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.

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, wavefront Gradient (slope) < 10 microradians RMS = 1.3-1.5x DL
High Performance RWE < 12.6 nm RMS for spatial scale lengths > 1 mm, wavefront Gradient (slope) < 4 microradians RMS =1.2-1.3x DL

Standard Grade

AOS Standard Grade is equivalent quality traditionally believed to conform to at λ/8 or λ/10 PV surface accuracy. AOS’ equivalent standard quality is re-specified as λ/10 RMS reflected wavefront error[1]. This is the most affordable option and provides good performance for many imaging applications. Data is provided using phase-measuring full aperture interferometry and data is characterized for low frequency spatial periods > 10mm.

[1] AOS Technical Note:

Precision Grade

AOS Precision Grade is 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 offers λ/20 RMS reflected wavefront error4 for spatial scale lengths > 1 mm. In addition, the Precision Grade specification includes a tolerance of < 10 micro-radian RMS gradient. This controls 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. Power Spectral Density (PSD) analysis also serves to qualify mid-spatial characteristics of AOS Precision Grade components.

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. In addition, the High Performance Grade specification includes a tolerance of < 4 micro-radian RMS gradient. This controls mid-spatial frequencies in the range of 1-10 mm, 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, Gradient Analysis, PSD Analysis, and Calculation of Focus Diameter. Our goal with precision grade is to wipe out the impact of linear periods to a negligible level.

* 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