Shot-noise-limited dual-beam detector for atmospheric trace-gas monitoring with near-infrared diode lasers

A dual-beam detector is used to measure atmospheric trace species by differential absorption spectroscopy with commercial near-infrared InGaAs laser diodes. It is implemented on the Spectromètre à Diodes Laser Accordables, a balloonborne tunable diode laser spectrometer devoted to the in situ monitoring of CH(4) and H(2)O. The dual-beam detector is made of simple analogical subtractor circuits combined with InGaAs photodiodes. The detection strategy consists in taking the balanced analogical difference between the reference and the sample signals detected at the input and the output of an open optical multipass cell to apply the full dynamic range of the measurements (16 digits) to the weak molecular absorption information. The obtained sensitivity approaches the shot-noise limit. With a 56-m optical cell, the detection limit obtained when the spectra is recorded within 8 ms is ~10(-4) (expressed in absorbance units). The design and performances of both a simple substractor and an upgraded feedback substractor circuit are discussed with regard to atmospheric in situ CH(4) absorption spectra measured in the 1.653-mum region. Mixing ratios are obtained from the absorption spectra by application of a nonlinear least-squares fit to the full molecular line shape in conjunction with in situP and T measurements.     

Electronic subtracter for trace-gas detection with InGaAsP diode lasers

An electronic noise-cancellation scheme has been developed and tested for second-harmonic (2f) detection with short-external-cavity and distributed-feedback InGaAsP diode lasers and wavelength modulation. The 2f background signal and noise from, e.g., optical feedback, optical fringes, and power-supply pickup are effectively reduced by subtraction of a measure of the signal-beam photocurrent from a measure of the reference-beam photocurrent. The dynamic range required for the lock-in amplifier is also reduced because the signal owing to modulation of the laser output at the first harmonic is canceled. Reduction of the 2f background and dynamic range are important for atmospheric-pressure detection where a large wavelength modulation is necessary. The detector noise was minimized by the use of zero-biased detectors in the subtraction circuit. A beam-noise level (defined as 2× the rms value) equivalent to a line-center absorption of 1.6 × 10(-6) was achieved with an equivalent-noise bandwidth of 1.25 Hz for 2f detection at 10 kHz. The electronic circuit is easy to construct and low cost.     

Fiber-amplifier-enhanced photoacoustic spectroscopy with near-infrared tunable diode lasers

A new approach to wavelength-modulation photoacoustic spectroscopy is reported, which incorporates diode lasers in the near infrared and optical fiber amplifiers to enhance sensitivity. We demonstrate the technique with ammonia detection, yielding a sensitivity limit less than 6 parts in 10(9), by interrogating a transition near 1532 nm with 500 mW of output power from the fiber amplifier, an optical pathlength of 18.4 cm, and an integration time constant of 10 s. This sensitivity is 15 times better than in prior published results for detecting ammonia with near-infrared diode lasers. The normalized minimum detectable fractional optical density, alphaminl, is 1.8 x 10(-8); the minimum detectable absorption coefficient, alphamin, is 9.5 x 10(-10) cm(-1); and the minimum detectable absorption coefficient normalized by power and bandwidth is 1.5 x 10(-9) W cm(-1)/square root Hz. These measurements represent what we believe to be the first use of fiber amplifiers to enhance photoacoustic spectroscopy, and this technique is applicable to all other species that fall within the gain curves of optical fiber amplifiers.     

Variable pressure infrared diode laser spectroscopy: a new method for trace-gas monitoring

A new method for measuring trace concentrations of atmospheric pollutants by infrared diode laser spectroscopy has been devised. This method relies on the increase of the signal as the pressure inside the cell increases, while the frequency of the diode is stabilized on the line, even if it is unresolved. Performances of this method were tested with N2O and with 1,3-butadiene. As an example of application, we measured the butadiene emitted by car exhausts. Sensitivity and rapidity of this method are equivalent to the usual scanning method in which the whole line is described, but this new method benefits from its simplicity and robustness.     

Frequency-modulation spectroscopy with blue diode lasers

Frequency-modulation spectroscopy provides ultrasensitive absorption measurements. The technique is especially adaptable to diode lasers, which can be modulated easily, and has been used extensively in the near-infrared and infrared spectral regions. The availability of blue diode lasers now means that the accessible wavelength region can be increased. We successfully demonstrate wavelength-modulation spectroscopy and two-tone frequency-modulation spectroscopy for the weak second resonance line of potassium at 404.8 nm and for the transition at 405.8 nm in lead, starting from the thermally populated 6(p)(2 3)P(2) metastable level. Information on the modulation parameters is obtained with a fitting procedure. Experimental signal-to-noise ratios at different absorption levels are compared with theoretical signal-to-noise ratios and show good agreement. Detection sensitivities of 2 x 10(-6) and 5 x 10(-6) for wavelength and two-tone frequency-modulation spectroscopy, respectively, for a 120-Hz bandwidth are demonstrated.     

