Frequency Response Equalization Using Fiber Bragg Grating Tilted Filter in RoF Systems

A uniform frequency response is highly appreciated in broadband wireless radio-over-fiber (RoF) systems. However, in conventional double sideband (DSB) modulation schemes, the RF gain tends to drop periodically due to the fiber chromatic dispersion effects, and the system's frequency response will deteriorate at high frequencies owing to the limited device bandwidth. In this paper, a novel scheme is described to overcome these problems by utilizing a fiber Bragg grating (FBG), which acts as a tilted filter to offer different suppression for optical carrier and the two sidebands. As the RF frequency increases, one sideband of the optically carried RF signal is more and more suppressed while the other one is just the contrary. Both theoretical analysis and experimental results show that the total RF power received at high frequency might even be greater than that at low frequency in this scheme. Therefore, it can compensate the RF gain drop at high frequency. Moreover, the dispersion effect is also mitigated by suppressing one sideband of the DSB signal. Thus, the system's frequency response is greatly improved and the delivery distance can be extended. The proposed scheme is highly tunable by tuning the central wavelength of the FBG.