Showing papers in "IEEE Journal of Quantum Electronics in 1966"

Resonant optical second harmonic generation and mixing

Abstract: Experimental and theoretical results are described on the enhancement of optical second harmonic generation (SHG) and mixing in KDP by the use of optical resonance. Both resonance of the harmonic and of the fundamental are considered. Large enhancements are possible for resonators with low loss. Using a planoconcave harmonic resonator containing 1.23 cm of KDP, the authors achieved a loss < 4 percent per pass. This resulted in an enhancement of ∼ 500 times the harmonic power internal to the resonator and ∼ 10 times external to the resonator. When resonating, the fundamental enhancements of ∼ 5 were observed. The theory includes the effect of double refraction. This results in a coupling coefficient of the generated harmonic power to the transverse modes of the harmonic resonator. The experimental results are in substantial agreement with the theory.

6C3 - Efficient pulsed gas discharge lasers

Abstract: A class of lasers which operates by cyclic excitation and relaxation in atomic vapor discharges is discussed. The required energy level structure for maximum efficiency is described. Pulsed laser action in neutral atomic copper vapor at 5105.54A (58 dB/m, 2 kW peak power) and at 5782.13A (42 dB/m, 0.6 kW peak power) provides experimental verification and promises high efficiency. Pulsed laser action was also observed in ionized calcium at 8542.09A and 8662.14A. Both have saturated gains of 58 dB/m and a total peak power output above 30 W.

6A5 - Possibility of self-focusing due to intensity dependent anomalous dispersion

Abstract: Close to an atomic resonance, the dependence of the real part of the susceptibility on power level (i.e., saturation effects) can be sizeable and therefore lead to self-trapping or self-focusing. It is shown here under what conditions this effect can occur. Order of magnitude estimates are given for the intensity dependent index change and the diameter of the trapped beam for some laser materials.

Effect of gain saturation on the oscillating modes of optical masers

Abstract: Because an active maser medium exhibits nonlinear gain saturation, the oscillating modes of an optical maser are expected to be somewhat different from those of the passive resonator. Statz and Tang [5] have obtained some numerical results for an active resonator with a pair of parallel-plane, infinite-strip mirrors. We have reformulated the problem for active resonators with circular mirrors of both parallel-plane and confocal geometries and have obtained numerical results using an iterative method of solution. We find the cardinal features of the active modes, such as mode patterns, diffraction losses, and resonant frequencies, to be essentially the same as those of the passive modes, even for unsaturated gains as high as three and a half dB per pass. The mode that predominates in an active Fabry-Perot resonator is found to be the lowest-order (TEM 00 ) mode. However, the predominating modes in an active confocal resonator are found to depend on the Fresnel number; the larger the Fresnel number, the higher is the mode order. The study includes computations of field distributions, diffraction losses, and phase shifts of the steady-state predominating modes and of their output intensities as functions of unsaturated gain, saturation parameter, mirror transmissivity, scattering loss, and resonator geometry.

Design of flashlamp driving circuits

Abstract: The problem of design of single mesh circuits for driving xenon flashlamps has been considered in detail. The normalized nonlinear differential equation I'\pm \alpha |I|^{1/2} + \int\min{0}\max{\tau} I d\tilde{\tau} = 1 for this system has been solved by digital computer and the solutions are presented. Since the equation is linear in time, though nonlinear in current, it is possible to provide explicit design equations. With them, for a given lamp type, energy input, pulse duration, and pulse shape factor, the inductance, capacitance, and operating voltage are easily determined. A procedure for estimating circuit losses is also presented.

Nonlinear theory of the internally loss modulated laser

Abstract: This paper presents a detailed nonlinear analysis of the internally loss-modulated laser including the effect of arbitrary atomic lineshape, saturation, and mode pulling. Results of the analysis are in part numerical and include a study of the spectral and time domain behavior of the laser output. The results include a determination of the minimum perturbation strength which is necessary to produce phase locking, peak pulse amplitude, and minimum pulsewidth as a function of perturbation strength, a consideration of the detuned case, and a comparison of AM- versus FM-type phase locking. Results are compared with the previously obtained linearized solutions of others.

