Reflectivity of steel at 10.6-µm wavelength

David M. Roessler and Victor G. Gregson, Jr. When this work was done both authors were with General Motors, Warren, Michigan 48090. D. M. Roessler is with the Research Laboratories, Physics Department, and V. G. Gregson, Jr., is now with GTE Sylvania, Mountain View, California 94042. Received 31 October 1977. 0003-6935/78/0401-0992$0.50/0. © 1978 Optical Society of America.

We report measurements with a pulsed CO2 laser of the reflectivity of AISI 1045 steel at 10.6 µm for pulse energy densities up to 25 J/cm2, corresponding to peak irradiances of up to 5 ×107 W/cm2.

Although CO2 lasers provide a convenient means to process metals, the high ir reflectivity of the latter allows only a small fraction of the incident power to be utilized. This reflectivity is expected to remain high as the intensity is increased until the threshold for surface damage is reached. This is typically above 107 W/cm2 for many materials.1–2 However, some re­cent work on copper3-4 at 1.06 µm showed a significant drop in the reflectivity at laser intensities believed to be below the permanent damage threshold. Further, an early study5 of the reflectivity of several steels, with pulsed CO2 radiation at 10.6 µm, showed large decreases occurring at intensities as low as 105 W/cm2. Our present data on AISI 1045 steel, which pre­viously showed the strongest effect,5 do not exhibit any such anomalous behavior.

The experimental arrangement is shown in Fig. 1 and is basically similar to that used in the earlier study5 but addi­tionally allows the absorptivity to be monitored. The pulsed CO2 TEA laser used to obtain high peak power densities yields a sharp spike of about 0.5-µsec duration followed by a long tail. The double mirror calorimeter AB absorbs only a few percent of the incident beam which is then focused by a potassium chloride lens. The double mirror calorimeter and the ab­sorbing calorimeter were used to monitor the pulse energy, thermopiles attached to them yielding a voltage-time plot from which the energy was computed. Electrical heaters wound around the calorimeters provided calibration. The potassium chloride lens was used to control the beam spot size at the sample surface, and the total range of power densities examined was covered by varying both the lens working dis­tance and the laser power supply voltage. In practice the maximum energy density used (25 J/cm2) was determined partly by air and near-surface breakdown problems. Beam

Fig. 1. Schematic of reflectivity measurement. The thermopile outputs from calorimeters A, B, and C are processed by a computer

to yield laser pulse energy.

patterns observed on photographic emulsions were slightly elliptical rather than circular and showed some nonuniformity of intensity. The spot size could be determined only to within a factor of 2 for diameters less than about 1 mm, and our es­timates of the area and hence the energy density have a min­imum uncertainty of about a factor of 4.

The experimental procedure was to calibrate the calorim­eters with respect to each other, with and without the sample in place. With the sample removed, the absorbing calorimeter C is positioned to intercept the laser beam as shown by the broken line of Fig. 1. When the sample is in place, C is rotated to pick up the reflected beam. The reflectance is then simply the ratio of the two energies measured by C for a given value of pulse energy as measured by the double mirror calorimeter. The calorimeter C had a 25-mm diam aperture and collected all light within a cone of semivertex angle 3°. In practice the received beam was never wider than 10 mm at C, at least for all measurements below 5 J/cm2. The instantaneous reflec­tance at any power density can only be measured by time-resolving the laser pulse, and a comprehensive set of such measurements on steel has recently been reported by Ready.6

The data presented here therefore refer to an energy reflec­tance in that they are not time-resolved and thus represent a reflectance averaged over the whole pulse length. If any large change in reflectivity occurred at some intensity reached during a pulse the averaged reflectivity should also show some change. It was under these conditions in fact that the earlier anomaly was reported.5 Any change in the reflectivity, other than merely a redistribution of the specularly and diffusely reflected light, should result in a corresponding change in the absorptivity. A thermopile was therefore buried near the sample surface to monitor any change in the thermal coupling between the laser and the steel.

