Method of Sellctive Intracavity Absorption Using Dye Lasers

The method of the selective intracavity absorption is based on the high sensitivity of the laser intensity on the absorption of the sample placed into the resonator. The method for Nd-glass laser with inhomogeneous broadening of the gain spectrum was grounded and experimentally checued by the authors of the papers^1,2. Later demands for the spectral region expanding of the method have led to application of dye lnsers for the intracavity measurements. The capability to broadrband emission in the different spectral regions makes of dye lasers the unique instruments for intracavity spectroscopy. A number of the successful experiments on the dye lasers applications to the sensitive detection of Na, da, Ho, Pr, I₂, Br₂ small concentrations was published until now^3–6 At the same time one cannot consider the theoretical analysis of the intracavity absorption characteristics of the dye lasers, i.e, the lasers with homogeneous spectral broadeninti of active media as sufficiently complete and reliable. The theoretical estimation of the sensitivity of the method for CW laser operation was given in papers^7,8. But analytical formulae for sensitivity in reference⁷ are valid only for near-threshold levels of pumping power which does not often correspond to experimental conditions and leaves open the question about potentialities of the method at the higher pumping power (two or more times exceeding the threshold). The authors of the reference⁸ consider the problem at any pumping levels, but their approach is based on the simplified model which seem to be doubtful and requires the special foundation. In particular they suppose the existence in the active media of the number of N_j particles, which interact only with one mode j and do not interact with other laser modes, and also of the number of N_o molecules (so called reservoir) equally interacting with every mode. In the case of the homogeneous spectral broadening such dividing the molecules into types does not seem to be rightful. In addition in the both the gain and loss of all modes (except one) are assumed to be of the same values so the total number of modes is given arbitrary and not connected with resonator, active media and pumping parameters. Because of this reason one cannot calculate correctly the value of the method sensitivity or analyze it's dependence on the parameters.     

Turnable Second Harmonic Generation from an Organic Dye Laser

Abstract

Ultraviolet light has been generated from a Rhodamine 60 organic dye laser by frequency doubling with a KDP crystal. The second harmonic was tuned from 2900Å to 3000Å with peak powers of 40 watts.

Organic dye lasers have now been reported with output wave-lengths extending throughout the visible and near infrared^1-12. The tunability^4,6,13 and output energies now available in the visible protion of the spectrum suggest frequency doubling as a means of obtaining a tunable source of ultraviolet light. We wish to report the observation of tunable second harmonic radiation centered at 2950Å generated from the laser output of a 10^?4 molar solution of Rhodamine 6G dissolved in ethyl alcohol.     

Direct observation of the two-photon generation of free excitonic molecules in CuCl

Two-photon absorption spectra due to excitonic molecule generation in thin CuCl samples at 4.2°K are directly measured using the intense light from a pulsed, frequency tunable dye laser. One narrow induced absorption peak is observed at 3893 Å. This peak exhibits a Fano interference line profile at low laser light intensities. High laser light flux densities result in a saturation, broadening, and blue shift of the induced absorption peak. The maximum value of the two-photon absorption cross section is directly measured to be 2 x 10^-43 cm⁴ sec, and the binding energy of the molecule is found to be 39 meV.     

Laser atomic photoionization spectral analysis of element traces

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.     

High-resolution spectroscopy of SO2 using a frequency-doubled,continuous-wave dye laser

The SO₂ molecule is of considerable interest in the context of atmospheric pollution, and in many laser monitoring techniques the ultraviolet absorption band at 300 nm is used to determine SO₂ concentrations in the atmosphere. Recent laboratory experiments with a resolution of 2 × 10^-3 nm showed that variations could occur in absorption cross-section measurements made with different laser bandwidths due to unresolved fine structure. We have investigated absorption spectra with a line width of 3 × 10^-6 nm, using a frequency-doubled continuous-wave dye laser, and have confirmed the existence of fine structure in the absorption even when collisionally broadened with an atmosphere of nitrogen. These measurements provide a data base from which valid absorption cross sections may be calculated for all monitoring laser bandwidths. We estimate the pressure broadening coefficient for nitrogen in this wavelength region as 83 ± 38 kHz Pa^-1 (11 ± 5 MHz torr^-1). The temperature dependence of the absorption cross-section was also investigated.     

