Deformable mirror

A deformable mirror, especially for the compensation of the influences of atmospheric interferences on the propagation or spreading of high energy laser beams, which incorporates a plurality of electrically actuatable actuators or adjusting elements engaging behind the mirror surface of the mirror. The actuators or adjusting elements is pressed against the rear side of a mirror plate, and is connected therewith along jacket or sheathing surfaces. Thereby, it is possible to achieve an almost point-like introduction of force for the regional deformation of the mirror; inasmuch as the pressure forces can act with a practically randomly small cross-section directly against the rear side of the mirror, while the tensile forces are taken up by the jacket or sheathing surfaces.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a deformable mirror, especially for the 
compensation of the influences of atmospheric interferences on the 
propagation or spreading of high energy laser beams, which incorporates a 
plurality of electrically actuatable actuators or adjusting elements 
engaging behind the mirror surface of the mirror. 
2. Discussion of the Prior Art 
A deformable mirror of the type referred to hereinabove has become known 
with respect to its typical constructional and utilization capabilities 
from the disclosure of FIG. 1 and FIG. 2 of the article by James E. Harvey 
and Gary M. Callahan "Wavefront Error Compensation Capabilities of 
Multiactuator Deformable Mirrors" (SPIE Vol. 141, Adaptive Optical 
Components, 1978, pages 50 through 57). 
In order to avoid complicated actions of the control system, required from 
such a compensating mirror is a most linearly possible relationship 
between an actuation of an actuator or adjusting element and the therewith 
associated localized deformation of the mirror surface. Consequently, an 
effort is made to provide for the most rigid possible mechanical and most 
pointedly possible coupling or connection of each adjusting element to the 
rear side of the mirror plate, whose front side serves as a compensation 
mirror surface which is located in the path of the beam. However, the 
mechanical coupling between an actuator or adjusting element and the rear 
side of the mirror plate, which is constructed as an adhesive connection, 
necessitates an extremely large contact surface in order to be able to 
also take up comparatively large tensile forces through the adhesive 
connection. Hereby, such forces are encountered when at least one 
adjusting element is deflected within a certain area, the mirror surface 
will then bulge; however, nearby located adjusting elements because of 
their actuation will be less or not at all deflected and thereby subjected 
to tension. Such a large-surfaced contact between of the adjusting element 
and the rear side of the mirror plate, in the interest of providing an 
adequate adhesive bond, stands opposite the requirement for localized 
defined deflecting capabilities in the finest possible distribution or 
arrangement across the mirror surface, in the interest of obtaining 
optimum correlation capabilities for compensating distortions in the 
cross-section of the path of the beam. With respect to the dimensioning of 
the control circuit for the electrical actuation of the actuators or 
adjusting elements, it is, moreover, undesirable in that such adhesive 
bonds or connections possess a certain elasticity in the direction of 
pressure transmission and, as a result, must be taken into consideration 
during the design of the control circuit must be considered as disturbing 
damping and even propagation time influences. 
SUMMARY OF THE INVENTION 
In recognition of these conditions, it is an object of the present 
invention to so design a compensating mirror of that type of construction, 
which at the lowest possible actuating demands on control technology, 
facilitates a finely arranged, defined compensation of the atmospheric 
propagation interferences across the cross-section of the path of the 
beam, and is thereby simply produceable and adjustable, as well as being 
dependably employable. 
The foregoing object is inventively achieved in that the compensating 
mirror of the type described herein has the actuators or adjusting 
elements pressed against the rear side of a mirror plate, and is connected 
therewith along jacket or sheathing surfaces. 
In accordance with this solution, it is possible to achieve an almost 
point-like introduction of force for the regional deformation of the 
mirror; inasmuch as the pressure forces can act with a practically 
randomly small cross-section directly against the rear side of the mirror, 
while the tensile forces are taken up by the jacket or sheathing surfaces; 
thus, for example, for adhesive connections over large-surfaced adhesive 
zones which are subjected to shear loads. Provided for this attachment are 
pivots or trunnions, which are preferably integrally formed on the rear 
side of the mirror plate (for instance, machined or without machining 
formed thereon), the mirror plate itself can be constructed as thin as 
possible; in essence, without the need for moving a large mass, to be 
deformed easily and in a locally defined manner. Herein, namely, the 
geometry of the introduction of the force from the actuators or adjusting 
elements to the mirror plate can be influenced by the cross-section of the 
connecting pivots or trunnions, which facilitate a mechanically stable and 
rigid attachment, in effect, for an adjusting element engagement without 
any interposes adhesive connection in the direction of pressure. As a 
result, there are also opened rotational possibilities for a precision 
adjustment with respect to the prestressing of the adjusting elements 
which are supported against the mirror plate; inasmuch as there will no 
longer occur any losses in sensitivity (with respect to the attachment of 
an adjusting element to the associated rearwardly facing region of the 
deformable mirror surface) caused by the elasticity and as a result of 
hysteresis influences of interposed adhesive layers.

