# Single and Multiple Phase Shifts Tilted Fiber Bragg Gratings

Authors: Andrea Cusano; Domenico Paladino; Agostino Iadicicco; Stefania Campopiano; Christophe Caucheteur
Journal: Research Letters in Optics
 (2009)

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## Abstract

The spectral behavior of single and multiple phase shifts tilted fiber Bragg gratings has been experimentally investigated. To this aim, a simple and cost-effective postprocessing technique based on local thermal treatment was used to create arbitrary phase shifts along the tilted grating structure. In particular, UV written tilted fiber Bragg gratings were treated by the electric arc discharge to erase the refractive index modulation in well-defined regions. We demonstrate that these defects give rise to interference pattern for all modes, and thus defect states can be achieved within all the attenuation bands, enabling a simple wavelength independent spectral tailoring of this
class of devices.


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## Body

## 1. Introduction

In the last decades, intense research
on fiber Bragg gratings (FBGs) as strain and temperature sensing elements has
been conducted: however, in order to increase the applicability of this
technology, new research paths have been opened. One of them consists in
studying structures far more complex than the widely used uniform FBG. Among
these, structures such as long-period grating [1], chirped FBG [2], and tilted
FBG (TFBG) [3] have been proposed for various sensing applications. In particular,
TFBGs have been recently investigated for various sensing applications ranging
form external refractive index, bending, and many others [4, 5], providing an
exciting technological platform to be employed in many industrial sectors. In
order to enlarge the potentialities of this class of devices, here, we
investigate the spectral behavior of single and multiple phase shifts TFBGs
fabricated by a low cost and simple postprocessing technique based on local
heat treatment. Different from common phase shift gratings [6], here the
electric arc discharge (EAD) was used as postprocessing technique to locally
erase the grating modulation, producing single and multiple phase shifts
without phase masks or patterned coatings [7] independently on the operating
optical wavelength [8].For the first time to the best of our
knowledge, we report on the spectral characteristics in weakly TFBG with
single- and multiphase shifts focusing the attention on the capability of the
proposed method to reshape and to force defect states in all attenuation bands.


## 2. Experiment

The investigated TFBG has been
manufactured into hydrogen-loaded Corning
single-mode fiber by means of frequency-doubled Argon ion laser emitting at 244 nm [9]. A 1060 nm uniform phase mask was mounted on a rotating stage in order
to apply the desired tilt. The transmitted spectrum of a 1 cm long 3° TFBG measured in
air is shown in Figure 1. TFBGs belong to the short-period grating family (period
of the refractive index modulation ~0.5 μm), and thus their transmitted spectra
exhibit an attenuation band related to the coupling with the contrapropagating
core mode (i.e., the Bragg peak at 1539.665 nm). However, since the index
modulation pattern is blazed with respect to the fiber axis, the coupling to
circularly and noncircularly symmetric cladding modes is enhanced while the
self-backward coupling of the core mode is reduced as compared with uniform
FBGs. According to TFBG theory, discrete resonance bands, related to the coupling
of discrete bounded cladding modes, are achieved for wavelengths shorter than
the Bragg resonance (see Figure 1, cladding mode dips are observed for
wavelengths shorter than 1537.100 nm) [3–9]. The resonance wavelengths depend
on the effective refractive indices of core and cladding modes and on the
grating pitch. An important feature of the weakly tilted FBGs is the presence
of a strong “ghost mode” resonance, immediately to the left of the Bragg
resonance. In Figure 1, it can be observed at 1537.365 nm. The ghost resonance
corresponds to a group of low-order asymmetric cladding modes which are well
confined in the core and less sensitive to the cladding-external medium
interface [5].Figure 1
Transmission spectrum of the
utilized3° TFBG.EAD technique has been already
demonstrated to be a valid low-cost postprocessing tool to induce localized
defects along uniform FBGs for their photonic bandgap engineering [8]. EAD
stage locally erases a well-defined region of the grating creating a
distributed phaseshift or equivalently a Fabry-Perot like structure [8]. In
this work, EAD approach, carried out by a commercial fusion splicer unit
(Fujikura FSM-50S), was applied to TFBGs for the first time to our knowledge.
Experimental training carried out on standard communication fiber allowed us to
determine the optimum processing parameters. As a result, fusion current and
arc duration were manually selected to 17.1 mA and 100 milliseconds, respectively, in
order to locally erase the grating structure while keeping the optical fiber
integrity.To investigate the effect of single-
and multiphase shifts along TFBGs, EAD treatment was repeated at various
precise locations along the grating length as schematized in Figure2. The first
EAD was applied in the grating center, while other EADs were forced at 3/4 and 1/4of the grating length. To this aim, the
grating area was localized with 1 mm resolution by observing the radiated light
when the grating is illuminated by red light.Figure 2
Schematic diagram of the
experimentally realized single- and multidefect TFBG (not in scale).TFBG transmitted spectra were recorded
at each EAD step by a simple optoelectronic setup with a wavelength resolution
of 1 pm involving a tunable laser source (Ando AQ4321D) and an optical spectrum
analyzer (Ando AQ6317C).


