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Journal ArticleDOI

Silicon photonic modulators for PAM transmissions

05 Jul 2018-Journal of Optics (IOP Publishing)-Vol. 20, Iss: 8, pp 083002

Abstract: High-speed optical interconnects are crucial for both high performance computing and data centers. High power consumption and limited device bandwidth have hindered the move to higher optical transmission speeds. Integrated optical transceivers in silicon photonics using pulse-amplitude modulation (PAM) are a promising solution to increase data rates. In this paper, we review recent progress in silicon photonics for PAM transmissions. Copyright (c) 2018 IOP. Personal use is permitted. For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.

Summary (6 min read)

1. Introduction

  • Multi-level pulse-amplitude modulation (PAM) provides an economically viable approach to higher data rates with affordable cost and complexity.
  • Short-reach applications, such as optical links in data centers, require high data rates, but have more stringent requirements on cost and energy consumption than other applications.
  • PAM provides a good compromise between data rate and complexity.
  • Optical interconnects are the major commercial application of silicon photonics.
  • The authors first discuss the figures of merit of modulators and considerations on optimizing silicon photonic modulators for PAM transmission (Section 2).

2. Performance and figures of merit

  • The authors discuss performance metrics of SiP modulators and crucial considerations for high-speed PAM transmissions.
  • To facilitate their discussion, the authors first briefly review fundamentals of silicon electro-optical phase shifters, including the phase modulation mechanisms amd commonly used phase shifter structures.
  • Then the authors discuss, in the context of a PAM transmission link, the power penalties induced by a SiP modulator due to its limited efficiency, insertion loss and limited bandwidth.
  • Based on the transmission link penalty, the authors discuss conventional figures of merit (FOM) of optical modulators and present a new FOM proposed for SiP PAM modulators.
  • At the end of this section, the authors discuss DSP options for PAM transmissions.

2.1.1. Phase modulation in silicon

  • Since the linear electro-optical effect (Pockels effect) is absent in unstrained silicon, the plasma dispersion effect is the most commonly used to achieved phase modulation in silicon [8].
  • For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.
  • Note that the free-carrer absorption is modulated in the same time.
  • Because the waveguide is typically designed to have a width (e.g., 500 nm) larger than than the height (e.g., 220 nm), the vertical junction has a larger depletion region overlapped with the optical mode and thus a better modulation efficiency.
  • Silicon photonic modulators for PAM transmissions 5 the modulation efficiency of SISCAP is an order-of-magnitude higher than that of the depletion-mode phase shifter using a lateral pn-junction, but also has a significantly higher absorption loss and capacitance.

2.2. Figure of merit of MZM design: efficiency, loss, and bandwidth

  • Consider the normalized transfer function of a typical Mach-Zehnder modulator (MZM) in figure 2.
  • Several impairments affecting PAM performance can be identified in the transfer function.
  • The authors refer to this affect as modulation loss, which is determined by the operating point, i.e., the input voltage excursion vis-a-vis Vπ.
  • The lower inset in figure 2 (in pink) is a typical electrical multi-level eye diagram, in this example PAM-4.
  • Limited bandwidth leads to a more closed eye, which the authors refer to as the intersymbol interference (ISI) penalty.

2.2.1. Efficiency as a FOM

  • Conventional modulators (such as LiNiO3) have low loss and, due to the linear electrooptic effect, the effective index change is linear in applied voltage.
  • Finally, the authors find the decrease in the eye opening on the upper most eye, as illustrated in Fig. 4, and use it as the nominal ISI penalty.
  • When the optical loss and limited extinction ratio are the only shortcomings of the modulator, higher extinction ratio leads to a lower (old) TPP, as seen in Fig. 2b.
  • 5. New TPP from (9), with y-axis at left, for different baud rates (solid lines) when Vpp = 4|Vb |= 3V; black dashed line is old TPP without bandwidth effects; pink dotted line is electro-optic bandwidth BWEO.as a function of L, with y-axis at right.
  • The insets show the noiseless eye diagrams for PAM-4 modulation format with gaussian shaped pulses with BWEO/BR = 0.9.

2.2.2. FOM for PAM

  • SiP modulators should be designed to have good extinction ratio and wide bandwidth.
  • Modulation loss is determined by the relationship between Vπ and L; this relationship is nonlinear due to the nonlinear variation of effective index of refraction with applied voltage.
  • Given a certain context of device-level design - a certain fabrication process, reference modulator design, RF and optical velocity matching, etc. - the authors would like to find the optimal value of L. Rather than using extinction ratio and bandwidth alone as quality indicators, they would ideally include system-level design choices such as modulation format, baud rate, driving strategy (linear region, CMOS drivers, low power) etc.
  • In [11, 12] the authors propose a figure of merit to capture the bandwidth impairments Copyright (c) 2018 IOP.
  • The authors see in figure 3 how Pout is found numerically, and how the MPP is related to contributing loss (material loss, extinction ratio and ISI).

