RF excitation of a flowing gas CO2 waveguide laser

Investigation of an RF excited CO₂ waveguide laser in flowing gas operation is reported. Power extraction of 0.8 W/cm with an efficiency of 10.3% has been achieved. Using W.W. Rigrod's analysis (1965), values of the small-signal gain α₀ and saturation parameter I_s have been determined for different excitation levels and for different pressures of the amplifying medium. The parameters α₀ and I_s, have been determined as 0.6%/cm and 10.4 kW/cm ², respectively, at 125 torr and 100 W/cm³ RF loading power. These values are close to those reported for sealed-off RF CO₂ waveguide lasers with xenon added to the gas mixture     

Effects of F- ions and F2 molecules on theoscillation process of a discharge-pumped ArF excimer laser

Time-resolved number densities of the fluorine negative ion in a discharge-pumped ArF excimer laser are measured by a dye laser absorption method. The peak density of F^- is 0.93 ×10 ¹⁵ cm^-3 at a total gas pressure of 2.5 atm, a gas mixture ratio of F₂-Ar-He=0.2-10.0-89.8, and a charging voltage of 28 kV for a 68-nF storage capacitor bank. The dependences of the peak F^- density and the ArF laser output power on the F ₂ gas fraction in F₂-Ar-He mixture are investigated. The effects of F^- ions and F₂ molecules on the ArF laser oscillation process are discussed by considering the F₂ mixture-ratio dependences of particle densities, laser output power, mean electron energy, and laser power extraction efficiency. With increasing F₂ mixing ratio, the ArF* excimer formation first increases as F^- increases, but in F₂-rich conditions the laser power decreases because of the laser photon absorption due to F^- ions and quenching of ArF* with F₂ molecules     

Diode-pumped ytterbium-doped Sr5(PO4)3F laser performance

The performance of the first diode-pumped Yb³⁺-doped Sr ₅(PO₄)₃F (Yb:S-FAP) solid-state laser is discussed. An InGaAs diode array has been fabricated that has suitable specifications for pumping a 3×3×30 mm Yb:S-FAP rod. The saturation fluence for diode pumping was deduced to be 5.5 J/cm ² for the particular 2.8 kW peak power diode array utilized in our studies. This is 2.5× higher than the intrinsic 2.2 J/cm ² saturation fluence as is attributed to the 6.5 nm bandwidth of our diode pump array. The small signal gain is consistent with the previously measured emission cross section of 6.0×10^-20 cm², obtained from a narrowband-laser pumped gain experiment. Up to 1.7 J/cm³ of stored energy density was achieved in a 6×6×44 mm Yb:S-FAP amplifier rod. In a free running configuration, diode-pumped slope efficiencies up to 43% (laser output energy/absorbed pump energy) were observed with output energies up to ~0.5 J per 1 ms pulse. When the rod was mounted in a copper block for cooling, 13 W of average power was produced with power supply limited operation at 70 Hz with 500 μs pulses     

Improved Ti:sapphire laser performance with new high figure ofmerit crystals

The continuous-wave (CW) laser performance of Ti:Al₂O ₃ crystals with high figure of merit is described. Using a 0.1% Ti:Al₂O₃ crystal (FOM=1000), output powers of 3.5 W at 800 nm are obtained, pumping with a 9.6 W argon ion laser operating multiline. Continuous tunability of the CW Ti:Al₂O ₃ laser extending from 665 to 1070 nm is also demonstrated for a 5 W pump power     

A CW CO2 laser with DC-discharge preionization

The application of the plasma-injection technique (involving DC-discharge preionization) to a CW CO₂ laser operating with recirculated gas is described. Measured characteristics of the discharges, the gain, and the output power are presented. An output of 110 W was obtained with a specific efficiency of 1.0 J/l^-1 of flowed gas at a pressure of 10 kPa. A simplified model to calculate a characteristic length of the main discharge, which is useful for design purposes, is developed. This type of laser may be scaled simply to higher output powers, requires a gas mixture with only 20% He, is compact and robust, and yet is simple in its electrical and mechanical requirements     

Spectroscopic Properties and Laser Performance of Er3+ and Yb3+ Co-Doped GdAl3(BO3)4 Crystal

An Er:Yb:GdAl₃(BO₃)₄ crystal was grown and room-temperature polarized absorption, emission, and gain spectra were investigated. Fluorescence decay curves of Er³⁺ at 1530 nm and Yb³⁺ at 1040 nm in the crystal were measured. Efficient laser operation of Er:Yb:GdAl₃(BO₃)₄ crystal at 1.5-1.6 mum was realized. Quasi-continuous-wave output powers of 1.8 W with slope efficiency of 19% and 0.78 W with slope efficiency of 14% were achieved in diode-pumped c-cut and c-cut and a-cut crystals, respectively. The output spectrum and polarization of Er:Yb:GdAl₃(BO₃)₄ laser were also investigated.     

