Patent Application
20080310840
The present invention relates to a communication network allowing transmission between nodes thereof by means of short and ultra short pulses, and to a method of routing in such a network.
JP 11215103A2 describes a device for multiplexing plural optical signals, in the form of short light pulse trains, on a dispersion axis. The multiplexing is provided by adding a variance (different amounts) of dispersion to respective different pulse trains at a transmitting side, and adding the same variance but of an opposite sign of dispersion at the receiving side, and applying multiple separation to them. The solution of JP 11215103A2 is applicable to the case of end-to end transmission.
US 20040208608 describes a dispersion compensation architecture for a switch ready optical network including an identified, switch-ready optical network region having a maximum propagation length, a dispersion section of the region having a section length, and dispersion compensation measures operably applied to said dispersion section, wherein the dispersion compensation measures are selected based on at least one determined regional target value of regional aggregated dispersion, the section length, and the maximum propagation length.
WO 0215441A2 describes a tunable dispersion compensation module which can be used to determine a desired dispersion compensation value for a transmission path. In one aspect, the tunable dispersion compensation module comprises at least one variable coupler that is capable of receiving one or more optical signals and providing the signals to a first path having a first dispersion, or second path having a second dispersion. Once the desired value is determined, a connectable compensation module is configured to provide the optimal dispersion compensation value determined using the tunable module. The connectable module may include a reconfigurable connector that is capable of receiving one or more optical signals and providing the signals to a first path having a first dispersion and a second path having a second dispersion. As is the case with the tunable module, multiple stages can be provided to the connectable module to achieve desired range and/or granularity of dispersion.
The above known techniques solve each its specific problem.
To the best of the Applicant's knowledge, the relevant prior art does not comprise references, which would approach the new task and the new solution which will be proposed below.
The object of the present invention is to provide a new technique for communication using ultra short pulses in an optical communication network, and a new telecommunication network enabling such communication.
The above object can be achieved by providing a network comprising a plurality of network nodes interconnected to one another by at least one optical fiber link along which an optical communication channel is extended, said channel being characterized by having a single optical wavelength, and wherein:
each of at least two network nodes out of said plurality being operative as a transmitting node capable of generating a first short pulse optical signal and transmitting it along said optical communication channel towards other network nodes of the plurality, and wherein each of the at least two network nodes being adapted to perform transmission at a different bit rate than the other;
at least one of the other network nodes being operative as a receiving node, is designated to receive transmissions from at least one of said transmitting nodes via said communication channel in a form of a second short pulse optical signal, and wherein said at least one receiving node being provided with a dispersion compensation module adapted to compensate dispersion created in the optical fiber along said optical channel extending between said at least one of said transmitting nodes and said at least one receiving node, thereby ensuring selective receipt, in a restored form, of transmissions comprised in said second optical signal and directed to said at least one receiving node.
According to one embodiment, the network is for example, a ring-like or a mesh-like network comprising a plurality of network nodes including transmitting nodes and receiving nodes interconnected to one another by optical fiber links and via an optical communication channel, said channel being characterized by a single optical wavelength; in the network:
each of the transmitting nodes being capable of generating a first short pulse optical signal and transmitting it via said optical communication channel towards other network nodes, different transmitting nodes being adapted to perform transmission at arbitrary bit rates;
each of said receiving nodes is assigned to receive a second short pulse optical signal from a particular transmitting node via said communication channel, and being provided with a dispersion compensation module adapted to compensate dispersion created in the optical fiber along a communication path via said optical channel between said particular transmitting node and said receiving node, thereby ensuring receipt of said second optical signal in the restored form at said receiving node, while any other short pulse optical signals to which said receiving node is not assigned are received as noise (preferably, low interference noise),
the network allows preventing interference of different short pulse signals having arbitrary bit rates, and detecting any of said signals at an arbitrary receiving node of the network without reconfiguring the network.
It is understood that any particular network node may constitute a transmitting node and/or a receiving node.
It should be noted that other receiving nodes of the network (not assigned to said second optical signal), will receive it as a low power noise due to not performing dispersion compensation matching for the specific distances of that nodes from the transmitting node.
