All-optical Orthogonal Frequency Division Multiplexing System Research Articles

Enhancing the Anti-Dispersion Capability of the AO-OFDM System via a Well-Designed Optical Filter at the Transmitter

This paper proposes a novel method to improve the anti-dispersion ability of the all-optical orthogonal frequency division multiplexing (AO-OFDM) system. By replacing the Sinc-shaped filter with a Gauss-shaped filter for sub-carrier generation and inserting a cyclic prefix (CP), the impact of dispersion on the system can be significantly mitigated. Formula derivation and numerical analysis of the pulse-shaping function of the AO-OFDM system in the time domain for each cycle indicated that the pulse-shaping function generated by the Gauss-shaped filter was less affected by the dispersion effect than that of the Sinc-shaped filter. Meanwhile, less inter-carrier crosstalk between carriers was also observed. After carrying out system transmission simulations employing these two different filters, we found that the AO-OFDM system based on the Gauss-shaped filter could greatly improve the anti-dispersion ability compared with the system based on a Sinc-shaped filter. When the parameter settings in both schemes were identical, that is, the number of subcarriers was 32 and the power of a single subcarrier was −13 dBm, the bit error rate (BER) of the system based on the proposed Gauss-shaped filter after 60 km SMF transmission was only 1.596 × 10−3, while the BER of the traditional Sinc-shaped filter based system scheme was as high as 8.545 × 10−2.

Improving the Resistance of AO-OFDM Signal to Fiber Four-Wave Mixing Effect Based on Insertion Guard Interval

In this paper, a method to suppress the impact of the nonlinear effects on an all optical orthogonal frequency division multiplexing (AO-OFDM) system is proposed. By inserting a guard interval (GI), the duty cycle of the optical signal in each symbol period of each subcarrier is decreased, thus generating a ‘zeroed’ temporal. By giving different time delays to the sub-carriers, these ‘zeroed’ temporals and optical signals of the adjacent sub-carriers are interleaved, which reduces the coincidence of the optical signals between the sub-carriers and eliminates the products of the four-wave mixing (FWM) effect to suppress the influence of the nonlinear effect on the system. The simulation results show that for an AO-OFDM system with 32 subcarriers, inserting GI and introducing different delays for each subcarrier can improve the transmission distance by 30 km or enhance the spectral efficiency by 16.7%, considering the 7% hard decision forward error correction (HD-FEC) threshold. Moreover, even when the number of subcarriers is up to 256 and the power of each subcarrier is as high as 0 dBm, our proposed signal optimization scheme can still guarantee that the BER can satisfy the 7% HD-FEC threshold.

Efficiency enhancement of phase-conjugated twin waves technique by shaping envelopes of subcarriers in all-optical OFDM systems

All-optical orthogonal frequency division multiplexing (AO-OFDM) transmission, which employs high-order modulation formats, opens up a new field of advanced optical transmission. However, a major limitation is the impact of Kerr nonlinearity on AO-OFDM systems that induces nonlinear phase noise (NPN). In this work, we proposed a new method for suppressing NPN in AO-OFDM systems based on shaping envelopes of phase-conjugated twin waves (PCTWs). In the proposed method, each subcarrier and its phase-conjugated replica are firstly modulated with m-array quadrature amplitude modulation (mQAM) format and then shaped by return-to-zero (RZ) coding format. Sequentially, the subcarriers are multiplexed to form twin AO-OFDM signals, which are sent into two similar fiber links. At receiver, both waveforms are separately detected and twin subcarriers are coherently added for suppressing the NPN. An analytical model that characterizes NPN in proposed system is also developed. In addition, the performance of proposed system with 15 subcarriers that modulated by a RZ-4QAM format is numerically demonstrated. The results show that the NPN is considerably suppressed and system performance is highly improved. A signal-to-noise ratio (SNR) is enhanced by 5.7 dB and transmission length is lengthened by 150% as compared to conventional 4QAM-AO-OFDM system.

Alleviating the nonlinear effect by narrowing down the differences between the modulus values of the constellation points in all-optical OFDM system

We use the iterative split-step Fourier method to simulate the eight quadrature amplitude modulation all-optical orthogonal frequency division multiplexing (OFDM) system employing different constellation design schemes. It is found that properly narrowing down the modulus difference between constellation points has an impact on the whole system’s ability to resist nonlinear effects. The results show that by means of changing the modulus values of the constellation points, the system bit error rate will be significantly reduced when the optical signal-to-noise ratio is high. Furthermore, by comparing four different constellation diagrams, we propose an alternative solution to reduce the distortion brought by the nonlinear effects on all-optical OFDM systems, that is, the constellation points can be appropriately redesigned to improve the performance of high nonlinear effect systems.