DETAILED DESCRIPTION 
As is shown in detail, located in the path 3 of a beam is the mirror 
surface 8 of an MDA compensating mirror 9. This mirror, through exerting 
an optical influence over the optical conditions in the applicable 
cross-sectional regions of the path 3 of the beam, serves to provide a 
compensation of distortions to which the beam geometry is subjected during 
passage through the atmosphere, in order to be able to concentrate the 
beam energy in the most narrowly restricted focal point on a target. 
Provided for the deformation of the mirror surface 8 (through displacement 
of individual of its regions with respect to each other in the direction 
of the normal towards the mirror surface 8) are rearward linear adjusting 
elements 18, which are clamped between the mirror surface 8 and an 
abutment 19. These actuator elements 18, in accordance with their 
electrical actuation, are subjected to a change in length for the 
respective bulging or curving of the region of the mirror surface 8 
located in front thereof. Preferably, the actuators or adjusting elements 
18 relate to piezo-columns, inasmuch as these possess good linear response 
characteristics at a high response limiting frequency, and allow for the 
introduction of high as well as reproducible pressure forces behind the 
mirror surface 8. 
For such a defined deformation of the mirror surface 8, an effort is made 
to obtain the possibly most point-like contact of the actuators or 
adjusting elements 18 behind the surface. For this purpose, the mirror 
surface 8 is formed on the front side of a thin mirror plate 31, on the 
rear side of which 32 there are also provided comparatively thin pivots or 
sleeve-shaped trunnions 33, in essence (further details thereof being set 
forth hereinbelow), are formed integrally therewith. The diameter of the 
trunnions 33, in the example of FIGS. 1 and 2, is somewhat greater than 
the thickness of the mirror plate 31; however, it is of the same magnitude 
(of a few millimeters in the instance of a diameter of a mirror plate 
being of the magnitude, for example, of about 150 mm to 250 mm). The 
points of pressure introduction by the adjusting elements 18 into the 
mirror plate 31 are also constructively given through relatively thin 
trunnions 33 with their end surface 34 freely rearwardly projecting. 
Actuator elements 18 possessing a small diameter can be set directly 
abutingly on the trunnion end surface 34. For actuator or adjusting 
elements 18 possessing a larger diameter it is expedient, as is considered 
in FIG. 1, to interpose conical coupling members 35 behind the mirror 
surface 8 for geometric correlation and thereby an optimum introduction of 
force. An effort is to be made that the cross-section of the introduction 
of force which is effective with respect to the mirror surface 8, due to 
the small diameter of the trunnions 33 in comparison with the diameter of 
the mirror plate 31, and in comparison with the mutual spacing of the 
trunnions 33, is as point-like as possible. 
In the direction of pressure transmission, between the adjusting elements 
18 and the rear side 32 of the mirror in effect, the associated trunnions 
33, there is no need to provide any nonrigid mechanical coupling means, 
and especially no adhesive medium, inasmuch as the mechanical connection 
along the generated contact surfaces between the adjusting element 18 and 
the trunnion 33 which is subjected to tension, can in this instance be 
formed along the inner generated surface of a sleeve 36. This sleeve 36, 
as illustrated in the drawing, can be shaped funicular in its axial 
longitudinal section, when a conical coupling member 35 is arranged 
between a trunnion 33 and the actual adjusting element 18 so as to provide 
for the correlation in the cross-section. The sleeve or bushing 36 can be 
shrink-fitted thereon; however, in the simplest case it is glued along the 
adjoining generated casing surface together with the adjusting element 18 
and the trunnion 33; wherein the connecting sleeve 36 (and the length of 
the trunnion 33) is designed to be so lengthy that there is afforded the 
necessary adhesive surface in the parallel shear direction for assuming 
the tensile force and transfer along the inner wall of the sleeve 36. The 
coupling of the actuator or adjusting element 18 to the trunnion 33 which 
is oriented in the direction of the pressure, and thereby towards the 
associated region of the mirror plate 31 or the mirror plate 8, thus 
remains free of any adhesive material due to the directly abuting 
contacting push; and consequently free of masses which would dampen the 
introduction of forces into the plate 31, which otherwise due to the 
requirements, must consider damping phenomena, and possibly also 
propagation time phenomena which would lead to a complicated structure of 
the control circuit for the compensating actuation of the mirror surface 
8. 
