Laser Axis Research Articles

Rotational diffusion and triplet state processes in dye laser solutions

A cw dye laser with a rotatable intracavity Brewster-angle plate can be forced to oscillate with its signal polarization rotated by an arbitrary angle β about the laser axis from the pumping laser polarization. The threshold pump power R(β) then increases as the signal polarization is rotated from parallel (β = 0°) to perpendicular (β = 90°). By measuring the ratio R(90°)/R(0°) and comparing it with the first excited singlet level fluorescence intensity ratio I(0°)/I(90°) measured in the same laser medium below threshold, it is possible to determine the important dye laser triplet state parameter KT ≡ kSTτTσT/σe. A detailed analysis of this new triplet measurement method is given. Both the orientational distribution of the excited state population and the gain for arbitrary polarizations and pumping geometry are calculated. Triplet state effects have been studied for Rhodamine 6G as a function of dye and soap concentration in H2O. The triplet state parameter KT is found to have the value 0.35 ± 0.02 at 5960 Å for a 5 × 10−4 M concentration of R6G in water with 2% by volume of Triton X-100, and 0.25 ± 0.02 for the same dye concentration with 4% by volume of Ammonyx LO. The very long rotational relaxation times measured (∼ 1–2 nsec) indicate rigid attachment of the R6G molecules to micelle soap complexes. Evidence of intermolecular energy transfer via dipolar coupling at higher dye concentrations is observed.

Electron Density Measurement in the HCN Laser Using Faraday Mode Splitting Technique

The Faraday mode-splitting, or self-heterodyne, technique has been used to measure electron density along the optical axis of the HCN laser (337 microm). The measurement indicates an afterglow electron density of 4.5 x 10(11) cm(-3); this result agrees with microwave measurements made across the laser axis. The sensitivity of the measurement is about 2 orders of magnitude better than an interferometric measurement at the same wavelength. The technique is easy to instrument, but care is required to interpret the results in the present application. The gain of the HCN laser is probably sufficient to permit use of this technique, in some cases, to measure the density of an external plasma.

Time variations in the far-field diffraction patterns of spatial modes from electron-beam-pumped semiconductor lasers

Detailed observations of the far-field radiation patterns from electron-beam-pumped GaAs and CdSe lasers reveal angular deviations of the patterns from the laser axis at angles greater than 25°. Moreover, these angular deviations are observed to decrease monotonically as a function of time during each output pulse. A simple model for the near-field laser modes predicts such angular deviations. Furthermore, the model allows the time-varying angular deviations to be accounted for in terms of a time-varying refractive index difference between the pumped and unpumped regions of the laser cavity. Two major sources of sufficient index variations are identified, viz., heating by the electron beam and index changes related to the presence of gain. The relative magnitudes of the two effects have been determined for GaAs and CdSe. The inherent time dependence of these effects proves adequate to account for the observed ``angular tuning'' of the spatial modes. Spatial variations in the refractive index have consequences other than angular tuning of the far-field modes. In particular, it is found that the diffraction losses associated with the spatial modes are strongly dependent on the refractive index difference. The impact on laser dynamics of the consequent time-varying total cavity loss is discussed.

Excitation of a long-pulse CO2 laser with a short-pulse longitudinal electron beam

A 3-nsec pulse of 600-keV electrons has been used to ionize a CO2–N2–He medium from which laser pulses lasting up to 120 μsec have been obtained. The electron beam is nearly collinear with the laser axis and normal to the sustainer field. Pulse energies of 11.5 J have been obtained from the 75-cm-long device.

OPERATION OF A LASER WITH A PLANAR RESONATOR AT HIGH PUMPING LEVELS

It is shown that the laser action in a resonator with plane-parallel mirrors is of single-mode nature if the pumping level is high. Stable transverse oscillation modes are characterized by waves which are generated near one edge of the resonator and travel at right angles to the laser axis.

High repetition rate TEA CO2laser

This correspondence reports on the successful operation of a TEA CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser at rates up to 1000 pps with an average output power up to 65 W over long periods of time. An essential feature of the present system is a rapid gas circulation transverse to the laser axis combined with a large volume of ambient gas. It is found that decomposition of the gas is one of the major problems at high repetition rates.

Electrode configuration and power output for a transverse flow CO2laser

The electrode configuration of a recently designed transverse flow laser comprised two parallel, water-cooled copper tubes, perpendicular to gas flow and parallel to the laser axis. The electrode distance was 60 mm and the electrode length 800 nm. The gas composition and pressure for maximum power output (700W) were He:7 torr; N2; 4 torr; CO2: 3.5 torr. The flow velocity of the gas was about 40 ms-1. The discharge voltage and current were 900 V and 12 A respectively, and the output mirror reflectivity 36%. It is shown that efficiency peaks with increase in input power and gas velocity.

