COVER Photonic implementations of signal processing have attracted a great deal of attention thanks to their potential to overcome the bandwidth and speed bottlenecks in electronic devices. Many all-optical signal processing techniques, which offer processing bandwidths as large as several THz, have been proposed and successfully used in applications such as ultrafast telecommunications, optical computing, microwave photonics and bio-photonics. The cover shows a time stretch microscopy technique that uses a multi-wavelength laser as a light source. By tuning the speed of the modulation signal, the frame rate of the imaging system can be raised to hundreds of MHz. In addition, the concept of anamorphic temporal imaging and its application to real-time optical analog data compression are reported for the first time. This new system performs time-bandwidth engineering and is designed based on a newly introduced mathematical function called the stretched modulation distribution. The background of the cover shows one such distribution reported in this issue (see the Special Topic: All-Optical Signal Processing).

Copyright Information For AuthorsAs soon as an article is accepted for publication, authors will be requested to assign copyright of the article (or to grant exclusive publication and dissemination rights) to Science China Press and Springer. This will ensure the widest possible protection and dissemination of information under copyright laws.

More information about copyright regulations for this journal is available at www.springer.com/11434.

For ReadersWhile the advice and information in this journal is believed to be true and accurate at the date of its publication, neither the authors, the editors, nor the publishers can accept any legal responsibility for any errors or omissions that may have been made. The publishers make no warranty, express or implied, with respect to the material contained herein.

All articles published in this journal are protected by copyright, which covers the exclusive rights to reproduce and distribute the article (e.g., as offprints), as well as all translation rights. No material published in this journal may be reproduced photographically or stored on microfilm, in electronic data bases, on video disks, etc., without first obtaining written permission from the publisher (respectively the copyright owner if other than Springer). The use of general descriptive names, trade names, trademarks, etc., in this publication, even if not specifically identified, does not imply that these names are not protected by the relevant laws and regulations.

Springer has partnered with Copyright Clearance Center’s RightsLink service to offer a variety of options for reusing Springer content. For permission to reuse our content please locate the material that you wish to use on link.springer.com or on springerimages.com and click on the permissions link or go to copyright.com, then enter the title of the publication that you wish to use. For assistance in placing a permission request, Copyright Clearance Center can be connected directly via phone: +1-855-239-3415, fax: +1-978-646-8600, or e-mail: info@copyright.com.

© Science China Press and Springer-Verlag Berlin Heidelberg

Abstracted/indexed in: Academic Search Alumni EditionAcademic Search CompleteAcademic Search EliteAcademic Search PremierASFA 1: Biological Sciences and Living ResourcesASFA 2: Ocean Technology, Policy and Non-Living ResourcesBiological AbstractsBiological SciencesBIOSIS PreviewsCAB AbstractsChemical AbstractsChemical and Earth SciencesCurrent Contents/PhysicalCurrent Mathematical PublicationsDigital Mathematics Registry

EMBioEnvironmental Engineering AbstractsEnvironmental Sciences and Pollution ManagementGoogle ScholarInspecMathematical ReviewsMathSciNetMeteorological and Geoastrophysical AbstractsPollution AbstractsScience Citation IndexSCOPUSWater Resources AbstractsZentralblatt MATHZoological Record

Volume 59 Number 22 August 2014

i

CONTENTS CONTENTS

www.scichina.com | csb.scichina.com | www.springer.com/scp | link.springer.com

Volume 59 Number 22 August 2014

I Towards Excellence in Science Chinese Academy of Sciences

SPECIAL TOPIC: All-Optical Signal Processing

EDITORIAL

2647 Preface MingLi•JoséAzaña•JianpingYao

INVITED ARTICLE

2649 Warped time lens in temporal imaging for optical real-time data compression MohammadH.Asghari•BahramJalali

PROGRESS

2655 All-optical signal processing for linearity enhancement of Mach–Zehnder modulators Xiao-PingZheng•Guo-QiangZhang•ShangyuanLi•Han-YiZhang•Bing-KunZhou

2661 Nanoscale all-optical devices based on surface plasmon polaritons JianjunChen•ChengweiSun•XiaoyongHu

2666 Analog-to-digital converters using photonic technology ZhiyaoZhang•HepingLi•ShangjianZhang•YongLiu

REVIEW

2672 Photonic generation of microwave signals with tunabilities HengyunJiang•LianshanYan•JiaYe•WeiPan•BinLuo•XihuaZou

ARTICLES

2684 Ultra-highsuppressionmicrowavephotonicbandstopfilters DavidMarpaung•BlairMorrison•MattiaPagani•RaviPant•BenjaminJ.Eggleton

