Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1998-11-13
2001-05-22
Chang, Audrey (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S565000, C359S569000, C359S559000, C359S016000, C359S742000
Reexamination Certificate
active
06236509
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the domain of laser cutting, particularly in view of dismantling and/or disassembly operations.
2. Discussion of the Background
The advantage of using laser techniques for carrying out dismantling operations is that it provides some process flexibility (possibility of remote cutting and variation of the cutting distance), and large potential benefits in terms of secondary cutting waste (less aerosols, less swarf, smaller cutting widths).
However, very few of these systems have been used, for three main reasons:
the most powerful laser sources, particularly CO
2
sources, require beam transport by mirrors since the wave length of CO
2
sources is incompatible with materials used for power optic fibers. These mirrors enable the necessary movements for cutting. The resulting mechanism is very complicated, particularly when it is necessary to cross confinements and biological shielding in nuclear installations.
the power of sources producing a beam that can be transported by optical fibers (for example Nd:YAG) did not exceed one kW until the last few years. Furthermore, the use of the optical fiber transmission technique causes problems which will be described below,
traditional laser cutting processes require the use of assistance gases to make cutting feasible (flushing of molten material, protection of optics), which consequently requires that the part to be cut should be followed very closely (at about 1 to 2 mm).
These constraints have made it very difficult to use lasers in cutting for dismantling for which confinement is compulsory, it is often necessary to pass through biological shielding, and proximity following is practically impossible since the geometry of objects to be cut is complex, not well known (since it is often difficult to measure, particularly in a nuclear environment) and very variable.
The use of optical fibers has also created a number of problems.
The emerging beam is disturbed due to its path, particularly at the exit from the fiber; for example, a beam with a Gaussian distribution about its center line has an annular shape at the exit from a conventional step-index fiber. Consequently, the maximum energy distribution is eccentric, and cutting performances are reduced.
For focal length of the order of one meter, beam disturbances can be corrected using conventional glass lenses, for example type BK7. These lenses are transparent to infrared (wave length 1.06 &mgr;m) emitted by an Nd:YAG laser. But the weight of a device based on this technology and for this focal length is around 15 to 20 kg. Furthermore, this type of assembly is as delicate as a telescope with the same aperture (20 cm) and, for example, must be protected from shocks that would modify the settings and affect operational safety.
Consequently, remote servocontrol of the focal length (zoom) necessary for cutting parts with complex geometry is practically impossible using conventional glass mirrors or lenses with reasonable size, fragility and weight parameters. Therefore, it is not really feasible to achieve focal lengths exceeding a meter, for laser cutting for dismantling using conventional optical systems that are more suitable for use in the laboratory and/or for short focal lengths (less than one meter) than for cutting on nuclear sites at long distances.
The use of mirrors to manufacture focusing lenses would further increase the size for an equivalent weight and fragility, taking account particularly of stiffening devices and mirror mountings.
SUMMARY OF THE INVENTION
The first objective of the invention is an optical focusing device with limited size and weight, which is fairly robust and is compatible with the use of a power laser, particularly for use in a laser cutting device. The optical focusing device must enable correction of defects caused by an optical laser beam transmission fiber.
The term “Fourier diffractive element” used below refers to a diffractive element that diffracts and divides an incident planewave into n planewaves. As is well known in the art, the mathematical relation between the complex amplitude transmission of a far field diffractive element and its diffraction pattern results from a Fourier transformation. Furthermore and as discussed below, the design algorithms for the diffractive element includes an iterative Fourier transformation. Consequently, persons of ordinary skill in the art often refer to such diffractive elements as “Fourier diffractive elements.”
More precisely, the first purpose of the invention is an optical laser beam focusing device, comprising:
a Fourier diffractive element that is capable of separating an incident beam into n beams along n directions symmetric about an optical axis of the device,
a diffractive element comprising n Fresnel lenses, capable of refocusing the n beams on the optical axis.
The advantages related to this device include the following.
The diffractive optical system corrects beam aberrations resulting from the fact that the beam was transported by an optic fiber. This correction may be made using a Fourier lens or Fresnel lenses, or partially corrected using both types of lenses.
This device is capable of synthesizing a large aperture using small components. It is a Fourier-Fresnel type assembly with a synthetic aperture, which separates the incident beam into n identical beams (using a Fourier diffractive element) and focuses these n beam portions (using n Fresnel lenses) and consequently replaces a lens with a large aperture, and therefore a large size in general, by a smaller assembly.
From the manufacturing point of view, the problem of manufacturing a large Fresnel diffractive component (100 mm minimal diameter) is reduced to the problem of machining n smaller components.
Furthermore, it is easy to use this type of device. The focal length is varied simply by moving one of the components parallel to the other. Since the chosen Fresnel-Fourier formula is not critical, their respective positions may be controlled with mechanically simple means.
This type of device is adapted to use with a power laser. Furthermore, it forms an optical component which is not very sensitive to environmental disturbances (vibrations, shocks, dust, etc.) and is therefore suitable for use on site, for example for a laser dismantling installation. Finally, this device may easily be replaced, for example in the form of modules; no critical optical positioning or realignment is necessary during the replacement.
The two elements (Fourier diffractive element and all Fresnel elements) may be laid out such that the device operates in transmission or in reflection. In the latter case, the two elements are laid out such that an incident beam is reflected by the Fourier diffractive element and broken down into n beams, each of these beams then being reflected by one of the Fresnel lenses towards the focus point.
Means, for example such as refractive elements, may also be provided for collimating the incident beam.
Another purpose of the invention is a device for creating focused laser radiation, comprising means of generating a laser beam and a focusing device like that described above.
The result is an assembly that operates in a fairly simple manner, with all the advantages described above in relation to the focusing device.
In particular, if the beam is transmitted by optical fiber at the exit from the laser, the focusing device can correct aberrations related to transport by optical fiber.
Finally, another purpose of the invention is a device for laser cutting, comprising a device for generating focused laser radiation as described above.
This focusing lens can be built into a device that can be used on a cutting site. Focal lengths of the order of one meter (or more) may easily be achieved, which is impossible with conventional optical systems (refractive components). The “zoom” function can be made in a very simple manner, and does not require the use of large and fragile glass mirrors and/or lenses; all that is necessary to effectively change the focal
Grandjean Jean-Paul
Kress Bernard
Meyrueis Patrick
Twardowski Patrice
Chang Audrey
Commissariat A l'Energie Atomique
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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