Multichannel wavelength multicasting for two QPSK signals based on FWM in SOA

We experimentally demonstrate multichannel wavelength multicasting for two nonreturn-to-zero quadrature phase-shift keying(NRZ-QPSK) channels based on four-wave mixing(FWM) in semiconductor optical amplifier(SOA). Through the interaction with the two pumps in SOA, the input two 25 Gb/s NRZ-QPSK channels are successfully simultaneously multicast to five and two new wavelengths, respectively. All the multicast channels are with a power penalty less than 2.5 d B at a bit error rate(BER) of 10-3. A characterization of the system performance using conversion efficiency and BER as figures-of-merit in terms of pump and signal powers is also presented. The results indicate that the pump and signal powers can be optimized to eliminate the introduced deleterious nonlinear components. The wavelengths of the two NRZ-QPSK channels and the two pumps need to be specified to avoid the crosstalk induced by high-order FWM.     

Parametric amplification in a microstructure fiber

We report an observation of parametric amplification with a gain greater than 30 dB in a 1.7-m microstructure fiber. It is found that there exists a peak pump power P_s. For peak pump powers less than P_s, the amplified twin (signal: longer wavelength, and idler: shorter wavelength) pulses are highly correlated, and the bandwidth of the amplified signal pulse broadens monotonically with pump power. For peak pump powers exceeding P_s, the correlation between the amplified twin pulses drops when the pump power increases, the bandwidth of the amplified signal pulse becomes narrower when the pump power increases, and the central wavelength of the amplified signal (idler) pulse shifts towards longer (shorter) wavelength, respectively, due to phase matching. PACS 42.65.Yj     

多泵浦光纤喇曼放大器的模拟分析

通过使用综合理论模型对不同配置条件下的多泵浦分布式光纤喇曼放大器的增益谱进行了数值模拟研究,该理论模型包含了瑞利散射、放大自发辐射和不同交互效应,包括泵浦与泵浦、泵浦与信号和信号与信号之间的交互与能量转移.模拟研究了泵浦源功率、泵浦源波长间隔及光纤损耗对喇曼增益谱的影响,结果表明设计多泵浦光纤喇曼放大器需要对泵浦源功率、泵浦源波长间隔光纤损耗谱进行综合考虑,需对泵浦源功率、泵浦源波长间隔进行合理配置.在本文的泵浦源波长设定条件下,考虑实际光纤的损耗谱特性,为了获得大的增益带宽和小的增益不平坦度,短波长泵浦源和最长波长的泵浦源需要更高的泵浦功率,中间波长泵浦源的功率应较低.     

Polarization insensitive wavelength multicasting of DPSK signal using four-wave mixing in a birefringent photonic crystal fiber

We demonstrate polarization-insensitive wavelength multicasting of differential phase-shift keying (DPSK) signal based on four-wave mixing (FWM) in a highly nonlinear, birefringent photonic crystal fiber (PCF). The 10-Gbit/s RZ-DPSK input signal is copied to 8 wavelength channels through FWM with three optical pumps. The pump wavelengths are unequally spaced to avoid undesirable crosstalk from pump-to-pump beatings. The dependence of conversion efficiency on input signal polarization is suppressed by exploiting the PCF birefringence together with control of the pump polarizations. All pump polarizations are aligned at 45 degrees to the principal axes of the PCF. Open eyes and error-free results are obtained in all the 8 multicast channels for polarization-scrambled RZ-DPSK input signal.     

电磁波在时变磁化等离子体信道中通信性能的实验研究

本文首先利用5.8 GHz微波实际测量了辉光放电等离子体源电子密度和碰撞频率随时间的变化规律. 然后搭建实验平台, 测试了多种通信调制体制的信号经过该等离子体源后的误码率, 实验发现BPSK调制方式的信号在该时间变化的等离子体信道中通信误码率最小. 最后加入磁场, 实验测试了L波段(1.5 GHz) BPSK调制信号和S波段(2.5 GHz) QPSK信号在该磁化等离子体中的衰减、相移以及眼图和星座图的变化, 通过与非磁化情况下对比发现, 加入磁场后, 信道的通信性能有所改善, 误码率显著降低, 可以有效地缓解时变等离子体引起的幅度和相位上的寄生调制效应.     

采用QPSK调制的50 Gbit/s高速大气激光通信传输特性研究

高级多电平调制格式在海上高速和高光谱效率的光通信中已经显示出巨大的潜力。将正交相移键控(QPSK)调制格式引入自由空间光(FSO)通信,实验证明可实现高达50 Gbit/s的高速FSO传输链路,能够清楚地观察到所发送的QPSK信号的眼图和星座图。通过比较传输之前和之后的误码率曲线,可以发现接收灵敏度都小于-36.7 dBm。当误码率为3.8×10-3时,它们的功率损失都小于4.2 dB。     

Optimum design of erbium-ytterbium codoped fibres for large-signal, high-pump-power applications

A comprehensive numerical fibre amplifier model has been used to optimise erbiumytterbium codoped active fibre for maximum gain and quantum conversion efficiency at large signal operation. The optimum cut-off wavelength of the LP₁₁ mode has been found to increase from 800 nm at low pump powers (≈ 50 mW) to 1400 nm at pump powers higher than 500 mW. While at low pump powers fibres with higher numerical aperture give higher quantum conversion efficiency, at high pump levels better large signal performance is achieved with fibres having lower numerical aperture. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998.     

