Chromatic dispersion compensation in coherent opticalcommunications

Chromatic dispersion compensation techniques in coherent transmission systems are reviewed and discussed for potential feasibility. The key compensation device is the wideband delay equalizer. It is shown that stripline-type delay equalizers have the potential for compensating distortion up to 10 Gb/s using a conventional 1.3-μm zero-dispersion single-mode fiber at 1.5 μm. Chromatic dispersion is successfully compensated with a stripline delay for CPFSK transmission at 4 and 6 Gb/s over 200 km of 1.3-μm zero-dispersion single-mode fiber at 1.55 μm. The bandwidth requirement of the compensation techniques for heterodyne detection is more than 10 GHz. However, it is difficult to realize such broadband receivers. Therefore, phase diversity detection with dispersion compensation is a promising scheme     

Performance limitations of amplified coherent optical transmissionsystems due to interfering echoes

Interfering echoes produced in semiconductor amplifier cascades are known to cause bit error rate (BER) degradations in optical transmission systems. An experimental investigation of the effect of interfering echoes in a coherent differential phase-shift keying (DPSK) system is described and results in good agreement with theoretical predictions are reported. It is concluded that interfering echoes limit the minimum allowable inter-amplifier attenuation, which in turn severely limits the number of semiconductor laser amplifiers that can be cascaded without optical isolation     

A signal-alignment theory in rolling-based lightwave transmissionsystems

A theory to support signal alignment in rolling-based lightwave transmission systems is developed. The theory originates from the transmission system environment, which consists of three basic building blocks-multiplexer, scrambler, and permuter. These building blocks are represented by mathematical operators, on the basis of which mathematical expressions of the whole transmission system become possible. The mathematical operators are directly translated into number operators which help detect and identify the received signals in the form of a table-the signal-detection table. The signal-detection tables play a central role in the signal-alignment theory, since any transmission system can be represented by its corresponding signal-detection table. The entries in the signal-detection tables are closely correlated so that the tables can be uniquely characterized by their corresponding characteristic expressions. Various types of signal-detection tables are examined, and solutions to align transmitted signals and to reduce signal-alignment time in rolling-based lightwave transmission systems are exemplified through scrambler-permuter configurations     

High-capacity coherent lightwave systems

Recent theoretical work on coherent optical detection systems is reviewed and experimental results in high-speed coherent transmission are summarized. The theoretical advantages and limitations of the various modulation and detection formats are discussed and experimental progress towards the implementation of these systems is reviewed. The most significant obstacles to the attainment of quantum-noise limited detection at higher speeds are seen to be the requirement of uniform frequency response from electronic components and the local oscillator laser power requirement, which increases as the square of the bit rate. To make full use of the single-mode fiber bandwidth, frequency-division multiplexing of many moderate-rate channels is a very promising technique for local systems. For long-distance applications, frequency multiplexing is still possible but is limited by the need for optical amplifiers or wavelength-selective multiplexers     

Amplifier-induced crosstalk in multichannel coherent lightwave systems

When an in line semiconductor laser amplifier is used to amplify several channels simultaneously, it can induce inter channel crosstalk if the amplifier gain is channel-dependent. It is shown that modulation of the carrier density at the beat frequency of two neighbouring channels can lead to considerable crosstalk even when the amplifier is operated well below the saturation level. An analytic expression for the channel gains of a travelling-wave amplifier is used to discuss and compare the crosstalk for ASK and FSK systems. The relatively short carrier lifetime in high-gain amplifiers may ultimately limit the channel spacing of such multichannel systems.     

Equalization in coherent lightwave systems using microwavewaveguides

The maximum bit rate/ distance product in recent single-frequency laser direct-detection lightwave system experiments has been limited by dispersion. An equalization technique, appropriate for coherent lightwave systems, that uses a microwave waveguide for overcoming the delay dispersion problem is considered. Results show that small low-loss waveguides can be used to greatly reduce dispersion. For example an 8 GHz bandwidth signal transmitted over 68 km of fiber can be equalized by a waveguide with a cross section of 6 mm×3 mm and a length of only 17 cm. With the waveguide equalizer, the dispersion-limited maximum bit rate/distance product for a standard fiber system can be increased to that of a dispersion-shifted fiber system at 1.55 μm, e.g., a 16-fold increase in maximum bit rate for 100 km transmission     

