Note: Descriptions are shown in the official language in which they were submitted.
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DIRECT WRITING OF OPTICAL DEVICES IN SILICA-BASED
GLASS USING FEMTOSECOND PULSE LASERS
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority to the United States Provisional Patent
Application Serial No. 60/146,274, filed July 29, 1999, entitled Direct
Writing of Optical
Devices In Silica-Based Glass Using Femtosecond Pulse Lasers of Nicholas F.
Borrelli
and Charlene Smith and the United States Provisional Patent Application Serial
No.
60/172,122, filed December 17, 1999, Femtosecond Laser Writing Of Glass,
Including
Borosilicate, Sulfide and Lead Glasses of Nicholas F. Borrelli, David L.
Morse,
Alexander Streltsov and Bruce Aitken.
BACKGROUND OF THE INVENTION
The present invention relates to methods for efficiently forming optical
devices
in glass. Specifically, the invention relates to direct-write methods of
forming light
guiding structures in glass compositions through light-induced refractive
index changes
using pulsed lasers having a pulse duration less than about 150 femtoseconds.
The
invention also relates to the optical devices made by the direct-write
methods.
Optical devices such as optical waveguides and Bragg diffraction gratings are
widely known in the telecommunications field. In an optical waveguide, a
higher
refractive index core surrounded by a lower refractive index cladding can
transmit a
large amount of optical information over long distances with little signal
attenuation.
The optical waveguide fiber is the prototype device of this type. The fiber is
produced
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by a method that, by virtue of its fabrication, gives the proper waveguiding
structure. A
Bragg grating is another type of optical device that can be used to isolate a
narrow
band of wavelengths from a broader signal. The most common materials used
commercially in telecommunications applications of light guiding devices are
doped
silica-based compositions.
It is known that pulsed laser sources can be used to effect both index changes
and to produce physical damage in glass. With regard to the former, the use of
pulsed
UV radiation sources for writing Bragg gratings is known. Recently, a "direct-
write"
laser method of forming optical waveguides within a glass volume that is
transparent to
the wavelength of a femtosecond laser has been disclosed. In this method, a
120 fs
pulsed 810-nm laser is focused within a polished piece of germania-doped
silica as the
glass is translated perpendicular to the incident beam through the focus.
Increases in
refractive index on the order of 10'2 were reported for a specific condition
in which the
focus was scanned ten times over the exposed area.
One potential problem with a direct write process of forming waveguides in
bulk
glass using short-pulse focused lasers is over-exposure. Irradiation with too
much
energy can lead to physical damage in the glass: Physical damage results in
undesired attenuation of optical signals transmitted through the glass.
Another problem in direct write methods of making optical structures relates
to
the trade-off between the dimensional stability of the writing device, e.g.,
the laser, and
the energy necessary to induce the desired refractive index change in the
substrate
material. Femtosecond lasers can be operated in three modes. Each of these has
advantages and disadvantages associated with it. The properties of each laser
configuration also make materials more or less desirable for a certain
application. The
table below presents some of the characteristics of these different modes.
Am lifier Oscillator Cavit Dum
Pulse Duration 40 -150 <40 <40
fsec
Energy Range 1 J -1 mJ 1 -10 nJ 10 - 50 nJ
Re Rate >kHz >MHz <1 MHz
Mode Qualit Poor Good Good
Stability Poor Good Not as stable as
oscillator s stem
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The table above illustrates the operational trade-offs as a consequence of how
the laser is configured. While it is relatively easy to obtain a 100 MHz
repetition rate
using the oscillator mode when the pulse energy is less than 10 nJ, at the uJ
level of
energy the repetition rate is traded off and drops to the several kHz range.
Mode
quality, which is qualitatively described by the temporal and spatial
integrity of the
beam, is relatively poor in the amplified system and improves when the
oscillator is
used. Similarly, the overall stability of the laser is found to be more robust
in the
oscillator case. These parameters tum out to be of practical importance in
direct-write
methods of making optical devices where one needs to control the dimensional
stability of the laser beam in order to write closely spaced optical
structures in the
substrate, such as diffraction grating lines.
