Fig. 1. Principle diagram of double Sagnac loop

Fig. 2. Simulated transmission spectra of double Sagnac loop filter under different conditions. (a) θ₁＝π/6, θ₂＝0；(b) θ₁＝0, θ₂＝π/6

Fig. 3. Structure diagram for measuring transmission spectrum of double Sagnac loop filter

Fig. 4. Transmission spectra of double Sagnac loop filter under different channel spacings. (a) 0.9 nm; (b) 0.35 nm；(c) irregular channel spacing

Fig. 5. Transmission spectra of double Sagnac loop filter under different conditions. (a) Different rotation angles of PC-1 with channel spacing of 0.9 nm; (b) different rotation angles of PC-1 with channel spacing of 0.35 nm; (c) different rotation angles of PC-1 with irregular channel spacing

Fig. 6. Experimental setup of channel spacing tunable multi-wavelength erbium-doped fiber laser utilizing double Sagnac loop filter and FWM effect

Fig. 7. Multi-wavelength output spectra. (a) Channel spacing of 0.9 nm; (b) channel spacing of 0.35 nm