Showing papers in "Applied Physics B in 1986"

Characterization of the noise in continuously operating mode-locked lasers

Abstract: An experimental and theoretical investigation of the fluctuations of the pulses from continuous-wave mode-locked lasers is presented. It is shown that these fluctuations can be detected and quantitatively characterized from measurements of the power spectrum of the light intensity. Such power spectra can be measured with great accuracy by shining the laser output on a suitable photodetector and by processing the detector signal with the use of an electronic spectrum analyzer. Different types of noise such as fluctuations of the pulse energy, pulse repetition time, and pulse duration, can be readily recognized from their characteristic spectral signature. Experimental results of noise measurements are presented for a synchronously mode-locked dye laser pumped by an acousto-optically mode-locked argon ion laser, and also for a colliding pulse passively mode-locked dye laser.

Coherent and Incoherent Reflection and Transmission of Multilayer Structures

Abstract: The complex-amplitude reflection and transmission coefficientsr andt of a pile of films are represented as a product of matrices. The matrices describe the transformation of two plane waves travelling in opposite directions between the films, and their development within the films.

Preionization and discharge stability study of long optical pulse duration UV-preionized XeCl lasers

Abstract: This paper presents a complete study of the temporal and spatial characteristics of the preionization of a XeCl rare-gas halide laser. The detailed study was made possible using the technique of laser-induced preionization which utilizes the uv radiation from a KrF laser to preionize a second rare-gas halide laser. In addition to the preionization study, high spatial and temporal resolution framing camera photographs have been used to investigate the growth of discharge instabilities which can lead to the premature termination of the XeCl optical pulse. The roles played by HCl, Xe, the buffer gas as well as the discharge energy loading in the development of discharge instabilities have been determined experimentally.

Optics of Floquet-Bloch waves in dielectric gratings

Abstract: The behavior of light in dielectric gratings is discussed in terms of the optical Floquet-Bloch waves (or modes). The emphasis is on the development of a good physical understanding of the nature of these waves, using the wavevector diagram to summarize their spatial dispersion and spectra. It is shown that Floquet-Bloch theory offers some advantages conceptually over the commonly used coupled-wave theory, because the rays of the Floquet-Bloch waves (given by their group velocities) play the same role in a periodic medium as do those of plane waves in isotropic or graded-index media. The effect on power conservation of truncating the Floquet expansions for the Floquet-Bloch waves is considered in detail. Using the greater intuitive power of Floquet-Bloch theory, it is shown (in contrast to recent claims to the contrary) how rigorous coupled-wave theory can be applied to symmetrical reflection gratings, and secondly how the light in these gratings can be viewed in terms of the multiple-beam interference of Floquet-Bloch waves, leading to behavior reminiscent of a low-finesse Fabry-Perot cavity.

Light-induced scattering in photorefractive crystals

Abstract: Light-induced scattering features in LiNbO3- and BaTiO3-crytals are compared with theories on holographic writing in photorefractive crystals. It is shown that they describe the experimental facts concerning the expected main scattering directions for a given incident polarization, the time development, the thickness and the wavelength dependence. Time records of the transmission offer a useful alternative for the determination of the photoconductivity. Furthermore, a new method for birefringence measurements is established. The high accuracy of this method is based on the automatic fulfillment of a phase matching condition by the anisotropically scattered radiation.

Dynamic phenomena in laser cutting and cut quality

Abstract: The quality obtained with laser cutting, one of the most important processes of laser machining, is characterized by a nearly periodic pattern of striations that cause a certain roughness of the cut surfaces Improvements of the cut quality, that would be of major technical importance, can only be obtained if the mechanism that leads to the formation of these striations is entirely understood The present author has argued already in 1984 that temporal fluctuations of laser power can induce oscillations of the liquid layer formed at the momentary end of the cut and its temperature, thus causing distortions of the cut surfaces due to the movement of that layer in cutting direction The numerical evaluation of the theoretical model yields satisfactory agreement of the “wavelength” of the striations and their dependance on the thickness of the workpiece with experimental values So far, no attempt has been made to calculate not only the wavelength but also the depth of these striations It is the purpose of this paper to determine the depth of the striations and to compare the numerical values with the experimental situation Moreover, the paper is devoted to the explanation of the fact that the laser cut surfaces are showing not only one but usually two different striation patterns, one with a finer structure adjacent to the upper surface and one with a coarser pattern adjacent to the lower surface of the workpiece

On some properties of axicons

Abstract: The radial and axial pump power distribution in the line focus of a cone (as simplest example of an axicon) is discussed. It is also shown that four conical surfaces covered with antireflection coatings can make an efficient polarizer for radial and tangential polarization of a light beam. Glass cones can be used as efficient pump optics for laser-pumped lasers. The advantages of the novel pumping scheme are discussed in detail for the cases of dye-laser and x-ray laser pumping.

