polarization-maintaining erbium-ytterbium co-doped fiber insidethe cavity

4. CONCLUSIONS

We demonstrated a simple technique to produce a medium powerspacing tunable multiwavelength fiber laser operating at roomtemperature using an unpumped erbium-doped fiber as a saturableabsorber to stabilize the lasing wavelengths and a Lyot-Sagnacloop filter was used as a tunable filter to produce different sepa-ration between lasing wavelengths. The laser could produce fivelasing wavelengths at maximum output power where the totaloutput power of the laser was 150 mW.

ACKNOWLEDGMENT

This research is supported financially by the Natural Sciences andEngineering Research Council of Canada.

REFERENCES

1. A.J. Poustie, N. Finlayson, and P. Harper, Multiwavelength fiber laserusing a spatial mode beating filter, Opt Lett 19 (1994), 716–718.

2. G. Das and J.W.Y. Lit, L-band multiwavelength fiber laser using anelliptical fiber, IEEE Photon Technol Lett 14 (2002), 606–608.

3. Z.G. Lu, F.G. Sun, G.Z. Xiao, P. Lin, and P. Zhao, High-power

multiwavelength Er3�-Yb3� codoped double-cladding fiber ring laser,IEEE Photon Technol Lett 17 (2005), 1821–1823.

4. G. Brochu, S. LaRochelle, and R. Slavik, Modeling and experimentaldemonstration of ultracompact multiwavelength distributed Fabry-Perotfiber lasers, J Lightwave Technol 23 (2005), 44–53.

5. C.S. Kim, Y.G. Han, R.M. Sova, U.C. Paek, Y.J. Chung, and J.U. Kang,Optical fiber modal birefringence measurement based on Lyot-Sagnacinterferometer, IEEE Photon Technol Lett 15 (2003), 269–271.

6. Y. Cheng, J.T. Kringlebotn, W.H. Loh, R.I. Laming, and D.N. Payne,Stable single-frequency traveling-wave fiber loop laser with integralsaturable-absorber-based tracking narrow-band-filter, Opt Lett 20(1995), 875–877.

7. C.S. Kim, R.M. Sova, and J.U. Kang, Tunable multi-wavelength all-fiber Raman source using fiber Sagnac loop filter, Opt Commun 218(2003), 291–295.

8. S. J. Frisken, Transient bragg reflection gratings in erbium-doped fiberamplifiers, Opt Lett 17 (1992), 1776–1778.

9. Y. Cheng, J.T. Kringlebotn, W.H. Loh, R.I. Laming, and D.N. Payne,Stable single-frequency traveling-wave fiber loop laser with integralsaturable-absorber-based tracking narrow-band-filter, Opt Lett 20(1995), 875–877.

© 2009 Wiley Periodicals, Inc.

DUAL-BAND-REJECTION FILTERBASED ON SPLIT RING RESONATOR(SRR) AND COMPLIMENTARY SRR

Xin Hu,1,2 Qiaoli Zhang,2 and Sailing He1,2

1 Division of Electromagnetic Theory, School of Electrical Engineering,Royal Institute of Technology, Stockholm S-100 44, Sweden2 Centre for Optical and Electromagnetic Research, State KeyLaboratory of Modern Optical Instrumentation, Zhejiang University,Hangzhou 310027, China; Corresponding author: sailing@kth.se

Received 7 January 2009

ABSTRACT: A novel concept of a compact, low insertion-loss dual-band-rejection filter (DBRF) is proposed, and its equivalent circuitmodel is given. The filter consists of single split ring resonators (SRRs)on the top of the host microstrip line and Complimentary SRR etched onthe back ground plane. The dimensions of the structure are as small as1.4 cm � 2 cm, while high frequency selectivity is achieved at bothband edges due to the presence of two transmission zeros. The filter hasan insertion loss of better than 1 dB, a return loss of larger than 10 dBin the passband from 3.3 to 4.0 GHz, and two rejections of greater than30 dB within 2.5–2.6 and 5.2–5.6 GHz. © 2009 Wiley Periodicals, Inc.Microwave Opt Technol Lett 51: 2519–2522, 2009; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24596

