Direct Optical Initiation (DOI), uses a moderate energy laser to shock initiate secondary explosives, via either a flyer
plate or exploding metal foil. DOI offers significant performance and safety advantages over conventional electrical
initiation. Optical fibres are used to transport the optical energy from the laser to the explosive device.
A DOI system comprises of a laser, one or more optical fibres, and one or more laser detonators. Realisation of a DOI
system is greatly eased by the use of fibre-to-fibre connections, allowing for easy integration into bulkheads or other
interfaces, such as firing tanks and environmental test chambers. Fibres to fibre connectors capable of transmitting the
required energy densities are not commercially available.
Energy densities in the region of 35 J cm-2 are required for initiation, above the damage threshold of typical optical
fibres. Laser-induced damage is typically caused by laser absorption at the input face due to imperfections in the surface
polishing. To successfully transmit energy densities for DOI, a high quality fibre end face finish is required.
A fibre-to-fibre connection utilizing micro-lens array injection into a large-core, tapered optical fibre, a hermetic fibre
bulkhead feedthrough, and a disposable test fibre has been developed. This permits easy connection of test detonators or
components, with the complex free-space to fibre injection simplified to a single operation. The damage threshold and
transmission losses of the fibre-to-fibre connection have been established for each interface.
A selection of commercially available high-power optical fibres have been characterised for radiation susceptibility in
Sandia’s Annular Core Research Reactor (ACRR). The fibres were subjected to a total gamma and neutron dose >2 Mrad(Si)
in a 7 ms pulse. The neutron fluence was >1015 n/cm2. Changes in the transmission characteristics of optical fibres carrying
high energy, short duration laser pulses (power densities of around 1.5 GW/cm2) were measured.
All fibres survived at least two consecutive radiation exposures, showing typical transient transmission losses of around 20%.
Post radiation exposure, the transmission characteristics returned to those of pristine fibres within one minute.
Direct Optical Initiation (DOI), uses a moderate energy Q-switched Nd:YAG laser to shock initiate secondary
explosives, via either a flyer plate or exploding metal foil. DOI offers significant performance and safety advantages over
conventional electrical initiation. Optical fibers are used to transport the optical energy from the laser to the explosive
Energy densities in the region of 35 J cm-2 are required for initiation, significantly above the damage threshold of typical
optical fibers. Laser-induced damage is typically caused by laser absorption at the input face due to imperfections in the
surface polishing. To successfully transmit energy densities for DOI, a high quality fiber end face finish is required.
Fiber assemblies were prepared by C Technologies Inc, NJ, USA, with Innovaquartz FG365UEC optical fiber, using a
variety of polishing methods, with both steel and zirconia ferrules. The quality of the fiber end faces was assessed using
non-contact optical profilometry. The damage threshold for each polishing method was then determined using a Q-switched
Nd:YAG laser and the optimal polishing method determined for each ferrule material. Significant performance
differences between zirconia and steel ferrules were observed, and a physical cause of this difference is proposed.
Laser-driven flyer plates offer a convenient, laboratory-based method for generating extremely high pressure shocks, in
excess of 30 GPa, in a variety of materials. They comprise of one or more thin layers forming a foil, coated onto a
transparent substrate. By irradiating the interface between foil and substrate with a moderate-energy, short-duration laser
pulse, it is possible to form a flyer plate, which can reach velocities in excess of 5 km/s. These flyer plates have several
applications, from micrometeorite simulation to initiation of secondary explosives.
The flyer plates considered here have up to four layers: an absorption layer, to absorb the laser energy; an ablation layer,
to form a plasma; an insulating layer; and a final, thicker layer that forms the final flyer plates.
By careful selection of both layer material and thickness, it is possible to increase the maximum velocity achieved for a
given laser pulse energy by increasing the proportion of laser energy coupled into flyer kinetic energy. Photonic Doppler
Velocimetry (PDV) is used to measure the flyer velocity.
