Optical: systems and elements – Polarization without modulation – Polarization using a time invariant electric – magnetic – or...
Reexamination Certificate
1999-11-10
2002-11-12
Chang, Andrey (Department: 2872)
Optical: systems and elements
Polarization without modulation
Polarization using a time invariant electric, magnetic, or...
C359S281000, C359S282000, C359S487030, C359S490020, C359S490020, 57, C385S006000, C385S024000, C385S027000, C385S031000, C385S034000, C385S119000
Reexamination Certificate
active
06480331
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarization independent optical isolator and, more particularly, to a reflection type of a polarization independent isolator.
2. Description of the Related Art
Optical fiber communication systems are now in practical use, and efforts are being made to advance research and development in this field. Accordingly, requirements for optical devices with more versatile functions have also increased.
An optical isolator is used as a functional component in a light transmission system such that light transmission therethrough is permitted in only one direction. A common use of optical isolators is as constituents of so-called “optical passive components” within optical amplifier systems, which are themselves important components of fiber-optic communication systems. Optical amplifier systems generally include optical isolators residing on both sides of an optical gain element such as an Er-doped fiber. Other optical passive components include Wavelength Division Multiplexers (WDM's) and signal monitors.
A polarization dependent optical isolator
100
is shown in
FIG. 1
as an example of a traditional and typical optical isolator of the prior art. As illustrated in
FIG. 1
, there is provided a 45-degree Faraday rotation element (which is also referred to as a Faraday rotator)
101
which always rotates light input thereto in one direction by virtue of a permanent magnet. A polarizer
102
and an analyzer
103
are respectively placed before and after the Faraday rotation element, with the polarizer
102
and analyzer
103
being maintained at relative positions rotated 45 degrees with respect to one another.
As shown in
FIG. 1
, light emitted from an optical fiber
104
is divided or separated into parallel beams by a lens
105
, and of the parallel beams, the polarizer
102
allows only polarized light oriented in a particular direction to pass through it; any other light is absorbed or reflected and eliminated. Polarized light that has passed through the polarizer
102
emanates from the Faraday rotation element
101
with its plane of polarization rotated by 45 degrees. The analyzer
103
is so arranged that polarized light with its plane of polarization rotated by 45 degrees passes through the analyzer
103
, is focused by a lens
106
and enters an optical fiber
107
.
On the other hand, and also as shown in
FIG. 1
, of light entering the polarization dependent optical isolator
100
in the reverse direction (from the optical fiber
107
), only polarized light that is rotated by 45 degrees relative to the polarizer
102
may pass through the analyzer
103
. Polarized light that has passed through the analyzer
103
will have its plane of polarization rotated by 45 degrees by the Faraday rotation element
101
, and then emanates therefrom. The resulting light is rotated by 90 degrees relative to the polarizer
102
and is eliminated. Because of this, light in the forward direction propagates forwardly while light in the reverse direction is eliminated.
However, the isolator
100
just described is polarization dependent, even with respect to light propagating in the forward direction. More particularly, only specific, polarized light can pass through the isolator
100
in the forward direction, and the remaining propagating light is not effectively utilized because it is eliminated. Typical optical fibers used in light wave communication and data transfer systems do not preserve optical polarization over long distances. Light emanating from such a fiber consists of a randomly mixed state of light polarized in all directions, regardless of the state of polarization of light input to the fiber. Polarization-preserving fiber is well known but is too expensive for general use over long distances. Polarization independent optical isolators have therefore found a wide variety of applications in fiber-optic light wave systems.
FIG. 2A
shows a well-known prior-art polarization independent optical isolator that is disclosed in U.S. Pat. No. 4,548,478. In the prior art polarization independent optical isolator
200
shown in
FIG. 2A
, tapered birefringent plates (tapered plates)
201
and
202
are placed on either side of a 45-degree Faraday rotator
203
. Referring now to
FIG. 2A
, when light emanates from the optical fiber
204
into the prior art polarization independent optical isolator
200
and enters in the forward direction into the first tapered plate
201
, the light is divided or separated into ordinary rays (o-rays) and extraordinary rays (e-rays) because of the differences in the index of refraction of the first tapered plate
201
due to polarization. These rays are refracted to different directions, and enter the 45-degree Faraday rotator
203
of FIG.
2
A.
Ordinary and extraordinary rays of which planes of polarization are rotated
45
degrees by the Faraday rotator
203
are caused to enter the second tapered plate
202
. The second tapered plate
202
is arranged such that an optical axis of the second tapered plate
202
is rotated 45 degrees around or about the light propagation direction relative to an optical axis of the first tapered plate
201
. Therefore, the foregoing ordinary and extraordinary rays correspond to ordinary and extraordinary rays in the second tapered plate
202
, respectively. Accordingly, ordinary rays and extraordinary rays that pass through the second tapered plate
202
emanate parallel to each other. These parallel beams of ordinary and extraordinary rays are focused onto the optical fiber
207
by the lens
206
.
On the other hand, light traveling in the reverse direction (emanating from fiber
207
and traveling toward the direction of fiber
204
as shown in
FIG. 2B
) is divided into ordinary rays and extraordinary rays after entering the second tapered plate
202
. The ordinary rays and the extraordinary rays are refracted to different directions by the second tapered plate
202
, enter the 45 degree Faraday rotator
203
, and are emitted therefrom with their plane of polarization rotated by 45 degrees.
For the light propagating in the reverse direction as shown in
FIG. 2B
, ordinary rays and extraordinary rays in the second plate
202
are converted to extraordinary rays and ordinary rays, respectively, in the first plate
201
by the Faraday rotator
203
, so that the direction of each of these rays after passing through the first tapered plate
201
is different from that of incident light. Accordingly, when these rays are converged by the lens
205
, focal points are formed outside the face of the fiber end
204
so that the light traveling in the reverse direction does not enter the optical fiber
204
.
Since optical isolators typically utilize Faraday rotators and since the angular polarization rotation of Faraday rotators typically depends on wavelength of the light propagating therethrough, the wavelength region that provides the 45-degree rotation is very narrow. Therefore, a high isolation is maintained only in a very limited wavelength region, unless deviation from 45-degree rotation is compensated for.
In U.S. Pat. No. 4,712,880, two optical isolators and two polarization rotation compensators which are incorporated into these optical isolators are disclosed. The first polarization rotation compensator described in U.S. Pat. No. 4,712,880 is shown in
FIG. 3A
as element
300
and is composed of a combination of a half-wave plate
301
whose principal axis is inclined at an angle of &thgr;/2 with respect to the plane of polarization of the incident light
302
and a quarter-wave plate
303
whose principal axis is inclined at an angle of &thgr; with respect to the plane of polarization of the incident light
302
, with the half-wave plate
301
and the quarter-wave plate
303
disposed in this order with respect to the forward light propagation direction.
The second polarization rotation compensator described in U.S. Pat. No. 4,712,880 (not shown) is similar except that the principal axis of the quarter-wave plate is paralle
Avanex Corporation
Chang Andrey
Curtis Craig
Staas & Halsey , LLP
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