| Optical fiber sensing technology is a new sensor technology by fiber as a media to measure and transfer signal. Optical fiber sensors have attracted considerable attention in recent years because of their high precision, remote detection capability, electric insulation and anti-electromagnetic interference, safety in high temperature, highly pressure, flammable, explosive and highly corrosive circumstance. Recently, optical fiber sensor are not only used in high-grade, precision and advanced area, but also in coal mine, bridge, building, petrifaction, electric, steel, nuke industry, plane, ship, medical science etc.. Optical fiber sensor is universally accepted as a leading developing direction of sensor technology, and it has an excellent development future.There are two parts in this dissertation:one is narrow linewidth low noise optical fiber laser. Spectral characteristics of phase-shifted fiber grating, and output characteristics of fiber laser are described in first part. Then a self-injection locking Er3+doped fiber laser and a novel composite structure Er3+doped DBR fiber laser with a π-phase shifted FBG are proposed with emphasis. Another is fiber gas sensor. Optical fiber gas detection method based on absorption spectrum is studied. And tunable laser absorption spectrum gas detection method and active fiber loop ring-down gas sensor based on dual wavelengths differential absorption method are described in detail. The main contents of this dissertation are as follows:1. Optical fiber grating and phase shift grating are simulated by using coupling-mode theory and transfer matrix method. Design parameters of fiber gratings are obtained which can optimize its design and manufacture. Optical fiber grating manufacture system based Argon ion laser and phase dither method are studied.2. A self-injection locking fiber laser is designed in this paper. By injecting a part of output laser back into its cavity to amplify again, it can realize a stable output. The degree of polarization (DOP) is improved from0.165to0.989, and its linewidth is compressed in half. The self-pulsation of output light is reduced from4dBm to0.1dBm.3. By self-injection locking, the relative intensity noise (RIN) of fiber laser can be reduced efficiently. When pump power is25mW, the RIN of the DFB fiber laser is suppressed about16dB/Hz by the self-injection method around the relaxation oscillation frequency. And the resonance peak of the DFB fiber laser shifts to lower frequencies obviously from74.5kHz to41kHz. By this method, the RIN are suppressed not only around relaxation frequency, but also at medium frequency. It can improve the quality of output laser, which is useful to select effective frequency range and improve the signal noise ratio in application.4. A novel composite structure fiber laser is proposed in this paper. By introducing a π-phase shifted FBG into the DBR cavity as a selective wavelength component, it can increase the length of active fiber and lasing in a single mode and narrow linewidth. Efficiency of pump is increased. The proposed composite structure fiber laser obtaines narrow linewidth of900Hz and low RIN of-95dB/Hz. The pump threshold is about several mW. The SMRS is more than67dB.5. Optical fiber gas detection system based tunable laser absorption spectrum is designed. A inside reference gas cell is introduced to wavelength positioning and adaptive adjustment for the methane gas detection. Normalizing for absorption spectrum by base line can correct the influence of light power and optical loss. Engineering applications of gas detection system are described.6. A dual-parameters optical fiber sensor is proposed by introducing a FBG into the gas cell, which can measure methane concentration and temperature simultaneously and revise them. Temperature can be obtained by the shift of FBG wavelength, which can regulate the deviation of methane detection caused by temperature changing, meanwhile, methane’s absorbing line can locate FBG’s wavelength in order to make temperature monitoring more accurate. 7. A novel active fiber loop CRD system based on dual wavelengths differential absorption method is designed. Two DFB Laser Diodes (LDs) with different wavelengths are employed, which are modulated by taking pulse signals. And they are controlled by a driver alternately to insure only one laser signal run in the loop. By comparing the sensing signal with reference signal, it can eliminate the influence the cavity loss variety and photoelectric device drift in the system efficiently. An EDFA with Automatic Gain Control is used to compensate the power loss of the laser in the ring-down cavity, in order to increase the cavity round trips and improve the precision of gas detection.. The method provides an effective way for fiber loop ring-down system application in industry, especially in long distance sensing. The main innovations of this dissertation are as follows:1. We investigated a simple self-injection locking (SIL) fiber laser. By injecting a part of output laser back into its cavity to amplify again, it can realize a stable single-polarization lasing. And its linewidth is compressed in half; RIN is suppressed about16dB/Hz and relaxation oscillation frequency of laser shifts to lower frequencies obviously from74.5kHz to41kHz.2. By introducing a π-phase shifted FBG into the DBR cavity as a selective wavelength component, it can increase the length of active fiber and lasing in a single mode and narrow linewidth. Efficiency of pump is increased. Several mW output power is realized using low Er3+doped fiber, and a narrow linewidth of900Hz and low RIN of-95dB/Hz is obtained, respectively.3. A dual-parameters optical fiber sensor is proposed by introducing a FBG into the gas cell, which can measure methane concentration and temperature simultaneously and revise them. Temperature can be obtained by the shift of FBG wavelength, which can regulate the deviation of methane detection caused by temperature changing, meanwhile, methane’s absorbing line can locate FBG’s wavelength in order to make temperature monitoring more accurate. 4. Optical fiber methane detection system is developed and used in coal mine gob, methane drainage, garbage plant and other industrial processes. It is insulation and intrinsic safety. Capacity of remote sensing makes duty officer can get enough information easily needn’t going to spot.5. A novel active fiber loop CRD system based on dual wavelengths differential absorption method is designed for the first time. It can calibrate the inherent loss of the system in real time by employing two DFB LDs with different wavelengths, which can improve the precision of gas detection. The method provides an effective way for fiber loop ring-down system application in industry, especially in long distance sensing. |
PDF Full Text Request