The fact that the annularly-shaped adhesive surface, which is subjected to 
shear during the introduction of tensile forces, possesses a certain 
degree of elasticity, is not disturbing, inasmuch as this is extremely low 
because of the thin adhesive gap; and moreover, may even provide a 
desirable mechanical uncoupling between an extensively actuated actuator 
or adjusting element 18 and a neighboring less intensely (and thereby 
subjected to tension) adjusting element 18, with a correspondingly 
constantly compensated or balanced extent of bulging of the mirror surface 
8 between the neighboring rearward engaging points of the adjusting 
elements 18. 
As is illustrated in FIG. 1, for introducing the pressure force of an 
actuator or adjusting element 18 into the mirror plate 31, there is 
suitably provided in series with a short modulation actuator element 18m a 
separate multiple lengthier correcting actuator element 18k, as well as a 
rear support 37. Located behind the latter is an adjusting component 38, 
preferably in the shape of a threaded bolt 39 with a lock nut 40, and 
which is guided in the mirror abutment 19, in order to be able to 
individually adjust the mechanical prestressing of the individual 
actuators or adjusting elements 18 and to thereby, for example, impart to 
the mirror surface 8 a stationary initial deformation in correlation with 
other optics influencing parameters located in the path 3 of the beam 
path. 
The mirror plate 31 is retained through the intermediary of an encompassing 
rubber seal 41 against an end flange 42 of a hollow cylinder 43 which, at 
its rear end supports the abutment 19 for adjusting elements 18 through 
the interposition of an elastic seal 44. The space within the hollow 
cylinder 43, in essence, about the adjusting elements 18 between the 
abutment 19 and the mirror plate 31, is preferably filled with 
electrically-insulating and relatively high-viscous, as well as good as 
possible heat-conductive liquid 45, for instance, such as oil. On the one 
hand, this serves to provide for a mechanical damping of the mechanical 
movements in the mirror plate 31 which occur under the influence of the 
actuation of the adjusting elements 18, and on the other hand, for 
dissipation the heat produced thereby and through the incidence of the 
beams into the mirror surface 8; and moreover, for the electrical 
insulation between the piezo-adjusting elements 18 which are operated with 
relatively high voltages. 
For effecting an uncoupling between the actuators or adjusting elements 18 
and for the absorption of the pressure fluctuations in the relatively 
incompressible liquid 45, there can serve gas cushions 46 which, for 
example, are arranged in the form of grids constituted of closed-porous 
plastic material between the adjusting elements 18. 
In the actual constructional embodiment pursuant to FIG. 3, the trunnions 
33 which are formed on the rear of on the mirror surface 31, each possess 
a coaxial hollow space 47 in their end surface 34, which preferably even 
extends into the plate 31 and thereby extends from rearwardly close to the 
mirror surface. Engaging into this hollow space 47 is a correspondingly 
geometrically conformed coupling member 35, such that its pressure 
transmitting tip 48 leads to a point-like introduction of force closely 
behind the mirror surface 8. 
The function of the sleeve 36 (pursuant to FIG. 1) is herein also taken 
over in the region of the trunnions 33 by the wall of its hollow space 47. 
Preferably, the hollow space 47 or the therein engaging region of the 
coupling member 35 is so configured in a longitudinal section, that there 
is obtained an adhesive gap 49, which conically widens towards the end 
surface 34, intermediate the hollow space 47 and the therein engaging 
coupling member 35. This provides the advantage that by means of a thin 
adhesive layer, as shown in cross section in gap 49, in the region of the 
rear side 32 of the plate, there can be attained a rigid connection of the 
coupling member 35, while opposite thereto in the direction towards the 
trunnion end surface 34 there is obtained an increasingly elastic 
connection; whereby there is reduced any tight mechanical coupling to a 
neighboring trunnion 33 through frictional forces along the outer wall of 
a trunnion 33 and a plate rear side 32. Herein, an effort is made to 
obtain a circularly-symmetrical formation of the bulging of the mirror 
surface 8 about the point 48 of force introduction, with a deflection or 
bulge amplitude which attenuates in cross-section pursuant to a gaussic 
distribution curve; however, which in actual practice is disrupted through 
the force introducing conditions from the neighboring trunnions 33. 
