IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 10, MAY 15, 2012
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Ultra-Wideband Microwave-Photonic Noise Radar Based on Optical Waveform Generation Daniel Grodensky, Daniel Kravitz, and Avi Zadok
Abstract— A microwave-photonic ultra-wideband (UWB) noise radar system is proposed and demonstrated. The system brings together photonic generation of UWB waveforms and fiber-optic distribution. The UWB noise waveform is generated using the amplified spontaneous emission that is associated with either stimulated Brillouin scattering in a standard optical fiber, or with erbium-doped fiber amplification. Waveforms of more than 1-GHz bandwidth and arbitrary radio-frequency carriers are generated, and distributed over 10-km fiber to a remote antenna unit. Laboratory experiments demonstrate ranging measurements with 10-cm resolution. Index Terms— Antenna remoting, broadband radar, microwave-photonics, nonlinear fiber-optics, stimulated Brillouin scattering, ultra-wideband communication.
I. I NTRODUCTION
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LTRA-WIDEBAND (UWB) radio is a transmission technology that is based on several GHz-wide waveforms [1]. UWB signals are free of sine-wave carriers, and their power spectral density is low. These characteristics provide UWB radio with unique advantages: improved immunity to multipath fading, increased ranging resolution, large tolerance to interfering legacy systems, enhanced ability for penetrating obstacles, and low electronic processing complexity at the receiver [1]. UWB technology is considered attractive for highspeed internet access, ground-penetrating and border-security radars, sensor networks, high accuracy localization, precision navigation, and through-the-wall imaging [1]. In many scenarios, UWB radio-based systems would need to extend their wireless transmission range by other distribution means. As the frequencies of UWB signals continue to increase, optical fibers become the preferable distribution medium. In particular, fiber-optic distribution of radar signals can replace bulky and lossy electrical cables needed for antenna remoting: the separation of the antenna and frontend elements of a radar system from the central office or unit, where waveforms are generated, processed and interpreted [2]. With radio-over-fiber (RoF) integration on the horizon, the generation of the UWB pulses by photonic methods becomes attractive. Microwave-photonic generation techniques can
Manuscript received December 6, 2011; revised February 9, 2012; accepted February 16, 2012. Date of publication February 24, 2012; date of current version April 18, 2012. This work was supported in part by the German-Israeli Foundation under Grant I-2219-1978.10/2009. The authors are with the Faculty of Engineering, Bar-Ilan University, RamatGan 52900, Israel (e-mail:
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2012.2188889
offer flexible tuning of high-frequency pulse shapes, inherent immunity to electromagnetic interference, and parallel processing via wavelength division multiplexing [3]. Driven by the promises of integration and flexibility, much research effort has been dedicated to the photonic generation of UWB waveforms in recent years [4]-[5]. Most microwave-photonic UWB generation schemes reported to-date had targeted impulse radio implementations: the transmission of tailored short pulses and their subsequent coherent detection. However, impulse radio requires elaborate pulse shaping and a detailed knowledge of the communication channel properties [1], [6]. A possible alternative is the transmission of modulated, broadband noise waveforms [7]-[9]. In particular, the use of noise instead of deterministic waveforms in UWB radar systems provides better immunity to interception and jamming [8]. UWB noise can be readily generated using electrical techniques, however optical methods are nonetheless appealing as part of a RoF integrated system. Although the separate potential advantages of UWB noise radars and of microwave-photonic UWB generation are largely recognized [3]-[5], [7]-[9], the two techniques had not yet been brought together, to the best of our knowledge. Note that fiber-optic correlation processing had already been employed in an UWB noise radar system [10]. In this letter, we propose and demonstrate an UWB-noise, microwave-photonic radar system that combines photonic waveform generation and fiber-optic antenna remoting. Noise is generated based on the amplified spontaneous emission (ASE) associated with the optical gain of stimulated Brillouin scattering (SBS), and of erbium-doped fiber amplifiers (EDFAs). A 10-km-long, two-way analog link is used in antenna-remoting. A ranging resolution of 10 cm is demonstrated in a laboratory experiment. II. P RINCIPLE OF O PERATION A simplified block diagram of the proposed microwavephotonic UWB noise radar system is shown in Fig. 1. A source of optical noise in a central office unit is coupled together with a continuous-wave (CW) local oscillator (LO), which is spectrally detuned form the noise central frequency by a few GHz. The combined optical field is split in two branches. The light in one path is detected by a broadband photo-diode within the central office. Upon detection, beating with the LO downconverts the optical noise to the radio-frequency (RF) domain. Heterodyne detection is necessary to spectrally separate the interference term of interest from base-band fluctuations due
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 10, MAY 15, 2012
