Fiber Bragg Grating sensors for harsh environments

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FACOLTÀ DISCIENZE MATEMATICHE, FISICHE E NATURALI

Corso di Laurea Magistrale in Fisica

M

ASTER

’

S

T

HESIS IN

P

HOTONICS

F

IBER

B

RAGG

G

RATING SENSORS FOR

HARSH ENVIRONMENTS

Candidate

Massimo M

ORANTE

Supervisors

Prof. Alberto D

I

L

IETO

Ing. Tiziano NANNIPIERI

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iii

Introduction

In the last few years an increased demand in the sensor market has been occurred and nowa-days the interest in harsh environment sensing application represents the driving force for the development of novel sensing elements based on fiber optic. Indeed, there are several industrial sectors (Oil&Gas, Aerospace and Automotive are few examples) that can benefit from the adoption of optical fiber based sensing systems to measure physical parameters such as temperature, deformation, pressure and humidity (as shown in Fig.1.1). Moreover, there are several requirements for the sensing element such as size, weight, electromagnetic immunity and very high temperature environment making the most common electronic sen-sors on the market useless.

FIGURE1.1: Examples of harsh environments for which there is a high

demand for innovative sensors. (Source: [1])

In this work, limits and the improvements about fiber optic sensors based on Fiber Bragg Grating Sensor (FBGS) subjected to high temperatures have studied in detail. The Fiber Bragg Grating (FBG), in its simplest form, is a permanent periodic refractive index modula-tion inscribed in the optical fiber core exploiting photosensitivity of optical fiber. FBG based sensors exploit the presence of a resonance condition for which they reflect incident light at the so-called Bragg wavelengthλB and this quantity can be monitored and traced back to

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parameters such as temperature and deformation of the FBG and therefore also the area of placing.

To give a brief idea of the capabilities of these photonic devices, the Bragg peak wavelength

variation (∆λB(∆T, ∆ϵ, ...)) is about 10pm each variation of 1◦C and about 1.2 pm for each small mechanical deformation of 1 µm/m. Therefore, using an appropriate interrogation equipment, it is possible to make sensors based on FBG with excellent sensing properties and apply them in several fields.

FBG 2 FBG 3 FBG 1 PI( ) T ( ) R( ) B,1 B,2 B,3 B,1 B,2 B,3 B,1 B,2 B,3 INPUT REFLECTED SIGNAL TRANSMITTED SIGNAL core cladding n3 n1 n2 n2 ncore n2 n3

z

FIGURE1.2: Principles of operation of an FBG. The inset figure shows a

sim-plified structure of an FBG inside an optical fiber

1.1 The project

This work has been carried out in collaboration withINFIBRATECHNOLOGIESSRL, an Italian company based in Pisa, focused on the engineering of innovative fiber optic sensors systems and their exploitation in industrial field. This work atINFIBRATECHNOLOGIESSRLallowed

me to understand the reality and the actual market demands for the FBG based fiber optic sensors. It has been noted that, for example, one of the major shortcomings in the market is a

stable dynamic strain measurement based on FBG sensor in high temperature environments. In fact,

the major manufacturers of FBG strain sensor gauge guarantee a maximum operating tem-perature of about + 80◦C and only a few manufacturers guarantee a maximum temperature not exceeding 150◦C-200◦C [2].

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• Analyze the behavior of commercial and low-cost FBGs beyond their operating

tem-perature limits and the physical principles describing their behavior. Hence, exploiting high temperature processes to define a method that improves the long-term stability of FBGs.

• Analyze and experimentally demonstrate the feasibility of FBG based optical fiber

sen-sor for dynamic strain measurements withstanding high temperatures.

Before investigating the above tasks, it is first necessary to provide the basis and the tools to understand how an optical fiber is made, how it works and what are the physical mecha-nisms involved (CHAPTER2). In theCHAPTER3an useful overview of FBG technology will be given to understand the physics of these photonic devices, how they are fabricated, the classification and the principles of operation as sensors of temperature and deformation.

Then in the CHAPTER 4, trying to address the first task, experimental tests have been carried out to observe the behavior of standard and no-standard FBG at temperatures much higher than their operating limits (tests up to 600◦C - 1000◦C). This allowed to observe and characterize mainly two thermal-optical processes for FBGs and fibers: the thermal annealing and the thermal regeneration. We did not try to do a deep study of these two phenomena, but the goal was to characterize and exploit them to bring beyond the operational limits of FBG naturally designed for low temperatures. CHAPTER 4 also includes the experimental

techniques that were used to interrogate FBGs: this is a fundamental part of the sensing and monitoring process, used for all subsequent experimental tests.

In the second part of this work, a possible simple strain sensor design for high tem-peratures was studied and tested. In CHAPTER 5 all the materials used and the assembly

procedures are then reported. The main objective was to observe what kind of problems can occur in this type of strain sensors at high temperatures (over 200◦C) trying to use the advantages of FBGs written in highly birefringent optical fiber. FBGs written in this type of fibers seem to be promising in the world of optical fiber sensors for the ability to monitor two signals simultaneously from the same sensor and link them, depending on the design of the sensor’s packaging, to the transversal deformation of the FBG or to the applied pres-sure. InCHAPTER5the design and construction of the high temperature sensor deformation apparatus is also reported. In fact, it was necessary to design a specific set-up for sensing element characterization within the laboratory furnace and according with the interroga-tion system. This set-up made it possible to apply static and dynamic strain to the FBG at different equilibrium temperatures. Here, the main difficulties were to ensure a correct con-version between the nominal deformation value and the real deformation induced to the FBG through the components of the set-up.

In the CHAPTER 6 the main results and analysis of the tests carried out on the sensor have been reported.

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In theCHAPTER7 CONCLUSIONS AND FURTHERRESEARCH, the main results and diffi-culties of this work have been summarized. Furthermore, a brief outlook for future research, possible improvements and applications is also reported.

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5

Chapter 2

Optical Fibers and Sensing

Technologies

Fiber Bragg Gratings are photonic devices designed and realized inside optical fibers and in this Chapter, all the fundamental and necessary concepts concerning optical fibers and their applications are provided. This will allow to understand how and which radiation it’s possible to guide inside a fiber, the materials involved and the manufacturing processes, which play a key role in the writing and thermal decay processes of FBGs.

2.1 Overview

ncore nclad ncoat a b r CLADDING COATING CORE CORE CLADDING COATING

FIGURE2.1: Cross section, transverse and refractive-index profile for a step-index fibers. (Adapted from [3])

The technological revolution of telecommunications has occurred thanks to the develop-ment of optical fibers during the last 40 years. An optical fiber (Fig.2.1), in its simplest form, consists of a circular dielectric waveguide and acts as a transmission cable that drives the light beam for kilometres away with a low signal attenuation (up to∼0.2 dB/km at telecom range) and guarantees a high transmission data rate (> 100 Gbits/s). The very low-loss propagation of confined optical modes was predicted [4] and realized [5] at the end of the

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sixties and the concentration of the absorbing impurities has been significantly reduced thus opening up the possibility to use optical fiber for optical communication purposes.

