Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02725804 2013-10-17
SYSTEM AND METHOD FOR OCDMA-BASED PHOTONIC LAYER SECURITY
ROBUSTNESS TO ARCHIVAL ATTACK
III. FIELD OF THE INVENTION
[0003] The present invention relates generally to optical networking, and
more
particularly to a system and method of providing security robustness against
archival attacks
in an optical CDMA-based photonic layer.
IV. BACKGROUND OF THE DISCLOSURE
[0004] The demand for bandwidth is scaling up to unprecedented levels.
With the 100
Gb Ethernet (100GbE) standard on the horizon, serial data communication rates
will
eventually outpace the single-channel capabilities of telecom transport
interfaces. In five
years, the transport of terabits of data generated from multitudes of data
gathering and
processing nodes, will need to be managed and delivered, on demand, to users
at
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secure campuses. The cost-effective use of existing public dark fiber, i.e.
unused
installed fiber, and the emerging transparent reconfigurable optical add-drop
multiplexer (ROADM)-based networks create a compelling case for photonic layer
security (PLS) for high bandwidth needs where advanced encryption systems
(ABS) is
not practical.
[0005] The success and widespread use of Code Division Multiple Access (CDMA)
in the
wireless domain has renewed interest in exploring the use of CDMA in the
optical
domain as well. However, optical CDMA (OCDMA) presents a very different set of
challenges. Initially, these challenges included spectral efficiency, which is
typically
low in OCDMA, inter-symbol interference, and susceptibility to network
impairments.
Significant progress has been made since the start of a Defense Advanced
Research
Projects Agency (DARPA) sponsored program addressing the shortcomings of
OCDMA. To the extent that spectral efficiency comparable to Wavelength-
Division
Multiplexing (WDM) networks is achieved, OCDMA networking has attractive
features based on the underlying property that connectivity is managed through
the use
of codes like telephone numbers as compared with wavelength exchange in WDM
networks.
[0006] Three distinct approaches have been the subject of much recent
research, a
potentially asynchronous time chips frequency bins or fast frequency hopping
(FFH), a
potentially asynchronous coherent time chips phase bins, and a synchronous
coherent
phase chips frequency bins or spectral phase encoding (SPE).
[0007] The attraction of SPE-OCDMA-based network lies in the prospect for
providing
security for multi-Gb/s data streams at the photonies layer. At the same time,
concerns
remain regarding quantifying the level of robustness of SPE-CDMA against
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eavesdropping. PLS offered by SPE-OCDMA has been historically viewed as
security
by obscurity, and thus the security robustness of SPE-OCDMA has been
controversial
since the theoretical paper by V. J. T. Shake entitled "Confidentiality
Perfounance of
Spectral-Phase-Encoded Optical CDMA," (published in IEEE Journal of Lightwave
Technology, April 2005, pp. 1652-1663) and experimental analysis by Weiner et
al.
(i.e., "Experimental investigation of security issues in OCDMA: a code-
switching
scheme," Elec. Lett., Vol. 41, No. 14, July 7, 2005.; and "Experimental
investigation
of security issues in OCDMA," JLT, vol.24, pp. 4228-4234, 2006), which
considered
limitations of SPE-OCDMA security in a particular case.
[0008] The case considered in the Shake and Weiner et al. references is
limited to a
situation where a single channel is present, and as stated by Shake "allowing
an
eavesdropper to isolate individual user signals".
V. SUMMARY OF THE DISCLOSURE
[0009] In the present invention the vulnerability of SPE-OCDMA to
eavesdropping is
addressed by emphasizing the need for the presence of multiple channels for a
successful phase scrambling technique that is the basis of the PLS. A
publication,
authored by the present inventors, detailing PLS, outlines for the first time
the
theoretical analysis that supports the robustness of phase scrambling to
exhaustive
search attack, and experimentally demonstrates the resistance of the disclosed
PLS
method to such attacks. The resilience of the PLS method to exhaustive search
attack
has been enhanced further by assigning multiple (quaternary) phase states to
frequency
bins. The usefulness of phase scrambling (also called phase masking) has also
been
confilmed in a conventional approach to SPE-OCDMA where the size of the search
space can be increased due to the availability of many frequency bins.
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MOM] In the present invention, a novel SPE approach to OCDMA is disclosed that
is
compatible with existing transparent reconfigarable optical networks, has high
spectral
efficiency, and is minimally affected by transmission impairments. The
underlying
technologies are based on the generation of stable combs of mode-locked laser
(MLL)
lines and the ability to access and change their relative phase with a
resolution of ¨1
GHz or better.
[0011] Essential to high spectral efficiency in any OCDMA system is the
suppression of
multi-channel interference (MUI). MUI rejection is accomplished in the present
invention by adopting an orthogonal set of codes for modifying the relative
phases of
the MLL lines. The present invention uses a synchronous operation to push the
MUI
away from the central clock position and an optical time gating suppresses the
MUI.
The initial experimental results, perfoimed during development of the present
invention, demonstrated a 12.5% spectral efficiency (0.125 bit/s/Hz) using on-
off
keying modulation and bulk optics coders. However, with the development and
use of
optically integrated integrated micro-ring resonator (MRR) phase coders, and
DPSK modulation
the present invention has demonstrated a 50% spectral efficiency. Moreover,
the
present invention, as described herein, has further increased the spectral
efficiency to
87% with eight tributaries at 10 Gb/s each using FEC and DQPSK modulation
technique.
[0012] To highlight the photonic layer security disclosed hereinbelow, the
OCDMA
concept has been extended to OCDM, where through inverse multiplexing
different
"channel" codes are assigned to all tributaries of a high-speed optical
signal. The
security of the high-speed signal then comes about from individual
tributaries, all of
which are present all of the time, obscuring each other.
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!..
