Resonant photoacoustic simultaneous detection of methane and ethylene by means. of a 1.63-μm diode laser

A compact multi-component trace-gas detector based on the resonant photoacoustic technique and a NIR external cavity diode laser has been developed. It has been characterized using a mixture of ethylene and methane diluted in ambient air. A spectroscopic investigation of combination bands and overtones between 5900 and 6250 cm^-1, obtained with an IR pulsed laser photoacoustic spectrometer, allowed us to find a wavelength region where the 2ν₃ overtone of CH₄ and the ν₅+ν₉ combination band of C₂H₄ show uncongested rotational lines. Using a single-mode scan of the diode laser in this region, around 6150 cm^-1, the sensitivity for the simultaneous detection of ethylene and methane is 8 ppm/mW and 40 ppm/mW respectively. Factors affecting the sensitivity and selectivity of the detection system and possible improvements suitable to reach the sub-ppm detection limit are discussed. Received: 1 August 2001 / Revised version: 28 November 2001 / Published online: 7 February 2002 An erratum to this article is available at .     

On-line monitoring of biogenic isoprene emissions using photoacoustic spectroscopy

Isoprene (C₅H₈) is one of the most important biogenic volatile organic compounds (VOCs) in the atmosphere. To calculate the impact of isoprene on atmospheric processes models have been developed that describe the isoprene release from plants. Measurements of this release require techniques for a fast, sensitive, on-line isoprene detection. Photoacoustic (PA) spectroscopy is applied here for the first time to monitor biogenic isoprene emissions. A CO overtone PA spectrometer is used for the detection, probing the C-H stretching vibrations in the 3 to 4 μm range. This allows us to detect isoprene down to a few ppb with a time resolution of one minute in a continuous gas flow. The number of laser lines can be adjusted to meet the requirements of the respective experiment in terms of time resolution and selectivity against other possibly interfering VOCs. This results in a highly versatile instrument for the isoprene detection in biological experiments. Furthermore, the infrared fingerprint offers the potential to detect different isoprene isotopomers simultaneously, thus allowing us to carry out on-line labelling experiments. The new apparatus was used to study the light dependence of isoprene emission from Eucalyptus globulus. The results demonstrate, that the detector system is a promising tool for the study of plant gas emissions. It allows the validation of existing emission models which are important for atmospheric processes. Received: 18 March 1999 / Revised version: 23 August 1999 / Published online: 30 November 1999     

Enhanced drilling using a dual-pulse Nd:YAG laser

A new method for improving the efficiency of laser drilling has been developed. Two synchronized free-running laser pulses from a tandem-head Nd:YAG laser are capable of drilling through 1/8-in-thick stainless-steel targets at a stand-off distance of 1 m without gas-assist. The combination of a high-energy laser pulse for melting with a properly tailored high-intensity laser pulse for liquid expulsion results in the efficient drilling of metal targets. We argue that the improvement in drilling is due to the recoil pressure generated by rapid evaporation of the molten material by the second laser pulse. Received: 29 August 2000 / Accepted: 18 December 2000 / Published online: 3 April 2001     

Tungsten microcone arrays grown using nanosecond pulsed-Nd:YAG laser in a low-pressure He-gas atmosphere

Tungsten microcone arrays with a high aspect ratio are formed by the cumulative nanosecond pulsed-Nd:YAG laser irradiation of single-crystal tungsten under low pressure in an inert atmosphere. The morphology of the microcones and their density were strongly affected by the number of laser pulses. The microcones grew to a length of 20 μm with a diameter of about 1.5 μm at the tip after irradiation with more than 1200 pulses under our experimental conditions. They may have potential applications for emission cathodes in a field-emission display (FED) and in microelectronic devices. Received: 8 January 2001 / Accepted: 13 June 2001 / Published online: 2 October 2001     

The laser pulse from a rf waveguide CO2 laser

The behaviour of a rf-excited waveguide CO₂ laser in the pulse regime is studied experimentally. The specific time sections of the pulse have been defined and described in detail. The output pulse evolution versus input power, pressure of the laser mixture, output coupling, repetition frequency and pulse width has been investigated. The spectral content of the pulse has been analysed. A model explaining the effect of laser tuning during the pulse duration is given. The ultrahigh pulse (UHP), defined as the pulse for which the bulge effect occurs, is predicted. Received: 22 October 2002 / Revised version: 13 January 2003 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. Fax: +48-71/3203-189, E-mail: eda@zr.ita.pwr.wroc.pl     

