Using the output of the photodiode as a current source, and then introduce an operational amplifier to convert the photo-generated current to voltage becomes a typical photodiode amplifier circuit. The seemingly simple circuit faces a difficult problem when detecting weak photo-generated current: the photosensitive surface of the diode and the resistor using for current-voltage conversion need to be enlarged as much as possible to increase the output amplitude. However, this will result in a reduction in the bandwidth of the photodiode amplifier circuit, a reduction in stability, and an increase in noise. It was therefore decided to introduce software to assist in circuit design for the improving of performances of circuit. To assessing the applicability, the photodiode amplifier circuit was matched with other optoelectronic devices such as laser driver, temperature controller and lock-in amplifier, and employed in wavelength modulation spectroscopy (WMS) for NH3 detection at its P(6) absorption line belonging to the ν3 + ν4 combination band. Under the experimental conditions of room temperature (296 K), normal pressure (1.01e5 Pa) and an effective absorption path length of 10 cm, the experiments were conducted using five samples with their concentrations ranging from 500ppm to 1%, and a 1.3ppm detection limit was deduced from the linear regression analysis. The results not only indicate that the photodiode amplifier circuit has great potential in other trace material detection applications based on absorption spectroscopy, but also reflect from the side that the method of using simulation software to assist circuit design is highly feasible.
KEYWORDS: Photodetectors, Gas sensors, Modulation, Sensors, Absorption, Signal detection, Optical fibers, Semiconductor lasers, Optical amplifiers, Signal to noise ratio
In this paper, we report on the utilization of a novel background-free method for trace gas sensing. The classical combination of wavelength modulation spectroscopy and phase sensitive detection is replaced by a balanced amplified photodetector based differential detection system. Through a beam splitter, the transmitted laser beam from the gas medium is divided into the real-time and delayed branches. An optic fiber delay line is introduced to create a time-delay between the separated branches, and the following differential detection yields the derivative of absorption spectrum that is equivalent to the first harmonic in wavelength modulation spectroscopy. Via this straight-forward approach, the quantitative analysis of gas is performed in the absence of the background which is caused by optical intensity modulation. The gas sensor is validated using ammonia and nitrogen mixture in a laboratory controlled environment (296 K in temperature and 1.01×105 Pa in total pressure), and yielding a detection limit of 6.4 ppm for ammonia. Experimental result shows the accuracy and potential utility of this gas sensor for industrial applications.
This study proposes a complex design for the recovery of the line shape function of gas absorption spectrum. Laser power passing through trace gas is divided into real-time and delayed components, and the difference between them, regarded as equivalent to the first-order derivative spectrum, is recorded and integrated to reconstruct the absorption line profile. As the real-time and delayed signals are derived from the same gas cell and photo detector, the elimination of the background is more convenient and effective than in the generally used double-beam detection that involves two gas cells and photo detectors. Compared with the first harmonic detection used in wavelength modulation spectroscopy (WMS), the generation of the derivative spectrum is achieved without modulating the injection current of the laser. Moreover, the expensive lock-in amplifier in WMS is replaced by a simply designed device composed of an all-pass filter and an instrumentation amplifier. Complexity and cost are thus reduced significantly, and stability improves. For system validation, the absorption spectroscopy of ammonia from 4986.5 cm-1 to 4987.5 cm-1 was recovered, and the obtained data agreed well with the theoretical calculations.
Conventional wavelength modulation spectroscopy is improved by logarithmic-transformed data processing and differential detection for trace gas sensing. The optic intensity modulation is isolated from the gas absorption induced laser power attenuation via the logarithmic-transformation, and then it is balanced out by differential detection. The theory is validated by the detection of P(6) ammonia absorption line belonging to the v3 + v4 combination band in a stainless tube with effective path length of 24.5 cm, under room temperature (296 K) and atmospheric pressure (1.01e5 Pa). The second harmonic is recovered while the residual amplitude modulation and distortion of harmonic are eliminated. A 0.7 ppm (part per million) detection limit is estimated on the assumption that the signal is equal with the noise. Excellent repeatability is observed from continuous measurements of 20 times during 240 hours. Standard deviation of the measured ammonia mole fraction and the simulation data is 1.1%. All of these inspire results indicate that our strategy is an ideal option for trace gas detection.
Conventional wavelength modulation spectroscopy is improved by logarithmic-transformed data
processing for trace gas sensing. Via the logarithmic-transformation, the optic intensity modulation is
isolated from the gas absorption induced laser power attenuation, and then the optic intensity modulation is
offset by its equivalent which is obtained from a referenced beam. The method is validated by the detection
of P(6) NH3-absorption line in a stainless tube with effective path length of 24.5 cm, under room
temperature (296 K) and atmospheric pressure (1.01e5 Pa). The second harmonic is recovered while the
residual amplitude modulation and distortion of curve are eliminated. A 0.7 ppm (part per million)
detection limit is inferred on the assumption that the signal is equal with the noise.
A versatile signal processing strategy for eliminating the residual amplitude modulation (RAM) and distortion in tunable diode laser wavelength modulation spectroscopy is theoretically demonstrated and experimentally validated. The strategy involves logarithmic transformation and differential detection, which are achieved using a homemade circuit. Through the logarithmic transformation, the optical intensity modulation of the laser, which performs as the source of RAM and distortion, is separated from the absorption-induced power attenuation and further balanced during the differential detection. The first harmonic, which is proportional to the first-order derivative of the absorption line profile in the case of a small modulation index, is extracted along with the second harmonic and is integrated for the recovery of the absorption line profile. The experiments are carried out for CH4 at its R(3) absorption line of the 2ν3 overtone for validation of the system, and the derived results are found to be in good agreement with the theoretical simulations. These promising results indicate the high potential of the strategy for absorption spectrum-based determination of gas properties.
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