Dye-laser amplified,atomic absorption flame spectroscopy

In this letter we report the detection of atomic absorption lines of Na, Li, Sr, Ba and Cs, by using a flashlamp-pumped dye laser with an acetylene-air flame inside the cavity. A new technique for a quantitative photo-electrical measurement is proposed. In case of Na (5890 Å), a sensitivity of 0.016 ppb/1% extinction and a minimum detectable concentration of 0.2 ppb are obtained. The sensitivity and the detectable limit are improved by a factor of 10³ and 10, respectively, compared with a conventional analysis by an atomic absorption flame spectrophotometer.     

Laser intracavity absorption spectroscopy

Emission spectra of multimode lasers are very sensitive to spectrally selective extinction in their cavity. This phenomenon allows the quantitative measurement of absorption. The sensitivity of measurements of intracavity absorption grows with the laser pulse duration. The ultimate sensitivity obtained with a cw laser is set by various perturbations of the light coherence, such as quantum noise, Rayleigh scattering, four-wave mixing by population pulsations, and stimulated Brillouin scattering. It depends on the particular laser type used, and on its operative parameters, for example pump power, cavity loss, cavity length, and length of the gain medium. Nonlinear mode-coupling dominates the dynamics of lasers that feature a thin gain medium, such as dye lasers, whereas Rayleigh scattering is more important in lasers with a long gain medium, such as doped fibre lasers, or the Ti:sapphire laser. The highest sensitivity so far has been obtained with a cw dye laser. It corresponds to 70000 km effective length of the absorption path. The ultimate spectral resolution is determined by the spectral width of mode emission, which is 0.7 Hz in this dye laser. High sensitivity and high temporal and spectral resolution allow various practical applications of laser intracavity spectroscopy, such as measurements and simulations of atmospheric absorption, molecular and atomic spectroscopy, process control, isotope separation, study of free radicals and chemical reactions, combustion diagnostics, spectroscopy of excited states and nonlinear processes, measurements of gain and of spectrally narrow light emission. Intracavity absorption in single-mode lasers shows enhanced sensitivity as well, although not as high as in multimode lasers. Received: 10 May 1999 / Published online: 29 July 1999     

宽带输出PM580掺杂聚甲基丙烯酸甲酯固体染料激光器研究

采用高效激光染料PM580作为掺杂物质,聚甲基丙烯酸甲酯(PMMA)作为基质,在对固体染料光谱特性研究的基础上,重点研究了调Q倍频Nd:YAG抽运下不同染料掺杂浓度的固体染料激光输出特性.研究结果表明掺杂浓度对输出激光波长影响明显,随着掺杂浓度的增加,激光输出波长红移,从激光增益出发,对该现象给出了理论解释.掺杂浓度对激光转化效率也有影响,当掺杂浓度为2×10^－4mol/L时,获得染料激光输出斜率效率最高达53.8%,抽运能量410mJ时,获得染料激光输出220mJ,激光带宽～8nm 关键词： 固体染料 宽带染料激光 PM580 聚甲基丙烯酸甲酯     

Nanoimprinted distributed feedback lasers comprising TiO2 thin films: Design guidelines for high performance sensing

Design guidelines for optimizing the sensing performance of nanoimprinted second order distributed feedback dye lasers are presented. The guidelines are verified by experiments and simulations. The lasers, fabricated by UV‐nanoimprint lithography into Pyrromethene doped Ormocomp thin films on glass, have their sensor sensitivity enhanced by a factor of up to five via the evaporation of a titanium dioxide (TiO₂) waveguiding layer. The influence of the TiO₂ layer thickness on the device sensitivity is analyzed with a simple model that accurately predicts experimentally measured wavelength shifts induced by varied superstrate refractive indices. The superstrate refractive index is additionally shown to determine which of the possible waveguiding modes dominates for lasing, indicating a method to flexibly select the polarization of the laser. The detection limit of the sensor system is further discussed, finding an optimum at 7.5· 10⁻⁶ RIU. Wavelength changes caused by dye bleaching must be taken into account for long‐term measurements.     

