# Induced Bragg Gratings in Optical Fibers and Waveguides Using an Ultrafast Infrared Laser and a Phase Mask

Authors: Stephen J. Mihailov; Dan Grobnic; Christopher W. Smelser; Ping Lu; Robert B. Walker; Huimin Ding
Journal: Laser Chemistry
 (2008)

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## Abstract

Since its development in 2003, the technique of Bragg grating inscription in optical fibers and waveguides with ultrafast infrared radiation and a phase mask has proven to be as simple as the standard UV-laser grating writing techniques but far more versatile. The ultrafast IR laser-based process allows for the creation of grating structures in glassy and crystalline materials that are not typically UV photosensitive.  In this article, we will review the studies that have been performed at the Communications Research Centre Canada on the grating formation processes as well as applications of the ultrafast laser technique to fabricate gratings in various optical fibers and waveguides.


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## Body

## 1. Introduction

High-power  femtosecond (fs) lasers  systems
are being used extensively for laser-material processing of glassy materials in
order to fabricate microfluidic and photonic devices. The ultrahigh peak power infrared (IR)
radiation generated by many of these systems has been used to induce large
index changes in bulk glasses for the fabrication of imbedded waveguides and
surface structures [1, 2]. In glass
optical fiber, femtosecond laser systems have been used to induce large index
changes and fabricate long-period fiber grating structures in a step-and-repeat
fashion [3, 4].The process of induced index change
in bulk glasses resulting from femtosecond-IR laser exposure is thought to
result from a multiphoton absorption/ionization process resulting in material
compaction and/or defect formation depending on the intensity of the exposure
[1]. Above the ionization threshold
intensity Ith, multiphoton ionization (MPI) induced
dielectric breakdown likely  results in localized melting and material
compaction causing an index change that is permanent up to the glass transition
temperature tg, of the material. The value of
Ith is
material-dependent.BelowIth, another regime of
induced index change has been observed that can be erased by annealing with
temperatures below the material tg [5]. In this regime, multiphoton absorption likely
results in defect formation similar to that seen for ultraviolet—(UV-) induced
index changes in photosensitive germanium doped silica glasses.Laser induced index change is the key feature of
most fiber Bragg grating (FBG) manufacturing. Typically FBGs are made by side exposure of the UV-sensitive fiber core
to a spatially modulated UV laser beam. The modulated UV beam is typically created by interfering two or more UV
beams using either a bulk interferometer or a zero-order-nulled phase mask
[6]. For femtosecond systems,
interferometric setups for generating beam modulation are nontrivial to align
since path lengths of the interfering beams need to be matched to within the
spatial location of the femtosecond-pulse [2], which for a 120-femtosecond
pulse would be ~36μm. The
phase mask technique solves the alignment problem since the optical path
lengths of the generated beams are automatically matched for an incoming beam
that is at normal incidence to the mask.Femtosecond UV laser inscriptions of fiber
gratings using silica phase masks have been performed by a number of research
groups [7–10]. Typically, silica-based
fibers are placed in close proximity to the mask, and large index changes have
been generated in suitably UV photosensitive waveguides. A limitation of this technique however is
that lower pulse intensities need to be employed since high nonlinear
absorption and group velocity dispersion would otherwise occur within the
mask. Such nonlinear absorption reduces
the amount of light transmitted through the mask that is available to induce an
index change in the fiber and simultaneously degrades the phase mask with
time.Combining the ultrafast IR laser and the phase
mask approach, we successfully demonstrated the efficient fabrication of retroreflecting
FBGs in standard telecom and pure silica core single mode fibers [11, 12]. The technique has proven to be as simple as
the standard UV-laser writing technique but is far more versatile as grating
inscription is not limited to materials that are traditionally
UV-photosensitive such as Ge-doped silica. Using the femtosecond-IR laser and the phase mask, we have successfully
inscribed high index-modulation grating structures in
pure silica core fibers [12], borosilicate glass waveguides [13], reverse
proton exchange LiNb03 waveguides [14], sapphire optical fiber [15, 16], fluoride glass fibers [17], actively doped silica fibers [18], phosphate
glass fibers [19], and pure silica photonic crystal fibers and tapers
[20]. In this paper, we will summarize
our investigations into the processes of ultrafast IR laser-induced FBGs with a
phase mask along with many of the applications that have arisen from the use of
this technique, such as direct fabrication of gratings through protective fiber
coatings, fabrication of fiber laser cavities, and the fabrication of high-temperature-stable grating sensors.


## 2. Phase Mask Design

The phase masks used in these studies are
similar to standard all-silica phase masks used for UV laser FBG inscription
however the etch depth is optimized for operation in the near IR (800 nm) as
opposed to the UV. Standard UV masks
used for fabrication of C-band FBGs typically have mask pitchesΛm of ~1.07μm with good zero-order transmission
suppression. The majority of the
transmitted energy then couples into the ±1 orders generated by the
mask. Even when optimized for operation
at 800 nm, a mask pitch of 1.07μm has considerable coupling of the incident
energy into the zero-order due to the proximity of the irradiating wavelength
to Λm [11, 12]. Nevertheless, high-quality gratings can be
inscribed using standard mask pitches as will be described below.In addition to standard mask pitches,
phase masks were fabricated withΛm values that would produce higher-order
Bragg resonances λB. At λB, the grating pitch in the
fiber, ΛG, is defined by mλB=neffΛG where m is the order
number and neff is the effective index of the fiber. As the
process for ultrafast IR laser-induced index change is a nonlinear one, it was
thought to be likely that higher-order mask pitches would easily produce Bragg
resonances in the telecom band. Four
phase masks were then produced by e-beam lithography with different pitches
such that higher-order Bragg resonances occurred at 1550 nm within standard
telecom Corning SMF-28 fiber, namely 4.284, 3.213, 2.142, and 1.071μm. Gratings with high index modulations, Δn, were easily written with each phase
mask. Gratings written with the higher-order
pitch phase masks (i.e., >1.07μm) had the additional advantage of having
grating structures that were directly observable under a standard optical
microscope.


## 3. Experimental Conditions

In these studies, Fourier transform limited
125 femtosecond duration laser pulses from a regeneratively amplified Ti: sapphire
laser operating at 800 nm were used to illuminate optical fibers and waveguides by passing the irradiating
beam through a cylindrical focusing lens and a phase mask as shown in Figure1. Gaussian beam diameters incident on
the focusing lens were typically 6.4 mm. Depending on the study that was being performed, pulse energies were
varied between 0.2 and 2 mJ/pulse, repetition rates were varied from 1 to 1000 Hz,
and various cylindrical focusing lenses were used (f=12 to 160 mm). When tight focusing geometries were used, it
was found that vertical scanning of the irradiating beam across the waveguide
normal to its axis eased alignment tolerances of the beam with respect to the
waveguide core as well as allowed for the fabrication of many interesting
grating structures. Typically, the
incident beam was centered onto the fiber core and then scanned vertically over
the fiber cross-section using a piezoactuated translation stage with a ±10μm
travel and a 20-second period. The
fiber-phase mask distance d also proved to be a very important parameter
as it would allow for the writing of pure two-beam interference gratings with a
phase mask, even one possessing poor zero-order suppression. Grating spectra were typically measured
using an Er+ white light source and an optical spectrum analyzer or
a tunable C-L band laser.Figure 1
Schematic representation of lens-fiber-phase mask geometry for waveguide exposures.Annealing experiments of
grating structures were typically performed in a Lindberg tube furnace equipped
with an internal quartz tube of 2.54 cm diameter centered within the furnace
tube. Gratings were loosely placed in
the inner tube so that no external stresses were applied to the grating. The experiments were performed in ambient air,
and the heating rate was limited to 10°C/min. A swept tunable laser system was
used to continually measure the grating spectral responses.


## 4. Ultrafast Laser-induced Grating Processes

### 4.1. Phase Mask-Order Interference:  Order Walk Off

Ultrafast laser pulses
interact differently with phase masks as compared to nanoseconds pulses or
continuous wave CW sources. The spatial localization of the ultrafast pulse
envelope incident on the mask is small (~40μm for a 125 femtosecond pulse).
As the diffracted pulses produced by the mask propagate away from the mask
surface, the propagation distance of each pulse, when projected onto the
optical axis normal to the mask, appears different, as is shown in Figure 2(a). The diffracted pulse envelopes have different
arrival times at a given distance d normal to the mask. For sufficiently
large d, the diffracted-order pairs (0, ±1, ±2, etc.) no longer overlap
resulting in a diffracted-order walk-off effect (see Figure 2(b)). For a mask with Λm=3.213μm and a 125 femtosecond pulse, the spatial
separation of the ±1 order from the zero and ±2 orders would occur at
fiber-mask distances d>1.3 mm. As the induced index change using ultrafast IR radiation is highly nonlinear
and highly localized within the target material, small d values (i.e.,
near the mask) produce multiple-order complex interference field patterns
within the waveguide [21]. For large d values, the two-beam interference
fringe patterns, caused by the diffracted-order walk-off effect, can be
produced inside the waveguide with only the high-energy ±1 orders from the mask
(see Figure 2(b)) [22]. A clear advantage of the walk-off effect is that for
short pulses, the phase mask need not be zero-order suppressed to produce a two-beam
interference pattern. Although not
investigated here, it is likely that this remote writing process would also be
advantageous for grating inscription with femtosecond UV sources. By using a tight focusing arrangement and a
large displacement between the phase mask and fiber, the power densities and
hence nonlinear absorption in the phase mask could be significantly reduced
thus prolonging the life of the phase mask and allowing for higher power
densities of UV light to interact with the fiber.Schematic of diffractive order walk-off (a) pulse envelope projection normal to phase mask, (b) two-beam interference from each order pair.
(a)(b)


### 4.2. Type I-IR versus Type II-IR

With the application of the order walk-off
effect and vertical scanning of the beam across the waveguide axis, fabrication
of gratings could be performed in a controlled way allowing for the study of
grating formation processes below and above the ionization thresholdIth. For femtosecond pulses interacting near the
phase mask, the complex interference field resulting from multiple beam
overlap, had high-peak intensities that would create gratings through an ionization
process likely accentuated by self-focusing. This process produced gratings
with the same pitch as the mask that were stable up to the glass transition
temperature of the fiber. Sudrie et al.
defined this intensity regime for index change in bulk silica glasses as type
II [5]. In order to avoid confusion with
type II UV-induced gratings, we defined this FBG formation regime as being type
II-IR [23].  At greater distances from
the mask, for the same pulse energy, the generated two-beam interference
pattern had reduced peak intensities that did not traverse the threshold value
for type II formation. Grating
structures with high  Δn's were created after a more lengthy
exposure that had a grating pitch that was half that of the mask.To
study the stability of theΔn of the gratings written in the type I-IR
and II-IR regimes, an isochronal annealing study was performed where the FBG
temperature was increased in 100°C increments and held at each
temperature for one hour. For comparison, a type I UV-induced FBG was
inscribed in H2-loaded SMF-28 fiber (3000 psi, 85°C, 24 hours) using a frequency doubled argon-ion laser and
a phase mask which produced a Δn=5×10−4. The estimated initial Δn for the femtosecond IR
gratings was 1.5×10−3. In Figure 3, the short-term performance of
the UV and  femtosecond IR FBGs with respect to temperature is presented where the
initial Δn of each of the FBGs is normalized to 1. With increasing
temperature, the Δn of
the UV grating, denoted by the circles, decreases and completely disappears
when the grating temperature approaches 1000°C. The annealing properties of the type I-IR
FBG, denoted by black squares in Figure 3, are very similar to those of the
type I UV gratings. For the fs type
II-IR FBG, the reflectivity increased slightly up to 500°C and then remained
stable. These annealing results suggest
that the type I-IR gratings are due to a highly nonlinear defect formation
process resulting from multiphoton absorption while the type II-IR gratings are
“damage” gratings resulting from a multiphoton ionization process. The increase in the Δn of the type II-IR grating with temperature
is likely the result of annealing of less stable index change that occurs in
the less intense trough regions of the interference field. As the intensities in these regions are below Ith, the index change is the result of a type I-IR
process and is erased at elevated temperatures.Figure 3
Short-term annealing of type II-IR (white square), type I-IR (black square), type I-UV (black cirlce), grating held at each temperature for 1 hour.


### 4.3. Type I-IR Scaling Behavior

Compared to techniques that measure femtosecond
IR-induced index changes in bulk glasses, the technique of Bragg grating
inscription in existing waveguides for measurement of index change is very
sensitive thus allowing for the detection of index changes as small as1×10−6. A series of experiments were performed in
Ge-doped SMF-28 fiber using the setup shown in Figure 1 in order to
determine the scaling behavior of the Δn growth of a type I-IR grating as a
function of the interference field intensity, where peak field intensity was
directly related to the pulse energy [23]. The 6.4 mm diameter beam from the IR-laser was focused with a f=30 mm focal length through the  Λm=3.21μm phase mask onto the fiber. The fiber was placed at distance d=5 mm behind the mask to ensure two-beam interference. The pulse repetition rate of the laser was
kept at 100 Hz. Several FBGs were then
fabricated using the different pulse energies of 850, 900, 950, 1000, 1100, and
1200μJ. The
transmission loss at λB for each grating was
continuously monitored. The spectra were
then modeled using a commercial FBG software package.The growth ofΔn as a function of exposure time and pulse
energy is presented in Figure 4(a) as well as the scaling behavior as a
function of energy, which is presented in Figure 4(b). Linear regression of the logarithm of the
scaling behavior versus the logarithm of the pulse energy yields a slope of 5,
meaning that the absorption process involved in the index growth scales with
the fifth power of the peak intensity of the interference field I5. A fifth power dependency on intensity
suggests that the absorption process is a 5-photon one.(a) Growth of the index modulation as a function of time and pulse energy. (b) Scaling behavior of theΔn growth rate as function of energy.
(a)(b)The 800 nm wavelength or
1.55 eV photon energy of the ultrafast IR source is far removed from the
typical bandgap of Ge-doped silica, which is thought to be ~7.1 eV [24]. A 5-photon absorption process amounting to
7.75 eV has sufficient energy to bridge the 7.1 eV bandgap of the Ge-doped
silica. Similar annealing behavior
between the type I-IR and type I-UV gratings also suggests that a similar
defect and/or compaction related mechanism of induced index change may be
occurring in the type I-IR FBGs.


### 4.4. Hydrogen Loading

The possibility that a similar mechanism of
induced index change was occurring in the Ge-doped fibers for both type I-UV
and type I-IR FBGs suggested that fiber photosensitization processes used for
UV exposures such as high Ge-core content of the waveguide or hydrogen loading
[25] might also enhance type I-IR grating formation in Ge-doped fibers. As femtosecond-IR exposures of Ge-doped bulk
silica generate GeE’ and germanium-oxygen-deficient centers [1], FBG formation
threshold intensity reduction with H2-loading may result from a
similar mechanism as seen in UV-induced index changes. Increased UV-photosensitivity due to H2-loading
of Ge-doped silica is likely due to higher formation rates of GeE’ centers
through photoinduced reactions of H2 with Ge-SiO2 [26].
This may be the case in the femtosecond-IR regime as well. Another series of experiments were then
performed to determine if there was a variation in the threshold pulse energies
needed to induce a Δn in standard telecom Ge-doped fibers
when loaded with high-pressure hydrogen. For comparison, measurements of the threshold intensity for grating
inscription in unloaded Ge-doped fibers as well as loaded and loaded pure
silica core single mode fibers were also performed [27]. Fibers were H2-loaded at 85°C,
3000 psi for 24 hours.While no variation in the intensity threshold for grating
formation was observed in either the H2-loaded or unloaded all-silica
core single mode fiber, the threshold intensity in the case of the SMF-28 fiber was reduced by a factor of
3. For comparison, isochronal annealing studies were preformed on UV-induced
and ultrafast-IR-induced FBGs written in H2-loaded SMF-28 fiber. Temperatures were increased in 100°C increments and allowed to
stabilize for one hour. The annealing
curves of low- and high-reflectivity gratings induced with either a frequency
doubled UV Ar+ laser or the femtosecond-IR laser show similar
responses to temperature (see Figure 5). For the FBG’s reported in [27], no
broadband IR absorption due to the formation of Ge-OH defects had been
observed. In subsequent experiments however that were not previously reported,
the induction of high Δn=5×10−3 in H2-loaded SMF-28 fibers
did in fact result in the formation of the Ge-OH absorption band in the 1400 nm
range suggesting that the photosensitivity enhancement due to hydrogen for
ultrafast IR may have similar mechanisms as those that occur for UV exposures.Figure 5
Isochronal annealing curves of low- (circles,Δn~10−5) and high- (squares, Δn~10−3) reflectivity FBGs written in H2-loaded SMF-28 fiber with CW UV laser light (white shape) or femtosecond-IR laser light (black shape).


