CVL-pumped dye laser for spectroscopic application

A spectroscopic dye laser system-pumped by a Copper Vapor Laser (CVL)-has been built and optimized for different dye solutions operating in the spectral range from 530 to 890 nm. Conversion efficiencies up to 40% were reached in broad band operation, 24% with a prism expander grating cavity, and 20% in grazing incidence configuration, which operated at typical band widths of 3 GHz. Second harmonic generation (SHG) of the dye laser output produced tunable uv-radiation between 260 and 408 nm at conversion efficiencies 5%. Fundamental and SHG output were used for resonant ionization experiments in molecular beams.     

Single-Longitudinal-Mode Pulsed Dye Laser with Coaxial End-Pumping and Self-Seeding

We report a single-longitudinal-mode pulsed dye laser scheme in grazing incidence geometry. The points are to incorporate coaxial end-pumping and self-seeding into the conventional grazing incidence geometry termed the Littman scheme (Appl. Opt. ²³ (1984) 4465). It is shown that the present scheme can considerably reduce the pump threshold and realize higher output power with better stability than the conventional Littman scheme.     

Single mode operation of a narrow bandwidth dye laser using a single prism, grazing incidence grating long cavity

The single mode pulsed dye laser is an attractive tool for many spectroscopic applications. Long cavity tunable dye lasers generally operate in multi-longitudinal modes within the bandwidth of gain profile. Single longitudinal mode oscillation can be obtained by either making the cavity short enough or introducing an additional loss mechanism, in which all modes but one have a gain less than their loss. A new technique to achieve single mode operation in a long cavity dye laser, based on Rhodamine 6G dye in ethanol and ethylene glycol solution, pumped by a high repetition rate copper vapor laser, is reported. This laser, which operates in three modes in grazing incidence grating configuration (cavity length of 16 cm), has been made to lase in single mode by increasing the loss in the resonator through beam walk-off.     

Efficient amplification of sub-picosecond pulses of a non-CW dye laser

Efficient amplification in a dye laser amplifier is investigated theoretically and experimentally. A five-level rate equation approach is considered including rotational relaxation of the dye molecules. The effects of the pump pulse duration and of the parameters of the input pulse are discussed. The results are compared with experimental data for 0.5 ps pulses of a pulsed dye laser. Conversion efficiencies >10% are achieved for a single pass amplifier using Nd:YAG pump pulses of 2 ns while an effective fluorescence lifetime of 1.7±0.2 ns is determined for the gain medium rhodamine 6G. The triple pass amplifier stage of the laser system achieves an energy conversion of 4% with 40 J output pulses.     

Efficient generation of tunable VUV laser radiation below 205 nm by SFM in BBO

Design and performance characteristics of a tunable dye laser system for sum frequency mixing (SFM) in a BBO crystal are presented. The system is composed of two tunable pulsed dye lasers pumped synchronously by the second harmonic of a commercial Nd:YAG laser. The radiation produced by the first dye laser is frequency doubled by second-harmonic generation (SHG) in KDP and subsequently mixed by SFM in BBO with the light of the second dye laser. The interest was focussed on generation of tunable laser radiation below 205 nm with high output power and long-time wavelength stability. High conversion efficiencies enable output energies of 100 J (20 kW) at 196 nm using only moderate Nd:YAG pump energies of 67 mJ. Altogether, a laser system with very good specifications for analytical application in the near VUV spectral region is reported.     

Demonstration of a nine lines distributed feedback dye laser using three pairs of pump beams

Controllable, maximum nine-wavelength operation of a wavefront divided, multiple beam pumped, distributed feedback dye laser (DFB), is reported for the first time. Equally spaced three to nine lines have been obtained by pumping a dye solution with three pairs of excitation beams derived from the same source. Experimental results lead to a nine line model of a distributed feedback dye laser (DFDL). The dye cell was excited by the 2nd harmonic of a laboratory built, cavity dumped, passively q switched and mode-locked Nd:YAG laser, to induce a temperature phase grating in the dye solution. Different features studied included threshold conditions, simultaneous induction of multiple gratings, impact of pump polarizations, and temporal and spectral characteristics of the emitted lines. This work on the DFDL is in agreement with most of the published results on semiconductor DFB lasers [1, 2, 3] and opens a new era of research. This multi-wavelength operation of a DFDL is based on the nonlinear effects of an overwritten multiple dynamic grating on a R6G dye solution in ethanol. PACS 42.60.By     

Application du contrôle de l'autofocalisation dans CS2 au raccourcissement d'impulsions laser picoseconde

Résumé Nous décrivons plusieurs techniques expérimentales qui permettent de maîtriser le phénomène d'autofocalisation d'un faisceau laser picoseconde dans un matériau présentant l'effet Kerr optique. Sous certaines conditions, ce phénomène est suffisamment prévisible et reproductible dans le temps et dans l'espace pour produire des impulsions picoseconde présentant certaines lois contrôlées de modulation de fréquences temporelles. Enfin, nous montrons comment des impulsions laser de durée initiale 30 ps modulées convenablement en fréquence par autofocalisation contrôlée dans CS₂ peuvent être raccourcies d'un facteur 30 environ par filtrage passif d'amplitude.

