In this Letter, we have proposed an in-fiber duplex optical antenna based on a 45° radiated titled fiber grating (RTFG), in which the 45° RTFG not only radiates the light from the fiber core to the free space, but also harvests the light from the free space back into the fiber core. Using the finite difference time domain method, we have theoretically analyzed the light recoupling efficiency of the RTFG. The simulated results have shown that the RTFG-based optical antennas have a maximum coupling efficiency of 10%. The recoupling wavelength and efficiency are related to the grating period and horizontal incidence angle. Furthermore, we demonstrate a programmable spectral filtering system based on the 45° RTFG antennas, which could achieve filtering with arbitrary spectral shapes. The spectral resolution is 0.4 nm and the insertion loss is around 20 dB. The proposed programmable spectral filtering system has a compact structure compared with the traditional filter.