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.
The characterization of mounted and/or bonded optical assemblies for survivability in harsh environments is crucial for
the development of robust laser-optical firing systems. Customized mounts, bonded assemblies and packaging strategies
were utilized for each of the laser resonator optics with the goal of developing and fielding a reliable initiation system
for use in extreme conditions. Specific components were selected for initial testing based on past experience, material
properties and optical construction. Shock, vibration and temperature testing was performed on three mounted optical
components; polarizing cube beam splitters, Q-switch assemblies and xenon flashlamps.
Previously, flashlamps of a solder-sealed construction type were successfully tested and characterized. This test regiment
characterized the more fragile glass-to-metal seal constructed flashlamps. Components were shock-tested to a maximum
impulse level of 5700 G's with a 1.1 millisecond long pulse. Vibration tests were performed to a maximum level of 15.5
grms for forty seconds in each of three axes. During each test, components were functionally tested and visually
inspected at a specified point to verify survival. Temperature tests were performed over a range extending 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.
Experimental results obtained from these tests will be discussed as will their impact on future component mounting
The next generation of low-cost smart munitions will be capable of autonomously detecting and identifying targets aided partly by the ability to image targets with compact and robust scanning rangefinder and LADAR capabilities. These imaging systems will utilize arrays of high performance, low-cost semiconductor diode lasers capable of achieving high peak powers in pulses ranging from 5 to 25 nanoseconds in duration. Aerius Photonics is developing high-power Vertical-Cavity Surface-Emitting Lasers (VCSELs) to meet the needs of these smart munitions applications. The authors will report the results of Aerius' development program in which peak pulsed powers exceeding 60 Watts were demonstrated from single VCSEL emitters. These compact packaged emitters achieved pulse energies in excess of 1.5 micro-joules with multi kilo-hertz pulse repetition frequencies. The progress of the ongoing effort toward extending this performance to arrays of VCSEL emitters and toward further improving laser slope efficiency will be reported.
Photodigm is developing long wavelength multiwatt grating outcoupled surface emitting (GSE) semiconductor lasers with continuous-wave (CW) output powers exceeding 1 W at 1310-nm and 0.4 W at 1550-nm wavelengths. These GSE lasers have full-width at half-maximum (FWHM) spectral bandwidth of less than 0.2 nm and a beam divergence of 0.5° x 3.2°(FWHM).
Experimental investigations at the Air Force Research Laboratory's Munitions Directorate required a firing set to initiate detonators in ultra-high-voltage experiments, but commercial firing sets with adequate high voltage isolation could not be found. To resolve this issue a Directorate team designed, fabricated a new firing set, verified that it met all design requirements, and obtained safety board approval for its use. The patented design uses all plastic fiber optic cables, pneumatic controls, battery isolation, and redundant safety devices to withstand voltage differences of hundreds of thousands of volts between the operator and the experiment. This paper describes the efforts to develop a very reliable firing set that met the high standards required by explosive safety procedures. Design features include Faraday shielding, redundant lock out features, multiple safing circuits to handle a variety of fault conditions and fail-safe bleed down circuits to eliminate concerns with residual stored electrical energy. The high-voltage-tolerant firing set allows research in unexplored areas such as electro-explosive effects.
The use of photosensitive materials for the development of integrated, refractive-index structures supporting telecom, remote sensing, and varied optical beam manipulation applications is well established. Our investigations of photosensitive phenomena in polysilanes, however, have been motivated by the desire to configure, or program, the photonic device function immediately prior to use. Such an operational mode imposes requirements on wavelength sensitivity, incident fluence and environmental conditions that are not typical of more conventional applications of photosensitive material. The present paper focuses on our efforts to understand and manipulate photosensitivity in polysilane thin films under different excitation wavelengths, local atmospheric compositions and thermal history in this context. We find that the photoresponse can be influenced through the control of such optical exposure conditions, thereby influencing the magnitude of the photoinduced refractive-index change attained.
In optical firing sets, laser light is used to supply power to electronics (to charge capacitors, for example), to trigger electronics (such as vacuum switches), or in some cases, initiate explosives directly. Since MEMS devices combine electronics with electro-mechanical actuators, one can integrate safe and arm logic alongside the actuators to provide all functions in a single miniature package. We propose using MEMS-activated mirrors to make or break optical paths as part of the safe and arm architecture in an optical firing set. In the safe mode, a miniature (~1 mm diameter) mirror is oriented to prevent completion of the optical path. To arm the firing set, the MEMS mirrors are deflected into the proper orientation thereby completing the optical path required for system functionality (e.g., light from a miniature laser completes the path to an optically triggered switch). The optical properties (i.e. damage threshold, reflectivity, transmission, absorption and scatter) of the miniature mirrors are critical to this application. Since Si is a strong absorber at the wavelengths under consideration (800 to 1064 nm), high-reflectivity, high-damage-threshold, dielectric coatings must be applied to the MEMS devices. In this paper we present conceptual MEMS-activated mirror architectures for performing arming and safing functions in an optical firing set and report test data which shows that dielectric coatings applied to MEMS-mirrors can withstand the prerequisite laser pulse irradiance. The measured optical damage threshold of polysilicon membranes with high-reflectivity multilayer dielectric coatings is ~ 4 GW/cm2, clearly demonstrating the feasibility of using coated MEMS mirrors in firing sets.
