Relative intensity noise induced by four-wave mixing in the case of two-wave transmission in an optical fiber

Four-wave mixing (FWM) is a significant nonlinear effect in wavelength division multiplexing (WDM) fiber-optic systems. For two-wave transmission, it is easily found that the FWM noise power decreases with frequency spacing and increases with signal power. However, the variation of relative intensity noise (RIN) with frequency spacing and signal power is only 2 dB at most. The intensity fluctuations induced by the energy exchange between the FWM generated new waves and the original ones are trivial and the influence of FWM on RIN can be neglected. It is also found that the increase of RIN with signal power is mainly attributed to stimulated Brillouin scattering (SBS) rather than FWM.     

Reduction of FWM noise in WDM-based QKD systems using interleaved and unequally spaced channels

To extensively deploy quantum key distribution(QKD) systems, copropagating with classical channels on the same fiber using wavelength division multiplexing(WDM) technology becomes a critical issue. We propose a user-based channel-interleaving WDM scheme with unequal frequency spacing(UFS-i WDM) to reduce the impairment on the quantum channels induced by four-wave mixing(FWM), and theoretically analyze its impact on quantum bit error rate(QBER). Numerical simulation results show that a UFS-i WDM can significantly reduce the FWM noise and improve QBER compared with the corresponding WDM scheme with equal frequency spacing(EFS), especially in the case of nonzero dispersion shifted fiber.     

Simultaneous four channel wavelength conversion of 50Gbps CSRZ–DPSK signals in S and C bands using HNLF without additional pump signals

Optical wavelength conversion is expected to be an important technique for future advanced dense wavelength division multiplexing systems. It enhances wavelength routing capabilities, improves network reconfigurability and eliminating the problem associated with wavelength reuse in network. Here, simultaneous 50Gbps four channel wavelength conversion is established in S and C bands of ITU grid using four wave mixing (FWM) technique in high nonlinear fiber (HNLF) without additional pump signals. Since the four channel wavelength conversion is to be performed, the frequency spacing between the pairs of signal in S and C bands should be maintained in order to avoid the signal degradation by the effect of higher order FWM. Thereby the best frequency spacing between the pairs of signals in S and C bands is estimated to maintain good BER over the wavelength converted signals of both bands. So the selected frequency spacing between the pairs fulfills the freedom of selecting any frequency spacing within a pair of wavelengths in S and C band signals. It is also shown that CSRZ–DPSK modulated input signal enhances the BER of wavelength converted signals over the RZ–DPSK. In addition to this, uniform wavelength conversion over a wide bandwidth with a reduced length of HNLF is achieved and also the best power range is estimated to obtain good conversion efficiency.     

Multi-wavelength fiber source with equal frequency spacing

A optical filter based on Sagnac interferometer was proposed to be acted as a comb filter with equal frequency spacing and good signal to noise ratio (SNR), which was composed of an 8.14 m stress-induced Hi-Bi (high-birefringence) PM (polarization-maintaining) fiber. Using this multi-wavelength Sagnac comb filter and a gain flattening Sagnac filter that made the output spectra flattening at different pump powers, a 25-channel multi-wavelength all-fiber source were successfully generated with channel spacing of 0.8 nm with respect to the center wavelength at 1550 nm and flattened gain about ±1 dB peak deviation. The channel spacing can be further reduced to 0.4 nm to produce a DWDM (dense wavelength division multiplexing) source, simply by increasing the Hi-Bi fiber to be 16.28 m. It can be used in many applications such as WDM (wavelength division multiplexing), optical amplifiers with a high SNR, narrow band filters and optical sensors.     

Simulation Results for DWDM Systems with Ultra-High Capacity

We present stimulation results for DWDM systems with an ultra-high capacity up to 1.28 Tbit/s and spectral efficiency approaching 0.4 bit/s/Hz. The impact of signal-to-noise ratio (SNR) on parameters such as channel spacing, length of fiber, dispersion, and number of channels has been investigated and the results obtained have been explained on the basis of fiber nonlinear effects. It has been shown that with an increase in channel spacing, the SNR increases to the maximum optimum value and then decreases to a steady value. With an increase in number of channels, the SNR decreases for small wavelength spacing. For large wavelength spacing, it becomes independent of the number of channels. Keeping channel spacing constant, the SNR decreases with an increase in the length of the fiber. The SNR also improves with a small increase in dispersion of the fiber. Further, it is observed that, with increase in length over dispersion-shifted fiber, the received power decreases and the bit error rate increases.     

