When femtosecond laser pulses are focused inside bulk transparent materials,
the intensity in the focal volume becomes high enough to produce permanent
structural modifications. This technique has been applied to fabricate
three-dimensional photonic structures for optical data storage, waveguides,
gratings, and couplers inside a wide variety of transparent materials.
We report micro-fabrication of optical elements in glasses with femtosecond
laser pulses, including fabrication of couplers, Bragg gratings, and
In recent years, micromachining or microfabrication by femtosecond laser
pulses in transparent materials has received much attention. When femtosecond
laser pulses are focused inside bulk transparent materials, the intensity
in the focal volume can become high enough to cause nonlinear absorption,
which leads to localized modification in the focal volume, while leaving
the surface unaffected. Recent demonstrations of three-dimensional micromachining
of glass using femtosecond laser pulses include waveguides, couplers,
gratings, binary data storage, lenses and channels . Direct writing
of optical devices using femtosecond laser pulses in glass has potential
applications in the telecommunication and optical signal processing
industries. The most important feature of the microfabrication technique
in glass is its ability for three-dimensional (3-D) integration of optical
or photonic devices inside transparent materials: Sequential direct
writing of individual devices inside bulk transparent materials simply
results in the integration of a 3-D photonic signal-processing system.
Although such an approach lacks in the speed of mass production compared
with the conventional lithographic process, the ability of 3-D integration
is priceless in that 3-D integration of photonic devices is quite difficult
to achieve by the conventional lithographic process or other related
techniques. The current prosperity of the semiconductor industry relies
on the progress of microelectronics technologies and no one can deny
the possibility that the present 3-D microfabrication techniques may
make some contributions to the industrial fabrication of integrated
|Fig. 1(a) Schematic of a three-dimensional
directional coupler. (b) Near-field patterns of coupler output when
coupling to a broadband light source (supercontinuum).
II. Waveguide and Coupler
The induction of permanent refractive-index change at the laser focal
point has been reported in bulk glasses to the order of 10-2 to 10-4.
By translation of the sample with respect to the focal point, fabrication
of waveguides and couplers has been demonstrated. Filamentation of femtosecond
laser pulses induces permanent refractive-index change in silica glass
. Filamentation occurs due to a balance between the Kerr self-focusing
of the laser pulse and the defocusing effect of the high-intensity plasma
generated in the self-focal region. Filamentation induces refractive-index
change. The location of a filament coincided with that of the refractive-index
change. The region of refractive index change was 10 to 500 micrometers
long due to the high numerical-aperture (NA) of the focusing lens used
on the silica glass. By translation of the sample parallel to the optical
axis, the 2-mm straight waveguide was fabricated.
We fabricated a directional coupler containing a 2-mm-long straight
waveguide and a curved waveguide connected to a different straight section
. The straight section was parallel to the straight waveguide and
was separated by a 4-µm center-to-center distance. Another straight
waveguide, which length is L mm, was then fabricated parallel to the
straight waveguide. The curved waveguide was fabricated from one end
of the straight section. The position of the filament in the curved
section was determined by computer-controlled positioning along an arc
of radius 17 mm. Coupling properties were investigated by focusing a
He-Ne laser beam having a wavelength of 632.8 nm into the straight waveguide.
Experimental results show that the splitting ratios of the directional
couplers with L = 1 mm and L = 0.5 mm are approximately 1:1 and 1:0.5,
respectively. By use of a 2-mm coupler, a beam can be divided into 1:1.
Producing real three-dimensional photonic devices is one of the salient
features of femtosecond micromachining. We demonstrate the realization
of three-dimensional directional couplers using filamentation . Figure
1 (a) shows the schematic of a three-dimensional directional coupler
consisting of three waveguides: a 2-mm-long straight waveguide (waveguide
I) and two curved waveguides that are connected to straight sections
(waveguide II and III). The curved waveguides have arc radii of 17 mm.
