U.S. patent number RE32,521 [Application Number 06/710,846] was granted by the patent office on 1987-10-13 for light demodulator and method of communication employing the same.
Invention is credited to James L. Fergason.
United States Patent |
RE32,521 |
Fergason |
October 13, 1987 |
**Please see images for:
( Certificate of Correction ) ** |
Light demodulator and method of communication employing the
same
Abstract
A light modulator for generating a beam of phase modulated light
including a source of polarized light, at least one and preferably
two liquid crystal cells having a thin layer of nematic liquid
crystals of positive dielectric anistropy through which the
polarized light is directed to produce a beam of light having a
phase shift corresponding to a modulating electrical signal which
is applied to each of the liquid crystal cells. The liquid crystal
cells in addition have a continuing electrical bias applied across
the layers in order to achieve the rapid response times necessary
to achieve modulation of the polarized light. Demodulation of the
polarized light occurs by splitting the modulated light beam into
its quadrature components and developing an electrical signal
corresponding to the phase difference between the two quadrature
components. A communication system involving its modulators and
demodulators delivers communication through light as a transmission
medium.
Inventors: |
Fergason; James L. (Atherton,
CA) |
Family
ID: |
27398674 |
Appl.
No.: |
06/710,846 |
Filed: |
March 12, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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121071 |
Feb 13, 1980 |
4385806 |
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913618 |
Jun 8, 1978 |
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Reissue of: |
235006 |
Feb 17, 1981 |
04436376 |
Mar 13, 1984 |
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Current U.S.
Class: |
398/152; 349/1;
359/900; 349/74; 398/125; 398/186; 398/184; 398/141 |
Current CPC
Class: |
G02F
2/00 (20130101); G02F 1/1393 (20130101); H04B
10/11 (20130101); H04B 10/25891 (20200501); G02F
1/133634 (20130101) |
Current International
Class: |
G02F
1/139 (20060101); G02F 1/139 (20060101); G02F
2/00 (20060101); G02F 2/00 (20060101); H04B
10/142 (20060101); H04B 10/142 (20060101); G02F
1/13 (20060101); G02F 1/13 (20060101); H04B
10/24 (20060101); H04B 10/24 (20060101); H04B
10/10 (20060101); H04B 10/10 (20060101); G02F
001/13 (); G02F 001/00 (); G02F 002/00 (); H01S
003/00 () |
Field of
Search: |
;350/332,334,346,347E
;455/605,611,616,617 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Niblack, W. & Wolf, E. "Polarization Modulation and
Demodulation of Light," Applied Optics, vol. 3, No. 2 (Feb. 1964)
pp. 277-279..
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Primary Examiner: Corbin; John K.
Assistant Examiner: Gallivan; Richard F.
Attorney, Agent or Firm: Murray; Thomas H.
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This application is a continuation-in-part of my copending U.S.
patent application Ser. No. 121,071 filed Feb. 13, 1980, U.S. Pat.
No. 4,385,806 which is a continuation-in-part of an earlier
application Ser. No. 913,618 filed June 8, 1978, now abandoned.
Claims
I claim:
1. A light modulator comprising at least one liquid crystal cell
comprising two parallel transparent plates; a transparent
electrical conductive layer applied to the confronting surfaces of
the said two plates; each said conductive layer having parallel
surface alignment, the said alignment of the two adjoining surfaces
being parallel; a continuous layer of nematic liquid crystal having
positive dielectric anisotropy between the two electrical
conducting coatings;
means for applying a fixed electrical bias to the two said
electrical conducting coatings;
means for applying an amplitude modulated oscillatory electrical
signal having a frequency greater than 10 hertz across the said
continuous layer;
a source of polarized light directed through the said two
transparent plates;
whereby the light which passes through the said two transparent
plates is a phase-shifted beam which is distinguishable from the
light from the said source in a manner which corresponds to the
said oscillatory electrical signal.
2. A light modulator of claim 1 wherein the said continuous layer
of nematic crystal has a thickness of 6 to 60 microns.
3. The light modulator of claim 1 wherein the said source of
polarized light is a source of unpolarized light having a wave
length from 4 microns to 240 millimicrons and a light polarizer is
interposed between the said light source and the said liquid
crystal cell.
4. The light modulator of claim 1 including two said liquid crystal
cells in series.
5. The light modulator of claim 4 wherein the said means for
applying an amplitude-modulated oscillatory electrical signal
across the continuous layer of nematic liquid crystal provides a
first signal to the first of said two said liquid crystal cells and
applies the same inverted electrical signal to the second of said
two said liquid crystal cells.
6. The light modulator of claim 1 wherein the said oscillatory
electrical signal has a frequency of 10 hertz to 500 kilohertz.
7. A communications system including a light modulator in
accordance with claim 1 and a light demodulator comprising:
a beam splitter which divides incident light into two separate 90
degree quadrature component beams;
a photosensitive transducer generating an oscillatory electric
signal corresponding to each of said separate component beams;
means for directing each of said separate component beams to a said
photosensitive transducer;
comparator means for generating an oscillatory electrical signal in
accordance with the phases difference of the electrical signals
from said transducers;
means for demodulating the said oscillatory electrical signal;
high pass filter means and low pass filter means;
means for delivering the demodulated signal through said filter
means;
transducer means responsive to the resulting filtered demodulated
signal;
and means for delivering the said phase-shifted beam from said
light modulator as the said incident light for said
demodulator.
8. The communications system of claim 7 wherein a quarter-wave
retardation plate is interposed between said light modulator and
said light demodulator.
