Zr0.8Sn0.2TiO4 ceramic substrate. The bandwidths of each pass-band quite agree with the desirable values for narrow band filter.In addition, for Zr0.8Sn0.2TiO4 ceramic substrate, the insertionlosses of dual-band filter are about 2.01 dB at the frequency of 2.35GHz and 2.9 dB at the frequency of 4.7 GHz. On the other hand,the return losses of designed filter are 27.8 dB (2.35 GHz) and 14dB (4.7 GHz). It was found that the dual-band properties exist forthe edge coupled filter.

The substrate composition of (Zr0.8Sn0.2)TiO4 was synthesizedvia a conventional solid-state reaction. On the other hand, the filteris fabricated using a silver microstip on dielectric substrate print-ing method. The measurements are performed with an HP8757Dnetwork analyzer. The measured performance is shown in Figure5. For Zr0.8Sn0.2TiO4 substrate, the measurements present dual-bandpass center frequencies of 2.475 GHz and 3 dB bandwidth of9.5%, and 4.875 GHz and 3 dB bandwidth of 6%, respectively,compared to simulated values. The measured maximum dual-bandinsertion losses are about 2.15 dB for the frequency of 2.475 GHzand 3 dB for the frequency of 4.875 GHz, and the measuredminimum return losses are about 17.77 dB at the frequency of2.475 GHz and 16.01 dB at the frequency of 4.875 GHz comparedto simulated values. A comparison between the measurement andsimulation is presented, so that the measured response is stronglyinfluenced by surface roughness of ceramic substrate, conductorthickness, and electric contact.

4. CONCLUSION

Miniaturized rectangular ring bandpass filter on the high permit-tivity ceramic substrates were implemented. The validity of theproposed method was confirmed through the design, fabrication,and measurement of compact rectangular ring bandpass filter at2.35 and 4.7 GHz on high permittivity Zr0.8Sn0.2TiO4 (�r � 37)ceramic substrates. The insertion losses of the implemented filterwith using Zr0.8Sn0.2TiO4 ceramic substrate are about 2.15 dB(2.475 GHz) and 3 dB (4.875 GHz), and the return losses of thedual-band filter are return losses are about 17.77 dB (2.475 GHz)and 16.01 dB (4.875 GHz). The 3dB passband widths of thenarrow band filter are about 9.5% for the frequency of 2.475 GHzand 6% for the frequency of 4.875 GHz. The pattern area is about17.35 � 7.3 mm2. The compactness in the circuit size make thedesign of rectangular ring filter using the microstrip miniaturizedhigh permittivity ceramic substrates attractive for further develop-ments and applications in the modern communication systems.

REFERENCES

1. C.C. You, C.L. Huang, and C.C. Wei, Single-block ceramic microwavebandpass filters, Microwave J 37 (1994), 24–35.

2. S.B. Cohn, Parallel-coupled transmission line resonator filters, IRETrans Microwave Theory Tech 6 (1958), 223–237.

3. S. Caspi and J. Adelman, Design of combline and interdigital filter withtapped-line input, IEEE Trans Microwave Theory Tech 36 (1988),759–763.

4. G.L. Matthaei, Comb-line bandpass filter of narrow or moderate band-width, IEEE Trans Microwave Theory Tech 6 (1963) 82–91.

5. U.H. Gysel, New theory and design for hairpin-line filters, IEEE TransMicrowave Theory Tech 22 (1974), 523–531.

6. C. Peixeiro, Microstrip rectangular ring bandpass filter elements forGSM, Proc Asia Pacific Microwave Conf 3(2000), 1273–1276.

7. C.C. Yu and K. Chang, Novel compact elliptic-function narrow-bandbandpass filters using microstrip open-loop resonators with coupled andcrossing lines, IEEE Trans Microwave Theory Tech 46 (1998), 952–958.

8. C.L. Huang, C.C. Yu, C.C. Wei, and B.C. Shen, Microwave dielectricresonator using doped (Zn,Sn)TiO4 ceramics, J Wave-Mater Interaction10 (1995), 1–14.

© 2007 Wiley Periodicals, Inc.

