divider and other SiGe ILFDs; the proposed ring-oscillator-based

ILFD has wider-locking-range percentage.

4. CONCLUSIONS

A new small die area ring-oscillator-based ILFD circuit has

been proposed and implemented in the 0.35 lm SiGe BiCMOS

technology. The ILFD consists of four-stage of differential am-

plifier to construct an eight-phase oscillator, and the injection

signal is applied to the tails of the differential amplifiers. The

proposed ILFD provides an eight-phase output and wide-locking

range from 12.7 to 15.25 GHz. The locking-range percentage of

the proposed ILFD is the largest among the published SiGe

HBT ILFDs.

REFERENCES

1. A. Joseph, J. Dunn, G. Freeman, D. Harame, D. Coolbaugh, R.

Groves, K. Stein, R. Volant, S. Subbanna, V. Marangos, S. St.

Onge, E. Eshun, P. Cooper, J. Johnson, J.-S. Rieh, B. Jagannathan,

V. Ramachandran, D. Ahlgren, D. Wang, and X. Wang, Product

applications and technology directions with SiGe BiCMOS, IEEE J

Solid State Circuits 38 (2003), 1471–1478.

2. A. Rylyakov and T. Zwick, 96-GHz static frequency divider in SiGe

bipolar technology, IEEE J Solid State Circuits 39 (2004),

1712–1715.

3. H. Knapp, M. Wurzer, T.F. Meister, K. Aufinger, J. Bock, S.

Boguth, and H. Schafer, 86 GHz static and 110 GHz dynamic fre-

quency dividers in SiGe bipolar technology, IEEE MTT-S Int Dig 2

(2003), 1067–1070.

4. J.C. Chien, C.S. Lin, L.H. Lu, H. Wang, J. Yeh, C.Y. Lee, and J.

Chern, A harmonic injection-locked frequency divider in 0.18 lmSiGe BiCMOS, IEEE Microwave Wireless Compon Lett 10 (2006),

561–563.

5. J.K. Nakaska and J.W. Haslett, An integrated 6.2–11GHz SiGe

inductorless injection locked frequency divider, Eur Microwave

Conf 1 (2005), 669–672.

6. S.-L. Jang, K.-C. Shen, C.-W. Chang, and M.-H. Juang, A 6-phase

�3 injection locked frequency divider in SiGe BiCMOS technology,

Microwave Opt Technol Lett 51 (2009), 1555–1557.

7. S.-L. Jang, C.C. Liu, and C.-W. Chung, A tail-injected divide-by-4

SiGe HBT injection locked frequency divider, IEEE Microwave

Wireless Compon Lett 19 (2009), 236–238.

8. F. Herzel and W. Winkler, A 2.5 GHz eight-phase VCO in SiGe

BiCMOS technology, IEEE Trans Circuits Syst II 52 (2005),

140–144.

9. X. Guan, H. Hashemi, and A. Hajimiri, A fully integrated 24-GHz

eight-element phased-array receiver in silicon, IEEE J Solid State

Circuits 39 (2004), 2311–2320.

10. J.J. Kim and B. Kim, A low-phase-noise CMOS LC oscillator with

a ring structure, IEEE ISSCC Dig Tech Papers 38 (2000), 430–431.

11. R. Tang and Y.-B. Kim, A novel 8-phase PLL design for PWM

scheme in high speed I/O circuits, IEEE Int SOC Conf (2006),

119–122.

12. Y.-H. Chuang, S.-H. Lee, S.-L. Jang, J.-J. Chao, and M.-H. Juang,

A ring-oscillator-based wide locking range frequency divider, IEEE

Microwave Wireless Compon Lett 16 (2006), 470–472.

VC 2009 Wiley Periodicals, Inc.

DEMONSTRATION AND ANALYSISOF DUAL-WAVELENGTH CLOCKRECOVERY BASED ON STIMULATEDBRILLOUIN SCATTERING

Ming Chen,1,2 and Shui-sheng Jian1,21 Key-Laboratory of All-Optical Networks and AdvancedCommunication Networks, Ministry of Education, Beijing 100044,China; Corresponding author: 05111023@bjtu.edu.cn2 Institute of Lightwave Technology, Beijing Jiaotong University,Beijing 10004, China

Received 29 April 2009

ABSTRACT: Dual-wavelength clock recovery (CR) based on

Stimulated Brillouin Scattering is realized. The maximum frequencyspacing of two channels is theoretically analyzed and experimentally

demonstrated. The total wavelength span of the CR scheme isinvestigated to be about 3.37 nm in experiment. Multiwavelength CRcan be implemented within the span. VC 2009 Wiley Periodicals, Inc.

