Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-01-25
2004-06-29
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S010000, C385S040000
Reexamination Certificate
active
06757461
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tunable dispersion compensation device for dynamically compensating for chromatic dispersion in an optical fiber which is a transmission path for use in an optical fiber communication system, an optical receiver including such a tunable dispersion compensation device, and an optical fiber communication system including such a tunable dispersion compensation device.
2. Description of the Prior Art
In recent years, as it has become desirable to utilize many channels (i.e., many optical signals) over a wider range of wavelengths to carry a lot of information via an optical fiber which is a transmission path in an optical fiber communication system, such as a wavelength division multiplexing (WDM) system, chromatic dispersion (group delay dispersion) in the optical fiber has required more precise compensation. Chromatic dispersion in an optical fiber causes spectral components of different wavelengths included in an optical signal to propagate through the optical fiber at different speeds, thereby inducing pulse broadening in the optical signal. For example, a single mode fiber used for optical fiber communication systems provides abnormal dispersion (negative group velocity dispersion) for an optical signal of a wavelength of 1550 nm, the chromatic dispersion having a positive sign and being typically equal to about 17 ps
m/km. In other words, spectral components of shorter wavelengths included in an optical signal propagate through the single mode fiber faster than other spectral components of longer wavelengths, and the pulse width of an optical signal having a spectral width of 1 nm increases only by about 17 ps every time the optical signal propagates through a 1 km length of the single mode fiber, for example. Two adjacent pulses in an optical pulse train that propagates through an optical fiber can thus overlap with each other at a high data rate. Such pulse overlapping can cause errors in data transmission.
In order to compensate for such chromatic dispersion in an optical fiber which is a transmission path, a dispersion compensation fiber and an optical waveguide, such as an optical fiber, including a chirped grating, which provide group velocity dispersion of a sign opposite to the dispersion in the optical fiber have been developed. On the other hand, there is a problem that chromatic dispersion in an optical fiber may vary with time because of a change in the temperature of the optical fiber, a change in the connection of the optical fiber, a change in the stress placed on the optical fiber due to external forces, and so on. Since those prior art dispersion compensation devices can only compensate for a fixed amount of chromatic dispersion, they cannot deal with such a problem. Particularly, in optical fiber communication systems that operate at 40 Gbit/s or higher, since a slight transition in the status of a transmission path changes the chromatic dispersion, it is forecast that a dynamic dispersion compensation is needed.
FIG. 17
is a diagram showing the structure of a prior art tunable dispersion compensation device as disclosed in Japanese patent application publications No. 10-221658, No. 2000-235170, and No. 2000-252920, to solve the above-mentioned problem. In the figure, reference numeral
2
denotes an optical waveguide in which a chirped grating having a grating pitch (i.e., grating period) that continuously changes along its optical axis is formed, reference numerals
3
-
1
to
3
-
n
denote a plurality of heaters for producing a desired temperature distribution in the optical waveguide
2
, respectively, and reference numerals
8
-
1
to
8
-
n
denote a plurality of electrodes via each of which an electric current flows into a corresponding heater, respectively.
In operation, since the nearer to an input/output end of the optical waveguide
2
the longer grating pitch and hence the longer Bragg reflection wavelength the grating has, spectral components having longer wavelengths in an optical signal are reflected back at locations nearer to the input/output end of the optical waveguide
2
and are output via the input/output end. In other words, spectral components of shorter wavelengths in an optical signal reach locations within the optical waveguide
2
, which are further from the input/output end of the optical waveguide
2
, and are reflected back at the locations corresponding to the Bragg reflection wavelengths decided by the grating pitches. Therefore, different spectral components in an optical signal are reflected back at different locations in the optical waveguide
2
and thus have different delays. As a result, when an optical signal with a broadened pulse width in which spectral components of shorter wavelengths exist at more forward parts thereof is incident on the optical waveguide
2
, the pulse width of the optical signal is compressed and is emitted out of the optical waveguide
2
.
The optical waveguide
2
is made of a material, such as silica glass, whose refractive index changes according to its temperature. A desired temperature distribution can be produced along the length of the optical waveguide
2
by adjusting the electric power applied to each of the plurality of heaters
3
-
1
to
3
-
n
by way of a corresponding one of the plurality of electrodes
8
-
1
to
8
-
n
. When the optical waveguide
2
is heated by the plurality of heaters
3
-
1
to
3
-
n
so as to have a desired temperature distribution, the grating pitch and refractive index of each segment of the chirped grating formed in the optical waveguide
2
which is heated by a corresponding one of the plurality of heaters change. As a result, the Bragg reflection wavelength of each segment of the chirped grating changes. The chromatic dispersion provided for an input optical signal by the optical waveguide
2
therefore changes.
Neither of the above-mentioned Japanese patent application publications discloses a concrete method of adjusting the electric power supplied to each of the plurality of heaters
3
-
1
to
3
-
n
for the purpose of dynamic dispersion compensation. For example, a method of adjusting the electric power to be applied to each of the plurality of heaters by changing the resistance value of a resistor connected in series to a corresponding one of the plurality of heaters can be devised. In this case, a variable resistor is connected to each of the plurality of heaters, and the resistance value of the variable resistor is changed and the electric power supplied to each of the plurality of heaters is therefore adjusted according to a desired temperature distribution to be produced in the chirped grating.
FIG. 18
is a diagram showing the structure of a prior art tunable dispersion compensation device that can dynamically compensate for chromatic dispersion, as disclosed in Japanese patent application publication No. 2000-137197, and
FIG. 19
is a diagram schematically showing the structure of an optical fiber communication system including the tunable dispersion compensation device
91
shown in
FIG. 18
, as disclosed in Japanese patent application publication No. 2000-244394. In
FIG. 18
, reference numeral
9
denotes a resistive thin film whose thickness changes linearly along the length of an optical waveguide
2
, reference numerals
27
a
and
27
b
denote electrodes via which an electric current is supplied to the resistive thin film
9
, and reference numeral
28
denotes a direct-current power supply for supplying the electric current to the resistive thin film
9
by way of the electrodes
27
a
and
27
b
. Furthermore, in
FIG. 19
, reference numeral
40
denotes an optical transmitter for multiplexing and transmitting a plurality of optical signals of different wavelengths each of which carries information, reference numeral
50
denotes an optical fiber transmission line via which the plurality of multiplexed optical signals are transmitted, reference numeral
90
denotes a dispersion compensation module provided with the tunable dispersion com
Hashimoto Minoru
Hirai Toshiyuki
Kato Akihiko
Matsuoka Ken
Burns Doane Swecker & Mathis L.L.P.
Mitsubishi Denki & Kabushiki Kaisha
Rojas Omar
Sanghavi Hemang
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