wideband true-time-delay unit for phased array beamforming using discrete-chirped fiber grating...

Wideband true-time-delay unit for phased arraybeamforming using discrete-chirped fiber grating prism

Yunqi Liu*, Jianping Yao, Jianliang Yang

Photonics Research Group, School of Electrical and Electronic Engineering, Nanyang Technological University,

Nanyang Avenue, Singapore 639798, Singapore

Received 8 November 2001; received in revised form 11 March 2002; accepted 23 April 2002

Abstract

A wideband true-time-delay (TTD) unit employing a novel fiber grating prism (FGP) for phased array beamforming

is proposed in this paper. The FGP consists of a single-mode fiber delay line, a chirped grating delay line and three

discrete fiber Bragg grating (FBG) delay lines. The first delay line is a length of single-mode fiber, which provides a fixed

time delay for all wavelengths. The second delay line, which employs a chirped grating, is used to provide small time

delays. The discrete FBG delay lines, which incorporate an array of 13 discrete FBGs, are used to provide large time

delays. A 5� 13 fiber-grating prism is constructed and experimented. The results show that the time delays produced bythe fiber grating delay lines are independent of the microwave frequency and agree well with the calculated time delays.

Based on the measured time delays, the radiation patterns of 5-element array antenna are calculated and analyzed. The

beampointing direction of the antenna is independent of the microwave frequency and can be controlled by tuning the

wavelength of the optical carrier. This 5� 13 TTD unit can provide phased array beamforming at microwave fre-

quencies up to 6 GHz. � 2002 Elsevier Science B.V. All rights reserved.

Keywords: Chirped grating; Fiber Bragg gratings; Fiber delay lines; True-time-delay

1. Introduction

High performance radars and communicationsystems require the use of phased-array antennas(PAAs). To overcome the squint problem, true-time-delay (TTD) units are usually used to keepthe beamforming direction stable at different mi-

crowave frequencies. Among different TTD tech-niques, photonic TTD system [1–4] has beenconsidered a promising technique for widebandphased array beamforming because of the advan-tages such as low loss, small size, lightweight andimmunity to electromagnetic interference. Pho-tonic technique can also provide the ability ofcontrolling several arrays using wavelength divi-sion multiplexing. Several configurations based onfiber grating delay lines have been proposedrecently [5–11]. One way to achieve widebandphotonic TTD beamforming is to use fiber grat-ing prisms (FGPs). The FGP design results in a

15 June 2002

Optics Communications 207 (2002) 177–187

www.elsevier.com/locate/optcom

*Corresponding author. Present address. School of Engi-

neering, City University, Northampton Square, London, EC1V

0HB, UK. Tel.: +44-20-70405060x3815; fax: +44-20-70408568.

E-mail address: [email protected] (Y. Liu).

0030-4018/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0030-4018 (02 )01529-8

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mail to: [email protected]

time-steered phased array processor, and thus of-fers the ability to achieve significant RF bandwidthin both transmit and receive operation withoutbeampointing or squint error. Two approaches areusually employed to construct an FGP beamste-erer, the one uses discrete fiber Bragg gratings(FBGs) [7,8] and the other one uses chirped grat-ings [7,9]. In the first approach, the prism is con-structed using a number of uniform FBGs withdifferent center wavelengths written at the differentlocations of the fiber delay lines. The spacingbetween any adjacent gratings determines the timedelay, and thus determines the beampointingdirection [7,8]. This approach assures the broad-band TTD operation, but only allows discretebeampointing. It can produce a minimum timedelay of about 9 ps and is suitable for beam-forming at microwave frequencies less than 3 GHz.In addition, it is difficult to control the positions ofthe FBGs during the delay line fabrication. Thesecond approach, which employs chirped gratings,allows continuous beamsteering and producessmaller time delays [7,9]. Therefore, chirped-grat-ing-based approach is suitable for TTD phasedarray beamforming at higher microwave frequen-cies. The difficulty related to these approaches isthat we need to provide a tunable multi-wave-length laser source with equally increased ordecreased wavelength spacing, or need to fabricatechirped fiber gratings with different chirp rate.In this paper, we propose and demonstrate a

