KEYWORDS: Laser sintering, Modulation, Digital signal processing, Signal detection, Receivers, Polarization, Transmitters, Optical amplifiers, Transient nonlinear optics, Single mode fibers
The capability of a directly modulated laser (DML) can be dramatically enhanced through precise control of the drive current waveform based on digital signal processing (DSP) and a digital-to-analog convertor (DAC). In this paper, a novel method to pre-compensate fiber dispersion for metro and regional networks is described for a bit rate of 10.709 Gb/s using a DML. A look-up table (LUT) for the drive current is optimized for dispersion mitigation. The entries of the LUT are determined based on the effects of the DML adiabatic and transient chirp on pulse propagation, the nonlinear mapping between the input current and the output optical power, and the bandwidth of the DML package. A DAC operating at 2 samples per bit (21.418 GSa/s with 6 bit resolution) converts the digital samples at the output of the LUT to an analog current waveform driving the DML. Experimental results for a bit rate of 10.709 Gb/s and on-off keying demonstrate a transmission reach of 252 km using a DML intended for 2.5 Gb/s operation and 608 km using a chirp managed laser intended for 10 Gb/s operation. Using this approach (DSP + DAC), the generation of 10.709 Gb/s differential phase shift keying (DPSK) and 56 Gb/s 16-ary quadrature amplitude modulation, sub-carrier multiplexed (QAM SCM) optical signals using the direct modulation of a passive feedback laser is also presented. 6-bit DACs operating at sampling rates of 21.418 GSa/s and 28 GSa/s, respectively, was used to generate the requisite analog current waveform.
KEYWORDS: Phase modulation, Signal processing, Optical amplifiers, Polarization, Receivers, Digital filtering, Digital signal processing, Telecommunications, Multiplexing, Optical filters
The performance of polarization multiplexed, quadrature phase shift keying (PM QPSK) and polarization multiplexed
16-ary quadrature amplitude modulation (PM 16-QAM) is considered with an emphasis on the signal processing
algorithms that compensate transmission impairments and implement key receiver functions.
KEYWORDS: Stochastic processes, Error analysis, Telecommunications, Monte Carlo methods, Statistical analysis, Interference (communication), Digital signal processing, Optical filters, Optical communications, Complex systems
Two different semi-analytical methods for error probability estimation in PDM-QPSK optical
communication systems are investigated. We consider the accuracy of a stochastic semi-analytical method
based on Gaussian noise statistics and a deterministic semi-analytical method where the noise probability
density function is estimated analytically against Monte-Carlo simulation. Linear coherent PDM-QPSK
systems with distortions induced by filtering only, and nonlinear coherent PDM-QPSK systems with or
without inline dispersion compensation are studied. Our results suggest that the stochastic semi-analytical
method based on Gaussian noise statistics works very well for practical fiber-optic communication systems.
We investigate the characteristics of 40-GHz all-optical clock recovery based on a distributed Bragg reflector
(DBR) self-pulsating laser. With the injection of a low timing jitter clock signal, the timing jitter characteristics
of the DBR self-pulsating laser are investigated using both time domain and frequency domain methods. The
results reveal that the cause of the timing jitter in the recovered clock signal depends on the injected clock signal
power. The system performance of the clock recovery is investigated by the injection of a 40 Gb/s return-to-zero
on-off key (RZ-OOK) signal with a 231 - 1 pseudo random bit sequence (PRBS) pattern.
KEYWORDS: Modulation, Transceivers, Receivers, Fiber to the x, Signal detection, Clocks, Eye, PIN photodiodes, Semiconductor lasers, Single mode fibers
In this paper we demonstrate the ability of offset sideband modulation (OSBM) to alleviate problems pertaining to the
electrical crosstalk between upstream and downstream signals in a transceiver used in the optical network unit of a FTTx
system. The OSBM signal consists of an optical carrier and an offset modulated sideband that are created using arbitrary
optical waveform generation. The performance of OSBM is investigated in the context of an inline transceiver, which is
a very simple, next generation, FTTx transceiver. The downstream OSBM bit rate was 2.5 Gbit/s and the upstream OOK
bit rate was 1.25 Gbit/s. A power penalty of only 0.2 dB with respect to an interference free system was observed with
an optical signal to interference ratio (OSIR) as small as -8 dB (the interfering signal power is 8 dB larger than the signal
power). This was the minimum OSIR available with the experimental setup, and it is expected that lower OSIRs can be
tolerated.
Wavelength-selective optical components used in WDM optical communication systems often exhibit a transfer function with amplitude and phase ripples. These ripples can lead to signal distortion and performance degradation. The pulse distortion induced by sinusoidal amplitude and phase response ripples is derived for Gaussian pulses, and concise results are presented for the power penalty in system performance. The analysis shows that the amplitude and phase response ripples have a similar impact on the transmitted signal, and the power penalty induced by these ripples depends on the chirp and pulse width of the transmitted signal. The combined effect of the characteristics of the transmitted signal and the ripple parameters on the system performance is discussed in detail. Numerical simulations show a good agreement with the theoretical analysis.
