Noise-equivalent Detection Limit Research Articles

Integrated fiber-optic Fabry–Perot vibration/acoustic sensing system based on high-speed phase demodulation

A fiber-optic Fabry–Perot (F-P) vibration/acoustic sensing system based on high-speed phase demodulation was developed. The demodulation part is mainly composed of a super luminescent diode (SLD), a miniature high-speed spectral module, and a field programmable gate array (FPGA) based spectral signal acquisition and processing circuit. The spectral acquisition rate is up to 20 kHz. Real-time spectral acquisition, fast Fourier transform (FFT), frequency estimation, phase calculation, and mode hopping suppression are all performed by FPGA to obtain absolute cavity length of a F-P interferometric sensor. As a result, the integrated design significantly improves the speed and stability of the phase demodulation. The performance of the system was verified by a fiber-optic F-P cantilever microphone. Experimental results show that the sensing system can realize highly sensitive detection of acoustic signals with the frequency between 20 Hz and 8 kHz, which basically covers the main range of human voice frequencies. The noise equivalent minimum detection limit of sound pressure reaches 2.8 μPa/Hz1/2 at 1 kHz. The designed fiber-optic acoustic sensing system has the features of resistance to electromagnetic interference, intrinsic safety, remote detection and small size. It can be used for vibration monitoring of transformers and large structures, voice communication in coal mines and nuclear magnetic resonance rooms.

Versatile Optofluidic Fabry-Perot Sensor for Multiple Physical Parameters in Microfluidic Chips

An optofluidic sensor, which is composed of a fiber Fabry-Perot interferometer (FPI) and a microfluidic chip, is experimentally demonstrated to measure aerostatic pressure, local temperature, and fluidic flow rate in microfluidic chips. The aerostatic pressure sensitivity is as high as −2393.3 nm/Bar. By demodulating the aerostatic pressure and temperature with a sensitivity matrix, both the physical parameters can be predicted simultaneously. The noise-equivalent detection limit (NEDL) of aerostatic pressure is as low as 48.4 μBar. In addition, the experiment results show that the fluidic flow rate sensitivity is −1.8574 nm/(μl/min) and the NEDL of fluidic flow rate is 97.1 nl/min, respectively. The miniature optofluidic sensor based on the optical fiber Fabry-Perot interferometer provides a promising cost-effective sensing platform for monitoring multiple physical parameters in the microfluidic chips, which is of great significance for on-chip biochemical reactions.

Intracavity absorption spectroscopy of HCl isotopes, H2O, CH4, C2H4, and C2H6 in the 3.1-3.4 µm spectral range using a Cr:CdSe laser.

We demonstrate the first application of a Cr:CdSe laser for highly-sensitive multicomponent intracavity absorption spectroscopy around λ = 3.1-3.4 µm. A detection scheme based on an integrated recording of multiple (∼70) individual Cr:CdSe laser pulses after a single pump-pulse excitation is reported. The sensitivity of our system corresponds to an effective absorption path length of Leff ≈ 850 m. Exemplary measurements of atmospheric H2O and CH4, and additionally introduced gas-phase HCl, C2H4, or C2H6 are presented. The achieved noise-equivalent detection limits are in the ppb range. Possibilities for further sensitivity enhancement by up to a factor of 104 are discussed.

Design and structural optimization of T-resonators for highly sensitive photoacoustic trace gas detection

