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Correlation holographic interferometry applied for studies of laser-induced erosion
1 July
1995
OPTlCS COMMUNICATIONS ELSEWIER
OpticsCommunications 118 (1995) 21-24
Correlation holographic interferometry applied for studies of laser-induced erosion G.V. Dreiden, I.V. Semenova A. F. loffe Physical Technical Institute, 26, Polytekhnischeskaya,
St. Petersburg,
194021, Russia
Received5 July 1994;revised version received 13 January 1995
Abstract The correlation holographic interferometry is applied to studies of laser-induced erosion of solid surface and, in particular, to determination of its damage threshold. Evaluations of the method accuracy are made. Preliminary experiments on the damage threshold determination for aluminum surface are performed.
1. Introduction
At present time it is an agreed-upon point of view that laser-induced erosion of metallic targets has a thermal nature. The damage severity depends on the parameters of the target material as well as on the laser pulse energy and power density. The material damage threshold is the parameter of crucial importance for any process of laser radiationmatter interaction. Its determination is based usually either on the visual or microscope-assisted recording of surface destruction or on the recording ot generation of a glowing torch caused by heated products of destruction. One can also use the enhanced scattering registration from an auxiliary continuous-type laser spot coincident with the irradiated spot [ 11, or the recording of acoustical disturbances in the target [ 21. For thin material layers one can use the method based on the recording of resonant frequency changes of piezoelectric resonator with the sensor being covered by the material under study [3]. For the targets to be placed on a liquid medium the method based on the recording of acoustical disturbances in this medium [ 41 is applicable. 0030-4018/95/$09.50 0 1995Elsevier Science B.V. All rights reserved SSD!OO30-4018(95)00183-2
All of these methods, however, are deficient to overcome the discrepancy of the data obtained; and a universally accepted test procedure for determination of laser-induced damage threshold has not been developed yet.
2. Research method and theory The changes in optical correlation of speckle patterns in relation to changes of a surface microstructure being caused by different processes at the surface, are used by a variety of research teams for quantitative analysis of these processes (see Refs. [ 5-71, where this technique was applied for studies of the phenomena of metallic oxidation, salt efforescence and plasticity, respectively). The correlation holographic interferometry [ 81 was also successfully applied in recent years for the studies of the processes leading to the surface microrelief changes, e.g., cavitation-induced erosion [ 91, corrosion [ lo], mechanical wear [ 8,101. In this method a change in the surface microrelief between two exposures results in the decorrelation of reconstructed light
22
G. V. Dreiden, I.V. Semenova /Optics
‘9’ 0.50 (l.mU 0.40
0.30
I\
0.20
0.10 I\ 0.001 000
I,
I,
I,
0 20
0.40
1,
I,
0.60
0.80
I.00
Y Fig. 1. Fringe contrast y plotted as a function of root-mean-square displacement (q) of a surface point.
waves, causing thereby a reduction of the fringe contrast. The microrelief of a surface subjected to laser irradiation of sufficiently high energy and power changes as a result of the removal of material. The distribution of the material removal rate over a surface is determined by several factors, such as an actual surface microrelief, a surface layer microstructure and an energy distribution along the irradiating laser beam cross-section. An analytical equation connecting the fringe contrast (7) with a root-mean-square displacement ((4)) of the surface point (normal to the surface) has been derived in Ref. [ 111 for the case of chemical corrosion of a metallic surface. After some transformations this equation can be rewritten in a following form: (4) =
hJlly_l 2rTcos 8 ’
Communications
118 (1995) 21-24
surface microstructure relief lead to a noticeable decrease of y. Thus, the method accuracy is the highest one just for small changes of surface microrelief caused by laser pulse close in energy to the damage threshold value. Higher measurement accuracy can be obtained if recording a correlation speckle photograph instead of a correlation holographic interferogram. According to the results of paper [ 81 the Young fringes contrast (in the method of speckle photography) is more sensitive to microrelief changes than that of interferometric fringes. However, the possible range of surface point displacements that can be measured by means of correlation speckle photography is considerably narrower. So, the technique of speckle photography can give more accurate results when measuring the damage threshold value, but it does not allow to follow the energy dependence of the process of laser-induced erosion.
