Note: Descriptions are shown in the official language in which they were submitted.
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OPTICAL CDMA USING A CASCADED MASK STRUCTURE
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to optical communication systems and, more
particularly, to
optical code-division multiple access (CDMA) communications systems. Aspects
of the
present invention find particular application in all-fiber optical
communications systems,
ranging from wide area networks to local area networks.
Description of the Related Art
Recent years have seen rapidly expanding demands for communications bandwidth,
resulting in the rise of technologies such as satellite communications, video
programming
distribution networks such as cable television, and spread-spectrum telephony
including, for
example, code-division multiple access telephony. Such technologies have
become common
and well integrated into everyday communications. Growing demand for
communications
bandwidth has brought significant investments in new communications
technologies and in
new communications infrastructure. For example, the cable television industry,
telephone
companies, Internet providers and various government entities have invested in
long
distance optical fiber networks and in equipment for fiber networks. The
addition of this
infrastructure has, in turn, spurred demand for bandwidth, resulting in demand
for yet
additional investment in new technologies and infrastructure.
Installing optical fibers over long distances is expensive. Additionally,
conventional
optical fiber or other optical communication networks utilize only a small
fraction of the
available bandwidth of the optical communication channel. There is
consequently
considerable interest in obtaining higher utilization of fiber networks or
otherwise increasing
the bandwidth used within optical fiber systems. Techniques have been
developed to
increase the bandwidth of optical fiber communication systems and to convey
information
from plural sources over a fiber system. Generally, these techniques seek to
use more of the
readily available optical bandwidth of optical fibers by supplementing the
comparatively
simple coding schemes conventionally used by such systems. In some improved
bandwidth
fiber systems, the optical fiber carries an optical carrier signal consisting
of a single, narrow
wavelength band and multiple users access the fiber using time-division
multiplexing (TDM)
or time-division multiple access (TDMA). Time division techniques transmit
frames of data
by assigning successive time slots in the frame to particular communication
channels.
Optical TDM requires high pulse rate diode lasers and provides only moderate
improvements
in bandwidth utilization. In addition, improving the transmission rates on a
TDM network
requires that all of the transceivers attached to the network be upgraded to
the higher
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transmission rates. No partial network upgrades are possible, which makes TDM
systems
less flexible than is desirable. On the other hand, TDM systems provide a
predictable and
even data flow, which is very desirable in mufti-user systems that experience
"bursty" usage.
Thus, TDM techniques will have continued importance in optical communications
systems,
but other techniques must be used to obtain the desired communications
bandwidth for the
overall system. Consequently, it is desirable to provide increased bandwidth
in an optical
system that is compatible with TDM communication techniques.
One strategy for improving the utilization of optical communication networks
employs wavelength-division multiplexing (WDM) or wavelength-division multiple
access
(WDMA) to increase system bandwidth and to support a more independent form of
multiple
user access than is permitted by TDM. WDM systems provide plural optical
channels each
using one of a set of non-overlapping wavelength bands provided over an
expanded portion of
the fiber's available bandwidth. Information is transmitted independently in
each of the
optical channels using a light beam within an assigned wavelength band,
typically generated
by narrow wavelength band optical sources such as lasers or light emitting
diodes. Each of
the light sources is modulated with data and the resulting modulated optical
outputs for all
of the different wavelength bands are multiplexed, coupled into the optical
fiber and
transmitted over the fiber. The modulation of the narrow wavelength band light
corresponding to each channel may encode a simple digital data stream or a
further plurality
of communication channels defined by TDM. Little interference will occur
between the
channels defined within different wavelength bands. At the receiving end, each
of the WDM
channels terminates in a receiver assigned to the wavelength band used for
transmitting data
on that WDM channel. This might be accomplished in a system by separating the
total
received light signal into different wavelengths using a demultiplexer, such
as a tunable
filter, and directing the separated narrow wavelength band light signals to
receivers assigned
to the wavelength of that particular channel. The number of users that can be
supported by
a WDM system is limited due to the difficulty in obtaining appropriately tuned
optical
sources. Wavelength stability, for example as a function of operating
temperature, may also
affect the operational characteristics of the WDM system.
As a more practical matter, the expense of WDM systems limits the application
of this
technology. One embodiment of a WDM fiber optic communication system is
described in
U.S. Patent No. 5,579,143 as a video distribution network with 128 different
channels. The
128 different channels are defined using 128 different lasers operating on 128
closely spaced
but distinct wavelengths. These lasers have precisely selected wavelengths and
also have the
well-defined mode structure and gain characteristics demanded for
communications systems.
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Lasers appropriate to the WDM video distribution system are individually
expensive so that
the requirement for 128 lasers having the desired operational characteristics
makes the
overall system extremely expensive. The expense of the system makes it
undesirable for use .
in applications such as local area computer networks and otherwise limits the
application of
the technology. As is described below, embodiments of the present invention
can provide a
video distribution network like that described in U.S. Patent No. 5,579,143,
and
embodiments of the invention can provide other types of medium and wide area
network
applications, making such systems both more flexible and more economical.
Embodiments of the present invention, as described below, use spread spectrum
communication techniques to obtain improved loading of the bandwidth of an
optical fiber
communication system in a more cost-effective manner than known WDM systems.
Spread
spectrum communication techniques are known to have significant advantages and
considerable practical utility, most notably in secure military applications
and mobile
telephony. There have consequently been suggestions that spread spectrum
techniques,
most notably code-division multiple access (CDMA), could be applied to optical
communications technologies. Spread spectrum techniques are desirable in
optical
communications systems because the bandwidth of optical communications
systems, such as
those based on optical fibers; is sufficiently large that mufti-dimensional
coding techniques
can be used without affecting the data rate of any electrically generated
signal that can
presently be input to the optical communications system. Different channels of
data can be
defined in the frequency domain and independent data streams can be supplied
over the
different channels without limiting the data rate within any one of the
channels. From a
simplistic point of view, the WDM system described above might be considered a
limiting
case of a spread spectrum system in that plural data channels are defined for
different
wavelengths. The different wavelength channels are defined in the optical
frequency domain
and time domain signals can be transmitted over each of the wavelength
channels. From a
CDMA perspective, the distinct wavelength channels of the WDM communication
system
described above provide a trivial, single position code, where individual code
vectors are
orthogonal because there is no overlap between code vectors.
There have been suggestions for optical CDMA systems that are generally
similar to
traditional forms of radio frequency CDMA, for example in Kavehrad, et al.,
"Optical Code-
Division-Multiplexed Systems Based on Spectral Encoding of Noncoherent
Sources," J.
