GRD Journals | Global Research and Development Journal for Engineering | International Conference on Innovations in Engineering and Technology (ICIET) - 2016 | July 2016
e-ISSN: 2455-5703
Four Wave Mixing suppression in CO-OFDM using Multiple Optical Phase Conjugate Modules 1R.
Mercy Kingsta 2B. Sakthi Tharani 3D. Mohana Priya 1,2,3 Assistant Professor 1,2,3 Department of Electronics and Communication Engineering 1,2,3 Mepco Schlenk Engineering College, Sivakasi, India Abstract Nonlinear effects in optical fibers occur due to the refractive index changes in the medium with optical intensity and inelasticscattering phenomenon. Four Wave Mixing (FWM) is caused by the third-order nonlinear susceptibility in Single-Mode Fibers (SMF). Optical Phase Conjugate (OPC) is an efficient technique to compensate the phase related impairments. OPC in the center of the channel defines especially a relationship between two coherent optical beams propagating in opposite directions with reversed wave front and identical transverse amplitude distributions. The most unique feature of OPC is that it automatically removes the aberration influenced on the forward beam passed through the disturbing medium. We design the dense WDM system with CO-OFDM signals at central wavelengths in the region of 1548 nm. This method offers almost negligible FWM by reducing the power level in optical spectrum. Through numerical simulation we report utilizing multiple OPC modules along fiber spans, which further improves the performance of communication systems in long-haul fiber-optic channels also it reduces the average power of FWM. Keyword- Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM); Four-Wave Mixing (FWM); Wavelength Division Multiplexing (WDM); Optical Phase Conjugate (OPC) __________________________________________________________________________________________________
I. INTRODUCTION In order to exploit optical bandwidth more efficiently OFDM was introduced in the optical domain [2]. The main reason is that OFDM has the ability to deal with large pulse spreads due to the chromatic dispersion [2],[3]. A main motivation for introducing OFDM in the optical domain is the possibility for high-speed data [4],[5] transmission over dispersive fiber without the need for costly optical dispersion compensation techniques. The basic concept behind the CO-OFDM is the division of a high bit rate data stream into several streams of low bit rate, which are simultaneously modulated onto orthogonal subcarriers with a conventional digital modulation scheme (such as QPSK, 16QAM, etc.) at low symbol rate.. Rather than transmit a high-rate stream of data with a single subcarrier, OFDM makes use of a large number of closely spaced orthogonal subcarriers that are transmitted in parallel. Fiber nonlinearity impairments such as FWM along the fiber degrades the signal strength of the CO-OFDM during transmission, this effect cannot be easily compensated [6]. In long haul communication, the multiple subcarrier frequencies propagate through the same fiber and they interact with each other, the mixing of the subcarrier frequencies leads to the additional frequencies which may have the same frequency as the original frequency are regarded as noise, which degrades the system capacity. FWM efficiency depends on signal power and dispersion, as well as channel separation in WDM systems. If the channel is close to the zero dispersion wavelength of the fiber, considerably high power can be transferred to FWM components.
II. SYSTEM DESCRIPTION A. Transmitter and Receiver Design In this the OFDM signal is generated as shown in Fig.1.OFDM Transmitter section the input data is converted to parallel signals using serial to parallel converter and they are modulated using quadrature amplitude modulator (4-QAM). The inphase and quadrature phase from QAM is fed as input to IFFT. The IFFT block produces a waveform which is a superposition of the QAM-modulated subcarriers. Then, cyclic prefixes (CPs) are added as a guard band to the IFFT signal, the real and imaginary parts of the IFFT signal combined together and fed into the optical modulators (MZM). A 90° phase shift provided at the laser to produce the I∕Q components. The output of the optical modulators is fed into the optical link containing one or multiple OPC modules before passing through the CO-OFDM coherent receiver. The reverse procedure is followed at the receiver to access the transmitted signal.CO-OFDM requires two photodetectors to convert the optical signal into electrical signal.
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Four Wave Mixing suppression in CO-OFDM using Multiple Optical Phase Conjugate Modules (GRDJE / CONFERENCE / ICIET - 2016 / 085)
Fig. 1: Proposed system model
B. OPC Modules The feature of the optical phase-conjugate beams is that the aberration influence imposed on the forward beam passed through the nonlinear medium, can be automatically removed for the backward beam passed through the same nonlinear medium [8]. General, a pair of optical waves are phase conjugated to each other if their complex amplitude functions are conjugated with respect to their phase factors. Optical phase-conjugate waves can be generated through various nonlinear optical processes (such as four-wave mixing, three-wave mixing, backward stimulated scattering, and others) as shown in Fig.2.
Fig. 2: Concept of phase conjugation of the signal
Here, the input and pump signal is fed into the nonlinear medium with ω signal & ω pump that interacts with each other by means of 3dB coupler to produce the FWM component ω conj [8],[9] as in the equation, ω conj = 2ω pump - ω signal The OPC modules are inserted in the middle of the two identical fibers generates the phase conjugated wave with spectral inversion structure. This technique is referred to as MSSI (MidSpan Spectral Inversion) or MOPC (Midway Optical Phase Conjugate). FWM process is not suitable practically if the fiber is polarization sensitive. The fiber can be made as polarization insensitive by means of two orthogonally polarized pump beams at different wavelengths, located symmetrically on the opposite sides of the zero-dispersion (λzd) fiber, are used [7],[10].By introducing OPC modules we can increase the robustness of the communication, most of the FWM products are eliminated as shown in the simulation results. C. FWM Introduction FWM is the degrading effects in WDM systems, if the channels are equally placed the newly generated wave leads to crosstalk. It is a parametric process, and it is a type of optical Kerr effect and occurs when light of two or more different wavelengths are injected into the fiber. It originates from third order nonlinear susceptibility (X 3). FWM results in BER fluctuations in WDM network which in turn affect the SNR and Quality of Service of the network. FWM effect is independent of the bitrate and is critically dependent on the channel spacing and fiber dispersion. Fig.2a shows the mixing of two waves at the wavelength of λ1 & λ2. When these waves mix up they generate sidebands at (λ1 ± λ2). FWM is an interchannel cross talk and induces interference of information between the channels.