Intensity noise in ASK coherent lightwave receivers

The effect of local oscillator intensity noise on the performance of two and three-branch ASK homodyne receivers and single-branch ASK heterodyne receivers is investigated and an optimum local oscillator power is found. At optimum local oscillator power, both the three-branch and heterodyne receivers are found to have a somewhat better sensitivity than the two-branch receiver. If the local oscillator power is high than the optimum value, the three-branch receiver is significantly less sensitive to intensity noise than the other two receivers     

Multigigabit subcarrier multiplexed coherent lightwave system

A wideband coherent detection optical communications system using two microwave subcarriers to transmit 8 Gb/s with a RF bandwidth efficiency of 1 b/s/Hz is described. It is demonstrated that two subcarriers each carrying 4 Gb/s QPSK, compactly spaced at just twice the data rate, can achieve a receiver sensitivity (ηP) of -26.5 dBm with as little as 0.5-dB crosstalk between subcarriers. How the various components affect the overall performance of QPSK-SCM systems, including a CNR model for multichannel systems is discussed     

Planar lightwave circuit dispersion equalizer

The authors report an integrated-optic dispersion equalizer fabricated on a planar lightwave circuit (PLC). This PLC dispersion equalizer is composed of several asymmetrical Mach-Zehnder interferometers cascaded in series. The dispersion equalizer has a high degree of design flexibility and can compensate for both normal and anomalous fiber dispersions at any center wavelength. Moreover, the equalizer can be applied to WDM transmission systems. This equalizer employing five asymmetrical interferometers is fabricated and its measured dispersion values are +834 and -1006 ps/nm. The effectiveness of the equalizer is demonstrated in a 2.5 Gb/s transmission experiment with a 1.3 μm zero-dispersion fiber at 1.55 μm. Also, its performance is evaluated theoretically     

Chromatic dispersion estimation methods using polynomial fitting in PDM-QPSK or other multilevel-format coherent optical systems

We propose a polynomial fitting algorithm based method for non-data-aided chromatic dispersion (CD) estimation in single carrier (SC) coherent optical systems with arbitrary modulation formats, and compare it with our previously proposed CD estimation method which is also based on the polynomial fitting algorithm but requires special modulation formats thus is a data-aided CD estimation method for systems with PDM-QPSK or other multilevel modulation formats. For the data-aided CD estimation method, an extra chirp-free OOK signal is transmitted. The curve of the average phase at the frequency ± f as a function of the frequency f is measured at the coherent receiver. The accumulated CD is then estimated with a polynomial fitting algorithm. In the simulation of a 50 Gbaud 50%-RZ OOK system through 12.5 × 80 km standard single mode fiber (SSMF), the estimation errors are within ± 50 ps/nm in 20 tests when the launch power is from −5 dBm to −1 dBm. Non-data-aided CD estimation for arbitrary modulation formats is achieved by measuring the differential phase between frequency f ± f_s/2 (f_s is the symbol rate) in digital coherent receivers. The estimation errors are within ± 200 ps/nm, in a 50 Gbaud PDM-QPSK system through 10 × 80 km SSMF with the launch power from −3 dBm to −1 dBm. The estimation accuracy can be potentially improved by averaging multiple results. The data-aided CD estimation method has an inherently bigger estimation range than that of the newly proposed non-data-aided method, while the newly proposed non-data-aided method can tolerate a much larger frequency offset between the transmitter and the local oscillator. These methods are promising for future optical fiber networks with dynamic optical routing and coherent detection.     

Subcarrier multiplexed coherent lightwave systems for videodistribution

The application of subcarrier multiplexing techniques to coherent lightwave systems designed for video transmission is described. The theory of the intrinsic receiver sensitivity of multichannel coherent SCM (subcarrier multiplexing) systems is presented. Specific implementations of SCM coherent systems transmitting 60 FM video channels and 20 100 Mb/s FSK (frequency shift keying) channels are described. It is shown that a simple phase noise canceling circuit can be implemented, which allows these systems to be built with wide-linewidth DFB (distributed feedback) lasers. Several applications of multichannel coherent SCM systems to video distribution networks are proposed     

