Title: Spread polarization transmitter and associated system and method of operation
Abstract: A spread polarization transmitter for transmitting at least one light signal comprises a spread-spectrum communication apparatus and a polarization modulator. The spread-spectrum communication apparatus modulates the at least one light signal according to a spread-spectrum modulation technique. The polarization modulator comprises a polarizer and a magnetic bubble waveguide. The polarizer is capable of polarizing the at least one spread-spectrum modulated light signal in a polarized direction. And the magnetic bubble waveguide, which is configured in accordance with a pseudo-random polarization code sequence such that the plurality of magnetic bubble domains assume a time varying position representative of the pseudo-random polarization code sequence, is capable of receiving at least one polarized, spread-spectrum modulated light signal. Thereafter, the magnetic bubble waveguide is capable of at least partially rotating the polarized, spread-spectrum modulated light signals by a predetermined angle from the polarized direction during transmission therethrough to create spread polarization modulated light signals.
Patent Number: 6,871,023 Issued on 03/22/2005 to Atmur, et al.
| Inventors:
|
Atmur; Robert J. (Whittier, CA);
Hunt; Jeffrey H. (Chatsworth, CA)
|
| Assignee:
|
The Boeing Company (Seattle, WA)
|
| Appl. No.:
|
005997 |
| Filed:
|
December 3, 2001 |
| Current U.S. Class: |
398/152; 398/184; 398/185; 398/200; 398/142; 398/43; 359/281; 359/246; 385/1; 365/22 |
| Intern'l Class: |
H04B 010//00; H04B 010//04; H04J 014//00; G02F 001//01; G11C 019//08 |
| Field of Search: |
398/152,184,185,200,142
359/237,238,246,276,278,279,281
|
References Cited [Referenced By]
U.S. Patent Documents
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| |
| 3711840 | Jan., 1973 | Copeland, III | 365/22.
|
| 3756690 | Sep., 1973 | Borrelli et al. | 385/1.
|
| 3764195 | Oct., 1973 | Blank et al.
| |
| 4040039 | Aug., 1977 | Hanson et al.
| |
| 4056812 | Nov., 1977 | Bobeck et al.
| |
| 4095279 | Jun., 1978 | Lins.
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| 4142247 | Feb., 1979 | Bobeck.
| |
| 4142249 | Feb., 1979 | Bobeck.
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| 4143419 | Mar., 1979 | Bobeck.
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| 4143420 | Mar., 1979 | Bobeck.
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| 4162537 | Jul., 1979 | Bobeck.
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| 4355373 | Oct., 1982 | Bobeck.
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| 4541072 | Sep., 1985 | Kikuchi.
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| 4669089 | May., 1987 | Gahagan et al.
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| 4671621 | Jun., 1987 | Dillon, Jr. et al.
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| 4728178 | Mar., 1988 | Gualtieri et al.
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| 4735489 | Apr., 1988 | Tolksdorf et al.
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| 4866698 | Sep., 1989 | Huggins et al.
| |
| 4985899 | Jan., 1991 | Walsh.
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| 5087984 | Feb., 1992 | Heiney et al.
| |
| 5245471 | Sep., 1993 | Iwatsuka et al.
| |
| 5287300 | Feb., 1994 | Stadler et al.
| |
| 5341396 | Aug., 1994 | Higgins et al.
| |
| 5657151 | Aug., 1997 | Swan et al.
| |
| 5841557 | Nov., 1998 | Otsuka et al.
| |
| 5867290 | Feb., 1999 | Dutt et al. | 398/43.
|
| 5973832 | Oct., 1999 | Bettman.
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| 6002512 | Dec., 1999 | Bergmann et al.
| |
| 6014475 | Jan., 2000 | Frisken.
| |
Other References
Garnet Films, GARNETS;
<http://www.iem.it/.about.magni/work/doc/garnet-phd/garnets.html>;
visited Aug. 8, 2000; pp. 1-48.
New Material for Magnetic Bubble Memory; STA TODAY; Aug. 1999; p. 1
<http://www.sta.go.jp/sonata/sonata/e9908_10.html>; visited Aug. 8,
2000.
|
Primary Examiner: Sedighian; M. R.
Assistant Examiner: Lee; David J.
Attorney, Agent or Firm: Alston & Bird LLP
Claims
What is claimed is:
1. A method of transmitting at least one light signal utilizing a magnetic
bubble waveguide comprising a plurality of magnetic bubble domains, said
method comprising:
polarizing at least one spread-spectrum modulated light signal in a
polarized direction;
configuring the magnetic bubble waveguide in accordance with a time-varying
pseudo-random code sequence such that the plurality of magnetic bubble
domains are in a time varying position representative of the pseudo-random
code sequence; and
transmitting the at least one polarized spread-spectrum modulated light
signal through the magnetic bubble waveguide such that a position of at
least one magnetic bubble domain at least partially rotates the at least
one spread-spectrum modulated light signal by a predetermined angle from
the polarized direction based upon the time-varying position of the
magnetic bubble domains.