### 4.5. Modeling Nonlinear Index Change:  the Rouard Method

Typically, the modeling of UV-induced
gratings using coupled mode theory, the matrix method or Rouard’s method
assumes that theΔn is sinusoidal in nature [28]. As the process for type I-IR-induced index change
has been shown to be highly nonlinear, this assumption is no longer valid. In order to model the type I-IR FBG spectral
responses the nonsinusoidal nature of the induced Δn needs to be taken into account.Rouard’s method is an effective
technique for modeling an arbitrary index profile [29–31] in a waveguide. By using Rouard’s method to model ultrafast
IR FBG grating responses, the nonsinusoidal nature of the type I-IR Δn was taken into account by subdividing
individual fringe planes into a series of Fresnel reflections [32]. This technique proved to be quite powerful as
it automatically generated for example, higher-order Bragg resonances when a
nonsinusoidal Δn was considered. Such higher-order
Bragg resonances occur when the Δn deviates from a sinusoidal profile
[33]. The model also provided subtle
insight into variations of growth rates of Δn as a function of incident
intensity. For peak fringe intensities
in SMF-28 fiber that were above the
threshold for grating formation but well below those that would produce a type
II-IR grating, the spectral response was most accurately modeled using an I6 scaling of the Δn (see Figure 6) [32].Figure 6
Comparison of modeled (black) and experimental (gray) spectra for an irradiation intensity below the type I-IR to type II-IR transition (2.7×1013 W/cm2).  Inset is the Δn profile used to model the spectrum; fringe spacing is not to scale in figure.For fringe intensities that
were approaching the type II-IR threshold, saturation in the growth rate of theΔn is observed which results in a more
“top-hat” like modulation profile. The
spectral response was most accurately modeled using a saturation function for
the normalized Δn where
(1)Δnnorm=I6C+I6, and C is a constant equaling 0.05. Figure 7 presents the experimental result
along with modeled response [32]. Limiting behavior and intensity clamping are common in highly nonlinear
absorption processes and may be responsible for this observed saturation effect
[34].Figure 7
Comparison of experimental (black) and modeled (gray) spectra for an irradiation intensity approaching the type I-IR to type II-IR transition (4×1013 W/cm2). Inset is the index modulation profile used to model the spectrum; fringe spacing is not to scale in figure.


### 4.6. High-Order Spectral Response Characteristics

We have shown that the five-photon absorption process
associated with the type I-IR gratings results in aΔn profile that is nonsinusoidal. Unlike UV laser-induced gratings, which in
the absence of saturation of the index change have only one Bragg resonance,
the type I-IR gratings possess a number of higher-order Bragg resonances [35].If the index modulation along
the grating in the waveguide no longer follows a sinusoidal profile,Δn(z) can be expanded as a
Fourier series [33, 36]: (2)Δn(z)=Δn02+∑m=1∞Δnmcos(2mπzNΛG), where m is the order number, NΛG is the grating periodicity in the
waveguide that is an integer number N multiplied by the first-order grating
pitch. ΛG and Δnm are the Fourier
coefficients of order m. Δn0 is the index change averaged
over the length of the grating and is often associated with the wavelength
shift of the Bragg resonance λB.In [36], the evaluation
of the Fourier components associated with each grating mask was determined for
the order resonance that occurred in the 1550 nm wavelength range for both
unloaded and H2-loaded SMF-28 telecommunication fiber, for example the 1550 nm resonance of a grating made
with the 4.28μm phase mask using the two-beam interference
technique corresponds to the Δn4 Fourier component.  By monitoring the wavelength shift that
occurred while inscribing gratings of a specific reflectivity with each of the
phase masks pitched 1.071, 2.142, 3.213, and 4.284μm, respectively, the relative amplitudes for
the Fourier coefficients were determined (see Figure 8).Figure 8
Comparison of the Fourier components for the pristine and hydrogen-loadedSMF-28 fiber. The magnitude of the first-order component has been set to 1.Using pulse intensities
that were below the threshold for grating formation in unloaded fiber, the
Fourier components of gratings written in H2-loaded SMF-28 fiber are consistent with those
that would be achieved through a 5-photon distortion of the sinusoidal
modulation. For gratings in unloaded
fiber written with intensities mid-way between the type I-IR threshold and the
type II-IR threshold, the Fourier components are consistent with a modulation
profile that is squarer.For gratings written in the
type II-IR regime, the induced index change is no longer the result of a
multiphoton absorption process but a multiphoton ionization process resulting
in localized damage. Using aCorning
980 Flexcorefiber with a cutoff wavelength of 980 nm, and the 4.28μm pitched phase mask, the induced type II-IR
grating generates a large number of higher-order Bragg resonances in the
1000–1800 nm range, many with almost equal coupling; see Figure 9. This suggests that in the type II-IR regime the induced index modulation
has a much more square profile than does the 5-photon absorption related
profile.Figure 9
Transmission spectrum of a type II-IR grating written in 980 nm cutoff single mode fiber with the 4.28μm pitched phase mask.  Inset depicts the calculated index modulations of each of the diffracted orders assuming that they were the fundamental resonance.


## 4.1. Phase Mask-Order Interference:  Order Walk Off

Ultrafast laser pulses
interact differently with phase masks as compared to nanoseconds pulses or
continuous wave CW sources. The spatial localization of the ultrafast pulse
envelope incident on the mask is small (~40μm for a 125 femtosecond pulse).
As the diffracted pulses produced by the mask propagate away from the mask
surface, the propagation distance of each pulse, when projected onto the
optical axis normal to the mask, appears different, as is shown in Figure 2(a). The diffracted pulse envelopes have different
arrival times at a given distance d normal to the mask. For sufficiently
large d, the diffracted-order pairs (0, ±1, ±2, etc.) no longer overlap
resulting in a diffracted-order walk-off effect (see Figure 2(b)). For a mask with Λm=3.213μm and a 125 femtosecond pulse, the spatial
separation of the ±1 order from the zero and ±2 orders would occur at
fiber-mask distances d>1.3 mm. As the induced index change using ultrafast IR radiation is highly nonlinear
and highly localized within the target material, small d values (i.e.,
near the mask) produce multiple-order complex interference field patterns
within the waveguide [21]. For large d values, the two-beam interference
fringe patterns, caused by the diffracted-order walk-off effect, can be
produced inside the waveguide with only the high-energy ±1 orders from the mask
(see Figure 2(b)) [22]. A clear advantage of the walk-off effect is that for
short pulses, the phase mask need not be zero-order suppressed to produce a two-beam
interference pattern. Although not
investigated here, it is likely that this remote writing process would also be
advantageous for grating inscription with femtosecond UV sources. By using a tight focusing arrangement and a
large displacement between the phase mask and fiber, the power densities and
hence nonlinear absorption in the phase mask could be significantly reduced
thus prolonging the life of the phase mask and allowing for higher power
densities of UV light to interact with the fiber.Schematic of diffractive order walk-off (a) pulse envelope projection normal to phase mask, (b) two-beam interference from each order pair.
(a)(b)


## 4.2. Type I-IR versus Type II-IR

With the application of the order walk-off
effect and vertical scanning of the beam across the waveguide axis, fabrication
of gratings could be performed in a controlled way allowing for the study of
grating formation processes below and above the ionization thresholdIth. For femtosecond pulses interacting near the
phase mask, the complex interference field resulting from multiple beam
overlap, had high-peak intensities that would create gratings through an ionization
process likely accentuated by self-focusing. This process produced gratings
with the same pitch as the mask that were stable up to the glass transition
temperature of the fiber. Sudrie et al.
defined this intensity regime for index change in bulk silica glasses as type
II [5]. In order to avoid confusion with
type II UV-induced gratings, we defined this FBG formation regime as being type
II-IR [23].  At greater distances from
the mask, for the same pulse energy, the generated two-beam interference
pattern had reduced peak intensities that did not traverse the threshold value
for type II formation. Grating
structures with high  Δn's were created after a more lengthy
exposure that had a grating pitch that was half that of the mask.To
study the stability of theΔn of the gratings written in the type I-IR
and II-IR regimes, an isochronal annealing study was performed where the FBG
temperature was increased in 100°C increments and held at each
temperature for one hour. For comparison, a type I UV-induced FBG was
inscribed in H2-loaded SMF-28 fiber (3000 psi, 85°C, 24 hours) using a frequency doubled argon-ion laser and
a phase mask which produced a Δn=5×10−4. The estimated initial Δn for the femtosecond IR
gratings was 1.5×10−3. In Figure 3, the short-term performance of
the UV and  femtosecond IR FBGs with respect to temperature is presented where the
initial Δn of each of the FBGs is normalized to 1. With increasing
temperature, the Δn of
the UV grating, denoted by the circles, decreases and completely disappears
when the grating temperature approaches 1000°C. The annealing properties of the type I-IR
FBG, denoted by black squares in Figure 3, are very similar to those of the
type I UV gratings. For the fs type
II-IR FBG, the reflectivity increased slightly up to 500°C and then remained
stable. These annealing results suggest
that the type I-IR gratings are due to a highly nonlinear defect formation
process resulting from multiphoton absorption while the type II-IR gratings are
“damage” gratings resulting from a multiphoton ionization process. The increase in the Δn of the type II-IR grating with temperature
is likely the result of annealing of less stable index change that occurs in
the less intense trough regions of the interference field. As the intensities in these regions are below Ith, the index change is the result of a type I-IR
process and is erased at elevated temperatures.Figure 3
Short-term annealing of type II-IR (white square), type I-IR (black square), type I-UV (black cirlce), grating held at each temperature for 1 hour.


## 4.3. Type I-IR Scaling Behavior

Compared to techniques that measure femtosecond
IR-induced index changes in bulk glasses, the technique of Bragg grating
inscription in existing waveguides for measurement of index change is very
sensitive thus allowing for the detection of index changes as small as1×10−6. A series of experiments were performed in
Ge-doped SMF-28 fiber using the setup shown in Figure 1 in order to
determine the scaling behavior of the Δn growth of a type I-IR grating as a
function of the interference field intensity, where peak field intensity was
directly related to the pulse energy [23]. The 6.4 mm diameter beam from the IR-laser was focused with a f=30 mm focal length through the  Λm=3.21μm phase mask onto the fiber. The fiber was placed at distance d=5 mm behind the mask to ensure two-beam interference. The pulse repetition rate of the laser was
kept at 100 Hz. Several FBGs were then
fabricated using the different pulse energies of 850, 900, 950, 1000, 1100, and
1200μJ. The
transmission loss at λB for each grating was
continuously monitored. The spectra were
then modeled using a commercial FBG software package.The growth ofΔn as a function of exposure time and pulse
energy is presented in Figure 4(a) as well as the scaling behavior as a
function of energy, which is presented in Figure 4(b). Linear regression of the logarithm of the
scaling behavior versus the logarithm of the pulse energy yields a slope of 5,
meaning that the absorption process involved in the index growth scales with
the fifth power of the peak intensity of the interference field I5. A fifth power dependency on intensity
suggests that the absorption process is a 5-photon one.(a) Growth of the index modulation as a function of time and pulse energy. (b) Scaling behavior of theΔn growth rate as function of energy.
(a)(b)The 800 nm wavelength or
1.55 eV photon energy of the ultrafast IR source is far removed from the
typical bandgap of Ge-doped silica, which is thought to be ~7.1 eV [24]. A 5-photon absorption process amounting to
7.75 eV has sufficient energy to bridge the 7.1 eV bandgap of the Ge-doped
silica. Similar annealing behavior
between the type I-IR and type I-UV gratings also suggests that a similar
defect and/or compaction related mechanism of induced index change may be
occurring in the type I-IR FBGs.


## 4.4. Hydrogen Loading

The possibility that a similar mechanism of
induced index change was occurring in the Ge-doped fibers for both type I-UV
and type I-IR FBGs suggested that fiber photosensitization processes used for
UV exposures such as high Ge-core content of the waveguide or hydrogen loading
[25] might also enhance type I-IR grating formation in Ge-doped fibers. As femtosecond-IR exposures of Ge-doped bulk
silica generate GeE’ and germanium-oxygen-deficient centers [1], FBG formation
threshold intensity reduction with H2-loading may result from a
similar mechanism as seen in UV-induced index changes. Increased UV-photosensitivity due to H2-loading
of Ge-doped silica is likely due to higher formation rates of GeE’ centers
through photoinduced reactions of H2 with Ge-SiO2 [26].
This may be the case in the femtosecond-IR regime as well. Another series of experiments were then
performed to determine if there was a variation in the threshold pulse energies
needed to induce a Δn in standard telecom Ge-doped fibers
when loaded with high-pressure hydrogen. For comparison, measurements of the threshold intensity for grating
inscription in unloaded Ge-doped fibers as well as loaded and loaded pure
silica core single mode fibers were also performed [27]. Fibers were H2-loaded at 85°C,
3000 psi for 24 hours.While no variation in the intensity threshold for grating
formation was observed in either the H2-loaded or unloaded all-silica
core single mode fiber, the threshold intensity in the case of the SMF-28 fiber was reduced by a factor of
3. For comparison, isochronal annealing studies were preformed on UV-induced
and ultrafast-IR-induced FBGs written in H2-loaded SMF-28 fiber. Temperatures were increased in 100°C increments and allowed to
stabilize for one hour. The annealing
curves of low- and high-reflectivity gratings induced with either a frequency
doubled UV Ar+ laser or the femtosecond-IR laser show similar
responses to temperature (see Figure 5). For the FBG’s reported in [27], no
broadband IR absorption due to the formation of Ge-OH defects had been
observed. In subsequent experiments however that were not previously reported,
the induction of high Δn=5×10−3 in H2-loaded SMF-28 fibers
did in fact result in the formation of the Ge-OH absorption band in the 1400 nm
range suggesting that the photosensitivity enhancement due to hydrogen for
ultrafast IR may have similar mechanisms as those that occur for UV exposures.Figure 5
Isochronal annealing curves of low- (circles,Δn~10−5) and high- (squares, Δn~10−3) reflectivity FBGs written in H2-loaded SMF-28 fiber with CW UV laser light (white shape) or femtosecond-IR laser light (black shape).


## 4.5. Modeling Nonlinear Index Change:  the Rouard Method

Typically, the modeling of UV-induced
gratings using coupled mode theory, the matrix method or Rouard’s method
assumes that theΔn is sinusoidal in nature [28]. As the process for type I-IR-induced index change
has been shown to be highly nonlinear, this assumption is no longer valid. In order to model the type I-IR FBG spectral
responses the nonsinusoidal nature of the induced Δn needs to be taken into account.Rouard’s method is an effective
technique for modeling an arbitrary index profile [29–31] in a waveguide. By using Rouard’s method to model ultrafast
IR FBG grating responses, the nonsinusoidal nature of the type I-IR Δn was taken into account by subdividing
individual fringe planes into a series of Fresnel reflections [32]. This technique proved to be quite powerful as
it automatically generated for example, higher-order Bragg resonances when a
nonsinusoidal Δn was considered. Such higher-order
Bragg resonances occur when the Δn deviates from a sinusoidal profile
[33]. The model also provided subtle
insight into variations of growth rates of Δn as a function of incident
intensity. For peak fringe intensities
in SMF-28 fiber that were above the
threshold for grating formation but well below those that would produce a type
II-IR grating, the spectral response was most accurately modeled using an I6 scaling of the Δn (see Figure 6) [32].Figure 6
Comparison of modeled (black) and experimental (gray) spectra for an irradiation intensity below the type I-IR to type II-IR transition (2.7×1013 W/cm2).  Inset is the Δn profile used to model the spectrum; fringe spacing is not to scale in figure.For fringe intensities that
were approaching the type II-IR threshold, saturation in the growth rate of theΔn is observed which results in a more
“top-hat” like modulation profile. The
spectral response was most accurately modeled using a saturation function for
the normalized Δn where
(1)Δnnorm=I6C+I6, and C is a constant equaling 0.05. Figure 7 presents the experimental result
along with modeled response [32]. Limiting behavior and intensity clamping are common in highly nonlinear
absorption processes and may be responsible for this observed saturation effect
[34].Figure 7
Comparison of experimental (black) and modeled (gray) spectra for an irradiation intensity approaching the type I-IR to type II-IR transition (4×1013 W/cm2). Inset is the index modulation profile used to model the spectrum; fringe spacing is not to scale in figure.


