Flat earthers claim that our planet is flat, despite any physical evidence and simple observation. Should we mock those who deny the obvious? Instead, let’s take the time to explain, and justify, on the basis of physical principles.

We have known for a long (very long) time, that earth is round: Aristotle provided first demonstration for a spherical earth in 4th century BC! By the 16th century, navigators had circled it and if scientific demonstrations were not enough, recent space observation of earth brings quite an easy -visual- proof of it.

Nevertheless, there are still a bunch of people to pretend that it is not. Should we mock them? In particular, as the physicists we are? I tend to think that it is a shortcut we should not take, if it’s not for other reasons, at least because it is not the most efficient way to convince and educate.

So when it comes to explain, again and again, that Shack Hartmann Wavefront Sensors are achromatic, I am very happy to do the same. Again, and again. And as physicist, to bring an explanation backed up by demonstration.

So here it goes.

Context

Shack Hartmann Wavefront sensors as we know them originally comes from an idea of Hartmann, in which a set of pinholes allowed to isolate rays and calculate their direction of propagation. Obviously, no effect of wavelength is questioned in the use of Hartmann masks of holes to reconstruct wavefronts.

Shack then proposed to use lenses instead of holes. As they concentrate light instead of blocking it, it is a much more efficient optical system. Combined with the advent of cameras and modern computers, it made the Shack Hartmann wavefront sensors the reference it is nowadays for wavefront sensing.

But here’s the thing: lenses are chromatic, hence Shack Hartmann wavefront sensors are limited by chromaticity! But that’s a simplification, and as always, the devil is in the details.

Simulations

So let’s leave faith or conspiracy theories aside and simulate the behavior of the microlenses array of a Shack Hartmann wavefront sensor at different wavelengths (representative of the sensor’s spectral range of use) and for different wavefront slopes (to mimic the sensor’s measurement dynamic of aberrations).

We use Zemax raytrace software to simulate the optical propagation of 3 incoming collimated beams corresponding to incidence angles of 0°, 1° and 3° sampled by a microlens (figure a). The distance between the microlens and the detector (wavefront sensor array of pixels) is optimized at 635nm.

We plot the various PSFs corresponding the centroid focused by the microlens for 4 different wavelengths: 450nm, 635nm, 850nm and 1050nm. (figure b). We can observe that the diameter of the PSFs increases significantly with wavelength, which is not surprising (we know that the diameter of the focal point is linearly proportional to the wavelength link). The visual effect of chromatism cannot be appreciated on the PSFs: 1/ longitudinally: the chromatic focal length of the microlens and the corresponding defocus do not compensate for the change in diameter of the centroid, 2/ laterally: no effect of the defocus on the shape of the PSF and on its symmetry can be appreciated (one could expect that the defocus combined with the lateral displacement of the centroid for high slopes values might affect the intensity distribution of the PSF).

We then plot the Spot Diagram for each slope and for each wavelength, together with the Airy disk (black circle in figure c) which represents the diffraction limit of the microlens at 635nm. All Spot Diagrams and all Spot Diagram variations due to wavelength changes are clearly well below the diffraction limit of the microlens, which represents its best ability to focus light. Also, the fact that for a given slope, all the Spot Diagrams continue to overlap at all wavelengths suggests that the effect of chromatism on lateral position of the centroid, if it exists, is minimal not only in relation with the Airy disk size but also in relation to the size of the pixels (corresponding to the gray grid in figure c).

Analysis

Even if the Spot Diagrams -and the absence of visual variations in PSF intensity distribution for the different slopes at different wavelengths- already tell us that the effect of chromatism is marginal in a design representative of a Shack Hartmann wavefront sensor, we continue the simulation analysis by calculating the barycentric position of each PSF. We then compare this position respect with the position of the PSF at 635nm, wavelength for which the system is optimized, and translate position errors in slope errors (see table below).

We can observe and quantify the error in slope measurements when combining high slopes values with spanning the spectral range of the sensor. Absolute value is found to be approx. 2urad at the limits of the spectral range, 450nm and 1050nm. Relative value is as small as approx. 0.004%, in the worst case.