Gas temperature dependent output of the atomic argon and xenonlasers

Laser power and energy have been measured for the transitions at 1.73 and 2.03 μm in atomic xenon, and 1.27 and 1.79 μm in atomic argon, for gas temperatures between 290-585 K. Nuclear excitation using ²³⁵U fission-fragments provided pump powers between 2-10 W/cm ³. Peak laser output in xenon was independent of gas temperature below 400 K. Laser output at 1.79 μm in argon decreased monotonically with increasing gas temperature while the 1.27 μm laser output was independent of gas temperature below 380 K. In all cases, the laser threshold increased as the gas temperature increased     

Spectroscopic properties and efficient diode-pumped laser operationof neodymium-doped lanthanum scandium borate

We report on the spectroscopic properties and laser performance of Czochralski-grown LaSc₃(BO₃)₄ (LSB) crystals with high neodymium concentrations up to 2.5·10²¹ cm^-3. The low-concentration quenching of the upper laser level of neodymium and the polarization dependence of the spectra indicate that LaSc₃(BO₃)₄ crystalizes in a huntite-type structure like Nd³⁺:YAl₃(BO₃)₄ (NYAB). With diode laser pumping at 808 nm, a multimode Nd³⁺:LaSc₃ (BO₃)₄ (NLSB) laser at 1063 nm is demonstrated. Optical slope efficiency was 64% with respect to absorbed pump power. The possibility of second-harmonic generation in the laser crystal is discussed     

A parametric study of the output of the optically pumped continuouswave CF4 laser

A parametric study of laser output versus CF₄ pressure and temperature was performed and correlated with a model for the gain in the system, which includes the relevant relaxation processes. Lasing in CF₄ was observed at temperatures below 170 K. Cooling the CF₄ gas, the output power of the laser increased from 3 mW at 142 K to 5 mW at 113 K, when 4% of the radiation was coupled out. Chopping the pump, the 16-μm signal consisted of a peak decaying in approximately 2 ms, superimposed on a CW background. This decay is caused by the slow relaxation in the CF₄ laser, resulting in filling of the lower laser level. For the CW CF₄ laser, vibrational relaxation from the laser lower level is even slower than diffusion to the cold cell walls. To increase the relaxation rate, HD was added. In this molecule, the J=1→3 rotational transition at 447 cm^-1 is almost resonant with the ν₂ vibration in CF₄. Maximum CW output was increased by 25% in a mixture containing 10% HD. At the same time, the lasing pressure range was extended     

High vibrational temperatures in optically-pumped CO2

A pulsed 4.3-μm CO₂ laser was used to optically pump mixtures of CO₂ and He, and create transient gain at 9 and 10 μm. A conventional continuous-wave CO₂ laser operating on both regular and sequence bands measures this transient gain, and determined the ν₃ (asymmetric stretching)-mode vibrational temperature T₃. The measured values of T ₃ are generally much higher than those attained in discharge-excited CO₂. It is shown that a Treanor distribution must be used to describe the populations in the ν₃ -mode when dilute mixtures of CO₂ in He are optically pumped to ν₃-mode temperatures of 3000 to 4000 K. Under these conditions the sequence-band gain coefficients are almost equal to those on the regular bands. The collisional relaxation of energy from the ν₃ mode shows evidence of fast V-T relaxation at high values of T₃, followed by a slower relaxation rate characteristic of the 00⁰1 population lifetime     

High-Power Laser Performance of Yb:YAl3(BO3)4 Crystals Cut Along the Crystallographic Axes

