KR20010072119A - Optical cdma system - Google Patents

Abstract

The optical communication system of the present invention uses Spread Spectrum Code Division Multiple Access (CDMA) to realize excellent bandwidth utilization, and a single encoding mask having a first code is used to encode the optical signal and spatially encoded by the mask. And temporarily modulated light using the data is transmitted on the fiber link and received by the decoder, and the polarization unresponsive separator is provided with two identical power components provided to the two decoding masks used to decode the received light. And one of the masks has a second code equal to the first code, the other mask has a third code which is the complement of the first code, and the output light beam filtered by the mask generates an output signal Differentially detected, the output signal is further processed for data recovery, and the electrical signal is low pass filtered. Highly electrically square detection, the first code is selected from a set of unipolar codes obtained from a set of balanced bi-orthogonal codes, the code being binary or analog.

Description

Optical CDMA system {OPTICAL CDMA SYSTEM}

Recently, the demand for communication bandwidth is rapidly expanding with the increased use of spread-spectrum telephony, including technologies such as satellite communications, video programming distribution networks such as cable TV, and code division multiple access telephony. . These technologies are generalized and integrate well with everyday communication. Increasing demand for bandwidth communications requires significant investment in new communications technologies and new communications infrastructures. For example, the cable TV industry, telephone companies, Internet providers, and various organizations have invested in equipment for long distance optical fiber networks and optical fiber networks. The addition of this foundation, which has an active demand for bandwidth usage, still requires additional investment in new technologies and infrastructure.

Installing optical fibers over long distances is expensive. In addition, conventional optical fibers or other optical communication networks are utilized only in a small portion of the available bandwidth of the communication system. Therefore, it is very important to obtain higher availability of the fiber optic network or to increase the bandwidth of the fiber optic system. Techniques for increasing the bandwidth of optical fiber communication systems and for transferring information from multiple sources through optical fiber systems have been developed. In general, these techniques seek to use the readily available optical bandwidth of the optical fiber by complementing the relatively simple coding structure conventionally used by such a system. In many optical fiber systems with improved bandwidth, optical fibers carry optical channels on an optical carrier signal of a single narrow wavelength band, and many users use time division multiplexing (TDM) or time division multiple access (TDMA). Access the optical fiber. Time division techniques transmit frames of data by allocating consecutive time slots within a frame to a particular communication channel. Optical TDMA requires short pulsed diode lasers and provides only a moderate improvement in the usefulness of the bandwidth. In addition, improving the transmission rate on a TDM network requires that all transceivers mounted in the network be upgraded to a higher transmission rate. Partial network upgrades are preferred for less flexible TDM systems. TDM systems, on the other hand, provide a predictable and uniform data flow, which is highly desirable for multi-user systems experiencing "bursty" use. Thus, TDM technology continues to be important in optical communication systems, but other techniques must be used to obtain the desired communication bandwidth for the entire system. Thus, it would be desirable to provide increased bandwidth to optical systems that are compatible with TDM communication technology.

One way to improve the utilization of optical communication networks is to increase system bandwidth than is permitted by TDM and to use wavelength division multiplexing (WDM) or wavelength division multiple access (WDMA) to support independent forms of multi-user access. Is to use. WDM systems provide a plurality of optical channels, each using one set of non-overlapping wavelength bands to provide extended bandwidth. Information is typically generated by a narrow wavelength band light source, such as a laser or light emitting diode, and transmitted independently to each optical channel using a light beam within the assigned wavelength band. Each light source is modulated with data, and the modulated light outputs obtained for all of the different wavelength bands are multiplexed, combined into an optical fiber and transmitted through the optical fiber. Modulation of narrow wavelength band light corresponding to each channel can encode a plurality of communication channels defined by simple digital data or TDM. There will be little interference between channels defined within different wavelength bands. At the receiving end, each WDM channel ends at a receiver assigned to the wavelength band used to transmit data on the WDM channel. This can be achieved within the system by separating the entire optical signal received at different wavelengths using a demodulator, such as a tunable filter, and irradiating a narrow wavelength band optical signal to a receiver assigned to the wavelength of a particular channel. At least in theory, the availability of properly tuned light sources limits the number of users supported by the WDM system. For example, the stability of the wavelength as a function of operating temperature can also affect the operating characteristics of the WDM system.

In a more practical matter, the cost of the WDM system limits the application of this technique. One embodiment of a WDM optical fiber communication system is disclosed in US Pat. No. 5,579,143 as a video distribution network using 128 different channels. The 128 different channels are confined using 128 different lasers, which are located close together but operate at separate wavelengths. These lasers have a correctly selected wavelength, also have a finite mode structure and have the gain characteristics required for a communication system. The lasers suitable for the WDM video distribution system are each expensive, and the demand for 128 lasers with the desired optical properties can be excessive for all systems. The cost of the system is undesirable for use in applications such as local area computer networks and also limits the application of the technology. As described below, embodiments of the present invention may provide a video distribution network as described in US Pat. No. 5,579,143, and one embodiment of the present invention is an adaptable and economical system for intermediate and wide area network applications. Other forms of can be provided. Unlike the WDM of many laser systems of US Pat. No. 5,579,143, one embodiment of the present invention provides sufficient adaptability and cost efficiency and can be used in at least several forms of a local network.

As described below, one embodiment of the present invention uses spread spectrum communication techniques to obtain an improved loading of the bandwidth of an optical fiber communication system at a lower cost than known WDM systems. Spread-spectrum communication technology is known to have significant advantages and practical utility, particularly for military security and mobile phone technology. Accordingly, spread spectrum techniques that can be applied to optical communication techniques have been proposed in code division multiple access (CDMA). Spread-spectrum technology is preferred for optical communication systems because the bandwidth of optical fiber based optical communication systems is large enough, and multi-dimensional coding technology can be used without affecting the data rate of the electrically generated signal that can be immediately input into the optical communication system. Different channels of data may be defined in the frequency domain, and independent data streams may be supplied through different channels without limiting the data rate within any of the channels. From a simple point of view, the above-described WDM system needs to consider the case where a plurality of data channels are limited to spread spectrum systems defined for different wavelengths. Different wavelength channels are limited to the optical frequency domain and time domain signals that can be transmitted through each of the wavelength channels. From CDMA, the separate wavelength channels of the aforementioned WDM communication system provide a single location code where each code vector is orthogonal because there is no overlap between the code vectors.

Suggestions for optical CDMA systems are described, 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. Similar to the traditional form of radio frequency CDMA of 534-545 (1995). In contrast to the WDM system described above, the proposed optical CDMA system uses a wide spectrum spectrum source and combines frequency (samely, wavelength) coding in addition to time domain coding. A schematic description of the theoretical optical CDMA proposed in Kavehrad's paper is shown in FIG. 1. The proposed optical CDMA system employs a broad spectrum, incoherent source such as an edge emitting LED, a super lumenescent diode or an erbium doped optical fiber amplifier. In the illustrated CDMA system, the wideband source is modulated into a time domain data stream 10, wherein the time domain modulated wide spectrum light 14 is generated by a mirror 18 or other steering optics. 16) to be investigated.

Within the spatial light modulator 16, the light beam 20 is incident on the grating 22 which spatially diffuses the spectrum of light to generate a light beam 24 having wavelengths of various components diffused through the spatial region. do. The spatially diffused spectral beam 24 then irradiates a beam onto a spatially patterned mask 28 that is incident on spherical lens 26 and filters incident light. The spatially filtered light by the mask 28 passes through a second spherical lens 30 and is directed onto a second diffraction grating 34 that recombines the light. The mask 28 is located midway between the pair of coordination lenses 26, 30, and the diffraction gratings 22, 34 are located on each focal plane of the confocal lenses 26, 30. The broad optical spectrum of the incoherent source is spatially enlarged in a spatially patterned mask 28, which spatially modulates the spread spectral light. Since the spectrum of the light is spatially enlarged, the spatial modulation results in a modulation of the wavelength of the light or an equivalent frequency of light. Thus, the modulated light has the frequency pattern characteristics of the particular mask used to modulate the mask. This frequency pattern can then be used to identify a particular user in the optical network or to identify a particular channel within a multichannel transmission system.

