WO2006058830A1

WO2006058830A1 - Method for compensating bit pattern dependent raman-induced crosstalk, optical wavelength multiplex transmission system, and optical wavelength converter

Info

Publication number: WO2006058830A1
Application number: PCT/EP2005/055914
Authority: WO
Inventors: Peter Krummrich
Original assignee: Siemens Aktiengesellschaft
Priority date: 2004-12-02
Filing date: 2005-11-11
Publication date: 2006-06-08
Also published as: DE102004058644A1

Abstract

The aim of the invention is to reduce bit pattern dependent Raman-induced crosstalk in an optical transmission path. Said aim is achieved by means of a wavelength conversion in which the channels operating at a smaller wavelength are converted in pairs to the channels operating at a greater wavelength, and vice versa. In one embodiment of the inventive method, a special dispersion compensation scheme is provided in which the accumulated dispersion upstream and downstream from the wavelength converter is adjusted for channel sections that are selected in pairs such that the Raman-induced energy transfer from channels operating at a smaller wavelength to channels operating at a greater wavelength is compensated taking into account the dispersion-related pulse walk-off.

Description

description

A method of compensating for bit pattern dependent crosstalk caused by stimulated Raman scattering, an optical wavelength division multiplex transmission system, and an optical wavelength converter

The invention relates to a method according to the preamble of patent claim 1, an optical wavelength division multiplex transmission system according to the preamble of patent claim 10 and an optical wavelength converter for a WDM system.

The rapid growth of the Internet is causing a rapid increase in traffic. For operators, wavelength division multiplexing, WDM for short, has proven to be a suitable technology to provide the appropriate transmission capacity. Distortion effects such as linear and nonlinear distortions in the transmission fiber limit the number of transmittable channels or, for a given number of channels or capacity, the possible ones

Transmission range. In the literature, for systems with many channels, bit-pattern dependent crosstalk through stimulated Raman scattering ("bit pattern dependent Raman induced cross talk"), abbreviated SRS-XT, is given as the ultimate limitation of the bandwidth-range product ,

The following section will explain SRS-XT in more detail. Stimulated Raman scattering, or SRS for short, causes an energy transfer from shorter, so-called "blue" wavelengths to longer, so-called "red" wavelengths and causes a coupling between the channels. In the following, a frequency range or wavelength range used for the transmission of a data signal is referred to as a channel. This frequency range corresponds approximately to the signal bandwidth and is for example 50 GHz in the 3rd transmission window. Due to the stimulated Raman scattering, the channels with the smaller wavelengths additionally undergo attenuation, while channels with the longer wavelengths have an attenuation. attenuation or gain experienced. This gain or additional damping is time-dependent. Since the Raman scattering takes place very rapidly, with time constants lying in the fs range and thus clearly below a bit duration, the strength of the Raman effect also depends on the bit sequences transmitted in the individual channels. The individual pulses in short wavelength channels experience a great deal of additional attenuation when many "1" bits are transmitted at the same time in many adjacent channels of longer wavelengths. When many "0" bits are transmitted in the other channels, there is little additional attenuation. In contrast, pulses in channels with long wavelengths gain gain when many "1" bits are transmitted at the same time in many channels with shorter wavelengths, and almost no change when many "0" bits are transmitted.

In addition to this bit pattern-dependent energy transfer, the dispersion-induced walkoff between the channels must also be taken into account for a more detailed analysis of the SRS-XT. A walkoff between the channels is due to the different transit times of the signals at the different wavelengths due to the fiber dispersion. This means that due to the dispersion, the pulses in the individual channels pass each other during the transmission. The amplification or attenuation by a transmitted "1" signal in one channel will affect the other channels due to the dispersion over several pulses. The dispersion consequently affects the position of the individual pulses in the channels, while the crosstalk caused by SRS alters the amplitude of the pulses.

By reducing or compensating for SRS-XT, which prevents Raman-induced energy transfer between the channels against the background of dispersion-based walk-off, larger transmission ranges would be achieved with simultaneously increasing bandwidth. So far, there are no known approaches to such compensation of SRS-XT from the literature.