Conclusion

We put a scale on the effect of chromatism even in presence of huge wavefront dynamic and demonstrated that their impact on centroid aspect and centroid lateral position is minimal. We can claim then Shack Hartmann wavefront sensors are achromatic on their whole spectral range such as our HASO4 BROADBAND.

We are happy to discuss how they suit your needs. Reach us at contact@imagine-optic.com .

Laser beam diagnostics:
What if Star Wars had deployed the ISO 11146 standard for laser beam diagnostics? We can’t rewrite history, but we can still delve into M2 measurement for your laser. (Image courtesy of Lucasfilm)

How is laser beam quality measured?

Several parameters are very useful to assess laser beam quality and properties. Among them are: M-squared factor (M2), divergence (), width at waist (w0), Beam Parameter Product (BPP), waist location, astigmatism, asymmetry, which can be calculated with conventional M2 beam propagation analyzers.

To reconstruct these parameters, such systems generally implement a camera that is moved along the laser propagation axis (z), either manually or by means of a translation stage, to acquire intensity maps of the beam. ISO 11146 standard defines that at least 10 observation planes are mandatory, half of them within one Rayleigh length (zR), the other half beyond 2 Rayleigh lengths.

What are the limitations of traditional M2 analyzers?

– Limitations include the fact that the translation has to be aligned respect with laser beam propagation, which takes time and limits the throughput in production. This is particularly true if laser manufacturing requires iterative steps and M2 measurements.

– Beam caustics must also be adapted to the available range of the translation stage, in order to acquire observation planes in and out the Rayleigh lengths. This is done by carefully choosing lenses of appropriate focal. Such lenses bring added aberrations to measured beam depending on their intrinsic quality and, again, quality of alignment. In addition, lens coating must also match the spectral properties of the laser. Such implementation results in voluminous benches, whose length is sometimes contained by internally folding the laser beam several times with the use of multiple mirrors. These mirrors also represent additional optical elements external to the laser.

– Last, the complete acquisition cycle requires time to scan the -at least- ten positions defined by the ISO 11146 standard. Any variation of the laser during this time affects the reconstruction of the M2 factor, which limits their use in case of dynamics effects.

New approach to laser beam diagnostics proposed by Imagine Optic

Imagine Optic proposes a new approach to conventional M2 beam propagation analyzers: in this method, the complete electromagnetic field of the laser is acquired from a single shot measurement and, from it, allows to access to as many observation frames within and out the Rayleigh lengths defined by the ISO 11146 standard and to fit the parameter M2.

Advantages of the method

Laser beam diagnostics is performed by Imagine Optic CAM SQUARED sensor:

+ It allows fast live acquisition thanks to its single shot acquisition (us to ms), making it perfect for alignment and characterization of laser or OPA dynamic changes

+ It is easy to set up, like a beam profiler, because there is no need to use a translation stage anymore, nor to align it on the laser beam ( 40 Seconds Demo Video )

+ This makes it a compact solution, with small footprint in the lab or on the production bench, and easy to handle for after-sales support and on-site service.

Avoid the path of Kylo Ren and don’t let a bad M2 factor ruin your day when there are quick and easy solutions at hand to test the quality of your laser beam. We are happy to discuss how they suit your needs. Reach us at sales@imagine-optic.com or through the contact form.

 Shack-Hartmann wavefront sensor .

If Charles Darwin wasn’t thinking of the Shack-Hartmann wavefront sensor when he wrote his theory of evolution in 1838, it’s only because they hadn’t yet been invented. Since then, wavefront analysis technique has followed a steady evolution, accumulating inherited advantages combined with rapid adaptive leaps over almost a century, to become an optimum solution for modern optical metrology.

1900, 1st original idea by Hartmann

Around 1900, Johannes Franz Hartmann developed the Hartmann plate, consisting of an array of holes, which he used in combination with photographic plates to interpolate wavefront slopes and align telescopes.

1971, improvement by Shack (& Platt)

In 1971, Roland Shack and Ben Platt developed an optimized design for the Hartmann plate in which the array of holes is replaced by a much more efficient array of microlenses that concentrates light instead of widely blocking it, as with the mask.

Þ Combined with the advent of CCD cameras, this new approach has made the technique much faster, easier to use and adapted to “low light” applications such as astronomy.

