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INTRODUCING: Mid-Spatial Frequency Surface Metrology

Measuring Mid-Spatial Frequency (MSF) is difficult. We have struggled with our vendors to provide adequate MSF control on our optics and high-end optics for semiconductor, high energy laser, X-Ray, and even autonomous vehicles require strict control of MSF’s
Now the Ä9|9MP Fizeau interferometer enables practical Mid-Spatial Frequency measurement. This is accomplish by the combination of high performance optical design, controlling spatial and temporal coherence, and rapid data acquisition during averaging.
An effective Mid-Spatial Frequency (MSF) measurement instrument needs to overlap and fill in the gap between both the form and roughness domains covered by other instruments, while accommodating substantial surface slopes with minimal retrace errors. Furthermore, it must achieve a low noise floor of tens of picometers of surface height accuracy, while remaining user-friendly.The Ä9|9MP fills the required gap spanning the spatial frequency range of 0.2 l/mm to 125 l/mm, see figure below.
For the Ä9|9MP a plano system was chosen, akin to a White Light Profilometer (WLP), but adopting a Fizeau design. A Fizeau design is enabled by ÄPRE’s SpectrÄ source. This architecture also allowed the integration of a super-polished reference flat, minimizing the size and magnitude of MSF errors within the reference optic.
The aim of measuring a surface radius of 120 mm even with a flat reference surface is possible with the 9 mm field of view. For radii <120 mm the field of view decreases, but continues to overlap with the resolution range of advanced Fizeau interferometers. Notably, the design achieves retrace error performance approaching λ/20, even at maximum slopes of 1,200 fringes across the field of view. Crucially, these minute retrace errors fall outside the typical spatial frequency range of Mid-Spatial Frequencies, rendering them filterable and thus inconsequential to the final measurements.
A low noise floor is accomplished by employing short temporal and spatial coherence. Temporal coherence control is accomplished using Spectrally Controlled Interferometry (SCI) technology. Alignment can be performed in the laser, long coherence mode. SCI electronically switches to the short coherence measurement mode. In the short coherence mode interference fringes are isolated solely to the measurement surface. This effectively eliminates interference fringes produced by other surface back reflections. To further eliminate any remaining coherent artifacts a spatial coherence buster is applied. During averaging of measurements spatial coherence is minimized. Thus averaging both removes speckles and spurious fringes, and other noise sources to minimize the measurement noise floor. This unique combination simulates the performance of a white light source within a Fizeau interferometer setup.
To achieve picometer noise floor measurements averaging is required. The Ä9|9MP system can acquire data with vibration tolerant Phase Measurement or Synchronous Phase acquisition. Both are electronically controlled by the spectrally controlled source. Synchronous phase acquisition is enabled by the SCI’s continuous and \textit{limitless} phase shifting, independent of cavity length. Consequently, it enables averaged acquisitions at over 10 times the speed of conventional methods, only limited by the camera’s frame rate. Averaging numerous frames of data can be performed rapidly, reducing measurement time significantly. Spectral Synchronous Phase acquisition is one more unique feature of SCI technology.
The Ä9|9MP is available in a fully automated system, or a bench top model.
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Measuring Material Homogeneity with SCI

Material Homogeneity

Optical index homogeneity is an important material parameter influencing lens performance. Laser interferometers have painstakingly measured this parameter which requires four separate measurements.  Laser interferometers have either required the use of contact plates with oft times poisonous index matching fluids. Or polishing a wedge into the test part to isolate the front from the back surface. In both cases the test part must be repeatedly tilted to isolate from test parts front and back surfaces as shown in figure 1. Tilting lowers the measurement accuracy as the test beams do not return over themselves (common path operation is lost), makes the test more complicated, and slows the measurement down exposing the test part to thermal variations increasing measurement uncertainty.

figure 1: Four measurements are required to measure homogeneity. With a laser source the test part must be tilted twice adding measurement errors and complicating the measurement.

Laser Wavelength Scanning Fourier Interferometer

Recently laser wavelength scanning Fourier (LWSF)  interferometry has been used for homogeneity measurements. LWSF enables the test part to be plane parallel polished and set up where no tilting during the measurement sequence is required. Once set up two measurement sets are required, one with the test part and one without. LWSF takes up to a minute to measure the cavity, therefore it exposes the measurement to environmental changes, limiting repeatability and accuracy. Plus vibration tolerant phase algorithms are not applicable.

