To investigate the role of LPG in a high-gain EDFA with full coverage in the C+L band, the gain and noise are tested in two separate scenarios, with and without an LPG inserted between ports 1 and 3 of circulator 2. These two structures are referred to as “comparison structure 1” and “comparison structure 2,” respectively. The amplifier structure (shown in Fig. 1) is designated as the “reported structure.”

Increasing the gain bandwidth of optical fiber amplifiers is the simplest and most effective method for improving the transmission capacity of optical fiber communication systems. Currently, C+L-band erbium-doped fiber amplifiers (EDFAs) are typically formed by parallel C- and L-band EDFAs, without any gaps between the two bands. Therefore, the research and development for an EDFA with a high gain bandwidth that fully covers the C+L band, have always garnered attention. However, the initially designed C+L band EDFA either fails to cover the gain bandwidth within entire C+L band or suffers from low gain levels and high noise, unable to fully meet the requirements of long-distance transmission systems. In this study, a C+L-band full-coverage high-gain EDFA is realized using a Bi/Er co-doped fiber as the main gain medium in a two-stage double-pass amplification structure with a long-period fiber grating inserted to shave the gain peak and fill the valley with 25 dB and 20 dB gain bandwidths corresponding to 78 nm (1529?1607 nm) and 85 nm (1527?1612 nm), respectively. The maximum gain reaches 51.2 dB, and the minimum noise is 3.9 dB, achieving a sufficiently high gain in a broad gain bandwidth covering the entire C+L-band, indicating a high potential for widespread use in future high-capacity long-haul optical fiber transmission systems.

To increase the gain level and control noise, the amplifier uses a two-stage double-pass amplification structure. The main amplifier has an 8 m long bismuth/erbium co-doped silica fiber (BEDF ) as the gain medium, which is bidirectionally pumped by a pair of 1480 nm laser diodes. The BEDF is fabricated using the atomic layer deposition (ALD) method in conjunction with modified chemical vapor deposition (MCVD). In addition to the co-doped elements of Bi (mole fraction of 0.004%) and Er (mole fraction of 0.109 %), Al, P, and Ge are also included. This fiber exhibits a background loss of 0.02 dB/m at 1200 nm and an absorption coefficient of 22 dB/m at 1531 nm. For the pre-amplifier, a 2 m high-concentration EDF is used as the gain medium, which is forward pumped by a 980 nm laser diode (LD) with a pump power of 100 mW. A fiber ring mirror consisting of an optical circulator (circulator 2) is connected at the end of the main amplifier, which is used to route the amplified input signals and the forward-amplified spontaneous emission (ASE) back into the amplifier system to achieve double-pass amplification. The gain bandwidth is expanded by inserting a wavelength-matched long-period fiber grating between the main amplifier and fiber ring mirror.

The experimental results on measurement gain and noise of the three EDFA structures under the same experimental conditions are displayed in Fig. 4. Clearly, the gain spectra of all three amplifier structures cover the C+L band region, extend to 1620 nm on the long-wavelength side, and provide a broader wavelength range than conventional EDFAs. Additionally, the gain level in the L-band, which is closely related to the broad-spectrum gain characteristics of the Bi/Er co-doped fibers, is enhanced. Among the three structures, the reported structure for the amplifier exhibits a significantly broader gain bandwidth than those of structures 1 and 2. Specifically, the 25 dB and 20 dB gain bandwidths reach 78 nm (1529?1607 nm) and 85 nm (1527?1612 nm), respectively, fully covering the entire C+L band. In terms of noise performance comparison, all three structures maintain a relatively low noise of below 6 dB within the wavelength range of 1545?1608 nm. This is primarily attributed to the incorporation of a preamplifier because the noise of the two-stage amplifier architecture is predominantly determined by the noise of the first-stage amplifier. Compared with the main performance parameters of C+L-band EDFAs reported in recent years (Table 1), the C+L-band EDFA demonstrated in this study exhibits the best comprehensive performance: a high gain level, low noise, and a 25 dB gain bandwidth that completely covers the C+L-band.

Using Bi/Er co-doped silica fiber as the primary gain medium and a two-stage double-pass amplification structure, this study employs long-period fiber gratings to expand the gain bandwidth. Consequently, a high-gain erbium-doped fiber amplifier with full coverage of the C+L band is achieved. The 25 dB and 20 dB gain bandwidths reach 78 nm (1529?1607 nm) and 85 nm (1527?1612 nm), respectively, with a maximum gain of 51.2 dB and minimum low noise of 3.9 dB. Compared with the Bi/Er co-doped fiber C+L band EDFA reported in recent years, this amplifier not only exhibits high gain and low noise but also achieves full coverage of the C+L band with 25 dB gain bandwidth. Its superior overall performance is expected to lead to important applications in future high-capacity long-haul optical fiber transmission systems.