loss bandwidth of 9 MHz (910�919 MHz). The recognition dis-

tance for the tag is obtained from the Friis transmission formula

[4] as follows:

R ¼ k4p

1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPtag

EIRP�Gtag�ð1�jCj2Þ�c

q (1)

where EIRP (effective isotropic-radiated power) is the maximum

power available for an RFID reader and is standardized as 36

dBm in many countries. The polarization mismatch loss, c, hasa value of �3dB for the proposed tag antenna with linear polar-

ization. Power sensitivity, Ptag, is proportional to the sensitivity

of the tag IC. Gtag is the antenna gain and 1 � | C |2 represents

the impedance mismatch loss between the tag antenna and the

tag IC. The recognition distance and wake-up sensitivity were

measured within an anechoic chamber. The measurement system

is comprised of a computer, an RFID reader (Mercury4, TM-

M4/W-NA-02), a reader antenna (EMW antenna, FSDC-07),

and variable attenuators. The wake-up sensitivity of the tag

antenna was �15.48 dBm at 915 MHz, and the measured maxi-

mum recognition distance was 6.3 m, as shown in Figure 9. The

simulated and measured radiation patterns in Figure 10 were

similar to those of a typical dipole antenna. The simulated and

measured peak gains of the tag antenna were 1.52 and 1.48 dBi

at 915 MHz, respectively.

4. CONCLUSION

We proposed a compact UHF RFID tag antenna using a para-

sitic element. The proposed tag antenna is miniaturized by uti-

lizing inductive coupling between the meandered dipole and the

parasitic element. The area (0.055k � 0.109k) of the proposed

antenna is only 30% of MLA [1] and IFA [1]. The peak gain of

the antenna is 1.52 dBi, and the measured maximum reading

distance is 6.3 m at 915 MHz. The performance factors for the

proposed tag antenna, such as size and gain, are better than

those of other small tag antennas [5–7].

ACKNOWLEDGMENT

This work was supported by the Seoul R&BD program (10848),

Republic of Korea.

REFERENCES

1. G. Marrocco, The art of UHF RFID antenna design: Impedance-

matching and size-reduction techniques, IEEE Antennas Propag

Mag 50, 66–79, Jan 2008.

2. Ansoft High Frequency Structure Simulator (HFSS), Ver.10.0,

Ansoft Corporation.

3. C.A. Balanis, Antenna Theory, 3rd ed., Wiley, New York, 2005.

4. Y. Choi, U. Kim, J. Kim, and J. Choi, Design of a modified folded dipole

antenna for UHF RFID tag, IEE Electron Lett 45 (2009), 387–389.

5. K. Finkenzeller, RFID handbook, Wiley, New York, 2000.

6. S.W. Bae, W.S. Lee, K.H. Chang, S.W. Kwon, and Y.J. Yoon, A

small RFID tag antenna with bandwidth-enhanced characteristics

and a simple feeding structure, Microwave Opt Technol Lett 50

(2008), 2027–2031.

7. G. Gonzalez, Microwave Transistor Amplifiers, 2nd ed., New

jersey, Prentice Hall, 1997.

VC 2010 Wiley Periodicals, Inc.

DOUBLE-CAVITY FIBER FABRY-PEROTTUNABLE FILTER

Yi Jiang,1 Caijie Tang,1 and Chong-Wen Wang2

1 School of Optoelectronic, Beijing Institute of Technology, Beijing100081, China; Corresponding author: bitjy@bit.edu.cn2 School of Software, Beijing Institute of Technology, Beijing,10081, China

