Patent Grant
7127178
The present invention relates to a system and method for optical transmission of communication and in particularly, to optical telecommunication systems in which periodic optical signals are transmitted.
One of the problems known in the art to be associated with transmission of optical signals via optical network, is a problem known as dispersion. Typically, the optical output at the transmitting end is modulated and transmitted in its modulated form. However, in the optical network the beam undergoes light dispersion, which causes a distortion of the transmitted signal.
Various solutions were suggested in the past to overcome the dispersion problem. They included electronic methods used at the receiver and/or transmitter, and the use of high-dispersion optical elements located along the fiber link. U.S. Pat. No. 5,371,625 is one such publication that tries to address the dispersion problem. The publication discloses a frequency modulation technique used instead of the intensity modulation to increase the path of the transmission. However, this requires a suitable optical receiver at the receiving end.
U.S. Pat. No. 4,979,234 teaches the use of a saturated semiconductor laser amplifier that can impose a chirp upon optical pulses having a pulse repetition rate in the range of 8 to 16 GHz, and having an optical carrier wave of wavelength in the near infra-red range. This solution ensures that degradation of the pulses because of dispersion does not commence until after an initial portion of the fiber segment has been traversed by the propagating pulses.
As known in the art, a temporal self-imaging effect in single mode fibers (also known as TALBOT effect) can be applied to reflection of periodic signals, from linearly chirped fiber gratings. This effect can be used for multiplying the repetition frequency of a given periodic pulse train without distorting the individual pulse characteristics. For a given fiber, the practical limit on the frequency multiplication factor depends only on the temporal width of the individual pulse.
Furthermore, a combination of techniques for short pulse mode generation such as pulse mode locking and the self-imaging effects allows to obtain short pulse trains with ultrahigh repetition rates, in the terabit regime as described for example in “Temporal self-Imaging in single mode fibers”, T. Jannson and J. Jannson, JOSA-A Vol. 71, No. 11, 1393–1376, November (1981); “Technique for multiplying the repetition rates of periodic trains of pulses by means of a temporal—self imaging effect in chirped fiber gratings”, J. Azanz and M. A. Muriel, Opt. Lett. Vol, 24, No. 23, 16721674, (1999); “Quasi Self Imaging using apreiodic sequences”, G. Indebetouw, JOSA-A Vol. 9, No. 4, 549–558, November (1992). The disclosures of all references mentioned above and throughout the present specification are hereby incorporated herein by reference.
It is an object of the present invention to provide an optical system having an increased quality of signals transmitted therethrough.
It is yet another object of the present invention to provide a device that is operative to restore periodic optical signals transmitted in optical telecommunication networks.
It is still another object of the present invention to provide method for improving quality of periodic signals transmitted in optical systems.
Other objects of the invention will become apparent as the description of the invention proceeds.
According to a first aspect of the invention there is provided an optical extending device for use in transmitting optical signals which comprise at least one sequence of periodic optical signals, said optical device comprising:
a first fiber optic having a characteristic dimensional propagation coefficient equal to β1 and adapted to be connected to a single mode second fiber optic having a length equal to L0 and a characteristic dimensional propagation coefficient equal to β0, wherein LP, the length of said first fiber optic is substantially equal to
{[T2/π−L0*β0]/β1}*{1−MOD(L0/{[T2/π−L0*β0]/β1}}
and the overall length of the first and second fiber optics is n ·LP
and wherein:
According to a preferred embodiment of the invention there is provided an optical apparatus that comprises an optical connector adapted to connect a single mode optical fiber having a length equal to L0 and a characteristic dimensional propagation coefficient equal to β0, with at least one optical extending device of the type described above and an optical signal egress means. When the extension fiber has the same physical characteristics as the single mode optical fiber, β0 in the above formula is equal to β1.
According to a preferred embodiment of the invention, the signals delivered via the device described herein are at a bit rate of at least 10 GHz.
According to another preferred embodiment of the invention, a transmission comprises periodic type signals and non-periodic signals. The periodic type signals may be signals used for synchronization purposes. Alternatively, such signals may be used as addresses/headers in packet-based networks such as IP networks, where preferably, the device of the invention is installed at routing nodes, allowing the retrieval of optical information used in the identification of routing addresses at much better reliability, e.g. by reducing routing errors. The non-periodic type signals are typically data signals and the like.
