• optical
• Oscillators
• photonics
• radar

In the last few years, Multistatic radar system have been extremely investigated. Such systems are better known as Multiple Input Multiple Output (MIMO) radars, since the same concept of MIMO telecommunication systems is employed. The basic idea is to exploit the advantages of Multiple Input Multiple Output systems in order to improve the performances of a radar system. In fact, enhancing the number of employed antennas, and positioning them in a proper way, increases the performances and the degrees of freedom of the system. In particular, MIMO Radars with widely separated antennas achieve the so called super-resolution, allowing improvements of the detection and imaging capability with respect to a single antenna-based (or antenna array) radar system.
Multispectral radar image processing is highly desirable in order to enhance the classification capability of an imaging radar system. This process is achievable if multiband radar is employed. In order to effectively perform a multispectral processing, RAW datas from the RF front-ends have to be delivered in a central processing unit. It involves low loss and high capacity links between the CPU and the RF front end.
As known, electronic devices are characterized by limited bandwidth and lossy links that avoids to achieve the featured just discussed (multiband and low loss distribution respectively). A solution for these problems could be represented by photonics technologies applied to microwaves. In the last few years, the research field of microwave photonics has been suggesting the possibility of exploiting the huge bandwidth and flexibility of photonics in order to generate RF signals that cover the frequency range from a few megahertz up to several tens of gigahertz. Several functions currently performed by electronic devices can be realized using photonics, overtaking all the limitations of electronic based systems. These functionalities are: up-and-down conversion, signal distribution, beamforming, filtering, local oscillators.
In recent years, the first photonics based radar system has been designed and realized by the CNIT’s photonics team. Such device exploits a multifrequency laser as optical clock, replacing the electronic ones, for generating multiple tunable radar signals intrinsically coherent each other and for receiving their echoes, avoiding noisy radio-frequency up-and downconversion and guaranteeing both the software-defined approach and high resolution. Its performance exceeds state-of-the-art electronics at carrier frequencies above two gigahertz.
An other fundamental feature of photonics-based radar systems is that all the signals can be delivered using the optical fibers. Such optical medium are characterized by extremely low losses (∼0,3 dB/km) allowing long distance links with low loss and no coherence degradation. This feature enables the possibility to realize a radar network where the RF signals are converted in optical domain and travels through the optical fiber (Radio over Fiber concept) from the network nodes (RF front-ends) to the network core (Central Processing Unit) and vice versa.
This work is inserted in an European project named ROBORDER, that foresee the realization of a complex sensor network that includes a multiband MIMO radar. This last unit will be characterized by a photonics-based core where photonics is used for up and down conversion, and widely separated antenna connected to each other employing optical fibers.
The objective of this thesis is the design and implementation of a highly stable multifrequency optical clock to be used as local oscillator of the multiband MIMO radar. This single optical clock should replace the different RF clocks required in the radar network working at different RFs. To this purpose, different techniques have been evaluated, based on Mode-locking laser, Phase Lock Loop and Injection Locking, looking for a trade off between the complexity of the system and flexibility in terms of carrier frequency. Two different architectures have been developed and tested. Both of them are based on injection locking technique and are particularly suitable for applications where flexibility in terms of carrier frequency is required (for example multiband radar systems).
The first solution consists in a photonic-based multifrequency RF oscillator that still require an external microwave reference signal. While the second architecture is an all optical solution.
In this work, the two architectures are analyzed, implemented and compared to identify the better solution in the different scenarios.
All the experiments reported in this thesis have been carried out in the Photonic Network and Technologies National Laboratory (PNTLab) of National Inter-university Consortium for Telecommunications (CNIT) with the collaboration of the Institute of Communication, Information and Perception Technologies (TeCIP) of Sant’Anna School of Advanced Studies (SSSA).