---

This paper deals with self-focusing of picosecond laser pulses due to the optical Kerr effect in CS₂. It describes experimental techniques by which the self-focusing process can be controlled with a good degree of spatial and temporal reproducibility. It includes applications to the frequency modulation of light pulses. Appropriate combination of this modulation with linear amplitude filtering results in pulse shortening of a 30 ps pulse by a factor of 30.

     

Group theory and experiments on helical and linear distributed feedback gas lasers

This study puts forward the concept of helical distributed feedback (DFB) lasers. The basic features of this new type of laser are derived by group theoretical considerations on cylindrical, circular linear periodic, and helical waveguide and laser structures. It is demonstrated that not only linear periodic structures but also helical structures show Bragg and DFB effects. Microwave and far-infrared experiments on passive helical metal waveguides reveal Bragg resonances in transmission. These results initiated the first experimental realization of a helical DFB gas laser, i.e. an optically pumped 496m CH₃F laser with a helical metal waveguide of a pitch close to 250m. This helical DFB laser shows higher-mode selectivity than the corresponding linear DFB laser. Finally, we show that the concept of helical DFB also applies to dye lasers with internal DFB incorporated by a mixture of the dye with a cholesteric liquid crystal.On leave from the Polish Academy of Sciences, Gdansk, Poland     

Development of a dual-wavelength dye-laser system for the UV and its application to simultaneous multi-element detection

In this paper, the design and performance characteristics of a pulsed tunable dye laser system for the simultaneous generation of two UV wavelengths are presented. The system is composed of an oscillator and an amplifier stage, pumped by the second harmonic of a commercial Nd:YAG laser. Dual-wavelength operation is achieved with one additional tuning mirror introduced to the prism expanded grazing incidence oscillator. The two obtained wavelengths are independently tunable, their separation is only limited by the gain profile of the dye. Both wavelengths are frequency doubled by Second-Harmonic Generation (SHG) in two KDP or BBO crystals. Performance characteristics such as bandwidth, efficiency, tuning range and wavelength separation are reported. As application two such systems are used for the simultaneous detection of the four elements cadmium, nickel, manganese and lead by Laser-Excited Atomic Fluorescence Spectrometry in a graphite furnace (ETA-LEAFS).     

Nd pentaphosphate lasers

Nd pentaphosphate is a crystalline material with very high Nd concentration (30 times higher than 1 % Nd doped YAG) and yet low concentration quenching. It is an excellent material to construct optically pumped, miniaturized, low threshold lasers, oscillating at 1.05sm. Material characteristics including spectroscopic properties relevant to laser properties are reviewed. Experiments on pulsed and CW laser action in this material including the recent achievement of a miniaturized fibre laser with only 12×12 um cross section are summarized.     

Robust estimation of Gaussian-noise variance in the presence of pulsed interference

A robust estimator of the mean power of a Gaussian noise signal in the presence of pulsed interference is proposed for remote sensing. The technique is based on the following rationale: under certain conditions, the histogram of a signal contaminated by pulsed interference is close to a normal distribution in a certain range of snapshot values, i.e., the histogram is an estimate almost insensitive to contaminating pulsed interference. Correspondingly, we propose a modified minimum ² method, which is called the method of robust approximation, for estimating the mean power of the useful Gaussian component. It is shown that the proposed estimate yields a higher accuracy compared with, e.g., median estimates and is close to the Cramér-Rao lower bound. The results of experimental testing of the technique using hydroacoustic data are presented.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 1, pp. 76–85, January 2005.     

A pulsed dye laser with grazing incidence and folded cavity

A novel design of a transversally pumped pulsed dye laser resonator using grazing incidence in a folded astigmatic cavity is described. The stability conditions, beam waist and output divergence of the oscillating mode are derived from the ray transfer matrix for this cavity. A diffraction limited gaussian beam is obtained. The linewidth can be adjusted down to 610 MHz without using any intracavity etalon.     

Nanosecond dye laser with a high-efficiency holographic grating for grazing incidence

By using a holographic grating specifically optimized for grazing incidence, it is possible to use an FP etalon in a dye laser resonator with grazing incidence. A spectral width of 4 pm with a power output of 31 kW is generated in stable operation. The influence of the mode of output coupling on the line/background ratio and the power output for grazing incidence have been investigated.     

