* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download OFC2016-TTD-v3 - School of Electrical Engineering and
Ellipsometry wikipedia , lookup
Dispersion staining wikipedia , lookup
Optical aberration wikipedia , lookup
Nonimaging optics wikipedia , lookup
Retroreflector wikipedia , lookup
X-ray fluorescence wikipedia , lookup
Magnetic circular dichroism wikipedia , lookup
Photonic laser thruster wikipedia , lookup
Ultraviolet–visible spectroscopy wikipedia , lookup
Interferometry wikipedia , lookup
Nonlinear optics wikipedia , lookup
Optical coherence tomography wikipedia , lookup
Photon scanning microscopy wikipedia , lookup
3D optical data storage wikipedia , lookup
Optical rogue waves wikipedia , lookup
Harold Hopkins (physicist) wikipedia , lookup
Optical amplifier wikipedia , lookup
Optical tweezers wikipedia , lookup
Silicon photonics wikipedia , lookup
Photonic True-Time Delay Beamforming Using a SwitchControlled Wavelength-Dependent Recirculating Loop Jiejun Zhang and Jianping Yao Microwave Photonics Research Laboratory, School of Electrical Engineering and Computer Science, University of Ottawa, Ontario K1N 6N5, Canada [email protected] Abstract: A photonic true-time delay beamforming network using a switch-controlled recirculating wavelength-dependent dispersive loop is proposed and demonstrated. By controlling the number of round trips in the dispersive loop, tunable progressive time delays are achieved. OCIS codes: (280.5110) Phased-array radar; (230.2035) Dispersion compensation devices; (060.5625) Radio frequency photonics. 1. Introduction Phased array antenna (PAA) plays a key role in modern radar systems as it can provide beam steering at a high speed without mechanical movement with ultra-high directivity. A beamforming network is required to feed a phase array antenna, which can be implemented using either phase shifters or true time delay lines. The advantage of using true time delay lines is the beam is squint free, thus it is suitable for broadband applications. The time delays can be realized using electronic delay lines, but with small bandwidth, large size and high loss. Photonics is considered a potential solution to implement true-time delays with large bandwidth, small size and low loss [1]. In the past few years, photonic true-time delay beamforming networks have been proposed and demonstrated based on free space optics [2], fiber optics such as fiber Bragg gratings (FBGs) [3] and linearly chirped fiber Bragg gratings (LCFBGs) [4], and integrated optics such as an integrated movable prism group [5] and a tunable optical filter to mimic the spectral response of a delay line [6]. A free space optics based beamforming network is very bulky [2]. A fiberoptics based beamforming network has a smaller size [3, 4], but a tunable laser source is required, making the system costly. An integrated optics based beamforming network has an ultra-smaller size, but only small the time delays are achievable due to the small size of the chip [5, 6]. In this work, we introduce a fiber-optic true-time delay beamforming network using a switch-controlled recirculating wavelength-dependent dispersive loop incorporating an LCFBG [7]. A laser array instead of a tunable laser source is used. Thus, the system is simpler. A microwave signal to be radiated to the free space is modulated on the multiple wavelengths from the laser array, which are sent to a switch-controlled recirculating wavelengthdependent dispersive loop [7]. Since the optical signals at different wavelengths are reflected from different locations of the LCFBG, different time delays are achieved. The tuning of the time delays are achieved by controlling the number of round trips in the loop, which is done by using a 2x2 optical switch. The proposed truetime delay beamforming network is experimentally demonstrated. A four true-time delay beamforming network with a time delay difference of 2.5 ns per round trip with a maximum round trip of 10 is demonstrated. 