Microwave Opt Technol Lett 52: 204–207, 2010; Published online in

Wiley InterScience (www.interscience.wiley.com).

DOI 10.1002/mop.24888

Key words: all-optical clock recovery; stimulated Brillouin scattering(SBS); optical signal processing

1. INTRODUCTION

All-optical signal processing is inevitably developed as the most

promising scheme for ultra-high-speed data transmission and

processing systems because of its potential of high speed

response. All-optical clock recovery (CR) is one of the corre-

sponding fundamental technology in signal processing for system

synchronization such as all-optical regeneration, all-optical time-

division switching systems, and all-optical demultiplexers. Sev-

eral optical timing extraction techniques suitable for high-speed

operations have been demonstrated using self-pulsating lasers [1],

optical passive tank circuit based on Fabry-Perot resonator [2, 3],

multisection laser diodes [4], two-photon absorption [5], and opti-

cal phase-lock loop [6, 7]. Most of these methods require prior

knowledge of the bit rate. An exception is an active optical filter

based on the comb-shaped gain spectrum according to Stimulated

Brillouin Scattering (SBS) [8, 9]. In Ref. 8, Kawakami et al. used

several extra continuous-wave (CW) lights with different center

frequencies as pumps to amplify multiple clock-related line spec-

tral components of the optical data signal; however, the center

frequencies of the CW laser must be tuned carefully in the band-

width range of Brillouin Gain (20–40 M) and automatic fre-

quency control (AFC) should be applied. In Ref. 9, Butler et al.

directly used the comb spectral components of the signal light as

the pump lights, and the clock can be recovered in optical domain

without the knowledge of the incoming data bit rate. Besides, as

the optical fiber in a dense wavelength-division-multiplexing sys-

tem hosts multiple wavelength channels, it is especially desirable

to extend this parallelism to signal processing by finding means

of all-optical CR. A multiwavelength clock-recovery device can

greatly simplify costs by eliminating the need to have a separate

regenerator for each wavelength, which has been researched in

Refs. 10–12.

In this study, multiwavelength all-optical CR based on SBS is

realized. According to the CR scheme, the maximum frequency

spacing is theoretical analyzed and demonstrated in our experi-

ment, correspondingly, for the first time, with low clock jitter and

204 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 1, January 2010 DOI 10.1002/mop

without pattern dependence. Meanwhile, the scheme presents an

idea for the measurement of the gain bandwidth of SBS.

2. PRINCIPLE OF OPERATION

SBS as the nonlinearity is utilized for all-optical CR and the

measurement of the gain bandwidth of SBS because it has a

long interaction time (narrow bandwidth) and a lower threshold

than other optical nonlinearities in fibers. The process of SBS

can be described classically as a nonlinear interaction between

the pump and the backward-propagating Stokes fields through

an acoustic wave, which is generated by the pump field through

the process of electrostriction. When the pump power is higher

than the SBS threshold, the stimulated scattering process trans-

forms most of the energy of the pump light to the counter-prop-

agating wave. The gain of each spectral component in the

Stokes signal is related to the power in the corresponding spec-

tral components of the pump. The result is an active filter that

selects and amplifies only the most intense spectral components,

facilitating the extraction of the appropriate frequency compo-

nents in the optical domain.

Here, an input data stream is split into two beams with oppo-

site directions using a directional coupler. The larger one works

as the pump; the other beam is downshifted by the Brillouin shift

of a dispersion shift fiber (DSF) using a modulator. As long as

the clock-related CW tones in the downshifted seed wave (Stokes

wave) have sufficient power to exceed the Brillouin threshold,

the data portion of the seed spectrum is attenuated relative to the

clock, and then the clock component can be extracted. Because

of the additional modulation, the variation of the pump detuning

due to the signal frequency has been discussed in Ref. 13. With

the extra modulation, the modulation frequency used to down-

shift the signal is designed to be fixed, so control over the fre-

quency of the seed signal is unnecessary. Furthermore, the

scheme based on SBS is without pattern effect. Meanwhile, as all

single wavelength Brillouin active filters can be introduced owing

to the narrow Brillouin gain bandwidth of the DSF fiber (about

20 MHz) and each seed signal is only allowed to interact with its

own unshifted pump signal, thus, multiwavelength CR is possi-

ble, if each channel’s clock tone power levels are all above the

Brillouin threshold, which can be satisfied by using high gain er-

bium-doped fiber amplifier (EDFA).

