Plastic Optical Fiber Sensors

Transactions of the Institute of Measurement and Control 22,5 (2000) pp. 431–457

Plastic optical fibre sensors anddevicesRebecca J. Bartlett, Rekha Philip-Chandy,Piers Eldridge, David F. Merchant, Roger Morganand Patricia J. ScullySchool of Engineering, Liverpool John Moores University, Liverpool, UK

This paper considers the impact of recent developments in polymer optical fibre (POF) andits application in optical fibre sensors and optical measurement. A discussion of techniquesdeveloped for POF-based sensors and devices, and their applications includes sensors to meas-ure flow, biofilm growth, turbidity, toxicity, humidity, rotation and fluorescence. Techniquesto photoinduce refractive index changes and physical gratings into POF, and chemically toremove the cladding and taper plastic optical fibres are described, along with the use of fluor-escent dye-doped plastic optical fibres (FPOF) in sensors and devices.

Key words: environmental measurement; flow; fluorescence; multimode; optical fibre sensor;particle concentration; polymer optical fibre; polymethylmethacrylate; turbidity.

1. Introduction

Plastic optical fibres (POF) are large diameter, flexible and durable multimodefibres made from polymers, including polymethylmethacrylate (PMMA), poly-styrene, polycarbonates and perfluorinated materials. POF suffers from a cheapand cheerful image; typically POF fibre has a 1-mm diameter core and 20-mmcladding, costs about 50p a metre, can be cut with a scalpel or hot knife, and

Address for correspondence: Patricia J. Scully, School of Engineering, Liverpool John Moores Uni-versity, Byrom Street, Liverpool L3 3AF, UK. E-mail: p.j.scullyKlivjm.ac.uk

2000 The Institute of Measurement and Control 0142-3312(00)TM029OA

432 Plastic optical fibre sensors and devices

polished using abrasive papers or metal polish. POF connectors and device hous-ings can be made out of plastic. Once the poor cousin of silica fibre, POF’s cheap-ness, ease of termination and robustness is rapidly increasing its credibility forlocal area networks (LANs), etc., as new graded index POF fibres become available(Polishuk, 1998), and advances in materials and manufacturing increase the POFbandwidth up to Gbits per second and reduce losses down to 10 dB/km (Kalishand Clayton, 1997).

POF was developed in 1968 by the US company Dupont, who sold the patentsto Mitsubishi Rayon, and since then, POF technology has developed in Japan,with recent rapid advances being made in reducing attenuation and increasingbandwidth for telecommunications applications. It is expected that as POFbecomes more widely accepted and its market share increases, its advantages inease of preparation and termination, and easy coupling to detectors and emitters,together with the low cost of terminations and devices, will make it highly attract-ive for sensing applications, particularly where throw-away, rugged front endsare required. The success of POF has led to a number of reviews of its telecommsapplications in trade magazines (Berman, 1998; Polishuk, 1998) and a book aimedat technicians (Club des Fibres Optiques Plastiques, 1997). POF is emerging asthe new standard for LAN cabling because the installation cost of POF is less thaneither copper or glass, although its material cost falls directly between the cost ofhigh-speed copper and glass (Berman, 1998).

POF has several advantages over glass fibre as far as sensing applications areconcerned, in that a number of enabling technologies have made it possible sig-nificantly and easily to enhance the performance of POF for sensors, using simpleand inexpensive techniques and equipment, compared with those required forglass. The only review in the literature of sensing applications is a brief trademagazine article by Graydon (1998), which deals solely with the sensorsdeveloped at Liverpool John Moores University, and consequently a more compre-hensive review was felt to be necessary.

The topics covered in this paper include a discussion of conventional POF fibrefor communications and dye-doped POF. Sensors based on conventional POF arebriefly described, followed by a detailed treatment of sensors based on dye-dopedPOF. Enabling technologies that allow POF to be sensitized to various measurandsor have its optical and physical properties modified to enhance its performance,are then detailed. These include evanescent field modulation by use of a chemi-cally stripped POF core exposed to the measurand (Chandy and Scully, 1998); coreelectric field modulation in a double-cladded inverse W high–low–high refractiveindex-profiled POF fibre, using tunnelling ray resonance (Zhang et al., 1996); andgrooving of fibres (Philip-Chandy et al., 1999). Refractive index profiling isobtained by dip-coating the stripped or cladded fibre with optical epoxy or sol-gels of defined refractive index (Zhang et al., 1996). Fluorescent core-doped POFhas been used for various sensor applications (Poisel et al., 1994; Klein et al., 1996;Chandy et al., 1998; Merchant et al., 1998). Physical surface and Bragg gratingshave been successfully photoinduced into the distal ends, sides and core of dopedand undoped plastic optical fibres and preforms, using laser radiation at variousultraviolet (UV) wavelengths (Schmitt et al., 1996; Peng et al., 1999), and tech-niques for chemical tapering of POF have been developed (Merchant et al., 1999).

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The sensors described in this review are in the research prototype stage and havenot been manufactured in quantity.

2. Types of POF fibre

The new types of POF emerging in the market-place demonstrate its commercialviability and encourage the marketing of POF connectors and associated optoelec-tronic devices, enabling POF-based sensor systems to be constructed easily andcheaply. Thus a discussion of POF types and their impact on sensors is instructive.

2.1 POF for communications

New types of POF have emerged in the race to reduce attenuation and increasebandwidth for telecomms applications. A discussion of the properties of telec-omms POF is important when considering its use for sensors and measurement,because the large market and high volume manufacture driven by the burgeoningcommunications industry ensures longevity in the market-place, and enables POFfibre and components to be affordable for sensors and control applications.

