OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 23 — Nov. 7, 2011
  • pp: 22961–22973
« Show journal navigation

Characteristics of laser absorption and welding in FOTURAN glass by ultrashort laser pulses

Isamu Miyamoto, Kristian Cvecek, Yasuhiro Okamoto, Michael Schmidt, and Henry Helvajian  »View Author Affiliations


Optics Express, Vol. 19, Issue 23, pp. 22961-22973 (2011)
http://dx.doi.org/10.1364/OE.19.022961


View Full Text Article

Acrobat PDF (1375 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

The nonlinear absorptivity of FOTURAN glass to ultrashort laser pulses is evaluated by experimental measurement and thermal conduction model at different parameters including energy and repetition rate of the laser pulse, translation speed and thermal properties of the sample. The mechanical strength of an embedded laser-melted sample and an overlapped weld sample is determined by a three-point-bending test and a shear test, respectively. The results are related to the average absorbed laser power Wab. We found the mechanical strength of an overlapped weld joint to be as high as that of the base material for low Wab, if the sample pair is pre-bonded to provide optical contact.

© 2011 OSA

1. Introduction

Localized internal processing of transparent materials by ultrashort laser pulses has been drawing much attention because of the wide range of available applications such as waveguide formation [1

1. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef] [PubMed]

], three-dimensional optical memory [2

2. M. Watanabe, H. Sun, S. Juodkazis, T. Takahashi, S. Matsuo, Y. Suzuki, J. Nishii, and H. Misawa, “Three-dimensional optical data storage in vitreous silica,” Jpn. J. Appl. Phys. 37(Part 2, No. 12B), L1527–L1530 (1998). [CrossRef]

] and fusion welding [3

3. T. Tamaki, W. Watanabe, J. Nishii, and K. Itoh, “Welding of transparent materials using femtosecond laser pulses,” Jpn. J. Appl. Phys. 44(22), L687–L689 (2005). [CrossRef]

,4

4. I. Miyamoto, A. Horn, J. Gottmann, D. Wortmann, and F. Yoshino, “Fusion welding of glass using femtosecond laser pulses with high-repetition rates,” J. Laser. Micro/Nanoeng 2, 57–63 (2007).

]. Consequently, it is important to evaluate the nonlinear absorptivity and the temperature field in commercially useful transparent materials, since they will affect the laser-matter interaction process and the quality of the modification especially for high pulse repetition rate processing.

Although several authors have modeled the absorption of the ultrashort laser pulse energy in distilled water using rate equations describing the free electron formation as a result of single laser pulse [5

5. J. Noack and A. Vogel, “Laser-induced plasma formation in water at nanosecond to femtosecond time scales: calculation of thresholds, absorption coefficient, and energy density,” IEEE J. Quantum Electron. 35(8), 1156–1167 (1999). [CrossRef]

,6

6. C. L. Arnold, A. Heisterkamp, W. Ertmer, and H. Lubatschowski, “Computational model for nonlinear plasma formation in high NA micromachining of transparent materials and biological cells,” Opt. Express 15(16), 10303–10317 (2007). [CrossRef] [PubMed]

], the model cannot be applied to the case of multi-pulse laser irradiation especially at high repetition rates. An experimental measurement procedure was developed to evaluate the nonlinear absorptivity of ultrashort laser pulses in bulk glass for high pulse repetition rates [4

4. I. Miyamoto, A. Horn, J. Gottmann, D. Wortmann, and F. Yoshino, “Fusion welding of glass using femtosecond laser pulses with high-repetition rates,” J. Laser. Micro/Nanoeng 2, 57–63 (2007).

,7

7. I. Miyamoto, A. Horn, and J. Gottmann, “Local melting of glass material and its application to direct fusion welding by ps-laser pulses,” J. Laser. Micro/Nanoeng 2, 7–14 (2007).

]. A thermal conduction model [8

8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

] was also developed to evaluate the laser energy absorbed in the laser-induced plasma at high pulse repetition rates. The model predicts that for a given pulse energy, the nonlinear absorptivity increases as the pulse repetition rate increases. This is due to increase in the density of the thermally excited free electrons, which seed electrons for avalanche ionization.

The mechanical strength of internally modified single glass samples that have been exposed to ultrashort laser pulses has been evaluated by different testing procedures including a four-point-bending test [9

9. K. Hirao, Y. Shimotsuma, J. Qiu, and K. Miura, “Femtosecond laser induced phenomena in gasses and photonic device application,” Mater. Res. Soc. Symp. Proc., 13–23 (2005).

], a double torsion test [10

10. N. Borrelli, J. Helfinstine, J. Price, J. Schroeder, A. Atreltsov, and J. Westbrook, “Glass strengthening with an ultrafast laser,” in Proceedings of the Int. Cong. Appl. Laser and Electro Optics (ICALEO) (2008), pp.185–189.

