Wavelength converter and wavelength converting apparatus

A wavelength converter comprising:

• a nonlinear optical medium having a phase modulated periodically modulated structure composed of a modulation unit structure and a phase modulation structure, the modulation unit structure having a structure in which a nonlinear optical coefficient is periodically modulated at a period substantially equal to a fundamental period Λ0 and the phase of the modulation varies nearly continuously, and the phase modulation structure having a structure in which the phase variation of the modulation unit structure is repeated at a phase modulation period Λph longer than the fundamental period Λ0;
• and
• means for launching light with one or two wavelengths of three wavelengths λ1, λ2 and μ3 including λ1=λ2, which satisfy the following equation, onto said nonlinear optical medium, 1 λ 3 = 1 λ 1 + 1 λ 2
• wherein
• said wavelength converter, utilizing a second order nonlinear optical effect occurring in said nonlinear optical medium, converts the input light into outgoing light with a wavelength equal to one of the three wavelengths, and different from at least one of the wavelengths of the incident light.