Coherent optical communication - OSA Publishing

Coherent optical communication: Towards realtime systems at 40 Gbit/s and beyond T. Pfau, S. Hoffmann, O. Adamczyk, R. Peveling, V. Herath, M. Porrmann and R. Noé University of Paderborn, EIM-E, Warburger Str. 100, D-33098 Paderborn, Germany [email protected] http://ont.uni-paderborn.de

Abstract: 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. ©2008 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communication; (060.1660) Coherent communications; (060.2840) Heterodyne; (060.5060) Phase modulation.

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S. Tsukamoto, D. -S. Ly-Gagnon, K. Katoh, and K. Kikuchi, " Coherent Demodulation of 40-Gbit/s Polarization-Multiplexed QPSK Signals with16-GHz Spacing after 200-km Transmission," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest (CD) (Optical Society of America, 2005), paper PDP29. C. Laperle, B. Villeneuve, Z. Zhang, D. McGhan, H. Sun, and M. O'Sullivan, " Wavelength Division Multiplexing (WDM) and Polarization Mode Dispersion (PMD) Performance of a Coherent 40Gbit/s DualPolarization Quadrature Phase Shift Keying (DP-QPSK) Transceiver," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper PDP16. G. Charlet, J. Renaudier, M. Salsi, H. Mardoyan, P. Tran, and S. Bigo, " Efficient Mitigation of Fiber Impairments in an Ultra-Long Haul Transmission of 40Gbit/s Polarization-Multiplexed Data, by Digital Processing in a Coherent Receiver," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper PDP17. K. Kikuchi, " Coherent Detection of Phase-Shift Keying Signals Using Digital Carrier-Phase Estimation," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest (CD) (Optical Society of America, 2006), paper OTuI4. M. Nakazawa, M. Yoshida, K. Kasai, J. Hongou, “20 Msymbol/s, 128 QAM Coherent Optical Transmission over 500 km Using Heterodyne Detection with Frequency-stabilized Laser,” Proc. ECOC 2006, Mo4.2.2, Sept. 24-28, 2006, Cannes, France. C.R.S. Fludger, T.Duthel, T. Wuth, C. Schulien, “Uncompensated Transmission of 86Gbit/s Polarization Multiplexed RZ-QPSK over 100km of NDSF Employing Coherent Equalisation,” Proc. ECOC 2006, Th4.3.3, Sept. 24-28, 2006, Cannes, France. S. J. Savory, G. Gavioli, R. I. Killey, P. Bayvel, “Electronic compensation of chromatic dispersion using a digital coherent receiver,” Opt. Express 15, 2120-2126 (2007). M. Nakazawa, J. Hongo, K. Kasai, and M. Yoshida, " Polarization-Multiplexed 1 Gsymbol/s, 64 QAM (12 Gbit/s) Coherent Optical Transmission over 150 km with an Optical Bandwidth of 2 GHz," in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper PDP26. S. Hoffmann, T. Pfau, O. Adamczyk, R. Peveling, M. Porrmann, and R. Noé, " Hardware-Efficient and Phase Noise Tolerant Digital Synchronous QPSK Receiver Concept," in Optical Amplifiers and Their Applications/Coherent Optical Technologies and Applications, Technical Digest (CD) (Optical Society of America, 2006), paper CThC6. J. Sitch, “Implementation Aspects of High-Speed DSP for Transmitter and Receiver Signal Processing,” Proc. SUM2007, Ma4.3, July 23-25, 2007, Portland, OR, USA.

