Single-mode Argon-ion Laser Research Articles

Fringe visibility of multimode laser light scattered through turbid water

Several years ago Swanson [Proc. SPIE 1750, 397 (1992)] performed a simple Michelson interferometric determination of the coherence length of a multimode argon-ion laser after the light passed through a tank of water. As colloidal particles were added to the water the observed coherence length (as measured by twice the distance the mirror moved for fringes to disappear) decreased. Subsequently, a series of careful experiments were performed with a single-mode laser to more accurately measure this change. In these experiments it was found that the 1.5-MHz width of the 514.5-nm line of a single-mode argon-ion laser broadened by as much as 1.3 +/- 0.2 MHz when small colloidal particles were added. At first glance such a broadening should not have resulted in any discernible change in the original Michelson experiment because the gain curve for the multimode laser is of the order of a few gigahertz. The zeros in the fringe visibility function depend on the spectral characteristics of the modes. Upon scattering, the spectral characteristics of the individual laser modes change from Voigt functions, containing both Lorentzian and Gaussian components, to primarily Gaussian. It is this change in the statistical properties of the modes, not the broadening, that accounts for the change in the fringe visibility for a multimode source.

Forward-scattered light: Spectral broadening and temporal coherence

Fabry-P\'erot spectroscopy was used to observe a spectral broadening of 1.3\ifmmode\pm\else\textpm\fi{}0.2 MHz in laser light forward scattered through a colloidal solution. Light from a single-mode argon-ion laser was collected after scattering through water to which measured amounts of diatomaceous earth or 0.08-\ensuremath{\mu}m-diam polystyrene spheres were successively added. The broadening is attributed to coupling between fluctuations in particle concentration and spontaneous thermal fluctuations. Though spontaneous fluctuations exist in all pure fluids, they are very weak in water. However, the presence of the particles induces temperature gradients in the fluid, which in turn induce fluctuations in particle concentration.

Gain and noise in subthreshold longitudinal laser modes.

A theoretical study is presented of the gain profile and noise spectrum in the vicinity of the longitudinal-mode frequency separation of a single-mode laser amplifier of class A or class B. The gain and the noise are associated with the adjacent subthreshold modes on either side of the lasing mode; coupled equations of motion for these three modes have been solved. The amplitudes of the contributions of each individual subthreshold mode to both the gain and the noise are generally anticorrelated. Thus striking cancellation effects should occur in self-heterodyne measurements from the superposition of these contributions. The theoretical predictions are compared with the results of a parallel experimental study on a single-mode argon-ion laser amplifier. Measurements of the gain profiles and noise spectra close to the longitudinal-mode separation are reported for a range of laser output powers. Results are presented for simultaneous detection of the contributions of the two adjacent modes; for detection of only one adjacent mode, the other being suppressed with a Fabry-P\'erot \'etalon; and again for detection of both modes but with their phase relative to the central laser beam modified by a Fabry-P\'erot phase shifter. The predicted cancellation effects are clearly confirmed, and the gain profiles and noise spectra are in good agreement with the theoretical expressions. The noise-cancellation effect is demonstrated by the generation of an optical ``antinoise'' beam that is nearly 180\ifmmode^\circ\else\textdegree\fi{} out of phase with respect to a broadband noisy input beam.

Noise and gain in sub-threshold laser modes

When have observed intensity-fluctuation noise centred around the frequency corresponding to the longitudinal mode separation for a single-mode argon-ion laser. The noise arises from beating between the lasing mode and spontaneous emission into the subthreshold modes on either side. The noise profiles are compared with gain profiles obtained for above-threshold laser amplification of an injected signal. Both sets of profiles broaden and shift with increasing laser power and appear identical in the case of symmetric sub-threshold modes. However, remarkable differences between noise and gain are observed for asymmetric tuning. The results are in good agreement with a theory for the gain which incorporates field coupling at signal and image frequencies via modulation of the population inversion.