2693 Serialtime-encodedamplifiedmicroscopyforultrafastimagingbasedonmulti-wavelengthlaser YeDeng•MingLi•NingboHuang•JoséAzaña•NinghuaZhu

2702 Non-blocking 2 × 2 switching unit based on nested silicon microring resonators with high extinction ratios and low crosstalks

JiayangWu•XinhongJiang•TingPan•PanCao•LiangZhang•XiaofengHu•YikaiSu

2709 All-optical wavelength converter using a microdisk resonator integrated with p-n junctions LinjieZhou•JingyaXie•JianpingChen

PROGRESS

Cell Biology2717 Progress in measuring biophysical properties of membrane proteins with AFM single-molecule force

spectroscopy MiLi•LianqingLiu•NingXi•YuechaoWang

LETTER

Ecology2726 Evaluatingtheinfluencesofmeasurementtimeandfrequencyonsoilrespirationinasemiaridtemperate

grassland BingweiZhang•ZhiqiangYang•ShipingChen•LimingYan•TingtingRen

Go To Website

ii

CONTENTS CONTENTS

www.scichina.com | csb.scichina.com | www.springer.com/scp | link.springer.com

ARTICLES

Atomic & Molecular Physics2731 Laser intensity induced transparency in atom-molecular transition process JieMa•YuqingLi•JizhouWu•LiantuanXiao•SuotangJia

High-Energy Physics2736 Super solar particle event around AD775 was found DazhuangZhou•ChiWang•BinquanZhang•ShenyiZhang•PingZhou•YueqiangSun•JinbaoLiang•GuangwuZhu• JiWu

Developmental Biology2743 Generationoftetraploidcomplementationmicefromembryonicstemcellsculturedwithchemicaldefined

medium ChunjingFeng•HaifengWan•Xiao-YangZhao•LiuWang•QiZhou

2749 Endoderm contributes to endocardial composition during cardiogenesis YanLi•XiaoyuWang•ZhenglaiMa•ManliChuai•AndreaMünsterberg•KennethKaHoLee•XuesongYang

Ecology2756 Sexual/aggressive behavior of wild yak (Bos mutusPrejevalsky1883)duringtherut:influenceoffemale

choice PaulJ.Buzzard•DonghuaXu•HuanLi

Geophysics2764 Identificationofthethick-layergreigiteinsedimentsoftheSouthYellowSeaanditsgeologicalsignificances JianxingLiu•XuefaShi•ShulanGe•QingsongLiu•ZhengquanYao•GangYang

Materials Science2776 FunctionalizationofPCLfibrousmembranewithRGDpeptidebyanaturallyoccurringcondensation

reaction WentingZheng•DiGuan•YuxinTeng•ZhihongWang•SuaiZhang•LianyongWang•DelingKong•JunZhang

Vol. 59 No. 22 August 5, 2014 (Published three times every month)

Supervised by Chinese Academy of SciencesSponsored byChineseAcademyofSciencesandNationalNaturalScienceFoundationofChinaPublished by Science China Press and Springer-Verlag Berlin HeidelbergSubscriptions

ChinaScienceChinaPress,16DonghuangchenggenNorthStreet,Beijing100717,ChinaEmail:sales@scichina.orgFax:86-10-64016350

North and South AmericaSpringerNewYork,Inc.,JournalFulfillment,P.O.Box2485,Secaucus,NJ07096USAEmail:journals-ny@springer.comFax:1-201-348-4505

Outside North and South AmericaSpringerCustomerServiceCenter,CustomerServiceJournals,Haberstr.7,69126Heidelberg,GermanyEmail:subscriptions@springer.comFax:49-6221-345-4229

Printed byBeijingArtownprintingCo.,Ltd.,ChuangyeyuanRoad,TaihuTown,TongzhouDistrict,Beijing101116,ChinaEdited byEditorialBoardofChineseScienceBulletin,16DonghuangchenggenNorthStreet,Beijing100717,ChinaEditor General Zuoyan Zhu

CN 11-1785/N广告经营许可证: 京东工商广字第0429号

邮发代号: 80-214 (英)国内每期定价: 120元

Artic le Optoelectronics & Laser

All-optical wavelength converter using a microdisk resonatorintegrated with p-n junctions

Linjie Zhou • Jingya Xie • Jianping Chen

Received: 1 December 2013 / Accepted: 28 February 2014 / Published online: 27 May 2014

� Science China Press and Springer-Verlag Berlin Heidelberg 2014

Abstract We explore all-optical wavelength conversion in

a microdisk resonator integrated with interleaved p-n junc-

tions. Numerical simulation based on temporal coupled

mode theory is performed to study the free-carrier dynamics

inside the cavity. It reveals that the detuning of pump and

probe frequencies and the carrier lifetime have a significant

effect on the device performance. Experimental result con-

firms that the conversion speed can be considerably

improved by applying a reverse bias on the p-n junctions.

