Analysis of optical DEMUX characteristics based on four-wave mixingin semiconductor optical amplifiers

We have analyzed the basic characteristics of all-optical demultiplexing (DEMUX) based on four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) by solving a modified nonlinear Schrodinger equation by the finite-difference beam propagation method. Amplified spontaneous emission noise was not included in our model. The optimum pump pulsewidth for obtaining the high ON-OFF ratio is 1~3 ps for 1 ps, 250 Gb/s probe pulses. The shorter limit of the pulsewidth is due to detuning between the pump and probe frequencies, which is determined by the gain bandwidth of the SOA. In order to achieve faster DEMUX operation, an SOA with broader gain bandwidth is required. We also simulated pattern effects in the FWM signal. Power fluctuation in the FWM signal can be reduced by using a strong energy pump pulse and/or weak energy probe pulse. The energy fluctuation of the FWM signal decreases to less than 1% for a 30-bit, 250-Gb/s input probe pulse train with a pulse energy of 0.01 pJ. This small fluctuation should not disturb DEMUX operation. We have also examined DEMUX from time multiplexed signals by repetitive pump pulses. Strong energy pump pulses decrease the FWM signal intensity. However, there is no pattern effect due to gain saturation because the pump pulses are injected continuously     

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     

Active optical pulse compression with a gain of 29.0 dB by usingfour-wave mixing in an optical fiber

The four-wave mixing gain in an optical fiber strongly depends on the nonlinear phase mismatch when the pump power itself causes a nonlinear index change. The peak of the pump pulse has a much higher gain than its wings, and so this process can be used to obtain efficient pulse compression. A 93-ps signal pulse at 1569.9 nm is compressed to as short as 20 ps with a gain of 29.0 dB. The wavelength-converted signal at 1559.9 nm is simultaneously compressed to 21 ps and amplified by 28.9 dB as compared with the original signal     

光子晶体光纤中双脉冲非线性传输的数值分析

采用分布傅立叶方法数值求解耦合的非线性薛定谔方程,模拟了双脉冲在光子晶体光纤中的非线性传输过程,计算和分析了离散效应和脉冲内拉曼散射对信号脉冲传输和压缩的影响。结果表明,当中心波长为800nm的泵浦脉冲在反常色散区进行泵浦,而中心波长为740nm的信号脉冲在正常色散区入射,在群速度色散,自相位调制及交叉相位调制联合作用下,信号脉冲在传输过程中不仅被压缩且存在最佳光纤长度。离散效应不仅导致脉冲压缩比的减小和压缩后峰值功率的降低,而且导致脉冲所需最佳光纤长度的增加以及压缩后的脉冲呈现不对称,还发现,脉冲内拉曼散射有利于改善脉冲压缩质量。     

Influence of probe depletion and cross-gain modulation on four-wavemixing of picosecond optical pulses in semiconductor optical amplifiers

The influence of probe depletion (PD) and cross-gain modulation (XGM) on four-wave mixing (FWM) of picosecond optical pulses in semiconductor optical amplifiers has been investigated numerically. The effects of PD and XGM become more significant for shorter input pulsewidth, higher pulse energy and smaller pump-probe frequency detuning. Results indicate that if the effects of PD and XGM are neglected the FWM efficiency can be seriously overestimated     

Generation of optical phase-conjugate waves and compensation forpulse shape distortion in a single-mode fiber

The generation of optical phase-conjugate waves and the application of optical phase conjugation (OPC) to optical communication systems is described. The method of pulse shape distortion compensation by OPC is outlined including distortion due to both fiber dispersion and the optical Kerr effect. The generation of a forward-going phase-conjugate wave in a third-order nonlinear medium is discussed and that by a nondegenerate forward four-wave mixing in a zero-dispersion single-mode fiber (SMF) is investigated. Suppressing the stimulated Brillouin scattering (SBS) of a pump wave in the fiber prevents saturation of the generation efficiency of the phase-conjugate wave even when the pump power exceeds the SBS threshold. In transmission experiments through a 200-km standard SMF with a 16-Gb/s intensity-modulated signal and a 5-Gb/s continuous-phase FSK (CPFSK) modulated signal, it is shown the applicability of OPC is modulation independent and that OPC effectively compensates for both chromatic dispersion and the optical Kerr effect     

线性源啁啾对孤子脉冲压缩的影响

通过线性源啁啾对孤子脉冲压缩影响的研究,发现泵浦脉冲和孤子脉冲的源啁啾参量地脉冲压缩产生重要影响,且孤子脉冲压缩存在最佳光纤长度。在泵浦功率一定的条件下,选取负啁啾的泵浦脉冲正啁啾的孤子脉冲可以获得高压缩因子的光脉冲;同时选取负啁啾的泵浦脉冲和正啁啾的孤子脉冲易于产生更短的压缩光脉冲。     

