bandwidths (|S11| < �10 dB) of the lower and upper bands are

35 and 150 MHz, respectively.

The simulated and measured normalized radiation patterns

of the antenna at 3.83 and 5.45 GHz are shown in Figure 3. It

can be seen that good radiation patterns are both excited at

the lower and upper bands, respectively. Figure 4 shows the

measured gain of the antenna in the broadside direction

against the frequency. The maximal gains within dual-band

are about 4.7 dB at 3.83 GHz and 6.2 dB at 5.45 GHz, which

are much higher than those of the antenna proposed in Refs.

10 and 11.

4. CONCLUSIONS

A novel design of dual band metamaterial antenna based on

CSRRs has been proposed. Because of the inclusion of CSRRs,

the conventional microstrip antenna previously resonating at one

frequency produces two working frequencies now. At each

working frequency, good matching and radiation characteristics

are both obtained. A good agreement between simulated and

measured results validates our design. The proposed CSRRs

antenna provides us a new way to design dual-band antennas.

Besides, thanks to the presence of the CSRRs, a size reduction

of microstrip antenna can be achieved.

ACKNOWLEDGMENTS

This work is supported partly by the Program for New Century

Excellent Talents in University of China, and partially supported

by the National Natural Science Foundation of China under Con-

tract No. 60601028, No. 60801040, National Key Laboratory

Foundation and the Fundamental Research Funds for the Central

Universities.

REFERENCES

1. V.G. Veselago, Electrodynamics of substrates with simultaneously

negative values of e and l, Sov Phys-Usp 10 (1968), 509–514.

2. J.B. Pendry, A.J. Holden, D.J. Robbins, and W.J. Stewart, Magne-

tism from conductors and enhanced nonlinear phenomena, IEEE

Trans MTT 47 (1999), 2075–2084.

3. D.R. Smith, W.J. Padilla, D.C. Wier, S.C. Nemat-Nasser, and S.

Schultz, Composite medium with simultaneously negative perme-

ability and permittivity, Phys Rev Lett 84 (2000), 4184–4187.

4. F. Martin, F. Falcone, J. Bonache, T. Lopetegi, R. Marques, and

M. Sorolla, Miniaturized coplanar waveguide stopband filters based

on multiple tuned split ring resonators, IEEE Microwave Wirel

Compon Lett 13 (2003) 511–513.

5. J.J. Ma, X.Y. Cao, and T. Liu, Design the size reduction patch

antenna based on complementary split ring resonators, In proceed-

ings of International Conference on Microwave and Millimeter

Wave Technology (ICMMT), pp. 401–402.

6. Y. Lee, S. Tse, Y. Hao, and C.G. Parini, A compact microstrip

antenna with improved bandwidth using complementary split-ring

resonant (CSRR) loading, IEEE Antennas Propagation Society

International Symposium, Honolulu, HI, pp. 5431–5434, 2007.

7. R.K. Baee, G. Dadashzadeh, and F.G. Kharakhili, Using of CSRR

and its equivalent circuit model in size reduction of microstrip

antenna, In proceeding of Asia-Pacific Microwave Conference

(APMC), pp. 1–4, 2007.

8. L. Ke, W. Guang-Ming, X. Tong, and X. He-Xiu, A novel circu-

larly polarilized antenna based on the single complementary split

ring resonator, In proceeding of Intenational Symposium on Signal

Systems and Electronics (ISSSE), vol. 2, pp. 1–4, 2010.

9. H. Zhang, Y.-Q. Li, X. Chen, Y.-Q. Fu, and N.-C. Yuan, Design

of circular polarization microstrip patch antennas with complemen-

tary split ring resonator, IET Microwave Antennas Propag 3

(2009), 1186–1190.

10. N. Ortiz, F. Falcone, and M. Sorolla, Dual band patch antenna

based on complementary rectangular split ring resonators, In pro-

ceeding of Asia-Pacific Microwave Conference (APMC), Singa-

pore, pp. 2762–2765, 2009.