Wavelength conversion at 10 Gb/s data rate is achieved with

a pump power of 5.41 dBm and a bias voltage of -6 V.

Keywords Resonators � Wavelength conversion �Optical signal processing � Integrated photonics

1 Introduction

All-optical wavelength conversion is one of the powerful

signal processing techniques that can be used to solve the

wavelength contention issues in future transparent wave-

length division multiplexing (WDM) optical communica-

tion networks. It is foreseeable that low-level signal

processing has to be performed in the optical domain

whenever possible in order to allow Tb/s data signals to

transparently route through complex network architectures

[1, 2]. The all-optical conversion provides a solution to

overcome the bottlenecks encountered at current power-

hungry and low-speed optical-electrical-optical (O\E\O)

approaches. There are multiple ways to realize wavelength

conversion including using semiconductor optical amplifi-

ers (SOAs) [3, 4], optical fibers [5, 6], chalcogenide

waveguides or fibers [7, 8], periodically-poled lithium

niobate (PPLN) waveguides [9], and GaAs [10] etc. Silicon

photonics based approaches have attracted much attention

due to their compactness and compatibility with the com-

plementary metal-oxide-semiconductor (CMOS) fabrica-

tion process [11]. All-optical wavelength conversion based

on four-wave mixing (FWM) in high-confinement silicon

waveguides has been demonstrated [12–16]. To obtain high

conversion efficiency with FWM, the waveguide group

velocity dispersion (GDD) needs to be carefully engi-

neered. A long waveguide and large pump power are

always required. By making use of the resonant enhance-

ment effect in silicon micro-resonators, low-power wave-

length conversion is achieved in micron-scale device

footprint [17, 18]. The optical intensity needed to excite the

Kerr nonlinear effect is usually high enough that free-car-

riers are generated via two photon absorption (TPA), which

is detrimental to FWM. On the other hand, however, the

free-carrier plasma dispersion (FCD) effect can also be

utilized for wavelength conversion [19, 20]. The original

optical signal (pump) is translated to free-carrier modula-

tion, which as a result enables the intensity modulation of

another optical beam (probe) via resonance shift. This

process can be regarded as all-optical modulation in con-

trast to the electro-optic modulation as widely investigated

in recent years [21]. Because the conversion is facilitated

by free-carriers, it does not need to satisfy the crucial phase

matching condition, which greatly eases the device design.

In this paper, we first numerically study the wavelength

conversion process in a microdisk resonator based on

SPECIAL TOPIC: All-Optical Signal Processing

L. Zhou (&) � J. Xie � J. Chen

State Key Laboratory of Advanced Optical Communication

Systems and Networks, Department of Electronic Engineering,

Shanghai Jiao Tong University, Shanghai 200240, China

e-mail: ljzhou@sjtu.edu.cn

123

Chin. Sci. Bull. (2014) 59(22):2709–2716 csb.scichina.com

DOI 10.1007/s11434-014-0405-4 www.springer.com/scp

temporal coupled mode equations. The effect of pump and

probe frequency detuning on the free-carrier dynamics and

the converted signal quality is analyzed. Simulation sug-

gests that reducing carrier lifetime can effectively improve

the conversion speed. We then present our experimental

results using a 6-lm-radius silicon microdisk resonator

integrated with interleaved p-n junctions. The small mode

volume and high Q-factor of the microdisk resonator are

critical for low power operation. Previous work used pas-

sive microring resonators and their operation speed is pri-

marily limited by the ns-long carrier lifetime [5, 6]. In our

device, we integrate interleaved p-n junctions in the reso-

nator so that the high electric field upon reverse bias can

considerably reduce the carrier lifetime, leading to an

increased conversion speed.

