Behavioral studies of gain and saturation energy density in a N2 laser with corona preionization

By the use of an oscillator-amplifier (OSC-AMP) TE N₂ laser system, both operating with corona preionizers, the laser parameters (small-signal gain g ₀ and saturation energy density E _s) have been measured at different N₂ gas pressure and for different states of the AMP preionizer. The details of our measurements are presented. In addition, the effect of He buffer gas on the laser parameters has also been investigated; it was found that both laser parameters remain almost constant up to 150 Torr of He gas pressure, indicating that He mainly affects the discharge uniformity. Finally, with the variation of the N₂ laser gain values in the literature, we found that the g ₀-N₂-laser parameter depends strongly on the length of the laser channel. Based on the most recent measurements, a graph showing this dependency is introduced.     

The small signal gain and the saturation intensity measurement of the nitrogen-ion laser

A series oscillator-amplifier has been fabricated and optimized in order to determine the small signal gain and the saturation intensity of the N₂⁺ amplifying medium. Two plasmas, which are composed of a mixture of helium and nitrogen gases, have been produced under pressures of 2–5 atm by applying an electrical transverse discharge. The time delay between the two plasmas has been controlled by the gas pressure. To optimize the gain, the entrance time of the oscillator's output pulse into the amplifier was synchronized with the maximum population inversion in the amplifier. The gain coefficient () was obtained from 6 to 124, by varying the intensity of the input beam at the optimum pressure. The small signal gain and the saturation intensity have been calculated as 0.04, 0.07, 0.10 cm⁻¹ and 7.77, 13.33, 19.97 (kW cm⁻²) at 1.25, 1.70, 2.20 atm, respectively. The dominant wavelength of the nitrogen-ion laser corresponding to the B²∑_u(v = 0) → X²∑_g(v′ = 1) transition, 427.8 (nm) wavelength, was used in this work.     

Gain and saturation intensity parameters in a transverse-flow CO2 laser

Details of an experimental investigation of the output characteristics of the 1.2 kW cw transverse-flow, electrically excited CO₂ laser are presented. They were used for estimation of the saturation intensity and the laser cavity loss values, as they follow from the Rigrod-type model of laser operation. The saturation intensity parameter was calculated from measurements of the output power and small signal gain performed with the same experimental conditions. Measurements of the small signal gain were conducted at different points along the gas flow direction for several laser operational parameters described by gas pressure and input electrical power.     

Dependence of the amplifying parameters on buffer gases in copper-vapor lasers

We use a pair of copper-vapor lasers in the oscillator–amplifier configuration for investigating the small-signal gain and the intensity saturation as the amplifying parameters, versus the pressure of various types of buffer gases. We show that the small-signal gain increases and the intensity saturation decreases with increase in the air pressure. Moreover, the values of these parameters are different for various gases used in the amplifier media. We show that both parameters are greater at the atmosphere of neon as a buffer gas than that of a mixture of helium and neon or air.     

Small signal gain and saturation intensity of a Yb:Silica fiber MOPA system

In this work, the small signal gain (γ₀) and signal saturation power (P_sat) of a continuous wave (CW) single-frequency ytterbium-doped (YD) double-clad (DC) fiber master oscillator power amplifier (MOPA) array has been determined in the optimum length. At first, we have obtained output versus input signal powers in the amplifier, using the coupled steady-state rate equations for the various end diode pumping such as co/counter propagation and bidirectional modes. On the other hand, the steady-state amplification relation was employed at a presumed γ₀ and P_sat as initial guess, to generate output signals. Then, using the least square method (LSM), those input/output signal powers were fitted by the best regression curve obtained from the amplification relation to determine the converged γ₀ and P_sat values. The analytical formulae for γ₀ and P_sat were derived using the rate equations as well.     

Dependences of small-signal gain and saturation-energy intensity on laser active length, buffer-gas type, and pressure variations in gold-vapor lasers

We measure the small-signal gain coefficient g ₀ and the saturation-energy intensity E _s of a gold-vapor laser by implementing an oscillator-amplifier on the base of the gold-vapor laser. The dependence of the gain and saturation properties of the laser on the length of the laser active medium, the buffer-gas pressure, and the buffer-gas type are studied. The measurements of g ₀ for the amplifier for different lengths of the laser active medium (60 and 75 cm) show that with increase in the active-medium length, g ₀ increases while E _s, vice versa, decreases. Such behavior of g ₀ and E _s is observed in the whole range of changing the total pressure of buffer gas in the He-Ne mixture.     

