Optical apparatus

Details Optical apparatus Optical apparatus

2003-08-14

2004-11-09

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S484010, C359S324000, C359S281000

Reexamination Certificate

active

06816300

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical element for use in an optical device utilizing the Faraday effect, and more particularly to a polarization independent optical isolator, an optical circulator, and a polarization beam combiner that are small and easy to assemble and require a very small number of components.
DESCRIPTION OF THE RELATED ART
FIG. 25
illustrates an optical circulator disclosed in Japanese Patent No. 2539563. The circulator includes an optical collimator that includes optical fibers
12
and
14
fixedly mounted to Ferrules
11
and
20
, respectively, and lenses
22
and
23
, and another optical collimator that includes an optical fiber
82
fixedly mounted to a Ferrule
81
and a lens
72
. There are provided two birefringent blocks
21
and
71
for branching a light beam into two optical paths or combining light beams into one optical path depending on the direction of travel and the polarization direction of the incident light beam(s). There are also provided a pair of wave plates
37
and
38
that cause the light to be polarized in the same direction. The wave plates
37
and
38
are positioned in parallel with respect to the light path. Positioned adjacent the wave plates
37
and
38
is a Faraday rotator
31
to which a magnet
32
applies an external magnetic field. Wave plates
67
and
68
are also positioned in parallel with resect to the light path. A Faraday rotator
61
is positioned adjacent the wave plates
67
and
68
, and a magnet
62
applies an external magnetic field to the Faraday rotator
61
. A birefringent block
41
is positioned in the middle of the optical circulator, so that the light passes through different optical paths depending on the direction of travel and the direction of polarization.
FIG. 26
illustrates another optical circulator disclosed in Japanese Patent No. 2539563. The optical circulator includes an optical collimator that includes optical fibers
12
and
14
fixedly mounted to Ferrules
11
and
20
, respectively, and lenses
22
and
23
, and another optical collimator that includes an optical fiber
82
fixedly mounted to a Ferrule
81
and a lens
72
. There are provided two birefringent blocks
21
and
71
between the two optical collimators, the birefringent blocks branching a light beam into two optical paths or combining light beams into one optical path depending on the direction of travel and polarization direction of the light beam(s). Provided between the two birefringent blocks
21
and
71
are a pair of Faraday rotators
31
and
34
and a pair of Faraday rotators
61
and
64
. The Faraday rotators
31
and
34
are positioned in parallel with respect to the light path. Permanent magnets
32
and
35
apply external magnetic fields to the pair of Faraday rotators
31
and
34
so that the polarization plane of the light passing through the Faraday rotators
31
and
34
is rotated by 45 degrees. The Faraday rotators
61
and
64
are positioned in parallel with respect to the light path. Permanent magnets
62
and
65
apply external magnetic fields to the pair of Faraday rotators
61
and
64
so that the polarization plane of the light passing through the Faraday rotators
61
and
64
is rotated by 45 degrees. A birefringent block
41
is positioned in the middle of the optical circulator, so that the light passes through different optical paths depending on the direction of travel and the direction of polarization of the light.
The optical circulator proposed in Japanese Patent No. 2539563 uses an optical collimator that is a combination of an optical fiber and a lens provided at each port. Therefore, individual optical components are of large sizes and therefore are not economical.
U.S. Pat. No. 5,991,076 in FIG.
27
and U.S. Pat. No. 6,049,426 in
FIG. 28
propose miniaturized optical circulators in which the optical components can be small. In these optical circulators, a birefringent block
21
branches light emitted from an optical fiber
12
into two light beams having polarization planes perpendicular to each other, or combines two light beams having polarization planes perpendicular to each other into a single beam. A birefringent block
71
branches the light emitted from an optical fiber
82
into two light beams having polarization planes at right angles with each other, or combines two light beams having polarization planes perpendicular to each other into a single beam.
Referring to
FIG. 27
, a pair of wave plates
37
-
38
, a pair of wave plates
67
-
68
, and Faraday rotators
31
and
61
are arranged between two birefringent blocks
21
and
71
. Each of the two pairs of wave plates
37
-
38
and
67
-
68
is positioned in parallel with respect to the light path and causes the light to be polarized in the same direction. Birefringent blocks
41
and
42
and lenses
51
and
52
are positioned symmetrically about the longitudinal middle of the optical circulator, so that the light passes through different light paths depending on the direction of travel of the light and the direction of polarization plane of the light.
The optical circulator in
FIG. 27
allows optical fibers to be positioned irrespective of the shape of a lens, thus lending itself to the miniaturizing of optical circulators. However, the requirement of positioning wave plates in parallel with respect to the optical path places certain limitations on the miniaturization of optical circulators. Wave plates used in these optical devices usually have a size of about several millimeters square and are cut into desired sizes by means of, for example, a dicing saw. However, the use of a dicing saw causes chipping in the range of several microns to several tens microns at cut surfaces and edges of the wave plates. When light passes through the chipped portions, the optical characteristics of the wave plate deteriorate. Thus, it is required to ensure that light paths are separated by at least a certain distance in designing an optical circulator. This is a barrier to the miniaturization of circulators. In addition, the number of optical parts is
12
, which is another factor behind increases in manufacturing cost.
FIG. 28
illustrates an optical circulator proposed in U.S. Pat. No. 6,049,426. This optical circulator includes two pairs of Faraday rotators, i.e., rotators
31
and
34
and rotators
61
and
64
, which retain their magnetization and do not require permanent magnets. The Faraday rotators
31
and
34
are positioned in parallel with respect to the optical path. The Faraday rotators
61
and
64
are also positioned in parallel with respect to the optical path. Wollaston prisms
45
and
46
are used to form oblique light paths so that lenses
22
and
72
are shared by two light beams, thereby configuring an optical circulator in which miniaturized optical components can be used.
The optical circulator in
FIG. 28
also requires two pairs of Faraday rotators, each pair including two Faraday rotators (
31
and
34
, or
61
and
64
) aligned in parallel with respect to the light path. Therefore, the mechanical structure of the optical circulator necessarily places limitations on the miniaturization of an optical circulator. Moreover, the inventors has found that because a Faraday rotator is a ferrimagnetic material, when two Faraday rotators having opposite magnetization directions are positioned in contact with each other, the magnetic characteristics of the two Faraday rotators can affect each other to cause them to be demagnetized. Thus, the structure in
FIG. 28
requires the two Faraday rotators to be somewhat spaced. This requirement implies that the light beams must further spaced apart.
In addition, the configuration in
FIG. 28
can incorporate Faraday rotators only of the magnetization retaining type that requires no external magnetic field. The temperature and wavelength dependencies of Faraday rotation determine the performance of an optical device that uses a Faraday rotator. Commercially available Faraday rotators of the magnetization retaining type are

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