To make the femtosecond laser direct-write method practical, large changes in
the refractive index (>10'3) of a material must be achieved in a reasonable
amount of
writing time. The formation of laser-induced physical damage should also be
avoided.
There continues to be a need for a practical direct write method of creating
silica-
based optical devices having a sufficiently increased refractive index at an
acceptably
high write rate. Such a method could be used to write continuous light-guiding
waveguide patterns connecting any two points within a continuous block of a
suitable
material, or make other optical devices, such as Bragg gratings.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide improved direct-write methods of
forming light guiding structures within a silica-based material substrate. In
particular, it
is desired to efficiently write three-dimensional light guiding structures in
glass, such a
waveguides and gratings, more quickly using lower pulse energy levels without
incurring physical damage of the glass.
It is a further object of the invention to write optical structures in silica-
based
materials using dimensionally stable operating modes of ultra-fast lasers.
In accordance with one aspect of the invention, it has been discovered that
soft
silica-based materials exhibit increased sensitivity to ultra-fast laser
writing of optical
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structures in the bulk. In particular, femtosecond laser-induced refractive
index
changes can be more easily produced in silica-based compositions having an
annealing point that is lower than that of the 5 mol.% germania (Ge02) - 95
mol.%
silica (Si02) system in that lower pulse energies and faster translation
speeds can
produce equivalent increases in refractive index as harder silica-based
materials.
In accordance with another aspect of the invention, a method is provided to
directly write light guiding structures in glass using short-pulse lasers with
substantially
no physical damage of the glass.
In accordance with another aspect of the invention, a method is provided to
write three dimensional optical structures in silica-based bulk glass.
Specifically, the
invention provides for translating the refractive index-increasing focus of an
ultra-fast
laser through a silica-based substrate in the x, y, and z-dimensions.
In accordance with still another aspect of the invention, a variety of optical
devices are disclosed which incorporate optical structures made by the methods
described herein.
These and other aspects of the invention will become apparent to those skilled
in the art in tight of this disclosure.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Fig. 1 is a schematic arrangement of equipment used in practicing the
invention.
Fig. 2(a) and Fig. 2(b) show the positioning of the incident laser beam
relative
to the scan direction in the top-write and axial-write orientations,
respectively.
Fig. 3(a) and Fig. 3(b) show the scanning beam profile and a photograph of
waveguides cross-sectional shape in the top-write orientations, respectively.
Fig. 3(c) and Fig. 3(d) show the scanning beam profile and a photograph of
waveguides cross-sectional shape in the axial-write orientations,
respectively.
Fig. 4(a) and Fig. 4(b) are perspective views of the top-write arrangement of
directly writing three dimensional optical devices in bulk glass.
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Fig. 5 is a schematic drawing of the equipment set-up for observing the far-
field
pattern.
Fig. 6 is a photograph of a far-field intensity pattern of a waveguide written
in a
silica-based material according to the invention.
5 Fig. 7 is a photograph of a far-field intensity pattern of a waveguide
written in
borate-doped silica according to the invention.
Figs. 8(a) - 8(b) are photographs of near-field intensity patterns of
waveguides
written in fused silica, germania-doped silica.
Fig. 8(c) is a trace of the intensity of the near field pattern and borate-
doped
silica.
Figs. 9(a) - 9(d) show various exemplary optical devices that can be made
using the invention.
Fig. 10 is a photograph of a Y-coupler written in silica using the invention.
DETAILED DESCRIPTION OF THE INVENTION
The direct-write method of forming light guiding structures in a bulk
substrate
according to the invention includes the steps of selecting a substrate made
from a
silica-based material in which the light guiding structure is to the written,
focusing a
pulsed laser beam at a position within the substrate effective to induce an
increase in
tiie refractive index of a portion of the irradiated material, and translating
the substrate
and focus with respect to one another to form a light guiding structure within
the
substrate along the scan path.
