WO2001013475A1

WO2001013475A1 - Method and apparatus for prevention of degradation of laser optical elements

Info

Publication number: WO2001013475A1
Application number: PCT/US2000/020776
Authority: WO
Inventors: Robert E. Basanese; Ted R. Weeden
Original assignee: Rofin-Sinar, Inc.
Priority date: 1999-08-13
Filing date: 2000-08-14
Publication date: 2001-02-22

Abstract

Degradation of an optical element (2), particularly one used in a high-power gas laser, is prevented by heating the optical element (2). The optical element (2) is maintained at a temperature that exceeds the dew point of gasses in the lasing medium (14) even when the laser is not operating to preclude formation of a condensate layer that can promote degradation of the optical element (2).

Description

METHOD AND APPARATUS FOR PREVENTION OF DEGRADATION OF LASER OPTICAL ELEMENTS

TECHNICAL FIELD This invention relates to maintaining performance of optical elements. In particular, the invention relates to the prevention of degradation of optical elements in high-power, gas-discharge lasers.

BACKGROUND ART

Prevention of decreases in the performance of optical elements is a known problem in the optical arts. In photography, for example, condensation of atmospheric moisture on a lens, or fogging, prevents the formation of a sharp image. It has been suggested that the lens be heated to prevent such condensation, as in US Patent

4,957,358 (Terada).

Reductions in the performance of optical components based on factors other than fogging are also known, such as "induced light absorption" caused by formation of

"F-centers" in certain refractory oxide materials, as taught in US Patent 4,740,988

(Knollenberg). This patent teaches that the formation of such F-centers can be prevented also by heating the optical elements.

Reductions in the performance of optical elements due to condensation of gasses present in a gaseous active medium are also known, and US Patent 5,224,110 (Hudson) teaches heating optical windows enclosing a gas-containing absorption cell to prevent such condensation. The absorption cell of Hudson is an accessory having a gas maintained at a particular vapor pressure to support selection of a tuned absorption line, the accessory being separate from the laser and located outside the laser cavity. Degradation of optical elements in a laser can be measured several ways. In high power gas lasers, one criterion (related to collimation of the laser light) is whether the system can focus the beam to a spot of a given size through an optical focusing element at a given distance. Using this criterion, the lifetime of the laser can be determined by the time over which such focus can be maintained.

Degradation in light collimation due to large scale fogging (i.e., drops formed on the surface leading to scattering of the light) caused by condensation is well known. Applicants have found, however, that laser beam collimation through optical elements in high-power gas lasers, such as lasers having power greater than 1 KW and preferably in the range of 5KW and higher, degrades even though large-scale fogging of the optical elements is precluded, for instance, because interaction with the laser beam heats the optical surface above the coating dew point.

SUMMARY OF THE INVENTION Applicants have discovered that reduction in the performance of optical elements of high-power gas lasers, such as the optics in 5KW and higher CO₂ lasers, can be caused by formation of absorbing components on the surface of the element that are not condensed gases. While the direct cause of formation of these components is not precisely known, applicants believe that this layer results from chemical decomposition of the optic element itself. For example, applicants have found that the absorbing layer for an optical element having a ZnSe coating includes very small (e.g., 50nm diameter) particles that are chemically high in Se, C, and O. Applicants believe that these particles are the result of a chemical reaction involving the ZnSe coating. This chemical reaction is believed to take place at an increased rate in a liquid environment created by condensation of gasses on the surface of the optical coating. Comprised of condensed vapors present in the optical cavity, this liquid coating creates an electrolyte that then provides the reaction environment for dissolving the optical coating by "trapping" ions present in the lasing gas.

Because the optical elements in high power lasers are heated by the laser light, it is believed that most of the condensation occurs when the laser is off or during a power-off or reduced-power part of a laser having power cycles. However, it is possible to sustain a small-scale condensation layer on the optic surface even though exposed to the full power beam, depending on the thermodynamic characteristics surrounding the optic surface and the degree of degradation on the optic surfaces. This small-scale layer could permit the beam to pass without significant scattering if the variation in thickness were less than the wavelength of the transmitted light.

