All-Fiber Frequency Shifter with a Lower Frequency Shift for the Coherent Detection

We demonstrate two kinds of all-fiber frequency shifters for optical coherence detection. The mode-selective coupler (MSC) and long period fiber grating (LPFG) are cascaded with the acoustically induced fiber grating (AIFG), respectively. Due to the frequency shifting characteristics of the AIFG, the low frequency shift of less than 1 MHz is achieved by both of them. Furthermore, the mode conversion occurs only within the frequency shifter, which is capable of implementing the input and output of the core LP01 mode. The LP11 mode converted by MSC and LPFG is finally converted back to LP01 mode by AIFG, and the frequency is shifted up. The structure of the two kinds of frequency shifters is simple and compact, and the signal-to-noise ratio (SNR) can reach 65 dB, which provides a feasible scheme for the coherence heterodyne detection technology of low frequency.