The method may be better understood by reference to a generalized
arrangement of an equipment set-up suitable for practicing the invention, as
shown in
Fig. 1. Laser 1 generates a pulsed laser beam 2 which is focused at a focus 3
positioned within a glass sample 4 by a lens 5. The sample is translated in
one or
more of the x-direction 6, y-direction 7, and z-direction 8 to effect
translation of the
sample with respect to the laser beam focus at a desired translation or scan
speed.
Such translation of the sample with respect to the focal point may be
accomplished by
a positioning or translation device (not shown), such as a computer controlled
XYZ
stage.
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Focusing of the laser beam significantly increases the peak intensity of the
beam compared to an unfocused beam. The high intensity of the focused beam
effects an increase in the refractive index of the glass along the path traced
by the
beam focus as it is translated through the sample. The resulting region of
increased
refractive index can guide light and therefore can function as an optical
waveguide.
A "top writing" method results from translating the sample in a scan direction
13
that is substantially perpendicular to the incident beam, as shown in Fig.
2(a). An
"axial writing" method results from translating the sample in a scan direction
13 that is
substantially parallel to the incident beam, as shown in Fig. 2(b). As the
skilled artisan
will readily appreciate, top-writing may also be accomplished by translating
the sample
in just the x-direction, just the y-direction, or both the x-direction and y-
direction
simultaneoulsy.
The focus profile and cross-sectional shape of top-written waveguides shown in
Fig. 3 (a) and Fig. 3(b) differ from those of axial-written waveguides, as
shown in Fig.
3(c) and Fig. 3(d). The beam profile in the vicinity of the focus relative to
the scan
direction 13 is shown for the top-write orientation in Fig. 3(a) and for the
axial-write
orientation in Fig. 3(c), respectively. When the. top-write focus is
translated through the
sample in the scan direction, a generally elliptoid cross-section of the
waveguide may
be produced, as indicated by Fig. 3(b). When the axial-write focus is
translated
through the sample in the scan direction, a generally circular cross-section
of the
waveguide often results, as indicated by Fig. 3(d). Accordingly, axially-
written
waveguides are generally preferred in order to produce waveguides having
substantially circular cross-sections. Top-writing may be desired in order to
write
continuous linear waveguides longer than the focal length of the focusing
lens.
The ability to write three-dimensional waveguides in a sample using the
present direct-write method is described further with reference to Figs. 4(a)
and 4(b).
The laser beam 2 can be focused by a lens 5 to a focus 3 positioned within
glass
sample 4. Translation of the sample in the x-, y-, and z-directions from a
first position
(x,,y,,z,) at depth D, to a second position (x2, y2, z2) at depth DZ causes an
increase in
the refractive index of the glass along the scan path 9 to form an optical
waveguide
extending in three dimensions between the first and second positions within
the
sample. If planar, i.e., two-dimensional, waveguides are desired, x, may be
the same
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as x2, y, may be the same as y2, or z, may be the same as z2. If linear
waveguides are
desired, x, and y, may be the same as x2 and y2, respectively, y, and z, may
be the
same as y2 and z2, respectively, or x, and z, may be the same as x2 and z2,
respectively.
The pulsed laser beam is characterized by several beam parameters. The
beam parameters include the wavelength, pulse duration or pulse width, pulse
energy,
and repetition rate. Preferably, the laser wavelength and sample are selected
to
minimize optical absorption of the beam energy by the sample. In the case of
both
doped and undoped silica-based glasses, the wavelength can fall within the
range of
about 400 nm to about 1100 nm, preferably from about 800 nm to about 830 nm.
Within this range of wavelengths, the optical absorption of the beam by a
silica-based
sample is virtually nonexistent. The glass materials intended to be used with
this
invention are substantially transparent to the wavelengths of interest.