Keywords: SRR; complimentary SRR; dual-band-rejection; filter

1. INTRODUCTION

MICROWAVE bandstop filters are widely used for distortionreduction in transmitters due to their effective suppression ofspurious signals in wireless communication applications. Dual-band-rejection filters (DBRFs) [1] are to choose two rejectionbands and keep a passband between them, and can be applied totreat the unwanted double-sideband spectrum of high power am-plifiers and mixers using one single filter to decrease the size andcost of the circuit. DBRFs are attractive also due to their lowertransmission loss than band-pass filters in the passband as theresonators in the DBRFs resonate not in the passband, but in therejection band. Furthermore, DBRF has a smaller physical sizethan a simple cascade of two conventional band-rejection filters

Figure 4 Output of the laser with a saturable absorber of length 0.55 m,(a) �� � 1.1 nm , PPump 1873 mW, Pout 129 mW, (b) �� � 3.4nm , PPump

1873 mW, Pout 128 mW

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 10, October 2009 2519

with different rejection bands. Effective approaches to DBRFs’design have been proposed in [2–4], such as applying compositeright/left-handed metamaterial transmission lines [2], different-length open stubs [3], stepped-impedance resonators [4], and soon.

This article presents a new approach to design DBRFs, wheresingle split ring resonators (SRRs) and complementary split ringresonator (CSRR) are utilized. SRRs [5–7] have attracted consid-erable attention due to their ability of allowing for artificial mag-netism at elevated frequencies. By virtue of the distributed capac-itance between concentric rings and overall rings inductance, SRRbehaves as an LC resonant tank that can be excited by an externalmagnetic flux [7]. Thereafter, the CSRR [8], which is the negativeimage of an SRR, has been proposed as another key particle formetamaterial design. Since SRRs and CSRRs are both planarconfigurations, SRRs and CSRRs (properly combined with shuntmetallic wires or series gaps) have been successfully applied to thedesign of novel planar microwave filters [9-11].

In this article, the proposed structure is presented and its circuitsynthesis procedure is described in Section II. Secondly, compar-isons between the results derived from the full wave simulationsand experimental result are given and verify the dual-band-rejec-tion with low-loss characteristics of the designed DBRF.

2. DESIGN OF DBRF

2.1. TheoryThe proposed DBRF is composed of two unit cells of the structuredepicted in Figure 1. We can see that CSRR is etched in the groundplane, underneath the conductor strip, while two single rectangularSRRs lay beside the upper conductor strip. The new DBRF isfabricated on Rogers RO3010 substrate with a relative dielectricconstant of 10.2 and a thickness of 50 mil. The upper conductorstrip has a width of 1.2 mm, which is corresponding to a charac-teristic impedance of 50 �. The dimensions are as follows: L � 5mm, W � 1.5 mm, t � c � g � 0.1 mm, d � s � 0.3 mm, andrext � 3 mm. The periodic length P is 7 mm. The size of DBRF isas small as 1.4 cm �2 cm.

Let us now focus on the propagation characteristics of theproposed structure through the analysis of the dispersion relation.This is inferred from the equivalent circuit of the unit cell (shownin Fig. 2), which is valid under the long-wavelength assumption,that is the electrical length of the unit cell is much smaller than thewavelength of the guided wave. L and C are the per-unit induc-tance and capacitance of the host microstrip line. The single SRRson the top and CSRR on the ground plane are modeled as parallelresonant circuits, with inductances LSRR, LCSRR and capacitancesCSRR, CCSRR, respectively. The single SRRs on the top are induc-tively coupled to the line through a mutual inductance, LM, whilethe CSRR on the ground plane is capacitively coupled to the linethrough a mutual capacitance, CM. By using the equivalent im-pedance of the series mutual inductance and the parallel mutualcapacitance [8], the circuit can be simplified to Figure 3(b), where

