Patent Publication Number: US-2004047543-A1

Title: Ocdma network architectures, optical coders and methods for optical coding

Description:
FIELD OF THE INVENTION  
       [0001] The invention relates to OCDMA network architectures and to optical coders. The invention equally relates to methods for multiplexing broadband signals originating from a plurality of users and to methods for optical coding of broadband signals.  
       BACKGROUND OF THE INVENTION  
       [0002] Optical fibres allow transmission of signals with a huge bandwidth. A regular sequence of short broadband light pulses supplied by a source can be used as binary signal that is to be transmitted. Each light pulse represents one bit of the signal. A light pulse can be on, representing a “1”, or off, representing a “0” of the binary signal. The distance in time from one light pulse to the next is one bit period. In order to be able to share the bandwidth for several connections, the pulses have to be encoded for transmission. To achieve such an encoding without the need for complicated electronic signal processing, optical code-division multiple access (OCDMA) to the optical fibres by optical coders was introduced. The most common OCDMA systems are coherent or incoherent, synchronous or asynchronous, and based on temporal or spectral coding or on frequency-hopping, which constitutes a combination of temporal and spectral coding. The present invention relates to such coding by frequency-hopping.  
       [0003] One advantages of an OCDMA is the gain from statistical multiplexing, the efficiency of statistical multiplexing increasing with the number of users. The number of users, however, can only be increased by employing longer codes, since in frequency-hopping OCDMA systems typically every user must employ an encoder for a unique coding. In frequency-hopping, the codes can be extended by.increasing the chip rate used for temporal coding or the number of the frequency bands used for frequency encoding. The number of the frequency bands can be increased by broadening the total frequency range or by compressing the single bands. The first alternative is limited by fibre dispersion and the second by component technology. A further problem lies in logistics as the number of users can be very high and each one requires a different coder. Therefore, a large amount of unique coders may have to be provided for each OCDMA network architecture that is to be employed for coding signals from a plurality of users and for multiplexing the coded signals to a single transmission fibre, making the construction rather expensive.  
       [0004] It is known in the state of the art to use Fibre Bragg Grating (FBG) technology or Arrayed Waveguide Grating (AWG) technology for optical coders that are to be employed for coding by frequency-hopping. The use of fibre gratings for frequency hopping coding is e.g. described in “Passive Optical Fast Frequency-Hop CDMA communications System”, H. Fathallah, L. A. Rusch, and S. LaRochelle, J.Lightwave Tecdh., 17. Pp.397-405 (1999).  
       [0005] Fibre gratings can be made very narrow, leading to narrow frequency bands, but they do not solve the logistics problem. In fibre gratings, there can also be problems with reflections. Moreover, systems with fibre gratings require an optical circulator for distributing a broadband signals to different fibre gratings, which can be relatively expensive. With AWG, on the other hand, the logistic problem can be eased slightly, but problems might arise with very dense frequency bands and the insertion loss is quite high with this technology.  
       SUMMARY OF THE INVENTION  
       [0006] It is an object of the invention to provide network architectures, optical coders and methods for optical coding that allow for a less expensive optical coding for a high number of users.  
       [0007] This object is reached according to the invention with an OCDMA network architecture, comprising a plurality of means for passband filtering and multiplexing broadband signals, each of said means being assigned to a group of users, and each of said means filtering a broadband signal provided by a user of the respective group with a different frequency passband and multiplexing the filtered signals of the users of one group into a single signal; a periodic optical coder assigned to each group of users for encoding the signals multiplexed by the means for filtering and multiplexing, each coder using a different code for encoding the signals originating from the different groups; and means for combining the signals output by the coders to a single broadband signal.  
       [0008] Periodicity of the to be employed coders means that the coder can code more than one wavelength division multiplexing (WDM) channel simultaneously. This is typically case in non-coherent temporal coding, but not always in coherent temporal coding or in frequency-hopping.  
       [0009] The proposed OCDMA network architecture can be employed for multiplexing the broadband signals from a plurality of users to a single fibre with a reduced total number of coders that have to be used for such a multiplexing. The users of one group are coded simultaneously with the same coder and therefore with the same code, but since they are assigned their own frequency band, the signals of the different users do not mix.  
       [0010] If the users provide a broadband signal that is so broad that it covers the total optical spectrum used in the network, i.e. by the multiplexing means and the coders, every user can use similar sources. The multiplexing means cut for each user only a narrow slice from the broadband spectrum, while the rest is not used for this user. This narrow slice is the actually required width of the spectrum for the user; it is also a broadband signal, even though not as broad as the original supplied broadband signal.  
       [0011] The object of the invention is equally reached with an OCDMA network architecture, comprising a periodic optical coder for each of a plurality of users for encoding a broadband signal originating from the respective user, wherein each user is assigned to one of a plurality of groups, and wherein the optical coders use a different code for the different users of the same group; means for combining the encoded signals of the users of each group into a single broadband signal; and means for filtering the combined signal of each group with a different frequency passband and for multiplexing the filtered signals of the different groups.  
       [0012] The second proposed network architecture equally relies on period coding of the signals provided by the different users. The coders in one group in this network architecture have to use a different code for each user, but the same coders can be used for each group. Therefore, the number of different coders can be reduces drastically, which simplifies logistics and installations.  
       [0013] Compared to the second proposed network architecture, in the first proposed network architecture, the number of coders can be reduced by 1/M, where M is the number users in one group. On the other hand, the second proposed network architecture requires N times less means for filtering and multiplexing, where N is the number of groups. Typically, the number of groups is smaller than the number of users in one group and WDM components that can be used as means for filtering and multiplexing are cheaper than coders, therefore the first proposed network architecture will usually be more cost effective. Still, this may vary with the distribution of users to the groups. In any case, both network architectures enables a reduction of costs compared to the state of the art, where each user uses a separate coder with a different code.  