2. Principle The schematic diagram of the proposed true-time delay beamforming network is shown in Fig. 1(a). The optical outputs from four laser diodes (LDs) with different wavelengths of 1 to 4 are combined at a wavelength-division multiplexer (WDM) and sent to a Mach-Zehnder modulator (MZM), where a microwave signal, generated by an electrical arbitrary waveform generator (AWG) to be radiated to free space, is modulated on the four wavelengths. The optical signals at different wavelengths at the output of the MZM is then launched into a switch-controlled wavelength-dependent recirculating loop through a 2x2 optical switch. In the loop, an LCFBG is adopted via an optical circulator (OC) to provide different time delays for different wavelengths. An erbium-doped fiber amplifier (EDFA) is also incorporated to compensate for the loss in the loop. Thus, that the optical signals can recirculate for multiple round trips in the loop. The number of round trips is controlled by the 2x2 optical switch. At the output of the loop, a second WDM is used to separate the time-delayed optical signals, which are converted to time delayed microwave signals at the photodetectors (PDs). If the optical switch is configured to make an optical signal at i recirculates in the dispersive loop multiple time, a series of time delayed replicas of the optical signal will be generated, with a time interval between two adjacent replicas determined by the wavelength-dependent time delay of the loop, given by Ti t0 i r (1) where t0 is the fixed time delay of the loop excluding the LCFBG; is the dispersion coefficient of the LCFBG and r is a reference wavelength. Now, we consider the time delay difference between two wavelengths i and i 1 recirculating in the loop after N round trips (2) t NTi 1 NTi N i 1 i N where is the wavelength spacing between two adjacent wavelengths. Note that the optical wavelengths are uniformly spaced. The time delay of an optical signal carried by a different wavelength relative to 1 as the round trip number N increases is shown in Fig. 1(b). It can be seen that a time delay difference t given by (2) can be achieved between the microwave signals carried by any two adjacent optical wavelengths. In our system, it is also noted that as N increases, an increasing amount of time delay can be achieved, which is required to scan the beam at different directions. The beam pointing angle can be tuned by changing the number of round trips, while the maximum number of round trips determines the largest beam pointing direction. N=0 LCFBG LD3 LD4 3 1 1 EDFA OC WDM LD2 AWG 2 WDM LD1 2 3 1 MZM 2x2 Switch 4 Antenna array 2 3 4 N=1 PD N=2 t 0 1 2 3 4 t t 2 PD t 3 N=3 PD PD Relative time delay to Channel 1 (a) (b) Fig. 1. (a) Schematic diagram of the true-time delay beamforming network. LD: laser diode; WDM: wavelength-division multiplexer; AWG: arbitrary waveform generator; MZM: Mach-Zehnder modulator; EDFA: erbium-doped fiber amplifier; OC: optical circulator; PD: photodetector; (b) the time delay of the signal in each channel relative to channel 1 as the number of round trips N increases. 3. Experiment A total of four channels are used for the proof-of-concept experiment. The central wavelengths of the four LDs are 1548.9, 1549.9, 1550.9 and 1551.9 nm, with a uniform wavelength spacing of 1 nm. The LCFBG is fabricated to have a dispersion coefficient of 2500 ps/nm within its 4-nm reflection band centered at 1550.5 nm. The spectrum of the optical carrier and the spectral response of the LCFBG are given in Fig. 2. It can be calculated that 2.5 ns , i.e., a true-time delay of 2.5 ns can be achieved between adjacent channels when the signal recirculates in the loop for one round trip. The AWG, which has a sampling rate of 10 Gb/s, is configured to generate a microwave signal to be transmitted. A 2x2 coupler is used instead of the 2x2 switch to simplify the experiment and to study the round-trip-by-round-trip behavior of the system. 