A TUNABLE PHOTONIC MICROWAVENOTCH FILTER WITH A NEGATIVE COEF-FICIENT USING NONLINEAR BEHAVIOROF A REFLECTIVE SEMICONDUCTOROPTICAL AMPLIFIER

Won-Bae Kwon,1 Yong-Kyu Choi,1 Dongsoo Lee,2

Geun-Yong Kim,2 and Chang-Soo Park1

1 School of Information and Communications, Gwangju Institute ofScience and Technology, 123 Cheomdan-gwagiro, Buk-gu,Gwangju 500-712, Republic of Korea; Corresponding author:csp@gist.ac.kr2 Honam Research Center, Electronics and TelecommunicationsResearch Institute, 1110-6 Oryong-dong, Buk-gu, Gwangju, 500-480, Republic of Korea

Received 23 September 2013

ABSTRACT: We propose and experimentally demonstrate a simple tun-

able photonic microwave notch filter with positive and negative coeffi-cients. The negative coefficient is obtained from a single input signal(positive coefficient) using cross gain modulation (XGM) in a reflective

semiconductor optical amplifier (RSOA). When the RSOA is operated inthe saturated region, the signal carried on the pump wavelength is

inversely copied to the probe wavelength by the XGM effect, showing anegative coefficient. This probe wavelength is easily generated by self-injection locking the RSOA through a wavelength-selective filter. Fur-

ther, variable time delay for filter tuning is achieved using wavelength-dependent time delay in a fiber medium by changing the Bragg wave-

length of the fiber Bragg grating. The proposed microwave notch filtershows notch dips of more than 33 dB and has a tuning range from 828to 905 MHz in a free spectral range with a wavelength change of 0.6

nm. VC 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett

56:1328–1331, 2014; View this article online at wileyonlinelibrary.com.

DOI 10.1002/mop.28315

Key words: photonic microwave notch filter; negative coefficient;reflective semiconductor optical amplifier; cross gain modulation

1. INTRODUCTION

Photonic microwave filters have attracted considerable attention

as alternatives to electrical filters in microwave and millimeter-

wave signal processing because they have several advantages

such as immunity to electromagnetic interference, wide band-

width, tunability, and reconfigurability. These filters can be eas-

ily implemented by introducing a time delay to each power

tapped from a single input and combining each delayed-power.

However, they have only positive coefficients and a resonance

peak at the baseband [1]. To overcome this limitation, a few fil-

ters with negative coefficients were introduced and demonstrated

[2, 3]. One drawback of this approach is the need for additional

optical sources to generate the negative coefficients. Recently,

Kim et al. [4] demonstrated a microwave notch filter with a sin-

gle optical source using a polarization converter. This method

shows high rejection levels and tuning characteristics; however,

the polarization states of a single optical source need to be care-

fully controlled to maintain the orthogonal characteristics

between the two polarization axes. In addition, its performance

largely depends on the stability of the polarization states in the

high-birefringence fiber.

In this article, we propose and experimentally demonstrate a

simple tunable photonic microwave notch filter with positive

and negative coefficients. The negative coefficient is obtained

from a single input signal (positive coefficient) using cross gain

modulation (XGM) in a reflective semiconductor optical ampli-

fier (RSOA). When the RSOA is operated in the saturated

region, the signal carried on the pump wavelength is inversely

copied to the probe wavelength by the XGM effect, showing a

negative coefficient. This probe wavelength is easily generated

by self-injection locking the RSOA through the wavelength-

selective filter. Also, variable time delay for filter tuning is

achieved using wavelength-dependent time delay in a fiber

medium by changing the Bragg wavelength of the fiber Bragg

grating (FBG), finally resulting in the change in a free spectral

range (FSR).

2. OPERATING PRINCIPLES

Figure 1(a) shows the schematic diagram of the proposed filter,

where an RSOA is used as a two-wavelength optical source.

The RSOA is a kind of SOA, and it has a highly reflective coat-

ing on one side and an antireflective coating on the other side

[5]. Originally, this RSOA has a broad spectrum under DC bias.

However, we can make it to produce two lasing lights with nar-

row spectral widths by injecting external seed lights to itself.

