Abstract:
A liquid crystal infrared light chopper includes a polarizer that receives and polarizes incoming infrared light, a layer of ferroelectric liquid crystal material switchable between at least two states, a pair of IR transparent, conductive substrates positioned on either side of the liquid crystal layer and an analyzer that blocks IR light of one polarization state and passes IR light of an opposite polarization state. The liquid crystal layer acts on polarized IR light by changing its polarization if the liquid crystal is in a first state and by not changing its polarization if the liquid crystal is in a second state. Voltages applied to the conductive substrates drive the liquid crystal layer to one of the two states. The analyzer blocks IR light when the liquid crystal layer is in one of the states and passes IR light when the liquid crystal layer is in the second state.

Description:
This application claims priority from U.S. Provisional Patent Application No. 60/195,885, filed Apr. 7, 2000, and entitled “Ferroelectric Liquid Crystal Infrared Chopper,” the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to infrared light modulators, and more particularly to infrared liquid crystal light modulators, including reflective and transmissive ferroelectric liquid crystal infrared “choppers.” 
     BACKGROUND OF THE INVENTION 
     Like visible light, infrared light is a form of electromagnetic radiation. It occupies the portion of the electromagnetic wavelength spectrum between 750 nm and 1000 μm. However, unlike visible light, infrared radiation is not visible to humans. Nevertheless, useful scientific information may be obtained by observing with specialized instruments the infrared radiation emitted from or reflected by objects from atoms to stars. Although the term “light” is often used to refer only to visible electromagnetic radiation, for ease of discussion in this application the term infrared light will be used along with infrared radiation, to refer to electromagnetic radiation in the infrared region. 
     One form of infrared measurement, known as radiometry, measures the intensity of infrared light a source emits, absorbs, or reflects by comparing the observed infrared light to a reference measurement. Radiometry is useful, for instance, in analyzing the chemical constituents in a gas sample. This subclass of radiometry is known as spectro-radiometry. 
     Spectro-radiometry makes use of the fact that all elements or compositions of matter have an infrared “signature” or “fingerprint”. Matter absorbs or reflects infrared radiation to some degree, depending on the wavelength of the radiation. By observing the pattern of infrared light reflected from or absorbed by a sample of unknown composition, it becomes possible to determine what elements and/or compounds are present in the sample. A spectro-radiometer used for such investigation directs various wavelengths of infrared light at the sample and observes to what degree the radiation is absorbed at each wavelength. Alternatively, a spectro-radiometer may direct infrared light containing a number of different wavelengths at the sample and observe the infrared light after passing through the sample using a number of infrared detectors, each configured to observe specific wavelengths. 
     Radiometers, like other types of infrared detectors that compare reflected infrared light to a reference measurement, generally require an alternating infrared source. Infrared light from the source is alternately directed at the detector through the sample and blocked from passing to the detector. The detector then compares the difference between the two measurements to produce an output signal representative of the difference. In order to increase the accuracy of such systems, it is important to have a consistent, and stable source of infrared light. That is, the alternating infrared waveform should have equal intensity and spectral composition from one ON pulse to the next, each OFF pulse should be equally dark, and the transitions between ON and OFF pulses should be consistent. 
     One method for providing an alternating infrared light source is to flash the actual source. However, the internal heat generated by infrared sources along with other factors limit the source&#39;s ability to emit infrared light of constant intensity or wavelength throughout a pulse when the source is pulsed ON and OFF. Therefore, this is an unreliable method of providing stable, alternating infrared light for many applications. 
     Another method for providing an alternating infrared source is through the use of mechanical “choppers”, such as shutters or rotating wheels with apertures to alternately block and pass the light. Mechanical choppers work well for many applications. However, many infrared sensor applications require very small choppers, for which mechanical choppers are not very well suited. Further, the moving parts in mechanical choppers may introduce unwanted vibration into the detection system, thus reducing the detector&#39;s usefulness. Additionally, components with moving parts are inherently less reliable than those with no moving parts. Finally, mechanical choppers are more difficult to control than similar electromechanical devices, making it more difficult to control the speed and accuracy with which they operate. 
     The use of mechanical choppers for visible light results in many of the same limitations and disadvantages. For that reason, it is well known to use liquid crystals for visible light choppers. However, liquid crystals have a number of limitations that make them potentially unsuitable for use in infrared choppers, and to applicants&#39; knowledge, liquid crystals have not been used in infrared choppers. Liquid crystal chopper components behave differently in infrared wavelengths in ways that make them potentially unsuitable for liquid crystal infrared choppers. First, inexpensive polarizers used in visible light applications are not effective at infrared wavelengths, and alternatives are expensive. Further, liquid crystals have high absorption of light in infrared wavelengths, thus substantially reducing the amount of infrared light available. Finally, the window material used to contain the liquid crystal must be something other than glass, which also absorbs infrared light, and alternatives to glass are also expensive. 