Growth mechanisms for single-wall carbon nanotubes in a laser-ablation process

Mechanisms proposed in the literature are compared with a current scenario for the formation of single-wall carbon nanotubes in the laser-ablation process that is based on our spectral emission and laser-induced fluorescence measurements. It is suggested that the carbon which serves as feedstock for nanotube formation not only comes from the direct ablation of the target, but also from carbon particles suspended in the reaction zone. Fullerenes formed in the reaction zone may be photo-dissociated into C₂ and other low molecular weight species, and also may serve as feedstock for nanotube growth. Confinement of the nanotubes in the reaction zone within the laser beam allows the nanotubes to be ‘purified’ and annealed during the formation process by laser heating. Received: 2 November 2000 / Accepted: 3 November 2000 / Published online: 23 March 2001     

Spectroscopic detection of biological NO with a quantum cascade laser

Two configurations of a continuous wave quantum cascade distributed feedback laser-based gas sensor for the detection of NO at a parts per billion (ppb) concentration level, typical of biomedical applications, have been investigated. The laser was operated at liquid nitrogen temperature near λ=5.2 μm. In the first configuration, a 100 m optical path length multi-pass cell was employed to enhance the NO absorption. In the second configuration, a technique based on cavity-enhanced spectroscopy (CES) was utilized, with an effective path length of 670 m. Both sensors enabled simultaneous analysis of NO and CO₂ concentrations in exhaled air. The minimum detectable NO concentration was found to be 3 ppb with a multi-pass cell and 16 ppb when using CES. The two techniques are compared, and potential future developments are discussed. Received: 1 November 2000 / Revised version: 19 January 2001 / Published online: 20 April 2001     

Wavelength-modulation spectroscopy using a frequency-doubled current-modulated diode laser

We have studied atomic absorption in an argon discharge by wavelength-modulation spectroscopy with a frequency-doubled diode laser. The tunable wavelength-modulated radiation at 430 nm was generated by frequency doubling a current-modulated 860-nm diode laser in a KNbO₃ crystal. 2f-, 4f- and 6f-harmonic spectra as a function of diode laser modulation depth were measured on a Doppler-broadened sample of excited argon atoms produced in a capacitively coupled plasma chamber. Characterisation of the harmonic signals was accomplished. Minimum detectable absorbances of 7.7×10^-5 and 1.9×10^-4 based on a 3σ criterion (σ being the standard deviation of the noise) were estimated for 2f- and 4f-harmonic detection of the frequency-doubled radiation with a time constant of 0.1 s. The concentrations of argon in the 1s₄ state were found to be in the range of 3×10⁸ to 1.2×10¹¹ cm^-3 for the experimental conditions studied. Received: 25 February 2002 / Revised version: 4 April 2002 / Published online: 14 May 2002     

Laser writing techniques for photomask fabrication using a femtosecond laser

Photomasks are the backbone of microfabrication industries. Currently they are fabricated by a lithographic process, which is very expensive and time consuming since it is a multi-step process. These issues can be addressed by fabricating photomasks by direct femtosecond laser writing, which is a single-step process and comparatively cheaper and faster than lithography. In this paper we discuss our investigations on the effect of two types of laser writing techniques, namely front- and rear-side laser writing, with regard to the feature size and the edge quality of a feature. It is proved conclusively that for the patterning of masks, front-side laser writing is a better technique than rear-side laser writing with regard to smaller feature size and better edge quality. Moreover the energy required for front-side laser writing is considerably lower than that for rear-side laser writing. Received: 22 May 2001 / Accepted: 14 September 2001 / Published online: 17 October 2001     

Scanning microscopy by mid-infrared near-field scattering

2 -laser radiation from an AFM tip. In the microscopic images we find and identify a new type of AFM-induced artifact (crosstalk via the tapping amplitude). Minimizing this by proper scan parameters we obtain evidence of true infrared contrast. The results demonstrate the material-sensitive potential of infrared-spectroscopic imaging and a spatial resolving power of better than 100 nm. Received: 18 November 1997/Accepted: 5 January 1998     

Off-axis continuous-wave cavity-enhanced absorption spectroscopy of narrow-band and broadband absorbers using red diode lasers