2 Device structure and theoretical model

Figure 1a shows the schematic of the microdisk resonator

integrated with interleaved p-n junctions along the disk rim

[22]. The microdisk resonator has high-Q whispering gallery

modes (WGMs). The optical power is greatly enhanced inside

the resonator on resonance. The enhancement factor is given by

(jF/p)2, where j is the waveguide-resonator coupling coeffi-

cient and F is the resonator finesse defined as the ratio between

the free spectral range (FSR) and the full-width half-maximum

(FWHM) width of the resonance [23]. If the resonator is criti-

cally coupled (all input power is coupled into the resonator on

resonance), then the enhancement factor becomes F/p. There-

fore, resonators with high Q-factors and small volumes favor

the resonant enhancement effect. When strong pump light is

coupled into the microdisk resonator, free-carriers are gener-

ated due to the TPA effect, which in turn detunes the resonance

frequency because of the FCD effect. The resonance Q-factor is

also varied by the accompanied free-carrier absorption (FCA).

Therefore, the resonance frequency can be modulated by the

pump light. If a continuous probe light is also coupled into the

resonator, then the resonance modulation can be translated into

intensity modulation of the probe light. This is the working

principle of all-optical wavelength conversion using the pump-

probe method. As the conversion is enabled via the intermediate

FCD effect, the operation speed is limited by the free-carrier

dynamics. To ensure a fast response, the free-carriers should

have a short lifetime. Besides the carrier lifetime, the cavity

photon lifetime is also a possible limiting factor to the response

speed, because the optical field needs time to build up and decay

in the cavity.

According to the temporal coupled mode theory, the

dynamic behavior of the pump (control) and probe energy

inside the resonator can be described by the following two

rate equations [23]:

d

dtac

r tð Þ ¼ �j xc � x0 � DxL0 tð Þ � DxNL

0 tð Þ� �

acr tð Þ

� 1

2r0 þ DrL

0 tð Þ þ DrNL0 tð Þ

� �ac

r tð Þ

� re

2ac

r tð Þ � jffiffiffiffire

pAc

i tð Þ; ð1Þ

d

dtap

r tð Þ ¼ �j xp � x0 � DxL0 tð Þ � DxNL

0 tð Þ� �

apr tð Þ

� 1

2r0 þ DrL

0 tð Þ þ DrNL0 tð Þ

� �ap

r tð Þ � re

2ap

r tð Þ

� jffiffiffiffire

pA

pi tð Þ; ð2Þ

where ar(t) is the energy-normalized amplitude in the

resonator, xc and xp are the pump and probe signal

frequencies respectively, x0 is the resonance frequency,

DxL0 tð Þ is the linear resonance frequency shift caused by

FCD, DxNL0 tð Þ is the nonlinear resonance frequency shift

caused by the Kerr effect, r0 is the resonator intrinsic delay

rate, DrL0 tð Þ is the linear delay rate due to FCA, DrNL

0 tð Þ is

the nonlinear delay rate due to TPA, re is the external delay

rate due to waveguide coupling, and Ai tð Þ is the power-

normalized input field amplitude. The output of the

transmitted probe amplitude is given by

Apo tð Þ ¼ A

pi tð Þ � j

ffiffiffiffire

pap

r tð Þ: ð3Þ

Based on the above theoretical model, we can study the

behavior of optical field and free-carriers in the resonator

with the presence of a pump signal. It should be noted that

the above theoretical model does not incorporate the

thermo-optic effect. In practice, the wavelength conversion

process is always accompanied by the thermo-optic effect

since the pump power is relatively high in the cavity.

However, the thermo-optic effect has a slow response in

the time scale of ls, several orders smaller than the ns free-

carrier response time. With the use of p-n junctions, the

carrier lifetime can be even shorter. Therefore, the slow

thermo-optic effect does not interfere significantly with the

fast wavelength conversion process if the optical signal

date rate is high. The effect of the thermal heating is only

to red-shift the resonance wavelengths. Hence, the pump

and probe wavelengths need to be tuned accordingly to

match the resonances.

We consider a microdisk resonator with a radius of 6

lm. The quality factor of the resonator is assumed to be

Q = 39104 (consistent with our experiment). The funda-

mental WGM has a confinement factor of about 0.9. The

TPA coefficient of silicon at 1.55 lm wavelength is

0.6 cm/GW and the Kerr coefficient is 6.3910-18 m2/W

[24]. The carrier lifetime is assumed to be 200 ps, which in

practice can be tuned by applying a reverse bias on the p-n

junctions. Figure 1b shows the transmission spectrum of

the waveguide coupled microdisk resonator. The coupling

2710 Chin. Sci. Bull. (2014) 59(22):2709–2716

123

coefficient is chosen such that the resonance is critically

coupled.