Received 24 April 2010

ABSTRACT: A double-cavity fiber Fabry-Perot tunable filter isproposed and experimentally demonstrated. There are two cavities in thetunable filter. The two cavities have close properties, and are driven by

same piezoelectric transducers. One cavity is used to scan wavelength,and another one is used to calibrate the wavelength. The double-cavityfilter is anticipated to be used to interrogate not only the spectra of

passive components, but also that of active light sources. VC 2010 Wiley

Periodicals, Inc. Microwave Opt Technol Lett 53:242–245, 2011; View

this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25752

Key words: fiber Fabry-Perot filter; fiber optic sensors

Figure 10 Simulated and measured radiation patterns of the proposed

tag antenna at 915MHz

242 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 DOI 10.1002/mop

1. INTRODUCTION

Optical fiber Fabry-Perot tunable filters (FFP-TF) play a critical

role in many fields, such as optical communication, fiber optical

sensing technology, and optical metrology. Of many innovative

tunable filters, all-fiber FFP-TFs [1] and MEMS tunable filters [2]

have been most commercially available. MEMS-based filters

have the wafer-scale manufacturing advantage. However, they

are susceptible to thermal–mechanical perturbation and nonlinear

optical properties. All-fiber FFP-TFs are driven by piezoelectric

transducers (PZTs), and they also exhibit nonlinear hysteresis

behavior and poor repeatability. When a FFP-TF is used to scan a

spectrum, the scanning wavelength must be calibrated by using

an etalon [3] or other standard wavelength calibrator. Thus, the

FFP-TF can only be used to interrogate passive components, such

as fiber Bragg gratings [4], extrinsic Fabry-Perot interferometric

sensors [5], wavelength division multiplexers, etc. FFP-TFs can-

not be used to interrogate the spectrum of active light sources,

such as light emission diodes, laser diodes, amplifier spontaneous

emission (ASE) sources, because the scanning wavelength cannot

be calibrated when it is used to scan the light from a light source.

There is no such a demonstration till now that a FFP-TF is used

to interrogate the spectrum of a light source. In this letter, we

demonstrate a double-cavity FFP-TF, which has two Fabry-Perot

cavities. One cavity is used as a general wavelength scanner, and

another cavity is used as a wavelength calibrator. Wavelength

scanning and wavelength calibration can be realized at the same

time. Thus, it can be used to interrogate the spectrum of a light

source. A new type of optical spectrum analyzer (OSA) is antici-

pated to be realized based on this new component.

2. CONFIGURATION OF DOUBLE-CAVITY FFP-TF

The configuration of the double-cavity FFP-TF is shown in Fig-

ure 1. The tuning structure is constructed with two PZT poles

and two aluminum blocks. Two Fabry-Perot filters with close

free spectrum range and close transmission wavelength are par-

allel fixed on the two aluminum blocks. The distances among

the PZTs and cavities should be as close as possible, so that the

Fabry-Perot cavities of the two filters keep same changes in

length. Besides, the two PZTs should be chosen carefully. The

two PZTs are selected to have close properties, for example, the

two PZTs should have same length, the displacements should be

the same when a voltage is applied on, and the two PZTs should

have close hysteresis behavior. Thus, the two cavities have same

movement when a sawtooth wave voltage is applied on.

The two filters used in the double-cavity tunable filter are

two high-finesse Fabry-Perot interferometers in which the cavity

is formed by depositing films on two fiber ends. But filters

formed with plane mirrors surfer from the diffraction loss of the

resonant cavity, which decreases the finesse, and also surfer

from the mismatch between fiber mode and resonator mode,

which increases the insertion loss [6, 7]. To overcome the prob-

lems, microlens were manufactured on the fiber ends to focus

the light in the cavity, and filters with high finesse and low-

insertion loss can be obtained in this way [8, 9].

In this letter, two filters with close FSR and transmission

wavelength were manufactured. The spectra of the two filters

are shown in Figure 2. For the first filter, shown in Figure 2(a),

the FSR is 69.84 nm, and the transmission wavelengths in the

wavelength range of 1515–1625 nm, the wavelength band of the

CþL ASE source, are 1547.00 and 1616.84 nm, respectively.