The above network thereby allows performing a so-called dispersion based routing by “broadcasting” of a short pulse optical signal from a transmitting node via an optical communication channel to all network nodes to which that channel arrives, and by further adjustment of dispersion compensation modules at one or more assigned receiving nodes to exact lengths of selected routes between the transmitter node and the assigned receiver node(s). The mentioned broadcasting in the network can be ensured, say, by branching optical fibers at the network nodes using optical couplers and splitters.
It should be emphasized that the proposed solution enables building the network without filters and add drop equipment at the nodes, which fact drastically simplifies the network and reduces its cost.
Picking only the required signal by a matching dispersion compensation module becomes possible owing to a very special feature of transmitting ultra short pulses: when affected by chromatic dispersion in a communication line, ultra short pulses become severely distorted and the power of the short pulses averages over the bit period so that when the dispersion matching condition does not exist, their energy become noise. In networks transmitting short pulses, exact dispersion matching allows for restoring the shape of ultra short pulses thereby recovering the desired signals. It should be noted, however, that the optical signals can be amplified along the single wavelength optical line quite easily, for example using remotely or locally pumped Erbium Doped Fiber Amplifiers.
The dispersion compensation module can be switcheable. Preferably, it is tunable to take into account possible fluctuations in dispersion of the fibers due to temperature, stress, aging, etc.
According to the preferred embodiment of the network, lengths of communication paths to be distinguished at any of said one or more receiving nodes by the dispersion compensation module of the node should differ from one another for of about 1 km or more. The minimal difference is pointed out as an example only, and should be specifically selected for each specific network, taking into account typical dispersion length of its links, depending on transmitters' parameters—pulse width, chirp, bit rate, etc.
The mentioned short pulse optical signals should be understood as formed by short or ultra short optical pulses of RZ (return to zero) type. For example, the pulses having duration in the order of picoseconds and extinction ratio (power of logical “1’/power of logical “0”) no less than 10 dB can be used for the proposed technology.
The short pulse signals in the network will preferably be transmitted at different bit rates, which is advantageous from the point of judicial bandwidth distribution taking into account bandwidth limitations and available equipment, from the point of minimizing prices of equipment, and from the point of maximally reducing noise in the network. Indeed, short pulse signals having low bit rate provide lower noise power to “neighboring signals” in the network during any specific time period (window), and require less expensive transmitting equipment than short pulse signals having higher bit rate.
Preferably, the network is provided with power equalization means ensuring that a group of short pulse optical signals incoming a specific network node and then leaving said network node in a form of a combined optical signal transmitted via a common optical fiber link, have substantially equal peak powers in the combined optical signal.
Since the peak energy of a short pulse optical signal can generally be written down as Pi av/BRi, where Pi av—average power of the incoming signal (i) and BRi is the bit rate of the incoming signal (i), the power equalizing means at each node, where a group of short pulse signals is joined into a combined outgoing signal, should be operative to equalize powers of these signals substantially according to the following equation:
P
1 av
/BR
1
=P
2 av
/BR
2
=P
3 av
/BR
3
. . . =P
n av
/BR
n., (1)
where BR1+BR2+BR3+ . . . +BRn=≦BRmax (2)
and BRmax is a design rule characteristic selected to minimize bit error rate (BER).
One group of the above-mentioned optical signals incoming the node should be understood as comprising short pulse optical signals incoming the node from various optical fiber links and/or short pulse signals added at that node (say, transmitted from that node); as has been mentioned, the optical signals of the group are to be joined i.e., egress from that node together, via a common optical fiber link (naturally, via a common optical channel in said link). In a mesh network, more than one groups of separate optical signals may be defined at one network node; the above-mentioned power equalizing rule is to be kept for each of said groups. Different groups may have different ratios of Pi/BRi, but within one and the same group the ratio Pi/BRi is to be maintained constant.
It should be noted that more than one short pulse optical signals may enter a network node via one and the same optical fiber link in the form of an incoming combined signal (i). In this case, the average power Pi av of such a combined incoming signal (i) is a sum of average powers of its component signals, and the BRi of this combined incoming signal (i) is a sum of bit rates of its component signals.
In the above case it should be kept in mind that a) peak powers of the component signals in such a combined signal are considered to be already equalized at a preceding node; b) that the total sum of the joined bit rates of any combined signal (incoming or outgoing) should be less than BRmax.
