Method for improving the resistance to laser flux of an optical component

Abstract

A method for improving the properties of resistance to laser flux of an optical component, comprising a step consisting in bringing the component into contact with an aqueous solution comprising at least one hydroxide of an alkaline metal or an alkaline earth metal in a quantity of between 5 and 30 mass % and having a temperature T of between 50 and 100° C.

Claims

1. Method for improving the properties of laser flux resistance of an optical component made of fused silica, comprising a step consisting of placing said optical component made of fused silica in contact with an aqueous solution comprising at least one hydroxide of an alkaline metal or alkaline-earth metal in an amount of between 5 and 30 mass % and having a temperature T of between 50 and 100° C., the damage probability on the backside in relation to the direction of propagation of the laser beam, for laser having a wavelength of 355 nm, pulse of 3 ns and fluence of between 19 and 22 J/cm.sup.2, of the optical component made of fused silica treated with said method being reduced by at least 10% compared with damage probability on the backside in relation to the direction of propagation of the laser beam of a non-treated optical component made of fused silica.

2. The method according to claim 1, wherein said aqueous solution is selected from among an aqueous solution of sodium hydroxide (NaOH), an aqueous solution of potassium hydroxide (KOH), an aqueous solution of calcium hydroxide (Ca(OH).sub.2), an aqueous solution of lithium hydroxide (LiOH), an aqueous solution of caesium hydroxide (CsOH) and mixtures thereof.

3. The method according to claim 1, wherein it comprises the following successive steps: a) preparing the aqueous solution comprising at least one hydroxide of an alkaline metal or alkaline-earth metal in an amount of between 5 and 30 mass % and having a temperature T of between 50 and 100° C.; b) contacting the optical component made of fused silica with the solution prepared at step (a), said contacting being conducted at said temperature T; c) rinsing the optical component made of fused silica with deionized water, distilled water or ultrapure water.

4. The method according to claim 1, wherein, for said contacting, said optical component made of fused silica is vertically immersed and vertically held in said aqueous solution comprising at least one hydroxide of an alkaline metal or alkaline-earth metal.

5. The method according to claim 1, wherein said contacting is performed under agitation.

6. The method according to claim 1, wherein throughout said contacting the volume of said aqueous solution comprising at least one hydroxide of an alkaline metal or alkaline-earth metal is permanently completed with ultrapure or demineralized water to offset the evaporation induced by the temperature of the solution higher than 50° C.

7. The method according to claim 1, wherein throughout said contacting the temperature of said aqueous solution comprising at least one hydroxide of an alkaline metal or alkaline-earth metal is maintained at said temperature T.

8. The method of claim 1, wherein the optical component is transparent following the step of contacting the optical component with the aqueous solution.

9. Method for improving the properties of laser flux resistance of an optical component, consisting of: (a) optionally preparing an aqueous solution comprising at least one hydroxide of an alkaline metal or alkaline-earth metal in an amount of between 5 and 30 mass % and having a temperature T of between 50 and 100° C.; (b) contacting said optical component with the aqueous solution, said contacting being conducted at said temperature T; (c) optionally rinsing the optical component with deionized water, distilled water or ultrapure water; the damage probability on the backside in relation to the direction of propagation of the laser beam, for laser having a wavelength of 355 nm, pulse of 3 ns and fluence of between 19 and 22 J/cm.sup.2, of the optical component treated with said method being reduced by at least 10% compared with damage probability on the backside in relation to the direction of propagation of the laser beam of a non-treated optical component of same type.

10. The method according to claim 9, wherein said optical component is selected from the group consisting in laser glass, polarizers, mirrors, lenses, diffractive optics such as phase gratings and wave plates, and view ports.

11. The method according to claim 9, wherein the material of said optical component is selected from the group formed by silicate glass, borosilicate glass, aluminosilicate glass, boro-aluminosilicate glass, fused silica and phosphate glass.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates characterization with the DMS system of samples either before (FIG. 1A) and after chemical treatment with an acidic solution of HF/HNO.sub.3 (FIG. 1B), or before (FIG. 1C) and after chemical treatment with a basic KOH solution (FIG. 1D).

