Laser-machined fibers as Fabry–Perot pressure sensors
Stuart Watson, Matthew J. Gander, William N. MacPherson, James S. Barton, Julian D. C. Jones,Thomas Klotzbuecher, Torsten Braune, Johannes Ott, and Felix Schmitz
Small size, all-optical fiber pressure sensors are desir-able for applications in which nonintrusive measure-ment techniques do not provide adequate spatialresolution, or for which direct optical access is not pos-sible. Some examples are measurement of unsteadyair flow within turbines and compressors, studies ofthe interaction of shock waves with structures in ex-plosive blasts, and pressure measurements within thebody for medical diagnostics. Optical methods can alsobe of benefit where electrical noise or hazards arisingfrom electrical discharge are problematic. Point sen-sors addressed by fiber optics can compete with theleading electrical pressure sensing technology becauseof their rapid response due to low diaphragm inertia.Their small size permits them to be operated withminimal intrusion and gives them the capability to beused in compact arrays, thus offering a degree of spa-tial resolution that is not possible with some of thelarger commercial sensors.
The general principle of many optical pressure sen-sors is the modulation of reflected light by the move-ment of a diaphragm exposed to differential pressure.Optical measurements are an attractive alternative
to electrical sensing of microelectromechanical sys-tems (MEMS) as the size scale (several tens to hun-dreds of micrometers) is compatible with opticalwavelengths and electrical connections are avoided.A number of miniature optical fiber pressure sensorslarger than a fiber have been reported previously,and their interrogation methods fall into two broadareas: intensity-based sensors and interferometricsensors. Strandman et al.1 described an intensity-based silicon sensor of 360 �m overall diameter usedduring the balloon dilation of constricted arteries. Amirror attached to a diaphragm was moved laterallyacross the fiber end face to give a pressure-dependentreflectivity. Tohyama et al.2 described an interferom-etric pressure sensor for a similar application. Zhouet al.3 formed a short-cavity Fabry–Perot sensor bybonding a 600 �m diameter silicon diaphragm to anetched glass substrate with a multimode fiber to de-liver and collect broadband light. The sensor outputwas a ratio of the reflected signals detected with andwithout a bandpass filter. Fiber Fabry–Perot pressuresensors offer several advantages over conventionalelectrical technology, one of which is their electricalisolation. Bing et al.4 used them for the detection ofpartial discharges inside high-voltage power trans-formers, where electrical immunity, chemical inert-ness, and small size were of great importance. We havepreviously reported low-finesse Fabry–Perot pressuresensors with copper diaphragms 50 to 100 �m in di-ameter on silicon sensors 500 �m square, fabricatedby deep reactive ion etching.5 The simultaneous illu-mination by three laser diodes at slightly differentwavelengths removed the phase ambiguity in theperiodic transfer function and avoided any restrictionon operating range. Up to three sensors were multi-
S. Watson, M. J. Gander, W. N. MacPherson, J. S. Barton ([email protected]), and J. D. C. Jones are with the School ofEngineering and Physical Sciences, Heriot-Watt University,Edinburgh EH14 4AS, UK. T. Klotzbuecher, T. Braune, J. Ott, andF. Schmitz are with the Institut für Mikrotechnik Mainz GmbH,Carl-Zeiss Strasse 18-20, D-55129 Mainz, Germany.
Received 6 October 2005; accepted 14 February 2006; posted 22February 2006 (Doc. ID 65199).
plexed simultaneously at a 1 MHz sampling rate foruse in transient aerodynamic experiments.
The limit for the miniaturization of optical pres-sure sensors is set by the scale of the diameter of thefiber itself. Abeysinghe et al.6,7 reported low-finesseFabry–Perot sensors fabricated on the end face of 200and 400 �m diameter multimode fibers. The cavitieswere formed photolithographically, and silicon dia-phragms, typically 7 �m thick, were anodicallybonded to the etched fiber. Totsu et al.8 also developeda method for fabricating Fabry–Perot pressure sen-sors for medical applications. The sensor ends weremicromachined on silicon wafers and were 125 �m indiameter, the same diameter as the addressing fibersto which they were individually coupled. The dia-phragm was a silicon dioxide mesa, 2.3 �m thick atthe center and 0.7 �m thick at the edges.
