Shiyou Fu

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

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.

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

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.

Research on laser wavelength measurement with rotating Fabry–Perot mirror

Abstract. A method using rotating Fabry–Perot (FP) mirror to measure CO2 laser wavelength was developed. The variation of FP transmittance changing with laser incident angle was calculated theoretically and the variation curve was given. The calculation illustrates that the variation of FP reflectance with incident angle 0 to 30 deg has little effect on the transmittance of FP. In the experiments, the CO2 laser transmittance variation of FP was measured at a wavelength of 9.27 μm. To improve the measurement precision of the laser wavelength, the method using the centrosymmetric peaks of FP transmittance curve in the range from −20 to +20  deg of laser incident angle was proposed. The precision of the measurement is about 0.01 μm. The experiment result is consistent with theoretical analysis, which demonstrates the feasibility of the laser wavelength measurement using rotating FP method.

Study of Offset Frequency Locking and Active Frequency Stabilization for Two RF-Excited Waveguide CO 2 Laser With Common Ground Electrode

In this paper, we designed the offset frequency locking and active frequency stabilization system for the continuous-wave (CW) and electro-optical Q-switched RF-excited waveguide CO2 lasers to provide an ideal light source for coherent radar. The CW laser is used as a local laser, and the pulsed laser is used as a main laser, which can produce Q-switched and cavity-dumped laser synchronously. A closed-loop laser frequency control system was used to stabilize the Q-switched laser at the gain curve center frequency and to lock the two lasers at a selected heterodyne frequency. The system includes two parts: 1) pulsed laser frequency stabilization and 2) offset frequency locking. The frequency stabilization part can keep the Q-switched laser oscillating at the peak of the gain curve by measuring the pulse build-up time continuously and controlling the cavity length. The offset frequency locking circuits keep the heterodyne frequency around the selected frequency by measuring the frequency of the beat signals and controlling the CW laser cavity length. In the experiment, the long-term frequency shift of the pulse laser was measured to be less than ±16 MHz and the offset frequency fluctuation was less than ±2 MHz when the frequency control system was in a closed-loop operation.

Study of stable laser cavity with hole-coupling output mirror

A new type of CO2 laser resonator with hole-coupling output is presented in this paper. The laser beam in the resonator is focused into the reflector assembly system composed of a reflector with a small central hole and an output mirror with a central coupling hole. According to the theory of diffraction optics, the laser optical field distribution in the cavity and the far-field distribution are analyzed by numerical calculation. In the experiment, the variations of the laser output power with continuous current discharged is measured. The maximum laser output power is 14.5W at the discharge current 12mA. The laser power distribution of the output beam at the distance of 1m is measured, and the result is in good accordance with numerical calculation. The laser cavity with hole-coupling can be used in intracavity pumped THz laser output and other types of intracavity pumped lasers.

Study on high power ultraviolet laser oil detection system

Laser Induce Fluorescence (LIF) is a widely used new telemetry technology. It obtains information about oil spill and oil film thickness by analyzing the characteristics of stimulated fluorescence and has an important application in the field of rapid analysis of water composition. A set of LIF detection system for marine oil pollution is designed in this paper, which uses 355nm high-energy pulsed laser as the excitation light source. A high-sensitivity image intensifier is used in the detector. The upper machine sends a digital signal through a serial port to achieve nanoseconds range-gated width control for image intensifier. The target fluorescence spectrum image is displayed on the image intensifier by adjusting the delay time and the width of the pulse signal. The spectral image is coupled to CCD by lens imaging to achieve spectral display and data analysis function by computer. The system is used to detect the surface of the floating oil film in the distance of 25m to obtain the fluorescence spectra of different oil products respectively. The fluorescence spectra of oil products are obvious. The experimental results show that the system can realize high-precision long-range fluorescence detection and reflect the fluorescence characteristics of the target accurately, with broad application prospects in marine oil pollution identification and oil film thickness detection.

Fluorescence instrument based on direct view holographic grating prism for remote sensing

We present an optical receiving system for LIF lidar using a direct view spectrometer based on holographic grating prism. The proposed receiving optical system consists of receiving telescope, slit, collimating lens, holographic grating prism, objective lens and ICCD camera. The receiving optical system based on this dispersion structure can not only reduces the optical distortion to offer a high optical efficiency, but also has a more compact structure which is very suitable for spectral dispersion of remote target. The system adopted an intensifier coupled a CCD to make up an ICCD camera. Based on real-time background subtraction algorithm, 60fps fluorescence spectrum can be obtained in real time. System validation experiment uses a semiconductor laser as excitation source to illuminate oil target to radiate fluorescence at a distance of 30 m. The fluorescent signal is received by the set up LIF lidar receiving optical system, and clear spectrum image is obtained. The designed in-line, direct view configuration holographic grating prism spectrometer owns the advantages of high light throughput, less optical distortions, compact structure, small volume and easy operation, which make a practical portable receiving optical system.

Study on Shipborne Video Electro-optical Tracking System Based on FPGA

Shipborne video electro-optical tracking system can be used for ocean target detection and recognition, marine rescue, etc. In this paper, an FPGA is selected as the core processor to realize the target detection and servo tracking. We propose a colour matching centroid tracking algorithm to solve the impact of complex water surface changes. Test results show that the system works normally and stably. In the 640 × 480 video image resolution, the tracking accuracy of the azimuth motor is 80 pixels, and the maximum tracking angular velocity is 30°/s. The tracking accuracy of the pitch motor is 50 pixels, and the maximum tracking angular velocity is 10°/s. The minimum size of target recognition is 16 × 16 pixels matrix.

A portable direct view configuration prism spectrometer using a double Amici prism

In this paper, we present a prism spectrometer that exploits a double Amici prism dispersion structure. The system consists of a slit, a collimating lens, a double Amici prism, an imaging lens and a CCD. The incident light enter into slit, and then is paralleled by a collimating lens to the double Amici prism. The double Amici prism is used to realize spectral dispersion. The dispersed light is collected by an imaging lens and image on the photosensitive surface of the CCD. The dispersion resolution is theoretical analyzed from the ray tracing point of view. In addition, the imaging position on CCD element at different wavelength is presented according to nonlinear curve of dispersion. The designed prism spectrometer can obtain a high light throughput and less optical distortion spectrum in the spectral range of 370-700nm. In experiment, we measured the spectral resolution of the designed prism spectrometer at five wavelength used a grating monochromator. The designed in-line, direct view configuration prism spectrometer owns the advantages of high light throughput, less optical distortions, compact structure, small volume and easy operation, which has important role in application of laser spectral measurement especially laser remote sensing spectral detection.

Laser output of radial-slab solid-state laser

In this letter, numerical simulation and experimental study of a radial-slab solid-state laser are presented. The laser includes four crossing-slabs pumped by four Xe flashlamps. The numerical simulation of coherent intensity in the near field and the far field indicates that the laser with the structure can improve the quality of output beam compared with incoherent beam combination. The radial-slab solid-state laser is fabricated, and initial experiments are carried out at a pulse repetition of 1 Hz. Nine beams in the near field and one combined beam in the far field are obtained in our initial experiment. The experimental results are consistent with the numerical analysis in the coherent condition. The results show that coherent beam combination is obtained by this laser.