Transmission of 2-ps optical pulses at 1550 nm over 40-km standardfiber using midspan optical phase conjugation in semiconductor opticalamplifiers

We demonstrate transmission of 2-ps optical pulses at 1550 nm over 40 km of standard fiber by employing midspan optical phase conjugation in semiconductor optical amplifiers (SOAs). The second-order group-velocity dispersion of the fiber is completely compensated and the third-order dispersion becomes a major transmission limitation. This experiment shows that the midspan optical phase conjugation system using SOAs is applicable to ultrahigh bit rates greater than 100 Gb/s     

基于高SBS阈值的HNLF产生高重复频率超短光脉冲

基于光纤中的四波混频(FWM)产生高重复频率超短光脉冲的原理,并为抑制光纤中的受激Brillouin散射(SBS),采用非均匀掺杂高SBS阈值非线性光纤,通过FWM对双拍频信号进行整形压缩,实验上获得了100GHz的高重复率超短光脉冲序列,进而分析了入纤功率对输出光脉冲的影响。     

A simple scheme for polarization insensitive four-wave mixing insemiconductor optical amplifiers

A simple scheme for polarization insensitive four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) is proposed, which utilizes just one conventional polarization-insensitive SOA and one input pump wave. The technique would be efficient in practical applications for frequency shifts about 0.5 THz for a device length of say 200 μm. The polarization insensitive operating principle of the FWM type is analyzed using an improved space-averaged model for describing FWM of copropagating and counterpropagating optical waves     

离散效应对光子晶体光纤中飞秒脉冲压缩影响

为了研究离散效应对光子晶体光纤中飞秒信号脉冲压缩的影响,采用分步傅里叶方法数值求解耦合的非线性薛定谔方程,模拟了双飞秒脉冲在光子晶体光纤中的非线性传输过程;计算和分析了离散效应和非同步耦合对信号脉冲压缩的影响。结果表明,当抽运脉冲在反常色散区进行抽运,而信号脉冲在正常色散区入射时,在群速度色散、3阶色散,自相位调制及交叉相位调制联合作用下,信号脉冲在传输过程中不仅被压缩且存在最佳光纤长度;离散效应导致信号脉冲压缩因子减小,所需最佳光纤长度增加以及压缩后的脉冲频谱呈现不对称,采用非同步耦合有利于改善信号脉冲的压缩质量。     

Active picosecond optical pulse compression in semiconductoroptical amplifiers

A novel technique is proposed to compress and amplify a weak picosecond optical pulse by utilizing a copropagating intense pump pulse in semiconductor optical amplifiers. It is shown that, simply by adjusting the time delay between the two pulses, a controlled compression of the pulsewidth by up to a factor of ~4 with pulse energy amplification of up to ~16 dB can be achieved simultaneously. Furthermore, a compressed pulse shape with strongly damped wings is also observed. The shortest achievable pulsewidth is demonstrated to be dependent on the frequency detuning and pump pulse properties     

Byte-Level Parametric Wavelength Exchange for Narrow Pulsewidth Return-to-Zero Signal

We investigate the feasibility of switching return-to-zero signals with 3-ps pulsewidth by byte-level parametric wavelength exchange (PWE) numerically and experimentally. Square-wave modulated pumps are used in PWE for pump gating. Simultaneous bit swapping for two signals at the same time slot is achieved. Error-free operation is achieved for both signal channels with ap3.5-dB power penalty at 10^-9 bit-error rate.     

Broad-band mid-span spectral inversion without wavelength shift of1.7-ps optical pulses using a highly nonlinear fiber Sagnacinterferometer

We demonstrate broad-band mid-span spectral inversion without wavelength shift of short optical pulses using a highly nonlinear fiber Sagnac interferometer. The 1.55-μm optical pulses with a 1.7-ps pulsewidth are transmitted over 40 km of standard fiber, showing the suitability of this technique for broad-bandwidth second-order dispersion compensation in high-bit-rate optical fiber transmission systems     

Characteristics of optical phase conjugation of picosecond pulsesin semiconductor optical amplifiers

Nondegenerate four-wave mixing (FWM) of picosecond optical pulses in semiconductor optical amplifiers is investigated theoretically, taking into account various carrier processes, probe depletion, and cross-gain modulation effects. Analytical solutions for greatly simplified coupled-wave mixing equations are obtained from which conditions where probe depletion and cross-gain modulation effects cannot be neglected are found. On the other hand, the frequency detuning dependence of optimum input pump pulse energy and shaping of output conjugate pulses are observed from numerical simulations. In particular, calculations show that the frequency symmetry of conjugate and probe pulses with respect to the pump pulse does not hold for optical pulse FWM. Two-photon absorption and ultrafast nonlinear refraction effects are shown to decrease the FWM conversion efficiency     