Considering that the Brillouin shift varies inversely with the

pump wavelength, we focus on the analysis of the maximum

frequency spacing for CR from two channel signals based on

SBS. Figure 1 shows the sketched analysis principle. This

pump-induced index grating scatters the pump light through

Bragg diffraction. Scattered light is downshifted in frequency

because of the Doppler shift associated with a grating moving at

the acoustic velocity mA. The antidirection Brillouin shift is, fB¼ 2nmA/kp, where n is the refractive index at the pump wave-

length kp. It is obvious that the Brillouin shifts are different

according to two channel signals with different central fre-

quency (expressed as f1 and f2). However, if f1 is close to f2, theinduced Brillouin shifts can be reasonably assumed to equal,

which means fB1 ¼ fB2. The seed signals are downshifted by the

modulation frequency fm, where fm ¼ fB1; thus, the multiwave-

length clocks can be recovered simultaneously without any diffi-

culty. As Figure 1(a) shows, although the Brillouin shift fB2becomes bigger going with the increase of f2, the downshifted

seed signal deviate from the centre of the Brillouin-gain band-

width for the DSF used in the scheme under the condition of fm¼ fB1; therefore, the SBS gain is reduced for this clock tone.

More direct current components exist in the extracted clock,

even make it impossible for CR. It is necessary to make sure

that the two seed signals with frequency of f1 � f 0m and f2 � f 0mare both adjusted to the range of gain bandwidth of SBS, so fmis changed to f 0m as shown in Figure 1(b). Then, the multiwave-

length clock can still be extracted even if the quality of the

clock is degraded. Figure 1(c) shows the ultimate situation that

both f1 � f 0m and f2 � f 0m totally exceed the Brillouin-gained

spectrum; it is unable to realize the multiwavelength CR. Here,

fm ¼ fB1 þ 1

2DmB ¼ fB2 � 1

2DmB; (1)

where DmB is the gain bandwidth of SBS, and fBi ¼ 2nmA/ki,fi ¼ c/ki, so the maximum frequency spacing is given by

f2 � f1 ¼ DmB � c=2nmA; (2)

if vA, n is set to be 5.96 km/s and 1.44, respectively, the values

appropriate for silica fibers, DmB ¼ 24 MHz at kp ¼ 1.55 lm,

the calculated maximum frequency spacing (f2�f1) is about 416

GHz, and the corresponding wavelength span for multiwave-

length CR is 3.4 nm.

Some points are necessary to illuminate: [1] The CR scheme

is based on SBS effect, and the Brillouin-gained bandwidth

depends on the fiber, accordingly the maximum frequency spac-

ing just relates to the fiber medium; [2] Only central frequency

of the signal is used for analysis as shown in Figure 1; if con-

sidering the spectral broadening induced by high-speed modula-

tion, the modulation rate fdm should subtract from f2�f1. For

example, in 10 Gbps optical system, the corresponding wave-

length span is changed to 3.3–0.08 nm [3]. The ideal upper

bound of data rate for the single channel clock extraction is also

about 416 Gbps because the pump is obtained directly from the

incoming signal [4]. The data rates of each wavelength need not

be the same. The only cost of adding each additional wave-

length channel is that the input power to the Brillouin active fil-

ter or the gain of the EDFA in the scheme must be increased by

3 dB [10] for each additional wavelength.

3. EXPERIMENTAL SETUP AND RESULTS

Figure 2 shows the experimental setup. Two channels 9.953 Gb/

s optical RZ data stream is generated by modulating the CW

output of a distributed feedback fiber laser with a cascaded

LiNbO3 Mach-Zehnder modulator (MZM) driven by pattern

generator (Agilent N4901B) with 223�1 bits length. A high gain

EDFA is used to increase the power of the signal above 100

Figure 1 Principle of the maximum frequency spacing

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 1, January 2010 205

mW to exceed the SBS threshold. Then, the multiwavelength

signals are injected to a directional coupler. As SBS is an inter-

action involving two counterpropagating waves, i.e., the pump

and the Stokes, that are separated in frequency from each other

by the acoustic phonon frequency of the medium, therefore, in

our system, we split the amplified data stream into two counter-

propagating signals and then downshift one of them by the

acoustic resonance to the Stokes frequency. Here, the clockwise

propagating signal is modulated by the Brillouin shift (10.480

GHz) of DSF using the MZM to be the Stokes wave, and the

anticlockwise signal with 95% of the total power functions as

the pump. The pump with frequency unshifted retains most of

the power of the input data stream so that the Stokes wave expe-

riences strong gain during the SBS interaction. The polarization

controllers maintained the same polarization state for the pump

and Stokes waves. Optical isolators are used to prevent the

Stokes signal from re-entering into the input or appear in the

forward direction. The performance of the system is investigated

by a digital sampling oscilloscope (Agilent DCA 86100 B).