Thanks to the available ultra-wide wavelength range compared with broadband laser sources, the use of blackbody radiators in photoacoustic spectroscopy features the simultaneous detection of multiple gas species in the presence of cross-interfering absorption lines. The major problem associated with broadband incoherent sources is less power and less stable intensity per wavenumber than lasers and leads to limited gas detection sensitivity. In this paper, the detectivity of a broadband double optical path differential photoacoustic system was enhanced with the development of dimension-optimized high-responsivity T-resonators for simultaneous multiple trace gas detection. Enhanced Q-factor and external noise suppression level constitute dual criteria for the optimization of T-resonators. Three digital signal processing algorithms were separately investigated which further improved gas dectectability. The capability of the multiple–trace-gas detection framework was verified by measuring CO2, C2H2 and H2O simultaneously. The spectral results processed by a wavelet denoising algorithm present the best performance in terms of background noise suppression and spectral feature fidelity. [Q2.2] With the absorption enhancement of the optimized T-cells and background suppression of the wavelet denoising algorithm, a broadband differential photoacoustic system was achieved with only a 30 mW globar source and normalized noise equivalent absorption coefficient value of 4.1 × 10−10 W·cm−1·Hz−1/2 which is two orders of magnitude improvement over the original T-cell-based photoacoustic configuration. The noise equivalent detection limits were found to be 223 ppbv for CO2, 625 ppbv for C2H2 and 865 ppbv for H2O, respectively.

A Versatile Multiple-Pass Raman System for Industrial Trace Gas Detection.

The fast and in-line multigas detection is critical for a variety of industrial applications. In the present work, we demonstrate the utility of multiple-pass-enhanced Raman spectroscopy as a unique tool for sensitive industrial multigas detection. Instead of using spherical mirrors, D-shaped mirrors are chosen as cavity mirrors in our design, and 26 total passes are achieved in a simple and compact multiple-pass optical system. Due to the large number of passes achieved inside the multiple-pass cavity, experiments with ambient air show that the noise equivalent detection limit (3σ) of 7.6 Pa (N2), 8.4 Pa (O2) and 2.8 Pa (H2O), which correspond to relative abundance by volume at 1 bar total pressure of 76 ppm, 84 ppm and 28 ppm, can be achieved in one second with a 1.5 W red laser. Moreover, this multiple-pass Raman system can be easily upgraded to a multiple-channel detection system, and a two-channel detection system is demonstrated and characterized. High utilization ratio of laser energy (defined as the ratio of laser energy at sampling point to the laser output energy) is realized in this design, and high sensitivity is achieved in every sampling position. Compared with single-point sampling system, the back-to-back experiments show that LODs of 8.0 Pa, 8.9 Pa and 3.0 Pa can be achieved for N2, O2 and H2O in one second. Methods to further improve the system performance are also briefly discussed, and the analysis shows that similar or even better sensitivity can be achieved in both sampling positions for practical industrial applications.

Sensitive multi-species trace gas sensor based on a high repetition rate mid-infrared supercontinuum source.

We present a multi-species trace gas sensor based on a high-repetition-rate mid-infrared supercontinuum source, in combination with a 30 m multipass absorption cell, and a scanning grating spectrometer. The output of the spectrometer is demodulated by a digital lock-in amplifier, referenced to the repetition rate of the supercontinuum source. This improved the detection sensitivity of the system by a factor 5, as compared to direct baseband operation. The spectrometer provides a spectral coverage of 950 cm-1 (between 2.85-3.90 µm) with a resolution of 2.5 cm-1 in 100 ms. It can achieve noise equivalent detection limits in the order of 100 ppbv Hz-1/2 for various hydrocarbons, alcohols, and aldehydes.

On-line analysis and in situ pH monitoring of mixed acid fermentation by Escherichia coli using combined FTIR and Raman techniques

