivers transmission was found to be <1.9dB for the 650km of standard SMF-28 fiber (No dispersion compensation). Dispersion penalty vs transmission distance data will be presented. Results of the study of nonlinear effects in multi channels link will be reported. We observed that there was self phase modulation effect leading to reduction in dispersion power penalty. This effect can be used to transmit single channel over distances longer then 650km. We showed that there was negligible crosstalk penalty and no FWM penalty. Threshold for stimulated Brillouin scattering was also investigated.
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