Cheng Zu-hai

An Efficient Pulsed CH3OH Terahertz Laser Pumped by a TEA CO2 Laser

An efficient pulsed CH3OH terahertz (THz) laser pumped by a TEA CO2 laser is investigated experimentally. To improve photon conversion efficiency and THz laser energy, two cavity configurations of the TEA CO2 laser, which is external and semi-external, are evaluated. The pump intensities are about 4.7 MW/cm2 and 1.2 MW/cm2, respectively. Higher pump intensity and more stable single lines are obtained in the external cavity. For the 3.8 J pump energy of the 9P(16) transition in the external cavity, the maximum terahertz output energy with 570.5 μm wavelength at 160 Pa is 431 μJ. With a 6 J energy pulse in terms of a semi-external cavity, a 353 μJ terahertz emission (570.5 μm) is produced. The corresponding photon conversion efficiencies are 1.36% and 0.705%, increasing by a factor of about 2.

Generation of Intense THz Pulsed Lasers Pumped Strongly by CO2 Pulsed Lasers

A theoretical method dealing with two intense laser fields interacting with a three-level molecular system is proposed. A discussion is presented on the properties of the solutions for time-independent and time-dependent absorption coefficients and gain coefficient on resonance for strong laser fields, based on analytic evaluation of the rate equations for a homogeneously broadened, three-level molecular system. The pump intensity range can be estimated according to the analytic expression of pump saturation intensity. The effects of pulse width, gas pressure and path length on the energy absorbed from pump light are studied theoretically. The results can be applied to the analysis of pulsed, optically pumped terahertz lasers.

Terahertz Generation in Nonlinear Crystals with Mid-Infrared CO2 Laser *

The terahertz (THz) generation based on difference frequency generation in nonlinear optical crystals pumped by mid-infrared CO2 laser has been investigated. We present a comprehensive study of the phase-matching conditions in the GaSe, ZnGeP2 and GaAs crystals. A comparison of the characteristics of these crystals as the THz frequency generator is also presented. The investigation of the conversion efficiency shows that GaSe and GaAs are the most promising nonlinear crystals for the efficient and widely tunable THz generation.

A Terahertz Wavemeter Based on a Fabry—Perot Interferometer Composed of Two Identical Ge Etalons

A simple and convenient terahertz wavemeter based on a Fabry—Perot interferometer (FPI) is presented. The interferometer is composed of two identical Ge etalons, which act as high-reflectance mirrors for terahertz waves. The transmission characteristics of the Ge FPI are analyzed using multiple-beam interference theory. The theoretical finesse of the FPI, defined as a ratio of 2π to the phase halfwidth of the transmission fringes, is larger than 12.5. Here, the wavemeter is used to measure the wavelengths of an optically pumped NH3 terahertz laser. The experimental results indicate that the measuring uncertainties are within ±1%. Higher accuracy can be expected if the power or pulse energy of the terahertz source is more stable.

An Efficient Photon Conversion Efficiency Ammonia Terahertz Cavity Laser

An efficient ammonia terahertz (THz) cavity laser is reported experimentally. Unlike the past design schemes such as hole couplers and freestanding mesh couplers, in our systems the input and output couplers are fabricated by depositing nickel capacitive metallic meshes on ZnSe and high-resistivity silicon substrates. Thus the couplers not only can be constructed as an F-P oscillator but also can be used as sealed windows that are easier to perform the adjustment of alignment with. To enhance THz laser output energy and photon conversion efficiency, the dominant factors such as pump intensity and gas pressure are investigated experimentally. Finally, a 1.35 mJ terahertz radiation of ammonia laser with 90 μm wavelength (3.33 THz) operating at 1.09 kPa pumped by a 402 mJ TEA CO2 laser with 9R (16) line is generated, and photon conversion efficiencies of 6.5% are achieved.

Pulsed CH3OH terahertz laser radiation pumped by 9P(36) CO2 lasers

An efficient pulsed CH3OH terahertz (THz) laser pumped by a TEA CO2 laser was investigated experimentally. A simple terahertz cavity and a TEA CO2 laser for the optically pumped THz radiation were studied experimentally. To improve THz laser energy and photon conversion efficiency, two different TEA CO2 lasers were developed to pump CH3OH. When CH3OH was pumped by the 9P(36) line with different powers of the CO2 laser, the generation of terahertz radiation with energy as high as 0.307mJ and 23.75mJ were obtained, respectively. The corresponding photon conversion efficiencies were 0.29% and 2.4%. The photon conversion efficiency increases by a factor of about 8. Meanwhile, higher peak power of pump laser effectively improves the photon conversion efficiency. And the optimum THz laser pressure increases with narrower pulse width of pump laser because of increasing absorptive gases molecules of CH3OH with higher peak power of pump laser.

Experimental Research on Plasma Induced by TEA CO2 Laser Propulsion

Results in the air-breathing propulsion experiments with a parabolic light craft and a self-made UV-preionized 100 J TEA CO2 laser device are presented. Air disturbance and the spectrum of the plasma after the interaction of pulsed laser radiation with the light craft were studied. It was found that the focal length of the parabolic light craft had a significant effect on the air-disturbance. Two shock waves were detected for the longer focal length, while only one shock wave detected for the short focal length. The spectrum of the laser-induced plasma, the distribution of the characteristic lines, and the temporal behaviors of the air plasma were studied in detail. The results showed that, the evolution of the laser-induced plasma lasted 20 μs, and the plasma spectrum would reach the maximum intensity at 7 μs.

Spectra of laser acoustic signals in water

To get the spectra of laser acoustic signal (LAS) in water induced by CO2 laser pulse, and the relation to the parameters of laser pulse (LP). A peak-tail approximation of the actual CO2 LP is used to get the theoretical thermoelastic LAS, after FFT(Fast Fourier Transformation), the affection of the waveform parameters to the LAS spectra is given. And the higher frequency part is affected mainly by the peak while the lower frequency part is affected by the tail. Affection of attenuation to the spectra of LAS is researched and showed spectra at different distance to the source. Experimental research shows that: The spectra of LAS remain unchanged at the same experiment condition and by reducing gas pressure to 5kPa, LAS with peak frequency at 16kHz is obtained.

Theoretical Design of a Wedged Output Coupler Coated by Capacitive Strip-Grating for Optically Pumped Terahertz Lasers

An output coupler for optically pumped terahertz laser consisting of capacitive strip-grating and wedged high-resistivity silicon substrate is designed theoretically and its transmittance performance is also discussed. The etalon effects frequently occurring in previous experiments are effectively suppressed, and thus a flat and accurate transmittance spectrum is obtained in a narrow wavenumber interval of 2 cm−1 near a particular center wavenumber. Furthermore the transmittance sensitivity to the slight shift of substrate thickness is also completely eliminated. The wedged output coupler is easy to fabricate and its substrate may be used repeatedly to meet various transmittance requirements.