Spot Polished Spherical and Flat Optical Components – Buyers Guide Chapter 5

In this blog the measurement of spherical and flat optics with a Fizeau interferometer that have been spot polished is discussed.

Spot polishers require improved performance over interferometers with standard 6X continuous zoom imaging

Spot polishing machines for rapid manufacture of standard and high accuracy spheres place new requirements on interferometer systems. The spot polishing method can create small ripple in the surface while shaping the overall form. Accurate positioning of the polishing spot is required to correct the surface errors to bring the surface into specification. To guide the spot polishers image distortion, resolution, and pixel scaling (calibration) are important These requirements primarily drive the imaging system of the interferometer. Continuous zoom system do not meet these requirements.

Modern Imaging Systems

Modern interferometers have discrete or fixed magnification imaging to improve resolution, and minimize distortion and ray tracing errors. All the interferometer optics are exposed to coherent laser light which highlights surface defects. Therefore the optics must be high quality to supress bulls eye artifacts (stray fringes) from scratches, pits, dust and reflections. These hight quality optics increase the system cost.

Interferometer Image Resolution

Increased resolution is required to measure mid-spatial frequency surface features. These features can be defined as the residuals present after the removal of 36 Zernike polynomial terms (see REVEAL analysis screen below, the image on the right are the residual mid-spatial frequencies). Mid-spatial frequencies in an optical surface scatters light degrading the image or lowers directed energy concentration. Therefore they must be measured and corrected.

Mid-spatial frequencies are measured with a high resolution imaging system. Typically greater than a megapixel camera is required. The resolution is limited by either the optical design or the camera resolution. If the camera limits then the smallest feature measurable is approximated by 80% of Nyquist frequency
(1-line/mm:2 Pixels), or ~400 lines/aperture for a megapixel camera. At 50 mm field of view, approximately 125 µm feature can be imaged. Continuous zoom interferometers are limited to <100 lines/aperture.

Interferometer Image Distortion

Image distortion maps a surface feature in the wrong position, and the polisher will move to the wrong position. In the best systems the camera limits resolution. As noted 400 lines/aperture is the practical limit of resolution in a megapixel camera, so distortion of 1/400 or 0.25% is required. The polishers polishing function might decrease this requirement, but with 0.25% the interferometer will not be the limiter. For higher resolution cameras the 80% Nyquist again drives distortion… more pixels better distortion requirements, yet practically the polishing function (the shape of the polishing spot) is the limiter. Continuous zoom systems can exhibit up to 2% distortion – 10X higher than modern interferometers.

Interferometer Ray Tracing Errors

When the test part deviates from a sphere many fringes are seen. These fringes indicate high slopes between the reference and test wavefronts. When high slopes occur the test and reference wavefronts traverse different paths through the imaging optics. These divergent paths create wavefront errors in an uncorrected interferometer system. This error can be measured by acquiring data with a null interference cavity, saving the data and then acquiring data with the maximum number of tilt fringes that can be measured and subtracting the two results. The residual error will primarily be due to ray tracing errors and is seen as coma and astigmatism, and sometime spherical aberration. In the old continuous zoom systems thes

e errors can be as large as a wave of error. Even for the small amount of slopes they can measure.

To speed the convergence of the polishing correction process minimizing these ray tracing errors are required. Only the highest quality systems are corrected for ray-trace errors.

Summary

For spot polishing of spherical and flat components a low distortion, high resolution interferometer with low ray trace errors is desired.

Next Post: Next we discuss special applications, starting with Harsh environments

February 1, 2016April 24, 2020

Optical System Alignment and Wavefront Measurement – Buyers Guide Chapter 4

In this blog the alignment of optical systems with a Fizeau interferometer is discussed.

The goal of a system wavefront test is alignment and confirmation of system performance. The measurement of optical system wavefront often requires a custom interferometer. When a fully reflective system is measured a standard HeNe laser Fizeau is sufficient as the wavelength does not matter. For refractive systems the wavelength often must match the optical system design, and optical system wavelengths vary from 10.6 um to 193 nm. Thus these systems are often “custom” except for a few wavelengths that are more common. For this discussion the wavelength is assumed to be matched to the system.

Zernike polynomials are often used to define system wavefront errors during alignment

Null Test

If a null, adjust until near-zero error is the goal then a standard continuous zoom system can be sufficient. At null ray trace errors are minimized and wavefront imaging distortion error minimal. Further mid-spatial frequency errors are not critical when measuring system alignment. Some of these measurements are made with a null corrector lens that matches the system under test wavefront with the interferometer expected wavefront, either spherical or plano.

