OTDR light reflection structure of optical sub-assembly for transceivers of PON system

An optical module comprising:

• a ferrule;
• a retainer provided with a hole therein receiving and fitting with a portion of said ferrule;
• and
• a light filter affixed to said retainer and configured to allow permeability of a wavelength of light, wherein an optical path passes through a hole in said ferrule and then through an empty space to said light filter, wherein said ferrule has a first surface sloping relative to said optical path and facing a second surface of said light filter, wherein said second surface is substantially vertical to said optical path, wherein a first axial distance between said first and second surfaces and parallel with said optical path is greater than a second axial distance between said first and second surfaces and parallel with said optical path, wherein said first axial distance is opposite to said second axial distance with respect to said optical path.