Wavelength selective optical device and method of tuning wavelength characteristics

1. A wavelength selective optical device connecting a first optical fiber with an end face to a second optical fiber, comprising:

a collimating lens having a first end surface located such that a divergent light emitted from the end face of a first optical fiber is incident, and having a second end surface, the collimating lens arranged to emit the incident divergent light as a parallel light beam emitted from the second end surface; and

an optical filter arranged to face the second end surface of the collimating lens so that the parallel light beam emitted from the collimating lens is incident on the optical filter,

wherein the collimating lens and the optical filter are arranged such that a reflected light by the optical filter is incident back to the lens and input to the second optical fiber which is located at a location in which the reflected light is focused by the lens,

wherein the collimating lens is provided with two gradient index rod lenses having different refractive index distribution constants relative to each other,

wherein said two gradient index rod lenses have substantially the same outer diameter, and are bonded to each other so that optical axes of said two gradient index rod lenses are aligned, and

wherein the refractive index distribution constants of said two gradient index rod lenses are √{square root over (A 1 )} and √{square root over (A 2 )}, respectively, and refractive indices on center axes are N 01 and N 02 , respectively, and respective lens lengths Z 1 and Z 2 of the lenses substantially satisfy:

tan(√{square root over ( A 1)}· Z 1)·tan(√{square root over ( A 2)}· Z 2)= N 01·√{square root over ( A 1)}/( N 02·√{square root over ( A 2)}).

2. A wavelength selective optical device connecting a first optical fiber with an end face to a second optical fiber, comprising:

a collimating lens having a first end surface located such that a divergent light emitted from the end face of a first optical fiber is incident, and having a second end surface, the collimating lens arranged to emit the incident divergent light as a parallel light beam emitted from the second end surface; and

an optical filter arranged to face the second end surface of the collimating lens so that the parallel light beam emitted from the collimating lens is incident on the optical filter,

wherein the collimating lens and the optical filter are arranged such that a reflected light by the optical filter is incident back to the lens and input to the second optical fiber which is located at a location in which the reflected light is focused by the lens,

wherein the collimating lens is provided with two gradient index rod lenses having different refractive index distribution constants relative to each other, and

wherein said two gradient index rod lenses have substantially the same outer diameter, and are bonded to each other so that optical axes of said two gradient index rod lenses are aligned; and

another gradient index rod lens having a first end surface thereof facing the optical filter, and

a third optical fiber arranged on a side of a second end surface of said another gradient index rod lens to which a light transmitted through the optical filter is coupled through the second gradient index rod lens.

3. A wavelength selective optical device according to claim 1 , wherein the optical filter is a multi-layered optical interference filter.

4. A wavelength selective optical device according to claim 3 , wherein the optical filter is formed directly on the second end surface of the collimating lens as a film.

5. A wavelength selective optical device connecting a first optical fiber with an end face to a second optical fiber, comprising:

a collimating lens having a first end surface located such that a divergent light emitted from the end face of a first optical fiber is incident, and having a second end surface, the collimating lens arranged to emit the incident divergent light as a parallel light beam emitted from the second end surface; and

an optical filter arranged to face the second end surface of the collimating lens so that the parallel light beam emitted from the collimating lens is incident on the optical filter,

wherein the collimating lens and the optical filter are arranged such that a reflected light by the optical filter is incident back to the lens and input to the second optical fiber which is located at a location in which the reflected light is focused by the lens,

wherein the collimating lens is provided with two gradient index rod lenses having different refractive index distribution constants relative to each other, and

wherein said two gradient index rod lenses have substantially the same outer diameter, and are bonded to each other so that optical axes of said two gradient index rod lenses are aligned; and

a cylindrical member arranged such that the collimating lens is inserted from one end of the cylindrical member and wherein the collimating lens is fitted without clearance to the cylindrical member, wherein the optical filter is provided on an end portion of the cylindrical member opposite the one end.

6. A method of tuning a wavelength characteristic of a wavelength selective optical device, in which a divergent light is collimated and incident on an optical filter through a lens, a predetermined wavelength range of the light incident thereon is reflected or transmitted by the optical filter, wherein the lens is provided with two gradient index rod lenses bonded to each other that have different refractive index distribution constants and the same outer diameter to each other to align optical axes of said two gradient index rod lenses, the method comprising the steps of:

fixing the optical filter closely adhered to one end of the lens for transmitting or reflecting a predetermined wavelength band;

fixing two optical fibers to the other end of the lens at a predetermined interval;

inputting light having the wavelength band reflected by the optical filter from one of the optical fibers to the lens;

coupling the light reflected by the optical filter to the other of the optical fibers by the lens;

adjusting relative locations of the two optical fibers fixed to the rod lenses at the predetermined interval such that an intensity of the coupled light becomes a maximum or a predetermined value or more, and

changing a combination of lens lengths of the two rod lenses of the fixed wavelength selective optical device to adjust a representative value of the reflected wavelength band in a predetermined range.