EP1101308A1 - Systeme optique a acces multiple par code de repartition (amcr) - Google Patents

Systeme optique a acces multiple par code de repartition (amcr)

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Publication number
EP1101308A1
EP1101308A1 EP99937636A EP99937636A EP1101308A1 EP 1101308 A1 EP1101308 A1 EP 1101308A1 EP 99937636 A EP99937636 A EP 99937636A EP 99937636 A EP99937636 A EP 99937636A EP 1101308 A1 EP1101308 A1 EP 1101308A1
Authority
EP
European Patent Office
Prior art keywords
light
code
optical
communication system
optical communication
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP99937636A
Other languages
German (de)
English (en)
Inventor
Birendra Dutt
Manouher Naraghi
James K. Chan
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
CodeStream Technologies Corp
Original Assignee
CodeStream Technologies Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by CodeStream Technologies Corp filed Critical CodeStream Technologies Corp
Publication of EP1101308A1 publication Critical patent/EP1101308A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/005—Optical Code Multiplex
    • H04J14/007—Orthogonal Optical Code Multiplex
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J13/00—Code division multiplex systems

Definitions

  • This invention relates to optical communication systems and, more particularly, to optical code-division multiple access communications systems that transmit data over optical fibers.
  • 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
  • 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.
  • improving the transmission rates on a TDM network requires that all of the transceivers attached to the network be upgraded to the higher transmission rates. No partial network upgrades are possible, which makes TDM systems less flexible than is desirable.
  • TDM systems provide a predictable and even data flow, which is very desirable in multi-user systems that experience "bursty" usage.
  • 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.
  • WDM wavelength-division multiplexing
  • WDMA wavelength-division multiple access
  • 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 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.
  • 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.
  • a demultiplexer such as a tunable filter
  • RECTIFIED SHEET (RULE 91) ISA/EP stability, for example as a function of operating temperature, may also affect the operational characteristics of the WDM system.
  • 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.
  • Lasers appropriate to the WDM video distribution system are individually 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.
  • 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 may be sufficiently flexible and cost effective to be used in at least some types of local area networks.
  • 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.
  • CDMA code-division multiple access
  • 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 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
  • RECTIFIED SHEET (RULE 91) ISA/EP 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.
  • 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.
  • 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," Lightwave Tech.. Vol. 13, No. 3, pp. 534-545 ( 1995).
  • the suggested optical CDMA system uses a broad-spectrum source and combines frequency (equivalently, wavelength) coding in addition to time-domain coding.
  • FIG. 1 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 source 12 such as an edge-emitting LED, super luminescent diode or an erbium-doped fiber amplifier.
  • 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 light modulator 16 by a mirror 18 or other beam steering optics.
  • 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 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, 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 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.
  • 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.
  • the mask 28 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.
  • 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 frequency or wavelength modulation characteristic of the 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 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 modulators 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.
  • FIG. 2 provides an illustration of the receiver circuitry in greater detail.
  • 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
  • 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 16 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.
  • 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.
  • 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.
  • 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.
  • bipolar code sequences i.e., sequences of +1 and - 1 values
  • Such codes include ⁇ -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.
  • 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) ⁇ -sequence or a unipolar form of a Hadamard 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
  • 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 O's and I'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 O's and I's into a first ON/OFF modulator 1 10 and through an inverter 1 12 into a second ON/OFF modulator 1 14.
  • the two modulators 1 10 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 1 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 O's and I'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
  • a spatial encoder with binary or analog encoding and a receiver.
  • a broad-spectrum light 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 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.
  • a beam separator which is in some particularly preferred embodiments a polarization insensitive splitter, diverts part of the beam in the fiber through a diffraction 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.
  • 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 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.
  • the masks in the encoder and decoder include unipolar binary codes comprising O's and I'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 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.
  • 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).
  • DSP digital signal processor
  • 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 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.
  • 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 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.
  • the wavelets are quantized or discrete spatial sine 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 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 into first and second channels of light with each channel having a different one of two orthogonal polarizations.
  • 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.
  • RECTIFIED SHEET (RULE 91) ISA/EP 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.
  • 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. 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 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.
  • 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. 10 A, B and C are continuous representations of discrete transparency functions for the mask of FIG. 9.
  • FIG. 1 1 is a graphical representation of a Fourier transform of light received from the fiber.
  • FIGS. 12A and B schematically illustrate an encoder and a decoder according to a third embodiment of the invention.
  • FIGS. 13 A, B and C present a graphical representation of a mask and mask functions according to a third embodiment of the 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 schematically illustrates a polarization insensitive beam separator that is
  • FIG. 16 illustrates in greater detail the optical detection circuitry schematically illustrated in FIG. 7.
  • the present inventors have investigated both the theoretical suggestions 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 systems are undesirable. As a preliminary matter, the systems are premised on the possibility of dividing received light into two different beams of light with equal power levels. As a practical matter, the polarization 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 Young patent and the coupler shown in the Kavehrad article are polarization sensitive, the power will not be equally divided between the two channels of the systems illustrated in those references.
