Control device of modulating signal and method

A control device that controls a modulating signal that takes a first level, a second level that is higher than the first level, and at least one third level that is higher than the first level and lower than the second level, the control device comprising:

• a high-side signal generating unit that generates a high-side signal, the high-side signal taking a level in accordance with a level of an alternating current component of an electrical signal generated from a modulated light when a polarity of the alternating current component is positive or magnitude of the alternating current component is zero, the high-side signal further taking a constant level when a polarity of the alternating current component is negative;
• a low-side signal generating unit that generates a low-side signal, the low-side signal taking a constant level when a polarity of the alternating current component is positive, the low-side signal further taking a level in accordance with a level of the alternating current component when a polarity of the alternating current component is negative or magnitude of the alternating current component is zero;
• a high-side peak value detecting unit that generates a high-side peak value signal that takes a fourth level in accordance with a first certain level out of levels that the high-side signal takes, the first certain level being a level with a greatest value of absolute values for levels that the high-side signal takes;
• a low-side peak value detecting unit that generates a low-side peak value signal that takes a fifth level in accordance with a second certain level out of levels that the low-side signal takes, the second certain level being a level with a greatest value of absolute values for levels that the low-side signal takes;
• a level adjusting unit that adjusts the at least one third level of the modulating signal on the basis of the fourth level taken by the high-side peak value signal and the fifth level taken by the low-side peak value signal,
• wherein the modulating signal causes an optical device to generate the modulated light by taking the first level, the second level, and the at least one third level while being applied to the optical device, the modulated light being a multivalued signal.