Off-axis parabolic mirrors are widely used in terahertz and infrared bands.

 They are referred to as OAP. Off-axis reflectors are one of the commonly 

used components for building terahertz and infrared optical paths. 

Based on the principle of geometric parabola, off-axis high-reflectivity

 parabolic mirrors can focus parallel incident collimated light beams/collimated

 THz waves to the focal point, and can also convert terahertz waves or infrared 

light emitted by point light sources into parallel transmission beams.