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Received 20 Aug 2007; revised 7 Nov 2007; accepted 7 Nov 2007; published 9 Jan 2008

21 January 2008 / Vol. 16, No. 2 / OPTICS EXPRESS 866

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1. Introduction Coherent optical communication is one of the most promising ways to achieve highest receiver sensitivity, excellent spectral efficiency and longest transmission distance for the next generation of optical communication systems. Already in the late 1980s and early 1990s coherent systems attracted a lot of attention as it was a promising way to improve the receiver sensitivity. Coherent PSK systems can in principle achieve a receiver sensitivity of 9 photons/bit, which is the best receiver sensitivity of all known modulation formats. But after the development of erbium-doped fiber amplifiers (EDFA) and the introduction of wavelength division multiplexing (WDM) the interest in coherent systems faded. In the last years, with the new possibilities offered by high-speed digital circuits, the interest in coherent optical communication systems revived. Apart from receiver sensitivity the interest lies now in the increase of the spectral efficiency as well as in the tolerance against dispersion effects and fiber nonlinearities. These are today the most limiting factors in ultralong haul communication systems. Intensity modulated systems with direct detection (IM/DD) can achieve a maximum spectral efficiency of only 1 b/s/Hz. Coherent systems allow to use multilevel modulation formats such as QPSK or QAM, which can raise the spectral efficiency up to several b/s/Hz. Additionally, in contrast to IM/DD systems or differential PSK systems, the received electrical signal in coherent receivers is proportional to the electrical field vector of the optical signal. Therefore the system becomes linear, which means that all linear distortions like chromatic dispersion as well as polarization mode dispersion can theoretically be compensated without any losses – and also nonlinear effects can be equalized very efficiently. However, these advantages have their price. A coherent optical receiver is much more complex than a simple direct detection receiver. High speed analog-to-digital converters (ADC) are needed to convert the received signal into the digital domain. Then these ADCs must be interfaced with a digital signal processing unit, which performs polarization control, equalization and finally the carrier and data recovery. To develop these components for the

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next generation of optical communication systems, which will run at 40 Gbit/s or even 100 Gbit/s poses a big challenge to scientists and engineers. This paper reviews the achievements made until today towards a realtime coherent communication system running at 40 Gbit/s. First the potential of the technology is demonstrated by showing some main results from offline coherent experiments. Then the components needed to realize these systems in realtime are discussed. In section 4 the milestones in realizing realtime coherent systems are resumed, and finally the measurement results of a realtime coherent synchronous QPSK system with a 3x3 coupler in the receiver are presented and discussed. 2. Potential of coherent optical communication systems Coherent optical communication promises ultimate performance and dispersion tolerance in upgraded or newly built fiber links. Many offline experiments have already been conducted to evaluate the possibilities offered by this technology. Most of them concentrated on the evaluation of polarization-multiplexed quadrature phase shift keying (QPSK) systems [1-3]. But also 8-level phase shift keying (8-PSK) [4] and quadrature amplitude modulation (QAM) have already been investigated [5]. The results of these experiments clearly demonstrate the potential of coherent optical communication. For polarization multiplexed QPSK transmission data rates up to 86 Gbit/s have been reached [6]. In another experiment 42.8 Gbit/s polarization multiplexed QPSK data was transmitted over 6400 km of single mode fiber (SMF) with purely electronic dispersion compensation [7]. This represents the longest reach achieved for transmission without optical dispersion compensation for any 10 GBaud system operating over standard fiber as well as for all 40Gbit/s systems. The highest number of bit/symbol achieved in a coherent transmission experiment was 12 bit/symbol in a polarization multiplexed 64 QAM testbed [8], reaching a spectral efficiency as high as 6 bit/s/Hz. 3. Components for realtime coherent QPSK transmission In the following the DSP components are described that are essential for the realization of a high-speed realtime coherent optical receiver. 3.1 High-speed analog-to-digital converter The coherent receiver requires a very fast analog-to-digital converter (ADC) with a sampling rate equivalent to the symbol rate to digitize the incoming signal. If CD compensation or PMD compensation should be applied a sampling rate even twice as high as the symbol rate is recommended. A very good architecture for such high-speed ADCs is the full-flash topology where 2N-1 parallel comparators are utilized to digitize the signal with a resolution of N bits in one step. The practical resolution of such flash ADCs is limited to 4 – 6 bits due to the loading of the input buffer with the 2N-1 comparators, which restricts the achievable bandwidth and sampling speed. Table 1 shows the specifications of a 5 bit 10 Gs/s ADC developed for the European synQPSK project. A fast circuit technology was necessary to accomplish the required sampling rate of >10 Gs/s, together with a fairly high level of integration to incorporate the encoding stages of the ADC. A 120 GHz SiGe hetero bipolar technology (IHP, Germany) turned out to be sufficient. The chip layout of the ADC is depicted in Fig. 1(a). 3.2 Digital signal processing unit To facilitate the signal processing functions for carrier recovery, polarization control and CD or PMD compensation, standard-cell based silicon CMOS technology is preferable due to its higher integration capability and lower power requirements compared to a bipolar technology. A drawback of CMOS standard-cell design is the limitation of the attainable data rates. Therefore, a fast demultiplexer must be built into the DSP unit to allow a high speed interface with the input signals from the ADCs. It must be realized with a full-custom demultiplexer circuit, integrated together with the standard-cell design into a single chip. The full-custom #86664 - $15.00 USD

