- Email: [email protected]
Cutting of glass by picosecond laser radiation
OPTICS COMMUNICATIONS
Optics Communications 106 ( 1994) 223-226 North-Holland
Cutting of glass by picosecond laser radiation M.B. Strigin and A.N. Chudinov Nonlinear Optics Laboratory, Technical University, Lenina ave., Chelaybinsk 454080, Russian Federation Received 3 November 1993
The cutting of glass plate is suggested and it is realized by using radiation of a passive mode-locked Nd:YAG laser. The present method is based on the possibility of forming a microcrack’s channel. A simple model of formation of channel and microcracks is
1. Introduction
We present a to our knowledge new method of cutting glass using laser radiation with a wavelength in the region of transparency for glass (0.3-2.5 pm). Let us consider the main idea of the suggested method. We are able to form a channel of microcracks in a transparent dielectric using the passively modelocked pulsed radiation of a Nd:YAG laser. The channel of microcracks started on the back side of a planar specimen, then it grew from pulse to pulse in the direction inside and ended on the face side. Driving the plate perpendicularly to the laser beam direction we formed a series of such channels. The stress is induced in the vicinity of each channel. The discharge of that stress results in the formation of a large common crack along the set of channels. This method of cutting a plate glass differs from traditional methods [ 11, since we do not use local evaporation or melting of the material, but it needs a polished face side of the specimen to let laser radiation into the bulk of the glass.
2. Experimental
A passive mode-locked Nd:YAG laser without a special system for extracting a single pulse from a laser train was used in our experiments. It operated in the TEMoo mode. The laser train consisted of about 25 pulses. The duration of each pulse was less than
100 ps. The energy of the laser pulse was 30 mJ. Laser radiation was focused by a lens (the focal length was 25 cm) in the bulk of specimen. The energy of the laser pulse was chosen to initiate the optical discharge on the back side of a sample. It is known, that the threshold of surface damage of a transparent dielectric for the back side of a plate is less than that of the face side, because there is interference of the laser wave in the material with the reflected wave from the exit side [ 21. On the other hand, the energy of the laser pulse must be not greater than that when the breakdown on the face side may be initiated. When the energy of the laser radiation lies in the range mentioned above, the formation of a channel of microcracks must always be initiated. This channel typically starts on the back side of a planar specimen; then it spreads from pulse to pulse in the direction inside and ends on the face side. We present the frequency dependence of the speed of the channel’s development in fig. 1. In all cases the energy of the laser pulses was kept the same when the pulse repetition frequency was varied. Thus far we have found from our experiments that there is an optimal pulse repetition frequency when the speed of the channel’s development is maximal. We investigated the shape of microcracks. For this reason, we obtained a common crack of a sample at which is studied the form of the channel and microcracks by means of a microscope. It had a magnification of 100 diameters. The channel was aligned along the pass of laser radiation and had the shape of a cylindrical tube. Its
0030-4018/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDZ 0030-4018(93)E0592-4
223
Volume 106, number 4,5,6
3.5
OPTICS COMMUNICATIONS
1
15 March 1994
0.5
0.4
L i 63 Lz 0 w Q 0.2
1.0 0.1 0.5
0.0 :,ii:', 0
5
10
15
20
FREQUANCY,
25
30
Hz.
Fig. 1. The speed of channel’s development as a function of laser repetition rate for a glass specimen; (open circles) when the focus was in the bulk of the specimen and (open squares) when the focus was at the back side of the specimen.
inner diameter was 200 pm. The ball-shape formation was furnished partly on the wall of the channel. Probably those balls are of melted and then cooled material of the sample. They were 50 pm and less in diameter. The microcracks were strung on the channel across a certain distance. They were cone-shaped with the top directed forward as the laser radiation does. The largest diameter of the cone’s base was about 800 pm. Let us denote the angle 20 of the cone on the plane of a common crack. Figure 2 shows the distribution of the crack’s probability having a given angle 8 for a glass sample. We used the opportunity of forming a channel of microcracks for cutting a planar specimen of glass. We could form the next channel if another laser ray did not pass through the cone of the previous channel. Thus far the minimal distance between two channels of microcracks must not be more than the largest diameter of the cone’s base, that is it is about 800 urn. When the common crack was formed during the growth of the next channel the maximum distance between channels was 1.5 mm. The formation of a common crack may take place some time after 224
0.0
ANGLE,
deg.
Fig. 2. Diagram of the probability of microcracks having a given angle 6. The number of microcracks under investigation was 100.
the end of the channels formation if the distance between neighbouring channels was not larger than 2.5 mm. Otherwise the common crack did not appear. Driving the sample perpendicularly to the laser beam direction we could cut out a piece of predetermined size from a glass sample. The plate sample could be rotated even at an angle 45” relative to the laser beam direction in the air.
