Optical isolator

Details Optical isolator Optical isolator

2000-07-07

2003-10-07

Bovernick, Rodney (Department: 2874)

Optical waveguides

Polarization without modulation

C385S034000

Reexamination Certificate

active

06631220

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of optical isolators and more particularly to the cost, size reduction, and improved manufacturability of optical isolators.
2. Brief Description of Related Art
Typical optical isolators include a pair of collimating elements, such as GRIN (graduated index) lenses at the input and output ports of the device, and a core assembly located between the pair of collimating elements. The core assembly is made up of a pair of optically birefringent devices disposed on either side of an optically active device, such as a Faraday rotator.
FIG. 1
shows the structure of a typical optical isolator
8
. Collimating elements
10
and
12
include metal collimator holders
25
,
45
, in which quartz collimator aligners
23
and
44
are positioned. Glass ferrules
22
,
42
for holding optical fibers
21
,
41
, respectively and GRIN (graduated index of refraction) lenses
24
,
43
are disposed within the quartz aligners
23
,
44
. GRIN lenses
24
,
43
act to collimate the light rays entering the device and to focus any light rays leaving the device.
The core assembly
14
includes a cylindrical permanent magnet
35
, birefringent wedges
31
,
34
and a rotator
33
, which is commonly made of yttrium iron garnet (YIG). The cylindrical permanent magnetic
35
generates a magnetic field for YIG rotator
33
. Birefringent wedges
31
and
34
separate incident light into two orthogonal rays that are parallel and perpendicular to the optical axis of each wedge and wedge
34
has its optical axis rotated relative to the optical axis of wedge
31
. The rotator
33
and birefringent wedges
31
,
34
are held in place within the magnetic cylinder
35
by rotator holder
32
.
There are two directions of operation, the forward direction in which light from fiber
21
enters the device and the reverse direction in which light from fiber
41
enters the device or reflections from light propagating in a forward direction enters the device.
Referring to
FIG. 2A
, in the forward direction of operation, light
60
from fiber
21
, diagram (a), enters the GRIN lens
24
and is collimated onto birefringent wedge
31
. The wedge
31
has two axes, a fast axis (a smaller index of refraction n
F
) and a slow axis (a larger index of refraction n
s
), that are orthogonal to each other. To simply the discussion, assume the fast axis is the horizontal and the slow axis is vertical, as shown. (In an actual implementation, the fast axis and slow axis can have a different angle with respect to the horizontal and vertical axes respectively.) Thus, regardless of the polarization of the input light, wedge
31
causes light emerging from the wedge to have a fast-axis component
64
and a slow-axis component
62
, each component being refracted differently by the wedge
31
, as shown in diagram (b) of FIG.
2
A. This light is then processed by the optical rotator
33
, which rotates the plane of polarization of both components in space by some angle &agr;, which depends on the thickness of the rotator
33
. This is shown in diagram (c) of
FIG. 2A. A
typical rotation angle is 45 degrees. The spatially rotated components then impinge on wedge
34
, which has its fast and slow axes rotated by an angle &bgr;. If &bgr; is the same angle as &agr;, the fast component of the light beam is aligned with the fast axis of the wedge
34
and the slow component is aligned with the slow axis of the wedge
34
. Because of this alignment, the light is refracted through wedge
34
without loss (ideally) to produce a collimated beam, as shown in diagram (d), that is focused by the GRIN lens
43
and accepted into the aperture of the fiber
41
.
Referring to
FIG. 2B
, in the reverse direction of operation, light of arbitrary polarization
66
from fiber
41
or reflected light from a forward traveling wave enters the device and is collimated by GRIN lens
43
so that substantially parallel rays impinge upon wedge
34
. Wedge
34
, like wedge
31
, has a fast axis and a slow axis, the axes rotated by the angle &bgr;, as described above. Light passing in the reverse direction through wedge
34
now has fast component
70
and slow component
68
in diagram (b) of
FIG. 2B
, orthogonal to each other and aligned with the rotated axes of wedge
34
. Next, the light beam passes, in the reverse direction, through the optical rotator
33
, which, being a non-reciprocal device, rotates the planes of polarization, shown in diagram (c), by an angle a in the same direction as the rotation in the forward direction of travel. This rotation causes the components,
70
,
68
of the light to be aligned with the vertical and horizontal axes. Light from the rotator is next processed by wedge
31
which has a horizontal fast axis and a slow vertical axis. However, because of the initial alignment of wedge
34
and the rotation of optical rotator
33
, the slow component
68
from wedge
34
is aligned with the fast axis of wedge
31
and the fast component
70
from wedge
34
is aligned with the slow axis of wedge
31
. The light beam is refracted by the wedge
31
, according to this alignment, causing a pair of divergent beams to emerge from the wedge, as shown in diagram (d) of FIG.
2
B. The divergent beams cannot be focused onto the aperture of the optical fiber
21
and the reverse-direction light is thus blocked from entering the fiber
21
.
Current optical isolators, such as the one in
FIG. 1
, have lengths in the range of 40 mm to 42 mm and outer diameters in the range of 5.3 to 5.5 mm. These dimensions result from an internal structure of the isolator and its packaging in order to meet the optical performance, reliability and manufacturability requirements placed on the isolator.
A measure of the optical performance of an optical isolator is the ratio of the insertion loss to the isolation, where the insertion loss is the reduction in intensity of the signal in the forward direction of propagation through the isolator and isolation is the reduction in intensity of a the signal in the reverse direction through the isolator. Ideally, the manufacture of the isolator is such as to minimize the insertion loss and maximize the isolation. To achieve this goal, the internal structure of the isolator must allow fine alignment adjustments of the collimators. Alignment of the isolator of
FIG. 1
is accomplished by quartz collimator aligners
23
and
44
. These components increase the outer diameter of the isolator of FIG.
1
.
Reliability is measured by the ability of the isolator to withstand certain environmental stresses such as temperature, humidity and vibration without a significant impact on the optical performance, i.e., the insertion loss and isolation ratio, of the isolator. In part, reliability is enhanced by an outer protective cover surrounding the holder
25
and cylindrical magnet
35
.
Finally, the manufacturability of the isolator is gauged by the manufacturing yield of isolators with good to superior optical performance characteristics. High yields of high performance devices translates into lower costs than with poor yields of high performance devices. An optical isolator may, in theory, be capable of superior performance that is not achievable in practice because the manufacturing process steps adversely affect the theoretical performance. An example of this is the use of high temperature solders to hold the collimating elements of the isolator in place. These solders can have a permanent and serious effect on the performance of the isolator by affecting the alignment of the collimating elements. This causes in irreparable loss in the performance of the isolator.
There is currently a demand for smaller and lower cost optical components such as optical isolators to reduce the overall size and cost of equipment using such components. However, reducing the size of an optical isolator precludes the use of currently available structures to meet the above-mentioned optical performance, reliab

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