Table of contents

A report is given of some of the results obtained at the Institute of Heat Physics, Siberian Branch of the Academy of Sciences of the USSR, on the establishment of a joint time and length standard. Attention is concentrated on the development of lasers with a stable emission frequency and of systems for frequency synthesis and measurement.

Results are presented of a theoretical and experimental investigation of a multipass laser cell. The optical system of the cell comprises two plane mirrors and one spherical. The simplicity of the cell design enables analytic calculations to be made of the pattern of light spots on the mirrors. The calculations are made allowing for aberrations of the focusing mirror to within the third order with respect to small deflections of an arbitrary beam from the axial one. By utilizing the quasiperiodicity of the beam motion, it is possible to reduce the finite-difference equations for the beam motion to a Hamiltonian system of differential equations. The problem of motion of light spots over the mirror is reduced to an appropriate dynamic problem which is solved using perturbation theory. The theoretical and experimental results are compared. The coordinates of a spot can be accurately determined (to within the spot size) from its number using the equations derived.

An analysis is made of a model of coherent Raman mixing which allows for the anti-Stokes and two Stokes stimulated Raman scattering components. A three-cell mixing system is proposed and it is shown that the infrared generation efficiency in this system, other conditions being equal, may be two or three orders of magnitude higher than that in single-cell systems.

An analysis is made of stimulated recombination of atoms in an electron–proton beam in a laser radiation field. An estimate is made of the current density of neutral hydrogen (deuterium) atoms which can be achieved in a nonrelativistic beam.

A theoretical study is made of the influence of the shot noise in the process of emission and absorption of photons on the statistical properties of a multimode solid-state laser. It is shown that at all stages of lasing the fluctuations

Δn_l

of the number of photons

n_l

in the individual modes are large:

Δn_l∼n_l

. The emission spectrum is then very jagged because of a weak correlation between the individual modes. These conclusions are in agreement with the results of published experimental data.

An analysis is made of conditions for achieving amplification due to transitions whose Doppler broadening is eliminated (under optical pumping) by light shifts which depend on the atomic velocity. An investigation is made of the spectral characteristics of this amplification process and of the features of lasing due to these transitions. The advantages of lasing due to this type of pumping are shown and possibilities for achieving unidirectional lasing in ring resonators are indicated.

The applicability of several photon antibunching criteria to transient optical fields is studied. The results are used to investigate resonance fluorescence from an atomic beam with a fluctuating number of atoms. It is shown that photon antibunching is observed provided that the average number of atoms and the background noise are sufficiently low.

It is found that allowance for the quadratic Doppler effect broadens and changes the resonance absorption line profile of particles trapped in an oscillator potential comprising a periodic lattice of resonances having an envelope in the form of a conventional Doppler profile, if the particle oscillation frequency is greater than the homogeneous line width. When the quadratic Doppler shift is small compared with the homogeneous line width, its influence is reduced to a shift of the resonances. A new possibility for eliminating thermal broadening of individual resonances is discussed.

A theory of atomic motion in arbitrary resonant laser fields is put forward. It is shown that when the natural line width of an atomic transition is greater than the recoil energy, the microscopic equations for the atomic density matrix can be reduced to the Fokker–Planck equation for the atomic distribution function and, in the case of slow atoms, the latter can be reduced to the Smoluchowski equation for the atomic density. The macroscopic Smoluchowski equation is used to find laser field configurations which ensure stable trapping of atoms. An extremely simple radiative atomic trap is described and its main parameters are given.

A study was made of the absorption and optoacoustic spectrum of dimethyl ether for line radiation from a low-pressure CO₂ laser. By pumping transitions having absorption coefficients of 10⁻²–10⁻³ cm⁻¹·Torr⁻¹, twenty submillimeter laser emission lines were obtained in the wavelength range 209–934 μ and passive Q switching of the CO₂ laser was achieved for 18 lines. An analysis of the experimental data revealed splitting of the investigated infrared and submillimeter transitions.

An investigation is made of optical pulse propagation in a two-photon-absorbing medium in the incoherent regime. Simple analytic expressions describing the initial stage of evolution of arbitrarily shaped radiation pulses are derived in the long-wavelength approximation. The steady-state pulse shape is determined. In cases of self-modulation instability, the steady-state pulse is a harmonic wave whose period is governed by the properties of the medium and does not depend on the intensity of the propagating radiation. A qualitative investigation is made of the formation of steady-state pulses.