## 4.6. High-Order Spectral Response Characteristics

We have shown that the five-photon absorption process
associated with the type I-IR gratings results in aΔn profile that is nonsinusoidal. Unlike UV laser-induced gratings, which in
the absence of saturation of the index change have only one Bragg resonance,
the type I-IR gratings possess a number of higher-order Bragg resonances [35].If the index modulation along
the grating in the waveguide no longer follows a sinusoidal profile,Δn(z) can be expanded as a
Fourier series [33, 36]: (2)Δn(z)=Δn02+∑m=1∞Δnmcos(2mπzNΛG), where m is the order number, NΛG is the grating periodicity in the
waveguide that is an integer number N multiplied by the first-order grating
pitch. ΛG and Δnm are the Fourier
coefficients of order m. Δn0 is the index change averaged
over the length of the grating and is often associated with the wavelength
shift of the Bragg resonance λB.In [36], the evaluation
of the Fourier components associated with each grating mask was determined for
the order resonance that occurred in the 1550 nm wavelength range for both
unloaded and H2-loaded SMF-28 telecommunication fiber, for example the 1550 nm resonance of a grating made
with the 4.28μm phase mask using the two-beam interference
technique corresponds to the Δn4 Fourier component.  By monitoring the wavelength shift that
occurred while inscribing gratings of a specific reflectivity with each of the
phase masks pitched 1.071, 2.142, 3.213, and 4.284μm, respectively, the relative amplitudes for
the Fourier coefficients were determined (see Figure 8).Figure 8
Comparison of the Fourier components for the pristine and hydrogen-loadedSMF-28 fiber. The magnitude of the first-order component has been set to 1.Using pulse intensities
that were below the threshold for grating formation in unloaded fiber, the
Fourier components of gratings written in H2-loaded SMF-28 fiber are consistent with those
that would be achieved through a 5-photon distortion of the sinusoidal
modulation. For gratings in unloaded
fiber written with intensities mid-way between the type I-IR threshold and the
type II-IR threshold, the Fourier components are consistent with a modulation
profile that is squarer.For gratings written in the
type II-IR regime, the induced index change is no longer the result of a
multiphoton absorption process but a multiphoton ionization process resulting
in localized damage. Using aCorning
980 Flexcorefiber with a cutoff wavelength of 980 nm, and the 4.28μm pitched phase mask, the induced type II-IR
grating generates a large number of higher-order Bragg resonances in the
1000–1800 nm range, many with almost equal coupling; see Figure 9. This suggests that in the type II-IR regime the induced index modulation
has a much more square profile than does the 5-photon absorption related
profile.Figure 9
Transmission spectrum of a type II-IR grating written in 980 nm cutoff single mode fiber with the 4.28μm pitched phase mask.  Inset depicts the calculated index modulations of each of the diffracted orders assuming that they were the fundamental resonance.


## 5. Applications of Ultrafast Induced Bragg Gratings in Optical Fiber and Aaveguides

### 5.1. Cladding Mode Suppression

An important application of FBG technology is for high
reflectivity narrowband reflectors in wavelength-division multiplexing (WDM)
systems of optical networks. Such FBGs
typically suffer from an out-of-band loss on the short wavelength side of the
Bragg resonance which is due to strong coupling from the forward propagating
core mode (LP01) into discrete backward-propagating cladding modes
[37]. Cladding mode coupling is caused
by a mismatch in the overlap between the LP01 mode and the fiber
core in which the FBG is inscribed. To
suppress cladding mode coupling, specialty fibers were developed which
possessed UV-sensitive cladding as well as core regions. When the FBG was inscribed uniformly across
both photosensitive regions of the fiber-cross section where the LP01 mode had nonzero power, coupling to cladding modes was eliminated [38]. The major drawback of this technique was that
these specialty fibers are costly to produce, and fiber photosensitivity also
needed to be enhanced by H2-loading.Initial FBG inscriptions
with the femtosecond-IR laser and the phase mask showed that structures could
be written with almost equal ease in either all-silica core or Ge-doped silica
core fibers without H2-loading [12]. By implementing the order-walk effect and beam scanning, high-quality
gratings were inscribed in both the core and cladding region of standard SMF-28 fiber in such a way that the LP01 mode was completely subtended by grating structure in the core and cladding
(see Figure 10). This resulted in an
almost complete suppression of the coupling into cladding modes as shown in Figure
11 [39]. In [39], a third-order grating
with ΛG=1.6μm was used in order to observe the structure
directly under an optical microscope. Similar FBGs written at the fundamental grating pitch of ΛG=0.535μm also demonstrated excellent cladding mode
suppression but the structure could not be directly observed under the optical
microscope as in Figure 10.Microscope images of photoinduced index modulation inSMF-28; (see Figure 10(a)). The fiber is imaged near the core and is viewed normal to the beam axis.
In Figure 10(b) the fiber is rotated 90° and is viewed along the beam axis.
(a)(b)Figure 11
Transmission of cladding mode suppressed grating inSMF-28 shown in Figure 10. Inset figure shows the cladding mode loss expressed as difference between the transmission spectrum and 1-R reflection spectrum.


### 5.2. Writing through the Fiber Jacket

Standard  UV-laser inscription processes of FBGs typically
require the removal of the protective polymer coating prior to exposure as
these coatings usually are highly absorbing in the UV. Coating removal and replacement are
time-consuming processes that threaten the mechanical integrity of the fiber
and are therefore undesirable from a manufacturing viewpoint.Recently, FBG inscription
through the standard polymer coating of a fiber was reported using femtosecond-IR radiation and the point-by-point method [40] where the tight focusing
geometry was able to produce a large intensity differential between the fiber
coating and the fiber core. Substantial
Δn values were induced in the core
without damaging the coating (Δn=3×10−4).Using the femtosecond-IR laser
and the phase mask technique, FBGs were successfully inscribed through the acrylate
coating of standardSMF-28 fiber,
when the fiber had been loaded with hydrogen [41]. A tight focusing lens was used with a focal
length f=12 mm, but with the fiber placed  at distance d=1 mm
behind the 1.071μm phase mask which resulted in a fundamental
period grating in the fiber of 0.5355μm when irradiated with 400μJ and 125 femtoseconds pulses. The resultant FBG had a Δn=3.5×10−4.Under identical exposure
conditions (100 Hz, 20 seconds cycle of ±10μm beam sweep and pulse energies of 220μJ), grating growth as a function of time was
compared for gratings written through the acrylate coating of the H2-loaded SMF-28 with growth rates obtained
when the loaded SMF-28 fiber had been
chemically stripped. In Figure 12(a),
significant grating growth is initially observed for FBGs written through the
jacket. However after ~5 minutes
exposure, no further increase in Δn was observed. For the stripped fiber, the grating growth
continues even after a 90-minute exposure (see Figure 12(b)). Observation of the exposed region of the
polymer jacket under the microscope revealed some damage to the jacket
surface. Several femtosecond IR
laser-based ablation studies of polymeric materials have shown that the
threshold for induced index change and polymer ablation decreases with the
number of incident pulses [42, 43]. It
is likely that the first several hundred pulses incident on the acrylate
polymer coating are creating “incubation sites” that eventually reduce the
transmission of the IR radiation through the jacket thus limiting the Δn growth. Using an optical fiber pull tester (Vytran
PTR-100), the breakage strength was compared to pristine SMF-28 fiber. For the 5
devices that were tested, the breakage strength was 4.5±0.5 GPa (690 ± 80 kpsi) as compared to 5.3±0.3 GPa (770 ± 40 kpsi) for the pristine fiber.(a) Increase inΔn as a function of exposure time for gratings written through the polymer coating with 250μJ pulses and a f=19 mm cylindrical lens (black squares) and gratings written on stripped SMF-28 fibre (gray triangles); (b) Long-term continued grating growth on stripped SMF-28 fiber.
(a)(b)In another set of
experiments, FBGs were inscribed through the acrylate coatings of high-Ge
content high NASM1500(4.2/125) fibers from FiberCore. Using exposure conditions similar to those
used with the SMF-28 fibers, but with
pulse energies up to 440μJ/pulse, high index modulations were induced
through the jacket of this high-NA fiber both with and without H2-loading
[44]. Figure 13 displays the growth
rate of the Δn as a function of exposure for H2-loaded
fiber. Index modulation values
approaching 10-2 are realized with the jacket removed. As with standard SMF fiber, saturation in the
induced index change through the jacket is observed. The inset image of Figure 13 displays the
transmission spectra of gratings written through the acrylate jacket with and
without hydrogen loading producing Δn of 1.4×10−3 and 7×10−4, respectively.Figure 13
Grating growth in H2-loaded FiberCore fiber through the jacket (white circles) and with jacket removed (black diamonds). Inset image is of transmission spectra of gratings written through thejacket with (top) and without (bottom) H2-loading.For downhole monitoring
applications in the oil and gas industry, FBG sensors are written into optical
fibers coated with polyimide, which is thermally stable up to 300°C. For standard UV grating inscription, the
polyimide coating needs to be removed, as it is strongly absorbing in the
UV. Polyimide is resistant to chemical
attack. Coating removal is then a
nontrivial exercise requiring immersion of the fiber in a hot sulfuric acid
bath. Using the femtosecond-IR laser
phase mask technique, we were able to directly inscribe gratings through the
polyimide jacket of commercially available high-NA fibers from OFS-Fitel (ClearLite CL-POLY-1310-21) after the fiber was H2-loaded.For unloadedOFS fiber,
the maximum FBG reflectivity achievable was 1%. A reflection spectrum of the device is shown in Figure 14(a). The Bragg resonance of the FBG, λB, is 1555.24 nm, which corresponds to an effective core mode
index neff=1.452. Similar FBGs written in SMF-28 result in a neff=1.447, implying that
there is a higher Ge-dopant concentration in the high-NA fiber. The
transmission spectrum of an FBG written in H2-loaded OFS fiber with pulse energies of 180μJ is
presented in Figure 14(b). The total
exposure time for the hydrogen loaded grating inscription was 5 minutes, after
which no further grating growth was observed. For both the loaded and unloaded OFS fibers, the induced-index
change for the gratings occurred in type I-IR regime. Using the Rouard method, the transmission
spectrum of the grating was modeled assuming a Gaussian index profile 6.4 mm in
length and a 5-photon process. The
modeled response is shown as the dotted line in Figure 14(b). From the model, the maximum index modulation
in the core corresponding to the experimental data is Δn=4.8×10−4. For the unloaded case, the reflectivity of
1% corresponds to a Δn=3×10−5.(a) Reflection spectrum of an FBG written through the polyimide jacket of theOFS fiber without H2-loading; (b) transmission spectrum of an FBG written through the polyimide jacket of H2-loaded OFS fibre.  Overlaid modeled spectrum shown with black dots.
(a)(b)Observation of the
exposed region of the polymer jacket under an optical microscope revealed some
damage to the jacket surface after the 5-minute exposure. Femtosecond-IR laser machining studies of
polyimide have shown that the ablation threshold for polyimide decreases with
the number of incident pulses [43]. It
is likely that the first several hundred pulses are inducing “incubation sites”
in the polyimide polymer coating that eventually reduces the transmission of
the IR radiation through the jacket thus limiting the Δn growth.To study the FBG temperature
stability, isochronal annealing studies were performed on the gratings at
elevated temperatures. In Figure15, the
reduction of Δn at various elevated temperatures is shown for a grating
with an initial Δn of 3.6×10−4. A portion of the Δn is
rapidly annealed out at 200°C but the remaining Δn is
relatively stable with increasing temperature up to 400°C. After 16 hours at 400°C, the grating had a Δn of
2×10−4 with no visible erasure. As the rated operating temperature of the
fiber coating is 300°C, it is likely that the grating reflectivity will remain
stable within the operating temperature of the coating. When the fiber temperature was increased in
100°C increments and annealed for 1 hour at each temperature, grating erasure
was observed with complete grating erasure occurring at 800°C. In a separate experiment, another device was
annealed at 360°C for 140 hours after being preannealed at 75°C for 36 hours in
order to out-gas any remaining hydrogen (see Figure 16). After 140 hours, a Δn≫2.2×10−4 remained.Figure 15
Isochronal annealing study of an FBG in the polyimide coated OFS fiber.Figure 16
Long-term annealing study at 360°C of an FBG in OFS fiber.  Inset is an expanded view of the initial annealing with Δn and temperature denoted by black and gray traces, respectively.


### 5.3. Gratings in Active Fibers for Fiber Lasers

Fiber lasers have become essential components for telecom
and sensing applications because of their high efficiency, gain, and good beam
quality. Writing Bragg gratings
reflectors directly into the core of active single mode optical fibers is a
convenient way to create compact, fiber compatible, and low-loss laser
resonator cavities. Traditional UV
laser inscription of fiber Bragg gratings (FBGs) however is not appropriate for
short cavity fiber lasers where high levels of Erbium–Ytterbium doping
(Er3+ and Yb3+) are required [45]. Germanium (Ge) codoping of the fiber core,
needed to enhance UV photosensitivity, causes ion clustering of Er3+ and Yb3+ core dopants resulting in low laser efficiency [46]. Glass substrates other than silica, for
example fluoride or phosphate glass, are more desirable host materials for
active dopants as higher concentrations of Er3+ and Yb3+ can be realized. Dissimilarity with
silica however makes coupling of these fibers to external resonator mirrors
difficult. Direct inscription of FBG
resonators in these fibers would be preferable.Recently, femtosecond-IR lasers were used to inscribe
grating reflectors directly into conventional, nonphotosensitive, silica-based Er:Yb-codoped
fiber by the point-by-point [47], or phase mask techniques [48] for the
manufacturing of fiber lasers. These
gratings successfully resulted in lasing of the fiber laser cavities. In our laboratories, short strong gratings
were induced in heavily doped Er-Yb phosphate glass fiber producing resonator
cavity reflectivities above 99.99% for a 6-mm-long grating corresponding to Δn>1.5×10−3 [15]. Femtosecond IR laser-induced gratings have also been fabricated in undoped [17] and doped fluoride
fibers [49].The impact, however, of high
active doping levels on femtosecond-IR laser inscription of FBGs was
unclear. In order to develop recipes for
laser cavity grating inscription, investigations to determine optimal exposure
conditions and active doping levels in silica-based fibers were performed [18]. High Δn FBGs were written in commercially available fibers with
different concentrations of Er3+ and/or Yb3+ in order to
study the magnitude of FBG reflectivity and spectral quality as a function of
active doping levels. For each fiber
that was studied, the saturation in induced Δn, the spectral quality and out-of-band
loss were evaluated and compared with gratings made in pure silica core single
mode fibers.Various active fibers with
different levels of doping were tested. Since Er3+ and Yb3+ dopant concentration values
were unavailable, it was assumed that they were proportional to the losses
given at telecommunication and pump wavelengths, respectively, as specified in
Table 1. The FBGs were written using a first-order 1.07μm
pitched phase mask positioned 1 mm away from the fibers. All exposures were
made using 1.2 mJ pulses of IR radiation with laser repetition rates set at 100 Hz.Table 1
Active fibers evaluated.
FiberDopantLoss (dB/m) atλ nmCore sizeλcutoffNA1532980(μm)(nm)INO EY302Er:Yb292274×611450.14INO EY305Er:Yb22477612000.18INO EY550Er:Yb11.25932.5×99000.17Coractive EY18-06Er:Yb584006na0.18INO Yb214Y06126.29000.12INO Er406Er37183.48600.2CorguideEr25106nanaBrown UniversityEr1248nanaGrating growth was very rapid,
producing Bragg resonances of –40 dB after only
a few seconds of exposure. All FBGs made in the
doped active fibers listed in Table1 share two characteristics: the initial
FBG growth rate exceeds that of pure silica core fibers (see Figure 17(a)) and
the growth of the Δn in all the FBGs made in active fibers
saturate well below the pure silica core fiber levels of 4×10−3. For FBGs in active fiber, the saturated Δn was estimated to be 2 to 3.5×10−3 (see Figure 17(b)).(a) Initial growth and saturated growth (b) ofΔn versus IR exposures for different active fibers and for pure silica core fiber (black dots). White triangles denote EY302, white squares denote EY305, and gray squares denote Er406.
(a)(b)The plot of the saturatedΔn versus the absorption at 1532 nm and
980 nm for the gratings was highly scattered (see Figure 18). Error
bars denote the average values of three gratings written in the fiber. A weak negative correlation between saturated
Δn’s and increasing Er and Yb content of
the active fibers was observed. The
large scatter of the saturated Δn’s may be due to the different glass
composition between the fibers that were studied. With all the doped fibers tested, spectral
quality and out-of-band insertion losses were similar to the cladding mode
suppressed gratings in standard fiber presented in Figure 11.Saturation levels of the active fibersΔn's as a function of (a) loss at 1550 nm and (b) loss at 980 nm.  Error bars denote the averaging of the values obtained from the three sequential exposures of the fiber.  Dotted lines denote the linear regression through the data points.
(a)(b)