2. A method according to claim 1 further comprising generating the
time-varying pseudo-random code sequence before configuring the magnetic
bubble waveguide.
3. A method according to claim 1 further comprising modulating at least one
electrical signal representative of the at least one light signal
according to a spread-spectrum modulation technique and thereafter
converting the at least one electrical signal to the at least one light
signal, wherein modulating the at least one electrical signal occurs
before polarizing the at least one spread-spectrum modulated light signal.
4. A method according to claim 3, wherein converting the at least one light
signal comprises passing the at least one electrical signal through a
light emitting transmitter to thereby generate the at least one light
signal.
5. A method according to claim 1, wherein the magnetic bubble waveguide is
a plurality of magnetic bubble waveguides arranged in a two-dimensional
array, said method further comprising:
passing the at least one spread-spectrum modulated light signal through a
light spreading element such that the at least one spread-spectrum
modulated light signal spreads into a plurality of spread-spectrum
modulated light signals, wherein transmitting comprises transmitting the
plurality of polarized spread-spectrum modulated light signals through the
array of magnetic bubble waveguides such that a position of at least one
magnetic bubble domain in each magnetic bubble waveguide at least
partially rotates at least one of the plurality of spread-spectrum
modulated light signals by a predetermined angle from the polarized
direction based upon the time-varying position of the magnetic bubble
domains, wherein transmitting the plurality of polarized spread-spectrum
modulated light signals through the array of magnetic bubble waveguides
generates a plurality of spread polarization modulated light signals; and
passing the plurality of spread polarization modulated light signals
through a light focusing element after transmitting the plurality of
polarized spread-spectrum modulated light signals such that the plurality
of spread polarization modulated light signals are focused into at least
one spread polarization modulated light signal.
6. A spread polarization transmitter for transmitting at least one light
signal comprising:
a spread-spectrum communication apparatus for spread-spectrum modulating
the at least one light signal; and
a polarization modulator comprising:
a polarizer capable of polarizing the at least one spread-spectrum
modulated light signal in a polarized direction; and
a magnetic bubble waveguide configured in accordance with a pseudo-random
polarization code sequence such that the plurality of magnetic bubble
domains assume a time varying position representative of the pseudo-random
polarization code sequence, wherein the magnetic bubble waveguide is
capable of receiving at least one polarized, spread-spectrum modulated
light signal and at least partially rotating the at least one polarized,
spread-spectrum modulated light signal by a predetermined angle from the
polarized direction during transmission therethrough based upon the
time-varying position of the magnetic bubble domains to thereby create at
least one spread polarization modulated light signal.
7. A spread polarization transmitter according to claim 6 further
comprising a transmission element capable of transmitting the at least one
spread polarization modulated light signal.
8. A spread polarization transmitter according to claim 6, wherein said
polarization modulator further comprises a pseudo-random polarization code
generator capable of generating the time-varying pseudo-random code
sequence.
9. A spread polarization transmitter according to claim 6, wherein said
spread-spectrum communication apparatus is capable of modulating at least
one electrical signal representative of the at least one light signal
according to a spread-spectrum modulation technique and thereafter
converting the at least one electrical signal to the at least one light
signal.
10. A spread polarization transmitter according to claim 9, wherein said
spread-spectrum communication apparatus includes a light emitting
transmitter capable of generating the at least one light signal as the at
least one electrical signal passes therethrough.
11. A spread polarization transmitter according to claim 6, wherein the
magnetic bubble waveguide comprises a plurality of magnetic bubble
waveguides arranged in a two-dimensional array and configured in
accordance with at least one pseudo-random polarization code sequence such
that the plurality of magnetic bubble domains of each magnetic bubble
waveguide assumes a time varying position representative of a respective
pseudo-random polarization code sequence, wherein said polarization
modulator further comprises:
at least one light spreading element capable of spreading the at least one
polarized, spread-spectrum modulated light signal into a plurality of
polarized, spread-spectrum modulated light signals for input into the
array of magnetic bubble waveguides, wherein each magnetic bubble
waveguide is capable of receiving at least one polarized, spread-spectrum
modulated light signal of the plurality of polarized, spread-spectrum
modulated light signals and at least partially rotating the at least one
polarized, spread-spectrum modulated light signal by a predetermined angle
from the polarized direction during transmission therethrough based upon
the time-varying position of the magnetic bubble domains, and wherein
transmitting the plurality of polarized spread-spectrum modulated light
signals through the array of magnetic bubble waveguides generates a
plurality of spread polarization modulated light signals; and
at least one light focusing element capable of focusing the plurality of
spread polarization modulated light signals into at least one spread
polarization modulated light signal.