The straight sections of waveguide II and III were parallel to the straight
waveguide at a 4 µm center-to-center separation. Lengths of the
straight sectors in waveguide II and waveguide III were 0.5 mm and 1.0
mm. The broadband supercontinuum was coupled to the straight waveguide
and near-field output patterns were monitored using the color CCD camera.
Figure 1 (b) shows the near-field output patterns of the coupler. Spectra
are different at the output because the coupling properties are dependent
on wavelength. The technique for fabricating spectroscopic couplers
in glass could thus have potential applications in wavelength division,
including in spectral filters. Beams of different wavelengths from different
lasers can also be coupled into a single waveguide.
|Fig 2(a) Fabrication of volume gratings
in silica glass. (b) Optical images of the fabricated gratings with
the period of 5 mm and thickness of 150 mm.
(c) Schematic of diffraction pattern. The fabricated grating with
the period of 3 mm and thickness of 150
III. Volume Grating
Figure 2 (a) shows the schematic of the fabrication of a volume grating.
When a 150-µm long filament was scanned for 300 µm in the
plane perpendicular to optical axis at the speed of 1 µm/s, a
layer of refractive-index change with a thickness of 2 µm was
induced. We stacked 60 layers with a sample displacement of 3 µm.
Figures 2 (b) and (c) show an optical image of the fabricated grating
and its magnified image, respectively. The maximum diffraction efficiency
of 74.8 % was obtained with TE (electric field perpendicular to the
plane of incidence) polarization of the He-Ne laser beam when the grating
was fabricated using the 0.10 NA focusing lens and with a period of
3 µm. The typical profile and image of the diffraction pattern
are shown in Fig. 2 (d). The angle between the transmitted zero-th-order
beam and the first-order diffraction beams was 12.2 degrees in the case
of the period of 3 µm. Volume gratings act as mirrors to reflect
beams in silica glass.
|Fig 3(a) Optical image of the fabricated
Fresnel zone plate by embedding the two-dimensional array of voids.
(b) Intensity distribution in the primary focal point when a cw-He-Ne
laser beam at the wavelength of 632.8 nm transmitted through the
IV. Fresnel Lens
Tightly focusing femtosecond laser pulses with high NA lenses produce
submicron-damage inside a wide variety of transparent materials. The
damage appears as cavities or voids with diameters of only 200 nm to
1µm, surrounded by densified material. We are presenting in this
section the report of the fabrication of a lens by embedding voids inside
Our designed Fresnel lens has the primary focal length of 3 mm at the
wavelength of 632.8 nm. The radius of the first odd zone was designed
to be 43.5 µm. The size of the zone plate was 400 µm ¥
400 µm. In this condition, the radius of the outer zone is 200
µm, which corresponds to the included number of the odd zone plate
The laser pulses were tightly focused by an objective lens with a NA
of 0.55 to create the voids inside silica glass. We fabricated the Fresnel
zone plate by embedding voids at the depth of 300 µm beneath the
sample of silica glass. The sample was displaced dot by dot in the plane
perpendicular to the laser propagation axis by steps of 1 µm with
a computer-controlled motor stage. Figure 3 (a) shows an optical image
of the fabricated Fresnel zone plate by embedding the two-dimensional
array of voids. The voids were embedded only in the even zones.
We investigated the focusing properties of the fabricated Fresnel zone
plate. The beam incident on the lens is diffracted and converges in
on the primary focal spot on the optical axis. Figure 3 (b) shows an
intensity distribution in the primary focal point when the He-Ne laser
beam at the wavelength of 632.8 nm was incident on the zone plate. The
measured spot size was 7.0 µm and agreed with the theoretical
value of 6.1 µm. The measured diffraction efficiency was 2.0 %.
In conclusion, we present microfabrication of optical devices by femtosecond
laser pulses in silica glass. This technique enables us to make integrated
optical circuits in bulk silica glass. The technique has potential applications
in the integration of a variety of micro optical elements inside a small
volume in bulk materials.
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