A method that involved the use of polymer dia-phragms was demonstrated by Cibula and Ðonlagic,9whereby a small section of multimode fiber wasspliced onto a single-mode fiber, which was thencleaved and its 62.5 �m diameter core chemicallyetched away to form a spacer of the desired length.The fiber end was then dipped in a polymer solution,and the resulting layer that sealed over the spacerwas cured to produce the diaphragm. The Fabry–Perot cavity was formed between the polymer dia-phragm and the single-mode fiber, which wasexposed during the etch process. Beard and Mills10
also used a polymer film in their sensors, althoughthe film itself ��50 �m thick) comprised the Fabry–Perot cavity. The low Young’s modulus of the filmenabled the sensor to operate successfully in an ul-trasound detection application. In this paper we de-scribe experiments to fabricate Fabry–Perot cavitiesin both conventional single-mode and multicoresingle-mode fibers approximately 125 �m in diame-ter by laser machining. These sensors use reflectivepolymer diaphragms and achieve the limit of minia-turization set by the size of the fiber.
2. Experiment
A laser-machined fiber-optic Fabry–Perot pressuresensor is illustrated in Fig. 1. The circular cavity isideally a few tens of micrometers in depth and wouldbe laser ablated in the fiber end, within the area ofthe cladding. In the case of a standard, single-modefiber containing only one core, the cavity is concentricwith the core. The diaphragm sealed over the top of
the cavity is partially reflective, thus producing alow-finesse Fabry–Perot interferometer in combina-tion with the Fresnel reflection at the air–core inter-face. The diaphragm flexes in response to pressurechanges in the environment, which has the effect ofchanging the cavity length and consequently the op-tical phase of the cavity. By illuminating the cavitywith light coupled into the addressing fiber, a mea-sure of the phase of the cavity can be calculated fromthe interference observed. The phase of the reflectedsignal is a function of the cavity length and thereforeprovides a relative measure of the difference in pres-sure between the external surroundings and thesealed cavity.
A. Laser Machining
An ArF excimer laser, operating at 193 nm, at a rep-etition rate of up to 100 Hz and a maximum pulseenergy of up to 200 mJ, was used to ablate the cavi-ties in the ends of the optical fiber.11 The beam wascollimated and shaped by crossed-cylinder lenses to aspot size of approximately 3 cm � 3 cm, which wasthen focused by a 6 � 6 microlens array onto an ap-erture for spatial filtering, before recollimation by asecond 6 � 6 microlens array. The resulting squareshaped, homogenized top-hat beam with an area ofapproximately 1 cm2, passed through a circular maskand was imaged onto the sample surface with a de-magnification of 36�, using a reflective Schwarzs-child objective. Two masks were available to definethe size of the ablation area, which could be either 30or 70 �m in diameter for these experiments.
Standard 125 �m diameter fibers were used, whichwere single mode either at 800 or 1550 nm. Since themost recent version of our optical interrogation sys-tem worked at commercial communication wave-lengths, the sensors described in this paper were allfabricated on 1550 nm single-mode fiber. The stan-dard fiber was Corning SMF-28, which will be re-ferred to here as single-core fiber (SCF). Novelmulticore fiber (MCF) was also used, which was alsosingle mode at 1550 nm. The MCF was supplied byFrance Telecom and by NASA. The France Telecomfiber had an approximately square cross section mea-suring approximately 150 �m across the diagonal.The NASA fiber had a circular cross section with adiameter of 125 �m, which is dimensionally similarto the conventional SMF-28 fiber (see Fig. 2). Bothtypes of MCF contained four cores that were ar-ranged in a square array with a 50 �m spacing be-tween them.