For this purpose, a laser diode (LD) with a wavelength of kpump

is used as one seed light and the other is obtained by selecting

wavelength (kprobe) of the continuous wave (CW) light (broad-

band) from the RSOA and returning it to itself. Selection and

feedback of the second wavelength are achieved using an FBG

filter with partial reflection at the Bragg wavelength. As a

result, two lasing lights of different wavelengths are generated

inside the RSOA at the pump and probe wavelengths. In case of

the high pump power, the modulation signal carried on the

pump wavelength induces XGM to the probe source inside

the RSOA which is gain-saturated. As a result, the CW light at

the probe wavelength is also inversely modulated by the input

data pattern. Thereby, an inverse pattern of the input signal can

be obtained, corresponding to the negative coefficients [5]. The

output at port #3 of the optical circulator is the combined

one of these two signals (noninverted and inverted signals). The

FSR of the filter is determined by the wavelength-dependent

time delay. Therefore, filter tuning is simply achieved by

Figure 1 (a) Functional schematic of the operating principles of the

proposed filter and (b) schematic of the direct form of the proposed filter

showing filter coefficients a0 and a1. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com]

1328 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 6, June 2014 DOI 10.1002/mop

changing the wavelength of the probe signal, that is, the Bragg

wavelength by thermal tuning.

Figure 1(b) shows the direct form of the proposed filter.

Positive and negative coefficients correspond to the nonin-

verted and inverted signals, and time delay between two sig-

nals is provided by the difference in the propagation delays.

Therefore, the output shows a notch filter characteristic, which

is given by

jHðf Þj / ja01a1e2j2pf ðTdÞj; (1)

where a0 and a1 are the filter coefficients, f is the modulation

frequency, and Td is the time delay.

To obtain sharp notch dips in the notch filter, the values of

each coefficient need be adjusted to be the same and to influ-

ence the XGM efficiency. This XGM efficiency can be carefully

optimized by controlling the modulated pump power, CW

power, and the injected current to the RSOA [6]. In our experi-

ments, a high XGM efficiency was obtained using low CW

power, high injected current to the RSOA, and high pump

power enough to operate the RSOA in the saturation condition.

The relationship between the FSR of the filter and the

time delay (Td) between the noninverted and inverted signals is

given by

1

FSR5Td5D3L3Dk; (2)

where D is the dispersion parameter, L is the fiber length, and

Dk is the wavelength difference between the inverted and the

noninverted signals. Therefore, by changing the time delay, the

FSR of the filter can be varied.

3. EXPERIMENTAL DEMONSTRATION

Figure 2 shows the experimental setup of the proposed filter. A

DC-biased LD with a center wavelength of 1546.5 nm is used

as the pump source. Its output is modulated by a Mach-Zehnder

modulator with the radiofrequency (RF) signal swept from 40

MHz to 2 GHz using a network analyzer. The optical power of

the modulated signal is controlled using an erbium-doped fiber

amplifier and a variable optical attenuator and is injected to the

RSOA through the optical circulator. The RSOA exhibits a las-

ing light with a narrow spectral width at the pump wavelength.

Conversely, the CW light from the RSOA is partially reflected

at FBG #1. The input wavelength is assigned to be outside the

bandwidth of FBG #1.

A train of cavity modes is generated in the RF domain as a

result of the resonance of the external cavity between FBG #1

and the RSOA [7]. In general, the cavity mode spacing (fspacing)

is given by

fspacing 5c

2nLcavity

; (3)

where c is the velocity of light in free space, n is the refrac-

tive index of the fiber, and Lcavity is the cavity length

between the RSOA and the FBG. Therefore, to prevent inter-

ference, a shorter cavity is ideally suited to set the cavity fre-

quencies above the frequency range of our interest to prevent

from the interference. However, owing to practical fiber con-

nection constraints, the external cavity length between the

RSOA and FBG 1 (reflectivity: 45%) was set to be approxi-

mately 26 cm, corresponding to a cavity mode spacing of

approximately 398 MHz. This can be improved by engraving

the FBG on a planar lightwave circuit external cavity laser

(PLC-ECL) [8].

After the XGM effect occurred in the RSOA, the noninverted

and inverted signals passed through the optical circulator and

entered into FBG #2 (reflectivity: 50%, center wavelength:

1546.5 nm), which was used to attenuate the noninverted signal

to equalize the amplitudes of both the signals. Following this,

the combined noninverted and inverted signals passed through a

10.18 km single mode fiber with a dispersion parameter of 17.5

ps/nm km for wavelength-dependent time delay and were

detected by the photodetector.

