measurement of incident intensity of uv radiation delivered by optical fibers

Measurement of Incident Intensity of UV Radiation Delivered by Optical Fibers

H O N G X U and J O N A T H A N E. K E N N Y * Department of Chemistry, Tufts University, Medford, Massachusetts 02165

Fiber optics are being used more and more frequently to deliver light to a sample for remote spectroscopic analysis, or to induce photochemistry or photoablation on an otherwise inaccessible sample, e.g., living tissue. For accurate quantitative results, the photon flux at the sample must be known. This paper presents two techniques for simple and accurate measurement of the intensity of ultraviolet light in the range 260 to 369 nm delivered to a distant sample by a fused-silica optical fiber. One technique uses a home-built evanescent wave coupler near the sample to couple excitation light into a monitoring fiber; the other uses a twin- fiber system, with equal amounts of source radiation launched into an excitation and a monitoring fiber. In each method, two Rhodamine B quantum-counter photomultiplier tube detectors are used simultaneously to assess the effectiveness with which the incident intensity is monitored. A critical evaluation of the results is presented. The twin fiber method is, theoretically, very simple; in practice, however, it is difficult to obtain a good wavelength-independent match between the two outputs. The evanescent wave coupling is also wavelength-dependent, but the observed results are in agreement with the predictions of simple evanescent field theory. Index Headings: Fiber optics; Evanescent wave couplers; Spectroscopic techniques.

INTRODUCTION

In any quantitative spectroscopic experiment or pho- tochemical procedure, accurate knowledge of the intensity of the light incident on the sample is important. The signal of interest is almost always dependent on this quantity, often in a linear fashion. Therefore, the incident intensity level is usually monitored so that fluctu- a t i o n s - d u e to changes in wavelength or temperature, electrical noise, or other variables--may be taken into account.

In conventional spectroscopic measurements, often used for analytical purposes, 1 this dependence is not the most interesting aspect of the experiment. However, if accurate information (e.g., regarding concentrations of analytes, or relative populations of molecular energy levels) is to be obtained, correcting raw signal levels for their dependence on incident intensity is necessary. Sometimes the desired information may be obtained by taking the ratio of two signals, both of which depend on the incident intensity in the same way. Of course, for signals that depend linearly on incident intensity, the simplest version of the ratio method is to divide the raw signal by another signal proportional to the incident intensity itself. This procedure is often referred to as power normalization.

In other spectroscopic experiments, sometimes the ob- ject is to determine the functional form of the depen-

Received 2 July 1990. * Author to whom correspondence should be sent.

dence of the observed signal on incident intensity. For example, in multiphoton ionization spectroscopy, reso- nance enhancement of at least one step in the absorption of multiple photons by a molecule usually occurs, giving a wavelength-dependent ionization signal which contains information about the molecular energy levels accessed in the resonant step. 2 Correct interpretation of the observed wavelength dependence requires a knowledge of whether, for example, the one- or two-photon absorption from the initial state is resonant, because of the different energies and selection rules involved.

In photochemistry, and especially in the rapidly growing field of photomedicine, knowledge of the dosage of light radiation delivered to the sample is of critical im- portance. 3 Laser ablation of tissue, photosensitization of intracellular chemical processes, and control of tanning and burning with sunscreens are important examples where accurate instantaneous and/or time-integrated measurements of exposure to light can make the differ- ence between beneficial and disastrous results of pho- totherapy. In these cases, the relation between incident intensity and the effect produced is (ideally) known be- forehand, and the incident intensity is sometimes the only important measurement made during the experiment.

Our own interest in this problem was initiated by our involvement in the development of instrumentation for in situ monitoring of groundwater contaminants using laser-induced fluorescence. 4-~ The measured fluorescence is normally the result of one-photon absorption by the analyte molecules of ultraviolet light delivered to the remote sample by multimode silica-core optical fibers. Of course, the same type of instrumentation is applicable to in vivo monitoring. The growing use of optical fiber systems to deliver light to specific parts of the body, for either spectroscopic or phototherapeutic purposes, sug- gests that methodologies developed for power normalization of our in situ fluorescence measurements may have a much wider applicability.

