This invention relates to high-power laser optics. More particularly, apparatus and method are provided for reflecting laser beams which may be used in material processing or other applications.

High-power carbon dioxide lasers for material processing and other applications have become widely used in recent years. It is desirable to focus the energy from such lasers to maximize effectiveness of cutting, drilling or other material processing applications. It is also desirable to be able to move the laser beam along the material being processed. The usual means of focusing lower-energy laser beams is by lenses made of zinc selenide or zinc sulphide, but the absorption of energy by these materials prevents their use with high-power lasers.

To overcome the limitations of transmitting optics for focusing, reflecting optics in the form of parabolic mirrors have been developed. Flat mirrors or other shapes of mirrors may also be used for directing a laser beam or moving the beam to a selected location. It has been found that copper is a suitable material to fabricate such mirrors, although other metallic materials can be used. Copper is easy to machine and polish and the high thermal conductivity of copper is an advantage in high-power laser applications, where the mirrors must be liquid-cooled. However, copper is a very soft material and it is easily damaged. The production environment in which the mirrors operate may contain fumes, humidity and, in some applications, molten particles from the material being processed. This environment can be very damaging to uncoated mirrors. In addition, as copper increases in temperature, it becomes prone to oxidation and loses its reflectivity. A protective coating on copper or any other metal used in such mirrors is needed.

A coating of molybdenum is presently used to increase the life of copper mirrors. The typical molybdenum coating protects copper from most of the damaging factors, but molybdenum coatings, in the thicknesses usually employed, absorb more than 2.5% of the laser power. For high-power lasers, having power outputs greater than 10 kilowatt, for example, the total power loss at the mirror is in the hundreds of watts.

Diamond has been suggested as a protective coating for copper mirrors. Diamo is an ideal material for coatings on mirrors to be used with high-power carbon dioxi lasers because it has low absorption of electromagnetic waves at a wavelength of 10 microns, the wavelength of energy from a carbon dioxide laser. Attempts have be made to coat such mirrors with diamond, the diamond being deposited by chemi vapor deposition, but adherence of the coating to the mirror has not been satisfacto U.S. Patent 4,987,007 describes a process for depositing a diamond-like coati on solids. This process of coating employs a very high energy laser beam which focused on a graphite target to cause ablation of carbon from the target. The high-ener carbon ions from the plume of ablated carbon are deposited on a substrate to form t coating or film. The resulting coating has many bonds corresponding to the atom bonds in diamond, along with other material which is not crystalline. Such material often called "diamond-like carbon." Coatings of such material are provided by Diamond Technology, Inc. of Houston, Texas as "AMORPHIC DIAMOND."

There is a need for an improved coating for copper or other metallic optic apparatus to provide protection for a reflecting surface against physical or chemic damage in the environment in which it is used. Such coating should adhere to t surface under conditions of use with very high-power lasers, and the coated mirror shou reflect a verv high percentage of incident energy.

According to one embodiment of the present invention, there is provided improved mirror of metallic material for laser applications wherein the improveme comprises a coating of diamond-like carbon on the reflecting surface of the mirror. I a preferred embodiment, the mirror is made of copper.

According to another embodiment of the present invention, there is provided a improved method for focusing and directing a laser beam wherein the improveme comprises focusing and directing the laser beam by a mirror, the reflecting surface of t mirror having a coating of diamond-like carbon.

Fig. 1 is a sketch of apparatus which may be used to form a coating of diamon like carbon on metal mirrors.

Referring to Fig. 1 , a schematic of apparatus used to deposit the coating of this invention is shown. Such apparatus is generally described in U.S. Patent 4,987,007, which is incorporated herein by reference for all purposes. The beam from laser 10 is reflected by mirror 12 to pass through window 14 in the wall of vacuum chamber 30. The beam then passes through lens 16 where it is focused on graphite target 20. Ring electrode 18 may be used to draw ions from the plume formed by the concentrated laser energy impinging on target 20. Optical components to be coated with the coating of this invention are attached to rotating substrate holders 24 so as to be directly within the plume of high energy carbon ions displaced from graphite target 12. Substrate holders 24 are preferably within a solid angle of about 60° as measured perpendicular to target 20, so as to be sufficiently within the plume. Preferably, substrate holders 24 are rotated during deposition of a coating.

A high-power Q-switched Nd.YAG laser may be used. The power of laser 10, focused by lens 16, should produce a power density on graphite target 20 of at least 10 ⁷ watts per square centimeter. Preferably, the power density is greater than 10 ¹⁰ watts per square centimeter on the graphite surface. Pressure in the vacuum chamber 30 is reduced by vacuum pumps to less than about 10 ^"5 torr, and preferably is in the range of about 10 ^'7 torr. Ring electrode 18 is preferably biased at a voltage in the range from about - 1 kilovolt to about -4 kilovolts, but is more preferably biased in the range from about -2 kilovolts to about -3.5 kilovolts.

