Waveleng-controlled hologram-waveguide modules for continuous beam-scanning in a phased-array antenna system Zhong Shi, Yongqiang Jiang, Brie Howley, Yihong Chen, Ray T. Chen Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas at Austin, 000 Burnet Road, PRC/MER/.606G, Austin, TX 78758 USA Abstract Waveleng-controlled true-time delay modules based on e dispersive hologram-waveguide are presented here to provide continuous beam-scanning for a X-band phased-array antenna system. The true-time delay modules operating in e 550nm region were fabricated wi continuously tunable time delays from 5ps to 64ps. All-optical waveleng conversion in e semiconductor optical amplifiers was proposed in e system to extend e beam-scanning scope from one dimension to two dimensions. The waveleng-controlled time delays were measured across e X-band (8-GHz) in e experiment. Keywords: Dispersion, waveleng conversion, semiconductor optical amplifier, true- time delay, phased-array antenna. Introduction The phased-array antenna offers various advantages such as accurate and quick beam-scanning wiout physical movement. It has numerous applications in bo military and civilian radar systems and in wireless communication systems. Optical true-time delay techniques have been extensively researched in e past few years because of eir promising potential applications in phased-array antenna systems. The main advantage of using e optical true-time delay technique in phased-array antenna systems is freedom from e frequency squint effect. This effect can cause change in e beam-scanning angle, an undesirable feature for e phased-array systems. The application of e optical true-time delay technique in phased-array antenna systems offers better performance compared wi e traditional electrical true-time delay technique. Optical true-time delay technique has much larger bandwid and is freedom from e electromagnetic interference, which is often a serious problem at needs to be considered in e design of an electrical scheme. Many different optical true-time delay schemes have been proposed and demonstrated in e past few years [,,3]. The acoustic-optic-based optical true-time delay scheme is compact and easy to integrate. However, its bandwid is quite limited. Chirped Bragg grating written into e fiber was used to achieve true-time delay for e antenna system. The problem associated wi chirped Bragg grating is at e time delay ripple is difficult to overcome. The tradeoff between e bandwid and e leng of e grating is also a problem. Chromatic dispersion in single-mode fiber was used to produce e desired time delay for e antenna systems. But e approach requires fiber leng up to several kilometers to get several picoseconds of time delay. The hologram-waveguide-based true-time-delay technique was researched because of its advantages of low cost, high packaging density, and simple fabrication process. The digitalized true-time-delay modules using hologram-waveguidebased technique were demonstrated [4, 5], in which time delay modules were designed for working in bo e 850 nm and e 550 nm waveleng regions. A quasi-analog true-time-delay scheme was proposed and demonstrated [6], an improvement compared wi digitalized schemes, but continuous time delays are available only in a narrow range. It is desirable for true-time-delay modules to provide continuously tunable time delays wi operating wavelengs in e 550 nm region, considering eir potential real applications. In is paper we present a waveleng-controlled scheme in hologram-waveguide true-time-delay modules to provide continuous beam scanning for a X-band phased-array antenna system. Time delays can be continuously tuned by varying e wavelengs in e various modules. The true-time-delay modules reported herein can provide continuous time delays from 5ps to 64ps. All-optical waveleng conversion in e 6 Photonic Integrated Systems, L. A. Eldada, A. R. Pirich, P. L. Repak, R. T. Chen, J. C. Chon, Editors, Proceedings of SPIE Vol. 4998 (003) 003 SPIE 077-786X/03/$5.00
