- Email: [email protected]
Investigation on four wave mixing effect in various optical Fibers for different spectral efficient orthogonal modulation Formats
Optics & Laser Technology 76 (2016) 64–69
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Investigation on four wave mixing effect in various optical Fibers for different spectral efficient orthogonal modulation Formats Surinder Singh n, Sukhbir Singh Department of Electronics and Communication Engineering, Sant Longowal Institute of Engineering & Technology, Sangrur, Longowal, Punjab 148106, India
art ic l e i nf o
a b s t r a c t
Article history: Received 17 February 2015 Received in revised form 11 July 2015 Accepted 21 July 2015 Available online 7 August 2015
The paper analyzes the four wave mixing (FWM) effect in different spectral efficient orthognal modulation formats at equal channel spacing of 100 GHz and 50 GHz to design long haul wavelength division multiplexing (WDM) optical system. Further, the comparison of reduction of FWM for existing and proposed modulation format have been analyzed by varying the laser input power from 10 dBm to 10 dBm. & 2015 Elsevier Ltd. All rights reserved.
Keywords: FWM Orthogonal modulation formats Channel spacing BER
1. Introduction In rapid growth of optical communication systems, the demand for higher information capacity is more because of growth in multimedia traffic such as internet usage, VoD (video on demand), cloud computing, and streaming videos and voice. In addition to pack more channels into limited bandwidth, orthogonal modulation formats and WDM multiplexing techniques are emerging and promising alternative [1,2]. Orthogonal modulation formats has high compact spectrum and more tolerance to phase noise. Therefore, it can go through less channel spacing to put more channels close to each other to improve spectral efficiency optical communication system and WDM technique for the long haul of high capacity optical transmission system in present time to fulfill demand of data rate above 40 Gb/s. These technologies are suggested to increase total information capacity of single fiber with less channel spacing of o100 GHz [3,4]. Orthogonal modulation techniques have better exploitation of the existing optical transmission network as emerging and promising technologies due to their large information carrying capacity, flexible scalability and enhanced security. Fiber nonlinearities such as self phase modulation (SPM), cross phase modulation (XPM) and four wave mixing (FWM) restrict the transmission distance and bandwidth efficiency of optical communication systems. These nonlinearities effects become major issue when optical power is very high and important to mitigate for better performance of long houl WDM systems. The FWM is
one of the major and significant degrading factors in WDM optical communication systems [5,6]. Four wave mixing effect represents the inelastic scattering that occurs when two or more frequency signal are mixed while propagating through the same fiber in the same direction and produce new frequency signals which leads to degradation of the performance of WDM systems [7]. In the case of FWM effect, three copropagating optical signals through the optical fiber in the same direction of frequencies say fp, fq and fr interact with each other and generate a fourth signal at frequency fpqr, where fpqr ¼ fp þ fq fr. These interactions become more severe in WDM system because a number of high optical power signals copropagate at different wavelengths. These newly generated frequency signals interact with the original frequency signals and there happens to be some frequency match between them, which leads to interchannel noise and degradation of the WDM system performance. When channels are equally spaced then probability of this frequency match increases [8]. The FWM effect can be more severe when in-line amplification used in optical link because by use of in-line amplification the effective area of fiber increases over which nonlinear interaction take place. FWM power generated at the frequency fpqr when inline amplifiers are used, is (assuming equal signal power P launched in all the wavelength channels) [9,10].
P (fpqr ) = k2P 3e−a . La ((M + 1) L e/A e )2ηpqr d2 pqr
(1)
Where k = (32π 3X ) /(n2cλ ). n
Corresponding author. E-mail address: [email protected] (S. Singh).
http://dx.doi.org/10.1016/j.optlastec.2015.07.013 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
ηpqr = [α 2/(α 2 + Δβ2 pqr )][1 + (4e−αL /(1 − e−αL )2) sin2 (Δβpqr L/2)]
(2)
S. Singh, S. Singh / Optics & Laser Technology 76 (2016) 64–69
65
Fig. 1. Architecture for WDM system employing orthogonal modulation.
