Fizeau Interferometer: Buy or Upgrade; what are the purchasing options?

Fizeau Interferometer Buyers Guide 9

This blog is about the performance and price options for laser Fizeau interferometers

Up to this point we have been discussing how various applications require specific interferometer sensor configurations and features. Now comes the key point when you need to do something. Do you need increased production capacity? Has a new process been introduced that requires better feedback? Is the IT department concerned about an old operating system? Has the computer failed, or the system has become unreliable but meets your production needs? Do you have a new program that requires a dedicated system? Or your customer is pushing for a result your system does not produce. What do you do? Do you buy a new interferometer? What are the options?

Buy New, Upgrade or Buy Refurbished?

Asking some questions can lead you to the purchasing options to consider.

Is the system to support a spot polishing process where slopes and mid-spatial frequencies are important?
- Yes – indicates a high performance imaging interferometer with a 2K X 2K level camera, low distortion and low ray-trace errors.
Is the environment harsh, with vibration and air turbulence present?
- Yes – indicates a SPMI (Multi-detector or Carrier Fringe acquisition) system
Does your application require custom fixturing, a special wavelength or non-standard aperture?
- A custom system is required – talk to the various providers regarding your best option
Does the system support a standard lap polishing process where form is the key measurand?
- Yes – indicates a classic interferometer

High Performance Imaging System laser Fizeau

These are the latest technology and will meet all the requirements for most situations, the problem is they tend to be expensive. Not only do they image mid-spatial frequencies well, the associated low ray trace errors mean they are suitable for carrier fringe data acquisition for operation in harsh environments. The major drawback is price. Most systems are 1.5X to 2X more expensive than a classic interferometer. If they were the same price everyone would probably buy one of these interferometers.

Harsh Environment laser Fizeau or laser Twyman-Green

There are several interferometers available that meet this need using both carrier fringe and multi-camera or multi-pixel configurations, Many of these applications have specific specifications that dictate size, weight, and result output. It is best to discuss these with the manufacturer to arrive the best choice.

Classic laser Fizeau Interferometer

With the classic Fizeau there are the most choices.

Buy New or Refurbished: Increase Production Capacity

If increased production capacity is required there are two choices: Buy new or refurbished. There are a few choices for new systems that have the same optical system as produced for the last 30 years – with extensively upgraded software! These new system are priced between $60,000 and $75,000 (USD).

Also available from time to time are refurbished interferometers with the same/similar classical optical design. These refurbished systems have the latest data acquisition and analysis software like a new system and are priced between $33,000 and $37,000.

Upgrade: Failed System, Old Operating System, New Acquisition and Analysis Software Required

There are thousands of interferometers installed worldwide that can be renewed by upgrading the electro-optics and software. These systems can operate as well as a new system, with the latest software, cameras and computer systems. Upgrading is cost effective and is priced between $22,000 and $27,000. Upgrading is usually the best choice when production capacity is not an issue, but systems are down. A side benefit is the increased efficiency of new electronics can also increase the throughput and therefore production capacity of existing systems.

Summary

There are several purchasing options available regarding laser Fizeau interferometers. Applications often drive the decision but price is also important. laser Fizeau Relative Pricing

March 23, 2016March 13, 2018

Measuring Thin Parallel Plates – Buyers Guide Chapter 8

In this blog the measurement of thin parallel plates with a Fizeau interferometer is discussed.

Filters, etalons and plane parallel components exhibit reflections from the front and back surface rendering them impossible to measure in a standard laser Fizeau interferometer. A standard interferometer uses a laser, making interference easy to create, no matter the interferometer cavity length. With parallel plates additional interferometer cavities are created adding confusing fringes: 1) Reference surface to the plate front surface, 2) reference surface to the plate back surface and 3) plate front surface to back surface.

A few techniques are available to overcome this confusion.

Spectrally Scanned Interferometry (SCI) – Click for more information

Laser Fizeau compared to SCI Fizeau – 1 cm substate with 250 um step in the middle.

ÄPRE has introduced a practical SCI source, a new source modality to Fizeau interferometry. SCI controls the coherence, fringe position (over 100’s of millimeters) and phase modulation of the fringes electronically. SCI aligns in high coherence mode, like a laser, isolates like a white light source, positions the fringes within the cavity, and phase modulates regardless the cavity size, even down to 50um. By isolating

the surface of interest, accuracy is improved and new applications are enabled. This is a new technology and its impact will emerge in the coming years.

Wavelength-Modulation + Fourier Analysis

Modulating laser wavelength will change the observed phase of the interference fringes. The rate of fringe modulation as the wavelength is changed is a function of the interferometer cavity length, long cavities modulate more rapidly than short cavities. Thus when a wavelength scan is performed and the results Fourier analyzed for modulation frequency the various cavities can be separated. By performing phase shifting interferometry analysis on the now separated surfaces thin parts can be measured, surface by surface. The drawbacks of this approach tend to be price and careful set up. To assure proper extraction, the surfaces must be positioned so the modulations are separable and not overlapped which is not always possible. Some versions also provide absolute position in space enabling millimeter length steps to be measured.

