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     

Intermodulation and harmonic distortions of laser diodes withoptical feedback

Analytical expressions for the second-order harmonic and the third-order intermodulation distortions of laser diodes with optical feedback are derived. It is shown theoretically and experimentally that optical feedback from the transmission fiber considerably enhances intermodulation and harmonic distortions of laser diodes. Third-order intermodulation distortions up to -15 dBc must be expected for two-tone modulation and low-intensity modulation indexes m≈7% per carrier, which affects the performance of subcarrier multiplexed (SCM) communication systems. To avoid additional distortions the external feedback level must be two orders of magnitudes lower than the critical external feedback level of the laser diode     

Wavelength conversion of SCM signals using semiconductor optical amplifiers: theory, experiments, and applications

In this paper, we deal with the subject of cross-gain modulation (XGM) and cross-phase modulation (XPM) semiconductor optical amplifier-based wavelength conversion of optical channels carrying subcarrier multiplexing signals in a comprehensive way. The equations and models that describe the conversion process and the resulting harmonic and intermodulation distortions are obtained showing the superior performance of XPM over XGM in terms of secondand third-order distortion and contrast ratio. Experimental results for XGM-based wavelength conversion that confirm the results predicted by our theoretical models are presented, and finally, we consider the specific application of wavelength conversion of optical channels carrying full frequency plans such as that of cable television applications.     

Formula for two-carrier intermodulation distortion in wavelengthconverted subcarrier multiplexed signals via cross gain modulation

We present, for the first time to our knowledge, closed expression for the computation of the harmonic and intermodulation distortions that appear on a wavelength converted two-tone subcarrier modulation (SCM) signal via cross gain modulation     

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     

Application of DBR mode-locked lasers in millimeter-wavefiber-radio system

An actively mode-locked laser (MLLD) integrated with a distributed Bragg reflector (DBR) was used as a light source for optical subcarrier transmission. The millimeter (mm)-wave power penalty due to fiber dispersion is suppressed within 4 dB by operating this laser in a symmetric four-mode configuration. The experimental results agree well with the calculation of dispersion-induced penalty for a four-component-lightwave source. Optical subcarrier transmission free from dispersion-induced penalty within experimental error is achieved by further suppression of the end-modes of the DBR-MLLD using a fiber Bragg grating (FBG)     

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     

Analysis of chromatic dispersion-induced second-harmonic distortions including laser dynamics to the second order

We perform analysis of chromatic dispersion-induced second-harmonic distortions including laser diode dynamics. Investigating laser diode dynamics fully to the second order, we derive analytical expressions for intensity and frequency modulation indices. Using these, we accurately model chromatic dispersion-induced second-harmonic distortions for signals produced by a directly modulated distributed feedback laser. The results agree well with numerical solutions, as well as with experimental results for a wide range of modulation frequencies and fiber lengths.     

On the nonlinear distortion of subcarrier multiplexed systems owing to stimulated Brillouin scattering

Analytical expressions are obtained for predicting the harmonic and intermodulation performance of subcarrier multiplexed systems owing to stimulated Brilloun scattering. These expressions are in terms of the ordinary Bessel functions with arguments depenedent on the modulation index.     

A subcarrier multiplexed CDM optical local area network, theory andexperiment

A hybrid scheme is proposed for the purpose of suppressing the effects of external modulation and/or laser nonlinearities in subcarrier multiplexing (SCM) fiber optic communications systems. Hybrid CDMA/FDMA combines two schemes in such a way that the resulting hybrid network is robust against interference and is much more spectrally efficient than a CDMA system. Several possible architectures are introduced for the hybrid CDMA/FDMA subcarrier fiber optic local area network (LAN). The networks utilize CDMA and SCM, an asynchronous multiple access scheme. Direct sequence spread spectrum multiple access (DS/SSMA), the most common form of CDMA is employed. It is shown that by using the code sequence sets for which the shift-and-add property holds, intermodulation products (IMPs) and harmonics have a similar interference-like effect as nonmatching sequences do. Owing to the fact that the shift-and-add property holds for conventional spreading sequences, suppression of nonlinear distortions is examined. An average error probability performance evaluation of the selected configuration is presented for a transceiver pair. In analysis of the system, the authors assume the interference term arising from other users is Gaussian distributed. The results are compared to that obtained by exact evaluation of interference distribution using Gauss quadrature rule integration method. The authors compare the performance of this scheme for two different code sequence lengths. They also present some preliminary experimental results on the proposed LAN implementation as well as its measured transmission performance     

