OPTICAL CDMA USING A CASCADED MASK STRUCTURE

Abstract: Optical CDMA transmitters and receivers include a spatial modulator using a fiber grating filter device encoded with CDMA code defined by the presence or absence of gratings at defined frequency positions. The presence of a fiber grating filter spanning a range near a particular chip frequency defines a "bit 0" chip and the absence of a fiber grating filter at another chip frequency designates the "bit 1" chip. The combination of gratings selectively provided over the sequence of chip positions along the fiber defines the CDMA code. This fiber grating filter device can be viewed as a cascade of many band-stop grating filters each corresponding to the CDMA code bit in the spectral domain. Consequently, a CDMA transmitter includes two components, a data modulator that modulates a broadband optical source, e.g., an erbidium doped fiber source (EDFS) or a super luminescent diode (SLD), and a fiber grating filter device to selectively pass and block portions of the spectrum of the broadband optical source. The optical CDMA-encoded output of this transmitter is then output to an optical network.

Description: OPTICAL CDMA USING A CASCADED MASK STRUCTURE BACKGROUND OF THE INVENTION Field of the Invention This invention relates to optical communication systems and, more particularly, to optical code-division multiple access (CDMA) communications systems. Aspects of the present invention find particular application in all-fiber optical communications systems, ranging from wide area networks to local area networks. Description of the Related Art Recent years have seen rapidly expanding demands for communications bandwidth, resulting in the rise of technologies such as satellite communications, video programming distribution networks such as cable television, and spread-spectrum telephony including, for example, code-division multiple access telephony. Such technologies have become common and well integrated into everyday communications. Growing demand for communications bandwidth has brought significant investments in new communications technologies and in new communications infrastructure. For example, the cable television industry, telephone companies, Internet providers and various government entities have invested in long distance optical fiber networks and in equipment for fiber networks. The addition of this infrastructure has, in turn, spurred demand for bandwidth, resulting in demand for yet additional investment in new technologies and infrastructure. Installing optical fibers over long distances is expensive. Additionally, conventional optical fiber or other optical communication networks utilize only a small fraction of the available bandwidth of the optical communication channel. There is consequently considerable interest in obtaining higher utilization of fiber networks or otherwise increasing the bandwidth used within optical fiber systems. Techniques have been developed to increase the bandwidth of optical fiber communication systems and to convey information from plural sources over a fiber system. Generally, these techniques seek to use more of the readily available optical bandwidth of optical fibers by supplementing the comparatively simple coding schemes conventionally used by such systems. In some improved bandwidth fiber systems, the optical fiber carries an optical carrier signal consisting of a single, narrow wavelength band and multiple users access the fiber using time-division multiplexing (TDM) or time-division multiple access (TDMA). Time division techniques transmit frames of data by assigning successive time slots in the frame to particular communication channels. Optical TDM requires high pulse rate diode lasers and provides only moderate improvements in bandwidth utilization. In addition, improving the transmission rates on a TDM network requires that all of the transceivers attached to the network be upgraded to the higher transmission rates. No partial network upgrades are possible, which makes TDM systems less flexible than is desirable. On the other hand, TDM systems provide a predictable and even data flow, which is very desirable in multi-user systems that experience "bursty" usage. Thus, TDM techniques will have continued importance in optical communications systems, but other techniques must be used to obtain the desired communications bandwidth for the overall system. Consequently, it is desirable to provide increased bandwidth in an optical system that is compatible with TDM communication techniques. One strategy for improving the utilization of optical communication networks employs wavelength-division multiplexing (WDM) or wavelength-division multiple access (WDMA) to increase system bandwidth and to support a more independent form of multiple user access than is permitted by TDM. WDM systems provide plural optical channels each using one of a set of non-overlapping wavelength bands provided over an expanded portion of the fiber's available bandwidth. Information is transmitted independently in each of the optical channels using a light beam within an assigned wavelength band, typically generated by narrow wavelength band optical sources such as lasers or light emitting diodes. Each of the light sources is modulated with data and the resulting modulated optical outputs for all of the different wavelength bands are multiplexed, coupled into the optical fiber and transmitted over the fiber. The modulation of the narrow wavelength band light corresponding to each channel may encode a simple digital data stream or a further plurality of communication channels defined by TDM. Little interference will occur between the channels defined within different wavelength bands. At the receiving end, each of the WDM channels terminates in a receiver assigned to the wavelength band used for transmitting data on that WDM channel. This might be accomplished in a system by separating the total received light signal into different wavelengths using a demultiplexer, such as a tunable filter, and directing the separated narrow wavelength band light signals to receivers assigned to the wavelength of that particular channel. The number of users that can be supported by a WDM system is limited due to the difficulty in obtaining appropriately tuned optical sources. Wavelength stability, for example as a function of operating temperature, may also affect the operational characteristics of the WDM system. As a more practical matter, the expense of WDM systems limits the application of this technology. One embodiment of a WDM fiber optic communication system is described in U.S. Patent No. 5,579,143 as a video distribution network with 128 different channels. The 128 different channels are defined using 128 different lasers operating on 128 closely spaced but distinct wavelengths. These lasers have precisely selected wavelengths and also have the well-defined mode structure and gain characteristics demanded for communications systems. -3- Lasers appropriate to the WDM video distribution system are individually expensive so that the requirement for 128 lasers having the desired operational characteristics makes the overall system extremely expensive. The expense of the system makes it undesirable for use in applications such as local area computer networks and otherwise limits the application of the technology. As is described below, embodiments of the present invention can provide a video distribution network like that described in U.S. Patent No. 5,579,143, and embodiments of the invention can provide other types of medium and wide area network applications, making such systems both more flexible and more economical. Embodiments of the present invention, as described below, use spread spectrum communication techniques to obtain improved loading of the bandwidth of an optical fiber communication system in a more cost-effective manner than known WDM systems. Spread spectrum communication techniques are known to have significant advantages and considerable practical utility, most notably in secure military applications and mobile telephony. There have consequently been suggestions that spread spectrum techniques, most notably code-division multiple access (CDMA), could be applied to optical communications technologies. Spread spectrum techniques are desirable in optical communications systems because the bandwidth of optical communications systems, such as those based on optical fibers, is sufficiently large that 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 system in that plural data channels are defined for different wavelengths. The different wavelength channels are defined in the optical frequency domain and time domain signals can be transmitted over each of the wavelength channels. From a CDMA perspective, the distinct wavelength channels of the WDM communication system described above provide a trivial, single position code, where individual code vectors are orthogonal because there is no overlap between code vectors. There have been suggestions for optical CDMA systems that are generally similar to traditional forms of radio frequency CDMA, for example in Kavehrad, et al., "Optical Code- Division-Multiplexed Systems Based on Spectral