NL2002213C2

NL2002213C2 - Deformable mirror with internal reflection.

Info

Publication number: NL2002213C2
Application number: NL2002213A
Authority: NL
Inventors: Gleb Vdovine
Original assignee: Flexible Optical B V
Priority date: 2008-11-17
Filing date: 2008-11-17
Publication date: 2010-05-18

Description

Deformable Mirror with Internal Reflection BACKGROUND OF THE INVENTION

5 1. Field of the Invention

This invention generally relates to the field of spatial light modulators and, more particularly to deformable mirrors for wavefront modulation and correction.

10 2. Description of the Prior Art A deformable mirror of the general kind is described in the article of R.H. Freeman and J. E. Pearson, “Deformable mirrors for all seasons and reasons” published in Applied Optics Vol. 21, No. 4, pp. 580 - 588, in 1982. The cross section of the mirror is shown 15 in Fig. 1. The mirror consists of a base plate 1.1, individual piezoelectric stack actuators 1.2 and a deformable surface 1.3. Each actuator is bonded to the substrate on one side and to the deformable surface on the other side. The substrate has openings 1.4 through which electrical wiring 1.5 is connected to the piezoelectric stack actuators.

The price of traditional deformable mirror is high because the mechanical design of the 20 mirror should be extremely stable, optical figure of the mirror should be fabricated with a very high precision, which is difficult to achieve for a deformable reflective plate. Piezoelectric actuators have relatively low reliability and very high cost which can be in the range $100 to $1000 per actuator. High stiffness of stack actuators can lead to destruction of the flexible mirror surface at full actuator stroke.

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Another approach to a spatial light wavefront modulator was reported in the article of K. Hees, R. Dandliker and R. Thalmann, “Deformable surface spatial light modulator” published in Optical Engineering Vol. 26, No. 5, p 418 (May 1987). The cross section of the modulator is shown in Fig. 2. The modulator consists of a transparent prism 2.1 30 coated with a conductive layer 2.2 over which a layer 2.3 of transparent dielectric deformable gel is deposited. All input light reflects from the interface between the gel and the air because the condition of full internal reflection is satisfied. The shape of the gel surface is deformed by a potential distribution created on the photoconductor layer 2.6 by the incident light 2.9 and external voltage source. The external light projects the 2 control pattern on the photoconductor. A comb structure 2.7 is used to create a periodic potential distribution on the photoconductor layer, resulting in periodic deformation of the dielectric gel surface caused by electrostatic forces. The gel surface is protected from the incoming light by the opaque layer 2.5. The transparent glass substrate 2.8 5 serves for the mechanical protection of the electrode structure. Small defomiations are inherent to the scheme shown in Fig. 2: the maximum displacement of the gel surface can reach only a small fraction of the total thickness of the gel layer, as large relative deformations will cause the mechanical destmction of the gel layer. On the other hand the gel thickness should be small to keep the control voltages low. Using the periodic 10 electrode structure further limits the maximum displacement of the gel surface.

Dielectric nature of the gel reduces the maximum deformation even further: although any dielectric experiences pressure forcing it to fill a gap, under the influence of the electric field, this force is much smaller than the force experienced by a conductor in the same conditions. Reported in the article maximum deformation of the gel layer 15 reached hundreds of nanometers, which is sufficient for image projection, but not enough for control of optical wavefronts in adaptive optics, where deformations of the order of microns to tens of microns are required.

Wavefront correctors based on liquid interfaces were reported by S. Kuiper and B.H 20 Hendriks “Variable-focus liquid lens for miniature cameras”, Applied Physics letters, vol. 85, No. 7, p. 1128 (Aug. 2004). The principle of operation of the device is based on electrovetting, causing change of the contact angle of a liquid drop. As only the edges of liquid surface are controlled, this principle is applicable to correction of a limited number of aberrations and is not usable for correction of random wavefronts.