Key developments are listed as follows: in 1992, Yoshiro Koike developedgraded index POF (GIPOF), and demonstrated a 1 Gbit/s capability over 30 musing a light-emitting diode (LED), and 2.5 Gbit/s transmission at 100 m usinga laser diode. Reseachers at Keio University in Japan developed a deuteratedPMMA fibre with losses of 20 dB/km and predict a theoretical attenuation limitat 1.3 and 1.5 mm of 0.25 and 0.2 dB/km respectively, comparable to that of silicafibre (Koike, 1998). Step index POF with a numerical aperture of 0.3 has demon-strated 155 Mbit/s transmission over 100 m. In 1995, Koike developed perfluori-nated GIPOF using a Teflon-type fluoropolymer called CYTOP, with low absorp-tion from visible wavelengths to 1300 nm (50 dB/km), enabling light sources anddetectors developed for silica fibre to be used with perfluorinated POF. A band-width of 1 GHz is anticipated for CYTOP fibre. Such improvements in POF per-formance in the telecommunications sector enhance sales and reduce the cost ofPOF components and enable it to be commercially viable for sensing applicationsas well.

To prove that POF is a serious contender, various countries are implementingPOF schemes and networks. As part of the US government commitment todevelop technologies strategically important to the US military and industry, theHigh Speed Plastic Network (HSPN) was established to develop GIPOF tech-nology. HSPN included Boeing, Honeywell, Hewlett Packard and Boston OpticalFibre (BOF) and worked for 3 years to bring POF technologies to the aerospace,automotive and datacomms marketplace. The experience gained and the tech-nology developed by HSPN members enabled the passage of the first POF indus-try standard set in May 1997.

POF-based LANs were developed by General Motors (US), Mercedes Benz(Germany) and Toyota (Japan). Boeing are considering POF for in-flight com-munications and entertainment for aircraft (Polishuk, 1998). European POF


activity takes place mainly in France and Germany. The Association of GermanElectrical Engineers (VDE) formed a POF subcommittee from members from thecommunications, automotive, cable and components industries as well as univer-sities and research institutes (Ziemann et al., 1997). In France, a plastic opticalfibres club was established to promote industrial development of POF. POFdevices in the research stage for telecomms and computer applications includeWDM (wavelength division multiplexing; Uehara and Mizusawa, 1998), two-dimensional array interconnects to VCSEL arrays (Neyer and Johnck, 1997) andGRIN (graded index) lenses have been made from GIPOF (Uodu and Hirota,1998).

The upper temperature limits of POF are important for industrial measurementand control applications and have been well documented for communicationsapplications. Conventional POF is limited to 85°C maximum, and when exposedto temperatures higher than this, the core polymer oxidizes (Hadjiloucas et al.,1996), creating a transmission loss. Conventional POF undergoes spectral changesin the UV and blue part of the spectrum (370–490 nm) due to ageing, which areinsignificant for short lengths of fibre (1.5 m), but create a problem for lengths of5 m or more. Accelerated ageing tests were performed for step-index POF equival-ent to environmental effects of temperature and humidity over 10–15 years (Daumet al., 1997), and it was found that the spectral changes induced affected the trans-mission of green light for traffic control. Considerable effort is being invested inproduction of POF which can withstand higher temperatures. Heat-resistant POFwith thermoplastic resin for the core and silicone elastomer for the cladding wasdeveloped by the Furukawa Electric Company in Japan, and exhibits a trans-mission loss of just 0.2 dB/m after 500 h of exposure to temperatures of 80°C and95% relative humidity (RH). Futjitsu have manufactured polymer fibres usingARTON, which are stable up to 150°C, and GIPOF fibre doped with triphenylphosphate (TPP) demonstrate high thermal stability at high humidity (80°C, 80%RH; Ishigure et al., 1997). Fibre refractive index profiles were stable at 85°C over5000 h. New dopants for GIPOF under investigation with Tg (glass transitiontemperatures) greater than 90°C and high humidities of 80% RH, are stable after600–700 h.

The properties of POF that are increasing its popularity and competiveness fortelecomms are exactly those that are important for optical measurement, sensorsand devices, and also for control applications where POF is merely acting as aconduit for information in the same way as a telecommunications fibre.

2.2 Dye-doped POF

In recent years, the incorporation of dyes into solid state materials has enableddevelopment of various optoelectronic devices ranging from tunable lasers andamplifiers to nonlinear optical devices such as switches and modulators. Thesedevices have applications in optical communications, photonics, material science,medicine and optical spectroscopy. Usually organic laser dyes are used, whichhave the advantages of high and fast nonlinearities, high quantity efficiency andbroad spectral range. PMMA doped with such dyes has advantages over its silica-

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based counterparts in that it has better efficiency, beam quality and superioroptical homogeneity than ORMOSIL (dye-doped organically modified silicates).

Further advantages of dye-doped POF include the fact that a wide range oforganic materials can be incorporated as dye molecules due to POF’s low pro-cessing temperature. Silica fibres are manufactured at temperatures in the range1800–2100°C, making it impossible to dope organics into them. Optical nonlinearorganic dyes begin to deteriorate at 300–500°C, but these temperatures are abovethe manufacturing temperature range of POF, enabling the incorporation of suchdyes into POF during manufacture.

Fluorescent-doped polymer fibre (FPOF) was developed in the late 1980s(Blumenfeld et al., 1986; 1987), originally for measurement of nuclear radiation bythe French Atomic Commission. Fluorescent POF is available with a fluorescentdye doping the core or the cladding. Cladding-doped FPOF is not available com-mercially but has been manufactured in research laboratories for specific experi-ments and applications (Muto et al., 1989).

Core-doped FPOF is commercially available in a range of dopants and withcircular or square cross section, and can be manufactured to specific requirements,by various commercial and research sources such as Optectron, France; Tven Poly-mer Optical Fibre Scientific Production Association, USSR and Poly-Optical, USA.The application of core-doped FPOF has been extended into the area of high-voltage discharges (Destruel et al., 1992), high-energy particle scintillators (Barniet al., 1996), optical amplifiers (Tagaya et al., 1995a,b), physical sensors and decor-ative lighting and displays.