As shown in the Fig.2.1, a common type of optical fiber consists of a cylindrical core sur-rounded by a cladding that has a slightly lower refractive index than the core. Both core and cladding are based on fused silica materials, a glassy and amorphous form of silicon dioxide SiO2. The first jacket that surrounds the cladding is called coating, a protective layer based on materials according to the specific requests of the fiber’s application [6]. The most com-mon coatings are made of polymeric materials such as acrylate, polyimide or even metallic coatings, for example, aluminum, copper or gold. As shown below, the coating has a fun-damental role in the mechanical characteristics and thermal expansion of fiber portions, in particularly when the fiber hosts a FBG sensor.

n₂ n₁ θmax π 2−θc θc θc θc θc

FIGURE 2.2: Light confinement in a step-index fiber. Rays for whichθ < θc are refracted out of the core

The increase in the refractive index of the core occurs through a doping of fused silica, based on compounds of Germanium (GeO2) or Phosphorus (P2O5). The simplest and most common optical fiber design is a step refractive index profile (called step-index fiber) in which the cladding has a refractive index equal to that of pure fused silica (nclad≃1.45 at 1550 nm), while the core, usually doped with GeO2, has a refractive index greater than cladding about ∆=ncore−nclad ≃4·10−3[3] (in Fig. 2.1the values of ncoreand ncladare exaggerated) .

This slight difference∆ enable a fundamental phenomenon for the operation of optical fibers: the total internal reflection (TIR). The TIR allows light to be guided and remain effectively confined within the fiber, according to the geometrical-optics theory [7]. In fact, by easily applying Snell’s law to the diagram shown in the Fig. 2.2, it is possible to derive which is the critical angleθc so that the radiation remains confined within the core and is not dispersed through the cladding. In practice, a dimensionless parameter is defined that quantifies the maximum angle of acceptance of the guided radiation, called the Numerical Aperture (NA).

NA≡sinθmax= √

n2

core−n2clad (2.1)

The value of NA is very important because it permit to quantify also, given the transmitted wavelengthλ and the radius of the core a, the type of guided radiation. Indeed, within an optical fiber are guided different optical modes together, as a direct solution of the Helmotz equation in cylindrical coordinates (LP modes, as shown in Fig2.3). At a fixed wavelength

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2.1. Overview 7

λ, the radiation could propagate with different spatial distributions of the electric and

mag-netic field in the medium depending on the boundary conditions of the system (such as NA and a, the diameter of the core).

FIGURE2.3: Examples of complex amplitude profiles of LP modes (Adapted

from [8])

The optical fibers could therefore be distinguished in Single Mode Fibers (SMF), gener-ally with a core diameter of about 4-10µm, or Multi Mode Fibers (MMF) with 2a≃100µm. Only one mode is propagated when the dimensions of NA, λ and a respect the following

single-mode condition, according to the solutions of the wave equation in the fiber:

V≡ 2πa_λ NA<2.405 (2.2)

To better clarify, the type of optical fiber used during all the experiments reported in this work is SMF and the radiation used to interrogate the FBGs isλ≃ 1550 nm, the core diam-eter of the fiber is about 2a = 9µm and the numerical aperture is NA=0.14 [9]. With these values, the parameter V ≃ 2.37 < 2.405 and therefore only one mode is supported within the fiber.

Not all electromagnetic radiation is guided in the same way inside an optical fiber. In fact, even if the dimension of V is optimal, there are some bands of the optical spectrum that are more attenuated than others. Inside the fiber different absorption processes occur, such as Rayleigth scattering (A∼1/λ4), IR and UV absorption, losses due to the waveguide defects, absorption due to resonances with the spectra of OH− defects and other processes. All these losses define the loss spectrum of the fiber optic, as shown in the Fig.2.4for a SMF. There is a particular band at 1550 nm where the attenuation is particularly low and with a flat shape, without resonances peaks (green band in Fig. 2.4. As a matter of fact, this is the most widely used band in long-range telecommunications and trans-oceanic submarine network cables, in which a very small loss of gain must be guaranteed (∼-0.2 dB/km). This frequency range is called C-band (also know as erbium window) and it is conventional defined

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0.8 1.0 1.2 1.4 1.6 1.8 1 0.01 0.05 0.1 0.5 5 10 50 100 Rayleigh scattering UV absorption Waveguide imperfections IR absorption SMF experimental Wavelength (µm) Los s (d B /k m ) _absorptionOH C-BAND

FIGURE2.4: Spectral loss profile of a single-mode fiber, several fundamental loss mechanism is shown (Adapted from [3])

as the wavelength range from 1530 nm to 1565 nm. All the optical fibers used in this work have working wavelengths centered around 1550 nm and therefore also the FBGs used have a Bragg peak that falls in this range.

2.2 Fabrication of Optical Fibers

The manufacture of optical fibers is a key passage in the understanding of the materials involved in the various physical processes, in particular as regards the writing processes of FBGs. In fact, during the manufacture of optical fibers several defects are formed in the fused silica glass matrix, which actually allow the production of FBGs, as shown in theCHAPTER3.

The production of optical fibers involves two stages: the first stage consists in the real-ization of the preform (Fig.2.5a) and the second consist in the drawn into an optical fiber (Fig.

2.5b) by a process that feeds the preform into a furnace at the proper speed and by the right heat and curing treatments.

The preform consists, at the begin of the treatment, in a tube of fused silica material of about 1 m length and diameter of a few centimeters. The preform is heated with a burner torch to a temperature of 1800◦C and filled with SiCl4 and O2 in gas form: this mix reacts and produces layers of fused silica SiO2 deposited in the inner walls of the tube. The fiber cladding is grown with this process and, to keep the desired refractive index under control, fluorine can be added to the tube (the tubes are shown in the Fig. 2.5a). Once the cladding has reached the desired thickness, it begins to add GeCl4gas which, reacting with the oxy-gen O2, create a deposit of doping material which increases the refractive index and form

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2.3. Birefringence and Polarization Maintaining Fibers 9

(A) MCVD process commonly used for cylindri-cal preform fabrication (Source: [10])

(B) Tower draw (Source: [11]) FIGURE2.5: The two main production stages of optical fibers. Manufacture

of the preform (2.5a) and spinning of the fiber through the draw tower (2.5b)

the core. By doping with P2O5, the refractive index of the fused silica can be decreased to create optical fibers with more complex refractive index profiles. Once the desired core thickness has been reached, the torch temperature is increased until the preform collapses and becomes a sharp pointed rod. The preform is then transferred to the drawn tower (Fig.

2.5b) where it is heated at the melting point of 2000◦C and drawn the melting preform into optical fiber with the appropriate external diameter. A coating layer is applied as the fiber is passed through a coating cup in the drawn tower and then it is cooled and treated with UV rays.

It is important to underline that during the growth processes of the cladding and of the core in the preform the very high temperatures cause violent reactions between the reagents and the vitreous matrix of the fused silica preform. It is not possible to have a 100% reac-tion, so the deposited chemicals have a proportion of suboxides and defects within the glass matrix [12]. The same thing happens during the melting and cooling process in the drawn tower. In particularly, in the core region the bond between oxygen, silicon and dopant-element (like germanium) does not occur correctly but through several meta-stable configu-rations [13].

In the next Chapter, the link between these defects and the formation of the FBGs is explained.

