Optical: systems and elements – Polarization without modulation – Polarization using a time invariant electric – magnetic – or...
Reexamination Certificate
2001-09-28
2004-07-13
Chang, Audrey (Department: 2872)
Optical: systems and elements
Polarization without modulation
Polarization using a time invariant electric, magnetic, or...
C359S282000, C359S490020, C359S490020, C359S900000, C372S703000, C385S011000, C385S027000, C385S033000, C385S036000, C385S039000
Reexamination Certificate
active
06762879
ABSTRACT:
BACKGROUND OF THE INVENTION
An optical circulator is a multi-ported passive device designed to receive as an input an optical signal on one port and transmit the optical signal to another port. Conventional optical circulators are employed in systems that require the transmission of an optical signal in a particular direction. For example, U.S. Pat. No. 4,650,289 by Kuwahara describes a conventional optical circulator.
FIG. 1
is a diagram of one such conventional optical circulator
10
. The conventional optical circulator
10
includes four ports, port A
12
, port B
24
, port C
32
, and port D
34
. The conventional optical circulator
10
also includes polarizer prisms
14
and
22
, mirrors
16
and
26
, Faraday rotators
18
and
28
, and optically active elements (e.g., half wave plates)
20
and
30
. Polarizer prisms
14
and
22
transmit light in different directions depending on the polarization of the light. Light polarized in a first direction is transmitted undeflected by the or polarizer prisms
14
and
22
. Light polarized in a second direction is transmitted at an angle of ninety degrees from the first direction. The mirrors
16
and
26
merely reflect light without a change in polarization. The Faraday rotators
18
and
28
rotate the direction of polarization of incident light by forty-five degrees in a particular direction regardless of the direction in which light traverses the Faraday rotators
18
and
28
. For example, the Faraday rotator
18
rotates the polarization of light from the mirror
16
in the same direction as light from the optically active element
20
. Optically active elements
20
and
30
rotate the polarization of incident light by forty-five degrees. However, the direction that the polarization is rotated depends upon the direction in which the light traverses the optically active elements
20
and
30
(i.e., optically active elements
20
and
30
are reciprocal devices). For example, optically active element
20
will rotate light from the Faraday rotator
18
by forty-five degrees in a particular direction. The optically active element
20
will rotate light from the polarizer prism
22
having the same polarization by forty-five degrees in the opposite direction. The Faraday rotator is an optically irreversible (i.e., non-reciprocal) element, that is, the rotation angle will double for light after a round trip through the Faraday rotator. Optically active elements
20
and
30
are reciprocal, that is, light after a round trip through these devices will not be rotated.
In operation, the Faraday rotator of
18
and optically active element
20
act as a function group that rotates polarization 90 degrees for light traveling from left to right (from
16
to
22
) but doesn't rotate polarization for the light passing through from right to left (from
22
to
16
). Similarly, optically active element
30
and faraday rotator
28
act as another function group with similar functionality. Input light has random polarization and includes two components. Polarizer prisms
14
or
22
reflect one component of the input light while another component passes through undeflected. For the purposes of this example, the first polarization P can be characterized as having a polarization that is in the incident plane (paper surface) and the second polarization S which polarization is perpendicular to the incident plane. The P components pass through polarizer prism
14
or
22
, but S components reflect 90 degrees at an intersection to the surface. More specifically, a light with random SOP (State of Polarization) input to port
1
and transmitted to prism
14
, divides into S and P components. The P components pass through to a second path (including components
12
,
30
,
28
,
26
and
22
), while the S components reflect to a first path (including components
14
,
16
,
18
,
20
and
22
).
For signal from port A to port B, the S components pass along the first path through the functional group of
18
and
20
, change polarization to be P, passes through polarizer prism
22
to port B. The P component from port A, passes through polarizer prism
14
, changes to be S polarization by functional group
30
and
28
, and then reflects at polarizer prism
22
to port B also. Accordingly, polarizer prism
14
acts as a splitter while polarizer prism
22
acts as a combiner, producing the full signal from port A to port B.
For signal from port B to port C, the S components arriving at port B are reflected to the second path, pass through functional group of
28
and
30
, maintain their S polarization, and are reflected at polarizer prism
14
to port C. The P component of the input light introduced at port B passes through polarizer prism
22
to the first path, passes through functional group of
18
and
20
, maintains the P polarization, and then passes through polarizer prism
14
to port C also. In this case, polarizer prism
22
is a splitter and polarizer prism
14
is a combiner. Thus the full signal from port B is received by port C. Similarly, the full signal from port C is delivered to port D and the full signal from port D to port A.
Optical circulators of this type are very difficult to manufacture. The difficulty arises in the perfectly parallel optical paths that must be maintained in the device (i.e., paths between polarizer prisms
14
and
22
). At the present time, no such devices are commercially offered.
SUMMARY OF THE INVENTION
In one aspect the invention provides a closed loop optical circulator including a first port, a last port and means for establishing a last optical path where the last optical path provides a path from the last port to the first port The means for establishing includes two pairs of complementary crystals. Each crystal of a respective pair transmits an optical signal of one polarization without deflection and deflects an optical signal of another polarization. The first pair of complementary crystals deflects optical signals of a second polarization in a direction perpendicular to a plane of a page and receives an optical signal from the last port and transmits the optical signal to the first port. The second pair of complementary crystals operable deflects optical signals of a first polarization in a direction along the plane of the page and is disposed between the first pair of complementary crystals. The optical circulator includes two pairs of complementary half wave plate rotators. Each pair of complementary half wave plate rotators is disposed between a crystal of the first pair and a crystal of the second pair of complementary crystals. Each half wave plate rotator includes a pair of half wave plate rotator groups where it each group includes a half wave plate and a glass portion. The optical circulator includes a half wave plate and a Faraday rotator disposed between crystals of the second pair of complementary crystals.
In another aspect, the invention provides a closed loop optical circulator including a first port, a last port and a path between the two including two pairs of complementary crystals. Each crystal of a respective pair transmits an optical signal of one polarization without deflection and deflects an optical signal of another polarization. The first pair of complementary crystals deflects optical signals of a second polarization in a direction perpendicular to a plane of a page and receives an optical signal from the last port and transmits the optical signal to the first port. The second pair of complementary crystals deflects optical signals of a first polarization in a direction along the plane of the page and disposed between the first pair of complementary crystals. The optical circulator includes two pairs of complementary half wave plate rotators. Each pair of complementary half wave plate rotators is disposed between a crystal of the first pair and a crystal of the second pair of complementary crystals. Each half wave plate rotator includes a pair of half wave plate rotator groups where each group includes a half wave plate and a glass portion. The optica
Chang Audrey
Curtis Craig
Fish & Richardson P.C.
Oplink Communications Inc.
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