There is little data on the temperature stability of commercial core-doped FPOF,but dye-doped fibres manufactured at Tver Research Production Centre for POFin Russia were assessed for temperature and ageing, and exhibited small effects.When FPOF samples were temperature cycled from 10 to 70°C, the intensity oftransmitted light at peak absorption or emission wavelengths changed by 5% butshifts in the peak absorption and emission wavelengths were not detected. How-ever, when exposed to sunlight, the absorption and emission spectra of certaindye-doped FPOFs shifted to longer wavelengths (Klein et al., 1996).

Work on laboratory-produced dye-doped POFs for optoelectronic devices com-menced in the early 1990s. Kuzuk et al. (1991), produced POFs doped with variousdye molecules and demonstrated highly nonlinear optical effects. Tagaya et al.(1995a,b) created active POF with laser dyes as dopants. Peng et al. (1994), workedon POF with b-carotene as the dopant for all-optical switching and laser dyes asdopants for optical fibre amplification. Dye-doped polymeric devices formed intoPOF have advantages over their waveguide or bulk optic counterpart, in thatuseful and usually weak effects can accumulate over long distances to build upa total effect, due to the confined core area and long interacting lengths. Appli-cations include all-optical switching and optical amplification.

Bulk polymer hosts impregnated with a certain dye have achieved 80% conver-sion efficiency from pump power to signal power with a tuning range close tothat in solution (Hermes, 1994). Dye-doped POFs can be formed into useful fibreamplifiers and lasers that operate at wavelengths other than the 1300 and 1550nm used in silica-based fibre lasers. Laser dyes used to dye-dope POF includedrhodamine 6G, rhodamine B, pyrromethane 650 and fluorescein. Innovative


devices have been designed based on single-mode core dye-doped POF fibresincorporating embedded electrodes (Welker et al., 1998), in which the electrodesare used to polarize the dye-doped core to give second-order susceptibility.

3. Sensors based on conventional plastic optical fibres

Sensors using classical, commercial (undoped or unmodified step-index PMMA)POF have been developed over the last 15 years and are based on ideas alreadyused in silica glass sensors, but exploiting the rugged and cheap nature of POFfor harsh environments and throw-away sensors. An example of this technologytransfer is a displacement sensor, developed by Ioannides et al. (1996), based onan array of three POF fibres, with the centre fibre emitting light that is reflectedfrom a mirror surface and captured by two outer fibres, whose distal ends arepositioned at different distances from the mirror. The range of the sensor extendsto 100 mm with an accuracy of 1%. A respiratory plethysmograph for use in amagnetic resonance (MR) scanner (Raza and Augousti, 1995), exploits the wellknown modulation mechanism of microbending by measuring the intensitychanges when a loop of POF is caused to expand and contract in sympathy withthe ribcage. A goniometer for monitoring limb position in a MR scanner via a40-m fibre-optic link, was developed by Scully et al. (1993), where the POF wasused to transfer light to and from an angled cylindrical rotating glass wedgemodulator. The goniometer had an angular range of 90°, with an average resol-ution of 29, and long-term accuracy of 61°, which is the specified accuracy forthe physiological application. The advantage of POF in a medical environment isthat the fibre can withstand rough handling by medical staff, and can easily bereplaced if broken by in-house technicians, as well as having the usual advantagesof immunity to electromagnetic interference, intrinsic safety and chemical immun-ity.

Hadjiloucas et al. (1996), developed a sensor for monitoring water uptake byplants. Again this was based on ideas from glass fibre sensor technology; a trans-mitting and reflecting fibre cut at optimum angles monitored the light reflectedfrom a meniscus in order to measure its position, to give an indication of wateruptake by a plant to a resolution of 2.5 × 10−9 l. The advantage of POF in thiscase is that it is easy to polish and cut to defined angles. The same sensor wasused to monitor the reflectivity of a mirror surface as a function of condensation,as part of a condensation dewpoint hygrometer.

Steiger (1998) describes the use of standard POF for a variety of applicationsincluding mechanical pressure (microbending), gas and liquid pressure, liquidlevel sensor (evanescent field sensing), radius (bending) sensor, stress, thermalswitch and rotation sensor, and colour detector. Niewisch (1997) describes POFsensors for measuring the level of LN2, detecting nitrogen bubbles and for quenchdetection applied to the challenging environment of a high-temperature supercon-ducting fault current limiter. The sensors were required to perform at tempera-tures as low as 77 K, have immunity to temperature variation between 77 and300 K, be unaffected by voltages up to 420 kV, and to be mechanically robust andcheap. The principle of the quench detector was to measure a voltage generated

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across a superconducting current limiter when short circuited by using the currentto illuminate LEDs. The light from the LEDs was transmitted through a length ofPOF to a detector and the reaction time was 350 ms compared with 1 ms usingelectromechanical elements. The bubble detector and liquid level sensor werebased on measurement of refractive index changes at the end of a cut length offibre. Telecomms devices based on classical POF include a 1 to N POF coupler(Faugeras et al., 1998) and a POF active coupler utilizing the ability to control theattenuation properties of liquid crystals to perform switching functions inresponse to an applied electric field (Irusta et al., 1997).

4. Sensors using dye-doped plastic optical fibres

4.1 Cladding dye-doped FPOF sensors

Cladding-doped FPOF enables light emitted at the excitation wavelength in thevicinity of the cladding, to generate fluorescent light that is coupled into the coreand propagated along to the end of the fibre for detection. To date, cladding-doped FPOF is not available commercially and has to be manufactured in the lab-oratory.

