Chirp switching circuit and optical transmission system

A chirp switching circuit, comprising:

• a Mach-Zehnder modulator having a Y-branched part for branching an incoming optical signal into first and second optical signals and an X-branched part merging said first and second optical signals with each other, said Mach-Zehnder modulator causing phase modulation in said first and second optical signals by a modulation signal;
• and
• a directional coupling optical switch that switches first and second optical output signals output from said X-branched part of said Mach-Zehnder modulator by merging said first and second output optical signals in response to a chirp switching control signal wherein
• said directional coupling optical switch outputting said first optical output signal in a first output optical waveguide of said X-branched optical switch to an output optical waveguide of said directional coupling optical switch when a control signal is applied with a first logic level, and said directional coupling optical switch outputting said second optical output signal in a second output optical waveguide of said X-branched optical switch to said output optical waveguide of said directional coupling optical switch when said control signal is applied with a second logic level.