1704 OPTICS LETTERS / Vol. 32, No. 12 / June 15, 2007

Multiple-element photonic microwave true-time-delay beamforming incorporating a tunable

chirped fiber Bragg grating with symmetricalbending technique

Young-Geun Han1,* and Ju Han Lee2

1Department of Physics, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea2Department of Electrical and Computer Engineering, University of Seoul, 13 Siripdae-gil, Dongdaemun-gu,

130-743, Korea*Corresponding author: yghan@hanyang.ac.kr

Received February 5, 2007; revised April 4, 2007; accepted April 9, 2007;posted April 18, 2007 (Doc. ID 79774); published June 6, 2007

We experimentally demonstrate a novel scheme for a multiple-element photonic microwave true-time-delaydevice with high tunability based on a tunable chirped fiber Bragg grating without center wavelength shift.We achieve the different true time delay by controlling the grating period of a chirped fiber Bragg gratingbased on the symmetrical bending technique as a multiwavelength signal source is applied to carry micro-wave signals. The proposed method does not require the complex structure of systems, wavelength tuning,and synchronization of optical devices such as tunable bandpass filters and optical input signals. We achievethe tunabilty of the time delay for a microwave signal carried over an optical signal in a range from 1 to230 ps. © 2007 Optical Society of America

OCIS codes: 060.2310, 060.2340.

Optical beamforming networks based on photonic mi-crowave true-time-delay (TTD) units have been ofhuge technical interest because of their great poten-tial for a phase-array antenna for modern radar andcommunication systems [1–3]. Compared with thetraditional electronic steering units, photonic beam-forming systems based on an optical microwave TTDunit have several advantages, including their immu-nity to the electromagnetic interference, small vol-ume, and light weight [1–3]. Versatile photonic TTDdevices based on a FBG utilizing static strain gradi-ent [4], moving strain perturbation [5], or a struc-tural gradient of fiber by tapering [6] have been re-ported. However, previous methods suffer from thecenter wavelength shift, which results in the systemcomplexity of the photonic TTD unit. In the previouswork [6], it also has significant reflectivity fluctuationof more than �3 dB for a given time-delay setting,which can induce the output power fluctuation of themicrowave signal.

In this Letter we experimentally demonstrate aflexible scheme for a continuous and multiple-element photonic microwave TTD device with hightunability based on a tunable chirped FBG (CFBG).Based on the symmetrical bending technique alongthe FBG, we can continuously control the time delayof the fiber grating without center wavelength shift.We then apply the proposed tunable CFBG to thecontinuous photonic TTD system and successfully ob-tain different time delays for three input wave-lengths corresponding to the variation of symmetri-cal bending curvature. The proposed method does notrequire the complex structure of systems, wavelengthtuning, and synchronization of optical devices suchas tunable bandpass filters and optical input signals.

Figure 1(a) shows the proposed experimental

scheme of a multiple-element photonic microwave

0146-9592/07/121704-3/$15.00 ©

TTD system based on the tunable CFBG. In order torealize a continuous photonic TTD system, the mul-tiplexed optical signals through the AWG are exter-nally modulated by a microwave signal through aLiNbO3 Mach–Zehnder modulator. The modulatedoptical signals are sent to the tunable CFBG via anoptical circulator and reflected by the tunable CFBGat different positions. The modulated optical signal isappropriately delayed by the tunable CFBG and con-verted to the time-delayed microwave signal througha photodetector.

The proposed tunable CFBG is based on the so-phisticated fiber bending technique to symmetricallyinduce a linear strains gradient in the center of the

Fig. 1. (Color online) Scheme of a (a) multiple-elementTTD configuration based on the tunable CFBG-based pho-tonic microwave TTD line and (b) the proposed tunableCFBG based on the symmetrical bending technique. (c)

Photograph of the proposed scheme.