Since then, the use of Shack-Hartmann sensors in optical metrology has continued to grow as they started to be used for laser testing, adaptive optics, ophthalmology for example.

1996, 1st commercial absolute wavefront measurement

Imagine Optic is established to materialize the vision of its cofounders: propose a reliable, easy to use and versatile tool for optical metrology at a time where only established alternative was the heavy, restrictive Fizeau interferometer.

Þ This is how the first Shack-Hartmann wavefront sensor offering live calibrated absolute measurement is commercialized as world premiere.

2001, first self-illuminated Shack-Hartmann

The LIP is born by combining our Shack-Hartmann wavefront sensor with an illumination feature in a built-in optomechanical platform.

Þ It optimizes optical quality and compactness of the solution in a ready to use to for transmission and reflection characterization of component.

2003, another patent

Imagine Optic turns the square apertures of the Hartmann plate, avoiding cross talk between adjacent sampling points.

Þ It allows to increase resolution and sampling of our HASO EUV Hartmann wavefront sensor operating at 248 nm.

2010, the advent of CMOS

Integration CMOS cameras as detectors for the wavefront sensors in replacement of the good old CCD.

Þ Customers benefit from faster and more sensitive sensors.

2015, broadband sensors

First Imagine Optic broadband Shack-Hartmann wavefront sensor offering absolute calibrated measurement in a wide 350-1100 nm spectral range with the HASO4 BROADBAND.

Þ Imagine Optic sensor precision of lambda/100 is now available for achromatic measurements at any wavelength

2017, instant setup feature

Invention of the SpotTracker  which avoid preliminary alignment procedure to make optical setup easier and quicker.

Þ It brings absolute tilt measurement and follow up over large variations unlike lateral shearing interferometers.

2020, resolution breakthrough

Imagine Optic moves/transposes and optimizes Onera’s experts research on LIFT technology to a commercial off-the-shelf solution

Þ Ultra High Resolution becomes available to Shack-Hartmann wavefront sensors with HASO LIFT 680 with a resolution of 680×504 phase points which is unheard of!

2022, an Optical Engineer Companion for the lab

Launch of the OEC : Imagine Optic makes its wavefront sensors, beam adapter modules, illumination platform, metrology sources interconnectable modules such as LEGO^® bricks.

Þ It provides versatility to create any optical configuration needed for metrology: any beam size, any divergence, any wavelength, transmission and reflection.

Imagine Optic metrology solutions then combines in a flexible optical lab tool that adapt to projects over time and can be upgraded function of future needs.

These innovations are only here to make your metrology more accurate, robust and easier to use. Stay tuned for more info about Shack-Hartmann wavefront sensor.  We are happy to discuss how they suit your needs. Reach us at sales@imagine-optic.com or through the contact form.

Celebrating Nobel Prize: Pierre Agostini, Ferenc Krausz and Anne L’Huillier, Nobel Prize laureates in Physics 2023 for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter

Like every year, the award of the Nobel prizes is a highly anticipated event for the entire scientific community. This year, we are delighted to celebrate the Nobel Prize in Physics 2023, awarded to Pierre Agostini, Ferenc Krausz and Anne L’Huillier. We are particularly proud to have provided support with Philippe Zeitoun (LOA) to Anne L’Huillier, who has used Imagine Optic’s HASO EUV Hartmann wavefront sensor in her groundbreaking research.

Dr. Anne L’Huillier is a physicist working on the interaction between short and intense laser fields with atoms: extremely short light pulses of a few tens or hundreds of attoseconds in the extreme ultraviolet spectral range are used to generate high-order harmonics in gases. She has pushed the boundaries of attosecond source development and its applications such as the study of ultrafast electron dynamics.

