Optical phase conjugation of multiwavelength signals in adispersion-shifted fiber

Optical phase conjugation (OPC) of multiwavelength signals in a dispersion-shifted fiber (DSF), which can be used for dispersion compensation in the wavelength division multiplexing communication system, is theoretically studied. The multiwavelength phase-conjugate signals are generated by four-wave mixing (FWM) in the dispersion-shifted fiber. There are the pulse-shape distortion and the induced frequency chirping in the phase-conjugate signals owing to pump depletion and the cross-phase modulation among the signals and phase-conjugate signals, respectively. The FWM among the pump wave, signals, and phase-conjugate signals causes unequal conversion efficiencies for the multiwavelength signals and enhances the induced frequency chirping in the phase-conjugate signals. The induced frequency chirping may deteriorate the restoration of the pulse shape. Both the pulse-shape distortion and induced frequency chirping increase with the signal and pump powers. The formula for the induced frequency chirping that is only caused by the signals through cross-phase modulation is derived. The requirement of the signal power in the dispersion-shifted fiber for the restoration of the pulse shape by the optical phase conjugation is estimated and numerically verified     

Unrepeated 40-Gb/s RZ single-channel transmission at 1.55 μmusing various fiber types

We investigate experimentally and theoretically the effect of signal power and dispersion compensation scheme in unrepeated return-to-zero single-channel 40-Gb/s 150-km transmission using standard single-mode fiber (SMF), nonzero dispersion shifted fiber [true wave fiber (TWF)] and dispersion shifted fiber (DSF). It is shown, that standard SMF allows significantly higher fiber-input power than nonzero dispersion shifted fiber or dispersion shifted fiber and, therefore, offers larger transmission spans     

Unrepeated transmission of 20-Gb/s optical quadrature phase-shift-keying signal over 200-km standard single-mode fiber based on digital processing of homodyne-detected signal for Group-velocity dispersion compensation

We demonstrate unrepeated optical transmission of 20-Gb/s quadrature phase-shift-keying (QPSK) signals over a 200-km-long standard single-mode fiber (SMF) without using any optical dispersion compensator. By employing optical homodyne detection, which can restore the entire information of the complex amplitude of the transmitted signal, group-velocity dispersion (GVD) of the SMF can be compensated electrically by a linear equalizer at the receiver. From off-line bit-error-rate measurements, we find that a simple transversal filter implemented in digital signal processing circuits after homodyne detection can effectively cancel the fiber GVD of up to 4000 ps/nm, enabling successful 20-Gb/s QPSK transmission.     

Simultaneous Cancellation of Fiber Loss, Dispersion, and Kerr Effect in Ultralong-Haul Optical Fiber Transmission by Midway Optical Phase Conjugation Incorporated With Distributed Raman Amplification

An alternative application of distributed Raman amplification (DRA) for ultralong-haul optical fiber transmission is proposed. In our study, the DRA is employed in a transmission system using midway optical phase conjugation (OPC) for amplifying an optical signal and, at the same time, for constructing signal power evolution, which is symmetrical with respect to the midpoint of the system where the OPC is performed. Then, the nonlinear signal waveform distortions that are caused by the Kerr effect, as well as fiber dispersion, are almost completely compensated by the OPC, whereas the fiber loss is compensated by the DRA. Three possible symmetrical signal power maps - a power map that has a reverse sign of the power map that is caused by lump amplification, a flat signal power map, and an arbitrary symmetrical signal power map - are numerically designed by using appropriate Raman pump powers. We show that the flat power map exhibits smaller difference from the target and a higher optical signal-to-noise ratio and requires lower pump power than the other two power maps. Numerical simulation results demonstrate that, by employing the flat power maps with a span of 40 km, a single-wavelength signal whose data rate is 160 Gb/s can be successfully transmitted over 5000 km, and the Kerr effect is sufficiently suppressed near limitation due to the nonlinear accumulation of noise. Finally, we study the feasibility of expanding our method to wavelength-division-multiplexed signal transmission by designing a DRA gain with multiple-wavelength pumping to simultaneously obtain a flat power map and a wide-and-flat gain bandwidth. By using four-wavelength Raman pumps while carefully choosing pump wavelengths and their powers, we achieve the DRA gain that simultaneously gives a fluctuation of the signal power of only 3.5%, a gain ripple of only 5.3%, and, at the same time, a gain bandwidth of as wide as 46 nm.     

