Patent ID: 7656911

Claim:
An external resonator type wavelength-variable laser capable of varying a wavelength within a range Δf comprising: a semiconductor light amplifier; a wavelength selection filter having periodic frequency characteristics; and a wavelength-variable filter, wherein the laser is configured so that the range Δf is divided into one or more regions, and regarding at least one region Δf i , where i is a number of regions the range Δf is divided into, a coefficient m j is derived from: m j =(j×Δf fs )/Δf FP where Δf fs : a period of the wavelength selection filter, Δf FP : a Fabry-Perot mode interval dependent on a length of the external resonator, and j: an integer not smaller than 1 and not larger than (Δf i /Δf fs )), a coefficient M j is an integer obtained by rounding off the first digit after decimal point of the coefficient m j , a coefficient N i,j is an integer obtained by discarding the first digit after decimal point of Δf i /(j×Δf fs ), and then relations of (M j −m j )>0, 0<N i,j ×2π(M j −m j )≦π are satisfied for at least one j, wherein the j=2; and the wavelength selection filter transmits a plurality of wavelength transmission bands including: a first wavelength transmission band having a center frequency matching a first frequency of a Fabry-Perot mode; and a second wavelength transmission band being directly adjacent to the first wavelength transmission band and having a center frequency with a phase difference of about π with respect to a second frequency of the Fabry-Perot mode.