U.S. patent application number 09/809936 was filed with the patent office on 2002-09-19 for secure wave-division multiplexing telecommunications system and method.
Invention is credited to Snawerdt, Peter.
Application Number | 20020131106 09/809936 |
Document ID | / |
Family ID | 25202537 |
Filed Date | 2002-09-19 |
United States Patent
Application |
20020131106 |
Kind Code |
A1 |
Snawerdt, Peter |
September 19, 2002 |
Secure wave-division multiplexing telecommunications system and
method
Abstract
A fiber optic data transmission system includes an optical fiber
and a data transmitter having a first laser having a first
wavelength, a first phase modulator for phase modulating light from
the first laser as a function of a first data input stream so as to
create a first phase-modulated output data stream, a second laser
having a second wavelength different from the first wavelength, and
a second phase modulator for phase modulating light from the second
laser as a function of a second data input stream so as to create a
second phase-modulated output data stream. The transmitter also
includes a combiner combining the first and second output data
streams into a phase-modulated optical signal for transmission over
the optical fiber.
Inventors: |
Snawerdt, Peter; (Melbourne
Beach, FL) |
Correspondence
Address: |
DAVIDSON, DAVIDSON & KAPPEL, LLC
485 SEVENTH AVENUE, 14TH FLOOR
NEW YORK
NY
10018
US
|
Family ID: |
25202537 |
Appl. No.: |
09/809936 |
Filed: |
March 16, 2001 |
Current U.S.
Class: |
398/91 ; 398/188;
398/40; 398/82 |
Current CPC
Class: |
H04B 10/505 20130101;
H04B 10/548 20130101; H04J 14/02 20130101; H04B 10/506
20130101 |
Class at
Publication: |
359/124 ;
359/154 |
International
Class: |
H04J 014/02; H04B
010/00 |
Claims
What is claimed is:
1. A fiber optic data transmission system comprising: an optical
fiber; and a data transmitter having a first laser having a first
wavelength, a first phase modulator for phase modulating light from
the first laser as a function of a first data input stream so as to
create a first phase-modulated output data stream, a second laser
having a second wavelength different from the first wavelength, and
a second phase modulator for phase modulating light from the second
laser as a function of a second data input stream so as to create a
second phase-modulated output data stream, the transmitter further
including a combiner combining the first and second output data
streams into a phase-modulated optical signal for transmission over
the optical fiber.
2. The system as recited in claim 1 further comprising a controller
controlling the first phase modulator as a function of an output of
a delayed-feedback exclusive-or gate having the first input data
stream as an input.
3. The system as recited in claim 2 wherein the controller controls
the second phase modulator as a function of another output of
another delayed-feedback exclusive-or gate.
4. The system as recited in claim 1 further comprising a receiver
receiving the phase-modulated optical signal from the optical
fiber, the receiver including a splitter for splitting the optical
signal into a first path with the first wavelength and a second
path with the second wavelength.
5. The system as recited in claim 4 wherein the receiver includes a
first interferometer receiving the first path and a second
intereferometer receiving the second path.
6. The system as recited in claim 1 further comprising a detector
for detecting a tap or loss of energy in the optical fiber.
7. A transmitter comprising: a first laser having a first
wavelength; a first phase modulator for phase modulating light from
the first laser so as to form a phase-modulated first optical data
stream; a second laser having a second wavelength different from
the first wavelength, a second phase modulator for phase modulating
light from the second laser so as to form a phase-modulated second
optical data stream, a combiner for combining the phase-modulated
first and second optical data streams, and a controller controlling
the first and second phase modulators as function of a first input
electronic data stream and a second input electronic data
stream.
8. The transmitter as recited in claim 7 wherein the controller
includes a first delayed-feedback exclusive-or gate and a second
delayed-feedback exclusive-or gate.
9. The transmitter as recited in claim 7 further comprising a
plurality of further lasers having different wavelengths.
