As the improvement of radar systems claims for digital approaches, photonics is becoming a solution for software defined high frequency and high stability signal generation. We report on our recent activities on the photonic generation of flexible wideband RF signals, extending the proposed architecture to the independent optical beamforming of multiple signals. The scheme has been tested generating two wideband signals at 10GHz and 40GHz, and controlling their independent delays at two antenna elements. Thanks to the multiple functionalities, the proposed scheme allows to improve the effectiveness of the photonic approach, reducing its cost and allowing flexibility, extremely wide bandwidth, and high stability.
© 2013 Optical Society of America
The rapid progress of digital processing is improving the functionalities of the electronic systems that permeate and serve our lives. In this run to digital, radar systems are no exception, and several new radar concepts have been recently imagined as the software defined radar (SDR), the multi-function radar, the arbitrary beamforming, or the shared aperture radar: all these novel system paradigms in the radar field would require to flexibly control the generation of one or more radio-frequency (RF) signals, and this would be conveniently managed in a digital fashion. In order to apply the digital trend to RF signals, high-speed digital electronics must be developed to surpass the need for the old (and hardly flexible) analog electronics. But as the carrier frequency increases, the task becomes more and more challenging, and nowadays direct digital synthesizers can only range up to few GHz.
Photonics is insistently proposing as the solution to fill the gap between the urgent need for digital solutions and the limitations of electronics at higher frequencies: the flexibility, wide bandwidth, and technological maturity of photonics in fact have led to numerous proposed architectures for the generation, manipulation, and detection of RF signals, also merging the radar applications with related fields as the wireless communications.
In this paper we report on our recent activities on the photonic generation of flexible wideband RF signals, focused (but not limited) to radar applications, and we extend the proposed architecture to the independent beamforming of multiple signals.
2. Flexible photonics-based generation of wideband RF signals
Coherent radars are requiring RF carriers with lower phase noise for improved sensitivity, and with higher frequency for smaller antennas. A modulation or coding of the phase of the radar pulse carrier is also necessary to implement pulse compression techniques for increased resolution without dangerous transmitted peak power . Moreover, the generation of multiple signals is sought for frequency agility and for multiband multifunctional radars . For high frequency carriers, the usual frequency multiplication processes reduce the phase stability of the original RF oscillators, and frequency diversity radars are commonly realized by using two or more radar transmitters, with increasing cost and power consumption .
In the last years, photonics has been suggested to effectively generate low phase-noise RF carriers, in particular at high frequency. Among other techniques [4,5], the heterodyning of modes from a mode locking laser (MLL) has proven to generate low phase noise RF carriers up to the EHF band (30-300GHz) . Wideband modulation and coding can also be applied in the photonic domain, avoiding to recur to frequency-specific electronic devices [7–11]. But most of these modulation schemes exploit optical interferometric structures which are hardly suitable for coherent radars with demanding frequency agility.
In  we have proposed a technique for optically generating multiple phase-coded RF signals with flexible carrier frequencies and with a phase stability suitable for coherent radar systems. The proposed scheme, as sketched in Fig. 1, modulates the spectrum of a MLL at intermediate frequency (IF), so that phase- and amplitude-modulated RF signals at any carrier frequency can be obtained by mixing a modulation sideband and a MLL mode in a photodiode (PD), thus realizing a stable photonic up-conversion. Therefore in principle a precise optical filtering device selecting only one sideband and one MLL mode would be necessary, but the filtering task in this case is easier and more efficient if realized in the RF domain. Thus, after the PD, an RF filter centered at the desired frequency allows to select the RF signal. The use of a precise direct digital synthesizer (DDS) for generating the coding signals at variable IF allows the implementation of a software-defined radar transmitter without losing the original phase stability of the MLL, matching the requirements of demanding surveillance systems. The scheme has been tested generating some of the most common pulse-compression techniques used in radar applications. As an example, Fig. 2 reports the results obtained by applying a 25MHz linear chirp to RF pulses at around 10GHz and 40GHz.
With the reported scheme, frequency agility can be implemented with a single MLL instead of a series of electronic oscillators. The proposed architecture allows to generate the desired RF signals either simultaneously or alternately, or even to continuously change their frequency by opportunely programming the DDS, provided a tunable RF filtering is available. The modulation as well can be changed meanwhile, thus implementing a waveform diversity technique. Besides its application to coherent radars, the proposed method can be helpful wherever phase- or amplitude-modulated RF signals are needed, as for example in the removal of the range ambiguity in radars, or in radio-over-fiber systems for the generation of complex modulation formats .
