44Gbit/s silicon Mach-Zehnder modulator based on interleaved PN junctions

ArticleinOptics Express 20(14):15093-9 · July 2012with50 Reads
Impact Factor: 3.49 · DOI: 10.1364/OE.20.015093 · Source: PubMed
  • 1st Hao Xu
    Capital Medical University
  • 30.78 · FiberHome
  • 21.82 · Chinese Academy of Sciences
  • 39.68 · Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China

A high speed silicon Mach-Zehnder modulator is proposed based on interleaved PN junctions. This doping profile enabled both high modulation efficiency of V(π)L(π) = 1.5~2.0 V·cm and low doping-induced loss of ~10 dB/cm by applying a relatively low doping concentration of 2 × 10(17) cm(-3). High speed operation up to 40 Gbit/s with 7.01 dB extinction ratio was experimentally demonstrated with a short phase shifter of only 750 μm.



Available from: Xi Xiao, Sep 17, 2014
High speed silicon Mach-Zehnder modulator
based on interleaved PN junctions
Hao Xu, Xi Xiao,
Xianyao Li, Yingtao Hu, Zhiyong Li, Tao Chu, Yude Yu, and
hong Yu
State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, P.
O. Box 912,Beijing 100083, China
Abstract: A high speed silicon Mach-Zehnder modulator is proposed based
on interleaved PN junctions. This doping profile enabled both high
modulation efficiency of V
= 1.5~2.0 cm and low doping-induced loss
of ~10 dB/cm by applying a relatively low doping concentration of 2 × 10
High speed operation up to 40 Gbit/s with 7.01 dB extinction ratio
was experimentally demonstrated with a short phase shifter of only 750 µm.
©2012 Optical Society of America
OCIS codes: (130.0250) Optoelectronics; (250.5300) Photonic integrated circuits; (250.7360)
Waveguide modulators.
References and links
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#168126 - $15.00 USD
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2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15093
Page 1
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1. Introduction
Silicon optical modulator is regarded as a principal component for chip-scale optical
interconnection as its intrinsic compatibility to monolithically integrate with complementary-
metal-oxide-semiconductor (CMOS) microelectronic circuits [1,2]. The depletion-mode
Mach-Zehnder modulator (MZM) can offer high modulation speed, broadband operation
spectral and high thermal tolerance [2]. It has played a critical role in high speed silicon
photonic integrated systems [3,4]. However, the depletion-mode MZM suffers low
modulation efficiency as the overlap between the optical mode and the depletion region is
relatively small. Simply increasing the doping concentration to enhance the modulation
efficiency will result in a high carrier absorption loss. Therefore, some methods have been
proposed to achieve both high modulation efficiency and low doping-induced loss such as
using doping compensation method [5] and employing a tilted p-n junction [6]. However,
these methods required either additional or high-precision processing steps which increased
the complexity. In previous research [7,8], we have reported a numerical simulation of the
depletion-mode silicon modulator based on interleaved PN junctions and its application with
microring resonator, which indicated and experimentally demonstrated that, benefited from
the enhanced overlap between the optical mode and the depletion region, high modulation
efficiency can be obtained by applying a relatively low doping concentration. Researches on
the similar structure have also been reported recently [9,10], however, high modulation
efficiency as well as low carrier induced loss was not achieved yet, and the modulation speeds
were limited to be 10 Gbit/s. In this paper, we propose a silicon Mach-Zehnder modulator
based on that doping profile fabricated in a standard 0.18µm CMOS processes. A figure of
merit of V
= 1.5~2.0 V·cm, low doping-induced loss of ~10dB/cm and 40 Gbit/s
dulation with 7.01 dB extinction were demonstrated with a short phase-shifter of 750µm.
2. Device structure and fabrication
Figure 1(a) shows the microscope image of the MZM, the device was fabricated in an
asymmetrical Mach-Zehnder interferometer (MZI) with 170 µm arm length difference. Multi-
mode interferences (MMI) and grating couplers were utilized for light splitting, combining
and coupling in and out. On the SOI wafer with a 340 nm thick top silicon layer and a 2 µm
thick buried oxide layer, the rib waveguide was optimized to be 450 nm wide with an 80nm
high slab. To balance the carrier induced optical loss, 750 µm long phase shifters were formed
in both arms. The geometry of periodically interleaved PN junctions embedded in the phase
shifter is shown in Fig. 1(b). The lengths of both P and N region were 300 nm making a
600 nm long period length.
