Fiber Bragg grating based four-bit optical beamformer

Transcription

1 Fiber Bragg grating based four-bit optical beaforer Sean Durrant a, Sergio Granieri a, Azad Siahakoun a, Bruce Black b a Departent of Physics and Optical Engineering b Departent of Electrical and Coputer Engineering Rose-Hulan Institute of Technology, 5500 Wabash Ave., Terre Haute, Indiana 47803, USA ABSTRACT In this paper we ipleent and characterize a two-channel optical prograable beaforer. The syste is designed to achieve four-bit resolution. The architecture of the prograable dispersion atrix is based on an array of four delay-lines each having two spliced fiber Bragg gratings. We have experientally investigated the optical signal processing perforance of the optical beaforer in receive and transit odes. Beapatterns for RF range MHz are presented along with the theoretical calculations. The ain lobe of the beapattern is shown to be independent of frequency for several target positions thus deonstrating a squint-free characteristic of this optical processor. Keywords: Optical signal processing, Phased-array antenna, Fiber Bragg gratings, True-tie delay beaforing. 1. INTRODUCTION Phased array antennas (PAA) along with beaforing systes are widely used in radar, satellite and obile counication systes. In such RF/icrowave systes, including both high-resolution PAA and signal processing electronics, true-tie delay (TTD) phase shifters are required. Individual transit/receive-eleent control allows the ipleentation of bea steering and shaping. However, it is desirable that antennas have wide scan angles, wide operational bandwidths and ultiple siultaneous independent beas. In conventional RF systes, TTD is achieved by switching between different lengths of electrical cable. However, these ipleentations tend to be bulky, heavy and susceptible to electroagnetic interference (EMI). Fiber-optic systes provide benefits since the beaforing syste becoes saller and lighter and can be controlled at high speed and it is iune to EMI. In the last decade, several optical techniques have been proposed for PAA control using fiber-optic systes [1]. In particular, systes using high-dispersion fibers [2-4] and fiber Bragg reflectors [5-7] for providing tie delays have been proposed and deonstrated. In this paper we design and experientally deonstrate a two-channel true-tie delay optical beaforer for controlling a phased array antenna using direct odulation of laser sources. The wideband processor has a resolution of four bits. In section 2 we describe in detail the theory of the beaforer for transit and receive odes. The beaforer easureents and analysis are discussed in section 3. Concluding rearks are given in section SYSTEM OPERATION A scheatic drawing of the beaforer architecture in the transit ode is shown in Figure 1(a). Two laser diodes provide optical carriers at λ 1 and λ 2 wavelengths (channels). An RF signal odulates the driving-currents (internal odulation) of the lasers which in turn odulates the optical carriers. RF cables and fiber patch-cords are set Corresponding author, phone: , fax: , e-ail: granieri@rose-hulan.edu

2 to ensure zero phase delay between the RF coponents after optical carriers are coupled. The odulated carrier feeds a prograable dispersion atrix (PDM), which perfors the true-tie delay processing. For each configuration of the PDM, λ 1 either leads or lags λ 2 by a tie-period. At the output of the PDM, after a proper phase difference is set, the channels are de-ultiplexed. Two broadband photo-detectors recover the delayed RF signals that are to feed the antenna eleents. Figure 1: Beaforer setup configuration for: (a) transit ode and (b) receive ode. The 4-bit architecture of the PDM, which is based on fiber Bragg grating arrays, is shown in figure 2. The N-bit version of two-channel architecture consists of an array of N delay lines. Each delay line is constructed by splicing two FBG. The center wavelength of each FBG atches one of the ultiplexed optical channels. The separation between Bragg reflectors is different for each delay line. Thus, the tie delay(s) between channels are proportional to these separations and are binary ultiples of the first line (iniu delay tie). The separation of two adjacent th gratings for the i line is given by L i =, (1) i 1 2 L1 where L1 is the iniu separation between gratings that corresponds to line 1. Using Eq. (1) the tie delay th provided by the i line can be written as neff Li τ i = 2, (2) c N where n eff is the effective refraction index of the fiber and c is the speed of light. Each of the 2 delay configurations of the PDM is an integer ultiple,, of iniu tie delay τ 1. The iniu tie delay associated with line 1 is directly related to the angular resolution and the iniu steering angle of any beaforer [8]. The steered angle θ is related to a characteristic paraeter of the PDM, that is τ 1, and a geoetrical paraeter of the antenna, the transit/receive eleent spacing Λ, by