Signal extraction with frequency arrangement (SEFA) andsuperimposed subcarrier modulation (SSM) schemes in fiber-orientedwireless access systems

This paper describes technologies for the improvement of the noise and nonlinearity characteristics of wireless access systems based on subcarrier optical transmission. First, the superimposed subcarrier modulation (SSM) technique is proposed to reduce the reflection noise induced by fiber connectors. We experimentally confirmed that the proposed method reduces this noise, and we found the optimum frequency and power of the superimposed subcarrier, SSM design methodology is also presented, taking both noise and nonlinearity into account. Second, signal extraction with frequency arrangement (SEFA) is proposed for increasing the optical modulation index, because the third-order intermodulation distortion (IM3) produced by the optical devices can be ignored. Simulation results show that SEFA greatly improves the transmission performance in terms of both the ratio of desired-to-undesired signal and the receiver sensitivity under Rayleigh fading. Combining the SSM and SEFA is also considered, for use with FP-LDs or DFB-LDs. These technologies will enable cheaper FP-LD implementation in fiber-oriented wireless access systems     

微波模拟信号调制下VCSEL腔结构参数的优化设计

以垂直腔面发射半导体激光器(VCSEL)为研究对象,采用Volterra方法结合速率方程理论,从器件设计角度,分析了在5 GHz微波信号模拟调制情况下出射面反射镜的反射率、有源区厚度和光限制因子对其二阶谐波失真、三阶谐波失真和三阶互调失真的影响.结果表明,对于垂直腔面发射半导体激光器,采取大反射率、小有源区厚度和小光限制因子有利于降低器件的谐波和互调失真.     

Impact of nonlinear distortion in radio over fiber systems with single-sideband and tandem single-sideband subcarrier modulations

In radio over fiber (RoF) systems, two subcarrier modulations (SCMs), i.e., single-sideband (SSB) and tandem single-sideband (TSSB), have been widely used. Both SSB and TSSB SCMs can be obtained using optical Mach-Zehnder modulators. In this paper, the authors theoretically investigate the impact of harmonic distortion and intermodulation distortion in RoF systems for one wavelength carrying two radio frequency (RF) signals with either SSB or TSSB SCM. It is found that nonlinear distortion can be reduced when the frequency difference of two RF signals is /spl sim/ 1 GHz compared with /spl sim/ 2 GHz and higher for both TSSB and SSB SCMs, particularly for large modulation indexes. When the frequency difference of RF signals is /spl sim/ 2 GHz and beyond, either a small modulation index or a medium modulation index combined with a minimum RF of 6 GHz must be used, and a large modulation index cannot be used due to serious nonlinear distortion. Comparison of SSB and TSSB SCMs with uniform distribution of optical signal subcarriers in a dense wavelength-division-multiplexing RoF system with channel spacing of 12.5 GHz shows by simulation that SSB SCM is better (worse) than TSSB SCM for small (large) modulation indexes.     

微腔效应对垂直腔面发射激光器谐波和互调失真的影响

采用速率方程与Volterra变换方法,分析了垂直腔面发射激光器在微波模拟调制情况下,频率、自发辐射因子以及自发辐射寿命对三阶谐波和三阶互调失真的影响.结果表明垂直腔面发射激光器的非线性失真依赖于自发辐射物理量的变化,对激光器的自发辐射进行适当控制,可以抑制非线性对性能的影响,扩大其最大调制频率,提高基于VCSELs器件的ROF(radio over fiber)系统的性能和应用范围.     