] and nano-indentation [11

11. Y. Bellouard, T. Colomb, C. Depeursinge, M. Dugan, A. A. Said, and P. Bado, “Nanoindentation and birefringence measurements on fused silica specimen exposed to low-energy femtosecond pulses,” Opt. Express 14(18), 8360–8366 (2006). [CrossRef] [PubMed]

,12

12. P. Kongsuwan, H. Wang, S. Vukelic, and Y. L. Yao, “Characterization of morphology and mechanical properties of glass interior irradiated by femtosecond laser,” J. Manuf. Sci. Eng. 132(4), 041009 (2010). [CrossRef]

], and it was found that the laser-irradiated glass samples is strengthened by creating the residual compressive stress. This is in contrast to the case of conventional nanosecond (ns) laser pulse processing where cracks form due to the development of residual stress that is in tension [13

13. T. Arai, N. Asano, A. Minami, and H. Kusano, “Inside process of glass with nanosecond pulsed laser,” in Proceedings of the Int. Cong. Appl. Laser and Electro Optics (ICALEO) (2008), pp.408–414.

]. There is limited study on the mechanical strength of laser-weld joints, because the weld quality is significantly dependent on sample preparations. The early work of fusion welding of glass samples [3

3. T. Tamaki, W. Watanabe, J. Nishii, and K. Itoh, “Welding of transparent materials using femtosecond laser pulses,” Jpn. J. Appl. Phys. 44(22), L687–L689 (2005). [CrossRef]

] showed that the joint strength is much lower than that of the base material, presumably because the sample preparation was not ideal. Recently a paper reported that the mechanical strength of laser-welded joint can be greatly enhanced by introducing a pre-bonding process step to the glass plates that are to be welded [14

14. K. Cvecek, I. Miyamoto, J. Strauss, M. Wolf, T. Frick, and M. Schmidt, “Sample preparation method for glass welding by ultrashort laser pulses yields higher seam strength,” Appl. Opt. 50(13), 1941–1944 (2011). [CrossRef] [PubMed]

]. The paper also indicated that the attractive force caused by van der Waals interaction [15

15. V. Greco, F. Marchesini, and G. Molesini, “Optical contact and van der Waals interactions: the role of the surface topography in determining the bonding strength of thick glass plates,” J. Opt. A, Pure Appl. Opt. 3(1), 85–88 (2001). [CrossRef]

] at the interface has to be taken into account in evaluating the mechanical strength of the pre-bonded welded joint. However, the effect of welding conditions on the mechanical strength of the weld joint has not yet been reported.

2. Nonlinear absorption properties of FOTURAN glass

2.1 Experimental measurement of nonlinear absorptivity

Ultrashort pulse laser system (Duetto from Time-bandwidth Products, wavelength λ = 1064nm, M2=1.1) with a pulse duration of 10ps is used to induce melting FOTURAN glass having a thickness of 1mm. The laser beam is focused by a microscope objective lens with a numerical aperture (NA) of 0.55. The exposure is done as the glass plate is transversely moved at a velocity region of 10~200mm/s. The pulse repetition rate f is varied over a wide range of 50kHz~8.2MHz, resulting in different energies per pulse. The thermal and physical properties of virgin FOTURAN are shown in Table 1

Table 1. Thermal and physical properties of FOTURAN [19] and borosilicate glass D263 [8]

table-icon
View This Table
. FOTURAN is a glass but upon heating and cooling can become a glass/ceramic composite.

The nonlinear absorptivity AEx can be determined by measuring the laser pulsed energy transmitted through the sample using the following equation [4

4. I. Miyamoto, A. Horn, J. Gottmann, D. Wortmann, and F. Yoshino, “Fusion welding of glass using femtosecond laser pulses with high-repetition rates,” J. Laser. Micro/Nanoeng 2, 57–63 (2007).

,8

8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

].
AEx=1QtQ01(1R)2
(1)
where Q0 is incident laser pulse energy, Qt is transmitted pulse energy and R is Fresnel reflectivity. The nonlinear absorptivity can be determined by this procedure with an uncertainly less than 3% [8

8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

], since the reflection and scattering by laser-induced plasma is negligible in transparent material [20

20. K. Nahen and A. Vogel, “Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses – Part II: Transmission, scattering, and reflection,” J. Sel. Topics Quant. El. 2(4), 861–871 (1996). [CrossRef]

]. A power meter with a rather slow response time of ≈1sec is used for measuring Qt. The experimental measurement of AEx was limited to the translation speeds below ≈30mm/s, because the sample size was limited to tens mm.

Figure 1
Fig. 1 Experimental and simulated nonlinear absorptivity of FOTURAN glass at a translation speed of 20mm/s at different pulse repetition rates with a focus position zh from the top surface of 640µm (NA0.55, τ = 10ps, average laser power 3W). The cross-sections of the sample are shown in Fig. 2.
shows the nonlinear absorptivity AEx plotted vs. pulse repetition rate f at a translation speed v=20mm/s and an average laser power of 3W. AEx is higher than 80% at f<700kHz, and quickly decreases for f>1MHz, reaching a value that is approximately 35% at f = 8.2MHz. Figure 2
Fig. 2 Cross-sections at different pulse repetition rates f at a constant average laser power of 3W in FOTURAN glass. (NA0.55, v=20mm/s, τ=10ps, zh=640µm).
shows the cross-sections of the internally melted glass sample for various pulse repetition rates. The modified structure consists of a teardrop-shaped inner structure and an elliptical outer structure like the case of borosilicate glass [9

9. K. Hirao, Y. Shimotsuma, J. Qiu, and K. Miura, “Femtosecond laser induced phenomena in gasses and photonic device application,” Mater. Res. Soc. Symp. Proc., 13–23 (2005).