First, the central wavelength of two channels is set to

1554.948 and 1555.754 nm, respectively, and the spectrums are

shown in Figure 3 with the dashed lines. Each channel’s clock

tone power levels are above the Brillouin threshold after ampli-

fied by a high gain EDFA. So, SBS interaction works as an

active filter to extract clock. The solid lines in Figure 3 show

the spectrum of the multiwavelength recovered clock. The tiny

frequency shift compared to the spectrum of input RZ signals is

due to the additional modulation at the Brillouin shift (10.480

GHz). Figure 4 shows the eye-diagrams of recovered clock at

1554.948 nm (the top trace) and 1555.754 nm (the bottom one).

The time jitters were 2.34 and 2.41 ps, respectively, which origi-

nated mainly from amplitude fluctuations. A broadband optical

filter for restraining amplified spontaneous emission from the

high gain EDFA should be applied. Besides, if the amplifier is

unsaturated, a small amount of noise can create substantial am-

plitude variations in the recovered clock signal because of the

exponential nature of the gain, and this may be overcome by

increasing the Stokes power. It is also helpful to utilize another

optical filter in the output. Or cascaded CR module based on

SBS is introduced.

Next step is to increase the frequency spacing by changing

the output wavelength of fiber laser. The multiwavelength clock

can still be extracted, even if the quality is degraded. However,

when the wavelengths are adjusted to the 1554.34 and 1557.71

nm, it became so hard to recover the two channel clocks. The

spectrum of output signals is shown in Figure 5. Here, we can

see, only one frequency component suffers the Brillouin-gain for

each channel, and not all clock tones are amplified; thus neither

clock can be recovered. This situation is homologous to upper

analysis illuminated in Figure 1(c). Then, we decrease the wave-

length span a little; the clock of one channel can be extracted as

shown in Figure 6, with high time jitter and huge noise.

The maximum wavelength span for two channels is demon-

strated to be 3.37 nm in our experiment. Once exceed this span,

Figure 3 Spectrums of multiwavelength RZ signal and extracted clock

Figure 4 The measured eye-diagram of multiwavelength recovered

clock. Time scale: 50 ps/div. [Color figure can be viewed in the online

issue, which is available at www.interscience.wiley.com]

Figure 5 The output spectrum of the clock recovery device

Figure 2 Experimental setup of multiwavelength clock recovery.

EDFA, erbium-doped fiber amplifier; ISO, isolator; PC, polarization con-

troller; DSF, dispersion shift fiber; MZM, M–Z modulator; AWG,

arrayed waveguide grating

206 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 1, January 2010 DOI 10.1002/mop

the clock tones of two channels are out of the Brillouin gain

spectrum and not able to experience the process of SBS. The

experiment results are in good agreement with the theoretical

analysis.

Although the measured wavelength span of our clock-recov-

ery device was about 3.37 nm, this span could be increased by

introducing strain into the DSF [10] or replaced by another fiber

loop with a larger Brillouin-gain bandwidth. Besides, multiple

modulators and corresponding wavelength filters that provide

different modulation shifts for different wavelength bands may

fundamentally extend the span.

Meanwhile, according to Eq. (2), a rough method for the

measurement of gain bandwidth of SBS can be presented, which

is given by

DmB ¼ f2 � f1ð Þ � 2nmA=c; (3)

The Brillouin-gain bandwidth can be calculated by using the ex-

perimental value of the maximum frequency spacing.

4. CONCLUSIONS

Maximum frequency spacing for dual-wavelength all-optical CR

based on SBS is demonstrated experimentally and understood

theoretically. The maximum wavelength span is 3.37 nm accord-

ing to the DSF we used in the experiment. Multiwavelength

clock can be recovered within the span. The all-optical CR

scheme can be extended to high speed and parallel all-optical

processing of multiple channels’ even more simultaneous data

rates (e.g., 160 Gbps in addition to 10 and 40 Gbps).

ACKNOWLEDGMENTS

This work is supported by Excellent Doctoral Scientific Innovation

Projects of Beijing Jiaotong University (No. 141051522), National

863 High Technology Projects of China (No. 2007AA01Z258,

2008AA01Z15). The authors acknowledge the optical transmission

group in Institute of Lightwave Technology in Beijing Jiaotong

University for their helpful discussion and support throughout the

work.

REFERENCES

1. P. Barnsley, H. Wickes, G. Wickens, and D. Spirit, All-optical clock

recovery from 5 Gbit/s RZ data using a self-pulsating 1.56 _m laser

diode, IEEE Photon Technol Lett 3 (1991), 942–945.