According to yet another embodiment of the present invention, the apparatus described comprises a plurality of optical fiber extensions. Each of these extensions has an appropriate length in accordance with the overall length of the link it is associated with and the parameters identifying the transmissions delivered therethrough. As an example, some of the plurality of optical fiber extensions may be adapted to convey telecommunication signals at different bit rates.
According to another aspect of the invention, there is provided a telecommunication system comprising:
at least one optical transmitter;
an optical link adapted to convey communication signals transmitted by said transmitter;
a self signal constructor adapted to convey said telecommunication signals transmitted along said optical link and forward them, comprising at least one optical extending device as described above; and
at least one optical receiver.
A self signal constructor according to the present invention should be understood to encompass any means that can reconstruct the signals, e.g. their image, their sequence, their pattern and the like, as explained herein.
According to an embodiment of the invention, there is provided a system comprising a plurality of optical transmitters each of which is linked to a wavelength multiplexer which is operative to receive outputs from said optical transmitters over a plurality of optical channels, a single mode optical fiber and an optical extending device associated therewith connecting the wavelength multiplexer with a wavelength demultiplexer which is operative to receive an input from said wavelength multiplexer, to de-multiplex the input received and transfer the plurality of de-multiplexed inputs to a plurality of optical receivers.
According to a further aspect of the invention there is provided a telecommunication router adapted to receive optical communication information from at least one transmitting device, where the router comprises at least one optical extending device of the type specified. Typically, the optical communication information comprises an address information and a payload information, where the address information comprises at least one sequence of periodic optical signals. According to this embodiment the addresses of the transmissions that are received by the router and should be forwarded is received at a high quality, and the number of cases where errors in destination occur due to the distortion of the address signals, are reduced.
By yet another aspect of the invention there is provided a network through which at least part of the optical information is transmitted in the form of periodic optical signals and the network comprises a plurality of the devices provided by the present invention. Typically, such a network may be considered as a combination of segments each of which defined between two neighboring elements. In each of such elements the optical information may either be converted into electrical information, or be further transmitted along a transmission path. Also, the information in its electrical form may be further processed, converted into optical information and forwarded along such a transmission path. In accordance with the present invention, each of these network elements may further comprise at least one optical fiber extension element of the type specified. The length of each of the at least one optical fiber extension element is calculated based on the length and properties of the optical link connecting this element to its preceding element and the rate by which the information is transmitted there.
As will be appreciated by a person skilled in the art, such a network may typically comprise optical devices they may affect the calculation of the length required for the fiber optic extension. Examples of such devices are in-line amplifiers, dispersion compensation fibers, etc. In order to take into account the presence of these devices along the transmission link, each such optical device is defined with an equivalent length and that equivalent length is added to the overall link length.
According to still another aspect of the present invention there is provided a telecommunication apparatus comprising:
a plurality of first optical transceivers operative to convey optical transmissions;
a second optical transceiver being in communication with said plurality of first optical transceivers, and connected to each of said plurality of first optical transceivers via a fiber optic having a length that is equal to a product of n and {[T2/π−L0*β0]/β1}. When an optical transmission is received at the second optical transceiver it may be detected and converted to its corresponding electrical form. Alternatively, the optical transmission received may be detected and wavelength converted.
According to a preferred embodiment of this aspect of the invention, the telecommunication apparatus further comprises a multiplexing/demultiplexing device adapted to operate in communication with any of said plurality of first optical transceivers.
The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:
Thus, when a detector is located at a distance that is equal to n*LP from the information source (n being an integer having the value of 1, 2, 3 . . . ), periodic signals will be detected by this detector in their self-reconstructed pattern, substantially free of distortion caused by the dispersion effect.
The mechanism described above is further illustrated in
Reference is now made to
Therefore, in accordance with the present invention, in order to reconstruct a periodic signal and obtain good operability for features such as time recovery, code detection, etc. the path along which that signal is transmitted, is extended by an additional optical fiber so that the overall extended path to become a product of n*LP. However, in the case that the extension portion is made of a different fiber (having a different β), the self signal reconstruction of the signals will occur every time the distance as measured from the signals' optical origin, is equal to a product of LP by an integer, where:
LP=[T2/π−L0*β0]/β1.
