The laser beam is the carrier for sensing the changes (displacement, length, thickness, surface topography, tomography) of the measured object, and the laser that generates the laser beam is the core of the precision measuring instrument. An instrument with excellent performance must use a light source with excellent performance—laser. For example, the laser interferometer makes full use of the high coherence, high polarization and high collimation of the laser beam, which takes the optical wavelength as the scale and realizes the displacement measurement of several meters, tens of meters and hundreds of meters with nanometer accuracy. Major manufacturing industries such as aviation, aerospace, automobiles and ships are inseparable from laser interferometers. The instruments listed in this book are “the ones you have not seen”, which needs to develop lasers with new principles and new technologies. The authors’ team invented the birefringence dual-frequency laser based on He–Ne laser as the light source for the instrument. Furthermore, the team developed a new functional microchip solid (neodymium-doped yttrium aluminum garnet (Nd:YAG) and neodymium yttrium vanadate (Nd:YVO4)) lasers and a multi-beam, common path (or quasi common path), frequency-stabilized birefringence dual-frequency laser, etc. In addition, the laser feedback instruments will also occupy a considerable space in this book. Therefore, this chapter will introduce the relaxation oscillation, as well as the control methods of the parameters related to the relaxation oscillation frequency. Relaxation oscillation plays an important role in the performance of the instrument based on laser feedback