After passing through the mask 28, the spatially modulated light passes through the lens 30, and then the wavelength modulated light beam 32 is spatially focused by the second grating 34. . The wavelength modulated and spatially focused light beam 36 passes out of the spatial light modulator 16 and is irradiated to the optical fiber network or transmission system 42 by a mirror 38 or other steering optics. Part of the CDMA system described herein represents the transmitter portion of the system, while the other part of the CDMA system shown below the optical path from the optical fiber network 42 is configured as a receiver for the illustrated system. The receiver is employed to identify a particular transmitter in a network that includes many users. This is accomplished by providing a characteristic space mask 28 in the transmitter and detecting at the receiver the spatial encoding characteristics of the transmission mask among many transmitted signals in the optical network. As described in the Kavehrad paper, it is important that the mask 28 is variable so that the transmitter can select various separate receivers on the network. In other words, frequency coding of the transmitted beam 40 when a particular user using the illustrated transmitter selects a particular receiver or when the user receives a transmitted data stream by changing the spatial pattern of the mask 28. Corresponds to the spatial coding characteristics of the receiver for which transmitter mask 0 (28) is intended.

The receiver shown in FIG. 1 detects data transmitted from a particular transmitter by detecting wavelength or frequency modulation characteristics of the transmitter mask 28 and rejecting signals with other characteristic frequency modulation patterns. Light received from the optical fiber network 42 is coupled into two different receive channels by the coupler 44. The first receiver channel comprises the same spatial light modulator 46 as the spatial light modulator 16, and the second receiver channel has a structure similar to that of the spatial light modulator 16 of the transmitter but of the transmitter mask 28 A spatial light modulator 48 having a "opposing" mask. Each spatial light modulator 46, 48 performs a filtering function on the received light signal and sends the filtered light out of the combined photodetectors 50, 52. Photodetectors 50 and 52 detect the filtered optical signal and provide an output signal to differential amplifier 54. The output of the differential amplifier 54 is provided to the low pass filter 56 and the original transmission data 58 is recovered.

2 is a detailed view of the receiver circuit. In this figure, spatial light modulators 46 and 48 are similar to the spatial light modulator 16 shown in FIG. 1, and individual components of the system are not shown separately. The received light 60 is input to the receiver and separated using a coupler 62 so that some of the light is irradiated into the spatial light modulator 46 and another portion of the light is mirrored in another space. Is irradiated into a conventional light modulator 48. Spatial light modulator 46 filters the received light 60 as used in the spatial light modulator 16 of the transmitter using the same spatial (frequency, wavelength) modulation function and filters the filtered light. The detector 50 is provided. Spatial light modulator 48 filters the received light using a complementary spatial filtering function and provides its output to the detector 52. Amplifier 54 subtracts the output signals from the two photodetectors. For the same filtering function effect of the transmitter with the spatial light modulator 16, the spatial light modulator 46 includes the same mask 66 as the transmitter mask 28. Spatial light modulator 48 includes a mask 68 that performs an auxiliary filtering function on masks 28 and 66, so that spatial light modulator 48 is assisted by the filtering function of spatial light modulators 16 and 46. FIG. Enable the fill tiling function. In Kavehrad's paper, each mask 16, 66, 68 is a liquid crystal element, and these masks are fully programmable.

The specific code embedded in the mask should be suitable for the proposed optical application. Although CDMA is widely used in radio frequency (RF) domain communication systems, its application in frequency (wavelength) domain encoding in optical systems is limited. This is because the success of the RF CDMA system is critically dependent on the use of well-designed bipolar code sequences (i.e., sequences of +1 and -1 values) with good correction characteristics. Such codes include M-sequences, gold sequences, casami sequences, and orthogonal Walsh codes. These bipolar codes can be used in the RF domain because electromagnetic signals containing phase information can be detected. RF CDMA technology is not widely applicable to optical systems in which incoherent light sources and direct detection (i.e. squared detection of intensity using a photodetector) are used because the optical system cannot detect phase information. Code sequences that define negative symbol values cannot be used in such optical systems. As a result, only unipolar codes, ie code sequences of values of 0 and 1, can be used for CDMA in direct detection optical systems.

The Kavehrad paper proposes the application of various bipolar codes to masks in the system shown in FIGS. 1 and 2, including masks provided with monopolar (zero and one thousand) M-sequences or monopolar forms of Hadamard codes. For these kinds of bipolar codes, the Kavehrad paper indicates that bipolar codes of length N should be converted to a monopolar code sequence of length 2N, and the system containing these codes should be able to support the entire N-1 users. The Kavehrad paper discloses only the theoretical application of CDMA systems with little discussion of the implementation of such a system.

More practical examples of optical CDMA systems including converted bipolar code sequences have been proposed for the transmission and detection of bipolar codes in monopolar systems. The system is described in "Optical CDMA with Spectral Encoding and Bipolar Codes," Proc., By L.Nguyen, B.Aazhang and JFYoung, et al. 29th Annual Conf. Information Sciences and System (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 is also summarized in US Pat. No. 5,760,941 to Young et al. The system referred to by Young's patent is schematically illustrated in FIG. 3, wherein the transmitter 80 uses a wide spectrum light source 82, which is driven by a beam splitter 84, with two spatial light modulators 90, 92. Are separated into two beams 86, 88 which are processed by < RTI ID = 0.0 > The first spatial light modulator 90 is placed on a dispersion grating 94 for spectral scattering the light beam 86 and on a first spatial encoding mask 96 that selectively passes or blocks spectral components of the light beam. And a lens 96 for collimating and irradiating the spectroscopy light. Lens 100 collects the spatial components of the spatially modulated light beam, and recombination grating 102 recombines the diffuse beam with encoded beam 104. The "pass" and "block" states of the encoding mask represent sequences of zeros and ones, i.e. binary numbers, monopole codes. The code 106 for the first mask 98 is code To have, here, U is the length of the N-pole cord, U ^★ his maintenance, Denotes the connection of two codes. The second encoder 92 (not shown in detail) has its encoding mask coded It is similar to the first encoder 90 except for having. The symbol source 108 outputs a sequence of phases representing zeros and ones to the first on / off modulator 110 and to the second on / off modulator 114 through the converter 112. The two modulators 110, 114 modulate the two spatially modulated light beams, which two beam splitters 116 to combine the two encoded light beams 118, 120. Are combined using. The modulated light beam is alternately coupled to an output port depending on whether the bits from the source are zero or one.

The system can then use the receiver as differential detection of two complementary channels as shown in the receiver of FIG. The receive channel has a code And code Are respectively provided, and a sequence of 0's and 1's is detected according to a channel receiving a signal associated with a mask of the channel. The system proposed in Young's US patent allows the use of bipolar codes developed for the RF CDMA technology used in optical CDMA systems. However, for a mask of length 2N, only N codes can be formed since code U and its complement U ^★ must appear connected on the mask.

It is therefore an object of the present invention to provide a frequency domain CDMA encoding / decoding structure and an optical communication system that embodies the structure when the number of users is maximized without excessively increasing interference. Another object of the present invention is to provide a system that uses a relatively simple system for encoding and decoding but effectively uses the spectrum.

The processing power ratio of the optical fiber based communication system is limited as a result of the data rate of each user timed the number of user pairs. The processing capability ratio of the optical fiber communication system is a function of the light source power of the user, the light source bandwidth, the user data rate, the number of users and the desired bit error rate (BER). In many such systems, the limiting element is a user-to-user interface that is independent of the light source power. The interface gives the maximum data rate when the user transmits information. It is an object of the present invention to increase the system throughput ratio of a spread spectrum CDMA communication system.

The present invention relates to an optical communication system, and more particularly, to an optical code division multiple access communication system for transmitting data through an optical fiber.