Simulation Results for DWDM Systems with Ultra-High Capacity

We present stimulation results for DWDM systems with an ultra-high capacity up to 1.28 Tbit/s and spectral efficiency approaching 0.4 bit/s/Hz. The impact of signal-to-noise ratio (SNR) on parameters such as channel spacing, length of fiber, dispersion, and number of channels has been investigated and the results obtained have been explained on the basis of fiber nonlinear effects. It has been shown that with an increase in channel spacing, the SNR increases to the maximum optimum value and then decreases to a steady value. With an increase in number of channels, the SNR decreases for small wavelength spacing. For large wavelength spacing, it becomes independent of the number of channels. Keeping channel spacing constant, the SNR decreases with an increase in the length of the fiber. The SNR also improves with a small increase in dispersion of the fiber. Further, it is observed that, with increase in length over dispersion-shifted fiber, the received power decreases and the bit error rate increases.     

Performance of Optical OFDM Receiver in the Presence of System Impairments

Abstract

Recently, there is an increasing interest in using orthogonal frequency division multiplexing schemes in advanced optical communication systems to compensate fiber dispersion. This article presents a comprehensive theoretical analysis to treat the optical orthogonal frequency division multiplexing scheme as a special case of optical subcarrier multiplexing system. An analytical expression is derived to calculate the laser power required to achieve a specific level of signal-to-noise ratio, as a function of various system and noise parameters. Simulations show that the presence of laser relative intensity noise may cause a signal-to-noise ratio floor, where the bit error rate cannot be improved further even when the laser power increases dramatically.     

Novel discretely tunable narrow linewidth fiber laser with uniform wavelength spacing

A novel configuration of the tunable fiber laser with uniform wavelength spacing in dense wavelength division multiplexing (DWDM) application is proposed. The ring type tunable fiber laser consists of an all-fiber comb filter which determines the wavelength spacing, and a piece of adjustable fiber grating to select the discrete lasing wavelength for WDM application. The proposed all-fiber ring type tunable laser has potential application in the DWDM and other optical systems due to its advantages such as narrow linewidth, easy tuning, uniform wavelength interval, etc..     

FWM minimization in WDM optical communication systems using the asymmetrical dispersion-managed fibers

Four wave mixing (FWM) induced power penalty is investigated theoretically for asymmetrical dispersion-managed fibers in wavelength division multiplexing (WDM) systems. The power penalty in these fibers is analyzed for various values of channel spacing, maximum dispersion and number of sections in different channels. Using numerical simulations, it is illustrated that the FWM induced power penalty is minimized for the case of two fiber sections with unequally special lengths and symmetrical dispersion values.     

基于光子晶体光纤中FWM的4×10 Gbit/s全光波长转换

 多波长转换对于增强波分复用光网络的灵活性具有重要意义。基于光子晶体光纤中的多四波混频原理,实现了4×10 Gbit/s全光波长转换,深入调查了泵浦功率、光纤长度、信号光与泵浦光偏振失配对波长转换信号质量的影响。结果表明：当泵浦光功率从6 dBm到20 dBm增长的过程中,转换信号Q因子随泵浦光功率增大而增大,最大为82.01,光纤长度从50 m到120 m变化过程中,转换信号Q因子最大为57.41,而随着信号光与泵浦光偏振失配角的增大,转换信号Q因子逐渐降低,当失配角大于60°后,转换信号质量急剧下降。     

Stable wavelength tunable fiber ring laser for true-time delay system

In this paper, we propose and experimentally demonstrate a novel wavelength tunable fiber ring laser source for a photonic beamforming system to control phased-array antenna. In this fabrication, a Sagnac loop composed of a polarization-maintaining (PM) coupler and a piece of high-birefringence (Hi-Bi) PM fiber is acted as a comb filter to make the frequency spacing equal. The wavelength of the output signal is controlled by the tunable filter outside the Sagnac loop. The intensities of the output signals with different wavelengths increased or decreased at the same frequency spacing are equal. A five-channel true-time delay system consisting of this tunable fiber source and five grating delay lines for discrete beamsteering has been demonstrated. In the experiment, the output signals of the tunable fiber ring with the equal frequency spacing have the same intensity of about 5.9 dBm and the same high signal to noise ratio (SNR) of 40 dB. If the tunable filter in this laser is replaced by a micro-electro-mechanical systems (MEMS) tunable filter, the speed of switching wavelength will increase rapidly.     

