Table of contents

The future trends in the development and design of high-power pulsed solid-state laser systems are considered in relation to the laser fusion problem. Attention is concentrated on neodymium-doped glass lasers, but the possibility of using other media, particularly ion-doped crystals, is also discussed. New methods for pumping such systems (both existing and future) are reviewed. Ways and means of tuning the emission wavelengths, methods for increasing the average power, and characteristic features of the design of systems as a whole are also discussed.

A description is given of a new method for investigating vibrational excitations of molecules in liquids: this method involves a preliminary brief excitation of a molecule by two picosecond pulses, differing in frequency by an amount which is in resonance with natural vibrations of the molecule, followed by a subsequent prolonged investigation of free relaxation of the excited system. The results are reported of an investigation of this kind made on C₆H₁₂ in the range 2875–2925 cm⁻¹. Lines not resolved in the usual spontaneous Raman spectrum were observed. The position of the well-known 2923 cm⁻¹ line was determined more accurately.

An ordinary matched filter responds with a peak signal to the input. The peak corresponds to the location of a target. In some pattern recognition tasks it is desirable not to find the location of a target but to identify it (for example, a printed character). It is therefore proposed that the concept of a matched filter is modified in such a way that the output pattern consists of vertical fringes. These fringes are quite suitable as a "recognition signature" because the now uninteresting vertical position of the input character becomes irrelevant: when the input is moving horizontally, the output fringes move correspondingly. A slit detector will therefore produce an electronic single-frequency signal. An even better performance can be expected from a detector with a fringe mask in front of it. Such a detector system is tuned resonantly to a particular moving fringe pattern. This modified matched filtering concept may incorporate such properties as multiplexing, principal component recognition, pupil replication, and incoherent inputs.

Double-heterojunction semiconductor diode lasers operating in an external cavity can be actively and passively mode-locked to generate continuous trains of picosecond pulses of about 1-W peak power. These lasers, which are also frequency-tunable, provide convenient and inexpensive sources of coherent ultrashort laser pulses for time-domain spectroscopy of semiconductor and molecular materials. Further developments in laser diode fabrication techniques should make it possible to extend the spectral range from the visible to the near-infrared (up to 4μ). Intracavity time-domain spectroscopy with increased sensitivity is also possible, particularly in the case of studies of semiconductor carrier dynamics. Other applications include investigation of coherent pulse propagation, two-photon spectroscopy, and development of laser oscillators exhibiting testability, phase conjugation, and other types of nonlinear behavior.

A description is given of the construction of laser systems (drivers) with neodymium glass and CO₂ gas active media designed for laser fusion research at the Institute of Laser Engineering at the Osaka University. The results are given of investigations of various physical processes in laser systems, such as the saturation effect in neodymium glass lasers and amplification of short light pulses in CO₂ lasers.

The influence of a saturation grating in the absorber of a passively mode-locked dye ring laser in the colliding pulse mode locking (CPM) regime on the pulse parameters is calculated and experimental results are presented. By changing from a Fabry–Perot resonator without a saturation grating in the absorber to a ring resonator in the CPM regime operating under similar experimental conditions, the pulse durations can be reduced from 0.4 to 0.1 psec. As a result of coherent interaction, approximately threefold reductions in the pulse duration and significantly broader regions of stability are obtained theoretically. If the active medium is shifted from its symmetrical position for equal amplification of the opposite (counterpropagating) pulses, numerical calculations give pulses having different energies and smaller regions of stability. Experimental results are reported.

An accurate analytic solution of the Vlasov equation in the one-dimensional case is given for plasma electrons in the potential electric field of a monochromatic high-frequency wave of arbitrary amplitude and spatial modulation allowing for a self-consistent field. The phase velocity of the plasma waves is assumed to be appreciably higher than the electron thermal velocity (the case of nonresonant diffusion).

The history of the development of high-power lasers for the production of dense high-temperature plasmas is described. The general laws governing the choice of active media for high-power lasers for laser fusion are noted and some comparative characteristics of neodymium, iodine, and CO₂–N₂ lasers are given. Possible methods of solving the energy problem are examined and prospects for the development of laser fusion research are analyzed.

A continuation is reported of the authors' earlier investigations in which the spectral behavior of gas lasers with strong homogeneous line broadening was analyzed numerically for two-mode lasing observed in an argon ion laser and for random types of multimode lasing. A separate analysis is made of the influence of the inhomogeneous line width and of the effect of different lifetimes of the upper and lower atomic levels on the profile and temporal evolution of the spectrum. It is shown that, in addition to the ratio of the homogeneous line width to the intermode spacing, these quantities also play an important part.

