In this Letter, we propose a simple structure of an orthogonal type double Michelson interferometer. The orthogonal detection method overcomes the problems of uneven ranging sensitivity and the inability of traditional interferometers to determine the displacement direction. The displacement measurement principle and signal processing method of the orthogonal double interferometer are studied. Unlike the arctangent algorithm, the displacement analysis uses the arc cosine algorithm, avoiding any pole limit in the distance analysis process. The minimum step size of the final experimental displacement system is 5 nm, which exhibits good repeatability, and the average error is less than 0.12 nm.

We propose a concept of wavelength synchronization to ensure the stability of ultra-dense channels in an ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) transmitter. A mode-locked laser is used to provide wavelength references for users. By injection locking the semiconductor laser, the separation of the wavelength reference is realized in an optical line terminal. The downlink and uplink wavelength references are interlaced and distributed to facilitate the synchronization of uplink carriers. In the optical network unit, the uplink optical carriers are filtered by injection locking semiconductor lasers, which achieve wavelength synchronization for the uplink users. In this Letter, an adaptive wavelength synchronization transmitter for UDWDM-PON is realized with a channel spacing of 5 GHz.