Note: Descriptions are shown in the official language in which they were submitted.
CA 02461368 2012-02-03
Doc. No.: 102-11 CA Patent
Bragg Grating and Method of producing a Bragg Grating
Using an Ultrafast Laser
Field of the Invention
[002] This invention relates generally to the direct writing of gratings in
light transmissive or
light absorbing optical media such as optical fiber and waveguides using a
diffraction grating
and femtosecond pulsed light; and, one aspect of this invention relates to a
method of
suppression of cladding modes in optical fiber or waveguides with gratings
therein.
Background of the Invention
[003] The fabrication of many photonic devices has been achieved through
exposure of
transmissive and absorbing materials to intense laser radiation in order to
change the optical
properties of said materials. For example, UV-induced photosensitivity of
germanium doped
silica glasses has been exploited in order to create permanent refractive
index changes in the
photosensitive Ge-doped silica cores of single mode optical fibers and
waveguides as opposed
to the undoped cladding. By creating a spatial intensity modulation of the UV
exposure either
by using a two-beam interference technique as disclosed in US patent
#4,807,950 by Glenn et
al. or by using a phase mask as disclosed in US patent #5,367,588 by Hill et
al., Bragg grating
structures can be produced in the photosensitive core of the waveguide.
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[0041 Bragg gratings in optical fiber and waveguides have developed into an
important
technology for wavelength division multiplexing (WDM) systems and other
applications for
fiber optic systems such as optical sensing because of the highly desirable
optical
characteristics the Bragg structures exhibit as well as the relative ease with
which they can be
fabricated. A large variety of optical devices have been fabricated using
Bragg gratings in
waveguides including optical add/drop multiplexing filters (OADM), gain
flattening filters,
band splitters and dispersion compensators.
[005] As disclosed by Glenn et al., permanent periodic gratings are provided
or impressed
into the core of an optical fiber by exposing the core through the cladding to
the interference
fringe pattern generated by two coherent beams of ultraviolet laser light that
are directed
against the optical fiber symmetrically to a plane normal to the fiber axis.
The material in the
fiber core is exposed to the resultant interference fringe intensity pattern
created by the two
overlapping UV beams creating permanent periodic variations in the refractive
index along
the length of the UV photosensitive core of the waveguide. The resultant
refractive index
variations are oriented normal to the waveguide axis so as to form the Bragg
grating.
[0061 A more popular method of photo imprinting Bragg gratings is taught by
Hill et al. in
US. Pat. No. 5,367,588 where an interference fringe pattern is generated by
impinging a
single UV light beam onto a transmissive diffractive optic known as a phase
mask. The
waveguide to be processed is placed immediately behind the phase mask and is
exposed to
the generated interference fringe pattern leading to the formation of the
Bragg grating
structure. In these prior art examples, optical fibers or waveguides having a
Ge doped
photosensitive core are irradiated with UV light at a predetermined intensity
and for a
predetermined duration of time sufficient to obtain a substantially permanent
Bragg grating
structure within the core of said waveguide.
[0071 These prior art gratings provide a useful function, however they are
known to suffer
from some limitations in terms of the out-of-band loss that results from
coupling of the
fundamental core mode LPo1 into backward-propagating lossy cladding modes. For
example,
a single mode optical fiber with a UV-photosensitive core and non UV-
photosensitive
cladding, will develop a Bragg grating structure using the techniques
disclosed by Glenn et
al. and Hill et al. only in the core region of the fiber. The fiber Bragg
grating will reflect
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light in a narrow band centered on the Bragg resonance wavelength, ABragg,
determined by the
well-known Bragg's diffraction condition
MAR-99 = 2ne.A (1)
[008] where neff is the effective refractive index seen by the fundamental
LP01 core mode, A
is the period of the grating and m is the order number. Re-expressing equation
(1) in terms of
the propagation constant of the fundamental mode,aol = 27132ef,/AB,agg yields
the phase
matching condition
2,80, = 27an (2)
[009] Because the refractive index modulated grating is localized to the core
region only,
but the mode field of the LP01 extends into the unmodulated cladding region,
some coupling
of the energy into cladding modes occurs due to a non-zero weighted overlap
integral
between the guided LP01 mode and the cladding modes. For optical fiber
waveguides that
comprise a core, a cladding surrounding the core and then air or a protective
coating
surrounding the cladding, several propagation modes can be supported in the
cladding region.
These modes may be guided or lossy depending on whether the outer layer
surrounding the
cladding has a lower or a higher refractive index than the cladding. These
modes are
commonly referred to as LPG,, cladding modes where ,uvis the mode number. If
the phase
matching condition
[0010]
flo1(2~,v) + J3~, (2 ) = 2n/A (3)
[0011] is satisfied, light propagating in the LP01 mode may couple into
cladding mode LPGõ
where J3~, = 27mm,,/A,,,, is the propagation constant of cladding mode LP,,,
at wavelength Acv,
n,,, is the modal refractive index of the cladding and fl01= 27Znefj/A,uv is
the propagation
constant of the fundamental mode LP01 at wavelength A,,,,. Since n,aõ is
always less than neff
for a single mode optical waveguide, the wavelength A,,, at which the phase
matching
condition in equation (3) is satisfied will always be less than the Bragg
resonance wavelength
IIBragg=
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[0012] Usually there are a series of wavelengths that meet this condition,
corresponding to a
series of cladding modes. Power that is coupled into the cladding modes is not
entirely
guided hence it is lost through absorption or scattering through the fiber
coating.
[0013] The strength of coupling between the LPD1 mode and the various cladding
modes
LPG,,, caused by the grating can be measured by a coupling coefficient,
containing an overlap
integral performed over the cross section of the fiber as discussed by Erdogan
in "Fiber
Grating Spectra " J. Lightwave. Tech. 15 (8), p.1277-1294 (1997)
K01, uv = f JdxdyE01(x, y)Euv (x, y)Anf (x,),) (4)
4 A-*
[0014] where K01,,,,, is the coupling coefficient between the guided LPD1 mode
and a cladding
mode LP,,,,. Aco, co, E01(x,y), E,,dx,y) and dnf are the fiber cross section,
angular optical
frequency, guided mode field, cladding mode field and photosensitivity profile
respectively.
