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Specialty Fibers and Relevant Technologies for Fiber-Optic Sensing

Koji Omichi,1 and Ryozo Yamauchi2

Fiber-optic sensing is one of the most important elements in photonic sensing technology. Novel specialty fibers and the relevant technologies have been developed for various application fields, such as avionics, civil infrastructures, atomic plants and oil&gas industries. In this paper, recent progress in the fiber optic sensing is reviewed with a focus on the specialty fibers.

1. IntroductionThe first study for the fiber-optic sensing was start-

ed shortly after the first realistic optical fiber was in-vented in 1970s, and 40 years have passed since then. From the beginning of the study, optical fibers were used not only as transmission medium, but also as sen-sors utilizing its response to strain, temperature, pres-sure, angular velocity, magnetic field, and so on. One of the representative examples is fiber-optic gyroscope used in avionics field. Currently, various sensing tech-niques have been proposed, and these efforts have re-sulted in success of novel technologies, such as shape sensing and fiber-optic scope. Fiber-optic sensing is widely used in various application fields, such as avi-onics, civil infrastructures (bridge, tunnel, etc), atomic plants and oil&gas industries. In order to meet new requirements raised from wide variety of application field, continuous efforts are being made for develop-ment of specialty fibers and the relevant technologies. In this paper, recent progress in the fiber-optic sens-ing is reviewed with a focus on the specialty fibers.

2. Polarization-maintaining fiberIn a normal single-mode fiber, axial asymmetry of a

core and/or other disturbance cause mode coupling between orthogonally polarized lights. Launched light propagates along the fiber changing its polarization state randomly. This random polarization state causes measurement noise in interferometric sensing. How-ever, in the case of polarization-maintaining fiber, no coupling occurs and polarization state is maintained due to its large mode birefringence. Table 1 shows cross sectional images of polarization-maintaining fi-bers. Elliptical core and side tunnel fibers are catego-rized geometrical birefringence type. Mode birefrin-gence of these fibers is originated from asymmetry of effective refractive index between two orthogonal po-larization states. On the other hand, PANDA (Polariza-

tion-maintaining AND Attenuation-reduced), bow-tie and elliptical jacket fibers are categorized stress-in-duced birefringence type. Their mode birefringence is originated from asymmetry of induced stresses be-tween two orthogonal polarization axes. Among these polarization-maintaining fibers, PANDA fiber is the most suitable in terms of optical coupling with conven-tional single-mode fibers because mode-field profile of this fiber is similar to that of single-mode fibers. More-over, polarization axial rotation can be aligned at splic-ing point by using a specialty fiber fusion splicer 1).

The fiber-optic gyroscope is one of the representa-tive examples of interferometric sensing. Since noises caused by fluctuation of polarization state and geo-magnetically-induced Faraday effect are well sup-pressed, measurement deviation is significantly re-duced when a PANDA fiber is used for interferometric fiber-optic gyroscopes 2). For the other type of gyro-scope using a PANDA fiber ring resonator, the polar-ization fluctuation is suppressed when 90˚ polarization axial rotation splicing is employed in the ring resona-tor 3). Among these gyroscopes, a high precision inter-ferometric gyroscope used for attitude control and in-ertial navigation of aircrafts/rockets employs all PANDA fiber interferometer. Its sensing head con-sists of a hundred to several thousand meter PANDA fiber with 50 - 200 mm diameter coiling. In order to

Table 1. Cross sectional images of polarization-maintaining fibers.

TypeGemetrical birefrin-

gence typeStress-induced birefringence

type

Polarization-maintaining

fiber

Elliptical core

Side tunnel

PANDA Bow-tieElliptical

jacket

1 Optics and Electronics Laboratory, Silicon Technology Department2 Corporate R&D, Development Planning Center

Fujikura Technical Review, 2015 11

prevent degradations of attenuation and polarization cross-talk due to temperature variation and/or fiber bending, optical parameters, such as relative refrac-tive index difference and mode birefringence have been optimized 4).

