Effect of modulator chirp and sinusoidal group delay ripple on the performance of systems using dispersion compensating gratings

The implications that the nonideal characteristics of a dispersion compensating grating have on system performance are determined, for the most part, by the group delay ripple (GDR) of the grating response over the modulated signal bandwidth. Since the GDR typically exhibits an irregular variation with wavelength that has periodic features, it is convenient to use a sinusoidal variation to assess the implications on system performance. The portion of the grating bandwidth occupied by the modulated optical signal is determined by the carrier signal wavelength, bit rate, modulation format, and modulator chirp. The implications of modulator chirp on the performance of 10-Gb/s dispersion compensated systems are considered. Using a LiNbO/sub 3/ Mach-Zehnder modulator, an electroabsorption modulator, and a multiple quantum-well Mach-Zehnder modulator with distinct chirp properties, the results demonstrate that to accurately assess the implications of GDR, the properties of the modulator chirp must be considered. In particular, results for chirp-free optical signals underestimate the implications of the GDR on system performance.     

Power penalty due to the amplitude and phase response ripple of a dispersion compensating fiber Bragg grating for chirped optical signals

A concise method is presented for rigorously calculating the power penalty due to the combined implications of the amplitude and phase response ripples of a dispersion compensating fiber Bragg grating and the chirp of the transmitted optical signal. By using trigonometric series to represent the ripples, the calculated penalty can be positive or negative, as obtained in numerical simulations and measurements, depending on the signal chirp and ripple within the modulated signal bandwidth. An approximate upper bound on the power penalty is also presented as an extension of earlier results that always yield positive penalties. Calculated and measured results are compared for two 10-Gb/s return-to-zero (RZ) signals with distinct chirp properties.     

Characterization of the chirp and intensity modulation propertiesof an SOA-MZI wavelength converter

Wavelength converters based on cross-phase modulation in semiconductor optical amplifiers (SOAs) in the arms of a Mach-Zehnder interferometer (MZI) provide a key network element for reconfigurable optical networks that incorporate wavelength routing. For system design purposes, the properties of the intensity and phase (chirp) modulation of the wavelength-converted signal for an SOA-MZI wavelength converter are important. In this paper, the small-signal α parameter, which characterizes the converter chirp, and the conversion frequency response are characterized experimentally for an integrated all-active SOA-MZI wavelength converter. For both co- and counterpropagating signals, the variation of the a parameter along the interference and conversion curves is considered in detail. Three optical modulators with quite different chirp properties are used to generate the modulated input signal to the wavelength converter. The results demonstrate that the chirp of the wavelength-converted signal is primarily determined by the device properties and the intensity of the modulated input signal. The a parameter of the wavelength-converted signal is negative for noninverting operation and positive for inverting operation. An important contribution of the paper is the detailed assessment of this key device characteristic. The experimental characterization of the wavelength converter is incorporated into a device model that can be used to obtain the pulse response. Calculated and measured results for the time dependence of the intensity and chirp of the wavelength-converted signal are in good agreement. The conversion frequency response for the intensity modulation is also measured along the conversion curve. The 3-dB bandwidth is found to be generally about 8-10 GHz     

Optimizing the bias and modulation voltages of MQW Mach-Zehndermodulators for 10 Gb/s transmission on nondispersion shifted fiber

The chirp and optical extinction ratio of a multiple quantum-well (MQW) Mach-Zehnder modulator depend on the device design and on the voltage waveforms applied to the arm electrodes. For 10 Gb/s transmission over nondispersion shifted fiber, joint optimization of the bias and modulation voltages is considered for a conventional modulator and a π-phase-shift modulator. Measured attenuation and phase constants for an optical signal propagating in the modulator waveguide are used to accurately model the Mach-Zehnder modulators. The influence of asymmetric Y-branch waveguides in the modulators is examined taking into consideration group velocity dispersion and self-phase modulation arising from the Kerr nonlinearity. When the modulators are operated with maximum optical extinction ratio, the dispersion limited transmission distance depends on the device design (phase-shift and Y-branch splitting ratio) and modulation format (dual drive or single drive). Optimization of the bias and modulation voltages reduces this dependence significantly, while also increasing the dispersion limited transmission distance     

Transmission performance of 10-Gb/s optical duobinary transmissionsystems considering adjustable chirp of nonideal LiNbO3Mach-Zehnder modulators due to applied voltage ratio and filterbandwidth