Nonlinear distortions and their cancellation in transistors

A linear two-port network model of the intrinsic transistor, which accounts for the nonlinear effects at the emitter-base junction, is developed in this paper. From this model nonlinear distortions are calculated and their variations with frequency and operating point are accounted for. With extrinsic parameters incorporated into this model it is shown how third-order distortion cancellation results from their effect. Two-signal injection, which permits the maximum conversion gain for the transistor as a mixer to be developed along with figures of merit for harmonic distortions, cross modulation, and intermodulation, is considered.     

Nonlinear distortion generated by DSF-based optical-phase conjugators in analog optical systems

In this paper, the nonlinear distortion induced by dispersion-shifted-fiber-based optical-phase conjugators (OPCs) in intensity-modulated subcarrier multiplexing optical systems is evaluated. In this study, it is shown that the nonlinear distortion mainly arises from the four-wave mixing (FWM) process during the phase conjugation, although there also exists influences from other nonlinear effects. Closed expressions for calculating the second- and third-order harmonic distortions due to FWM, self-phase modulation, cross-phase modulation and group-velocity dispersion effects in the dispersion-shifted-fiber-based OPC are also reported for the first time. The influence of several system design parameters, such as the optical modulation index, the number of channels, the input optical powers, and the effective area of the dispersion-shifted fiber on the compensation of fiber-induced nonlinear distortions employing the optical-phase conjugation technique, is considered.     

Intermodulation properties of double- and triple-circuit parametric amplifiers with nonlinear barrier capacitance

There have been performed an experimental investigation of intermodulation effects in narrowband regenerative double- and triple-circuit parametric amplifiers (RDPA and RTPA) with nonlinear barrier capacitance. It has been obtained that their single-signal and double-signal intermodulation characteristics are of the conventional form, and their phase shifts in the vicinity of the resonance frequency are close to 90°, that is not enough for the compensation of nonlinear distortions in the case of amplifiers cascading. It has been demonstrated that the introduction of the additional oscillatory circuit at the double signal frequency into RDPA does allow effective compensation of the intermodulation interferences of odd order in the single-stage parametric amplifier.     

Improvements in fiber-optic transmission of multicarrier TV signal

When the number of TV channels is high, a significant harmonic and intermodulation distortion arises. These distortions can be reduced by improving the modulation linearity of the laser or by applying a more appropriate modulation method. Both solutions are discussed and evaluated in detail. The linearity has been significantly improved by applying an active matching technique. The improvement achieved by the new driving circuit over the conventional one is 15 dB considering the third harmonic distortion. The sensitivity of QPSK TV transmission to disturbing interferences has been investigated. A so-called group modulation method offers a better approach for optical multicarrier TV transmission     

Transmitter linearity considerations for digital TV broadcasting

An introduction is given to the principles of digital modulation as it is being applied to advanced television by comparison with the way in which chrominance information modulates the NTC color subcarrier. The concept of the unit circle in constellation diagrams is explained for both signal-to-noise ratio and signal-to-interference. Using constellation diagrams, the effects of AM/AM and AM/PM and their combined effects are shown in terms of reducing noise/interference margins. it is seen that the dynamic range of the ATV transmitter must be sufficient to handle the transient peak power which results from filtering to constrain the data to the 6-MHz channel. Out-of-channel spectral components are generated by intermodulation in the high-power amplifier, as are in-channel intermodulation components which are responsible for reducing the eye opening. This results in reduction in noise/interference margins. It is shown that nonlinear amplification of the transient peaks results in intermodulation products that may cause interference to a signal on adjacent channels and reduce expected coverage     

啁啾高斯脉冲在光纤中传输的脉冲展宽研究

通过求解非线性薛定谔方程，研究了线性光纤中色散导致的具有初始频率啁啾的高斯脉冲展宽的详细物理过程。得到高斯脉冲在光纤中色散所致的脉冲展宽的特性，啁啾因子对脉冲展宽的影响，并讨论了光纤色散对不同宽度脉）中的影响，对色散补偿等技术的研究具有一定的参考价值。