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

15 June 2002 Optics Communications 207 (2002) 177 187 www.elsevier.com/locate/optcom Wideband true-time-delay unit for phased array beamforming 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 by the 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 frequencies up to 6 GHz. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Chirped grating; Fiber Bragg gratings; Fiber delay lines; True-time-delay 1. Introduction * Corresponding author. Present address. School of Engineering, City University, Northampton Square, London, EC1V 0HB, UK. Tel.: +44-20-70405060x3815; fax: +44-20-70408568. E-mail address: Y.Liu@city.ac.uk (Y. Liu). High performance radars and communication systems require the use of phased-array antennas (PAAs). To overcome the squint problem, truetime-delay (TTD) units are usually used to keep the beamforming direction stable at different microwave frequencies. Among different TTD techniques, photonic TTD system [1 4] has been considered a promising technique for wideband phased array beamforming because of the advantages such as low loss, small size, lightweight and immunity to electromagnetic interference. Photonic technique can also provide the ability of controlling several arrays using wavelength division multiplexing. Several configurations based on fiber grating delay lines have been proposed recently [5 11]. One way to achieve wideband photonic TTD beamforming is to use fiber grating prisms (FGPs). The FGP design results in a 0030-4018/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S0030-4018(02)01529-8

178 Y. Liu et al. / Optics Communications 207 (2002) 177 187 time-steered phased array processor, and thus offers the ability to achieve significant RF bandwidth in both transmit and receive operation without beampointing or squint error. Two approaches are usually employed to construct an FGP beamsteerer, the one uses discrete fiber Bragg gratings (FBGs) [7,8] and the other one uses chirped gratings [7,9]. In the first approach, the prism is constructed using a number of uniform FBGs with different center wavelengths written at the different locations of the fiber delay lines. The spacing between any adjacent gratings determines the time delay, and thus determines the beampointing direction [7,8]. This approach assures the broadband TTD operation, but only allows discrete beampointing. It can produce a minimum time delay of about 9 ps and is suitable for beamforming at microwave frequencies less than 3 GHz. In addition, it is difficult to control the positions of the FBGs during the delay line fabrication. The second approach, which employs chirped gratings, allows continuous beamsteering and produces smaller time delays [7,9]. Therefore, chirped-grating-based approach is suitable for TTD phased array beamforming at higher microwave frequencies. The difficulty related to these approaches is that we need to provide a tunable multi-wavelength laser source with equally increased or decreased wavelength spacing, or need to fabricate chirped fiber gratings with different chirp rate. In this paper, we propose and demonstrate a TTD unit employing a novel FGP. The FGP consists of a single-mode fiber delay line, a chirped grating delay line and three discrete FBG delay lines. The first delay line with a length of singlemode fiber provides a fixed time delay for all wavelengths. The second delay line employing a chirped grating is used to provide small time delays that discrete FBG delay lines may not be able to produce. The discrete FBG delay lines, which consist of an array of discrete FBGs, are used to provide large time delays. The combination of chirped and discrete fiber gratings ensures a higher time delay resolution without increasing the complexity of the TTD system. A 5 13 TTD unit using the proposed FGP is constructed and experimented. The results show that the time delays produced by the 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 a 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 frequencies up to 6 GHz. To the best of our knowledge, this is the first experimental demonstration of a TTD unit using single-mode fiber delay line, a chirped grating delay line and discrete FBG delay lines that can work at the microwave frequencies up to 6 GHz. 2. Theory The system configuration of the proposed N M ðn ¼ 5; M ¼ 13Þ TTD unit is illustrated in Fig. 1. The critical design parameters of the FGP are the minimum steer angle step h min and the minimum achievable time delay T min. Analysis in [7] shows that h min and T min can be expressed as: h min ¼ arcsinð4nf m d min =cþ; ð1þ T min ¼ 2nd min =c; ð2þ where n ¼ 1:5 is the refractive index of the fiber core, d min is the minimum grating spacing, f m is the maximum microwave frequency, and c is the freespace speed of light. A limit on the minimum attainable time delay is set by the minimum distance allowed between two adjacent discrete FBGs. Therefore discrete FBG array may not be able to produce the required time delays. From Eq. (1), we can also deduce that discrete FBG delay lines cannot be used at high microwave frequencies in order to keep the minimum steering angle step. It is shown that to achieve a 10 angle resolution at 3 GHz, a maximum delay step of about 9.09 ps is required. This is close to the practical lower limit on the time delay step that can be produced by discrete FBGs [12]. In this paper, we use a chirped grating instead of a series of discrete FBGs as the second delay line to produce smaller time delays and thus to increase the maximum operating frequency. This design produces better beam steering resolution at