LWSF Surface Confusion

Plus there can be confusion as to what surfaces are measured since the Fourier algorithm “finds” the six sets of fringes present in the cavity of which only four are of interest. 

SCI Simplifies Homogeneity Measurements for Improved Accuracy

SCI, similar to LWSF, uses a plane parallel polished test part to measure homogeneity. Therefore the test part is easy to manufacture. In the laser alignment mode SCI enables easy alignment of the four surfaces. Starting with the empty cavity nulled alignment, then adding the test part and minimizing fringes to complete alignment. 

With SCI only the cavity to be measured is seen, there is no confusion. After setting up, switch to the short coherence, white light, mode:

  1. Select the transmission flat (TF) to test part S1. Measure with vibration tolerant (VT) PSI in < 1 second.
  2. Then electronically select TF to S2 and measure with VTPSI. 
  3. Now select the TF to RF cavity – through the test part – measure. 
  4. Finally remove the test part and measure the TF to RF cavity. 

REVEAL homogeneity application leads the user through these sets, see figure 2. 

figure 2: REVEAL leads the user step by step through the process. Enable the next measurement to be made and proceed.

Once the four measurements are complete the homogeneity result computed automatically, see figure 3.

figure 3: The final result of four measurements with the homogeneity mapped on the right and PV result of 19.107 ppm shown in the middle bottom.

See the link below for a ÄPRE technical paper regarding the computing the uncertainty of measurement of homogeneity measurements.

Summary

Homogeneity is an important performance parameter for optical materials. Spectrally Controlled Interferometry (SCI) simplifies the measurement, improves accuracy and ÄPRE REVEAL software leads the user step by step through the process.

Homogeneity Measurement Uncertainty Technical Paper >

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Measuring Optical Domes: Faster and More Accurate with SCI

Optical domes are an important component in environmentally challenging imaging environments. Accurate metrology is required to achieve good imaging system performance. 

Applications of Dome Optics

Examples of Optical Domes

Dome optics are the last component in a growing number of imaging and guidance applications. Domes protect the imaging optics from the surrounding environment without degrading optical performance. Therefore the dome’s transmitted wavefront quality is important to control as it directly influences the final image quality.  Dome applications include: 

  • Underwater Camera
  • Submersibles for robotic and manned exploration
  • Unmanned aerial vehicle
  • Remotely Piloted Vehicle
  • Space applications
  • Missile Guidance System
  • Security systems

Manufacturing Parameters to Control

Transmitted wavefront is degraded by several parameters which must be controlled during manufacture:

  • Individual surface irregularity
  • Front-to-back surface thickness variation (TTV)
  • Front-to-back surface decenter (X-Y)
  • Center of radii shift in Z
  • Transmitted wavefront

Typical Metrology Methods

Laser Fizeau Interferometer

A laser Fizeau interferometer appears as an obvious choice to measure domes. Transmitted wavefront and surface irregularity are typical parameters measured with a Fizeau interferometer. The long coherence of the laser Fizeau frustrates these measurements as both surfaces mutually interfere with the reference sphere as well as the front and back surfaces creating three interference patterns overlaid making accurate surface measurement impossible. [Optically a dome is equivalent to a flat parallel plate and encounter the same measurement difficulties] Tilt can be introduced in an attempt to visually separate the surfaces, or one surface is painted black to lower the fringe contrast from that surface, but in the end the measurements are not reliable.

5-Axis CMM with Point Probe or Depth Probe

5-Axis Coordinate Measuring Machines (CMM) swings a probe

figure 1: 5 Axis CMM Geometry [Vahidi Pashaki, Pooyan & Pouya, Milad. (2016). VOLUMETRIC ERROR COMPENSATION IN FIVE-AXIS CNC MACHINING CENTER THROUGH KINEMATICS MODELING OF GEOMETRIC ERROR. Advances in Science and Technology Research Journal. 10. 207-217. 10.12913/22998624/62921.]
around the dome, measuring as it moves (see figure 1). If the CMM has ample range in the B Axis it can measure the full dome hemisphere. Two types of probes for surface sensing are used: A surface probe or a depth probe. The surface probe only measures one surface at a time and thus does not measure TTV or decenter. The depth probe measures both surfaces simultaneously providing measurement of all parameters except transmitted wavefront. 

