Abstract:
A variable optical device for controlling the propagation of light has a body of liquid crystal optical material with a center and a periphery, a heating system including an electrically controllable heat source and a thermal radiator arranged at the periphery for cooling a portion of the body of material. The heating system is operative to generate a spatially modulated temperature gradient and to provide a desired light propagation behavior.

Description:
TECHNICAL FIELD 
     The present invention relates to the field of tunable liquid crystal optical devices. 
     GENERAL DESCRIPTION 
     The refractive index of a material is usually proportional to the density of the material. Accordingly, since most materials expand upon heating, the refractive index of most materials decreases with increasing temperature. This is not the case for specific ordered materials, such as oriented liquid crystals (LC), since the ordering of LC molecules can drastically change their optical properties. Thus, the extraordinary polarized light will usually see different refractive index n e  compared to the ordinary polarized light, which will see ordinary refractive index n o . Consequently, the degradation of that ordering due to the heat may also generate corresponding modulations of its refractive index. In the case of positive anisotropy (n e &gt;n o ) the heat may reduce the n e  but increase n o . In fact, different LCs will have different behaviors in different temperature ranges (e.g., in some cases, both n e  and n o  may be decreasing with increasing temperature, etc.). The corresponding refractive index variations (with temperature) in those kinds of materials may be very strong. Differentially heating a uniform body of material will lead to a gradient in the refractive index of the material corresponding to the temperature gradient in the material. More specifically, in the above mentioned example, the refractive index n e  will be lowest where the greatest increase in temperature occurs. 
     It follows that, for materials with temperature sensitive optical properties, such as liquid crystals, a lensing effect can be created by taking advantage of the temperature dependence of the refractive index of the material. 
       FIG. 1  illustrates a typical example of the temperature dependence of the refractive index (n) of a liquid crystal material. The experimental results reveal the material&#39;s dn/dT, which is higher for some materials than for others. 
       FIG. 2  illustrates an example of the temperature-dependent refractive indices for two different liquid crystal materials. In this case, both materials are high temperature-gradient refractive index (dn/dT) liquid crystal materials. The experimental results reveal that, for such materials, an important change in refractive index can be obtained for a relatively small change in temperature. 
     The following are rough estimations of temperature dependent optical power, taking for example certain predefined material parameters: 
     Material Data:
         Going from room temperature T to isotropic phase, we can change the n by almost Δn=0.1   Let us suppose we use the half of the above mentioned range, the slope becomes: 0.05/20° C.=2.5×10 −3 /° C.   LC film thickness is h=50 um       

     Optical Basics:
         Optical Power (OP)=2 (Δn h)/r 2          

     We need to have 10 diopters (OP=1/0.1 m=1/100 mm). 
     For r=0.87 mm, between 40 and 80° C. (giving a total of Δn=40° C.×2.5×10 −3 /° C.≈0.1), OP=2×0.1×0.05 mm/0.757 mm 2 =0.0132/mm=13 Diopters. 
     Thus the non uniform heating of a high dn/dT material in order to produce a lensing effect could be a cost-effective way of producing tunable lenses. 
     SUMMARY 
     The present invention provides a novel heating system for generating a thermally actuated optical lens, and the use of this heating system to create a tunable liquid crystal lens, as well as modules and devices made thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by way of the following detailed description of embodiments of the invention with reference to the appended drawings, in which: 
         FIG. 1  illustrates a prior art example of the temperature-dependent refractive index of a liquid crystal material; 
         FIG. 2  illustrates a prior art example of the temperature-dependent refractive indices of high do/dT liquid crystal materials; 
         FIG. 3  is a schematic representation of a novel liquid crystal (LC) lens configuration, according to a non-limiting example of implementation of the present invention; 
         FIG. 4  illustrates the differential temperature gradient across the lens aperture, for the LC lens configuration shown in  FIG. 3 ; 
         FIGS. 5 to 7  illustrate variant homeotropically aligned LC providing a positive tunable lens configurations in the case of positive optical anisotropy, according to non-limiting examples of implementation of the present invention, in which the liquid crystal cell director is in the normal direction to the substrates. 
         FIGS. 8 to 9  schematically illustrate alternative variants of LC lens configurations, according to non-limiting examples of implementation of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to a novel heating system for generating a thermally actuated optical lens, and the use of this heating system to create a tunable liquid crystal lens, as well as modules and devices made thereof. 
     In order to have differential refractive index modulation and corresponding lensing effect when heating a liquid crystal body of material arranged in a lens configuration, we need:
         1—Appropriate thermal conduction and diffusion across the lens aperture; and   2—different temperatures across the aperture.       