For the second filter, the FSR is 70.56 nm, and the transmission

wavelengths are 1540.04 nm and 1610.60 nm, as shown in Fig-

ure 2(b). There is 7-nm difference between the transmission

wavelengths of the two filters. The finesse of the two Fabry-

Perot filters is 280, and the bandwidth is 0.25 nm. The loss of

the two filters is close to 3 dB. After the two filters are fixed on

the tuning structure, the FSRs of the two filters change to be

67.92 and 69.84, and the difference between the two transmis-

sion wavelengths becomes � 17 nm. We attrib the change of

the wavelength shifts to the shrinkage of the glue during the

curing duration. The transmission wavelengths of the two filters

are 1546.22 nm and 1529.18 nm in the C-band wavelength

range, respectively, when there is no driving voltage applied.

3. EXPERIMENTS

The double-cavity FFP-TF was tested by applying a sawtooth

wave voltage on PZT drivers. The experimental setup is shown

in Figure 3. An ASE source is connected to the two paralleled

Figure 1 Configuration of double-cavity FFP-TF (a) and (b)

Figure 2 Spectra of the two parallel filters, (a) first filter, (b) second

filter

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 243

cavities though a coupler. To ensure only one resonance wavelength

existed within the wavelength range of light source, we use a C-

Band ASE source in the experiment, with wavelength range of less

than 50 nm. The outputs of the two filters are wavelength-scanning

light with bandwidth of 0.25 nm. The two beams of the scanning

light are injected into two etalons, which wavelengths are located

on ITU standard. The etalon has a FSR of 0.8 nm (100 GHz), and

finesses of 14. The wavelength thermal stability of the etalon is bet-

ter than 0.7 GHz from 0�C to 70�C. A FBG with bandwidth of 0.7

nm is serially connected with the etalon to remove one peak from

the spectrum of the etalon, making a marker at 1549.31 nm to iden-

tify the fixed wavelength of all other peaks. The resonance wave-

lengths between the two etalon have a wavelength difference of

only 2 pm. Then, the scanned spectra of the two etalon are photode-

tected and sampled into a computer with a two-channel A/D card.

The sampled datum arrays are shown in Figure 4 when a

sawtooth wave is applied on the double-cavity FFP-TF. The

sawtooth wave is a linear voltage, from �5 to 26 V, ensuring a

whole FSR can be scanned in one scanning. For the first filter, a

voltage range from �3 to 16 V is required for scanning a whole

FSR, as shown in Figure 4(a). For the second filter, the voltage

is 10 to 22 V for scanning a whole FSR, as shown in Figure

4(b). The difference of the driving voltage is caused by the dif-

ferent range of FSRs. This difference should be as small as

enough to reduce the calibration error, although it seems larger

in our experiment. From Figure 4, we can obtain the relationship

between the two filters, and we can obtain the scanning wave-

length of the second filter from the wavelength of the first filter

in which the wavelength can be calibrated with an etalon.

To illustrate the application of the double-cavity FFP-TF in

analyzing the spectrum of a light source, an initial experiment

was carried out to measure the spectrum of a DFB laser, which

wavelength is at ITU standard, 1550.9 nm. The laser light is

injected into the second filter and detected with a photodiode.

The first filter is used as a wavelength calibrator, which is con-

nected with an ASE source and etalon, and detected with another

photodiode. When the double-cavity FFP-TF is scanned, the sam-

pling index of the second filter is calibrated by the sampling index

of the first filter, and, thus, by the wavelength of the etalon. A set

of arithmetic is designed to realize the wavelength calibration,

which is discussed in Ref. [3]. Then, we can obtain the spectrum

of the laser, as shown in Figure 5. The central wavelength in Fig-

ure 5 is close to that of the laser. We find that, in the experiment,

the measured wavelength has a variation of � 60.3 nm. We attrib

this variation to the nonconsistency of the two filters. We regard

that the wavelength accuracy can be increased by making the two

cavities to have same property. We also find that the bandwidth

of the measured light, � 0.25 nm, is larger than the actual value,

which is less than 0.1 nm. In fact, the measured bandwidth is

close to the bandwidth of the scanning filter. The resolution of the

spectrum analyzer is limited by the bandwidth of the scanning

filter, 0.25 nm. This shortage can be limited by increasing the

finesse of the filter, thus, decreasing the bandwidth of the filter.

