Laser investigations at 269 nm for XeF(D-X) in Ne crystals

From line narrowing in amplified spontaneous emission at the D-X transition (269nm) of XeF in solid Ne a gain coefficient of 3.4 cm^–1 has been derived and ground-state losses of 2.8 cm^–1 have been determined by variation of the absorption length. A dielectric laser cavity has been optimized with the reflectivities R₁=100% and R₂=70% for 1 cm long crystals; laser action has been achieved.     

Flexible laser diode trace gas sensor based on cavity ringdown spectroscopy with an optical fiber-coupled high-finesse external-cavity diode laser

A flexible and portable trace nitrogen dioxide sensor based on cavity ringdown spectroscopy using an optical fiber-coupled high-finesse cavity was successfully demonstrated. Tailoring the spatial mode matching condition of the core of an optical fiber and high-finesse external cavity allows for effective optical feedback into an antireflection-coated laser diode for stable resonant enhancement of the external cavity. The external cavity, which works as a ringdown cavity, could be remotely located from the light source and receiver section by only a single mode optical fiber. The sensitivity was found to be 1.0×10⁻⁷ cm⁻¹ in a compact 1-cm³ ringdown cavity volume.     

Threshold and saturation properties of a solid-state XeF (C-A) excimer laser

A dielectric laser cavity of 1 cm length has been optimized for high gain (7 cm^–1) operation which is achieved in XeF doped Ar crystals. Mode structures on the C-A spectral distribution around 540 nm and far field transverse mode patterns are reported. Photchemical gain burning is observed in the spectral mode structures. The dependence of the laser threshold on pump energy, pumped length and on cavity losses is studied. XeF densities of 7×10¹⁷ cm^–3 and distributed losses of 1.2 cm^–1 are derived. The measured quantum efficiency of 14% and the saturation behaviour are consistently described. Losses by transient aborption and two photon absorption are discussed.     

Diode laser spectroscopy of overtone bands of acetylene

Overtone absorption lines of acetylene in the regions around 12700 and 11800 cm^–1 have been examined by the use of tunable diode lasers in free-running mode. The diode laser emission wavelength was scanned around the gas resonances by simply sweeping its injection current, permitting a direct observation of the absorption line-shapes. Weak overtone absorption lines have been detected by using the wavelength modulation (WM) spectroscopy with 2nd harmonic detection technique and the collisional broadening and shift coefficients have been obtained. The high resolving power and accuracy of the spectrometer permitted a wavenumber error of less than 0.01 cm^–1. The correct interpretation of the absorption signals when detecting the second harmonic in the presence of a sloping background is discussed.     

Coherent tunable far infrared radiation

Tunable, cw, far infrared (FIR) radiation has been generated by nonlinear mixing of radiation from two CO₂ lasers in a metal-insulator-metal, (MIM) diode. The FIR difference-frequency power was radiated from the MIM diode antenna to a calibrated indium antimonide bolometer. Two-tenths of a microwatt of FIR power was generated by 250 mW from each of the CO₂ lasers. Using the combination of lines from a waveguide CO₂ laser, with its larger tuning range, with lines from CO₂, N₂O, and CO₂ isotope lasers promises complete coverage of the entire far infrared band from 100 to 5000 GHz (3–200 cm^–1) with stepwise-tunable cw radiation.Contribution of the National Bureau of Standards, not subject to copyright     

Waveguide CO2 lasers on sequence-band transitions

Two waveguide CO₂ lasers, a quartz waveguide and an alumina waveguide, have been studied on the 00°2–[10°l,02°1]_I,II sequence bands. The use of an intra-cavity hot CO₂ cell, which is a part of the waveguide, suppresses the regular-band transitions. The quartz waveguide laser has a total of 58 lines lasing on both the 9.4 µm and 10.4 µ,m sequence bands. The alumina waveguide laser has 36 lines lasing on the 10.4 µm sequence band and twice the output power of the quartz waveguide laser, whereas lasing on the 9.4 µm sequence band is difficult. The lasers can be operated on the selected single line without line jumping problem. The frequency tuning range of the strong lines is limited by the free spectral range of the cavity.     

Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking

A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEM_oo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm⁻¹) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼5×10⁻¹⁰ cm⁻¹ for 1 s averaging over scans, with a dynamic range of four orders of magnitude.     