5A2 - Theory of the optical parametric oscillator

Abstract: A formalism for describing optical parametric oscillation is developed. The theory is applied to the derivation of the oscillation threshold condition, power output, the Manley-Rowe conditions, index matching, and frequency tuning.

Selection of Raman laser materials

Abstract: In order to select Raman laser materials for a specific purpose, it is important to know which frequency shifts and excitation power thresholds can be expected from various substances. The thresholds are strongly dependent on the peak scattering cross sections of the corresponding incoherent Raman lines, and these cross sections can vary by several orders of magnitude between Raman lines from different molecules as well as between Raman lines from a single molecule. This paper points out how the rules concerning frequencies, intensities, degrees of depolarization, and line widths, established for incoherent Raman scattering, can be used as a guide for the prediction of Raman laser performance for new materials. Subsequently, all substances and participating vibrations which have been reported to exhibit stimulated Raman scattering to date are listed and discussed.

8C2 - Fluid flow measurements with a laser Doppler velocimeter

Abstract: The basic principles of the laser Doppler velocimeter, a new instrument for measurement of localized flow velocities in gases and liquids, are discussed in detail. Velocity measurements are made by detecting the Doppler shift in monochromatic laser light which is scattered from small contaminant particles in the fluid. A CW gas laser serves as the light source, and the Doppler shift is detected by optical heterodyne techniques. The latest fluid flow measurements obtained with the laser Doppler velocimeter are presented.

9A9 - Pulsed laser holograms

Abstract: The limited coherence of most pulsed lasers is the principle difficulty in making pulsed holograms. This paper describes techniques used for the production of quality holograms of back lighted subjects, using ordinary pulsed lasers. The arrangements used achieve a temporal and spatial matching of the scene and reference beams to the degree required by the laser coherence cell. Techniques are described which prevent the destruction of the spatial match when using a diffusing screen. The use of lenses to permit the recording of large subjects on small holograms with limited pulsed energy is described. The limitations on subject motion are described. Experimental examples of the techniques are illustrated by holographic recordings of bullets in flight. The technique of double exposure holographic interferometry is also illustrated.

8A2 - The self-focusing of light of different polarizations

Abstract: In an effort to verify the hypothesis that molecular reorientation (ac Kerr effect) is responsible for the self-focusing of light beams in certain liquids, we have measured and compared the thresholds and other characteristics of self-focusing for circularly and linearly polarized beams incident on these liquids. We show that for plane waves propagating in a homogeneous, isotropic, ensemble of molecules having anisotropic polarizability tensors, the nonlinear index should be four times as great for linearly as for circularly polarized waves. The hope that this difference in indexes would be reflected in a four-fold increase in the threshold power for self-trapping when circularly instead of linearly polarized light is incident was not realized. In practice, the increase was always found to be much less. However, in every case studied, the trapped light from a beam, circularly polarized to better than 1 part in 200, was markedly, if not completely, depolarized as soon as self-trapping could be detected. We show qualitatively that this polarization instability should exist for all but linearly polarized light and for a variety of nonlinear mechanisms. However, in the absence of even an approximate quantitative theory of the self-trapping of light that is not linearly polarized, the comparative measurements of thresholds cannot be said either to verify or disprove the hypothesis of molecular reorientation.

The output power of a 6328-&#197; He-Ne gas laser

Abstract: Expressions are derived for the output power of a gas laser in single-mode operation. This theory is then extended to multimode operation where the mode spacing is less than the pressure-broadened "natural" atomic linewidth. Using measured values for the linewidth and saturation parameters, these formulas are used to predict 6328-A output power for a He-Ne laser as a function of laser gain, cavity loss, and output coupling. Experiments are reported which show good agreement with the theoretical predictions, and it is concluded that the theory presented here is adequate to describe the main features of the power output behavior of a 6328-A He-Ne gas laser.

9A4 - Laser action by enhanced total internal reflection

Abstract: Oscillation and amplification have been observed in a fiber in which the core is passive glass and the cladding is neodymium-doped glass. As in conventional fiber optics, the refractive index of the core is greater than that of the cladding, so that light is carried in the core by total internal reflection. The results are interpreted as meaning that the reflection coefficient at the interface between passive core and laser cladding is greater than unity. Expressed in waveguide terminolgy, modes which are guided by the high index passive core, but have a substantial portion of their energy in the cladding, can be amplified by means of an active cladding.