(2) FIG. 2 illustrates the surface topography and corresponding roughness values obtained with the samples before («New»; FIG. 2A) and after chemical treatment either with a basic KOH solution («KOH treated»; FIG. 2B), or with an acidic solution of HF/HNO.sub.3 («Treated with HF/HNO.sub.3»; FIG. 2C).

(3) FIG. 3 illustrates the probability of sample damage before («New control sample») and after chemical treatment either with an acidic solution of HF/HNO.sub.3 («HF/HNO.sub.3»), or with a basic KOH solution («KOH»).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(4) I. Preparation of the Corrosive Solution.

(5) To prepare a corrosive potassium hydroxide solution (KOH) with a concentration of 30 mass %, 150 g of KOH pellets (AnalaR NORMAPUR 26688.296 or equivalent) are dissolved under heat with 350 ml of ultrapure or distilled water in a beaker of 500 ml capacity. To promote this operation, and improve the homogeneity of the solution, manual or magnetic stirring is used in addition to heating of the solution. The solution is ready for use when all the pellets have been dissolved. This volume of solution is specially adapted for the tests described below on optical components of circular shape and diameter of 50 mm.

(6) The solution thus prepared is brought to the desired treatment temperature. In the examples below this temperature is 100° C. The solution is therefore termed a «corrosive treatment solution» in the remainder hereof. The temperature of this solution must be maintained at the desired temperature throughout the following operations.

(7) II. Treatment of the Optical Ccomponent.

(8) The optical component to be treated is inserted in an adapted holder system, allowing handling without any risk of contamination (strap). The use of a device in Teflon© is recommended, taking care that the optical component is handled the least possible. The whole is called a «strapped component» below.

(9) The strapped component is immersed vertically in the corrosive treatment solution. This step can lead to a variation in temperature of the corrosive treatment solution. It is then necessary to adapt the set temperature of the heating means. The strapped component is held in position throughout the treatment time, under agitation.

(10) For example, to erode 1 μm of silica, the strapped component remains in place about 45 min in a corrosive treatment solution with 30 mass % KOH held at 100° C.

(11) Throughout the treatment time, ultrapure or demineralized water is added to the corrosive treatment solution to offset the evaporation induced by the temperature of the solution higher than 50° C. For this purpose, a reflux assembly is used.

(12) Once the treatment time is reached, the strapped component must be lifted from the solution and abundantly rinsed in ultrapure or distilled water. To dry the optical component, the strapped component is left in vertical position in a clean environment.

(13) III. Advantages of the Method.

(14) III.1. Material and Methods.

(15) To demonstrate the advantages, the results obtained on 3 samples are given below. The analysed samples were three disks of polished fused silica 50 mm in diameter. The first was surface eroded for a time of between 7 and 10 h, using a chemical solution of hydrofluoric acid HF and nitric acid HNO.sub.3. This solution was an aqueous solution having ultrapure water as solvent, and having a HF/HNO.sub.3 mass ratio of 1:8. The second was eroded for a time of between 7 and 10 h with a potash KOH solution conforming to the protocol described under item II above. The last was kept intact to be used as control sample.

(16) The surface condition of these samples was characterized in different manners with: observations of scatter defects using the DMS system; roughness measurements using an optical roughness meter; roughness measurements using an atomic force microscope AFM; measurements of surface energy and wettability; and measurements of laser flux resistance.

(17) III.2. Defects Observed with the DMS System.

(18) The system called DMS, abbreviation for «Defects Mapping System», entails illuminating a sample via the edges using a LED light ring and taking a picture of the surface of the sample. Any defects on the surface scatter LED light and appear white in the photo.

