Optical: systems and elements – Light interference – Electrically or mechanically variable
Reexamination Certificate
2001-08-28
2003-11-11
Juba, John (Department: 2872)
Optical: systems and elements
Light interference
Electrically or mechanically variable
C359S615000, C359S199200
Reexamination Certificate
active
06646805
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for wavelength dispersion and generation of a wavelength dispersion slope, and an apparatus for compensating for the wavelength dispersion accumulated in an optical fiber transmission network, and more specifically to an apparatus using a virtually imaged phased array for generating a wavelength dispersion and a wavelength dispersion slope.
2. Description of the Related Art
In the conventional fiber optical communications system for transmitting information through an optical system, a transmitter transmits a pulse to a receiver through an optical fiber. However, the wavelength dispersion of an optical fiber deteriorates the quality of a signal of a system.
To be more practical, as a result of wavelength dispersion, the transmission speed of a signal of an optical fiber depends on the wavelength of the signal. For example, if a pulse having a long wavelength (for example, the pulse of the wavelength indicating a red color pulse) is transmitted at a higher speed than a pulse having a short wavelength (for example, the pulse of the wavelength indicating a blue color pulse), it is normal dispersion. On the other hand, if a pulse having a short wavelength (for example, the pulse of the wavelength indicating a blue color pulse) is transmitted at a higher speed than a pulse having a long wavelength (for example, the pulse of the wavelength indicating a red color pulse), then it is abnormal dispersion.
Therefore, when a pulse contains red and blue pulses and is transmitted from a transmitter, the pulse is divided when it is transmitted through an optical fiber into a red pulse and a blue pulse which are received by a photodetector at different times. If a red pulse is transmitted at a higher speed than a blue pulse, it is normal dispersion.
If there are continuous wavelength components from blue to red as another example of pulse transmission, a pulse is extended in an optical fiber and distorted by wavelength dispersion because a red component and a blue component are transmitted at different speeds. Since all pulses contain finite wavelength extension, such wavelength dispersion frequently occurs in the fiber optical communications system.
Therefore, it is necessary for the fiber optical communications system to compensate for wavelength dispersion to obtain a higher transmission capacity.
To compensate for the wavelength dispersion, the fiber optical communications system requires an inverse dispersion device. Normally, an inverse dispersion device provides inverse dispersion for a pulse to nullify the dispersion generated by transmission through an optical fiber.
There are several devices which can be used as an inverse dispersion device. For example, a dispersion compensation fiber has a specific sectional index profile, thereby functioning as an inverse dispersion device and providing the inverse dispersion for nullifying the dispersion generated by the optical fiber. However, the dispersion compensation fiber is expensive in production cost, and has to be sufficiently long enough to successfully compensate for the wavelength dispersion. For example, when an optical fiber is 100 km long, the dispersion compensation fiber is approximately 20 through 30 km. Therefore, there is the problem of a large loss and size.
FIG. 1
shows the chirp fiber grating used as an inverse dispersion device to compensate for the chromatic dispersion.
As shown in
FIG. 1
, a ray is transmitted through an optical fiber, wavelength-dispersed, and then provided for an input port
48
of an optical circulator
50
. The optical circulator
50
provides the ray for a chirp fiber grating
52
. The chirp fiber grating
52
returns the ray to the optical circulator
50
such that different wavelength components can be reflected by the channel fiber grating at different distances, different wavelength components can travel different distances, and the wavelength dispersion can be compensated for. For example, the chirp fiber grating
52
can be designed such that a long wavelength component can be reflected at a long distance, and travel a longer distance than a short wavelength. Then, the optical circulator
50
provides the ray reflected to an output port
54
from the chirp fiber grating
52
. Therefore, the chirp fiber grating
52
can add inverse dispersion to a pulse.
However, the chirp fiber grating
52
has very narrow band for a reflected pulse. Therefore, a sufficient wavelength band cannot be obtained to compensate for the ray containing a number of wavelengths such as a wavelength division-multiplexed light. A number of chirp fiber gratings can be cascaded for a wavelength division-multiplexed signal. However, the resultant system becomes costly. The chirp fiber grating obtained by incorporating a circulator is appropriate for a single-wavelength fiber optical communications system, etc.
FIGS. 2 and 3
shows the conventional diffraction grating used to generate wavelength dispersion.
As shown in
FIG. 2
, a diffraction grating
56
has a grating surface
58
. Parallel rays
60
having different wavelengths are input into the grating surface
58
. The rays are reflected by each stage of the grating surface
58
, and interferes each other. As a result, rays
62
,
64
, and
66
having different wavelengths are output at different angles from the diffraction grating
56
. The diffraction grating can be used in the spatial grating pair array described later to compensate for the wavelength dispersion.
FIG. 3A
shows a spatial grating pair array used as an inverse dispersion device to compensate for wavelength dispersion.
As shown in
FIG. 3A
, a ray
67
is diffracted from a first diffraction grating
68
, and becomes a ray
69
for a short wavelength and a ray
70
for a long wavelength. These ray
67
and ray
70
are diffracted by a second diffraction grating
71
, and travel in the same direction. As shown in
FIG. 3A
, the wavelength components having different wavelengths travel different distances, thereby compensating for the wavelength dispersion. A long wavelength (such as the ray
70
, etc.) travels a longer distance than a short wavelength. Therefore, the spatial grating pair array shown in
FIG. 3A
indicates abnormal dispersion.
FIG. 3B
shows another spatial grating pair array used as an inverse dispersion device to compensate for chromatic dispersion.
As shown in
FIG. 3B
, lenses
72
and
74
are positioned between the first and second diffraction gratings
68
and
71
. A long wavelength (such as the ray
70
) travels a shorter distance than a short wavelength (such as the ray
69
). Therefore, the spatial grating pair array shown in
FIG. 3B
indicates normal dispersion.
The spatial grating pair array as shown in
FIGS. 3A and 3B
are normally used to control the dispersion using a laser resonator. However, an actual spatial grating pair array cannot provide sufficient dispersion to compensate for a relatively large amount of chromatic dispersion generated by the fiber optical communications system. To be more practical, the angular dispersion generated by diffraction grating is normally very small, that is, approximately 0.05°
m. Therefore, to compensate for the wavelength dispersion generated in the fiber optical communications system, the first and second diffraction grating
68
and
71
have to be largely apart. Accordingly, such a spatial grating pair array is not practical at all.
FIG. 4
shows the conventional technology of an inverse dispersion device using a VIPA.
In the above mentioned conventional technology, in the patent application numbers 10-534450 and 11-513133, the ‘Virtually Imaged Phased Array’ as shown in
FIG. 4
, that is, the device containing the portion referred to as VIPA
1
, is suggested as an inverse dispersion device. The VIPA transmits from the VIPA the rays having different wavelengths spatially discriminated. This device includes an optical return device
2
for generating multiple reflection in the VIPA.
The above mentioned device can be realized by
Kawahata Yuichi
Mitamura Nobuaki
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