Most Pulsed Fiber Lasers (FLs) are built on a Master Oscillator - Power Amplifier (MOPA) architecture, as this
configuration has the advantage, among others, of exploiting direct modulation of the diode laser seed (the MO) to reach
high repetition rates and high peak-power pulsed operation. To enhance the FL global performance and reliability, high
power single-lateral-mode 1064 nm diodes with outstanding long-term behavior are needed. The reliability of these
devices at high power has been a challenge for years, due to the high built-in strain in the Quantum Well (QW). In this
paper, we present excellent reliability results obtained, in both cw and pulsed conditions, on the latest generation of 1064
nm single-lateral-mode diodes developed at 3S PHOTONICS. Aging tests in cw conditions prove the intrinsic robustness
of the diode even at very high junction temperatures, while specific tests in pulsed operation at 45 °C heat-sink
temperature, and high repetition rates of several hundred kHz, confirm the stability of the devices in accelerated
conditions directly derived from real applications. Both free-running and wavelength stabilized (by means of a Fiber
Bragg Grating (FBG)) packaged devices show very stable performances under pulsed conditions. Reliable operation at
higher average power than currently commercially available diode lasers seeds is demonstrated.
We demonstrate very high reliability level on 980-1060nm high-power single-mode lasers through multi-cell tests. First,
we show how our chip design and technology enables high reliability levels. Then, we aged 758 devices during 9500
hours among 6 cells with high current (0.8A-1.2A) and high submount temperature (65°C-105°C) for the reliability
demonstration. Sudden catastrophic failure is the main degradation mechanism observed. A statistical failure rate model
gives an Arrhenius thermal activation energy of 0.51eV and a power law forward current acceleration factor of 5.9. For
high-power submarine applications (360mW pump module output optical power), this model exhibits a failure rate as
low as 9 FIT at 13°C, while ultra-high power terrestrial modules (600mW) lie below 220 FIT at 25°C. Wear-out
phenomena is observed only for very high current level without any reliability impact under 1.1A.
For the 1060nm chip, step-stress tests were performed and a set of devices were aged during more than 2000 hours in
different stress conditions. First results are in accordance with 980nm product with more than 100khours estimated
MTTF. These reliability and performance features of 980-1060nm laser diodes will make high-power single-mode
emitters the best choice for a number of telecommunication and industrial applications in the next few years.
Spatially resolved photo-luminescence (PL) line-scans were performed in a specific optical micro-probe to determine the soldering-induced local stresses in GaAs/GaAlAs laser diode arrays designed for high-power operation at 808 nm. In this approach, the sign and magnitude of the local stress are deduced from the spectral shift associated with band-to-band transitions in the GaAs substrate. The sensitivity (minimal equivalent hydrostatic stress that can be detected) is better than 10 MPa. The spatial resolution of the micro-PL technique (of the order of 1 micrometer), together with the short acquisition times, allows for detailed investigations of the stress profiles along the whole laser bars with a large number of data points. Different aspects of the mechanical stress distribution at the various steps of the process could thus be revealed. Finally, correlations between solder-induced stress distribution and estimated lifetimes were established. In particular, << V-shaped >> defects, which are known as a failure mechanism on this type of devices, were observed only on the laser bars for which the micro-PL indicates the strongest compressive stress. This leads to consider the micro-PL approach proposed here as a cost- effective screening technique for the high-power GaAs/GaAlAs laser diode arrays.
For DPSSL applications at low duty cycle (typically 150 microsecond(s) /20 Hz), we have developed a new concept of `High Brightness' stacked arrays, which leads to power density of 10 kW/cm2, and to 40 kW/cm2 when directly coupled to a lens-duct. The high intensity is made possible thanks to a new proprietary stacking technology that permits a stacking pitch of only 100 micrometers : the principle is to directly stack the arrays without any heat spreaders other than the arrays themselves. When compared to standard commercial QCW stacked arrays, the benefits of using these high brightness laser diodes pumping sources are the following: (1) an important cost reduction related to a drastic simplification in the assembling process, and (2) an improved pumping efficiency associated with an improved brightness (approximately 1 factor 4), consequence of a reduced pitch between linear bar arrays. High-efficiency, frequency-quadrupled, end-pumped 8 mJ, 12-ns-long UV pulses 0.266 micrometers Nd:YAG laser has been developed using these high brightness pump sources. This air-cooled laser is an attractive alterative to the more conventional millijoules-range UV source. A compact high energy 300 mJ Q-switched diode-pumped laser has been developed. Laser performances and integration level demonstration contribute to a preliminary laser design for future airborne laser applications.
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