Figure 2a shows the pump and probe signals input to the

access waveguide. The pump is a square-like pulse with 8

mW optical power and 1 ns pulse width, and the probe is a

continuous wave (CW) with 0.8 mW optical power. The

pump and probe wavelengths are both set at the resonance

frequency. Due to the resonant enhancement effect, the 8

mW power is high enough to induce sufficient resonance

shift. The probe power is 10 times less leading to negligible

nonlinear effect in the cavity. Once the pump signal is

turned on, the optical energy trapped in the cavity gradually

Fig. 1 (Color online) Wavelength conversion process in a microdisk resonator. a Schematic graph illustrating the microdisk resonator integrated

with interleaved p-n junctions; b Transmission spectra before (solid line) and after (dashed line) optical pumping

Fig. 2 (Color online) Simulated wavelength conversion process in the microdisk resonator. a Waveforms of the pump and probe input signals;

b Variation of optical energy stored in the cavity; c Variation of the free-carrier concentration in the cavity and detuning of the resonance

frequency; d Waveform of the output probe signal

Chin. Sci. Bull. (2014) 59(22):2709–2716 2711

123

increases. As a result of TPA, free-carriers are generated.

Figure 2b shows the pump and probe optical energy accu-

mulated in the cavity. It is noticeable that there is a sharp

peak at the leading edge of the pump energy curve, resulted

from the interplay between external coupling and resonance

shift by free-carriers. After the peak, the pump energy is

stabilized to 0.1 pJ. On one side, the pump power is con-

tinuously fed into the cavity via waveguide coupling, and

the stored energy in the cavity could reach the maximum if

the resonance were not shifted. However, on the other hand,

the increasing energy generates free-carriers which inevita-

bly blue-shift the resonance frequency. The blue-shifted

wavelength reduces the stored energy until the system is

stabilized. For the probe light, the initially stored energy in

the cavity begins to decay following the pump pulse. It

recovers to the original level when the pump is turned off.

The free-carrier concentration overall follows the

stored energy in the cavity as shown in Fig. 2c. Because

the carrier lifetime is not short enough, it cannot resolve

the narrow dip in the pump energy profile. The resonance

frequency detuning is determined by both FCD and Kerr

effects. The FCD effect is much stronger than the Kerr

effect, and therefore, resonance shift almost follows the

free-carrier concentration curve. Figure 2d shows the

output probe signal. It is worth mentioning that the peak

optical power (1 mW) at the leading edge exceeds the

input power (0.8 mW) due to the resonance dynamic

tuning. At the steady state, the probe output power is 0.55

mW, a little lower than the input power because of the

Lorentzian spectral profile of the resonance.

From the above discussion, one sees that the pump

signal may not necessarily be set at the original resonance

frequency in order to obtain the maximum resonance

shift. Blue-shift of the pump signal helps to form positive

feedback so that more power can be coupled and stored in

the cavity. Figure 3a and b show the variation of free-

carrier concentration and the resultant output probe

waveforms, respectively, when the pump is slightly blue-

shifted from the original resonance frequency. The blue-

detuning of the pump first increases the steady-state car-

rier concentration until it reaches the maximum around

dxc = 0.003 FSR. Although the leading peak is always

present in the carrier concentration curves, it is gradually

diminished in the output probe signals. This is because

the probe transmission is less sensitive to the resonance

shift when it is at the on-state. The rising edge of the

output signal becomes slower with the increasing pump

detuning as it needs more time to build up the pump

energy in the cavity. With further detuning after the

maximum point, the coupled power decreases consider-

ably, resulting in low free-carrier concentration and weak

distorted output signal.

In the proceeding simulation, the resultant probe signal

has the same polarity with the pump signal. If we set the

Fig. 3 (Color online) Effect of the pump frequency detuning on the device performance. a Variation of free-carrier concentration; b Output

waveforms of the probe signal

2712 Chin. Sci. Bull. (2014) 59(22):2709–2716

123

probe to the blue-shifted resonance frequency, then the

output probe signal becomes inverted as shown in Fig. 4.

When the pump is at the original resonance frequency (dxc

= 0), the steady-state free-carrier concentration is 291016

cm-3 and the resonance blue-shift is 0.0025 FSR (Fig. 2c).

Therefore, we can set the probe detuning to be

dxp = 0.0025 FSR. The corresponding output signal is

shown in the top panel of Fig. 4. The leading peak in the

free-carrier concentration is translated into a small peak at

the off-state of the output signal. When the pump is set at

dxc = 0.003 FSR, the probe needs to be detuned to dxp =

0.005 FSR as shown in the middle panel of Fig. 4. To

reduce the impact of the leading peak, we can set dxp =

0.0055 FSR at the expense of more ripples at the off-state

as shown in the bottom panel of Fig. 4.

