WO2000007054A1

WO2000007054A1 - Apparatus and method for producing an improved laser beam

Info

Publication number: WO2000007054A1
Application number: PCT/US1999/015421
Authority: WO
Inventors: John A. Hopkins; Frederick A. Schwartz; Mary Helen Mccay; Thurman Dwayne Mccay; Narendra B. Dahotre; John Brice Bible
Original assignee: University of Tennessee Research Foundation
Current assignee: University of Tennessee Research Foundation
Priority date: 1998-07-31
Filing date: 1999-07-09
Publication date: 2000-02-10
Anticipated expiration: 2001-01-31
Also published as: JP2002521833A; WO2000007054A9; US6016227A; CA2338422A1; IL140883A0; AU4867599A; CZ2001216A3; CN1149418C; KR20010072161A; CN1311870A; IL140883A; EP1101140A1; BR9912667A

Abstract

This invention relates to an improved laser beam useful in material surface processing and an apparatus for producing the improved laser beam. The improved laser beam of the present invention comprises a power distribution that is more intense at the outer regions than in the central region. The invention includes an apparatus comprising a power spherical and cylindrical optical elements aligned to focus and/or shape a laser beam into a novel profile useful in material surface processing. The invention further includes a method of using this apparatus to focus and/or shape a laser beam for producing Laser Induced Surface Improvements, hereinafter referred to as 'LISI'.

Description

Title: APPARATUS AND METHOD FOR PRODUCING

AN IMPROVED LASER B.EAM

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved laser beam useful in material

surface processing and an apparatus for producing the improved laser beam.

The improved laser beam of the present invention comprises a power

distribution that is more intense at the outer regions than in the central region.

The invention includes an apparatus comprising optical elements aligned to

focus and/or shape a laser beam into a novel profile useful in material surface

processing. The laser beam and apparatus of the present invention are useful

for producing Laser Induced Surface Improvements, hereinafter referred to as

"LISI".

2. Description of the Prior Art

Laser beams have been used to treat and/or process a workpiece, such

as a piece of metal. Prior art devices employing laser beams for materials

processing have comprised a beam delivery system and a movement system

for moving the workpiece relative to the beam. The process effect that the

beam has upon the workpiece is a function of many variables, including but not limited to, power distribution and beam shape. Uniform processing of the

workpiece surface by the beam is normally desired.

One family of prior art beam processing devices have the disadvantage

of nonuniform surface processing because the beams used by such devices

have Gaussian or normal power distributions. Jones et al., "Laser-beam

Analysis Pinpoints Critical Parameters", Laser Focus World (January 1993).

Another type of prior art laser beam has an equalized or uniform power

distribution across the cross sectional area of the beam. Such a beam is

described in U.S. Patent No. 5,124,993. Such an equalized power distribution

can result in less energy deposition at the outer regions of the laser beam

irradiation zone due to increased heat transfer in these regions.

This processing nonuniformity results in surface irregularities in the

processed workpiece and nonuniform processing depth under the surface

area of the workpiece irradiated by the beam. Prior art beam configurations

are typically circular. The power density of such circular beams is greatest at

the center, thereby resulting in a maximum energy deposition at the center of

the beam. The power distribution of typical prior art beams is shown in Figure

1A.

In LISI processes, where the power at point A is constant as a function

of time, the energy deposition at point A, ED_A, is proportional to the product of the power at point A, P_A, and the length of time, T, that point A was

irradiated as shown. Where the power distribution is spatially nonuniform, the

resulting energy deposition distribution is proportionally nonuniform.

Another drawback of the Gaussian power distribution of prior art beams

is that the power density and corresponding energy deposition decline as a

function of radial distance from the beam center. Accordingly, workpieces

processed with such prior art beams are overprocessed in the central beam

region and underprocessed in the outer beam regions. The depth of

processing is proportional to the power distribution. The processed region is

referred to herein as "the melt region". A typical melt region of a prior art

beam having a Gaussian distribution is shown in Figure 1 B. The temperature

history resulting from the melt region profile depicted in Figure 1 B and the

power distribution in Figure 1A, is shown in Figure 1 C.

