Convergent Polishing: A Simple, Rapid, Full Aperture Polishing Process of High Quality Optical Flats & Spheres

Summary

A novel optical polishing process, called “Convergent Polishing”, which enables faster, lower cost polishing, is described. Unlike conventional polishing processes, Convergent Polishing allows a glass workpiece to be polished in a single iteration and with high surface quality to its final surface figure without requiring changes to polishing parameters.

Abstract

Convergent Polishing is a novel polishing system and method for finishing flat and spherical glass optics in which a workpiece, independent of its initial shape (i.e., surface figure), will converge to final surface figure with excellent surface quality under a fixed, unchanging set of polishing parameters in a single polishing iteration. In contrast, conventional full aperture polishing methods require multiple, often long, iterative cycles involving polishing, metrology and process changes to achieve the desired surface figure. The Convergent Polishing process is based on the concept of workpiece-lap height mismatch resulting in pressure differential that decreases with removal and results in the workpiece converging to the shape of the lap. The successful implementation of the Convergent Polishing process is a result of the combination of a number of technologies to remove all sources of non-uniform spatial material removal (except for workpiece-lap mismatch) for surface figure convergence and to reduce the number of rogue particles in the system for low scratch densities and low roughness. The Convergent Polishing process has been demonstrated for the fabrication of both flats and spheres of various shapes, sizes, and aspect ratios on various glass materials. The practical impact is that high quality optical components can be fabricated more rapidly, more repeatedly, with less metrology, and with less labor, resulting in lower unit costs. In this study, the Convergent Polishing protocol is specifically described for fabricating 26.5 cm square fused silica flats from a fine ground surface to a polished ~λ/2 surface figure after polishing 4 hr per surface on a 81 cm diameter polisher.

Introduction

The major steps in a typical optical fabrication process include shaping, grinding, full aperture polishing, and sometimes small tool polishing^1-3. With increasing demand for high quality optical components for imaging and laser systems, there have been significant advancements in optical fabrication over the last several decades. For example, precision, deterministic material removal is now possible during the shaping and grinding processes with advancements in Computer Numerical Controlled (CNC) glass shaping machines. Similarly, small tool polishing technologies (e.g., computer controlled optical surfacing (CCOS), ion figuring, and magneto-rheol....

Protocol

1. Preparation of Polisher and Slurry

First prepare the Convergent Polishing System (specifically called Convergent, Initial Surface Independent, Single Iteration, Rogue Particle-Free Polisher or CISR (pronounced ‘scissor’))^7,17 by installing the pad & septum, conditioning the pad, diluting & chemically stabilizing the slurry, and incorporating the slurry within the filtration system.

1. On the CISR polisher, adhere a polyurethane pad ....

Results

The Convergent Polishing protocol described above allows a ground fused silica workpiece (in this case a 26.5 cm square) to be polished, in a single iteration of 4 hr per surface, to a peak-to-valley flatness of ~λ/2 (~330 nm) for low aspect ratio workpieces and ~1λ (~633 nm) for high aspect ratio workpieces (see Figure 3). Again, this process repeatedly converges workpieces to the same final surface figure without requiring changes to polishing parameters and is independent of the initial surface figure.......

Discussion

As discussed in the Introduction, successful implementation of Convergent Polishing with respect to surface figure involves eliminating or minimizing all the phenomena that influence spatial material non-uniformity except that of workpiece-lap mismatch due to workpiece shape. If any one of these phenomena is not appropriately mitigated, either through process control or through appropriate engineering of the polisher, then the desired convergence point cannot be achieved or maintained; hence essentially every mitigation .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
MHN 50 mil Polyurethane Pad	Eminess Technologies	PF-MHN15A050L-56
Cerium oxide polishing slurry	Universal Photonics	HASTILITE PO
Septum Glass (waterjet cut)	Borofloat ; Schott	NA
Diamond conditioner	Morgan Advanced Ceramics	CMP-25035-SFT
Ultrasonic Cleaner	Advanced Sonics Processing System	URC4
Purification Optima Filter cartridge	3M	CMP560P10FC
Blocking Pitch	Universal Photonics	BP1
Blocking Tape	3M	#4712
Cleanroom Cloth	ITW Texwipe	AlphaWipe TX1013
Single Particle Optical Sensing	Paritcle Sizing Systems	Accusizer 780 AD