     Therefore, the need exists for an infrared chopper that is small, reliable, and as free of moving parts as possible. If this is to be done with liquid crystals, there are many technical challenges to be solved before a chopper can be produced that is sufficiently economical to be used in various systems. It is against this background and a desire to solve the problems of the prior art that the present invention has been developed. 
     SUMMARY OF THE INVENTION 
     The present invention relates generally to a liquid crystal infrared light modulator that can be driven by electrical signals to modulate incoming infrared light. The modulator includes a polarizer receptive of the incoming light that produces polarized light and a layer of liquid crystal material switchable between at least two states, the liquid crystal layer acting on polarized IR light from the polarizer to provide a first polarization state of polarized light if the liquid crystal is in a first state and to provide a second polarization state of polarized light if the liquid crystal is in a second state. It also includes a pair of IR transparent, conductive substrates positioned on either side of the liquid crystal layer that are suitable for having voltages applied thereto to drive the liquid crystal layer to one of the two states. It further includes an analyzer that substantially blocks polarized light of the first polarization state when the liquid crystal layer is in the first state and substantially passes polarized light of the second polarization state when the liquid crystal layer is in the second state. 
     The liquid crystal material may be ferroelectric. The ferroelectric liquid crystal material may have a birefringence greater than 0.17 at a wavelength of 589 nm. The ferroelectric liquid crystal material may have an average transmissivity across the electromagnetic spectrum from 8-13 μm greater than 50%. At least one of the conductive substrates may include Germanium. The Germanium may be doped to increase the conductivity thereof. At least one of the polarizer and the analyzer may be constructed by a lithographic process. The polarizer and analyzer may be in a crossed-polarizer configuration relative to one another. 
     The present invention is also related to a liquid crystal infrared light modulator that can be driven by electrical signals to modulate incoming infrared light. The modulator includes a polarizer receptive of the incoming light that produces polarized light and a layer of liquid crystal material switchable between at least two states, the liquid crystal layer acting on polarized IR light from the polarizer to provide a first polarization state of polarized light if the liquid crystal is in a first state and to provide a second polarization state of polarized light if the liquid crystal is in a second state. It also includes an analyzer that substantially blocks polarized light of the first polarization state when the liquid crystal layer is in the first state and substantially passes polarized light of the second polarization state when the liquid crystal layer is in the second state. It further includes a first conductive substrate positioned on one side of the liquid crystal layer, the first substrate having a reflector thereon that reflects IR light such that incoming polarized light passes twice through the liquid crystal layer before passing through the analyzer and a second conductive substrate transparent to IR light positioned on the opposite side of the liquid crystal layer from the first substrate, the first and second substrates cooperating to modulate the liquid crystal layer between the two states. 
     The present invention also relates to a liquid crystal infrared light modulator that modulates incoming polarized IR light such that the IR light is either substantially blocked or passed by an analyzer after traveling through the modulator. The modulator includes a pair of conductive substrates and a layer of liquid crystal material positioned between the substrates, the liquid crystal layer operating between at least two states, a first state that provides a first polarization state of polarized light from the incoming polarized IR light and a second state that provides a second polarization state of polarized light from the incoming polarized IR light. The conductive substrates can be driven by electrical signals to modulate the liquid crystal between the two states, the substrates and the liquid crystal layer configured in either of two configurations, a transmissive configuration, wherein the IR light passes through the liquid crystal layer only once, and a reflective configuration, wherein the IR light passes through the liquid crystal layer twice. 
     The present invention also relates to an infrared detection system that analyzes the characteristics of a composition of matter. The detection system includes a source of infrared light and an IR chopper that alternately substantially passes and substantially blocks light from the IR light source. The chopper includes a polarizer receptive of incoming infrared light that produces polarized light, a layer of liquid crystal material switchable between at least two states, the liquid crystal layer acting on polarized IR light from the polarizer to provide a first polarization state of polarized light if the liquid crystal is in a first state and to provide a second polarization state of polarized light if the liquid crystal is in a second state, a pair of IR transparent, conductive substrates positioned on either side of the liquid crystal layer for modulating the liquid crystal layer between the two states, and an analyzer that substantially blocks polarized light of the first polarization state when the liquid crystal layer is in the first state and substantially passes polarized light of the second polarization state when the liquid crystal layer is in the second state. The detection system also includes an IR detector that senses the IR light after the IR light passes through the composition of matter. The composition of matter is located such that the IR light passes therethrough either before or after the IR light passes through the liquid crystal IR chopper. 