2.2.3. FOM performance

  • The authors examine four different baud rates, and see a bowl shape for all plots.
  • For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.
  • At high L the minima are different, however the bowl is very shallow, so the MPP associated with the minima are almost equal.
  • The authors new FOM is a simple closed form expression of easily identified and/or measured quantities.
  • Thus optimal modulators can be designed for a given system target (modulation level M and baud rate BR).

2.3. Energy consumption

  • The energy consumption per unit bit is a crucial performance metric for accessing an optical link.
  • For a depletion-mode modulator using traveling-wave electrodes (e.g., a single TW- MZM or a phase-shifter segment applying a TW electrode in the BWS driving scheme), Copyright (c) 2018 IOP.
  • For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.
  • For a given baud rate, the energy per bit of a TW-MZM decreases with the PAM order.
  • In case where more than one modulation elements in a modulator, the total power consumption are simply the their summation.

2.4. Linearity

  • Linear amplitude response is desired to achieve equally spaced eyes in PAM signal since the BER is limited by the least-open eye.
  • It is tested using two closely spaced frequency tones as inputs (here defined as f1 and f2) and measuring the linear outputs (at f1 and Copyright (c) 2018 IOP.
  • SiP MZMs showed good linearity [18–20].
  • Up to 88.9 dB·Hz1/2 of second-order SFDR and 113.3 dB·Hz2/3 of third-order IMD-SFDR were measured using a SiP MZM [20].
  • In [15], in order to achieve equally spaced optical eye in a high-speed PAM MRM, the electrical driving signal was pre-distorted with the extent depending on the bias and frequency detuning.

2.5. Chirp

  • MZMs in push-pull operation generally have very low chirp.
  • MRMs have a stronger chirp that can also be suppressed by using the RA-MZI structure [27, 28].
  • Up to 20-km 112-Gb/s transmission of PAM-4 using an O-band SiP modulator has been achieved without dispersion compensation [30].
  • C-band PAM transmission suffers from strong chromatic dispersion of Copyright (c) 2018 IOP.
  • For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.

2.6. Timing impairments

  • In addition to the compression of eye opening due to the previously discussed imperfections (due to insertion loss, voltage-limited ER, and limited BW), eye diagram skew may also cause power penalties and increase the DSP burden.
  • Skew makes determination of the optimum sampling point ambiguous, as it varies from eye to eye.
  • The eye skew is becoming a crucial issue for VCSELs [32].
  • There is little research into the origin of eye skew in SiP modulators; for SiP MZMs eye skew is much less significant than it is for VCSELs.
  • In segmented MZMs or cascaded architectures using compact MRMs (e.g., DAC-less driving configurations to be discussed in the next section), the skew will be exacerbated in the presence of delay mismatch between their driving signals [34, 35].

2.7. DSP options

  • DSP can significantly increase the baud rate and the transmission distance of an optical link.
  • Bandwidth limitations in particular can be overcome by pulse shaping in the transmitter side and equalization in the receiver side.
  • Application specific integrated circuits for DSP might be larger with equalization, but that is only a small increase in complexity.
  • For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.
  • The FEC complexity depends on the decoding technology used; higher performance decoding is iterative.

3.1. Mach-Zehnder Modulator

  • For generation of optical PAM signals, the conventional method is to generate the multilevel signals in the electrical domain.
  • For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.
  • In the segmented modulator, the driving signals are applied sequentially to the segments with precisely tunable timing circuits to match the optical signal delay between different segments.
  • The other segmentation approach uses weighted binary codes to determine the length of each segment, as illustrated in figure 5(c).
  • PAM-4 transmission at 20 Gb/s used a segmented MZM with a siliconinsulator-silicon capacitive junction [47].

3.2. Microring modulators

  • In addition to MZMs, microring modulators (MRM) [50–56] are another popular solution widely examined for integrated optical transmitters.
  • MRMs provide a scalable solution for optical interconnects and are attractive for highbandwidth-density chip-level communications [57].
  • A phase shifter is inserted in the ring cavity for intra-cavity modulation.
  • Amplitude modulation of an MRM can also be achieved by modulating the coupling between the microring cavity and the bus waveguide.
  • For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.

