A system and a method for providing more gain while minimizing the potential for parasitic oscillation and amplified spontaneous emissions in an optically pumped optical amplifier or laser system, utilizing a partitioned monolithic gain element. Each fan blade comprises a root portion and an aerofoil portion. A surface-active layer 7 made of a surfactant is formed on surfaces of ribs 1a on a belt body 1. A joint portion of the tether to the vehicle’s inner side wall is located at lower level than a joint portion of the tether to the vehicle’s outer side wall as viewed in a vertical section taken along the vehicle’s transverse direction.