A Technique to improve the Spectral efficiency by Phase shift keying modulation technique at 40 Gb/s in DWDM optical systems. A.V Ramprasad and M.Meenakshi Reserach scholar and Assistant professor, Department of Electroncis and Communication Engg. College of Engg, Anna unversity, Guindy,Chennai -25,INDIA. avramprasad2002@yahoo.co.in ABSTRACT Fiber non linear effects play a major role in the limitations of optical system band width. The traffic demands can utilize the available optical system band width either by increasing the data rate or by multiplexing the wavelength channels. This utilization can be measured in terms of its spectral efficiency (b/s/hz) which can be improved by decreasing the channel spacing and increasing the data rate. In this paper an extensive study has been carried on PSK modulations with Bragg filtering technique and hybrid Raman amplification at a data rate of 40 Gb/s with 16 channels. It is found that the CSRZ-DQPSK is more tolerant to non linear and dispersive effects with Bragg filtering with 1.6b/s/Hz spectral efficiency. Keywords: Modulation formats, Bragg filtering, specctral efficiency, Hybrid Raman technique. 1 INTRODUCTION Spectral efficiency is a key issue to enhance aggregate capacity and cost-effectiveness of optical systems. The evolution of these systems is presently following three major technological trends regarding capacity increase (1) an increase in channel bit rate (systems with 40Gb/s per channel), (2) a widening of the transmission bandwidth of the optical fiber (S and L-bands optical amplifiers), and (3) a narrowing of the channel spacing (from 200 GHz to 25 GHz)[1][2][3].This narrowing technique can be implemented by a better filtering technique and an optimal modulation format. In this paper for the first time of the author s knowledge we have considered the implementation of asymmetric pre-filtering by Bragg filtering technique. The spectral efficiency of this system can also be improved by hybrid Raman amplifier technique in the Carrier suppressed Return to zero format (CSRZ-DQPSK) differential quadrature phase shift keying modulation format. Since detailed studies on the benefit of asymmetric pre-filtering have been already reported for OOK( on off keying) carrier-suppressed return-to-zero (CS- RZ) signals[4]. However, regarding differential quadrature phase-shift keying (DQPSK) signals, which is a promising format for high bit rate ULH system applications, similar study has not been reported so far, whereas remarkable demonstrations of 40Gbit/s-based DWDM transmission have been already reported by using symmetrically pre-filtered DQPSK signals. The highlight of this work is the application of asymmetric Bragg filtering in CSRZ DQPSK modulation format towards the improvement of spectral efficiency. The discovery of photosensitivity in optical fiber by Hill et al. in 1978 has opened the doors for the development of a new class of components namely called fiber Bragg gratings (FBG) devices. The filtering capabilities of Bragg gratings combined with its all-fiber configuration and flexibility make them an ideal candidate for components such as spectrallydesigned complex filters, dispersion compensators and add/drop filters. This paper is comprised as follows section 2 brief about the improvement of the spectral efficiency with the Bragg filtering in the presence of non linear effect. Section 3 discusses about the impact of non linear effects in the presence of Hybrid Raman amplifiers for other modulation formats like Duo binary and CSRZ DQPSK techniques. Section 4 brings about the results and discussion and finally section 5 concludes our work. 2 IMPLEMENTATION OF BRAGG FILTERING TECHNIQUE IN CSRZ DQPSK In this section, we have described our novel technique of combining the CSRZ DQPSK modulation format with the dispersion compensating Bragg filter that will improve the spectral efficiency of 40 Gb/s optical system. In the modern dense wavelength division multiplexed (DWDM) systems, characterized by small channel spacing and high bit rates, selective band pass filters are a key component for the implementation of fundamental functions, such as channel add-drop, channel selection, de multiplexing, multi channel filtering and inter leaver. Fig 1 is the transmitter set up for generating CSRZ- DQPSK. To achieve this we require high selectivity(ability to separate two adjacent channels),high out of band rejection in order to guarantee a low cross talk between the channels, flat in band response and low insertion loss[5]. Ubiquitous Computing and Communication Journal 1
