The need for powerful reflective optics operating over a broad bandwidth has motivated the development of off axis parabolas with higher imaging performance then ever before. Within the last five years everything about OAP’s has evolved – tolerances for quality and performance, manufacturing methods, testing methods, thin film coatings and alignment technologies. AOS has re-engineered the Off-Axis Parabola for today’s applications.
Achieving the highest quality requires a comprehensive approach to mechanical design, optical quality, and mounting & alignment.
In Mechanical design we incorporate datum surfaces that allow us to calibrate the off-axis angle, focal length, and clocking of the OAP to its parent axis. AOS, provides toleranced drawings to all of our customers free of charge. Optical Quality at AOS takes into consideration not just surface form error, but wavefront quality, gradient error, mid-spatial frequency tolerances, and relates the quality of the wavefront directly to the focus quality you can expect to receive. Finally, it doesn’t matter how good the OAP is, if it deforms in its mount, and you can’t optimize its alignment. Therefore, we provide Mounting Solutions with the alignment positioning resolution you need to optimize the performance of your mirror.
To learn more click on the “Specifications”, “Dimensions” and “Ordering Info” tabs, or contact us today and speak with one of our applications engineers. You don’t need to become and OAP expert – that’s why we’re here. Contact us now.
Specifying an OAP is key for ensuring that you get the performance you need. Our applications engineers will do this for you. In fact, we provide our customers with a free design service. We’ll produce a fully toleranced drawing for you and will work with your systems engineers to make sure the design gives you the performance you need for the best value. We also have a number of helpful technical notes to assist you in specifying an off-axis parabola yourself.
|Sizes||50 mm – 800 mm|
|Materials||Fused Silica, Zerodur / Clearceram, SiC, Nexcera|
|Off-Axis Angles||0-90 degrees|
|F/#||F/1 – F/20|
|Reflected Wavefront Quality||< 10 nm RMS|
|Mid-Spatial Error Tolerance||Standard, Precision, & High Performance (see tech note)|
AOS offers multiple quality grade options depending on the application desired. These quality grades can differ significantly in price to allow customers to select the most affordable option for their application and budget. Conventional ways of specifying optical quality in aspheric optics often do not address mid-spatial error tolerances, which are key to predicting imaging quality.
|Grade||AOS Definition||Diffraction Limit|
|Standard||RWE < 63 nm RMS for spatial scale lengths > 10 mm||N/A|
|Precision||RWE < 32 nm RMS for spatial scale lengths > 1 mm
|= 1.3-1.5x DL|
|High Performance||RWE < 12.6 nm RMS for spatial scale lengths > 1 mm
Diffracted features in image plane < 10-6 in relative intensity to central focus peak
The most common tolerances for specifying the optical quality of aspheric mirrors such as off axis parabolas are surface accuracy and reflected wavefront error. Surface error is the deviation of the surface from its perfect form. Wavefront error is the deviation of the resulting reflected or transmitted wavefront from its perfect shape.
At first glance, the decision to specify optics based on its wavefront quality or surface form may seem purely one of preference. One might be tempted to assume they are merely related to each other by a scale factor. However, this is not correct. One can convert between the two – but not by a simple scale factor like in a flat or spherical optic. It turns out that the decision can be quite important when one considers how precision optics are measured. It may surprise some people to know that interferometers measuring aspheres in typical test configurations do not measure surface form error at all – only wavefront error and there is no single scale factor that can extract surface error for an off-axis parabola. The consequence of incorrect scaling is that those specifying optical quality based on surface accuracy may not be getting the quality they believe.
Metallic Coatings: Metals provide the most efficient means to enhancing the reflectivity of an optical mirror. Metallic coatings, are readily manufacturable, provide broadband performance and are less sensitive to incident angle, group velocity dispersion (GVD) and polarization than dielectric coatings. Due to high absorption, metallics, in general, often have lower laser damage threshold than dielectrics. Because metallic coatings are relatively soft, protective dielectric overcoat(s) are applied to enhance the metal’s durability, cleanability, and resistance to environmental damage. With proper care, protected metallic coatings can be cleaned with lens tissue and standard solvents.
Multi-layer Dielectric Coatings (MLD): MLD coatings provide lower absorption reflectors than metallics, which can often result in higher laser damage resistance. MLD coatings can be tuned for very specific performance requirements for reflectors, partial reflectors, leaky mirrors, beamsplitters, polarization control coatings, and anti-reflective coatings. AOS, through our partner Okamoto Optics, provides standardized coatings for high-energy laser applications – but most MLD coatings are user specific. Please contact us for more details.
The definitions of Off-Axis Parabola geometry is not universal. The graphic below and the definitions to follow are the conventions adopted by AOS.
|Parent Axis||The optical axis normal to the center of the parent parabola.|
|Segment Axis||The segment axis is parallel to the parent axis and corresponding to the centerline of the relected (or incident) collimated beam.|
|*Parent Focal Length (fp)||The distance from the vertex to the focus along the parent axis. This spec is fundamental to all parabolas – even those off-axis.|
|Segment Focal Length (fs)||The distance from the intersection of the OAP surface and segment axis to the focal point.|
|*Off-Axis Angle (OAA)||The angle subtended between the segment axis and the segment focal length.|
|*Off-Axis Distance (OAD)||The distance between the parent axis and the segment axis. Some manufacturers define this as the distance from the parent axis to the inner edge of the segment – we do not because measuring this quantity is less reliable and prone to higher errors. The center of the parabola is a more reliable datum point.|
|Sag (Z)||The displacement of surface from the vertex, at the radial distance (r) from the parent axis.|
|Departure from Sphere (d)||It is often useful to determine the difference in sag between the parabola surface and the sag of the best-fit sphere. This quantity is known as the “departure from sphere” and determines the degree of apherization. This often has a bearing on the level of difficulty in manufacturing.|
|Form Error||Also called “Figure Error” or “Surface Irregularity”. This is the departure from the perfect intended surface contour.|
|Mid-Spatial Frequency (MSF) Error||Periodic surface ripples of spatial scale length between 1 mm and 10 mm (some definitions extend this to 33 mm). See AOS Technical Note on MSF Error for a more detailed description. See also ISO 10110 for general definitions.|
|Roughness||Surface errors of scale length 0.0025 mm to 0.080 mm. See also ISO 10110-8-2010 for more general definitions.|
Send us your drawings or specifications and we’ll begin preparing a quote for you.
The table below lists the information we’ll need to fully specify your OAP (no worries if you’re not sure about the specs. we can work it out with you). You can find information on definitions and other technical data under the “Specifications” and “Dimensions” tabs.
|Size of the incident or reflected beam: this tells us how to set the dimensions of the OAP|
|Focal Length (either the segment focal length or the parent focal length)|
|Off-Axis Angle or Off Axis Distance|
|Quality needed (reflected wavefront error & cosmetic quality)|
Contact us today to speak with an applications engineer.
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