1. Field of the Invention
The invention relates generally to optical modulators, specifically optical retarder and polarization modulator assemblies that are used in optical systems such as lasers or narrow-band quasi-monochromatic beams that exhibit a relatively long coherence length (many waves).
2. Description of the Related Art
The phenomenon of phase interference is well-known and is described in standard optical texts such as Born and Wolf, Principles of Optics or Hecht and Zejak, Optics. While these present a fuller and more accurate treatment of the topic, an exemplary situation is summarized here as follows. When light is incident upon a structure that exhibits reflection at two or more nearly-parallel surfaces, there is interference between beams which have reflected from the different surfaces involved, or which have experienced multiple reflections between the surfaces involved. If the light is quasi-monochromatic or monochromatic (such as a laser beam) and is phase-coherent over distances corresponding to the differential path lengths involved, interference will result in spatially resolved light and dark fringes corresponding to regions of constructive and destructive interference. The specific fringe pattern, arising as it does from the relative phase between the beams, varies with the wavelength of light. So the interference pattern may be seen as varying with location for light of a given wavelength, or varying with wavelength for a given location.
In some structures such as the well-known Fabry-Perot interferometer, Fizeau interferometer, and so on, the fringe pattern is desired as a means of selecting wavelength, measuring wavelength, or spectrally filtering an optical beam. However, in most imaging or modulation systems, such an interference pattern would be undesirable, and components are designed to minimize or eliminate such effects.
Known techniques for doing so include mounting the optical components at non-normal incidence, so that beams reflecting from the various surfaces can be spatially separated, which eliminates the interference; or use of highly-efficient anti-reflection coatings, so that the energy in the various reflections beams is minimized; or by incorporating lossy elements between the reflecting surfaces, so that multiple-pass reflections are damped. If the system is viewed as a resonator, the latter approach effectively reduces its Q factor.
Wedged substrates have been used to construct a liquid crystal variable retarder, to defeat interference arising from reflections at the exterior faces of the device. Similarly, devices have been built wherein the liquid crystal layer is bounded by high-reflection mirrors, to produce a liquid-crystal tunable etalon, and these have generally been constructed using wedged substrates to eliminate fringing from reflections at the outer face of the device. However, use of wedged substrates is incompatible with prevailing liquid crystal fabrication methods, which are designed to use flat, relatively thin sheet glass instead. In the normal process, one produces a large panel containing many liquid crystal cells, which are subsequently cut into individual pieces, filled, and sealed. In contrast, the use of wedged substrates forces one to assemble the cells singly, at greatly increased cost. Also, the process is considerably more labor-intensive, so the number of cells a given facility can produce is much lower than if the panelized approach is used. Further, several of the steps involved in liquid crystal cell fabricationxe2x80x94such as spin-coating, alignment-layer buffing, and adhesive deposition xe2x80x94cannot be performed as easily when wedged substrates are used, nor can comparably tight quality control be achieved. Thus, liquid crystal cells that use wedged substrates are inherently more expensive, require non-standard production equipment, have lesser quality, and are difficult to provide in volume.
It is common practice to provide a liquid crystal cells with anti-reflection coated faces, either by laminating it to polarizers or similar materials that have such a coating; or by cementing the cell to glass windows that have antireflection coatings on their outer faces. The quality of coating which can be produced on glass is superior to that produced on polarizers, but even under ideal conditions it is difficult to produce less than 0.25% reflection per surface reliably.
While this is a small amount, which one might expect would have a negligible effect on the overall assembly, this is not actually the case. The intensity of the fringes produced when two phase-coherent beams interfere is:
xe2x80x83Ifringe=4(IA*IB)xc2xdxe2x80x83xe2x80x83[1]
where IA and IB are the intensities of the two beams. In a liquid crystal cell that operates in reflection-mode, beam IA might be the primary beam, and beam IB might be an unwanted reflection from the anti-reflection coated outer face of the device. If the intensity of the beam incident upon the cell is termed I0, the two intensities are then
IA=0.9975 I0xe2x80x83xe2x80x83[2a]
IB=0.0025 I0xe2x80x83xe2x80x83[2b]
where we make the approximation that the cell is otherwise lossless. Thus, the fringe intensity is
Ifringe=4(0.0024938 I02)xc2xd=0.1998 I0xe2x80x83xe2x80x83[3]
or nearly 20 percent of the intensity of the primary beam. So even surfaces or interfaces that produce what one might expect to be negligible reflections, based on the reflection coefficients involved, yield quite significant interference patterns when they interfere with a bright beam. This is because, loosely speaking, such interference is proportional to the strength of the electric field of the weaker beam, while intensity is a measure of the square of the electric field. In the present example, the intensity of the beam reflected from the coated surface is {fraction (1/400)} as great as that of the incident beam, but the electric field is {fraction (1/20)}th as great. When the reflected beam interferes with the main beam, it alters the electric field up or down by 5 percent, which produces an intensity change of plus or minus 10 percent, for a total peak-to-valley fringe depth of 20 percent.
