Cladding-pumped laser and amplifier for E- and S-bands based on multimode bismuth-doped graded-index fibers: toward "watt-level" output power.

In this Letter, we investigated the potential scalability of output power of a cladding-pumped laser and a power amplifier (booster) based on a multimode Bi-doped fiber (BDF) using the mode-selection approach. We fabricated the multimode double-clad graded-index (GRIN) fiber with a confined Bi-doped germanosilicate glass core with a diameter of ≈30 and ≈60 µm. Using femtosecond (fs) inscription technology with high spatial resolution, Bragg gratings of a special transverse structure allowing the selection of low-order modes were written into the core of BDFs. The operation features of the cladding-pumped multimode bismuth-doped GRIN fiber lasers with the inscribed Bragg gratings with various reflection coefficients were investigated. In addition, the behavior of the output power and the beam quality (M2 parameter) of the optical radiation of the developed devices was studied. The CW laser and booster operating at nearly 1.45 µm with maximum output powers of ≈0.8 and ≈1 W, respectively, based on the 60-µm-core BDF under pumping by multimode laser diodes at 808 nm were developed, which are, to the best of our knowledge, the most powerful cladding-pumped BDF devices to date. Near single-mode lasing (M2 <1.3) is demonstrated for a 30-µm-core fiber. The experimental data open new possibilities to achieve higher powers in cladding-pumped BDF sources, which are more cost-effective compared to core-pumped counterparts.

Cladding pumped bismuth-doped fiber amplifiers operating in O-, E-, and S-telecom bands.

Bismuth-doped fibers (BDFs) are considered nowadays as an essential part of the development of novel optical amplifiers, which can provide a significant upgrade to existing fiber optic telecommunication systems, securing multiband data transmission. In this paper, a series of BDF amplifiers (BDFAs) for O-, E-, and S-telecom bands based on a cladding pumping scheme using low-cost multimode semiconductor laser diodes at a wavelength of 0.7-0.8 µm were demonstrated for, it is understood, the first time. The developed BDFAs are characterized by a high peak gain of >25-30 dB in the corresponding telecom bands and a relatively low noise figure of 5-6 dB. Comparative analysis shows that most of the parameters of cladding pumped BDFAs are close to those of the best core pumped ones. This research opens up new opportunities for utilizing Bi-doped fibers as a key element of cost-effective and ready-to-work BDFAs for various practical applications.

Cladding-pumped bismuth-doped fiber laser.

For the first time, to the best of the authors' knowledge, a cladding-pumped bismuth-doped fiber laser (BDFL) is demonstrated. A "home-made" Bi-doped germanosilicate fiber with a 125 µm circular outer cladding made of fused silica and coated by a low refractive index polymer is used as an active medium pumped by commercial multimode laser diodes with a total output power of 25 W at 808 nm. We find that the BDFL with a free-running cavity (when feedback is provided by ≈4% back reflection from two bare right-angle cleaved fiber ends) composed of a 100-m-long bismuth-doped fiber is capable of emitting at a wavelength of 1440 nm. A slope efficiency of 0.5% with respect to the absorbed pump power with a maximum output power of ≈50 mW is obtained in a BDFL with a cavity formed by a highly reflective Bragg grating at 1461 nm and a right-angle cleaved fiber end. The beam quality factors (M2) of the output BDFL in the horizontal and vertical directions are measured to be 1.18 and 1.13, respectively. The processes affecting the efficiency of the BDFLs are also discussed. The possible improvements for the output power scaling and increasing the efficiency of the cladding-pumped BDFLs are proposed.