Encoding of Noncoherent Sources," J. Lightwave 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 - J- schematic illustration of the theoretical optical CDMA suggested in the Kavehrad article is presented in FIG. 1. The suggested optical CDMA system uses a broad-spectrum (broadband), incoherent source 12 such as an edge-emitting LED, super luminescent diode or an erbium-doped fiber amplifier. In the illustrated CDMA system, the broad-spectrum source is modulated with a time-domain data stream 10 and the time domain modulated broad-spectrum light 14 is directed into a spatial light modulator 16 by a mirror 18 or other beam steering optics. Within the spatial light modulator 16, light beam 20 is incident on a grating 22, which spatially spreads the spectrum of the light to produce a beam of light 24 having its various 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. After passing through the mask 28, the spatially modulated light passes through the lens 30 and the wavelength modulated light beam 32 is then spatially condensed by the second grating 34. The wavelength modulated and spatially condensed light beam 36 passes out of the spatial light modulator 16 and is directed by mirror 38 or other beam steering optics into a fiber network or transmission system 42. The portion of the CDMA system described to this point is the transmitter portion of the system and that portion of the illustrated CDMA system down the optical path from the fiber network 42 constitutes the receiver for the illustrated system. The receiver is adapted to identify a particular transmitter within a network including many users. This is accomplished by providing a characteristic spatial mask 28 within the transmitter and detecting in the receiver the spatial encoding characteristics of the transmission mask from among the many transmitted signals 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 receiver or user to receive the transmitted data stream by altering the spatial pattern of the mask 28, and hence the frequency coding of the transmitted beam 40, so that the transmitter mask 28 corresponds to a spatial coding characteristic of the intended receiver. The receiver illustrated in FIG. 1 detects data transmitted from a particular transmitter by detecting the spatial (frequency or wavelength) modulation characteristic of the transmitter mask 28 and rejecting signals having different characteristic spatial modulation patterns. Light received from the optical fiber network 42 is coupled into two different receiving channels by coupler 44. The first receiver channel includes a spatial light demodulator 46 that has a mask identical to one used in the spatial light modulator 16 and the second receiver channel includes a spatial light modulator 48 of similar construction to the transmitter's spatial light modulator 16, but having a mask the "opposite" of the transmitter mask 28. Each of the spatial light demodulators 46, 48 performs a filtering function on the received optical signals and each passes the filtered light out to an associated photodetector 50, 52. Photodetectors 50, 52 detect the filtered light signals and provide output signals to a differential amplifier 54. The output of the differential amplifier is provided to a low pass filter 56 and the originally transmitted data 58 are retrieved. FIG. 2 provides an illustration of the receiver circuitry in greater detail. In this illustration, spatial light demodulators 46 and 48 are generally similar to the spatial light modulator 16 shown in FIG. 1 and so individual components of the systems are not separately described. Received light 60 is input to the receiver and is split using coupler 62, with a portion of the light directed into spatial light demodulator 46 and another portion of the light directed into the other spatial light modulator 48 using mirror 64. Spatial light demodulator 46 filters the received light 60 using the same spatial (frequency, wavelength) modulation function as is used in the transmitter's spatial light modulator 16 and provides the filtered light to photodetector 50. Spatial light demodulator 48 filters the received light using a complementary spatial filtering function and provides the output to the detector 52. Amplifier 54 subtracts the output signals from the two photodetectors. To effect the same filtering function as the transmitter's spatial light modulator 16, the spatial light demodulator 46 includes a mask 66 identical to the transmitter mask 28. Spatial light demodulator 48 includes a mask 68 that performs a filtering function complementary to masks 28 and 66 so that spatial light modulator 48 performs a filtering function complementary to the filtering function of spatial light modulator 16. In the Kavehrad article, each of these masks 16, 66, 68 is a liquid crystal element so that the masks are fully programmable . The particular codes embodied in the masks must be appropriate to the proposed optical application. Although CDMA has been widely used in radio frequency (RF) domain communication systems, its application in frequency (wavelength) domain encoding in optical systems has been limited. This is because the success of the RF CDMA system depends crucially on the use of well-designed bipolar code sequences (i.e., sequences of + 1 and -1 values) having good correlation properties. Such codes include -sequences, Gold sequences, Kassami sequences and orthogonal Walsh codes. These bipolar codes can be used in the RF domain because the electromagnetic signals contain phase information that can be detected. RF CDMA techniques are not readily applicable to optical systems in which an incoherent light source and direct detection (i.e., square-law detection of the intensity using photodetectors) are employed, because such optical systems cannot detect phase information. Code sequences defining negative symbol values cannot be used in such optical systems. As a result, only unipolar codes, i.e., code sequences of 0 and 1 values, can be used for CDMA in a direct-detection optical system. The Kavehrad article suggests the adaption of various bipolar codes for the masks within the system illustrated in FIGS. 1 & 2, including masks provided with a unipolar (only 0's and l's) -sequence or a unipolar form of a Hadamard code. For these sorts of bipolar code, the Kavehrad article indicates that the bipolar code of length N must be converted into a unipolar code sequence of length 2N and that a system including such codes could support a total of N-l users. The Kavehrad article primarily addresses theoretical aspects of a CDMA system, providing predictions of system performance on the basis of assumptions as to how such a system might operate. Little description is provided of how to implement a general bipolar code so as to be effective in an optical system. A more practical example of an optical CDMA system, including an example of a converted bipolar code sequence, has been proposed for transmission and detection of bipolar code sequences in a unipolar system. This system is described in a series of papers by L. Nguyen, B. Aazhang and J.F. Young, including "Optical CDMA with Spectral Encoding and Bipolar Codes," Proc. 29th Annual Conf. Information Sciences and Systems (Johns Hopkins University, March 22-24, 1995), and "All-Optical CDMA with Bipolar Codes", Elec. Lett., 16th March 1995, Vol. 3, No. 6, pp. 469-470. This work is also summarized in U.S. Patent No. 5,760,941 to Young, et al., and this work is collectively referenced herein as the Young patent. In the Young patent's system, schematically illustrated in FIG. 3, the transmitter 80 employs a broad-spectrum light source 82 the output of which is split by a beam splitter 84 into two beams 86 and 88 that are processed by two spatial light modulators 90 and 92. The first spatial light modulator 90 comprises a dispersion grating 94 to spectrally disperse the light beam 86 and a lens 96 to collimate and direct the dispersed light onto a first spatial encoding mask 98 which selectively passes or blocks the spectral components of the light beam. Lens 100 collects the spectral components of the spatially modulated light beam and recombination grating 102 recombines the spread beam into encoded beam 104. The "pass" and "block" states of the encoding masks represent a sequence of 0's and l's, i.e., a binary, unipolar code. The code 106 for the first mask 98 has a code U®U*, where U is a unipolar code of length N, U* is its complement and "®" denotes the concatenation of the two codes. The second encoder 92 (details not shown) is similar in structure to the first encoder 90 except that its encoding mask has a code U*®U. Symbol source 108 outputs a sequence of pulses representing 0's and l's into a first ON/OFF modulator 110 and through an inverter 112 into a second ON/OFF modulator 114. The two modulators 110 and 114 modulate the two spatially modulated beams of light and the two beams are combined using a beam splitter 116 to combine the two encoded light beams 118 and 120. The modulated light beams are alternately coupled to the output port depending on whether the bit from the source is 0 or 1. This system can then use a receiver with differential detection of two complementary channels, as