  • receivers Without equal 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.
  • particularly preferred receivers according to the present invention receive light beams and divide the light beams into different channels using a polarization insensitive beam separator. Incorporation of a polarization insensitive beam separator at the input to the receiver makes the implementation of an optical CDMA system more practical and significantly increases the number of users provided on the communication system.
  • the Kavehrad article describes an optical CDMA system in which coding presented on masks is switched at the data rate of each channel of the system.
  • RECTIFIED SHEET (RULE 91) ISA/EP 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- l channels available to N-l users.
  • the 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.
  • 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.
  • 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.
  • receivers in accordance with preferred aspects of the present invention provide a polarization insensitive beam splitter.
  • a polarization insensitive beam splitter A polarization insensitive beam splitter.
  • RECTIFIED SHEET (RULE 91) ISA/EP 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.
  • Signals within the two channels of the receiver are preferably detected in a differential fashion, for example 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.
  • 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 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 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 H , s, 2 , s_,, s 22 , . . . s N court s ⁇ are connected to an optical fiber medium 130.
  • the first group of users s, prescription s 2 combat . . . s N may be proximately located and coupled to the fiber 130 in a star configuration, and the second group of users s, 2 , s- 2 , .
  • . . s N2 may be proximately located but remote from the first group and coupled to the fiber 130 in a star configuration.
  • 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. 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 S j ,, s j2 communicate with each other using a channel of the optical
  • Each pair of users (s j s j2 ) 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 j assigned to the user pair (S j ,, s j2 ), and the receiving user s )2 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.
  • 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)
  • SLD super luminescent diode
  • EDFS erbium-doped fiber source
  • 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 16 shown in FIG. 1 with the exception of the mask and coding scheme, then spatially encodes the modulated broad-spectrum light beam.
  • the encoder 150 includes a diffraction grating 152 that spatially spreads the spectrum of the modulated light beam along an axis.
  • the spatially spread light beam is collimated 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.
  • 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 184 is, in this illustrated preferred embodiment, identical to the encoding mask 156.
  • Light passed through the decoding mask 184 is passed through a collimating lens
  • RECTIFIED SHEET (RULE 91) ISA EP 188 and a diffraction grating 192 recombines the spatially spread light into a broad spectrum beam.
  • 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.
  • this second decoding mask 188 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 170 may then be supplied to a photodetector 196 to convert the light into an electrical signal.
  • 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, 196 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 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.
  • FIG. 8 shows another embodiment of the decoder 210.
  • 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 bit position.
  • the detector signal from each detector in the array is multiplied by either " I " or "- 1 " 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.
  • T is digital processing can be performed in discrete logic hardware or in a DSP 220 using software.
  • the outputs of the detectors may also be multiplied by numbers other than "1" or "- 1". 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
  • unipolar codes means code sequences comprising I's and O'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 code sequences comprising -I's and I'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 u is (1-u), i.e. its bit-wise complement in which O's are substituted by I's and I's are substituted by O's.
  • a complement of a unipolar analog code f is (1-0- Unipolar binary codes are used as an example in the following description.
  • the basic requirement for a spectral encoding/decoding scheme is that the decoding apparatus at a receiving user be able to recover data signals from the corresponding transmitting user while reducing or eliminating interference from signals from all other users.
  • the receiving masks will be fixed as a particular receiver always receives the same channel of data.
  • the receiving masks will be programmable so that different signal sources can be selected from the many possible sources.
  • 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 unipolar binary code may be represented by a sequence of binary digits, such as 1001111010101 1, where subscript i designates the '* user pair (or channel).
  • the number of digits in the sequence, N, is referred to as the length of the code.
  • 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.
  • the codes are preferably chosen such that they are orthogonal, or:
  • each transmitting user may transmit signals using a single encoding mask, and the corresponding receiving user may use a single decoding mask identical to the encoding mask to recover the signal from the corresponding transmitting user
  • a set of unipolar codes may be used such that although a code u, in the set is not orthogonal to other codes U j 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 u, and its complement u.*, i.e.
  • the decoders of embodiments of FIGS. 7 and 8 implement the principle of Eq. (2).
  • 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 j
  • the second channel having the complementary mask 172 generates a light beam representing u,* ⁇ *
  • the differentially arranged detectors 62 and 63 generates the difference signal u,*(u J -u J *).
  • the array of detectors 73 outputs signals representing u, and the DSP 74 calculates ⁇ •(U j -U j *) based on the outputs of the detector array 73.
  • the difference signal u,»(u J -u.*) is non-zero only for the signal from the user that uses a mask having a code u,. Consequently, such decoders are able to recover the signals from the transmitting user i
  • 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:
  • the unipolar codes u are derived from the bipolar codes v, by substituting the -I's in v, with O's, or
  • the bipolar code v is "balanced" in that they have equal numbers of I's and -I's (eq. (4)). These particularly preferred unipolar codes u, thus have equal numbers of I's and O's. As a result, half of the light power may be transmitted as signals, thereby promoting the efficient utilization of the source power.
  • a Hadamard matrix is a square matrix the elements of which are I's or -I's such that all rows are orthogonal to each other and all columns are orthogonal to each other.
  • a 4x4 Hadamard matrix may be:
  • RECTIFIED SHEET (RULE 91) ISA/EP
  • 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).
  • a set of unipolar codes u tract u 2 , . . ., 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+1 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 j by replacing all - I's with O's.
  • the above 4x4 Hadamard matrix may be used to generate the following codes:
  • Hadamard matrices having a size N that is a power of 2 may be constructed from.H- using a recursive algorithm
  • RECTIFIED SHEET (RULE 91) ISA/EP near balanced when, for example, u,*l is substantially smaller than N.
  • N the length of the codes
  • changing a few digits of some codes in the set may result in a near orthogonal or near balanced code set.
  • 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.