Nonlinear effect on sinc-shaped optical filter demultiplexing all-optical OFDM system

We propose a novel all-optical orthogonal frequency division multiplexing (OFDM) systems without the requirement for discrete fast Fourier transform (FFT). The liquid crystal on silicon (LCOS) programmable wavelength selective switch (WSS) is employed in this all-optical OFDM for filtering, which can generate multiple orthogonal sub-carriers on the modulated optical source of super-continuum spectrum. Based on the proposed all-optical OFDM, the error-free transmission with high spectral efficiency can be achieved in the case of no dispersion and nonlinear effect. Furthermore, we systematically analyze the impact of nonlinear effect on the all-optical OFDM transmission systems, known as the main limiting factor in dense multi-carriers systems. And system simulation indicates that in the absence of local oscillating laser and extra dispersion and nonlinear compensation algorithms, our proposed all-optical OFDM can achieve 5 km single mode optical fiber transmission with less than 5*10−3 bit error rate (BER) and 1*10−2 BER when employing no more than 16 and 64 sub-carriers, respectively. From the perspective of spectral efficiency and system complexity, our proposed all-optical OFDM system shows a huge superiority over single sideband modulation in short distance transmission.

PAPR reduction in all-optical OFDM based on time interleaving odd and even subcarriers

In this paper, a new all-optical approach for reducing peak-to-average power ratio (PAPR) in all-optical orthogonal frequency division multiplexing (AO-OFDM) system based on time interleaving odd and even subcarriers (TIOES) is proposed and analytically modeled. In this approach, all subcarriers are initially modulated with higher modulation format such as m-array quadrature amplitude modulation (mQAM) and encoded by return-to-zero (RZ) coder. Then, odd and even subcarriers are temporally interleaved. An analytical model for estimating the impact of the interleaved time on PAPR reduction and induced nonlinear phase noise (NPN) is developed. The effect of reducing PAPR on performance of AO-OFDM system is also numerically investigated using VPItransmissionMaker® commercial software. This system utilizes an optical coupler-based inverse fast Fourier transform/fast Fourier transform. The results reveal that minimum PAPR is occurred when interleaved time between odd and even subcarriers equal to half symbol duration where the PAPR has been reduced by 3 dB. In addition, by employing proposed technique, the signal to noise ratio (SNR) is improved by about 2.5 dB at transmission distance of 800 km as compared with 4QAM AO-OFDM without PAPR technique.

Performance evaluation of an efficient RZ-MQAM modulation scheme in all-optical OFDM transmission systems

All optical orthogonal frequency division multiplexing (AO-OFDM) technique is a promising technique and employed in many military applications for data transmission over long distance. But AO-OFDM signals are very sensitive to nonlinear distortion and these systems suffer from nonlinear impairments. This paper describes a novel nonlinear phase noise reduction technique by decreasing interaction time between subcarriers in AO-OFDM transmission systems. This technique mitigates the effects of nonlinearity on all-optical OFDM transmission systems and improves performance of transmission systems. The proposed AO-OFDM transmission system has been successfully simulated and analyzed for estimating phase noise due to cross-phase modulation, self-phase modulation and four-wave mixing. The proposed RZ-8QAM AO-OFDM system with 29 subcarriers is examined and the transmission distance is fixed at 550km.

Banded all-optical OFDM super-channels with low-bandwidth receivers.

We propose a banded all-optical orthogonal frequency division multiplexing (AO-OFDM) transmission system based on synthesising a number of truncated sinc-shaped subcarriers for each sub-band. This approach enables sub-band by sub-band reception and therefore each receiver's electrical bandwidth can be significantly reduced compared with a conventional AO-OFDM system. As a proof-of-concept experiment, we synthesise 6 × 10-Gbaud subcarriers in both conventional and banded AO-OFDM systems. With a limited receiver electrical bandwidth, the experimental banded AO-OFDM system shows 2-dB optical signal to noise ratio (OSNR) benefit over conventional AO-OFDM at the 7%-overhead forward error correction (FEC) threshold. After transmission over 800-km of single-mode fiber, ≈3-dB improvement in Q-factor can be achieved at the optimal launch power at a cost of increasing the spectral width by 14%.

