FR2620239A1 - OPTICAL SYSTEM FOR RADIATION SENSOR - Google Patents

Abstract

An optical system 18 for infrared radiation sensor is made from crystalline semiconductor material with a cap or arc-shaped section and is inserted into a mounting ring 46 of the housing leaving a peripheral annular gap 47 which receives a ring 48 in sintered powder for soldering the glass. The melting of this sintered ring for soldering the glass 48 causes, after cooling, a hermetic junction between the periphery 45 of the optical system and the mounting ring 46 of the housing, for which an iron alloy is preferably chosen. nickel, for the convenience of machining.

Description

OPTICAL SYSTEM FOR RADIATION SENSOR

  The present invention relates to an optical system for a radiation sensor, in particular for an infrared radiation sensor, with attachment of at least one lens body to the sensor housing. Such an optical system for sensors intended for the detection of targets on the basis of radiation

  infrared, is known from German patent 33 26 876.

  To the. Unlike the embodiment shown there in the drawing, the sensor can also include a hollow cylinder-shaped housing which can be folded down around a pivot axis parallel to the axis of the munition from the inside. of

  this in an active position, outwards.

  Depending on the existing space conditions, provision may also be made, in addition, to mount the sensor coaxially, therefore to mount the detector element without deflecting the beam in the hollow cylindrical housing of the sensor axially after the optical system with lenses inserted in the opening. This gives, with a more compact structure, simpler adjustment possibilities with regard to the desired optical geometry and generally makes it possible to produce the sensor so that it can be subjected to greater stresses, therefore that it is more resistant. shooting. However, sealing the interior of the sensor housing on the side of the aperture with the optical lens system can cause problems. Indeed, the assembly into a complete sensor of separate modules, manufactured by being hermetically encapsulated, is very complex. After mounting the detector and the hybrid circuit, as well as after checking their operation, it is certainly possible to securely close the hollow cylindrical housing of the sensor towards the rear by hermetically sealing a cover, but the functionally critical closure remains from the sensor housing to the connection between the body of the

lens and housing wall.

  Attempts to radially bond the lens body with the housing wall using epoxy resin, as is known per se in industrial optics for bonding an optical component to a frame, have failed, since such junctions do not have not been found to be vacuum-tight; they give off gas, which is detrimental to the operation of the semiconductor components contained in the sensor, and they are hygroscopic, which can cause disturbances in electrical operation due to corrosion of the conductive tracks. Radial embedding with O-rings has not been shown to be airtight either, since in the event of strong axial accelerations, large deformations of these annular seals occur, which can

  destroy the optical system.

  In view of these facts, the object of the invention is to design an optical system of the type indicated so that they satisfy higher mechanical requirements from the point of view of airtightness and

optical quality.

  To achieve this objective, according to the invention, an optical system with arc section lenses made of crystalline semiconductor material is embedded in a metal mounting ring of the housing with the interposition of a sintered powder ring for the

glass welding.

  According to this solution, it is possible to admit a relatively large annular interval between the periphery of the lens and the frame ring, an interval which houses a sintered ring, easy to handle, constituted by the solder material of the pulverulent glass per se, that the '' then melts, after positioning the optical lens system inside the sensor housing. By cooling, a hermetic junction is obtained between the lens body and the gas-tight frame ring, devoid of water molecules and capable of undergoing

high mechanical stresses.

  As a choice of material for the mounting ring (or for the sensor housing as a whole, when the latter is made in one piece>, an iron-nickel alloy meets the requirements for agreement of the temperature gradient between the body lens crystal and the metal frame, although the temperature curve over the extended interval between the melting temperature of the sintered ring of glass solder material and the surrounding storage temperature, however, exhibits considerable differences between the two. However, it has surprisingly turned out that, in the case of a convex cross-sectional geometry, in the shape of a cap, of the body of the optical system with lenses pressed along its periphery, it accepts the residual tensions without destruction. still present after a cooling treatment, which must be attributed in particular to differences in temperature curves; further despite the two domed lens body surfaces s differently necessary due to the geometrical optics of the converging lenses, there are practically no more after cooling of the ring of molten solder material, optical defects, which may be due to the fact that the deformations on one side of the lens are very largely compensated, from the point of view of geometric optics, by deformations on the surface of

opposite lens.