Fig. 2. Simulation setup for single channel (a) NRZ/DQPSK/PolSK orthogonal modulated transmitted and receiver, (b) NRZ/PolSK orthogonal modulated transmitted and receiver (c) DPSK modulated transmitted and receiver. Table 1 Different parameters of SMF and DCF considered for demonstration of FWM in different orthogonal modulation format. Fiber type
Fiber length
Attenuation (dB/km)
Dispersion (ps/nm/km)
Dispersion slope (ps/nm2/km)
Effective area (μm2)
Differentia group delay (ps/km)
SMF DCF
40 8
0.2 0.5
17 85
0.075 0.3
70 22
0.2 0.2
Table 2 Parameter of EDFA gain amplifier considered for demonstration of FWM in different orthogonal modulation format. Amplifier type Gain (dB) Noise figure (dB) EDFA (Gain control)
20
4
Noise center frequency (THz)
Noise bandwidth (THz)
193.4
13
Δβpqr = [(2πλ2)( fp − fr fp − fr )] × {D + dD/dλ (λ2/2c )( fp − fq fq − fr )}
(3)
Where P is input laser power, a is fiber attenuation coefficient and M is number of amplifiers. Le is effective length of system, Ae is effective area of fiber, α is total fiber attenuation, L is system length n is refractive index of the fiber, λ is center wavelength, X is thirdorder nonlinear electric susceptibility, dpqr is degeneracy factor, D is dispersion coefficient, and ηpqr is FWM efficiency. FWM efficiency depends on phase mismatch between channels, also depends on channel separation and fiber dispersion coefficient.
In literature, various techniques has been reported for suppressing the FWM effect, such as the use of nonzero dispersion fibers, dispersion management and unequal-channel spacing techniques [11–13]. In this paper, we investigate the effect of FWM in spectral effeicient orthognal modulation formats with variable laser input power to design the WDM optical communication system. This paper organized as follows: Section 1 introduced the WDM techniques, optical nonlinearities and orthogonal modulation techniques and Section 2 describes the general discussion of WDM system description, followed by discussion of generation and detection of two-dimension orthogonal signal and threedimension orthogonal signal. In Section 3, the results of effect FWM in WDM system is presented and in Section 4 the conclusions have been made.
2. System setup Fig. 1 shows the architecture of WDM system employing orthogonal modulation formats. As shown in Fig. 1, the optical signals are generated from CW laser sources that are modulated as 2D/3D orthogonal modulation as shown in Fig. 2(a) and (b) and transmitted over 40 km fiber link. The driving input power of all
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S. Singh, S. Singh / Optics & Laser Technology 76 (2016) 64–69
Fig. 3. Spectrum and eye diagram of DPSK singal at (a) 100 GHz channel spacing and (b) 50 GHz channel spacing (for input power of 10 dBm).
BER
10
0
-50
Without Compensation
10
With Compensation
DPSK DPSK`` DPSK
-100
10
DPSK``
-16 -14 -12 -10 -8
-6
-4
-2
0
2
4
6
8
10
Received Power (in dBm)
channel laser sources (with linewidth of 10 MHz) is variable to analyze the effect of FWM. The generated orthogonal modulated signals from transmitter are then fed into optical multiplexer where they are combined to transmitt through the optical link and erbium doped fiber amplifier (EDFA) compensate the linear loss occurring during propagation through optical link and noise figure of amplifier is kept at 4 dB [14]. At receiver side, the multiplexed signals are passed through optical demultiplexer, Mach–Zehnder interferometer (MZI), PIN detector and filter for BER measurement. The values of dark current and responsivity of PIN detector is set to be 10 nA and 1 A/W, respectively. The simulation setup for demodulation of orthogonal modulated signal is shown in Fig. 2(a) and (b). The system is simulated by input power to light source to visualize the effect of FWM in orthogonal modulated signal and results such as optical spectra, eye diagram, BER and quality factor are measured.
-20
10
3. Results and discussion
-40
10
To analyze the effect of FWM in orthogonal modulation formats for WDM systems, different parameters for SMF, DCF and EDFA present in Table 1 and Table 2. In addition, various cases are given below.
Without Compensation
-60
BER
10
With Compensation -80
Case 1. DPSK modulated channels at channel spacing of 100 GHz and 50 GHz with variable laser input power.
10
DPSK
-100
10
DPSK`` DPSK DPSK``
-120
10
-15
-10
-5
0
5
10
Received Power (in dBm) Fig. 4. BER performance of two-channel DPSK modulation at channel spacing of (a) 100 GHz and (b) 50 GHz with compensation and without compensation (First channel data formats represented as DPSK and second channel as DPSK``).
In this case, two DPSK modulation are employed channel at channel spacing of 100 GHz and 50 GHz with variable laser input power range from 10 dBm to 10 dBm. Central frequencies at 100 GHz channel spacing are 193.1 THz, 193.2 THz and at channel spacing of 50 GHz central frequencies are 193.1 THz and 193.15 THz. Fig. 3(a) and (b) represent the results for channel spacing of 100 GHz and 50 GHz respectively, where 40 km single span of SMF
S. Singh, S. Singh / Optics & Laser Technology 76 (2016) 64–69
67
Fig. 5. Spectrum and eye diagram of two-dimension orthogonal modulated singal at (a) 100 GHz channel spacing and (b) 50 GHz channel spacing (for input power of 10 dBm).
