Minimum Detectable Absorption Research Articles

Detection of oxygen using short external cavity GaAs semiconductor diode lasers

High sensitivity detection of O(2) in the near infrared with short external cavity (SXC) AlGaAs semiconductor diode lasers is reported. The mode control provided by the SXC enhances the performance of these diode lasers by allowing a number of single laser modes (up to ten) to be individually selected and continuously scanned over extended frequency intervals. At a constant laser heat sink temperature, this provides nearly complete spectral coverage of up to approximately 40 cm(-1) for overlapping modes. When using second harmonic detection techniques, these SXC controlled lasers were found to provide high sensitivity detection of O(2) at atmospheric pressure. A minimum detectable absorbance of approximately 4 x 10(-6) has been achieved with a SNR of 1. The stability of these laser systems has also allowed the continuous monitoring of an absorption signal over extended time intervals. Absorbances of 1 x 10(-2) have been continuously monitored for durations of up to 15 h with an rms uncertainty of +/-6 x 10(-5). These results are reproducible for measurements made on all the laser modes that could be selected with the SXC.

Tunable diode laser frequency modulation spectroscopy through an optical fiber: High-sensitivity detection of water vapor

Frequency modulation spectroscopy through a single-mode optical fiber has been demonstrated for monitoring H2O vapor at 7665 cm−1. Using a 2.5 mW distributed feedback laser and 100 m of optical fiber, a minimum detectable absorption of 5×10−7 was achieved. To obtain this sensitivity limit, very large spurious signals and technical noise were suppressed using a dual-channel detection scheme.

Measurements in the HCl 3 ← 0 band using a near-IR InGaAsP diode laser

Hydrogen chloride gas is measured by absorption in the second overtone band near 1.2microm using an InGaAsP diode laser. The strength and air-broadening coefficient of the H(35)Cl P(3) line are measured. The line strength is ~18% higher than the previous grating spectrometer measurement and suggests that the commonly used experimental value for the 3 ?0 HCl vibrational moment may be low. High-frequency two-tone FM detection is also used in this study to measure trace concentrations of HCl. Using a multimode laser, the sensitivity limit is 3-parts per million HCl in air at 50 Torr, corresponding to a minimum detectable fractional absorption (multimode) of 4 x 10(-6). Optimization of the detection method should permit real-time measurement of concentrations below 0.1 ppm. The low cost and convenient operating characteristics of InGaAsP diode lasers make them attractive for many applications in high-resolution near-IR spectroscopy and trace species detection.

Trace gas detection using 13–μm InGaAsP diode laser transmitter modules

The detection of small absorptions using a laser absorption spectrometer (LAS) based on InGaAsP diode laser transmitter modules has been investigated. The modules are normally employed in optical communication systems and as such operate at 1.3 microm and have single-mode-fibor pigtails to couple the light out. The minimum detectable absorption of the LAS wasfound tobe ~5 x 10(-5) with harmonic detection and ~1 x 10(-4) with sweep integration. The dominant noise source was caused by reflections off the cleaved end of the fiber pigtail. The strength and number of absorption lines in the 0.7-1.6-microm spectral region which are free from interference is considered for the major constituents of the atmosphere. It is found that there arc sufficient strong isolated lines for trace gas detection and monitoring purposes usinga LAS based on InGaAsP lasers and the reported minimum detectable absorption.

Ultratrace Determination of Metals with Dithizone by Thermal Lens Spectrophotometry

A pump-probe thermal lens instrument was constructed, evaluated, and applied to the determination of metals after extraction with dithizone in carbon tetrachloride. An argon-ion laser operated at 514.5 nm, with a power of just 22 mW at the cuvette position and chopped at 2 Hz, and a He-Ne laser were used as a pump and probe, respectively. The thermal lens effect was measured by a pinhole-photodiode and a lock-in amplifier. Evaluation of the setup with an iodine solution in carbon tetrachloride gave a minimum detectable absorbance of 5 × 10−6 (S/N = 3), the upper limit of the dynamic range being 5.7 × 10−3. Relative standard deviation was about 3%. Calibration curves and limits of detection for several metal dithizonates were obtained. From extractions of cadmium dithizonate at concentrations lower than 2 ng mL−1, a LOD of 8 pg mL−1, corresponding to an absolute LOD of 4 fg of cadmium in the probe volume, was achieved.

Optimisation of a compact single-laser-dual-beam thermal lens apparatus

The experimental parameters that determine the thermal lens signal and the length of the optical system are considered in order to develop a simple and compact single-laser-dual-beam set-up for thermal lens spectrometry. The results, expressed in terms of signal amplitude and signal to noise ratio, show that they depend on the focal length of the focusing lens, the distance between the sample cell and the detector, the size of the aperture used to sample the thermal lens signal and the relative power of the pump and probe beams. The performance of the apparatus is optimised in the shortest configuration for which the length of the optical system is less than 0.9 m from the laser head. A minimum detectable absorbance of 8.4 × 10–6 is obtained in ethanol with only 4 mW of excitation power available at the sample cell.