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2.3 Birefringence and Polarization Maintaining Fibers

The core of an optical fiber, if perfectly uniform and homogeneous, can support linearly polarized modes both long x and long y directions(the transverse directions of the fiber). The refractive index is the same in any transverse directions x, y along the z direction of the fiber and in this case the modes are degenerate. This degeneration is removed if there is a

non-uniformity of mechanical stress in the core along z, given that a variation of mechanical

stress is associated with a change in the refraction index, as a consequence of photoelasticity in the fiber (for details APPENDIXA). A change in the refractive index in the two directions x and y breaks the degeneration between the two modes, separating them temporally [3]. The magnitude which describes this difference in uniformity between the index of refraction along x and y is called birefringence of the fiber, defined as

B≡nx−ny (2.3)

In a standard fiber, birefringence is very low and has a value of B ∼10−7. There are instead optical fibers in which can be very high (B ∼ 10−4) and are called high-birefringence (Hi-Bi) fibers, used in part of this work. This value of B is obtained thanks to the presence of Stress Applying Parts (SAP) in the cladding that induce permanent mechanical stresses and therefore a permanent high birefringence along the fiber. Examples of these fibers are the

panda fiber and the bow-tie fiber (Fig. 2.6a), in which there are stressful elements between the core and the cladding with different geometries.

Stress Applying Parts (SAPs)

Bow-Tie Panda

(A) Cross section of a bow-tie (left) and a panda

fiber (right) BEAT L ENGTH FAST MODE SLOW MODE

(B) State of polarization in a Hi-Bi fiber

(Adapted from [3]) FIGURE2.6: Example of SAPs in Hi-Bi fiber’s cross sections (A) and

propaga-tion of radiapropaga-tion inside a Hi-Bi fiber (B)

Hi-Bi in optical fibers allows to define a phase shift between the two modes that propa-gate along x and y and therefore, if the radiation is launched perfectly linearly polarized, it remains linear, if instead it is launched with a general polarization state, it passes from

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2.4. Fiber Sensing Technologies 11 linear, elliptical and then again linear periodically (as illustrated in the Fig. 2.6b), forming

beats in the fiber of length

LB =λ/B (2.4)

LB is called beat length, and the propagation axis in which the index of refraction is smaller is called the fast axis (y) and in which it is greater slow axis (x).

The fibers used in part of the experiments inCHAPTER5andCHAPTER6are panda fibers

with a B∼3·10−4interrogated atλ∼1550 nm and the beat length is then LB ∼5 mm.

2.4 Fiber Sensing Technologies

Telecommunications are not the only applications of optical fibers and, for over forty years, optical fibers are used as sensor in various engineering and research fields. The sensing designs are not based on a single concept, but on a variety of optical phenomena that can be used to measure several physical and chemical parameters [14], as temperature, strain, pressure, EM field and also bio-chemical parameters.

Based on their topology and configuration, fiber optic sensors can be classified as

single-point, multipoint (quasi-distributed), or distributed.

In a single-point sensor, the sensing portion of the fiber is typically located at a specific spatial point and it’s possible to monitoring one or more parameters of this point. In a

quasi-distribuited fiber optic sensor two or more sensing regions along the length of a fiber are

monitored at the same time and these sensing regions can be physically spaced from a few millimeters to several meters away, depending on the requirements. FBGs are fiber-optic sensors that can fall into either of two categories: in one optical fiber there can be placed one or an array of FBGs. A distributed fiber optic sensor, on the other hand, bases its sensitivity by exploiting the entire length of the optical fiber. It is a unique capability of optical fibers, in which the infrastructure itself is used as a sensitive element and the sensitive measure-ment length can even reach tens of kilometers. Distributed fiber optic sensors are generally sensors that are based on effects related to scattering phenomena within the fiber, such as Raman scattering and Brillouin scattering. This set of techniques are called Distributed Tem-perature Sensing (DTS) or Distributed Temperature and Strain Sensing (DTSS).

It is important to define the multiplexing technique of optical fiber sensors. By

multiplex-ing we mean the techniques and the mechanism of how the optical signals from sensors are

combined with each other within the same optical fiber. The multiplexing techniques are divided into three following different categories:

• Space-Division Multiplexing (SDM): one fiber host one sensor and this is monitored

choosing every fiber channel using a common optical switch, as shown in Fig. 2.7. This multiplexing technique is the most costly in terms of installation and the avail-able bandwidth is not fully utilized.

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RECEIVER LASER SOURCE t t t t t1 t2 t3 t4 RECEIVER LASER SOURCE 3 1 2 4

WDM Wavelength Division Multiplexing

OPTICAL SWITCH

RECEIVER LASER SOURCE

SDM Space Division Multiplexing

TDM Time Division Multiplexing

FIGURE2.7: Schematic diagram of the SDM technique

• Time-Division Multiplexing (TDM): it is based on the ability to separate the response of

a sensor from another in the time domain, thanks to a pulsed source and a broadband reception system (Fig. 2.8). This interrogation technique is ideal for monitoring DTS-based fibers or even FBGs made with the same wavelength. This technique requires a powerful temporal resolution capability to reach high distances and the required resolution (both in quasi-distributed and distributed fiber optic sensors).

RECEIVER LASER SOURCE t t t t t1 t2 t3 t4 RECEIVER LASER SOURCE 3 1 2 4

FIGURE2.8: Schematic diagram of the TDM technique

• Wavelength-Division Multiplexing (WDM): this technique is based on the ability to solve

different wavelengths simultaneously, therefore, as can be seen in the Fig. 2.9, in the same optical fiber it is possible to placed a large number of sensors. Care must be taken to avoid band overlap between the various sensor spectra. The WDM is the most used interrogation technique for FBG sensing solutions and, in this work, during all the experimental tests, the FBG’s interrogators used are WDM based.RECEIVER

LASER SOURCE t t t t t1 t2 t3 t4 RECEIVER LASER SOURCE 3 1 2 4

FIGURE2.9: Schematic diagram of the WDM technique

The node that connects the optical fiber of the input laser and the output fiber of the receiver is called optical circulator. This passive optical device is widely used in photonic

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2.4. Fiber Sensing Technologies 13 applications and allows the optical signal to be guided from one port to another and in only one direction. LASER PD FBG 1 2 3

FIGURE2.10: Schematic diagram of a 3-port circulator

The basic configuration of the optical circulator consists of three ports. Referring to the Fig. 2.10, in a 3-port circulator a signal is transmitted from port 1 to port 2, another signal is transmitted from port 2 to port 3 and, finally, a third signal can be transmitted from port 3 to port 1. In practice, one or two ports are used as inputs and the third port is used as the output. This allows, in the case of interrogation techniques, that the reflected radiation is correctly acquired by the receiver and avoids perturbations induced by the reflected light coming from the FBG.

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15

Chapter 3

Fiber Bragg Grating: theory and

technology

In this Chapter, the theory, fabrication and the State of the Art of FBGs are investigated. After a brief definition of optical gratings and Fiber Bragg Gratings, the physical phenom-ena for which it is possible to induce and write gratings within fibers are explained.

Different writing methods, based on different physical principles, allow the production of numerous types of FBGs for a wide range of applications.

Finally, defined the physical principles, the spectral properties of FBGs are explained using the couple-wave theory. This analysis allows to quickly understand the major spectral pro-prieties of an FBG and how it is possible to exploit these results to develop temperature, deformation and other type of sensors.

