Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-04-19
2001-04-24
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S037000, C385S046000
Reexamination Certificate
active
06222957
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to an inexpensive dispersion slope compensator able to equalize the energies of output optical signals.
2. Description of the Related Art
The development and use of optical fibers has greatly increased the ability of people around the world to communicate. Due to the swelling requirements of wide bandwidth, current trends in the designs of the optical fiber networks tend towards increasing volume, reliability and economy. With such requirements, a wavelength division multiplexing (WDM) transmission system is the best choice.
It has been noticed that too large a dispersion or a long-distance signal transmission easily causes signal interference in any a wave band. Furthermore, the extent of accumulated dispersions for each optical signal in a WDM transmission system is always different, so using a single dispersion compensation fiber to simultaneously compensate the accumulated dispersions for all the optical signals perfectly is impossible. Thus, various techniques of controlling the compensation for dispersion are being actively developed.
FIG. 1
depicts a dispersion slope compensator according to a first example of the prior art, wherein mixed optical signals of wavelengths &lgr;
1
, &lgr;
2
propagating in an optical fiber
11
are transmitted to optical filters
15
,
16
through a fiber coupler
12
. The optical filters
15
,
16
are used to select the optical signals. For example, the optical filter
15
only allows the optical signal of wavelength &lgr;
1
to propagate into a dispersion compensation fiber
17
, while the other optical filter
16
only allows the optical signal of wavelength &lgr;
2
to propagate into another dispersion compensation fiber
18
. The two dispersion compensation fibers
17
,
18
are of different lengths so that the respective dispersions of the optical signals of wavelengths &lgr;
1
and &lgr;
2
are compensated perfectly. Then, the two compensated optical signals of wavelengths &lgr;
1
and &lgr;
2
are combined by another fiber coupler
19
.
In the first example, three optical filters and dispersion compensation fibers will be used if three optical signals of different wavelengths &lgr;
1
, &lgr;
2
and &lgr;
3
are mixed and simultaneously transmitted. By analogy, N optical filters and dispersion compensation fibers are used when N optical signals of different wavelengths &lgr;
1
, &lgr;
2
. . . &lgr;
N
are mixed and simultaneously transmitted.
FIG. 2
depicts another dispersion slope compensator according to a second example of the prior art, which includes two arrayed waveguide gratings (AWGs)
21
,
22
and a plurality of dispersion compensation fibers
23
connected therebetween. An optical fiber for propagating mixed optical signals of wavelengths &lgr;
1
, &lgr;
2
. . . &lgr;
N
is connected to one of the inputs
1
,
2
, . . . , N of the arrayed waveguide grating
21
. The arrayed waveguide grating
21
separates the mixed optical signals into the dispersion compensation fibers
23
. The dispersion compensation fibers
23
are of different lengths so that the respective dispersions of the optical signals of wavelengths &lgr;
1
, &lgr;
2
. . . &lgr;
N
are compensated perfectly. Then, the compensated optical signals are combined by the other arrayed waveguide grating
22
and sent out from one of the outputs
1
′,
2
′, . . . , N′ of the arrayed waveguide grating
22
.
The disadvantages of the first and second examples mentioned above are now discussed. In the first example, the use of optical filters
15
,
16
results in a great deal of energy loss. The more channels used, the more energy of the transmitted optical signals lost. The energy loss of the transmitted optical signals would be 6 dB for two channels and 9 dB for three channels.
Furthermore, both of the two examples mentioned above fail to equalize the energy of each channel. To solve this problem, erbium-doped fiber amplifiers (EDFAs) generally are provided after the dispersion compensation fibers for equalizing the strengths of each optical signals. However, EDFAs are very expensive, the use of which greatly increases the cost of building the WDM transmission system.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a dispersion slope compensator that solves the above-mentioned problems.
In accordance with the object of the present invention, a dispersion slope compensator is provided. The dispersion slope compensator includes a dispersion compensation fiber, an optical circulator and a plurality of fiber gratings. The optical circulator is connected between an optical fiber and the dispersion compensation fiber for introducing mixed optical signals propagating in the optical fiber into the dispersion compensation fiber. The fiber gratings are provided at predetermined positions of the dispersion compensation fiber so as to respectively reflect the mixed optical signals back to the optical fiber through the optical circulator. Thus, accumulated dispersions of the mixed optical signals are fully compensated due to different propagating distances of each of the mixed optical signals in the dispersion compensation fiber.
Furthermore, the reflectivity of each fiber gratings in the present invention is predetermined by controlling the time period for exposing the germanium doped silicon fiber to ultraviolet rays in the manufacturing process. The optical signal propagating a long distance in the dispersion compensation fiber corresponds to the fiber gratings of high reflectivity. On the other hand, the optical signal propagating a short distance in the dispersion compensation fiber corresponds to the fiber grating of low reflectivity. In this way, the energies of respective optical signals are made equal to each other.
Furthermore, the optical circulator, the dispersion compensation fiber and the fiber gratings used in the present invention are all commercially available and inexpensive. Therefore, the cost of building the WDM transmission system can be greatly reduced.
REFERENCES:
patent: 5748349 (1998-05-01), Mizrahi
patent: 6097861 (2000-08-01), Kim et al.
Chi Sien
Dung Jeng-Cherng
Jang Win-Yann
Lin Samuel I-En
Bednarek Michael D.
Palmer Phan T. H.
Pittman Shaw
Uconn Technology Inc.
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