We also interrogate the spectrum of a light source, which is

combined with two DFB lasers, by using the double-cavity tuna-

ble filter. The wavelengths of the two lasers are 1549.3 nm and

1550.9 nm, respectively, which are all located on ITU standard.

The obtained spectrum is shown as in Figure 6. It is clear that

the wavelengths of the two lasers are correctly recovered.

Figure 3 Experimental setup (a) and (b). [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com]

Figure 4 The sampled data arrays, (a) first filter, (b) second filter

Figure 5 Spectrum of a DFB laser

Figure 6 Spectrum of two DFB lasers. [Color figure can be viewed in

the online issue, which is available at wileyonlinelibrary.com]

244 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 DOI 10.1002/mop

4. CONCLUSIONS

In conclusion, we have proposed and experimentally demon-

strated a double-cavity FFP-TF, which is ready to be used as a

new type OSA. There are two Fabry-Perot cavities in the filter,

and the two cavities have close properties. One cavity is used to

scan wavelength, and another one is used to calibrate the wave-

length. The two cavities undergo same tuning procedure, thus,

the nonlinear hysteresis behavior and poor repeatability of the

scanning filter can be mostly removed by using the calibration

filter. The light from DFB lasers is scanned using this double-

cavity filter, and the spectrum is correctly recovered. The initial

experimental results show the feasibility of this component.

ACKNOWLEDGMENTS

This work was supported by the Program for New Century Excel-

lent Talents in the University (NCET) of China and Chinese 863

Project (2008AA04Z406).

REFERENCES

1. C.M. Miller and F.J. Janniello, Passively temperature-compensated

fiber Fabry-Perot filter and its application in wavelength division

multiple access computer network, Electron Lett 26 (1990),

2122–2123.

2. P. Tayebati, et al, Widely tunable Fabry-Perot filter using

Ga(A1)As-AlOx deformable mirrors, IEEE Photon Technol Lett 10

(1998), 394–396.

3. Y. Jiang, High-resolution interrogation technique for an EFPI by

peak-to-peak method, Appl Opt 47 (2008), 925–932.

4. A.D. Kersy,T.A. Berkoff, and W.W. Morey, Multiplexed fiber

Bragg grating strain-sensor system with a fiber Fabry-Perot wave-

length filter, Opt Lett 18 (1993), 1370–1372.

5. Y. Jiang, Fourier transform white-light interferometry for the mea-

surement of fiber-optic Fabry-Perot interferometric sensors, IEEE

Photonics Technol Lett 20 (2008), 75–77.

6. V. Bhatia,M.B. Sen,K.A. Murphy, and R.O. Claus, Wavelength-

tracked white light interferometry for highly sensitive strain and

temperature measurements, Electron Lett 32 (1996), 247–249.

7. B. Yu,G. Pickrell, and A. Wang, Thermally tunable extrinsic

Fabry-Perot filter, IEEE Photonics Technol Lett 16 (2004),

2296–2298.

8. Y. Jiang and C.J. Tang, High-finesse microlens optical fiber Fabry-

Perot filters, Microw Opt Technol Lett 50 (2008), 2386–2389.

9. Y. Jiang and C.J. Tang, High-finesse micro-lens fiber-optic extrin-

sic Fabry-Perot interferometric sensors, Smart Mater Struct 17

(2008), 055013.

VC 2010 Wiley Periodicals, Inc.

