This PDF file contains the front matter associated with SPIE Proceedings Volume 6662, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.
The design and characterization of small, ruggedized laser and optical subsystems is required for the continued
development of robust laser driven optical firing systems. Typically, these subsystems must be capable of generating the
needed optical energy, delivering that energy via fiber-optical cables and occupying a volume as small as possible. A
novel beam splitting and fiber injection scheme has been proposed as a way to further reduce the volume of an earlier
laser-optical firing systems. The new design approach incorporates two diffractive optical components; a beam
homogenizer and a grating-splitter. Other modifications incorporated into this system included a custom designed fiber
optical connector ferrule, capable of holding three optical fibers, a laser subsystem utilizing a dual-flashlamp pumping
scheme and changes to the power supply to effectively drive the new flashlamp assembly. A set of prototype hardware
was obtained and initial assembly and characterization has begun. The proposed alignment strategy will be discussed
along with expected performance in the areas of: laser beam splitting efficiency, fiber injection efficiency, channel-to-channel
energy balance, air breakdown margin and fiber-optical cable interchangeability. The information and
performance obtained to date will be compared and contrasted with that from earlier systems on which this newly
designed prototype is based. These earlier or "Baseline" systems, utilized more traditional beam splitting and fiber
injection schemes. Earlier systems successfully utilized partially reflecting, dielectric coated mirrors, for beam splitting,
plano-convex lenses, for fiber injection and discrete SMA-905 connectors fiber-optical cables.
A growing number of applications involve the transmission of high-intensity laser pulses through optical fibers.
Previously, our particular interests led to a series of studies on single-fiber transmission of Q-switched, 1064 nm pulses
from multimode Nd:YAG lasers through step-index, multimode, fused silica fibers. The maximum pulse energy that
could be transmitted through a given fiber was limited by the onset of laser-induced breakdown or damage. Breakdown
at the fiber entrance face was often the first limiting process encountered, but other mechanisms were observed that
could result in catastrophic damage at either fiber face, within the initial "entry" segment of the fiber, and at other
internal sites along the fiber path. These studies examined system elements that can govern the relative importance of
different damage mechanisms, including laser characteristics, the design and alignment of laser-to-fiber injection optics,
fiber end-face preparation, and fiber routing. In particular, criteria were established for injection optics in order to
maximize margins between transmission requirements and thresholds for laser-induced damage. Recent interests have
led us to examine laser injection into multiple fibers. Effective methods for generating multiple beams are available, but
the resulting beam geometry can lead to challenges in applying the criteria for optimum injection optics. To illustrate
these issues, we have examined a three-fiber injection system consisting of a beam-shaping element, a primary injection
lens, and a grating beamsplitter. Damage threshold characteristics were established by testing fibers using the injection
geometry imposed by this system design.
Exposure of optical materials to transient-ionizing-radiation fields can give rise to transient and/or
permanent photodarkening effects. In laser materials, such as YAG, such induced optical loss can result in
significant degradation of the lasing characteristic of the material, making its selection for optical device
applications in radiation environments unfeasible. In the present work, the ionizing-radiation response of
Nd:YAG laser rods of varying composition and microstructure are examined. The optical properties of the
materials are examined using a variety of optical spectroscopies and observations are correlated with the
results of the ionizing-radiation studies. It is found that radiation damage in these materials is strongly
influenced by the material microstructure.
The characterization of laser-optical subsystems packaged for survivability in harsh environments is crucial for the
development of robust laser-optical firing systems. Previously, custom mounts and bonded optical assemblies were
environmentally tested to ensure their survivability1. The results verified the sub-assemblies would enable the design of
a laser-optical initiation system that could be fielded for use in extreme conditions.
A design package, which utilized the proven opto-mechanical sub-assemblies, was selected. This design was based on
past experience and desired performance criteria. The packaged laser-optical assembly was tested to the same
environmental levels as the sub-assemblies. This test regiment encompassed the harshest environments currently
utilized. Temperature tests were performed ranging from a maximum of +75 degrees C to a minimum of -55 degrees C,
allowing for a two hour soak at each temperature set point. Vibration tests were performed to a maximum level of 15.5
grms for forty seconds in each of three critical axes. Shock tests were performed until failure which was an impulse
level of 5700 G's with a 1.1 millisecond long pulse. The laser-optical assembly was visually inspected and functionally
tested before and after each test to verify survival. As designed, the system was intended to be hermetically sealed via
laser welding. Therefore the visual inspection of the interior was performed post mortem. Experimental results obtained
from the environmental tests will be discussed as will their impact on future packaging strategies.
A firing set capable of charging a 0.05 μF capacitor to 1.7 kV is constructed using a 2.5 mm diameter Series Connected
Photovoltaic Array (SCPA) in lieu of a transformer as the method of high voltage generation. The source of illumination
is a fiber coupled 3 W 808 nm laser diode. This paper discusses the performance and PSpice modeling of an SCPA used
in a firing set application.
