Design And Analysis Of High-power Flattened L-band Erbium-doped Fiber Superfluorescent Sources

Erbium-doped fiber super fluorescent (Er-SFS) have attracted a wide range of applications such as optical spectrum analysis, optical communication, optical sensor and optical coherence tomography (OCT) due to their merits of broad bandwidth, spectral stability, high output power, simple structure, low cost, long lifetime and ease of fiber coupling among other broadband sources. Traditional C-band (1520nm-1560nm) Er-SFS cannot satisfy the fast growth of communication. Expanding bandwidth to L-band (1565nm-1610nm) is an effective way. Besides, high-power, long wavelength infrared source can offer OCT better penetration depth and image contrast. However, L-band Er-SFSs operate at the tail of gain spectrum and their efficiencies are typically low. Most reported output power is less than150mW. In addition, the bandwidth is referred to the integral bandwidth of optical fiber gyroscope and cannot intuitively indicate the spectral flatness. We aim to comprehensively develop a high-performance L-band Er-SFS with hundreds of output power, spectrum flatness, high pump optical-to-optical conversion efficiency as well as an efficient way for power scaling. The main achievements are summarized as follows:1. We apply the theory of spectral shaping to analyze and compare the performance of four different core-pumped dual-stage double-pass L-band Er-SFS configurations (no midway isolator introduced) affected by the pumping ratio, fiber length arrangement and reflectivity of fiber loop. The results show that only the backward-pumped dual-stage double-pass configuration can simultaneously provide high flatness and high output power. A SFS with314-mW output power,32.41-nm bandwidth,1584.84nm center wavelength and2.23-dB flatness can be demonstrated when the length arrangement of21m and pump ratio of750mW are6:1and2:3, respectively. For fixed reflectivity, the change of total active fiber length will almost not affect its output characteristics. The increase of reflectivity will increase the output power output and bandwidth. The mean wavelength will shift toward longer wavelength.2. We apply the principle of "seed+MOPA (master oscillator power amplifier)" to analyze the performance of four similar core-pumped dual-stage double-pass configurations with midway isolator instead affected by the pumping ratio, fiber length arrangement and reflectivity of fiber loop. The results show that it is difficult to obtain optimal L-band Fr-SFS with fixed total active length and reflectivity. But if the limit of total length of active fiber is removed, the output bandwidth will increase with the increase of reflectivity of fiber loop while the output power is insensitive to the change.The key contributions of this thesis are as the followings:A polarization multiplexing technique is applied to offer single-mode Walt-level pump power. A bandwidth based on spectrum flatness is defined to intuitively reflect its flatness. Finally, an efficient way to scale traditional low power L-band Er-SFS to hundreds of milliwalt is demonstrated with high flatness and high optical conversion efficiency.