Non-Null Testing

Subsystem testing can produce non-null wavefronts in the final alignment. For non-null system an interferometer with low ray-trace errors is important. With high fringe density, or high slopes, ray trace errors grow. Ray-trace errors are developed when the test and reference wavefront traverse diverging paths to the camera and are seen primarily as coma and astigmatism, with sometimes spherical errors. If the final “aligned” condition is at 10 waves of spherical aberration then unless the interferometer is well corrected for ray-tracing errors the data will exhibit errors in final alignment.

Precision alignment is important for optimal optical system performance, especially with more complex optical paths.

Small systems

A bench top system test is similar to measuring an optical component and standard phase shifting data acquisition is sufficient.

Real Time Adjustment

Recently the introduction of widely available simultaneous data acquisition interferometers have enabled near real time phase. So alignments can be adjusted continuously for more rapid convergence on alignment.

Large Systems

For large systems, typically telescopes, vibration and turbulence become an issue. If an issue then only a simultaneous phase measuring system will be able to acquire data.

Summary

In most cases a standard interferometer with near matching wavelength is sufficient to test optical system wavefront. Where large or non-nulled cavities are involved a high performance interferometer with low ray-trace errors and/or simultaneous phase measurement need to be used.

Next Post: Small tool polishing applications

January 29, 2016March 13, 2018

Lap Polished Flats, Spheres and Prismatic Components – Buyers Guide Chapter 3

In this blog the measurement of optics with a Fizeau interferometer that have been lap polished is discussed.

What interferometer is needed to produce good parts? Each manufacturing process requires a specific measurand, the quantity to be measured, to provide the feedback necessary to control the process and produce good parts. In the following posts several applications will be discussed with optional systems highlighted.

Random, Near of Full Sized Tool Polishing

The historic method to manufacture optics has been random polishing on a spindle polisher. The procedure of rough forming the surface

shape, grinding to near polish and then lap polishing has been used for hundred of years. The beauty of lap polishing is the random nature, averaging over large areas of the sphere, which self corrects, and lead to high quality surfaces. Further the mid-spatial frequency ripples, the residual surface features remaining after the removal of 36-Zernike polynomials, tends to be suppressed due to averaging. There are limits and caveats as always, yet in general these are reasonable assumptions. Thus the measurand is simply the shape of the surface as defined by 36 Zernike polynomial coefficients.

The Old Zygo® Mark II Optics Sufficient

Since the meaurand is simply low spatial frequency shape, in this application, a continuous zoom imaging interferometer, with the inclusion of phase measurement is sufficient. This is why the Mark II architecture as been useful for over 35 years, it was sufficient for nearly all optics produced until this century. The low spatial resolution of the imaging system (no matter the camera resolution), and inherent image distortion of the zoom lens up to ~2%, and slope induced errors have little effect on measurement uncertainty of flats and spheres when measured at a null fringe condition.

Vibration Tolerant Data Acquisition Important

More important than the optical system for this application is the data acquisition and analysis software. This starts with vibration tolerant phase data acquisition as found in modern systems to report phase data without the influence of the production environment vibration. (We plan to discuss the history and development of vibration tolerant PSI in a future blog.) Further the software must be compatible with standard industry standards including ISO and data formats (.dat formats), and be easy to use.

Upgrading an Old Interferometer Is a Good Option

To stay current and meet the the requirements of lap polished optics there are two choices: Buying new or upgrading. The first option is purchasing a newly constructed system with classic optical components that has a new data acquisition system. The second is simply upgrading a classic system to a modern data acquisition system (with vibration tolerant algorithms). The performance of each will be equivalent, with the upgrade being much less expensive.

Summary

Lap polished Flats, Spheres and Prisms in a normal production environment are sufficiently measured with a continuous zoom system, where the value choice is often a system upgrade.

In the next post we’ll explore what is an appropriate interferometer for transmitted wavefront measurement.

January 22, 2016January 11, 2021

Brief History of the Laser Fizeau Interferometer – Buyers Guide Chapter 2

Click here to read an detailed history

Hippolyte Fizeau, Inventory of the Fizeau Interferometer

In this blog a brief historical outline of the Fizeau interferometer is presented.

Understanding the history of the commercial laser Fizeau interferometer helps to understand the choices available in today’s market. Some systems available today are remnants of historic designs.

