Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
1. (" field of tbe,~nvention
This invention relates to optical communication systems and, more
particularly, to
optical code-division multiple access communications systems that transmit
data over optical
fibers.
2. DescriQlion 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
progamming
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 use, 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 communication system. There
is
consequently considerable interest in obtaining higher utilization of fiber
networks or
otherwise increasing the bandwidth of 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 .channel on an optical carrier signal consisting of a
single, narrow
wavelength band and multiple users access the fiber using time-division
multiplexing
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(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 TDMA requires short-pulsed 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 ri~ansmission
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,
l0 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
15 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 to provide expanded
bandwidth.
Information is transmitted independently in each of the optical channels using
a light beam
20 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
25 data stream or a further plurality of communication channels defined by
TDM. Little
interference wil! 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
30 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. At least theoretically, the availability of appropriately
tuned optical
sources limit the number of users that can be supported by a WDM system.
Wavelength
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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 l28 different channels are defined using i28 different lasers operating on
l28 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. Lasers appropriate to the WDM video distribution system are
individually
l0 expensive so that the requirements for 128 lasers having the desired
operational
characteristics make 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.
1 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. Unlike the WDM many laser system of U.S. Patent No. 5,579, i43,
embodiments of the present invention may be sufficiently flexible and cost
effective to be
used in at least some types of local area networks.
20 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
25 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 multi-dimensional coding techniques can be
used without
30 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
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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
S 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 Enoding of Noncoherent
Sources," ,('Z
I O Leg twavr,~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
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,
incoherent
I S source l2 such as an edge-emitting LED, super luminescent diode or an
erbium-doped feber
amplifier. In the illustrated CDMA system, the broadband source is modulated
with a time-
domain data stream 10. The time domain modulated broad-spectrum light 14 is
directed into
a spatial (fight modulator 16 by a mirror 18 or other beam steering optics.
Within the spatial light modulator l6, light beam 20 is incident on a grating
22,
20 which spatially spreads the spectrum of the light to produce a beam of
light 24 having its
various 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 fclters the incident light. Light
spatially filtered by
the mask 28 passes through a second spherical lens 30 onto a second
diffractive grating 34,
25 which recombines the light. Mask 28 is positioned midway between the pair
of confocal
lenses 26, 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
30 modulation effects a modulation in the wavelength of the light or,
equivalently, in the
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
mufti-channel transmission system.
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After passing through the mask 28, the spatially modulated light passes
through the
lens 30 and the wavelength modulated light beam 32 is then spectrally
condensed by the
second grating 34. The wavelength modulated and spectrally 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
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
l0 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 within the
optical network. As set forth in the Kavehrad article, it is important for the
mask 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
1 S 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 frequency or wavelength modulation characteristic
of the
20 transmitter mask 28 and rejecting signals having different characteristic
frequency
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
modulator 46 identical to the spatial tight modulator 16 and the second
receiver channel
includes a spatial light modulator 48 of similar construction to the
transmitter's spatial light
25 modulator 16, but having a mask the ''opposite" of the transmitter mask 28.
Each of the
spatial light modulators 4b, 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 tight 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
30 originally transmitted data 58 are retrieved.
FIG. 2 provides an illustration of the receiver circuitry in greater detail.
In this
illustration, spatial light modulators 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
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coupler 62, with a portion of the light directed into spatial light modulator
46 and
another portion of the light directed into the other spatial light modulator
48 using
mirror 64. Spatial light modulator 46 filters the received light 60 using the
same
spatial (frequency, wavelength) modulation function as is used in the
transmitter's
spatial light modulator l6 and provides the filtered light to photodetector
50. Spatial
light modulator 48 filters the received light using a complementary spatial
filtering
function and provides the output to the detector 52. Amplifier 54 provides
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 modulator
46 includes
a mask 66 identical to the transmitter mask 28. Spatial light modulator 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 modulators 16, 46. In the Kavehrad article, each of
these
masks 16, 66, 68 is a liquid crystal element so that the masks are fully
programmable.
l S The particular codes embodied in the masks must be appropriate to the
proposed
optical application. Although CDMA has been widely used in radio frequency
(ItF) 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
~0 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 tv optical
systems in which
an incoherent light source and direct detection (i.e., square-law detection of
the intensity
25 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 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
30 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 ZN~and that a system including such codes
could support
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a total of N-1 users. The ICavehrad article addresses only a theoretical
application of a
CDMA system, with little discussion of the implementation of such a system.
A more practical example of an optical CDMA system including 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", E(ec. 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 a(., and this work is collectively referenced herein as
the Young
patent. In this system, schematically illustrated in FIG. 3, the transmitter
80 employs a broad
spectrum light source 82 which is split by a beam splitter 84 into two beams
86 and 88 to be
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
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" state 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 unipo(ar 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 1 l0 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 l 18
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
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
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2N, only N codes can be defined since the code U and its complement U* must be
concatenated on the mask.
Therefore, it is an object of the invention to provide a frequency-domain CDMA
encoding/decoding scheme and an optical communication system incorporating
such a
scheme where the number of users is maximized without raising interference
unduly. It is
another object of the invention to provide a system providing a relatively
simple system for
encoding and decoding the light but effciently using the entire spectrum
available.
Summary of the Preferred Embodiments
These and other objects are obtained by using a spatial encoder with binary or
analog
l0 encoding and a receiver. in particular, a broad-spectrum tight source is
modulated with data
to be transmitted. The modulated light beam is then dispersed using, for
example, a
diffraction grating and passed through a spatial spectrum-coding mask. The
spatial coding
mask presents a unipolar code belonging to a set of unipolar codes that are
preferably derived
from a set of balanced bipolar orthogonal codes. The dispersed frequencies of
the encoded
I 5 modulated light beam are then recombined to provide a modulated, encoded
spread spectrum
optical signal for injection into an optical fiber or another optical
communication system.