We investigate interplay between the residual and applied chirp of optical duobinary modulated signals in order to improve transmission performance. To find the best performance for 10-Gb/s optical duobinary transmission systems, we consider the residual chirp accompanying from the finite extinction ratio, the applied chirp adjusted by the applied voltage ratio (the chirp parameter) between two electrodes of LiNbO₃ modulators, and the bandwidth of electrical low-pass filters used in duobinary transmitters. The simulation results suggest that nearly zero chirp during the mark (`1') period and large peak chirp at the middle of the space (`0') provide the best transmission performance. This zero chirp around marks and high peak chirp at the middle of each space can be controlled by the applied voltage ratio between two electrodes of modulator and the filter bandwidth, respectively     

LiNbO/sub 3/ Mach-Zehnder modulators with fixed negative chirp

Negative chirp modulation of optical transmitters is needed for the optimal performance of high bit-rate transmission in optical fiber systems operating at nonzero dispersion window at 1.5 /spl mu/m. In this paper several approaches to achieving any fixed chirp value for LiNbO/sub 3/ modulators are proposed, Experimental results of impedance matched devices on x-cut lithium niobate with chirp values of /spl plusmn/0.7 and /spl plusmn/1.0 are presented. By tuning the modulators bias point, the chirp value can be adjusted linearly around the designed value without degrading other performances.     

Simple measurement of fiber dispersion and of chirp parameter ofintensity modulated light emitter

We report a novel method to measure two important parameters for optical fiber intensity modulated transmission systems: dispersion of optical fibers and chirp parameter of modulated light emitters. The method is easy, quick, and accurate for chirp parameter in the -10-to-10 range     

Dispersion Effects in Millimeter-Wave Fiber-Radio Systems Employing Direct-Sequence Code Division Multiple Access

We investigate the impact of fiber chromatic dispersion on the performance of an externally modulated millimeter-wave fiber-radio system incorporating a broadband radio direct-sequence code division multiple access (DS-CDMA) scheme. In particular, we investigate the effect of dispersion on DS-CDMA signals with regard to CDMA code rate, center frequency, and modulator chirp. We show that the spreading of the signal spectrum using DS-CDMA can result in significant reductions in data amplitude variations normally experienced in externally modulated millimeter-wave fiber-radio systems. We also demonstrate the successful transmission of a 10-Mb/s data channel over a 39-GHz fiber-radio link employing DS-CDMA and consisting of 25 km of standard optical fiber and a 1-m radio link.     

Modulation characteristics of semiconductor Mach-Zehnder opticalmodulators

The transmission and chirp characteristics are described for two types of semiconductor Mach-Zehnder modulators, distinguished by the differential phase shift between the two arms of the interferometer in the unbiased state. The conventional modulator has a differential phase shift of 0 radians, while the π-shift modulator has a differential phase shift of π radians. The nonlinear dependence on the applied voltage of the attenuation and phase constants of the optical signal propagating in the p-i-n waveguide leads to different characteristics for the two modulators. The influence of the splitting ratio of the Y-junctions is considered for single-arm and dual-arm (push-pull) modulation formats. The π-shift modulator is shown to yield better transmission performance for 10 Gb/s systems compared to the conventional modulator     

Generation of 40 GHz phase stable optical short pulses using intensity modulator and two cascaded phase modulators