Prior art processing methods employ beam overlap during processing

of the workpiece to increase the processing depth of the underprocessed

outer regions. For LISI processes, surface characteristics such as

smoothness and homogeneity of composition are important, particularly for

hardness and corrosion resistant properties. The beam overlap required with

prior art beam processing methods results in nonuniformities in processing depth, composition and surface profile. All of these nonuniformities are

undesirable.

In LISI processes employing the deposition of a precursor material on

the workpiece prior to irradiation, the low power densities in the outer region

of the circular beam can result in the melting and removal of the precursor

material before it is incorporated into the substrate melt. This can result in

nonprocessed irradiated regions.

The efficiency of beam processing systems is a function of many

variables, including but not limited to beam shape, size and power distribution.

Prior art beam processing devices have the disadvantage of inefficient

processing rates, resulting in part from, small beam area, nonuniform power

distribution and circular beam shape.

The present invention offers the advantages of (a) improved power

distribution, (b) improved surface quality, and (c) improved surface processing

rate. These advantages are achieved through the use of a beam shaping and

focusing apparatus that produces an improved laser beam with a novel power

distribution that is increased at the outer regions relative to the central region.

Improved surface quality results from the improved power distribution of the

present invention. The improved processing rate results from the larger,

noncircular beam surface area, and the more uniform power distribution. SUMMARY OF THE INVENTION

The present invention comprises an improved laser beam for material

surface processing as well as an apparatus for producing this improved laser

beam.

The laser beam further comprises a power distribution that is

substantially constant in the central region, increased at the outer regions, and

which decreases in a substantially step function fashion at the end regions.

The term "substantially constant in the central region", as used herein, means

that the maximum range of power distribution variance in the central region

is less than 7.5% of the maximum power level in the central region.

This power distribution resembles the head of a bat and is hereinafter

referred to as a "bat ears" distribution. The ratio of the peak intensity or

magnitude of the power in the outer regions to the average intensity or

magnitude of power in the central region is greater than or equal to 1.2.

The substantially constant distribution in the central region provides

substantially uniform energy deposition and material processing in the central

region. The increased power distribution at the outer regions compensates

for the increased energy or heat flux in this region.

The present invention may be used for LISI processing of workpieces

having sufficient volume such that the region of the workpiece outside the energy deposition region (the nonenergy deposition region) acts as a large

heat sink. The heat sink properties of the workpiece causes the increased

energy or heat flux at the interface between the energy deposition region and

the nonenergy deposition region.

The substantially step function decrease in power distribution at the end

regions provides for controlled material processing in a region approximately

equal to the width of the power distribution. This provides improved material

processing where the laser beam is applied to the workpiece along contiguous

parallel paths. In such processing, it is desirable to minimize beam overlap

because such overlap increases the energy deposition in overlapping regions

relative to the energy deposition in the central region of the beam, and

decreases area coverage processing rates.

An apparatus of the present invention is also directed toward a

combination of optical elements for producing the laser beam of the present

invention. This combination of optical elements comprises a collimating

optical element, a first cylindrical optical element, and a second cylindrical

optical element.

The apparatus is comprised of optical elements oriented to obtain

described characteristics of dimension and energy distribution. The apparatus

provides for appropriate focusing of collimated or uncollimated input laser beams and may be configured to occupy minimal volume. The focal length

of the apparatus may be modified to meet existing or desired performance

requirements.

Because operation will typically occur at conditions other than those for

a corresponding circular beam, the amount of back-reflected energy and

subsequent likelihood of damage may be minimized for applications with a

fiber optically delivered input laser beam. The apparatus may be designed

with active or passive cooling of optical elements to allow continuous

operation at high power throughput conditions.