The measurement reference or metrology frame is created by controlling the CMM’s motions. Motion control of a point in space (the probe) is technically difficult especially with 5 non-orthogonal axes are concerned. Therefore the measurement accuracy is degraded compared to a Fizeau interferometer that has a fixed reference (metrology frame). 

The 5-Axis CMM is slow to measure (minutes to hours depending on the resolution of data to acquire), typically expensive if state-of-the-art measurement uncertainty is achieved (~$750,000 [ USD]) and even then they do not approach optical metrology accuracy required.

SCI Fizeau: Fixed Metrology Frame all Parameters Measured

The Fizeau configuration is optimal considering accuracy. With ÄPRE’s Spectrally Controlled Interferometry (SCI) the coherence is controlled enabling the measurement of all parameters. 

figure 2: Dome setup in Fizeau measurement configuration

Just like for a plane parallel plate the SCI coherence can be adjusted to create fringes on each surface individually and between the front and back surfaces. In this way an accurate, fast method exists to measure all dome parameters. The following interference cavities provide the required results:

  • Surface Irregularity 1: Fizeau Sphere Reference to Surface 1 (PVr & RMS: Tilt & Power removed)
  • Surface Irregularity 2: Fizeau Sphere Reference to Surface 2 (PVr & RMS: Tilt & Power removed)
  • TTV: Surface 1 to 2 (PVr result)
  • Decenter (X & Y): Surface 1 to 2 (Tilt result)
  • Center of radii shift in Z: Surface 1 to 2 (Power result)
  • Transmitted Wavefront: Fizeau Sphere Reference to Additional Reference Sphere (PVr & RMS: Tilt & Power removed)
figure 3: Typical SCI Dome results without the multiple interference which frustrates measurement

The region of measurement is determined by the Fizeau sphere f#. A faster (lower number) sphere the more coverage. To cover a full hemisphere an f# of 0.5 is required and these do not exist. Tilting the sphere to the edges enables coverage of the full hemisphere over a series of measurements. 

Summary

Domes are an important element in imaging systems used in environmentally challenging environments. ÄPRE’s SCI technology provides rapid and accurate measurements of all parameters in one setup. 

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Applied Optics Paper >

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Measuring Prisms: S1, S2, Wavefront + Relative Angle

Prisms are complex optical components. Transmitted wavefront performance is controlled by multiple surfaces, the angle between them, and the material homogeneity.  SCI makes interferometric prism measurement fast, easy and more accurate than conventional methods.

Conventional Approach: Multiple Instruments, Delayed Shipments

Prisms are typically measured on multiple instruments and in several setups. A goniometer measures angles, and an interferometer measures the surface flatness of S1 and S2, and transmitted wavefront (which includes the homogeneity). And in a separate set up, the relative angles are measured in an interferometer. Multiple instruments and setups increase the time of measurement, slowing manufacturing and delaying the shipment of finished parts.

Laser Fizeau: a Compromised Measurement

Multiple surface interference prevents direct measurement of prism without work arounds

Complicating the laser Fizeau measurement are multiple back surface reflections between surfaces S1 and S2 and the transmission flat (TF) and reference flat (RF). Workarounds exist that add time and degrade the measurement accuracy while also increasing risk of component damage.

Surface Flatness with Laser Fizeau

Back reflections from S1 and S2 force the user to frustrate the fringes on the opposite surface by painting or greasing. The procedure is to paint S2, measure S1, strip the paint off S2 and paint S1, the measure S2, clean S1 and continue processing; this cycle is repeated until the prism meets specification.

Transmitted Wavefront with Laser Fizeau

The Prism must be tilted to measure transmitted wavefront with a laser Fizeau interferometer

Transmitted wavefront measures the wavefront passing through the prism, and consequently through S1 and S2. To suppress multiple reflections the prism must be rotated so the light reflected off these surfaces falls outside the interferometer’s acceptance angle. Thus the transmitted wavefront measured in a laser Fizeau is not in the “as used” unrotated orientation.

SCI Simplifies the Measurement

Spectrally Controlled Interferometry (SCI) simplifies and speeds up the measurement process. Now all four parameters can be measured in one setup with no preparation for increased accuracy, faster cycle time and lower risk of damage.

SCI with Fizeau interferometer setup. All important parameters are measured in one setup.

