Table of contents

A review is given of the work published in 1977-1984 on synchronous pumping of tunable lasers. The first part provides a comparison of various theoretical approaches used to describe the mechanism of formation of ultrashort light pulses in synchronously pumped lasers. The second part reviews the recent work reporting experimental investigations of the main types of synchronously pumped tunable lasers.

A study was made of a high-pressure pulsed laser utilizing 3d–4p transitions in the Ar atom and emitting at wavelengths of 1.27, 1.79, 2.08 and 2.40μ. When a 10-liter He–Ar active mixture was pumped by a beam of fast electrons, the output energy was 0.8J and the laser efficiency was > 1%. The electron-beam-controlled excitation method was realized for the first time in the case of He–Ar and Ne–Ar mixtures and the output energy was 1.5J. A study was made of the spectral composition of the radiation as a function of the experimental conditions.

Three-component mixtures are recommended for increasing the output energy characteristics of high-pressure lasers pumped by an electron beam and emitting as a result of infrared transitions in ArI, KrI, and XeI. It is shown experimentally that dilution of helium (used as the buffer gas) with neon in the ratio [Ne]/([Ne+[He])=0.35–0.45 makes it possible to increase the output energy by a factor of 2–3. An analysis of the experimental results is given.

Measurements were made of the delay time of plasma formation on irradiation of metal and insulator targets with microsecond CO₂ laser radiation pulses. An investigation was made of the dependences of the energy losses in forming a surface plasma on various parameters of the laser radiation. The experiments were performed in a one-dimensional arrangement with three pulse durations. The results were analyzed on the basis of an evaporation mechanism of low-threshold optical breakdown.

A pulse-periodic picosecond erbium-doped garnet (Y₃Al₅O₁₂:Er³⁺) laser with synchronous electrooptic modulation of losses in the resonator containing a partial polarizer was investigated. The spectral and time characteristics of the fourth harmonic of the laser radiation (λ = 0.73μ) and also of the up-converted signal obtained by mixing with neodymium laser radiation (λ = 0.78μ) were determined as a function of the detuning between the modulation and intermode beat frequencies. Under optimal conditions the laser operated at a repetition frequency of 1–1.5Hz and it generated a train of 25 ultrashort pulses with an energy of 0.5MJ (± 3%) per spike and of 40 ± 10 psec duration at the fourth harmonic frequency. The relationship between the duration of the pulses τ and the width of the spectrum Δν of the fourth harmonic was τΔν∼2. A parametric superradiance signal formed in a ZnGeP₂ crystal pumped with the radiation from this erbiumlaser, operated in the ultrashort pulse regime, provided a reliable indication of the precision of optimization of the optical length of the resonator.

Attention is drawn to the important role played by vibrationally excited oxygen molecules in the kinetics of an oxygen-iodine laser. It is suggested that the presence of vibrationally excited molecules in chemically produced oxygen may be largely responsible for rapid dissociation of molecular iodine added to oxygen. Possible experimental ways, based on coherent anti-Stokes Raman scattering spectroscopy, are suggested for testing this hypothesis.

An experimental investigation was made of modulation frequency shifts of radiation from stabilized He–Ne/¹²⁷I₂ lasers (λ=633 nm) which occurred when the frequency deviation changed. It was found that the lens-like properties of the absorbing medium were not the dominant cause of the observed shifts. It was concluded on the basis of a theoretical analysis that the most probable cause of an asymmetry of the nonlinear resonances and of the modulation shifts resulting from them in He–Ne/I₂ lasers was an asymmetry of the absorption coefficient caused by collisions involving a velocity change.

The results are given of theoretical and experimental investigations of frequency-modulation saturated-absorption and saturated-dispersion resonances in a linear He–Ne laser employing an intracavity methane absorption cell. The ultimate sensitivity of the method of frequency-modulation spectroscopy of an intracavity absorbing gas was shown to be 10⁻¹¹cm⁻¹·Hz^1/2. Studies were made of the stability and of the frequency shifts of a linear He–Ne/CH₄ laser stabilized by frequency-modulation resonances of the saturated dispersion and saturated absorption. A frequency stability of 8×10⁻¹⁵ for an averaging time of 1 sec and a frequency reproducibility of 10⁻¹³ were obtained.

The characteristics of the mode intensity and conditions for occurrence of optical bistability in a ring laser with a saturable absorber are considered. The rate equation approximation method is used to find analytic expressions for these characteristics. It is shown that changes in such physical parameters as the homogeneous and inhomogeneous broadening, coefficients of relaxation of amplifying and absorbing cells, etc. cause changes in the range of existence of optical bistability or a hysteresis cycle. The influence of inhomogeneous broadening and of spontaneous emission near the lasing threshold on the appearance of optical bistability is demonstrated for the single-mode regime.

