Femtosecond laser joining of transparent materials by dfgh4bnmu

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									Femtosecond laser joining of
   transparent materials

  Sinisa Vukelic, Jason Lau and Y. Lawrence Yao
 Manufacturing Research Lab, Columbia University




                 Manufacturing Research Laboratory
                                                     1
           Introduction
Possible to process any material
Precision much higher than nanosecond
laser
Heat conduction and hydrodynamic
motion negligible
  thermal damage (microcracks) and HAZ
  (heat-affected-zone) greatly reduced
Especially effective for interaction with
dielectrics

                                            2
                Applications
                                   Cutting teflon coated
                                    fused silica tube with
                                   (a) Nanosecond laser
                                   (b) Femtosecond laser


   (a)                  (b)


Tooth drilling using:
(a) Nanosecond laser
(b) Femtosecond laser


                              Perry et al. (1999)

                                                    3
 Motivation and Objectives
Wide variety of use of dielectric
materials in various fields:
  Lab on chip
  Optics and fiber optics
  MEMS, etc.
Reliable joining technique not available
Laser welding flexible, easy and
environmentally friendly way to join
dielectrics

                                           4
Nanosecond laser glass welding
Excimer (deep UV
wavelength)
  Absorption through
  entire thickness
  Complicated optics
                         Ho et al. (2005)
Nd:YAG
  Poor absorption –
  intermediate coating
  layer needed


                         Luo and Lin (2001)

                                              5
     Theoretical background

Laser material interaction
highly non-linear
Two types of femtosecond
                             Multiphoton ionization
ablation:
  Multiphoton ionization
  Avalanche ionization


                              Avalanche ionization




                                                      6
   Laser-glass interaction

Due to the non-linear absorption only
glass in focal volume affected
Ablation done through laser induced
breakdown
Free electrons reach critical density
Material transforms into absorbing
plasma with metallic properties


                                        7
                Laser system
               Mai Tai

                    Mode-locked
Diode-pumped        Ti:Sapphire
   cw laser         Laser


                         Spitfire

  Evolution              Stretcher/compressor     Output:
Nd:YLF Laser                                      Energy 0.75mJ

                         Regenerative amplifier   Pulse width:
                                                  130 fs




                                                         8
         Experimental setup
Hurricane laser
1 kHz repetition rate
Wavelength: 800 nm
Pulse width 130 fs
TEM00 mode
Spatial mode <1.5 x
diffraction limited



                        Objective lens 10x and 40x
                        Numerical aperture (NA):
                        0.25 and 0.65 respectively



                                              9
Experimental setup (continued)

Simulated joining experiments
Focal point in interior of the specimen
Explosive plasma expansion occurs within
focal volume
Soda lime glass plates used with 1 and 3 mm
thickness
Energy and Numerical Aperture effect studied



                                          10
    Numerical aperture dependence
Numerical aperture responsible
for focusing quality of incoming
beam
Loose focusing cause larger
region to be affected                            1.5 mm
                                 1mm
Plasma produced by such pulse
not energetic enough to produce
explosive expansion
Structural changes cannot take
place
                                       NA 0.65            NA 0.25




                                                             11
                  Energy dependence


1mm




      10 uJ/pulse 20 uJ   30 uJ   40 uJ   50 uJ   60 uJ   65 uJ

         Feature size slightly increases
         Inner region produced directly by laser pulse
         Outer region result of heat conduction
         Material near the focal region has higher density
                                                                  12
        Glass joining experiments
Feature length                                                      180
proportional to laser pulse                                         160




                                   T e a rd ro p le n g th (u m )
energy                                                              140
                                                                    120
Feature width related to                                            100
the repetition rate                                                  80
    laser energy less absorptive                                     60
                                                                     40
   at lower rep. rate
                                                                     20
Structural changes take                                               0
place at ~400 J/cm^2                                                      0   200   400   600   800   1000   1200   1400   1600
                                                                                          Fluence (J/cm^2)




                                                                                                                    13
                Future work
Actual welding two
surfaces
  Cylindrical surfaces
  Similar materials
  Dissimilar materials
                         Tamaki et al. (2006)
Investigation of
change of refractive
index
Numerical modeling
                         Tamaki et al. (2005)


                                                14
Future work - numerical modeling
 Modify existing ablation model
 Boundary conditions significantly
 different
 Consider:
   Explosive plasma expansion
   Plastic deformation
   Heat transfer



                                     15
Future work - numerical modeling
           continued
 Governing equations
   Free electron generation
       ∂ne
           = (wmpi + ptunn )⋅ na + wava ne
        ∂t
   Coulomb explosion
        ∂ 2u   ∂ 2u    ∂Te
       ρ 2 = λ 2 + CTe
        ∂t     ∂x      ∂x
   Two temperature model
               ∂Te     ∂qe
      ce (Te )     =−       − G (Te − Tl ) + Qlaser
                ∂t      ∂x
          ∂qe               ∂Te
      τe       + qe = − K e
           ∂t               ∂x
                                                      16
        Possible research interests


Arnaud et al. (2003)   Joglekar et al. (2003)

                                                Glezer et al. (1996)

   Optical waveguides
   Nanomachining
   Three dimensional optical data storages


                                                                 17

								
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