Intracavity spectrometer with a ring travelling-wave dye laser: Reduction of detection limit

In a ring travelling-wave dye laser a parasitic structure in the intracavity absorption spectrum is fully eliminated and a detection limit as low as 10^-10 cm^-1 is obtained. The sensitivity dependence of the intracavity laser spectrometer with travelling-wave cavity on the pumping level is also investigated.     

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     

Two-photon pumping of random lasers by picosecond and nanosecond lasers

We experimentally demonstrated two-photon pumping of random lasers using picosecond and nanosecond pump lasers. The picosecond laser pumping experiment was performed with 400 ps laser pulses at 770 nm, and the gain media was a Coumarin 480D dye solution doped with TiO₂ nanoparticles. Onset of laser action was observed at a pump laser pulse energy below 500 μJ. The nanosecond laser pumping experiment was performed with 7 ns laser pulses at 1064 nm, and the gain media was a Rhodamine 640 dye solution doped with TiO₂ nanoparticles. Onset of laser action was observed at a pump laser energy ∼18 mJ. Our results suggest that there exists an optimal pulse duration of the pumping laser in two-photon pumped random lasing that leads to minimum photodamage of the gain media and still keeps a high pumping efficiency. PACS 33.50.Dq; 42.55.Mv; 42.55.Zz     

Spectral characteristics of a Yb-doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic laser

We report the experimental observation of random wavelength emission and intensity-dependent central-wavelength shift in a diode-pumped Yb³⁺-doped Y₂O₃ ceramic laser. We show experimentally that, like conventional lasers, the emission of the laser has fixed well-defined transverse modes; however, its instantaneous emission wavelengths change randomly with time. The central wavelength of the laser emission also shifts with the intracavity light intensity. A model was developed to describe the spectral behavior of Yb³⁺-doped lasers. We show that the observed random wavelength emission and central lasing wavelength shift of the laser could be well explained based on the strong reabsorption of light in the gain medium. PACS 42.55.Rz; 42.60.Mi; 42.55.Xi     

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.     

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.     

Optimization of the pulse energy of GaAs saturable absorber Q-switched lasers

By taking into account the single-photon absorption (SPA) and two-photon absorption (TPA) processes of GaAs, the Gaussian spatial distributions of the intracavity photon density and the initial population-inversion density, and the pumping and the spontaneous emission during the pulse formation, the new normalized rate equations of a GaAs saturable absorber Q-switched laser are solved. The key parameters of an optimally coupled GaAs saturable absorber Q-switched laser are determined, including the optimal normalized coupling parameter and the optimal normalized GaAs saturable absorber parameters, which can maximize the pulse energy, and a group of general curves are generated for the first time, which clearly show the dependence of the optimal key parameters on the parameters of the gain medium, the GaAs saturable absorber, and the resonator. In addition, the influence including the space variation, the pumping and the spontaneous emission is also shown. Sample calculations for a diode-pumped Nd³⁺:YVO₄ laser with a GaAs saturable absorber are presented to demonstrate the use of the curves and the relevant formulas.     

Dye-Laser Fluorescence of 4-Methylumbelliferone and Related Coumarins

During tha eight years since phthalocyanine solution was used by Sorokin and Lankard in the original dye laser¹ and DTTC bromide and iodide were used in the work of Schafer, Schmidt and Volze², laser emission has been reported³ from close on 400 combinations of laser dye and solvents covering a wavelength range extending from 340 to 1175 nm. Many hundreds or even thousands of attempts to induce laser action in a vast number of available chemicals have proved unsuccessful, either by pulsed or continuous laser pumping, or by flashlamp techniques. In fact, relatively few combinations have established themselves as highly efficient, reliable and entirely satisfactory dye laser media. The compound rhodamine 6G is probably the most widely used in the range 540–640 nm. However, some ten different solvents and a variety of excitation techniques have been necessary to maintain maximum fluorescence intensity over this range. A t shorter wavelengths an acidic solution of 4,6- dimethyl-7-methylamino-coumarin in ethanol was observed to lase⁴ over the range 430–530 nm and a particular solution of 4-methylumbelliferone to lase from 391 to 567 nm⁵. This offered the possibility of achieving narrow linewidth tunability using dye lasers based on single solutions. It represented a significant advance on the first experiments² in which it was possible by changing the solvent, varying the solute concentration or changing the reflectivity of the resonator mirror, to Shift the wavelengths by up to 60 nmm in the infra-red region.     