The integral in equation (3) is nulled, ie Ko1,uv = 0, if dnf is constant over
the area where the
guided LP01 mode field is confined. This is usually not the case in standard
photosensitive
core fibers since the cladding is far less photosensitive.
[0015] In a method disclosed by E. Delevaque et al, Conference on Fiber
Communication,
Technical Digest Series, Vol 8, No. 6, pp 343-346 (1995), the cladding is
rendered
photosensitive as well as the core, so that the refractive index grating is
recorded in both the
core and, to an extent, in the cladding. When a UV-induced index grating is
written into the
core and the intermediate cladding region, suppression of cladding modes
results.
[0016] There are several examples of prior art where different fiber designs
and fiber
chemistries are employed to render the cladding photosensitive to UV light, as
disclosed in
US patents #5,627,933 and #5,790,726 by Ito et al., US Patent #6,005,999 by
Singh et al., US
patent 6,009,222 by Dong et al., US Patent #6,221,555 by Murakami et al. and
US Patent
#6,351,588 by Bhatia et al. These prior art fiber designs provide a useful
function however
they suffer from some limitations. Often the complicated fiber chemistry
results in a fiber
that possesses many internal stresses, which makes the fiber more fragile.
Mode fields of the
guided LPOi are not identically matched to standard Ge-doped telecom fiber
(for example
SMF-28 from Corning Inc.) hence high splice losses result. The fiber is often
more
expensive to produce.
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[0017] These prior art gratings produced in these UV-photosensitive cladding
fibers often
suffer from some limitations in terms of the amount of induced index change
that is possible.
In order for high refractive index modulated Bragg grating structures to be
written in these
optical fibers, the optical fiber often needs to be photosensitized to UV
light by exposing
such an optical fiber to hydrogen or deuterium gas at elevated pressures and
temperatures as
taught by Atkins et al. in US. Pat. No. 5,287,427 or by hydrogen flame
brushing as taught be
Bilodeau et al. in US. Pat. No. 5,495,548. After exposure, it is preferable to
subject the UV
written structures to annealing at elevated temperatures in order to remove
any remaining
interstitial hydrogen or deuterium present in the waveguide core. As taught by
Erdogan et al.
in US patent #5,620,496, this annealing step is often implemented in order to
stabilize by
accelerated aging, the induced index change. These extra processing steps to
the optical fiber
or waveguide complicate the manufacturing of photonic devices and reduce
yield.
[0018] Recently processes that employ high-intensity laser pulses in the
femtosecond pulse
duration regime for creating permanent changes in the refractive indices of
glasses have been
explored by several groups of researchers. K.M. Davis et al. disclose a
technique for inducing
index change in bulk glasses with ultra-high peak power femtosecond infra-red
radiation in
Opt. Lett. 21, 1729 (1996). The physical process that appears to cause the
refractive index
change in the materials need not be due to the dopant dependant mechanisms
occurring with
UV-induced index change, namely color center formation. Instead the refractive
index
change is due to the creation of free electrons through non-linear absorption
and multi-
photon ionization of bound charges, followed by avalanche ionization and
localized dielectric
breakdown as these free electrons are accelerated by the intense but short
time duration laser
field. Also, this leads to a localized melting and restructuring of the
material and a concurrent
increase in the index of refraction. The creation of waveguides in bulk
glasses using this
technique is taught by Miura et al. in US Patent #5,978,538 while the
modification or
trimming of existing waveguide structures is taught by Dugan et al. in US
Patent #6,628,877.
[0019] In order to photo imprint retroreflective Bragg structures into the
core of optical
fibers or waveguides using high-intensity femtosecond time duration radiation,
it is
advantageous to generate an interference fringe pattern originating from a
single femtosecond
laser pulse either using a holographic technique or a diffractive optic.
Hosono et al. in US
patent #6,633,419 disclose an apparatus for producing a hologram using a two-
beam laser
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interference exposure process, comprising the steps of using a femtosecond
laser having a pulse
width of 10 to 900 femtoseconds and a peak output of 1 GW or more that is
capable of
generating a pulse beam at or close to the Fourier transform limit. The beam
from the laser is
divided into two beams using a beam splitter, controlled temporally through an
optical delay
circuit and spatially using plane and concave mirrors each having a slightly
rotatable reflection
surface to converge the beams on a surface of or within a substrate for
recording a hologram at
an energy density of 100 GW /cm2 or more with keeping each polarization plane
of the two
beams in parallel so as to match the converged spot of the two beams
temporally and spatially,
whereby a hologram is recorded irreversibly on the substrate formed of a
transparent material,
semiconductor material or metallic material. The volume hologram is optionally
layered so as to
provide a multiplex hologram recording that is permanent unless it is heated
to a temperature to
cause the structural change in the atomic arrangement of the substrate in
which the hologram is
inscribed.
[0020] Miller et al., in US patent 6,297,894 teach a method for utilizing a
diffractive optic to
generate an interference fringe pattern in order to induce refractive index
changes in materials
using femtosecond time duration laser radiation. An exemplary embodiment of
the invention of
Miller et al. comprises a femtosecond laser source for providing light to a
diffractive optical
element. Light propagating from the diffractive optical element is incident on
a curved mirror,
which acts to focus the light into a lens or another curved mirror and then
into a target.
[0021] Mihailov et al. in US Patent Publication No. 2004/0184731, from which
this application
claims priority, disclose a technique for fabrication of Bragg grating
structures in optical media
such as optical fibers and waveguides with an ultrafast (< 500 ps ) laser
source and a zero-order
nulled phase mask using a direct writing technique. The resultant grating
structures have high
induced-index modulations (> 1 x 10-3) which were achieved without any special
fiber
sensitization process such as those taught be Atkins et al. in US. Pat. No.
5,287,427. Since the
refractive index change need not be dependent on the dopant in the
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core or cladding of the waveguide, refractive index changes can be induced in
both regions of
the waveguide.
[0022] It is an object of this invention to overcome the aforementioned
limitations within the
prior art systems for fabrication of cladding mode suppressed Bragg gratings
in optical fiber
and waveguides by inducing refractive index change in optical fibers and
waveguides using
femtosecond time duration laser radiation. Additionally, it would be
beneficial to provide a
simple method of producing high quality fiber Bragg gratings (FBGs) that are
robust, do not
require specialty optical fiber and are not subject to photosensitization
techniques or
annealing.