The time duration of each pulse, a.k.a., the pulse width, is preferably less
than
about 150 fs. Lasers having pulse widths of this duration or shorter are
referred to as
femtosecond or ultra-fast lasers. More preferably, the pulse duration is less
than about
100 fs. Most preferably, the pulse width is about 40 fs to about 60 fs. Lasers
having
pulse widths as short as 18 fs have been used to practice the invention. The
energy
per pulse, or pulse energy, can be from about 1 nJ to about 10 pJ, preferably
within
the range of about 0.1 to about 10 ~.J. More preferably, the pulse energy
falls within
the range of about 1 to about 4 pJ. The repetition rate or pulse frequency
preferably
falls within the range extending from about 1 kHz to about 250 kHz for
amplified laser
systems, but can be as high as 80 MHz.
The laser can be any device capable of generating a pulsed laser beam
characterized by the desired beam parameters. The laser may be, for example, a
Ti:Sapphire amplifier system. One suitable laser is a Quantronix Odin
multipass
amplifier seeded with a mode-locked Tiaapphire oscillator.
A suitable focusing lens includes a microscope objective having a
magnification
power of about 5x to about 20x. The focusing lens preferably has a numerical
aperture (NA) of about 0.16 to about 0.25. An especially preferred focusing
lens is a
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10x, 0.16 NA aspheric lens. A diffraction limited spot size of the focused
laser beam
was achieved using this lens.
The translation device may be any device capable of translating the sample
with respect to the beam focus at the translation speeds of interest.
Preferably, the
translation speed lies in the range of about 5 p.m/s to about 500 p.m/s or
faster. For
example, a computer controlled XYZ positioning device, available from the
Newport
Co., can be used.
While the examples below refer to moving the glass sample with respect to a
fixed focus, the skilled artisan will readily appreciate that alternatively
the laser focus
could be moved relative to a fixed sample, or both the laser focus and sample
could be
moved simultaneously with respect to a fixed reference point to achieve the
desired
relative translation speed between the sample and focus.
While the drawings have depicted the glass samples suitable for use in the
present invention as having substantially planar surfaces oriented at right
angles to
one another, the skilled artisan will recognize that the invention is not
limited to such
regular solid geometries. Rather, the invention can be used to direct-write
optical
waveguides in virtually any regular- or irregular-shaped three-dimensional
sample. It
is preferred, however, that the sample be positioned relative to the incident
laser beam
such that the beam is substantially perpendicular to the surface of the sample
through
which the incident beam passes.
The composition of the substrates in which the light guiding structures may be
written by this inve~~tion are silica-based materials, including undoped fused
silica and
doped binary and ternary silica systems. Silica-based materials are preferred
in light
of their various desirable optical properties as well as their widespread use
in
telecommunication device applications.
By "silica-based materials" we mean glass compositions that include silica and
which are essentially free of alkali, alkaline earth, and transition metal
elements, as
well as other impurities which would cause absorption in the 1300 -1600 nm
range. If
present at all, such impurities will typically not be found in the silica-
based materials
used in this invention at levels higher than 10 ppb (parts per billion).
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In accordance with this disclosure, waveguides can be written more easily in
bulk substrates made from soft silica-based glass compositions using lower
pulse
energies and/or faster translation speeds than in hard silica-based materials
without
sacrificing the magnitude of the induced index change. Soft silica-based
compositions
appear to be more sensitive to direct writing of light guiding structures
using ultra-fast
(femtosecond) lasers than hard silica-based composition glasses.
For the purposes of this disclosure, "soft" silica-based materials are defined
as
doped or undoped silica-based materials having an annealing point less than
that of 5
mol.% Ge02 - 95 mol.% Si02, i.e., silica-based materials having an annealing
point
less than about 1380°K. The preferred silica-based glasses are undoped
and doped
binary or ternary silica-based materials having an annealing point less than
about
1380°K, more preferably less than about 1350°K, and most
preferably within the range
of about 900°K to about 1325°K. The annealing point is defined
as the temperature at
which the viscosity of the material is 10~3~8 poise.