       FIG. 3  is a schematic representation of a liquid crystal (LC) lens configuration, according to an example of implementation of the present invention. A planar layer of liquid crystal is arranged on top of a bottom substrate, and is itself coated with a top substrate. An optically transparent, electrically controlled local heater is arranged between the LC layer and the bottom substrate, positioned centrally with respect to the LC layer. A ring-shaped thermal radiator is arranged on top of the top substrate, proximate to a periphery of the LC layer. This thermal radiator, which may be implemented by any appropriate heat-radiating material (e.g. metal), act as a heat sink for cooling the top substrate and the LC layer. 
       FIG. 4  illustrates an example of the variable temperature gradient across the LC lens that can be achieved by heating and cooling the substrate-coated LC layer with the heating system of  FIG. 3 . As shown, a peak temperature T C  is obtained at the center of the lens aperture, while the temperature of the material at the border, T B , is significantly lower as a result of the cooling by the thermal radiator. 
     Advantageously, the use of an annular thermal radiator to cool a portion of the substrate-coated LC layer, in combination with an electrically controllable local heater for applying heat directly to the center of the LC layer, provides for an improved control over the optical properties of the LC lens. 
       FIGS. 5 to 9  are schematic representations of variant LC lens configurations using the novel heating system described above, according to non-limiting examples of implementation of the present invention. 
     In  FIG. 5 , a patterned fixed conductive structure, formed of a fixed conductor electrode and an integrated, electrically-controllable heater, is arranged between the bottom substrate and the LC layer. This conductive structure may perform multiple functions, including heating of the electrode and thus of the LC material, as well as a thermal sensing function (to help with the heating function, etc). Advantageously, this configuration would not require two cross layers of LC to handle light polarization. 
     In  FIG. 6 , an additional top transparent electrode (e.g. ITO layer) is added to the lens configuration of  FIG. 5 . The electrode system formed of the top and bottom electrodes is operative to generate an electric field acting on the LC layer, in response to an applied drive signal. Advantageously, this configuration would not require two cross layers of LC to handle polarization. Furthermore, this configuration would allow a double control of the optical properties of the lens, notably electric and thermal control. In a specific example, for an initial planar alignment of the LC molecules and no electric field, a negative tunability can be achieved (by heating the electrode from the center). In another specific example, for an initial planar alignment of the LC molecules and a strong electric field (vertical alignment), positive tunability can be achieved. 
     In  FIG. 7 , there is shown a simplified fabrication of the LC lens, without the alignment layers. In this case, the centrally-positioned local heater is separate from the bottom electrode (which is optional) and can itself perform multiple functions, including heating of the electrode and a thermal sensing function. 
     In  FIG. 8 , the patterned fixed conductive structure of  FIG. 5  is sandwiched between a pair of cross layers of LC and provides all of the above-described advantages. This configuration may provide useful modes of manipulation of light depending upon its polarization. 
     In  FIG. 9 , the LC cross layer lens configuration of  FIG. 8  is adapted to include top and bottom electrodes. Accordingly, the two cross oriented LC layers compensate for the light polarization dependency and the patterned fixed conductive structure combined with the electrode structure provide for a double control (electric and thermal) of the lens optical properties. In a specific example, for an initial planar alignment of the LC molecules and no electric field, a negative tunability can be achieved (by heating the electrode from the center). In another specific example, for an initial planar alignment of the LC molecules and a strong electric field (vertical alignment), positive tunability can be achieved. The choice of the type of the liquid crystal (particularly of the temperature dependence of its refractive indexes n e  and n o ) and the control zone of temperature changes will provide very rich control possibilities. 
     It is important to note that the above-described embodiments of the present invention have been presented for illustration purposes but that additional variants and modification are possible and should not be excluded from the scope of the present invention. 
     It should also be appreciated by the reader that various optical devices can be developed using the device described above.