[0023] It is a further object of this invention to provide a method of writing
a grating in the
cladding of a standard optical fiber that has not been photosensitized and
that is not
necessarily sensitive to actinic radiation.
Summary of the Invention
[0024] A method for inducing a spatially modulated refractive index pattern in
an at least
partially light transmissive or absorbing material, comprising the steps of:
providing the at least partially light transmissive or absorbing material;
disposing a mask to be used as an interferometer, adjacent the partially light
transmissive
material such that light incident upon the mask is transmitted directly into
said material; and,
providing electromagnetic radiation on a surface of the mask, the
electromagnetic
radiation having a predetermined wavelength range and having a pulse duration
of less than
or equal to 500 picoseconds, wherein the mask is disposed to permit a portion
of the
electromagnetic radiation to interact with the mask and be incident on the at
least partially
light transmissive or absorbing material, the interaction of the
electromagnetic radiation with
the mask for producing a spatial intensity modulation pattern within the least
partially light
transmissive or absorbing material, the electromagnetic radiation incident on
the least
partially light transmissive or absorbing material being sufficiently intense
to cause a change
in an index of refraction of the at least partially light transmissive or
absorbing material,
wherein magnetic radiation interacting with the surface of the mask having a
sufficiently low
intensity to not significantly alter produced spatial intensity modulation
properties of the
mask.
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[0025] In accordance with another aspect of the invention, there is provided a
method of
inducing a spatially modulated refractive index pattern in a cladding of an
optical waveguide,
comprising the steps of
providing the an optical waveguide having a cladding and a core;
disposing a mask to be used as an interferometer, adjacent the cladding such
that light
incident upon the mask is transmitted directly into said cladding; and,
providing electromagnetic radiation on a surface of the mask, the
electromagnetic
radiation having a predetermined wavelength range and having a pulse duration
of less than
or equal to 500 picoseconds, wherein the mask is disposed to permit a portion
of the
electromagnetic radiation to interact with the mask and be incident on the
cladding, the
interaction of the electromagnetic radiation with the mask for producing a
spatial intensity
modulation pattern within the cladding, the electromagnetic radiation incident
on the
cladding being sufficiently intense to cause a change in an index of
refraction of the cladding,
the electromagnetic radiation interacting with the surface of the mask having
a sufficiently
low intensity to not significantly alter produced spatial intensity modulation
properties of the
mask.
[0026] In accordance with another aspect of the invention, there is provided
method for
inducing a spatially modulated refractive index pattern in at least a
partially transmissive
material, comprising the steps of:
providing the at least partially transmissive material;
disposing and orienting a mask adjacent to the at least partially transmissive
material at a
distance "d" such that group velocity walk-off results in pure 2-beam
interference within the
at least partially transmissive material when irradiated with a pulse of light
of less than or
equal to 100 picoseconds, wherein the distance "d" is chosen such that the
difference in times
of arrival of the order pairs due to group velocity walk-off results in the
pure 2-beam
interference pattern of subbeams of said pulse of light that have passed
through or reflected
off of the mask; and,
[0027] irradiating the mask with pulsed light having a duration of 100 ps or
less to generate
the index modulated pattern in the at least partially light transmissive
material.
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[0028] In all aspects of the invention, a mask or diffraction grating is
placed near the object
in which the modulation pattern is to be written and a direct writing
technique is used
wherein the mask or grating is irradiated with pulsed light having a pulse
duration of 100
picoseconds or less.
[0029] In accordance with another aspect of the invention, there is provide an
optical
waveguide grating comprising:
a core having a refractive index n1;
a cladding provided around an outer periphery of said core, said cladding
having a refractive
index n2 different than the refractive index n1 of said core, wherein the
cladding is not
substantially photosensitive to actinic radiation (UV); and,
a grating written in the cladding.
[0030] Within this specification, direct writing is defined as positioning of
the target material
with reference to the mask such that the mask is used an interferometer and
the interference
field produced by the mask impinges on the target. This distinguishes from
systems where
the diffractive element is used to produce multiple beams which are being
redirected and
further processed.
Although in a preferred embodiment the mask is a phase mask it can be a
diffractive optic
element, an amplitude mask or a reflection diffraction grating.
Brief Description of the Drawings
[0031] The invention is now described with reference to the drawings in which:
[0032] Fig. 1 is a prior art system for holographic imaging;
[0033] Fig. I a is a transmission spectrum of a Bragg grating device UV-
inscribed in standard
single mode telecom fiber that suffers from cladding mode coupling loss;
[0034] Fig. 1b is a prior art system of single mode optical fiber which
possesses a UV-
photosensitive core and inner cladding region;
[0035] Fig. 2 is a diagram of a system according to an embodiment of the
invention;
[0036] Fig 3a is a representative view of the time of arrival at a given
distance from the
phase mask of the fs duration pulse envelopes in each of the 0, +1 and +2
orders;
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[0037] Fig 3b is a representative view of interference fringes generated by
overlapping fs
pulse envelopes a given distance from the phase mask;
[0038] Fig. 4a is a representative top view of an optical fiber being written
to using the
apparatus of Fig. 2;
[0039] Fig. 4b is a representative side view of an optical fiber being written
to using the
apparatus of Fig. 2;
[0040] Fig 5a is an optical microscope image of the index modulation induced
in the core
and cladding of a standard single mode telecom fiber (SMF-28) using the
femtosecond laser
and the phase mask. The view is normal to the fs beam plane;
[0041] Fig 5b is an optical microscope image of the index modulation induced
in the core
and cladding of a standard single mode telecom fiber (SMF-28) using the
femtosecond laser
and the phase mask that has been rotated 90 with respect to the image in Fig.
5a;
[0042] Fig. 6 is a production system according to an embodiment of the
invention;
[0043] Fig. 6a is the transmission spectrum of the cladding mode suppressed
grating that was
induced with the femtosecond laser and the phase mask and corresponds to the
grating shown
in Fig. 5;
[0044] Fig. 6b is the reflection spectrum of the cladding mode suppressed
grating that was
induced with the femtosecond laser and the phase mask and corresponds to the
grating shown
in Fig. 5;
[0045] Fig. 6c is the cladding mode loss spectrum expressed as difference
between the
transmission spectrum Fig 6a and 1-R reflection spectrum Fig 6b;
[0046] Fig. 7 is a diagram of a system according to an embodiment of the
invention for
writing gratings through a jacket of a waveguide; and,
[0047] Fig. 8 is a diagram of a system according to an embodiment of the
invention featuring
a precision alignment stage.
