Broadly tunable difference-frequency spectrometer for trace gas detection with noncollinear critical phase-matching in LiNbO3

3 is reported. Using an external cavity diode laser, tunable from 795 to 825 nm, and a cw diode-pumped Nd:YAG ring laser at 1064 nm, we produced narrowband mid-ir laser light between 3.16 and 3.67 μm. This broad tuning range of 440 cm^-1 can be critically phase-matched by changing the external crystal angle within a range of only 0.4°. Even for a fixed crystal angle broadband phase-matching over 230 cm^-1 was achieved. No realignment was required when tuning the laser over the whole wavelength range, which enabled the use of a compact 36-m multipass cell as gas chamber. The conversion efficiency could be improved by almost a factor of 3 by applying noncollinear instead of collinear phase-matching, resulting in 30 nW of idler power with good beam quality. Spectra of methane were recorded in laboratory air, which demonstrate the rapid and continuous broad tunability at high sensitivity, enabling sub-ppm detection. Finally, wavelength modulation spectroscopy at high resolution was applied as a promising tool for further inproving the performance of this laser spectrometer. Received: 16 March 1998/Revised version: 1 July 1998     

High efficiency second harmonic generation with a low power diode laser

\valunit{8}{mW} ( corrected for the output mirror reflection) of the cw coherent blue light around by frequency doubling of only from a diode laser. With IR power of we reach the doubling efficiency of . The overall conversion efficiency from the electrical power into the blue light is . By the way of careful analysis of the Blue Light Induced IR Absorption (BLIIRA) in the potassium niobate based external doubling cavity we obtain good agreement with the theoretical conversion efficiency. Received: 29 November 1996     

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     

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 .     

A tunable light source in the 370 nm range based on an optically stabilized,frequency-doubled semiconductor laser

A tunable harmonic output power of 18 W at a wavelength of =370 nm is obtained by resonance-enhanced frequency doubling of an optically-stabilized semiconductor laser. A commercially available AlGaAs laser diode which emits a maximum power of 10 mW at =740 nm is operated in an extended-cavity configuration. Dispersion prisms are used in the extended cavity to obtain longitudinal-mode selection with low loss of optical power. The output is focussed into an optically isolated high-finesse ring resonator which contains a LiIO₃ crystal for second-harmonic generation. One potential application of this laser source is the optical excitation and laser cooling of ytterbium in an ion trap. In a related demonstration experiment, the frequency-doubled diode laser is applied to excite the =369.5 nm ² S _1/2-² P _1/2 transition of ytterbium ions in a hollow-cathode discharge.     

Continuous-wave light source at 317 nm based on frequency doubling of a diode laser

Received: 16 October 1997     

Analytical descriptions of mode switching characteristics of diode lasers

Using perturbation analysis to solve the rate equations of semiconductor lasers, the analytical expressions for the evolution of the carrier and photon densities have been derived when the laser is switched from one oscillation mode (or wavelength) to another. The solutions show that the deviations of the carrier and photon densities are damped and oscillatory. The decay rate of these deviations depends not only on the carrier lifetime, but also on the photon lifetime, which is different from that of an ordinary diode laser experiencing switch-on processes. Received: 5 December 2000 / Revised version: 5 March 2001 / Published online: 7 June 2001     

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     

Oxygen measurements at high pressures with vertical cavity surface-emitting lasers

Measurements of oxygen concentration at high pressures (to 10.9 bar) were made using diode-laser absorption of oxygen A-band transitions near 760 nm. The wide current-tuning frequency range (>30 cm^-1) of vertical cavity surface-emitting lasers (VCSELs) was exploited to enable the first scanned-wavelength demonstration of diode-laser absorption at high pressures; this strategy is more robust than fixed-wavelength strategies, particularly in hostile environments. The wide tuning range and rapid frequency response of the current tuning were further exploited to demonstrate wavelength-modulation absorption spectroscopy in a high-pressure environment. The minimum detectable absorbance demonstrated, ∼1×10^-4, corresponds to ∼800 ppm-m oxygen detectivity at room temperature and is limited by etalon noise. The rapid- and wide-frequency tunability of VCSELs should significantly expand the application domain of absorption-based sensors limited in the past by the small current-tuning frequency range (typically <2 cm^-1) of conventional edge-emitting diode lasers. Received: 26 July 2000 / Revised version: 2 January 2001 / Published online: 20 April 2001     

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     

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     

Intracavity photoacoustic gas detection with an external cavity diode laser

The first use of an external cavity diode-laser light source in combination with a photoacoustic detector for high-sensitivity gas detection is described. This combined system is applicable for detecting gases with absorption coefficients as low as 5 x 10^–8 cm^–1 by operating the photoacoustic cell in an intracavity mode. Measurements were made on the 1.13 m absorption lines of water vapour. For quantitative measurements, it was found to be necessary to introduce a reference cell into the system.     