Undoped soft silica-based materials include, for example, commercial grade
fused silica, such as Corning 7980 glass, which can have an annealing point in
the
range of about 1261 °K to about 1323°K. As for the doped
systems, the preferred
dopants which may be used to soften silica include oxides of the elements
boron,
phosphorous, aluminum, and germanium, such as borate (B203), phosphate (P205),
alumina (A/203), and germania (GeOz), respectively.
In binary boron-doped silica-based systems, the borate content may comprise
up to 20 wt.% or more borate. For example, the binary glass systems 9 wt.%
B203-91
wt.% SiOz and 20 wt.% B203-80 wt.% SiOz may be used to practice the invention.
The
annealing point of the 9 wt.% Bz03-91 wt.% SiOz composition is about
1073°K. The
annealing point of the 20 wt.% B203-80 wt.% SiOz composition is about
999°K.
In binary phosphorous-doped silica-based systems, the phosphate content may
also comprise up to 20 wt.% or more phosphate. For example, the binary glass
systems 10 wt.% P205-90 wt.% Si02 and 7 wt.% PZOS-93 wt.% SiOZ may be used to
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practice the invention. The annealing point of the 7 wt.% P205-93 wt.% Si02
composition is about 1231 °K.
In binary aluminum-doped silica-based systems, the alumina content may
5 comprise up to 20 wt.% or more alumina. For example, the binary glass
systems 10
wt.% AI203-90 wt.% SiOZ may be used to practice the invention.
In binary germanium-doped silica-based systems, the germania content may
comprise up to about 22 wt.% or more germania. For example, the binary glass
10 systems 20 wt.% GeOZ-80 wt.% Si02 and 22 wt.% Ge02-78 wt.% Si02 may be used
to
practice the invention. The annealing point of the 20 wt.% Ge02-80 wt.% Si02
composition is about 1323°K while that of the 22 wt.% Ge02-78 wt.% Si02
composition
is about 1311 °K.
"Hard" silica-based materials are defined as doped or undoped silica-based
materials having an annealing point higher than that of the 5 mol.% GeOz - 95
mol.%
Si02 system, i.e., higher than about 1380°K. Examples of hard silica-
based materials
include dry fused silica which has an annealing point of about 1425°K.
As is generally
known in the art, "dry" fused silica has virtually no residual hydroxyl
groups, while
commercial grade fused silica may have higher levels, for example, about 800
ppm
hydroxyl groups.
The skilled artisan will readily appreciate that many other silica-based
compositions could be used to practice the invention.
The silica-based materials used in this invention are preferably made by a
flame hydrolysis process. In such a process, silicon-containing gas molecules
are
reacted in a flame to form SiOz soot particles. These particles are deposited
on the
hot surface of a rotating body where they consolidate into a very viscous
fluid which is
later cooled to the glassy (solid) state. In the art, glass making procedures
of this type
are known as vapor phase hydrolysis/oxidation processes or simply as flame
hydrolysis processes.
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The induced refractive index changes reported below in connection with the
examples were determined by the beam spread method assuming a step index
profile.
A schematic of the experimental set-up for estimating the radiation-induced
change in
the refractive index in the waveguides made according to the invention by this
method
is shown in Fig. 5. After writing a waveguide 16 in sample 4, by using a
spatial filter
20, collimating lens 19, beam splitter 17, telescope 18, and lens 5, light
from a HeNe
laser 21 was coupled into the waveguide 16 and the numerical aperture (NA) of
the
cone of light that emerged was measured. Since the length of the waveguides
made
in the example below was typically 1 cm, unguided light from the HeNe
interfered with
the light coupled out the waveguide. This interference resulted in an
inten'erence
pattern of concentric rings in the far field as recorded by a digital camera
14 and
personal computer 15. A recorded image of the interference pattern is shown in
Fig. 6.
The radius at which the fringes died out, R,~~9e, was measured. The distance
from the exit of the waveguides to the viewing surface, L, was fixed at 75 cm.