In a traditional fluorimeter with conventional optics, a beamsplitter is used to pick off a fraction of the excitation light and direct it to a suitable detector. 1 If the light source is tunable, wavelength-dependent aspects of the excitation (and pickoff) optics must be considered, in general. However, in such a system, the largest of the wavelength-dependent effects is usually the response of the detector, which may be effectively flattened by the use of a quantum counter. 7 This results in a near-constant relationship between the number of photons at each wavelength that are incident on the sample and the output of the quantum-counter (QC) detector. Other residual wavelength-dependent effects (e.g., the reflectivity

Volume 45, Number 3, 1 9 9 1 ooo3-7o2s/9v45o3-o42952.oo/o APPLIED SPECTROSCOPY 429 © 1991 Society for Applied Spectroscopy

of the beamsplitter) may be ignored, or measured and accounted for if necessary.

In a similar instrument which uses fiber optics to deliver light to the sample, additional factors must be considered. Slight differences in the focal length of the launch lens with wavelength can result in big differences in the fraction of light that enters the fiber; so too can small mechanical instabilities which affect the relative position of the fiber with respect to the focused beam. The internal and external transmittances of the fiber both depend steeply on wavelength in the ultraviolet part of the spectrum; this effect is especially important when long fibers are used. Also, fiber bending losses can affect the efficiency of light delivery in the optical fiber. These effects destroy the simple relationship between the sample illumination and the output of the pre-launch quantum counter. Because of these problems, the strategy of taking ratio-based measurements has been strongly rec- ommended for designing fiber-optics-based analytical techniques, s

Given a sufficiently stable optical system, all the effects described above could be held constant, and a single calibration could be performed and used to correct all subsequent measurements of source intensity at the remote sample. However, in many applications, it may be difficult or impossible to ensure the validity of a previous calibration, and some more direct method of accurately measuring the source intensity at the sample is needed.

An example of such an application is provided by remote fluorescence measurements which utilize pulsed la- sers to provide excitation light in the ultraviolet region of the spectrum. 4 Even if the optical system were free of short- or long-term mechanical instabilities, the relationship between pre-launch power or energy measurements and the illumination of the sample at a given wavelength is subject to change because of changes in the transmission of the excitation fiber with time. For example, fiber-optic sensors are often used to make in situ measurements in an environment that is "hostile" (i.e., subject to harsh temperature, pressure, or chemical stresses). In such an environment, it is possible that slight changes in the physical properties of the fiber could occur. Even in an environment free of such problems, fibers repeatedly exposed to high-energy UV light pulses are known to undergo "solarization" (i.e., a loss in transmission due to light-induced damage)2 The useful life- time of a sensor would be enhanced if mild solarization could be tolerated without compromising the quality of the measurement.

One technique that has been used in remote fluorescence measurements using fiber optics is based on the use of back-scattered excitation radiation (Rayleigh scat- tering) for power normalization? ° Under certain conditions, this method gives adequate results. But it fails when the solution is not sufficiently dilute or the scattered light is not completely separated from the emission light. 6

In this paper, we describe two methods for monitoring the intensity of light delivered to a remote sample using optical fibers. One method involves the use of an evanescent wave coupler located near the sample to couple a known fraction of the excitation light to a quantum- counter detector. The other is a twin-fiber method: equal amounts of excitation light are launched into two iden-

tical fibers, one of which is used for illumination of the sample and the other for power normalization.

GENERAL THEORY

The evanescent wave, which extends from the core of an optical fiber into the cladding (or the surrounding medium), is the basis of many fiber-optic measurement schemes, s An evanescent wave coupler can couple some of the light from one optical fiber to another when the two fiber cores are brought close enough to each other. These couplers have been used in optical fiber commu- nication systems and other optical fiber systems in the visible and IR regions of the spectrum, n Probably because the penetration depth of the evanescent wave is much shorter and the attenuation of the fiber is much larger in the UV region, the use of evanescent wave couplers in the UV region has not, to our knowledge, been previously reported.

The evanescent field phenomenon has been discussed frequently in the literature, s,12 When a ray is totally in- ternally reflected at the interface between two materials of different refractive index (e.g., the core and cladding of an optical waveguide), the intensity of the evanescent field extending into the medium of lower index decays exponentially with distance from the boundary: s

Iev = I0exp - (1)

where z is the distance normal to the optical interface, I0 is the intensity at z = 0, and dp is the penetration depth, given by s

dp ~- ~ (n12s in20 - n22) -'/2 (2)

where 0 is the angle of incidence, k is the wavelength in vacuum, and n~ and n2 are the refractive indices of the higher- and lower-index media (the fiber core and the medium surrounding the core), respectively.