A certain reduction in the strength of the influence of SRS-XT can be achieved by using transmission fibers with high dispersion coefficients. The aforementioned walkoff i. the propagation time differences of the signals at different wavelengths is increased with strong dispersion and there is a faster passing together of the pulses in the channels with different wavelengths. On average, the power fluctuations and thus the crosstalk through SRS during transmission are reduced, but not compensated. The fibers with the highest dispersion coefficients occurring in practical use are standard single-mode fibers (SSMF). Fibers with higher dispersion coefficients have distinct disadvantages with regard to other transmission properties (pulse distortions, phase distortions). In addition, transmission systems with the already deployed SSMF must be able to achieve long ranges. Even with SSMF, SRS-XT limits the possible range in systems with many channels or wide channel spacing.

The invention has for its object to provide a method and arrangements, which allows an improvement of the transmission properties in WDM systems by compensation of SRS-XT.

This object is achieved by a method having the features of patent claim 1 and by arrangements having the features of patent claims 9 or 11.

The advantage of the invention is that at least partial compensation of SRS-XT is achieved by converting the wavelengths within the transmission path, which leads to a greater transmission range.

In an advantageous embodiment of the invention according to claim 2, a dispersion curve is used, through which a ne temporal synchronization of the bit sequences is to be achieved in order to allow the best possible compensation of the SRS-XT.

Another advantage of the invention is that the method is easy to implement. In principle, only one additional unit for wavelength conversion is required in a WDM system.

Furthermore, in an advantageous embodiment of the invention, dispersion overcompensation is used in a first part of the transmission path and dispersion undercompensation is used in a second part of the transmission path or vice versa. This has the particular advantage that in addition a suppression of the cross-phase modulation is achieved.

In another advantageous embodiment of the invention, a dispersion full compensation is performed after each link section. This has the particular advantage that at any point after each section of a wavelength converter can be used. For example, a wavelength converter could be switched on dynamically.

An additional advantage of the invention is that according to dependent claim 6, the transmission path can be divided into more than two parts, between which the wavelength converter is inserted. A multiple wavelength conversion has the advantage that the gain or attenuation of the bits is better regulated. In addition, it may be advantageous for already existing transmission links due to their existing structure to split the transmission path several times.

Further advantageous embodiments of the invention are specified in the remaining subclaims.

It is particularly advantageous that by exchanging the channels in conjunction with the improved dispersion management probably non-linear perturbations such as the SRS-related Übersprechen, as well as linear interference effects are eliminated and thus an optimal system performance is achieved.

The invention will now be explained with reference to an embodiment with the aid of Figures 1 to 4.

Show it

FIG. 1 shows a sketch which is intended to show the effects of the bit pattern-dependent crosstalk due to the Raman effect during the transmission in a section of a WDM transmission system (without consideration of dispersion).

FIG. 2 shows the block diagram of a WDM transmission system with a plurality of link sections and a wavelength converter according to the invention in the middle of the transmission system.

Figure 3 shows a possible arrangement of the wavelength converter according to the invention.

FIG. 4 shows a representation of a possible dispersion compensation scheme for the individual route sections.

In FIG. 1, for three channels with different wavelengths λi to λ ₃ (with λi <X2 <λ ₃ ), three example selected bit sequences ((0010101) in the channel at λi, (0010100) in the channel at X2 and (1010000) in the channel at λ ₃ ) on the transmitting side before and on the receiving side after passing through an optical transmission fiber shown. At the beginning of the transmission path all "1" bits have the same power, and at the end of the transmission path the power of some "1" bits has changed due to Raman scattering. The bit-pattern-dependent energy transfer of channels with a smaller wavelength to channels with a greater wavelength can be recognized from the reception-side changes in the powers of the "1" bits in Fig. 1. Thus, the power of the "1" bit with the number 3 at λi increases on the receiver side as it increases at λ ₃ . The same holds true for the power of the "1" bit numbered bit 5 at λi and λ _2. However, the power of the "1" bit numbered 1 (at λ ^) or 7 (at λi) remains constant since in the other channels, only "O" bits occur at the same time, in simple terms this means that amplification only occurs when "1" bits in at least two channels are transmitted simultaneously. However, this presentation does not take into account the effects of dispersion. Due to the dispersion-related walkoff and the concomitant

Passing the pulses, the gain or attenuation of a bit is distorted by a transmitted "1" signal, since now the bits are no longer "simultaneously" transmitted. From this it becomes clear that for optimal compensation of SRS-XT two effects (Raman effect + dispersion) have to be considered.

