Enhancing Signal Quality of Evanescent-Wave MultimodeFiber Optic Sensor Using Diffuse Excitation Light

Jianjun Ma, Wojtek J. BockCentre de recherche en photonique, Departement d'informatique et d'ingenierie,

Universite du Quebec en Outaouais, P.O. Box 1250, Station B, Gatineau, Quebec J8X 3X7, Canada,ma.jianjunguqo.ca, 1-819-5953900 ext 1619wojtek.bockWuqo.ca, 1-819-5953900 ext 1623

AbstractIn a typical evanescent-wave-based optical device,

whether using fiber optics, free space or a lab-on-a-chip, thesignal is not only weak but often mixed with a strong strayexcitation (Ex) light. This leads to a requirement for a high-performance filter and detector assembly for signal recoveryand thus adds a major cost to the entire system. We propose anon-Lambertian type light diffuser, offering a highly efficientnovel use of Ex power to simultaneously achieve a strongerfluorescent signal and reduce the stray Ex power level.Particular attention is given to how this universal concept canbe applied to a lab-on-a-chip device.Key Words: Spectroscopy, Fluorescence and luminescence,Diffusers, Sources, Fiber-optic components, Fiber-opticsensors, Fiber-optic sources and detectors, Lab-on-a-chip.

1 IntroductionEvanescent-wave (EW) based devices are often preferred

for surface event assays. The signal achieved via such asystem reflects only what occurs within the thin active layerclose to the sample-device interface on the sample side. Thethickness of this layer is approximately several hundrednanometers or one wavelength. Any events taking placebeyond this layer will not affect the signal. This offers anextremely high signal-to-noise ratio. This unique feature isparticularly appreciated for chemical, biological and clinicalsample analysis.

However, a problem in most EW-based systems, includingthose using free space [1], fiber optics [2] and the lab-on-a-chip concept [3], is that a weak signal mixed with strong strayexcitation (Ex) light appears in the detecting end. Although in[3], the CCD detector is normal to the direction of the Ex lightrays and in principle will completely suppress the stray Exlight, full suppression will only be achieved with a perfectwaveguide and uniform samples. The inevitable defectsexisting in the waveguide and the inhomogeneity of thesample, whether in liquid or in solid thin-film form, willrandomly scatter part of the stray light towards the detectionsystem. The strength of this stray light can vary from a levelsimilar to that of the signal to several orders of magnitudehigher than the achievable signal. For a liquid form sample,we have observed such a phenomenon. [4] The patterncreated by multiple tiny sample spots on one side of thewaveguides implies even less signal strength since thecollectable signal strength is highly dependent on the size ofthe interactive area. Accordingly, although this lab-on-a-chipproposal features simultaneous multiple sample assaycapability, the signal-to-noise ratio before the filter is notexpected to be very high, especially when the aforementioneddefective waveguide and

inhomogeneous samples are involved. Unfortunately, thesefactors are common in practice since no control can beexercised over sample homogeneity. Turning to a costly high-performance detector and filter assembly set for proper signalextraction becomes an inevitable choice for such a device.

However, we found that a highly efficient use of Expower is possible, allowing us to simultaneously achieve astronger fluorescent signal and reduce the stray Ex powerlevel. This is obviously the best solution for the problems wejust described.

Instead of dealing with the sensing or detectingcomponents of the system, as seen in many articles, in thispaper we mainly address the modification of the light sourcefor overall performance enhancement. We first present thetheoretical and experimental investigations based on atransmission-type multimode (MM) EW fiber-optic sensor.Then we show that the concept of modifying the light sourcefor performance enhancement can be extended to a lab-on-a-chip device, establishing its universal significance to an EW-based system.