L�SRR � �2LM2 CSRR, C�SRR �

LSRR

�2LM2 (1)

L�CSRR �CCSRR

�2CM2 ; C�CSRR � LCSRR�2CM

2 (2)

The series impedance and shunt admittance are

ZSE � j�L �j�L�SRR

1 � �2/�SRR2 , YSH � j�C �

j�C�CSRR

1 � �2/�CSRR2 (3)

where �SRR and �CSRR are the resonate frequencies of the SRRsand CSRR.

The dispersion relation can be obtained:

cos��l � � 1 � ZSEYSH

� 1 � �� j�L �j�L�SRR

1 � �2/�SRR2 � � � j�C �

j�C�CSRR

1 � �2/�CSRR2 ��

Figure 1 Unit cell of microstrip line loaded with CSRRs (etched in theground plane) and SRRs (laid on the top) (a) top view, (b) bottom view.Gray area represents the metallization

2520 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 10, October 2009 DOI 10.1002/mop

� 1 � �2�LC �L�SRRC

1 � �2/�SRR2 �

LC�CSRR

1 � �2/�CSRR2

�L�SRRC�CSRR

�1 � �2/�SRR2 ��1 � �2/�CSRR

2 �� (4)

From the dispersion relations, we can see the structure shows dualband-stop property at the vicinity of �SRR and �CSRR.

2.2. ResultTo validate the design concept, the DBRF with two unit cells of thestructure shown in Figure 1 is fabricated. The simulation is com-pleted using IE3D, and the measurement is accomplished using anHP8722 network analyzer. Figure 3 plots the simulated and mea-sured responses, indicating that both responses have good agree-ment.

The measured result shows that there are two transmissionzeros on both sides of the passband. They are 36 dB and 40 dBat frequencies of 2.55 GHz and 5.05 GHz, respectively. Within2.5–2.6 and 5–5.5 GHz the rejection is greater than 30 dB. Thepassband insertion losses are less than 1 dB within 3.3–4.0 GHz.The circuit area in the fabricated DBRF was 14 � 20 mm2.

3. CONCLUSION

This study presents a new type of compact microstrip DBRF.Dual-band performance and compact size are attained simulta-neously by using CSRR and single SRRs. The equivalent circuitmodels are developed, and the performance of the filter is verifiedby simulated and measured results. The two transmission zeros arelocated at 2.55 and 5.05 GHz with levels of 36 dB and 40 dB,

respectively. The new filter requires only 14 � 20 mm2 space.Further size reduction can be achieved by applying multiple SRR[12], which can achieve miniaturized linear dimensions of theorder of �0/30-�0/40. The compact-size, and low-cost filter can beused for reducing the interference in full duplex systems in satellitecommunications.

ACKNOWLEDGMENTS

This work was supported by the Swedish Research Council (VR)under Project No. 2006-4048 and the National Basic ResearchProgram (973) of China (NO.2004CB719802).

REFERENCES

1. R.J. Cameron, M. Yu, and Y. Wang, Direct-coupled microwave filterswith single and dual stopbands, IEEE Trans Microw Theory Tech 53(2005), 3288–3297.

2. C.-H. Tseng and T. Itoh, Dual-band bandpass and bandstop filtersusing composite right/left-handed metamaterial transmission lines,IEEE MTT-S Int Dig, San Francisco, CA (2006), 931–934.

3. Z. Ma, K. Kikuchi, Y. Kobayashi, T. Anada, and G. Hagiwara, Novelmicrostrip dual-band bandstop filter with controllable dual-stopbandresponse, Proc Asia-Pacific Microw Conf, Bangkok, Thailand (2007),1177–1180.

