Optical dispersion compensation

Abstract: The present invention relates generally to the field of optical communications and particularly to optical communication techniques which compensate for dispersion such as that in optical fibers. Compensation of optical dispersion is achieved with a polarisation beam splitter with first, second and third ports, a dispersion compensation fiber, a first return means that changes the mode of polarisation of a returned signal and a second return means. One end of the dispersion compensation means is connected to the third port of the polarisation beam splitter, the other end is connected to the first return means. The second return means is connected with the second port of the polarisation beam splitter, while the first port of the polarisation beam splitter forms input and output. As the signal passes through the disperion compensating fiber four times, the length of the dispersion compensating fiber can be greatly reduced.

Description: TECHNICAL FIELD The present invention relates generally to the field of optical communications and particularly to optical communication techniques which compensate for dispersion such as that in optical fibers. BACKGROUND OF THE INVENTION Optical communications systems using optical fibers to couple a light source, such as laser, and a photodetector are now widely used for high speed (for example, Gbit/sec data rates) and long distance (for example, trans-Atlantic or trans-Pacific) communications. Many technical problems have had to be overcome for these systems to reach their present state of development. Perhaps the most widely known problem was caused by the lossy nature of the first silica based optical fibers. The loss in such fibers was greatly reduced, to the order of a tenth of a dB/km or even less, by the development of fibers fabrications techniques that greatly reduced the presence of loss creating impurities in the fibers. After low loss optical fibers had been developed, other system parameters became important to the further development of optical communications systems. For example, fibers have chromatic dispersion; that is, the propagation velocity of the radiation depends upon its frequency. Narrow band light sources in the form of solid state lasers were developed. These lasers typically radiated in several relatively closely spaced modes which propagated at different velocities. The presence of multiple modes and the existence of chromatic dispersion, limited either the data transmission rate or the transmission distance. Radiation sources, such as distributed feedback (DFB) lasers, that emitted only a single mode were developed to overcome these problems. However, even the single mode modulated light of a DFB-laser has a finite bandwidth which causes a pulse to spread when chromatic dispersion is present. One approach to solving this problem developed dispersion shifted fibers, which are often referred to by the acronym DSF. Dispersion shifted fibers have a region of very low or no chromatic dispersion. However, the use of such fibers suffers from several drawbacks. Firstly, the laser must be selected to emit at the frequency at which the fiber has no chromatic dispersion. Secondly, much non-dispersion shifted fiber has already been installed. Other techniques that compensate for fiber chromatic dispersion are desirable if they overcome the previously discussed limitations imposed by non-dispersion shifted fibers. One technique inserts, at an arbitrary point in the transmission path between the transmitter and the receiver, a dispersion compensating fiber (DCF). The length of fiber is selected to provide dispersion compensation for a certain transmission length and therefore enable transmission over either an extended distance or at a higher data rate. This approach suffers from the added cost of the DCF and, more significantly, the losses introduced by such fibers. These losses are at least comparable to the losses in the system fibers and limit the system capabilities. From EP 0 658 988 A1 an apparatus which reduces the costs of the DCF and compensates for the losses introduced by the DCF is known. The apparatus has an optical circulator with at least first, second and third ports. The apparatus also has return means and a dispersion compensating waveguide, such as a DC-fiber, connecting the return means to the second port. An amplifier is connected to the circulator. The amplifier has a pump laser, a multiplexer, and a doped fiber. The pump laser is connected to the multiplexer, and the fiber amplifier is connected between the return means and the second port of the circulator. European patent application EP-A-0595595 discloses apparatus for adapting an optical signal passing through an optical communication system comprising: optical signal adapting means; input means for coupling the optical signal from the communication system into the adapting means and for passing said optical signal through said adapting means for a first time; a first return means comprising polarisation rotating means connected to said adapting means, for returning said optical signal back through said means a second time to the input means with a changed polarisation; a second return means is connected to said input means for returning said changed polarisation optical signal back through said adapting means for at least a third time before said optical signal is passed, output means for coupling said optical signal back into said communication system. Summary of the Invention The present invention provides apparatus according to claim 1. The present invention also provides a method according to claim 5. The present invention advantageously provides an apparatus for the compensation of optical dispersion. It is one aim of the invention to further reduce the costs of the optical dispersion compensation. An advantage of the present invention is that it allows further reduction in the length of the dispersion compensating fiber needed to compensate for the dispersion of the optical transmission line. In particular, the object is achieved by providing an apparatus for compensation of optical dispersion which has a polarisation beam splitter with a first, second and third port, a dispersion