12. A spread polarization communication system comprising:
a transmitter device comprising:
a spread-spectrum modulation apparatus for spread-spectrum modulating at
least one light signal;
a polarization modulator comprising a plurality of magnetic bubble domains,
and configured in accordance with a time-varying pseudo-random
polarization code sequence such that the plurality of magnetic bubble
domains assume a time-varying position representative of the pseudo-random
polarization code sequence, wherein said polarization modulator is capable
of polarizing the at least one spread-spectrum modulated light signal in a
polarized direction and thereafter polarization modulating the at least
one polarized spread-spectrum modulated light signal during transmission
therethrough, wherein transmitting the at least one spread-spectrum
modulated light signal through said polarization modulator creates at
least one spread polarization modulated light signal; and
a transmission element capable of transmitting the at least one spread
polarization modulated light signal; and
a receiver capable of receiving the at least one spread polarization
modulated light signal and thereafter demodulating the at least one spread
polarization modulated light signal in accordance with modulation provided
by the polarization modulator and thereafter with modulation provided by
the spread-spectrum modulation apparatus to thereby obtain a
representation of the at least one light signal.
13. A spread polarization communication system according to claim 12,
wherein the at least one light signal includes an original polarization,
wherein said receiver is further capable of polarization filtering the at
least one spread polarization modulated light signal after demodulating
the at least one spread polarization modulated light signal in accordance
with the polarization modulator to thereby obtain a representation of the
original polarization of the at least one light signal.
14. A spread polarization communication system according to claim 12,
wherein the at least one light signal includes an original polarization,
and wherein said receiver comprises:
a reception element capable of receiving the at least one spread
polarization modulated light signal;
a polarization demodulator comprising a plurality of magnetic bubble
domains, and configured in accordance with a time-varying position
representative of an inverse of the time-varying pseudo-random
polarization code sequence such that the plurality of magnetic bubble
domains assume a time-varying position representative of the inverse of
the time-varying pseudo-random polarization code sequence, wherein said
polarization demodulator is capable of polarization demodulating the at
least one spread polarization modulated light signal during transmission
therethrough, wherein transmitting the at least one spread polarization
modulated light signal through said polarization demodulator recreates the
at least one spread-spectrum modulated light signal;
a polarization filter capable of polarization filtering the at least one
spread-spectrum modulated light signal as the at least one spread-spectrum
modulated light signal passes therethrough to thereby obtain a
representation of the original polarization of the at least one light
signal; and
a spread-spectrum demodulation apparatus capable of spread-spectrum
demodulating the at least one spread-spectrum modulated light signal to
thereby recreate the at least one light signal.
15. A spread polarization communication system according to claim 14,
wherein said spread-spectrum demodulation apparatus is capable of
converting the at least one spread-spectrum modulated light signal into at
least one spread-spectrum modulated electrical signal representative of
the at least one spread-spectrum modulated light signal and thereafter
demodulating the at least one spread-spectrum modulated electrical signal
to thereby recreate at least one electrical signal representative of the
at least one light signal.
16. A spread polarization communication system according to claim 15,
wherein said spread-spectrum demodulation apparatus includes a light
detecting receiver capable of converting the at least one spread-spectrum
modulated light signal.
17. A spread polarization communication system according to claim 12,
wherein said polarization modulator further comprises a pseudo-random
polarization code generator capable of generating the time-varying
pseudo-random code sequence.
18. A spread polarization communication system according to claim 12,
wherein said spread-spectrum modulation apparatus is capable of
spread-spectrum modulating at least one electrical signal representative
of the at least one light signal and thereafter converting the at least
one electrical signal to the at least one light signal.
19. A spread polarization communication system according to claim 18,
wherein said spread-spectrum modulation apparatus includes a light
emitting transmitter capable of generating the at least one light signal
as the at least one electrical signal passes therethrough.
20. A spread polarization communication system according to claim 12,
wherein said polarization modulator comprises a plurality of polarization
modulators disposed in a two-dimensional array, wherein said transmitter
device further comprises:
at least one light spreading element capable of spreading the at least one
polarized, spread-spectrum modulated light signal into a plurality of
polarized, spread-spectrum modulated light signals for input into the
array of polarization modulators, wherein transmitting the at least one
spread-spectrum modulated light signal through said plurality of
polarization modulators creates a plurality of spread polarization
modulated light signals; and at least one light focusing element capable
of focusing the plurality of spread polarization modulated light signals
into at least one spread polarization modulated light signal.
Description
FIELD OF THE INVENTION
The present invention relates generally to systems and methods for optical
communication and, more particularly, to systems and methods for optical
communication using spread-spectrum technology and the polarization of
optical signals.
