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• Abstract

SECTION I

INTRODUCTION

The realization of high-speed digital signal processors (DSPs) and digital-to-analog converters (DACs) has allowed advanced signal processing to offer substantial improvements in the performance and functionality of transmitters for optical fiber communication systems. The synthesis of the appropriate drive signal(s) for an optical modulator or a directly modulated laser permits the generation of modulated optical signals with unprecedented control of the time-varying amplitude and phase. In this paper, we describe notable achievements reported in 2012 on the generation of modulated optical signals using both offline and real-time digital signal processing.

Two approaches for generating modulated optical signals using a DSP and DAC(s) are illustrated in Fig. 1(a) and (b) using dual-drive and single-drive modulation, respectively. For the case of dual-drive modulation, the continuous-wave signal from a laser is applied to an IQ Mach–Zehnder modulator, which allows the in-phase (I) and quadrature (Q) components of the optical field to be generated independently. For the case of single-drive modulation, a single-drive optical modulator or directly modulated laser is used to generate the modulated optical signal. This approach is fundamentally different from the case of the IQ modulator since the drive signal simultaneously determines both the amplitude and phase of the optical field.

Fig. 1. Transmitter utilizing (a) an IQ Mach–Zehnder modulator and (b) a single-drive modulator or directly modulated laser.(c) Real-time transmitter module incorporating a DSP and four DACs for a DP IQ modulator (from [15] with permission of IEEE).

Most of the exploratory research in this area uses offline signal processing whereby sample values for the desired drive signals are precalculated and then stored in the memory of an arbitrary waveform generator. The sequence of stored sample values is repetitively read out and applied to a DAC to generate the prescribed analog drive signal. Real-time implementations have been addressed by combining FPGAs and DACs and by ASICs that provide a high level of integration for the DSP and DACs. The former approach allows implementation issues to be more directly addressed compared with offline processing but is not viable for practical systems. Due to the expense, the latter approach is only feasible as part of a product development activity.

SECTION II

OFFLINE PROCESSING

The performance of spectrally efficient optical fiber transmission systems is ultimately limited by the per-channel power that can be launched into each span. As the launch power increases, the optical signal-to-noise ratio of the received signal increases, and but so do the effects of fiber nonlinearities, which adversely affect signal quality. Techniques have been developed to precompensate for the effects of intra- and interchannel fiber nonlinearities. A perturbation-analysis-based technique that mitigates self-phase modulation and intrachannel cross-phase modulation and four-wave mixing by altering the transmitted symbols based on estimates of the changes that will be imposed by the nonlinearities during transmission has been demonstrated [1]. Using two 6-bit 28 GSa/s DACs, an IQ modulator, and real-time DSP in the receiver, the system reach was increased from 1400 to 2000 km for a 112 Gb/s dual-polarization (DP) RZ-QPSK signal.

The effects of fiber nonlinearities have been mitigated for a 537.6 Gb/s DP 64-ary QAM signal comprised of eight frequency-locked subcarriers spaced by 6.25 GHz [2]. The insertion of a pilot tone allowed for receiver-based compensation of the phase noise created by interchannel fiber nonlinearities and the neighboring channels (both subcarriers and WDM channels). The DSP for the eight-level modulator drive signals included generating raised-cosine pulses with a rolloff factor of 0.1, the addition of a pilot tone, and precompensation for the band-limiting effects of the transmitter and receiver. A 10-bit 11.2 GSa/s arbitrary waveform generator and IQ modulator were used to generate the individual 62.7 Gb/s signals.

The spectral efficiency of an optical signal can be increased by the combination of spectral shaping using a minimum bandwidth Nyquist pulse shape (sinc pulse) and a high-order modulation format. A 12-bit 12 GSa/s arbitrary waveform generator and IQ modulator have been used to generate a 54 Gb/s DP 512-ary QAM signal with a near-ideal Nyquist pulse shape having a rolloff factor of only 0.0024 (as limited by the finite time window for the pulse generation) [3]. The 3 Gsym/s signal exhibited a modulated signal bandwidth of 3 GHz and was transmitted over 44 km of ultralarge effective area fiber.

Significant effort has been devoted to increasing the per-channel symbol rate beyond what can be achieved with current and foreseen DAC capabilities. A superchannel uses more than one optical carrier but is considered as a single entity with the subcarriers being routed through a network as a single group. This allows the subcarriers to be more closely spaced together compared with the individual modulated carriers in a conventional WDM network, which are routed separately. Two approaches to implement a superchannel are orthogonal frequency division multiplexing (OFDM) and Nyquist WDM.

OFDM has emerged as a key enabling technology for Tb/s superchannel generation with high spectral efficiency. Forty 117.6 Gb/s DP 16-ary QAM OFDM signals with a net spectral efficiency of 4.7 bits/s/Hz have been generated using 12 GSa/s arbitrary waveform generators and digital precompensation for the responses of the drive amplifiers and IQ modulator [4].

High-speed DACs have enabled the all-electronic generation of OFDM signals beyond 100 Gb/s. A 1.5 Tb/s guard-banded superchannel, consisting of eight 30 Gsym/s pilot-free DP 16-ary QAM OFDM subchannels with a net spectral efficiency of 5.75 bits/s/Hz has been demonstrated [5]. Two novel 10-bit 50 GSa/s DACs were used to generate each of the eight 233 Gb/s OFDM subchannels.