3.3. Driving topologies using binary amplitude modulators

  • In the driving architectures discussed previously, the optical PAM signal is achieved, in general, through phase modulation and optical interference or resonance.
  • Figure 6 shows two architectures for PAM-4. Each AM produces a different OMA: one acts as MSB and the other as LSB.
  • In the parallel architecture, figure 6(b), the response of the PAM modulator is the vector addition of the transfer function of each AM.
  • This could offer a better power penalty than the series architecture if the AM has an insertion loss greater than 3 dB.

4. State-of-the-art demonstrations

  • The authors review some of the recently demonstrated SiP modulators and integrated transmitters.
  • A new figure of merit for SiP modulators was presented recently in [14] that takes into account system parameters such as peak-to-peak drive voltage, bit rate, modulator rise-fall time, and relative optical modulation amplitude, similar to the objectives of this paper.
  • Various simulations are presented to demonstrate the utility of the FOM.
  • The authors use lateral pn junction doping densities of NA=5×1017 cm-3 and ND=3×1017 cm-3 and a wavelength of = 1550nm, unless otherwise noted.
  • Let Vin be the RF input voltage that can be positive or negative and falls between –Vpp/2 and Vpp/2.

4.1. MZMs for PAM

  • Among various phase shifter configurations, the lateral pn-junction in a depletion mode is the most popular for high-speed modulators in the generic silicon photonics processes.
  • For any other purposes, permis ion must be obtained from the IOP by emailin permissions@iop.org.
  • For PAM-4, up to 168 Gb/s generation and 128 Gb/s transmission over 1 km have been demonstrated [48] .
  • Figure 3(a) illustrates the measured phase shift for each segment of the ME-MZM and the TWMZM.
  • On the r ceive side, the authors replace the DCA with a 50 GHz, 0.65 A/W photodetector for optical to electrical conversion and a 63 GHz real time oscilloscope (RTO) to capture the modulated signals and store it for the offline receiver DSP and error counting.

4.2. Resonator-based modulators for PAM

  • Resonator-based modulators are appealing for optical interconnects due to their low energy consumption and high bandwidth density.
  • For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.
  • Cascaded resonator structures can also provide a wider bandwidth, which decreases the temperature sensitivity of the modulator.
  • Dual-MRM in serial/ Lateral-pn PAM-4 60 (7%) B2B Rx 2 (LSB)/ 5 (MSB) 45 [87] PAM-4 transmission results up to 30 Gbaud (60 Gbit/s) were achieved.

4.3. EAM for PAM transmission

  • As a CMOS compatible material system, GeSi has a strong electro-absorption (EA) effect which is intrinsically an ultrafast process, enabling high-speed modulation.
  • For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.
  • Typically, Ge EAMs work in Lband (around 1600 nm) and could be shifted to C-band (1550 nm) with the incorporation of Si during Ge epitaxial growth.
  • At these wavelengths, the relatively large chromatic dispersion coefficient may induce notches in the frequency response of optical link which limits the maximal fiber span without chromatic distortion compensation techniques.
  • To obviate the electrical DAC and power-hungry DSP, Verbist, et al., [65] generated 112 Gb/s PAM-4 by the vector addition of two binary driven EAMs in parallel.

4.4. Electronic-photonic integrated transmitters

  • Co-design and integration of optical modulators and CMOS or BiCMOS drivers for high-speed silicon photonic transmitters may reach their full potential for low-cost, lowpower electronic-photonic integrated systems.
  • For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.
  • In [94], a flexible transmitter employing segmented MZM and its distributed CMOS driver in 32 nm CMOS is reported, where on-chip optical equalization was performed to improve transmitter performance.
  • Higher voltage-swing and faster transistors available in BiCMOS pave the way for demonstration of BiCMOS driver and SiP modulator co-design for PAM-M modulation format [97, 98].
  • While its power consumption (30 fJ/bit) is higher than the CMOS solutions [91–93, 96], a very large extinction ratio (up to 13 dB) was achieved with a long MZM concept.

5. Outlook and conclusion

  • Through years of intensive research and development, researchers and engineers have developed in-depth understating of the capacity, design and performance trade-offs of Silicon photonic modulators for PAM transmissions 26 SiP modulators on generic integration platforms for high-speed PAM transmissions.
  • For next generations of optical links, targeting terabits or even petabits, putting aside wavelength and spatial multiplexing, optical modulators should run at much higher baud rates with higher modulator formats.
  • III-V compound materials are likely to be first adopted in a generic silicon photonics process due to the urgent need for on-chip amplifiers and lasers.
  • SiP modulators will continue advancing along with the process and platform, as well as further advances in low-power CMOS and packaging technologies.
  • For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.