Recovery of the transmitted signal is through the use of a special matched
filter. At
any receiver, a beam separator, which is in some particularly preferred
embodiments a
polarization insensitive sputter, diverts part of the beam in the fiber
through a diffraction
20 grating to spatially separate the spectrum of the light in the fiber. The
spatially spread signal,
potentially comprising of spread spectrum optical communications, is passed to
a receiver
providing signal recovery. Most preferably, the optical signal is converted to
an electrical
signal by differential detection, the resulting electrical signal is
preferably low pass filtered
and then, in particularly advantageous embodiments, the electrical signal is
provided to a
25 limiting element that removes the negative components of the electrical
signal. The
differential detection of the receiver can be implemented in a number of ways.
In one embodiment, the masks in the encoder and decoder include unipolar
binary
codes comprising 0's and 1's such as Walsh codes. The spatially spread light
will pass
through two decoding masks. One decoding mask is the same as the encoder mask
while the
30 other decoding mask is the bit-wise complement of the encoder mask, or
otherwise the
complement of the encoder mask if the code is, for example, analog. The
spatially spread
decoded light signals are combined, and the two channels of optical signals
are preferably
converted to electrical signals by differential detection. Within the
illustrative embodiments
described here, it is possible to provide~an L position mask to define L-1
communication
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channels for a total of L-1 users.
In another embodiment, the spatially spread light can be detected by an array
of
detectors. Each detector in the array measures the light power of the
corresponding optically
spread wave length and outputs a corresponding array of electrical signals.
The array of
electrical signals will then be processed by a digital signal processor (DSP).
The digital
signal processing includes multiplying the signal from each detector in the
array by a positive
or negative one depending on whether the encoder mask bit is a one
(transparent) or a zero
{opaque). The resulted bit products are then summed before thresholding for
data recovery.
This digital processing corresponds to multiplying the signal from individual
detectors in the
l0 array by the corresponding bit in a Hadamard code, which is the bipolar
version of the Walsh
encoder code.
The coding might also use unipolar analog codes. Here, analog coding means
that
the spatial encoder uses variable opacity masks as opposed to digital coding
(i.e., where the
spatial encoder uses masks with cells that are either transparent or opaque).
The code
15 preferably should use one of a set of unipolar wavelets derived from a set
of unique,
orthogonal wavelet functions such as cosine and/or sine waves, rectangular
waves or
Chebyshev polynomials. The unipolar wavelets are of course discrete functions
as opposed
to continuous functions due to the fact that the masks are not continuous but
comprise a
plurality of cells. In this embodiment, the wavelets are quantized or discrete
spatial sine
20 waves of various harmonic frequencies, which permits decoding to be done by
using a spatial
Fourier transform on the detected spatially spread light pattern. The limit on
the number of
codes is only based upon the effects of using discrete as opposed to
continuous harmonic sine
waves and the resolution of the receiver.
Particularly preferred receivers according to the present invention include at
the
25 input to the receiver a polarization insensitive beam separator. 'These
preferred receivers
separate the received light beam into two beams of sufficiently equal power
levels to allow
the preferred differential detection scheme of the optical CDMA receivers to
effectively
detect a desired user channel. An embodiment of a polarization insensitive
beam separator
might consist of a first polarization sensitive element that divides the
received light beam
30 into first and second channels of light with each channel having a
different one of two
orthogonal polarizations. For example, one channel of light might include the
vertically
polarized component of the received light beam and the other channel might
include the
horizontally polarized component of the received light beam. The polarization
of one of the
channels is then converted to the polarization of the other light beam. For
linearly polarized
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light this might consist of rotating the polarization of the light. The two
channels of light are
then recombined and provided to a beam splitter. This beam splitter is
typically a
polarization sensitive element that accurately splits the combined beams into
two beams of
substantially equal power because the polarization of the combined beams is
well defined
and predictable.
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
l0 used in the system of FIG. 1.
FIG. 3 illustrates an encoder for using bipolar codes in an optical CDMA
system.
FIGS. 4 & 5 present different configurations of an optical fiber network
according to
the present invention.
FIG. 6 is a block diagram of a first embodiment of an encoder according to the
1 S present invention.
FIG. 7 is a block diagram of a first embodiment of a decoder according to the
present
invention.
FIG. 8 is a block diagram of a second embodiment of a decoder according to the
present invention.
20 FIG. 9 is a sketch of a liquid crystal mask for use in a third embodiment
of an
encoder according to the present invention.
FIGS. I OA, B and C are continuous representations of discrete transparency
functions for the mask of FIG. 9.
FIG. 11 is a graphical representation of a Fourier transform of light received
from the
25 fiber.
FIGS. 12A and B schematically illustrate an encoder and a decoder according to
a
third embodiment of the invention.
FIGS. 13A, B and C present a graphical representation of a mask and mask
functions
according to a third embodiment of the invention.
30 FIG. l4 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 schematically illustrates a polarization insensitive beam separator
that is
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preferred in accordance with prcfcrred embodiments of the present invention.
FIG. 16 illusu'atos in greater detail the optical detcctlon circuitry
schematically
illustrated in FIG. 7.
Related Applications
The following applications arc related to the present application and are each
incorporated by reference in their entirety into this application:
"High Capacity Spread Spectrum Optical Communications System," 1'CT
application No.
WO-A-98!023057, published on August 8, 1997.
"Optical CDMA System using Sub-Band Coding," PCT application ao. WO-A-
00107319,
published on February l0, 2000.
