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Optics Communications 281 (2008) 1476–1479

Novel tunable microwave photonic notch filter usinga 3 � 3 coupler based Sagnac loop

Yan Gao a,*, Liang Dou a, Anshi Xu a, Ping Shum b, Ming Tang b

a State Key Laboratory of Advanced Optical Communication Systems and Networks, Peking University, Beijing 100871, Chinab Network Technology Research Center (NTRC), Research TechnoPlaza, Nanyang Technological University,

50 Nanyang Drive, Singapore 637553, Singapore

Received 28 June 2007; received in revised form 26 October 2007; accepted 22 November 2007

Abstract

We propose a simple and stable structure to realize a tunable negative coefficients microwave photonic notch filter. A 3 � 3 couplerbased Sagnac loop interferometer with an asymmetrically placed phase modulator is used to acquire the notches. Our system is simplyimplemented and with good stability. Theoretical analysis and experimental results are presented and show a good agreement. The struc-ture is proved to be a robust filter with more than 35 dB rejection deep notches.� 2007 Elsevier B.V. All rights reserved.

Keywords: Microwave photonic filter; Notch filter; Sagnac loop; Phase modulator; Optical signal processing

1. Introduction

Microwave photonic filters (MPF) bring significantadvantages inherent to photonics such as low loss, largebandwidth, immunity to electromagnetic interference(EMI), tunability and reconfigurability. They can be usedin various wireless communications and radar applications.Until now different kinds of MPF have been reported inliteratures [1–8], and most of the conventional incoherentMPF [1–3] have positive coefficients of their impulseresponses, which exhibit a resonance at baseband. Thuspeople have been trying different ways to reach negativecoefficients. These methods are based on differentialdetection [4], intensity inversion caused by gain saturationin a semiconductor optical amplifier (SOA) [5], FBG basedSagnac loop [6] and p phase inversion in a Mach–Zehndermodulator [7]. In this paper, we propose a novel tunablenegative coefficients photonic notch filter using a 3 � 3coupler based Sagnac loop. With a broad-band sourceand a polarization controller, we can achieve stable

0030-4018/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.optcom.2007.11.064

* Corresponding author. Tel.: +86 10 62755136; fax: +86 10 62751763.E-mail address: camellia.yan@gmail.com (Y. Gao).

performance easily. Experimental results and theoreticalanalysis are presented to verify the proposed configurationand deep notches with more than 35 dB rejection areobtained.

2. Implementation scheme

The proposed MPF topology is shown in Fig. 1. TheSagnac loop consists of two sections of single mode fiber(SMF) with different length L1 and L2, a polarization con-troller (PC) and a phase modulator. Continuous-wave lightfrom a broad-band source is fed into port 2 of the 3 � 3coupler and split equally into three parts: clockwise (CW)light from port a, counterclockwise (CCW) light from portc and the rest is terminated at port b, respectively. The cou-pler used here is a fully symmetric one [9], whose S-matrixwill be given in the following section. The CW light travelsthrough the SMF with length L1 first, and then passes thePC, phase modulator and SMF with length L2 in turn. TheCCW light passes these components in the opposite order.Because the arrival moments of the CW and the CCWbeams at the phase modulator are different, the input RFsignal changes, and so does the RF signal induced phase

Fig. 1. Topology of negative tap photonic notch filter using a 3 � 3 coupler based Sagnac loop.

Y. Gao et al. / Optics Communications 281 (2008) 1476–1479 1477

modulation upon the two beams. This difference in phasemodulation of the two counter-propagating beams will beconverted into intensity modulation when these beamsrecombine at the 3 � 3 coupler to interfere together. Sincethe fiber used in this scheme is all standard single modefiber, a polarization controller (PC) is needed to counteractthe birefringence in the loop. The optical signals from twooutput ports (1 and 3) of the 3 � 3 coupler are detected byphotodetectors separately, and then are subtracted in thereceiving module. Hence, the phase difference inducedintensity modulation can be observed. A network analyzeris used to measure the frequency response of the system inthe RF regime. Since MPF is used as a device instead ofbeing used for tele-communications, the broadband sourcedoes not limit its practical use [10]. Further, our schemecan be easily extended to a tunable MPF by adding a tun-able optical fiber delay line into the loop. By tuning thelength of the optical fiber delay line, the free spectral range(FSR) of the MPF can be controlled accordingly.