[0013] An aspect of the present invention includes an optical system for
transporting
encrypted data. The optical system has a transmitter for transmitting an
encrypted
signal and a receiver for receiving and decrypting the encrypted signal. The
transmitter
includes a source for generating a sequence of optical pulses, each optical
pulse having
a plurality of spectral lines uniformly spaced in frequency with fixed
absolute
frequency and relative phase; a passive optical splitter for copying the
sequence of
optical pulses and outputting a plurality of channels, each channel being
identical to
the sequence of optical pulses; a plurality of data modulators, each
associated with a
respective channel of the plurality of channels, each data modulator
modulating the
respective channel using data to generate a modulated data signal; a
respective spectral
phase encoder coupled to each of the plurality of data modulators, the
spectral phase
encoder encoding respective modulated data signals using a plurality of
mutually
orthogonal phase codes that are individually associated with the respective
spectral
phase encoder; an optical combiner for combining the encoded data signals; and
a
spectral phase scrambler for code-scrambling the combined data signals using a
scramble code as an encryption key to generate the encrypted signal.
[0014] The receiver includes a spectral phase descrambler for descrambling the
encrypted
signal signals using the scramble code as a key to generate a descrambled data
signal;
a plurality of spectral phase decoders for applying an inverse of the phase
codes to the
descrambled data signal to generate a decoded signal, each spectral phase
decoder
being a conjugate match to one of the respective spectral phase encoder; a
respective
optical time gate coupled to each of the plurality of spectral phase decoders,
for time
gating the decoded signal to isolate a desired data signal; and a demodulator
coupled to
the optical time gate for detecting and demodulating the desired data signal
to retrieve
the data.
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[0015] Another aspect of the present invention is a method for transporting
encrypted
optical data. The method includes the steps of generating a sequence of
optical pulses,
each optical pulse having a plurality of spectral lines uniformly spaced in
frequency
with fixed absolute frequency and relative phase; copying the sequence of
optical
pulses and outputting a plurality of channels, each respective channel being
identical to
the sequence of optical pulses; modulating the respective channel using data
to
generate modulated data signals; encoding respective modulated data signals
using a
plurality of mutually orthogonal phase codes to generate a plurality of
encoded data
signals; and combining the plurality of encoded data signals; code-scrambling
the
combined plurality of encoded data signals using a scramble code as an
encryption key
to generate an encrypted signal.
VI. BRIEF DESCRIPTION OF THE DRAWINGS
[00161 These and other features, aspects, and advantages of the present
invention will
become better understood with regard to the following description, appended
claims,
and accompanying drawings wherein:
FIG. 1 illustrates a flow diagram of the process for performing signal
encoding in
accordance with an embodiment of the present invention;
FIG. 2 illustrates a flow diagram of the process for perfouning signal
decoding in
accordance with an embodiment of the present invention;
FIG. 3 illustrates a block representation of a micro-ring resonators (MRR)
based
coder in accordance with an embodiment of the present invention;
FIG. 4a illustrates a matrix representation of Hadamard-8 orthogonal codes in
accordance with an embodiment of the present invention;
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FIG. 4b illustrates a transmission spectrum of a coder after applying the
Haddamard-8 codes shown in FIG. 4a;
FIG. 5 illustrates a block representation of a network system in accordance
with an
embodiment of the present invention;
FIG. 6 illustrates representative signals during three stages of an embodiment
of
the present invention;
FIG. 7 illustrates a block representation of another encoding/decoding system
in
accordance with an embodiment of the present invention.
VII. DETAILED DESCRIPTION OF DISCLOSURE
[0017] Coding and decoding are based on modifying the relative spectral phases
of a set
of well-defined phase-locked optical frequencies that are the output of a mode-
locked
laser. An embodiment of the present invention uses 8 or 16 equally spaced
phase-
locked laser lines confined to an 80-Gliz window depending on the data rate
for
individual channels. Within the scope of the present invention, this 80-GHz
window is
considered to comprise 8 or 16 frequency bins, each bin being phase encode
using a
coder based on an ultrahigh resolution optical de-multiplexer.
[0018] Compared with the prior art SPE work that had used the very broad
optical
bandwidth of an ultra-short pulse source, the present invention has the
advantage of
coding individual frequency lines occupying a small tunable window. The
narrower
spectral extent of the coded signal in an embodiment of the present invention
also
limits the impact of transmission impairments, such as dispersion, and results
in
compatibility of the present invention with transparent WDM optical networks.
[0019] The encoding process of an embodiment of the present invention, shown
in FIG. I,
begins with generation of a train of short pulses using return-to-zero (RZ)
line coding
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=
=
in step 101. The spectral content of the pulse train includes a stable comb of
closely
spaced phase-locked frequencies generated by a mode-locked laser (MLL), and
have a
frequency spacing equal to the MLL pulse repetition rate.
[0020] An example of an appropriate MLL for use in the present embodiment is a
harmonic MLL commercially available from Calmar Opticom that has been modified
to include a thermally stable Fabry-Perrot filter inside the cavity. As a
result of the
modification to the MLL, the comb stability has been reduced substantially to
approximately 10 MHz over the duration of data taking. The MLL, as described,
can
be limited to a 80 GHz total spectral width using an optical band pass filter,
and can be
positioned spectrally within any desired WDM window by tuning the optical
filter, or
the coder. The phase-locked addition of these frequencies generates a pulse
train with
a pulse width of 12.5 ps, inversely proportional to the 80-GHz spectral width
of the
window. These pulses can subsequently be modulated to carry data.
[0021] The pulse train is passively split into a plurality of channels in step
103. Each of
the channels is an identical copy of the pulse train and of each other. In
step 105, each
channel is separately and simultaneously modulated to embed data therein. Once
modulated, each channel is spectral phase encoded in step 107. For example, in
a
simple illustrative case, the phase can be shifted by either 0 or n, as
prescribed by the
binary code selection in step 109. The selected binary codes are converted to
phase
codes in step 111, for example binary 0 is converted to phase angle 0 and
binary 1 is
converted to phase angle n.