To induce the XGM effect, peak powers of the pump and

probe signals were set to be approximately 26.76 and 21.33

dBm, respectively. Further, to obtain the inverse pattern, the

RSOA was biased at 70 mA to satisfy the saturation

condition.

Figure 2 Experimental setup of the proposed filter. LD, laser diode; PC, polarization controller; MZM, Mach-Zehnder modulator; EDFA, erbium-

doped fiber amplifier; VOA, variable optical attenuator; CIR, circulator; RSOA, reflective semiconductor optical amplifier; FBG, fiber Bragg grating;

SMF, single mode fiber; PD, photodetector; and E-AMP, electrical amplifier

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 6, June 2014 1329

4. RESULTS AND DISCUSSION

To confirm the inverse data pattern, a signal pattern of

“10101010” was applied to the modulator from the pulse pattern

generator and a tunable optical band pass filter was used to sep-

arate one of the signals after FBG #2. The inverted signal is

shown in Figure 3. A slower rising and falling time compared to

that of the input is attributed to the bandwidth (<1.3 GHz) of

the RSOA.

Figure 4 shows the optical spectra of the noninverted and

inverted signals measured using an optical spectrum analyzer.

Owing to external injection locking, the RSOA with an initial

broadband spectrum shows two wavelength lasing spectra at

1546.5 nm (pump wavelength) and 1552.7 nm (probe wave-

length), respectively. A power difference between the two sig-

nals was observed owing to the DC level of the inverted signal,

which was filtered out using a DC block at the receiver side.

For wavelength tuning, the Bragg wavelength of FBG #1 was

thermally adjusted with a thermoelectric cooler controller with a

wavelength range of up to 0.6 nm. A wavelength spacing of 6.2,

6.5, and 6.8 nm was observed between the two signals, as

shown in Figure 4.

Figure 5 shows the frequency response of the proposed filter

with the notch dips more than 33 dB. The solid line represents

the experimental result, and the dotted line represents the result

obtained through simulation. An FSR of approximately 905

MHz was obtained from a wavelength spacing of 6.2 nm. The

measured frequency response showed good correlation with the

simulated frequency response. Further, owing to the negative

coefficient, no resonance peak was observed in the filter charac-

teristic at the baseband.

Figure 6 shows the measured frequency responses of the fil-

ter. An FSR of approximately 905, 865, and 828 MHz corre-

sponds to a wavelength spacing of 6.2, 6.5, and 6.8 nm,

respectively. A train of cavity modes with mode spacing of

approximately 398 MHz was observed at approximately 800

MHz and 1.6 GHz. However, it was observed that the peak of

the tone frequency of the cavity mode was so small that it did

not affect the filter performance.

5. CONCLUSION

A simple tunable photonic microwave notch filter with a nega-

tive coefficient using an RSOA was proposed and demonstrated.

This negative coefficient was obtained through the XGM effect

Figure 6 Measured frequency responses of the proposed notch filter.

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com]

Figure 3 Data patterns of the noninverted and inverted signals meas-

ured after FBG #2. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com]

Figure 5 Measured and simulated frequency responses of the proposed

filter. [Color figure can be viewed in the online issue, which is available

at wileyonlinelibrary.com]

Figure 4 Optical spectra of the noninverted and inverted signals meas-

ured after FBG #2. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com]

1330 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 6, June 2014 DOI 10.1002/mop

in the RSOA. The optical source for the negative coefficient

was generated by the RSOA and an external FBG. Further, vari-

able time delay to tune the FSR was achieved by thermally

changing the Bragg wavelength of the external FBG. The pro-

posed notch filter showed notch dips of more than 33 dB and a

tuning range of an FSR from 828 to 905 MHz.

ACKNOWLEGMENTS

This research was supported by the “Basic Research Projects in

High-tech Industrial Technology” Project through a grant pro-

vided by GIST in 2013 and by the MSIP (Ministry of Science,

ICT and Future Planning), Korea, under the C-ITRC (Conver-

gence Information Technology Research Center) support

program (NIPA-2013-H0401-13-2006) supervised by the NIPA

(National IT Industry Promotion Agency).

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VC 2014 Wiley Periodicals, Inc.