The mirror plate 31 together with the trunnions 33 which are formed on the 
rear thereof, and its mirror surface 8 can be produced either through 
machining (for example, milled from a solid) or without machining (for 
example, from a casting); preferably it is constituted of copper, inasmuch 
as this possesses favorable reflective properties with regard to infrared 
radiation and with good temperature dissipating properties. 
Extraordinarily good reflective properties are evidenced by a metallic 
mirror plate 31 in which no grain boundaries are present; for instance, 
which is propagated as a monocrystal, and which in the most expedient 
instance does not even require polishing of its mirror surface 8. In this 
instance it is particularly expedient to allow for the concurrent growth 
of the trunnions 33 on the rear surface of the plate 31; in order to also 
avoid any grain boundaries to be formed in the trunnions 33; inasmuch as 
in this manner are there avoided any lattice distortions which, 
conceivably, could lead on the mirror surface 8 to an adverse influence 
over the reflective effects; and such a monocrystalline structure of the 
plate 31 with the trunnions 33 on the rear thereof also evidences 
particularly satisfactory properties with respect to the regional 
deformation pursuant to the extent of localized rearward introductions of 
force. At a monocrystalline growth of the trunnion 33 on the rear side 32 
of the plate, suitably, through applicable nuclei orientation deflective 
measures, there are formed curvilinear root areas 50 at the transitions 
from the rear side 32 of the plate to the cylindrical outer wall of the 
trunnions 33, in order to all possibly avoid herein any sharp kink 
locations and thereby any material fatigue phenomena caused by the 
alternating introductions of force from the actuators or adjusting 
elements 18. Preferably, such a curvilinear root area 50 does not extend 
circularly symmetrically about the axis of a trunnion 33, but is overally 
distorted in different directions and thereby applied adhesive sheet like 
on the rear side 32 of the plate, as is illustrated by the overall 
representation of FIG. 4. As a consequence, the introduction of the 
(tensile) force from one coupling element 35 through the surrounding 
hollow trunnion 33 into the rear side 32 of the mirror plate 31 is not 
circularly symmetrical; in view of which the anisotropy of the mechanical 
properties (especially the modulus of elasticity) of the monocrystalline 
metal plate 31 allows itself to be extensively compensated, which 
notwithstanding the point-like introduction of force can lead to a 
non-pointlike symmetrical deformation, and as a result not to the desired 
bulging or deflecting geometry of the mirror surface 8. The formation and 
orientation of such non-circular root areas 50 are to be so designed as to 
be oriented relative to the crystal matrix-elementary cubics in the 
concrete instance of the monocrystalline-propagated plate 31 (which can be 
determined through the intermediary of crystallography and material 
information). 
As is illustrated in FIG. 3 through the cross-hatching, for a further 
increase in the mechanical strength of the trunnions 33, notwithstanding 
the most possibly soft, and easily deformable material for the plate 31, 
and especially to avoid brittle failure phenomena in the root area 50, 
provision can be made that after the monocrystalline growth of the plate 
31 with the still pure edge region of the mirror surface 8, to add to the 
smelt an alloy material such as, for example, especially beryllium (in the 
magnitude of between 1% and 2%) whereby, about the engaging point of the 
tip 48 of the coupling element, and thereby in the trunnion root area 50 
as well as for the trunnions 33 themselves, to obtain an alloy and thereby 
a mechanically higher stressable material on the matrix structure of the 
material of the mirror surface 8. 
This also enhances the compensating effect of a structured root area 50 
which deviates in plan view from the circular shape (FIG. 4). 