A gas delivery system may be incorporated with the apparatus to assist

the laser processing, cool the apparatus, or provide a means of preventing

contamination of the optical elements. The apparatus may be made of any

structural material, but would likely include opaque walls for beam safety

considerations. The inner and outer wall surfaces may be coated to increase

absorption of any spurious reflections. A protective cover glass could also be

used to protect optics from contamination without altering the effectiveness of

the apparatus. The apparatus could be designed so that it would breakaway

from a mounting bracket rather than break itself.

DESCRIPTION OF DRAWINGS

Figure 1 A depicts a Gaussian power distribution of a prior art beam. Figure 1 B depicts a typical melt region of the prior art beam depicted in

Figure 1A.

Figure 1C depicts a temperature history of a prior art beam resulting

from the melt region profile depicted in Figure 1 B and the power distribution

depicted in Figure 1A.

Figure 2A depicts the power distribution in the width dimension of the

laser beam of the present invention.

Figure 2B depicts the power distribution in the length dimension of the

laser beam of the present invention.

Figure 2C depicts a top view of the rectangular cross sectional profile

of a laser beam of the present invention.

Figure 3 depicts a side view of the apparatus embodiment of the

present invention.

Figure 4 depicts a top view of the apparatus embodiment of the present

invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment, the laser beam of the present invention has

a rectangular cross sectional profile 20 comprising a width dimension 22 and

a length dimension 24, as shown in Figure 2C. In a preferred embodiment,

the width dimension of said beam is at least 8 times the size of the length dimension of said beam. The ratio of the width dimension to the length

dimension measured at location of half maximum intensity is hereinafter

referred to as "aspect ratio".

As shown in Figure 2A, the laser beam of the present invention

comprises a power distribution in the width dimension that is substantially

constant in the central region 10, increased at the outer regions 12, and which

decreases in a substantially step function fashion at the end regions 14 of the

width dimension. The ratio of the intensity or magnitude of the power

distribution in the outer regions to the intensity or magnitude in the central

region is greater than or equal to 1.2. This ratio is hereinafter referred to as

the "outer to central intensity ratio".

In a preferred embodiment, as shown in Figure 2A, the central region

extends at least 50% of the width of the beam. In another preferred

embodiment, as shown in Figure 2A, the central region extends in a range of

60%-75% of the width of the beam.

In a preferred embodiment, the power distribution in the length

dimension is substantially constant as shown in Figure 2B. In a preferred

embodiment, as shown in Figure 2B, the size of the length dimension is less

than one millimeter. In a preferred embodiment, the laser beam of the present invention is

selectively moveable along an axis parallel to the length dimension of said

beam.

An apparatus of the present invention is also directed to shaping a laser

beam emitted from a laser source to have a peak outer to average central

intensity ratio greater than or equal to 1.2. This apparatus is depicted in

Figures 3 and 4. It comprises a collimating optical element 32 position to

collimate a laser beam emitted from a source. The invention further

comprises a first cylindrical optical element 34 having a first curved outer

surface 33. The first cylindrical optical element is positioned to receive a

collimated laser beam from the collimating optical element. The first

cylindrical optical element is spaced 0.1 mm to 5,000.0 millimeters from the

collimating optical element.

The invention further comprises a second cylindrical optical element 36

having a second curved outer surface 35 that is oriented in a range of 89.5

degrees to 90.5 degrees out of rotational alignment with the first curved

surface. The second cylindrical optical element is positioned to receive a

laser beam from the first cylindrical optical element. In a preferred

embodiment, the first and second curved surfaces face each other.

As shown in Figures 3 and 4, a preferred embodiment of this invention

further comprises a biconcave lens 30 positioned between a laser beam source and the collimating optical element, so as to project a more divergent

laser beam on said collimating optical element than would exist without the

biconcave lens.