We present an application of continuous-wave (cw) cavity-enhanced absorption spectroscopy (CEAS) with off-axis alignment geometry of the cavity and with time integration of the cavity output intensity for detection of narrow-band and broadband absorbers using single-mode red diode lasers at λ=687.1 nm and λ=662 nm, respectively. Off-axis cw CEAS was applied to kinetic studies of the nitrate radical using a broadband absorption line at λ=662 nm. A rate constant for the reaction between the nitrate radical and E-but-2-eneof (3.78±0.17)×10^-13 cm³ molecule^-1 s^-1 was measured using a discharge-flow system. A nitrate-radical noise-equivalent (1σ≡ root-mean-square variation of the signal) detection sensitivity of 5.5×10⁹ molecule cm^-3 was achieved in a flow tube with a diameter of 4 cm and for a mirror reflectivity of ∼99.9% and a lock-in amplifier time constant of 3 s. In this case, a noise-equivalent fractional absorption per one optical pass of 1.6×10^-6 was demonstrated at a detection bandwidth of 1 Hz. A wavelength-modulation technique (modulation frequency of 10 kHz) in conjunction with off-axis cw CEAS has also been used for recording 1f- and 2f-harmonic spectra of the ^RR(15) absorption of the b¹Σ_g ⁺-X³Σ_g ^- (1,0) band of molecular oxygen at =14553.947 cm^-1. Noise-equivalent fractional absorptions per one optical pass of 1.35×10^-5, 6.9×10^-7 and 1.9×10^-6 were obtained for direct detection of the time-integrated cavity output intensity, 1f- and 2f-harmonic detection, respectively, with a mirror reflectivity of ∼99.8%, a cavity length of 0.22 m and a detection bandwidth of 1 Hz. Received: 24 June 2002 / Revised version: 12 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1865/275410, E-mail: vlk@physchem.ox.ac.uk     

A tunable diode laser system for the remote sensing of on-road vehicle emissions

2 O, and NO₂ emissions have been measured with this instrument. The system is also capable of measuring CO, NH₃, H₂CO, CH₃OH, and other small molecules in vehicle exhaust. Received: 18 May 1998/Revised version: 1 July 1998     

Thermal tuning and modulation of a DFB fibre laser with a thin-film heater

Results are given for thermal tuning and modulation of a 1556-nm distributed feedback fibre laser by resistive heating of a thin silver film chemically deposited on the fibre. Without reaching the limits of performance, linear tuning is demonstrated at a rate of 1.72 pm/mW up to about 200 pm, and a peak-to-peak modulation of 100 MHz up to modulation frequencies of 60 Hz. The heat flow is analyzed, and the coated fibre is characterized in terms of the static and dynamic wavelength response to the applied electric power. The performance of the scheme is tested by recording part of the ν₁+ν₃ combination band spectrum of ¹³C₂H₂ with thermal modulation and scanning of the fibre laser. Received: 12 March 2002 / Revised version: 24 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +45/4593-1137, E-mail: sus@dfm.dtu.dk     

Detection of biogenic CO production above vascular cell cultures using a near-room- temperature QC-DFB laser

We report the first application of pulsed, near-room-temperature quantum cascade laser technology to the continuous detection of biogenic CO production rates above viable cultures of vascular smooth muscle cells. A computer-controlled sequence of measurements over a 9-h period was obtained, resulting in a minimum detectable CO production of 20 ppb in a 1-m optical path above a standard cell-culture flask. Data-processing procedures for real-time monitoring of both biogenic and ambient atmospheric CO concentrations are described. Received: 24 October 2001 / Published online: 29 November 2001     

Cavity-enhanced absorption: detection of nitrogen dioxide and iodine monoxide using a violet laser diode

We present an application of cavity-enhanced absorption spectroscopy with an off-axis alignment of the cavity formed by two spherical mirrors and with time integration of the cavity-output intensity for detection of nitrogen dioxide (NO₂) and iodine monoxide (IO) radicals using a violet laser diode at λ=404.278 nm. A noise-equivalent (1σ≡ root-mean-square variation of the signal) fractional absorption for one optical pass of 4.5×10^-8 was demonstrated with a mirror reflectivity of ∼0.99925, a cavity length of 0.22 m and a lock-in-amplifier time constant of 3 s. Noise-equivalent detection sensitivities towards nitrogen dioxide of 1.8×10¹⁰ molecule cm^-3 and towards the IO radical of 3.3×10⁹ molecule cm^-3 were achieved in flow tubes with an inner diameter of 4 cm for a lock-in-amplifier time constant of 3 s. Alkyl peroxy radicals were detected using chemical titration with excess nitric oxide (RO₂+NO→RO+NO₂). Measurement of oxygen-atom concentrations was accomplished by determining the depletion of NO₂ in the reaction NO₂+O→NO+O₂. Noise-equivalent concentrations of alkyl peroxy radicals and oxygen atoms were 3×10¹⁰ molecule cm^-3 in the discharge-flow-tube experiments. Received: 4 February 2003 / Revised version: 10 March 2003 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +44-1865/275-410, E-mail: vlk@physchem.ox.ac.uk     