(19) DMS photos grouped together in FIG. 1 were taken on chemically treated samples before (FIG. 1A and FIG. 1C) and after treatment (FIGS. 1B and 1D) to observe the effect thereof on the number of surface defects.

(20) The points in the centre and at the top of the samples are deliberate indents intended as reference points. They are not to be taken into consideration when assessing changes in surface condition after the chemical treatments.

(21) It can be seen that the surface condition of the sample treated with a hydrofluoric acid solution is strongly degraded (FIG. 1B). Treatment with HF/HNO.sub.3 led to the onset of a highly scattering white veil made extremely visible with this system of observation. On the contrary, the surface condition of the sample treated with the basic KOH solution was not degraded (FIG. 1D). The traces that can be seen at the bottom and top right of the sample after treatment are traces left by cleaning performed after treatment.

(22) III.3. Roughness.

(23) A. Examined Using an Optical Roughness Meter.

(24) Roughness measurements were taken before and after treatment, using an optical roughness meter.

(25) The device has three different magnification lenses allowing observation of defects having a spatial period ranging from 1 mm to 1 μm. The values given in Table 1 below are RMS roughness values.

(26) TABLE-US-00001 TABLE 1 RMS roughness measurements obtained with optical roughness meter on samples before and after treatment X1 X10 Spatial Spatial period X100 period between between Spatial period between 1 mm and 100 μm 100 μm and 10 μm 10 μm and 1 μm Treatment with HF/HNO.sub.3 Before 0.4 nm 0.2 nm 0.4 nm After 1.3 nm 1.2 nm 3.2 nm Treatment with KOH Before 0.4 nm 0.2 nm 0.4 nm After 0.3 nm 0.2 nm 0.2 nm

(27) Treatment with the acidic solution HF/HNO.sub.3 degraded roughness over the three ranges of spatial period. The strongest increase was observed with the ×100 lens, i.e. for small periods. On the contrary, the basic treatment with KOH did not degrade roughness. This treatment even improved roughness when measured with the ×100 lens. The term «chemical smoothing» effect can be given to the treatment subject of the present invention.

(28) B. Examined Using an Atomic Force Microscope (AFM).

(29) Roughness measurements on nanometric scale were taken using an atomic force microscope (AFM) on the two chemically treated samples and on the new control sample. The analysis area measured 5 μm×2.5 μm. The results are given in FIG. 2 where the value indicated in each image is the RMS roughness value in the observed area.

(30) Treatment with KOH (FIG. 2B) doubled surface roughness compared with the non-treated surface (FIG. 2A), whereas acidic treatment with HF/HNO.sub.3 (FIG. 2C) multiplied roughness by 20. These characterizations are coherent with the roughness measurements given under item III.3.A above which show a very strong increase in roughness with this type of acid chemical treatment and in particular in defects of small spatial periods (×100 lens).

(31) III.4. Surface Tension and Wettability.

(32) Surface tension was measured for each sample 2 h30 after chemical treatment. Surface tension (surface energy of the glass) after treatment with KOH (64 mN/m) was significantly higher than for the sample treated with acid HF/HNO.sub.3 (56 mN/m), indicating the better wettability of the KOH-treated surface.

(33) III.5. Laser Flux Resistance.

(34) UV (355 nm) laser flux resistance tests were performed on these samples. The counted defects relate to damage created by the laser on the exit side of the component.

(35) The test procedure followed was 10:1; i.e. to be declared «non-damaged» each test site must undergo 10 laser pulses at constant energy density per surface unit (fluence) without the onset of a defect.

(36) The damage curves are given in FIG. 3.

(37) It can be seen that the sample treated with the HF/HNO.sub.3 solution did not improve in performance regarding laser flux resistance. On the contrary, the sample treated with the KOH basic solution reduced its damage probability compared with the new control sample (curve shift towards strong fluence). Treatment with KOH therefore distinctly improved performance in terms of laser flux resistance.

BIBLIOGRAPHIC REFERENCES

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