Abstract— We review the state-of-the-art in photonic crystal fiber (PCF) based mechanical sensing. We then introduce our recently developed highly birefringent PCF that allows pressure and force sensing when combined with a fiber Bragg grating sensor. We clarify the measurement principle and explain how the optimized microstructure of the PCF yields its record-high sensitivity. In addition we demonstrate the selectivity of our bare fiber sensor, which remains insensitive to temperature changes or axial strain. We explain how the integration of this specialty fiber sensor technology with biocompatible polymer based micro-technology provides great opportunities for flexible and stretchable photonic sensor skins. Such touch sensitive devices can therefore play an important role in the domain of biorobotic instrumentation.

I. INTRODUCTION HE use of sensors in the processing industry, material manufacturing, civil engineering, transport and energy

production becomes more and more important for collecting data on the influence of physical quantities. These quantities typically include temperature, pressure, strain or force. In many cases conventional electromechanical sensors, for example electrical strain gauges, are perfectly qualified for these tasks. Electrical sensors may nevertheless experience difficulties in performing properly under challenging circumstances, for example in the presence of sources of electromagnetic radiation such as high voltage lines or lightning strikes. Their intrinsic temperature sensitivity can also disturb the sensor signal when the physical quantity of interest is measured in transient temperature regimes. Finally, electromechanical sensors can exhibit low fatigue

Manuscript received January 31, 2012. This work was financially supported by the European Commission (EU) FP7 project “PHOSFOS” – Photonic Skins for Optical Sensing. Vrije Univ. Brussel also wishes to acknowledge financial support from the EU FP7 IAPP project “SMARTSOCKET”, the Agency for Innovation by Science and Technology (IWT), the Research Foundation–Flanders (FWO), the Methusalem and Hercules Foundations Flanders, and the Interuniversity Attraction Poles (IAP)–Belgian Science Policy. The COST TD1001 action “OFSESA” is acknowledged as well.

T. Geernaert, C. Sonnenfeld, S. Sulejmani, T. Baghdasaryan, F. Berghmans and H. Thienpont are with the Brussels Photonics Team of the Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium; phone: 0032-2-629-1381; fax: 0032-2-629-3450; e-mail: tgeernae@vub.ac.be)

B. Van Hoe and G. Van Steenberge are with IMEC-Universiteit Gent-CMST, Technologiepark 914A, B-9052 Zwijnaarde, Belgium;

P. Mergo is with the Marie Curie-Sklodowska University, Pl. M. Curie-Sklodowskiej 3, 20-031 Lublin, Poland;

W. Urbanczyck is with the Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland;

M. Becker is with the Institute of Photonic Technology, Albert-Einstein-Straße 9, 07745 Jena, Germany;

life or drift, which calls for frequent replacements or recalibrations. When traditional sensors fail optical fiber sensors (OFS) can provide a solution. An optical fiber sensor encodes a measurand of interest in one (or more) properties of an optical signal that is guided within an optical fiber [1], [2]. OFS feature many advantages over conventional sensors: they are small and lightweight; they are chemically inert; they can be multiplexed and allow distributed and/or quasi-distributed sensor configurations; they allow absolute measurements because they can be self-referencing; and they can have a very linear response. The implementation and commercial success of this sensor technology has therefore considerably increased in the last 25 years and it is now being considered steadily in the biomedical and biorobotic field as well [3] [4]. Reasons for this include for example the minimally invasive nature of optical fibers which makes them very suited for integration within flexible actuators or surfaces and the soft natural feel offered by flexible photonic skins when compared to rigid and stiff electrical components. We present two technological trends in the field of optical fiber sensors that are very much likely to push the adoption of OFS for biorobotics. First, the use of photonic crystal fibers which allow for unprecedented design flexibility of waveguide and sensing features. Second, the embedding of OFS in flexible and stretchable polymer materials that provides solutions for wearable sensor devices.