Effectiveness of phase-conjugated twin waves on fiber nonlinearity in spatially multiplexed all-optical OFDM system

Abstract In this paper, we investigate the effectiveness of using phase-conjugated twin waves (PCTWs) technique to mitigate fiber nonlinear impairments in spatially multiplexed all-optical orthogonal frequency division multiplexing (AO-OFDM) systems. In this technique, AO-OFDM signal and its phase-conjugated copy are directly transmitted through two identical fiber links. At the receiver, the two signals are coherently superimposed to cancel the phase noise and to enhance signal-to-noise ratio (SNR). To show the effectiveness of proposed technique, a spatially multiplexed AO-OFDM system is demonstrated by numerical simulation. AO-OFDM signal and its phase conjugated copy are optically generated by using optical coupler-based inverse fast Fourier transform (OIFFT)/fast Fourier transform (OFFT). The generated signal includes 29 subcarriers where each subcarrier is modulated by 4-quadrature amplitude modulation (4QAM) format at a symbol rate of 25 Gsymbol/s. The results reveal that transmission performance is considerably improved where the transmission distance of the proposed system is increased by ∼45% as compared to that of original system without PCTWs technique.

All‐optical wavelet‐based orthogonal frequency division multiplexing system based on silicon photonic integrated components

In this study, the authors discuss in detail the implementation of all-optical orthogonal frequency-division multiplexing system based on discrete wavelet packet transform (DWPT) by using silicon-on-insulator (SOI) cascaded Mach–Zehnder interferometers (MZIs). DWPT can be used in the multiplexing and de-multiplexing of data channels in coherent optical communication in order to improve spectral efficiency excluding a cyclic prefix. DWPT and inverse-DWPT may be realised in an integrated-photonic MZI platform. They demonstrated experimentally the post-fabrication tuning of 1 × 4 cascaded MZI de-multiplexing filters. Tuning procedures effectively overcome fabrication tolerances and meet the target transfer function of the devices. The fabrication platform represents an essential stepping-stone toward the all-optical realisation of DWPT-based multiplexing.

Time and frequency synchronisation in all‐optical orthogonal frequency division multiplexing systems

In all-optical orthogonal frequency division multiplexing (AO-OFDM) systems, symbol time misalignment (STM) and subcarrier frequency offset (ScFO) result in intercarrier-interference and intersymbol interference, leading to degradation of bit error rate (BER) performance. In this study, an analytical study of the impacts of STM and ScFO on AO-OFDM system performance is conducted. In the presence of STM and ScFO, AO-OFDM system performance is evaluated with the number of subcarriers (N SC), cyclic prefix requirements and receiver bandwidth (BWRX ). For AO-OFDM system without CP, results show that the STM gives higher BER than the ScFO. To keep the BER below 10 − 3 in AO-OFDM systems with on-off keying-modulated three optical subcarriers and homodyne detection with BWRX = 30 GHz, the maximum acceptable relative deviations for the STM and the ScFO are 11 and 14%, respectively.

Performance analysis of an all-optical OFDM system in presence of non-linear phase noise.

The potential for higher spectral efficiency has increased the interest in all-optical orthogonal frequency division multiplexing (OFDM) systems. However, the sensitivity of all-optical OFDM to fiber non-linearity, which causes nonlinear phase noise, is still a major concern. In this paper, an analytical model for estimating the phase noise due to self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM) in an all-optical OFDM system is presented. The phase noise versus power, distance, and number of subcarriers is evaluated by implementing the mathematical model using Matlab. In order to verify the results, an all-optical OFDM system, that uses coupler-based inverse fast Fourier transform/fast Fourier transform without any nonlinear compensation, is demonstrated by numerical simulation. The system employs 29 subcarriers; each subcarrier is modulated by a 4-QAM or 16-QAM format with a symbol rate of 25 Gsymbol/s. The results indicate that the phase variance due to FWM is dominant over those induced by either SPM or XPM. It is also shown that the minimum phase noise occurs at -3 dBm and -1 dBm for 4-QAM and 16-QAM, respectively. Finally, the error vector magnitude (EVM) versus subcarrier power and symbol rate is quantified using both simulation and the analytical model. It turns out that both EVM results are in good agreement with each other.

