Laser shock peening apparatus with a diffractive optic element

A laser peening apparatus, including a laser generator to generate a laser beam having a first cross-sectional shape; and a diffractive optic element. The diffractive optic element changes the laser beam to a second cross-sectional shape. The apparatus also includes demagnifying and magnifying lenses. The diffractive optic element may create a second cross-sectional shape such as rectangular, hexagonal, or even split the laser beam into multiple beams. The diffractive optic element may also create a second cross-sectional shape of the laser beam varying in intensity thereacross or varying in energy distribution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the use of coherent energy pulses as from 
high power pulsed lasers in the shock processing of solid materials, and 
more particularly to a device and method for generating a particularly 
formed laser beam profile for use in the process. The invention is 
especially used for enhancing on creating desired physical properties such 
as hardness, strength, and fatigue strength in solid materials. 
2. Description of the Related Art 
Known methods for the shock processing of solid materials, particularly 
known methods for the laser shock processing or peening of solid 
materials, typically use coherent energy from a laser beam oriented normal 
to a workpiece. The cross sectional shape of the laser beam may be 
circular for efficient laser beam creation, or for slab type laser rods, 
it may be rectangular. 
Such circular cross sectional laser beams create circular laser peened 
spots on a workpiece surface when oriented normal thereto. As previously 
known in the art, to adequately laser shock the desired area in the 
workpiece, such geometry, i.e., circular, of the laser spot necessitated 
an overlap of adjacent laser shocked spots on the workpiece to 
sufficiently create a residual compressive field within the workpiece. 
Such overlap of the peened spots, reduces the speed of the process. 
Lenses utilized within high powered laser systems have historically used 
only refractive lenses. Such lenses utilize a material such as glass or 
plastic having varying thicknesses to bend or refract the laser energy. 
What is needed in the art, is the ability to change the circular cross 
section of the high power laser beam to that of an alternate shape, such 
that a majority of the laser energy created may be applied to the 
workpiece without substantial loss. Further, needed in the art, is an 
energy efficient method to alter a cross sectional shape of the incoming 
laser beam to permit efficient processing of the workpiece without 
necessitating overlap of the laser peened spots. 
Also needed is a technique to provide more uniform energy distribution 
independent of spot shape. 
SUMMARY OF THE INVENTION 
According to the present invention an apparatus method is provided of laser 
shock processing, that can be used in a production environment to control 
and alter the cross sectional shape of the applied laser beam. 
The invention, in one form thereof, utilizes a diffractive optic member 
designed to change the incoming, normally circular, cross sectional laser 
beam to that of a desired shape. Desired shapes may include that of a 
square, torus, eccentric circle, hexagon, or triangle shape. In addition, 
a single laser beam may be split up into multiple beams. One feature of 
the invention is that such diffractive optics may create a laser beam with 
a cross sectional shape having substantially shaped edges, particularly 
straight edges. 
Additionally, use of diffractive optics in the present invention and 
method, may control the applied energy of the laser beam in which the 
transition area between a laser peened spot and the non-laser peened area 
may gradually drop in intensity, such that high residual surface tensile 
forces in the area about the laser peened spot may be reduced. 
In another form of the invention, demagnifying and magnifying optics are 
utilized about the diffractive optic element to enable use of such 
diffractive optic elements without exceeding the element's damage 
threshold. Traditional diffraction gradings, have a low damage threshold 
of approximately 350 milijoules per centimeter squared. Other diffractive 
optics have different thresholds, but typically not as high as refractive 
optics. 
An advantage of the present invention is that a laser beam with circular 
cross section may be formed to any desired shape to enable efficient laser 
processing. 
Another advantage of the present invention is that utilization of 
diffractive optics within the laser peening system may change its spatial 
profile applied energy in a controllable fashion to decrease residual 
surface tensile forces in the laser shock peened workpiece.

DETAILED DESCRIPTION OF THE INVENTION 
The improvements in fatigue life produced by laser shock processing are the 
results of residual compressive stresses developed in the irradiated 
surface that retard fatigue crack initiation and/or crack propagation. 
Changes in the shape of a crack front and slowing of the crack growth rate 
when the crack front encounters the laser shocked zone have been shown. 