3.1 Bragg Gratings and Fiber Bragg Gratings

FBGs andFBGSs are periodic structure inside an optical fiber and, to carry out an electromag-netic analysis of the propagation of optical wave inside this kind of structures, it’s necessary to study some basics scattering and kinematic properties of gratings, in general. This basic kinematic proprieties are fundamental to all waves and to all kind of period structure [15] and a common starting point for the model of these structures is the Bragg law.

The Bragg’s law describes the constructive interference condition of the interacting radia-tion with a periodic structure and, by the conservaradia-tion of energy and momentum, it could be expressed in the form in Eq.3.1.

2k sinθ= mkG (3.1)

Where θ is the incident angle, m is an integer and represent the diffraction order, k is the

wavenumber of the incident light beam k=2πn/λ, kG= 2_Λπ is the grating wavenumber and n is the index refraction of the grating material (Figure3.1).

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first order (m=1) and the3.1becomes:

λB =2nΛ (3.2)

This equation directly provides a qualitative estimate of the wavelength reflected by an FBG, givenΛ the grating period and the characteristics of the medium (represented by the index refraction n). Bragg grating is essentially a narrow band filter and the Eq.3.2gives the central

✓i ✓r ✓t 0 +1 +2 +3 -1 -1 0 +1 +2 +3

⇤

(A) Sketch of interaction of the light beam with a Bragg grating

m = 1 m = 0 ✓1

✓2

(B) A simplified scheme of the radiation guided in an optical fiber that interacts with a FBG FIGURE3.1: Examples of Bragg gratings

wavelength of this filter, but there is no information about the bandwidth and the shape of

the spectrum: these details are provided by the couple-wave theory applied to the FBG system, briefly explained in the Sec. 3.4

For waveguides and also gratings inscribed in waveguides, the index of refraction n is not just a material property, but also a quantity that depends strictly on the design of the waveguide and on the propagating modes considered. For this reason, for an optical fiber grating an effective refractive index neff is defined, which is calculated numerically according to the design of the waveguide.

To give an useful example, for a grating inscribed within the core of a SMF, the effective refractive index seen by a the propagating light of λ = 1550 nm is neff ≃ 1.45. Thus, ac-cording to the Eq. 3.2, the grating periodΛ for a reflected λB falling into the C-band (1530 nm-1565 nm) is usually∼500 nm.

3.2 Photosensitivity in Optical Fibers

FBGs are written inside optical fibers by numerous techniques which allow to induce con-trolled variations of refractive index along the axial direction of the fiber z. By writing with a suitable refractive index profile neff(z)withΛ period, it is possible to create FBGs with a very high reflectivity peak.

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3.2. Photosensitivity in Optical Fibers 17 In this section, the main physical phenomena which allow to permanently modify the refrac-tive index of fused silica are reported.

The reasons of the formation of grating within optical fibers are not yet fully understood [12], but the scientific community agrees that the two main phenomena responsible for the change of the refractive index are the photosensitivity of defects in germanosilicate fibers and/or non linear processes such as Multiphoton Ionization Process (MIP) and laser-induced damages. It is useful to clarify that in the case of photosensitivity, grating is just a variation of refractive index, while in the case of damage processes, the FBG is a physical corrugation (and therefore also a refraction index variation), observable with an optical microscope as shown in Fig.3.7.

Historical perspective

The first FBG was created by a Canadian research group by B. S. Kawasaki and K. O. Hill in the 1978 [16] [17]. This group was studying non-linear optic effects on a Ge-doped silica fiber with an Argon visible light source (∼488 nm), as reported in the original scheme of the apparatus (Fig.3.2).

FIGURE3.2: Historical experimental apparatus in which the first fiber grating was created in the 1978 (Source: [16])

The laser was distributed and was steady oscillating within the fiber. Hill et al. noted, by monitoring the output signal spectrum, an increase in reflectivity/lowering in trasmittivity at the oscillation frequency of the laser (488nm). In about 10 minutes the reflectivity passed from 4% to over 50% as shown in Figure3.3.

The most surprising thing was that this effect was permanent over time and they didn’t observe any decay in reflectivity for several days. The bandwidth of the reflectivity peak was very narrow (∆ν ∼ 200MHz) indicating a grating length of ∼1m. This was the first experiment that led to the production of FBGs and special optical fiber filters.

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FIGURE3.3: Back reflected light in time from the fiber in the experiment sets in Figure3.2. Insets show respectively typical reflection and transmission

spec-tra of the fiber. (Source: [17])

Subsequent experiments were performed [18][19][20] to understand the origin of the phenomenon observed in [17] and other filters were also made with 242-248 nm lasers or doubled, which soon suggested that the phenomenon of the formation of the refractive index modification is linked to a band of UV spectrum close to 5 eV.

Most of the scientific community suggested the theory that link the refraction index mod-ification to a UV absorption mechanism related to the Germanium defects contained within the fiber and with absorption band close to 5 eV (∼248 nm). A second theory instead links these refractive index changes to stress induced by multi-ionization of matter.

Nowadays it is believed that the two theories are both valid and closely related to the type of writing of FBG.

Defects in Optical Fibers

As mentioned in CHAPTER 2, optical fibers are made of fused silica (an amorphous form of SiO2). During the fabrication of the preform, inside the fiber core, germanium doping replace silicon and this process, although well controlled, give rise to a large number of

defects within the vitreous matrix. Defect means an anomalous configuration of the bonds

between Silicon and Oxygen or dopant (Germanium) and Oxygen.

The defect connected to the 244nm band, as noted above, is believed to be the GeO defect (or Germanium Oxygen Deficiency Centers (GODC)), a germanium atom coordinated with another Si and Ge atom [21]. A representation of this defect is shown in the Figure3.4b, in comparison with the ordinary bond of the doped fused silica in Figure3.4a.

Under UV illumination, the bond in the Figure3.4bbreaks, creating a new meta-stable bond called GeE’ center. It is thought that the electron from the GeE’ center is liberated and is

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3.2. Photosensitivity in Optical Fibers 19 O O O O O O O O O O O _O O O O O O O O O O O O O Si Si Si Si Si Si Si Si Ge

(A) Representation of an ordinary chemical bond between germanium and oxygen in a

preform of an optical fiber

+

h⌫

GODC Ge(3) or GeE’ hole center

Ge Si O O O O O O Ge O O O O O O Si

(B) Photon absorption of GeO defect (Adapted from [12])

FIGURE3.4: Representation of photosensitivity mechanism

free to move within the glass matrix via hopping or tunneling, or by two-photon excitation into the conduction band. This electron can be retrapped at the original site or at some other defect sites, as also noted by [22]-[23]. The removal of this electron, it is believed, causes a reconfiguration of the shape of the GeE’ center, possibly also changing the density of the material, as well as the absorption spectrum [12].

In fact, a variation of the absorption spectrum of the optical fiber exposed to UV radiation was measured by [20] (Figure3.5a) and, in terms of the Kramers and Kröning’s theory, this change in absorption spectrum is related to a local change of refractive index [24], as shown in Eq. 3.3. This new configuration of writing defects so created are also called drawing-induced defects (DID) [25].

