Cavity ring-down spectroscopy of molecularly thin iodine layers

Cavity ring-down spectroscopy (CRDS) has so far mostly been used for measurements in the gas phase. Only in 1999 was a first spectrum of condensed phase published. This spectrum was measured by using a coated plate between the cavity mirrors. Rather than using this method, our measurements were made using the cavity mirrors as a substrate. This way, the scattering losses could be reduced by approximately a factor of 100. In our measurements we investigated molecularly thin layers of iodine. The iodine spectra were taken in the frequency range from 16200 to 17200 cm^-1 using pulsed CRDS. Received: 14 April 2000 / Revised version: 26 July 2000 / Published online: 22 November 2000     

Control of transverse mode pattern in a helium-neon laser using the cat’s eye cavity

This paper introduces an innovative method to control and select transverse mode pattern in a He-Ne laser. The cat’s eye cavity is established by using a cat’s eye reflector as the reflecting mirror. Laser transverse mode pattern varies continuously when the distance between the convex lens and the concave mirror is changed, which form the cat’s eye reflector. By Newton-Cotes method, the numerical solution of two-dimensional diffraction integral equation in the cat’s eye cavity is obtained, the laser operating modes under various parameters are analyzed, and the eigenvalue η_mn and diffraction power loss corresponding with every transverse mode are calculated. The parameters assuring fundamental transverse mode output are figured out, which match the experiment results well. Therefore, a new convenient real-time means for the control and selection of the laser transverse mode is realized.     

Compact diode-side-pumped Nd:YVO<Subscript>4</Subscript> laser in grazing-incidence configuration

The analysis of compact CW diode-side-pumped grazing-incidence-geometry Nd:YVO₄ laser designs is presented. An output power of 5 W (λ=1064 nm) was produced at 17 W of diode pump (conversion efficiency of 30%) in single transverse TEM₀₀ mode operation at high laser beam quality (M_x ²≈1.05 and M_y ²≈1.01). The resonator geometry was analyzed by applying generalized 4×4 matrix modeling of the spatial mode size, including the impact on the laser operation of cavity astigmatism and a thermal lens in the laser slab. The simplicity and compactness of the laser cavities allow their use for technological applications. Received: 31 July 2002 / Published online: 22 January 2003 RID="*" ID="*"Corresponding author. Fax: +44-20/7594-7744, E-mail: m.damzen@ic.ac.uk     

Intracavity laser absorption spectroscopy and cavity ring-down spectroscopy in low-pressure flames

Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) have been used for measurement of the NH₂-radical spectrum near 643 nm. NH₂ was obtained in low-pressure methane/air flat flames doped with minor amounts of ammonia (as low as 0.023%). The NH₂ concentration was measured both by CRDS and ICLAS in the same conditions. This enables us to compare the practical sensitivity of the two methods. Both methods were also used for measurements in a sooting acetylene/air flame (ϕ = 2.6). The comparative advantages of the methods and their complementarities are discussed.     

Detection of N2O by photoacoustic spectroscopy with a compact, pulsed optical parametric oscillator

A pulsed optical parametric oscillator (OPO) operated in an optical cavity with a grazing-incidence grating configuration (GIOPO) was used for sensitive photoacoustic detection of trace quantities of dinitrogen oxide (N₂O). The (ν₁+ν₃) combination vibration band of N₂O was excited with the idler beam of the GIOPO at 2.86 μm using an optical cavity optimized for the idler beam. The linewidth of the GIOPO could be reduced to 0.4 cm^-1, allowing the rotational structure of the absorption spectrum to be resolved. A concentration sensitivity (signal-to-noise ratio=3) of 60 parts in 10⁹ by volume (60 ppb V) N₂O in synthetic air was obtained. This may be sufficient for continuous monitoring of N₂O in the atmosphere. Received: 29 April 2002 / Revised version: 4 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +49-6221/54-4255, E-mail: peter.hess@urz.uni-heidelberg.de     

