Wavefront Coding (WFC) imaging systems can be considered as an effective and feasible technique to reduce retroreflection while maintaining good image quality, based on its advantage of high-quality imaging along with a wide range of defocusing. In this work, an anti-retroreflection imaging technique based on WFC with cubic Phase Mask (CPM) is proposed and an optical design example is also demonstrated to verify the feasibility of our proposal. Both the simulated Peak Normalized Cross-Correlation (PNCC) and the Optical Transfer Function (OTF) show that the optical system with CPM can achieve good image quality as well as substantial retroreflection reduction. Compared with the conventional optical system with no CPM, it is possible to dramatically reduce the reflected beam of the cat-eye by two orders of magnitude.
The work concerning the laser-induced damage under long-range (km-class) outdoor testing is very limited due to the difficult, laborious and time-consuming process. In this paper, we demonstrate a 1.5 km outdoor experiment of observing the laser-induced damage of CMOS imaging sensor and predicting the required pulse energy at other propagation lengths and visibility conditions theoretically. The outdoor experiment verifies that large amount of pixel permanent damage on a commercial visible-band CMOS camera can be produced by using the 532 nm pulsed laser with an output energy of higher than 50 mJ at a propagation range of 1.5 km and the atmospheric visibility of 10km. Meanwhile, the laser power density travelling through turbulent atmosphere is estimated theoretically by using a phase screen model. The required laser energy to damage the sensor for different propagation length or visibility condition could be predicted. The simulation results indicates that the atmospheric effects lead to significant impact to the spatial profile of laser beams at long propagation range, which should be considered and analyzed delicately when one is designing a laser countermeasure device.
In order to enhance the detection capability of “cat’s eye” effect existing in the photoelectronic equipment at long-range, we develop a laser active detection system in this paper. The parameters of laser transmitter and imaging system are well-designed based on the requirement of long-range active detection. The laser light is produced by a high energy pulsed laser at 532 nm, assembled with a beam expander which can achieve a tunable laser divergence angle from 0.03° to 5°, and the reflected light can be detected by a CMOS assembled with an long-focal-length optical lens. An experimental demonstration system is build up according to the system design, which can successfully detect a camera (Aperture-50 mm) at a detection range of 5.7 km, with no extra sophisticated signal processing schemes required. An evident “cat’s eye” target can be observed in the raw image from the digital camera (exposure time-3 ms) when the laser divergence angle is 3°, whose intensity is more than 3 times of the background. The results indicate that this work meets the demand of long-range laser active detection based on the “cat’s eye” effect.
This paper presents a new design method of infrared search and track systems, that is, optical-digital joint optimization design method for IR detection of unresolved target. It is different from the traditional design concept of optics-detector-preprocessor sequential and independent optimization. The merit function suitable for infrared unresolved target detection is constructed for the whole infrared search and track system, and the design parameters of optical lens, infrared detector and digital preprocessor are taken as optimization variables, and then the global optimization is carried out through effective algorithm to find the overall optimization results. The proposed method belongs to an advanced form of computational optical imaging, which relaxes the local design constraints, and has a greater degree of design freedom. The research results show that it is significantly better than the traditional method.
We report the results of our recent experimental investigation of the modulation frequency detuning effect on the output pulse dynamics in a pulse modulated actively mode-locked ytterbium doped fiber laser. The experimental study shows the existence of five different mode-locking states that mainly depend on the modulation frequency detuning, which are: (a) amplitude-even harmonic/fundamental mode-locking, (b) Q-switched harmonic/fundamental mode-locking, (c) sinusoidal wave modulation mode, (d) pulses bundle state, and (e) noise-like state. A detailed experimental characterization of the output pulses dynamics in each operating mode is presented.
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