10. A receiver comprising: an optical splitter for splitting light
into a first wavelength and a second wavelength different from the
first wavelength, and a first interferometer receiving light at the
first wavelength and a second interferometer receiving light at the
second wavelength.
11. The receiver as recited in claim 10 wherein the optical
splitter includes a Bragg grating.
12. The receiver as recited in claim 10 further comprising a
detector for detecting a tap or loss of energy in the optical
fiber.
13. The receiver as recited in claim 10 further comprising a
plurality of further interferometers.
14. A method for transmitting secure data is also provided
comprising the steps of: transmitting light from a first laser at a
data transmitter; phase modulating light from the first laser at
the data transmitter as a function of a first electronic data input
stream so as to create a first output data stream, a first binary
bit being represented by a first phase and a second binary bit
being presentated by a second phase different from the first phase;
transmitting light from a second laser having a wavelength
different from the first laser; phase modulating light from the
second laser at the data transmitter as a function of a second
electronic data input stream so as to create a second output data
stream; and combining the first and second output data streams.
15. The method as recited in claim 14 wherein the phase modulated
data is a function of outputs of delayed-feedback exclusive-or
gates.
16. The method as recited in claim 14 wherein all of the light from
the first and second lasers is phase-modulated.
17. The method as recited in claim 14 further including receiving
the phase-modulated combined optical signal in a receiver, and
splitting the combined output signal into a first and second path
as a function of wavelength.
18. The method as recited in claim 17 further comprising passing
each of the first and second paths through an interferometer.
19. The method as recited in claim 14 further including monitoring
a fiber for intrusion.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to
telecommunications and more particularly to improving security and
data transmission over wave-division multiplexed fiber optic
networks.
[0003] 2. Background Information
[0004] In current fiber optic networks, an electronic data stream
is fed to a laser amplitude modulator. The laser amplitude
modulator typically pulses or alters the laser output to create an
amplitude-modulated optical signal representative of the electronic
data stream. The laser amplitude modulator and laser thus define a
transmitter for transmitting the optical signal over an optical
fiber, which is then received by a receiver. The receiver for the
amplitude-modulated optical signals of the optical data typically
includes a photodiode to convert the optical signals back into the
electronic data stream.
[0005] The reading of the amplitude-modulated optical data signals
using a photodiode is straightforward: the optical signals either
produce an electric output at the photodiode or they do not. As a
result, an output electronic data stream of zeros and ones is
generated.
[0006] However, optical fiber may be tapped. The optical fibers can
be spliced or even merely clamped so as to obtain optical signals
from the fiber. It also may be possible to tap fibers without
physically touching the optical fiber, for example by reading
energy emanating or dissipating along the fiber.
Amplitude-modulated optical signals, with their ease of detection
from a photodiode, require that only a small amount of energy be
tapped and passed through the photodiode in order to be converted
into a tapped electronic data stream.
[0007] To confront non-secure optical and non-optical data lines,
it has been known to use public key/private key encryption so that
the data stream being transmitted is encoded in a format that makes
it difficult to decode. Encryption however has several drawbacks,
including the need for extra processing steps and time. Moreover,
public key/private key encrypted data can be cracked, and the
devices and algorithms for doing so are constantly improving.
[0008] U.S. Pat. No. 5,455,698 purports to disclose a secure fiber
optic communications system based on the principles of a Sagnac
interferometer. A data transmitter is a phase modulator for
modulating counter-propagating light beams sent by a receiver round
a loop. The receiver includes a light source, a beamsplitter for
splitting light from the light source into counter-propagating
light beams and for receiving the phase-modulated light beams, and
an output detector. U.S. Pat. No. 5,223,967 describes a similar
Sagnac-interferometer-based system operating over a single optical
fiber.
[0009] The Sagnac-interferometer-based systems described in these
patents have the disadvantage that they require the light to travel
over a loop, whether back and forth in a single fiber or over a
long length looped fiber. As a result, either the link budget for
the single fiber must be doubled, reducing the data carrying
capacity for a single fiber, or else a looped fiber with
significant and expensive extra length of at least twice that of a
single fiber must be laid between the transmitter and the receiver.