For this field of research, our HASO EUV proved one more time to be an indispensable tool for the characterization, adjustment and alignment of such light sources as they provide single-shot, simultaneous detection of amplitude and phase, over a wide spectral range. Check our publication “Single-shot extreme-ultraviolet wavefront measurements of high-order harmonics”, DOI: 10.1364/OE.27.002656, co signed by G. Dovillaire, Imagine Optic CTO, for more insights on the potential of the wavefront sensor!  Celebrating Nobel Prize 

Since Imagine Optic was founded, we’ve been very proud to support the research and groups of several Nobel Prize winners over the years, to whom we’ve supplied adaptive optics solutions:

 + Stephan W. Hell and Eric Betzig, Nobel prize co-laureates in Chemistry 2014 for the development of super-resolved fluorescence microscopy (AO KIT BIO)

 + Gérard Mourou Nobel Prize co-laureate in Physics 2018 for their method of generating high-intensity, ultra-short optical pulses (HASO and ILAO STAR)

Congratulations to Dr. L’Huillier, Dr. Agostini and Dr. Krausz for the Nobel Prize in Physics 2023 and, through them, to the entire scientific research community. Their achievements inspire us to continue serving cutting-edge technologies for the betterment of science and humanity.

plane optics quality control

NIGHTMARE, a rather average 1981 horror movie that yet perfectly recreates the TERROR of an optician confronted with a reflection on the back surface of a sample under test

Windows, prisms, filters, mirrors, crystals…from the (so) precious and fragile thin screens of our cell phones to the large tough airborne pod windows, plane-parallel optics are omnipresent in our daily life and professional applications.

It is therefore no surprising that the optical metrology of these components is of the utmost importance. It is more surprising, however, that the testing of such components, with no power, no complicated shape requiring huge dynamic range such as aspheric or freeform optics, can result in a tricky use case if not a NIGHTMARE. Isn’t it?

That’s why plane optics has been the subject of much effort, frustration, additional costs and technical patches for many years. We reviewed some of them in this white paper [Link to the white paper]. In this blog post, we propose to tell the story of how our production technicians stopped having BAD DREAMS and COLD SWEAT when it comes to characterizing and validating thin plane-parallel optics (glass substrate) before we mount them.

plane optics quality control

Unwanted interference, the TERROR of optical testing

When it comes to perform the metrology of plane optics, it might happen that you get multiple reflections from your sample under test:

– for example, you need to deliver a control sheet to your customer together with the nice protective windows you just produced. Bad news for you, it’s an AR coated, both surfaces are parallel and reflect the same amount of signal back to your instrument, with no way to filter one from another.

– lucky you, you need to characterize a coated mirror, with excellent reflectance at 1030 nm. Sadly, your metrology tool is based on a HeNe laser at 633.8 nm, where the AR coating is no longer reflective: you then get a GHOST signal from the back reflection of your mirror that disturbs the one you are interested in.

plane optics quality control

WAKE UP: the Parallel Optics Procedure (POP) will chase away your BAD DREAMS!

At Imagine Optic, we sell metrology solutions and we are also the first ones to need and use them: that’s one of our motivations to develop and perfect them.

In particular, we need to test the optical quality of glass substrate we implement in our products, custom metrology benches: dichroic, filters, beam splitters, windows, as surface shape and mid-spatial-frequency errors could impact the final performance of the systems.

To this end, we use a proprietary (patented) technique, POP -standing for Parallel Optics Procedure-, that allows for the characterization of large thin plane-parallel optics in two simple steps:

– step 1 > measurement in reflection on the glass substrate: our instrument MESO acquires the combined aberrations of the test beam reflected by the front and back surfaces of the sample

– step 2 > measurement in transmission on the glass substrate: MESO gets the transmitted aberrations of the sample as the test beam pass through the front and back surfaces (in double pass).

From these two measurements, it is possible to retrieve the surface figure error of both surfaces:

Surface Figure Error (SFE) of the front surface (left column) and back surface (right column) of a 200 mm glass substrate characterized before coating with POP, Imagine Optic proprietary Parallel Optics Procedure

Conveniently, the user also obtains the transmitted wavefront error (TWE) as a bonus, with no extra effort! If we filter the low-order aberrations, we can even identify the pattern generated by the polishing tool, as shown in picture below:

Transmitted Wavefront Error (TWE) of the same 200 mm glass substrate characterized before coating with POP, Imagine Optic proprietary Parallel Optics Procedure

Whether or not THERE IS A MONSTER IN YOUR CLOSET, we’ll be happy to discuss with you what solution best suits your needs. Reach us at contact@imagine-optic.com  .