### 5.4. Gratings in Photonic Crystal Fibers

Inscription of FBG
structures in photonic crystal fibers (PCFs) enhances the flexibility of the
PCF for telecommunication as well as microfluidic sensing related
applications. Direct inscription of
FBGs in pure silica PCF has been achieved using ArF excimer laser irradiation
as well as femtosecond UV irradiation producingΔn~2×10−4 after a 3.8-hour write time
and ~1×10−4 after a one hour exposure time
[9, 50].Using the femtosecond-IR
laser-phase mask approach, FBGs were written in pure silica PCF and fiber
tapers [20]. Scattering of the writing
beam by the PCF cladding holes can be reduced by selecting fiber geometries
with smaller number of cladding holes between the fiber surface and the
core. With such a PCF, for example the ESM-12-01 fiber from Blaze Photonics/Crystal Fibre, which has 3 rows of air
holes surrounding the silica core, a Δn value of ~4×10−4 was obtained after a 5-minute exposure with a Λm=3.12μm mask, tight focusing (f=30 mm), and high
pulse energy (1200μJ/pulse at 200 Hz). The spectral response of the grating in the ESM-12-01 fiber is shown in Figure 19. For other
PCF with larger number of air holes, scattering of the incident IR light did
not result in any grating fabrication in the core, even at pulse energies that
resulted in damage to the fiber. By
tapering such PCF in a H2/O2 flame, the fiber diameter
could be reduced causing the bandgap generated by the cladding holes as seen by
the transversal IR beam to shift to a wavelength that was not resonant with the
IR source. The resultant narrower hole
diameter and spacing in the tapered PCF allowed partial transmission of the IR
radiation into the core resulting in FBGs in the tapered regions of the PCF
with Δn~3×10−4 after an 80-second exposure. Alternatively, the holes of the PCF could be collapsed altogether. Gratings inscribed in the collapsed region
across the entire cross section of the taper diameter produced multimode
transmission/reflection responses.Figure 19
(a) Cross section ofESM-12-01 fiber. (b) Transmission (gray), reflection (black), and modeled (dotted) spectra of a grating written in ESM-12-01 fiber with 1200μJ pulse energy 200 Hz. Total exposure time 5 minutes.The tapering
of the PCF by the H2/O2 flame such that the air holes was
not collapsed but still produced a high level of birefringence in the tapered region [51]. LMA-15   PCF from Crystal Fibre A/S was tapered from its initial 230μm outer diameter to a taper waist diameter of
45μm and 10 mm length. The fiber taper still guided the fundamental
mode without any loss. By inscribing a
weak grating in the PCF core of the taper region, the birefringence of the PCF
taper was quantified. In Figure 20, the
grating was probed with polarized light from a tunable laser source and the
spectral characteristics of the gratings made in the tapered PCF show a strong
dependence on the signal polarization. When the polarization of the signal was rotated using an inline fiber
polarizer, it was possible to switch the main reflection spectrum from the
Bragg resonance wavelength at 1525.2 nm to a lower wavelength of 1524.1 nm. For a birefringent Bragg grating,
the wavelength shift between the two orthogonal polarizations ΔλB will be
related to the difference between the neff of the two modes Δneff by: ΔλB/λB=Δneff/neff which results in a Δneff or birefringence value
of 1.05×10−3.Figure 20
(a) Scanning electron microscope image of 45μm diameter LMA-15 taper, (b) reflection spectra of grating in 45μm LMA-15 fiber taper for two orthogonal polarizations.


### 5.5. Induction of High Birefringence

For some applications, a given amount of birefringence in an
optical device or system is desirable, among the most important being the
single polarization fiber lasers [52] and Bragg grating sensing [53].
Typically, highly birefringent fibers of complex design are used in these
specialized applications. The birefringence, however, could be introduced
locally by processing the optical substrate with high-energy laser pulses. Small amounts of birefringence (4×10−5) can be created with UV exposure of Ge-doped
fibers [54] by controlling the polarization of the UV-writing source so that it
was normal to the optical fiber axis.Similarly, type I-IR
FBGs induced with a phase mask uniformly about the core have been shown to
produce similar levels of birefringence when the polarization of the IR beam
was also polarized normal to the fiber axis (~10-5) [55].
In the type II-IR regime, FBGs written uniformly about the fiber core produced levels
of birefringence approaching 10-4, an order of magnitude higher than the birefringence
resulting from the type I-IR exposure. Although the type II-IR gratings potentially
produce enough birefringence for most applications, one draw back of these
structures is that there is typically increased insertion loss for gratings
with stronger Δn's. For applications such as short cavity single
polarization lasers, the increased loss associated with the birefringent laser
resonator mirror would reduce the laser performance.In our laboratories, we have shown that if a type II-IR grating structure is made
in the cladding close to but not overlapping the core, such that there is no
retroreflectance from the structure, a large amount of
birefringence is locally induced in the core region without generating excess
loss [56].  Birefringence values as high
as ~8×10−4 were obtained in the core of SMF-28 fiber when type II-IR grating
structures were inscribed in the cladding of the fiber on both sides of the
core in close proximity to the core/cladding interface. The splitting of the reflection spectrum of a
weak probe grating written in the core is shown in Figure 21 along with a
schematic of the exposure which produced the high birefringence result and a
photomicrograph of the grating induced in the cladding. The birefringence is likely due to induced
stress rather than direct asymmetric refractive index modification.The spectra of the probe grating before and after the cladding exposure are denoted by black and grey traces, respectively.  Inset photomicrograph shows a type II-IR exposure inSMF-28 fiber in the cladding proximate to the core.  Image taken along the exposure beam axis  (b) shows a schematic representation of the exposure conditions that produced the high birefringent spectral result.
(a)
Wavelength (nm)(b)
Wavelength (nm)When a strong type I-IR
grating (~95% reflectivity) is present in the core, the type II-IR cladding
exposures resulted not only in increased birefringence of the effective core
mode, as measured by the induced polarization dependent wavelength shift, but
also resulted in a large grating strength difference between orthogonal
polarization axes (ΔTr)
for the Bragg resonance. As can be seen
in Figure 22, the ordinary polarization corresponds to a 15 dB strong grating
while the extraordinary polarization results in a secondary resonance of 5 dB
located 540 pm away from the main resonance (~5×10−4 birefringence) resulting in ΔTr=10 dB. The difference in the Δn with polarization based on the
evaluation of the refractive index modulation of the two gratings resulted in a
value of ~2 × 10-4 assuming the approximation of a fundamental (first-order) grating. Since the spectrum of the original grating
measured before the cladding exposure did not display a large ΔTr at the Bragg resonance, it can be assumed that the initial Δn was similar for the two principle
states of polarization and the change in ΔTr was a result of the cladding exposure.Figure 22
Large birefringence of the refractive-index modulation induced by a symmetric cladding exposure of a strong type I-IR core grating. The two orthogonal polarization states are denoted by the gray and black traces, respectively.


### 5.6. High-Temperature Stable Sensor Gratings

A very important application of FBG technology is its usage as sensing elements. FBGs are capable of monitoring both temperature and strain through variations in the spectral
response of the grating, which can be directly correlated to strain and
temperature imposed on the grating structure [57]. The applicability of traditional UV
laser-induced FBGs is restricted to lower temperatures (<600°C) because of the instability of the UV generated grating structure at
higher temperatures. High-temperature
grating stability is especially important for sensing applications such as
monitoring furnaces, combustion situations, and so forth.Femtosecond IR laser-induced
gratings in standardSMF-28 and all
silica core single mode fiber have displayed high long-term thermal stability
at 1000°C when the FBGs were inscribed in the type II-IR regime [12]. Above 1000°C, the silica substrate itself is
susceptible to structural transformations that limit the functionality of the
fiber [58]. For temperatures >1000°C, fiber substrates other than silica
need to be considered. The most
successful optical fiber used for high-temperature sensor applications is the
single crystal sapphire fiber that has a glass transition temperature tg of ~2030°C. Unlike conventional
silica-based single mode optical fibers, sapphire fibers are made in the form
of rods absent a cladding layer. With
fiber diameters commercially available, beam propagation within the fiber is
highly multimode at the 1550 nm telecommunication wavelengths. Present sapphire fiber sensors are mostly
based on Fabry-Perot structures within the fiber producing a broadband
interferometric signal that varies with temperature [59].Femtosecond IR-laser induced gratings in sapphire fiber show no degradation of the grating
strength at high temperatures up to 1500°C [15, 16]. The sapphire FBGs (SFBG) have definite
advantages over other sapphire fiber sensors that rely on Fabry-Perot etalons
at the fiber tip. Unlike Fabry-Perot
sapphire sensors, SFBG sensors with their discrete resonant wavelength could
potentially be used as distributed optical sensor arrays up to 2000°C. The SFBGs that are inscribed in sapphire
fiber using femtosecond-IR lasers and a phase mask are naturally multimode
devices. The multimode reflection
spectrum observed with the retroreflective SFBGs is characterized by a large
bandwidth having a complicated structure that consists of a superposition of
different modes reflected by the grating. Although the SFBG sensor can be used in the multimode regime,
propagation of a single mode is preferable due to stability and the detection
advantages that a narrowband single mode reflection response offers. To produce a single mode response, the
multimode SFBGs were probed using tapered single mode fibers. Single and low-order mode
reflection/transmission responses are produced when the mode field of the
launched single mode is matched with the fundamental mode supported by the
sapphire fiber. For a 150μm diameter sapphire fiber, this would
correspond to an LP01 mode field ~150μm in diameter. When single mode optical fiber is tapered such that the normalized
frequency or V number of the core is <1, the fundamental LP01 mode is no longer core guided but is guided by the cladding-air interface
resulting in a mode field with the same diameter as the tapered fiber. Matching taper and sapphire fiber outer
diameters may excite only a single mode of the sapphire fiber. The evanescent tapers were made from fibers
with different cutoff wavelengths λcut.Although, theoretically less
efficient for mode matching, the expanded fundamental mode of a single mode
fiber can also be used to reduce the number of modes excited by the probing
signal. As in the case of tapers, the
single mode probe signal from a fiber collimator can be launched from a single
mode 1550 nm optical circulator into the SFBG, and the reflected signal can be
detected at the third port of the circulator instead of using the less
efficient fiber couplers. A
configuration made of an input single mode tapered fiber and multimode silica
fiber used for output coupling was also tested and has delivered a filtered
multimode transmission spectrum. The tapered coupling improved the spectral
resolution of the sapphire FBG as compared to its multimode responses. Such improvements facilitate the utilization
of the sapphire FBG as a high-temperature sensor.


#### 5.6.1. Thermally Stable FBGs inSIO2 Based Fibers

FBGs written with ultrafast IR radiation exhibit different
thermal stability characteristics depending upon the pulse duration and
energy. Above a low-intensity threshold
for grating formation (type I-IR) [23], Δn values with similar thermal stability to UV-induced
gratings can be generated as was shown in Figure 3. At higher power levels that are consistent
with white light generation in the fibers, thermally ultra-stable
defects/damage sites could be produced (type II-IR).A long-term study of type II-IR grating
stability was performed by inscribing largeΔn type II-IR FBGs in SMF-28 fiber which were then heated to 1000°C in steps of 100°C for 1 hour at each
temperature [58]. Once 1000°C was
reached, the grating was then kept at constant temperature for 150, hours and
the grating reflectivity and resonant wavelength were monitored (see Figure 23). There was no noticeable degradation
of the increased grating strength for the duration of the test and the grating
maintained Δn=1.7×10−3. The initial increase in the femtoseconds
grating Δn, shown in Figure 3 previously, is likely a result of the two kinds of index change being written
simultaneously. The peaks of the complex
interference pattern are sufficiently intense to ultimately ionize the glass in
the fiber producing an index change that is durable with temperature. In the valleys of the interference pattern,
the intensity is below the type II-IR threshold, however some type I-IR index
change is generated. As the device is
annealed, the permanent type II-IR index change remains while the annealable
type I-IR index change is erased resulting in a higher Δn. The temperature of the femtoseconds grating
was subsequently increased and kept at 1050°C for 100 hours (see Figure
23). The Δn of the femtoseconds
grating decreased slightly from 1.7×10−3 to 1.6×10−3 during 100 hours at 1050°C. A drift of the Bragg resonance to
longer wavelengths of 0.2 nm was detected at the end of experiment. Spectra taken initially at room temperature
and after 100 hours at 1050°C are shown in inset of Figure 23.Figure 23
Long-term annealing at 1000 and 1050°C of a femtosecond IR FBG.  Inset displays the initial room temperature transmission spectrum and the final spectrum at 1050°C for 100 hours.


#### 5.6.2. High-Temperature Multiparameter Sensor inSIO2

As presented in Section4.6 above, the type II-IR FBG
written in Corning 980 Flexcore fiber with a cutoff wavelength of 980 nm
and the 4.28μm pitched phase mask has many Fourier
components in its spectral response that produces Bragg resonances at various
wavelengths (see Figure 9). The large
number of spectral resonances generated by a single grating structure is
available to be used for multiparameter sensing. Aside from strain and temperature, other
parameters like bending, pressure, and so forth could be simultaneously
measured using only one grating. The availability of supplementary spectral
characteristics could also be used to improve the accuracy of the
strain-temperature discrimination.Wavelength shifts of the various diffracted
orders as a function of the temperature in the absence of strain are shown in Figure24(a). The temperature was changed from ~20°C to ~800°C. The room temperature 1550 nm Bragg resonance wavelength was monitored
in order to confirm that the thermal equilibrium was reached after ~30 minutes
at each temperature. After the thermal
equilibrium was reached, all the resonances of the Bragg grating were measured
at constant temperature. At room
temperature, the gratings were subjected to controlled stress in a specially
designed jig. The strain was applied
incrementally from 0 to 3000με. After each strain increment, the wavelength
shift was monitored in the 1550 nm range until the measurement confirmed that
the Bragg wavelength was stable, as measured by an optical spectrum analyzer. The Bragg wavelength of each diffraction
order was then evaluated by scanning a 10 nm spectral range around each high-order
resonance. Wavelength shifts of the
various diffracted orders as a function of strain are shown in Figure 24(b).(a) Bragg-order wavelength variation as a function of temperature for the grating shown in Figure9, (b) Bragg-order wavelength variation as a function of strain. Orders m=7,8,9,10,11, and 12 are denoted by white circles, black squares, white squares, triangles, diamonds, and black circles, respectively.
(a)(b)Under the application of strain and
temperature and of another arbitrary parameterP, the grating spectrum of order m will undergo a wavelength shift: (3)Δλm(ε,T,P)=KεmΔε+KTmΔT+KPmΔP,  where Δλm are the wavelength shifts
when the ambient parameters; stress (ε),
temperature (T), and a generic
general parameter (P) are changing by
the amount of Δε,ΔT, and ΔP. Considering temperature and strain, the
variations in Δλm differ from order to order
as shown in Figures 24(a) and 24(b). Assuming that the resonance shifts are linear and independent of each
other, the slopes or K values of the Δλm versus T or ε curves
can be used to setup a 2×2K matrix which will allow for the discrimination between temperature and strain
using a single Bragg grating [60]. By
employing more than two resonances, the problem becomes one that is over
defined, potentially resulting in reduced measurement error. Alternatively, other parameters in addition
to temperature and strain could also be monitored.


#### 5.6.3. High-Temperature Stable FBGs in  Sapphire Fiber: Multimode Response

FBGs were fabricated in crystalline sapphire fiber with
diameters of 60, 100, 120, 150, and 180μm by exposing the fibers to the 125 femtoseconds
pulsed-IR radiation. The tests were made
using the 4.28μm period phase mask that resulted in a 5th-order Bragg
resonance in the sapphire fiber at 1490 nm. Alternatively, a 1.747μm period phase mask was used that gives a second-order
Bragg resonance in sapphire fiber at 1523 nm. The first phase-mask, although producing lower reflectivity high-order
resonances, has the advantage of producing grating structures in sapphire fiber
that are easily observed under an optical microscope. The shorter pitched phase
mask was specially designed to generate a low-order resonance in the sapphire
fiber in the telecom wavelength range. In this way, the grating growth could be conveniently monitored with an
erbium white light source and a spectrum analyzer.The sapphire FBGs (SFBGs) were fabricated using
the setup shown in Figure1. The femtosecond-IR
pulses were focused with a 25-mm-focal length lens through the phase mask and
onto the sapphire fiber. Since the
sapphire fibers have a hexagonal cross-section, they were placed in a rotation
jig that would allow the fiber to be turned along its axis in order to optimize
the exposure. The femtosecond-IR beam
was focused inside the fiber and then scanned normal to the fiber using a
precision vertical stage in order to expose the whole cross section of the
sapphire fiber to the laser beam. As the
sapphire fiber is essentially a sapphire rod absent a cladding layer, the
spectral response from an inscribed grating is highly multimode. To measure the multimode response from the
SFBG, an S-C band tunable laser was passed through a 3-dB-multimode coupler
made in-house with 105/125μm core/clad silica fiber. The output port of the coupler was mounted on
5-axis precision stages and butt-coupled to the sapphire fiber with index
matching oil having a refractive index of 1.46. The reflected response was measured using a fast detector.SFBGs
were fabricated in 150μm diameter fibers with both the 4.28 and 1.747μm phase masks producing maximum reflectivities
of 8%. There is a limitation on the
maximum reflectivity of a multimode fiber grating due to the large number of
modes that the fiber will support. In
105μm core multimode silica fiber, using standard
writing conditions, the maximum reflectivity obtained using a first-order
phase mask was 30%, which corresponded to a Δn~3×10−3. This reflectivity was not reached in gratings
made in sapphire with similar diameters of 100μm or even in the case of
smaller diameters of 70 and 60μm. It is
possible that the lower reflectivity obtained in sapphire fibers as compared to
silica is the result of a lower saturation level of the Δn that can be induced in sapphire fiber
using the method of ultrafast radiation and the phase mask.The magnitude of the SFBG reflectivity also
depends on the position of the grating along the sapphire fiber length. The multimode
response of an SFBG written in a 120-cm-long sapphire fiber with 150μm core at ~20 cm from one end of the fiber is
presented in the Figure 25. In the figure, the multimode reflection spectrum
measured from the short end is presented together with the spectral response of
the same grating measured from the long side of the sapphire fiber. There is systematic 4–6 dB difference in the
strength of the reflected signal when it propagates through 1 meter of the
sapphire fiber. This loss is likely related
to the propagation of the mode as well as mode conversion within the
fiber. In addition to the reflectivity,
the bandwidth of the grating response is also reduced which implies that higher-order
modes are lost as the signal propagates along the length of the fiber.Figure 25
Multimode reflection spectra of the gratings written in the 150μm core diameter sapphire fiber measured from the short side (gray line) and from the long side (black line) of a 120 cm-long sapphire fiber.


