Optical waveguides – Polarization without modulation
Reexamination Certificate
1999-03-31
2002-05-21
Lee, John D. (Department: 2874)
Optical waveguides
Polarization without modulation
C385S015000, C385S024000, C385S039000
Reexamination Certificate
active
06393168
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for maintaining an incident optical signal having a low degree of polarization in a specific state of polarization at a terminal of an optical waveguide.
In the technical field of transmitting a light beam while maintaining its state of polarization, a special optical fiber capable of maintaining the state of polarization, i.e., a polarization maintaining optical fiber is often used. “Introduction to Optical Fiber Communications” (Yasuharu Suematsu et.al., OHM-sha, Ltd., pp. 197-199, Mar. 10, 1991, Third Edition) describes polarization maintaining optical fibers. An optical fiber having increased elliptical deformation and axially asymmetrical side pits in the gradient index to increase structural axial asymmetry, an elliptical jacket fiber having a double cladding structure and an elliptically deformed intermediate cladding to apply stress to the core, and the like are available in addition to the PANDA fiber (Polarization maintaining and absorption reducing fiber) having a structure in which stress applying portions are formed in the cladding to apply anisotropic stress to the core.
The most popular PANDA fiber has only two directions of the plane of polarization allowed to be maintained, as shown in FIG.
16
. The directions of the plane of polarization of the remaining polarization maintaining optical fibers described above are also limited to specific directions.
As shown in
FIG. 16
, reference numeral
101
denotes a cross-section of the PANDA fiber. Stress applying portions
103
a
and
103
b
are present on the cross-section
101
so as to sandwich a core
102
through which a light beam passes. Directions
104
and
105
along which the polarization maintaining optical fiber can maintain the plane of polarization are a direction connecting the centers of the stress applying portions
103
a
and
103
b
, and a direction perpendicular to the above direction, as indicated by chain lines. No incident light beam having the plane of polarization in a direction except the above two directions can be maintained.
To solve this problem in a conventional arrangement, as shown in
FIG. 17
, a light source
106
a
having a high degree of polarization like a semiconductor laser is used. A polarization controller
108
serving as a means for arbitrarily changing the plane of polarization is arranged in front of a polarization maintaining optical fiber
107
. The plane of polarization of a laser beam must be aligned in a polarization maintainable direction of the polarization maintaining optical fiber
107
, and the aligned laser beam must be guided to the polarization maintaining optical fiber
107
.
Referring to
FIG. 17
, reference numeral
109
denotes a single-mode optical fiber for connecting the light source
106
a
and the polarization maintaining optical fiber
107
; and
110
, a connecting portion between the polarization maintaining optical fiber
107
and the single-mode optical fiber
109
. The connecting portion comprises an optical connector or may be fusion-spliced. Reference numeral
111
denotes an output terminal of the polarization maintaining optical fiber
107
. The polarization controller
108
is located midway along the single-mode optical fiber
109
in FIG.
17
. However, the polarization controller
108
may be arranged between the single-mode optical fiber
109
and the polarization maintaining optical fiber
107
.
FIG. 18
shows the state of a propagation light beam in the optical circuit system shown in FIG.
17
. Assume that a light beam emitted by the light source
106
a
having a high degree of polarization is an elliptically polarized light beam
112
a
(reference numeral
113
denotes its plane of polarization) (linear polarization and circular polarization correspond to special cases of elliptical polarization). In this case, the polarization controller
108
is inserted midway along the single-mode optical fiber
109
to convert the elliptically polarized light beam
112
a
into a linearly polarized light beam
114
parallel to a polarization maintaining plane
104
of the polarization maintaining optical fiber
107
, and the linearly polarized light beam
114
is incident. The state of polarization in the polarization maintaining optical fiber
107
and the state of polarization of an exit light beam from the output terminal
111
can be kept constant.
FIG. 19
shows the state of a propagation light beam when the polarized light beam
112
a
emitted by the light source
106
a
having a high degree of polarization is incident on the polarization maintaining optical fiber
107
such that the plane
113
of polarization of the polarized light beam
112
a
is entirely different from the polarization maintaining direction of the optical fiber. That is, this is equivalent to the case in which the polarization controller
108
is omitted from the arrangement in FIG.
18
. In this case, the polarization maintaining optical fiber
107
has no longer the polarization maintaining function. A polarized light beam
112
b
different from the polarized light beam
112
a
propagates through the polarization maintaining optical fiber
107
and emerges from the output terminal
111
in a state
115
of polarization different from the plane
113
of polarization. When the light beam receives perturbation
116
at any position in the polarization maintaining optical fiber
107
, the state of polarization of the exit light beam changes from
115
to
117
in accordance with the perturbation
116
. The perturbation is disturbance such as a dynamic stress, stress based on temperature or humidity, or the like.
Consider an optical system for allowing a polarization analyzer
118
to observe a light beam controlled by a polarization controller
108
, as shown in FIG.
20
. This system is obtained by adding the polarization analyzer
118
to the output terminal
111
in FIG.
17
. The observation result is expressed on the Poincaré sphere using a state of polarization as the Stokes parameter, as shown in FIG.
21
. Reference numeral
1101
denotes clockwise circular polarization;
1102
, −45° linear polarization;
1103
, vertical linear polarization;
1104
, +45° linear polarization;
1105
, horizontal linear polarization; and
1106
, counterclockwise circular polarization. When the polarization controller
108
aligns the plane of polarization of the elliptically polarized light beam
112
a
with the plane of polarization of the polarization maintaining optical fiber
107
, the plane of polarization observed by the polarization analyzer
118
is observed on an equator
1107
of the Poincaré sphere. That is, the observed light beam is any one of the linearly polarized light beams. When the plane of polarization of the elliptically polarized light beam
112
a
is not aligned with the plane of polarization of the polarization maintaining optical fiber
107
, the state on the equator
1107
of the Poincaré sphere cannot be maintained upon receiving the perturbation in an unstable state of polarization. The state of polarization irregularly changes, as indicated by reference numeral
1108
.
As described above, in light beam propagation using a light source having a high degree of polarization like a semiconductor laser while maintaining a state of polarization, the plane of polarization of an exit light beam from the semiconductor laser must be converted into the polarization maintaining direction of the polarization maintaining optical fiber using a means (e.g., a polarization controller) capable of arbitrarily changing the state of polarization. The converted light beam must be guided to the polarization maintaining optical fiber. The means for aligning the state of polarization with the polarization maintaining plane of the polarization maintaining optical fiber is generally expensive and complex. It is also possible to incorporate the polarization maintaining optical fiber as a semiconductor laser module while aligned with the plane of polarization of the exist light beam from
Atoji Motoyuki
Sugai Eiichi
Blakely & Sokoloff, Taylor & Zafman
Lee John D.
NTT Advanced Technology Corporation
LandOfFree
Method and apparatus for maintaining optical signal having... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for maintaining optical signal having..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for maintaining optical signal having... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2865495