The dual-frequency coherent lidar (DFCL) has advantages of anti-interference, stability, and can obtain low Doppler frequency shift in high-speed dynamic target detection. A performance evaluation model of DFCL is established for remote Gaussian rough object detection. The detection ability is closely related to laser echo characteristics, especially the intensity and coherence. The laser beam radius on the far field increases with the decay of the emitted laser pulse coherence, and the atmospheric turbulence reduces the coherence further. The intensity utilization factor is defined and calculated. The decoherence effects of rough surfaces are calculated via the complex coherence degree under typical roughness parameters and laser wavelength. Moreover, the Doppler frequency shift is proportional to dual-frequency difference ∆f, but the signal-to-noise ratio (SNR) decreases with larger ∆f duo to the coherence reduction of dual-frequency laser, and the optimal dual-frequency difference ∆fm selection criteria is determined for practical applications; and the system efficiency reduction factor are calculated and compared under typical detection parameters. Finally, the combined effects of laser source coherence, atmospheric turbulence, optical parameters and ∆f on the SNR improvements are analyzed considering dual-frequency and single frequency lidar systems. This research is of significance to reveal the dual-frequency coherent detection process and the optimization method of coherent lidar systems.
Rough target can cause the wavefront distortion of laser return, which shows the decoherence phenomenon and reduces the detection performance of heterodyne lidar systems. In fact, the decoherece process includes both the laser source and the rough target. The actual laser beams are usually partially coherent, and the atmospheric turbulence aggravates the coherence of laser spots on the rough target and the backscattered laser return. The backscattered laser field of rough plane is derived based on the GSM beam and the generalized Huygens-Fresnel principle. And the beam truncation effect of actual optical transceiver is also analyzed by using the hard edge aperture function. The laser return intensity variations are obtained by considering the laser beam coherence, the rough surface height fluctuation and the atmospheric turbulence. Then decoherence effects are calculated via the complex coherence degree under typical roughness parameters and laser wavelengths. For practical target, the complex coherence degree can be approximated by the Dirac delta function, and then the system efficiency and the effective coherent solid angle can also be used for further analyses. The results show a positive correlation between the decoherence effect and the roughness. The research on the scattering characteristics of rough planes expands the scattering theory and provides a reference for the design and analysis of long-range and high-precision heterodyne lidar system.
The space environment is becoming increasingly complicated; therefore, precise detection and identification of space targets are critical to preserving space security. Compared to optical and radar imaging, obtaining space target information using laser echo waveform is more efficient for detection. Based on the skew-normal distribution decomposition, the connection between sub-echoes and target scale following the decomposition of the space target pulse laser echo waveform is explored, and an inversion approach is suggested to identify the scale information of the solar panel utilizing the intersection of skewness and kurtosis contours in the coordinate system of the variables to be solved, based on the high-order moment characteristics of waveforms such as skewness and kurtosis. Using the proposed method, we realized the scale inversion of a 60-cm solar panel of a cube satellite inclined at 45 deg, with the turntable and the detection system placed 80 m apart. The results show that the skewness and kurtosis of the decomposed echo can represent the target size information, and the approach used here can successfully extract the solar panel scale information of a typical satellite, providing methods and data support for space target detection and classification.
At present, the technical research of lidar used in unmanned vehicle driving mainly focuses on continuously improving the density of lidar point cloud under the working mode of lidar with single wavelength, but the detection of echo is limited to single echo, missing a lot of details. Although the increase of laser point cloud density can improve the object recognition ability based on the geometric features of the point cloud, it also has a decreasing effect and many additional system requirements, which cannot fundamentally solve the problem of the lack of physical property detection ability caused by the single wavelength of lidar. To promote cross-country environment physical properties of the laser radar detection ability and help the laser radar's ability to obtain the information such as target state, in this paper, based on the calculation results of typical target spectral characteristics and lidar echo characteristics, a wavelength selection method for unmanned multi-wavelength lidar in off-road environment is proposed, which uses principal component analysis of typical target spectral features to determine the characteristic wavelengths that can distinguish the target by spectral features. Besides, the degree of waveform splitting is discussed through the simulation calculation of laser echo waveform, which helps finding the spectral wavelengths to distinguish targets in the same distance.
The model of laser return field backscattered by the atmospheric aerosols is established based on the Huygens-Fresnel diffraction integral formula in weak turbulence environment. The laser transmission properties in turbulent atmosphere provide the foundation to study the laser return field. The characteristics of intensity, complex coherence degree and the backscattering enhancement effect of laser return are analyzed under typical laser source coherence, turbulence, optical transceiver diameters and beam truncations. The laser source partial coherence reduces the coherence of laser return in turbulent environments. For the laser source beams with the same coherence degree, the laser intensities on the aerosols plane change relatively less with the increase of turbulence and detection range when the optical aperture is smaller; however, the laser return complex coherence degree is higher using larger optical transceiver in turbulent atmosphere. Moreover, the backscattering enhancement effect is mainly related to the turbulence, especially in the far field. The research is of significance to reveal the heterodyne detection process and the optimization method of coherent lidar systems.
