Study On Thresholds For Femtosecond Laser Three Dimensional Micromachining Of Silica Glass And Its Application

Femtosecond laser micromachining has not only high precision, good quality and great spatial selectivity, but also unique ability of 3D microprocessing inside transparent materials. To realize controllable 3D machining, the incident laser energy should be optimized according to the refractive index change and damage thresholds that are dependent on both pulse parameters and focusing conditions, which needs systematic investigation. In this thesis, we studied the influence of focusing depth, numerical aperture (NA), pulse duration and polarization on the thresholds of fused silica and fabricated single mode waveguides, multi-mode interference (MMI) couplers, waveguide arrays, photonic bandgap structures, waveguide gratings.We found that the refractive index change threshold, damage threshold and their difference increased with the focusing depth, which rose from 100, 110, and 10 nJ at depth of 200μm to 300, 450, 150 nJ at depth of 2100μm. This was caused by the spherical aberration induced by the mismatch of the refractive indexes. The aspect ratio of the waveguide cross section can be selected by changing the focusing depth.It was found that the damage threshold was influenced not only by the polarization and the pulse duration of the incident femtosecond laser beam, but also by the focusing conditions. The damage threshold for circularly polarized femtosecond laser pulses was higher than that for linearly polarized laser pulses in the tight focusing case (NA> 0.4) , but the former was lower than the later in the loose focusing case (NA<0.4). The reverse might be due to different damage processes: laser induced damage at high NA and the self-focusing breakdown at low NA; We also found even the same linear polarization has different influence on the damage threshold when multiple pulses overlapped. The pulses whose polarizations were parallel to the waveguide direction, were easier to induce the damage inside the sample comparing with the pulses whose polarization were vertical to. For the case of the single pulse interaction with the glass, the difference was negligible; when the pulse duration was longer than 260 fs, the damage threshold for negatively chirped pulses was lower than that for positively chirped pulses.Taking into account the influence of several parameters, we controlled the cross section of the waveguides and fabricated various optical elements based on waveguides, A 5mm long waveguide was written at the depth of 2100μm, which was single mode at 632.8nm and exhibited propagation loss of 0.56dB/cm. The refractive index change was calculated to be ².47×10^-3; We fabricated MMI waveguides using transverse writing method and obtained 3 and 5 beams when the widths were 30 and 48μm, which matched the simulation by the beam propagation method; We wrote a 3×3 waveguides array to realize the coupling; The band-gap guidance of light was also demonstrated in waveguide arrays with a negative defect.