Fiber induced distortion in a subcarrier multiplexed lightwavesystem

The harmonic and intermodulation distortions induced by fiber dispersions are analyzed. The concept of the common frequency (or common period) is introduced for the analyses of intermodulation. It is shown that both the magnitude and shape of the sum terms of the intermodulation are quite similar to those of the harmonic distortions of the same order. The difference terms of the intermodulation are much smaller if the subcarrier frequencies are not far apart from one another. It is concluded that the influence of dispersion-induced distortions depends on the subcarrier frequency, the dispersion constant, the length of the fiber, and the modulation indexes. Accompanied by the development of high-frequency and large capacity systems and single-mode lasers with extra narrow spectrum, the dispersion-induced distortions may become one of the most important factors in determining information capacity and transmission distance     

Ultimate performance of optical DSB signal-based millimeter-wavefiber-radio system: effect of laser phase noise

It is theoretically shown that the phase noise of laser light source can be automatically eliminated by compensating the differential group delay due to the fiber dispersion between the two sidebands of the optical DSB signal. The bit error rate (BER) measurement of 60 GHz millimeter (mm)-wave subcarrier multiplexed optical double-sideband (DSB) signal transport in dispersion-compensated optical fiber link using fiber Bragg grating (FBG) mill show that the effect of the laser phase noise on the BER is as small as a few % relative to the other additive noise effect, thus realizing almost the ultimate performance     

A compensation method for dispersion-induced third-orderintermodulation distortion using an etalon

A method to compensate dispersion-induced third-order intermodulation distortion (IM3_CD) in subcarrier multiplexing (SCM) optical transmission systems is proposed. It is confirmed theoretically and experimentally that IM3_CD can be compensated by interference-induced third-order intermodulation distortion (IM3_IN). Simple analytic expressions of IM3_CD and IM3_IN are derived from the same expression of optical electric field which includes exact phase term taking into account the phase of laser chirp. From the theoretical analyses, we clarified the conditions of IM3_CD-compensation. IM3_IN can be easily generated by an etalon which is a kind of interferometer and a optical glass plate. More than 30 dB of IM3_CD -compensation was achieved, and IM3_CD-compensation was achieved regardless of the transmission distance by adjusting tilt angle of the etalon. Using this method, high quality signal transmission can be realized even though the wavelength of laser diode (LD) is not coincident with the fiber zero dispersion wavelength     

Self-Referencing Fiber-Optic Intensity Sensors Using Ring Resonators and Fiber Bragg Gratings

An improved ring resonator self-referencing technique in a new reflection configuration for remote fiber-optic intensity sensors is demonstrated using fiber Bragg gratings. Sensor sensitivity doubles and a single fiber lead is used. The sensor is interrogated at two subcarrier frequencies having a high rejection of interference from laser source intensity fluctuations and loss in the fiber lead. We experimentally demonstrate the efficiency of the new reflection configuration, the usefulness of the theoretical model proposed, and discuss design parameters for optimum insertion losses     

Signal distortion and noise in AM-SCM transmission systemsemploying the feedforward linearized MQW-EA external modulator

In subcarrier multiplexing (SCM) optical video distribution systems, the nonlinear signal distortion generated by the combined action of laser chirp and fiber dispersion limits the transmission distance. This paper for the first time shows that low-chirp MQW (multiquantum well)-EA (electroabsorption) external modulators are applicable to AM-SCM transmission systems. The feedforward linearization technique is used to compensate the signal distortion due to the nonlinearity in the modulator's L-V (light power versus voltage) characteristic. The effectiveness of this type of modulator for suppressing dispersion-induced distortion is clarified experimentally. A feedforward linearized transmitter with composite second order (CSO) intermodulation distortion <-62 dBc, composite triple beat (CTB) intermodulation distortion <-59 dBc and a carrier-to-noise ratio (CNR) >50.5 dB is successfully constructed. It is shown that the transmitter can achieve a CSO <-59 dBc, a CTB <-57 dBc and a CNR >49.5 dB for a 32 channel AM signal and 10 km transmission. Furthermore, estimations of the signal distortion cancellation and the noise characteristic achieved with feedforward circuit modeling are shown. The results obtained here emphasize that MQW-EA external modulators are applicable to AM-SCM transmission systems     

On the use of tapered linearly chirped gratings asdispersion-induced distortion equalizers in SCM systems

We report on the use of tapered linearly chirped fiber gratings to equalize dispersion-induced distortion in long distance/broad-band subcarrier multiplexed (SCM) systems. Different apodization functions for the coupling factor of the fiber grating have been considered and their equalizing capability of fiber-induced distortion in a subcarrier system have been compared. The results of the theoretical analysis show that both harmonic and intermodulation distortions are significantly reduced when the equalizer parameters have been properly designed. Higher reduction of the nonlinear distortion is obtained when the length of the filter increases, even though a saturation effect on this reduction figure appears when the equalizer length is about four or five times its minimum value to achieve equalization, which is derived theoretically based on the SCM optical system parameters. The performance of a Tanh-type tapered linearly chirped grating equalizer on a realistic broadband SCM system is also considered. The results reveal that the dispersion-induced distortion can be neglected when the equalizer is employed     

