KEYWORDS: Heterodyning, Photodetectors, Signal detection, Signal processing, Signal attenuation, Multiplexing, Time correlated single photon counting, Background noise, Sensors, LIDAR
Laser heterodyne detection can obtain multidimensional information such as range, velocity, and micro-vibration. However, the traditional linear detection mode using PIN and APD photodiodes faces challenges of better sensitivity, especially in remote target detection. Developments of single photon detectors (SPD) such as the GM-APD array and SNSPD array provide the foundations for more robust and sensitive detection. Photon heterodyne detection (PHD) combines the advantages of heterodyne detection and single photon detection, which can obtain target multidimensional information with high sensitivity. The PHD performance is closely related to SPD working mode, and SPD single pixel detection mode requires a long accumulation time and the maximum photon count rate (PCR) is limited due to the dead time. In this paper, we propose a GM-APD pixel unit multiplexing (GPUM) method to improve the PHD performance. Theoretical analyses are conducted including the detection probability and SNR. The influence factors of coherent efficiency, photon number, and accumulative number are discussed in detail considering the PHD waveform recovery and SNR improvement. In addition, the heterodyne detection experiments are carried out using linear detection and photon detection, and the velocity extraction performance are analyzed in detail. This research is of significance to reveal the PHD process and optimize the coherent lidar systems design.
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 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.
In recent years, biotechnology has been widely used in production and living. The research on processing biomaterials and developing them into new functional types has also been gradually carried out. In this paper, the influence of water content of microbial material on the extinction properties of infrared band was studied from the aspects of composition and structure. The sample of moisture content of microbial was established. The qualitative law between the water content of microbial particle and its absorption property was given. The quantitative relationship between the water content of microbial particle and its scattering property was calculated. Under the conditions of strong attenuation, the optimal solution of microbial materials based on water content is designed. The results show that when the transmittance is less than 10%, the complex refractive index n of microbial particles can be changed within the range of 0 < Δn ≤ 0.072 by controlling the amount of intracellular bound water. The controllability and variability of complex refractive index n can improve the extinction performance of microbial materials in the infrared band by up to 50%.
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.
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