Erbium-doped fiber amplifier second-order distortion in analog links and electronic compensation

The authors study the second-order distortion when an erbium-doped fiber amplifier (EDFA) is used to amplify the analog optical AM cable TV (CATV) multiple carrier signal from a directly modulated distributed feedback (DFB) semiconductor laser. Experimentally, it was seen that this second order distortion depends critically on the gain of the EDFA fiber amplifier. The authors attribute this distortion to the interaction between the frequency chirping of the DFB laser and the variable gain with wavelength of the amplifier. The authors describe an electronic predistorter that compensates the nonlinearity produced by the DFB-laser-EDFA combination. As a result, the high power advantage of the EDFA can be fully realized in spite of the potential for second-order distortion in the system.<>     

Second-order distortion of amplified intensity-modulated signalswith chirping in erbium-doped fiber

Second-order distortion produced in an erbium-doped fiber amplifier (EDFA) is investigated theoretically and experimentally. Rate equations for intermodulation products are led by the perturbation method. From numerical calculation, it is shown that optimum combinations of input power to EDFA and signal wavelength exist that give a distortion-free condition, Improvement of the composite second-order (CSO) distortion, which results from the cancellation of the laser distortion and the gain tilt induced distortion, was confirmed for VHF multichannel transmission. Furthermore, CSO distortion-free transmission in cascaded EDFA's has been demonstrated for a 42-channel AM-SCM signal by experimentally determining the optimum operating condition of the EDFA such that each amplifier generates no additive distortion     

Compensation for fiber-induced composite second-order distortion inexternally modulated lightwave AM-SCM systems using optical-phaseconjugation

Compensation for composite second-order (CSO) distortion due to the interplay of fiber chromatic dispersion and self-phase modulation in externally modulated amplitude modulated subcarrier multiplexing (AM-SCM) systems employing optical-phase conjugation (OPC) is investigated. The simulation results show that significant reductions (>20 dB) of fiber-induced CSO may be achieved using OPC, which results in an increase of the transmission distance in AM-SCM video distribution systems operating near 1550 nm. In addition, a comparison with another compensation technique, the phase modulation technique, is established, showing that OPC is much less sensitive to variations in the fiber length. However it is also calculated that for practical implementation of the OPC-based compensation technique, 25 to 30 dB reduction in the amplified spontaneous emission (ASE) associated with the OPC will be required     

Cancellation of second-order distortion of directly modulated laserin erbium-doped fiber amplifier

Experimental verification of cancellation of second-order distortion from a directly modulated laser diode by amplification in an erbium-doped fiber is demonstrated. This cancellation occurs when the distortion of laser diode and the distortion generated in the erbium-doped fiber amplifier are out of phase     

Polarization dependent distortion in AM-SCM video transmissionsystems

The second-order intermodulation distortion product, namely composite second-order (CSO) distortion, of an AM-SCM video transmission system can be caused by the optical isolators, cascaded fiber amplifiers, or the fiber transmission cable employed. Moreover, the absolute distortion value depends on the polarization angle of light input to the optical isolators, cascaded fiber amplifiers, and fiber transmission line. The degraded distortion values are measured and their mechanisms are clarified     

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     

Numerical simulation of clipping induced distortion in externallymodulated lightwave AM-SCM systems

For the first time to our knowledge the results of numerical simulation, and experimental verification, of clipping induced distortion in externally modulated lightwave AM-SCM CATV systems are presented. Comparison of these results with clipping induced distortion in directly modulated systems show a remarkable outcome: under the same modulation conditions the composite triple beat (CTB) distortion component in externally modulated systems is 6 dB higher than in directly modulated systems     