EXPERIMENTAL CHARACTERIZATIONOF FSS FOR WLAN APPLICATIONSWITH LOW-COST UWB ELLIPTICALMICROSTRIP MONOPOLE ANTENNAS

Clarissa de L. N�obrega,1 Marcelo R. da Silva,1

Paulo H. da F. Silva,2 and Adaildo. G. D’Assunc~ao1

1 Federal University of Rio Grande do Norte, UFRN-CT-DCO, CaixaPostal 1655, CEP 59078-970, Natal, RN, Brazil; Correspondingauthor: adaildo@ct.ufrn.br2 Federal Institute of Education, Science and Technology of Paraiba,IFPB, CEP 58015-430, Jo~ao Pessoa, PB, Brazil

Received 26 September 2013

ABSTRACT: In this work, we propose an elliptical microstrip monop-ole antenna with ultrawideband (UWB) characteristics to be applied in

low cost measurements of frequency selective surfaces (FSSs). The 210-dB impedance bandwidth of the elliptical antenna is 1.0–13.5 GHz,which is about 172% broader. To demonstrate the antenna application

in FSS measurements, we also propose a dual-band band-stop FSS with

metallic patches based on teragon fractal geometry designed to reject

wireless local area network signals. Theoretical analysis of the ellipticalmonopole and FSS is done using Ansoft HFSS and Ansoft DesignerTM

commercial softwares, respectively. Prototypes of designed antenna and

FSS were built and measured. Through the use of UWB monopole anten-nas, we obtained an excellent agreement between simulated and meas-ured FSS transmission coefficients. VC 2014 Wiley Periodicals, Inc.

Microwave Opt Technol Lett 56:1331–1333, 2014; View this article

online at wileyonlinelibrary.com. DOI 10.1002/mop.28371

Key words: monopole antenna; frequency selective surfaces; FSS, tera-gon fractal; WLAN applications

1. INTRODUCTION

The ultrawideband (UWB) monopole antennas have been the

subject of extensive studies in recent years for applications in

3.1–10.6 GHz frequency range, according to Federal Communi-

cations Commission as reported in 2002. Due to the increase of

requirements on the manufacturing of low cost and low profile

antennas, the microstrip planar monopoles which operates in

UWB systems, have been widely used in modern communica-

tion devices because of its desired features, such as: light

weight; wide impedance bandwidth; good radiation characteris-

tic; simple manufacturing process; and easy to integrate with

radio frequency devices [1–4].

Conversely, frequency-selective surfaces (FSSs) have

received considerable attention as electromagnetic spatial fil-

ters for microwave and millimeter-wave signals and have been

investigated over the years for widespread applications, such

as: microwave absorbers; reflector antenna systems; band-pass

radomes; and in low probability of intercept systems [5]. In

recent applications, a FSS periodic structure can be used to

modify an indoor propagation environment through alternative

techniques known as “wireless building.” Thus, a conventional

reflect/transmit FSS may be placed in the walls and/or win-

dows of a building to reduce interferences between adjacent

WLAN radios and protect people against electromagnetic radi-

ation [6–8]. In this context, the use of geometric fractals as

FSS metallic patches provide interesting features in a single-

layer structure: compact design; stable resonance frequencies

at oblique incidence; and multiband responses, allowing a

more effective filtering of more than one frequency band of

interest [9–11].

In this work, we propose an UWB elliptical microstrip

monopole antenna to be applied in low-cost measurements of

FSSs at normal incidence [12]. This elliptical monopole was

designed by means an easy and cheap manufacture process.

Usually, the commercial horn antennas used for FSS measure-

ments are expensive devices depending on the operating fre-

quency range, which is often quite limited.

To validate the use of the proposed antenna in the FSS char-

acterization, we designed a band-stop FSS with metallic patches

based on teragon fractal geometry. The shapes presented by

fractal patches allow one to design an efficient FSS spatial filter

with dual-band behavior for WLAN applications, rejecting sig-

nals in the bands of 2.4–2.5 GHz (IEEE 802.11b) and 5.0–6.0

GHz (IEEE 802.11a).

The simulations of the elliptical monopole and FSS were

performed using Ansoft HFSSTM and Ansoft DesignerTM com-

mercial softwares, respectively. We measured the transmission

coefficient of FSS and the return loss of monopole antenna

using a vector network analyzer from Agilent Technologies

(model N5230A). Two monopole antenna prototypes operating

in the range of 1.5–8.0 GHz were used in the FSS experimental

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 6, June 2014 1331