Liehtwave Tech., Vol. 13, No. 3, pp. 534-545 (1995). As opposed to the WDM
system
described above, the suggested optical CDMA system uses a broad-spectrum
source and
combines frequency (equivalently, wavelength) coding in addition to time-
domain coding. A
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schematic illustration of the theoretical optical CDMA suggested in the
Kavehrad article is
presented in FIG. 1. The suggested optical CDMA system uses a broad-spectrum
(broadband), incoherent source 12 such as an edge-emitting LED, super
luminescent diode or
an erbium-doped fiber amplifier. In the illustrated CDMA system, the broad-
spectrum
source is modulated with a time-domain data stream 10 and the time domain
modulated
broad-spectrum light 14 is directed into a spatial light modulator 16 by a
mirror 18 or other
beam steering optics.
Within the spatial light modulator 16, light beam 20 is incident on a grating
22, which
spatially spreads the spectrum of the light to produce a beam of light 24
having its various
l0 component wavelengths spread over a region of space. The spatially spread
spectrum beam
24 is then incident on a spherical lens 26 which shapes and directs the beam
onto a spatially
patterned mask 28, which filters the incident light. Light spatially filtered
by the mask 28
passes through a second spherical lens 30 onto a second diffractive grating
34, which
recombines the light. Mask 28 is positioned midway between the pair of
confocal lenses 26,
15 30 and the diffraction gratings 22, 34 are positioned at the respective
focal planes of the
confocal lens pair 26, 30. The broad optical spectrum of the incoherent source
is spatially
expanded at the spatially patterned mask 28 and the mask spatially modulates
the spread
spectrum light. Because the spectrum of the light is spatially expanded, the
spatial
modulation effects a modulation in the wavelength of the light or,
equivalently, in the
20 frequency of the light. The modulated light thus has a frequency pattern
characteristic of
the particular mask used to modulate the mask. This frequency pattern can then
be used to
identify a particular user within an optical network or to identify a
particular channel within
a multi-channel transmission system.
After passing through the mask 28, the spatially modulated light passes
through the
25 lens 30 and the wavelength modulated light beam 32 is then spatially
condensed by the
second grating 34. The wavelength modulated and spatially condensed light beam
36 passes
out of the spatial light modulator 16 and is directed by mirror 38 or other
beam steering
optics into a fiber network or transmission system 42. The portion of the CDMA
system
described to this point is the transmitter portion of the system and that
portion of the
30 illustrated CDMA system down the optical path from the fiber network 42
constitutes the
receiver for the illustrated system. The receiver is adapted to identify a
particular
transmitter within a network including many users. This is accomplished by
providing a
characteristic spatial mask 28 within the transmitter and detecting in the
receiver the spatial
encoding characteristics of the transmission mask from among the many
transmitted signals
35 within the optical network. As set forth in the Kavehrad article, it is
important for the mask
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28 to be variable so that the transmitter can select from a variety of
different possible
receivers on the network. In other words, a particular user with the
illustrated transmitter
selects a particular receiver or user to receive the transmitted data stream
by altering the
spatial pattern of the mask 28, and hence the frequency coding of the
transmitted beam 40,
so that the transmitter mask 28 corresponds to a spatial coding characteristic
of the intended
receiver.
The receiver illustrated in FIG. 1 detects data transmitted from a particular
transmitter by detecting the spatial (frequency or wavelength) modulation
characteristic of
the transmitter mask 28 and rejecting signals having different characteristic
spatial
modulation patterns. Light received from the optical fiber network 42 is
coupled into two
different receiving channels by coupler 44. The first receiver channel
includes a spatial light
demodulator 46 that has a mask identical to one used in the spatial light
modulator 16 and
the second receiver channel includes a spatial light modulator 48 of similar
construction to
the transmitter's spatial light modulator 16, but having a mask the "opposite"
of the
transmitter mask 28. Each of the spatial light demodulators 46, 48 performs a
filtering
function on the received optical signals and each passes the filtered light
out to an associated
photodetector 50, 52. Photodetectors 50, 52 detect the filtered light signals
and provide
output signals to a differential amplifier 54. The output of the differential
amplifier is
provided to a low pass filter 56 and the originally transmitted data 58 are
retrieved.
2o FIG. 2 provides an illustration of the receiver circuitry in greater
detail. In this
illustration, spatial light demodulators 46 and 48 are generally similar to
the spatial light
modulator 16 shown in FIG. 1 and so individual components of the systems are
not
separately described. Received light 60 is input to the receiver and is split
using coupler 62,
with a portion of the light directed into spatial light demodulator 46 and
another portion of
the light directed into the other spatial light modulator 48 using mirror 64.
Spatial light
demodulator 46 filters the received light 60 using the same spatial
(frequency, wavelength)
modulation function as is used in the transmitter's spatial light modulator 16
and provides
the filtered light to photodetector 50. Spatial light demodulator 48 filters
the received light
using a complementary spatial filtering function and provides the output to
the detector 52.
Amplifier 54 subtracts the output signals from the two photodetectors. To
effect the same
filtering function as the transmitter's spatial light modulator 16, the
spatial light
demodulator 46 includes a mask 66 identical to the transmitter mask 28.
Spatial light
demodulator 48 includes a mask 68 that performs a filtering function
complementary to
masks 28 and 66 so that spatial light modulator 48 performs a filtering
function
complementary to the filtering function of spatial light modulator 16. In the
Kavehrad
-$r,
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article, each of these masks 16, 66, 68 is a liquid crystal element so that
the masks are fully
programmable.
The particular codes embodied in the masks must be appropriate to the proposed
optical application. Although CDMA has been widely used in radio frequency
(RF) domain
communication systems, its application in frequency (wavelength) domain
encoding in optical
systems has been limited. This is because the success of the RF CDMA system
depends
crucially on the use of well-designed bipolar code sequences (i.e., sequences
of + 1 and -1
values) having good correlation properties. Such codes include M-sequences,
Gold sequences,
Kassami sequences and orthogonal Walsh codes. These bipolar codes can be used
in the RF
domain because the electromagnetic signals contain phase information that can
be detected.
RF CDMA techniques are not readily applicable to optical systems in which an
incoherent
light source and direct detection (i.e., square-law detection of the intensity
using
photodetectors) are employed, because such optical systems cannot detect phase
information.
Code sequences defining negative symbol values cannot be used in such optical
systems. As a
t5 result, only unipolar codes, i.e., code sequences of 0 and 1 values, can be
used for CDMA in a
direct-detection optical system.
The Kavehrad article suggests the adaption of various bipolar codes for the
masks
within the system illustrated in FIGS. 1 & 2, including masks provided with a
unipolar (only
0's and 1's) M-sequence or a unipolar form of a Hadamard code. For these sorts
of bipolar
code, the Kavehrad article indicates that the bipolar code of length N must be
converted into
a unipolar code sequence of length 2N and that a system including such codes
could support
a total of N-1 users. The Kavehrad article primarily addresses theoretical
aspects of a CDMA
system, providing predictions of system performance on the basis of
assumptions as to how
such a system might operate. Little description is provided of how to
implement a general
bipolar code so as to be effective in an optical system.
A more practical example of an optical CDMA system, including an example of a
converted bipolar code sequence, has been proposed for transmission and
detection of bipolar
code sequences in a unipolar system. This system is described in a series of
papers by L.