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Four Wave Mixing suppression in CO-OFDM using Multiple Optical Phase Conjugate Modules (GRDJE / CONFERENCE / ICIET - 2016 / 085)
Fig. 2 (a): Mixing of two waves
III. SIMULATION SETUP Simulations of the proposed model Fig.3 are conducted using Optsim v5.0. In sample mode, a 100-Gb/s pseudo-random binary sequence (PRBS) at four parallel lanes is modulated by 4QAM. A 8-point IFFT block is used in order to maintain the orthogonality between the OFDM signals. Cyclic Prefix (CP) is added and they act as a guard band for subcarrier frequencies to avoid overlapping. A Mach-Zehnder modulator (MZM) linear modulator is used to modulate the laser source by means of an applied electrical signal and the inphase and quadrature phase signals are combined and given as an input to the 32 & 64 WDM channels and they are transmitted through the optical nonlinear fiber (Length, L=600km) with the Loss of 0.1dB/km, Dispersion of 2 ps/nm/km and other parameters are also included in the fiber properties and the coherent detection is done at the receiver. The optical signal with the central wavelength of 1548nm with the channel spacing of 50 GHz (64 channels) is launched into the OPC modules (2 & 4) which are placed in the different spans of fiber. In order to have the long distance communication to be efficient, EDFA amplifiers (boosters) are added at the regular intervals. It will increase the signal strengths which are faded due to the various nonlinear effects. The results of the model are analyzed by means of optical spectrum at the input and output of the fiber.
Fig. 3: OFDM Transmitter and Receiver
IV. WDM TRANSMISSIONS AND DISCUSSIONS A. Channel Setup The WDM channels at the input side are used to describe the FWM effect in optical communication as shown in Fig.4. with the multiple OPCs in the fiber link.
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Four Wave Mixing suppression in CO-OFDM using Multiple Optical Phase Conjugate Modules (GRDJE / CONFERENCE / ICIET - 2016 / 085)
Fig. 4: WDM Channel Setup-With OPCs
In 32WDM setup- 32 OFDM transmitters are used and for 64 WDM setup- 64 OFDM transmitters are used inside the compound block (Wavelength ranges from 1562-1537 nm). By increasing the number of users within the bandwidth leads to high FWM effects.
Fig. 5: channel OFDM Output
The OFDM output signal is represented as input in Fig.4.a.1, 4.a.2 while this signal is transmitted through the nonlinear fiber it results in FWM and that effect is shown in Fig.4.b.1, 4.b.2
Fig. 6: channel OFDM Output
The induced FWM effects are reduced by means of two OPCs as explained in the OPC concept and it is shown as output in Fig.4.c. Here two OPCs are placed in between the fiber spans, so that the effect induced in the first half of the fiber is eliminated by the second half by the phase conjugated signal.
Fig. 7: Fiber Output (32 Channels)– Without OPCs
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Four Wave Mixing suppression in CO-OFDM using Multiple Optical Phase Conjugate Modules (GRDJE / CONFERENCE / ICIET - 2016 / 085)
Fig. 8: Fiber Output (64 Channels)-Without OPCs
Fig. 9: 32- channel (2OPCs) Output
In Fig.4.d 32-channels with four OPCs are shown. As similar to the WDM 32 channels with two OPCs the FWM effect of this setup is compensated by four OPCs.
Fig. 10: 32- channel (4 OPCs) Output
The effect of FWM in 64 WDM channels with two OPCs are shown in Fig.4.e as discussed earlier FWM effects is high in 64 channels. And they are compensated by the OPCs present in between the fiber spans. In turn it reduces the interference between the subcarrier frequencies and the smooth reception of the original carrier frequencies occur.
Fig. 9: 64- channel (2OPCs) Output The same 64 WDM channel with four OPCs output are shown in Fig.4.f. By introducing four OPCs in the fiber link it reduces the noise in the sidebands compared to two OPCs and the original frequencies are detected by the receiver.
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Four Wave Mixing suppression in CO-OFDM using Multiple Optical Phase Conjugate Modules (GRDJE / CONFERENCE / ICIET - 2016 / 085)
Fig. 10: 64- channel (4 OPCs) Output
V. CONCLUSION In this proposed model, FWM effects of the nonlinear fiber are reduced by means of OPCs in between the fiber spans. In the long haul fiber communication, there will be more FWM effects due to the mixing of subcarrier frequencies. This model was done using Numerical Simulations. By comparing the outputs of all setup it was clear that by introducing more number of OPCs in the nonlinear fiber increase the efficiency of transmission by reducing the FWM noise. In the fiber span, the FWM noise in the output by two OPCs is further reduced by means of introducing four OPCs. This was also efficient when we increase the number of channels.
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