An investigation was made of the spectral properties of four-photon parametric oscillation when the frequency of the generated field is tunable near an atomic resonance line. An ω₄ = ω₁–ω₂+ω₃ generation process involving coherent anti-Stokes scattering in thallium vapor was achieved experimentally. Biharmonic pumping was provided by resonant excitation of a 6P_1/2–6P_3/2 two-photon transition at the frequency ω₁–ω₂. Measurements were made of the oscillation power when the frequency ω₃ was tunable near the frequency of a 6P_3/2–7D_3/2 transition at various thallium vapor pressures. It was found that the spectral properties of resonant parametric oscillation depend strongly on the concentration of active-mixture atoms and also on various saturation effects.

A prototype device was made for measuring the concentration of carbon monoxide in the atmosphere with the aid of pulsed injection lasers. It is suitable for operation under urban and field conditions, and has a sensitivity of 0.01 ppm. A description is given of the characteristics of the device and of the results of using it continuously for long periods.

An experimental study was made of the contrast ratio of a neodymium laser using wavefront reversal by stimulated Brillouin scattering. It was found that the contrast ratio can be as high as ~10⁸, its limiting value can be determined by scattering on the surfaces of the optical elements, and that it is practically independent of the contrast ratio of the master oscillator.

A study was made of the dependences of the efficiency of resonant parametric frequency conversion in sodium vapor on the infrared radiation wavelength, vapor density, and detuning from two-photon pump resonance. It was found in this conversion system that for each infrared wave the conversion efficiency had three maxima on tuning the frequency of two-photon pumping over the region of a forbidden transition. The principal maximum was realized for accurate two-photon resonance, and the two other maxima resulted from a rise in the cubic susceptibility close to single-photon sum-frequency resonances. A quantum efficiency of 35% was achieved for a 9.26 μ infrared wave.

A spectral method is used in an attempt to find a unified approach to describe several variants of wavefront reversal in stimulated scattering: in a waveguide and when spatially inhomogeneous and single-mode radiation is focused by a lens. It is found that this involves allowing for a specific correlation between the spatial spectral components in the expansion of the pump field. Approximate solutions are found for the scattered field in these cases.

The causality concepts are used to prove a generalized radiation principle applicable to absorbing and amplifying media. It is shown that the reflection coefficients of homogeneous plane waves incident from a transparent medium on an interface with an amplifying medium do not exceed unity. This is used to discuss the published experiments. An integral equation is derived and solved for the modes of a laser in which amplification is due to the reflection of light from an active layer.

An investigation is made of the feasibility of absolute measurements of the brightness of light from the signal-to-noise ratio M of a parametric frequency converter with reflection and absorption in a nonlinear crystal. A model of a plane-parallel layer and a constant monochromatic pump source of frequency ω₀ is used. Equations are derived to describe the transmission and reflection of waves with frequencies ω₁ and ω₂=ω₀–ω₁ in the layer. The resultant four-dimensional scattering matrix determines the quantum noise of the converter in accordance with the "nonlinear" Kirchhoff law. It is found that the number of photons per mode N in the incident light is equal to M multiplied by a correction factor close to unity, which depends on the absorption coefficient and on the refractive index.

An investigation is reported of the spectral, luminescence, and lasing properties of a Gd₃Ga₅O₁₂ crystal activated with chromium and neodymium ions. The high efficiency of the energy transfer process from chromium to neodymium ions is demonstrated. For example, the probability of an elementary Cr³⁺–Nd³⁺ interaction event in a Gd₃Ga₅O₁₂ crystal was 12 times higher than that in a Y₃Al₅O₁₂ crystal. It was found that sensitization of neodymium ion luminescence by chromium ions can increase severalfold the energy characteristics of cw and pulsed neodymium lasers. An investigation of the free-lasing parameters shows that the ultimate differential lasing efficiency of neodymium in a Gd₃Ga₅O₁₂:Cr:Nd crystal is 3.6 times higher than that for a YAG:Nd crystal under comparable conditions.

An experimental investigation was made of an ultraviolet I₂ laser with an energy of 13 J in a 12-μsec pulse and an average specific output energy of ~10 J/liter. An analysis was made of the processes occurring in the Using cycle, the density of the excitation spectrum was estimated, and rates were obtained of the relaxation of the vibrational energy of the excited states and of the depopulation of the lower active state. The conditions under which a steady-state population inversion can exist in an I2 laser during continuous pumping were established. The maximum attainable efficiency was estimated to be ~1% for the pump source employed. The efficiency of conversion of the pump radiation energy in the I2 absorption band into laser radiation energy amounted to at least 6%. The quantum efficiency of lasing was not less than 11%. When the fluorescence was included, it was found that the quantum efficiency for the D' state of I₂ was several times higher.