Nguyen, B. Aazhang and J.F. Young, including "Optical CDMA with Spectral
Encoding and
Bipolar Codes," Proc. 29th Annual Conf. Information Sciences and Systems
(Johns Hopkins
University, March 22-24, 1995), and "All-Optical CDMA with Bipolar Codes",
Elec. Lett.,
16th March 1995, Vol. 3, No. 6, pp. 469-470. This work is also summarized in
U.S. Patent
No. 5,760,941 to Young, et al., and this work is collectively referenced
herein as the Young
patent. In the Young patent's system, schematically illustrated in FIG. 3, the
transmitter 80
employs a broad-spectrum light source 82 the output of which is split by a
beam splitter 84
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into two beams 86 and 88 that are processed by two spatial light modulators 90
and 92. The
first spatial light modulator 90 comprises a dispersion grating 94 to
spectrally disperse the
light beam 86 and a lens 96 to collimate and direct the dispersed light onto a
first spatial
encoding mask 98 which selectively passes or blocks the spectral components of
the light
beam. Lens 100 collects the spectral components of the spatially modulated
light beam and
recombination grating 102 recombines the spread beam into encoded beam 104.
The "pass"
and "block" states of the encoding masks represent a sequence of 0's and 1's,
i.e., a binary,
unipolar code. The code 106 for the first mask 98 has a code U~U*, where U is
a unipolar
code of length N, U* is its complement and "~" denotes the concatenation of
the two codes.
The second encoder 92 (details not shown) is similar in structure to the first
encoder 90
except that its encoding mask has a code U*~U. Symbol source 108 outputs a
sequence of
pulses representing 0's and 1's into a first ON/OFF modulator 110 and through
an inverter
112 into a second ON/OFF modulator 114. The two modulators 110 and 114
modulate the
two spatially modulated beams of light and the two beams are combined using a
beam
splitter 116 to combine the two encoded light beams 118 and 120. The modulated
light
beams are alternately coupled to the output port depending on whether the bit
from the
source is 0 or 1.
This system can then use a receiver with differential detection of two
complementary
channels, as illustrated in the receiver of FIG. 2. The receiving channels are
equipped with
2o masks bearing the codes U*~U and U~U*, respectively, and sequences of 0's
and 1's are
detected according to which channel receives a signal correlated to that
channel's mask. The
system proposed in the Young patent allows the use of the bipolar codes
developed for RF
CDMA technologies to be used in optical CDMA systems. However, for a mask of
length 2N,
only N codes can be defined since the code U and its complement U* must be
concatenated
and represented within each mask.
Systems like that shown in the Kavehrad article and the Young patent typically
perform optical CDMA encoding using one or more spatial light modulators (I6
or 90, 92).
Such spatial light modulators are comprised of a relatively complex
combination of optics,
i.e., dispersion gratings (22, 34 or 94, 102), lenses (26, 30 or 96, 100) and
a spatial pattern
mask (28 or 98), which typically require critical alignment and opto-
mechanical mounts to
perform properly. Vibrations and misalignments are highly detrimental to
performance.
Non-ideal optical characteristics, such as Gaussian beam shapes, can introduce
cross talk or
otherwise impair system signal-to-noise ratios. Additionally, two such
structures are
typically required to decode a received CDMA code (see spatial light
modulators 46, 48 in
FIGS. 1 and 2). Accordingly, the production costs for such devices can be
undesirably high.
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Even with aggressive optical designs, these optical systems are large and
heavy. Therefore, it
is an object of the invention to provide a simplified, cost-effective
structure, e.g., one that
that is compact and does not require alignment, for modulators and
demodulators within an
optical CDMA communication system.
SUMMARY OF THE PREFERRED EMBODIMENTS
These and other objects may be obtained with optical CDMA transmitters and
receivers include a fiber grating filter device encoded with a CDMA code by
selectively
providing filter elements within a series of possible narrow bandwidth filter
positions in the
device. Individual code bits might be stored, for each frequency band within
the filter, by
assigning the presence of a fiber grating filter at a chip frequency a "bit 0"
chip and the
absence of a fiber grating filter a "bit 1" chip. The fiber grating filter
device provides a
cascade of many band-stop grating filters, with the presence or absence of
each filter
corresponding to a value the CDMA code bit in the spectral domain.
Consequently, a CDMA
transmitter may be formed from a data modulator to modulate a broad-spectrum
(broadband) optical source, e.g., an erbium-doped fiber source (EDFS) or a
super luminescent
diode (SLD), and a fiber grating filter device as described above to
selectively pass (or
conversely block) multiple portions of the spectrum of the broad-spectrum
optical source.
The optical CDMA-encoded output of this transmitter is then output to an
optical network
that contains the outputs of many such devices. An appropriate CDMA receiver
may include
a beam splitter that receives an optical signal from the optical network and
feeds split optical
signals to two fiber grating filter devices as described above. One of the two
fiber grating
filter devices provides the CDMA code of the filter of a corresponding
transmitter and the
other filter provides the complement of that CDMA code. Outputs from the two
fiber grating
filter devices are connected to a differential detector that recovers the
transmitted data
signal. The described architecture is conducive to a low manufacturing cost
since it can be
implemented by a fusion splice of a piece of fiber in series with the data
modulator (in the
transmitter) and/or the outputs of the splitter (in the receiver).
A transmitter responsive to a data signal for use in an optical communication
system
might include a broadband light source having a frequency spectrum comprised
of multiple
frequency portions, a data modulator for selectively modulating the broadband
light signal in
response to the data signal to provide a modulated broadband light signal, and
a fiber grating
filter device for selectively passing frequency portions of the modulated
broadband light
signal to an optical network. The order of the data modulator and the fiber
grating filter
device can be reversed with essentially equivalent results.
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A receiver for use in an optical communication system for selectively
retrieving a
CDMA-encoded signal from a light signal received from an optical network might
include a
splitter for splitting the received light signal into first and second split
light signals, a first
fiber grating filter device for selectively passing portions of the first
split broadband light
signal to thereby generate a first filtered light signal, a second fiber
grating filter device for
selectively passing complementary portions of the second split broadband light
signal to
thereby generate a second filtered light signal, and a differential detector
for generating a
received data signal from the difference between the first and second filtered
light signals.
The first fiber grating device passes portions of the received light signal
corresponding to
those portions of a desired CDMA-encoded signal.
Transmitting and receiving devices in accordance with aspects of the invention
might
be combined in an optical communication system. A source generates a broadband
light
signal comprising multiple frequency portions and provides the signal to a
transmitter, for
the optical communication system. The transmitter includes a data modulator
for selectively
passing the broadband light signal in response to a transmit data signal to
generate a data
modulated broadband light signal. The transmitter further includes a first in-
fiber Bragg
grating that passes frequency-separated portions of the broadband light
signal. The receiver
for the optical communication system includes a splitter that splits a
received broadband
light signal into first and second split broadband light signals. A second in-
fiber Bragg
grating selectively passes portions of the first split broadband split light
signal to generate a
first filtered split signal. A third in-fiber Bragg grating selectively passes
complementary
portions of the second split broadband light signal to generate a second
filtered light signal.