implemented to suppress or avoid this effect. Fig 1. Transmitter set up for CSRZ DQPSK Fiber Bragg gratings (FBGs), arrayed waveguide gratings (AWGs), and cascaded Mach Zehnder filters are excellent devices for filtering purposes but their complexity or dimensions strongly increase for applications in high- density WDM systems. Compared to Fiber Bragg grating the AWGs are difficult to realize when channels are closely spaced. The optical signal and associated ASE noise output from the EDFA is passed through an optical band pass filter bandwidth The FBG filter shape is designed for 25-GHz channel spacing UDWDM by optimizing the tradeoff between cross talk and dispersion[6]. After passing through the optical band pass filter, the signal is received by MZI interferometric detection, a p-i-n with 15-GHz bandwidth. The p-i-n is followed by a selectable RF low-pass filter, a limiting amplifier (LA), and an error detector. The decision threshold at the input to the LA, as well as error detector threshold, and sampling instant are all optimized before measuring bit error rate (BER). Filters allow light to transmit in a very narrow spectral range and reflect light in other spectral ranges[7]. In this paper we have simulated CSRZ DQPSK modulation format at 40 Gb/s with 16 channels of separation 25 Ghz,with wavelengths varied from 1556.4nm -1559.4 nm and with 8 spans of EDFA and NZDSF of 80 kms each with the total distance of 640 kms.at the spectral efficiency of 1.6b/s/hz. The non linear effects and the dispersive effects are compensated as we have adapted Bragg technique for filtering actions. The channels went through a Lithium Niobate Mech zender modulator driven at 10 Ghz clock cycle. The duty cycle of CSRZ modulation is 60 % and second modulator produces DQPSK modulation with the quadrature shift. Bragg filter is chromatic dispersion tolerant as mentioned in the literatures the self phase modulation can be tolerated. The transmission line has a total length of 640 kms and consists of 08 spans. Each span has an EDFA, approximately 15 kms of SMF and 65 kms of dispersion shifted fiber for a net dispersion of approximately 0.8 ps/nm/km at 1557 nm. A single adjustable gain-flattening filter is installed at the halfway point of the transmission line. We are using Chirped Bragg grating filter in our paper where the spectral broadening due to SPM can be narrowed that depends up on the initial chirp induced by Brag grating. Several schemes can be Fig 2.back to back measurement of Q factor Table 1..fiber coefficients NZDSF fiber Attenuation Coefficient (α) 0.22 Chromatic dispersion (D c ) 3 Dispersive Slope (S) 50 Nonlinearity Coefficient ( γ) 1.8 PMD Coefficient 0.1 Effective core area f ) 72 Using the uniform grating for dispersion compensation the Q factor is improved as shown in the table 4. Fig 2 shows the Eye diagram Q factor back to back with the simulation parameters in Tables 1 and 5. Table 2: Bragg filtering parameters Parameter Multi channel filter Reflected bandwidth f c ±10 Drop channel Insertion loss Drop Channel IL Uniformity Group Delay across Reflected Band Adjacent & No n-adjacent Channel Isolation PDL of Dropped & Transmitted Channels Express Channel insertion Loss Cladding Modes GHz < 1.5 db <0.1 db <12.0 ps >20 db, 22 db <0.02 db < 1.0 db < 0.2dB Ubiquitous Computing and Communication Journal 2
Drop Channel Isolation in Express >25 db Low pass filter B/w 1.875 Ghz. (ITU± 10 GHz) Apodization profile Table 3: obtained OSNR value. OSNR values Sinc function BER values 3 HYBRID RAMAN AMPLIFICATIONS IN NON LINEAR EFFECTS. 22 in the center channel 10-12 Table 4: Meaured Q factor with and without filtering Distance in Q with out Q with kms comp comp Back to Back 37. 