Showing papers in "Applied Physics B in 1983"

Laser phase and frequency stabilization using an optical resonator

Abstract: We describe a new and highly effective optical frequency discriminator and laser stabilization system based on signals reflected from a stable Fabry-Perot reference interferometer. High sensitivity for detection of resonance information is achieved by optical heterodyne detection with sidebands produced by rf phase modulation. Physical, optical, and electronic aspects of this discriminator/laser frequency stabilization system are considered in detail. We show that a high-speed domain exists in which the system responds to the phase (rather than frequency) change of the laser; thus with suitable design the servo loop bandwidth is not limited by the cavity response time. We report diagnostic experiments in which a dye laser and gas laser were independently locked to one stable cavity. Because of the precautions employed, the observed sub-100 Hz beat line width shows that the lasers were this stable. Applications of this system of laser stabilization include precision laser spectroscopy and interferometric gravity-wave detectors.

Frequency modulation (FM) spectroscopy

Abstract: Frequency modulation (FM) spectroscopy is a new method of optical heterodyne spectroscopy capable of sensitive and rapid measurement of the absorption or dispersion associated with narrow spectral features. The absorption or dispersion is measured by detecting the heterodyne beat signal that occurs when the FM optical spectrum of the probe wave is distorted by the spectral feature of interest. A short historical perspective and survey of the FM spectroscopy work performed to date is presented. Expressions describing the nature of the beat signal are derived. Theoretical lineshapes for a variety of experimental conditions are given. A signal-to-noise analysis is carried out to determine the ultimate sensitivity limits.

Tunable room-temperature cw laser action in Cr 3+ : GdScGa-Garnet

Abstract: We report for the first time on tunable room temperature cw laser action of a transition metal laser material In Cr3+:GdScGa-garnet crystals almost the total fluorescence is channelled into the broad band4 T 2→4 A 2 transition The fluorescence lifetime is τ=120 μs Laser pumping in the blue-green and yellow-red spectral range yields pump thresholds around 250 mW and slope efficiencies up to 11% The wavelength of the free running Cr3+ laser is centered at 777 nm

Repetitive passive Q-switching and bistability in lasers with saturable absorbers

Abstract: We report detailed experimental data on the passive Q-switching operation in a CO2 laser with CH3I saturable absorber, and on the transient behaviour in the near-Q-switching situation. Under suitable operating conditions, we found bistability in the output power. In some cases, we observed the simultaneous presence of bistability and passive Q-switching. The theoretical part of the paper starts from the four-level model of laser with saturable absorber, as formulated by other authors. By adiabatically eliminating the variables of the resonant levels, we reduce the problem to a set of three differential equations, from which we derive explicit analytical conditions for the rise of passive Q-switching. These conditions turn out to be in good qualitative and partially quantitative agreement with our experimental findings as well as with other experimental data previously obtained by other authors. Finally we classify the possible combinations of passive Q-switching and bistability that one can find in this type of experiments.

A stroboscopical gas-electron diffraction method for the investigation of short-lived molecular species

Abstract: A new stroboscopical electron diffraction technique is suggested for studying shortlived molecular species such as free radicals and vibrationally or/and electronically excited molecules in the gaseous phase. The preliminary results are reported for a diffraction study of the multiphoton dissociation of ir-laser irradiation by the CF3I molecule.

Nonlinear waves guided by a dielectric slab

Abstract: We consider dispersion relations and field structure of TE-polarized guided waves travelling along an asymmetric dielectric slab surrounded by two different nonlinear media. For a given configuration there are four types of guided waves. Three of this four types possess at least one field maximum outside the slab region and have no counterpart in linear waveguide optics. The solutions of the dispersion relations depend now on an additional parameter making them more flexible with respect to the linear limit.

Dye stability under excimer-laser pumping

Abstract: The stability of polymethine dyes under XeCl laser excitation has been investigated (For these dyes emitting in the ir the difference between absorbed and emitted photoenergy is 2 eV) The stability was measured in an amplifier device operating in the saturated regime While the influence on dye concentration is negligible, the fluence dependence shows the importance of two photon absorptions A meausure for the stability is the number of photons which can be emitted per dye molecule before the dye solution has degraded to 50% of the initial value This value is in the range of 150–280 The stability is clearly related to the formation of photoproducts absorbing at the pump and the laser wavelength A model starting from the change in the dye solution absorption spectra is in very good agreement with the observed energy decays

Picosecond dye laser emission in the infrared between 1.4 and 1.8 μm

Abstract: The stimulated emission of new infrared dyes was investigated. The dyes have shortS 1-lifetimes between 5 and 12 ps with corresponding low fluorescence quantum efficiencies of 10−3 to 10−4. In spite of this, 2% of the pumping energy were converted to an intense infrared pulse with frequencies between 5550 cm−1 (1.8 μm) and 7150 cm−1 (1.4 μm). The dye solutions were excited by single picosecond pulses of a Nd:glass laser in a travelling wave system consisting of a diffraction grating and a tranversely pumped dye cell. The pulse duration at 6250 cm−1 was measured to be 6 ps.