Moreover, the receiver contains the light source, as opposed to the
current installed base where the transmitter has the light
source.
[0010] The Sagnac-interferometer-based systems thus are expensive
to build and operate, and do not work particularly well with
existing systems. Moreover, because a broadband light source is
desired for Sagnac-intereferometer based systems (see U.S. Pat. No.
5,455,698 at col 1, lines 66 et seq.), these systems do not work
well with wavelength division multiplexed (WDM) systems in which
data is transmitted over different wavelengths. The U.S. Pat. No.
5,455,698 patent describes splitting a wavelength division
multiplexed system with three different wavelengths. However, two
of the waevelengths are guard bands strictly used for alarm
dteection and not for information transmitting. See, e.g., the '698
patent at col. 13, lines 44-55.
[0011] U.S. Pat. No. 6,072,615 purports to describe a method for
generating a return-to-zero optical pulses using a phase modulator
and optical filter. The RZ-pulse optical signal transmitted over
the fiber is easily readable by a detector.
[0012] U.S. Pat. No. 5,606,446 purports to describe an optical
telecommunications system employing multiple phase-compensated
optical signals. Multiple interferometric systems are combined for
the purpose of multiplexing various payloads on the same optical
transmission path. The patent attempts to describe a method for
providing fiber usage diversity using optical coherence length
properties and a complex transmit/receive system. Each transmitter
has a splitter, a plurality of fibers and a plurality of phase
modulators to create the multiplexed signal, which is then
demultiplexed at the receiver. This system is complex and
expensive.
[0013] U.S. Pat. No. 5,726,784 purports to describe a WDM optical
communications system with remodulators and diverse optical
transmitters. An external modulator is used to impart an
amplitude-modulated output signal for each wavelength, as described
in colum 6, lines 14 to 36 of the '784 patent. Optoelectronic
detectors can easily read these amplitude-modulated signals. The
entirety of U.S. Pat. No. 5,726,784 is hereby
incorporated-by-reference herein.
SUMMARY OF THE INVENTION
[0014] An object of the present invention is to provide an improved
security WDM transmission system and device. Yet another alternate
or additional object of the present invention is to provide a
simple yet secure phase-modulated optical data transmission system
usable in a WDM system.
[0015] The present invention provides a fiber optic data
transmission system comprising a transmitter having a first laser
having a first wavelength, a first phase modulator for phase
modulating light from the first laser as a function of a first data
input stream so as to create a first phase-modulated output data
stream, a second laser having a second wavelength different from
the first wavelength, and a second phase modulator for phase
modulating light from the second laser as a function of a second
data input stream so as to create a second phase-modulated output
data stream. A combiner combines the first and second output data
streams into a phase-modulated optical signal, which is transmitted
over an optical fiber.
[0016] Preferably, a controller controls the first phase modulator
as a function of an output of a delayed-feedback exclusive-or gate
having the first input data stream as an input, as described in
co-owned and co-pending U.S. patent application Ser. No. ______,
entitled "Secure Fiber Optics Telecommunications System and Method"
and filed on Jan. 17, 2001, the entire disclosure of which is
hereby incorporated by reference herein. The controller also
preferable controls the second phase modulator as a function of an
output of another delayed-feedback exclusive-or gate.
[0017] The present system also includes a receiver receiving the
optical signal from the optical fiber. The receiver includes a
WDM/DWDM splitter for splitting the optical signal into a first
path with the first wavelength and a second path with the second
wavelength. A first delayed-arm interferometer receives the first
path and a second delayed-arm intereferometer receives the second
path. The interferometers may be similar to those described in
incorporated-by-reference U.S. patent application Ser. No. ______,
entitled "Secure Fiber Optics Telecommunications System and Method"
and filed on Jan. 17, 2001.