], and the size of the outer structure varies in accordance with the nonlinear absorptivity.

2.2 Determination of characteristic temperature of modified structure

The simulation model used to evaluate the nonlinear absorptivity is briefly presented here. A detailed account can be found in Ref [8

8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

]. The temperature field in laser-irradiated glass sample is simulated assuming a line heat source with continuous heat delivery w(z) appears in an infinite solid moving at a constant speed of v along the x-axis. The relevant equation is
T(x,y,x)=14πK0lw(z')sexp{v2α(x+s)}dz'+T0
(2)
where s2 = x2 + y2 + (z-z’)2, K is the thermal conductivity and α is thermal diffusivity given by the relation K/cρ (c = specific heat and ρ=density) and T0 is room temperature. In this model, it is assumed that the thermal properties of the material are constant for the sake of simplicity. For w(z), we introduced a simple function of the form
w(z)=azm+b,0<z<l
(3)
where l is the length of the laser-absorbed region along z-axis (light propagation axis), and a, b and m are positive constants. The function w(z) can be determined by fitting the isotherm of the maximum cycle temperatures attained in (y,z) frame at dT/dx=0 to the cross-section of the experimental modification structures, assuming the characteristic temperature of the modified structure is known. Then the nonlinear absorptivity ACal can be derived by the Eq. (4).
ACal=1fQ00lw(z)dz,
(4)
where f is pulse repetition rate, and Q0 incident laser pulse energy. It was shown the simulated nonlinear absorptivity agrees with the experimental nonlinear absorptivity with an accuracy of 3%, assuming that the characteristic temperature of the outer structure of the modified zone in borosilicate glass is the forming temperature with a viscosity η = 104dPa.s [8

8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

].

In this study the characteristic temperature of the outer structure is determined from the experimental measurement of nonlinear absorptivity, since viscosity data of FOTURAN glass is not available. Figure 3(a)
Fig. 3 (a) Cross-section of FOTURAN glass (f=500kHz, Q0=5µJ, v=20mm/s, zh=250µm), (b) Simulated isothermal lines for m=0.5, 1.0 and 2.0 (characteristic temperatures: assumed to be Tout = 900þC and Tin = 3,000þC), (c) Intensity distribution w(z) for different values of m.
shows a typical cross-section of internally melted FOTURAN glass sample obtained at Q0 = 5µJ, f = 500kHz and v = 20mm/s. Figure 3(b) shows the simulated isothermal lines fitted to the experimental structures for m = 0.5, m = 1 and m = 2, where optimized values of a, b and l are used and the characteristic temperatures of the outer and the inner structures are assumed to be Tout = 900°C and Tin = 3,000°C, respectively. Figure 3(c) shows w(z) corresoponding to each value of m. It is seen that the shape of the inner structure and its relative position with respect to the outer structures are sensitive to the value of m, and that m = 2 provides the best result. A value of m = 2 is reasonable considering that the radius of the laser beam spot is proportinal to z2 at locations away from the focus. It should also be noted that the size and shape of the outer isotherm depend little on m, suggesting that the nonlinear absorptivity can be determined even without an exact distribution of w(z), if the integrated value of w(z) is known. Figure 4
Fig. 4 Relationship between characteristic temperature of the outer structure Tout and the nonlinear absorptivity ACal (f = 500kHz, Q0 = 5µJ, v = 20mm/s, m = 2).
shows the effect of Tout on the evaluated value of ACal. The simulated value of ACal varies linearly with Tout. The characteristic temperature can be determined from this figure if the nonlinear absorptivity is known.

In order to determine the value of Tout, the experimental nonlinear absorptivity AEx is determined at the same conditions as that in Fig. 3(a). The result is AEx = 70.5%. Using AEx = 70.5%, a value of Tout = 900°C is obtained for the characteristic temperature. The data indicates the effect of the accuracy of Tout on ACal is approximately 0.075%/°C.

2.3 Effect of laser processing parameters on nonlinear absorptivity

In this section, the nonlinear absorptivity of FOTURAN glass is determined by fitting the simulated isotherms at Tout = 900°C using different parameters including laser pulse repetition rate, average laser power and translation speed. For an average laser power of 3W, the focus position measured from the top surface is zh = 640µm. Figure 1 shows the nonlinear absorptivity at a translation speed of v = 20mm/s as a function of pulse repetition rate. The simulated value of ACal agrees with experimental values of AEx over a wide range of pulse repetition rate of f = 50kHz~8.2MHz within an uncertainty of 3% similar to that shown in Ref [8

8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

].

The nonlinear absorptivity ACal simulated at different average laser powers at f = 1MHz at 20mm/s is plotted in Fig. 5(a)
Fig. 5 Nonlinear absorptivity of FOTURAN glass (zh=640µm) plotted vs. (a) average laser power in comparison with D263 (f = 1MHz, v = 20mm/s), and (b) translation speed (at average laser power of 3W, f=1MHz).
. ACal increases with increasing laser power, because the rate of multiphoton ionization increases with increasing the laser intensity. Figure 5(b) shows ACal plotted vs. translation speed for a pulse repetition rate of f=1MHz. These results show that ACal decreases with increasing translation speed. This can be explained by the fact that the cooling rate is faster at higher translation speeds so that the laser-irradiated region cools down faster between the laser pulses. Thus as the translation speed increases, the density of the thermally excited free electrons is decreased at the moment of the laser pulse impingement, so that the density of seed electrons for avalanche ionization is decreased [8

8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

].