BACKGROUND OF THE INVENTION
In optical communication systems, such as those operating according to
spread-spectrum techniques, a transmitted signal is spread over a
frequency band that is much wider than the bandwidth of the information
being transmitted. Two techniques commonly used in spread-spectrum systems
are frequency hopping and direct sequence (DS) modulation. Frequency
hopping involves shifting the carrier frequency in discrete increments, in
a pattern dictated by a pseudo-random code. In direct sequence modulation,
each bit of an information-bearing signal is modulated by a higher
frequency, pseudo-random code signal. The modulation may simply comprise
reproducing the input code signal when the information bit is one, and
inverting the code signal when the information bit is zero. Each bit of
the code signal, or each bit of the product signal obtained by modulating
the information-bearing signal with the code signal is referred to as a
"chip."
In a system using direct sequence modulation, the chip rate, i.e., the
frequency of the pseudo-random code signal, is typically much higher than
the bit rate of the information-bearing signal. The bandwidth occupied by
the transmitted signal is directly determined by the chip rate. A receiver
in a direct sequence modulated communication system includes means for
producing the same pseudo-random code signal as that used by the
transmitter, in the same time epoch. The code signal is employed to decode
the transmitted data and extract the information-bearing signal, even in
the presence of noise or jamming.
Applications of spread-spectrum systems are various and generally depend
upon characteristics of the codes being employed for band spreading and
other factors. In direct sequence systems, for example, where the code is
a pseudo-random sequence, the transmitted signal acquires the
characteristics of noise, making the transmission indiscernible to any
eavesdropper who is incapable of decoding the transmission. In this
regard, system sensitivity to interference is fundamentally altered. The
use of noise-like modulation carrier signal, occupying the same frequency
spectrum as all other users, creates effective noise that equals the sum
of all the other user signals. Thus, the effective signal-to-noise (S/N)
ratio at the receiver is increased because the noise is no longer a
worst-case interference signal (as previously required), but instead the
average interference of the summed signals from the other users.
In addition to the benefits of making the transmission indiscernible to
eavesdroppers, and decreasing the sensitivity to system receivers,
spread-spectrum techniques can also increase the data channel density
available in a frequency channel. By spreading each bit of an
information-bearing signal over a bandwidth of frequencies determined by
the pseudo-random code signal, the amount of data that can be transmitted
over a given frequency channel is increased over traditional narrow-band
systems.
Whereas spread-spectrum communications provide a large number of benefits
over traditional communication techniques, conventional spread-spectrum
communications systems also have their limitations. In this regard, just
as the effective noise in a channel is the sum of signals on the channel,
the energy density of each channel has an upper maximum where the
waveguide for the channel becomes saturated. Additionally, in optical
transmission, a phenomenon known as polarization dispersion occurs when
optical signals travel over long distances. Polarization dispersion is an
effect caused in light that travels in multiple polarization modes. When a
waveguide, such as an optical fiber, is asymmetric in all directions, the
light traveling along one polarization can end up traveling at a speed
different than light traveling in another direction. If the light spreads
enough, the signal can overlap with other light signals and, thus, corrupt
the both signals.
SUMMARY OF THE INVENTION
In light of the foregoing background, the present invention provides a
spread polarization transmitter and an associated system and method of
operation that increases data channel density over conventional
spread-spectrum systems, while reducing the problems associated with
polarization dispersion. In this regard, the spread polarization
transmitter and associated system and method of operation modulate light
signals according to a polarization technique as well as a spread-spectrum
technique. By further modulating a spread-spectrum modulated signal, the
data channel density is increased by the amount of data that can be added
to each frequency at different polarization directions. Additionally, by
modulating the polarization of signals, polarization dispersion is
eliminated because the signal only has one polarization direction.
According to one embodiment, a system for spread polarization communication
includes a transmitter and a receiver. The transmitter comprises a
spread-spectrum modulation apparatus for spread-spectrum modulating at
least one light signal. In a further embodiment, the spread-spectrum
modulation apparatus is capable of modulating at least one electrical
signal representative of the at least one light signal according to a
spread-spectrum method and thereafter converting the electrical signals to
the light signals, such as via a light emitting transmitter.
The transmitter also includes a polarization modulator that includes a
plurality of magnetic bubble domains, and is configured in accordance with
a time-varying pseudo-random polarization code sequence such that the
magnetic bubble domains assume a time-varying position representative of
the pseudo-random polarization code sequence. In a further embodiment, the
polarization modulator further comprises a pseudo-random polarization code
generator capable of generating the time-varying pseudo-random code
sequence. The polarization modulator is capable of polarizing the
spread-spectrum modulated light signals in a polarized direction and
thereafter polarization modulating the polarized spread-spectrum modulated
light signals during transmission therethrough. Thus, by transmitting the
spread-spectrum modulated light signals through the polarization
modulator, the system creates at least one spread polarization modulated
light signal. Additionally, the transmitter includes a transmission
element, such as a waveguide, capable of transmitting the spread
polarization modulated light signals.