Production of a nuclear spin polarized23Na-beam by optical pumping

Nuclear spin polarization of an atomic²³Na-beam was produced by a combination of optical pumping with a dye laser and a sextupole magnet and, alternatively, by optical pumping with two dye lasers. The maximum value measured for the vector polarization wasP _z=0.86+0.08, using beam foil spectroscopy. Further improvements of polarized ion sources based on this principle are discussed.     

Dual-wavelength operation of two coupled dye lasers

The analysis of the influence of an intracavity absorbing dye cell on the spectral characteristics of a free running dye laser leads to the operation of two coupled dye lasers. A powerful flashlamp-pumped dye ring laser pumps a secondary intracavity dye laser with a conversion efficiency of 30 %. Both wavelengths are independently tunable and cover the spectral range from 580 nm to 652 nm in the case of the dye combination rhodamine 6G and cresyl violet.     

Growth and optical properties of a new self-Q-switched laser crystal: Cr:Yb3Al5O12

Cr⁴⁺:YbAG (Cr⁴⁺:Yb₃Al₅O₁₂) crystal with a size up to Φ24 mm×30 mm was grown by the Czochralski method. In the absorption spectrum, there are two absorption bands at 939 and 969 nm, respectively, which are suitable for InGaAs diode laser pumping, and there is an absorption band at 1030 nm, which is suitable for passive Q-switched laser output at 1.03 μm. A broad emission spectrum from 970 to 1100 nm was exhibited from 940 nm wavelength pumping. This crystal is promising as a self-Q-switched laser crystal used for compact, efficient, highly stable, passive self-Q-switched thin chip solid-state lasers.     

Sensitivity of Narrow-Band Intracavity Laser Spectroscopy in Generation of the Second Harmonic

We carried out an analysis of the sensitivity of the method of intracavity laser spectroscopy with intracavity generation of the second harmonic to small frequency-independent losses evaluated by measuring the energies of laser-radiation pulses. We consider laser behavior with a constant and modulated quality factor of the cavity resonator. It is shown that the sensitivity of the method may be improved by an order of magnitude or higher by transforming the frequency of radiation and selecting the optimal parameters of the active medium pumping, nonlinear crystal, Q-factor of the cavity, and the means of its modulation. Measurement of the radiation power at the frequency of the second harmonic expands the range of laser-operation time regimes in which an increase in the sensitivity of the method is ensured.     

Dye Laser for Absorption Trace Analysis of Sodium

The advent of organic dye lasers made possible many experiments in physics, chemistry and biology. Most of these applications are based on the possibility to tune dye laser emission almost at any wavelength between 3400 Å and 12000 Å.This quality combined with high light power made feasible multiphotone absorption, selective absorption and fluorescence experiments. Recently a group of authors published the papers: “Enhancement of absorption spectra by dye laser quenching I and II”^1,2 which start a new field of applications of dye lasers. Namely if a weak absorber is placed inside the broad band laser cavity the absorption is enhanced and this results in the same absorption bands or lines observable in the spectrally resolved laser output. This effect can be used for detection of the traces of elements as a complementary method to the classical absorption spectroscopy but with much higher sensitivity. Same method could be also very useful for detection of transient species with a very short lifetimes.     

Simultaneous dual-wavelength emission at 0.90 and 1.06 µm in Nd-doped laser crystals

Continuous-wave (CW) simultaneous laser emission on the 0.9-μm ⁴ F _3/2 → ⁴ I _9/2 transition and the ⁴ F _3/2 → ⁴ I _11/2 transition at 1.06 μm is obtained in Nd-based laser crystals of thin-disk geometry and using a multi-pass pumping scheme. A Nd:Y₃Al₅O₁₂ (Nd:YAG) thin disk emitted simultaneous laser radiation at 946 and 1064 nm with 5.1 W output power, and Nd:YVO₄ and Nd:GdVO₄ thin-disk lasers with more than 3 W output power at 0.91 and 1.06 μm were realized. The ratio between the output power at one of the wavelengths and the total output power could be varied by the laser resonator design. An intracavity frequency-doubled Nd:YVO₄ thin-disk laser with alternate green at 532 nm and “deep-blue” at 457 nm generation of high average output powers is demonstrated.     

