1. Introduction

With the ever-increasing demands for data transmission capacity, intensive research efforts are being made to increase per-channel line rates in wavelength-division-multiplexed (WDM) optical transmission systems. While current commercial 100-Gb/s systems use single-carrier polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK), the majority of studies aiming to achieve 400-Gb/s and 1-Tb/s-class transmission have employed multicarrier (or superchannel) approaches, in which optical frequency-division multiplexing (FDM) is used to overcome the bandwidth limitation of electronics [1–4]. On the other hand, single-carrier high-symbol-rate transmission technologies have also been studied extensively. Traditionally, high symbol rates have been pursued by using optical time-division multiplexing (OTDM) [5–9]. However, most OTDM experiments have relied on pulsed lasers (or discrete pulse carvers) and/or fiber delay lines, which are not suitable for practical system implementation. Several integrated OTDM modulators without fiber delay lines have been reported [10, 11], although they can support only binary modulation formats. Recently, high-symbol-rate quadrature amplitude modulation (QAM) technologies leveraging high-speed electronics are attracting much interest as simple and potentially cost-effective solutions for increasing line rates [12–14]. Essential components for implementing high-symbol-rate QAM transmitters are high-speed electronic digital-to-analog converters (DACs). The record single-carrier line rate with a single optical modulator of 864 Gb/s (72-Gbaud PDM-64QAM) was achieved by using SiGe DACs with a sampling rate of 72 GSample/s and a 6-dB bandwidth of 23 GHz [14]. However, a line rate of 1 Tb/s has yet to be achieved with high-speed DACs and a single modulator.

In this paper, we report single-carrier 1-Tb/s (125-Gbaud) PDM-16QAM signal generation and transmission using high-speed InP multiplexer-DAC (MUX-DAC) modules and an integrated OTDM modulator with external PDM emulation. The MUX-DAC module contains 2:1 time-domain MUXs and a DAC and has a bandwidth of larger than 40 GHz [15, 16]. In the modulator, a pulse generator and IQ modulators for the two OTDM tributaries are integrated in a hybrid silica-LiNbO₃ optical circuit [17]. The pulse generator generates two orthogonal carrier-suppressed return-to-zero (CSRZ) pulses [18], which enable a spectrally-efficient OTDM. The combination of two-fold electronic TDM (2ETDM) with the MUX-DACs and 2OTDM with the integrated modulator enabled us to achieve a high symbol rate of 125 Gbaud. The signal was transmitted over 80-km standard single-mode fiber (SSMF).

2. Device technologies

2.1 MUX-DAC module

The MUX-DAC IC was fabricated with 0.5-μm-emitter InP heterojunction bipolar transistor (HBT) technology, which yields a cutoff frequency of 290 GHz and a maximum oscillation frequency of 320 GHz [19]. The IC includes six 2:1 MUXs and a 6-bit DAC. Each pair of the twelve half-rate digital input signals are multiplexed by a MUX, and then the six full-rate digital outputs of the MUXs are converted into an analog signal by the DAC. The output analog 6-dB bandwidth of the MUX-DAC module, in which the IC is packaged with RF connectors, is larger than 40 GHz [15, 16].

2.2 OTDM modulator

Figure 1(a) is the optical-circuit diagram and configuration of the OTDM modulator, which was originally designed and fabricated as a dual-carrier modulator [20]. The modulator circuit consists of a pulse-generator, which is a dual-parallel Mach-Zehnder modulator (MZM) with a 2x2 coupler at the output, a quad-parallel IQ modulator, and a polarization-multiplexing (PM) circuit. Four IQ modulators correspond to the two OTDM tributaries for the two orthogonal polarizations, though only one polarization channel was used in this transmission experiment as explained later. To obtain a final output symbol rate of B, the two MZMs in the pulse generator are biased to the null points and driven with sinusoidal clock signals with a frequency of B/4 and a relative delay of 1/B. The relative optical phase between the outputs from the two MZMs is set to zero. With these conditions, the generator outputs two CSRZ pulse trains from the two output ports, A and B, respectively. As shown in the Fig. 1(b), an intensity peak at port A always coincides with extinction at port B and vice versa. Thus, the two CSRZ pulse trains can be used for 2OTDM without inter-symbol interference. In the frequency domain, the spectrum of a CSRZ pulse train has a twin peak with a spacing of B/2, neglecting higher-order components for simplicity. Supposing each IQ modulator is driven with a B/2-baud NRZ data signal synchronized with the intensity-peak timing of the CSRZ pulse, the spectrum of the output signal from the IQ modulator is represented by a convolution of the twin peak and a sinc waveform with a main-lobe bandwidth of B and has the total spectral bandwidth of 1.5B. The final output of the OTDM modulator is the sum of the two modulated CSRZ signals and has a spectral bandwidth of 1.5B, too.

