High-speed low-chirp PAM-4 transmission based on push-pull silicon photonic microring modulators

We report a silicon photonic modulator based on the use of dual parallel microring modulators (MRMs) inserted in a Mach-Zehnder interferometer (MZI). It is operated in a push-pull configuration for low-chirp transmission at approximately 1550 nm. The chirp parameters of the device are measured using 10 Gb/s on-off keying (OOK) transmission over 20 km of standard single mode fiber (SSMF), and they are less than 0.01, showing the lowchirp characteristic of the modulator. We further demonstrate four-level pulse amplitude modulation (PAM-4) transmission at 92 Gb/s over 1 km of SSMF and at 40 Gb/s over 20 km of SSMF. The measured bit error rates (BERs) are below the hard-decision (HD) forward error correction (FEC) threshold of 3.8 × 10. © 2017 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (200.4650) Optical interconnects; (250.4110) Modulators. References and links 1. D. A. B. 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Introduction
From small scale micro-processors in mobile phones and laptops, to large-scale data centers, ultra-high speed data transmission at low cost is becoming an urgent need.It enables multimedia applications and cloud-based services to millions of users, which benefits their daily life.However, limited by bandwidth and power consumption, electrical components and wires are experiencing a performance bottleneck.Optical communications based on silicon photonic devices, which can overcome these limitations [1][2][3], is a promising solution.Silicon photonics (SiP) is compatible with the existing CMOS fabrication processes, and hence is capable of large volume production of photonic devices at low cost [4,5].Reports on a variety of high speed SiP devices, such as modulators [6][7][8][9][10][11] and photodetectors (PDs) [12,13], further show the potentiality of SiP.
Among SiP devices, the MRM is a key component.It has a compact size, and its filtering ability makes it favorable for wavelength-division-multiplexing systems.Recently, MRMs have been applied in a micro-processor [14] and data center interconnects [15].However, the frequency chirp of MRMs [16] limits their application in optical transmission links where chromatic dispersion (CD) is not negligible.Digital signal processing (DSP) and dispersion compensation fibers (DCFs) are two typical ways to compensate CD.DSP-based CD compensation requires access to the modulated phase information [17], therefore it cannot be applied to systems employing intensity modulation and direct detection.The use of DCFs will introduce excess loss in the transmission link and thus the link will require optical amplification [18].Recently, MRMs with low-chirp using OOK modulation at 10 Gb/s [19] and 60 Gb/s [20] have also been reported.
In this paper, we report a silicon photonic modulator for high-speed low-chirp PAM-4 transmission in the C-band (approximately 1550 nm).The modulator is designed by inserting two MRMs in a balanced MZI, and it is operated in a push-pull configuration.Its low-chirp characteristic is theoretically analyzed using a system level simulation, and experimentally demonstrated by the measured constellation diagrams using 10 Gb/s OOK transmission over 20 km of SSMF.We further present PAM-4 transmission at 92 Gb/s over 1 km of SSMF and at 40 Gb/s over 20 km of SSMF with BERs below the HD FEC threshold of 3.8 × 10 −3 .

Device design, simulation and fabrication
The schematic structure of the device is shown in Fig. 1.The silicon waveguide has a height of 220 nm and it is on a 90 nm thick slab.The MRMs are designed to be identical with the same radius of 20 µm and the same gap of 200 nm from the MZI waveguide.The balanced MZI has a length of 230 µm.On one of the MZI arms, a heater with a 125 µm length is designed for phase tuning to make the MZI perfectly balanced.Around 75% of the MRM waveguide is doped to form a symmetric lateral pn junction for carrier-depletion modulation using the same doping levels as those in [21].The remaining 25% is n doped as a heater for resonant wavelength tuning.In our design, light travels through the MZI, while differential radio frequency (RF) signals are applied on the PN junctions of the MRMs simultaneously.A DC voltage is applied on the heater of MRM 1 to align the resonances of the two MRMs.The device was fabricated in a multi-project-wafer run at IME A*STAR.As shown in Fig. 1, while applying RF data levels 0, 1, 2, 3 on MRM 1, the inverted RF data levels 3, 2, 1, 0 are applied on MRM 2 simultaneously to achieve the push-pull operation of the device.To clearly demonstrate the operating principle, we simulate the power transmission and phase responses of the device in Lumerical Interconnect.As shown in Fig. 2(a), when the four levels of differential RF data applied on the two MRMs, the transmitted powers of the device at the operating wavelength λ are 0.033, 0.173, 0.327 and 0.477, respectively.They have nearly even spacing, which is reasonable for generating PAM-4 optical signals.
in which the modulated optical intensity I and the phase Ф are both functions of the applied voltage V.As shown in Fig. 2(b), the phases at λ are all zero, regardless of the RF data value that is applied.Therefore, dФ/dV in Eq. ( 1) is zero and the simulated α-parameter is also zero, which proves the device is capable of zero-chirp modulation.

