Objective With the rapid development of 5G communications, global cloud computing, ultrahigh-definition videos, and internet of things, the demand for data communications has increased exponentially. Moreover, the traditional communication systems are unable to meet the current data transmission requirements. The communication capacity of a single channel is close to Shannon's limit and is challenging to increase. Therefore, expanding the transmission bandwidth, particularly for the L-band, is currently an effective solution based on the existing transmission systems.
Methods A modified chemical vapor deposition (MCVD) technology was used to fabricate the extended L-band erbium-doped fibers. To extend the L-band gain bandwidth, P/Al was introduced into the fiber core to suppress the excited state absorption (ESA) of Er 3+. In addition, the optical parameters of the fibers were measured and analyzed, and the L-band amplification performance was investigated based on the two-stage amplification structure.
Results and Discussions The erbium-doped fiber has the core and cladding diameters of 5.4 μm and 125 μm, respectively. The fiber's numerical aperture is approximately 0.2, the absorption coefficients at 980 nm and 1535 nm are 11.04 dB/m and 38.8 dB/m, respectively, and the background loss at 1200 nm is 15 dB/km. Figure 2 shows the absorption and emission cross-sections of the extended L-band erbium-doped fiber. The difference between emission and absorption in the L-band region reflects the gain capability of the fiber in the L-band. Ostensibly, when the wavelength is longer than 1622 nm, the difference between emission and absorption is negligible, indicating that the ESA of the erbium-doped fiber is well suppressed and it has a strong gain ability in the extended L-band region. Based on the two-stage amplification structure in Fig. 3, the input signal power is -1 dBm, the 20 dB gain output is extended to 1622 nm, the maximum noise figure is 5.3 dB, and the saturated output power is 24.5 dBm.
Conclusions We demonstrated an extended L-band erbium-doped fiber fabricated via the MCVD technology. Based on the two-stage amplification structure for the first and second stages with lengths of 11 m and 25 m , respectively, the long-wavelength of the 20 dB gain output was extended to 1622 nm under a 980 nm excitation. The maximum noise figure was 5.3 dB, and the saturated output power was 24.5 dBm.
