We investigated optical damage (surface and bulk) in one of the most promising wide bandgap nonoxide nonlinear crystals, HgGa 2 S 4 , that can be used in ∼1-μm pumped optical parametric oscillators (OPOs) and synchronously pumped OPOs (SPOPOs) for generation of idler pulses above 4 μm without two-photon absorption losses at the pump wavelength. The optical damage has been characterized at the pump wavelength for different repetition rates using uncoated and antireflectioncoated (mainly with a single layer for pump and signal wavelengths) samples. HgGa 2 S 4 is the most successful nonlinear crystal (both in terms of output energy and average power) for such OPOs, but optical damage inside the OPO has a lower threshold and represents at present the principal limitation for the achievable output. It is related to peak pulse and not to average intensity, and bulk damage in the form of scattering centers occurs before surface damage. Such bulk damage formation is faster at higher repetition rates. Lower repetition rates increase the lifetime of the crystal but do not solve the problem. The safe pump fluence in extracavity measurements is <1 J∕cm 2 , which corresponds to ∼100 MW∕cm 2 for the 8-ns pulse duration (both values peak on-axis). In the OPO, however, peak on-axis fluence should not exceed 0.3 J∕cm 2 limited by the formation of bulk scattering centers in orange-phase HgGa 2 S 4 . In the nanosecond OPO regime, the damage resistivity of Cd-doped HgGa 2 S 4 is higher and that of the almost colorless CdGa 2 S 4 is roughly two times higher, but the latter has no sufficient birefringence for phase-matching. In SPOPOs operating in the ∼100 MHz regime, the damage limitations are related both to the peak pulse and the average intensities, but here HgGa 2 S 4 seems the best nonoxide candidate to obtain first steady-state operation with Yb-based mode-locked laser pump sources.

Synthetic aperture radar is a remote sensing technology finding applications in a wide range of fields, especially related to Earth observation. It enables a fine imaging that is crucial in critical activities, like environmental monitoring for natural resource management or disasters prevention. In this picture, the scan-on-receive paradigm allows for enhanced imaging capabilities thanks to wide swath observations at finer azimuthal resolution achieved by beamforming of multiple simultaneous antenna beams. Recently, solutions based on microwave photonics techniques demonstrated the possibility of an efficient implementation of beamforming, overcoming some limitations posed by purely electronic solutions, offering unprecedented flexibility and precision to RF systems. Moreover, photonics-assisted RF beamformers can nowadays be realized as integrated circuits, with reduced size and power consumption with respect to digital beamforming approaches. This paper presents the design analysis and the challenges of the development of a hybrid photonicintegrated circuit as the core element of an X-band scanon-receive spaceborne synthetic aperture radar. The proposed photonic-integrated circuit synthetizes three simultaneous scanning beams on the received signal, and performs the frequency down-conversion, guaranteeing a compact 15 cm 2 -form factor, less than 6 W power consumption, and 55 dB of dynamic range. The whole photonics-assisted system is designed for space compliance and meets the target application requirements, representing a step forward toward a deeper penetration of photonics in microwave applications for challenging scenarios, like the observation of the Earth from space.

This paper presents the design and the performance analysis of a photonics-based beamformer for a spaceborne synthetic aperture radar implementing the scan-onreceive functionality. The considered device is a hybrid photonic integrated circuit composed of actives in InP and passives in TriPleX™, realizing the fast beamforming of three receiver beams out of 12 radio-frequency input signals and providing their simultaneous down-conversion to intermediate frequency. The analysis considers as main performance indicators the gain, the noise figure, and the dynamic range of the photonics-based beamformer, and demonstrates the device compliance to the application requirements and its suitability for satellite missions.

Multi-static SARs from LEO orbits allow the single-pass high-resolution imaging and detection of moving targets. A coherent MIMO approach requires the generation of multi-band, thus orthogonal, signals, the fusion of which increases the system resolution. Up to now the synchronization capability of SAR signals of different satellites is critical. Here, we propose the use of photonics to generate, receive and distribute the radar signals in a coherent multi-static SAR constellation. Photonics overcomes issues in the implementation of MIMO SAR, allowing for the flexible generation of multi-band signals and centralized generation in a primary satellite with coherent distribution to all the secondary satellites of the SAR signals over FSO links. The numerical analysis shows the proposed system has a NESZ < −29.6 dB, satisfying the SAR system requirements. An experimental proof of concept based on COTS, for both signal up- and down-conversion, is implemented to demonstrate the system functionality, showing performance similar to the simulations. The implementation of the proposed systems with integrated technologies could reduce the system SWaP and increase robustness to vibrations. A design based on the consolidated SOI platform with the transfer printing-based hybrid integration of InP semiconductor optical amplifiers is proposed. The amplifiers compensate for the losses of the passive SOI waveguides, decreasing the overall conversion loss. The polarization multiplexing of the modulated and unmodulated combs to be sent from (to) the primary to (from) the secondary satellite over the FSO links avoids complex space-consuming optical filters requiring several control signals.

No abstract