3 c τ 1 θ = arcsin. (3) Λ It is iportant to note that in the event that the RF frequency changes instantaneously the angular direction of the radiated bea will not drift. This property is also known as squint free beaforing. Figure 2: Two-channel 4-bit prograable dispersion atrix. SW: optical switch, B: optical attenuator (balancer), OC: optical circulator. The scheatic for the receive-ode configuration is shown in figure 1(b). An incoing RF signal fro a target is received by the two antenna array eleents. The phase difference at the antenna eleents depends on the target angle. The received RF signal at each eleent independently odulates one of the optical carriers. The tie delay between the ultiplexed optical channels is corrected by the PDM and detected with a single photodetector. The output power of the photodetector is a function of the corrected phase difference between the RF signals ( K + φ ) P ( db) = 10log K cos 1 2, (4) where φ is the phase difference of RF signals and K 1, K 2 are the proportionality constants. Thus, the output power is related to the target angular position via this phase difference. When the PDM properly corrects for the phase difference at the antenna eleents, a axiu power will be detected for a given target position. 3. EXPERIMENTAL RESULTS Our experients are perfored using a 4-bit PDM that provides 16 optical delay configurations. These possible delays are: τ = τ1, = 0,1, K, 15. Two 8 W DFB seiconductor lasers odules, Ortel 1541A, with wavelengths of n and n provide the optical carriers. Optical sources are internally odulated with an RF signal of 17 db average power. The central wavelengths of the fiber Bragg gratings atch the wavelength of optical carriers. All the gratings have reflectivity fro 97.7% to 99.8% and FWHM of 0.3 n to 0.42 n. As in all delay lines the n channel lag the n channel, only asyetrical beapatterns can be received or

4 transitted. That is, the PDM can be prograed to steer only positive or negative angles fro antenna broadside [9]. The iniu separation between FBGs is L = corresponding to line 1. Separations in successive lines are: 0.36, 0.72 and The position of FBG is easured with accuracy of ±2. The theoretical iniu tie delay of the PDM, calculated fro Eq. (2), is τ = ns. Optical circulators are used to route the signals to/fro the array lines. In order to get all the delay configurations, two 1x2 and three 2x2 E-Tek optical switches are prograed. The switches are controlled by eans of an Agilent 34970A data acquisition unit. Note that because of the nuber of coponents is not the sae for each of the paths the optical signals will undergo different levels of attenuation depending on the delay configuration. Thus, as the insertion loss of PDM is path dependent, spurious power variations fro changes in constants K 1 and K 2 for different configurations can affect the easureents. In order to balance the optical power in the PDM, in-fiber air-gap echanical attenuators are placed in each of the paths that bypass the delay lines, as shown in Figure 2. Since the reflectivity of each FBG is different in each line, the optical power output of the PDM is different for each WDM channel. This unbalanced inline power is different for each delay line. To copensate for these changes one of the lasers is coupled to an EXFO FVA3100 prograable optical attenuator. After a coplete balancing, the insertion loss of the syste is approxiately 11.7 db. The ain sources of loss are connectors and optical circulators in the PDM. Figure 3: Beapatterns obtained in transit ode at frequencies 50 MHz, 100 MHz, and 200 MHz. 3.1 Tie delay easureents In order to characterize the tie delays produced by the syste, the configuration in Figure 1(a) is used. The laser diodes are fed with an RF signal out of port #1 of a vector network analyzer. Port #2 of network analyzer detects the RF signal out of the Thorlabs DC400FC photodetectors, one at a tie. Therefore, the phase and agnitude of s- paraeter s 21 are easured for each channel. For a given frequency, the tie delay introduced by the PDM can be obtained by subtracting the phase values associated with the paraeter S 21 of the two channels according with φ = φ + 0 2π ν RF τ, where φ is the phase difference for the th delay configuration, ν RF is the center frequency

5 of RF, and φ 0 is an arbitrary constant phase shift. Experiental data is obtained by sweeping the RF signal of the vector network analyzer between 100 MHz and 1 GHz. The tie delay is calculated fro the slope of the linear fit to the data. Calculated tie delays have errors of less than 1% copared with the expected values. These discrepancies can be attributed to grating spacing errors and phase noise. 3.2 Beapattern characterization For beapattern easureent and characterization of transit ode, the configuration shown in Figure 1(a) is slightly odified. The output signal fro the PDM is detected by a single photodetector and a Tektronix 2782 RF Spectru Analyzer. Thus, the RF phase shift created by the PDM is transfored to power variations according to Eq. (4). In order to obtain the transit beapattern for a desired signal frequency, the PDM is stepped through each of the 16 possible tie delay configurations. Figure 3 shows the experiental and theoretical beapatterns for signals at frequencies 50 MHz, 100 MHz, and 200 MHz. These figures illustrate that the beaforer angular steering range is 0 o (broadside) to 62 o. Theoretical curves are calculated fro Eqs. (3) and (4). Figure 4: Beapatterns obtained for an incoing RF signal of 40 MHz in receive ode for: no delay, 4τ 1, 8τ 1, and 15τ 1. These cases correspond with optical carriers passing though: none of the delay lines, third delay line, forth delay line, and all delay lines respectively. For the receive ode easureents the incoing RF signal fro a target is siulated by a HP odel 83650A RF synthesizer. The output of the RF synthesizer is split and sent to the laser diodes shown in Figure 1(b). To siulate the phase difference between two antenna eleents due to a non-broadside target, an RF phase shifter is introduced before one of the laser inputs. Beapatterns are constructed by sweeping the RF phase shifter and easuring the RF output power fro a single photodetector using the RF spectru analyzer. Figure 4 illustrates typical bea-patterns easured for an incoing RF signal at 40 MHz and four different PDM delay configurations: =0, 4, 8 and 15. Angular position of the ain lobe (arked by an arrow) changes with the switch configuration according with Eq.(3). Note that beapatterns for =0 and =15 appear to overlap because the phase difference introduced by the PDM for