The NA
of the waveguide was calculated from the relation
NA = R~,~9e / L
Assuming a step index profile, the induced refractive index change On was then
calculated based on the relation 0n = (NA)2 / 2n.
In order that the invention may be more readily understood, reference is made
to the following examples, which are intended to be illustrative of the
invention, but are
not intended to be limiting in scope.
Example 1
Pulses from a Ti:sapphire multi-pass amplifier which were 60-fs in duration
and
had pulse energies of approximately 1 ~J were focused with a 10x (0.16 NA)
microscope objective into fused-silica glass samples mounted on a computer
controlled high-precision 3-D translation stage. The fused-silica samples were
translated through the focal point of the beam at a rate of 30 p,m/s.
Waveguide
structures were created within the bulk material.
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Example 2
A 830 nm laser was used to deliver 40 fs pulses at a 1 kHz repetition rate.
The
energy per pulse was from about 1 p,J to about 5 ~.J. The beam was focused
into the
glass below the surface with a lens with a numerical aperture of 0.16 in air.
The
sample was moved under the beam at a rate of about 5 ~m/s to about 100 ~m/s.
The
experimental conditions were kept constant for exposure to samples of fused
silica and
for 14 wt.% Ge02 - 86 wt.% Si02. The beam was focused about 1 mm below the
surface of the glass. For samples irradiated at the same exposure conditions,
the
diameter of the laser-affected region of the germania-silica sample was about
twice
that of the fused silica sample. From this result, we concluded that the
germania-silica
material was more sensitive to refractive index changes induced by ultra-fast
laser
exposure than fused silica.
Example 3
Substrates of various glass compositions, i.e., SiOZ (Corning product 7980),
22
wt.% Ge02-78 wt.% SiOz and 9 wt.% 8203-91 wf.% Si02, were exposed to focused
laser radiation by the axial write method. The laser wavelength was 830 nm.
The
pulse duration was 40 fs. The energy per pulse was 1.0 uJ. The repetition rate
was 1
kHz. The scan speed was 20 wmls. After exposure, the induced refractive index
change at 633 nm was estimated from the far-field pattern of the waveguide
produced.
The induced refractive index change results are tabulated below in Table 1.
The
annealing point of each of these materials is also reported in Table 1.
Table 1: Induced refraction index change (Example 3)
Glass CompositionAnnealing Induced Refractive
(% based on weight)Point Index Change
( ~K )
Si02 (Coming 7980)1261 - 1323 0.0003
78%SiOz-22%GeOz 1311 0.0009
91 %Si02-9%8203 1073 0.0030
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Example 4
A sample of 9 wt.% Bz03-91 wt.% Si02 glass was exposed to focused
laser radiation by the axial write method. The laser wavelength was 830 nm.
The pulse duration was 40 fs. The energy per pulse was 1.0 pJ. The repetition
rate was 1 kHz. The scan speed was 20 p,m/s. A photomicrograph of the far
field pattern of this sample is shown at Fig. 7. The double lobed pattern is
indicative of the propagation of a higher order mode. Insofar as the silica
and
germania samples of Example 2 showed single lobe patterns, the effective
refractive index change of the borate sample must have been greater than that
of the other two samples to support the additional mode.
Example 5
Each of the glass compositions listed in Table 1 were exposed to focused laser
radiation. The laser wavelength was 830 nm. The pulse duration was 40
femtoseconds. The energy per pulse was 0.5 p,J. The scan speed was 10 pm/s.
After
exposure, the samples were photographed through a microscope at a
magnification of
400x. The resulting photomicrographs of the Si02, 22 wt.% Ge02 - 78 wt.% Si02,
and
9 wt.% B203 - 91 wt.% Si02 samples at Fig. 8(a) to 8(c), respectively, show
increasing
spot sizes for the softer glass compositions. These results indicate the
increased
sensitivity of the softer glass compositions to 40 fs pulsed laser irradiation
at 830 nm.
