We report on a refractive index modification (▵n) induced by femtosecond irradiation and evaluation of the
profile for created lines inside the different types of optical glasses, i.e., silicate or borate glass containing the metal
oxides such as BaO, TiO2, or La2O3 and silica glass. The lines are fabricated by scanning a stage and focusing the
femtosecond laser pulses, 800nm wavelength, a 250 kHz repetition rate and 200 fs pulse duration, from the Ti:sapphire
regenerative amplifier system. The ▵n profiles of modification were obtained with Qualitative Phase Microscopy
technique and presented systematically for a different input power and a variety of glasses. The ▵n profile changed with
focusing condition using 10× (N.A.=0.3) or 40× (N.A.=0.85), and input power in a single glass. However, the ▵n and a
trend of the sign was different depending on glass types. For example, silicate glass containing TiO2, exhibited negative
▵n trend the ▵n became smaller in the modified region. Furthermore, the glass showed relatively large negative ▵n, < -
0.01 decrease of the ▵n, ▵n < -0.01, in the investigated power range. These results could be useful for a design or use of
glasses for micro optics, such as grating, diffractive lens or lens array, produced by femtosecond laser fabrication.
We report an invisible two-dimensional (2D) barcode embedded into a synthetic fused silica by femtosecond laser
processing using a computer-generated hologram (CGH) that generates a spatially extended femtosecond pulse beam in
the depth direction. When we illuminate the irradiated 2D barcode pattern with a 254 nm ultraviolet (UV) light, a strong
red photoluminescence (PL) is observed, and we can read it by using a complementary metal oxide semiconductor
(CMOS) camera and image processing technology. This work provides a novel barcode fabrication method by
femtosecond laser processing using a CGH and a barcode reading method by a red PL.
Using femtosecond laser processing with glass-hologram, fabrication of 1cm-long straight waveguide and X-coupler is
reported in this paper. We design and fabricate 4-level glass-hologram which generates 1cm-long straight line intensity.
We fabricate 1cm-long waveguides inside fused silica at one shot exposure with the glass-hologram. We investigate the
waveguide performance of near field pattern and propagation loss at wavelength of 1550nm. The near field pattern is
almost circular shape. The propagation loss at 1550nm is estimated to be < 1.0 dB/cm. As an example of an optical
device consisting of straight waveguides, we fabricate X-coupler or 2x2 coupler using straight line waveguides, and
observe the output power ratio depending on crossing angle.
We report on a curved waveguide fabrication using femtosecond laser processing with a glass hologram. We design and
produce a glass hologram that transforms femtosecond laser beam into a half-ring beam. The half-ring beam generated
by the glass hologram is patterned inside fused silica with one laser shot. The guided light whose bending radius is larger
than 1mm is observed at wavelength of 635nm. As a simple application, a directional coupler consisting of a straight-line
waveguide and a half-ring waveguide is fabricated by two laser shots. Its basic functionality as a coupler is confirmed.
We also develop a hologram that simultaneously produces a straight-line and a half-ring. Using it, we demonstrate a
directional coupler fabrication inside crown glass with one laser shot.
In accordance with the development of various optical devices, an urgent need for innovative 3D microfabrication
method arises. It requires not only rapid processing time or high energy efficiency but also high flexibility in designing
3D structure. Hence we established new 3D microfabrication method to satisfy all of these seemingly-contradictory
factors. This method uses only single femtosecond laser pulse and phase CGH (computer generated hologram); the phase
distribution of the pulse is controlled by the CGH and an arbitrary 3D microstructure is fabricated inside transparent
material by multi photon absorption. It means that this method costs extremely short time and low power for the
fabrication of an arbitrary complex 3D microstructure. In this report, the microfabrication of 3D spiral array which
consists of 24 dot elements is demonstrated. It is very difficult to process multiple elements at different depths
simultaneously, because the light intensity depends on the numerical aperture number of the objective lens and the
distance from the CGH. Hence we improved the CGH calculation by considering these dependencies so that the light
intensity of each element could be controlled separately. By this intensity adjustment, the shape of all elements becomes
homogeneous. The other side of this intensity control is that it is able to process different shape elements intentionally by
varying the intensity of each element. This intensity control is confirmed by the microfabrication with another CGH
which forms 7 dot elements of different shapes. This result proves the high flexibility in designing 3D structure of this
method.
We report on a new waveguide fabrication method with femtosecond laser pulses shaped by Computer-Generated
Hologram (CGH). We design and make CGH's that generates a straight-line intensity distribution from an input laser
intensity distribution. We fabricate a waveguide inside a fused silica sample with exposing the line intensity beam
generated by the CGH, without translating the sample. The fabricated waveguide is 5.1 mm long and 6μm width. We also observe guided-light passing through the waveguide that is butt-coupled to a single mode fiber, at wavelength of 635 nm. The near field pattern is nearly circular cross section. This is the first achievement of waveguide fabrication using a CGH.
We demonstrated a simple chromatic dispersion reduction method of 3-dimensional (3D) patterning of femtosecond pulses using a multi-level phase type diffractive optical element (DOE) and a focusing objective lens. Our method increases flexibility of femtosecond laser microprocessing. With appropriate focal length of the DOE and distance between the DOE and the focusing lens, large chromatic dispersion of the DOE resulting from spectral bandwidth of a femtosecond pulse can be reduced, and 3D focusing pattern of femtosecond pulse can be obtained not only controlled in focal plane but also in focal depth. The method was verified through optical and processing experiments with laser pulses of 400 fs duration and of 40 nm bandwidth. The focal length of the DOE and the objective lens was 1600 mm and 10 mm, respectively. Partially periodical structure of focusing points was formed at designed position and its focal depth were much smaller than that focused with only the DOE. By irradiating the constructed beam, microstructure was formed precisely inside SiO2 glass. The processed points are clearly separated each other with a separation of 5 mm and the spot sizes were almost same as those irradiated without the DOE.
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