Original sentence: Optical fibres have evolved into many forms since the practical breakthroughs that saw their wider introduction in the 70s as conventional step index fibres   and later as single material fibres where propagation was defined by an effective air cladding structure
Edited sentence: Optical fibres have evolved into many forms since the practical breakthroughs that saw their wider introduction in the 70s as conventional step index fibres    and later as single material fibres where propagation was defined by an effective air cladding structure