Guided acoustic Brillouin scattering measurements in optical communication fibers.

Guided acoustic Brillouin (GAWBS) noise is measured using a novel, homodyne measurement technique for four commonly used fibers in long-distance optical transmission systems. The measurements are made with single spans and then shown to be consistent with separate multi-span long-distance measurements. The inverse dependence of the GAWBS noise on the fiber effective area is confirmed by comparing different fibers with the effective area varying between 80 µm2 and 150 µm2. The line broadening effect of the coating is observed, and the correlation between the width of the GAWBS peaks to the acoustic mode profile is confirmed. An extensive model of the GAWBS noise in long-distance fibers is presented, including corrections to some commonly repeated mistakes in previous reports. It is established through the model and verified with the measurements that the depolarized scattering caused by TR2m modes contributes twice as much to the optical noise in the orthogonal polarization to the original source, as it does to the noise in parallel polarization. Using this relationship, the polarized and depolarized contributions to the measured GAWBS noise is separated for the first time. As a result, a direct comparison between the theory and the measured GAWBS noise spectrum is shown for the first time with excellent agreement. It is confirmed that the total GAWBS noise can be calculated from fiber parameters under certain assumptions. It is predicted that the level of depolarized GAWBS noise created by the fiber may depend on the polarization diffusion length, and consequently, possible ways to reduce GAWBS noise are proposed. Using the developed theory, dependence of GAWBS noise on the location of the core is calculated to show that multi-core fibers would have a similar level of GAWBS noise no matter where their cores are positioned.

The crystal structure of D-mandelate dehydrogenase reveals its distinct substrate and coenzyme recognition mechanisms from those of 2-ketopantoate reductase.

D-Mandelate dehydrogenases (D-ManDHs), belonging to a new d-2-hydroxyacid dehydrogenase family, catalyze the conversion between benzoylformate and d-mandelate using NAD as a coenzyme. We determined the first D-ManDH structure, that of ManDH2 from Enterococcus faecalis IAM10071. The overall structure showed ManDH2 has a similar fold to 2-ketopantoate reductase (KPR), which catalyzes the conversion of 2-ketopantoate to d-pantoate using NADP as a coenzyme. They share conserved catalytic residues, indicating ManDH2 has the same reaction mechanism as KPR. However, ManDH2 exhibits significant structural variations in the coenzyme and substrate binding sites compared to KPR. These structural observations could explain their different coenzyme and substrate specificities.