Performance and limitations of dual-comb based ranging systems.

Dual-comb LiDARs have the potential to perform high-resolution ranging at high speed. Here, through an implementation involving electro-optic modulators and heterodyne detection, we quantify the ranging systems trade-off between precision and non-ambiguity range (NAR) using a unique performance factor. We highlight the influence of the comb amplitude envelope on the precision with a distance measurement limited by the repetition rate of the optical comb. The influence of the combs repetition rate on the NAR and on the precision is illustrated through a setup allowing distance measurement with a tunable NAR. Finally, we demonstrate the impossibility to resolve different targets, quantify the impact on the measured distance and develop on the conditions in which non-linear effects of the interference make the measurement impossible.

Free space optical communication receiver based on a spatial demultiplexer and a photonic integrated coherent combining circuit.

Atmospheric turbulence can generate scintillation or beam wandering phenomena that impairs free space optical (FSO) communication. In this paper, we propose and demonstrate a proof-of-concept FSO communication receiver based on a spatial demultiplexer and a photonic integrated circuit coherent combiner. The receiver collects the light from several Hermite Gauss spatial modes and coherently combine on chip the energy from the different modes into a single output. The FSO receiver is characterized with a wavefront emulator bench that generates arbitrary phase and intensity patterns. The multimode receiver presents a strong resilience to wavefront distortions, compared to a monomode FSO receiver. The system is then used to detect an analog modulation of an optical beam through a random wavefront profile to mimic the transmission of a signal on a degraded optical link.

Comparison of different active polarimetric imaging modes for target detection in outdoor environment.

We address the detection of manufactured objects in different types of environments with active polarimetric imaging. Using an original, fully adaptive imager, we compare several imaging modes having different numbers of polarimetric degrees of freedom. We demonstrate the efficiency of active polarimetric imaging for decamouflage and hazardous object detection, and underline the characteristics that a polarimetric imager aimed at this type of application should possess. We show that in most encountered scenarios the Mueller matrices are nearly diagonal, and sufficient detection performance can be obtained with simple polarimetric imaging systems having reduced degrees of freedom. Moreover, intensity normalization of images is of paramount importance to better reveal polarimetric contrast.

Active polarimetric imager with near infrared laser illumination for adaptive contrast optimization.

We designed and built an active polarimetric imager with laser illumination at 1.5 µm wavelength for adaptive polarimetric contrast optimization. It can generate and analyze any polarization state on the Poincaré sphere in order to best adapt to the polarimetric properties of the scene. Polarimetric contrast optimization is performed by analyzing the scene with an ultrafast active-contour-based segmentation algorithm. This device is, to the best of our knowledge, the first fully adaptive imager controlled by image processing algorithms for polarimetric contrast enhancement. Its capabilities are illustrated in some examples of real-world decamouflage applications.

Excited state absorption: a key phenomenon for the improvement of biphotonic based optical limiting at telecommunication wavelengths.

Spectroscopic properties, two-photon absorption (TPA) and excited state absorption (ESA), of two organic cyanine dyes and of a ruthenium based organometallic cyanine are compared in order to rationalize their similar ns-optical power limiting (OPL) efficiency in the telecommunication wavelength range. The TPA contribution to the ns-OPL behavior is higher for both organic cyanines, while the main process is a TPA-induced ESA in the case of the organometallic system, in which the ruthenium induces a broadening of the NIR-ESA band and resulting in a strong spectral overlap between TPA and ESA spectra.

Performance evaluation for long-range turbulence-detection using ultraviolet lidar.

Clear-air turbulence could be detected at long range using a UV lidar. Because the vertical speed cannot be retrieved from Doppler shift analysis at long range, the turbulence detection is based on the measurement of molecular density fluctuation associated with the turbulent wind. After an optimization of the characteristics of the candidate UV lidar, we present an evaluation of the detection range and of the false alarm rate and missed alarm rate depending on the altitude and vertical velocity root mean square. This study shows that 96% of turbulence with vertical velocity leading to dislodging of unsecured objects in the airplane can be detected at 15 km using a 2 W laser at 355 nm with a false alarm rate of 0.18 per flight hour.

Photochromic compounds as optical limiters in the nanosecond time range: the example of mercury dithizonate complex.

Although being an efficient photochromic compound which absorbs in the blue in its stable form and in the orange in its photoactivated form, the mercury dithizonate complex is shown to be a poor optical limiter for nanosecond laser pulses at the wavelengths where both isomers absorb. Optical limiting effect, which is a consequence of reverse saturable absorption due to the photoactivated form, is demonstrated to be weak because of the back photobleaching of this form, which is important all the more as the laser intensity is high. Numerical integration of the spatiotemporal evolution of the laser beam intensity across the solution helps the understanding of the respective roles of the laser fluence and pulse duration. Finally, we draw the conclusion that photochromic compounds can only be used as optical limiters if the time constant for the back photochemical reaction is slow compared to the pulse duration.