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demultiplexer can use a differential circuit topology (source coupled FET logic) to achieve the necessary data rates for the high speed interface. Fig. 1(b) shows the layout of two full-custom 1:8 demultiplexers combined with a standard-cell DSP unit for carrier & data recovery of a synchronous QPSK transmission system at 10 Gbaud. The standard-cell based DSP unit is clocked at 625 MHz to enable the complex processing in 16 parallel units in a PLL-free approach [9]. At the output of the DSP unit 32 recovered data bits (16 channels, I&Q) are available at a data rate of 625 Mbit/s. Table 1 lists the specifications of the combined full-custom demultiplexer and the standardcell DSP unit.

Fig. 1. Layout of 5 bit SiGe ADC chip (a) and CMOS chip with combined full-custom design for demultiplexing and standard-cell design for carrier & data recovery (b).

Table 1. Specifications of SiGe ADC and CMOS chip

Technology

SiGe ADC 0.25 μm SiGe:C BiCMOS, fT = 120 GHz, fmax = 140 GHz

Technology

> 15 Gs/s

Data input (differential) Input clock frequency

0.5 – 5.0 GHz

4.5 @ 15 Gs/s

Input data rate

1 – 10 Gbit/s

Resolution

5 bit

Sampling rate Effective number of bits SINAD

28.9 dB @ 15 Gs/s

Full scale range Power consumption Supply voltages

500 mV 4.3 W -4 V, +1.8 V

Number of transistors

3296

Number of transisitors/gates

Chip size

6.4 mm2

Chip size

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CMOS chip 120 nm CMOS, fT = 90 GHz, VT = 0.3 V

Max. interface bandwidth Input amplitude Power consumption Supply voltages

2 x 5 bit

100 Gbit/s 400 mV ~1.2 W @ 10 GHz +1.2 V, +1.8 V, +3.3 V 5145 transistors (full-custom demux), 63187 gates (DSPU) 7.2 mm2