Additive-pulse mode locking of a diode-pumped Nd:KGd(WO4)2 laser

Self-starting additive-pulse mode locking of a diode-pumped Nd:KGd(WO₄)₂ laser is demonstrated for the first time. An output power of 0.85 W is achieved with 2.3-ps pulses at a repetition frequency of 76.5 MHz. The shortest pulse generated had a duration of 1.9 ps. Received: 2 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +44-141/5482891, E-mail: a.major@phys.strath.ac.uk     

Real-time measurement of nitrogen dioxide in vehicle exhaust gas by mid-infrared cavity ring-down spectroscopy

The application of pulsed cavity ring-down spectroscopy (CRDS) was demonstrated for the measurement of nitrogen dioxide (NO₂) in automotive exhaust gas. The transition of the ν ₃ vibrational band assigned to the antisymmetric stretching mode of NO₂ was probed with a thermoelectrically cooled, pulsed, mid-infrared, distributed feedback, quantum cascade laser (QCL) at 6.13 μm. The measurement of NO₂ in the exhaust gas from two diesel vehicles equipped with different aftertreatment devices was demonstrated using a CRDS-based NO₂ sensor, which employs a HEPA filter and a membrane gas dryer to remove interference from water as well as particulates in the exhaust gas. Stable and sensitive measurement of NO₂ in the exhaust gas was achieved for more than 30 minutes with a time resolution of 1 s.     

等效电路计算同轴输出腔TM310模式的间隙阻抗

设计了外Q值较小,工作于高阶横磁TM310模的X波段速调管单间隙同轴输出谐振腔。用微波等效电路理论计算了高阶横磁TM310模情形六个漂移管的等效间隙中心位置,由此计算腔内等效间隙中心到输出波导内横向膜片的等效长度。以MATLAB编程计算得到同轴谐振腔TM310模式加载矩形波导滤波器输出回路的间隙阻抗,其结果与传统冷测模拟法计算结果吻合。验证了等效长度计算方法的正确性,用于圆柱腔基模的传统微波等效电路理论能用于分析同轴谐振腔高阶横磁模式输出回路,且比传统的冷测模拟法及场分析法更为快捷。     

High-precision pulsed photoacoustic spectroscopy in NO2-N2

A method for high-precision pulsed photoacoustic spectroscopy applied to a simple system for detection of NO₂ traces in nitrogen is presented. The acoustic signal from a closed cell containing NO₂/N₂ samples irradiated by a pulsed visible laser is analyzed in the frequency domain. A signal-processing method to obtain a high-resolution Fourier spectrum of the signal was developed. An accurate fitting of the resonance peaks with Lorentzian profiles gives high-precision determination of the amplitude and width of the resonance peaks. The resonance maximum is proportional to the absorbed energy; therefore, the choice of the laser wavelength, linewidth and frequency stability are critical for a precise calibration due to the fine structure of the NO₂ optical spectrum. The method also allows high-accuracy measurement of the Q of the acoustic cavity. The dependence of Q on the buffer gas pressure is characteristic of an acoustic cavity where energy losses near the walls predominate. Consequently, an important enhancement of sensitivity takes place at high N₂ pressure. Received: 1 June 2001 / Revised version: 27 July 2001 / Published online: 7 November 2001     

Pulsed cavity ring-down spectroscopy of NO and NO2 in the exhaust of a diesel engine

The application of pulsed cavity ring-down spectroscopy has been demonstrated for the in situ quantitative determination of NO and NO₂ in the exhaust of a diesel engine. NO absorption has been monitored at the transition from the Χ²Π ground state to the A²Σ⁺ state at 226 nm. For NO₂, absorption bands in the spectral region from 438 nm to 450 nm were used. At the selected engine conditions, concentrations of 212±22 ppm and 29±4 ppm have been measured for NO and NO₂, respectively, in good agreement with separate chemical exhaust gas analysis. The method is sensitive enough to meet the European Euro V standard directive on NO_x emissions. This communication discusses the relatively simple setup needed for this type of measurement, the problems encountered, as well as the prospects for single-stroke, simultaneous measurements of both NO and NO₂ at the sub-ppm level. Received: 30 November 2001 / Revised version: 18 February 2002 / Published online: 14 March 2002     

Transverse mode locking of Stokes radiation in diode end-pumped Nd:YVO<Subscript>4</Subscript> laser passively Q-switched by Cr<Superscript>4+</Superscript>:YAG

Transverse mode locking of the Stokes component generated in the mode of stimulated Raman scattering self-conversion by a pulsed diode end-pumped Nd:YVO₄/Cr:YAG laser is implemented for the first time under conditions of frequency degeneracy of cavity modes.     