High-power continuous-wave laser operation near 1 mum was demonstrated at room temperature with c-cut and a-cut Yb:YAl₃(BO₃)₄ crystals end-pumped by a fiber-coupled diode laser. Using a 2-mm-thick c-cut crystal, 10.6 W of output power was generated with an optical-to-optical efficiency of 65% and a slope efficiency of 72%. The complex polarization state of the generated laser radiation was also studied. With a 2-mm-thick a-cut crystal, sigma-polarized laser oscillation was obtained, producing a maximum output power of 8.1 W; the optical-to-optical and slope efficiencies were 56% and 61%, respectively. Complete absorption and emission cross section spectra of the Yb:YAl₃(BO₃) ₄ crystal were also derived, revealing very strong anisotropy of the spectroscopic properties     

Measurement of temporal behavior of electron density in adischarge-pumped ArF excimer laser

Time-resolved number densities of electrons in a discharge-pumped ArF excimer laser are measured by an interferometric method. The peak electron density is 6.7×10¹⁵ cm^-3 at a total gas pressure of 2.5 atom, a gas mixture ratio of F₂-Ar-He=0.2-10.0-89.8, and a charging voltage of 24 kV for a 68-nF storage capacitor bank. The dependences of the electron density and laser output power on the Ar and F₂ fractions in Ar-F₂-He mixture and on the Ne-He mixing ratio in Ar-F₂ -He-Ne mixture are investigated, and the effects of Ar-F₂ -He-Ne mixing ratio on the ArF laser discharge are discussed. The experimental data of the peak electron density are also compared with the results of a computer simulation. A good agreement between them was obtained by considering the fact that the actual discharge volume occupied only part of the electrode width     

A pulsed N2O waveguide laser with extended tunability

An air cooled N₂O waveguide laser with pulsed longitudinal DC excitation is described. It can oscillate in 70 lines, and, at a pressure of 80 mbar, it can be tuned single line and single mode over a tuning range of 350 MHz in a number of stronger lines. The laser performance is studied for different gas mixtures, involving He, N ₂, and CO, and for various resonator configurations and cooling schemes. Operation at the required pressure is possible only if CO is used in the gas mixture. The potential of the laser for Doppler limited spectroscopy is illustrated through measurements of NH₃      

Simultaneous emission of the HF and N2 lines from aplasma cathode TEA laser

Simultaneous laser action from HF and N₂ is obtained, from a plasma cathode TEA laser, for the first time. The sliding discharge along the surface of a dielectric is used as a plasma cathode, for the main volumetric discharge. The laser operates at atmospheric pressure, with a gas mixture of SF₆:C₃H₈:N₂:He. For a typical flow rate ratio of 0.27:0.024:0.2:19.8 1 min^-1, it produces simultaneously 160 mJ HF and 0.6 mJ N₂ laser outputs at 0.43% and 1.4×10^-3% efficiencies respectively, at the moderate charging voltage of 28.5 kV. These output characteristics are obtained from a small active discharge volume and length of 10⁶ cm³ and 38 cm respectively. These values extend the performance, recently reported in the literature, of a sliding discharge HF/N₂ laser with corresponding simultaneous energy outputs of 12 mJ HF and 1.1 mJ N₂, to a higher energy output level, thus making the device suitable for a broader range of applications. This novel dual wavelength HF/N₂ laser system presented, can be particularly convenient for medical experiments, where the IR beam can be used for tissue ablation, while the UV beam can be used as the excitation source for fluorescence spectroscopic measurements, for the evaluation of the ablation process. Details are presented on the dependence of the laser performance parameters, such as output energy, discharge voltage and current and structure of the laser output pulses on the mixture composition and the circuit parameters     

Fluorescence and laser operation in single-mode Ti-diffusedNd:MgO:LiNbO3 waveguide structures

The spectral properties of the guided-wave Nd fluorescence and results of laser oscillation in Ti-indiffused single-mode Nd:MgO:LiNbO ₃ waveguides and waveguide cavities, respectively, are reported. The splitting and polarization behavior of the fluorescence lines around 0.9, 1.08, and 1.37 μm were studied. Using a single-mode diode laser as a pump source (λ_p=814.6 nm), an oscillation threshold in an 8-mm-long structure of 2.1-mW absorbed pump power has been obtained. An output power up to 310 μW (limited by the available pump power), a slope efficiency of 16% at power levels >150 μW, and an emission linewidth of 0.21 nm (at λ_s=1085 nm) have been measured     

Theoretical and experimental investigations of a diode-pumpedquasi-three-level laser: the Yb3+-doped Ca4GdO(BO3)3 (Yb:GdCOB) laser