1 is a view showing a conventional optical fiber that mediates a CDMA communication system;

FIG. 2 is a more detailed view of one receiver outline used in the system of FIG. 1;

3 illustrates an encoder using a bipolar code in an optical CDMA system;

4 and 5 is a view showing another appearance of the optical fiber network according to the present invention,

6 is an exploded view of the assembly of the first embodiment of the encoder according to the present invention;

7 is an assembled exploded view of a first embodiment of a decoder according to the present invention;

8 is an assembled exploded view of a second embodiment of a decoder according to the present invention;

9 is a structural diagram of a liquid crystal mask used in the third embodiment of the encoder according to the present invention;

10A, 10B and 10C show the discrete transparency function for the mask of FIG. 9 continuously;

11 is a Fourier type graph of light received from a fiber,

12A and 12B schematically show an encoder and a decoder according to a third embodiment of the present invention;

13A, 13B and 13C are graphs of a mask function and a mask according to a third embodiment of the present invention,

14 is a diagrammatic representation of tools for creating an array of N wide-spectrum optical sources with sufficient intensity to generate light rays for N channels of communication on a fiber using the method according to the present invention;

15 is a diagrammatic representation of a preferred polarizing unreacted optical splitter in accordance with a preferred embodiment of the present invention, and

16 is a diagram illustrating the optical detection circuit illustrated in FIG. 7 in more detail.

This and other objects are achieved using a spatial encoder with binary or analog encoding and a receiver. In particular, a wide-spectrum light source is modulated using the data to be delivered. The modulated light beam is dispersed using, for example, a diffraction grating and passes through a spatial spectral-coding mask. The spatial coding mask represents a monopole code that belongs to the preferred monopole code set derived from the orthogonal code set of balanced anodes. The distributed frequencies encoding the changed light beams are recombined to provide transformed and encoded spread spectrum optical signals that are injected into an optical fiber or optical communication system.

The recovery of the transmitted signal is particularly through the use of a matched filter. In some receivers, in some particularly preferred embodiments, the optical splitter, which is a polarization unreacted splitter, separates and converts light in the fiber through a diffraction grating to spatially separate the spectrum of light from the fiber. Spatially spread signals, potentially including spread spectrum optical communication, pass through a receiver providing signal recovery. Most preferably, the optical signal is converted into an electrical signal by differential detection and the generated electrical signal is preferably low pass filtered and in a particularly advantageous embodiment a limiting element is provided which removes the negative component of the electrical signal. Differential detection of the receiver can be implemented in many ways.

In one embodiment, the masks of the encoder and decoder comprise unipolar binary codes, including zeros and ones, such as Walsh codes. Spatially diffuse light will pass through two decoding masks. One decoding mask is the same as the encoder mask, so long as the other decoding mask is a bit-wise supplement of the encoder mask, or if the supplement of the encoder mask is opposite if the code is for example analog. The spatially spread decoded light signal is recombined and the two channels of the optical signal are preferably converted into electrical signals by differential detection. Within the described embodiment described herein, it is possible to provide an L location mask to define the L-I communication channel for the entire L-I user.

In another embodiment, spatially diffused light can be detected by an array of detectors. Each detector in the array optically measures the intensity of light that matches the length of the diffuse wave and outputs a matching array of electrical signals. The arrangement of the electrical signals will be processed by digital signal processing (DSP). Digital signal processing involves varying the signal obtained from each detector in an array by positive or negative depending on whether the encoder mask bit is 1 (transparent) or 0 (opaque). The generated bit products are combined before data recovery. The digital processing coincides with the diversification of the signals obtained from the individual detectors in an arrangement by the corresponding bits in the Hadamard code, which is the bipolar form of the Walsh encoder code.

Coding may also use unipolar analog code. Here, analog coding means that the spatial encoder uses various opacity masks as opposed to digital coding (ie, the spatial encoder uses a mask of transparent or opaque cells). The code preferably uses one set of unipolar wavelets derived from a set of unique orthogonal wavelet functions, such as cosine and / or sine wave, square wave or Chebyshev polynomial. Unipolar wavelets are separate functional processes as opposed to continuous functions due to the fact that the mask is not persistent and contains many cells. In this embodiment, the wavelets are separated from spatially sinusoidal frequencies of various harmonics that are allowed to be decoded to be done using spatial Fourier transforms in the form of quantized or detected spatially diffuse light. The limitation of the number of cords is only based on the effect of using components that oppose the continuous harmonic sine wave and the disassembly of the receiver.

Particularly preferred receivers according to the invention comprise a polarizing unresponsive optical splitter at the input of the receiver. A more preferred receiver is that the optical CDMA receiver that effectively detects the required user channel splits the received light beam into two light beams at a sufficiently equal level of intensity to allow for the desired differential detection scheme. An embodiment of a polarization unresponsive optical splitter consists of a primary polarization sensitive element that divides the light received by the primary and secondary channels of the light of each channel with the other of two orthogonal polarizations. For example, one channel of light contains a component that vertically polarizes the received light and the other channel contains a component that horizontally polarizes the received light. The polarization of one channel is converted to the polarization of another ray. For light polarized in a straight line this involves rotating the polarization of the light. Two channels of light are recombined and provided to the beamsplitter. This beamsplitter is typically a polarization sensitive element that actually separates the combined light into two lights of substantially the same intensity, since the polarization of the combined light can be well defined and tidy.

Related application

The following applications are related to this application and are each made by reference in their entirety:

US Patent No. 08 / 752,211, "High Capacity Spread Spectrum Optical Communications System," filed November 19, 1996.

US Patent No. 09 / 126,217, "Optical CDMA System Using Sub-Band Coding," filed July 30, 1998.

US Patent No. 09 / 127,343, entitled "Method and Apparatus for Reduced Interference in Optical CDMA, filed July 30, 1998,"

We have studied both the theoretical proposals of the Kavehrad paper discussed above and the systems described in Young's patents and related patents discussed above. Many aspects of the system are inadequate. As a preliminary issue, the system is described above with the possibility of dividing two different rays of light at the same intensity level. In practical matter, the polarization of the light beams obtained in optical communication systems such as optical fiber networks will be unknown. Since beamsplitters such as those shown in Young's patent and couplers shown in Caberard's paper are polarization sensitive, the intensity will not be equally divided between the two channels of the system described in the above reference. Unless the same or nearly the same intensity level is in the two channels of the receiver, the subtraction of the two signals will not properly separate other user signals and the receiver will not be effective. Given this problem, a particularly preferred receiver according to the invention receives the light beam and divides the light beam into different channels using a polarizing unreacted optical splitter. Making a polarized unresponsive optical splitter at the input of the receiver allows a more practical optical CDMA system to be implemented and significantly increases the number of users provided to the communication system.

The Caberard paper describes that the coding optical CDMA system shown in the mask changes with the data rate of each channel of the system. In practical matter, the implementation requires a mask that can be changed at least 500 MHz and preferably higher. As described in the Caberard paper, suitable masks do not exist in the implementation of this structure and coding scheme. Preferred embodiments of the present invention provide a simpler coding scheme using slowly changing masks with high-speed data adjustments performed at fixed or input light sources.

Both Caberard and Young systems propose implementation of a bipolar cord in a method that requires a 2N-position mask to provide N-I users to the system. A particularly preferred embodiment of the invention uses a coding scheme and a detection scheme that allows N-position masks to define N-I channels useful for N-I users. The preferred coding scheme produces a modified binary (0, 1) Hadamard code array in the form of a monopole used in the encoding mask for the transmitted data. If the data is reproduced at the appropriate receiver, the data is certainly reproduced. The receiver comprises two detection channels: one channel containing a mask like that of the delivery device and a second channel with a mask that is a bit-wise supplement of the mask of the delivery device. Most preferably the receiver comprises a polarizing unresponsive optical splitter defining two receiving channels in the receiver. This system has been observed to provide reliable optical data transfer in many user systems. Thus, some preferred embodiments of the present invention have the advantage of bipolar coding schemes as described in the Young patent, but even shorter code arrangements.