Interference effects arising from reflections at the opposite faces of the liquid crystal layer itself are in some sense unavoidable, since there is always a finite reflection, and one usually wishes the liquid crystal to have a uniform thickness, to yield a retardance that is the same for all points within the aperture. Thus, one inevitably forms a parallel resonant cavity structure. However, because the liquid crystal layer is relatively thin (typically 4-25 microns) and well-controlled, the effects of this fringe pattern are often acceptable.
One reason for this is that the spectral period between successive peaks is relatively wide. This is an interference of relatively low order, where order denotes the path difference between interfering beams, counted out in wavelengths of light in the intervening medium.
Low-order interference has a wide spectral separation, while high-order interference has a narrow spectral separation. This may be quantified and calculated if one desires. When interference occurs from reflection at opposite faces of a slab of material, the spectral separation between successive peaks in the fringe pattern is given by
xcex4xcex=xcex2/(2nd)xe2x80x83xe2x80x83[4]
where xcex is the wavelength of light involved, n is the refractive index of the material between reflective surfaces, and d is the thickness of the slab. For a liquid crystal layer 10 microns thick with an index of 1.50, operated at 1.5 microns, the spectral separation between successive fringes is 75 nm. When such cells are used to make tunable filters, attenuators, switches, or other components which control or transmit light over a bandwidth narrower than 75 mn, the fringes do not significantly distort the bandpass of the system. In contrast, interference arising from parallel surfaces that are more widely spaced, will have a narrower spectral period, and will introduce ripple or distortion within the passband of the system. Consequently, interference between such elements is more deleterious to the system than interference across the liquid crystal layer itself.
At the other extreme, reflection between elements that are separated by a distance which exceeds the coherence length of the beam involved, does not lead to interference since the beams are not phase-coherent. One can estimate the coherence distance of a polychromatic beam by the equation
L=xcex2/xcex4xcexxe2x80x83xe2x80x83[5]
In this case, the bandwidth of interest xcex4xcex is that of the limiting element in the system, whether that be the source, the liquid crystal assembly, or some other spectral filtering element. It is often impractical when working with relatively coherent sources (or in narrowband instruments) to construct a liquid crystal assembly using components that are so thick that all reflecting surfaces (or interfaces) are so far apart as to greatly exceed L. Even when possible, this approach results in excess bulk, weight, and cost.
It is common to combine an optical retarder with a liquid crystal cell, to effect an optical retardance that is variable by means of the electro-optic action of the cell; and which spans a range that is not readily available by use of the liquid crystal cell alone. Basically, an optical retarder is placed in series with the cell, and oriented with its slow axis either parallel to, or perpendicular to, the retarder axis of the liquid crystal cell. The retardances of the two elements are summed or differenced thereby. One common reason for using this arrangement is to compensate for the residual retardance of the cell (since some retardance remains, even at high applied voltages), and in this way to enable providing a retardance of zero. This is exhaustively described in the prior art as a way to obtain enhanced contrast or extinction ratio when such a system is placed between polarizers. Such assemblies of a cell and retarder are availably commercially from Meadowlark Optics (Longmont, Colo.).
Another situation where optical retarders are used in combination with liquid crystal cells is in the construction of tunable filters using Lyot and Solc design. In applications such as solar astronomy, telecommunications channel selection, and Raman imaging, such filters are used to produce narrow, tunable passbands. Complete filters of this type are available from Cambridge Research and Instrumentation, Inc. (Boston, Mass.).
In a retarder-compensated cell, it is impossible to match the index of the retarder to that of the other materials involved, if only because the retarder is made of birefringent material and thus has two distinct optical indices, no and nc. Even if one or the other index matches that of the other components involved, it is impossible to match both. Further, the optimum materials for retarder selection may not have indices that are well-matched to those of the other materials. Retarder choice is governed by factors such as aperture, thickness, cost, and amount of retardance needed.