KEYWORDS: Target detection, Signal to noise ratio, Entangled states, Signal attenuation, Photons, Light sources and illumination, Signal detection, Radar, Interference (communication)
Quantum illumination makes use of the strong correlation between entangled photons, making it possible to detect targets that break through physical limits. The Gaussian entangled state is a continuous variable entangled state with a high number of Hilbert spatial modes, which is currently a viable alternative to quantum lighting. In order to explore the target detection ability of Gaussian entangled state, this paper starts from the Gaussian entanglement model of Lloyd, establishes the target detection error probability boundary model under Gaussian entangled quantum illumination, and explores the optimal detection condition of Gaussian entangled state based on the detection signal-to-noise ratio and the number of measurements. The results show that when the Gaussian entangled state is optimal for the coherent state, the number of measurements is inversely proportional to the signal-to-noise ratio. Under the same error probability, the difference between the detection distances of the two states is determined by the atmospheric attenuation coefficient. The smaller the attenuation coefficient is, the smaller the attenuation coefficient is, the difference is greater.
Range-resolved laser reflective tomography is of great potential application in obtaining image information about an object with non-imaging laser radar system. The resulting time-dependent return signal which collected by non-imaging laser radar system provides one-dimensional raw projection data for reconstructing target image. However, this return signal can be regarded as the multi-convolution between the distribution function of target reflectivity, atmospheric transmission, detection circuit response and acquisition circuit response with the emitted laser pulse signal. In order to efficiently improve the reconstructed target image quality using short pulse laser, this paper presents a method used to restore the impulse response of target reflectivity modulation from the resulting time-dependent return signal so as to improve the reconstructed target image quality. This method is based on sending the laser and making it vertically irradiate to a profile of target in the front view to obtain basic wave pulse, then used it to recover the response of target reflectivity modulation. The experiment results show that this method is feasible and efficient.
Focusing on the underwater exploration potential of lidar and the limitations of physical limit on its detection sensitivity and detection accuracy, this paper focuses on modeling and analyzing the ultra-sensitive detection performance of entangled Fock state under seawater attenuation environment. Based on the LCMMS state, the entanglement detection model under seawater attenuation environment is established. The minimum phase error accuracy of LCMMS under the uniform distribution of photon number and the interference fringe contrast formula are derived. The simulation results show that the ultra-sensitive detection distance of the entangled Fock state under clear seawater can reach a distance of more than ten meters under water. The high photon number entangled state can make the distance higher; because LCMMS contains more high In the M and M' state of the photon number, the ultra-sensitive detection range of the quantum interferometric target detection of LCMMS under seawater loss is nearly 1.3 times that of the M and M' state.
The precise target identification is significant for commanding decisions and enemy identification. The micro-Doppler effect (MDE) can reflect the subtle movement characteristics of the target, which provides a new way for the target detection and recognition. However, the current research is mainly on the micro-motion feature extraction and classification of the targets, which is not capable for identifying the targets of the same type. This also reduced the application of the MDE. In fact, by accurately estimating the micro-motion parameters and combining sufficient prior knowledge, the target can be accurately identified. Further, the MDE detected by laser in infrared band has higher sensitivity and resolution than microwave detection, especially for the MDE generated by weak vibrations. Thus, in this paper, the photocurrent model of the laser detected MDE echo signal is established. The all-fiber coherent laser detection system for target micro-motion is designed. The detection sensitivity of and resolution requirements of the multicomponent micro-Doppler features are analyzed. Based on the time varying auto-regression (TVAR) model, the precise parameter estimation method for micro-motions are proposed, which provides the basis for target identification. The validity of the theoretical analysis and estimation method is verified through simulation. This research is helpful for extending the application of MDE from classification to precise identification in the future.
