Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-11-09
2003-10-28
Kim, Robert H. (Department: 2871)
Optical waveguides
With optical coupler
Plural
C385S037000, C385S028000
Reexamination Certificate
active
06640024
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an apparatus for measuring stress, and more particularly to a fiber-optic add-drop wavelength filter which drops specific channels or adds specific channels in a wavelength division optical communication system while having low insertion loss.
BACKGROUND ART
In general, a passive device is required which may drop or add channels at any points in line in order to realize a wavelength division optical communication system. In particular, an element is required which can select narrow wavelength range less than 1 nm while having low insertion loss in order to increase the number of channels. In an optical fiber, fiber-optic Bragg gratings have a characteristic of a good wavelength selectivity and low loss like this.
The simplest fiber-optic add-drop wavelength filter is made by combining fiber-opticr Bragg gratings and a fiber-optic directional coupler. In the fiber-optic add-drop wavelength filter, output optical signals are obtained by inputting optical signals to a port of the fiber-optic directional coupler, reflecting output optical signals therefrom at the fiber-optic Bragg gratings, and then causing the signals to pass through the fiber-optic directional coupler again. This type of add-drop wavelength filter may be very simply manufactured but has a disadvantage of always giving an insertion loss of at least 6 dB. To overcome this, for instance, a non-reciprocal optical circulator is connected to optical Bragg gratings so that input optical signals, after being reflected on the Bragg gratings via the optical circulator, are output through a remaining port through which the optical signals did not proceed in an optical circulator. Although advantageously this is very stable and has small amount of reflection loss and interference between channels, this has disadvantages that there exists a certain insertion loss, an expansion to an integrated optical device is impossible, and the cost is high because this is not an all-fiber device. Therefore, an add-drop wavelength filter is required which is an all-fiber structure while having low loss, and several types of all-fiber add-drop wavelength filters have been proposed.
FIGS. 1A
to
1
C show functions of structural parts of general add-drop wavelength filters.
FIG. 1A
shows the function of a wavelength dropping device
10
,
FIG. 1B
shows the function of a wavelength adding device
10
′, and
FIG. 1C
shows the function of an add-drop wavelength filter
10
″. Referring to
FIG. 1A
, when optical signals
60
comprising of multi-wavelengths including a wavelength &lgr;
i
are introduced to an input port
20
of the wavelength dropping device, signals of the specific wavelength &lgr;
i
propagate to a drop port
40
and the remaining signals excluding the wavelength &lgr;
1
propagate to an output port
30
. Referring to
FIG. 1B
, signals
90
excluding the specific wavelength &lgr;
i
are introduced into an input port
20
′, added to the signals
110
of &lgr;
i
introduced to an add port
50
′, and then the added signals
100
propagate to an output port
30
′. Referring to
FIG. 1C
, when optical signals
120
are introduced into an add-drop wavelength filter
10
″ through an input port
20
″, signals
140
of &lgr;
i
in the optical signals are separated to propagate to a drop port
40
″, and the remaining signals with the signals of &lgr;
i
being separated are joined with new signals
150
of &lgr;
i
to propagate to an output port
30
″ as new signals
130
.
Several configurations of conventional add-drop wavelength filters performing functions like these are illustrated as follows:
FIG. 2
shows a configuration of an add-drop wavelength filter in which Bragg gratings
220
and
222
are written respectively within both arms of a Mach-Zehnder interferometer
205
comprising two optical couplers
210
and
212
. When input optical signals are introduced into an input port I, the signals which are separated after the reflection by the Bragg gratings are output through a drop port D, the remaining output signals propagate to an output port D, and optical signals to be added are input into an add port A. An add-drop wavelength filter of this type has a low insertion loss and an excellent wavelength selectivity however requires accurate finish processes about the length or the refractive index of the optical fiber in manufacturing. Furthermore, even after completion, the filter is sensitive to external temperature variation and thus has a poor stability in operation.
FIG. 3
shows a configuration of an add-drop wavelength filter of the type which combines a fiber-optic polarization splitter
230
, polarization controllers
240
and
242
, and fiber-optic Bragg gratings
250
and
252
.
FIG. 4
to
FIG. 6
show configurations of fiber-optic add-drop wavelength filters of the type in which a fiber-optic Bragg grating
270
is written within a fiber-optic directional coupler
260
. Herein, same components are represented as same reference numerals. As described above, although the way using a polarization divider or using a directional coupler, within which a Bragg grating is written, is relatively more stable than an interferometer type, each components should be controlled very accurately in order to attain desired wavelength characteristics and it is difficult to obtain good wavelength characteristics.
Besides, as illustrated in
FIG. 7
, an add-drop wavelength filter was also presented, in which a titled Bragg grating
310
is written within a fused-type fiber-optic directional coupler
300
which is made with two different optical fibers
280
and
290
. In turn, as illustrated in
FIG. 8
, an add-drop wavelength filter was also presented which was made by using a dual core optical fiber
320
, directional couplers
330
and
332
, and a Bragg grating
350
which is written within only one core
340
of the dual core optical fiber. But in these cases, they are disadvantageously difficult to manufacture because the used fiber-optic devices can not be made or gained easily.
Furthermore, recently, an add-drop wavelength filter using two different dual mode optical waveguides
360
and
370
, mode discriminating directional couplers
380
and
382
and titled Bragg gratings
390
,
392
and
394
was proposed by Strasser et al, as shown in FIG.
9
. This add-drop wavelength filter has a low loss and is stable in general, but includes several things difficult to be realized in practice. In order to make a mode discriminating directional coupler which is used in the proposed add-drop wavelength filter, two different dual mode optical fibers or dual mode optical waveguides are required in which effective refractive indexes of LP
11
mode are same each other with an accuracy of at most 0.0001 and effective refractive indexes of LP
01
modes are different from each other, which are difficult to manufacture. Furthermore, two Bragg gratings having same reflective spectra and same mode conversion features are required to be written within two different waveguides respectively, which is also difficult to be realized because a very high accuracy is demanded.
Therefore, in the prior art, it is hard to recognize that the optimum fiber-optic add-drop wavelength filter for a wavelength division optical communication system exists which is easy to manufacture and has low insertion loss and good stability, and therefore there exist difficulties to select a suitable fiber-optic add-drop wavelength filter according to situations.
DISCLOSURE OF INVENTION
Accordingly, it is an object of the present invention to provide an all-fiber add-drop wavelength filter which may be easily manufactured while having excellent wavelength selectivity, good stability, and low insertion loss.
The fiber-optic add-drop wavelength filter of the present invention to achieve the above mentioned object basically serves to separate optical signals having at least one wavelength &lgr;
i
(1≦i≦n) from input optical signals consisting of multiple wavelengths of
Kim Byoung Yoon
Park Hee Su
Yun Seok Hyun
Kim Robert H.
Korea Advanced Institue of Science & Technology
Suzue Kenta
Wang George Y.
Wilson Sonsini Goodrich & Rosati
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