High Efficient Optically Pumped CH₃F Gas Terahertz Laser At 192μm

Optically pumped gas THz laser is a powerful way to generate THz laser with high energy and beam quality. It can be applied in numerous of fields including THz coherent detection, THz spectroscopy and plasma diagnostics. The optically pumped gas THz laser has developed continuously in the past decades, while it has some difficulties to overcome such as the bulky structure and low efficiency. Especially, a high-performance dichroic mirror in infrared/terahertz band is difficult to manufacture, which limits the development of a high efficient optically pumped gas THz laser. In addition, 192μm THz wave can transmit many materials with its frequency within the atmospheric window, and this attracts much attention in recent years. Consequently, investigation on optically pumped CH₃ F gas THz laser is carried on theoretically and experimentally, in order to obtian high efficient THz laser at 192μm.Theoretically, a four-level rate equation model is established to describe the dynamic process of the optically pumped CH₃ F gas THz laser at 192μm. The pump laser is simulated with a short pulse width of 100 ns, and the Runge-Kutta method is used to solve the rate equations numerically. The influences of pump energy, gas pressure, cavity length and output coupling transmittance on laser output characteristics are discussed, including the pulse delay time, pulse width, THz laser energy and energy conversion efficiency（ECE）. The results can provide a theoretical derection for the system design and the experimental investigation.Experimentally, by investigating absorption and gain characteristics of the CH₃ F medium, the condition to achieve stimulated amplification is discussed for 192μm radiation. Based on the reflection of the pump input window at 192μm, double-pass amplified THz radiation is obtained from a mirror-less cavity. The influences of gas pressure, pump energy and gain medium length on THz output energy are discussed. When the pump energy is 600 m J, CH₃ F pressure is 1350 Pa and the gain medium length is 185 cm, THz pulse energy of 0.48 m J is obtained from the double-pass amplification process, which is 5.3 times in comparison with the output energy from the single-pass amplification process. The mechanism of the optically pumped CH₃ F mirror-less THz laser is also discussed by investigating the pressure dependence of the THz intensity, pulse delay time and pulse width. It is indicated that 192μm THz radiaton is mainly generated from ASE process. Furthermore, a Ge F-P interferometer is used to measure THz wavelength, and the measured value is about 187.2μm.With a Zn Se window and a crystal quartz output coupler, THz laser oscillator is achieved to generate 192μm laser. The influences of pump energy, cavity length and output coupling transmittance on the THz laser energy are discussed respectively. When the pump energy is 600 m J, the cavity length is 100 cm, and the output coupling transmittance is 78%, pulse energy of 0.6m J is obtained for 192μm THz laser, with an ECE of 0.1%. Based on the knife-edge technique, it is found that the transverse intensity distribution of the 192μm laser accords with the Gaussian distribution, which indicates the 192μm laser is a fundamental transverse modal one. The horizontal and vertical divergence angle is measured to be 17 mrad and 17.3mrad respectively. When the room temperature is 25℃and the air humidity is 50%, the atmospheric attenuation coefficient at 192μm is measured to be 0.012cm-1. Furthermore, 192μm THz laser is able to pass through papers, plastics and organic glass. These characteristics indicate 192μm THz laser is a potential source for THz detection and imaging in short distance.Based on an anti-reflection coated Ge mirror serve as the dichroic beam splitter（DBS）, a high efficient optically pumped CH₃ F gas THz laser at 192μm is presented. The 30-inclined Ge DBS can provide a dichroic performance of 98% transmitted to the 10.17μm laser and 76% reflected to the 192μm laser simultaneously, and this efficiently decreases the intrinsic loss in THz cavity. When the pump energy is 600 m J, the cavity length is 100 cm and the output coupling transmittance is 64%, pulse energy of 0.92 m J is obtained for 192μm THz laser with an ECE of 0.16%. It is the highest ECE for an optically pumped gas THz laser at 192μm.By tuning the pump laser lines and the optimal incident angles of the Ge DBS, another 4 efficnet THz laser lines are obtained, with wavelengths of 181μm, 261μm, 360μm and 496μm. The 261μm and 360μm line can both give higher ECE in comparison with the previously reported work. It is indicated that Ge DBS can be efficiently applied to optically pumped gas THz lasers with wavelengths from 100μm to 500μm.