A model of a holographic memory for recording a one-dimensional sequence of bits is used to investigate the reasons for crosstalk and loss of information. It is shown that the noise between neighboring bits (elements of an image) is primarily due to diffraction and aberrations of lenses, system failure because of displacements of components, and errors in the alignment of the optical part of the system. The noise can be reduced considerably by a suitable selection of a reconstructing beam and exact (though not excessively so) alignment of the optics. The most important source of error is a local damage to a hologram, which results in local distortions even in Fourier holography. A satisfactory agreement between the theory and measurements is obtained. The proposed theoretical approach may serve as the basis of model calculations.

The coefficient of cyclotron absorption in a finite-density Maxwellian plasma is evaluated both analytically and numerically for temperatures and harmonic numbers requiring the finite electron gyro radius to be taken into account to terms of order higher than the lowest significant one commonly used. A comparison is made with results obtained to lowest signficant order and an exponential approximation is proposed which provides a good fit for a fairly wide range of temperatures and harmonic numbers. The emphasis of the numerical analysis is on the case of waves propagating perpendicularly to the magnetic field.

A review is given of investigations of the physics of high-current pulsed discharges. The idea of employing these for the optical pumping of lasers was advanced by N. G. Basov in the mid sixties. High-power optical radiation in a broad band of wavelengths (up to the far vacuum ultraviolet) is used for pumping lasers, including the high-power lasers employed for thermonuclear investigations, for studies in photochemistry and of the interaction of radiation with matter, and in the physics of shock waves and heat waves.

A new proposal is made and discussed that it should be possible to operate a chemical oxygen–iodine laser not with the aid of a cooled trap for freezing out water vapor and hydrogen peroxide, but by deep cooling of a chemical generator. This may improve considerably the operating characteristics of such a laser.

Transverse stimulated thermal Rayleigh scattering was observed at a threshold power density 10 MW/cm² in ether and pentane vapors containing absorbing admixtures of complex organic molecules; the scattering was observed near the critical point. It is suggested that gases near the critical point be used for low-threshold nonlinear conversion of wavefronts and frequencies of incident radiation.

An investigation is made of the excitation kinetics of polyatomic molecules in the quasicontinuum. An approach is proposed to allow for a gradual transfer of molecules from lower levels to the quasicontinuum without making a detailed study of their population kinetics. A solution of the diffusion equation in the quasicontinuum is used to derive analytic expressions for the vibrational energy distribution function of the molecules under the conditions of collisionless photodissociation experiments. Expressions are also derived for the dissociation yield and the average energy absorbed by the molecules as a function of the laser radiation energy flux. The results of the calculations show good agreement with the experimental data for CF₃I and SF₆ molecules.

An investigation was made of the spectral, luminescence, and lasing properties of gadolinium scandium gallium garnet (GSGG) crystals activated with neodymium and chromium ions in which the efficiency of transfer of excitation from chromium to neodymium ions is considerably higher than in YAG:Nd³⁺:Cr³⁺. It was found that the parameter characterizing the probability of an elementary Cr³⁺–Nd³⁺ interaction event for GSGG was twenty times higher than that for YAG and twice that for gadolinium gallium garnet. Investigations were made of the free lasing parameters of GSGG:Nd³⁺:Cr³⁺ and it was found that the maximum differential lasing efficiency was twice that of GSGG:Nd³⁺. It is concluded that efficient Cr³⁺→Nd³⁺ energy transfer can improve severalfold the energy characteristics of cw and pulsed neodymium lasers.

An experimental investigation was made of fluctuations in the frequency of a helium-neon laser containing a nonlinear methane absorption cell and emitting at the wavelength of 3.39 μ. Those measured were shown to be natural fluctuations. A reduction was achieved in the natural fluctuations by introducing a nonlinearly absorbing gas into the resonator cavity. The observed reduction in the spectral density of the frequency fluctuations was found to be quantitatively determined by the dynamic self-stabilization effect. It was also found that, for observation frequencies comparable with the relaxation constants of methane, an effective reduction in the spectral density took place when the radiation frequency was tuned not only close to the center of the methane transition but also within its entire Doppler absorption profile. The results of the experiments were in agreement with the conclusions of a previously performed analysis.

An analysis is made of several models of lasers with a selective saturable absorber. The emission spectrum is calculated on the assumption of spectral homogeneity of the active medium and absorber. It is shown that in the limiting case of a very narrow absorption line the lasing may be bistable.