Case 2. Two dimentional (NRZ/PolSK) orthogonal modulated channels at channel spacing of 100 GHz and 50 GHz with variable laser input power. In this case, two NRZ/PolSK orthogonal modulation employed channel at channel spacing of 100 GHz and 50 GHz with variable laser input power range fromn 10 dBm to 10 dBm. Central frequencies at 100 GHz channel spacing are 193.1 THz, 193.2 THz and at channel spacing of 50 GHz central frequencies are 193.1 THz and 193.15 THz. Extinction ratio of NRZ is kept at 13 dBm for proper detection of PolSK modulated data. Fig. 5(a) and (b) represent the optical spectrum and eye diagram of two-dimensional orthogonal modulated signal for channel spacing of 100 GHz and 50 GHz respectively due to FWM effect, each wavelength channel contributes to interchannel crosstalk from adjacent and non-adjacent channel.
-20
10
BER
Without Compensation
-40
10
NRZ PolSK NRZ`` PolSK`` NRZ PolSK NRZ`` PolSK``
With Compensation
-60
10
-10
-5
0
5
10
15
Received Power (in dBm)
-20
10
Without Compensation
BER
fiber. As shown in the optical spectrum of DPSK due to FWM effect each wavelength channel contributes to interchannel crosstalk from adjacent and non-adjacent channel. For 100 GHz channel spacing eye diagram is clear but in case of 50 GHz channel spacing eye diagram shows the severe impairments due to FWM degenerate case. Fig. 4(a) and (b) shows the BER performance of point-to-point DPSK signal for WDM system at channel spacing of 100 GHz and 50 GHz with variable launch powers respectively. In the presence of thermal, shot and FWM noises, it is observed from Fig. 4(a) and (b) that in case of 100 GHz channel separation and low transmitted power, BER performance improved with an increase in laser launch power. As the input laser power increases, after 1 dBm power level the FWM noise power becomes dominant and the BER performance degrading. In case of 50 GHz channel spacing, similar variation BER performance of DPSK with input launch power but FWM effect is more dominant than 100 GHz channel separation. After 40 km transmission, the power penalties are less than 1.5 dB and 0.5 dB observed with and without compensation respectively.
-40
10
With Compensation
NRZ PolSK NRZ`` PolSK`` NRZ PolSK NRZ`` PolSK``
-60
10
-10
-5
0
5
10
15
Received Power (in dBm) Fig. 6. BER performance of two channel employing NRZ/PolSK orthogonal modulation at channel spacing of (a) 100 GHz (b) 50 GHz with compensation and without compensation (first channel data formats represented as NRZ, PolSK and second channel as NRZ``, PolSK``).
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S. Singh, S. Singh / Optics & Laser Technology 76 (2016) 64–69
Fig. 7. Spectrum and eye diagram of three-dimension orthogonal modulated singal at (a) 100 GHz channel spacing and (b) 50 GHz channel spacing (for input power of 10 dBm).
compensation), BER perfromance is improved with an increase in laser launch power. -3
10
-4
10
-5
10
-6
10
-7
10
-8
NRZ PolSK DQPSK NRZ``
Case 3. Three dimensional (NRZ/DQPSK/PolSK) orthogonal modulated channels at channel spacing of 100 GHz and 50 GHz with variable laser input power.
-10
PolSK`` DQPSK``
-5
0
5
10
15
Received Power (in dBm) 100 NRZ PolSK
-2
10
DQPSK NRZ`` PolSK``
-4
10 BER
In this case, two NRZ/DQPSK/PolSK orthogonal modulation employed channel at channel spacing of 100 GHz and 50 GHz with variable laser input power range from 10 dBm to 10 dBm. Central frequencies at 100 GHz channel spacing are 193.1 THz, 193.2 THz and at channel spacing of 50 GHz central frequencies are 193.1 THz and 193.15 THz. Extinction ratio of NRZ kept at 13 dBm for proper detection of DQPSK and PolSK modulated data. Fig. 7(a) and (b) shows the optical spectrum and eye diagram of three-dimensional orthogonal modulated signal for channel spacing of 100 GHz and 50 GHz respectively and it is observed that due to FWM effect, each wavelength channel contributes to interchannel crosstalk from adjacent and non-adjacent channel as compared to DPSK and two-dimensional orthogonal modulation format. Fig. 8(a) and (b) shows the BER performance of point-to-point three-dimensional orthogonal modulated signal for WDM system at channel spacing of 100 GHz without and with compensation respectively. In the presence of thermal, shot and FWM noises, it is observed from Fig. 8(a) and (b) that in case of 100 GHz channel separation and low transmitted power (without and with
10
BER
To analyze the effect of FWM in two-dimension orthogonal modulated signal we also consider the effect of SRS with thermal and shot noise. From Fig. 6(a) and (b), it is observed that BER improves with increase in the input laser power but at certain input power level, BER degrades due to SRS and as the FWM noise become dominant. It is obseved that at channel spacing of 100 GHz BER performance of NRZ/PolSK is better as commpared to 50 GHz spaced channel because with the decrease in channel spacing, FWM-induced noise power increases linearly while the SRS gain decreases linearly which leads to the degradation of the BER performace. After 40 km transmission, the power penalties are less than 2.34 dB and 0.75 dB observed with and without compensation respectively.
