1. Introduction

Fiber amplifiers and lasers have received much attention because of their stability, easy of use, and high quality. Silica-based rare-earth-doped fibers are widely used for amplifiers and lasers. Because most conventional optical fiber components are made of silica-based fibers, silica-based rare-earth-doped fiber is considered easy to connect with optical fiber components, however, for advanced applications improved optical properties are desired. For example, wavelength division multiplexing (WDM) systems require broadband amplification. Short pulse amplifiers or high peak power lasers require fibers with lower nonlinearity to prevent nonlinear distortion. However, silica-based rare-earth-doped fiber has limited amplifier bandwidth and doping concentration of rare earth ions. To overcome the challenges of rare-earth-doped silica-based fiber, many types of fibers using non-silicate glass materials have been studied. For example, telluride glass has been studied for broadband amplifiers and L-band amplifiers [1–3]. Fluoride glass has been studied for thulium-doped fiber to improve power conversion efficiency at the S-band [4], because it has small phonon energy and prevents phonon relaxation into the intermediate energy level of Tm₃₊. Phosphate glass is studied for highly erbium-doped fiber [5]. Our group has reported bismuth-oxide-based erbium-doped fibers (BIEDF) for broadband amplifiers [6,7], short pulse amplifications [8,9], and widely tunable fiber lasers [10]. We also show the fusion splice-ability of BIEDF to silica fiber.

In recent years, highly Yb₂O₃-doped fibers, which enable the prevention of the nonlinear effect, have attracted significant interest. The nonlinear effect is proportional to fiber length, and highly Yb₂O₃-doped fiber has shorter required fiber lengths, and thus suppresses the nonlinear effect. As the host material of highly Yb₂O₃-doped fiber, phosphate glass [11,12], and phosphosilicate glass [13] have been developed.

In this paper, we report the absorption and emission properties of highly Yb₂O₃-doped Bi₂O₃-based glass. A ytterbium-doped Bi₂O₃-based fiber is also fabricated, and a fiber laser is demonstrated.

2. Optical characteristics of ytterbium-doped bismuth-oxide-based glass

Bi₂O₃-based glasses doped with Yb₂O₃ were prepared by melting. Several oxide materials (SiO₂, Al₂O₃, etc.) were added to obtain thermal stability during fiber drawing. The Yb₂O₃ was added externally in mol-% to Bi₂O₃-based glasses. The absorption spectra of Yb₂O₃-doped glasses with Yb₂O₃ concentration of 0.15 to 3.0 mol-% are shown in Fig. 1(a). The absorption band around 977 nm arises from the electronic transition from ²F_7/2 to ²F_5/2 in Yb³⁺ ions. The absorbance dependence of the Yb₂O₃ concentration is plotted in Fig. 1(b) at the peak wavelength of 977 nm. No decline is observed even at 3 mol-% with an absorbance of 7800 dB/m. From the slope of the absorbance, the mol fraction of the absorbance is estimated at 2600 dB/m/mol. The absorption cross section at 977 nm is 2.1 × 10^-20 /cm².

Absorption spectrum deconvolution was done, as shown in Fig. 2. The energy levels of Yb³⁺ are shown in the figure’s inset. The four Stark levels in ²F_7/2 are labeled a-d, and three Stark levels in ²F_5/2 are labeled e-g. The absorption is deconvolved by four Lorentzian curves. The center wavelengths are determined by a second-order derivation of the absorption spectrum, as shown on the right-hand side of Fig. 2. The absorption spectrum is well fitted with four Lorentzian curves. The narrow line at 977 nm is attributed to the transition labeled a to e. The transitions a to f and a to g are also observed, with center wavelengths of 953 nm and 921 nm, respectively. The transition labeled a to g strongly appeared in Bi₂O₃-based glass, but is weakly observed in silica-based glass [14]. Because of these transitions, the absorption shows a relatively flat profile compared to that of silica-based glass. This is the same tendency as sodium borate and bismuthate glasses [15].

 

Fig. 1. (a) Absorption spectra of Yb₂O₃-doped Bi₂O₃-based glass with different Yb₂O₃ concentrations. (b) Absorption as a function of Yb₂O₃ concentration.

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Fig. 2. Absorption spectrum of Yb³⁺-doped Bi₂O₃-based glass. Dotted lines show Lorentzian curves. Right-hand graph shows the second-order derivation of the absorption spectrum.

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An emission spectrum is shown in Fig. 3. The energy level was determined by second-order derivation. The emission spectrum was fitted with four Lorentzian curves. From the spectral analysis, the obtained Stark levels are described in the figure’s inset. The transitions from e to a, b, c, and d are 977 nm, 1005 nm, 1029 nm, and 1060 nm, respectively. The emission spectrum is almost exactly expressed with these Lorentzian curves. Slight differences are believed to be transitions with one or two phonons.