6A1 - A new class of trapped light filaments

Abstract: A new class of self-trapped light filaments has been discovered and its properties investigated. These filaments have a very small diameter, ( \sim 5 \mu ), contain a few ergs of energy, and last a very short time, ( \lsim 1/ns ). Stimulated Raman emission occurs in these small filaments with high initial gain at the beginning of the filament and apparent saturation towards its end. Possible mechanisms for the creation, containment, and termination of these filaments are discussed.

7A11 - Laser induced emission of electrons, ions, and X rays from solid targets

Abstract: We have first studied the spatial distribution of electrons and ions emitted from a solid target when hit by a focused, Q -switched laser beam. The particle emission is highly anisotropic. In a second experiment we have analyzed the ion energy for various targets such as beryllium, carbon, and molybdenum. For each target element, evidence for multicharged ions has been shown as a function of laser intensity. With a laser flux at the target surface of 6 \times 10^{10} W/cm2, the maximum ion energy obtained is about 2 keV. Finally, we have shown that particle emission is accompanied by X-ray emission.

7A5 - Evaporation and heating of a substance due to laser radiation

Abstract: Gas-dynamic equations are studied for determination of heating, vaporization, and expansion of substance when a radiation of Q -switched laser is collimated onto a surface of solids. A possibility of existence of the self-regulating regime of vaporization and heating in one-dimensional case is proved for the case when an optical density value of the vapored gas is about unity in all the stages of this process. The values for temperature and velocity of the substance expansion are obtained. An approximate consideration of this problem is carried out when a spherical case of expansion takes place describing substance vaporization by a strong-focused radiation. The experimental investigation data of gas dynamic values are presented for the case of "plume" arising from collimated laser radiation interactions with the energy of several hundred MeV. Results obtained in experiments permit to calculate temperature, expansion velocity, and optical transparency of vapored substance for different stages of this process.

9C6 - Experimental studies of saturable optical absorption

Abstract: The optical absorption of several substances has been found to depend in a reversible way on the incident light flux [1]-[5] of a high intensity laser. Moreover, it has been observed [6]-[7] that the absorption decreases not only at the laser wavelength but simultaneously within a large band around this wavelength. These effects may be explained by a time-dependant population distribution among the different excited states of absorbing molecules during the strong laser excitation [8]. The following experimental results enable us to determine the different time constants of importance in this phenomenon.

1A4 - Photocount distributions and field statistics

Abstract: The statistical properties of a radiation field (either Gaussian or laser) have been investigated by photon counting distributions. Two methods (nonlinear and linear) are here introduced and compared from the viewpoint of the accuracy of the measurement and of the associated information capacity. Finally, the method of the "joint photocount distributions" is here introduced as a new tool for the investigation of the time evolution of a random field.

5C3 - GaAs as an electrooptic modulator at 10.6 microns

Abstract: The electrooptic properties of a number of semiconductors were investigated. Of particular interest was the possibility of using these materials for modulation of infrared radiation, since many of the efficient modulation materials for the shorter wavelengths, such as KTN and KDP, are opaque in this region. We have investigated experimentally the modulation potential of a number of semiconducting materials. These include ZnS and GaAs of the noncentrosymmetric 43m class. The electrooptic coefficients were determined by using a Co 2 , 10.6μ and a He-Ne 3.39 μ laser as the radiation source. Based on our experiments, GaAs appears as a suitable material for infrared modulation at \lambda > 10\mu .

Characteristics of a single-frequency Michelson-type He-Ne gas laser

Abstract: A new mode suppression configuration is described for producing a single-frequency output from gaseous lasers. This configuration has the form of a modified Michelson interferometer where the basic modification is in the introduction of a third feedback mirror for coupling the two branches of the Michelson. Small path length differences between the two branches of the Michelson are responsible for the mode suppression properties of the device which come about through interference phenomena at the beam splitter. This results in a frequency-dependent power loss out of the beam splitter which can provide for large amounts of mode discrimination. The device is made into an oscillator by introducing gain into each of the two branches of the Michelson. A detailed analysis of the device is given based on the scattering matrix formalism. The results of the analysis give the oscillation frequencies, the degree of mode discrimination, the internal laser radiation intensity, and equations for the output power. Experimentally, a 10 mW single-mode 6328 A He-Ne Michelson-type gas laser is described. Data on the frequency spectrum, output power, and amplitude stability of a free running laser are given and correlated with theory. While it is shown that single-frequency operation is easily obtained, the device in its present form does not include a means for stabilizing the frequency.