With SCI, no surface preparation is required. Setup the prism as shown in Figure 1. With SCI, the prism measurement is first set up in a long coherence mode, similar to that of a laser Fizeau. This allows fringes to be quickly observed and nulled for multiple cavities at once, making setup fast and precise. Next, the SCI source is placed in the short coherence mode to localize fringes to a single cavity. Individual measurements can now be made by electronically selecting each of the four cavities, one at a time. First, surface figure is evaluated by selecting the interference between the transmission flat ( TF reference surface) and S1 is selected and measured, followed by the transmission flat and S2. Then the S1-S2 cavity, the relative prism angles, is selected and measured. Finally the TF to RF cavity is measured for transmitted wavefront.

All these measurements were made in one setup without preparation and can be done in less than five minutes including setup, a greater than 10X savings in time, with increased accuracy and lower chance of damaging the part.

Summary

Tradition methods to measure a prism are time consuming, with lower accuracy, potential causing damage to the part. New Spectrally Controlled Interferometry (SCI) maintains the ease of setup with long coherence fringes and then measures all critical parameters without surface preparation and too higher accuracy.

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Glass Substrates: Total Thickness Variation (TTV)

Thickness variation (TTV), and often wedge, is specified in optical windows and substrates. There are two methods for measuring TTV: Front-to-Back and Direct.

Front-to-Back

TTV can only be calculated if the front and back measurements are performed without translating or moving the part. While this restriction poses challenges for traditional laser Fizeau systems, this is accomplished easily with SCI as the fringes are moved rather than the part.

  1. The front surface is measured, named, and stored in REVEAL’s clipboard
  2. The back surface is measured, and a correction is applied for the index of refraction.
  3. The two measurements are added to report the TTV.

An advantage of Front-to-Back TTV parts as thin as 100 µm OPL can be characterized.

Direct

The Direct measurement is accomplished by locating the SCI interference fringes inside the substrate itself. These fringes are called “internal Fizeau” fringes because the fringes are between the front and back surfaces or internal to the substrate. Once the SCI has localized fringes inside the substrate, the interference phase is directly measured and hence the TTV acquired. Phase measurement of internal Fizeau fringes is only possible by wavelength modulation (wavelength phase shifting) and SCI phase shifting. 

The OPL limitations for Direct TTV measurement are:

  • For wavelength modulation: OPL must be >2 mm and phase measurements can take more than 60 seconds
  • For SCI: OPL of 500 µm are possible with measurement times measured in milliseconds.

Summary

Measurement of a glass plate has many complications mostly due to interfering back reflections as found in a laser interferometer. SCI solves this problem and expands what can be measured on a substrate in one setup and with no preparation.

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Measuring Plane-Parallel Substrates

Plane-Parallel Substrates

What could be easier than measuring a plate of glass? Actually, almost all interferometric measurements are easier! Measuring a plate of glass, or a substrate, is one of the most difficult interferometric measurements. In a laser interferometer, the two surfaces of the plate simultaneously interfere with the reference surface and each other creating three sets of fringes. When multiple sets of fringes exist it is impossible to make a measurement.

Laser Fizeau a Compromised Measurement

The laser Fizeau interferometer produces fringes off every surface. Therefore, back reflections off the unwanted surfaces must be suppressed in order to take an accurate measurement. The typical suppression method is to apply a foreign material to the back surface to reduce the magnitude of the back reflection and decrease these fringe’s contrast. This foreign material might be blue paint, grease, index matching fluid or tape.

Accuracy Degraded

These methods cannot completely diminish the fringe; their contrast is just reduced. By setting a modulation (contrast) threshold the laser Fizeau nominally ignores these fringes during measurement. Unfortunately thresholding is not foolproof. The low contrast secondary surface fringe intensities add to the higher contrast front surface fringes distorting the phase measurement. Therefore even though the results can improve, the back surface fringes still degrade the measurement.

The back surface coating can also warp the part, depending on the thickness to diameter ratio of the part. Most users assume the paint does not warp the front surface, but drying paint induces stresses and hence it degrades the accuracy, the larger the diameter to thickness ratio the more severe the potential warping.

Time Wasted/Costs Increased

The largest associated cost of measuring plano optics is the time wasted during sample preparation. Painting can take minutes to a few hours for the paint to dry1 and while the measurement can be fast, removing the paint can add hours to the processing.

Substrate Measurement with SCI

Front Side Measurement with SCI

Spectrally Controlled Interferometry (SCI) solves this problem. First, in the long coherence “laser mode” SCI allows for easy alignment, just like in a laser Fizeau. Then by electronically controlling the spectrum of the illumination, the coherence of the Fizeau interferometer is narrowed and positioned on the surface of interest, thereby limiting the interference to only one cavity. Therefore, substrates can be measured without preparation and to higher accuracy.