According to yet another embodiment of the invention, LP, the length of the extending portion is chosen arbitrarily but when in operation, it is subjected to a voltage that causes a change in the characteristic dimensional propagation coefficient of the extending portion (β) so that the following equation is satisfied:
β1=[T2/π−L0*β0]/LP
As would be appreciated by a man skilled in the art, such an extension device may be applied in various optical communication systems, where periodic pulse sequence of signals are transferred. Examples of such applications can be found where periodic signals are used for synchronization purposes, as addresses for routing packets, for classification purposes, for monitoring purposes and the like.
The example schematically illustrated in
As can be appreciated by a person skilled in the art, this configuration can be extended for use in a case of multiple transmitters, as presented in
The length of the extension fibers 26, 36 and 46 is such that allows the self imaging of periodic pulse sequence in each of the respective transceiver units 28, 38 and 48.
Let us now consider a case where a periodic sequence of signals is transmitted at a bit rate of 40 GHz, via an optical fiber 80 Km long and having the characteristic dimensional propagation coefficient, β, equal to 0.1 psec2/m. In such a case, the sequence will be reproduced every 1,989 meters. The length of the fiber extension will therefore be equal to:
1989*[1−MOD(80000/1989)]=1989*0.779=1549 m long.
Reference is now made to
The length of each of extension fibers 128i may be determined by any one of a number of methods. One such method may be described as follows: a transmitting light source, e.g. LED is connected at one end of optical fiber 126i and is used to transmit periodic signals at a constant and pre-determined power P(0). The power of signal P(L0i) received at the other end of a L0i Km long optical fiber 126i is measured, and the values of these two parameters are substituted in the following formula:
L0i=(10/(α)log[P(0)/P(L0i)]
where α is the attenuation coefficient of the optical fiber i and has the dimensions of [dB/Km].
Once L0i is determined, the length of its corresponding extension fiber, Lei, is evaluated for the expected transmission rate in accordance with
Lei={[T2/π−L0*β0]/β1}*{1−MOD(L0/{[T2/π−L0*β0]/β1}}
as described above.
An optical fiber 128i having the length of Lei is then connected to optical fiber 126i by any method for connecting two optical fibers as known in the art per se. According to another embodiment of the invention, substation 120 further comprises a multiplexer/demultiplexer 130. Incorporating such a multiplexing/demultiplexing device in the substation, may be used for example to multiplex signals received from subscribers 122, as well as signals received from subscribers using other technologies e.g. digital pair gain devices, digital loop carriers, remote switching units and the like, if applicable. In such a case, the optical signals received in substation 120 are converted into their corresponding electrical form and all the signals received are multiplexed for transmission over a single communication medium 132. Communication medium 132 may be a fiber optic, standard POTS telephone line, xDSL, wireless or any other medium as appropriate for use with devices 130 as is known in the art. Inverse multiplexing (demultiplexing) is used in the opposite direction to split the transmission received along communication medium 132 into a number of transmissions, each directed to a subscriber connected to substation 120.
It will be appreciated that the above-described methods may be varied in many ways, including but not limited to, changing the exact implementation used. It should also be appreciated that the above described description of methods and networks are to be interpreted as including network in which the methods are carried out and methods of using the network components.
The present invention has been described using non-limiting detailed descriptions of preferred embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all the features shown in a particular figure. Variations of embodiments described will occur to persons of the art. Furthermore, the terms “comprise”, “include”, “have” and their conjugates, shall mean, when used in the claims “including but not necessarily limited to”.
| Number | Date | Country | Kind |
|---|---|---|---|
| 137083 | Jun 2000 | IL | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IL01/00594 | 6/28/2001 | WO | 00 | 12/30/2002 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO02/01764 | 1/3/2002 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4979234 | Agrawal et al. | Dec 1990 | A |
| 5371625 | Wedding et al. | Dec 1994 | A |
| 5847863 | Galvanauskas et al. | Dec 1998 | A |
| 20020163688 | Zhu et al. | Nov 2002 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20040008922 A1 | Jan 2004 | US |