An important aspect of a particularly preferred embodiment of the present invention provides a polarizing unreacted optical splitter that accepts light that is polarized or unpolarized in the receiver in unpredictable ways and reliably receives two received lights of the same intensity. In order to perform other desirable detection according to some embodiments of the present invention, it is desirable to ensure that the light received by the two lights at about the same intensity level is reliably divided despite the polarization of the input light. In a general problem, conventional beamsplitters are polarization sensitive and if the polarization of the input light is not known in advance the input beams cannot be divided into equally intensity light. Most practical optical fiber delivery systems use optical fibers that do not protect the polarization of light rays. As such, it is impossible to predict the polarization of light rays obtained in optical CDMA fiber delivery systems in most systems.

A particularly preferred embodiment of the receiver according to the preferred aspect of the present invention provides a polarization unresponsive beamsplitter. A polarization analyzer or polarization sensitive beamsplitter obtains light rays from an optical communication system. The output from the polarization analyzer or polarization beamsplitter consists of two orthogonal polarized light rays and light rays provided along two different optical paths. The polarization of one of the optical paths is modified to the other polarization of the light. Both beams containing the same polarized light of the now known polarization are both combined and separated two equal intensity light using a conventional polarization sensitive beamsplitter. These particularly preferred aspects of the invention are described in more detail below with reference to FIG. 15 in particular and to the discussions that follow.

In both channels of the receiver the signal is preferably detected by combining the light from each channel to a different pair of photodiodes in a different form, for example back-to-back. The electrical output from the photodiode will be a different measurement of the signal from the two channels. In a particularly preferred embodiment of the invention, the electrical output signal is low pass filtered and an electrical square circuit element such as a diode is provided. This square element or limiter is preferably used to eliminate the decreasing rate of the received electrical signal and also to amplify the increasing rate of the received electrical signal. The decreasing rate of the electrical signal is equal to the noise immediately, so the signal can be removed from the noise rate of the entire system.

The CDMA encoding / decoding scheme according to the invention can be used in optical communication systems such as, for example, communication systems, cable TV systems, local area networks (LANs), fiber backbone links and other high bandwidth applications in communication networks. 4 shows a structural diagram of an optical communication system in which the present invention is used. A plurality of users pairs S ₁₁ , S ₁₂ , S ₂₁ , S ₂₂ ,..., S _N1 , S _N2 are connected to the optical fiber medium 130. The first group of users, S _11, S _21, ..., S _N1, are located in close proximity, it may be coupled to the fiber 130 to the specific form, the second user group, S _12, S _22, ... S _N2 is located close to the first group, but slightly apart, and is coupled to the fiber 130 in the form of a star. Optionally, users in the first or second group, or users in both, may be coupled to the fiber 130 at each distributed point as disclosed in FIG. 5. The structure of FIG. 4 is very suitable for a fiber backbone, while the structure of FIG. 5 is more suitable for a telephone system.

The user pairs S _j1 and S _j2 are connected to the other using an optical fiber channel, and different user pairs are simultaneously connected on the same optical fiber. Each pair of users S _j1 , S _j2 is designated with a code u _j to transmit and receive data between two users, and another pair of users is preferably assigned a different code. The transmitting user (eg S _j1 ) in the user pair encodes the optical signal using the code U _j designated as the user pair S _j1 , S _j2 , and the receiving user S _j2 in the user pair has the same Code U _j is used to decode the optical signal. The structure can be used, for example, as an optical fiber backbone of a communication network. Embodiments of the present invention have been described above that they may be used in a network environment, and other system architectures in which the present invention may be used are described below.

6 shows a first embodiment 140 of a CDMA modulator / encoder. As the broadband light source 142, for example, a superluminescent diode (SLD) or erbium-doped fiber source (EDFS) was coupled to the optical modulator 144. The optical modulator regulates light at light source 142 based on data or other information, for example, at data source 146 using a key or pulsecode module. And an encoder 150 similar to the spatial light modulator 16 disclosed in FIG. 1 excluding the mask and coding scheme spatially encodes the modulated wideband beams. The encoder 150 includes a diffraction grating 152 in which the spectrum of light modulated along the axis is spatially spread. The spatially diffused light beam is aimed by the collimating lens 154 and the collimated beam passes through the encoding mask 156. The encoding mask provides a spatially encoded and modulated ray collected by collimating lens 158 and is a unitary optical fiber combined behind the broad spectral beam by diffraction grating 160 injected into fiber 162. . Optocouplers 164, such as star couplers, Y couplers, and the like, are used to couple the encoded beams to the fiber 162. Optionally the beam is first encoded by encoder 150 and adjusted by modulator 144.

7 shows a compatible decoder with two channels 170 and 172. An optical signal comprising as many spread spectrum signals as possible is converted in fiber 162 using an optocoupler (not initiated) and split into two beams through beam splitter 174. The beam splitter is most preferred as shown in FIG. 15 and described below with reference to the drawings. One incident light is spatially diffused along the axis by the diffraction grating 176 and is aimed by the collimating lens 180 before passing through the detection or decoding mask 184. In the preferred embodiment described above, the decoding mask 184 is the same as the encoding mask 156. Light passing through the decoding mask 184 passes through the collimating lens 188, the diffraction grating 192 spatially recombines the diffused light into a broad spectrum beam. In the other channel, the second component of the split receiving beam is spatially diffused by the diffraction grating 178 and aimed by the collimating lens 182 before passing through the second decoding mask 186. Most preferably in the switched-binary Hadamard code unipolar embodiment of the decoder, the second decoding mask 188 is the bit format complement of the encoder mask 184. After passing through the second decoding mask 186, the beam passes through the collimating lens 190 and the diffraction grating 194 to eliminate spatial diffusion. The output of the first decoder channel 170 is supplied to the photodetector 196 to convert light into an electronic signal. Similarly, output from decoder channel 172 is supplied to photodetector 198 to convert light into an electronic signal. And two electrical signals are attenuated by the anti-parallel arrangement of the two detection diodes 196 and 198 and the clock recovery hardware and / or software 200 supplied to the data. Two electrical signals are also processed with two gain control circuits, respectively, to adjust for different loss differences in the two detector channels 170 and 172 before another cycle is performed. Fine electrical signals are then detected for data recovery. Data recovery for digital data streams includes, for example, integration and square detection of other signals. Data recovery for similar signals provided by the instant code mask embodiment of the present invention includes, for example, low pass filtering of other signals.

8 shows another embodiment of the decoder 210. In this embodiment, the light rays received from the fiber are not split into two channels with two masks, but are diffused by the grating 212 and aimed by the lens 214. The collimated light is then intercepted by the arrangement of the detector 216. The number of detectors in the array is equal to the number of bits in the encoder mask. Each detector position corresponds to an encoder mask bit position. The detector signal at each detector in the arrangement is multiplied by "1" or "-1" depending on whether the corresponding encoder mask bit is "transparent" or "opaque". The results of all multiplier outputs are summed. The sum is then compared to threshold 218 for data recovery. The digital processing is performed in discrete logic hardware or in DSP 220 using software. When a pseudo mask is used for encoding, the detector's output is multiplied by a number other than "1" or "-1". In the embodiments of Figures 6 and 7, only one encoder mask is used for the transmission data, and no concatenated code is required in contrast to conventional designs.

A preferred encoding and decoding scheme according to the invention is described next. As used herein, "unipolar code" means a code sequence having a value between 0 and 1 in the case of a binary code or a code sequence comprising 1 or 0 in the case of binary code. Means a code function having a value between -1 and 1 in the case of a pseudo code or a code sequence comprising -1 or 1 in the case of binary code. The complement of the unipolar binary code u is (1-u), for example a bit format complement where 0 is replaced by 1 and 1 is replaced by 0. The complement of the unipolar pseudo code f is (1-f). Unipolar binary code is used as an example in the following description.