We introduce an experimental setup allowing continuous monitoring of bacterial fermentation processes by simultaneous optical density (OD) measurements, long-path FTIR headspace monitoring of CO2, acetaldehyde and ethanol, and liquid Raman spectroscopy of acetate, formate, and phosphate anions, without sampling. We discuss which spectral features are best suited for detection, and how to obtain partial pressures and concentrations by integrations and least squares fitting of spectral features. Noise equivalent detection limits are about 2.6 mM for acetate and 3.6 mM for formate at 5 min integration time, improving to 0.75 mM for acetate and 1.0 mM for formate at 1 h integration. The analytical range extends to at least 1 M with a standard deviation of percentage error of about 8%. The measurement of the anions of the phosphate buffer allows the spectroscopic, in situ determination of the pH of the bacterial suspension via a modified Henderson-Hasselbalch equation in the 6–8 pH range with an accuracy better than 0.1. The 4 m White cell FTIR measurements provide noise equivalent detection limits of 0.21 μbar for acetaldehyde and 0.26 μbar for ethanol in the gas phase, corresponding to 3.2 μM acetaldehyde and 22 μM ethanol in solution, using Henry’s law. The analytical dynamic range exceeds 1 mbar ethanol corresponding to 85 mM in solution. As an application example, the mixed acid fermentation of Escherichia coli is studied. The production of CO2, ethanol, acetaldehyde, acids such as formate and acetate, and the changes in pH are discussed in the context of the mixed acid fermentation pathways. Formate decomposition into CO2 and H2 is found to be governed by a zeroth-order kinetic rate law, showing that adding exogenous formate to a bioreactor with E. coli is expected to have no beneficial effect on the rate of formate decomposition and biohydrogen production.

All-Optical Photoacoustic Multigas Analyzer Using Digital Fiber-Optic Acoustic Detector

We present an all-optical photoacoustic (PA) multigas analyzer by using a digital fiber-optic acoustic detector, which consists of a spectrum demodulation based fiber Fabry–Perot (F–P) pressure sensor and a digital virtual lock-in amplifier. The concentration of multigas is quantified by using an intensity-modulated infrared thermal source and two wavelength-modulated laser diodes. Light beams from the three sources are coupled to a single nonresonant PA cell. The generated PA pressure is detected by the fiber F-P pressure sensor, which is demodulated by a near-infrared spectrometer. A lock-in white-light interferometer is developed for detecting first-harmonic ( $1f$ ) and second-harmonic ( $2f$ ) signals. The noise equivalent detection limits with a 100-s averaging time are 37, 9, 6, 17, 4, and 60 ppb for CH4, C2H2, C2H4, C2H6, CO, and CO2, respectively.

Parallel gas spectroscopy using mid-infrared supercontinuum from a single Si3N4 waveguide

Efficient third-order nonlinear optical processes have been successfully integrated on silicon nitride (Si3N4) waveguides. In particular, owing to Si3N4 wide transparency window spanning from the visible to the middle-infrared (mid-IR), efficient mid-IR dispersive-wave (DW) generation from a fiber laser has been recently demonstrated, and its potential as a source for absorption spectroscopy of a single gas has been established. Here we show that the system can be further engineered to broaden the coverage of a single DW without losing efficiency, as to enable simultaneous and discrete detection of several gas-phase molecules within the 2900 and 3380cm-1 functional group region. We demonstrate quantitative detection of acetylene, methane, and ethane using a simple direct-absorption spectroscopy scheme, achieving a several hundreds of parts-per-million noise-equivalent detection limit with a 5 cm long gas cell.

Multiple‐pass enhanced Raman spectroscopy for fast industrial trace gas detection and process control

AbstractWe report an advanced multiple‐pass Raman spectroscopy setup with enhanced sensitivity for industrial in situ monitoring and process control applications. A multiple‐pass cavity with 20 total passes is constructed using one concave mirror and two high‐reflection mirrors. The multiple‐pass cavity is fully compatible with gas chamber with dead volume as small as 2.5 ml. The setup is simple, reliable, and robust, which is important for practical industrial applications. With this system, gas samples in static cells are tested to show the analytical potential of this multiple‐pass setup. The results show that the noise equivalent detection limit (3σ) of 12.0 (N2), 13.3 (O2), 12.2 (CO2), and 5.8 Pa (hydrogen isotopologues), which corresponds to relative abundance by volume at 1 bar total pressure of 120, 133, 122, and 58 ppm can be achieved in 1 s with a 1.5‐W red laser. We further demonstrate a modified version of current multiple‐pass Raman spectroscopy, which can afford an even lower detection limit for nonhazardous gas samples. The sensitivity of the current setup can be further increased, and different approaches are discussed in detail. Based on the experience gained in this prototype setup, various improvements are currently being investigated and implemented, aiming at higher sensitivity and stability. The results obtained with this newly designed Raman system are very promising, as sensitive subsecond monitoring and process control of low‐pressure hydrogen isotopologues based on Raman spectroscopy seems to be within reach in a fusion reactor. Besides, the analysis and monitoring of other important gas species (e.g., environmental gas) can also be benefited from the current design.

