Three Dimensional Simulation of Gas-Radiation Interactions in Gas Lasers

The lineshape of the I 2 P 1/2 → 2 P 3/2 transition provides a means to ascertain a variety of useful information regarding the performance of the chemical oxygen-iodine laser (COIL). The value at the center of the lineshape, commonly referred to as the 'line center,' is proportional to the laser amplification on the I 2 P 1/2 → 2 P 3/2 transition. The infinite integral of the lineshape is proportional to the number density of the ground and excited states of atomic iodine in the gas, indicating the degree of I 2 dissociation. And the width of the lineshape indicates the amount of broadening of the transition, both due to collisional and Doppler shift effects. As the Doppler shift is proportional to velocity, the width of the transition can be used to estimate the degree of random molecular motion in the gas, expressed in macroscopic terms as temperature. A Doppler shift to the frequencies in the transition can also occur through the straight-line, bulk motion of the gas, and this can be used to examine the velocity field of the gas. However, the flow may experience rotation through the presence of eddies carried within the gas, and these too may contribute to the Doppler shift of the lineshape frequencies. Given that eddies by virtue of their positive and negative velocity components can induce positive and negative Doppler shift, the widening of the lineshape is similar to thermal motion which also includes positive and negative velocities. Thus, when interpreting transition lineshapes, if some account is not made for both thermal and rotational motion, the effect of either physical process will be over-estimated. The work discussed here is oriented toward examining the interplay between the gas dynamics and the lineshape of the I 2 P 1/2 → 2 P 3/2 transition, and in turn determine the ramifications for the use of spectroscopic lineshape based diagnostics and interpretation of their data. These efforts in turn are directly linked to efforts improve the understanding of the physical processes underlying chemical lasers, as excursions outside the traditional operational parameter space become increasingly necessary.

Great success has been obtained in the R&D of a chemical oxygen-iodine laser (COIL) operating on the electronic transition of the iodine atom, which gets an excitation from the energy donor -singlet delta oxygen (SDO). The latter is normally produced in a chemical SDO generator using very toxic and dangerous chemicals, which puts a limit for civilian applications of COIL that is still a very unique apparatus. Totally new non-chemical SDO generator is needed to allow oxygen-iodine laser to achieve its full potential as a non-hazardous efficient source of high-power laser radiation. There was interest in producing SDO in electric discharge plasma since the 50's long before COIL appearing. The idea of using SDO as a donor for iodine laser was formulated in the 70's. However, the injection of iodine molecules into a low- pressure self-sustained discharge did not result in iodine lasing. One of the main factors that could prevent from lasing in many experiments is a rather high threshold yield approximately 15 percent at 300K, which is needed for obtaining an inversion population. An analysis of different attempts of producing SDO in different kinds of electric discharge plasma has been done which demonstrates that high yield at gas pressure of practical interest for modern COIL technology can be obtained only in non-self sustained electric discharge plasma. The reason is that the value of relatively low reduced electrical field strength E/N approximately 1E-16 V.cm 2 , which is an order of magnitude less than that for the self-sustained discharge, is extremely important for the efficient SDO production. Although different kinds of non-self sustained discharges can be used for SDO production, we got started experiments with e-beam sustained discharge in gas mixtures containing oxygen. High specific input energy up to approximately 3 - 5 kJ/ has been experimentally obtained. Theoretical calculations have been done for different experimental conditions indicating a feasibility of reasonable SDO yield. Experimental and theoretical research of self-sustained electric discharge in SDO produced in a chemical generator, which is very important for getting plasma-chemical kinetic data needed for an estimation of SDO yield, is also discussed.

Using the premixed isothermal model of the chemical oxygen iodine laser (COIL) the analytical expression is derived, which shows the dependence of power of the (COIL) on the parameters of active media, resonator and gas velocity. The two-dimensional numerical modeling of COIL was done in order to study the influence of mixing on the power characteristics of COIL. The results of calculations are compared with analytical predictions and experimental results.