A NOVEL SWITCHABLE LOWPASSFILTER CONSTRUCTED WITHELETROMAGNETIC BANDGAP ANDILLUMINATION-SENSITIVE CADMIUMSULFIDE

Jack Wu,1S. N. Qiu,2C. X. Qiu,2 and I. Shih1

1Department of Electrical and Computer Engineering, McGillUniversity, Montreal, QC, Canada H3A 2T52CIS Scientific, Montreal, QC, Canada J4Z 1W6

Received 12 February 2007

ABSTRACT: A novel construction of microwave switchable lowpassfilter based on electromagnetic bandgap (EBG) and cadmium sulfide(CdS) thin film is proposed. Linearly tapered slot-array EBG structureis well known for its exceptional stopband ability and compact physicalsize. When the illumination-sensitive CdS thin film is deposited on theperforations of the EBG structure, the stopband phenomenon induced bytapered slot-array can be controlled by optical means. Innovative “con-ductive islands” were introduced to reside in the center of each perfora-tion in order to enhance the optical switching. It has been demonstratedin both simulation and experimental measurement that the transmissioncoefficient is switched from 31.3 to 5.6 dB at 13 GHz when the gapwidth between the conductive island and the perforation is 30 �m. Asthe gap width decreases, the insertion loss can be reduced. Simulationresult indicates that the insertion loss can be improved to around 1 dBat 13 GHz when the gap width is decreased to 5 �m. © 2007 WileyPeriodicals, Inc. Microwave Opt Technol Lett 49: 2238–2242, 2007;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.22665

Key words: cadmium sulfide; electromagnetic bandgap; lowpass filter;microstrip transmission line; tapered array pattern

1. INTRODUCTION

The term photonic bandgap was initially acknowledged in late1980s where it is used to describe the blockage of travelingelectromagnetic (EM) waves in optical regime. Such blockage inthe transmission is caused by a periodically changing pattern of thedielectric layers with different index of refractions [1–3]. In the

Figure 5 Measured frequency responses for Zr0.8Sn0.2TiO4 ceramic sub-strate

2238 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 9, September 2007 DOI 10.1002/mop

microwave applications, the structures that exhibit this phenome-non are referred to as electromagnetic bandgap (EBG) structures.The most advantageous aspects of the microwave EBG structuresare their extremely simple implementation and effective stopbandability. Planar EBG structures can be implemented easily in theground plane of a microstrip transmission line with periodic per-forations. These perforations, with appropriate spacing, provideinterference for the traveling EM waves at a certain range offrequency [4–6]. It has been reported that that the ratios ofperforation dimensions to the structure period are closely linked tothe stopband frequency [7].

Ordinary periodic planar EBG structures have S11 ripples in thepassband region and therefore the transmission quality in thisregion is degraded [8, 9]. However, this drawback of the planarEBG structures can be solved with a special tapered array (TA)pattern [10]. TA patterns not only provide defined passband andstopband regions, but also the position of cutoff frequency can becontrolled very easily. The physical length of TA patterned EBGstructure does not increase with a decrease of cutoff frequency likethe typical EBG structures. Furthermore, the stopband region ofthe TA patterns is much wider than regular periodic patterns.Figure 1 shows a typical EBG structure with linearly TA patterninside of a ground plane of a microstrip transmission line. Thesecharacteristics are ideal for the construction of a switchable low-pass filter utilizing the illumination-sensitive cadmium sulfide(CdS) thin film. It has been reported that CdS thin film depositionby the chemical bath deposition (CBD) method yields the bestresults in terms of photoconductivity and photosensitivity [11, 12].

The CBD method is also a low-cost and a simple way to depositCdS over a large area.

In this work, CdS thin film and EBG structure were utilizedtogether to realize the optical switchable microwave filter. Highlysensitive CdS thin films were deposited on the ground plane of theEBG structures to cover up the perforations. Meanwhile, a novelfeature of the perforation pattern called “conductive island” isintroduced to enhance the switching performance of the EBGstructures. The additional conductive islands in the center of theperforations were found to improve the insertion loss significantlywhen the structure was under illuminated condition. At the sametime, the added feature does not affect the EBG structure’s stop-band ability when CdS thin film was under dark condition. Thedesign of the TA patterned EBG structure is presented first. Then,the construction of the optically switchable structure is discussed.At the end, both simulations and experimentally measurements aregiven to demonstrate and validate the proposed switchable filter.

