Showing papers on "Laser published in 1968"

Self‐Pulsing in Lasers

Abstract: The origin of pulse formation in lasers is investigated. The main features of this phonomenon are explained in terms of a homogeneously broadened, two‐level ring laser model. First, it is found that a stationary (cw) solution cannot exist for certain choices of laser parameters. By a numerical integration of the laser equations, it is shown next that a buildup of a pulse occurs for these parameters. The final (steady‐state) pulse shape and its velocity are determined. Furthermore, it is found that a multistable operation is possible. A simple approximate equation is derived, by which most of the features of the final pulse are given analytically.

Gas Lasers

Abstract: A review is given of the present status of gas discharge lasers, with particular attention to developments reported in 1965 and early 1966. Following a brief history, gas lasers are classified by types--neutral atom, ion, and molecular--and a comparison is given of the properties of the various types. A short discussion is given of noise and coherence properties. Detailed descriptions are given of three recent developments of particular interest--the CO 2 laser, the argon-ion laser, and pulsed self-terminating lasers. Finally, brief mention is made of the most important present applications of gas lasers.

Vaporization of Metal Absorbing Laser Radiation

Abstract: The problem of vaporization of a light-absorbing metal into vacuum is considered. It is assumed that the density of the light energy flux is not excessively large so that there is no significant absorption of light by the vapor. The expansion of the vapor thus occurs in a centered rarefaction wave. The obtained boundary conditions relate the values of the hydrodynamic variables in the rarefaction wave with the surface temperature of the metal. This is accomplished by an approximate solution of the gaskinetic problem of vapor motion within a thin film directly adjacent to the phase interface. The velocity of the vaporization front, the surface temperature of the metal, the temperature and velocity of the vapor, and the recoil momentum are calculated.

A laser Doppler velocimeter employing the laser as a mixer-oscillator

Abstract: A laser Doppler velocimeter is described in which the laser is employed not only as a light source, but also as a mixer-oscillator, in order to give a simpler optical system. The efficiency is comparable to that of a con-ventional system, such as a Michelson interferometer, and coherence requirements do not impose very severe restrictions. The frequency response of the system is also considered. Measurements of the coherence and frequency response of the system are also given.

The laser illuminated absorptivity spectrophone: a method for measurement of weak absorptivity in gases at laser wavelengths.

Abstract: A spectrophone measures absorptivity by sensing thermal expansion in a confined sample gas. Laser source excitation provides sufficient radiation to measure precisely very weak absorptivities at laser wavelengths. This paper describes the theoretical capability, design considerations, and experimental testing of a pulsed ruby laser absorptivity spectrophone and a cw CO2 laser absorptivity spectrophone. A spectrum of the water vapor line at 6943.8 A was obtained. The peak absorptivity was 3 × 10−6 cm−1. In the vicinity of 9.6 μ, absorptivities of CO2−N2 mixtures were measured down to 1.2 × 10−7cm−1.

Some properties of the plasmas produced by irradiating light solids by laser pulses

Abstract: A theory is proposed for the process of irradiation of finite solid targets of light atoms (H2, D2, Li) by giant laser pulses. The properties of the plasma produced are studied and the conditions to be satisfied in order to obtain an isotropic plasma are given.

Flashlamp‐Pumped Organic‐Dye Lasers

Abstract: The results of recent theoretical and experimental studies on flashlamp‐pumped organic‐dye lasers are presented. The importance of rapid pumping is considered in detail. Constructional aspects of a suitable flashlamp assembly are given, and lasing characteristics of various organic dyes excited by this lamp are summarized.