[0022] The phase shifted channels are then temporally synchronized and
passively
combined to produce a coded signal in step 113. To further secure the data
prior to
transmission in the present invention, the coded signal generated in step 113
is
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scrambled using a spectral phase scrambler in step 115. The spectral phase
scrambler
applies a scramble code to the coded signal. The scramble code is generated in
step
117 where a random phase setting is applied to phase codes. At this point, the
signals
are ready for transmission outside of a secured area in step 119. The process
shown in
FIG. 1 is perfolined simultaneously on all the channels. Each channel using an
individual phase code for SPE and an individual scrambler code.
[0023] When the relative phases of the frequencies are shifted, the set of
frequencies is
unaltered, but recombination of the frequencies results in a different
temporal pattern:
e.g., a pulse shifted to a different part of the bit period, multiple pulses
within the bit
period, or more noise-like distribution of optical power. Each OCDMA code of
the
present invention is defined by a unique choice of spectral phase shifts. The
set of
codes are defined to make efficient use of the spectrum within a given window,
and
which can be separated from each other with acceptable error rates, even when
a
maximum number of codes occupy the window.
[0024] In the present embodiment, a set of Hadamard codes has been selected,
which are
orthogonal and binary. However, use of Hadamard codes within the present
invention
is intended for illustrative purposes only, and the present invention should
not be
construed as limited to only these codes. Binary Hadamard codes are converted
to
phase codes by assigning to +1's and -1's phase shifts of 0 and n,
respectively. This
choice is based on the goal of achieving high spectral efficiency with minimal
multi-
channel interference (MUI).
[0025] Unlike many optical coding schemes that have been proposed, the process
in the
present embodiment offers true optical orthogonality, in the sense that MUI is
zero at
the time that the decoded signal is at a maximum. The number of orthogonal
codes is
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equal to the number of frequency bins; and if all codes were used, high
spectral
efficiency is possible.
[0026] After data modulation, the initial comb of frequencies produced by the
MLL is
spectrally broadened by the spectral content of the modulating data. Thus, it
is
1
necessary to define frequency bins centered on each of the MLL frequencies.
Encoding
the data-modulated signal then consists of applying the phase shift associated
with a
frequency to the entire bin. The output of the phase encoder is then a signal
obtained
by summing the phase-shifted frequency components of the modulated signal, or
equivalently, by convolving the modulated optical signal at the input of the
phase
encoder with the inverse Fourier transform of the phase code.
[0027] Applying any of these orthogonal codes (except for the trivial case of
Hadamard
Code 1, which leaves all phases unchanged) results in a temporal pattern which
has
zero optical power at the instant in time where the initial pulse would have
had its
maximum power. Decoding is accomplished by using a matched, complementary
code. For simplicity the binary codes used herein are there own complements,
and
consequently the coder and decoder are identical. The decoded signal has the
pulses
restored to their original position within the bit period and restores the
original pulse
shape. The repetition rate of the MLL is an inverse of the frequency bin
spacing and
the pulse duration is an inverse of the window size. Decoding using an
incorrect
decoder results in a temporal pattern that again has zero optical power at the
center of
the bit period and the majority of the energy for that pulse is pushed outside
the time
interval where the desired pulse lies. The desired decoded signal can then be
separated
from all other channel signals by appropriate optical time gating.
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[0028] Although this choice of orthogonal codes implies synchrony as a system
requirement, since de-synchronization will move unwanted optical power into
the
desired signal's time slot, careful code selection allows some relaxation of
this
requirement. Simulations indicate that for four simultaneous channels
transmitting at
2.5 Gb/s and using a suitably chosen set of four codes among the set of 16
Hadamard
codes of length 16, up to 15 ps of relative delay can be tolerated with a
power penalty
within 1 dB at a BER of 10-9. Better resiliency to asynchronism may be
achieved by
using multiphase codes.
[0029] FIG. 2 shows the general steps involved for decoding a signal received
from the
encoding method shown in FIG. 1. A scrambled signal is received in step 201.
In step
203, the signal is descrambled using a spectral phase descrambler. The
spectral phase
descrambler selects a scramble code in step 205 that is the compliment of the
code
used to scramble the original signal in step 115 of FIG. 1. Generally
speaking, the
scramble code can be considered a private key analogue, similar to private
keys known
in the art of digital data encryption. The descrambled signal is decoded in
step 207
using a spectral phase decoder that applies a compliment of the phase codes
used in the
encoding process (step 107 of FIG. 1). The phase codes used in step 207 are
dependent
on the decoder selected in 209.
[0030] Once decoded, the signal is then passed through an optical time gating
device in
step 211 in order to isolate the decoded signal from all the other signals.
The decoded
signal corresponds to one of the charmels discussed above, thus in step 213
the
decoded channel is detected and demodulated to extract the embedded data. At
this
point the extracted data is ready for output in step 215.
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IMPLEMENTATION OF SPE-OCDMA
[0031] High-resolution manipulation of the optical phase has been achieved
interferometrically by bulk optics and through optically integrated micro-ring
resonators (MRR). The MRR technology is compatible with optical integration,
making it advantageous for reducing cost as well as creating novel
functionalities for
optical signal processing. FIG. 3 shows the optical circuit for the MRR-based
coder
300 used in an embodiment of the present invention. The coder 300 consists of
a
number of stacks 308 of 4 rings 306. Each stack 308 is, in turn, coupled to an
input bus
302 and an output bus 304.
[0032] Each stack 308 can be tuned thermally to any one of the MLL lines and
their
arrangement ensures that all lines experience the same optical path length.