2.4 Effect of thermal properties on nonlinear absorptivity

To gain more insight on the nonlinear absorption process in FOTURAN glass, a comparison has been done with another glass material that has different thermal properties. The thermal properties of the material should affect the nonlinear absorptivity because heat accumulates at high pulse repetition rates. In order to simplify the problem, our comparison was with a glass material that has similar band gap energy Eg as shown in Table 1. By placing this restriction we can possibly overlook the effects of the multiphoton ionization (MPI) rate which should be similar in the two materials. The band gap energy of FOTURAN glass was determined by a Tauc plot based on optical transmission spectroscopy data. A value of Eg = 3.6eV is obtained, which is in good agreement with the value reported in Ref [16

16. B. Fisette and M. Meunier, “Three-dimensional microfabrication inside photosensitive glasses by femtosecond laser,” J. Laser. Micro/Nanoeng 1, 7–11 (2006).

]. In this experiment, the material D263 (Schott Glass Corp., Mainz, Germany) serves as the reference material, because the band the gap energy of Eg = 3.7eV is close to that of FOTURAN glass and the nonlinear absorptivity has been well-studied [8

8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

].

In Fig. 5(a), the nonlinear absorptivity ACal of D263 simulated at f=1MHz and v=20mm/s [8

8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

] is plotted vs. average laser power, showing ACal of FOTURAN glass is smaller than that of D263 when compared at the same average laser power. This is attributed to the larger thermal conductivity K of FOTURAN glass, since both materials have approximately the same band gap energy and . Assuming the same amount of laser energy is absorbed by MPI, D263 must reach a higher temperature during laser irradiation due to a smaller thermal conductivity. This should result in a larger density of thermally excited free electrons to seed avalanche ionization.

Figure 6
Fig. 6 Cross-sectional area S in the isothermal line of Tout for FOTRUAN glass and D263 at pulse duration of 10ps and 400fs at translation speed of v=20mm/s.
is a plot of the cross-sectional area S within the isotherm of Tout as a function of average absorbed laser power Wab at v=20mm/s for both FOTURAN glass and D263. The data points include different pulse repetition rates f for D263 [8

8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

] and FOTURAN glass. It is interesting to see that data points of the both materials fall nearly on a single line, in spite of the fact that both materials have different thermal conductivities K (see Table 1). We postulate that the effect of a lower Tout in FOTURAN glass is compensated by the effect of a larger K for D263. The figure also includes a data point taken at pulse duration of τ = 400fs [4

4. I. Miyamoto, A. Horn, J. Gottmann, D. Wortmann, and F. Yoshino, “Fusion welding of glass using femtosecond laser pulses with high-repetition rates,” J. Laser. Micro/Nanoeng 2, 57–63 (2007).

]. Note that this point also falls on the same line, suggesting avalanche ionization is the predominant absorption process at high pulse repetition rates even for 400fs.

As will be discussed in section 3, the average absorbed laser power Wab plays an important role in defining the mechanical strength of the laser-irradiated sample. We have shown the nonlinear absorptivity of FOTURAN glass can be evaluated by the experimental measurement and the application of a thermal conduction model. It is also possible to determine Wab through the measurement of the molten cross-sectional area for FOTURAN or D263.

3. Mechanical strength of weld joint

3.1 Mechanical strength of internally melted single glass sample

The mechanical strength of a weld joint is affected not only by the laser-irradiation conditions such as pulse repetition rate, pulse energy and translation speed, but by the geometry of the weld joint and the preparation of the glass plates. In order to ascertain the major factors affecting the mechanical strength of a weld joint in the laser-irradiation conditions, influences that might result from geometry of the weld joint of the glass plates were minimized. The internally melting of the single glass sample was made at an average laser power of 2.5W at different pulse repetition rates f and the translation speeds v. The embedded melted samples were lapped and polished parallel to the melt line to expose the maximum width of the melt line to the surface. These samples were then cut perpendicularly to the melt line having a width of 15mm. The mechanical strength of the molten region was determined with a three-point-bending test by applying the maximum stress at the molten region as schematically shown in the inset of Fig. 7
Fig. 7 Mechanical strength of internally melted single FOTURAN sample determined by a three-point-bending test at different f and v at a constant average laser power of 2.5W. A thick dotted line shows the average strength of the base material.
.

The mechanical strength data determined at different pulse repetition rates f and translation speeds v is shown in Fig. 7. A total of five samples were tested for each condition. The strength of the virgin material having no laser irradiation dose falls within the region of 115~200MPa (average strength: 152MPa). On the other hand, the average value of all the internally melted samples (60 data points in total) is approximately 145MPa. This is nearly equivalent to the virgin or base material. However, the strength of the internally melted single plate is approximately 30MPa lower than that of the base material for the conditions v = 20mm/s and f = 0.2MHz. The data show that the value of the strength tends to increase with increasing v and f, reaching as high as or even above that of the base material. There is some uncertainty scattering in the data set, but the general observed trend is valid.

