Optics: measuring and testing – By polarized light examination – With polariscopes
Reexamination Certificate
2001-10-23
2004-07-13
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With polariscopes
C356S364000, C359S249000
Reexamination Certificate
active
06762837
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarization compensator and a wavelength division multiplexing (WDM) apparatus, and in particular to a polarization compensator for compensating a polarization state of a lightwave and a wavelength division multiplexing apparatus using same.
Characteristics of optical phenomena differ, in many cases, depending on a polarization state such as a reflection and a refraction. Also, various optical circuits utilizing a polarization dependency have been considered in which a change of an electric quantity, a magnetic quantity, distortion, or the like is converted into a change of a refractive index. In such optical circuits, it is important to input a lightwave in a predetermined polarization state.
Also, in an optical transmission, a wavelength division multiplexing method has been adopted responsive to a recent demand for a large-capacity transmission. In this wavelength division multiplexing method, it is important to transmit many lightwaves whose wavelengths are different from each other through an optical fiber.
2. Description of the Related Art
[1]
FIGS. 7A and 7B
show an optical isolator
90
as an example of an optical circuit having a general polarization dependency. The optical isolator
90
is composed of a polarizer
91
, a magneto-optical rotator
92
with 45° taking advantage of the Faraday effect, and an analyzer
93
arranged on z-axis in series. The directions of the polarizer
91
and the analyzer
93
are respectively set to have 0° and 45° with respect to x-axis.
As shown in
FIG. 7A
, a lightwave
64
of a liner polarization, inputted from the polarizer
91
, whose polarization plane P
1
is on xz plane, is rotated by 45° at the rotator
92
to pass through the analyzer
93
as an output lightwave
65
without losses. Oppositely, as shown in
FIG. 7B
, a polarization inputted from the analyzer
93
and rotated by 45° is further rotated by 45° at the rotator
92
, and a polarization plane P
2
becomes a linear polarization on yz plane, so that it can not pass through the polarizer
91
.
Thus, the optical isolator
90
has a function of blocking that the linear polarization outputted from e.g. the analyzer
93
is reflected by e.g. an optical fiber to be passed through in the opposite direction.
Also, in order to pass through the optical isolator
90
without losses, the lightwave
64
must be a linear polarization whose polarization plane P
1
is on the xz plane. Accordingly, in order for a lightwave of an arbitrary polarization to pass through the optical isolator
90
, the polarization plane P
1
is required to be converted into a linear polarization on the xz plane.
[2]
FIG. 8
shows an arrangement of a wavelength division multiplexing (hereinafter, occasionally abbreviated as WDM) optical transmission apparatus
201
. Lightwaves whose wavelengths are different from each other transmitted from optical transmitters
30
_
1
-
30
_n (hereinafter, occasionally represented by a reference numeral
30
) are respectively inputted to a coupler
35
through optical fibers
32
_
1
-
32
—
n (hereinafter, occasionally represented by a reference numeral
32
), optical amplifiers
31
_
1
-
31
_n (hereinafter, occasionally represented by a reference numeral
31
), and optical fibers
32
′_
1
-
32
′_n (hereinafter, occasionally represented by a reference numeral
32
′).
The coupler
35
couples the inputted lightwaves to be outputted to an optical fiber
36
. An optical amplifier
38
amplifies the lightwaves to which the wavelength division multiplexing is performed, and relays the same to the subsequent stage. Thus, a wavelength division multiplexing is a technology capable of increasing a transmission capacity of a single optical fiber by transmitting many lightwaves whose optical frequencies are different from each other through the single optical fiber.
For increasing the number of wavelengths in the wavelength division multiplexing, it is effective to narrow an interval between optical frequencies whose wavelengths are adjoining. However, to narrow the interval between the optical frequencies causes the following problems: (1) a problem of crosstalk between signals, especially of coherent crosstalk between signals caused by a spread spectrum upon a signal modulation, and (2) a problem of crosstalk between wavelengths caused by a non-linear effect of an optical fiber.
[1] Although various optical circuits for converting a lightwave into a linear polarization of a predetermined direction have been devised, there has been no simple optical circuit for converting a lightwave of an arbitrary polarization into a linear polarization of a predetermined direction only with an electric signal without a mechanical operation.
[2] On the other hand, as a method of overcoming the WDM problems, a transmission method of orthogonal polarization has been proposed. In this method, polarization directions of signal lights for odd even channels are orthogonalized to be transmitted. Namely, a transmitter sets the polarization directions of the optical signals for the odd even channels to be orthogonal and transmits the same. The orthogonal state is almost maintained in an optical transmission line.
In the WDM optical transmission apparatus
201
with n-channels shown in
FIG. 8
, the polarization states from the optical transmitter
30
to the coupler
35
are not managed, so that the polarization relationship between the channels is random.
The optical fibers
32
and
32
′ of the WDM optical transmission apparatus
201
are connected with a polarization holding fiber, thereby enabling the orthogonal state to be held. However, there is a problem that the optical amplifier
31
composed of an erbium-doped polarization holding fiber has a low manufacturability and is expensive.
Also, in case of a WDM optical transmission apparatus
202
where the distance between the transmitters
30
_
1
-
30
_n and the coupler
35
is long, the arrangement where the optical transmitters
30
_
1
-
30
_n and the coupler
35
are respectively connected with the optical amplifiers
31
_
1
-
31
_n, dispersion compensating fibers
33
_
1
-
33
_n, and optical amplifiers
34
_
1
-
34
_n is essential, and the dispersion must be compensated by the polarization holding fiber, so that there is a problem that the practicability of this arrangement is low.
Thus, in the prior art, there has been no WDM optical transmission apparatus having a method and an arrangement of a practical level on which polarizations of a plurality of modulated optical signals are managed, and odd even channels are mutually orthogonalized to be coupled.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a polarization compensator for converting a lightwave only a linear polarization or at least one of a linear polarization, a circular polarization, and an elliptical polarization into a linear polarization having a polarization plane of a predetermined direction, and a wavelength division multiplexing apparatus for coupling lightwaves so as to prevent polarization planes of the lightwaves whose wavelengths are adjoining from coinciding with each other by using the polarization compensator.
It is to be noted that the “polarization compensation” in the specification of the present invention means that a lightwave of an arbitrary polarization is converted into a linear polarization having a polarization plane of a predetermined direction.
In order to achieve the above-mentioned object, a polarization compensator according to the present invention comprises: a rotator for rotating an input lightwave by an angle designated with a control signal and providing an output lightwave; a polarizer, having a polarization plane set in a reference direction, for inputting the output lightwave; and a controller for outputting the control signal which gives instructions to make a polarization plane of the input lightwave the reference direction based on a polarization sig
Font Frank G.
Fujitsu Limited
Punnoose Roy M.
Staas & Halsey , LLP
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