Common retarder materials include calcite, lithium niobate, polyvinyl alcohol, polycarbonate, polyethylene terapthelate (Mylar), mica, and quartz. All of these mismatch the index of the standard liquid crystal cell glass such as Corning 7059 or 1737F by 0.05 or more. Among other retarder materials such as liquid crystal polymers, stressed glass, KDP (potassium dihidrogen phosphate) and its isomorphs, some have the potential to provide a better match but all suffer one or more of the following limitations: they are not readily available, are subject to hygroscopic attack, cannot be produced in large aperture, have limited retardance range, or are very costly.
As this indicates, it is not practical to eliminate reflection between a retarder and a liquid crystal cell by choice of component material.
The incorporation of a retarder into the overall assembly increases the number of reflective interfaces by at least one (and in most cases two or more), and thus increases the number of interfering beams. Consequently, all the problems recited earlier with regard to liquid crystal cells in coherent beams, apply with even greater force when constructing assemblies of liquid crystal cells with retarders.
Thus there is no device or method of construction at present which provides a liquid crystal cell, either alone or in series with a fixed retarder, that does not suffer from significant interference between two or more beams, in addition to the interference from the opposite faces of the liquid crystal layer itself.
It is an object of the present invention to provide a liquid crystal cell that can be used with essentially no optical interference other than that produced by the opposite faces of the liquid crystal layer. It is a further object to provide such an assembly in a manner that is compatible with conventional, low-cost manufacture of the liquid crystal element involved, and specifically to be compatible with the high-volume, panelized cell construction described above. This compatibility is achieved without adding significant cost anywhere else in the assembly process. Fixed retarders may be incorporated for purposes of compensation or of providing a desired range of retardance values.
It is yet another object of the invention to provide both reflective and transmissive embodiments that share the aforementioned benefits over the prior art. Finally, it is an object of this invention to attain this without restricting in any way the choice of materials that can be used as retarders, substrates, or liquid crystal materials. Thus it is possible to span the entire range of retardances, aperture, and the like which have heretofore been possible, while enjoying the novel benefit of eliminating high-order interference.
The invention consists, in the simplest embodiment, of a reflective or transmissive cell of ordinary manufacture to which a wedged glass spacer is attached on one or both faces of the cell by index-matching cement or epoxy. The spacer material and the epoxy are well-matched in optical index to the substrate glass used in the liquid crystal cell, since any mismatch at the interfaces will produce a reflected beam that will interfere with the primary beam. However, it is relatively straightforward to match the refractive index of the wedged spacer to the substrate within 0.005 over a wide spectral range, by simply using the same glass type as is employed in the substrate. For example, if Corning 1737F is used as the substrate, the spacers could be cut from sheets of the same material, then ground and polished to achieve a wedge of 0.5 degrees. Alternatively, one can use an optical glass that matches the index of Corning 1737F. Optical epoxies and gels are available which have an index in the proper range (n=1.49-1.54) and are thus an excellent match. In this way, reflections at the spacer/cell interface are kept to as little as 0.0003 percent or less.
Windows are then mounted to the wedged face of the spacer, and such a window typically will have its outer face anti-reflection coated so there is low reflection at the glass/air interface. In a reflective mode cell, there is only one set of spacers and windows; in a transmissive mode cell, two sets may be employed, one on each side of the cell.
A compensating retarder may be mounted to the wedged face of the spacer when this is required, and a window may then be mounted to the exterior face of the retarder, if desired. Or, the exterior face of the retarder may itself have an anti-reflection coating so it functions as the window. Or, the overall assembly may be joined with other optical elements using index-matching adhesives, eliminating the need for anti-reflection coatings on outermost faces. A retarder is normally planar, in order to produce uniform retardance across its aperture. Thus, reflections at the interface between the retarder and spacer, and retarder and window (if present), are canted at an angle to the plane of the liquid crystal material. This angle is chosen so that the divergence between the main beam and the unwanted reflections is great enough that they are spatially separated before encountering the detector, film, or measurement apparatus to which the light beam involved is ultimately presented.
The result is that interference is defeated, just as when one constructs the liquid crystal cell using wedged substrates. However, the construction is enormously simplified with the present invention, since the cell can be fabricated using high-quality, low cost methods including panelized assembly. The only additional cost of the present invention compared to a prior-art fringing assembly, is that of the wedged spacer element, which is readily made and attached using conventional optical device manufacturing methods, with special attention to matching the optical indices of the adhesive, spacer, and cell substrates.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.