Measurement of the stimulated thermal Rayleigh scattering instability

The first clear experimental demonstration, to our knowledge, is reported of large amplification of small-scale spatial perturbations by stimulated thermal Rayleigh scattering of a cw laser beam propagating through an absorbing medium in a context normally associated with thermal blooming. A single-mode argon-ion laser beam with λ = 488 nm was propagated vertically downward through a 1.2-m cell filled with CCl4 that was doped with an absorber to have optical depths in the range 0.5–2.3. A shear-plate interferometer near the cell input generated the perturbation. Fringe growth was rapid and visually obvious, as was competing growth from dust specks, etc. The measured growth rate is in good agreement with the asymptotic rate from analytic stimulated thermal Rayleigh scattering theory.

Single-mode resonator incorporating an internal multimode optical fiber and a phase-conjugate reflector

Phase-conjugate mirrors (PCM’s) can be used to correct for phase distortion introduced by optical elements included within laser resonators, enabling diffraction-limited output beams to be produced. A severe example of a phase-distorting medium that could be included within a resonator is a multimode optical fiber. We describe the operation of a resonator that contains such a fiber and uses a PCM to restrict the output from the fiber to the lowest-order transverse mode. The system thereby enables the output from a laser to be transported to a remote location through the multimode fiber without loss of mode quality. The PCM was a high-gain passive PCM made from a barium titanate crystal pumped by a 60-mW single-mode argon-ion laser.

Laser-induced fluorescence modulation techniques for velocity measurements in gas flows

Broadband fluorescence of iodine, excited at 514.5 nm by a single-mode argon-ion laser tuned to the quasi-linear part of an absorption line, was used to detect the Doppler shift and hence the velocity of iodine molecules seeded in a nitrogen jet flow. The slope of the absorption line profile was measured directly using a frequency shift introduced by acoustooptic modulators (AOMs). A velocity of 36 m/s was measured in a jet of N2 at 60 Torr in 2 ms with an accuracy of 11%. To reduce experimental noise, the laser beams were switched at 125 KHz and signal-tuned amplification was used.

High power second harmonic generation of 257 nm radiation in an external ring cavity

Continuous wave high power frequency doubling of a stabilized, single mode argon ion laser in a resonant external ring cavity is discussed and experimental results are presented. The second harmonic is generated in a Brewster cut ADP crystal which is 90° temperature phase matched. Greater than 80 millwattts of stable, usable radiation at 257 nm is generated. Successful operation of the ring cavity with an internal harmonic beamsplitter to extract the UV radiation is reported.

Holocamera for 3-D micrography of the alert human eye

A holocamera that safely records holograms of the full depth of the alert human eye with a spatial resolution of ~20 microm is described. A single-mode argon-ion laser generating 2 W at 5145 A serves as the illuminating source. Holographic exposure times of 0.3 msec are achieved by means of a fail-safe electromechanical shutter system. Integrated retinal irradiance levels are well under the American National Standards Institute safety standards. Reconstructed real images are projected directly onto the vidicon faceplate of a closedcircuit TV system, enabling convenient scanning in the x-y-z dimensions of the reconstructed eyeball. Serially reconstructed holograms of cataractous rabbit eyes and normal human eyes are presented.

Brillouin scattering in systems of biological significance

Brillouin scattering spectra, with a single mode argon-ion laser and triple-pass Fabry-Perot interferometer, have been measured for natural biological fibres and for films of synthetic polypeptides. For wet unstretched rat tail collagen, with the scattering vector parallel and perpendicular to the fibre axis, single Brillouin shifts of ± 7.9 GHz and ± 6.3 GHz respectively were observed. With the scattering vector at 45° to the fibre axis, two peaks ( ± 6.35 GHz and ± 7.7 GHz) were found. For horse hair keratin the shifts for the scattering vector parallel and perpendicular to the fibre axis were ± 8.7 GHz and ± 7.7 GHz respectively. A single peak only was found with the vector at 40° to the fibre axis. Two synthetic polypeptides poly(DL-caprylic acid) and poly(Y-ethyl-L-glutamate), known to have the a-helix conformation, gave shifts of ± 6.36 and ± 6.1 GHz. The results are discussed.

