Abstract

Mechanically polished fused silica surfaces were heated with continuous-wave CO2 laser radiation. Laser-damage thresholds of the surfaces were measured with 1064-nm 9-nsec pulses focused to small spots and with large-spot, 1064-nm, 1-nsec irradiation. A sharp transition from laser-damage-prone to highly laser-damage-resistant took place over a small range in CO2 laser power. The transition to high damage resistance occurred at a silica surface temperature where material softening began to take place as evidenced by the onset of residual strain in the CO2 laser-processed part. The small-spot damage measurements show that some CO2 laser-treated surfaces have a local damage threshold as high as the bulk damage threshold of SiO2. On some CO2 laser-treated surfaces, large-spot damage thresholds were increased by a factor of 3–4 over thresholds of the original mechanically polished surface. These treated parts show no obvious change in surface appearance as seen in bright-field, Nomarski, or total internal reflection microscopy. They also show little change in transmissive figure. Further, antireflection films deposited on CO2 laser-treated surfaces have thresholds greater than the thresholds of antireflection films on mechanically polished surfaces.

© 1982 Optical Society of America

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References

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  1. P. A. Temple, D. Milam, W. H. Lowdermilk, “CO2-laser polishing of fused silica surfaces for increased laser damage resistance at 1.06 μm,” Natl. Bur. Stand. (U.S.) Spec. Publ. 568 (1979), p. 229.
  2. P. A. Temple, M. J. Soileau, “1.06 μm laser-induced breakdown of CO2-laser-polished fused SiO2,” Natl. Bur. Stand. (U.S.) Spec. Publ. 620 (1980), p. 180.
  3. P. A. Temple, Appl. Opt. 20, 2656 (1981).
    [CrossRef] [PubMed]
  4. J. Swain, Lawrence Livermore National Laboratory; private communication.
  5. R. B. Sosman, The Phase of Silica (Rutgers U. Press, N.J., 1965), p. 52, “The first effect of high temperature on pure silica, whether in the form of crystalline quartz or vitreous silica, is to produce cristobalite.”

1981 (1)

1980 (1)

P. A. Temple, M. J. Soileau, “1.06 μm laser-induced breakdown of CO2-laser-polished fused SiO2,” Natl. Bur. Stand. (U.S.) Spec. Publ. 620 (1980), p. 180.

1979 (1)

P. A. Temple, D. Milam, W. H. Lowdermilk, “CO2-laser polishing of fused silica surfaces for increased laser damage resistance at 1.06 μm,” Natl. Bur. Stand. (U.S.) Spec. Publ. 568 (1979), p. 229.

Lowdermilk, W. H.

P. A. Temple, D. Milam, W. H. Lowdermilk, “CO2-laser polishing of fused silica surfaces for increased laser damage resistance at 1.06 μm,” Natl. Bur. Stand. (U.S.) Spec. Publ. 568 (1979), p. 229.

Milam, D.

P. A. Temple, D. Milam, W. H. Lowdermilk, “CO2-laser polishing of fused silica surfaces for increased laser damage resistance at 1.06 μm,” Natl. Bur. Stand. (U.S.) Spec. Publ. 568 (1979), p. 229.

Soileau, M. J.

P. A. Temple, M. J. Soileau, “1.06 μm laser-induced breakdown of CO2-laser-polished fused SiO2,” Natl. Bur. Stand. (U.S.) Spec. Publ. 620 (1980), p. 180.

Sosman, R. B.

R. B. Sosman, The Phase of Silica (Rutgers U. Press, N.J., 1965), p. 52, “The first effect of high temperature on pure silica, whether in the form of crystalline quartz or vitreous silica, is to produce cristobalite.”

Swain, J.

J. Swain, Lawrence Livermore National Laboratory; private communication.

Temple, P. A.

P. A. Temple, Appl. Opt. 20, 2656 (1981).
[CrossRef] [PubMed]

P. A. Temple, M. J. Soileau, “1.06 μm laser-induced breakdown of CO2-laser-polished fused SiO2,” Natl. Bur. Stand. (U.S.) Spec. Publ. 620 (1980), p. 180.