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Silicon photonic modulators for PAM
transmissions
Wei Shi, Yelong Xu, Hassan Sepehrian, Sophie LaRochelle, and Leslie A. Rusch
IOP Journal of Optics, (Volume 20, Issue 8) (2018)
Doi: 10.1088/2040-8986/aacd65
http://iopscience.iop.org/article/10.1088/2040-8986/aacd65/meta
© 2018 Institute of Physics Publishing (IOP). Personal use of this material is
permitted. However, permission to reprint/republish this material for advertising
or promotional purposes or for creating new collective works for resale or
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work in other works must be obtained from the IOP and/or its partner(s).

Silicon photonic modulators for PAM transmissions
Wei Shi, Yelong Xu, Hassan Sepehrian, Sophie LaRochelle, and
Leslie A. Rusch
1) Department of Electrical and Computer Engineering and 2) Center for Optics,
Photonics, and Lasers (COPL), Universit´e Laval, Qu´ebec, QC, Canada, G1V 0A6.
E-mail: wei.shi@gel.ulaval.ca
March 2018
Abstract. High-speed optical interconnects are crucial for both high performance
computing and data centers. High power consumption and limited device bandwidth
have hindered the move to higher optical transmission speeds. Integrated optical
transceivers in silicon photonics using pulse-amplitude modulation (PAM) are a
promising solution to increase data rates. In this paper, we review recent progress
in silicon photonics for PAM transmissions.
Copyright (c) 2018 IOP. Personal use is permitted. For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.

Silicon photonic modulators for PAM transmissions 2
1. Introduction
Multi-level pulse-amplitude modulation (PAM) provides an economically viable
approach to higher data rates with affordable cost and complexity. Conventional non-
return-to-zero (NRZ) modulation cannot support higher data rates required in the next
generations of optical links in data centers. This is simply due to the fact that the analog
bandwidth of optical and electrical components cannot keep pace with the growth in data
traffic. Increasing the number of fibers (e.g., achieving 25 Gb/s 400 Gb/s via VCSELs by
16 separate multimode fibers) could meet demand, but is not economically viable. For
reach greater than a few hundred meters, single-mode fibers are used, but fiber costs
go up. Higher order modulation formats, such as quadrature amplitude modulation
(QAM), and coherent detection can effectively increase single-channel data rates within
a given analog bandwidth, and are exploited in commercial, long-haul transmission
systems. However, coherent detection is expensive and requires complex optical and
electronic components, such as local oscillator and fast digital signal processors (DSP).
Short-reach applications, such as optical links in data centers, require high data
rates, but have more stringent requirements on cost and energy consumption than other
applications. PAM provides a good compromise between data rate and complexity.
PAM allows for direct direction of optical intensity signals without requiring complex
DSP, although good signal-to-noise ratio is required. The IEEE 400 Gb/s Ethernet
task force (IEEE 802.3bs) has converged on 4-level PAM (PAM-4) for links from 500 m
to 10 km [1]. This can be realized using 50 Gb/s by 8 wavelengths or 100 Gb/s by 4
wavelengths in a grid of coarse wavelength division multiplexing (CWDM) or local area
network (LAN) WDM. Future terabit links may require higher-order PAM and a finer
WDM grid for higher spectral efficiency and transmission capacity.
Silicon photonics (SiP) has quickly emerged as an enabling technology for large-
scale integrated photonic circuits [2, 3] Capable of manipulating electrons and photons
on the same platform, silicon photonics promises to pack more functionality on a single
chip [4]. It uses the advanced manufacturing process of microprocessors, known as
the CMOS process, by which tens of billions of transistors have been integrated on
a single chip. This disruptive technology is based on high-index-contrast materials
allowing for extremely strong optical confinement on the nanometer-scale, leading to
orders-of-magnitude reduction in footprint and cost. Optical interconnects are the major
commercial application of silicon photonics. They are driven by the quickly increasing
demand for high-speed optical links in data centers [5].
Optical modulation is an essential function in an optical link. Since the first GHz
demonstration [6], silicon optical modulators have attracted tremendous interest [7].
Silicon photonics foundries are now widely accessible to academic researchers and small
industry groups. This has considerably accelerated the research and development of
SiP modulators in the last few years. Various devices and driving strategies have been
examined for optical PAM signal generation on silicon. Significant effort has led to
record-breaking demonstrations of single-channel data rates beyond 100 Gb/s and ultra-
Copyright (c) 2018 IOP. Personal use is permitted. For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.

