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Off-Axis Parabolic Mirror Selection Guide
Edmund Optics Inc.

Off-Axis Parabolic Mirror Selection Guide

Introduction to Off-Axis Parabolic Mirrors

Parabolic mirrors have the ability to focus collimated light without introducing spherical aberration. An Off-Axis Parabolic (OAP) mirror is simply a side section of a parent parabolic mirror (Figure 1). Collimated light that is incident to an OAP mirror is focused to a point. However, unlike a centered parabolic mirror, an OAP mirror has an advantage in that it allows more interactive space around the focal point without disrupting the beam as shown in Figure 2.

Gold-coated off-axis parabolic mirrors
Figure 1: Gold-coated off-axis parabolic mirrors
Schematic of an OAP showing how their unique shape lets the focal point of the mirror be accessible
Figure 2: Schematic of an OAP showing how their unique shape lets the focal point of the mirror be accessible

Depending on which section of a parabolic shape an OAP mirror is replicating, the angle between the focal point and the central ray axis can be large or small. Figure 3 represents this phenomenon by modeling a 15° and 45° off axis mirror respectively. Note: It is important to keep the incident beam parallel to the optical axis, any angular displacement will produce comatic aberration.

Diagrams of 15 Degrees and 45 Degrees OAP mirrors
Figure 3: Diagrams of 15° and 45° OAP mirrors

OAP Mirror Coatings

Edmund Optics offers off-axis parabolic mirrors with a wide variety of coating options off-the-shelf and available for immediate shipping (Figure 4). Fully custom design and manufacturing is also available.

Reflectance curves for OAP mirror coating options
Figure 4: Reflectance curves for OAP mirror coating options

Impact of Surface Roughness on Performance

Surface roughness describes high spatially frequency errors on optical surfaces. It impacts how much light is unintentionally scattered off of the surface into different directions (Figure 5). This deviation from the ideal surface shape is often measured in Angstroms (Å) and Edmund Optics® OAPs are specified as having either <50Å or <100Å RMS surface roughness. You can learn in our Roughness of Diamond Turned Off-Axis Parabolic Mirrors application note.

Typical Scatter Profiles
Figure 5: Typical scatter profiles showing that standard-grade OAPs (left) result in more scattered light while comparable TECHSPEC® 50Å OAPs (right) reduce the amount of unwanted scatter

OAP Mirror Selection Guide

The following selection guide provides helpful information detailing the various OAP mirror options available from Edmund Optics®.

TECHSPEC® Off-Axis Parabolic Mirrors
 Coating OptionsOffset Angle OptionsSurface Roughness OptionsAlignment Through Holes IncludedDesigned for Ultrafast Laser Pulses
Gold OAPs Bare and Protected Gold
Ravg ≥94% @ 700 - 800nm
Ravg ≥97% @ 800 - 2,000nm
Ravg ≥98% @ 2,000 - 12,000nm
15°, 30°, 45°, 60°, and 90° <50Å and <100Å No No
Aluminum OAPs Protected Aluminum
Ravg ≥85% @ 400 - 700nm Ravg ≥97% @ 2,000 - 12,000nm

Enhanced Aluminum

Ravg >89% @ 250 - 450nm
Ravg >85% @ 250 - 700nm
15°, 30°, 45°, 60°, and 90° <50Å and <100Å No No
Silver OAPs Protected Silver
Ravg ≥98% @ 2,000 - 12,000nm
90° <100Å No No
Laser Line OAPs Laser Line Coatings
>99.5% Reflectivity at Nd:YAG Design Wavelengths
90° <50Å No No
Ultrafast-Enhanced Silver OAPs Ultrafast-Enhanced Silver
Ravg >99% @ 600 - 1000nm, 0°
Rs >99% @ 540 - 1000nm, 45°
Rp >98.5% @ ~730 - 870nm, 45°
90° <50Å No Yes, low group delay dispersion of 0 ±20fs2 @ 600 - 1050nm
OAPs with Alignment Through Holes Enhanced Aluminum, Protected Aluminum, and Protected Gold 90° <50Å Yes No
Table 1: Comparison of the different off-the-shelf OAPs available from Edmund Optics

Custom OAP Capabilities

Edmund Optics® engineers design and diamond turn custom OAPs, off-axis elliptical mirrors, and other optics. Watch the video below to learn about our in-house diamond turning and visit the link below to learn more and get a quote.

Learn More

Additional Resources

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