The rest of this paper is structured as follows: In Section II, we introduce photonic crystal fibers, the sensing potential that they offer and the particular fiber Bragg grating-based sensor that we have developed. Then we present the integration technologies that are available to turn such smart optical fiber sensors into flexible and (sometimes) stretchable polymer sensing foils in Section III. In Section IV we show the sensing properties of the fabricated sensor skins. Finally we conclude in Section V.

II. PHOTONIC CRYSTAL FIBER BASED SENSING PCFs are a relatively new type of optical fiber that can be

optimized for a large range of applications by tailoring the number, the size (air hole diameter d or the lattice constant Λ), the position and the geometry of a pattern of air holes that form a confining microstructure around a (sometimes doped) fiber core region (see Fig. 1) [6][7]. This is in contrast with conventional step-index fibers that are composed of two concentric cylinders off glass. PCFs indeed introduced an unprecedented series of novelties in every

Towards Flexible Photonic Sensing Skins with Optical Fiber Sensors Thomas Geernaert, Member, IEEE, Camille Sonnenfeld, Sanne Sulejmani, Tigran Baghdasaryan,

Geert Van Steenberge, Bram Van Hoe, Pawel Mergo, Waclaw Urbanczyck, Martin Becker, Francis Berghmans, Member, IEEE, Hugo Thienpont, Member, IEEE

T

The Fourth IEEE RAS/EMBS International Conferenceon Biomedical Robotics and BiomechatronicsRoma, Italy. June 24-27, 2012

978-1-4577-1198-5/12/$26.00 ©2012 IEEE 628

subfield of optical fiber technology including non-linear optics, optical telecommunications, optical fiber lasers.

Fig. 1: Schematic representation of the cross-section of a photonic crystal fiber with a hexagonal lattice of air holes, denoting common

parameters such as air hole diameter d, lattice constant Λ.

A. Sensing with Photonic Crystal Fibers Not in the least they have received increasing interest for

their potential in sensing applications [8]-[10]. Various types of PCF sensors have already been proposed, including for example interferometric and polarimetric sensors for different physical parameters (see Fig. 2), as well as evanescent field sensors for monitoring specific chemical compounds in gases [11][12] and liquids [13]. Even biophotonic sensors are considered [14].

Moreover the first demonstration of Bragg grating inscription in PCFs brought grating-based sensor technology to these fibers [15]. Over the last years, sensing with FBGs in PCF was reported several times [16] [17]. Strain sensing in a two-mode PCF was for example shown by Martelli et al. [18][19]. They showed that the response of the Bragg peak wavelengths corresponding to two different optical modes can be exploited for temperature insensitive strain sensing.

Fig. 2: Scanning Electron Microscope Images of cross-sections of

fabricated highly birefringent PCFs.

The sensing applications of birefringent PCFs, targeting for example the measurement of hydrostatic pressure, strain, bending and lateral force has been reported as well in attempts to separate different measurands or to improve state-of-the-art sensitivities. Frazao et al. for example reported on an FBG in a birefringent PCF and in a standard step-index fiber to simultaneously measure axial strain and temperature with resolutions of 1.5 °C and 10.7 µε over a measurement range of 100 °C and 2000 µε [21]. Other examples can be found in [22][23]. Exploring the potential of LPGs soon followed [24] with e.g. a Mach-Zehnder interferometer in a PCF with LPGs [25]. Strain and temperature sensing with LPGs has been reported by Zhu et al.[26][27] and a more general study of the sensitivities of

LPGs in PCF to temperature, strain, and external refractive index was published in 2007 by Petrovic et al. [28].

Future sensing devices relying on PCFs and gratings therein could lead for example to an advanced lab-on-a-fiber, as proposed by Canning [29]. The combination of multiple sensing technologies in one fiber will obviously require complex fabrication techniques and can therefore be considered as an incentive for the development of PCF-based optical fiber sensors, because the potential market for such devices could make them a commercial success story.