Silicon photonic modulators for PAM transmissions 3
low energy consumption down to 1 fJ/bit. Substantial progress in CMOS-photonics
co-design and integration has been made towards fully integrated CMOS-driven optical
PAM transmitters.
Here, we review recent progress on silicon photonic modulators and transmitters
for PAM transmissions. While there have been significant progress and impressive
demonstrations using new materials and hybrid structures, we will focus on materials
(doped silicon and germanium) and technologies available in generic large-wafer CMOS-
compatible photonics processes. The rest of this paper is organized as follows. We
first discuss the figures of merit of modulators and considerations on optimizing silicon
photonic modulators for PAM transmission (Section 2). In Section 3, we review driving
strategies to achieve optical PAM signals. Section 4 reviews some of the sate-of-the-art
devices, including modulators and CMOS-photonic PAM transmitters. In Section 5, we
discuss the challenges and opportunities for future terabit optical links.
2. Performance and figures of merit
In this section, we discuss performance metrics of SiP modulators and crucial
considerations for high-speed PAM transmissions. The section is organized as follows.
To facilitate our discussion, we first briefly review fundamentals of silicon electro-optical
phase shifters, including the phase modulation mechanisms amd commonly used phase
shifter structures. Then we discuss, in the context of a PAM transmission link, the
power penalties induced by a SiP modulator due to its limited efficiency, insertion loss
and limited bandwidth. Based on the transmission link penalty, we discuss conventional
figures of merit (FOM) of optical modulators and present a new FOM proposed for SiP
PAM modulators. Afterwards, we review other important characteristics need to be
considered in designing a PAM link using SiP modulators, such as energy consumption,
chirp, linearity, and timing impairments. At the end of this section, we discuss DSP
options for PAM transmissions.
2.1. Silicon electro-optical phase shifters
2.1.1. Phase modulation in silicon
Since the linear electro-optical effect (Pockels effect) is absent in unstrained silicon, the
plasma dispersion effect is the most commonly used to achieved phase modulation in
silicon [8]. Using the Drude model [9], the change of refractive index n
Si
and the
excess loss α
Si
due to the free-carrier absorption are functions of density variations in
free electrons and holes ( n and p) and are given by
n
Si
= 3.64 × 10
10
λ
2
0
n 3.51 × 10
6
λ
2
0
p
0.8
(1)
where the wavelength (λ
0
) is given in meter, the carrier densities per cubic centimeter,
and the change in absorption in per centimeter.
α
Si
= 3.52 × 10
6
λ
2
0
n + 2.4 × 10
6
λ
2
0
p (2)
Copyright (c) 2018 IOP. Personal use is permitted. For any other purposes, permission must be obtained from the IOP by emailing permissions@iop.org.