To machine the cavities in the ends of the opticalfibers, they were mounted in an xy translation stage,facing upward to the incident laser beam. A stablemounting was required to minimize the movement ofthe fiber during laser ablation. Two fiber mountingmethods were employed. In the first, the fibers werestripped of their protective acrylic buffer layer, leav-ing approximately 10 to 15 mm of bare fiber length,which was supported in a ferrule with an inner di-ameter of 127 �m, with the fiber face located just
Fig. 1. Schematic of fiber-optic Fabry-Perot pressure sensor.
below the surface of the ferrule end. The secondmethod involved packing bundles of 25 fibers intoglass capillary tubes held firmly in place by dentalwax. These fibers were not stripped of their acrylicbuffer layer and therefore retained their outside di-ameter of 250 �m. The benefit of this technique wasthat the fiber sensor end was more robust because ofthe full protective buffer. Also, the close proximity ofeach fiber in the bundle meant that the laser-machining process was quicker since the translationstage did not have to be moved so far to locate thenext fiber to ablate. Although the first technique pro-duced a more fragile sensor because of the bare silicaend, it did enable us to use cleaved fibers. The highquality surface finish of a cleaved end face was ad-vantageous in producing a uniform ablation surfacefree of inhomogeneities and thereby maximizing theFresnel reflection at this interface. In the case of thefiber bundle, the entire bundle and the glass capillarywere polished to achieve the necessary surface finishon each fiber end.
A series of experiments showed that pulse fluen-ces of approximately 7 J cm�2 (the maximum achiev-able value) produced the best cavities. The quality ofthe ablation surface at the bottom of the cavity wasthe most critical factor because inhomogeneities herewould adversely affect the Fresnel reflection at thefiber–air interface and thus affect the performanceof the Fabry–Perot interferometer. The number ofpulses defined the depth of the cavity and the ablatedsurface was observed to deteriorate in quality witheach successive pulse. For this reason, cavity depthswere kept to a minimum. Cavities were producedwith well-defined circular cross sections either 30 or70 �m in diameter and were typically 14 �m deep(see Fig. 3).
The SCFs were laser machined so that the cavitywas concentric with the core. Even when the coreposition was not easily identified when viewed withthe camera of the laser system, an adequate align-ment could be achieved simply by ensuring that thelaser spot size was concentric with the face of thefiber. To identify the cores of multicore fibers, it wasoften necessary to couple visible light into the fiber
and thereby illuminate the ends of each core. How-ever, some of these fibers had their end faces coatedwith zinc selenide in order to use one coated core as atemperature sensor, as has previously been reportedfor SCF12; illumination did not aid with locating thecores. For these fibers, we used a low-fluence series oflaser pulses to ablate the coatings while leaving theunderlying fiber unaffected. By removing just onequadrant of the coating on the fiber face, it was pos-sible to identify at least one core, and it was therebyeasy to establish the likely position of the other three.Up to four cavities were ablated in the ends of theFrance Telecom MCF, as demonstrated in Fig. 4. Forthe MCF supplied by NASA, it was necessary for thecore and the ablated cavity to be off center with re-spect to each other, whether a 30 or a 70 �m diameterlaser spot size was used. Even the smaller spot sizewould have produced a centered cavity whose circum-ference would have been practically too close to theedge of the 125 �m diameter fiber.
Single fibers were held in their ferrules with alow-adhesion putty and were easily removed afterablation. To remove the bundled fibers, they wereheated in their capillary tubes until the dental waxmelted, and they were then forced upward out of thetube so that the wax did not enter the ablated cavi-ties. Any residual wax was then scraped away.