Operation of pulsed dye laser with an intracavity phase step

Effects of an intracavity phase step on the spectral and spatial characteristics of pulsed dye laser radiation are studied. In the broadband operation regime of the dye laser, the lasing spectrum was strongly, periodically modulated. In the narrowband regime, periodic alteration between TEM₀₀ and TEM₀₁-like beam profiles is observed on tuning the laser wavelength. Two different spectral periods of these effects are recognized. The shorter period corresponds to double passage and the longer one to apparent single passage of intracavity light through the phase step on a cavity round trip. Depending on the position of the phase step along the laser cavity, single-passage period, double-passage period, or a superposition of both periods appeared in the broadband emission spectrum and in the beam profile alteration sequence. These observations demonstrate several possibilities for the control of spectral, spatial, and phase characteristics of laser beams by simple intracavity phase elements.     

Intracavity frequency-doubled diode-pumped Nd : LaSc3(BO3)4 lasers

 An intracavity frequency-doubled 10%Nd : LaSc₃(BO₃)₄ (Nd : LSB) laser was investigated in different resonator configurations and in different operation modes under continuous wave (cw) and quasi-cw laser-diode pumping. With a Cr⁴⁺ : YAG passive modulator and a KTP crystal the second-harmonic output power at 531 nm amounted to 190 mW in Q-switched TEM₀₀ mode at 750 mW of pump power. In a sandwich resonator, when all the optical elements were in direct contact with each other, 0.8 W of green output power was obtained in cw mode under 2.7 W of pump power with a slope efficiency of 44%. In the same setup under fiber-coupled diode-laser array pumping (5.6 W of incident power), 1.2 W of green output power was achieved in cw mode and 1.4 W in quasi-cw mode. Received: 30 April 1996/Revised version: 1 July 1996     

Intra-cavity spectroscopy with diode lasers

A multi-mode diode laser with an external cavity is studied experimentally and theoretically for its application to intra-cavity spectroscopy. One facet of a typical Ga_0.89Al_0.11As laser diode was antireflection-coated by deposition of HfO₂ such that 10^–3 residual reflectivity was left over. This diode was placed in an external optical cavity. The emission spectrum of this diode laser is highly sensitive to any frequency-dependent loss in the cavity, and the detectivity of such a loss grows with the pump rate. Even close to threshold, the absorption at 780 nm of Rb atoms with a density of 5×10¹⁰ cm^–3 has been detected. An adequate model for diode lasers based on rate equations and including frequency-dependent gain saturation is developed and applied to the calculations of output spectra. The sensitivity of these spectra to intra-cavity absorption is determined by the overall cavity loss — which is rather high — and the fraction of spontaneous emission in the total emission, in contrast with dye lasers where it is limited by nonlinear mode coupling. Various criteria for the sensitivity are suggested. The smallest detectable absorption with a perfectly antireflection-coated laser is 10^–6 cm^–1. Improvement of the characteristics of the laser diode would increase the sensitivity.     

Experimental study of nonlinear mode mixing in dual-wavelength semiconductor lasers

New schemes for the generation of the sum, difference, and double frequencies in dual-wavelength injection lasers, based on an intracavity nonlinear mode mixing owing to a bulk nonlinearity, are experimentally investigated. Namely, the operation of the dual-wavelength laser diodes at the TE₀ and TE₁ modes, butt-joint laser diodes, and interband cascade lasers with tunnel junction are described.     

Wide-band intracavity microwave cells for laser-microwave double resonance spectroscopy

The design and use of two wide-band microwave cells suitable for double resonance experiments within the resonator of a cw CO₂-laser are described. The high sensitivity of the intracavity arrangement allowed very strong double-resonance and two-photon transition signals to be observed with high microwave-frequency scan-rates (1 GHz/min), and low microwave pumping powers (≦10mW/cm²). Both cells were used over a range 15 to 63 GHz, with transmission and VSWR measurements made over the range 18 to 26.5 GHz.     

A single mode flashlamp-pumped dye laser

A flashlamp-pumped dye laser capable of single transverse and longitudinal mode operation is described. The maximum untuned power output is 2 MW cm^-2, TEM₀₀ mode. By employing three intracavity Fabry—Pérot etalons, a minimum spectral width of 4 MHz, stable to within 12 MHz, has been obtained.