Circa 1850: Fizeau Interferometer Invented

Hippolyte Fizeau invented this interferometer configuration for an 1851 experiment that at the time supported the either-drag theory, later disproven by Michelson, leading the way to Einstein’s theory of relativity

1960’s: Lasers

Laser Invented paving way for the laser Fizeau, an unequal path interferometer

1970’s: The architecture is established

The first commercial laser Fizeau interferometer, named for George Hunter the designer

Zygo® invents basic system architecture with sensor (mainframe) and reference (transmission sphere/flat) – the GH Interferometer in 1974.
Zygo® employs twin-spot alignment, with continuous zoom/rotating ground glass imaging design to accommodate low resolution (Vidicon camera) and off the shelf zoom lens (Mark II, in 1978).
Fringe following data acquisition introduced to replace visual fringe evaluation.
Bell Labs invents phase measuring interferometry on a 32X32 array data acquisition using a Vidicon camera, and PDP 8 rack mounted computer.

1980’s: Data acquisition improvements

Digital detectors (up to 256X256 by the late 1980’s), CCD/CID, and workstation computers introduced.
Carrier fringe and four camera simultaneous phase measurement invented (SPMI) for vibration insensitive data acquisition.

1990’s: Software advances

The First Phase Measuring Interferometer at Bell Labs

Enhanced graphics and additional results with faster processing in lower cost personal computers, primarily driven by Wyko and followed by Zygo®
CCD’s with 512×512 pixels introduced – no changes to the ground glass-continuous zoom historic imaging design

2000’s: Data Acquisition and illumination Improved

4D Technologies and ESDI successfully commercialize SPMI
Asphere interferometer introduced: Stitching (QED), Fizeau Scanning (Zygo®)
1KX1K custom imaging driven by deterministic, computer controlled spot polishing
Push to better imaging systems in custom designs
Lower spatial coherence illumination (ring) introduced to improve measurement accuracy

2010’s: MegaPixels + Low Ray Trace Errors + Improved imaging

First newly designed interferometer optical systems that break the ground glass-continuous zoom historic design for high-resolution, low distortion imaging plus minimized ray-trace errors for improved accuracy with steeply sloped wavefronts from mild asphere optics
Sub-nyquist (ESDI) asphere interferometer introduced

2020 and Beyond: Light Sources and Software

Lasers have dominated since the 1960’, but their limitations are catching up with them. Vertex back reflection fringes limit measurement accuracy, spurious fringe mask mid-spatial frequencies, and lasers are unable to measure domes, plates, prisms due to back surface reflections.

To overcome these limitations low coherent sources have been introduced. Delay line white light sources minimize the back reflections and spurious fringe problem but are nearly impossible to align, like a standard white light source.

ÄPRE’s Spectrally Controlled Interferometry (SCI) retains the ease of use of a laser with a long coherence mode and then switches to short coherence (white light) mode at camera frame rates. SCI can also measure the cavity distance enabling the measurement of radius of curvature, without the external radius rail, at higher accuracy. SCI looks like the source of the future.

Confused fringes with laser source (left), clean fringes with SCI (right)

SCI source Spectrum (left) produces Interference fringes (right)

Software

The control of the entire measurement process will become possible with every interferometer using the same measurement setup via centralized control without variation user to user. Software will guide the user as to whether a part passes or fails, and databases will track parts through manufacture and data will be reported consistently, all enabling better process control and thus improved parts.

Summary

The Fizeau interferometer has a long history and promising future. Contact ÄPRE to discuss your interferometery needs to apply the best source + interferometer to your application.

In the next post we’ll explore what is an appropriate interferometer for lap polished flats, spheres and prismatic components.

January 21, 2016March 13, 2018

Fizeau Interferometer Buyers Guide – Chapter 1

The many laser Fizeau interferometer choices make selection confusing.

You don’t want an laser Fizeau interferometer. You just want optics that meet specification, and your Fizeau interferometer helps you do just that. Yet choosing the right interferometer can be confusing. Just consider the choices:

Data Acquisition

PSI, IPMI, or Carrier Fringe, or wavelength tuning, vibration insensitive, vibration tolerant, multi-surface, scanning Fizeau, stitching…

Illumination

high-coherence, low-coherence or partial coherence (ring)

Wavelength

633nm, 1.06um, 3.39um, 1550nm, 10.6um, 650nm, and many others.

Imaging

zoom, discrete, and fixed magnification, plus ground glass or coherent

Detectors

256K, Mega or multi-mega pixel detectors, and then their is CCD or CMOS

System Configurations

Common path, off-axis, high-slope, steep surface, mid-spatial frequencies

Software Analysis

Zernike, slope, ISO, masking, frequency filters, box filters, and a plethora of others

Applications

wavefront, surface, radius of curvature, homogeneity, wedge, corner cubes and …

The combinations are hard to keep track of let alone configure to your specific requirements and budget. So over the next few weeks we will be running a series of blogs to offer some guidance and insight regarding what is important depending on your application.

In the next post we’ll set the stage with a brief history of optical test interferometry.