Theoretical investigation of 8 /spl times/ 10-Gb/s WDM signal transmission performance based on gain-equalized SOAs using backward Raman pumping at DCF

We have theoretically investigated 8 /spl times/ 10-Gb/s wavelength-division multiplexing (WDM) signal transmission characteristics based on semiconductor optical amplifiers (SOAs) with equalized gain using discrete Raman amplification (DRA). Gain equalization and low noise figures have been obtained by adjusting the backward Raman pumping power and wavelength at a dispersion compensating fiber (DCF) for each span. Bit-error-rate characteristics were calculated for 8 /spl times/ 10-Gb/s WDM signal transmission over 6 /spl times/ 40-km single-mode fiber (SMF) + DCF links with gain-equalized SOAs using DRAs at DCF. Approximately a 2.5-dB improvement of the receiver sensitivity was achieved by using SOAs and DRAs with optimized Raman pumping. One can easily upgrade the transmission length of a link based on SOAs with an appropriate backward pump laser at each DCF.     

Fiber nonlinearity and dispersion mitigation in 40-Gb/s NRZ WDM transmission using a multichannel optical equalizer

We investigate the mitigation of distortion due to self-phase modulation and dispersion in 40-Gb/s nonreturn-to-zero wavelength-division-multiplexed transmission using a multichannel optical equalizer. The ability of the equalizer to reduce the signal degradation due to fiber nonlinearities was demonstrated. We achieved 21-channel transmission at BER <10/sup -9/ without forward error correction over 750 km of true wave reduced slope fiber with 107-km amplifier span lengths.     

40-Gb/s RZ transmission over a transoceanic distance in a dispersion managed standard fiber using a modified inline synchronous modulation method

This paper analyzes in detail numerically a 40-Gb/s return-to-zero (RZ) transmission system over a transoceanic distance in a strongly dispersion managed line composed of standard single-mode fiber (SMF) and dispersion compensation fiber (DCF). We derived a periodically steady-state pulse (a DM soliton) in a DM line. Since the pulse width of a steady-state pulse is too broad for a 40 Gb/s system, the conventional in-line synchronous modulation technique cannot greatly improve the transmission quality. However, we found that the modified inline synchronous modulation technique, which is reported as the black-box optical regenerator, can effectively extend the transmission distance even in such a strongly DM line. We discuss the mechanism of the modified synchronous modulation technique with respect to a steady-state pulse in a transmission line, and show that a 40-Gb/s RZ signal can be transmitted over 20 000 km.     

Simultaneous suppression of third-order dispersion and sideband instability in single-channel optical fiber transmission by midway optical phase conjugation employing higher order dispersion management

In optical phase conjugation (OPC) systems, the third-order dispersion (TOD) of optical fibers and the nonlinear resonance at well-defined signal sideband frequencies called sideband instability (SI) mainly limit the transmission performance. We propose, for the first time, a scheme for simultaneous suppression of both TOD and SI in OPC systems using a periodic higher order dispersion-managed link consisting of standard single-mode fibers (SMFs) and reverse dispersion fibers (RDFs). Computer simulation results demonstrate the possibility of 200-Gb/s data transmission over 10 000 km in the higher order dispersion-managed OPC system, where the dispersion map is optimized by our system design strategies.     

Dispersion-Compensation Schemes for 160-Gb/s 1200-km Transmission by Optical Phase Conjugation

Appropriate dispersion management possesses the superiority to relax the limitation of nonlinearities in high-speed transmission systems employing optical phase conjugation (OPC). In this paper, several dispersion-compensation schemes have been analytically and numerically investigated in a 160-Gb/s OPC system. A comprehensive performance comparison of these schemes over 1200-km transmissions has been carried out to present a reference for future OPC system designs. The scheme with a gradient dispersion map is recommended, particularly for G.655 fiber with a relatively low local dispersion.     