Monitoring of the stratospheric ozone layer by laser radar

Abstract: Monitoring of the height distribution of atmospheric ozone up to 50 km is being performed with a ground-based lidar system, which has been in operation since October 1982 on the summit of Zugspitze in the German Alps. Daily and monthly averages of the ozone profile are being obtained with high precision. Furthermore, stratospheric aerosol layers, originating from the eruption of El Chichon volcano in Mexico in sping 1982 are being recorded.

Experimental measurements of non-Gaussian scattering by a fractal diffuser

Abstract: We report measurements of the statistics of intensity scintillations of 10.6 μm CO2 radiation scattered by a two-dimensional fractal phase screen. The surface profile of the phase screen is measured and shown to have a Hausdorff-Besicovitch dimension of approximately 1.4. Measurements of the on-axis intensity fluctuations as a function of illuminated spot size in both the Fresnel and Fraunhofer region are presented. This fractal scatterer does not produce the high contrast values found in speckle patterns of smoothlyvarying surfaces. Both direct and heterodyne detection results are reported. Measurements of the average intensity as a function of angle are also presented, and illustrate the failure of theoretical models which assume a Gaussian phase autocorrelation function.

Picosecond pulse shortening by travelling wave amplified spontaneous emission

Abstract: A new scheme for travelling-wave excitation of amplified spontaneous emission (ASE) employing transversal pumping is presented. ASE pulses emitted in the forward direction had a duration of 6 ps, corresponding to one half of the pump pulse duration. The spectrum was strongly structured, with individual components having a width of 0.1–1.0 A. Essential characteristics of longitudinal and transversal excitation of travelling wave ASE are compared.

Complete and reversible absorption of radiation

Abstract: Kirchhoff's law has caused confusion in the discussion about the theoretical maximum work from radiation because it permits only infinitesimal or irreversible absorption. With the aid of an optical circulator, non-Kirchhoff behaviour is possible in principle which permits at the same time complete and reversible radiation absorption. The key to overcome Kirchhoff's law seems to be the magnetic field, a condition explicitly excluded by Kirchhoff. The work extracted in complete reversible conversion of radiation is calculated. The irreversibilities connected with the emission process are discussed.

Laser Atomic Photoionization Spectral Analysis of Element Traces

Abstract: A new method for determining trace amounts of elements in a substance has been developed. It is based on thermal evaporation and atomization of a substance in vacuum and detection of the released trace atoms by the laser stepwise photoionization technique. A rather effective way of detecting atoms is their multistep excitation to Rydberg states by the radiation of tunable dye lasers with their subsequent ionization by electric field pulses. The analytical laser spectrometer consisting of several tunable, pulsed dye lasers, a vacuum chamber with an atomizer and a recording system is described. The microimpurities of elements have been detected in pure materials: the contents of Na impurities in CdS crystals (2×10−6%) and Al impurities in germanium (2×10−7%). The detection limit of about 10−9% was determined. A technique for direct determination of the content of elements in natural water and human blood is described. A high sensitivity, universality and other merits of the method enable it to be used in determining traces of many important elements at a level of 10−8 to 10−11% almost in any matrix.

Amplification and measurement of single 1.6–3.5 ps Pulses generated by a distributed feedback dye laser

Abstract: We have shown that distributed feedback dye lasers are capable of generating single transform-limited pulses of 1.6–3.5 ps duration when pumped by a mode-locked Nd: YAG laser. A short-pulse amplifier has been constructed in which the level of amplified spontaneous emission could be kept below 10−3 by properly taking into account the polarization anisotropy of the gain and by precise timing of the pump pulses. A sensitive autocorrelator is described which allows to measure pulse duration in a single shot.

Preionization kinetics of an X-ray preionized XeCl gas discharge laser

Abstract: The influence of preionization conditions on the performance of a XeCl gas discharge laser preionized by a short, high intensity x-ray pulse, has been studied. The laser output energy and optical pulse temporal characteristics have been used to determine the roles of initial electron density and of electron attachment to HCl and to impurities. Although the short pulse preionization technique functions well for XeCl when the laser voltage pulse has a short risetime (≲20 ns), it is less well suited to XeCl lasers using slowly rising voltage pulses (>50 ns), or to F2 containing gas mixtures (KrF and XeF lasers).

