Research And Application Of Fiber Acousto-optic Tunable Filter

With the coming of all optical fiber network era, the optical fiber communication which develops rapidly, has raised a higher requirement for the optical fiber filter. It is necessary to research an optical fiber filter with the characteristics of simple structure, wide tuning range and fast tuning speed. In this thesis, optical fiber acousto-optic filter and its application for the field of optical fiber sensor are studied. Highly sensitive axial strain fiber laser sensor based on single-mode fiber based acousto-optic filter (SMF-AOTF) and fiber Bragg grating (FBG) demodulation system based on SMF-AOTF are demonstrated, respectively. The main achievements of this thesis are listed as follows.1. The axial strain and temperature characteristics of SMF-AOTF are analyzed in the theory and experiment. The SMF-AOTF has a linear wavelength shift response to the axial strain and temperature, and the SMF-AOTF with smaller fiber diameter is more sensitive to the axial strain and temperature. In the experiment, the axial strain and temperature characteristics of SMF-AOTF with the fiber diameter of 39μm and 67μm are measured. The experimental results accord well with the simulated ones.2. A highly sensitive axial strain fiber laser sensor based on SMF-AOTF is proposed and demonstrated. In the laser sensing system, SMF-AOTF acts as a band-rejection filter of the fiber laser senor and the sensing head used for axial strain measurement. The proposed fiber laser sensor combines the advantages of SMF-AOTF with linear wavelength shift and high sensitive to axial strain and fiber laser with high optical signal-to-noise ratio and narrow bandwidth. Experimental results show that a good linearity is obtained between the axial strain and central wavelength of the proposed fiber laser sensor, and a high sensitivity of-0.148 nm/με can be successfully achieved, which is much higher than the other types of fiber sensor.3. A FBG demodulation system based on SMF-AOTF is proposed and demonstrated. Due to band-stop characteristics of the SMF-AOTF, the demodulation system can measure the axial strain which is loaded in the FBG, by measuring the peak power of the reflection of FBG after the SMF-AOTF. Meanwhile, by controlling the output voltage of the signal general of SMF-AOTF, the system response of the proposed FBG demodulation system can be tuned.4. The characteristics of FBG based acousto-optic filter (FBG-AOTF) are studied, including uniform FBG-AOTF and non-uniform FBG-AOTF with the effects of Gaussian-apodization, phase shift and linear chirp. With the building of related theory model, the transmission characteristics of the FBG-AOTF are analyzed under different acoustic frequencies and acoustically induced strains. In the experiment, the uniform FBG-AOTF with shear-mode PZT in the acoustic frequencies of 563kHz and 885.5kHz, and the uniform FBG-AOTF with ordinary PZT in the acoustic frequencies of 390kHz and 700kHz, are measure respectively. The experimental results show that the transmission characteristics of the uniform FBG-AOTF with two different types PZT, are the same, and the various trendy of the experimental results accord well with the simulated ones.5. A fiber acousto-optic tunable filter based on superimposed fiber Bragg gratings (SFBG-AOTF) has been demonstrated and studied. Compared to the normal fiber Bragg gratings based all fiber acousto-optic tunable filter, SFBG-AOTF can modulate the two optical resonant wavelength of the gratings synchronously. The spectrum of SFBG-AOTF under various acoustic frequencies and acoustically induced strains, has been analyzed theoretically. In the experiment, the spectrum of SFBG-AOTF with the acoustic frequencies of 390kHz and 710kHz, are measured. The variation trend of the experimental results accord well with the simulated ones.