11. H. Zhang, Y.-Q. Li, X. Chen, Y.-Q. Fu, and N.-C. Yuan, Design

of circular/dual-frequency linear polarization antennas based on the

anisotropic complementary split ring resonator, IEEE Trans Anten-

nas Propag 57 (2009), 3352–3355.

VC 2012 Wiley Periodicals, Inc.

MULTIMODE INTERFERENCE IN OUTERCLADDING LARGE-CORE, AIR-CLADPHOTONIC CRYSTAL FIBER

Luıs Coelho,1 Jens Kobelke,2 Kay Schuster,2

and Orlando Frazao1

1 INESC Porto, Rua do Campo Alegre 687, 4169-007 Porto,Portugal; Corresponding author: lcoelho@inescporto.pt2 Institute of Photonic Technology, Albert-Einstein-Strasse 9,07745 Jena, Germany

Received 29 June 2011

ABSTRACT: It is described a large-core air-clad photonic crystal

fiber-based sensing structure using the outer cladding as a lightguide, which is highly sensitive to refractive index. The sensing

head is based on multimodal interference, and relies on a singlemode/large-core air-clad photonic crystal fiber/single mode fiberconfiguration. Using this configuration and controlling the light to

travel in a segment of the outer cladding multimode fiber, it waspossible to implement a sensing head and the results were obtained

independently from variations of temperature, strain and refractiveindex. VC 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett

54:1009–1011, 2012; View this article online at

wileyonlinelibrary.com. DOI 10.1002/mop.26726

Key words: optical fiber sensors; multimode interference

1. INTRODUCTION

Multimode interference (MMI) in optical fiber structures has

been studied to develop novel optical devices since MMI-

based devices have desirable advantages such as high sensitiv-

ity, immunity to electromagnetic interference, compact size,

and low cost [1, 2]. Usually, the MMI-based device consists

of a step-index multimode fiber (MMF) section spliced

between two single mode fibers (SMF) forming a SMF-MMF-

Figure 6 Measured peak gain versus frequency of the fabricated

antenna

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 4, April 2012 1009

SMF structure which exhibit unique spectral characteristics

making them suitable for signal transmission and sensing

where different applications namely, temperature, displace-

ment, curvature, or refractive index have been studied by sev-

eral researchers [3–5].

In this article, a novel structure with a large-core air-clad

photonic crystal fiber (PCF) is presented using the outer clad-

ding as optical waveguide. This structure uses the outer cladding

from a large-core air-clad PCF segment spliced between stand-

ard single mode fibers (SMF).

2. EXPERIMENTAL RESULTS

In Figure 1(a), the scheme of the sensing head used is presented.

Two single mode fibers (SMF) are spliced to a large-core air-

clad PCF with 5-cm long being the SMF core aligned with the

outer cladding PCF, which produce the resonances that could be

used as sensing element.

Figure 1(b) shows a picture of the PCF end output and it’s

possible to observe that the entire ring of the outer cladding is

illuminated although the light input occurs in a small lateral

area. The large-core air-clad PCF has a core diameter of 85 lmand a total cladding diameter 204 lm. In the microstructured

region, the holes have a diameter of �17 lm and the bridges

width inner holes of 2.5 lm.

The sensing head was characterized in temperature, strain,

and refractive index. For temperature measurement, the struc-

ture was placed in a tube furnace, and submitted to increas-

ing values of temperature in the range 0–400�C, at 50�C-steps.

Temperature variations up to 400�C were applied when the

sensor head was slightly stretched and the results are presented

in Figure 2. The wavelength shift indicates very close sensitiv-

ities, 14.6 pm/�C and 16.4 pm/�C, for k1 and k2. Inset of Figure2 the spectra for temperature variation of 400�C are shown and

the result shift has dependence with wavelength. This result is

expected, due to its composition being only single material

(pure silica). The small difference between the two temperatures

sensitivities could be justified with the temperature dependence

on the wavelength measurement.

