Diffration Based Alignment for Lasers

An innovative diffraction-based alignment system to achieve beam alignment through a laser system with increased accuracy and flexibility.

 

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DESCRIPTION

 

The use of alignment references is common practice to assist in the building and in the day-to-day alignment process of laser chains. Traditionally, alignment references for large-aperture laser chains consist of cross-hairs of adequate size placed in the object planes of the beam diagnostics system in order to achieve a sharp image of cross-hairs on the diagnostic camera sensor (image-relying configuration). Two or more alignment references are placed in different locations along the beam path in such a way that, if the laser chain is properly aligned, the images of alignment references overlap on the camera sensor. If the laser chain is not aligned, appropriate optical elements are adjusted in order to achieve reference image overlap. The main drawback of this system is represented by a strong limitation both on possible locations where the alignment references can be placed and on where the inspection of the beam can be carried out. Indeed, if alignment references are not placed at the object planes of the diagnostic system, a blurred image surrounded by a diffraction pattern is formed on camera sensors, thus preventing an accurate alignment process. For the same reason, if beam inspection is not carried out at the image plane, the images of references become blurred. It is also worth noting that nowadays many laser chains rely on spatial filters to remove random fluctuations from the beam intensity profile. In this case, alignment carried out using prior art alignment references becomes less accurate, since the high-frequency filtering performed by pinholes causes blurring in the cross-hair image.

 

This new system developed at STFC overcomes all issues stated above by deliberately placing alignment references, characterised by an appropriately designed shape, away from the object planes of the beam diagnostics system. The alignment of the laser chain is checked by using the resulting superposition of diffraction patterns instead of the image of the alignment references (diffraction-based configuration). As a result, this alignment apparatus is no more limited by image-relying requirements, thus greatly increasing the number of possible locations in which alignment references can be placed and allowing more flexibility on beam diagnostics and visual inspection of the beam. The shape of diffraction-based alignment references has been designed in order to achieve optimised diffraction patterns. If the laser chain is aligned, the resulting superposition of diffraction patterns is symmetrical and characterised by distinctive features. As soon as misalignment occurs, a break in the symmetry pattern becomes clearly visible and the aforementioned distinctive features suddenly disappear, thus improving the accuracy of the alignment process compared to traditional methods. If alignment has to be carried out in laser chains containing spatial filters, the feature size of references can be optimised according to the diameter of pinholes in order to avoid degradation in alignment accuracy. This method also allows controlling the beam near-field position with unprecedented accuracy.

 

KEY FEATURES

 

•       The exploitation of diffraction patterns created by innovative alignment references, rather than the images of alignment cross-hairs.

•       An increase of possible locations in which references can be positioned and in which the laser beam can be inspected, higher accuracy in beam alignment and near-field positioning, alignment of laser chains employing spatial filters without degradation in alignment accuracy.

•       A high degree of flexibility, enabling its application in a wide range of high-aperture laser chains.

 

DOMAIN OF APPLICATION

 

Scientific and Industrial Lasers

 

Patent Information:
Country Serial No.
United Kingdom 1604940.5
Patent Cooperation Treaty PCT/GB2017/050603
For Information, Contact:
Liam Brennan
STFC Innovations
liam.brennan@stfc.ac.uk
Keywords:
03. OTHER INDUSTRIAL TECHNOLOGIES
03.c. Apparatus Engineering
05.f. Physics
09. MEASUREMENTS AND STANDARDS