     The present invention also relates to a liquid crystal infrared light modulator that can be driven by electrical signals to modulate incoming infrared light. The modulator includes a reflective polarizer receptive of the incoming light that produces polarized light and a liquid crystal cell. The cell includes a layer of liquid crystal material switchable between at least two states, the liquid crystal layer acting on polarized IR light from the polarizer to provide a first polarization state of polarized light if the liquid crystal is in a first state and to provide a second polarization state of polarized light if the liquid crystal is in a second state. The cell also includes a first conductive substrate positioned on one side of the liquid crystal layer, the first substrate having a reflector thereon that reflects IR light such that incoming polarized light passes twice through the liquid crystal layer before passing through the analyzer, and a second conductive substrate transparent to IR light positioned on the opposite side of the liquid crystal layer from the first substrate, the first and second substrates cooperating to modulate the liquid crystal layer between the two states. The modulator also includes a reflective analyzer that substantially blocks polarized light of the first polarization state when the liquid crystal layer is in the first state and substantially passes polarized light of the second polarization state when the liquid crystal layer is in the second state. At least two of the polarizer, the liquid crystal cell, and analyzer are positioned in a substantially coplanar relationship with one another. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a gas monitoring system employing the ferroelectric liquid crystal (FLC) chopper of the present invention. 
     FIG. 2 is a diagram of the FLC chopper of FIG.  1 . 
     FIGS. 3 a  and  3   b  are illustrations of the operation of the FLC chopper of FIG.  2 . 
     FIG. 4 is a collection of some useful FLC compounds that could be employed in the FLC chopper of FIG.  2 . 
     FIG. 5 is a diagram of a second embodiment of an FLC chopper of the present invention. 
     FIGS. 6 a  and  6   b  are the transmittances of an FLC mixture in the smectic C and isotropic phases, respectively. 
     FIG. 7 is a first reflective embodiment of an FLC chopper of the present invention. 
     FIG. 8 is a second reflective embodiment of an FLC chopper of the present invention. 
     FIG. 9 is a third reflective embodiment of an FLC chopper of the present invention. 
     FIG. 10 is a fourth reflective embodiment of an FLC chopper of the present invention. 
    
    
     DETAILED DESCRIPTION 
     An invention is described herein for a ferroelectric liquid crystal chopper. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. Based on the following description, however, it will be obvious to one skilled in the art that the present invention may be embodied in a variety of specific configurations. In addition, well-known processes for producing various components and certain well-known optical effects of various optical components will not be described in detail in order not to unnecessarily obscure the present invention. 
     A system  20 , shown in FIG. 1, includes an infrared (IR) light source  22 , a chopper  24 , a gas mixture  26  to be tested, and a detector  28 . The light source  22  may provide continuous light across a broad portion of the infrared spectrum or it may step through portions of the infrared spectrum in a controlled fashion, either based on a sequence programmed internally or based on external control. The chopper  24 , which will be discussed in greater detail below, modulates the continuous IR light into a series of alternating, sequential time periods with the light at either a first, higher intensity level or a second, lower intensity level. This modulated light is then passed through the gas mixture  26  to be tested. As is well known in the art of analysis of gas components, the gas mixture will absorb a particular percentage of the IR light passing therethrough, with the percentage varying with the wavelength of the light. The detector  28  detects the light not absorbed by the gas mixture. This spectral fingerprint of the gas mixture can be used to determine the components thereof in a fashion well known to those having skill in this art. It is also well known that most IR detectors such as the detector  28  are preferably operated to detect modulated rather than continuous IR light. 
     The chopper  24  includes a first polarizer  32 , a first transparent electrode  34 , a layer of ferroelectric liquid crystal (FLC) material  36 , a second transparent electrode  38 , a second polarizer  40 , and a voltage source  42 , in a simplified embodiment shown in FIG.  2 . As is known in the art of driving FLC devices, the voltage source  42  can control the optic axis of the FLC layer  36  to be generally in either a first orientation when an electric field of one polarity is applied or a second orientation when an electric field of an opposite polarity is applied. This may be accomplished by applying a square-wave voltage waveform to the electrodes  34  and  38  to create the electric fields of alternating polarity across the FLC layer  36 . If the thickness of the FLC layer  36  is properly selected relative to the wavelength of the light passing therethrough, then the FLC layer  36  will act as a half-waveplate and incoming linearly-polarized light will be converted to outgoing linearly-polarized light that has been rotated about the optic axis. The first polarizer  32  creates the linearly-polarized incoming light from unpolarized light and the second polarizer  40  either blocks or passes the outgoing linearly-polarized light, depending on the relative orientations of the polarizers  32  and  40  and the optic axis of the FLC layer  36 . The combination of the two electrodes  34  and  38  and the FLC layer  36  can be seen to be a simplified FLC cell  39 . 
