Detailed analysis of the R1f/ΔI1 WMS technique and demonstration of significantly higher detection sensitivity compared to 2f WMS for calibration-free trace gas sensing.

Recognizing that wavelength modulation spectroscopy (WMS) is particularly important in the development of high-sensitivity gas sensing systems, this paper presents a detailed analysis of the R 1f /Δ I 1 WMS technique that has recently been successfully demonstrated for calibration-free measurements of the parameters that support detecting multiple gases under challenging conditions. In this approach, the magnitude of the 1f WMS signal (R 1f ) was normalized by using the laser's linear intensity modulation (Δ I 1) to obtain the quantity R 1f /Δ I 1 that is shown to be unaffected by large variations in R 1f itself due to the variations in the intensity of the received light. In this paper, different simulations have been used to explain the approach taken and the advantages that it shows. A 40 mW, 1531.52 nm near-infrared distributed feedback (DFB) semiconductor laser was used to extract the mole fraction of acetylene in a single-pass configuration. The work has shown a detection sensitivity of 0.32 ppm for 28 cm (0.089 ppm-m) with an optimum integration time of 58 s. The detection limit achieved has been shown to be better than the value of 1.53 ppm (0.428 ppm-m) for R 2f WMS by a factor of 4.7, which is a significant improvement.

A Fiber Bragg Grating-Based Sensor for Passive Cavitation Detection at MHz Frequencies.

Fiber Bragg gratings (FBGs) are a potential alternative to piezoelectric ultrasound sensors for applications that demand high sensitivity and immunity to electromagnetic interference (EMI). However, limited data exist on the quantitative performance characterization of FBG sensors in the MHz frequency range relevant to biomedical ultrasound. In this work, we evaluated an FBG to detect MHz-frequency ultrasound and tested the feasibility of measuring passive cavitation signals nucleated using a commercial contrast agent (SonoVue). The sensitivity, repeatability, and linearity of the measurements were assessed for ultrasound measurements at 1, 5, and 10 MHz. The bandwidth of the FBG sensor was measured and compared to that of a calibrated needle hydrophone. The FBG showed a sensitivity of 0.99, 0.769, and 0.818 V/MPa for 1, 5, and 10 MHz ultrasound, respectively. The sensor also exhibited linear response ( 0.975 ≤ R -Squared ≤ 0.996) and good repeatability with a coefficient of variation (CV) less than 5.5%. A 2-MHz focused transducer was used to insonify SonoVue microbubbles at a peak negative pressure of 175 kPa and passive cavitation emissions were measured, in which subharmonic and ultraharmonic spectral peaks were observed. These results demonstrate the potential of FBGs for MHz-range ultrasound applications, including passive cavitation detection (PCD).

Light-induced in situ active tuning of the LSPR of gold nanorods over 90 nm.

We demonstrate an easy and controllable method for light-induced active tuning of the longitudinal surface plasmon resonance (LSPR) of gold nanorods (AuNRs) over ∼94nm. The red-shift of the LSPR can be controlled by varying the time of exposure to a 532 nm laser. The tuning is achieved by photo-induced dissolution of individual AuNRs by sodium dodecyl sulfate (SDS) under continuous illumination. The dissolution of the AuNRs increases the aspect ratio, and consequently the LSPR exhibits a gradual but large redshift. A key feature is that it is possible to selectively tune the LSPR of a specific AuNR in a group while leaving the others totally unaffected. Such controllable, light-induced, post-synthesis fine-tuning of the LSPR is useful for tailoring the plasmonic response of individual AuNRs for a wide range of applications.

Measurement of atmospheric carbon dioxide and water vapor in built-up urban areas in the Gandhinagar-Ahmedabad region in India using a portable tunable diode laser spectroscopy system.

This paper reports open-path in situ measurements of atmospheric carbon dioxide at Gandhinagar (23.2156°N, 72.6369°E) and Ahmedabad (23.0225°N, 72.5714°E) in the heavily industrialized state of Gujarat in western India. Calibration-free second harmonic wavelength modulation spectroscopy (2f WMS) is used to carry out accurate and fully automated measurements. The mean values of the mole fraction of carbon dioxide at four locations were 438 ppm, 495 ppm, 550 ppm, and 740 ppm, respectively. These values are much higher than the current global average of 406.67 ppm. A 1 mW, 2004-nm vertical cavity surface-emitting laser is used to selectively interrogate the R16 transition of carbon dioxide at 2003.5 nm (4991.2585 cm-1). The 2f WMS signal corresponding to the gas absorption line shape is simulated using spectroscopic parameters available in the HITRAN database and relevant laser parameters that are extracted in situ from non-absorbing spectral wings of the harmonic signals. The mole fraction of carbon dioxide is extracted in real-time by a MATLAB program from least-squares fit of the simulated 2f WMS signal to the corresponding experimentally obtained signal. A 10-mW, 1392.54-nm distributed feedback laser is used at two of the locations to carry out water vapor measurements using direct absorption spectroscopy. This is the first instance of a portable tunable diode laser spectroscopy system being deployed in an urban location in India to measure atmospheric carbon dioxide and water vapor under varying traffic conditions. The measurements clearly demonstrate the need to adopt tunable diode laser spectroscopy for precise long-term monitoring of greenhouse gases in the Indian subcontinent.

Absolute noninvasive measurement of CO2 mole fraction emitted by E. coli and S. aureus using calibration-free 2f WMS applied to a 2004 nm VCSEL.