In another preferred embodiment, the invention further comprises a

laser source capable of emitting laser light having a wave length of 1.06

microns and further comprising a 600 micron diameter fiber from which said

laser beam exits. In a preferred embodiment, the laser beam exiting the

biconcave lens has a divergence angle of 4.4 degrees. In this preferred

embodiment, the lens may be constructed of fused silica glass and would

further comprise an antireflection coating suitable for use with normal

incidence radiation having a wave length of 1.06 microns.

In another preferred embodiment, the biconcave lens has a focal length

of 75 mm and is positioned 98 mm from the effective point source location of

the laser beam. The collimating optical element is a spherical optical element

having a focal length of 100 mm and located 41.3 mm from the biconcave

lens.

In a preferred embodiment, the first cylindrical optical element is a

cylindrical lens having a focal length of 200 mm, located 22 mm from the

collimating optical element. In a preferred embodiment, the second cylindrical

optical element is a cylindrical lens having a focal length of 152.4 mm and

located 5 mm from the first cylindrical optical element. The second cylindrical optical element is rotated 90 degrees around the axis of beam travel with

respect to the first cylindrical optical element and is oriented to have its curved

surface facing the first cylindrical optical element. In a preferred embodiment,

the focal plane at which the desired energy distribution is obtained is located

167.2 mm from the second cylindrical optical element.

The foregoing disclosure and description of the invention are illustrative

and explanatory. Various changes in the size, shape, and materials, as well

as in the details of the illustrative construction may be made without departing

from the spirit of the invention.

Claims

WHAT IS CLAIMED IS:

1. A laser beam comprising a central region and two outer regions, each

of said outer regions comprising an outer edge, said laser beam

comprising a power distribution that is substantially constant in the

central region, increased at the outer regions, and which decreases in

a substantially step function fashion at the outer edges such that the

peak outer to average central intensity ratio of said beam is greater

than or equal to 1.2.

2. The laser beam of claim 1 wherein said laser beam has a rectangular

cross sectional profile comprising a width dimension and a length

dimension.

The laser beam of claim 2 wherein said central region extends at least

50% of the width of said beam.

The laser beam of claim 3 wherein said central region extends 60-75%

of the width of said beam.

5. The laser beam of claim 2 wherein said beam has an aspect ratio

greater than or equal to 8.

6. The laser beam of claim 2 wherein the power distribution in the length

dimension is substantially constant.

7. The laser beam of claim 6 wherein the size of said length dimension is

less than one millimeter.

8 The laser beam of claim 1 wherein said beam is selectively movable

along an axis parallel to the length dimension of said beam.

9. A laser beam having a rectangular cross sectional profile comprising a

width dimension and a length dimension, said width being at least eight

times the size of said length dimension, said beam further comprising

a power distribution in the width dimension, said power distribution

comprising a central region and outer regions comprising outer edges,

said distribution being substantially constant in the central region,

increased at the outer regions, and decreasing in a substantially step

function fashion at the outer edges of said width dimension.

10. The laser beam of claim 9 wherein said central region extends at least

50% of the width of said beam.

11. The laser beam of claim 9 wherein said beam has an aspect ratio

greater than or equal to 8.

12. The laser beam of claim 9 wherein the power distribution in the length

dimension is substantially constant.

13. An apparatus for shaping a laser beam emitted from a laser beam

source to have a peak outer to average central intensity ratio greater

than or equal to 1.2 comprising:

a. a collimating optical element positioned to collimate a laser beam

emitted from a source;

b. a first cylindrical optical element having a first curved outer

surface, said first cylindrical optical element being positioned to

receive a collimated laser beam from said collimating optical

element, wherein said first cylindrical optical element is spaced

0.1 to 5,000.0 millimeters from said collimating optical element;

and

c. a second cylindrical optical element having a second curved

outer surface that is oriented 89.5 - 90.5 degrees out of

rotational alignment with said first curved surface, said second

cylindrical optical element being positioned to receive a laser

beam from said first cylindrical optical element.