3. Discussion The present method of cutting glass is based on the possibility of forming channel microcracks. Therefore the physical nature of the origin of the channel and microcracks will be discussed in this part of article. Taking into account the frequency dependence of the channel propagation speed, one can come to the conclusion of the wave-like nature of this phenomenon. A similar wave-like process is the spread of optical discharge in gases when the photodetonation wave regime (PD-wave) takes place [ 31. For our case to estimate this process let us assume that the PD-wave is provoked in the bulk of a glass specimen by a picosecond pulse from a laser train.
Volume 106, number
4,5,6
OPTICS
COMMUNICATIONS
The energy that reaches the front of the PD-wave during the time dt is S, dt where S, is the intensity of the laser pulse. This energy is absorbed in a glass volume pv dt where v is the speed of the leading edge of the PD-wave and p is the density of glass. Taking into account that the propagation of the PD-wave inside the sample results in a local evaporation of glass we may assume that the laser energy will be mainly converted into thermal energy of ions. Thus far we consider that the thermal energy of the electrons is very small and it will be ignored. The equation of a energy conservation takes the following form pvdte(T)=S,,dt.
(1)
Here t(T) is the kinetical energy of ions per unit of volume of glass at the temperature T. Assuming the value of v to be of the same order of magnitude as the thermal speed of ions, that is v-m, one gets the estimation of the PD wave speed, v= (s,/p)“3
.
(2)
For our case the estimated channel spread speed is vx lo6 cm/s. Thus far the front of the PD-wave travels a distance of about 1 urn during the time of 100 ps being approximately equal to the duration of a single pulse. Assuming that the PD-wave is damped completely during the laser resonator round-trip time about 3 ns and it is generated again by the next single pulse from the laser train, the average speed of the channel development is approximately equal to 1 mm/s, when the pulse repetition frequency is 15 Hz. It is of the same order of magnitude as the speed measured in the experiment (fig. 1). Let us consider in details the expression for the average speed of channel development:
s
15 March
1994
linear), loss due to scattering and self-focusing loss. The coefficient (Ydepends on the intensity of the laser light, on the time t and on the frequency v as well, CY=(Y(S, t, v). Let us take this coefficient in the following rough approximation (Y= a0 (S, t ) + a, (S, t ) v, and substitute it into eqs. (3) and (4). Then
(5) We see that the average speed depends on the repetition rate as v- /I1 v exp( -/?zv). That was observed in our experiments (fig. 1). A photodetonation wave induced by the picosecond train propagates inside the material of the sample towards the source of the laser radiation forming the channel. If the energy provided from the laser is not sufficient for maintaining a PD-wave it is converted into a flux of phonons, and as a result a lot of small cracks will be produced. Their orientation will be arbitrary in the angular range from 0 to 2n rad. It may be said that an interface of solid/gas type was formed in the material. The orientation of originating microcrack is determined by the condition that the flux of laser radiation through this interface per unit of square of rising microcrack is maximal. This condition can be satisfied if the laser wave enters the solid/gas interface at an angle near the angle of total internal reflection. It that case the emergent ray is nearly parallel to the glass surface and hence it will be absorbed completely by the hot gas thus providing a fast growth of microcrack. In our case, thus far, the orientation of the microcracks are found at angles near 40” with respect to the direction of the laser radiation, as we observed in the experiment (fig. 2).
T
v,, = v
v(t,z)+@)]‘3$
(3)
0
where z is the longitudinal coordinate of a front PDwave; r is the duration of the laser train and S( t, z) is the intensity of the laser train for which we may write S(t,z)=So(t,z)exp(-az).
(4)
4. Conclusion We suggested a new method of cutting transparent dielectrics which is based on the possibility of formation of a microcrack’s channel. We presented a simple model of formation of a channel and microcracks, which is in a reasonable agreement with experimental data.
Here (Y is the loss coefficient for laser radiation. It includes the loss due to absorption (linear and non225
Volume 106, number 4,5,6
OPTICS COMMUNICATIONS
Acknowledgements
The authors are grateful to Prof. B. Ya Zel’dovich and Dr. V.V.V. Schkunov for many helpful discussions and critical reading of this manuscript.
