An experimental laser ignition system has been developed for the M230 cannon used on the Apache helicopter. This high-rate of fire (625 rounds/minute) chain gun provided many design challenges including a complex optical path, limited available space, high levels of shock and vibration and a maximum allowable cartridge functioning time of 4 ms. In order to satisfy this time limit with laser ignition, a subprogram was developed to explore laser sensitive, fast-acting energetic materials with good ignition transfer properties. Annual consumption rates in excess of 500,000 rounds per year in training dictated that ultimate ammunition production costs be considered in all design alternatives. Environmentally benign ("green") materials are being required by federal and state environmental regulations and standards and were factored into the new optical ignition system design. Both newly developed nano energetics and other pyrotechnic materials have been explored to fill this requirement. The development of the system and components is described in this paper from the firing of Mann barrel (single shot) fixtures to optimize cartridge performance, single-shot firing of the cannon using fiber-optic and optical-train light paths, culminating in a demonstration firing of the automatic cannon at its full rate of fire for a 10-round burst. The remaining technical challenges and future direction of the program are presented.
Medium caliber cannons, such as the Army's M230 chain gun, currently utilize a high current electrical pulse to initiate the propellant. While electrical ignition is reliable, electrical based primers are susceptible to premature ignition from EMI, EMP, or other stray or directed electromagnetic sources. Laser ignition of medium-caliber cannon systems has several advantages over the current electrically initiated ignition system. In addition to removing hazards due to electrostatic or radiated electromagnetic energy, lasers are an ideal ignition source for new primer compounds, such as Metastable Intermolecular Composites (MIC), that are potentially environmentally friendly replacements for lead styphnate containing compounds. This paper will describe our efforts to develop a laser source suitable for the M230 medium-caliber automatic cannon as used on the Apache AH-64 helicopter. We will describe early proof of concept laser systems including a fiberoptic-coupled flashlamp-pumped Nd:YAG source, direct Nd:YAG laser sources, and a full rate of fire demonstration laser that was mounted directly on the M230 housing. We will also discuss our plans and designs for a direct semiconductor laser ignition source for the M230.
Firing systems typically incorporate isolation-based architectures that are established by the safety themes of particular weapon systems. Robust electrical diversion barriers are implemented to isolate energy from detonation-critical components until the event of intended use of the system. An optical trigger assembly is being developed to enhance the safety of new firing systems. It couples a fast trigger signal through an exclusion region barrier without compromising the integrity of the barrier in abnormal environment situations. A laser diode generates an optical pulse that is coupled through a sapphire stub to a photoconductive semiconductor switch (PCSS). The PCSS drives a vacuum switch tube to complete the triggering chain in the firing system. A general discussion and comparison of triggering technology options, and the design characteristics and performance parameters of the specific optical trigger point design are presented in this paper.
A complete packaged optical firing system was designed and fabricated to be used to trigger optical detonators. The system consists of a specially designed Nd:YAG laser operating at 1064 nm, attenuator module, and splitter module that divide the beam into multiple high power, and one low power channels. The beams are then focused into fibers to transmit the optical pulse to the optical detonators. The low power channel can be used for monitoring timing and/or energy. No active cooling is required since it is designed for low repetition rates. The system is designed to be rack mounted, requiring only 110 V AC power and rugged enough to be transported to various test sites without further alignment. One of the unique aspects of this system is the incorporation of numerous safety features to prevent accidental release of optical or electrical energy, mirroring safety themes required for electrical firing systems.