∆n(λ) = 1 2π2 P (_∫ +∞ 0 ∆α(λ′) 1− (λ′/λ)2dλ ′ ) (3.3)

Furthermore, there is a more effective way to enhance the photosensitivity up to∆n ∼ 10−2_{, called hydrogen loading or hydrogen diffusion. The fiber, before writing the grating, is} commonly placed in an high pressure H2-atmosphere (>150 atm) and at a temperature lower than 100◦C [27]. After the writing, an increase in the absorption at the∼244nm UV band, related to the GODCs, is observed as measured by [26] [28] (Fig. 3.5b). It is assumed that the high pressure allows the hydrogen gas to penetrate in the vitreous matrix and, given the significant concentration of Ge-O-Ge bonds, the H2 reacts with these bonds resulting in the formation of Ge-OH, which absorbs UV radiation and therefore increasing the pho-tosensitivity and/or the internal stress [29]. However, the writing of gratings in hydrogen

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(A) Attenuation spectrum of an optical fiber be-fore (solid line) and after (dashed line) to UV ex-posure (242nm). It is evident a decrease of attenu-ation (dot line) due to an absorption change in the

240nm band. (Source: [20])

(B) Optical absorption spectra of a germanosil-ica preform rod heated in a H2-atmosphere at

500◦C for different exposure times [26]

FIGURE3.5: Attenuation spectrum before and after the UV exposure (left) and the effect of hydrogenation on the H2spectrum

loaded fibres undoubtedly involves multiple mechanisms which are still investigated in lit-erature. It has been noticed that the hydrogenated gratings are much more stable and resistant in temperature [30].

The change in the refractive index is positive and is often referred to as Type I index change and then to Type I FBGs. The FBGs used in this work were written by exploiting this type of photosensitivity in a germano-silicate fiber preloaded with hydrogen, as stated by the manufacturer. In theCHAPTER4, the behavior of these FBGs in temperature and the effects

on the modification of the refractive index are measured.

Multiphoton Ionization Processes (MIP) and grating damage

By increasing the flow of photons that affect the fiber it is possible to realize FBGs also outside the mentioned UV band, obtaining gratings with excellent spectral properties and resistant to high temperatures, compared to the Type I FBGs.

With sufficiently intense radiation it is possible to induce several linear and non-linear light-matter interaction processes, such as avalanche ionization processes of the material from the va-lence and conduction band, tunneling phenomena and absorption of free carriers [31] (as schema-tized in Figs.3.6a-3.6b). The intensity of the radiation is usually achieved thanks to the use of pulsed lasers with I >1013W/cm2. This intensity involves the production of free electrons and absorption results in a highly localized deposition of energy into an electron plasma that is formed within the focal volume of the radiation. The energy of the laser-excited elec-tron plasma is then transferred to the lattice of the bulk material after several picoseconds, which then leads to material modification that is permanent. This modification can be in the form of material compaction/densification, the creation of macroscopic defects, localized melting,

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3.3. Methods to induce FBGs 21

(A) Multiphoton and tunneling ionization generates free electrons

(B) Free electrons absorb radiation and impact-ionize surrounding mate-rial resulting in avalanche ionization FIGURE3.6: Schematic of free electron plasma formation with high intensity

pulses [31]

nanograting or microvoid formation. Which type of modification that is created depends on

the I of the laser pulse and the writing technology. These types of gratings thus formed have a higher temperature resistance, very close to the melting temperature of the fused silica (tg∼1700◦C).

FIGURE3.7: FBG fabricated with a point-by-point technique (Source: [32])

FBGs based on these phenomena are called Type II FBGs and, specifically, many FBGs are written with femtosecond pulsed lasers at 800 nm radiation and are called fs-IR FBGs gratings or Type II-IR.

3.3 Methods to induce FBGs

In the last decades, several FBGs writing techniques within optical fibers have been devel-oped [33]. This section illustrates the historical and most used writing techniques, based on the physical principles described in the previous section.

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Internal writing

Historically the first gratings were written through a radiation injected directly into the fiber to obtain stable patterns of UV radiation which, thanks to the photosensitivity, has induced the desired refractive index variation (Fig. 3.2). This type of technique is also know as

internal writing and the grating is traditionally called Hill grating, discovered by Hill [17] and Kawasaky [16] in 1978. This technique is limited to the frequency of the laser used and to the UV-defects photosensitivity band. In fact, these kind of techniques were soon abandoned and the research has focused on an FBG external writing that allows to choose the reflected Bragg wavelength and to produce different filters and FBGs.

UV radiation fiber inte rfer ence pat tern UV mirror 50% beam splitter 50% beam splitter

compensating platecompensating plate

cylindrical lens cylindrical lens

2'

UV mirror

FIGURE 3.8: Holographic technique for UV writing FBGs (Adapted from [12]/[18])

Holography

The first external writing technique is the holographic technique and was demonstrated, for the first time, by G. Meltz in 1989 [18] with a interferometer set-up similar to the one shown in the Fig. 3.8. The effectiveness of this method, in addition to allowing a transverse writing, is the possibility of writing gratings at wavelengths chosen independently from the writing

λUV, varying the mutual angleφ of the two UV beams, as

λB(φ) = neffλUV

nUVsin(φ)

(3.4) Where nUVis the refractive index of fused silica in the UV band.

Both internal and holographic methods have been largely superseded by the diffusion of the

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3.3. Methods to induce FBGs 23

Phase Mask

(A) Phase mask technique (Source: [34]) (B) Example of a phase mask widely used in the industrial

pro-duction of FBG (Source: [35])

A phase-mask is a diffraction grating used to split a single laser beam into several diffrac-tive orders. In particular, a FBG’s writing phase mask is a relief grating etched in a silica plate. This type of component is very similar to a photo mask used in the semiconductor wafer industry. As shown in Figures3.9a,3.9b, a UV laser beam is directed on the phase mask, placed in front of the fiber without coating, ready for the writing. The laser beam is perpendicular to the phase mask and the fiber. The radiation is then diffracted in several diffractive orders, in particular the order +1 and -1, interfering with each other, originate an interference pattern on the bare part of the fiber. The order 0 is suppressed through a direct block of the radiation or through a suitable optic set-up. Through photosensitivity [36] of the fiber or even exploiting pulsed lasers and MIP processes [37], it is possible to write very pre-cise gratings without high technical attentions. The major drawback of this technique is that for each phase mask only oneλB is associated, since the pitch of the mask Λpm univocally determines the pitch of the written FBGΛ as Λ= Λpm/2.

Point-by-Point writing

The point-by-point writing technique is one of the most precise and versatile technique to pro-duce gratings resistant to high temperatures. As shown in the Fig. 3.10, a femtosecond pulsed IR laser with high intensity is focused on the fiber core through a microscopic lens. The radiation cause a compaction or a controlled damage on the fiber core, as described in the previously Section. The alternation of voids and areas of non-exposure, constitutes the grating within the fiber, as already shown in the Fig.3.7. It is essential to use a very precise translation stage, which allows very fine shifts of the fiber. The FBGs thus created have a very high thermal resistance [38] because the degradation of the grating occurs only if the

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fused silica softening temperature is reached tg>1600◦C. With the right focus technique, it is possible to write FBG even without removing the coating from the fiber.

fs-laser shutter PC shutter controller translation controller microscope objective fiber translation stage FBG

FIGURE 3.10: A typical set-up for point-by-point writing technique (Adapted from [38])

3.4 FBG’s spectral theory

As anticipated in the section3.1, it is possible to derive an analytical expression of the spec-tral shape of an FBG, depends on its spatial distribution of the effective refractive index. This model is based on the Couple Wave Analysis (CWA) and leads to interesting results that allow to correctly correlate spectrum changes to macroscopic parameters of FBG, such as the length L of the FBG and its refractive index variation shape δn(z), that is related to the writing process, the density of defects and the aging state of FBG, as observed in the

CHAPTER4.