For ordnance system and testing applications in which safety is paramount, laser detonators and firing systems are strong candidates. Both low-power (1 W) and high-power (~1 MW) laser-driven explosive devices provide safety against stray current and electrostatic discharges, including lightning. This article addresses only one class of high-power laser-driven detonators that provide prompt detonation - the laser-driven analog of electrical exploding bridgewire (EBW) detonatorsm which we call a "laser EBW." Coupling of laser power into a plasma and then to the explosive powder will be described. Drawing upon current initiatives within DOE laboratories, this talk will emphasize similarities between high-power laser detonators and high-power electrical detonators in terms of firing power requirements and development of deonation. In explosive testing applications, laser detonators provide separation of noisy electrical firing systems from diagnostic sensors that may be embedded in an experimental assembly. Laser detonators can be made without any metallic content, and that is desirable for radiography experiments. Feasibility of reliable transmission of a firing pulse through optical fibers is a key element in applications for missile ordnance, warhead firing, and other mobile systems. The preparation and characterization of fibers, and their capabilities and limitations are also discussed briefly.
Los Alamos National Laboratory is currently designing a series of direct optically initiated (DOI) detonators. The primary purpose of this series of detonators is to achieve a level of safety in the face of unintentional initiation from an electrical source. The purpose of these experiments is to determine the minimum spotsize that will initiate the low density initial pressing in these laser detonators. With this information it is expected that a more robust optically initiated detonator can be designed and manufactured. Results from a series of experiments will be discussed. First a range of small core diameter fiber optics with varying energy injection levels will be tested to find the minimum energy level necessary to achieve reliable initiation. Second, a range of apertures will be employed to trim the spotsize down to a minimum size that will still maintain reliable initiation. This information will help to understand whether the initiation criteria for the DOI Laser Detonator are dominated by energy density, total energy or a combination of these criteria.
Evaluation of laser initiated explosive trains has been an area of extreme interest due to the safety benefits of these systems relative to traditional electro-explosive devices. A particularly important difference is these devices are inherently less electro-static discharge (ESD) sensitive relative to traditional explosive devices due to the isolation of electrical power and associated materials from the explosive interface. This paper will report work conducted at Sandia National Laboratories' Explosive Components Facility, which evaluated the initiation and deflagration-to-detonation characteristics of a Laser Driven Exploding Bridgewire detonator. This paper will report and discuss characteristics of Laser Exploding Bridgewire devices loaded with hexanitrohexaazaisowurtzitane (CL-20) and tetraammine-cis-bis-(5-nitro-2H-tetrazolato-N2) cobalt (III) perchlorate (BNCP).
OASYS Technology has an ongoing program to design and fabricate high-energy laser
detonation devices for Los Alamos National Laboratories. The purpose of this program is
to demonstrate the feasibility of detonating high explosive (HE) by focusing the energy
from a fiber-coupled, high energy laser onto a metallized window surface. A high
explosive package integrated with the metallized window keeps the high explosive
material isolated from the ambient environment until activated by the vaporization of the
window metal film by the laser. The package constraints for these designs require a right
angle turn that must be achieved by reflection, since the minimum bend radius of the
fiber is substantially larger than the desired package size. Four optical designs were
proposed and fabricated to compare survival and detonation efficiency. This paper will
discuss the design requirements, fabricated designs, test methods and results, and conclusions.
We report measurements of the performance of mode-filtered, Yb-doped, double-clad fiber amplifiers (30 μm core
diameter) seeded with passively Q-switched, Nd:YAG microchip lasers operating at 1 kHz repetition rate and pulse
durations of 0.38 ns and 2.3 ns. The amplified pulses were fully characterized spectrally, temporally, and spatially. The
output beam quality was diffraction-limited (M2 < 1.2). With the 0.38 ns seed laser, we obtained a maximum peak
power of 1.27 MW, corresponding to a peak in-fiber irradiance of 440 GW/cm2. With the 2.3 ns seed laser, we obtained
a maximum pulse energy of 1.1 mJ, corresponding to an in-fiber fluence of 410 J/cm2. This irradiance and fluence are
the highest observed, to our knowledge, for ns-duration pulses, which bodes well for further power scaling with largercore
fibers. Nonlinear processes resulted in spectral broadening and distortion of the pulse temporal profiles. At 1.27
MW peak power, the linewidth containing 80% of the pulse energy was 0.55 nm, and the linewidth at 1.05 mJ pulse
energy was 1.0 nm. The experimental measurements were compared with the initial results of a pulse-amplification
model containing no adjustable parameters. The model predictions are in excellent agreement with the measured pulse
energies and small-signal gains. The laser systems employed a simple architecture, consisting of passively Q-switched
oscillators; single-stage, single-pass, cw-pumped amplifiers; conventional, solid fibers; and < 6 cm coil diameters. This
approach is suitable for use in practical applications requiring compact, rugged, and efficient laser sources.
