Double-Path Wavefront Sensing and Interferometer: Comparing Approaches to Optical Surface Measurement 

An interferometer is a phase-sensitive optical instrument that converts differences in optical path length (OPL) into measurable intensity variations through the principle of interference. Its operation is fundamentally based on the superposition of coherent electromagnetic waves.

Fizeau Interferometer: Physical Principle

For a monochromatic plane wave, the electric field can be expressed as:

E(r,t) = E0 *cos(kr – ωt + φ)

When two coherent wave E1 and E2 overlap, the detected intensity is proportional to the time-averaged square of the total field:

This expands to:

where Δϕ is the phase difference between the two beams. The phase difference is directly related to the optical path length difference (OPD):

Δϕ= 2π/ λ*Δ(OPL)

with:

OPL = n*L 

where n is the refractive index and L is the geometric path length.

Thus, a path length change of only a fraction of the wavelength produces measurable fringe shifts. For visible light (λ ≈ 633 nm), a phase shift of 2π corresponds to a path change of one wavelength, enabling sub-nanometer displacement sensitivity through phase interpolation techniques. 

Here is a schematic of the Fizeau interferometer, one of the classical interferometers used in optical surface testing: interference occurs between a reference surface and the test surface.

Fig 1: Scheme of a Fizeau interferometer

Its main advantage is its high resolution. However, it has limited dynamic range: while it detects very small variations with high sensitivity, it struggles with large surface errors or steep features due to phase ambiguity. It is also highly sensitive to vibration which blurs the interference pattern and temperature changes, often requiring controlled environments.

A common implementation is the Fizeau interferometer, widely used for testing optical flats and mirrors because of its stability and high accuracy in surface figure measurements.

While interferometers remain the standard for sub-nanometer metrology, high performance and factory calibrated Shack–Hartmann wavefront sensors (SHWFS) typically achieve ~3 nm height resolution. The SHWFS-based system offers three advantages over single-wavelength interferometry: larger dynamic range, reduced sensitivity to environmental noise, and faster, simpler operation suitable for industrial settings.

Double-Path Wavefront Sensing Solution: MESO system

• Multi-wavelength wavefront sensing

• Tolerance to air turbulence and vibrations

• Overcoming back surface reflection artifacts

• Double surface characterization with one click

• Optical zoom maintains full resolution across 1.5–6″ optics

MESO is a compact, effortless metrology system for plane-parallel optics and wavefront sensing based on Shack-Hartmann technology, providing a robust alternative to traditional interferometry. Unlike conventional wavefront sensors, which require an external beam, MESO integrates illumination in a double-pass configuration, allowing the wavefront to be measured after propagation through the optical element. A microlens array measures wavefront slopes from centroid positions to reconstruct a full phase map, while the advanced LIFT (Linearized Focal Plane Technique) enhances resolution up to 16× by also analyzing centroid intensity distribution. With single-frame acquisition (~20 ms), strong vibration resistance, and a very short optical path, MESO delivers fast, stable, and high-precision optical measurements in demanding environments.

Fig 2: Scheme of the MESO solution

In this blog, we will highlight the main advantages of MESO double path wavefront sensing compared to traditional Fizeau interferometry, illustrated with live experimental results conducted by Imagine Optic France.

MESO’s Core Strength: Multi-Wavelength Wavefront Sensing

The first and most fundamental advantage is the multi-wavelength measurement capability of the MESO system. 

 Fig 3: MESO – wavefront metrology system operating live at a trade show, measuring the surface of a flat window, no optical table needed

MESO’s interference-free measurement principle eliminates the need for long-coherence light sources, enabling the use of multiple compact and cost-efficient light sources -available at almost any wavelength of the spectrum- within a single system (up to four wavelengths). 

This flexibility allows users to perform measurements at the exact wavelength for which their optical components were designed, avoiding assumptions about spectral behavior.

It also provides adaptability of the instrument to the spectral characteristics of the object under test: one of MESO’s significant advantages is its ability to characterize coated flat optics such as dichroic components. Instead of struggling with weak reflections from certain surfaces, users can simply select the wavelength best suited for the measurement. For example, one wavelength can be used to measure the transmitted wavefront error (TWE), while another can measure the reflected wavefront error (RWE), all with a simple click.

MESO supports this multi-wavelength testing capability thanks to an implementation providing an achromatic response, operating from 405 nm to 1064 nm. In the example below, several wavefront measurements were acquired on the same component using three different wavelengths (402 nm, 635 nm, and 785 nm) without any system realignment. The results demonstrate excellent consistency across all measurements.

Fig 4: Similar surface measurements acquired at 3 different wavelengths with MESO showing the achromatic behavior of the system.

Vibration? Air Turbulence? SHWFS Handles It, Interferometers struggle

Air turbulence introduces random fluctuations in the refractive index, causing tiny but rapid changes in the optical path. Interferometers, which rely on a stable phase across the entire aperture, are highly sensitive to these fluctuations: if the optical path difference shifts by even a fraction of a wavelength, interference fringes wash out, and any vibration or airflow can destroy coherence.

Wavefront sensing–based solutions, such as Shack–Hartmann wavefront sensing, are well suited for operation in hostile environments, including those affected by vibrations or turbulence generated by external sources such as motors, pumps, blowers, or even personnel.

Unlike many interferometric techniques (like Fizeau), these disturbances do not corrupt the raw signal that must be analyzed. This robustness comes from the extremely fast measurement cycle of wavefront sensors, which can be on the order of tens of microseconds (e.g., ~20 µs). Because each measurement is captured so quickly, the sensor effectively “freezes” the instantaneous wavefront, preventing environmental perturbations from significantly degrading the raw data.