Extended-cavity diode lasers with tracked resonances

We present a painless, almost-free upgrade to present extended-cavity diode lasers (ECDLs) that improves the long-term mode-hop-free performance by stabilizing the resonance of the internal cavity to the external cavity. This stabilization is based on the observation that the frequency or amplitude noise of the ECDL is lowest at the optimum laser diode temperature or injection current. Thus, keeping the diode current at the level where the noise is lowest ensures mode-hop-free operation within one of the stable regions of the mode chart, even if these should drift due to external influences. This method can be applied directly to existing laser systems without modifying the optical setup. We demonstrate the method in two ECDLs stabilized to vapor cells at 852 and 895 nm wavelengths. We achieve long-term mode-hop-free operation and low noise at low power consumption, even with an inexpensive non-antireflection-coated diode.     

Lithium isotope separation with tunable diode lasers

A laser-isotope-separation study of lithium has been performed with two-step excitation involving UV laser radiation and a visible tunable-diode laser. The method yields a high degree of selectivity by tuning the narrow-linewidth diode laser to the D1 or D2 levels of the lithium atom. Selective laser excitation is simplified by the use of the tunable diode laser and the overall approach benefits from the application of a compact mass selector that includes a precision magnetic sector and an ion beam that is designed specifically for light atoms such as lithium.     

Frequency-resolved absorption tomography with tunable diode lasers

A tunable diode laser was used for absorption tomography in an axisymmetric atmospheric pressure flat-flame burner. A rapid tomographic inversion algorithm was used to facilitate the many reconstructions at a relatively sparse set of projections typical of laser absorption tomography. Profiles of temperature and CO2 mole fraction were measured simultaneously in methane-air flames. Absorption measurements were made near the R-branch bandhead at 4.17 microm to minimize interferences with other species, while providing good temperature and concentration sensitivity at flame conditions. The procedure showed the advantage of reconstructing detailed spectra at each radial node.     

Dual interband cascade laser based trace-gas sensor for environmental monitoring

The development of an interband cascade laser (ICL) based spectroscopic trace-gas sensor for the simultaneous detection of two atmospheric trace gases is reported. The sensor performance was evaluated using two ICLs capable of targeting formaldehyde (H2CO) and ethane (C2H6). Minimum detection limits of 3.5 ppbV for H2CO and 150 pptV for C2H6 was demonstrated with a 1 s integration time. The sensor was deployed for field measurements of H2CO, and laboratory quantification of both formaldehyde and ethane are reported. A cross comparison of the atmospheric concentration data for H2CO with data collected by a collocated commercial H2CO sensor employing Hantzsch reaction based fluorometric detection was performed.These results show excellent agreement between these two different approaches for trace-gas quantification. In addition, laboratory experiments for dual gas quantification show accurate, fast response with no crosstalk between the two gas channels.     

Properties of diode lasers with intensity noise control

The effect of the reduction of intensity noise by electronic feedback on the frequency stability of a GaAlAs laser is investigated. Only a small increase (approximately 30%) in stability was observed because the intensity and frequency noise were found to be not completely correlated. The technique described can also be used to estimate the coherence function gamma2 between the various noise properties of the laser's emission without using a correlator.     

Multispecies in situ monitoring of a static internal combustion engine by near-infrared diode laser sensors

A multispecies near-infrared diode laser spectrometer has been constructed for measurements of carbon monoxide, carbon dioxide, and methane directly in the exhaust of a static internal combustion engine. A wavelength modulation-division multiplexing scheme was implemented for the two distributed feedback diode lasers. Gas concentration variations were observed for changes in operating conditions such as increasing and decreasing the throttle, adjusting the air-fuel ratio, and engine start-up.     

Wavelength-modulation detection of acetylene with a near-infrared external-cavity diode laser

An external-cavity diode laser operating at 1500 nm was used to record the combination band of acetylene (C(2)H(2)). By combination of wavelength-modulation spectroscopy with a noise-canceler detection circuit, a minimum detectable absorbance of 4.8 × 10(-4) with a 300-ms time constant was achieved, although this result was limited by étalon fringes. When combined with this detection technique, continuous, widely tunable output from an external-cavity laser is ideally suited for high-resolution absorption spectroscopy with excellent sensitivity.     