Laser applications

Abstract: When lasers were new many people seemed to expect that this exciting development would rapidly revolutionize civilization. More than seven years have now passed since the advent of the first laser and the world has changed only slightly. As a result, it has been said that the laser is a solution in search of a problem. It is the purpose of this article to show that, in spite of comments to the contrary, there are some problems for which lasers are a solution. It is appropriate to do this now because the state of the art has reached the point at which useful energy and power outputs can be achieved with sufficiently high efficiency and reliability to make some applications practical. The emphasis here is on industrial and medical applications, as opposed to the many present and potential military applications. Despite the progress made during the past few years, much more research and development will be required before the full potential of lasers is realized.

Compression of optical pulses

Abstract: A scheme, similar to one used in the chirp radar, is proposed to compress in time, without loss of energy, the light pulse generated by a mode-locked laser. On the basis of preliminary experimental results obtained with a mode-locked He-Ne laser and a LiNbO 3 phase modulator, it is predicted that compression of 50-ps pulses derived from a Nd:YAG laser to 0.4 ps or less is feasible.

Frequency mixing in the infrared and far‐infrared using a metal‐to‐metal point contact diode

Abstract: Metal‐to‐metal point contact diodes were used to obtain the 54‐GHz beat notes between two adjacent 10.6‐μ CO2 laser transitions. The speed of the diodes in the far‐infrared is at least 1000 GHz. This was tested with a 337‐μ HCN laser.

MnBi Thin Films: Physical Properties and Memory Applications

Abstract: Thin films of MnBi possess many unusual physical properties and are particularly suitable for memory applications using laser Curie‐point writing and magneto‐optical readout. Films prepared on mica substrates have the easy direction of magnetization perpendicular to the film plane. They typically have a specific Faraday rotation of 5×105 deg/cm, and an absorption coefficient of 3.8×105 cm−1 for 6328 A wavelength, permitting a readout rate of 109 bits/sec at 1 mW laser power level using available detectors. Curie‐point writing and erasing of a spot a few microns in diameter in a 1000 A thick film with a 4 μsec pulse from a 13 mW Gaussian laser beam have been achieved. For a 106 bits/cm2 packing density, heating at adjacent spots is negligible. Calculations show that the writing magnetic field can be reduced to less than 10 Oe by using a memory plane consisting of discrete squares of MnBi. Analytical and experimental results as well as memory design considerations are presented.

CONTINUOUS 0.532‐μ SOLID‐STATE SOURCE USING Ba2NaNb5O15

Abstract: A continuous 0.532‐μ solid‐state source utilizing the nonlinear material Ba2NaNb5O15 inside the cavity of a 1.064‐μ YAlG:Nd laser has generated 1.1 W of coherent green power. This represents 100% conversion to the green of the available infrared radiation from the YAlG:Nd laser utilized.

Vibrational Energy Transfer in Methane

Abstract: The asymmetric stretching vibration ν3 of methane has been excited by a chopped He–Ne laser. The phase shift of infrared emission from the asymmetric stretch and from the infrared‐active bend ν4 has been measured as a function of chopping frequency and gas pressure. Vibrational relaxation times have been found for the removal of energy from the asymmetric stretch (pτ = 7.0 ± 1 nsec·atm), for the appearance of this energy in the ν4 bend (pτ = 5 + 3, −2 nsec·atm), and for the relaxation of vibrational energy into translational and rotational energy (pτ = 1.90 ± 0.10 μsec·atm). The mechanism and rates of V → V energy transfer are compared to calculations for a repulsive intermolecular potential. The general applicability of the laser‐excited vibrational fluorescence method and the interpretation of phase‐shift data are discussed.

Absolute Raman Scattering Cross-Section Measurement of the 992 cm −1 Line of Benzene*

Abstract: An accurate measurement has been made of the Raman scattering cross section of the 992 cm−1 line of benzene. Previous cross-section measurements were either made indirectly by comparison with the Rayleigh scattering cross section or by using a pulsed light source. In the present measurement the cross section was measured directly using a cw argon laser as the light source. The results give a peak differential cross section of 1.05(±0.08)×10−29 cm2 per molecule, per steradian, per wavenumber of linewidth, per plane of polarization for an incident light beam with a wavelength of 4880 A, and a linewidth of 2.3 (±0.05) cm−1. Allowing for the wavelength dependence and the optical dispersion, our total cross section agrees to within 20% with two other results. The Raman scattering cross sections of other liquids have been measured by use of the 992 cm−1 line of benzene as a reference. These values, together with details of the absolute cross-section measurement, are presented.