## 3. Results

In this section, the effects of single
and multiple phase shifts created by successive EAD steps are investigated with
regards to all the coupling mechanisms exhibited by TFBGs.Figure3 compares the transmitted
spectra of the pristine grating (black solid line) and the structure with a
single phase shift (dashed line). The attention was first focused on the
attenuation bands related to cladding modes bounded at the cladding-air
interface. The specific wavelength range was chosen in correspondence of the
deepest attenuation bands (high-order cladding modes). Here, attenuation bands
related to azimuthally (dips of about −39 dB) and nonazimuthally (dips of
about −29 dB) symmetric cladding modes can be observed. As evident, a single
phase shift is able to produce the formation of an allowed band or defect state
inside the attenuation band associated to each cladding modes. This means that
the local grating erasure forms an interferometric structure for cladding
modes. The principle of operation can be explained as follows from Figure 2. The
TFBG section located at the right of the phase shift region couples light at
specific wavelengths toward contrapropagating cladding modes. These cladding
modes delayed by the phase shift region reach the first TFBG section where
recoupling toward the core mode occurs. A continuous interference can thus be
observed due to the interaction between the direct core mode and part of core
mode due to cladding modes recoupling. The main effect is the creation of
attenuation bands characterized by a single and narrow range of allowed optical
wavelengths. Its location depends on the phase shift amount and thus on the
erased region length and the effective index of the cladding modes.  Besides  the defect state formation, the
EAD stage significantly affects the depth of all attenuation bands too. In the
investigated spectral range (see Figure 3), attenuation bands depth related to azimuthally
symmetric cladding modes passes from −39 dB to −33 dB; whereas those related to
nonazimuthally symmetric modes decrease of about 7 dB.Figure 3
Spectra of the pristine,
single-defect, and multidefect TFBG in the wavelength range involving few
cladding modes attenuation bands.The spectrum of a multidefect
structure is also plotted in Figure3 (grey solid curve). The multiphase shift
mechanism significantly reshapes the grating spectrum. The new coupling regime,
in fact, leads to well-defined optical wavelengths which in turn are able to
propagate without significant losses when compared with the pristine structure.
Moreover, for each attenuation band, we can observe the formation of two weak
dips on both sides of the pristine symmetric and asymmetric resonance bands
characterized by a depth of approximately 15 dB. As matter of fact, the multi-EAD treatment forces a drastic peak-to-peak amplitude decrease from
approximately 30 dB (pristine grating) to 15 dB in the investigated spectral
range. However, we believe that the spectral location and the amplitude of new
dips can be manipulated by acting on the phase shift amount of each defect and defect position along the grating. Hence, from these preliminary results, EAD
represents a simple tool enabling complete photonic bandgap engineering with
regards to the cladding mode response in UV-written TFBGs.Figure4 reports the same analysis as
Figure 3 focusing this time the attention on the Bragg resonance. The spectrum of
the grating subjected to single EAD clearly shows the formation of a 4 dB deep
defect state at 1539.581 nm inside the Bragg band; whereas the modulation on the
short wavelength side was already present in the unperturbed spectrum. The
grating erasure, in fact, acts as a Fabry-Perot cavity for the core mode
reflected from the two TFBG sections. This effect has been widely investigated
and reported elsewhere [8].Figure 4
Spectra of the pristine,
single-defect, and multidefect TFBG in the wavelength range involving the Bragg
attenuation band.Additionally the introduction of multiphase shift along the grating structure decreases the strength of the
unperturbed grating mirrors and induces a multi-interference regime. The former
effect is responsible for a significant amplitude decrease (the band depth
decreases of 8.6 dB) and bandwidth enlargement with consequent loss in
wavelength selectivity. On the contrary, multibeating regime forces the
formation of multiple fringes inside the original Bragg stop-band.Finally, Figure5 shows the spectral
effects due to successive EAD operations on the attenuation band related to the
so-called “ghost mode.” Here, we observe a negligible effect on the spectral
shape of the ghost band during the first two EAD steps. Finally, we observe the
formation of allowed bands also for the ghost band when the third EAD was
created.Figure 5
Spectra of the pristine,
single-defect, and multidefect TFBG in the wavelength range involving the ghost
band.In particular, two allowed bands of 8 dB and 6 dB at 1537.459 nm and 1537.549 nm,  respectively, can be clearly
remarked. The presence of a more complex allowed bandwidth within the ghost
band could be explained by recognizing from [5] that this attenuation band is
related not to a single but to a group of cladding modes well confined in the fiber
core. The leaky nature of the ghost modes at the core cladding interface and
the spatial location of the phase shift section able to produce a defect state
modification in the ghost band are actually under investigation to a full
comprehension of the phase shift mechanism in this case.On the base of these results, the
proposed method seems to work correctly as low cost, postprocessing, and
wavelength independent tool capable to completely reshape the spectrum of
weakly TFBGs. However, it is reasonable to believe that spectral signature can
be ad hoc tailored by acting on number, distribution, and size of the heat
treated regions with benefices in practical applications. For instance, finer
spectral features lead to advantages in terms of lower resolution when the
device is interrogated by using wavelength shift measurements. The
core/cladding mode recoupling and the consequent interefernce regime, instead,
could be exploited in new configurations to enhance the sensing performances of
this class of devices. For instance, for chemical application where the small
changes in the surrounding refractive index need to be monitored, the defect
states should exhibit a relative shift inside each cladding mode band with
consequence enhancement in sensitivity against the refractive index as compared
to the cladding mode dips shifts. However, experimental investigations on this
topic are currently in progress. Additional, the core/cladding mode recoupling
regime due to localized perturbations [8] could be involved to improve the
performance of new sensing scheme based on reflected light monitoring [5].Also, spectral features tailoring
could enable new filtering functionalities for communication applications.


## 4. Conclusion

The spectral characteristics of single-
and multiphase shift weakly tilted FBG have been investigated. EAD was used as
low-cost tool to achieve local and distributed phase shift sections along the
grating. Spectral modifications involving cladding modes dips, the Bragg band,
and also the ghost resonance have been observed, demonstrating the possibility
to tailor the spectra of this class of devices by using simple postprocessing
and wavelength independent fabrication. On the basis of these results, we
believe that playing on defects number and shape, their longitudinal
positioning, new all fiber photonic devices can be achieved and optimized for
specific applications.



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*Source: 481010-2009-03-30.xml*