While turbulence or vibrations may still introduce temporary effects, such as a small tilt component in the measured wavefront, the measurement itself remains valid. These fluctuations can then be mitigated by averaging multiple acquisitions, allowing the noise contributions from environmental disturbances to be reduced while preserving the underlying optical information.

As an example, the testing of a mirror in a vacuum chamber with the MESO instrument is reported.

Fig 5:  Zernike coefficients: observation of high order aberrations before and after vacuum is established in the vacuum chamber where the mirror is placed.

Fig.6 Reflected wavefronts of the mirror in the vacuum chamber before (left, at time A) and after (right, at time B) the pump is switched ON.

As expected from the Zernike plot, both wavefront maps match perfectly, as the wavefront error is 87 nm RMS with pump off and 84 nm RMS with pump on. This corresponds to a difference of 3 nm, or λ/170, situated well within the accuracy and repeatability of the instrument.

As explained, RAW signal integrity is not compromised by vibrations as centroid size and intensity distribution are similar whether the pump is off or on.

Fig. 7 Centroids corresponding to the Reflected Wavefronts of the mirror in the vacuum chamber before (left, at time A) and after (right at time B) the pump is switched On.

MESO: Solving Back-Surface Reflection in Interferometry

Speaking with real data, as well known parallel optics represent a challenging type of optics for many of the established optical metrology solutions, such as laser interferometers. 

Unwanted back surface reflection participates in the interference pattern.

  Fig.8 Interferogram of a plane parallel optics with 3-beam interference due to reflection from back surface, creating artifacts in the reconstructed phase map (from Zygo youtube channel ‘Overcoming challenges of measuring Plane Parallel Optics’) 

Examples of the Optical Metrology Approaches Referenced Above:

• Laser interferometry: provides high-precision surface measurements, but struggles with parallel optics due to fringe ambiguity and strong sensitivity to alignment errors. 

• Multi-wavelength / tunable interferometry: resolves phase ambiguity by combining multiple wavelengths, but increases system complexity and sensitivity to environmental noise. 

• Deflectometry: measures surface slopes instead of height, reducing fringe issues; however, it requires complex reconstruction and has limited absolute height accuracy. 

• Scanning probe methods: deliver very high accuracy and direct surface measurement, but are slow and not suitable for large-area optics. 

• Optical coherence–based techniques: improve depth discrimination and reduce ambiguity, but come with higher cost and limited lateral resolution. 

• Confocal microscopy: enables depth-resolved measurements with good vertical resolution, but has limited field of view and slower acquisition for large surfaces.

The “Click-and-that’s-it” Wavefront Sensing Solution

Wavefront sensing can be an effective solution for the characterization of parallel optics thanks to a new feature POP (Plane Parallel Optics) developed by Imagine Optic. It consists of a new patented method which takes advantage of and uses the back reflection from the second surface of the optics instead of trying to cancel it out. The procedure is described as follows (see fig. below): first, a measurement is performed, in reflection, on the optics to be tested (which has already been aligned in double pass). The signal (centroids) acquired is a combination of light reflected by the first and the second surface of the parallel optics. This is not a problem because the wavefront sensor uses a temporally incoherent source, so there is no generation of destructive interference that would compromise the integrity of the signal. Then, a second measurement is conducted in transmission and in double-pass using a flat reference mirror. By combining the two measurements and assuming the homogeneity of the substrate, it is possible to retrieve the profile of the front and rear surfaces (RW) and the transmitted wavefront (TW, which corresponds directly to the second measurement). There is no need to move the sample between the two measurements, avoiding the risk of differential aberrations coming from the handling (inversion) of the sample. It also has the advantage that the different acquisitions are taken with the same referential, avoiding the need for fiducials to register the different measurements.

Fig 9. two-steps method to characterize (plane) parallel optics with MESO instrument

MINI MESO Solution for Large Curvature Surface Measurements 

The MINI MESO offers a dynamic curvature range from approximately 0.008 m to infinity (plane wave). This wide dynamic range, enabled by compatible wavefront sensors, allows the system to measure not only plane-parallel optics but also strongly curved surfaces.

In conventional interferometry, highly curved optics present significant challenges. As the curvature increases, the number of interference fringes becomes very dense. This leads to fringe crowding, reduced fringe contrast, and increased difficulty in phase unwrapping and reconstruction. In extreme cases, the fringe density exceeds the spatial resolution of the detector, making accurate phase retrieval unreliable or even impossible.

By contrast, MINI MESO does not rely on fringe density for phase reconstruction. Instead, it directly captures the wavefront slope/shape within its dynamic range, enabling robust measurements even for steep or strongly curved surfaces. This eliminates the fringe crowding limitation inherent to interferometric methods and significantly simplifies alignment and data processing.

Additionally, the MINI MESO is a very compact and lightweight setup. This is because it does not require a large collimated beam at the input, eliminating the need for long focal length optics. As a result, the overall system footprint is significantly reduced while maintaining high measurement capability.

Conclusions

The MESO system utilizes a double-path wavefront sensing configuration to deliver measurement reliability and precision that has been proven through actual experimental data. This performance is achieved by combining multi-wavelength wavefront sensing, robust performance against vibrations and air turbulence, and the ability to overcome back-surface reflection artifacts. Flexible measurement distances and seamless double-surface characterization with a single click further simplifies complex optical testing. These capabilities, validated by real-world measurements, place this platform at the forefront of optical metrology, providing a powerful, versatile, and user-friendly solution that elevates performance well beyond conventional interferometry.

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