The development and improvement of holographic sight is continuing with better accuracy, reduced size and greater
simplicity. In this work, for a given requirement, a holographic sight was designed and developed to get an optimum
performance of the parameters like: range, field of view, elevation, and windage adjustments of the stable image of the
reticle for zeroing in on the target. It is achieved by considering the critical aspects of optical, mechanical and electronic
circuitry for controlled light output in the device. The design incorporates a laser diode light source and a pair of
holographic elements (reticle and collimator) to make the sight achromatic, thus avoiding sighting errors due to laser
diode wavelength drift. The complete assembly of the holographic sight consists of only four components namely, laser
diode, holographic collimator, plane mirror and the reticle hologram. The virtual image of the reticle can be adjusted to
coincide with the impact point of the bullet by adjusting the laser diode position.
Laser initiation of energetic materials has been a topic of interest almost since the invention of the first laser in 1960.
Since then, a wide range of lasers, and an even wider range of energetic materials, ranging from sensitive primary
explosives such as lead azide, to very insensitive explosives such as Triamino Trinitrobenzene (TATB) have been
investigated. With the continuous reduction in laser size, and increase in laser energies and powers, using lasers to
initiate energetic materials is becoming easier and more practical to implement in a system environment.
In this paper we examine the development of the concept of laser initiation, from its early days using large Ruby lasers,
to the more modern use of Nd:YAG lasers. We collate and present here the open source literature published in this field
in order to produce a concise and accurate historical overview of the research published to date, and make a prediction of
future trends where possible. We also examine research presented in enabling technologies, such as laser-driven flyer
plates and high-energy optical fibers.
If a HMX-based explosive is subjected to an insult then there is a potential for the insulted β-HMX to undergo a
phase change to the more sensitive δ form. AWE has an ongoing programme to develop a science-based model
of the response of HMX-based explosives to potential insults. As part of this programme there is a need to
identify whether δ-HMX has been formed, as this would subsequently affect the intrinsic safety properties of the
δ-HMX, unlike the more stable β form, exhibits unusual optical properties for an explosive, as it acts as a
frequency-doubling material. When illuminated by a high-energy laser pulse areas of the explosive charge that
contain δ-HMX emit frequency doubled light. This non-linear optical phenomenon allows for a non-invasive
diagnostic to be developed to study creation of the more sensitive δ phase within HMX based formulations.
AWE has developed a portable diagnostic system based on this concept to investigate the behaviour of HMX-based
explosives after low-speed impacts. The results of the commissioning trials are presented; using both an
inert simulant, KDP, to align and prove the system and HMX samples from low-speed impact experiments. The
results of these experiments are compared to initial calculations using the Hydrocode EDEN.
We have developed a suite of optical diagnostics and analyses for
probing the velocity and spatial distribution of ablatively launched
metal with nano-scale precision. We utilize a nanosecond laser
pulse to launch a thin layer of metal and then use optical and
opto-electronic devices to diagnose the velocity and topography.
Our Photonic Doppler Velocimeter (PDV) utilizes the heterodyne
principle that allows us to track multiple velocity components. We
have investigated a number of different methods for analyzing this
data to provide increased velocity and temporal resolution. We also
discuss the possibilities to extend the sensitivity of the PDV
system to provide a compact diagnostic with a broad range of
capabilities. Our topographer is based on the Shack-Hartmann
interferometer that can resolve the changing shape of the ablated
metal surface as it is launched. We compare the experimental data
to hydrodynamic simulations to provide a feedback loop to improve
our theoretical models. The ultimate goal is to develop a
well-understood laser-based firing set for direct optical initiation
(DOI) of explosives.
Recent advances over the last five years in high-speed digitizing oscilloscopes and high-bandwidth photodiodes, driven
primarily by the telecommunications industry, have enabled the development of a new type of interferometer for
measuring high velocities, such as those found in detonics experiments.
The heterodyne velocimeter can be visualized as a fiber-based Michelson interferometer. The beam from a single-mode
fiber laser at 1550 nm is passed through a circulator, acting to separate bi-directional light. The beam is then reflected via
free-space optics from the surface of interest, and then focused back into the same fiber. This reflected light is mixed
with an approximately equal amount of non-reflected light, and the resulting interference is recorded using a high-bandwidth
photodiode and oscilloscope. In contrast to more traditional velocimetry techniques such as VISAR, only a
single data channel is required per probe.
The uses of heterodyne velocimetry have, to date, been primarily in the multi-microsecond time regime, i.e. explosively driven
metal plates. In this paper, we present a four-channel, ultra-high bandwidth system designed for use in the sub-microsecond
time regime, and present the results obtained from laser-driven flyer plates traveling in excess of 3 km s-1.
We have developed analysis software suited to use in this time regime, where a relatively small displacement is recorded.
The original heterodyne velocimeter relied on back-reflectance from the probe to obtain the non-reflected light. This
limits both the flexibility of the system and the efficiency of the probes. We have overcome this issue by introducing a
beam splitter into the system prior to the circulator. This allows the probing system to be designed for maximum
efficiency, and we are then able to tune the non-reflected light on a shot-to-shot basis.