Tube-cantilever double resonance enhanced fiber-optic photoacoustic spectrometer

An ultra-high sensitive trace gas detection method based on tube-cantilever double resonance enhanced fiber-optic photoacoustic spectroscopy (PAS) is proposed. The first-order resonant frequencies of the acoustic resonant tube and the fiber-optic cantilever microphone were both equal to the frequency of the photoacoustic pressure signal. This method combines the amplitude amplification of the photoacoustic pressure wave in an acoustic resonant tube with the response enhancement of the photoacoustic signal by the cantilever, making the gas detection extremely sensitive. An experimental double resonance enhanced photoacoustic spectrometer was built for trace acetylene detection at the wavelength of 1532.83 nm. A noise equivalent detection limit (1σ) was achieved to be 27 ppt with a 200-s averaging time, which is the best value reported so far. In addition, the normalized noise equivalent absorption (NNEA) coefficient reached 4.2 × 10−10 cm−1 W Hz−1/2.

Highly sensitive photoacoustic multi-gas analyzer combined with mid-infrared broadband source and near-infrared laser

A highly sensitive photoacoustic (PA) multi-gas analyzer was developed in combination with a mid-infrared broadband source and a near-infrared laser. A multi-pass cylindrical PA cell was designed to efficiently couple the two PA excitation sources. The mid-infrared thermal radiation light was transmitted into the longitudinal direction of the cell for detecting CO, CO2, CH4, C2H6, C2H4 and H2O gas. The fundamental-frequency intensity modulation (1f-IM) technique was used to measure the generated PA signal. The collimated near-infrared laser beam was incident from the side hole of the cell for detecting C2H2 gas. The second-harmonic wavelength modulation spectroscopy (2f-WMS) technique was used to extract the laser-excited PA signal. The noise equivalent detection limits (1σ) were achieved to be 27 ppb, 10 ppb, 94 ppb, 24 ppb, 20 ppb and 37 ppb for C2H2, CO, CO2, CH4, C2H6 and C2H4 with an averaging time of 60 s, respectively. The proposed method takes full advantage of the multi-component gas measurement capability of mid-infrared broadband source based PA spectroscopy (PAS) and the high precision of laser-based PAS.

Multiple‐pass‐enhanced Raman spectroscopy for long‐term monitoring of hydrogen isotopologues

AbstractWe report a variant of multiple‐pass Raman spectroscopy with enhanced sensitivity for engineering applications. A four‐mirror multiple pass cell is introduced and compared with the conventional two (concave)‐mirror configuration. With this system, gas samples in static cells are measured and monitored. The results show that the noise equivalent detection limit (3σn) close to 100 ppm for hydrogen isotopologues can be achieved within two seconds with a 1.5‐W red laser. The relative precision is characterized by long‐term measurement, and the results indicate high system stability. The signal‐to‐noise ratio (SNR) in different configurations are investigated, and results show that high (greater than nine) SNR gain is achieved in the configuration of 12 passes. This newly designed Raman system is highly stable and suitable for fast and precise in‐line monitoring of hydrogen isotopologue mixtures, as well as other (hazardous) gas samples.

Optofluidic refractive index sensor based on asymmetric diffraction.