Optical firing sets need miniature, robust, reliable pulsed laser sources for a variety of triggering functions. In many cases, these lasers must withstand high transient radiation environments. In this paper we describe a monolithic passively Q-switched microlaser constructed using Cr:Nd:GSGG as the gain material and Cr4+:YAG as the saturable absorber, both of which are radiation hard crystals. This laser consists of a 1-mm-long piece of undoped YAG, a 7-mm-long piece of Cr:Nd:GSGG, and a 1.5-mm-long piece of Cr4+:YAG diffusion bonded together. The ends of the assembly are polished flat and parallel and dielectric mirrors are coated directly on the ends to form a compact, rugged, monolithic laser. When end pumped with a diode laser emitting at ~807.6 nm, this passively Q-switched laser produces ~1.5-ns-wide pulses. While the unpumped flat-flat cavity is geometrically unstable, thermal lensing and gain guiding produce a stable cavity with a TEM00 gaussian output beam over a wide range of operating parameters. The output energy of the laser is scalable and dependent on the cross sectional area of the pump beam. This laser has produced Q-switched output energies from several μJ per pulse to several 100 μJ per pulse with excellent beam quality. Its short pulse length and good beam quality result in high peak power density required for many applications such as optically triggering sprytrons. In this paper we discuss the design, construction, and characterization of this monolithic laser as well as energy scaling of the laser up to several 100 μJ per pulse.
Applications requiring injection of a high-power multimode laser into multiple fibers with equal energies, or specific energy ratios, provide unique design challenges. As with most all systems, engineering trades must balance competing requirements to obtain an optimal overall design. This is particularly true when fabrication issues are considered in the design process. A few of these competing design requirements are discussed in this conceptually simple system. This fiber injection system consists of three components; a refractive beam homogenizer, a diffractive beamsplitter, and a fiber array. We show the design process, starting with first-order design, for an example fiber injection system that couples a high-power YAG laser into seven fibers. Design goals include high efficiency, good beamsplitting uniformity, compact overall size, maximum mode filling of the fibers, and low cost of fabrication and assembly.
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 study, the effects of ionizing radiation on the optical response of undoped and 1.1% Nd-doped single-crystal and polycrystalline YAG have been investigated. In the undoped materials it is seen that both laser materials exhibit significant loss at the 1.06 μm lasing wavelength following exposure to a 40 krad, 30 nsec pulse of gamma radiation. In the undoped single-crystal samples, the transmission loss is initially large but exhibits a rapid recovery. By contrast, the undoped polycrystalline YAG experiences an initial 100% loss in transmission, becoming totally opaque at 1.06 μm following the radiation pulse. This loss is slow to recover and a large residual permanent photodarkening effect is observed. Nd-doping improves the optical response of the materials in that the radiation-induced optical loss is substantially smaller in both the polycrystalline and single-crystal YAG samples. Preliminary results on the radiation response of elevated-temperature samples will also be reported.
The initiation of explosives by laser illumination has been known for many years. In this paper we will discuss the development of a working detonator design that reduces the energy required for detonation in a low-density secondary explosive by vaporizing a thin metal coating. We present data on the development of the design for a workhorse laser detonator that provides enhanced safety over existing exploding bridgewire detonators (EBWs). Comparison of this laser initiated data to an exploding-bridgewire (EBW) provides insight into the mechanism of initiation of detonation in low-density PETN by the plasma source. A novel diagnostic technique to determine the run-distance to detonation also known as the apparent Center-of-Initiation (COI) will also be discussed.
The challenges of developing laser ignition for an artillery cannon in which the black powder (BP) of the charges must be ignited are described. Laser ignition threshold behavior for BP is quantified for hot and cold temperatures and compared to ambient behavior. The threshold behavior for single grains of BP is compared to that of multiple grains more similar to that encountered in the gun. In addition to characterization of the BP ignition parameters, the influence of other materials is discussed briefly.
Metastable intermolecular composites (MIC) consisting of nanometer-scale aluminum and molybdenum trioxide have been proposed as fast initiators. A compound of this class of material was evaluated as a potential environmentally friendly replacement pyrotechnic material for lead styphnate for use in the primer of the M230 medium-caliber automatic cannon. In addition to removing the lead hazard, laser ignition would also reduce or remove certain hazards due to electrostatic or radio frequency radiation. This study was conducted with both a flashlamp-pumped Nd+3:YAG laser and a fiber-coupled diode laser. The measured threshold ignition energies of the MIC and two other inorganic pyrotechnic compounds are presented. The low ignition threshold, advances in diode laser technology, and compact size of the diode laser indicated that laser diode technology could be an ideal candidate ignition source for the M230 cannon. The candidate pyrotechnic compounds were also evaluated for suitability in laser initiation via measurement of time-to-first-light. This metric provided a measurement of the potential for achievement of the necessary action time required for proper cannon operation.