Phase
shifters 310 based on theimal expansion are placed between the stacks 308.
These
phase shifters 310 change the optical paths encountered by different
wavelengths, thus
altering the phase of the wavelength.
[0033] FIG. 4a shows a matrix representation of Hadamard-8 orthogonal codes.
The 0 and=
=
7t phase shifts are represented by +1 and -1, respectively. Each row (or
column)
represents a code that is orthogonal to every other row (or column). FIG. 4b
shows the
transmission spectrum of a coder when it is set to code 2 of the Haddamard-8
set of
codes, with TE phase shifts between neighboring frequency bins as shown in the
inset.
The low fiber-to-fiber insertion loss of ¨4dB is mostly attributable to the
coupling to
input-output fibers. The flattop and sharp drop at the bin edges are the
result of the
high quality of the MRR.
[0034] FIG. 5 shows system architecture 500 of an embodiment of the present
invention.
The RZ pulsed output of the MLL 502 is shared by all channels each with its
own data
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modulator 504 followed by its respective Spectral Phase Encoder (SPE) 506. The
SPE
506 spreads the pulse energy from the center of the bit interval. A delay line
ensures
that all channel signals enter the fiber combiner 508 in synchrony. The plots
at the top
of FIG. 5 show time and frequency representations of the signal at the
positions
marked by the vertical arrows. Prior to exiting the secure area, the combined
signal
passes through a spectral phase scrambler 508a, which scrambles the combined
code
using a private key (i.e., scramble codes), thus providing a further layer of
encryption
to the transmission.
[0035] On the receiving side of the network, the now encrypted signal is
received by a
spectral phase descrambler 509a. The spectral phase descrambler applies the
private
key to the scrambled signal, thus descrambling the signal. The descrambled
signal is
multiplexed at an optical multiplexer 509 (such as a beamsplitter). Each of
the signal
copies is processed by a spectral phase decoder (SPD) 510. The SPD reassembles
the
pulse at the center using an orthogonal set of codes to minimize the energy
from other
channel signals in the sampling window. Use of the orthogonal codes coupled
with -
time gating provided by the Optical Time Gate (OTG) 512, suppresses multi-
channel
interference. The OTG 512 isolates the decoded signal form the remaining
signals in
the signal copy. Once the signal has been decoded and isolated, a detection
and
modulation unit 514 extracts the data embodied in the signal.
[0036] FIG. 6 shows the combination of four encoded signals in the plot
referenced as
602, and a decoded signal in the presence of three other codes (channel
signals) is
shown before gating in the plot referenced as 604. In plot 604, an area
referenced as
606 shows a region of low error. This area is termed an eye, thus in the pre-
gating plot
604, the eye is beginning to open (become discernable). However, after gating,
the
optical power associated with the interfering channel signals is present but
is displaced
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in time from the decoded data, as seen in the post-gating plot 608. In plot
608, the eye
606 is significantly more defined, thus after the gating process, the decoded
signal is
easily separable from the interfering channel signals.
[0037] As a result, in the presence of multiple channels, the present
invention uses optical
time-gating provided by, for example, an semiconductor optical amplifier (S0A)-
based Terahertz optical asymmetric de-multiplexer (TOAD). The optical time
gating
extracts the desired, correctly decoded pulse, and identifies and extinguishes
the
remaining multi-channel interference, resulting in the plot referenced as 608.
WDM COMPATIBILITY
[0038] Emerging networks that are optically transparent can, in principle,
allow for a
variety of signal types, modulation foiniats, and bit rates to be transported
over a
common infrastructure. The present invention provides compatibility with
conventional WDM networks.
[0039] As described in detail earlier, the present invention utilizes coherent
spectral phase
encoding and decoding of a tightly spaced comb of phase-locked wavelengths. In
comparison to phase coding approaches known in the art, the present invention
applies
a specific spectral phase component to each distinct line of the multi-
wavelength comb
rather than to multiple lines. In addition to confining a modulated MLL line
to the
center of a frequency and avoiding bin-edge overlap problem, the present
invention
allows for minimization of the spectral width of the OCDMA signal while still
allowing for a larger code set, or potentially a greater number of channels.
For
example, using ultra-fine filtering technology in the embodiments described
above, up
to 16 distinct spectral phase components can be applied at a 5 GHz
granularity.
Therefore, the multi-channel composite OCDMA spectrum is confined to
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approximately 80 GHz, which is compatible with filtering and transmission
components commonly used for conventional DWDM systems aligned to either a 100
GHz or 200 GHz ITU grid.
[0040] In the remainder of this section, experimental results are presented
demonstrating
compatibility of the present invention with a transparent reconfigurable
optical
network that is simultaneously transporting conventional DWDM signals, both in
another and in the same WDM window. The set up utilizes a multi-wavelength
transparent DWDM network similar to the Advanced Demonstration Technology
Network (ADTNet). This network is designed on a 200 GHz channel grid and can
dynamically reconfigure the ¨150 GHz optical pass bands across the network.
[0041] For the OCDMA channels, a multi-wavelength signal is first generated
using a
mode locked laser (MLL1) operating at a 5 GHz repetition rate, which produces
a
comb of phase-locked wavelengths spaced on a 5 GHz grid and spanning over 100
GHz. By filtering the spectrum to approximately 80 GHz, a continuous stream of
¨12.5 ps pulses results in the time domain. These pulses are then data-
modulated at 2.5
Gb/s using independent Mach-Zehnder modulators (MZM) for each O-CDMA
channel. Each channel is assigned one of a set of mutually orthogonal 16-
element
spectral phase codes. The encoded signals from the two OCDMA channels (which
each occupy the same ¨80 GHz bandwidth) are then passively combined and added
to
a single WDM channel on the optical network using a reconfigurable optical
add/drop
multiplexer (ROADM). In addition to the two OCDMA channels, an additional 6
WDM channels are also added to the network as well as a second primary
reference
synchronization MLL, which provides a global synchronization reference
throughout
the network. The composite optical signal is transmitted to a second ROADM
over a
25-km SMF fiber link followed by the appropriate dispersion compensation.