This mechanism has been exploited by doping the cladding of POF with fluor-escein, which has a fluorescence spectrum susceptible to humidity. The intensityof fluorescent light is proportional to the humidity of the air surrounding thefibre. The fluorescein is excited by a He–Cd laser of wavelength 441 nm (Mutoet al., 1989; 1990; 1994), and measured using a band-pass filter and photomultipliertube to an accuracy of 63% RH over a range of 5–85% RH. Response time is lessthan 1 s. Researchers at Liverpool John Moores University (LJMU) developed asensor based on the same principles but using a cheap blue LED as light source,and silicon photodetector (Hasseinibalim, 1997), with a range of 8–94% RH, accu-racy of 61.6% and sensitivity of 2 nW/% RH. The work performed by Muto usedfibre manufactured by an in-house fibre extrusion facility, but at LJMU the clad-ding-doped fibre was prepared in the laboratory by stripping step-index POF(described in section 5.2) and then painting the stripped core with an aqueoussolution of sodium fluorescein (uranine) which was allowed to dry in air. How-ever, the salt flaked off the fibre surface. A more robust sensor was developedusing sodium fluorescein salt added to an optical epoxy which was applied tothe stripped fibre.

Morisawa and Muto (1998) have cladded an ARTON fibre core with tetrophen-ylporphine (TTP) dye-doped polymer. When pumped using a He–Cd blue laser,fluorescence took place proportional to the oxygen concentration for both gaseousand dissolved oxygen. The response time of the sensor was 2.5 min, and it demon-strated a linear calibration curve in the range 0–10 mg/l of dissolved oxygen. Thedevice could detect up to 100 ppm of oxygen with 0.01% resolution.

4.2 Sensors based on commercial core dye-doped FPOF

Exciting developments in sensor technology are possible with commercial coredye-doped FPOF. The cheapness of the fibre (£2–3/m) and ease of terminationenables innovative but cheap and disposable sensors to be constructed.


Commercially produced FPOF has been applied to a number of physical sensorssuch as a slip ring by researchers at Nuremburg (Poisel et al., 1994; 1997). FPOFwas used to detect the data transmitted from an LED rotating round a circularaxis, by shaping it to an annular ring of radius 1.25 m, as shown in Figure 1. Thebandwidth is limited by the fluorescence lifetime of the dye contained within theFPOF, which was about 7 ns giving a system bandwidth of up to 155 MHz. Theangular position of the light source could be measured to an accuracy of 1%.Conventional mechanical sliprings consist of electrically conductive metal ringsand contact brushes, and transmission rate is limited to 5 Mbit/s. Abrasive wearleads to a limited lifetime, and time-consuming maintenance. The researchersdeveloped a three-colour sensor using circles of FPOF core doped with three dif-ferent types of fluorescent dye to cover a spectral range of 320–720 nm. This wasused to measure the emission spectrum of different broad-band light continuous-wave (cw) and pulsed light sources (Klein et al., 1996). Other sensors based oncommercial FPOF include an ambient light detector, optical potentiometer, opticalposition detector and intruder detector (Laguesse, 1993). Grattan and Kalymnios(1998), describe the possibility of doping POF with temperature-sensitive fluor-escent dyes and measuring their decay-time. An optical fibre dipstick has beendeveloped by Augousti et al. (1990), based on a green fluorescent POF fibre dopedwith commercial organic dye and dipped into a liquid. The intensity of fluor-escence guided along the fibre was used as a measure of the depth of the liquidin which the fibre was placed.

4.3 FPOF distributed photodetectors

The use of commercial FPOF has been demonstrated as distributed photodetec-tors, with a spectral response defined by the absorption spectrum, and as wave-length shifters, leading to applications in spectroscopy and spectrofluorimetry,negating the need for gratings and diffractive elements (Merchant et al., 1998).Together with diffuse, wraparound light sources such as electroluminescent films,the distributed detecting properties of fluorescent POF have enabled turbiditymeasurement by multiangle illumination and detection of light scattered fromparticles in aqueous suspension (Chandy et al., 1998).

Figure 1 Optical fibre slip ringSource: Poisel et al. (1994)

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The sensors exploit the FPOF as follows: the core of the FPOF acts to redirectlight incident through the side walls of the fibre into guided light along the coresix times more efficiently than conventional fibres. The combination of FPOF injec-tor and POF link to a photodetector forms an extended cylindrical optodetector.The absorption spectrum of the dopant is selected preferentially to detect thewavelength of light scattered by or emitted by the fluid under test and rejectany stray light. An example of an application of this arrangement to measure thefluorescence of a liquid is discussed in section 4.4.

4.4 Fluorimeter

The sensor comprises a transparent cylindrical tube through which the waterunder investigation is continually pumped. An electroluminescent film (ELF) sur-rounds the exterior surface of the tube at one end as an excitation source. Anarray of FPOFs is embedded into the exterior tube wall some distance from thefilm (Figure 2).

The ELF is chosen to emit a wavelength range matched to the application. Filmsare available commercially from 400 to 800 nm in broad band (60–100 nmbandwidth), or multiband ‘white’ spectra (Ono, 1995). They are low-power(several mW per cm2), cool, and emit a uniform omnidirectional flux. The planar

Figure 2 Schematic of optical fibre fluorimeter (courtesy of D.F.Merchant)


construction of the ELF gives optimal sensor performance, in that its large surfacearea contact offers uniform illumination over the maximum volume of the tube.

Light generated by the light source is not internally reflected when it passesthrough the tube due to symmetry. If it is redirected while passing through thewater, it may be converted into a guided ray and thus propagate along the tube(Figure 3). This may be due to scattering (turbidity) or fluorescence events. Ifscattered, the wavelength of the re-directed ray is unchanged. If generated byfluorescence, the ray is shifted to a longer wavelength. The light is confined withinthe outer tube walls and so can interact with the embedded optical fibres andexcite modes within them. The end section of these fibres is doped with a fluor-escent material (FPOF) to enhance this coupling by efficiently converting lightpassing into the fibres through the sides into guided modes (Figure 4).

