We report a simple and compact all-solid-state laser generating 488 nm light with continuously variable output power in the range from 1 mW to over 120 mW. We frequency double single frequency radiation from an external cavity semiconductor laser in a periodically poled MgO:LiNbO3 ridge waveguide. The laser maintains a high quality TEM00 circular beam with M2 < 1.1 and very low r.m.s. noise of less than 0.06% over the entire range of output power. Less than 0.1% peak-to-peak output power variation was measured during 14 hours of operation. No degradation of the conversion efficiency has been observed for operation at an output power of 70 mW for 3.5 months. The prototype laser has a small footprint of 5x8 cm.
Cavity ring-down spectroscopy (CRDS) can provide high sensitivity, high precision, and absolute calibration in a wide range of environments. We report on a compact cavity ring-down spectrometer that can measure atmospheric toxic industrial compounds such as hydrides and hydrazines. The ring-down spectrometer is fully contained in two 5 ¼" tall, 19" wide rack mount enclosures and utilizes a robust, near-infrared, fiber-coupled tunable diode laser. The instrument has a baseline sensitivity of 8 x 10-11 cm-1/Hz½. We will present the results of this study, which demonstrates the capability to detect toxic gases such as arsine, silane, and hydrazine (simulated using ammonia) in air at parts per billion (ppb) concentrations in less than 1 minute. We will also present results on CRDS instrument performance, including zero drift, precision, absolute accuracy, and linearity over a wide range of environmental operating conditions.
We report on cavity-enhanced second-harmonic generation of 488 nm radiation in a 5 mm long periodically poled KTiOPO4 (PPKTP) crystal pumped by the output of a single-mode 976 nm semiconductor external cavity laser. At a pump laser output power of 660 mW, a mode-matching efficiency into an enhancement cavity of 65 % was observed. A maximum power of 156 mW at 488 nm was generated in the enhancement cavity of which 130 mW was coupled out. Under these pump laser conditions an overall optical conversion efficiency of 20 % and an overall electrical to optical efficiency of 9 % was measured. Both the spatial and spectral properties of the 488 nm beam are of very high quality. Typically, a near-diffraction-limited beam with M2<1.1 is produced with low astigmatism and little ellipticity.
We recently demonstrated trace detection using Cavity Ring Down Spectroscopy (CRDS) coupled with telecom DFB diode lasers. Our scheme exploits optical feedback from a V-shaped cavity back to the laser. We built trace-gas detectors for CH4 and HF, characterized by a low cost, simplicity, compactness and sensitivity. Operating wavelength are 1.312 micrometers for HF and 1.65 micrometers for methane. The optical setup includes a distributed feed-back (DFB) diode laser, temperature stabilized by a Peltier, a collimating lens, 2 steering mirrors, a V-shaped optical resonator and a photodiode. The V-cavity is made of three low-cost super mirrors R 99.995%) and contains the air sample to be analyzed (20cm3). In standard atmospheric conditions the detection limits for 1 second integration time are of 50 ppbv for HF and 200 ppbv for methane. We present an analysis of the mechanisms of cavity injection and laser feedback, allowing to estimate the influence of various parameters on the performances of this type of apparatus. Calculations and results are given, with particular emphasis on the detection limit and the dynamic range.
A high resolution and high sensitivity spectroscopic technique is developed by utilizing a Bruker IFS 120HR Fourier-transform spectrometer to detect the absorption spectra of samples inside a Ti:Sapphire laser cavity. A trigger-sampling system makes it possible to investigate the evolution of intra-cavity laser absorption spectra with the generation time. Owing to the multiplex advantage of FTS, the quantum noise in ICLAS can be significantly reduced in FT-ICLAS. The absorption of atmospheric water vapor in the range of 12450 - 12700 cm-1 was recorded at a resolution of 0.05 cm-1 to examine the performance of our FT-ICLAS apparatus. With 200 scans on the FT-ICLAS within 2 hours, a sensitivity of 10-9 cm-1 was obtained. It is also demonstrated that the FT-ICLAS is able to record both positions and intensities in a wide spectral range. This technique was applied to measure the HOD vOD equals 5 overtone in the range of 12550 - 12900 cm-1 with a resolution of 0.05 cm-1. The spectrum was analyzed with the ground states combination differences method and the spectroscopic parameters were obtained. For 81.1% rotational levels in this band, the difference between calculate and experimental energy was less than 0.01 cm-1 which indicated that both the recording and analyzing of this spectrum were successful.