Tunable sub-100 femtosecond dye-laser pulses generated with a nanosecond pulsed pumping

Subpicosecond pulses at a fixed wavelength produced with a low-Q cavity dye laser pumped by a single, nanosecond laser (Q-switched Nd:YAG) are converted into tunable high-power sub-100 femtosecond pulses by generation, spectral selection, amplification and compression of a supercontinuum. The tunable, chirped, high-energy pulses obtained are compressed with a prism pair. Energies up to 50 J in sub-100 fs pulses were obtained in the 540 to 650 nm range using 40 mJ of the Nd: YAG-laser pumping pulses at 532 nm. The whole sub-100 fs system including the low-Q dye laser uses only one Nd:YAG laser as a pump source.     

Rapid thermal annealing of electrically-active defects in virgin and implanted silicon

Chemical etching of single-crystalline (100)Si induced by pulsed laser irradiation at 308, 423, and 583 nm has been investigated as a function of the laser fluence and C1₂ pressure. Without laser-induced surface melting, etching requires Cl radicals which are produced only at laser wavelengths below 500 nm. With low laser fluences ((308 nm)<100 mJ/cm²) etching is non-thermal and based on direct interactions between photocarriers and Cl radicals. For fluences which induce surface melting ((308 nm)>440 mJ/cm²) etching is thermally activated. In the intermediate region both thermal and non-thermal mechanisms contribute to the etch rate.     

Ultrabroadband laser using prism-based “spatially-dispersive” resonator

An ultrabroadband (up to 35 nm FWHM) generation by a pulsed dye laser is reported. The principle of laser operation is based on a recently proposed cavity configuration, which we have termed spatially-dispersive resonator. By means of a prism pair this spreads the various spectral components into different spatial areas in the active medium and thus reduces the competition between them. A generation of a spectrum containing a number of equally spaced lines is also demonstrated.     

Attaining the wavelength range 189–197 nm by frequency mixing inβ-BaB2O4

Wavelengths in the range from 188.9 to 197 nm have been obtained by type-I sum frequency generation (SFG) in -BaB₂O₄. The fundamental beams were supplied by pulsed dye lasers one of which tuned between 780 and 950 nm and the other frequency-doubled at 497 nm. The possibility of shifting the excimer wavelength 248.5 nm to the excimer wavelength 193 nm has been demonstrated, replacing the frequency-doubled dye laser by KrF excimer lasers of different beam properties.     

Indomethacin nanoparticles directly deposited on the fluidized particulate excipient by pulsed laser deposition

Nanoparticles of indomethacin (IM), a sparingly soluble drug in water, were prepared by pulsed laser deposition with Nd: YAG laser at 1064 nm. Variation of the deposition rate (DR) with various experimental conditions, such as species and pressure of the background gas, and laser fluence, was discussed. We obtained highest DR, 2.7 g/cm²min, under He at 100 Pa with the laser fluence of 25 J/cm². In the deposited solid product, no trace of drug decomposition was observed by HPLC. Deposition of IM nanoparticles was achieved on the fluidized excipient, potato starch particles of 20 m regime. By TEM observation and zeta potential distribution measurement, we confirmed that surface of excipient particles was fully covered by nanoparticles of IM. Thus, the present method enables us a new method of one-step preparation of drug-excipient nanocomposites to eliminate tedious problems associated with nanoparticles handling.     

Effects of excited-state absorption and stimulated emission in UV dyes on the shortening of laser pulses interacting with saturable absorbers

Saturation properties of the dyes in the presence of excited-state absorption and amplified spontaneous emission in the dye are studied theoretically. In the analysis the above two effects are included separately and it is shown that they are deleterious to the pulse-shortening capabilities of the dyes. It is also shown that in the conventional single-pass shortening of laser pulses with saturable dyes large pulse shortening ratios (20) may be expected if the dye saturation is combined with a properly delayed dye generation.     

Laser ablation-induced gas and flyer ejection from composite films applied to regulation of a liquid micro-stream

We have developed laser-ignitable gas generating films to control a liquid stream for use in micro processing that can be ignited with both continous wave (CW) and pulsed lasers. Films of 100 m thick polyethylene terephthalate (PET) were coated with mixtures of nitrocellulose, trimethylolethanetrinitrate, and carbon black. A light beam from a Nd:YAG laser diode, or an Ar⁺ laser was focused onto the nitrocellulose layer through the PET substrate to ignite the layer. In the case of CW irradiation the delay time between the onset of irradiation and the ignition of the films was established by measuring the transmission of the laser light passing through the films. Gas generation dynamics were studied as a function of film thickness and irradiation conditions. In the case of pulsed laser irradiation, the dynamics were followed by imaging using a conventional double pulse pump-probe technique, with which it was possible to observe material ejection and the transient disturbance of a micro liquid stream in the path of emitted debris.PACS 79.20.Ds; 41.75.Jv; 61.80.Ba