The characteristics of the mode intensity and conditions for occurrence of optical bistability in a ring laser with a saturable absorber are considered. It is shown that the condition for the occurrence of optical bistability is common to single-mode and multimode lasers with a saturable absorber. The range of optical bistability is transformed as a result of changes in the homogeneous broadening, in the difference between the two relaxation coefficients of the upper levels in amplifying and absorbing parts of a laser, and in the rates of pumping of these parts.

A theoretical analysis is made of a chemical NF–IF laser emitting in the visible range. Its amplifying properties are studied and the energy characteristics are calculated as a function of laser parameters. It is shown that the specific output energy of a cw IF laser can exceed 120J/g and the overall chemical efficiency can be ≈ 5%. Specific practical recommendations are given on the achievement of high energy parameters of a cw IF laser. Estimates indicate that the laser energy density available from a pulsed form of this laser can exceed 60J/liter (for an efficiency ≈ 6.5%).

An analysis is made of the cooling of a gas containing water vapor as a result of absorption of CO₂ laser radiation of 9.4–10.4μ wavelengths. A numerical study is reported of the influence of the parameters of the gas and of the radiation intensity on the magnitude of the cooling effect and its lifetime.

The dependences of the intensities of the radiation emitted by iodine atoms in the region of 1.3μ on the concentrations of Ar (up to 70 atm), Xe (up to 45 atm), and CF₃I (up to 3 atm) made it possible to estimate the equilibrium constants of processes of the I(²P_1/2)+M = I(²P_1/2)·M type and the probabilities of radiative transitions resulting in the satellites. The data on the relative intensities of the satellites were obtained for several other gases. An analysis of the results indicated that photodissociation of molecules of the C_kF_2k=1 I type with 1 ≤ k ≤ 4 in a compressed rare gas at pressures above 16 atm(Xe) or 28 atm(Ar) resulted in a higher gain in the case of a short-wavelength satellite than in the case of the 1315 nm atomic line.

An analysis is made of the diffusion of molecules excited by a laser field through a porous plate in the approximation of a single channel represented by a capillary. It is shown that the first few collisions of a molecule with the walls of the capillary, which cause deactivation if the molecule is excited, have a considerable influence on the ratio of the permeabilities of the capillary to excited and unexcited molecules. Several conclusions are drawn on the permeability of a capillary under various conditions and these may be confirmed or contradicted by experiments.

A report is given of the discovery of new infrared fluorescence bands observed on excitation of concentrated solutions of the two polymethine dyes which were the first to be synthesized. The conditions (concentration and nature of excitation) for the appearance of these infrared fluorescence bands were determined and the possible mechanisms of their formation were considered. A discussion is given of the prospects for constructing efficient polymethine lasers emitting in the infrared.

A theoretical analysis is made of the mutual influence of a high-power CO₂ laser radiation pulse propagating in a gaseous medium and an electron density wave originating from this pulse. It is shown, in terms of a simplified model for ionization by electron collisions, that this influence can lead to a substantial shortening of nanosecond and picosecond pulses. Two mechanisms contribute to this process: an active mechanism involving absorption of the radiation energy by the electrons and a reactive mechanism for which compensation of the phase modulation caused by the electron density wave is necessary. The efficiencies of these mechanisms depend in different ways on the gas pressure. It is shown that there is an optimal pressure at which the pulse duration is the shortest.

Single-frequency and biharmonic laser pumping of a few-mode fiber waveguide was used in a study of the spectral width of parametric amplification under conditions of intermode phase matching. It was found that this width was governed by the pump power and by imperfections of the fiber waveguide. Experimental values of the spectral width of parametric amplification were used to estimate variation of the diameter of a fiber along its length. The results indicated that under intermode phase matching conditions the region of spectral tuning of a narrow-band anti-Stokes wave obtained as a result of biharmonic pumping corresponded to the spectrum of parametric amplification in the case of single-frequency pumping.

A calculation is made of the temperature field of a continuous optical discharge in air, sustained by CO₂ laser radiation. The refraction of a laser beam in the discharge plasma and the radiative heat transfer are taken into account. Good agreement with experiment is obtained. It is established that the refraction leads to a strong broadening of the caustic and to a shift of the beam constriction toward the laser so that the discharge becomes localized in the region of the highest radiation intensity. This explains why a high-power optical discharge is never localized at the geometric focus, even when there is sufficient radiation power in that region. The threshold and upper limits for a discharge to occur are derived as a function of the power and the focal length of the lens.

