Vol. 24, No. 17 | 22 Aug 2016 | OPTICS EXPRESS 19467
Segmented silicon MZM for PAM-8 transmissions
at 114 Gb/s with binary signaling
A. D. SIMARD,1 B. FILION,1 D. PATEL,2 D. PLANT,2 AND
S. LAROCHELLE1,*
1
Centre d'optique, photonique et laser (COPL), Department of Electrical and Computer Engineering,
Université Laval, Québec, QC, G1V 0A6, Canada
2
Photonic Systems Group, Department of Electrical and Computer Engineering, McGill University,
Montréal, Québec, H3A 0E9, Canada
*
sophie.larochelle@gel.ulaval.ca
Abstract: We experimentally demonstrate PAM-8 generation from binary electrical signals
driving a silicon multi-electrode Mach-Zehnder modulator acting as an optical digital-toanalog converter. Measured BER in back-to-back configuration is used to evaluate signal
quality. We demonstrate 38 GBd PAM-8 transmission below the forward error correction
(FEC) threshold using minimum mean square error (MMSE) equalization. The results show
that modulators with segmented phase shifters can be advantageously used to eliminate the
need for high bandwidth electronic digital-to-analog converters in the generation of multilevel
signals. These modulators, that can be designed and fabricated with standard CMOS
compatible tools and processes, are of high interest for short range high-speed data links.
©2016 Optical Society of America
OCIS codes: (130.3120) Integrated optics devices; (250.7360) Waveguide modulators.
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#267499
Journal © 2016
http://dx.doi.org/10.1364/OE.24.019467
Received 13 Jun 2016; revised 3 Aug 2016; accepted 8 Aug 2016; published 12 Aug 2016
Vol. 24, No. 17 | 22 Aug 2016 | OPTICS EXPRESS 19468
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1. Introduction
The growing popularity of video streaming and cloud computing is pushing the development
of high bandwidth short optical links used in data centers. The commonly used 10G
technologies for these links, such as parallel vertical-cavity surface-emitting lasers (VCSELs),
are considered impractical for next generation systems. Different technologies are suggested
as a replacement, such as silicon modulators, indium phosphide modulators, directly
modulated distributed feedback lasers or long wavelength VCSELs [1,2]. Integrated silicon
photonics (SiP) is a technology with great potential for these applications due to its high level
of device integration, low-power operation and cost effectiveness [2,3]. The main types of SiP
modulator are the Mach-Zehnder modulators (MZMs), the microring modulators (MRMs) and
Bragg grating modulators (BGMs). MRMs, that show potential for low power consumption
and high density, have been demonstrated at 66 Gb/s with a power consumption of 15 fJ/bit
[4]. Recently, OOK modulation at bit rates reaching 55 Gb/s has been achieved with BGMs
[5] demonstrating that the performance of BGMs is now approaching the transmission rates of
MRMs. However, the sensitivity to temperature and fabrication variations limit the power
efficiency of these two resonant structures [6]. Although they are less compact, MZMs offer a
reliable temperature insensitive option that shows high performance, for example, a 70 Gb/s
modulator with a power consumption of 350 fJ/bit was demonstrated in [7].
In parallel to modulator technology development, higher-order modulation formats such as
pulse-amplitude modulation (PAM) or discrete multi-tone are also being investigated [8].
These modulation schemes are preferred due to their high spectral efficiency and
compatibility with direct detection, which are two properties of interest for short reach
interconnect data links in which dispersion is not an issue [9–11]. Moreover, multilevel
formats can also present an advantage in terms of the modulator power consumption provided
that the driving voltage requirement is not increased. This aspect has been investigated by
comparing OOK and PAM-4 implemented with the same driving voltage, using either MRM
[4] or MZM [7], demonstrating a 1.5-3 reduction in the energy per bit consumed by the
modulators when using PAM-4.