The results are reported of computer calculations of the threshold current of oxide-insulated stripe laser diodes with various values of the sheet resistance and stripe width. The optical stripe widths are determined as a function of the sheet resistance.

Stimulated Raman scattering in a fiber converter pumped by a pulsed YAG:Nd laser made it possible to investigate backscattering in fiber waveguides over a wide range of wavelengths. The attenuation of light found by the backscattering method in a multimode fiber waveguide at wavelengths 0.53–0.8μ and 1.06–1.37 μ was in good agreement with the results obtained by the usual break-off method.

Fibers were prepared from glassy chalcogenide semiconductors and their properties investigated. It was possible to vary the refractive index over the transverse cross section of the fibers by recording on them an annular profile using radiation of wavelength λ =0.51 μ. It was discovered that optical darkening and bleaching of fibers having an As₂S₃ core occurred under the action of λ =0.63 μ laser radiation. The changes in the optical parameters of fibers under the action of radiation were attributed to transformations in the optical structure of the glassy chalcogenide semiconductor materials and a redistribution of nonequilibrium charge carriers in traps.

Stimulated scattering of light by oscillations of the shape of aerosol particles, excited resonantly by a train of high-intensity laser pulses, was detected and investigated. The excitation of natural oscillations of fog drops could be used to determine their size, to induce selective fragmentation, and to generate monochromatic microwave radiation in the presence of an external electric field.

The wave approach is used to solve the problem of design of a fiber-optic line consisting of a high-coherence source emitting a linearly polarized spherical wave, a fiber waveguide, and a threshold photodetector. The intensity of the radiation emitted by the source is modulated by a simple binary code. The optimization criteria are the requirement of minimum distortion of the signal waveform at the exit from the fiber and the maximum stability of the communication process. Allowance is made for all types of dispersion (material, waveguide, and mode), and also for the coupling between the modes because of slight irregularities of the fiber resulting in distortion of the pulse shape. The dependences of the duration of the input pulses and of the photodetector threshold on the wavelength of the radiation emitted by the source are given and analyzed, together with the dependence of the reliability of reception of the transmitted signal on the fiber waveguide length.

An investigation was made of the distribution of color centers along a glass optical fiber and a single-fiber optical cable with the fiber lying freely in a tube. The induced optical attenuation of the investigated fibers and cables was used as a measure of the number of color centers. The fibers and cables were irradiated with γ rays from a ⁶⁰Co source and the dose was up to 10⁷ rad. The dependence of the induced attenuation on the dose was determined for each investigated segment. The attenuation was measured at the wavelength of 0.82 μ.

It is shown that an instability of the rotation of the direction of polarization of the output radiation may occur in double-pass amplifiers operating under saturation conditions. This instability appears because of a nonlinear interaction between the opposite waves in an amplifier. A model of relaxation constants for different types of transition is used to derive criteria for the occurrence of such an instability.

The efficiencies of various methods of detection of an optical signal were compared over an atmospheric 500-m path at the wavelength of 10.6 μ. The methods investigated were direct detection, heterodyne detection without compensation of the influence of temporal phase fluctuations, and heterodyne detection with such compensation. The measurements were carried out for a single aperture and also in the case of spatially separate detection by two apertures. It was found that compensation reduced considerably the probability of an error.

Millisecond lasing was observed as a result of a self-terminating ⁴I_11/2–⁴I_13/2 transition in erbium ions present in Lu₃Al₅O₁₂:Er crystals. It was found that such lasing was due to efficient transfer of the erbium ions from a lower active level.

A theoretical study is made of the diffraction of a guided mode at a contact between two electric waveguides. A closed system of equations is obtained for the scattering coefficients and this system is solved by the method of successive approximations. A rapid convergence of the method makes it possible to use just the first two approximations in the interpretation of the experimental data.

Picosecond pulses of 0.7–0.8 μ wavelengths were generated in an alexandrite laser as a result of electronic-vibrational transitions ⁴T₂→⁴A₂+hω_phonon. Passive mode locking was ensured by the use of DS1 and DTTS saturable absorbers. The duration of the pulses generated using DS1 was 8 psec at wavelengths of 0.725–0.745 μ, whereas the duration of the pulses generated using DTTS was 90 psec in the range 0.75–0.775μ.

A study was made of the deformation of the absorption spectrum of ZnSe single crystals excited with dye laser radiation of wavelengths 446–458 nm. It was found that the form of the bleaching (transmission-enhancement) spectrum was independent of the exciting light wavelength, indicating that the broadening of the bleached absorption band was homogeneous.