In another embodiment, the polarization modulator comprises a plurality of
polarization modulators disposed in a two-dimensional array. In this
embodiment, the transmitter further comprises at least one light spreading
element and at least one light focusing element, such as at least one
gradient index (GRIN) lens. The light spreading element is capable of
spreading the at least one polarized, spread-spectrum modulated light
signal into a plurality of polarized, spread-spectrum modulated light
signals for input into the array of polarization modulators. Transmitting
the at least one spread-spectrum modulated light signal through the array
of polarization modulators creates a plurality of spread polarization
modulated light signals. Following transmission of the spread-spectrum
modulated light signal through the array of polarization modulators, the
light focusing element is capable of focusing the plurality of spread
polarization modulated light signals into at least one spread polarization
modulated light signal.
To receive the spread polarization modulated light signals, the system
includes a receiver. Upon reception of the spread polarization light
signals, the receiver is thereafter capable of demodulating the spread
polarization modulated light signals in accordance with modulation
provided by the polarization modulator and thereafter with modulation
provided by the spread-spectrum modulation apparatus to thereby obtain a
representation of the at least one light signal. In a further embodiment,
the light signals include an original polarization. In this embodiment,
the receiver is further capable of polarization filtering the spread
polarization modulated light signals after demodulating the spread
polarization modulated light signals in accordance with the polarization
modulator to thereby obtain a representation of the original polarization
of the light signals.
In another embodiment where the light signals include an original
polarization, the receiver includes a reception element and a polarization
demodulator, a polarization filter and a spread-spectrum demodulation
apparatus. In this embodiment, the reception element is capable of
receiving the spread polarization modulated light signals. The
polarization demodulator, which comprises a plurality of magnetic bubble
domains, is configured in accordance with a time-varying position
representative of an inverse of the time-varying pseudo-random
polarization code sequence such that the plurality of magnetic bubble
domains assume a time-varying position representative of the inverse of
the time-varying pseudo-random polarization code sequence. The
polarization demodulator is capable of polarization demodulating the
spread polarization modulated light signals during transmission
therethrough, where transmitting the spread polarization modulated light
signals through the polarization demodulator recreates the spread-spectrum
modulated light signals.
The polarization filter is capable of polarization filtering the
spread-spectrum modulated light signals as the spread-spectrum modulated
light signals pass therethrough to thereby obtain a representation of the
original polarization of the light signals. And the spread-spectrum
demodulation apparatus is capable of spread-spectrum demodulating the
spread-spectrum modulated light signals to thereby recreate the light
signals. In one embodiment, the spread-spectrum demodulation apparatus is
capable of converting the spread-spectrum modulated light signals into
spread-spectrum modulated electrical signals representative of the
spread-spectrum modulated light signals, such as via a light detecting
receiver. Upon converting the spread-spectrum modulated light signals into
spread-spectrum modulated electrical signals, the spread-spectrum
demodulation apparatus is capable of demodulating the spread-spectrum
modulated electrical signals to thereby recreate at least one electrical
signal representative of the light signals.
In operation, in a method of transmitting the light signals,
spread-spectrum modulated light signals are polarized in a polarized
direction. In a further embodiment, before polarizing the spread-spectrum
modulated light signals, electrical signals representative of the light
signals are modulated according to a spread-spectrum modulation technique.
The spread-spectrum modulated electrical signals are then converted to the
light signals.
After polarizing the spread-spectrum modulated light signals, the magnetic
bubble waveguide is configured in accordance with the time-varying
pseudo-random code sequence such that the magnetic bubble domains are in a
time varying position representative of the pseudo-random code sequence.
In one embodiment, the time-varying pseudo-random code sequence is
generated before configuring the magnetic bubble waveguide. Next, the
polarized spread-spectrum modulated light signals are transmitted through
the magnetic bubble waveguide such that the position of at least one
magnetic bubble domain at least partially rotates the spread-spectrum
modulated light signals by a predetermined angle from the polarized
direction based upon the time-varying position of the magnetic bubble
domains.