DQPSK``
10-6 10-8 10-10 -15
-10
-5
0
5
10
Received Power (in dBm) Fig. 8. BER performance of two channel employing NRZ/DQPSK/PolSK orthogonal modulation (a) without compensation (b) with compensation at channel spacing of 100 GHz (first channel data formats represented as NRZ, DQPSK, PolSK and second channel as NRZ``, DQPSK``, PolSK``).
S. Singh, S. Singh / Optics & Laser Technology 76 (2016) 64–69
haul bandwidth efficient WDM system at 100 GHz and 50 GHz spaced channels. Results are also presented for variable laser input powers indicating the range of the validity of the WDM system. From analysis of FWM effect, it is concluded that WDM system performance is very much dependent on FWM noise and system performance degrade at positive power level in all considered cases. It is observed that three-dimension orthogonal modulated channels are affected more by FWM as compared to two-dimension orthogonal. DPSK modulation formats and FWM effect are more dominant at 50 GHz channel spacing than 100 GHz. Due to FWM effect more on three-dimensional orthogonal modulation format, it is difficult to design practical long houl WDM communication system as compared to other modulation formats.
-2
10
NRZ PolSK DQPSK NRZ``
-4
10
69
PolSK``
BER
DQPSK``
-6
10
-8
10
-10
-5
0
5
10
15
Received Power (in dBm)
Acknowledgment The authors would like to thank TEQIP-II, SLIET, Longowal for their funding to research projects no: TEQIP/905 dated: 30.10.13.
-2
10
NRZ PolSK -4
DQPSK
10
References
NRZ`` PolSK``
BER
-6
DQPSK``
10
-8
10
-10
10
-12
-10
-8
-6
-4
-2
0
2
4
6
8
10
Received Power (in dBm) Fig. 9. BER performance of two channel employing NRZ/DQPSK/PolSK orthogonal modulation (a) without compensation (b) with compensation at channel spacing of 50 GHz (first channel data formats represented as NRZ, DQPSK, PolSK and second channel as NRZ``, DQPSK``, PolSK``).
As the input laser power increases, after certain power level the FWM noise power becomes more dominant and the BER performance DQPSK degrades more as compared to NRZ and PolSK when there is no compensation techniques are used. In case of fully compensated system, similar variation in BER is observed. From Fig. 9(a) and (b) it is observed that at 50 GHz channel spacing, similar variation BER performance of three-dimension with input launch power but FWM effect is more dominant than 100 GHz channel separation.
4. Conclusion In this paper, an analysis for interchannel FWM in a single span orthogonal modulated link is presented. Results are reported for the different orthogonal modulation formats for designing of long
[1] Y. Shao, N. Chi, High spectral-efficiency 100 Gb/s transmission using DRZ, DQPSK and PolSK three dimension orthogonal modulation, Opt. Commun. 285 (6) (2012) 1049–1052. [2] Pierpaolo Boffi, et al., 20 Gb/s differential quardrature phase shift keying transmission over 2000 Km in 64-channel WDM system, Opt. Commun. 237 (2004) 319–323. [3] A. Sheetal, A.K. Sharma, R.S. Kaler, Simulation of high capacity 40 Gbps long haul WDM system using different modulation formats and dispersion compensated schemes in presence of Kerr’s effect, Optik 121 (8) (2010) 739–749. [4] Y. Shao, et al., Novel optical orthogonally modulation schemes for superimposing DPSK Signal on dark RZ signal, Opt. Commun. 281 (4) (2008) 3658–3667. [5] K. Inoue, H. Toba, Fiber four wave mixing in multi- amplifier systems with non uniform chromatic dispersion, J. Lightwave Technol. 13 (1995) 89–93. [6] C.T. Shuxian Song, K.R. Allen, Rongqing Hui Demarest, Intensity dependent phase matching effects on four wave mixing in optical fiber, J. Lightwave Technol. 17 (1999) 2285–2290. [7] A. Yu, M.J. O’Mahony, Effect of four-wave-mixing on amplified multiwavelength transmission systems, Electron. Lett. 30 (11) (1994) 876–878. [8] R. Kaler, R.S. Kaler, Investigation of four wave mixing effect at different channel spacing, Optik 123 (2012) 352–356. [9] M.J. O’Mahony, D. Simeonidou, A. Yu, J. Zhou, The design of a European optical network, J. Lightwave Technol. 13 (5) (1995) 817–828. [10] M.W. Maeda, W.B. Sessa, W.I. Way, A. Yi-Yan, L. Curtis, R. Spicer, R.I. Laming, The effect of four wave mixing in fibers on optical frequency division multiplexed systems, J. Lightwave Technol. 8 (9) (1990) 1402–1408. [11] K. Nakajima, M. Ohashi, K. Shraki, T. Horiguchi, Four wave mixing suppression effect of dispersion distributed fibers, J. Lightwave Technol. 