Cascade Fourier Domain Optical Coherence Tomography for the Metrology of Optical Components Including Freeform


Recent advances in deterministic grinding and polishing of optical quality surfaces have enabled the manufacturing of optical components with increasingly more complex surface shapes. Rotationally variant, i.e. freeform, optical components are of increasing interest to the optical design community as they allow for substantial advances in optical performance and/or packaging footprint and form factor compared to conventional systems.


A survey of recent optical designs employing freeform optical surfaces show that freeform sag departure from best fit sphere (defined as the base sphere that minimizes the RMS sag departure) may range from < 100 microns (mild) 600 microns or more (extreme). This scale of sag departure is far outside the typical dynamic range of interferometry for conventional, rotationally invariant optical surfaces. Moreover, tolerance on the optical surface figure often calls for error less than a half to quarter wave (PV) (316 nm to 158 nm at the 633 nm He-Ne wavelength typical for interferometry, respectively). The metrology instrument must be capable of measurements with accuracy and precision at least an order of magnitude better than the specified tolerance. Furthermore, as optical surfaces are ground and polished to tens of nanometer RMS roughness or better, non-contact measurement techniques are typically preferred if not required especially in later stages of fabrication.


There currently exists no instrument capable of non-contact, nanometer scale accuracy and precision measurements for both reflective and transmissive freeform optical components while remaining versatile across different freeform shapes.



We have invented a metrology system based on cascaded interferometers which has the following advantages:


1) Is non-contact,

2) Has nanometer scale accuracy and precision,

3) Is capable of mm-scale sag departure and up to tens of degrees of slope,

4) Achieves direct depth profile, decoupling systematic errors,

5) Performs all-optical Fourier transform reducing data processing time,

6) Enables simultaneously decoupled dual-surface measurement as well as sub-surface feature measurement for transmissive components,

7) Permits simultaneous measurement of fiducial markers,

8) Is capable of measuring both rough and smooth optical surfaces,

9) Has wide measurable spatial frequency range covering figure to mid-spatial frequencies,

10) Can accommodate component diameters up to 2" without adjustment of lens or sample,

11) Is capable of fast data acquisition speed on the order of minutes,

12) Has a low-cost and compact form factor, and

13) Allows for in situ implementation as optical sensor probe.


Additional information can be found in the following publications:


Di Xu, Andres Garcia Coleto, Benjamin Moon, Jonathan C. Papa, Michael Pomerantz, and Jannick P. Rolland, "Cascade optical coherence tomography (C-OCT)," Opt. Express 28, 19937-19953 (2020).


Di Xu, Andres Garcia Coleto, Zhenkun Wen, Benjamin Moon, Jonathan C. Papa, Michael Pomerantz, John C. Lambropoulos, Jannick P. Rolland, "Cascade optical coherence tomography (C-OCT) towards freeform metrology," Proc. SPIE 11490, Interferometry XX, 114900K (21 August 2020).


Di Xu, Romita Chaudhuri, and Jannick P. Rolland, "Telecentric broadband objective lenses for optical coherence tomography (OCT) in the context of low uncertainty metrology of freeform optical components: from design to testing for wavefront and telecentricity," Opt. Express 27, 6184-6200 (2019).


URV Reference Number: 2-18049
Patent Information:
For Information, Contact:
Curtis Broadbent
Licensing Manager
University of Rochester
Jannick Rolland-Thompson
Di Xu