Cavity-enhanced absorption spectroscopy with a rapidly swept diode laser

Cavity-enhanced absorption spectroscopy is explained in terms of the transmission function of a rapidly swept interferometer, and the integrated transmission is shown to be proportional to the cavity ringdown time. The technique is demonstrated on the b¹Σ_g ⁺-X³Σ_g ^-  (1,0) band in molecular oxygen at 687 nm using a tunable diode laser and a relative-ly high-Q optical cavity (finesse ≈4000). A detection limit of 3×10^-8 cm^-1 s^1/2 is achieved for a 0.8 cm^-1 scanning range. Received: 24 June 2002 / Revised version: 5 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1865/275410, E-mail: peverall@physchem.ox.ac.uk     

Local fuel concentration measurements in internal combustion engines using spark-emission spectroscopy

Spectrally resolved visible and ultraviolet emissions are investigated as a basis for wide-range, individual-cycle measurement of the local fuel concentration in spark-ignition engines. The 388-nm CN emission intensity, normalized by the spark-discharge energy during the observation interval (typically 150 μs at the start of the glow discharge), is found to be the most useful measure of fuel concentration when data are required over a wide range. Calibration data for homogeneous propane–air and isooctane–air mixtures over a wide range of cylinder gas conditions at the time of ignition collapse to a single curve when the fuel concentration is expressed in terms of the number density of carbon atoms. The carbon number densities measured in this study correspond to fuel–air equivalence-ratios in the range 0–3 at 95% throttle conditions. Random and systematic errors are 10% or less. Applied to an engine in which liquid fuel is injected directly into the cylinder, the technique reveals substantial cyclic fluctuations in the fuel concentration at the spark gap for early fuel injection (intended to produce a homogeneous fuel–air mixture in the combustion chamber) and large fuel-concentration fluctuations for late fuel injection (which produces a highly stratified mixture). The results also show that for stratified operation with a fixed fuel-injection timing, a spark timing that is later than optimum leads to incomplete combustion in many cycles due to fuel–air ratios that are too lean for good ignition and rapid flame development. Received: 6 November 2001 / Revised version: 6 May 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-586/986 0176, E-mail: todd.fansler@gm.com     

Amplitude noise suppression in high-repetition-rate pulse train generation from a frequency-modulated Er-Yb laser

A detailed experimental analysis of the amplitude noise in high-repetition-rate picosecond pulse trains generated by spectral filtering of a frequency-modulated Er-Yb:glass laser is reported. Two distinct sources of noise are identified, and stabilization techniques for noise suppression are proposed and experimentally demonstrated. Intensity noise suppression of ∼20 dB, corresponding to less than 2% residual amplitude fluctuations of the pulse train, has been achieved at repetition rates of 2.5 GHz and 5 GHz with pulse durations of ∼50 ps. Received: 28 July 1999 / Revised version: 13 September 1999 / Published online: 20 October 1999     

Short-wavelength far-infrared laser cavity yielding new laser emissions in CD3OH

A significantly improved far-infrared laser has been used to generate optically pumped laser emissions from 26 to 150 μm for CD₃OH. Using an X–V-pumping geometry, several new laser emissions have been found for CD₃OH. In addition, an increase in power, by factors from 10 to 1000, for many of the previously known shorter-wavelength laser lines, below 100 μm, has been observed. Frequency measurements for several lines have also been performed and have been reported to a fractional uncertainty up to ±2×10^-7, permitting the spectroscopic assignment of the laser transition. One of the frequency-measured lines, 44.256 μm observed using the 10R34 pump, has confirmed the assignment of the previously reported FIR emission (n,K;J)=(1,7;20)?(0,8;20)A in the ground vibrational state. Received: 26 October 2000 / Published online: 7 February 2001