A tunable coherent light source includes a pump laser for generating a pump beam and an optical parametric oscillator including a crystal exhibiting an output curve for a pump beam of a defined wavelength, the output curve defining wavelengths of signal and idler outputs based on periodically poled grating periods of the crystal. The crystal has a plurality of segments associated with a plurality of grating periods of the output curve, each segment of the plurality of segments having a different crystal structure. At least one of the plurality of segments comprises a crystal structure combining at least two of the grating periods A heating device heats the crystal to an elevated operating temperature and is adjustable for adjusting the output wavelengths of each of the segments.