Effect of diffusion on gain saturation in gas lasers

A theoretical model is developed to account for the effect of diffusion on gain saturation in gas lasers. It is shown that the effect can be characterized by the functionK(k ₂ a, k ₂ b), which depends on pressure, temperature, radius of the beama, and the radius of the cylindrical cavityb. It is concluded that diffusion can be very effective whenk ₂ a is small andb is several times larger thana.     

Pressure Dependence of the Small-Signal Gain and Saturation Intensity of a Copper Bromide Laser

A pair of copper bromide lasers in an oscillator-amplifier configuration is used to investigate the small signal gain and saturation intensity as amplifying parameters and output power of lasers, versus pressure of buffer gas. It is shown that the amplifying parameters and laser output power have a maximum value at optimum buffer gas pressure of 11 Torr. The challenge between microscopic parameters such as stimulated emission cross section, laser upper level lifetime, and population inversion, which determine the values of laser characteristics respective to the operational pressure of buffer gas, are investigated. Thus an optimum delay time of about 10 ns is determined, and a maximum output power equivalent to about 12 W is extracted. The amplifying parameters and measured output power of laser versus delay times show some local maxima and minima at the delay time interval of 6-43 ns.     

Nitrogen lasers: Optical devices of variable gain coefficient

A N₂-laser system, consists of an oscillator and an amplifier of different electrode lengths, ranged from 5 to 31 cm, operating under the optimized gas pressures conditions and 14 kV input voltage, was used to measure small signal gain, g₀, and saturation energy density, E_s. It was found that g₀-parameter follows a profile of the type m + n/l_AMP, where l_AMP is the effective electrode length of the laser amplifier, and (m, n) are some constants. So, by reducing the active length a gain value as high as ∼1.2 cm⁻¹ for l_AMP = 5 cm was obtained. The proposed function, with a rather good approximation, can be used to explain almost all the reported g₀-values of laser systems with moderate current densities. For explaining more details of the reported gain coefficients, the g₀l_AMP-parameter was introduced, where it was realized that with a good approximation a Gaussian profile is a suitable function for explaining the reported g₀l_AMP-products. Furthermore, we found that at a constant input voltage the saturation energy density and output energy density, E_out, regardless of the type of N₂-lasers and their operational conditions, are linearly correlated. Based on this observation, and by operating the system at different input voltages, the functional dependence of the measured E_s/E_out-values with respect to the operational voltages is introduced. Details of our present observations, along with the previously reported g₀, and E_s-measurements are presented graphically, or tabulated. The results are giving some interesting features of TE/TEA N₂-lasers for lasers g₀, E_s predictions which are valuable for laser designs and also for further theoretical investigations.     

Frequency dependence of amplifying parameters of a copper vapor laser using air as a buffer gas

We use a pair of copper vapor lasers in an oscillator–amplifier configuration to investigate amplifying parameters such as the small signal gain and the saturation intensity versus the pulse repetition frequency when two different types of buffer gases are employed. We show that the values of these parameters are not the same if different gas mixtures are used in the gain medium. We show that the values of the parameters are estimated to be higher if a He–Ne buffer gas is used than in the case of air. The laser output power is relatively high and has fairly good stability at some special pulse repetition frequencies when air is used as a buffer gas.     

Polarization-dependent gain and saturation energy density in a TE N2-laser amplifier

The polarization-dependent gain, g₀, and saturation energy density, E_s, in a TE N₂-laser amplifier were measured, using an oscillator-amplifier laser system for different amplifier electrode gap separations, d_AMP, of 7, 9 and 4 mm and gas pressure of p = 77, 60, and 165 Torr, respectively. It was realized that for the amplifier with the gap separation of 7 and 9 mm, where the pd_AMP-value has its optimum-value of 54 Torr cm, the gain-coefficient for the input beam with the polarization parallel to the discharge electrodes (P-polarized beam) is slightly higher than the case when the beam polarization is perpendicular to the discharge electrodes (S-polarized beam). In this case, the depolarization ratio for d_AMP = 7 mm is the range of ∼0.998 to ∼0.962 as the input voltage increases from 12 to 15.5 kV, having a minimum of 0.937 around 14 kV. For the E_s-parameter, the reversed order is true. Also, it was found that the saturation energy densities for three states of polarization are linearly related to the output energy densities, having different slopes of 0.11, 0.14, and 0.17 for R (randomly), P- and S-polarization, respectively. The present measurement supports qualitatively the prediction of polarization-inhomogeneity model for the stimulated emission cross-section, showing that randomly oriented dipoles exhibit slightly larger gain on the direction of the electric field.     