17 (1999) 1814–1822. [12] C.F. Wehmann, L.M. Fernandes, C.S. Sobrinho, J.L.S. Lima, M.G. da Silva, E.F. de Almeida, J.A. Medeiros Neto, A.S.B. Sombra, Analysis of the four wave mixing effect (FWM) in a dispersion decreasing fiber (DDF) for a WDM system, Optic. Fiber Technol. 11 (2005) 306–318. [13] F. Forghieri, R.W. Tkach, A.R. Chraplyvy, WDM systems with unequally spaced channels, J. Lightwave Technol. 13 (5) (1995) 889–897. [14] S. Singh, R.S. Kaler, Flat gain l-band Raman-EDFA hybrid optical amplifier for dance wavelength division multiplexed system, IEEE Photon. Technol. Lett. 25 (3) (2013) 250–252.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Investigation on four wave mixing effect in various optical Fibers for different spectral efficient orthogonal modulation Formats Surinder Singh n, Sukhbir Singh Department of Electronics and Communication Engineering, Sant Longowal Institute of Engineering & Technology, Sangrur, Longowal, Punjab 148106, India
art ic l e i nf o
a b s t r a c t
Article history: Received 17 February 2015 Received in revised form 11 July 2015 Accepted 21 July 2015 Available online 7 August 2015
The paper analyzes the four wave mixing (FWM) effect in different spectral efficient orthognal modulation formats at equal channel spacing of 100 GHz and 50 GHz to design long haul wavelength division multiplexing (WDM) optical system. Further, the comparison of reduction of FWM for existing and proposed modulation format have been analyzed by varying the laser input power from 10 dBm to 10 dBm. & 2015 Elsevier Ltd. All rights reserved.
Keywords: FWM Orthogonal modulation formats Channel spacing BER
1. Introduction In rapid growth of optical communication systems, the demand for higher information capacity is more because of growth in multimedia traffic such as internet usage, VoD (video on demand), cloud computing, and streaming videos and voice. In addition to pack more channels into limited bandwidth, orthogonal modulation formats and WDM multiplexing techniques are emerging and promising alternative [1,2]. Orthogonal modulation formats has high compact spectrum and more tolerance to phase noise. Therefore, it can go through less channel spacing to put more channels close to each other to improve spectral efficiency optical communication system and WDM technique for the long haul of high capacity optical transmission system in present time to fulfill demand of data rate above 40 Gb/s. These technologies are suggested to increase total information capacity of single fiber with less channel spacing of o100 GHz [3,4]. Orthogonal modulation techniques have better exploitation of the existing optical transmission network as emerging and promising technologies due to their large information carrying capacity, flexible scalability and enhanced security. Fiber nonlinearities such as self phase modulation (SPM), cross phase modulation (XPM) and four wave mixing (FWM) restrict the transmission distance and bandwidth efficiency of optical communication systems. These nonlinearities effects become major issue when optical power is very high and important to mitigate for better performance of long houl WDM systems. The FWM is
one of the major and significant degrading factors in WDM optical communication systems [5,6]. Four wave mixing effect represents the inelastic scattering that occurs when two or more frequency signal are mixed while propagating through the same fiber in the same direction and produce new frequency signals which leads to degradation of the performance of WDM systems [7]. In the case of FWM effect, three copropagating optical signals through the optical fiber in the same direction of frequencies say fp, fq and fr interact with each other and generate a fourth signal at frequency fpqr, where fpqr ¼ fp þ fq fr. These interactions become more severe in WDM system because a number of high optical power signals copropagate at different wavelengths. These newly generated frequency signals interact with the original frequency signals and there happens to be some frequency match between them, which leads to interchannel noise and degradation of the WDM system performance. When channels are equally spaced then probability of this frequency match increases [8]. The FWM effect can be more severe when in-line amplification used in optical link because by use of in-line amplification the effective area of fiber increases over which nonlinear interaction take place. FWM power generated at the frequency fpqr when inline amplifiers are used, is (assuming equal signal power P launched in all the wavelength channels) [9,10].