2007 Optical Society of America

June 15, 2007 / Vol. 32, No. 12 / OPTICS LETTERS 1705

grating. The symmetrical bending technique can pre-vent center wavelength shift of a fiber grating. Thereare versatile methods to induce symmetrical bendingalong the fiber grating based on thermal tuningmethods or structural bending techniques [7–10].The thermal tuning techniques [6,7] are expensiveand require two heaters to suppress the center wave-length shift and to induce chirp ratio variation alongthe grating. Their small tuning range is an addi-tional drawback. For structural bending techniquesin previous reports [5,8], it is not easy to control thebending shape and to achieve the linear tunability ofthe time delay. In the proposed apparatus, we cancontrol the arbitrary bending shape and the lineartunability because the position of moving pivots ischanged. It is also cost-effective compared with pre-vious methods. When the left translation stage ismoved by the micrometer (+y direction), its gear ro-tates the sawtooth wheel, while the right translationstage is moved oppositely by the rotary motion of thesawtooth wheel (−y direction). Then the position oftwo pivots on two translation stages is changed oppo-sitely, and the symmetrical bending along the flexiblecantilever beam can be created by two translationstages that are moved oppositely by the interactionbetween two gears and a sawtooth wheel. Conse-quently, the tension and compression strain along theCFBG through the symmetrically curved cantileverbeam can be induced, corresponding to the bendingdirection. Therefore, the properties of the CFBG,such as bandwidth and group delay, can be continu-ously controlled by the tension and compressionstrain at each side of the CFBG without the centerwavelength shift. The flexible cantilever beam ismade of a spring steel with high resistance againstfatigue and corrosion. The length and thickness ofthe cantilever beam are 15 cm and 0.2 mm, respec-tively. The CFBG was carefully attached to themiddle of the cantilever beam by using the UV curingadhesive (Op.4-206541, Dymax Inc.) as shown in Fig.1(c). The CFBG was apodized by the Blackman pro-file to reduce the sidelobes and the group-delayripple. The grating length was 10 cm.

Since the beamforming angle in the TTD system isproportional to the time-delay difference ��t� be-tween adjacent channels, the direction of beampoint-ing can be determined by the time-delay difference,which can be changed by the variation of the time de-lay within the fiber grating [5]. The beampointing ofa phase-array antenna is determined by the bendingof the grating. The strain variation ���, chirped reflec-tion bandwidth ����, and grating period variation���� of the grating based on the symmetrical bendingcan be written as [8–10]

� =6d

L3 y�L − 2x�,

�� = �max − �min = �p�1 − ��6d

Lsin �,

�� =��

, �1�

2neffL

where �p and neff are the center wavelength of thegrating and the effective index, respectively. L is thetotal length of the cantilever beam, and d is the dis-tance between the grating axis and neutral axis,which is equal to the half-thickness of the cantileverbeam. � is the photoelastic coefficient, and � is theangle between the axis of the grating and the neutralaxis of the cantilever beam when the grating wasbent. The tension and compression strain can be in-duced in the region where x�L /2 and L /2�x�L, re-spectively. As seen in Eq. (1), the grating period of thegrating can be changed by the tension and compres-sion strain induced by the symmetrical bending, butthe center wavelength is not changed because the ef-fect of tension and compression strain on the centerwavelength shift should be compensated mutually.

Figure 2(a) shows the experimental results of re-flection spectra of the tunable CFBG with the varia-tion of a moving stage. It was clearly obvious that thehigh optical extinction ratio of three-element TTDunit was more than �25 dB. Compared with previousworks [4–6], it shows superior performance in termsof reflectivity fluctuations ���0.8 dB� for a givenchirp setting induced by the symmetrical bending.The reflectivity reduction for different bending statuswas measured to be �3.5 dB because of the bending-induced insertion loss. When the left translationstage is moved by the micrometer in the range from 0to 10 mm, the chirped reflection bandwidth of thegrating was changed in a range from 3 to 7.46 nm be-cause of the enhancement of tension and compressionstrain. Figure 2(b) shows the variation of the spectralbandwidth of the tunable fiber grating with the mov-ing distance of a translation stage. The spectralbandwidth of the grating becomes broad as the mov-ing distance increases, because the grating periodwas symmetrically changed by the tension and com-pression strain. It is clearly evident that the centerwavelength of the tunable fiber grating is notchanged because of the symmetrical bending. As the

Fig. 2. (Color online) (a) Optical reflection spectra of thetunable CFBG for the variation of a moving stage, (b) thespectral bandwidth change as a function of the moving dis-tance of a translation stage, (c) the time-delay spectral re-sponse of the fiber grating as a function of a moving dis-tance, and (d) the measured group-delay ripple of the fiber

grating at the moving distance of 6 mm.

