Method and transmission system for optimization of the transmission characteristics of an optical wavelength division multiplex system

The transmission capacity of optical transmission systems can be increased by multiplexing channels, which have a number of different wavelengths, of an optical wavelength division multiplex (WDM) signal. Due to the wavelength dependency of optical components and effects, the channels have different levels, signal-to-noise ratios and/or signal distortion at the end of the transmission path; for example, in the form of nonlinearities. Ideally, the channels should have a flat gain spectrum after being amplified one or more times along the transmission path. By way of example, a transmission system having a number of cascaded Raman amplifiers has been proposed in Patent Application DE 10057659.1, in order to achieve the flat gain spectrum.

In conventional WDM transmission systems, identical channel levels are set for all the channels at the start of the transmission path via variable attenuaters. However, the wavelength dependency of the optical components results in channel level differences increasing virtually continuously along the path.

Better utilization of the system resources is achieved by the use of pre-emphasis for the channel levels at the start of the transmission path. The distribution of the channel levels is chosen at the start of the path such that all the channels have the same signal-to-noise ratios OSNR at the end of the path. This OSNR pre-emphasis with signal-to-noise ratio control is highly suitable when there is little signal distortion or for so-called noise-limited systems.

A further improved method is the Q-pre-emphasis proposed in Patent Application DE 10047342.3, in which the level distribution of the channels at the start of the transmission path is chosen such that the channels have the same values of the Q factor at the end of the transmission path (see “Optical Fiber Telecommunications”, IIIA, I. P. Kaminow, T. L. Kich, p. 316, 1997, ISBN 0-12-395170-4). The channels with a poor Q factor are raised at the start of the transmission path in order to compensate for the poor signal quality by improved signal-to-noise ratios OSNR at the path end. Since the Q-factor represents a direct measure of the signal quality, the Q-pre-emphasis has the advantage over the previously cited OSNR pre-emphasis that, in addition to the OSNR compensation, a large number of additional limiting effects are taken into account, such as nonlinear signal distortion, different dispersion compensation between the channels, different transmitter or receiver characteristics, and crosstalk due to multiple reflections.

In addition to the Q-factor as a measure of the signal quality, equivalent quality parameters also can be used, such as the bit error rate BER or the number of corrected bits in systems using forward error correction (FEC).

In systems with high channel levels at the start of the transmission path, OSNR-pre-emphasis or Q-pre-emphasis results in high levels of signal distortion due to nonlinear effects such as Four Wave Mixing, Self Phase Modulation, Cross Phase Modulation, Stimulated Raman Scattering (see “Fiber-Optic Communication Systems”, G. P. Agrawal, 2nd Edition, pp. 323-328). This nonlinear distortion is not compensated for by improving the signal-to-noise ratios OSNR when increasing individual channel levels. FIG. 1 shows the Q-factor for optimal dispersion in a transmission system having 8 channels (100 GHz channel separation, 10 Gb/s data rate and NRZ coding), as a function of the input power P in of the channels. The channel quality Q initially rises linearly as the input power P in increases, but enters saturation with the nonlinear effect becoming increasingly significant at high input power levels and then falls again for even higher power levels.

An object of the present invention is, therefore, to ensure an improvement in the transmission quality for a wavelength division multiplex system with broadband cascaded Raman amplifiers; in particular, in the presence of high levels of nonlinear signal distortion.

The method according to the present invention for optimization of the transmission characteristics of a wavelength division multiplex system having cascaded Raman amplifiers, which each have a number of pump sources, for Raman amplification of a WDM signal which has a number of channels has an optimized quality parameter for each channel of the WDM signal at the transmission path end. The quality parameter is suitable for determining signal distortion. Depending on the spectrum of the quality parameter, different Raman gain profiles are newly selected for at least one of the Raman amplifiers in order to compensate for, and optimize, the quality parameters for all the channels.

The Q-factor or bit error rate is calculated via an appropriate measurement device as a suitable quality parameter. In the case of secure data transmission via an error-correcting code, the bit error rate is determined, for example, by the number of corrected bits.

There is no need for pre-emphasis of the channels of the WDM signal at the start of the transmission path for initial adjustment of the quality parameter and/or of the signal-to-noise ratios. Since the channel levels are correspondingly low at the start of the transmission path, there is only little nonlinear signal distortion. Any required pre-emphasis can be arranged at the end of the transmission path for matching to the low channel levels in order that no nonlinear effects occur at high levels.