  • 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, 184, 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.
  • 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 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.
  • 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.
  • control is binary, and Walsh codes (unipolar
  • the masks can be implemented by LCD pixel arrays or by a photonic integrated circuit such as a solid state amplifier array.
  • the masks may be fixed and formed on glass blanks.
  • Such fixed masks are most preferably binary masks embodying unipolar Hademard codes.
  • the glass may be BK7 or quartz and the reflective regions could be gold.
  • the glass may still be BK7 or quartz and the blocking regions could be chrome.
  • 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 ? derived from balanced bipolar orthogonal wavelet functions g; using Eqs. (2) -
  • RECTIFIED SHEET (RULE 91) ISA/EP (4), which are illustrated in the context of binary codes, apply equally to analog codes.
  • 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 beam passing through a cell.
  • a first encoder mask transparency function shown in FIG. 10A may have a spatial frequency of 1 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 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. 10B.
  • a third encoder may have a frequency three times the frequency of the first encoder as shown in FIG. IOC.
  • 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 for higher usage systems.
  • the maximum number of harmonics or Walsh code bits (and therefore, the 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.
  • Chebyshev polynomials could also be used as they are orthogonal with respect to each other.
  • Using cosine waves for the encoding function also permits an easier decoder design.
  • the received signal can be separated through a spatial filter for the frequency of the desired signal and then that signal can be recovered.
  • FIG. 1 shows the Fourier transform of a signal received from a fiber where the separate encoded signals include 1 L, 2/L, 4/L and 8/L. Any one of these signals may be readily obtained by filtering for that particular spatial frequency in the received signal.
  • an alternative method may be used for modulating signals using two codes as is shown in FIG. 12 A.
  • the optical path for the spatially spread light source 240 is switched between a first mask 242 and a second mask
  • RECTIFIED SHEET (RULE 91) ISA/EP 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 (not shown).
  • Receiving data proceeds in the converse manner as shown in FIG. 12B.
  • a decoder 260 receives light from the communications channel and generates the 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 from the receiver 268 may then be processed by a digital signal processor 270 for recovery of the data.
  • FIG. 13A shows one alternative embodiment of the masks appropriate for coding where two different masks are used for transmitting ones and zeros.
  • 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.
  • the discrete transparency functions for parts 282, 284 are the complements of each other such as shown in FIG. 13B, where the ordinate represents spatial frequency and the abscissa represents intensity. For transmitting the other possibility (i.e. the zero), as shown in FIG.
  • 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.”
  • first halves 282, 286 can be a first polynomial such as a sine wave and the second halves 284, 288 can be a second polynomial such as a Chebyshev function.
  • RECTIFIED SHEET (RULE 91) ISA/EP of the invention
  • other embodiments of the invention are also possible.
  • discrete wavelet functions are used for encoding
  • the masks may be formed photographically.
  • the optical systems 150, 170 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.
  • the optical chambers are implemented with diffraction gratings, collimating lenses and an optical mask having a code, but other implementations are also possible.
  • encoders and decoders can also be applied to analog modulation of the optical signal.
  • 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.
  • the codes may be shared on a time sharing basis to provide for time division multiplexing.
  • 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.
  • 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.
  • various network algorithms may also be implemented.
  • 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 su s 2 , . . . s are connected to an optical fiber medium 130 and each user S j may communicate with any other user s t over the optical fiber.
  • Each user or node S j is assigned a code U j for
  • RECTIFIED SHEET (RULE 91) ISA/EP receiving data from other users, and different users are preferably assigned different codes.
  • the transmitting user s 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 j5 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.
  • the code that had been used by that 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.
  • token passing techniques may be used for gaining codes for securing one of the code division channels.
  • the disclosed embodiments permit an increase in the number of simultaneous users.
  • the maximum number of simultaneous users that are permitted for the same number of codes is 2 N Z where N is the maximum number of codes.
  • the maximum number of codes with holding everything else constant is 2 N .
  • total system throughput is dramatically increased, thereby permitting a system throughput of at least one half of a terabit, with the total system throughput being determined by the maximum number of simultaneous users, and the users data rate.
  • 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 sufficient intensity for driving a channel of the optical communications system.
  • a single erbium- 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 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
  • RECTIFIED SHEET (RULE 91) ISA/EP which splits the input source signal and provides the output over four fibers to an array of four fiber amplifiers 304.
  • 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.
  • this process is repeated through several further hierarchical stages.
  • the outputs from the four fiber amplifiers 304 are provided over fibers to a corresponding set 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.
  • 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.
  • the 128 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 fixed 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.
  • the combined light signals are split, 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.
  • the receivers which may have the structure shown in FIG. 7, 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.
  • 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.
  • FIG. 15 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 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.
  • one channel of light might include the vertically polarized component of the
  • ISA/EP 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.
  • 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.
  • 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.
  • the rotation element may be a '/_ waveplate or an appropriate Faraday rotator.
  • 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 at an end of the fiber of the second optical path.
  • the two beams on the two optical paths have had their polarizations properly oriented, the two beams are recombined and then split into a pair of substantially equal power beams to propagate along two additional beam paths.
  • a typical polarization sensitive beam splitter 366 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
  • 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, 172 of FIG. 7 where the two channels are analyzed through the masks shown in FIG. 7.
  • 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.
  • 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 1 6, 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.
  • the electrical output signal 200 from the diode pair 196, 198 is then low pass filtered by filter 380. The low pass filtering is performed to remove high frequency noise signals.
  • 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
  • RECTIFIED SHEET (RULE 91) ISA/EP 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.