2. DESIGN OF TA EBG STRUCTURES

TA pattern consists of a periodic array of slots, whose lengths arelinearly tapered toward the ends. This critical characteristic allowsthe traveling EM waves to be exposed to periodic interferencesprogressively and therefore it is possible to obtain much definedregions of passband and stopband in a frequency response. Thisindicates that the transmission and reflection performances of thestructure are very good. The parameters for the preliminary designare shown in Figure 2.

To understand the TA patterned EBG structure better, the slotscan be modeled with inductor–capacitor (LC) resonator equivalent

Figure 2 A 7-slot TA pattern showing preliminary design dimensions

Figure 3 Equationuivalent circuit representation for a 7-slot TA pat-terned EBG structure. (L4 � 1.10 nH, C4 � 0.20 pF, L3 � 0.80 nH, C3 �0.14 pF, L2 � 0.50 nH, C2 � 0.08 pF, L1 � 0.20 nH, and C1 � 0.02 pF)

Figure 4 Comparison of S21/S11 between equivalent circuit model (EC)and measurement (M) results for a 7-slot TA pattern EBG structure withL � 7.7 mm

Figure 1 An EBG structure constructed with 7-slot linearly TA patternimplemented in the ground plane

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 9, September 2007 2239

circuits. Figure 3 illustrates a circuit representation of a 7-slot TApatterned EBG structure with microstrip transmission lines inbetween. The grey components represent the microstrip transmis-sion line in the TA patterned EBG structure, which can be char-acterized by series inductors and shunt capacitor. The slots aremodeled with LC network where L4C4 represents the center slot.

Figure 4 shows the comparison of S-parameters between theequivalent circuit model and the actual measurement of the sam-ple. It is observed that the S-parameters match very well. There-fore, the equivalent circuit model serves as a good preliminarydesign tool and representation for the actual TA patterned EBGstructure.

3. SIMULATIONS OF SWITCHABLE STRUCTURES

When CdS is deposited entirely on the ground plane of an EBGstructure, the periodic perforations are covered up. Thus, the CdSthin film’s conductivity becomes a critical factor in the operationof the microwave structure. When the thin film is in the resistivestate (Dark condition), the traveling wave experiences a discon-tinued ground plane and a filter response is produced accordingly.When the thin film is in the conductive state (illuminated condi-tion), the traveling wave experiences a continuous ground planeand therefore the structure behaves like an ordinary microstrip line.Effectively, the responses of the constructed microwave filter canbe turned on and off by the illumination. Figure 5 shows an EBGstructure with the CdS thin film deposited on the ground plane tocover up the perforations.

To enhance the switching performance, a conductive island isinserted in the center of each EBG perforations. Thus, the EBGpattern is now defined by boundaries of gaps instead of perfora-tions (Fig. 6). The conductive islands are the same as the rest ofground plane, which are gold conductors.

The proposed structure reduces the area that is covered by CdSwhile maintains the same periodically induced stopband effect. Byadding the conductive islands, the overall conductivity in theperforation regions is increased under the illuminated state becausethe gold metallization has a much higher conductivity than the CdSthin film.

Agilent Advanced Design System’s full-wave Momentum isused to simulate the proposed structures, which have the insertedconductive islands. The gap width was varied from 5 to 100 �m todemonstrate the significance of the conductive islands. The simu-lation parameters are presented in Table 1.

The proposed structures were simulated with CdS thin film,which is assumed to exhibit both high and low sheet resistances topredict the optical switching performance. In the following simu-lations, the sheet resistances for the illuminated and dark stateswere assumed to be 300 �/square and 109 �/square respectively.Figure 7 shows the S-parameters for three cases under dark state:g � 5 �m, g � 100 �m, and the structure without the conductiveislands.