Another possible receiver design is to combine the high-speed ADC and the signal processing unit in a single chip in a BiCMOS or CMOS technology [10]. This has the advantage that the system has lower power consumption and the high-speed interface between the ADCs and the CMOS chip can be omitted. However, highest bipolar transit frequency fT and smallest CMOS feature size can so far not be combined in the same BiCMOS technology. And in CMOS it is very difficult to achieve the needed ADC input bandwidth. 4. Recent realtime coherent transmission experiments The first realtime coherent synchronous QPSK transmission was published in 1991 [11]. 100 Mbit/s QPSK data was received in a digital receiver using a phase-locked-loop (PLL) to compensate for the intermediate frequency and phase offset between the signal laser and the local oscillator laser. The drawback of the PLL approach is that external cavity lasers (ECL) are required. The PLL can only lock, if the laser linewidth times symbol rate ratio is below 0.0001. Another approach which is more tolerant against laser phase noise and allows to use standard DFB lasers is the feed-forward carrier recovery [12]. The first realtime coherent transmission system using this kind of receiver was demonstrated in 1992 [13], with a throughput of 565 Mbit/s of PSK modulated data. The data was recovered synchronously in a coherent receiver with analog carrier recovery and standard DFB lasers. More than a decade later, in June 2006, the first realtime QPSK transmission with standard DFB lasers was published [14]. This time a digital synchronous coherent receiver with feed-forward carrier recovery was applied for a data rate of 800 Mbit/s. The system was realized using commercially available ADCs interfacing with a field programmable gate array (FPGA) for signal processing. In November 2006 the maximum data rate for coherent synchronous QPSK transmission was already pushed to 4.4 Gbit/s [15]. Another half year later the spectral efficiency for realtime systems was doubled by a polarization-multiplexed QPSK transmission system with fast electronic polarization tracking at a data rate of 2.8 Gbit/s [16], still realized with commercially available electronic components. But in July 2007 Nortel announced the first realtime implementation of a polarization multiplexed coherent QPSK system with a data rate of 40 Gbit/s, implemented this time in an application specific integrated circuit (ASIC) [17]. 5. Realtime coherent QPSK transmission applying a 3x3 coupler in the receiver Novel transmission formats are not only assessed by their performance, but also by their costs and complexity. Therefore the realization of coherent QPSK transmission with standard components, which can be produced at low cost, is essential. One milestone to reach this goal was the development of a phase noise tolerant carrier recovery scheme, which allowed the use of standard DFB lasers [12]. Another possibility to reduce the component costs is the replacement of the optical 90° hybrid, which is in general realized with integrated optics [18] or free-space micro-optics, by a fused symmetric 3x3 coupler. This is advantageous compared to the use of asymmetric 3x3 couplers, which can be operated as 90° hybrids as well, but without suppressing direct detection terms. This section describes how the I&Q components can be extracted from the 3x3 coupler output signals and presents the first measurement results for a realtime QPSK transmission with a 3x3 coupler applied in the receiver. 5.1 Mathematical description From a complex I&Q signal X obtained in a coherent receiver with 90° hybrid, a 3-phase signal (x1, x2, x3) similar to the output of a 3x3 coupler can be obtained as

(

)

x1 = Re{X } , x 2,3 = −(1 2 ) Re{X } ± 3 2 Im{X } . (1) It contains the same information as X and fulfills the additional condition x1 + x2 + x3 = 0 . But in receivers with 3x3 coupler the latter is not the case because of thermal noise. Nevertheless Re{X} and Im{X} can be obtained by linear combinations of the output signals #86664 - $15.00 USD

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x1, x2 and x3 of a 3x3 coupler. In order to minimize the influence of thermal, quantization, direct detection, ASE and intensity noise, each of x1,2,3 must be weighted proportional to the contained amplitude of the respective component. A suitable transformation is X = Re {X } + j Im{X } ~ 2 x1 − x 2 − x 3 + j 3 (x 2 − x 3 ) .

(2)

This is superior to the simple assignment Re{X } ~ x1 , Im{X } ~ x2 − x3 since subsequent application of first (2), then (1) enforces x1 + x2 + x3 = 0 , as recommended in [19]. The approximation 3 ≈ 7 4 = 1.75 can be used for a hardware-efficient implementation of (2). The weighting proportional to the desired amplitude component can also be automated, which is especially advantageous if coupler properties are not known exactly, as is the case in practice. The input transformation matrix can be adaptively optimized by correlating the data before and behind the decision circuit, similar to the polarization control algorithm described in [12]. By setting the complex input vector to ⎛ X1 ⎜ ⎜X ⎝ 2