3.2 Mechanical strength of overlapped weld joint

Prior to the overlap welding, the glass plates have to be faced with intimate contact to confine the laser-induced hot plasma within material [8

8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

]. It was reported [14

14. K. Cvecek, I. Miyamoto, J. Strauss, M. Wolf, T. Frick, and M. Schmidt, “Sample preparation method for glass welding by ultrashort laser pulses yields higher seam strength,” Appl. Opt. 50(13), 1941–1944 (2011). [CrossRef] [PubMed]

] that using sample pairs with optical contact, cracks can be prevented and that the mechanical strength of the weld joint can reach much higher than the case without optical contact [3

3. T. Tamaki, W. Watanabe, J. Nishii, and K. Itoh, “Welding of transparent materials using femtosecond laser pulses,” Jpn. J. Appl. Phys. 44(22), L687–L689 (2005). [CrossRef]

]. The optical contact is marked by a lack of light reflections from the interface, and can be normally obtained with the glass surfaces having roughness and flatness found in float glass, if the glass surfaces are carefully cleaned [23]. Lapping and polishing are, however, needed to provide optical contact in FOTURAN glass plates, since the surface roughness and flatness of as-received FOTURAN plate are not as good as float glass. Two FOTURAN glass plates of 1mm in thickness in optical contact were overlap-welded by irradiating a focused laser beam. The focus position is placed a little below the interface to provide the maximum width of the molten region at the interface.

In evaluating the mechanical strength of the weld joint, the effect of the optical contact force has to be evaluated [14

14. K. Cvecek, I. Miyamoto, J. Strauss, M. Wolf, T. Frick, and M. Schmidt, “Sample preparation method for glass welding by ultrashort laser pulses yields higher seam strength,” Appl. Opt. 50(13), 1941–1944 (2011). [CrossRef] [PubMed]

], because the measured strength of the weld joint overestimates as the result of the attractive force due to van der Waals interactions [15

15. V. Greco, F. Marchesini, and G. Molesini, “Optical contact and van der Waals interactions: the role of the surface topography in determining the bonding strength of thick glass plates,” J. Opt. A, Pure Appl. Opt. 3(1), 85–88 (2001). [CrossRef]

]. The optical contact force can be evaluated by inserting a blade with measured thickness between the contact surfaces and measuring the length of the “air wedge” extending to the contact line [24

24. W. P. Maszara, G. Goetz, A. Caviglia, and J. B. McKitterick, “Bonding of silicon wafer for silicon-on-insulator,” J. Appl. Phys. 64(10), 4943–4950 (1988). [CrossRef]

]. In the present study, the optical contact force was evaluated by applying the shear force to break the optical contact, because the glass sample having a thickness of 1mm are too thick to insert a blade. Glass samples with a width of 15mm were used for evaluating the shear force to break the optical contact at different length L as schematically shown in Fig. 9
Fig. 9 Shear force to break optical contact FOC plotted vs. contact area SOC. The optical contact force per unit area σOC is also plotted.
. The sample with the minimum length of L = 1mm was produced by HF-etching of masked glass sample.

Overlap welding was performed using the pre-bonded sample with L=10mm corresponding to FOC≈88.2N (σOC≈0.6MPa), which is strong enough to keep optical contact in handling the sample for the welding experiment. Although σOC≈0.6MPa is approximately two orders smaller than the strength of FOTURAN glass, L=10mm is also two orders larger than typical bead width. Therefore the strength of the weld joint σW was determined by subtracting the optical contact force FOC from the rupture load FRUP, and then dividing by the cross-sectional area of the weld bead SW using

σW=FRUPFOCSW.
(5)

Overlap welding was performed at an average laser power of 3W at different pulse repetition rates and different translation speeds. In Figs. 10(a)
Fig. 10 Effect of (a) translation speed at f=1MHz and (b) pulse repetition rate at v = 20mm/s on the rupture strength and weld area of the overlap-weld samples. (L=10mm, length of weld path = 8~9mm)
and 10(b), the rupture load FRUP and the welded area SW are plotted as a function of translation speed v with a constant pulse repetition rate of f=1MHz, and as a function of pulse repetition rate at a constant translation speed of v = 20mm/s, respectively.

Figure 8 also presents σW data for overlap-welded samples, where the data points for (a) f=1MHz and (b) v=20mm/s are shown. In the figure, the nonlinear absorptivity of the overlap welding was assumed to be equal to the values obtained at zh = 640µm, which are given by Figs. 1 and 5(b). In spite of the fact that the nonlinear absorptivity is actually affected by zh, most data points of the overlap weld are included within the aforementioned colored region that highlights the data from the internal melt single plate studies. It is noted that one single data point taken at translation speed of 100mm/s lies outside this region. Furthermore the data measured at different v but constant f has a steep dependence with translation speed. We believe this result is as a consequence of the nonlinear absorptivity for the overlap welding (zh≈1mm) which is actually smaller than that of the single plate studies at zh = 640µm. This difference is due to the larger spherical aberration which results in a lower laser power density at the focus, and the decrease in the nonlinear absorptivity due to the spherical aberration tends to increase with increasing translation speed.

