Study Of Optical Fiber Delay Line Based On Fiber Bragg Grating

Phased-Array Antenna using conventional electrical phase-shifter technique introduces beam squinting and signal broadening problem, which seriously limits its performance in wide-band scanning. However, optical true-time delay technology based on optical fiber delay line can be applied to overcome this problem effectively.Fiber Bragg grating(FBG), as an important fiber device, has become more and more popular in optical communication and fiber sensor area. It is well known that optical signal with wavelength that satisfies Bragg resonant condition(resonant wavelength) will be reflected at different positions in FBG and then acquire different time delays. Furthermore, the varying temperature and strain can change the reflectivity and time-delay characteristic of FBG, which leads to different time delay for optical signal with a specific resonant wavelength. Therefore, FBG has become of great importance in fabricating fiber delay line, even continuously tunable time delay can be achieved.First of al, this thesis studies transmission characteristic of FBG by using coupled mode theory and transmission matrix method, which involves reflectivity and time-delay characteristic of uniform FBG and linear chirped fiber bragg grating(LC FB G) respectively. With the help of Matlab software, it is figured out that how the parameters like refractive index modulation depth, length and chirp value affect the characteristic of the two kinds of FBG above. Then six common apodization function is further studied, which shows that apodization can restrain the ripple wave of reflective spectrum and improve the linearity of time delay curve of FBG, especially, the function of “sinc” and “tanh” are more effective when applied to LCFBG.Secondly, the time-delay characteristic of LCFBG is studied under the effect of changing temperature and strain. Analyzed from theory, it is found that thermal expansion and thermo-optic effect caused by temperature, shape changing and elasto-optic effect caused by strain, both bring about the shift of Bragg reflective wavelength of LCFBG. As a result, an optical signal with specific wavelength in reflective spectrum of LCFBG will experence a different time-delay compared with unaffected situation. Both theoretical and experiment study prove that there exists linear relationship between the shift value of Bragg reflective wavelength and the changing value of temperature or strain. During the experiment study, LCFBG is packged on a copper sheet to enhance its temperature sensitivity, the results indicate that the temperature sensitivity is 3.1 times higher than that of bare LCFBG in the temperature range of 0~70℃, and cantilever beam of unifor m strength is utilized to stretch or compress LCFBG, the shift value of bragg reflective wavelength has reached 6.45 nm. Another conclusion is that the dispersion of LCFBG is changing with the variation of temperature and strain, but not a lot.In the end, optical fiber delay line(OFDL) based on fiber Bragg grating is studied. For the study of OFDL based on uniform FBG, three structures of 4-bit delay line are proposed successively. Both structure 1 and 2 contain cascaded FBGs, which cause the problem of crosstalk and optical power disagreement, and also make the time-delay step very large. However, structure 3 with all FBGs connected in parallel, overcomes these problem effectively. As for the study of OFDL based on LCFBG, on the one hand, by using a partially apodized LCFBG, a 5-bit delay line is developped, which achieves a time-delay scope of 0~168.6ps, with average time-delay step of 5.39 ps, the resolution is about 2.46 ps. On the other hand, by using the strain-tuning technology, a delay line based on tunable LCFBG is experimently studied, the result indicates that a time-delay scope of-64.5ps~60ps with minimum time-delay step of amost 0.1ps is achieced, which means qusi-continuous time delay is obtained.