P (fpqr ) = k2P 3e−a . La ((M + 1) L e/A e )2ηpqr d2 pqr
(1)
Where k = (32π 3X ) /(n2cλ ). n
Corresponding author. E-mail address: [email protected] (S. Singh).
http://dx.doi.org/10.1016/j.optlastec.2015.07.013 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
ηpqr = [α 2/(α 2 + Δβ2 pqr )][1 + (4e−αL /(1 − e−αL )2) sin2 (Δβpqr L/2)]
(2)
S. Singh, S. Singh / Optics & Laser Technology 76 (2016) 64–69
65
Fig. 1. Architecture for WDM system employing orthogonal modulation.
Fig. 2. Simulation setup for single channel (a) NRZ/DQPSK/PolSK orthogonal modulated transmitted and receiver, (b) NRZ/PolSK orthogonal modulated transmitted and receiver (c) DPSK modulated transmitted and receiver. Table 1 Different parameters of SMF and DCF considered for demonstration of FWM in different orthogonal modulation format. Fiber type
Fiber length
Attenuation (dB/km)
Dispersion (ps/nm/km)
Dispersion slope (ps/nm2/km)
Effective area (μm2)
Differentia group delay (ps/km)
SMF DCF
40 8
0.2 0.5
17 85
0.075 0.3
70 22
0.2 0.2
Table 2 Parameter of EDFA gain amplifier considered for demonstration of FWM in different orthogonal modulation format. Amplifier type Gain (dB) Noise figure (dB) EDFA (Gain control)
20
4
Noise center frequency (THz)
Noise bandwidth (THz)
193.4
13
Δβpqr = [(2πλ2)( fp − fr fp − fr )] × {D + dD/dλ (λ2/2c )( fp − fq fq − fr )}
(3)
Where P is input laser power, a is fiber attenuation coefficient and M is number of amplifiers. Le is effective length of system, Ae is effective area of fiber, α is total fiber attenuation, L is system length n is refractive index of the fiber, λ is center wavelength, X is thirdorder nonlinear electric susceptibility, dpqr is degeneracy factor, D is dispersion coefficient, and ηpqr is FWM efficiency. FWM efficiency depends on phase mismatch between channels, also depends on channel separation and fiber dispersion coefficient.
In literature, various techniques has been reported for suppressing the FWM effect, such as the use of nonzero dispersion fibers, dispersion management and unequal-channel spacing techniques [11–13]. In this paper, we investigate the effect of FWM in spectral effeicient orthognal modulation formats with variable laser input power to design the WDM optical communication system. This paper organized as follows: Section 1 introduced the WDM techniques, optical nonlinearities and orthogonal modulation techniques and Section 2 describes the general discussion of WDM system description, followed by discussion of generation and detection of two-dimension orthogonal signal and threedimension orthogonal signal. In Section 3, the results of effect FWM in WDM system is presented and in Section 4 the conclusions have been made.
2. System setup Fig. 1 shows the architecture of WDM system employing orthogonal modulation formats. As shown in Fig. 1, the optical signals are generated from CW laser sources that are modulated as 2D/3D orthogonal modulation as shown in Fig. 2(a) and (b) and transmitted over 40 km fiber link. The driving input power of all
66
S. Singh, S. Singh / Optics & Laser Technology 76 (2016) 64–69
Fig. 3. Spectrum and eye diagram of DPSK singal at (a) 100 GHz channel spacing and (b) 50 GHz channel spacing (for input power of 10 dBm).
BER
10
0
-50
Without Compensation
10
With Compensation
DPSK DPSK`` DPSK
-100
10
DPSK``
-16 -14 -12 -10 -8
-6
-4
-2
0
2
4
6
8
10
Received Power (in dBm)
channel laser sources (with linewidth of 10 MHz) is variable to analyze the effect of FWM. The generated orthogonal modulated signals from transmitter are then fed into optical multiplexer where they are combined to transmitt through the optical link and erbium doped fiber amplifier (EDFA) compensate the linear loss occurring during propagation through optical link and noise figure of amplifier is kept at 4 dB [14]. At receiver side, the multiplexed signals are passed through optical demultiplexer, Mach–Zehnder interferometer (MZI), PIN detector and filter for BER measurement. The values of dark current and responsivity of PIN detector is set to be 10 nA and 1 A/W, respectively. The simulation setup for demodulation of orthogonal modulated signal is shown in Fig. 2(a) and (b). The system is simulated by input power to light source to visualize the effect of FWM in orthogonal modulated signal and results such as optical spectra, eye diagram, BER and quality factor are measured.