In order to deduce the difference in the nonlinear absorptivity between zh=640µm and zh≈1mm (overlap welding), we measured the width of the welded region in the samples ruptured in the shear test, and compared this with the simulated width of the molten zone. It was found that the width of the weld region was approximately γ≈75% of the simulated width at 100mm/s. The measured width of the fractured region were γ≈81% and γ≈95% of the values at zh = 640µm at v = 50mm/s and 10 = mm/s, respectively, supporting the conclusion that the effect of zh on the nonlinear absorptivity increases with increasing translation speed. Assuming it is possible to revise the value for the average absorbed laser power W’ab by the relation W’ab = γ*Wab, the data values of overlap weld joint in Fig. 8 that are at higher translation speeds move to lower Wab, and approach the data taken at v = 20mm/s. Further experiments are needed to quantify this analysis.

Figure 8 also contains data of the strength of overlap-welded D263 [14

14. K. Cvecek, I. Miyamoto, J. Strauss, M. Wolf, T. Frick, and M. Schmidt, “Sample preparation method for glass welding by ultrashort laser pulses yields higher seam strength,” Appl. Opt. 50(13), 1941–1944 (2011). [CrossRef] [PubMed]

]. The data point falls in the same region of FOTURAN data, suggesting that the overlap-welded samples provide the equivalent strength to the internally melted single plate, if the sample pair is appropriately pre-bonded to provide optical contact.

Figure 11
Fig. 11 SEM photographs of ruptured face of overlap welded sample and strength of the weld joint at 3W at f = 1MHz.
shows fracture faces observed by SEM at different translation speeds at a laser pulse repetition rate of f = 1MHz. The sample at 20mm/s (11a) with the lower joint strength appears as an irregular fracture face, suggesting that a complicated stress field is produced in the weld bead possibly as a result of the local compensation of the tensile stress on the compressive stress field. The sample at 100mm/s (11c) with the higher joint strength appears as a smooth fracture face, suggesting the compensation of the stress field due to tensile stress is smaller. For better understanding of the mechanism, further study is needed by simulating the stress field.

4. Summary and conclusions

The nonlinear absorptivity of FOTURAN material to incident ps laser pulses has been experimentally measured and simulated by a model that includes the effects of different processing parameters including pulse energy, the repetition rate of the laser pulse, the sample translation speed and thermal properties. The mechanical strength of an internally melted single sample and overlapped welded sample were evaluated by a three-point bending test and a shear test, respectively. The strength of the overlap weld joint is found to be as high as the internally melted single glass plate when the sample pair is prepared such that optical contact is achieved. The results show that the mechanical strength of the both single and overlap weld joint samples decreases with increasing the average absorbed laser power Wab, and is as strong as the base material when Wab<1.2W.

Acknowledgments

The authors wish to thank Dr. J. Gottmann and Dipl.-Phys. D. Schaefer, Lehrstuhl für Lasertechnik LLT, RWTH Aachen University, for their measurement of band gap energy of the glass sample. This work was partially supported by Erlangen Graduate School in Advanced Optical Technologies (SAOT).

References and links

1.

K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef] [PubMed]

2.

M. Watanabe, H. Sun, S. Juodkazis, T. Takahashi, S. Matsuo, Y. Suzuki, J. Nishii, and H. Misawa, “Three-dimensional optical data storage in vitreous silica,” Jpn. J. Appl. Phys. 37(Part 2, No. 12B), L1527–L1530 (1998). [CrossRef]

3.

T. Tamaki, W. Watanabe, J. Nishii, and K. Itoh, “Welding of transparent materials using femtosecond laser pulses,” Jpn. J. Appl. Phys. 44(22), L687–L689 (2005). [CrossRef]

4.

I. Miyamoto, A. Horn, J. Gottmann, D. Wortmann, and F. Yoshino, “Fusion welding of glass using femtosecond laser pulses with high-repetition rates,” J. Laser. Micro/Nanoeng 2, 57–63 (2007).

5.

J. Noack and A. Vogel, “Laser-induced plasma formation in water at nanosecond to femtosecond time scales: calculation of thresholds, absorption coefficient, and energy density,” IEEE J. Quantum Electron. 35(8), 1156–1167 (1999). [CrossRef]

6.

C. L. Arnold, A. Heisterkamp, W. Ertmer, and H. Lubatschowski, “Computational model for nonlinear plasma formation in high NA micromachining of transparent materials and biological cells,” Opt. Express 15(16), 10303–10317 (2007). [CrossRef] [PubMed]

7.

I. Miyamoto, A. Horn, and J. Gottmann, “Local melting of glass material and its application to direct fusion welding by ps-laser pulses,” J. Laser. Micro/Nanoeng 2, 7–14 (2007).

8.

I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express 19(11), 10714–10727 (2011). [CrossRef] [PubMed]

9.

K. Hirao, Y. Shimotsuma, J. Qiu, and K. Miura, “Femtosecond laser induced phenomena in gasses and photonic device application,” Mater. Res. Soc. Symp. Proc., 13–23 (2005).

10.

N. Borrelli, J. Helfinstine, J. Price, J. Schroeder, A. Atreltsov, and J. Westbrook, “Glass strengthening with an ultrafast laser,” in Proceedings of the Int. Cong. Appl. Laser and Electro Optics (ICALEO) (2008), pp.185–189.

11.

Y. Bellouard, T. Colomb, C. Depeursinge, M. Dugan, A. A. Said, and P. Bado, “Nanoindentation and birefringence measurements on fused silica specimen exposed to low-energy femtosecond pulses,” Opt. Express 14(18), 8360–8366 (2006). [CrossRef] [PubMed]

12.