Polarization insensitive frequency converter with the capability ofchirp removal

We report on a new type of polarization insensitive frequency conversion technique using four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). In this technique the input signal is spectrally duplicated by a Mach-Zehnder type phase modulator. Then, the resulting two waves generate the frequency converted output signal by FWM with use of an additional pump wave. The converted output wavelength is independent of the wavelength of the input signal. In addition to the frequency conversion, the chirp of the input signal is removed by the FWM process bit-error-rate (BER)-measurements after transmission of the converted signal over standard communication fiber are reported     

High-speed picosecond optical pulse compression from gain-switched 1.3-µm distributed feedback-laser diode (DFB-LD) through highly dispersive single-mode fiber

Picosecond optical pulse compression characteristics of chirped pulses from gain-switched distributed feedback-laser diodes (DFB-LD) transmitting through highly dispersive media are studied theoretically and experimentally. It is clarified theoretically that gain-switched chirped pulses can be compressed to about a 0.7-time bandwidth product by normal dispersion of the dispersive media and that the optimum dispersion value to obtain a minimum compressed pulse is proportional to the square of original pulsewidth. Through a dispersion, shifted single-mode fiber with -48-ps/nm normal dispersion at a 1.3-μm wavelength, gain-switched 30-ps (FWHM) pulses from a directly modulated 1.3-μm DFB-LD at a 4.4-GHz repetition rate have been successfully compressed to 6.4-ps optical pulses with a 0.86-time bandwidth product. Experimental results agree with the theoretical analysis.     

110-pJ and 410-fs pulse at 10 GHz generated by single-stage external fiber compression of optically injection-mode-locked semiconductor optical amplifier fiber laser

We demonstrate the high-power amplification and single-stage fiber-based pulse compression of a traveling-wave semiconductor optical amplifier (TWSOA) fiber ring laser (SOAFL), which is optically mode-locked by backward injecting the TWSOA with a dark-optical-comb at repetition frequency of 10 GHz. A negatively chirped SOAFL pulsewidth of 5.4 ps is compensated and shortened to 3.9 ps by a 75-m-long dispersion compensating fiber. With a single-stage pulse compression in a single-mode-fiber spool, the SOAFL pulses with input average power of 1.7 W can further be compressed to a pulsewidth of 410 fs, corresponding to a pulse compressing ratio of nearly ten. The maximum average output power of the amplified SOAFL pulse is up to 1.1 W, corresponding to the peak power and pulse energy of >0.27 kW and 110 pJ, respectively.     

Phase dynamics of semiconductor optical amplifiers at 10-40 GHz

The phase dynamics that occur in bulk InGaAsP-InP semiconductor optical amplifiers (SOAs) in response to picosecond pulse excitations at 10 and 40 GHz are studied experimentally and numerically for various amplifier lengths. The time dependencies of the phase changes and of the absolute gain of the amplifier are measured simultaneously. The total phase shifts induced by 1.5-ps pulses at 10 GHz are higher than /spl pi/ in SOAs with active region lengths between 0.5 and 2 mm and exceed 2/spl pi/ in a 1.5-mm-long amplifier. Phase shifts above /spl pi/ are measured at 40 GHz in 1.5- and 2-mm-long SOAs. The dependence of the total phase shift on the amplifier bias current and length and on pump pulse energy is investigated. Numerical simulations based on a comprehensive time-domain SOA model allow us to confirm the experimental results for a wide range of amplifier parameters. In particular, SOAs with lengths up to 5 mm have been modeled, and the calculations suggest that the maximum phase shifts occur in amplifiers of approximately 2-mm length. The phase dynamics measurements are illustrated at the example of an optical time division multiplexing add-drop multiplexer, based on a SLALOM switch, gated by 10- or 40-GHz control pulses. We find that simultaneous good dropping and clearing is possible if the length and the operating conditions of the SOA in the switch are chosen such as to induce a full /spl pi/ phase shift.     

Time-resolved optical gating based on dispersive propagation: a newmethod to characterize optical pulses

We introduce the technique of time-resolved optical gating (TROG) based on dispersive propagation (DP), a new noninterferometric method for characterizing ultrashort optical pulses in amplitude and phase without the need for a short optical gating pulse. TROG is similar to frequency-resolved optical gating except that the role of time and frequency is interchanged. For the DP-TROG geometry, we show that measurements of the autocorrelation trace of the pulse after propagation through a medium with variable dispersion together with a single measurement of its intensity spectrum contain sufficient information to reconstruct the pulse in amplitude and phase. Pulse reconstruction for this DP-TROG geometry works very well even for the case of a nonlinearly chirped double pulse. Compared with other methods, DP-TROG does not introduce an ambiguity in the direction of time for the pulse. Due to its simplicity and improved sensitivity. DP-TROC is expected to be useful in characterizing low-energy pulses