Substrate holders 24 may be electrically biased or not biased, but preferably it is biased by pulsed RF voltage which is synchronized with the laser pulses. The voltage may be greater than - 100 volts, but is preferably in the range of -200 to -300 volts.

The time of deposition is varied to produce films of suitable thickness. Preferably, the film thickness is in the range from 0.25 to about 0.5 microns, but films having any desired thickness up to several microns may be used.

After coating, the coated mirror is removed from vacuum chamber 30 and used in apparatus requiring reflection of high-energy laser beams.

The mirrors of the present invention may generally be any that are utilized to reflect laser beams. Some embodiments of this invention will be particularly useful in

the practice of machining or processing of materials by high-power lasers. The mirr may be made of copper, aluminum, stainless steel or other metals or alloys. Wh parabolic mirrors are used, the focal point of the mirror is selected to provid concentrated beam of energy at the desired location, such as at the location wh drilling or cutting of material is to be performed.

Copper mirrors are available from II- VI, Inc. of Saxonburg, Pennsylvan Machining or cutting apparatus employing high-power laser energy is available fr many sources in industry. The coated mirrors of the present invention may be utiliz in apparatus for cutting or materials processing or may be used in any other apparat in which laser energy is to be reflected and where the lifetime of the reflecting surface of importance.

A diamond-like coating of the present invention may be applied by any meth that will produce such coating. The term "diamond-like carbon" generally includ carbon materials having both amorphous and microαystalline atomic structures a having a hydrogen concentration from about 0% to about 40%. A preferred form diamond-like carbon is "AMORPHIC DIAMOND," produced by the method of U. Patent No. 4,987,007 and available from SI Diamond Technology, Inc. of Housto Texas.

The absorption of energy of such materials is less in the range of wave-lengt from about 8 microns to about 14 microns than at wave-lengths outside this rang Diamond-like coatings will be particularly useful on mirrors used to reflect energy in t band of wave-length.

Example 1 A one-inch diameter copper mirror was obtained from Laser Power Optics of S Diego, California. The mirror was placed in a vacuum chamber such as chamber 30 Fig. 1. Pressure in the chamber was reduced to about 10 ^'7 torr. Laser power w concentrated on a graphite target to produce an energy density or about 10 ¹⁰ watts p square centimeter. A ring electrode such as electrode 18 was biased to -2.8 kilovolts. rotating substrate holder such as 24 was biased with RF pulses synchronized with t

laser pulses at a voltage of -250 volts. A coating having a thickness in the range from 0.25 to 0.5 microns was formed.

The coated mirror was removed from the vacuum chamber and standard tests, prescribed in MIL-C-48497, were performed to determine the durability of the coating.

For adhesion testing, the adhesive surface of a one-inch wide cellophane tape was firmly pressed against the coated surface and quickly removed at an angle normal to the coated surface. There were no effects of the tape on the coating surface.

A humidity test was performed by placing the mirror in an environmentally controlled chamber for 24 hours at a temperature of 120°F. and a relative humidity from 95 to 100%. There were no visible effects on the coating after the test.

Moderate and severe abrasion tests were performed on the mirror. In the moderate test a clean, dry laundered cheese cloth, ^! Λ-inch thick and 3/8-inch wide, was rubbed across the surface for at least 50 times with one pound of bearing force. In the severe abrasion test a standard eraser was rubbed 20 times on the coated surface with a bearing force of 2.5 pounds. Neither test produced any visible effect on the coating.

A salt fog test was performed by immersing the coated optical component for 24 hours in a solution of sodium chloride containing 6 ounces of salt in 1 gallon of water. Subsequent to this test the coated optical component was removed from the solution and dried with clean, laundered cheesecloth and then evaluated for performance. There was no visible effect on the coating.

Absorption of the laser beam from a carbon dioxide laser was measured using the "gradient method," prescribed by the International Standards Organization as No. ISO TC-172-SC 9 WG 6. In this method, the rates of change in temperature of the minor and holder assembly during irradiation of the mirror with a laser and during the period after the laser power is turned off are measured. Use of a procedure having both heating and cooling phases eliminates or minimizes the error due to losses from convection and thermal conduction. The following formula was used for calculating absorptance of energy:

WWtniά ΛbmUΨp mce .— — { AV-Wt heating phase ♦ άV t cootinf phase]

where, m is mass, c is specific heat (summed over all materials, i, present), P is po T is temperature and t is time.

The coated copper mirror was exposed to radiation from a 1.2 kilowatt carb dioxide laser for one minute. Longer exposure was not possible because the mirror not water cooled. The test showed 1.6% absorption on reflection of the laser beam, b before and after the environmental tests outlined above. Similar tests usi molybdenum coated copper mirrors showed 2.5% absorption of the energy.

It will be appreciated that while the present invention has been primar described with regard to the foregoing embodiments, it should be understood t variations or modifications may be made in the embodiments described herein witho departing from the broad inventive concept disclosed above or claimed hereafter.