Noise Equivalent Absorption Sensitivity Research Articles

Sensitivity enhancement in off-axis integrated cavity output spectroscopy.

We report on a detailed model of an improved three mirror off-axis integrated cavity output spectroscopy (OA-ICOS) setup, which re-injects the light reflected by the optical cavity. The model simulates the impact of design parameters on instrument sensitivity and can be used for any off-axis configuration. We demonstrate the application of this model for the real-time detection of ethylene with a pulsed quantum cascade laser (QCL). The three mirror OA-ICOS scheme provides a 10 times increase in signal-to-noise ratio as compared to standard OA-ICOS, resulting in a noise equivalent absorption sensitivity of 1.5 x 10(-8) cm(-1) Hz(-1/2).

Three mirror off axis integrated cavity output spectroscopy for the detection of ethylene using a quantum cascade laser

We demonstrate the performance of a compact and robust gas sensor based on a pulsed distributed feedback quantum cascade laser (QCL, laser pulses 30ns, repetition rate 1MHz) in combination with off-axis integrated cavity output spectroscopy (OA-ICOS) in an improved 3 mirror configuration. The room temperature laser (wavelength 915cm−1) was wavelength tuned over ∼0.25cm−1 with a 4kHz repetition frequency. A detection limit of 10 ppbv (part-per-billion volume) for regular OA-ICOS is demonstrated for ethylene in 2min averaging time, which is equal to a noise equivalent absorption sensitivity (NEAS) of 1×10−8cm−1Hz−1/2. An improved three mirror configuration OA-ICOS scheme showed a four times increase in sensitivity as compared to standard OA-ICOS, resulting in a NEAS of 2.5×10−9cm−1Hz−1/2. The sensor was used for ethylene detection from apples, stored under controlled atmosphere conditions.

Very high finesse optical-feedback cavity-enhanced absorption spectrometer for low concentration water vapor isotope analyses

So far, cavity-enhanced absorption spectroscopy (CEAS) has been based on optical cavities with a high finesse F that, however, has been limited by mirror reflectivity and by cavity transmission considerations to a few times 10,000. Here, we demonstrate a compact near-infrared optical-feedback CEAS instrument for water vapor isotope ratio measurements, with F>140,000. We show that this very high finesse can be effectively exploited to improve the detection sensitivity to the full extent predicted by the increased effective path length to reach a noise equivalent absorption sensitivity of 5.7×10(-11) cm(-1) Hz(-1/2) for a full spectrum registration (including possible effects of interference fringes and fit model inadequacies).

Off-beam quartz-enhanced photoacoustic spectroscopy with LEDs

A photoacoustic ozone sensor based on quartz-enhanced photoacoustic spectroscopy is presented. Instead of a laser, a UV-LED at 285 nm is utilized as light source. Using an acoustical resonator in off-beam configuration, it is feasible to align the light of the LED through the resonator tube. The sensor is integrated in a miniaturized flow-through gas cell which allows fast gas exchange. The sensor performance and the influence of the speed of sound on the measurement signal are investigated. For the detection of ozone in the Hartley band, a detection limit of S = 1.27 ± 0.08 ppmv (1σ) and a noise equivalent absorption sensitivity of D = (3.02 × 10−8 ± 1 × 10−10) cm−1W(Hz)−1/2 have been achieved.

Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy

A combination of optical feedback self-locking of a continuous-wave distributed feedback diode laser to a V-shaped high finesse cavity, laser phase modulation at a frequency equal to the free spectral range of the V-cavity and detection of the transmitted laser beam at this high modulation frequency is described for possible application in cavity-enhanced absorption spectroscopy.In order to estimate an absorbance baseline noise of laser intensity and frequency modulated light triplet passed through the V-cavity in open air, a 1.5-cm long optical cell filled by C2H2 at low pressure was placed behind the cavity output mirror. The performance of the setup was evaluated from the experimental bandwidth normalised relative intensity noise on the cavity output and the frequency modulation absorption signals induced by C2H2 absorption in the 1.5-cm cell. From these data we estimate that the noise-equivalent absorption sensitivity of 2.1*10^-11 cm^-1 Hz^-1/2 by a factor of 11.7 above a shot-noise limit can be achieved for C2H2 absorption spectra extracted from the heterodyne beat signals recorded at the transmission maxima intensity peaks of the successive TEM00 resonances.

Absolute density measurement of SD radicals in a supersonic jet at the quantum-noise-limit

The absolute density of SD radicals in a supersonic jet has been measured down to (1.1 ± 0.1) × 10(5) cm(-3) in a modestly specified apparatus that uses a cross-correlated combination of cavity ring-down and laser-induced fluorescence detection. Such a density corresponds to 215 ± 21 molecules in the probe volume at any given time. The minimum detectable absorption coefficient was quantum-noise-limited and measured to be (7.9 ± 0.6) × 10(-11) cm(-1), in 200 s of acquisition time, corresponding to a noise-equivalent absorption sensitivity for the apparatus of (1.6 ± 0.1) × 10(-9) cm(-1) Hz(-1/2).

