Studies On Dispersion Characteristics Of Fiber Bragg Grating Fabry-Perot Cavity

In recent yeas, fiber Bragg grating F-P cavities have been applied in the field offiber optic sensing and optical communication because of its excellent characteristics.With its new continuous development in science and technology, the new applicationsof fiber Bragg grating F-P cavity are emerging and therefore it becomes so importantto study its transmission spectra and dispersion characteristics here.The main results of this research are as follows:1. Firstly, for the F-P cavity using totally identical parameters of the two ends offiber Bragg gratings, we studied variation of transmission spectra, group delay andchromatic dispersion versus wavelength changes when the grating parameters change.It is found that with the increase of the refractive index modulation depths, fringevisibility of Bragg gratings and granting lengths of Bragg gratings, and cavity length ofF-P cavity, minimum transmission of F-P cavity increases. But grating period and corerefractive index has less effect on minimum transmission of F-P cavity. Transmissionbandwidth of the F-P cavity decreases with increase of the grating length and therefractive index modulation depth. Other parameters have no effect on the transmissionbandwidth. The numbers of resonance peaks in the transmission bandwidth becomemore with larger refractive index modulation depth and the F-P cavity length and otherparameters have no effect on it. When the cavity length is reduced to a certain extent,that is the threshold cavity length, there exist only one mode in the transmissionbandwidth. Here the parameter of the threshold cavity length is chosen as0.9mm. Theinterval between the resonant peaks within the transmission bandwidth increases withenlarging the grating length and the cavity length and decreases with enlarging therefractive index modulation depth, keep no change with enlarging the Grating period,the refractive index modulation depth and the core refractive index.Transmission lines can be red-shifted to increase parameters of the refractive index modulation depth ofgrating, cavity length of F-P cavity, granting length but the linewidths of resonancepeaks will decrease by increasing the three parameters plus that of the fringe visibilityof Bragg grating.2. For fiber Bragg grating F-P cavity formed by two identical Bragg gratings, weinvestigate how various parameters of Bragg grating F-P cavity affect the maximumgroup delay of the probe field and positive and negative extreme values of thedispersion. We find that the maximum value of group delays and the positive andnegative extreme values of the dispersion increases with enhancing granting length, therefractive index modulation depth, the fringe visibility of Bragg grating and the cavitylength but there are less effect of core refractive index and the grating period. Groupdelay and dispersion curve red-shifted with grating period, the refractive indexmodulation depth and the core refractive index to be increased. We also draw thecurves of the ratio of group velocity to speed of light in vacuum versus wavelength.With parameters of granting length, the refractive index modulation depth, the fringevisibility of Bragg grating and the cavity length to be increased, the probe light withwavelength at smallest transmission is transmitted from subluminally to superluminally.Group velocity is changed slower to the probe fields with resonant peaks. With therefractive index modulation depth, grating period and core refractive index increase,the corresponding wavelength position of the maximum and minimum group velocitywill be red-shifted. The effect of the cavity length on group velocity of the transmittedlight is limited.3. For fiber Bragg grating F-P cavity with different grating parameters at two ends,the minimum transmission decreases and the maximum group delay increases with theslight increase for the differences of the length difference of the two gratings, thefringe visibility of Bragg grating, the refractive index modulation depth. However,minimum transmission increases and maximum group delay decrease with slightincrease two grating period difference. Transmission of the resonant peaks in thetransmission bandwidth decreases with these values increase. This is disadvantageous for mode selection of F-P cavity. With increase the value of the difference of the twograting period and refractive index modulation depth, transmission lines red-shiftedand transmission bandwidth changes wider.4. In order to confirm resolution and dispersion of fiber Bragg grating F-P cavity,we compared the properties of transmission and dispersion of the F-P cavity with thoseof fiber Bragg gratings, in the condition that the two Bragg gratings are identical. Wefind that the numbers of the resonant peak within the transmission bandwidth willincrease in proportional to the length of F-P cavity. These resonant peaks are in thetransmission bandwidth of Bragg grating which are used for the F-P cavity. Theinterval of these resonant peaks become smaller comparing with the same Bragggratings. So compared with fiber Bragg grating, the resolution of F-P cavity should behigher in practical applications. From the dispersion curves of group delay of fiberBragg grating F-P cavity, we find that there are many negative values, so it can be usedfor dispersion compensation. Compared with fiber Bragg grating, the values fordispersion compensation by F-P cavity have large increase. And there is an additionalchoice for the cavity length of F-P cavity.The results of this study can be used for actual experiments and applications. Itprovides a better way using F-P cavity.