Should the channels of the start of the transmission path also be subjected to pre-emphasis for initial adjustment of the quality parameter and/or of the signal-to-noise ratios, the measurement device which is arranged at the end of the transmission path will be able to compensate for high levels of signal distortion and high signal-to-noise ratios between the channels by readjustment of the Raman gain profiles.

In a transmission system with little nonlinear signal distortion, a second measurement device may be arranged at the end of the transmission path in order to determine the signal-to-noise ratios of each of the channels. Furthermore, different Raman gain profiles may be newly set for each of the Raman amplifiers, via an additional controller, in order to compensate for and to maximize the signal-to-noise ratios (OSNR).

Suitable selection and control of individual pump currents in the broadband Raman amplifiers makes it possible to set different Raman gain profiles and, thus, to optimally match the quality parameter and/or the signal-to-noise ratios of the channels at the path end.

Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.

FIG. 1 shows the Q-factor for optimal dispersion in an transmission system having eight channels as a function of the input power of the channels.

FIG. 2 shows the basic profile of the method according to the present invention for optimization of the transmission quality.

FIG. 3 shows a number of Raman gain profiles which can be set differently.

FIG. 4 shows the transmission system based on the method according to the present invention.

FIG. 2 shows the basic profile of the method according to the present invention for optimization of the transmission quality of the channels K i of a WDM signal S over a transmission path which has a wavelength division multiplex system with broadband cascaded Raman amplifiers RV 1 , RV 2 , . . . . At the end of the transmission path, a quality parameter Q or BER is determined (as a Q-factor Q or as a bit error rate BER) for each channel K in order to produce a spectrum. If the spectrum of the quality parameter Q or BER has particular signal distortion or non uniform signal-to-noise ratios between the channels K i , then different Raman gain profiles are newly selected for at least one of the Raman amplifiers RV 1 , RV 2 , . . . in order to compensate for and optimize the quality parameter Q or BER, and they are set via a controller. This controller for the corresponding Raman gain profiles of the Raman amplifiers RV 1 , RV 2 , . . . set the pump currents of the pump source of the Raman amplifiers RV 1 , RV 2 , . . . such that all the channels K i have approximately the same, and optimized, quality parameters Q or BER at the end of the transmission path.

FIG. 3 shows a number of Raman gain profiles G which can be adjusted differently, as a function of the wavelength λ, which can be newly selected in order to compensate for and optimize the quality parameter Q or BER. This readjustment is carried out by appropriate control of the pump currents of the pump source, which has a number of different wavelengths, on at least one of the Raman amplifiers RV 1 , RV 2 , . . . . This readjustment compensates firstly for signal distortion, for example originating from nonlinear effects, and secondly for gain irregularities caused by channel level OSNR differences, such as misalignments along the transmission path. The Raman gain profiles are not necessarily linear and/or constant in comparison to conventional amplification methods, but may have an adapted variable spectrum depending on the distribution of the pump currents of the pump source.

FIG. 4 shows a transmission system based on the method according to the present invention with N cascaded Raman amplifiers RV 1 , RV 2 , . . . , RV N . At the start of the transmission path, the channels K i (0<i<m) (which originate from m transmitters T x ) of the WDM signal S are combined via a multiplexer MUX into an optical fiber LWL and are also preamplified by via a booster B before being transmitted through N serial sections, each having a Raman amplifier RV i . At the end of the transmission path, the channels K i are subjected to preamplification and are passed through a demultiplexer DEMUX to m corresponding receivers R x , where they are received. The m receivers R x are followed by a measurement device M which determines the quality parameter Q or BER for each channel K i . A spectrum of the quality parameter Q or BER is produced, and corresponding Raman gain profiles obtained from this spectrum are allocated to the respective Raman amplifiers RV 1 , RV 2 , . . . , RV N in order to compensate for and optimize the quality parameter Q or BER. A controller R for setting the different Raman gain profiles is connected to the measurement device M and to each of the Raman amplifiers RV 1 , RV 2 , . . . , RV N in order to make it possible to control the corresponding pump currents for the pump source for the Raman amplifiers RV 1 , RV 2 . . . , RV N .

Since no pre-emphasis is required for the WDM signal S at the start of the transmission path, the signal distortion remains limited, and can be compensated for correctly by controlling the respective Raman gain profiles based on the analysis of the spectrum of the quality parameter Q or BER.

Although the present invention has been described to reference with specific embodiments, those of skilled in the art will recognized that changes may be made thereto without departing from the spirit and scope of the present invention as set forth in the hereafter appended claims.