14. The apparatus of claim 13 wherein said collimating optical element is

a collimating lens.

15. The apparatus of claim 13 wherein said first and second cylindrical

optical elements are cylindrical lenses.

16. The apparatus of claim 13 wherein said first and second cylindrical

elements are integrally formed.

17. The apparatus of claim 13 wherein the focal length of said first

cylindrical optical element is 200 millimeters.

18. The apparatus of claim 13 wherein the focal length of said second

cylindrical optical element is 152.4 millimeters.

19. The apparatus of claim 13 wherein said first and second curved

surfaces face each other.

20. The apparatus of claim 13, further comprising a biconcave lens

positioned between a laser beam source and said collimating optical

element so as to project a more divergent laser beam on said

collimating optical element.

AMENDED CLAIMS

[received by the International Bureau on 15 October 1999 (15.10.99); original claims 6, 8, 12, 16 and 20 cancelled; original claims

1, 7, 9,11 and 13 amended; remaining claims unchanged

(5 pages)]

1. A laser beam comprising a central region and two outer regions,

each of said outer regions comprising an outer edge, said laser

beam comprising a power distribution that is substantially constant

in the central region, increased at the outer regions, and which

decreases at the outer edges such that the peak outer to average

central intensity ratio of said beam is greater than or equal to 1.2.

2. The laser beam of claim 1 wherein said laser beam has a

rectangular cross sectional profile comprising a width dimension and

a length dimension.

3. The laser beam of claim 2 wherein said central region extends at

least 50% of the width of said beam.

4. The laser beam of claim 3 wherein said central region extends

60-75% of the width of said beam.

5. The laser beam of claim 2 wherein said beam has an aspect ratio

greater than or equal to 8.

6. Deleted

7. The laser beam of claim 2 wherein the size of said length dimension

is less than one millimeter.

8. Deleted

9. A laser beam having a rectangular cross sectional profile comprising

a width dimension and a length dimension, said width being at least

eight times the size of said length dimension, said beam further

comprising a power distribution in the width dimension, said power

distribution comprising a central region and outer regions comprising

outer edges, said distribution being substantially constant in the

central region, increased at the outer regions, and decreasing at the

outer edges of said width dimension.

10. The laser beam of claim 9 wherein said central region extends at

least 50% of the width of said beam.

11. The laser beam of claim 9 wherein the peak outer to average central

intensity ration of said beam in the width dimension is greater than

or equal to 1.2.

12. Deleted

13. An apparatus for shaping a laser beam emitted from a laser beam

source to have a peak outer to average central intensity ratio greater

than or equal to 1.2 comprising:

a. a collimating optical element positioned to collimate a laser

beam emitted from a fiber optic source;

b. a first cylindrical optical element having a first curved outer

surface, said first cylindrical optical element being positioned

to receive a collimated laser beam from said collimating

optical element, wherein said first cylindrical optical element

is spaced 0.1 to 5,000.0 millimeters from said collimating

optical element; c. a second cylindrical optical element having a second curved

outer surface that is oriented 89.5 - 90.5 degrees out of

rotational alignment with said first curved surface, said second

cylindrical optical element being located 5 millimeters from

said first cylindrical optical element; and

d. a bioconcave lens positioned between a laser beam source

and said collimating optical element so as to project a more

divergent laser beam on said collimating optical element.

14. The apparatus of claim 13 wherein said collimating optical element

is a collimating lens.

15. The apparatus of claim 13 wherein said first and second cylindrical

optical elements are cylindrical lenses.

16. Deleted

17. The apparatus of claim 13 wherein the focal length of said first

cylindrical optical element is 200 millimeters.

18. The apparatus of claim 13 wherein the focal length of said second

cylindrical optical element is 152.4 millimeters.

19. The apparatus of claim 13 wherein said first and second curved

surfaces face each other.

0. Deleted