In accordance with the invention, degradation in the performance of an optical element in a high-power gas laser is prevented by heating the surface of the optical element to prevent condensation while simultaneously providing a suitable heat sink and thermal control to prevent overheating of the element. In accordance with a further aspect of the invention, the surface of the optical element is heated even after the laser power has been turned off or reduced. Thus, contrary to the teachings in the prior art, applicants provide a method and an apparatus to prevent condensation continuously, not because the condensed gasses are themselves the direct cause of reduction in optical performance as in the case of lens fogging, but, instead, because they are an indirect cause of degradation that must be continuously prevented.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective of an optical cell for a gas-discharge laser in accordance with the invention. Figure 2 is a partial cross section of a prior art optical cell and resonator. Figure 3 is a partial cross section of an optical cell such as that shown in figure 1 attached to a resonator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 illustrates an optical cell of the type generally used in a gas discharge laser and modified in accordance with the invention. The optical cell includes an optical element 2 that is held between a front mounting ring 4 and a rear mounting ring 6. A chiller ring 8 is mounted to the rear mounting ring and is in thermal contact with the optical element 2 as will be described in more detail with reference to figures 2 and 3. Circulating water 16 provided through inlet conduit 10 and passing out through outlet conduit 12 cools the chiller ring 8.

In use, the assembly shown in figure 1 is attached to a resonator 22 (see figures 2 and 3) by bolts (not shown) that pass through holes 13 in the assembly and into the bulk metal of the resonator housing.

Figure 2 illustrates the operation of the prior art optical cell. In this system, the optical element 2, which is heated by contact with the lasing gasses and passage of the high-power laser beam, is cooled by the chiller 8 to provide a suitable heat sink. In this arrangement, the temperature of the optical element 2 is typically less than that of the lasing gas and greater than that of the chiller ring 8. This allows the chiller ring to remove heat from the optical element to maintain its temperature. The problem with the system shown in figure 2 and operated as described above is that a component of the gas 14 can condense on the surface S1 when the temperature of S1 falls below the dew point of that component. This is not typically a problem during operation of the laser because the heat supplied by the laser beam elevates the temperature of the element and because the gas 14 is usually uncontaminated by such gasses as water vapor. A problem arises, however, in the operation of high-power lasers because of the need to heat elements inside the chamber. As these elements reach elevated temperatures, they contribute impurities to the gasses, in correspondence with their vapor pressure behavior. These contaminant gasses reach a saturated equilibrium concentration inside the chamber with dew points corresponding to the temperature of each heated element. These impurities can then condense on S1. This is particularly possible after the laser has been turned off, because the optical element cools faster than do the gasses, resulting in condensation of the gasses onto S1. Then, the condensate dissolves some of the gas to create an electrolyte that has a highly corrosive effect on the optical coating. Recognizing this, applicants propose to maintain the temperature of the surface of the optical element such that this condensation is prevented. In the preferred embodiment, applicants heat the optical element, such as by an annular, flexible electric heating element 18. The heating element is connected to a source of electrical power (not shown) through leads 19. The heating element 18 is preferably held in place by a circumferential clamp 20, and a clamping strip 21 may be placed between the clamp and the heater.

To ensure good thermal contact between the heating element and the optic 2, a thin layer 24 of thermally conductive material, such as Indium, is placed between the heating element and the optical element. Because this material is "dead-soft," it is placed in a v-groove formed between an angled face of the heater 18 and the edge of the optic 2. The layer 24 is annular and may be wedge-shaped in cross section as well to conform to the shape of the groove. The layer is pressed against the edge of the optic by being continually urged into the groove by a resilient o-ring 26. The o-ring is carried in a channel 28 in the front mounting ring 4. Other resilient elements, such as springs, may be used and other arrangements for urging the thermally conductive element 24 against the optic may be used. For example the clamp 20 may have a resilient component.

Other heating elements can be used, such as fluid or chemically heated elements. Heating element 18 is preferably arranged to heat the optical element such that its temperature is maintained above that of the lasing medium 14. At the same time, the temperature of the chiller ring is maintained lower than that of the optical element. By this arrangement, the chiller ring withdraws heat from the optical element and does not add heat to the mounting ring or resonator. Thus the mounting ring and other components of the resonator structure remain stable and not distorted. In a preferred embodiment, the heating strip is heated by providing 3.8 amperes of electrical current at 22 volts. The optical element is preferably maintained at 65°C. This temperature may be higher but does not exceed 140°C. The upper limit is specified to prevent accelerated formation of surface oxides.

