| The sensor based on optical fiber grating laser, as a type of novel sensor, has been developing significantly in recent years. The key sensing element is optical fiber grating laser with obvious advantages like high SNR(signal to noise ratio) and narrow linewidth. Compared to the traditional electric sensor, the sensor based on optical fiber grating laser has more distinct advantages, such as high sensitivity, immunity to the electromagnetic interference, compactness, etc. Compared to the sensor based on passive optical fiber grating, optical fiber grating laser sensor still has its own unique advantages of narrow linewidth and high SNR. In terms of signal demodulation, the sensor based on optical fiber grating laser can also skillfully demodulate sensing signals by means of phase and frequency demodulation, besides wavelength demodulation method. For example, measurands can be detected by demodulating the laser signal in frequency domain, which not only improves sensitivity significantly, but also reduces the demodulating system's cost, because the electric demodulation technology for analyzing frequency domain signal is maturer than traditional wavelength demodulation method. What's more, according to the absorption spectroscopy, sensors based on optical fiber grating laser can detect the concentration of environmentally pollutional gases like CO2, CO and CH4 in the light of the absorption property of these gases in NIR(Near Infrared) and MIR(Middle Infrared). Therefore, it can develop into a simple absorption-type gas sensing system, which provides a very effective method for monitoring environmental health.On the basis of different mothods of signal demodulation and sensing applications, two types of sensors based on optical fiber grating laser were designed and achieved in this dissertation:magnetic field sensor based on orthogonal dual-frequency distributed Bragg reflector(DBR) fiber laser and gas sensor based on tunable Thulium(Tm)-doped fiber grating laser. The former measured magnetic field and electric current with high sensitivity by measuring the beat frequency signal of DBR fiber laser, and the latter completed the effective concentration detection of CO2 gas with high sensitivity utilizing a compact 2?m tunable Tm-doped fiber laser. The main research work and achievements of this dissertation are as follows:1 Fiber Bragg grating, fiber grating laser and their separate developing status of sensing applications were introduced, and their sensing properties were also compared. Two different types of sensors based on fiber grating laser were presented in detail:beat frequency modulation sensor and absorption sensor. As for sensors modulated in frequency domain, the beat frequency signal of fiber laser is employed to sense measurands, which has advantages of simple modulation method and high sensitivity. With regard to absorption sensors, as per the absorption spectroscopy, concentrations of gases are detected with the method of measuring different absorptive properties of different gases in the regions of UV and IR.2 A novel magnetic field sensor based on orthogonal dual-frequency DBR fiber laser was investigated. FBGs were fabricated by scanning method according to the phase mask technology. In scanning method, a 193nm ArF excimer laser was used successfully to inscribe serially high reflective and low reflective gratings in the same one Erbium(Er)-doped fiber, with a proper distance between these two gratings. DBR fiber grating laser with a short cavity was created. A wire with electric current among magnetic field can produce Ampere's force, which was applied transversely into this DBR fiber laser to make beat frequency signal change, and the measurement of magnetic field and electric current was completed through measuring the change of beat frequency signal. This is the basic principle of magnetic field sensor based on DBR fiber laser. Experimental results (-258.92kHz/mT and?1.08727MHz/A) indicated that responsive sensitivity was agree well with the theoretical calculation. Minimum detectable magnetic field was up to the order of Gause, which provided a new option for the weak magnetic field detection.3 On account of realizing on-site concentration detection of CO2 gas with high sensitivity, the design and optimization of an all-fiber linear 2?m Tm-doped fiber grating laser was achieved. FBGs were fabricated by fixed point method according to the phase mask technology. In fixed point method, a 248nm KrF excimer laser was employed to inscribe high reflective and low reflective gratings in the normal single-mode photosensitive fiber. Er-doped or Tm-doped fiber was spliced between these two gratings to create Er-doped or Tm-doped fiber grating laser. The former laser was put to use as the pump source of the latter laser. Tm-doped fiber laser was optimized, and the pump source——1600nm Er-doped fiber laser was firstly optimized, mainly in two aspects of grating pairs and Er-doped fiber. Pump power is up to 173.5mW after optimization, which was employed as the pump source to create Tm-doped fiber laser. The same mothod was utilized to optimize Tm-doped fiber laser, and output power were 35.5mW and 10.6mW at 1874nm and 1995nm respectively, which was the preparation for the further gas sensing.4 With a view to the improvement in sensitivity and accuracy of 2?m Tm-doped fiber grating laser in CO2 gas sensing application, its tunability was achieved by tuning low reflective grating(high reflective end is broadband silver mirror coated at the end of optical fiber by Tollen's reaction) of laser's configuration, in light of the mechanical property that bending grating produces strain. The range of tunability at both1874nm and 1995nm were 30nm. What's more, the wavelength of laser operating at 1995nm could be tuned up to 2004nm, at which CO2 gas possesses the optimal absorptive property in the region of 2?m. During the whole process of tuning, the 3dB bandwidth of output laser was observed and investigated, and an effective method of preventing 3dB bandwidth from broadening was proposed. Both output power and tunable range satisfy realization of the steady gas sensing. Finally, the theoretical calculation and analysis aiming at this tunable fiber laser's application in CO2 gas sensing were completed, combining with our previous experiments. Minimum detectable concentration,3.73x1016 molecule/cm3, amounting to 1254ppm, shows the excellent property and great potential in gas sensing. |
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