3. Theoretical analysis

According to the structure shown in Fig. 1 along withthe transmission matrix of the 3 � 3 fully symmetric cou-pler [9]

1ffiffiffi3p

1 ej23p ej23p

ej23p 1 ej23p

ej23p ej23p 1

264

375

the output RF voltage is given by

V ¼ R0 � g� ðP 1ðtÞ � P 3ðt � DtÞÞ ð1Þwhere R0 is the resistor of the receiving circuit, g is the pho-todiode responsivity, P1 and P3 are powers of the two out-put arms, respectively. Dt is the time difference caused bythe length difference of the fibers between the coupler out-puts and the photodetectors. Based on the coupler’s trans-mission matrix the items in the bracket can be rewritten asfollows:

P 1ðtÞ � P 3ðt � DtÞ ¼ 2

9DP 0 cos

2

3p� uðtÞ

� ��

� cos2

3pþ uðt � DtÞ

� ��ð2Þ

In this equation, D is the power transmission coefficient ofthe loop, P0 is the power coupled into the Sagnac loop and/ is the phase shift difference caused by the asymmetricallyplaced phase modulator, and it is defined as

u ¼ M sinðxRFtÞ �M sin xRF t � nðL1 � L2Þc

� �� �

¼ 2M cos xRF t � DLn2c

� �� �sin

xRFDLn2c

� �ð3Þ

M is modulation coefficient of the phase modulator, i.e.

M ¼ pV RF=V p

Vp is the phase modulator switching voltage, VRF is the RFsignal amplitude, xRF is the RF signal angular frequency, n

is the fiber refractive index and c is the speed of light invacuum.

To be noted, the switching voltage is different for CWand CCW waves if the frequency is quite high, e.g. severalGHz. We make an approximation of their equality herebecause the phase modulator is operating at several hun-dreds of MHz in our current scheme. Under such condi-tion, the mismatch of the two sections of output fiberscan be neglected as it is easy to control the length of thetwo fibers at several centimeters scale. Here we assume /is small, and Eq. (1) can be simplified to the followingform:

V � 4ffiffiffi3p

9DR0P 0gM

� sinxRFDLn

2c

� �cos xRF t � Dt

2� nDL

2c

� �� �ð4Þ

In the experiment the input RF signal should be small en-ough compared with the switching voltage of the phase

1478 Y. Gao et al. / Optics Communications 281 (2008) 1476–1479

modulator to guarantee the effectiveness of the small-angleapproximation.

From Eq. (4), it is clear that the filter’s FSR equals toc

DL�n. Finally the transmission function of the MPF can bederived according to (4)

Hðf Þ � 4

27D2R2

0P 20g

2M0 1� cosxRFDLn

c

� �� �ð5Þ

where M0 ¼ M2

P rf-in

Due to the inherent symmetric property of the Sagnacloop, the two counter-propagating light beams travelthrough the same light path [8], thereby the phase noisepresented in the M–Z interferometer based structure canbe eliminated and high SNR can be obtained.

According to Ref. [11], the output suffers from polariza-tion noise if polarized light is used and suffers from sensi-tivity noise if unpolarized light is used. As for the Sagnacinterferometer with a 3 � 3 coupler, similar results can alsobe concluded. According to the output equation in [11], abroadband source can significantly reduce the mode cou-pling induced sensitivity noise because of its short coherentlength. So in order to get stable performance with singlemode fibers, we choose a broadband source. In addition,we put a polarization controller (PC) in the loop to com-pensate for the polarization states difference of CW andCCW waves at the phase modulator, which is induced bythe imperfectness of the source. Through adjustment ofthe PC the best signal contrast can easily be reached.

4. Results and discussion

The experiment configuration shown in Fig. 1 is carriedout. It comprises a broad-band source produced by anEDFA, a fiber-based polarization controller, a phase mod-ulator and an equally divided 3 � 3 coupler. The spectrumof the ASE broad-band light from EDFA is shown inFig. 2. The phase modulator used is a LiNbO3Y waveguidephase modulator (PM51544-Q). Its insertion loss is around3 dB and the switching voltage is 4 V. Two output beams

Fig. 2. The spectrum of the ASE broad-band light from EDFA.