Another method of accomplishing the objective of maintaining the temperature of the optical element is to increase the temperature of the water 16 circulating in the chiller ring such that the temperature of the optic is greater than that of the lasing medium. In that case, the temperature of the chiller ring would be raised so that the temperature of the optic is greater than that of the lasing medium, but that might distort the mounting ring and the resonator 22 by raising their temperatures. Thus, an alternative would be to provide two fluid circuits, one to heat the optic to the desired temperature and another to maintain the desired temperature of the resonator. Or, an element that increases the thermal resistance between the optic and the chiller could be used, such as an insulating element. This solution presents a control problem, however, because the amount of thermal resistance must be a function of the laser power. The invention further contemplates the use of electronic control circuits to control the electric heating of the heater 18 by measuring such parameters as laser power, laser medium temperature, laser medium gas pressures, optic temperature, resonator temperature, chiller temperature, and the like. Measurement of these parameters can be used to determine whether condensation will occur and to adjust the parameters to avoid such condensation.

In accordance with a method of the invention, the heater element 18 is activated whenever condensation could occur. For example, the heater element is activated even after the laser itself has been inactivated to preclude condensation and resulting degradation as described above. Further, the heater element is activated during reduced-power periods of lasers controlled to provide a power cycle. Thus, the heater is activated to prevent condensation on the optical element at all times to prevent chemical degradation of the element as described above.

The heater can be activated to heat the optic continuously or in accordance with the laser's power cycle. Further, the heater can be activated for a predetermined period of time after the laser has been turned off. That period of time can be ascertained by analysis of the thermal cooling of the laser or empirically. As well, the length of time can be determined by measuring any of the noted parameters that indicate the potential for condensation and turning off the heater when condensation will no longer occur.

Modifications within the scope of the appended claims will be apparent to those of skill in the art.

Claims

We claim:

1. A gas laser of the type having at least one optical element in contact with a lasing medium, wherein the improvement comprises means for maintaining continuously the temperature of said optical element above the dew point of gasses in said medium.

2. A laser according to claim 1 wherein said heater comprises a resistance heater.

3. A laser according to claim 1 wherein said heater comprises a fluid heater.

4. A laser according to claim 1 wherein further comprising means for cooling said optical element.

5. A laser according to claim 4 wherein said means for cooling comprises a chiller ring.

6. A laser according to claim 1 wherein said heater comprises an electric heater and a thermally conductive element between said heater and said optical element, and wherein said thermally conductive element fits in an annular groove formed by said heater and said optical element.

7. A laser according to claim 6 wherein said annular groove is v-shaped in axial cross section and further comprising a resilient element urging said thermally conductive element into said groove.

8. A laser according to claim 7 wherein said resilient element is an o-ring.

9. An optical cell comprising an optical element, a mounting ring, a chiller in thermal contact with said optical element, and a heating element in thermal contact with said optical element.

10. An optical cell according to claim 9 wherein said optical element is adapted to form part of a laser cavity.

11.An optical cell according to claim 10 further comprising means for attaching said optical cell to a laser resonator.

12. An optical cell according to claim 11 wherein said heating element is adapted to continuously prevent condensation of laser gasses on said optical element.

13. An optical cell according to claim 9 wherein said heating element comprises an electric heater and a thermally conductive element between said heater and said optical element, and wherein said thermally conductive element fits in an annular groove formed by said heater and said optical element.

14. A laser according to claim 13 wherein said annular groove is v-shaped in axial cross section and further comprising a resilient element urging said thermally conductive element into said groove.

15. A laser according to claim 14 wherein said resilient is an o-ring.

16. A method for preventing degradation of an optical element in a gas laser comprising continuously preventing condensation of gasses on said element.

17. A method according to claim 16 wherein said step of preventing comprises the step of heating at least a portion of said optical element.

18. A method according to claim 17 wherein said step of heating comprises heating a surface of said optical element in contact with said gasses.