9. A communications system including a light modulator in
accordance with claim 1 which delivers a said phase-shifted beam to
a receiver unit, said receiver unit including a polarized light
splitter, a photosensitive transducer for generating separate
electrical signals corresponding to the quadrature light beams
obtained from said splitter,
comparator means for generating an oscillatory electrical signal
related to the phase difference between the two said quadrature
signals;
demodulator means and filter means for reproducing an electrical
signal which corresponds to the phase shift of said phase-shifted
light beam.
10. The communications system of claim 9 wherein the said
phase-shifted beam traverses a gas to the said receiver unit.
11. The communications system of claim 10 wherein the said gas is
the atmosphere.
12. The commmunications system of claim 9 wherein the said
phase-shifted beam traverses a transparent solid to the said
receiver unit.
13. The communications system of claim 12 wherein the said
transparent solid is an optical glass fiber.
14. A multi-directional communicating device comprising multiple
light modulators as defined in claim 1, each being parallel to a
plane which is normal to a radial from a common, fixed focus; a
light source for directing polarized light through each of said
multiple light modulators; means for providing a distinct phase
modulation to the light beam transmitted through each of the said
light modulators.
15. The directional communications system of claim 14 wherein the
said light modulators are arrayed about a common light source
located at the said focus.
16. A locating system including at least two communication devices
as described in claim 14, said devices being arranged so that
multiple phase-shifted light beams from one source intersect at
least one phase-shifted light beam from the other source, and the
regions of intersection identify preestablished locations with
respect to the said two communication devices;
receiving means in any location being responsive to the single
light beam received from each source at the said location.
17. A communications system including
a receiving location having a light source and a light demodulator
adapted to convert a phase modulated light beam into a
corresponding electrical signal;
a transmitting location including a light modulator as described in
claim 1, a reflective surface, a source of a modulated electrical
signal, means for applying the said modulated electrical signal to
the said light modulator and a polarizing plate disposed between
said modulator and said reflected surface;
said receiving location and said transmitting location being
arranged so that light from said light source can be directed
through said modulator and said polarizing plate to said reflective
surface; and
a reflected light beam from said reflected surface is directed
through said polarizing plate and through the said modulator to
develop a phase modulated light beam which is directed to said
demodulator at the said receiving location.
18. The communications system of claim 17 wherein the said
reflective surface is a corner reflector.
19. The communication system of claim 17 wherein the said reflector
surface is a reflecting prism.
20. The communications system of claim 19 wherein the said
reflecting prism is a tetrahedron.
21. The communications system of claim 17 wherein the said
phase-shifted beam traverses a gas to the said receiver unit.
22. The communications system of claim 21 wherein the said gas is
the atmosphere.
23. The communications system of claim 17 wherein a quarter-wave
retardation plate is interposed between said receiving location and
the said transmitting location.
24. A communicating system including
a receiving location having a light source and a light demodulator
adapted to convert a phase modulated light beam into a
corresponding electrical signal;
a transmitting location including a light modulator according to
claim 1, a reflective surface, a source of a modulated electrical
signal, means for applying the said modulated electrical signal to
the said light modulator and a polarizing plate disposed between
said modulator and said reflected surface;
said receiving location and said transmitting location being
arranged so that light from said light source can be directed
through said modulator and said polarizing plate to said reflective
surface; and
a reflected light beam from said reflected surface is directed
through said polarizing plate and through the said modulator to
develop a phase modulated light beam which is directed to said
demodulator at the said receiving location.
25. A communications system including a light modulator in
accordance with claim 1,
means for transmitting the said phase shifted beam to a
demodulator;
responsive means within said modulator which are responsive to an
electrical oscillating signal; and
light responsive means within said demodulator for converting said
phase shifted beam to an electrical oscillating signal.
26. A light modulator comprising three juxtaposed parallel
spaced-apart transparent plates having a transparent electrically
conductive coating over all confronting surfaces;
a layer of nematic liquid crystal composition having positive
dielectric anisotropy in the space between confronting surfaces of
said plates; parallel a polarizing plate juxtaposed and parallel to
said plates;
said three plates and said polarizing plate being secured in fixed
relation; means for applying a fixed electrical bias to the
confronting coatings;
means for applying an amplitude modulated oscillatory electrical
signal across each said layer of the nematic liquid crystal
composition;
said modulator being adapted to develop a phase shift in a beam of
light passing through the said modulator from the said polarizing
plate and thence through the said layer of nematic liquid crystal
composition.
27. The light modulator of claim 26 wherein the said three plates
and the said polarizing plate are secured in a common housing.
.Iadd.
28. A light modulator according to claim 1 wherein the said
amplitude-modulated oscillatory electrical signal corresponds to
separate color components of a light source. .Iaddend. .Iadd.29.
The light modulator of claim 1 including means for converting the
said phase-shifted light to a corresponding amplitude-modulated
signal. .Iaddend. .Iadd.30. The light modulator of claim 29 wherein
the said amplitude-modulated signal is fully-modulated. .Iaddend.
.Iadd.31. The light modulator of claim 4 wherein the oscillatory
electrical signal applied to a first of said liquid crystal cells
is opposite in phase to the electrical signal applied to the second
of said liquid crystal cells. .Iaddend. .Iadd.32. A method for
controlling polarized light comprising:
(a) establishing a film of nematic liquid crystal having positive
dielectric anisotropy between two parallel surfaces and imparting a
first generally parallel surface orientation to the said liquid
crystal film at the said surfaces;
(b) establishing a second bulk orientation of the bulk of the
liquid crystal in a direction which is different from the said
first generally parallel orientation of the liquid crystal at the
said surfaces;
(c) applying a varying electrical signal to said surfaces to alter
the relative direction of the alignment of the liquid crystal
between the said surface orientation and the said bulk
orientation;
(d) passing polarized light through said film to generate light
which is phase shifted in a manner corresponding to the said
varying electrical
signal. .Iaddend. .Iadd.33. The method of claim 32 wherein the
phase-shifted light corresponds to a separation of separate color
components of a light source. .Iaddend. .Iadd.34. The method of
claim 33 wherein the said phase-shifted light corresponds to green
light components from a light source. .Iaddend. .Iadd.35. The
method of claim 33 wherein the said phase-shifted light corresponds
to red light components from a light source. .Iaddend. .Iadd.36.
