Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-03-02
2001-11-13
Pascal, Leslie (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C385S140000
Reexamination Certificate
active
06317233
ABSTRACT:
BACKGROUND OF THE INVENTION
1 Field of the Invention
The present invention relates to an optical power equalizer in a wavelength division multiplexing(WDM) optical communication system, and more particularly, to an optical power equalizer in a WDM optical communication system which can tune an optical power down to a desired extent and a variable optical attenuator for use therein.
2. Background of the Related Art
Having overcome a level in which a light is merely taken as a transmission medium in place of electricity, currently the optical communication has come to a level in which an optical signal is converted into an electrical signal at a final reception terminal after amplifying the optical signal for a longer distance transmission or processing the optical signal while maintaining a form of the optical signal without converting the optical signal into an electrical signal. And, recently, the wavelength division multiplexing technique is rapidly developed and spread for increasing a transmission capacity, in which an individual light source is provided for each of wavelengths in an optical signal, multiplexed before transmission, demultiplexed at a reception terminal, split into the individual wavelengths, thereby receiving the optical signal. And, sometimes, an optical amplifier is provided in the middle of the transmission for a long distance transmission. An optical communication network is sometimes provided with an add-drop function in which portions of the optical signal are allocated to required places and optical signals from the places are added thereto.
FIG. 1
schematically illustrates a related art WDM add-drop system, in which an optical signal transmitted through an optical amplifier
10
is demultiplexed through a demultiplexer
11
into individual wavelengths &lgr;
1
,&lgr;
2
,- - -,&lgr;
n
, add-drops are made for the individual wavelengths through optical switches
12
, multiplexed through a multiplexer
14
, amplified through an optical amplifier, and transmitted through one optical fiber. In this instance, as shown in
FIG. 2A
, the optical signal before the optical signal is multiplexed exhibits a non-uniform state between different wavelengths. Though there are many reasons which cause this non-uniform state, this non-uniform state typically comes from composite factors, such as differences of amplification gains between the wavelengths when the optical signal is amplified through the optical amplifier, differences in the wave splitting performances of the multiplexer
11
between the individual wavelengths, non-uniform switching characteristics of the optical switches
12
, which cause the non-uniform state between the individual wavelengths in the optical signal at an input terminal of the multiplexer
14
so serious that a signal characteristic is deteriorated too much unable to make a long distance communication. Consequently, it is required to make powers of the optical signals uniform before the multiplexing, for which the optical attenuators
13
as shown in
FIG. 1
are provided, owing to which outputs of the individual wavelengths are made uniform as shown in FIG.
2
B. The optical attenuator, generally used currently with optical fibers handled manually, is not responsive to continuous signal changes since the system is fixed once adjusted initially. Therefore, a variable optical attenuator is required, which can be used continuously while monitoring an optical signal. Such a variable optical attenuator is disclosed, for example, in U.S. Pat. No. 4,644,145. Though the U.S. Pat No. 4,644,145 discloses variable optical attenuators used in optical waveguides for compensating variations of signals received at an optical receiver, with the object of the U.S. Pat. 4,644,145 lying on improvement of a receiver performance merely by making sizes of signals at a reception terminal uniform, it is very difficult to employ the optical attenuator in the WDM optical communication because the optical attenuator is one used before the optical communication system technology is matured, in which each of fast optical signals received in succession is attenuated at a fast speed for compensating time basis non-uniformity of the optical signals. That is, for modulation of the fast optical signals, though the variable optical attenuator in the U.S. Pat. No. 4,644,145 uses LiNbO
3
as a substrate for utilizing an electrooptical effect or GaAs, a semiconductor material, a device using LiNbO
3
as a substrate is expensive despite fast modulation is allowed, and not suitable for insertion in the middle of transmission for use, not in a few GHz range of fast modulation, but in the add-drop multiplexing system at a few kHz range of frequency because of a low coupling efficiency of the optical fiber and a device using GaAs, a semiconductor material, can not be put into practical use because of a further lower coupling efficiency and expensive material. And, the related art variable optical attenuator has a micrometer which uses an optical fiber for manual mechanical manipulation of the micrometer, or a micrometer fitted with a motor for electric control. However, the related art variable optical attenuator with a micrometer fitted with a motor, not only has many problems in actual application to the WDM optical communication system because it has a large power consumption, high driving voltage over 10V, relatively bulky, and a slow tuning speed of 50-1,400 msec, but also has a problem of high cost because the device is not suitable for mass production in view of the device structure.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to an optical power equalizer in a WDM optical communication system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an optical power equalizer in a WDM optical communication system, in which a tuning for maintaining an output of an optical signal in an optical communication system constant is allowed.
Other object of the present invention is to provide an optical power equalizer in a WDM optical communication system, which can provide a stable operation and has a low material cost and a simple fabrication process suitable for mass production; and a variable optical attenuator for use therein.
Another object of the present invention is to provide an optical power equalizer in a WDM optical communication system, which is operative with easy and has a low insertion loss; and a variable optical attenuator for use therein.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the optical power equalizer in a WDM optical communication system includes a variable optical attenuator for reducing a power of an optical signal to a given level, an optical coupler for detecting a portion of signal proportional to an output of the variable optical attenuator, and an optical power monitor for receiving an output of the optical coupler and generating an electrical control signal for controlling an output of the variable optical attenuator.
In another aspect of the present invention, there is provided a variable optical attenuator including two asymmetric optical waveguides adjacent to each other to form a directional coupler, and a thermooptic electrode for varying the asymmetry of the asymmetric optical waveguides, thereby attenuating an optical power of an optical communication system.
The optical power equalizer in a WDM optical communication system of the present invention not only allows to maintain a performance of an optical signal at an
Bello Agustin
Fleshner & Kim LLP
LG Electronics Inc.
Pascal Leslie
LandOfFree
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