The CWA is a mathematical formulation that has a wide application in photonics [15], especially in the field of gratings and periodic structures (such as Distributed Feedback Lasers (DFB) and Distributed Bragg Reflector Lasers (DBR)). This approach is very similar to the

perturbation theory in Quantum Mechanics. FBGs are optical gratings and therefore is

possi-ble to use CWA approach successfully.

Consider a FBG as a spatial variation of the dielectric constantε in the homogeneous opti-cal fiber medium,

ε(x, y, z) =ε¯(x, y) +∆ε(x, y, z) =ε¯(x, y) +

∑

m̸=0 εm(x, y)exp ( −im2π Λz ) (3.5)

In Eq. 3.5the dielectric perturbation component ∆ε has been developed in Fourier’s series since the grating is a periodic structureΛ.

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3.4. FBG’s spectral theory 25 The radiation E(x, y, z, t)that propagates in the unperturbated optical fiber is essentially a linear combination of the normal modes Em (LP modes in a step-index fiber) , as already mentioned inCHAPTER2.

E=

∑

m

Am(z)Em(x, y)ei(ωt−βmz) (3.6)

Where Am andβm = neff· 2_λπ_m are the amplitudes and wavenumbers of the modes Em that satisfy the wave equation (∇ ·E=0). Hence, to solve the wave equation in the perturbated medium (Eq.3.5) it has to substitute Eq. 3.6in∇ ·E=0 obtaining:

[

∇2₊_ω2_µ₍_ε_¯₍_{x, y}_{) +}_∆ε₍_{x, y, z}₎₎]_E₌₀ _(3.7) The Eq. 3.7leads to a set of differential equations difficult to solve in general, in particular because of the second derivative of Amalong z and then the∆ε could be general functions of x, y, z and have a non-trivial coupling with the modes.

Slowly Varying Amplitude and Coupling approximations

The type of grating to analyze can be approximated with a standard FBG in which the vari-ation of the refractive index has a amplitude of modulvari-ation ofδn ∼ 10−4 _{that is much less} than the effective refractive index of the optical fiber neff =1.44−1.45 [39].

Hence, given thatδn≪neff, the dielectric perturbation is obviously weak and the variation of

0 0.2 0.4 0.6 0.8 1 normalized FBG z-axis [z/L] 0 0.2 0.4 0.6 0.8 1 Modes power [|F| 2 , |B|

2 ] forward incident power_{back reflected power}

L z x y Cladding Core FBG

~

FIGURE 3.11: Contradirectional case: forward and reflected power at the Bragg condition

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the modes amplitudes are considered slow [15], as described in Eq.3.8 d2

dz2Am≪ βm

d

dzAm (3.8)

The condition in Eq.3.8is also called the Slowly Varying Amplitude (SVA) condition. Another simplification of the Eq. 3.7 is to consider the part of the spectrum close to the Bragg wavelengthλB in which only two modes amplitudes are strongly coupled instead of the infinite number of mode amplitudes Am, as shown in the Eq. 3.9 and according to the Bragg condition in Eq. 3.1, Essentially, the two modes could travel in the same z direction

β1β2 >0 (codirectional) or in the opposite directionβ1β2 <0 (contradirectional).

β1−β2=m 2π Λ :    β1 =β2≡ β : codirectional β1 =−β2 ≡ β : contradirectional (3.9)

For an FBG whose back reflectivity is to be calculated, it is valid the contradirectional

coupling caseβ− (−β) =m2_Λπ in which there is a forward amplitude F(z)and a back-reflected

amplitude B(z)(Fig. 3.11). The Eq.3.7can be reduced as follow:    d dzF(z) =−iκB(z)e+i∆βz d dzB(z) = +iκ∗F(z)e−i∆βz ∆β≡β1−β2−m 2π Λ κ = ω 4 ∫ E∗ϵm(x, y)Edx dy (3.10)

The solution of this coupled system is described graphically in the Fig. 3.11. The forward radiation⃗β that interact with the grating of length L, yields energy by bringing out reflected radiation−⃗β which has the maximum value at the input of FBG B(z=0)and lowering the amplitude of the radiation transmitted at the output of the FBG F(z =L). Withκ is indicated the coupling coefficient of the grating, which is calculated by projecting the propagation modes on the amplitudes of the dielectric perturbation termεmas shown in Eq.3.10.

The simplest case that allows to describe very realistically the spectrum R(λ)of an FBG is to consider the modulation of the refractive index along the fiber axis n(z)as a simple oscillation n(z) =neff+δn cos ( 2π Λ z ) (3.11) Replacing and resolving the Eq.3.10, the explicit form for the reflection spectrum of the FBG is obtained R(ω) =B(0) F(0) 2 = |κ| 2 sinh2(sL) s2_cosh2₍_sL_{) + (}_∆β/2₎2_sinh2₍_sL₎ (3.12) ∆β=4πneff ( 1 λ− 1 λB ) s ≡ √ |κ|2₋ ( ∆β 2 )2 κ≃ πδn_λ (3.13)

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3.4. FBG’s spectral theory 27 1539.5 1540 1540.5 1541 1541.5 0 10 20 30 40 50 60 Reflectivity [%] L = 1 L=2.8mm n=1.7 10-4 1539 1539.5 1540 1540.5 1541 1541.5 1542 -80 -60 -40 -20 0 Reflectivity [dB] simulated FBG 1539.5 1540 1540.5 1541 1541.5 0 10 20 30 40 50 60 Reflectivity [%] real FBG 1539 1539.5 1540 1540.5 1541 1541.5 1542 -80 -60 -40 -20 0 Reflectivity [dB] real FBG -3dB Rmax FWHM

FIGURE3.12: Comparison between a spectrum of a simulated FBG and the corresponding real spectrum

The shape of the reflectivity has been shown in the Fig. 3.12 , comparing it with the real spectrum of a commercial FBG used during the experiments. It should be noted, especially on the logarithmic scale, that the side lobes at the edge of the peak, as predicted by the theory, are actually attenuated. This is due to processes during the writing of the grating, called

apodization techniques: these processes are used to attenuate the gain of the side lobes and

avoid cross-talks between different gratings.

In addition to confirming the shape of the spectral peak, a first important result of the theory is provide a link between the maximum reflectivity peakvalue Rmax, the length of the grating L and the modulation amplitude of the refractive indexδn. The bandwidth of the peak is also closely related to the amplitudeδn, as shown below

Rmax=tanh2(κL) FWHM≃2λB

_nδn_eff (3.14) These results are very important for understanding the behavior of Type I FBGs at high temperatures.

