A laser light source is disclosed consisting of first and second lasers that respectively generate laser beams of wavelengths .lamda..sub.1 and .lamda..sub.2. Wavelength .lamda..sub.1 is in the range of 0.9-1.0 .mu.m. A nonlinear optical crystal uses the laser beams of wavelengths .lamda..sub.1 and .lamda..sub.2 as inputs, and outputs a coherent beam having a wavelength .lamda..sub.3 of a difference frequency that satisfies a relationship of 1/.lamda..sub.1-1/.lamda..sub.2=1/.lamda..sub.3. The nonlinear optical crystal has a periodically poled structure of a single period. The wavelength .lamda..sub.3 of the difference frequency varies between 3.1 .mu.m and 2.0 .mu.m when the wavelength .lamda..sub.2 varies between 1.3 .mu.m and 1.8 .mu.m.