The separation of fluorescent light from the excitation signal is provided byselective wave-guiding of the fluorescence within the tube walls. No optical filters,gratings or selective detectors are required making the sensor inexpensive, reliableand versatile. The absorption spectrum of the FPOF dopant is selected to preferen-tially detect the fluorescent light wavelengths of the fluid under test and rejectany stray light from the excitation source. By appropriate choice of and positioningof FPOFs, the device can measure both fluorescence and ‘colour’ in terms of broad-band (50–200 nm) wavelength absorption without recourse to separate and lossyoptical wavelength filters or dispersive elements, and can also measure turbidity.Using pure (single parameter) samples the sensor has been shown to measure:

1) fluorescence: 0.2 ppb up to 100 ppm fluorescein sodium (uranine) concen-tration (Figure 5). Linear from 0.2 ppb to 80 ppm with resolution of 0.15 ppbfor samples up to 1 ppm and sensitivity of 13 nW/ppm with accuracy of 0.6ppb at 10 ppb. Long-term drift was less than 4% in 12 months;

2) turbidity: 0.1 to 500 FTU. Linear from 0.1 to 100 FTU (Figure 6 shows the lowturbidity characteristic). Resolution 0.1 FTU for samples up to 100 FTU.

Figure 3 Preferential waveguiding of fluorescent light byangular changes (courtesy of D.F. Merchant)

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Figure 4 Fluorescent injector (courtesy of D.F. Merchant)

3) absorption: 0.1–100% (measured as percentage transmission at peak wave-length through a 10-mm path), with resolution less than 1%.

The device shows almost zero drift from calibration when in use. The outputis monitored and changes corrected for by reference monitoring. Water is exposedonly to the inside of the tube so that fouling is reduced to a minimum. The sensortube is simple to clean when required, and offers no restriction to flow. Themajority of the system cost and complexity is in the detection and processingelectronics so the tube assembly can be replaced if damaged. The tube dimensionsare chosen for the flow used and ranges required.

The advantages of the LJMU sensor design over fibre–fibre or lensed fibreprobes for absorption measurement, or excitation and 90° detection for fluor-escence measurement, include the fact that the inherently low coupling efficiencyof sensors, using the distal end-face of fibres to collect light from illuminatedfluids, increases the complexity of detection and the power required from theexcitation source. Thus high-powered lasers and photomultiplier/avalanchediode detectors are required, resulting in high system cost and increased size. Useof conventional undoped fibres for light collection would result in the need forseparate optical filters and gratings. The ability of FPOF to collect and guide lightincident over its whole surface area, renders it more effective than fibre evanescentwave absorption/excitation techniques. Finally the requirement that the fibres insuch systems be in direct contact with the test fluid can lead to fibre surface foul-ing, and damage if particulate matter is present.


Figure 5 Response of optical fibre fluorimeter to fluorescence(courtesy of D.F. Merchant)

Figure 6 Response of optical fibre fluorimeter to turbidity(courtesy of D.F. Merchant)

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4.5 Toxicity sensor

The toxicity biosensor (Merchant et al., 1998) uses the fluorosensor describedabove in section 4.4 to measure the fluorescence generated by a toxicity assay,which produces fluorescein as the indicative measurand. The fluorescence is gen-erated when living micro-organisms (Chlorella spp. algae) in aqueous suspensionhydrolyse fluorescein diacetate (FDA) to fluorescein, which fluoresces at 515 nmwhen excited at wavelengths below 490 nm. The hydrolysis reaction is pro-portional to the metabolic rate of the organisms and is inhibited by the presenceof toxins. Therefore, measurement of the rate of increase of fluorescent intensityindicates the toxicity of the cell environment. Electroluminescent film, radiatingat around 490 nm, is wrapped round a glass tube containing the cells in water,to which 0.1 ml of stock solution of FDA is added. The fluorescent light is collectedby scattered coupling into the walls of the glass tube, in which are embedded anarray of 1-mm diameter FPOF fibres, forming a bundle to transfer the light to adetector, as described in section 4.4.

4.6 Turbidity sensor

Fluorescent POFs have been used for turbidity measurements for water qualitymonitoring. A flow-through sensor has been developed with a linear responseover a range 0.1–200 FTU, with a sensitivity of 3.3 nW/unit FTU, and a resolutionof 0.1 FTU for 0–1.0 FTU (Chandy et al., 1998; Figure 7). It uses a diffuse, wrap-around electroluminescent film as a light source, to illuminate at all angles, andfluorescent fibre to detect at all angles. It is known that particles of different sizesscatter light at preferential angles, enabling particle sizing to be performed(Naimimohasses et al., 1995). Traditional turbidity measurement involves thedetection of light scattered at 90° to the illumination direction, and has the disad-vantage of being unable to detect particle sizes below 5 mm, allowing filter break-

Figure 7 Turbidity sensor characteristic: plot of light intensityversus turbidity for sensing and reference fibre


through to occur and the nondetection of dangerous spores such as Cryptosporid-ium. Tests have shown that the LJMU turbidity sensor has the potential to detectparticles that cannot be detected by traditional and conventional means.

5. Enabling technologies

Many researchers have manufactured their own specialist POF fibre, usingresearch laboratory facilities, for sensing applications (Muto et al., 1989; 1990; 1994;Peng et al., 1998; 1999). However, there are methods of simply altering the opticalproperties of commercially available POF fibre without recourse to polymer manu-facture or fibre extrusion facilities, by stripping the cladding, adding an extra layerof epoxy, by grooving or by chemical reaction to create a sol-gel. Refractive indexprofiling is obtained by dip-coating the stripped or cladded fibre with opticalepoxy or sol-gels of defined refractive index. Epoxies have been doped with fluor-escent compounds to create a fluorescent dye-doped cladding. Surface physicalgratings have been sucessfully photoinduced into the distal ends and sides ofplastic optical fibres, using UV laser radiation at 248 nm. Bulk refractive indexchanges have been photoinduced in PMMA. In addition, techniques have beendeveloped for chemically tapering POFs. These techniques will be described indetail as follows:

5.1 Photoinduced refractive index changes and gratings in POF

The technology of Bragg gratings, formed in silica optical fibres using UV laserlight to create photoinduced refractive index changes, is well established and hasfound numerous applications in communications and metrology. Gratings in POFlag far behind, with grating formation being reported by just two groups in Liver-pool and Australia.