#### 5.6.4. High-Temperature Stable FBGs in  Sapphire Fiber: Single Mode Response

Multimode
responses from FBG sensors are not as desirable as single mode responses since
the spectral bandwidth is broad and the dynamic range of the reflection signal
is low. In order to achieve a
quasi-single mode response from the SFBG, it was necessary to investigate other
approaches to probe the grating aside from the multimode coupler. The maximization of single mode coupling to
the multimode sapphire fiber can be achieved by the generation of a large mode
field diameter (MFD) single mode by for example, a fiber taper [16].For single mode fibers, the normalized
frequency orV number is given by  (4)V=2πrλ(nco2−ncl2), where r is the core radius, λ is
the wavelength, and ni is
the refractive index of the core or cladding. For single mode operation, V≤2.405. When a fiber is tapered, the ratio of cladding/core radii remains
constant however V decreases. The
transition of the fundamental LP01 mode from a core-cladding guided into a cladding-air guided mode occurs when
[61] (5)V<Vcc≅2lnS{1+0.26lnS}−1/2, where S is the
ratio of the cladding to core diameters.Once condition (5) is met, the fundamental mode
expands into the cladding resulting in a large area mode that can be better
coupled into the sapphire fiber. As the
expanded mode field is limited by the outer diameter of the taper, maximization
of the single mode coupling occurs when the difference between the taper and
the sapphire fiber diameters is minimized. Fiber tapers were fabricated using
Corning Puremode HI 780 single mode fiber with an outer diameter ϕ=125μm and a cutoff wavelength λcut<720 nm using a swept hydrogen flame while the fiber was pulled on
precision translation stages. When
pulled to taper diameters ≤70μm, the fundamental mode was completely guided
by the cladding air interface as shown schematically in Figure 26. Commercially available 50μm and 60μm diameter
fiber collimators were also used for testing.Figure 26
Schematic of the LP01 mode field expansion resulting from fiber tapering and coupling of the taper into the sapphire fiber.In order to obtain a
single mode response from the sapphire grating, a probe signal from the swept
tunable laser system was passed through a 1550-nm-range optical circulator and
into the tapered fiber/fiber collimator. The output of the collimator and the
tapers were mounted on 5-axis precision stages and butt-coupled to the sapphire
fiber with index matching oil having a refractive index of 1.46.The same SFBG that produced the multimode response
shown in Figure25 was also probed with a 60μm diameter taper (see Figure 27). The reflection spectrum measured with the 60μm
taper has a 3 dB bandwidth of 0.33 nm similar to that of an FBG in single mode
fiber, which is compared to the multimode spectrum that has a 3 dB
bandwidth of 6 nm. Compared to the
multimode response, there is also a 10 dB improvement in the signal-to-noise
ratio of the 60μm taper response. The loss at the
taper/sapphire splice is 4.5 dB per pass. This loss may be due to the mismatch
of excited modes generated by and collected by the tapers.Figure 27
Reflection spectra of SFBG with a 100-μm-silica core multimode fibre (dotted line) and a 60-μm-taper (black line) as the input couplers.To measure the response of the SFBG to
temperature, the portion of the sapphire fiber that contained the Bragg grating
was inserted into a high-temperature ceramic microfurnace (NTT model
CMH-7022). In order to position the
Bragg grating within the microfurnace, an He-Ne laser was
connected to the input port of the multimode coupler. The red light scattered
by the Bragg grating was then used to position the grating accurately within
the center of the furnace where the temperature distribution was at its
maximum. The microfurnace temperature
was monitored at its center using a platinum-rhodium
thermocouple with a measurement range up 1700°C. A gradient with a 20% decrease in the
temperature from the center to edge of the 20-mm-long microfurnace was
observed. The wavelength shift of the
SFBG as a function of temperature for both the single and multimode spectral
responses is presented in Figure 28. Both the single and multimode responses are similar. For comparison, the wavelength shift at 1550 nm of the high-temperature stable type II-IR grating written in SMF-28 is also shown. Up to 1000°C, the Bragg resonances of both
the sapphire and Ge-doped silica fibers vary linearly with temperature, 26 pm/°C
and 14 pm/°C, respectively.Figure 28
Variation of single mode (white squares) and the multimode SFBG wavelength response (black circles,) with temperature.  Wavelength variation of the Bragg resonance of a type II-IR FBG written inSMF-28 fiber is also shown (black triangle).


### 5.7. Gratings in Other Crystalline Materials

#### 5.7.1. Bragg Gratings in Lithium Niobate Waveguides

Along with sapphire, the femtosecond-IR laser grating
inscription technique has also been demonstrated in other crystalline
waveguides and fibers. Lithium niobate
(LiNbO3), because of its ferroelectric nature, is an attractive
material for fabrication of active integrated optical devices. The versatility of the waveguides made in
this material can be enhanced through the creation of efficient permanent
grating structures. Due to the electro-optic properties of the substrate, the
lithium niobate Bragg gratings have the potential to satisfy the rapid tuning
requirements needed by telecommunication components, such as tunable add/drop
filters (ROADMs), tunable dispersion compensators, switches, and so forth that
are used for dynamic control of the signal in the network nodes of agile
all-optical communication networks.In our laboratory, gratings were fabricated in
40 mm long reverse proton exchange (RPE) LiNbO3 channel waveguides
using the femtosecond-IR laser-phase mask approach [14]. Using the setup shown in Figure 1 with an f = 30 mm focusing lens and the 4.28μm pitched phase mask in the two-beam
interference regime and 600μJ pulse energies, grating responses up to -3.5 dB in transmission for exposures at 100 Hz for 1 minute were achieved. The peak field intensity corresponding to the
pulse energies used was 2.0 to 2.5×1013 W/cm2, which is of the same order of magnitude as previously
established IR ultrafast laser ablation thresholds of LiNbO3 [62]. The spectral responses of the
grating are shown in Figures 29(a) and 29(b). The measured Bragg reflection corresponds to a sixth-order resonance for
an effective index of the propagation mode through the waveguide of 2.135.Figure 29
(a) Transmitted spectral response of a lithium niobate grating made with femtosecond IR-radiation. (b) Reflected spectral response.Bragg
gratings made in the LiNbO3 waveguide corresponding to the spectral
responses presented in Figure 29 were inspected visually with an optical
microscope. As can be seen in Figure 30(a), a surface relief corrugated
grating has formed on the waveguide that is a typical ablation effect.
Measurement of the surface relief grating with an atomic force microscope
revealed a 300 nm corrugation depth.Optical microscope image of the grating induced (a) on the surface and (b) below the surface of the lithium niobate waveguide.
(a)(b)In the regions were the ablation process is incomplete, a
secondary grating could be seen a few microns below the crystal surface
displaying a different physical structure as compared to that of the surface
grating (Figure30(b)). The surface and
subsurface gratings are in phase suggesting that they are being created by the
same laser exposure. The surface
ablation threshold is less than that of the damage threshold to the subsurface
material.  The core of the waveguide is
positioned near the surface and within the Rayleigh range of the 30 mm focal
length lens (~20μm). It
is expected therefore that the surface and subsurface regions see the same
threshold intensity. The subsurface
refractive index change does not have the morphology of catastrophic
damage. The surface and subsurface
gratings develop simultaneously. Surface
grating development ultimately arrests the evolution of the subsurface grating
due to scattering of the femtosecond-IR radiation by the surface layer.


#### 5.7.2. YAG Fiber Bragg Grating

The femtosecond-IR laser phase mask technique was used to
inscribe Bragg gratings into 100μm diameter yttrium aluminium garnet (YAG) fiber
rod manufactured by MicroMaterials Inc. Using a 4.28-μm-pitch phase mask and 900μJ pulse energies and an f=30 mm focal
length cylindrical lens, a grating structure was observed under the optical
microscope as shown in Figure 31.Figure 31
Optical microscope image of the grating induced in 100μm diameter YAG fiber.


## 5.1. Cladding Mode Suppression

An important application of FBG technology is for high
reflectivity narrowband reflectors in wavelength-division multiplexing (WDM)
systems of optical networks. Such FBGs
typically suffer from an out-of-band loss on the short wavelength side of the
Bragg resonance which is due to strong coupling from the forward propagating
core mode (LP01) into discrete backward-propagating cladding modes
[37]. Cladding mode coupling is caused
by a mismatch in the overlap between the LP01 mode and the fiber
core in which the FBG is inscribed. To
suppress cladding mode coupling, specialty fibers were developed which
possessed UV-sensitive cladding as well as core regions. When the FBG was inscribed uniformly across
both photosensitive regions of the fiber-cross section where the LP01 mode had nonzero power, coupling to cladding modes was eliminated [38]. The major drawback of this technique was that
these specialty fibers are costly to produce, and fiber photosensitivity also
needed to be enhanced by H2-loading.Initial FBG inscriptions
with the femtosecond-IR laser and the phase mask showed that structures could
be written with almost equal ease in either all-silica core or Ge-doped silica
core fibers without H2-loading [12]. By implementing the order-walk effect and beam scanning, high-quality
gratings were inscribed in both the core and cladding region of standard SMF-28 fiber in such a way that the LP01 mode was completely subtended by grating structure in the core and cladding
(see Figure 10). This resulted in an
almost complete suppression of the coupling into cladding modes as shown in Figure
11 [39]. In [39], a third-order grating
with ΛG=1.6μm was used in order to observe the structure
directly under an optical microscope. Similar FBGs written at the fundamental grating pitch of ΛG=0.535μm also demonstrated excellent cladding mode
suppression but the structure could not be directly observed under the optical
microscope as in Figure 10.Microscope images of photoinduced index modulation inSMF-28; (see Figure 10(a)). The fiber is imaged near the core and is viewed normal to the beam axis.
In Figure 10(b) the fiber is rotated 90° and is viewed along the beam axis.
(a)(b)Figure 11
Transmission of cladding mode suppressed grating inSMF-28 shown in Figure 10. Inset figure shows the cladding mode loss expressed as difference between the transmission spectrum and 1-R reflection spectrum.


## 5.2. Writing through the Fiber Jacket

Standard  UV-laser inscription processes of FBGs typically
require the removal of the protective polymer coating prior to exposure as
these coatings usually are highly absorbing in the UV. Coating removal and replacement are
time-consuming processes that threaten the mechanical integrity of the fiber
and are therefore undesirable from a manufacturing viewpoint.Recently, FBG inscription
through the standard polymer coating of a fiber was reported using femtosecond-IR radiation and the point-by-point method [40] where the tight focusing
geometry was able to produce a large intensity differential between the fiber
coating and the fiber core. Substantial
Δn values were induced in the core
without damaging the coating (Δn=3×10−4).Using the femtosecond-IR laser
and the phase mask technique, FBGs were successfully inscribed through the acrylate
coating of standardSMF-28 fiber,
when the fiber had been loaded with hydrogen [41]. A tight focusing lens was used with a focal
length f=12 mm, but with the fiber placed  at distance d=1 mm
behind the 1.071μm phase mask which resulted in a fundamental
period grating in the fiber of 0.5355μm when irradiated with 400μJ and 125 femtoseconds pulses. The resultant FBG had a Δn=3.5×10−4.Under identical exposure
conditions (100 Hz, 20 seconds cycle of ±10μm beam sweep and pulse energies of 220μJ), grating growth as a function of time was
compared for gratings written through the acrylate coating of the H2-loaded SMF-28 with growth rates obtained
when the loaded SMF-28 fiber had been
chemically stripped. In Figure 12(a),
significant grating growth is initially observed for FBGs written through the
jacket. However after ~5 minutes
exposure, no further increase in Δn was observed. For the stripped fiber, the grating growth
continues even after a 90-minute exposure (see Figure 12(b)). Observation of the exposed region of the
polymer jacket under the microscope revealed some damage to the jacket
surface. Several femtosecond IR
laser-based ablation studies of polymeric materials have shown that the
threshold for induced index change and polymer ablation decreases with the
number of incident pulses [42, 43]. It
is likely that the first several hundred pulses incident on the acrylate
polymer coating are creating “incubation sites” that eventually reduce the
transmission of the IR radiation through the jacket thus limiting the Δn growth. Using an optical fiber pull tester (Vytran
PTR-100), the breakage strength was compared to pristine SMF-28 fiber. For the 5
devices that were tested, the breakage strength was 4.5±0.5 GPa (690 ± 80 kpsi) as compared to 5.3±0.3 GPa (770 ± 40 kpsi) for the pristine fiber.(a) Increase inΔn as a function of exposure time for gratings written through the polymer coating with 250μJ pulses and a f=19 mm cylindrical lens (black squares) and gratings written on stripped SMF-28 fibre (gray triangles); (b) Long-term continued grating growth on stripped SMF-28 fiber.
(a)(b)In another set of
experiments, FBGs were inscribed through the acrylate coatings of high-Ge
content high NASM1500(4.2/125) fibers from FiberCore. Using exposure conditions similar to those
used with the SMF-28 fibers, but with
pulse energies up to 440μJ/pulse, high index modulations were induced
through the jacket of this high-NA fiber both with and without H2-loading
[44]. Figure 13 displays the growth
rate of the Δn as a function of exposure for H2-loaded
fiber. Index modulation values
approaching 10-2 are realized with the jacket removed. As with standard SMF fiber, saturation in the
induced index change through the jacket is observed. The inset image of Figure 13 displays the
transmission spectra of gratings written through the acrylate jacket with and
without hydrogen loading producing Δn of 1.4×10−3 and 7×10−4, respectively.Figure 13
Grating growth in H2-loaded FiberCore fiber through the jacket (white circles) and with jacket removed (black diamonds). Inset image is of transmission spectra of gratings written through thejacket with (top) and without (bottom) H2-loading.For downhole monitoring
applications in the oil and gas industry, FBG sensors are written into optical
fibers coated with polyimide, which is thermally stable up to 300°C. For standard UV grating inscription, the
polyimide coating needs to be removed, as it is strongly absorbing in the
UV. Polyimide is resistant to chemical
attack. Coating removal is then a
nontrivial exercise requiring immersion of the fiber in a hot sulfuric acid
bath. Using the femtosecond-IR laser
phase mask technique, we were able to directly inscribe gratings through the
polyimide jacket of commercially available high-NA fibers from OFS-Fitel (ClearLite CL-POLY-1310-21) after the fiber was H2-loaded.For unloadedOFS fiber,
the maximum FBG reflectivity achievable was 1%. A reflection spectrum of the device is shown in Figure 14(a). The Bragg resonance of the FBG, λB, is 1555.24 nm, which corresponds to an effective core mode
index neff=1.452. Similar FBGs written in SMF-28 result in a neff=1.447, implying that
there is a higher Ge-dopant concentration in the high-NA fiber. The
transmission spectrum of an FBG written in H2-loaded OFS fiber with pulse energies of 180μJ is
presented in Figure 14(b). The total
exposure time for the hydrogen loaded grating inscription was 5 minutes, after
which no further grating growth was observed. For both the loaded and unloaded OFS fibers, the induced-index
change for the gratings occurred in type I-IR regime. Using the Rouard method, the transmission
spectrum of the grating was modeled assuming a Gaussian index profile 6.4 mm in
length and a 5-photon process. The
modeled response is shown as the dotted line in Figure 14(b). From the model, the maximum index modulation
in the core corresponding to the experimental data is Δn=4.8×10−4. For the unloaded case, the reflectivity of
1% corresponds to a Δn=3×10−5.(a) Reflection spectrum of an FBG written through the polyimide jacket of theOFS fiber without H2-loading; (b) transmission spectrum of an FBG written through the polyimide jacket of H2-loaded OFS fibre.  Overlaid modeled spectrum shown with black dots.
(a)(b)Observation of the
exposed region of the polymer jacket under an optical microscope revealed some
damage to the jacket surface after the 5-minute exposure. Femtosecond-IR laser machining studies of
polyimide have shown that the ablation threshold for polyimide decreases with
the number of incident pulses [43]. It
is likely that the first several hundred pulses are inducing “incubation sites”
in the polyimide polymer coating that eventually reduces the transmission of
the IR radiation through the jacket thus limiting the Δn growth.To study the FBG temperature
stability, isochronal annealing studies were performed on the gratings at
elevated temperatures. In Figure15, the
reduction of Δn at various elevated temperatures is shown for a grating
with an initial Δn of 3.6×10−4. A portion of the Δn is
rapidly annealed out at 200°C but the remaining Δn is
relatively stable with increasing temperature up to 400°C. After 16 hours at 400°C, the grating had a Δn of
2×10−4 with no visible erasure. As the rated operating temperature of the
fiber coating is 300°C, it is likely that the grating reflectivity will remain
stable within the operating temperature of the coating. When the fiber temperature was increased in
100°C increments and annealed for 1 hour at each temperature, grating erasure
was observed with complete grating erasure occurring at 800°C. In a separate experiment, another device was
annealed at 360°C for 140 hours after being preannealed at 75°C for 36 hours in
order to out-gas any remaining hydrogen (see Figure 16). After 140 hours, a Δn≫2.2×10−4 remained.Figure 15
Isochronal annealing study of an FBG in the polyimide coated OFS fiber.Figure 16
Long-term annealing study at 360°C of an FBG in OFS fiber.  Inset is an expanded view of the initial annealing with Δn and temperature denoted by black and gray traces, respectively.