The outputs of the second and third filters are provided to a differential
detector that
generates a received data signal corresponding to the difference between the
first and second
filtered light signals. Most preferably, the first in-fiber Bragg grating
defines a first code, the
second in-fiber Bragg grating is identical to the first in-fiber grating and
the third in-fiber
Bragg grating defines a second code complementary to the first code.
Such a system is particularly useful when the one or more transmitters, i.e.,
devices
comprised of a data modulator and a first fiber grating device, place data-
modulated, CDMA-
encoded, broadband light signals onto an optical network and one or more
receivers, i.e.,
devices comprised of a splitter, second and third fiber grating filter devices
and a differential
detector, retrieve a broadband light signal from the optical network to
selectively retrieve
data-modulated, CDMA-encoded broadband light signals contained within and
consequently
the data originally provided to the transmitters.
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The invention may be better understood from the following description when
read in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a conventional optical fiber mediated CDMA communication
system.
FIG. 2 provides a more detailed view of one receiver configuration that might
be used
in the system of FIG. 1.
FIG. 3 illustrates an encoder for using bipolar codes in an optical CDMA
system.
FIGS. 4A-4D illustrate operational characteristics of certain elements of the
modulation or encoding system in accordance with the present invention.
FIG. 5 provides an example of the transmission through a single Bragg grating.
FIG. 6 illustrates the structure of a typical optical fiber.
FIG. 7 illustrates the structure of one type of Bragg grating within a optical
fiber
waveguide.
FIGS. 8 & 9 present different configurations of an optical fiber network
according to
the present invention.
FIG. 10 is a block diagram of a mufti-channel encoder according to the present
invention.
FIG. 11 is a diagram showing the correspondence between an exemplary CDMA
code,
a fiber grating filter device of the present invention and the resulting
bandpassed spectrum
FIG. 12 is a simplified diagram of an integrated fiber grating device for CDMA
coding.
FIG. 13 is a block diagram of a decoder according to the present invention.
FIG. 14 schematically illustrates an apparatus for generating an array of N
broad-
spectrum optical sources having sufficient intensity to generate light beams
for N channels of
communication over a fiber using methods in accordance with the present
invention.
FIG. 15 illustrates in greater detail the optical detection circuitry
schematically
illustrated in FIG. 13.
FIG. 16 illustrates a modification to the source generation mechanism of FIG.
14.
CROSS-REFERENCED APPLICATIONS
The following applications are each incorporated by reference in their
entirety into
this application:
"Optical CDMA System," application Serial No. 09/126,310, filed July 30, 1998.
2. "Optical CDMA System Using Sub-Band Coding," application Serial No.
09/126,217,
filed July 30, 1998.
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3. "Method and Apparatus for Reduced Interference in Optical CDMA",
application
Serial No. 09/127,343, filed July 30, 1998.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
These and other objects are obtained with CDMA transmitters and receivers
which
include a wavelength or frequency modulator formed using a fiber grating
filter device
encoded in wavelength or frequency with a CDMA code such that the "bit 0" chip
is encoded
as the presence of a fiber grating filter spanning a frequency range near that
chip frequency
and the "bit 1" chip is encoded as the absence of a fiber grating filter
spanning that
frequency range at that chip frequency within the sequence of chip frequency
positions along
the fiber. This fiber grating filter device may be arranged as a cascade of
many band-stop
grating filters each corresponding to a different CDMA code bit in the
frequency domain.
Consequently, a CDMA transmitter may be formed from a broad-spectrum
(broadband)
optical source, e.g., an erbium doped fiber source (EDFS) or a super
luminescent diode (SLD),
a data modulator that responds to a data signal to modulate the optical source
and an
appropriate fiber grating filter device to selectively pass (or conversely
block) portions of the
spectrum of the broad-spectrum optical source. The optical CDMA-encoded output
of this
transmitter may be coupled into an optical network along with the outputs of
other such
devices. Similarly, an optical CDMA receiver may be formed from a beam
splitter that
receives an optical signal from the optical network and feeds split signals to
two receiving
fiber grating filter devices. One of the receiving filter devices includes a
filter encoded with
the same CDMA code defined within the filter of a corresponding transmitter
and the other
receiving filter device is encoded with the complement of that same CDMA code.
The two
receiving filter devices couple their output to a differential detector that
recovers the
transmitted data signal. Such an architecture is conducive to low cost
manufacturing since it
can be implemented by a fusion splice of a piece of fiber in series with the
data modulator (in
the transmitter) and/or the outputs of the splitter (in the receiver). This
optical system is
generally immune to misalignments and vibrations. For this reason, the optical
system is
durable and capable of use in practical, non-ideal environments.
Preferred embodiments of the present invention provide an optical CDMA system
that is more compact, is more readily aligned and is less susceptible to
misalignment than
conventional optical CDMA systems such as those described in the Kavehrad
article and the
Young patent, discussed above in the Background. Aspects of the invention
provide at least
one transmission channel that encodes broadband light with an identifying code
and outputs
the encoded light signal to an optical communication link such as an optical
fiber. This
system may encode broadband light by passing the input light through a mask
that
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modulates the broadband light as a function of wavelength across an
operational portion of
the spectrum of the input light. An appropriate encoding mask in accordance
with the
present invention might consist, for example, of a number of wavelength
dependent filters
arranged sequentially along the length of an optical fiber with each
wavelength dependent
filter attenuating, substantially uniformly, light within a range of
wavelengths around a
central wavelength. The effect of this sequence of wavelength dependent
filters is to
selectively attenuate a corresponding number of wavelength bands across the
operational
portion of the spectrum, leaving other wavelength ranges unattenuated. The
pattern of
attenuated wavelength bands dispersed over the spectrum and interleaved with
unattenuated wavelength bands represents the code for that channel.
Reception of the encoded signal is performed using a technique that detects
the
pattern of attenuated wavelength bands and which accommodates the fact that
the received
optical signal can reliably provide only amplitude and wavelength (or
frequency) information.
As discussed above in the Background, CDMA techniques such as those used in
cellular
I S telephones rely on the use of a well designed set of codes which take
advantage of phase to
reduce noise in the system. Phase information is generally not available in
optical systems
where transmission occurs over most types of optical fibers or over sufficient
distances.
Preferred embodiments of the present invention overcome the lack of phase
information by a
differential detection and noise cancellation process.