B. Fiber Bragg Grating Sensors One of the most well-known possibilities to turn an

optical fiber into a strain sensor is to use a so-called fiber Bragg grating (FBG) [30][31]. This device is a wavelength-selective filter fabricated inside the core of an optical fiber for which the reflected wavelength changes under the influence of external perturbations. (see Fig. 3).

Fig. 3: Illustration of the working principle of a fiber Bragg grating. When a broadband optical source is coupled into a fiber that is equipped with a FBG, a narrow spectral band centered around the Bragg wavelength will be reflected while the rest of spectrum is simple transmitted. Although these sensors have many advantages they still show a number of shortcomings. For example, a FBG in a conventional step-index optical fiber is sensitive to both temperature changes and axial strain leading to cross-sensitivity problems. Furthermore the sensitivity of FBGs in conventional optical fiber to transverse strain remains considerably lower than to axial strain, while monitoring transverse strains is most important in many applications.

C. FBG Sensor in Butterfly PCF To overcome the current limitations of FBG-based

sensors, we have designed and fabricated a dedicated highly birefringent photonic crystal fiber to replace the conventional single-mode fibers that are most often used. Because of the particular shape of its microstructure we refer to this fiber as the Butterfly PCF (see Fig. 4) [33]. The central part of the core region is doped with GeO2 to allow the inscription of FBGs using conventional ultraviolet grating writing techniques [34],[35].

This PCF is highly birefringent and therefore contains two orthogonally polarized modes that can operate individually.

!

λ

Pin

λ

Ptransmitted

λ

Preflected

Fibre cladding Fibre core

λB ≈ 1500 nm

Bragg wavelength

1-10 mm

Bragg grating = Zone with periodically modulated neff

λB = 2neff Λ neff : effective refractive index Λ : grating period

Λ ≈ 530 nm

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A Bragg grating inscribed in this PCF then yields two Bragg reflection peaks, one corresponding to each mode, and encodes pressure or transverse mechanical load in the spectral distance between those peaks. That spectral distance Δλ is linked with the PCF’s modal birefringence, which is practically insensitive to temperature and to longitudinal strain [36]-[38]. This principle is illustrated in Fig. 5.

Fig. 4: Scanning Electron Microscope image of the cross-section of a fabricated Butterfly PCF.

Fig. 5: Measurement principle: Longitudinal strain and temperature changes (Left) induce identical wavelength shifts for the two Bragg peaks while transverse line loads or hydrostatic pressure (Right) appear as an increase or decrease of the the spectral distance Δλ.

Fig. 6 shows reflection and transmission spectra of a FBG

fabricated in such fibre: the two narrow reflection bands around the central Bragg wavelengths are clearly visible, can easily be distinguished and the grating strength is adequate for most sensing applications. This allows the interrogation of our Butterfly sensors with commercially available FBG read-out hardware such as spectrometer-based or laser scanning based systems with a typical Bragg wavelength accuracy of 1 pm.

Moreover, as can be seen in Fig. 7, the fabrication process of FBGs in the Butterfly PCF has achieved the level of maturity that allows the reliable and repeatable fabrication of wavelength-multiplexed sensor arrays along one single PCF sample. Such FBG arrays allow quasi-distributed sensing since they contain multiple sensing points along a single

optical fiber connection.

Fig. 6: Reflection and Transmission spectrum of a FBG inscribed in the Butterfly PCF.

Fig. 7: Reflection spectrum of an array of 5 wavelength multiplexed FBGs in Butterfly PCF.