All data were taken in a normal laboratory air atmosphere at near-normal incidence and on both freshly polished and old surfaces. Measurements were repeated over a period of several months.

These data are shown in Fig. 2 for both the reflectivity and the thermal coupling coefficient. The latter was obtained from the thermopile reading which was calibrated by assuming that, at low incident energy densities on a smooth surface, the coupling coefficient is simply the absorptivity. Although the data scatter is large, the basic trend is quite clear as shown by the lines drawn as visual aids. The reflectivity remains high, near 90%, until energy densities of above 5 J/cm2 are attained; the thermal coupling coefficient similarly shows little change. For comparison with the earlier work,5 we have used an ef­fective pulse width of 0.5 µsec to estimate the peak power in­tensity reached during the pulse. Thus the optical properties do not change appreciably until a peak intensity of above 107

W/cm2 is reached. At higher levels there is a drastic decrease in the reflectivity and an associated increase in the coupling coefficient. Examination of the surface also revealed that damage occurs at these levels. A recent very comprehensive study6 of the reflectivity of stainless steel near 10 µm for a variety of atmospheres also suggests that the reflectivity is high until intensities above 107 W/cm2 are reached. We have not explored the mechanism for the observed reflectivity change, but we can estimate the laser intensity required for surface damage from melting. If we take a mean value of 0.1 cm2/sec for the thermal diffusivity of steel, for a laser pulse width of 0.5 µsec, the diffusion length L is about 2 µm, i.e., much less than the laser spot diameter (>1 mm) at the metal surface. We can therefore approximate the temperature in­crease ΔT produced by laser beam of intensity I as follows:

992 APPLIED OPTICS / Vol. 17, No. 7 / 1 April 1978

Fig. 2. The reflectivity and thermal coupling coefficient of AISI 1045 steel at 10.6 µm. The peak intensity is estimated on the basis of a

0.5-µsec pulse duration.

For a thermal conductivity K of 0.4 W/cm K and a reflectivity R of 0.9, we thus have ΔT = 5.6 × 10–5 I K. The melting point of AISI 1045 steel is about 1700 K, and thus ΔT is about 1400 K. Thus the required laser intensity for melting is about 2.5 × 107 W/cm2, which is consistent with an interpretation of surface damage for the observed reflectivity change.

At laser intensities above 107 W/cm2 a variety of other in­teractions at the surface can occur such as breakdown in un-filtered air, plasma formation and so on. Therefore, the ob­served changes in reflectivity and thermal coupling coefficient may not necessarily be ascribed solely to surface melting. The origin of the reported anomaly5 is not clear, particularly as we were able to use some of the original samples but were unable to repeat the data. We can only speculate that it was an ar­tifact associated with data scatter. The latter was reduced in the present work by very extensive repeated calibration and by collecting data sets in continuous time spans. Our over-all conclusion is that the reflectivity of steel shows no anomalous behavior and is about 0.9 or above, depending on the surface condition, until energy densities above 5 J/cm2 (for 0.5-µm pulses) are reached.

We greatly appreciate the competent technical assistance of John M. Paavola and Paul L. Frechette throughout this work.

References 1. N. G. Basov, V. A. Boiko, O. N. Krokhin, O. G. Semenoy, and G.

V. Sklizkov, Sov. Phys. Tech. Phys. 13, 1581 (1969). 2. A. M. Bonch-Bruevich, Ya. A. Imas, G. S. Romanov, M. N. Li-

benson, and L. N. Maltsev, Sov. Phys. Tech. Phys. 13, 640 (1968).

3. T. E. Zavecz, M. Saifi, and M. Notis, Appl. Phys. Lett. 26, 165 (1975).

4. J. C. Koo and R. E. Slusher, Appl. Phys. Lett. 28, 614 (1976). 5. B. A. Sanders and V. G. Gregson, Jr., in Proceedings Electro-

Optical Systems Design Conference, New York (September 1973), pp. 24-29.

6. J. F. Ready, IEEE J. Quantum Electron. QE-12, 137 (1976).

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