Doppler effects due to thermal macromotion of ions in an rf quadrupole trap

Abstract: The second-order Doppler shift is an important source of systematic error in rf quadrupole trapped ion frequency standards. This shift can be reduced by cooling the secular motion of the ion cloud with a light background gas at low pressure. Using a thermalized ion cloud model, it is possible to relate the Doppler shift to the temperature of the ion cloud. It is shown that, in practice, the measured frequency of the first-order Doppler sidebands can be used to determine the ion cloud temperature.

Persistent spectral holes in external fields

Abstract: Experimental results and model calculations of the effects of an external electric field on persistent spectral holes are reviewed. Hole-burning spectroscopy of impurity centers in crystals and amorphous solids yields information on various properties of the centers and their interaction with the electric field. The applications of persistent spectral hole burning presented in this paper include: optical-data storage in the electric-field dimension, modulation and pulse shaping of light beams, determination of the Debye-Waller factor of host-guest systems with strongly inhomogeneously broadened transitions, detection of ultra-sound and high-frequency acoustic phonons. The effects of a magnetic field and external pressure on persistent spectral holes are discussed.

Undamped undulation superposed on the passive Q-switching pulse of a CO2 laser

Abstract: An undamped undulation superposed on the pulse tail of the passive Q-switching is observed using HCOOH gas as a saturable absorber. The pulse shapes with the undulation are nicely reproduced through the rate-equation analysis in which the laser gain medium is described as a three-level system. Good agreements between the observation and the calculation are also obtained in the dependence of the period and the width of passive Q-switching pulse on laser parameters. The mechanism of the undulation is interpreted as the relaxation oscillation attributed to the relaxation from the lower laser level. The collisional rate constant of HCOOH molecule is also obtained.

A laser infrared source of nanosecond pulses tunable from 1.4 to 22 μm

Abstract: A reliable source of coherent ns pulses of infrared radiation continuously tunable between 1.4 and 22 μm has been designed and built with the aim of developing a time-resolved infrared vibrational spectroscopy for species adsorbed on surfaces. The system is based on a Nd: YAG-laser and dye-laser combination which drive difference mixing processes in a sequence of nonlinear optical crystals (two LiNbO3, and a CdSe or AgGaS2). The system operates at MW peak power levels above 2500 cm−1, at kW power levels from 1000–2500 cm−1 and at 10–100 W levels down to 450 cm−1. These power levels are certainly sufficient for spectroscopic purposes, and at shorter wavelengths molecular pumping and applications requiring high-power should be possible. Vibrational spectra of a monolayer of CO adsorbed on Pt in an electrochemical cell have been obtained in an initial application of this source.

Dye-laser pulse shortening by transient absorption following excited-state intramolecular proton transfer

Abstract: Molecules undergoing excited-state intramolecular proton transfer often show transient absorption after pulsed optical excitation, due to the tautomeric form in the singlet ground state. This absorption may be used to suppress, after a short time, the laser action of dyes emitting in the transient absorption band. The method is designed for high photochemical stability. Lasing from PBBO, pumped 10 times above threshold by an excimer pumped PTP dye laser, was suppressed after 3.3ns by the addition of 2-(2′-hydroxyphenyl)-benzoxazole, which absorbed 30% of the pump energy. At higher concentrations, intense and stable 80ps PBBO laser pulses were obtained. The pulse evolution is simulated by model calculations. A hypothetical super-dye, consisting of chemically linked laser- and absorber-moities, is also discussed. Here Forster energy transfer should result in particularly efficient laser pulse shortening.

Heterodyne of optically pumped fir molecular lasers and direct frequency measurement of new lines

Abstract: Direct frequency measurements of optically pumped far infrared (fir) molecular lasers are reported, also for new lines. This heterodyne technique is versatile and simple. By controlling some parameters of resonator and pump source, possible causes of error are evidenced in the precise frequency determination. The implications for molecular spectroscopy are also discussed.

