Assembly method for coherent receiver

A method of assembling an optical module that provides a housing, an input port, and an optical component, the housing having one side and another side, the input port receiving a signal light that has two polarization components at right angles each other, a polarization dependent beam splitter (PBS) that splits the signal light into two portions depending on the polarizations contained in the signal light, a beam splitter (BS) that splits the local light into two portions, a multi-mode interference (MMI) device that interferes between one of the two portions of the signal light and one of the two portions of the local light, optical components provided between the PBS and the MMI device, the method comprising steps of:

• placing a tool on a rotation stage with a reference wall, the tool having a pair of sides extending in parallel to each other and a reference side making the angle θ against one of the sides, the tool in one of the paired side being abutted against the reference wall of the rotation stage;
• arranging the PBS between the signal port and the MMI device;
• arranging at least of the components between the PBS and the MMI device;
• inputting a test light of a second wavelength range outside a first wavelength range of the signal light to the input port;
• and
• adjusting at least of the components to become the maximum which a magnitude of the test light received by MIMI device,
• wherein a shortest wavelength of the first wavelength range is 1,565 nm, and
• the second wavelength range includes the wavelength that is shorter than 1,565 nm.