Detailed Description of the Invention
[0048] Referring to Fig. 1, the prior art of Miller et al. is shown. A
femtosecond laser
pulse 8 is incident on a diffractive element 10. The laser pulse 8 is
diffracted and propagates
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to a curved mirror 12. The laser pulse is reflected and propagates into a lens
16. The laser
pulse then propagates from the lens to the target.
[0048] Unfortunately, the technique taught by Miller et al. is very sensitive
to the
alignment of the various optical components. This technique is an
interferometric technique,
which incorporates a diffractive optic to split a beam to produce a plurality
of beams. The
optical path lengths of the diffracted beams should be the same length within
a tolerance
corresponding to the physical presence of the electromagnetic radiation i.e.
approximately 30
m for 125 femtosecond laser pulses. Although not impossible, it is often
difficult to provide
such highly accurate path lengths, even with costly equipment. Consequently,
when such a
technique is used to produce a Bragg grating in an optical fiber even a small
misalignment in
any of the optical components almost certainly results in a poor grating.
Indeed, the "pulse-
to-pulse" stability of the optical system as taught by Miller et al. should be
very consistent or
the fringe contrast produced by the overlapping beams is "washed out". Indeed,
small
vibrations and air currents are sufficient to reduce the "pulse to pulse"
stability in an optical
system as taught by Miller et al. Additionally, the filtering characteristics
such as
apodization, chirp, and phase-shifts are difficult to image remotely.
Therefore, it would be
difficult to incorporate the teachings of Miller et al. in a system for
producing Bragg grating
structures in optical fiber.
[0049] As a person of skill in the art will be aware, the relatively short
duration of a
femtosecond laser pulse provides a laser pulse that is not monochromatic.
Additionally, a
person of skill in the art will be aware that a diffractive element, such as a
phase mask,
angularly deflects light according to the wavelength of the light. In US
6,297,894 issued to
Miller et al. beginning at column 2 line 49, states that, "Providing a hybrid
technique of
utilizing standard phase masking techniques in combination with using ultra
short high
power femto-second pulses is problematic, since close coupling a phase mask to
create an
interferometric pattern in a sample is not feasible; the mask will experience
optical damage
due to the high peak intensity of light required at the sample position.
Hence... the mask
must be located remotely...". Embodiments of the invention presented
hereinbelow
demonstrate that the reasoning of Miller et al. need not be the case. Further,
the prior art of
Miller et al. states that remotely locating the phase mask will protect it
from optical damage
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but will not produce interference fringes because of the dispersive spreading
of the high
spectral content ultra short laser pulse. Ultra short duration pulses of light
having long
wavelengths are known to have very high intensity and consequently it was
reasoned that this
type of pulse would damage a phase mask positioned in close proximity to a
target of such a
pulse.
[0050] Referring to Fig 1 a, the transmission spectrum of a fiber Bragg
grating written in
standard telecom fiber with a UV-source is presented. Note the coupling to
cladding modes
that is visible on the short wavelength of the reflection wavelength of the
grating.
[0051] Referring to Fig. lb, the prior art of Ito et al, Singh et al, and
Murakami et al is
shown. Optical fiber gratings were UV-induced using the techniques of Glenn et
al or Hill et
al. in the photosensitive fiber described in Figs ibA and lbB. Doping SiO2
with a
photosensitive dopant Ge02 for increasing the refractive index forms a core 1
of an optical
fiber 5. The addition of Ge02 dopant also increases the photosensitivity of
the core 1 with
respect to the outer cladding 3. In order to achieve a good suppression of the
cladding mode
coupling, Bhatia et al. teach in US Patent #6,351,588 that matching of the
photosensitivity of
the inner cladding and core must be closely matched, hence the inner cladding
layer 2 must
have the same photosensitivity as the core 1. The different prior art examples
teach how
different fiber chemistries, different dopants added to the core, inner and
outer cladding
regions can achieve the single mode refractive index profile and the
photosensitivity core-
cladding profile in order to suppress cladding mode coupling when a UV-induced
grating is
inscribed in the fiber.
[0052] Unfortunately, the special fiber designs taught by Ito et al, Singh et
al, Murakami et
al and Bhatia et al. are complex processes which often produce fibers which
are expensive
compared to standard single mode telecom fiber, and may suffer from modal
field
differences or internal stresses that make them difficult to splice to
existing fiber networks
without incurring a high splice loss penalty. Gratings that are often
fabricated in this fiber
require high index changes therefore it is often necessary to photosensitize
the fiber by the
techniques taught by Atkins et al. or Bilodeau et al. and subsequently anneal
the fiber grating
devices by the techniques taught by Erdogan et al. What would be preferable is
a technique
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for fabrication of high index modulated gratings directly in the core and
cladding regions of
standard telecom fiber such as SMF-28 without using special fiber designs or
photosensitization
techniques.
[0053] Mihailov et al. in US Patent Publication No. 2004/0184731 disclose a
technique for
fabrication of Bragg grating structures in fibers and waveguides with an
ultrafast (< 500 ps )
laser source and a zero-order nulled phase mask, thereby not requiring
photosensitive optical
fiber sensitive to actinic radiation for imprinting a grating therein. Actinic
radiation is
electromagnetic radiation that can produce photochemical reactions in
photosensitive treated or
doped optical fibers. The structures of Mihailov et al. have high induced-
index modulations (> 1
x 10-3) that were achieved without any special fiber sensitization. Mihailov
et al. also disclosed
in a paper, which was published in 2003 in the Optical Society of America
Technical Digest and
entitled "Fabrication of fiber Bragg gratings (FBG) in all-S102 and Ge-doped
core fibers with
800 nm picosecond radiation ", that induced index modulations could be
generated in both the
core and cladding region of Ge-doped and all-silica core fibers. Since the
refractive index
change is not dependent on the dopant in the core or cladding of the
waveguide, refractive index
changes can be induced in both regions of the waveguide. It is disclosed here
that by controlling
the exposure location and number of interfering beams produced by the phase
mask that
generate the interference pattern, pure 2-beam interference patterns were
created in the core and
extended into the cladding region immediately surrounding the core in standard
Ge-doped step-
index single mode telecom fiber without the necessity of hydrogen loading and
annealing.
