Cover Page Title: Laser marking with graded contrast micro crack inside transparent material using UV ns pulse laser Authors: Futoshi MATSUI*(1,2), Masaaki ASHIHARA(1), Mitsuyasu MATSUO (1), Sakae KAWATO(2), and Takao KOBAYASHI(2) 1 Industrial Technology Center of Fukui Prefecture 2 Graduate School of Engineering, University of Fukui Corresponding author: Futoshi MATSUI* Industrial Technology Center of Fukui Prefecture Postal address: 61-10 Kawaiwashiduka, Fukui, Fukui 910-0102 Phone: +81-776-55-0664 Fax: +81-776-55-0665 e_mail: futoshi_gov@fklab.fukui.fukui.jp 1
Title: Laser marking with graded contrast micro crack inside transparent material using UV ns pulse laser Authors: Futoshi MATSUI*(1,2), Masaaki ASHIHARA(1), and Mitsuyasu MATSUO (1), Sakae KAWATO(2), Takao KOBAYASHI(2) Keyword: laser-induced crack marking, high-contrast level, UV laser, borosilicate glass Abstract: We have developed an efficient laser-induced marking system using the UV (ultraviolet) ns (nanosecond) pulse laser for inscribing the laser-induced crack image inside transparent BK-7 glass plates. The crack area was found to be proportional to the total laser energy and number of laser shots and also related to the scattering intensity of the image points. The 16 steps or 4-bit contrast level could be recorded in one point and resolved by this method. This laser-induced crack marking technique with high-contrast level can be used for high quality, two and three-dimensional memory and image inside the transparent materials. 2
1. Introduction The laser marking on the surface of metals and semi-conductor materials are widely used in industry as an efficient laser processing technique. The laser-induced cracking inside transparent materials is also used for inscribing the image of objects [1], which is sometimes referred to as laser crystal art [2]. The laser marking inside the transparent materials is also useful in chemical and medical applications for protecting erosion, friction and correction of the marked surfaces. In this technique, the image contrast is controlled roughly by a crack number density using pulsed lasers and the image contrast level is limited to only one-bit (white and black) scale or so. In this research we report a new technique of the laser-induced crack marking for high contrast level imaging inside a glass by controlling the fluence of ns (nanosecond) pulse UV (ultraviolet) laser beam. More than 16-contrast level of scattering cracks was obtained and high quality and clear imaging was realized inside transparent glasses. 2. Crack formation and efficient laser processing A schematic of the laser-induced crack marking process inside transparent materials is shown in Fig. 1. By focusing the pulse laser beam, high intensity optical field is created near the focal point and generates high temperature and high shock wave pressure inside the transparent solid materials and induces cracks along the laser beam. The crack also expands toward transverse direction at 3
higher fluence of the laser energy. As a transparent material we used BK-7, borosilicate crown glass which has several advantages over silica glass in the laser induced crack marking. The BK-7 glass has absorption band edge wavelength of about 350nm and small absorption coefficient of α =2.81 10-2 cm -1 at the wavelength of 355nm of the third-harmonic beam of the Nd: YVO 4 laser. This close wavelength relation between the material absorption and the laser results in significantly lower laser-induced bulk damage threshold (LIDT) of the BK-7 glass than the silica glass by single photon absorption effect [3], and the UV 355nm third-harmonic beam is useful for efficient production of laser-induced cracks than using the fundamental beam at 1064nm or second-harmonic beam at 532nm wavelength. The ns (nanosecond) pulse laser for was used for marking BK-7 glasses. Fs (femtosecond) ultrashort pulse lasers are often used for micro processing [4,5]. Although it has high peak power enough to process wide band gap materials, the pulse energy and the average power are significantly lower than nanosecond lasers. In the case of laser marking application of transparent materials, large cracking volume is required and the nanosecond pulse laser is suitable for highly efficient laser marking processing. 3. Experimental system In the laser-induced crack marking system, the V pulse third-harmonic of Nd:YVO 4 laser at 4
355nm wavelength of 30 ns pulse width (Coherent, model AVIA 355-7000) was used as the laser source and the total block diagram is shown in Fig. 2. The pulse repetition frequency is selectable from 10kHz to 100kHz. The laser beam is scanned by galvanometer mirrors (GSI, model HMPM10) and focused by a 120mm-diameter f-θ lens with 106mm focal length. The scanning and focusing system has 16bit angle resolution and 1.2µm spatial resolution on a 75mm square transparent target material. The laser pulse repetition frequency, the number of laser shots and laser beam position are controlled by using a personal computer. The laser pulse energy is kept constant at 50 µj and the number of laser shots for single crack position is changed for controlling the crack size. The crack area was measured automatically by using a transmission camera and a micro-computer image processing system. 4. Experimental results In Fig. 3, the transmission microscope images of the laser-induced cracks observed along the laser beam direction are compared for single shot per point in Fig. 3 (a) and 16 laser shots per point in Fig. 3 (b). The spacing between the cracks is 150µm. It is shown that the crack size clearly increases by 16 laser shots. The images of the laser-induced cracks observed from transverse direction to the laser beam are shown in Fig. 3 (c) and (d), single and 16 shots respectively. The depth of the longitudinal crack along the laser beam was approximately 1.2mm. It is evident that the 5