Y. Liu et al. / Optics Communications 207 (2002) 177 187 179 Fig. 1. A TTD unit employing a novel FGP. a given microwave frequency. In addition, the system complexity is reduced and the difficulty in producing closely spaced discrete FBGs is solved. In the TTD unit shown in Fig. 1, the conversion from electrical to optical is realized using an electro-optic modulator, to which a light source from the tunable laser is applied. The conversion from optical to electrical is achieved via high-speed photodetectors. A polarization controller is inserted between the source and the modulator to control the polarization state. The modulated light feeds a group of N single-mode fibers through an equal-path 1:N power divider. The first fiber delay line consists of a length of single-mode fiber. The length of the first delay line is so short that the dispersion of the fiber itself can be negligible. Therefore the time delay is fixed and identical for all wavelengths. The second delay line includes a chirped grating, and the third to the fifth fiber delay lines include a spatially distributed array of 13 FBGs. The different peak-reflection wavelengths k 1, k i ði ¼ 2; 3;...; 6Þ, k 7, k j ðj ¼ 8; 9;...; 12Þ, k 13 of the 13 gratings and the chirped grating are produced to be within the tuning range of the tunable laser source Dk. The optical signals are sent to the delay lines through the circulators and reflected back by the gratings at different locations of the delay lines. Different time delays are obtained in accordance with the particular grating address. So for the grating delay lines, each wavelength is associated with a different roundtrip time delay. For the proposed unit, the time delays for k 7 are identical in all the four fiber grating delay lines and equal to the time delay produced by the first delay line. The photodetector recovers the M individually time delayed signals. The signals are amplified and then sent to the N antenna-radiator elements. The time delays can be controlled by tuning the wavelength of the optical carrier, which leads to the beamscaning of the antenna systems. To ensure an acceptable signal to noise ratio at the outputs of the photodetectors, the system insertion loss should be compensated. For the TTD unit shown in Fig. 1, the overall insertion loss, including the loss from the delay lines, the optical circulator, the 1:5 optical splitter, the electro-optic modulator, the splices, the connectors, and the polarization controller, is about 20 db. In the experiment, an erbium-doped fiber amplifier