Recently, new applications, such as strain and tem-perature simultaneous measurement have been pro-posed utilizing sensitivity-difference in strain and tem-perature, between two orthogonally polarized lights 5)

6). It is indispensable for fiber-optic sensors to compen-sate optical fluctuations due to temperature variation of atmospheric air and/or objectives being measured. Therefore, the simultaneous measurement technique is expected to be a novel solution to realize the tem-perature compensation. Currently, a high mode bire-fringence PANDA fiber has been proposed to improve measurement accuracy for this application. This fiber consists of a small mode-field diameter core and high-ly B2O3 concentrated stress applying parts, and the stress applying parts are arranged close to the core in order to enhance mode birefringence 7). Figure 1 shows strain and temperature dependences of Bragg wavelength difference for the proposed high birefrin-gence PANDA fiber and a conventional telecom grade PANDA fiber. A steeper slope of the temperature de-pendence is obtained from the high birefringence PANDA fiber, as we expected. Since the magnitude of sensitivity difference between temperature and strain means discrimination accuracy of temperature and strain, this result indicates that superior temperature and strain separation is obtained when we use the high birefringence PANDA fiber.

3. Radiation-resistant fibersOptical fibers have been highly appreciated as infor-

mation transmission medium in radiation circum-stances, but at the same time, continuous efforts have been made for improving their radiation durability properties 8). When the fiber is irradiated by radiation, network defect is induced in the glass, such as E’ center(∫Si•) and non-bridging oxygen hole center (∫Si-O∞). They are generated from oxygen deficient center (∫Si-Si∫) and/or covalently-bonded silica (∫Si-

Panel 1. Abbreviations, Acronyms, and Terms.

PANDA fiber–Polarization-maintaining AND Atten-uation-reduced fiber A PANDA fiber maintains polarization state of transmitted light due to difference in propagation constants between two orthogonally polarized lights.

Mode birefringence The magnitude of effective refractive index differ-ence between two orthogonal polarization states (x polarization and y polarization). Mode birefrin-gence B is given by B = nx -ny = (nx0-ny0) + C(sx - sy) where, nx and ny are effective refractive indices for x and y polarizations, nx0 and ny0 are effective refractive indices for x and y polarizations origi-nated from core geometry, C is photoelastic coefficient, sx and sy are induced stresses for x and y polarization axes.

FBG–Fiber Bragg Grating An FBG reflects only a certain wavelength which satisfies Bragg s condition.

RFBG–Regenerated Fiber Bragg Grating Refractive index modulation of this grating is regenerated during an anneal process. The RFBG has strong durability for high temperature up to around 1000 °C.

PCF–Photonic Crystal Fiber An PCF confines transmitted light into a core surrounded by a hole-assisted cladding.

PBGF–Photonic Band-Gap Fiber A PBGF has a periodic structure in cross-section. Transmitted light is confined into a partially deformed field in the periodic structure.

MCF–Multi-Core Fiber A MCF has several cores in one cladding. Indepen-dent signals can be transmitted through each of the cores.

Rel

ativ

e B

rag

g w

avel

eng

th d

iffer

ence

(p

m)

-40

0

40

-80

-1205025 10075 150125

Temperature (°C)

Strain (µε)

2500 750500 12501000

High birefringence

Fiber

Telecom grade

Str

ain

Tem

p.

,,

Fig. 1. Strain and temperature dependences of Bragg wavelength difference for PANDA fiber gratings.

12

O-Si∫) which usually exist in the fiber. These seeds of the defects have absorption peak around UV region to visible region, then a tail from the absorption peak ap-pears in the transmission wavelength range. Its effect is not ignorable when an amount of the defect is huge. It increases the fiber loss and sometimes ends up a transmission problem. It is known that an appropriate amount of OH doping to the fiber core prevents these problems. An OH ion acts as a terminator for the non-bridging oxygen hole center and generates termina-tion of Si-OH. These fibers have been proposed to use for temperature measurement and inner wall observa-tion of the nuclear reactor 9) 10). Figure 2 shows struc-ture of the fiber-optic scope developed for observing inner wall of nuclear reactors. Both a light-guide fiber and image transmission fiber have a silica core doped with OH and fluorine-doped cladding.