     The general operation of the chopper  24  is illustrated in FIGS. 3 a  and  3   b . In FIG. 3 a , incoming unpolarized light  50  is directed to the first polarizer  32  which is oriented such that only vertically-polarized light  52  passes therethrough to the FLC cell  39 . In this case, the FLC cell  39  has been oriented such that when a first electric field is applied thereto, the optic axis of the FLC cell  39  is vertical. This causes the vertically-polarized light  52  to be passed therethrough without change to its polarization state and orientation, resulting in vertically-polarized light  54 . When this vertically-polarized light  54  strikes the second polarizer  40 , no light is passed therethrough because the second polarizer is oriented such that only horizontally-polarized light is passed therethrough. It can be appreciated that this case produces an OFF state for the chopper  24 . 
     In FIG. 3 b , the vertically-polarized light  52  passing through the first polarizer  32  is rotated 90 degrees by the FLC cell  39  to create horizontally-polarized light  56 . This rotation occurs because a second electric field, of opposite polarity to the first electric field, has been applied to the FLC cell which causes the optic axis of the FLC cell to rotate by approximately 45 degrees. Since the FLC cell is configured to behave as a half-waveplate, linearly-polarized light is converted to linearly-polarized light oriented at an angle rotated by twice the difference between the incoming light orientation and the optic axis of the FLC cell  39 . Since the optic axis is now 45 degrees from the incoming light orientation, the light is rotated by 90 degrees to become horizontally-polarized light. This horizontally-polarized light  56  passes through the second polarizer  40  resulting in horizontally-polarized light  58  passing out of the chopper  24 . It can be appreciated that this case produces an ON state for the chopper  24 . 
     Some suitable FLC compounds for use alone or in combination in the FLC layer  36  of the chopper  24  are shown in FIG.  4 . As will be discussed in further detail below, it has been discovered that it is desirable to use FLC compounds with relatively lower absorption in the mid-infrared wavelength region as compared to other compounds and with a relatively higher birefringence in this wavelength region as compared to other compounds. These compounds in FIG. 4 have been determined to be better in these two characteristics than many other compounds. 
     FIG. 5 shows an embodiment of an FLC IR chopper  70  having multiple layers of anti-reflective (AR) coating. The chopper  70  is held in place within a main fixture body  72  by an insulated support spring  74  and a fixture lid  76 . The layers of the chopper  70  include an FLC layer  78  that is sandwiched between a pair of zinc selenide (ZnSe) AR layers  80  applied to the inner sides of a pair of germanium window/electrodes  82 . The next layer (moving from the middle to the outer portion of the chopper  70 ) is another zinc selenide AR layer  84  applied to the outer sides of the germanium window/electrodes  82 . To these layers  84 , a layer of gold wire-grid polarizer  86  is applied. On the outside of the polarizer layers  86  is an AR layer of lanthanum fluorine (LaF)  88 . The AR coatings were applied by Spectrum Thin Films of Bohemia, N.Y. 
     It has been discovered empirically that these various AR layers increase the transmission efficiency through the chopper  70 . Generally, the effectiveness of an AR coating increases as the number of layers is increased in the dielectric stack of the AR coating. In this case, however, additional layers on the inside of each window/electrode adds to the capacitance between the electrodes which requires larger voltages to drive the FLC cell. Simulations showed that a single layer of ZnSe yielded acceptable transmittance without increasing the capacitance by an unacceptable amount. In addition, it was discovered that at least one type of photoresist material was highly transparent in the mid-infrared region and could be used to make an AR coating and protect the gold wire polarizer layer from mechanical damage. Nevertheless, because the thickness of this layer could not be suitably controlled, the photoresist was not used. 
     In addition, while it was determined that the order of the gold wire polarizer layer and the LaF layer could be reversed, it was found to be less desirable than in the order as described above because the gold wire layer appears to adhere better to the ZnSe than the LaF. Further, having the LaF on the outside served to protect the gold wire layer from mechanical damage. 
     Conducting lead wires are attached to each germanium electrode/window by silver epoxy. The wires exit the fixture via apertures in the fixture body or lid radially or axially near or at the perimeter of the fixture. 