TTD unit employing a novel FGP. The FGPconsists of a single-mode fiber delay line, a chirpedgrating delay line and three discrete FBG delaylines. The first delay line with a length of single-mode fiber provides a fixed time delay for allwavelengths. The second delay line employing achirped grating is used to provide small time de-lays that discrete FBG delay lines may not be ableto produce. The discrete FBG delay lines, whichconsist of an array of discrete FBGs, are used toprovide large time delays. The combination ofchirped and discrete fiber gratings ensures a highertime delay resolution without increasing the com-plexity of the TTD system. A 5� 13 TTD unitusing the proposed FGP is constructed and ex-perimented. The results show that the time delaysproduced by the delay lines are independent of the

microwave frequency and agree well with the cal-culated time delays. Based on the measured timedelays, the radiation patterns of a 5-element arrayantenna are calculated and analyzed. The beam-pointing direction of the antenna is independent ofthe microwave frequency and can be controlled bytuning the wavelength of the optical carrier. This5� 13 TTD unit can provide phased array beam-forming at microwave frequencies up to 6 GHz.To the best of our knowledge, this is the first ex-perimental demonstration of a TTD unit usingsingle-mode fiber delay line, a chirped grating de-lay line and discrete FBG delay lines that can workat the microwave frequencies up to 6 GHz.

2. Theory

The system configuration of the proposedN �M ðN ¼ 5; M ¼ 13Þ TTD unit is illustrated inFig. 1. The critical design parameters of the FGPare the minimum steer angle step hmin and theminimum achievable time delay Tmin. Analysis in[7] shows that hmin and Tmin can be expressed as:

hmin ¼ arcsinð4nfmdmin=cÞ; ð1ÞTmin ¼ 2ndmin=c; ð2Þwhere n ¼ 1:5 is the refractive index of the fibercore, dmin is the minimum grating spacing, fm is themaximum microwave frequency, and c is the free-space speed of light.A limit on the minimum attainable time delay is

set by the minimum distance allowed between twoadjacent discrete FBGs. Therefore discrete FBGarray may not be able to produce the required timedelays. From Eq. (1), we can also deduce thatdiscrete FBG delay lines cannot be used at highmicrowave frequencies in order to keep the mini-mum steering angle step. It is shown that toachieve a 10� angle resolution at 3 GHz, a maxi-mum delay step of about 9.09 ps is required. Thisis close to the practical lower limit on the timedelay step that can be produced by discrete FBGs[12]. In this paper, we use a chirped grating insteadof a series of discrete FBGs as the second delayline to produce smaller time delays and thus toincrease the maximum operating frequency. Thisdesign produces better beam steering resolution at

178 Y. Liu et al. / Optics Communications 207 (2002) 177–187

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a given microwave frequency. In addition, thesystem complexity is reduced and the difficulty inproducing closely spaced discrete FBGs is solved.In the TTD unit shown in Fig. 1, the conversion

from electrical to optical is realized using an elec-tro-optic modulator, to which a light source fromthe tunable laser is applied. The conversion fromoptical to electrical is achieved via high-speedphotodetectors. A polarization controller is in-serted between the source and the modulator tocontrol the polarization state. The modulated lightfeeds a group of N single-mode fibers through anequal-path 1:N power divider. The first fiber delayline consists of a length of single-mode fiber. Thelength of the first delay line is so short that thedispersion of the fiber itself can be negligible.Therefore the time delay is fixed and identical forall wavelengths. The second delay line includes achirped grating, and the third to the fifth fiberdelay lines include a spatially distributed array of13 FBGs. The different peak-reflection wave-lengths k1, ki ði ¼ 2; 3; . . . ; 6Þ, k7, kj ðj ¼ 8; 9; . . . ;12Þ, k13 of the 13 gratings and the chirped gratingare produced to be within the tuning range of thetunable laser source Dk. The optical signals are