KEYWORDS: Signal to noise ratio, Heterodyning, LIDAR, Signal detection, Polarization, Fiber lasers, Pulsed laser operation, Digital signal processing, Monte Carlo methods, Sensors
Coherent Doppler lidars (CDL) and coherent differential absorption lidars are widely applied in the measurements of atmospheric wind and constituents respectively. To improve the detection range of heterodyne lidars, the demands for laser linewidth are studied based on the statistical theory and Monte Carlo simulations. The signal to noise ratio (SNR) and the spectrum of intermediate frequency (IF) signal are analyzed under different laser power and linewidth. When the detection range is beyond the coherent length, the IF signal can still be measured, and the power spectrum of IF signal will be broadened, which results in the peak value decrease in the power spectrum. In heterodyne Doppler lidars, the frequency extraction errors of IF signal fluctuate with SNR. To realize the velocity measurement performance for wind and other moving targets, detection performances with various laser linewidth are analyzed according to the 3σ criterion. The calculations indicate that better results can be obtained with larger powers when the laser linewidth is relatively wider and that the effective detection range of lidar can be longer than the coherent length for lasers with certain linewidth. To verify the analysis, heterodyne experiments are carried out based on the fiber delay lines and fiber lasers with different linewidths, and the SNR is controlled by a variable optical attenuator. The results show that measurements with large laser power can reduce the errors caused by the power spectrum broadening of IF signal. The analysis may aid the determination of laser power and linewidth in heterodyne lidars.
In order to grasp the information of wind field and disturbance in the airport in real time, and to ensure the safety of flight, a method of detecting wind field disturbance using coherent laser is presented. A model to solve the vector velocity of the wind field disturbance is established in this paper. Based on the radial velocity simulation of coherent laser echo signal, a reliable and effective radial velocity data is provided for inversing the vector velocity of the wind field disturbance. Actually, the radial wind velocity appears relatively large fluctuations due to the distribution inhomogeneity of aerosol particles and sensor noise in actual measurement. Therefore, the purpose of adding random noise into the above-mentioned inversion of the radial wind velocity is to simulate the measured radial wind velocity data. In the case of noise interference, the damping least square algorithm is proposed to solve the numerical optimal vector velocity of the wind field disturbance to verify the solving model. In addition, the vector velocity of the wind field disturbance is compared and analyzed under different scanning azimuth interval. Through the simulation results, it shows that the mean square error(MSE) of inversion result is smaller with the decrease of scanning azimuth interval. When the scanning azimuth interval is less than 60°, the mean squared error of the vector velocity of the wind field disturbance is less than 1.14m/s, horizontal direction disturbance quantity is less than 4°, which lays a good theoretical basis for the follow-up field tests.
Spore is an important part of bioaerosols. The optical characteristics of spore is a crucial parameter for study on bioaerosols. The reflection within the waveband of 2.5 to15μm were measured by squash method. Based on the measured data, Complex refractive index of Aspergillus oryzae spores within the waveband of 3 to 5μm and 8 to 14 μm were calculated by using Krames-Kronig (K-K) relationship. Then,the mass extinction coefficient of Aspergillus oryzae spores within the waveband of 3 to 5μm and 8 to 14μm were obtained by utilizing Mie scattering theory, and the results were analyzed and discussed. The average mass extinction coefficient of Aspergillus oryzae spores is 0.51 m2/g in the range of 3 to 5μm,and 0.48m2/g in the range of 8 to 14μm. Compared with common inorganic compounds, Aspergillus oryzae spores possesses a good extinction performance in infrared band.
Quantum Sensors like Quantum Radar and Lidar based on the interference of non-classical states can achieve super-sensitivity beyond the Standard Quantum Limit (SQL). But as the photons transporting in atmosphere, the environmental interaction causes quantum de-coherence and results in the reduction of super-sensitivity range of the quantum sensors. The most significant effect of atmospheric transmission is photon loss along with phase fluctuation. In this letter, we introduce both the photon loss and phase fluctuation by adding a fictitious beam splitter in the signal arm of Mach- Zehnder interferometer (MZI). The density matrix with N00N and M&M' entangled states being the input states under the condition of photon loss and phase fluctuation is given respectively. Then as the optimal detection schemes parity operator is used as the detector and the formula of the sensitivity is derived. The super-sensitivity range of M&M’ and N00N states with de-coherence are simulated. As a consequence, with high photon loss M&M’ states shows the better phase sensitivity than N00N states but the N00N state is better when the loss is smaller than 20%. And with pure phase fluctuations N00N states get the longer range. M&M’ states is sensitive to the transmittance difference between two arms of the interferometer.
The k-nearest-neighbor simplex (kNNS) based on an adaptive C-mutual proportion standard deviation metric for classification of hyperspectral remote sensing imagery was proposed. By analyzing spectral characteristics on the samples of the same and different classes, a C-mutual proportion standard deviation metric is put forward which innovates a novel metric on distance and similarity measures for pattern recognition. Combined with the adaptive adjusting algorithm, this metric is used for the classification of hyperspectral remote sensing imagery. The traditional kNNS classification algorithm is improved by this metric and the adaptive adjusting algorithm, and its classification accuracy is enhanced. Three experiments with different types of hyperspectral imagery are conducted to evaluate the performance of the proposed algorithm in comparison to the other five classification algorithms. The experimental results demonstrate that the proposed algorithm is superior to other algorithms on overall accuracy and kappa coefficient.
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