The foregoing results strongly suggest that softness of the exposed glass
compositions is a key parameter in determining the magnitude of the laser
induced
refractive index change.
Example 6
Optical waveguides were written in various bulk glasses using femtosecond
laser irradiation. A Ti-sapphire laser irradiating at 830 nm with a pulse
width within the
range of about 40 fs to about 50 operating at pulse energies of 0.5 - 10 uJ.
The pulse
repetition rate was 1 kHz. The beam was focused with a 0.15 NA lens into the
block of
glass that was translated at linear speeds of 5 - 100 p,m/s. Assuming a
diffraction
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limited Gaussian beam, the estimated spot size of the focal point of the beam
was 5
Vim. The glass was exposed to the beam by translating the block relative to
the focal
point in the axial direction, i.e., in the direction of the beam. The nominal
intensities
used for the exposure therefore ranged from 0.05 - 1 x 10'5 W/cmz. The induced
refractive index changes (10-3) are reported in Table 2.
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Table 2: Induced refractive index change (10-3) (Example 6)
Glass Pulse Scan
Speed
(~.m/s)
(% based on weight)Energy
(uJ)
100 50 20 10 5
2 0.4 - - - -
Si02 1 0.2 1.2 2.4 0.8 -
0.5 - 0.03 0.2 0.4 0.6
2 0.5 2.6
9%Ge02 - 91 %Si02 1 - - - 1 0.5
0.5
2 0.16 0.9 6 16 -
22%Ge02 - 78%SiOz 1 0.01 0.2-0.50.9 4-10 -
0.5 - - 0.5 1-2 5
2 1 4* 8
9%Bz03 -91%SiOz 1 1 3-4 10* 10*
0.5 0.04 0.1
2 _ _ _ _ _
Si02 (hydrogen loaded)1 - - 0.3 1.3 -
0.5 - - - - -
" double lobed pattern
5 Examale 7
Optical waveguides were written in fused silica using femtosecond laser
irradiation. The Ti-sapphire laser irradiated at 830 nm with a 150 fs pulse
width. The
pulse energy was 5, 10, and 20 ~J. The pulse repetition rate was 1 kHz. The
beam
10 was focused with a 0.1 NA lens. The glass substrate was translated at
linear speeds
of 15, 50, and 500 ~mls. The glass was exposed to the beam by translating the
substrate relative to the focus in the "top-write" orientation, i.e., in a
direction
perpendicular to the beam. The induced refractive index changes (10-3) for
Example 7
are reported in Table 3.
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Table 3: Induced refractive index change (10'3) (Example 7)
Glass Pulse EnergyScan Speed
(~mls)
500 50 15
20 0.1
Si02 10 0.9 3
5 0.4
Example 8
Femtosecond laser pulses were produced by a Quantronix Odin multipass
amplifier which was seeded with a mode-locked Tiaapphire oscillator. The
operating
wavelength was 830 nm. The system produced 60-fs pulses at a 1 kHz repetition
rate.
The laser beam was focused into a sample of fused silica using a 10x (0.16 NA)
single
aspheric-lens microscope objective. Photonic structures were written by
translating
the sample with respect to the focal region using computer controlled three-
dimensional stages which had a resolution of 200 nm. By using this objective
having
this relatively long working-distance, waveguides as long as 2 cm parallel to
the beam
were written. Using the NA measurement techniques described above, values for
the
induced refractive index change (10'3) of the silica were determined, as
reported in
Table 4. In all cases, the diameter of the waveguides was approximately 3 ~,m.
The
waveguide diameter appeared to have minimal dependence on the incident pulse
energy or the translation speed.
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Table 4: Induced refractive index change (10'3) in fused silica (Example 8)
Pulse Energy Scan
Speed
(pm/s)
(~,J) 400 200 100 50 20 10 5
4.0 - - 0.83 3.3*
2.0 - - 0.03 0.3 2.1* 2.5*
1.0 - - - - 0.065 1.1 0.97
0.5 - _ _ _ _ - 0.53
" double-lobed tar tieia intensity pattern that is characteristic of a second
mode
- too small to measure
Example 9
The experimental conditions of Example 8 were repeated, but the sample was
made of the softer glass composition 9 wt.% BZO3 - 91 wt.% Si02 rather than
fused
silica. The values for the induced refractive index change (10'') of the boron-
doped
silica material are reported in Table 5.
