Filter with variable transmission character, optical transmission equipment and method of optical transmission

#The present invention relates to optical transmission equipment and more particularly, to a wavelength gain characteristic shift slope filter, optical transmission equipment and optical transmission method, which are suitable for wavelength division multiplexing.

#In order to cope with rapid increase of traffic involved by rapid spread of the Internet, dense wavelength division multiplexing (DWDM) has been implemented in North America. The implementation of the DWDM is due to the fact that an optical amplifier for use in collective amplification of wavelength division multiplexing light has an amplification band. However, the gain of the optical amplifier depends on wavelength and further the wavelength dependency also varies depending on its input level. For this reason, the current wavelength division multiplexing cannot use the full amplification band of the optical amplifier and can use only a band called “red band” having a relatively flat gain wavelength dependency.

#In this connection, the articles associated with the invention of the present application are U.S. Pat. Nos. 5,675,432 and 5,987,200 and U.S. application Ser. No. 09/191,051 (JP-A-11-150526), the description of the articles being hereby incorporated by reference. Described in the U.S. Pat. No. 5,987,200 is a multiplexing/demultiplexing device of a narrow-band Bragg grating type which can change add/drop wavelengths by changing the temperature of a fiber grating.

#Preferred embodiments of the present invention will now he described in conjunction with the accompanying drawings, in which:

#FIG. 1 is a block diagram of optical transmission equipment in accordance with an embodiment of the present invention;

#FIG. 2 is a block diagram of an optical demultiplexer applied to the embodiment of the present invention;

#Embodiments of the present invention will be explained by referring to the attached drawings.

#FIG. 1 is a block diagram of optical transmission equipment in accordance with an embodiment of the present invention.

#In the present embodiment, a wavelength band of a optical signal supplied from a light transmitter 1 has λ1=1531±1.5 nm, λ2=1534±1.5 nm, λ3=1537±1.5 nm, λ4=1549.5±11 nm, and λ5=1510±10 nm. The λ5 is a wavelength band of monitor light standardized by the International Telecommunications Union-Telecommunications standardization sector (ITU-T), and the λ1 to λ4 are wavelength bands of optical signal. It is assumed therein that a boundary wavelength between the wavelength bands of the optical signal provides an inflection point to a wavelength-gain characteristic of Er-doped fiber (EDF) of an optical amplifier, the reason of which will be explained later. The bands λ1 to λ5 are convenient divisions merely for the purpose of explaining the effects of the present invention, and thus the respective wavelength bands are not necessarily limited to the shown divisions but may be further divided into sub-divisions. Further, more or less signal wavelengths may be multiplexed into each band. Furthermore, the band λ5 may be a band having a center wavelength of 1480 nm.