Laser feedback interference (also known as self-mixing interference) is substantially different from traditional laser interference. The former occurs in the laser (light source), and the laser medium gain plays an important role in the interference effect (fringe form). The latter occurs only in the optical path outside the laser. In this paper, from the perspective of application and comparing with the traditional laser interferometry, the key technologies are discussed of feedback interferometer of microchip-laser common-path frequency-modulation, including the "relaxation oscillation" which has special significance for the feedback of solid-state microchip lasers, the formation of high light sensitivity (completely non-contact), the improvement of measurement accuracy, measurement speed, frequency stabilization technology, etc. The results of the team's research on the application of microchip laser common-path frequency-modulation feedback interferometers will also be presented, including feedback confocal microscopy, surface measurement, vibration (and sound) measurement, in-plane displacement measurement, thermal expansion coefficient measurement, refractive index measurement, etc.