[0054] Referring to Fig. 2, a simplified diagram of an apparatus for producing
fiber Bragg
gratings (FBGs) according to an embodiment of the invention is shown. The
apparatus
comprises: a laser source 20 for providing ultra-short duration pulses of
laser light; a cylindrical
lens 21 for focusing the ultra-short duration pulses at a target 24; a
diffractive optical element
22; and, an optical waveguide 23 having a target portion 24. In use, the
optical waveguide 23 is
biased against a fixture and the target portion 24 of the optical waveguide 23
is stripped of any
external jacket. The diffractive optical element 22 is positioned adjacent to
and aligned with the
target portion 24. When the laser source 20 is activated it emits an ultra-
short duration pulse of
laser light. The ultra-short duration pulse
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propagates from the laser source 20 and is directed to pass through the
cylindrical lens 21.
The ultra-short duration pulse then propagates from the cylindrical lens 21 to
the diffractive
element 22. The diffracted ultra-short duration pulse of laser light then
propagates into the
optical fiber wherein an interference fringe pattern is generated. The
intensity peaks of the
interference fringe pattern are spatially oriented along a length of the
optical fiber to cause
periodic index changes within the fiber at predetermined intervals, thus
forming a Bragg
grating therein. Although this embodiment of the invention relies upon a
cylindrical lens 21
for focusing the ultra-short duration pulses of light this need not be the
case. In an
alternative embodiment of the invention, means for providing optical power in
the form of a
focusing mirror is used to focus the ultra-short duration pulses of light
instead of the
cylindrical lens 21. Although this invention is particularly suited to writing
gratings, such as
Bragg gratings in the core or cladding of standard telecom optical fiber
without the
requirement of photosensitizing the waveguide or fiber, it is also suited to
photoresist
patterning in optical material and to direct patterning of glasses,
semiconductor materials,
non-linear crystalline materials such as LiNO3. Such surface and volume
holograms are
optionally used for optical encoding and data storage. Similarly taps can be
generated by
writing Bragg gratings at an angle in the form of a blazed grating as
described by Hill et al in
United States Patent 6,385,369. The invention as described in reference to
Fig. 2 improves
on remote imprinting of interference fringes generated by a phase mask by
using peak
powers that are below the damage threshold of the phase mask and; below the
threshold of
supercontinuum generation that has been correlated with nonlinear self-
focusing processes,
which lead to damage. The intensity levels incident on the fiber waveguide
that are required
to create photoinduced index changes in Ge-doped fibers are not as high as
previously
thought as there seems to be preferential multi-photon absorption in the Ge-
doped region as
opposed to the undoped cladding. It is unclear if the preferential
multiphotonic absorption in
the core is due to the presence of the Ge dopant or is more generally due to a
multiphotonic
self-focusing process that is accentuated as a result of the step index change
at the circular
core cladding interface that is a lensing effect of the core. Embodiments of
the invention
featuring a silica phase mask permit the placement of the silica phase mask in
close
proximity to the target waveguide without damaging the silica phase mask, when
laser
intensities below the damage threshold of the silica are used. Thus, in an
embodiment of the
14
CA 02461368 2004-03-19
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invention, the diffractive element is a silica phase mask. Indeed, the phase
mask is
optionally made of any material that is transmissive to the range of
wavelengths of the ultra
short duration pulse of laser light. Suitable materials include BK7 glass,
soda lime glass,
plastic, and UV transmissive glasses such as silica, calcium fluoride, and
magnesium
fluoride. Alternative embodiments of the invention feature a phase mask that
is not
transmissive to the incident ultra short duration pulse of laser light. For
example, the phase
mask is optionally a frequency-doubling medium like a crystal with an
antireflection coating
in the infra red so that visible light from an ultra short duration pulse is
generated in the
phase mask and the generated light diffracted but the IR light is reflected.
[0055] In the embodiment of the invention shown in Fig. 2, the alignment of
the various
components is very important to ensure that a good quality Bragg grating is
produced,
however the process of aligning the components is relatively simple in
comparison with the
prior art of Miller et al. Specifically, the fiber is positioned near a
predetermined position
relative to a focusing point of the laser source 20 and cylindrical lens 21
combination. The
diffractive element is positioned within the optical path of a laser pulse and
at a
predetermined distance from the optical fiber. The impact of the adversity of
the angular
dispersion of the diffracted beams as taught by Miller et al. is greatly
reduced by disposing
the silica glass phase grating mask adjacent and parallel to an optical
medium. Since the
beam intensity is also below the damage threshold of the phase mask, the mask
need not be
placed and aligned remotely as taught by Miller.
[0056] Although this invention relates broadly to writing index changes in
optical media
by using a diffractive element such as a phase mask, by direct writing
techniques, unlike
Miller's indirect remote techniques, it is been found to be particularly
useful for use in
writing gratings in the cladding of a standard unphotosensitized optical
fiber. This is very
useful in creating gratings with cladding mode suppression.
[0057] In order to obtain good cladding mode suppression it is important that
the index
change across the cross section of the fiber be uniform, and that the index
modulation be
continuous across the core-cladding interface. As taught by Dyer et al. in
"Analysis of
grating formation with excimer-laser irradiated phase masks", Optics
Communications, Vol.
CA 02461368 2004-03-19
Doe. No. 102-11 CA
115, pg. 327-334 (1995), when multiple beam interference occurs the resulting
field pattern
generated by the phase mask becomes complex. Mills et al. disclose in "Imaging
of free-
space interference pattern used to manufacture fiber Bragg gratings", Applied
Optics, Vol.
39, pg. 6128-6135 (2000) measurements of the field generated by a phase mask.
For a phase
mask with a poor zero order, that multiple beam interference of the 0 and 1
orders
generated by the phase mask produce sinusoidal fringe patterns at a distance
from the phase
mask known as the Talbot pitch. At the distance corresponding to half the
Talbot pitch
another set of sinusoidal fringe patterns is generated that are 180 degrees
out of phase with
the pattern at the Talbot pitch. In order to produce a continuous index
modulation across the
core and cladding region this phase shift in the generated grating planes is
undesirable and
should be avoided.
