Xiaoyong Hu

Rapidly tuning miniature transversely excited atmospheric-pressure CO2 laser.

An experimental study of a rapidly tuning miniature transversely excited atmospheric-pressure CO2 laser is reported. To rapidly shift laser wavelengths over selected transitions in the 9-11 microm wavelength region, we have utilized a high-frequency stepping motor and a diffraction grating. The laser is highly automated with a monolithic microprocessor controlled laser line selection. For the achievement of stable laser output, a system of laser excitation with a voltage of 10 kV, providing effective surface corona preionization and allowing one to work at various gas pressures, is utilized. Laser operation at 59 emission lines of the CO2 molecule rotational transition is obtained and at 51 lines, the pulse energy of laser radiation exceeds 30 mJ. The system can be tuned between two different rotational lines spanning the wavelength range from 9.2 to 10.8 microm within 10 ms.

Simple lidar detecting wake profiles

A simple lidar to detect wake profiles based on laser backscattering by bubbles at 180° is reported. The monostatic optical geometry is adopted in this lidar and a theoretical model for measuring light scattered backwards by bubbles at 180° is given. The profiles of the wakes produced by a two-blade propeller with a diameter of 46 mm at 6000 rpm (revolutions per minute) and 8000 rpm are achieved. Thus the wake region is identified and the fine structure within wakes is detected. Also, the repeatability of the simple lidar is tested through many experiments. Results show that it is possible to detect wakes using laser backscattering by bubbles at 180°.

Passively Q-switched laser with single longitudinal mode based on the frequency selection of grating and F-P etalon in twisted-mode folded cavity

A passively Q-switched side-pumped laser with folded resonator is specially constructed for singlelongitudinal-mode smooth pulse output. Nd:YAG is chosen as the laser active medium and Crlsupg4+l/supg:YAG as the saturable absorber medium. Additionally, the method of frequency selection by grating with 1200 line/mm and Fabry-Perot (F-P) etalon is used in the twisted-mode cavity. The single-frequency smooth pulses are produced with 10-Hz repetition rate, 20-ns pulse width, and 1.064-\mu m wavelength. The probability of single-frequency laser output measured is over 99% by using the methods of Fourier analysis and F-P etalon multiple-beam interferometry at the threshold voltage. The measured near-field and far-field angles of divergence are 1.442 and 1.315 mrad, respectively. The values of Mlsupg2l/supg are 1.32 and 1.31 separately with the knife-edge method. Single pulse at 1.064 \mu m with the energy of 8.8 mJ is achieved in TEMlsubg00l/subg mode.

Rapidly tuning miniature TEA CO2 laser – rotating mirror and grating mechanism

Abstract In this research, directed toward using differential absorption lidar (DIAL) for measuring concentrations of pollutant gases, a device for rapidly tuning a transversely excited atmospheric-pressure (TEA) CO 2 laser is presented. It is shown that it is possible to utilize a rotating six-sided scanning mirror and a fixed diffraction grating to rapidly switch wavelength over randomly selected lasing transitions in the 9–11 μm region of the spectrum. The scanning mirror and an optical encoder are driven by a hysteresis synchronous motor at a speed of 1500 rpm. A surface-wire-corona preionization was utilized in a cavity. The laser system is highly automated with microprocessor-controlled laser line selection. Single-branch emission at two wavelengths with time interval ⩽10 ms has been obtained from a single cavity TEA CO 2 laser. An accurate line selection has been demonstrated in over 40 transitions at a pulse repetition frequency of up to 100 Hz. The laser energy at first-order couple output was up to 20 mJ per pulse and the pulse width is about 60 ns in an active volume of 36 cm 3 .

Five temperature mathematical modeling of TEA CO2 laser

Five temperature model is used to describe the process of the dynamics preferably in the transversely excited atmospheric-pressure (TEA) CO2 laser. In this paper five temperature vibrational dynamics and vibrational-rotational dynamics are analyzed and calculated. All physical constants and relaxation rates related to this model are examined. The laser pulse waveform can be calculated when varied lasers parameters. The output power and energy can be obtained from calculated intensity, it can provide theoretical basis for laser design. Theoretical arithmetic shows a good agreement with the experimental result.