Therefore, by modulating the polarization of the light signals as well as
spread-spectrum modulating the light signals, the spread polarization
transmitter and an associated system and method of operation of the
present invention increase data channel density over conventional
spread-spectrum systems. Also, by further modulating the polarization of a
spread-spectrum modulated signal, the data channel density is increased by
the amount of data that can be added to each frequency at different
polarization directions. Further, modulating the polarization of signals
eliminates polarization dispersion because the signal only has one
polarization direction.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus described the invention in general terms, reference will now be
made to the accompanying drawings, which are not necessarily drawn to
scale, and wherein:
FIG. 1 is a block diagram of a spread polarization transmitter according to
one embodiment of the present invention;
FIG. 2 is a block diagram of a receiver for a spread polarization
communication system such as would accompany the spread polarization
transmitter illustrated in FIG. 1, according to one embodiment of the
present invention;
FIG. 3 is a block diagram of a polarization modulator according to one
embodiment of the present invention;
FIG. 4 is a block diagram of one embodiment of a polarization demodulator;
FIG. 5 is a schematic perspective view of a portion of a magnetic bubble
device waveguide according to one embodiment of the present invention;
FIGS. 6A and 6B are schematic top views of a portion of a magnetic bubble
device waveguide of a polarization modulator and a polarization
demodulator, respectively, according to one embodiment of the present
invention;
FIG. 7 is a schematic side view of a portion of a two-dimensional array of
magnetic bubble device waveguides;
FIG. 8 is a schematic perspective view of two of the magnetic bubble device
waveguides of the array illustrated in FIG. 7 taken along line 8; and
FIG. 9 is a schematic top view of the portion of the two-dimensional array
of magnetic bubble device waveguides including a light spreading element
and a light focusing element, according to one embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention now will be described more fully hereinafter with
reference to the accompanying drawings, in which preferred embodiments of
the invention are shown. This invention may, however, be embodied in many
different forms and should not be construed as limited to the embodiments
set forth herein; rather, these embodiments are provided so that this
disclosure will be thorough and complete, and will fully convey the scope
of the invention to those skilled in the art. Like numbers refer to like
elements throughout.
FIGS. 1 and 2 illustrate one arrangement for a spread polarization
communication system, including a spread polarization transmitter 10 and a
receiver 12. FIG. 1 illustrates the spread polarization transmitter 10,
which includes a spread-spectrum modulation apparatus 14, polarization
modulator 16 and a waveguide 18. The spread-spectrum modulation apparatus
includes a digital data source 19, which produces the electrical digital
information-bearing signal I(t) to be transmitted. The spread-spectrum
modulation apparatus can employ any of a number of different
spread-spectrum modulation techniques, such as direct sequencing or
frequency hopping, without departing from the spirit and scope of the
present invention. For illustrative purposes, however, reference herein
will only refer to the direct sequence (DS) technique of spread-spectrum
modulation.
The spread-spectrum modulation apparatus further includes a DS modulator 20
that combines the electrical information-bearing signal with a code signal
C(t) produced by a pseudo-random number (PN) code generator 22. Code
signal C(t) is typically a pseudo-random binary sequence that has many of
the characteristics of random noise, but which is deterministic and
reproducible by intended receivers. Such signals may be produced by
relatively simple circuitry employing shift registers with feedback at
selected taps.
Each bit of code signal C(t) is referred to as a "chip". The number of
chips per second (the "chip rate") in the code signal C(t) is chosen to be
higher than the data rate (bits per second) of the information-bearing
signal I(t). Because the chip rate is higher than the data rate, the
output of DS modulator 20, comprising the DS modulated electrical signal
I(t)C(t), has a broader frequency spectrum than information-bearing signal
I(t). The number of chips used to modulate each bit of signal I(t)
represents a fundamental design parameter of the system, since it
represents the degree by which the bandwidth of the information-bearing
signal will be spread for transmission.
The DS modulated signal 24 is input to an intermediate frequency (IF)
modulator 26, and modulated onto an IF carrier produced by an IF
oscillator 28. The result is an IF electrical signal 30 that is input to a
transmitter block 32, such as a laser diode or light emitting diode. In
general, IF modulator 26 may employ virtually any type of signal
modulation, such as phase shift, frequency shift, and minimum shift keying
(MSK) modulation. In phase shift modulation, each chip of DS modulated
signal 24 controls the phase of the IF carrier, while in frequency shift
modulation, each chip controls the frequency of the IF carrier. In MSK
modulation, the DS modulated signal is separated into even and odd chip
sequences, and the even and odd chip sequences are then modulated onto a
pair of IF carriers that are in quadrature relationship with one another.
The transmitter block converts the IF electrical signal 30 into an IF light
signal 33, and transmits the signal to the polarization modulator 16. The
polarization modulator, in turn, modulates the polarization of the IF
light signal with a pseudo-random polarization code signal to thereby
obtain a spread polarization modulated light signal 35, as described
below. The spread polarization signal is then transmitted from the spread
polarization transmitter via the waveguide 18, which can comprise any of a
number of objects such as a fiber optic cable.
Attention is now drawn to FIG. 2, which illustrates a receiver 12 for
receiving the data transmitted by the spread polarization transmitter 10
of FIG. 1. The receiver includes a waveguide 34, a polarization
demodulator 36 and a spread-spectrum demodulation apparatus 38. The
spread-spectrum demodulation apparatus includes a receiver block 40, a
demodulator 42, a local IF oscillator 44, a correlator 46, and a PN code
generator 48. The spread polarization modulated light signal is received
by waveguide 34 and thereafter passed to the polarization demodulator. The
polarization demodulator essentially demodulates the polarization of the
spread polarization signal with a pseudo-random code that is the inverse
of the pseudo-random code applied to the IF light signal in the
transmitter to thereby retrieve a representation of the original IF light
signal 50, as described below.
