Off-Axis Parabolic Mirrors and Their Characteristics

OAP mirrors are smaller than parabolic mirrors and are easier to incorporate into complex optical systems with minimal obstruction. Their small footprint and unique shape causes the mirror’s focal point to be readily accessible.

By definition, an OAP mirror is cut from the parent parabola at an off-axis: it is not centered on the parent’s optical axis. Because of this, the optical axis of the derivative mirror will be displaced from the optical axis of the parabolic mirror. The focal point, however, will be the same.

Off-set angle: The angle between the optical axis and the direction of the incoming or outgoing light beam in an off-axis parabolic mirror.

Parent focal length: The focal length of the ideal on-axis parabolic mirror with the same shape as the off-axis mirror.

Effective focal length: The actual focal length of the off-axis parabolic mirror, accounting for the offset angle and deviation from the ideal configuration.

Choosing an Off-Axis Parabolic Mirror

Key parameters of an off-axis parabolic mirror are the off-axis angle (OAA), the off-axis distance (OAD), and the focal length. 

Choose an appropriate OAP mirror by deriving the desired reflected focal length from your desired beam diameter. The small angle approximation leads to a very simple relationship between these two parameters:

Beam Diameter = 2 x sin(Θ) x Reflected Focal Length 

Where Θ is the divergent half-angle of the incident light. In fiber optics, when collimating from an optical fiber,  you may replace sin(Θ) by NA (fiber).

How to Use an OAP Mirror

An OAP mirror can be used to derive collimated light from a point source or spherical wave by placing the source or origin of the wave at the OAP mirror’s focal point. It can also be used to focus collimated light to a point by directing the light along the optical axis of the off-axis parabolic mirror.