Dynamics of the CO2 lower laser levels as measured with a tunable diode laser

Abstract: A tunable diode laser operating in the 4.3 μm region is used to probe a conventional cw CO2 laser discharge. Vibrational populations in the 1000, 0200, 0220, and 0110 levels of CO2 are measured under lasing conditions, i.e., in the presence of intense 10.4 and 9.4 μm fields. The tunable diode laser is also used to monitor the energy transfer processes between the four levels after the passage of an intense 10.4 μm pulse. The detailed information provided by the tunable probe laser enables us to determineseparately all the vibration-vibration (V-V) and vibration-translation (V-T) rate constants of importance in the relaxation of the lower laser levels in CO2. The V-V rate constants are found to vary from a low value of 4.5×104 s−1 Torr−1 for the coupling of 0110 to 1000 to a high value of 8.0×105 s−1 Torr−1 for the coupling of 0110 to 0220.

Measurement of third-order nonlinear susceptibilities by non-phase matched third-harmonic generation

Abstract: The third-order susceptibilities χ (3) (−ω3;ω1,ω1,ω1) of some liquids, solids and air are determined by third-harmonic generation. The samples are placed behind the focal region of a laser beam in an evacuated environment to avoid third-harmonic generation of the surroundings. For fluid media the sample cell is made out of two thin fused quartz plates and oriented to an angle of zero net third-harmonic production in each window (minimum Maker fringe position).

Diode-laser spectroscopy of supersonic free jets

Abstract: A supersonic-free-jet infrared spectrometer has been constructed for investigation of molecular vibrational spectra at low rotational and vibrational temperatures. The sensitivity of measurement in a pulsed jet is increased by employing a phase-sensitive detection method synchronized with the pulse frequency. The performance of the spectrometer is examined for the absorption lines of the NH3v2 band. A rotational temperature as low as 16K is attained when seeded in He. Cold-jet spectra are demonstrated for thev3 bands of PF5,34SF6, and182WF6.

Cylindrical Gaussian-Hermite modes in rectangular waveguide resonators

Abstract: Approximate solutions to the wave equation inside a rectangular metallic waveguide has been obtained in terms of cylindrical Gaussian-Hermite functions.E mn π modes propagate with negligible straight guide losses in the far-infrared region. A new type of low-loss optical resonator suitable for FEL operation at long wavelengths is presented here.

Generation of tunable narrow-bandwidth VUV radiation by anti-Stokes SRS in H2

Abstract: Thirteen tunable anti-Stokes components have been observed for the first time starting with a dye laser. Energies in the long-wavelength VUV region (around 190 nm) reach values of 50 μJ (∼10 kW) corresponding to a conversion factor η∼0.5×10−3. The shortest wavelength observed is at 138 nm where up to 40 nJ (∼8 W) are measured.

Short uv laser pulse generation by quenching of resonator transients

Abstract: Short (<0.5 ns) ultraviolet laser pulse generation by quenching of resonator transients is described. An output energy of 100–150 nJ at λ=340 nm is obtained using a pump energy of only 5–7 mJ at 308 nm. This pulse was amplified up to 100 μJ by a two stage amplifier.

Oxidation-state-specific detection of uranium in aqueous solution by photoacoustic spectroscopy

Abstract: By means of photoacoustic spectroscopy it has been possible to detect U(IV) and U(VI) in aqueous solution in the 10−6 M/L range. Piezoelectric transducer detection, pulsed laser radiation and a differential method to reduce solvent and window absorption has been used. Photoacoustic spectroscopy seems to be a suitable method for the low level detection of actinide elements.

Laser dye stability

Abstract: A figure-of-merit (FOM) consisting of the initial laser slope efficiency times the dye lifetime constant is used to compare the effect of a Pyrex light filter and the cover gas upon the output characteristics of six laser dye solutions. The FOM is found to improve using the Pyrex filter from 2-fold (coumarin 102 under air) up to 3800-fold (rhodamine 6 G under argon). In most cases, the use of argon in place of air for a coumarin dye not only improves the laser output, but also increases the lifetime constant. However, two of the coumarin dyes had an increase in lifetime under air as compared to under argon. This difference is explained in terms of the relative energy levels of the singlet and triplet states of these dyes. In order to measure long-lived dye solutions, a separate output-monitored linear flashlamp was used to degrade the solutions. The dye lifetime constants were corrected for changes in the output of this linear flashlamp. The relative output of this dye degradation flashlamp was found to depend upon the wavelength of the measurement and the position of the sensor along the length of the lamp. The variation in the output of several flashlamps is shown for lamps operating up to a total of 30 million flashes. The lifetime constants of the dye LD-490 measured by using the coaxial laser flashlamp was found to be comparable with, but lower than, the constants obtained from using the linear dye-degradation flashlamp.