The angle 0 in Eq. 2 can be distributed between ~r/2 and the critical angle for total internal reflection, 0°:

sin 0c = n-A (3) rt2

The wavelength dependence seen in Eq. 2 above indicates that the amount of light detected by a QC used to monitor the intensity of the evanescently coupled light in the second fiber will not be in constant proportion to the amount of light incident on the sample, even if the length of fiber in each leg of the system beyond the coupling point is the same. Rather, the ratio of intensity at the sample to that at the evanescent wave QC is wavelength-dependent, and this dependence must be known (i.e., a calibration curve must be available) so that raw data may be corrected.

The theoretical basis for the twin-fiber method is simple: place two fibers in the focal plane of the excitation beam, in symmetrically equivalent positions, and equal amounts of light will be launched into both. One may be used to illuminate the sample, the other to illuminate a QC. If wavelength-independent launch optics are used, the output of the QC may be taken to be equal to the

430 Volume 45, Number 3, 1991

EXCITATION OFF-AXIS MONOCHROMATOR REFLECTOR

A R C ~ R O i M ~ ~ ~ f E X Fi,BER ~ FIBER HOLDER

PICOAMMETER 1

FIG. 1. Block diagram of the apparatus used to assess the evanescent wave coupler for monitoring of incident intensity at a sample illuminated with the use of an optical fiber. Quantum counter (QC) 2 is the monitoring detector, while QC i is placed at the position normally occupied by the sample to be illuminated. In the twin-fiber method, a second fiber identical to the excitation fiber is positioned with one end in the fiber holder and the other in QC 2.

intensity of the incident light at the sample at all wavelengths. In theory, then, no calibration curve is needed.

EXPERIMENTAL

Materials. The plastic-clad silica (PCS) fibers were obtained from Fiberguide Industries. The fibers have a 600-#m-diameter core, 660-gin cladding, and 785-gm- o.d. Teflon ® jacket. The fiber numerical aperture is 0.4.

Apparatus. The block diagram of the fiber-optic-based fluorimeter for the evanescent wave technique is shown in Fig. 1. The radiation from a 150-W Xenon compact arc lamp (PTI) was dispersed by the [/8 double monochromator (Bausch & Lomb 505) with 5-nm bandpass. The PTI lamp housing uses front surface ellipsoidal and parabolic optics for focusing, which produces a donut- shaped beam profile. An off-axis paraboloidal reflector (1.5-in. diameter) was used to couple the light to the 1-m-long excitation fiber. The use of reflective optics everywhere in the excitation system eliminates variations in focal position and angle of launch for the light entering the fiber as wavelength is changed. A Newport five-axis fiber-optic positioner was used for positioning the fiber at the image produced by the mirror. The evanescent wave coupler, placed about 0.7 m from the launch end of the excitation fiber, coupled some of the excitation light to the other fiber--called the coupled fiber--which was 0.3-m long. The light transmitted by each fiber was measured by a quantum counter utilizing a concentrated solution (about 5 g per L) of Rhodamine B in front of a photomultiplier tube (PMT). In the twin-fiber method, the ends of two 1-m-long fibers were both inserted into the chuck of the fiber positioner; the other ends of the fibers were inserted into the QCs as shown in Fig. 1. Otherwise, the apparatus was the same as for the evanescent wave method.

The output signals from the two PMTs (1P28 Ha- mamatsu) were separately processed by two picoammeters (Keithley 485) which were interfaced to an IBM PC computer by a Metrabyte DASH-8 interface. The

o

0.39 80

]co / hx :'" ., ..,-.,.?',i " ' ' i \ ' ' ' v '

60 .;y" ~

0.35 i ,.:.t.kfi" Icox300 ,, :~.

• ~ / " 5 : ] " . ~ x - - 40 .~

0.31 ~ N 20

0.27 i i I t i 0 260 280 300 320 340 360

Wavelength (nm) FIG. 2. Intensities of the light incident on the monitoring detector, /co, and on the sample, Io., in the evanescent wave method (right-hand scale), and their ratio (left-hand scale) as a function of excitation wavelength in the ultraviolet region of the spectrum.

computer interface also controlled the stepper motor for the monochromator. A program was written to control data acquisition and storage on disk for later analysis.