The strain response up to 1500�le for k1 and k2 are pre-

sented in Figure 3 and the results show different sensitivities for

the wavelengths studied, �7.5 pm/le for k1 and �0.59 pm/;lefor k2. Inset of Figure 3 shows the spectra for strain variation of

1500 le and it’s possible to observe the significant wavelength

dependence. For this case, two different sensitivities are pre-

sented. When the strain is applied in the SMF, the high differ-

ence of the young modulus between the two fibers creates an

asymmetry in the longitudinal strain of the large-core air-clad

PCF.

To check the capability of this sensor head for simultane-

ous measurements, the relationship between the wavelength

shifts Dk1 and Dk2, as induced by changes in temperature (KT)

and strain (Ke) may thus be expressed in matrix form where

KT, and Ke denote temperature and strain sensitivities for Dk1and Dk2.

The relation between temperature and strain could be

expressed in the equation bellow where D ¼7.38 � 10�3 is the

determinant of the matrix. Using this form is possible to confirm

the possibility of the simultaneous measurement of these two

physical parameters.

DTDe

� �¼ 1

D�5:9� 10�3 7:5� 10�4

�1:64� 10�2 1:46� 10�2

� �Dk1Dk2

� �

Inducing refractive index variations of 0.065 in water by

adding small quantities of ethylene glycol and maintaining once

again the sensor head slightly stretched it is possible to observe

that it has very high sensitivity (Fig. 4). For k1 and k2, the

Figure 1 Schematic diagram of the measurement setup: (a) large-core

air-clad PCF segment spliced between standard SMF; (b) cross section

of a large-core air-clad PCF (illuminated by a SMF28)

Figure 2 Temperature response in air considering k1 and k2. Inset,

optical spectra shift with a temperature variation of 400�C

Figure 3 Strain response in air considering k1 and k2. Inset, opticalspectra shift with a strain variation of 1500 le

1010 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 4, April 2012 DOI 10.1002/mop

sensitivity reached was 214.74 nm/RIU and 322.08 nm/RIU and

a resolution of 6.14 � 10�4 and 7.20 � 10�4 was achieved. In

the inset of Figure 4, there are given the spectra for refractive

index variation of 0.0415, and they present the high dependence

on wavelength.

This sensing head can be used as an optical refractometer

with low dependence in temperature due to high difference

between the sensitivities of those two physical parameters. Con-

sidering real applications with a variation of 50�C the wave-

length shift is 0.8 nm (for the high sensitivity wavelength peak

k2), which is a very low value when compared with 322.08 nm/

RIU.

3. CONCLUSIONS

Summarizing a large-core air-clad PCF-based sensing structure

was demonstrated. A MMI is generated in the outer cladding

and present different sensitivities to temperature, strain and re-

fractive index in liquids. Different sensitivities to temperature

and strain are obtained in the both wavelength peaks. Because

of this behavior, it is possible to obtain a sensing head for si-

multaneous measurement for strain and temperature using the

matrix method. The same structure can also be used as an opti-

cal refractometer with low temperature sensitivity.

REFERENCES

1. W.S. Mohammed, P.W.E. Smith, and X. Gu, All-fibre multimode

interference bandpass filter, Opt Lett 31 (2006), 2547–2549.

2. O. Frazao, J. Viegas, P. Caldas, J.L. Santos, F.M. Ara�ujo, L.A.

Ferreira, and F. Farahi, All-fiber Mach–Zehnder curvature sensor

based on multimode interference combined with a long-period gra-

ting, Opt Lett 32 (2007), 3074–3076.

3. Q. Wang and G. Farrell, All-fibre multimode-interference based re-

fractometer sensor: proposal and design, Opt Lett 31 (2006),

317–319.