P. Kongsuwan, H. Wang, S. Vukelic, and Y. L. Yao, “Characterization of morphology and mechanical properties of glass interior irradiated by femtosecond laser,” J. Manuf. Sci. Eng. 132(4), 041009 (2010). [CrossRef]

13.

T. Arai, N. Asano, A. Minami, and H. Kusano, “Inside process of glass with nanosecond pulsed laser,” in Proceedings of the Int. Cong. Appl. Laser and Electro Optics (ICALEO) (2008), pp.408–414.

14.

K. Cvecek, I. Miyamoto, J. Strauss, M. Wolf, T. Frick, and M. Schmidt, “Sample preparation method for glass welding by ultrashort laser pulses yields higher seam strength,” Appl. Opt. 50(13), 1941–1944 (2011). [CrossRef] [PubMed]

15.

V. Greco, F. Marchesini, and G. Molesini, “Optical contact and van der Waals interactions: the role of the surface topography in determining the bonding strength of thick glass plates,” J. Opt. A, Pure Appl. Opt. 3(1), 85–88 (2001). [CrossRef]

16.

B. Fisette and M. Meunier, “Three-dimensional microfabrication inside photosensitive glasses by femtosecond laser,” J. Laser. Micro/Nanoeng 1, 7–11 (2006).

17.

Y. Cheng, K. Sugioka, M. Masuda, K. Toyoda, M. Kawachi, K. Shihoyama, and K. Midorikawa, “3D microstructuring inside Foturan glass by femtosecond laser,” RIKEN Review 50, 101–106 (2003).

18.

H. Helvajian, P. D. Fuqua, W. W. Hansen, and S. Janson, “Nanosatellites and MEMS fabrication by laser microprocessing,” in Proceedings of the 1st Int. Symp. On Laser Precision Microfabrication- LPM2000 (2000), pp. 319–326.

19.

http://www.mikroglas.com/index.php?PAGE_ID=544#.

20.

K. Nahen and A. Vogel, “Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses – Part II: Transmission, scattering, and reflection,” J. Sel. Topics Quant. El. 2(4), 861–871 (1996). [CrossRef]

21.

I. Miyamoto, A. Horn, J. Gottmann, D. Wortmann, I. Mingareev, F. Yoshino, M. Schmidt, P. Bechtold, Y. Okamoto, Y. Uno, and T. Herrmann, “Novel fusion welding technology of glass using ultrashort pulse lasers,” in the Proceedings of the Int. Cong. Appl. Laser and Electro Optics (ICALEO) (2008).

22.

C. Hnatovsky, R. S. Taylor, P. P. Rajeev, E. Simova, V. R. Bhardwaj, D. M. Rayner, and P. B. Corkum, “Pulse duration dependence of femtosecond-laser-fabricated nanogratings in fused silica,” Appl. Phys. Lett. 87(1), 014104 (2005). [CrossRef]

23.

http://labaccessories.mellesgriot.com/pdfs/Cleaning_Methods.pdf.

24.

W. P. Maszara, G. Goetz, A. Caviglia, and J. B. McKitterick, “Bonding of silicon wafer for silicon-on-insulator,” J. Appl. Phys. 64(10), 4943–4950 (1988). [CrossRef]

25.

S. S. Kachkin and Y. V. Listsyan, “Optical-contact bonding strength of glass components,” Sov. J. Opt. Technol. 47, 159–161 (1980).

OCIS Codes
(140.3390) Lasers and laser optics : Laser materials processing
(140.3440) Lasers and laser optics : Laser-induced breakdown
(140.7090) Lasers and laser optics : Ultrafast lasers
(160.2750) Materials : Glass and other amorphous materials
(190.4180) Nonlinear optics : Multiphoton processes
(350.3390) Other areas of optics : Laser materials processing

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: August 31, 2011
Revised Manuscript: September 22, 2011
Manuscript Accepted: September 24, 2011
Published: October 28, 2011

Citation
Isamu Miyamoto, Kristian Cvecek, Yasuhiro Okamoto, Michael Schmidt, and Henry Helvajian, "Characteristics of laser absorption and welding in FOTURAN glass by ultrashort laser pulses," Opt. Express 19, 22961-22973 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-23-22961