illustrated in the receiver of FIG. 2. The receiving channels are equipped with masks bearing the codes U*®U and U®U*, respectively, and sequences of 0's and l's are detected according to which channel receives a signal correlated to that channel's mask. The system proposed in the Young patent allows the use of the bipolar codes developed for RF CDMA technologies to be used in optical CDMA systems. However, for a mask of length 2N, only N codes can be defined since the code U and its complement U* must be concatenated and represented within each mask. Systems like that shown in the Kavehrad article and the Young patent typically perform optical CDMA encoding using one or more spatial light modulators (16 or 90, 92). Such spatial light modulators are comprised of a relatively complex combination of optics, i.e., dispersion gratings (22, 34 or 94, 102), lenses (26, 30 or 96, 100) and a spatial pattern mask (28 or 98), which typically require critical alignment and opto-mechanical mounts to perform properly. Vibrations and misalignments are highly detrimental to performance. Non-ideal optical characteristics, such as Gaussian beam shapes, can introduce cross talk or otherwise impair system signal-to-noise ratios. Additionally, two such structures are typically required to decode a received CDMA code (see spatial light modulators 46, 48 in FIGS. 1 and 2). Accordingly, the production costs for such devices can be undesirably high. Even with aggressive optical designs, these optical systems are large and heavy. Therefore, it is an object of the invention to provide a simplified, cost-effective structure, e.g., one that that is compact and does not require alignment, for modulators and demodulators within an optical CDMA communication system. SUMMARY OF THE PREFERRED EMBODIMENTS These and other objects may be obtained with optical CDMA transmitters and receivers include a fiber grating filter device encoded with a CDMA code by selectively providing filter elements within a series of possible narrow bandwidth filter positions in the device. Individual code bits might be stored, for each frequency band within the filter, by assigning the presence of a fiber grating filter at a chip frequency a "bit 0" chip and the absence of a fiber grating filter a "bit 1" chip. The fiber grating filter device provides a cascade of many band-stop grating filters, with the presence or absence of each filter corresponding to a value the CDMA code bit in the spectral domain. Consequently, a CDMA transmitter may be formed from a data modulator to modulate a broad-spectrum (broadband) optical source, e.g., an erbium-doped fiber source (EDFS) or a super luminescent diode (SLD), and a fiber grating filter device as described above to selectively pass (or conversely block) multiple portions of the spectrum of the broad-spectrum optical source. The optical CDMA-encoded output of this transmitter is then output to an optical network that contains the outputs of many such devices. An appropriate CDMA receiver may include a beam splitter that receives an optical signal from the optical network and feeds split optical signals to two fiber grating filter devices as described above. One of the two fiber grating filter devices provides the CDMA code of the filter of a corresponding transmitter and the other filter provides the complement of that CDMA code. Outputs from the two fiber grating filter devices are connected to a differential detector that recovers the transmitted data signal. The described architecture is conducive to a low manufacturing cost since it can be implemented by a fusion splice of a piece of fiber in series with the data modulator (in the transmitter) and/or the outputs of the splitter (in the receiver). A transmitter responsive to a data signal for use in an optical communication system might include a broadband light source having a frequency spectrum comprised of multiple frequency portions, a data modulator for selectively modulating the broadband light signal in response to the data signal to provide a modulated broadband light signal, and a fiber grating filter device for selectively passing frequency portions of the modulated broadband light signal to an optical network. The order of the data modulator and the fiber grating filter device can be reversed with essentially equivalent results. A receiver for use in an optical communication system for selectively retrieving a CDMA-encoded signal from a light signal received from an optical network might include a splitter for splitting the received light signal into first and second split light signals, a first fiber grating filter device for selectively passing portions of the first split broadband light signal to thereby generate a first filtered light signal, a second fiber grating filter device for selectively passing complementary portions of the second split broadband light signal to thereby generate a second filtered light signal, and a differential detector for generating a received data signal from the difference between the first and second filtered light signals. The first fiber grating device passes portions of the received light signal corresponding to those portions of a desired CDMA-encoded signal. Transmitting and receiving devices in accordance with aspects of the invention might be combined in an optical communication system. A source generates a broadband light signal comprising multiple frequency portions and provides the signal to a transmitter, for the optical communication system. The transmitter includes a data modulator for selectively passing the broadband light signal in response to a transmit data signal to generate a data modulated broadband light signal. The transmitter further includes a first in-fiber Bragg grating that passes frequency-separated portions of the broadband light signal. The receiver for the optical communication system includes a splitter that splits a received broadband light signal into first and second split broadband light signals. A second in-fiber Bragg grating selectively passes portions of the first split broadband split light signal to generate a first filtered split signal. A third in-fiber Bragg grating selectively passes complementary portions of the second split broadband light signal to generate a second filtered light signal. The outputs of the second and third filters are provided to a differential detector that generates a received data signal corresponding to the difference between the first and second filtered light signals. Most preferably, the first in-fiber Bragg grating defines a first code, the second in-fiber Bragg grating is identical to the first in-fiber grating and the third in-fiber Bragg grating defines a second code complementary to the first code. Such a system is particularly useful when the one or more transmitters, i.e., devices comprised of a data modulator and a first fiber grating device, place data-modulated, CDMA- encoded, broadband light signals onto an optical network and one or more receivers, i.e., devices comprised of a splitter, second and third fiber grating filter devices and a differential detector, retrieve a broadband light signal from the optical network to selectively retrieve data-modulated, CDMA-encoded broadband light signals contained within and consequently the data originally provided to the transmitters. - 7 - The invention may be better understood from the following description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a conventional optical fiber mediated CDMA communication system. FIG. 2 provides a more detailed view of one receiver configuration that might be used in the system of FIG. 1. FIG. 3 illustrates an encoder for using bipolar codes in an optical CDMA system. FIGS. 4A-4D illustrate operational characteristics of certain elements of the modulation or encoding system in accordance with the present invention. FIG. 5 provides an example of the transmission through a single Bragg grating. FIG. 6 illustrates the structure of a typical optical fiber. FIG. 7 illustrates the structure of one type of Bragg grating within a optical fiber waveguide. FIGS. 8 & 9 present different configurations of an optical fiber network according to the present invention. FIG. 10 is a block diagram of a multi-channel encoder according to the present invention. FIG. 11 is a diagram showing the correspondence between an exemplary CDMA code, a fiber grating filter device of the present invention and the resulting bandpassed spectrum. FIG. 12 is a simplified diagram of an integrated fiber grating device for CDMA coding. FIG. 13 is a block diagram of a decoder according to the present invention. FIG. 14 schematically illustrates an apparatus for generating an array of N broad- spectrum optical sources having sufficient intensity to generate light beams for N channels of communication over a fiber using methods in accordance with the present invention. FIG. 15 illustrates in greater detail the optical detection circuitry schematically illustrated in FIG. 13. FIG. 16 illustrates a modification to the source generation mechanism of FIG. 14. CROSS-REFERENCED APPLICATIONS The following applications are each incorporated by reference in their entirety into this application: 1. "Optical CDMA System," application Serial No. 09/126,310, filed July 30, 1998. 