[0018] The first laser preferably is a continuous wave laser, for
example a semiconductor laser operating at 1550.92 nm, with the
second laser being for example a continuous wave semiconductor
laser operating at 1546.12 nm. However, other wavelengths are
possible.
[0019] The receiver may include detectors for converting the output
optical signals from the interferometers into electronic output
data streams. Filters may be provided to reduce any noise at the
output signal.
[0020] The system preferably includes a detector for detecting a
tap or loss of energy in the optical fiber. Most preferably, the
detector is an energy sensor, which may or may not include
programmable "trip" levels, which can monitor the amplitude of the
light in the fiber. If a tap occurs, it must couple off a
significant amount of energy to pass through an interferometer with
a low bit error rate, thus making detection of the tap by the
detector highly likely. The energy detector preferably is located
upstream from the WDM/DWDM splitter.
[0021] Depolarizers preferably are located between the lasers and
the respective phase modulators, and in one arm of the
interferometers of the receiver.
[0022] The present invention also provides a transmitter comprising
a first laser having a first wavelength, a first phase modulator
for phase modulating light from the first laser so as to form a
phase-modulated first optical data stream, a second laser having a
second wavelength different from the first wavelength, a second
phase modulator for phase modulating light from the second laser so
as to form a phase-modulated second optical data stream, a combiner
for combining the phase-modulated first and second optical data
streams, and a controller controlling the first and second phase
modulators as function of a first input electronic data stream and
a second input electronic data stream. The controller preferably
includes a first delayed-feedback exclusive-or gate and a second
delayed-feedback exclusive-or gate.
[0023] In addition, the present invention also provides a receiver
comprising an optical splitter for splitting light into a first
wavelength and a second wavelength different from the first
wavelength, and a first interferometer receiving light at the first
wavelength and a second interferometer receiving light at the
second wavelength.
[0024] The optical WDM/DWDM splitter preferably includes a Bragg
grating.
[0025] A method for transmitting secure data is also provided
comprising the steps of: transmitting light from a first laser at a
data transmitter; phase modulating light from the first laser at
the data transmitter as a function of a first electronic data input
stream; transmitting light from a second laser having a wavelength
different from the first laser; phase modulating light from the
second laser at the data transmitter as a function of a second
electronic data input stream; and combining light from the first
and second lasers so as to create a combined output signal with
phase-modulated data.
[0026] Preferably, the phase modulated data is a function of
outputs of delayed-feedback exclusive-or gates. Preferably, all of
the light from the first and second lasers is phase-modulated.
[0027] The method further may include receiving the phase-modulated
combined optical signal in a receiver, and splitting the combined
output signal into a first and second path as a function of
wavelength. The first and second paths are then each passed through
an interferometer.
[0028] The method preferably includes monitoring a fiber for
intrusion. The monitoring preferably includes monitoring an energy
level in the fiber with programmable trip levels.
[0029] While the invention has been described with two different
wavelength lasers, more lasers are of course possible.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] A preferred embodiment of the present invention is described
below by reference to the following drawings, in which:
[0031] FIG. 1 shows a transmitter of the present invention;
[0032] FIG. 2 shows a receiver of the present invention; and
[0033] FIG. 3 shows details of the control system of FIG. 1.
DETAILED DESCRIPTION
[0034] FIG. 1 shows a preferred embodiment of a secure
telecommunications system 1 with wave division multiplexing
according to the present invention. The system 1 includes a
transmitter 2 with continuous wave coherent lasers 11, 12, 13 and
14, each for example a semiconductor laser emitting a narrow band
of light at 1557 nm, 1554 nm, 1551 nm and 1548 nm, respectively.