A cw AgGaS2 difference frequency spectrometer with diode lasers as pump sources

2 and two diode lasers as pump sources and experiments with this setup are presented. In contrast to the majority of known applications of the difference frequency mixing with solid state or gas lasers, we apply two single mode diode lasers with emission wavelengths of 690 nm and 805 nm, respectively. By fixing the emission wavelength of one diode laser and tuning the wavelength of the second laser, by changing temperature or excitation current, we can cover typically 0.87 cm^-1 (FWHM) in good agreement with recently published data and theory. With an input power of 20 mW and 6 mW we achieved an output power in the nW-range. To demonstrate the capabilities of the spectrometer we scanned CO and OCS absorption lines near 2107 cm^-1. From these experiments we deduce an overall signal-to-noise ratio of 1000:1 and a spectral resolution better than 30 MHz. With such parameters a trace gas detection of CO at sub-ppm level will be possible. Received: 19 August 1996/Revised version: 5 November 1996     

Picosecond duration laser pulse generation by simultaneous active-passive mode-locking in Ar+ ion and dye lasers

We use a rate equation model to analyse a technique for the generation of picosecond duration laser pulses in argon pumped dye lasers. The system is based on passive mode-locking of the Ar⁺ laser by saturable absorption of the dye which is inserted in the Ar⁺ laser cavity. Simultaneously, the dye is forced to oscillate in a regime of synchronously pumped mode-locking. The system is relatively simple, does not require the use of an acousto-optical light modulator and can be easily implemented in commercially available lasers. Pulses shorter than 100 ps and having an average power of 20 mW were obtained at a repetition rate of 110 MHz. Good agreement between the model and experimental results was also obtained.     

Detection and Quantitation Of Several Atomic Species By Intra-cavity Quenching Of Laser Emission

Until recently laser research has been the province of the physicist wherein lasers have been utilized extensively as sources of coherent, highly monochromatic energy. The thrust of this research has been to employ the organic solution laser output as an analytical signal from which information about a particular system may be extracted. Preliminary investigations in this laboratory showed that a great number of variables are active in the achievement of lasing from an organic solution. More significantly, concurrent work in this laboratory produced some anomalous results which were subsequently attributed to a cavity defect. This suggested that small energy losses at discrete wavelengths within the resonant cavity of an organic solution laser could result in quenching of broad band laser emission at those specific wavelengths. These considerations led to investigations in which atoms and mlecules were purposefully introduced into the resonant cavity of an organic solution laser.¹ A search of the literature revealed that investigators at The National Bureau of Standards had previously observed this phenomenon and had reported on the intra-cavity absorption of a pulsed rhodamine 6G laser emission by sodium vapor.² In a follow-up paper Keller and co-workers demonstrated the enhancement of absorption for Eu(NO₃)₃) when placed within the cavity of a rhodamine 6G laser. Concurrently absorption was observed from Ba and Sr in an air-acetylene flame within a dye laser cavity by, Thrash et al.⁴ Hansch and co-workers⁵ duplicated the intra-cavity absorption experiment with iodine vapor and compared the sensitivity of this result with measurements obtained from conventional absorption techniques. At about the same time Latz, Wyles, and Green¹ reported data which dennnstrated that the extent of intra-cavity absorption for nitrogen dioxide was linearly related to its concentration. Investigation into the use of a laminar flow burner with an air-acetylene flame within a dye laser cavity showed part per billion (ppb) detection limits for sodium as well as the detection of barium and mercury. The completion of these intracavity absorption studies in the visible region of the spectrum yielded the results which are reported here as well as quantitative intyacavity absorption data for Eu⁺³. In an independent study Konjevic also reported detection of sodium by intra-cavity absorption from an airnatural gas flame⁶.     

Generation of narrowband tunable VUV radiation at the Lyman-α wavelength

Tunable narrowband VUV radiation has been generated at the Lyman-α wavelength $\text{λ =}\text{1216}\text{\&}\text{A}\text{ring}$; by frequency tripling in krypton the frequency-doubled output of a powerful dye laser system which is excited with the second harmonic of a Nd-YAG laser. 5 ns long UV dye laser pulses ($\text{λ =}\text{3646}\text{A}\text{?}$) of 1.8 MW peak power yielded VUV light pulses of 2.2 W (5.4 × 19⁹ photons/pulse). The bandwidth of the dye laser radiation could be narrowed to 8.7 × 10^-3cm^-1 (4.6 × 10^-3 Å). The expected bandwidth of the VUV is less than 5.2 × 10^-2cm^-1 (7 × 10^-4 Å). The tunable VUV radiation is used for the recording of the absorption spectra of the Lyman-α resonance transitions in atomic hydrogen and deuterium with doppler-limited resolution.     