As the wavelength conversion is enabled by the free-

carrier dynamics in the cavity, the conversion speed is

ultimately limited by the carrier lifetime as well as the

photon lifetime. Figure 5 shows the variation of free-car-

rier concentration in the cavity and the corresponding

output waveforms for three different carrier lifetimes.

There is no frequency detuning for both the pump and

probe signals. Two consequences are resulted for shorter

carrier lifetime. First, the trailing edge of the output signal

becomes sharper as is expected. Second, the on-state output

power is reduced as the free-carrier concentration becomes

lower for shorter carrier lifetime.

3 Experimental results

Figure 6a shows the optical microscope image of the fab-

ricated device. The access waveguide is 300 nm wide and

its separation from the microdisk is 0.25 lm. The period of

the interleaved p-n junctions is about 1 lm with a half duty

cycle. The junction width along the radial direction is 2.3

lm. Compared to a single circular p-n junction, the inter-

leaved p-n junctions have larger fabrication tolerance [25].

Moreover, the electric field of the p-n junctions has larger

overlap with the WGMs, so that the free-carriers generated

by TPA can be fast swept out. The doping concentration of

the p and n regions is about 1017 cm-3 to minimize the

absorption loss. The doping concentration of the inner n?

and outer p? regions is about 1020 cm-3 to form good

ohmic contact. The inner doping radius is 4.2 lm so that it

does not deteriorate the first radial-order WGM [26]. The

device was first patterned using 248-nm deep ultra-violet

(DUV) photolithography followed by plasma dry etch.

Then a 1.5 lm thick silicon dioxide layer was deposited

Fig. 4 (Color online) Inverted output waveform when the probe

frequency is blue-detuned from the resonance frequency

Fig. 5 (Color online) Effect of the free-carrier lifetime on the device performance. a Variation of free-carrier concentration in the cavity;

b Output waveforms of the probe signal

Chin. Sci. Bull. (2014) 59(22):2709–2716 2713

123

using plasma-enhanced chemical vapor deposition (PEC-

VD). Finally, aluminum (Al) layer was sputtered and pat-

terned. The entire process is CMOS compatible.

Figure 6b shows the measured transmission spectrum of

the microdisk for transverse electric (TE) polarization. The

device fiber-to-fiber insertion loss is about 20 dB. Three

pronounced WGMs are observed in the spectrum. We

selected the resonances at k1 = 1,530.6 nm and k2 = 1,548.8

nm (first radial-order mode) for our wavelength conversion

experiment. The Q-factor of these resonances is 39104 and

the extinction ratio is about 8 dB. There is also another

resonance mode (second radial-order mode) with a higher

extinction ratio but a lower Q-factor of 4,600 that can also

be utilized for wavelength conversion. The third radial-

order mode has significant overlap with the central high-

doping region, and therefore it has the lowest Q-factor of

only 700. The insets show the magnified resonance spectral

profiles around k1 and k2. The photon lifetime of this mode

is s = Q1k1/2pc = 25.2 ps, determined by the microdisk

resonator internal loss and external coupling strength.

Figure 7 shows the experimental setup for wavelength

conversion. The pump and probe light waves are coupled

into the microdisk from the two ends of the access

waveguide. Their wavelengths are tuned to the left

shoulder of the resonances as indicated in the insets of

Fig. 6. We note that the resonance is red-shifted due to

the thermo-optic effect after pump power is coupled into

the resonator. The pump and probe wavelengths are

adjusted accordingly following the resonance red-shift.

The pump signal is generated by modulating a CW light

using a LiNiO3 amplitude modulator (AM) driven by a

pulse pattern generator (PPG) followed by a microwave

Fig. 6 (Color online) Fabricated device and its transmission spectrum. a Optical microscope image of the fabricated device; b Transmission

spectrum of the microdisk resonator. The insets show the zoom-in of the resonance dips. The pump and signal wavelengths (kc and kp) are

marked

Fig. 7 (Color online) Experimental setup for wavelength conversion. EDFA Erbium-doped fiber amplifier; BPF Band pass filter; PC

Polarization controller; AM Amplitude modulator; MA Microwave amplifier; PPG Pulse pattern generator; PD Photodetector

2714 Chin. Sci. Bull. (2014) 59(22):2709–2716

123

amplifier (MA). Tunable erbium-doped fiber amplifiers

(EDFA) are used to adjust the pump and probe light

power. Polarization controllers are used to set light waves

to TE polarization. The pump and probe signals are

separated by two circulators.