## 5.3. Gratings in Active Fibers for Fiber Lasers

Fiber lasers have become essential components for telecom
and sensing applications because of their high efficiency, gain, and good beam
quality. Writing Bragg gratings
reflectors directly into the core of active single mode optical fibers is a
convenient way to create compact, fiber compatible, and low-loss laser
resonator cavities. Traditional UV
laser inscription of fiber Bragg gratings (FBGs) however is not appropriate for
short cavity fiber lasers where high levels of Erbium–Ytterbium doping
(Er3+ and Yb3+) are required [45]. Germanium (Ge) codoping of the fiber core,
needed to enhance UV photosensitivity, causes ion clustering of Er3+ and Yb3+ core dopants resulting in low laser efficiency [46]. Glass substrates other than silica, for
example fluoride or phosphate glass, are more desirable host materials for
active dopants as higher concentrations of Er3+ and Yb3+ can be realized. Dissimilarity with
silica however makes coupling of these fibers to external resonator mirrors
difficult. Direct inscription of FBG
resonators in these fibers would be preferable.Recently, femtosecond-IR lasers were used to inscribe
grating reflectors directly into conventional, nonphotosensitive, silica-based Er:Yb-codoped
fiber by the point-by-point [47], or phase mask techniques [48] for the
manufacturing of fiber lasers. These
gratings successfully resulted in lasing of the fiber laser cavities. In our laboratories, short strong gratings
were induced in heavily doped Er-Yb phosphate glass fiber producing resonator
cavity reflectivities above 99.99% for a 6-mm-long grating corresponding to Δn>1.5×10−3 [15]. Femtosecond IR laser-induced gratings have also been fabricated in undoped [17] and doped fluoride
fibers [49].The impact, however, of high
active doping levels on femtosecond-IR laser inscription of FBGs was
unclear. In order to develop recipes for
laser cavity grating inscription, investigations to determine optimal exposure
conditions and active doping levels in silica-based fibers were performed [18]. High Δn FBGs were written in commercially available fibers with
different concentrations of Er3+ and/or Yb3+ in order to
study the magnitude of FBG reflectivity and spectral quality as a function of
active doping levels. For each fiber
that was studied, the saturation in induced Δn, the spectral quality and out-of-band
loss were evaluated and compared with gratings made in pure silica core single
mode fibers.Various active fibers with
different levels of doping were tested. Since Er3+ and Yb3+ dopant concentration values
were unavailable, it was assumed that they were proportional to the losses
given at telecommunication and pump wavelengths, respectively, as specified in
Table 1. The FBGs were written using a first-order 1.07μm
pitched phase mask positioned 1 mm away from the fibers. All exposures were
made using 1.2 mJ pulses of IR radiation with laser repetition rates set at 100 Hz.Table 1
Active fibers evaluated.
FiberDopantLoss (dB/m) atλ nmCore sizeλcutoffNA1532980(μm)(nm)INO EY302Er:Yb292274×611450.14INO EY305Er:Yb22477612000.18INO EY550Er:Yb11.25932.5×99000.17Coractive EY18-06Er:Yb584006na0.18INO Yb214Y06126.29000.12INO Er406Er37183.48600.2CorguideEr25106nanaBrown UniversityEr1248nanaGrating growth was very rapid,
producing Bragg resonances of –40 dB after only
a few seconds of exposure. All FBGs made in the
doped active fibers listed in Table1 share two characteristics: the initial
FBG growth rate exceeds that of pure silica core fibers (see Figure 17(a)) and
the growth of the Δn in all the FBGs made in active fibers
saturate well below the pure silica core fiber levels of 4×10−3. For FBGs in active fiber, the saturated Δn was estimated to be 2 to 3.5×10−3 (see Figure 17(b)).(a) Initial growth and saturated growth (b) ofΔn versus IR exposures for different active fibers and for pure silica core fiber (black dots). White triangles denote EY302, white squares denote EY305, and gray squares denote Er406.
(a)(b)The plot of the saturatedΔn versus the absorption at 1532 nm and
980 nm for the gratings was highly scattered (see Figure 18). Error
bars denote the average values of three gratings written in the fiber. A weak negative correlation between saturated
Δn’s and increasing Er and Yb content of
the active fibers was observed. The
large scatter of the saturated Δn’s may be due to the different glass
composition between the fibers that were studied. With all the doped fibers tested, spectral
quality and out-of-band insertion losses were similar to the cladding mode
suppressed gratings in standard fiber presented in Figure 11.Saturation levels of the active fibersΔn's as a function of (a) loss at 1550 nm and (b) loss at 980 nm.  Error bars denote the averaging of the values obtained from the three sequential exposures of the fiber.  Dotted lines denote the linear regression through the data points.
(a)(b)


## 5.4. Gratings in Photonic Crystal Fibers

Inscription of FBG
structures in photonic crystal fibers (PCFs) enhances the flexibility of the
PCF for telecommunication as well as microfluidic sensing related
applications. Direct inscription of
FBGs in pure silica PCF has been achieved using ArF excimer laser irradiation
as well as femtosecond UV irradiation producingΔn~2×10−4 after a 3.8-hour write time
and ~1×10−4 after a one hour exposure time
[9, 50].Using the femtosecond-IR
laser-phase mask approach, FBGs were written in pure silica PCF and fiber
tapers [20]. Scattering of the writing
beam by the PCF cladding holes can be reduced by selecting fiber geometries
with smaller number of cladding holes between the fiber surface and the
core. With such a PCF, for example the ESM-12-01 fiber from Blaze Photonics/Crystal Fibre, which has 3 rows of air
holes surrounding the silica core, a Δn value of ~4×10−4 was obtained after a 5-minute exposure with a Λm=3.12μm mask, tight focusing (f=30 mm), and high
pulse energy (1200μJ/pulse at 200 Hz). The spectral response of the grating in the ESM-12-01 fiber is shown in Figure 19. For other
PCF with larger number of air holes, scattering of the incident IR light did
not result in any grating fabrication in the core, even at pulse energies that
resulted in damage to the fiber. By
tapering such PCF in a H2/O2 flame, the fiber diameter
could be reduced causing the bandgap generated by the cladding holes as seen by
the transversal IR beam to shift to a wavelength that was not resonant with the
IR source. The resultant narrower hole
diameter and spacing in the tapered PCF allowed partial transmission of the IR
radiation into the core resulting in FBGs in the tapered regions of the PCF
with Δn~3×10−4 after an 80-second exposure. Alternatively, the holes of the PCF could be collapsed altogether. Gratings inscribed in the collapsed region
across the entire cross section of the taper diameter produced multimode
transmission/reflection responses.Figure 19
(a) Cross section ofESM-12-01 fiber. (b) Transmission (gray), reflection (black), and modeled (dotted) spectra of a grating written in ESM-12-01 fiber with 1200μJ pulse energy 200 Hz. Total exposure time 5 minutes.The tapering
of the PCF by the H2/O2 flame such that the air holes was
not collapsed but still produced a high level of birefringence in the tapered region [51]. LMA-15   PCF from Crystal Fibre A/S was tapered from its initial 230μm outer diameter to a taper waist diameter of
45μm and 10 mm length. The fiber taper still guided the fundamental
mode without any loss. By inscribing a
weak grating in the PCF core of the taper region, the birefringence of the PCF
taper was quantified. In Figure 20, the
grating was probed with polarized light from a tunable laser source and the
spectral characteristics of the gratings made in the tapered PCF show a strong
dependence on the signal polarization. When the polarization of the signal was rotated using an inline fiber
polarizer, it was possible to switch the main reflection spectrum from the
Bragg resonance wavelength at 1525.2 nm to a lower wavelength of 1524.1 nm. For a birefringent Bragg grating,
the wavelength shift between the two orthogonal polarizations ΔλB will be
related to the difference between the neff of the two modes Δneff by: ΔλB/λB=Δneff/neff which results in a Δneff or birefringence value
of 1.05×10−3.Figure 20
(a) Scanning electron microscope image of 45μm diameter LMA-15 taper, (b) reflection spectra of grating in 45μm LMA-15 fiber taper for two orthogonal polarizations.


## 5.5. Induction of High Birefringence

For some applications, a given amount of birefringence in an
optical device or system is desirable, among the most important being the
single polarization fiber lasers [52] and Bragg grating sensing [53].
Typically, highly birefringent fibers of complex design are used in these
specialized applications. The birefringence, however, could be introduced
locally by processing the optical substrate with high-energy laser pulses. Small amounts of birefringence (4×10−5) can be created with UV exposure of Ge-doped
fibers [54] by controlling the polarization of the UV-writing source so that it
was normal to the optical fiber axis.Similarly, type I-IR
FBGs induced with a phase mask uniformly about the core have been shown to
produce similar levels of birefringence when the polarization of the IR beam
was also polarized normal to the fiber axis (~10-5) [55].
In the type II-IR regime, FBGs written uniformly about the fiber core produced levels
of birefringence approaching 10-4, an order of magnitude higher than the birefringence
resulting from the type I-IR exposure. Although the type II-IR gratings potentially
produce enough birefringence for most applications, one draw back of these
structures is that there is typically increased insertion loss for gratings
with stronger Δn's. For applications such as short cavity single
polarization lasers, the increased loss associated with the birefringent laser
resonator mirror would reduce the laser performance.In our laboratories, we have shown that if a type II-IR grating structure is made
in the cladding close to but not overlapping the core, such that there is no
retroreflectance from the structure, a large amount of
birefringence is locally induced in the core region without generating excess
loss [56].  Birefringence values as high
as ~8×10−4 were obtained in the core of SMF-28 fiber when type II-IR grating
structures were inscribed in the cladding of the fiber on both sides of the
core in close proximity to the core/cladding interface. The splitting of the reflection spectrum of a
weak probe grating written in the core is shown in Figure 21 along with a
schematic of the exposure which produced the high birefringence result and a
photomicrograph of the grating induced in the cladding. The birefringence is likely due to induced
stress rather than direct asymmetric refractive index modification.The spectra of the probe grating before and after the cladding exposure are denoted by black and grey traces, respectively.  Inset photomicrograph shows a type II-IR exposure inSMF-28 fiber in the cladding proximate to the core.  Image taken along the exposure beam axis  (b) shows a schematic representation of the exposure conditions that produced the high birefringent spectral result.
(a)
Wavelength (nm)(b)
Wavelength (nm)When a strong type I-IR
grating (~95% reflectivity) is present in the core, the type II-IR cladding
exposures resulted not only in increased birefringence of the effective core
mode, as measured by the induced polarization dependent wavelength shift, but
also resulted in a large grating strength difference between orthogonal
polarization axes (ΔTr)
for the Bragg resonance. As can be seen
in Figure 22, the ordinary polarization corresponds to a 15 dB strong grating
while the extraordinary polarization results in a secondary resonance of 5 dB
located 540 pm away from the main resonance (~5×10−4 birefringence) resulting in ΔTr=10 dB. The difference in the Δn with polarization based on the
evaluation of the refractive index modulation of the two gratings resulted in a
value of ~2 × 10-4 assuming the approximation of a fundamental (first-order) grating. Since the spectrum of the original grating
measured before the cladding exposure did not display a large ΔTr at the Bragg resonance, it can be assumed that the initial Δn was similar for the two principle
states of polarization and the change in ΔTr was a result of the cladding exposure.Figure 22
Large birefringence of the refractive-index modulation induced by a symmetric cladding exposure of a strong type I-IR core grating. The two orthogonal polarization states are denoted by the gray and black traces, respectively.


## 5.6. High-Temperature Stable Sensor Gratings

A very important application of FBG technology is its usage as sensing elements. FBGs are capable of monitoring both temperature and strain through variations in the spectral
response of the grating, which can be directly correlated to strain and
temperature imposed on the grating structure [57]. The applicability of traditional UV
laser-induced FBGs is restricted to lower temperatures (<600°C) because of the instability of the UV generated grating structure at
higher temperatures. High-temperature
grating stability is especially important for sensing applications such as
monitoring furnaces, combustion situations, and so forth.Femtosecond IR laser-induced
gratings in standardSMF-28 and all
silica core single mode fiber have displayed high long-term thermal stability
at 1000°C when the FBGs were inscribed in the type II-IR regime [12]. Above 1000°C, the silica substrate itself is
susceptible to structural transformations that limit the functionality of the
fiber [58]. For temperatures >1000°C, fiber substrates other than silica
need to be considered. The most
successful optical fiber used for high-temperature sensor applications is the
single crystal sapphire fiber that has a glass transition temperature tg of ~2030°C. Unlike conventional
silica-based single mode optical fibers, sapphire fibers are made in the form
of rods absent a cladding layer. With
fiber diameters commercially available, beam propagation within the fiber is
highly multimode at the 1550 nm telecommunication wavelengths. Present sapphire fiber sensors are mostly
based on Fabry-Perot structures within the fiber producing a broadband
interferometric signal that varies with temperature [59].Femtosecond IR-laser induced gratings in sapphire fiber show no degradation of the grating
strength at high temperatures up to 1500°C [15, 16]. The sapphire FBGs (SFBG) have definite
advantages over other sapphire fiber sensors that rely on Fabry-Perot etalons
at the fiber tip. Unlike Fabry-Perot
sapphire sensors, SFBG sensors with their discrete resonant wavelength could
potentially be used as distributed optical sensor arrays up to 2000°C. The SFBGs that are inscribed in sapphire
fiber using femtosecond-IR lasers and a phase mask are naturally multimode
devices. The multimode reflection
spectrum observed with the retroreflective SFBGs is characterized by a large
bandwidth having a complicated structure that consists of a superposition of
different modes reflected by the grating. Although the SFBG sensor can be used in the multimode regime,
propagation of a single mode is preferable due to stability and the detection
advantages that a narrowband single mode reflection response offers. To produce a single mode response, the
multimode SFBGs were probed using tapered single mode fibers. Single and low-order mode
reflection/transmission responses are produced when the mode field of the
launched single mode is matched with the fundamental mode supported by the
sapphire fiber. For a 150μm diameter sapphire fiber, this would
correspond to an LP01 mode field ~150μm in diameter. When single mode optical fiber is tapered such that the normalized
frequency or V number of the core is <1, the fundamental LP01 mode is no longer core guided but is guided by the cladding-air interface
resulting in a mode field with the same diameter as the tapered fiber. Matching taper and sapphire fiber outer
diameters may excite only a single mode of the sapphire fiber. The evanescent tapers were made from fibers
with different cutoff wavelengths λcut.Although, theoretically less
efficient for mode matching, the expanded fundamental mode of a single mode
fiber can also be used to reduce the number of modes excited by the probing
signal. As in the case of tapers, the
single mode probe signal from a fiber collimator can be launched from a single
mode 1550 nm optical circulator into the SFBG, and the reflected signal can be
detected at the third port of the circulator instead of using the less
efficient fiber couplers. A
configuration made of an input single mode tapered fiber and multimode silica
fiber used for output coupling was also tested and has delivered a filtered
multimode transmission spectrum. The tapered coupling improved the spectral
resolution of the sapphire FBG as compared to its multimode responses. Such improvements facilitate the utilization
of the sapphire FBG as a high-temperature sensor.


