Coherent light generators – Particular beam control device – Scanning
Reexamination Certificate
2002-07-10
2004-07-13
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Scanning
C372S098000
Reexamination Certificate
active
06763045
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a method of laser-cutting or marking an article and to an apparatus for laser beam targeting.
BACKGROUND OF THE INVENTION
Current lasers scanning technology involves two techniques:
1) Post-objective targeting using galvanometric motor scanning mirrors, where the laser beam first passes through an objective or “z” lens in order to keep the beam at optimum focus at a target plane, upstream of the X and Y scanning mirrors, wherein the focused laser beam, by linear displacement, follows the shape of a structure, such as a label to be marked or cut. Linear displacement of the focused laser beam is controlled by digital signals, representing X and Y laser beam positions on the target plane and directly corresponding to signals taken from the X and Y galvanometric motors, which in turn allowed a laser beam to be directed onto the article; or
2) Pre-objective targeting using galvanometric motor scanning mirrors, where the beam is kept in focus at the target plane by passing through a scanning or flat field or f-theta lens down-stream of the galvanometric motor controlled mirrors.
In both of the above systems, the laser beam is directed on a X galvanometric motor controlled mirror, which deflects the laser beam onto an Y galvanometric motor controlled mirror, which in turn reflects the laser beam towards the target plane. The X and Y galvanometric motors each comprise a central spindle which is held within a magnetic field and onto which a deflecting mirror is mounted. By sending a signal to increase or decrease the magnetic field, the spindle can be rotated clockwise and anti-clockwise at high speeds and with great accuracy and repeatability. By controlling an X and an Y galvanometric motor in combination with laser source switching, an image can be scanned onto the target area.
Disadvantageously, current laser beam scanning targeting techniques commonly involve the switching of the laser source in direct conjunction to the laser beam being scanned across a target plane. Because e.g. a carbon dioxide laser source cannot produce plasma energy or lase at the lowest ten percent of maximum laser energy, when the scanning speed at the target is lower than 10% of the maximum scanning speed, the resulting laser process will have more energy striking the target than is required to effect a uniform depth of processing. This problem is illustrated with respect to FIG.
5
.
FIG. 5
shows a graph illustrating the problem of acceleration, maximum speed and deceleration of a galvanometric motor determining the speed of movement of the laser across the target to laser beam intensity. As shown in
FIG. 5
, during the acceleration and the deceleration phase of the galvanometric motor the speed of movement of the laser beam across the target plane is inherently slower than at maximum speed. Accordingly, the laser beam output power has to be increased and decreased in proportion to the motor speed to guarantee a uniform laser beam intensity on the target surface. This will not be possible with the common carbon dioxide laser which at minimum speed generates a laser beam intensity which will be either zero or too high.
Further, scanning targeting techniques using current carbon dioxide lasers commonly suffer from larger than required laser beam spot diameters at the target plane. This is due to the relationship between the diameter of the unfocused laser beam, the size of the X and Y mirrors, the mass of the X and Y mirrors in relation to the galvanometric motors, which control positioning and positional tolerance and speed, the focusing ability of the f-theta lens and the wavelength of the carbon dioxide generated laser beam energy.
Current alternative carbon dioxide laser beam scanners use, for example, the SK1020 scan head manufactured by SCANLAB AG of Puchheim, Germany, which when used in conjunction with a laser source, commonly controls that laser source by simply switching on the laser beam energy to a pre-determined power output level when a mark is required to be scanned onto the target. When the scan reaches the end or completion of that mark the laser beam energy is simply switched off again. Although, electronic delays and pulse suppression can be effected to assist the uniformity of the required mark, there are no accounts for scanning acceleration or deceleration at the target plane.
Disadvantageously, the above mentioned scan heads have no controls to ramp the laser beam power at the target in direct relationship to the speed of the combined X and Y galvanometric scanning, as explained above with respect to FIG.
5
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Further, the current scan heads have relatively large laser beam spot sizes on the target plane. The input laser beam diameter should be kept relatively small in order to minimize the size and mass of the X and Y scanning mirrors thereby restricting the inertia that said X and Y scanning mirrors will load onto the galvanometric motors.
Also, the current scan heads have no control over the target plane laser beam profile, therefore there is no accounting for or control over the natural Gaussian profile of the laser beam. Disadvantageously, this natural Gaussian profile is detrimental to the accurate processing of fine graphics art and other materials because it will produce a laser beam at the target plane of higher intensity in the center of said laser beam and lower intensity at the edges.
Another disadvantage of the prior art scan heads, controlling laser beam energy by switching is that the switching on and off of a laser source to match galvanometric scanning is detrimental to the longevity of that laser source and compounds attempts to maintain that laser source's power output stability.
Finally, the above method of laser source control by switching to match galvanometric scanning only allows for the duplication of scanning heads processing from a single laser source, and does not allow for any configuration of scanning heads to form an array whereby each scanning head can take a part of the total target area.
U.S. Pat. No. 4,952,034 discloses a method of writing a liquid crystal display with a laser beam, wherein the intensity of the laser beam applied to the liquid crystal is controlled by a laser beam varying unit. The intensity of the laser beam irradiated on the liquid crystal is changed with the deflection speed of the optical axis, wherein the change of laser beam intensity is obtained by a acousto-optical modulation. Modulating the laser energy into ultrasonic pulses of varying frequency is used for thermal writing of a liquid crystal display at constant laser beam energy.
WO 89/11 948 relates to a method of laser beam steering to optimize the efficiency of laser welding and other laser material processing. According to this reference, the polarization of laser beam energy is steered in the same direction as the processing path across a target material; further, this document also relates to the benefits of striking the material at or near the Brewster angle for said material being processed.
EP 0 393 676 discloses a solid imaging system, wherein a laser beam is modulated in accordance with the laser beam sleep speed to maintain a substantially constant layer depth at all speeds. The modulator preferably uses an acousto-optical switch or an electro-optical switch. The electro-optical switch is based on a crystal that changes optical polarity when voltages are applied.
It is the object of the present invention to provide a method of laser cutting or marking an article and a laser beam targeting apparatus whereby the transmission of a beam of laser energy is finely controlled in direct relationship to the speed at which two galvanometric motor driven mirrors deflect said laser beam and scan that laser beam onto a target plane. According to a further aspect, the present invention relates to a method and an apparatus for decreasing the laser beam spot diameter at the target plane.
SUMMARY OF THE INVENTION
To solve the above object the present invention provides a method of laser cutting or marking an article,
Duane Morris LLP
Jr. Leon Scott
Raylase AG
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