Single crystals of AgCl and AgBr and of their solid solution KRS-13 were grown. The lowest absorption was exhibited by the solid solution: it amounted to 6×10⁻⁵cm⁻¹ at 10.6μ and 1.4×10⁻⁴cm⁻¹ at 5–6μ. An investigation of the edges of many-phonon absorption yielded estimates of the minimum intrinsic optical losses, which were 10⁻¹–10⁻² dB/km at wavelengths in the range 5–10μ. Extrusion was used in the preparation of polycrystalline fiber waveguides from AgCl, AgBr, and KRS-13. The optical losses at 10.6μ in the waveguides made of AgCl (0.5 dB/m) and in KRS-13 (1.3 kB/m) were governed by the absorption, whereas in the AgBr waveguide (3.5 dB/m) they were governed by the scattering. A polycrystalline KRS-13 waveguide was prepared for the first time and it exhibited a high breaking strength (∼150 MPa and a low minimum radius of elastic bending (∼5 cm).

A study was made of the emission spectra of a cooled single-mode homojunction laser used in scanning interferometers under conditions when a sinusoidal component of frequency ƒ_m up to 1.2 GHz was superimposed on a constant injection current. It was found that a strong frequency modulation could be achieved for a fairly small depth of amplitude modulation amounting to M ≲ 10%. The frequency modulation coefficient K_fm was within the range 0.1–0.8 GHz/mA, depending on the degree of excess above the threshold and on the frequency ƒ_m. The physical mechanisms governing the dependence K_fm(ƒ_m) were analyzed.

An experimental study was made of the kinetics of emission of individual modes of a multimode cw dye laser. Fluctuations of the output radiation in the modes and reduction in the characteristic time of these fluctuations were observed when excess of the pumping rate above the threshold was increased. The only factor that determined the sensitivity of intracavity laser spectroscopy was the duration of emission of radiation in each mode.

Numerical modeling method and simultaneous solution of the equations describing vibrational kinetics, hydrodynamics, and laser kinetics are used to study refraction of laser radiation by boundary thermal self-interaction waves in a pulsed electron-beam-controlled CO₂ laser with a telescopic resonator. An analysis is made of the influence of this refraction on the deterioration of the divergence of laser radiation as a function of such parameters as an increase in the resonator length, pulse duration, and composition of the mixture. A calculation is made of the optimal values of the parameters for which the energy emitted in a given solid angle has its maximum value.

An experimental investigation was made of optical systems in which polarizers eliminated the influence of an opposite wave on the angular characteristics of the radiation emitted by a laser with a telescopic resonator. The feasibility and advantages of the use of an unstable resonator were analyzed for a generator of reference waves in systems for a degenerate four-wave interaction at the wavelength of 10.6μ.

An analysis is made of the transmission of a monochromatic signal with a random spatial modulation through a nonlinear medium with a local response. It is shown that in the case of a weak nonlinearity the distortions of a partly modulated field are regular and can be used to determine the nonlinear refraction constant.

The first liquid laser of a new type was constructed using ether solutions of coumarin 6 mixed with diphenylpolyenes. The laser was operated in the regime of a standing photobleaching wave with a lasing zone moving in the solution at a velocity comparable with that of sound (∼1 km/sec). The excited region of 50 cm³ volume generated radiation of 2.5J energy for 15μsec in the green part of the spectrum (517±3 nm). It was found experimentally that lasing was quasicontinuous and steady, and that it could be scaled up in time and in volume. It was established experimentally that 1,4–diphenylbutadine and stilbene can quench effectively the triplet states of the molecules of coumarin 6 in ether solutions, which makes it possible to achieve prolonged lasing as a result of wide-band optical pumping.

A new system for the excitation of a resonator-free laser is considered. In contrast to the familiar regime of a wave traveling along the optic axis, the point at which the scanning occurs is not the point of local deposition of the pump energy, but the point where the threshold conditions are satisfied. When the phase matching conditions are obeyed by the distribution of the pump power along the optic axis, the reverse laser flux may be suppressed and the radiation energy is then coupled out only in one direction. The law of longitudinal modulation of the pump power is then satisfied in the laser. A report is given of an experimental study of a resonator-free Ar–N₂ laser excited by a high-intensity proton beam. The reverse flux is reported to be reduced more than threefold. The main radiation flux is observed in the predicted direction.

A theoretical explanation is given of the polarization aberrations exhibited by geodesic waveguide lenses in LiNbO₃:Ti. The dependences of the magnitude of the aberrations on the parameters of the geodesic lenses are obtained.