Silicon photonic modulators for PAM transmissions 4
Using the plasma dispersion effect, various modulation mechanisms (such as carrier
accumulation, carrier injection, and carrier depletion) and their implementation
structures (namely, phase shifters) have been examined [7]. In all the mechanisms,
the essential idea is to vary the silicon waveguide’s effective index by changing the free-
carrier distributions. The phase modulation is proportional to the change of the effective
index n
eff
as function of the applied voltage V , which can be calculated performing
the mode overlap between the optical field (E) and the free-carrier distributions:
n
eff
(V ) =
dn
eff
dn
Si
Z Z Z
E
· n
Si
(V )E
E
· E
dxdydz (3)
The term dn
eff
/dn
Si
describes the dependence of the effective index n
eff
on the material
index n
Si
and is very close to one. Here we define x and y to be the vertical and lateral
direction, respectively, and z the longitudinal direction of the waveguide. Note that the
free-carrer absorption is modulated in the same time.
α
pn
(V ) =
Z Z Z
E
· α
Si
(V )E
E
· E
dxdydz (4)
Clearly, increasing the carrier densities and their overlap with the optical field leads
to a higher modulation efficiency but also causes higher absorption. The design of a
silicon phase shifter is a compromise between efficiency and loss. The carrier transport
dynamics (present as RC elements in the circuit level) also needs to be considered as it
limits the modulation speed. These tradeoffs will be further discussed in the following
subsection in examining the modulator FOM.
Figure 1 shows cross-sectional schematics of three phase-shifter structures the most
explored for SiP PAM modulators. The first applies the mechanism of carrier depletion.
It uses a lateral pn-junction embedded in a rib waveguide (figure 1a). The diode works
under reverse bias where the width of depletion region changes as function of the voltage
across the diode. Since there exists no free carrier in the depletion region, the effective
index of the waveguide also changes as the voltage.
The second structure also works in the depletion mode but uses a vertical pn-
junction (figure 1b) where the optical mode overlaps with the depletion region along
the lateral direction. Because the waveguide is typically designed to have a width
(e.g., 500 nm) larger than than the height (e.g., 220 nm), the vertical junction has a
larger depletion region overlapped with the optical mode and thus a better modulation
efficiency. However, for the same reason, it also has a higher abortion and a bigger
junction capacitance that limits the operating bandwidth.
The third structure is based on a silicon-insulator-silicon capacitor (SISCAP)
structure [10] with a very thin oxide layer (typically 5 to 10 nm) between a SOI layer
in the bottom and a poly-Si on the top. The accumulated charges (free carriers) on
the top and bottom of the oxide capacitor are changed by the applied voltage for
phase modulation. Due to the better mode overlap and higher free-carrier densities,
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Figures (15)
Citations
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Journal ArticleDOI
Xiaodan Pang1, Weisheng Hu2, Gunnar Jacobsen, Sergei Popov3  +9 moreInstitutions (4)
15 Jan 2020
TL;DR: This article focuses on IM/DD transmissions, and provides an overview of recent research and development efforts on key enabling technologies for 200 Gbps per lane and beyond, and expects high-speed IM/ DD systems will remain advantageous in terms of system cost, power consumption, and footprint for short reach applications in the short- to mid- term perspective.
Abstract: Client-side optics are facing an ever-increasing upgrading pace, driven by upcoming 5G related services and datacenter applications. The demand for a single lane data rate is soon approaching 200 Gbps. To meet such high-speed requirement, all segments of traditional intensity modulation direct detection (IM/DD) technologies are being challenged. The characteristics of electrical and optoelectronic components and the performance of modulation, coding, and digital signal processing (DSP) techniques are being stretched to their limits. In this context, we witnessed technological breakthroughs in several aspects, including development of broadband devices, novel modulation formats and coding, and high-performance DSP algorithms for the past few years. A great momentum has been accumulated to overcome the aforementioned challenges. In this article, we focus on IM/DD transmissions, and provide an overview of recent research and development efforts on key enabling technologies for 200 Gbps per lane and beyond. Our recent demonstrations of 200 Gbps short-reach transmissions with 4-level pulse amplitude modulation (PAM) and discrete multitone signals are also presented as examples to show the system requirements in terms of device characteristics and DSP performance. Apart from digital coherent technologies and advanced direct detection systems, such as Stokes–vector and Kramers–Kronig schemes, we expect high-speed IM/DD systems will remain advantageous in terms of system cost, power consumption, and footprint for short reach applications in the short- to mid- term perspective.

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Abstract: Abstract The tremendous growth of data traffic has spurred a rapid evolution of optical communications for a higher data transmission capacity. Next-generation fiber-optic communication systems will require dramatically increased complexity that cannot be obtained using discrete components. In this context, silicon photonics is quickly maturing. Capable of manipulating electrons and photons on the same platform, this disruptive technology promises to cram more complexity on a single chip, leading to orders-of-magnitude reduction of integrated photonic systems in size, energy, and cost. This paper provides a system perspective and reviews recent progress in silicon photonics probing all dimensions of light to scale the capacity of fiber-optic networks toward terabits-per-second per optical interface and petabits-per-second per transmission link. Firstly, we overview fundamentals and the evolving trends of silicon photonic fabrication process. Then, we focus on recent progress in silicon coherent optical transceivers. Further scaling the system capacity requires multiplexing techniques in all the dimensions of light: wavelength, polarization, and space, for which we have seen impressive demonstrations of on-chip functionalities such as polarization diversity circuits and wavelength- and space-division multiplexers. Despite these advances, large-scale silicon photonic integrated circuits incorporating a variety of active and passive functionalities still face considerable challenges, many of which will eventually be addressed as the technology continues evolving with the entire ecosystem at a fast pace.

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  • ...Challenges, design, and optimization of silicon modulators for general applications and, in particular, for pulse amplitude modulation form a broad and rich topic and have been detailed in recent review papers [107, 108]....

    [...]

  • ...A bandwidth-aware figure of merit (FOM) was recently proposed for silicon modulators in a pulse-amplitude modulation (PAM) link [107, 110] to minimize this penalty....

    [...]

  • ...Bandwidths ranging from 30 to 60 GHz [107, 111, 125, 126] have been reported for all-silicon TW-MZMs, depending on the bias and the length of the modulator....

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  • ...A numerical approach was used for modulator design optimization (minimum MPP) for coherent transmissions at 400 Gb/s and beyond [111]....

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  • ...This binary-weighted segmentation (BWS) scheme has been examined for short-reach links with direct detection [107, 125, 127], where a PAM signal can be achieved using binary inputs directly applied to the RF drivers without a digital-to-analog converter (DAC)....

    [...]