### 5.6.1. Thermally Stable FBGs inSIO2 Based Fibers

FBGs written with ultrafast IR radiation exhibit different
thermal stability characteristics depending upon the pulse duration and
energy. Above a low-intensity threshold
for grating formation (type I-IR) [23], Δn values with similar thermal stability to UV-induced
gratings can be generated as was shown in Figure 3. At higher power levels that are consistent
with white light generation in the fibers, thermally ultra-stable
defects/damage sites could be produced (type II-IR).A long-term study of type II-IR grating
stability was performed by inscribing largeΔn type II-IR FBGs in SMF-28 fiber which were then heated to 1000°C in steps of 100°C for 1 hour at each
temperature [58]. Once 1000°C was
reached, the grating was then kept at constant temperature for 150, hours and
the grating reflectivity and resonant wavelength were monitored (see Figure 23). There was no noticeable degradation
of the increased grating strength for the duration of the test and the grating
maintained Δn=1.7×10−3. The initial increase in the femtoseconds
grating Δn, shown in Figure 3 previously, is likely a result of the two kinds of index change being written
simultaneously. The peaks of the complex
interference pattern are sufficiently intense to ultimately ionize the glass in
the fiber producing an index change that is durable with temperature. In the valleys of the interference pattern,
the intensity is below the type II-IR threshold, however some type I-IR index
change is generated. As the device is
annealed, the permanent type II-IR index change remains while the annealable
type I-IR index change is erased resulting in a higher Δn. The temperature of the femtoseconds grating
was subsequently increased and kept at 1050°C for 100 hours (see Figure
23). The Δn of the femtoseconds
grating decreased slightly from 1.7×10−3 to 1.6×10−3 during 100 hours at 1050°C. A drift of the Bragg resonance to
longer wavelengths of 0.2 nm was detected at the end of experiment. Spectra taken initially at room temperature
and after 100 hours at 1050°C are shown in inset of Figure 23.Figure 23
Long-term annealing at 1000 and 1050°C of a femtosecond IR FBG.  Inset displays the initial room temperature transmission spectrum and the final spectrum at 1050°C for 100 hours.


### 5.6.2. High-Temperature Multiparameter Sensor inSIO2

As presented in Section4.6 above, the type II-IR FBG
written in Corning 980 Flexcore fiber with a cutoff wavelength of 980 nm
and the 4.28μm pitched phase mask has many Fourier
components in its spectral response that produces Bragg resonances at various
wavelengths (see Figure 9). The large
number of spectral resonances generated by a single grating structure is
available to be used for multiparameter sensing. Aside from strain and temperature, other
parameters like bending, pressure, and so forth could be simultaneously
measured using only one grating. The availability of supplementary spectral
characteristics could also be used to improve the accuracy of the
strain-temperature discrimination.Wavelength shifts of the various diffracted
orders as a function of the temperature in the absence of strain are shown in Figure24(a). The temperature was changed from ~20°C to ~800°C. The room temperature 1550 nm Bragg resonance wavelength was monitored
in order to confirm that the thermal equilibrium was reached after ~30 minutes
at each temperature. After the thermal
equilibrium was reached, all the resonances of the Bragg grating were measured
at constant temperature. At room
temperature, the gratings were subjected to controlled stress in a specially
designed jig. The strain was applied
incrementally from 0 to 3000με. After each strain increment, the wavelength
shift was monitored in the 1550 nm range until the measurement confirmed that
the Bragg wavelength was stable, as measured by an optical spectrum analyzer. The Bragg wavelength of each diffraction
order was then evaluated by scanning a 10 nm spectral range around each high-order
resonance. Wavelength shifts of the
various diffracted orders as a function of strain are shown in Figure 24(b).(a) Bragg-order wavelength variation as a function of temperature for the grating shown in Figure9, (b) Bragg-order wavelength variation as a function of strain. Orders m=7,8,9,10,11, and 12 are denoted by white circles, black squares, white squares, triangles, diamonds, and black circles, respectively.
(a)(b)Under the application of strain and
temperature and of another arbitrary parameterP, the grating spectrum of order m will undergo a wavelength shift: (3)Δλm(ε,T,P)=KεmΔε+KTmΔT+KPmΔP,  where Δλm are the wavelength shifts
when the ambient parameters; stress (ε),
temperature (T), and a generic
general parameter (P) are changing by
the amount of Δε,ΔT, and ΔP. Considering temperature and strain, the
variations in Δλm differ from order to order
as shown in Figures 24(a) and 24(b). Assuming that the resonance shifts are linear and independent of each
other, the slopes or K values of the Δλm versus T or ε curves
can be used to setup a 2×2K matrix which will allow for the discrimination between temperature and strain
using a single Bragg grating [60]. By
employing more than two resonances, the problem becomes one that is over
defined, potentially resulting in reduced measurement error. Alternatively, other parameters in addition
to temperature and strain could also be monitored.


### 5.6.3. High-Temperature Stable FBGs in  Sapphire Fiber: Multimode Response

FBGs were fabricated in crystalline sapphire fiber with
diameters of 60, 100, 120, 150, and 180μm by exposing the fibers to the 125 femtoseconds
pulsed-IR radiation. The tests were made
using the 4.28μm period phase mask that resulted in a 5th-order Bragg
resonance in the sapphire fiber at 1490 nm. Alternatively, a 1.747μm period phase mask was used that gives a second-order
Bragg resonance in sapphire fiber at 1523 nm. The first phase-mask, although producing lower reflectivity high-order
resonances, has the advantage of producing grating structures in sapphire fiber
that are easily observed under an optical microscope. The shorter pitched phase
mask was specially designed to generate a low-order resonance in the sapphire
fiber in the telecom wavelength range. In this way, the grating growth could be conveniently monitored with an
erbium white light source and a spectrum analyzer.The sapphire FBGs (SFBGs) were fabricated using
the setup shown in Figure1. The femtosecond-IR
pulses were focused with a 25-mm-focal length lens through the phase mask and
onto the sapphire fiber. Since the
sapphire fibers have a hexagonal cross-section, they were placed in a rotation
jig that would allow the fiber to be turned along its axis in order to optimize
the exposure. The femtosecond-IR beam
was focused inside the fiber and then scanned normal to the fiber using a
precision vertical stage in order to expose the whole cross section of the
sapphire fiber to the laser beam. As the
sapphire fiber is essentially a sapphire rod absent a cladding layer, the
spectral response from an inscribed grating is highly multimode. To measure the multimode response from the
SFBG, an S-C band tunable laser was passed through a 3-dB-multimode coupler
made in-house with 105/125μm core/clad silica fiber. The output port of the coupler was mounted on
5-axis precision stages and butt-coupled to the sapphire fiber with index
matching oil having a refractive index of 1.46. The reflected response was measured using a fast detector.SFBGs
were fabricated in 150μm diameter fibers with both the 4.28 and 1.747μm phase masks producing maximum reflectivities
of 8%. There is a limitation on the
maximum reflectivity of a multimode fiber grating due to the large number of
modes that the fiber will support. In
105μm core multimode silica fiber, using standard
writing conditions, the maximum reflectivity obtained using a first-order
phase mask was 30%, which corresponded to a Δn~3×10−3. This reflectivity was not reached in gratings
made in sapphire with similar diameters of 100μm or even in the case of
smaller diameters of 70 and 60μm. It is
possible that the lower reflectivity obtained in sapphire fibers as compared to
silica is the result of a lower saturation level of the Δn that can be induced in sapphire fiber
using the method of ultrafast radiation and the phase mask.The magnitude of the SFBG reflectivity also
depends on the position of the grating along the sapphire fiber length. The multimode
response of an SFBG written in a 120-cm-long sapphire fiber with 150μm core at ~20 cm from one end of the fiber is
presented in the Figure 25. In the figure, the multimode reflection spectrum
measured from the short end is presented together with the spectral response of
the same grating measured from the long side of the sapphire fiber. There is systematic 4–6 dB difference in the
strength of the reflected signal when it propagates through 1 meter of the
sapphire fiber. This loss is likely related
to the propagation of the mode as well as mode conversion within the
fiber. In addition to the reflectivity,
the bandwidth of the grating response is also reduced which implies that higher-order
modes are lost as the signal propagates along the length of the fiber.Figure 25
Multimode reflection spectra of the gratings written in the 150μm core diameter sapphire fiber measured from the short side (gray line) and from the long side (black line) of a 120 cm-long sapphire fiber.


### 5.6.4. High-Temperature Stable FBGs in  Sapphire Fiber: Single Mode Response

Multimode
responses from FBG sensors are not as desirable as single mode responses since
the spectral bandwidth is broad and the dynamic range of the reflection signal
is low. In order to achieve a
quasi-single mode response from the SFBG, it was necessary to investigate other
approaches to probe the grating aside from the multimode coupler. The maximization of single mode coupling to
the multimode sapphire fiber can be achieved by the generation of a large mode
field diameter (MFD) single mode by for example, a fiber taper [16].For single mode fibers, the normalized
frequency orV number is given by  (4)V=2πrλ(nco2−ncl2), where r is the core radius, λ is
the wavelength, and ni is
the refractive index of the core or cladding. For single mode operation, V≤2.405. When a fiber is tapered, the ratio of cladding/core radii remains
constant however V decreases. The
transition of the fundamental LP01 mode from a core-cladding guided into a cladding-air guided mode occurs when
[61] (5)V<Vcc≅2lnS{1+0.26lnS}−1/2, where S is the
ratio of the cladding to core diameters.Once condition (5) is met, the fundamental mode
expands into the cladding resulting in a large area mode that can be better
coupled into the sapphire fiber. As the
expanded mode field is limited by the outer diameter of the taper, maximization
of the single mode coupling occurs when the difference between the taper and
the sapphire fiber diameters is minimized. Fiber tapers were fabricated using
Corning Puremode HI 780 single mode fiber with an outer diameter ϕ=125μm and a cutoff wavelength λcut<720 nm using a swept hydrogen flame while the fiber was pulled on
precision translation stages. When
pulled to taper diameters ≤70μm, the fundamental mode was completely guided
by the cladding air interface as shown schematically in Figure 26. Commercially available 50μm and 60μm diameter
fiber collimators were also used for testing.Figure 26
Schematic of the LP01 mode field expansion resulting from fiber tapering and coupling of the taper into the sapphire fiber.In order to obtain a
single mode response from the sapphire grating, a probe signal from the swept
tunable laser system was passed through a 1550-nm-range optical circulator and
into the tapered fiber/fiber collimator. The output of the collimator and the
tapers were mounted on 5-axis precision stages and butt-coupled to the sapphire
fiber with index matching oil having a refractive index of 1.46.The same SFBG that produced the multimode response
shown in Figure25 was also probed with a 60μm diameter taper (see Figure 27). The reflection spectrum measured with the 60μm
taper has a 3 dB bandwidth of 0.33 nm similar to that of an FBG in single mode
fiber, which is compared to the multimode spectrum that has a 3 dB
bandwidth of 6 nm. Compared to the
multimode response, there is also a 10 dB improvement in the signal-to-noise
ratio of the 60μm taper response. The loss at the
taper/sapphire splice is 4.5 dB per pass. This loss may be due to the mismatch
of excited modes generated by and collected by the tapers.Figure 27
Reflection spectra of SFBG with a 100-μm-silica core multimode fibre (dotted line) and a 60-μm-taper (black line) as the input couplers.To measure the response of the SFBG to
temperature, the portion of the sapphire fiber that contained the Bragg grating
was inserted into a high-temperature ceramic microfurnace (NTT model
CMH-7022). In order to position the
Bragg grating within the microfurnace, an He-Ne laser was
connected to the input port of the multimode coupler. The red light scattered
by the Bragg grating was then used to position the grating accurately within
the center of the furnace where the temperature distribution was at its
maximum. The microfurnace temperature
was monitored at its center using a platinum-rhodium
thermocouple with a measurement range up 1700°C. A gradient with a 20% decrease in the
temperature from the center to edge of the 20-mm-long microfurnace was
observed. The wavelength shift of the
SFBG as a function of temperature for both the single and multimode spectral
responses is presented in Figure 28. Both the single and multimode responses are similar. For comparison, the wavelength shift at 1550 nm of the high-temperature stable type II-IR grating written in SMF-28 is also shown. Up to 1000°C, the Bragg resonances of both
the sapphire and Ge-doped silica fibers vary linearly with temperature, 26 pm/°C
and 14 pm/°C, respectively.Figure 28
Variation of single mode (white squares) and the multimode SFBG wavelength response (black circles,) with temperature.  Wavelength variation of the Bragg resonance of a type II-IR FBG written inSMF-28 fiber is also shown (black triangle).


## 5.6.1. Thermally Stable FBGs inSIO2 Based Fibers

FBGs written with ultrafast IR radiation exhibit different
thermal stability characteristics depending upon the pulse duration and
energy. Above a low-intensity threshold
for grating formation (type I-IR) [23], Δn values with similar thermal stability to UV-induced
gratings can be generated as was shown in Figure 3. At higher power levels that are consistent
with white light generation in the fibers, thermally ultra-stable
defects/damage sites could be produced (type II-IR).A long-term study of type II-IR grating
stability was performed by inscribing largeΔn type II-IR FBGs in SMF-28 fiber which were then heated to 1000°C in steps of 100°C for 1 hour at each
temperature [58]. Once 1000°C was
reached, the grating was then kept at constant temperature for 150, hours and
the grating reflectivity and resonant wavelength were monitored (see Figure 23). There was no noticeable degradation
of the increased grating strength for the duration of the test and the grating
maintained Δn=1.7×10−3. The initial increase in the femtoseconds
grating Δn, shown in Figure 3 previously, is likely a result of the two kinds of index change being written
simultaneously. The peaks of the complex
interference pattern are sufficiently intense to ultimately ionize the glass in
the fiber producing an index change that is durable with temperature. In the valleys of the interference pattern,
the intensity is below the type II-IR threshold, however some type I-IR index
change is generated. As the device is
annealed, the permanent type II-IR index change remains while the annealable
type I-IR index change is erased resulting in a higher Δn. The temperature of the femtoseconds grating
was subsequently increased and kept at 1050°C for 100 hours (see Figure
23). The Δn of the femtoseconds
grating decreased slightly from 1.7×10−3 to 1.6×10−3 during 100 hours at 1050°C. A drift of the Bragg resonance to
longer wavelengths of 0.2 nm was detected at the end of experiment. Spectra taken initially at room temperature
and after 100 hours at 1050°C are shown in inset of Figure 23.Figure 23
Long-term annealing at 1000 and 1050°C of a femtosecond IR FBG.  Inset displays the initial room temperature transmission spectrum and the final spectrum at 1050°C for 100 hours.


## 5.6.2. High-Temperature Multiparameter Sensor inSIO2

As presented in Section4.6 above, the type II-IR FBG
written in Corning 980 Flexcore fiber with a cutoff wavelength of 980 nm
and the 4.28μm pitched phase mask has many Fourier
components in its spectral response that produces Bragg resonances at various
wavelengths (see Figure 9). The large
number of spectral resonances generated by a single grating structure is
available to be used for multiparameter sensing. Aside from strain and temperature, other
parameters like bending, pressure, and so forth could be simultaneously
measured using only one grating. The availability of supplementary spectral
characteristics could also be used to improve the accuracy of the
strain-temperature discrimination.Wavelength shifts of the various diffracted
orders as a function of the temperature in the absence of strain are shown in Figure24(a). The temperature was changed from ~20°C to ~800°C. The room temperature 1550 nm Bragg resonance wavelength was monitored
in order to confirm that the thermal equilibrium was reached after ~30 minutes
at each temperature. After the thermal
equilibrium was reached, all the resonances of the Bragg grating were measured
at constant temperature. At room
temperature, the gratings were subjected to controlled stress in a specially
designed jig. The strain was applied
incrementally from 0 to 3000με. After each strain increment, the wavelength
shift was monitored in the 1550 nm range until the measurement confirmed that
the Bragg wavelength was stable, as measured by an optical spectrum analyzer. The Bragg wavelength of each diffraction
order was then evaluated by scanning a 10 nm spectral range around each high-order
resonance. Wavelength shifts of the
various diffracted orders as a function of strain are shown in Figure 24(b).(a) Bragg-order wavelength variation as a function of temperature for the grating shown in Figure9, (b) Bragg-order wavelength variation as a function of strain. Orders m=7,8,9,10,11, and 12 are denoted by white circles, black squares, white squares, triangles, diamonds, and black circles, respectively.
(a)(b)Under the application of strain and
temperature and of another arbitrary parameterP, the grating spectrum of order m will undergo a wavelength shift: (3)Δλm(ε,T,P)=KεmΔε+KTmΔT+KPmΔP,  where Δλm are the wavelength shifts
when the ambient parameters; stress (ε),
temperature (T), and a generic
general parameter (P) are changing by
the amount of Δε,ΔT, and ΔP. Considering temperature and strain, the
variations in Δλm differ from order to order
as shown in Figures 24(a) and 24(b). Assuming that the resonance shifts are linear and independent of each
other, the slopes or K values of the Δλm versus T or ε curves
can be used to setup a 2×2K matrix which will allow for the discrimination between temperature and strain
using a single Bragg grating [60]. By
employing more than two resonances, the problem becomes one that is over
defined, potentially resulting in reduced measurement error. Alternatively, other parameters in addition
to temperature and strain could also be monitored.