sent to the delay lines through the circulators andreflected back by the gratings at different locationsof the delay lines. Different time delays are ob-tained in accordance with the particular gratingaddress. So for the grating delay lines, eachwavelength is associated with a different round-trip time delay. For the proposed unit, the timedelays for k7 are identical in all the four fibergrating delay lines and equal to the time delayproduced by the first delay line. The photodetectorrecovers the M individually time delayed signals.The signals are amplified and then sent to the Nantenna-radiator elements. The time delays can becontrolled by tuning the wavelength of the opticalcarrier, which leads to the beamscaning of theantenna systems.To ensure an acceptable signal to noise ratio at

the outputs of the photodetectors, the system in-sertion loss should be compensated. For the TTDunit shown in Fig. 1, the overall insertion loss,including the loss from the delay lines, the opticalcirculator, the 1:5 optical splitter, the electro-opticmodulator, the splices, the connectors, and thepolarization controller, is about 20 dB. In the ex-periment, an erbium-doped fiber amplifier

Fig. 1. A TTD unit employing a novel FGP.

Y. Liu et al. / Optics Communications 207 (2002) 177–187 179

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(EDFA) is incorporated into the system to com-pensate for the insertion loss.The far-field pattern of an N-element phased

array with the element spacing of dPAA is given by[13]

AF ðhÞ ¼ jf ðwÞj ¼ sinðNw=2ÞN sinðw=2Þ

��

��; ð3Þ

wðhÞ ¼ �bdPAA sin h þ a; ð4Þwhere a ¼ bn2Dd, b ¼ 2p=km, km is the wavelengthof the RF signal, h is the angle of radiation, Dd isgrating spacing difference between adjacent delaylines. For the proposed TTD unit, Dd is equal tothe center-to-center spacing between the locationsof the chirped grating delay line at k7 and otherwavelengths. The distance dPAA between two ad-jacent antenna elements must be chosen to avoidthe existence of more than one main lobe in theradiation pattern. Exactly one period of the arrayfactor appears in the visible region when the ele-ment spacing is one-half wavelength [13]. Eqs. (3)and (4) give the normalized array factor of thephased array antenna using the FGP as phaseshifters. From Eq. (3), the radiation pattern attainsthe maximum when wðhÞ ¼ 0, thereforedPAA sin h0 ¼ 2nDd: ð5ÞSo the beampointing angle corresponding to themain lobe of the array antenna, h0, can be ex-pressed in terms of the grating spacing difference,

sin h0 ¼2nDddPAA

ð6Þ

Eq. (6) states that the beampointing direction isdetermined by the grating spacing difference and isindependent of the microwave frequency. There-fore, the FGP is a TTD beamformer and is suit-able for wideband applications.

3. Experiment and results

A prism consisting of a length of single-modefiber delay line, a chirped grating delay line andthree discrete FBG delay lines, shown in Fig. 1, isconstructed and experimented. The quality of thechirped grating and the discrete FBGs affectsgreatly the performance of the TTD unit. In the