In Liverpool, surface physical grating formation was demonstrated in commer-cial POF supplied by Toray Industries (PGR-FD1000) using a UV excimer laserat 248 nm. A number of incubation pulses (about 90 at 1–2 Hz) in the preablativeregime (below 90 mJ/cm2) were applied to the fibre using a phasemask to formfringes (Schmitt et al., 1996; Schmitt, 1999). The gratings were revealed by washingwith isopropyl alcohol (IPA), which reacts with photoproducts or oligomers ofPMMA, to remove the fragments and expose the periodically perturbed surface.This procedure was used to create gratings with a 524-nm period and depth of250 nm on fibre distal ends, and 10-mm long gratings with period 535 nm, on theside of a stripped cladding POF. Such gratings can be used as fibre-based gratingcouplers for sensing refractive index changes, as well as wavelength monitoring.The disadvantage of the technique is that because of the revealing process, opticalfeedback to enable control and optimization of the procedure is not possible, andin-fibre refractive index changes were not achieved.

Recent work at LJMU has created refractive index changes within PMMA at avariety of UV wavelengths. The aim of this work is to develop sensing devicesin POF using gratings, and refractive index profiling by optimizing the effect of

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various wavelengths on photochemical reactions and refractive index modulationin PMMA and other polymers such as polystyrene and CYTOP mixtures usedfor POF cores and cladding. Refractive index modification is of interest to manu-facturers of graded index fibres because index profiles can be created duringfibre manufacture.

In Australia, Peng et al. (1999) reported a study of the photosensitivity effectin dye-doped polymer optical fibre, and the fabrication of gratings in these fibresand their preforms, and demonstrated two types of photosensitivity due to eitherthe dopant or the inherent properties of PMMA.

Peng et al. (1999) manufactured double-cored monomode PMMA fibres in thelaboratory, and doped the 3-mm diameter twin core with 170 ppm fluorescein. Acw argon ion laser emitting at 514 nm was launched into one of the cores of thePOF fibre to induce a refractive index change, and the fundamental mode wasinterrogated using a He–Ne laser beam at 633 nm at 1 mW, which was launchedequally into both cores. The far-field intensity distribution from each fibre formeda fringe pattern, and its fringe shift was recorded for various argon ion laserpowers and converted into phase shift. A total phase shift of 15p, correspondingto an induced refractive index change of 3.3 × 10−5, was recorded for 0.36 mW for9 min and 1.08 mW for 53 min.

Further work by Peng used an OPO (optical parametric oscillator) to generatepulses of 5 ns width at 10 Hz at wavelengths of 325, 280 and 248 nm, in order toilluminate the twin-cored fibre transversely. Fringe shifts up to 10p were observed,due to the photosensitivity of PMMA polymer rather than the fluorescein dopant.

Peng et al. (1999) also reports the formation of gratings in a polymer fibrepreform. A grating was written using a phase mask with period of 1.06 mm. Twowavelengths of 248 and 325 nm were used to illuminate at a 10-ns period at 5 mJ.At 248 nm, a surface grating was formed. At 325 nm, a bulk grating was formedwithin the core, due to periodic change in refractive index with photosensitivityeffect. Gratings were also formed in multimode laboratory-manufactured POFfibre drawn from a preform using 325 nm and a Sagnec interferometer arrange-ment. The multimode POF formed by Peng et al. (1999) has seven modes as indi-cated by the seven peaks in the reflection spectrum of the grating (Figure 8).

Photoinduced refractive index changes have applications in altering the refrac-tive index profile of POF to form GI-POF, and have been investigated alongsideother methods such as using a centrifugal field to generate a composition gradientof monomers (Van Duijnhoven et al., 1998), or using CF4 + H2 plasma surfacefluorination of PMMA (Bartoli et al., 1998). Miyata et al. (1997) have describeddoping PMMA with 4-nitrophenyl-N-butylnitrone and illuminating with 330-nmUV light to achieve a 2% refractive index after 60 s. Kumar et al. (1997) describethe formation of surface and volume gratings in polymer optical fibres. PMMAdoped with benzoquinone can be volume written at blue and green wavelengths.

5.2 Stripping the cladding

Removing the cladding from an optical fibre and replacing with the measurandis a well known technique used with polymer clad silica (PCS) fibre to form


Figure 8 Reflection spectrum of multimode polymer fibre gratingSource: Peng et al. (1999)

evanescent field sensors (Deboux et al., 1995; Macraith et al., 1995). The evanescentportion of the electric field travelling within the core of the fibre penetrates intothe cladding layer, and changes in the absorption or refractive index of the clad-ding perturb the evanescent field, thus affecting the intensity of the guided modeswithin the fibre core. The cladding was easily removed from polymer-clad silica(PCS) fibre using chemical solvents, but initial attempts to remove the claddingfrom POF were by hand polishing or scraping with a scalpel, causing unevenremoval. Eventually a method was devised for removing the cladding from POFusing an acetone/water mixture by Bayle and Mateo (1996) and further refinedby Merchant et al. (1999). The CYTOP-based cladding on PMMA-cored POF[Raytela POF from Toray Industries (PFU-FB1000)] was removed by soaking inan acetone/water solution. The fibre to be stripped was supported on clamps,but with no tension applied. About five drops of acetone were applied to the fibreusing a 2 × 5 cm folded lint-free lens tissue which was hung on the fibre. Thetissue was immediately rotated and slid along the treatment region to expose thecladding to the solvent, using the fingers. It is possible to detect the removal ofthe cladding in the form of granules and a corresponding increase in surface fric-tion; 5 or 10 s after the acetone application, about three drops of distilled waterare applied to the tissue, and the rubbing continued. When the core is fullystripped, the surface tension reduces again, and the exposed region can be washedin water to remove the acetone.