Analysis of optical phase-conjugate characteristics of picosecondfour-wave mixing signals in semiconductor optical amplifiers

We have analyzed for the first time the optical phase-conjugate characteristics of picosecond four-wave mixing (FWM) signals in semiconductor optical amplifiers (SOAs) using the finite-difference beam propagation method (FD-BPM). We show that the optical phase-conjugate characteristics of the FWM signals are strongly dependent on input pump pulsewidths. As a typical example, we have demonstrated that SOAs act as an ideal phase-conjugator, within the confines of reversing the chirp of optical pulses, for a 10-ps input pump pulse and a ~2.2-ps linearly chirped input probe pulse. When the pulsewidth of pump pulse becomes short, the minimum compressed pulsewidth is obtained by using a fiber shorter in length than the input fiber, but having the same group velocity dispersion as the input fiber. For a much shorter pump pulse such as 1 ps, the short FWM signal can be obtained via the gating characteristics of the FWM. However, only a part of the phase information is copied to the FWM signal due to such gating characteristics. The phase information is also degraded due to the fast nonlinear effect in the SOA. Thus, the pulsewidth is not compressed by propagation through a dispersive medium     

Electronic Dispersion Compensation

The performance of different electronic equalization and processing schemes for 40- and 10-Gb/s optical transmission over single-mode fiber (SMF) are discussed, from the point of their ability to compensate chromatic dispersion (CD) and polarization mode dispersion (PMD). In addition, the impact of fiber nonlinearity and modulation format on equalization is also investigated. The main objective of this paper is to present an overview and a comparison of the performances rather than a detailed explanation of the principles of the different equalization schemes. The equalizers which will be covered are analog equalizer (feedforward and decision feedback type), maximum likelihood sequence estimator (MLSE), electronic precompensation, coherent/intradyne detection with digital signal processing (DSP) equalization, DSP-based optical orthogonal frequency division multiplexing (OFDM), and turbo equalization.     

中间链路光学相位共轭补偿相干光正交频分复用系统的光纤非线性损伤

由于相干光正交频分复用(CO-OFDM)系统具有很高的峰均功率比(PAPR)以及非常近的子载波间隔,使得光纤非线性损伤成为系统的决定因素。提出中间位置光学相位共轭(OPC)补偿算法补偿CO-OFDM的Kerr损伤,由于OPC两端链路对称,可以最大限度地保证满足补偿条件,具有很好的非线性补偿效果。而且无链路色散补偿和有链路色散补偿系统均适用。该算法能使单信道40 Gb/s CO-OFDM的最大Q因子提高3 dB,非线性阈值提高4 dB;波分复用(WDM)系统的最大Q因子能提高1.1 dB,非线性阈值提高1 dB。     

Experimental Demonstration of 1.25-Gb/s Radio-Over-Fiber Downlink Using SBS-Based Photonic Upconversion

We experimentally demonstrate a radio-over-fiber downlink system using a stimulated Brillouin scattering (SBS)-based photonic upconversion technique. The Brillouin selective amplification characteristic of SBS is incorporated to generate the 11-GHz band radio-frequency (RF) carrier. The dual-electrode Mach-Zehnder optical modulator, which is used to carry the broadband data in the optical carrier instead of the optical sideband, is adopted along with the SBS-based carrier generation setup. To vindicate the broadband capabilities of the proposed scheme, 1.25-Gb/s pseudorandom bit sequence data is carried in the optical carrier. Error-free operation of the 1.25-Gb/s downlink is achieved without critical power penalties after the 13-km fiber transmission.     

Comparison of nonlinear pulse interactions in 160-Gb/s quasi-linear and dispersion managed soliton systems

The understanding and development of 160-Gb/s transmission systems requires the study of the impact of different dispersion compensation schemes on pulse propagation in nonlinear fiber. In this paper, we present an investigation of 160-Gb/s optical transmission systems, focusing on optimal propagation regimes, and in particular, we analyze different transmission limitations and dominant nonlinear effects by comparing quasi-linear and dispersion managed soliton systems. Two quasi-linear systems, one using nonzero dispersion-shifted fiber (NZDSF) and the other single-mode fiber (SMF), and one short-period (1 km) dispersion managed soliton (DMS) system are studied, both for single-channel and wavelength-division-multiplexed (WDM) transmission. First, the performance of the two quasi-linear systems in single-channel transmission are compared and it is shown that the NZDSF and SMF systems allow similar error-free transmission distances with only small differences in the intrachannel four-wave mixing (IFWM) induced amplitude jitter. The effect of pulsewidth on transmission performance in this regime was investigated and the use of shorter pulses was found to result in lower amplitude jitter. We analyzed the behavior of the DMS system and showed that the reduced pulse broadening during transmission allowed a significantly longer single-channel transmission distance with a smaller impact of nonlinearities compared to quasi-linear propagation. The sensitivity of the DMS system performance to statistical fluctuations in the fiber dispersion was studied and the results show the level of accuracy in the dispersion management map which must be ensured in these systems. Finally, the performance of the DMS in WDM transmission was investigated and it was found that it was subject to very large penalties increasing the minimum channel spacing possible because of the strong impact of interchannel cross-phase modulation (XPM).     

