Revolution in Optical Detection: From the "Delicate" Reference Mirror to the "Robust" Four-Wavefront Shearing Interferometry

Introduction:

How to accurately measure the 'wavefront' shape of a beam of light is a core issue that determines imaging, laser processing, and astronomical observation. Traditional interferometers rely on reference mirrors, are sensitive to vibrations, and have bulky structures. The new generation of four-wavefront lateral shearing interferometry technology, with the advantages of 'common optical path, no reference, and anti-interference', is bringing high-precision wavefront detection out of the laboratory and embracing broader application scenarios.

Part 1: What is a wavefront sensor? Why is it important?

1. Wavefront: The 'shape' of light

Light is not only a particle that propagates in a straight line, but also a wave. When an ideal light wave (such as a plane wave or spherical wave) passes through media such as lenses, air disturbances, or surface imperfections of optical components, its equiphase surface undergoes distortion - just like ripples on a calm lake surface after a stone is thrown in. This equiphase surface is called the 'wavefront'. Measuring the shape of this wavefront is known as 'wavefront sensing'.

II. Why do we need a wavefront sensor?

Wavefront distortion can directly lead to blurred imaging, reduced laser focusing, and increased measurement errors. The task of a wavefront sensor is to quantify the distortion and provide a basis for correction and quality control.

III. 'Troubles' of traditional wavefront detection

Before the emergence of wavefront sensors, the most commonly used method was interferometry, such as the Twyman-Green interferometer and the Fizeau interferometer.

Main limitations:

◆ Dependence on reference surface: The error of the reference mirror itself will directly affect the measurement results, and it is extremely difficult to manufacture an absolutely ideal reference mirror.

◆ Sensitive to the environment: Requires stringent conditions such as vibration isolation tables, constant temperature and humidity, making it difficult to apply in workshop settings.

◆ Complex structure and large size: not suitable for integration into compact systems.

Part 2: Four-wavefront lateral shearing interferometry - a new generation of wavefront detection technology

Ⅳ. Lateral shear interference: light beam 'compares itself with itself'

The wavefront to be measured is divided into two beams, which overlap and interfere after being translated in the vertical direction. Both beams originate from the same wavefront, eliminating the need for a reference mirror and ensuring a common optical path, thus providing strong anti-interference capability.

Advantages: no reference surface error, vibration resistance, compact structure.

Limitation: Traditional methods can only obtain information in a single direction, requiring two collections.

Schematic diagram of transverse shear interference principle

Ⅴ.Four wavefront transverse shear interferometry: one imaging, two directions

Scientists have introduced a two-dimensional grating to simultaneously generate ± 1st order diffracted light in the X and Y directions (a total of four beams), which overlap and interfere with each other during propagation,

A composite interferogram obtained from a single imaging with bi-directional shear information.

Core advantages: single collection, complete information, and good real-time performance.

Ⅵ. From improving Hartmann templates to randomly encoding hybrid gratings

Early improvements to Hartmann templates (MHM) resulted in higher-order diffraction and Talbot effects, which limited the flexibility of the system.

Our FIS4 wavefront sensor uses a Random Encoding Hybrid Grating (REHG):

◆Almost eliminate unnecessary diffraction orders, with high signal-to-noise ratio

◆ No Talbot effect, continuously adjustable shear rate

◆ Flexible setting of sensitivity and dynamic range

Schematic diagram of the optical path of a randomly encoded hybrid grating four wavefront transverse shear interferometer

Ⅶ. The core value of four wavefront shear interferometry

◆ Common optical path, self interference: No need for reference mirror, anti vibration and anti-interference, can work without isolation table.

◆Single collection of bidirectional information: suitable for dynamic process measurement.

◆Extremely compact structure: The core components are only one grating and one camera, with a small volume.

◆Large dynamic range: By adjusting the shear rate, wavefront distortion can be measured from nanometer to hundreds of micrometers.

Four-wavefront lateral shearing interferometry sensor

Conclusion

From relying on reference mirrors to self interference, from unidirectional to bidirectional simultaneous, wavefront detection technology continues to break through. Random Encoding Hybrid Grating (REHG) pushes four wavefront shearing interferometry to new heights - purer diffraction, more flexible adjustment, and wider application scenarios.