Interleaved PN junctions were realized by p-type doping and n-type compensation. The
waveguide was firstly P-type doped with a background doping concentration of 2 × 10
gional N-type compensation with higher density of 4 × 10
was employed to form the
abrupt interleaved junctions. Highly doped P + and N + regions were respectively defined
1 µm away from the edge of the rib waveguide to ensure low carrier absorption loss. A 1 µm
thick aluminum coplanar waveguide (CPW) electrode was designed for drive signal
transmission. The device was fabricated by commercial 0.18 µm CMOS process in the
Semiconductor Manufacturing International Corporation using the similar processes as
Ref. [8].
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Received 8 May 2012; revised 8 Jun 2012; accepted 10 Jun 2012; published 20 Jun 2012
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2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15094
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Fig. 1. (a) Microscope image of the MZM. (b)Schematic view of the phase shifter with
periodically interleaved PN junctions.
3. Measurement results and discussion
3.1 DC performance
The transmission spectra of the MZM at different reverse bias voltages is shown in Fig. 2(a),
while a MZI composed of same optical structures without ion implantation was measured for
comparison. The recorded spectra in Fig. 2(a) were normalized to a waveguide of the same
length. It can be calculated from the spectral curves that the device insertion loss is ~2 dB and
carrier induced loss of the phase shifter is ~10 dB/cm by the cut back method. The modulation
efficiency V
is 1.5~2.0 cm under the bias voltages varying from 0 V to 8
V. These
measured results show good agreement with the simulation results of the similar structures
proposed in Refs [7] and [8]. In order to evaluated the performance of a phase shifter, a
figure-of-merit (FOM) Loss· Efficiency, the product of carrier-induced loss and V
, was
fined in Ref [5]. For the phased shifter shown in Fig. 1(b), This FOM is not over 20 dB·V
which is comparable with the result optimized by doping compensation method proposed in
Ref [5]. Figure 2(b) presents the comparison of the measured V
of the MZM based on the
proposed structure and those based on the lateral PN junction. The lateral PN junctions were
designed to be with 50 nm and 100 nm offset in the rib waveguide respectively as described in
Ref [11]. It is observed that interleaved PN junctions provided higher modulation efficiency
with the same doping concentration and optical structures. Moreover, as the interleaved PN
junctions are oriented cross the rib waveguide, the overlap of the depletion regions and the
optical mode is insensitive to the location of the PN junctions in the rib waveguide, which
enables much higher misalignment tolerance for the fabrication process.
Fig. 2. (a) Normalized transmission spectra of the MZM and MZI. (b) Comparison of the V
of the MZM based on interleaved PN junctions and lateral PN junctions with offset.
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Received 8 May 2012; revised 8 Jun 2012; accepted 10 Jun 2012; published 20 Jun 2012
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3.2 Dynamic performance
This doping profile is predicted to have ~35 GHz intrinsic bandwidth based on our simulation
[7]. However, the high-speed performance of MZM depends on not only the intrinsic device
speed governed by motion of carries, but also the issues associated with the relative large
parasitic effects of PN junction and electrode [1]. Since the depletion-mode MZM can be
considered as a capacitive-load CPW transmission line [12], an equivalent circuit model based
on the device’s structure dimensions and material parameters is proposed to characterize the
high speed performance of the MZM.
Figure 3(a) shows the distributed circuit model proposed to account for the propagation
effects of the depletion-mode MZM. The circuit includes elements from the model in Refs
[13] and [14], which was extracted to characterize CPW on loss silicon substrate and
experimental validated over a broad bandwidth. In Fig. 3(a), these parameters were defined as
follow: R
and L were the series resistance and inductance of the aluminum conductors of the
PW electrode. R
represented longitudinal current loss in the silicon substrate, C
and C
re the signal line to ground line and signal line to silicon substrate capacitance, C
and G
were used to describe the relaxation between slow–wave at lower frequency and quasi-TEM
modes at higher frequency. Approximate expressions of these parameters can be extracted by
using the conformal mapping method and partial capacitance approach. All these parameters
above were taken per unit length and described in detail in Refs [13] and [14]. For a
depletion-mode MZM working under reverse bias, the depletion capacitance is significant for
the PN junction [15], therefore, the junction capacitance C
(F/m)can be expressed by
( )
2( )( )
r A D
qN N
C v
L N N V v
ε ε
where A is the total cross-section area of the PN junctions, L is the length of the phase shifter,
and ε
are the permittivity of free space and dielectric constant of silicon, q is the
elementary charge, N
and N
are the doping concentration of P-type and N-type, V
and v are
the built-in potential and reverse bias voltage. R
(·m) is the resistance in series with the
junction capacitance which represents the contact resistance of the doping regions and the
annealed alloy at the interface between metal and semiconductor. It can be extracted by curve-
fitting the measured characteristic parameters, namely, the characteristic impedance,
attenuation constant and the effective index of the microwave. For a depletion-mode MZM
working at a certain reverse bias, the complex characteristic impedance Z
and complex
propagation coefficient γ can be evaluated as function of frequency f from [13]
Re Im
Z Z jZ Z Y
= + =
j Z Y
= + =
γ α β
1 1
R R j L
1 1 1 1
( ) ( )
j C j C j C G j C
ω ω ω ω
+ +
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In Eq. (4), α is the attenuation constant, β is the phase constant, and the effective index of the
microwave can be expressed as n
= (c·β)/ω, where c is the light speed in free space and ω =
2πf is the angular frequency. Figure 3(b)–3(d) illustrate the measured characteristic
parameters of the MZM based on the lateral PN junction using the on wafer de-embedding
techniques described in [16]. The S parameter of the MZMs with different phase shifter length
of 1000 µm, 750 µm and 500 µm from DC to 20 GHz were measured by a signal integrity
twork analyzer (SPARQ) from Lecroy at 3V bias. The testing system, including network
analyzer, cables, probes, the bias-Tee and the DC block, was calibrated using short-open-
load-through calibration on Impedance Standard Substrate. As shown in Fig. 3(b)–3(d),
excellent curve fitting was achieved by setting R
= 0.035· m and C
= 200 fF/mm which
was ~30 fF/mm higher than that calculated from Eq. (1). It is believed this discrepancy results
from the simplified method to extract Eq. (1) without considering doping process conditions.