(port 1 and port 3) from the coupler are detected by twophotodetectors separately and then differentiated. A net-work analyzer (HP8510C) is used to acquire the filter’sresponse. The highest RF amplitude to be filtered is limitedby the phase modulator’s switching voltage. Ours is 4 Vand the maximum RF power should not exceed�7.9 dB m to satisfy the approximation in Eq. (4). In theexperiment the input RF signal was set to �20 dB m andthe result is shown in Fig. 3 together with the simulationresponse. The lowest frequency of the used network ana-lyzer limits the frequency starting point to be 45 MHz.However, it can be extrapolated to zero frequency easilyaccording to the periodic property. As shown in Fig. 3the measured first notch is at 86.7 MHz and it is equal tothe filter’s FSR, which is determined by a length differenceof 2.30 m between the two SMF sections from the theoret-ical analysis. In this curve, we can see that deep notcheswith more than 35 dB rejection are achieved.

Furthermore, after putting a tunable fiber delay line(MDL-001, General Photonics) into the loop, the corre-sponding response of a FSR tunable filter is shown inFig. 4. The delay line has a tunable range of about 9 cmcorresponding to about 1.05 MHz FSR difference shownin the experiment result which is also perfectly matchedwith our theoretical computation.

As a symmetric 3 � 3 coupler is used in our scheme, theSagnac interferometer can automatically operate at quad-rature status without any other nonreciprocal componentand the broad-band source guarantees a stable response.But when a 2 � 2 coupler is used, a nonreciprocal phasebias component should be inserted into the loop to intro-duce p/2 phase shift between CW and CCW light.

We have measured the EDFA’s relative intensity noisewe used in experiment, getting a result about �120 dB/Hz. Although it is higher than the conventional narrow-band lasers such as DFB laser we still can get high enoughSNR in our structure. In fact, the broadband source hasbeen widely used in various microwave photonic filters,

Fig. 3. Measured and simulated response of the 3 � 3 coupler basedSagnac loop negative tap photonic notch filter.

Fig. 4. Frequency responses of the tunable photonic notch filter.

Y. Gao et al. / Optics Communications 281 (2008) 1476–1479 1479

such as [10,12]. Therefore we believe that the broadbandsource’s relatively higher RIN is not a limiting factor.

Due to the lack of photo detectors with high frequencyresponse, we currently just proved our scheme of photonicnotch filter at relative lower frequency. For higher fre-quency applications, e.g. in the range of GHz, the switch-ing voltage difference between CW and CCW lightcannot be neglected. Both the response of the phase mod-ulator and the mismatch of the two output fibers shouldbe reconsidered.

The two traces are measured at different states of thetunable optical delay line, and the delay is 0 and 9 cm,respectively. The inset shows the frequency shift of the

two traces, and it is clearly four times of the difference ofthe FSR.

5. Conclusions

In summary, we have demonstrated a novel structure toachieve a tunable negative tap photonic notch filter for pho-tonic signal processing theoretically and experimentally. A3 � 3 coupler based Sagnac loop interferometer operatingat quadrature is used with an asymmetrically placed phasemodulator. It is easily configured with good stability.Experimental results proved that our structure is a robustnotch filter with more than 35 dB rejection deep notch.

References

[1] A.J. Seeds, IEEE Trans. Microwave Theory Tech. 50 (2002) 877.[2] J. Capmany, B. Ortega, D. Pastor, J. Lightwave Technol. 24 (2006) 201.[3] R.A. Minasian, IEEE Trans. Microwave Theory Tech. 54 (2006) 832.[4] S. Sales, J. Capmany, J. Marti, D. Pastor, Electron. Lett. 31 (1995)

1095.[5] F. Coppinger, S. Yegnanarayanan, P.D. Trinh, B. Jalali, Electron.

Lett. 33 (1997) 973.[6] G. Ning, S. Aditya, P. Shum, L.H. Cheng, Y.D. Gong, Chao Lu,

IEEE Photonics Technol. Lett. 17 (2005) 1935.[7] B. Vidal, J.L. Corral, J. Marti, IEEE Photonics Technol. Lett. 17

(2005) 666.[8] E.H.W. Chan, R.A. Minasian, IEEE Photonics Technol. Lett. 17

(2005) 1740.[9] J. Pietzsch, J. Lightwave Technol. 7 (1989) 303.

[10] J. Capmany, J. Mora, B. Ortega, D. Pastor, Opt. Lett. 30 (2005) 2299.[11] W. Burns, C.L. Chen, R. Moeller, J. Lightwave Technol. 1 (1983) 98.[12] Daniel Pastor, Jose Capmany, Beatriz Ortega, IEEE Photonics

Technol. Lett. 13 (2001) 726.