Color signal correlation detection by matched spatial filtering

Abstract: A polychromatic correlation detection technique by complex spatial filtering is presented. This technique utilizes a diffraction grating and three co-lineared (red, green, and blue) coherent sources. The technique of complex spatial filter synthesis for the polychromatic corelation detection is demonstrated. This technique offers true color correlation detection which is very suitable for color signal recognition and identification. The correlation diffraction efficiency with this technique is generally higher than that of the wavelength-multiplexed technique. Several interesting experimental demonstrations of this color signal correlation detection scheme are provided. Finally we note that this color signal detection technique is a simple and versatile processing technique which has broad range of applications of complex color signal detection, recognition and identification.

On the thermodynamics of the photoacoustic effect of condensed matter in gas cells

Abstract: The photoacoustic (PA) effect of condensed matter measured in a gas-microphone cell can be interpreted by the Rosencwaig-Gersho-model. This model developed originally for thermally thick gas columns is extended to arbitrary gas lengths. The periodic variation of temperature varies the internal energy of the total volume of the gas leading to a pressure oscillation by an isochoric process. Further, taking into account a residual volume as introduced by Tam and Wong, the description leads finally to an extended Rosencwaig-Gersho model (ERG). Measurements with argon (γ=1.67) and Freon 13 (CClF3, γ=1.17) for thermally thin and thick gas colomns confirm the isochoric character of the PA effect at frequencies far below the acoustic cell resonance. Experimental results of other groups can be interpreted very well with our model. Furthermore, the extended Rosencwaig-Gershomodel leads just in the low frequency region to the same results as the model of McDonald and Wetsel.

Laser-induced nonlinear resonances in the continuum at third-harmonic generation in Na vapor

Abstract: A theoretical and experimental study of light-induced autoionizing-like resonance in the continuum at THG of λ=191 nm in sodium vapor is presented. A gain of the output THG power as much as 18 times due to autoionizing-like resonance has been obtained.

Investigations on the thermionic diode: The ionisation probability of Rbn 2 P J atoms by noble gas collisions

Abstract: By using the frequency-doubled light of a single-mode dye laser the effect of noble gas collisions (Ne and Xe) on the intensities of the Rb principal series lines 5S-nP (n≧15) has been studied in a thermionic diode. The signal intensities of the low-lying lines of the series could be increased considerably by applying higher noble-gas pressures. The intensities of the high-lying lines were found to be pressure independent indicating a constant ionisation probability of highly excited atoms by collisions. By comparing the diode signals of the discrete lines with the ionisation continuum it is shown that the ionisation probability of high-lying levels is unity within the experimental error.

New single-pulse generation technique for distributed feedback dye lasers

Abstract: The distributed feedback dye laser (DFDL) generates a train of picosecond pulses when pumped well above threshold. This DFDL emission can be quenched by injecting a laser pulse into DFDL. By proper timing of the quencher laser pulse, only the first DFDL pulse is generated while the successive pulses are suppressed. Operational characteristics and practical design considerations of such a quenched DFDL are given. With 2.5 ns long pump pulses from a N2 laser, a shortest DFDL pulse of 17 ps was obtained at 380 nm.

Picosecond light-pulses tunable from the violet to the near infrared

Abstract: System parameters of an optical parametric generator pumped by the third harmonic of a Nd:glass laser are presented. The tuning range extends from 5700 to 22,700 cm−1; i.e., it covers a frequency range of 17,000 cm−1. Experimental data on spectral bandwidths, conversion efficiencies, and pulse shape are discussed. A pump beam with a divergence of four degrees simultaneously generates the entire visible spectrum (blue to red) separated by the angle of emission.

A molecular beam laser-microwave double-resonance spectrometer for precise measurements of high temperature molecules

Abstract: A laser-microwave double-resonance experiment has been developped basing on the principle of the known laser-rf double-resonance technique. This novel beam experiment allows high-resolution measurements of rotational transitions in a molecular ground state. The method was applied to the high-temperature radical CaCl to study the hyperfine structure of transitions between low-lying rotational states. Microwave scans yielded linewidths of 15–20 kHz limited only by time-of-flight broadening and residual Zeeman splitting.