To perform the power equalization, the following measures can be undertaken: average power Pi av of a particular signal can be increased or decreased by affecting its transmitter, using fixed or variable optical splitters, optical couplers, optical attenuators or amplifiers; bit rate BRi of a specific signal can be changed at the transmitting node as required by the bandwidth allocation.
The network preferably comprises a network management system (NMS) responsible for controlling said power equalization means at the network nodes. The NMS can be of a centralized type or of a distributed type, and is to be operative to obtain information on different short pulse signals incoming and outgoing each particular network node. The information comprises at least bit rate BRi of each particular short pulse signal, and average power Pi av of a single or combined signal (i) incoming a node via a specific incoming link. According to one embodiment, such information can be provided by preliminarily tagging different short pulse signals having different BRi say, by modulating thereof at the transmitting nodes using different low frequency tones, and by further detection of the modulation tones at the intermediate and receiving network nodes for creating and updating a traffic map of the network.
Owing to the broadcast character of transmission in the proposed network, the network intrinsically comprises alternative paths (at least a first and a second paths) via which a short pulse optical signal from a particular transmitting node may arrive to a particular receiving node. In one-preferred-embodiment, the dispersion compensation module for detecting the required short pulse optical signal at a receiving node may comprise a number of sections respectively tuned to a number of the alternative paths to simultaneously receive from them a number of copies of said required signal. The receiving network node may further comprise equipment for comparison and/or averaging, and/or determining Bit Error Rate (BER), and/or phase matching, and/or mutual correlating of the received copies—so as to finally select the best quality signal copy (say, from the point of Signal to Crosstalk ratio, from the point of BER) and/or to finally obtain a copy of the required signal, being inspected and repaired from the point of errors. This embodiment is the most advantageous since it allows obtaining highly reliable traffic protection in the network.
In another embodiment, the dispersion compensation module can be selectively adjustable to compensate either dispersion created in a required short pulse optical signal when passing along a main communication path, or dispersion created in the required short pulse optical signal when passing along any alternative communication path.
Selection of a new communication path may be triggered by failing to detect the required optical signal expected from the communication path to which the dispersion compensation unit is currently matched.
In yet a further embodiment, the dispersion compensation module can be adapted to iteratively adjust the dispersion so as to match any possible and preliminarily known path via which the required signal still arrives to the receiving node (“scanning the network”). If the required signal is received via more than one path, the best quality signal is to be selected at the receiving node.
Preferably, the receiving node can be equipped with at least one so-called searchable absorber, capable of suppressing signals weaker than a pre-determined power threshold, if such are incidentally restored by the dispersion compensation module. Such weak signals are considered to be so-called looped signals.
For arranging the described type communication, including the dispersion based routing in a mesh network, at least one optical communication channel should be provided and dedicated for such a purpose.
However, the optical fiber links in the network may support a multi-channel optical transmission system, where each of the optical channels can be used in the way described above, to transmit at least one short pulse optical signal from each transmitting node of the network. Such a network would present a new kind of hybrid “WDM-dispersion communication” network.
In one alternative modification of the network, the network comprises means for time division multiplexing of the optical signals transmission. In one embodiment, the means for time division multiplexing enable transmission of the short pulse optical signals from different transmission nodes at respective different time slots. Alternatively or in addition, the means for time division multiplexing may enable transmission of a number of different short pulse signals from one and the same transmitting node in different time slots.
As has been mentioned above, each of the transmission nodes of the network can be adapted to transmit a number of short pulse optical signals via one and the same said optical communication channel. To allow separation (distinguishing) of said signals at the time of launching, the node may be either provided with time division multiplexing means for transmitting different signals in different time slots. Alternatively, the node may comprise units inserting different starting values of dispersion into “n” different short pulse signals, for example to achieve widening of “n−1” signals immediately at the transmitting node, thus preventing overlapping of peak pulses of different outgoing optical signals in the common combined signal.