4. K.-S. Chin, J.-H. Yeh, and S.-H. Chao, Compact dual-band bandstopfilters using stepped-impedance resonators, IEEE Microwave WirelessComp Lett 17 (2007), 849–851.

5. J.B. Pendry, A.J. Holden, D.J. Robbins, and W.J. Stewart, Magnetismfrom conductors and enhanced nonlinear phenomena, IEEE TransMicrowave Theory Tech 47 (1999), 2075–2084.

6. D.R. Smith, W.J. Padilla, D.C. Vier, S.C. Nemat-Nasser, and S.Schultz, Composite medium with simultaneously negative permeabil-ity and permittivity, Phys Rev Lett 84 (2000), 4184–4187.

7. S. Linden, C. Enkrich, M. Wegener, J.F. Zhou, T. Koschny, and C.M.Soukoulis, Magnetic response of metamaterials at 100 terahertz, Sci-ence 306 (2004), 1351.

8. J.D. Baena, J. Bonache, F. Martín, R. Marques, F. Falcone, T. Lope-tegi, M.A.G. Laso, J. García-García, I. Gil, M. Flores, and M. Sorolla,Equivalent circuit models for split ring resonators and complementarysplit ring resonators coupled to planar transmision lines, IEEE TransMicrow Theory Tech 53 (2005), 1451–1461

9. J. Bonache and I. Gil, Complementary split ring resonators for mi-crostrip diplexer design, Electron Lett 41 (2005), 810–811

10. J. Bonache and I. Gil, Novel microstrip bandpass filters based oncomplementary split-ring resonators, IEEE Trans Microw TheoryTech 54 (2006), 265–271.

Figure 2 (a) The equivalent circuit of the unit cell of the designedstructure and (b) its simplification

1 2 3 4 5 6-50

-40

-30

-20

-10

0

Frequency(GHz)

dB

SimulatedExperimental

S11 S21

Figure 3 Simulated and measured responses of the fabricated DBRF

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 10, October 2009 2521

11. F. Martin, F. Falcone, J. Bonache, R. Marques, and M. Sorolla Min-iaturized coplanar waveguide stop band filters based on multiple tunedsplit ring resonators, IEEE Microwave Wireless Comp Lett 13 (2003),511–513.

12. F. Bilotti, A. Toscano, and L. Vegni, Design of spiral and multiplesplit-ring resonators for the realization of miniaturized metamaterialsamples, IEEE Trans Antennas Propag 55 (2007), 2258–2267.

© 2009 Wiley Periodicals, Inc.

ALL OPTICAL MULTI-TAP MICROWAVEFILTER WITH HIGH SIDELOBESUPPRESSION USING PEAK PROFILEOF ASE AND ONE MULTIWAVELENGTHFBG

Li Xia, S. Aditya, P. Shum, and Junqiang ZhouNetwork Technology Research Centre, Nanyang TechnologicalUniversity, Singapore 639798, Singapore; Corresponding author:xiali@ntu.edu.sg

Received 28 January 2009

ABSTRACT: A new all optical 10-tap microwave filter is proposedusing a broadband amplified spontaneous emission (ASE) light sourceand a single specially designed multiwavelength FBG. Measured resultsshow that the sidelobe suppression is larger than 20 dB. The filter re-sponse is insensitive to the polarization state of light. This techniqueenables one to obtain a simple and low cost photonic microwave filterwith a stable response. © 2009 Wiley Periodicals, Inc. Microwave OptTechnol Lett 51: 2522–2524, 2009; Published online in Wiley Inter-Science (www.interscience.wiley.com). DOI 10.1002/mop.24666

Key words: multi-tap microwave filter; amplified spontaneous emission(ASE); multiwavelength fiber Bragg grating (FBG)