Exact compensation for both chromatic dispersion and Kerr effect ina transmission fiber using optical phase conjugation

We propose a new method to compensate exactly for both chromatic dispersion and self-phase modulation in a transmission fiber, where the light intensity changes due to fiber loss and amplifier gain. This method utilizes optical phase conjugation (OPC). The pulse shape is precompensated before OPC by transmission through a fiber with large dispersion. A computer simulation demonstrates effective compensation for waveform distortion in a 40 Gb/s NRZ intensity-modulated light transmission     

Tunable coherent IR and FIR sources utilizing modulational instability

A new tunable coherent infrared (IR) source is presented. It utilizes the sideband produced by modulational instability of an optical signal in a fiber which results from a combination of anomalous dispersion and the nonlinear Kerr effect. The generated frequency is in proportion to the square root of the optical pump signal. A coherent IR signal is generated by extracting the sideband.     

Nonlinear noise amplification in optical transmission systems with optical phase conjugation

Taking into account the influence of group-velocity dispersion (GVD) and the nonlinear Kerr effect, the nonlinear amplification of the amplified spontaneous emission (ASE) noise in fiber transmission systems using optical phase conjugation (OPC) is studied. Under a path-averaged power approximation for long-haul transmission systems, an equivalent system is developed to evaluate ASE noise amplification and accumulation in OPC systems. Combining the theoretical analysis and numerical simulations, the noise suppression effect in OPC systems is demonstrated and discussed. By using the numerical calculation method, the power variation along the system is involved in the evaluation of noise amplification. It is shown that the power variation through the system results in an imperfect compensation of the modulation instability (MI) effect, which furthermore causes the degradation of the noise suppression performance in OPC systems with anomalous dispersion.     

Multiband Signal Generation and Dispersion-Tolerant Transmission Based on Photonic Frequency Tripling Technology for 60-GHz Radio-Over-Fiber Systems

We designed and experimentally demonstrated an efficient photonic frequency-tripling technology for 60-GHz radio-over-fiber systems to simultaneously realize millimeter-wave (mm-wave), microwave, and baseband signal generation. The technique utilizes vestigial sideband filtering in combination with optical carrier suppression to generate novel alternate subcarrier modulation for high tolerance of fiber dispersion. Experimental verification of the proposed scheme is presented with generation and error-free transmission of 2.1-Gb/s data on the 63-GHz mm-wave and 21-GHz microwave carriers over 50-km single-mode fiber (SMF-28) without dispersion compensation. The power penalty for both signals is less than 1.0 dB.      

Nonlinear fibers for signal processing using optical Kerr effects

This paper reviews nonlinear optical-fiber designs for signal processing using optical Kerr effects. The requirements for designing nonlinear fibers are described first. Then, the design concept is discussed and design examples are shown to illustrate the tradeoffs among the different fiber properties such as effective area, dispersion, and attenuation. Furthermore, fiber designs with distributed Brillouin frequency shift to mitigate the effect of simulated Brillouin scattering (SBS) in nonlinear fibers are discussed in detail. An SBS-threshold increase of 7 dB over conventional nonlinear fibers is experimentally demonstrated.     

High-Resolution Optical Sampling of 640-Gb/s Data Using Four-Wave Mixing in Dispersion-Engineered Highly Nonlinear As$_2$S $_3$ Planar Waveguides

We present the first demonstration of an optical sampling system, using the optical Kerr effect in a chip-scale device, enabling combined capability for femtosecond resolution and broadband signal wavelength tunability. A temporal resolution ${ ≪ }500$ fs is achieved using four-wave mixing in a 7-cm-short chalcogenide planar waveguide. The use of a short length, dispersion-shifted waveguide with ultrahigh nonlinearity ($10^4;{rm W}^{-1}{cdot}{rm{km}}^{-1}$) enables high-resolution optical sampling without the detrimental effect of chromatic dispersion on the temporal distortion of the signal and sampling pulses, as well as their phase mismatch. Using the device, we successfully monitor a 640-Gb/s optical time-division multiplexing (OTDM) datastream, showcasing its potential for integrated chip-based monitoring of signals at bitrates approaching and beyond Tb/s. We discuss fundamental limitations and potential improvements.