Pulse sources based on lithium niobate modulators are very attractive for optical time division multiplexing (OTDM) transmission systems because the modulators are now commercially available,qualified for system use,and can operate up to very high speeds and over a wide wavelength range.In this paper,we describe the principles of operation and performance of the pulse source based on lithium niobate modulators.The pulse source is based on a Mach-Zehnder intensity modulator (IM) and two phase modulators (PMs).The continuouswave (CW) light is modulated in an IM and then strongly phase modulated in two cascaded PMs.The chirped pulses are subsequently compressed to desired width using dispersion compensation technology.This method has the advantage of acquiring larger chirp using normal PM rather than that special designed PM of very low Vπ.It can also generate shorter pulses than conventional methods incorporating only one PM driving by a radio frequency (RF) signal with the power larger than 1 W which may damage the device.Generation of 40 GHz optical pulses shorter than 2 ps is theoretically illustrated,simulated and experimentally verified.Experimental results show that 40 GHz phase stable optical pulses with pulse-width of 1.88 ps,extinction ratio (ER) larger than 20 dB,the timing jitter of 57 fs and signal-to-noise ratio (SNR) of 32.8 dB can be achieved.This is also a cavity-less pulse source whose timing jitter is determined only by the RF source rather than by the actively controlled cavity.In the experiment,the phase noise of the RF source we used is as low as -98.13 dBc/Hz at a 10 kHz offset frequency which resulting very low timing jitter of generated pulses.The pulses are then modulated at 40 Gbaud/s with an inphase/quadrature (l/Q) modulator and multiplexed to 160 Gbaud/s with less interference between each other.After back-to-back demultiplexing by an electro-absorption modulator (EAM) to 40 Gbaud/s and demodulation by a delay interferometer (DI),clear and opened eye diagrams of 40 Gbaud/s I and Q tributary signals are obtained which verify the good performance of generated pulses in the 160Gbaud/s differential quadrature phase shift keying (DQPSK) OTDM system and further prove the phase stability and high quality of generated pulses.     

30-gb/s signal transmission over 40-km directly modulated DFB-laser-based single-mode-fiber links without optical amplification and dispersion compensation

Based on a recently proposed novel optical-signal-modulation technique of adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM), numerical simulations of the transmission performance of AMOOFDM signals are undertaken in directly modulated DFB laser (DML)-based single-mode-fiber (SMF) links without optical amplification and dispersion compensation. It is shown that a 30-Gb/s transmission over a 40-km SMF with a loss margin of greater than 4.5 dB is feasible in the aforementioned simple configuration using intensity modulation and direct detection (IMDD). In addition, the DFB-laser frequency chirp and the transmission-link loss are identified to be the key factors limiting the maximum achievable transmission performance of the technique. The first factor is dominant for transmission distances of < 80 km and the second one for transmission distances of > 80 km. It is also observed that fibers of different types demonstrate similar transmission performances, on which fiber nonlinear effects are negligible.     

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     

Chromatic dispersion tolerance of new duobinary transmitters based on two intensity modulators without using electrical low-pass filters

This paper describes an investigation of the effect of the extinction ratio and the chirp parameter of the first intensity modulator on the transmission performance of a proposed duobinary transmitter based on two intensity modulators without using electrical low-pass filters. This modulation scheme can generate duobinary signals using only two-level electrical signals without the conversion process to three-level electrical signals using electrical low-pass filters. The simulation results suggest that the appropriate chirp and extinction ratio of the first intensity modulator provide larger dispersion tolerance, compared with the conventional duobinary transmitter. The duobinary transmitter based on two intensity modulators can increase the dispersion tolerance without using electrical low-pass filters.     

Transmission performance of chirp-controlled signal by usingsemiconductor optical amplifier

We examine the fiber transmission performance of the optical signal whose chirp is controlled by utilizing phase modulation in semiconductor optical amplifier (SOA) with both simulations and experiments. This chirp control technique converts a positive chirp created by electroabsorption (EA) modulator into negative chirp, which reduces the waveform degradation due to the chromatic dispersion in transmission over standard single-mode fiber (SMF). It also provides an optical gain that is sufficient to compensate the insertion loss of the EA modulator. We investigate how the chirp control is affected by the input power to the SOA and the carrier lifetime of the SOA. As the SOA input power increases, the negative chirp becomes large, while the waveform is largely distorted due to gain saturation. However, the waveform distortion at high SOA input powers can be shaped by using a frequency discriminator. The acceleration of the carrier lifetime also reduces the waveform distortion due to gain saturation. We demonstrate that the chirp control technique is effective even for a high bit rate optical signal up to 10 Gb/s, when the carrier lifetime is expedited by optical pumping     

Chirp and system performance of integrated laser modulators

Time-resolved chirp measurements and laser-threshold measurements are used to understand the chirp performance of integrated laser electroabsorption modulators. The different effects of the intrinsic modulator chirp component and chirp caused by optical feedback, from reflection at the output facet into the laser, are observed. A figure of merit is introduced to assess the chirp performance, which shows good correlation with dispersion penalty measurements. The effect of reflection-induced chirp is considerably reduced by operating the modulator with negative chirp. An optimized device is reported, which delivers purely negative chirp with reasonable optical power     