High power laser systems have a number of uses in a variety of scientific and defense applications, for example laser
induced breakdown spectroscopy (LIBS) or laser-triggered switches. In general, high power optical fibers are used to
deliver the laser energy from the source to the target in preference to free space beams. In certain cases, such as nuclear
reactors, these optical systems are expected to operate in ionizing radiation environments. In this paper, a variety of
modern, currently available commercial off-the-shelf (COTS) optical fiber designs have been assessed for successful
operation in the transient gamma radiation environment produced by the HERMES III accelerator at Sandia National
The performance of these fibers was evaluated for high (~1 MW) and low (<1 W) optical power transmission during
high dose rate, high total dose gamma irradiation. A significant reduction in low optical power transmission to 32% of
maximum was observed for low OH- content fibers, and 35% of maximum for high OH- fibers. The high OH- fibers were
observed to recover to 80% transmission within 1 μs and 100% transmission within 1 ms. High optical power
transmission losses followed generally similar trends to the low optical power transmission losses, though evidence for
an optical power dependent recovery was observed. For 10-20 mJ, 15 ns laser pulses, around 46% was transmitted
coincident with the radiation pulse, recovering to 70% transmission within 40 ns of the radiation pulse.
All fibers were observed to completely recover within a few minutes for high optical powers. High optical power
densities in excess of 1 GW/cm2
were successfully transmitted during the period of highest loss without any observed
damage to the optical fibers.
Detonators are used to convert electrical or other energy into an explosive output. This output can then be used to initiate further explosive charges. To aid in the development of explosive systems, it is important to characterize the output of detonators, in particularly the pressure produced.
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 Photonic Doppler Velocimeter (PDV) can be visualized as a fiber-based Michelson interferometer. The light from a single-mode fiber laser at 1550 nm is passed through a circulator, which acts to separate bi-directional light. The beam is then reflected via free-space optics off the surface of interest, and then focused back into the same fiber. This reflected light is then 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.
We have used our PDV system to investigate the performance of optical and electrical detonators. The detonators examined are the commercially available RISI RP-80, and an AWE DOI (Direct Optical Initiation) detonator. The RP-80 is an exploding bridgewire (EBW) detonator, utilizing Pentaerythritol Tetranitrate as the initiating explosive and a RDX output pellet. The DOI detonator uses an aluminum flyer to initiate a Hexanitrostilbene (HNS) pellet. Both detonators are canned in aluminum and the velocity of the can was measured, and from this, the output pressure of the detonator has been determined. This is compared to calculated values.
Direct Optical Initiation (DOI) of explosives offers significant safety advantages over traditional electrical initiation of
explosives, primarily by removing the electrically conducting pathway to the explosive material. A firing system
typically consists of three main components: the fireset, which provides the energy to initiate the explosive; the cable,
which transmits or conducts this energy; and the detonator, which uses this energy to initiate the explosive charge.
Electrical firing systems used to fire secondary explosives typically use voltages of 500 volts and upwards, with currents
of 500 amps and upwards. The technology to transmit such signals over the short distances required is mature and well-proven.
However, an optical initiation system requires optical powers in excess of 10 MW, and the technology to deliver
such powers is relatively immature. Optical fibers are used to transmit the firing energy, which require very high
tolerances to ensure the beam is successfully coupled into the fiber without damage. Fiber optic tapers offer a method to
relax these tolerances, and hence reduce system cost and complexity, by providing a larger area into which to couple this
beam. We present here our initial results from a series of tests aimed at establishing the feasibility of using tapered
optical fibers for this purpose. The transmission loss and beam profiles are reported as a function of the beam position on
the input face of the optical fiber.