-20
10
3. Results and discussion
-40
10
To analyze the effect of FWM in orthogonal modulation formats for WDM systems, different parameters for SMF, DCF and EDFA present in Table 1 and Table 2. In addition, various cases are given below.
Without Compensation
-60
BER
10
With Compensation -80
Case 1. DPSK modulated channels at channel spacing of 100 GHz and 50 GHz with variable laser input power.
10
DPSK
-100
10
DPSK`` DPSK DPSK``
-120
10
-15
-10
-5
0
5
10
Received Power (in dBm) Fig. 4. BER performance of two-channel DPSK modulation at channel spacing of (a) 100 GHz and (b) 50 GHz with compensation and without compensation (First channel data formats represented as DPSK and second channel as DPSK``).
In this case, two DPSK modulation are employed channel at channel spacing of 100 GHz and 50 GHz with variable laser input power range from 10 dBm to 10 dBm. Central frequencies at 100 GHz channel spacing are 193.1 THz, 193.2 THz and at channel spacing of 50 GHz central frequencies are 193.1 THz and 193.15 THz. Fig. 3(a) and (b) represent the results for channel spacing of 100 GHz and 50 GHz respectively, where 40 km single span of SMF
S. Singh, S. Singh / Optics & Laser Technology 76 (2016) 64–69
67
Fig. 5. Spectrum and eye diagram of two-dimension orthogonal modulated singal at (a) 100 GHz channel spacing and (b) 50 GHz channel spacing (for input power of 10 dBm).
Case 2. Two dimentional (NRZ/PolSK) orthogonal modulated channels at channel spacing of 100 GHz and 50 GHz with variable laser input power. In this case, two NRZ/PolSK orthogonal modulation employed channel at channel spacing of 100 GHz and 50 GHz with variable laser input power range fromn 10 dBm to 10 dBm. Central frequencies at 100 GHz channel spacing are 193.1 THz, 193.2 THz and at channel spacing of 50 GHz central frequencies are 193.1 THz and 193.15 THz. Extinction ratio of NRZ is kept at 13 dBm for proper detection of PolSK modulated data. Fig. 5(a) and (b) represent the optical spectrum and eye diagram of two-dimensional orthogonal modulated signal for channel spacing of 100 GHz and 50 GHz respectively due to FWM effect, each wavelength channel contributes to interchannel crosstalk from adjacent and non-adjacent channel.
-20
10
BER
Without Compensation
-40
10
NRZ PolSK NRZ`` PolSK`` NRZ PolSK NRZ`` PolSK``
With Compensation
-60
10
-10
-5
0
5
10
15
Received Power (in dBm)
-20
10
Without Compensation
BER
fiber. As shown in the optical spectrum of DPSK due to FWM effect each wavelength channel contributes to interchannel crosstalk from adjacent and non-adjacent channel. For 100 GHz channel spacing eye diagram is clear but in case of 50 GHz channel spacing eye diagram shows the severe impairments due to FWM degenerate case. Fig. 4(a) and (b) shows the BER performance of point-to-point DPSK signal for WDM system at channel spacing of 100 GHz and 50 GHz with variable launch powers respectively. In the presence of thermal, shot and FWM noises, it is observed from Fig. 4(a) and (b) that in case of 100 GHz channel separation and low transmitted power, BER performance improved with an increase in laser launch power. As the input laser power increases, after 1 dBm power level the FWM noise power becomes dominant and the BER performance degrading. In case of 50 GHz channel spacing, similar variation BER performance of DPSK with input launch power but FWM effect is more dominant than 100 GHz channel separation. After 40 km transmission, the power penalties are less than 1.5 dB and 0.5 dB observed with and without compensation respectively.
-40
10
With Compensation
NRZ PolSK NRZ`` PolSK`` NRZ PolSK NRZ`` PolSK``
-60
10
-10
-5
0
5
10
15
Received Power (in dBm) Fig. 6. BER performance of two channel employing NRZ/PolSK orthogonal modulation at channel spacing of (a) 100 GHz (b) 50 GHz with compensation and without compensation (first channel data formats represented as NRZ, PolSK and second channel as NRZ``, PolSK``).
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S. Singh, S. Singh / Optics & Laser Technology 76 (2016) 64–69
Fig. 7. Spectrum and eye diagram of three-dimension orthogonal modulated singal at (a) 100 GHz channel spacing and (b) 50 GHz channel spacing (for input power of 10 dBm).
compensation), BER perfromance is improved with an increase in laser launch power. -3
10
-4
10
-5
10
-6
10
-7
10
-8
NRZ PolSK DQPSK NRZ``
Case 3. Three dimensional (NRZ/DQPSK/PolSK) orthogonal modulated channels at channel spacing of 100 GHz and 50 GHz with variable laser input power.