In a CDMA system, the basic condition for the spectral encoding / decoding scheme is that the decoding device at the receiving user recovers the data signal at the corresponding transmitting user, while reducing or eliminating interference from signals from all other users. In some systems, the reception mask will be fixed if a particular receiver receives data on the same channel. In other systems, the reception mask can be programmed such that other signal sources can be selected from many possible sources. In a spread spectrum CDMA system using an incoherent light source, face information is not available because the incoherent optical system only transmits a positive signal, and only a monopole code can be used for encoding. The monopolar binary code is represented by a sequence of binary digits, for example u _i = 110011110101011, where the subscript i represents the i th user pair (or channel). The number N of digits in the sequence is represented by the length of the code. Indeed, for certain preferred binary unipolar code masks, each code value is a fixed spacing slot that is transparent or opaque in a spatially patterned mask that corresponds to a fixed frequency or wavelength spacing in a broad spectrum of light modulated spatially. Corresponds to

If a single mask is used for encoding and decoding, the code is preferably chosen such that it is perpendicular.

(In Equation 1, "占" represents a bit-type dot producer of two codes, and M is a constant.)

When orthogonal codes are used, each transmitting user transmits a signal using a single encoding mask, and the corresponding receiving user uses the same single decoding mask as the encoding mask to recover the signal from the corresponding transmitting user. Eliminate interference from other users. However, the desired result only appears if the code is chosen as a binary base vector:

u ₁ = 000 ....... 001

u ₂ = 000 ....... 010

u _N = 100 ....... 000

Since the set of codes is only one digit of the total code, only one frequency bin of the mask is undesirable because power passes through while most of the bins are shielded. The system may be referred to as an incoherent wavelength division multiple access (WDMA) system. The code is not desirable because only about 1 / N of the light source is transmitted and the rest is discarded.

In the encoding and decoding systems described in Figures 6 and 7, a single mask is used for encoding, two masks are used for decoding, and a set of monopolar codes is defined such that the code u _{i in} the set is orthogonal as above. Depending on the other code u _j in the set, it may be used. It is also chosen that the code u _i is orthogonal to the difference between the particular other code u _j and its complement u _j ^* .

(In Equation 2, M ^j is a constant)

It can be seen that the decoders of the embodiments of FIGS. 7 and 8 represent the principle of equation (2). In the embodiment of FIG. 7, the light rays received at user j include signals from all transmitting users i encoded with code uj. The first channel 170 with the mask 56 generates light rays represented by u _i · u _j , and the second channel with the auxiliary mask 172 generates light rays represented by u _i · u _j ^* , and differential The arranged detectors 62 and 63 generate fine signals u _i. (U _j -u _j ^* ). In the embodiment of FIG. 8, the arrangement of the detector 73 outputs the signal represented by u _i , and the DSP74 calculates u _i · (u _j −u _j ^* ) based on the output of the detector arrangement 73. In Equation 2, the fine signal u _i (u _j -u _j ^* ) is not zero for a signal from a user using a mask having a code u _i . As a result, the decoder can recover the signal from the transmitting user i and remove the signal from all other users.

A set of monopole codes that satisfy Equation 2 can be derived from a set of balanced bipolar binary right angle codes (vi) that satisfy the following conditions:

; And

(In Equations 3 and 4, "1" represents a code in which all digits are one.)

The monopolar code (u _i ) is derived from the bipolar code (vi) by substituting -1 for 0 in vi, or

The bipolar code v _i is " equilibrium " with the same number of 1 and -1 in equation (4). The particularly preferred monopole cord (ui) is thus the same number as 1 and 0. As a result, half of the optical power is transmitted as a signal, facilitating efficient use of the source source.

An example of a balanced bipolar right angle code set is a code set based on a Hadamard matrix. A Hadamard matrix is a rectangular matrix component of 1 or -1 where every column is perpendicular to the other and every row is perpendicular to the other. For example, a 4 × 4 Hadamard matrix is:

The row (or column) vector, excluding the first row (or column) of the Hadamard matrix, provides a set of balanced bipolar binary right-angle codes that satisfy equations (3) and (4). As such, a set of unipolar codes u ₁ , u ₂ ,..., U _n used in the preferred monopolar-dipole spread spectrum CDMA system according to the present invention may be made by first constructing an n + 1 or more Hadamard matrix. Can be. Except for the first row (or column), all rows (or columns) of the Hadamard matrix can be used to make unipolar code u _j by replacing all -1's with zeros.

For example, for three user systems, the 4 × 4 Hadamard matrix can be used to form the following code:

u ₁ = [1 0 1 0]

u ₂ = [1 1 0 0]

u ₃ = [1 0 0 1]

Although there are no rules for forming a normal Hadamard matrix of arbitrary size, this is a known method for achieving a Hadamard matrix of a certain size. For example, a Hadamard matrix with size N, the power of ₂ , can be made from H ₂ using a regression algorithm.

The rules for forming a matrix with size N which is a factor of four are also known.

Although the bipolar cord sets used to generate the monopole codes in equations (3) and (4) are orthogonal and balanced, in practice the bipolar cord sets are not desirable but may be used with "near orthogonal" or "nearly balanced" codes. Can be. Code set is U _j U _j (i ≠ j) is substantially U _j Is less than orthogonal when U _j . Code is U _j It is almost balanced when 1 is substantially less than N. For example, if the code length N is large, the result is that some digits of a code change in a nearly orthogonal or nearly balanced code set. If the code is not completely orthogonal or completely balanced, as well as nearly orthogonal or nearly balanced, interference from another user is increased and system execution worsens, and this deterioration continues until full system execution. As such, codes that are substantially orthogonal or nearly balanced are considered to be orthogonal or balanced within the scope of the present invention for the purposes of the present invention.

In Figures 6 and 7, the coding masks 156, 184, 186 are not only transparent but also reflective. However, the inventor has noted that the reflective mask is difficult to make and does not have an extinction ratio of the desired size. In some embodiments, the mask is made of a liquid crystal material that is divided into a plurality of cells from " a " to " L " as shown in FIG. 9, where L is any integer and is the maximum allowable length of the code. Such LCD masks are commercially available or can be easily made using conventional techniques. The cells form a one-dimensional array arranged along the axis 230 of the spreading spatial spectrum generated by the diffraction grating 152. In one embodiment, the control of the cell is analog, which means that not only can the opacity of each cell be infinitely adjusted, but also in at least three or more controllable steps. A large number of defined steps are preferred, and the level of opacity is preferably used at levels 64 or greater. In another embodiment, the control is binary and Walsh code (unipolar Hadamard) is used. Such masks may be implemented by photonics integrated circuits such as LCD pixel arrays or solid state amplifier arrays. Currently, a system is preferred where the signal is multiplexed on the fiber and the mask is formed and fixed on the glass blank. As such a fixed mask, a binary mask that specifically realizes unipolar Hadamard code is most preferable. In the reflective mask the glass can be BK7 or quartz and the reflective area can be gold. In the most preferred fixed, binary and transmission masks at present, the glass is BK7 or quartz and the blocking area is chromium. In general, a mask is an easy and useful technique having approximately 1 to 2 inches to form a mask of 128 different, equally sized and continuous positions on the mask, which is observed in the OC-12 application of the present invention. Masks with good granularity and 256 or 512 positions are easily formed using this useful technique.

Preferred formation of analog coding uses a set of monopolar wavelength function f _i derived from a balanced bipolar vertical wavelength function g _i using f _i = (g _i +1) / 2. Equation 2-4 described in the environment of binary code applies an analog code. In other words, if the anode wavelength function satisfies Equations 3 and 4, the derived monopole wavelength function satisfies Equation 2. In one embodiment, the wavelength function is a discontinuous harmonic spatial sine wavelength as shown in FIG. The axis of ordinate is the axis at which the frequency of the beam is spread and the axis of abscissa is the axis of transparency of the beam passing through the cell. In particular, the first encoder mask transparency function shown in FIG. 10A has a spatial frequency of 1 / L, where L is the number of cells. The mask of the first encoder is a discontinuous (continuously opposite) cosine wavelength by transparency having one cycle above the frequency spectrum of L, whereby the lowest and highest frequency regions of the encoded spectrum have maximum intensity and intermediate spectral frequencies. Has the lowest strength. The second encoder mask has a spatial frequency intensity mask that has two full cycles and is twice the first encoder frequency in the length of encoder L of FIG. 10B. The third encoder also has a frequency that is three times the first encoder frequency as shown in FIG. 10C. Other harmonics may be preferably used and may minimize the material throughput of the system, the maximum number of cords may be more than one hundred and preferably hundreds or more for a multiuse system.