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Ferrofluid mirror was patented by William Shuter in the US patent 5650880. In this invention the shape of rotating mirror was controlled by magnetic fields. Rotation of the mirror limits the aberrations to axi-symmetrical only, making the mirror not suitable for wavefront correction. Further, ferrofluid is not transparent and the surface reflectivity is 30 low.

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SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a low-cost deformable mirror that is scalable to large number of control channels N, (N=l .... 106) and can be 5 electrostatically controlled by a matrix of integrated electrodes fabricated by using printed board technology for small number of control channels N, and by integrated microelectronics technology such as CMOS active matrix for large number of control channels.

10 It is a further objective of this invention to provide a deformable mirror with optical surface fabricated without using grinding and polishing operations. This objective is achievable as the surface of the liquid naturally takes a flat horizontal shape, with no special forming or polishing operation required.

15 It is a further objective of this invention to provide a deformable mirror with integrated actuator connections, having no separate wires inside the mirror. This object is reached by a deformable mirror of the kind referred to above wherein the electrical connections to the actuators are provided by at least one printed circuit board or CMOS or bipolar active matrix of control electrodes.

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Another object of this invention is to provide a deformable mirror with thermal compensation, reducing its thermal defomiations due to changing ambient temperature and under high power laser load . This objective is easily achievable with our invention as the shape of the liquid surface does not depend on the temperature, securing perfect 25 thermal compensation.

Still another object of this invention is to provide a deformable mirror with highly reliable actuator operation. This aim is reached by using actuators that actuate the liquid surface by electrostatic field. Since the field is not a material object, it can not be 30 damaged, thus increasing the reliability.

Still another object of this invention is to provide a deformable mirror with a very large number of control channels. This aim is reached by using bipolar or CMOS active matrix for control of the shape of surface of conductive liquid. Unlike gels or dielectric 4 liquids, conductive liquid surface has a very high sensitivity to external electrostatic actuation, making possible operation with large number of relatively low-voltage control actuators. Further, the surface of liquid has no stiffness, therefore profiles with very high spatial frequency are readily obtainable.

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Another object of this invention is to provide a deformable mirror with actuators sufficiently protected against the incident high power radiation.

This aim is reached by using full internal reflection from the deformable surface of a 10 transparent liquid or gel layer, wherein no radiation is transmitted through the liquid surface to the actuating electrodes.

Still another object of this invention is the increase of the wavefront correction range with respect to ordinary deformable mirror. Indeed, it can be shown that if the 15 deformation of liquid interface is φ then the full deformation of the wavefront after it leaves the output window will be equal to 2φη where η > 1 is the refraction index of liquid.

Still another object of this invention is to provide a mechanically stable design of a 20 deformable mirror with good damping and a high frequency of the first mechanical resonance. This aim is reached by adjusting the liquid viscosity to a level that makes the mirror insensitive to external mechanical perturbations. Although the scope of working liquids for this invention is not limited to glycerol and water, we can give an example that the viscosity of a solution of water in glycerol can be controlled in a wide range by 25 adjusting the amount of added water.

The advantages of this invention arc achieved by using the internal reflection on the electrostatically deformable surface of a transparent conductive liquid or a colloidal solution.

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The operation of the proposed wavefront corrector is explained in Fig, 3, that also illustrates the preferred implementation of the corrector. The incident light goes through the input window formed by the left cathetus of the right-angle prism 3.1. The prism is fabricated from a transparent material with refraction index of nl > 1. The hypothenuse 5 of the prism with some extra borders that form a reservoir filled with transparent liquid, colloidal solution with refractive index n2, that is close to nl so that the reflection on the interface between the prism and the liquid is weak and can be neglected. The size of the hypothenuse is sufficiently large to eliminate the influence of 5 the edge capillary effects on the surface flatness in the central part of the hypothenuse.