     One very important design criterion for this chopper  24  is that it must include sufficiently practical and economical components so that it can be produced and sold for a reasonable price to make it feasible for use in consumer and commercial applications. Preferably, it should be suitable for mass production. In order to achieve this, several technical obstacles were overcome. First of all, it was recognized that optical materials suitable for the mid-infrared wavelength region (approximately 8 to 13 μm.) were necessary. Many of the optical materials typically used for optical systems operating in the visible region do not perform adequately in the infrared region. For example, glass which is used for lenses, windows, and as a transmissive substrate with films applied thereto (such as in a beamsplitter) is highly transmissive to visible wavelengths, but is much less transmissive in the mid-infrared region. Another example is the type of polarizers used. Polarizers for visible light do not work well for infrared light. The polarizers commercially available for use with infrared light are very expensive, and thus not practical. 
     Second, because of the absorptive nature of all organic compounds in the mid-infrared region, it was discovered that not only should compounds that are relatively less absorptive be used, but also it would decrease the total absorption if the thickness of the FLC layer  36  were decreased. This decrease in the thickness requires a proportional increase in the birefringence of the FLC layer  36 . This can be appreciated from the following equation that expresses the intensity of light I passing through the chopper  24  when the incident light is unpolarized.              I   =       1   /   2                     I   0            sin   2          (       πΔ                 nd     λ     )              sin   2          (     2      θ     )                 (   1   )                                
     where I 0  is the incoming light intensity, Δn is the FLC layer&#39;s optical birefringence, d is the FLC layer thickness, λ is the light&#39;s wavelength, and θ is the angular orientation of the FLC layer&#39;s optic axis. Normally the FLC cell will be oriented so that θ=0 in the shutter&#39;s OFF state (where sin 2 (2θ)=0), and θ=45° in its ON state (so that sin 2 (2θ) attains its maximum value of 1). 
     As can be seen, the intensity is also a function of wavelength, so that with the longer wavelengths of mid-infrared, which are in the range of 10 to 30 times longer than visible wavelengths, the product of Δnd must be 10 to 30 times larger than with visible wavelengths. Unless the birefringence can be increased by that amount, the thickness of the LC layer will have to be dramatically increased. This exacerbates the absorption problem of FLC materials at these wavelengths. 
     The goal was to make an FLC mixture with improved transmission in the mid-infrared wavelength range. In addition to this requirement, the mixture also had to meet normal requirements for FLC mixtures: easy alignment, wide temperature range (operating temperature at 50° C.), and appropriate tilt angle and switching speed. The approach to reducing IR absorption included two methods. The first was to decrease IR absorption of the individual FLC compounds or components. While predicting the spectrum of a given molecule in the 8 μm.-13 μm. region is difficult, since this is normally termed the “fingerprint region,” certain empirical ties between structure and spectrum were made. For instance, it appears that oxygens, especially in the form of ether linkages, tend to decrease a molecule&#39;s transmissivity. Such empirical observations were used when designing new FLC compounds. The molecular design process was iterative—meaning we would design, synthesize, and evaluate one compound, and then use that information to help in designing new compounds. The second method to reduce IR absorption was increasing the birefringence of the materials. The higher the birefringence of the mixture, the thinner the FLC layer will be, and thus the shorter the light pathlength through the material. Altering the birefringence usually required increasing electron delocalization leading to higher birefringence materials. The goal was to have materials with both higher birefringence and lower IR absorption than average. By high birefringence, we refer to materials having a birefringence greater than or equal to approximately 0.17 when measured at approximately 589 nanometers (nm.). 
     In designing a new material, both the birefringence and the transmissivity of the new material had to be considered. The range of birefringence, from the smallest to the largest birefringence FLC compounds, varies by about a factor of three. In contrast, the range of transmissivity, particularly for a given wavelength (as opposed to an average over the entire region of interest) could be closer to a factor of fifty. Thus, the risks and rewards for designing a high transmissivity material were much greater than those for designing a high birefringence material. If a compound&#39;s low transmissivity could be directly tied to one functional group, that type of functional group would no longer be considered for future-generation compounds. 
     As part of the design step, we attempted to estimate the IR spectrum of potential new materials using molecular modeling programs such as MacSpartan and HyperChem. However, the estimates provided by these programs proved not to be helpful in this case. Our iterative empirical method ultimately proved to be much more useful. 
     Many new potential FLC compounds were synthesized, along with about 15 previously known compounds. Many of these new compounds have proven to have very high birefringence and very high IR transmissivity. These new compounds lay the groundwork for an FLC mixture optimized for IR use. 
