1. FIELD OF THE INVENTION
This invention relates generally to explosive igniters and, more particularly, toward fiber optic coupled, light responsive explosive igniters that are resistant to strong electromagnetic pulses.
2. HISTORY OF THE PRIOR ART
There are many practical applications in which it is necessary to have one or more well-timed explosions. For example, explosive bolts are used on spacecraft to separate successive stages of a vehicle, sequentially actuated shaped charges are often utilized to bring down old dilapidated buildings, and precisely timed initiating explosions are utilized to set off larger quantities of working explosives underground for the generation of strata cleavage in the mining of crude oil and subterranean water sources. In this context, the working explosive usually is a relatively large amount of relatively insensitive explosive material and the initiating explosive (which explodes first in time) is a relatively small quantity of explosive material. Military applications include explosive weapons, triggering of guns, launching of rockets, and the setting off of explosive mines with precise timing. In all of these cases, it is absolutely essential that the initiation of the explosion be a controlled event that is not subject to random inputs such as extraneous noise, radio signals or other electromagnetic pulse inputs, or disruptive signals deliberately generated by saboteurs.
The use of electrical wiring between controls operated by a user and the location of the explosive device itself is particularly vulnerable to weather related electrical potential gradients, extraneous radio signals, proximity to high voltage electrical lines, and to physical damage whether accidental or intentional. There is the additional danger of a possible ground loop through wet terrain leading to an unintended or ill-timed firing of the explosive device. In applications requiring concealment, the wiring may provide a means of easy detection of the explosive device. There is, therefore, considerable interest in utilizing optical fibers or other optical links in place of electrical wiring to an explosive component. With optical communication between the user and the device it becomes somewhat easier to circumvent inadvertent or covert tampering with such devices as well, generally through the use of temporal and spectral coding techniques.
While direct initiation of powerful explosions by a pulse of light is sometimes used, this requires either light intensity levels whhich are difficult to generate and communicate over useful distances or optically sensitive primary explosive materials. Such materials tend to be very sensitive to mechanical inputs, and this tends to defeat any safety advantages gained from optical coupling between the user operated controls and the explosive devices. Although efforts have been made to directly initiate intermediate explosives with intense light pulses, the energy required to accomplish this function appears generally to be well above what one can envision in a practical component design.
For all the above-discussed types of applications, it is convenient to have a self-contained device that contains a small quantity of explosive material that can be set off by an externally provided signal. Where the small quantity of explosive so exploded is to initiate the explosion of a much larger quantity of working explosive material, the device must be attached to a quantity of the explosive material, preferably a container of the same. For those applications where the explosion of a small quantity of explosive material contained within the device will suffice, the device must be attached to a conduit that will carry the force of the explosion to achieve the desired purpose. For either type of use, a secure and convenient technique for attaching the device where it is to be used is to employ a positive mechanical linkage.
Examples of known devices known include, for example, one in which the explosive is ignited by high level, monochromatic, radiant energy derived from some form of laser and conveyed to the explosive through an optical fiber, as in U.S. Pat. No. 3,408,937. In U.S. Pat. No. 3,528,372, a device utilizes an infrared laser to ignite a heat-sensitive small charge in a frangible thin-bottomed container from which metallic fragments are propelled at high velocities to set off a relatively insensitive high explosive charge. A device in U.S. Pat. No. 4,149,466 uses an intermediate source of light in the ignition process for precise timing but requires a primary connection to the explosive device through electrical wiring. A somewhat different approach in U.S. Pat. No. 3,724,383 utilizes a succession of secondary explosives, e.g., KHND and PETN initiated by a low energy laser beam to give a higher order detonation. For blasting operations where a plurality of blast holes must first be drilled and an explosive loaded into each together with a primer and a detonating cord for each such blast hole, one technique proposed in U.S. Pat. No. 4,455,941 has an optical fiber coupled to each electrical wire leading to the blasting caps so that if there is a break in the wire there is also a contemporaneous break in the optical cable, which can be detected by the user prior to setting off synchronized plural explosions. The described prior art requires the use of either light sensitive explosive or a very high intensity light source, e.g., a laser. Such light-sensitive explosives may be subject to ill-timed explosions in response to extraneous X-ray pulses or intense shock waves. Deliberate interference with the fiber optic cable could be used for sabotage purposes merely by coupling a laser or other bright light source to the cable.
There is therefore a need, in both commercial and military applications, for an explosion-initiating device that is insensitive to extraneous electromagnetic radiation, requires no electrical wiring that may be subject to the formation of a ground loop leading to unintended firing of the explosive device, and utilizes low-intensity light transmission through conventional optical fibers, with optional safety coding, to arm and then to set off an explosive device in response to an optically transmitted command.