Reinhold Noe

Hardware-Efficient Coherent Digital Receiver Concept With Feedforward Carrier Recovery for

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.

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This synchronous quadrature-phase-shift keying (QPSK) receiver concept allows us to process signals in parallel after electronic demultiplexing. An automatic electronic polarization control with a control time constant in the low microsecond range is provided. It is followed by a phase-locked loop free carrier recovery. An intermediate frequency linewidth tolerance of up to 10/sup -3/ times the QPSK symbol rate is expected. Commercially available distributed feedback lasers shall, therefore, suffice.

-QAM Constellations

Quadrature phase-shift keying (QPSK) is attractive to increase transmission lengths and capacity, especially when it is combined with polarization division multiplex. Baseband processing at the symbol rate allows to keep the required electronic bandwidth low. So far, external cavity lasers seemed to be indispensable for such transmission systems due to linewidth requirements. We propose a feedforward carrier recovery scheme based on regenerative intradyne frequency dividers, i.e., the well-known regenerative frequency divider is extended to process baseband in-phase and quadrature (I and Q) signals. An IF linewidth tolerance of up to 0.001 times the QPSK symbol rate is predicted, 2 decades more than for an optical phase locked loop with a realistic loop delay. This means that commercially available DFB lasers shall suffice for synchronous optical QPSK/BPSK transmission.

PLL-free synchronous QPSK polarization multiplex/diversity receiver concept with digital I&Q baseband processing

In coherent optical systems or sensors, polarization matching between the superposed beams must be assured. The tracking range of automatic polarization control systems should be endless, i.e. any resets of finite range retarders, which transform the polarization, should cause no significant intensity losses. A variety of experimental systems including a computer as feedback controller are described. They include the minimum configuration of three fixed eigenmode retarders, i.e. the orientation of birefringence cannot be changed. These retarders are realized by fiber squeezers. Error-tolerant systems which contain more than the minimum number of elements, however, are better suited to cope with time variant retarder transfer functions, etc. A fourth fiber squeezer allows the losses of a nonideal systems to be kept to only 0.07 dB. Finally, for the first time, a closed loop system with two integrated optical retarders is described. These retarders have variable eigenmodes, i.e. adjustable birefringence orientation. An optimization procedure helps to idealize the device behavior. The system has less than 0.15 dB intensity losses, coupling and attenuation not included. >

Phase noise-tolerant synchronous QPSK/BPSK baseband-type intradyne receiver concept with feedforward carrier recovery

We develop an analytic model of Coherent Optical Orthogonal Frequency Division Multiplexing (OFDM) propagation and detection over multi-span long-haul fiber links, comprehensively and rigorously analyzing the impairments due the combined effects of FWM, Dispersion and ASE noise. Consistent with prior work of Innoe and Schadt in the WDM context, our new closed-form expressions for the total FWM received power fluctuations in the wake of dispersive phase mismatch in OFDM transmission, indicate that the FWM contributions of the multitude of spans build-up on a phased-array basis. For particular ultra-long haul link designs, the effectiveness of dispersion in reducing FWM is far greater than previously assumed in OFDM system analysis. The key is having the dominant FWM intermodulation products due to the multiple spans, destructively interfere, mutually cancelling their FWM intermodulation products, analogous to operating at the null of a phased-array antenna system. By applying the new analysis tools, this mode of effectively mitigating the FWM impairment, is shown under specific dispersion and spectral management conditions, to substantially suppress the FWM power fluctuations. Accounting for the phased-array concept and applying the compact OFDM design formulas developed here, we analyzed system performance of a 40 Gbps coherent OFDM system, over standard G.652 fiber, with cyclic prefix based electronic dispersion compensation but no optical compensation along the link. The transmission range for 10-3 target BER is almost tripled from 2560 km to 6960 km, relative to a reference system performing optical dispersion compensation in every span (ideally accounting for FWM and ASE noise and the cyclic prefix overhead, but excluding additional impairments).

Endless polarization control systems for coherent optics

Coherent optical communication systems promise superior performance, but their realization in real time also poses big technical challenges. After an introduction the potential of coherent optical transmission systems is shown as manifested in offline experiments. Then we present key components that are necessary to realize these systems in real time. We review recent achievements in realtime coherent communication and finally present the results of a realtime QPSK transmission system with a 3x3 coupler in the receiver. The achieved BER at a data rate of 1.4 Gbit/s is well below the FEC threshold.

Phased-array cancellation of nonlinear FWM in coherent OFDM dispersive multi-span links

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

Coherent optical communication: Towards realtime systems at 40 Gbit/s and beyond

The authors compare three polarization handling methods in coherent optical systems. These methods are: endless polarization control, polarization diversity, and data-induced polarization switching. These methods are also compared with active, data-synchronous polarization switching. It is seen that endless polarization control is potentially the most powerful candidate, however, the choice of polarization control devices remain questionable. Polarization diversity is as versatile as polarization control and is potentially the fastest method; however, it yields lower receiver sensitivity. Endless control or a well-designed diversity receiver should be used for coherent trunk systems. Data-induced polarization switching is restricted to frequency-shift-keying (FSK) systems. It promises a loss span similar to that of diversity, but is far simpler, which makes it recommendable for FSK distribution systems. >

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

Polarization-mode dispersion (PMD) prevents the cost-effective upgrading of fiber networks to 40 and sometimes even to 10 Gbit/s. This paper reviews recent progress in its mitigation and compensation and points out where more research is needed. Electronic PMD mitigation is preferable at 10 Gbit/s, due to its low cost, even though it is accompanied by a considerable residual penalty. A lot of work takes place in the field of optical PMD compensation. Among the numerous detection methods for first-order PMD, we prefer a purely electronic, hence low-cost, arrival time detection method, with a linear readout and ps-sensitivity. Surprisingly, the most easily detectable higher order of PMD is the third order, indicated by a slope steepness difference. Both methods rely on a polarization scrambler at the transmitter side, which can be shared. Regarding PMD compensators, LiNbO/sub 3/ devices are probably needed to guarantee a sufficient speed. A distributed PMD compensator allows to integrate a number of polarization transformers and differential group delay sections on one chip, thereby exactly emulating the way how the fiber accumulates, but in reverse order and orientation. We report on progress in using these devices, including their use for PMD compensation in a 40-Gbit/s carrier-suppressed return-to-zero differential phase-shift keying experiment. More work is needed to perfection the device, and to implement a fast endless polarization control. The theory of the distributed PMD compensator lends itself to a new definition of higher order PMD by a Fourier expansion of mode conversion, as an alternative to the familiar Taylor expansion of the PMD vector.

Comparison of polarization handling methods in coherent optical systems

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.