180 Y. Liu et al. / Optics Communications 207 (2002) 177 187 (EDFA) is incorporated into the system to compensate for the insertion loss. The far-field pattern of an N-element phased array with the element spacing of d PAA is given by [13] AF ðhþ ¼jf ðwþj ¼ sinðnw=2þ N sinðw=2þ ; ð3þ wðhþ ¼ bd PAA sin h þ a; ð4þ where a ¼ bn2dd, b ¼ 2p=k m, k m is the wavelength of the RF signal, h is the angle of radiation, Dd is grating spacing difference between adjacent delay lines. For the proposed TTD unit, Dd is equal to the center-to-center spacing between the locations of the chirped grating delay line at k 7 and other wavelengths. The distance d PAA between two adjacent antenna elements must be chosen to avoid the existence of more than one main lobe in the radiation pattern. Exactly one period of the array factor appears in the visible region when the element spacing is one-half wavelength [13]. Eqs. (3) and (4) give the normalized array factor of the phased array antenna using the FGP as phase shifters. From Eq. (3), the radiation pattern attains the maximum when wðhþ ¼0, therefore d PAA sin h 0 ¼ 2nDd: ð5þ So the beampointing angle corresponding to the main lobe of the array antenna, h 0, can be expressed in terms of the grating spacing difference, sin h 0 ¼ 2nDd ð6þ d PAA Eq. (6) states that the beampointing direction is determined by the grating spacing difference and is independent of the microwave frequency. Therefore, the FGP is a TTD beamformer and is suitable for wideband applications. 3. Experiment and results A prism consisting of a length of single-mode fiber delay line, a chirped grating delay line and three discrete FBG delay lines, shown in Fig. 1, is constructed and experimented. The quality of the chirped grating and the discrete FBGs affects greatly the performance of the TTD unit. In the experiment, three FBG delay lines with 13 discrete FBGs are fabricated in hydrogen-loading radiation mode suppression single-mode photosensitive fiber using a 244-nm frequency-doubled argon ion laser source. Phase masks with different periods are used to produce the gratings with different center wavelengths at different locations of the fiber delay lines. The locations of the gratings are controlled during the fabrication using a highprecision translation stage. The core and the cladding of the fiber are photosensitive and the gratings are written into both the core and the inner cladding. This feature results in the suppression of the loss peak in transmission spectrum at the wavelength several nanometers below the central reflection peak. Each grating has a length of 3 mm, a full-width at half-maximum (FWHM) bandwidth of about 0.6 nm and a peak reflectivity of higher than 99%. The transmission spectrum of delay line 3 (the first discrete FBG delay line) is shown in Fig. 2(a). The center wavelengths of the gratings 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. It can be seen that the FBGs have little bluewavelength radiation loss. So the reflected light from the short-wavelength gratings may not experience such radiation loss. In the experiment, we calibrate the time delays of all the fiber grating delay lines at k 7 to be equal to the time delay of the first delay line. Table 1 shows the center-to-center spacing between Grating 7 and the other gratings in the three discrete FBG delay lines. From Table 1 we can calculate the mean spacing differences between adjacent delay lines for a given grating Dd Gi ¼ f½ðd Gi Þ line 5 ðd Gi Þ line 4 Š þ½ðd Gi Þ line 4 ðd Gi Þ line 3 Šg=2 ¼½ðd Gi Þ line 5 ðd Gi Þ line 3 Š=2; ð7þ where Dd Gi is the mean spacing difference between adjacent delay lines for grating i ði ¼ 1; 2;...; 6; 8;...; 13Þ, d Gi is the center-to-center spacing between Grating 7 and the other gratings shown in Table 1. So the mean grating spacing differences are )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.

Y. Liu et al. / Optics Communications 207 (2002) 177 187 181 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 )5.1 Line 3 (mm) )34.7 )28.0 )23.9 )18.9 )14.4 )8.1 Line 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 Thanks to the wavelength selectivity of the gratings, different wavelengths are reflected from different gratings at different physical points along the fiber. In order to produce TTDs with significant smaller duration, a chirped grating is employed in the second delay line to get the desired small time delays. The chirped grating has a length of 50 mm with a broad bandwidth from 1528.8 to 1561.1 nm and reflectivity of more than 95%. The transmission spectrum of the chirped fiber grating is shown in Fig. 2(b). The minimum time delay step which can be created using a chirped grating delay line is determined by the wavelength tuning step and the chirp rate of the grating. If the wavelength tuning step of the tunable laser source is 0.01 nm, the chirped grating can produce 0.15 ps time delay across the grating bandwidth. Therefore, the chirped grating can be suitable for beamforming at microwave frequencies of higher than 3 GHz, up to approximately 18 GHz. In this experiment, a section of the spectrum from 1545 to 1560 nm is used, which corresponds to a length of about 25 mm of the chirped grating. To compensate for the overall insertion loss in the TTD unit, an EDFA is incorporated into the system. The amplified lightwave signal is split into five channels by an optical tree splitter. The tree splitter consists of a 20:80 broadband coupler and a1 4 broadband coupler, as shown in Fig. 3. In the system, the amplitude errors of the tunable Fig. 3. Configuration of the optic tree splitter.