It is shown that an allowance for the absorption in the reflecting layers is needed in calculations of the frequency characteristics of laser mirrors. We shall report numerical calculations for typical mirrors operating in the visible range and consisting of constant-thickness layers with the ratio of the refractive indices amounting to 1.7. This makes it possible to find the dependence of the absorption coefficient A on the frequency ω for any ratio of the bulk extinction coefficients of dielectrics. The operating energy of lasers is ω_max/ω_min≈1.3 between the values of the absorption coefficient A differing by a factor of two. Outside this range the losses rise steeply (partially when the number of layers is large) and they may exceed the nominal value by tens of times.

Problems associated with the observation of generation of electron-positron pairs in a laser plasma are considered. A simple method is suggested for detecting single positrons against the background of other particles emitted by a plasma: it involves the use of a magnetic device based on a current-carrying linear conductor (this device can be called a drift magnetic separator). The operating principle and construction of a drift magnetic separator are described and the results of its application are given.

An yttrium aluminum garnet laser emitting single pulses in the regime of mode self-locking was used to study the response time of prototype photomultipliers fitted with two microchannel plates. The best photomultipliers with a gain of at least 10⁶ generated anode pulses with a rise time not exceeding 0.4 nsec and duration not exceeding 0.6 nsec at midamplitude.

It is shown that fast thermalization, characterized by a time constant τ_T ≲ 1 psec, of the Stark components of the levels of the active transition giving rise to the 1.06μ emission from Nd³⁺ ions in glass is responsible for the high efficiency of amplification of 1–100 nsec pulses in high-power neodymium laser systems.

A study is made of the possibility of using spatial filtering of the speckle pattern of radiation from a single-fiber sensor for the determination of the pressure in an acoustic wave. It is shown that a modification of the initial speckle pattern caused by a dynamic pressure on a fiber waveguide makes the sensor signal a linear function of the change in pressure. It is shown that an increase in the light wave intensity at the entry to a sensor results in a slight reduction in the signal/noise ratio.

A description is given of a tunable dye laser with a grazing-incidence grating and a three-mirror resonator, operating near its stability limit, which made it possible to increase considerably the size of the TEM₀₀ mode. Conversion of 10% of the pump was achieved and the width of the emission line was 0.1 cm⁻¹.

Passive mode locking was achieved in a laser made of a gadolinium scandium gallium crystal doped with Cr and Nd. The duration of ultrashort pulses was 4.2 ± 0.4 psec.

A study was made of the lasing characteristics of two new dyes belonging to the naphthalimide and benzanthrone classes, when they were pumped with the second harmonic of ruby laser radiation. The emission wavelength could be tuned over a range of ∼100 nm (from 500 to 600 nm in the case of naphthalimide and from 640 to 740 nm in the case of benzanthrone). The use of these compounds in lasers increased the output energy compared with the widely used coumarin 7 and uranin dyes in the green region or cresyl violet in the red region. Moreover, the new dyes ensured a considerably wider wavelength-tuning range.

A study is made of the possibility of increasing the efficiency of a mixing gasdynamic laser by cooling hot N₂ with a cold gas and creation of a thermal nonequilibrium state in the subsonic part of the nozzle. The necessary amount of CO₂ and H₂O are added beyond the nozzle edge. Calculations are reported for a mixing gasdynamic laser and it is shown that promising results can be expected if two-stage mixing of gases is used.

Optical components were constructed for focusing monochromatic visible light into arbitrary lines with a given intensity distribution. Two-gradation focusing components fabricated by photolithographic methods from photoresist films were found to have an efficiency of about 30% when producing lines of width close to the diffraction limit.

A study was made of new classes of ultraviolet dyes for use as bleachable filters of radiation of the 308 nm wavelength. The saturation characteristics were determined for some dyes belonging to the oxazole and imidazole series. It was found that some of them could be used effectively as saturable absorbers for the radiation emitted by XeCl lasers.

A model is proposed of chemical pumping of the B³Π₀⁺ state of the IF molecules in F–H₂–NF₂– IF mixtures. This model is used in numerical calculations of the kinetics of the relevant reactions. It is shown that the B→X transition in IF can ensure a high small-signal gain which should be sufficient for lasing. The processes which limit the rate of pumping of the upper active level are identified. Methods for removing such limitations are suggested.

A study was made of the divergence of amplified spontaneous radiation from an extended source. An Ar–N₂ mixture was excited by a high-intensity proton beam and it emitted ultraviolet radiation as a result of transitions in the second positive system of the N₂ molecule. The pump power density reached 2.2×10⁷W·atm⁻¹·cm⁻³ and the length of the excited region was 30 cm. An anomalously strong directionality of amplified spontaneous radiation was observed: this directionality was 20 times higher than that expected on geometric grounds.