an opo with very fast and accurate tuning . the angle of the crystals in the opo is controlled by converting the linear motion of a voice coil into rotational motion . in preferred embodiments one or two opo crystals are mounted as a crystal unit that can rotate around an axis such that the angle of the crystals with respect to the beams &# 39 ; direction can be varied to generate the desired wavelengths . the crystal unit has a lever that is connected to the shaft of the voice coil such that as the shaft extend or retracts the level is pulled or pushed and the linear motion of the shaft is converted to an angular motion of the crystal unit . the position of the voice - coil shaft is controlled in a close - loop based on a built - in encoder . the relation between the reading of the encoder and the crystals &# 39 ; angle is recorded and provides the calibration of the unit . preferably calibration is done by measuring the output wavelength of the opo as a function of the encoder position .