We present an analysis of a reliability assessment tailored specifically to fuzes based on laser diode assemblies. Fuzes
are required to deliver high energy in a single short pulse (micro- to milliseconds) after prolonged storage (tens of years)
in thermally non-stabilized environments. The temperature variation could easily exceed 100 degrees, and the transition
from one extreme to the other could be slow or rapid, depending on a particular application. The operating requirements
for diode laser fuzes are dramatically different from the majority of other diode laser applications and thus a reliability
assurance program for laser fuzes should reflect these differences in usage. In this paper we demonstrate that it is
possible to build accelerated aging conditions based on thermal cycling. As parameters in the accelerated thermal aging,
we used the total temperature difference between the lowest and the highest points in the cycle, and the average rate of
temperature change between the extreme points. This accelerated aging technique based on thermal cycling can predict
the performance deterioration over time after storage in thermally non-stabilized environments. The basis of this
approach can be extended to the analysis of reliability in environments with high vibration and radiation levels.
With the maturing of high-power diode laser technology, studies of laser-assisted ignition of a variety of substances are
becoming an increasingly popular research topic. Its range of applications is wide - from fusing in the defense,
construction and exploration industries to ignition in future combustion engines. Recent advances in InP-based
technology have expanded the wavelength range that can be covered by multi-watt GaAs- and InP-based diode lasers to
about 0.8 to 2 μm. With such a wide range, the wattage is no longer the sole defining factor for efficient ignition.
Ignition-related studies should include the interaction of radiation of various wavelengths with matter and the reliability
of devices based on different material systems. In this paper, we focus on the reliability of pulsed laser diodes for use in
ignition applications. We discuss the existing data on the catastrophic optical damage (COD) of the mirrors of the GaAsbased
laser diodes and come up with a non-destructive test method to predict the COD level of a particular device. This
allows pre-characterization of the devices intended for fusing to eliminate failures during single-pulse operation in the
field. We also tested InP-based devices and demonstrated that the maximum power is not limited by COD. Currently,
devices with >10W output power are available from both GaAs- and InP-based devices, which dramatically expands the
potential use of laser diodes in ignition systems.
One of the recent advances in solid-state laser (SSL) defense technology is the 100W level Er-doped "eye-safe" laser
with low quantum defect pumping at 1.53μm. Major technical challenges in achieving high-wattage devices include
increasing the system power conversion efficiency and arranging the removal of heat from both the crystal and the
pumps. It is known that performance of the crystal can be improved dramatically by cryogenic cooling. Hence, it is
desirable to have cryo-cooled pumps to realize ergonomic and efficient diode-pumped SSL with unified cryogenic
cooling.
In this paper we report on the development of LN2-cooled InP-based λ~1.5-1.6 μm diode pumps. The broad area lasers
demonstrated 11W in continuous-wave (CW) regime at an operating current of 20A. Despite the highest CW power
measured to date from an InP-based emitter, we did not observe catastrophic optical mirror damage. The spectral width
of the radiation from a cooled device decreased 1.5-2 from its room-temperature value, which will significantly improve
pumping efficiency.
We show that laser diode design has to be optimized for performance at cryogenic temperatures. Reviewing the data on
LN2 cooled lasers emitting in the wavelength range of 1.13 - 1.8 μm, we discuss the route to increase the power
conversion of the LN2 cooled InP-based pumps to greater than 60% and further narrow and stabilize the laser emission
spectrum.
Recent progress in rare-earth doped fibers has allowed Yb-doped fiber lasers to be power-scaled to several kW's.
Remarkably, the continued rise of the fiber laser output power into multi-kW range is being limited by the pump diodes
rather than the fibers themselves.
In this article we discuss our recent progress in the development of high-brightness fiber-coupled laser diode modules for
pumping Yb and Er doped lasers. Pumps based on laser diode arrays as well as on multiple single emitter platforms will
be described. The prospects of power scaling as well as expected limitations to different designs will be discussed.
We demonstrate 976 nm pump module with 55W ex-fiber output power from 105 μm core diameter fiber. The coupling
efficiency was 58%. Similar approach was used for realization of 1450 nm diodes and as a result over 15 W CW power
was achieved from the fiber with the same aperture.
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