ULTRA-SENSITIVE INTRACAVITY SPECTROSCOPY WITH MULTIMODE LASERS

Intracavity laser spectroscopy is characterized by extreme sensitivity of the emission spectrum to narrow spectral perturbations such as absorption, gain or light injection. Intracavity absorption spectra obey a modified Lambert-Beer law, where the length of the absorption cell is substituted by 1 = c.t, where c is the velocity of light, and t is the duration of the laser pulse. The time resolution of intracavity measurements is limited only by the sensitivity required for the detection of the extinction k, such that i ≥ 1/kc. With the minimum detectable absorption being 10-5 cm-1, e.g., the resolvable time is on the order of a microsecond. The ultimate sensitivity of intracavity spectroscopy with a cw laser is limited by one of two competing factors : spontaneous emission, and non-linear mode interaction, such as stimulated Brillouin scattering. Depending on laser parameters, minimum detectable extinction is in the range 10-7-10-12 cm-1. Non-linear mode interaction can also give rise to distortion of the line shapes observed with intracavity spectroscopy. High sensitivity along with time resolution opens a wide field of practical application for intracavity spectroscopy such as pollution detection, detection of forbidden and non-linear transitions, combustion and plasma diagnostics, and the study of kinetics of molecules and radicals.

Construction of a Highly Sensitive Spectrometer in the Visible Region

A sensitive spectrometer was constructed in order to observe very weak visible absorption lines of molecules using a ring dye laser, a multiple-pass absorption cell, and a Stark modulation technique. The minimum detectable absorption coefficient is 3×10-9/cm and is limited by excess laser-radiation noise of the ring dye laser. This system has been successfully applied for the observation of very weak overtone transitions of NH3 with Δv=5.

An ultrasensitive photoacoustic spectrophotometer for liquids.

The radius of a cylindrical photoacoustic cell for liquids and the phase of the amplification system were optimized, based on the theory of photoacoustic signal detection. As a result of the optimization, the minimum detectable absorption coefficient, calculated from calibra tion curves obtained for cobalt 1-nitroso-2-naphthol chelate xylene solutions was 8.7×10-6cm-1 and the detection limit absorption coefficient of this photoacoustic spectrophotometer, expressed in twice the signal to noise ratio, was 3.8×10-8 cm-1.

Electronic analyzing light signal subtraction and measuring device for transient absorption spectrophotometry

An electronic system is described that subtracts the analyzing light signal level from the analyzing light signal modulated by a transient absorption signal at the output of a photodetector in a transient absorption spectrophotometer. The system enables sensitive measurement of the transient with simultaneous measurement of the analyzing light level immediately before the transient. The shortest rise time is 50 ns. The time window for the measurement can be selected to a maximum of 20 ms with a droop of 20 μV. Input signals of 10 V of either polarity are permitted. The minimum detectable absorption signal is determined by the noise of the photomultiplier used and can be about 0.2% for an experiment without signal averaging.

A secondary laser cavity system for spectrophotometry of trace elements based on intracavit6y quenching and thermal lens effects

A system based on correlated cavities is described for quantitative measurements of small absorbances by means of intracavity quenching and the thermal lens effect. Theory suggesting the possibility of such measurements was confirmed experimentally. Methods for determining traces of arsenic and phosphorus in semiconductor silicon were developed; the procedures are based on the formation of reduced 12-molybdophosphoric and 12-molybdoarsenic heteropoly acids and measurement of absorbances either for aqueous solutions or for extracts into isobutanol. The total effect of intracavity quenching and thermal lensing was applied. The detection limits were 100 pg cm −3 for arsenic and 10 pg cm −3 for phosphorus. Layer-by-layer determination of phosphorus in silicon wafers was possible over a concentration range of 10 15–10 20 atoms cm −3, with a sample area of 1 cm 2 and layer thicknesses of 0.35–0.6 μm. The minimum detectable absorbance via the thermal lens effect was 5 × 10 −5, and that via the total effect of intracavity quenching and thermal lensing was 6 × 10 −6.

LASER-GAS SPECTROPHONE MEASUREMENTS WITH A FIBER-OPTIC INTERFEROMETER

A fiber-optic interferometer is the acoustic sensor in a gas spectrophone. A minimum detectable absorption capability of 1.6 x 10-7 cm-1·W/√HZ has been demonstrated.

Two-wavelength and power-balanced oscillation of a CO2 laser for application to differential absorption measurements

A new technique of alternate two-wavelength oscillation of a CO2 laser is discussed for application to various differential absorption spectroscopic measurements. Power-balanced, two-adjacent branch oscillation using a single CO2 laser was achieved by modulating the angle of a mirror inside the laser cavity and adjusting automatically the cavity length. The two-wavelength modulation frequency was extended up to about 1.2 kHz. Line tuning and power modulation characteristics were studied. The laser was used in long-path differential absorption measurements of ethylene air pollution molecules to demonstrate the capability of this power-balanced, two-wavelength oscillation method. The minimum detectable absorption was nearly 1×10−3 in a short-path cell experiment and 3×10−3 in a long-path experiment.

