CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2019/009846 filed Mar. 12, 2019, claiming priority based on Japanese Patent Application No. 2018-048868 filed Mar. 16, 2018, the disclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present invention relates to a variable equalizer and a control method for a variable equalizer, and particularly relates to a variable tilt equalizer in which a tilt of a wavelength characteristic of the equalizer is variable, and a control method therefor.

BACKGROUND ART

In order to ensure transmission quality of a long-distance optical submarine cable system, a tilt of a wavelength characteristic (optical profile) of an intensity of an optical signal propagating through an optical transmission path is desirably flat (tilt level is 0 dB) in a period from a BOL to an EOL of the system. The BOL represents a beginning of life, and the EOL represents an end of life. However, a tilt level of the optical profile fluctuates due to repair or aged deterioration of a submarine cable and a repeater. Accordingly, a variable tilt equalizer device capable of adjusting a tilt level is necessary for keeping a tilt level flat during system operation.

In connection with the present invention, PTL 1 discloses a technique for adjusting an optical gain profile in an optical amplification repeater transmission system. PTL 2 discloses a technique for compensating a dispersion slope in an optical transmission system.

CITATION LIST

Patent Literature

[PTL 1] Japanese Unexamined Patent Application Publication No. 2001-237776[PTL 2] Japanese Unexamined Patent Application Publication No. 2008-154123

SUMMARY OF INVENTION

Technical Problem

FIG. 8is a diagram illustrating a configuration of a general variable equalizer900. The variable equalizer900includes two 1×9 optical switches and nine tilt equalizers. The two 1×9 optical switches select one tilt equalizer, and give a loss based on a predetermined wavelength characteristic to an input wavelength division multiplexing (WDM) optical signal. The tilt equalizer gives a loss varying depending on a wavelength to an optical signal, and thereby, a tilt level of the optical signal is adjusted. WDM is an abbreviation for wavelength division multiplexing, and a wavelength division multiplexing optical signal is referred to as “WDM signal” hereinafter.

FIGS. 9 to 11are diagrams illustrating a wavelength characteristic of a tilt equalizer. The tilt equalizer is a device in which a loss changes monotonically with respect to a wavelength in a predetermined wavelength range from λ1 to λ2 (λ1<λ2). As a commercially available tilt equalizer, one fabricated with a fiber grating is known.FIG. 9illustrates an example of a wavelength characteristic of a tilt equalizer of +n dB. A loss at the wavelength λ2 in the tilt equalizer of +n dB is higher by n (dB) than a loss at the wavelength λ1. For example, assuming that a loss at the wavelength λ1 in the tilt equalizer is 0.5 dB, a loss at the wavelength λ2 in the tilt equalizer of +n dB is n+0.5 dB.FIG. 10illustrates an example of a wavelength characteristic of a tilt equalizer of −n dB. A loss at the wavelength λ1 in the tilt equalizer of −n dB is higher by n (dB) than a loss at the wavelength λ2.FIG. 11illustrates an example of a wavelength characteristic of a tilt equalizer of 0 dB. In the tilt equalizer of 0 dB, a loss in a range from the wavelength λ1 to the wavelength λ2 is constant (e.g., 0.5 dB). InFIGS. 9 to 11, change in loss in a range from the wavelength λ1 to the wavelength λ2 is monotonic.

The variable equalizer900has a problem that a large number of the tilt equalizers are necessary for expanding an adjustment range of a tilt level. For example, in the variable equalizer900, nine tilt equalizers are necessary for setting tilt levels at an interval of 1 dB in a range from −4 dB to +4 dB. Since a communication device to be installed on the seabed is required to be smaller in size, there is limitation on the number of devices that can be mounted in the communication device. Thus, in a multi-core system in which many signal lines are used, a large number of tilt equalizers are necessary, thereby a system price is caused to rise.

Object of Invention

An object of the present invention is to provide a technique for implementing a small-sized variable equalizer with a wide adjustment range of a tilt level, and a control method for the variable equalizer.

Solution to Problem

A variable equalizer according to the present invention includes: a first optical equalizer group including a plurality of first optical equalizers whose tilt amounts are different from each other; a second optical equalizer group including a plurality of second optical equalizers; and an optical element forming an optical path of an optical signal in such a way that the input optical signal passes through the selected first optical equalizer and the selected second optical equalizer and is output, wherein at least one of a plurality of the second optical equalizers has a tilt amount different from that of any of a plurality of the first optical equalizers.

A method of controlling a variable equalizer according to the present invention includes: selecting a first optical equalizer from a first optical equalizer group including a plurality of the first optical equalizers whose tilt amounts are different from each other; selecting a second optical equalizer from a second optical equalizer group including a plurality of the second optical equalizers in which at least one of a plurality of the second optical equalizers has a tilt amount different from that of any of a plurality of the first optical equalizers; and forming an optical path of an optical signal in such a way that the input optical signal passes through the first optical equalizer and the second optical equalizer and is output.

Advantageous Effects of Invention

The present invention provides a small-sized variable equalizer with a wide adjustment range of a tilt level.