experiment, three FBG delay lines with 13 discreteFBGs are fabricated in hydrogen-loading radia-tion mode suppression single-mode photosensitivefiber using a 244-nm frequency-doubled argon ionlaser source. Phase masks with different periodsare used to produce the gratings with differentcenter wavelengths at different locations of thefiber delay lines. The locations of the gratings arecontrolled during the fabrication using a high-precision translation stage. The core and thecladding of the fiber are photosensitive and thegratings are written into both the core and the in-ner cladding. This feature results in the suppres-sion of the loss peak in transmission spectrum atthe wavelength several nanometers below thecentral reflection peak. Each grating has a lengthof 3 mm, a full-width at half-maximum (FWHM)bandwidth of about 0.6 nm and a peak reflectivityof higher than 99%. The transmission spectrum ofdelay line 3 (the first discrete FBG delay line) isshown in Fig. 2(a). The center wavelengths of thegratings from left to right are 1545.7, 1547.0,1548.2, 1549.3, 1550.2, 1551.5, 1553.1, 1554.9,1556.0, 1556.8, 1557.7, 1558.6 and 1559.8 nm. Itcan be seen that the FBGs have little ‘‘blue-wavelength’’ radiation loss. So the reflected lightfrom the short-wavelength gratings may not ex-perience such radiation loss. In the experiment, wecalibrate the time delays of all the fiber gratingdelay lines at k7 to be equal to the time delay of thefirst delay line. Table 1 shows the center-to-centerspacing between Grating 7 and the other gratingsin the three discrete FBG delay lines.From Table 1 we can calculate the mean spac-

ing differences between adjacent delay lines for agiven grating

DdGi ¼ f½ðdGiÞline 5 � ðdGiÞline 4þ ½ðdGiÞline 4 � ðdGiÞline 3g=2

¼ ½ðdGiÞline 5 � ðdGiÞline 3=2; ð7Þ

where DdGi is the mean spacing difference betweenadjacent delay lines for grating i ði ¼ 1; 2; . . . ;6; 8; . . . ; 13Þ, dGi is the center-to-center spacingbetween Grating 7 and the other gratings shown inTable 1. So the mean grating spacing differencesare )11.5, )9.5, )7.9 )6.2, )4.7, )2.6, 0, 3.0, 4.7,5.9, 7.2, 8.4 and 10.4 mm.


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Thanks to the wavelength selectivity of thegratings, different wavelengths are reflected fromdifferent gratings at different physical points alongthe fiber. In order to produce TTDs with signifi-cant smaller duration, a chirped grating is em-ployed in the second delay line to get the desiredsmall time delays. The chirped grating has a lengthof 50 mm with a broad bandwidth from 1528.8 to1561.1 nm and reflectivity of more than 95%. Thetransmission spectrum of the chirped fiber gratingis shown in Fig. 2(b). The minimum time delaystep which can be created using a chirped gratingdelay line is determined by the wavelength tuningstep and the chirp rate of the grating. If thewavelength tuning step of the tunable laser sourceis 0.01 nm, the chirped grating can produce �0.15ps time delay across the grating bandwidth.Therefore, the chirped grating can be suitable forbeamforming at microwave frequencies of higherthan 3 GHz, up to approximately 18 GHz. In thisexperiment, a section of the spectrum from 1545 to1560 nm is used, which corresponds to a length ofabout 25 mm of the chirped grating.

To compensate for the overall insertion loss inthe TTD unit, an EDFA is incorporated into thesystem. The amplified lightwave signal is split intofive channels by an optical ‘‘tree’’ splitter. The treesplitter consists of a 20:80 broadband coupler anda 1� 4 broadband coupler, as shown in Fig. 3. Inthe system, the amplitude errors of the tunable

Fig. 3. Configuration of the optic ‘‘tree’’ splitter.

Fig. 2. Transmission spectrum of the fiber grating delay lines. (a) Delay line 3 (discrete FBG delay line); (b) delay line 2 (chirped fiber

grating).

Table 1

Center-to-center spacing between Grating 7 and the other gratings

G1 G2 G3 G4 G5 G6

Line 2 (mm) )22.8 )18.9 )15.6 )12.3 )9.3 )5.1Line 3 (mm) )34.7 )28.0 )23.9 )18.9 )14.4 )8.1Line 4 (mm) )45.8 )38.0 )31.4 )24.8 )18.8 )10.4