"vlethod and Apparatus for Reduced Interference in Optical CDMA," PCT
application
no. WO-A-OOI07317, published on July 29, 1999.
i 5 Detailed Description of the Prelerr~ed Ernbodimenta
The present inventors have investigated both the thzoretical su~estions of the
Kavehrad article discussed above and the system described in the Young patent
and the related
articles discussed above. A number of aspects of these systeuis are
undesirable. As a
preliminary matter, the systems arc premised on the possibility of dividing
received light into
ZO two different beams of light with equal power levels. As a practical
matter, the polarizatioa of
the light beam received from an optical communication system such as an
optical fiber network
will not be known. Because beam splitters such as those shown in the Youno
patent and the
couple shown in the ICavehrad article are polarization sensitive, the power
will not be equally
divided between the two channels of the systems illustrated in those
refertnces. Without equal
2S or substantially equal power levels within the two channels of the
receiver, the subtraction of
the two signals will not adequately separate different user signals and the
receivers will not be
effective. Tod address this problem, particularly preferred receivers
according to the present
invention receive Iight beams and divide the light beams into differe:~t
channels using a
polarization insensitive beam separator. Incorporation of a polarization
insensitive beam
30 separator at the input to the rectiver makes the implementation of an
optical CDMA system
more praccical and significantly increases the number of users prnvlded on the
communication
system.
The Kavehrad anicle describes as optical CDbIA system in which coding pnsonted
on
masks is switehcd at the data rate of each channel of the system. As a
practical matter,
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such an implementation would require masks capable of switching at rates of at
least 500
MHz and preferably higher. As stated in the Kavehrad article, appropriate
masks do not exist
to implement this architecture and coding scheme. Preferred embodiments of the
present
invention provide a simpler coding scheme utilizing fixed or slowly changing
masks with
high-speed data modulation performed on input light sources.
Both the Kavehrad and Young systems suggest implementation of bipolar codes in
manners that require 2N-position mask to provide N-1 users on a system.
Particularly
preferred embodiments of the present invention utilize a coding scheme and a
detection
scheme that allows an N-position mask to define N-1 channels available to N-1
users. The
I O preferred coding scheme incorporates binary (0, 1 ) Hadamard code
sequences modified to a
unipolar form used in an encoding mask for transmitting data. The data can be
reliably
recovered if the data are recovered in an appropriate receiver. The receiver
should include
two detection channels with one channel including a mask identical to that of
the transmitter
and the second channel including a mask which is the bit-wise complement of
the
transmitter's mask. Most preferably, the receiver includes a polarization
insensitive beam
separator for defining the two receiving channels within the receiver. This
system has been
observed to provide reliable optical data transmission in a many user system.
Consequently,
some preferred embodiments of the present invention gain the advantages of a
bipolar coding
scheme like that described in the Young patent, but do so with a shorter code
sequence,
effectively doubling the bandwidth of the communication system.
A significant aspect of particularly preferred embodiments of the present
invention
provides receivers with a polarization insensitive beam separator that accepts
beams that
might be polarized in an unpredictable manner or might be unpolarized and
reliably splits the
received beams into two beams of equal power. To accomplish the differential
detection
preferred according to certain embodiments of the present invention, it is
desirable that the
received beam be reliably divided into two beams with substantially equal
power levels,
regardless of the polarization of the input beam. As a general matter,
conventional beam
splitters are polarization sensitive and will not reliably divide input light
beams into equal
power beams unless the polarization of the input beam is known beforehand.
Most practical
optical fiber transmission systems use optical fibers that do not preserve the
polarization of a
light beam. As such, it is not possible in most systems to predict the
polarization of a light
beam that is received from an optical CDMA fiber transmission system.
Particularly preferred embodiments of receivers in accordance with preferred
aspects
of the present invention provide a polarization insensitive beam sputter. A
polarization
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analyzer or a polarization sensitive beam splitter receives the light beam
from the optical
communication system. The outputs from the polarization analyzer or
polarization beam
splitter consist of two orthogonal polarization light beams and the light
beams are provided
along two different optical paths. The polarization of one of the optical
paths is altered to be
the other polarization of light. The two light beams, both now consisting of
light of the same
polarization with the polarization known, are then combined and split into two
equal power
beams using a conventional, polarization sensitive beam splitter. This
particularly preferred
aspect of the present invention is described in greater detail below and in
particular with
reference to FIG. 15 and its accompanying discussion.
Signals within the two channels of the receiver are preferably detected in a
differential fashion, for e~cample by coupling the light from each channel to
different ones of
a pair of photodiodes in a back-to-back configuration. The electrical output
from the
photodiodes will then be a difference measurement of the signal received in
the two
channels. In particularly preferred embodiments of the present invention, the
electrical
output signal is low pass filtered and then provided to an electrical square
law circuit element
such as a diode. This square law element or limner 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 signs!
are immediately identifiable as noise and so can be removed to improve the
signal to noise
ratio of the overall system.
The CDMA encoding/decoding scheme according to the present invention may be
applied in optical communication systems such as telecommunications systems,
cable
television systems, local area networks (LANs), as fiber backbone links within
communication networks, and other high bandwidth applications. FIG. 4
illustrates the
architecture of an exemplary optical communications system in which the
present invention
may be applied. A plurality of pairs of users s", s,~, sZ,, sZZ, . . . sN,,
sN2 are connected to an
optical fiber medium 130. The first group of users s", s=,, _ . . sN, may be
proximately located
and coupled to the fiber 130 in a star configuration, sad the second group of
users s,I, sZ=, . . .
sN= 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 l30 at separate and distributed points, as shown
in FIG. 5. The
architecture of FIG. 4 may be more appropriate, for example, for a fiber
backbone, whereas
the architecture of FIG. 5 may be more appropriate for a telephone system.