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.     

All-diode-laser ultraviolet absorption spectroscopy for sulfur dioxide detection

Ultraviolet radiation around 302 nm was produced by sum-frequency generation in beta-barium borate employing a blue and a near-infrared diode laser. The diode-laser-based spectrometer has a mode-hop-free tuning range of 50 GHz and was used for high-resolution spectroscopic detection of sulfur dioxide. Differential measurements of several gas concentrations, at low pressure as well as at ambient atmospheric pressure, were demonstrated. The detection limit in the first implementation of the instrument was about 20 ppm m at atmospheric pressure, essentially limited by interference fringes. Methods for greatly improving this limit are suggested.     

Fundamental noise sources in a high-sensitivity two-tone frequency modulation spectrometer and detection of CO2 at 1.6 μm and 2 μm

2 , and its sensitivity is 7(2)×10^-8 in a 1-Hz bandwidth. The corresponding minimum detectable concentration of CO₂ in air has been estimated to be 1 ppm · m. This opens the possibility of a detection at ppb levels at 2 μm, where a two orders of magnitude increase in the CO₂ absorption signal is demonstrated. Received: 06 April 1998/Revised version: 02 July 1998     

Tunable frequency-controlled laser source in the near ultraviolet based on doubling of a semiconductor diode laser

Continuously tunable ultraviolet laser radiation at 397 nm was generated by doubling the output of a semiconductor diode laser. The fundamental radiation was provided by a 150 mW AlGaAs laser diode injected by a low-power AlGaAs laser diode which was frequency stabilized by optical feedback using a new scheme of a miniature external cavity. Second-harmonic generation was produced in a lithium-triborate crystal placed in a compact enhancement cavity. The fundamental radiation was used for sub-Doppler spectroscopy of the Ar I 4s ³ P ⁰ ₀–4p ¹ P ₁ transition at 795 nm; the second-harmonic radiation was used for spectroscopy of the Ca II 4² S _1/2–4² P _1/2 transition at 397 nm.     

Second-harmonic generation of 405-nm light using periodically poled KTiOPO4 pumped by external-cavity laser diode with double grating feedback

A frequency-doubled laser diode system for generation of blue–UV light is described. The system is based on an external-cavity high-power laser diode with double feedback from the zeroth and the first orders of a diffraction grating. Light at 405 nm is generated in a single-pass configuration using periodically poled KTiOPO₄. We show that the double grating feedback improves the second harmonic conversion efficiency by several orders of magnitude as compared to the freely running laser. The conversion efficiency may be improved further such that higher second-harmonic powers may be generated.     

Remote sensing of volcanic gases with a DFB-laser-based fiber spectrometer

We demonstrate monitoring of H₂O and CO₂ emitted in a volcanic area, using a spectrometer equipped with two distributed feedback (DFB) semiconductor diode lasers. Each laser is resonant with a molecular species and is fiber-coupled to allow remote operation of the spectrometer. Recordings of H₂O and CO₂ lines made at the Solfatara volcano, in southern Italy, are shown, and the application of such a spectrometer as a new tool for the continuous monitoring and surveillance of volcanoes is discussed. Received: 28 June 1999 / Revised version: 20 December 1999 / Published online: 23 February 2000     

Extended-cavity grating-tuned operation of mid-infrared InAsSb diode lasers

2 cryostat that permits placement of both the collimation optics and the extended-cavity elements within a vacuum chamber. We have observed tuning of the laser frequency over a 1.44 THz (48 cm^-1) range by rotation of the external grating. This result was obtained without anti-reflection coating of the laser output facet. Received: 8 July 1997/Revised version: 31 July 1997     

Narrow bandwidth operation of high-power broad-area diode laser using cascaded phase-conjugate injection locking

A broad-area laser is injection-locked by another broad-area laser that is also injection-locked by a single-mode diode laser. Two double-phase conjugate mirrors of photorefractive BaTaO₃ are used to couple the master laser beams to the first slave laser, and the first slave laser output to the second slave laser. One of the double-phase conjugate mirrors is built up with the beams from two broad-area lasers. Two slave lasers are oscillating in single longitudinal mode at 808.5 nm and the spectral width is the same as that of the master laser. Final single-mode output power from the second slave broad-area laser is 840 mW, which is limited by the power of the injection beam. This work verifies the possibility of the multi-stage cascaded injection locking of high-power diode lasers with phase-conjugate injection. Received: 18 November 1998 / Revised version: 29 January 1999 / Published online: 7 April 1999