       [0014] Also corresponding OCDMA network architectures for demultiplexing a broadband signal for different users reach the object of the invention, if they comprise the same means as the respective architecture for multiplexing, but wherein all means are employed in a reversed manner. Equally, corresponding methods for multiplexing broadband signals originating from a plurality of users reach the object of the invention.  
       [0015] There might be different lengths of fibres (0 to N km) between the different components—users, coders, multiplexers/demultiplexers and couplers/splitters—of the proposed network architectures. This is of particular importance between the employed multiplexers and couplers, so different multiplexing stages can be located in different places.  
       [0016] A variety of coders can be used for the proposed network architectures. At least some of them can be temporal coders, which may both comprise serial and/or parallel delay lines, or coherent temporal-and-phase fibre:Bragg grating coders. Alternatively or additionally, at least some of the coders may be spectral phase coders and/or frequency-hopping coders, wherein the frequency hopping coders may comprise AWG, interleavers and/or FBG. All coders must be periodic, i.e. able to code in different WDM channels.  
       [0017] In a preferred embodiment of the network architectures, at least some of the employed components, i.e. of the coders couplers/splitters and multiplexers, can be used bidirectionally in order to enable a bi-directional use of the OCDMA network architecture.  
       [0018] The proposed network architectures can be used as an upgrade to an existing WDM network. This means e.g. that an existing WDM component is one of the WDM components of the new network, or many existing WDM components can be combined to a single fibre. When a network is to be upgraded in order to support more users, additional coders and couplers are inserted to network. The employed coders have to be periodic, or at least to able to code within each passband of WDM multiplexer. Major advantage of such an OCDMA upgrade compared to a normal WDM upgrade is that the WDM components are similar. In normal WDM upgrades, in contrast, the channel spacing may be the same, but the centre frequencies are shifted.  
       [0019] Any of the proposed network architectures can be included in a mixed network architectures, where the outputs of the different network architectures are multiplexed to one single fibre.  
       [0020] The object of the invention is further reached with an optical coder for coding broadband signals, comprising at least one optical interleaver for receiving a broadband signals and for splitting the frequency spectrum of the signal into at least two frequency sets with an interleaved frequency distribution; means for separate coding of at least two of the frequency sets; and means for combining the split and coded frequency sets provided by the means for coding.  
       [0021] The object is equally reached with a corresponding method for coding a broadband signal, comprising:  
       [0022] receiving a broadband signal;  
       [0023] splitting the broadband signal spectrally with at least one optical interleaver into different frequency sets with interleaving frequencies;  
       [0024] coding separately at least two of the frequency sets; and  
       [0025] combining the coded frequency sets to a single broadband signal.  
       [0026] With respect to this coder and this method, the invention proceeds from the idea that optical interleavers can be used for splitting broadband signals into different frequency sets before coding the frequency sets separately. An interleaver is able to separate a broadband signal into at least two separate, interleaved frequency sets with at least twice the channel target spacing. Equally, an interleaver is able to combine at least two separate frequency sets. Interleaver technologies are known for achieving narrow channel spacings of 50 GHz and narrower.  
       [0027] The interleaver technology allows for very dense frequency bands, enabling an efficient use of the available total frequency range. At present, bands of down to 2,5 GHz have been achieved. At the same time, the optical properties of interleavers are comparable to those of FBG and AWG. In addition, the involved all-fibre technology allows for potentially low prices. If interleavers are manufactured with planar technology, several components could even be integrated to. form one coder on a single chip. In the whole, a reduced cost in coding can be expected when employing the above proposed coder and method. In addition, the coder of the invention can be used in different WDM bands, so that the necessary number of different coders can be decreased significantly, leading to cheaper production and easier logistics and installations. In addition, the flexibility for code design is increased with the employment of interleavers for splitting broadband signals that are to be coded.  
       [0028] Interleavers are typically based on different kind of interferometers, like Mach-Zehnder, Michelson, etc., but also fiber bragg grating structures can be used.  
       [0029] The term coding or coder is to be understood to include equally encoding or encoder and decoding or decoder, since the difference consists only in the code that is applied to the respective signal. E.g., if delay lines are used for coding, in a decoder with delay lines that are a time-reversed version of the delay lines used in an encoder, the original broadband signal send to the encoder is recovered. Only if the codes mismatch, the chips are spread along the bit period. The coders can in particular also be employed simultaneously in one direction as encoder and in the opposite direction as decoder.  
       [0030] Preferably, the optical interleaver used in the optical coder of the invention comprises cascaded optical interleavers. The first stage of such a cascade should comprise a single interleaver for receiving a broadband signal and for splitting it into two frequency sets. Each following stage should then comprise the double number of interleavers of the preceding stage so that each frequency set output by one stage is split into two in the next stage, thereby doubling the channel spacing. A cascade of interleavers comprises at least two stages. The stages of the cascade of interleavers can also be combined in a way that a broadband signal input to the cascade is directly interleaved to at least four frequency sets.  
       [0031] The means for separate coding of at least two of the frequency sets can use any coding method. Particularly suited are means for temporal coding, e.g. delay lines. The means for separate coding of at least two of the frequency sets can moreover be suited for coding the frequency sets coherently or incoherently.  
       [0032] Instead of coding all frequency sets, one or several frequency sets can be removed, a coding only being applied to the remaining frequency sets. This can be achieved by preventing one or several of the frequency sets from reaching the means for combining the coded frequency sets, e.g. simply by providing an interruption in the connection to those means.  
       [0033] Just like the at least one interleaver for splitting the incoming broadband signal, the means for combining the split and coded frequency sets can be at least one interleaver, and in particular a cascade of interleavers. In this case, the coder can even be used bi-directionally, each at least one interleaver combining the frequency sets that were provided by the other at least one interleaver after coding. Alternatively, those means for combining can comprise at least one coupler.  