Standard SCI can isolate surfaces separated as thin as 150 µm Optical Path Length (OPL), where:

OPL = n (index of refraction) X d (physical thickness)

With custom designed SCI systems even thinner substrates are possible to be measured. Once the fringes are found a standard phase measurement is performed taking only milliseconds.

Back Side Measurement with SCI

Without moving the part, the fringes are electronically moved to the back of the substrate. Now the front surface is invisible and only the backside measured. The distance the fringes are moved is the OPL, making finding the second surface easy. A standard phase measurement is again performed.

This measurement was made through the material so the index of refraction must be accounted for. ÄPRE’s back side measurement profile (application in REVEAL) makes this correction. The REVEAL profile compensates for the index of refraction correcting the surface heights, while reversing the sign of the surface heights, as if it was measured face-on (air to glass).

Summary

Spectrally controlled interferometer coupled with a standard Fizeau interferometer enables direct measurement of the front and back surfaces of a substrate without requiring treating the surfaces to suppress back reflection interference. With easy alignment in a coherent “laser” mode and electronically isolated fringes SCI lowers the cost of measurement and improves accuracy.

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1 Users of SCI have noted they are saving hours-per-part switching from painting to SCI.

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Failing Good Parts? Maybe the Problem is Retrace Error

Failing Good Parts? Maybe the Problem is Retrace Error

Retrace error is rarely specified for an interferometer, often ignored and can cause good parts to fail. Knowing how to minimize, decreases scrap, saves time, and lowers manufacturing cost.

Transmitted Wavefront Measurements

Recently an ÄPRE upgrade customer using a 6X zoom interferometer noticed unexplained user-to-user variations. The setup was a window within a Transmission Flat (TF)/Reference Flat (RF) cavity. With the window in place, the operator adjusted the tilt fringes using the reference flat to a user selected number of tilt fringes.

  • User-1 observed variations up to 0.04 fr.
  • User-2 observed variations up to 0.50 fr, on the same parts!

User-2 was scraping good parts with a 0.1 fr specification!

Root Cause: Measurements Vary with Cavity Tilt

Investigation revealed the root cause of the variation. User-1 adjusted to <3 tilt fringes to near null. User-2 adjusted to 7 to 10 tilt fringes. 7 to 10 fringes was enough to introduce measureable retrace error to cause the variation and falsely failed parts.

In Classic Continuous Zoom Systems, Tilt Fringes Degrade Results

Retrace error is not controlled in zoomed imaging classical interferometers. Consequently tilt fringes degrade results. Strangely even most high-end interferometers today do not specify retrace error. Why not? Retrace error is important to everyday measurements.

Recipe for Best Results: Window Transmitted Wavefront

  • Align the transmission flat (TF), ÄPRE Application Note: How to Align a TF
  • Align the reference flat (RF) until the fringes are nulled
    • This minimizes Retrace Errors
  • Measure the cavity without the window
  • Save the resulting measurement to the Clipboard
  • Subtract Reference (Analysis Tools) with the cavity measurement selected
    • Removes TF/RF cavity residual errors
  • Place the window into the cavity
  • Tilt the window just enough so no window fringes are seen
  • Measure and report the results

Retrace Errors also Influence Spherical Surface Testing

The presence of tilt fringes in a classical zoom system will degrade any measurement, even spheres and flats. Only the ÄPRE S-Series minimizes retrace error. An ÄPRE S-Series interferometer, exhibits <λ/20 Retrace Error at 500 fringes! With the S-Series retrace error induced user to user variations are nearly nonexistent.

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Spectrally Controlled Interferometry – Wins Prestigious Award

ÄPRE’s Spectrally Controlled Interferometry was awarded one of Laser Focus World’s highest awards for innovation in photonics, a Gold-Level Honoree.

We are proud to be recognized by this leading industry journal for the innovation that SCI brings to optical testing.

Even more exciting is hearing from our customers how SCI saves time and money as plates do not require painting or wedging to measure, and miniature optics are finally measurable with an interferometer!

Plus it can be added to ÄPRE’s S-Series interferometers now or in the future as they are SCI ready, or your old interferometer can be upgraded with an SCI source.

Contact us to learn more.