Preparation of Evanescent Wave Coupler. The two ends of each fiber were polished with 600-grit emery paper (3- M or Math Associates). A knife was used to mark the region (~1.5 cm) on each fiber where the jacket and cladding needed to be removed; then the Teflon ® jacket and cladding were melted in the marked region with the use of a soldering iron. In order to remove all the residual cladding, the stripped parts of the fibers were immersed in a hot chromic acid bath for about 30 min. The two fibers were put into a home-made grooved fiber holder plate, which formed a parallel coupling region about 5 mm long, with the two fiber cores just touching, as shown in Fig. 1. A cover plate which held the fibers in position was attached to the grooved plate with screws.

Acquisition of Spectra. For each power normalization method, the excitation monochromator was scanned in 1-nm steps from 260 to 369 nm. At each wavelength, the output voltage of the excitation and power normalization picoammeters were read sequentially 100 times. The average and standard deviation for each channel were cal- culated and stored. For the evanescent wave method, the [ number of the launch optics was varied by changing the distances from the exit slit of the monochromator to the parabolic reflector, and from the parabolic reflector to the launch end of the excitation fiber.

RESULTS AND DISCUSSION

Evanescent Wave Method. The spectral output of the excitation and coupled fibers, Ie, and/co, for one data set (for an approximately f/2.7 input beam) and their ratio are plotted in Fig. 2. The latter curve is nonlinear. In- spection of Eqs. 1 and 2 indicates that, if (n12sin20 - n22) -1/2 is approximately constant over the wavelength range of interest, the ratio of the evanescent wave intensity to the incident wave intensity should depend exponentially on 1/k. Therefore, we plotted ln(Ie./Ico) vs. l/X, as shown in Fig. 3. We found a good straight line fit, with a correlation coefficient of 0.98. Repetitions of this

APPLIED SPECTROSCOPY 431

5.9

5.8

0

"-- 5.7

5.6

5.5 I I I I I 2710 2 9 3 0 3 1 5 0 3 3 7 0 3 5 9 0 3810

106/X (nm)

FIG. 3. Na tu ra l logar i thm of t he ratio of exci ta t ion in tens i ty inc ident on the sample to moni to r channe l in tens i ty m e a s u r e d with the use of evanescen t wave coupler vs. reciprocal wavelength. Indiv idual da t a po in t s are connec ted by shor t line segments ; the resul t of a l eas t - squares fit to Eq. 9 is also shown.

experiment gave consistent results, with slopes and intercepts that were constant to within much less than 1%. Measurements taken one week later showed variations of 1-2 % in slope and intercept. We expected that such variations might be due to small shifts in the mechanical positioning of the fiber tip in the image plane of the excitation optics, so we systematically varied the vertical position of the launch end of the excitation fiber and repeated the experiment. For a 1-mm change in fiber position, the slopes ranged from 2.18 to 2.11, and the intercepts from 5.05 to 5.03.

In a separate experiment to measure the effect of changing launch f number, two or three repeat scans were done at each of three different f numbers, for which linear fits of similar quality were obtained, as summarized in Table I.

Now we attempt a semi-quantitative analysis of these results. First we assess the approximation

1 ~(n~2sin20 - n22) -'/2 = k. (4)

For a given launch geometry, the value of 0 varied by less than a degree from its average value over the donut- shaped beam profile; the small amount of variation in 0 due to chromatic dispersion of the core over the wavelength range studied was 0.1 °. Thus, sin20 was constant to about 1 part per thousand. Essentially all the wavelength dependence of the quantity k in Eq. 4, then, is due to the wavelength dependence of the refractive index of fused silica, which accounts for a change of less than 4 % (nonlinear but monotonic) in k from k = 260 nm to 369 nm. Our approximation then ignores an effect which is less than 10% as large as the explicit dependence of dp on k in Eq. 2. Therefore, over this wavelength range, the approximation made in Eq. 4 is reasonable. It follows, then, that

I o z In ~ kk" (5)