Effects of Drift and Diffusion of Excited States on Spatial Hole Burning and Laser Oscillation

The rate equation for the population inversion of homogeneously broadened 4-level lasers is solved for the cases of small and large mobility of excited states. Considered are drift and diffusion, i.e., collective and random motions along the laser axis. Both drift and diffusion smooth spatial holes which are burned into the population inversion by standing-wave fields. A drift results also in a drag effect, i.e., a phase lag between the population inversion and the standing-wave field. For homogeneously broadened lasers, sufficient smoothing of spatial holes will result in spontaneous full-power single-frequency operation. This is examined by calculating the unsaturated gains of secondary modes as a function of the excitation, drift, and diffusion parameters. Spatial hole burning is insignificant for gas lasers because of their large carrier diffusion. Furthermore, ionic drift in gas-ion lasers alone could effectively quench spatial hole burning. This is in contrast to dye lasers where drifts, i.e., longitudinal flow rates of the order of the sound velocity, would be required to obtain a single-frequency output. In solid-state lasers like ruby and Nd: YAG, energy diffusion between lattice sites is shown to significantly reduce spatial hole depths although the matrix elements for the exchange are quite small. An experiment is described which confirms the theory and allows the measurement of small diffusion constants of excited laser states. For the 4F3/2 level in Nd: YAG a value of 5×10−7 cm2/sec is obtained. This energy exchange among Nd3+ sites contributes 0.3 GHz to the homogeneous linewidth. Implications for axial mode selection and single-frequency oscillation of various lasers are discussed.

TRANSVERSELY EXCITED ATMOSPHERIC PRESSURE CO2 LASERS

Laser action at 10. 6 μm has been achieved in CO2–N2–He gas mixtures at all pressures up to atmospheric by means of fast electrical discharges transverse to the laser axis. The atmospheric pressure of operation is very promising as the laser energy per pulse increases with pressure and the average power per unit volume is proportional to the square of the operating pressure. A brief description of a simple laser of this type, operating at atmospheric pressure is given along with some of the major achievements to date using this technique.

Resonant modes of GaAs junction lasers - II: High-injection level

Previous work on the resonant modes of GaAs junction lasers operating near threshold is extended to include spectra obtained at currents 1.5 to 3 times the threshold level. At these currents, two families of modes are shown to oscillate simultaneously with different center frequencies. The modal families are demonstrated to correspond to different mode numbers in the direction perpendicular to the junction plane. These observations are found to be in agreement with a previously reported theory for the GaAs laser cavity, which allows a dielectric constant variation in both directions transverse to the laser axis. The general characteristics of the observed spectra are explained qualitatively in terms of a derived mode dependence of the steady-state gain function. It is suggested that the strong frequency dependence of the spatial gain distribution perpendicular to the junction plane can allow two modal families with different mode numbers perpendicular to the junction to oscillate simultaneously with different center frequencies.

A spatially nonuniform laser model

The Statz-deMars rate equations for a four-level laser are replaced by equations that allow for variation of the photon densities and inversion along the laser axis. Analytic solutions of these first order partial differential equations are found for the incubation region of the laser spike, i.e., for the time before stimulated emission can affect the inversion. Analytic solutions of the Statz-deMars equations are also derived for the incubation region. The spatially dependent equations are solved numerically in the spiking region for normal and Q -switched systems. After integration of these solutions to obtain the total photon number, comparison with the Statz-deMars solutions shows that there are differences if the cavity Q is low. The effect of stimulated scattering processes within the laser cavity is studied. It is found that these processes can be effective self- Q -switching mechanisms. The spatial variation plays a vital role in such Q -switching.

CaF&amp;lt;inf&amp;gt;2&amp;lt;/inf&amp;gt;:Dy&amp;lt;sup&amp;gt;2+&amp;lt;/sup&amp;gt;lasers