The method of claim 33 wherein the said phase-shifted light
corresponds to yellow light components from a light
source. .Iaddend. .Iadd.37. The method of claim 32 wherein the said
film is established between transparent conducting electrodes.
.Iaddend. .Iadd.38. The method of claim 37 wherein the said
transparent conducting electrodes are formed predominantly from
oxides of metals selected from the class consisting of tin and
indium. .Iaddend. .Iadd.39. The method of claim 32 including the
step of converting the phase-shifted light to a corresponding
amplitude-modulated signal. .Iaddend. .Iadd.40. The method of claim
39 wherein the said phase-shifted light is converted in a
polarizing filter. .Iaddend. .Iadd.41. The method of claim 39
wherein the said step of converting the phase shifted light
develops a fully amplitude-modulated signal. .Iaddend. .Iadd.42.
The method of claim 32 wherein the said amplitude-modulated light
corresponds to a separation of separate color components of a light
source. .Iaddend. .Iadd.43. The method of claim 32 including the
step of separately modulating the red components and the green
components of the said light source. .Iaddend.
.Iadd.44. A method for controlling polarized light comprising:
(a) establishing a film of nematic liquid crystal having positive
dielectric anisotropy between two parallel surfaces and imparting a
first generally parallel surface orientation to the said liquid
crystal film at the said surfaces;
(b) establishing a second generally parallel bulk orientation of
the bulk of the liquid crystal in a direction which is different
from the said first generally parallel surface orientation of the
liquid crystal at the said surfaces;
(c) applying a varying electrical signal to said surfaces to change
the alignment of the liquid crystal between the said surface
orientation and the said bulk orientation;
(d) passing polarized light through said film to generate light
which is phase shifted in a manner corresponding to the said
varying electrical signal. .Iaddend. .Iadd.45. In the method for
controlling polarized light according to claim 44, maintaining the
direction of orientation of the bulk of the liquid crystal
generally perpendicular to the said surfaces of
the film. .Iaddend. .Iadd.46. The method of claim 44 wherein the
phase-shifted light corresponds to a separation of separate color
components of a light source. .Iaddend. .Iadd.47. The method of
claim 46 wherein the said phase-shifted light corresponds to green
light components from a light source. .Iaddend. .Iadd.48. The
method of claim 46 wherein the said phase-shifted light corresponds
to red light components from a light source. .Iaddend. .Iadd.49.
The method of claim 46 wherein the said phase-shifted light
corresponds to yellow light components from a light
source. .Iaddend. .Iadd.50. The method of claim 44 wherein each of
the said spaced-apart films is established between transparent
conducting electrodes. .Iaddend. .Iadd.51. The method of claim 50
wherein the said transparent conducting electrodes are formed
predominantly from oxides of metals selected from the class
consisting of tin and indium. .Iaddend. .Iadd.52. A method for
controlling polarized light comprising:
(a) establishing two spaced apart films of nematic liquid crystal
having positive dielectric anisotropy, each said film contained
between two parallel surfaces; imparting a generally parallel
alignment of each liquid crystal film at its surfaces;
(b) applying a varying electrical signal to the said surface of
each said film to align the direction of each liquid crystal film
at its surfaces in directions which are different from the
orientation of the bulk of each liquid crystal film;
(c) passing polarized light through each said film in sequence to
generate light which is phase shifted in a manner corresponding to
the said varying electrical signals, wherein the resultant phase
shift of the two said films corresponds to the vector summation of
the phase shift resulting
from each of the said two spaced apart film. .Iaddend. .Iadd.53. In
the method of claim 52 applying a said electrical signal applied to
one of said films which is opposite in phase to the electrical
signal applied to the other of said films. .Iaddend. .Iadd.54. In
the method of claim 52 including the step of converting the
phase-shifted light to a corresponding amplitude-modulated signal.
.Iaddend. .Iadd.55. The method of claim 54 wherein the said
phase-shifted light is converted in a polarizing filter. .Iaddend.
.Iadd.56. The method of claim 54 wherein the said step of
converting the phase shifted light develops a fully
amplitude-modulated signal. .Iaddend. .Iadd.57. The method of claim
52 wherein the said amplitude-modulated light corresponds to a
separation of separate color components of a light source.
.Iaddend. .Iadd.58. The method of claim 52 including the step of
separately modulating the red components and the green components
of the said light source. .Iaddend.
.Iadd.59. A method for controlling polarized light comprising:
(a) establishing a film of nematic liquid crystal having positve
dielectric anisotropy between two parallel surfaces and imparting a
first generally parallel surface orientation to the said liquid
crystal film at the said surfaces;
(b) establishing a second bulk orientation of the bulk of the
liquid crystal in a direction which is different from the said
first generally parallel orientation of the liquid crystal at the
said surfaces;
(c) altering the relative direction of the alignment of the liquid
crystal between the said surface orientation and the said bulk
orientation;
(d) whereby the phase shifting property of the film corresponds to
the said relative direction. .Iaddend.
Description
TERMINAL DISCLAIMER
The term of any patent issuing on this patent application shall
still expire with the expiration of applicant's co-pending patent
application Ser. No. 121,071, filed Feb. 13, 1980, U.S. Pat. No.