More accurately C
could be calculated by the commercial semiconductor device-modeling
package such as Silvaco [17]. As the capacitance is proportional to the total cross-section area
of the PN junctions, the junction capacitance C
of the phase shifter shown in Fig. 1(b) can be
proximated by C
= 615/(0.9-v)
fF/mm and it decreases to be 290 fF/mm at 3.5 V bias
accordingly. The CPW electrode induced capacitance is calculated as 156 fF/mm based on the
proposed circuited model.
Fig. 3. (a) Equivalent circuit model of the depletion-mode MZM. (b)–(c) Curve-fitting of the
measured transmission-line parameters of the MZM based on the lateral PN junction at 3V
Figure 4(a) shows the measured transmission data (S21) of this MZM. Since the depletion
capacitance of the PN junction decreased with the increasing reverse bias voltage, the
electrical 6 dB roll off frequency raised from 11 GHz at 0 V bias to be over 20 GHz at 3 V
bias. However, with the increasing bias voltage, the decreasing slope of junction capacitance
declined rapidly as shown in Fig. 4(b), so that the transmission line property of the MZM
changed slightly when the bias voltage was further raised from 3 V to 5 V.
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Received 8 May 2012; revised 8 Jun 2012; accepted 10 Jun 2012; published 20 Jun 2012
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Fig. 4. (a) Transmission parameters S21 of the MZM at different bias voltages. (b) Junction
capacitance C
of the MZM at different bias voltage.
Optical eye diagrams were measured to demonstrate the high speed performance of this
vice by applying the non-return-zero pseudorandom binary sequence (PRBS) signal with
-1 pattern length. The PRBS signal of 30 Gbit/s, 40 Gbit/s and 44 Gbit/s were amplified to
be of 7 V peak-to-peak (V
) amplitude and biased at 3.5 V to drive the MZM. A standard
50 SMA terminal resistance and a DC block were used to terminate the MZM. Continuous-
wave laser beam at ~1550 nm was coupled into the MZM through a grating coupler. The
output light from the grating coupler on the other side was amplified by Erbium-doped fiber
amplifier and transmitted through a band pass filter. Finally, the modulated optical signal was
detected by an optical module of a Tektronix digital scope DSA8300. Figure 5(a)–5(c) shows
the output eye diagrams measured at 3.5 V bias. It is observed that over 7 dB extinction
radio was measured at the modulation speed of 30 Gbit/s and 40 Gbit/s. At the modulation
speed of 44 Gbit/s, which is the maximum bit rate of the pattern generator can supply, an open
optical eye diagram was achieved with 5.68 dB extinction radio. Without considering the
terminal resistance and the optical energy, the power consummation of this device working at
40 Gbit/s and 3.5 V bias is estimated as 4.1 pJ/bit using the equation of
(1 / 4)
bit PP
[18], where the C is the modulator capacitance and equals to the sum of the junction
capacitance and electrode induced capacitance.
Fig. 5. Eye diagrams measured at (a) 30 Gbit/s, (b) 40 Gbit/s and (c) 44 Gbit/s.
