Fig. 1. The basic structure of short fiber delay self-heterodyne system in visible light band

Fig. 2. Peak-valley position and serial number of short fiber delay self-heterodyne beat spectrum signal

Fig. 3. The spectrum simulation of delay self-heterodyne beat signal. (a) The simulation of delay self-heterodyne beat signal S(f) and its components when the linewidth is 70 kHz and the delay fiber length is 127 m; (b) the spectrum of the beat signal of the 30 kHz laser under different length delay fibers; (c) the frequency spectrum of the beat signal of the laser with different linewidths passing through a delay self-heterodyne system with a delay fiber length of 2 km

Fig. 4. The variation trend of the second peak-to-valley value ΔS₁₀ with the linewidth Δf under different length delay fibers

Fig. 5. Single-frequency laser linewidth measurement system in visible light band based on short fiber delay self-heterodyne

Fig. 6. The single-frequency laser linewidth measurement device in visible light band based on dual-beam heterodyne method

Fig. 7. Smoothing process of spectrum signal with noise

Fig. 8. Delayed self-heterodyne beat signal spectrum of 635 nm external-cavity diode laser with 127 m delay fiber

Fig. 9. Delayed self-heterodyne beat signal spectrum of 635 nm external-cavity diode laser with 500 m delay fiber

Fig. 10. Frequency spectrum of two-beam heterodyne beat signal of 635 nm external-cavity diode laser

Fig. 11. The variation trend of the second peak-to-valley value ΔS₁₀ of the beat frequency signal spectrum of different linewidth lasers with the fiber length L

Fig. 12. Background noise of photodetectors