Tunable Er{sup}3+-doped fiber amplifiers covering S- and C + L-bands (1490 ～ 1610 nm) using discrete all-fiber ASE suppressing filters

摘要

S-band (1480 ～ 1520 nm) has been recently studied as a new frequency band for fiber-optic communication. So far, the most promising S-band EDFAs employ the EDFs with depressed inner cladding to achieve fundamental-mode cutoff λ{sub}c at the longer wavelengths [1]. The depressed inner cladding in EDFs modifies the waveguide dispersion, which in terms varies the refractive index dispersion (RID) n(λ) curves. The effective indices of the longer (shorter) wavelengths become lower (higher) than the index of the outer silica cladding respectively. The ASE at the longer wavelengths can then be substantially suppressed so that in the shorter wavelengths (S-band) higher population inversion and sufficient amplification can be obtained. The λ{sub}c can be tuned toward shorter wavelengths by bending the fiber and the total distributed loss for wavelengths longer than the λ{sub}c can be > 200 dB through an entire 15-m-long EDF. However, a specially designed EDF is required for S-band amplification and the cutoff efficiency and insertion loss become worse and higher, respectively, when the radius of bending curvature gradually decreases [1]. In contrast to the λ{sub}c induced by waveguide dispersion, we have demonstrated widely tunable (1250 ～1650 nm) side-polished fiber short-pass filters based on material dispersion [2]. The λ{sub}c is thermo-optically tunable (no moving part) and the high cutoff efficiency (deep stopband and sharp filter skirt) can still be maintained while tuning to different wavelengths. The short-pass filter can be further incorporated into the ring cavity of an EDF to locally suppress the unwanted wavelengths and achieve a high efficiency tunable fiber laser. However, a single local λ{sub}c is inefficient for the standard EDFs (no depressed inner cladding) to be operated as an amplifier at the shorter wavelengths (S-band) of the gain bandwidth. Consequently, we employ multistage tunable fused-tapered fiber short-pass filters discretely located in the standard silica-based EDF to achieve S-band amplification [3] in this work.