Singlemode Fiber-optic collimator lenses are made from high index glass to ensure low aberration.
Standard collimators are optimized for telecom applications, but can be adjusted to other wavelengths/applications.
These collimators use high-index plano-convex spherical lenses to collimate light from singlemode fibers. The angled flat side of these lenses assures low back reflection. The curved side, due to high refractive index material, provides remarkable imaging capabilities.
Those can be used for fiber-optic switches, DWDMs, fiber-optic attenuators, optical isolators and other applications.
NOTE : There is also an option to use aspheric lenses for collimators. Typically, aspheric lens give better beam quality and lower insertion loss.