  Other variants, developments, characteristics and advantages of the invention

  will emerge from the detailed description which follows,

  of an exemplary embodiment shown in the accompanying drawing, limited to the essentials, and not to the scale, the single figure of which represents in longitudinal axial section the mounting of a converging lens in the opening of the housing 'a radiation sensor. The radiation sensor 11 represented comprises a detector 12 for capturing radiated energy 10 in the infrared range of the spectrum of electromagnetic waves. The detector 12 comprises at least one detector element 13 <produced in the form of a semiconductor detector or a pyrometric detector) which

  is sealed in a housing 15.

  The detector element 13 receives the radiated energy 10 through a detection window 16 via a sensor opening 17 which includes an optical system & lenses 18 mounted in the housing 19 of the sensor, to transform the energy radiated 10 into electrical signals. For this, there is provided in a housing 21 which is hermetically connected to the housing 15 of the detector, a hybrid assembly electrically connected to the detector element 13, as described in detail in German application 36 36 422

dated 25/10/1986.

  The interior cavity 41 of the housing 19 of the sensor is filled with a hermetically sealed protective gas. The inclusion takes place on the one hand by the insertion of the housing 21 of the hybrid circuit under a housing cover 42 and, on the other hand, by the embedding of the optical lens system 18 in the opening 17. When in particular the sensor 11

  is part of a research rocket for a (under) -

  ammunition which can be fired from tube weapons, the mount of the optical system must be able to accept the high acceleration forces occurring during firing and still guarantee, even afterwards, the exact geometric positioning of the optical system with lenses 18 and

  hermetically closing the interior cavity 41.

  For this, the optical lens system 18, comprising at least one converging lens, has a generally curved section (therefore has a surface

  convex outer surface and a concave outer surface).

  A marginal zone 43 of the optical lens system 18 is arranged coaxially inside the hollow cylindrical housing 19 of the sensor, that is to say pressed axially against a support shoulder 44. It thus remains between the periphery 45 of the body of the optical lens system and the inner lateral surface of the part of the housing 19 of the sensor constituting a mounting ring 46, a peripheral annular slot 47 into which is inserted a ring of sintered material 48 consisting of glass welding powder as commonly found in commerce, for example in the form of borosilicate

  at Firma Schott Glaswerke).

  Preferably, the centering of the lens optical system 18 is not carried out by means of this ring of sintered material 48, but on frustoconically inclined surfaces of the support shoulders 44 of the frame and of the marginal zone 43 of the lens , so that the glass solder ring 48 can be inserted with a slight play fit. The solder ring 48 can be sintered without problems at a constant wall thickness, which guarantees a thickness of uniform solder layer for the junction of the periphery 25 of the lens with the mounting ring 46 via the annular slot 48, when the ring is melted

  of sintered material 48 after insertion.

  The body of the lens optical system 18 consists of a material transparent for the spectral range of interest, in the case of infrared radiated energy 10, a monocrystalline or polycrystalline semiconductor material such as silicon or germanium. The housing 19 of the sensor or, in the case of a divided housing 19, at least its part serving as a mounting ring 46, must have over the extended temperature range between the melting temperature of the ring of sintered material of glass welding 48 (of the order of + 700 ° C.) and the lowest permissible surrounding temperature of the sensor [11 (of the order of -40 ° C.)], a concordance of the coefficient of thermal expansion with the material of the system, lens optics 18, therefore in particular with silicon. Metals such as tungsten or molybdenum are therefore a particularly suitable material for the mounting ring 46. In fact, they are not particularly suitable for mass production, because they are difficult to handle, for example they are poorly welded with other components or elements of the sensor housing 19 (such as for example a detector base 49 produced separately). An iron-nickel alloy, such as for example that which is marketed under the name of "Vacon 10" by the company Vakuumschemelze, represents a particularly good compromise between the conditions imposed on the temperature curves and the conditions imposed on manufacturing. This material can be welded without problems with common machine steel from which the detector base 49 can be manufactured in turn, as shown schematically in the schematic diagram of the drawing by the weld bead

  peripheral 50 of a divided sensor unit 19.