The maximum number of harmonics or Walsh code bits (the maximum number of codes) is limited by the number of cells in the mask. In analog masks, the number of different levels of opacity allowed in the mask is due to quantization noise at the encoder. Alternatively, Chebyshev can be used when they are perpendicular to each other, rather than using cosine wavelengths.

Cosine wavelengths for the encoding function make it easier to design a decoder. In particular, if a spatial Fourier transform of the received signal occurs, the received signal can be separated through a spatial filter for the frequency of the desired signal and then the signal is recovered. 11 shows the Fourier transform of a signal received from a fiber where the separated encoded signal comprises 1 / L, 2 / L, 4 / L and 8 / L. One such signal can be easily obtained by filtering a specific spatial frequency in the received signal.

In a third preferred embodiment of the disclosed encoder, rather than a pulse coder modulating the data, two codes are used to modulate the signal as shown in FIG. 12A in an alternative manner. In this embodiment of the encoder 238, the light path for the spatially diffused light source 240 is diverted between the first mask 242 and the second mask 244, which is the source of the data source 248. Implemented in a first mask 242 by a switcher 246 that reacts with data, the first mask encoding light provides a digital " 1 " signal and the second mask encoding light is digital for the same code channel. Provide a "0" signal. The modulator uses one liquid crystal to switch the optical path between two different encoder masks in a manner similar to a binary mask receiver. The light in both masks then combines by the summer 250 and then provides an optical communication channel, such as an optical fiber (not shown).

The received data is processed in the opposite way as shown in Fig. 12B. Decoder 260 receives light from the communication channel and generates a spectrum of received light that is spatially spread while receiving input light 262 through masks 242 and 244 through the same mask 264 and 266, respectively. In masks 264 and 266 light is then provided to other receivers 268 in the manner described in the binary receiver embodiment. The signal at the receiver 268 is then processed by the digital signal processor 270 to recover the data.

13A shows one alternative embodiment of a mask for proper coding where two different masks are used to transmit ones and zeros. In the first version, the mask formed in the L cell in the liquid crystal mask 280 is divided into four parts 282, 284, 286 and 288. Portions 282 and 284 each comprise L / 2 cells along a first linear array arranged along the spreading axis of the spectrum on the first column for encoding " 1 " for a particular code channel in the second column. 286 and 288 include L / 2 cells arranged along the same axis to encode "0" in the same channel. Preferably, the discrete transparency functions for portions 282 and 284 are complemented with each other as shown in FIG. 13B, where the ordinate represents spatial frequency and the abscissa represents intensity. As shown in FIG. 13C, in order to transmit another possibility (eg, 0), the complement of the discrete strength function for the portions 286 and 288 is reversed. In other words, the portion of the mask in section 282 is the same as the mask portion at "288" and the mask portion at "284" is the same as the portion of the mask at "286".

In addition, a mask is provided where the coding is to be repaired, which provides coding where the first portion 282 of the mask is a quadrature wavelength function and the second half is opaque to both "0" 284 and the second level. And the first half 286 is all opaque and the second half has the same pattern as the first half 282 to make "1". Alternatively, the first half 282, 286 may be a first polynomial such as a sinusoidal wavelength, and the second half 284, 288 may be a second polynomial such as a Chebyshev function.

Although specific embodiments of encoders and decoders in accordance with embodiments of the present invention are described, other embodiments may be possible in the present invention. For example, although a discontinuous wavelength function is used to encode, it can have a mask that can allow a continuous function for coding. For example, the mask can be formed by photography.

The optical systems 150, 170 and 172 in the encoder of FIG. 6 and the decoder of FIG. 7 are generally referred to as light chambers. An optical chamber, which may be a set of discrete light or integrated optical devices, encodes an input diffuse optical signal by selectively attenuating components of the spectrum of the signal according to the code. The code, which can be binary or analog, determines the degree of attenuation of the spectral components of the input signal. As will be explained in the embodiment, the light chamber is implemented with a light mask having a diffraction grating, a collimating lens and a cord, and another implementation is also possible.

It will also be appreciated that the disclosed embodiments of the encoder and decoder may apply analog modulation of the optical signal.

Similarly, only CDMA techniques are described above, but those skilled in the art can easily understand these techniques by system parameters. The system can also be used with wavelength (frequency) division multiplexing and time division multiplexing. For example, it may be used in other parts of the light spectrum such that wavelength division multiplexing may be used in other coding schemes. In addition, the code can be partitioned on a time division basis to provide time division multiplexing. In addition, optical spectrum (frequency domain) CDMA can be combined with city-domain optical CDMA to increase the number of codes and users on the network. In the time domain wide spectrum example, some users are provided with different city-domain wide spectrum codes to encode the data before the data is provided to the optical encoder. However, these users can be divided into the same wavelength encoding design discussed above. Of course, once the optical information received at the decoder is converted to the electrical digital domain, the digital signal is processed according to the time domain wide spectrum code to recover the transmitted information.

In addition, the design of a combination of various multiplexing schemes is possible, and various network algorithms can be implemented. For example, in the present invention, a plurality of users s ₁ , s ₂ , ... s _N are connected to the optical fiber medium 130 and each user s _j , as shown in the network environment shown in FIG. 5. Is applicable to a variety of fiber communication system architecture that can communicate with any user (s _i ) over the optical fiber. Each user or node s _j is assigned a code u _j to receive data from another user, and another user is assigned a different code. When the user s _i transmits data to the user s _j , the transmitting user s _i encodes the optical signal using a code assigned to the receiving user s _j , and the receiving user Decode the signal using the code assigned to the transmitting user. This necessitates changing the code so that the transmitting user transmits data according to the receiving user's code. The code for either node must be allocated from one or more nodes distributed over the network. Then, when a node is online in the network, a code is needed to encode to select one wide spectrum channel to communicate with. When a node leaves the network, the code is used by a particular code that is assigned to another node on the network. Various designs can be used to create this CSMA / CD technology or permanently assigned channels. Alternatively, this technique is used for gain codes to fix one code division channel.

In addition, the disclosed embodiments allow for increasing the number of concurrent users. In particular, the prior art design described above allows the same number of codes to be 2 ^{N / 2} when the maximum number of concurrent users is N the maximum number of codes. However, in the disclosed embodiment the maximum number of codes is ^2N . This increases the overall system throughput, thereby allowing for at least half the system output of terabits. Overall system throughput is determined by the maximum number of concurrent users and the user data rate.

Specific preferred implementations of the entire optical fiber communication system according to the present invention are now described and described. This entire system is used to increase capacity, for example bandwidth, and the optical communication system is connected with a plurality of users of the fiber optical connection. FIG. 14 illustrates a plurality of diffusion-spectrum sources at low cost using one erbium-doped fiber source and erbium-doped fiber amplifier to provide a source of channels and respective sufficient strength to drive a channel of an optical communication system It describes a device for generating a. As shown, one erbium-doped fiber source 300 outputs light with an acceptable spread spectrum, and typically provides a bandwidth of about 28 nanometers in which the intensity of the source varies 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, known as a light emitting fiber source, is provided on the fiber in a splitter, such as a star coupler 302, which splits the input source signal and provides an output on an array of four fiber amplifiers 304.

As the output of the fiber source 300 is divided into four different sources, the intensity drops. Each of the four divided sources is amplified by four fiber amplifiers to provide four wide-spectrum light beams that provide the same intensity as the original source 300 intensity. In the 128 channel system described, this process is repeated through several hierarchical steps. As such, the outputs of the four fiber amplifiers 304 are provided on the fiber in a corresponding set of four splitters 306, which may be star couplers. Splitter 306 splits the output into a plurality of outputs of reduced strength in the fiber amplifier. The split output at splitter 306 is then provided on another array of fiber amplifiers 308 that amplify the intensities of the multiple channels of diffuse-spectrum light to provide the next set of source light beams 310 with appropriate intensities. Lose. This process is repeated until the number of wide-spectrum sources with adequate intensity has been generated for 128 independent sources for 128-channel fiber communication systems. This hierarchical arrangement preferably utilizes a single source source and multiple fiber amplifiers capable of obtaining the desired set of broad spectrum light sources, which can result in lower cost fiber amplifiers compared to fiber sources.