High resolution Raman spectroscopy of gases with laser sources. XIV—The pure rotational spectra of dimethylacetylene and dimethyldiacetylene

AbstractThe pure rotational Raman spectra of dimethylacetylene (DMA), dimethylacetylene‐d6 (DMA‐d6) and dimethyldiacetylene (DMDA) vapor excited by the λ0 = 4579 Å,4765 Å,4880 Åand 5145 Å lines of a single mode argon ion laser have been photographed under high resolution. The spectra were recorded on Kodak IIIa‐J spectroscopic plates hypersensitized by baking in nitrogen. The spectra of DMA and DMA‐d6 are those of symmetric top rotors showing the R‐ and S‐branches, whereas in the spectrum of DMDA only the S‐branch is observed. The spectra of DMA and DMA‐d6 were analyzed by including an Hj (J + 3/2)5 term in the frequency formula for the S‐branch, while for DMDA the conventional expression was employed. The results for the molecular constants are, in cm−1,magnified image These constants are average over the ground and excited vibrational states. The quoted errors are five standard deviations. The rCC and rCC bond lengths in dimethylancetylene are related by the expression magnified image.

High resolution Raman spectroscopy of gases with laser sources. XIII—the pure rotational spectra of 1,3,5,7‐cyclooctatetraene and 1,5‐cyclooctadiene

AbstractThe pure rotational Raman spectra of 1,3,5,7‐cyclooctatetraene (C8H8) and 1,5‐cyclooctadiene (C8H12) vapor at pressures of 4 and 2 torr, respectively were photographed under high resolution. The 4765 Å, 4880 Å and 5145 Å radiations of a single mode argon ion laser were used to generate the spectra. The molecular constants found from the analysis of the spectra are, for C8H8: B = 0.090392(15) cm−1, DJ = 1.34(10) × 10−8 cm−1 and, for C8H12: B = ½(A + B) = 0.090345(110) cm−1, DJ = 7.12(1.80) × 10−10, cm−1, where the errors are five standard deviations. These constants are averages over the ground state, the low‐lying vibrational states which are sufficiently populated at room temperature, as well as over the possible conformers concurrently present in the sample.

Selective isotope excitation of fluorescence of CuI

The resonance fluorescence spectra of CuI vapor excited by single-mode argon ion laser lines have been obtained and analyzed. Isotopic molecules of cuprous iodide, Cu 63I and Cu 65I, have been selectively excited. The transitions involved are found to involve the C 1Σ- X 1Σ and A 1Σ- X 1Σ systems. The rotational constant of the A-band system of Cu 65I is determined to be 0.0670 cm −1. The other molecular constants obtained are in good agreement with previously reported data. Vibrational energy transfer is evident, but no significant rotational broadening or new rotational lines have been observed over the temperature range studied.

Resonance fluorescence of I2 at 5145 Å

Resonance fluorescence of I2 at 5145 Å has been studied with a single mode argon ion laser. Three previously reported transitions have been reexamined under high resolution with a Fabry-Perot interferometer and three new strong lines have been identified in the far wings of the laser gain curve.

Molecular constants and potential curves of Na2 from laser-induced fluorescence

The fluorescence spectrum of the Na2 transition B1Πu - X1Σg+ induced by several single mode argon ion laser lines has been analyzed, using a Fabry-Perot interferometer crossed with a grating monochromator. This work improves earlier measurements on Na2 and extends the fluorescence analysis to very high vibrational and rotational quantum numbers close to the dissociation limit. The molecular constants obtained by a Dunham expansion from measured wavenumbers allow construction of the potential curves for upper and lower state, using the RKR-procedure. The rotationless potential curve in the B1Πu state displays a hump of 550 ± 120 cm−1 over the dissociation limit.