Excellent radiation durability is also obtained when fluorine is slightly doped in the core instead of OH 11).

Fluorine has an effect to terminate both the E’ center and the non-bridging oxygen hole center. Figure 3 shows attenuation variation of radiation resistant fi-bers for Co-60g ray irradiation. Transmitted light of a silica core fiber is monotonically attenuated with in-crease of the irradiation dose. On the other hand, at-tenuation saturation is observed at low irradiation dose for the fluorine-doped core fiber. There is low at-tenuation increase of only 2 dB/km when the irradia-

tion dose is up to 10,000 Gy. In order to apply the fluo-rine-doped core fiber to fiber-optic sensing, Bragg grating inscription into the fiber has been proposed by using ultra-short pulsed laser 12).

4. Fiber-optic temperature sensors used in harsh environmentsSince silica based optical fibers have excellent

chemical stability for harsh environments from cryo-genic temperature to high temperature of around 1000 °C, some researches have applied the fibers to tem-perature sensors for such harsh environments.

In high temperature environments, there was a common challenge in thermal decay of refractive in-dex modulation of FBGs inscribed by an ultraviolet la-ser. Recently, some papers reported regeneration phe-nomenon of the refractive index modulation during an anneal process 13). Figure 4 shows the regeneration process of a regenerated FBG (RFBG). Although the reflectivity becomes smaller up to 0.01%(-40 dB) when the FBG was thermally treated at 900 °C, shortly thereafter it goes up to 3%(-15 dB) at the same treat-ment temperature. Since the RFBG has strong ther-mal durability up to its regeneration temperature, con-tinuous efforts are being made for application of the sensor to high-temperature containers and heat ex-changers for power generation.

In the cryogenic temperature, a metal coating on a fiber has been proposed to enhance temperature sen-sitivity. Bragg wavelength shift DlB of FBG due to tem-perature variation is given by,

DλB =2n DTaL +n

(dn/dT )

where, n is an effective refractive index of a fiber, L is a grating period, a is a thermal expansion coeffi-cient, dn/dT is temperature dependence of effective refractive index. The thermal expansion coefficient a

0

-20

-10

-30

-40

-5060 80 100 120 140 1600 20 40

Time (min.)

Ref

lect

ivity

(d

B)

1000

600

800

400

200

0

Tem

per

atur

e (°

C)

TemperatureReflectivity

Regeneration

Fig. 4. Regeneration process of an RFBG.

Light sourceProtection tube

Imagingdevice

Light-guide fiber

Eyepiece lensImage fiberObject Objective lens

Fig. 2. Schematic diagram of a fiber optic scope.

20

10

15

5

06000 8000 100000 2000 4000

Dose (Gy)

Rel

ativ

e at

tenu

atio

n (d

B/k

m)

Fluorine doped core fiberSilica core fiber

Measured at 1310 nm

Fig. 3. Attenuation variation of a radiation resistant fiber for Co-60g ray irradiation.

Fujikura Technical Review, 2015 13

is expressed as the following equation when some ma-terial is coated on a fiber.

a =Afiber Efiber+Acoating Ecoating

Afiber Efiber a fiber+Acoating Ecoating acoating

where, A is cross-sectional area, E is Young’s modu-lus and suffix terms mean fiber or coating parameters. It was necessary for an FBG to improve temperature sensitivity because silica based glass has small ther-mal expansion coefficient a fiber, as well poor tempera-ture dependence dn/dT in cryogenic temperatures 14). Therefore, coating of a silica based fiber with large Young’s modulus and thermal expansion coefficient materials has been proposed in order to enhance tem-perature sensitivity. Figure 5 shows temperature de-pendences of Bragg wavelength shift for a nickel coat-ed FBG and an uncoated FBG. A larger Bragg wavelength shift is obtained in the nickel coated FBG compared with the uncoated FBG. This result indi-cates a metal coating on a fiber has a potential to real-ize precise temperature resolution even in cryogenic temperatures. Moreover, it is possible to control tem-perature sensitivity by changing coating diameter once we know Young’s modulus and thermal expan-sion coefficient of the coating material.

