Ari Olafsson

Photoacoustic detection of NH3 in power plant emission with a CO2 laser

The paper describes a photoacoustic spectrometer for detection of NH3 in power plant emission with a detection limit below 1 ppm. The radiation source is a tunable CO2 waveguide laser, and detection is performed at reduced pressure, where the vibration-rotation lines of NH3 are essentially Doppler broadened. Immunity against interference is ensured by recording a characteristic spectral profile, and problems associated with the high concentration of CO2, and the associated line center absorption are eliminated by utilizing the effect of kinetic cooling on the photoacoustic phase. A computerized spectrometer has been constructed and tested under realistic conditions at a Danish power plant operating a test facility for selective non-catalytic reduction of NOx. Results of this field test are given.

Mode structure of hollow dielectric waveguide lasers

The mode structure of hollow dielectric waveguide lasers with free space sections and flat mirrors is studied theoretically and experimentally. The study covers the fundamental mode and the three most important higher order modes, and graphs are given which identify regions of high mode discrimination in the parameter space. Calculated coupling losses are verified experimentally by detailed studies of the output power of CO2 lasers as a function of resonator geometry. The intensity profile inside and outside the resonator is calculated, and the profile outside the resonator is compared with experiments for the fundamental mode as well as for the higher order modes. It is shown that in general the fundamental mode is non-Gaussian, and that drastically different output characteristics are obtained for different choice of output plane. The paper identifies design criteria for obtaining single line and single mode oscillation over a wide tuning range, even in the densest region of the CO2 laser line spectrum, and this is exemplified by a spectroscopic application.

A study of the methanol laser with a 500 MHz tunable CO2 pump laser

The absorption spectrum of CH3OH is measured optoacoustically over a 500 MHz tuning range in 84 different CO2 laser lines, and 19 assigned CO-stretch absorptions are used for establishing the linearity of the measurements over two decades of absorption coefficient. 40 new FIR laser lines originating from large offset absorption, are reported in the wavelength range between 40 and 400 μm, and their wavelengths are given with a relative accuracy of 5×10−5. 30 lines are assigned, involving CO-stretch transitions in torsionaln=0, 1, and 2 states, as well as transitions inn=3 of the vibrational ground state, pumped by transitions to Fermi interacting states.

Cavity enhanced absorption spectroscopy in the 10 μm region using a waveguide CO2 laser

Abstract The cavity enhanced absorption (CEA) technique is extended into the 10 μm region using a line-tunable continuous wave CO 2 laser. Part of the laser beam is deflected by an acousto-optical modulator (AOM), and is used to excite a mechanically unstable high-finesse optical cavity. In order to assure a stable and optimal transmittance of light through the cavity, the laser frequency and the cavity eigenfrequencies are modulated independently. The time-integrated intensity of the light exiting the cavity, which is inversely proportional to the cavity losses, is measured using a lock-in detection scheme. An absorption detection sensitivity of 1.5×10 −6 cm −1 Hz −1/2 is readily obtained with a rather simple setup.

Photoacoustic spectroscopy of O3 with a 450-MHz tunable waveguide CO2 laser

Abstract Photoacoustic absorption signatures have been obtained for ozone in 450-MHz tuning windows of a waveguide CO 2 laser. Out of 42 observed absorption lines, 31 are assigned to the ν 1 and the ν 3 bands, and 7 to various hot bands of 16 O 16 O 16 O. Two lines are assigned to the ν 3 band of the isotopomers 16 O 16 O 18 O and 16 O 18 O 16 O, which were present in their natural abundance. Precise collision broadening measurements are reported for two lines of the 16 O 16 O 16 O ν 3 band.

Photoacoustic study of kinetic cooling

Abstract Kinetic cooling is studied through photoacoustic CO 2 laser monitoring on gas mixtures containing CO 2 in quantities typical of smoke. The photoacoustic response is recorded in the time domain as well as in the frequency domain, and results are given for the change in responsivity induced by small quantities of H 2 O. The importance of phase information is illustrated for a number of cases.

Photoacoustic spectroscopy of C 2 H 4 with a tunable waveguide CO 2 laser

A photoacoustic gas spectrometer using a wave-guide CO(2), laser, tunable over 500-MHz windows in 70 lines, is described. The spectrometer is used for mapping Doppler-limited spectral signatures of ethylene and for measuring pressure-broadening rates for collisions with N(2), Ar, and He. A total of 41 observed transitions are assigned to the nu(7), nu(10), and nu(4) bands of normal ethylene and to the nu(7) band of the (13)C(12)CH(4) isotopomer.

Intraline tunable CO2 waveguide lasers and applications in high resolution spectroscopy

Abstract Although early publications on the CO 2 waveguide laser reported tuning ranges close to and exceeding 1 GHz in individual laser lines, this potential has not been widely exploited for spectroscopy. In this paper we outline the difficulties which have hampered spectroscopic applications, and we point out their theoretical background and the avenues for solution. Following this, we review the work that has been done, including fundamental as well as applied spectroscopic work.

Trace Gas Detection with Infrared Gas Lasers

The growing concern about our environment has led to a demand for methods and equipment which can perform a wide variety of monitoring tasks. The detection of specific molecules in the atmosphere may be motivated by the need for monitoring the emission of toxic chemicals used by industry, or by the need to perform a general control of the air we breathe, in particular in areas which are subject to large scale emission caused by human activities, such as traffic, livestock breeding, and energy production. In addition, it may be motivated by the need to get a better understanding of global trends in the concentration of molecules which are of importance in connection with the greenhouse effect and ozone destruction in the stratosphere, or it may simply be necessary in order to get a deeper insight into the huge number of physical and chemical processes which occur in the atmosphere, and which act together to produce what we commonly denote as weather and climate.

FTIR photoacoustic trace gas detection

A new design of a multipass photoacoustic cell intended for FTIR measurements is reported. In spite of the intracell reflections the wall signal is negligible. Comparison with transmission measurements using a fundamental band of COS show a sensitivity equivalent to a 3 m transmission cell. Detection limit for COS is estimated 0.1 ppm corresponding to αmin=10−5/cm.