2. "Optical CDMA System Using Sub-Band Coding," application Serial No. 09/126,217, filed July 30, 1998. 3. "Method and Apparatus for Reduced Interference in Optical CDMA", application Serial No. 09/127,343, filed July 30, 1998. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS These and other objects are obtained with CDMA transmitters and receivers which include a wavelength or frequency modulator formed using a fiber grating filter device encoded in wavelength or frequency with a CDMA code such that the "bit 0" chip is encoded as the presence of a fiber grating filter spanning a frequency range near that chip frequency and the "bit 1" chip is encoded as the absence of a fiber grating filter spanning that frequency range at that chip frequency within the sequence of chip frequency positions along the fiber. This fiber grating filter device may be arranged as a cascade of many band-stop grating filters each corresponding to a different CDMA code bit in the frequency domain. Consequently, a CDMA transmitter may be formed from a broad-spectrum (broadband) optical source, e.g., an erbium doped fiber source (EDFS) or a super luminescent diode (SLD), a data modulator that responds to a data signal to modulate the optical source and an appropriate fiber grating filter device to selectively pass (or conversely block) portions of the spectrum of the broad-spectrum optical source. The optical CDMA-encoded output of this transmitter may be coupled into an optical network along with the outputs of other such devices. Similarly, an optical CDMA receiver may be formed from a beam splitter that receives an optical signal from the optical network and feeds split signals to two receiving fiber grating filter devices. One of the receiving filter devices includes a filter encoded with the same CDMA code defined within the filter of a corresponding transmitter and the other receiving filter device is encoded with the complement of that same CDMA code. The two receiving filter devices couple their output to a differential detector that recovers the transmitted data signal. Such an architecture is conducive to low cost manufacturing since it can be implemented by a fusion splice of a piece of fiber in series with the data modulator (in the transmitter) and/or the outputs of the splitter (in the receiver). This optical system is generally immune to misalignments and vibrations. For this reason, the optical system is durable and capable of use in practical, non-ideal environments. Preferred embodiments of the present invention provide an optical CDMA system that is more compact, is more readily aligned and is less susceptible to misalignment than conventional optical CDMA systems such as those described in the Kavehrad article and the Young patent, discussed above in the Background. Aspects of the invention provide at least one transmission channel that encodes broadband light with an identifying code and outputs the encoded light signal to an optical communication link such as an optical fiber. This system may encode broadband light by passing the input light through a mask that modulates the broadband light as a function of wavelength across an operational portion of the spectrum of the input light. An appropriate encoding mask in accordance with the present invention might consist, for example, of a number of wavelength dependent filters arranged sequentially along the length of an optical fiber with each wavelength dependent filter attenuating, substantially uniformly, light within a range of wavelengths around a central wavelength. The effect of this sequence of wavelength dependent filters is to selectively attenuate a corresponding number of wavelength bands across the operational portion of the spectrum, leaving other wavelength ranges unattenuated. The pattern of attenuated wavelength bands dispersed over the spectrum and interleaved with unattenuated wavelength bands represents the code for that channel. Reception of the encoded signal is performed using a technique that detects the pattern of attenuated wavelength bands and which accommodates the fact that the received optical signal can reliably provide only amplitude and wavelength (or frequency) information. As discussed above in the Background, CDMA techniques such as those used in cellular telephones rely on the use of a well designed set of codes which take advantage of phase to reduce noise in the system. Phase information is generally not available in optical systems where transmission occurs over most types of optical fibers or over sufficient distances. Preferred embodiments of the present invention overcome the lack of phase information by a differential detection and noise cancellation process. At the receiver, the channel to be detected is discriminated on the basis of the wavelength modulation that represents the code for that channel. The signal is received over an optical fiber and the received signal is split into two equal components and provided to two receiver fibers. The two split off components pass through decoding masks in each of the two receiver fibers. Most preferably, the decoding masks provided in the respective fibers are constructed in a manner similar to that of the encoding mask and thus in this embodiment the decoding masks each preferably consist of a series of wavelength dependent filters that selectively attenuate wavelength bands within the received light. In accordance with the preferred detection technique, the first of the decoding masks is identical to the encoding mask and the second of the decoding masks is the opposite of the encoding mask. The signals from the two decoding channels are then input to the differential inputs of a differential detector such as an arrangement of back to back diodes. Here, a mask that is the opposite of the encoding mask may be one that attenuates the wavelength bands not attenuated by the encoding mask and passes those wavelength bands attenuated by the encoding mask. An "opposite" mask is also known as a complementary mask here. Such a detection scheme, when coupled with an appropriate code set, provides highly effective - Λ2 - discrimination of a desired channel from among other channels and with good noise rejection characteristics. Embodiments of an optical communication system in accordance with the present invention use at least one or, more preferably, a plurality of matched broadband optical sources, with one source provided for each of the channels desired for the optical communication system. Each transmission channel of the optical communication system is defined by a code unique to that particular channel. The code that identifies the channel is embodied in an encoding mask that modulates the broadband input light signal as a function of wavelength when the input light signal passes through the mask. This mask may, for example, be similar to the spatial modulators illustrated in FIGS. 1 and 3 in that it imparts an amplitude modulation to the broadband light as a function of wavelength. Masks in accordance with the present invention are different from the encoding and decoding masks of FIGS. 1-3 in that the encoding and decoding masks preferred here do not operate on a spatially spread optical signal. Rather, encoding and decoding masks in accordance with preferred embodiments of the present invention operate on broad spectrum beams of light without spreading or dispersion in frequency or wavelength. The spatial modulators illustrated in FIGS. 1 and 3 modulate all of the wavelengths of the broadband light in parallel. By contrast, preferred embodiments of the present invention modulate the various wavelengths of this invention's broadband light source in series by passing the light source through a series of cascaded filters. Each of the filters may be specific to a particular narrow band of wavelengths and may, in a particularly simple example, either allow the selected wavelength band to pass or strongly attenuate the light within the wavelength band. The total filter array is made up of, for example, 128 filters, each covering a band of wavelengths adjacent to the wavelength band covered by at least one adjacent filter so that the spectrum of the broadband light source is covered by the series of filters. The cascaded filter assembly is illustrated simplistically in FIGS. 4A-4D, in which a small portion of the operationally available spectrum is shown schematically (FIG. 4A) before passing through a mask (FIG. 4B) which modulates the input signal with a code defined as a binary function of the wavelength (FIG. 4C). The modulated signal of FIG. 4D is produced by passing the input signal of FIG. 4A through the mask illustrated in FIG. 4B. Note that the intensity of the modulated optical signal varies from a high level of I 1? less than the input intensity of I 0 , due to the losses associated with the unattenuated portions of the beam passing through the cascaded series of filters. The low level of I 2 is reduced by 25 dB or more with respect to ϊ γ . As illustrated in FIG. 4C, the code defined by the cascaded filter array can be viewed as a sequence of binary values defined across wavelength ranges