Liquid crystal cell for use in coherent beams

Details Liquid crystal cell for use in coherent beams Liquid crystal cell for use in coherent beams

2001-04-25

2003-03-18

Sikes, William L. (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C349S196000, C385S034000

Reexamination Certificate

active

06535257

ABSTRACT:

BACKGROUND OF THE INVENTION
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 & 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 fabrication—such as spin-coating, alignment-layer buffing, and adhesive deposition —cannot 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:
 
I
fringe
=4(
I
A
*I
B
)
½
  [1]
where I
A
and I
B
are the intensities of the two beams. In a liquid crystal cell that operates in reflection-mode, beam I
A
might be the primary beam, and beam I
B
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 I
0
, the two intensities are then
I
A
=0.9975
I
0
  [2a]
I
B
=0.0025
I
0
  [2b]
where we make the approximation that the cell is otherwise lossless. Thus, the fringe intensity is
I
fringe
=4(0.0024938
I
0
2
)
½
=0.1998
I
0
  [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
&dgr;&lgr;=&lgr;
2
/(2
nd
)  [4]
where &lgr; 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

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