Development of a cw-laser-based cavity-ringdown sensor aboard a spacecraft for trace air constituents

The progress in the development of a sensor for the detection of trace air constituents to monitor spacecraft air quality is reported. A continuous-wave (cw), external-cavity tunable diode laser centered at 1.55 μm is used to pump an optical cavity absorption cell in cw-cavity ringdown spectroscopy (cw-CRDS). Preliminary results are presented that demonstrate the sensitivity, selectivity and reproducibility of this method. Detection limits of 2.0 ppm for CO, 2.5 ppm for CO₂, 1.8 ppm for H₂O, 19.4 ppb for NH₃, 7.9 ppb for HCN and 4.0 ppb for C₂H₂ are calculated. Received: 3 April 2002 / Revised version: 3 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-202/994-5873, E-mail: Houston@gwu.edu     

Cavity ring-down spectroscopy of CH and CD radicals in a diamond thin film chemical vapor deposition reactor

A mixture of H₂ and CH₄ is passed over a hot-wire tungsten filament in a diamond thin film chemical vapor deposition reactor. The resulting CH radicals are measured in absorption using cavity ring-down spectroscopy (CRDS). The concentration of the CH radicals increases as the filament is approached. The rotational temperature measurements indicate a large temperature discontinuity between the filament and the CH in the gas phase. The pathways for CH production were investigated by replacing H₂ by D₂ in the feed gas mixture, which resulted in the exclusive production of CD. From this observation it is concluded that rapid H/D isotope exchange dominates in the gas phase. Nonperiodic temporal oscillations in the CH concentration are observed when a rhenium filament is used in place of a tungsten filament. The oscillations are attributed to the nonperiodic changes in the amount of carbon at the filament surface. Received: 21 August 2000 / Accepted: 23 August 2000 / Published online: 23 May 2001     

Coherent coupling of a single <Superscript>40</Superscript>Ca<Superscript>+</Superscript> ion to a high-finesse optical cavity

We demonstrate coherent coupling of the quadrupole S_1/2D_5/2 optical transition of a single trapped ⁴⁰Ca⁺ ion to the standing wave field of a high-finesse cavity. The dependence of the coupling on temporal dynamics and spatial variations of the intracavity field is investigated in detail. By precisely controlling the position of the ion in the cavity standing wave field and by selectively exciting vibrational state-changing transitions the ion’s quantized vibration in the trap is deterministically coupled to the cavity mode. We confirm coherent interaction of ion and cavity field by exciting Rabi oscillations with short resonant laser pulses injected into the cavity, which is frequency-stabilized to the atomic transition. Received: 23 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. E-mail: christoph.becher@uibk.ac.at RID="**" ID="**"Present address: Time and Frequency Division, National Institute of Standards and Technology, Boulder, CO 80305, USA     

Cavity ring down spectroscopy: detection of trace amounts of substance

We describe several applications of cavity ring-down spectroscopy (CRDS) for trace matter detection. NO₂ sensor was constructed in our team using this technique and blue-violet lasers (395–440 nm). Its sensitivity is better than single ppb. CRDS at 627 nm was used for detection of NO₃. Successful monitoring of N₂O in air requires high precision mid-infrared spectroscopy. These sensors might be used for atmospheric purity monitoring as well as for explosives detection. Here, the spectroscopy on sharp vibronic molecular resonances is performed. Therefore the single mode lasers which can be tuned to selected molecular lines are used. Similarly, the spectroscopy at 936 nm was used for sensitive water vapour detection. The opportunity of construction of H₂O sensor reaching the sensitivity about 10 ppb is also discussed.     