centered about increasing central wavelengths. The complement to the mask illustrated within FIGS. 4B and 4C replaces the zeros with ones (gratings replaced by an absence of a grating) and ones with zeros (absence of a grating replaced by a grating effective at that wavelength). It should also be noted that the distribution of the gratings is illustrative of how the gratings are spread in wavelength. As a practical matter, there is no need for the gratings to be spaced apart as illustrated. The input spectrum illustrated in FIG. 4A is a simplification of the spectral intensity distribution of optical sources such as erbium doped fiber diodes, super luminescent diodes or other broadband light sources appropriate for use in the optical communication system. The mask shown in FIG. 4B includes a set of optical elements intended to attenuate a set of the input wavelengths including λj, λ 3 , λ 4 and λ 7 . Most preferably the optical elements attenuate the light within the wavelength band essentially uniformly across the wavelength band. Appropriate attenuating optical elements include Bragg gratings formed within the optical fiber and designed to block a wavelength band centered on the wavelength λ N . The transmission characteristics of Bragg gratings are illustrated schematically in FIG. 5, which shows the characteristics of a single fiber grating of what is typically an array of fiber gratings within the fiber. Fibers having arrays of gratings with the illustrated characteristics are commercially available from Uniphase Corporation of San Jose, California. As illustrated, Bragg gratings can provide a high level of attenuation, for example of 25 dB or more, within a well-defined wavelength band. It should be noted that Bragg gratings are used either in a transmission mode or in a reflection mode. Either mode of operation can produce acceptable performance within embodiments of the present invention. Bragg gratings typically are formed as a regular array of step-wise variations in the index of refraction within the core of the optical fiber. The core and cladding of an optical fiber, illustrated schematically in FIG. 6, are distinguished by differences in the index of refraction between the core and cladding, typically caused by differences in the composition of the core and cladding. FIG. 7 schematically illustrates the step-wise variations in the index of refraction associated with the grating. The illustrated variations in the index of refraction are with respect to the average index of refraction through the core of the fiber. The formation of Bragg gratings is well understood in the art, as reflected by U.S. Patent No. 5,235,659 to Atkins, et al., U.S. Patent No. 5,287,427 to Atkins, et al., U.S. Patent No. 5,327,515 to Anderson, et al., U.S. Patent No. 5,457,760 to Mizrahi and U.S. Patent No. 5,475,780 to Mizrahi. Each of these patents is incorporated herein by reference for their teachings in the use and formation of gratings within waveguides including optical fibers. As is well known in the art, the central wavelength (λ N in FIG. 5) at which the Bragg grating is effective is determined by the spacing between the individual variations in the index of refraction (FIG. 7). The width of the wavelength band is defined by the sharpness of the transitions in the index of refraction. Together these considerations define the number of wavelength bands that can be defined across the spectrum of the input beam. If the optical techniques of defining Bragg gratings described in the listed patents and those commercially available are inadequate to define a sufficient number of wavelength bands for a desired number of channels on the system, then other waveguide configurations might be used. For example, very fine features can be defined either in silica/silicon systems or in doped lithium niobate systems using x-ray and e-beam lithography techniques. Such techniques can improve the density of modulations in the index of refraction and the sharpness of the individual variations within the core of the fiber. After encoding with the mask, the modulated broadband signal is passed to the optical communication system, which is typically an optical fiber. Signals encoded in this manner may be transmitted over a variety of optical systems including, for example, single mode fibers, multi-mode fibers, polarization preserving fibers and through free space. As a practical matter, most applications presently available for this system are over fiber networks using single mode fibers and so that operating environment is primarily described herein. Decoding or demodulation is performed by coupling a fiber carrying at least the desired or target signal into a receiver or demodulator. The received signal is split into two parts by a splitter. The received light signal passes through two decoding masks. One decoding mask is the same as the encoding mask while the other decoding mask is the bitwise complement of the encoding mask, or otherwise the complement of the encoder mask if the code is, for example, analog. The two channels of decoded light signals are preferably converted to electrical signals by differential detection. For example, the decoded light signals are provided to a differential detector such as a pair of back to back diodes. The differentially detected signal is then subjected to square law detection to remove noise signals. Within the illustrative embodiments described here, it is possible to provide an L position mask (defined in terms of wavelength bands) to define L-l communication channels for a total of L-l users. Preferably, the mask is a binary mask of the type that can be analyzed as a series of optical states corresponding to a series of 0's and l's. In some embodiments, the masks in the encoder and decoder include unipolar binary codes comprising 0's and l's such as Walsh codes. The desirable properties of such codes and the design of such codes are also discussed in the applications identified and incorporated by reference above. Other codes are believed to be viable in this system, but the referenced Walsh codes are believed to provide particularly significant advantage to the present invention. The optical CDMA communication system according to the present invention may be applied in optical communication systems such as telecommunication systems, cable television systems, local area networks (LANs), as fiber backbone links within communication networks, and other high bandwidth applications. FIG. 8 illustrates the architecture of an exemplary optical communication system in which the present invention may be applied. A plurality of pairs of users s n , s 12 , s 2 ι, s 22 , • • • s N1 , s N2 are connected to an optical fiber medium 130. The first group of users s n , s 21 , . . . s N1 may be proximately located and coupled to the fiber 130 in a star configuration, and the second group of users s 12 , s 22 , • . . s N2 may be proximately located but remote from the first group and coupled to the fiber 130 in a star configuration. Alternatively, the users in the first group or the second group or both may be coupled to the fiber 130 at separate and distributed points, as shown in FIG. 9. The architecture of FIG. 8 may be more appropriate, for example, for a fiber backbone, whereas the architecture of FIG. 9 may be more appropriate for a computer system interconnected over a medium area network. Pairs of users s jl} s j2 communicate with each other using a channel of the optical fiber, and different pairs of users may simultaneously communicate over the same optical fiber. Each pair of users (s jl; 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 jl; encodes the optical signal using the code u, assigned to the user pair (s^, s j2 ), and the receiving user s j2 in the pair decodes the optical signal using the same code u,. This architecture may be used, for example, for a fiber optic backbone of a communication network. The embodiments of the present invention are described as they may be applied in this network environment; other system architectures in which the invention is also applicable are described later. FIG. 10 shows an embodiment of a CDMA modulator/encoder 140. A broad-spectrum light source 142, such as a super luminescent diode (SLD) or erbium-doped fiber source (EDFS), is coupled to an optical modulator 144. The optical modulator 144 modulates the broad-spectrum (broadband) light 146 from the optical source 142 based upon data or other information from a data source 148 using, for example, keying or pulse code modulation. In the case of a digital data source, the optical modulator 144 either passes or blocks the light 146. Alternatively, an optical modulator 144 can be constructed that will pass portions of the -/6 - light 146 in response to an analog source 148. Encoder 150 then wavelength encodes, i.e., CDMA encodes, the modulated broad-spectrum light beam 152 for injection into the fiber 162, which may be a single mode optical fiber. An optical coupler 164 such as a star coupler, a Y coupler or the like is used to couple the encoded beam into the fiber 166, which may be a portion of an optical network. Alternatively, the light beam may be first encoded with the encoder 150 and then modulated