FIG. 2 shows the block diagram of a WDM transmission system. On the transmitting side, the carrier signals emerging from the transmittors Ti to T _{M are} modulated with data signals either directly or by means of modulators. The resulting optical signals OSi to 0S _M are then combined with a multiplexer MUX to a transmission-side WDM signal WS. The WDM signal WS accordingly comprises M channels at the center wavelengths (λi, λ ₂ ,..., Λ _M ). The WDM signal WS is now transmitted in a transmission path which consists of N route sections SA. Each track section SA contains a piece of an optical waveguide SSMF, for example a standard monomode fiber, an optical amplifier OAl, a piece of dispersion-compensating fiber DCF and a further optical amplifier 0A2. To compensate for SRS-XT, the transmission link is divided into two parts in this embodiment. Between two link sections SA, a wavelength converter WU is inserted. In a preferred embodiment of the method according to the invention, the wavelength converter WU is introduced in the middle of the transmission path if the number of route sections is even (for example 2N). The Transmission line does not have to be divided into two equal lengths.

In a preferred embodiment of the method according to the invention, the wavelength converter WU converts the channel having the shortest wavelength λi to the one having the longest wavelength λ _M and, in turn, the channel having the longest wavelength λ _M to the one having the shortest wavelength λi. With the other channels it proceeds analogously in pairs, ie the channel with the second shortest wavelength λ ₂ is converted to the one with the second longest λ (Mi) and so on. With an odd number of channels, one channel can be converted to an adjacent, previously unused wavelength. By the arrangement of the wavelength converter WU in the middle of the transmission system, the distortions that occur in the first half of the transmission system by SRS-XT are reversed in the second half. Bits transmitted in short wavelength channels in the first half of the transmission system and experiencing additional attenuation are transmitted in long wavelength channels in the second half and gain. In order for the compensation of the SRS-XT to take place completely, the level distributions of the channels at the fiber inputs in front of and behind the wavelength converter WU must be the same, at least in pairs, for the same so-called Raman efficiencies of the fibers, ie for each section in the second half of the wavelength Systems should set up a stretch of track in the first half with the same level distribution. The term Raman efficiency refers to the quotient of the Raman coefficient divided by the effective closed-field field. It is a size that is material-dependent and characterizes the strength of the Raman scattering and that also includes fiber parameters over the mode field area. Thus, if a fiber has a higher Raman efficiency in the first half of the transmission path than the fibers in the second half, then the stronger influence of the SRS-XT on the first half fiber is due to a higher input level of fiber in the second half compensate again.

The wavelength-converted WDM signal WS _K is divided after transmission in the second half of the transmission path on the receiving side with a demultiplexer DMUX in the individual channels and demodulated according to the transmission-side occupancy, so that the transmitted data signals receivers Ri to R _{M are} supplied.

If an even number of wavelength converters are used in a transmission path, there is the particular advantage that the channels at the output of the path have the same wavelength as at the input of the path.

FIG. 3 illustrates a preferred embodiment of the wavelength converter WU. A WDM signal WS is fed to a first band switch BW1, which splits the WDM signal into two subbands with channels of larger and smaller wavelengths TB1 and TB2. Subband TB1, which contains the channels with the longer wavelengths, is supplied to a wavelength converter WK1. The wavelength converter WK1 has the task of converting the channels with the longer wavelengths contained in sub-band TB1 to free, yet unoccupied channels with smaller wavelengths. In this case, the channel with the largest wavelength λ _M is preferably converted to a free channel with the smallest wavelength λ i, and the channel with the second largest wavelength λ ( _M i) is converted to a free channel having the second smallest wavelength λ ₂ The wavelength converter WK is preferably made of periodically poled lithium niobate (PPLN), for example by the non-linear process of four-wave mixing (FWM) in a pumped waveguide structure In this case, the pump radiation generated by the pump sources PQ via a beam splitter ST in front of the waveguide WL in the signal path If sufficiently powerful pump sources are available, the pump radiation can also be generated with the aid of a single source for both wavelength converters WK1 and WK2 and split into the signal path with a power divider before coupling into the signal path and fed to both beam splitters ST , which it's the channels containing the smaller wavelengths, the procedure is reversed accordingly. In a wavelength converter WK2, the channels with the smaller wavelengths of TB2 are converted into unoccupied channels with longer wavelengths. Here, too, the wavelengths are preferably mirrored with respect to a mean wavelength, analogous to the procedure at WK1. A second band filter BW2 adds the converted sub-bands Tbl ^ _k and TB2 together again. To avoid crosstalk, BW2 filters can be inserted in front of the second band filter to suppress the remaining portions of the original channels and the remaining pump radiation.