Laser shock processing is an effective method of increasing fatigue life 
in metals by treating fatigue critical regions. For a more thorough 
background and the prior history of laser shock processing and that of 
high powered processing of engineered materials, reference can be made to 
U.S. Pat. No. 5,131,957. This patent shows the type of laser and laser 
circuit adaptable for use with the present invention. Another type of 
laser adaptable for use with the invention, is that with a ND-Glass Laser 
manufactured by LSP Technologies, of Dublin, Ohio. 
Use of transparent and opaque overlays is known with the standard laser 
shock processing or laser peening. Such descriptions of transparent and 
opaque laser peening overlays used with the process are not discussed 
here. 
The present invention includes utilization of a diffractive optic element 
or sometimes called a binary optic to generate a custom, specific shape 
for laser peening. Such an optic may take a relatively uniform beam or 
uniformally circular beam and map such beam shape and energy distribution 
to any format desired. 
For instance, as shown in FIG. 1, a laser (12) creates a laser beam 10 
having a circular cross section as shown therebelow. Laser beam 10 passes 
through a demagnifying objective lens 20, thereby passing into a 
diffractive optic element 30. Based on the design of the diffractive optic 
element, to be discussed later, the altered laser beam 10 passes into a 
magnifying or collecting lens 40, then onto workpiece 50, which is the 
workpiece to be laser shock processed. Directly prior to impact with 
workpiece 50, laser beam 10 cross sectional shape may be that indicated as 
rectangular (specifically square) or that of many other shapes as 
indicated, such as toroidal, hexagonal, bar, and triangular. 
Demagnifying and magnifying lenses 20, 40 are utilized to maintain the 
laser beam 10 intensity less than the maximum limit of the diffractive 
optic element 30. Numerous types of magnifying and demagnifying lenses may 
be utilized as known in the art, but the preferred lens is a spherical A/R 
coated lens. 
Diffractive optic element 30 of the present invention works by breaking up 
waves of incoming light energy into a large number of waves which 
recombine to form a completely new wave or waves on the opposite side of 
optic element 30. The new waves can move in a different direction than the 
incoming light pulse. Additionally, diffractive optics can convert a 
single beam into several output beams and focus the input beam into a 
point or into a pattern while homogenizing or defusing the beam into a 
controlled area. 
Diffractive optics break-up the light waves by forcing them through 
microscopic patterns, typically etched into the surface of the lens or 
member, through photolithography. Once a diffractive element is created, 
it may be reproduced through embossing plastic injection molding, or batch 
photolithographic processing. 
Design of such diffractive optics is known in the art, and particular 
companies are available for designing such. Such diffractive optics may 
eliminate hotspots or places of excessive laser energy within the incoming 
laser beam to create a homogenous output laser beam. Additionally, such 
diffractive optics may be designed to blur the outer edges of the laser 
output beam, thereby decreasing the amplitude or power as applied to the 
workpiece in laser shock processing. 
By altering the applied energy, particularly decreasing the amplitude of 
the power applied in the outer edges of the beam, a reduction of residual 
surface tensile stresses in the workpiece may be created. Designs and 
functions of diffractive elements may create any output image desired from 
any known input image. 
Use of diffractive optics within a laser peening process enables efficient 
beam shaping and control of the spatial profile energy amplitude to either 
create a uniform laser beam from a non-uniform input laser beam or simply 
change the cross sectional shape of the output laser beam as compared to 
the input laser beam or both. 
Particular input parameters necessary for design of diffractive optics are 
the wavelength of the input laser beam, spatial profile, cross sectional 
shape, and the desired output cross sectional shape along with the desired 
energy amplitude profile. Such diffractive optics may homogenize and 
defuse such beams and edges as necessary. 
While this invention has been described as having a preferred design, the 
present invention can be further modified within the spirit and scope of 
this disclosure. This application is therefore intended to cover any 
variations, uses, or adaptations of the invention using its general 
principles. Further, this application is intended to cover such departures 
from the present disclosure as come within known or customary practice in 
the art to which this invention pertains and which fall within the limits 
of the appended claims.