Interferometry and wavefront sensing Illustration: Roger Federer, 20 Grand Slams and Rafa Nadal, 22 Grand Slams still counting

Tom and Jerry, Roger Federer and Rafa Nadal, Peter Pan and Captain Hook, some would say France and England, history is full of ‘meilleurs ennemis’. Could it be that Interferometry and Wavefront Sensing fall in this historical category?

Interferometry has long been adopted as a reference in optical metrology. Generation of interference is as old as optics and its use is therefore well known and widespread. Fringe interference pattern, this pattern of dark and bright stripes is directly linked to the wavelength, which has the good taste of being small in the case of light waves, and therefore represents a very good indicator of small defects of surface shape and flatness or transmitted wavefront.

Wavefront sensing -which refers to the act of measuring aberrations with sensors that do not require an unaberrated reference beam to interfere with- is slightly newer. Although it is easy to argue that it may already have a similar capillary age to Rafa… Shack-Hartmann Wavefront Sensors measure wavefront derivatives by sampling with an array of lenslets and measuring the displacement of centroids they focus on a 2D photon sensor (typically a CMOS camera). Their accuracy is no longer contested and their use in optical metrology, optical system alignment or adaptive optics correction is generalized.

So, what is the current status of the two techniques?

Well, there is no question that interferometry is a standard. Any optical manufacturer will have a Fizeau interferometer or a Twyman-Green interferometer for quality control. However, in some situations, they might not be the best fit. This is precisely where Shack Hartmann wavefront sensors and their new brothers the high resolution LIFT sensors represent a winning alternative.

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When it comes to measuring in the presence of vibrations or atmospheric turbulences, metrology solutions based on Shack-Hartmann or LIFT principle are easily implemented on the shop floor with no need of additional, bulky and expensive equipment.

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If it comes in handy to measure at specific wavelengths, with Shack Hartmann wavefront sensors it is easy to get away from the typical interferometer’s HeNe laser and accommodate with the specificity of the coating -or material- of the optics to be tested.

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For situations where reflections from the back surface of optics creates unwanted interference, Imagine Optic’s patent for (thin) plane parallel optics characterization offers an alternative to the preparation of the substrate with in-house secret recipe coatings. Browse our website to learn more about interferometry and wavefront sensing. Browse our website to learn more about interferometry and wavefront sensing 

Like Rafa Nadal and Roger Federer, both Shack-Hartmann wavefront sensors and Fizeau interferometers have their own field of choice.

Whether or not you enjoy tennis or historical rivalries, we’ll be happy to discuss with you what solution best suits your needs.

Reach us at sales@imagine-optic.com or through the contact form.

NANOLITE, the novel optical metrology platform for extreme UV enters operational phase.

Nanolite, established January 2020, is a joint collaboration and laboratory between the LIDYL CEA laboratory (CEA-CNRS) and Imagine Optic, focusing on innovative optical metrology and imaging solutions at short wavelengths, in particular in the Extreme-UV (EUV, typically between 10 and 100nm) range. Along the Nanolite roadmap, a major milestone is the availability of a novel EUV source providing large photon flux, high stability and beam quality, based on the use of an original laser source. This first milestone has recently been successfully passed.

This high-performance beamline will now serve as a key device to advance the next Nanolite objectives. On top of being an ultra-precise calibration source for current EUV wavefront sensors and future developments, it will enable the development of “at lambda” metrology solutions, in particular for the qualification of X-EUV optics. Such optics, e.g. used in Synchrotron beamlines, ideally require accurate quality control before installation, which is currently not possible with the required level of precision when based on measurements in the visible range. At lambda wavefront sensing in a context approaching its final working conditions will provide both increased accuracy and more relevant results. Moreover, the source will also contribute to the next developments on ultrafast nanometric imaging mainly driven by LIDYL, with applications focused on the study of ultrafast magnetization – a possible key tool to drive the electronics of the future.

By providing their expertise in the characterization and the generation of “made-to-measure wavefronts” Imagine Optic is happy to contribute to the definition of novel metrology solutions in the short wavelengths, on top of our current range of wavefront sensors such as HASO-EUV or HASO HXR. 

Read the full announcement by CEA hereunder (French only).