P. A. Temple, D. Milam, W. H. Lowdermilk, “CO2-laser polishing of fused silica surfaces for increased laser damage resistance at 1.06 μm,” Natl. Bur. Stand. (U.S.) Spec. Publ. 568 (1979), p. 229.

Appl. Opt. (1)

Natl. Bur. Stand. (U.S.) Spec. Publ. 568 (1)

P. A. Temple, D. Milam, W. H. Lowdermilk, “CO2-laser polishing of fused silica surfaces for increased laser damage resistance at 1.06 μm,” Natl. Bur. Stand. (U.S.) Spec. Publ. 568 (1979), p. 229.

Natl. Bur. Stand. (U.S.) Spec. Publ. 620 (1)

P. A. Temple, M. J. Soileau, “1.06 μm laser-induced breakdown of CO2-laser-polished fused SiO2,” Natl. Bur. Stand. (U.S.) Spec. Publ. 620 (1980), p. 180.

Other (2)

J. Swain, Lawrence Livermore National Laboratory; private communication.

R. B. Sosman, The Phase of Silica (Rutgers U. Press, N.J., 1965), p. 52, “The first effect of high temperature on pure silica, whether in the form of crystalline quartz or vitreous silica, is to produce cristobalite.”

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Figures (12)

Fig. 1
Fig. 1

Single-pass CO2 laser-polishing apparatus. The sample is moved left and right by a variable-speed motor and up and down by hand.

Fig. 2
Fig. 2

Multipass CO2 laser-polishing apparatus.

Fig. 3
Fig. 3

Raster scan pattern used to treat 1.5-in. diam fused-silica samples in the multipass CO2 laser-polishing scheme.

Fig. 4
Fig. 4

Appearance of the surface of fused silica before and after single-pass laser polishing as seen using TIRM.

Fig. 5
Fig. 5

Two TIRM photographs showing the lack of glazing in parts processed by the multipass technique. The part shown is a 200-W (Schedule B) sample.

Fig. 6
Fig. 6

Replica electron micrograph of damage pits caused by a 1064-nm 1-nsec laser pulse on the surface of a single-pass CO2 laser-polished surface.

Fig. 7
Fig. 7

A 1064-nm 1-nsec damage threshold on fused silica surfaces as a function of treatment schedule used in multipass CO2 laser polish.

Fig. 8
Fig. 8

Bar graph showing the number of sites which survived 100 shots at each of two intensities on four multipass CO2-polished samples and on a typical mechanically polished sample. The shots were illuminated on a 0.5-mm2 grid with only one exposure per site.

Fig. 9
Fig. 9

Beam energy required to cause damage as a function of beam waist position. The solid lines are not mathematical fits but are hand-sketched to aid in interpretation.

Fig. 10
Fig. 10

Residual strain present in three parts processed as per Table I. These photographs are the appearance as seen through a polariscope.

Fig. 11
Fig. 11

Transmissive or bulk figure generated by interference between the front and rear surfaces of three parts produced as per Table I.

Fig. 12
Fig. 12

A schematic representation of the mechanism for producing strain in a CO2 laser-processed sample. In (a) the part is being irradiated from above. The result is a nonuniform temperature distribution causing the hot upper surface to bow due to greater expansion. In (b), a critical flow temperature, Tc, has been exceeded, causing the irradiated surface to flow outward. (c) When the part is cooled, the irradiated surface will go into tension.

Tables (4)

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Table I CO2 Laser-Polishing Treatment Schedulesa

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Table II Fused Silica Entrance Surface Damage

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Table III Number of Undamaged Sites (Out of Thirty-Three Exposures) on Mechanically Polished Fused Silica Surfaces

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Table IV Large-Spot Laser-Damage Threshold of Antireflectlon Films Deposited on Mechanical- and Laser-Polished Surfaces

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