59 ------------ 100kms 21.80702 26.70 200kms 19.026 21.948 300kms 15.42509 18.7878 400kms 13.2275 15.4994 Fiber Bragg grating with chirp and apodization is used in this paper. After selecting the mono mode SMF fiber, we have selected the grating properties whose order is one with sine function shape, linear periodic chirp of 101 segments with total chirp 2 nm, length 50000, index modulation is 0.0006, The user defined function for apodization is the fourth order Gaussian function. The grating used for the filtering action is that each grating has the different period The device can use as 16 channel WDM narrow band selective filter, As shown in Table 4 the Bragg filter performance can be evaluated for dispersion compensation and the Q factor is calculated for various distance using EDFA. Table 3 shows the obtained OSNR values and calculated it error rate.table 2 shows the Bragg filtering parameters. used in our simulation. Table 5 Simulation parameters used in CSRZ DQPSK. Values Continuous wave laser Non linear dispersive fiber Optical filter trapezoidal filter Photo detector 7 dbm/channel Attenuation 0.22 db/km Zero db bandwidth = 45 Ghz,Cut off bandwidth -50 Ghz,cut off magnitude -30 db. Darkcurrent 10 na Fig 3 hybrid Raman set up. The gain medium here is the transmission fiber itself and Raman amplification by the pump will produce less noise due to distributed amplification[8]. The noise figure approximately is 0 db for the hybrid condition In this section using the parameters in tables 2 and 6 with and with out the presence of Raman hybrid amplification the Q factors and the OSNR values are measured for the fixed number of fiber spans and the dispersion compensation ratio is plotted for the Q factors the eye opening penalty is calculated for the hybrid conditions. In the case of multiplexed channels 16 channels of spacing 25 GHz is used for our simulation for the first time with Bragg filtering asymmetrical to determine the EYE PENALTY.Fig 4 shows the plot between the calculated eye opening penalty when the optical filter bandwidth is varied for multi channel 16 in numbers. From the above figure it is clear that by assuming the optimal value of Eye opening penalty around 1 db and analyzed the performance of the multiplexed channels. Due to narrow spectral width duo binary are expected to provide improvements in terms of tolerance to GVD and should allow increase in the spectral efficiency whereas CSRZ DQPSK system was initially proposed for its resilience to SPM [11].However the relative performance of these modulation formats strongly depends on the type of the system (fiber type, amplifier spacing, hybrid Raman amplification etc..) Table 6: hybrid setup simulation parameters. Values PRBS 40 Gb/s data rate,2 32 Continuous wave Laser Mechzender Modulator EDFA Number of loops length 1550nm 15 mw power 100 db ext ratio Gain 5 db Noise figure 6dB 4 spans of EDFA and Ubiquitous Computing and Communication Journal 3
Bessel optical filter Pin diode Nonlinear dispersive fiber 1550 nm with the varying band width Responsivity 1 A/w Dark current 10 na. Fig 4. Eye opening penalty with Bragg filters at the receiver for 16 channels. Table 7: Q factor comparison for a distance of 640 kms. Modulation Q with out Q with hybrid hybrid RZ 27.67 30.88 NRZ 18.78 21.66 CS RZ DQPSK 20.44 24.44 Duo binary 19.88 20.88 3.1 Multi channel For a multi channel of 16 channels with 12 dbm /channel the duo binary modulation format and CSRZ DQPSK is tolerant to non linear effects and narrowing the filter bandwidth does not affect the eye opening penalty with the filter bandwidth in increased and on observing the other modulation formats like RZ and CSRZ-DQPSK where the eye opening penalty is reduced gradually for CSRZ DQPSK whereas drastically for RZ.The value of the OSNR improvement in the presence of the hybrid amplifier depends on the Raman amplifier gain ratio. Table 7 shows the measured OSNR values with and with out Hybrid amplifiers. Table 8: Q factor comparison for a distance of 640 kms. Modulation Q with EDFA Q with hybrid CSRZ- DQPSK 14.67 24.44 4 RESULTS AND DISCUSSION In this paper we have observed the improvement in the spectral efficiency to 1.6b/s/hz at 40 Gb/s with 25 Ghz channel spacing. To achieve this improvement we have applied CSRZ-DQPSK modulation technique with asymmetric Bragg filtering for channel separation. This kind of filtering action will suppress the non linear effects like SPM and reduces the GVD which will improves the OSNR and Q factor. In the second half of our paper we have also compared the performance for other modulation formats like RZ, Duobinary with CSRZ DQPSK using Bragg filtering technique nce can be improved by the Hybrid Raman amplifier. From