Photovoltaic power converters transform optical power into electrical power, which is inherently immune to RF, EMI, high voltage, and lightning effects. Capable of powering electronic circuitry directly over optical fiber in a wide variety of applications, this technology has been validated in industries such as electric power, communications, remote sensing and aerospace. From no more than a laboratory curiosity less than fifteen years ago, power-over-fiber, or photonic power, has established itself in thousands of industrial operations worldwide. Optical energy for pre-amplifiers or low-power transmitters as well as switches and relays can be efficiently delivered through noise immune and non-conductive optical fiber. These advantages are also readily available for safe and arm applications since optical fiber is immune to electrical noise, magnetic fields and conduction of unexpected electrical currents. Since it is made from glass, a dielectric fiber is impervious to electromagnetic interference. High optical power is readily delivered through fiber, and conversion of optical to electrical energy at the remote site with efficient photovoltaic converters is routine.
In this work, we report a highly efficient Photovoltaic Power Converter (PPC) suitable for 920 nm to 970 nm InGaAs MQW lasers for the first time. The epitaxial layers were grown by low pressure MOCVD on the semi-insulting GaAs substrate. The epi layers consist of a p-n junction of In0.12Ga0.88As and the window layer of p+ AlInGaAs. The device is made of seven or eight pie-segments of equal area series-connected by means of air-bridges. Under 500mW of 940nm laser illumination, the open-circuit voltage of the eight-segment InGaAs chip is 6.7V. The short-circuit current is 29.7mA. Its maximum delivered electrical power is 171.2mW, equal to a 34.2% overall power conversion efficiency. We also demonstrate the high temperature characteristic and stability of the device.
We establish an empirical model to project the highest power output from a photovoltaic power converter (PPC). This model helped us achieve over one watt electrical output power from a single fiber channel. A total of 1.2W electrical power output from two parallel connected 8-segment devices was obtained from a well heat-sunk package with 4W laser illumination from a single fiber. To the best of our knowledge, this is the first time that, over one watt electrical power has been delivered by a single fiber channel. Over 30% power conversion efficiency was maintained in this high power conversion process, whereas the power conversion efficiency was over 40% at low laser input power. This high electrical power output enables more applications in sensing, safing, or arming that could not be achieved before due to less available power. It also further strengthens the position of this unique solution of providing isolated power in harsh, noisy and high-voltage environments.
This report describes the features of monolithic, series connected silicon (Si) photovoltaic (PV) cells which have been developed for applications requiring higher voltages than obtained with conventional single junction solar cells. These devices are intended to play a significant role in micro / mini firing systems and fuzing systems for DOE and DOD applications. They are also appropriate for other applications (such as micro-electro-mechanical-systems (MEMS) actuation as demonstrated by Bellew et. al.) where electric power is required in remote regions and electrical connection to the region is unavailable or deemed detrimental for whatever reason. Our monolithic device consists of a large number of small PV cells, combined in series and fabricated using standard CMOS processing on silicon-on-insulator (SOI) wafers with 0.4 to 3 micron thick buried oxide (BOX) and top Si thickness of 5 and 10 microns. Individual cell isolation is achieved using the BOX layer of the SOI wafer on the bottom. Isolation along the sides is produced by trenching the top Si and subsequently filling the trench by deposition of dielectric films such as oxide, silicon nitride, or oxynitride. Multiple electrically isolated PV cells are connected in series to produce voltages ranging from approximately 0.5 volts for a single cell to several thousands of volts for strings of thousands of cells.
An optical polyspectral sensor has been developed and tested which calculates the magnitude and directional velocity of an incoming projectile to queue a reactive countermeasure. This paper describes the sensor modeling, sensitivity analysis, and experimental results of a sensor consisting of four sheets of light. Sensor application could be extended to all projectiles that present a measurable laser radar cross section to the sensor.