compensation means, a first return means that changes the direction of polarisation of the returned signal and a second return means. One end of the dispersion compensation means is connected to the third port of the polarisation beam splitter, the other end is connected to the first return means. The second return means is connected with the second port of the polarisation beam splitter. The first port of the polarisation beam splitter forms input and output of the apparatus for input and output signals of different polarisation mode. In a preferred embodiment an optical circulator with a first, second and third port is connected with its second port to the first port of the polarisation beam splitter. First and third port of the optical circulator are connected to optical fibers which form an optical transmission line. The present invention will become more fully understood from the detailed description given hereinafter and further scope of applicability of the present invention as defined by the appended claim will become apparent. However, it should be understood that the detailed description is given by way of illustration only, since various changes and modifications within the scope of the invention as defined by the appended claim will become apparent to those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS The following detailed description is accompanied by two Figures showing schematic representations of apparatus according to this invention. DETAILED DESCRIPTION Depicted in Fig. 1 are transmitters L 1 to L n , an optical multiplexer 1, an optical circulator 2, a polarisation beam splitter 4, a dispersion compensation means 5, a first return means 6, a second return means 3, a receiver 9 and optical fibers A to F, interconnecting the components mentioned above. The transmitters L 1 to L n provide optical signals, modulated radiation, typically from semiconductor lasers. The optical multiplexer 1 may be in particular a wavelength selective multiplexer. It combines the signals of the transmitters L 1 to L n . The circulator 2 has a first 21, second 22 and third 23 port. The receiver 9 may be built up in particular from an optical demultiplexer and photodetectors that detect radiation and convert it into electrical signals. The depicted optical fibers A to F may be silica based single mode fibers. The characteristics of the fibers depend upon the wavelength of the optical signals. The polarisation beam splitter 4 allows a optical signal or a radiation of a first polarisation mode, e. g. the TE mode, to pass through it via a first port 41 to a third port 43 and vice versa. An optical signal or a radiation with a polarisation being rotated by 90 degrees, e. g. the TM mode, is passed from either its first port 41 or its third port 43 to a second port 42. The first and second return means 6 and 3 may be each made of a dielectric mirror or a grating which may be written into the fibers F and D. The use of either linear or chirped gratings as return means 6 and 3 is within the scope of this invention. The first return means 6 includes, in addition, a Faraday rotator 7 which rotates the polarisation of a signal or a radiation applied to it by 90 degrees. A mirror or grating 8 and the Faraday rotator 7 of the first return means 6 can be either separate, discrete elements or their functions may be combined in a single element. The dispersion compensation means 5 may be a dispersion compensating fiber (DCF). The length of the DCF required depends on its own characteristics and the length and the characteristics of the optical fibers A and B. From the art it is well known that, in principle, the length of the DCF will be selected in such a way that the dispersion of the optical fibers A and B, forming an optical transmission line, will be compensated completely. The length of the DCF necessary for this invention will become apparent from the following. As mentioned above, the optical signal is produced by the transmitters L 1 to L n and the optical multiplexer 1. Within the transmitters L 1 to L n , lasers provide the modulated radiation. It is known that the radiation of lasers has preferred modes of polarisation. To ensure that the radiation of all transmitters L 1 to L n has only one polarisation mode, e. g. the TE mode, a polarisation controller or a polarisation maintaining fiber can be used. After the multiplexer 1 has combined the signals of the transmitters L 1 to L n being generated in such a manner as explained above, the optical signal is coupled into the optical fiber A. The optical fiber A is connected to the first port 21 of the optical circulator 2 which couples the optical signal into the optical fiber C, which is in turn connected to the second port 22 of the optical circulator 2. The other end of the optical fiber C is connected to the first port 41 of the polarisation beam splitter 4. As explained above, the optical signal with the predetermined polarisation mode TE passes through the polarisation beam splitter 4 and is coupled into the optical fiber E which is connected to the third port 43 of the polarisation beam splitter 4. The other end of the optical fiber E is connected to the DCF 5 which is connected with an other optical fiber F to the first return means 6. It is also possible to directly connect the DCF 5 to the output 43 of the polarisation beam splitter 4 and the input of the first return means 6. The optical signal is reflected by the mirror 8 of the first return means 6 and its polarisation is rotated by the Faraday rotator 7, which is part of the first return means 6, by 90 degrees to the second polarisation mode TM. The optical signal again passes the DCF 5 and again comes to the polarisation beam splitter 4, via the third port 43. As the polarisation mode of the optical signal has been changed by the Faraday rotator 7 of the first return means 6, the optical signal is coupled into optical fiber D which is connected to the second port 42 of the polarisation beam splitter 4. The other end of the optical fiber D is connected to the second return means 3 which reflects the optical signal. The optical signal enters the polarisation beam splitter 4 at its second port 42 again and passes through the DCF 5 for the third