     Representative examples of the new compounds made in this project are shown in FIG.  4 . The first compound, Compound A, is a biphenylpyrimidine, which was used as the basis for the new mixtures. This material is almost ideal for this application. Due to its three delocalized rings, it has very high birefringence. Since there are no non-aromatic heteroatoms in the system, the IR transmissivity is very high and it has low viscosity. Several homologues were made of this material, where n and m each varied from 5 to 10 carbons. For example, in one useful compound n=7 and m=6, in another n=8 and m=9, and in a third n=7 and m=9. In addition, other variants of this material could be prepared. These compounds could be used for up to half of a mixture, but other materials are also needed to broaden phases and to provide other required liquid crystalline properties, depending on the intended application and operating environment, as is well known to those having skill in the art. In general the FLC material may be any single or mixture of FLC material that exhibits at least two optical states in response to electrical fields. Such materials are well known to those skilled in the art of FLC materials. The specific FLC material or mixture of FLC materials is highly dependent upon the operating environment of the device and/or specific characteristics desired in the device. For instance, environmental factors (e.g., temperature) and characteristics such as switching speed, switching voltage and contrast, substantially affect the choice of FLC material or mixture to be used. Materials best suited for each of these and other characteristics are well known to those skilled in the art of FLC materials, as are the methods used to combine FLC materials into suitable mixtures that best satisfy specific objectives. 
     Compound B is an example of a compound that was made but rejected for use in this project. This compound has a cinnarnate group (the combination of phenyl-ethene-carboxylate) coupled with a naphthalene via an alkyne group, and has a further alkyne group attached to the naphthalene. This very long delocalization results in a material with extremely high birefringence. Although its birefringence was high, the naphthalene group had a fair number of absorptions in the fingerprint region, giving this compound low IR transmissivity. Again, higher birefringence is good, but only if transmissivity is also high. In this case, the low transmissivity more than offsets any gains due to higher birefringence. 
     Compound C is similar in structure to Compound B, has a slightly lower birefringence, and its phenyl ring makes it much more transparent to IR light. If its birefringence and transmissivity were its only important properties, this would be an ideal constituent of an LC material for IR applications. Unfortunately, this compound is relatively insoluble in other liquid crystals. Other similar compounds, designed to have higher solubility, were also synthesized, but tended to have lower transmissivity than Compound C. 
     Compound D is representative of another promising class of materials. The only heteroatoms in this compound are the aromatic nitrogens and the sulfur link to the tail. Materials with sulfur linkages have much higher transmissivities than the equivalent oxygenated species, and have comparable birefringences. Thus, this compound, and several others like it, proved to be quite useful in the new mixtures. 
     The last type of compound shown in FIG. 4 is Compound E, which is a fluorinated tolane compound. The tolane system (phenyl-acetylene-phenyl) imparts very high birefringence, and substituting fluorines for hydrogens was expected to both reduce the amount of C-H absorbencies in the IR, and increase the tendency of the compound to form tilted smectic phases. It appears that, in regards to the first expectation, fluorination simply changed the type of absorbencies in the IR, without resulting in a net reduction. This material, like many of the other tolanes we looked at, still tended to broaden a mixture&#39;s N phase while decreasing the desired smectic C phase. 
     The IR spectra of the new compounds were measured under standardized conditions. In a typical measurement, a 50 μm thick sample of a compound was melted, and the melt&#39;s spectrum was taken. The transmission was then averaged over several portions of the spectrum, including several specific notches which coincide with peaks in the IR spectra of the anesthetic gases. The averaged spectra were then compared with each other, and the compounds with the best results were either used in mixtures or structurally modified to give even better compounds. 
     It was discovered that the spectra of a mixture changed with increasing temperature. The most likely explanation for this is that the change is not due to the temperature per se, but instead due to the phase change which accompanied the temperature change. Thus, the fact that the mixture&#39;s spectrum in the smectic C phase altered when it was melted may simply indicate that the degree and type of molecular interactions had changed. Since it is well-known that, in solution IR spectroscopy, the solvent used influences a material&#39;s IR spectra, this is not overly surprising. An example of this is shown in FIGS. 6 a  and  6   b , where the top spectrum was run at 125° C. (in the isotropic phase), whereas the bottom spectrum was run at room temperature (in the smectic C phase). The differences between the two spectra are directly attributable to the different liquid crystal phases. The room temperature material is not aligned; were it aligned, the difference could potentially be noticeably larger. 