Short Coherence Balanced Arms

An incoherent source, like a diode, can produce fringes at a location in front of a Fizeau reference flat IF a secondary interferometer system is coupled to it. By adjusting the coupling cavities length the interference fringes are placed in space in front of the Fizeau reference surface. This method produces high quality, single surface fringes where the front and back surfaces of a thin part can be measured. Another short coherence approach is to use a Twyman-Green equal path interferometer and a diode. The main drawbacks of these systems are price and set up. The fringes exist at a specific point in space and the test part must be moved to within micrometers in tip and tilt and Z to simply see fringes. This can be challenging if separate alignment aids are not provided.

Grazing Incidence

By producing a steeply grazing illumination beam and varying the spatial coherence of the illumination, the back surface of a thin part can be eliminated from measurement. This techniques has been used successfully to measure semiconductor masks and sapphire wafers, though it is not used often in optical testing. The steep grazing incidence angle enable even rough surfaces to reflect specularly, with desensitized fringes (5um equivalent wavelength) and therefore the accuracy is not sufficient for most optical work.

February 23, 2016March 13, 2018

Photonics West 2016

Zygo® GPI™ Upgrades Introduced

The First Phase Measuring Interferometer at Bell Labs

At Photonics West we introduced a new product, upgrading Zygo® GPI™ and Mark IV™ interferometers. This also includes the VeriFire™ line of interferometers. All these system are basically the same with minor variations and which we can accommodate.

We are pleased by the growing excitement about our ÄPRE REVEAL™ software. Its backward compatibility with MetroPro® file types and unique traceable metrology make it the production software of choice.

Also several other exhibitors displayed products run by REVEAL. Davidson Optronics again displayed their 12″ Fizeau interferometer, designed in concert with Äpre Instruments, along with the PSM micro-profiler both using REVEAL. M3 displayed their new 3.39um IR interferometer using REVEAL software.
See you in Frankfurt at Optatec 2016.®™

February 23, 2016March 13, 2018

Harsh Environment Testing: Optical Towers and In-situ Metrology – Buyers Guide Chapter 6

In this blog measurement of optics in a harsh environment with a Fizeau interferometer is discussed.

For large systems, typically telescopes, vibration and air turbulence hinder or prevent the measurement phase by temporal phase shifting. Also interferometers placed on machine tools to measure in situ experience a vibrating environment. When vibration and turbulence hinder measurement then only a simultaneous phase measuring system (SPMI) will be able to acquire data. These systems acquire phase data fast enough to freeze fringe motion due to vibration and turbulence.

SPMI Data Acquisition: Multi-Camera and Carrier Fringe

There are two primary SPMI data acquisition architectures: Multi-camera¹ and carrier fringe² . Multi-camera uses polarization to split the intensity data into multiple images with shifted phases which are analyzed for wavefront phase. Carrier fringe uses tilt in the wavefront coupled with several different analytical approaches to extract phase. Both approaches are successfully employed commercially, and are functionally and performance equivalent.

Tower for testing large mirrors at the University of Arizona

Averaging is Required

The SPMI system enables phase to be acquired, but the phase is changing rapidly and by large amounts and any single measurement is meaningless. To achieve stable data, averaging must be employed. The amount of averaging required is a function of the frequency and amplitude of the vibration and turbulence. A useful method to determine how much averaging is required is to acquire 100 data sets in a series as would be performed in an average. Calculate the RMS or P-V of this data set, divide the RMS or P-V by the measurement repeatability desired to obtain a ratio. Square this ratio and set the average to this squared value. For example, if the single measurement repeatability is 6,000 nm P-V and desired is 60 nm P-V then 1002 averages must be taken, at a minimum, to achieve 60 nm P-V repeatability, assuming a gaussian distribution. It is often practical to stir the air with a fan to improve convergence, which can take many hours for large cavities with slowly moving fringes. SPMI systems can often acquire and calculate phase in seconds so averaging can be rapid.

Repeatability is Not Accuracy

Finally, note that repeatability is not low measurement uncertainty, or accuracy. Measurement uncertainty is primarily driven by the optical design of the interferometer, temperature variations in all the optics, mechanical stresses in mountings and optical misalignments and null lenses errors – a la Hubble. Controlling these is much harder than averaging and is a topic unto itself.

SPMI Interferometers are Non-Common Path – Hence Lower Accuracy

Also note SPMI systems are non-common path systems – the polarization paths are different or tilt exists between the test and reference wavefronts. These differing paths degrade the optical performance compared to PMI which can be common path, i.e. the test and reference beams perfectly overlap when a sphere or flat is measured in a nulled condition. So from an accuracy point of view PMI- nulled can outperform SPMI, but when data cannot be acquired due to turbulence or vibration then SPMI is required and an accuracy compromise is acceptable.

Summary

Acquiring data in harsh, vibrating and turbulent environments requires an SPMI data acquisition interferometer. Achieving high accuracy results requires careful attention to the all measurement parameters and is typically of lower accuracy than a nulled laser Fizeau phase shifting interferometer in a quiet environment.

¹ Smythe, R & Moore, R; “Instantaneous Phase Measuring Interferometry”, Opt Eng, Jul/Aug 1984, 25(4):361 – 364

² Takeda, M. “Spatial-carrier fringe-patter analysis and its applications to precision interferometry and profilometry: an overview, Ind Metrol 1990;1(2):79-99

February 9, 2016April 24, 2020

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.