A variety of optical systems use high optical powers or energies, for example, power transport. These systems may be expected to operate in harsh and challenging environments, which may include ionizing radiation. In this paper, several different types of modern, commercially available optical fiber designs have been assessed for reliable operation in a transient gamma radiation environment. The fibers tested are large core multimode silica fibers optimized for the delivery of high power infrared laser light. Some of the fibers are specifically designed to operate in harsh radiation environments, and these are compared against designs of varying radiation resilience from other manufacturers. It was found that fibers were able to successfully transmit a laser pulse of up to 0.375 MW in peak power within a few hundred nanoseconds after irradiation, with less than a 10% loss in transmission.
Laser-driven flyer plates comprise of one or more thin layers forming a foil coated onto a transparent substrate. Irradiation of the foil/substrate interface with a Q-switched laser pulse produces a plasma, the expansion of which forms a flyer plate, which can reach velocities in excess of 5 km/s. These plates impart shocks in excess of 50 GPa, with duration of less than a nanosecond. This shock is sufficient to initiate secondary explosives such as Hexanitrostilbene (HNS) and Pentaerythritol Tetranitrate (PETN).
Thresholds of detonators based on laser-driven flyer plates are typically measured in terms of energy. By using a Photonic Doppler Velocimeter (PDV) we measure the velocity of the flyer plate at the threshold energy. This allows calculation of the shock pressure and duration imparted to the explosive.
By initiating HNS with a variety of flyer thicknesses, from 3 to 5 &mgr;m, we are able to evaluate Pn&tgr; in this extreme shock regime. The calculated value of n is compared to published values and discussed for similar systems. We are also able to use the James Criterion to analyze the initiation, with values of Ec and &Sgr;c being determined from experimental data, providing a predictive capability to model other configurations such as different flyer thicknesses and materials.
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.
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.
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.
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.
A system for launching flyers using a Q-switched Nd: YAG laser has been developed for shock initiation of secondary
explosives. Flyers have been launched at velocities approaching 6 km s-1. Optical fibers are used to transport the optical
energy from the laser to the detonator.
The launch of these flyers with sufficient velocity requires a fluence in the region of 35 J cm-2, significantly above the
damage threshold of most optical fibers. This damage is typically caused by laser absorption at the input face due to
imperfections in the surface polishing. A variety of optical fibers with high quality input faces have been tested at
fluences up to 50 J cm-2, and their damage thresholds and beam profiles have been measured.
The standard fiber used in this system is a low hydroxyl (-OH) content, 400μm diameter core silica fiber, with CO2 laser
polished faces. In addition to this, fibers tapering down to 300μm and 200μm core diameter were investigated, as a
means of increasing the efficiency of the system, along with mechanically polished fibers.
The fiber currently enters the detonator body from the rear. Depending on the application, it may be required for the
fiber to enter from the side. To facilitate this, fibers with a machined output face, designed to produce an output at
approximately 90 degrees to the fiber axis were tested.
Finally, a 2:1 fiber splitter was tested, as a first step to enable simultaneous firing of several detonators. Multiple
initiation points are desirable for applications such as programmable initiation, and it is intended to study fiber splitters
with a higher split ratio, such as 4:1 and 8:1.
The results of these experiments are presented, and assessments made of suitability for transmission of high-power Qswitched
Nd:YAG laser pulses.
Raman spectroscopy measures molecular vibrations by analyzing the frequency components of scattered laser light. It will provide information about the composition, crystallinity, stress, and spatial distribution of the various types of carbon found in diamond films. The general composition of diamond films can be investigated by monitoring bands in the Raman spectrum at 1332 cm-1 for crystalline diamond, 1580 cm-1 for graphite, and a broad band around 1350 cm-1 for amorphous material. The bandwidth of the 1332 cm-1 band is indicative of the crystal quality. Stress variations in diamond result in wavenumber shifts of the 1332 cm-1 band in the Raman spectrum. The internal stress in differently oriented diamond films has been investigated using Raman microscopy and was found to vary along the length of a crystallite. Using Raman mapping, it is possible to determine the spatial distribution of diamond and non-diamond carbon on the surface of a diamond film. The resulting gray scale images allow the regions of high diamond concentration to be identified.