The influence of fiber dispersion on the transmission performance of a quadruple-frequency optical millimeter wave with two signal modulation formats

This paper reports investigations of the transmission performance of a 40 GHz dual-tone optical millimeter (mm) wave by quadrupling a radio-frequency local oscillator via a single dual-electrode Mach–Zehnder modulator based on the transmission function of the dispersive fiber. The theoretical analysis and simulation results show that, although the fiber chromatic dispersion does not cause a fading effect on the optical mm wave with the two signal modulation formats, the bit walk effect caused by the time shift of the sidebands due to fiber dispersion limits its transmission distance as the signal is modulated on the two tones. The limitation of transmission distance caused by the bit-walk effect can be overcome if the signal is modulated on only one of its two tones, and, in this way, the transmission distance is greatly extended. Moreover, the dual-tone data modulation format is more suitable for a radio-over-fiber link with transmission distance less than 40 km owing to the good signal performance and ease of implementation, while single-tone data modulation can be applied for a longer fiber link, even up to 120 km.     

Novel and simple model of 10-Gb/s electroabsorption modulated lasers and its experimental validation of transmission performance due to overshoot of optical signals

We have experimentally and theoretically investigated the transmission performance of 10-Gb/s electroabsorption modulated lasers (EMLs) due to the overshoot of optical pulses. When a highly negative bias voltage is applied to EMLs, the overshoot becomes larger due to nonlinear transfer curves of EMLs. In order to further understand the overshoot effect of optical pulses from EMLs on transmission performance, we propose a novel and simple EML model based on the frequency response (magnitude and phase) and the transfer curves (P-V and /spl alpha/-V) of EMLs. Although the model does not solve the rate equations and the wave equations, it can accurately predict output pulse shapes and the frequency chirp as well as the transmission performance with reducing simulation time. Using the EML model, we can calculate the overshoot and dispersion power penalty due to modulation bandwidth and group delay difference in 10-Gb/s EMLs. Our results suggest that the overshoot should be considered to accurately predict the transmission performance of 10-Gb/s EMLs.     

Simultaneous compensation of laser chirp, Kerr effect, anddispersion in 10-Gb/s long-haul transmission systems

The effects of chirp and fiber nonlinearity in a directly modulated 10-Gb/s intensity-modulated direct-detection (IM-DD) fiber transmission system are investigated by simulation, and a simple and flexible technique for compensating these effects is discussed. Self-phase-modulation (SPM) in optical fiber can be equalized by an anomalous dispersion fiber, whereas pulse broadening caused by laser transient chirp can be compensated by normal dispersion. Using these characteristics, laser transient chirp, SPM, and fiber dispersion can be simultaneously compensated by equalizing fibers inserted within certain intervals. Optimum compensation is always realizable for such fixed equalizing fibers, since the magnitude of SPM can be controlled by changing the optical power in the fiber. Simulation suggests that this technique enables 10-Gb/s, 100-km fiber transmission by direct modulation     

Tunable Chirp Managed Laser

We demonstrate a compact tunable chirp managed laser (CML) comprising a radio-frequency switch, a directly modulated distributed feedback laser array, a microelectromechanical systems mirror, and an optical filter and obtain error-free transmission performance over 30 nm at 10.7 Gb/s up to 200 km without electrical dispersion compensation (EDC) or optical dispersion compensation. Transmission distance is stretched to 300 km over the same wavelength range by adding a standard EDC to the receiver.     

基于级联马赫-曾德尔调制器的太赫兹通信系统

为了有效解决太赫兹通信系统中信号难以调制,影响通信系统性能的问题,提出了一种基于级联马赫-曾德尔调制器的太赫兹通信系统。将要传递的伪随机不归零码与频率为10GHz的射频本振信号混频后调制到级联马赫-曾德尔调制器上,通过调节两个调制器的偏置电压,使其分别偏置在最大传输点和最小传输点上,得到的光载波信号经过光放大器放大,结合高非线性光纤的四波混频效应,利用相移布喇格光栅进行模式选择,经过光电转换后的太赫兹信号通过基带数据恢复,可以得出该太赫兹通信系统传输的误比特率。结果表明,基于级联马赫-曾德尔调制器结构可以将太赫兹信号的产生与调制结合到一起。该研究对太赫兹通信系统的实用化有一定借鉴意义。