Compact add/drop optical device

A compact add/drop optical device comprising:

• a first glass ferrule having a first hole and grasping a pair of optical fibers by the first hole;
• a first GRIN lens having a first end surface and a second end surface, wherein the first end surface is combined with the first glass ferrule, and a predetermined interval is formed between the first GRIN lens and the first glass ferrule;
• a second glass ferrule having a second hole and grasping an optical fiber by the second hole;
• a second GRIN lens having a third end surface and a fourth end surface, wherein the third end surface is combined with the second glass ferrule, and another predetermined interval is formed between the second GRIN lens and the second glass ferrule;
• a filter positioned between the first GRIN lens and the second GRIN lens;
• a first spacer ring sandwiched between the first GRIN lens and the filter;
• a second spacer ring sandwiched between the second GRIN lens and the filter;
• a third spacer ring sandwiched between the first GRIN lens and the first glass ferrule;
• and
• a fourth spacer ring sandwiched between the second GRIN lens and the second glass ferrule.