KEYWORDS: Single photon detectors, Reflectivity, Signal to noise ratio, Single photon avalanche diodes, Sensors, Image processing, Single photon, Reconstruction algorithms, Imaging systems, Detection and tracking algorithms
The Geiger area array (Avalanche Photo Diode) APD single photon detector is mainly used in the fields of long-distance remote sensing detection and target recognition due to the high sensitivity and high time resolution. However, as the laser imaging system is used for long-distance detection, the signal-to-noise ratio of the original image data collected by the detector is very small. Thus, the reconstruction of an accurate target image is often used, requiring a large number of raw data stacks, and resulting in extended acquisition times. Based on photon reflectivity estimation, in this paper an image reconstruction algorithm was proposed to address the issue of prolonged image acquisition time. The photon response number is first correlated with the photon flight time by establishing a 3-dimensional space-time matrix. the maximum likelihood estimation on the matrix was performed to realize image reconstruction with a few frames structure. The simulation experiment of the algorithm shows that the Root Means Square Error (RMSE) of the reconstructed image is only 0.1029 and the signal-to-noise ratio is 2:100. In a real scene, an InGaAs area array APD detector is employed to build a single-photon detection system, and the original image data of the target outside 1.3KM is collected. By comparing the imaging results of 200 frames and 10,000 frames of original data, it is confirmed that our method can be used in the condition of a small amount of photon response, and the quality of imaging can be effectively improved.
The broadband reverse absorption in a multi-branched conjugated compound TPPh is investigated. Transient absorptive spectra of TPPh solution is recorded and a broadband excited-state absorption (475~780 nm) is discovered. The lifetime of this broadband excited-state absorption was measured to be about 20 ns. Transient fluorescence experiment was conducted to confirm that the long-lived broadband excited-state absorption is established on the first singlet state. Optical limiting with extremely high linear transmittance is achieved under the excitation of 532 nm, 21 ps pulses. Theoretically analysis showed that both two-photon absorption and excited-state absorption played a part in it, optical limiting capability is thus enhanced via cooperating effects of two-photon absorption and excited-state absorption. All of the findings suggested that TPPh is a good broadband nonlinear absorptive material and could be further optimized for optical limiting applications.
Meso-tetrakis(4-cyanophenyl)N-confused porphyrin [NCTPP(CN)4] and its two metallized derivatives with Cu2+ and Zn2+ ligand in the central position of the macrocycle are synthesized and spectroscopically characterized. Their excited-state dynamics are investigated with transient absorption (TA) spectroscopy upon excitation by 190 fs laser pulses at 420 nm within their Soret band region. A global and target analysis for the TA spectra of each porphyrin is performed via a four-level model including singlet (S) and triplet (T) states to extract the photophysical parameters at a variety of absorption wavelengths. Furthermore the corresponding excited-state lifetimes are extracted and discussed.
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