A silicon photonic integrated circuit is provided, which includes a first optical power splitter, a second optical power splitter, a first grating coupler and a second grating coupler. The first optical power splitter has an input, a first output and a second output, in which the input is configured to receive an inputted beam, and the first output is configured to output a returned beam. The second optical power splitter has an input, a first output and a second output, in which the input is coupled to the second output of the first optical power splitter. The first and second grating couplers are respectively coupled to the first and second outputs of the second optical power splitter, and are configured to optically couple two opposite ends of a fiber coil, respectively.