Gallium sulfide glasses

Two compsns. for transparent glass exhibiting transmission far into the IR region of the electromagnetic radiation spectrum are (in mol. % based on the sulphide): (A) 40-80% Ga2S3, 0-35% RSx, where R is at least one network forming cation selected from Al, Sb, As, Ge and In, 1-50% Ln2S3, where Ln is at least one cation selected from a rare earth metal and Y, 1-45% MSx , where M is at least one modifying cation selected from Ba, Cd, Ca, Pb, Li, Hg, K, Ag, Na, Sr, Tl, and Sn and 0-10% total Cl and/or F: (B) 5-30% Ga2S3, 0-10% R2S3, where R is 55-94.5% GeS2, 0.5-25% MSx, where M is at least one modifying cation selected from Ba, Cd, Ca, Pb, Li, Hg, K, Ag, Na, Sr, Tl, Sn, Y and a rare earth metal of the lanthanide series, 0-10% total selenide, 0-25% total Cl and/or F, where the S and/or Se content can vary between 85-125 % of the stoichiometric value. Waveguide structures using gallium sulphide glass are also claimed in which the core glass has a different compsn. to the cladding glass to create a difference in refractive index between them.