by the modulator 144. Due to the orthogonal nature of the CDMA codes selected, a plurality of CDMA modulator/encoders 140-1, 140-N can be coupled to the same star coupler 164 and their outputs can be distinguished, i.e., demodulated or decoded, at a distant site. With reference to FIG. 11, dotted line 168 corresponds to the broadband spectrum of the light beam 146. To perform CDMA encoding, the encoder 150 must selectively pass (or, conversely, reflect or block) portions of the broadband spectrum. Embodiments of the present invention perform this task with an encoder 150 that is implemented as a fiber grating filter device 170. The fiber grating filter device 170 is encoded with a desired CDMA code such that the "bit 0" chip is encoded as a fiber grating filter for that chip frequency and the absence of a fiber grating filter designates the "bit 1" chip in the sequence of chip positions along the fiber. This fiber grating filter device 170 can be viewed as a cascade of many band-stop grating filters each corresponding to the CDMA code bit in the spectral domain. For example, FIG. 11 shows a 9-bit CDMA code 172 where the presence of a grating filter, e.g., filter 174, causes a particular wavelength or frequency band, e.g., WI, corresponding to that grating filter to be reflected, and the absence of a grating filter allows a wavelength or frequency band, e.g., W3, to pass. Consequently, an exemplary bandpassed spectrum 176 can be achieved corresponding to the exemplary CDMA code 172. Such a fiber grating filter device 170 can be achieved from a series of grating filters, e.g., fusion spliced in series onto output fiber 162, formed in situ in a single filter assembly or they can be integrated together as one composite filter 178 (exemplified in FIG. 12). The mask or filter assembly illustrated in FIG. 12 represents the Fourier transform of the cascaded filter assembly illustrated in FIG. 11. Decoding may also be accomplished with an integrated mask or filter assembly as illustrated in FIG. 12 by using the illustrated filter within one of the receiver fibers and the complement (Fourier inverse) of the FIG. 12 filter in the other of the receiver fibers. The complement of the FIG. 12 filter might alternately be calculated as the Fourier transform of the complement of the filters of FIG. 11. An encoder 150 of the present invention, i.e., one implemented as a fiber grating filter device 170, avoids the critical alignment and opto-mechanical mounts typically required by the devices described in reference to FIGS 1-3. Accordingly, the present invention may facilitate reduced manufacturing costs. Because the optical system is integrated wholly within fibers, the typical optical losses associated with passing through multiple interfaces and inserting optical signals into fibers are significantly reduced. Consequently, the system exhibits very low levels of optical loss, typically less than 0.2 dB. The device is low weight and compatible with a range of commercial environments. To the extent that lengths of fiber are needed to accomplish a particular modulation or demodulation function, the fiber can be coiled so that the required optical path takes up very little actual space. The low weight, compact size and commercial, rugged nature stands in real contrast to systems such as those illustrated in FIGS. 1-3, which are largely limited to use in laboratory environments. The implementation and use of similar grating filters (restricted to a WDM environment) is described in U.S. Patent No. 5,457,760 to Mizrahi which is incorporated herein by reference. Mizrahi describes formation of a Bragg grating filter by exposing an optical fiber to an interference pattern of actinic, typically ultraviolet, radiation to form refractive index perturbations in the fiber core. Alternatively, the optical fiber is exposed to an interference pattern created by impinging a single actinic beam on a phase mask. Typically, such phase masks are manufactured by reactive ion etching of a fused-quartz substrate through a chromium mask pattern by electron-beam lithography. However, since Mizrahi only shows the use of such devices in a WDM environment, Mizrahi only shows devices manufactured for passing a single portion of a broadband signal. In constrast, embodiments of the present invention typically pass multiple portions of a broadband signal. For example, in a preferred implementation, the broadband signal is divided into 128 different wavelength or frequency band portions, e.g., and each fiber grating filter device 170 is fabricated to pass two or more of these frequency band portions. FIG. 13 shows a compatible CDMA decoder 180. Light signals 182 containing a potential plurality of spread spectrum signals are diverted from the fiber 166 using an optical coupler 184 (e.g., a star coupler), and split into two beams 186 and 188 through a beam splitter 190. The beam splitter 190 need not be a polarization insensitive element since the input beams are unpolarized on exiting the fiber network. A first split off beam 186 is filtered by a first fiber grating filter device 192 and the second split off beam 188 is filtered by a second fiber grating filter device 194. Preferably, in a binary, unipolar embodiment of the decoder, this second fiber grating filter device 194 is formed as the bit-wise complement of the first fiber grating filter device 192, i.e., grating filter elements exist in the second fiber grating filter device 194 corresponding to each frequency that the first fiber grating filter device 192 passes and, conversely, fiber grating elements are absent for each frequency that the first fiber grating filter device 192 blocks. The beam 196, after being passed through the first fiber grating filter device 192 may then be supplied to a first photodetector 198 to convert the light into an electrical signal. Similarly, the beam output 200 from the second fiber grating filter device 194 is supplied to a second photodetector 202 to convert the light into an electrical signal. Preferably, the two electrical signals are then subtracted by the back-to-back arrangement of the two photodetector diodes, 198 and 202, for being supplied to data and clock recovery hardware and/or software 204. Alternatively, an operational amplifier or equivalent device can be used as a differential detector 206 to generate a difference signal from the two fiber grating filter devices 192 and 194. The two electrical signals may also be separately processed by two gain control circuits, respectively, to adjust for different losses in the two channels 186 and 188, before a difference calculation is performed. The differential electrical signal is then detected for data recovery. Data recovery for digital data streams may include, for example, integrating and square-law detecting the difference signal. Data recovery for analog signals provided by analog code mask embodiments of the invention may include, for example, low- pass filtering the difference signal. As discussed above, it is preferred that the optical communication system be provided with a plurality of different broadband optical sources, with a single source preferably provided for each channel of the optical transmission system. Most preferably this does not mean that an EDFS or SLD is provided for each channel. These sources are expensive and it is possible to provide the desired set of matched sources in a more economical manner. FIG. 14 illustrates a particularly preferred configuration for generating a sufficient number of optical sources having well matched intensity distributions in a cost-effective manner using a single erbium-doped fiber source. A single erbium doped fiber source 300 outputs light with an acceptably broad spectrum, generally providing a bandwidth of about 28 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 which splits the input source signal and provides the output over four fibers to an array of four fiber amplifiers 304. As the output of the fiber source 300 is split into four different sources, the intensity drops in the expected manner. Each of the four split off sources is thus amplified by the four fiber amplifiers to provide four broad-spectrum light beams preferably each having an intensity approximately equal to the original source 300 intensity. For the illustrated 128 channel system, this process is repeated through several further hierarchical stages. Thus, the outputs from the four fiber amplifiers 304 are provided over fibers to a corresponding set of four splitters 306, which may also be star couplers. The splitters 306 split the output from the fiber amplifiers into a plurality of outputs also of reduced intensity. The split off output from the splitters 306 are then provided to a further array of fiber amplifiers 308, which preferably amplify the intensity of the plural channels of broad-spectrum light to provide a next set of source light beams 310 having an appropriate intensity. This process is repeated until a sufficient number of broad-spectrum sources having an appropriate intensity are generated, for example 128 independent sources for the illustrative 128-channel fiber communication system. This hierarchical arrangement is preferred as using a single