longitudinal cracks are induced by single shot exposure and transverse cracks expand by increasing the number of shots or laser fluence. The area of the laser-induced cracks observed from longitudinal direction is plotted as a function of total laser energy or number of laser shots per point in Fig. 4. It is shown that the crack area is almost linearly proportional to the total laser energy or number of laser shots N. The scattering image and the transmission image of the laser-induced cracks observed from the longitudinal direction to the laser beam are compared in Fig. 5 (a) and (b). The scattering image was recorded by irradiating a halogen-lamp light from the transverse direction to the laser beam direction. It was checked the scattering intensity of the cracks is proportional to the crack area observed from the longitudinal direction. As an example of the laser-induced crack marking technique, 16 different contrast square images were marked in a BK-7 glass by changing the laser shots and the scattering images are shown in Fig.6. This result indicate each image contrast could be resolved and 16 step imaging contrasts can be demonstrated for high density two and three dimensional imaging inside the transparent glass. 5. Conclusion The efficient laser marking system has been realized using the UV ns pulse laser for inscribing the laser-induced crack image inside transparent BK-7 glass plates. The crack area was found to be 6
proportional to the total laser energy and the number of laser shots and also related to the scattering intensity of the image points. The 16 steps or 4-bit contrast level could be recorded and resolved by this marking method. This high-contrast level crack marking technique can be used for high-quality, two and three-dimensional memory and image inside the transparent materials. References [1] F. Dahmani, A. W. Schmid, J. C. Lambropoulos, and S. Burns, "Dependence of birefringence and residual stress near laser-induced cracks in fused silica on laser fluence and on laser-pulse number", Appl. Opt., Vol. 37, No. 33, pp.7772-7784 (1998) [2] H. Niino, A. Narasaki, T. Sato, Y. Kawaguchi, Micro-processing of transparent material using Laser ablation (in Japanese) Journal of Japan Laser Processing Society Vol.9, pp.10-14 (2002) [3] N. Kuzuu, K. Yoshida, K. Ochi, Y. Tsuboi, T. Kamimura, Laser-induced bulk damage of various types of silica glasses at 532 and 355nm Jap. J. Appl. Phys., Vol.43, pp.2547-2548 (2004) [4] A. Marcinkevičius, S. Juodkazis, M. Watanabe, M. Miwa, S. Matsuo, H. Misawa, and J. Nishii, "Femtosecond laser-assisted three-dimensional microfabrication in silica," Opt. Lett. 26, pp. 277-279 (2001) [5] N. Takeshima, Y. Kuroiwa, Y. Narita, S. Tanaka, and K. Hirao, "Fabrication of a periodic structure with a high refractive-index difference by femtosecond laser pulses," Opt. Express 12, pp.4019-4024 (2004) 7
Fig. 1 Laser-induced crack marking process Fig. 2 Experimental system of laser-induced crack marking Fig. 3 Microscope images of laser-induced cracks; (a) and (b) are transmission images observed along the laser beam direction, (c) and (d) are images observed from transverse the laser beam direction. The number of laser shots N=1 for (a)and (c), N=16 (b) and (d), respectively. Fig. 4 Crack area as a function of total laser energy or number of laser shots per point Fig. 5 The scattering image and the transmission image of the laser-induced cracks observed from the longitudinal direction to the laser beam Fig. 6 Scattering images of 16 different contrast squares marked in a BK-7 glass by changing the number of laser shots 8
Pulse laser beam Focusing lens Transparent solid material Laser-induced cracks Fig. 1 Laser-induced crack marking process Galvanometer mirrors F-θ focusing lens UV Laser Control signal Transparent Scanning material control Z axis stage PC Fig. 2 Experimental system of laser-induced crack marking 9
(a) (b) (c) (d) Fig. 3 Microscope images of laser-induced cracks; (a) and (b) are transmission images observed along the laser beam direction, (c) and (d) are images observed from transverse the laser beam direction. The number of laser shots N=1 for (a)and (c), N=16 (b) and (d), respectively. 10
Crack area [µm 2 ] 1500 1000 500 Number of laser shot : N 0 5 10 15 BK-7 glass λ = 355nm Single pulse energy:50µj 0 0 200 400 600 800 Total laser energy [µj] Fig. 4 Crack area as a function of total laser energy or number of laser shots per point (a) Scattering image (b) Transmission image Fig. 5 The scattering image and the transmission image of the laser-induced cracks observed from the longitudinal direction to the laser beam 11
Fig. 6 Scattering images of 16 different contrast squares marked in a BK-7 glass by changing the number of laser shots 12