Iodine spectrometer for low-shifted light scattering

Using an iodine-vapor cell and a single-mode argon–ion laser, we have constructed a spectrometer for low-shifted light scattering, which has resolution better than 0.3 GHz (0.01 cm−1). A method has been proposed to deconvolute the spectra.

High resolution Raman spectroscopy of gases with laser sources. The rotational spectrum of cyanuric fluoride-C 3N 3F 3

The pure rotational Raman spectrum of cyanuric fluoride vapor was photographed using a high resolution plane grating spectrograph. The spectrum was excited with the λ = 4880 A ̊ radiation emitted by a single-mode argon-ion laser. Two sets of molecular constants were determined from the R and S branches. The preferred results are those determined from the S-branch data. These are: B 0 = 0.0655954 ± 14 × 10 −7 cm −1, D J = (2.52 ± 0.17) × 10 −9 cm −1 and H J = (−1.59 ± 0.59) × 10 −14 cm −1, where the uncertainties are one standard deviation. Possible effects of line shifts due to unresolved K structure and the presence of hot bands on the accuracy of the values of the molecular constants are discussed. The B 0 value is compared to the rotation constant computed with the structural parameters determined with the electron diffraction technique; the agreement between these two rotation constants is only fair.

Laser excited fluorescence spectrum of nitrogen dioxide

The fluorescence spectrum of NO 2 excited by a single mode argon-ion laser was studied at a spectral resolution of 0.1 cm −1. In particular the Stokes and the anti-Stokes ν 2 band excited by the 5145 Å line of an argon-ion laser were examined in detail. Some of the vibrational and rotational parameters of the ground electronic state and the rotational constants of the B 2 vibronic state were obtained.

Velocity of hypersonic waves in liquid oxygen

Velocities of hypersonic (3–5 GHz) waves in liquid oxygen have been determined by thermal Brillouin scattering techniques. The incident radiation (λ = 5145 Å) was obtained from a stabilized single-mode argon-ion laser, and the scattered radiation was analyzed at 90° with a piezoelectrically-scanned Fabry-Perot spectrometer. Measurements cover the temperature range from the triple point (54.35°K) to the normal boiling point (90.18°K), and in this range the velocities were observed to decrease linearly with increasing temperature from 1193 to 909 m/sec. These results are compared with ultrasonic (1.2 MHz) velocities as measured by Van Itterbeek and Van Dael at temperatures above 61°K, and no dispersion effects are evident at least up to 3.6 GHz.

Brillouin Scattering in Simple Liquids: Argon and Neon

Recent theoretical work has suggested that at high frequencies, there should be significant departure from classical hydrodynamic behavior in simple fluids. In particular, the frequency dependence of transport coefficients is no longer negligible and may introduce observable effects into the propagation of high-frequency sound. We have measured the sound velocity of high-frequency phonons (1-3 GHz) in liquid argon and liquid neon along their vaporpressure equilibrium curves using the Brillouin scattering technique. The Brillouin spectra were excited with a single-mode argon-ion laser operating at 5145 or 4765 and were analyzed and detected with a Fabry-Perot interferometer and standard photoelectric techniques. Hypersonic (3 GHz) velocities observed in argon decrease linearly from 850 m/sec at 85°K to 742 m/sec at 100°K and uniformly exhibit a small departure from low-frequency (1 MHz) data obtained under the same thermodynamic conditions. This effect is in qualitative agreement with theoretical-model predictions of a negative velocity dispersion at high frequencies. Our measurements of the sound velocity in liquid neon are the first in this material by any technique, and hance cannot be compared with ultrasonic values. The hypersonic velocity in neon decreases not quite linearly from 620 m/sec at 24.9°K to 508 m/sec at 32°K. When compared with results in other noble-gas liquids through corresponding-state arguments, these data suggest the existence of measurable quantum effects in the hypersonic velocity of liquid neon. In addition, an interesting change in slope of the velocity-versus-temperature curve (of 17%) is observed at 28°K. © 1969 The American Physical Society.