Pairs of users sJ,, sj~ communicate with each other using a channel of the
optical
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fiber, and different pairs of users may simultaneously communicate over the
same optical
fiber. Each pair of users (s~,, 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~,, encodes the optical signal
using the code u~
assigned to the user pair (s~,, s~~), 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. 6 shows a first embodiment 140 of a CDMA modulator/encoder. A broadband
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
modulates the light
from the optical source 142 based upon data or other information from the data
source 146,
using, for example, keying or pulse code modulation. Encoder 150, which is
similar to the
spatial light modulator l6 shown in FIG. 1 with the exception of the mask and
coding
scheme, then spatially encodes the modulated broad-spectrum light beam. The
encoder i 50
includes a diffraction grating 152 that spatially spreads the spectrum of the
modulated light
beam along an axis. The spatially spread light beam is coilimated by a
collimating lens 154
and then the collimated beam is passed through the encoding mask 156. 'The
encoding mask,
provides a spatially encoded, modulated beam of light that is collected by
collimating lens
158 and combined back to a broad spectrum beam by a diffraction grating 160
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 162.
Alternatively, the light beam may be first encoded with the encoder 150 and
then modulated
by the modulator 144.
FIG. 7 shows a compatible decoder, which has two channels 170 and 172. Light
signals containing a potential plurality of spread spectrum signals are
diverted from the fiber
162 using an optical coupler (not shown), and split into two beams through a
beam separator
174. The beam separator is most preferably a polarization insensitive element
like that
illustrated in FIG. 15 and discussed below with reference to that figure. One
incoming beam
is spread spatially along an axis by a diffraction grating 176 and is then
collimated by a
collimating lens 180 before being passed through a detection or decoding mask
184. The
decoding mask l84 is, in this illustrated preferred embodiment, identical to
the encoding
mask 156. Light passed through the decoding mask I 84 is passed through a
collimating lens
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l 88 and a diffraction grating 192 recombines the spatially spread light into
a broad spectrum
beam. In the other channel, the second component of the split, received beam
is spread
spatially by a diffraction grating 178 and is then collimated by a collimating
lens 182 before
being passed through a second decoding mask 186. Most preferably, in this
converted-binary
Hadamard code, unipolar embodiment of the decoder, this second decoding mask t
88 is the
bit-wise complement of the encoder mask 184. The beam, after being passed
through the
second decoding mask 186, is passed through the collimating lens 190 and a
diffraction
grating 194 to remove the spatial spreading. The output of the first decoder
channel ! 70 may
then be supplied to a photodetector 196 to convert the light into an
electrical signal.
l0 Similarly, the output from decoder channel 172 is supplied to a photo
detector 198 to convert
the light into an electrical signal. The two electrical signals are then
subtracted by the back-
to-back arrangement of the two detector diodes, l96 and 198 for being supplied
to data and
clock recovery hardware and/or software 200. The two electrical signals may
also be
separately processed by two gain control circuits, respectively, to adjust for
different losses
l5 in the two detector channels 170 and 172, 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.
20 FIG. 8 shows another embodiment of the decoder 210. In this embodiment, the
beam
of light received from the fiber is not split into two channels with two
masks, but is instead
spread by the grating 212 and is collimated by a lens 214. The collimated
light is then
intercepted by an array of detectors 216. The number of detectors in the array
is equal to the
number of bits in the encoder mask. Each detector position corresponds to the
encoder mask
25 bit position. The detector signal from each detector in the array is
multiplied by either "l" or
"-i" depending on whether the corresponding encoder mask bit is a
"transparent" or
"opaque." The results of all the multiplier outputs are then summed. The sum
is then
compared with a threshold 218 for data recovery. This digital processing can
be performed
in discrete logic hardware or in a DSP 220 using software. When an analog mask
is used for
30 encoding, the outputs of the detectors may also be multiplied by numbers
other than "1" or "-
I ". It should be noted that in both embodiments of FIGS. 6 and 7 only one
encoder mask is
used for transmitting data and no concatenated code is required in contrast
with prior art
designs.
The preferred encoding and decoding scheme according to the present invention
is
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explained next. As used in this specification, ''unipolar codes" means code
sequences
comprising 1's and 0's in the case of binary codes, or code functions having
values between 0
and 1 in the case of analog codes. "Bipolar codes" means codc sequences
comprising -1's
and 1's in the case of binary codes, or code functions having values between -
1 and 1 in the
case of analog codes. A complement of a unipolar binary code a is ( 1-u), i.e.
its bit-wise
complement in which 0's are substituted by 1's and 1's are substituted by 0's.
A complement
of a unipolar analog code f is (I-f). Unipolar binary codes are used as an
example in the
following description.
In a CDMA system, the basic requirement for a spectral encoding/decoding
scheme
l0 is that the decoding apparatus at a receiving user be able to recover data
signals firom the
corresponding transmitting user while reducing or eliminating interference
from signals from
all other users. For some systems, the receiving masks wilt be fixed as a
particular receiver
always receives the same channel of data. For other systems, the receiving
masks will be
programmable so that different signal sources can be selected from the many
possible
1 S sources. fn a spread spectrum CDMA system using an incoherent light
source, because an
incoherent optical system can only transmit positive signals (light
intensities), and no phase
information is available, only unipolar codes may be used for encoding. A
unipoiar binary
code may be represented by a sequence of binary digits, such as
u;=11001111010101 l, where
subscript i designates the i''' user pair (or channel). The number of digits
in the sequence, N,
20 is referred to as the length of the code. In practice, for the particularly
preferred binary
unipolar code mask, each of the code values corresponds to a fixed interval
slot, either
transparent or opaque, on the spatially patterned mask that in turn
corresponds to a fixed
frequency or wavelength interval in the spatially modulated broad spectrum
beam of light.