semiconductor optical amplifier was proposed to extend e beam-scanning scope from one dimension to two dimensions [7]. The waveleng-controlled time delays were measured across e X-band in e experiment.. The principle and e structure The general configuration of e proposed scheme for waveleng-controlled time delay modules is shown in Fig.. The system has N discrete waveguide stripes wi icknesses of h, h, and h N, respectively. They are controlled independently by N discrete wavelengs,,, N. All waveguide stripes have e same hologram grating structure on e top surface in order to provide surface-normal fanouts. Therefore, all diffracted beams have e same diffraction angle θ when e incident beams have e same waveleng. The ( m + ) fanouts ( m : bouncing number in e substrate) between e j module and e ( j + ) module have a time delay T m+ at e incident center waveleng. If e incident waveleng for e j module is tuned from e center waveleng to +, e diffraction angle becomes θ + θ, and θ is determined by e dispersion equation, which in turn is derived from e Bragg condition [8]: θ = tan( θ / ) () This situation introduces a time delay τ between + for e following equation determines τ m+ : ( m + ) fanout in e module j. The mnh j τ m + =. () c cos( θ + θ ) cos( θ ) Here n is e refractive index of e glass substrate. When e waveleng is tuned from to + in e ( j + ) module, e diffracted beam + has a diffraction angle of θ + θ, and θ is also determined by Equation (). The time delay τ m+ between + for e ( m + ) fanout in e ( j + ) module is determined by e same equation as (), except at e substrate ickness h j is replaced by h j+. The total time delay T m+ for e ( m + ) fanout between adjacent modules can be continuously tuned according to e following equation depending on e waveleng tuning direction in e different modules: T τ + τ m+ ) T m+ T + ( τ + τ m ). (3) m+ ( m+ m+ m+ + For e time delays to be continuously tuned, e maximum time delay from e m fanout has to be equal to or larger an e minimum time delay from e ( m + ) fanout. In at case, e following equation should be satisfied: T m + τ + τ m) T ( τ + τ m+ ). (4) ( m m+ m+ Wi different delay combinations, e proposed scheme is capable of generating continuous time delays for RF beam scanning. Table shows e designed time delay results for e different fanouts at e different tuning wavelengs for e two strips having eight fanouts each, wi a ickness of 4.04mm and 4.48mm, respectively. Proc. of SPIE Vol. 4998 63
The structure of a waveleng-controlled two-dimensional phased-array antenna system is shown in Fig.. Wavelengs from e tunable lasers are intensity modulated and en sent to e two hologram-waveguide true-timedelay modules, respectively. The wavelengs,, togeer wi CW wavelengs are injected into e semiconductor optical amplifiers, in which waveleng conversion occurs. The time delay information in is transferred to via waveleng conversion whose principle will be explained in e next section. The wavelengs,, are injected into e time delay modules, which is shown in Fig.. The outputs from e time delay modules in e second stage are detected by photo detectors and en electrically amplified. The amplified electrical signals are eier analyzed by e network analyzer to determine e time delay information or connected to an antenna system to find e radiation patterns. By tuning e lasers in e first stage or in e second stage, e beamscanning direction in e elevation direction or in e azimu direction can be controlled separately. Figure : Configuration of e waveleng-controlled hologram-waveguide true-time delay modules 64 Proc. of SPIE Vol. 4998
Table : Designed true-time delay results at different incident wavelengs All-optical waveleng conversion refers to transferring data information from one waveleng to anoer waveleng wiout optical-electrical-optical (O-E-O) conversion. Since waveleng conversion is in e optical domain, e potential complexity of O-E-O conversion wiin e communication systems could be greatly reduced. There are several different schemes for realizing waveleng conversion, such as cross-gain modulation (XGM) in semiconductor optical amplifier, cross-phase modulation (XPM) in semiconductor optical amplifier, and four-wave mixing (FWM) in semiconductor optical amplifier [9], [0], []. The waveleng conversion based on cross-gain modulation in semiconductor optical amplifier (SOA) was used in our experiment because of its ease of operation. Cross-gain modulation uses e nonlinear gain saturation effect in semiconductor optical amplifier to transfer data information from modulated light to CW light. The working principle may be briefly summarized as follows: e incoming intensitymodulated light modulates e gain of e semiconductor optical amplifier via e gain saturation effect. The CW light at e desired output waveleng is modulated by gain variation, which is in turn caused by e modulated light. So e data information is transferred from modulated light to CW light via gain variation in e semiconductor optical amplifier. The detailed analysis of e working mechanism of cross-gain modulation in semiconductor optical amplifier can be found in [9]. The converted signal in is phase-inverted compared wi e original signal in. But phase-inversion does not affect e true-time delays between e modules since all e signals have phase-inversion. Proc. of SPIE Vol. 4998 65
