Optical isolator

Details Optical isolator Optical isolator

2001-09-21

2003-09-16

Bruce, David V. (Department: 2882)

Optical waveguides

Polarization without modulation

C385S034000

Reexamination Certificate

active

06621946

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical isolators, and particularly to compact isolators having low insertion loss and low vulnerability to external environmental conditions.
2. Description of Prior Art
In present-day optical communications technology, optical signals are typically passed through a plurality of optical interfaces. All interfaces produce reflected signals. Reflected signals which return to a light source through a primary optical route cause the light source to become unstable and noisy. Optical isolators are used to block these reflected signals from reaching the light source. Ideally, optical isolators permit all light rays to move in a forward direction only, and block light rays from moving in a reverse direction.
As shown in
FIG. 1
, a conventional isolator
1
includes first and second optical collimators
10
, an isolated center core
30
and an outer tube
40
enveloping the collimators
10
and the isolated center core
30
. Each collimator
10
comprises a stainless steel tube
11
, a sleeve
12
, a ¼ pitch Graded Index (GRIN) lens
13
, and a ferrule
14
which accommodates an optical fiber
15
. The collimators
10
convert input optical signals into parallel rays, for providing sound coupling between two optical devices. The isolated center core
30
is stationed between the two collimators
10
and comprises a first polarizer
31
, a Faraday rotator crystal
32
, a second polarizer
33
, and a toroidal magnetic core
34
. The magnetic core
34
envelops the two polarizers
31
,
33
and the Faraday rotator crystal
32
to protect them, and provides a magnetic field for the Faraday rotator
32
.
Light signals pass in a forward direction from an end of the right-hand input fiber
15
to the right-hand Graded Index (GRIN) lens
13
. The GRIN lens
13
collimates the light, and the collimated light from the GRIN lens
13
is then transmitted through the first polarizer
31
. The first polarizer
31
is a birefringent crystal wedge. The first polarizer
31
separates incident light from the GRIN lens
13
into an ordinary ray polarized perpendicularly to an optical axis of the first polarizer
31
, and an extraordinary ray polarized along the optical axis of the first polarizer
31
.
Separation occurs because the birefringent crystal wedge has two indexes of refraction, one for the light polarized along the optical axis and another for the light polarized perpendicularly to the optical axis. The polarized light from the first polarizer
31
is then rotated 45° by the Faraday rotator
32
. The Faraday rotator
32
is typically formed from garnet doped with impurities, or alternatively YIG, and is placed in the magnetic core
34
.
The rotated light rays then enter the second polarizer
33
, sometimes called an analyzer. Like the first polarizer
31
, the second polarizer
33
typically is a birefringent crystal wedge. An optical axis of the birefringent crystal of the second polarizer
33
is oriented by 45° with respect to the optical axis of the birefringent crystal of the first polarizer
31
. Thus the ordinary ray from the first polarizer
31
is also an ordinary ray of the second polarizer
33
, and the extraordinary ray from the first polarizer
31
is also an extraordinary ray of the second polarizer
33
. The result is that after having traveled from the first polarizer
31
through the second polarizer
33
, the two polarized rays are recombined by the second polarizer
33
. The two polarized rays are then refocused by the left-hand GRIN lens
13
to a point on an end of the left-hand fiber
15
.
In the reverse direction, light from the left-hand fiber
15
is separated by the second polarizer
33
into two rays: an ordinary ray polarized perpendicularly to the optical axis of the second polarizer
33
, and an extraordinary ray polarized along the optical axis of the second polarizer
33
. When passing through the Faraday rotator
32
, the light of both rays is rotated 45°. This rotation is nonreciprocal with the rotation of light in the forward direction. The ordinary ray from the second polarizer
33
is polarized along the optical axis of the first polarizer
31
, and the extraordinary ray from the second polarizer
33
is polarized perpendicularly to the optical axis of the first polarizer
31
. The ordinary and extraordinary rays from the second polarizer
33
have swapped places incident upon the first polarizer
31
. Thus the light, having passed through the first polarizer
31
, does not leave the polarizer
31
in parallel rays. The non-parallel light is focused by the right-hand GRIN lens
13
to a point not at the end of the input fiber
15
. Thus, light traveling in the reverse direction is not passed back into the right-hand fiber
15
.
The conventional optical isolator
1
has its isolated core
30
between the two collimators
10
. The left-hand sleeve
12
is within the stainless steel tube
11
. The left-hand GRIN lens
13
has a protruding end
131
protruding out of the sleeve
12
into the magnetic core
34
. In assembly, the first polarizer
31
, the rotator crystal
32
and the second polarizer
33
of the isolated core
30
are stationed within the magnetic core
34
. End portions of the polarizers
31
,
33
and the rotator crystal
32
are glued to an inner surface of the magnetic core
34
. Then the protruding end
131
of the left-hand GRIN lens
13
is glued to the inner surface of the magnetic core
34
. The isolated core
30
is thus securely connected with the left-hand collimator
10
.
When the magnetic core
34
of the isolated core
30
is glued to the protruding end
131
of the left-hand GRIN lens
13
, excess glue may contaminate the GRIN lens
13
and the adjacent surface of the adjacent second polarizer
33
. Such contamination reduces the performance of the GRIN lens
13
, and results in a large insertion loss of the isolator
1
. In addition, such contamination on gelatine surfaces of the GRIN lens
13
and the second polarizer
33
is difficult to remove. Furthermore, the left-hand collimator
10
is fixedly connected with the isolated core
30
. It is difficult to adjust the relative position of the collimator
10
and the isolated core
30
, so as to accurately focus output light on the end of the left-hand fiber
15
. Moreover, the components of the isolator
1
are unduly large. This adds to costs, particularly the cost of the isolated core
30
. Finally, the isolated core
30
is located outside the two sleeves
12
. Thus the isolated core
30
is vulnerable to changes in temperature of the external environment, which may adversely affect the operation of the isolator
1
.
Accordingly, an improved isolator is needed to overcome the many disadvantages of conventional isolators.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide an optical isolator with low insertion loss and low cost.
Another object of the present invention is to provide an optical isolator which is less vulnerable to changes in its surrounding environment.
A further object of the present invention is to provide an optical isolator which has no contamination caused by excess glue, and which has an isolated core which is effectively insulated from external contaminants both during and after assembly.
To solve the problems of the prior art and achieve the objects set out above, an optical isolator of the present invention comprises a first optical collimator, an isolated core, a second optical collimator and an outer tube. The second collimator has a long sleeve which entirely accommodates the isolated core. The isolated core comprises a first polarizer, a Faraday rotator crystal and a second polarizer stationed in sequence within a toroidal magnetic core. An axial length of the toroidal magnetic core is equal to or slightly less than an overall length of the two polarizers and the rotator crystal. The two polarizers and the rotator crystal are sized such that an overall diameter of the isolated core is less than an inner diameter of th

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