According to a second aspect of the invention, there is provided a method for communication in an optical network, preferably a mesh or ring-like optical network, comprising a plurality of network nodes including network nodes operative as transmitting nodes and network nodes operative as receiving nodes, the plurality of network nodes being interconnected by one or more optical fiber links and via an optical communication channel provided along said one or more links, wherein said channel is characterized by a single optical wavelength,
the method comprises:
assigning for each node operative as a transmitting node one or more nodes operative as receiving nodes, and for each node operative as a receiving node—at least one communication path extending between said node operative as a receiving node and a specific node operative as a transmitting node;
transmitting, from at least two nodes operative as transmitting nodes, short pulse optical signals via said optical communication channel nodes at different bit rates;
compensating, at each of the nodes operative as receiving nodes, dispersion accumulated in a short pulse optical signal transmitted from one of said at least two nodes operative as transmitting nodes, upon passing one of said at least one assigned communication paths. The method thereby enables restoration of the short pulse optical signal at the receiving node, while not restoring and thus neglecting other undesired optical signals due to non-compensated dispersion thereof.
Preferably, the method comprises preventing mutual interference of the different short pulse signals in the network;
The step of preventing mutual interference of the different short pulse signals can preferably be provided by performing power equalization at one or more network nodes, by ensuring that a group of short pulse optical signals incoming a specific network node and then leaving said network node in a form of a combined optical signal transmitted via a common optical fiber link, have substantially equal peak powers in the combined optical signal.
The power equalization can be performed substantially according to the following equation (1):
P
1 av
/BR
1
=P
2 av
/BR
2
=P
3 av
/BR
3
=P
i av
/BR
i
= . . . =P
n av
/BR
n., (1)
where BR1+BR2+BR3+ . . . +BRi+ . . . +BRn=≦BRmax (2)
and where: Pi av/BRi is the peak power of a short pulse optical signal;
Pi av—is average power of the incoming signal (i),
BRi is the bit rate of the incoming signal (i), and
BRmax is a design rule characteristic selected to minimize bit error rate (BER).
According to a most practical version of the method, at least one of the incoming shot pulse optical signals (i) is a combined incoming signal (i), and the average power Pi av of such a combined incoming signal (i) is a sum of average powers of its component signals, and the bit rate BRi of said combined incoming signal (i) is a sum of bit rates of its component signals.
To perform the power equalization, the following steps can be undertaken: increasing or decreasing average power Pi av of a particular optical signal (i) by affecting its transmitter(s) or using an amplifier; bit rate BRi of a specific signal (i) can be changed.
For the purpose of network management the method may comprise a step of preliminarily tagging different short pulse signals (say, by modulating thereof at the transmitting nodes using different low frequency tones), and a consequent step of detecting the modulation tones at network nodes for creating and updating a traffic map of the network.
The method may comprise detection of two or more copies of a required short pulse optical signal incoming a specific receiving node from two or more alternative paths in the network, by providing a selectively tunable said dispersion compensation module.
Said detection of the two or more copies can be substantially simultaneous, and be followed by processing such as: comparison from the point of Signal to Noise ratio (Signal to Crosstalk ratio), from the point of Bit Error Rate (BER), and/or averaging and/or correlating said two or more copies—for obtaining a best quality copy, an averaged copy and/or even a copy “with errors corrected” of the required short pulse signal.
Alternatively, said detection of the two or more copies can be performed in sequence (iteratively). In other words, the method may comprise a step of adjusting the dispersion compensation in a pre-determined order to detect arrival of the optical signal via any of the communication paths which are preliminarily known (assigned) to the receiving node. This step can be compared with scanning the network for the signal to be received by iteratively adjusting dispersion compensation means so as to match any possible path via which the signal still arrives to the receiving node.
Still further, detection of a next copy can be performed whenever one of the alternative paths in the network fails.
According to another version of the method, the step of preventing interference of different optical signals in the network comprises performing time division multiplexing in the network. The time division multiplexing may comprise enabling transmission of the short pulse optical signals from different transmission nodes at respective different time slots. Alternatively, or in addition, the time division multiplexing may allow transmission of a number of different short pulse optical signals from one and the same transmitting node in different time slots.
For networks with long communication distances (paths longer than 100 km), the method preferably comprises fine tuning of the dispersion at the receiving nodes, to overcome dispersion fluctuations due to temperature, stress, fiber aging, bending, etc.
Both with respect to the proposed method and the described network, the following should be noted. If, for example, two or more transmitting nodes are equally spaced from a particular receiving node, and one of the transmitting nodes should not interfere the particular receiving node, the distances can artificially be made different, by intentionally introducing, say, different spare portions of optical fiber or dispersion compensation modules between the transmitting nodes and the receiving node, for changing dispersion of optical fiber links there-between and induce the required dispersion difference.