various plots and results we see CSRZ DQPSK has better performance with the hybrid Raman amplifier and OSNR improvement is also observed. An improvement in the transmission performance of 4-5 db of OSNR and Q factor at the receiver side has been achieved with and with out applying the bragg filtering as shown in table 4. As shown in the Fig.4 for the DWDM systems the eye penalty remains nearly one db for the CSRZ-DQPSK scheme in the narrow band filtering. As a result we can conclude the hybrid Raman amplification will improve the spectral efficiency with CSRZ -DQPSK format using the SMF fiber with the sufficient dispersion compensating parameters as shown in the Table 1. The improvement in the OSNR is obtained based on the Raman gain introduced by the pumping in the distributed amplification. In this paper for the first time simulation is carried out for 16 channels at the distance of 640 kms using Dispersion compensator Bragg filter where the spectral efficiency is improved to 1.6 b/s/hz at the rate of 40 Gb/s and the channel spacing is 25 Ghz.The Q factor performance is improved where the non linear and dispersive effects are tolerated at the high input power 12 dbm /channel The Q factors are measured and found that at 640 kms it is showing better performance when hybris raman amplifier is applied rather than doped amplifiers as shown in Table 8.. 6. Conclusion An improvement in spectral efficiency is carried out at 40 Gb/s with 25 Ghz spacing of 16 channels with the input power of 12 dbm / channel by using hybrid raman amplifier with carrier suppressed DQPSK modulation technique using Bragg filtering technique to suppress the linear and non linear effects References [1] S. D. Personick, Receiver design for digital fiber optic communication systems, I, Bell Syst. Tech. J., vol. 52, pp. 843 874, July 1973. [2] D. Marcuse, Calculation of bit-error probability for a lightwave system with optical amplifiers and post-detection Gaussian noise, J.LightwaveTechnol., vol. 9, pp. 505 513, Apr. 1991. Ubiquitous Computing and Communication Journal 4
[3] Takeshi Hoshida, Optimal 40 Gb/s Modulation Formats for Spectrally Efficient Long-Haul DWDM Systems, journal of lightwave technology, vol. 20, no. 12, deceber 2002 [4] Anes Hodžic et al, Optimized Filtering for 40-Gb/s/Ch- Based DWDM Transmission Systems Over Standard Single- Mode Fiber, IEEE photonics technology letters, VOL. 15, NO. 7, JULY 2003 [5] Joseph M. Kahn, Spectral Efficiency Limits and Modulation/Detection Techniques for DWDM Systems, EE journal of selected topics in quantum electronics, vol. 10, no. March/April 2004 [6] Antonio Mecozzi,, Analysis of Intrachannel Nonlinear Effects in Highly Dispersed Optical Pulse Transmission, IEEE photonics technology letters, vol. 12, no. 4, April 2000 [7] ZERVAS (M.) and MURIEL (M.A.), An efficient inverse scattering algorithm for the design of nonuniform fiber Bragg gratings, Journal of Quantum Electronics, 35, no 8, pp. 1105-1115, August 1999. [8] Raman/erbium- doped inline amplifiers, in Proc. OFC 2000 [9] T. N. Nielsen et al., 3.28-Tb/s (82_40 Gb/s) transmission over 3_100 km nonzero-dispersion fiber using dual C- and L-band hybrid Raman/erbium- doped inline amplifiers, in Proc. OFC 2000. [10] T. Ito et al., 3.2 Tb/s 1,500 km WDM transmission experiment using 64 nm hybrid repeater amplifiers, in Proc. OFC 2000. [11] Y. Zhu et al., 1.28 Tbit/s (32_40 Gbit/s) transmission over 1000 km with only 6 spans, in Proc. ECOC 2000 [12] A.V. Ramprasad and M.Meenakshi, A Study of ASK and PSK modulation formats towards optimal performance in optical communications ICOCN 2005, December 2005, 13-18 Thailand [13] A.V. Ramprasad and M.Meenakshi, A study of various modulation formats against the non linear distortion in DWDM systems, jyothirgamaya 05, conference on condensed matter physics laboratory, SN college, Kollam August 2005. [14] A. Sano and Y. Miyamoto, Performance evaluation of prechirped RZ and CS-RZ formats in high-speed transmission systems with dispersion management, IEEE J. Lightwave Technol., vol. 19, no. 12, pp. 1864-1871, Dec. 2001. [15] B. SchmauB, D. Werber, and P. Paschke, Nonlinearity tolerant dispersion compensation scheme (NTDCS) at 40 Gbit/s NRZ and RZ transmission on different fiber types, in Proc. ECOC 98, pp. 513-514, 1998. Ubiquitous Computing and Communication Journal 5