4,385,806.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to modulating polarized light by
transmitting the polarized light through at least one liquid
crystal cell containing a thin layer of nematic liquid crystal
composition having positive dielectric anisotropy.
2. Description of the Prior Art
Liquid crystal cells containing a layer of nematic liquid crystal
composition between a pair of parallel transparent plates are
employed in a variety of optical installations. Such devices are
used principally in digital display devices.
In my copending application Ser. No. 121,071 aforesaid, an improved
liquid crystal cell is described wherein a conductive transparent
film is provided on each side of the transparent flat plates which
confine the liquid crystal composition and an electrical bias
voltage, AC or DC, is applied to the two thin films to permit the
liquid crystal composition to respond rapidly to additional
electrical voltages applied across the two conductive coatings.
It is also known that thin layers of liquid crystal composition
will bring about a phase shift in a beam of polarized light which
is transmitted through the liquid crystal composition. Heretofore,
the devices have been employed as light shutters which block the
polarized light in the regions where an electrical field is
established across the liquid crystal composition. Typical turn-off
times of 250-300 milliseconds are common with existing liquid
crystal light shutters which employ nematic liquid crystals. Such
response time is much too large to permit the use of such devices
in high speed communication.
SUMMARY OF THE INVENTION
According to the present invention, liquid crystal cells as
described in the aforesaid U.S. patent application Ser. No. 121,071
can achieve remarkably rapid response times. Switching speeds of 10
microseconds have been achieved. Such high speed switching permits
the application of oscillatory electrical signals of the order of
10 hertz to 500 kilohertz to be applied effectively to such liquid
crystal cells. In accordance with this invention, an oscillatory
carrier wave signal, for example, a 30 kilocycle signal, is
modulated with a lower frequency communication signal, for example,
an audio frequency signal or a series of signal pulses. The
resulting modulated carrier wave is applied across the conductive
surfaces of the electrically biased liquid crystal cell. A beam of
polarized light, transmitted through such liquid crystal cell, will
experience a phase shift in accordance with the applied modulated
carrier wave. The resulting transmitted light beam has an
identifiable phase shift which can be detected at a detection
location which is remote from the light source. The detection
station has facilities for separating the transmitted light beam
into its quadrature components. Each of the quadrature components
activates an appropriate photo responsive transducer to generate an
electrical analog signal corresponding to the observed phase of
each quadrature component of the light beam. The analog electrical
signals are compared and a differential signal is demodulated to
reproduce at the receiving location an electrical signal
corresponding to the original input audio signal or chain of pulse
chains introduced at the transmitting location.
The liquid crystal cell in this invention consists of a pair of
transparent flat plates which are coated with an electrically
conductive transparent coating and some parallel surface alignment
treatment such as a polyvinyl alcohol coating which has been rubbed
uniaxially. The parallel alignment of both transparent plates is
parallel in this surface mode modulation invention.
A preferred embodiment of the present invention employs two such
liquid crystal cells in series whereby the polarized light beam is
transmitted through each of the two liquid crystal cells, each of
which introduces its own independent phase shift into the light
beam. The phase shifts are vectorially added whereby the two liquid
crystal cells function with respect to the light beam in a manner
which is analogous to the functioning of a push-pull amplifier
acting upon an oscillatory electrical signal. As a consequence, the
linearity of response of the resulting phase shifted light beam is
remarkably increased.
It is a particular feature of the present invention that the phase
shifted light beam can be delivered along with other light from the
same source or from other sources to a remote detector. The phase
shifted light beam can be successfully demodulated by a detector
where the phase shifted light beam energy comprises a nearly
insignificant portion of the total light including background light
energy at the detector location, e.g., less than two percent of the
incident light energy can be effective.
Accordingly, it is a principal object of this invention to provide
method and apparatus for generating a phase modulated beam of
polarized light.
A further object of this invention is to employ two liquid crystal
cells in series to achieve remarkable linearity of response in a
phase modulated polarized light beam.
It is a further object of this invention to provide a method and
apparatus for phase modulation of separate components of a light
source, such as red light energy, green light energy, yellow light
energy, and the like.
It is a still further object of this invention to provide the phase
modulating apparatus and method for any light source including by
way of example natural light, fluorescent lamps, battery operated
flashlights, ultraviolet light sources, infra-red light sources,
laser light sources, monochromatic light sources, and the like.
A further object of the invention is to provide a method and
apparatus for demodulating a phase modulated polarized light beam
to reproduce a communication delivered by the phase modulated
polarized light beam.
Another object of this invention is to provide a method of
apparatus for remote communication systems involving transmission
of light energy from a receiving location to a transmitting
location and reflection communications using the same light energy
back from the transmitting installation to the receiving
installation.
It is a further object of the invention to provide a method and
apparatus for communicating between a source and a receiver by
means of a light beam .
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is schematic illustration of the light modulator of this
invention.
FIG. 2 is a schematic illustration of a demodulator for the
modulated light beam of this invention.
FIG. 3 is a cross-section illustration taken through a light
modulation liquid crystal cell unit according to one embodiment of
this invention.
FIG. 4 is a cross-section illustration, similar to FIG. 3, taken
through a light modulating liquid crystal cell unit according to an
alternative embodiment of the present invention.
FIG. 5 is a graphical illustration of a modulated electrical signal
resulting from a single liquid crystal cell according to this
invention.
FIG. 6 is a graphical illustration of the modulated electrical
signal resulting from two sequential liquid crystal cells according
to a preferred embodiment of this invention.
FIG. 7 is a schematic illustration of a light modulating
installation according to this invention.