It can be seen from Figure 7 that the S-parameters for the threecases are almost identical. The 3 dB cutoff frequencies of the S21

are all located at around 9 GHz. When the CdS thin film operatesunder dark state, the sheet resistance is high and therefore theground plane that contains the slot-array patterns is not continuous.

Figure 5 EBG with linearly tapered slot-array pattern and the CdS thinfilm deposited on the ground plane to facilitate optical switching

Figure 6 A ground plane with (a) regular tapered slot-array perforationsand (b) conductive islands located in the center of each slot

TABLE 1 Simulation Parameters for the Structures withConductive Islands

Structural DimensionsSubstrate type Alumina (Al2O3)Dielectric constant (�r) 9.8Substrate thickness (h) 250 �mMicrostrip line width (w) 250 �m

EBG Pattern DimensionsNumber of slots (n) 7Center slot length (L) 7.7 mmStructure period (a) 2.0 mmSlot length step (x) 1.2 mmSlot width (t) 0.5 mmMinimum slot length (v) 0.5 mmGap width (g) 5–100 �m

Figure 7 S-parameters under dark state for g � 5 �m, g � 100 �m, andthe structure without the conductive islands (NO CI)

2240 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 9, September 2007 DOI 10.1002/mop

Thus, the structures maintained their stopband properties and theresults shown in Figure 7 are expected. When the CdS thin film isexposed to illumination, the sheet resistance reduces significantlyand renders the film to be conductive. Thus, the ground planebecomes continuous and the stopband effect of the EBG structureis eliminated. Figure 8 shows the simulated S21 of the structureswith the gap widths (g) vary from 5 to 100 �m under illuminatedstate.

The importance of the conductive islands and the gap width (g)can be observed in Figure 8. As g is decreased, the insertion loss(S21) reduces significantly. At 13 GHz, the insertion loss reducedfrom 16 to 1 dB when compared between the case of g � 5 �m andthe case of no conductive islands. Therefore, by adding the con-ductive islands into the EBG perforations, it facilitates the possi-bility of making an optically switchable device. Figure 9 shows acomparison of S21 between illuminated and dark states for the caseof g � 5 �m. The dark to illumination insertion loss ratio at 13GHz is about 40 (40–1 dB).

4. EXPERIMENT RESULTS AND COMPARISONS

Actual samples of tapered slot-array EBG structures with andwithout the conductive islands were fabricated according to thedimensions shown in Table 1. The gap widths (g) in the groundplane of the EBG structure were constructed with standard photo-lithography steps. The case of g � 30 �m was used to demonstratethe effect of the conductive islands. After the EBG structures wereconstructed, the CdS thin film was then deposited on the groundplane to cover up the perforation patterns. The deposition of the

CdS layer is presented in Table 2, which is repeated six times todecrease the illuminated sheet resistance.

The vector network analyzer was used to measure the S-parameters of the samples in both illuminated and dark conditions.A green-light (532 nm) laser pointer (50 mW/cm2) was used as theincident illumination to switch the proposed structure. Figure 10shows photos of the fabricated samples for measurements. Theresults are compared to simulated ones and they are presented inthe following section.

Figure 11 shows the comparisons of simulated and measuredS21 results for the structures that do not contains the conductiveislands. It is seen that the results for the dark state match very well.For the illuminated state, the measured S21 has a slightly lowerinsertion loss. The reason for this occurrence might be because thesheet resistance of the actual deposited CdS thin film under illu-minated condition was lower than 300 �/square, which is the sheetresistance value used in the simulations. However, the insertionloss for the illuminated state is still large when the conductive

Figure 8 Simulated S21 with g varies from 5 to 100 �m under illumi-nated condition. (No IC � No conductive islands present)

Figure 9 Comparison of S21 between illuminated and dark states (thecase of g � 5 �m)

TABLE 2 Preparation of CBD Solution and Procedure ofDeposition

Steps Actions

1 Add 11.2 g of 0.5 M cadmium acetate solution into the beaker2 Add 2.8 g of 99% TEA and stir thoroughly3 Add 5 g of 30% ammonia solution and stir thoroughly4 Add 2.8 g of 0.5 M thiourea solution and stir thoroughly5 Add 20 g of DI water and stir thoroughly for 1 min6 Place the beaker in water bath (95°C) and set the stir rate (600

rpm), wait for 15 s7 Immerse the glass substrate into the solution for 30 min8 Remove the substrate and rinse with DI water for 1 min9 Place the substrate with DI water in ultrasonic bath for 2 min

