Rongqing Tan

Theoretical model and simulations for a cw exciplex pumped alkali laser.

The Exciplex Pumped Alkali Laser (XPAL) system, which is similar to DPAL (Diode Pumped Alkali vapor Laser), has been demonstrated in mixtures of Cs vapor, Ar, with and without ethane. Unlike DPAL, it uses the broadband absorption blue satellite of the alkali D2 line, created by naturally occuring collision pairs. For example, Cs-Ar collision pairs have an absorption width which is as wide as the one of commercial semiconductor diode lasers. A continuous wave XPAL four-level theoretical model is presented in this paper. More factors are considered, such as the spectral dependence of pumped laser absorption for broadband pumping and the longitudinal population variation. Some intra-cavity details, such as longitudinal distributions of pumped laser and alkali laser, can also be solved well. The predictions of optical-to-optical efficiency as a function of temperature and pumped laser intensity are presented. The model predicts that there is an optimum value of temperature or pumped laser intensity. The analysis of the influence of cell length on optical-to-optical efficiency shows that a better performance can be achieved when using longer cell. The prediction of influence of Ar concentration and reflectivity of output coupler shows that higher optical-to-optical efficiency could be achieved if lower reflectivity of output coupler and higher Ar concentration are used. The optical-to-optical efficiency as high as 84% achieved by optimizing configuration with the pumped intensity of 5 × 10⁷ W/cm² presented shows that broadband pumped four-level XPAL system has a potential of high optical-to-optical efficiency.

Air pressure effect on propulsion with transversly excited atmospheric CO2 laser

The assessment of energy partition between air and solid propellant has been conducted using a transversely excited atmospheric CO2 laser. The experiments were performed by focusing output pulses of the laser (200-ns pulsewidth at 10.6-µ mw avelength and ∼10.6-J pulse energy) on aluminum targets mounted on a ballistic pendulum. Coupling coefficients and mass removal rates were determined as functions of air pressure, which varied from 1 atm to 3.5 mtorr. The data from both coupling coefficients and mass removal rates show that there is a sharp transition region ranging between 1.0 and 10 torr. In this region, the momentum imparted to the target via air breakdown appears comparable to the momentum due to the breakdown on the target surface. At pressures exceeding 10 torr, the coupling to the target due to air breakdown dominates the ablation.

Experimental Study of Coupling Coefficients for Propulsion on TEA CO2 Laser

The original purpose of this study was to address a partition of propulsive energy between air and metal, when the breakdown is initiated at the metal surface and/or in adjacent air space. Coupling coefficient as a function of air pressure varied in the range 4 mTorr – 1 atm is presented. The experiments were conducted by focusing output pulses of a TEA CO2 laser system (0.2‐μs pulsewidth at 10.6 μm wavelength and ∼ 10.0 J energy) on aluminum targets. Coupling coefficients were derived from the pendulum displacements.

Experimental Study on Laser Propulsion of Air‐breathing Mode

A series of experiments were done to investigate the effect of laser parameters on laser propulsion. A high power high repetition rate TEA CO2 laser was employed in the experiments. The output energy of the laser is up to 15J and the repetition rate is up to 150 pps. The light craft models in the experiments were two parabolic aluminum shells with the exit diameter of 50 mm, the focus lengths were 5mm and 10mm respectively. The effect of the laser’s repetition rate on coupling coefficient was mainly investigated. The value of optimal repetition rate for laser propulsion of air‐breathing mode is discussed. On the basis of our investigation, a setup of laser propulsion demonstration experiment was designed. And laser‐powered free flight demonstration was realized. The 4.2g parabolic craft with 10mm focus length was boosted by the TEA CO2 laser to the altitude of more than 2.6m (limited by the ceiling of the laboratory) in a flight lasting 1.75s. The output pulse energy is 13J and the repetition rate is 50 pps.

High-repetition rate industrial TEA CO2 laser with average output power of 1.5 kW

High power high repetition rate TEA CO2 laser has potential importance in material processing such as shock hardening, glazing, drilling, welding, and cutting for high damage threshold materials, as well as in chemical reaction and isotope separation. This paper describes a transverse-flow closed-cycle UV-preionized TEA CO2 laser with peak pulse power of 20 MW, maximum average power of 1.5 KW at repetition rate of 300 HZ. The laser has compact constructure of gas flow circulation system using tangential fans. With addition of small amounts of H2 and CO to the normal CO2-N2-He gas mixture, one filling sealed operating lifetime is up to millions of pulses. A novel spark gap switch has been developed for very high repetition rate laser discharge in the condition of high pulse power.

Quasicontinuous wave linearly polarized rubidium vapor laser pumped by a 5-bar laser diode stack

Abstract. We report a quasicontinuous wave (CW) linearly polarized rubidium vapor laser. The pumping source consists of five laser diode bars and its linewidth is reduced from the raw 1.8 to 0.2 nm by a bulk volume Bragg grating. Instead of adopting the “quasi-waveguide structure” gain cell, the pumping light of the rubidium vapor laser propagates freely in the vapor cell. The pumping light with polarization perpendicular to one of the rubidium laser is coupled into the resonator cavity by the polarized beam splitter. This laser configuration is suitable for a convection-cooling diode-pumped alkali vapor laser.

A sequential discharge TEA CO2 laser with high repetition rate and high output power

Abstract In this paper, a novel excitation method named as sequential discharge is realized in a two-module TEA CO2 laser by using a special rotating spark gap. It is demonstrated that the repetition rate and the output power of a laser can be multiplied through this method. For the two-module TEA CO2 laser in the experiment, the repetition rate is 300 Hz and the average power is 356 W when each module discharges; the repetition rate is 600 Hz and the average power is 713 W when the two modules discharge sequentially.

Air-breathing mode laser propulsion with a long-pulse TE CO 2 laser

Air-breathing mode laser propulsion experiment with a long-pulse transversely excited (TE) CO2 laser is carried out, and its ignition problem is solved with the ignition needle of lightcraft. Owing to the ignition needle, an order of magnitude reduction in the ignition threshold is demonstrated. The result is compared with previous study. The momentum coupling coefficient is also measured in the experiment and its dependence upon laser pulse energy (6-14 J) and pulse width (20, 32, and 40 \mu s) is discussed.

Rotating spark gap switched discharge TEA CO2 laser with average power up to 12 kW

A high average power TEA CO2 laser employing rotating spark gap switch is described. Average power up to 12kW has been achieved at the repetition rate of 400Hz.

Comparative study of diode-pumped alkali vapor laser and exciplex-pumped alkali laser systems and selection principal of parameters

Abstract. A theoretical model based on common pump structure is proposed to analyze the output characteristics of a diode-pumped alkali vapor laser (DPAL) and XPAL (exciplex-pumped alkali laser). Cs-DPAL and Cs-Ar XPAL systems are used as examples. The model predicts that an optical-to-optical efficiency approaching 80% can be achieved for continuous-wave four- and five-level XPAL systems with broadband pumping, which is several times the pumped linewidth for DPAL. Operation parameters including pumped intensity, temperature, cell’s length, mixed gas concentration, pumped linewidth, and output coupler are analyzed for DPAL and XPAL systems based on the kinetic model. In addition, the predictions of selection principal of temperature and cell’s length are also presented. The concept of the equivalent “alkali areal density” is proposed. The result shows that the output characteristics with the same alkali areal density but different temperatures turn out to be equal for either the DPAL or the XPAL system. It is the areal density that reflects the potential of DPAL or XPAL systems directly. A more detailed analysis of similar influences of cavity parameters with the same areal density is also presented.