Planar lightwave circuit polarization-mode dispersion compensator

In this letter, we fabricated an integrated polarization-mode dispersion compensator on a silica-based planar lightwave circuit that incorporated an endless polarization controller and a fixed polarization dependent delay line. The compensator consisted of polarization beam splitters, thermooptic phase shifters, tunable couplers, and polarization converters. We experimentally confirmed its operation at a data rate of 43 Gb/s     

Fading in lightwave systems due to polarization-mode dispersion

System fading caused by polarization-mode dispersion is investigated at 1.7 Gb/s using highly-birefringent, dispersion-shifted fiber at 1.55 μm. The observed fading, which is manifested by random fluctuations of the bit error rate for a fixed receiver power, is observed to depend on the environmental conditions of the fiber, with the time constant for fading varying from minutes to hours depending on the rate of change of the ambient temperature. The mean dispersion penalty inferred from the observed fluctuations in the bit error rate is consistent with a square-law dependence on the polarization-mode dispersion for small penalties     

Equalization in coherent lightwave systems using a fractionallyspaced equalizer

The performance of a fractionally spaced analog tapped delay line equalizer that has the advantages of being adaptive, being capable of equalizing other linear distortions such as polarization dispersion and nonideal receiver response, and eliminating chromatic dispersion over any distance if a sufficient number of taps are used is discussed. It is shown how this equalizer can be implemented at intermediate frequency and at baseband (for homodyne detection). Results show that an N-tap equalizer increases the maximum bit rate distance approximately (N-1)/twofold (e.g. a threefold increase in distance with a seven-tap equalizer)     

All-optical 2R repeater for coherent lightwave communicationsystems

The intensity noise reduction capability of an all-optical 2R repeater using an injection-locked laser diode (ILL) is examined, and the effectiveness of the use of the repeater is verified in coherent CPFSK long-haul transmission experiments with in-line optical amplifiers     

Chromatic dispersion measurement in graded-index multimode opticalfibers

The chromatic dispersion observed in a graded-index multimode optical fiber is a function of the radial position of the launched light. When a graded-index multimode optical fiber is overfilled spatially and angularly, the chromatic dispersion observed is an average (composite) chromatic dispersion over all of the possible launches. Under certain assumptions, the relationship between the chromatic dispersion observed for the overfilled fiber and the chromatic dispersion of any restrictive launch can be determined. This relationship can be used to predict the composite graded-index multimode optical fiber chromatic dispersion from the measurement of the chromatic dispersion of a single simplified launch, such as a single-mode fiber launch. Preliminary experimental results confirm this approach     

Mitigation of polarization-mode dispersion in multichannel lightwave transmission systems

We propose a novel polarization-mode dispersion (PMD) compensation scheme for wavelength-division-multiplexing (WDM) systems, in which many WDM channels share one or a few PMD compensators at receiver site. It is shown, both analytically and numerically, that this scheme can achieve virtually the same performance as that using per-channel-based PMD compensation.     

Performance of coherent ASK lightwave systems with finiteintermediate frequency

The impact of finite intermediate frequency (IF) on the performance of heterodyne ASK lightwave systems is examined and quantified in the presence of laser phase noise and shot noise. For negligible linewidths, it is shown that certain finite choices of IF (R _b,3R_b/2,2R_b,5R_b/2, etc.) lead to the same ideal bit-error-rate (BER) performance as infinite choices of IF. Results indicate that for negligible linewidths the worst case sensitivity penalty is 0.9 dB for proper heterodyne detection and occurs when f_IF=1.25 R_b. For nonnegligible linewidths (e.g., when ΔνT⩾0.04) the sensitivity penalty is always less than 0.9 dB for finite choices of IF. The analysis presented does lead to a closed-form signal-to-noise ratio (SNR) expression at the decision gate of the receiver which can readily be used for BER and sensitivity penalty computations. The SNR expression provided includes all the key system parameters of interest such as system bit rate (R_b), the peak IF SNR (ξ), laser linewidth (Δν), and the IF filter expansion factor (α). The findings of this work suggest that the number of channels in a multichannel heterodyne ASK lightwave system can be increased substantially by properly choosing a small value for the IF at the expense of a small penalty <1 dB. On the negative side, IF frequency stabilization becomes a more critical requirement in multichannel systems employing small values of IF