In addition to the wavelength conversion, in which channels with short and long wavelengths are interchanged in pairs, the method according to the invention for suppressing SRS-XT uses a special dispersion compensation scheme in order to ensure temporal synchronization of the bits in the individual channels. If the wavelength converter WU is arranged, for example, in the middle of the transmission path, the dispersion compensation scheme must be selected such that there are pairs of path sections in front of and behind the wavelength converter WU, at whose inputs the same bits in the individual channels meet one another at a time , This is achieved if the same value of the accumulated dispersion is set for two selected route sections. The same pairwise arrangement of the accumulated dispersion in front of and behind the wavelength converter must also be selected at the inputs of the dispersion-compensating fibers DCF, since SRS-XT can also occur in these. If the same value for the accumulated dispersion is not set at the inputs of the dispersion compensating fibers of paired sections, the input levels to the DCF must be chosen so low that no SRS-XT occurs in them. Time synchronized bits at the fiber inputs can be achieved by a dispersion compensation scheme in which the dispersion of a link through a dispersion-compensating fiber completely compensates DCF becomes (full compensation). Thus, no accumulated dispersion occurs at the beginnings of the sections. In this dispersion compensation scheme, the bits that pass the input of the first link at a given time also arrive at the inputs of the other links simultaneously. An advantage of the full-compensation scheme is that the splitting of the transmission path and the arrangement of the wavelength converter WU is irrelevant. A drawback of the full-compensation scheme is that it can lead to large cross-phase modulation (XPM) distortion of the signals. To suppress said XPM, the accumulated dispersion should increase or decrease slightly from link to link. An example of such a dispersion compensation scheme, which allows suppression of the cross-phase modulation and at the same time the inventive compensation of the SRS-XT, is shown in FIG. In Fig. 4, the accumulated dispersion is plotted against the length of the transmission path for individual sections of the route with the numbers 1 to 12. In section No. 1 at the beginning of the transmission path, the value of the accumulated dispersion initially drops below zero. This is called pre-compensation. In the following sections of the first half of the transmission path, the dispersion accumulated during transmission is greater than the dispersion compensated in the DCF, resulting in undercompensation per section. The accumulated dispersion at the beginning of each segment thus increases. In the second half of the transmission system overcompensation and post-compensation are used. This means that per dispersion section the dispersion compensation in the DCF is greater than the dispersion accumulated in the transmission fiber. The size of the over- and undercompensation and the compensation in the middle of the transmission system are selected such that pairs of path sections occur in front of and behind the wavelength converter WU, which have the same accumulated dispersion at the input. In the Chosen scheme these are the sections 1 and 12, 2 and 11 and so on.

In the following, a numerical example will be given, which further clarifies the method according to the invention. A configured according to FIG. 2 WDM system consists of 20 sections SSMF with a length of 100 km. In these 160 channels are to be transmitted, 80 in the C and 80 in the L band. The channels have a bit rate of 10 GBit / s and a channel spacing of 50 GHz. The wavelength converter WU according to the invention is arranged after 10 route sections. The dispersion compensation scheme has an undercompensation of 50 ps / nm per section ahead of the wavelength converter WU and over-compensation of -50 ps / nm. The pre-compensation is -200 ps / nm, the after-compensation is -1200 ps / nm. The input level into the DCF before the first section is chosen so low (total power less than 10 dBm) that no noticeable SRS-XT occurs. The other input levels can be selected as high as the route layout allows. In this case, the pairs of path sections in front of and behind the wavelength converter, which have the same accumulated dispersion, also have equal distributions of the channel levels at the fiber input with the same Raman efficiency.

