Method for controlling wavelength-tunable laser

A method for controlling a wavelength-tunable laser comprising a wavelength detection unit having an etalon, the method comprising:

• a first step of acquiring from a memory a first driving condition for laser-oscillating the wavelength-tunable laser at a first wavelength;
• a second step of computing a second driving condition for oscillating the wavelength-tunable laser at a second wavelength with reference to the first driving condition and a wavelength difference between the first and second wavelengths;
• and
• a third step of detecting with the wavelength detection unit an output wavelength of the wavelength-tunable laser driven according to the second driving condition and performing according to a result thereof feedback control for correcting the driving conditions of the wavelength-tunable laser;
• the method further comprising a step, before the third step, of determining whether or not to reverse a correction sign for the correction in the feedback control with respect to a correction sign in the first driving condition.