As the next generation of ultra-fast optical transmission systems, the optical time division multiplexing (OTDM) systems with a bit rate up to 160 Gb/s are actively being researched. Several experimental ultra-fast OTDM transmission systems with all-optical 3R-regeneration have been reported. With the development of ultra-fast OTDM technologies, techniques for rigorously evaluating the system performance through the calculation of bit error ratio (BER) have become increasingly important. We report a novel analytical model for estimating the performance of a 160 Gb/s OTDM receiver. The BER of the OTDM system is evaluated based on the calculation of the noise probability density function using the moment generating function. The optical pulse broadening by the fiber dispersion is compensated through the dispersion-managed approach and both nonlinearity and dispersion of the fiber channel are taken into account. Noise (ASE, shot and thermal) and performance-impairing factors (intrachannel interactions and timing jitter) are included in the calculation of the BER. A variational analysis approach is used to solve the optical pulse evolution over a periodical, dispersion-managed, nonlinear fiber channel as this yields an analytical expression for the received optical pulses. The OTDM demultiplexer is modeled as an optical gate controlled by an optical or electrical clock signal and the cyclostationary characteristic of the ASE noise after passing the OTDM demultiplexer is considered. Also the timing jittering between the signal pulse and the gate window and its effects on the signal decision are taken into account. Based on the proposed model, calculated results for the performance of the 160 Gb/s OTDM transmission systems are presented.
A technique is presented for measuring the optical phase transfer function of optoelectronic devices for stimulus frequencies from 100 MHz up to the modulation bandwidth of the device. Using high-resolution optical spectra and a novel instrument setup that makes use of RF signal processing to obtain the stimulus signals, the change in phase of an optical signal is obtained as a function of a time-varying electrical stimulus for electrical-to-optical devices or an optical stimulus for optical-to-optical devices. In the technique, the modulation frequency of the stimulus can be varied over a wide range (e.g., 100 MHz to 10 GHz for a 10 Gb/s device). Thus the proposed technique complements low-frequency and stepped measurements of the optical phase transfer function. The technique is demonstrated by considering the change in phase of the output signal from an electroabsorption modulator as a function of the applied voltage.
The performance of an all-optical regenerator utilizing self-phase modulation in a highly nonlinear fiber and offset optical filtering is assessed using computer simulation. By varying the bandwidth and offset of the optical filter, the Q-factor performance of the regenerator is near-optimized for systems impaired by ASE noise and systems impaired by both residual dispersion and ASE noise. Generally, the near-optimum bandwidth and offset of the optical filter differs for these two types of systems. It is found that the selection of the bandwidth and offset is more stringent for systems with ASE noise only. For systems with residual dispersion and ASE noise, the selection of the filter bandwidth and offset depends on the amount of residual dispersion with quite different trends being observed. The regenerator is more effective when the residual dispersion is negative.
The regenerative properties of using a clock-modulated pump for higher-order four-wave mixing in a highly nonlinear fiber are demonstrated through simulations and experimental measurements, at
10 Gb/s, for 3R optical regeneration and NRZ-to-RZ format conversion. The use of a clock-modulated pump for higher-order four-wave mixing enables retiming and enhances the reshaping properties of the signal, thereby improving the regeneration of the signal, relative to a continuous-wave pump.
A model for a multiple quantum well electroabsorption modulator is developed based on measurements of the escape time of photogenerated carriers, the dependence of the fiber-to-fiber loss on the applied voltage, the dependence of the α-parameter of the modulated signal on the applied voltage, and the intensity modulation
frequency response. The accuracy and computational efficiency of the model in describing the intensity and phase modulation properties of optical signal make it suitable for computer simulations aimed at system design and performance evaluation.
The results of a detailed experimental characterization of a three-port, all-active SOA-MZI wavelength converter are presented. It is shown that for counter-propagating input signals, the ASE noise is intensity modulated due to gain saturation in the output SOA. Consequently, the waveform for the wavelength-converted signal is determined by both cross- phase modulation and self-gain modulation. Time- and frequency-domain measurements are used to characterize the properties of the modulated ASE noise, wavelength-converted signal and total signal. For both co- and counter- propagating signals, the small-signal chirp properties of the wavelength converted along the conversion curve are considered. The measured results for the small-signal chirp and optical conversion are incorporated into a device model that can be used to obtain the large-signal pulse response. Based on this model, good agreement is demonstrated between calculated and measured results for the time dependence of the intensity and chirp of the wavelength-converted signal.
The variation in system performance ofmulti-span lightwave systems that use a cascade ofdispersion compensating gratings
is characterized in terms of the maximum peak to peak group delay ripple over the -1 dB bandwidth of the grating. Results
are presented that specify the requirement for the group delay ripple in order for the system performance to remain within a
specified penalty.
For 10 Gb/s and 40 Gb/s systems, the implications of the reflection and group delay spectra of a non-ideal dispersion compensating grating on the receiver sensitivity are examined. The range of fiber lengths for which a grating provides adequate compensation is determined.
The chirp and optical extinction ratio of multiple quantum well Mach-Zehnder modulators and electroabsorption modulators depend on the device design and the modulating voltage waveform. For transmission over non-dispersion shifted fiber, 10 Gb/s system performance is reviewed by considering joint optimization of the bias and modulation voltages. The implications of modulator design are examined taking into consideration group velocity dispersion and self-phase modulation arising from the Kerr nonlinearity. The results demonstrate that optimization of the bias and modulation voltages is effective for increasing the transmission distance, reducing the variation in performance obtained when Mach-Zehnder modulators of different designs are operated with maximum optical extinction ratio, and reducing the variation in performance obtained with different transmitted powers when electroabsorption modulators are operated with maximum optical extinction ratio.
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