Pulsed dye laser absorption spectroscopy with metastable and excited mercury atoms

A nitrogen-pumped tunable dye laser is used as a spectrometer. It is especially well suited for spectral profile study of absorption lines starting from short-lived atomic levels. Up to now, in order to illustrate its feasibility, we have measured absorption profiles for the lines 4047 Å (7³S₁ ← 6³P₀) and 4358 Å (7³S₁ ← 6³P₁) of mercury.     

Resonance Ionization Mass Spectrometry (RIMS) with Pulsed and CW-Lasers on Plutonium

The detection of long-lived plutonium isotopes in ultra-trace amounts by resonance ionization mass spectrometry (RIMS) is a well-established routine method. Detection limits of 10⁶ to 10⁷ atoms and precise measurements of the isotopic composition have been achieved. In this work multi-step resonance ionization of plutonium atoms has been performed with tunable lasers having very different output intensities and spectral properties. In order to compare different ways for the resonance ionization of plutonium broadband pulsed dye and titanium:sapphire lasers as well as narrow-band cw-diode and titanium:sapphire lasers have been applied for a number of efficient excitation schemes. It has been shown, that for identical excitation schemes the optical isotope selectivity can be improved by using cw-lasers (bandwidths < 10 MHz) instead of pulsed lasers (bandwidths > 2 GHz). Pulsed and cw-laser systems have been used simultaneously for resonance ionization enabling direct comparisons of pulsed and continuous ionization processes. So far, a three-step, three-color laser excitation scheme has been proven to be most practical in terms of efficiency, selectivity and laser wavelengths. Alternatively a newly discovered three-step, two-color excitation scheme which includes a strong two-photon transition from an excited state into a high-lying autoionizing state yields similar ionization efficiencies. This two-photon transition was characterized with respect to saturation behavior and line width.     

Parasitic selection in intracavity laser detection spectroscopy

The origin of the parasitic structure in the spectra of multimode dye lasers in the presence of additional optical surfaces in the cavity has been investigated. Methods are proposed to diminish the influence of parasitic structures in real experiments on the results of measurements by intracavity laser spectroscopy when studying low absorption with coefficients down to ～10^-9cm^-1.     

He-Ne Laser for Intra-Cavity Enhanced Absorption Measurement

It has been shown recently that when a relatively weak absorber is placed within a laser cavity an enhacement of absorption occurs^1–3. This method has been succesfully used for trace analysis of Na^1,4, I₂ ^3,5, Sr and Ba^{+ 6}, Eu(NO₃)₃ ^2,8, Pr(NO₃)₃, NdCl₃ and HoCl₃.⁸ In these experiments the absorbing species were placed inside the cavity of: flashlamp-pumped dye lasers^1–4,6 continous wave dye lasers^3,5 and dye lasers pumped by a ruby laser⁸.     

Electronic tuning of dye lasers by an electrooptic birefringent Fabry-Perot etalon

An electrooptic birefringent Fabry-Perot etalon is described for a wavelength selector of dye lasers. The laser emission was electronically tuned with a linewidth of several angstroms over 135 Å. This was achieved by changing the applied voltages from -1.2 to +1.2 kV to the etalon constructed from a 3.6 mm long, LiTaO₃ crystal.     

Observations of four-wave mixing processes in barium vapour

Emission at fifty discrete wavelengths is observed when Ba I is excited by two laser beams, a dye laser tuned to the λ 7911 Å intercombination line, 6s²¹S₀-6s6p ³P₁ and a ruby laser. The wavelengths range from 2312 Å to 8027 Å. Most of the emission lines can be attributed to four-wave mixing processes in barium.     

Absorption line narrowing (A.L.N.) in a three-level system of neon under interaction with two quasi-resonant lasers

We report the observation of Absorption Line Narrowing (A.L.N.) in a Doppler-broadened three-level system of neon under interaction with two quasi-resonant single-mode lasers. The resonance signals are obtained with the laser beams propagating either in the same direction or in opposite direction. The resonance width is found to be smaller in the latter case. The isotopic shifts ²²Ne-²⁰Ne of the lines λ = 1.15μ, λ = 6096 Å and λ = 5945 Å have been measured using the A.L.N. method.