Device characterization
The direct current (DC) characterization spectra of the device are shown in Fig. 3.They are measured by applying 0 to 10 V reverse bias voltages on the RF pad of each MRM.The total on-chip insertion loss is approximately 14 dB.Though the two MRMs are designed to be identical, the measured resonant wavelengths are different due to the deviations between the design and fabrication.The quality factor, extinction ratio and the resonance shift at −10 V bias of the two MRMs are measured to be same, which are ~11k, 18 dB and 4 pm/V, respectively.Figure 4 shows the EE S11 responses and the electro-optic (EO) S21 responses of the two MRMs measured at 50 pm and 100 pm away from their resonant wavelengths (as marked by the dots in Fig. 3).As shown in Fig. 4(a), the EE S11 responses are the same at different detuned wavelengths for each MRM.However, the measured 3-dB EE bandwidth of MRM 1 is 29.9 GHz, which is 7.7 GHz less than that of MRM 2. The difference is also due to the variations in the fabrication process, such as the implantation levels and the misalignment of the pn junction.These variations result in different junction capacitances and resistances for the two MRMs, and therefore the measured 3-dB EE bandwidths are different.As shown in Fig. 4(b), the measured EO S21 responses at the detuned wavelengths 50 pm away from the resonances are flat, but those at 100 pm detuning have peaking at high frequencies.This is because the light at larger wavelength detuning from the resonance is more easily released from the microring resonator, and thus it has a shorter photon life-time [23,24].This results in a peaking effect and an enhancement of the 3-dB EO bandwidth [23,24]   The chirp of the device is evaluated by measuring the constellation diagrams and the chirp parameters of the modulated optical signals.The experimental setup is shown in Fig. 5(a).The device was driven by differential 16 bits' pseudo random binary sequences (PRBS) generated by an SHF 12103A bit pattern generator (BPG).After RF amplification and signal synchronization using the RF delay line, the peak-to-peak voltages of the RF signals were both measured to be 1.8 V pp in a 50 Ω system, and DC bias voltages of −2 V were applied using 65 GHz bias tees.A DC voltage of 1.08 V was applied on the MRM 1 heater to align the resonances of the two MRMs.A laser output at 14 dBm was coupled in to the chip and modulated by the device.Next, the optical signals were transmitted over SSMF ranging from 0 to 20 km and amplified by an erbium-doped fiber amplifier (EDFA).An APEX AP-2443B complex optical spectrum analyzer (OSA) was used to obtain the constellation diagrams and the chirp parameters.As a clock input at 10 GHz is necessary for the measurement using the complex OSA, we modulated the device at 10 Gb/s.The measured results are shown in Figs.

5(b)-5(d).
Compared to previously reported α-parameters of push-pull MZMs [25][26][27][28], the measured chirp parameters of the device from back-to-back (B2B) to 20 km of SSMF transmission are all very small.This shows that the push-pull operation of the dual parallel MRMs is capable of low-chirp modulation.

PAM-4 transmission
Figure 6(a) illustrates the experimental setup of PAM-4 transmission by operating the device in the push-pull configuration.The differential RF data were generated by a 70 GSamples/s 8bit digital-to-analog converter (DAC) using offline DSP shown in Fig. 6(b).After RF amplification and synchronization, the peak-to-peak voltages of the differential data, which were measured in a 50 Ω system, were 1.6 V pp .Together with −3 V bias voltages, the differential RF data modulated the two MRMs respectively.A DC voltage of 1.08 V was applied on the heater of MRM 1 to tune its resonant wavelength closer to that of the MRM 2. A 14 dBm laser output was coupled into the device.The modulated PAM-4 optical signals were transmitted over SSMF ranging from 0 to 20 km, and were amplified by an EDFA.A digital communication analyzer (DCA) was used to obtain the optical eye diagrams.A 35 GHz photodetector (PD) with a trans-impedance amplifier (TIA) received the signals at a fixed average power of 0.7 dBm.A 160 GSamples/s 62 GHz 8-bit real-time oscilloscope (RTO) was used to capture the received data for offline DSP processing.
Figure 6(b) depicts the sequential steps of the offline DSP.On the transmitter side, a pseudo random integer sequence (PRIS) was generated.A raised-root cosine (RRC) filter with a roll-off factor of 0.5 was applied for pulse shaping.Then the sequence was resampled to the DAC sampling rate of 70 GSamples/s by inserting (70/B -1) zeros, where B is the baudrate in Gbaud.A pre-emphasis filter was obtained by a least-mean-squares algorithm using a training sequence [29].It was used to compensate the limitations of the DAC and the RF amplifiers, but not that of the modulator.Afterwards, the DAC generated the differential RF data based on the sequence.On the receiver side, the data captured by the RTO were first resampled to 2 samples/symbol.A filter matched to the RRC filter on the transmitter side was then applied.An equalizer, whose coefficients were obtained from a training sequence, was used to compensate for the remaining frequency response degradation.Afterwards, the data were compared to the transmitted PRIS after synchronization to measure the BERs and to plot the eye diagrams.The measured BER results are shown in Fig. 8. Soft-decision (SD) FEC threshold of 2 × 10 −2 , HD FEC threshold of 3.8 × 10 −3 and KP4 FEC threshold of 2.2 × 10 −4 are all listed for performance evaluation.The device in the push-pull operation has a BER smaller than 10 −6 in B2B transmission at 70 Gb/s.After transmission over 1 km of SSMF, the BER at 92 Gb/s is below the HD FEC threshold, and the BER at 100 Gb/s is below the SD FEC threshold.Moreover, 40 Gb/s PAM-4 transmission over 20 km of SSMF with BER lower than 3.8 × 10 −3 is also successfully demonstrated.

