Revolutionizing Precision Optics Measurement: Sydor Optics Harnesses Äpre Spectrally Controlled Interferometry (SCI)

Sydor Optics is widely regarded as a world leader in processing and delivering precision flat-surfaced, parallel, and wedged glass optical components. Patrick Drury, Director, Quality & Operational Excellence, Tom Grambo, QA Technician, and John Mandelaro, Precision Optics Manufacturing Technician Apprentice, share how they are using Äpre Spectrally Controlled Interferometry (SCI) to automate and speed the measurement process of the entire part.

When and why did you start working with Äpre?

We’ve been working with Äpre for around seven years. We use interferometers so often in our process, and we have so many pieces of equipment they approached us to beta-test an early version of their SCI interferometer. Apre leads the charge on scanning coherence. Other systems followed suit, but the competing technologies just aren’t the same.

Their service is better than the service of any other system that we have. Äpre has a small group we can collaborate directly with – they’re responsive and eager to try new things. Their team is really good at getting back to us if we have questions on anything software related. Bob visits us to talk about the systems – he’s an owner. We don’t get that level of service from others.

What volumes do you work in?

On a given month, we check thousands of parts. Annually, I’d estimate it at 100,000 parts or more. We have three Apre systems that streamline our processes. On some programs, we test EVERY part, not just a sample, and this type of interferometer saves us 30 minutes per part in some cases. You can imagine how that adds up over time in these volumes.

On some programs, we test EVERY part, not just a sample, and this type of interferometer saves us 30 minutes per part in some cases. You can imagine how that adds up over time in these volumes.

How else does the interferometer save time?

A big time-saver is having the ability to set up and save part-specific parameters using the software. Say you’re measuring a wedge you might forget to change the refractive index. If missed our customers will demand a retest. The software saves us that step because it automatically loads all set up parameters when that specific analysis configuration is loaded. Plus scripting makes the multi-step process 5X faster by taking 4 separate measurements and doing them in one automatic sequence.

Scripting makes the multi-step process 5X faster by taking 4 separate measurements and doing them in one automatic sequence.

Tell us more about the scripting.

We hired John out of MCC’s Optical Systems Technology program. As an apprentice, he moves through every single department. He faced a big learning curve when he landed in the inspection department.

The apprentice program allowed him the time he needed to learn how to use the interferometer. And he just went further down the rabbit hole – he learned it and went the extra mile. He’s been working with Artur at Äpre on writing and implementing code to automate the measurement process so any one of the operators can load the part and run the script.

It’s been a huge benefit to the whole company and has also added to his skillset.

John Mandelaro, an apprentice at Sydor, works on automating their metrology for faster inspection.

How has it changed your experience in optics?

The problem-solving makes it exciting. Just seeing it work, understanding why, and empowering other people to keep projects moving. When you combine all these different skill sets: optics, manufacturing, quality assurance, and even sprinkling in some programming and scripting knowledge, it’s not just a job anymore. It’s a career. You’re changing the space that you’re in.

“When you combine all these different skill sets: optics, manufacturing, quality assurance, and even sprinkling in some programming and scripting knowledge, it’s not just a job anymore. It’s a career.”

Tom Grambo, Sydor Optics

At Äpre we’re keeping our eyes on the horizon. Let us provide you with state-of-the-art systems to advance your process. Contact us today to get started.

September 14, 2023

Game-Changing Interferometry

Spectrally Controlled Interferometry (SCI) revolutionizes traditional Fizeau interferometry, and empowers optical manufacturers with enhanced process improvement and manufacturing control. But how?

Spectral temporal coherence control gives you the best of both worlds – both high and low temporal in one source.
Spectral cavity selection gives you a live view of the fringes on the surface of interest, vs. the traditional Fourier methods.
Spectral synchronized phase shifting minimizes environmental influences and measures mid-spatial frequencies.

Today’s optics industry demands this level of optimal performance and precision. Read our feature in Laser Focus World to see how you can use SCI to your advantage.

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October 22, 2020September 30, 2022

Optical Surface Radius of Curvature

Optical surface radius is a primary control parameter. It must be measured to confirm the optical element meets specification. Spectrally Controlled Interferometry promises a faster and more accurate method to measure surface radius.

Multiple methods are presently used to measure radius.

Test plates: A test plate has a pre-measured radius to which the test surface is compared, the difference is power fringes is reported.
Mechanical probe: The sagitta of the lens surface is measured over a fixed diameter, from which radius is calculated. Small changes in sagitta measurement can lead to large errors in reported radii.
Interferometer “Cateye to Confocal”: An interferometer with a transmission sphere (TS) is set up at the catseye position off the surface of interest, and position noted. Then the lens is moved to the confocal position and the position noted. The difference in positions is the radius. PMI interferometers also compensate for any residual power in the measurements due to imperfect positioning. The positions are shown in figure 1.