Other wavelengths however are possible. Light emitted from lasers
11, 12, 13, 14 is depolarized and then passes through phase
modulators 21, 22, 23, 24, respectively, each for example a
Mach-Zender phase modulator. An electronic controller 18 controls
phase modulators 21, 22, 23, 24 as a function of four input
electronic data streams 31, 32, 33, 34, respectively. Controller 18
is also programmable to control the optical power output of light
emitted by lasers 11, 12, 13, 14. Preferably, the power output of
the lasers is set as low as possible for a given optical span,
while maintaining a low bit error rate. This reduces the light
available for any tap.
[0035] Depending on the binary output 51 of controller 18, phase
modulator 21 either imparts a first phase shift to the light (for
example, zero) or a second phase shift different from the first
phase shift (for example, 180 degrees) on the light passing through
phase modulator 21, thus creating a phase-modulated optical signal
41, which represents a stream of binary bits. The first phase shift
for example represents a binary zero and the second phase shift a
binary one. Likewise, phase modulators 22, 23, 24, as a function of
outputs 52, 53, 54 of controller 18 respectively, impart a first
phase shift or second phase shift on light from lasers 12, 13, 14,
respectively. Thus independent data streams 41, 42, 43, 44 are
created.
[0036] FIG. 3 shows a respresentation of the control circuit of
controller 18. Each input electronic data stream 31, 32, 33, 34 is
fed through a delayed-feedback exclusive-or gate 131, 132, 133,
134, respectively. Each feedback delay of each gate 131, 132, 133,
134 may equal, for example, a number of bits. The feedback delays
may be the same or, preferably, differ among the gates 131, 132,
133, 134. The binary output streams 51, 52, 53, 54 are used to
phase modulate the light from lasers 11, 12, 13, 14, respectively,
to create the phase-modulated optical data streams 41, 42, 43,
44.
[0037] Optical data streams 41, 42, 43, 44 are combined in a
combiner 60 which combines the light at the different wavelengths
and sends it over a fiber 70.
[0038] Light from fiber 70 is received in a receiver 3 according to
the present invention. A splitter 80 splits off a portion of the
light, directing part of the optical energy to the light monitoring
detector 82 and passing the remaining light to a wave division
splitter 90 preferably having a Bragg grating 91.
[0039] A detector 82, for example a light energy detector, monitors
the light energy in the fiber 70 via the light energy coupled to
the detector by splitter 80, the light energy being a function of
the amplitude. If the amplitude drops, most likely from a tap, the
detector alerts the receiver and can, for example, sound an alarm
or alert network maintenance personnel. Additionally, since the
receiver 3 is generally part of a component box, which also
includes a transmitter, the component box transmitter can send a
signal back to the component box containing transmitter 2 so as to
instruct transmitter 2 to stop sending data, or to send data over a
standby fiber. Detector 82, while preferably part of receiver 3,
also could be located separately from receiver 3, for example where
fiber 70 enters a building or other secure environment.
[0040] WDM splitter 90 splits the light into the four wavelengths
originally sent by lasers 11, 12, 13 and 14 to paths 71, 72, 73,
74. Each path 71, 72, 73, 74 enters a delayed-arm interferometer
92, 93, 94, 95. The delay loop of each interferometer 92, 93, 94,
95 corresponds to the electronic feedback delay in each of the
circuits 131, 132, 133, 133, respectively.
[0041] The phase-modulated data as it passes through the respective
interferometer 92, 93, 94, 95 either constructively interferes or
destructively interferes so as to create signals read by detectors
101, 102, 103, 104, the signals being representative of input data
31, 32, 33, 34, respectively. This process is decribed in more
detail in incorporated-by-reference U.S. patent application No.
______, entitled "Secure Fiber Optics Telecommunications System and
Method" and filed on Jan. 17, 2001.
[0042] Filters 111, 112, 113 and 114 are provided to compensate for
any slight mismatch between the optical delay in the
interfermometer and the electronic delay, and for other noise.
[0043] System 1 provides a secure method for transmitting multiple
streams of data over a single optical fiber, which is difficult to
decode if tapped, and also permits excellent detection of the
existence of a tap.
* * * * *