QWLSI: a powerful wavefront technology overcoming limitations of Shack-Hartmann wavefront sensors and Fizeau interferometers

What is a wavefront sensor ?

A wavefront sensor is a device designed to perform measurements of optical wavefronts. The term "wavefront sensor" applies to wavefront measurement instruments that do not require any reference beam to interfere with such as Fizeau or Twyman-Green interferometers. Shack-Hartmann and lateral shearing interferometry are the two major technologies used to manufacture wavefront sensors.

Wavefront sensors are used in a large range of applications such as optics testing and alignment (surface measurement, transmitted wavefront error measurement, modulation transfer function …), laser and optical systems qualification and control with adaptive optics, and material inspection, as well as quantitative phase imaging. Most photonics-related industries can benefit from wavefront sensing. Visit our application markets page and learn more about the broad range of uses of wavefront sensors for each industry.

In this page, you will find more information about Phasics' patented wavefront sensing technology and how it compares with other wavefront measuring devices such as Shack-Hartmann wavefront sensors and Fizeau interferometers.

QWLSI a unique and patented wavefront sensing technology

Quadriwave lateral shearing interferometry (QWLSI), also known as modified Hartmann mask technique, is a patented wavefront sensing technology. It stands out for its high spatial resolution, its ability to measure diverging beams with no relay lens and its achromaticity. This technology was introduced on the market in 2004 by Phasics and is now internationally recognized for its performance and ease of integration.

Phasics quadriwave lateral shearing interferometer called SID4 wavefront sensor with an interferogram in the background

The latest innovation in wavefront sensing: QWLSI is a powerful alternative to Shack-Hartmann and Fizeau interferometry

QWLSI technology was developed to overcome the lack of resolution of the Shack-Hartmann (SH) technique. It uses a smart diffractive grating design instead of the holes used in the Hartmann test and the microlenses proposed by Shack in the 1960s , which led to the Shack-Hartmann wavefront sensor.

Quadriwave lateral shearing interferometry (QWLSI) offers

  • High resolution: 4x more than Shack-Hartmann
  • Achromaticity in the entire spectral range of the detector
  • Large dynamics allowing direct measurement of diverging beams
  • No reference arm unlike classical Fizeau interferometry
  • Compactness and ease of implementation
Timeline representation of the evolution of wavefront sensing technologies - Hartmann test  (1880), Shack-Hartmann 1960), Phasics QWLSI (2000)

Quadriwave lateral shearing interferometry (QWLSI principle)

Quadriwave lateral shearing interferometry measures the phase and intensity of a beam with nanometric sensitivity and very high spatial resolution. This innovative technology relies on a diffractive grating that replicates the incident beam into 4 identical waves. After a few millimeters of propagation, the 4 replicas overlap and interfere, creating an interferogram on the detector. The phase gradients are encoded in the interference fringe deformation.

iconepublications    Visit our publication page

Schematic of the principle of Phasics quadriwave lateral shearing interferometry wavefront sensing technology.

Wavefront sensing measurements technologies comparison: Shack-Hartmann vs Fizeau interferometry vs QWLSI

Phasics QWLSI wavefront sening technology vs Shack Hartmann vs Fizeau

What is a Shack-Hartmann wavefront sensors (SHWFS) ?

Shack-Hartmann wavefront sensors are based on 2 main components: an array of microlenses and an image sensor located at the focal plane of the microlenses array. The Shack-Hartmann operation principle is to track the position of the focal spots on the detector. When an incoming wavefront is plane, all focal spot images are located in a regular grid defined by the lenslet array geometry. As soon as the wavefront contains aberration, the images of the focal spots become displaced from their initial positions.

Shack-Hartmann technology's strengths are the compactness and light reconstruction algorithms that allow high-speed wavefront reconstruction. Also Shack-Hartmann are self-referenced and thus insensitive to environment perturbation.

Shack-Hartmann wavefront sensors' limitations are related to the microlenses: first, the spatial resolution is determined by the size and number of the microlenses (limited to 100 x 100 measurement points with hundreds of microns of spatial resolution); second, the microlenses are chromatic, which means the Shack-Hartmann wavefront sensor has to be optimized at a certain wavelength that will define the detector-microlenses distance. When the Shack-Hartmann wavefront sensor is used at a different wavelength, the accuracy of the measurement decreases. Last, in case of strong wavefront distortion, there is a risk that the focal spots end up outside of the detector area (in case of strong curvature) or associated to the wrong microlens (cross-talk) leading to measurement errors.

What is a Fizeau interferometer ?

The Fizeau interferometer measurement principle is based on an interferometric arrangement that relies on interference phenomenon between 1) a reference surface (reference mirror or reference sphere) and 2) a test surface. Interference fringes are recorded on a detector and any deviations between the surface shapes lead to distortions of the fringes. From the fringe distortions, the wavefront is then computed.

The advantages of Fizeau interferometers are the very high resolution that is only limited by the detector, and the high wavefront sensitivity.

The main weakness of Fizeau interferometers is the limited dynamic range of the measurement; with large wavefront departures, stitching methods or computed generated hologram (CGH) integration must be implemented. Also, Fizeau interferometers' lack flexibility regarding the test wavelength, as it is limited to the integrated laser. Finally, the Fizeau interferometer tends to be bulkier and more sensitive to the environment than self-referenced wavefront sensors.