We explained earlier that our Butterfly sensor is designed

for measurements of hydrostatic pressure or transverse loads. For the non-embedded sensor samples we first provide more detail on their pressure sensing capability. Fig. 8 shows the change of the Bragg peak separation Δλ, which is our sensor signal, as a function of applied hydrostatic pressure P for two different designs of the Butterfly PCF. Both negative (dΔλ/dP < 0 – the peak separation decreases with pressure) and positive (dΔλ/dP > 0 – the peak separation increases with pressure) sensitivities can be obtained by controlling the fabrication process and the resulting microstructure deformations and air filling factor in the core region. We obtained experimental values of -15 pm/MPa and 33 pm/MPa for respective fibers, in very good agreement with the numerically simulated sensitivity values [32]. Finite element simulations were carried out using commercially available COMSOL Multiphysics software and rely on analyzing the hydrostatic pressure sensitivity of the modal birefringence. To do so changes in the stress distribution and deformations of the microstructure are taken into account. The details of the different steps in the simulation process have been explained in Martynkien et al. [33]. The optimization of the microstructure of a PCF thus lead to record-high mechanical sensitivities, more than 20

Wavelength

Ref

lect

ed P

ower

Wavelength

Ref

lect

ed P

ower

εz

ΔT

Ft

P

630

times higher than in conventional birefringent optical fibers, that can be relatively easily addressed via fiber Bragg gratings.

Fig. 8: Bragg peak separation Δλ versus hydrostatic pressure for two Butterfly PCFs with opposite pressure sensitivities. Triangles (dΔλ/dP > 0) and dots (dΔλ/dP < 0) are measurement values, while dotted lines are the corresponding numerical simulation results.

In addition to the record high pressure sensitivities the

sensitivity to temperature T expressed as dΔλ/dT is below 0.02 pm/°C.

III. SENSOR INTEGRATION Embedding the Butterfly sensors described above in

polymer materials brings a realm of applications in the field of healthcare within range. One can for example embed FBGs in standard silica fibers in a 2D flexible bio-compatible polymer surface to form pressure sensing pads [5]. Pressure on the pad is then transformed into axial strain detected by the FBG. The achieved operational and mechanical properties meet the requirements of various bio-mechanical applications targeting the measurement of pressure exerted on human skin, such as for example in amputee sockets, shoe sensors, wheelchair seating-system sensors, hospital-bed monitoring sensors, rehabilitation robots and wearable exo-skeletons. Single or multiple FBGs can be embedded in polymer pads to form either small single pressure sensor elements or larger quasi-distributed pressure sensor surfaces [42]-[44].

To enable the embedding of fiber sensors in polymer pads we have developed different technologies that each come with their respective advantages [45][46]. The first embedding approach was to use straightforward injection molding in a polymethyl methacrylate (PMMA) or glass mold, during which the fibers are supported in channels located at the edge of the mold. The second approach was to use laser ablation to create tracks in a cured sheet of flexible host polymer. The main advantage of this approach is the flexibility in terms of fiber embedding design, which allows using designs from the field of micro-technology for stretchable (opto)electronics, incorporating meander shapes of non-stretchable materials (e.g. fibers) to allow for a certain level of elasticity. The third method is based on a

soft lithography process for which the position of the optical fiber is defined even more accurately, by transferring a well-defined negative master mold into the host material. For the master mold we used SU-8 (a hard epoxy material) patterning on a silicon wafer, since this allows for high accuracies in 3 dimensions, highly compatible with the requirements for fiber embedding. After patterning the SU-8 as part of the master mold, a controlled amount of Polydimethylsiloxane (PDMS) material is poured on the silicon wafer, which can be easily removed from the master mold after a moderate thermal curing step. The planar sheets that can be fabricated with these technologies are illustrated in Fig. 9 (Left).

Fig. 9: Integration technology for both planar polymer sensor skins (Left) as cylindrical polymer tubes is available.

Embedding technology is not limited to planar two-

dimensional pads. The fiber sensors can also be embedded in other shapes of polymer materials, such as for example tubes (see Fig. 9 (Right)). This again brings new possibilities to assemble sensor systems dedicated to medical applications such as for example gastro-intestinal pressure sensing. Pressure applied on the tube is detected as strain acting on the fiber sensors in amounts that depend on the fiber location within the tube and on the diameter of the latter.