Optimization of polarization spectroscopy for rubidium D lines

Polarization spectroscopy of the D lines of rubidium atoms is investigated experimentally,especially with different pump powers and cell temperatures.We find that there are four candidate transitions suitable for frequency stabilization,and optimal pump powers and cell temperatures are also presented to obtain a perfect signal with maximal amplitude and slope.The optimal signal is insensitive to the fluctuations of laser power and the temperature,which can enhance the performance of frequency locking.     

High-performance optical modulators bias control system without dither

A software-based bias control system of push–pull Mach–Zehnder modulators (MZMs) without dither tone is described in this paper. The slope of the average optical output power respect to the bias voltage is used to control the operating bias points in the automatic control system. The paper analyses the theory of automatic bias control intensively and verifies the effectiveness of the proposed technique which is a modulation-format-free method under the RF signal through testing and contrasting the BER (bit error rate) between the new technique and manual measure on the experimental platform. Based on this, an automatic bias control system set up by an optical QPSK composed by double MZMS on the LabVIEW platform is described.     

Performance analysis and improvement of spectrally amplitude encoded/decoded OCDMA system

This paper describes a performance analysis of an incoherent optical code-division multiple-access scheme based on wavelength/time (W/T) codes. The system supports 16 users operating at 2.5Gb/s/user while maintaining bit-error rate (BER) < 10⁻¹¹ for the correctly decoded signal. It has been observed that there are two major problems giving rise to performance degradation of the system in terms of number of users and type of code.In this paper we have studied the optical simulator Encoding/Decoding for different lengths & gain in terms of Quality factor (Q) and Bit Error Rate (BER) performance. The system supports 16 users while maintaining bit-error rate (BER) < 10⁻¹¹ for the correctly decoded signal. Our aim is to design and simulate a Tree Network Topology Optical Code Division Multiple Access System for large number of users using wavelength–time code and to analyze the performance of the system based on BER and Eye Diagram under the influence of number of simultaneous users with different received powers.     

掺铒光纤非线性耦合器的实验研究

提出一种耦合参量可用光调控的掺铒光纤非线性耦合器.用熔融拉锥法熔合两根掺铒光纤,拉制成工作波长为155nm的3dB掺铒光纤耦合器.通过调变输入耦合器一臂的980nm泵浦光功率,可以改变两臂的传播常量差,从而改变耦合器两臂信号光的相对输出功率.通过测量输入泵浦光功率和两臂信号光输出功率,得到直通臂耦合比依赖于泵浦光功率的实验曲线.实验研究表明,当泵浦光功率从0 mW变化至20 mW时,耦合比的变化可达到40％.与理论模拟的结果一致.     

掺铒光纤放大器中上转换过程的影响

掺杂浓度的提高影响了掺铒光纤放大器(EDFA)的性能,对经典Giles模型提出了新的要求。首先研究了高掺杂下铒离子距离较小引起的上转换过程的机理,通过优化速率方程模型分析了上转换过程对EDFA的影响,得出了在泵浦光和信号光远远大于ASE光的假设下泵浦光和信号光沿光纤长度变化的近似表达式,并将该近似分析解与解析解进行了比较。仿真结果表明上转换过程会降低随光纤传输的信号光和泵浦光,影响EDFA的放大性能,并得出了当掺铒浓度大于2.95×1025m-3时,上转换过程的影响不能忽略的结论。     

On BER assessment of conventional- and wavelet-OFDM over AWGN channel

Orthogonal wavelets enclose more potential of dropping the ISI- and ICI-sway amid the carriers due to multipath propagation over the free-space fading channels. Moreover, wavelet-OFDM is more bandwidth efficient as compared to conventional OFDM and is less affected by Doppler shift. Accordingly, a BER estimation of discrete Fourier transform (DFT) based OFDM system and discrete wavelet transform (DWT) based OFDM system is carried out in this work. The assessment is demonstrated by means of diverse modulation schemes explicitly BPSK, QPSK, 8-PSK, QAM, 8-QAM, 16-QAM for both conventional- and wavelet-OFDM wireless system over AWGN channels. Higher orders of PSK schemes involve more SNR requirement to target an acceptable BER of 10⁻⁵ over AWGN channels in both the cases. Furthermore, the efficiency of W-OFDM increases with decreasing the required value of BER.     