       [0034] The connection between the at least one interleaver and the means for coding are realised advantageously by a separate fibre, waveguide or free space optics for each output frequency set.  
       [0035] The number of necessary interleavers can be reduced by reflecting the frequencies back after coding to the at least one interleaver, which is used in this case in addition for combining the frequency sets again to a single broadband signal. A direction selective component like a circulator should then be provided for separating broadband signals entering and leaving the at least one interleaver. Instead of a circulator, also a directional coupler can be used. If such an embodiment of a coder is to be used bi-directionally and if a circulator is used as direction selective component, the signals from the different directions have to be provided in parallel to the circulator by suitable means, since a circulator is not a bi-directional device.  
       [0036] As explained above, the above proposed coders of the invention can also be used bi-directionally. A bi-directional use of coders halves the number of coders compared to a unidirectional use. Moreover, fewer fibres are required, as long as the available fibre capacity is sufficient for both directions. Another benefit is that with the number of coders and of fibres, also the number of fibre connections is reduced, which makes the system easier to install, since only one fibre is going to each location and it is not possible to misconnect the directions.  
       [0037] However, in the coders proposed above, the chip rate and the pulse length of the broadband signals must be the same in both directions. This requires unnecessary fast transmitters and receivers for one of the directions if an asymmetric connection would be enough.  
       [0038] Therefore, in a preferred embodiment of the coders of the invention, the at least one optical interleaver is suited to be used at the same time as means for combining split and coded frequency sets. Moreover, the means for combining the split and coded frequency sets are formed by a corresponding at least one optical interleaver. The frequency sets provided by any of the two at least one interleavers are then combined after coding by the respective other one of the two at least one interleavers. These features correspond to one of the above mentioned coders that can be used bi-directionally. In addition, however, means are provided for coding each frequency set output by one of at least one interleaver with a separate path of coding. Finally, means for forwarding each output frequency set to a predetermined one of the separate paths of coding depending on their origin are provided.  
       [0039] Alternatively to the at least one interleavers, any other kind of frequency selection and of frequency selective components, like AWGs, WDM filters or FBGs, could be used, as far as they are able to separate both directions simultaneously and as long as they are bi-directional.  
       [0040] A corresponding method comprises:  
       [0041] receiving a broadband signal with a characteristic indicative of the origin of the broadband signal;  
       [0042] splitting the broadband signal spectrally with at least one interleaver into different frequency sets;  
       [0043] determining the origin of the frequency sets;  
       [0044] coding each frequency set separately with a code assigned to the determined origin; and  
       [0045] combining the coded frequency sets to a single signal.  
       [0046] In contrast to the first described coders, this bi-directional coder is able to determine for each formed frequency set from where it originates. For each direction, separate means for coding are provided to which the frequency sets are distributed according to their origin, while sharing means for separating a broadband signal and for combining frequency sets in the respective opposed function. Therefore, this coder allows that different directions may have different bit and chip rates and pulse lengths. Accordingly, this coders enables an efficient employment as bi-directional coders in asymmetric systems. The same applies for the proposed method.  
       [0047] The means for distinguishing between the different origins of a frequency set can be frequency selective components, like WDM components, or direction selective components, like circulators; directional coupler cannot be employed.  
       [0048] Directive selective components enable the use of the same frequencies in both directions. With WDM components, in contrast, different directions have to use-different frequencies. As WDM component, e.g. a band WDM, a course WDM, a dense WDM, an AWG, a FBG or some similar component can be used. Probably most convenient is to use red-band WDM components for one direction and blue-band WDM components for the other direction, or C-band WDM components for one direction and L-band WDM components for the other direction. This kind of coder could probably even be fabricated on single chip.  
       [0049] For a further reduction of frequency splitting components in a coder, also the bi-directional coder of the invention can employ reflection. In this case, the function of the two at least one interleavers are combined in one at least one interleaver, just like the means for distinguishing the origin of frequency sets. Signals from both origins enter the single at least one interleaver. Since both types of signals use the same direction inside of the coder, the means for distinguishing the origin have to be frequency selective. After coding, the signals are simply reflected back to the means for separating and combining signals via the means for distinguishing their origin, which first means are used then for combining the frequency sets. At the output, a direction selective component should be employed for separating incoming and output broadband signals. Such a structure minimises the number of means for splitting the broadband signals, because one interleaver is used four times: to separate and to combine frequency bin chips from both directions.  
       [0050] When a circulator is used as direction selective component. in a bi-directional coder employing reflection, again the signals from the different directions have to be provided in parallel to the circulator by suitable means.  
       [0051] In case delay lines are used for asymmetric bi-directional coding, the length of the delay lines inside the coder can be shorten by using some of the delay lines in common in both directions, wherein the common delay should be equal to the shorter one of the two delays. This would mean that between the employed circulators or WDM components, one branch would be as short as possible and the length of the other branch would be responsible for the difference between the two total delays. Such a combined use of delay lines may be realised by employing a first fibre Bragg grating between a first delay line and a second delay line and a second fibre Bragg grating at the end of the second delay line. A frequency set from either direction enters the delay lines via the first delay line. The first fibre Bragg grating is designed to reflect the frequency band of the frequency set that is to be coded with a shorter delay only by the first delay line and to pass through all other frequency bands. The second fibre Bragg grating is designed to reflect the frequency band of the frequency set that is to be coded with a longer delay by the first and the second delay lines. Therefore, a signal of the first frequency band passes the first delay line twice, and a signal of the second frequency band passes both delay lines twice.  
       [0052] The object of the invention is also reached with an optical coder, comprising a plurality of cascaded coherent coders, wherein each of the cascaded coders is wavelength selective and reflects signals of a corresponding wavelength divisional multiplexing channel back with different amplitudes and phases and passes other wavelength divisional multiplexing channels through.  