2. M. Jinno and T. Matsumoto, Optical tank circuits used for all-opti-

cal timing recovery, IEEE J Quantum Electron 28 (1992), 895–900.

3. G. Contestabile, A. D’Errico, M. Presi, and E. Ciaramella, 40-GHz

all-optical clock extraction using a semiconductor-assisted Fabry-

Perot filter, IEEE Photon Technol Lett 16 (2004), 2523–2525.

4. I. Kim, C. Kim, G. Li, P. LiKamWa, and J. Hong, 180-GHz clock

recovery using a multisection gain-coupled distributed feedback

laser, IEEE Photon Technol Lett 17 (2005), 1295–1297.

5. R. Salem and T.E. Murphy, Broad-band optical clock recovery sys-

tem using two-photon absorption, IEEE Photon Technol Lett 16

(2004), 2141–2143.

6. S. Kawanishi and M. Saruwatari, Ultrahigh-speed PLL-type clock

recovery circuit based on all-optical gain modulation in travelling-

wave laser diode amplifier as a 50 GHz detector, Electron Lett 24

(1993), 1714–1715.

7. O. Kamatani and S. Kawanishi, Ultrahigh speed clock recovery with

phase locked loop based on four-wave mixing in a traveling-wave

laser diode amplifier, J Lightwave Technol 14 (1996), 1557–1568.

8. H. Kawakami, Y. Miyamoto, T. Kataoka, and K. Hagimoto, All-op-

tical timing clock extraction using multiple wavelength pumped

Brillouin amplifier, IEICE Trans Com E78-B (1995), 694–701.

9. D.L. Butler, J.S. Wey, M.W. Chbat, et al., Optical clock recovery

from a data stream of an arbitrary bit rate by use of Stimulated Bril-

louin Scattering, Opt Lett 20 (1995), 560–562.

10. C. Johnson, K. Demarest, C. Allen, R. Hui, K.V. Peddanarappagari,

and B. Zhu, Multiwavelength all-optical clock recovery, IEEE Pho-

ton Technol Lett 11 (1999), 895–897.

11. D.V. Kuksenkov, S. Li, M. Sauer, and D.A. Nolan, Nonlinear fiber

devices operating on multiple WDM channels, 31st Eur Conf Opt

Commun (ECOC) Mo3.5.1 (2005), 51–54.

12. T. von Lerber, J. Tuominen, H. Ludvigsen, S. Honkanen, and F.

Kueppers, Multichannel and rate all-optical clock recovery, IEEE

Photon Technol Lett 18 (2006), 1395–1397.

13. X. Zhou, H.H.M. Shalaby, L. Chao, et al., A performance analysis

of all-optical clock extraction circuit based on stimulated Brillouin

scattering, J Lightwave Technol 18 (2000), 1453–1466.

VC 2009 Wiley Periodicals, Inc.

STABLE OPERATION OF MICROWAVEPHOTONIC FILTERS CONSTRUCTEDWITH TWO CASCADED HI-BI FIBERS

H. Dong,1 J. Q. Zhou,2 S. Aditya,2 Y. D. Gong,1 and P. Shum2

1 Institute for Inforcomm Research, 1 Fusionopolis Way, # 21-01Connexis, Singapore 138632; Corresponding author:hdong@i2r.a-star.edu.sg2Network Technology Research Centre, Nanyang TechnologicalUniversity, Singapore 637553

Received 6 April 2009

ABSTRACT: For the microwave photonic filters constructed with twocascaded Hi-Bi fibers, we demonstrate theoretically and experimentally

a new technique that uses a dual- or tri-wavelength spacing-tunablelaser source to greatly suppress the sensitivity of the filter response toenvironmental perturbations without the introduction of the phase-

induced intensity noise. VC 2009 Wiley Periodicals, Inc. Microwave Opt

Technol Lett 52: 207–212, 2010; Published online in Wiley InterScience

(www.interscience.wiley.com). DOI 10.1002/mop.24844

Key words: microwave photonic filter; Hi-Bi fiber; differential groupdelay; multi-wavelength laser

1. INTRODUCTION

Microwave and millimeter-wave signal processing using micro-

wave photonic filters (MPF) offer various advantages like large

time–bandwidth product, electromagnetic interference immunity,

and inherent low loss due to the use of optical components [1].

To achieve a stable operation free from environmental

Figure 6 The measured eye-diagram of degraded recovered clock.

Time scale: 50 ps/div. [Color figure can be viewed in the online issue,

which is available at www.interscience.wiley.com]

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 1, January 2010 207