Elsevier

Optics & Laser Technology

Volume 44, Issue 6, September 2012, Pages 1823-1828
Optics & Laser Technology

Polarized reflectance of aluminum and nickel to 532, 355 and 266 nm Nd:YAG laser beams for varying surface finish

https://doi.org/10.1016/j.optlastec.2012.02.014Get rights and content

Abstract

Polarized reflectances of Al and Ni treated by two types of surface finish, namely lapping and electro-polishing, for 532, 355, and 266 nm wavelength laser beams, are measured and compared with the theoretical polarized reflectance. For metals treated by lapping, there exists a distinct trend coming from the surface directionality that results from the mechanical surface finish. The reflectance for the case where the surface directionality is parallel to the electric field of incident beam is higher than the case where surface directionality and electric field are perpendicular to each other. This trend is not observed in metals treated by electro-polishing having no surface directionality. Our studies provide reference to the surface finish effects on the reflectance of Al and Ni for various wavelength beams, providing insight into the improvement of laser processing efficiency by adjusting surface directionality of metals and polarization of beam.

Highlights

► Linearly polarized reflectances of Al and Ni for 532, 355, and 266 nm beams are measured. ► Two different surface finishes (lapping and electro-polishing) are considered. ► Absorptance is lower when surface directionality is parallel to electric field of incident beam. ► Absorptance is higher when surface directionality is perpendicular to electric field of beam.

Introduction

In the studies about reflectance being dependent on the surface roughness of metals, topics that include reflectance of stainless steel and nickel to HeNe laser beam [1], reflectance of nickel, aluminum, copper and brass specimens to HeNe laser and Ar laser [2], hemispherical reflectance of metals as functions of wavelength and surface roughness [3], and absorptances of stainless steel and nonferrous metals (Al, Cu, and Zn) to Nd:YLF and Nd:YAG laser beams according to surface conditions by integrating sphere [4], [5] have been reported in the literature. Other studies have focused on the temperature-dependent absorptance of aluminum, stainless steel and titanium with paint coatings to Nd:YAG laser and CO2 laser [6] and absorption of metallic alloys to Nd:YAG laser beam by thermocouples [7].

In the laser cutting of metals, a-priori knowledge on the absorptance of metals is an important factor of the laser process. Various studies of laser cutting of metals include topics such as the three-dimensional (3-D) theory of laser cutting influenced by beam polarization [8], the theoretical and experimental features of laser cutting with plane polarized radiation [9], the effects of radial tangential polarization in laser material processing [10], and laser cutting with P-polarized beam at the cut front [11]. Laser cutting with absorption efficiency of materials has been principally dealt with in these previous works.

In this paper, two types of surface finish methods are used: first being lapping and the other being electro-polishing. Lapping is a mechanical method widely used in a number of precision manufacturing applications [12]. Electro-polishing is the anodic method used for the surface finish of metals to make a fine surface with brightness [13]. We measured linearly polarized reflectance of Al and Ni with lapping and electro-polishing for 532 nm, 355 nm, and 266 nm wavelength beams. We chose angle of incidence 45° because the experimental and theoretical values of S-polarized and P-polarized reflectances were the most obvious.

This study provides reference data of polarized reflectance of Al and Ni for various wavelength beams according to the surface finish. Furthermore, a relationship between surface directionality and polarization of laser beam is provided, and from this we suggest the possibility of improving laser cutting efficiency in regard to the surface directionality of metals. This paper is about experimental and theoretical studies where there are no previous works dealing with reflectance for both surface directionality and polarization. The information provided would serve useful to the laser processing community.

Section snippets

Theory of reflectance

Reflectance or absorptance of metals is described by Fresnel's law, and theoretical linearly polarized reflectance is considered in two different ways. The electric vector of the incidence beam being perpendicular to the plane of incidence is termed transverse electric (TE) polarization, perpendicular polarization, or S-polarization. Another case where the electric vector of the incidence beam is parallel to the plane of incidence is called transverse magnetic (TM) polarization, parallel

Experimental setup

We use the energy sensors (J-50MB-YAG, Coherent) to measure the polarized reflectance at 45°. Although use of integrating sphere for measuring reflectance is a common method, it suffers from known shortcomings such as light losses through beam port, directional dependency of beam scattering, diffraction effects in apertures and imperfect reflection of inner surface [4], [5], [21], [22]. A different approach taken in the study is similar to the method for measuring specular reflectance, and it,

Results and discussion

The polarized reflectance of metals was measured by the approach outlined previously, and we compared the measurements with the theoretical values. Fig. 4 shows theoretical and experimental reflectances of metals treated by lapping and electro-polishing, and error bars are represented by maximum and minimum values in the experiments.

Theoretically, S-polarized reflectance is higher than P-polarized reflectance as confirmed by the present experiment for Al and Ni treated by both lapping and

Conclusion

In this study, the polarized reflectances of Al and Ni treated by both lapping and electro-polishing for three wavelengths are measured and compared with the theoretical reflectances. Although roughness of samples with electro-polishing is higher than the ones with lapping, the reflectance of sample treated by electro-polishing is higher than the one by lapping because of the steepness of the peaks and valleys on the surface. Also, for the samples with lapping, we found an unusual trend

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