The paper describes a study of pulse-periodic operation of a chemical oxygen-iodine laser. The generator in use was of the jet type with high chlorine utilization degree (• 97%). Atomic iodine was released by dissociating methyl-iodide ( KG 3 I) in volumetric electric discharge. Steady lasing was achieved at a repetition rate up to 30 Hz. The emission energy attained per individual pulse in a train was 1.1 J at a pulse energy repeatability of (3÷ 5) % and a specific energy extraction from the active medium of 1.7 J/L. The lasing pulse duration depended on the concentra-tion of methyl-iodide and the energy deposited into discharge. The minimal half-height duration of pulses was achieved as 10 µ m at a concentration of atomic iodine in the laser cavity ~1*10 15 cm -3 . Key words: chemical oxygen-iodine laser, pulse-periodic, methyl-iodide, volumetric electric discharge The pulse-periodic chemical oxygen-iodine laser (COIL) is considered by us as a likely applicant to technologies requiring the conditions that are unattainable so far with the aid of solid-state pulse-periodic lasers. It means the range of average lasing power from 1 to 10 kW at a per-pulse lasing energy up to 10 J. There are at least two approaches possible to get COIL in pulse-periodic mode of operation. The first consists of Q-switching in the laser cavity or gain modulation in the active medium. Along with this, the active medium is produced by mixing singlet oxygen with molecular iodine just as it takes place in cw lasing case. Application of this method to formation of large-volume active media has well-known restrictions applied to relaxation processes occurred during the interaction between singlet oxygen and iodine and we’d not describe it here. Another method proposed by scientists of the Lebedev Institution and confirmed by specialists in China assumes using an initially stable mix of singlet oxygen and iodine dissociating by one or another method upon filling the laser cavity by appropriate gases [1, 2, 3].

For pt.II, see ibid., vol.29, no.3, p.944-53 (1993). The results from a combined experimental/theoretical investigation of the influence of a magnetic field on the gain and polarization in a supersonic chemical oxygen laser are described. Theoretical calculations are presented for the normalized gain as a function of magnetic field strength and are shown to be in good agreement with the experimental data. Experimental results are given for the gain suppression for P and S polarization over the range 0-1000 G. Without polarizing elements internal to the laser cavity the degree of P polarization was measured for field strength from 0 to 200 G, and a qualitative explanation is offered for the observed behavior. >

Photodetachment neutralization (PDN) has been proposed as a major improvement to the gas cell neutralization utilized on current neutral beam heating systems for magnetic containment fusion devices. This PDN system will use a Chemical Oxygen Iodine Laser (COIL) to produce a light beam with photons of a near optimal wavelength which can be reflected back and forth across the path of the ion beam to create the necessary high density light ''cloud'' required for the photodetachment process. In a fusion device the neutral beam goes directly into the process vacuum chamber and therefore the oxygen iodine gas mixture in the laser must be isolated by a window from the neutral beam channel without loss of too much light or leakage of miniscule quantities of laser gas. The aerodynamic windows that have traditionally been used with chemical lasers are viewed as undesirable for the fusion application where any contamination of the fusion vacuum chamber by laser gas would be a big problem. It was concluded that the technological issues were uncertain enough that a verification by designing, fabricating, and testing of a demonstration window would be required before feasibility of such a window could be considered certain. The statement of work defined for this study consisted of two tasks: determination of the absorption characteristics of the proposed sapphire window material and the fluorocarbon coolant, and design and analysis of the proposed double disk heat exchanger window and demonstration test hardware.

Simulations of gas lasers are performed to examine high power gas laser components. These simulations are performed in a manner that illustrates the application of high performance computing to achieve an increased understanding of the physics underlying gas lasers, and improve their operation. Computational fluid dynamic (CFD) simulations coupled to a geometric optics resonator model for the chemical oxygen-iodine laser (COIL) are executed to provide this information.