Dynamics of injection lasers

Abstract: The generation of picosecond pulses by means of semiconductor lasers is reviewed, citing results of an investigation of the characteristics in spike and bistable regimes. Self-synchronization of axial modes in a laser with an external mirror is also discussed.

Laser induced underwater sparks

Abstract: Measurements have been taken of the radiative flux density, the spectrum, and the temporal behavior of laser induced underwater sparks. When a 30‐MW Q‐switched ruby laser was focused into water, the resulting spark revealed a blackbody‐like spectrum with a temperature of 15 000°K. The spark was delayed in time by 15 nsec with respect to the laser pulse and had a temporal behavior very similar to the laser pulse producing it.

IMAGE CONVERSION FROM 1.6 μ TO THE VISIBLE IN LITHIUM NIOBATE

Abstract: Image information at 1.6 μ has been converted to the green by mixing it with highly collimated ruby laser radiation in lithium niobate. About 50 lines resolution has been achieved, limited only by the laser beam quality. A tenfold increase on this should be attainable with a single transverse mode laser.

EFFICIENT CW LASER OSCILLATION AT 4416 Å IN Cd (II)

Abstract: A gain of 20%/m and a power output of 50 mW have been measured on the 5s2 2D5/2 → 5p2P03/2 4416‐A laser transition in Cd (II) using a single isotope of Cd114. The system was operated in a tubular furnace at temperatures in the range of 200°C with He as a buffer gas. The laser output with respect to He pressure, discharge current, and furnace temperature are discussed along with a proposed excitation mechanism. The system appears to have the prospect of being one of the most efficient, high power lasers operating in the visible region of the spectrum.

A spectroscopic study of the plasma generated by a laser from polyethylene

Abstract: This paper describes a spectroscopic study of the plasma produced when a giant pulse ruby laser of 5 J energy and 17 ns duration is focused into a 0.3 mm diameter spot on a polyethylene target in vacuum. Quantitative measurements of spectral intensities in the range from 20 Angstrom to 6000 Angstrom have yielded estimates of the electron and ion densities, the electron temperature and the streaming velocities of the ions. From these results it is estimated that the electron temperature, the ionization energy, the energy lost by radiation and the energy of recoil of the target account for only about 10% of the incident laser energy. The ion kinetic energy accounts for 70% of the incident energy with an experimental uncertainty of ± 70%. It is calculated that there are about 1016 ions produced by the laser beam striking the target. It is shown that the electrons contained in the plasma expanding away from the target cool adiabatically. In the course of the work there have been identified several new lines hitherto unclassified.

Theory of Two-Photon Spectroscopy in Solids

Abstract: The coefficient of absorption of two photons is calculated for a two-band model of a solid. The frequency and polarization dependence is evaluated for the case that one photon (laser) frequency is fixed while the second is varied. Wannier exciton states are explicitly included in both the intermediate and final states. Depending upon the symmetry of the two energy bands, the final state may either be an exciton $s$, $p$, or $d$ state. All three cases have different frequency and polarization dependences. The final formulas for the matrix element are complicated, and approximate formulas are given which are useful for many applications. It is shown that some of the two-photon spectroscopy experiments in semiconductors and alkali halides which have been reported so far can be explained by these calculations.

Measurements on the raman component of laser atmospheric backscatter

Abstract: Density profiles of the gaseous atmosphere up to 3 km have been measured using laser backscatter. The ambiguity in the return at the laser frequency (6943 A) due to the aerosol scatter component was avoided by monitoring the frequency‐shifted Raman vibrational‐rotational band of the nitrogen component centered at 8285 A.

