Optical waveguides – Polarization without modulation
Reexamination Certificate
2001-04-11
2004-08-24
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Polarization without modulation
C385S015000, C385S027000, C359S490020, C359S490020, C359S490020
Reexamination Certificate
active
06782145
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical technology, and more particularly to a method and system for providing an in-line optical circulator.
BACKGROUND OF THE INVENTION
Conventional optical circulators are used for many purposes. For example, conventional optical circulators may be employed in systems transmitting optical signals in order to transmit optical signals in a particular direction. In a three port optical circulator, an optical signal input at the first port will be transmitted to the second port. An optical signal input at the second port will be transmitted to the third port. However, optical signals will not be transmitted in the reverse direction. For example, an optical signal input at the second port will not be transmitted to the first port. Optical circulators can also come in a variety of configurations. One desirable configuration is an in-line optical circulator in which the first and third ports are adjacent, while the second port is at the opposing side of the system.
One prior art optical circulator is described in U.S. Pat. No. 5,909,310 by Li, et al and shown in FIG.
1
A. This conventional optical in-line circulator
10
includes a first port
12
, a second port
14
and a third port
16
. The conventional optical in-line circulator
10
also includes a first collimator
18
, a first birefringent crystal
20
, a first pair of half wave plates
22
A and
22
B, a first Faraday rotator
24
, a conventional Wollaston prism
26
, a second birefringent crystal
28
, a second Faraday rotator
30
, a second pair of half wave plates
32
A and
32
B, a third birefringent crystal
34
, a second collimator
36
and the fiber for the second port
14
.
This conventional optical in-line circulator suffers from disadvantages. First, the optical axes half wave plates
22
A and
22
B in the first pair of wave plates and the first birefringent crystal
20
need to be aligned to each other. Similarly, the optical axes of the half wave plates
32
A and
32
B in the second pair of wave plates and the second birefringent crystal
34
also need to be aligned to each other. These alignment procedures that are required in the manufacturing process are complicated and difficult. Therefore, the tolerance of the relative orientation of the directions of the optical axes of the wave plates
22
A,
22
B and
32
A,
32
B are relatively high, which yields a lower isolation. Thus, manufacturing is made more complex and expensive. In addition, temperature dependent phase retardation for the half wave plates
22
A,
22
B,
32
A and
32
B gives the circulator a narrower temperature bandwidth for isolation.
U.S. Pat. No. 6,049,426 by Xie et al. (“Xie”) describes another conventional in-line optical circulator.
FIG. 2
depicts a conventional in-line optical circulator
50
in accordance with the teachings of Xie. It does not utilize any half wave plates and eliminates one birefringent crystal. However, the optical circulator of Xie uses an additional Wollaston prism
52
having wedges
52
A and
52
B. One of ordinary skill in the art will readily realize that the conventional in-line optical circulator
50
is relatively difficult to manufacture with higher cost. The optical circulator
50
suffers from two drawbacks. First, the optical circulator
50
uses two Wollaston prisms
26
′ and
52
. As described above, Wollaston prisms
26
′ and
52
are relatively difficult and expensive to manufacture. The cost is thus increased by the additional number of Wollaston prism. Thus, although the half-wave plates
22
A,
22
B,
32
A and
32
B have been eliminated, the addition of a Wollaston prism still renders the optical circulator of Xie expensive and difficult to manufacture. Second, since the beam deflection angular tolerance introduced by Wollaston prisms is accumulated with the number of Wollaston prisms used, the beam deflection angular tolerance introduced by Wollaston prisms
26
′ and
52
in circulator
50
is doubled compared with the circulator with only one Wollaston prism, making optical alignment and, therefore, manufacture more difficult and complex.
Accordingly, what is needed is a system and method for providing an optical circulator that is simpler to manufacture with a lower cost. The present invention addresses such a need.
SUMMARY OF THE INVENTION
The present invention provides a method and system for providing an optical circulator. The optical circulator comprises a first port, a second port and a third port adjacent to the first port. The optical circulator also comprises a first birefringent material, a first rotator pair, a polarization beam deflector, a second birefringent material, a second rotator pair and a third birefringent material. The first birefringent material is adjacent to the first and third ports. The first rotator pair, second birefringent material, second rotator pair and third birefringent material follow in order, with the third birefringent material being closest to the second port. The first birefringent material is optically coupled to the first port and the third port and has a longitudinal axis, a transverse direction perpendicular to the longitudinal axis, a first displacement direction and a first length. The first displacement direction is at a first oblique angle from the transverse direction. The polarization beam deflector changes the direction of the optical signal without introducing a walk-off in the optical signal. The second birefringent material having the longitudinal axis and a second displacement direction, the second displacement direction being perpendicular to the longitudinal axis. The third birefringent material has the longitudinal axis, the transverse direction perpendicular to the longitudinal axis, a third displacement direction and a second length. The third displacement direction is at a second oblique angle from the transverse direction. As a result, a first optical path is established from the first port to the second port, and a second optical path is established from the second port to the third port such that when an optical signal is input at the first port the optical signal travels along the first optical path to the second port and when the optical signal is input to the second port the optical signal travels along the second optical path to the third port.
According to the system and method disclosed herein, the present invention provides an in-line optical circulator which can be more easily and cheaply manufactured than conventional in-line optical circulators. In particular, the optical circulator can be made with only a single polarization beam deflector and without the use of any half-wave plates, making the optical circulator in accordance with the present invention more economical, simpler to fabricate and have better performance for isolation.
REFERENCES:
patent: 6366402 (2002-04-01), Li
Li Yiqiang
Wang Yongjian
AC Photonics, Inc.
Rahll Jerry T
Sawyer Law Group LLP
Ullah Akm Enayet
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