226
15 March 1994
References [ 1] W.W. Duley, Laser processing and analysis of materials (Plenum Press, New York, 1983). [2] A.A. Golubzov, N.F. Pilipetskii, A.N. Sudarkin, A.N. Chudinov and V.V. Schkunov, Dokladii AN SSSR 282 (1985) 861 (in russian). [ 31 Yu. P. Raiizer, Spark by laser and spread of discharges (Science Press, Moscow, 1974 )
Optics Communications 106 ( 1994) 223-226 North-Holland
Cutting of glass by picosecond laser radiation M.B. Strigin and A.N. Chudinov Nonlinear Optics Laboratory, Technical University, Lenina ave., Chelaybinsk 454080, Russian Federation Received 3 November 1993
The cutting of glass plate is suggested and it is realized by using radiation of a passive mode-locked Nd:YAG laser. The present method is based on the possibility of forming a microcrack’s channel. A simple model of formation of channel and microcracks is
1. Introduction
We present a to our knowledge new method of cutting glass using laser radiation with a wavelength in the region of transparency for glass (0.3-2.5 pm). Let us consider the main idea of the suggested method. We are able to form a channel of microcracks in a transparent dielectric using the passively modelocked pulsed radiation of a Nd:YAG laser. The channel of microcracks started on the back side of a planar specimen, then it grew from pulse to pulse in the direction inside and ended on the face side. Driving the plate perpendicularly to the laser beam direction we formed a series of such channels. The stress is induced in the vicinity of each channel. The discharge of that stress results in the formation of a large common crack along the set of channels. This method of cutting a plate glass differs from traditional methods [ 11, since we do not use local evaporation or melting of the material, but it needs a polished face side of the specimen to let laser radiation into the bulk of the glass.
2. Experimental
A passive mode-locked Nd:YAG laser without a special system for extracting a single pulse from a laser train was used in our experiments. It operated in the TEMoo mode. The laser train consisted of about 25 pulses. The duration of each pulse was less than
100 ps. The energy of the laser pulse was 30 mJ. Laser radiation was focused by a lens (the focal length was 25 cm) in the bulk of specimen. The energy of the laser pulse was chosen to initiate the optical discharge on the back side of a sample. It is known, that the threshold of surface damage of a transparent dielectric for the back side of a plate is less than that of the face side, because there is interference of the laser wave in the material with the reflected wave from the exit side [ 21. On the other hand, the energy of the laser pulse must be not greater than that when the breakdown on the face side may be initiated. When the energy of the laser radiation lies in the range mentioned above, the formation of a channel of microcracks must always be initiated. This channel typically starts on the back side of a planar specimen; then it spreads from pulse to pulse in the direction inside and ends on the face side. We present the frequency dependence of the speed of the channel’s development in fig. 1. In all cases the energy of the laser pulses was kept the same when the pulse repetition frequency was varied. Thus far we have found from our experiments that there is an optimal pulse repetition frequency when the speed of the channel’s development is maximal. We investigated the shape of microcracks. For this reason, we obtained a common crack of a sample at which is studied the form of the channel and microcracks by means of a microscope. It had a magnification of 100 diameters. The channel was aligned along the pass of laser radiation and had the shape of a cylindrical tube. Its
0030-4018/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDZ 0030-4018(93)E0592-4
223
Volume 106, number 4,5,6
3.5
OPTICS COMMUNICATIONS
1
15 March 1994
0.5
0.4
L i 63 Lz 0 w Q 0.2
1.0 0.1 0.5
0.0 :,ii:', 0
5
10
15
20
FREQUANCY,
25
30
Hz.
Fig. 1. The speed of channel’s development as a function of laser repetition rate for a glass specimen; (open circles) when the focus was in the bulk of the specimen and (open squares) when the focus was at the back side of the specimen.
inner diameter was 200 pm. The ball-shape formation was furnished partly on the wall of the channel. Probably those balls are of melted and then cooled material of the sample. They were 50 pm and less in diameter. The microcracks were strung on the channel across a certain distance. They were cone-shaped with the top directed forward as the laser radiation does. The largest diameter of the cone’s base was about 800 pm. Let us denote the angle 20 of the cone on the plane of a common crack. Figure 2 shows the distribution of the crack’s probability having a given angle 8 for a glass sample. We used the opportunity of forming a channel of microcracks for cutting a planar specimen of glass. We could form the next channel if another laser ray did not pass through the cone of the previous channel. Thus far the minimal distance between two channels of microcracks must not be more than the largest diameter of the cone’s base, that is it is about 800 urn. When the common crack was formed during the growth of the next channel the maximum distance between channels was 1.5 mm. The formation of a common crack may take place some time after 224
0.0
ANGLE,
deg.
Fig. 2. Diagram of the probability of microcracks having a given angle 6. The number of microcracks under investigation was 100.
the end of the channels formation if the distance between neighbouring channels was not larger than 2.5 mm. Otherwise the common crack did not appear. Driving the sample perpendicularly to the laser beam direction we could cut out a piece of predetermined size from a glass sample. The plate sample could be rotated even at an angle 45” relative to the laser beam direction in the air.