2 A common model valid for EW field description of botha slab waveguide and a multimode fiber

As seen in many publications, the model for rays travelingin a slab waveguide is also valid for describing what occurs ina MM fiber core-cladding interface. Such modeling makes theanalysis simple and clear in concept. For both waveguides,the EW strength in the cladding side highly depends on thepower carried by high-order guiding mode groups. The raypicture in Fig. 1 indicates that higher-order modes take theform of steeper rays. Rays with an incident angle approachingthe critical angle O, will extend into the cladding muchfurther. Using subscript "1" and "2" to represent core andcladding, respectively, the EW field is expressed by:E2 = E20 exp(-y2 .x|)exp(-i/Jz), (1)

which propagates only in the z direction and decreases inamplitude when |x| increases. Here Y2 is the attenuationcoefficient of the guiding mode in the cladding side and / isthe mode phase propagation constant. The coefficient Y2 is:

( \2

Y2 =n2ko n) sin2O1 1, (2)

where n1 and n2 are the refractive indices of the core andcladding, respectively. 01 is the incident angle of a ray at thecore-cladding interface for total internal reflection (TIR)formation and ko = 2;T I 2A is the wavenumber in free space.

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From Eq. (2), a larger incident angle 01 leads to a tighterconfinement of the wave within the fiber core, suggesting thatthe emission power level strongly connects with those Ex raygroups close to the critical angle.x

Fig.1. Ray picture representation of the EW field,indicating deeper penetration of EW field when rayincident angles approach the critical angle.

3 Distinction between a multimode fiber and a slabwaveguide

A major distinction exists between a slab waveguide and aMM fiber. For a MM fiber, the rays discussed in thepreceding section fall into the meridian ray group, and willalways cross the fiber axis during propagation. A MM fiberallows skew rays to travel within the fiber, but they neverpass the fiber axis. A skew ray follows a helical trajectorywhen propagating, forming a closed loop when projected ontothe fiber cross-section. Skew rays can be divided into twocategories: first, a group of rays under TIR that form guidingmodes, and second, a group of rays that experience low loss,forming tunneling leaky modes, or tunneling modes. Unlikeother leaky modes radiating their power in an extremely shortdistance, tunneling modes can travel within the fiber coreover a distance of many kilometers without significant lossand actually can be treated as guiding modes. The formationof skew rays occurs because of the curved surface of the fiber,suggesting that a slab waveguide will not have these raygroups.

4 Key role of tunneling modes for EW powerenhancement

Unlike guiding modes, which are derived from thereal , solutions of the eigenvalue equation with 6 as thepropagation constant or eigenvalue of the mode, tunnellingmodes are found from certain complex , solutions of theeigenvalue equation and associated only with the modegroups that operate just below cutoff. Therefore, the mostcritical difference between them is that the modal parametersof tunnelling modes appearing in the fields are complex,which leads to the radiation of the energy. In terms of rayoptics, from one perspective, like leaky rays, tunnelling rays[5] also undergo partial reflection. However, they do notexperience refraction. Instead, their attenuation is dueexclusively to the curvature of the fiber cross-section. Theserays tunnel from the core-cladding boundary through theevanescent region adjacent to the core and emerge somedistance from the boundary and lose their power, aphenomenon known as radiation caustic. In direct contrast towhat occurs with the slab waveguide, a significant portion ofthe radiation field can persist for enormous distances withinthe circular fiber waveguide for a MM fiber with V>> 1.

Furthermore, unlike a higher-order guiding mode with onlyan EW field residing in the EW layer, a tunnelling mode notonly has a longer EW field in the cladding, but also has thisEW tail cascaded with an additional oscillating wave fieldfurther extending into the cladding. This characteristicendows tunnelling modes with a capability superior to that ofhigher-order guiding modes to contribute large amounts of Expower and to receive fluorescent signals tunnelled back fromthe sample. This is particularly significant for an EW fibersensor, whose fiber length is typically in meters.

More tunnelling modes than guiding modes are allowed ina step-indexed MM fiber, [5, Ch. 36]. For a weakly guidedMM fiber with A <<1, there is roughly the same number oftunnelling modes as guiding modes. This suggests thattunnelling modes play an important role in delivering lightwithin an MM fiber since they are easily excited.