[0058] In accordance with this invention, a more practical approach to writing
a grating in
the core and the cladding is to utilize the self-aligning nature of the phase
mask to match the
path lengths. Referring now to Fig 3a, the femtosecond beam 30 is normally
incident on the
phase mask 33. The pulse envelope 31of the beam is quasi-Gaussian. The 1/e
spatial width
32 of the pulse envelope for example for a 120 fs pulse would be 36 m. When
the pulse 31
propagates through the phase mask, the pulse is split and diffracted into
various orders (0,
1, 2 etc.). For clarity only the 0 order (36), +1 order (35), and +2 order
(34) paths are
shown. For a given time, the zero order pulse 42 will propagate a distance D
(37) from the
phase mask, The +1 order pulse 41 will propagate a distance D (37) along the
first order
beam path 35 which is at an angle 38 with respect to zero order beam path 36.
Similarly the
+2 order pulse 40 will propagate a distance D (37) along the first order beam
path 34 which
is at an angle 39 with respect to zero order beam path 36. The angles 38 and
39 are
calculated using sing = i2/11 where 6 is the angle due to the ith order, A is
the wavelength of
the femtosecond beam and A is the mask pitch. For a given propagation distance
37 of the
zero order, the corresponding projections d of the pulses 41 and 42 on to the
zero order beam
36 will be less than D (37) where d = D cos6i. When D - d is larger than the
pulse width 32,
the orders have "walked-off 'each other and will no longer interfere.
[0059] Referring to Fig 3b, since the time of arrival of the pulses 40, 41,
and 42 will be
different at distance D (37) from the phase mask, this difference causes a
spatial separation
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CA 02461368 2004-03-19
Doc. No. 102-11 CA
of the order pairs due to group velocity walk-off resulting in a pure 2-beam
interference
pattern rather than multiple beam interference patterns observed near the
phase mask. For
the time it takes the zero order pulse 42 to propagate the distance 37, the
1 order pulses 41
will propagate and interfere in region 43. The pitch of the interference
pattern is half the
pitch of the phase mask. The region where the 1 orders does not overlap,
that is where
there is transverse walk-off of the beams, there are no interference pattern
generated. Since
the femtosecond source has very good spatial coherence any overlap of the 1
orders will
result in a 2-beam interference pattern. A similar effect will occur for the
2 orders 40
producing an interference fringe pattern 44. Since the index change due to
femtosecond
irradiation is a threshold multi-photon ionization process, only the order
pairs that create an
interference fringe patterns with intensities that are above threshold will
result in an induced
index modulation. If the phase mask has been designed to be zero-order nulled
as taught by
by Hill et al. in US patent #5,367,588, the majority of the energy is coupled
into the 1
orders (-70%).
[0060] Referring to Fig. 4a a top view diagram representative of a fiber
receiving a pulse
as described with reference to Fig. 2. Fig. 4a includes an optical fiber 43,
having a
waveguide cladding 44 and a waveguide core 45, which is placed a distance 47
from the
phase mask such that the difference in times of arrival of the order pairs due
to group
velocity walk-off results in a pure 2-beam interference pattern; a diffractive
element 42 in the
form of a phase mask; and a lens 41. As the 1 orders propagate through the
cross-section of
the fiber 43, they will generate the index modulated pattern 46 in both the
cladding 44 and
core 45 of the optical fiber. {in Fig 5 we show the experimental results that
we have obtained
with a 3.213 m mask.
[0061] Referring to Fig. 4b, a side view of the components presented in Fig.
4a is shown.
This diagram includes indications of rays 40 and 41 that are indicative of the
extent of an
optical pulse. The rays 40 and 41 are shown converging as they approach the
diffractive
element 43. The separation of the rays 40 and 41 within the diffractive
element 43 is shown
to be larger than their separation within the waveguide next to the waveguide
core 45. Thus,
it is apparent that the maximum intensity of the light pulse is proximate or
near the
waveguide core 45. It should be noted that the rays are nearly symmetric about
the axis 47.
17
CA 02461368 2004-03-19
Doc. No. 102-11 CA
Tight focusing of the optical pulse will also permit smaller focal spot sizes,
less than the
waveguide core, which will allow for fabrication for selective exposure of
specific regions of
the core and cladding. In the preferred embodiment of the invention, the
cylindrical lens 42
has a short focal length so that the beam focus is much smaller than the
waveguide cladding
44. Considering Gaussian beam optics, the half width of the focal line would
be w A-V 2f/i w0
where ? is the wavelength and f is the focal length of the cylindrical lens.
For example a
cylindrical lens with a 19 mm focal length would have a focal line width of -
4 m. The
corresponding free space Rayleigh range for the focused beam is ZR = n w2/a,.
The beam will
remain approximately collimated over twice this length. For the example of the
19 mm focal
length cylindrical lens the Rayleigh range is -15 4m which corresponds to a
beam waist -30
m in length which is greater than the waveguide core 45 but less than the
waveguide
cladding 44. By scanning the lens 42 vertically (48), the focal spot generated
by the rays 40
and 41 can be swept along the axis 47. In this fashion the induced refractive
index
modulation can be swept vertically across the cross-section of the fiber 46.
In an alternative
embodiment, the diffractive element 43 and the optical fiber 46 can be scanned
vertically
rather than the lens 42.
[0062] Optical microscope images of the gratings fabricated in standard non-
photosensitized single mode fiber (SMF-28) with a 125 femtosecond pulse
duration 800 nm
beam, through a 3.213 m pitch phase mask using the techniques disclosed here
are
presented in Fig 5. A distance 47 of 3 mm was used, however the 1 order walk-
off from the
zero order occurred at distances > 1.3 mm from the mask for a 125 fs pulse.
[0063] The image of Fig. 5a was taken normal to a plane defined by the
femtosecond
beam. Fig. 5b was taken with the grating device rotated 90 to Fig 5a. Since
the grating
structure is continuous across the core-cladding interface of the optical
fiber and penetrates
several microns into the cladding layer, good suppression of cladding modes
result.