Characterization of wavelength interleaving in radio-over-fiber systems employing WDM/SCM

A radio-over-fiber (RoF) distribution system incorporating both sub-carrier multiplexing and wavelength division multiplexing (WDM) technologies is presented. This signal is directly modulated onto three high-speed lasers. Bragg filters are employed at the receiver base station in order to both demultiplex the required optical channel, and ensure that the detected signal is single side band (in order to overcome dispersion limitations of the link). System spectral efficiency is optimised by wavelength interleaving. The channel spacing between the WDM channels is varied and the system performance for different values of channel spacing and spectral efficiencies is investigated. The results show that wavelength interleaving is a reliable technique that could be used to increase the spectral efficiency of RoF systems.     

Phase noise characteristics induced by stimulated Brillouin scattering in the presence of four-wave mixing in an interferometric fiber sensing system

Phase noise characteristics induced by stimulated Brillouin scattering (SBS) in the presence of four-wave mixing (FWM) in an interferometric fiber sensing system are investigated. It is found that the phase noise mainly results from SBS and the influence of FWM on the phase noise is negligible. It is due to that the channels are not perfectly equally spaced in the wavelength division multiplexing (WDM) system and the induced beat noise is filtered by the ??kHz/MHz photodetector. The phase noise caused by SBS rather than FWM should be focused on in the practical applications of interferometric fiber sensing systems.     

Priority-based parameter optimization strategy for reducing the effects of four-wave mixing on WDM system

In order to meet the ultra high speed and ultra long-haul transmission distance in wavelength division multiplexing (WDM) systems, the nonlinear impairment affecting the overall spectral efficiency and system performance should be minimized. This paper proposes a strategy to mitigate the four-wave mixing (FWM) effect in WDM system. The strategy determines the effect of both single and combined effects of second, third, and fourth optimization priority parameters such as fiber length, input power, dispersion, channel spacing, and effective area on FWM power. A comparison study was made under different types of optical fiber such as single-mode fiber (SMF), dispersion shifted fiber, non-zero dispersion fiber, and non-zero dispersion shifted fiber. In addition, the system performance in term of bit-error-rate was calculated in the case of single priority (impact of effective area) and combined priority (impact of effective area, input power, fiber length and channel spacing). The results show that the FWM effect was reduced based on the transmission parameters order of optimization, i.e., priority selection proposed. Moreover, the results indicated that increasing sequentially the effective area, fiber length; channel spacing and decreasing the input power provide the most significant sequence in suppressing the effects of FWM. This priority sequence brought the suppression ratio to approximately 26.3% in SMF, which suppressed the FWM effects up to −50 dBm. In term of BER; the combined priority introduces improvement in BER of 2.31 × 10⁻²⁵ in comparison with single priority that has value of BER 4 × 10⁻¹⁴. Finally, this work suggests that the proposed priority-based parameter optimization strategy is an ideal solution for optimum performance of WDM system.     

4 × 25 GHz uni-traveling carrier photodiode arrays monolithic with In P-based AWG demultiplexers using the selective area growth technique

We use the selective area growth(SAG) technique to monolithically integrate In P-based 4-channel arrayed waveguide gratings(AWGs) with uni-traveling carrier photodiode arrays at the O-band. Two kinds of channel spacing demultiplexers of 20 nm and 800 GHz are adopted for potential 100 Gbps coarse wavelength division multiplexing and local area network wavelength division multiplexing systems, with an evanescent coupling plan to facilitate the SAG technique into device fabrication. The monolithic chips in both channel spacings exhibit uniform bandwidths over 25 GHz and a photodiode responsivity of 0.81 A/W for each channel, in agreement with the simulated quantum efficiency of 80%. Cross talk levels are below-20 d B for both channel spacing chips.     