  This iron-nickel material certainly does not necessarily have over a wide range a good agreement of coefficient of thermal expansion with the crystalline material of system 18; this is why it is not possible to join, with insertion of the ring of solder material 48, a flat plate to the ring of mount 46, because this junction loses its tightness during cooling of the solder temperature to the surrounding temperature and can even break the crystal plate. However, the tensions still appearing even after cooling, due to the still insufficient agreement of the coefficients of thermal expansion, are supported without problems when the system 18 has, as shown schematically in the drawing, upper surfaces and lower curves in the same direction, in section, between the radial embedding points. The fusion of the ring of sintered material for welding glass 48 then directly leads, without flow of the glass welding material, to a hermetically sealed weld of the system 18 with the metal ring 46, while retaining the optical quality of the system 18. used, both during cooling and during and after significant axial accelerations. The Junction is gas and moisture tight and is well suited for mass production both with regard to the welding of the glass in the ring 46 and with regard to the Junction of this ring 46 to obtain the housing sensor 19 complete; even along the periphery 45 of the lens, therefore on the annular slot 47, it is possible to admit greater tolerances than in the case of a common bonding joint, which are flattened while ensuring sealing and mechanical resistance by

  the ring of sintered material for welding glass 48.

  The posterior hermetic closure of the internal cavity 41 of the sensor can be carried out directly by adjusted insertion of the housing 21 of the hybrid circuit which is fixed to it, or even, more simply, by a rear cover 51. As symbolically shown on the drawing, this is first, suitably, screwed to check the operation of the completed sensor 11, to compensate for axial tolerances of the inserted components; optionally, after the completion of the assembly and adjustment operations, it is hermetically connected to the rest of the hollow cylindrical casing 19 of the sensor by a

peripheral weld bead.

  To keep the body of the optical system 18 opposite to its peripheral seat on the support shoulder 44 protected against impact, it is possible, as has also been shown in the drawing, to introduce into the clearance of the opening 17, a support ring 52, for example screwing it until

  contact with the optical system 18.

Claims (6)

  1. Optical system (18) for a radiation sensor (11), in particular for an infrared radiation sensor (11), with attachment of at least one lens body to the housing (19) of the sensor, characterized in that '' an optical system of lenses (18), of arc section, constituted by a crystalline semiconductor material, is embedded in a ring of mount (46) of metal of the case (19) with interposition of a ring (48) sintered powder for glass welding.   2. Optical system according to claim 1, characterized in that the ring of sintered material for welding the glass (48) is inserted in an annular gap (47) between the periphery (45) of the body of   lens and ring (46) of the housing.   3. Optical system according to any one of   claims 1 and 2, characterized in that the system   lens optic (18) is supported axially by a marginal zone (43) against a hollow frustoconical support shoulder (44) at the peripheral surface   inside the ring (46) of the housing.   4. Optical system according to claim 3, characterized in that, opposite the support shoulder (44), a support ring (52) is supported axially inside the ring of frame <46) against the body   of the lens optical system (18).   5. Optical system according to any one of   previous claims, characterized in that it is   incorporated in a multi-part sensor housing (19), one part of which constitutes the mounting ring (46) of the lens body.   6. Optical system according to any one of   previous claims, characterized in that   the mounting ring (46) for the crystalline body of the lens optical system (18) consists of a iron-nickel alloy.