Temporal behavior of the high-power pulsed gas terahertz laser pumped by a fundamental mode TEA CO2 laser.

Optically pumped gas molecular terahertz (THz) lasers are promising for generating high-power and high-beam-quality coherent THz radiation. However, for pulsed gas THz lasers, the temporal behavior of the output THz pulse has rarely been investigated. In this study, the temporal behavior of a pulsed gas THz pumped by a fundamental-mode TEA CO2 laser has been presented for the first time both in simulation and experiment. A modified laser kinetics model based on the density matrix rate equation was used to simulate the temporal behavior and output pulse energy of a pulsed gas THz laser at different gas pressures. The results clearly show that the working gas pressure and pump pulse energy have critical influences on the output THz pulse shape. Three typical pulse shapes were obtained, and the THz pulse splitting caused by gain switching was quantitatively simulated and explained based on the laser dynamic process. Besides, with an incident pump pulse energy of 342 mJ, the maximum output THz pulse energy of 2.31 mJ was obtained at 385 µm, which corresponds to a photon conversion efficiency of approximately 56.1%, and to our knowledge, this is the highest efficiency for D2O gas THz laser. The experimental results agreed well with those of the numerical simulation for the entire working gas pressure range, indicating that our model is a powerful tool and paves the way for designing and optimizing high-power pulsed gas lasers.

Thermal infrared and broadband microwave stealth glass windows based on multi-band optimization.

With the rapid development of detection technologies, compatible stealth in the infrared and radar ranges has become increasingly essential not only for military application but also for personal privacy protection. In this study, we design a metamaterial window that possesses stealth ability in both the thermal infrared and broadband microwave ranges, using a particle swarm optimization algorithm to realize multi-band optimization. We experimentally verify that the proposed structure can achieve over 90% microwave absorption in the range 5.1 to 19.2 GHz (covering the X and Ku bands), with low infrared emissivity (∼0.15), and also maintain visible transmittance above 60%. Moreover, the window retains good performance up to 200 °C owing to the intrinsic properties of the material. Our multi-band optimization method enables the application of the transparent metamaterial windows in electromagnetic shielding and stealth and can potentially be applied in smart window related industries.

A Sensitive Carbon Dioxide Sensor Based on Photoacoustic Spectroscopy with a Fixed Wavelength Quantum Cascade Laser.

A photoacoustic spectroscopy (PAS) based carbon dioxide (CO2) sensor with a fixed wavelength quantum cascade laser (FW-QCL) was demonstrated. The emission wavelength of the FW-QCL at 4.42 µm in the mid-infrared spectral region matched a fundamental CO2 absorption line. Amplitude modulation of the laser intensity was used to match the resonant photoacoustic (PA) cell. The noise from the background was reduced with the correlation demodulation technique. The experimental results showed that the sensor had excellent signal stability and a concentration linear response. When the integration time was 1 s, a 1σ minimum detection limit (MDL) of 2.84 parts per million (ppm) for CO2 detection was achieved. The long-term stability of the sensor was evaluated by means of an Allan deviation analysis. With an integration time of ~100 s, the MDL was improved to 1 ppm. This sensor was also used to measure the CO2 concentration from some common emission sources, such as cigarette smoking, automobile exhaust, and the combustion of some carbon-containing materials, which confirmed the stability and robustness of the reported FW-QCL based CO2-PAS sensor system.

High energy, widely tunable Si-prism-array coupled terahertz-wave parametric oscillator with a deformed pump and optimal crystal location for angle tuning.

A high energy, widely tunable Si-prism-array coupled terahertz-wave parametric oscillator (TPO) has been demonstrated by using a deformed pump. The deformed pump is cut from a beam spot of 2 mm in diameter by a 1-mm-wide slit. In comparison with a small pump spot (1-mm diameter), the THz-wave coupling area for the deformed pump is increased without limitation to the low-frequency end of the tuning range. Besides, the crystal location is specially designed to eliminate the alteration of the output position of the pump during angle tuning, so the initially adjusted nearest pumped region to the THz-wave exit surface is maintained throughout the tuning range. The tuning range is 0.58-2.5 THz for the deformed pump, while its low frequency end is limited at approximately 1.2 THz for the undeformed pump with 2 mm diameter. The highest THz-wave output of 2 µJ, which is 2.25 times as large as that from the pump of 1 mm in diameter, is obtained at 1.15 THz under 38 mJ (300 MW/cm2) pumping. The energy conversion efficiency is 5.3×10-5.

Si-prism-array coupled terahertz-wave parametric oscillator with pump light totally reflected at the terahertz-wave exit surface.

A Si-prism-array coupled terahertz (THz)-wave parametric oscillator with the pump totally reflected at the THz-wave exit surface (PR-Si-TPO) is demonstrated by manufacturing an 800 nm air gap between the crystal and the Si-prism array. Influence on the total reflection of the pump from the Si prisms is eliminated and efficient coupling of the THz wave is ensured by using this air gap. When the THz-wave frequency varies from 1.8 to 2.3 THz, compared with a Si-prism-array coupled TPO (Si-TPO) with the pump transmitting through the crystal directly, the THz-wave output energy is enhanced by 20-50 times, and the oscillating threshold is reduced by 10%-35%. Furthermore, the high end of the THz-wave frequency tuning range of the PR-Si-TPO is expanded to 3.66 THz compared with 2.5 THz for the Si-TPO.

Heterodyne Doppler velocity measurement of moving targets by mode-locked pulse laser.

In this study, heterodyne detection is adopted to measure the velocity of a target simulated by a rapidly rotating plate by using a mode-locked pulse laser as the resource. The coherent beat frequency of the signal light reflected by target and local oscillation light occurred on the surface of the detector. Then the waveform of beat frequency was processed by filtering to obtain the Doppler frequency shift of the signal light induced by target. With this frequency shift, the velocity of target could be obtained by calculation. Results indicate that the measurement has a high precision. The error on average is within 0.4 m/s.

Passive frequency stabilization in Nd:YAG pulse laser using reflective volume Bragg grating.

We demonstrate a stable Q-switched single-longitudinal-mode (SLM) Nd:YAG laser using a volume Bragg grating as the output coupler. The reflective volume Bragg grating, serving as a longitudinal selector and passive frequency stabilizer, effectively eliminates the mode hopping effect of the laser. The maximum output energy of the SLM obtained from the current experimental setup is 18.5 mJ. The maximum separation of frequencies is significantly less than the longitudinal mode separation, indicating that a stable SLM laser is achieved.

Single longitudinal mode oscillation from a multi-interferometric cavity.

A method to generate stable single longitudinal mode (SLM) radiation from a multi-interferometric cavity configuration that can be considered as the combination of one Michelson cavity and two Fox-Smith cavities is presented. A numerical model of the interferometric cavity is investigated to optimize the laser for mode selection, and experimental verification has been carried out in a tunable TEA CO(2) laser. Pulse output energy of 300 mJ at 10.6 mum has been obtained at repetition rate of 20 Hz, corresponding to a repetition of SLM operation of 100%. This result shows that this interferometric cavity gives better performance in mode selection than other cavities based on multibeam interference.

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.