In the streak tube laser imaging system, there are two conflicts: the first is between high spatial resolution and wide field of view (FOV). The second is between high temporal resolution and deep depth of field (DOF). In this paper, a new non-scanning streak tube laser imaging system is presented. A microlens array with three different apertures is used to non-uniformly collect data from the image plane so the conflict between high spatial resolution and wide FOV is relieved and the detectable range of system is also increased. A remapping fiber optics with special design is used to realign the image plane on the two photocathodes of streak tubes to realize the operation mode of the two streak tubes so the conflict between high temporal resolution and deep DOF is relieved. The mathematical model of the entire imaging system is established based on the range equation. The structure parameters of the receiving optical system are optimized in order to achieve the optimal utilization rate of light energy. In the third, three simulated contrast experiments are organized, and the experiment results demonstrate that the imaging system proposed in this paper possesses properties of higher spatial resolution, wider FOV, higher temporal resolution, deeper DOF, and larger detectable range.

Design and modeling of three-dimensional laser imaging system based on streak tube

Investigation on the thermohaline structure of the stratified wake generated by a propagating submarine

The challenges associated with acquiring the clear images of objects in underwater environment are difficult to overcome due to the absorptive and scattering nature of seawater. Recently, the research community has focused on mitigating these effects. The recent developments in image enhancement algorithms and strategies of signal light enhancement have brought improvement in some application areas. In this work, we review the six most common methods based on signal light enhancement. We present the individual working mechanisms, latest representative advances, and suitable application conditions. Moreover, we also present a detailed comparison of these techniques. In each technique, we present their applicable environments and conditions according to the following indicators: operating distance (from 2 attenuation lengths (AL) to 13.5 AL), resolution (from centimeter to millimeter), and field of view (FOV). By summarizing and analyzing the existing problems that restrict the underwater optical imaging techniques, the future development trends are prospected.

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Underwater Optical Imaging: Key Technologies and Applications Review

In a longitudinally excited CO2 laser that had a 45 cm-long discharge tube with a 1:1:2 mixture of CO2/N2/He gas at a pressure of 3.0 kPa, we realized the generation of a short laser pulse with a spike pulse width of about 200 ns and a pulse tail length of several tens of microseconds, control of the energy ratio of the spike pulse part to the pulse tail part in the short laser pulse, the generation of a long laser pulse with a pulse width of several tens of microseconds, and control of the pulse width in the long laser pulse, by using four types of excitation circuits in which the capacitance was adjusted. In the short laser pulse, the energy ratio was in the range 1:14–1:112. In the long laser pulse, the pulse width was in the range 25.7–82.7 μs.

Control of laser pulse waveform in longitudinally excited CO 2 laser by adjustment of excitation circuit

Brillouin-based LiDAR is an alternative remote sensing technique for measuring the distribution profiles of temperature, salinity, and sound speed in the upper ocean mixed layer. Its principle is based on the dependence of Brillouin frequency shift on the temperature, salinity, and depth of ocean. Therefore, it is necessary to investigate the effect of various seawater parameters on Brillouin frequency shift for ocean remote sensing by using the Brillouin LiDAR. Here we theoretically and experimentally investigate the influence of temperature, salinity, and pressure (depth) of seawater on Brillouin frequency shift in the upper ocean for the first time. Numerical simulations of the distribution profiles of temperature, salinity, and Brillouin frequency shift in the upper-ocean mixed layers of East China Sea and South China Sea were performed, respectively, by employing the Brillouin equations and the World Ocean Atlas 2018 (WOA18). A special ocean simulation system was designed to carry out the stimulated Brillouin scattering (SBS) experiments for validating the numerical simulations. The results show that the seawater temperature is the most important factor for the Brillouin frequency shift in the upper-ocean mixed layer compared with the salinity and pressure. At the same salinity and pressure, the frequency shift increases by more than 10 MHz for every 1 °C increase in temperature. Also, the differences of Brillouin frequency shift between experimental and theoretical values at the same parameter conditions were analyzed. The experimental results coincide well with the theoretical simulations. This work is essential to future applications of Brillouin LiDAR in remote sensing of the temperature, salinity, or sound velocity profiles of ocean.