Table 5: Induced refractive index change (10'3) in boron-doped silica (Example
9)
Pulse Energy Scan
Speed
(pm/s)
(w1) 400 200 100 50 20 10 5
4.0 1.2 1.7 4.03*
2.0 1.1 1.4 2.5 4.03* 3.3* 4.8*
1.0 - 1.1 1.4 3.3* 4.83* 3.3*
0.5 - - 0.13 0.83 2.1
* double-lobed far-field intensity pattern that is characteristic of a second
mode
- too small to measure
In most cases, the same write conditions, including pulse energy and scan
speed produced a larger induced refractive index increase in the boron-doped
silica
glass than in the fused silica glass. Accordingly, the exposure required to
produce the
same degree of index change is significantly less for the boron-doped silica
material
then for the fused-silica material.
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The increased sensitivity of the boron-doped glass compared to the fused
silica
glass is illustrated also by comparing the exposure required to produce the
characteristic double-lobed far-field pattern as shown in Fig. 6. This pattern
appears to
correspond to the onset of a second mode.
It is interesting to note that the onset of the second mode for a simple step-
index waveguide is given by the equation 2~rNA/~. = 2.4. We measure that at
the
onset of the double-lobed pattern the NA is 0.08, and with the measuring
wavelength
of 633 nm this value would correspond to a waveguide radius of about 3 pm,
which is
approximately the size of the observed guide. In both the boron-doped silica
and the
pure fused silica glasses, the response of the material appears to saturate,
and
attempts to produce index changes larger than the saturation value by either
increasing the pulse energy or by reducing the translation speed resulted in
damaged
waveguides that did not efficiently guide light.
A wide variety of optical devices in bulk glass can be made using the
presently
described materials and methods. Example 10 describes the fabrication and
performance of a Y-coupler device.
Example 10
A Y-coupler was written in a bulk sample of pure fused silica at the
conditions
of Example 1. A photograph of the structure shows the guiding of light from an
argon
laser, as shown in Fig. 10. The vertical dimension of the photograph is
magnified with
respect to the horizontal dimension for clarity. The splitting angle was
measured as
approximately 0.5°. It was observed that approximately half of the
514.5 nm light was
coupled into each of the two branches of the coupler.
The present invention can also be used to make a wide variety of other optical
devices, such as the star coupler having central guide 22 surrounded by a
plurality of
peripheral guides 23, as shown in Fig. 9 (a). The invention can also be used
to make a
passive Mach-Zehnder coupler including a pair of Mach-Zehnder guides 26, as
shown
in Fig. 9(b). An active Mach-Zehnder coupler including Mach-Zehnder guides 26
and
a thermal or other type activator 24, as shown in Fig. 9(c), could also be
made using
this invention.
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The present invention can also be used to make Bragg or the type diffraction
gratings in bulk glass, as shown in Fig. 10. Waveguide 16 leads to grating
lines 25.
Line spacings of 0.5 ~,m are possible using this invention.
It will be understood that the above described preferred embodiments) of the
present invention are susceptible to various modifications, changes, and
adaptations,
and the same are intended to be comprehended within the meaning and range of
equivalents of the appended claims.
Further, although a number of equivalent components may have been
mentioned herein which could be used in place of the components illustrated
and
described with reference to the preferred embodiment(s), this is not meant to
be an
exhaustive treatment of all the possible equivalents, nor to limit the
invention defined
by the claims to any particular equivalent or combination thereof. A person
skilled in
the art would realize that there may be other equivalent components presently
known,
or to be developed, which could be used within the spirit and scope of the
invention
defined by the claims.