[0064] Fig 6a presents the transmission spectrum of a grating written in
standard single
mode fiber (SMF-28) with a 125 femtosecond pulse duration 800 nm beam, through
a 3.213
m pitch phase mask. Fig 6 b presents the reflection spectrum of the device,
while the
transmission loss due to cladding modes is presented Fig 6c. The spectrum Fig
6c) is the
18
CA 02461368 2004-03-19
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difference of the short wavelength side of the measured transmission spectrum
Fig 7a) with
the transmission inferred from the reflection spectrum Fig 6b). The grating
device presented
in Fig 6a, 6b and 6c displays excellent optical performance and cladding mode
suppression.
At the Bragg resonance ABragg, the pitch of the grating structure in the fiber
A is defined by
mA.Bragg=2neffA where m is the order number and neffis the effective index of
the fiber
resulting in a third order retro-reflecting grating. The grating device has a -
30 dB
transmission at 2Bragg with cladding mode coupling induced loss is - 0.1 dB
(Fig. 7b). The
out-of-band insertion loss measured on the long wavelength side of the grating
was < 0.05
dB.
[0065] As a person of skill in the art will be aware, the invention need not
be limited to the
writing of cladding mode suppressed Bragg gratings within the waveguide core
and cladding
of an optical fiber. Any application that presently is facilitated by the use
of UV-
photosensitive cladding fiber can be envisaged without using any special fiber
type and the
ferntosecond laser-phase mask technique disclosed here. For example in some
cases it is
preferable to provide a tap by writing a grating within the cladding of the
optical waveguide
without providing a set of interference fringes in the core. In this instance
it was found to be
advantageous writing the grating at a distance where 3-beam interference
occurred by being
near to the phase mask, grating planes can be induced at "Talbot" planes that
can be on either
side of the core without being in the core. Here the 0, 1 orders would be
utilized. Using
precision rotation stages, the fiber could then be rotated and reexposed.
Alternatively the
fabrication of a tilted or blazed grating written in the core and cladding of
a fiber could be
used to produce doped-fiber amplifier gain equalizing filters. By generating
an asymmetry in
the photosensitive profile, gratings written only in the cladding region that
are not tilted
could be used to create cladding modes to produce doped-fiber amplifier gain
equalizing
filters. Furthermore, long period gratings can be written in standard non-
photosensitive
telecommunications fiber by the method of this invention.
[0066] A person of skill in the art will easily envision the modification of
embodiments of
the invention for use in a wide variety of applications. There are several
different
applications and waveguide architectures into which index modulated structures
in cores and
cladding could be inscribed to produce useful devices. Grating structures
could be easily
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CA 02461368 2004-03-19
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inscribed across the cross-section of tapered optical fibers or waveguides for
cladding mode
suppression. The technique disclosed here would greatly facilitate the
fabrication of these
devices, as photosensitization through hydrogen loading would not be required
and the
grating structure could be impressed across the entire cross section of the
taper irrespective
of photosensitive doping.
[0067) The instant invention can be used for the fabrication of grating
assisted fused
biconic tapered coupler devices wherein a grating assisted coupler device is
written across
the entire the fusion region of a fused biconic tapered coupler. The invention
obviates the
complex costly process of photosensitization through hydrogen loading during
photoinscription in photosensitive clad fiber that has been made into a fused
biconic tapered
coupler.
[00681 A person of skill in the art will easily envision the modification of
embodiments of
the invention for use in a wide variety of applications. For example, the
invention is equally
applicable to planar waveguide structures and buried waveguide structures.
[00691 Clearly, the use of a higher order phase mask permits more variation in
the physical
positioning of the target waveguide when it receives a diffracted pulse.
Referring to Fig. 6, a
production system for producing FBGs according to another embodiment of the
invention is
shown. This embodiment of the invention is specifically intended to take
advantage of the
reduced tolerances of a system according to the invention. In a first stage 61
of the system
60, a portion 62 of an optical fiber 63 is stripped, exposing the cladding.
The stripped
portion 62 is disposed in a package 67. The package includes a transparent
portion 67a. In a
second stage 64, the stripped portion 62 is brought into close proximity with
a diffractive
optical element. The diffractive optical element receives a femtosecond pulse
of laser light.
The femtosecond pulse is diffracted and propagates through the transparent
portion 67a and
forms an interference pattern within stripped portion 62 of the optical fiber
63. The intensity
peaks of the interference pattern are sufficiently intense to cause a
permanent change in the
index of refraction in the optical fiber 63. This production method is highly
advantageous
because it helps to protect the exposed optical fiber during processing.
Additionally, it
provides flexibility in that the first stage 61 is optionally performed at a
first location while
CA 02461368 2004-03-19
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the second stage 65 is performed at a second location. Optionally, an opaque
covering, such
as paint, is applied to the transparent portion 67a.
[00701 Referring to Fig. 7, an apparatus for writing a Bragg grating into an
optical fiber
according to an embodiment of the invention is shown. The apparatus comprises
a
femtosecond laser source 71, a lens 72, a diffraction element 73, and a target
waveguide 74.
The target waveguide 74 includes a jacket 75, a cladding 76 and a waveguide
core 77. The
femtosecond laser source 71 provides pulses of laser light having a
predetermined range of
wavelengths. The material of the jacket 75 is transparent to this
predetermined range of
wavelengths. In use, the femtosecond laser source 71 provides a pulse of laser
light. The
pulse of laser light propagates to the lens 72 that focuses the pulse of laser
light. The pulse
of laser light is incident a surface of the diffractive element 73. A portion
of the pulse laser
light propagates to the target waveguide 74 and is incident thereon. Since the
jacket 75 is
transparent to the range of wavelengths of the femtosecond laser source 71 the
portion of the
pulse of laser light penetrates the jacket 75 and forms an interference fringe
pattern in the
waveguide core 77. The intensity of the peaks of the interference fringe
pattern is sufficient
to cause a change in the index of refraction of the waveguide core 77. Thereby
forming a
Bragg grating. There are prior-art examples of fabrication of UV-induced fiber
gratings
where special polymer jackets are used that are transmissive to the UV-light.
Aspell et al in
US Patent 5,620,495 disclose a method of fabrication of UV-induced fiber Bragg
gratings
through a single UV-transmissive polymer coating while Starodubov in US patent
6,222,973
teaches a similar method of UV-induced grating fabrication but with optical
fiber being
protected by a multi-layer polymer coating. In this embodiment of the
invention no special
polymer coating is required and standard coatings with are transmissive to the
infrared
radiation are used.