用于CO2激光传输的10.6μm波段空心布拉格光纤

空心布拉格光纤是具有一维光子晶体(1DPC)包层和空心芯区的新型光子带隙光纤。针对它在CO2激光传输中的应用,设计和制备了传输波段中心波长在10.6μm的空心布拉格光纤样品。利用傅里叶变换红外光谱仪(FTIR)可以观察到光纤样品在10.6μm具有明显的透射峰。使用CO2激光,通过截断法测量得到光纤样品在10.6μm的传输损耗为2.35dB/m。测量了不同弯曲曲率下光纤样品的弯曲损耗,结果表明弯曲损耗系数随曲率的增大而线性增长。在接近光纤输出端处,弯曲半径为10cm的光纤90°弯曲引入的附加损耗约为2dB。实验结果论证了光纤样品的CO2激光低损耗传输特性,展现了空心布拉格光纤在提升CO2激光操作灵活性上的应用潜力。     

一种新颖的宽带光纤光栅透射谱测量方法

本文在理论分析基础上,用一种新颖的波长扫描光源,并采用自相关、波长解调装置,实现了宽带光纤光栅透射谱的测量。实验结果与精密光谱仪测量比较,误差小于5%,并与理论分析较好地符合。     

光纤OFDM系统中的色散补偿技术研究

为了减小光纤的色度色散对光纤正交频分复用（OFDM）系统性能的影响,提出了将无线通信中的信道估计器引入光纤OFDM系统进行信道估计的色散补偿方法。在理论上分析了光纤的色度色散对于传输OFDM信号的影响,并在实验中采用梳状导频的形式,在每个OFDM码元的特定的子载波上插入导频,在接收端通过基于最小平方（LS）原则的LS估计器进行信道估计,得到了OFDM光纤传输的信道幅度响应和相位响应,通过使用LS估计器,直接调制的光OFDM信号在单模光纤中传输200km,误比特率低于10-6 ,功率代价小于2dB。结果表明,高频子载波较低频子载波更容易受到色散的影响,在光纤OFDM系统中引入信道估计器进行信道估计能够有效补偿由色度色散带来的相位偏移和幅度衰减。     

Reduction of dispersion-induced distortion in SCM transmissionsystems by using predistortion-linearized MQW-EA modulators

The application of low-chirp MQW (multiquantum well)-EA (electro-absorption) modulators to subcarrier multiplexing (SCM) optical transmission systems is studied. The authors show that a third-order predistortion circuit is feasible for compensating the nonlinearity of this type of modulator. The degree of frequency chirping per optical intensity modulation depth of the modulator is theoretically determined to be about 1.4 MHz. A 42-channel AM-SCM signal transmitter with the predistortion circuit realizes both composite second-order distortion (CSO) and composite triple beat distortion (CTB) values under -57 dBc after 200-km-long transmission. The authors confirm dispersion-induced distortion of the MQW-EA modulator is as small as that of the LiNbO₃ modulator. Carrier-to-noise ratios (CNRs) of 45.2 dB for channel 1 and 43.8 dB for channel 42 were measured after 100 km transmission. It is found that the deterioration of a CNR is caused by the cascaded erbium doped fiber amplifiers (EDFAs), Rayleigh backscattered power and the optical phase noise. Theoretical CNRs show that the output power of the modulator should be higher to improve CNR     

光纤传输系统中基于相位预调制的信号整型

利用相位预调制技术解决高速长距离光纤传输系统中面临的接收灵敏度降低和色散容限问题。通过在发射端对非归零(NRZ)的光信号进行比特同步相位预调制,使非归零码在传输过程中得到波形重整,演变为归零(RZ)的波形,从而提高眼图开启度。实验观测了普通非归零码和相位预调制的非归零码在不同相位调制深度和色散下的光谱、眼图和功率代价。10Gb/s的传输结果表明,链路色散绝对值小于1000ps/nm时,施加相位预调制的非归零信号功率代价小于1dB,比普通非归零信号具有更高的接收灵敏度和更低的色散功率代价。因此,基于相位预调制的信号整型技术可减轻系统对光信噪比和色散管理的要求,延长传输距离。     

Extended Transmission Reach Using Optical Filtering of Frequency-Modulated Widely Tunable SSG-DBR Laser

We propose a novel transmitter consisting of a frequency-modulated widely tunable super-structure-grating distributed Bragg reflector (SSG-DBR) laser and an optical filter. The SSG-DBR laser acts as a frequency modulating light source with a constant output power by modulating the reverse bias voltage in the phase control (PC) region. By optically filtering the output light from the frequency-modulated laser, we have demonstrated 60- and 180-km transmissions for 20- and 10-Gb/s nonreturn-to-zero (NRZ) signals, respectively. The power penalty was 2.2 dB after the 180-km transmission of a 10-Gb/s NRZ signal as determined by bit-error-rate measurements. Furthermore, an extended transmission reach was achieved with a wide tuning range without controlling the bias and modulating voltages in the PC region.     