When a single mask is used for encoding and decoding, the codes are preferably
25 chosen such that they are orthogonal, or:
M iji j
(1) "' '; 0 ~j ~=i
where '' ~" denotes the bit-wise dot product of two codes, and M is a
constant. When
orthogonal codes are used, each transmitting user may transmit signals using a
single
encoding mask, and the corresponding receiving user may use a single decoding
mask
30 identical to the encoding mask to recover the signal from the corresponding
transmitting user
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while rejecting interfering signals from all other users. This desirable
outcome, however,
occurs only when the codes are chosen to as the binary basis vectors:
u~ = 000......001
ui = 000......010
uN = 100......000
This set of codes is undesirable in that, since only one digit of the entire
code is l, only one
frequency bin of mask passes power through it while the great majority of the
bins are
S blocked. Such a system can be viewed as an incoherent wave division multiple
access
(WDMA) system. Such codes are undesirable as only about 1/N ofthe source power
is
transmitted and the rest of it is wasted.
In the encoding and decoding system described in FIGS. 6 and 7, in which a
single
mask is used for encoding and two masks are used for decoding, a set of
unipolar codes may
be used such that although a code u; in the set is not orthogonal to other
codes u~ in the set
according to the definition of orthogonality set forth above. Rather, the code
u; is selected to
be orthogonal to the difference between any other code ui and its complement
ui*, i.e.
_ _ M~ iji=j
u' ~ (e a =) ~ 0 ij i~j
where M' is a constant.
l 5 It can be seen that the decoders of embodiments of FIGS. 7 and $ implement
the
principle of Eq. (2). In the embodiment of FIG. 7, the received light beam at
a user j contains
signals from all transmitting users i encoded with codes u;. The first channel
170 having the
mask 56 generates a light beam representing u;~u~, while the second channel
having the
complementary mask I72 generates a light beam representing ui~u~*, and the
dit~erentially
arranged detectors 62 and 63 generates the difference signal u;~(u~ u~*). In
the embodiment
of Fig. 8, the array of detectors 73 outpuu signals representing u;, and the
DSP 74 calculates
u;~(u~ u~*) based on the outputs of the detector array 73. According to Eq.
(2), the difference
signal u;~(u~ ua*) is non-zero only for the signal from the user that uses a
mask having a code
u;. Consequently, such decoders arc able to recover the signals from the
transmitting user i
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and reject signals from alt other users.
A set of unipolar code that satisfy Eq. (2) may be derived from a set of
balanced
bipolar binary orthogonal codes v; that satisfy the following conditions:
S (3)
o ~l ~ pi
and
(4)
v~ t = 0
where ''1" represents a code in which every digit is 1. The unipolar codes u;
are derived from
the bipolar codes v; by substituting the -1's in v, with 0's, or
IO (5)
u, = 2 (v~ ~ t )
The bipolar code v, is "balanced" in that they have equal numbers of 1's and -
1's (eq. (4)).
These particularly preferred unipolar codes u, thus have equal numbers of 1's
and 0's. As a
result, half of the light power may be transmitted as signals, thereby
promoting the efficient
utiliTation of the source power.
t 5 An example of balanced bipolar orthogonal code set is a code set based on
Hadamard
matrices. A Hadamard matrix is a square matrix the elements of which are 1's
or -1's such
that all rows are orthogonal to each other and all columns are orthogonal to
each other. For
example, a 4x4 Hadamard matrix may be:
t 1 1
t
t -tt
-t
1 l -t
-t
t -t-1
1
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The column (or row) vectors except the first column (or row) of a Hadamard
matrix provide
a set of balanced bipolar binary orthogonal codes satisfying Eqs. (3) and (4).
Thus, a set of
unipo[ar codes u,, uz, . . ., u" used in the uni-bipolar spread spectrum CDMA
system preferred
in accordance with the present invention may be constructed by first
constructing a
Hadamard matrix of size n+i or greater. Except the first column (or row),
every column (or
row) of this Hadamard matrix may be used to generate a unipolar code u~ by
replacing all -i's
with 0's.
For example, for a three-user system, the above 4X4 Hadamard matrix may be
used
to generate the following codes:
a~ _ [1 0 I 0]
u= _ [1 I 0 O]
ul = [1 0 0 11
1 I l
H ,I=
1 1
Although rules for constructing a general Hadamard matrices of arbitrary size
do not exist,
there are known methods for constructing Hadamard matrices of certain sizes.
For example,
Hadamard matrices having a size N that is a power of 2 may be constructed
from.Hz
using a recursive algorithm
N~ N
Hs~ = X. _N~
Rules for constructing matrices having a size N that is a factor of 4 are also
known.
l5 Although Eqs. (3) and (4) indicate that the bipolar code set used to
generate the
unipolar codes should be orthogonal and balanced, in practice, it may be
acceptable although
not desired to use code sets that are "near orthogonal" or "near balanced." A
code set is near
orthogonal when, for example, u;~u~ (i~j) is substantially smaller than u;~u;.
The codes are
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near balanced when, for example, u,~l is substantially smaller than N. For
example, when
the length N of the codes is large, changing a few digits of some codes in the
set may result
in a near orthogonal or near balanced code set. When the codes are not
perfectly orthogonal
or balanced but only near orthogonal or near balanced, interference from other
users may
increase and the system performance may deteriorate, but such deterioration
may be
acceptable so long as the overall system performance is acceptable. Thus, such
near
orthogonal or near balanced codes may be considered orthogonal or balanced for
the
purposes of the present invention and are within the scope thereof.