We first investigate the effect of junction bias on the

wavelength-converted signal. Figure 8a shows the resultant

output probe waveforms in response to a 100 MHz square

wave pump signal. The waveguide-coupled optical power of

the pump is 5 dBm. The open circuit configuration has the

slowest response, as there is no closed loop for the free-

carriers to recombine. The signal transition edge becomes

much sharper once the short circuit loop is formed. Reverse

bias of the p-n junctions can form strong electric field across

the depletion region, and therefore the TPA generated free-

carriers can be fast swept out of the cavity, leading to reduced

carrier lifetime and hence faster transition. At 28 V bias, the

transition time is as short as 100 ps, which is almost two

orders smaller than the open circuit case. Hence, it demon-

strates that using high electric field to deplete free-carriers is

an effective way to speed up the all-optical modulation. We

also perform wavelength conversion at 10 Gb/s using a

custom non-return-to-zero (NRZ) waveform pattern as

shown in Fig. 8b. The average optical powers of the pump

and probe light waves in the waveguide are 5.41 and 25.89

dBm, respectively. The bias voltage is set at 26 V. As we set

the pump and probe wavelengths to the left shoulder of the

resonances, the converted bit sequence is complementary to

the original one in consistence with the simulation shown in

Fig. 4 (bottom panel). As the waveforms were recorded by

an 8 GHz oscilloscope (Tektronix Digital Serial Analyzer),

the conversion speed is partially limited by the oscilloscope

bandwidth.

In our experiment, wavelengths of the pump and probe

signals are carefully tuned to follow the red-shift of reso-

nances after thermal heating by the pump light. As silicon

has a relatively large thermo-optic coefficient, it is hard to

circumvent self-heating in our current device. However, it

is possible to implement temperature-independent silicon

photonic devices by upper-cladding materials with a neg-

ative thermo-optic coefficient such as polymethyl meth-

acrylate (PMMA) and titanium dioxide (TiO2) [27, 28]. In

this way, the resonances can be stabilized, and hence the

operation wavelengths are independent of the pump power

level.

4 Conclusion

In summary, we theoretically studied and experimentally

demonstrated wavelength conversion in a silicon micro-

disk resonator integrated with interleaved p-n junctions.

The wavelength conversion is enabled by the free-carrier

plasma dispersion effect in the cavity. The interplay

between external optical feeding and intra-cavity free-

carrier dynamics is critical to the quality of wavelength

conversion. In particular, we analyzed the influence of

pump and probe frequency detuning and the carrier

lifetime on the converted signal. The experiment using a

6 lm-radius microdisk resonator with a Q-factor of

39104 also confirms that reverse bias can significantly

shorten free-carrier lifetime and improve conversion

speed. We successfully realized wavelength conversion of

a 10 Gb/s bit sequence with 5.41 dBm pump power and

-6 V bias.

Fig. 8 (Color online) Experimental results. a Wavelength-converted square waveforms at 100 MHz speed for various biases. i: open circuit; ii:

short circuit; iii: -4 V bias; iv: -8 V bias; b Optical waveform conversion using a 10 Gb/s bit sequence. i: input pump signal; ii: wavelength-

converted output probe signal. The bias is set at -6 V

Chin. Sci. Bull. (2014) 59(22):2709–2716 2715

123

Acknowledgments This work was supported in part by the National

Basic Research Program of China (ID2011CB301700), the National

High Technology Research and Development Program of China

(2013AA014402), the National Natural Science Foundation of China

(61007039, 61001074, 61127016, 61107041), the Science and

Technology Commission of Shanghai Municipality (STCSM) Project

(12XD1406400). We also acknowledge IME Singapore for device

fabrication.

References

1. Ramaswami R, Sivarajan K, Sasaki G (2009) Optical networks: a

practical perspective. Morgan Kaufmann Publishers Inc., San

Francisco

2. Zhang Y, Chowdhury P, Tornatore M et al (2010) Energy effi-

ciency in telecom optical networks. IEEE Commun Surv Tut

12:441–458

3. Durhuus T, Mikkelsen B, Joergensen C et al (1996) All-optical

wavelength conversion by semiconductor optical amplifiers.

