Optical module and method for assembling the same

Details Optical module and method for assembling the same Optical module and method for assembling the same

2002-08-05

2004-08-24

Glick, Edward J. (Department: 2882)

Optical waveguides

With optical coupler

Input/output coupler

C385S035000, C385S052000

Reexamination Certificate

active

06782162

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical module for use in a field of an optical communication, and more particularly, to an optical module including a collimator comprising an optical fiber and a lens.
In a field of an optical communication, when it is required to give an action to a light transmitted through an optical fiber, an optical device is inserted in an optical path. Since a light emitting from the optical fiber diverges, a collimator which converts a diverging light to a parallel light is used in order to efficiently introduce the light to the optical device. The collimator and the optical device are practically modularized to be used as the optical module.
FIG. 1
shows, as one example of the optical module, a filter module
200
which separates a light of a multiplexed wavelength in the optical communication of a wavelength multiplex system.
Now, a light having two multiplexed wavelengths &lgr;1, &lgr;2 enters a lens
121
from an optical fiber
111
. The lens
121
is a gradient index rod lens, whose refractive index is distributed in a radial direction of a circular section perpendicular to an optical axis. When a lens length of the lens
121
is adequately designed, a diverging light entered from the optical fiber
111
is converted to a parallel light. A spectral separation optical filter
150
which is an optical device is arranged in contact with an end face opposite to a light receiving surface (an end face opposed to the optical fiber
111
) of the lens
121
. The optical filter
150
reflects the light having a wavelength &lgr;1 and transmits the light having the wavelength &lgr;2. The light reflected by the optical filter
150
is converged by the lens
121
and enters an optical fiber
113
.
The light (wavelength &lgr;2) transmitted through the optical filter
150
enters a lens
122
, is converged therein and then enters an optical fiber
112
. Generally, the lens
121
and the lens
122
have the same characteristics. An optical system is integrated into one housing
170
to constitute the optical filter module
200
.
A basic optical system of the collimator used for the optical module will be described in accordance with FIG.
2
. Now, consider a case where point light sources
211
,
213
are arranged on one focal surface
281
of a first convex lens
221
, and on an optical axis
201
and at a position &Dgr;P separated from the optical axis
201
, respectively, as shown in FIG.
2
A. In view of geometrical optics, a light from the light source on the focal surface
281
is converted to a parallel light by the convex lens
221
. However, unless the position of the light source exists on the optical axis
201
, the direction of the parallel light inclines with reference to the optical axis
201
in accordance with the position &Dgr;P of the light source. When the parallel light beam enters a second convex lens
222
having the same focal distance as the first convex lens
221
, images
212
,
214
are formed at positions symmetrical with respect to the light sources
211
,
213
regarding both the lenses
221
,
222
.
A traveling state of the light beam is the same as shown in
FIG. 2B
, even when the lenses are gradient index rod lenses
321
,
322
. The rod lenses
321
,
322
are cylindrical, whose refractive indexes are distributed along radial directions from the cross sectional center. A refractive index distribution n(r) is ideally represented by the following equation:
n
(
r
)=
n
0
(1−(
A/
2)r
2
)
wherein r is a distance from a center axis of the lens, n
0
is a refractive index on a center axis of the lens, A
1/2
is a refractive index distribution constant. A meandering period (pitch) P of the light beam in the gradient index rod lens is represented by P=2&lgr;/A
1/2
. Here, for simplification, the gradient index rod lens having a lens length of 0.25 P is illustrated. In the lens of 0.25 P, a light generated from the point source light on the end face is converted to a parallel light beam and then emitted.
A case where point light sources
311
,
313
are arranged on one end face of a first convex lens
321
on an optical axis
301
at a spaced-apart position &Dgr;P from the optical axis
301
respectively is considered. However, unless the position of the light source exists on the optical axis, the direction of the parallel light inclines with reference to the optical axis depending upon the position &Dgr;P of the light source in the same manner as in the convex lens. When the parallel light enters a second convex lens
322
having the same pitch as the first lens
321
, images
312
,
314
are formed at the symmetrical position of the light source relative to both lenses
321
,
322
.
Generally, when two lenses having limited effective diameters are arranged on the same optical axis, the optical axis inclines when a distance L between the lenses increases, consequently there is a case where a part of the light can not enter the second lens.
In the case of the optical filter module
200
shown in
FIG. 1
, since each of optical fibers
111
,
113
has a limited diameter, at least one optical fiber is arranged apart from the center axis of lens. The light entering and emitted from the optical fiber arranged apart from the center axis of lens has predetermined angle with respect to the center axis at another end face of the lens.
For example, when the optical axis of the optical fiber
111
is coincident with the center axis of the lens
121
, the incident light toward the lens becomes parallel relative to the center axis of the lens at the emitting end of the lens and is emitted perpendicularly with reference to the end face of the lens. In this case, however, since a part of the light which is reflected at the end face of the lens returns to the optical fiber
111
, the reflected return light is undesirable for the optical communication. When in manufacturing the optical module, it is not always easy to frequently adjust the position of the incident light to perpendicularly emit the light from the end face of the lens, even if a reflected return light problem may not be considered.
Actually, as shown in
FIG. 1
, the end face of the optical fiber
111
and the face of the lens opposed to the optical fiber are normally formed to incline relative to the optical axis in order to prevent the reflected return light. In addition, even when the light enters from the lens face inclined with respect to the optical axis, the outgoing light from the lens inclines with respect to the optical axis.
A method will be considered to incline the center axis of the lens
122
relative to the optical axes of the optical fiber
111
and the lens
121
in order to converge the light having a given angle relative to the center axis of the lens
121
and to introduce thereto with a low loss. According to the method, the center axes of lenses
121
and
122
are arranged with a predetermined angle in the optical filter module
200
in FIG.
1
. In this case, it is necessary for an inside diameter of a sleeve
160
to be largely formed than that of a glass holder
144
to keep a sufficient space for adjusting the angle.
Now, the assembly process of the optical filter module
200
will be described below.
First, the collimator
201
including two optical fibers
111
,
113
, the lens
121
and the optical filter
150
is assembled. Distal ends of two optical fibers are inserted into a capillary
131
having, for example, two holes and secured thereto with an adhesive, and then the end face of the capillary
131
is polished. The capillary with optical fibers
131
is inserted into a glass holder
140
capped with a cylindrical metal tube and secured thereto with the adhesive.
The optical filter
150
is adhered to the end face of lens
121
. The lens
121
is inserted into the glass holder
142
, and secured thereto with the adhesive. In this case, the light having the wavelength &lgr;1 enters the lens
121
from the optical fiber
111
, and the capillary
131
and the lens
121
are adjusted so that the reflecte

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