Fibre-optic communications systems have traditionally carried data using binary (on-off) encoding of the light amplitude. However, next-generation systems will use both the amplitude and phase of the optical carrier to achieve higher spectral efficiencies and thus higher overall data capacities(1,2). Although this approach requires highly complex transmitters and receivers, the increased capacity and many further practical benefits that accrue from a full knowledge of the amplitude and phase of the optical field(3) more than outweigh this additional hardware complexity and can greatly simplify optical network design. However, use of the complex optical field gives rise to a new dominant limitation to system performance-nonlinear phase noise(4,5). Developing a device to remove this noise is therefore of great technical importance. Here, we report the development of the first practical ('black-box') all-optical regenerator capable of removing both phase and amplitude noise from binary phase-encoded optical communications signals.

/pdf/all-optical-phase-and-amplitude-regenerator-for-next-3wnsxhkves.pdf

All-optical phase and amplitude regenerator for next-generation telecommunications systems

We have experimentally compared the performances of optical millimeter-wave generation or up-conversion using external modulators based on different modulation schemes. The generated or up-converted optical millimeter wave using the optical carrier suppression (OCS) modulation scheme shows the highest receiver sensitivity, highest spectral efficiency, and smallest power penalty over long-distance delivery. Moreover, the OCS modulation scheme has a simple configuration and low-frequency bandwidth requirement for both electrical and optical components. Employing an OCS modulation scheme, 16-channel dense wavelength-division multiplexing signals at 2.5-Gb/s per channel have been up-converted to a 40-GHz carrier simultaneously.

Optical millimeter-wave generation or up-conversion using external modulators

This book, published in 2007, provides comprehensive coverage of the theory and practice of OPAs and related devices, including fiber optical parametric oscillators (OPOs). After introducing the field, the theory and techniques behind all types of fiber OPAs are covered starting from first principles - topics include the scalar and vector OPA theory; the nonlinear Schrodinger equation; OPO theory; and quantum noise figure of fiber OPAs. Challenges of making fiber OPAs practical for a number of applications are discussed, and a survey of the state-of-the-art in feasibility demonstrations and performance evaluations is provided. The capabilities and limitations of OPAs; the potential applications for OPAs and OPOs, and prospects for future developments in the field are discussed. Theoretical tools developed in this text can also be applied to other areas of nonlinear optics. This is a valuable resource for researchers, advanced practitioners, and graduate students in optoelectronics.

Fiber Optical Parametric Amplifiers, Oscillators and Related Devices: The nonlinear Schrödinger equation

Recent progress in coherent optical communication, a field revived by advances in digital signal processing (DSP), is reviewed. DSP-based phase and polarization management techniques make coherent detection robust and practical. With coherent detection, the complex field of the received signal is fully recovered, allowing compensation of linear impairments including chromatic dispersion and polarization-mode dispersion using digital filters. In addition, fiber nonlinearities can also be compensated by using backward propagation in the digital domain.

Recent advances in coherent optical communication

Preface 1. Planck's radiation law and the Einstein coefficients 2. Quantum mechanics of the atom-radiation interaction 3. Classical theory of optical fluctuations and coherence 4. Quantization of the radiation field 5. Single-mode quantum optics 6. Multimode and continuous-mode quantum optics 7. Optical generation, attenuation and amplification 8. Resonance fluorescence and light scattering 9. Nonlinear quantum optics Index

The quantum theory of light.

All-optical regeneration of differential phase-shift keying signals based on phase-sensitive amplification is described. Nearly ideal phase regeneration can be achieved in the undepleted-pump regime, and simultaneous amplitude and phase regeneration can be realized in the depleted-pump regime.

All-optical regeneration of differential phase-shift keying signals based on phase-sensitive amplification.

– DPSK phase-and-amplitude regeneration with a NOLM-based phase-sensitive amplifier is demonstrated experimentally. For a highly degraded input signal, maximum differential phase errors were reduced from 82° to 41°, while the SNR was improved by more than 5-dB. Differential phase Q-factor improvement was better than 6-dB. The PSA was operated free of excess noise due to stimulated Brillouin scattering by using a binary phase modulated pulse train as the pump. The impact of pump fluctuations on regeneration performance is clarified. The regenerated signal was characterized by measurement of the constellation diagram by linear optical sampling, giving the first directly measured evidence of DPSK phase regeneration.

Phase-and-amplitude regeneration of differential phase-shift keyed signals using a phase-sensitive amplifier

Amplification and simultaneous phase regeneration of DPSK signals is demonstrated using a phase-sensitive amplifier. Phase-sensitive gain is achieved in a Sagnac fiber interferometer comprised of nonpolarization maintaining, highly nonlinear fiber operating in the un-depleted pump regime. Both the pump and signal are RZ-DPSK pulse trains. The amplifier is capable of producing greater than 13 dB of phase-sensitive gain for an average pumping power of 100 mW, and easily reduces the BER of the regenerated DPSK signal by two orders of magnitude compared to the un-regenerated signal, corresponding to a negative power penalty of 2 dB. Careful optimization of the regenerator reveals much stronger BER Improvement.

Demonstration of phase-regeneration of DPSK signals based on phase-sensitive amplification

Efficient systems, devices, apparatus and methods to generate, transmit and detect differentially encoded 8-level phase-modulated optical signals for spectrally efficient optical communication systems is invented. It includes an electrical encoder and an optical encoder for generation of differentially encoded 8-level phase-modulated optical signals and optical demodulators and balanced detectors for detection of the optical signals. The optical signals are transmitted through optical fiber links or air. The electrical encoder maps three independent data channels into three differentially-encoded data sequences. In the optical encoder, the encoded data sequences from the electrical encoder drive optical modulators to generate differentially-encoded 8-level phase modulated optical signals at a symbol rate equal to the bit rate of each input data channel. After transmission through optical fiber, the optical signals are demodulated optically and the original data are recovered by the balanced detectors with direct detection.

/pdf/direct-detection-optical-differential-8-level-phase-shift-e3ur4tw8yj.pdf

Direct-detection optical differential 8-level phase shift keying (OD8PSK) for spectrally efficient transmission

All-optical clock recovery from 180-Gb/s data streams has been demonstrated using a self-pulsing multisection gain-coupled distributed feedback laser. The laser produces self-pulsations with a tuning range of more than 230 GHz. The recovered clock has a jitter of less than 410 fs over a dynamic range of 7 dB.

Cheolhwan Kim

Papers

All-optical regeneration of differential phase-shift keying signals based on phase-sensitive amplification.

Phase-and-amplitude regeneration of differential phase-shift keyed signals using a phase-sensitive amplifier

Demonstration of phase-regeneration of DPSK signals based on phase-sensitive amplification

Direct-detection optical differential 8-level phase shift keying (OD8PSK) for spectrally efficient transmission

180-GHz clock recovery using a multisection gain-coupled distributed feedback laser