3. Discussion The present method of cutting glass is based on the possibility of forming channel microcracks. Therefore the physical nature of the origin of the channel and microcracks will be discussed in this part of article. Taking into account the frequency dependence of the channel propagation speed, one can come to the conclusion of the wave-like nature of this phenomenon. A similar wave-like process is the spread of optical discharge in gases when the photodetonation wave regime (PD-wave) takes place [ 31. For our case to estimate this process let us assume that the PD-wave is provoked in the bulk of a glass specimen by a picosecond pulse from a laser train.
Volume 106, number
4,5,6
OPTICS
COMMUNICATIONS
The energy that reaches the front of the PD-wave during the time dt is S, dt where S, is the intensity of the laser pulse. This energy is absorbed in a glass volume pv dt where v is the speed of the leading edge of the PD-wave and p is the density of glass. Taking into account that the propagation of the PD-wave inside the sample results in a local evaporation of glass we may assume that the laser energy will be mainly converted into thermal energy of ions. Thus far we consider that the thermal energy of the electrons is very small and it will be ignored. The equation of a energy conservation takes the following form pvdte(T)=S,,dt.
(1)
Here t(T) is the kinetical energy of ions per unit of volume of glass at the temperature T. Assuming the value of v to be of the same order of magnitude as the thermal speed of ions, that is v-m, one gets the estimation of the PD wave speed, v= (s,/p)“3
.
(2)
For our case the estimated channel spread speed is vx lo6 cm/s. Thus far the front of the PD-wave travels a distance of about 1 urn during the time of 100 ps being approximately equal to the duration of a single pulse. Assuming that the PD-wave is damped completely during the laser resonator round-trip time about 3 ns and it is generated again by the next single pulse from the laser train, the average speed of the channel development is approximately equal to 1 mm/s, when the pulse repetition frequency is 15 Hz. It is of the same order of magnitude as the speed measured in the experiment (fig. 1). Let us consider in details the expression for the average speed of channel development:
s
15 March
1994
linear), loss due to scattering and self-focusing loss. The coefficient (Ydepends on the intensity of the laser light, on the time t and on the frequency v as well, CY=(Y(S, t, v). Let us take this coefficient in the following rough approximation (Y= a0 (S, t ) + a, (S, t ) v, and substitute it into eqs. (3) and (4). Then
(5) We see that the average speed depends on the repetition rate as v- /I1 v exp( -/?zv). That was observed in our experiments (fig. 1). A photodetonation wave induced by the picosecond train propagates inside the material of the sample towards the source of the laser radiation forming the channel. If the energy provided from the laser is not sufficient for maintaining a PD-wave it is converted into a flux of phonons, and as a result a lot of small cracks will be produced. Their orientation will be arbitrary in the angular range from 0 to 2n rad. It may be said that an interface of solid/gas type was formed in the material. The orientation of originating microcrack is determined by the condition that the flux of laser radiation through this interface per unit of square of rising microcrack is maximal. This condition can be satisfied if the laser wave enters the solid/gas interface at an angle near the angle of total internal reflection. It that case the emergent ray is nearly parallel to the glass surface and hence it will be absorbed completely by the hot gas thus providing a fast growth of microcrack. In our case, thus far, the orientation of the microcracks are found at angles near 40” with respect to the direction of the laser radiation, as we observed in the experiment (fig. 2).
T
v,, = v
v(t,z)+@)]‘3$
(3)
0
where z is the longitudinal coordinate of a front PDwave; r is the duration of the laser train and S( t, z) is the intensity of the laser train for which we may write S(t,z)=So(t,z)exp(-az).
(4)
4. Conclusion We suggested a new method of cutting transparent dielectrics which is based on the possibility of formation of a microcrack’s channel. We presented a simple model of formation of a channel and microcracks, which is in a reasonable agreement with experimental data.
Here (Y is the loss coefficient for laser radiation. It includes the loss due to absorption (linear and non225
Volume 106, number 4,5,6
OPTICS COMMUNICATIONS
Acknowledgements
The authors are grateful to Prof. B. Ya Zel’dovich and Dr. V.V.V. Schkunov for many helpful discussions and critical reading of this manuscript.
226
15 March 1994
References [ 1] W.W. Duley, Laser processing and analysis of materials (Plenum Press, New York, 1983). [2] A.A. Golubzov, N.F. Pilipetskii, A.N. Sudarkin, A.N. Chudinov and V.V. Schkunov, Dokladii AN SSSR 282 (1985) 861 (in russian). [ 31 Yu. P. Raiizer, Spark by laser and spread of discharges (Science Press, Moscow, 1974 )