1. INTRODUCTION

Nowadays, all optical microwave filters are attractive in variousareas, due to their large time-bandwidth products, insensitivity toelectromagnetic interference, ability to directly process microwavesignals in the optical domain, etc [1]. Among these, the multi-tapmicrowave filters offer the advantage of high selectivity and thushigh signal to noise suppression ratio. Most previous work onmulti-tap microwave filters incorporated an array of opticalsources with dispersive media [2]. Although these structures havethe advantages of independently tuning the optical wavelength andoutput power, the complexity and cost of the whole scheme in-creases significantly when a high tap number is targeted. Spectrumslicing of broadband sources using a number of fiber Bragg grat-ings (FBGs) have been reported, e.g., [3], but the usage of severaluniform gratings has limitation in terms of achieving a largenumber of taps and adjustment of tap-weights. A multi-tap filterdesign based on a single optical source and a single superstruc-tured FBG was reported in [4], but the wavelength spacing in thiskind of FBG is hard to tune; this affects the tunability of themicrowave response.

In this article, we demonstrate a new all optical multi-tapmicrowave filter design using a broadband ASE light source, asingle specially designed multiwavelength FBG, and a length ofsingle mode fiber (SMF). This technique exploits the Gaussian-likeof peak profile of ASE in order to reduce the sidelobe level in thefilter response. The FBG is designed for equal reflectivity ofseveral wavelengths; details of design and fabrication of FBG areprovided in Section 2. This leads to a very simple multi-tap filter

architecture and offers the prospect of tuning the free spectralrange (FSR) through mechanical tuning of the FBG [5]. Measuredradio frequency (RF) response is presented to demonstrate a 10-tapfilter. The measured response matches well with calculated results.A higher tap-count can be achieved by designing the FBG fornarrower wavelength spacing.

2. EXPERIMENTAL SETUP

The all optical multi-tap filter setup is shown in Figure 1. First,ASE light comes from a broadband light source. One speciallydesigned FBG is used to spectrum-slice the Gaussian-like profileof the first peak of ASE spectrum around 1531 nm, shown inFigure 2(a), to achieve a multiwavelength spectrum with suitableweight distribution. The semiconductor optical amplifier (SOA)guarantees enough light power into the intensity modulator, andalso keeps the profile of the multiwavelength spectrum unchanged,as shown in Figure 2(b). The polarization controller can change thepolarization state of input light in order to test the polarizationsensitivity of the whole microwave filter structure. An RF signalfrom a network analyzer drives a Mach-Zehnder (MZ) intensitymodulator. The modulated light passes through a certain length ofsingle mode fiber (SMF) to utilize its dispersion to obtain differenttime delays for different optical wavelengths. The final output lightis fed to an optical detector, which is followed by a networkanalyzer.

Multiwavelength FBGs have attracted considerable interest foruse in wavelength reference or comb filters in WDM systems [6,7]. To fabricate the multiwavelength FBG in a cost-effective andflexible way, here we use an approach presented in [8]. Theapproach is based on a strongly chirped phase mask with discretephase shifts. Advantages of this approach include: (1) Arbitrarychannel spacing can be achieved in principle; (2) the spatialinterval between the neighboring phase shifts is large and therequired phase shifts can be obtained by a precise translation stagesuch as a piezoelectric transducer (PZT); (3) the channel count canbe increased by simply increasing the grating length; (4) thereflectivity at each wavelength can be equal if the exposure inten-sity is kept constant during the writing process.

A strongly chirped phase mask with a central pitch of 1057.85nm and the chirp coefficient of 2.18 nm/cm is used in our FBGfabrication keeping in view the peak position of the ASE spectrum.The FBG is fabricated on UV-sensitive fiber using the phase maskscanning method. The phase profile distribution is designed toachieve a channel spacing of about 0.8 nm. The phase shift isrealized by detuning the relative position between phase mask andthe photosensitive fiber at the proper grating position using piezo-electric ceramic translation stage. The measured transmissionspectrum of the fabricated FBG is shown in Figure 3; the central

Figure 1 Schematic diagram of the all optical multi-tap microwave filter

2522 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 10, October 2009 DOI 10.1002/mop