IV. FLEXIBLE SENSOR SKINS FBGs in highly birefringent PCFs as discussed in section

II can be embedded in the polymer pads described above. Pressure or transverse strain is then directly detected by the sensor gratings, which are insensitive to axial strain. This is illustrated in Fig. 10 that shows the results of two-dimensional finite element simulations carried out using commercially available COMSOL multiphysics software.

With this level of sensitivity, it is difficult to measure loads typically encountered in biomechanical applications which are on the order of tens of kPa, as these lead to very small changes only in the Bragg peak separation. Although the butterfly PCF has proven its importance for hydrostatic pressure sensing and transverse load sensing in its bare state, the results indicate that its high transverse load sensitivity is just not sufficient for most biomechanical applications when it is directly embedded in a flexible polymer pad. This is due first to the relatively small loads that are considered in this field and second, because the stress induced by the load is distributed over the entire pad. Hence the portion of stress actually transferred to the embedded fiber is limited.

glue joint protection

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Fig. 10: Material birefringence in a Butterfly PCF oriented at 30° due to a 1 MPa transverse load applied to a polymer pad with elastic modulus E = 615 kPa and Poisson coefficient ν = 0.49.

Decreasing the mismatch in material properties between

fiber and host material one possibility to increase the sensitivity of the pad. Another possibility is to structure the polymer pad in order to maximize the induced stress in the region around the optical fiber sensor. This is illustrated in Fig. 11 which shows the strain anisotropy that is present in a polymer pad with a Butterfly PCF embedded in the center when a distributed load is applied on the top surface while the bottom surface is fixed. Red regions are regions where the strain anisotropy is high, and thus regions that are favorable for positioning the fiber as its material birefringence will be most influenced there. Fig. 11 depicts the anisotropic strain when grooves are fabricated (e.g. with laser ablation) nearby the embedded fiber. Due to the presence of these macrostructures stress is concentrated resulting in an increased response to pressure.

Fig. 11: Strain difference εx - εy present in a polymer pad with an embedded PCF. (a) Plain structure. (b) With a groove macrostructure.

Fig. 12: Overview of the possible improvement to the pressure response dΔλ/dP of embedded Butterfly PCF sensors when different types of macrostructures are applied.

Fig. 12 gives an overview of the possible improvement of the sensitivity of the Bragg peak separation of a butterfly PCF when embedded in a polymer pad with various types of

macrostructures. When no macrostructure is present (such as in Fig. 11 (top) the pressure sensitivity would be -21 pm/MPa. However by introducing a macrostructure (such as in Fig. 11 (bottom), this sensitivity can be increased with at least 73%. This brings the response up to the level of biomechanical application requirements, i.e. from 35 to 50 pm/MPa.

V. CONCLUSION First we exploited the unprecedented design flexibility

offered by photonic crystal glass fiber technology to design and fabricate Bragg grating based pressure sensor devices that operate in a temperature insensitive manner and that have a pressure sensitivity exceeding that of state-of-the-art bow-tie fiber based sensors with a factor of 20. We then embedded these PCF sensor devices in polymer pads to form sensor elements that can be applied in various biomechanical applications requiring distributed measurements of pressure exerted on the human skin with the intention to reliably address the interaction of the human body with external forces and hence to provide, for example, increased patient comfort or to avoid the development of pressure ulcers. We have developed different fiber embedding techniques based on molding, laser ablation or soft lithography to integrate multiplexed fiber sensors in stretchable polymer host materials. We proposed means to improve and to tune the sensitivity of the PCF sensor pads by structuring the polymer host.

Whereas there is still progress to be made in the field of PCF based sensors, we believe that these offer real advantages on different fronts. We are therefore confident that there is a future for this optical fiber sensor technology in a wide range of medical applications. To ensure this future we will continue tackling essential issues such as packaging, fully-fledged system integration, optical coupling and interfacing, dependable strain transfer and reliability.

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