A novel optofluidic refracrtive index (RI) sensor was proposed based on asymmetric Fraunhofer diffraction. In-plane optofluidic lens, light source, slit, diffraction pattern visualization zone and optical path were integrated into the microfluidic networks to avoid the manual alignment of the optical components as well as to reduce the cost of external bulky components. Unlike the conventional RI sensor, this device visualizes the bulk refractive index change of the liquid through a diffraction image, which is readily read-out for clinical diagnosis right at the point-of-care or on-site security check. In the experiment, the device can measure a RI change of as low as ~10-5 RIU. A low noise-equivalent detection limit (NEDL) of ~10-6 refractive index unit (RIU) and high sensitivity of ~1.1 × 104/RIU were achieved. The new device is practical and suitable to be extended for high throughput applications by simultaneously reading multiple chips with an 2D-array image sensor.

Diode laser photoacoustic spectroscopy of CO2, H2S and O2 in a differential Helmholtz resonator for trace gas analysis in the biosciences and petrochemistry

Photoacoustic spectroscopy in a differential Helmholtz resonator has been employed with near-IR and red diode lasers for the detection of CO2, H2S and O2 in 1 bar of air/N2 and natural gas, in static and flow cell measurements. With the red distributed feedback (DFB) diode laser, O2 can be detected at 764.3 nm with a noise equivalent detection limit of 0.60 mbar (600 ppmv) in 1 bar of air (35-mW laser, 1-s integration), corresponding to a normalised absorption coefficient α = 2.2 × 10−8 cm−1 W s1/2. Within the tuning range of the near-IR DFB diode laser (6357–6378 cm−1), CO2 and H2S absorption features can be accessed, with a noise equivalent detection limit of 0.160 mbar (160 ppmv) CO2 in 1 bar N2 (30-mW laser, 1-s integration), corresponding to a normalised absorption coefficient α = 8.3 × 10−9 cm−1 W s1/2. Due to stronger absorptions, the noise equivalent detection limit of H2S in 1 bar N2 is 0.022 mbar (22 ppmv) at 1-s integration time. Similar detection limits apply to trace impurities in 1 bar natural gas. Detection limits scale linearly with laser power and with the square root of integration time. At 16-s total measurement time to obtain a spectrum, a noise equivalent detection limit of 40 ppmv CO2 is obtained after a spectral line fitting procedure, for example. Possible interferences due to weak water and methane absorptions have been discussed and shown to be either negligible or easy to correct. The setup has been used for simultaneous in situ monitoring of O2, CO2 and H2S in the cysteine metabolism of microbes (E. coli), and for the analysis of CO2 and H2S impurities in natural gas. Due to the inherent signal amplification and noise cancellation, photoacoustic spectroscopy in a differential Helmholtz resonator has a great potential for trace gas analysis, with possible applications including safety monitoring of toxic gases and applications in the biosciences and for natural gas analysis in petrochemistry.Graphical abstract

Ultrahigh Q Polymer Microring Resonators for Biosensing Applications

The capability of an ultrahigh Q polymer microring resonator as a biosensing system is systematically investigated. By optimizing the device design and fabrication process, the device operates in the slightly under-coupled regime with a record intrinsic Q factor of 8.0 × 105, which offers a high sensitivity and low detection limit. In surface mass detection, a surface mass density of 12.7 pg/mm2 of bovine serum albumin is detected due to the physical adsorption. The noise-equivalent detection limit is approximately 5.3 pg/mm2 in a wavelength-shift scheme and 55.9 fg/mm2 in an intensity-variation scheme. These results show that the best sensing performance with the on-chip polymer ring resonator system is achieved.

Cavity-enhanced absorption sensor for carbon monoxide in a rapid compression machine

