Retroreflection, which is also known as the cat-eye effect, occurs in many imaging systems. The laser beam that irradiates on the focused photodetector would be reflected along its entranceway, by means of which the focused imaging system could be easily discovered and located. The paper analyzes the present research status of retroreflection reduction techniques. It shows that the related techniques mainly focus on four aspects: defocusing, masking, filtering, and one-way optical device. However, there are still some apparent disadvantages among the above technical schemes, which are generally manifested in the severe loss of imaging quality and the restricted application scope. Along with the rapid development of parallel computing platforms in recent years, computational imaging is widely applied in many fields. The paper proposes two novel configurations of anti-laser reconnaissance imaging systems based on the wavefront coding and the light-field rendering. Applying the computational imaging techniques will hopefully achieve a substantial retroreflection reduction without severely degrading the imaging quality, which shows us a broad prospect.
<p>Based on previous anti-cat-eye effect imaging techniques sacrificing too much imaging quality to achieve substantial retroreflection reduction, an anti-cat-eye effect imaging technique based on light-field imaging is proposed. Relevant studies have been carried out regarding the mechanism and effectiveness of both antilaser reconnaissance and blinding for this technique. By applying the Fresnel–Kirchhoff diffraction theory and defining the microlens array as a superposition of a series of microlens units, the retroreflection formation of the light-field imaging system is theoretically modeled. Based on the physical model, the influences of defocusing on the intensity distributions of spots on both the light-field detector and observation planes are further studied. The results show that, compared with a conventional system with the defocus invariant and flexible reconstruction properties, the light-field imaging system not only increases the interference and blinding thresholds by nearly one order of magnitude but also reduces both the retroreflection maximum intensity and the echo-detector receiving power by more than one order of magnitude, which sufficiently manifests the superior antilaser reconnaissance and blinding performances of the light-field imaging system.</p>
Diode pumped alkali laser (DPAL) with hydrocarbon buffer gases has the features of low threshold and high efficiency. The chemical reaction between alkali and hydrocarbon gases affects the life time of DPAL. In this paper, a method based on Fourier transform infrared spectroscopy and Lambert-Beer law is adopted to find a safe temperature at which DPAL runs for a long term. A theoretical model is established to figure out ways to reduce the peak temperature in the cell window. The results indicates that 170 °C is a safe temperature. Although the absorbance of the cell window to the pump light and alkali laser is lower, there is temperature increase. Small light-transmitting area and air blowing on the windows can reduce the peak temperature effectively. Cooling the cell window is essential and critical in a long-term running DPAL.
A self-heated diode-pumped alkali laser (SDPAL) with a microfabricated alkali cell is proposed. Based on Beach’s model and finite-element analysis theory, the output characteristics of a cesium self-heated laser are studied. The results indicate that an SDPAL with a cell length of 2 mm is feasible. The output power of a typical SDPAL is ∼Watt level. Rapid heat convection around the mini cell can increase the output power. At the same time, the utilization ratio of the pumping light will decrease. A heating experiment is also conducted to validate the theoretical model. When pumping power of 0.69 W is illuminated on the light absorber, the cell temperature can reach 76.4°C with a single-side heated structure. The results show that with a mini vapor cell, SDPAL can be portable and competitive when ∼Watt-level laser with wavelength of alkali D1 line is required.
Thermal control of the volume Bragg grating (VBG) in the LD with the external cavity is critical for the tuning of the wavelength and the narrowing of the bandwidth. Based on finite element theories, thermal properties of the VBG were researched under different conditions of LD illumining area, laser power, gratings’ working temperature and heat convection. Both the VBGs in the external cavity of LD bar and LD stack were considered in the experiments. The results show that higher working temperature of the VBG and adopting better heating convection cooling methods is beneficial to realize the uniformity of the VBG temperature distribution.
Thermal control of the volume Bragg grating (VBG) in the laser diode (LD) with the external cavity is critical for the tuning of the wavelength and the narrowing of the bandwidth. Based on finite element theories, thermal properties of the VBG were researched under different conditions of the LD illuminated area, laser power, gratings’ working temperature, and heat convection. Both the VBGs in the external cavity of the LD bar and stack were considered in the experiments. The results show that higher working temperature of the VBG and adopting better heat convection cooling methods are beneficial to realize the uniformity of the VBG temperature distribution.
In light of the difficulties to directly measure plume gas concentration by existing methods, the paper proposed an
inversion algorithm based on multivariate regression analysis. We first of all built up a multivariate regression model of
plume gas concentration by dividing the plume into several homogeneous layers along the observation direction. Then a
group of discrete spectral data was sampled out from plume infrared radiation curve at the intervals of certain wave
numbers. Thus the spectroscopic data without atmospheric attenuation could be obtained when the discrete spectral data
was divided by the atmospheric transmittances at corresponding wave numbers. After that, we worked on the
temperature profile of the plume, figuring out the average temperature of each layer of plume through integration
according to the outcomes of plume layering. At the same time, supported by the High Resolution Temperature Gas
Spectral Database (HITEMP), we also computed out the average absorption coefficient of each layer of plume. Thereby,
the triplicity of the spectroscopic data without atmospheric attenuation, the average temperature of each layer of plume
and the average absorption coefficient of each layer of plume, as the input parameters for the multivariate regression
model of plume gas concentration, could finally enable us to work out the concentration distribution of the plume gas
along the observation direction by least squares method which, however, only took into consideration the effect of vapor
and carbon dioxide. The comparison with the concentration distribution acquired through numerical computation of
plume flow field proves the feasibility of the inversion algorithm.
The analytical models were adopted to rapidly find out and simulate rocket plume apparent infrared radiation
intensity observed by satellite in the wave bands of 2.7 um and 4.3 um at various altitudes at boosting stage. Specifically,
a rocket plume flow field was first of all divided into three regions-highly under-expanded initial region, free turbulent
efflux transitional region and free turbulent efflux main region-to build up a simplified flow field model of rocket
plume. Then by simplifying the atmospheric emission and absorption factors into vapor and carbon dioxide only, we
divided the heterogeneous plume into several homogeneous layers along the observation direction, which enabled us to
construct a layered infrared radiation integration model of rocket plume. After that, we formulated a spectral
transmittance model of each layer of plume by use of the
Curtis-Godson approximation HITEMP database. The final
step was the modeling of atmospheric spectral transmittance by means of the Combined Atmospheric Radiative Transfer
(CART) software. Simulated curve for the intensity of rocket plume infrared radiation bears high similarity to the one
measured by satellite.