Rayleigh Scattering of Ruby Laser Light in Neutral Gases.

Abstract: Measurements of Rayleigh scattering from atoms and molecules in the gaseous state at 1-atm pressure are described. The use of a Q-switched ruby laser of 8-MW average power and care in minimizing spurious light permitted the determination of very small depolarizations. No depolarization could be detected in the scattering from argon and helium. However, finite depolarization ratios ρν (for vertically polarized incident light) were measured for xenon and methane: 1.55(±0.25) × 10−4 and 1.27 (±0.23) × 10−4, respectively. Departures from ideal-gas behavior provide the most plausible explanation for these findings.Depolarization ratios were also measured in hydrogen, deuterium, nitrogen, and nitrous oxide, and were found to be lower than generally accepted values. Measured differential-scattering cross sections at 60° for He, Ar, Xe, CH4, H2, D2, N2, and N2O were within experimental error of values calculated from known indices of refraction. The angular dependence of Rayleigh scattering in N2 as a function of the polarization states of both incident and scattered radiation was studied from 30° to 150°, and was found to be in agreement with theory.

Laser-Excited Rotation–Vibration Raman Scattering in Ultra-Small Gas Samples*

Abstract: The focused beam of an argon–ion laser was used as a light source for Raman-scattering experiments in gases. Photoelectric recording of rotation–vibration Raman spectra has been observed from about 1011 gas molecules in the focal region of the laser beam. The focusing geometry used to illuminate the gas samples is discussed and the important parameters of the optical system for collecting the Raman-scattered light from the small volumes are considered. The pulse-counting detection system is described and the new experimental results are presented.

The Gaussian mode in optical resonators with a radial gain profile

Abstract: The dependence of the parameters of the Gaussian mode in laser resonators on the properties of the medium in the cavity is studied. Experimental verification of the theoretical results is presented. It is found that the modes in a high‐gain laser may differ widely from the usual free space resonator results. Also, resonator configurations which in free space are unstable may, with a suitable medium, support low‐loss Gaussian modes.

Spectroscopy of K2 Using Laser-Induced Fluorescence

Abstract: When potassium vapor contained in a cell of aluminosilicate glass at 300°C is irradiated with the 6328‐A line of the He–Ne laser, several fluorescence series are excited which originate from the B 1Πu state of the 39K2 molecule. Analysis of the fluorescence allows the spectroscopic constants (cm−1) of the X 1Σg+ ground state as measured by Loomis and Nusbaum to be corrected, giving ωe = 92.021, Be = 0.056743, ωexe = 2.829 × 10−1, αe = 1.65 × 10−4, ωeye = −2.055 × 10−3, γe = − 7.2 × 10−6, De = 8.63 × 10−8, δe = +1.5 × 10−7. The laser line is found to coincide with several molecular transitions. Three of these were determined from analysis of the spectrum to be (v″ = 0, J″ = 82) → (v′ = 7, J′ = 81), (v″ = 0, J″ = 18) → (v′ = 6, J′ = 17), and (v″ = 1, J″ = 72) → (v′ = 8, J′ = 72). Radiation from K atoms accompanying K2 excitation is also observed. It is proposed that the mechanism primarily responsible for the atomic potassium emission is the excitation transfer reaction K2*+K → K2 + K*, leaving the K2 molec...

Methods for Detecting Weak Light Signals

Abstract: Theoretical and experimental evaluation of three low-level photodetection techniques, namely, lock-in, noise-voltage, and electron-pulse counting are presented. The 992 cm−1 Raman band of benzene excited by a 4.2-mW He–Ne laser operating at 6328 A provided a weak light signal which was detected by an EMI 9558 A, cooled photomultiplier. The electron-pulse-counting method employing a pulse-height discriminator was found to be superior to the other methods with regard to both signal sensitivity and signal-to-noise ratio.