Method of fabricating a wavelength division multiplexed (WDM) unit

A method of fabricating a WDM unit comprising the steps of:

• attaching a first and second GRIN lenses to opposite surfaces of a WDM filter by glue, wherein the WDM filter has two flat faces and the first and second GRIN lenses both have a flat face adjacent to the two flat faces of the WDM filter, wherein the two flat faces of the WDM filter are larger than the corresponding flat faces on the first and second GRIN lenses so that the glue is applied to outer joint periphery of junctions between the WDM filter and the first and second GRIN lenses;
• securing two glass tubes inside two inner metal tubes by glue respectively;
• securing a dual fiber pigtail and a single fiber pigtail in the glass tubes by glue respectively;
• positioning the second GRIN lens in one of the glass tube having the dual fiber pigtail, and adjusting the second GRIN lens to obtain an optimized result measured by a testing instrument;
• securing the second GRIN lens in one of the glass tubes by glue;
• positioning the first GRIN lens in another glass tube having the single fiber pigtail, and adjusting the first GRIN lens to obtain an optimized result measured by the testing instrument;
• and
• securing the first GRIN lens in the glass tube having the single fiber pigtail by glue.