       [0053] The coherent coders can be coherent coders as proposed in the applications titled “Method for optical coding, optical coder, and OCDMA network architecture” of the same filing date by the same applicant.  
       [0054] Similarly, the object of the invention is also reached with an optical coder, comprising a plurality of cascaded periodic fibre Bragg gratings (FBG interleavers) composing a periodic frequency hopping coder, wherein each of the cascaded FBG interleavers is designed to reflect a specific frequency set of a provided broadband signal and for passing other frequency sets of the provided broadband signal through, and wherein at least between some. of the FBG interleavers delay lines are provided. The functionally of such a coder is the same as the functionality of an interleaver coder, i.e. it can be used for a periodic frequency-hopping coding. Filters that produces similar frequency sets can be made with FBGs described e.g. in M.Ibsen et. al: “Sinc-Sampled FBG for Identical Multiple Wavelength Operation”, IEEE Photonics Tech. Lett. No.6 June 1998. In periodic frequency hopping coders there would be one such “FBG interleaver” for each frequency set. The fibre Bragg grating FBG reflects a predetermined frequency set and passes other frequencies through. The delays achieved between the FBGs determine the code. Coding over multiple bit periods is especially advantageous here.  
       [0055] The object of the invention is equally reached with a corresponding method. for coding a broadband signal, comprising:  
       [0056] a) receiving a broadband signal;  
       [0057] b) reflecting a first frequency set of the broadband signal with a first FBG grating and passing on all remaining frequency sets of the broadband signal;  
       [0058] c) delaying all passed on frequency sets;  
       [0059] d) reflecting a further frequency set of the broadband signal with a further FBG grating and passing on all remaining frequency sets of the broadband signal;  
       [0060] e) repeating steps c) and d) for all further frequency sets of the broadband signal desired in the coded broadband signal; and  
       [0061] f) combining the reflected frequency sets to a single broadband signal.  
       [0062] The delaying in step c) may be of a length of zero for some repetitions.  
       [0063] Preferred embodiments of the methods of the invention correspond to the above described preferred embodiments of the coders and the network architectures.  
       [0064] The coders of the invention can be employed in particular in the OCDMA network architectures according to the invention. Both, the network architectures and the coders and methods of the invention are based on the periodicity of coding.  
       [0065] Preferred embodiments of the coders, the network architecture and the methods of the invention are moreover included in the subclaims.  
       [0066] The coders and the methods of the invention can be used in particular in frequency-hopping OCDMA systems, especially in IP over fibre networks.  
       [0067] The network architectures, coders and methods according to the invention can be suitably combined with the network architectures, coders and methods proposed in applications of the same filing date by the same applicant, titled “Method and optical coder for coding a signal in an optical fibre network” and the above already mentioned “Method for optical coding, optical coder, and OCDMA network architecture”, both incorporated by reference herewith. 
     
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
     [0068] In the following, the invention is explained in more detail with reference to drawings, of which  
     [0069]FIGS. 1 a, b  illustrate the functioning of an embodiment of an interleaver;  
     [0070]FIG. 2 illustrates the use of interleavers in an embodiment of an optical coder according to the invention;  
     [0071]FIG. 3 shows a first embodiment of a coder according to the invention;  
     [0072]FIG. 4 shows a second embodiment of a coder according to the invention;  
     [0073]FIG. 5 shows a third embodiment of a. coder according to the invention;  
     [0074]FIG. 6 shows a fourth embodiment of a coder according to the invention;  
     [0075]FIG. 7 shows a fifth embodiment of a coder according to the invention;  
     [0076]FIG. 8 shows a sixth embodiment of a coder according to the invention;  
     [0077]FIG. 9 shows a first embodiment of a network architecture according to the invention;  
     [0078]FIG. 10 illustrates the filtering of frequencies in the network architecture of FIG. 9;  
     [0079]FIG. 11 shows a second embodiment of a network architecture according to the invention;  
     [0080]FIG. 12 illustrates the filtering of frequencies in the network architecture of FIG. 11; and  
     [0081]FIG. 13 shows a mixed network architecture according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0082]FIG. 1 a  illustrates the functioning of an interleaver used for combining two separate frequency sets and FIG. 1 b  illustrates the functioning of an interleaver used for separating a broadband signal into two separate frequency sets.  
     [0083] In FIG. 1 a , a first frequency set with channels Ch1,3,5, etc. and a channel spacing of 100 GHZ and a second frequency set with channels Ch2,4,6, etc. and a channel spacing of 100 GHz are fed to port  1  and port  2  respectively of an optical interleaver PMP. As indicated by the numbering of the channels, the frequencies for the different channels are distributed alternatingly to the two frequency sets. The interleaver PMP combines the two frequency sets to a single frequency set with channels Ch1,2,3, etc., the combined frequency set having a target channel spacing of 50 GHz. The combined frequency set is output at the output common of the interleaver PMP. The interleaver in this figure work therefore as multiplexer MUX.  
     [0084] In contrast to the interleaver PMP of FIG. 1 a , the interleaver PMP in FIG. 1 b  is used for splitting signals, i.e. as a demultiplexer DEMUX. A signal with channels Ch1,2,3 etc. and a channel spacing of 50 GHz is fed to the input common of the interleaver PMP. For splitting the received signal, the interleaver PMP distributes the channels Ch1,2,3 etc. alternatingly to two outputs port  1  and port  2 , leading to a first frequency set with channels Ch1,3,5, etc. and a second frequency set with channels Ch2,4,6, etc., each frequency set having twice the channel target spacing of 50 GHz. The interleavers of FIGS. 1 a  and  1   b  may be the same, only employed in opposite direction.  
     [0085] For transmission of a signal in an optical network, a user outputs short broadband pulses representing a sequence of bits of said signal. In frequency-hopping coding, the temporal frequency pattern constitutes together with the possible empty time slots a 2-dimensional code. It represent optical power values in the time-frequency space. The spectral separation of a broadband pulse for temporal coding can be achieved by the described interleavers.  