An experimental investigation was made of the influence of the relative delay time τ and of the intensity ratio R₂₁, of the spectral components emitted by a copper vapor laser on the energy and spectral characteristics of lasing in rhodamine 6G. For certain values of τ and P₂₁ lasing in the dye was disrupted. A clamping effect was discovered between the rhodamine 6G laser emission spectrum and the yellow line of the copper vapor laser. The results obtained were used to determine the parameters of an interference filter for suppressing the yellow line from the copper vapor laser, and this made it possible to raise the efficiency of conversion of the pump radiation into lasing in the dye.

An investigation was made of changes in the permittivity ε of liquid insulators (benzene and nitrobenzene) in strong optical field. The change in the permittivity Δε was deduced from a change in the capacitance C of a capacitor filled with the investigated liquid and subjected to illumination with laser radiation of 100–250 kW/cm² power density. The corresponding electric field intensity of laser radiation was 10 kV/cm and the change produced by this field Δε was positive and amounted to about 10⁻⁵. The influence of various effects contributing to the permittivity change was analyzed.

Vacuum ultraviolet (VUV) laser systems for the 60–80 nm range are proposed on the basis of utilization of collisional energy transfer from metastable levels of rare gases to alkali metal atoms and excitation of the latter to long-lived quartet autoionizing states. Either the atoms are transferred from these states to upper active levels by a pulse of strong laser radiation or stimulated Raman scattering is accompanied by the emission of VUV photons.

A study was made of the influence of homogeneous and circular magnetic fields on a single-mode optical fiber exhibiting linear birefringence and wound into a coil with a circumference equal to the beat length. It was found that the sensitivity of a current sensor based on such a fiber can be as high as that of sensors utilizing an isotropic fiber.

An analysis is made of the polarization anisotropy of a single-mode fiber with a twisted elliptic core. The Jones matrix is obtained and the complex function of the state of polarization of light in a fiber is investigated. The results are reported of measurements of the linear and circular birefringence of a borosilicate single-mode glass fiber.

An analysis is made of the published results of an experimental study of the excitation of rotational states of molecules by an external resonance field. Some features of such excitation can be explained if the external field is switched on and off not instantaneously but adiabatically. In view of the adiabatic conditions an increase in the detuning of an external field should reduce the observed signal at a rate faster than that proportional to the reciprocal of the square of the detuning.

Mode birefringence of an elliptic fiber is described by a formula valid in the case of a small ellipticity of the core and an arbitrary difference between the refractive indices of the core and cladding. This formula is derived using approximate vector solutions of the Maxwell equations for two dominant orthogonally polarized modes. It is shown that there are considerable changes in the mode birefringence due to changes in the parameters of the single-mode regime. A study is made of the difference between the group velocities of the dominant modes. It is shown that the ranges of the parameters of a single-mode elliptic fiber in which the mode birefringence is maximal and the intermode dispersion minimal overlap near the limit of the singlemode regime. A comparison is made with the published results.

An analysis is made of the conditions of propagation of opposite waves in a ring Raman fiber laser excited by an external periodic signal whose duration is less than the time needed for a round trip in the ring resonator. It is shown that an allowance for nonlinear properties of the fiber gives rise to an optical nonreciprocity in respect of the opposite waves. A calculation is made of the frequency difference between the opposite waves in a ring laser as a function of the laser parameters.

A calculation is made of the polarization at the doubled frequency on the assumption that the inversion symmetry of a nematic liquid crystal is D_h. Explicit expressions are obtained for the dependence of the effective susceptibility on the molecular hyperpolarizabilities and on the orientational order parameters.

An experimental investigation was made of a copper vapor laser having a sectioned transverse discharge. Optically controlled semiconductor switches were employed to switch an excitation pulse generator. Their switching stability was ∼0.01 nsec. The rate of development of the discharge in a Cu–Ne mixture was found to depend on the temperature in the active zone of the laser. A study was made of the influence of the temperature drop in the individual laser sections on the scatter of the discharge development rates. It was shown to be advantageous to use optically controlled semiconductor switches rather than gas-discharge spark gaps. When using the former, the lack of synchronism in exciting the copper atoms along the sectioned discharge gap was reduced from 20 to 5 nsec, the peak radiated pulse power was increased by almost a factor of four for the λ₁=510.6 nm green line and was more than doubled for the λ₂=578.2 nm yellow line in the copper vapor laser emission spectrum.

A review is given of papers presented at the Fifth International Conference on Laser Spectroscopy. The main topics at the Conference were: fundamental applications of laser spectroscopy; applications of lasers in spectroscopy; double resonance; collisional phenomena; nonlinear processes; Rydberg states; methods for investigating unstable states; cooling, trapping, and control of motion of atoms and molecules; surfaces and solids; vacuum ultraviolet; development of new lasers.