| With the development of the national pillar industries, such as petrochemical engineering, deep-sea exploration and aerospace, and the development of the national economy and people’s livelihood industries, such as food safety, electric power and gas pollution, more and more gas sensors are needed for both sensing of combustible, explosive, poisonous gases and water vapor. Besides, the applications are much more extended. As a typical example of optical fiber sensing, optical fiber gas sensor has obtained attractions in the fields of electricity, construction, traffic, petrol and oil, chemical industry, medical treatment and environmental protection because of its specific advantages of high sensitivity, high precision, wide measurement range and fast response.In addition to national economy and people’s livelihood industries, it has always been one important research project in the field of science using interaction between laser and gas molecular to measure and transmit gas information (species, concentration, pressure, temperature and flow velocity etc). As for water vapor molecular, its strongest fundamental absorption line lies in the range of mid infrared. However, light source and photoelectric material are very expensive and hard to control, which are bad for the industrialization. The light source adopted in our research is DFB diode laser. Its output wavelength is around 1370 nm and the absorption lines in this range are overtone transitions, not the strongest one. Fortunately, the chosen DFB diode laser has perfect performance (narrow line width, MHz, high spectral power density, easy to control) and fabrication of photoelectric material in this range is very mature. Thus our research of water vapor absorption focuses on the absorption lines in this area.In the research and development even the production of water vapor sensors, long-term reliability has become one of the most important limitations of product improvement. This dissertation focuses on the research of key techniques to improve the performance of optical fiber water vapor sensors, specially the parameters of detection resolution, reliability etc. Moreover, we also study multi-parameters gas sensor, and achieved the simultaneously measurement of water vapor concentration and gas pressure inside gas cell. The results of this dissertation are very important to accelerate the industrialization of optical fiber water vapor sensors. The main research contents are listed as follows.(1) A method based the differential value of two adjacent absorption peaks was proposed introduced to measure water vapor concentration. When absorption spectroscopy corresponding to single absorption line is used measure water vapor concentration, amplitude of absorption strength is always used. The amplitude of absorption strength is determined by the differential value of absorption peak and the base line (without absorption) of absorption spectroscopy. However, choice of position of baseline uncertain, and random noise can affect the baseline more than the absorption peak. The proposed method can be used to solve the problem of uncertainty of baseline choice when determining the amplitude of absorption strength and can further improve the anti-interference ability of optical fiber water vapor sensor.(2) An improved Ebers Moll model-based BRD circuit was introduced. This circuit is a new electric noise canceller, which can be mainly used to cancel the shot noise of PIN photoelectric detector, laser intensity noise and PIN photoelectric detector etc. The Ebers Moll model-based BRD circuit can be used in dual-beam optical fiber gas sensing system to demodulate gas information. In the detection, BRD circuit can directly normalize the photocurrents without the tedious process of transforming photocurrents into voltage information and normalizing them using subtraction and division. The usage of BRD circuit can also avoid some phase difference and electric noise introduction during the photocurrent to voltage conversion. BRD circuit can suppress the effect of laser intensity variation on measurement. As a result of measurement test, this improved BRD circuit has improved the suppression ration from 53 dB to 88 dB. We also apply this improved BRD circuit to water vapor concentration detection. As a result a high precision of 71.8 ppbv has been achieved, which provides the possibility of designing high-precision water vapor sensor using low cost demodulation circuit.(3) Technical proposal combining wavelength scanning and intensity modulation was studied. This proposal combines the technical advantages of direct TDLAS with scanning spectroscopy and high-frequency intensity modulation. This is to say, the biggest advantage of direct TDLAS with scanning spectroscopy is that the measured signal profile directly corresponds the absorption spectroscopy for the chosen absorption line without any tedious process. The biggest advantage of high-frequency intensity modulation is that measured signal under high frequency (equal to modulation frequency) has low system 1/f noise, and the application of lock-in amplifier can further suppress the noise outside the pass band. Based on the proposal above, we design and assemble experiment apparatus for water vapor measurement. As a result, the measured absorption spectroscopy agrees the simulated absorption very well with an error within 1%, and a detection precision of 43 ppbv for water vapor concentration is achieved using only a 10-cm gas cell.(4) Extra absorption induced interference in optical fiber water vapor sensors was elucidated. We analyze the package structures of butterfly packaged DFB diode laser, coaxial packaged PIN photoelectric detectors, and transmission type optical fiber collimators. There exist some interspaces inside their structures because of the need of proper functioning, and these interspaces offer the possibility for permeation of probed gas molecular into the optical path. The permeation can result in extra absorption during the gas detection, which should not happen according to our traditional understanding. For this reason we propose two methods to suppress the extra absorption induced interference on detection measurement, and the two methods are PIN photoelectric detectors matching method and long optical path dilution method, and the suppression results are from 288 ppmv to 4 ppmv and from 727 ppmv to 25.2 ppmv respectively.