To generate PAM signals, different schemes can be implemented to eliminate the need for
expensive digital-to-analog converters (DACs) and lower the system complexity. For
example, segmented MZMs used directly as optical DACs have been proposed as early as
1980 [12]. Following the initial demonstration that used LiNbO3 technology, Ehrlichman et
al. theoretically investigated the linearity of these optical DACs [13]. More recently, the study
of segmented MZMs has found renewed interest in the context of high-capacity short reach
data links using integrated silicon photonic transmitters for multi-level formats [14].
Demonstrations of PAM-4 transmissions at 10 GBd (20 Gbit/s) have thus been performed
using a silicon-insulator-silicon capacitive (SISCAP) junction [15] and the same technology
Vol. 24, No. 17 | 22 Aug 2016 | OPTICS EXPRESS 19469
was applied to QAM-16 generation [16]. PAM-4 was also demonstrated up to 100 Gb/s using
a depletion based phase shifter [17]. Similar performances were achieved using a different
optical configuration which prevents the use of expensive digital-analog converters, namely
dual-parallel Mach-Zehnder interferometers [18]. Finally, very recently, 64-QAM at 32 GBd
was demonstrated using an indium phosphide IQ segmented Mach-Zehnder modulator [19].
In this paper, we present a silicon MZM DAC with multiple phase shifters in a series
push-pull (SPP) configuration achieving PAM-8 generation for transmissions above 100 Gb/s.
In section 2, we detail the modulator design and in section 3, we present characterization
results of the optical and electrical frequency responses. We also investigate the analog
bandwidth (BW) of the electrodes. Finally, in section 4, we perform system test under PAM-8
modulation. To the best of our knowledge, bit error rates below the forward error correction
(FEC) threshold of 3.8x10−3 up to 114 Gb/s and PAM-8 transmission without precompensation are reported here for the first time with a SiP modulator.
2. Modulator design
Figure 1 shows the schematic of the MZM designed to operate in the C-band. In order to
generate multi-level optical output while driving the modulator directly with binary data
streams (removing the need for a DAC), the phase shifter on each arm is segmented. The
segmented phase shifter design simplifies considerably the generation of multi-level optical
signals. To generate a PAM-8 signal, three data streams are sent to the three pairs of phase
shifters operating in SPP configuration with integrated 50 Ω terminations. The SPP
configuration has been chosen because it reduces the required voltage and the modulationinduced chirp. Furthermore, the integrated 50 Ω terminations were necessary to test the device
without packaging. The two arms of the MZM have an intentional asymmetry that results in a
free spectral range of 5.6 nm. This length imbalance allows adjusting the operation point by
changing the optical wavelength. In a packaged design, a thermal phase shifter would be
added to perform this function. The 3-dB couplers at the input and output are multimode
interferometers (MMI) identical to those described in [20].
The three phase shifters lengths are L, 2L, and 4L, where L is 500 μm. Assuming a linear
optical phase shift as a function of the phase shifter length, the segment lengths were chosen
to generate a multilevel optical signal with similar reverse bias and RF power applied to each
segment. However, due to fabrication errors and non-linearity in the electrode phase response,
fine tuning in RF power and bias voltage applied to each segment must be done. The shorter
and longer phase shifters are used for the least and most significant bit respectively.
Fig. 1. Schematic of the segmented MZM with SPP electrodes.
As described in [7], the transmission lines are co-planar strip (CPS) waveguides with Tshaped extensions to reduce the speed of the microwave signal and match the group velocity
of the optical mode. The modulator was modeled with the RLGC circuit model of the
transmission line augmented with a reverse biased PN junction modeled by a series RC circuit
in shunt with the transmission line. The values of the RLGC circuit elements of the standalone
CPS transmission line were extracted from a driven terminal s-parameter simulation using
HFSS software. The PN junction was subsequently added in MATLAB, from which the
propagation constant and the characteristic impedance were calculated. The SPP configuration
halves the PN junction capacitance seen across the transmission line leading to a reduction in
the microwave loss. However, this reduction in capacitance also results in a decrease of the
microwave index. Relative to regular CPS electrodes, these T-shaped extensions increase both
the inductance and capacitance of the transmission line, therefore increasing the index of the
Vol. 24, No. 17 | 22 Aug 2016 | OPTICS EXPRESS 19470
microwave. The capacitance is determined by the spacing between the edges of the T-shaped
extensions, while the inductance is controlled by the width of the stem of the T-shaped
extension. The stem and the pad areas were chosen to be the smallest allowed by the
fabrication design rules. The modulator electrodes were designed on the top 2-um thick metal
layer, the thickest of the metal layers available, to reduce the skin effect loss. Since the
characteristic impedance also changes as the ratio of the inductance to the capacitance, a set
of geometrical parameters giving the largest microwave index while maintaining an
impedance matching close to 50 Ω were chosen by performing various sweeps. The
impedance was also designed to match the 50 Ω of the RF drivers and the on-chip resistance.
