AOS Quality Grades for Aspheres

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. Defining the specification for gradient limits the amplitudes of such features and enables the highest energy densities for image resolution criteria such as point spread function or encircled energy.

Standard Grade

STANDARD GRADE is comparable quality to what may be found in many catalogs or manufactured by traditional technologies. AOS’ “Standard Grade” is the lowest cost option and guarantees a reflected wavefront error < λ/10 RMS. Standard Grade provides good performance for many imaging applications but does not control mid and high spatial frequency amplitudes in the wavefront and does not include a PV spec. Standard Grade may include periodic features or “ripples” in the surface and wavefront resulting from residual tool path textures used in deterministic finishing. Data is provided using phase-measuring full aperture interferometry and data is characterized for spatial scale lengths > 1mm.

[1] AOS Technical Note: www.apertureos.com/wp-content/uploads/2016/07/AP_note-Wavefront-vs-Surface2.pdf

Precision Grade

AOS Precision Grade is most often applied for high-energy lasers, beam expanders and other precise imaging applications. Precision grade applies high-resolution interferometry to more stringently characterize mid-spatial content and provide a higher level of image quality.

AOS Precision Grade guarantes a reflected wavefront error <  λ/20 RMS for spatial scale lengths > 1 mm.

Some mid-spatial error is apparent in the interferometry, but amplitudes and frequencies are limited by tolerances. Precision Grade specification includes a PSD envelope tolerance and focus quality specification based on the amount of concentrated focal spot intensity vs. the diffraction limited prediction. These two metrics control mid-spatial waviness in the range of 1-10 mm, and ensures higher focused energy concentrations. Focused Spot size is guaranteed to be within 1.3x – 1.5x the theoretical diffraction limit.

Fianlly, we place a tolerance on the maximum intensity of diffracted features in the focus. Features such as satellite focii are limited to < 10-6of the central peak in the point-spread function.

RWE < 32 nm RMS for spatial scale lengths > 1 mm

Focused Spot size is guaranteed to be within 1.3x – 1.5x the theoretical diffraction limit.

Relative Diffracted Feature Intensity < 1×10-6

PSD Threshold:   f(x)=a/(1+(x/b)^2

 a: 10^6,    b: 100,   c:2

High Performance Grade

High Performance Grade is the current state of the art in our precision aspheres. This grade is used for only the most demanding imaging applications.

AOS guarantees λ/50 RMS reflected wavefront error* for spatial scale lengths > 1 mm.

Even in High Performance grade, some mid-spatial and high spatial frequencies are visible, however the tolerance envelope allowed by our high performance grade is much more stringent than in Precision Grade. In addition, the High Performance Grade specification includes a PSD tolerance to control mid-spatial frequencies, and enables the highest focused energy concentrations approaching the diffraction limit. This grade is the closest we can get to diffraction-limited performance with spot sizes ranging from 1.2x – 1.3x the theoretical diffraction limit. High Performance Grade includes all the same analysis of Precision Grade including Phase Plots, PSD Analysis, and Calculation of Focus Diameter. Fianlly, we place a tolerance on the maximum intensity of diffracted features in the focus. Features such as satellite focii are limited to < 10-7of the central peak in the point-spread function.

RWE < 16 nm RMS for spatial scale lengths > 1 mm

Focused Spot size is guaranteed to be within < 1.3x the theoretical diffraction limit.

 

Relative Diffracted Feature Intensity < 1×10-7

PSD Threshold:   f(x)=a/(1+(x/b)^2

 a: 10^5,    b: 100,   c:2

* note: AOS quotes reflected wavefront instead of surface accuracy for off axis parabolas as this is how OAPs are tested and used. Surface Accuracy (although commonly appearing in many publications) cannot be directly measured interferometrically or easily scaled from reflected wavefront data. More explanation of this is available in our application note, “Specifying Wavefront or Surface Error in Aspheres”.

Posted: August 28, 2016