[00711 Referring to Fig. 8, another embodiment of the invention is shown. This
embodiment comprises a femtosecond laser source 81, a lens 82, an phase mask
83, a target
waveguide 84, and an alignment stage 85 supporting precision alignment. The
target
waveguide 84 includes a waveguide core 86 and a waveguide cladding 87. In use,
the
femtosecond laser source 81 provides a pulse of laser light. The pulse of
laser light
propagates to the lens 82 that focuses the pulse of laser light. The focused
pulse of laser light
21
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is incident a surface of the phase mask 83. A portion of the pulse propagates
to the target
waveguide 84 and is incident thereon. The portion of the pulse forms an
interference fringe
pattern in the target waveguide 84. The intensity of the peaks of the
interference fringe
pattern is sufficient to cause a change in the index of refraction of the
target waveguide 84.
The alignment stage 85 is sufficiently precise to vary the location of the
interference fringe
pattern by setting the alignment stage 85 prior to producing pulses from the
femtosecond
laser source 81. Thus, in a first mode of operation a set of fringes
corresponding to an
interference fringe pattern is written into the waveguide core 86 and the
waveguide cladding
87 of the target waveguide 84. In a second mode of operation a set of fringes
is written to
the waveguide cladding 87 absent writing fringes to the waveguide core 86. In
a third mode
of operation, the alignment stage provides an angle between the phase mask 83
and the
waveguide core 86 thereby producing a grating that is blazed. In a fourth mode
of
operation, the focused pulse of laser light is incident a surface of the phase
mask 83. A
portion of the pulse propagates to the target waveguide 84 and is incident
thereon. The
portion of the pulse does not form an interference fringe pattern in the
target waveguide 84
but instead produces an intensity modulation based on the binary output from
the phase
mask.
[0072] The embodiment of the invention described with reference to Fig. 8 is
also highly
beneficial because it supports a "step and repeat" production process. The
step and repeat
production process is useful in the production of very long fiber gratings.
Such gratings are
commonly used in wavelength dispersion compensation systems used in DWDM
optical
networks. The step and repeat production process is also useful for producing
localized
grating structures along a length of the fiber, such as sampled gratings.
Other suitable
applications will be apparent to one of skill in the art. A person of skill in
the art will be
aware that other embodiments of the invention such as, for example, the
embodiment
described with reference to Fig. 5 will also support a step and repeat
process.
[0073] Due to the rapid but short-lived multi-photon ionization process, very
high index
variations are possible without prior sensitization of the fibers or
waveguides with, for
example, hydrogen or deuterium. In addition, the highly localized index
changes produced by
the interference fringes from the phase mask are similar to single shot UV-
induced damage
22
CA 02461368 2011-06-21
gratings in that the refractive index change is permanent up to the glass
transition
temperature of the fiber. Thus, in a process according to the invention, high
spectral quality
gratings with very high index modulations are easily produced. Beneficially,
these gratings
are also robust at high temperatures. These advantages make such a process
particularly well
suited to high volume manufacturing as it reduces the need for additional
processing steps.
Additionally, since the index changes produced are relatively high a very high
quality Bragg
grating is produced.
[00751 A wide variety of alternative embodiments of the invention are easily
envisioned by a
person of skill in the art. For example, other embodiments of the invention
feature an
amplitude mask instead of a diffractive optical element. If the amplitude mask
is provided
sufficiently close to the target waveguide then a grating is produced absent
diffraction of the
electromagnetic radiation.
[00761 Instead of using a phase mask to inscribe pure 2-beam interference
patterns
selectively in the core and cladding of waveguide, a holographic technique of
inducing a
refractive index change in materials could be used to selectively induce index
changes in the
core and cladding of optical fibers and waveguides as disclosed herein.
[00771 Of course the grating can be a single contiguous uniform grating that
spans the core
and substantially most or all of the cladding or, alternatively the grating
can be comprised of
two gratings having same or similar characteristics or wherein the gratings
have different
characteristics, such as a different index modulation, position or pitch from
the cladding
region.
[00781 Within this specification, actinic radiation is to be understood to be
radiation that
causes a chemical photorefractive index change in an optical material as
opposed to a
physical process that appears to cause the refractive index change in the
materials not due to
the dopant dependant mechanisms occurring with UV-induced index change, namely
color
23
CA 02461368 2011-03-23
center formation. In accordance with this invention, the predominant
refractive index change
is due to the creation of free electrons through non-linear absorption and
multi-photon
ionization of bound charges; this may be followed by avalanche ionization and
localized
dielectric breakdown as these free electrons are accelerated by the intense
but short time
duration laser field. This leads to a localized melting and restructuring of
the material and a
concurrent increase in the index of refraction.
[0079] Numerous other embodiments of the invention will be apparent to one of
skill in the
art without departing from the spirit and scope of the invention. For example,
a person of
skill in the art will be aware that the technique according to the invention
is optionally
combined with the teachings of Miura et al. in US patent 5,978,538, and Dugan
et al. in US
Patent #6,628,877. Miura et al. teach the writing of waveguide in bulk glasses
while Dugan
et al. teach the modification or trimming of existing waveguide structures
with ultrafast
radiation. Specifically, the techniques taught by the invention, and the
techniques of Miura
et al. are optionally combined to provide a Bragg grating in a photosensitive
material. For
example, it is known to use sapphire fiber for high power beam delivery in
industrial and
medical applications. Typically the fibers are multimode with core diameters
no smaller than
150 microns. Since this fiber is typically a single crystal, it is extremely
difficult to draw it
down to a 5 micron diameter where it would act as a single mode fiber. Since
the sapphire
fiber has a very high glass transition temperature (2000 C) it has sometimes
been used for
high temperature sensing applications. Using a phase mask according to the
techniques
described with reference to the invention in combination with the method of
Miura et al, a
refractive index induced core is optionally written in the center and along
the length of the
sapphire fiber. The resulting device provides a high temperature stress-
temperature sensor
based on retro-reflecting Bragg gratings. There are similar fiber grating
sensors in standard
fiber that are used at lower temperatures in the oil and gas industry. This
new sensor is
optionally incorporated into smart skin structures that are exposed to high
temperatures, for
example the Space Shuttle.
24