 

Fig. 1 (a) Optical-circuit diagram and (b) principle of the OTDM modulator

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As shown in Fig. 2, the modulator has a hybrid configuration of silica planar lightwave circuits (PLCs) and a LiNbO₃ (LN) chip. The LN chip has ten push-pull pairs of straight phase modulators in an array, which corresponds to the ten MZMs (MZM1-10 from top to bottom): two for the pulse generator and eight for the four IQ modulators. All other passive components, such as couplers, static phase shifters and the PM circuit, are fabricated in the PLCs. The pulse generator and the IQ modulators are connected via U-turn waveguides, and the input and output ports of the module are laid on the same side. The modulator’s static insertion loss for the both polarization is 9.5 dB at 1550-nm wavelength. All the MZMs have electro-optic 3-dB bandwidths of ~23 GHz. The half-wave voltage, V_π, of each MZM is ~3.5V.

 

Fig. 2 Configuration of the OTDM modulator.

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3. Experimental setup

Figure 3 shows the experimental setup for 1-Tb/s single-carrier transmission. Continuous-wave (CW) light from an external-cavity laser (ECL) with a wavelength of 1550 nm and a linewidth of ~30 kHz was modulated with the integrated OTDM modulator. The modulator was driven with two 31.25-GHz clocks and four 62.5-Gbaud four-level signals generated by two MUX-DACs, each of which was driven with four decorrelated 31.25-Gb/s 2¹⁵-1 pseudo-random bit sequences (PRBSs) from a bit-pattern generator (BPG). A relative delay of 32 symbols at 62.5 Gbaud (512 ps) was added between the two complementary outputs of each MUX-DAC for decorrelation, and a relative delay of 8000 symbols at 31.25 Gbaud was added between each adjacent channels of four PRBSs. The peak-to-peak swing voltages for the pulse generator and each IQ modulator were around 2.0 and 0.3 times the half-wavelength voltage (V_π) of the MZMs. Since only two MUX-DACs were available for this experiment, we used only two IQ modulators corresponding to one of the two polarization channels (those corresponding to MZM3, 4, 7, and 8 in Fig. 2) in the modulator. The output 125-Gbaud single-polarization 16QAM signal from the modulator was converted to a PDM signal by an emulation circuit, which consists of a 50% splitter, a fiber delay line of ~40 m, and a polarization-beam combiner (PBC). A programmable optical equalizer (OEQ) was optionally inserted between the modulator and the PDM emulator. The parameters of the OEQ were set so that its transmittance spectrum is the difference between the ideal Nyquist-shaped spectrum with a roll-off factor of 0.01 and the measured spectrum of the output signal of the transmitter. The PDM signal was transmitted over 80-km SSMF.

 

Fig. 3 Experimental setup.

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At the receiver side, we used an offline digital coherent receiver consisting of another ECL as a local oscillator (LO), a dual-polarization optical hybrid (DPOH), four 70-GHz balanced photodiode modules (BPDs), and two synchronized digital storage oscilloscopes (DSOs), each with two input channels operating at 160 GSample/s with an analog bandwidth of 63 GHz. No optical time-domain demultiplexing was used. The stored digital data with a per-channel length of 3M samples were analyzed by using a similar offline processing as the one described in [21]. In this experiment, an adaptive equalizer (AEQ) with 43-tap half-symbol-spaced (T/2-spaced) adaptive finite-impulse-response (FIR) filters with a butterfly configuration was used to demultiplex the polarization channels and equalize the channel responses. To control the adaptive filters, we used the joint constant-modulus and multi-modulus algorithm (CMA-MMA) [22] for the pre-convergence at startup before we switched to the decision-directed least-mean-square (DD-LMS) algorithm. The carrier frequency offset and phase fluctuation were compensated by a digital phase-lock loop (PLL). The bit error ratio (BER) was calculated from the 2M bits of demodulated data.