We are developing a robust and compact photonic proximity sensor for munition applications. Successful implementation of this sensor will provide a new capability for direct fire applications. The photonic component development exploits pioneering work and unique expertise at ARDEC, ARL, and Sandia National Laboratories by combining key optoelectronic technologies to design and demonstrate components for this fuzing application. The technologies employed in the optical fuze design are vertical cavity surface-emitting lasers (VCSELs), the p-i-n or metal-semiconductor-metal (MSM) photodetectors, and miniature lenses optics. This work will culminate in a robust, fully integrated, g-hardened component design suitable for proximity fuzing applications. This compact sensor will replace costly assemblies that are based on discrete lasers, photodetectors, and bulk optics. It will be mass manufacturable and impart huge savings for such applications. The specific application under investigation is
for gun-fired munitions. Nevertheless, numerous civilian uses exist for this proximity sensor in automotive, robotics and aerospace applications. This technology is also applicable to robotic ladar and short-range 3-D imaging.
This paper describes the photonic component development taking place at Sandia National Laboratories, ARDEC and the Army Research Laboratory in support of an effort to develop a robust, compact, and affordable photonic proximity sensor for munitions fuzing applications. Successful implementation of this sensor will provide a new capability for direct fire applications. The technologies under investigation for the optical fuze design covered in this paper are vertical-cavity surface-emitting lasers (VCSELs), vertical-external-cavity surface-emitting lasers (VECSELs), integrated resonant-cavity photodetectors (RCPDs), and refractive micro-optics. The culmination of this work will be low cost, robust, fully integrated, g-hardened components suitable for proximity fuzing applications. The use of advanced photonic components will enable replacement of costly assemblies that employ discrete lasers, photodetectors, and bulk optics. The integrated devices will be mass produced and impart huge savings for a variety of Army applications. The specific application under investigation is for gun-fired munitions. Nevertheless, numerous civilian uses exist for this proximity sensor in automotive, robotics and aerospace applications. This technology is also applicable to robotic ladar and short-range 3-D imaging.
In our present world the Crystal Growth Technology does not have the necessary and sufficient conditions to manufacture large sizes; especially in the Sapphire Crystal world. We have a theoretical and methodological development for growing gigantic Sapphire Crystal Lenses. Our gigantic Sapphire Crystal Lenses have a unique optical characteristic which will be used in the Global System of Laser Weapons (GSLW); hence solving one of the crucial problems in the Relay Mirror System; where it captures the Laser beam from the earth surface, cleaning the beam in the Satellite and redirecting the laser energy to the precise desired target.
Developed and solution for the temperature and heat-elasticity fields in growth systems are considered theoretical, in order to assess their effects on the optical symmetry of the growing crystal. The process is modeled using three-dimensional curvilinear coordinates to describe a closed, low-strain heat-elasticity system, with allowance made for the temperature variations of the thermal properties of the multilayer growth system, and nonlinear and unsteady-state process with arbitrary boundary conditions.
The results presented as plots of the strain, stress, displacement, and temperature fields; demonstrate the potential of the method for designing new growth units and improving the existing ones and suggesting that crystals, in general, without frustration of optical symmetry can, in principle, be grown. In order to solve generalized problem for large optics. It is required to have super and correct mathematical computing calculations, and using basic fundamental laws of nature regarding optical symmetry in the crystal, and discovering the radical "new wave method" for crystal growth technology.
Nd:YAG lasers have been successfully used to demonstrate laser ignition of howitzer propellant charges including bag, stick, and the Modular Artillery Charge System (MACS). Breech Mount Laser Ignition Systems (BMLIS) have been designed, installed 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). The NLOS-C incorporates advanced weapon technologies, to include a BMLIS. United Defense's Armament Systems Division has recently designed and built a NLOS-C System Demonstrator that uses a BMLIS that incorporates Kigre's patented square pulse technology. NLOS-C is one of the weapon systems being developed for use with the US Army's "systems of systems" Future Combat System (FCS), Manned Ground Vehicles (MGV) program, and is currently undergoing development testing at Yuma Proving Grounds. In this paper we discuss many technical aspects of an artillery laser ignition system and present BMLIS test data obtained from actual gun firings conducted with a number of different US Army howitzer platforms.