Laser ignition of energetic materials is an attractive technology for replacement of low energy electro-explosive devices
which pose a safety hazard. The development of this technology has historically been based on go/no-go threshold
testing using off-the-shelf laser diodes and solid state lasers. Here we seek to build a more fundamental understanding of
the laser ignition process by analyzing the interactions and response of the energetic material to the incident laser beam.
We begin with a radiative heat transfer model of the laser-beam-assisted heating of a homogeneous energetic material
with given optical properties. An analytical solution of the 2-flux model equations is developed and this expression for
the volumetric absorption of laser energy in an absorbing and isotropically scattering medium is coupled to the
conservation of energy equation. Two limiting cases-minimum power and minimum energy thresholds for ignition - are discussed, and the minimum energy threshold is calculated directly from the energy equation in the limit of zero
dissipative losses. The effects of power density and beam shape are of particular interest and two common
configurations are analyzed. Although the applicability of thermal models is limited by large uncertainties in the optical
properties of energetic materials, the analysis provides a qualitative understanding of the ignition process and a
correlation between ignition thresholds and the various material properties and design parameters.
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.
In this study, we examine processes limiting the performance of 4 micron superlattice pin photodiodes for different temperature and mesa size regimes. We show that the performance of large mesa photodiodes at low temperature is most severely limited by a trap-assisted tunneling leakage current (x300), while small mesa sizes are additionally limited by perimeter leakage (x20). At room temperature, large mesa photodiodes are limited by the diffusion current, and small
mesa photodiodes are further limited by the perimeter leakage (x100). To reduce or eliminate the impact of perimeter leakage, we have tried passivating the mesa sidewalls with SiN, an approach that was only minimally successful. We have also laid the groundwork for another approach to elimination of perimeter leakage currents, namely, elimination of the sidewalls altogether through planar processing techniques. Planar processing schemes require the deposition of a
thick, wide bandgap semiconductor or "window layer" on top of the homojunction. We compare the performance of two otherwise identical InAs/GaSb superlattice homojunction detectors, except one with a GaSb window layer, and one without. We show that inclusion of the thick GaSb window layer does not degrade detector performance.
In this paper we demonstrate high-power GaAs-based and InP-based superluminescent diodes (SLD) with tilted waveguides emitting in 8xx nm and 15xx nm spectral ranges respectively. The analysis of devices with different cavity lengths emphasizes the tradeoff between output power and spectral width. Power levels of about 200 mW for 820 nm SLDs and about 100 mW for 1590 nm SLDs have been demonstrated for longer cavity devices. Spectral modulation was less than 6-7% at 70-80 mW output power for both 8xx and 15xx SLDs. Simple model proposed for evaluation of spectrum modulation for both GaAs and InP devices based on semi-empirical approach is in agreement with experimental observations.
In this paper we summarize the results on the development of high power 1300 nm ridge waveguide Fabry-Perot and distributed-feedback (DFB) lasers. Improved performance of MOCVD grown InGaAsP/InP laser structures and optimization of the ridge waveguide design allowed us to achieve more than 800 mW output power from 1300 nm single mode Fabry-Perot lasers. Despite the fact that the beam aspect ratio for ridge lasers (30 degree(s) x 12 degree(s)) is higher than that for buried devices, our modeling and experiments demonstrated that the fiber coupling efficiency of about 75-80% could be routinely achieved using a lensed fiber or a simple lens pair. Fiber power of higher than 600 mW was displayed. Utilizing similar epitaxial structures and device geometry, the 1300 nm DFB lasers with output power of 500 mW have been fabricated. Analysis of the laser spectral characteristics shows that the high power DFB lasers can be separated into several groups. The single frequency spectral behavior was exhibited by about 20% of all studied DFB lasers. For these lasers, side-mode suppression increases from 45 dB at low current up to 60 dB at maximum current. About 30% of DFB lasers, at all driving currents, demonstrate multi-frequency spectra consisting of 4-8 longitudinal modes with mode spacing larger than that for Fabry-Perot lasers of the same cavity length. Both single frequency and multi frequency DFB lasers exhibit weak wavelength-temperature dependence and very low relative intensity noise (RIN) values. Fabry-Perot and both types of DFB lasers can be used as pump sources for Raman amplifiers operating in the 1300 nm wavelength range where the use of EDFA is not feasible. In addition, the single-mode 1300 nm DFB lasers operating in the 500 mW power range are very attractive for new generation of the cable television transmission and local communication systems.
