Title of Invention	A SYSTEM FOR IMPROVING OPTICAL SIGNAL TO NOICE RATIO AND BIT ERROR RATIO OF A TRANSMISSION SYSTEM
Abstract	The present invention provides a system for improving Optical Signal to Noise Ratio (OSNR) of a transmission system using non gain-flattened optical amplifiers and also provides an optically amplified Dense Wavelength Division Multiplexed (DWDM) transmission system that incorporates aforesaid system and has improved channel OSNR.

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Technical Field The present invention relates to a system for improving Optical Signal to Noise Ratio (OSNR) of a transmission system using non gain-flattened optical amplifiers. The present invention also relates to an optically amplified Dense Wavelength Division Multiplexed (DWDM) transmission system that incorporates aforesaid system and has improved channel OSNR. Background Art In DWDM transmission systems, optical amplifiers are an integral part. In general, erbium doped fiber amplifiers (EDFA) are used to amplify multiple channels. The use of optical amplifiers results in the generation of noise. This generation is intrinsic to the amplification process. The ratio of the optical signal power to the optical noise power is called the Optical Signal to Noise Ratio (OSNR) and is a measure of the quality of the signal transmission. The intrinsic gain spectrum of an EDFA consists of several peaks and valleys. In a chain of cascaded amplifiers the signal near the peak of the gain will grow at the expense of other signals. Hence the optical signal to noise ratio (OSNR) for different channels will be different even if at the input to the link, they were same. Quite a few ways have been demonstrated over the years to flatten the spectral gain characteristics and hence^to effectively improve the relative OSNR variation between the channels. These methods can be categorized under three categories a) Glass composition method, b) Spectral equalizer method) c) Hybrid amplifier method. In all these methods one has to use either special materiaLfor the optical fiber instead of silica or optical filters with special spectral characteristics, which are not very cost effective for multi span DWDM transmission system with multiple amplifiers. It has also been shown that OSNR can be improved by signal pre-emphasis at the beginning of the link. In practice it might not be always possible to control the transmitter power in order to implement this scheme. A good description of the above-mentioned schemes can be found in " Erbium-Doped Amplifiers: Fundamentals and Technology " by P.C Becker et al, Academic Press, 1999. In one of the interesting schemes, it has been shown that OSNR of the system can be improved by demultiplexing the signal channels in the middle of the link and carrying out the spectral equalization by using separate amplifier for each channel and multiplexing them by an optical multiplexer for onward transmission. A publication by L.Eskildsen et al, IEEE Photon. Tech. Lett 6,1321 (1994) gives a description of a similar scheme. The drawback of such a scheme is that as the channel count increases the system will become expensive due to the use of separate optical amplifiers for each channel. Objects of the Invention The main object of the present invention is to provide a system to improve the OSNR of channels of a transmission system. Another object of the present invention is to provide a system which uses non gain-flattened EDFAs in a multichannel transmission system for reducing the relative variation in the OSNR across the channels. Yet another object of the present invention is to provide a system for increasing the number of spans in a multichannel transmission system using non gain-flattened EDFAs. Still another object of the present invention is to provide a system for alleviating the OSNR limitation on the link length in a multichannel transmission system using non gain-flattened EDFAs. One more object of the present invention is to provide an optically amplified Dense Wavelength Division Multiplexed (DWDM) transmission system that incorporates aforesaid system and has improved channel OSNR. Summary of the Invention Accordingly, the present invention provides a system for improving Optical Signal to Noise Ratio (OSNR) of a transmission system using non gain-flattened optical amplifiers. The present invention also provides an optically amplified Dense Wavelength Division Multiplexed (DWDM) transmission system that incorporates aforesaid system and has improved channel OSNR. Detailed Description of the present Invention Accordingly, the present invention provides a system for improving Optical Signal to Noise Ratio (OSNR) of a transmission system using non gain-flattened optical amplifier, said system comprising a WDM Band Splitter (101) connected to plurality of Variable Optical Attenuators (VOAs) (102), outputs from all VOAs are connected to a WDM Band Combiner (103) and output of said WDM Band Combiner being passed through a non gain-flattened optical amplifier (104). In an embodiment of the present invention, the WDM Band splitter splits the incoming optical signal into plurality of multi-channel signal bands. In another embodiment of the present invention, the WDM Band Splitter splits the incoming optical signal into two multi-channel signal bands. In yet another embodiment of the present invention, the WDM Band Splitter splits the optical signal into two multi-channel signal bands, one having longer wavelengths and the other having shorter wavelengths. In still another preferred