6 =15 at this particular frequency is approxiately 2π. Figure 5 shows bea-pattern easureents for three RF signals of 40 MHz, 70 MHz, and 100 MHz. Figure 6(a) corresponds to zero delay, i.e., the optical carriers do not pass through any of the FBG array lines. Hence the target is detected at the broadside position. In figure 6(b) through (d) the carriers pass through different cobinations of delay lines in order to provide the desired tie delay. In these cases, the PDM is detecting the target at angular positions of 13.62º, 28.11º, and 62.07º. For all the above figures, the experiental data are fit to Eq. (4) by using a nonlinear curve-fit routine. Notice that Eq. (3) suggests that the antenna steering-angle is a function of phase delay and separation of antenna eleents but it is independent of the transitted/received signal frequency. The position of the ain lobe in Figure 5 is shown to be independent of frequencies between MHz deonstrating the squint-free characteristic of the syste. Figure 5: Beapatterns easureent for receive ode at 40 MHz (square), 70 MHz (circle) and 100 MHz (triangle) frequencies for target at angular position of: (a) 0º (broadside), (b) 13.62º, (c) 28.11º, and (d) 62.06º. The ain-lobe angular positions, arked by arrows, are independent of RF operating frequencies which confirs a squint-free beaforing. 4. CONCLUSIONS We have constructed and characterized a 2-channel 4-bit optical bea-forer syste operating at 1310n using a fiber-optic prograable dispersion atrix. The working prototype is used to deonstrate the accuracy of generated bea-patterns in the transit and receive odes for the RF range of MHz. Beapatterns are obtained for

7 steering angles up to 62. Theoretical and experiental values for ain-lobes for different delay configurations are in good agreeent. Our optical beaforer exhibits squint-free radiation pattern in RF band of MHz. Measureents are liited to 100 MHz in order to obtain a reasonable nuber of data points per bea lobe. Otherwise, the upper liit is set by the 2 GHz bandwidth of the photodiode. The optical processor can be easily scaled for a large nuber of antenna eleents. Additional channels require only stacking up the sae nuber of FBGs on each array. While iproving syste resolution to N-bit will require N arrays of FBGs. ACKNOWLEDGMENTS The authors would like to acknowledge Daniel Purdy of Office of Naval Research for his support of this project under the contract nuber N REFERENCES 1. N. Riza, Ed., Selected papers on photonic control systes for phased array antennas MS-136, SPIE Milestone Series, Washington, R. Soref, Optical dispersion technique for tie-delay bea steering, Applied Optics, 31, , R. Esan, M. Frankel, J. L. Dexter, L. Goldberg, M. G. Parent, D. Stilwell and D. G. Cooper, Fiber-optic pris true tie-delay antenna feed, IEEE Photon. Technol. Lett., 5, , S. T. Johns, D. A. Norton, C. W. Keefer, R. Erdann and R. Soref, Variable tie delay of icrowave signals using high dispersion fibre, Electronics Letters, 29, , R. Soref, Fiber grating pris for true tie delay beasteering, Fiber and Integrated optics, 15, , H. Zuda, R. Soref, P. Payson, S. Johns and E. Toughlian, Photonic beaforer for phased array antennas using a fiber grating pris, IEEE Photon. Technol. Lett., 9, , D. Tong, and M. Wu, Prograable dispersion atrix using Bragg gratings for optically controlled phased array antennas, IEEE Photon. Technol. Lett., 10, , B. Black, A. Siahakoun, L. Slaybaugh, J. Chestnut, and D. Thelen, Coponent-level siulation of optical beaforing systes Proc. SPIE , S. Palit, S. Granieri, A. Siahakoun, B. Black, K. Johnson and J. Chestnut, Binary and Ternary architectures for a two-channel optical receive beaforer Technical Digest of International topical eeting on icrowave photonics (MWP), 273-6, IEICE Electronics Society, Awaji, 2002