4. Results

Figure 4 shows eye diagrams and optical signal spectra of the two 62.5-Gbaud OTDM tributaries (A and B) and the final 125-Gbaud output signal. The horizontal axis in each spectrum is relative optical frequency with respect to the carrier frequency. All data except the spectrum with OEQ were measured at the main output port of the modulator, and the data for each tributary was obtained by turning off the driving signals for the other tributary. The spectrum with OEQ was measured at the output of the OEQ. The eye diagrams of the two tributaries show alternating pulses, each with a repetition rate of 62.5 GHz, while the final output waveform is not peaky. All spectra without OEQ show an almost identical twin-peak shape, while those of the two tributaries have fine fringes corresponding to the relative delay of 32 sub-rate symbols between the complementary outputs of each MUX-DAC. These fringes were not observed in the spectrum of the final output, because the I signal lagged behind Q signal in one tributary while the Q lagged behind I in the other. The side peaks observed at around ± 93.75 GHz (three times the clock frequency of 31.25 GHz) correspond to a relatively large swing voltage of 2.0V_π for the pulse generator.

 

Fig. 4 Eye diagrams (left column) and optical signal spectra.

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The BER versus optical signal-to-noise ratio (OSNR) curves measured in a back-to-back configuration are shown in Fig. 5. The data were obtained by bypassing the 80-km SSMF and loading optical noise to the transmitted signal using an amplified-spontaneous-emission (ASE) source. Constellations for the two polarizations with an OSNR of 37 dB are also shown. We successfully demodulated the 125-Gbaud 1-Tb/s PDM-16QAM signal without using any optical or electrical time-domain demultiplexing. This is because the relative optical phase between the OTDM tributaries is stable, which is a significant advantage of using the integrated OTDM modulator instead of conventional fiber-connected discrete modulators. The required OSNR is 28.4 dB without OEQ at the BER of 2.7x10⁻², which is the threshold of the low-density parity-check convolutional forward error correction (FEC) code using a layered decoding algorithm with 20% overhead [23]. Thus, the OSNR penalty with respect to the theoretical limit is about 6.4 dB. The use of OEQ reduced the penalty by about 1 dB. We consider this improvement was obtained because the OEQ equalized the channel response before the optical noise was added to the signal and thus avoided noise enhancement at the AEQ in the receiver. The curves are not smooth where BERs are relatively small (≤1.5x10⁻²). We consider this behavior is caused by drift in the bias condition of the modulator, to which the signal quality is sensitive.

 

Fig. 5 Back-to-back BER curves and constellations of the 1-Tb/s PDM-16QAM signals.

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The result of the 80-km SSMF transmission experiment is shown in Fig. 6, where the received Q-factor is plotted against the optical power launched to the 80-km SSMF. This data was obtained without using the OEQ. The received power at the DPOH was kept constant ( + 4.2 dBm) by using a variable optical attenuator (VOA). The LO input power into the DPOH was 15.6 dBm. At the launched power of about + 9 dBm, the received Q-factor is 6.5 dB, which corresponds to a margin of 1.8 dB from the threshold of the FEC mentioned above. The Q-factor drastically degrades when the launched power exceeds + 14 dBm, probably as a result of nonlinear effects in SSMF.

 

Fig. 6 Received Q-factor versus launched optical power into the 80-km SSMF.

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5. Conclusion

We demonstrated 1-Tb/s PDM-16QAM transmission using high-speed InP MUX-DACs and an integrated OTDM modulator. A signal with a high symbol rate of 125 Gbaud was generated from 31.25-Gbaud data signals by using 2ETDM with the MUX-DAC and 2OTDM with the modulator. Unlike conventional OTDM experiments, the signal was successfully demodulated without using any time-domain demultiplexing. To the best of our knowledge, this is the first demonstration of a single-carrier 1-Tb/s transmission with a single modulator with external PDM emulation.