As is considered in FIG. 3, it can be expedient to regionally vary the 
cross-sectional geometry and thereby the mass of the mirror plate 31, in 
order to thereby achieve a deattenuation coupling for the mechanical 
vibrations which spread out from the individual trunnions 33 to the 
neighboring trunnions 33; for instance, which are reflected at the plate 
rim 51. Such uncoupling masses consist especially of beadings or 
reinforcements 52 which extend across the rear side 32 of the plate along 
the rim 51 and about the trunnions 33. Hereby, these uncoupling beadings 
52, as is also illustrated in FIG. 4 need not be constructed in a 
continuous manner; it is sufficient to provide the formation in the shape 
of a series of individual humps or protuberances 53. As is also 
ascertainable from FIG. 4, it is expedient that in the connecting line 
between mutually neighboring trunnions 33, there be formed more extensive 
uncoupling measures in the form, for example, of higher or wider humps or 
protuberances 53 than in regions in which the nearest neighboring 
trunnions 33 are more remote from each other. A precise mechanical 
correlation can be simply effected through regional mass or weight 
reductions; for example, by the working down of predetermined 
protuberances 53. In comparison with the simplified representation in FIG. 
4, the arrangement of the uncoupling reinforcements 52 need not extend 
circularly concentrically about the applicable trunnions 33; in 
conformance with the geometry of distribution of the trunnions 33 on the 
rear side 32 of the plate it can be even more expedient to form a 
polygonal array, and as a result a honey combed structure from the 
entirety of the uncoupling reinforcements 52 on the rear side 32 of the 
plate. 
As is illustrated in FIG. 3, at least a few of such beadings or 
reinforcements 52 or protuberances 53 are expediently so designed and 
utilized as to contain the grid structures for the gas cushions 46, for 
the mechanical uncoupling of the actuators or adjusting elements 18 
through the damping liquid 45. 
Instead of the additional damping masses in the form of the individual 
humps or protuberances 53 or the encompassing beadings 52 on the rear side 
32 of the plate, or in addition to these measures, for the mechanical 
uncoupling of the introductions of force taking place at the neighboring 
trunnions 33, there can be also provided weakenings in the material of the 
plate 31 such as, for example, grooves 54 extending in the rear side 32 of 
the plate. Also these, in contrast with the simplified representation in 
FIG. 4, can possess a honey combed pattern in accordance with the extent 
of the geometric distribution of the trunnions 33. 
In the interest of a mechanically, and essentially also with respect to the 
localized alternating loads, most possibly stable mirror plate 31, at 
possibly lowest masses which are to be moved by the adjusting elements 18, 
pursuant to the representation of FIG. 5, there can also be provided a 
composite or sandwich-structured mirror plate 31. In this plate, a foil 55 
constituted of metal which serves as the mirror surface 8, preferably 
again constructed of copper, can be rigidly fastened on the front surface 
56 of a support plate 31 having pivots or trunnions 33 formed on the rear 
side thereof. 
The foregoing is produced from fiber-reinforced structural material, in 
which the more advantageous heat dissipating properties are provided from 
reinforcing materials of metal fibers or carbon fibers rather than glass 
fibers. Through the conical orientation 57 and the annular orientations 58 
of the fibers, especially in the root area 50 of the transition from the 
trunnion 33 to the plate 31, there can be influenced the geometric 
characteristics of the introduction of forces, and thereby the deformation 
of the mirror surface 8 pursuant to the measure of the granular or matrix 
orientation of the foil 55, and thereby concurrently realized a 
predeterminate uncoupling of the influencing magnitudes which originate 
from the neighboring trunnion 33. 
For the remainder, there can also be provided for this uncoupling a 
structure on the rear side 32 of the plate pursuant to FIG. 3; in essence 
with beadings 52 and grooves 54 constituting mass barriers between 
neighboring trunnions 33 and towards the rim 51. 
A still more extensive uncoupling can be occasioned when the mirror surface 
8 consists of facetted adjoiningly positioned foils 55, which are each 
associated with a trunnion 33. The thin slits between the individual foils 
55, in effect, the applicable facet boundary 59, does somewhat disrupt the 
normal dispersion of the attenuation of a point-like bulging in the center 
in front of a trunnion 33; however, on the other side it acts especially 
as a barrier against deforming influences caused by a neighboring trunnion 
33. 
Inasmuch as this position of the facet boundaries 59 relative to the 
arrangement of the trunnions is fixedly predetermined, then for the 
remainder the amplitude disruptions can be extensively compensated for 
through applicable orientations 57, 58 of the extent of the reinforcing 
fibers within the structural material of the mirror plate 51.