The first application of the stripped fibre was a humidity sensor, described pre-

Bartlett et al. 447

viously in section 4.1, where the cladding was replaced with optical epoxy dopedwith fluorescein salt.

A second application was for use as a biofilm sensor, where the stripped coreof the fibre was exposed to biofouling and scaling conditions in a closed-loopwater supply. The sensor consisted of a 1-mm diameter multimode plastic fibrewith a PMMA core of 0.980 mm, surrounded by a thin cladding layer of fluori-nated PMMA, with its cladding removed over a 5-cm length to form the sensingregion. Light from a 660-nm laser diode source was transmitted through the fibreand its intensity measured using an ANDO power meter with silicon detector.

Biofilms were grown under laboratory conditions by using nutrient solutionsprepared by adding either 1% yeast extract or 1% glucose nutrients to tap water,which was left over a few days for biofilms to form. The fibre was immersed innutrient solution and left for about 220 h for biofilm to form (Figure 9). For thefirst 60 h, no significant change occurred in the signal, but over the 60–170-hperiod, the optical intensity change was 2.5 mW/h. After this the signal levelledoff. The ‘S’ shaped curve is a product of (a) the penetration depth of the evan-escent field; (b) the build-up of biomass of the biofilm to a constant level of about2 mm.

The effect of changing the refractive index of the cladding on the far field modaldistribution of a stripped cladding POF is shown in Figure 10. The POF was illumi-nated using light from a He–Ne laser, focused down using ×20 microscope objec-tive to illuminate the fibre distal end off-axis, so that the outer order modes wereexcited to form a doughnut shape when projected onto a screen. As the refractiveindex of the cladding is varied, the modal pattern is redistributed. Figure 10 showsa cross-section of the centre of the modal pattern, with transverse distance plottedagainst modal intensity distribution. The two peaks of the intensity distributionvaried with refractive index.

Uchiyama (1997) describes a POF sensor where the cladding of an Artone

fibre with norbornene core was removed and replaced with polyisoprene. Whenimmersed in acetone or alkanes, the refractive index of polyisoprene changes from

Figure 9 Biofilm sensor characteristic: plot of intensity of lighttransmitted through the sensor versus time for declad POF biofilmsensor (courtesy of R. Philip-Chandy)


Figure 10 Graph of far-field distribution of 1-mm POF, withoutouter modes preferentially excited, as a function of refractiveindex of cladding (courtesy of P. Eldridge)

1.52 to 1.46, causing coupling of leaky modes to guided modes within the POFfibre, when illuminated with a green LED. The sensor detects acetone concen-tration from 10 to 100% and applications include detection of gasoline and oilleaks.

5.3 Tapers in POF

Tapering is a technique well developed in silica fibre and glass optical fibre sensorsand devices using tapers are numerous. Replacing the cladding with a measurand,and exploiting the increased penetration of the evanescent field with decreasedfibre diameter, causes an enhanced interaction with measurands or analytespresent at the core–cladding interface, or immobilized in this region with a sol-gel substrate (Macraith et al., 1995) at the tip of a fibre probe. Fibres can be reducedto diameters of 1 mm, enabling them to penetrate into single cells (Uttamchandaniand McCulloch, 1996). Such configurations have been extended to POF to takeadvantage of its ease of coupling and termination, and low cost leading to throw-away devices. POF fibres are typically 1 mm diameter which is very large formedical and biological measurement applications, but tapering reduces the widthof the fibre tip and enables efficient coupling to glass or PCS fibre if necessary.Silica fibres are traditionally tapered using heat and pull techniques, but for POFtapers, an alternative technique of chemical etching, has been developed(Merchant et al., 1999) using organic solvents.

POF is tapered using chemical solvents to form both end tapers and bi-tapers

Bartlett et al. 449

(Figure 11). Reduction of the POF core diameter increases the evanescent fieldpenetration of guided modes, so applications include evanescent field sensing ofparameters such as refractive index, and core surface adsorption of chemicals andenzymes, as well as all the other well defined applications of glass fibre tapers.The POF tapers can be recoated with a polymer of defined refractive index, orcontaining fluorescent dyes, for sensing applications (Yamakawa, 1997).

The CYTOP-based cladding on PMMA cored POF (Raytela POF from TorayIndustries (PFU-FB1000)) is removed by soaking in an acetone/water solution.The exposed core of the fibre is tapered using a mixture of 60% acetone, 20%distilled water and 20% methyl isobutyl ketone (MIBK). The fibre region thatremains below the solvent surface is uniformly dissolved, producing a linear pro-file waist region, so that a tapered region is created if the exposure time to thesolvent is varied. A continuous change in fibre core diameter is produced byslowly removing the fibre from the solvent during the etching, as the fibre isimmersed in the solvent for different times depending on its depth below thesurface. Controlling the time and rate of immersion generates tapers with radialdiameter profiles that vary linearly with length, or which have discrete steps.

Biconical fibre tapers can be generated by immersing the middle of a length offibre in the solvent. The profile of the bi-conical tapers can be varied from smoothto step profile by controlling immersion. Experiments have shown that 1-mmdiameter fibres can be reduced to 0.1 mm over a 30-mm length.

Applications of POF tapers include mode redistributors, and reduction of thediameter of POF to enable effective coupling to glass fibre.

5.4 Physical perturbation of the fibre: grooving

POF fibre can easily be sensitized to bending or deflection by cutting grooves inthe radial fibre surface, to form an optical fibre strain sensor.

