Add-drop multiplexer with signal amplification ability

Details Add-drop multiplexer with signal amplification ability Add-drop multiplexer with signal amplification ability

2001-05-14

2004-04-20

Ullah, Akm Enayet (Department: 2874)

Optical waveguides

With optical coupler

Plural

C385S037000, C385S043000, C385S047000

Reexamination Certificate

active

06724955

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention is related to an optical coupler using a grating filter in which a grating (periodic perturbation of refractive index) is formed in a core of an optical fiber and a portion adjacent the core.
The present invention contributes in the field of optical communications. It is useful technology when only specific wavelength is separated out or added in wavelength multiplex transmission.
A conventional optical transmission type filter is described below.
Since it is generally common for the transmission optical filter to be required in the field of optical communications, an optical fiber grating filter is used in combination with optical parts including an optical circulator in order to transmit a predetermined wavelength band.
FIG. 8
illustrates a schematic diagram of an optical fiber grating filter
22
. Moreover,
FIG. 9
illustrates the general reflective properties of the optical fiber grating filter
22
. The optical fiber grating filter
22
formed in an optical fiber
21
is a filter of a reflection type which reflects light of a predetermined wavelength, allows transmission of other wavelengths, and is excellent in wavelength selection properties for narrow band use, and has little insertion loss. Although a usual reflective bandwidth is approximately 1 nm, if a special process is used, it can also realize a reflective bandwidth of 0.2 mm to 10 mm.
FIG. 10
illustrates an example an optical transmission filter formed by combining an optical fiber grating filter
32
and optical circulator
33
in the course of an optical fiber
31
. Although a signal incident from a port
1
is. outputted to a port
2
if the optical fiber grating filter
32
is formed on the route to the port
2
, only a specific wavelength (it is called wavelength &lgr;B hereupon) is reflected and outputted from a port
3
after it returns back to the optical circulator
33
side. If two or more signals by which wavelength multiplex is carried out from the port
1
, only a signal corresponding to wavelength &lgr;B will be outputted from the port
3
and all the signals corresponding to other wavelength will be outputted from the port
2
. That is, the multiplex separation of the signal of specific wavelength LB can be carried out.
Moreover,
FIG. 11
illustrates optical transmission filter formed by combining an optical fiber grating filter
42
and an optical fiber coupler
43
in the course of an optical fiber
41
. In this example, the signal inputted from the port
1
is outputted to the ports
3
and
4
by halves by the optical fiber coupler
43
. At the port
3
the signal is reflected by the optical fiber grating filter
42
and the signal corresponding to wavelength &lgr;B reinputs to the optical fiber coupler
43
, and is outputted to the ports
1
and
2
by halves again. Therefore, one fourth of the inputted signals will be outputted in the port
2
. If two or more signals are inputted to the port
1
by which wavelength multiplex is done, only the signal corresponding to wavelength &lgr;B will be outputted from the port
2
, and all the signals corresponding to other wavelength will be outputted from the port
3
.
In addition,
FIG. 12
shows a grating filter
52
formed in extension part of an optical fiber coupler
53
which functions as an optical transmission filter in the course of an optical fiber
51
. In this example, a signal input to port
1
of the optical fiber coupler
53
is reflected by a grating filter
52
is formed in an extension portion of the optical fiber coupler
53
, if the signal is of specific wavelength &lgr;B, and will be outputted to the port
2
. In this case, if two or more signals by which wavelength multiplex is done are inputted, the signal corresponding to the wavelength &lgr;B is outputted to the port
2
, all signal corresponding to another wavelength is outputted to the port
4
and the multiplex separation of the signal of specific wavelength &lgr;B can be done.
The problem in the above-mentioned conventional technology is as follows.
At first, in the method shown in
FIG. 10
, in case of using the combination of the optical fiber grating filter
32
and the optical circulator
33
, although the insertion loss is in approximately 2 dB between the port
1
and port
2
, it is excelled in properties, the problem is that the optical circulator
33
is expensive. Moreover, in the method shown in
FIG. 11
by the combination of the optical fiber grating filter
42
and optical fiber coupler
43
, if the optical fiber coupler
43
is compared with the optical circulator
33
, although it is a cheap device, as for insertion loss, the minimum is 6 dB (namely, ¼). Furthermore, a signal outputted from the port
3
has an insertion loss of 3 dB (namely, ½).
Furthermore, by the combination of the optical fiber grating filter
52
and optical fiber coupler
53
shown in
FIG. 12
, the signal of specific wavelength &lgr;B that is outputted from the port
2
to the port
1
and
4
, the signal of specific wavelength &lgr;B outputted has a 0.4 dB (namely, {fraction (9/10)}) insertion loss.
SUMMARY OF THE INVENTION
In light of the forgoing, a multiplexer has a grating built-in type optical coupler and an optical amplifier, and effects not only multiplex separation of a signal but addition of a specific wavelength, and the excitation light which became unnecessary can be removed effectively and it is enabled to suppress a fall of the signal light efficiency. Furthermore, since the device does not use optical parts, such as the above-mentioned optical circulator, and is formed by an optical fiber, transmission is good. Accordingly it is an object of the present invention to provide an inexpensive wavelength multiplex transmission system having a low insertion loss.
According to the invention, there is provided an add-drop multiplexer with signal amplification ability, comprising two optical couplers of a grating built-in type with the two same structures and two optical amplifiers.
Moreover, the optical coupler of the grating built-in type of the present invention including molten extended portion of fiber coupler formed as a fiber grating, the pitch of the grating is a uniform structure in the length direction, apodization is carried out, and grating length is 2.0 mm and change in induced refractive index is 0.001. Here, apodization uses a window function for induced refractive index change of grating in the length direction of an optical fiber as shown in FIG.
13
.


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