G13 G12 G11 G10 G9 G8

Line 2 (mm) 21.3 17.4 15.0 12.2 9.6 6.0

Line 3 (mm) 31.4 25.5 21.9 17.9 14.3 9.3

Line 4 (mm) 41.4 33.6 28.8 23.4 18.6 12.0


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laser source arises during the wavelength tuning,and the variability between five different channelscan be compensated using equalization techniquewhich is often adopted in the standard PAAs.The time delays with the optical wavelength

tuned from k1 to k13 for microwave frequenciesranging from 1 to 10 GHz are measured. Fig. 4shows the experimental setup for the time delaymeasurement. The light from the tunable lasersource is sent to the electro-optic modulatorthrough the polarization controller. The micro-wave signal from the signal generator is applied tothe modulator. The modulated light is reflected byone of the fiber gratings of the delay line. The re-flected light is splitted into two beams. One is sentto an optical spectrum analyzer. The other is am-

plified by the EDFA and then converted to elec-trical signal by the high-speed photodetector. Thedetected signal is then sent to the oscilloscope(Tektronix CSA 8000). The microwave signalgenerated by the signal generator is also sent to theoscilloscope to compare the time delay with thedetected signal. Different time delays are measuredwhen the wavelength of the tunable laser source istuned.Table 2 shows the measured time delays of de-

lay line 3 at microwave frequencies of 2, 4, 6 and10 GHz. The results show that the delay lineprovides identical time delays for different micro-wave frequencies. The small deviations of the ex-perimental time delays away from the theoreticalcalculations could be attributed to uncertainties in

Fig. 4. Experimental setup for the time delay measurement.

Table 2

Measured time delays of delay line 3 at different microwave frequencies

Grating Grating space (mm) Theoretical time

delays (ps)

Experimental time delays (ps)

2 GHz 4 GHz 6 GHz 10 GHz

G1 )22.8 )228 )232 )226 )235 )233G2 )18.9 )189 )180 )193 )199 )195G3 )15.6 )156 )138 )139 )172 )169G4 )12.3 )123 )112 )120 )129 )128G5 )9.3 )93 )80 )92 )100 )96G6 )5.1 )51 )52 )49 )52 )53G7 0 0 0 0 0 0

G8 6.0 60 59 60 59 69

G9 9.6 96 94 94 92 94

G10 12.2 122 122 136 112 118

G11 15.0 150 144 150 141 147

G12 17.4 174 182 194 163 169

G13 21.3 213 203 219 211 210


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the positioning of the gratings during fabricationand the errors of time delay measurements. It isclearly seen that the time delays produced by thefiber gratings are independent of the microwavefrequencies. The measured time delays of the fourfiber grating delay lines with respect to the opticalwavelength at 6 GHz are plotted in Fig. 5. Thetime delay of the first delay line is also shown inthe figure. The measured dispersion rates for thefiber grating delay lines are 14.4, 31.9, 46.0, 61.9,respectively. The experimental results are consis-tent with the theoretical results calculated from themean grating spacing differences. The TTD unitusing the FGP is suitable for phased array beam-forming at the microwave frequencies up to 6GHz.The array factors of a 5-element antenna are

calculated from the grating spacing differencesmeasured in the experiments. To avoid the existence

Fig. 6. Radiation patterns of a 5-element phased array antenna steered by the TTD unit at 6 GHz.

Fig. 5. Measured time delays for the TTD unit operating at

6 GHz.


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of more than one main lobe in the radiation pat-terns, the fixed element spacing of the phased arrayantenna is set to be half the wavelength of the mi-crowave frequency [13]. For 6-GHz microwavefrequency, the element spacing is dPAA ¼ 25 mm.

Only nine grating wavelengths of the delay lines(from k3 to k11, corresponding to gratings3; 4; . . . ; 11) can be used for the beamforming. Theother four wavelengths lead to the wrong radiationdirections because of the long grating spacing with

Fig. 7. Radiation patterns of a 5-element phased array antenna steered by the TTD unit at 3 GHz.