Chemical oxygen-iodine laser (COIL) is unique chemical laser based on electronic transition of iodine atom with wavelength 1.315 μm. Measurement of gas-dynamic parameters of the active medium by pneumometric methods looks like the basic at present. Accuracy of these methods is not always high enough. Laser spectrometry methods give an additional opportunity of active medium parameters measurement. Distant methods allow making measurements without penetration inside the laser active medium. Additionally, several parameters like pressure, temperature and velocity can be detected simultaneously with small response time and high accuracy. The data processing algorithm for such measurements is presented.

First Page

The basic principles of the laser Doppler velocimeter, a new instrument for measurement of localized flow velocities in gases and liquids, are discussed in detail. Velocity measurements are made by detecting the Doppler shift in monochromatic laser light which is scattered from small contaminant particles in the fluid. A CW gas laser serves as the light source, and the Doppler shift is detected by optical heterodyne techniques. The latest fluid flow measurements obtained with the laser Doppler velocimeter are presented.

Effect of AlGaN quantum barrier thickness on electron-hole overlapping in deep-ultraviolet laser diode

The statistical model of scattered by flowing Brownian particles coherent radiation is suggested. The model includes the random Doppler shifts caused by particle Brownian motion and the speckle fluctuations caused primarily by the flow motion of particles. Analytical expressions are obtained for the correlation function, power spectrum, and spectral width of scattered radiation in the imaging geometry typically used in optical coherence tomography (OCT). It is shown that the spectral density has the Voigt shape, a well-known spectral profile from atomic and molecular spectroscopy. The approach enables the choice of the experimental regimes for the measurement of Brownian particle motion parameters even in the presence of flow. These regimes are characterized by the dominant contribution of Brownian motion in the spectral width of the flow-caused Doppler shift component. Further, the new formalism suggests that prior attempts to extract transverse flow velocity are only valid at near-perpendicular geometry. The impact of the small scattering volume contributing to the OCT signal is also discussed.

In the last decades, diode laser systems conquered the spectral range step-by-step from conventional gas lasers, wherever they can match or outperform in optical specifications. Although highly anticipated in the ultraviolet wavelength range, for instance in high-resolution lithography, biological and medical fluorescence applications or holography, cw single frequency operation of sufficient power has been a challenge for diode or other solid state laser systems. Currently this scope is still dominated by the HeCd gas laser, emitting at 325 nm with powers of up to 100 mW. In this paper we present a diode laser system emitting at 325 nm offering the same output power by efficient second harmonic generation (SHG) of a master oscillator power amplifier (MOPA) at 650 nm. For the master oscillator a ridge waveguide diode is anti-reflection coated and used in an external cavity diode laser (ECDL) with grating feedback in Littrow configuration. This setup features a MHz line width (coherence length of 100m), a coarse tuning range from 649 nm to 657 nm and a mode hope free tuning of 20 GHz. In a second step, we use a tapered amplifier to boost the output from the ECDL to a level of 400 mW, for powering an efficient second harmonic generation process in an enhancement cavity. Faraday isolators on both ends of the amplifier stage prevent back reflection and stabilize the single mode operation of the system. Together with astigmatism compensation this yields to a high spatial quality (M²<1.5) of the amplified beam. The frequency doubling is achieved by using a four mirror bow-tie enhancement resonator fitted with a Beta-Barium Borate (BBO) crystal. The cavity length is actively locked to the laser frequency using the Pound-Drever-Hall method. With this set-up, stable and reliable laser operation is achieved. After a few minutes warm-up time, fixed frequency and tunable UV output power of more than 100 mW could be generated, opening this important wavelength range for future product development.

We demonstrate the Kerr-lens mode-locked Pr3+:LuLiF4 (Pr:LLF) laser pumped by a blue laser diode (LD). By theoretical calculation of the group velocity dispersion in the laser gain, the compensation was employed for the realization of the continuous-wave mode-locked laser at the wavelength of 604nm with the pulse width of 1.1ps which, to the best of our knowledge, is the shortest pulse width in the Pr3+ ion doped crystal lasers pumped with LDs. It can be believed that the present Pr:LLF laser should provide some inspiration for the development of the blue LD pumped visible lasers, especially in the mode-locking laser operation.

Doppler shift attenuation method analysis with the narrow gate on transitions below procedure