KEYWORDS: Quantum cascade lasers, Carbon monoxide, Digital signal processing, Absorption, Signal detection, Gas lasers, Optical engineering, Signal processing, Laser systems engineering, Calibration
We report on the design, development, and field deployment of a 10-mW, 4320-nm distributed-feedback quantum cascade laser-based tunable diode laser spectroscopy (TDLS) system in India for in situ measurement of atmospheric CO2. The portable system was deployed at Mount Abu (24.5926° N, 72.7156° E), a hill station in western India, to carry out week-long measurements of background atmospheric CO2 using direct detection. The mean mole fraction was estimated to be 396 ± 8 ppm. The system was then deployed in Gandhinagar (23.2156° N, 72.6369° E), the capital of the state of Gujarat, to make foreground measurements over the next week. The mean mole fraction at this location was 503 ± 27 ppm. The difference between the background levels in Mount Abu and foreground levels in Gandhinagar is evident. The detection limit of the system, as measured from an Allan variance analysis, was determined to be 260 ppb for an integration time of 58 s and a path length of 20 cm, which is sufficient for such measurements. Another compact and light-weight TDLS system was also deployed for water vapor measurement. It consisted of a 1392.54-nm distributed feedback laser driven by custom electronics and a digital signal processor to carry out waveform generation, data acquisition, and postprocessing tasks.
This paper demonstrates the interrogation of a fiber Bragg grating with a flat-topped reflection spectrum centred on 1649.55 nm using only a single mode tunable 1651.93 nm semiconductor laser and a fiber ring resonator. The Bragg shift is accurately measured with the fiber-optic ring resonator that has a free spectral range (FSR) of 0.1008 GHz and a broadband photo-detector. Laser wavelength modulation and harmonic detection are used to transform the gentle edges of the flat-topped FBG spectrum into prominent leading and trailing peaks, either of which can be used to accurately measure spectral shifts of the FBG reflection spectrum with a resolution of 0.9 pm. A Raspberry Pi-based low-cost embedded processor is used to measure the temperature-induced spectral shifts over the range 30˚C – 80˚C. The shift was linear with a temperature sensitivity of 12.8 pm/˚C. This technique does not use an optical spectrum analyzer at any stage of its design or operation. The laser does not need to be pre-characterized either. This technique can be readily extended to all types of tunable diode lasers and is ideally suited for compact field instruments.
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.
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.
Quantitative tunable diode laser spectroscopy (TDLS) has established itself as a very powerful technique for the
detection of gases in field applications such as industrial process control. Recent calibration-free techniques have
made field measurements more robust. However, in many situations, the significant levels of laser intensity
modulation gives rise to background signals that either limit detection sensitivity or distort the target signals, thereby
making it difficult to extract useful information. This paper outlines the recent trends in calibration-free wavelength
modulation spectroscopy (WMS) and focuses on the elimination of the undesirable effects of both linear as well as
nonlinear intensity modulation. The approach is generic and should be useful with newer types of lasers that have
shown significantly nonlinear power-current characteristics.
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.
KEYWORDS: Digital filtering, Nonlinear filtering, Signal to noise ratio, Temperature sensors, Spatial resolution, Electronic filtering, Analog electronics, Linear filtering, Quantization, Data acquisition
This article discusses a digital filtering technique to improve the temperature resolution of a Raman fiber optic distributed temperature sensor. A frequency-domain digital filtering algorithm has been used for this typically time-domain task to show that by reducing the effect of noise introduced by analog electronics and the quantization noise of the analog-to-digital converter in the detection and data acquisition stages, superior temperature resolution is achievable. An important feature of the filtering scheme used is that no spatial inaccuracy is introduced into the ranging of hot zones, despite the highly nonlinear phase response of the filter. Digital filtering used for postprocessing of data shows clearly and unambiguously a temperature resolution of 5 K in the present setting of our experiment, with the possibility of much greater improvement. Merely using analog signal conditioning does not provide the same clarity and uniqueness in temperature resolution and spatial location that digital filtering offers. This digital filtering scheme offers greater flexibility and versatility than mere signal-averaging approaches to improve the SNR of such systems. The filter's ability to reject deterministic interfering frequencies of appreciable energy content is also demonstrated by simulating such frequencies of much greater energy than the actual signal.
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