Corrugated neat thin-film conjugated polymer distributed-feedback lasers

Wave-guided thin-film distributed-feedback (DFB) polymer lasers are fabricated by spin coating a PPV-derived semiconducting polymer, thianthrene-DOO-PPV, onto oxidised silicon wafers with corrugated second-order periodic gratings. The gratings are written by reactive ion beam etching. Laser action is achieved by transverse pumping with picosecond laser pulses (wavelength 347.15 nm, duration 35 ps). The DFB-laser surface emission and edge emission are analysed. Outside the grating region the polymer film is used for comparative wave-guided travelling wave laser (amplified spontaneous emission (ASE)) studies. The pump pulse threshold energy density for wave-guided DFB-laser action (4–9 μJ cm^-2) is found to be approximately a factor of two lower than the threshold for wave-guided travelling wave laser action. The spectral width of the DFB laser (down to Δλ_DFB≈0.07 nm) is considerably narrower than that of the travelling wave laser (Δλ_TWL≈14 nm). The DFB-laser emission is highly linearly polarised transverse to the grating axis (TE mode). Only at high pump pulse energy densities does an additional weak TM mode build up. The surface-emitted DFB-laser radiation has a low divergence along the grating direction. For both the DFB lasers and the travelling wave lasers, gain saturation occurs at high excitation energy densities. Received: 7 January 2002 / Revised version: 15 February 2002 / Published online: 14 March 2002     

Controlled two-pulse generation in Q-switch mode from a single laser using Q-modulator with frustrated total internal reflection

In this work a cavity design for double-pulse generation in Q-switched mode from a single laser is proposed, based on the construction of a second laser channel using a FTIR (frustrated total internal reflection) Q-modulator. A time interval between the two pulses from 500 ns to 120 μs is obtained in a Nd:YAG laser. A comparison with other methods and cavity designs for double-pulse generation is presented. The case when this technique is applied on a tunable laser with metastable upper laser level (Cr:LiCAF, Cr:LiSAF, Alexandrite, Co:MgF₂ or other) is also considered. Even though the method presented in the paper does not rely only on the cavity configuration proposed, some advantages can be obtained – both polarization and wavelength-independent tuning without polarization and wavelength restrictions in combination with the possibility of different wavelengths and polarizations in each pulse. Moreover, by using an active medium generating wavelengths around and up to 3 μm, the Pockels-cell-Q-switch optical transmission problems can be avoided. Received: 9 May 2001 / Revised version: 2 August 2001 / Published online: 15 October 2001     

Effective action approach to the Leggett's mode in two-band superconductors

We investigate a collective excitation (Leggett's mode) corresponding to small fluctuations of the relative phase of two condensates in two-band superconductor using the effective “phase only” action. We consider the possibility of observing Leggett's mode in MgB₂ superconductor and conclude that for the known at present values of the two-band model parameters for MgB₂ Leggett's mode arises above the two-particle threshold. Received 10 May 2002 Published online 19 November 2002     

Cavity-ring-down spectroscopic studies of NO2 in the region around 613 nm

Catalytic degradation and diffusion processes of NO₂ were followed by cavity-ring-down spectroscopy (CRDS) at 612.9 nm. The suitability of this absorption method for quasi-continuous, direct quantitative measurements over extended periods of time is demonstrated. The high sensitivity of the method is reflected by the fact that NO₂ concentrations as low as 200 ppb were detected at wavelengths at which the absorption of NO₂ is 12-fold lower than at the absorption maximum at 413 nm. Absorption coefficients of less than 1×10^-7 cm^-1 were measured. Received: 25 February 2000 / Revised version: 4 August 2000 / Published online: 5 October 2000     

Accurate method for the measurement of absorption cross sections of solid-state saturable absorbers

A rigorous approach taking into account both the temporal and spatial distributions of the pulsed probe beam used for transmission measurements of solid-state saturable absorbers is applied to characterise these absorbers accurately. The use of this method can significantly influence the resulting value for the ground-state absorption cross section. It also permits us to avoid the introduction of an artificial excited-state absorption in systems where, according to energy-level diagrams, no such loss should occur. This is illustrated in the case of passive Q-switchers for 1.5-μm lasers such as Co²⁺-doped LaMgAl₁₁O₁₉ and MgAl₂O₄ and Co²⁺- or Cr²⁺-doped ZnSe and ZnS. Received: 21 November 2001 / Revised version: 22 January 2002 / Published online: 14 March 2002