3. Extension to flexible optical beamforming
The photonics-based RF generation method described above can be conveniently exploited also to manage the beamforming of multiple RF signals in phased array antennas (PAAs).
PAAs allow to steer the transmitted RF beam without physically moving the antenna, and are used in an increasing number of applications such as multifunctional radars, electronic warfare, and communications. Common PAAs use electronic phase shifters at each antenna element to control the viewing angle of the array, but when steering broadband signals this approach suffers the squint phenomenon which causes different frequencies of the signal spectrum to aim at a different angle. This can be avoided if the phase shifters are substituted by true-time delays (TTDs). The approach based on TTDs is actually implemented in high-performance applications and requires complicated signal processing techniques.
The TTD functionality can be easily implemented exploiting photonics, thanks to its capability of realizing controllable delays with wide bandwidth, and with the additional advantages of electro-magnetic interference (EMI) insensitivity. Optical tunable TTDs have been demonstrated through optical path switching , wavelength tuning or switching in conjunction with dispersive elements [15,16], or slow light . Experimental results are reported covering up to 16 antenna elements , with total delays up to 2.5ns  and scanning angles up to 90° at millimeter waves . Few techniques also report the capability for continuous beam steering .
The photonics-based solutions listed above consider the optical beamforming function as an additional functional block of the RF transceiver, that needs to convert the signal from the RF domain to the optical domain in order to implement the TTD functionality, and then back to RF. Although the use of photonics simplifies the task of beamforming with respect to RF solutions, this double conversion represents a significant increase of system cost and complexity. Here we propose to include into the same photonics-based functional block both the beamforming through TTD and the precise generation of the RF signal, extending the scheme presented in the previous section. This approach combines the high performance of the photonics-based RF generation with the effectiveness of the optical beamforming taking advantage of the broad spectrum of the MLL, thus optimizing the impact of the photonics subsystem in the RF transmitter.
3.1 Photonics-based RF signal generation and beamforming
The scheme of principle of the proposed approach is shown in Fig. 3(a). To allow the generation of multiple RF signals, the modulator is driven by multiple signals at different IFs so that the modulation sidebands are clearly separated in the optical spectrum. The optical signal is sent to each element of the arrayed antenna through a spool of optical fiber which introduces a wavelength-dependent delay through chromatic dispersion. A special tunable optical filter (tunable multi-pair bandpass filter, TMP-BPF) is then added at each antenna element to opportunely select in principle a pair of optical signals (one MLL mode and one sideband) for each RF signal to be generated. The spectral region where the optical pairs are taken will control the delay of the generated RF signals, according to the chromatic dispersion induced by the remoting fiber. Changing the filter position induces a delay Δt on the optical signal given by Δt = D·Δλ, where D is the value of the accumulated chromatic dispersion and Δλ the wavelength difference of the selected optical pairs. At each array element, after the TMP-BPF the filtered spectrum is sent to a PD which produces the RF signals as the beatings between the input spectral lines. In case the TMP-BPF is not able to select only the signal pair of interest, electrical bandpass filters (BPFs) can be added after the PD, as reported in Fig. 3(a), to isolate the appropriate RF signals to be transmitted through a multi-band (or several single-band) PAA element. From the formula above it is evident that the delay induced on the RF signals changing the filter position is independent from the RF signal frequency and bandwidth. Figure 3(b) sketches the optical and electrical spectra in the case of simultaneously generating two different RF signals.
3.2 Experimental setup and results
To demonstrate the broadband TTD capability of the proposed beamforming scheme, the setup depicted in Fig. 4(a) has been implemented. The optical path is composed of a fiber MLL with a repetition rate of about 10GHz (namely 9953MHz) and a 3-dB bandwidth of about 0.7nm, and a spool of dispersion compensating fiber (DCF) with a chromatic dispersion of −320ps/nm. The TMP-BPF is emulated here by a liquid-crystal-on-silicon programmable filter (Finisar WaveShaper 4000S, WS) configured to operate as a single 50GHz-bandwidth BPF selecting 5 adjacent lines of the MLL. The optical signal is detected by a 40GHz-bandwidth PD, generating an RF signal made of components at about 10, 20, 30, and 40GHz. The PD output is split into two paths, and two electrical BPFs centered exactly at 9953MHz and 39812MHz isolate the spectral components. The RF signals are then acquired by a dual-channel sampling oscilloscope. First, the WS is centered at 194.165THz, −50GHz offset from the MLL center wavelength, and then moved by 10GHz steps in order to select different groups of modes, up to 194.265THz. The filtered spectra at the two extreme positions of the tuned range are reported in Fig. 5(a). Figure 5(b) reports the measured delays for the 10GHz and 40GHz components, as a function of the optical BPF offset, i.e. the frequency difference from the initial position, and the theoretical delay curve. As can be seen, the results at 10GHz and 40GHz present an identical linear trend, and fit the theory very well. This experiment thus emulates the TTD of a signal spanning over 30GHz and confirms the effectiveness of the scheme. Since the MLL presents a discrete spectrum, the available delays are discrete as well, and the steps are determined by the lines spacing (i.e. the MLL repetition rate) and the amount of chromatic dispersion. In this work the step is 25.2ps.