     [0086]FIG. 2 demonstrates the employment of such interleavers for a frequency-hopping coder according to the invention.  
     [0087] A broadband pulse source  20  is connected to the input of a first interleaver  21 . One of the two outputs of the first interleaver  21  is connected to a second interleaver  22  and the other one of the two outputs is connected to a third interleaver  23 , the three interleavers  21 - 23  thus forming a two-stage cascade. The two outputs of the second and the two outputs of the third interleaver  22 ,  23  are connected each to a separate fibre  24 - 27 .  
     [0088] For indicating the data bit “1”, the broadband pulse source  20  sends a broadband light pulse to the first interleaver  21 . The first interleaver  21  divides the spectrum of the received broadband pulse into two frequency sets, as explained with reference to FIG. 1. The second and the third interleaver  22 ,  24  each receive one of the frequency sets output by the first interleaver  21  and divide the respective received frequency set in the same manner again into two frequency sets, thus forming in the whole four different frequency sets. Each frequency set is fed to the fibre  24 - 27  connected to the respective output of the second and the third interleaver  22 ,  23 . The frequencies are distributed alternatingly to the four frequency sets so that any chosen frequency band comprising four consecutive frequencies comprises one frequency in each of the four sets. The frequency sets constitute chipsets with a chip present for every frequency of the respective frequency set for each broadband pulse that is input to the cascade of interleavers  21 - 23 . The four frequency sets are indicate in FIG. 2 as four sequences of chips with a frequency increasing from the left to the right side, each frequency set being assigned to one of the outputs of the cascade.  
     [0089] In more general terms, the cascaded interleavers  21 - 23  divide the spectrum into N f  frequency sets. Since each interleaver divides a received signal into two frequency sets, N f  is fixed by the employed number (n) of interleaver stages to N f =2 n . Therefore, the number of stages of the cascade are determined depending on the spectral code length that is to be achieved. In the cascade, the first interleaver has an input channel spacing of SP, and in the following stages, the interleavers of the nth stage have a spacing of 2 n−1 *SP. The frequencies in the fibres are thus separated by N f *SP. The number of the frequencies in each frequency-set depends on the bandwidth of the broadband pulse source  20 . Sometimes, however, the employed interleavers may constitute a limiting factor.  
     [0090]FIG. 3 shows a first embodiment of a complete coder  30  including the interleavers  21 - 23  of FIG. 2 for spatial splitting of an incoming broadband signal.  
     [0091] The coder  30  comprises the cascaded interleavers  21 - 23  of FIG. 2, to the input of which an optical fibre  35  is connected. Each output of the second and the third interleaver  22 ,  23  is connected via a separate fibre  24 - 27  to a delay line  31 - 34  causing a different temporal delay. The delay line  33  connected to the first output of the third interleaver  23  comprises an interruption. The delay lines  34 - 37  are connected again via separate fibres  24 ′- 27 ′ to a second cascade of interleavers  21 ′- 23 ′ which is formed mirror-wise to the first cascade of interleavers  21 - 23 . That means, the two fibres  24 ,  25  connected to the outputs of the second interleaver  22  are connected via delay lines  31 , 32  and fibres  24 ′,  25 ″ to two inputs of a forth interleaver  22 ′ and the two fibres  26 ,  27  connected to the outputs of the third interleaver  23  are connected via delay lines  33 ,  34  and fibres  26 ′,  27 ′ to two inputs of a fifth interleaver  23 ′. The outputs of the fourth and the fifth interleavers  22 ′,  23 ′ are connected to two inputs of a sixth interleaver  21 ′. The sixth interleaver  21 ′ has a single output connected to a single fibre  35 ′.  
     [0092] If a short broadband pulse originating from a broadband pulse source  20  (not shown in FIG. 3) is inserted to the coder  30  via fibre  35 , the pulse is first divided into N f frequency sets by the first cascade of interleavers  21 - 23  as explained with reference to FIG. 2.  
     [0093] After the spectral split, the frequencies of the frequency sets are to be temporally coded either coherently or incoherently. To this end, each of the frequency sets is delayed individually by the delay line  31 - 34  connected to the output of the second and the third interleaver  22 ,  23  outputting the respective frequency set. The different lengths of the delay lines  31 - 34  leading to a different temporal coding of the frequency sets are indicated by different numbers of loops in three of the delay lines  31 ,  32 ,  34 . Since the delay line  33  between the first output of the third interleaver  23  and the first input of the fifth interleaver  23 ′ is interrupted, the first frequency set output by the third interleaver  23  is not coded but removed.  
     [0094] The coded frequency sets are forwarded to the interleavers  21 ′- 23 ′ of the second cascade via the respective fibres  24 ′- 27 ′. The second cascade functions in an exactly reversed way as the first cascade. Accordingly, the second cascade of interleavers  21 ′- 23 ′ combines the coded frequency sets again and feeds the combined signal to the single fibre  35 ′, the combined signal having a minimal channel spacing of SP. In the resulting signal, each of the chipsets propagates in the single fibre  35 ′ in different time slots, while each chipset consists of many frequencies. Therefore, the input short broadband pulse was coded in frequency and in time. Because an optical interleaver is a periodic device, the broadband pulse can be located at any place in the frequency band, if the width of the input pulse is greater than or equal to N f *SP.  
     [0095] The bits are decoded and detected incoherently.  
     [0096] The described coder of FIG. 3 can be used bi-directionally.  
     [0097]FIG. 4 shows a second embodiment of a complete coder similar to the one in FIG. 3, but which requires less interleavers by making use of reflectors.  
