Optical networks are undergoing significant changes, fueled by the exponential growth of traffic due to multimedia services and by the increased uncertainty in predicting the sources of this traffic due to the ever changing models of content providers over the Internet. The change has already begun: simple on-off modulation of signals, which was adequate for bit rates up to 10 Gb/s, has given way to much more sophisticated modulation schemes for 100 Gb/s and beyond. The next bottleneck is the 10-year-old division of the optical spectrum into a fixed "wavelength grid," which will no longer work for 400 Gb/s and above, heralding the need for a more flexible grid. Once both transceivers and switches become flexible, a whole new elastic optical networking paradigm is born. In this article we describe the drivers, building blocks, architecture, and enabling technologies for this new paradigm, as well as early standardization efforts.

Elastic optical networking: a new dawn for the optical layer?

Digital filters underpin the performance of coherent optical receivers which exploit digital signal processing (DSP) to mitigate transmission impairments. We outline the principles of such receivers and review our experimental investigations into compensation of polarization mode dispersion. We then consider the details of the digital filtering employed and present an analytical solution to the design of a chromatic dispersion compensating filter. Using the analytical solution an upper bound on the number of taps required to compensate chromatic dispersion is obtained, with simulation revealing an improved bound of 2.2 taps per 1000ps/nm for 10.7GBaud data. Finally the principles of digital polarization tracking are outlined and through simulation, it is demonstrated that 100krad/s polarization rotations could be tracked using DSP with a clock frequency of less than 500MHz.

Digital filters for coherent optical receivers.

This paper presents a novel digital feedforward carrier recovery algorithm for arbitrary M-ary quadrature amplitude modulation (M-QAM) constellations in an intradyne coherent optical receiver. The approach does not contain any feedback loop and is therefore highly tolerant against laser phase noise. This is crucial, especially for higher order QAM constellations, which inherently have a smaller phase noise tolerance due to the lower spacing between adjacent constellation points. In addition to the mathematical description of the proposed carrier recovery algorithm also a possible hardware-efficient implementation in a parallelized system is presented and the performance of the algorithm is evaluated by Monte Carlo simulations for square 4-QAM (QPSK), 16-QAM, 64-QAM, and 256-QAM. For the simulations ASE noise and laser phase noise are considered as well as analog-to-digital converter (ADC) and internal resolution effects. For a 1 dB penalty at BER = 10-3, linewidth times symbol duration products of 4.1 x 10-4 (4-QAM), 1.4 x 10-4 (16-QAM), 4.0 x 10-5 (64-QAM) and 8.0 x 10-6 (256-QAM) are tolerable.

Hardware-Efficient Coherent Digital Receiver Concept With Feedforward Carrier Recovery for $M$ -QAM Constellations

The drive for higher performance in optical fiber systems has renewed interest in coherent detection. We review detection methods, including noncoherent, differentially coherent, and coherent detection, as well as a hybrid method. We compare modulation methods encoding information in various degrees of freedom (DOF). Polarization-multiplexed quadrature-amplitude modulation maximizes spectral efficiency and power efficiency, by utilizing all four available DOF, the two field quadratures in the two polarizations. Dual-polarization homodyne or heterodyne downconversion are linear processes that can fully recover the received signal field in these four DOF. When downconverted signals are sampled at the Nyquist rate, compensation of transmission impairments can be performed using digital signal processing (DSP). Linear impairments, including chromatic dispersion and polarization-mode dispersion, can be compensated quasi-exactly using finite impulse response filters. Some nonlinear impairments, such as intra-channel four-wave mixing and nonlinear phase noise, can be compensated partially. Carrier phase recovery can be performed using feedforward methods, even when phase-locked loops may fail due to delay constraints. DSP-based compensation enables a receiver to adapt to time-varying impairments, and facilitates use of advanced forward-error-correction codes. We discuss both single- and multi-carrier system implementations. For a given modulation format, using coherent detection, they offer fundamentally the same spectral efficiency and power efficiency, but may differ in practice, because of different impairments and implementation details. With anticipated advances in analog-to-digital converters and integrated circuit technology, DSP-based coherent receivers at bit rates up to 100 Gbit/s should become practical within the next few years.