B. Diaphragm Attachment
The sensor diaphragms were produced from largermembranes of 2 �m thick, aluminized polycarbonatefoil. Prior to coating with aluminum, the foil wasstretched over an approximately 25 cm diameterring. A second ring was fitted flush around the firstand secured the foil in a taut condition. Smaller ringswere coated with epoxy and placed on the foil insidethe larger ring, producing a tight membrane undertheir weight. Once the epoxy had cured, the smallerrings were cut out of the larger one and their foilswere coated with aluminum by thermal evaporationto a thickness of approximately 100 nm.
To seal the diaphragms over the ends of the laser-machined fibers, the fiber ends were brought intocontact with the reflective side of the polycarbonatemembrane until the film was under slight tension,
Fig. 2. Multicore fiber cross section showing the four cores illu-minated (MCF provided by NASA).
Fig. 3. 70 �m diameter cavity ablated into the end of a single-corefiber.
indicating a sealed contact between the two (seeFig. 5). These procedures involved the use of a roboticarm to maneuver the fiber and a microscope to viewit. UV curable epoxy was carefully applied betweenthe fiber end and the film. The epoxy was then curedwith UV light, producing a continuous bond with thefilm around the circumference of the fiber end. Fi-nally, the fiber was pushed through the membrane,puncturing it and leaving a sealed diaphragm overthe end of the fiber, as can be seen in Fig. 6.
C. Optical Interrogation
The method by which the phase of the cavity was mea-sured has been reported previously.5,13 It used threelaser wavelengths, approximately evenly spacedacross the cavity transfer function, the reflected sig-nals of which were recorded individually and combinedin an algorithm to determine the phase. This overde-termination improves upon single wavelength tech-niques by reducing phase ambiguities and eliminatingthe common-mode losses and noise to which anintensity-based measurement technique is highly sus-ceptible. It is also useful in applications where rapidchanges go beyond one full cycle of the transfer func-tion. Figure 7 shows the lines that represent each ofthe three laser wavelengths used in the optical inter-rogation system superimposed on a typical cavitytransfer function, which was obtained as described inSubsection 3.A.
Figure 8 illustrates the optical interrogation of asingle sensor. The sensor was illuminated with allthree laser wavelengths (1532, 1547, and 1563 nm)and the reflected light was filtered by means of fiberBragg gratings so that the signal for each wavelengthcould be recorded individually. Each laser source de-livered up to 10 mW, sufficient to multiplex up to sixsensors by distributing the power via a directionalcoupler network. The detectors were high-bandwidthInGaAs photodiodes, and their voltage outputs wererecorded by an analog-to-digital (A�D) data acquisi-tion system. Two data acquisition systems were used:one operated at up to a 100 MHz sample rate with8-bit resolution, while the other was a 1 MHz systemwith, 12-bit resolution. A three-wavelength algo-rithm was used to construct the phase data fromthese signals.13 A more comprehensive explanation ofthe system can be found in Ref. 5.
3. Results
A. Initial Testing
Fibers were examined with a microscope to ensurethat the diaphragm was both sealed and intact. The
Fig. 4. Four cores ablated with 30 �m diameter cavities in FranceTelecom MCF.
Fig. 7. Cavity transfer function for a SCF sensor. The dashedlines represent the three laser wavelengths.
Fig. 5. Diaphragm attachment procedure.
Fig. 6. Photograph of the side view of a single-core fiber sensor;the inset shows the top view.
cavity was then illuminated with a broadband sourceand the reflected light spectrum was displayed byusing an optical spectrum analyzer (OSA) to verifythat a Fabry–Perot cavity existed there. The cavitytransfer function for a SCF sensor is shown in Fig. 7.The free spectral range, measured as the wavelengthdifference �� between successive peaks on the trans-fer function, is related to the cavity depth d by
d ��0
2
2��, (1)
where �0 is the mean wavelength and the refractiveindex of the cavity medium (air) has been assumed tobe 1.0. Equation (1) gives a value of 18 �m for thedepth of this cavity, whereas the physical depth mea-sured with a calibrated microscope after ablation was14 � 1 �m. This discrepancy was likely due to dia-phragm lift or bulging, a phenomenon that was ob-served in some sensors examined under a microscope.This effect may have resulted due to epoxy movementor relaxation effects in the diaphragm.