Conclusion
In this paper, we experimentally present a silicon photonic modulator for high-speed lowchirp PAM-4 transmission.The device is based on dual parallel MRMs inserted in a balanced MZI, and it is operated in a push-pull configuration for low-chirp transmission in C-band.We use a system level simulation for chirp analysis, and then present the measured constellation eye diagrams using 10 Gb/s OOK transmission up to 20 km of SSMF.The measured chirp parameters are all less than 0.01, further proving the low-chirp characteristic of the device.PAM-4 transmission at 92 Gb/s over 1 km of SSMF and at 40 Gb/s over 20 km of SSMF are successfully demonstrated, with measured BERs below the HD FEC threshold of 3.8 × 10 −3 .

Funding
Natural Sciences and Engineering Research Council of Canada (NSERC) (CRDPJ430446-12).

Figure 2 (
b) shows the simulated phases when various differential RF data levels are applied.The chirp of the modulator is quantitatively estimated by the chirp parameter [

Fig. 2 .
Fig. 2. Simulated (a) power transmission and (b) phase responses of the device in the push-pull operation.

Fig. 3 .
Fig. 3. Measured DC spectra when applying reverse bias voltages on the RF pad of (a) MRM 1 and (b) MRM 2.
Figure4shows the EE S11 responses and the electro-optic (EO) S21 responses of the two MRMs measured at 50 pm and 100 pm away from their resonant wavelengths (as marked by the dots in Fig.3).As shown in Fig.4(a), the EE S11 responses are the same at different detuned wavelengths for each MRM.However, the measured 3-dB EE bandwidth of MRM 1 is 29.9 GHz, which is 7.7 GHz less than that of MRM 2. The difference is also due to the variations in the fabrication process, such as the implantation levels and the misalignment of the pn junction.These variations result in different junction capacitances and resistances for the two MRMs, and therefore the measured 3-dB EE bandwidths are different.As shown in Fig.4(b), the measured EO S21 responses at the detuned wavelengths 50 pm away from the resonances are flat, but those at 100 pm detuning have peaking at high frequencies.This is because the light at larger wavelength detuning from the resonance is more easily released from the microring resonator, and thus it has a shorter photon life-time[23,24].This results in a peaking effect and an enhancement of the 3-dB EO bandwidth[23,24].The measured 3-dB EO bandwidths of MRM 2 are 16.8 GHz at 50 pm detuning and 23.3 GHz at 100 pm detuning.They are both larger than those of MRM 1, which are 15.1 GHz at 50 pm detuning and 21.4 GHz at 100 pm detuning.Since the two MRMs have the same Q-factor, or

Fig. 4 .
Fig. 4. Measured (a) EE S11 responses and (b) EO S21 responses of the two MRMs measured at 50 pm and 100 pm away from their resonant wavelengths.

Fig. 5 .
Fig. 5. (a) Experimental setup for chirp characterization, measured constellation diagrams and chirp parameters using 10 Gb/s OOK signals after (b) B2B, (c) 2 km and (d) 20 km of SSMF transmission.(x axis: real part of the electric field, y axis: imaginary part of the electric field.)

Fig. 7 .
Fig. 7. (a) Optical eye diagrams at 43. 75 Gb/s after B2B transmission obtained by the DCA, and eye diagrams obtained using offline DSP at (b) 80 Gb/s after 2 km, (c) 56 Gb/s after 10 km and (d) 34 Gb/s after 20 km of SSMF transmission.

Figure 7 (
Figure 7(a) shows the B2B 43.75 Gb/s optical eye diagram obtained by the DCA without applying the offline DSP on the receiver side.The four levels of modulated optical signals are clearly distinguished.Figures 7(b)-7(d) show the eye diagrams obtained using the offline DSP on the receiver side.The eye diagrams at 80 Gb/s after 2 km transmission, at 56 Gb/s after 10 km transmission, and at 34 Gb/s after 20 km transmission are all clearly open.As shown in