     It should be noted that, while the assessment spectra taken for compounds and mixtures were all taken in a 50 μm thick sample, the actual IR cells made with the mixtures had thicknesses proportional to their birefringence. For instance, in a cell tuned for optimal operation at 10.9 μm, a mixture with a birefringence of 0.22 would lead to a cell thickness of 25 μm, or half the “standard” thickness. From that we can estimate that a cell made with the mixture tested in FIGS. 6 a  and  6   b  would have half the absorption of the spectrum from FIG. 6 b.    
     Uniform alignment of FLC materials is usually achieved through a process known as “surface stabilization.” In order for this process to work, the thickness of the FLC layer has to be less than one quarter the pitch (length of a full rotation of the SmC* helix). The helical pitch of an FLC mixture may be in the range of 50 μm. The thickness of the cell may be roughly half this number. Therefore, strong surface stabilization may not be achieved. The resulting non uniformity in the FLC layer is not significant with respect to operation of an FLC IR chopper since it is the modulation of the intensity of incident radiation that is of primary importance, not the preservation of its spatially varying characteristics. However, in order for this FLC material to be useful in IR imaging or IR scene generation systems, the spatial uniformity problem would have to be addressed. 
     One approach to optimizing spatial uniformity is in the selection of an alignment layer material. Initially, we chose a material which, after cell fabrication, was observed to yield the best spatial uniformity. In the end, however, this was not how the optimal alignment layer material was chosen. Of the many materials tried, two polyamides yielded superior results: SE610 and SE5291 (Nissan Chemical Industries, Ltd., Japan). It was observed subsequently that the uniformity of cells incorporating SE610 would improve as cells were driven whereas the uniformity of cells incorporating SE5291 remained unchanged. SE5291 is known to have strong polar surface interactions with the FLC with which it is in contact; SE610 has much weaker interactions with the FLC. We speculate that the weaker interaction with SE610 allows the FLC to align itself when driven whereas the strong interaction with SE5291 prevents such “self alignment”. 
     The polarizers  32  and  40  were fabricated with a lithographic manufacturing technology that was substantially less expensive and better suited to volume manufacturing than traditional manufacturing processes. For example traditional methods may cost on the order of $5000 per polarizer, while the method used to fabricate the polarizer of the present invention may be in the range of $100. Lift-off lithography was used to create wire grids 0.7 μm. wide on a pitch of 1.4 μm., resulting in a spacing between wires of 0.7 μm. The lift-off lithography process was performed by Digital Optics Corporation of Charlotte, N.C. 
     The transparent electrodes  34  and  38  may be composed of doped germanium. Typical FLC cells for use with visible light may include glass windows and a transparent electrode layer composed of a coating of indium tin oxide (ITO) deposited on the glass. Unfortunately, ITO is not transparent in the mid-infrared region, and neither are other conducting oxides that are transparent in the visible region. Germanium is fairly transparent in the mid-infrared region. In addition, the germanium can be doped to make it an n-type material. This allows the 4 layers of 2 windows and 2 electrode layers in conventional FLC cells to be replaced with 2 layers, since the window and electrode functionality are now in the same layer. N-doping was discovered to be preferable to p-doping because it provides for better ohmic contacts, although p-doping could be used as well. A thickness of 2 mm. for the germanium electrodes  34  and  38  was found to be suitable. Because the window material is conducting, it is necessary to house the chopper  24  such that the two electrodes  34  and  38  are insulated from each other. 
     An alternative embodiment to an FLC IR chopper  100  is shown in FIG.  7 . This embodiment includes a pair of reflective polarizers  102  and  104  placed in a co-planar fashion on a single substrate  106  and a reflective FLC cell  108  spaced apart therefrom. Unpolarized light  110  strikes the first reflective polarizer  102  and is reflected as linearly-polarized light  112  of a particular orientation. This light  112  is directed to the reflective FLC cell  108  which includes a window/electrode  114 , a layer of FLC material  116 , and a reflective electrode  118 . The reflective electrode  118  may be composed of gold. The FLC cell  108  is configured so that it acts as a half waveplate only when the light passes twice therethrough due to the reflection. The result is linearly-polarized light  120  that has an orientation based on the orientation of the incoming linearly-polarized light  112  and the orientation and state of the FLC cell  108 . This light  120  is then directed to the second reflective polarizer  104  which is oriented such that it will reflect linearly-polarized light of only a single orientation. If the incoming light  120  is linearly-polarized at an orientation orthogonal to the orientation of the second polarizer  104 , then no light will be reflected, resulting in an OFF state for the chopper. Otherwise, linearly-polarized light  122  of the orientation of the second polarizer  104  will be reflected, resulting in an ON state for the chopper  100 . One of many reasons a reflective embodiment may be desirable is because the number of window/electrodes is reduced from two to one, along with any associated AR coatings. 