From the polarization demodulator, the IF light signal 50 is transmitted to
the receiver block 40 of the spread-spectrum demodulation apparatus. The
receiver block, which can comprise any of a number of devices such as a
photodiode, down-converts the IF light signal to produce an IF electrical
signal 52. Demodulator 42 receives the IF electrical signal and, using the
local IF oscillator 44, essentially inverts the modulation produced by IF
modulator 26 in the transmitter to produce DS modulated electrical signal
54. DS modulated signal 54 is then input to correlator 46, which compares
the incoming waveform to the code produced by PN code generator 48. This
code is identical to the code produced by PN code generator 22 in the
transmitter. The output of correlator 46 is the information-bearing
electrical signal I(t).
PN code generators 22 and 48 produce pseudo-random sequences that are
characterized by a "length," the length being the number of bits produced
by the generator before the bit sequence begins repeating. In a typical
application, the code sequences are quite long, such that the bit sequence
repeat time is measured in hours or days. The receiver includes
synchronization circuitry (not shown) for synchronizing code generator 48
with the code produced by code generator 22. The synchronization circuitry
is omitted from FIG. 2 for simplicity, and because synchronization per se
does not form part of the present invention.
Reference is now made to FIGS. 3 and 4, which highlight the polarization
modulator and polarization demodulator, respectively. The polarization
modulator includes a polarizer 56, a magnetic bubble device waveguide 58
and a polarization pseudo-random (PN) code generator 60. The polarizer,
which can comprise any of a number of conventional devices, inputs the IF
light signal and alters the polarization of the IF light signal in a
polarized direction, such as in a linear direction. From the polarizer,
the fixed polarization IF light signal enters the magnetic bubble device
waveguide.
The magnetic bubble device waveguide 58, which is made of a highly
anisotropic magnetic material such as Bismuth Iron Garnet, includes a
plurality of magnetic bubble domains 59, as shown in FIG. 5. As is known
to those skilled in the art, materials such as Bismuth Iron Garnet have a
large uniaxial anisotropy, which results in a very simple magnetic domain
structure orienting the magnetization of the material normal to the
surface of the film. When materials such as Bismuth Iron Garnet are
arranged in a thin film and exposed to a constant magnetic field
perpendicular to the film, the film supports small regions of cylindrical
magnetic domains called "magnetic bubbles," which have a magnetization
opposite in direction from the rest of the material. By applying a very
brief, but high, external field pulse to the film, the magnetic bubbles
can be nudged along the film. Tracks can then be laid on the film to focus
the movement of the magnetic bubbles by depositing magnetic material on
the surface of the film.
If the tracks are laid in continuous loops, and motive force is applied to
the magnetic bubbles with a pair of wire coils (not shown) wrapped around
the film and energized with a two-phase voltage, a form of magnetic memory
can be formed. In this regard, the presence of a magnetic bubble in a
defined region of a loop can represent a binary "1," while the presence of
a magnetic bubble in another defined region (or absence of a magnetic
bubble altogether) can represent a binary "0," as shown in FIG. 6A. Data
can be read and written in a chain of moving magnetic bubbles as they
passed by a predefined point in the bubbles' path. In this regard, the
magnetic bubble device waveguide has characteristics similar to a
conventional magnetic bubble memory device, as such is known to those
skilled in the art. For an example of one type of a conventional magnetic
bubble memory device that could be implemented as a magnetic bubble device
waveguide, see U.S. Pat. No. 3,711,840 issued Jan. 16, 1973 to Copeland,
III and assigned to Bell Telephone Laboratories, Inc., the contents of
which are hereby incorporated by reference in their entirety.
To utilize the properties of the magnetic bubble device waveguide 16 that
allow for reading and writing data, the magnetic bubble device waveguide
includes the polarization PN code generator 60, which has similar
characteristics to the PN code generator 22. In this regard, the
polarization PN code generator generates a time-varying code signal
typically comprising a pseudo-random binary sequence that has many of the
characteristics of random noise, but which is deterministic and
reproducible by intended receivers. Such signals may be produced by
relatively simple circuitry employing shift registers with feedback at
selected taps. Similar to the chip rate of the code signal, C(t), the
time-varying code signal generated by the polarization PN code generator
is preferably chosen to be higher than the data rate (bits per second) of
the fixed polarization IF light signal.
As the fixed polarization IF light signal enters the magnetic bubble device
waveguide 16, a phenomenon known as the Faraday effect causes the
polarization of the fixed polarization IF light signal to rotate by a
predetermined, Faraday angle. The Faraday effect is the field-induced
difference in refraction of the left and right circularly polarized
components of the fixed polarization IF light signal that is incident
parallel to the magnetic field. By passing the fixed polarization IF light
signal through the magnetic bubble device waveguide, the Faraday effect
causes the rotation of the polarization plane of the fixed polarization IF
light signal by the predetermined Faraday angle, which is typically less
than 180.degree. with respect to the magnetic bubble device waveguide.