On the stabilization of a high-power single-frequency laser

Abstract: A single-mode 6328A He-Ne laser is described which has a single-frequency power output of 50 mW. Amethodof stabilizing the output of this laser is described, and experimental results are quoted to show that the output can be stabilized to the resonance frequency of a stable reference cavity to within ± 1 Mc/s. This stabilization scheme has two advantages over an earlier scheme used with such a single-mode laser: 1) the output is neither amplitude nor frequency modulated by the feedback system; and 2), the output coupling may be continuously varied so that optimum output may be obtained without experimenting with mirrors of various transmissivities.

7B2 - Laser brightness gain and mode control by compensation for thermal distortion

Abstract: In the limit of excellent optical quality of laser rods, the beam divergence will be determined by the optical distortion produced in the rod brought about by nonuniform absorption of pump light. For ruby lasers this has been shown to be approximately spherical and reproducible from firing-to-firing. Thus it should be possible to compensate for this by means of an external mirror having a curvature opposite to that induced in the rod, and by this technique reduce the beam divergence. The distortion produced in the rod is described in terms of a ray matrix of a thick lens whose focal length is a function of the input energy to the rod. A procedure is given for measuring this focal length to within ± 3 percent; it involves measuring the beat frequencies between transverse modes of a flat-mirror cavity containing the laser rod as an active element. After compensating an excellent optical quality rod with an external mirror, a brightness gain of about 100 was achieved simultaneously with single transverse mode operation (TEM 00 ) at pump energies as high as 40 percent over threshold. This procedure for achieving single transverse mode operation is advantageous compared to other methods since 1) no extra elements are needed in the cavity, 2) the beam divergence is near the diffraction limit for the rod holder, and 3) the TEM 00 mode is not only selected but enhanced in volume, thus increasing the power output in this mode and the mode stability of the laser.

On the optimum geometry of a 6328&#197; laser oscillator

Abstract: Approximate calculations of the maximum output power of a 6328 A He-Ne gas laser are made for a number of laser tubes and the results compared with experimentally observed values. Good agreement is obtained. Similar calculations are then used to determine the maximum output power, optimum mirror transmission, and optimum laser geometry (length and diameter of laser tube, for a number of special cases.

1A1 - Photoelectron statistics produced by a laser operating below and above the threshold of oscillation

Abstract: A simple semiclassical derivation is given of the statistics of the photoelectrons emitted from a photosurface illuminated by light with statistically time dependent intensity. Comparison is made with the quantum mechanical expressions. Experiments are described that yield the probability of emission of n photoelectrons within a fixed time interval from a photocathode illuminated by a laser below and above threshold. The experimental results are compared with the theory developed in this paper and expressions derived by Glauber.

9C1 - Single transverse and longitudinal mode Q-switched ruby laser

Abstract: Single transverse and longitudinal modes have been observed in the output of a passive Q -switched ruby laser when two spherical mirrors are used for the resonator. This result has been consistently obtained with several rods, with two sets of mirrors of different radius of curvature, and with a few values of reflectivity of the mirrors which proves that it is typical of a spherical resonator. The saturable absorber is a solution of vanadium phtalocyanine in nitrobenzene. To have a single transverse mode, the mirror alignment and the position of the rod within the cavity is critical. When the laser oscillates in a single mode, the output pulse is very reproducible and the fluctuations of peak amplitude and width of the pulse are less than ∼3 percent. Furthermore, as expected, the output beam has a very good spatial coherence and its divergence approaches the limits set by diffraction. As far as peak output power and width of the pulse, the best results have been obtained with one concave and one convex mirror of ∼ 5-m radius of curvature, one mirror being 99 percent and the other 50 percent reflecting. A pulse of 1 MW peak power and 30 ns half-width has been obtained.