0 -10 100 (a) Reflectivity (%) Power (dBm) -30 -40 -50 -60 10 80 8 60 6 40 4 20 2 Group Delay (ns) -20 (b) -70 -80 1548 1549 1550 1551 Wavelength (nm) 1552 0 0 1547 1548 1549 1550 1551 1552 1553 1554 Wavelength (nm) Fig. 2. (a) Optical spectrum of the laser array, and (b) the magnitude and group delay responses of the LCFBG. To demonstrate the operation of the system, an electrical pulse with a temporal width of 1 ns is generated by the AWG. The waveforms detected at the outputs of the four PDs following the second WDM are shown in Fig. 3. Multiple time-delayed replicas of the electrical pulse can be detected at the output of each channel. In addition, the pulses from the four channels overlap in time only for N=0 and an increasing time shifts between the time delayed pulses can be observed for N>0, indicating that a true-time delay has been achieved between the channels when the pulse starts to recirculate in the loop. A reduced amplitude can be seen as N increases, as the EDFA cannot be configured to fully compensate for the loop loss to avoid lasing. Nevertheless, the pulses can still recirculates for more than 10 round trips before being imbedded in noise. The reduced amplitude may be compensated by a power amplifier that is commonly used in phased array antenna. More detailed waveforms are shown in Fig. 4 for different number of round trips of 0, 3 and 6. The time spacing between two pulses from adjacent channels are measured to be 7.6 and 15.1 ns for N=3 and N=6, respectively, which agree well with the theoretical values of 7.5 and 15 ns. Voltage (mV) 2 N=1 N=0 1 2 3 4 1.5 N=2 N=3 1 N=4 N=5 0.5 0 -0.5 0 0.5 1 N=6 1.5 2 Time (ms) N=7 2.5 3 3.5 Fig. 3. Measured time-delayed signals at the outputs of the four PDs when an electrical pulse is applied to the MZM. 1 2 3 4 Voltage (mV) 0.8 Voltage (mV) (a) 1.5 1 0.5 0 -40 -20 0 20 Time offset (ns) 40 1 2 3 4 (b) 0.6 0.3 0.4 0.2 0 -40 1 2 3 4 (c) 0.25 Voltage (mV) 2 0.2 0.15 0.1 0.05 0 -20 0 20 Time offset (ns) 40 -40 -20 0 20 Time offset (ns) 40 Fig. 4. The detailed waveforms at the output of the true-time delay beamforming network. (a) N=0; (b) N=3 and (c) N=6. 4. Conclusion We have proposed and demonstrated a photonic true-time delay beamforming network based on a switch-controlled recirculating wavelength-dependent dispersive loop. By controlling the number of round trips that the optical signals recirculate in the dispersive loop, different time delays were achieved. A four-channel true-time delay beamforming network with a time delay difference of 2.5 ns per round trip and a maximum number of round trips of 10 was demonstrated. 5. References [1] J. Yao, "Microwave photonics," J. Lightwave Technol. 27(3), 314-335 (2009). [2] D. Dolfi, P. Joffre, J. Antoine, J.-P. Huignard, D. Philippet, and P. Granger, "Experimental demonstration of a phased-array antenna optically controlled with phase and time delays," Appl. Opt. 35(26), 5293-5300 (1996). [3] Y. Liu, J. Yao, and J. Yang, "Wideband true-time-delay unit for phased array beamforming using discrete-chirped fiber grating prism," Opt. Comm. 207(1), 177-187 (2002). [4] Y. Liu, J. Yang, and J. Yao, "Continuous true-time-delay beamforming for phased array antenna using a tunable chirped fiber grating delay line," IEEE Photonics Technol. Lett. 14(8), 1172-1174 (2002). [5] J. Wang, P. Hou, H. Cai, J. Sun, S. Wang, L. Wang, and F. Yang, "Continuous angle steering of an optically-controlled phased array antenna based on differential true time delay constituted by micro-optical components," Opt. Express 23(7), 9432-9439 (2015). [6] R. Bonjour, S. A. Gebrewold, D. Hillerkuss, C. Hafner, and J. Leuthold, "Continuously tunable true-time delays with ultra-low settling time," Opt. Express 23(5), 6952-6964 (2015). [7] J. Zhang and J. Yao, "Time-stretched sampling of a fast microwave waveform based on the repetitive use of a linearly chirped fiber Bragg grating in a dispersive loop," Optica 1(2), 64-69 (2014).