     Another reflective embodiment of an FLC chopper  130  is shown in FIG. 8 the chopper includes a first reflective polarizer  132 , a reflective FLC cell  134 , and a second reflective polarizer  136  all located in a co-planar fashion on a single substrate  138 . A pair of reflectors  140  and  142  (which could be replaced with a single reflector) are spaced apart from the substrate  138 . Unpolarized light  144  strikes the first reflective polarizer  132  and is reflected as linearly-polarized light  146  of a particular orientation. This light  146  is reflected off of reflector  140  and directed as light  148  to the reflective FLC cell  134  which includes a window/electrode  150 , a layer of FLC material  152 , and a reflective electrode  154  which may be composed of gold. The FLC cell  134  is configured so that it acts as a half waveplate only when the light passes twice therethrough due to the reflection. The result is linearly-polarized light  156  that has an orientation based on the orientation of the incoming linearly-polarized light  148  and the orientation and state of the FLC cell  134 . This light  156  is then directed to the reflector  142  where it is directed as light  158  to the second reflective polarizer  136  which is oriented such that it will reflect linearly-polarized light of only a single orientation. If the incoming light  158  is linearly-polarized at an orientation orthogonal to the orientation of the second polarizer  136 , then no light will be reflected, resulting in an OFF state for the chopper. Otherwise, linearly-polarized light  160  of the orientation of the second polarizer  136  will be reflected, resulting in an ON state for the chopper  130 . Because of the length of the light path, it may be necessary for the incoming unpolarized light  144  to be collimated. 
     A third reflective embodiment of an FLC chopper  170  is shown in FIG.  9 . The chopper  170  includes a polarizer  172 , a window/electrode  174 , an FLC layer  176 , and a reflective electrode  178 . Incoming unpolarized light  180  is directed to the FLC cell  170  where it first passes through a transmissive polarizer  172 , allowing linearly-polarized light to pass through the window/electrode  174  and the FLC layer  176  where it is reflected by the reflective electrode  178  back through the FLC layer  176  and the window/electrode  174 . The FLC cell  170  is configured such that it acts as a half waveplate when the light passes twice therethrough in this fashion. The polarizer  172  then either allows reflected light  182  to pass therethrough or it does not, depending on the orientation and state of the FLC layer  176 . Note that in this parallel polarizer arrangement, the contrast ratio between the ON and the OFF states may be somewhat reduced from a crossed polarizer arrangement. It may also be possible to use a beam splitter arrangement or a polarized beamsplitter arrangement with this FLC cell  170  or in any of the other embodiments shown. 
     A fourth reflective embodiment of an FLC chopper  190  is shown in FIG.  10 . The chopper  190  includes a first transmissive polarizer  192  and a second transmissive polarizer  194 , both located on a single substrate  196  that is, or has portions that are, transmissive to IR light. An FLC cell  198  includes a window/electrode  200 , an FLC layer  202 , and a reflective electrode  204 . Incoming unpolarized light  206  first passes through the first transmissive polarizer  192 , allowing linearly-polarized light to pass through the window/electrode  200  and the FLC layer  202  where it is reflected by the reflective electrode  204  back through the FLC layer  202  and the window/electrode  200 . The FLC cell  198  is configured such that it acts as a half waveplate when the light passes twice therethrough in this fashion. The second transmissive polarizer  194  then either allows reflected light  208  to pass therethrough or it does not, depending on the orientation and state of the FLC layer  202 . Note that the polarizers  192  and  194  could be oriented in a parallel polarizer arrangement or in a crossed polarizer arrangement. 
     The foregoing description is considered as illustrative only of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and process shown and described above. For example, the principles of the present invention may be equally applicable to various types of liquid crystal devices, such as those including nematic liquid crystals, ferroelectric liquid crystals, polymer dispersed liquid crystals, and others. Furthermore, the particular order, position, and relationship of the various layers and optical components of the chopper of the present invention could be changed as needed. Additionally, while the FLC chopper of the present invention has been described with regard to an application for a gas monitoring system, this is but one example of possible applications for the chopper. Also, for many applications sufficient performance may be obtained even without the optimal orientations, rotations, and retardances described. For example, the polarizers might not need to be exactly parallel or perpendicular, the waveplate might not need to be exactly a half waveplate, the rotation of the optical axis might not need to be exactly 45 degrees, and so forth. Accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention as defined by the claims which follow.