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f-
[0042] At the second ROADM, the conventional WDM channels as well as the OCDMA
signals are dropped from the network. The dropped OCDMA signals are clearly
difficult to resolve individually due to the encoding process. The OCDMA data
signals
are then sent to an OCDMA receiver, which consists of a spectral phase decoder
and
SOA-based interferometric optical time gate (OTG). The OTG filters out multi-
channel interference by temporally extracting only the desired channel using
the
dropped primary reference laser signal for synchronization. The desired data
channel is
faithfully recovered.
[0043] Finally, at this point, a corm-nercially available OC-48 optical
receiver, or other
appropriate optical receiver, is used to convert the RZ pulse stream into a
2.5 Gb/s
NRZ electrical signal. Data transmission over the above network has been
demonstrated with a l dB power penalty compared to the back-to-back
configuration.
[0044] Intra-WDM compatibility of the present invention can be exploited for
taking
advantage of unused capacity and/or delivering multilevel security in a given
WDM
window. The OCDMA systems of the present invention described to this point
have
had the phase-locked comb frequency line contiguous within the DWDM window.
However, because of the phase coherence between the comb lines, it is possible
to
perfolin phase coding on disjointed comb lines. This possibility allows the
present
invention to simultaneously transmit multiple OCDMA channels and a
conventional
OOK channel within the same ITU window. The phase encoded CDMA offers the
unique capability of conveying broadband signals over non-contiguous frequency
bands, thus allowing the transmission of several OCDMA channels in the unused
bandwidth of a single WDM channel where the SONET signal is actually present.
The
experimental confirmation that spread spectrum signals can be conveyed over
disjointed frequency bands, demonstrate a truly OCDMA-overlaid WDM system.
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[0045] For the experiments demonstrating the performance of disjointed
frequency bins,
suitably modified two coder phase masks are used in order to also allow for
insertion
of an 0C-192 SONET signal in the middle of the set of MLL lines. In
particular, the
phase mask is modified in order to allow for: 1) de-multiplexing of 20 MLL
lines (a
total pass-band of 100 GHz); 2) phase encoding of the first 8 and the last 8
MLL lines
=
using Hadamard codes of length 16; 3) notching of the 4 central MLL lines.
This
creates a 20 GHz "spectral hole" in the middle of the OCDMA band. Short dips
in the
frequency response arise from destructive interference caused where two
adjacent
frequency bins differ in phase by Ir. The purpose of creating this "spectral
hole" is to
allow for the insertion of a 10 Gb/s OOK signal. It is worth pointing out that
the
disclosed OCDMA system is spectrally spreading channels over a band
characterized
by a non-contiguous frequency range. It is also worth noting that, despite the
disjointed
frequency range, the orthogonality of the two Hadamard codes is fully
preserved. At
comparable powers for an OOK and two OCDMA signals, the results show
negligible
effect from OOK signal on the OCDMA and 1 dB power penalty on the OOK signal
when two OCDMA signals occupy the same WDM window.
OCDMA-BASED PHOTONIC LAYER SECURITY
[0046] The present invention is easily extendable to OCDMA-based photonie
layer
security and applications that can support where digital encryption is not a
possibility.
In any case, PLS is not always intended to replace the conventional digital
encryption,
but it can complement and augment it. It can be effectively applied in a
"nested
encryption" capability that avails itself where needed. However, in the coming
five
years period, PLS may be the only cost-effective means that can provide secure
communications for bandwidths in excess of several terabits for applications,
such as,
multimedia "virtual theaters" and "on-demand grid-based supercomputing".
17
APP 154R CA 02725804 2010-11-25
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j.
[0047] Since orthogonal codes are used herein, the maximum number of
simultaneous
channels is equal to the number of frequency bins. Coders have been
constructed for
use in the present invention with 16 bins, and can be extend to 32 bins.
Consequently,
the coders are capable of supporting 16 or 32 codes. The current limitation on
the
maximum number of frequency bins comes from the ability to remove heat used
for
phase adjustment and frequency tuning of the micro-ring resonators. For
Hadamard
codes of order N (HN) number of possible orthogonal code states so generated
is N. An
eavesdropper equipped with an adjustable decoder would have to guess only on
the
order of Npossible code settings in order to tune in on any given
transmission. For
increased data obscurity/scrambling, it would be desirable if the eavesdropper
were
required to search through a far larger number of possible codes.
[0048] A useful technique for increasing the space of code states of a set of
orthogonal
codes is to apply an orthogonal matrix WAr generated from Hy by pre-
multiplying by a
monomial matrix MN of order N (a monomial matrix in a matrix in which there is
only
one non-zero element in each row and in each column) where the non-zero
elements
are arbitrarily chosen phase shifts. An interesting special case of a monomial
matrix
MN is a diagonal matrix DN of order N with all of the on-diagonal elements
being
arbitrarily chosen phase shifts. In the context of the present invention, the
general pre-
multiplication process is referred to as code-scrambling and the monomial or
diagonal
multiplier as the scrambling code. Scrambling based on a diagonal matrix is an
interesting special case because such scrambling can be implemented in shared
physically separate SPE scramblers and descramblers as shown in FIG. 5. For
the more
general monomial case, the scrambled codes WN themselves must be implemented
in
the individual phase encoders/decoders as shown in FIG. 7, but monomial
scrambling
significantly increases the number of scrambler settings.