Recent progress towards the realization of high-power, non- cryogenic (quasi-)cw mid-IR lasers based on the `W' configuration of the active region is reported. Type-II diodes with AlGaAsSb broadened-waveguide separate confinement regions are the first III-V interband lasers to achieve room-temperature pulsed operation at a wavelength longer than 3 micrometers . For cw operation, Tmax was 195 K and Pout equals 140 mW was measured at 77 K. Optically- pumped W lasers recently attained the highest cw operating temperatures (290 K) of any semiconductor laser emitting in the 3 - 6 micrometers range. For a (lambda) equals 3.2 micrometers device at 77 K, the maximum cw output power was 0.54 W per uncoated facet. In order to maximize the absorption of the pump in the active region, an optical pumping injection cavity structure was used to create an etalon cavity for the 2.1 micrometers pump beam. The pulsed incident pump intensity at threshold was only 8 kW/cm2 at 300 K for this edge- emitting mid-IR laser. The differential power conversion efficiency was 9% at 77 K and 4% at 275 K, which indicates promising prospects for achieving high cw output powers at TE-cooler temperatures following further optimization.
In this work the spectral characteristics of a new type of mid-infrared diode laser are discussed and an application for CO trace gas detection is demonstrated. The InGaAsSb/AlGaAsSb QW diode lasers operating in the spectral range of 2.0 - 2.7 micrometer in continuous wave (CW) regime at room temperature (RT) were developed last year. Earlier, the spectral range of RT CW operation for diode lasers was limited by 2.0 - 2.1 micrometer. The extension of wavelength to 2.7 micrometer was achieved for InGaAsSb/AlGaAsSb quantum well (QW) lasers by employing for QWs new quasi-ternary InGaSb(As) compositions that are out of the miscibility gap for InGaAsSb materials. Single spatial mode ridge lasers emitting at 2.2 - 2.7 micrometer have parameters similar to those of the infrared lasers with (lambda) less than 2 micrometer widely used for spectroscopic application. At operating currents about 80 - 200 mA and temperatures up to +50 degrees Celsius, these lasers emit CW output power of several milliwatts. Investigation of the laser spectra has revealed the current and temperature ranges where a single longitudinal mode dominates with side mode suppression of 22 - 25 dB. The dominant mode can be tuned in wavelength by varying current or temperature. The lasers were used to record high-resolution CO absorption lineshapes (2v band near 2.3 micrometer) in a static cell (14.9-cm path). Probed CO transitions were selected for applications to in situ measurements in high- temperature combustion flows. In general, the measured CO absorption lineshapes agreed with theoretical Voigt profiles calculated using the HITRAN database to within 2%. For a minimum detectable absorbance of 0.01% and a 1-meter long path, the CO measurement sensitivity for the probed R30 transition near 2.302 micrometer was 5 - 10 ppm at 1000 K. This value is about two orders of magnitude better than the sensitivity reported for CO detection with conventional diode lasers that probe transitions in the 3v band near 1.56 micrometer.
In the work we continue our studies of broadened waveguide separate confinement InGaAsSb/AlGaAsSb quantum well diode lasers grown by MBE on n-GaSb substrates. To avoid the structure degradation associated with the miscibility gap in the 2.3 - 2.7 micrometer wavelength range, we used highly strained, 'quasi-ternary' InxGa1-xSb1-yAsy compounds with 0.25 < X < 0.38 and y approximately equals 0.02 as the material for QWs. From spontaneous emission measurements we have identified that the Auger process determines the rate of recombination in quantum well active region over the entire temperature range studied (15 - 110 degrees Celsius) for 2.6 micrometer lasers and at temperatures higher than 65 degrees Celsius for 2.3 micrometer lasers. Under these conditions, strong temperature dependence of Auger coefficient leads to the rapid increase of threshold current density with temperature (T0 approximately 40 degrees Celsius). In the range of 15 - 65 degrees Celsius for 2.3 micrometer devices we believe monomolecular non-radiative mechanism dominates and T0 is about 110 degrees Celsius. In addition, single-mode CW room temperature ridge-waveguide lasers with wavelength of 2.3 - 2.55 micrometer have been fabricated for the first time. The lasers display threshold currents around 50 mA with CW output powers of several milliwatts. Switching of the peak lasing position has been observed for both CW and pulsed operation and is related to second sub-band transitions. These results show that excess carrier energy distribution and their concentration are current dependent above threshold.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.