In the case of the MCF sensors, the sensor fiber wasspliced onto the fan-out system so that each corecould be interrogated through individual fibers(Corning SMF-28). The fan-out was constructed toallow four SCFs to address the four cores in the MCFindividually. A detailed description of this system canbe found in Ref. 14. The OSA was used to establishthe existence of a sensor cavity at the end of one of thefour cores before proceeding with further experimen-tation on that pressure sensing core.
B. Pressure Cycling
Sensors were tested for their response to pressurechanges by pressure cycling them in a chamber withnitrogen. A SenSym SX150DN piezoresistive pres-sure sensor was used to measure the chamber pres-sure. The results from a pressure cycling experimentare shown in Fig. 9. The pressure was raised to nomore than 3 bars before venting to atmospheric pres-sure and repeating this process to remove any hys-teresis in the response of the sensor. It is evident thatthe phase measured by the optical sensor and thepressure measured by the piezoresistive sensor donot exactly follow each other. This is most obvious
during the constant pressure parts of each cycle, atwhich times the phase recorded by the optical sensorexhibits a drift. This may be a consequence of epoxymovement or may suggest significant temperaturesensitivity.
Cycling was also used to calibrate the optical phaseof the cavity with respect to the pressure measuredby the piezoresistive sensor and thereby determineits sensitivity. The SCF sensor of Fig. 9 was cycledand the phase was plotted against the pressure tocalculate the interferometric phase sensitivity, whichwas measured to be 0.36 � 0.04 rad bar�1 from alinear data fit. The calculated sensitivity for adiaphragm radius a, thickness h is15
��
�P �4
�
3�1 � �2�a4
16Eh3 . (2)
For the polycarbonate film diaphragm h � 2 �m,Young’s modulus E � 2.3 GPa Poisson’s ratio �� 0.37, a � 15 �m, and wavelength � � 1550 nm,yielding a sensitivity of 0.36 rad bar�1 in close agree-ment with the fit to the experimental data.
A MCF sensor was pressure cycled in a chamberpressurized with nitrogen gas up to 2 bars. The indi-vidual core that addressed the pressure-sensing cav-ity was interrogated via the fan-out system. For thistest, in which the pressure change was known andvaried slowly, it was sufficient to use a single wave-length to interrogate the sensor, allowing the opticalphase to be determined simply by fitting a sinusoid tothe response. The phase sensitivity of the sensorwhose data are shown in Fig. 10 was 11.7 rad bar�1.The calculated sensitivity determined by usingEq. (2) and the same values as those used for the SCFcalculation, with the exception that a � 35 �m and� � 1532 nm, was 10.7 rad bar�1. The calculation issensitive to the actual diaphragm radius through thea4 term; a cavity radius only 2% larger would accountfor this discrepancy.
Fig. 8. Schematic of a sensor interrogation system.
Fig. 9. Pressure cycling a reference electrical pressure sensor(top), a SCF sensor (middle), and the resulting pressure calibrationcurve for the optical sensor (bottom).
Dynamic pressure experiments were conducted in ashock tube in which pressurized nitrogen was used toburst a diaphragm and deliver a shock to the sensorsunder test. The sensors were positioned at the end ofthe shock tube, as illustrated in Fig. 11, such thatthey were facing the incident shock wave along theaxis of the tube. A SenSym SX150DN piezoresistivepressure sensor was positioned close to the end of thetube where the optical sensors were located, but in aside-on orientation. The lowest resonant frequency ofthe sensor diaphragm is16
f �10.21
2a2 � Dh
, (3)
where
D �Eh3
12�1 � �2�, (4)
and where � 1100 kg m�3 is the density of thepolycarbonate film, and all other variables and valuesare the same as for Eq. (2). This yields 1.2 MHz forthe 70 �m diameter diaphragm and 6.5 MHz for the30 �m diameter diaphragm.