Fiber-Coupled Ozone Sensor Based on Tuning Fork-Enhanced Interferometric Photoacoustic Spectroscopy

In this paper, a completely fiber-coupled ozone sensor based on tuning fork (TF)-enhanced photoacoustic spectroscopy is presented. Crystalline silicon micro-TFs are processed and characterized. Their photoacoustically induced motion is monitored by a compact intensity interferometer. An acoustic microresonator for this new type of TFs is optimized. A frequency-doubled Nd:YAG laser is used as an excitation source. The sensor performance is assessed by calibration measurements. It reaches a detection limit of S = 2.13 ppm ± 0.02 ppm and a noise-equivalent absorption sensitivity of D = (1.27 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> ± 1.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-11</sup> ) cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> W(Hz) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/2</sup> .

Optical parametric oscillator based off-axis integrated cavity output spectroscopy for rapid chemical sensing

An optical parametric oscillator (OPO), pumped by a fiber-amplified diode laser, is combined with off-axis integrated cavity output spectroscopy (OA-ICOS). The cw OPO (power 1.2 W, tunability 3-4 μm, 5 cm(-1) mode-hop-free tuning) has a tuning speed of 100 THz/s, which is ideal for rapid and sensitive trace gas detection. Combined with OA-ICOS, a detection limit of 50 parts per trillion by volume (1×10(12)) of ethane (C(2)H(6)) in nitrogen was obtained in 0.25s at 2997 cm(-1), corresponding to a noise equivalent absorption sensitivity of 4.8×10(-11) cm(-1) Hz(-1/2). The system demonstrates real-time measurements of methane and water in exhaled human breath.

Rapid and sensitive trace gas detection with continuous wave Optical Parametric Oscillator-based Wavelength Modulation Spectroscopy

A fiber-amplified Distributed Bragg Reflector diode laser is used to pump a continuous wave, singly resonant Optical Parametric Oscillator (OPO). The output radiation covers the 3–4 μm with ability of rapid (100 THz/s) and broad mode-hop-free tuning (5 cm−1). Wavelength Modulation Spectroscopy is combined with the OPO to take optimal advantage of the spectral scan speed. The sensitivity of the system was determined as 0.8 ppbv (parts-per-billion by volume) for ethane (C2H6) for the absorption peak at 2996.9 cm−1 recorded in 1.3 seconds, corresponding to a noise equivalent absorption sensitivity (NEAS) of 1.2×10−9 cm−1/Hz1/2. A comparison between results using the 1st, 2nd and 4th harmonic derivative signal from wavelength modulation was performed. The broad continuous tunability was demonstrated by covering 35 cm−1 while recording absorption features of ethane, methane and water.

Multipass optical cell based upon two cylindrical mirrors for tunable diode laser absorption spectroscopy

A compact multipass optical cell based upon a front cylindrical mirror with a central input/output hole and a rear cylindrical mirror is described for application to tunable diode laser absorption spectroscopy. Rotation of the rear cylindrical mirror and small variations of the cell base length (∼110 mm) have been used to achieve different spot patterns and path lengths. CO2 and CH4 direct absorption measurements using the developed cell with a fibre-coupled distributed-feedback laser at 1605.5 nm have been performed. A noise-equivalent absorption sensitivity of 2.5×10-8 cm-1 Hz-1/2 has been determined for 138 passes in the cell and averaging of 2000 consecutive scans within 1.93 s.

Gas-phase photoacoustic sensor at 8.41 μm using quartz tuning forks and amplitude-modulated quantum cascade lasers

We demonstrate the performance of a novel infrared photoacoustic laser absorbance sensor for gas-phase species using an amplitude-modulated quantum cascade (QC) laser and a quartz tuning fork microphone. The photoacoustic signal was generated by focusing 5.3 mW of a Fabry–Perot QC laser operating at 8.41 μm between the tines of a quartz tuning fork which served as a transducer for the transient acoustic pressure wave. The sensitivity of this sensor was calibrated using the infrared absorber Freon 134a by performing a simultaneous absorption measurement using a 31-cm absorption cell. The power and bandwidth normalized noise equivalent absorption sensitivity (NEAS) of this sensor was determined to be D=2.0×10-8 W cm-1/Hz1/2. A corresponding theoretical analysis of the instrument sensitivity is presented and is capable of quantitatively reproducing the experimental NEAS, indicating that the fundamental sensitivity of this technique is limited by the noise floor of the tuning fork itself.

An off-axis cavity-enhanced absorption spectrometer at 1605 nm for the 12CO2/13CO2 measurement

A distributed feedback diode laser sensor based upon off-axis cavity-enhanced absorption spectroscopy at 1605.5 nm has been developed for 13C16O2/12C16O2 isotope ratio measurements in synthetic air and human breath. A noise-equivalent absorption sensitivity of 3.9×10-10 cm-1 Hz-1/2 has been determined for a cavity base length of 28.2 cm and averaging 4000 scans within 8.688 s. For 5% CO2 concentration measurements, δ13C standard deviations of 1.8 ‰ and 3.7 ‰ have been estimated for five successive measurements based on peak height and integrated area estimations at 107.9 Torr, respectively. The contributions of amplified spontaneous emission of the laser and a radiation that is spatially uncoupled into the cavity mode have been described for cavity transmittance measurements. The limitations of the developed sensor and further steps towards precision and accuracy improvements are discussed.