5. Micro-structured fiber, Multi core fiberMicro-structured fibers, including a holy fiber, a

photonic crystal fiber (PCF) and a photonic band-gap fiber (PBGF), have some unique features, such as (1) broadband transmission, (2) low non-linearity, (3) high mode birefringence, originated from their own cross-sectional design. Table 2 shows cross sectional images of micro-structured fibers. The total internal reflection type PCF confines transmitted light into a core surrounded by a hole-assisted cladding. Specialty fiber sensors have been proposed utilizing their unique responses to strain, pressure and temperature, which depend on the cladding structure 15). A PBGF has a periodic structure in cross-section, and transmit-ted light is confined into a partially deformed field in the periodic structure. In general, a fiber-optic gyro-scope utilized a low-coherence broadband light source in order to reduce interferometric noise due to Ray-leigh backscattering. On the other hand, an air core type PBGF can be utilized with a high-coherence laser light source because Rayleigh backscattering is sig-nificantly suppressed in the air core 16). As a result, to-tal measurement deviation caused by several types of optical noise is reduced compared with conventional fibers.

A multi-core fiber (MCF) has several cores in one cladding, and independent signals can be transmitted through each of the cores. The MCF has been consid-ered as a breakthrough technology to overcome a transmission capacity limit of current optical telecom-munication networks. Actually, the world record trans-mission capacity of 1.01 Pb/s/fiber was achieved in 2012 17). Table 3 shows cross sectional images of MCFs. The hexagonal close-packed core structure with a simple step refractive index profile was pro-posed at an early stage. Then, a trench refractive index profile has been demonstrated in order to reduce cross-talk between cores. As well, unique core ar-rangements, such as binary pitch structure and circu-larly-arrangement structure also have been proposed. Recently, new applications, such as fiber-optic shape

1000

-1000

-2000

0

-3000

-4000150 200 250 3000 50 100

Temperature (K)

Bra

gg

wav

elen

gth

shi

ft (p

m)

Nickel coated FBGUncoated FBG

Fig. 5. Temperature dependence of Bragg wavelength shift for a nickel coated FBG.

Table 2. Cross sectional images of micro-structured fibers.

Type Total internal reflection type Photonic bandgap type

Micro-structured fiber

Hole-assisted typePolarization-main

taining typeLarge Aeff type High NA type Air core type All solid type

Features,Application

• Low bending loss • Polarization-maintaining device

• Sensor

• Broadband transmission

• Low non- linearity

• Optical amp- lifier• Fiber laser

• High power transmission• Fiber-optic gyroscope

• Fiber laser

y

14

sensing have been proposed utilizing sensitivity-differ-ence for bending direction of each core 18). The fiber shape can be measured along longitudinal direction of the fiber based on bending information on each fiber position. A fan-in/fan-out device and a multi-core con-nector which are used in this application have also been reported 19). Fan-in/fan-out device combines and splits signals of single core fibers into one MCF, and the multi-core connector enables us to easily mate the fan-in/fan-out device with the MCF based sensor.

These examples indicate that micro-structured fi-bers and MCFs have a potential to realize new sensing technologies employing their unique fiber designs.

6. ConclusionFiber-optic sensing technology, which is an impor-

tant element of the photonic sensing have been dis-cussed, and the recent progress have been reviewed with a focus on the specialty fibers. Proposed specialty fibers are not limited to what we discussed on this ar-ticle, and many fibers have been demonstrated in or-der to meet new requirements raised from a variety of application fields. Specialty fibers and the relevant technology will progress, and contribute to innovation of the fiber-optic sensing.