embodiment of the present invention, spectral equalization is carried out on the two multi-channel signal bands. In a further embodiment of the present invention, spectral equalization of the multi-channel band is performed using individual non gain-flattened optical amplifiers. In one more embodiment of the present invention, the two multi-channel signal bands are separately transmitted through two Variable Optical Attenuators (102a) and (102b). In one another embodiment of the present invention, the two VOAs (102a and 102b) provide fine tuning required to obtain optimum link performance. In one further embodiment of the present invention, the attenuation provided by the two VOAs are set to provide equal lowest signal / channel powers in both bands. In an embodiment of the present invention, the two multi-channel signal bands are combined by the WDM Band Combiner. In another embodiment of the present invention, the combined multi-channel signal is passed through a non gain-flattened optical amplifier (104). In still another embodiment of the present invention, the non-gain flattened optical amplifier is an Erbium Doped Fiber Amplifier (EDFA). In yet another embodiment of the present invention, the EDFA is set for constant gain operation. In a further embodiment of the present invention, the EDFA is set to provide gain more than the insertion losses due to the WDM Band Splitter, VOAs and the WDM Band Combiner and also to provide the signals with additional power. In one more embodiment of the present invention, the system is optionally provided with one or more Optical Spectrum Analyzers (OSA) to view the spectra of the multi-channel signal bands. The present invention also provides an optically amplified Dense Wavelength Division Multiplexed (DWDM) transmission system having improved channel OSNR, said transmission system comprising an Array of Transmitters (201) whose output is multiplexed using a Multiplexer (202), the multiplexed signal is amplified using a Booster Amplifier (203) and launched into a number of spans, one or more systems as herein described before (208) connected in between the spans to improve the OSNR of the transmission system, the signal from the last span is given to a Demultiplexer (209) and the demultiplexed signal is detected using an array of receivers (210). In an embodiment of the present invention, the transmitter array consists of lOGbps externally modulated lasers (EML). In another embodiment of the present invention, the transmitter array includes 16 channels from ITU- T grid no. 22 to 37. In still another embodiment of the present invention, the Booster Amplifier is a non gain-flattened EDFA operating under constant power configuration. In yet another embodiment of the present invention, the transmission system comprises of twelve spans. In a further embodiment of the present invention, each span consists of 80 Km of ITU-T G. 652 compliant Single Mode Fibers (SMF) (206), a Dispersion Compensation Fiber (DCF) (204), two Inline Amplifiers ILA1 (207) and ILA2 (205). f In one more embodiment of the present invention, the DCF (204) compensates the accumulated dispersion of each span. In one another embodiment of the present invention, the Inline Amplifier (ILA2) (205) makes up the nominal loss in the DCF. In one further embodiment of the present invention, the Inline Amplifier(ILAI) (207) makes up for the nominal loss in the SMF. In an embodiment of the present invention, the Inline Amplifiers(ILA1 andlLA2) are non gain-flattened EDFAs. In another embodiment of the present invention, ILA1 and ILA2 are operated under constant gain conditions. In still another embodiment of the present invention, the system to improve the OSNR (208) is implemented after the fourth span. in yet another embodiment of the present invention, the system (208) splits the 10 channel optica! signal into 2 eight-channel signal bands, one having longer wavelengths and the other having shorter wavelengths. In a further embodiment of the present invention, the band having signals of longer wavelength comprises ITU-T grid Nos. 22 to 29. In one more embodiment of the present invention, the band having signals of shorter wavelength comprises ITU-T grid Nos. 30 to 37. Accordingly the present invention provides a system for improving Optical Signal to Noise Ratio (OSNR) and Bit Error Ratio (BER) of a transmission system, said system comprising a WDM Band Splitter (101) for optically splitting the incoming signal into a plurality of bands each with multiple channels said plurality of bands are being connected to a plurality of variable optical attenuators (102) for providing variable attenuation, characterized in that the attenuations of the variable optical attenuators are set such that the channel with lowest power in each of the bands are made to have equal power, outputs from the said Variable Optical Attenuators are being connected to a WDM Band Combiner (103) for combining the plurality of bands and the output of the WDM Band Combiner (103) are being connected to a non-gain flattened optical Amplifier (104) for amplification. The present invention shall now be fully described with reference to the accompanying drawings in which, Figure 1 is a schematic configuration of the system, used to improve the OSNR. Figure 2 is a schematic diagram of the DWDM transmission system employing the system of the present invention after the fourth span to improve the OSNR. Figure 3 is»the illustration of the spectrum of the signal after the Booster Amplifier Figure 4 is the illustration of the spectrum of