By definition, Io is the intensity at z = 0. Since only a

TABLE I. Least-squares fits of Eq. 9 in text for three different values of launch f number.

f No. N.A. Slope (nm) Intercept C

2.3 0.21 223 4.54 1.07 x 10 -2 2.6 0.19 216 4.73 8.83 x 10 -3 2.9 0.17 230 5.16 5.74 x 10 -3

fraction of the light in the excitation fiber contributes to the evanescent wave at the core boundary, and since the coupled fiber is in close contact with only a part of this boundary, we may write

Io = c1Io, (6)

and

Ico = C2Iev (7)

with Cl, c2 < 1. Now we can use Eqs. 6 and 7 in Eq. 5 to get

in CIe" = ~ (8) Ico k~

where C = Cl x c2. It is easy to see that C is related to the fiber coupling efficiency: the larger the value of C, the larger the coupling efficiency. Rewriting Eq. 8, we obtain

In I e x - z 1 lnC. (9) Ico k ~

This equation is now in the same form as that used to analyze the experimental data in Table I. The slope is equal to z / k and the intercept is equal to - l n C. The results in Table I show the dependence of the coupling efficiency on the [ number of the incident beam. When the [ number increases, the coupling efficiency, C, de- creases. Since the smaller f number beam can fill the higher-order modes in the fiber more efficiently, the evanescent wave coupling is favored for higher-order modes, as expected. TM

The analysis presented above highlights the qualita- tive features of evanescent wave coupling, and shows that working calibration curves may be related to the simplest and best known expressions for evanescent wave intensity. In our experiment, the excitation light is launched into many modes of the fiber, each one characterizable (theoretically, at least) by the fraction of the total intensity it carries and other geometrical parameters. A more complete analysis of the coupling would consider each mode separately, integrating the evanescent wave coupling over the entire interaction length and summing with appropriate weighting factors over all modes. At a given wavelength, the exponential factor (see Eqs. 1 and 2) would be essentially constant for all modes, for reasons already given, so it can be factored outside these sums and integrations. The quantity C in the equations above represents, in some sense, the net result of these sum- mations and integrations of pre-exponential factors for all modes. A very thorough description of evanescent wave coupling in two identical multimode step-index fibers, with cores touching, is available elsewhere. 14

One interesting result that may be obtained from our simplified treatment is that we can estimate an effective


1.1 , 1.2 8 0

.¢.

a~

~9

O

FIG. 4.

1.0

0.9-- 260

[ I I [ I 280 300 320 340 360

Wavelength (nm) R a t i o of c o r r e c t e d i n c i d e n t i n t e n s i t y m o n i t o r o u t p u t to t r u e

incident intensity, for the evanescent wave coupling method. The raw monitor output was corrected with the use of the ratio of IJl¢o cal- culated from Eq. 9 with the use of best-fit values of slope and intercept from a previous calibration•

z value, which is the distance between the fibers in the evanescent wave coupler. For an f /2.7 incident beam, 0 is about 83 °. F rom Eqs. 6 and 2 we can est imate k = 0.072 and dp = 20 nm at k = 275 nm. From the experi- menta l result tha t z /k = 224 nm, we get z ~ 16 nm.

Finally, we show, in Fig. 4, the result of correcting the evanescent wave QC ou tpu t with the use of a stored calibration in the form of Eq. 9. T h a t is, the ou tpu t of the evanescent wave QC, I¢o, at a given wavelength X, was mult ipl ied by the value of IeJI¢o obtained from Eq. 9 with the use of previously de termined values of z /k and C. The ratio of this number to the t rue I~. at the same wavelength was very nearly unity: the average value of the ratios graphed in the figure is 1.000, with a s tandard deviation of 0.003. Thus, the evanescent wave technique provides an accurate way to moni tor the intensi ty of UV light delivered to a remote sample using fiber optics.

With our evanescent wave coupler, a convenient frac- t ion (about 0.3 %) of the excitat ion light is diverted to the monitor ing channel, so tha t most of the available source intensi ty may be utilized for the in tended purpose (spectroscopy, photochemist ry , ablation). The design we have presented utilizes equal lengths of fiber f rom the point of coupling to both the sample and the monitoring detector , so tha t equal t ransmission losses occur in bo th arms at all wavelengths. Obviously, with different frac- tions of excitation light travelling through these two arms, solarization effects in each would be different. Thus, to be used successfully with high-power systems, the coupler should be located far enough downstream from the point where the source radiat ion is launched tha t the intensi ty has been a t t enua ted to well below power damage levels.