Each section of the modulator was biased through a thin line to provide some inductance and
avoid the high speed RF signal from shorting to ground.
The modulator structure was fabricated at IME using 193 nm deep UV photolithography.
The wafer had a 220 nm top-silicon layer and a BOX thickness of 3 μm. The optical
waveguides outside the phase shifters are full-etched 500 nm wide strip waveguides. The
modulator ridge waveguides have a width of 500 nm and a 90 nm slab thickness. The PN
junction was positioned at the center of the waveguides. Finally, optical coupling is made
with focusing grating couplers optimized for 1550 nm.
3. Modulator characterization
In this section, the optical, electrical and electrical/optical (E/O) characteristics of the
modulator are investigated. The modulator insertion loss (spectral transmission) is measured
using a commercial wavelength-swept interferometric system (OVA, Luna Technology). A
typical spectral measurement is shown in Fig. 2(a). The 12 dB insertion loss caused by the
grating couplers were removed. This coupling loss could be reduced significantly for example
by using edge couplers with on-chip subwavelength gratings inverted tapers [21] or Si3N4
multi-rod [22]. The on-chip modulator loss (~5 dB) is detailed as follow: the background
waveguide propagation loss is about 1.5 dB, the loss due to doping is evaluated to be around
2.5 dB, while waveguide converters and MMI excess loss is ~1 dB.
Fig. 2. (a) Insertion loss measurement of the MZM without any voltage applied to the
electrodes (b) Normalized E/O |S21| for the long phase shifter with reverse bias voltages from
0 V to 8 V.
The electro-optical (E/O) characteristics of the device were measured with a 40 GHz
vector network analyzer. The normalized E/O S21 measurements are shown in Fig. 2(b) as a
function of reverse bias voltage for the long phase shifter. The E/O bandwidth is greater for
the short phase shifter since the microwave loss and the impact of the velocity mismatch
between the optical and the electrical wave are reduced. The 3 dB bandwidth is ~38 GHz for
the medium and long phase shifters while it is higher than 40 GHz for the short phase shifter.
Figure 2(b) shows that the 3 dB electrical bandwidth slightly increases with the reverse bias
voltage. However, due to the nonlinear dependence of the depletion width variation, and its
overlap with the optical mode field, as the reverse bias voltage changes, a larger eye opening
(i.e. separation between the levels in the vertical direction) is observed when the reverse bias
voltage approaches 0 V. This is expected because, at low reverse bias, modulation of the
Vol. 24, No. 17 | 22 Aug 2016 | OPTICS EXPRESS 19471
carrier density will occur in the center of the waveguide where the mode field is higher
resulting in a larger phase shift. Consequently, careful optimization of the reverse bias voltage
is required to obtain the best compromise between frequency response and eye opening. There
is also a trade-off between the modulator extinction ratio and the optical modulation
amplitude that, in the present case, is optimized by wavelength tuning.