FIG. 8 is a schematic illustration of a demodulating installation
for the modulated light beam of this invention.
FIG. 9 is a perspective illustration of a building room sharing one
embodiment of the communication system of this invention.
FIG. 10 is a plan view of a multidirectional light communication
transmitting installation.
FIG. 11 is a schematic illustration of two multi-directional light
communication transmitting installations and an airborne light
communication receiver.
FIG. 12 is a cross-section illustration similar to FIG. 3 taken
through a light modulating liquid crystal cell unit according to a
further embodiment of the present invention.
FIG. 13 is a schematic illustration of a communications
transmitting and receiving installation according to a further
embodiment of this invention.
FIG. 14 is a schematic illustration of a communication transmitting
installation according to a still further embodiment of this
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, a beam of polarized light
can be phase modulated in a predictable and reproduceable fashion.
It is essential that the source of light be polarized initially and
that the polarized light be transmitted through at least one liquid
crystal cell having a thin layer of a nematic crystal composition
having positive dielectric anisotropy. The liquid crystal layer is
confined between parallel transparent plates, such as glass plates,
which are coated with a transparent electrically conductive coating
such as tin oxide or indium oxide and which are aligned by any
appropriate alignment techniques such as uniaxial rubbing of a
polyvinyl alcohol coating or by means of uniaxial drying techniques
for other coatings. In accordance with the teachings of the
aforesaid copending U.S. patent application Ser. No. 121,071, an
electrical signal is applied as a bias to the two transparent
electrically conductive coatings in order to align the
preponderance of the liquid crystal layer except for the liquid
crystal molecules immediately adjacent to the electrical conductive
coatings. Thereafter the magnitude of an applied electrical signal
across the two electrical conductive coatings will determine the
amount by which a polarized light beam will have its phase shifted
in traversing the liquid crystal cell.
Referring to the drawings, FIG. 1 shows a light source 10 and a
polarizer 11 in line with two liquid crystal cells 12, 13. A light
beam 50 from source 10 is delivered from polarizer 11 as a
polarized beam 51 to the cells 12, 13. The liquid crystal cells 12,
13, develop a phase shift in the polarized light beam 51 which is
transmitted as indicated at 14. The light beam 14 is delivered, as
shown in FIG. 2, preferably through a quarter wave plate 95
(hereinafter more fully described) and thence to a polarizing cube
15 which delivers separate light beams 16, 17 related to each of
the quadrature components of the incident light beam 14. The
separate light beams 16, 17 are delivered to photosensitive
transducers 18, 19 which develop an electrical signal related to
the light beam 16, 17, respectively. The electrical signals from
the transducers 18, 19 are delivered to a comparator 20 which
develops an output electrical signal related to the phase
difference between the two light beams 16, 17. The output signal
from the comparator 20 is demodulated in a demodulator 21. An
output signal is delivered at a terminal 22 corresponding to the
phase shift appearing in the incident light beam 14.
The amount of modulation of phase shift introduced into the light
beam by the liquid crystal cells 12, 13 may be related to an input
signal from a source 23 which is modulated in a modulator 24 to
produce a modulated signal which is applied through conductors 53,
54 as a bias to the liquid crystal cells 12, 13.
A typical liquid crystal cell for the present purposes is
illustrated in FIG. 3 wherein a pair of transparent plates 25 is
spaced apart. Each of the transparent plates 25 has a transparent
electrically conductive coating 26 such as tin oxide or indium
oxide. A thin layer 27 of nematic liquid crystal composition having
positive dielectric anisotropy is provided between the two layers
26. The nematic liquid crystal 27 is preferably from about 6 to 60
microns in thickness. An appropriate perimeter seal such as a
collar 28 is provided to confine the liquid crystal layer 27 and to
retain the plates 25 in fixed relationship. Two electrical
conductors 29, 30 are connected, one to each of the electrically
conductive coatings 26 for applying electrical signals across the
nematic liquid crystal layer 27.
In the assembly shown in FIG. 1, two individual crystal cells of
the type shown in FIG. 3 may be employed. The transparent plates 25
preferably are optically clear glass although other glass may be
employed as well as appropriate transparent plastic substances.
Liquid crystal compositions in general are well known. The liquid
crystal cell should not have excessive absorption for the light
which is being transmitted. For example, where infrared light is
employed as a light source 10, the selection of the liquid crystal
composition should take this factor into consideration and avoid
compositions which have an absorption spectrum which includes
infrared wave lengths.
By way of example, the nematic liquid crystals may include
phenyl-cyclohexanes, cyano-phenyl-alkyl-benzoates and dialkyl
benzoates. No problems have been encountered with the use of Shiff
bases as an ingredient of the liquid crystal composition. The
liquid crystal composition may be a pure single liquid crystal or
may be mixtures of different liquid crystal compositions.
What is important in the present invention is the application of an
electrical bias between the conductors 29, 30 which will have the
effect of biasing the preponderance of the liquid crystal
ingredients in the layer 27 so that the application of incremental
electrical signals across the conducting layers 26 will achieve a
rapid on-off cycle for the cell. The principle of applying an
electrical bias is fully disclosed in copending U.S. patent
application Ser. No. 121,071 aforesaid. The electrical bias which
is applied to the conductors 29, 30 may be a DC bias or an AC bias.
If an AC signal is applied as the bias for the liquid crystal cell,
its frequency should be dissimilar to the frequency of any carrier
signals which are employed as a modulating signal. By providing an
electrical bias, the liquid crystal cell may be rapidly and
reliably regulated whereby the phase shift experienced by a
polarized light beam traversing through the liquid crystal cell
will be dependent upon the instantaneous amplitude of the applied
modulating signal.
