Arup Lal Chakraborty

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 couplers output from which the absolute absorption profile is directly recovered with high accuracy using 1f detection.

Detection of CH

This paper demonstrates detection of methane using tunable diode laser spectroscopy (TDLS) through difference frequency generation (DFG) in order to address fundamental rotational-vibrational absorption lines, located around 3404 nm. Direct detection confirms that wavelength referencing of recovered lineshapes, developed for Near infra-red (Near-IR) systems, has been successfully transferred to the presented Middle infra-red (Mid-IR) system. Traditional 1f and 2 f TDLS with WMS detection regimes are also functionally confirmed analogous to their Near-IR equivalents.

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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.

in the Mid-IR Using Difference Frequency Generation With Tunable Diode Laser Spectroscopy

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 lasers 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.

This paper reports the extraction of concentration and pressure of methane using a 1650-nm laser diode operated at its phase quadrature frequency (fq) and using the calibration-free residual amplitude modulation (RAM) method. Although the RAM method is the simplest calibration-free technique, it has low signal levels compared with the phasor decomposition (PD) method for small values of the phase difference ψ between the laser intensity modulation and frequency modulation (FM). For the laser diode used in this paper, ψ turned out to be 90° at a very modest modulation frequency of 125.5 kHz, which is an order of magnitude lower than values reported elsewhere. The RAM signal and FM signal are at phase quadrature at this frequency and each can be fully and simultaneously recovered along a detection axis of a lock-in amplifier free from distortion by the other. The absolute absorption profile is accurately recovered with the signal-to-noise ratio (SNR) maximized for this laser. The gas parameters are extracted by fitting a Voigt line shape to the experimental data. These results show that when operating at fq, the PD method offers no advantage over the RAM method and is therefore redundant. In addition, the background RAM is eliminated by an automatic fiber-optic RAM nulling technique. Finally, the RAM method is also implemented at high values of modulation index to increase the SNR ratio. The time resolution of measurements is currently 10 s. The prospect of using the RAM method with a 2004-nm vertical cavity surface emitting laser is also explored.

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

Residual Amplitude Modulation Method Implemented at the Phase Quadrature Frequency of a 1650-nm Laser Diode for Line Shape Recovery of Methane

A limiting factor of tuneable diode laser spectroscopy (TDLS) with wavelength modulation spectroscopy (WMS) is the presence of background residual amplitude modulation (RAM) on the recovered 1st harmonic signal. The presence of this background term is due to direct modulation of the source laser power. This work presents a novel method to optically remove the unwanted background, with the major benefit being that measurement sensitivity can be increased. The recently developed phasor decomposition method1 (PDM), is a near IR (NIR) TDLS analysis technique that is used with the addition of the new RAM nulling method to recover gas absorption line-shapes. The PDM is a calibration free approach, which recovers the gas absorption line-shape and the isolated 1st derivative of the line-shape from the 1st harmonic signal. The work presented illustrates and validates the new RAM nulling procedure with measurements examining the 1650.96nm absorption line of methane (CH4) with comparisons to theory.

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

The concentration of atmospheric pollutants and greenhouse gases needs to be precisely monitored for sustainable industrial development and to predict the climate shifts caused by global warming. Such measurements are made on a continuous basis in ecologically sensitive and urban areas in the advanced countries. Tunable diode laser spectroscopy (TDLS) is the most versatile non-destructive technology currently available for remote measurements of multiple gases with very high selectivity (low cross-sensitivity), very high sensitivity (on the order of ppm and ppb) and under hazardous conditions. We demonstrate absolute measurements of acetylene, methane and carbon dioxide using a fielddeployable fully automated TDLS system that uses calibration-free 2f wavelength modulation spectroscopy (2f WMS) techniques with sensitivities of low ppm levels. A 40 mW, 1531.52 nm distributed feedback (DFB) diode laser, a 10 mW, 1650 nm DFB laser and a 1 mW, 2004 nm vertical cavity surface emitting laser (VCSEL) are used in the experiments to probe the P9 transition of acetylene, R4 transition of methane and R16 transition of carbon dioxide respectively. Data acquisition and on-board analysis comprises a Raspberry Pi-based embedded system that is controllable over a wireless connection. Gas concentration and pressure are simultaneously extracted by fitting the experimental signals to 2f WMS signals simulated using spectroscopic parameters obtained from the HITRAN database. The lowest detected concentration is 11 ppm for acetylene, 275 ppm for methane and 285 ppm for carbon dioxide using a 28 cm long single-pass gas cell.

Recent advance in tunable diode laser spectroscopy with background RAM nulling for industrial applications

Recovery of the absolute absorption line shape is demonstrated for methane with a 1650nm edge-emitting laser and for carbon dioxide with a 2004nm VCSEL. The potential for direct detection and wavelength modulation spectroscopy are explored.

High-sensitivity remote detection of atmospheric pollutants and greenhouse gases at low ppm levels using near-infrared tunable diode lasers

An electronically-controlled fibre-optic RAM nulling method is presented for tunable diode laser spectroscopy (TDLS) of gases. An electronic variable optical attenuator and a 1x2 optical switch are used to demonstrate the cancellation of the background RAM signal that limits the detection sensitivity in 1f WMS. This is an significant improvement upon the generic RAM nulling method that has recently been shown to be well suited to direct recovery of the absolute gas absorption line shapes of gases that commonly encountered in process control applications.