AcknowledgmentA part of studies on the radiation-resistant fibers

was a result of collaboration with Japan Atomic Energy Agency (JAEA). Collaboration with Railway Technical Research Institute (RTRI) helped the research of the fiber-optic cryogenic temprature sensor. Research on the above-mentioned multi-core fibers was partially supported by National lnstitute of Information and Communication Technology (NICT), Japan under “R&D of Innovative Optical Communication Infra-structure”. The authors wish to acknowladge all of the involved parties.

References

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2) K. Okamoto, et al.: “All-PANDA-Fiber Gyroscope with Long Term Stability,” Electron. Lett., Vol. 20-10, pp.429-430, 1984

3) X. Wang, et al.: “Reduction of Polarization-Fluctuation In-duced Drift in Resonator with Twin 90˚ Polarization-Axis Rotated Splices,” Optics Express, Vol. 18, No.2, pp.1677-1683, 2010

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6) W. Zou, et al.: “High-Accuracy Discriminative Sensing of Strain and Temperature by Use of Birefringence and Brill-ouin Scattering in a Polarization-Maintaining Fiber,” OFS-19, Proc. of SPIE, 7004-105, 2008

7) K. Hayashi, et al.: “High Performance Polarization-main-taining Optical Fiber,” IEICE Technical Report, Vol. 114, No.64, OFT2014-6, pp.25-30, 2014 (in Japanese)

8) E.J. Friebele, et al.: “Radiation-induced optical absorption bands in low loss optical fiber waveguides,” J. Non-Cryst. Solids, Vol. 38 & 39, pp.245-250, 1980

9) T. Kakuta, et al., 2002 Fall Meeting of the Atomic Energy Society of Japan, I-34, 2002 (in Japanese)

10) H. Naito, et al., 2010 Fall Meeting of the Atomic Energy So-ciety of Japan, J-40, 2010 (in Japanese)

11) K. Aikawa, et al.: “Radiation-Resistant Single-Mode Optical Fibers,” Fujikura Technical Review, No.37, pp.9-13, 2008

12) D. Grobnic, et al.: “Radiation sensitivity of Bragg gratings written with femtosecond IR lasers,” Fiber Optic Sensors and Applications VI, Proc. of SPIE, Vol. 7316, 73160C, 2009

13) J. Canning, et al.: “Helium and Regenaration in Optical Fi-bres,” OFS-22 PDP, OF200-20, 2012

14) M. Frövel, et al.: “Multiplexable fiber Bragg grating tem-perature sensors embedded in CFRP structures for cryo-genic applications,” Proc. of the 3rd EWSHM, pp.938-945, 2006

15) W. Urbanczvk,: “Birefriengent microstructured fibers: new opportunities for sensing,” OFS-20, OF101-04, 2009

16) M.J.F. Digonnet, et al.: “Coherent backscattering noise in a photonic-bandgap fiber optic gyroscope,” OFS-20, Proc. of SPIE, Vol. 7503, 750302, 2009

17) H. Takara, et al.: “1.01-Pb/s(12 SDM/222 WDM/456 Gb/s) Crosstalk-managed Transmission with 91.4-b/s/Hz Aggre-gate Spectral Efficiency,” Proc. of ECOC 2012, Th.3.C.1, 2012

18) C.G. Askins, et al.: “Bend and Twist Sensing in a Multiple-Core Optical Fiber,” OFC/NFOEC2008 Technical Proceed-ings, OMT3, 2008

19) K. Omichi, et al.: “Multi-core to 7 single-core-fibers fan-out device with multi-core fiber pigtail connector,” OFS-23, Proc. of SPIE, 9157-501, 2014

Table 3. Cross sectional images of multi core fibers.

Multi-core fiber

Hexagonal close-packed stucture

Binary pitch

structure

Circularly-arranged structure

Refractive index profile

Step Trench Trench Trench

Number of cores

7 7 10 12