18
APP its CA 02725804 2010-11-25
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PCT/US2009/048877
[0049] In other words, when random phase settings corresponding to the
scrambling code
are imposed upon all the conventional Hadamard codes, a new set of N distinct
orthogonal codes is produced, referred to here as the modified Hadamard codes.
Each
panel is the simulated temporal intensity variation for two bit periods as
might be seen
by an eavesdropper. The left panel shows the result of encoding with the
original
Hadamard-32 codes 6, 7, 9, and 12. The spiky nature of the patterns and their
discrete
appearance in the time domain would appear to render the codes vulnerable to
detection by an eavesdropper.
[0050] However, using the corresponding set of scrambled Hadamard-32 results
in a
substantially different time-dependent signal. The modified Hadamard-32 has
been
created by a diagonal scrambler using random 0 and n phase shifts for each
element.
Using this technique, not only suppresses the peak amplitude of the variation,
but the
energy in a bit is now spread throughout the bit period. This degree of signal
obscuration coupled with the potentially large number of possible scrambler
states and
the ability to dynamically change the scrambler code setting at will all
contribute to the
obscurity of the composite signal. The large code space makes eavesdropping by
exhaustive search for the scrambler key a practically impossibility in a brute
force
attack.
[0051] The basis for an optical communication channel that is secure for all
practical
purposes results from the inability of an attacker to guess the descrambling
code before
the code is changed by the channels. While there are still only N distinct
orthogonal
codes available at any one time, the number of possible WI,/ code
configurations is
governed by the number of different arbitrarily chosen scrambler matrices that
can
exist and that number can be quite large. For the case of a diagonal
scrambling matrix
with p possible phase states at each of the N diagonal elements, the number of
distinct
19
APP 1545 CA 02725804 2010-11-25
WO 2010/033286 PCT/US2009/048877
DN matrices is pN. For example, a case with N=32 and p=4 possible phase
states, the
number of distinct Div matrices is 432 (-1018). Invoking the more general
monomial
scrambling brings an NT! increase in this number. The claim of robustness of
the PLS
approach lie in the analyses of the numbers involved: assuming a 1000 bits
flow
temporal resolution time trace needed to look for opening of an eye by
changing
different phase settings, one needs 10000 flops of a teraflops computer per
guess of the
setting. Considering the 1018 possibilities, that translates to a very long
time to identify
an eye and start the process of decoding the signal compared to the short time
required
to change the phase settings. The search space can be made larger if a
monomial
ma
lY
1 0 0 0 0 O 0 O1 1111111
1111.111
fr
triX 01000000
111111141'1111111
0010004)0
1111111111111111
is 000low i iii iiii 11111,111 t.
O 0 0 0 1 ci0 0 1 1 1 1 1 1 1 ,1 =1 1 1 1 1 ill
use 0 0 0 0 0 1 0 0 1 1 1 1 1 / 1 1 1111:11
O0000 0 10 11111111 111111111
UOUUL3U1.4 1_1111111-1 III
.11111]
in addition to the scrambling diagonal matrix to create the orthogonal
modified codes.
[0052] Code scrambling is achieved by applying a common random phase setting
to the
original Hadamard phase codes IIN. The elements of the scrambler phase
settings can
have any arbitrary phase shift (co) between [0, 2n]. The concept is
illustrated in the
following matrix: The matrix, above, uses a Hadamard-8 as the scrambling code
DN,
represented as a diagonal matrix. The scrambled codes WN are found in the
columns of
the resulting matrix product. In this example, a diagonal scrambling code DN
was
constructed using binary phase shifts 0 and Jt only. The [1,-1] elements of
the codes are
represented in teuus of the phase shifts as e*, where 1 and -1 indicate phase
shifts of
zero and IT, respectively. Since the coders can be configured with arbitrary
spectral
APP 1R4R CA 02725804 2010-11-25
WO 2010/033286 PCT/US2009/048877
phase settings, they can be used for de/encoding and de/scrambling operations
by
applying the appropriate phase codes.
[0053] As mentioned previously, the total number of possible scrambling codes,
which
determines the total number of possible modified Hadamard code configurations,
is
given bypN for a diagonal scrambler where p is the number of phase states and
N is the
number of spectral components. For example, in a setup with two-phase states
of0 and
and N = 8, the total number of possible scrambling codes is 28. The example
shown
in the above matrix uses only one particular scrambling code out of the
possible 28
choices, resulting in one realization of orthogonal modified Hadamard codes.
The
scrambling code can be viewed as the secret key used in conventional secure
communications. With the scrambling code hidden from an eavesdropper the
possible
search space for guessing the code settings becomes exponentially larger than
the
conventional N Hadamard codes.
[0054] The scrambler (descratnbler) corresponding to DN can be implemented in
a
physically separate SPE while the channels continue to use IIN for encoding or
decoding. Alternatively, the scrambler (descrambler) and encoder (decoder) can
be
combined and implemented in a single SPE directly using the columns of DN.
Given a
situation where a group of OCDMA channels is used to send data to a common
destination, it is advantageous to separate the two functions so that all
channels share
one scrambler thereby simplifying the synchronization of scrambler code
changes.
However, the DN codes can be implemented directly in each coder unit.
[0055] The scrambling spreads the energy of the pulse more unifointly over the
pulse
interval, thus reducing the peak powers. Once encoded, the signal can be
descrambled
and then decoded to recover the original pulse in the correct time-slot by
applying a
21
API) 1R4.R CA 02725804 2010-11-25
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matched spectral phase code. Conversely, incorrect decoding leaves the energy
of the
pulse distributed away from the central position at which the correctly
decoded signal
is at its peak. Effective scrambling for enhanced confidentiality requires
multiple
codes to operate in parallel.
