Off-Axis Parabolic Mirror

June 21, 2024

An off-axis parabolic mirror is a specialized optical mirror cut from a larger parabolic mirror. Parabolic mirrors collimate light, and an off-axis parabolic (OAP) mirror will do the same, and can also focus incident collimated light to a point. At Avantier Inc, we excel in delivering custom off-axis parabolic mirror design and manufacturing solutions for a wide range of applications

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, it 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.

As shown on the diagram below, this is an example of focusing collimated light to a single point after light beam is reflected on the surface of the off-axis parabolic mirror.

In order to increase the reflectance on the reflective surface, Avantier usually will coat additional gold or silver layer, thus making sure customer’s application could achieve a higher efficiency

There are three important specifications when we try to describe an off-axis parabolic mirror, which are off-set angle, parent focal length, and effective focal length:


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.

Centered Parabolic Mirror: This diagram shows a traditional parabolic mirror where parallel light rays converge at a single focal point (F). It’s ideal for applications where the light source or target is on the optical axis.

Off-axis Parabolic Mirror (OAP): This diagram depicts an OAP mirror that focuses collimated light to an off-center focal point, allowing unobstructed access to the focal plane. It’s useful when the optical axis must be kept clear for other components or when a wide off-axis angle is needed.


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).

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