time. At the first return means 6 the optical signal is reflected again by the mirror 8 and its polarisation is rotated by 90 degrees for the second time by the Faraday rotator 7, so that the optical signals polarisation mode is again TE. The optical signal passes the DCF 5 for the forth time and now passes through the polarisation beam splitter 4 via third port 43 to first port 41. The optical signal is coupled into the second port 22 of the optical circulator 2 through the optical fiber C. The optical circulator 2 couples the optical signal into the optical fiber B which is connected to its third port 23. Finally the optical signal is received by the receiver 9 for further processing. As it can be seen from the above description, the length of the DCF 6 necessary for the present invention is only a quarter of the length usually required or half of the length as known from the state of the art, as discussed in the above mentioned EP 0 658 988 A1. It is another advantage that because of the rotation of the polarisation by 90 degrees, compensation for polarisation mode dispersion caused by the DCF 5 is achieved, as the optical signal passes the DCF 5 four times in different polarisation modes. To compensate for the losses of the optical signal caused by the DCF 5, fiber amplifiers can be used, for example, Erbium Doped Fiber Amplifiers (EDFA). One or more fiber amplifiers may be inserted anywhere along the path of the optical signal, e. g. at the positions denoted by A, B, C, D, E and F in the optical fibers. It is also possible to cut the DCF 5 at an arbitrary position 51 and place a fiber amplifier to connect the two parts of the cut DCF 5. It is another possibility to use the DCF 5 itself as fiber amplifier by pumping light into the DCF 6. Such an amplifier is also called Raman Fiber Amplifier. It also is advantageous to insert a polarisation sensitive filter at any of the positions D, E, F or 51 to equalise the power level along the propagation way. The foregoing describes a unidirectional signal transmission. It is obvious, however, that the invention is also applicable to bidirectional transmission as shown in Fig. 2. Depicted are transceivers 10 and 11 which are connected to first 21 and third 23 port of an optical circulator 2. Transceivers 10 and 11 have first transmitter and second receiver and first receiver and second transmitter, respectively. The transmitters and receivers are not shown individually for reasons of clarity. For bidirectional transmission the optical circulator 2 requires four ports, forth port 24 being used to connect a second apparatus comprising further polarisation beam splitter 4', DCF 5' and returning means 3' and 6', while the apparatus as described above is connected to second port 22. For more detail reference to EP 0 658 988 A1 can be made. By way of example the present invention has been described using a optical signal composed of a plurality of single signals, combined by wavelength division multiplexing (WDM). It is obvious that the present invention is not limited to WDM, and is applicable to other optical signals.

Claims: Apparatus for compensating for optical dispersion of an optical signal passing through an optical communication system (1,A,2,B,9) comprising: optical signal dispersion compensating means (5); input means (4) comprising a polarisation sensitive beam splitter for coupling the optical signal from the communication system into the dispersion compensating means (5) and for passing said optical signal through said dispersion compensating means for a first time; a first return means (6,7,8) comprising polarisation rotating means (7) connected to said dispersion compensating means (5), for returning said optical signal back through said dispersion compensating means (5) a second time to the input means (4) with a changed polarisation; a second return means (3) is connected to said input means (4) for returning said changed polarisation optical signal back through said dispersion compensating means (5) a third time; the first return means (6,7,8) in use returning said optical signal with a changed polarisation back through said dispersion compensating means (5) a fourth time so as to be provided to output means for coupling said optical signal back into said communication system; and the output means also comprises the polarisation sensitive beam splitter (4). Apparatus according to claim 1 characterised in that the polarisation beam splitter has first (41), second (42) and third (43) ports, the first port being connected to the communication system, the second port being connected to the second return means, and the third port being connected to the dispersion compensating means. Apparatus according to claim 1 or claim 2 characterised in that the first return means (6,7,8) comprises a Faraday rotator (7) which rotates the polarisation of the optical signal to form a rotated optical signal, and a mirror (8). Apparatus according to claim 3 characterised in that the Faraday rotator and mirror are combined in a single element. Method of compensating for the optical dispersion of an optical signal passing through an optical communication system (1,A,2,B,9) comprising the steps of: coupling (4) via a polarisation sensitive beam splitter (4), the optical signal from the communication system into dispersion compensating means (5) and passing said optical signal through said dispersion compensating means (5) for a first time, rotating the polarisation of (6,7) and returning (6,8) said optical signal and passing said optical signal back through said dispersion compensating means for a second time, returning said optical signal to pass back through said dispersion compensating means (5) for a third time, returning said optical signal with a changed polarisation back through said dispersion compensating means (5) a fourth time, before coupling said optical signal back into said communication system, the polarisation sensitive beam splitter (4) coupling said optical signal back into said communication system.

Inventors: MOLLER LOTHAR BENEDIKT ERHARD, MOLLER, LOTHAR BENEDIKT ERHARD JOSEF
Applicants: LUCENT TECHNOLOGIES INC, LUCENT TECHNOLOGIES INC.
Application year: 1997
Main classification: H04B10
IPC codes: H04B10