Fig. 1. Principle of dual-frequency laser generation^[10]. (a) Acousto-optic frequency-shift; (b) dual longitudinal modes to select the frequency

Fig. 2. Zeeman-birefringence dual-frequency laser^[10]

Fig. 3. Frequency stabilization via equal intensity method

Fig. 4. Frequency difference drift in a long period

Fig. 5. The schematic of dual-frequency interference

Fig. 6. Drift test results for designed dual-frequency laser interferometers

Fig. 7. 70 m linearity test in National Institute of Metrology, China

Fig. 8. 70 m linearity test results of Zeeman-birefringence dual-frequency laser interferometer. (a) Linearity; (b) measurement errors

Fig. 9. Measuring device of nonlinear errors

Fig. 10. Comparison of nonlinear errors of two dual-frequency laser interferometer^[32]. (a) Agilent dual-frequency laser interferometer; (b) Zeeman-birefringence dual-frequency laser interferometer

Fig. 11. Precise measurement applications with Zeeman-birefringence dual-frequency laser interferometer. (a) Test of satellite electric propulsion system; (b) CNC machine calibration; (c) coordinate measuring machine calibration

Fig. 12. Zeeman-birefringence dual-frequency laser used in Nikon NSR mask aligner

Fig. 13. Zeeman-birefringence dual-frequency laser interferometer

Fig. 14. The schematic of three-mirror model for laser feedback interferometry^[46]

Fig. 15. Schematic of laser feedback effect. (a) Zero frequency feedback; (b) frequency-shifted feedback

Fig. 16. Laser frequency-shifted feedback optical system

Fig. 17. Output characteristics of solid-state microchip laser. (a) Fundamental transverse and longitudinal mode; (b) wavelength and power stability

Fig. 18. Laser power spectra under different feedback levels. (a)--(c) Simulation results; (d)--(f) experimental results

Fig. 19. Gain function curve

Fig. 20. Laser frequency-shifted feedback optical system

Fig. 21. Flow chart of the phase demodulation of laser frequency-shifted feedback interferometer

Fig. 22. Test results of the laser frequency-shifted feedback interferometer. (a) Short-period drift; (b)displacement resolution

Fig. 23. Laser frequency-shifted feedback interferometer

Fig. 24. Single-spot two-dimensional displacement measurement based on laser frequency-shifted feedback interferometry^[52]

Fig. 25. Two-dimensional displacement resolution. (a) In-plane displacement; (b) off-plane displacement

Fig. 26. Two-dimensional displacement test results. (a) Random motion; (b) circle motion

Fig. 27. Rotation measurement method based on double-beam frequency-shifted feedback interferometry

Fig. 28. Frequency-shifted signals^[76]. (a) S1; (b) S2

Fig. 29. Remote eavesdropping system based on laser frequency-shifted feedback^[77]

Fig. 30. The spectrograms of the test sound recovered in the different distances^[77]. (a) Test sound spectrogram; recovered spectrograms at (b) 100 m, (c) 150 m, and (d) 200 m

Fig. 31. Schematic of liquid refractive index measurement^[62]

Fig. 32. Measurement system for materials’ coefficient of thermal expansion. (a) System diagram; (b) device

Fig. 33. Laser confocal frequency-shifted feedback imaging system

Fig. 34. Two-dimensional longitudinal view of microfluidic channels. (a) LFCT system imaging result at 0.02 mW; (b) LCT system imaging result at 0.02 mW; (c) LCT system imaging result at 0.73 mW; (d) microfluidic chip structure diagram

Fig. 35. Laser ultrasound frequency-shifted feedback imaging system^[89]