The scheme of principle of Fig. 3(a) has been experimentally tested through the setup reported in Fig. 4(b). Due to laboratory limitations, the test is run exploiting an on-off keying baseband modulation, so before applying the chromatic dispersion, the MLL output is modulated with 1ns-long pulses (1GHz modulation bandwidth) given by a bit pattern generator synchronous with the MLL. Two output ports of the WS are used, and the WS is programmed to route to each port a pair of 10GHz-spaced modes and a pair of 40GHz-spaced modes (Figs. 6(a) and 6(b)). Two wide-bandwidth PDs detect the optical signals at each WS port, and the generated signals are acquired by the oscilloscope (Figs. 6(c) and 6(d)). Due to the baseband modulation, there are no spectral components at IF and the BPFs after the PDs can be avoided. In order to analyze the two RF signals at the oscilloscope, the WS is set to alternately block one of the mode pairs in each port, so that the signal generated by the other pair can be studied. In Figs. 6(c) and 6(d) it can be seen that in Port A the signal at 10GHz is anticipated with respect to Port B, while the signal at 40GHz is delayed. This situation emulates the opposite steering of two independent signals in a two-element PAA. The 40GHz signal at Port A or B appears slightly distorted due to the non-ideality of the WS, which cannot completely extinguish the adjacent modes, giving an overmodulation at 10GHz. Using IF modulations and proper RF filtering (or better optical filters) would easily avoid this problem.
4. Comments and conclusions
The experimental results reported above highlight the effective unique features of the proposed multi-function photonics-based architecture, which simultaneously generates multiple RF signals, and steers them independently controlling their delays to an array of antenna elements. In the proof-of-concept experiment, two wideband signals at about 10GHz and 40GHz have been generated at two antenna elements, with a maximum delay of about 400ps and a delay resolution of 25.2ps. In a practical PAA implementation, the system parameters are related to the specific considered application (number of array element, carrier frequencies, array dimension, turning range), thus a trade-off should be sought within the several parameters of our proposed system. Longer delays can be obtained increasing the chromatic dispersion, or increasing the filters detuning using a MLL with larger bandwidth (i.e., shorter pulses). Improved delay resolution is achievable reducing the chromatic dispersion (thus reducing the maximum delay) or using a MLL with lower repetition rate. Very large arrays (up to hundreds of elements) can also be managed resorting to a low repetition rate, short pulsewidth MLL. In the above tests, the TMP-BPF has been realized using a WS. A field implementation of the proposed scheme should exploit advanced photonic integration techniques instead, which would improve the filtering functionality also avoiding the use of RF filters after the PDs. Moreover, it would allow a reduction of system cost and encumbrance while simplifying the management of large antenna arrays.
In conclusion, we have proposed and reported a photonics-based scheme integrating the functions of RF signal generation and TTD beamforming. The photonics-based multi-functional block is composed of a MLL and a tunable filtering matrix. The chromatic dispersion, necessary to generate the delays, can be provided by the optical fiber used for feeding the PAA. The scheme has been experimentally validated by photonically generating two wideband signals at about 10GHz and at 40GHz, and controlling their independent delays at two antenna elements, thus emulating the independent TTD beamforming of the generated signals. Thanks to the multiple functionality, the proposed scheme allows to improve the effectiveness of the photonic approach, reducing its cost and allowing flexibility, extremely wide bandwidth, and high stability. These features, added to the EMI immunity, low losses, and potentials for low weight and power consumption, make the proposed photonics-based scheme a promising solution for advanced RF transmitters with beamforming capability.
This work has been supported by the ERC projects PHODIR (contract n. 239640) and PREPARE (contract n. 324629), and by the Italian Defense Ministry project SOPHIA (contract n. 20008).
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