DESIGN OF A COMPACTULTRAWIDEBAND SLOT ANTENNA WITHDUAL BAND-NOTCHED FUNCTION

Jin-Yuan Xue, Shu-Xi Gong, Peng Fei, Wei Wang,and Fei-Fei ZhangNational Key Laboratory of Antennas and Microwave Technology,Xidian University, Xi’an, Shanxi 710071, People’s Republic ofChina; Corresponding author: xue_xd@126.com

Received 28 April 2010

ABSTRACT: A compact ultrawideband (UWB) microstrip-fed slotantenna with dual band-notched function is presented. The prototype

consists of an L-shaped microstrip line, a modified ground plane withT-shaped slot and a small stub embedded on the ground for impedancematching. The designed antenna satisfies the return loss requirement of

larger than 10 dB in UWB frequency range from 3.1 to 10.6 GHz,

whereas shows the wireless local area network and worldwideinteroperability for microwave access band notched in the frequency

band of 5.1–6 GHz and 3.5–4.1 GHz by the inserted rectangle strips.Detailed analysis and experimental results for the design are studiedand investigated. VC 2010 Wiley Periodicals, Inc. Microwave Opt

Technol Lett 53:245–249, 2011; View this article online at

wileyonlinelibrary.com. DOI 10.1002/mop.25739

Key words: dual band-notched function; slot antenna; UWB antenna

1. INTRODUCTION

Ultrawideband (UWB) technology has been widely used in com-

munication systems, because of its attractive characteristics,

such as low complexity, low cost, wide impedance bandwidth,

and omni-directional radiation pattern. The frequency band of

UWB communication systems is 3.1–10.6 GHz, which was

approved by the Federal Communications Commission [1]. This

band causes interference with the existing wireless communica-

tion systems, for example the wireless local area network

(WLAN) operating in 5.15–5.35 GHz together with 5.725–5.825

GHz bands, worldwide interoperability for microwave access

(WIMAX) in 3.4–3.7 GHz, and C-band (3.7–4.2 GHz) satellite

communication systems. To avoid possible electromagnetic in-

terference, it is desirable to design UWB antennas with notched

band.

In the last few years, band-notched UWB antennas based on

various techniques have been proposed. Such as the use of the

split-ring resonator [2] and electric-LC resonator [3], the com-

plementary split-ring resonator [4], and complementary electric-

LC resonator [5], cutting a slot (U-shaped, arc-shaped and

pi-shaped slot) on the patch [6], I-shaped parasitic element

printed on the rear side of the substrate [7]. However, most of

these antennas can generate only one notched frequency band.

Moreover, it is not easy to adjust the centre frequencies of the

notched bands.

In this article, a compact design of dual band-rejected slot

UWB antenna has been proposed. L-shaped feedline, T-shaped

slot ground, and a small stub have been formed on the ground

edge to achieve impedance matching and wideband impedance

characteristic for UWB applications. The dual notched bands of

5.5 GHz and 3.8 GHz are obtained by inserting two strips on

ground plane. Details of the proposed antenna design are pre-

sented. The experimental results of return loss, radiation pat-

terns, and dispersion characteristic in frequency domain are also

presented and investigated. The proposed antenna structure is

simulated using the Ansoft High Frequency Structure Simulator

(HFSS 11) software, with lumped port excitation.

2. ANTENNA DESIGN

Figure 1 depicts the geometry of the proposed antenna. As is

shown, the proposed slot UWB antenna is mainly composed of

a ground plane with T-shaped slot, L-shaped microstrip feed

line, and two rectangle strips with different length on the

ground. The T-shaped slot in ground plane consists of one open-

ended x-directed slot, one open-ended y-directed slot, and a

small stub with dimension W7 � L4 embedded at the left bottom

corner of the slot for improving impedance matching. The

printed open slot on ground plate has been designed to produce

wideband impedance characteristic in small size antenna [8].

The width of the L-shaped microstrip feed line is fixed at Wt ¼1.5 mm, which makes the feedline’s characteristic impedance Zo¼ 50 X. Thus, it can be connected with a 50 X SMA connector

directly. The band rejection function of the proposed slot

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 2, February 2011 245