We report the first demonstration, to the best of our knowledge, of accurate real-time noninvasive measurement of the absolute cumulative mole fraction of metabolic carbon dioxide emitted by Escherichia coli and Staphylococcus aureus over a period of several hours of their life cycles using a recently developed calibration-free wavelength modulation spectroscopy technique. A 1 mW vertical-cavity surface-emitting laser is used to interrogate a single rotational vibrational absorption line of carbon dioxide at 2003.5 nm. The measurements are immune to laser intensity fluctuations and variable optical coupling that is inevitable in such free-space coupled experiments that run over 10-18 h. The cumulative carbon dioxide mole fraction follows the characteristic modified Gompertz model that is typical of bacterial growth in batch cultures. The characteristic growth parameters are extracted from this curve. The technique can be readily extended to study multiple volatile organic compounds that bacteria are known to emit.

Fiber Bragg grating interrogation using wavelength modulated tunable distributed feedback lasers and a fiber-optic Mach-Zehnder interferometer.

This paper demonstrates a technique of high-resolution interrogation of two fiber Bragg gratings (FBGs) with flat-topped reflection spectra centered on 1649.55 nm and 1530.182 nm with narrow line width tunable semiconductor lasers emitting at 1651.93 nm and 1531.52 nm, respectively. The spectral shift of the reflection spectrum in response to temperature and strain is accurately measured with a fiber-optic Mach-Zehnder interferometer that has a free spectral range of 0.0523 GHz and a broadband photodetector. Laser wavelength modulation and harmonic detection techniques are used to transform the gentle edges of the flat-topped FBG into prominent leading and trailing peaks that are up to five times narrower than the FBG spectrum. Either of these peaks can be used to accurately measure spectral shifts of the FBG reflection spectrum with a resolution down to a value of 0.47 pm. A digital signal processing board is used to measure the temperature-induced spectral shifts over the range of 30°C-80°C and strain-induced spectral shifts from 0 µÏµ to 12,000 µÏµ. The shift is linear in both cases with a temperature sensitivity of 12.8 pm/°C and strain sensitivity of 0.12 pm/µÏµ. The distinctive feature of this technique is that it does not use an optical spectrum analyzer at any stage of its design or operation. It can be readily extended to all types of tunable diode lasers and is ideally suited for compact field instruments and for biomedical applications in stroke rehabilitation monitoring.

Calibration-free 2f WMS with in situ real-time laser characterization and 2f RAM nulling.

This Letter demonstrates a new calibration-free 2f wavelength modulation spectroscopy (WMS) technique to measure gas concentration and pressure without the need for laser precharacterization. A 1650-nm laser diode is used for methane concentration and pressure measurements for pressures up to 4 bar and for a modulation index (m) of 2.2. All laser parameters such as the intensity, linear and nonlinear intensity modulation (IM), frequency modulation (FM) characteristics, the phase difference ψ1 between the FM and the linear IM, and the phase difference ψ2 between the FM and the nonlinear IM are accurately estimated in situ and in real time. This technique accounts for variations in these parameters that arise due to scanning of the laser's center wavelength, laser temperature variations, and aging of the laser. The laser is modulated at its phase quadrature frequency at which the linear IM and the FM are orthogonal to each other (ψ1=90°). This ensures that the two linear IM-dependent distorting Fourier components are orthogonal to the detection axis, and the undistorted 2f signal is recovered. This simplifies the simulation and gas parameter-extraction process. Finally, 2f RAM nulling is implemented to remove the significant absorption-independent 2f residual amplitude-modulation (RAM) signal that is seen to cause significant distortion of the 2f signal and detector saturation.

Suppression of intensity modulation contributions to signals in second harmonic wavelength modulation spectroscopy.

Recovery of the full 2f wavelength modulation spectroscopy (WMS) signal in isolation from the 2f residual amplitude modulation (RAM) due to nonlinear intensity modulation (IM) and distortion due to linear IM is demonstrated. The 2f RAM is eliminated using a fiber delay line, while the linear IM-induced distortion is eliminated by a phasor decomposition approach. This generic and robust two-pronged strategy removes the need to separately measure the 2f RAM in high-modulation-index calibration-free 2f WMS. It is also important for widely tunable 2f WMS using nontelecom diode lasers with highly nonlinear characteristics leading to high-2f RAM levels.

Influence of the wavelength-dependence of fiber couplers on the background signal in wavelength modulation spectroscopy with RAM-nulling.

Recently a technique to optically eliminate the background residual amplitude modulation in 1f wavelength modulation spectroscopy was demonstrated, where perfect elimination throughout the scan range was not achieved due to the wavelength-dependence of couplers and that of the laser intensity modulation. This paper theoretically analyzes the technique and experimentally demonstrates that the elimination can be perfect for one of three possible experimental configurations, making this important for potential applications with some recently-developed laser sources. For the other configurations a non-zero background slope is predicted, experimentally verified, and the anomalous nature of signals is thereby explained. A common signal normalization method is devised that is independent of the signal slope, a fact that is important for industrial deployment of such systems.

Elimination of residual amplitude modulation in tunable diode laser wavelength modulation spectroscopy using an optical fiber delay line.

A new fiber-optic technique to eliminate residual amplitude modulation in tunable diode laser wavelength modulation spectroscopy is presented. The modulated laser output is split to pass in parallel through the gas measurement cell and an optical fiber delay line, with the modulation frequency / delay chosen to introduce a relative phase shift of pi between them. The two signals are balanced using a variable attenuator and recombined through a fiber coupler. In the absence of gas, the direct laser intensity modulation cancels, thereby eliminating the high background. The presence of gas induces a concentration-dependent imbalance at the coupler's output from which the absolute absorption profile is directly recovered with high accuracy using 1f detection.