182 Y. Liu et al. / Optics Communications 207 (2002) 177 187 laser source arises during the wavelength tuning, and the variability between five different channels can be compensated using equalization technique which is often adopted in the standard PAAs. The time delays with the optical wavelength tuned from k 1 to k 13 for microwave frequencies ranging from 1 to 10 GHz are measured. Fig. 4 shows the experimental setup for the time delay measurement. The light from the tunable laser source is sent to the electro-optic modulator through the polarization controller. The microwave signal from the signal generator is applied to the modulator. The modulated light is reflected by one of the fiber gratings of the delay line. The reflected light is splitted into two beams. One is sent to an optical spectrum analyzer. The other is amplified by the EDFA and then converted to electrical signal by the high-speed photodetector. The detected signal is then sent to the oscilloscope (Tektronix CSA 8000). The microwave signal generated by the signal generator is also sent to the oscilloscope to compare the time delay with the detected signal. Different time delays are measured when the wavelength of the tunable laser source is tuned. Table 2 shows the measured time delays of delay line 3 at microwave frequencies of 2, 4, 6 and 10 GHz. The results show that the delay line provides identical time delays for different microwave frequencies. The small deviations of the experimental time delays away from the theoretical calculations 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 Experimental time delays (ps) delays (ps) 2 GHz 4 GHz 6 GHz 10 GHz G1 )22.8 )228 )232 )226 )235 )233 G2 )18.9 )189 )180 )193 )199 )195 G3 )15.6 )156 )138 )139 )172 )169 G4 )12.3 )123 )112 )120 )129 )128 G5 )9.3 )93 )80 )92 )100 )96 G6 )5.1 )51 )52 )49 )52 )53 G7 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

Y. Liu et al. / Optics Communications 207 (2002) 177 187 183 Fig. 5. Measured time delays for the TTD unit operating at 6 GHz. the positioning of the gratings during fabrication and the errors of time delay measurements. It is clearly seen that the time delays produced by the fiber gratings are independent of the microwave frequencies. The measured time delays of the four fiber grating delay lines with respect to the optical wavelength at 6 GHz are plotted in Fig. 5. The time delay of the first delay line is also shown in the figure. The measured dispersion rates for the fiber grating delay lines are 14.4, 31.9, 46.0, 61.9, respectively. The experimental results are consistent with the theoretical results calculated from the mean grating spacing differences. The TTD unit using the FGP is suitable for phased array beamforming at the microwave frequencies up to 6 GHz. The array factors of a 5-element antenna are calculated from the grating spacing differences measured 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.

184 Y. Liu et al. / Optics Communications 207 (2002) 177 187 of more than one main lobe in the radiation patterns, the fixed element spacing of the phased array antenna is set to be half the wavelength of the microwave frequency [13]. For 6-GHz microwave frequency, the element spacing is d PAA ¼ 25 mm. Only nine grating wavelengths of the delay lines (from k 3 to k 11, corresponding to gratings 3; 4;...; 11) can be used for the beamforming. The other four wavelengths lead to the wrong radiation directions 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.

Y. Liu et al. / Optics Communications 207 (2002) 177 187 185 Grating 7. The radiation angles for the microwave frequencies 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 k 7 (1553.1 nm), which is consistent with the theoretical analysis. The radiation patterns calculated using Eqs. (3) and (4) for a 5-element PAA steered by the proposed TTD unit at 6 GHz is shown in Fig. 6. The 5-element phased array beamforming system provides beamforming at nine different radiation angles at the frequencies up to 6 GHz. For microwave frequency at 3 GHz, the element spacing is set to be d PAA ¼ 50 mm. The beampointing angles determined by the 13 wavelengths 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 to the large element spacing of the array antenna. Fig. 8. Configuration of a 9-channel FGP extended from the 5-channel FGP.

