Optical device with a pig tail optical fiber and its production method

A method of producing an optical device including an optical waveguide, having a rectangular cross-section and having a terminal, and a pig tail optical fiber including a first optical fiber portion having a circular cross-section with a specific refractive index difference substantially equal to or higher by up to 1% than that of said optical waveguide, and having a first terminal and a second terminal, the first terminal being connected to the terminal of said optical waveguide, and including a second optical fiber portion, having a circular cross-section with a specific refractive index difference lower by up to 0.5% than that of said optical waveguide and having a terminal connected to the second terminal of said first optical fiber portion, said method comprising the steps of:

• producing said first optical fiber portion with a core having a dopant for refractive index control and a cladding surrounding the core, so that the core has a mode field diameter substantially equal to that of said optical waveguide;
• and
• diffusing the dopant of the core in the neighborhood of said second terminal of said first optical fiber portion by applying heat thereto until the mode field diameter of said first optical fiber portion is substantially the same as that of said second optical fiber portion at said second terminal of said first optical fiber portion.