Spectral Properties of the Radiation of Injection Lasers at Prethreshold Excitation Levels

The spectral properties of the radiation of injection lasers at low (prethreshold) levels of excitation were investigated experimentally by the method of optical heterodyning of the Doppler signal and also with the use of a KSVU23 spectral setup. It is established that these lasers may serve as sources of the optical radiation of TE and TMpolarizations with a retuned length of coherence within the range 1–10 mm in spectrometric facilities of photoheterodyne type. A procedure for determining the inversion current in an injection laser is suggested.     

Increase in the Gain of a Laser with Autoinjection of Priming Radiation in the Formation of Giant Pulses

Optical schemes of lasers that ensure the formation of giant pulses from narrowband, freeoscillation priming radiation with a change in the Qfactor and simultaneous increase in the gain coefficient and in the coefficient of filling a resonator with an active substance are developed. The results of experimental investigations of the parameters of singlepulse radiation of ruby lasers with electrooptical switching of a linear composite resonator are given.     

Radiation Source in the Ranges 2.1–2.3 and 3.7–4.3 μm for the Spectroscopy of the Atmosphere

To expand the possibilities of spectroscopic systems based on a CO₂ laser, we have mixed the frequencies of its radiation with the radiation of erbium lasers. Simultaneous use of Er³⁺:CaF₂ ( = 2.76 m) and Er³⁺:YAG lasers ( = 2.96 m) makes it possible for the spectrum of generated sum and difference frequencies to overlap the ranges 2.2–2.3 and 3.6–4.3 m that coincide with the atmospheric transparency windows. Generation of the difference frequencies of radiation of erbium lasers and of the 10.6-m radiation of a CO₂ laser were observed in CdSe at a phase-matching angle of 72°40. In a bleached CdSe crystal of length 40 mm the efficiency of transformation has attained 12%. To sum up the frequencies, 4.5-mm ZnGeP₂ crystals were used, with the efficiency of transformation being 3% and the phase-matching angle being equal to 52°.     

Continuously tunable VUV radiation (129–210 nm) by anti-Stokes Raman scattering in cooled H2

The generation of tunable vacuum ultraviolet radiation by anti-Stokes Raman scattering of tunable ultraviolet dye-laser radiation in cold hydrogen has been investigated. The scattering efficiency of XeCl laser and Nd:YAG laser pumped commercial dye lasers and the influence of different beam profiles has been studied. Up to 12 anti-Stokes orders down to 129 nm were observed with output powers between about 20 kW at =191 nm and somewhat less than 100 W at =129 nm. The efficiency of transversely pumped lasers with an intensity peaked in the center of the beam profile was found to be higher than doughnut shaped intensity distributions. The cooling of the active gas to liquid nitrogen temperatures improved the output pulse energies 3 to 5 times on average. It was found that this intensity increase was caused mainly by the narrowing of the Raman linewidth upon cooling.     

Method of Measuring Electron Density in Pulse Metal Vapor Lasers Using Microwave Technique

A method of measuring the electron density kinetics in the plasma of pulse-periodic metal vapor and metal compound vapor lasers using microwave technique within the wavelength region of 78–142 GHz is realized. The method is based on the dependence of the absorption efficiency of a probe microwave radiation on the frequency of electron collisions and also on the dependence of the cut-off or the critical density on the frequency of the probe radiation. In a quasioptical translucence scheme the plasma of a copper bromide vapor laser is studied within an interpulse time interval in a gas-discharge tube of 20 mm in inner diameter and with a 300 mm long active heated region excited by a capacitor discharge with a pulse repetition rate of 10 kHz. It is shown that under operational conditions typical for metal vapor lasers the absorption of the probe radiation within the interpulse intervals is totally determined by Coulomb electron–ion collisions. This allows one to measure both the density kinetics and the electron temperature. Probable measurement errors are analyzed and it is shown that they result in a systematic error for the absolute value of the electron density but have a slight influence on the density kinetics.     

Two‐Color Laser Photodynamic Therapy of Tumors

A method of photodynamic therapy of tumors (neoplasms) based on simultaneous action of radiation from two lasers with different generation wavelengths is proposed and implemented in an experiment on animals. The antineoplastic effect was evaluated by the dimensions of the necrosis region determined using the method of vital staining of tissues. The effect of photodynamic therapy in the case of simultaneous use of the two lasers (₁ = 627.8 nm and ₂ = 671 nm) turned out to be higher than in the case of their separate application. The hypothetical reason for the phenomenon is oxyhemoglobin photodissociation that leads to the enrichment of blood with free oxygen.     