After a sufficient channel of source light has been generated, the channel of source light is provided to an array of spatial light modulators or encoders as shown in FIG. 127 different encoders use 128 bin masks to spatially encode the input optical signal, where each of the 127 masks provides a different unipolar Hadamard code vector generated in the manner described above. . More preferably, each of the masks is a fixed mask for use in the transfer mode, where the mask has a total of 128 equally sized bins, which span the usable width of the linear mask. Thus, 128 bins have a total bandwidth of approximately 3.5 THz (28 nanometers), with each adjacent bin defining a subsequent frequency interval that provides a bandwidth of about 25 kHz. Each of the same-sized bins of the fixed mask is assigned according to a code vector having one or two other 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 unique spatial encoding function, each of which is also modulated into a time domain signal using a modulator 144 such as shown in FIG. 6, for example. After the various channels are modulated spatially (equivalently, frequency) and temporally, 128 channels are combined and scanned into the fiber.

This long distance transmission for a fiber communication system is operated in a manner similar to the manner in which other conventional fiber communication systems are operated. It is typical to use single mode fiber as is conventional. Further, the signal on the fiber is preferably amplified at regular intervals, for example every 40 to 80 kilometers, using conventional fiber doped amplifiers.

At the other end of the transmission fiber, the combined optical signal is separated and amplified and provided to 128 receiver arrays, 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 extend the usage or loading for the fiber so that the receiver includes a fixed mask to dedicate each receiver for a single channel of 128 channels. Receivers that may have the structure shown in FIG. 7 may each be dedicated to a particular channel defined by a particular transmitter by including within the receiver one mask that is identical to the transmitter mask and a second mask that is in bit direction complementary relationship to the transmitter mask. . As described above, it is possible to provide a variable mask to the receiver or transmitter, such as using a programmable LCD element, which is preferred in other embodiments. However, in the illustrated embodiment, the use of fixed masks at both the transmitting and receiving ends of the communication system reduces the system cost and significantly improves the bandwidth for large fiber links.

As mentioned above, the recovery of the optical signal from the fiber communication link is accomplished using a receiver that splits the light beam received from the optical system into two components having substantially similar power levels. A particularly preferred form of the present invention is shown in FIG. 15, which preferably shows a beam splitter used at the time of input to the receiver. The beam splitter according to the present invention can split the received light beam into two beams of sufficiently equal power level, allowing the good differential detection mechanism of the optical CDMA receiver to effectively detect the desired user channel.

One embodiment of a polarization insensitive beam splitter may be comprised of a first polarization sensitive element that separates a received light beam into first and second optical channels each having one of two different orthogonal polarizations. For example, one light channel may comprise a vertical polarization component of the received light beam and another channel may comprise a horizontal polarization component of the received light beam. Next, the polarization of one of the channels is converted to the polarization of the other light beam. For linearly polarized light, this may consist of rotating the polarization of the light. The two optical channels are then recombined and provided to the beam splitter. This beam splitter is typically a polarization sensitive element that accurately splits a combined beam into two beams of substantially the same power, since the polarization of the combined beam is well defined and predictable.

A specific embodiment of receiving light from the single mode fiber 350 is described with reference to FIG. 15. Fiber 350 is generally non-polarization preservative, and it is convenient to use a conventional linear polarizer as beam splitter 352 or polarization analyzer because the light in fiber 350 is probably linearly polarized in any direction. Polarization sensitive element 352 preferably separates the input signal beam into two orthogonal polarization components and provides the two components to two different optical paths 354 and 356. In general, different power levels will be provided along each path. The illustrated optical path may propagate through free space or may travel through polarization preservation fiber. In either case, the polarization of the light within each arm will be a uniform polarization until the polarization is changed.

One component of the light is provided along the light path 354 and maintains vertical linearly polarized light 358 throughout the light path 354. The polarization along the other optical path 356 is initially horizontal 360, and then the polarization is rotated by 90 ° by the rotating element 362 so that the polarization of the light of the second optical path is referred to by reference numeral " It is intended to be linear vertical as indicated by 364 ". When the second optical path 356 propagates through free space, the rotating element may be a half wave plate or a suitable Faraday rotator. Most preferably, the rotating element 362 performs mechanical rotation of the fiber by 90 ° when the second optical path 356 propagates through the polarization preserving fiber. Most commonly, the rotation of the fiber will proceed continuously over the length of the fiber. Of course, it is also possible to perform the rotation via other means, such as inserting a rotating element at the end of the fiber of the second optical path.

Once the two beams on the two optical paths are properly oriented in their polarization, the two beams are recombined and then split into a pair of substantially equal power beams to propagate along two additional beam paths. It is possible to use a typical polarization sensitive splitter 366 to split the beam into two substantially equal power beams after the beams from paths 354 and 356 have been combined. Two desired output beams are preferably provided along the optical paths 368 and 370 through the single mode optical fiber with linear polarization of the illustrated embodiment. 7 and 15, the input fiber 350 of FIG. 15 corresponds to the input fiber 162 of FIG. 7, and the output beams along the optical fiber paths 368, 370 (FIG. 15) are the same as those of the embodiment of FIG. 7. Corresponds to two optical paths propagating from element 174. Next, the divided and received beams are provided to the filtering elements 170 and 172 of FIG. 7 which analyze two channels through the mask shown in FIG.

In the illustrated optical CDMA system, it is highly desirable to reduce the interference between different channels of a user or between different channels having different multiple signals so that a larger number of channels can be provided on a single fiber. Various mechanisms for carrying out the work are known and are described in the present application and in other applications incorporated by reference. The basic way the system reduces interference is to only inject light into the optical communication system to indicate one binary state. The source is modulated such that the source generates an output intensity for indicating one logical binary state, for example logical value one. No light is provided to indicate a logical value of zero. This has the effect of reducing the overall interference in the system. Of course, particularly good coding schemes, including receiving systems comprising different channels with complementary filtering functions, provide a very remarkable and basic device for interference reduction.

The preferred electrical system shown schematically in FIG. 16 also provides a mechanism for reducing interference. The subsystem shown in FIG. 16 provides further details for a back-to-back diode device, denoted by reference numerals 196 and 198 in FIG. 7. Two complementary filtered optical signals are provided to back-to-back diodes 196, 198 that perform differential amplification as well as square light detection. Other combinations of photo detectors, differential detections and electrical amplification are known and can be used instead of the above functions. In a particularly preferred embodiment of the present invention, the electrical output signal 200 from diode pairs 196 and 198 is low pass filtered by filter 380. Low pass filtering is performed to remove the high frequency noise signal. In the illustrated system capable of receiving one of the multi-channel video data at a data rate of approximately 622 MHz from the optical communication system, the filtering may pass a frequency of 630 to 650 MHz. The filtered electrical signal is then provided to an electric square circuit element 382, such as a diode. The square element or limiter may preferably be used to eliminate the positive rising portion of the received electrical signal and also to amplify the negative falling portion of the received display signal. The negative falling portion of the electrical signal is immediately identifiable as noise and can therefore be removed to improve the signal-to-noise ratio of the entire system. Next, the electrical signal output from the limiter 382 is analyzed to detect a signal above a threshold, which is recognized as a transmission signal.

While the invention has been described with particular emphasis on certain preferred embodiments of the invention, the invention is not limited to the specific embodiments described herein. Those skilled in the art will appreciate that certain changes or modifications may be made to the specific embodiments of the present invention while remaining within the teachings of the present invention. For example, although the above embodiments are described in terms of a fiber-based communication system, the form of the present invention can be used immediately in an optical system through air. As such, the scope of the invention should be determined by the claims that follow.

