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Mounting Adapter Installation

Features

• >95% Average Reflectivity Over a Broadband Range from 800 nm to 10 µm
• Collimates a Point Source or Focuses a Collimated Beam without Spherical or Chromatic Aberrations
• Effective Focal Lengths: 1", 2", 4", 6", or 9" (25.4 mm, 50.8 mm, 101.6 mm, 152.4 mm, or 228.6 mm)
• Surface Roughness: <100 Å (RMS)
• Clear Aperture: >90% of Diameter
• Two Kinds of Mounting Adapters for Ø1/2", Ø1", and Ø2" Versions:
• Post Mounting Adapters with 8-32 (M4) Taps
• Rounded Adapters for Stable Three-Point Contact in Mirror Mounts

Thorlabs' off-axis parabolic (OAP) mirrors are first-surface mirrors whose reflective surfaces are equivalent to segments of a parent parabolic mirror. They achromatically focus a collimated beam or collimate a broadband point source, and their off-axis design spatially separates the focal point from the rest of the beam path. The all-reflective design eliminates phase delays and absorption losses introduced by transmissive optics.

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The angle between the focused beam and the collimated beam, which is called the "off-axis angle", is 90°. As shown to the left, the propagation axis of the collimated beam should be normal to the bottom of the substrate to achieve a proper focus. The OAPs sold here are fabricated using aluminum substrates, and the diamond-turned parabolic surface has a protected gold coating that provides >95% average reflectivity from 800 nm - 10 µm. The non-optical surfaces are black anodized and laser-engraved with the item number for easy identification.

Thorlabs also offers off-axis parabolic mirrors with holes for applications that require two beams to be overlapped.

Mounting Adapters
The bottom of each off-axis parabolic mirror contains three tapped mounting holes in a triangle pattern and an alignment hole. Together with our Mounting Adapters for Ø1/2", Ø1", and Ø2" OAPs, which contain three corresponding counterbores and an alignment pin, these features allow our OAPs to be repeatably and securely mounted. The tapped holes are also useful in OEM applications.

We offer two types of mounting plates, both of which provide three-point mechanical stability. The first type is designed to be mounted in any Ø1", Ø2", or Ø3" mirror mount, depending upon the diameter of the OAP. The second type, designed for post mounting, contains an 8-32 (M4) tapped hole on all four sides for direct mechanical compatibility with Ø1/2" Posts.

Our Ø1/2" and Ø1" OAPs can also be adapted to our SM-threaded rotation mounts by placing them into our SM Thread to Double Bore Adapters.

All data shown below is for unpolarized light, unless otherwise stated. The shaded regions in the graphs denote the ranges over which we recommend using these mirrors.

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Excel Spreadsheet with Raw Data for Protected Gold, 45° AOI

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Excel Spreadsheet with Polarization-Dependent Raw Data for Protected Gold, 45° AOI

Selecting a Mirror Based on Desired Output Beam Diameter

When using an off-axis parabolic mirror to collimate a point source, selection of the appropriate mirror is often done based on the desired output beam diameter. Beam diameter can be calculated very easily using the divergence half-angle of the incident light (Θ) and the reflected focal length of the OAP. To calculate the beam diameter in the small angle approximation, use the following equation:

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

If you are collimating from a fiber, the equation can be rewritten as:

Beam Diameter = 2 x NA (Fiber) x Reflected Focal Length

The table below lists the output beam diameter as a function of the reflected focal length of the mirror and the divergence half-angle of the incident light. Not listed here is the diameter of the OAP. The clear aperture of the OAP you select should be larger than the desired beam output diameter.