-10
PolSK`` DQPSK``
-5
0
5
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15
Received Power (in dBm) 100 NRZ PolSK
-2
10
DQPSK NRZ`` PolSK``
-4
10 BER
In this case, two NRZ/DQPSK/PolSK orthogonal modulation employed channel at channel spacing of 100 GHz and 50 GHz with variable laser input power range from 10 dBm to 10 dBm. Central frequencies at 100 GHz channel spacing are 193.1 THz, 193.2 THz and at channel spacing of 50 GHz central frequencies are 193.1 THz and 193.15 THz. Extinction ratio of NRZ kept at 13 dBm for proper detection of DQPSK and PolSK modulated data. Fig. 7(a) and (b) shows the optical spectrum and eye diagram of three-dimensional orthogonal modulated signal for channel spacing of 100 GHz and 50 GHz respectively and it is observed that due to FWM effect, each wavelength channel contributes to interchannel crosstalk from adjacent and non-adjacent channel as compared to DPSK and two-dimensional orthogonal modulation format. Fig. 8(a) and (b) shows the BER performance of point-to-point three-dimensional orthogonal modulated signal for WDM system at channel spacing of 100 GHz without and with compensation respectively. In the presence of thermal, shot and FWM noises, it is observed from Fig. 8(a) and (b) that in case of 100 GHz channel separation and low transmitted power (without and with
10
BER
To analyze the effect of FWM in two-dimension orthogonal modulated signal we also consider the effect of SRS with thermal and shot noise. From Fig. 6(a) and (b), it is observed that BER improves with increase in the input laser power but at certain input power level, BER degrades due to SRS and as the FWM noise become dominant. It is obseved that at channel spacing of 100 GHz BER performance of NRZ/PolSK is better as commpared to 50 GHz spaced channel because with the decrease in channel spacing, FWM-induced noise power increases linearly while the SRS gain decreases linearly which leads to the degradation of the BER performace. After 40 km transmission, the power penalties are less than 2.34 dB and 0.75 dB observed with and without compensation respectively.
DQPSK``
10-6 10-8 10-10 -15
-10
-5
0
5
10
Received Power (in dBm) Fig. 8. BER performance of two channel employing NRZ/DQPSK/PolSK orthogonal modulation (a) without compensation (b) with compensation at channel spacing of 100 GHz (first channel data formats represented as NRZ, DQPSK, PolSK and second channel as NRZ``, DQPSK``, PolSK``).
S. Singh, S. Singh / Optics & Laser Technology 76 (2016) 64–69
haul bandwidth efficient WDM system at 100 GHz and 50 GHz spaced channels. Results are also presented for variable laser input powers indicating the range of the validity of the WDM system. From analysis of FWM effect, it is concluded that WDM system performance is very much dependent on FWM noise and system performance degrade at positive power level in all considered cases. It is observed that three-dimension orthogonal modulated channels are affected more by FWM as compared to two-dimension orthogonal. DPSK modulation formats and FWM effect are more dominant at 50 GHz channel spacing than 100 GHz. Due to FWM effect more on three-dimensional orthogonal modulation format, it is difficult to design practical long houl WDM communication system as compared to other modulation formats.
-2
10
NRZ PolSK DQPSK NRZ``
-4
10
69
PolSK``
BER
DQPSK``
-6
10
-8
10
-10
-5
0
5
10
15
Received Power (in dBm)
Acknowledgment The authors would like to thank TEQIP-II, SLIET, Longowal for their funding to research projects no: TEQIP/905 dated: 30.10.13.
-2
10
NRZ PolSK -4
DQPSK
10
References
NRZ`` PolSK``
BER
-6
DQPSK``
10
-8
10
-10
10
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-8
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-4
-2
0
2
4
6
8
10
Received Power (in dBm) Fig. 9. BER performance of two channel employing NRZ/DQPSK/PolSK orthogonal modulation (a) without compensation (b) with compensation at channel spacing of 50 GHz (first channel data formats represented as NRZ, DQPSK, PolSK and second channel as NRZ``, DQPSK``, PolSK``).
As the input laser power increases, after certain power level the FWM noise power becomes more dominant and the BER performance DQPSK degrades more as compared to NRZ and PolSK when there is no compensation techniques are used. In case of fully compensated system, similar variation in BER is observed. From Fig. 9(a) and (b) it is observed that at 50 GHz channel spacing, similar variation BER performance of three-dimension with input launch power but FWM effect is more dominant than 100 GHz channel separation.
4. Conclusion In this paper, an analysis for interchannel FWM in a single span orthogonal modulated link is presented. Results are reported for the different orthogonal modulation formats for designing of long
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