Optopyrotechnic initiator

Abstract

An optical initiator of a pyrotechnic charge, including: a body including a cavity, which contains the pyrotechnic charge and a mechanism for igniting the charge by absorption of a laser radiation, the ignition mechanism being placed in contact with the charge; a laser radiation source; an optical fiber for transporting laser radiation from the source to the ignition mechanism. The ignition mechanism includes a metal plate and the metal plate and the laser radiation source are configured for a laser radiation issuing from the laser radiation source to be absorbed by the metal plate and converted into thermal energy, so that thermal conduction of the thermal energy from the metal plate to the pyrotechnic charge causes the ignition of the pyrotechnic charge. The metal plate can include perforations arranged periodically.

Claims

1. An optical initiator of a pyrotechnic charge, comprising: a body including a cavity in which the pyrotechnic charge is situated; an ignition means for igniting the charge by absorption of a laser radiation, the ignition means being placed in contact with the charge; a laser radiation source; an optical fiber for guiding a laser radiation from the source to the ignition means; and wherein the ignition means includes a metal plate that includes a plurality of perforations, the metal plate and the laser radiation source being configured so that a laser radiation issuing from the laser radiation source is absorbed by the metal plate and converted into thermal energy, so that thermal conduction of the thermal energy from the metal plate to the pyrotechnic charge causes ignition of the pyrotechnic charge.

2. An optical initiator according to claim 1, wherein the perforations are arranged periodically to define, in the metal plate, a plurality of identical elements connected together by bridges.

3. An optical initiator according to claim 1, wherein the metal plate is made from a metal chosen from platinum, gold, tungsten, or an alloy of at least two of these metals.

4. An optical initiator according to claim 1, further comprising a lens for focusing the laser radiation that is interposed between a first end of the optical fiber and the ignition means, a second end of the optical fiber being connected to the laser radiation source, the focusing lens being chosen from a spherical lens and a cylindrical lens.

5. An optical initiator according to claim 1, wherein the plate has a thickness of between 0.02 mm and 0.1 mm.

6. An optical initiator according to claim 1, wherein the plate is covered with a dichroic coating.

7. An optical initiator according to claim 1, wherein the laser source emits ultraviolet radiation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) This description will be given with regard to the accompanying drawings, among which:

(2) FIG. 1 depicts a schematic view in longitudinal section of an optical initiator according to the invention;

(3) FIG. 2 depicts a front view of a multiperforated plate according to a first preferred embodiment of the present invention; and

(4) FIG. 3 depicts a front view of a multiperforated plate according to a second preferred embodiment of the present invention.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

(5) With reference to FIG. 1, this shows an initiator 1 that comprises a body 2 provided with a cavity 3, wherein there are disposed a pyrotechnic charge 4 and a metal plate 5 placed in contact with the charge.

(6) An optical fibre 6 guides a laser beam from a laser source (not shown) towards the metal plate 5.

(7) In a known fashion, a connecting piece 7 serves as a support for the optical fibre 6 and thus makes it possible to place one end 8 of the optical fibre in contact with the metal plate 5, the other end 9 being connected to the laser source. The connecting piece here has a thread facilitating connection thereof to the body 2.

(8) It is also possible to improve the focusing of the laser beam on the metal plate and thus to increase the efficacy of the optopyrotechnic initiation by placing a focusing lens (not shown) between the end 8 of the optical fibre and the metal plate 5.

(9) In a known fashion, the initiator may serve to form a pyrotechnic chain, the body of the initiator then forming the first stage of the chain, the second, third, etc. stages of the pyrotechnic chain comprising pyrotechnic charges less and less sensitive and more and more energetic than the charge of the initiator.

(10) Since a laser beam is a coherent beam forming a small-diameter laser spot, the metal plate does not need to be large. In fact, it is preferable for the plate to be small in order to accelerate heating thereof by the laser beam. It is however preferable for the size of the plate to be sufficiently great for it to be easy to align the laser beam on the plate. Likewise, the smaller the thickness of the plate, the more rapidly is this thickness heated by the laser beam and the less easily manipulatable is this plate. In the end, the choice of the dimensions of the metal plate is a compromise between speed of heating of the metal plate, the ease of alignment of the laser beam and the ease of manipulation of the metal plate. For example, for a laser beam having a laser spot 1 mm in diameter, a metal plate having the form of a pellet approximately 3 mm in diameter and a thickness of approximately eighty hundredths of a millimeter can be chosen.

(11) It should be noted that the thickness of the metal plate also depends on the power of the laser source that is used.

(12) In preferred embodiments of the invention, the metal plate comprises multiple perforations arranged periodically. Two examples of possible geometries of perforations are illustrated in FIGS. 2 and 3.

(13) With reference to FIG. 2, the metal plate 5 is a circular-shaped pellet, and the perforations 10 are circular and identical and are situated at the vertices of contiguous hexagons forming a honeycomb structure. In this way identical elements 11 are obtained, connected together by bridges 12 or ligaments of material. The central spot 13 represents a circular focal spot, which may for example be obtained by using a spherical lens.

(14) Another possible architecture of the perforations is shown in FIG. 3, in which, unlike FIG. 2, the perforations 10 have a form resulting from the intersection of three branches, each branch following the direction of a hexagon wall. The central bar 14 represents the area of impact of the laser on the plate, which is here a rectangular focal spot that can be obtained by passing the laser beam through a cylindrical lens for example.

(15) The perforations can be obtained by proceeding with a laser machining or by photoetching of the metal plate. It should be noted that the circular form of the perforations in FIG. 2 is easier to achieve than the perforations in FIG. 3.

(16) The objective of the perforation of the plate is to reduce the surface area of the plate that the laser beam must heat by isolating elements 11 between which the thermal conduction is minimised by bridges 12, the role of which is to provide the structuring and cohesion of all the elements 11 in a single plate 5. The advantage of the elements 11 thus obtained in the plate 5 is to reduce the metal mass to be heated and consequently to reduce the heating time and therefore to increase the ignition dynamics of the pyrotechnic charge in contact with the plate 5. The multiplicity of elements 11 that are heated also increases the number of grains of the powder that constitutes the pyrotechnic charge, which are raised to the ignition temperature, that is to say the temperature at which they react. The pyrotechnic initiation of the pyrotechnic charge is thus less punctiform and more homogeneous, which increases the reliability of initiation and reduces the risks of long fire.

(17) Another advantage of the periodic multi-perforation is that, as the perforation patterns are repeated over the entire surface of the plate, the elements 11 and the bridges 12 are all identical. Consequently a misalignment of the focal spot does not significantly influence the heat transfer. For example, in FIG. 2, seven elements 11 or sub-targets are illuminated by the focal spot of the laser beam (the non-illuminated elements 11 not being heated, because of the presence of the bridges 12, which limits the thermal conduction). A misalignment of this focal spot would mean that it would still illuminate the equivalent of seven sub-targets. Thus, even in the case of optical misalignment, the pyrotechnic composition would be heated in the same way. In FIG. 3, there are eight elements that are illuminated and, even in the case of axial or angular misalignment of the laser beam, eight sub-targets always remain illuminated.

CITED REFERENCES

(18) [1] EP 2 508 838 A1

(19) [2] EP 1742 009 A1

(20) [3] FR 2 831 659 A1