Polarisation behaviour of diode lasers with frequency selective feedback

The paper reviews recent results obtained with diode lasers used in external hybrid cavities with frequency selective feedback. Such cavities attract continuing interest for several reasons. They generate a tunable single laser mode with very low linewidths (usually a few tens of kilohertz). Very wide discrete tunable ranges over 100 nm for Fabry-Perot type and over 200 nm for quantum well lasers are achieved. They can be made to oscillate in a tunable mode having the desired polarization state,TE orTM and, in some cases, simultaneously atTE andTM. This is done by designing a cavity that increases strongly theTM/TE intensity ratio and by using coatings on one laser facet that greatly lower bothTE andTM reflectivities. High-speed polarization switching in the gigahertz range is possible by inserting passive or active polarization selecting elements in the cavity. For all these reasons hybrid external cavities are attractive for applications in optical metrology, spectroscopy and optical communications. Moreover, the external cavity configuration allows the study of physical mechanisms in the laser diode by inducing on purpose phenomena that would have been otherwise impossible to achieve with free-running lasers.     

External-cavity diode lasers with different devices and collimating optics

Comparative operating characteristics of external-cavity diode lasers (ECDL's) with either a channel substrate planar device or a multi-quantum-well (MQW) device are presented. These include the output beam profile, which is significantly altered depending on the collimating lens used (either multielement or graded index), power versus injection-current characteristics, and the optical frequency and the rf spectra. The coherence lengths of the different laser diode-collimating-lens combinations in the ECDL are measured, and a new method for calculating the coupling coefficient and the coupled values of the internal quantum efficiency and the internal lumped loss is demonstrated for the MQW device.     

Fast tuning of external-cavity diode lasers

We report on a method for characterizing the tuning capabilities of small-bandwidth external-cavity diode lasers. The step response of a piezo-driven Littman-Metcalf external-cavity diode laser and the compensation of its optoelectromechanical frequency response is investigated. When an approximately 50-V compensated Gaussian pulse is applied to the piezo element, a detuning of 13.6 GHz is observed. This modification of the laser is useful for several spectroscopical applications and as a tunable seed laser for lidar applications.     

Single-exposure, photorefractive holographic surface contouring with multiwavelength diode lasers

We studied the use of multiwavelength diode lasers for surface profilometry through holographic recording in sillenite Bi12TiO20 crystals. When such lasers are used, the holographic image from single-exposure recordings appears covered with interference fringes providing information on the surface relief of the object. By taking advantage of the narrow interference fringes due to the multiwavelength emission of the laser, we obtained interferograms by holographic recording with two reference beams, which improves the surface analysis by visual inspection and enhances the profilometry sensitivity.     

2.2 μm cw single-mode diode lasers with thermoelectric cooling

Continuous-wave lasing in the 2.2 μm region is reported for single-mode GaAlAsSb/GaSb/GaInAsSb double-heterostructure diode lasers with thermoelectric cooling. The temperature dependences of the threshold current near room temperature are investigated for the cw and pulsed pumping regimes. A record characteristic temperature T₀=132 K has been achieved. Pis’ma Zh. Tekh. Fiz. 23, 39–45 (August 12, 1997)     

Quantitative species measurements in microgravity flames with near-IR diode lasers

Absolute concentrations of water vapor are measured in microgravity (μ-g), nonpremixed methane, and propane jet flames with diode-laser wavelength modulation spectroscopy. These experiments are performed in the 2.2-s μ-g drop facility at the NASA Lewis Research Center. Abel inversion methods are used to determine time-dependent radial profiles from eight line-of-sight projections across the flames. At all measured heights above the nozzle, water vapor spatial distributions in μ-g flames are much wider than their 1-g counterparts. Radial growth of the water signal continues throughout the drop, verifying earlier suggestions that a steady state is not reached during the duration of the test, despite a quasi-steady flame shape. Large amounts of water vapor are observed at larger radii, at odds with visual (video) observations and numerical predictions.     

Power-scalable system of phase-locked single-mode diode lasers

The direct use of diode lasers for high-power applications in material processing is limited to applications with relatively low beam quality and power density requirements. To achieve high beam quality one must use single-mode diode lasers, however with the drawback of relatively low optical output powers from these components. To realize a high-power system while conserving the high beam quality of the individual emitters requires coherent coupling of the emitters. Such a power-scalable system consisting of 19 slave lasers that are injection locked by one master laser has been built and investigated, with low-power diode lasers used for system demonstration. The optical power of the 19 injection-locked lasers is coupled into polarization-maintaining single-mode fibers and geometrically superimposed by a lens array and a focusing lens. The phase of each emitter is controlled by a simple electronic phase-control loop. The coherence of each slave laser is stabilized by computer control of the laser current and guarantees a stable degree of coherence of the whole system of 0.7. An enhancement factor of 13.2 in peak power density compared with that which was achievable with the incoherent superposition of the diode lasers was observed.