Figure 11 Photograph of tapered POF (courtesy of D.F.Merchant)


Perturbing the surface of POF fibre by cutting radial grooves into its surfaceenhances the bending loss when the fibre is curved, so that light transmittedthrough the fibre is attenuated as a function of the deflection of the fibre (Figure12). Thus, an optical fibre strain gauge is formed where the deflection or extensionof the fibre is related to the opening of each groove. A number of grooves wereformed radially into the fibre, using a hot scalpel, and the grooves extended intothe core. Six grooves were determined as the optimum number to achieve acompromise between insertion losses and sensitivity. The depth to which thegrooves extend into the core of the fibre was an important factor in determiningthe sensitivity of the device, and this depth is also ultimately limited by its effecton insertion loss and on the inherent strength of the plastic optical fibres. Thedepth of each groove was 0.5 mm. In order to maximize the fibre sensor sensitivity,the grooves were located where the beam exhibits its maximum deflection, i.e.,approximately at one half of the width of the beam from its base, as in the caseof conventional resistance strain gauge location.

As the cantilever bent in the presence of the airflow, the angle of the groovesvaried. The groove angle increased when the airflow was facing the sensor andvice versa. These changes of groove angle caused an intensity modulation of thelight transmitted through the fibre, because light was attenuated at each groove.Changes in intensity can be related to changes in the angle caused by the forceinducing the bending of the cantilever, and therefore to the velocity of the fluid.

The POF strain sensor described was used to form a flow sensor (Chandy et al.,1996; Philip-Chandy et al., 1999) mounted on a square cross-sectioned beamdeflected by fluid flow. Two optical fibre strain sensors measured the strain gener-ated in two orthogonal directions, when placed on adjacent sides of the deflectedbeam (Figure 13). Experiments verified that strain magnitude was proportionalto the square of velocity magnitude. Vectorial addition of the orthogonal strain

Figure 12 Diagram of grooves cut into POF

Bartlett et al. 451

Figure 13 Schematic of flow sensor (courtesy of N.F. Schmitt)

components gave the strain magnitude and direction. The optical fibre strainsensor consisted of a grooved piece of plastic 1-mm diameter fibre, with corediameter of 0.980 mm, and cladding thickness 20 mm, attached to the beam. Asthe beam deflected, the grooves opened and light was attenuated as a functionof the strain experienced by the beam surface (Figure 14). A flow range of 0–30m/s can be measured, with a sensitivity of 0.005 units of optical strain per metre.The flow magnitude resolution was 1.4 m/s, with repeatability of 0.3% and negli-gible hysterisis. Flow direction resolution was 5.9°.

5.5 Tailoring the refractive index profile of the fibre

The ability to couple light transversely, through the sides of a POF optical fibre,can be enhanced by coating the cladding with a layer of higher refractive index,of value matching the core (optical epoxy), over a sensitized region of around6 cm. This causes the fibre to be leaky, and for light to be coupled out, or trappedinto the core, if it penetrates into the outermost layer. This is, in fact, core electric


Figure 14 Characteristic of flow sensor; optical root strain versusmagnitude of wind speed for various orientation angles (courtesyof R. Philip-Chandy)

field modulation in a double-cladded inverse W high–low–high refractive indexprofiled POF fibre, using tunnelling ray resonance (Zhang et al., 1996).

Fibres treated as described were used to form a particle concentration sensor.Two, parallel plastic 1-mm diameter fibres were placed in the measurand solutioncontaining an aqueous suspension of particles. Light travelling within the trans-mitting fibre leaks out over the sensitized region, undergoes multiple scatteringafter interaction with the suspended particles and enters the receiving fibre, whichis similarly sensitized by an extra cladding layer (Figure 15). The light entering

Figure 15 Diagram of particle concentration sensor (courtesy ofF.H. Zhang)

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the receiving fibre is in the order of picowatts, but is measured using a 270-Hzchopped 850-nm light source to remove ambient light. The ratio of forward andbackscattered light in the receiving fibre gives a parameter proportional to particleconcentration. The sensor can be used in a point probe or a distributed mode andhas measured yeast concentration in the range of 0–16 g/l with an accuracy of3% and sensitivity of 3.4 pW/(g/l) (Figure 16).

5.6 Sol-gels in POF

Yamakawa (1997) developed a chemical sensor made out of two pieces of POFwith claddings removed and ends fused together. The ends were made pointedby drawing while heating, and then a hydrogel (hydrolysed PMMA) was createdby immersing the PMMA tip in concentrated sulphuric acid. The sensor end wasdipped in fluorescein solution, so that the dye was immobilized in the sol-gel.When excited using blue light (488-nm argon ion laser or 470-nm blue LED),the fluorescein dye was shown to be sensitive to pH over a range from 9 to 2.5pH units.

6. Conclusion

The recent advances in POF technology have enabled it to be considered as aserious contender for communications applications, and increased sales havereduced the cost of POF connectors and optoelectronic devices. Mirroring the shiftfrom communications into sensors and devices for glass based fibre in the 1980s,

Figure 16 Characteristic of particle concentration sensor(courtesy of F.H. Zhang)


research groups have developed enabling technologies to sensitize POF to measur-ands, or to change its characteristics in order to create POF-based sensors anddevices. The advantages shown by POF over silica-based fibre, such as ease oftermination, simple coupling to detectors and emitters, coupled with ruggednessand low cost, combine with the ability to modify POF characteristics using inex-pensive techniques, to enable low-cost development of POF sensors and devices.

A wide range of attractive sensor technologies have been developed in researchlabs, and the time may now be ripe for commercial exploitation of their cheapand robust characteristics.

Acknowledgements

The authors would like to acknowledge the financial support provided by Liver-pool John Moores University research funds (Physics and Electrical EngineeringUnits of Assessment), The Royal Society Paul Instrument Fund, The EuropeanCommission (CLOOPT N ENV4-CT97–0634 On-line measurement for preventingfouling when closing industrial process water circuit), EPSRC ROPA GR/M48031,as well as BICC-Corning and Yorkshire Water Services.

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