the signal at the end of the 5'"span, without any spectral reshaping. Figure 5 is the illustration of the spectrum of the signals just after implementing the system of the present invention at the end of the4th Span. Figure 6 is the illustration of the spectrum of the signals after the5th span after implementing the system of the present invention at the end of the4th span. Figure 7 is the illustration of the OSNR map of an ordinary DWDM system (without implementing the system of the present invention). Figure 8 is the illustration of the OSNR map when the system of the present invention is implemented. Brief Description of the Accompanying Tables In the tables accompanying the specification, Table 1 provides a list of parameters used to simulate the DWDM link, as detailed in figure 2, using VPItransmissionmaker™ WDM software. Table 2 provides the numbers corresponding to the graphical representation of the OSNR of all channels from spans 1 through 12 and at the output of the system 208 as illustrated by Figure 8. Table 3 provides the data showing the improvement in the OSNR in each of the individual channels over the entire span, once the system 208 is implemented after the fourth span. The foregoing and other aspects and advantages will be better understood from the following detailed description of preferred embodiments of the invention which are given by way of illustration and therefore should not be construed to limit the scope of the present invention in any manner. The preferred embodiments are described in detail with reference to the drawings for a multiple span DWDM link consisting of 16 channels and several spans. Detailed Description of Preferred Embodiments Referring to Figure 1, a system is shown, through which the OSNR improvement is achieved. The signal is transmitted to a WDM Band Splitter 101. The function of the band splitter is to split the incoming signals into two bands. One band consists of the longer wavelengths and the other band consists of the shorter wavelengths. While the figure specifically refers to two bands, this can be generalized to having more bands. The spectra of the two bands may be viewed in an Optical Spectrum Analyzer (OSA). Each of the bands has peaks and valleys of signals. The bands are separately transmitted through two separate Variable Optical Attenuators 102a and 102b. The VOAs are set such that the minimum signal or channel powers in the two bands are more or less equal. It should be noted that the VOAs 102a and 102b are subsequently fine-tuned for optimum link performance when the scheme is implemented in a DWDM transmission system. The bands are subsequently combined using a WDM band combiner 103. The combined signals are passed through a non gain-flattened EDFA 104. The EDFA is set for constant gain operation. The EDFA 104 is used to overcome the insertion losses due to the WDM Band Splitter, VOAs and the WDM Band Combiner and also to provide the signals with additional power. Figure 2 is illustrating the use of the scheme to improve the OSNR in a multi-span optically amplified DWDM transmission system. The output of a Transmitter Array 201 is multiplexed using a Multiplexer 202. The signal is then boosted by a non gain-flattened Booster Amplifier 203 and launched into the first span. For the sake of clarity only span number one, four, five and twelve are illustrated. The Dispersion Compensating Fibers (DCF) in span numbers one, four, five and twelve are denoted by 204a, 204b, 204c, and 204d, respectively. The ITU-T G.652 compliant Single Mode Fiber (SMF) in span numbers one, four, five and twelve are denoted by 206a, 206b, 206c, and 206d respectively. In each span the accumulated dispersion is more or less compensated by the DCF over the signal band. The non gain-flattened Inline Amplifiers used to make up for the nominal loss in the SMF is denoted by ILA1 and are represented in the figure in span number one, four, five and twelve by 207a, 207b, 207c and 207d, respectively. The non gain-flattened Inline Amplifiers used to make up for the nominal loss in the DCF is denoted by ILA2 and are represented in the figure in span number one, four, five and twelve by 205a, 205b, 205c and 205d, respectively. The scheme to improve the OSNR 208 is implemented after the fourth span. The detailed working of the same has been explained earlier with reference to Figure 1. The signal coming out of the multiplexer is introduced to the next span, namely the fifth span and it gets transmitted to the subsequent spans. The signal is demultiplexed using the Demultiplexer 209. The demultiplexed signals are detected by an array of receivers 210. The simulation parameters used to simulate the link using VPItransmissionmaker WDM are provided in Table 1. The transmitter array includes 16 Channels from ITU-T grid no. 22 to 37 consisting of lOGbps externally modulated lasers (EML). The signals are multiplexed using a multiplexer and thereafter boosted by a non gain-flattened booster EDFA operated under a constant power configuration. Each span consists of 80 km of ITU-T G.652 compliant fibers. Link loss is compensated by a non-gain flattened EDFA operating under constant gain condition. The accumulated dispersion of each span is compensated by a Dispersion Compensating Fiber (DCF) and the loss incurred in the DCF length is compensated by another non-gain flattened EDFA operating under constant gain condition. The scheme to improve the OSNR as has been detailed in Figure 1 has been implemented after the fourth span. The two bands that are split consist of ITU-T grid 22-29 