4. System analysis
The experimental setup is shown in Fig. 3. To generate the PAM-8 signal, each segment of
the MZM is electrically driven by a two-level PRBS of length 215-1 generated by a bit pattern
generator (BPG). Two tunable delays (not shown) are used for decorrelation and also for skew
adjustment to ensure a perfect phase match. Each binary stream is fed into a RF amplifier. For
all experiments, the modulation peak-to-peak voltage is 4 V (VRF,3 in Fig. 3) for the long
phase shifter and 3.75 V for the middle and short segments (VRF,2 and VRF,1 in Fig. 3); our
experimental setup imposed a limitation on the middle and short segments voltage amplitudes
that could not be adjusted independently. Additionally, the reverse bias voltage for all the PN
junctions is set to 2 V for the three phase shifters (VDC,1 = VDC,2 = VDC,3 in Fig. 3) for all the
experiments. The signal wavelength is set to 1551.18 nm and its polarization is adjusted with
a polarization controller (PC) to maximize coupling into the fundamental TE mode. We
evaluate that ~1 dBm is injected into the MZM. After modulation, the signal is fed into an
erbium-doped fiber amplifier (EDFA) followed by a tunable optical bandpass filter. A
variable optical attenuator (VOA) is used to sweep the received power at the photodetector
(PD), which has a 70 GHz bandwidth but no integrated transimpedance amplifier. Finally, the
detected signal is captured with a real-time oscilloscope (RTO) (63 GHz bandwidth, 160
GSa/s sampling rate, Keysight DSOZ634A) for offline signal processing and BER
calculations.
Fig. 3. Experimental setup used to characterize PAM-8 transmission with the segmented MZM.
The digital signal processing is as follows. First, a second order Gaussian low-pass filter is
applied. Retiming and resampling at the symbol rate are next performed and a MMSE
equalizer with 50 taps trained over 2000 symbols is then applied. Finally, the vertical decision
thresholds are optimized to minimize the BER. For each measurement, the BER is calculated
over 50 000 symbols. Figure 4 shows the eye diagrams captured with an optical sampling
oscilloscope with 5 dBm received optical power and Fig. 5 shows measured BER against the
received power for 10, 20, 30, and 38 Gbaud. Assuming a FEC threshold of 3.8x10−3, errorfree transmission is achieved at a received power of approximately −5.4 dBm, −2.2 dBm, −0.6
dBm, and 2.4 dBm for 10 Gbaud (30 Gbps), 20 Gbaud (60 Gbps), 30 Gbaud (90 Gbps), and
38 Gbaud (114 Gbps), respectively.
Vol. 24, No. 17 | 22 Aug 2016 | OPTICS EXPRESS 19472
Fig. 4. Eye diagrams with 5 dBm received power.
Fig. 5. BER as a function of the received power.
The power consumption of this modulator with SPP electrodes, dominated by resistive
3
2
power loss, can be estimated as in [7] according to P =  Vrms
R where R is the
,i
i =1
termination resistance and Vrms,i are the rms driving voltages of the three electrode segments.
This estimate implies that the line impedance is matched to the source and termination (50 Ω).
The calculated energy per bit is Eb ≈7.7 pJ/bit. It should be noted that the modulator was
specifically designed to be driven by the same voltages for all three segments and therefore
was not optimized to minimize power consumption. Since the bandwidth is limited by the
longest electrode, using same length electrodes for all segment could help to reduce power
consumption but at the expense of higher optical insertion loss. This aspect requires further
investigation as optimization of power consumption should also consider equalizer
requirements and the fact that the segmented MZM removes the need for an electronic DAC.
5. Conclusion
We achieved for the first time, to the best of our knowledge, 114 Gbps PAM-8 modulation
with measured BER below the FEC threshold (3.8x10−3) with a multi-electrode silicon MZM
DAC and MMSE equalization. Our results demonstrate that silicon multi-electrode MZM
DACs offer a practical solution for the needs of short reach 100G and 400G links.
Funding
The SPEED research project (Silicon Photonic Electrically Engineered Devices) is funded by
NSERC (RDCPJ438811-12), PROMPT (PJT-2011-17), Ciena and the Canada Research Chair
in Advanced photonic technologies for communications (APTEC).
Acknowledgments
We acknowledge the scientific and technical support of CMC Microsystems and Keysight.