PROBLEM TO BE SOLVED: To provide a wavelength variable laser enabling stable wavelength selection by accurately controlling the frequency of an RF signal of acoustic optical filter.
SOLUTION: The wavelength variable laser having a resonator formed with two reflection faces is provided with a gain medium for generating a laser beam; a first filter generating a surface acoustic wave to a light guide by applying the RF signal and selectively transmitting light of a frequency region corresponding to the frequency of the RF signal; and a second filter having a cyclic transmissive frequency region in the resonator. The laser has a frequency control means for controlling the frequency of the RF signal in response to the size of a beat signal having the frequency of N times (N is an integer including 1) of the RF signal included in output light output from the resonator. Since the beat signal of the output light is the minimum when controlled into the optimum RF signal frequency, control of the frequency of the RF signal corresponding to a desired frequency can be accurately performed.
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