FM dye lasers

Abstract: We have investigated the detailed operation of a frequency modulated dye laser (FML). The FML consists of a standing wave Rh6G dye laser with an intracavity transverse ADP phase modulator which is driven at a frequency close to the cavity mode spacing. An ideal FML output consists of a laser beam which is constant in amplitude but sinusoidally varying in frequency. This provides a source of many laser modes which are equally spaced by the modulation frequency. Several dye laser configurations have been investigated. Measurements of the mode intensities, total power, amplitude modulation and rf beat amplitudes have been made as a function of the rf driving frequency of the phase modulator. The FM laser obtained has been frequency stabilised by locking it to a reference interferometer and also by frequency offset locking it to a single-frequency dye laser.

Dead time correction of photon correlation functions

Abstract: The dynamic range of single photon counting measurements in quasi elastic light scattering is restricted by detector and counter dead time effects. While distortions of single interval statistics have been treated at great length, only lowest order corrections or very special cases of dead time effects on temporal correlation functions were computed in the past. Dead times result in a direct distortion of correlograms on time scales comparable to the dead time. This effect exists even at low count-rates. It is independent of the count rate for paralyzable systems. Nonparalyzable systems show a count rate dependence with increasing correlation times at high count rates. Furthermore, counting saturation produces additional distortions extending to all lag times. These distortions are computed for the rather general case of Γ-distributed intensities with arbitrary shape of the photon correlation function. Such signals are commonly found in multiparticle homodyne experiments with a finite size detector, i.e. arbitrary value of the intercept or contrast of the correlogram. Exact results are provided for the paralyzable system including the effect of fluctuating dead times. The latter case is then used to compute a useful approximation for nonparalyzable systems as well.

Spectral and temporal study of picosecond-pulse propagation in a single-mode optical fibre

Abstract: The power-dependent pulsewidth variations for input 85 ps pulses from a cw mode-locked Nd: YAG laser propagating through 125 m of single-mode optical fibre have been examined using a Synchroscan streak camera. Simultaneous spectral and temporal measurements provide information as to the optimum parameters for pulse compression in fibres.

Optical harmonic generation in media with positive dispersion

Abstract: Third-harmonic generation in gaseous media with positive dispersion is investigated. Using the example of the 351.7–117.2 nm conversion process in rare gases the possibility of optical harmonic generation in media with positive dispersion was first experimentally obtained and theoretically substantiated. Unusual dependence of the thirdharmonic generation efficiency on the intensity of incident beam (η∼W4) is explained due to the second-order Kerr effect.

Microphotometric investigations of thick refractive index gratings in photorefractive crystals

Abstract: Transmission microphotometry is used as a new tool for the investigation of the holographic writing process of thick refractive index gratings. Theoretical results on the origin of the contrast formation are discussed. They are experimentally confirmed by rocking curves of thick refractive index gratings in doped LiNbO3 crystals.

Frequency doubling of cw ring-dye-laser radiation in lithium iodate crystals

Abstract: We report on frequency doubling of cw ring-dye-laser radiation in lithium iodate crystals (LiIO3). The conversion coefficient and the matching angle were measured as a function of the fundamental wavelength in the range 586.3–624 nm. For Type I phasematching the matching angle θm=90° for noncritical matching was found at 586.3 nm. At this wavelength the conversion coefficient reaches the maximum value which was observed to be as high as 1.6×10−2 W−1 for a LiIO3 crystal of 1 cm length. From the dependence of the matching angle on the wavelength the refractive induces were determined for the ordinary as well as for the extraordinary ray in the wavelength range from 293 nm up to 355 nm. The influence of focussing on the conversion coefficient is discussed.

Detection of atomic oxygen by laser-induced fluorescence spectroscopy at 130 nm

Abstract: VUV radiation generated by stimulated Raman scattering from H2 and tunable around 130 nm is applied to the detection of atomic oxygen produced in a flow-tube. Concentrations in the range to the detection of atomic oxygen produced in a flow-tube. Concentrations in the range of 1010 to 1011 O-atoms per cm3 lead to fluorescence signals that can easily be detected on a nanosecond timescale. We deduce that oxygen impurities generated by plasma-wall interaction in present-day tokamak experiments should be measurable with spatial and temporal resolution applying this vuv source.

Intensity stabilisation of an argon laser using an electro-optic modulator —Performance and limitations

Abstract: An intensity-stabilisation system for an argon laser using an electro-optic modulator is described. We achieved a degree of stabilisation of up to 40 dB (factor of 100) for intensity fluctuations below a few kHz. The performance in this frequency range did not reach that expected from the available loop gain. We have identified two serious limiting factors to the performance of our system at low frequency: geometrical fluctuations and frequency fluctuations of the laser light.