Performance analysis of dense wavelength division multiplexing secure communications with multiple chaotic optical channels

The performance of dense wavelength division multiplexing secure communications with multiple chaotic optical channels is numerically analyzed in this paper. Taking the multiplexing of three chaotic optical channels as an example, we investigate the effects of second-order dispersion coefficient and nonlinear coefficient of fiber, channel spacing, message amplitude and bit rates on chaotic synchronization and multiplexing communications. Chaotic synchronization quality and Q-factor of the recovered message decrease with the increasing fiber length. A 1.25 Gbits/s non-return-to-zero (NRZ) sequence can be securely transmitted up to 60 km under the influence of the other two chaotic optical channels. Compared with the fiber dispersion, the cross-phase modulation is the primary factor which deteriorates the quality of communications. The results also show that the quality of communications is unlimited to the channel spacing as long as chaotic synchronization can be maintained. In addition, the effect of the amplitude of encrypted message on Q-factor and the confidentiality is demonstrated.     

Optimum Compensator Positioning to Reduce Four-Wave Mixing in Wavelength Division Multiplexing Optical Communication Systems Using Dispersion-Managed Fibers

Abstract

The impact of the compensator's location along the optical fiber on a variation of four-wave mixing power penalty is investigated theoretically and numerically for dispersion-managed fibers in wavelength division multiplexing systems. The power penalty is analyzed for fibers with different attenuation coefficients and lengths, and the optimum positions for the compensator along the fiber is discussed in cases of both dispersion compensation of each fiber section between two amplifiers and whole-compensation.     

Phase-matching analysis of four-wave mixing induced by modulation instability in a single-mode fiber

Four-wave mixing induced by modulation instability in a single-mode fiber is analyzed from the phase-matching point of view. For the two-channel transmission, a method is proposed to select the four-wave-mixing-induced sidebands, which is based on the proper use of a continuous-wave and a pulse as light sources. We find that a mass of sidebands are generated in the modulation instability resonance region, and the power of the sideband increases with not only the peak power of the pump pulse but also the continuous-wave power which acts as a seed. The research will provide guidance for fiber communication and sensing systems using wavelength division multiplexing technology.     

A comparative study of modified PUSCA and paired PUSCA strategies in WDM system

In G.653 compliant fiber, four-wave mixing (FWM) is a dominant non-linear effect. Performance of optical wavelength division multiplexing (WDM) network based on G.653 fiber is affected by the generated FWM components. The paper aims to discuss the methods used to reduce the number of FWM components and make efficient utilization of bandwidth. We have earlier proposed two methods (a) modified periodically unequally spaced channel allocation (modified PUSCA) and (b) paired PUSCA. In this paper, we have compared both the methods by considering the ratio of increase in weighted average signal to FWM noise ratio to increase in bandwidth as a figure of merit. Further, the performance of paired PUSCA scheme on G.653 fiber has been compared with equally spaced channel allocation (ESCA) scheme on G.652 fiber in which FWM is not a dominant non-linear effect. We demonstrate that paired PUSCA on G.653 fiber performs better than ESCA on G.652 fiber when the number of channels is less than 30.     

偏振模色散及非线性效应对40 Gbit/s密集波分复用系统的影响

高速率、大容量的密集波分复用系统是光纤通信系统的最终发展方向 ,单信道速率达到 4 0Gbit/s时 ,光纤的非线性效应、偏振模色散现象对系统的影响更加突出。在综合考虑群速度色散、自相位调制、交叉相位调制、四波混合、偏振模色散等因素的基础上 ,推导了密集波分复用系统中任意信道的耦合非线性薛定谔方程组。利用扩展的分步傅里叶方法对该方程进行了数值计算 ,通过对 8× 4 0Gbit/s密集波分复用系统的仿真 ,分别研究了非线性效应和偏振模色散对密集波分复用系统的影响。发现由于交叉相位调制和四波混合作用 ,多波长的密集波分复用系统比单波系统受非线性效应影响严重 ;系统受偏振模色散与非线性效应的影响程度与输入信号功率有关 ,在入射光单信道平均功率较低 0 .1mW时 ,偏振模色散是影响系统性能的主要因素 ;当入射光单信道平均功率较高1mW时 ,系统受非线性效应影响严重。而偏振模色散在使信号脉冲展宽的同时 ,类似于非零色散位移光纤中的微小色散 ,对非线性效应又有一定的抑制作用。