Optical device

Details Optical device Optical device

2002-04-25

2004-10-19

Bovernick, Rodney H. (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

C385S033000, C385S031000, C385S047000

Reexamination Certificate

active

06807337

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical device used in a communication system using optical fibers and particularly relates to an optical device using a gradient index rod lens provided with a reflecting layer.
In recent years, increase in capacity of an optical fiber communication network has been strongly demanded because of the rapid advance of popularization of Internet. Development of wavelength division multiplexing (WDM) communication as a method for increasing the capacity has been advanced rapidly. In WDM communication, light beam components with slightly different wavelengths are demodulated individually and mixed into an optical signal so that the optical signal can be transmitted through one optical fiber. At an end point of transmission, the optical signal in which the light beam components with different wavelengths are mixed is separated into light beam components by wavelengths so that the light beam components with different wavelengths can be received. The mixing of light beam components into an optical signal is referred to as “multiplexing” and the separation of the optical signal into light beam components is referred to as “demultiplexing”. A multiplexer/demultiplexer using an optical filter is used as a method for performing such multiplexing/demultiplexing.
An upper half of
FIG. 6
shows an example of the multiplexer. As shown in the upper half of
FIG. 6
, the wavelength multiplexer has three optical fibers, a pair of lenses, and a filter. That is, light with a wavelength of &lgr;1 is output from an optical fiber
101
. The light is input to a rod lens
103
. The light with a wavelength of &lgr;1 reaches a filter
104
while converted into a parallel beam by the rod lens
103
. The filter
104
reflects the light with a wavelength of &lgr;1. The reflected light is input to the rod lens
103
again and converged by the rod lens
103
. The converged light is coupled to an optical fiber
102
. On the other hand, light with a wavelength of &lgr;2 is output from an optical fiber
111
. The light with a wavelength of &lgr;2 reaches the filter
104
while converted into a parallel beam by a rod lens
113
. The filter
104
transmits the light with a wavelength of &lgr;2. The light transmitted through the filter
104
is input to the rod lens
103
and converged by the rod lens
103
. The converged light is coupled to the optical fiber
102
. In this manner, a light component with a wavelength of &lgr;1 output from the optical fiber
101
and a light component with a wavelength of &lgr;2 output from the optical fiber
111
are multiplexed so that the multiplexed light is coupled to the optical fiber
102
.
Demultiplexing is performed as shown in a lower half of FIG.
6
. That is, light components with wavelengths of &lgr;1 and &lgr;2 are output from the optical fiber
102
. The light components are input to the rod lens
103
. The light components reach the filter
104
while converted into parallel beams by the rod lens
103
. The filter
104
reflects the light component with a wavelength of &lgr;1. The reflected light component is input to the rod lens
103
again and converged by the rod lens
103
. The converged light component is coupled to the optical fiber
101
. On the other hand, the light component with a wavelength of &lgr;2 reaches the filter
104
while converted into a parallel beam by the rod lens
103
. The filter
104
transmits the light component with a wavelength of &lgr;2. The light component transmitted through the filter
104
is input to the rod lens
113
and converged by the rod lens
113
. The converged light component is coupled to an optical fiber
112
. In this manner, light components with wavelengths of &lgr;1 and &lgr;2 output from the optical fiber
102
are demultiplexed into the optical fibers
101
and
112
.
When the optical system shown in
FIG. 6
is used practically, a filter
4
may be brought into contact with an end surface
43
b
of a left rod lens
3
as shown in FIG.
7
. Incidentally, a right rod lens is not shown in FIG.
7
. When the filter
4
is disposed as shown in
FIG. 7
, it is unnecessary to position and fix the rod lens and the filter separately for forming the optical system as a module. There is an advantage in that the long-term stability of the optical system can be improved as well as the optical system can be assembled easily. This is a configuration effectively using the characteristic that the rod lens has a planar end surface.
In
FIG. 7
, an output optical fiber
1
and an input optical fiber
2
are disposed in parallel to each other, similarly to those in FIG.
6
. End surfaces
41
and
42
of the two optical fibers
1
and
2
are disposed so as to face an end surface
43
a
of the rod lens
3
through a gap of a suitable distance. The gap may be formed as an air layer or may be filled with a medium
5
such as matching oil or an adhesive agent.
For example, the index distribution of the rod lens
3
is given by the following formula (see Japanese Patent Laid-Open No. 91316/1985):
n
(
r
)
2
=n
0
2
·{1−(
g·r
)
2
+h
4
(
g·r
)
4
+h
6
(
g·r
)
6
+h
8
(
g·r
)
8
+ . . . }
in which r is a radial distance measured from the optical axis of the rod lens, r
0
is the effective radius of the rod lens, n
0
is the refractive index of the rod lens on the optical axis of the rod lens, g is a gradient index distribution coefficient of second order, and h
4
, h
6
, h
8
. . . are gradient index distribution coefficients of fourth, sixth, eighth . . . order respectively.
The periodic length P of the rod lens is equal to 2&pgr;/g. When the length Z of the rod lens
3
on the optical axis of the rod lens
3
is set to be slightly smaller than 0.25P, a luminous flux output from the optical fiber
1
is collimated into approximately parallel light rays at the end surface provided with the filter
4
. Hence, the luminous flux reflected by the filter
4
is converged again and returned to the optical fiber
2
.
When the position of the end surface
41
of the optical fiber
1
is adjusted both in a direction of the optical axis
21
and in a direction perpendicular to the optical axis
21
while the two optical fibers
1
and
2
are disposed in parallel to the optical axis
23
of the rod lens
3
, the luminous flux output from the optical fiber
1
is focused on the end surface
42
on the optical axis
22
of the optical fiber
2
so that high coupling efficiency can be obtained.
In the arrangement shown in
FIG. 7
, however, a principal beam component (defined as a beam component of maximum intensity forming a symmetrical center of Gaussian beams) of the light output from the optical fiber
1
disagrees with the optical axis
22
of the optical fiber
2
. Hence, an inclination &thgr;d is generated in an XZ plane as shown in FIG.
8
. As a result, a coupling loss corresponding to the size of the inclination &thgr;d is produced.
The inclination &thgr;d can be eliminated if the length of the rod lens
3
is set to be 0.25 pitches while the end surfaces
41
and
42
of the two optical fibers are brought into contact with the end surface
43
a
of the rod lens. In such a design, there is however a disadvantage in that the degree of freedom for delicate adjustment of focusing and positioning the fibers is spoiled. If the lens length is shortened, the adjustment can be made easily because the distance between each of the end surfaces
41
and
42
of the optical fibers
1
and
2
and the end surface
43
a
of the lens
3
becomes long, but there is a problem that the loss due to the inclination &thgr;d becomes large.
SUMMARY OF THE INVENTION
The invention is to provide a condition for suppressing the loss due to the inclination &thgr;d to be in a practically allowable range.
An optical device includes an optical system constituted by a combination of an output optical fiber, an input optical fiber and a gradient index rod lens, the output optical fiber and the input optical fiber being arranged so that optical axes of the two optical fibe

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