[0056] An embodiment of the present invention is shown in FIG. 7 for
demonstrating
OCDMA-based PLS. It should be noted that while FIG. 7 shows two paths in both
the
transmission section and receiver section, in actuality the embodiment shown
in FIG. 7
is constructed of any plurality of paths. Therefore, for simplicity the
present
embodiment will be described from the stand point of two encoding and decoding
paths. The output of a 10 GHz MLL 702 is split into two separate paths by a
splitter
704. Each path is independently modulated by 5 Gb/s data streams consisting of
215-1
pseudorandom bit sequences (PRBS) in the data modulator 706 to generate two
independent OCDMA signals in a single polarization. The modulated signals are
then
encoded in respective spectral phase encoder/scrambler 708. The two encoded
signals
are scrambled using a randomly chosen scrambling code, for example
[-1, 1, 1, 1, -I, I, -1, -1]. The power of each of the scrambled OCDMA signals
is
equalized by variable optical attenuators 710, and connected to fiber delay
lines 712
for synchronization. The output from the delay lines 712 are then passively
combined
at a combiner 716. In this embodiment, all channels occupy the same 80 GHz (8
frequency bins x 10 GHz) spectral bandwidth, making this system compatible
with
existing WDM systems.
[0057] At the receiver, a splitter 720 passively splits the incoming signal
into a plurality of
identical copies of the incoming signal. A decoder 722 is configured to
correctly
descramble and decode either of the two OCDMA signals by selecting the
corresponding phase code. An optical time gate 724 rejects the multi-channel
22
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= interference by extracting the desired decoded channel from the remaining
incorrectly
decoded signals. A detector 726 detects the decoded channel from among the
plurality
of channels in the signal and demodulates the channel, extracting the data.
[0058] In FIG. 7, the encoding (decoding) and the scrambling (descrambling)
operations
are shown as being performed in a single coder, however, the encoding and
scrambling
may be implemented in separate devices.
[0059] Apparatus shown in FIG. 7 can be readily modified in the context of two
5 Gb/s
tributaries in each polarization resulting in four 5 Gb/s tributaries of a 20
Gb/s data
stream once polarization multiplexed by a polarization beam splitter (not
shown). The
only additional care is to ensure that the arrival of each tributary in the
fiber is within
the same bit period using integrated delay lines. Alternatively, such time
compensation
can be done electronically after the detection of each tributary.
[0060] Resilience to archival attack arises from the need for an eavesdropper
to record the
coded data with vastly higher time, amplitude, and optical phase resolution
than the
authorized user needs to employ. For example, if an eavesdropper attempts to
record a
scrambled waveformõ for post processing, he would need to measure the optical
field
of the complex signal, which would need to be recorded in real time with
temporal
resolution at least 16 times that needed to receive the decoded data and with
analog (as
opposed to digital) amplitude sampling. The high-speed temporal sampling
requires a
much higher-bandwidth receiver, which will introduce significantly more noise,
corrupting the analog measurement of the signal amplitude. An effort to
capture the
different lower-bandwidth signals on each MLL frequency will reduce the
requirement
for temporal resolution but at the expense of multiple parallel measurements,
which
must be synchronized to sub-nanosecond timescales.
23
ADD 42452
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;-
DEPLOYMENT OF OCDMA-BASED PLS
[0061] A deployment scenario of the present invention consists of two secure
areas within
which OCDMA coding would be used for signal routing/addressing purposes.
However, before these signals leave the secure areas, they would be scrambled
by
passing through a shared scrambling stage. At the entrance to the secure area
received
signals would be collectively de-scrambled and finally decoded by the end
user. This
architecture satisfies obscurity conditions for practically secure, point-to-
point
transmission by precluding access to any single code. Specifically, it must be
assumed
that the groups of codes leaving the secure areas are always sent
simultaneously, as
would be the case if a large bandwidth signal were being sent as multiple sub-
rate data
streams on several codes in parallel.
[0062] Inverse multiplexing can be used to reach 100 GbE. The key aspect of
inverse
multiplexing is that either all codes (for example four codes) are
simultaneously in use
or none are; and when the codes are present, they form a self-obscuring group.
The
role of the scramblers, which apply an arbitrarily chosen set of phase shifts
to the
ensemble of codes, is to present an eavesdropper with a very large search
space if he is
to guess the phase codes in use. Ideally, the state of the scrambler is
updated on a
regular basis in synchronized fashion at the two ends of the link.
MULTILEVEL SECURITY
[0063] Multilevel security has been used in both military and commercial
arenas where
over the same transmission medium such as optical fiber, communication
channels
with different levels of security coexist. For example, in a DWDM network
additional
security for a given channel is provided by digital electronic encryption.
Therefore,
secure channels are protected from inadvertent or malicious access to their
data.
24
APP 1848 CA 02725804 2010-11-25
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[0064] Compatibility with DWDM networks is utilized in the OCDMA scheme of the
present invention to provide multilevel security. A use of the OCDMA-based PLS
described above is for multilevel security for avionics applications where
space,
weight, and power (SWaP) are of prime importance. Currently, dedicated fibers
are
used in order to provide multilevel security. Recent efforts to standardize
WDM
networking to limit the excessive use of fiber infrastructure are a welcome
improvement. However, in order to provide multilevel security digital
encryption has
to support designated channels. The OCDMA-based PLS of the present invention
is an
all-optical means that can readily be incorporated because of the availability
and the
SWaP figure of merits.
[0065] A set of AES signals can be transported on an OCDMA system as a means
of
denying an eavesdropper ready access to the digital cipher text bits and
thereby
frustrating an archival attack. AES creates an initial state Di0K0 by
computing the
XOR sum of the data D and round key Ko. ABS then performs three operations on
D6Ko: a non-linear substitution followed by row shift and column mixing. This
is
repeated for 10 to 14 iterations, depending on key size. Each round requires a
key
created through key expansion.