Interferometer radius measurement is the “gold standard” method today. To achieve this measurement the following additional equipment is needed:

Lens mount that allows for free motion along the axis of the lens (Z axis) to micrometer position accuracy
Free Z motion over up to a meter, with micro-radian angular alignment to minimize cosine errors.
A separate Z position metrology tool. Either a glass scale or a displacement measuring interferometer (DMI) The DMI is the most accurate as it minimizes Abbe offset errors, though it adds environmental uncertainty if not calibrated.

Figure1: The distance between catseye and confocal is the surface radius. Note extra mechanical fixturing and metrology instruments are needed to measure the positions accurately

Spectrally Controlled Interferometry (SCI) Radius Measurement

SCI has a unique combination of four properties:

Easy alignment in laser mode
Electronically switch to white light mode to isolate fringes to the surface of interest
Measures the cavity phase electronically (no mechanical motions)
Measures the absolute distance of the measured cavity

SCI Absolute Position Measurement

The SCI source parameters that determine the absolute distance measurement are:

λ₀ = The nominal wavelength of the SCI source
Δλ = A tunable source parameter

The absolute distance from the transmission sphere to the optical surface is:

l_c = λ²₀ / 2Δλ (1)

This combination means SCI can provide all the measurements required for surface radius with neither an additional DMI, nor the precision mechanics to move from Catseye to Confocal position.

SCI Radius Measurement

Since SCI can measure absolute position there is no requirement to measure at two positions. For the first time surface irregularity and radius can be measured at the same time. This also improves measurement accuracy by the elimination of several error sources as discussed below.

SCI Radius is measured by first calibrating the radius of the TS = RoC_TS. Then directly measuring the cavity length l_c with SCI and calculating the part RoC_x from:

RoC_x = RoC_TS – l_c (2)

Figure 2: Measuring surface radius of SCI requires calibrating the TS radius and then directly measuring the cavity length and calculating the Test Optic radius.

Measurement Accuracy

Repeatability

The foundation of accuracy is repeatability. If a measurement process is repeatable it can then be calibrated to yield accurate results. Repeatability of measurement has been shown to be <0.5 µm for radii in the 20 mm to 175 mm range. This repeatability is better the best measurements with DMI’s.

Accuracy NIST Study

In 2001 NIST ran an experiment to determine the error sources and limits of accuracy for surface radius¹. The results of that experiment and error sources are shown in figure 3, along with estimates from Typical DMI and Typical Scale (Glass) radius benches and SCI.

A key observation is most of the error sources are due to measuring the distance between catseye and confocal. In the NIST study fully 90% of the errors are in this part of the measurement. The same is true for the DMI and Glass Scale. The only difference is the magnitude of the errors that are 16X and nearly 500X greater, respectively, than the NIST study.

These values are a precaution to users of a DMI or glass scale radius bench to not underestimate the errors in the measurements.

Figure 3: Surface radius error budget for several methods of measurement.

SCI Eliminates the Z Axis Motion Error Sources

No Z motion eliminates the first six error sources (green). Leaving the balance of the error sources that are equal to the Typical DMI set up. SCI can potentially improve the uncertainty to better than the NIST study.

An additional error source is the calibration of the TS radius that also must be added. Since it can be bootstrapp calibrated to the same level a simple doubling of the errors to ~50 nm is expected.

As of October 2020 SCI can measure surface radius up to 250 nm cavity lengths, and potentially out to 500 mm cavity lengths.

Summary

SCI promises to replace traditional catseye-confocal lens surface radius measurement by saving time and greatly improving accuracy. It is the go to method for high-performance lens radius metrology.

¹ T. Schmitz, A. Davies, C. Evans; Uncertainties in interferometric measurements of radius of curvature, Appeared in Optical Manufacturing and Testing IV, H. Philip Stahl, Editor, Proceedings of SPIE Vol. 4451, 432-447, 2001

July 17, 2019

Measuring Prisms: Multiple Wavefront Channels In One Setup

Prism Stacks Produce Multiple Wavefronts

Prism stacks are difficult or even impossible to measure with a laser interferometer, yet are easy to measure with Spectrally Controlled Interferometry (SCI).

Examples of prism stacks for projector imaging systems, displacement measuring, monolithic or assembled component interferometers, and RGB combiners.