An appropriate modulating signal may be applied to the electrically
conductive layers 26 through electrical conductors 31, 32 (FIG. 3)
or maybe added to the bias conductors 29, 30. By providing a liquid
crystal cell having a response cycle time less than 30
milliseconds, the liquid crystal cell can react to oscillatory
electrical impulses having frequencies of 300 kilohertz.
In the installation illustrated in FIG. 1, there are two liquid
crystal cells, 12, 13 each of which is constructed similarly to
that shown in FIG. 3. It is feasible to combine two or more liquid
crystal cells for the purposes of the present invention into a
single unit as shown in FIG. 4 wherein the liquid crystal cell unit
35 includes three spaced-apart transparent plates 36, 37, 38. The
central plate 38 has a transparent electrical conducting coating 39
on both surfaces. The outer transparent plates 36, 37 have an
electrical conducting 39 on their inner surface. Two layers 40, 41
of liquid crystal composition are provided between the plates 36,
38 and 37, 38. An appropriate collar 42 functions as a perimeter
seal and support for the liquid crystal cell unit 35. A pair of
conductors 43, 44 provides an electrical bias across the liquid
crystal layer 41. A pair of electrical conductors 45, 46 provides
an electrical bias across the liquid crystal layer 40. A modulating
electrical signal may also be applied across the conductors 43, 44
and across the conductors 45, 46, superimposed upon the electrical
bias.
The dual liquid crystal cell unit 35 provides a compact unit which
will carry out the present invention in a preferred embodiment,
i.e., an embodiment which employs two liquid crystal cells 12,
13.
Operation of the Device
Referring to FIG. 1, a light source 10 having a wave length from
about 4 microns to about 240 millimicrons delivers a beam of light
50 through a polarizer 11 in FIG. 1 to develop a polarized light
beam 51. The polarized light beam 51 is delivered through the
transparent liquid crystal cell 12 to generate a first phase
shifted light beam 52 which is in turn delivered through a second
liquid crystal cell 13 to produce the phase shifted modulated light
beam 14. The polarized light beam 51 is a light beam having waves
which vibrate primarily in parallel planes. As is well known, the
light waves which appear to be in the polarization planes are
resultant waves which can be considered to have a fast axis
component and a slow axis component at right angles to each other,
also known as quadrature components. The polarized light beam 51,
when passing through the liquid crystal cell 12, will experience a
retardation of the slow axis component by an amount which is
dependent upon the intensity of the electrical charge on the liquid
crystal cell 12. Thus the light beam appearing at 52 can be
distinguished from the polarized light beam at 51 in that the slow
axis components of the beam have been retarded by a measurable
amount which is less than a quarter wave length of the light in the
beam 50. The beam 52 may be employed as a modulated light beam for
delivering communications if desired. However, it is preferred to
deliver the phase modulated light beam 52 through a second liquid
crystal cell 13 to increase the linearity of the phase modulated
beam at 14. Two such sequential liquid crystal cells function in a
manner analogous to that of a push-pull amplifier.
The linearity benefits can be illustrated by referring to FIGS. 5
and 6. In a test installation, a sawtooth input signal having a
frequency of 100 Hertz was modulated with a carrier signal having a
frequency of 20 Kilohertz. The modulated electric signal was
applied to a single liquid crystal cell containing a layer of
nematic liquid crystal mixture as follows:
43.2% by weight of the reaction product of 4-heptyl benzoylchloride
and a mixture of 2-methyl-1, 4-benzene diol and 4-pentyl benzoyl
chloride;
22.2% by weight p-cyanophenyl-p-n-pentyl benzoate; and
34.1% by weight p-cyanophenyl-p-n-heptyl benzoate.
The resulting light beam at 51 was demodulated in apparatus of the
type shown in FIG. 2 and produced an output signal as shown in FIG.
5 which is similar to a sine wave signal.
The signal of FIG. 5 has the same frequency as the input sawtooth
wave, but exhibits many sub-harmonic components of the input
sawtooth wave.
By employing two identical liquid crystal cells 12, 13 as shown in
FIG. 1, the light wave 52 was further modulated to produce the
light wave 14 which was modulated in the same apparatus to produce
a signal of the type shown in FIG. 6 wherein the sawtooth linearity
of the output signal is apparent. This significant improvement in
linearity of output results from the use of two liquid crystal
cells in series, which is analogous to a push-pull amplifier.
Push-pull amplifiers achieve a higher distortion free output power
than a single ended amplifier. Push-pull amplifiers employ two
input signals which are equal in ampplitude and 180 degrees out of
phase. Accordingly the signals applied to the liquid crystal cells
12, 13 for push-pull operation through the conductors 53, 54
respective are equal but 180 degrees out of phase.
A Particular Example
As shown in FIGS. 7 and 8 an operating example at the present
communication system is described. A transmitter station 60 of FIG.
7 includes an input signal source 61 which was the audio signal
received from a public radio broadcasting station. The audio signal
is delivered through an amplifier 62 to an amplitude modulator 63.
A carrier wave oscillator 64 generates a 300 khz carrier wave
signal delivered to the amplitude modulator 63. The resulting
amplitude modulated signal is delivered from the modulator 63
through an amplifier 59 to a liquid crystal cell unit 65 including
two cells of the type described in FIG. 3. A light source 66 was a
sealed beam headlight obtained from an automobile parts store
operating at 12 volts DC.
The liquid crystal cell unit 65 had a diameter of 4 inches. The
laminates had a total thickness of about one quarter-inch. A
polarizer 67 was positioned between the light source 66 and the
liquid crystal cell unit 65. A bias voltage from a source 68 was
also applied to the liquid crystal cell 65 in accordance with the
principles set forth in the aforesaid U.S. patent application Ser.