A monolithic crystalline Si photovoltaic device, developing a potential of 2,120 Volts, has been demonstrated12. The monolithic device consists of 3600 small photovoltaic cells connected in series and fabricated using standard CMOS processing on SOI wafers. The SOI wafers with trenches etched to the buried oxide (BOX) depth are used for cell isolation. The photovoltaic cell is a Si pn junction device with the n surface region forming the front surface diffused region upon which light impinges. Contact is formed to the deeper diffused region at the cell edge. The p+ deep-diffused region forms the contact to the p-type base region. Base regions were 5 or 10 μm thick. Series connection of individual cells is accomplished using standard CMOS interconnects. This allows for the voltage to range from approximately 0.5 Volts for a single cell to above a thousand volts for strings of thousands of cells. The current is determined by cell area. The voltage is limited by dielectric breakdown. Each cell is isolated from the adjacent cells through dielectric-filled trench isolation, the substrate through the SOI buried oxide, and the metal wiring by the deposited pre-metal dielectric. If any of these dielectrics fail (whether due to high electric fields or inherent defects), the photovoltaic device will not produce the desired potential. We have used ultra-thick buried oxide SOI and several novel processes, including an oxynitride trench fill process, to avoid dielectric breakdown.
The optical transfer of power is becoming important for military and industrial applications. The powering of electrical circuitry, sensors and actuators over optical fiber offers immunity from RF, EMI, voltage breakdown, lightning and high voltage hazards. Optical power transfer is being employed in industries such as electric power, communications, remote sensing, and aerospace. In this paper we address issues associated with the illumination of Series Connected Photovoltaic Arrays (SCPA). SCPAs are extremely sensitive to the uniformity of illumination. The performance of a photovoltaic array is dominated by the least illuminated cell. We introduce an analytical model that predicts the performance of a photovoltaic array for an arbitrary illumination. Experimental data on array performance is presented, and general issues associated with the problem of producing uniform illumination are discussed.
Photodigm is developing high brightness grating outcoupled surface emitting (GSE) semiconductor lasers with
continuous-wave (CW) output power exceeding 1 W at 1064-nm wavelength. The GSE lasers have full-width at halfmaximum
(FWHM) spectral bandwidth of less than 0.2 nm and a beam divergence of 1° x 3.4° (FWHM).
The laser pulse shape should be varied to meet different requirements of the inertial confinement experiments. Chirped pulse-stacking is an effective method to obtain a long pulse with desired pulse shape using short pulses. We introduce a method to obtain a long pulse shaped by chirped pulse stacking in fiber time-delay lines. We demonstrate an all fiber pulse shape generator that can generate arbitrary pulse shapes by stacking a set of 20-ps pulses output from fiber mode-locking laser. The device offers a 2.2ns arbitrary waveform optical pulse with power up to 30dBm, bandwidth of 0.2nm and a rise time less than 50ps. The output optical pulse has a temporal adjustment precision of 32bit and an amplitude adjustment precision of 10bit. Experimental and theoretical results show that the generator provided the required stability, flexibility, fast rise time and high contrast pulse for laser fusion research.
Pockels cell driven by one-pulse process (OPP) is a new technique different from the regular plasma electrode Pockels cell (PEPC). In the OPP regime, only the positive and negative switching pulses will be supplied on opposite sides of the crystal. The OPP Pockels cell developed in our laboratory can work well at high working pressure. In this working pressure range, the gas is steady broken at the start of the switching pulse flattop. When the working gas is evacuated to low pressure, the discharge is unsteady. In an invariable electric field, the gas discharge is determined by the initial free electrons. At low pressure, the low density of initial electrons leads to the instability of gas discharge. The ultraviolet light conduces to increasing the density of the initial free electrons, and reducing the randomness of the initial free electrons. In this experiment, the ultraviolet light is used to irradiate the cathode of the Pockels cell. The experimental results show that, at low pressure, the breakdown of the working gas is steady, and the breakdown delay is shorter with ultraviolet irradiation.