Claims

claims

1. A method for compensating the caused by stimulated Raman scattering bit pattern dependent crosstalk in a multi-channel optical wavelength multiplex transmission system (or WDM system) whose transmission path comprises a plurality of sections, characterized in that the transmission path in at least two Divide the parts that after the first part of the transmission path, a wavelength conversion is performed, such that at least a part of the optical signals transmitted in the first part of the transmission path in channels with a smaller wavelength after wavelength conversion in channels with a longer wavelength and vice versa, and that the dispersion profile with respect to the two parts of the transmission path is designed so that the accumulated dispersion in the first part of the transmission path with the accumulated dispersion in the second n part of the transmission path at least approximately canceled.

2. The method according to claim 1, characterized in that the dispersion profile with respect to the two parts of the transmission path is designed such that in each case pairs of path sections of the first and the second part of the transmission path have the same accumulated dispersion.

3. The method according to claim 1 or 2, characterized in that is designed for an even number of link sections of the dispersion course of the two parts of the transmission path such that the accumulated dispersion at the entrance of symmetrically arranged to subdivide the transmission path sections is the same.

4. The method according to any one of the preceding claims, characterized in that the dispersion profile with respect to the two parts of the transmission path is designed such that in one part of the transmission path undercompensation is performed and in the other part overcompensation or vice versa.

5. The method according to any one of the preceding claims, characterized in that after each link section of the transmission line, a full compensation is performed.

6. The method according to any one of the preceding claims, characterized in that when dividing the transmission path in more than two parts between the individual parts, a wavelength conversion takes place.

7. The method according to any one of the preceding claims, characterized in that the wavelength conversion is performed such that the channel with the smallest wavelength of the WDM system is converted to the channel with the largest wavelength and the channel with the second smallest wavelength of the WDM system the channel with the second largest wavelength is implemented and so on.

8. The method according to claim 7, characterized in that the wavelength conversion from the spectral C-band is in the L-band and vice versa.

9. The method according to any one of the preceding claims, characterized in that are used as optical signals binary optical signals.

10. Optical wavelength multiplex transmission system (or WDM system) with a plurality of optical channels whose transmission path has a plurality of sections, characterized in that between two sections a wavelength converter (WU) is provided which is designed such that at least one Part of the supplied signals transmitted in channels having a smaller wavelength are emitted into channels with a greater wavelength and vice versa, and that the path sections have means for dispersion compensation, which are designed so that the accumulated dispersion in the first section with the accumulated dispersion in the at least approximately neutralizes the second section of the route.

11. An optical wavelength division multiplex transmission system according to claim 10, characterized in that the wavelength converter (WU) is designed as an optical wavelength converter.

12. An optical wavelength converter (WU) for a WDM system, which is configured such that the input side, a WDM signal (WS) of a first band switch (BWL) can be fed, which the WDM signal (WS) in subbands with larger and smaller Wavelengths (TBl and TB2) divides that the first output of the first band-pass filter (BWL) is connected to a first wavelength converter (WKl), which converts the sub-band with the larger wavelengths (TBl) in the sub-band with the smaller wavelengths (TBl _k ) in that the second output of the first band-pass filter (BW1) is connected to a second wavelength converter (WK2) which integrates the sub-band with the smaller wavelengths (TB2) into the second band-pass converter (BW1)

Sub-band with the larger wavelengths (TB2 _k ) transferred, and that the outputs of the first and the second wavelength converter (WKl and WK2) to a second band-pass filter (BW2) ange- are closing, which reassembles the two wavelength-converted subbands (TBl _k and TB2 _k ) and outputs at its output.

13. Optical wavelength converter (WU) for a WDM system according to claim 12, characterized in that the first or the second wavelength converter (WK1, WK2) each consist of a pump source (PQ), a beam splitter (ST) and a conversion element (WL) and is configured such that a first input of the beam splitter (ST) is supplied to the optical signal to be converted and a second input of the beam splitter (ST) a pump signal from the pumping source (PQ) is supplied, that the output of the beam splitter ( ST) leads both signals to a conversion element (WL), in which a non-linear process for wavelength conversion takes place and from which a wavelength-converted optical signal emerges.

14. An optical wavelength converter (WU) for a WDM system according to claim 12 and 13, characterized in that each of the wavelength converter (WKl, WK2) is designed as a pumped waveguide structure of periodically poled lithium niobate.