Fig. 1. (a) Design of the fiber preforms: RIP formed by variation of GeO2 content. (b) Typical view of the surface of the polished slab (central region with a higher GeO2 content is brighter than the peripheral one; black color corresponds to the silica glass cladding) cut out from the preform. (c) Typical RIP of the fabricated preforms (solid line) and fibers (dashed line), fundamental mode intensity profile calculated using the fiber RIP (dashed–dotted line). (d) Bi ions concentration (symbols) and relative BACs concentration (solid line) as a function of radial coordinate from the center of the preforms (Samples A, B, and C).

Fig. 2. Absorption and luminescence properties of the studied BDFs. (a) Small-signal absorption spectra. (b) Schematic energy-level diagram of BACs-Si indicating the absorption (arrow up lines), emission (arrow down solid line), and non-radiative (dotted and wavy lines) transitions. (c) Luminescence spectra of Sample B under various excitation wavelengths.

Fig. 3. Power dependences of the optical loss at 1310 nm (balls) and luminescence at 1430 nm (squares) of the studied samples. Inset: residuals indicating difference between experimental and calculation data.

Fig. 4. (a) Detailed view of the absorption (σa) and emission (σe) cross-sections spectra of BACs (solid lines) and their Gaussian decomposition (dashed and dotted lines), correspondingly. (b) Conversion factor of the total Bi content into the active centers of the BDFs versus the radial coordinate.

Fig. 5. (a) Small-signal gain spectra of a 30 m long BDF (Sample C) core-pumped at λp=1310  nm (gray band) for various input powers. Red band indicates the selected region used for achieving lasing. (b) Typical output emission spectra of the CW Bi-doped fiber laser operating at λL∼1461  nm and detailed view of the laser lines measured with a resolution bandwidth of 0.07 nm (inset) with different output powers at λp=1310  nm.

Fig. 6. (a) Output power of the lasers based on Sample C as a function of the absorbed pump power at λp=1310  nm and 1340 nm. (b)–(d) Dependences of slope efficiency (η), output power (PL), pump power threshold (Pth), and unabsorbed pump power (Punabs) of the lasers based on Samples A, B, C on the length of the active fibers, correspondingly.

Fig. 7. Experimental setup for the measurement of the gain in BDFs.

Fig. 8. Schematic of Bi-doped fiber laser experiments. OSA is an optical spectrum analyzer; LPF is a 1400 nm long-pass filter; HR FBG is a highly reflective fiber Bragg grating.