Journal of Science and today's world 2013, volume 2, issue 6, pages:

Transcription

1 Scholar Journal Available online: Journal of Science and today's world ISSN 3-36X Research Article Use of optical filters in production of VSB modulation in the multiplex systems of wavelength multiplexing Fahimeh Darabi, Mohammad Mehdi Karkhanehchi University of Razi, Kermanshah, Iran Received: 14 February 013 / Accepted: 19 May 013 / Published: 7 June 013 Copyright 013 F. Darabi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract The ever-increasing data traffic on every telecommunications network and the Internet leads to a unique explosion of intense research in order to increase channel capacity. One such research effort is on 40 Gb/s per channel in optical Dense Wavelength Division Multiplexing (DWDM ) communications systems.one way to increase the information capacity that could be transmitted 10 1 over a single fibers is to utilize VSB Modulation format, so that Bit-error-rate(BER) of or better can be achieved across all channels. In this paper optical filters are used to generate VSB Modulation and the design of the band-pass elliptic optical filter is described. The simulation results with MATLAB software are presented.hence gives summary of the most common modulation format for 40 GB/s optical s stems and comparison are made between. Keywords: VSB modulation, wavelength multiplexing, multiplex systems Correspondence should be addressed to F. Darabi, University of Razi, Kermanshah, Iran. 76 P a g e

2 1. Introduction WDM is used to increase the capacity of optical transmission system. In this way, different channels on different wavelengths are sent on a single fiber and separated in the receiver. In WDM systems the capacity of filtering a wavelength among a number of wavelengths is significantly necessary and important. There are various methods of filtering and most of them works based on the interference, diffraction and absorption principles and are fixed and variable. Optical filters are a key tool for WDM systems. The clearest of them is separation of those channels that are far too close together. Another usage of optical filters is in de multiplexes. Multiplexing of different wavelengths is a simple job and is performed by some components such as a star coupler but de multiplexing operation needs to optical filters []. Another application of optical filters is in VSB modulation generation in the multiplexing systems of wavelength multiplex. For this purpose, a pass by filter must be used. This filter can be a member of this list: Bessel, Chebyshev, elliptic and Batrorth filters. In this paper, elliptic filters are used, since they have the steepest pass area between pass-band and stop-band without loss of stability [3].. The increase of optical filter capacity Increment of the transmitted information over a fiber needs to meet two requirements: 1- The minimum amount of the useful bandwidth of the transmitted signal - Removal of optical carriers to reduce the average optical power of all channels leads to increase transmitted power. Therefore, when optical carriers of several channels are multiplexing with together, the average optical power is increased due to the optical power carriers and result in nonlinear dispersion effects. Thus, removal of optical carriers seems necessary. To maximize the installed fibers capacities and meet the above objectives, the VSM modulation is used. The characteristics of this modulation are as follows: narrow bandwidth, removal of optical carriers and high spectral efficiency. 3. Optical Filter Using an optical filter is necessary to obtain VSB modulation format signals. In this paper an elliptic pass by optical filter has been used. One of the important characteristics of elliptic filter is the minimum passage bandwidth to limit the ripple in the pass band and attenuation on the minimum in the stop-band. The spectral response of an N-order elliptic filter can be expressed by the following equation [3]: 763 P a g e

3 w H Lp 1 ( jw) 1 E ( w) N, Where E N ( ) is the rational Chebyshev function. Similarly, the elliptic filter transfer function in the S domain is as follows: 1, where S=jW and bands clearly. H LP 1 D( s) r 0i i 1 s B1 is B0i r N 1 for even Ns and s A N r for odd Ns. Figure 1 shows all of the filter Figure1. The VSB modulation spectral characteristics in contrast with DSB The characteristics of the designed filter are as follows: Pass band ripple less than 0.5 DB, minimum attenuation stop-band at 10 DB, Passage region between 0 to 40 GHZ, and a GHZ stop-band from both sides of pass band. Optical frequency response of the designed filter is shown in figure. These filters can be made of by grating approaches of filter Bragg and or Mach Zender delay interferometer. In the Mach Zender delay interferometers the group delay is nearly constant so there is no dispersion which can penetrate the system via the optical filter. 764 P a g e

4 Figure. Frequency response of designed filter 4. Modulation Modulation type of optical systems is the transformation method of electrical signals to optical signals. Common modulation formats is optical systems with bit rates of 40 GB/s are: NRZ, RZ. As can be seen in figure 3 in RZ type, each optical pulse which indicates the bit 1 is shorter than the bit distance and the pulse altitudes goes back to zero before the end of the bit time. But in NRZ type, the optical pulse remains constant till the end of bit time and its altitude does not go back to zero between bits representing 1 [4]. Figure 3. RZ and NRZ modulation formats One of the advantages of NRZ modulation is that its pulse bandwidth is less than that in RZ modulation (i.e., half of the RZ) but adversely is affected by the nonlinear effects such as XPM and SPM. The advantages of RZ modulation in relation with NRZ modulation include higher maximum power, higher signal to noise ratio, lower bit error ratio, greater security against the effects of fiber nonlinearity and polarization mode dispersion. Thus, RZ modulation is a more suitable choice for terabit long distance optical communication systems. 5. RZ DSB modulation 765 P a g e

5 In this modulation, the top and bottom edges are transmitted with each other. RZ DSB modulation shows a higher dispersion tolerance and security against nonlinear effects. But because of the transmission of two edge bands with exactly the same information, it does not have high spectral efficiency in speeds higher than 40 GB/s [5]. If we consider the message signal as an RZ random pulse train [ ], which is depicted in figure 4, corresponding to equation (3), the spectrum of optical power by a modulation factor of 80% is given in figure 5. E( t) P ( t) exp( j( w t )) out c 3 Where, P out (t) is the signal power, wc optical carrier frequency and phase shift produced by modulator. Figure 4. RZ signal Figure 5. RZ DSB optical spectrum modulation 766 P a g e

6 6. RZ VSB modulation In the speeds higher than 40 GB/s, spectral efficiency is the main factor and is regarded as a key point for the next generation optical systems. Therefore, to enhance the efficiency of spectrum RZ VSB modulation is used. To produce this modulation the optical filter designed in the previous section is applied and some part of the lower band of the RZ DSB modulation will be deleted. This is shown in figure 6. As can be seen RZ DSB bandwidth in 40 GB/s bit rate is 80 GHZ and that in the RZ VSB is 40 GB/s. Figure 6. RZ DSB and RZ VSB modulation optical spectrum 7. Conclusion In this paper VSB modulation is proposed to increase the transmitted information over a fiber in a 40 BG/s transmission in wavelength multiplexing systems. In order to produce VSB modulation an elliptic optical pass by filter with 40 GHZ pass band is designed and via this filter the lower side band of RZ DSB modulation optical spectrum will be deleted. Simulation results using MATLAB software are presented and a comparison between the conventional modulations in 40 GB/s transmission systems is performed and the advantages and disadvantages of each one are proposed. References [1] V.Kartalopulos,"DWDM Network,Device and Technology",John Wiley & Sons,003 [] Ottmar Krauss,"DWDM and Optical Networks", P a g e

7 [3] L.N.Binh,"DWDM VSB modulation format optical transmission",jornal of optical communication,monash university,june 008 [4] L.N.Binh and D.Wong,"DWDM optically amplified transmission systems"depatment of E & CSE,,monash university,005 [5] L.N.Binh and Z.Csematony,"Double Side Band Carrier suppressed RZ and NRZ Modulation Format for Ultra-high Capacity 40Gb/s DWDM Optical communications systems",department of E & CSE,Technical Report MEC-3,October P a g e