An EDFA-Gain Equalizer Based On a Sagnac Loop With an Unpumped Erbium-Doped Fiber

We have demonstrated an erbium doped fiber amplifier (EDFA) gain equalizer for flattening the amplified spontaneous emission gain spectrum using a Sagnac loop incorporated with a single-section unpumped erbium-doped fiber. The gain spectra output from the transmission port and the reflection port of the Sagnac loop are studied in detail. The absorption effect of unpumped erbium-doped fiber and the resonance mode caused by the birefringence beat length in the loop are combined to make the gain spectrum be partially or completely flattened by adjusting the polarization controllers properly in the loop. On the one hand, the part of the spectrum is flatted within +/- 0.15 dB and +/- 0.145 dB in the wavelength range of 16 nm and 14 nm at the reflection port and transmission port respectively. On the other hand, the total spectrum is flatted within +/- 1.225 dB over a bandwidth of 36.5 nm in the whole C-band at the transmission port, while the two-peak with 29.6 nm interval occurred at the reflection port because of the 0.15-1.55 m birefringence beat length of single-mode erbium-doped fiber in the loop. The results are consistent with theoretical simulations. This flat technique has the potential to be used in communication technology, especially in wavelength division multiplexing to adjust multichannel gain simultaneously and multi-wavelength narrow-linewidth fiber lasers to achieve a wide range of stable light output.