3. Experimental arrangement Fig. 2 presents the optical scheme of experimental setup, that includes a holographic interferometer, an acting laser and a device for laser pulse energy control. A double-exposure holographic interferogram of the target (T) surface was recorded on the photo plate
(1)
where h is recording radiation wavelength, 8is an angle formed by a normal to the surface and radiatingirecording direction. Since formula ( 1) was derived basing on the only assumption that the probability density of a random magnitude q distribution follows the Gaussian distribution, such the relationship can be applied also for the valuation of laser-induced erosion of the surface. Since the dependence of y against (q) ( 1) exhibits a substantially nonlinear behaviour (see Fig. 1, where it is calculated for the given experimental conditions), even the small changes (of the order of lo-’ km) of
Fig. 2. Experimental set-up, where HeNe and Rb are the corresponding lasers, EM is energy meter, M are mirrors, BS are beamsplitters, PP is photoplate, T is target, W is wedge, P is prism, F is a set of filters.
G. V. Dreiden. I. V. Semenova / Optics Communications
(PP) by means of a HeNe laser radiation. During the time between two exposures the target surface was subjected to the action of pulsed Ruby laser (Rb) radiation (E=0.3 J, 7= 20 ns). The laser pulse energy was changed by a set of filters placed on the beam way and was controlled by energy meter (EM). The carrier fringes on the interferogram were obtained via the wedge (W) rotation at the time between two exposures. The fringe contrast was measured in the reconstructed real image of the target surface by means of a photomultiplier.
118 (1995) 21-24
23
0.50
1
0.20
0.10 1
4. Results and discussion
O.~i,
0.00
When irradiating the object surface by laser pulse of sufficiently high power, its microrelief changes due to laser-induced melting or evaporation of the material. If acting by laser radiation with sequentially rising energy on the material surface one can follow the energy dependence of the process of laser-induced erosion of this surface and determine the damage threshold as a power magnitude beginning from which the carrier fringes contrast decrease occurs. In our experiments we used an aluminum target (surface roughness R, = 28 p.m) and subjected it to the laser pulse series with the energy rising sequentially from 0.02 J up to 0.3 J. Since the energy distribution over the irradiated spot (6 mm in diameter) was made to be constant, one can use the power density units. Fig. 3 presents a fragment of a typical holographic interferogram of the target surface being subjected to the laser pulse series. The four spots on this interferogram correspond to the following power density magnitudes: 4.8 X 106; 8.9 X 106; 2.7 X 10’ and 4.3 X 10’ W/cm’, respectively. The power density magnitude
10.00
20.00
30.00
4000
50.00
P (MW/c& Fig. 4. Root-mean-square displacement (q) of a surface point plotted as a function of laser pulse power density P.
close to damage threshold value was obtained as one corresponding to a minimal noticeable decrease of fringe contrast (spot 1 in Fig. 3) and was measured thus to be equal to 4.8 X lo6 W/cm*. The fringes disappear completely in the irradiated region at the power density magnitudes higher than 4.3 X 1O7W/cm2 (spot 4 in Fig. 3). The interference fringes contrast in the irradiated spots was determined for the central cross section of the specimen, the values thus obtained were normalized then to the contrast in the area not subjected to laser radiation. Taking into account the ‘y-(q) relationship (Fig. 2) and the experimental data on fringe contrast as a function of laser pulse power density one can plot the surface point displacement (which corresponds to a degree of laser-induced erosion) versus pulse power density (Fig. 4). 5. Conclusions
Fig. 3. Holographic interferogram of the aluminum target surface subjected to the laser pulse series. Spots l-4 correspond to the following pulse power densities: 4.8 X 106; 8.9 X 106; 2.7X 10’ and 4.3X 10’ W/cm*, respectively.
In the present paper we have shown the opportunities of correlation holographic interferometry as a tool to study the laser-induced erosion of solid surfaces as well as to determine their damage threshold. The dependence of surface point displacement against laser pulse power density was obtained. The damage threshold value for aluminum target was determined.