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Abstract

La présente invention concerne un système de communications par fibre optique utilisant des techniques d'accès multiple par étalement du spectre afin d'obtenir une meilleure utilisation de largeur de bande. Le signal optique est codé au moyen d'un masque unique muni d'un premier code. La lumière, codée dans l'espace à l'aide du masque et modulée dans le temps avec des données, est émise sur une liaison de fibre et reçue par un décodeur. Un séparateur insensible à la polarisation décompose la lumière reçue en deux composantes de puissance égale qui sont dirigées vers deux masques décodeurs, utilisés afin de décoder le signal. L'un des masques comprend un deuxième code identique au premier code et l'autre masque comprend un troisième code complémentaire du premier code. Les faisceaux lumineux de sortie filtrés par les masques sont détectés de manière différentielle afin de générer un signal de sortie, traité ultérieurement afin de récupérer des données. Le signal électrique passe à travers un filtre passe-bas et sa détection quadratique est effectuée électriquement. Les premiers codes sont choisis parmi un ensemble de codes unipolaires dérivés d'un ensemble de codes orthogonaux bipolaires équilibrés. Ces codes peuvent être binaires ou analogiques.
EP99937636A 1998-07-30 1999-07-29 Systeme optique a acces multiple par code de repartition (amcr) Withdrawn EP1101308A1 (fr)

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US12631098A 1998-07-30 1998-07-30
US126310 1998-07-30
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US7773882B2 (en) * 2005-05-26 2010-08-10 Telcordia Technologies, Inc. Optical code-routed networks

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US5867290A (en) * 1996-11-19 1999-02-02 Rdl Commercial Technologies Corporation High capacity spread spectrum optical communications system

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