Spectroscopic characterization and laser performance of diode-laser-pumped Nd: GdVO4

We have carried out a detailed spectroscopic characterization of Nd: GdVO₄, a new laser crystal with high effective absorption and emission cross sections. The accidental degeneracy of the upper⁴ F _3/2 laser level decreases the number of emission lines and creates -together with the anisotropic crystal field - high emission cross sections (7.6 × 10^–19 cm² at 1.06 µm and 300 K). In addition, the lines are strongly homogeneously broadened (1.6 nm for the 808.4 nm diode-laser pump transition). The temperature dependences of lifetime, linewidths, and cross sections have been determined. Slope efficiencies up to 57% with respect to the absorbed diode-laser pump power and output powers up to 0.8 W have been achieved at 1.06 µm from a 2 mm long crystal. Intracavity second-harmonic generation, using a KTP crystal, is demonstrated.     

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.     

Selective optothermal detection of NO2 and H2O with a frequency-tuned CO waveguide laser

The results are reported of the CO-laser optothermal (OT) detection of impurity gases when their absorption spectra overlap with those of an interfering gas. The influence of the latter was avoided using low gas pressures corresponding to a maximum of the OT sensitivity. Frequency tuned in the 5.2–6.3 m wavelength range, ¹²C¹⁶O and ¹³C¹⁶O waveguide lasers were used. The fine frequency tuning at 490 MHz was achieved for 150 laser transitions of both molecules. The OT sensitivity was estimated by NO₂ detection in the presence of water vapor. The minimal detectable concentration proved to be 60 ppb at P _19–18(14) transition of a ¹²C¹⁶O laser for NO₂ and 75 ppb on P _12–11(13) transition of a ¹³C¹⁶O laser for H₂O.     

Infrared diode laser spectroscopy

Three types of lasers (double-heterostructure 66 K InAsSb/InAsSbP laser diode, room temperature, multi quantum wells with distributed feedback (MQW with DFB) (GaInAsSb/AlGaAsSb based) diode laser and vertical cavity surface emitting lasers (VCSELs) (GaSb based) have been characterized using Fourier transform emission spectroscopy and compared. The photoacoustic technique was employed to determine the detection limit of formaldehyde (less than 1 ppmV) for the strongest absorption line of the v₃ + v₅ band in the emission region of the GaInAsSb/AlGaAsSb diode laser. The detection limit (less than 10 ppbV) of formaldehyde was achieved in the 2820 cm⁻¹ spectral range in case of InAsSb/InAsSbP laser (fundamental bands of v₁, v₅). Laser sensitive detection (laser absorption together with high resolution Fourier transform infrared technique including direct laser linewidth measurement, infrared photoacoustic detection of neutral molecules (methane, form-aldehyde) is discussed.     

Compact tunable mid-infrared laser source by difference frequency generation of two diode-lasers

Two continuous-wave single mode diode-lasers (Hitachi HL 7851G and Toshiba TOLD 9150) are applied as signal and pump sources for difference frequency generation (DFG) in an AgGaS2 crystal with a length of 30 mm. For 90° type I phase matching tunable mid-infrared laser radiation around 5 µm is obtained with an output power of up toP _DFG = 0.2 µW while the diode lasers are operated with powers of 30 and 50 mW at the center wavelengths 682 and 791 nm, respectively. The performance of the diode-laser-DFG system is shown as the absorption spectrum of CO for the P(28) rotational line around 2023 cm^–1 is probed in a 10cm long cell and in the exhaust of an engine.This research was supported by the Technologiestiftung Schleswig-Holstein under contract 94-35i.     

Detailed vibrational population distributions in a CO2 laser discharge as measured with a tunable diode laser

A tunable diode laser (TDL) operating in the 2150–2350 cm^–1 wavenumber region is used to probe a conventional cw CO₂ laser discharge. Absorption lines in more than 25 different vibrational bands are observed, enabling us to determine absolute vibrational populations inall levels of concern to the dynamics of the 10 m CO₂ laser. Levels in thev ₃ mode of CO₂ as high as 00°9 are monitored, and it is found that anharmonic effects play a significant role in the populations of such levels. Thev ₁ andv ₂ mode populations are also investigated in detail, and it is found that these modes are strongly coupled and maintain a common vibrational temperature under all discharge conditions. The use of a TDL is shown to be a powerful technique for investigating the dynamics of infrared molecular lasers.This work was supported in part by the National Science and Engineering Research Council, Canada     

