Differential-phase-shift keying (DPSK) has recently been used to reach record distances in long-haul lightwave communication systems. This paper will review theoretical, as well as implementation, aspects of DPSK, and discuss experimental results.

Optical phase-shift-keyed transmission

Tunable semiconductor lasers have been listed in numerous critical technology lists for future optical communication and sensing systems. This paper summarizes a tutorial that was given at OFC '03. It includes some discussion of why tunable lasers might be beneficial, an outline of basic tuning mechanisms, some examples of tunable lasers that have been commercialized, and a discussion of control techniques. More extensive data is given for the widely-tunable sampled-grating distributed-Bragg-reflector (SGDBR) type of laser, including data for such lasers integrated monolithically with modulators to form complete transmitter front ends. A summary of reliability data for the SGDBR laser is also given. It is concluded that tunable lasers can reduce operational costs, that full-band tunability is desirable for many applications, that monolithic integration offers the most potential for reducing size, weight, power and cost, and that sufficient reliability for system insertion has been demonstrated.

/pdf/tunable-semiconductor-lasers-a-tutorial-u60opvuxtv.pdf

Tunable semiconductor lasers: a tutorial

Signaling using oDQPSK offers great potential for application in 10 and 40 Gb/s DWDM transmission systems, offering increased spectral efficiency, relaxed dispersion management and improved PMD tolerance. Practical realization of encoder and decoder functionality through integration should lead to wide system application.

Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical transmission

Information-theoretic limits to spectral efficiency in dense wavelength-division-multiplexed (DWDM) transmission systems are reviewed, considering various modulation techniques (unconstrained, constant-intensity, binary), detection techniques (coherent, direct), and propagation regimes (linear, nonlinear). Spontaneous emission from inline optical amplifiers is assumed to be the dominant noise source in all cases. Coherent detection allows use of two degrees of freedom per polarization, and its spectral efficiency limits are several b/s/Hz in typical terrestrial systems, even considering nonlinear effects. Using either constant-intensity modulation or direct detection, only one degree of freedom per polarization can be used, significantly reducing spectral efficiency. Using binary modulation, regardless of detection technique, spectral efficiency cannot exceed 1 b/s/Hz per polarization. When the number of signal and/or noise photons is small, the particle nature of photons must be considered. The quantum-limited spectral efficiency for coherent detection is slightly smaller than the classical capacity, but that for direct detection is 0.3 b/s/Hz higher than its classical counterpart. Various binary and nonbinary modulation techniques, in conjunction with appropriate detection techniques, are compared in terms of their spectral efficiencies and signal-to-noise ratio requirements, assuming amplified spontaneous emission is the dominant noise source. These include a) pulse-amplitude modulation with direct detection, b) differential phase-shift keying with interferometric detection, c) phase-shift keying with coherent detection, and d) quadrature-amplitude modulation with coherent detection.

/pdf/spectral-efficiency-limits-and-modulation-detection-3x3484f2pl.pdf

Spectral efficiency limits and modulation/detection techniques for DWDM systems

A wireless communication system can comprise two or more antennas that interfere with one another via free space coupling, surface wave crosstalk, dielectric leakage, or other interference effect. The interference effect can produce an interference signal on one of the antennas. A cancellation device can suppress antenna interference by generating an estimate of the interference signal and subtracting the estimate from the interference signal. The cancellation device can generate the estimate based on sampling signals on an antenna that generates the interference or on an antenna that receives the interference. The cancellation device can comprise a model of the crosstalk effect. Transmitting test signals on the communication system can define or refine the model.

Method and system for antenna interference cancellation

The practicability of using phase modulation in a 16 channel 10 Gbit/s system over 1600 km of standard SMF with simple dispersion compensation and the robustness in presence of ASK modulated channels has been shown for the first time.

Robustness of DPSK direct detection transmission format in standard fibre WDM systems

A highly symmetric monolithically integrated balanced photodetector module is presented. Revealing a bandwidth of 34 GHz and a broadband common-mode rejection ratio of more than 20 dB, the module is suitable for 40 Gbit/s systems.

High-speed balanced photodetector module with 20 dB broadband common-mode rejection ratio

Signal performance monitoring by a new histogram method is proposed for the identification of crosstalk. It is based on a deconvolution method. An investigation into the measured histograms for signals with inband crosstalk is presented and the BER successfully evaluated.

Histogram method for identification and evaluation of crosstalk

A new subcarrier technique for optical performance monitoring and transport of channel overhead independent of the client data and without optoelectronic conversion in the client data path has been verified experimentally performing loop tests with PRBS bit error measurements for the example of a 10 Gb/s client channel and a 50 Mb/s control channel converted to an RF subcarrier above the client modulation spectrum. Strict correlation between the client channel the ASE noise of optical amplifiers and nonlinear distortion due to self phase modulation (SPM). An early warning function of the control channel is realised by proper choice of the modulation depth. A pronounced BER degradation in the control channel indicates a beginning degradation in the client channel. This feature enables preemptive performance management, i.e. management can take measures in good time to prevent the network from outages and maintain the quality of service. It was shown for the measurement example that the control signal together with the client signal can be transmitted error free over distances up to 1653 km. This enables transport of channel overhead or labelling for optical burst and packet networks. The overhead or label maybe protected against degradation of optical transmission by implementing a forward error correction (FEC) in the control channel. The activity of the error correction algorithm can be used as a quality measure instead of a direct BER measurement.

Control modulation technique for client independent optical performance monitoring and transport of channel overhead

A single-polarization, 160 Gb/s RZ-DPSK-signal is transmitted over 240 km fiber, processed in a network node converting RZ-DPSK into OOK, 160 Gb/s into 40 Gb/s and the wavelength from 1551 nm to 1559 nm The 40 Gb/s OOK-signal is transmitted error-free over additional 160 km SSMF

M. Rohde

Papers

Robustness of DPSK direct detection transmission format in standard fibre WDM systems

High-speed balanced photodetector module with 20 dB broadband common-mode rejection ratio

Histogram method for identification and evaluation of crosstalk

Control modulation technique for client independent optical performance monitoring and transport of channel overhead

160 Gb/s regenerating conversion node