J Lightwave Technol 14:942–954

4. Liu Y, Tangdiongga E, Li Z et al (2007) Error-free 320-Gb/s all-

optical wavelength conversion using a single semiconductor

optical amplifier. J Lightwave Technol 25:103–108

5. Mao W, Andrekson PA, Toulouse J (2005) All-optical wave-

length conversion based on sinusoidal cross-phase modulation in

optical fibers. IEEE Photon Technol Lett 17:420–422

6. Murray RT, Kelleher EJ, Popov SV et al (2013) Widely tunable

polarization maintaining photonic crystal fiber based parametric

wavelength conversion. Opt Express 21:15826–15833

7. Pelusi M, Luan F, Madden S et al (2010) Wavelength conversion

of high-speed phase and intensity modulated signals using a

highly nonlinear chalcogenide glass chip. IEEE Photon Technol

Lett 22:3–5

8. Ta’Eed VG, Fu L, Pelusi M et al (2006) Error free all optical

wavelength conversion in highly nonlinear As-SE chalcogenide

glass fiber. Opt Express 14:10371–10376

9. Hu H, Nouroozi R, Ludwig R et al (2010) Polarization-insensitive

all-optical wavelength conversion of 320 Gb/s RZ-DQPSK sig-

nals using a Ti: PPLN waveguide. Appl Phys B 101:875–882

10. Absil P, Hryniewicz J, Little B et al (2000) Wavelength con-

version in GaAs micro-ring resonators. Opt Lett 25:554–556

11. Reed GT, Knights AP (2008) Silicon photonics. Wiley Online

Library

12. Pu M, Hu H, Peucheret C et al (2012) Polarization insensitive

wavelength conversion in a dispersion-engineered silicon wave-

guide. Opt Express 20:16374–16380

13. Foster MA, Turner AC, Salem R et al (2007) Broad-band con-

tinuous-wave parametric wavelength conversion in silicon

nanowaveguides. Opt Express 15:12949–12958

14. Rong H, Kuo Y-H, Liu A et al (2006) High efficiency wavelength

conversion of 10 Gb/s data in silicon waveguides. Opt Express

14:1182–1188

15. Turner-Foster AC, Foster MA, Salem R et al (2010) Frequency

conversion over two-thirds of an octave in silicon nanowave-

guides. Opt Express 18:1904–1908

16. Hu H, Ji H, Galili M et al (2011) Ultra-high-speed wavelength

conversion in a silicon photonic chip. Opt Express 19:19886–

19894

17. Turner AC, Foster MA, Gaeta AL et al (2008) Ultra-low power

parametric frequency conversion in a silicon microring resonator.

Opt Express 16:4881–4887

18. Morichetti F, Canciamilla A, Ferrari C et al (2011) Travelling-

wave resonant four-wave mixing breaks the limits of cavity-

enhanced all-optical wavelength conversion. Nat Commun 2:296

19. Xu Q, Almeida VR, Lipson M (2005) Micrometer-scale all-

optical wavelength converter on silicon. Opt Lett 30:2733–2735

20. Li Q, Zhang Z, Liu F et al (2008) Dense wavelength conversion

and multicasting in a resonance-split silicon microring. Appl

Phys Lett 93:081113

21. Reed GT, Mashanovich G, Gardes F et al (2010) Silicon optical

modulators. Nat Photon 4:518–526

22. Zhu H, Zhou L, Sun X et al (2013) Photocurrent generation in a

microdisk resonator integrated with interleaved pn junctions.

Paper presented at CLEO: Science and Innovations, Optical

Society of America, San Jose, California, June 9-14, 2013

23. Manolatou C, Lipson M (2006) All-optical silicon modulators

based on carrier injection by two-photon absorption. J Lightwave

Technol 24:1433–1439

24. Tsang HK, Liu Y (2008) Nonlinear optical properties of silicon

waveguides. Semicond Sci Technol 23:064007

25. Li Z-Y, Xu D-X, McKinnon WR et al (2009) Silicon waveguide

modulator based on carrier depletion in periodically interleaved

pn junctions. Opt Express 17:15947–15958

26. Zhou L, Poon AW (2006) Silicon electro-optic modulators using

pin diodes embedded 10-micron-diameter microdisk resonators.

Opt Express 14:6851–6857

27. Zhou L, Okamoto K, Yoo SJB (2009) Athermalizing and trim-

ming of slotted silicon microring resonators with UV-sensitive

pmma upper-cladding. IEEE Photon Technol Lett 21:1175–1177

28. Djordjevic SS, Shang K, Guan B et al (2013) CMOS-compatible,

athermal silicon ring modulators clad with titanium dioxide. Opt

Express 21:13958–13968

2716 Chin. Sci. Bull. (2014) 59(22):2709–2716

123