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to the optical noise intensity. In addition, heterodyne detection provides freedom to select a central radio frequency of choice. The resulting electrical noise is sampled and stored as a reference. Light in the other branch is transmitted over fiber towards a remote antenna unit, where heterodyne beating is used once again to generate a replica of RF noise that is stored at the central office. The resulting electrical waveform is amplified and transmitted by a directive antenna towards potential targets. Reflected signals are collected by a second directive antenna, amplified and carried back over fiber towards the central office via direct modulation of a distributed feedback (DFB) laser diode source. The received waveform is detected, sampled and correlated against the stored reference using digital signal processing. The distances of targets are recovered based on the timing of correlation peaks. The system can be calibrated by initially placing a known target at a known distance. A. Optical Noise Sources The ASEs of both SBS and EDFAs were used as sources of optical noise in the proposed system. In SBS, a strong pump wave and a weaker, counter propagating probe wave optically interfere to generate, through electrostriction, a traveling longitudinal acoustic wave. The acoustic wave couples these optical waves to each other [11]. In the absence of a seed input probe, SBS could be initiated by thermally-excited acoustic vibrations [11]. The naturally occurring vibrations scatter a fraction of the incident pump into a preliminary probe, which is further amplified. In this scenario SBS acts as a generator of ASE. The frequency of SBS-ASE is lower than that of the incident pump by the Brillouin shift B ∼ 2π·11 GHz. UWB generation requires a substantial spectral broadening of the inherently narrowband SBS process, that is only ∼ 2π·30 MHz-wide for a CW pump. Over the last five years, direct modulation of laser diodes that are used as pump waves is routinely employed to broaden the SBS bandwidth to several GHz [12]. It has been shown that the SBS gain coefficient g (ω), where ω is the frequency of the probe, scales approximately with the power spectral density (PSD) of the broadened pump wave Pp (ω + B ) [12]. Carefully synthesized pump modulation could provide a uniform PSD within a bandwidth of interest, and consequently generate a uniform PSD of SBS-ASE [11]. Figure 2 shows a schematic illustration of an SBS-ASE noise source block [13]. Alternatively, EDFAs could provide ASE with a nearuniform PSD over a bandwidth of several THz. In the UWB
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Fig. 1. Simplified block diagram of a microwave-photonic, UWB noise radar system. FO: fiber-optic.
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Fig. 3. Top-PSD of the directly modulated SBS pump wave. Center-PSD of the down-converted SBS-ASE noise. Bottom-histogram of the down-converted SBS-ASE noise (bar), alongside a zero-mean Gaussian distribution of equal variance (line).
noise generation experiments reported below, fiber Bragg gratings (FBGs) were used to select a spectral portion of the EDFA-ASE that was only several GHz-wide. Note that in principle, the photonic noise source block in Fig. 1 may be replaced by an electrical one that would modulate a light source for RoF distribution towards the remote unit. III. E XPERIMENTAL R ESULTS Figure 3 (top) shows a heterodyne measurement of the PSD of the modulated DFB laser diode used as an SBS pump wave [13]. A nearly uniform PSD was obtained within a range of over 1 GHz. The SBS pump was launched into a 25 km long section of standard fiber through a circulator. Figure 3 (center) shows the PSD of the down-converted, UWB RF noise, generated through SBS-ASE [13]. The PSD was nearly uniform within a range of 1 GHz. Figure 3 (bottom) shows a histogram of the UWB RF noise, which is of Gaussian statistics. The transmission and reception horn antennas at the remote unit had a directive gain of 20 dBi. The transmitted RF power was 5 dBm. Figure 4 shows the measured cross-correlations between the reflections from a 40 cm × 40 cm metallic target and the reference waveform stored at the central office, for several target distances. The full width at half maximum (FWHM) of the main auto-correlation peak was equivalent to a ranging resolution of 20 cm, in agreement with the noise bandwidth. The received power varied between measurements due to the manual placement of antennas and target.
Relative Corr. Mag. [Lin.]
GRODENSKY et al.: UWB MICROWAVE-PHOTONIC NOISE RADAR
the order of 5 m. It is scalable with higher transmission power and larger, more directive antennas.
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Fig. 4. Measured correlation between SBS-ASE noise waveform reflected from a metal target and a reference replica. The distances to the target were 1.5 m (blue, dotted), 1.95 m (red, solid), and 2.85 m (black, dashed) [9]. -40
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A microwave-photonic, UWB noise radar system was demonstrated. The waveforms were generated using the ASE associated with SBS or an EDFA. The system combines the antenna remoting capabilities, broad bandwidth and flexibility of reconfiguration provided by microwave photonics, together with the potentially superior resilience against jamming and interception of noise radars. Photonic noise generation could scale towards high frequencies of electrical noise in the millimeter waves range, provides a high degree of randomness based on a physical process, and readily integrates into a RoF system. The ranging resolution is not affected by the length of the fiber link. Lastly, the sidelobes of the correlation function may be suppressed using signal processing methods at either the receiver [14] or the transmitter [15] end. R EFERENCES
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Fig. 6. Measured correlation between EDFA-ASE noise waveform reflected from a metal target and a reference replica. The distances to the target were 1.43 m (red, solid), 1.9 m (green, dashed), and 2.16 m (blue, dashed–dotted).
The PSD and histogram of RF UWB noise generated using EDFA-ASE are shown at the top and bottom panels of Fig. 5, respectively. A 5 GHz-wide FBG was used for slicing the optical spectrum of the ASE, and the RF waveform was further filtered to accommodate the 2 GHz bandwidth of the antennas. Once more, the generated noise is of Gaussian statistics. Here a pair of 10-km-long fibers connected the central office and remote unit. Cross-correlations between reflected and reference waveforms are shown on Figure 6. The ranging resolution was on the order of 10 cm. The measurement range, for our experimental parameters and target size, is estimated on
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