Fundamental limitations in EDFA-based subcarrier-multiplexed AM-VSBCATV systems

The authors present and discuss fundamental limitations for erbium-doped fiber amplifier (EDFA)-based subcarrier-multiplexed AM-VSB CATV distribution systems. The impairments they consider are shot noise, amplifier spontaneous emission noise, and laser clipping-induced nonlinear distortion. Treating the amplifier input signal power and pump power as resources, they obtain limits on the number of receivers to which one can deliver, say, 60 (or 80) channels at a carrier-to-noise ratio (CNR) of 55 dB (or 48 dB). A simple approach that can be used to recalculate the results presented here for different system and amplifier fiber parameters is presented     

人眼安全高重频窄脉宽单模全光纤激光器特性研究

介绍了基于主振荡功率放大结构的人眼安全全光纤激光器.首先对比了电光调制及直接调制产生的种子激光在百kHz重复频率、纳秒级脉冲宽度的激光放大器中优缺点,综合系统需求选择直接调制方式;之后对窄脉冲单模放大中出现的脉冲分裂现象进行了研究,选用10 m纤芯的双包层铒镱共掺光纤,仅通过两级放大即获得了1 550 nm,重复频率为200 kHz,脉冲宽度为4.07 ns,峰值功率为1.02 kW的单模激光输出.具有结构紧凑、稳定可靠的特点,可用于三维视频激光雷达.     

Reduced intermodulation distortion in 1300 nm gain-clamped MQWlaser amplifiers

A 1300 nm gain-clamped DFB multiple quantum well laser amplifier with negligible pass band ripple, 20 dB fiber to fiber gain, and 10 dB reduction in gain saturation is demonstrated. The remaining gain saturation is attributed to longitudinal hole burning. After some modifications the reduction in gain saturation is improved to more than 30 dB for an input signal having the same polarization state as the lasing mode. From these experiments and a theoretical analysis it is concluded that there is a potential for realizing highly linear 1300 nm CATV semiconductor laser amplifiers using gain-clamping with less intermodulation distortion than today's directly modulated linear semiconductor lasers     

Cumulative waveform distortion in cascaded optical amplifierrepeaters for multigigabit IM/DD systems

An analysis was conducted of a cumulative pattern-dependent waveform distortion in cascaded semiconductor laser and Er³⁺-doped fiber amplifiers. At 2.5 Gb/s, cumulative waveform distortion limits the number of cascaded amplifiers to about 20 for the semiconductor amplifiers. The Er³⁺-doped fiber amplifier is relatively unaffected-over 100 stages can be cascaded. The Er³⁺ amplifier is seen to be the better choice for long-haul multigigabit systems     

基于啁啾脉冲放大的掺铒全光纤结构激光器

为了使全光纤结构啁啾脉冲放大实现超短激光的脉冲输出,采用色散补偿光纤对种子脉冲进行了展宽,利用普通单模光纤来实现放大后脉冲的压缩。对整体激光系统进行了理论分析和实验验证,获得了420fs的超短激光脉冲,平均功率达到1.81W。并利用周期极化的铌酸锂晶体对放大激光脉冲进行倍频,将激光波长拓展到近红外的780nm附近,光谱半峰全宽达到11nm。结果表明,在全光纤结构的掺铒光纤激光系统中,可以通过改变色散光纤的插入量对脉冲展宽和压缩过程中光谱畸变进行有效的控制。这一结果对于实现高功率的全光纤超短脉冲放大系统是有帮助的。     

High-power Nd-doped fiber amplifier for coherent intersatellitelinks

Intersatellite optical communication links require laser beams of at least 1 W of power modulated at gigahertz frequencies. They can be delivered by a master oscillator power amplifier system which consists of a stable single-frequency Nd:YAG laser, a high-speed phase modulator, and an optical amplifier. To allow coherent detection, the polarization state must be stable and controllable at the amplifier output and the phase noise must be kept low. We report here the design and the experimental results of a diode laser-pumped double clad Nd-doped fiber amplifier with polarization control, 30-dB gain, and about 1.3 W of output power     

Using equalizers to offset the deterioration in SCM videotransmission due to fiber dispersion and EDFA gain tilt