At the receiver, the channel to be detected is discriminated on the basis of
the
wavelength modulation that represents the code for that channel. The signal is
received over
an optical fiber and the received signal is split into two equal components
and provided to
two receiver fibers. The two split off components pass through decoding masks
in each of the
two receiver fibers. Most preferably, the decoding masks provided in the
respective fibers are
constructed in a manner similar to that of the encoding mask and thus in this
embodiment
the decoding masks each preferably consist of a series of wavelength dependent
filters that
selectively attenuate wavelength bands within the received light. In
accordance with the
preferred detection technique, the first of the decoding masks is identical to
the encoding
mask and the second of the decoding masks is the opposite of the encoding
mask. The
3o signals from the two decoding channels are then input to the differential
inputs of a
differential detector such as an arrangement of back to back diodes. Here, a
mask that is the
opposite of the encoding mask may be one that attenuates the wavelength bands
not
attenuated by the encoding mask and passes those wavelength bands attenuated
by the
encoding mask. An "opposite" mask is also known as a complementary mask here.
Such a
detection scheme, when coupled with an appropriate code set, provides highly
effective
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discrimination of a desired channel from among other channels and with good
noise rejection
characteristics.
Embodiments of an optical communication system in accordance with the present
invention use at least one or, more preferably, a plurality of matched
broadband optical
sources, with one source provided for each of the channels desired for the
optical
communication system. Each transmission channel of the optical communication
system is
defined by a code unique to that particular channel. The code that identifies
the channel is
embodied in an encoding mask that modulates the broadband input light signal
as a function
of wavelength when the input light signal passes through the mask. This mask
may, for
example, be similar to the spatial modulators illustrated in FIGS. 1 and 3 in
that it imparts
an amplitude modulation to the broadband light as a function of wavelength.
Masks in
accordance with the present invention are different from the encoding and
decoding masks of
FIGS. 1-3 in that the encoding and decoding masks preferred here do not
operate on a
spatially spread optical signal. Rather, encoding and decoding masks in
accordance with
preferred embodiments of the present invention operate on broad spectrum beams
of light
without spreading or dispersion in frequency or wavelength.
The spatial modulators illustrated in FIGS. 1 and 3 modulate all of the
wavelengths of
the broadband light in parallel. By contrast, preferred embodiments of the
present invention
modulate the various wavelengths of this invention's broadband light source in
series by
passing the light source through a series of cascaded filters. Each of the
filters may be
specific to a particular narrow band of wavelengths and may, in a particularly
simple
example, either allow the selected wavelength band to pass or strongly
attenuate the light
within the wavelength band. The total filter array is made up of, for example,
128 filters,
each covering a band of wavelengths adjacent to the wavelength band covered by
at least one
adjacent filter so that the spectrum of the broadband light source is covered
by the series of
filters.
The cascaded filter assembly is illustrated simplistically in FIGS. 4A-4D, in
which a
small portion of the operationally available spectrum is shown schematically
(FIG. 4A) before
passing through a mask (FIG. 4B) which modulates the input signal with a code
defined as a
binary function of the wavelength (FIG. 4C). The modulated signal of FIG. 4D
is produced
by passing the input signal of FIG. 4A through the mask illustrated in FIG.
4B. Note that
the intensity of the modulated optical signal varies from a high level of I1,
less than the input
intensity of Io, due to the losses associated with the unattenuated portions
of the beam
passing through the cascaded series of filters. The low level of IZ is reduced
by 25 dB or more
with respect to I1. As illustrated in FIG. 4C, the code defined by the
cascaded filter array can
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be viewed as a sequence of binary values defined across wavelength ranges
centered about
increasing central wavelengths. The complement to the mask illustrated within
FIGS. 4B
and 4C replaces the zeros with ones (gratings replaced by an absence of a
grating) and ones
with zeros (absence of a grating replaced by a grating effective at that
wavelength).
It should also be noted that the distribution of the gratings is illustrative
of how the
gratings are spread in wavelength. As a practical matter, there is no need for
the gratings to
be spaced apart as illustrated.
The input spectrum illustrated in FIG. 4A is a simplification of the spectral
intensity
distribution of optical sources such as erbium doped fiber diodes, super
luminescent diodes or
other broadband light sources appropriate for use in the optical communication
system. The
mask shown in FIG. 4B includes a set of optical elements intended to attenuate
a set of the
input wavelengths including ~,,, 7~9, ~,4 and ~..,. Most preferably the
optical elements attenuate
the light within the wavelength band essentially uniformly across the
wavelength band.
Appropriate attenuating optical elements include Bragg gratings formed within
the optical
fiber and designed to block a wavelength band centered on the wavelength ~.N.
The
transmission characteristics of Bragg gratings are illustrated schematically
in FIG. 5, which
shows the characteristics of a single fiber grating of what is typically an
array of fiber
gratings within the fiber. Fibers having arrays of gratings with the
illustrated
characteristics are commercially available from Uniphase Corporation of San
Jose,
California. As illustrated, Bragg gratings can provide a high level of
attenuation, for example
of 25 dB or more, within a well-defined wavelength band. It should be noted
that Bragg
gratings are used either in a transmission mode or in a reflection mode.
Either mode of
operation can produce acceptable performance within embodiments of the present
invention.
Bragg gratings typically are formed as a regular array of step-wise variations
in the
index of refraction within the core of the optical fiber. The core and
cladding of an optical
fiber, illustrated schematically in FIG. 6, are distinguished by differences
in the index of
refraction between the core and cladding, typically caused by differences in
the composition
of the core and cladding. FIG. 7 schematically illustrates the step-wise
variations in the
index of refraction associated with the grating. The illustrated variations in
the index of
refraction are with respect to the average index of refraction through the
core of the fiber.
The formation of Bragg gratings is well understood in the art, as reflected by
U.S. Patent No.
5,235,659 to Atkins, et al., U.S. Patent No. 5,287,427 to Atkins, et al., U.S.
Patent No.
5,327,515 to Anderson, et al., U.S. Patent No. 5,457,760 to Mizrahi and U.S.
Patent No.
5,475,780 to Mizrahi. Each of these patents is incorporated herein by
reference for their
teachings in the use and formation of gratings within waveguides including
optical fibers.
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As is well known in the art, the central wavelength (7~N in FIG. 5) at which
the Bragg
grating is effective is determined by the spacing between the individual
variations in the
index of refraction (FIG. 7). The width of the wavelength band is defined by
the sharpness of
the transitions in the index of refraction. Together these considerations
define the number
of wavelength bands that can be defined across the spectrum of the input beam.
If the
optical techniques of defining Bragg gratings described in the listed patents
and those
commercially available are inadequate to define a sufficient number of
wavelength bands for
a desired number of channels on the system, then other waveguide
configurations might be
used. For example, very fine features can be defined either in silica/silicon
systems or in
l0 doped lithium niobate systems using x-ray and e-beam lithography
techniques. Such
techniques can improve the density of modulations in the index of refraction
and the
sharpness of the individual variations within the core of the fiber.
After encoding with the mask, the modulated broadband signal is passed to the
optical
communication system, which is typically an optical fiber. Signals encoded in
this manner
may be transmitted over a variety of optical systems including, for example,
single mode
fibers, multi-mode fibers, polarization preserving fibers and through free
space. As a
practical matter, most applications presently available for this system are
over fiber
networks using single mode fibers and so that operating environment is
primarily described
herein.