24
G.V. Dreiden. I.V. Semenova /Optics
Acknowledgements The research described in this publication was made possible in part by Grant N R56000 from the International Science Foundation.
References [ 11 I.Y. Milev, S.S. Dimov, D.V. Terziev, J.I. Iordanova, L.B. Todorova and A.B. Gelkova, J. Appl. Phys. 70 (1991) 4057. [2] IA. Konovalov, K.S. Sklyarenko and SK. Sklyarenko, Techn. Phys. Lett. 20 (1994) 226. [3] V.P.Veiko,V.N.Bankov,M.N.LibensonandB.M.Yurkevich, Method for determination of the energy threshold for damage to a material under a concentrated energy flow, Certified invention, Certificate #635414 (USSR), 1978.
Communications
118 (1995) 21-24
[4] V.P. Veiko, G.V. Dreiden, Yu.1. Ostrovsky, IV. Sernenova
and E.A. Shakhno, Sov. Phys. Tech. Phys. 35 ( 1990) 499. [5] M. Muramatsu, G.H. Guedes and N.G. Caggioli, Optics Laser Techn. 26 (1994) 167. [6] P. Zanettaand M. Facchhii, Optics Comm. 104 (1993) 35. [7] J.S. Steckenrider and J.W. Wagner, Exp. Mech. 31 (1991) 8. [ 81 Yu.I.0stmvskyandV.P. Shchepinov, ProgmssinOptics XXX, ed. E. Wolf (1992) pp. 87-135. [Q] A.P. Drnitriev, G.V. Dmiden, A.V. Osintsev, Yu.1. Ostrovsky, V.P. Shchepinov, M.I. Etinberg and V.V. Yakovlev, Sov. Phys. Tech. Phys. 34 (1989) 192. [lo] A.V. Osintsev, Yu.1. Ostrovsky, Yu.P. Presnyakov and V.P. Shchepinov, Sov. Phys. Tech. Phys. 37 (1992) 879. [ 1l] K.N. Petrov and Yu.P. Preanyakov, Optika i Spectr. (Optics Spectr.) 44 (1978) 309 (in Russian).
1995
OPTlCS COMMUNICATIONS ELSEWIER
OpticsCommunications 118 (1995) 21-24
Correlation holographic interferometry applied for studies of laser-induced erosion G.V. Dreiden, I.V. Semenova A. F. loffe Physical Technical Institute, 26, Polytekhnischeskaya,
St. Petersburg,
194021, Russia
Received5 July 1994;revised version received 13 January 1995
Abstract The correlation holographic interferometry is applied to studies of laser-induced erosion of solid surface and, in particular, to determination of its damage threshold. Evaluations of the method accuracy are made. Preliminary experiments on the damage threshold determination for aluminum surface are performed.
1. Introduction
At present time it is an agreed-upon point of view that laser-induced erosion of metallic targets has a thermal nature. The damage severity depends on the parameters of the target material as well as on the laser pulse energy and power density. The material damage threshold is the parameter of crucial importance for any process of laser radiationmatter interaction. Its determination is based usually either on the visual or microscope-assisted recording of surface destruction or on the recording ot generation of a glowing torch caused by heated products of destruction. One can also use the enhanced scattering registration from an auxiliary continuous-type laser spot coincident with the irradiated spot [ 11, or the recording of acoustical disturbances in the target [ 21. For thin material layers one can use the method based on the recording of resonant frequency changes of piezoelectric resonator with the sensor being covered by the material under study [3]. For the targets to be placed on a liquid medium the method based on the recording of acoustical disturbances in this medium [ 41 is applicable. 0030-4018/95/$09.50 0 1995Elsevier Science B.V. All rights reserved SSD!OO30-4018(95)00183-2
All of these methods, however, are deficient to overcome the discrepancy of the data obtained; and a universally accepted test procedure for determination of laser-induced damage threshold has not been developed yet.