Conventional OFDM suffers from a high peak-to-average power ratio (PAPR), which results in a low tolerance to fiber nonlinearities. Discrete Fourier transform spread (DFTS) OFDM has been shown to reduce the PAPR considerably [6]. A DFTS OFDM transmitter, which utilized a unique word (UW) embedded at both ends of the OFDM symbol enabled the compensation of both linear and nonlinear phase noise at the receiver. Transmission of a 1 Tb/s DP QPSK UW-DFTS-OFDM superchannel over 8000 km of standard single-mode fiber was demonstrated.

OFDM has also been applied to gigabit passive optical networks. Using a 8-bit 12 GSa/s DAC and an electroabsorption modulator, a 40 Gb/s optical double sideband OFDM signal was transmitted over 20 km of standard single-mode fiber with a 21.5 dB link budget [7]. To compensate for the frequency chirp of the modulator, subcarrier-adaptive modulation and preemphasis were used at the transmitter.

Nyquist WDM or quasi-Nyquist WDM allow for closely spaced unlocked optical subcarriers to be used at the transmitter provided each subchannel has been digitally or optically prefiltered to minimize crosstalk. The transmission of a 30 Tb/s superchannel over 6630 km of standard single-mode fiber has been achieved using 294 subcarriers, each consisting of 100 Gb/s DP 16-ary QAM signal with a nearly rectangular spectrum [8]. The net spectral efficiency was 6.1 bits/s/Hz. The offline DSP involved filtering using a raised cosine filter with a rolloff factor of 0.001 and a precompensation for the responses of the DACs, drive amplifiers, and IQ modulator.

A net spectral efficiency of 8.25 bits/s/Hz has been demonstrated using time-domain hybrid QAM [9]. Each time division multiplexed frame allows for m-ary QAM signals with two different values of m. Using five frequency-locked subcarriers, the 494.85 Gb/s signal exhibited a bandwidth of 50 GHz. The individual 9.7 Gsym/s 32/64-ary hybrid QAM signals used a raised-cosine pulse shape with a rolloff factor of 0.01 and were precompensated for the DAC response.

To perform Nyquist digital pulse shaping, a DAC would normally be operated at 2 samples per symbol to allow full control over the spectral content of the modulated signal up to the symbol rate frequency. A sampling rate of 1.5 samples per symbol (23.4375 GSa/s) has been used to generate 15.625 Gsym/s DP 16-ary QAM signals with a raised cosine pulse shape having a rolloff factor of 0.01 [10]. The combination of a sampling rate of 1 sample per symbol and analog electrical filtering enabled the generation of Nyquist 28 Gsym/s coherent DP QPSK [11].

The use of Nyquist digital pulse shaping has also been applied to a directly modulated transmitter with a direct detection receiver for very short reach applications [12]. A 56 Gb/s 16-ary QAM signal was generated using RF subcarrier modulation with a subcarrier frequency of half the symbol rate, Nyquist pulse shaping, and a directly modulated passive feedback laser. A 6-bit 28 GSa/s arbitrary waveform generator was used to implement the digital pulse shaping, upconversion, and precompensation for the responses of the DAC and laser.

SECTION III

REAL-TIME PROCESSING

Real-time transmitters have been reported that use both FPGAs and ASICs. A novel FPGA platform that included two 6-bit 32 GSa/s DACs has been used to generate a 224 Gb/s DP 16-ary multicarrier offset-QAM signal with seven RF subcarriers [13]. At the FEC limit of $\hbox{BER} = 3.8 \times 10^{-3}$, the implementation penalty was only 3.1 dB for offline processing of the received signal.

A pulse-shaping algorithm with dynamic precision allowed improving the performance of the real-time generation of a 150 Gb/s DP 64-ary QAM signal using an FPGA-based transmitter [14]. By dynamically adjusting the effective word length from 6 to 11 bits, transmission over 150 km of standard single-mode fiber was achieved using 6-bit 25 GSa/s DACs without an increase in the signal processing complexity.

Finally, a real-time transmitter incorporating an ASIC for the DSP and DACs has been used for spectral shaping and the generation of a 1 Tb/s superchannel comprised of five subcarriers [15]. Fig. 1(c) illustrates the transmitter module. The ASIC has 22 million gates and was implemented in 65-nm CMOS technology. It enables soft decision FEC encoding, pulse shaping for Nyquist WDM, reconfigurability of the bit rate and modulation format for symbol rates of 35 Gsym/s, and includes four 6 bit DACs for a DP IQ modulator.

SECTION IV

CONCLUSION

Recent advances in transmitter technology allow for innovations that dramatically improve the performance and functionality of high bit-rate optical fiber communication systems. From a technology perspective, further advances in this area would benefit from increases in the resolution, bandwidth, and sampling rates of DACs, but more so from the development of flexible real-time signal processing platforms that facilitate exploratory research without a reliance on FPGA-based platforms or offline signal processing. Towards this end, the implementation complexity of proposed signal processing algorithms used in transmitters needs to be more explicitly addressed. It is anticipated that reconfigurable transmitters that support different transmission distances through selection of the modulation format will be extended to yet more flexible transmitters required for elastic networks, and that electrical-to-optical converters other than IQ modulators will receive increased attention as the technology matures. Ingenuity will continue to enrich the field.

Footnotes

Corresponding author: J. C. Cartledge (e-mail: john.cartledge@queensu.ca).

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