4. A. Kumar, R.K. Varshney, C.S. Antony, and P. Sharma, Transmis-

sion characteristics of SMS fiber optic sensor structures, Opt Com-

mun 219 (2003), 215–219.

5. Q. Wang, G. Farrell, and W. Yan, Investigation on single-mode –

multimode – single-mode fiber structure, J Lightwave Technol 26

(2008), 512–519.

VC 2012 Wiley Periodicals, Inc.

RADIATION EFFICIENCY ENHANCEMENTUSING LUMPED INDUCTORS FORDUAL-MODE MOBILE PHONE

Seung-Jun Lee,1,2 Sung-Won Park,1 Chul-Woo Park,2 andYoung-Sik Kim2

1Mobile Communication Division, Samsung Electronics Co., Ltd.,Suwon-Si, Gyeonggi-do 443-742, Korea2Department of Computer and Radio CommunicationsEngineering, Korea University, Seoul 136-713, Korea;Corresponding author: yskim@korea.ac.kr

Received 4 July 2011

ABSTRACT: This letter proposes a radiation efficiency enhancementtechnique using a lumped inductor for a dual-mode mobile phone. Twoplanar inverted-F antennas (PIFAs) with a collinear arrangement are

used for dual-mode operation at the CDMA band of 824–894 MHz andGSM band of 880–960 MHz. Each antenna is separately connected to a

lumped inductor of 4.7 nH. The electric field is more confined on eachinverted-F section at its operating band in this configuration and thecoupling between PIFAs is less than �10 dB. The simulated and

measured results show that radiation efficiency of the proposed antennais improved by up to 33%. The radiation patterns are similar to those ofa dipole antenna with slightly tilted main beam directions due to the

PIFA placement. VC 2012 Wiley Periodicals, Inc. Microwave Opt

Technol Lett 54:1011–1013, 2012; View this article online at

wileyonlinelibrary.com. DOI 10.1002/mop.26735

Key words: coupling; dual-mode; mobile phone; PIFA; radiationefficiency

1. INTRODUCTION

In recent years, multiantennas, which can easily be integrated into

wireless communication systems, have been investigated for

multiple-mode mobile phones [1]. A mobile phone needs dual

standby and talk status for dual-mode application, because the

phone has to receive and to transmit two similar frequency sig-

nals simultaneously at the CDMA and GSM bands. Two inde-

pendent radiators (antennas) are used widely [2]. Many studies

have recently been conducted on reducing the mutual coupling by

means of improving the isolation between two antennas that are

closely located due to the limited size of a mobile phone. Techni-

ques include the suspended line [1], the decoupling network

between two antennas [3], adding an LC circuit for the feed line

[4], and the shorting port of antenna facing each other [5]. In gen-

eral, once two antennas are strongly coupled, most of the signal

from the antenna can be received or absorbed by the other. Thus,

the transmitting signal of the antenna cannot be efficiently radi-

ated and the radiation efficiency may be decreased. This letter

presents a radiation efficiency enhancement technique using a

lumped inductor for a dual-mode mobile phone. The antenna

design and parametric study results are also discussed.

2. ANTENNA DESIGN

As shown in Figure 1, two planar inverted-F antennas (PIFAs) are

placed collinearly at the end of the PCB, and two inductors are

inserted at the shorting port of each antenna. The total size of the

proposed antenna is 100 � 50 mm2, about the same as most com-

mercial smart phones. The region under the radiating plate is not

filled with a metal layer to achieve good radiation performance.

The proposed structure is fabricated on a 1-mm-thick FR4 sub-

strate with a dielectric constant of 4.3. The antenna radiators are

made of a 0.2-mm-thick copper plate. The width of both PIFA

meander-lines is 1.5 mm. The length of the CDMA antenna is

Figure 4 Refractive index response considering k1 and k2. Inset, opti-cal spectra for a refractive index variation of 0.0415 RIU

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 54, No. 4, April 2012 1011