It is also possible that the reservoir is much larger than the prism hypotenuse, to further reduce the influence of edge capillary effects. The incident light passes the interface between the prism and the liquid and undergoes a partial or full reflection on the interface between the liquid and the ambient gas. It is advisable, but not necessary, that 10 he partial pressure of liquid vapour in the gas should be high enough, to slow down or prevent the evaporation of the liquid.

The electrode structure 3.3 with N electrodes is placed at a distance d over the surface of the liquid. When a constant voltage V is applied to the i-th electrode, the liquid surface is statically locally deformed, the by the electrostatic forces. The nature of 15 electrostatic forces is different for dielectric and conductive liquids, in general conductive liquids will experience larger deformations than dielectrics. If the liquid is dielectric, the deformation depends on the dielectric constant of the liquid, the higher the constant, the higher the deformation. In many cases dielectric liquid can be made conductive by dissolving a small amount of inorganic salt, making it electrolyte.

20 The liquid surface can be only horizontal. This property limits the applicability of the mirror. However, in many cases the liquid can be converted to a colloidal solution which would be able to preserve its flat surface in a non-horizontal state.

25 BRIEF DESCRIPTION OF THE DRAWINGS

Subsequently the present invention will be elucidated with the help of the accompanying drawings wherein: FIG. 1 illustrates the Prior Art, representing a not to scale cross section of a 30 deformable mirror with stacked piezoelectric actuators based on direct piezo effect; FIG. 2 illustrates the Prior Art, representing a not to scale cross section of a spatial light modulator based on full internal reflection in a layer of dielectric gel.

FIG. 3 represents an example cross section of the preferred configuration of the proposed invention.

6 FIG. 4 represents an example implementation of the proposed invention with the second cathetus of the prism coated with mirror coating, resulting in doubling of the wavefront deformation introduced by the mirror.

FIG. 5 represents an example implementation of the proposed invention with the input 5 and output window formed by a plurality of prisms.

DETAILED DESCRIPTION OF THE INVENTION

While the invention is described below with reference to illustrative embodiments for a 10 particular application, it should be understood that the invention is not limited to these embodiments. Fig. 3 shows the schematic cross section of a deformable mirror according to this invention. The mirror includes a prism 3.1 with input and output windows positioned on the catheti of the prism. The hypothenuse of the prism is covered by a plurality of layers of non-mixable liquids 3.2, whereas the reflection 15 happens on the interface between different liquid layers and/or on the interface of the top liquid layer with the ambient. The surface of liquid is deformed by applying potentials between liquid and control electrodes 3.3. The mechanisms of electrostatic deformation of dielectric and conductive liquids are physically different, with maximum deformation achieved when conductive liquid is used.

20 The static shape U (x, // ) of liquid interface is described by the equation: y) = te°vy)2/2(P ~ P9U(x> y) -kU(a:> y) where Δ is the Laplace operator, p is the liquid density, g is the gravitational constant, e is the dielectric constant of the gas filling the gap between the liquid and the electrode, k is the elastic constant (k > 0 for gels and k = 0 for liquids), and T is the 25 surface tension of the liquid.

Accordingly, the surface shape is determined by: - gravity, represented by the term -pgU(x, //); - surface tension, represented by the factor T; - electrostatic control, represented by the term η,,Γι.λ y)2/2d2; 30 - elastic reaction of the gel deformation —kU (x. y), k — 0 for liquids; \item Dirichlet U\L or Neumann <)U/0n\L boundary conditions along the edges L of the liquid pool.

The maximum deformation of the liquid surface can be estimated by assuming T=0: 7

Umax = KoVZaj2(pg + k)d2.

Assuming the electrode is positioned at the distance of 0.5 mm over the water surface, p = 1000 kg/m3, V=100 V and k = 0, we obtain for maximum surface displacement: Umax=18 micron. This result proves that optically significant deformations of the 5 liquid surface can be obtained with relatively low voltages and with relatively large gaps between liquid and the control electrode structure. For comparison, membrane deformable mirrors use voltages of the order of 300 V and gaps of the order of 50 micron.