Influence of temperature-salinity-depth structure of the upper-ocean on the frequency shift of Brillouin LiDAR.

In this paper, a new method of measuring a water-stimulated Brillouin scattering (SBS) frequency shift by optical coherent detection is presented, in order to remote-sense the underwater temperature of the ocean. A single longitudinal mode, passively Q-switched pulsed Nd:YAG laser is used as the light source, the water SBS beam is used as the signal beam, and a portion of the incident laser beam is used as the local oscillator. The heterodyne is detected by a high-speed photodetector, and the heterodyne frequency is the Brillouin frequency shift. Therefore, the underwater temperature can be determined according to the relationship between the Brillouin frequency shift and the water temperature. To test and verify its practicability, the heterodyne waveforms at different water temperatures are recorded in the laboratory with a wide-band oscilloscope, and the Brillouin frequency shifts are deduced by a Fourier transform. The experimental results are consistent with the theoretical analysis. This work provides the foundation for the development of a water temperature measurement system based on coherent Brillouin scattering.

Theoretical and experimental investigations on measuring underwater temperature by the coherent Brillouin scattering method.

Optically pumped terahertz (THz) lasers with high pulse repetition frequency are designed. Such a laser includes two parts: the optically pumping laser and the THz laser. The structures of the laser are described and analyzed. The rate equations for the pulsed THz laser are given. The kinetic process and laser pulse waveform for this kind of laser are numerically calculated based on the theory of rate equations. The theoretical results give a helpful guide to the research of such lasers.

Optically pumped terahertz lasers with high pulse repetition frequency: theory and design

In this letter, we develope a control and image processing system for Streak Tube Imaging Lidar (STIL). In the system, the data acquisition card control and the software interface are programmed in Visual Basic (VB) while the image processing is finished by MATLAB. A STIL imaging experiment is carried out in the laboratory. We obtained the intensity and range images of targets with pseudo color by image processing and reconstruction for a set of raw streak images of targets at different distances acquired by STIL. The range resolution is better than 2 centimeters.

Control and image processing for streak tube imaging lidar based on VB and MATLAB

A new method for laser-frequency stabilization by controlling the pulse setup time is presented. The frequency-stabilization system monitors the pulse setup time continuously, and controls it by adjusting the cavity length. Laser frequency is stabilized to the center of the gain curve when the setup time is the shortest. The system is used to stabilize a radio-frequency-excited waveguide CO2 laser tuned by grating, and the shift of laser frequency is estimated to be less than §25 MHz for an extended period. The system has the advantages of compact structure, small volume, and low cost. It can be applied for frequency stabilization of other kinds of pulsed lasers with adjustable cavity.

Frequency stabilization of pulsed CO2 laser using setup-time method

The invention relates to an area array charge coupled device (CCD) prism spectrograph and a space-time accumulation data processing method thereof. The area array CCD prism spectrograph comprises a direct vision prism decomposition system, a convergent lens, an area array CCD and a computer. The direct vision prism decomposition system scatters light signals to be measured into monochromatic light with different wavelengths, and the monochromatic light is focused through the convergent lens to form a continuous linear spectrum with wavelengths arranged in a progressive increase mode. The area array CCD outputs corresponding charge quantity signals to the computer, and the computer is used for conducting data processing through space-time accumulation to generate a spectrogram to achieve display and storage of the spectrum to be measured. By means of the method, the sensitivity and signal to noise ratio of a spectrograph are effectively improved, and the method is especially suitable for fast measuring of weak spectrum signals. The area array CCD prism spectrograph is simple in structure, easy to machine and adjust, low in production cost, small in size, convenient to operate and wide in application range.

Shiyou Fu

Papers

Theoretical and experimental investigations on measuring underwater temperature by the coherent Brillouin scattering method.

Optically pumped terahertz lasers with high pulse repetition frequency: theory and design

Control and image processing for streak tube imaging lidar based on VB and MATLAB

Frequency stabilization of pulsed CO2 laser using setup-time method

Area array charge coupled device (CCD) prism spectrograph and space-time accumulation data processing method thereof