The decayed emission intensity at the ²F_5/2 level was measured. The exponential decayed emission intensity of ytterbium-doped Bi₂O₃-based glass is shown in Fig. 4(a). The lifetime is plotted as a function of Yb₂O₃ concentration in Fig. 4(b). The lifetime can be calculated from the absorption cross section [16]. The calculated lifetime from the absorption spectrum is 0.55 ms. A measured lifetime of 0.55 ms is obtained with a Yb₂O₃ concentration of 0.15 mol-%, which corresponds to the calculated value. This means there are no defects between the upper ²F_5/2 level and lower ²F_7/2 level, which would cause reduction of the lifetime accompanying non-radiate relaxation. With increasing Yb₂O₃ concentration, the measured lifetime is slightly increased. Energy migration between ytterbium ions is considered to occur [16].

 

Fig. 3. Emission spectrum of Yb³⁺-doped Bi₂O₃-based glass pumped by a laser diode at 975 nm. Dotted lines show Lorentzian curves.

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Fig. 4. (a) Decayed emission intensity of Yb³⁺-doped Bi₂O₃-based glass. (b) Lifetime of the ²F_5/2 level of Yb³⁺ as a function of Yb₂O₃ concentration.

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3. Bismuth-oxide-based ytterbium-doped fiber

A single-mode Bi₂O₃-based ytterbium-doped fiber (BIYDF) was fabricated with core diameter of 5.3 μm and a cladding diameter of 125 μm with plastic coating. As the core, Bi₂O₃-based glass doped with 0.5 mol-% of Yb₂O₃ was used. The Yb³⁺ concentration was 6,600 wt-ppm (1.6 × 10²⁶ /m³). The refractive index of the core and the numerical aperture of the fiber at 1304 nm are 2.03 and 0.13, respectively. The absorption at 977 nm is estimated to be 1140 dB/m from the confinement factor of light and the absorption of the core glass. A propagation loss of 0.24 dB/m was measured at a wavelength of 1310 nm using the cut-back method. The BIYDF was fusion-spliced to single-mode silica fiber (Nufern 1060-XP) using a commercial fusion-splicer. The splice loss was estimated to be 0.6 dB/point. Angled-cleaving and mix-angle splicing was applied to suppress the reflection due to the large refractive index difference between the BIYDF and silica fiber, using the same technology developed for Bi₂O₃-based erbium-doped fiber [8].

The experimental setup for the fiber laser is shown in Fig. 5. The fiber laser was obtained using a Fabry-Perot configuration. Fiber Bragg gratings (FBGs) were used as mirrors. A 0.50-m length of BIYDF inserted between two FBGs was pumped at 979 nm using laser diodes connected to the cavity through 975/1064-nm WDM couplers. The pump power was measured through the WDM coupler. The output signals were measured using an optical spectrum analyzer and a power meter. The reflectance and the full width at half maximum (FWHM) of FBG1 were 99.9% and 0.3 μm, respectively. The FWHM of FBG2 was 0.1 μm. The total loss of WDM couplers and BIYDF was 1.9 dB in this experiment.

The output spectra of fiber lasers using FBG2 with reflectance of 11% are shown in Fig. 6(a). The output spectra’s FWHM were 0.03 nm. The pump power dependence of the output power on different reflectances of FBG2 is plotted in Fig. 6(b). The slope efficiency was 14% with reflectance of 50%, 25% with reflectance of 29%, and 36% with reflectance of 11%. By optimizing the laser configuration and reducing losses, the efficiency will be much improved.

 

Fig. 5. Experimental setup for fiber laser.

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Fig. 6. (a) Emission spectrum of Yb³⁺-doped Bi₂O₃-based glass. (b) Pump power dependence of the output power on FBG2 reflectance.

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4. Conclusion

Bi₂O₃-based ytterbium-doped glasses were developed. The absorption of Yb³⁺ increased linearly with the concentration of Yb₂O₃, and 7800 dB/m was obtained with Yb₂O₃ of 3 mol-%. The absorption and emission spectra were analyzed, and Stark levels were investigated. Absorption to three Stark levels of ²F_5/2 from the ground state is strongly observed, and it shows a relatively broad profile from 920 nm to 960 nm. We also fabricated Bi₂O₃-based ytterbium-doped fiber using the newly developed host glass with a fiber loss of 0.24 dB/m. We also demonstrated a fiber laser, which showed a slope efficiency of 36%.