Twin-Fiber Method. The spectral ou tpu t of each of the twin fibers is shown in Fig. 5, along with their ratio. Despite a t t empts to position the two fibers symmetrically in the image plane of the source, the ratio varies by about 20 % over the wavelength range studied, indi- cating tha t the two parts of the source imaged onto the fibers are not identical " twins."

Suppose fiber 1 is used for exciting the sample and fiber 2 for intensi ty monitoring. Since there is no obvious

I1 / 12 : ..•;....,..,,,,.......~..y....,'e" +'" 60 1.1 - , ........; . . . .

= - - - Z 40 "~ 1.0 .i'..',:" - - -

7./v;\:-~ = o O

0.9 20

0.8 , I I I ' 0 260 280 300 320 340 360

Wavelength (nm) FIG. 5. Intensities of the light incident on the monitoring detector, 12, and on the sample, I~, in the twin-fiber method (right-hand scale), and their ratio (left-hand scale) as a function of excitation wavelength in the ultraviolet region of the spectrum•

theoret ical basis for the variat ion in the ratio IJI2, a calibration curve cannot be described by a small number of parameters as in the case above. I f an entire set of wavelength-dependent ratios is stored, and subsequent /2 readings are mult ipl ied by the s tored ratios, a corrected value of incident intensi ty may be obtained. The ratio of this corrected value to the t rue incident in tens i ty /1 is shown in Fig. 6. Averaged over the entire spectrum, the value is equal to 1.002, with a s tandard deviation of 0.005. Th e result is reasonably close to the ideal value of unity, bu t not as accurate as tha t obta ined with the evanescent wave method.

This me thod appears to be simple and accurate. Bu t it has limitations: the incident beam has to be large enough and symmetr ical enough to couple to two fibers, and care mus t be taken in setup to ensure tha t equal launching conditions are achieved for bo th fibers. In practice, this is not easy to achieve, so a full calibration curve, con- sisting of a rat io a t every wavelength of interest, mus t be collected and stored. Fur thermore , if this me thod were used under high-power conditions, equal amounts of

1.1

O

©

O

FIG. 6•

1.0 . A ~ n J b 4 . A ^ . ~ A A . k = _ / k - IV v "V "v vv'J v',r~"-,,-'v w - ' v " v w v w -

0.9 ~ ~ ~ ~ ~ 260 280 300 320 340 360

Wavelength (nm)

Ratio of corrected incident intensity monitor output to true incident intensity, for the twin-fiber method. The raw monitor output was corrected with the use of the ratio of I1/I2 stored for each wavelength from a previous calibration.

APPLIED SPECTROSCOPY 433

power damage to the two fibers would not be guaranteed. Under low-power illumination conditions, the experi- menter might not want to sacrifice half the available source intensity to a monitoring channel.

CONCLUSIONS

We have developed two power normalization methods for fiber-optic-based instruments. The evanscent wave coupling of two fibers in the deep UV region was observed for the first time, an accomplishment made possible by developing a way to remove the jacket and cladding in the middle of the fiber and a clamp which holds the two cores together over an extended interaction length. Thus it becomes possible to bring two fibers together very closely, which is necessary to permit coupling in the UV region. For a given evanescent wave coupler and launch geometry, we can find a simple calibration equation which relates the intensity of the coupled light to that incident on the sample, and which agrees with simple evanescent wave theory with respect to the observed wavelength dependence.

The two-fiber method is very simple to use, but it requires a reasonably large, symmetrical source beam and careful initial alignment. It reduces the potential sample illumination by a factor of two. It is suitable for use in systems in which the launched light intensity is far below the threshold for solarization. Even in a seem-

ingly favorable case, it required the use of a stored calibration curve because of the observed dependence of the ratio of the two fibers' output on wavelength.

ACKNOWLEDGMENTS

This work was supported by the Environmental Protection Agency through a grant to the Center for Environmental Management at Tufts University. We would like to thank Dr. George B. Jarvis for helpful discussions.

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measurement of incident intensity of uv radiation delivered by optical fibers

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