OOK-to-QPSK modulation-format conversion based on two gain-transparent SOAs

In this paper, the system scheme of on–off keying (OOK) to quadrature phase-shift keying (QPSK) format conversion is proposed, which is based on two gain-transparent semiconductor optical amplifiers (GT-SOA). GT-SOA is used as a simple all-optical phase modulator, which is no effects of cross gain modulation and four-wave mixing existing in conventional SOA. The assistant light is used to speed up the SOA carrier recovery time. The principles of OOK to QPSK format conversion are expounded. The influences of the assistant light power and signal light power on the conversions are numerically analyzed. The OOK signal can still be exactly converted to the QPSK signal with the power of OOK signal being changed within a certain range.     

Quantitative analysis of optical power budget of bismuth oxide-based erbium-doped fiber

We investigated optical power budget of Bi₂O₃-based erbium-doped fiber (BIEDF). Lateral spontaneous emissions and scattering laser powers in the BIEDF were measured quantitatively by using an integrating sphere. Compared with the power of amplified spontaneous emission and signal detected at the output fiber end, it was found that considerable powers were consumed by the laterally emitting lights. As an optically undetected loss limits power conversion efficiency (PCE) of the fiber amplifier, the effect of nonradiative decay from the termination level of pump excited state absorption (pump ESA) was estimated from decay rate analyses of the relevant levels. The nonradiative loss was comparable to amplified signal power in the BIEDF when pumped with a 980-nm LD. Nonradiative decay following cooperative upconversion (CUP) process is also discussed using rate equations analysis.     

基于光纤四波混频效应的新型组播方法

组播是波长路由光通信网络中的一项重要功能.提出并实验实现了基于高非线性光纤四波混频效应的全光组播方法,将一束载有数据的输入信号光和一束抽运光一起入射到一段高非线性光纤中,通过两个简并四波混频过程产生了两个携带该数据信息的闲频光,从而实现了单一信号的两信道组播,同时抽运光也会受到数据信息的调制.该方法的优点在于组播的实现只需要单一的抽运光源,而不需要再提供其他输入光,结构简单,组播光信号功率均匀、波长可调,并且具有进一步增加组播信道的能力. 关键词： 组播 四波混频 高非线性光纤     

Phase sensitive amplifier for PSK signals based on non-degenerate four-wave-mixing in the optical fiber

A phase-sensitive-amplifier (PSA) using only one pump channel based on frequency non-degenerate four-wave-mixing (FWM) process for phase-shift-keying (PSK) signals is proposed and demonstrated. Theoretical analyses are given first. By optimizing power levels of three channels and parameters of the highly nonlinear fiber (HNLF), the PSA is able to restore the signal fidelity (down to a BER of 10^? 9) as the input signal shows an error-floor with the BER of 10^? 6. Moreover, the PSA for the multi-level PSK signals could be achieved by adjusting the phase relationship between two input signals.     

An effective numerical method for gain profile optimizations of multi pumped fiber Raman amplifiers

In this paper, we have solved propagation equations of multi-pump fiber Raman amplifier using Runge–Kutta (RK 4th order) numerical method and pump power evolutions along with the fiber length. They are used to calculate the net gain and gain ripple by varying the input signals powers for different fiber lengths. The pump powers are optimized by genetic algorithm and resulting net gain and gain ripple are reported graphically as well as in tabular form. The optimum minimum gain ripple is 0.26 dB for 1 mW input signal powers for 50 km fiber length. By increasing the fiber length gain ripple increases to 0.5 dB for 0.1 mW input signal power. In comparison to other methods reported in the literature, our method is simple to implement and efficient for numerical design of Raman amplification in optical communication systems.     

Generation of coherent optical multi-carriers using concatenated,dual-drive Mach-Zehnder and phase modulators

The generation of coherent optical subcarriers based on a concatenated dual-drive Mach-Zehnder intensity modulator（IM） and two phase modulators（PMs） is proposed and experimentally demonstrated.The modulation index and DC bias of PM+IM modulation are theoretically investigated.Theoretical analysis and numerical study are also carried out to examine the proposed scheme.We use 25-GHz RF synchronous sinusoidal signals to drive cascaded two-stage PMs and IM,through which we generate 28 subcarriers with peak power fluctuations less than 4 dB.The measured tone-to-noise ratio of the subcarrier is higher than 40 dB.The experimental results show that for 100-Gb/s polarization multiplexing QPSK signal,the receiver sensitivity of the back-to-back signal is -28.6 dBm,and the power penalty is lower than 1 dB after 100-km transmission at the BER of 1×10^-9.     

LD泵浦掺铥(Tm3+)光纤激光器的数值分析

首先从掺铥光纤激光器的速率方程和光传输方程出发,建立数学模型,通过Matlab软件进行数值计算,分析了泵浦光和激光沿光纤的分布以及各能级离子数的变化.在不同掺杂浓度下,研究了小信号增益与入纤泵浦功率的关系以及泵浦光和激光功率与增益介质长度的关系.在不同泵浦功率下,研究了输出功率与输出耦合镜反射率的关系.进一步对不同泵浦吸收系数,研究了斜率效率和泵浦阈值与光纤长度的关系.分析结果表明存在最佳光纤长度和最佳耦合输出透过率,使得激光输出功率达到最佳值.