⎞ ⎟ ⎟ ⎠

⎛ x1

= ⎜⎜ ⎝

+ jx 2 ⎞ ⎟ x 3 ⎟⎠

(3)

where X1 and X2 are originally the received signals in two polarizations, the algorithm [12] can also be used to automatically adapt the transformation matrix to the 3-phase input signal. In doing so the algorithm even compensates for non-ideal 3x3 coupler characteristics. 5.2 Experimental setup The transmitter uses a DFB signal laser with a specified linewidth of 1 MHz and a QPSK modulator driven with 2x700 Mbit/s precoded PRBS data with a sequence length of 223-1 (Fig. 2). After transmission through 80 km of standard single mode fiber, the signal is optically preamplified and filtered by a ~20 GHz wide bandpass. The coherent receiver features a second DFB laser as its local oscillator, and manual polarization control. The two optical signals are superimposed in a fused 3x3 coupler and detected with three photodiodes. The resulting electrical I&Q signals are amplified before being sampled with 6-bit analog-todigital converters (ADCs). The ADCs interface with a Xilinx Virtex 4 FPGA where the three phase signal is converted to an I&Q signal according to (2) or (3) and the carrier and data are recovered according to [9]. The FPGA also generates a clock phase error signal and a LO frequency error signal for NRZ clock recovery and LO frequency control.

Fig. 2. Setup for 1.4 Gbit/s coherent QPSK transmission with 3x3 coupler used in the receiver.

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5.3 Measurement results and discussion Fig. 3 shows the averaged BER of the received I&Q channels against the preamplifier input power at the receiver. The minimum BER was 2.8·10−5 for the receiver configuration with fixed transformation (2). With adaptive transformation ((3) combined with [12]) the best measured BER was 7.3·10−5. For both receivers the PRBS could be detected until the preamplifier input power was set below -52 dBm. For comparison the BER performance of a receiver with a 90° hybrid is also shown in Fig. 3. It can be seen that the receiver sensitivity is about 1 dB higher than for the 3x3 coupler and fixed transformation function in the receiver. This results from non-ideal coupling ratios in the 3x3 coupler. In contrast, the 90° hybrid allows control of coupling ratios as well as phase shift. For the 3x3 coupler receiver with adaptive transformation the receiver sensitivity is the same as with 90° hybrid because the algorithm compensates for the non-ideal 3x3 coupler – but only for low preamplifier input power levels. The BER floor is higher than for the receiver with hard-wired transformation function. Very recently, the same effect showed up after the system was re-engineered to track fast polarization fluctuations in a polarizationmultiplexed QPSK transmission. A hardware error in the ADC-FPGA interface was then found, and eliminated. This has brought down the BER floors. We are confident that the same would hold for this 3x3 coupler experiment but can not test it because the system is needed for ongoing other experiments. -1,0 3x3 coupler hard-wired

-1,5

3x3 coupler adaptive

log(BER)

-2,0

90° hybrid

-2,5 -3,0 -3,5 -4,0 -4,5 -5,0 -52

-48

-44 -40 -36 preamplifier input power [dBm]

-32

-28

Fig. 3. Measured BER vs. preamplifier input power for 1.4 Gbit/s synchronous QPSK transmission.

6. Summary We have summarized the recent achievements in coherent optical communication, considering offline and realtime experiments. Electronic components were presented which are necessary to realize a realtime coherent QPSK transmission system at 10 Gbaud. Additionally the measurement results for the realtime implementation of a synchronous QPSK transmission system with a data rate of 1.4 Gbit/s and a 3x3 coupler in the receiver were shown. The minimum achieved bit error rate of 2.8·10−5 is close to the one achieved with a 90° hybrid and is well below the FEC threshold. Acknowledgments The authors acknowledge support from the European Commission under contract number FP6-004631 (synQPSK-project, http://ont.upb.de/synQPSK). Additionally the authors wish to thank Y. Achiam from CeLight Israel, H. Porte from Photline Technologies and F. J. Tegude from the University of Duisburg-Essen for their excellent cooperation. #86664 - $15.00 USD

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