Journal ArticleDOI
Abstract: Silicon photonics has enormous potential for ultrahigh-capacity coherent optical transceivers. We demonstrate an in-phase and quadrature (IQ) modulator using silicon photonic traveling-wave modulators optimized for higher order quadrature amplitude modulation (QAM). Its optical and RF characteristics are studied thoroughly in simulation and experiment. We propose a system-orientated approach to optimization of the silicon photonic IQ modulator, which minimizes modulator-induced power penalty in a QAM transmission link. We examine the tradeoff between modulation efficiency and bandwidth for the optimal combination of modulator length and bias voltage to maximize the clear distance between adjacent constellation points. This optimum depends on baud rate and modulation format, as well as achievable driving voltage swing. Measured results confirm our prediction using the proposed methodology. Without precompensating bandwidth limitation of the modulator, net data rates up to 232 Gb/s (70 Gbaud 16-QAM) on single polarization are captured, indicating great potential for 400+ Gb/s dual-polarization transmission.

16 citations


Cites background from "Silicon photonic modulators for PAM..."

  • ...Most of the previously reported works [2] focused on maximizing the electro-...

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  • ...This is one of the major driving forces for research in silicon photonics (SiP) [1], [2]....

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  • ...formats, with direct detection [2]–[6] or coherent detection [7]–[10], there exists a gap between optimization efforts in the design of SiP modulators and in the system-level performance....

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Journal ArticleDOI
TL;DR: A compact direct current injection thermo-optic switch based on a Mach-Zehnder Interferometer configuration that is suitable for autonomous vehicle applications as it has a low heating resistance value, a rapid 2.16 μs switching time constant, and a Pπ of 28 mW.
Abstract: In this paper we present a compact direct current injection thermo-optic switch based on a Mach-Zehnder Interferometer configuration that is suitable for autonomous vehicle applications as it has a low heating resistance value of 97 Ω, a rapid 2.16 μs switching time constant, and a Pπ of 28 mW. The device relies on multimode interference to achieve low optical insertion losses of less than 1.1 dB per device, while allowing direct current injection to heat the waveguide and achieve fast operation speeds. Furthermore, the total resistive value can be tailored as the heating elements are placed in parallel.

15 citations


References
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Journal ArticleDOI
Ming Liu1, Xiaobo Yin1, Erick Ulin-Avila1, Baisong Geng1  +6 moreInstitutions (2)
02 Jun 2011-Nature
TL;DR: Graphene-based optical modulation mechanism, with combined advantages of compact footprint, low operation voltage and ultrafast modulation speed across a broad range of wavelengths, can enable novel architectures for on-chip optical communications.
Abstract: Graphene, the single-atom-thick form of carbon, holds promise for many applications, notably in electronics where it can complement or be integrated with silicon-based devices. Intense efforts have been devoted to develop a key enabling device, a broadband, fast optical modulator with a small device footprint. Now Liu et al. demonstrate an exciting new possibility for graphene in the area of on-chip optical communication: a graphene-based optical modulator integrated with a silicon chip. This new device relies on the electrical tuning of the Fermi level of the graphene sheet, and achieves modulation of guided light at frequencies over 1 gigahertz, together with a broad operating spectrum. At just 25 square micrometres in area, it is one of the smallest of its type. Integrated optical modulators with high modulation speed, small footprint and large optical bandwidth are poised to be the enabling devices for on-chip optical interconnects1,2. Semiconductor modulators have therefore been heavily researched over the past few years. However, the device footprint of silicon-based modulators is of the order of millimetres, owing to its weak electro-optical properties3. Germanium and compound semiconductors, on the other hand, face the major challenge of integration with existing silicon electronics and photonics platforms4,5,6. Integrating silicon modulators with high-quality-factor optical resonators increases the modulation strength, but these devices suffer from intrinsic narrow bandwidth and require sophisticated optical design; they also have stringent fabrication requirements and limited temperature tolerances7. Finding a complementary metal-oxide-semiconductor (CMOS)-compatible material with adequate modulation speed and strength has therefore become a task of not only scientific interest, but also industrial importance. Here we experimentally demonstrate a broadband, high-speed, waveguide-integrated electroabsorption modulator based on monolayer graphene. By electrically tuning the Fermi level of the graphene sheet, we demonstrate modulation of the guided light at frequencies over 1 GHz, together with a broad operation spectrum that ranges from 1.35 to 1.6 µm under ambient conditions. The high modulation efficiency of graphene results in an active device area of merely 25 µm2, which is among the smallest to date. This graphene-based optical modulation mechanism, with combined advantages of compact footprint, low operation voltage and ultrafast modulation speed across a broad range of wavelengths, can enable novel architectures for on-chip optical communications.