## 5.6.3. High-Temperature Stable FBGs in  Sapphire Fiber: Multimode Response

FBGs were fabricated in crystalline sapphire fiber with
diameters of 60, 100, 120, 150, and 180μm by exposing the fibers to the 125 femtoseconds
pulsed-IR radiation. The tests were made
using the 4.28μm period phase mask that resulted in a 5th-order Bragg
resonance in the sapphire fiber at 1490 nm. Alternatively, a 1.747μm period phase mask was used that gives a second-order
Bragg resonance in sapphire fiber at 1523 nm. The first phase-mask, although producing lower reflectivity high-order
resonances, has the advantage of producing grating structures in sapphire fiber
that are easily observed under an optical microscope. The shorter pitched phase
mask was specially designed to generate a low-order resonance in the sapphire
fiber in the telecom wavelength range. In this way, the grating growth could be conveniently monitored with an
erbium white light source and a spectrum analyzer.The sapphire FBGs (SFBGs) were fabricated using
the setup shown in Figure1. The femtosecond-IR
pulses were focused with a 25-mm-focal length lens through the phase mask and
onto the sapphire fiber. Since the
sapphire fibers have a hexagonal cross-section, they were placed in a rotation
jig that would allow the fiber to be turned along its axis in order to optimize
the exposure. The femtosecond-IR beam
was focused inside the fiber and then scanned normal to the fiber using a
precision vertical stage in order to expose the whole cross section of the
sapphire fiber to the laser beam. As the
sapphire fiber is essentially a sapphire rod absent a cladding layer, the
spectral response from an inscribed grating is highly multimode. To measure the multimode response from the
SFBG, an S-C band tunable laser was passed through a 3-dB-multimode coupler
made in-house with 105/125μm core/clad silica fiber. The output port of the coupler was mounted on
5-axis precision stages and butt-coupled to the sapphire fiber with index
matching oil having a refractive index of 1.46. The reflected response was measured using a fast detector.SFBGs
were fabricated in 150μm diameter fibers with both the 4.28 and 1.747μm phase masks producing maximum reflectivities
of 8%. There is a limitation on the
maximum reflectivity of a multimode fiber grating due to the large number of
modes that the fiber will support. In
105μm core multimode silica fiber, using standard
writing conditions, the maximum reflectivity obtained using a first-order
phase mask was 30%, which corresponded to a Δn~3×10−3. This reflectivity was not reached in gratings
made in sapphire with similar diameters of 100μm or even in the case of
smaller diameters of 70 and 60μm. It is
possible that the lower reflectivity obtained in sapphire fibers as compared to
silica is the result of a lower saturation level of the Δn that can be induced in sapphire fiber
using the method of ultrafast radiation and the phase mask.The magnitude of the SFBG reflectivity also
depends on the position of the grating along the sapphire fiber length. The multimode
response of an SFBG written in a 120-cm-long sapphire fiber with 150μm core at ~20 cm from one end of the fiber is
presented in the Figure 25. In the figure, the multimode reflection spectrum
measured from the short end is presented together with the spectral response of
the same grating measured from the long side of the sapphire fiber. There is systematic 4–6 dB difference in the
strength of the reflected signal when it propagates through 1 meter of the
sapphire fiber. This loss is likely related
to the propagation of the mode as well as mode conversion within the
fiber. In addition to the reflectivity,
the bandwidth of the grating response is also reduced which implies that higher-order
modes are lost as the signal propagates along the length of the fiber.Figure 25
Multimode reflection spectra of the gratings written in the 150μm core diameter sapphire fiber measured from the short side (gray line) and from the long side (black line) of a 120 cm-long sapphire fiber.


## 5.6.4. High-Temperature Stable FBGs in  Sapphire Fiber: Single Mode Response

Multimode
responses from FBG sensors are not as desirable as single mode responses since
the spectral bandwidth is broad and the dynamic range of the reflection signal
is low. In order to achieve a
quasi-single mode response from the SFBG, it was necessary to investigate other
approaches to probe the grating aside from the multimode coupler. The maximization of single mode coupling to
the multimode sapphire fiber can be achieved by the generation of a large mode
field diameter (MFD) single mode by for example, a fiber taper [16].For single mode fibers, the normalized
frequency orV number is given by  (4)V=2πrλ(nco2−ncl2), where r is the core radius, λ is
the wavelength, and ni is
the refractive index of the core or cladding. For single mode operation, V≤2.405. When a fiber is tapered, the ratio of cladding/core radii remains
constant however V decreases. The
transition of the fundamental LP01 mode from a core-cladding guided into a cladding-air guided mode occurs when
[61] (5)V<Vcc≅2lnS{1+0.26lnS}−1/2, where S is the
ratio of the cladding to core diameters.Once condition (5) is met, the fundamental mode
expands into the cladding resulting in a large area mode that can be better
coupled into the sapphire fiber. As the
expanded mode field is limited by the outer diameter of the taper, maximization
of the single mode coupling occurs when the difference between the taper and
the sapphire fiber diameters is minimized. Fiber tapers were fabricated using
Corning Puremode HI 780 single mode fiber with an outer diameter ϕ=125μm and a cutoff wavelength λcut<720 nm using a swept hydrogen flame while the fiber was pulled on
precision translation stages. When
pulled to taper diameters ≤70μm, the fundamental mode was completely guided
by the cladding air interface as shown schematically in Figure 26. Commercially available 50μm and 60μm diameter
fiber collimators were also used for testing.Figure 26
Schematic of the LP01 mode field expansion resulting from fiber tapering and coupling of the taper into the sapphire fiber.In order to obtain a
single mode response from the sapphire grating, a probe signal from the swept
tunable laser system was passed through a 1550-nm-range optical circulator and
into the tapered fiber/fiber collimator. The output of the collimator and the
tapers were mounted on 5-axis precision stages and butt-coupled to the sapphire
fiber with index matching oil having a refractive index of 1.46.The same SFBG that produced the multimode response
shown in Figure25 was also probed with a 60μm diameter taper (see Figure 27). The reflection spectrum measured with the 60μm
taper has a 3 dB bandwidth of 0.33 nm similar to that of an FBG in single mode
fiber, which is compared to the multimode spectrum that has a 3 dB
bandwidth of 6 nm. Compared to the
multimode response, there is also a 10 dB improvement in the signal-to-noise
ratio of the 60μm taper response. The loss at the
taper/sapphire splice is 4.5 dB per pass. This loss may be due to the mismatch
of excited modes generated by and collected by the tapers.Figure 27
Reflection spectra of SFBG with a 100-μm-silica core multimode fibre (dotted line) and a 60-μm-taper (black line) as the input couplers.To measure the response of the SFBG to
temperature, the portion of the sapphire fiber that contained the Bragg grating
was inserted into a high-temperature ceramic microfurnace (NTT model
CMH-7022). In order to position the
Bragg grating within the microfurnace, an He-Ne laser was
connected to the input port of the multimode coupler. The red light scattered
by the Bragg grating was then used to position the grating accurately within
the center of the furnace where the temperature distribution was at its
maximum. The microfurnace temperature
was monitored at its center using a platinum-rhodium
thermocouple with a measurement range up 1700°C. A gradient with a 20% decrease in the
temperature from the center to edge of the 20-mm-long microfurnace was
observed. The wavelength shift of the
SFBG as a function of temperature for both the single and multimode spectral
responses is presented in Figure 28. Both the single and multimode responses are similar. For comparison, the wavelength shift at 1550 nm of the high-temperature stable type II-IR grating written in SMF-28 is also shown. Up to 1000°C, the Bragg resonances of both
the sapphire and Ge-doped silica fibers vary linearly with temperature, 26 pm/°C
and 14 pm/°C, respectively.Figure 28
Variation of single mode (white squares) and the multimode SFBG wavelength response (black circles,) with temperature.  Wavelength variation of the Bragg resonance of a type II-IR FBG written inSMF-28 fiber is also shown (black triangle).


## 5.7. Gratings in Other Crystalline Materials

### 5.7.1. Bragg Gratings in Lithium Niobate Waveguides

Along with sapphire, the femtosecond-IR laser grating
inscription technique has also been demonstrated in other crystalline
waveguides and fibers. Lithium niobate
(LiNbO3), because of its ferroelectric nature, is an attractive
material for fabrication of active integrated optical devices. The versatility of the waveguides made in
this material can be enhanced through the creation of efficient permanent
grating structures. Due to the electro-optic properties of the substrate, the
lithium niobate Bragg gratings have the potential to satisfy the rapid tuning
requirements needed by telecommunication components, such as tunable add/drop
filters (ROADMs), tunable dispersion compensators, switches, and so forth that
are used for dynamic control of the signal in the network nodes of agile
all-optical communication networks.In our laboratory, gratings were fabricated in
40 mm long reverse proton exchange (RPE) LiNbO3 channel waveguides
using the femtosecond-IR laser-phase mask approach [14]. Using the setup shown in Figure 1 with an f = 30 mm focusing lens and the 4.28μm pitched phase mask in the two-beam
interference regime and 600μJ pulse energies, grating responses up to -3.5 dB in transmission for exposures at 100 Hz for 1 minute were achieved. The peak field intensity corresponding to the
pulse energies used was 2.0 to 2.5×1013 W/cm2, which is of the same order of magnitude as previously
established IR ultrafast laser ablation thresholds of LiNbO3 [62]. The spectral responses of the
grating are shown in Figures 29(a) and 29(b). The measured Bragg reflection corresponds to a sixth-order resonance for
an effective index of the propagation mode through the waveguide of 2.135.Figure 29
(a) Transmitted spectral response of a lithium niobate grating made with femtosecond IR-radiation. (b) Reflected spectral response.Bragg
gratings made in the LiNbO3 waveguide corresponding to the spectral
responses presented in Figure 29 were inspected visually with an optical
microscope. As can be seen in Figure 30(a), a surface relief corrugated
grating has formed on the waveguide that is a typical ablation effect.
Measurement of the surface relief grating with an atomic force microscope
revealed a 300 nm corrugation depth.Optical microscope image of the grating induced (a) on the surface and (b) below the surface of the lithium niobate waveguide.
(a)(b)In the regions were the ablation process is incomplete, a
secondary grating could be seen a few microns below the crystal surface
displaying a different physical structure as compared to that of the surface
grating (Figure30(b)). The surface and
subsurface gratings are in phase suggesting that they are being created by the
same laser exposure. The surface
ablation threshold is less than that of the damage threshold to the subsurface
material.  The core of the waveguide is
positioned near the surface and within the Rayleigh range of the 30 mm focal
length lens (~20μm). It
is expected therefore that the surface and subsurface regions see the same
threshold intensity. The subsurface
refractive index change does not have the morphology of catastrophic
damage. The surface and subsurface
gratings develop simultaneously. Surface
grating development ultimately arrests the evolution of the subsurface grating
due to scattering of the femtosecond-IR radiation by the surface layer.


### 5.7.2. YAG Fiber Bragg Grating

The femtosecond-IR laser phase mask technique was used to
inscribe Bragg gratings into 100μm diameter yttrium aluminium garnet (YAG) fiber
rod manufactured by MicroMaterials Inc. Using a 4.28-μm-pitch phase mask and 900μJ pulse energies and an f=30 mm focal
length cylindrical lens, a grating structure was observed under the optical
microscope as shown in Figure 31.Figure 31
Optical microscope image of the grating induced in 100μm diameter YAG fiber.


## 5.7.1. Bragg Gratings in Lithium Niobate Waveguides

Along with sapphire, the femtosecond-IR laser grating
inscription technique has also been demonstrated in other crystalline
waveguides and fibers. Lithium niobate
(LiNbO3), because of its ferroelectric nature, is an attractive
material for fabrication of active integrated optical devices. The versatility of the waveguides made in
this material can be enhanced through the creation of efficient permanent
grating structures. Due to the electro-optic properties of the substrate, the
lithium niobate Bragg gratings have the potential to satisfy the rapid tuning
requirements needed by telecommunication components, such as tunable add/drop
filters (ROADMs), tunable dispersion compensators, switches, and so forth that
are used for dynamic control of the signal in the network nodes of agile
all-optical communication networks.In our laboratory, gratings were fabricated in
40 mm long reverse proton exchange (RPE) LiNbO3 channel waveguides
using the femtosecond-IR laser-phase mask approach [14]. Using the setup shown in Figure 1 with an f = 30 mm focusing lens and the 4.28μm pitched phase mask in the two-beam
interference regime and 600μJ pulse energies, grating responses up to -3.5 dB in transmission for exposures at 100 Hz for 1 minute were achieved. The peak field intensity corresponding to the
pulse energies used was 2.0 to 2.5×1013 W/cm2, which is of the same order of magnitude as previously
established IR ultrafast laser ablation thresholds of LiNbO3 [62]. The spectral responses of the
grating are shown in Figures 29(a) and 29(b). The measured Bragg reflection corresponds to a sixth-order resonance for
an effective index of the propagation mode through the waveguide of 2.135.Figure 29
(a) Transmitted spectral response of a lithium niobate grating made with femtosecond IR-radiation. (b) Reflected spectral response.Bragg
gratings made in the LiNbO3 waveguide corresponding to the spectral
responses presented in Figure 29 were inspected visually with an optical
microscope. As can be seen in Figure 30(a), a surface relief corrugated
grating has formed on the waveguide that is a typical ablation effect.
Measurement of the surface relief grating with an atomic force microscope
revealed a 300 nm corrugation depth.Optical microscope image of the grating induced (a) on the surface and (b) below the surface of the lithium niobate waveguide.
(a)(b)In the regions were the ablation process is incomplete, a
secondary grating could be seen a few microns below the crystal surface
displaying a different physical structure as compared to that of the surface
grating (Figure30(b)). The surface and
subsurface gratings are in phase suggesting that they are being created by the
same laser exposure. The surface
ablation threshold is less than that of the damage threshold to the subsurface
material.  The core of the waveguide is
positioned near the surface and within the Rayleigh range of the 30 mm focal
length lens (~20μm). It
is expected therefore that the surface and subsurface regions see the same
threshold intensity. The subsurface
refractive index change does not have the morphology of catastrophic
damage. The surface and subsurface
gratings develop simultaneously. Surface
grating development ultimately arrests the evolution of the subsurface grating
due to scattering of the femtosecond-IR radiation by the surface layer.


## 5.7.2. YAG Fiber Bragg Grating

The femtosecond-IR laser phase mask technique was used to
inscribe Bragg gratings into 100μm diameter yttrium aluminium garnet (YAG) fiber
rod manufactured by MicroMaterials Inc. Using a 4.28-μm-pitch phase mask and 900μJ pulse energies and an f=30 mm focal
length cylindrical lens, a grating structure was observed under the optical
microscope as shown in Figure 31.Figure 31
Optical microscope image of the grating induced in 100μm diameter YAG fiber.


## 6. Conclusions

In this paper, we have summarized our work on femtosecond laser
inscription of Bragg gratings in various materials. The use of high-peak-power femtosecond infrared
radiation and a phase mask for the inscription of Bragg gratings in optical
fiber and waveguides has been shown to be as easy to implement as standard
ultraviolet laser grating inscription techniques. The true advantage of the femtosecond
technique over the ultraviolet laser approach is that Bragg gratings can be
inscribed in many more optical materials that need not be sensitive to
ultraviolet radiation. In fact, to date,
we have successfully inscribed grating structures in any solid material that is
transparent to the infrared radiation. As a result, not only traditional Bragg grating sensor and
telecommunication applications can be realized using our approach but also
applications where grating inscription was formerly not possible. Some of these applications include grating
inscription in exotic waveguide structures such as crystalline sapphire optical
fiber and lithium niobate waveguides, glasses such as pure silica,
borosilicate, actively doped silica, fluoride, phosphate glasses, and photonic
crystal fibers. Depending on the
intensity regime used, high spectral quality gratings can be easily
manufactured for telecommunication type applications such as dense wavelength
division multiplexing or fiber laser cavity mirrors. High reliability gratings can be easily
fabricated by direct writing through the standard protective coatings of
fibers, removing extra processing steps in grating manufacturing. With higher intensities, high spectral quality
gratings can be written that are thermally stable up to the glass transition
temperature of the material into which they are written. Such gratings show potential for high
temperature sensing applications (1000°C). Gratings in sapphire fiber could conceivably
be used in high-temperature sensor applications up to 2000°C.



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*Source: 416251-2008-09-21.xml*