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Grating 7. The radiation angles for the microwavefrequencies up to 6 GHz are �70:4�, �48:1�,�34:3�, �18:2�, 0�, 21:1�, 33:9�, 44:6� and 59:8�.The broadside radiation (0� beampoint direction)occurs at k7 (1553.1 nm), which is consistent withthe theoretical analysis. The radiation patternscalculated using Eqs. (3) and (4) for a 5-elementPAA steered by the proposed TTD unit at 6 GHz isshown in Fig. 6. The 5-element phased array

beamforming system provides beamforming at ninedifferent radiation angles at the frequencies up to 6GHz. For microwave frequency at 3 GHz, the ele-ment spacing is set to be dPAA ¼ 50 mm. Thebeampointing angles determined by the 13 wave-lengths are�43:4�,�34:8�,�28:1�,�21:8�,�16:4�,�9:0�, 0�, 10:4�, 16:2�, 20:6�, 25:6�, 30:3� and 38:4�.All the 13 wavelengths can be used thanks tothe large element spacing of the array antenna.

Fig. 8. Configuration of a 9-channel FGP extended from the 5-channel FGP.


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Fig. 7 shows the radiation patterns of a 5-ele-ment PAA steered by the proposed TTD unit at3 GHz.It should be noted that the grating position

errors will lead to a relative larger time delay errorat higher microwave frequencies because the timedelay difference between two adjacent gratingsrequired for a given beampointing angle is muchsmaller for higher microwave frequencies. In ad-dition, the dispersion property of the FBGs willaffect the time delay accuracy at high microwavefrequencies and should also be considered.It should also be noted that for the proposed

TTD unit, the main lobe of the radiation pattern isquite wide because only five radiation elements areused. As the element number increases, a muchnarrower main lobe would be achieved [13]. In theexperiment, the TTD unit consists of five delaylines has been demonstrated. The 5-channel FGPcan be easily extended to a 9-channel FGP byadopting the configuration shown in Fig. 8. In thefigure, the single-mode fiber delay line is used asthe fifth delay line. The fiber grating delay lines inchannel 1, 2, 3 and 4 use the same fiber gratings asthose in channel 9, 8, 7 and 6. But the orders of thegratings are reversed. For the FGP shown in Fig.1, we can reverse the order of the grating to avoidthe cladding mode loss. However, it is necessary touse the grating cladding mode suppression tech-nique for the FGP shown in Fig. 8. Based on theTTD unit, a beamforming system with a largenumber of delay lines can be constructed usingdifferent feed geometry [14].

4. Conclusion

In conclusion, a TTD unit for widebandphased-array beamforming has been proposed andexperimented. The TTD unit was constructed us-ing an FGP consisting of a single-mode fiber delayline, a chirped fiber grating delay line and threefiber delay lines with 13 discrete FBGs. The timedelays produced by the delay lines with the opticalwavelength tuned from k1 to k13 were measured.The results showed that the measured time delayswere independent of the microwave frequenciesand were consistent with the theoretical calcula-

tions. The radiation patterns of a 5-element arrayantenna were calculated and analyzed using themeasured time delays. The 5� 13 TTD unit issuitable for phased array beamforming at micro-wave frequencies up to about 6 GHz. To furtherincrease the operating frequency, the chirp rate ofthe chirped grating should be increased, and thecenter-to-center spacing of the discrete FBGs hasto be further reduced, which would lead to thesame problem as for the second delay line. Tosolve this problem, we may replace all the discreteFBGs by chirped gratins with different chirp rates.A TTD unit using chirped gratings that couldoperate at frequencies up to 18 GHz is now underinvestigation.

Acknowledgements

The authors wish to thank Dr. Chao Lu andMr. Jun Hong Ng for their support in this re-search. The help from Ms. Xin Guo, Dr. XiufengYang, Dr. Yong Wang and Mr. Zhenrong Wangon the experiments is also gratefully acknowl-edged.

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wideband true-time-delay unit for phased array beamforming using discrete-chirped fiber grating...

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