     [0098] The first cascade of interleavers  21 - 23  and the delay lines  34  are arranged identically as in FIG. 3. Each delay line  34  is terminated, however, by a respective mirror or reflector  50 . A second cascade of interleavers is not provided. Instead, the first interleaver  21  of the cascade of interleavers  21 - 23  is connected via a circulator  51  to both, an input and an output fibre  35 ,  35 ′.  
     [0099] When a short broadband pulse originating from a broadband pulse source  20  (not shown) arrives on fibre  35 , it is forwarded by the circulator  51  to the first stage of the cascade of interleavers  21 - 23 . As described with reference to FIG. 3, the broadband signal is first divided into N f  frequency sets, and then each frequency set is temporally delayed by the respective delay line  34 . When reaching the end of the delay lines  34 , however, the frequency sets are now reflected by the respective reflector  50 . The reflected frequency sets therefore pass the respective delay line  34  a second time in reversed direction, until they reach the single cascade of interleavers  21 - 23  again. This cascade is then used in addition for combining the delayed frequency sets to a single signal, which is forwarded by the circulator  51  to the output fibre  35 ′.  
     [0100] The coder of FIG. 4 cannot be used bi-directionally without further supplements, but it requires only one cascade of interleavers  21 - 23  by using it for dividing and for combining of signals.  
     [0101] A third embodiment of a frequency-hopping coder according to the invention is shown in FIG. 5. The structure of the coder is similar to the one depicted in FIG. 3, but in order to allow for an independent bi-directional coding, some additional components have been included.  
     [0102] A first cascade of interleavers  21 - 23  like the one described with reference to FIG. 2 is connected on the one hand via its first-stage interleaver  21  to a generator/receiver of broadband light pulses (not shown). On the other hand, both connections of each of the two interleavers  22 ,  23  of the second stage facing away from the cascade are coupled to a respective WDM component  40 . A second shown cascade of interleavers  21 ′- 23 ′ has an identical structure, inclusive WDM components  41 . Each WDM component  40  of the first cascade is provided with two connections facing away from the first cascade, each connection being connected to corresponding connections of the WDM components  41  of the second cascade via fibres and delay lines  34 ,  34 ′, of which only the delay lines connected to the second output of the third interleaver  23  are provided with reference signs. All used WDM components  40 ,  41  are identical. Instead of the WDM components  40 ,  41 , circulators can be employed.  
     [0103] Short broadband pulses entering the first cascade via the first interleaver  21  are separated by the corresponding interleavers  21 - 23  into four different frequency sets as described with reference to FIG. 2. The same interleavers  21 - 23  combine frequency sets coming from the opposite direction and output them as single signal. Equally, short broadband pulses entering the second cascade via the sixth interleaver  21 ′ are separated by the corresponding interleavers  21 ′- 23 ′ into four different frequency sets, while the same interleavers  21 ′- 23 ′ combine frequency sets coming from the opposite direction.  
     [0104] The WDM components  40 , 41  are used for distinguishing between different directions. Frequency sets output by the first cascade are directed by the respective WDM components  40  to the respective first ones of the delay lines  34 . Those frequency sets are received by the respective WDM components  41  of the second cascade and forwarded to interleavers  21 ′- 23 ′. Frequency sets output by the second cascade are directed by the respective WDM components  41  to the respective second ones of the delay lines  34 ′. Those frequency sets are received by the WDM components  40  of the first cascade and forwarded to interleavers  21 - 23 . Accordingly, frequency sets coming from different directions can be delayed by different delay lines  34 ,  34 ′, and therefore be temporally coded independently.  
     [0105] If circulators are used instead of WDM components  40 ,  41 , the same frequencies can be used in both directions, since a circulator is a direction selective component. If WDM components are used, different directions have to use different frequencies, since these components are only frequency selective.  
     [0106] Because interleavers are periodic the described coders are also periodic, but with the coder of FIG. 5, now different codes can be used in different directions.  
     [0107]FIG. 6 shows a fourth embodiment of a frequency-hopping coder according to the invention. It constitutes an alternative bi-directional coder that is also able to code broadband pulses from both directions independently, like the coder of FIG. 5. But additionally, it comprises reflectors similar to the coder of FIG. 4.  
     [0108] A single cascade of interleavers  21 - 23  composed of a first interleaver  21  in the first stage and two interleavers  22 ,  23  in the second stage is connected via WDM components  40  for each output of the interleavers  22 ,  23  of the second stage to two different delay lines  34 ,  34 ′ respectively. Again only the delay lines connected to the outputs of the third interleaver  23  are provided with reference signs. Each delay line  34 ,  34 ′ is terminated by a reflector  50 . The interleaver  21  of the first stage of the cascade is connected on the side facing away from the cascade to a circulator  51 , which is in turn connected to means for supplying pulses from different directions in parallel. Those means comprise four WDM components  52 - 55 . WDM component  52  is connected on the one hand to a first broad band pulse source (not shown) and on the other hand via WDM component  53  and via WDM component  54  to the circulator  51 . WDM component  55  is connected on the one hand to a second broad band pulse source (not shown) and on the other hand via WDM component  53  and via WDM component  54  to the circulator  51 .  
     [0109] The two broadband pulse sources provide signals from two different directions. The use of the WDM components  40 ,  52 - 54  requires that the different directions have different frequencies.  
     [0110] All signals originating from the first broadband pulse source are referred to in the figure by A and all signals originating from a second broadband pulse source are referred to by B. Incoming broadband light pulses are first arranged in parallel by the WDM components  52 - 55 , because the circulator  51  is not a bi-directional device. Broadband light pulses A have to pass WDM component  52  before reaching the circulator  51  via WDM component  53  and broadband light pulses B have to pass WDM component  55  before reaching the circulator  51  via the same WDM component  53 . Signals A, B from both directions are fed via the WDM components  52 - 54  and the circulator  51  to the first stage of the cascade of interleavers  21 - 23 .  