Gain and saturation measurements in a discharge excited F2 laser using an oscillator amplifier configuration

The small-signal gain coefficient and the saturation intensity of a F₂ pulsed discharge molecular laser at 157 nm have been measured using two discharge devices in an oscillator-amplifier configuration. The small signal gain coefficient was measured to be 5.2±0.4% cm^–1 at 3 atm total pressure and 1.5 cm electrode spacing and 4.1±0.4% cm^–1 at 2 atm total pressure and 2 cm electrode spacing while the values of the saturation intensity were 5 MW/cm² and 4.6 MW/cm², respectively.     

Effect of thermal nonlinearity in high-absorption media on the parameters of the photoacoustic signal detected by the gas microphone method: The fundamental and second harmonics

A perturbation theory is put forward that describes the effect of thermal nonlinearity due to the temperature dependence of the thermophysical parameters of high-absorption systems with a low thermal conductivity on the parameters of the photoacoustic signal detected by the gas microphone technique. It is found that the dependence of the photoacoustic signal amplitude on incident beam intensity I ₀ stems from the dependence of the illuminated surface temperature on I ₀. This dependence is a complicated function instead of being a simple quadratic function as was expected. In the limiting cases (μ_sβ ? 1 and μ_sβ ? 1), this contribution to the photoacoustic signal amplitude is described by simple expressions, which are convenient for determining the thermal coefficients of the thermophysical parameters of the medium. It is found that the thermal nonlinearity significantly affects the photoacoustic signal phase in the frequency region meeting the condition μ_sβ ～ 1. In the above limiting cases, its effect is insignificant. A theory of generation of the photoacoustic signal second harmonic is proposed. The second harmonic is related to the temperature dependence of the thermophysical parameters of the buffer gas and sample. It is shown that the amplitude of the signal is a quadratic function of the incident beam intensity and varies with its frequency as ω^?3/2 for μ_sβ ? 1 and ω^?5/2 for μ_sβ ? 1.     

Amplification and saturation in a CO2 waveguide amplifier

An ir CO₂, dc current pumped, optical waveguide (WG) amplifier has been built, and its active medium optical parameters measured for several CO₂ emission lines, and their dependence from active medium total pressure, discharge current and temperature was investigated.High gain is found which, coupled with relatively high saturation power in the WG fundamental mode and ease of fabrication with this technology in long (up to 1.5 m) lengths, indicates promising use to efficiently amplify high spectral and spatial purity output of a short, highly tunable WG laser up to power levels suited for nonlinear spectroscopy and optical pumping. The dependence of the small signal gain coefficient and of the saturation parameter for individual rotational lines on the radiation intensity was computed using experimentally known parameters of the discharge plasma. The computation was carried out using the two mode rate equation approach for CO₂–N₂–He gas mixtures. A satisfactory agreement between theoretical and experimental results was obtained.Work supported by G.N.S.M.-C.N.R. and M.P.I.     

Synchronization effect on the small-signal gain and saturation intensity of a CuBr laser

A pair of copper bromide laser in an oscillator–amplifier configuration was used to investigate the small signal gain and saturation intensity as amplifying parameters, versus delay time of triggering between lasers. The maximum amplifying parameters and output power of 0.07 cm^?1, 43 μJ cm^?2, and 9 W were measured, respectively at optimum delay time of about 10 ns. The amplifying parameters and measured output power of laser versus delay times between intervals of 6–43 ns, shows some local maxima and minima. The output power extracted from the amplifier exceeds that achieved with the same device as the oscillator by more than 60%.     