Chromatic dispersion monitoring and adaptive compensation using pilot symbols in an 8 x 125 Gbit/s all-optical OFDM system

We propose and experimentally demonstrate a novel technique for chromatic dispersion (CD) monitoring and adaptive compensation in an 8 x 12.5 Gbit/s all-optical orthogonal frequency-division multiplexing (AO-OFDM) system by using two pilot symbols and a virtually imaged phased array (VIPA) for a tunable CD compensator. The two pilot symbols are added to the first and the last sub-channels of the OFDM signal, and their relative time delay is detected and used for CD estimation at the CD monitoring circuit. The monitored CD value is fed to VIPA for CD compensation. In the experiments, the relative time delay between the two pilot symbols was successfully observed, and the adaptive CD compensation drastically improved the bit-error-rate (BER) from over 10(-5) to under 10(-9). The estimated CD values showed less than 10 ps/nm difference from the values measured by a photonic dispersion analyzer, which is accurate enough since the AO-OFDM system can keep BER<10(-9) upto 20 ps/nm residual CD.

Transmission performance of a 400 Gbit s−1 all-optical orthogonal frequency division multiplexing system

The performance of a 400 Gbit s−1 all-optical orthogonal frequency division multiplexing (AO-OFDM) transmission system is researched with the effects of chromatic dispersion, fiber nonlinearities and amplified spontaneous emission (ASE) noise. The numerical simulation results show that the AO-OFDM system can provide a higher spectral efficiency (SE) and a better sensitivity than a dense wavelength division multiplexing (DWDM) system. The accumulated dispersion tolerance of the system reaches 330 ps nm−1. When transmitted over single-span 80 km single-mode fiber (SMF), AO-OFDM signals have a 1.5 dB power penalty at BER=10−3 due to the fiber Kerr nonlinearities, and the receiver sensitivity of the AO-OFDM system is obviously degraded with increasing incident optical power. In multispan transmission, the interaction of the fiber Kerr nonlinearity with the ASE noise is analyzed. A 1320 km maximum transmission distance is realized at 0 dBm incident optical power. The transmission discount due to the ASE noise and fiber nonlinearities in the AO-OFDM system is calculated. Fiber Kerr nonlinearities impose a greater limitation on the performance of the AO-OFDM system for long-distance transmission. All results clearly indicate the feasibility of AO-OFDM technology for next generation 400 Gbit s−1 fiber communication and multiservice networks.

Optical orthogonal frequency division multiplexed transmission using all‐optical discrete Fourier transform

Orthogonal frequency division multiplexing (OFDM) can provide spectrally efficient communication channels because it can utilize carrier orthogonality and various impairment mitigation methods. An optical OFDM signal can be generated electronically to multiplex lower-rate carriers. In recent advancements, OFDM signals are also shown to be generated and demultiplexed by all-optical discrete Fourier transform (DFT), overcoming the speed limit of electronics for >Tbps capacity. High-performance DFT devices, such as arrayed waveguide grating (AWG) or planar lightwave circuit (PLC), are critically required to obtain strong orthogonality for scalable all-optical OFDM (AO-OFDM) system implementations. Advanced techniques such as coherent modulation and detection with digital impairment mitigation are also important for long-reach AO-OFDM transmissions. More recently, optical superchannel schemes have been introduced utilizing coherent detection for multi-Tbps AO-OFDM transmissions. This paper reviews the device and system aspects for the AO-OFDM technology, including a generalized theoretical model to provide an indepth understanding.

PAPR Reduction in All-optical OFDM Systems Based on Phase Pre-emphasis

This paper investigates the peak-to-average power ratio (PAPR) theory in all-optical orthogonal frequency division multiplexing (OFDM) optical fibre communication systems. We find out that phase pre-emphasis could effectively reduce PAPR in all-optical OFDM communication systems which employ intensity modulation-direct detection (IM-DD) method. An equation is developed and proposed to calculate suitable phasing values for pre-emphasis. Furthermore, we find out that phase pre-emphasis cannot reduce PAPR effectively in all-optical OFDM systems that employ Phase Shift Keying (PSK) or Quadracture Amplitude Modulation (QAM) method.

全光采样OFDM系统性能分析

The performance of a novel all-optical sampling orthogonal frequency division multiplexing (OFDM) system is proposed and analyzed. Time delays and phase shifters are used to realize all optical forward/inverse discrete Fourier transform (DFT/IDFT). Different system configurations are tested and analyzed to optimize the performance, including the system capacity, modulation formats, DFT/IDFT constructions, and the width of the sample pulse. The 50- and 100-Gb/s real-time all-optical sampling (AOS) OFDM systems are investigated. All results are analyzed, and useful suggestions are offered for future high-speed applications.