Optically-Pumped, Low-Noise, High-Power Semiconductor Optical Amplifiers

The EDFA has revolutionized optical fiber communication systems. Actually, the semiconductor optical amplifier (SOA) existed before the EDFA and yet has so far limited to niche applications despite continued efforts aimed at improving its performances. It is safe to say that any meaningful advance in SOA technology would require a fundamentally new approach or a paradigm shift.  In Phase I, we successfully demonstrated the new concept of optically-pumping SOA (OP SOA) to improve the performances of SOAs over that of the EDFA. The most important performance metrics that conventional SOAs lag behind EDFAs are 1) noise figure (NF), 2) saturation power and 3) polarization-independent gain. The noise figure of an amplifier increases with the population inversion factor, internal loss and the fiber coupling loss. For electrically-pumped SOAs, i) maximum doping concentration and thermal effects limit injected carrier density and how low the population inversion factor can be; and in the meantime, ii) high doping concentration leads to high internal loss due to free carrier absorption. All of the above constraints, limitations and tradeoffs of conventional SOAs can be simultaneously and effectively mitigated, if not completely removed by using optical pumping. In OP SOAs, carriers are generated locally and proportional to the pump intensity, which can be increased beyond the level of electrical pumping, improving NF, gain coefficient and gain bandwidth.  Furthermore, since there is no carrier transport, thermal effects will be greatly reduced. The entire heterostructure of the OP SOA is completely undoped, and hence 1) there will be no free carrier absorption in the cladding regions and 2) the mode size can be increased without much restriction which reduces fiber-to-waveguide coupling loss and thus the NF and increases the saturation power. The material composition can be chosen independently to achieve both ultrawide gain bandwidth and polarization-independent gain. In Phase I, we designed the heterostructure and waveguide geometry and then fabricate the OP SOAs. We then measured the performance of the SOA and achieved > 30 dB small-signal gain, 3-dB bandwidth of > 60 nm, < 5 dB noise figure, and > 20 dBm saturation power. I. In addition, without constraints of electrical pumping, designed a QW material with nearly identical TE and TM gain. In Phase II, we will further optimize wafer design for OP SOAs with a goal is of achieving < 4 dB noise figure and >27 dBm saturation power for a fiber pigtailed SOA for the S+C+L bands and demonstrate polarization-independent operation of the OP OSA. In addition, we will demonstrate system applications of OPSOAs in optical communication, specifically, as a preamplifier for receivers, and microwave photonics.We have assembled a strong team with outstanding expertise in photonic devices and optical communication systems, as well as technology transfer and commercialization to carry out the proposed STTR research.