in the first band and ITU-T grid 30-37 in the second band. Figure 3 illustrates the spectrum after the Booster Amplifier. In the 1530 nm region, the gap in the spectrum is attributed to the amplified spontaneous emission (ASE) rejection filter used with each amplifier in order to prevent the saturation of subsequent amplifiers in the link by ASE noise. It can be observed from the figure that the spectrum of the transmitters is more or less flat after the booster amplifier. Figure 4 illustrates the spectrum after the fifth span wherein the scheme to improve the OSNR is not implemented. It can be observed that there are peaks and valleys of the amplifier in the signal band. The valleys degrade the OSNR considerably. Figure 5 illustrates the spectrum after the implementation of the scheme to improve the OSNR. The spectrum is noted at the point where the signal is launched into the fifth span. In this figure it should be noted that there is a spectral reshaping done so that the minimum channel powers in both bands namely ITU-T grid 22-29 and ITU-T grid 30-37 are more or less equal. It should be noted that VOAs 103a and 103b are used to fine-tune the settings to get optimum link performance. Figure 6 illustrates the spectrum at the end of the fifth span where the scheme to improve the OSNR is carried out at the end of the fourth span. As had been mentioned earlier with reference to figure 5 the spectral reshaping done at the end of the fourth span can be observed. The OSNR map, when channels are transmitted across all twelve spans without the implementation of the scheme to improve the OSNR, is illustrated in Figure 7. The improvement in the OSNR after the implementation of the scheme can be seen in Figure 8. The corresponding data is tabulated in Table 2. The data showing the improvement in the OSNR in each of the individual channels over the entire span, once the system 208 is implemented after the fourth span is shown in Table 3. There is a substantial improvement in the OSNR of the transmitted channels up to 12 spans. The implementation of the scheme to improve the OSNR results in all channels having a Bit Error Rate (BER) of less than 1 in 1015 even at the end of the eighth span. We claim 1. A system for improving Optical Signal to Noise Ratio (OSNR) and Bit Error Ratio (BER) of a transmission system, said system comprising a WDM Band Splitter (101) for optically splitting the incoming signal into a plurality of bands each with multiple channels said plurality of bands are being connected to a plurality of variable optical attenuators (102) for providing variable attenuation, characterized in that the attenuations of the variable optical attenuators are set such that the channel with lowest power in each of the bands are made to have equal power, outputs from the said Variable Optical Attenuators are being connected to a WDM Band Combiner (103) for combining the plurality of bands and the output of the WDM Band Combiner (103) are being connected to a non-gain flattened optical Amplifier (104) for amplification. 2. The system as claimed in claim 1, wherein the WDM Band splitter splits the incoming optical signal into plurality of signal bands each comprising multiple channels. 3. The system as claimed in claim 1, wherein the WDM Band Splitter splits the optical signal into multi-channel signal bands, one having longer wavelengths and the other having shorter wavelengths. 4. The system as claimed in claim 1, wherein the spectral reshaping of the multi channel band is performed using individual variable optical attenuators. 5. The system as claimed in claim 1, wherein the two multi-channel signal bands are separately transmitted through Variable Optical Attenuators. 6. The system as claimed in claim 1, wherein the attenuations of the variable optical attenuators are set such that the channel with the lowest power in each of the bands are made to have equal power after amplification. 7. The system as claimed in claim 1, wherein the VOAs provide the required fine tuning to obtain optimum link performance. 8. The system as claimed in claim 1, wherein the multi-channel signal bands are combined by the WDM Band Combiner. 9. The system claimed in claim 1, wherein the combined multi-channel signal is passed through an Erbium Doped Fiber Amplifier (EDFA). 10. The system as claimed in claim 1, wherein the EDFA is set for constant gain operation. 11. The system as claimed in claim 1, wherein the EDFA is set to provide gain more than the insertion losses due to the WDM Band Splitter, VOAs and the WDM Band Combiner and also to provide the signal with additional power. 12. The system as claimed in claim 1, wherein the system is optionally provided with one or more Optical Spectrum Analyzers (OSA) to view the spectra of the multi-channel signal bands. 13. The system as claimed in claim 1, wherein Optical Signal to Noise Ratio (OSNR) and Bit Error Ratio (BER) for all the channels in a multi-span DWDM transmission link are improved for the signals transmitted over multiple spans. 14. An optically amplified Dense Wavelength Division multiplexed (DWDM) transmission system using a system for improving Optical Signal to Noise Ratio (OSNR) and Bit Error Ratio (BER) (208) as claimed in claims 1-13 wherein the said the system (208) is being implemented at an intermediate stage. 15. A system for improving optical signal to noise ratio (OSNR) and bit error ratio of a transmission system substantially as herein described with reference to the accompanying drawings.

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