originating source and a number of fiber amplifiers to obtain the desired set of broad- spectrum light sources, which advantageously takes advantage of the lower price of fiber amplifiers as compared to the fiber source. After sufficient channels of source light have been generated, the channels of source light are provided to an array of spatial light modulators or encoders like that shown in FIG. 11. The 128 different encoders use a 128-bin mask to spatially encode the input light signal, with each of the 128 masks presenting a different one of a unipolar Hadamard code vector generated in the manner discussed in the applications referenced and incorporated by reference above. Most preferably, each of the masks is a fixed mask for use in a transmission mode, with the mask having a total of 128 equal sized bins, with the bins spanning the usable spectrum of the cascaded filter mask. Thus, the 128 bins span a total of about 3.5 THz (28 nanometers) in bandwidth, with each adjacent bin defining a subsequent wavelength or frequency interval providing about 25 GHz of bandwidth. Each of the equal sized bins of the fixed mask is assigned according to the code vector to have one or the other of two binary values. Each of the 128 channels of the communication system is then defined by a distinct wavelength or frequency encoding function and each of the channels is also modulated with a time-domain signal, for example using a modulator 144 like that shown in FIG. 10. After the various channels are modulated both in wavelength (equivalently, frequency) and temporally, the 128 channels are combined and injected into a fiber. Long distance transmission for this fiber communication system is managed in a manner similar to the manner other conventional fiber communication systems are managed. As is conventional, it is typical to use a single mode fiber. In addition, the signals on the fiber will undergo dispersion and losses. It is preferable that the signals on the fiber be amplified using a conventional fiber doped amplifier at regular intervals, for example, every forty to eighty kilometers. At the other end of the transmission fiber, the combined light signals are split, amplified, and provided to an array of 128 receivers, each corresponding to one of the fixed mask channels defined by the 128 transmitters coupled into the fiber. The primary purpose of the illustrated embodiment is to expand the usage or loading on the fiber, so the receivers also include fixed masks so that each receiver is dedicated to a single one of the 128 channels. The receivers 140, as shown in FIG. 13, are each dedicated to a particular channel defined by a particular transmitter by including within the receiver one mask identical to the transmitter mask and a second mask that is the bit-wise complement of the transmitter mask. In the illustrated optical CDMA system, it is very desirable to reduce the interference between different channels of users or of different multiplexed signals so that a greater number of channels can be provided over a single fiber. Various mechanisms have been identified to perform this task and are described in the present application and in the other applications incorporated by reference herein. A fundamental way in which the present system reduces interference is by injecting light into the optical communication system only to indicate one binary state. The source is modulated so that the source produces an output intensity to indicate one logical binary state, for example, a logical 1. No light is provided to indicate a logical 0. This has the effect of reducing the overall interference in the system. Of course, the particularly preferred coding scheme, including the receiving system including different channels with complementary filtering functions, provides a very significant and basic mechanism for reducing interference. The preferred electrical system, illustrated schematically in FIG. 15, also provides a mechanism for reducing interference. The subsystem illustrated in FIG. 15 provides further detail on the back-to-back diode arrangement indicated at 198, 202 in FIG. 13. The two complementarily filtered optical signals are provided to the back-to-back diodes 198, 202, which effect both a square law optical detection and also a differential amplification function. Other combinations of optical detectors, difference detection and electrical amplification are known and might well be substituted for these functions. In particularly preferred embodiments of the present invention, the electrical output signal 200 from the diode pair 198, 202 is then low pass filtered by filter 380. The low pass filtering is performed to remove high frequency noise signals. In the illustrated system which might receive one of plural channels video data from the optical communication system at a data rate of approximately 622 MHz, the filtering might pass frequencies below about 630-650 MHz. The filtered electrical signal is then provided to an electrical square law circuit element 382 such as a diode. This square law element or limiter preferably removes the negative going portions of the received electrical signal and might also be used to amplify the positive going portions of the received electrical signal. The negative going portions of the electrical signal are -Λ - immediately identifiable as noise and so can be removed to improve the signal to noise ratio of the overall system. The electrical signal output from the limiter 382 is then analyzed to detect signals above a threshold value, which signals are recognized as transmitted logical ones. A particularly desirable and particularly economical implementation of multiple sources having desirable spectral similarities is to provide a single originating light source that is coupled to a fiber, where the output of the source is split, for example into four components by a star splitter. Each of the split off components is then amplified to an appropriate level and then each of the split off and amplified components is provided to a separate star splitter. A hierarchical structure of an original source that is split and amplified, with each successive source channel being split and amplified, can be used to develop a great many sources having essentially identical spectral characteristics. Another method of reducing interference is to reduce the correlation between different noise signals. A difficulty observed by the present inventors when implementing the source strategy shown in FIG. 14 is an undesired level of temporal correlation between the different sources. This level of correlation can give rise to undesirable levels of correlation of noise sources or of correlation between the different communication channels associated with the different sources. Consequently, preferred embodiments decorrelate the different sources. This might be accomplished by inserting different optical delays along each of the output paths of the different source channels. A large number of distinct sources 400-403 are defined, for example using the technique illustrated in FIG. 14 and discussed above, so that the sources provide similar optical outputs with similar spectral bandwidths and spectral power distribution. While four sources are shown, the system will typically include 128 or more total sources corresponding to 128 or more users. The outputs of each of the sources 400-403 are passed through a delay to reduce the temporal correlation between the different sources. Such optical delays could consist of optical delay lines or extended optical propagation paths. Causing each of the sources to pass through different lengths of fiber delay lines is the most preferred mechanism for providing appropriate delays. Delays might alternately be generated using free space propagation through different optical paths. Fiber delays are preferred since they can be implemented using only minimal space, so that the overall optical system can be provided in a sufficiently small space as to allow a wider range of implementations for systems embodying this aspect of the present invention. Referring once again to FIG. 16, appropriate delays are effected by passing the output of each of the sources 400-403 through different lengths of single mode fibers 404-407. The different length fibers are selected to impose a delay of between about one and about two or more times the data rate on successive sources. Considering a data rate of approximately 622 Mbt/sec, an appropriate delay can be fashioned by adding about one and a half feet of optical fiber (equivalent to —1.5 GHz) for each desired delay. Thus, for the first source 400, no additional length of fiber would be added as this represents the baseline. For the second source 401, 1.5 feet of additional fiber 405 would be included in the output path and for the third source 402, a three foot length of fiber 406 beyond the baseline length of fiber 404 is provided. Similarly, the output from source 403 is coupled through a fiber 407 that is about 4.5 feet (—4.5 GHz) longer than fiber 404. Each of the users within a system, which may total 128 users or more or equivalently might total 128 channels of multiplexed data, is provided with a source originating from a central