     [0111] In the cascade of interleavers  21 - 23 , the broadband pulses are split into four different frequency sets as described with reference to FIG. 2. The WDM components  40  are able to distinguish between the frequency sets A, B coming from different directions. Frequency sets A originating from the first direction are forwarded by the WDM components  40  to the respective delay lines  34  connected to the first connections of the respective WDM component  40 . Frequency sets B originating from the second direction are forwarded to the delay lines  34 ′ connected to the second connections of the respective WDM component  40 . The frequency sets are therefore delayed independently, leading to a different temporal coding of the frequency sets A, B of the different directions. At the end of each of the delay lines  34 ,  34 ′, the reflectors  50  reflect the frequency sets A, B back to the connection of the WDM component  40  from which they were output. The WDM components  40  pass the encoded and reflected frequency sets on to the cascade of interleavers  21 - 23 . The interleavers  21 - 23  of the cascade combine the frequency sets A or B again and form a broadband signal A, B for output to the circulator  51 . The direction selective circulator  51  separates the incoming signals from the signals output by the cascade. The WDM components  52 - 55  direct the output signals A, B again to opposite directions. Broadband signal B originating from the second source is forwarded to the first source via WDM components  54  and  52  and broadband signal A originating from the first source is forwarded to the second source via WDM components  54  and  55 .  
     [0112] The proposed encoder minimises the number of interleavers, because each interleaver  21 - 23  is used four times.  
     [0113] The means  52 - 55  for supplying pulses from different directions in parallel to the direction selective component can be used equivalently with the encoder of FIG. 4 for enabling a bi-directional use of the coder with the same codes in both directions.  
     [0114]FIG. 7 shows an embodiment of another coder according to the invention. This coder is composed of several separate coherent coders  42   a  to  42   c.    
     [0115] A circulator  51  is connected to two fibres  35 ,  35 ′ and to a cascade of N coherent coders  42   a  to  42   c , of which only the first two and the N th  are shown. Each of the coherent coders  42   a  to  42   c  is formed by fibre Bragg gratings. Each coder  42   a  to  42   c  is designed to code another one of N different WDM channels ch1-chN.  
     [0116] When a broadband signal arrives via the first one of the fibres  35 , it is forwarded by the circulator  51  to the first coder  42   a , where a first WDM channel ch1 included in the signal is reflected with different amplitudes and phases. All other WDM channels ch2-chN are passed through to the second coder  42   b . The second coder  42   b  reflects a second WDM channel ch2 included in the signal with different amplitudes and phases and passes through all remaining WDM channels to the following coders, each reflecting a specific WDM channel and passing the other received channels through to the next coder until the Nth coherent coder  42   c  is reached. Finally, the Nth coder  42   c  reflects an N th  WDM channel chN included in the signal with different amplitudes and phases. The order of the coders provided for the different channels ch1-chN can be chosen arbitrarily.  
     [0117] The reflected WDM channels are forwarded as coded signal by the circulator  51  to the second fibre  35 ′.  
     [0118]FIG. 8 shows an embodiment of a coder according to the invention, which constitutes a coder composed of several fiber Bragg grating interleavers, and which can be employed for periodic frequency-hopping coding.  
     [0119] A circulator  51  is connected to two fibres  35 ,  35 ′ and to a first FBG interleaver  43   a . The first interleaver  43   a  is connected via a delay line  34 , a second interleaver  43   b , a second delay line  34 , a third interleaver  43   c  and a third delay line  34  to a fourth interleaver  43   d . Each interleaver  43   a  to  43   c  is designed to reflect another one of 4 different frequency sets  1 - 4 . The lengths of the employed delay lines  34  is indicated by the different numbers of loops in each line.  
     [0120] Like in the example of FIG. 7, a broadband signal arriving via the first one of the fibres  35  is forwarded by the circulator  51  to the first interleaver  43   a , where a first frequency set  1  included in the broadband signal is reflected. All other frequency sets  2 - 4  are passed through to the first delay line  34 , delaying those passed through frequency sets  2 - 4  with a first delay. The delayed frequency sets  2 - 4  are then forwarded to the second interleaver  43   b , where again a certain frequency set  2  included in the broadband signal is reflected, while all other frequency sets  3 , 4  are passed through to the second delay line  34 . Two further frequency sets  3 , 4  included in the broadband signal are additionally delayed in the same way by the second delay line  34  and the third interleaver  43   c  and the third delay line  34  and the fourth interleaver  43   d . Frequency set  4  is reflected in the interleaver  43   c  and frequency set  3  in the interleaver  43   d . In case there are more frequency sets included in the broadband signal, additional delay lines and interleavers may be provided.  
     [0121] The reflected signals are delayed again by each delay line  34  they pass on their way back to the circulator  51 , which leads to the complete temporal coding of the different frequency sets of the broadband signal. The complete delays of each frequency set, i.e. the order of the interleavers and the lengths of the delay lines between the interleavers, constitute the code applied to the complete signal for coding by frequency-hopping. The reflected coded signals are forwarded by the circulator  51  to the second fibre  35 ′ as a single frequency-hopping coded signal. The same time slot in a code cannot be used twice.  
     [0122] In the following, a possible employment of a coder according to the invention in a network architecture according to the invention is described.  
     [0123]FIG. 9 shows a first possibility of an integration of encoders according to the invention in a network architecture used for encoding the signals originating from a plurality of users  60 .  
     [0124] Each of a plurality of users  60  is connected to a separate encoder  61 . The users  60  are grouped into M groups, each group comprising N users. The output of the encoders  61  belonging to the users  60  of one group are connected to the inputs of one of M couplers  62 . The output of each coupler  62  is connected to one of M inputs of a WDM multiplexer  63 . The single output of the WDM multiplexer is. connected to a single optical fibre  64 .  
