Abstract: An aspect of the present disclosure enables prediction of a refractive index profile of a transparent glass preform obtained in a production stage of a glass particulate deposit by a VAD method. The glass preform production apparatus includes a gas supply system, a burner, and a signal processing device. The signal processing device includes an imaging device that images a particle flow of glass fine particles, and a calculation unit. The calculation unit extracts, at any one or more time points during a period from the start of production to the end of production of the glass particulate deposit, image data representing a state of at least the flame or the particle flow from an image obtained by the imaging device, and regressively predicts a refractive index profile of the transparent glass preform serving as an objective variable from an explanatory variable including the image data.

Abstract: A highly hygroscopic polyamide fiber formed from a polyamide compound containing a polyvinylpyrrolidone (in an amount of 3-15 wt %) which is synthesized in isopropyl alcohol as a solvent, contains pyrrolidone in an amount less than 0.1 wt %, and has a K-value of 20-70. This fiber contains pyrrolidone in an amount less than 0.1 wt % of the amount of polyvinylpyrrolidone. This fiber is characterized by a yellowness index lower than 10 before dyeing, a large difference between the maximum moisture absorptivity and the standard moisture absorptivity, and a low content of water-soluble components that are subject to leaching.