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3.5 Sensing principles

The FBG, as shown in the Section3.4, is a practical spectral filter, but the most widespread and surprising use is in the field of sensors. The FBG is extremely sensitive to temperature

gradients and external deformations: the thermal response arises due to the inherent thermal

expansion of the fiber and the temperature dependence of the refractive index, whereas the strain response arises due to both the physical elongation of the FBG and the change in neff due to the photo-elastic effect [40]. In the Fig. 3.13is shown a simple and clear diagram of how the central Bragg peak shift according to the variation of T and of the applied axial deformationϵz.

tensile strain

compressive strain

temperature increase

temperature decrease

FIGURE3.13: Schematic representation of the effect of the axial strian and of the temperature on the reflected optical power of the FBG

These terms are easily obtained with a first-order development of the Bragg condition

λB:

λB =2neffΛ (3.15)

Considering the first order expansion with T temperature and ∆l or ϵ ≡ ∆l_L relative strain or,

simply, strain: ∆λB(T,∆l) =∆λB temp. +∆λ_B strain = = ∂λB(T,∆l) ∂T l ∆T+ ∂λB(T,∆l) ∂l T ∆l

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3.5. Sensing principles 29

Temperature dependence in FBGs

Considering the part dependence on temperature, using the3.15: ∆λB temp. = ∂λB(T,∆l) ∂T l ∆T= ∂ ∂T[2neffΛ]∆T= =2neff ∂Λ ∂T∆T+2Λ ∂neff ∂T ∆T= = λB Λ ∂Λ ∂T∆T+ λB neff ∂neff ∂T ∆T (3.16)

Hence, the relative variation of Bragg wavelength on temperature is: ∆λB λB temp. =∆T [ 1 Λ ∂Λ ∂T + 1 neff ∂neff ∂T ] =∆T(α+η) (3.17)

It has been defined two coefficients, the relative thermal expansion coefficient of the fiberα and the relative thermal index refraction dependency coefficientη. The value of these two coefficients are tabulated in the literature [41][42]:

α≃0.55·10−6_◦1 C ; η≃ 1 neff 7.8·10−6_◦1 C (3.18)

Tte estimated temperature sensitivity atλ∼1550 nm isαT ∼10 pm/◦C.

It is possible to develop higher orders of the Braggs law in Eq.3.15, especially to describe the behavior of FBGs for measurements in a wide temperature range.

Strain dependence

Considering the part dependence on strain, using the3.15: ∆λB strain = ∂λB(T,∆l) ∂l T ∆l= ∂ ∂l[2neffΛ]∆l= =2neff ∂Λ ∂l ∆l+2Λ ∂neff ∂l ∆l= = λB Λ ∂Λ ∂l ∆l+ λB neff ∂neff ∂l ∆l

Therefore, the relative variation of Bragg wavelength on strain is: ∆λB λB strain = ∆l [ 1 Λ ∂Λ ∂l + 1 neff ∂neff ∂l ] = = ϵ [ L Λ ∂Λ ∂l + L neff ∂neff ∂l ]

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If axial strainϵz is applied to a portion of the fiber containing the FBG, the full length of the strained fiber L and the total length of the Bragg grating LFBG ≡ NFBG×Λ carry the same relative strain contribution, because it is possible to approximate the strain along the full strained fiber as the strain in a small portion of the same fiber LFBG, as follows

δl L ≃ δLFBG LFBG = δΛ Λ Hence, the first term in the brackets is almost one:

L Λ ∂Λ ∂l ≃ L Λ δΛ δl ≃1

The relative variation of Bragg grating’s wavelength on strain becomes: ∆λB λB strain =ϵ_z [ 1+ 1 neff ∂neff ∂ϵ ] ≡ ϵz[1−pϵ] (3.19)

Where p_ϵis the strain-optic coefficient given by the axial deformation (z axis) of the fiber ([43] and AppendixA).

pϵ = 1 2n

2

eff(p12−ν(p11+p12)) (3.20) Considering values for ordinary fibers p12 ≃ 0.252 and p11 ≃ 0.113 are the photo-elastic coefficients for the(x, y)and(x, x)transverse directions respectively.ν≃0.16 is the Poisson

number of the material. With these values it is possible to estimate the FBG’s sensitivity to

the longitudinal strainϵzat 1550 nm as follows

∆λB = λB[1−pϵ]ϵz ≡αϵϵz ≃1.2pm/µϵ·ϵz (3.21)

✏

z

✏

z

⌫✏

z

⌫✏

z ⌫✏z

ˆ

x

ˆ

z

ˆ y

FIGURE3.14: Visualization of the Poisson’s effect in a cylindrical fiber portion

The value of p_ϵgives a negative contribution because the deformation tends to decrease the density of the fiber and therefore the refractive index, causing a negative shift of the central wavelength. The analytical expression of the p_ϵis derived from the theory of

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31

Chapter 4

FBGs in high temperature

environments

Before describing the development of an FBG based dynamic strain sensor for harsh en-vironments, it is necessary to take into account the main high temperature phenomena to which the sensing element is subjected as well as the response to these high temperature environments for different types of FBGs. To this end, it is important to define what harsh

environments mean. In this context, we refer to an environment with temperatures above

300◦C - 400◦C, pressures and mechanical vibrations values close to those achieved in appli-cation fields such as railway, oil & gas and civil environments, where high temperatures and stresses are reached.

In this chapter, the main tests at high temperatures and the results are reported, such as the verification of thermal annealing and the phenomenon of regeneration of optical fibers and FBGs.

In the next section, the FBGs interrogators used to characterize the sensors response will be described.

4.1 FBG’s interrogators

The interrogation technique is a fundamental part of optical fiber sensor technology. It’s possible to refer to a interrogation technique not only for FBG, but also for any fiber optic component or even for fiber portions, as in the DTS technique.

In this section, the operating principles of the two interrogators used in this work are de-scribed, which are both based on the WDM technique (mentioned inCHAPTER2).

The first interrogator used for the first experimental tests is the SmartScan by SmartFibres, a dynamic and very compact interrogator, for field use and with not too high demands. The second interrogator used is the MicronOptics’ Hyperion si255. This interrogator has a higher precision and resolution than the first one and it’s much suitable for research and development measurements.

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SMARTFIBRES- Smart Scan

(A) Photo of the Smart Scan used during the experiments (Source [44])

(B) Example of MG-Y-L (mod. Finisar S7500)[45] FIGURE4.1: The Smart Scan interrogator (A) and a scheme of the tunable

MG-Y laser (B)

Smart Scan by SmartFibres [44] is a WDM-based interrogator, ultra-compact and very suitable to every applications fields (shown in Fig.4.1a). It’s possible to measure part of the C-band spectrum from 1528 nm to 1568 nm (40 nm bandwidth) sampling the optical spec-trum with 400 steps separated by approximately 100 pm. Moreover, the interrogator is able to acquire four channels and 16 FBGs per channel in parallel.

The interrogation in the wavelength domain is allowed thanks to a tuning of the wave-length of the laser in a triangular waveform. The laser is a Modulated-Grating Y (branch) laser (MG-Y-L) [46], a single semiconductor chip in which there is a laser that is constructed by substituting the reflecting section of a DBR laser by an arrangement of two electrically tunable reflection sections (modulated gratings) and a Multimode Interference (MMI) cou-pler which couple the two reflectors’s spectra together and guide them to a gain section, as shown in Fig. 4.1b. The tuning process of the MG-Y laser exploits the Vernier effect [47] for which the spectral distance between the reflective peaks of reflectors is chosen slightly

different, such that if both are superimposed by the MMI, only one reflection peak of each

reflector can overlap with a peak of the other reflector. This makes it possible to operate the laser at any wavelength of the spectrum where the responses of the two reflectors can be aligned [48]. In the case of Smart Scan, the separation between the two adjacent peaks is 12.5 GHz which, at 1550 nm corresponds to 100 pm. In fact, the number of wavelength that can be acquired per single laser scan is 400. Furthermore, to ensure a good FBG spectrum recon-struction it is recommended from the manufacturer to use FBG with a bandwidth greater than 500 pm to have enough number of points to measure the peak properly.