Theoretical investigation of Kleinman symmetry in molecules

Abstract: From measurements of the electric-field-induced second-harmonic generation with different polarizations of incident light, direct information should be obtainable on the extent to which Kleinman symmetry is obeyed in single molecules.

Resonant light absorption by the ordered structures of ions stored in a trap

Abstract: The ordered structures of ions stored in a trap are considered It is shown that light absorption of ions has a resonance with a homogeneous width

Calculations of TM-polarized nonlinear waves guided by thin dielectric films

Abstract: We found the dispersion relations for TM-polarized nonlinear waves guided by a dielectric film of thicknessd bounded on one side by a nonlinear uniaxial medium characterized by a dielectric tensorɛ xx =ɛ y =ɛ c ,ɛ zz =ɛ c +α c |E z |2,α c >0 (self-focusing medium),E z being the electric-field component perpendicular to the surfaces. Numerical calculations are given for the power dependence of the propagation wave vector. For sufficiently larged/λ (λ: wavelength) we have found regions with multiple solutions corresponding to the same power flow. This suggests possible applications to optical devices.

Polarization Effects of Picosecond CARS in Liquids

Abstract: Coherent Raman scattering of delayed probing pulses after ultrafast excitation is investigated under generalized polarization conditions. Three factors are shown to contribute to the scattering signal: (i) scattering off the isotropic part of the resonant material excitation via the isotropic component of the Raman polarizability (ii) scattering via the anisotropic part of the Raman polarizability from a second, oriented component of the resonant material excitation (iii) four-wave mixing via the non-resonant part χNR of the third-order susceptibility. We demonstrate theoretically and experimentally that different polarization conditions lead to drastic changes of the signal transients in liquids. For the ring breathing mode of C6H5Br the ratio of non-resonant to resonant contributions is measured to be χNR/χres=0.037±0.015.

Multiplex rotational CARS of N2, O2, and CO with excimer pumped dye lasers: Species identification and thermometry in the intermediate temperature range with high temporal and spatial resolution.

Abstract: Pure rotational CARS spectra of N2, O2, air, and CO have been obtained using excimer laser pumped dye-lasers. The combination of the folded BOXCARS phase matching geometry with the broad-band laser multiplex method allowed high spatial and temporal resolution. Species and concentration analysis as well as thermometry up to 700 K is demonstrated, and possible applications are discussed.

Spectral and temporal investigations of nonlinearities in a non-polarization preserving single-mode optical fibre

Abstract: Intense green and red components generated in a non-polarization preserving optical fibre using a Q-switched and mode-locked cw Nd: YAG laser have been examined spectrally and temporally with picosecond resolution. In addition to the red (four-photon interaction) and green (second-harmonic) components, many other frequency mixing processes were detected. From theoretical considerations, the red component (anti-Stokes) which is generated through parametric four photon mixing should be in the same propagation mode as the Stokes component (ir) and this was confirmed experimentally.

Phase conjugation by reflection gratings in electrooptic crystals

Abstract: Phase conjugation by reflection gratings in electrooptic crystals in investigated in a nonlinear model proposed by Kukhtarev and Semenets [1] which takes into account not only photoconductivity but also dark conductivity. The coupled-wave equations of the model can be solved analytically. To take the boundary conditions into account, numerical calculations are unavoidable. These can be performed, however, without any approximation. Phase mismatch and self-oscillation effects are dealt with in the undepleted pump approximation.

Coherent anti-Stokes Raman spectroscopy (CARS) of thev 3 band of methane in supersonic molecular beams

Abstract: Supersonic molecular beams of methane are investigated in the expansion region using coherent anti-Stokes Raman scattering (CARS). Raman spectra of thev3 vibration with resolved rotational structure at low temperatures are reported. Comparison with calculated CARS spectra shows that the rotational distribution in the beam may be well described by a Boltzmann distribution. Temperatures are the same for all three nuclear spin modifications within the experimental error.

Attachment kinetics and life time of perionization electrons in rate-gas halogen laser mixtures

Abstract: Experiments on homogeneous initiation of pulsed avalanche discharges in typical XeCl laser mixtures show that the x-ray preionization pulse remains effective after a time delay of the order of 20 μs even at very high gas pressures (∼15 atm). Detailed kinetics analyses indicate that dissociative attachment of HCl is relatively slow during and after the preionization period so that the long effective delay time cannot be attributed to the storage effect of Cl− ions. Instead, the observed phenomenon can be attributed to the long survival time of the free electrons due to relatively slow diffusion and electron-ion recombination. Some problems arising from quantitative interpretation of the experimental results are also noted and discussed.