光纤Bragg光栅透射谱测量中的激射现象

本文报道用LED作为宽带光源测量光纤Bragg光栅透射谱时观测到了的反射激射峰现象,并根据实验结果给出了合理的解释,峰值的出现是由于LED和光纤光栅耦合形成了外腔激光器,工在光纤光栅的Bragg波长处产生激射而形成,激射峰的峰值功率随LED驱动电流变化过程中明显的阈值行为有力地证明了这一点。     

An all-optical 2R regenerator using a compact self-seeded Fabry-Pe/spl acute/rot laser diode incorporated in a bidirectional EDFA

An all-optical 2R regenerator, based on a compact self-seeded Fabry-Pe/spl acute/rot laser diode with a 10-mm-long embedded fiber Bragg grating cavity, and a bidirectional erbium-doped fiber amplifier, is proposed and experimentally demonstrated to execute all-optical 2R regeneration at 10 Gb/s. Compared with the conventional 1R regeneration, the proposed scheme has achieved significant 6.4-dB improvement of power penalty at bit-error ratio =10/sup -9/ in the transmission experiment over 100-km standard single-mode fiber.     

A cladding-pumped fiber laser with pump-reflecting inner-cladding Bragg grating

A fiber Bragg grating (FBG) was inscribed in the inner cladding of a double-clad fiber, in order to reflect, and thus, double-pass, the pump radiation in a cladding-pumped fiber laser. The effective pump reflectivity of the inner-cladding FBG was /spl sim/46.4%. This increased the slope efficiency of a Yb-doped cladding-pumped fiber laser by up to 8.5%, and the output power increased by 45% with a 3-W pump source. The FBG provides stable and robust pump reflection, while leaving the fiber end free, for example, so that a signal transmission fiber can be spliced to the double-clad fiber.     

Transmission characteristics of DFB laser modules for analogapplications

Analog transmission characteristics of highly linear 1.3- and 1.55-μm distributed feedback (DFB) laser modules are described. To obtain high-power and high-performance DFB lasers, the bias dependence of distortion characteristics is discussed. The carrier-to-noise ratio, the composite second-order distortion, and the composite triple beat of the DFB laser module which is developed for analog CATV applications are presented. The analog transmission characteristics, such as CNR degradation, after fiber transmission, dispersion-induced distortion, and the influence of reflection, have been made clear. The stable high linearity of the DFB laser also has been confirmed by the composite distortion measurement under an accelerated aging test     

基于窄带光纤光栅的微波载波单边带调制

利用窄带光纤光栅简单而有效地实现了RoF系统中单边带调制,克服长距离传输时的色散损耗.分析了RoF系统中双边带调制对系统带来的影响,以及单边带滤波对窄带光纤光栅反射光谱和反射率的要求.研究了采用基于窄带光纤光栅的单边带调制后,对不同调制方式、不同调制速率、不同传输距离的RoF传输系统性能的改善情况.实验研究表明,光纤光栅单边带调制在以16QAM调制的15GHz RoF系统中传输25km后,信噪比与双边带调制相比可提高约4dB.     

888nm半导体抽运的60W皮秒激光振荡器模型

为了建立一个基于888nm半导体抽运的高平均功率和高效率皮秒激光振荡器的理论模型,采用模拟计算方法和谐振腔理论、ABCD定律、自洽条件以及连续被动锁模条件,利用激光晶体Nd:YVO4对波长888nm半导体抽运源的吸收特性和元件的相关参量、合适的腔模参量、实现稳定锁模的参量进行了理论分析和计算,并通过以上研究和模拟计算得到了皮秒振荡器模型的相关数据。结果表明,在120W的抽运功率下,激光器可以输出约61.5W的皮秒激光,光光转换效率51.3%。这一理论模型的建立对高平均功率和高效率皮秒激光振荡器的实验研究起着指导作用。     

用级联光纤光栅补偿色散在G.652光纤上4×10Gbps无电中继低代价传输640km

用计算机控制扫描法写入的低偏振模色散(PMD＜1.2ps)切趾啁啾光纤光栅级联补偿了4×10Gbps在G.652光纤上传输640km的色散,功率代价不大于1.5dB,光栅引入的损耗小于4dB,并研究了光源波动时系统功率代价的变化.实验表明总的功率代价不与光栅组的数目或光栅组数目的平方根成比例关系,为长距离无中继在G.652光纤传输用光栅补偿色散提供了实验依据,光栅的时延纹波不是各组单个光栅纹波的简单叠加,而是与光栅的位置及EDFA增益有关.     

一种新型的高功率双波长自相似抛物线脉冲激光器

基于正色散单模光纤、掺Yb3+增益光纤和多模光纤光栅,提出了一种可调高功率双波长自相似抛物线脉冲激光器。采用非线性薛定谔方程描述双波长自相似抛物线脉冲的传输和演化过程,并利用分步傅里叶方法进行数值模拟。输入脉冲在正色散单模光纤与掺镱增益光纤中产生自相似抛物线脉冲,多模光纤布拉格光栅对激光器进行双波长选择,通过调节偏振控制器实现双波长稳定运行。数值模拟分析结果表明该激光器的运行是可行且有效的。