The coding masks 156, l 84, 186 in FIGS. 6 and 7 may be either transmissive or
reflective. As a practical matter, however, the present inventors have
observed that reflective
masks are harder to make and do not usually have a desirably large extinction
ratio. In some
embodiments, the masks are made of liquid crystal material as shown in FIG. 9
divided into a
plurality of cells "a" through "L", with L an arbitrary integer and being the
maximum .
permitted length of the code. Such LCD masks are commercially available or are
readily
l5 made using known technology. The cells form a one-dimensional array
arranged along the
axis 230 of spatial spectrum spreading caused by the diffraction grating 152.
In one
embodiment, the control of the cells is analog, meaning that the opacity of
each cell is either
infinitely adjustable or is adjustable in at least three or more separately
controllable stages.
Preferably a large number of finite stages, preferably sixty-four or greater
levels of opacity
should be used. In another embodiment, the control is binary, and Walsh codes
(unipolar
Hadamard) are used. These masks can be implemented by LCD pixel arrays or by a
photonic
integrated circuit such as a solid state amplifier array. Alternately, and
presently preferred
for systems where multiplexing of signals onto a fiber is most desirable, the
masks may be
fixed and formed on glass blanks. Such fixed masks are most preferably binary
masks
embodying unipolar Hademard codes. For a reflective mask, the glass may be BK7
or quartz
and the reflective regions could be gold. For the presently most preferred
fixed, binary,
transmission mask, the glass may still be BK7 or quartz and the blocking
regions could be
chrome. Generally masks are on the order of one to two inches across so that
readily
available technology can be used to define a mask having 128 different, equal
sized and
contiguous positions on the mask, as is presently contemplated for an OC-12
application of
the present invention. Masks with finer granularity with 256 or 512 positions
are readily
defined using available technology.
A preferred form of analog coding is using a set of unipolar wavelet functions
fi
derived from balanced bipolar orthogonal wavelet functions g; using
f;~g;+1)12. Eqs. (2) -
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(4), which are illustrated in the context of binary codes, apply equally to
analog codes. tn
other words, if the bipolar wavelet functions satisfy Eqs. {3) and (4), then
the derived
unipolar wavelet functions satisfy Eq. (2). In one embodiment, the wavelet
functions are
discrete harmonic spatial sine waves (represented for purposes of illustration
as continuous
functions) as shown in FIG. 5. The ordinate axis is the axis along which the
frequencies of
the beam are spread and the abscissa is the relative transparency of the beans
passing through
a cell. In particular, a ferst encoder mask transparency function shown in
FIG. l0A may have
a spatial frequency of I/L, where L is the number of cells. The mask of that
first encoder is a
discrete (as opposed to continuous) cosine wave in terms of transparency
having one cycle
0 over the frequency spectrum of L, such that the lowest and highest frequency
portion of the
encoded spectrum have the maximum intensity and the mid-range spectral
frequencies have
the lowest intensity. A second encoder mask may for example have a spatial
frequency
intensity mask of twice the frequency of the first encoder with two full
cycles across the
length of the encoder L of FIG. 108. Still further a third encoder may have a
frequency three
l 5 times the frequency of the first encoder as shown in FIG. l OC. Other
higher harmonics are
preferably used, and preferably to maximize the system throughput, the maximum
number of
codes should be over one hundred and preferably over several hundred far
higher usage
systems.
The maximum number of harmonics or Walsh code bits (and therefore, the
20 maximum number of codes) is limited only by the number of cells in the
mask. For the
analog mask, the number of different levels of opacity permitted in the mask,
results in the
quantization noise in the encoder. Alternatively, rather than using cosine
waves, Chebyshev
polynomials could also be used as they are orthogonal with respect to each
ocher.
Using cosine waves for the encoding function also permits an easier decoder
design.
25 In particular, if one takes the spatial Fourier transform of the received
signal, the received
signal can be separated through a spatial filter for the frequency of the
desired signal and
then that signal can be recovered. As a simple example, FIG. I l, shows the
Fourier
transform of a signal received from a fiber where the separate encoded signals
include I/L,
2/L, 41L and 8/L. Any one of these signals may be readily obtained by
filtering for that
30 particular spatial frequency in the received signal.
In a preferred third embodiment of the disclosed encoder, rather than pulse
code
modulate the data, an alternative method may be used for modulating signals
using two codes
as is shown in FIG. 12A. In this embodiment of an encoder 238, the optical
path for the
spatially spread light source 240 is switched between a first mask 242 and a
second mask
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244, which is complementary to the first mask 242, by a switcher 246
responsive to data
from a data source 248, the first mask encoding the light to provide a digital
"one" signal and
the second mask encoding the light to provide a digital "zero" signal for the
same code
channel. The modulator switches the light path between two different encoder
masks using
one liquid crystal in a manner similar to the binary mask receiver embodiment,
The light
from both masks is then summed by a summer 250 and then provided to the
optical
communications channel such as a optical fiber (noi shown).
Receiving data proceeds in the converse manner as shown in FIG. 12B. A decoder
260 receives light from the communications channel and generates ttie
spatially spread
spectrum of the received light with receiving input optics 262 through masks
264, 266 which
are identical to the mask 242 and the mask 244, respectively. The light from
the masks 264
and 266 is then provided to a differential receiver 268 in the manner
described above in the
binary receiver embodiment. The signal firom the receiver 268 may then be
processed by a
digital signal processor 270 for recovery of the data.
t 5 FIG. 13A shows one alternative embodiment of the masks appropriate for
coding
where two different masks are used for transmitting ones and zeros. In a first
version, the
mask formed of L cells in a liquid crystal mask 280 is divided into four
parts, 282, 284, 286
and 288. Parts 282 and 284 comprise L/2 cells each along a first linear array
arranged along
the axis of spreading of the spectrum on a first row to encode a "one" for
this particular code
channel and at a second column, cells 286 and 288 also comprise L/2 cells
arranged along the
same axis for encoding a "zero" for this same channel. Preferably, the
discrete transparency
functions for parts 282, 284 are the complements of each other such as shown
in FIG. 138,
where the ordinate represents spatial frequency and the abscissa represents
intensity. For
transmitting the other possibility (i.e. the zero), as shown in FIG. 13C, the
complements of
the discrete intensity functions for parts 286 and 288 are reversed. In other
words, the
portion of the mask in section 282 is identical to the portion of the mask in
288 and the
portion of the mask in 284 is identical to the portion of the mask in 286.