High-Speed Multi-Frame Laser Schlieren is used for visualization of a range of explosive and non-explosive events.
Schlieren is a well-known technique for visualizing shock phenomena in transparent media. Laser backlighting and a
framing camera allow for Schlieren images with very short (down to 5 ns) exposure times, band pass filtering to block
out explosive self-light, and 14 frames of a single explosive event.
This diagnostic has been applied to several explosive initiation events, such as exploding bridgewires (EBW), Exploding
Foil Initiators (EFI) (or slappers), Direct Optical Initiation (DOI), and ElectroStatic Discharge (ESD). Additionally, a
series of tests have been performed on "cut-back" detonators with varying initial pressing (IP) heights. We have also
used this Diagnostic to visualize a range of EBW, EFI, and DOI full-up detonators. The setup has also been used to
visualize a range of other explosive events, such as explosively driven metal shock experiments and explosively driven
microjets. Future applications to other explosive events such as boosters and IHE booster evaluation will be discussed.
Finite element codes (EPIC, CTH) have been used to analyze the schlieren images to determine likely boundary or initial
conditions to determine the temporal-spatial pressure profile across the output face of the detonator. These experiments
are part of a phased plan to understand the evolution of detonation in a detonator from initiation shock through run to
detonation to full detonation to transition to booster and booster detonation.
A system for launching flyer plates using a Q-switched Nd:YAG laser has been developed for shock initiation of
secondary explosives. Flyer plates have been launched at velocities exceeding 4 km s-1. These flyers produce sub-nanosecond
duration shocks in excess of 30 GPa on impact.
Flyer planarity and integrity have been studied by impacting polymethylmethacrylate (PMMA) windows and using a
high-speed streak camera to record the light generated. Analysis of this data has provided an insight of the key
mechanisms and enabled the system attributes to be controlled and optimized for explosive initiation.
Pentaerythritol Tetranitrate (PETN) has been tested with specific surface areas (SSA) ranging from 12,700 cm2 g-1 to
25,100 cm2 g-1 and the effect of SSA on initiation threshold in this extremely short duration shock regime is examined. A
minimum surface area size for initiation is evident. Calculations show that the pulse width is on the order of the particle
size. We observed partial reactions in some firings, and we propose a mechanism to explain this.
The normalized initiation thresholds are compared to electrical slapper thresholds on the same explosives, and these data
have been used to evaluate P2τ for both laser driven flyer plates and electrically driven flyer plates. The critical energy
fluence calculated is compared to published values and discussed for similar systems.
Fundamental studies on laser ignition have been performed by the US Department of Energy under ARES (Advanced Reciprocating Engines Systems) and by the California Energy Commission under ARICE (Advanced
Reciprocating Internal Combustion Engine). These and other works have reported considerable increases in fuel efficiencies along with substantial reductions in green-house gas emissions when employing laser spark ignition. Practical commercial applications of this technology require low cost high peak power lasers. The lasers must
be small, rugged and able to provide stable laser beam output operation under adverse mechanical and
environmental conditions. New DPSS (Diode Pumped Solid State) lasers appear to meet these requirements. In this work we provide an evaluation of HESP (High Efficiency Side Pumped) DPSS laser design and performance with regard to its application as a practical laser spark plug for use in internal combustion engines.
Breech Mounted Lasers (BMLs) have been successfully used to demonstrate laser ignition of howitzer propellant
charges including bag, stick, and the Modular Artillery Charge System (MACS). BMLs have been integrated and tested
on many artillery systems, including the US Army's M109A6 Paladin, M198, M777 Light Weight, Crusader, and Non-Line-of-Sight Cannon (NLOS-C). Until now, these lasers have been relatively large and inefficient systems based on a
flashlamp pumped Nd:YAG laser design. Modern vehicle platforms will require smaller, more efficient lasers that can
operate under increased shock and vibration loads. Kigre's new DPSS (Diode Pumped Solid State) lasers appear to meet
these requirements. In this work we provide an evaluation of HESP (High Efficiency Side Pumped) DPSS laser design
and performance with regard to its application as a practical artillery laser ignition system.
A multichannel fiber-optic intensity based sensor for remote inclination measurement has been developed based on light
beam displacement by a transparent free hanging parallel sided deflector glass plate. The deflector plate is mounted onto
a gimbal suspension to avoid interference between orthogonal tilt directions. In order to enable vibration resistant
measurement the deflector is damped by a transparent viscous liquid. Light intensity drift compensation is achieved
through the comparison of two output signals obtained from two spaced measuring channels integrated in the sensing
element. The specifications of the sensor are as follows: threshold sensitivity - 0,01 deg, dynamic range - 25 dB,
measuring angle range ±4 deg. Sensor output signal variation corresponding to 30% input light intensity decrease does
not exceed ±4%. Due to the utilization of standard low-loss multimode optical fibers the sensing element can be removed
from the light generation and processing units to a distance of up to several kilometers. High sensitivity and stability of
the proposed technique allow its wide spread application in structural health monitoring as well as in machine building,
seismology and other areas where precision angular positioning or monitoring is required.