(5) The reasons of second harmonic signals’distortion in TDLAS/WMS were analyzed and a method based on BRD circuit was proposed for recovery of pure wavelength modulation second harmonic signal in dual-beam TDLAS/WMS detection system. This method includes BRD circuit technique and TDLAS/WMS technique. According to the analysis, second harmonic signals’distortion derives from residual amplitude modulation of DFB diode laser and laser power variation. In the test of BRD based dual-beam TDLAS/WMS detection system, the factors accounting for the distortion have been well suppressed. This is because the perfect normalization of residual amplitude modulation and laser power variation by application of BRD circuit in form of photocurrent without causing any other phase difference. This research offers important reference for recovery of pure wavelength-modulation harmonic signals to simplify the tedious waveform analysis and to improve the detection resolution of water vapor sensing. What is more, this technique makes it possible to reach the full potential of TDLAS/WMS method.(6) As the absorption line of water vapor at 1368.597 nm, its FWHM will increase as gas pressure. For example, when the situation is 8 atm/296 K, the full width at half maximum of water vapor absorption is FWHM=278.4 pm. However for most DFB diode laser with this wavelength, the wavelength scanning range will be 200-300, because of which the whole absorption line can’t be fully scanned. We research the gas detection technique when scanning wavelength range is limited. This technique is based on direct TDLAS with DFB diode laser, and adopts Levenberg Marquardt algorithm meanwhile. Simultaneously measurement of water vapor concentration and gas pressure inside the gas cell can be achieved using this technique. With Levenberg Marquardt algorithm, whole absorption spectroscopy can be simulated with the recorded incomplete absorption spectroscopy using direct TDLAS. After that, the differential value between absorption peak and baseline can be used to demodulate water vapor concentration, as a result a detection precision of 20 ppmv has been achieved; the obtained FWHM value can be used to demodulate gas pressure inside the gas cell based on the ratio of obtained FWHM to FWHM under standard conditions (1 atm/296 K), as a result a detection precision of 5% has been achieved.During the accomplishment of my work during the past years, some innovations were proposed and proved to be right. The main innovations are listed as follows.(1) A sensing method using the differential value of two adjacent absorption lines are proposed for the first time. For spectral absorption type gas sensors, absorption intensity by probed gas molecule is the most important parameter used to demodulate probed gas concentration. Generally absorption intensity can be determined by the differential value between absorption peak and base line of absorption spectrum. However, for single absorption line there exists uncertainty during determining base line of absorption spectrum, especially when the signal to noise ratio is bad or the FWHM is wide. Absorption lines at 1368.597 nm and 1367.8621nm are chosen as research subjects, and the differential value of two adjacent absorption lines is used to detect water vapor concentration.(2) Water vapor inside optical devices is considered for the first time. Influence of its existence on trace water vapor sensing is analyzed and some techniques to remove or suppress the influence are discussed. Influence of water vapor inside optical devices is defined as extra absorption induced interference in this dissertation. The mechanical structures of main optical devices (DFB dioede laser, PIN photoelectric detector and optical fiber collimator) are analyzed and the sources of extra absorption induced interference are provided. Two feasible methods are proposed to suppress the influence of water vapor inside optical devices. The two proposals can be applied practical design of optical fiber water vapor sensors.(3) We have improved a BRD detection circuit to achieve sensitive optical fiber water vapor sensing. This improvement is based on the idea of conductivity matching of the two PIN photoelectric detectors. This circuit has been applied in dual-beam optical water vapor sensing system realizing elimination of photocurrent noise in a wide band range, normalizing the two beams in the form of photocurrent, avoiding some problems caused in the process of photocurrent to voltage conversion.(4) A BRD-based TDLAS/WMS technique was proposed to realize recovery of pure wavelength-modulation second harmonic signal. Second harmonic signal’s distortion mainly derives from residual amplitude modulation and variation of output power of the DFB diode laser. During the experiment test using this BRD-based TDLAS/WMS technique, these interference factors have been successfully removed, which simplify the analysis of recorded distorted absorption profile when spectral absorption type gas sensors are used.(5) Simultaneous measurements of water vapor concentration and gas pressure inside gas cell has been realized. In the implementation we combine TDLAS technique and Levenberg Marquardt algorithm to achieve simulating the whole absorption line profile using recorded incomplete absorption line profile. After that water vapor concentration and gas pressure inside gas cell have been simultaneously measured using the simulated absorption line profile.
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