Fig.. Structure of a waveleng-controlled two-dimensional phased-array antenna system 3. Experimental results A wide-waveleng tuning range is important to get e desired time delays since waveleng-tuning is used in e truetime delay modules. The gain bandwid of e semiconductor optical amplifier has to be wide enough to make possible a sufficient waveleng tuning range. The measured gain vs. waveleng is shown in Fig. 3. The 3-dB gain bandwid of semiconductor optical amplifiers is about 60nm, quite desirable for waveleng conversion wiin such a wide gain bandwid. The measured frequency response and e measurement setup of cross-gain modulation in semiconductor optical amplifier are shown in Fig. 4. The measurement was made using cross-gain modulation wi a counterpropagation scheme in which optical filters were eliminated in e setup. The electrical signals from e photo detectors were analyzed by a microwave spectrum analyzer (MSA). To verify e designed result of time delay, we used e experimental setup shown in Fig. 5 to measure e RF phase vs RF frequency curves. An HP network analyzer (850C) was used to provide an X-band RF signal and to measure e time delays in e experiment. A Santec tunable laser was modulated by a LiNbO 3 external modulator and e modulated signal was fed into one of e true-time delay modules h (4.04mm) or h (4.48mm). After desired time delays wiin e module, e output was fed into e high-speed p-i-n photodetector connected wi post-amplifier (PAs). The phase of e electrical signal from e post amplifier was measured by e HP network analyzer. We measured e 7 (m=6) fan-out for bo modules in e experiment. The results of RF phase vs RF frequency are shown in Fig. 6. The time delays obtained from e above measurement are 8ps at =580nm for h and =50nm for h, 38ps at =550nm for h =550nm for h, 48ps at =50nm for h and =580nm for h. 4. Conclusion 66 Proc. of SPIE Vol. 4998
Waveleng-controlled hologram-waveguide true-time delay modules were presented herein. The application to a twodimensional X-band phased-array antenna system is proposed in e paper. The waveleng-controlled time delays were measured across e X-band in e experiment. Acknowledgement The support from AFOSR and MDA is acknowledged. Special anks to Ms. Wanda Miller for her polishing e text of e whole paper. References. L. H. Gesell, R. E. Feinleib, J. L. Lafuse, T. M. Turpin, Acoustooptic control of time delays for array beam steering, Proc., SPIE, Optoelectronic Signal Processing for Phase-Array Antenna IV, Vol. 55, 994, pp. 94-04.. A. Molony, C. Edge, I. Bennionm, Fiber grating time delay element for phased array antennas, Electronics Letters, 3, 485, 995. 3. A. M. Levine, Use of fiber optical frequency and phase determining element in radar, Proc. of e 33 rd Annual Symposium on Frequency Control, IEEE, 436, 979. 4. R. Li, Z. Fu, R. Chen, High Packing Density.5-THz True-Time-Delay Lines Using Spatially Multiplexed Substrate Guided Waves in Conjunction wi Volume Holograms on a Single Substrate, J. L. T., Vol. 5, pp. 53-58, 997. 5. Y. Chen, R. T. Chen, A fully packaged true time delay modules for phase array antenna demonstration, IEEE Photonics Technology Letters, Vol. 4, pp. 75-77, Aug. 00. 6. Z. Fu, C. Zhou, R. T. Chen, Waveguide-hologram-based waveleng-multiplexed pseudoanalog true-time-delay module for wideband phased-array antennas, Applied Optics, Vol. 38, No. 4, pp. 3053-3059, May 999. 7. S. Yegnanarayanan, B. Jalali, Waveleng-selective true time delay for optical control of phased-array antenna, IEEE Photonics Technology Letter, Vol., No. 8, 049-05, August 00. 8. H. Kogelnik, Coupled wave eory for ick hologram gratings, The Bell System Technical Journal, Vol. 48, pp. 909-947, Nov. 969. 9. T. Durhuus, B. Mikelsen, C. Joergensen, S. L. Danielsen, K. E. Stubkjaer, All-optical waveleng conversion by semiconductor optical amplifiers, Vol. 4, No. 6, 94-954, June 996. 0. T. Durhuus, C. Joergensen, B. Mikkelsen, R. J. S. Pedersen, and K. E. Stubkjaer, All optical waveleng conversion by SOA s in a Mach-Zehnder configuration, IEEE Photonics Technol. Lett., Vol. 6, pp. 53-55, Jan. 994.. G. Grosskipf, R. Ludwig, R. Schnabel, and H. G. Weber, Frequency conversion wi semiconductor laser amplifiers for coherent optical frequency division switching, Proc. IOOC 89, Kobe, Japan, July 989, Paper 9C4-4 Proc. of SPIE Vol. 4998 67
8 6 4 Gain (db) 0 8 6 4 0 500 50 540 560 580 600 Waveleng (nm) Fig. 3. The measured results of gain vs. waveleng Fig. 4. The measured results of e frequency response 68 Proc. of SPIE Vol. 4998
Fig. 5. The setup for measuring time delays Fig. 6. The measured RF phase vs. RF frequencies Proc. of SPIE Vol. 4998 69