source and delayed by an amount different from all of the other sources. It will of course be appreciated that different mechanisms for achieving optical delays are known and could be practiced to achieve similar results. Another method of reducing interference, and one that has been observed to be particularly effective, is the use of a data modulation scheme that limits the amount of time that the source is maintained in an on state. Time domain modulated data are provided to the optical communication system by modulating the sources. Sources may be directly modulated or may be modulated by passing the source light through an element that can modulate the source. In preferred embodiments of the present invention, modulation is accomplished so that a light pulse of predetermined intensity is provided to the optical system when one binary value is to be transmitted and no light is provided to the optical system when the other binary value is to be transmitted. A schematic example of the modulation of a source with a data stream is shown in FIG. 10. While the present invention has been described with particular emphasis on certain preferred embodiments of the present invention, the present invention is not limited to the particular embodiments described herein. Those of ordinary skill will appreciate that certain modifications and variations might be made to the particular embodiments of the present invention while remaining within the teachings of the present invention. For example, while the above embodiments have been presented in terms of communications systems mediated over fiber, aspects of the present invention are immediately used in an over the air optical system. Additionally, while the description has primarily described the use of fiber grating filter devices in transmitting unipolar CDMA-encoded signals, the use of these fiber grating filter devices is equally applicable to a bipolar system. As such, the scope of the present invention is to be determined by the following claims. -A3 -

Claims: 1. A transmitter responsive to a data signal for use in an optical communication system, comprising: a light source outputting a broadband light signal having a frequency spectrum comprised of multiple frequency portions; a data modulator for selectively modulating the broadband light signal in response to the data signal to provide a modulated broadband light signal; and a fiber grating filter device for receiving the modulated light signal and selectively passing a characteristic plurality of the multiple frequency portions of the modulated broadband light signal to an optical network so that the light signal passed to the optical network is modulated in a frequency pattern that corresponds to a code characteristic of the transmitter. 2. The transmitter of claim 1 wherein the data modulated broadband light signal is binary encoded in successive frequency portions according to the fiber grating filter device. 3. The transmitter of claim 1, wherein the fiber grating filter device comprises a plurality of discrete grating filters spaced along the fiber. 4. The transmitter of claim 1, wherein said fiber grating filter device passes approximately one half of the broadband light signal. 5. The transmitter of claim 1, wherein the fiber grating filter device passes two or more of the frequency portions. 6. The transmitter of claim 1, wherein the frequency spectrum is divided into 128 frequency portions and the fiber grating filter device passes two or more of the frequency portions. 7. The transmitter of claim 6, wherein the fiber grating filter device passes approximately one half of the incident light. 8. The transmitter of claim 2, wherein the data modulator uses a duty cycle substantially less than 50%. ^ - 9. The transmitter of claim 1, wherein the code characteristic of the transmitter is a Walsh code. 10. A transmitter responsive to a data signal for use in an optical communication system, comprising: a light source outputting a broadband light signal having a frequency spectrum comprised of multiple frequency portions; an in-fiber filter selectively attenuating portions of the frequency spectrum of the broadband light signal to thereby frequency-modulate the broadband light signal; and a data modulator time domain modulating the broadband light signal in response to the data signal, so that the transmitter outputs a time domain and frequency modulated broadband light signal, with the frequency modulation representing a code identifying the transmitter, the code defined as selective attenuations within the multiple frequency portions, each attenuated frequency portion subject to an attenuation approximately constant over the frequency portion. 11. The transmitter of claim 10 wherein the data modulated broadband light signal is CDMA encoded by the fiber grating filter device. 12. The transmitter of claim 10 wherein the in-fiber filter device comprises a plurality of discrete grating filters. 13. The transmitter of claim 10 wherein the in-fiber filter device passes approximately half of the broadband light signal. 14. The transmitter of claim 10 wherein the in-fiber filter passes two or more of the frequency portions. 15. The transmitter of claim 10 wherein the frequency spectrum is divided into 128 frequency portions and the in-fiber filter passes two or more of the frequency portions. 16. A receiver for use in an optical communication system for decoding a CDMA- encoded broadband light signal received from an optical network, comprising: a splitter that receives and splits the broadband light signal into first and second split broadband light signals; a first in-fiber Bragg grating filter selectively passing frequency portions of the first split broadband light signal to thereby generate a first filtered light signal; a second in-fiber Bragg grating filter selectively passing complementary frequency portions of the second split broadband light signal to thereby generate a second filtered light signal; and a differential detector generating a received data signal corresponding to a difference between the first and second filtered light signals, and wherein the first fiber grating filter device passes frequency portions of the broadband light signal corresponding to those of a desired CDMA-encoded signal. 17. The receiver of claim 16 wherein the in-fiber Bragg grating filter device comprises a plurality of discrete grating filters. 18. The receiver of claim 16 wherein the in-fiber Bragg grating filter device defines a Walsh code in frequency. 19. The receiver of claim 16 wherein the in-fiber Bragg grating filter passes two or more of the frequency portions. 20. The receiver of claim 16 wherein the frequency spectrum is divided into 128 frequency portions and the in-fiber Bragg grating filter is formed to pass two or more of the frequency portions. 21. An optical communication system, comprising: a light source providing broadband light signal having a frequency spectrum comprised of multiple frequency portions; a data modulator selectively passing a broadband light signal in response to a transmit data signal to thereby generate a data modulated broadband light signal; a first fiber grating filter device by selectively passing portions of the frequency spectrum of the data modulated broadband light signal; a splitter for splitting the data modulated broadband light signal into first and second split broadband light signals; -λl>' a second fiber grating filter device selectively passing portions of the first split broadband split light signal to generate a first filtered split signal corresponding to those portions of the frequency spectrum passed by the first fiber grating filter device; a third fiber grating filter device for selectively passing complementary portions of the second split broadband light signal to generate a second filtered light signal; and a differential detector for generating a received data signal corresponding to the transmit data signal formed from the difference between the first and second filtered light signals. 22. The optical communication system of claim 21 wherein the data modulator and the first fiber grating device comprise a transmitter that places the data-modulated, CDMA-encoded, broadband light signal on an optical network and the splitter, second and third fiber grating filter devices and the differential detector comprise a receiver that retrieves the data modulated broadband light signal from the optical network to retrieve a data-modulated CDMA-encoded signal. 23. The optical communication system of claim 21 wherein at least one of the fiber grating filter devices comprises a plurality of discrete grating filters. 24. The optical communication system of claim 21 wherein the fiber grating filter devices pass two or more of the frequency portions. 25. The optical communication system of claim 21 wherein the frequency spectrum is divided into 128 frequency portions and the fiber grating filter devices pass two or more of the frequency portions.

Inventors: CHAN JAMES K, DUTT BIRENDRA, CHAN, JAMES, K., DUTT, BIRENDRA
Applicants: CODESTREAM TECHNOLOGIES CORP, CODESTREAM TECHNOLOGIES CORPORATION
Application year: 2000
Main classification: H04J13
IPC codes: H04B10, H04J14