We present a theoretical and experimental analysis of a diode-pumped Yb³⁺-doped Ca₄GdO(BO₃)₃ (Yb:GdCOB) laser. A new model for a diode-pumped quasi-three-level laser is described. The effects of absorption saturation, temperature profile, and the beam quality M² factor of the pump diode have been taken into account, for the first time to our knowledge. We have obtained a good agreement between experimental measurements and theoretical calculations with two different pump wavelengths, 902 and 976 nm. Our model has given good predictions of the laser performances for different crystal temperatures and different M² factors of the pump beam. As much as 440 mW of output power (at 1082 nm) have been achieved for 640 mW of absorbed pump power at 976 nm, corresponding to one of the highest slope efficiencies (81%) ever obtained with Yb-doped lasers     

A theoretical study of a Ti-diffused Er:LiNbO3 waveguidelaser

A detailed theoretical procedure of determining the mode sizes of fundamental mode field distribution in a coaxially pumped Ti-diffused Er:LiNbO₃ waveguide laser has been described. The mode sizes, effective refractive indexes, effective pump area, and coupling efficiency as a function of the initial Ti stripe width W were numerically calculated for the waveguide with 95- and 100-nm initial Ti stripe thicknesses, respectively. The results indicated that the threshold pump power is severely affected by the stripe width, while slope efficiency is hardly changed as W; both show little difference between 95- and 100-nm stripe thickness. In addition, the stimulated emission cross section of Er³⁺ in Er:LiNbO₃ channel waveguide versus the wavelength were calculated directly from its fluorescence spectra using the β-τ method. Subsequently, threshold pump power and slope efficiency were evaluated     

Frequency-tunable optically pumped carbon monoxide laser

Single-line frequency-tunable lasing was observed in an optically pumped, repetitively pulsed, room-temperature CO laser for the first time. The R(0) and R(7) ro-vibrational transitions in the (2,0) overtone of CO at 2.3 μm were optically pumped with a high-energy optical parametric oscillator. Single-line lasing was observed on (2,1) P(2)-P(17) transitions and R(0)-R(11) transitions (covering wavelengths within the range 4.6-4.9 μm) when using a diffraction grating as the spectrally selective reflector of the laser resonator. The observed CO laser pulse lengths were ~10^-7 s with peak power up to 10⁴ W. The influence of CO pressure, the addition of buffer gas (He, Ar), Q-factor of the laser resonator, and the pump pulse energy on CO laser pulse temporal characteristics and output energy spectral distribution was studied experimentally     

Study of etch rate characteristics of SF6/He plasmas byresponse-surface methodology: effects of interelectrode spacing

The characteristics of SF₆/He plasmas which are used to etch Si₃N₄ have been examined with experimental design and modeled empirically by response-surface methodology using a Lam Research Autoetch 480 single-wafer system. The effects of variations of process gas flow rate (20-380 sccm), reactor pressure (300-900 mtorr). RF power (50-450 W at 13.56 MHz), and interelectrode spacing (8-25 mm) on the etch rates of LPCVD (low-pressure chemical vapor deposition) Si₃N₄, thermal SiO₂, and photoresist were examined at 22±2°C. Whereas the etch rate of photoresist increases with interelectrode spacing between 8 and 19 mm and then declines between 19 and 25 mm, the etch rate of Si₃N ₄ increases smoothly from 8 to 25 mm, while the etch rate of thermal SiO₂ shows no dependence on spacing between 8 and 25 mm. The etch rates of all three films decrease with increasing reactor pressure. Contour plots of the response surfaces for etch rate and etch uniformity of Si₃N₄ as a function of spacing and flow rate at constant RF power (250 W) display complex behavior at fixed reactor pressures. A satisfactory balance of etch rate and etch uniformity for Si₃N₄ is predicted at low reactor pressure (~300 mtorr), large electrode spacing (12-25 mm), and moderate process gas flow rates (20-250 sccm)     

Lossless stripe waveguide optical beam splitter: modeling of theY-structure

We present a theoretical model of a loss-compensated symmetric Y-junction acting as an optical beam splitter. We consider silica (SiO ₂) channel waveguides which are assumed to be highly doped with Er³⁺. The model was developed using the beam propagation method (BPM) and a fast-Fourier-transform (FFT)-based algorithm. The analysis showed that considerable gain levels, about 4.2 dB/cm at each port of the Y-junction, can be achieved for erbium concentration 2.5×10²⁰ ions/cm³, signal power 1 μW and pump power 250 mW