In addition to having masks where the coding is complementary, it is also
possible to
provide coding where a first portion 282 of the mask is the orthogonal wave
function and the
second half is all opaque for a "zero" 284 and the second level, the first
half 286 is all opaque
and the second half is the same pattern as the first half 282 to make a "one."
Alternatively,
the first halves 282, 286 can be a f rst polynomial such as a sine wave and
the second halves
284, 288 can be a second polynomial such as a Chebyshev function.
Although specific embodiments of encoders and decoders according to
embodiments
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of the invention are disclosed, other embodiments of the invention are also
possible. For
example, while discrete wavelet firnctions arc used for encoding, it is
possible to have masks
that permit continuous functions for coding. For example, the masks may be
formed
photographically.
The optical systems 150, I 70 and 172 in the encoder of FIG. 6 and decoder of
FIG. 7
may be generally referred to as optical chambers. An optical chamber, which
may be a set of
discrete optics or an integrated optical device, spectrally encodes an input
broad band optical
signal by selectively attenuating the spectral components of the signal
according to a ''code."
The code, which may be binary or analog, determines the degree of attenuation
of each
spectral components of the input signal. In the illustrated embodiments, the
optical chambers
are implemented with diffraction gratings, collimating lenses and an optical
mask having a
code, but other implementations are also possible.
Furthermore, it should also be understood that all of the disclosed
embodiments of
encoders and decoders can also be applied to analog modulation of the optical
signal.
Similarly, while only CDMA techniques have been described above, those of
ordinary skill in the field will readily understand that depending upon system
parameters, the
system may also be used with wavelength (frequency) division multiplexing and
time
division multiplexing. For example, different coding schemes may be used for
different
portions of the optical spectrum so that wavelength division multiplexing may
be used. In
addition, the codes may be shared on a time sharing basis to provide for time
division
multiplexing. .Also, optical spatial (frequency domain) CDMA can be combined
with time-
domain optical CDMA to increase the number of codes and the users in the
network. In the
time domain spread spectrum embodiments, several users are provided with
different time
domain spread spectrum codes for encoding the data before the data is provided
to the optical
encoder. However, these users can share the same wavelength encoding schemes
discussed
above. Of course, at the decoder, once the received optical information is
converted back
into the electrical digital domain, the digital signal must be processed
according to the time
domain spread spectrum code to recover the desired transmitted information.
In addition to the various different possible types of combinations of
multiplexing
schemes that are possible, various network algorithms may also be implemented.
For
example, the present invention may be applied to various fiber communication
system
architecture, such as a network environment shown in FIG. 5, in which a
plurality of users s,,
sZ, . . . sN are connected to an optical fiber medium 130 and each user s~ may
communicate
with any other user s; over the optical fiber. Each user or node s~ is
assigned a code u~ for
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receiving data from other users, and different users are preferably assigned
different codes.
When a user s; transmits data to a user s~, the transmitting user s; encodes
the optical signal
using the code assigned to the receiving user s~, and the receiving user
decodes the signal
using its assigned code. This may require that the transmitting user be able
to dynamically
vary the code it uses to transmit data depending upon the code of the intended
recipient user.
The codes for any one node may be assignable from one or more master nodes
distributed
throughout the network. Hence, when a node in a network comes on line, it
requests a code
or codes for encoding for selecting one of the possible spread spectrum
channels over which
to communicate. When that node leaves the network, the code that had been used
by that
l0 particular node may be reassigned to a different node in the network.
Various schemes may
be used for making such requests such as CSMA/CD technique or token passing on
a
permanently assigned channel. Alternatively, token passing techniques may be
used for
gaining codes for securing one of the code division channels.
In addition, the disclosed embodiments permit an increase in the number of
l5 simultaneous users. In particular, in prior art schemes such as those
discussed above, the
maximum number of simultaneous users that are permitted for the same number of
codes is
2"n where N is the maximum number of codes. However, in the disclosed
embodiment, the
maximum number of codes with holding everything else constant is 2". Thus,
total system
throughput is dramatically increased, thereby permitting a system throughput
of at (east one
20 half of a terabit, with the total system throughput being determined by the
maximum number
of simultaneous users, and the users data rate.
A particularly preferred implementation of an overall optical fiber
communication
system in accordance with the present invention is now described and
illustrated. This
overall system may be used for adding capacity, i.e., increasing the
bandwidth, of an optical
25 communication system that connects plural users of an extended f ber optic
connection. FIG.
14 illustrates a preferred apparatus for generating a plurality of broad-
spectrum sources in a
cost effective manner using a single erbium-doped fiber source and a hierarchy
of erbium-
doped fiber amplifiers to provide enough channels of sources, each with
su~cient intensity
for driving a channel- of the optical communications system. As shown, a
single erbium-
30 doped fiber source 300 outputs light with an acceptably broad spectrum,
generally providing
a bandwidth of about 28 nanometers 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 TH2. 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
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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 origins! 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 ampiifters 304 are provided over fibers to a
corresponding set of
four splatters 306, which may also be star couplers. The splatters 306 split
the output from
the fiber amplifcers into a plurality of outputs also of reduced intensity.