The simultaneous interrogation of multiple channels is possible thanks to the presence of a 1×4 beam splitter and a circulator for each channel. The radiation is detected by a InGaAs-based photodiode. In order to increase the interrogator dynamic range, a gain level

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4.1. FBG’s interrogators 33

Swept Laser

1x4

Beam Splitter Circulators

PC CH1 CH2 CH3 CH4 FBG arrays Detection Circuit Photodiodes Comms Processing

FIGURE4.2: Block diagram of Smart Scan (Adapted from [49])

for each channel can be tuned, allowing measurements in a dynamic range of 27 dB divided by 9 levels, 3dB for each level. The maximum acquisition rate is 25 kHz, as also shown in the next Chapter. It is worth noting, from the block diagram, that no absolute wavelength reference is integrated within Smart Scan hardware architecture so that the FBG’s peaks are determined by the dynamics of the laser sweep.

MICRONOPTICS- Hyperion si255

The Hyperion si255 [50] is more sophisticated than the Smart Scan. The laser technology is based on a proprietary tuning architecture and allows a very large bandwidth spanning from 1460 nm to 1620 nm (160 nm) with a nominal resolution of 1 pm. Combining large source bandwidth and 16 channels it is possible to monitor hundreds of FBG sensors. Unlike the Smart Scan, the Hyperion si255 allows to have, in a single acquisition, the full dynamic range of the absolute reflected optical power.

As shown in the block diagram4.4and referring to the graph in Fig.4.3bthe wavelength and the optical power of the Hyperion si255 is referred to four types of reference sources for an accurate internal spectral calibration and a certified optical power value. In partic-ular, the internal architecture is provided with the following references: a Power Reference

Channel to measure the power from swept spectrum optical source resolved during a single

source sweep, a Fabry-Pérot Channel is used to control and calibrate the entirety of the source sweep, a FBG reference (thermally monitored) for a first raw association between time and wavelength and an Acetylene Absorption Reference (gas cell) module for accurate calibration

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(A) Photo of the Hyperion si255 used during the experiments

(Source: [50])

Power Ref.

Fabry-Perot

Gas Cell

FBG Ref.

(B) Spectrum of the four reference sources of the Hyperion si255 (Source: [51])

FIGURE4.3: The Hyperion si255 interrogator (A) and four reference sources (B)

between wavelength and sweep time.

For both interrogators, a proprietary software is provided that allows the acquisition of the spectra and the Bragg wavelength charts in time of the interrogated FBGs. The accurate determination of the Bragg peak over time is obtained through a peak detection algorithm that can be customized according to the spectral shape of the interrogated FBG. For each mea-surement or data processing, the name of the interrogator used to perform this meamea-surement is reported in brackets in the figure’s caption.

Swept Laser 90/10 Beam Splitter PC CH15 CH16 Photodiodes Circulators Power Ref. Gas Cell FBG Ref. Fabry-Perot Detection Circuit CH1 CH2 FBG arrays Processing Comms Photodiodes

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4.2. Preliminary preparations 35

4.2 Preliminary preparations

The FBGs

During the following tests, a Type I FBG is main used. The FBG was purchased by a commer-cial company [52] and, as stated by the manufacturer, the FBG was written with a standard phase mask technique in a germano-silicate fiber (SMF- 28e) preloaded with hydrogen. Hydro-genation, as mentioned in the Sec.3.2, allows to achieve very high reflectivity. The FBG was written for a length L of 5 mm and aλBof∼1545 nm. The coating is in acrylate, a transparent polymer resistant up to about 85◦C- 120◦C. All other technical data are shown in the Table

4.1. 1542 1543 1544 1545 1546 1547 1548 1549 1550 Wavelength [nm] 0 20 40 60 80 100 Reflectivity [%] Acrylate FBG HR

FIGURE4.5: Spectrum of the FBG-1 at room temperature (Smart Scan)

Name: FBG-1

Type: Single FBG I-type FBG length: 5 mm λc 1545.5±0.5 nm FWHM 0.50±0.05 nm Rmax >90 % Coating: Acrylate Fiber: SMF-28e TABLE 4.1: FBG-1 features

Therefore, the FBG chosen is standard, low cost and with an operating temperature up to 100◦C, limited by the fiber coating. With the right precautions, the fusion of the coating

is not a problem for the FBG, but this makes the fiber bare (only core and cladding) and much

more fragile, increasing the risk of breaking, as shown in the next Chapter.

Coating removal

(A) Before removing coating (B) After removing coating FIGURE4.6: FBG decoating

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As shown in the Sec.3.4, FBGs are very sensitive to thermal expansion and deformation. It is therefore appropriate, before making temperature or strain measurements, to remove a portion of coating at the FBG point to avoid coating interference during bare FBG expansion and contraction compromising the measurement during acrylate melting at elevated tem-peratures. Usually∼1 cm of FBG re-coating is removed on each side of the FBG, as shown in Fig. 4.6b, using isopropyl alcohol, or, if necessary, with a chemical decoating technique based on a CH2Cl2solution.

Figures4.6aand4.6bshow the portion of fiber in which the FBG (L = 5 mm) has been written, between the two blue signs. In the Fig.4.6bthe part of the coating surrounding the FBG has been removed and it is evident the change in the fiber diameter (from 250 µm to 125µm of the bare fiber).

The furnace

The thermal tests were performed using a NABERTHERML3/11 furnace [53] (Fig.4.7), with internal dimensions of 16cm×14cm×10cm [w×d×h]. It has two ventilation holes, one in

the furnace-door and one in the back side: thanks to these it is possible to insert the optical fibers, as described below. The furnace temperature control electronics allows to heat up with a maximum thermal gradient of ∼ 18◦ C/min using a PID based process controller with temperature feedback given by a thermocouple positioned in the upper part of the inner chamber furnace. The temperature measurements reported in this work are based on the reading temperature of the furnace thermocouple.

Fiber Bragg Grating sensors for harsh environments

M

’

T

P

F

IBER

B

RAGG

G

RATING SENSORS FOR

HARSH ENVIRONMENTS

Candidate

Massimo M

Supervisors

Prof. Alberto D

L

Ing. Tiziano NANNIPIERI

Contents

Chapter 1

Introduction

z

1.1

The project

Chapter 2

Optical Fibers and Sensing

Technologies

2.1

Overview

2.2

Fabrication of Optical Fibers

2.3

Birefringence and Polarization Maintaining Fibers

2.4

Fiber Sensing Technologies

Chapter 3

Fiber Bragg Grating: theory and

technology

3.1

Bragg Gratings and Fiber Bragg Gratings

⇤

3.2

Photosensitivity in Optical Fibers

3.3

Methods to induce FBGs

3.4

FBG’s spectral theory

∑

∑

~

~

3.5

Sensing principles

✏

✏

⌫✏

⌫✏

ˆ

x

ˆ

z

Chapter 4

FBGs in high temperature

environments

4.1

FBG’s interrogators

4.2

Preliminary preparations