Decoding or demodulation is performed by coupling a fiber carrying at least
the
desired or target signal into a receiver or demodulator. The received signal
is split into two
parts by a splitter. The received light signal passes through two decoding
masks. One
decoding mask is the same as the encoding mask while the other decoding mask
is the bit-
wise complement of the encoding mask, or otherwise the complement of the
encoder mask if
the code is, for example, analog. The two channels of decoded light signals
are preferably
converted to electrical signals by differential detection. For example, the
decoded light
signals are provided to a differential detector such as a pair of back to back
diodes. The
differentially detected signal is then subjected to square law detection to
remove noise
signals.
Within the illustrative embodiments described here, it is possible to provide
an L
position mask (defined in terms of wavelength bands) to define L-1
communication channels
for a total of L-1 users. Preferably, the mask is a binary mask of the type
that can be
analyzed as a series of optical states corresponding to a series of 0's and
1's. In some
embodiments, the masks in the encoder and decoder include unipolar binary
codes
comprising 0's and 1's such as Walsh codes. The desirable properties of such
codes and the
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design of such codes are also discussed in the applications identified and
incorporated by
reference above. Other codes are believed to be viable in this system, but the
referenced
Walsh codes are believed to provide particularly significant advantage to the
present
invention.
The optical CDMA communication system according to the present invention may
be
applied in optical communication systems such as telecommunication systems,
cable
television systems, local area networks (LANs), as fiber backbone links within
communication networks, and other high bandwidth applications. FIG. 8
illustrates the
architecture of an exemplary optical communication system in which the present
invention
may be applied. A plurality of pairs of users s11, s~z, szu szz, . . . sN~,
sNZ are connected to an
optical fiber medium 130. The first group of users sll, szl, . . . sNl may be
proximately located
and coupled to the fiber 130 in a star configuration, and the second group of
users slz, szz, . . .
sNZ may be proximately located but remote from the first group and coupled to
the fiber 130
in a star configuration. Alternatively, the users in the first group or the
second group or
both may be coupled to the fiber 130 at separate and distributed points, as
shown in FIG. 9.
The architecture of FIG. 8 may be more appropriate, for example, for a fiber
backbone,
whereas the architecture of FIG. 9 may be more appropriate for a computer
system
interconnected over a medium area network.
Pairs of users s~,, sz communicate with each other using a channel of the
optical fiber,
and different pairs of users may simultaneously communicate over the same
optical fiber.
Each pair of users (s~l, s~z) is assigned a code u~ for transmitting and
receiving data between
the two users, and different pairs of users are preferably assigned different
codes. The
transmitting user in a user pair, e.g., s~l, encodes the optical signal using
the code u~ assigned
to the user pair (s~l, s~2), and the receiving user s~z in the pair decodes
the optical signal using
the same code u~. This architecture may be used, for example, for a fiber
optic backbone of a
communication network. The embodiments of the present invention are described
as they
may be applied in this network environment; other system architectures in
which the
invention is also applicable are described later.
FIG. 10 shows an embodiment of a CDMA modulator/encoder 140. A broad-spectrum
light source 142, such as a super luminescent diode (SLD) or erbium-doped
fiber source
(EDFS), is coupled to an optical modulator 144. The optical modulator 144
modulates the
broad-spectrum (broadband) light 146 from the optical source 142 based upon
data or other
information from a data source 148 using, for example, keying or pulse code
modulation. In
the case of a digital data source, the optical modulator 144 either passes or
blocks the light
146. Alternatively, an optical modulator 144 can be constructed that will pass
portions of the
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light 146 in response to an analog source 148. Encoder 150 then wavelength
encodes, i.e.,
CDMA encodes, the modulated broad-spectrum light beam 152 for injection into
the fiber
162, which may be a single mode optical fiber. An optical coupler 164 such as
a star coupler,
a Y coupler or the like is used to couple the encoded beam into the fiber 166,
which may be a
portion of an optical network. Alternatively, the light beam may be first
encoded with the
encoder 150 and then modulated by the modulator 144. Due to the orthogonal
nature of the
CDMA codes selected, a plurality of CDMA modulator/encoders 140-1, 140-N can
be coupled
to the same star coupler 164 and their outputs can be distinguished, i.e.,
demodulated or
decoded, at a distant site.
1o With reference to FIG. 11, dotted line 168 corresponds to the broadband
spectrum of
the light beam 146. To perform CDMA encoding, the encoder 150 must selectively
pass (or,
conversely, reflect or block) portions of the broadband spectrum. Embodiments
of the
present invention perform this task with an encoder 150 that is implemented as
a fiber
grating filter device 170. The fiber grating filter device 170 is encoded with
a desired CDMA
15 code such that the "bit 0" chip is encoded as a fiber grating filter for
that chip frequency and
the absence of a fiber grating filter designates the "bit I" chip in the
sequence of chip
positions along the fiber. This fiber grating filter device 170 can be viewed
as a cascade of
many band-stop grating filters each corresponding to the CDMA code bit in the
spectral
domain. For example, FIG. 11 shows a 9-bit CDMA code 172 where the presence of
a grating
20 filter, e.g., filter 174, causes a particular wavelength or frequency band,
e.g., Wl,
corresponding to that grating filter to be reflected, and the absence of a
grating filter allows a
wavelength or frequency band, e.g., W3, to pass. Consequently, an exemplary
bandpassed
spectrum 176 can be achieved corresponding to the exemplary CDMA code 172.
Such a fiber
grating filter device 170 can be achieved from a series of grating filters,
e.g., fusion spliced in
25 series onto output fiber 162, formed in situ in a single filter assembly or
they can be
integrated together as one composite filter 178 (exemplified in FIG. 12). The
mask or filter
assembly illustrated in FIG. 12 represents the Fourier transform of the
cascaded filter
assembly illustrated in FIG. 11. Decoding may also be accomplished with an
integrated mask
or filter assembly as illustrated in FIG. 12 by using the illustrated filter
within one of the
30 receiver fibers and the complement (Fourier inverse) of the FIG. 12 filter
in the other of the
receiver fibers. The complement of the FIG. 12 filter might alternately be
calculated as the
Fourier transform of the complement of the filters of FIG. 11.
An encoder 150 of the present invention, i.e., one implemented as a fiber
grating filter
device 170, avoids the critical alignment and opto-mechanical mounts typically
required by
35 the devices described in reference to FIGS 1-3. Accordingly, the present
invention may
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facilitate reduced manufacturing costs. Because the optical system is
integrated wholly
within fibers, the typical optical losses associated with passing through
multiple interfaces
and inser ting optical signals into fibers are significantly reduced.
Consequently, the system
exhibits very low levels of optical loss, typically less than 0.2 dB. The
device is low weight
and compatible with a range of commercial environments. To the extent that
lengths of fiber
are needed to accomplish a particular modulation or demodulation function, the
fiber can be
coiled so that the required optical path takes up very little actual space.