No. 121,071. The amplitude modulated carrier wave causes the light
from the source 66 to experience a phase shift as it traverses the
liquid crystal cell unit 65 so that the light wave 69 resulting
from the phase shift is a phase modulated light wave wherein the
components of the light wave in the slow axis are retarded by a
reproduceable amount from the corresponoding components in the fast
axis. The light wave 69 in one test was directed against a painted
concrete block wall 75 feet away. The reflected light from the
concrete block wall was received in a receiving station shown in
FIG. 8.
The reflected light wave 69' was received in an appropriate
polarized light splitter 70 which was a Brewster's-angle beam
splitter. Any optical prism device might be employed which will
separate the two quadrature components of the light beam 69' and
deliver them separately to photodetector 71, 72 which can generate
an analogous electrical signal for comparison in a comparator 73.
The differential between the two quadrature electrical signals
varies in accordance with the phase relation of those two signals.
Hence the output signal from the comparator 73 corresponds to the
modulated input signal which was applied from the amplifier 59 to
the liquid crystal cell unit 65 in the transmitter station 60.
The differential signal from the comparator 73 is delivered through
a demodulator 74 and the resulting audio frequency signal is
filtered in a high pass, low pass filter 75 to re-establish the
original input signal analogous to the signal from the source 61.
The output of filter 75 is amplified in an amplifier 76 and the
amplified signal is delivered to an output device 77 which was a
radio speaker.
So long as the light beam 68' was directed into the splitter 70, it
was possible to hear the radio broadcast at the loudspeaker 77.
When the light beam 69' was interrupted, the loudspeaker 77 output
was reduced or eliminated.
Alternative Modifications
By employing the described system illustrated in FIGS. 7 and 8, a
number of different communication systems can be achieved. For
example, the light source 66 may be a fluorescent light source in a
room ceiling as illustrated in FIG. 9 wherein a room 80 has a
ceiling 81, sidewalls 82 and outside wall 83 and a floor 84. The
ceiling 81 is equipped with familiar fluorescent fixtures 85 which
contain fluorescent lamps 86 behind appropriate light diffusers. In
this installation, a light modulator 87 may be applied to a small
portion of the area of one or more of the ceiling fixtures 85.
Electrical conductors (not shown) are connected to the light
modulator 87. A portion of the light from the ceiling fixtures thus
is delivered through the light modulator 87 which includes a
polarizer and one or preferably two liquid crystal cells in
accordance with this invention. The light within the room 80 thus
includes a small percentage of light which has a phase shift
resulting from the controlled modulation achieved within the
modulator 87.
An appropriate communication receiving station 88 is provided
somewhere within the room 80, for example, on the top of a desk 89.
The receiving station 88 has a light receiving opening 90 which
receives light from the room 80. The light received by the light
opening 90 is in part natural outdoor light from a window 91, in
part fluorescent light from the ceiling fixtures 85, and in part
additional light from any other light sources which may be within
the room 80 for example, incandescent table lamps, etc. A portion
of the light received by the light-receiving opening 90 is the
modulated light which is delivered into the room through the light
modulator 87. The modulated light within the room 80 may comprise
two percent or less of the total light. However, all of the light
which is not part of the modulated signal will be cancelled in the
beam splitter; the differential resulting from the modulated light
can be readily detected within the receiving station 88. Thus, as
shown in FIG. 9, it is possible to deliver communications within a
room 80 from a transmitter 87 to a receiver 88. The performance of
the system is certainly enhanced if the modulated light from the
transmitter 87 is concentrated as a beam against the light
receiving opening 90. However, it has been found to be sufficient
that the modulated light constitutes as little as two percent or
less of the light which is observed by the light receiving opening
90.
In a particularlly preferred embodiment of the present invention,
the light received at a receiving station, for example, FIG. 2, is
initially delivered through a quarter wave length retardation plate
95 which functions to optimize the intensity of the two quadrature
components of the modulated light wave 14. The retardation plate 95
is not essential, but the use of the retardation plate assures that
the two quadrature components of the modulated wave will have the
same intensity. While a quarter wave length retardation plate is
optimum, the system will also benefit from any retardation plate
approximating a quarter wave length.
In this regard, it should be noted that each of the individual
color components of a light wave has its own independent wave
length. This a 140 nanometer retardation plate corresponds to a
quarter wave length retardation for green light but is not a
precisely quarter wave length retardation for red light. However, a
140 nanometer retardation plate has been found to be a useful
device when employing white light as the communications medium.
Further Embodiments of the Invention
Referring to FIG. 10 there is illustrated a multidirectional
communications installation employing the present communication
system. As shown in FIG. 10, an omni-directional homing and
communication device 100 includes a light source 101 surrounded by
a plurality of light modulators 102, each of which includes a
polarizer and at least one, preferably two, liquid crystal cells of
the type illustrated in FIG. 3. The light from each of the light
modulators 102 develops a modulated light beam which is
schematically illustrated by the numeral 103 which radiates
outwardly covering a sector 104 of the available 360.degree. range.
For example, the light modulator 102A generates a modulated light
wave 103A over the sector identified by the numeral 104A.
Thus at any point in the 360.degree. range outside the device 100
there is available one light modulated wave 103 generated from a
corresponding light modulator 102 except for small radial sectors
105 which exist between adjacent sectors 104. Each individual light
modulator 102 generates its unique light wave 103 in accordance
with modulation signals derived from a corresponding modulator 106.