10 High sensitivity and low tolerances make the liquid deformable mirror suitable for realization of large numbers of control channels, indeed a CMOS active matrix with maximum voltage of 50 V positioned 0.1 mm over liquid surface would be able to produce maximum displacements of the order of 180 micron, which is more than sufficient for adaptive optics.

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The properties of liquid mirror can be controlled in a wide range by using soluble and non-mixable mixtures of different liquids such as solution of glycerine in water or non-mixable layers of water and oil, where water would provide necessary conductivity as it can be made electrolyte by dissolving salts in it, while a thin layer of oil on top of water 20 would provide the necessary optical quality and dynamic damping of surface waves.

The properties of the deformable mirror could be further modified by using one or several layers of non-mixable colloidal solutions and gels.

25 The properties of the modulator can be further amended by submerging the electrode structure in the top layer of liquid, providing the electrostatic control is performed on one or a plurality of intermediate layers that separate different non-mixable liquids.

The properties of the modulator can be even further amended by using a combination of 30 electrostatic control of the surface by means of electrode structure as shown in Fig. 3 and electrovetting control of the contact angle at the edges L of the liquid pool.

The properties of the modulator can be further amended by using a sandwich deformable stmeture including interchanging liquid and gel layers.

5 8

The properties of the modulator can be further amended by using a sandwich deformable stmcture including interchanging dielectric and conductive layers kept at different electrical potentials.

The amplitude of wavefront correction can be doubled as shown in Fig. 4 by making one cathetus of the prism reflected, so that the incoming light undergoes two reflections on the deformable surface of the liquid.

10 The mirror can be made more compact by using a plurality of prisms for input and output window as shown in Fig. 5.

The correction range can be doubled by coating one side of all prisms with a reflective coating, so that the incoming light undergoes two reflections on the deformable surface 15 of the liquid.

Water, glycerol, NaCl (salt) and, their mixtures and solutions can be used as deformable conductive liquids. Silicon gels as well as gels based on water and gelatine, with additions of salt can be used as dielectric and conductive gel layers. These 20 materials arc listed here as an example, the scope of this invention is not limited to these materials.

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Claims

A deformable mirror for modulation and correction of optical wave fronts, comprising: 5. a transparent optical holder comprising the input and output windows; - at least one layer of an immiscible, transparent, electrically conductive, deformable substance incorporated in the container; and - an electrostatic control structure provided with electrodes, characterized in that the condition of internal reflection on the surface of the substance for optical wave fronts entering through the input window is met; and the wavefront exiting through the output window is modulated by electrostatic potentials applied to the control structure by electrostatic distortion of the surface of the substance.

Deformable mirror according to claim 1, characterized in that one or more layers of the transparent substance is liquid.

Deformable mirror according to claim 1 or 2, characterized in that one or more layers of the transparent substance is a gel.

4. Deformable mirror as claimed in claim 1, 2 or 3, characterized in that the input window and the output window are formed by the two adjacent sides of a rectangular prism, while the liquid layer covers the prism's hypothesis.

5. Deformable mirror according to claim 1,2 or 3, characterized in that the input window and the output window are both located in an adjacent side of the prism, while the second adjacent side is a mirror.

A deformable mirror according to any one of the preceding claims, characterized in that the control structure is represented by a matrix of integrated electrodes.

7. Deformable mirror according to one of the preceding claims, characterized in that the optical holder comprises a number of prisms with a common hypothesis, while the input window is represented by the number of prisms on one side and the output window is represented by the number of 5 prisms on the other side.

A deformable mirror according to any one of claims 1-6, characterized in that both the entrance windows and the exit windows are represented by adjacent sides on one side, while the other prisms are mirrors.

A deformable mirror according to any one of the preceding claims, characterized in that the electrode structure is represented by an active matrix made in CMOS technology or in bipolar technology. 15