A miniature tunable TEA CO2 laser using isotope 13C16O2 and 12C16O2

A miniature tunable TEA CO2 laser using isotope 13C16O2 as the active medium is developed to extend the spectral range of CO2 lasers for further application. The optimization of the energy parameters of the tunable TEA 13C16O2 laser and the same laser using 12C16O2 are studied. When a gas mixture (13C16O2: N2: He = 1: 1: 3) at a total pressure of 6.4 × 104 Pa is used, the TEA 13C16O2 laser of a 45-cm3 active volume obtains 51 emission lines in the [0001–1000] and [0001–0200] bands. The maximum pulse energy of the TEA 13C16O2 laser is about 357 mJ. The same laser using the conventional gas mixture (12C16O2: N2: He = 1: 1: 3) at a pressure of 6.66 × 104 Pa is measured to obtain 69 laser emission lines and the maximum pulse energy of laser radiation is about 409 mJ.

Efficient CO2 frequency doubling with Hg1-xCdxGa2S4

As it is known well, using of frequency doubling in differential absorption CO2 laser lidars increases the number of atmospheric gases under the control. Additional improvement in potential and gas analyses accuracy, technical and exploitation parameters has to be waited for with design of frequency doublers with more efficient nonlinear crystals. In this work for the first time detailed investigation results are represented on physial properties of new mixed nonlinear crystals grown in accordance with diagram HgGa2S4:CdGa2S4→Hg1-xCdxGa2S4, phase-matching and potential efficiencies of second harmonic generation, so as on results of experimental investigation of TEA and mini TEA CO2 laser frequency doubling with it use. In spite of two time lower nonlinear susceptibility coefficients of both parent crystals in comparison with most efficient middle IR crystals high efficiency of frequency doubling is fixed in mixed crystals at room temperature. It is tree time in comparison with frequency doubling with, for example, popular ZnGeP2 and 5.5 time with AgGaSe2. These advantages are reached because realization of optimal non-critical phase-matching by choose of mixing ratio x=0.5, so as 2.3 time higher damage threshold, lower phonon absorption at CO2 laser wavelengths and lower meanings of refractive indexes at fundamental and second harmonic wavelengths. Exploitation parameters of mixed Hg1-xCdxGa2S4 crystal doublers are not worse than parameters of doublers with well-known crystals.

Second harmonic generation in tunable TEA CO2 laser

3~5um mid-infrared laser displaying good atmospheric transmitting properties is mostly used as laser source, in many applications such as remote sensing for air pollution determining. And it is usually obtainable in Second harmonic generation (SHG) in CO2 laser. In the course of investigation, a homemade nonlinear optical crystal AgGaSe2, of the size 7×8×12mm3, was used for SHG in tunable TEA CO2 laser with different wavelengths. And, 12 coherent laser sources in 3~5μm from CO2 spectrum region 10.6μm~9.2μm were obtained. In our results, phase-matching angles aqre in good accordance with the crystals cutting. Theory calculating on conversion was made as well, to compare with experimental data which present changes of energy conversion under pumps rising. And in the interest of enhancing energy conversion by means of peaking pulse through cutting tail, a plasma shutter, argon charged in body under normal air pressure, was effectively arranged in experiment. As a result, about 50nS width peak pulse was generated, and up to 10% energy conversion (2mJ, 5.3μm) was achieved against 1% without shutter, both pumping in 10P(20) line.

Processing methods in frequency domain for bubble laser scattering signals

For the detection of the bubble-laser-scattering property, it is the key to improve the signal to noise ratio. The various noises in the bubble-laser-scattering signal, the background light, power frequency of 50Hz, radio frequency noise and inherent noise within the electronic system, are analyzed in detailed. Two frequency-domain methods used to process the bubble scattering signal, the Fourier transform and the power spectral estimation, are focused on. These two methods can reduce the noise, isolate the DC component and eliminate the impact of the power frequency of 50Hz, thus the pulse of bubble laser scattering could be detected. The physical meaning for the power spectral estimation is much clearer than that of the Fourier transform. The power spectrum curve obtained by using the Yule-Walker AR (regression model) method is much smoother and its frequency resolution is higher than the other methods. The results show that the Yule- Walker AR spectrum estimation is the most reasonable and most effective method.

The backscattering property experiment research of wake bubbles

The bubble backscattering intensity between 140 and 160 degree is achieved utilizing bistatic method, and the bubble scattering signal is detected, analyzed and processed. The photo multiplier tube is used to detect the weak signal. Threshold detection distinguishes between the bubble backscattering and water backscattering. Bubble scattering pulses are stationary random signals and have statistic properties. So the sample average for an enough long time may be as the scattering intensity at a certain angle. Experimental results have the same trend as the theoretical results. The experimental research on the bubble backscattering properties is importance to the ocean sensing and wake homing.