Accordingly, a receiver positioned anywhere outside the device 100,
so long as it is not immediately within one of the blank sectors
105, can immediately detect its position relative to the device 100
by the nature of the information which is received from the light
wave 103 which is directed to the related sector 104.
The device 100 can be employed to deliver unique communications to
receivers within each of the selected sectors 104.
In a further embodiment as shown in FIG. 11, similar devices 110A,
100B can be mounted on the ground 111 with multiple light
modulators 112 each of which receives light from a source 113. Each
of the light modulators 112 includes a polarizer and at least one,
preferably two, liquid crystal cells of the type shown in FIG. 3.
Each of the modulators 112 generates a light wave which radiates
outwardly over a sector 114. Thus sets of the devices 110 may be
employed to deliver intersecting light beams which can be received
by a remote airborne receiver, for example, in an airplane 115. The
receiver can determine its location with respct to the two devices
110A and 110B.
Other Embodiments
Heretofore the present invention has been described in terms of a
single liquid crystal cell or a single pair of cooperative liquid
cells for establishing a phase shift in a beam of polarized light.
It is possible that multiple phase shifts can be introduced into a
single beam of polarized light and that each of the multiple phase
shifts can be employed to carry distinct communication. All that is
required is additional independently modulated liquid crystal cells
or pairs of liquid crystal cells in series at the transmitter
station and corresponding additional liquid crystal cells or pairs
of liquid crystal cells at the receiving station for restoring the
light wave to zero.
A further means for delivering multiple communications in a beam of
light is the use of separate modulators for different color
components of the light beam. The color components may be
separated, modulated and recombined at the transmitter station.
Thereafter at the receiving station, the combined modulated light
beam may be separated into the same color constituents, each of
which can be demodulated.
A particularly useful version of the present transmitter is
illustrated in FIG. 12 which is similar to the cell of FIG. 4. The
unit 35' has three spaced-apart juxtaposed transparent plates 36',
37', 38'. The central plate 38' has a transparent electrical
conducting coating 39' on both surfaces. The outer transparent
plates 36', 37' have an electrical conducting coating 39' on their
inner surface. Two layers 40', 41' of liquid crystal composition
are provided between the plates 36', 38' and 37', 38' respectively.
Electrical conductors (not shown) provide electrical bias across
the liquid crystal layer 40' and across the liquid crystal layer
41'.
The liquid crystal cell unit 35' of FIG. 12 also includes within
the collar 42' a polarizing plate 47.
In a practical construction embodying a liquid crystal cell of the
type shown in FIG. 12, the thickness of the laminate including
polarizing plate 47, the glass plates 36', 37', 38' and the two
liquid crystal films 40', 41' has been about one quarter inch. The
laminate was tested in the form of a circular disc having a
diameter of about four inches.
The principle advantage of the structure illustrated in FIG. 12 is
that the unit contains a complete modulating device with a
polarizing plate and two liquid crystal cells in a common housing,
namely a collar 42'. The device shown in FIG. 12 will function as a
communications transmitter by transmitting light through the
laminate from the right to the left as shown in FIG. 12, that is,
through the polarizing plate 47 and thence through the two liquid
crystal films 40', 41' in series. An appropriate modulated
electrical signal is applied to each of the liquid crystal films
40', 41' as already described in the discussion of FIG. 4.
The device of FIG. 12 has a further interesting application in
communications as shown in FIGS. 13, 14. Referring to FIG. 13,
there is illustrated a communications including a receiving
location 120 and a transmitting location 121. The receiving
location includes a light source 122 and a light demodulator 123
similar to that shown in FIG. 8. The transmitting location 121
includes a reflective surface 124 such as a mirror, a shiny metal
surface, a bright surface, or an appropriate reflecting prism. An
electrical cable 125 delivers a modulated electrical signal from a
modulator 126 to the unit 35' as shown in FIG. 7.
In the operation of the communications system of FIG. 13, light
energy is delivered from the light source 122 to the unit 35' as
indicated schematically by the broken line 127. The light passes
through the unit 35' in the direction from left to right as shown
in FIG. 12. The light thus passes sequentially through the liquid
crystal film 41', liquid crystal film 40', and thereafter through
the polarizing plate 47. The polarizing plate 47 transmits only
polarized light. Thus the incident light at 127' on the reflective
surface 124 is polarized light. Reflected light 128 passes through
the liquid crystal cell unit 35' in the direction from right to
left as seen in FIG. 12 and experiences the phase shifts which are
introduced into the beam by the liquid crystal films 40', 41. The
transmitted light beam 128' thus is modulated in accordance with
the signals applied to the liquid crystal cell 35' from the
modulator 126 and conductors 125. The phase modulated light beam
128' impinges on a demodulator 123 which develops a signal at the
receiving location 120 corresponding to the signal introduced by
the modulator 126 of the transmitting location.
A principal advantage of the communication system of FIG. 13 is
that the transmitting location does not require significant power
consumption, i.e., no light source is required at the transmitting
location. The system further has excellent security features for
restricting the information from the transmitting location 121 to
the receiving station 120. The security of the transmitting
location also is improved as a result of the absence of any light
source.
A further refinement of the communication system of FIG. 13 is
illustrated in FIG. 14 wherein the transmitting location 121'
includes a liquid crystal cell unit 35" and a modulator 126'
connected by a cable 125' to the liquid crystal cell unit 35". The
incident light traverses the cell unit 35' from left to right in
FIG. 14 and impinges upon corner reflector 129, for example, a
tetrahedron prism which has the property of reflecting an incident
beam in a parallel direction. The incident beam 130 is reflected
from the corner reflector as a reflected beam 131 which is parallel
to the incident beam 130. This system, employing a corner
reflector, provides further security for the communications
system.
* * * * *