10 Remove the substrate and rinse with DI water for 5 min11 Repeat from step 1 if multiple numbers of depositions is

required

Figure 10 Photos of fabricated samples (a) without and (b) with theconductive islands (CI). CdS thin films were deposited on the ground planeof the samples to cover up the perforations

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 9, September 2007 2241

islands are not presented. The dark to illumination insertion lossratio for the measured results at 13 GHz is 1.9 (24.1–12.7 dB). Theratio is equal to 2.4 for the simulated results.

Figure 12 illustrates the comparisons of simulated and mea-sured S21 for the structures that contain the conductive islands. Thegap width (g) is equal to 30 �m in this case. The insertion loss forthe illuminated case improved significantly when it is compared tothe structure without the conductive islands. The dark to illumi-nation insertion loss ratio for the measured results at 13 GHz is 5.6(31.3–5.6 dB). The ratio is equal to 8.6 for the simulated results.Therefore, by inserting the conductive islands, the effective ratioincreased almost three times. Moreover, from the simulation anal-ysis, this performance can be improved even further by reducing g.

The switching of EBG structures by means of illumination hasbeen demonstrated successfully. Also, the switching performancecan be enhanced significantly with the addition of conductiveislands. Therefore, a novel EBG structure that is suitable foroptical switching has been constructed and achieved.

5. CONCLUSION

In this work, the possibility of building a switchable structure wasexplored and studied. The illumination-sensitive CdS thin film wasutilized as the switching medium. Therefore, it is crucial to find theappropriate microwave structure to complete this task. It has beenshown that the EBG structures can be enhanced and modified byadding the conductive islands in the perforations. The greatestadvantage of this feature is to make the EBG structures suitable forswitching and thus the insertion loss is improved significantly. At

the same time, the conductive islands do not affect EBG structures’ability to create stopband regions. The switching ratio of theenhanced EBG structure is almost three times higher than theordinary EBG structure. Therefore, the conductive islands are anintegral part of an optically switchable microwave structure.

REFERENCES

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7. J. Wu, I. Shih, S.N. Qiu, C.X. Qiu, P. Maltais, and D. Gratton,Photonic bandgap microstrips with controlled bandstops, 2ndCanSmart Workshop: Smart Materials and Structures, 2002, pp. 171–179.

8. M. Bozzetti, A. D’Orazio, M. De Sario, V. Petruzzelli, F. Prudenzano,and F. Renna, Tapered photonic bandgap microstrip lowpass filters:Design and realization, IEE Proc Microwave Antennas Propag 150(2003), 459–462.

9. T.Y. Yun and K. Chang, Uniplanar one-dimensional photonic-bandgap structures and resonators, IEEE Trans Microwave TheoryTech 49 (2001), 549–553.

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© 2007 Wiley Periodicals, Inc.

MULTIWAVELENGTH RAMAN FIBERLASER INSERTING A SAMPLEDCHIRPED FIBER BRAGG GRATING

G. Ning, P. Shum, J. Q. Zhou, and L. XiaNetwork Technology Research Center, Nanyang TechnologicalUniversity, 50 Nanyang Drive, Singapore 637553

Received 21 February 2007

ABSTRACT: A simple multiwavelength Raman fiber ring laser withequalized peak power is proposed and demonstrated experimentally byinserting a sample chirped fiber Bragg grating. By adjusting the polar-ization controller (PC) in the ring cavity, up to ten stable wavelengthswith equalized peak power are achieved at room temperature. The Ra-man fiber ring laser has many advantages such as simple structure, lowloss, and multiwavelength lines with moderate output power. © 2007Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2242–2245,

Figure 11 Comparisons of simulated (S) and measured (M) S21 for thecase where no conductive islands were presented in the structures

Figure 12 Comparisons of simulated (S) and measured (M) S21 for g �30 �m

2242 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 9, September 2007 DOI 10.1002/mop