Optics: measuring and testing – By light interference – Having wavefront division
Reexamination Certificate
2000-06-13
2003-06-10
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having wavefront division
Reexamination Certificate
active
06577403
ABSTRACT:
BACKGROUND OF THE INVENTION
1—Field of the Invention
The invention relates to the analysis of the wavefront of a light beam.
2—Description of the Prior Art
This type of analysis is used to test optical components, to qualify optical devices and to control deformable optical components used in active or adaptive optics. It is also used to study physical phenomena which cannot be measured directly, such as variations of the optical index within turbulent media encountered on passing through the terrestrial atmosphere or in a wind tunnel. It is also used to test the flatness of electronic components, for example matrix focal planes, and for shaping power laser beams.
The type of wavefront analysis to which the invention relates is based on the use of a diffraction grating positioned on the path of the beam to be analyzed.
To make the following description easier to understand, a grating of the above kind is defined as an optical system introducing periodic variations of phase, intensity, or phase and intensity. Any grating is therefore characterized by the multiplication of two functions: the one, called as a phase function, representing periodic variations of phase introduced by the grating and the other one, called as an intensity function, representing periodic variations of intensity introduced by the grating.
French patent application No. 2,712,978 outlines the mode of construction and definition of a two-dimensional grating. A set of points disposed regularly in two directions constitutes a plane meshing. The points define an elementary mesh. The elementary mesh is the smallest surface for paving the plane without gaps in the two directions defining it. The polygon of the elementary mesh is the minimum surface polygon whose sides are supported by the mediatrices of the segments connecting any point of the set to its nearest neighbors. A two-dimensional grating is the intentional repetition of an elementary pattern disposed in accordance with a plain meshing. A plain meshing can define hexagonal or rectangular elementary meshes (square meshes are merely a special case of rectangular meshes).
When a diffraction grating is illuminated with a light beam, called as an incident beam, the light beams diffracted by the grating, called as emergent beams, can be described using two equivalent approaches.
The first approach considers the emergent beams as replicas of the incident beam. They are referred to as sub-beams, each corresponding to one order of diffraction of the grating.
The second approach considers the emergent beams as beams diffracted by each mesh of the grating. They are referred to as secondary beams.
When an intensity function is introduced by a grating, each secondary beam originates from a sub-pupil.
The Hartmann-Shack analyzer is known by the article “PHASE MEASUREMENTS SYSTEMS FOR ADAPTIVE OPTICS”, J. C. WYANT, AGARD Conf. Proc., N
o
300, 1981. The general principle of its operation is optical conjugation of the phase error to be analyzed with an analysis plane containing an array of microlenses which defines a two-dimensional grating made up of a phase function, each microlens delimiting a secondary beam. In the common plane of the foci of the microlenses, referred to as the observation plane, two-dimensional meshing of spots deformed according to the slopes of the wavefront is observed. For active or adaptive optical control applications, the preferred meshing is rectangular. The foregoing description is based on a conventional description of the Hartmann-Shack analyzer and on the approach of decomposition into secondary beams. Another interpretation based on decomposition into sub-beams diffracted by the array of microlenses is outlined in the publication “Variations on a Hartmann theme”, F. Roddier, SPIE, TUXON 1990.
That type of analyzer has the advantage of working in polychromatic light, provided that the path difference error to be detected does not depend on the wavelength. It is very simple to use, consisting of a single optical component, and its optical efficiency is very high. However, its sensitivity and dynamic range can be adjusted only by changing the array of microlenses. In an analysis mode referred to as the undersampled mode it can also be used to analyze the wavefront obtained from low-intensity light sources. This analysis mode uses a small number of microlenses, regardless of the wavefront to be measured, in order to concentrate the low usable flux at a few points where the slope of the wavefront is measured.
French patent application No. 2,712,978, already mentioned, describes a three-wave lateral shear interferometer using a two-dimensional phase and/or intensity grating and a spatial filtering system. Using the approach of decomposition into sub-beams, the grating optically subdivides the incident beam to be analyzed into three sub-beams in a conjugate plane of the error. Particular optical processing of the three sub-beams obtained in this way produces an interferogram consisting of a hexagonal meshing of light spots whose contrast does not vary, regardless of the observation plane used. The interferogram is sensitive to the slopes of the wavefront, and offers the possibility of continuous adjustment of the dynamic range and sensitivity. The observation distance is defined as the distance between the observation plane and the zero sensitivity plane, which is a plane conjugate with the plane of the grating downstream of the spatial filter. In the article “Achromatic three-wave (or more) lateral shearing interferometer”, Journal of Optical Society of America A, volume 12, N
o
12, December 1995, pages 2679-2685, there is an outline description of a modification of the above interferometer toward a four-wave lateral shear interferometer in which the two-dimensional meshing of the light spots observed in the interferogram is rectangular and therefore better suited to active or adaptive optical control applications.
This type of analyzer is achromatic and its luminous efficiency is close to that of the Hartmann-Shack analyzer. On the other hand, it is more complex to use because of the insertion of the spatial filtering system for selecting the sub-beams between the grating and the observation plane of the interference fringe system. Also, the spatial filtering system imposes limitations on measuring light beams with a high level of interference or a very large bandwidth. It therefore cannot use the undersampled analysis mode referred to in connection with the Hartmann-Shack analyzer.
It would therefore appear to be strongly desirable to have an interferometer combining the simplicity of use and operating capacity of the Hartmann-Shack analyzer, from low-intensity light sources with high levels of interference or very large bandwidth, and the flexible adjustment of the dynamic range of the three-wave lateral shear interferometer described in French patent application No. 2,712,978 or the four-wave lateral shear interferometer outlined in the article “Achromatic three-wave (or more) lateral shearing interferometer”.
OBJECT OF THE INVENTION
The object of the present invention is to make progress in this direction.
SUMMARY OF THE INVENTION
The invention can be considered in the form of a method or a system.
The method of analyzing the wavefront of a light beam according to the invention is of the type wherein a two-dimensional diffraction grating with rectangular meshing is placed in a plane which is perpendicular to the light beam to be analyzed and which is optically conjugated with a plane of analysis of the wavefront, thereby multiplying an intensity function by a phase function and causing through these functions the beam to be diffracted into different beams emergent from the grating. The intensity function defines a rectangular meshing of sub-pupils in the two-dimensional meshing transmitting the light from the light beam to be analyzed to form a plurality of secondary beams disposed in accordance with the rectangular meshing. The phase function introduces a phase shift between two adjacent secondary beams such that the two adjacent sec
Guerineau Nicolas
Primot Jerome
Laubscher, Sr. Lawrence E.
ONERA (Office National d'Etudes et de Recherches Aerospatia
Turner Samuel A.
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