5 Launching conditions of guiding and tunneling modesA guiding mode can be formed by a group of meridian

rays or skew rays under TIR.To launch meridian rays, the incident angle is restricted by

its numerical aperture NA [6]:

NAndn =no sinlO.dn n n2 (3)

where 0Omdn is the maximum half-angle of the acceptancecone of the meridian rays and no, n1 and n2 are the refractiveindices of media outside the fiber core entrance surface, fibercore and fiber cladding, respectively. For the air medium,nO =1.

To launch skew rays under TIR, the ray incident anglefrom outer surface of the fiber core Os, must lie in a planehaving an angle y with a meridian plane, where meridian raysare found. The following relationship exists under theassumption of TIR:

cos)y sinOt = cos 0 = 1ni

(4)

where n0 sin OS, = n1 sin Ot and Ot is the refractive angle. s isthe incident angle of the skew ray.

The general requirement governing the incident angle O,kof skew rays can hence be simply derived:

2n2l NAmdn/IT nZ,7 c ; n q>Vn1 2 d-n ~~~~1Y- JT - / -

mdn 2COSy COSy 2 y)

So the numerical aperture of meridian rays is only thespecial case of Eq. (5) when y= 0. Clearly, Oskw > Omdn,suggesting that the minimum incident angle of skew raysstarts from the maximum acceptance angle of meridian rays.In other words, the guiding modes associated with skew rayscan be excited only by rays outside the maximum acceptancecone of meridian rays, including rays with incident anglesclose to 2T/ 2. Furthermore, each meridian plane is associatedwith many skew planes since y can vary within a wide rangeof values. In this regard, there are far more skew rays thanmeridian rays in MMF waveguide. In particular, this alsoimplies that there are far more skew rays with steeper incidentangles. They are associated with the mode group that has a

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no Sill U,, -

greater proportion of its EW field extending into the claddingand thus their effects on the sensor performance are far moreimportant, especially when a fiber with a large mode volumeis involved.

To launch tunneling rays, we have to be aware that theserays do not follow the condition of TIR. Instead, in terms ofmode theory, the associated tunnelling modes belong to agroup of leaky modes just below cutoff. The radialcomponent of the propagation constant of a tunnelling mode,represented by Ir , must be real within the core, formingtrapped power, imaginary in the cladding side adjacent to thecore-cladding surface, forming an EW field, and again realbeyond this EW layer, forming radiated power. Similar to theprocedure of derivation of the acceptance angle of skew raysunder TIR, the acceptance angles of tunnelling modes obeythe following relationship [7]:

2 2

1 -(rp)os2 -p >n sinO > n (6)

or NAmdn > no sin >NAmdn (7)d1 -(r Ip) cos2

where p is the fiber core radius, r is the incident pointrelative to the fiber center and rp is the azimuthal anglereflecting the skewness of the ray.

In much the same way as a skew ray is formed under TIR,a tunnelling ray, which is also skew, is formed by an incidentray with the minimum incident angle starting from themaximum acceptance angle of meridian rays. Its maximumacceptance angle, however, has a limited value depending onthe combination of incident point r and the azimuthal angle9. In particular, the incident rays associated with skew raysunder TIR cannot excite tunnelling rays since tunnelling raysdo not follow the restriction of TIR. The incident ray anglesfor meridian rays, skew rays under TIR and tunnelling raysare all different. The possible entering positions are locatedacross the entire fiber core end-face. The possible launchingangles for these rays can vary within a vast range of0.<OT/2.

6 Enhancing the EW power level by exciting meridian andskew ray groups simultaneously

The preceding analysis indicates that both enhancement ofEx power in the EW layer and the tunneled-back signal levelheavily rely on the high-order guiding modes and tunnelingmodes, with tunneling modes playing a more important role,especially for a short fiber such as normally employed in fibersensor systems. Further optimization has to include thesuppression of power in lower-order modes. The reliance onthe source to achieve this goal now becomes clear. Theincident ray angles must obey the following conditions of:(a) confining the incident ray angles to the rangesin' NAndn < 01. < /T /2 (8)

with no =1;(b) allowing the incident rays to hit different positions acrossthe core entrance surface and, to the extent possible, touniformly excite all the desired modes.