Jeffrey Steinfeld, Robert Field, Matthew Gardner, Manjula Canagaratna, Shengfu Yang, Arturo Gonzalez-Casielles, Scott Witonsky, Pankaj Bhatia, Barclay Gibbs, Brian Wilkie, Stephen Coy, Alexander Kachanov
The ability to detect a broad range of chemical species in the atmosphere, geosphere, and hydrosphere, over a wide range of ambient levels and conditions, is a key enabling technology both for environmental monitoring and for sensing the presence of hazardous materials such as explosives or chemical agents. In this paper we describe several recent developments in the area of high-sensitivity chemical monitoring, including Cavity RingDown Spectroscopy (CRDS) and Raman spectroscopy. Significant improvements in the sensitivity of both ultraviolet and infrared CRDS have been made in our laboratory during the past year. Infrared CRDS, employing tunable infrared optical parametric oscillators, multilayer dielectric high-reflectivity mirrors, and membrane preconcentrator technology, may have the potential for detecting vapors from concealed explosives as well as a wide variety of volatile organic compounds present at parts-per-billion levels or below. Sensitivity enhancement techniques for Raman spectroscopy, such as Surface-Enhanced Raman Spectroscopy, may achieve similar sensitivities for samples in ultra-dilute solutions. We also address the relationship of novel monitoring techniques, such as these high-sensitivity detection methods, to current and future regulatory issues.
Ultrasensitive optical spectroscopy technologies for environment monitoring, and in general, for gas analysis in the near Infrared are mainly based on the following spectroscopic methods: diode laser spectroscopy with multipass cells, with or without frequency modulation, photoacoustic spectroscopy, and since very recently, difference frequency spectroscopy with diode lasers. One of the most important issues of any monitoring technology, as important as the sensitivity, is its ability to provide absolute absorption coefficients without the need of complicated and cumbersome calibration procedures. Until now, two of the most sensitive optical spectroscopic technologies capable of providing this absolute information, Cavity Ringdown Spectroscopy and Intracavity Laser Absorption Spectroscopy have practically no use in this field. Due to recent advances, these two methods can now provide low-cost very compact field instruments working in the spectral range from 0.8 to 2.5 microns, with the smallest detectable absorption down to 10-10 (one over 10 billions) per one centimeter of the absorption path. This would result in the sub-ppb detection limit for moisture for example. Experimental results obtained with prototype field instruments developed by our group will be presented. Future perspectives will be discussed.
Optical spectroscopic methods based on direct absorption offer a quantitative measurement of the absorbance, which is the product of the concentration, the molar absorption coefficient of the transition being observed and the length of the absorption path. An absorption sensitivity adequate for trace detection may be achieved by increasing the path length. One solution is offered by cavity ringdown spectroscopy (CRDS), attractive for its simplicity. We recently demonstrated that an external cavity diode laser (ECDL) can be conveniently employed for CRDS instead of a pulsed laser, contrary to previous applications. Here we extend this result to distributed feed-back (DFB) diode lasers. Paying special attention to the coupling of the laser source to the cavity, we developed an extremely simplified CRDS scheme with a sensitivity of about 10-8/cm/(root)Hz. We then built detectors for methane and HF, working close to the optical wavelengths 1.65 and 1.31 micrometer, respectively With an optical assembly of about 50 cm length and a response time of about 1 s, these devices accurately measure atmospheric methane concentrations in the range 0.5 to 200 ppmv, and HF concentrations from 0.1 to 50 ppmv.
Intracavity laser absorption spectroscopy was used to study concentration profiles of several radicals in flames. Very high sensitivity of ICLAS enables quantitative measurements of absolute concentrations of atoms and radicals in flames. MOst of the experiments were done with a flat flame burner placed inside the cavity of a broad band dye laser. The spectra of HCO radicals and CH2 radical, in the singlet electronic state were measured with a high signal-to-noise ratio at different position above the burner.In this work we report the measurement of the concentration profiles of HCO and CH2 radical sin methane/air flame. The spectra of these two radicals can be measured simultaneously which is advantageous in combustion diagnostics. Cavity ring-down laser spectroscopy was used to measure the OH concentration profiles of the HCO and OH radicals are in reasonable agreement with computer simulation results. However, a rough estimation of the CH2 absolute concentration indicates a much higher concentration than that which can be predicted based on the model calculation.
Emission dynamics of a multimode broadband dye laser have been investigated experimentally. Cavity dispersion properties are shown to essentially affect the character of emission. Regular self-excited oscillations of emission intensity in singular modes have been observed in a cw ring unidirectional dye laser. The period of these oscillations is shown to depend on the cavity dispersion. These oscillations are supposed to be due to a four-wave parametric mixing of non uniformly spaced modes in active media. When the cavity dispersion is compensated no intensity fluctuations of emission in single modes are observed up to as long lasing duration at approximately 10-1 s. With such a laser employed the highest sensitivity of an intracavity laser spectroscopy has been achieved which provides the detection threshold of this technique by the absorption coefficient as low as 10-11 cm-1.
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