Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-10-14
2003-03-18
Pascal, Leslie (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06535309
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical multiplexing/demultiplexing apparatus and method therefor for use in an optical communication network to which wavelength division multiplexing (WDM) techniques are applied, and more particularly, to optical multiplexing/demultiplexing apparatus and method therefor, in which an optical power balance of each wavelength light of a wavelength division multiplexed signal light can be retained at input and output ports of the optical multiplexing/demultiplexing apparatus.
2. Description of the Related Art
A WDM system which utilizes band characteristics of optical fibers is a transmission system that is suitable for constructing an optical network which is flexible in taking in and out signal lights. Such an optical network permits not only the transmission of a wavelength division multiplexed signal from a point to a point but also the selective transmission of only signal light of a certain wavelength in a wavelength division multiplexed signal light and the branching or insertion of signal light of the other wavelengths by an optical multiplexing/demultiplexing apparatus provided at some midpoint on a transmission path.
FIG. 10
illustrates an example of an optical communication system (namely, a WDM network system), which is networked by using conventional optical multiplexing/demultiplexing apparatuses.
In the conventional system of
FIG. 10
, at least a pair of optical fibers are used as transmission paths for upstream and downstream communication lines (or circuits). Further, a plurality of optical amplifier repeaters are placed for compensating losses in optical fibers. Each of the optical amplifier repeaters is provided with at least two optical amplifiers for the upstream and downstream communication lines. Moreover, a plurality of signal lights (namely, WDM signal lights) of different wavelengths are transmitted from each terminal device to a single optical fiber. Furthermore, the transmitted WDM signal lights are assigned transmission paths thereto according to the wavelength thereof, respectively, by the optical multiplexing/demultiplexing apparatuses, then, sent to a receiving terminal device.
The conventional optical multiplexing/demultiplexing apparatuses used in such a WDM network system are constituted by combining optical add-drop multiplexer (OADM) circuits as illustrated in FIG.
11
.
In the OADM circuit of
FIG. 11
, among WDM signal lights propagating in a main optical fiber, signal lights of some wavelengths are branched to branch optical fibers. Further, signal lights of the remaining wavelengths are multiplexed with a signal light input from an insertion optical fiber, then to be output to the main optical fiber. This OADM circuit can be constituted by using wavelength multiplexing/demultiplexing elements, for instance, a dielectric multilayer filter, a WDM coupler, a fiber grating and an arrayed waveguide grating (AWG). Hitherto, there have been proposed ADM circuits of various constitutions (see, for example, Miyakawa et al., “A Study of Optical ADM using Dielectric Filter”, Denshi Joho Tsushin Gakkai Sogo Taikai, vol. B-10-234, 1997; “Optical ADM and Wavelength Routing Based on WDM System”, by the applicant of the instant application, Denshi Joho Tsushin Gakkai Sogo Taikai, vol. B-1158, 1996; and Mizuochi et al., “A Study on Optical Add-Drop Multiplexers Using Fiber Grating Filters”, Denshi Joho Tsushin Gakkai Tsushin Society Taikai, vol. SB-9-5, 1995).
In an actual WDM network system, there is the necessity for using at least one optical fiber pair paths for the upstream and downstream lines. Therefore, the optical multiplexing/demultiplexing apparatus is constituted by at least two OADM circuits as illustrated in FIG.
11
.
FIG. 12
shows an example of the conventional optical multiplexing/demultiplexing apparatus constituted by two OADM circuits.
The aforementioned conventional optical multiplexing/demultiplexing apparatus, however, has encountered the problem that there is caused a difference in optical power level between signal light (hereunder sometimes referred to as through light) passing through the OADM circuit and signal light (hereunder sometimes referred to as add light) inserted in the OADM circuit.
In the case that the optical power level is balanced between through light and add light included in signal light passing through the OADM circuit as shown in FIG.
13
(A), the WDM signal light is normally transmitted between the terminal devices. However, the balance between the optical power levels of the through light and the add light included in the signal light passing through the OADM circuit is lost when the power of the WDM signal light or the add light input to the OADM circuit decreases owing to an increase in loss in the optical fiber provided in a pre-stage of the optical multiplexing/demultiplexing apparatus or to lowering of an output of the optical amplifier repeater. For instance, in the case where the power level of the add light is extremely smaller than that of the through light as illustrated in FIG.
13
(B), the add light is buried in noises caused by the optical amplifier repeater at a repeater section provided in a post-stage of the optical multiplexing/demultiplexing apparatus. Consequently, the transmission characteristics are extremely deteriorated.
As a system for reducing the difference in optical power level between through light and add light, there have been proposed systems for regulating the optical power of add light transmitted from a terminal device to an optical multiplexing/demultiplexing apparatus, by controlling an operation of an optical amplifier repeater placed between the terminal device, which transmits add light, and the optical multiplexing/demultiplexing apparatus. Practically, an example of such systems is such that an amplifying operation of an optical amplifier repeater is controlled by transmitting a supervisory control signal from a terminal device to an optical multiplexing/demultiplexing apparatus. Alternatively, another example of such systems is that a dummy light of different wavelengths is transmitted from the terminal device in addition to add light, and the optical power level of the add light output from the optical amplifier repeater is controlled by regulating a power level of the dummy light. These systems, however, have encountered the problems that the systems cannot preserve the balance between the optical power levels of the through light and the add light in the case that an abnormality, such as, increase in loss of optical fiber, occurs between a terminal device and an optical amplifier repeater or between an optical amplifier repeater and an optical multiplexing/demultiplexing apparatus, and that there is a limit to the range of the optical power of add light, which can be controlled by an optical amplifier repeater.
SUMMARY OF THE INVENTION
The present invention is accomplished in view of the aforementioned problems. Accordingly, an object of the present invention is to provide optical multiplexing/demultiplexing apparatus and method therefor in which an optical power of each wavelength signal light of a wavelength division multiplexed signal light can be retained at input and output ports by controlling a power level of an insertion light in the optical multiplexing/demultiplexing apparatus.
To achieve the foregoing object and in accordance with an aspect of the present invention, there is provided an optical multiplexing/demultiplexing apparatus comprising a branching/inserting device connected to a transmission path where a wavelength division multiplexed signal light is transmitted, and being capable of branching a signal light of at least one wavelength from a signal light transmitted on the transmission path, and being capable of inserting a signal light of at least one wavelength into a signal light transmitted on the transmission path, wherein the optical multiplexing/demultiplexing apparatus further comprises an insertion-light power control device for controlling an
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