This thesis investigates fundamental transmission limitations in both ultra dense wavelength division multiplexing (U-DWDM) digital systems and subcarrier multiplexing lightwave systems. The main research results are organized in four parts as follows.
First of all, transmission performance of ultra-dense 2.5 and 10 Gbps NRZ IM/DD wavelength division multiplexing systems in various single-mode fibers is investigated. Fundamental limiting factors and their remedies by using optimum dispersion compensation for periodically amplified systems in C-band are presented.
In order to increase the transmission capacity of a DWDM optical system, one can either increase the transmission data rate per wavelength, or increase the number of wavelengths while keeping proper transmission granularity. The first approach can be illustrated by the recent increase in data rate per wavelength from 2.5 Gbps to 10 and 40 Gbps. The second approach is to significantly increase the number of wavelengths in a fixed optical spectrum (e.g., C-band) by decreasing the spacing between neighboring wavelengths. By using this approach, capacity increase can be achieved without resorting to high-speed (e.g., > 40 Gbps) electronics, while keeping compatibility with existing 2.5/10 Gbps SONET/SDH equipment. Along this line, the focus of this paper is on ultra-dense wavelength division multiplexing (U-DWDM) transmission systems. Examples of U-DWDM systems include 25 GHz-spaced 10 Gbps and 6.25 GHz-spaced 2.5 Gbps transmission systems.
It should be noted that, even though a few U-DWDM system experiments have been carried out recently [1-5], the fundamental limiting factors and their remedies in such systems remain unclear. It is obvious that there are different transmission issues to be dealt with in the above-mentioned two distinct approaches.
When the transmitting data rate is higher than 40 Gbps, severe chromatic dispersion and polarization mode dispersion problems will have to be resolved even before dealing with optical nononlinearity-induced penalties. On the other hand, U-DWDM systems intuitively should have optical nonlinearity-induced system limitations such as four-wave mixing (FWM) and cross-phase modulation (XPM) penalties. The first part of this thesis considers various nonlinear distortions/interferences to determine the fiber nonlinearity limited maximum transmission distances in U-DWDM systems. Optimum launched power and dispersion compensation ratio (DCR), and the dominant optical nonlinearities in different systems are
also discussed.
In Section 2.1, we provide an overview of the main nonlinear distortions/interferences in U-DWDM systems. In Section 2.2, we analytically calculate and numerically simulate the capacity and distance limitations of U-DWDM systems. Discussions and conclusions are provided in Sections 2.3 and 2.4, respectively.
Second, we found that the combined effects of self- and external-phase modulations must be considered in order to precisely predict the CSO distortions in a long-distance 1550 nm externally-modulated AM-CATV system, especially when the applied phase modulation index and modulating tone frequency to the integrated phase modulator are high. This result has important implications to the optimum design of 1550 nm transmitter for long-distance AM- and QAM-CATV systems.
Recently, there has been intense interests in long-distance 1550 nm external-modulation AM-CATV systems based on conventional single-mode fibers [6-13]. When the optical power launched into these long-distance systems is below the stimulated Brillouin scattering (SBS) threshold, it has been found that the transmission distance is mainly limited by the fiber dispersion-induced composite second order (CSO) distortions. These CSO distortions in turn were believed to be caused by self-phase modulation (SPM) [8-10], and by the residual intensity modulation from an imperfect phase modulator [11]. However, analyses developed for either of the above mechanisms were not accurate in predicting the resultant CSO values in long-distance AM-CATV transmission systems [10-12].
It is noted that all commercially available 1550 nm CATV transmitters have an integrated phase modulator and a Mach-Zehnder interferometer (MZI) modulator. In order to increase the SBS threshold, a ~2 GHz tone (or a few tones > ~2GHz) is usually applied to the phase modulator, and a high SBS threshold can be obtained by using a high phase modulation (PM) index (β) [13]. However, the resultant high launched optical power into the system can unavoidably increase the SPM effects. In addition, when the applied PM index β or the PM modulating tone frequency are high, or when the transmission distance is long, both SPM and external phase modulation (EPM) can be mixed with intensity modulation in a nonlinear dispersive optical fiber system. The resultant CSO distortions due to their combined effects, however, have not yet been thoroughly investigated. Section 3 of this thesis presents both experimental and numerical results on the above subject.
Third, the validity of using multiple CW tones as the signal source to test the linearity of a multichannel M-ary quadrature-amplitude-modulation (M-QAM) subcarrier multiplexed (SCM) lightwave system was investigated. We consider the following representative optical fiber system nonlinearities: (1) laser clipping, and (2) the combined effect of laser frequency chirp and fiber dispersion. The results show that, if all orders of nonlinear distortions (NLDs) in a signal bandwidth are included in the total NLD power, the error caused by replacing M-QAM signals with CW tones can be within measurement uncertainty.
It is believed that SCM lightwave systems can be used to transport multichannel M-QAM signals to provide broadband digital services such as Internet access, digital video, IP telephony, etc. [14,15]. In the past, the linearity characteristics of such systems were investigated by using multiple CW tones [16-18], mainly because of the practical difficulty in generating multiple distinct M-QAM channels. However, the spectral distributions of nonlinear distortions (NLDs) caused by multiple CW tones and multiple wideband M-QAM signals are quite different. In the former case, NLDs consist of various distinct beats such as composite second orders (CSOs) and composite triple beats (CTB’s). In the latter case, NLDs are spread over several channels and are like white noise. Fig.1 (a) and (b) illustrate the spectra of the two cases. To our knowledge, there is no report discussing the validity of using CW tones to replace the actual M-QAM signals. In section 4 of this thesis, we study this validity by performing spectral analysis, numerical simulation and experimental verification. We chose two representative optical fiber nonlinearities in SCM lightwave systems to study: (1) laser clipping, and (2) the combined effect of laser chirping and optical fiber dispersion.
Fig. 1 Illustrations of typical NLDs generated from (a) CW tones and (b) QAM. Signal level were normalized to 0 dB for comparison.
Finally, a long-distance 1550 nm subcarrier multiplexed lightwave trunk system which transported 78 channels of 64-QAM signals was demonstrated in a recirculating loop experiment. Each channel can
achieve a carrier-to-(noise + nonlinear distortion) ratio of 30 dB after 740 km transmission through conventional single-mode fiber without dispersion compensation.
A subcarrier multiplexed (SCM) lightwave system transporting multi-channel M-ary quadrature-amplitude-modulation (M-QAM) signals can have transmission features such as high system capacity and long transmission distance [19-20]. This is due to the fact that the carrier-to-noise ratio (CNR) and carrier-to-nonlinear distortion ratio (CNLD) requirements of M-QAM signals are lower than those of AM-VSB signals. In addition, M-QAM signals have a high spectral efficiency, which makes multi-gigabit/sec data transmission feasible when using conventional CATV optical transceivers.
Therefore, multi-channel M-QAM SCM trunk systems have a great potential to be used for interconnecting CATV headends and delivering various digital communication services.
It was found that the fundamental M-QAM system capacity of either a laser diode- or a linearized external modulator-based transmitter could be as high as tens of gigabit/sec [22, 23]. However, the transmission distances of all reported M-QAM SCM systems are still rather limited. In section 5 of this thesis, we experimentally demonstrated that the transmission distance of an M-QAM external modulation SCM system carrying an equivalent data capacity of 2.34 Gb/s could exceed 740 km. In addition, for the first time, an optical fiber recirculating loop was implemented in an SCM system experiment.