Stimulated electronic Raman scattering in Cs vapour: a simple tunable laser system for the 2.7 to 3.5 μm region

Stimulated electronic Raman scattering (SERS) in atomic vapours provides a simple method of extending the tuning ranges of pulsed dye lasers well into the infrared region. The special advantages of this technique in comparison with other types of tunable infrared lasers are discussed, and are illustrated by describing a SERS system which uses a modest nitrogen laser-pumped dye laser ( 20 kW). This produces infrared radiation tunable from 2.67 to 3.47m by SERS in caesium vapour, which is contained in a heat pipe oven. Photon conversion efficiencies of up to 50% are obtained. The design of the heat pipe oven, operation of the system and optimization of experimental parameters are described in detail.     

Optical systems for high-power laser applications: principles and design aspects

Starting from the optical properties of laser beams, the requirements of optical systems for manipulating laser radiation in industrial applications are derived. The relevant parameters, relations to the diffraction limit and the state-of-the-art design techniques are discussed. The three important types of lasers for use in industrial materials processing operate at wavelengths ranging from the infrared (10.6 m, CO₂ laser; 1.06 m, Nd:YAG) to the ultraviolet region (excimer lasers). Each wavelength range is associated with specific design challenges. The scarcity of suitable refractive materials for the 10 m wavelength range and the ultraviolet below 300 nm is a major constraint. Reflective systems are used widely at the longer wavelength, but some designs suffer from coma. The 1.06 m radiation from the Nd:YAG laser can make use of many well-developed optical means for handling visible light. Energy transport by optical fibres is commonly used. Optical systems for excimer laser applications are specific in that they image a mask onto a workpiece, and use the high photon energy and the high definition possible with the short wavelength for precision micro-machining.     

Short-pulse high-intensity excimer lasers — A powerful tool for the generation of coherent VUV and XUV radiation

Different approaches for the generation of coherent VUV and XUV radiation with a 400 fs KrF excimer-laser system are studied. In nonlinear optical experiments it is shown that four-wave difference-frequency mixing in Xe, using a near two-photon resonance with the KrF laser radiation, is well suited for the generation of tunable VUV radiation in the range 130–200 nm. Conversion efficiencies of 2% and output energies up to 260 J have been demonstrated. Further prospects to achieve J energies are discussed. Using this VUV source and the KrF laser, powerful XUV radiation can be generated by different low-order frequency mixing processes. In first experiments on this subject, direct frequency tripling of the KrF laser pulse has resulted in 14 J XUV radiation at 83 nm.For the realization of soft-X-ray lasers, specific advantages of short-pulse KrF drivers are discussed. Novel scenarios based on a hybrid KrF/Ti: sapphire laser system and multiphoton resonant excitation are considered.Prof. F. P. Schäfer on the occasion of his 65th birthday.     

Optically-Pumped Far-Infrared Laser Lines in Hydrazine, Methanol, Heavy Water, and Ammonia: New Laser Lines and Frequency Measurements

A far-infrared laser cavity designed to favor short-wavelength laser lines was used to generate optically-pumped far-infrared laser radiation. New far-infrared laser lines were discovered in hydrazine, heavy water, ammonia, and several short-wavelength lines previously discovered in methanol were observed. Wavelength, frequency, and relative intensity measurements were performed on laser lines in the wavelength range 42.4 to 253.7 m. Each far-infrared frequency measurement was obtained by mixing the far-infrared radiation with radiation from two reference CO₂ lasers and from a microwave synthesizer in a metal-insulator-metal diode. The pump laser was a high-Q Fabry Perot resonator oscillating on 275 grating-selected laser lines including regular, sequence, and hot band lines.     

Generation of short-pulse VUV and XUV radiation

Starting from intense short-pulse KrF (248 nm, 25 mJ, 400 fs), ArF (193 nm, 10 mJ, 1 ps), and Ti:sapphire (810 nm, 100 mJ, 150 fs) laser systems, schemes for the generation of fixed-frequency and tunable VUV and XUV radiation by nonlinear optical techniques are investigated. With the KrF system, a four-wave mixing process in xenon yields tunable radiation in the range of 130–200 nm with output energies of, so far, 100 J in less than 1 ps. For the XUV spectral range below 100 nm, nonperturbative high-order harmonic generation and frequency mixing processes in noble gas jets are considered. To achieve tunability, the intense fixed-frequency pump laser radiation is mixed with less intense but broadly tunable radiation from short-pulse dye lasers or optical parametric generator-amplifier systems. In this way, tunability down to wavelengths of less than 40 nm has been demonstrated.     

Active media for tunable blue–green lasers based on aminocoumarins in polymethylmethacrylate

The lasing characteristics of 17 aminocoumarins in solutions and polymer polymethylmethacrylate (PMMA) matrix pumped by exciplex XeCl-laser radiation (_pump=308 nm) are investigated. It is demonstrated that 4-CF₃ and 3-substituted coumarins are promising for the preparation of solid-state active media for tunable blue–green lasers. PACS 42.55.Mv; 42.55.Px     

The effect of selective chemical etching on the directivity pattern for radiation of a semiconductor laser

The effect of the selective wet chemical etching of the emitting surface on the directivity pattern of radiation in the plane normal to the p-n junction is studied for InGaP/GaAs lasers with InGaAs quantum wells. It is found by atomic-force microscopy that the cylindrical lens (converging or diverging, depending on the type of etchant) is self-formed on the emitting surface due to the different etching rate of wide-gap layers (InGaP) and active layers (GaAs, InGaAs) of the lasers. By adjusting the corresponding etching time, the aperture angle of the laser radiation pattern in the plane normal to the p-n junction can be changed in the range of 57°–82° at the initial aperture angle at the half maximum level of 6°.     