Nitrogen lasers, optical devices of variable gain coefficient: Theoretical considerations

Based on our previous measurements on the gain values by employing an oscillator amplifier (OSC-AMP) N₂-laser system of variable AMP electrode lengths, a calculation has been made for evaluation of the gain coefficient using rate equations. It is shown both numerically and analytically that small signal gain, g₀, is following our experimental observations of g₀=m+n/l_AMP, where m and n are some constants, and l_AMP is the length of the amplifier. For simplifying the calculation in the OCS-AMP circuit, an experimental condition of imposing the OSC-open-circuit operation has been applied, where the voltage waveforms from the relevant sections of the Blumlein circuit have been used for evaluation of the circuit parameters. Due to the fact that during the past years different cross-sections for the electron-impact excitations from the ground to the upper, N₂(C), and lower, N₂(B), have been introduced, our experimental observations have also been applied to examine the effect of introduced electron impact-excitation cross-sections on the g₀(l_AMP) behavior.     

Experimental investigation of the effect of collisions and temperature on degenerate four-wave mixing

An experimental investigation (including a comparison with a simple theoretical model) of the effect of buffer-gas composition, pressure and temperature on resonant Degenerate Four-Wave Mixing (DFWM) has been performed. The DFWM signal from NO in a quartz cell was measured and the effect of quenching as well as elastic (phase-changing) collisions was studied by varying the total and partial pressures of N₂ and CO₂ as buffer gases. It was found that the DFWM signal first slowly increased with buffer-gas pressure (up to 10 mbar) and then rapidly decreased. It was furthermore found that the DFWM signal was considerably less sensitive to quenching collisions as compared to Laser-Induced Fluorescence (LIF) (for laser intensities approximately equal to half the DFWM saturation intensity of the transition). On the other hand, while LIF is virtually insensitive to elastic collisions, DFWM displays a larger sensitivity to elastic collisions than to quenching collisions. The DFWM saturation intensity was found to increase with buffer-gas pressure (although slower than expected). When varying the temperature of the gas composition, it was found that the DFWM signal decreased markedly with increasing temperature. This decrease is too fast to be explained solely by a change in the population of the molecular state probed by the laser.     

Application of obliquely interfering TE10 modes for electron energy gain

Two fundamental TE₁₀ modes are considered to interfere at a small angle θ and then propagate along the z-axis in an evacuated rectangular waveguide. The electron trajectory in the resultant field and the expressions for energy gain and the acceleration gradient are obtained, when the electron is injected along the z-axis. A 50 keV electron gains 718 keV energy in a 4.0 cm × 2.5 cm waveguide, when the microwave with intensity of 1 × 10¹⁰ W/cm² and frequency 5.577 GHz is used and the modes superpose at an angle of 10°; here the maximum acceleration gradient is obtained as 251 MeV/m. The energy gain and acceleration gradient are decreased with increasing width of the waveguide and microwave frequency. Higher gradient and larger energy gain are obtained for the higher microwave intensity, smaller angle of superposition and also when the electron is injected with larger initial energy.     

Population inversion on transitions to the ground state of atoms upon nonresonance absorption of laser radiation

Lasing at the resonance transitions (D _1? and D _2?lines) of sodium was observed in the superradiance mode upon nonresonance optical excitation in the presence of a buffer gas. The dependences of the lasing intensity on the exciting radiation intensity and on the detuning of its frequency from the frequencies of resonance transitions were studied. It is found that, under specific conditions of the experiment (high pressure of a buffer gas and a rather high radiation intensity), in the case of a large positive detuning of the exciting radiation frequency from the resonance (“working”) transition frequency, the population inversion is produced at the “ working” transition, which results in lasing.     

Analytical studies of gain optimization in CO2-N2 gasdynamic lasers employing two-dimensional wedge nozzles

An analytical method has been proposed to optimise the small-signaloptical gain of CO₂-N₂ gasdynamic lasers (gdl) employing two-dimensional (2D) wedge nozzles. Following our earlier work the equations governing the steady, inviscid, quasi-one-dimensional flow in the wedge nozzle of thegdl are reduced to a universal form so that their solutions depend on a single unifying parameter. These equations are solved numerically to obtain similar solutions for the various flow quantities, which variables are subsequently used to optimize the small-signal-gain. The corresponding optimum values like reservoir pressure and temperature and 2D nozzle area ratio also have been predicted and graphed for a wide range of laser gas compositions, with either H₂O or He as the catalyst. A large number of graphs are presented which may be used to obtain the optimum values of small signal gain for a wide range of laser compositions without further computations.