The low weight,
compact size and commercial, rugged nature stands in real contrast to systems
such as those
illustrated in FIGS. 1-3, which are largely limited to use in laboratory
environments.
The implementation and use of similar grating filters (restricted to a WDM
environment) is described in U.S. Patent No. 5,457,760 to Mizrahi which is
incorporated
herein by reference. Mizrahi describes formation of a Bragg grating filter by
exposing an
optical fiber to an interference pattern of actinic, typically ultraviolet,
radiation to form
refractive index perturbations in the fiber core. Alternatively, the optical
fiber is exposed to
an interference pattern created by impinging a single actinic beam on a phase
mask.
Typically, such phase masks are manufactured by reactive ion etching of a
fused-quartz
substrate through a chromium mask pattern by electron-beam lithography.
However, since
Mizrahi only shows the use of such devices in a WDM environment, Mizrahi only
shows
devices manufactured for passing a single portion of a broadband signal. In
constrast,
embodiments of the present invention typically pass multiple portions of a
broadband signal.
For example, in a preferred implementation, the broadband signal is divided
into 128
different wavelength or frequency band portions, e.g., Wl-W,~, and each fiber
grating filter
device 170 is fabricated to pass two or more of these frequency band portions.
FIG. 13 shows a compatible CDMA decoder 180. Light signals 182 containing a
potential plurality of spread spectrum signals are diverted from the fiber 166
using an optical
coupler 184 (e.g., a star coupler), and split into two beams 186 and 188
through a beam
splitter 190. The beam sputter 190 need not be a polarization insensitive
element since the
input beams are unpolarized on exiting the fiber network. A first split off
beam 186 is
filtered by a first fiber grating filter device 192 and the second split off
beam 188 is filtered
by a second fiber grating filter device 194. Preferably, in a binary, unipolar
embodiment of
the decoder, this second fiber grating filter device 194 is formed as the bit-
wise complement
of the first fiber grating filter device 192, i.e., grating filter elements
exist in the second fiber
grating filter device 194 corresponding to each frequency that the first fiber
grating filter
device 192 passes and, conversely, fiber grating elements are absent for each
frequency that
the first fiber grating filter device 192 blocks. The beam 196, after being
passed through the
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first fiber grating filter device 192 may then be supplied to a first
photodetector 198 to
convert the light into an electrical signal. Similarly, the beam output 200
from the second
fiber grating filter device 194 is supplied to a second photodetector 202 to
convert the light
into an electrical signal. Preferably, the two electrical signals are then
subtracted by the
back-to-back arrangement of the two photodetector diodes, 198 and 202, for
being supplied to
data and clock recovery hardware and/or software 204.
Alternatively, an operational amplifier or equivalent device can be used as a
differential detector 206 to generate a difference signal from the two fiber
grating filter
devices 192 and 194. The two electrical signals may also be separately
processed by two gain
control circuits, respectively, to adjust for different losses in the two
channels 186 and 188,
before a difference calculation is performed. The differential electrical
signal is then detected
for data recovery. Data recovery for digital data streams may include, for
example,
integrating and square-law detecting the difference signal. Data recovery for
analog signals
provided by analog code mask embodiments of the invention may include, for
example, low-
pass filtering the difference signal.
As discussed above, it is preferred that the optical communication system be
provided
with a plurality of different broadband optical sources, with a single source
preferably
provided for each channel of the optical transmission system. Most preferably
this does not
mean that an EDFS or SLD is provided for each channel. These sources are
expensive and it
is possible to provide the desired set of matched sources in a more economical
manner. FIG.
14 illustrates a particularly preferred configuration for generating a
sufficient number of
optical sources having well matched intensity distributions in a cost-
effective manner using a
single erbium-doped fiber source. A single erbium doped fiber source 300
outputs light with
an acceptably broad spectrum, generally providing a bandwidth of about 28
manometers in
wavelength over which the intensity of the source varies by less than about 5
dB. The 28-
nanometer bandwidth corresponds to a system bandwidth of about 3.5 THz. The
output of
the erbium-doped fiber source, also known as a super luminescent fiber source,
is provided
over a fiber to a splitter such as a star coupler 302 which splits the input
source signal and
provides the output over four fibers to an array of four fiber amplifiers 304.
As the output of the fiber source 300 is split into four different sources,
the intensity
drops in the expected manner. Each of the four split off sources is thus
amplified by the four
fiber amplifiers to provide four broad-spectrum light beams preferably each
having an
intensity approximately equal to the original source 300 intensity. For the
illustrated 128
channel system, this process is repeated through several further hierarchical
stages. Thus,
the outputs from the four fiber amplifiers 304 are provided over fibers to a
corresponding set
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of four splitters 306, which may also be star couplers. The splitters 306
split the output from
the fiber amplifiers into a plurality of outputs also of reduced intensity.
The split off output
from the splitters 306 are then provided to a further array of fiber
amplifiers 308, which
preferably amplify the intensity of the plural channels of broad-spectrum
light to provide a
next set of source light beams 310 having an appropriate intensity. This
process is repeated
until a sufficient number of broad-spectrum sources having an appropriate
intensity are
generated, for example 128 independent sources for the illustrative 128-
channel fiber
communication system. This hierarchical arrangement is preferred as using a
single
originating source and a number of fiber amplifiers to obtain the desired set
of broad-
spectrum light sources, which advantageously takes advantage of the lower
price of fiber
amplifiers as compared to the fiber source.
After sufficient channels of source light have been generated, the channels of
source
light are provided to an array of spatial light modulators or encoders like
that shown in FIG.
11. The 128 different encoders use a 128-bin mask to spatially encode the
input light signal,
with each of the 128 masks presenting a different one of a unipolar Hadamard
code vector
generated in the manner discussed in the applications referenced and
incorporated by
reference above. Most preferably, each of the masks is a fixed mask for use in
a transmission
mode, with the mask having a total of 128 equal sized bins, with the bins
spanning the usable
spectrum of the cascaded filter mask. Thus, the 128 bins span a total of about
3.5 THz (28
nanometers) in bandwidth, with each adjacent bin defining a subsequent
wavelength or
frequency interval providing about 25 GHz of bandwidth. Each of the equal
sized bins of the
fixed mask is assigned according to the code vector to have one or the other
of two binary
values. Each of the 128 channels of the communication system is then defined
by a distinct
wavelength or frequency encoding function and each of the channels is also
modulated with a
time-domain signal, for example using a modulator 144 like that shown in FIG.
10. After the
various channels are modulated both in wavelength (equivalently, frequency)
and
temporally, the 128 channels are combined and injected into a fiber.
Long distance transmission for this fiber communication system is managed in a
manner similar to the manner other conventional fiber communication systems
are
managed. As is conventional, it is typical to use a single mode fiber. In
addition, the signals
on the fiber will undergo dispersion and losses. It is preferable that the
signals on the fiber
be amplified using a conventional fiber doped amplifier at regular intervals,
for example,
every forty to eighty kilometers.