Xe2Cl fluorescence and absorption in self-sustained discharge XeCl lasers

Fluorescence at 490 nm from the triatomic excimer Xe₂Cl^* has been investigated to determine the 308 nm absorption due to this species in an x-ray preionized, self-sustained gas discharge XeCl laser. The dependence of Xe₂Cl^* density on laser intensity (at 308 nm), buffer gas and Xe and HCl partial pressures has been determined for discharges with a peak electrical power deposition of 2.5 GWl^–1. Xe₂Cl^* absorption is estimated to reach 0.6% cm^–1 under non-lasing conditions but decreases to a non-saturable 0.2% cm^–1 for intracavity laser intensity>1 MW cm^–2. XeCl^* and Xe₂Cl^* fluorescence intensities were found to be a similar for both helium and neon buffer gases but laser output was a factor of two greater with a neon buffer.     

Development of a tunable mid-IR difference frequency laser source for highly sensitive airborne trace gas detection

The development of a compact tunable mid-IR laser system at 3.5 μm for quantitative airborne spectroscopic trace gas absorption measurements is reported. The mid-IR laser system is based on difference frequency generation (DFG) in periodically poled LiNbO₃ and utilizes optical fiber amplified near-IR diode and fiber lasers as pump sources operating at 1083 nm and 1562 nm, respectively. This paper describes the optical sensor architecture, performance characteristics of individual pump lasers and DFG, as well as its application to wavelength modulation spectroscopy employing an astigmatic Herriott multi-pass gas absorption cell. This compact system permits detection of formaldehyde with a minimal detectable concentration (1σ replicate precision) of 74 parts-per-trillion by volume (pptv) for 1 min of averaging time and was achieved using calibrated gas standards, zero air background and rapid dual-beam subtraction. This corresponds to a pathlength-normalized replicate fractional absorption sensitivity of 2.5×10^-10 cm^-1. Received: 29 April 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-303/497-1492, E-mail: dr@ucar.edu     

Relaxation-oscillations in the C → A laser emission of XeF in Ar crystals

Transversely pumped 1 cm long crystals are operated in a plan-parallel cavity with dielectric mirrors of 100% and 30% reflectivity. Relaxation-oscillations in the laser emission at 540 nm with frequencies in the range of (1 to 1.8)·10⁹ s^–1 are observed. A square root dependence on pump rate is shown and an internal loss coefficient of 5.4 cm^–1 is derived.On leave from the Polish Academy of Sciences IF-FM, Fiszera 14, PL-80-952 Gdansk, Poland     

Modeling of high-pressure 12-μm NH3 lasers

Experimental measurements of small-signal gain in an optically-pumped NH₃ amplifier are carried out at pressures ranging from 40 Torr to 760 Torr, and the results are used to validate a rate-equation model describing the amplifier dynamics. The gain measurements show that dilute mixtures of <0.5% NH₃ in N₂ are reqired to minimize the problems of gas heating due to pump absorption. The model is used to extrapolate the results to gas pressures of several atmospheres, and to demonstrate the potential for highpressure operation of optically-pumped NH₃ lasers. For a pump intensity of 100 MW/cm², calculations indicate that operation of an NH₃–N₂ laser is feasible up to a pressure of 10 atm, which would provide a maximum continuous tuning range of 4 cm^–1. High-resolution spectroscopy reveals that gain on a few NH₃ transitions is eliminated at high pressures due to the presence of overlapping absorptions in other NH₃ bands.     

Luminescence of rare gas crystals at high excitation densities for VUV laser applications

Exciton densities of the order of 10¹⁸ cm^–3 are generated in 0.1–0.3 mm thick surface layers in an area of 10×20 mm² of optically clear rare gas crystals. The quantum efficiencies at 126 nm (Ar), 145 nm (Kr), and 172 nm (Xe) remain near 0.5 even for the highest excitation densities. The corresponding gain coefficients of 2.6 cm^–1 (Ar) to 18 cm^–1 (Xe) exceed those of high pressure gas lasers by a factor of 20. Stimulated emission is inferred by observing the line narrowing, the dependence of intensities and time courses on excitation density and amplification measurements. The net gain coefficient is reduced however to 0.5–1 cm^–1 by transient absorption of excited centers and scattering by irradiation induced defects. The results are analysed by a system of rate equations for the excitation, relaxation, quenching, and amplification processes. A peculiar temperature dependence of the quantum efficiencies and time courses is attributed to electron trapping at grain boundaries.     

Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 μm

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS₂. IR frequency tuning between 3076 and 3183 cm^–1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)^–1/2.