Laser Interferometer Back Reflections

A laser light source interferometers (for example a laser Fizeau) create fringes from every back reflecting surface. In some prism assemblies these are created in multiple channels and are superimposed creating confused interference patterns that are impossible to measurement.

Spectrally Controlled Interferometry Eliminates Back Reflection

Spectrally Controlled Interferometry (SCI) can selectively isolate the interference cavity of interest for easy and accurate measurement. And as opposed to white light “delay line” interferometers, SCI is easy to align alignment prior to measurement using its long coherence alignment mode.

Imaging System Prism Stack

In an imaging prism stack the transmitted wavefront passes through several back reflecting surfaces, see figure 1. In this case there are four potential back reflection surfaces. SCI can measure each of these independently if the surface is of interest, or only the transmitted wavefront without corruption. This allows a straight on measurement without tilting or distorting the wavefront.

Figure 1: Schmidt-Pechan Prism. The circled interfaces are areas where back reflections can prevent accurate measurements with a laser interferometer. (ref. https://commons.wikimedia.org/wiki/File:Schmidt-Pechan_prism.svg)

Displacement Measuring Interferometer Components

Interferometers for displacement measurement typically have multiple channels for the test and reference beam path. SCI enables the measurement of each channel independently, without the degrading influence of multiple entry and exit surfaces. To select the measurement channel cavity the polarization is rotated by 90º from one to the other, which is accomplished at the interferometer. [A linear polarized interferometer, where the polarization can be rotated, is used for this application. Contact ÄPRE for more information.]

Figure 2: a-WOW Interferometer for measuring displacement and angle. (ref. Loughridge, Russell & Abramovitch, Daniel. (2013). A tutorial on laser interferometry for precision measurements. Proceedings of the American Control Conference. 3686-3703. 10.1109/ACC.2013.6580402.)

RGB Prism Combiners

RGB (red-green-blue) prism combiners are particularly difficult to measure. The three channels must be separated to measure each path’s transmitted wavefront, yet optically they are co-axial. With a laser interferometer this is impossible, as all the wavefronts overlap, see figure 3. Plus there are multiple back reflecting surfaces that must be eliminated for an accurate measurement. Each channel can be aligned independently in the long coherence align mode. Then selected in the short coherence mode for accurate measurement.

Figure: 3: Dichroic Prism for RGB selection. The red circle indicate surfaces that reflect back into the interferometer confusing or suppressing the measurement.

Interferometer resolution

Many RGB prism combiners are classed as micro-optics with a less than 6 mm clear aperture. Therefore a standard 4 inch (100 mm), zoomed imaging interferometer with ~400µm image resolution or worse at all zoom magnifications, has insufficient image resolution. A minimum of 60 µm image resolution is required to measure the wavefront of a 6 mm clear aperture prism. 60 µm resolution yields a 100 X 100 image resolution across a 6 mm clear aperture.

Figure4: Image resolution demonstrated for 100 mm aperture interferometer. Airy disk size properly sampled at ≥Nyquist limit is required

NOTE: This is not 100 X 100 pixels but image resolution. The camera array must be 2X this or a minimum of 200 x 200 pixels AND the imaging optics must produce an airy disk size of 60 µm. At 100 X 100 image resolution 36 Zernike polynomials are accurately calculated. See Figure 4.

Shown in figure 5 is ÄPRE’s S6|HR interferometer with 15 µm image resolution capable of measuring the transmitted wavefront in components to 1.5 mm clear aperture.

Figure 4: ÄPRE S6|HR interferometer with SCI light source for measuring micro-optics including prisms and prism assemblies.

Spectrally Controlled Interferometry Separates the RGB Channels

Again with a laser interferometer the three channels create back reflections simultaneously making the measurement impossible. In the measurement set up (see figure 5) each six prism assembly cavity has three cavity of different optical path lengths. Therefore the distance between the test and reference flat is different for each channel. With SCI each channel is selected one-at-a-time. Typical measurement results with SCI and S6|HR are shown in Figure 6.

Figure 5: RGB Prism assembly setup with S6|HR Interferometer.

Figure 6″ Typical results measuring an RGB prism assembly with S6|HR and SCI. One setup three results in seconds.

Summary

Prisms assemblies are difficult optical elements to measure. Back reflections from multiple surfaces and multiple channels make laser interferometry incapable of making these measurements. SCI offers an easy to use, accurate method to measure these components for the first time.

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June 18, 2019April 30, 2021

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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April 11, 2019October 16, 2024

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.

The front surface is measured, named, and stored in REVEAL’s clipboard
The back surface is measured, and a correction is applied for the index of refraction.
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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April 2, 2019June 18, 2019

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.

June 14, 2018

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.