Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-12-31
2003-06-24
Mullen, Jr., Thomas J. (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06583900
ABSTRACT:
This application incorporates the following related applications by reference: Japanese Patent Application Nos. 07-075787; 07-214733; 05-198674, related to U.S. Pat. Nos. 5,696,614, 5,602,666, 5,612,807, 5,636,046; 08-098704, related to U.S. Ser. No. 08/752,516, now U.S. Pat. No. 5,877,881; 09-044406; 09-044407; 10-040113; 10-038908; 10-090383; 10-072810; 10-020615; 09-216050; 09-201,825, related to U.S. Ser. No. 09/017,692, now U.S. Pat. No. 6,104,847; 09-240934; and 09-287489, related to U.S. Ser. No. 09/080,399, now U.S. Pat. No. 6,351,323.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical wavelength division multiplexing network.
2. Description of the Related Art
For a future multimedia network, a super-long and large capacity optical communications system and a light wave network using the system is demanded with research and development proceeding vigorously.
A conventional system for realizing large-capacity data communications can be a time-division multiplexing (TDM) system, an optical time-division multiplexing (OTDM) system, a wavelength-division multiplexing (WDM) system, etc.
Among these systems for realizing the functions of the above described light wave network, the WDM system can utilize the broadband and large capacity of optical fibers, and can select, branch, and insert an optical transmission signal independent of a modulation system or speed using an optical wavelength multiplexer/demultiplexer (optical filter).
That is, the light wave network requires an add/drop multiplexer (ADM) for adding/branching signals as necessary, and optical routing and cross-connecting functions for selecting a transmission line.
An add/drop multiplexer has been studied and developed for adding/branching an optical signal. The add/drop multiplexer can be a fixed-wavelength type for adding/branching optical signals having fixed wavelengths; and an optional wavelength type for adding/branching optical signals having optional wavelengths.
A device of the fixed wavelength type includes, for example, a circulator and a fiber grating, and reflects one of the transmitted optical signals, which has a specific wavelength, on the fiber grating to branch it through the circulator. When an optical signal is added, the optical signal to be added is temporarily transmitted to the fiber grating through the circulator. A specific wavelength is reflected on the fiber grating, and the optical signal is multiplexed with an optical signal passing through a transmission line.
In such a device of the fixed wavelength type, the wavelengths of added/branched optical signals are determined when the system is produced. Consequently, there is the problem that a large number of requests to the light wave network cannot be completely satisfied.
On the other hand, since the wavelengths of added/branched optical signals in a device of the optional wavelength type can be changed through a remote operation even after the system has been produced, a request to change added/branched wavelengths (channels) can be easily satisfied.
FIG. 1
shows an example of the configuration of an optical ADM device using an optical switch.
A wavelength multiplexed light having the wavelengths &lgr;
1
through An is input from the input terminal to a demultiplexer (DMUX), and is branched into optical signals having respective wavelengths. An optical signal having each wavelength is input to a 2×2 optical switch provided for each wavelength. The 2×2 optical switch passes each optical signal through, or drops it.
The optical signal dropped by the 2×2 optical switch is transmitted to a tributary station (branch station). An optical signal passing through the 2×2 optical switch is input to a multiplexer as is, multiplexed into a wavelength multiplexed light, and then output. An optical signal dropped by the 2×2 optical switch is transmitted to a tributary station. The tributary station first multiplexes the dropped optical signal through a wavelength multiplexer/demultiplexer, and then branches the multiplexed optical signal to provide an optical signal to an optical receiver OR provided for each channel. Although not shown in
FIG. 1
, the optical receiver OR is provided with a wavelength selection filter, selects an optical signal having a predetermined wavelength from among optical signals branched by a wavelength multiplexer/demultiplexer, and receives the selected signal.
Thus, an optical signal having a specified wavelength can be dropped by demultiplexing, by the OADM device, the optical signal wavelength-multiplexed into signals of respective wavelengths and dropping each optical signal. At the terminal of a tributary station, an optical signal of a specified wavelength (channel) can be received by selecting a desired wavelength from among dropped optical signals and receiving an optical signal having the selected wavelength. Especially, when dropped wavelengths are different from each other, the wavelength of an optical signal received by, for example, the first optical receiver can be variable if a wavelength selection filter provided before the optical receiver OR can select variable wavelengths.
An electric signal converted from an optical signal by an optical receiver OR is processed by an electric ADM device (E ADM) for performing an add/drop multiplexing using an electric signal. A signal to be transmitted from a tributary station is output from the E ADM, and is converted into an optical signal by an optical transmitter OS for output. The wavelength of the optical signal output from each of the optical transmitters OS of the tributary station shown in
FIG. 1
is one of the wavelengths dropped by the OADM device, and is output to an optical switch. An optical switch switches the optical path of an optical signal transmitted from an optical transmitter OS, and transmits an optical signal having a corresponding wavelength to a 2×2 optical switch which performs a dropping process. Each 2×2 optical switch for performing a dropping process receives an optical signal having the same wavelength as the dropped optical signal from the tributary station, and transmits the signal to a multiplexer MUX. Thus, the optical signal transmitted from the tributary station is multiplexed with the optical signal passing through the OADM device, and is output as a wavelength multiplexed optical signal.
An OADM device of an optional wavelength type can normally be the above described device using an optical switch. However, it does not operate quickly. Furthermore, when an optical network is operated by a system using a smaller number of wavelengths than the maximum number of multiplexed wavelengths, it has output/input ports of a multiplexer and a demultiplexer, which are not required, and therefore has unnecessary equipment. Additionally, when a 2×2 optical switch is equipped from the beginning, it is an unnecessary optical switch consuming the initial investment. In the above described system, since an optical signal is branched by the multiplexer to optical signals having each wavelength, the multiplexer has the characteristic of a band pass filter for optical signals having each wavelength. If devices having such a characteristic of a band pass filter are connected in series, small differences in pass band are accumulated and cause the problem that the pass band of the entire system becomes very narrow for each wavelength. Therefore, to solve the problem, the pass bands of optical devices should strictly match each other, thereby imposing severe restrictions on the system design and mounting operations.
Furthermore, since the optical signal is AM-modulated, a side band is generated in the component of a wavelength. If such an optical signal is propagated through a system having a very narrow pass band, then the wavelength is undesirably varied, and the receiving unit may not be able to receive an optical signal. In the worst case, the system cannot propagate an optical signal.
The above described problem occurs when the system is designed
Chikama Terumi
Kai Yutaka
Miyata Hideyuki
Nakazawa Tadao
Onaka Hiroshi
Fujitsu Limited
Mullen Jr. Thomas J.
Staas & Halsey , LLP
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