[0066] OCDMA encryption performs analogous operations in a single pass. Each
of n data
bits modulates a set of m coherent MLL lines. This represents an m-fold
expansion of
the signal bandwidth, whereas AES operates with essentially the same fixed
bandwidth
as the plain text signal. The seeming loss in spectral efficiency represented
by this
expansion is recovered by the ability to transport several differently encoded
signals in
this same bandwidth.
APP 1848 CA 02725804 2010-11-25
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[0067] A set of orthogonal signals is created by applying a Hadamard phase
mask to each
set of modulated MLL lines. State mixing is first accomplished by combining
the
orthogonal signals, which remain separable via the Hadamard coding. A keyed
pseudo-random phase rotation is finally performed on each wavelength of the
mixed
signal, which creates mixed, but still mutually orthogonal, states that are no-
longer
separable by a Hadamard phase mask, While separable states can be recovered by
a
complementary set of keyed phase rotations, further mixing occurs when the
phase
rotation is improperly corrected by an eavesdropper. Sampling that is the
result of time
gating constitutes one of the non-linear operations in OCDMA "encryption". The
OCDMA system of the present invention is linear in the fields, but ordinarily
the
intensities have to be measure using a square law detector. Therefore, the
measured
signal is nonlinear to the injection of coherent noise created by multi-
channel
interference (MUI) in the gated time interval. The role of this "analog-based"
nonlinearity as opposed to the digital-based nonlinearly in providing PLS is
currently
under investigation.
COMPARISON WITH ONGOING ALTERNATIVE RESEARCH
[0068] Currently there are two spectral phase-scrambling based approaches to
PLS,
OCDMA as described herein and an approach, which relies on the ability to
slice the
spectral content of a single-channel modulation-broadened laser spectrum with
ultrahigh resolution and applies phase scrambling to individual spectral
slices, referred
to hereinafter as "Essex approach".
[0069] In the Essex approach, a typical 10 Gb/s ON-OFF modulated optical
signal
occupying a bandwidth of approximately 20 GHz is partitioned using a spectral
phase
mask into roughly 128 different bins and a random phase shift is applied to
each
26
APP 1848 CA 02725804 2010-11-25
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spectral bin. Each bin has a spectral width on the order of 0.16 GHz (for
comparison,
in the OCDMA PLS approach of the present invention, the width of the phase
bins are
on the order 5-10GHz). At the receiver, a conjugate phase mask is applied to
restore
the signal spectrum to its original state and thereby recover the original
digital signal.
The high-resolution of the phase mask translates into an optical impulse
response that
is tens of times longer than the bit interval of the original signal. The
phase mask
thereby gives rise to considerable inter-symbol interference (ISI), obscuring
the
original single-channel signal.
[0070] The "secret key" in the Essex approach describes the state of the
random phase
mask much as the OCDMA system relies on "secret" scrambler spectral phase
masks.
A difference between the Essex encoder and the OCDMA PLS system of the present
invention is that the former operates on a single, modulated optical signal to
introduce
large ISI on an individual channel whereas the OCDMA system operates on an
inverse-multiplexed combination of several orthogonally encoded signals that
coherently interfere to obscure one another.
[0071] The ability to recover the original signal from the encoded version
relies on the
fidelity with which the product of the encoder and decoder spectral phase
masks
approaches unity across the signal bandwidth. However, one critical but
unavoidable
consequence of spectral phase encoding is the presence of a pattern of
intensity dips in
the transfer function of encoders and decoders wherever the phase of two
adjacent bins
differs. The intensity dips can result in complete cancellation of the signal
spectrum
near bin edges where the phases of two adjacent bins differ by 7r. Such dips
can be
seen in the spectral response of the coders used in prior OCDMA systems.
27
APP 1848 CA 02725804 2010-11-25
WO 2010/033286 PCT/US2009/048877
[.
[0072] For comparison, while the OCDMA encoders described earlier also exhibit
these
intensity dips, the number of bin edges here is far smaller (8-16 versus 128).
In
principle, the pattern of intensity dips can reveal information about the
scrambling key.
However, when multiple codes are simultaneously present, the absence of a dip
in one
code can cover the dip in another code. More importantly, the data modulation
is
designed such that little power is placed near the bin edges of the signal
spectrum,
further obscuring phase-mask-induced intensity dips. Such tailoring of the
data
modulation is not possible with the single-channel Essex approach.
[0073] In addition to the spectral phase encoding approaches discussed, there
are other
PLS techniques that effectively apply phase modulation in the time domain
instead of
the frequency domain. For example, consider a single-channel approach
originally
Ir
developed by Northwestern University and often referred to as Alpha Eta. In a
typical
phase-based implementation, a pseudorandom M-ary phase rotation is temporally
applied on a bit-by-bit basis on the top of a transmitted DPSK signal. An
intended
receiver, who holds a proper decoding key, applies the proper conjugate
rotation,
allowing it to make a nearly optimal binary decision on the bit value. In
contrast, an
eavesdropper without the proper key is placed at a significant disadvantage in
terms of
detecting the more complex M-ary signal. One of the uniqueness of the Alpha
Eta
approach is that by limiting the initial transmitter power to appropriate
levels and
having sufficient number of phase states, the Alpha Eta approach can force the
eavesdropper to be in a quantum noise-limited detection regime. Prototypes of
Alpha
Eta have already been demonstrated at speeds and distances that approach those
of
interest for real-world systems. For example, in transmission at 622 Mb/s has
been
demonstrated over distances up to 850 km on the BoSSNET research link, and the
28
CA 02725804 2013-10-17
experiments have shown some of the requirements that would enable a network to
support
this format.
[0074] The described embodiments of the present invention are intended to
be
illustrative rather than restrictive, and are not intended to represent every
embodiment of the
present invention. Various modifications and variations can be made.
29