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Coherent detection in optical fiber systems

Achieving on-chip optical signal isolation is a fundamental difficulty in integrated photonics1. The need to overcome this difficulty is becoming increasingly urgent, especially with the emergence of silicon nano-photonics2,3,4, which promises to create on-chip optical systems at an unprecedented scale of integration. Until now, there have been no techniques that provide complete on-chip signal isolation using materials or processes that are fundamentally compatible with silicon CMOS processes. Based on the effects of photonic transitions5,6, we show here that a linear, broadband and non-reciprocal isolation can be accomplished by spatial–temporal refractive index modulations that simultaneously impart frequency and wavevector shifts during the photonic transition process. We further show that a non-reciprocal effect can be accomplished in dynamically modulated micrometre-scale ring-resonator structures. This work demonstrates that on-chip isolation can be accomplished with dynamic photonic structures in standard material systems that are widely used for integrated optoelectronic applications. The realization of a chip-based, broadband optical isolator is of considerable interest for integrated photonics. To date, no technique has been shown to be able to do this using materials and processes that are CMOS-compatible. Now, scientists propose that the use of direction-dependent photonic mode transitions in silicon nanophotonic structures could be the solution.

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Complete optical isolation created by indirect interband photonic transitions

Various attempts have been made to fabricate waveguide-type isolators in III/V material by implanting magnetic materials, but none of them has so far resulted in a commercial product. Here, we report for first time on an integrated optical isolator implemented in III/V material. It consists of a single-sideband electrooptic modulator where traveling electrical waves make the transmission direction-dependent. Isolation is 30 dB, excess insertion loss is 8 dB. Residual rms ripple is 7% for peak-to-peak RF driving amplitudes of 3.5 V at 4.0 GHz. The estimated transmission penalty for 40 Gb/s return-to-zero differential phase shift keying (RZ-DPSK) signals is 0.2 dB (0 dB measured).

Novel nonmagnetic 30-dB traveling-wave single-sideband optical isolator integrated in III/V material

For the first time, synchronous quadrature phase-shift keying data is recovered in real-time after transmission with standard distributed feedback lasers using a digital inphase and quadrature receiver. Forward-error-correction-compatible performance is reached at 800 Mb/s after 63 km of fiber. Self-homodyne operation with an external cavity laser is error-free

First Real-Time Data Recovery for Synchronous QPSK Transmission With Standard DFB Lasers

Travelling electrical waves make the transmission direction-dependent in a single-sideband electro-optic modulator. Isolation is 30 dB; excess insertion loss is 8 dB. Residual r.m.s. ripple is 7% for RF driving amplitudes of 3.5 Vpp at 4.0 GHz. Transmission penalty for 40 Gbit/s RZ-DPSK signals is 0.2 dB estimated, 0 dB measured.

Non-magnetic 30 dB integrated optical isolator in III/V material

The combination of return-to-zero differential quadrature phase-shift keying with polarization-division multiplex, a 16-ary modulation scheme, allows for ultimate spectral efficiency. We raise C-band fiber capacity with phase-shift keying transmission beyond previously reported figures, achieving forward-error correction limit performance over four fiber spans.

5.94-Tb/s 1.49-b/s/Hz (40/spl times/2/spl times/2/spl times/40 Gb/s) RZ-DQPSK polarization-division multiplex C-band transmission over 324 km

This letter presents a coherent digital polarization diversity receiver for real-time polarization-multiplexed synchronous quadrature phase-shift keying transmission with distributed feedback lasers at a data rate of 2.8 Gb/s. The tolerance against fast polarization changes and polarization-dependent loss is evaluated for different filter widths in the carrier recovery circuit. The minimum achieved bit-error rate is 3.4 times 10-7.

Selwan K. Ibrahim

Papers

Novel nonmagnetic 30-dB traveling-wave single-sideband optical isolator integrated in III/V material

First Real-Time Data Recovery for Synchronous QPSK Transmission With Standard DFB Lasers

Non-magnetic 30 dB integrated optical isolator in III/V material

5.94-Tb/s 1.49-b/s/Hz (40/spl times/2/spl times/2/spl times/40 Gb/s) RZ-DQPSK polarization-division multiplex C-band transmission over 324 km

Coherent Digital Polarization Diversity Receiver for Real-Time Polarization-Multiplexed QPSK Transmission at 2.8 Gb/s