A comparative study of laser tissue interaction at 2.94 μm and 10.6 μm

Laser cutting of gelatin and tissue with Er and CO₂ lasers is explained by combined action of evaporation, ejection of liquid and elastic deformation of the region of radiation impact. It is shown that the ejection mechanism is more pronounced at 2.94 m than at 10.6 m. The use of high speed photography has revealed the influence of the temporal pulse shape. The experimental results are explained by a thermo-mechanical model.     

Far-infrared detection by GaAs-IREDs and GaAs0.6P0.4-LEDs

In GaAs-IREDs and GaAs_0.6P_0.4-LEDs a photoconductive and a bolometric FIR response has been observed at low temperatures by applying different pulsed optically pumped FIR lasers. The short circuit current responsivity peaks at =400 m being about 1.5 mA/W. By monitoring the rapid modulation of the FIR radiation due to mode beating of the pump laser, an upper limit of 1 ns was established to the time constant of the photoconductive signal.     

Dynamics of the Emission of Light by C2 and C3 Molecules in a Laser Plasma Produced by Two-Pulse Irradiation of the Target

The laser plasma produced by irradiation of a graphite target simultaneously by the first and second harmonics of two Nd³⁺:YAG lasers has been studied by emission spectroscopy methods. The delay between radiation pulses () varied from 0 to 700 sec. It is established that in the absence of a delay between pulses ( = 0) the increase in the intensity of plasma emission at the wavelengths corresponding to the radiative transitions of the C₂ and C₃ molecules is of nonradiative character. The plasma produced by laser radiation at the wavelength = 1064 nm exerts its influence on the radiation spectrum of the plasma produced by laser radiation at the wavelength = 532 nm, if the magnitude of the delay between laser pulses does not exceed 30 sec. The most probable reason for this character of influence of the prepulse on the laser plasma radiation spectrum is sublimation of soot particles caused by laser radiation at = 532 nm.     

Estimates for the efficient production of antihydrogen by lasers of very high intensities

Starting from the non-linear relativistic equations of motion for charged particles in the very high intensity fields of laser radiation, the maximum kinetic energy _kin of the resulting oscillation is derived exactly. In non-relativistic conditions _kin agrees with the well-known valuee ² E _v ²/(2m₀²|n|), showing a dependence on the rest massm ₀ of the particle. In the relativistic case, the mass dependence vanishes. The multipole radiation is calculated on the basis of Sommerfeld's formula for relativistic conditions. It is shown that this radiation is not important for oscillation energies up to =70m ₀ c ² for electrons in neodymium glass laser radiation and up to higher values for CO₂ lasers and for protons. With the limitationm ₀ c ² < _kin < , the formula for _kin is used to calculate the pair production (a) for singly oscillating particles in vacuum without collisions and (b) for plasmas with collisions. Taking into account the local increase of the effective electric laser field near the cut-off density due to the decrease of ¦n¦ (n is the complex refractive index), there is the possibility of efficient proton pair production at intensities of 10¹⁹ W cm^–2 for neodymium glass lasers and of 10¹⁷ W cm^–2 for CO₂ lasers, besides electron pair production.     

Calculation of beam divergence of pulsed electron-beam-discharge lasers (EBL)

The beam divergence in electron-beam-discharge lasers is determined by the refraction of the radiation by the gradients of the electron and gas densities. At short pump pulse durations ( 1 sec) and, accordingly, at high charged-particle densities (n_e 10¹⁴ cm^-3) the divergence is due to the inhomogeneity of the ionization of the active medium. At long durations ( > 10 sec) the divergence is determined by the refraction of the radiation by the gas-density gradients produced by the hydrodynamic motion of the active medium during the course of the pump pulse. By using unstable telescopic resonators with dimensions smaller than the energy-input region, it is possible to considerably increase the energy of the radiation whose divergence is due to refraction.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 116, pp. 181–187, 1980.     

Analysis of sensitivity and low-frequency intensity noise characteristics of Bragg reflector lasers induced by reflected light

An analysis of the sensitivity and low-frequency intensity noise characteristics of Bragg reflector lasers is given. It is found that Bragg reflector lasers have increased sensitivity to reflected light for low grating coupling strength (l) and is similar to that of Fabry-Perot lasers except for highl values. Also, the intensity noise can be reduced by operating the device at high injection level.