The method may additionally comprise a step of suppressing short pulse optical signals weaker than a pre-determined threshold, if such weaker signals are restored by the dispersion compensation module, since they are considered to be either random or looped signals.
It should be noted that the invention is most advantageous in middle range networks where so-called optical fiber distances between nodes are of about 100 km, and where the minimal difference between the distances is of about 1 km. However, the given numerical values are exemplary, depend on parameters of optical fibers, laser transmitters etc., and are to be pre-selected for each specific network.
When necessary, optical amplifiers (preferably remotely or locally pumped EDFA or Raman amplifiers) can be used for amplifying all the signals in the optical channel to overcome losses in links, splitters, couplers and DCMs (dispersion compensation modules).
Details of the invention will become clearer as the description proceeds.
The invention will further be described and illustrated with the aid of the following non-limiting drawings.
a illustrates a time diagram of a short pulse signal S1 detected at a receiving node N4 upon matching its dispersion compensation module to the path N1-N3-N4. No power equalization is provided in the network.
b illustrates a time diagram of an original short pulse optical signal S1 transmitted from a transmitting node N1, as shown in
a is a time diagram of the short pulse signal S1 detected at a receiving node N4 upon matching its dispersion compensation module to the path N1-N3-N4, when power equalization is provided in the network (namely at the network node N3).
b is equal to
a is a time diagram of the short pulse signal S1 detected at a receiving node N4 upon matching its dispersion compensation module to the alternative path N1-N2-N3-N4, and when power equalization is provided in the network.
b is equal to
a is a time diagram of a short pulse signal S2 detected at a receiving node N4 upon matching its dispersion compensation module to the path N3-N4, and when power equalization is provided at the node N3.
b—illustrates a time diagram of the original short pulse signal S2 shown in
In
b illustrates the short pulse optical signal S1 outgoing from node N1 and having bit rate BR1. The peak power of the binary “1” pulses is the maximal available (the signal is not yet distorted).
a illustrates how the signal S1 is received at N4 upon performing dispersion compensation by the module 12, according to the main path N1-N3-N4 of the signal S1 (the dispersion compensating module 12 is adjusted to the length of the main path being 80 km). It can be seen that though the signal S1 is generally restored, considerable noise and even possible errors are created by the dispersion distorted signals S2 and the signal S1 arriving from the alternative path N1-N2-N3-N4.
a illustrates how the received signal S1 is improved upon performing power equalization of interfering incoming signals at node N3. At node N3, ratios of average powers of the incoming signals to their respective bit rates are taken into account:
{[Pav of S1(main)]/BR1=[P av of S1(alternative)]/BR1=[P av of S2]/BR2}
to create the outgoing combined signal in the fiber link N3-N4.
It is kept in mind that BR1+BR1+BR2 (in the link N3-N4)≦BRmax. To perform the power equalization, the following measures can be undertaken: average power Pi av of a particular signal can be increased or decreased by affecting its transmitter or controlling an amplifier/an attenuator, by using fixed or variable optical splitters or couplers; bit rate BRi of a specific signal can be changed at its transmitting node, taking into account bandwidth allocation.
It can be seen, that the resulting detected signal S1 is much more clear and clean than that of
In practice, it should be noted that:
b illustrates that the signal S1 can be received at the node N4 also from the alternative path N1-N2-N3-N4, if the dispersion compensation module 12 is adjusted to the distance of this alternative path (120 km). The power equalization is performed as before, and the signal seems clear though peak powers' values are differently distributed over the signal.
b illustrates a time diagram of the optical signal S2 having bit rate BR2 four times higher than the bit rate BR1.
a illustrates that upon adjustment of the dispersion compensation module 14 at the receiving node N4 to the path N3-N4 (40 km), and when performing power equalization at N3 (with the aid of the NMS 16), the signal S2 is received at N4 with quite satisfying quality.
While the invention is disclosed in the detailed description with reference to a limited number of specific implementations, it should be appreciated that other means and measures exist for preventing interference and ensuring reliable detection of short pulse signals in the proposed dispersion compensation based optical network, and these measures are to be considered part of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 172856 | Dec 2005 | IL | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IL2006/001313 | 11/14/2006 | WO | 00 | 9/2/2008 |