Measurements of DF laser absorption by methane using an intracavity spectrophone

Absorption coefficient measurements for trace amounts of methane are reported, in a 1-atm argon balance, at seventeen deuterium-fluoride (DF) laser line frequencies. An optoacoustic detector (spectrophone) operating within the DF laser optical cavity yielded a minimum detectable absorption of 0.007 km(-1) (7 x 10(-8) cm(-1)) for a 10:1 SNR. Results are compared with previous measurements and with calculated absorption coefficients obtained using the AFGL atmospheric absorption parameter compilations. A 2.5-fold increase in agreement results when the 1980 AFGL methane parameters are used instead of the 1978 parameters.

A two-wavelength oscillation CO2 laser for differential absorption measurements of atmospheric trace molecules

A two-wavelength oscillation C02 laser was constructed for the differential absorption measurement of low concentrations of atmospheric trace molecules and pollution molecules. A diffraction grating was alternately angle-modulated for the two-branch oscillation using a single C02 laser. The output power of the two wavelengths was balanced to be equal. With these characteristics, the phase-sensitive detection technique can be applied to give highly sensitive detection of the received power difference, which is proportional to the absorption in the optical path of the transmitted beams. A differential absorption spectrometer was used to demonstrate the usefulness of this two-wavelength laser oscillation method incorporating the output power balancing technique. The concentration of ethylene molecules was detected in a calibration cell and was also measured in real time in the exhaust of a combustion engine. From these results it has been shown that this differential absorption scheme offers a minimum detectable absorption of approximately 0.8%.

A study of the K = 3 nA1 - A2 centrifugal splitting in the 3 ν2 and 4 ν2 states of PH 3 using a difference-frequency laser system

A frequency tunable infrared source has been constructed by using the (Ar-laser) - (dyelaser) difference frequency method developed by Pine and applied to the observation of the overtone bands of PH 3 3 ν 2 ← 0 and 4 ν 2 ← ν 2 in the 3.4 μm region and 4 ν 2 ← 0 in the 1.6-μm region. A Stark modulation method was used to increase the sensitivity of detection. For transitions which were well modulated, the minimum detectable absorption coefficient was estimated to be ∼3 × 10 −7 cm −1 using a 3-m cell. Emphasis was placed on the observation of the A 1- A 2 splitting for K = 3 n rotational levels. For the 3 ν 2 state splittings were observed for K = 3, 6, and 9 because PH 3 is a very nearly spherical top in this state. The magnitude and the J dependence of the observed K = 3 n splittings have been analyzed by using a normal symmetric rotor Hamiltonian and a centrifugal distortion term of the form τ xxxz[(J + 3 + J − 3)J z + J z(J + 3 + J − 3)] 4 .

Calibrated Sensitivity Measurements of Nuclear Magnetic Resonance Spectrometers

A definition of sensitivity for NMR spectrometers is proposed. This definition relates sensitivity directly to the minimum detectable steady state NMR absorption χ″. Furthermore, the definition is expressed in a form which makes explicit the sequence of measurements appropriate to its determination. A new solid state calibrator circuit is presented which is both simple and reliable. Its use in performing sensitivity measurements is described.

Scanning Microwave Echo Box Spectrometer

A resonant cavity microwave spectrometer for wavelengths between 3 and 30 cm, which uses radar echo boxes as absorption cells, is described. The minimum detectable absorption coefficient at 10 cm wavelength is 10−10 cm−1, and this sensitivity is maintained during frequency scans by automatic feedback tuning of the echo box. The spectrometer is well suited to studies of gaseous chemical reactions, and some preliminary observations are reported, including the negative results of a search for the 10 cm spectrum of CH, and measurements of OH spectra at 4.7 and 6.0 GHz.

Microwave Spectra of SPCl3, Experimental

The microwave spectra of thiophosphoryl chloride, SPCl 3 , were studied within the frequency region from 2,600 Mc to 2,900 Mc with the cavity spectrometer, of which minimum detectable absorption coefficient was about 5×10 -9 cm -1 . Several absorption lines of the rotational transition of the symmetric and asymmetric top molecules were observed and their temperature effect examined.

Variable-Frequency Microwave Cavity Spectrometer

A variable-frequency, resonant cavity spectrometer was developed for the investigation of molecular gases in the 10-cm spectral region. Either Stark or Zeeman modulation could be applied. A single coaxial cavity, operated in the TEM mode, served as the Stark or Zeeman modulation cell and as the frequency tuning and stabilizing mechanism. The rf power transmitted through the cavity was detected by a bolometer and the signal component at the fundamental modulation frequency amplified and passed to a phase sensitive detector. The minimum detectable absorption coefficient for the spectrometer was determined to be αmin = 1×10−10 cm−1.