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett.21(21), 1729–1731 (1996). [CrossRef] [PubMed]
  2. M. Watanabe, H. Sun, S. Juodkazis, T. Takahashi, S. Matsuo, Y. Suzuki, J. Nishii, and H. Misawa, “Three-dimensional optical data storage in vitreous silica,” Jpn. J. Appl. Phys.37(Part 2, No. 12B), L1527–L1530 (1998). [CrossRef]
  3. T. Tamaki, W. Watanabe, J. Nishii, and K. Itoh, “Welding of transparent materials using femtosecond laser pulses,” Jpn. J. Appl. Phys.44(22), L687–L689 (2005). [CrossRef]
  4. I. Miyamoto, A. Horn, J. Gottmann, D. Wortmann, and F. Yoshino, “Fusion welding of glass using femtosecond laser pulses with high-repetition rates,” J. Laser. Micro/Nanoeng2, 57–63 (2007).
  5. J. Noack and A. Vogel, “Laser-induced plasma formation in water at nanosecond to femtosecond time scales: calculation of thresholds, absorption coefficient, and energy density,” IEEE J. Quantum Electron.35(8), 1156–1167 (1999). [CrossRef]
  6. C. L. Arnold, A. Heisterkamp, W. Ertmer, and H. Lubatschowski, “Computational model for nonlinear plasma formation in high NA micromachining of transparent materials and biological cells,” Opt. Express15(16), 10303–10317 (2007). [CrossRef] [PubMed]
  7. I. Miyamoto, A. Horn, and J. Gottmann, “Local melting of glass material and its application to direct fusion welding by ps-laser pulses,” J. Laser. Micro/Nanoeng2, 7–14 (2007).
  8. I. Miyamoto, K. Cvecek, and M. Schmidt, “Evaluation of nonlinear absorptivity in internal modification of bulk glass by ultrashort laser pulses,” Opt. Express19(11), 10714–10727 (2011). [CrossRef] [PubMed]
  9. K. Hirao, Y. Shimotsuma, J. Qiu, and K. Miura, “Femtosecond laser induced phenomena in gasses and photonic device application,” Mater. Res. Soc. Symp. Proc., 13–23 (2005).
  10. N. Borrelli, J. Helfinstine, J. Price, J. Schroeder, A. Atreltsov, and J. Westbrook, “Glass strengthening with an ultrafast laser,” in Proceedings of the Int. Cong. Appl. Laser and Electro Optics (ICALEO) (2008), pp.185–189.
  11. Y. Bellouard, T. Colomb, C. Depeursinge, M. Dugan, A. A. Said, and P. Bado, “Nanoindentation and birefringence measurements on fused silica specimen exposed to low-energy femtosecond pulses,” Opt. Express14(18), 8360–8366 (2006). [CrossRef] [PubMed]
  12. P. Kongsuwan, H. Wang, S. Vukelic, and Y. L. Yao, “Characterization of morphology and mechanical properties of glass interior irradiated by femtosecond laser,” J. Manuf. Sci. Eng.132(4), 041009 (2010). [CrossRef]
  13. T. Arai, N. Asano, A. Minami, and H. Kusano, “Inside process of glass with nanosecond pulsed laser,” in Proceedings of the Int. Cong. Appl. Laser and Electro Optics (ICALEO) (2008), pp.408–414.
  14. K. Cvecek, I. Miyamoto, J. Strauss, M. Wolf, T. Frick, and M. Schmidt, “Sample preparation method for glass welding by ultrashort laser pulses yields higher seam strength,” Appl. Opt.50(13), 1941–1944 (2011). [CrossRef] [PubMed]
  15. V. Greco, F. Marchesini, and G. Molesini, “Optical contact and van der Waals interactions: the role of the surface topography in determining the bonding strength of thick glass plates,” J. Opt. A, Pure Appl. Opt.3(1), 85–88 (2001). [CrossRef]
  16. B. Fisette and M. Meunier, “Three-dimensional microfabrication inside photosensitive glasses by femtosecond laser,” J. Laser. Micro/Nanoeng1, 7–11 (2006).
  17. Y. Cheng, K. Sugioka, M. Masuda, K. Toyoda, M. Kawachi, K. Shihoyama, and K. Midorikawa, “3D microstructuring inside Foturan glass by femtosecond laser,” RIKEN Review50, 101–106 (2003).
  18. H. Helvajian, P. D. Fuqua, W. W. Hansen, and S. Janson, “Nanosatellites and MEMS fabrication by laser microprocessing,” in Proceedings of the 1st Int. Symp. On Laser Precision Microfabrication- LPM2000 (2000), pp. 319–326.
  19. http://www.mikroglas.com/index.php?PAGE_ID=544# .
  20. K. Nahen and A. Vogel, “Plasma formation in water by picosecond and nanosecond Nd:YAG laser pulses – Part II: Transmission, scattering, and reflection,” J. Sel. Topics Quant. El.2(4), 861–871 (1996). [CrossRef]
  21. I. Miyamoto, A. Horn, J. Gottmann, D. Wortmann, I. Mingareev, F. Yoshino, M. Schmidt, P. Bechtold, Y. Okamoto, Y. Uno, and T. Herrmann, “Novel fusion welding technology of glass using ultrashort pulse lasers,” in the Proceedings of the Int. Cong. Appl. Laser and Electro Optics (ICALEO) (2008).
  22. C. Hnatovsky, R. S. Taylor, P. P. Rajeev, E. Simova, V. R. Bhardwaj, D. M. Rayner, and P. B. Corkum, “Pulse duration dependence of femtosecond-laser-fabricated nanogratings in fused silica,” Appl. Phys. Lett.87(1), 014104 (2005). [CrossRef]
  23. http://labaccessories.mellesgriot.com/pdfs/Cleaning_Methods.pdf .
  24. W. P. Maszara, G. Goetz, A. Caviglia, and J. B. McKitterick, “Bonding of silicon wafer for silicon-on-insulator,” J. Appl. Phys.64(10), 4943–4950 (1988). [CrossRef]
  25. S. S. Kachkin and Y. V. Listsyan, “Optical-contact bonding strength of glass components,” Sov. J. Opt. Technol.47, 159–161 (1980).

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited