Optical waveguides – Polarization without modulation
Reexamination Certificate
2001-10-17
2003-04-29
Uliah, Akm E. (Department: 2874)
Optical waveguides
Polarization without modulation
C385S015000, C385S034000, C359S494010, C359S490020, C359S490020
Reexamination Certificate
active
06556733
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical isolators, and particularly to optical isolators with birefringent crystals which have optical axes that must have precise relative alignment to yield optimal optical performance.
2. Description of Prior Art
In present-day optical communications technology, optical signals frequently pass through a plurality of optical interfaces. At each interface, reflected signals are generated from the optical signals. If the reflected signals travel back to the signal source through the primary optical route, the source becomes unstable and noisy. Optical isolators are used to block these reflected signals from reaching the source. Ideally, optical isolators transmit all of the light of an optical signal in the forward direction, and block all of the reflected light in the reverse direction.
FIG. 1
, illustrates an optical isolator
110
as disclosed in U.S. Pat. No. 5,446,813. The isolator
110
includes a first optical collimator
120
, an optical isolated core
130
, and a second optical collimator
140
. The first optical collimator
120
collimates input optical signals from an input optical fiber
121
into the isolated core
130
. The first optical collimator
120
comprises a ferrule
122
retaining the input optical fiber
121
therein, and a graded index (GRIN) lens
123
. The ferrule
122
and the GRIN lens
123
are both secured into a tube
124
, which in turn is further secured into a stainless steel tube
125
. The second optical collimator
140
has a structure which is identical to that of the first optical collimator
120
. The second optical collimator
140
is used to collimate optical signals from the isolated core
130
into an output optical fiber
141
. The second optical collimator
140
is secured into a stainless steel tube
145
. The isolated core
130
comprises a first birefringent crystal
131
, a second birefringent crystal
133
, and a Faraday rotator
132
stationed between the two crystals
131
,
133
. The elements of the isolated core
130
are adhered to each other, and then secured into a tube
134
. The isolator
110
also has a stainless steel tube
150
, with the first and the second optical collimators
120
,
140
and the isolated core
130
inserted therein.
In operation, the first birefringent crystal
131
separates incident optical signals into two beams having polarization planes perpendicular to each other. Then the Faraday rotator
132
rotates the two polarized beams a specific angle &thgr;, such as 45 degrees. The second birefringent crystal
133
recombines the two separated beams, and the optical collimator
140
converges the recombined beams into the output optical fiber
141
. Because the Faraday rotator
132
is optically nonreciprocal, any returning optical signals from the output optical fiber
141
cannot be converged into the input optical fiber
121
. As a result, the isolator
110
ensures one-way signal transmission.
Insertion loss and isolation are the two most important criteria in determining performance of the isolator
110
. The most decisive factor regarding isolation is whether the angle between optical axes of the two birefringent crystals
131
,
133
is equal to the rotating angle &thgr; by which the Faraday rotator
132
rotates forward singles transmitted therethrough. Furthermore, if the angle between the optical axes is equal to &thgr;, insertion loss of the isolator
110
is decreased.
I t is difficult to control relative positions of the two birefringent crystal
131
,
133
during assembly of the isolator
110
. Accordingly, it is difficult to control precise adjustment of the angle between the optical axes of the two crystals such that the angle is equal to the rotating angle of the Faraday rotator.
Furthermore, the isolated core
130
of the isolator
110
is formed by adhering the two birefringent crystals
131
,
133
and the Faraday rotator
132
together as a unit. Therefore, if the isolated core
130
is found to not meet required optical performance standards, it is necessary to discard the entire isolated core
130
.
There is a need for an improved optical isolator which can overcome the disadvantages of the prior art.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an optical isolator which allows easy and precise relative alignment of optical axes of birefringent crystals during assembly of the isolator.
Another object of the present invention is to provide an optical isolator which allows easy and precise readjustment of relative alignment of optical axes of birefringent crystals of the isolator.
To solve the problems of the prior art and achieve the objects set out above, an optical isolator in accordance with a preferred embodiment of the present invention comprises a first optical collimator, a first birefringent crystal, a Faraday rotator, a second birefringent crystal, and a second optical collimator. The first and second collimators have the same structure and configuration. Each first and second collimator comprises a ferrule, an optical fiber retained in the ferrule, and a collimating lens, all of which are secured in a tube. The first and second birefringent crystals are respectively fixed to the first and second collimators. The Faraday rotator is stationed between the first and second collimators, and fixed onto an end of the first collimator. In assembly, the first and second collimators and the Faraday rotator are all secured in a stainless steel outer tube. The second collimator is rotated within the outer tube until correct relative alignment of optical axes of the birefringent crystals is attained.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 4375910 (1983-03-01), Seki
patent: 5317655 (1994-05-01), Pan
patent: 5689360 (1997-11-01), Kurata et al.
patent: 5734762 (1998-03-01), Ho et al.
patent: 5889904 (1999-03-01), Pan et al.
patent: 6212305 (2001-04-01), Pan
patent: 2002/0159149 (2002-10-01), Zhu et al.
Chen Chien-Cheng
Dy Ja Jn
Lee Chun Yu
Yu Tai-Cheng
Chung Wei-Te
Uliah Akm E.
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