186 Y. Liu et al. / Optics Communications 207 (2002) 177 187 Fig. 7 shows the radiation patterns of a 5-element PAA steered by the proposed TTD unit at 3 GHz. It should be noted that the grating position errors will lead to a relative larger time delay error at higher microwave frequencies because the time delay difference between two adjacent gratings required for a given beampointing angle is much smaller for higher microwave frequencies. In addition, the dispersion property of the FBGs will affect the time delay accuracy at high microwave frequencies and should also be considered. It should also be noted that for the proposed TTD unit, the main lobe of the radiation pattern is quite wide because only five radiation elements are used. As the element number increases, a much narrower main lobe would be achieved [13]. In the experiment, the TTD unit consists of five delay lines has been demonstrated. The 5-channel FGP can be easily extended to a 9-channel FGP by adopting the configuration shown in Fig. 8. In the figure, the single-mode fiber delay line is used as the fifth delay line. The fiber grating delay lines in channel 1, 2, 3 and 4 use the same fiber gratings as those in channel 9, 8, 7 and 6. But the orders of the gratings are reversed. For the FGP shown in Fig. 1, we can reverse the order of the grating to avoid the cladding mode loss. However, it is necessary to use the grating cladding mode suppression technique for the FGP shown in Fig. 8. Based on the TTD unit, a beamforming system with a large number of delay lines can be constructed using different feed geometry [14]. 4. Conclusion In conclusion, a TTD unit for wideband phased-array beamforming has been proposed and experimented. The TTD unit was constructed using an FGP consisting of a single-mode fiber delay line, a chirped fiber grating delay line and three fiber delay lines with 13 discrete FBGs. The time delays produced by the delay lines with the optical wavelength tuned from k 1 to k 13 were measured. The results showed that the measured time delays were independent of the microwave frequencies and were consistent with the theoretical calculations. The radiation patterns of a 5-element array antenna were calculated and analyzed using the measured time delays. The 5 13 TTD unit is suitable for phased array beamforming at microwave frequencies up to about 6 GHz. To further increase the operating frequency, the chirp rate of the chirped grating should be increased, and the center-to-center spacing of the discrete FBGs has to be further reduced, which would lead to the same problem as for the second delay line. To solve this problem, we may replace all the discrete FBGs by chirped gratins with different chirp rates. A TTD unit using chirped gratings that could operate at frequencies up to 18 GHz is now under investigation. Acknowledgements The authors wish to thank Dr. Chao Lu and Mr. Jun Hong Ng for their support in this research. The help from Ms. Xin Guo, Dr. Xiufeng Yang, Dr. Yong Wang and Mr. Zhenrong Wang on the experiments is also gratefully acknowledged. References [1] W. Ng, A.A. Walston, G.L. Tangonan, J.J. Lee, I.L. Newberg, N. Bernstein, J. Lightwave Technol. 9 (1991) 1124. [2] H. Zmuda, E.N. Toughlian, Photonic Aspects of Modern Radar, Artech House, Boston, MA, 1994. [3] I. Frigies, A.J. Seeds, IEEE Trans. Microwave Theory Tech. 43 (1995) 2378. [4] D.T.K. Long, M.C. Wu, IEEE Photon. Technol. Lett. 8 (1996) 812. [5] G.A. Ball, W.H. Glenn, W.W. Morey, IEEE Photon. Technol. Lett. 6 (1994) 741. [6] A. Moloney, C. Edge, I. Bennion, Electron. Lett. 31 (1995) 1485. [7] R.A. Soref, Fiber Integrated Opt. 15 (1996) 325. [8] H. Zmuda, R.A. Soref, P. Payson, S. Johns, E.N. Toughlian, IEEE Photon. Technol. Lett. 9 (1997) 241. [9] J.L. Cruz, B. Ortega, M.V. Andres, B. Gimeno, D. Pastor, J. Capmany, L. Dong, Electron. Lett. 33 (1997) 545. [10] B. Ortega, J.L. Cruz, J. Capmany, M.V. Andres, D. Pastor, IEEE Trans. Microwave Theory Technol. 48 (2000) 1352. [11] Y. Liu, J. Yang, J.P. Yao, Wideband true-time-delay unit using discrete-chirped fiber Bragg grating prism, CLEO/

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