A 6-mode pre-amplifier for turbulence-resistant free-space optical communication

A few-mode pre-amplifier with high gain and high coupling efficiency was suitable for the turbulence-resistant free-space optical (FSO) communication. Here, we first built and experimentally demonstrated a forward- and core-pumped 6-mode (6 M) pre-amplifier. By optimizing the 6 M erbium-doped fiber (EDF) length, we achieved over 28 dB of gain and a noise figure of less than 7 dB with an input intensity of-30 dBm. Moreover, we experimentally evaluated the 6 M pre-amplifier of 2.5 Gbps On-Off Keying (OOK) signal transmission with a homemade tunable turbulence emulator. The advantages of the 6 M pre-amplifier over the single-mode (SM) preamplifier varied with channel turbulence conditions in terms of communication performances. Compared with the SM pre-amplifier, the coupling efficiencies increased by 4.5 dB and 5.8 dB on average, and the median bit error rates (BERs) of the communication system improved by 2 and 5 orders of magnitude, respectively, in the presence of channels with the scintillation index sigma I 2 of 0.3978 and 1.0553 (moderate to strong turbulence).