However, it is difficult to use light sources available onthe market directly to achieve this goal since most of theiroutput power distribution, as mentioned before, is suited tolower-order mode group excitation. The emission pattern ofmost sources can be expressed as:B(O)= B, cos' 0 (9)

where n > 1 , Bo is the radiance along the normal to theradiating surface.

Typically, an LED has n 1 which is commonly called aLambertian or diffuse source, and it shows relatively widerand more uniform angular distribution. An LD, on the otherhand, having n > 100, [8] delivers energy in a much narrowercone. Both, however, have their power concentrated mainlyon the normal to the radiating surface. Obviously, directcoupling in a traditional manner will favor the lower order-mode group.

Tilting the conventional light source described by Eq. (9)to feed the main power to the maximum acceptance angle ofthe fiber might help to achieve the goal. The major drawbackof a conventional source is that the source surface is notdirectly in touch with the fiber core. Instead, another opticalcomponent such as a lens is inserted in between to enhancethe coupling efficiency. The limited numerical aperture of thelens hence blocks most of the rays capable of exciting thesedesired modes, indicating a limited possibility of improving tothese modes through tilting of the source. We can expect thata collimated light beam tilted to the fiber axis will also havelimited effect on the desired mode coupling for the samereason, although it will contribute to suppression of lower-order modes.

The above analysis suggests that a source emitting lightrays with their direction changing randomly and withisotropic intensity distributed in each direction might help toachieve the goal. Additionally, the source should send thelight as directly as possible to the fiber core without theassistance of any NA-limited devices. We demonstrate asource that has these features and highlight its remarkableeffect on performance enhancement via our experiments inthe next section.

7 Experimental verificationWe propose a fiber-optic side-emitting diffuser as a light

source with characteristics close to the requirements describedat the end of the last section. A short segment of jacket at thefiber tip of a 3-meter MMF with core / cladding /jacket sizeof 400 / 430 / 730 gm is removed with a copper wire stripper.The exposed cladding has a thickness of only 15 pm, forminga thin and roughened layer surrounding the fiber core. Part ofthe light in the fiber core will penetrate into the cladding andradiate into the surrounding space. The roughened cladding,with many irregular local sites, maneuvers light rays in alldirections and forms a light diffuser such as described inSection 6. The side-emitting effect of such a diffuser is shownin Fig. 3(a). Obviously, the output profile from this uncladdedfiber segment is that of a non-Lambertian light diffuser,sending light rays randomly in all directions.

The light from an Argon laser is coupled to this fiber atthe other end. To enhance the diffuser light strength, the laser

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is focused off the center of the fiber axis, forming a brightring when the far-field mode field pattern is observed at theother end of the fiber. The intensity at the fiber exit isadjusted to 3 mW.

As illustrated in Fig. 2, an EW-based fiber-optic sensorsystem is established. The flow-cell is filled with a water-diluted Rodamine 6G (R6G) sample having a concentration of29 ptg / ml. A 2-m MM fiber of the same type is decladded inthe middle portion to form a 10-cm sensing segment, which issealed in the flow-cell and surrounded by the R6G sample.One end of this r-fiber is connected to a USB 2000spectrometer-computer system via a linear variable filter(LVF). LVF is a light duty filter to block part of the stray Exlight. A 2-m MM fiber of another type connects LVF with thespectrometer. Two fiber holders with translation and rotationstages hold one end of the r- and i- fibers, respectively. Exlight from an Argon laser arrives at the entrance of the r-fiberin the form of diffused light via the i-fiber. The two-stagearrangement ensures that the r-fiber can follow the i-fiber'stip position for best signal quality and simultaneouslymaintains the angle in between unchanged.

Flow cell Uncladded fiber segment

nltlitl\ / Inlet LVF

significantly enhance the EW power level and suppress thestray Ex light and thus dramatically improve the overallsystem performance. In fact, the proposed diffuser can alsoimprove the performance of an EW-based lab-on-a-chipdevice.

i-fiber'k:

I'i-fiber

(a) oc=130° (b) Definition of alignment angle

Fig.3. Photo for a typical alignment angle at oc=130°, which isassociated with the experimental result illustrated in Fig. 3(b).