     [0125] Each user  60  outputs short broadband light pulses representing binary data that is to be transmitted via the optical fibre  64 . The short broadband pulses are encoded separately for each user  60  in the corresponding encoder  61 . Each encoder  61  corresponds to the encoder described with reference to FIG. 3 and outputs a signal with N f chipsets. Within each group of users  60 , each encoder  61  apply a different code to the respectively received broadband pulses. The maximum number of users for each group depends on the codes.  
     [0126] In order to extend the total number of users using one fibre  64 , the coded signals from N users are combined respectively by one of the couplers  62 , each coupler  62  outputting a WDM channel Ch1-ChM. Even though the same frequency sets are used for all users of one group, their signals can be differentiated within one channel Ch1-ChM because of the different temporal coding employed by the encoders  61  of one group.  
     [0127] The channels Ch1-ChM are then forwarded to the WDM multiplexer  63 . In the WDM multiplexer  63 , to each channel Ch1-ChM there is assigned a dedicated frequency passband. For each channel Ch1-ChM, the multiplexer  63  passes through only those frequencies from the combined frequency sets that belong to the passband of the respective channel Ch1-ChM. The filter is designed in a way that it passes exactly one frequency of each of the N f  frequency sets for each channel Ch1-ChM. All other frequencies are removed for that channel.  
     [0128] The filter can be in any part of the frequency set, if it only selects N f  adjacent frequencies for each channel Ch1-ChM.  
     [0129] The selection of frequencies from the frequency sets is illustrated in FIG. 10, where the chips of the different chipsets of the first channel Ch1 pass through the WDM multiplexer  63  if they lie in the frequency band of the first grey area. Correspondingly, the chips of the second channel Ch2 pass through the WDM multiplexer  63  if they lie in the frequency band of the second grey area etc.  
     [0130] After multiplexing, only N f  adjacent frequencies originate from one encoder  61 . If some frequency sets have been removed by the encoder  61 , even less frequencies originate from that encoder. If the coders and the WDM multiplexers are not perfectly aligned, there might be not exactly N f  frequencies, but e.g. N f −1 full power frequencies and two half power frequencies. But this is compensated at the receiving end, since these two half power frequencies are delayed equally in the coders, so they form one full power frequency at the receiver.  
     [0131] Since for every channel Ch1-ChM and therefore for every group of users  60  only a limited frequency band is admitted to the common fibre  64 , the encoders  61  have to be different within the same group, but the same set of encoders  61  can be used in all groups.  
     [0132] For regaining the pulses output by the users, which is not illustrated here, the WDM channels are separated again by demultiplexing, and signals with N f  adjacent chips are routed to a decoder for each user, which combine the chips originating from a corresponding encoder to pulses again. If bi-directional coders are employed, the same network architecture can be used for encoding and for decoding.  
     [0133]FIG. 11 shows a second possibility of an integration of encoders according to the invention in another network architecture according to the invention used for encoding the signals originating from a plurality of users  80 .  
     [0134] Like in the network architecture of FIG. 9, the users  80  are grouped in N groups, each group comprising M users  80 . But in contrast to the architecture of FIG. 9, the users  80  of each group are first connected to a separate WDM multiplexer  81 . Only the output of each of the N multiplexers  81  is connected to a separate encoder  82 . The output of each of the N encoders  82  is connected in turn to one of the N inputs of a coupler  83 . The coupler  83  has a single output connected to a single optical fibre  84 .  
     [0135] Each user  80  of each group outputs broadband pulses as a channel Ch1-ChM with a width greater than or equal to N f *SP. N f  is again the number of frequency sets generated in each encoder  82  for encoding, and SP is the channel spacing of the first interleaver of the first cascade in the encoders, as described with reference to FIG. 2. The channels Ch1-ChM of all M users  80  of one group are fed to the inputs of the WDM multiplexer  81  assigned to the respective group.  
     [0136] Each multiplexer  81  combines the signals from the M users  80  of one group, but passes through for each user  80  only those frequencies that lie within the range of a passband reserved for the respective user  80  or channel Ch1-ChM. Each passband should have a width of about N f *SP. Accordingly, each multiplexer  81  outputs a combined signal with a separate frequency band for each user  80  of one group, as illustrated in FIG. 12. Each grey area highlights in the different chipsets the frequencies of the frequency passband assigned to one user  80  of one group.  
     [0137] Now, the signal with the filtered and multiplexed channels are fed to the coder  82  assigned to the respective group of users  80 . The coder  82  encodes the input signal as described with reference to FIG. 3.  
     [0138] Each coder  82  encodes the signals of the M users  80  of one group simultaneously with the same code. But since within their group the M users  80  have their own frequency band, the signals of the different users  80  do not mix. After coding, the signals from the different user groups are combined by the coupler. 83 . Because the same frequencies may be assigned to users  80  of different user groups, the N coders  82  must apply different OCDMA codes. The maximum number of user groups depends therefore on the number of provided coders  82  with different codes.  
     [0139] Demultiplexing and decoding occurs with a similar network architecture in the opposite direction. If bi-directional coders are employed, the same network architecture can be used for encoding and for decoding.  
     [0140] As became apparent, the second embodiment of a network architecture has the advantage over the first embodiment of a network architecture that each coder can be used for a whole group of users, thereby reducing the required number of coders drastically.  
     [0141] As demonstrated in FIG. 13, different embodiments of network architectures  101 - 104  can be combined in a mixed network architecture, which permits a very flexible network design. The mixed architecture of FIG. 13 combines one network architecture  102  of the first described embodiment and two different network architectures  101 ,  104  of the second described embodiment. In addition, a multiplexer  103  outputting multiplexed signals without encoding is comprised. In the mixed architecture, rectangles represent couplers, circles represent coders and trapeziums represent WDM components. The signals output by each network architecture  101 - 104  are fed to a multiplexer  105 , which multiplexes all provided signals in a suitable manner to a single optical fibre  106 .