Claims (34)

A data source for providing data; And A light source for providing a first light beam, the encoder being connected to the data source to cause the first light beam to be modulated according to the data, The encoder A first spectral filtration assembly comprising a first cord, and An optical coupler for coupling the modulated and encoded light beam with the optical fiber, The code is a sequence of N digits each having one of at least two values, and the optical chamber separates the light beam into N spectral components, each corresponding to one digit of the code, Attenuate each spectral component according to a value, and use the code to spectrally encode the input light beam by recombining the spectral components to produce an encoded output light beam, The first code is selected from a set of unipolar codes, where each code in the set is orthogonal to the difference between any other code in the set and the complement of the other code. Optical communication system, characterized in that. The method of claim 1, A user side receiving a decoder to decode the optical signal received from the optical fiber and recover the data transmitted by the transmitting user, And the decoder comprises a phase insensitive optical power separator for dividing a portion of the optical signal transmitted by the optical fiber into approximately equal power components. The method of claim 2, Second and third spectral filtration assemblies connected to receive the first and second components of the received light; And A photodetector provided to receive the first and second filtration components of the received light, The second spectral filtration assembly comprises a third spectral filtration assembly comprising a first cord and a complement of the first cord, wherein the second and third spectral filtration assemblies contain the first and second filtration components of the received light. Output, And the photodetector provides an electrical signal output. The method of claim 3, wherein The electrical signal indicative of a differential measurement between the first and second filtration components of the received light. The method of claim 4, wherein The electrical signal output is provided to a limiting circuit for removing an electrical noise signal having an opposite sign of the recovered data. The method of claim 4, wherein And said electrical signal output is provided to a limiting circuit comprising an electric square law detector. The method of claim 6, And the limiting circuit comprises a diode in series with the electrical signal. The method of claim 1, Further comprising a decoder for recovering data transmitted by a user receiving and transmitting light from the optical fiber, The decoder A phase unreacted optical power separator for dividing a portion of the optical signal transmitted by the optical fiber into first and second components of approximately equal power; Second and third spectral filtration assemblies connected to receive the first and second components of the received light; And A photodetector provided to receive the first and second filtration components of the received light, The second spectral filtration assembly comprises a third spectral filtration assembly comprising a first cord and a complement of the first cord, wherein the second and third spectral filtration assemblies contain the first and second filtration components of the received light. Output, And the photodetector provides an electrical signal output. The method of claim 1, Further comprising a decoder for recovering data transmitted by a user receiving and transmitting light from the optical fiber, The decoder Second and third spectral filtration assemblies connected to receive the first and second components of the received light; A photodetector provided to receive the first and second filtration components of the received light; And A limiting circuit connected to receive an electrical signal output, The second spectral filtration assembly comprises a third spectral filtration assembly comprising a first cord and a complement of the first cord, wherein the second and third spectral filtration assemblies comprise first and second filtration components of the received light. Output The photodetector provides an electrical signal output, The limiting circuit removes an electrical noise signal having an opposite sign of recovered data from the electrical signal output. The method of claim 9, And said limiting circuit comprises a diode through which an electrical signal flows. The method of claim 1, Further comprising a user side receiving a decoding apparatus for recovering a data signal transmitted by a user who decodes and transmits the optical signal transmitted by the optical fiber, The decoding device An optical power splitter for switching a part of the optical signal transmitted by the optical fiber; A spatial frequency diffractor that spatially diffuses the spectrum of the beam of light diverted along one axis in one plane into an N spectral component corresponding to the N spectral component separated by the first optical chamber; An array of N photodetectors each corresponding to a digit of a first code and disposed in the plane along the axis; And Circuitry for processing output electrical signals from an array of photodetectors, Each detector detects the spectral components of the diffuse beam and outputs an electrical signal indicative of the intensity of the detected spectral components, The circuit multiplies the number according to the value of the corresponding digit in the first code by the output signal from each detector and adds the multiplied electrical signal to produce an output signal, indicating that the output signal is to be recovered. An optical communication system characterized by the above. The method of claim 1, The value of each digit of the first code is 0 or 1, and the first spectral filtration assembly blocks spectral components when the value of the corresponding code digit is 0, and the value of the corresponding code digit is 1 An optical communication system, characterized in that passing at least a portion of the spectral components. The method of claim 1, The value of each digit of the first code is between 0 and 1, and wherein the first spectral filtration assembly attenuates each spectral component such that the attenuated intensity is proportional to the value of the corresponding code digit. Optical communication system. The method of claim 1, And said data signal is a digital signal. The method of claim 1, And the data signal is an analog signal. The method of claim 1, The optical communication system has a plurality of users for simultaneously transmitting optical signals on a fiber, and the encoders for different users simultaneously transmitting have different first codes. The method of claim 2, The phase non-reactive optical power separator A first polarization sensitive element arranged to receive an optical signal and to separate the optical signal into first and second optical components; A first beam path along which the first light component moves and a second beam path along which the second light component moves accordingly; A polarization modifier disposed along the second beampath; And A beam splitter that receives the first and second light components and splits the first and second light components into third and fourth light components, As the output from the first polarization reaction element, the first light component has a first polarization, the second light component has a second polarization, and the polarization modifier has a polarization of the second light component mainly being a first polarization. Optical communication system, characterized in that for changing to be. The method of claim 17, And the first and second beam paths remain polarized. The method of claim 18, And the first and second beam paths propagate through an optical fiber. The method of claim 19, And the polarization modifier is a rotation of the fiber of the second optical path. The method of claim 17, The beam splitter is an optical communication system, characterized in that the polarization reaction element. The method of claim 17, And the third and fourth optical components have the same power level. The method of claim 17, The decoder Second and third spectral filtration assemblies connected to receive the third and fourth light components; And A photodetector provided to receive said first and second filtered light components, The second spectral filtration assembly comprises a third spectral filtration assembly comprising a first cord and a complement of the first cord, wherein the second and third spectral filtration assemblies output first and second filtration light components; , And the photodetector provides an electrical signal output. The method of claim 23, wherein And said electrical signal output is indicative of the difference amount between the first and second filtered optical components. The method of claim 24, And the electrical signal output is provided to a limiting circuit for removing an electrical noise signal having an opposite sign of the recovered data. The method of claim 24, And said electrical signal output is provided to a limiting circuit comprising an electric square detector. The method of claim 3, wherein Each spectral filtration assembly A spatial frequency diffracter that spatially spreads the spectrum of the input light beam along one axis in one plane; A mask divided into an array of N cells arranged in the plane along the axis for optically processing a diffuse light beam; And A spatial diffusion spectrum recombiner for recombining the spatial spread spectrum of the light beam processed by the mask to produce an output light beam, Wherein said array of N cells corresponds to a sequence of N digits of code in an optical chamber, each cell in an optical state in accordance with a value of a corresponding code digit. The method of claim 27, Wherein each cell of the mask is transparent or opaque. The method of claim 27, Wherein each cell of the mask has one of two levels of opacity. The method of claim 27, And said mask is a liquid crystal element. In a method of optically communicating between a plurality of users, For each user communication the method is: Providing a data signal; Providing a light beam; Modulating the intensity of the light beam according to the data signal; Spectral encoding the light beam using a first code; And Connecting the modulated and encoded light beam with the optical fiber, The first code is a sequence of N digits each having one of at least two values, wherein the first code is selected from a set of unipolar codes, wherein each code in the set is any other code in the set. And orthogonal to the difference between the complement of the other code and the plurality of users. The method of claim 31, wherein For each user communication: Diverting a portion of the light beam from the optical fiber; Dividing the converted light beam into first and second light beams; Spectral decoding the first light beam using the first code; Spectral decoding the second light beam using a code that is the complement of the first code; And Differentially detecting the spectrally decoded first and second light beams to produce an output signal. The method of claim 31, wherein And wherein said spectral encoding step comprises splitting a light beam into a plurality of spectral components using a fixed mask. The method of claim 33, wherein Wherein the spectral encoding step comprises separating the first and second light beams into a plurality of spectral components using the first and second fixed masks.