Semi-insulating Gallium Arsenide (GaAs) provides an alternative to ZnSe in medium and high-power CW CO2 laser systems for lenses and rear mirrors. GaAs is particularly useful in applications where toughness and durability are important. Its hardness and strength make GaAs a good choice where dust or abrasive particles tend to build up or bombard the optical surface. Softer substrates allow particles to imbed in the optic even when the best coatings are used.