Optics: measuring and testing – Lamp beam direction or pattern
Reexamination Certificate
2001-11-02
2004-06-15
Adams, Russell (Department: 2851)
Optics: measuring and testing
Lamp beam direction or pattern
C356S123000
Reexamination Certificate
active
06750957
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and a device for analysing an optical wavefront. It represents an improvement of the methods of wavefront analysis based on local measurement of the slope of the wavefront.
2. Description of the Related Arts
Analysis of a wavefront by local measurement of the slope (corresponding to the local derivative of the phase of the wavefront) is for example the principle of wavefront analysers called “Shack-Hartmann array” wavefront analysers. Generally they have an array of spherical microlenses and a array detector, each microlens focusing the surface element of the wavefront intercepted by the subaperture corresponding to the microlens, thus forming a light spot on the detector. The local slope of the surface element is determined from the position of the spot on the detector. Actual analysis of the wavefront surface, i.e. reconstruction of the phase of the wavefront for example on a base of polynomials, can be obtained by integration of the local measurements of the slope. Other types of analysers work on a line of the wavefront. Cylindrical microlenses arranged linearly and a detector with linear geometry are, for example, used in this case. In the same way as in the Shack-Hartmann array type, the local slopes of the wave line are measured from the positions of the spots formed by the microlenses.
SUMMARY OF THE INVENTION
Generally, the method according to the invention applies to any type of wavefront analysers based on measuring the local slope of the wavefront. The term “array of microlenses” will be used hereinafter for any set of microlenses for use in this type of analyser, it being possible to arrange the microlenses linearly or according to a two-dimensional array. Similarly, we shall talk of analysis of a “wavefront”, and this analysis can relate equally to a part of the surface of the wavefront, in particular a line of the wavefront or the complete surface of the wavefront.
FIG. 1
shows an assembly ML of microlenses L
i
and a detector DET for implementing a method of wavefront analysis as described above. When a wavefront F
1
enters the system, each microlens forms a spot T
i
on the detector. To determine the position of the spots, generally it is assumed that a spot T
i
formed by a given microlens L
i
is within an assumed localization zone Z
i
. This localization zone is for example defined by the projection on detector DET of the subaperture SP
i
corresponding to the microlens L
i
, as shown in FIG.
1
. This assumption offers the advantage of considerably simplifying the circuit for localization of the spots, thus making the system faster. Sometimes the structure of the array of microlenses is not perfect and may have local defects, for example defects in arrangement of the microlenses or defects relating to the size of one microlens relative to another. This introduces an error in the position of the spot formed. To overcome this type of problem, generally the positions of the spots formed from a reference beam that is known perfectly are subtracted from the positions of the spots formed from the wavefront to be analysed. Of course, to avoid introducing any error during this operation, it is necessary for the positions of the two spots formed by the same microlens to be subtracted from one another. If it is assumed a priori that a spot detected in a given localization zone has come from the subaperture that defines this zone, there is a risk of introducing an error during the subtraction operation when a wavefront has a considerable deflection, for example. Thus, as can be seen for example in
FIG. 1
, if a wavefront F
2
has considerable deflection, the spot T
i
formed by lens L
i
is within the assumed localization zone Z
i+1
corresponding to lens L
i+1
. There is a displacement of a subaperture (in the chosen example) between the subaperture SP
i
from which spot T
i
originates and the subaperture Sp
i+1
defining the localization zone Z
i+1
in which the spot T
i
is actually located.
Of course, we always try to obtain perfect arrays of microlenses and the technology is advancing in this direction. However, the problem of knowing with certainty the correspondence between a detected spot and the subaperture from which it originated always arises, for example when we require exact measurement of the deflection using a device that is required to have a wide dynamic range, i.e. a device capable of analysing wavefronts possessing large deflections, among other things. In this case, to know this correspondence with certainty, it is necessary to be able to measure the displacement between the subaperture from which the spot originated and the subaperture that defines the assumed localization zone in which the spot is located.
A solution has been proposed in this direction by the company Adaptive Optics Associates (AOA, Cambridge, Mass.). This solution, applied to a wavefront analyser of the Shack-Hartmann array type, is explained in the article “Hartmann sensors detect optical fabrication errors” (LASER FOCUS WORLD, April 1996). It consists, in the course of measurement, of bringing the detector of the array of microlenses closer, in such a way that, regardless of the local slope of the wavefront being analysed, all the flux collected by a subaperture is located totally within the assumed localization zone defined by this subaperture. Then the detector is moved farther away from the array of microlenses as far as its normal working position while following the position of the spot. It is thus possible to detect whether it changes zone. This solution has some drawbacks. In particular, it necessitates movement of the detector, which involves mechanical constraints in the system and the risk of introducing an error in the measurement, because of possible deflection of the detector, or poor axial repositioning during movement. Furthermore, this calibration operation must be repeated for each analysis of a new wavefront. And even in the course of analysis of a wavefront, since the correspondence between a spot and the microlens from which it originated is determined by following the position of this spot, if this position is lost (for example because the flux is cut off momentarily), the correspondence is no longer certain and recalibration becomes necessary.
To overcome these drawbacks, the present invention proposes another solution permitting exact measurement of the parameters of the wavefront and in particular of its deflection. It consists of choosing an array of microlenses having one or more local variations of its structure. According to one example of implementation, each local variation can be a difference in positioning of one or of several microlenses. This variation can be an unwanted defect of the array or a local variation introduced in a controlled manner during manufacture. Comparing the positions of the spots formed starting from a wavefront to be analysed with the positions of the spots formed for example starting from a known reference wavefront, it is possible, owing to the presence of the local variation of structure which for example introduces variations in the positions of certain spots, to measure any displacement between the subaperture from which a detected spot originated and the subaperture that defines the assumed localization zone in which the spot is located.
More specifically, the invention relates to a method of wavefront analysis based on local measurement of the slope of the wavefront, the method comprising a stage of wavefront acquisition consisting of:
a stage of detection of the wavefront especially by means of an array of microlenses, a detector and means for processing the signal; each microlens defines an indexed subaperture, and focuses a surface element of the wavefront, intercepted by the said subaperture; a spot is formed on the detector which delivers a signal; an assumed localization zone of the spot on the detector is defined for each subaperture.
a stage of processing of the signal supplied by the d
Bucourt Samuel Henri
Levecq Xavier Jean-Francois
Adams Russell
Esplin D. Ben
Imagine Optic
Young & Thompson
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