1706 OPTICS LETTERS / Vol. 32, No. 12 / June 15, 2007

micrometer moves further to the left translationstage, the bending curve along the cantilever beambecomes strong, which can increase the amount ofthe tension and compression strain corresponding tothe bending direction. A large amount of strain gra-dient continuously changes the grating period alongthe uniform CFBG without center wavelength shift.Accordingly, the center wavelength should be keptconstant even if the grating period of the grating ischanged. As shown in Fig. 2(a), the center wave-length shift over the whole tuning range was mea-sured to be less than 0.02 nm. Figure 2(c) shows thetime-delay spectral response of the fiber grating as afunction of a moving distance. We clearly achievedthe linear characteristic of the complete time-delayspectral response of the fiber grating as a function ofa moving distance. The group-delay ripple was mea-sured to be less than �±3 ps when the translationstage was moved to 6 mm as shown in Fig. 2(d).

In order to measure the time delay of the micro-wave signal based on the proposed technique, the in-put light signal was externally modulated by RF sig-nals with different frequencies and entered thetunable CFBG. All input signals were reflected fromthe tunable CFBG at different locations, which caninduce the time delay. Then they were filtered by theAWG and sent to a high-speed photodetector to con-vert the time-delayed optical signals to the time-delayed microwave signals. By using an oscilloscope,we measured the time delays. Figure 3 shows themeasured time delay as a function of moving distanceof a translation stage for the different microwave fre-quencies of 5, 7.5, and 10 GHz at the different inputwavelengths of 1558, 1558.5, and 1559 nm, respec-tively. We could continuously control the time delayfor the microwave frequency below 10 GHz, becauseour system is only for the low frequency signal of lessthan 10 GHz. For high RF frequency application of

Fig. 3. (Color online) Measured time delay with the varia-tion of a moving stage for the different microwave frequen-cies at the different optical input wavelengths.

the proposed method, it is necessary to improve thegrating quality such as low sidelobe, low group-delayripple, etc. The time delay was reduced by the varia-tion of a moving stage because of the reduction ofchirp ratio corresponding to the symmetric bendingof the fiber grating. The overlapped spectral region inthe reflection spectrum in Fig. 2 can provide the pos-sible operating wavelength for the microwave truetime delay. We could flexibly control the time delay ofa microwave signal carried over an optical signal in arange from 1 to 230 ps. We believe that the maximumdelay time can be further improved by optimizing thegrating parameters such as initial chirp ratio, disper-sion, apodization profile, and grating length. The ex-perimental resolution was estimated to be ±5.2 ps.We measured the time-delay variation of the pro-posed method by repeating the measurement of thetime delay and found no significant variation.

In conclusion, we experimentally demonstrated aflexible scheme for the continuous and multiple-element photonic microwave TTD device with hightunability based on a tunable fiber grating. We effec-tively controlled the time delay of a fiber gratingbased on the symmetrical bending technique withoutcenter wavelength shift, which was less than 0.02 nmover the whole tuning range. The fluctuation of re-flectivity of the proposed tunable CFBG over thewhole tuning range was less than 0.8 dB. We obvi-ously achieved the continuous operation of photonicTTD for operating microwave frequencies below10 GHz. We think that it is necessary to improve thequality of our TTD unit for high RF frequency up to40 GHz in terms of the group-delay ripple, reflectivityfluctuation, and system stability. The time delay of amicrowave signal carried over an optical signal wascontrolled in a range from 1 to 230 ps.

This work was supported by the Korea Science andEngineering Foundation through Quantum PhotonicScience Research Center at Hanyang University.

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