The absolute quantum efficiency η of the luminescence from the upper active level of neodymium in an NdP₅O₁₄ crystal was determined. The result (η=0.35±0.03) indicated that it should be possible to obtain η=1 for a crystal of La_1–xNd_xP₅O₁₁ with x<0.1.

The results are presented of an experimental investigation of the instability of the optical length of waveguides having rectangular and parabolic refractive index profiles, when they are subjected to bending and heating.

It is shown that fast waves of absorption of laser radiation with the propagation mechanism dominated by the role of the intrinsic radiation emitted from the resultant plasma may appear at flux densities at least an order of magnitude lower than those at which they have been observed experimentally. An expanding plasma generates a shock wave where the pressure is much higher than atmospheric. Numerical calculations relating to the spectral radiation-dynamic problem are used to study the evolution of such subsonic radiation waves traveling from the initial plasma layer and to determine the time dependences of their main parameters (up to the quasisteady propagation stage). It is shown that these effects may occur when the flux (power) density exceeds 0.1 MW/cm² for CO₂ lasers and 1 MW/cm² for neodymium lasers.

The hydrogenic atom model is used to calculate the electron polarizabilities of an F center in KCl. The two-photon absorption cross section found by calculation is in agreement with the experimental results.

A description is given of a computer-controlled wavelength meter designed for the 500–650 nm range. It consists of four Fizeau interferometers, multichannel photodetectors with a control system, and a microcomputer. The resolution of this instrument is 10⁷. The minimum detectable energy is 0.1 mJ. It can deal with continuous and pulsed laser radiation.

Some lasing characteristics were determined for a new Na–Al phosphate glass of the LGS-T type characterized by a higher thermal conductivity and thermal stability than the known phosphate glasses and by a fairly high thermal expansion coefficient, which made it possible to synthesize athermal glasses. No concentration quenching of the neodymium luminescence was observed when this glass contained up to 6 wt.% Nd₂O₃. Active elements in the form of rods 8 mm in diameter and 100 mm long were capable of an average output power in excess of 20 W at a repetition frequency of 15 Hz. The investigated glass had good technological characteristics and could be manufactured on a large scale.

An investigation was made of a photoinitiated chemical D₂–F₂–CO₂ laser. When the mixture composition was D₂:F₂:CO₂:He = 1:5.5:4:9.5 ([O₂]/[F₂]=0.03, p=1 atm) and the degree of initiation was Δ[F₂]/[F₂] = 0.05%, the specific output energy was 37 J/liter, which corresponded to a technical laser efficiency of 18%.

An investigation was made of the energy characteristics of a chemical HF laser utilizing an H₂–SF₆ mixture and excited by a beam of relativistic electrons. Efficient lasing was obtained at gas mixture pressures up to 4.5 atm, a specific output energy of 50 J/liter being obtained at 1.5 atm. It should be possible to tune continuously the lasing frequency in a range 2.2 cm⁻¹ centered on each vibrational-rotational line and thus build a highpower tunable laser of the kind required in laser-chemical investigations.

A study was made of the energy parameters of an electron-beam-excited H₂–HF laser at active medium pressures up to 3 atm, a specific output energy of 180 J/liter being obtained at an efficiency (referred to the electron beam energy deposited in the active volume) of 500%. An increase in the degree of initiation achieved on substituting sulfur hexafluoride in place of helium enabled a specific output energy of 400 J/liter to be obtained from a mixture at atmospheric pressure, with an efficiency of ~140% and a chemical efficiency of 22%.

A study was made of the dependences of the parameters of a chemical H₂–F₂ laser on the degree of initial initiation of the mixture by an electron beam. The conditions for the attainment of the maximum efficiency were different from those needed to achieve the maximum output energy.

It is shown that in the case of stimulated Brillouin scattering pumped by a small number of nonaxial (transverse) modes there is practically no wavefront reversal. In a theoretical analysis of this situation one has to allow for the non-Bragg terms in the equations for the interaction between the waves which show no oscillations along the longitudinal coordinate.

The concept of zigzag propagation of light rays in a dielectric waveguide is used to show how to determine the coefficient of attenuation of light representing its escape from the waveguide and the coupling constant of interacting waves in a periodically corrugated waveguide. It is also shown that this concept makes it possible to provide a simple explanation of some of the observations made earlier in relation to integrated optics.

It was established that the frequencies of relaxational oscillations exhibited by a solid-state traveling-wave ring laser depended on the phase nonreciprocity of the resonator. The results of a theoretical analysis were confirmed by an experiment on a cw YAG:Nd laser.