At the other end of the transmission fiber, the combined light signals are
split,
amplified, and provided to an array of 128 receivers, each corresponding to
one of the fixed
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mask channels defined by the 128 transmitters coupled into the fiber. The
primary purpose
of the illustrated embodiment is to expand the usage or loading on the fiber,
so the receivers
also include fixed masks so that each receiver is dedicated to a single one of
the 128 channels.
The receivers 140, as shown in FIG. 13, are each dedicated to a particular
channel defined by
a particular transmitter by including within the receiver one mask identical
to the
transmitter mask and a second mask that is the bit-wise complement of the
transmitter
mask.
In the illustrated optical CDMA system, it is very desirable to reduce the
interference
between different channels of users or of different multiplexed signals so
that a greater
number of channels can be provided over a single fiber. Various mechanisms
have been
identified to perform this task and are described in the present application
and in the other
applications incorporated by reference herein. A fundamental way in which the
present
system reduces interference is by injecting light into the optical
communication system only
to indicate one binary state. The source is modulated so that the source
produces an output
intensity to indicate one logical binary state, for example, a logical 1. No
light is provided to
indicate a logical 0. This has the effect of reducing the overall interference
in the system. Of
course, the particularly preferred coding scheme, including the receiving
system including
different channels with complementary filtering functions, provides a very
significant and
basic mechanism for reducing interference.
The preferred electrical system, illustrated schematically in FIG. 15, also
provides a
mechanism for reducing interference. The subsystem illustrated in FIG. 15
provides further
detail on the back-to-back diode arrangement indicated at 198, 202 in FIG. 13.
The two
complementarily filtered optical signals are provided to the back-to-back
diodes 198, 202,
which effect both a square law optical detection and also a differential
amplification function.
Other combinations of optical detectors, difference detection and electrical
amplification are
known and might well be substituted for these functions. In particularly
preferred
embodiments of the present invention, the electrical output signal 200 from
the diode pair
198, 202 is then low pass filtered by filter 380. The low pass filtering is
performed to remove
high frequency noise signals. In the illustrated system which might receive
one of plural
channels video data from the optical communication system at a data rate of
approximately
622 MHz, the filtering might pass frequencies below about 630-650 MHz. The
filtered
electrical signal is then provided to an electrical square law circuit element
382 such as a
diode. This square law element or limiter preferably removes the negative
going portions of
the received electrical signal and might also be used to amplify the positive
going portions of
the received electrical signal. The negative going portions of the electrical
signal are
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immediately identifiable as noise and so can be removed to improve the signal
to noise ratio
of the overall system. The electrical signal output from the limiter 382 is
then analyzed to
detect signals above a threshold value, which signals are recognized as
transmitted logical
ones.
A particularly desirable and particularly economical implementation of
multiple
sources having desirable spectral similarities is to provide a single
originating light source
that is coupled to a fiber, where the output of the source is split, for
example into four
components by a star splitter. Each of the split off components is then
amplified to an
appropriate level and then each of the split off and amplified components is
provided to a
l0 separate star splitter. A hierarchical structure of an original source that
is split and
amplified, with each successive source channel being split and amplified, can
be used to
develop a great many sources having essentially identical spectral
characteristics.
Another method of reducing interference is to reduce the correlation between
different noise signals. A difficulty observed by the present inventors when
implementing
the source strategy shown in FIG. 14 is an undesired level of temporal
correlation between
the different sources. This level of correlation can give rise to undesirable
levels of
correlation of noise sources or of correlation between the different
communication channels
associated with the different sources. Consequently, preferred embodiments
decorrelate the
different sources. This might be accomplished by inserting different optical
delays along
each of the output paths of the different source channels. A large number of
distinct sources
400-403 are defined, for example using the technique illustrated in FIG. 14
and discussed
above, so that the sources provide similar optical outputs with similar
spectral bandwidths
and spectral power distribution. While four sources are shown, the system will
typically
include 128 or more total sources corresponding to 128 or more users.
The outputs of each of the sources 400-403 are passed through a delay to
reduce the
temporal correlation between the different sources. Such optical delays could
consist of
optical delay lines or extended optical propagation paths. Causing each of the
sources to pass
through different lengths of fiber delay lines is the most preferred mechanism
for providing
appropriate delays. Delays might alternately be generated using free space
propagation
through different optical paths. Fiber delays are preferred since they can be
implemented
using only minimal space, so that the overall optical system can be provided
in a sufficiently
small space as to allow a wider range of implementations for systems embodying
this aspect
of the present invention. Referring once again to FIG. 16, appropriate delays
are effected by
passing the output of each of the sources 400-403 through different lengths of
single mode
fibers 404-407. The different length fibers are selected to impose a delay of
between about
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CA 02338027 2001-O1-17
WO 00/76102 PCT/US00/12235
one and about two or more times the data rate on successive sources.
Considering a data
rate of approximately 622 Mbt/sec, an appropriate delay can be fashioned by
adding about
one and a half feet of optical fiber (equivalent to --1.5 GHz) for each
desired delay. Thus, for
the first source 400, no additional length of fiber would be added as this
represents the
baseline. For the second source 401, 1.5 feet of additional fiber 405 would be
included in the
output path and for the third source 402, a three foot length of fiber 406
beyond the baseline
length of fiber 404 is provided. Similarly, the output from source 403 is
coupled through a
fiber 407 that is about 4.5 feet (--4.5 GHz) longer than fiber 404. Each of
the users within a
system, which may total 128 users or more or equivalently might total 128
channels of
multiplexed data, is provided with a source originating from a central source
and delayed by
an amount different from all of the other sources. It will of course be
appreciated that
different mechanisms for achieving optical delays are known and could be
practiced to
achieve similar results.
Another method of reducing interference, and one that has been observed to be
particularly effective, is the use of a data modulation scheme that limits the
amount of time
that the source is maintained in an on state. Time domain modulated data are
provided to
the optical communication system by modulating the sources. Sources may be
directly
modulated or may be modulated by passing the source light through an element
that can
modulate the source. In preferred embodiments of the present invention,
modulation is
accomplished so that a light pulse of predetermined intensity is provided to
the optical
system when one binary value is to be transmitted and no light is provided to
the optical
system when the other binary value is to be transmitted. A schematic example
of the
modulation of a source with a data stream is shown in FIG. 10.
While the present invention has been described with particular emphasis on
certain
preferred embodiments of the present invention, the present invention is not
limited to the
particular embodiments described herein. Those of ordinary skill will
appreciate that certain
modifications and variations might be made to the particular embodiments of
the present
invention while remaining within the teachings of the present invention. For
example, while
the above embodiments have been presented in terms of communications systems
mediated
over fiber, aspects of the present invention are immediately used in an over
the air optical
system. Additionally, while the description has primarily described the use of
fiber grating
filter devices in transmitting unipolar CDMA-encoded signals, the use of these
fiber grating
filter devices is equally applicable to a bipolar system. As such, the scope
of the present
invention is to be determined by the following claims.
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