IF (Counts)I~nnn

270(

1 801

901

U

O Fluorescent Peak

0 Strong & Distorted Sign alNoise from Sp, trum

0 Ex Soulrce sn a

0 Missing Gap-400 450 500 550 600 650 700 750

Wavelength (nm)

(a)IF (Counts)

0/-Inn

Fig.2. Experimental setup. i-fiber: illuminating fiber; r-fiber: receiving fiber.

Fig. 3 (a) is a photo for a typical angle at u=130° with a

defined in Fig. 3 (b). The corresponding results, illustrated inFig. 4, indicate surprising contradictions to the intuitivethinking. The angle at oc=20°, thought to have a higherfluorescent signal level because the r-fiber receives muchstronger Ex power from the i-fiber, is associated withseriously distorted spectra for both the signal and Ex light,which is shown in Fig. 4 (a). However, much larger angles,for example, oc=100° and 130°, yield dramatically improvedsignal quality, which is highlighted in Fig. 4 (b). Obviously,this is because the diffuse light source shown in Fig. 3 (a) hasmuch higher probability of distributing its power among therays with incident angles associated with the meridian, skewand tunnelling ray groups. In terms of mode theory, largenumbers of higher-order modes and tunneling modes areexcited, enabling the higher level of EW power for sampleexcitation. On the other hand, the much weaker power in alldirections ensures much less power being delivered into thelower-order modes, dramatically suppressing the power levelof stray Ex light.

8 A proposed alternative to a lab-on-a-chip deviceSo far, for a MM-fiber-based EW fluorometer, we have

successfully demonstrated that a non-Lambertian diffuser can

270(

1 80(

90(

Normal Spectrum) Coc=100°O 1300O rmNoral Signal

0 Normi_GapJ

400 450 500 550 600 650 700 750

Wavelength (nm)(b)

Fig.4. Experimental curves associated with different fiberalignment angles. (a) Poor signal quality when r-fiber istaking the strong Ex power directly launched from thecore of i-fiber; (b) Dramatic improvement of signalquality when r-fiber is illuminated by diffuse lightscattered from the coarse fiber cladding.

Today's intensive research efforts based on the lab-on-a-chip concept have delivered a large number of new devices,meeting the exploding demands from the chemical,biological, medical and clinical sectors. EW-based technologyis often preferred in since EW power is more uniform and canovercome problems due to lighting variations.

Different versions of EW-based lab-on-a-chip deviceshave been proposed, many of them designed to achieve

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IODu

MDUL

multiple sample analysis. One outcome is the appearance ofarray biosensors, such as that described in [3]. However, asindicated in the Introduction section, because each individualsample in such a sensor array occupies only a very small area,the collectable fluorescent power is limited. Since the signallevel from a MM fiber can build up over the fiber lengthunder the sensing architecture of Fig. 2, a bundle of shortsegments of MM fibers, with a properly designed micro-fluidic system surrounding them, can also realize the goal ofmultiple sample analysis. With one end of the bundleilluminated by the proposed light diffuser, and a proper filterat the other end, we can expect a higher level of signal fromeach fiber. This way the expensive CCD array may bereplaced by a low-cost CMOS array as a signal detector. Theproposed light diffuser is also expected to enhance theperformance of a similar system where the MM fiber isreplaced by a bundle of capillaries. This design may simplifythe system since the hollow core of the capillary serves as thefluid channel.

8. G. Keiser, Optical Fiber Communications (McGraw-HillHigher Education, third edition, 2000), Ch. 5.

ConclusionsIn conclusion, the significance of this work is that a

properly designed diffuse power source will not onlydramatically enhance the performance of an EW-basedsystem, but will also greatly lower the performancerequirements of the light source, filter and detector, resultingin a dramatic drop of the system cost. The results presentedare applicable to all EW-based measurement platforms,including fiber-optic and lab-on-a-chip based devices.

AcknowledgementThe authors gratefully acknowledge support for this

project from the Natural Sciences and Engineering ResearchCouncil of Canada and from the Canada Research ChairsProgram.

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