Fig. 1. Principle of laser feedback and laser output intensity diagram^[2]. (a) Principle of laser feedback; (b) laser feedback fringe, curve of laser intensity induced by mirror displacement

Fig. 2. Principle diagram of optical feedback frequency modulation and phase heterodyne measurement method for micro-chip laser^[6]

Fig. 3. Output transverse mode of Nd∶YVO₄ laser^[7]

Fig. 4. Optical power spectrum and relaxation oscillation of the microchip laser^[8]. (a) Single-longitudinal mode laser power spectrum, f_RO; (b) dual-longitudinal mode laser power spectrum, f_RO'; (c) optical power spectrum of single-longitudinal mode laser frequency shift feedback, modulated frequency f

Fig. 5. Relaxation oscillation frequency of Nd∶YVO₄ laser and Nd∶GdVO₄ laser as a function of relative pump level

Fig. 6. Relationship of relaxation oscillation frequency with relative pump level at different cavity lengths

Fig. 7. Variation of relaxation oscillation frequency with relative pump level under different reflectances of output mirror

Fig. 8. Frequency stabilization of the microchip laser (size is 30 mm×30 mm×40 mm)^[9]

Fig. 9. System configuration of the quasi common path, frequency multiplexing laser self-mixing interferometer^[8]

Fig. 10. Results of performance verification. (a) Zero drift characteristics of the system; (b) tested data of the system's displacement resolution

Fig. 11. Micro-chip laser feedback interferometer, quasi-common-path dual microchip frequency multiplexing technology

Fig. 12. Schematic diagram of two laser beams generated by two LD's directly pumping a piece of Nd∶YVO₄ microchip laser

Fig. 13. Laser transverse modes of the two beams

Fig. 14. Resolution of the quasi-common-path frequency-multiplexing microchip laser interferometer

Fig. 15. Zero drift measurement at the working distance of 10 m

Fig. 16. Total frequency shift f is less than the relaxation oscillation frequency (oscilloscope display)

Fig. 17. Total frequency shift f is greater than the relaxation oscillation frequency (oscilloscope display)

Fig. 18. Optical path structure of Nd∶YVO₄ laser feedback confocal system

Fig. 19. One-dimensional, two-dimensional, and three-dimensional tomography

Fig. 20. Frequency domain and time domain signal of feedback light obtained by microchip feedback interferometer (target is sound box) ^[19]

Fig. 21. Waveform and spectrum of sound signal^[19]. (a) Measured signal; (b) original signal

Fig. 22. Schematic diagram of two-dimensional displacement measurement by laser feedback of the micro-piece^[20]

Fig. 23. Measurement results of two-dimensional displacement^[20]. (a) In-plane displacement resolution; (b) out-of-plane displacement resolution; (c) two-dimensional displacement range; (d) Lissajous graph trajectory

Fig. 24. Measurement device of material thermal expansion based on Nd∶YAG laser feedback interferometer^[21]

Fig. 25. Refractive index and thickness measured by quasi common laser feedback interferometry^[22]