Figure 12 shows the simultaneous response of aSCF laser-machined sensor and an electrical sensor(SenSym) when exposed to a shock in the shock tube.The electrical sensor was originally intended only fortriggering purposes in these experiments and not forany means of making comparisons with the fiber sen-
sor because of its relatively low bandwidth response.However, over the 60 ms time scale shown here,these graphs adequately demonstrate the response ofthe calibrated fiber sensor to the initial and reflectedshock waves, which follows the response of the elec-trical sensor. It is evident that there is significantringing in the signal of the latter; and, due to itsside-on orientation, the measured pressures are alsolower than those measured by the SCF sensor.
The experiments were designed to compare laser-machined fiber sensors with a high-performance elec-trical sensor, but, unfortunately, a failure in theexperiment prevented a direct comparison in thesame shock experiment. However, independent shockexperiments were made by using a fiber sensor, andthey were repeated with a high-performance electri-cal sensor. Figure 13 shows the result from an exper-iment with a Kulite XCQ-080 piezoresistive pressuresensor, which was positioned at the end of the tubeand facing the incident shock, as described in Fig. 11.Figure 14 shows the result from a calibrated opticalsensor in a similar experiment. These two results,although from separate experiments where the peakpressures may have differed, demonstrate the rapidresponse capability of the optical sensor comparedwith the Kulite sensor. Their performance can bemeasured in terms of their rise time measured across10%–90% of the amplitude of the shock front. In the
Fig. 11. Shock tube used to make dynamic pressure measure-ments.
Fig. 12. Measurement of the differential pressure of a shock waveand the successive reflected shocks, recorded simultaneously by aSCF laser-machined sensor and a piezoresistive sensor.
Fig. 13. Differential pressure of a shock front measured by anelectrical (Kulite) sensor.
Fig. 10. MCF pressure cycling experiment for a single interroga-tion wavelength.
case of the Kulite this was 8 �s, whereas the rise timeof the SCF sensor was just 3 �s. No MCF sensorswere shocked in these experiments.
4. Conclusions
Fabry–Perot cavities were successfully fabricated inthe ends of both single-mode fibers and novel, multi-core fibers and were shown to operate as miniaturepressure sensors. The laser ablation of the fibers pro-duced highly consistent features; however, themethod by which the diaphragms were attached wasmore problematic. The difficulties that were experi-enced at this stage included membrane tearing,rather than puncturing, and the occasional dia-phragm that did not adhere well to the fiber. Thediaphragms of some sensors were also observed tobulge slightly, an effect that was more common withthe fully buffered fibers.
Pressure sensors were also fabricated by using in-dividual cores in multicore fibers, with up to fourcavities laser machined in four-core fiber. The pres-sure sensing core was addressed by a single laserwavelength, and the optical phase of the cavity wasdemonstrated to change with respect to pressure.Some of these fibers were coated with ZnSe to illus-trate the possibility of a dual function device on onefiber, potentially capable of pressure and tempera-ture sensing on adjacent cores. Further work is re-quired to produce films of sufficient quality fortemperature measurement in this configuration.
The interferometric pressure sensitivities of bothsingle core and multicore sensors were in good agree-ment with the values expected from the diaphragmproperties, and dynamic experiments on single-coresensors showed rise times of 3 �s in response to in-cident shocks of the order of 3 bars.
The authors acknowledge support from the Euro-pean Commission’s Improving Human Potential Pro-gramme and from the UK Engineering and PhysicalSciences Research Council. W. N. MacPherson ac-knowledges the latter for the provision of funding viaits Advanced Fellowship Programme. We are grateful
to A. J. Moore for useful discussions on multicorefiber and to France Telecom and Gary Fleming,NASA Langley, for the supply of multicore fiber.
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Fig. 14. Differential pressure of a shock front measured by anoptical (SCF) sensor.