2. Research method and theory The changes in optical correlation of speckle patterns in relation to changes of a surface microstructure being caused by different processes at the surface, are used by a variety of research teams for quantitative analysis of these processes (see Refs. [ 5-71, where this technique was applied for studies of the phenomena of metallic oxidation, salt efforescence and plasticity, respectively). The correlation holographic interferometry [ 81 was also successfully applied in recent years for the studies of the processes leading to the surface microrelief changes, e.g., cavitation-induced erosion [ 91, corrosion [ lo], mechanical wear [ 8,101. In this method a change in the surface microrelief between two exposures results in the decorrelation of reconstructed light
22
G. V. Dreiden, I.V. Semenova /Optics
‘9’ 0.50 (l.mU 0.40
0.30
I\
0.20
0.10 I\ 0.001 000
I,
I,
I,
0 20
0.40
1,
I,
0.60
0.80
I.00
Y Fig. 1. Fringe contrast y plotted as a function of root-mean-square displacement (q) of a surface point.
waves, causing thereby a reduction of the fringe contrast. The microrelief of a surface subjected to laser irradiation of sufficiently high energy and power changes as a result of the removal of material. The distribution of the material removal rate over a surface is determined by several factors, such as an actual surface microrelief, a surface layer microstructure and an energy distribution along the irradiating laser beam cross-section. An analytical equation connecting the fringe contrast (7) with a root-mean-square displacement ((4)) of the surface point (normal to the surface) has been derived in Ref. [ 111 for the case of chemical corrosion of a metallic surface. After some transformations this equation can be rewritten in a following form: (4) =
hJlly_l 2rTcos 8 ’
Communications
118 (1995) 21-24
surface microstructure relief lead to a noticeable decrease of y. Thus, the method accuracy is the highest one just for small changes of surface microrelief caused by laser pulse close in energy to the damage threshold value. Higher measurement accuracy can be obtained if recording a correlation speckle photograph instead of a correlation holographic interferogram. According to the results of paper [ 81 the Young fringes contrast (in the method of speckle photography) is more sensitive to microrelief changes than that of interferometric fringes. However, the possible range of surface point displacements that can be measured by means of correlation speckle photography is considerably narrower. So, the technique of speckle photography can give more accurate results when measuring the damage threshold value, but it does not allow to follow the energy dependence of the process of laser-induced erosion.
3. Experimental arrangement Fig. 2 presents the optical scheme of experimental setup, that includes a holographic interferometer, an acting laser and a device for laser pulse energy control. A double-exposure holographic interferogram of the target (T) surface was recorded on the photo plate
(1)
where h is recording radiation wavelength, 8is an angle formed by a normal to the surface and radiatingirecording direction. Since formula ( 1) was derived basing on the only assumption that the probability density of a random magnitude q distribution follows the Gaussian distribution, such the relationship can be applied also for the valuation of laser-induced erosion of the surface. Since the dependence of y against (q) ( 1) exhibits a substantially nonlinear behaviour (see Fig. 1, where it is calculated for the given experimental conditions), even the small changes (of the order of lo-’ km) of
Fig. 2. Experimental set-up, where HeNe and Rb are the corresponding lasers, EM is energy meter, M are mirrors, BS are beamsplitters, PP is photoplate, T is target, W is wedge, P is prism, F is a set of filters.
G. V. Dreiden. I. V. Semenova / Optics Communications
(PP) by means of a HeNe laser radiation. During the time between two exposures the target surface was subjected to the action of pulsed Ruby laser (Rb) radiation (E=0.3 J, 7= 20 ns). The laser pulse energy was changed by a set of filters placed on the beam way and was controlled by energy meter (EM). The carrier fringes on the interferogram were obtained via the wedge (W) rotation at the time between two exposures. The fringe contrast was measured in the reconstructed real image of the target surface by means of a photomultiplier.