Abstract A sensor based on cavity-enhanced absorption spectroscopy (CEAS) was implemented for the first time in a rapid compression machine (RCM) for carbon monoxide concentration measurements. The sensor consisted of a pulsed quantum cascade laser (QCL) coupled to a low-finesse cavity in the RCM using an off-axis alignment. The QCL was tuned near 4.89 µm to probe the P(23) ro-vibrational line of CO. The pulsed mode operation resulted in rapid frequency down-chirp (6.52 cm−1/µs) within the pulse as well as a high time resolution (10 µs). The combination of rapid frequency down-chirp and off-axis cavity alignment enabled a near complete suppression of the cavity coupling noise. A CEAS gain factor of 133 was demonstrated in experiments, resulting in a much lower noise-equivalent detection limit than a single-pass arrangement. The sensor thus presents many opportunities for measuring CO formation at low temperatures and for studying kinetics using dilute reactive environments; one such application is demonstrated in this work using dilute n-heptane/air mixtures in the RCM. The formation of CO during first-stage ignition of n-heptane was measured over 802–899 K at a nominal pressure of 10 bar. These conditions correspond to the NTC region of n-heptane and such results provide useful metrics to test and compare the predictions of low-temperature heat release by different kinetic models.

Detection of shock-heated hydrogen peroxide (H2O2) by off-axis cavity-enhanced absorption spectroscopy (OA-CEAS)

Cavity-enhanced absorption spectroscopy (CEAS) is a promising technique for studying chemical reactions due to its desirable characteristics of high sensitivity and fast time-response by virtue of the increased path length and relatively short photon residence time inside the cavity. Off-axis CEAS (OA-CEAS) is particularly suited for the shock tube applications as it is insensitive to slight misalignments, and cavity noise is suppressed due to non-overlapping multiple reflections of the probe beam inside the cavity. Here, OA-CEAS is demonstrated in the mid-IR region at 1310.068 cm−1 to monitor trace concentrations of hydrogen peroxide (H2O2). This particular probe frequency was chosen to minimize interference from other species prevalent in combustion systems and in the atmosphere. The noise-equivalent detection limit is found to be 3.25 × 10−5 cm−1, and the gain factor of the cavity is 131. This corresponds to a detection limit of 74 ppm of H2O2 at typical high-temperature combustion conditions (1200 K and 1 atm) and 12 ppm of H2O2 at ambient conditions (296 K and 1 atm). To our knowledge, this is the first successful application of the OA-CEAS technique to detect H2O2 which is vital species in combustion and atmospheric science.

Graphene-Enhanced Brillouin Optomechanical Microresonator for Ultrasensitive Gas Detection.

Chemical sensing is one of the most important applications of nanoscience, whose ultimate aim is to seek higher sensitivity. In recent years, graphene with intriguing quantum properties has spurred dramatic advances ranging from materials science to optoelectronics and mechanics, showing its potential to realize individual molecule solid-state sensors. However, for optical sensing the single atom thickness of graphene greatly limits the light-graphene interactions, bottlenecking their performances. Here we demonstrate a novel approach based on the forward phase-matched Brillouin optomechanics in a graphene inner-deposited high Q (>2 × 106) microfluidic resonator, expanding the "electron-photon" interaction in conventional graphene optical devices to the "electron-phonon-photon" process. The molecular adsorption induced surface elastic modulation in graphene enables the Brillouin optomechanical modes (mechanical Q ≈ 43,670) extremely sensitive (200 kHz/ppm) in ammonia gas detection, achieving a noise equivalent detection limit down to 1 ppb and an unprecedented dynamic range over five orders-of-magnitude with fast response. This work provides a new platform for the researches of graphene-based optomechanics, nanophotonics, and optical sensing.

Intracavity absorption spectroscopy of formaldehyde from 6230 to 6420 cm−1

We apply intracavity absorption spectroscopy for measurements of the absorption spectrum of formaldehyde, CH2O, from 6230 to 6420 cm−1, of which only a small fraction (6351–6362 cm−1) has been recorded elsewhere. The measurements are performed in the cavity of a broadband Er3+-doped fiber laser, with a sensitivity corresponding to the effective absorption path length of 45 m and a spectral resolution of 0.1 cm−1. The noise-equivalent detection limit of CH2O achieved with the strongest absorption line at 6252.64 cm−1 is estimated to be 5 ppm. High tolerance to broadband losses and the accessible time resolution of 50 µs make it possible to apply this detection system for time-resolved monitoring of CH2O together with other molecules in harsh combustion environments, e.g., in combustion engines.