Four polarization maintaining optical fiber ferrule and optical coupler using same

An optical coupler, comprising:

• (a) a cylindrical sleeve having an inside surface and first and second ends;
• (b) a pair of four polarization maintaining optical fiber ferrules, one of said ferrules positioned at each of the first and second ends of said sleeve, and each of said ferrules having a cylindrical body with an outer surface, a central axis, an end-face perpendicular to the central axis, a throughhole at the center of the end-face extending along the central axis, said throughhole having a diameter at the end-face, at least one first reference portion on the outer surface of the cylindrical body of the ferrule, and four polarization maintaining optical fiber elements fastened within said throughhole, each of the polarization maintaining optical fiber elements having a polarization maintaining plane and a diameter which is related to the diameter of said throughhole by the following formula:
• d=(2.sup.1/2 +1)d.sub.1 +.delta.
• where
• d:
• Diameter of the throughhole at the center of the end-face of said ferrule,
• d.sub.1 :
• Diameter of the polarization maintaining optical fiber element,
• and
• .delta.:
• Allowance for error in .mu.m;
• and
• wherein the polarization maintaining planes of at least one pair of polarization maintaining optical fiber elements arranged symmetrically with respect to the central axis of said ferrule are fastened at certain angles with respect to said first reference portion;
• (c) a lens unit positioned within said sleeve, comprising first and second graded index rod lenses, each of said graded index rod lenses having a first end facing the first end of the other graded index rod lens and a second end adjacent to the end-face of one of said ferrules;
• and
• (d) a thin film located between the first ends of said first and second graded index rod lenses within said sleeve which partially transmits incident light and reflects the remainder of the light.