The carrier-to-noise ratio (CNR) and distortion of a 1.55 μm amplitude-modulated subcarrier-multiplexed (AM-SCM) video transmission system are worsened by the dispersion of the 1.3 μm zero-dispersion fiber. Distortion is also increased by the EDFA gain tilt. Both problems are shown to be corrected by the use of dispersion equalizers and gain tilt equalizers. The application of the equalizers to SCM video transmission systems is proposed     

AC and DC gain tilt of erbium-doped fiber amplifiers

AC gain tilt and DC gain tilt are defined as the variable gain of an erbium-doped fiber amplifier (EDFA) with input laser wavelength chirp due to modulation, and with different static laser unmodulated wavelengths of no modulation. This paper proposes a novel experimental setup for measuring AC gain tilt. This paper also clarifies experimentally and theoretically the dependency of the AC gain tilt on the input light modulation frequency from near DC to high frequency. The measured AC gain tilt values at high frequency resemble the measured spontaneous gain tilt values for input wavelengths from 1530 to 1580 nm. The influence of the AC gain tilt effect on AM-SCM video transmission systems is evaluated     

Simultaneous suppression of stimulated Brillouin scattering andinterferometric noise in externally modulated lightwave AM-SCM systems

Externally modulated AM-SCM lightwave systems employing optical power amplifiers, small linewidth DFB lasers and long optical feeder lines, face low stimulated Brillouin scattering threshold powers and phase-to-intensity noise conversion by multipath interference through Rayleigh scattering. We show that these problems can be effectively counteracted simultaneously without introducing additional intermodulation distortion, both by single tone phase modulation and by dithering of the laser's optical frequency     

Implementation and evaluation of a dual parallel linearizationsystem for AM-SCM video transmission

An externally modulated transmitter was linearized for AM subcarrier multiplexing (AM-SCM) video transmission. Linearization was accomplished by combining the outputs of two lithium niobate Mach-Zehnder interferometers which were simultaneously modulated. By controlling the optical and RF power at each modulator, more than 20 dB of third-order distortion cancellation was demonstrated for a multitone video spectrum. Results are presented for both NTSC and PAL B/G frequency plans. The effects of modulator nonideality on system performance are analyzed and confirmed experimentally. In particular, overall system performance is shown to be very sensitive to acoustic effects and to optical and electrical multipath interference. System modeling is used to provide tolerances on various critical parameters     

Multifrequency laser having integrated amplified output coupler forhigh-extinction-ratio modulation with single-mode behavior

We present a 16-channel waveguide grating router multifrequency laser that can be modulated with a 250-MHz bandwidth, a 50-dB extinction ratio, and constant single-mode behavior. We demonstrate a means for immunizing the singlemode lasing against unwanted backreflections from the integrated modulation amplifier. The laser is especially suitable for fiber dispersion-map measurements     

澄清关于有线电视放大器的几种模糊认识

目前有线电视技术人员对有线电视放大器的模糊认识主要有：放大器的增益可以调节；放大器的非线性失真只与输出电平有关，与输入电平无关；放大器也能放大模糊的电视信号；放大器倾斜输出能改善有线电视系统的CTB指标。对此，经过分析加以澄清，帮助从业人员正确做好放大器的调试、选用与更换工作，确保有线电视网络质量。     

Multichannel AM-VSB television signal transmission using anerbium-doped optical fiber power amplifier

The use of an erbium-doped optical fiber power amplifier in a multichannel amplitude-modulated vestigial-sideband CATV transmission system for potential subscriber loop applications is discussed. Using a color-center laser pump at 1.48 μm, a fiber-to-fiber gain of 11.5 dB (at 1.539 μm), which is 10 dB compressed from the small-signal gain, and a total system power margin of 15 dB were achieved. No intermodulation distortion was introduced by the fiber amplifier. The received video SNR of 40-46 dB was limited by the linearity of the distributed-feedback laser diode used