By manipulating the positions of the magnetic bubbles 59 within the
magnetic bubble device waveguide with a polarization PN code sequence
generated by the polarization PN code generator 60, the rotation of the
polarization of the IF light signal can be controlled such that the fixed
polarization IF light signal is polarization modulated to form a spread
polarization modulated light signal 62.
Referring now to FIG. 4, after the spread polarization light signal 62 has
been received by the receiver 12 (illustrated in FIG. 2), the spread
polarization light signal is passed from the waveguide 34 to the
polarization demodulator 36. The polarization demodulator includes a
magnetic bubble device waveguide 64, a polarization PN code generator 66
and a polarizer 68. As illustrated in FIGS. 3 and 4, the organization of
the polarization demodulator is essentially the reverse of the
organization of the polarization modulator. As such, the polarization
demodulator inputs the spread polarization light signal and demodulates
the spread polarization light signal to thereby retrieve a representation
of the original fixed polarization IF light signal.
Magnetic bubble device waveguide 64, like magnetic bubble device waveguide
58, includes a plurality of magnetic bubbles 70. Also, in the same manner
as magnetic bubble device waveguide 64, by manipulating the positions of
the magnetic bubbles 70 within magnetic bubble device waveguide 64 with
polarization PN code generator 66, the rotation of the polarization of the
spread polarization light signal 62 can be controlled such that the spread
polarization modulated light signal is polarization demodulated as the
spread polarization light signal passes therethrough to thereby obtain the
fixed polarization IF light signal, as shown in FIG. 6B. In this regard,
polarization PN code generator 66 generates a time-varying polarization
code sequence that is the inverse of the code signal generated by
polarization PN code generator 60, as illustrated by comparing FIGS. 6A
and 6B.
Polarization PN code generators 60 and 66 produce pseudo-random sequences
that are characterized by a "length", the length being the number of bits
produced by the generator before the bit sequence begins repeating. In a
typical application, the code sequences up to 32 or more bits long.
Additionally, polarization PN code generators 60 and 66 can clock the
polarization PN code sequence according to a number of different methods,
including a synchronous clock signal, or time-sliced clock signal to allow
higher modulation frequencies.
After the spread polarization light signal 62 has been polarization
demodulated into the fixed polarization IF light signal, the fixed
polarization IF light signal travels through polarizer 68, which is
configured opposite polarizer 56. In this regard, polarizer 68 converts
the fixed polarization IF light signal into a representation of the
original IF light signal, which is then transmitted into the receiver
block 40, as described above.
In another advantageous embodiment of the present invention, each magnetic
bubble device waveguide 58, 64 can comprise a two-dimensional array of
magnetic bubble device waveguides 71, as shown in FIGS. 7-9 with reference
to the magnetic bubble device waveguide of the transmitter 10. In this
embodiment, the two-dimensional array of magnetic bubble device waveguides
include a light spreading element 72, such as a gradient index (GRIN)
lens, disposed adjacent one end of the array of magnetic bubble device
waveguides, as shown in FIG. 9. In this regard, as the fixed polarization
IF light signal is transmitted to the array of magnetic bubble device
waveguides, in the case of the polarization modulator, the fixed
polarization IF light signal first passes through the light spreading
element. The light spreading element, in turn, spreads the fixed
polarization light signal into a plurality of fixed polarization IF light
signals 73 for input into the two-dimensional array of magnetic bubble
device waveguides. The individual fixed polarization IF light signals are
then passed through the array of magnetic bubble device waveguides.
Following polarization modulation, the individual spread polarization
modulated light signals are passed through a light focusing element 74,
such as a GRIN lens oriented reverse the light spreading element 72. The
light focusing element focuses the individual spread polarization
modulated light signals into an aggregated spread polarization modulated
light signal. The spread polarization modulated light signal is then
passed to the waveguide 18, in the case of the spread polarization
transmitter 10.
Therefore, by modulating the polarization of the light signals as well as
spread-spectrum modulating the light signals, the spread polarization
transmitter and an associated system and method of operation of the
present invention offer benefits over conventional spread-spectrum
communication systems. In this regard, the present invention increases
data channel density. Further, the present invention eliminates the
polarization dispersion associated with conventional spread-spectrum
communication systems because the signal only has one polarization
direction.
Many modifications and other embodiments of the invention will come to mind
to one skilled in the art to which this invention pertains having the
benefit of the teachings presented in the foregoing descriptions and the
associated drawings. Therefore, it is to be understood that the invention
is not to be limited to the specific embodiments disclosed and that
modifications and other embodiments are intended to be included within the
scope of the appended claims. Although specific terms are employed herein,
they are used in a generic and descriptive sense only and not for purposes
of limitation.
*