The split off output
from the splatters 306 are then provided to a further array of fiber
amplifiers 308, which
preferably amplify the intensity of the plural channels of broad-spectrum
tight 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
I S 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.
6. The 127 different encoders use a 128-bin mask to spatially encode the input
light signal,
with each of the 127 masks presenting a different unipolar Hadamard code
vector.generated
in the manner discussed 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 width of the linear mask. Thus, the l28 bins span a total
of about 3.5
THz (28 nanometers) in bandwidth, with each adjacent bin defining a subsequent
frequency
interval providing about 25 GHz of bandwidth. Each of the equal sized bins of
the f xed
mask is assigned according to the code vector to have one or the other of two
binary values.
One of the two binary values is identified by a blocking chrome stripe on the
glass substrate
of the mask and the other binary value is identified by an unblocked,
transparent stripe on the
glass substrate. Each of the 128 channels of the communication system is then
defined by a
distinct spatial 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. 6.
After the
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various channels are modulated both spatially (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 conventibnal 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,
l0 amplified, and provided to an array of 128 receivers, each corresponding to
one of the fixed
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. T'he receivers, which may have the structure shown in FIG. 7, are
each dedicated
l 5 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. As discussed above, it is possible and in
other
embodiments desirable to provide either the receiver or the transmitter with a
variable mask
such as one using a programmable LCD element. For the illustrated embodiment,
however,
20 the use of fixed masks on both the transmitting and receiving ends of the
communication
system provides a reduced cost system that provides significantly improved
bandwidth for a
high volume fiber link.
As discussed above, recovery of optical signals from the fiber communication
link is
accomplished using a receiver that separates the light beam received from the
optical system
25 into two components that should have substantially similar power levels. A
particularly
preferred aspect of the present invention is illustrated in FIG. I5, which
shows a beam
separator that is preferably used at the input to the receiver. A beam
separator in accordance
with the present invention is capable of separating the received light beam
into two beams of
sufficiently equal power levels to allow the preferred differential detection
scheme of the
30 optical CDMA receivers to effectively detect a desired user channel.
An embodiment of a polarization insensitive beam separator might consist of a
first
polarization sensitive element that divides the received light beam into first
and second
channels of light with each channel having a different one of two orthogonal
polarizations.
For example, one channel of light might include the vertically polarized
component of the
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received light beam and the other channel might include the horizontally
polarized
component of the received light beam. The polarization of one of the channels
is then
converted to the polarization of the other light beam. For linearly polarized
light this might
consist of rotating the polarization of the light. The two channels of light
are then
recombined and provided to a beam splitter. This beam splitter is typically a
polarization
sensitive element that accurately splits the combined beams into two beams of
substantially
equal power because the polarization of the combined beams is well defined and
predictable.
Referring to FIG. l5, a specific embodiment is described in which light is
received
from a single mode fiber 350. Since the fiber 350 is generally not
polarization preserving
and the light within the fiber 350 is likely linearly polarized in an
arbitrary direction, it is
convenient to use a conventional linear polarizer as a beam splitter 352 or
polarization
analyzer. The polarization sensitive element 352 preferably separates the
input light beam
into two orthogonal polarization components and provides those two components
to two
different optical paths 354, 356. Generally different power levels will be
present along each
path. The illustrated optical paths may propagate through free space or may
proceed through
polarization preserving fibers. In either case, the polarization of the light
within each arm
will be of a uniform polarization until the polarization is altered.
One component of the light is provided along optical path 354 and maintains a
vertical linear polarization 358 throughout the optical path 354. Along the
other optical path
- 20 356, the polarization is initially horizontal 360 and then the
polarization is rotated by 90° by
a rotation element 362 so that the polarization of the second optical path's
light becomes
linear vertical as indicated at 364 in FIG. 15. When the second optical path
356 propagates
through free space, the rotation element may be a %Z waveplate or an
appropriate Faraday
rotator. When the second optical path 356 propagates through a polarization-
preserving
fiber, the rotation element 362 is most preferably effected by a mechanical
rotation of the
fiber by 90°. Most generally the rotation of the fiber will proceed
continuously over a length
of the fiber. Of course, it is possible to perform the rotation through other
means, such as by
inserting a rotation element ai an end of the fiber of the second optical
path.
Once the two beams on the two optical paths have had their polarizations
property
oriented, the two beams are recombined and then split into a pair of
substantially equal
power beams to propagate along two additional beam paths. After the beams from
paths 354
and 356 are combined, it is possible to use a typical polarization sensitive
beam splitter 366
to divide the beams into two substantially equal power beams. The two desired
output beams
are provided along optical paths 368 and 370 preferably through single mode
optical fiber
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with linear polarizations in the illustrated embodiment. Comparing FIG. 7 and
FIG. l 5, the
input fiber 350 of FIG. 15 may correspond to the input fiber 162 of FIG. 7 and
the output
beams along optical fiber paths 368, 370 (FIG. 15) correspond to the two
optical paths shown
propagating from element 174 of the FIG. 7 embodiment. The split, received
beams are then
provided to the filtering elements 170, I 72 of FIG. 7 where the two channels
are analyzed
through the masks shown in FIG. 7.
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
tight is provided to
I S 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. 16, also
provides a
mechanism for reducing interference. The subsystem illustrated in FIG. 16
provides further
detail on the back-to-back diode arrangement indicated at 196, 198 in FIG. 7.
The two
complementarily filtered optical signals are provided to the back-to-back
diodes 196, 198,
which effect both a square law optical detection but 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
196, 198 and 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 of 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 ones.
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. As such, the scope of the present invention is to be determined by the
following
claims.
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