2,764 citations


Journal ArticleDOI
Richard A. Soref1, Brian R. BennettInstitutions (1)
Abstract: A numerical Kramers-Kronig analysis is used to predict the refractive-index perturbations produced in crystalline silicon by applied electric fields or by charge carriers. Results are obtained over the 1.0-2.0 \mu m optical wavelength range. The analysis makes use of experimental electroabsorption spectra and impurity-doping spectra taken from the literature. For electrorefraction at the indirect gap, we find \Delta n = 1.3 \times 10^{5} at \lambda = 1.07 \mu m when E = 10^{5} V/cm, while the Kerr effect gives \Delta n = 10^{-6} at that field strength. The charge-carrier effects are larger, and a depletion or injection of 1018carriers/cm3produces an index change of \pm1.5 \times 10^{-3} at \lambda = 1.3 \mu m.

2,428 citations


Journal ArticleDOI
19 May 2005-Nature
TL;DR: Electro-optic modulators are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures, and here a high-speed electro-optical modulator in compact silicon structures is experimentally demonstrated.
Abstract: Metal interconnections are expected to become the limiting factor for the performance of electronic systems as transistors continue to shrink in size. Replacing them by optical interconnections, at different levels ranging from rack-to-rack down to chip-to-chip and intra-chip interconnections, could provide the low power dissipation, low latencies and high bandwidths that are needed. The implementation of optical interconnections relies on the development of micro-optical devices that are integrated with the microelectronics on chips. Recent demonstrations of silicon low-loss waveguides, light emitters, amplifiers and lasers approach this goal, but a small silicon electro-optic modulator with a size small enough for chip-scale integration has not yet been demonstrated. Here we experimentally demonstrate a high-speed electro-optical modulator in compact silicon structures. The modulator is based on a resonant light-confining structure that enhances the sensitivity of light to small changes in refractive index of the silicon and also enables high-speed operation. The modulator is 12 micrometres in diameter, three orders of magnitude smaller than previously demonstrated. Electro-optic modulators are one of the most critical components in optoelectronic integration, and decreasing their size may enable novel chip architectures.

2,207 citations


Journal ArticleDOI
TL;DR: The techniques that have, and will, be used to implement silicon optical modulators, as well as the outlook for these devices, and the candidate solutions of the future are discussed.
Abstract: Optical technology is poised to revolutionize short-reach interconnects. The leading candidate technology is silicon photonics, and the workhorse of such an interconnect is the optical modulator. Modulators have been improved dramatically in recent years, with a notable increase in bandwidth from the megahertz to the multigigahertz regime in just over half a decade. However, the demands of optical interconnects are significant, and many questions remain unanswered as to whether silicon can meet the required performance metrics. Minimizing metrics such as the device footprint and energy requirement per bit, while also maximizing bandwidth and modulation depth, is non-trivial. All of this must be achieved within an acceptable thermal tolerance and optical spectral width using CMOS-compatible fabrication processes. This Review discusses the techniques that have been (and will continue to be) used to implement silicon optical modulators, as well as providing an outlook for these devices and the candidate solutions of the future.

1,894 citations


Journal ArticleDOI
Ansheng Liu1, Richard Jones1, Ling Liao1, Dean A. Samara-Rubio1  +4 moreInstitutions (1)
12 Feb 2004-Nature
TL;DR: An approach based on a metal–oxide–semiconductor (MOS) capacitor structure embedded in a silicon waveguide that can produce high-speed optical phase modulation is described and an all-silicon optical modulator with a modulation bandwidth exceeding 1 GHz is demonstrated.
Abstract: Silicon has long been the optimal material for electronics, but it is only relatively recently that it has been considered as a material option for photonics1. One of the key limitations for using silicon as a photonic material has been the relatively low speed of silicon optical modulators compared to those fabricated from III–V semiconductor compounds2,3,4,5,6 and/or electro-optic materials such as lithium niobate7,8,9. To date, the fastest silicon-waveguide-based optical modulator that has been demonstrated experimentally has a modulation frequency of only ∼20 MHz (refs 10, 11), although it has been predicted theoretically that a ∼1-GHz modulation frequency might be achievable in some device structures12,13. Here we describe an approach based on a metal–oxide–semiconductor (MOS) capacitor structure embedded in a silicon waveguide that can produce high-speed optical phase modulation: we demonstrate an all-silicon optical modulator with a modulation bandwidth exceeding 1 GHz. As this technology is compatible with conventional complementary MOS (CMOS) processing, monolithic integration of the silicon modulator with advanced electronics on a single silicon substrate becomes possible.

1,551 citations


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