118 (1995) 21-24
23
0.50
1
0.20
0.10 1
4. Results and discussion
O.~i,
0.00
When irradiating the object surface by laser pulse of sufficiently high power, its microrelief changes due to laser-induced melting or evaporation of the material. If acting by laser radiation with sequentially rising energy on the material surface one can follow the energy dependence of the process of laser-induced erosion of this surface and determine the damage threshold as a power magnitude beginning from which the carrier fringes contrast decrease occurs. In our experiments we used an aluminum target (surface roughness R, = 28 p.m) and subjected it to the laser pulse series with the energy rising sequentially from 0.02 J up to 0.3 J. Since the energy distribution over the irradiated spot (6 mm in diameter) was made to be constant, one can use the power density units. Fig. 3 presents a fragment of a typical holographic interferogram of the target surface being subjected to the laser pulse series. The four spots on this interferogram correspond to the following power density magnitudes: 4.8 X 106; 8.9 X 106; 2.7 X 10’ and 4.3 X 10’ W/cm’, respectively. The power density magnitude
10.00
20.00
30.00
4000
50.00
P (MW/c& Fig. 4. Root-mean-square displacement (q) of a surface point plotted as a function of laser pulse power density P.
close to damage threshold value was obtained as one corresponding to a minimal noticeable decrease of fringe contrast (spot 1 in Fig. 3) and was measured thus to be equal to 4.8 X lo6 W/cm*. The fringes disappear completely in the irradiated region at the power density magnitudes higher than 4.3 X 1O7W/cm2 (spot 4 in Fig. 3). The interference fringes contrast in the irradiated spots was determined for the central cross section of the specimen, the values thus obtained were normalized then to the contrast in the area not subjected to laser radiation. Taking into account the ‘y-(q) relationship (Fig. 2) and the experimental data on fringe contrast as a function of laser pulse power density one can plot the surface point displacement (which corresponds to a degree of laser-induced erosion) versus pulse power density (Fig. 4). 5. Conclusions
Fig. 3. Holographic interferogram of the aluminum target surface subjected to the laser pulse series. Spots l-4 correspond to the following pulse power densities: 4.8 X 106; 8.9 X 106; 2.7X 10’ and 4.3X 10’ W/cm*, respectively.
In the present paper we have shown the opportunities of correlation holographic interferometry as a tool to study the laser-induced erosion of solid surfaces as well as to determine their damage threshold. The dependence of surface point displacement against laser pulse power density was obtained. The damage threshold value for aluminum target was determined.
24
G.V. Dreiden. I.V. Semenova /Optics
Acknowledgements The research described in this publication was made possible in part by Grant N R56000 from the International Science Foundation.
References [ 11 I.Y. Milev, S.S. Dimov, D.V. Terziev, J.I. Iordanova, L.B. Todorova and A.B. Gelkova, J. Appl. Phys. 70 (1991) 4057. [2] IA. Konovalov, K.S. Sklyarenko and SK. Sklyarenko, Techn. Phys. Lett. 20 (1994) 226. [3] V.P.Veiko,V.N.Bankov,M.N.LibensonandB.M.Yurkevich, Method for determination of the energy threshold for damage to a material under a concentrated energy flow, Certified invention, Certificate #635414 (USSR), 1978.
Communications
118 (1995) 21-24
[4] V.P. Veiko, G.V. Dreiden, Yu.1. Ostrovsky, IV. Sernenova
and E.A. Shakhno, Sov. Phys. Tech. Phys. 35 ( 1990) 499. [5] M. Muramatsu, G.H. Guedes and N.G. Caggioli, Optics Laser Techn. 26 (1994) 167. [6] P. Zanettaand M. Facchhii, Optics Comm. 104 (1993) 35. [7] J.S. Steckenrider and J.W. Wagner, Exp. Mech. 31 (1991) 8. [ 81 Yu.I.0stmvskyandV.P. Shchepinov, ProgmssinOptics XXX, ed. E. Wolf (1992) pp. 87-135. [Q] A.P. Drnitriev, G.V. Dmiden, A.V. Osintsev, Yu.1. Ostrovsky, V.P. Shchepinov, M.I. Etinberg and V.V. Yakovlev, Sov. Phys. Tech. Phys. 34 (1989) 192. [lo] A.V. Osintsev, Yu.1. Ostrovsky, Yu.P. Presnyakov and V.P. Shchepinov, Sov. Phys. Tech. Phys. 37 (1992) 879. [ 1l] K.N. Petrov and Yu.P. Preanyakov, Optika i Spectr. (Optics Spectr.) 44 (1978) 309 (in Russian).