4. Conclusion
Benefited from the enhanced overlap between the depletion region and the optical mode by
the interleaved PN junctions’ structure, a high speed of 40Gbit/s and low doping-induced loss
of ~10 dB/cm silicon MZM was experimentally demonstrated. It was fabricated in a
commercial 0.18 µm CMOS process with a relatively low doping concentration of 2 ×
and a short phase-shifter of 750 µm. Further optimization should be carried out to
realize optimal trade-off among various FOMs of the MZM, including modulation speed and
efficiency, insertion loss, power consumption and area efficiency. The interleaved PN
junctions presented can offer flexible designs for the improvement in future.
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Received 8 May 2012; revised 8 Jun 2012; accepted 10 Jun 2012; published 20 Jun 2012
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The authors thank Tektronix for the instrument support to our devices measurement and
Semiconductor Manufacturing International Corporation (SMIC) for the fabrication support to
our Silicon photonics research. This work is supported by the National Basic Research
Program of China (Grant No. 2011CB301701, No. 2012CB933502 and No. 2012CB933504),
the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KGCX2-
EW-102), and the National Natural Science Foundation of China (Grant No. 61107048 and
No. 60877036).
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Received 8 May 2012; revised 8 Jun 2012; accepted 10 Jun 2012; published 20 Jun 2012
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Page 7
    • "Therefore, we fix these parameters in this paper. Other methods considered for larger φ are employing interleaved p/n junction [7, 10, 18, 19, 25] and increasing doping concentration. The former increases the overlap between the depletion layer and waveguide mode. "
    [Show abstract] [Hide abstract] ABSTRACT: In this study, we investigated the performance of Si lattice-shifted photonic crystal waveguide (LSPCW) Mach-Zehnder modulators theoretically and experimentally. The LSPCW increases the phase shift in modulator to 2.3 - 2.5 times higher, which allows for size reduction and high performance. On-chip passive loss was reduced to less than 5 dB by optimizing each component. We obtained 25 Gbps clear open eye and 3 dB extinction ratio at a drive voltage of 1.5 – 1.75 V for 200 μm phase shifter with linear p/n junction when we added a modulation loss of 7 dB. This modulation loss was reduced to 0.8 dB, maintaining other performance, by employing interleaved p/n junction and optimizing doping concentrations.
    Preview · Article · Nov 2014 · Frontiers in Physics
    • "It can be seen that the performance of phase modulators has improved dramatically in recent years. Devices with operation in excess of 40 Gbit/s [25, 33, 41, 45, 48], phase efficiencies below 1 V.cm [50] , power consumption down to 2 fJ/bit [51] and loss at 1 dB/mm and below [35, 43, 45] have been demonstrated although not with all these parameters together. Continual improvements in the device performance are still regularly being reported. "
    [Show abstract] [Hide abstract] ABSTRACT: The majority of the most successful optical modulators in silicon demonstrated in recent years operate via the plasma dispersion effect and are more specifically based upon free carrier depletion in a silicon rib waveguide. In this work we overview the different types of free carrier depletion type optical modulators in silicon. A summary of some recent example devices for each configuration is then presented together with the performance that they have achieved. Finally an insight into some current research trends involving silicon based optical modulators is provided including integration, operation in the mid-infrared wavelength range and application in short and long haul data transmission links.
    Preview · Article · Aug 2014 · Nanophotonics
    • "However, most of the reported high-bandwidth silicon carrier-depletion MZMs were demonstrated by using devices with rather short phase shifters with an extremely high or impractical V π . A typical V π for > 25 Gb/s operation is larger than 7 V353637383940 . Compared with LiNbO 3 or InP modulators where a V π of 2–3 V can be achieved, the V π of silicon MZMs is much higher. "
    [Show abstract] [Hide abstract] ABSTRACT: Silicon photonic devices and integrated circuits have undergone rapid and significant progresses during the last decade, transitioning from research topics in universities to product development in corporations. Silicon photonics is anticipated to be a disruptive optical technology for data communications, with applications such as intra-chip interconnects, short-reach communications in datacenters and supercomputers, and long-haul optical transmissions. Bell Labs, as the research organization of Alcatel-Lucent, a network system vendor, has an optimal position to identify the full potential of silicon photonics both in the applications and in its technical merits. Additionally it has demonstrated novel and improved high-performance optical devices, and implemented multi-function photonic integrated circuits to fulfill various communication applications. In this paper, we review our silicon photonic programs and main achievements during recent years. For devices, we review highperformance single-drive push-pull silicon Mach-Zehnder modulators, hybrid silicon/III-V lasers and silicon nitrideassisted polarization rotators. For photonic circuits, we review silicon/silicon nitride integration platforms to implement wavelength-division multiplexing receivers and transmitters. In addition, we show silicon photonic circuits are well suited for dual-polarization optical coherent transmitters and receivers, geared for advanced modulation formats. We also discuss various applications in the field of communication which may benefit from implementation in silicon photonics.
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