The fabrication and operation of CaF 2 :Dy2+lasers are considered in detail. Laser action occurs at 4238.5 cm-1(extrapolated to 0°K) between the states T 1 (5I 7 ) \rightarrow T_{2} (5I 8 ).Chemical or electrolytic reduction techniques produce the most stable laser rods. The fluorescent lifetime of the (5I 7 ) states depends primarily upon the total Dy concentration in the crystal and varies from over 50 ms at low concentrations to 10 ms at 0.2 percent dysprosium. This lifetime is essentially temperature independent. The operation of the laser in a magnetic field is analyzed. By aligning the magnetic field and the laser axis parallel to the crystalline [100] axis, modulation of the laser output can be obtained at rates up to 1 MHz with a few tens of gauss modulating field. The saturation in laser output power at high levels is due to the finite lifetime of the (5I 7 ) multiplet, which is estimated to be several ms. The fluorescent linewidth of the laser transition varies from 4 cm-1at 200°K to 0.024 cm-1at 4.2°K. The absolute energy of the transition shifts 2.0 cm-1over the same temperature range with a slope of -0.009 cm-1/°K at 77°K. A technique of cooling the crystal with flowing, nonboiling liquid N 2 permits high-level laser operation near 77°K. Power outputs up to 1.21 watts have been obtained from CaF 2 :Dy2+when the crystal is pumped optically with two 1-kW tungsten lamps in a double ellipse. Under these conditions, the crystal temperature rises 15 to 20° above the coolant due to the high thermal resistivity of the crystal-liquid nitrogen interface. Typical pulse thresholds at 77°K are 20 joules into an FT524 lamp. The threshold drops rapidly at lower temperatures, but is strongly dependent on the frequency separation between the narrow fluorescent line and the cavity modes. Measurements at 4.2°K are particularly difficult because of this effect. Continuous operation at 27°K has been obtained at electrical inputs to the tungsten lamp as low as 20 watts. Laser output consists of a continuous output modulated at 20 to 100 kHz, with spherical reflectors or a pulsed, on-off output with flat reflectors. With spherical reflectors, magnetic feedback can be applied to reduce the amplitude of the modulation to less than 10 percent. The same type of magnetic modulation can be used to obtain a series of output pulses phase locked to the modulating frequency. The modulating frequency can be varied from a few kHz up to almost a MHz. The most effective modulation was obtained with an inhomogeneous modulating magnetic field and with the laser axis parallel to the crystalline [100] direction. At low modulation frequencies and with high fields, quasi- Q - switched operation was obtained. Q -switching was also observed with an external rotating mirror, which gave a series of pulses of 20 ns rise time. These did not have the characteristic high power of Q -switched pulses but peak powers of only a few tens of watts. This is assumed to be caused by the absorption of the metastable (5I 7 )level. The single-pass gain of a laser amplifier was measured; it corresponded to \alpha=0.039 cm-1.

TIME AND SPATIALLY RESOLVED INTERFEROMETRY ON PULSED-LASER-INDUCED PLASMAS

A technique is described for the measurement of plasma density. Multiple beam interferometry gives simultaneous time and position information on the state of the laser-induced plasma. Experiments using a single interferometric probe at various distances from the target surface along the incident laser axis indicates that the leading edge of the blowoff absorbs a portion of the incident energy and hence additional fringe shifts are observed.

Breadth of Decay Quanta in Gas Lasers

The spectrum of the radiation emitted spontaneously along the axis of a gas laser can be expected to exhibit a fine structure of narrow, Doppler-shifted lines which are substantially free of normal thermal broadening. The widths of these lines depend on the energies and lifetimes of all three of the atomic levels involved in the successive stimulated and spontaneous transitions and on the nonlinear response of the atoms to the laser radiation. Expressions are given for these dependencies for laser oscillations near and far above threshold in the limits that the quanta of energy emitted spontaneously are either much larger or much smaller than the quanta emitted coherently. It is suggested that the fine structure might be useful for precise spectroscopic determinations of a few atomic energy level separations and natural lifetimes and for studies of broadenings arising from atomic collisions and interactions with intense radiation.

Exfocal Pumping of Optical Masers in Elliptical Mirrors

An efficiency near unity can be achieved in optical pumping devices for lasers, where unabsorbed light is refocused at the light source, leading to multiple passes of pumping energy through source and laser. In properly dimensioned elliptical mirrors this can, in principle, be done by placing source and laser outside of the focal points. Furthermore, in these “exfocal” elliptical designs the light density of the source is transformed into the laser at a ratio of 1:1, resulting in very low-threshold energies. The lowest value observed for ruby lasers up to 7.6 cm in length was 50 W-sec at room temperature in a rotational-ellipsoidal mirror where the axis of source and laser are oriented in the rotational axis of the ellipsom, while their lengths are equal to the distance between focus and wall. Alternative designs are exfocal elliptical and circular cylinders. In exfocal ellipsoids the pump light distribution is of exactly rotational symmetry which leads to symmetrical absorption of pumping light in the laser. As a result, quasi-periodic relaxation oscillations of 5000-μsec duration and a component of continuous emission have been observed at room temperature with 300 W-sec pumping energy.