Clinical Potential of the Side-Glowing Optical Fibers

 

Janis Spigulis and Daumants Pfafrods*

 

University of Latvia, Raina Blvd. 19, Riga, LV-1586, Latvia

*) ANDA OPTEC Ltd., PO Box 326, Livani, LV-5316, Latvia

 

 

ABSTRACT

 

   Unconventional designs of silica-core optical fibers providing efficient lateral emission through the cylindrical side surface have been developed and manufactured. A physical model of the side-glowing fibers, available experimental data and potential clinical applications of this type of lightguides are discussed. Promising clinical applications can be expected in following areas:

1. Phototherapy, including photo-dynamic therapy, soft-laser therapy and visible/UV broadband irradiation therapy.

2. Single-fiber laser Doppler flowmetry.

3. Laser-tissue dosimetry.

4. Linear and planar "cold light" illumination.

5. Optical sensing of mechanical pressure.

 

Keywords: medical lightguide instruments, specialty optical fibers, linear and planar illumination/irradiation sources.

 

 

1. INTRODUCTION

 

   Optical fibers originally are supposed to transmit the introduced light energy  with minimum losses to their distal ends which then emit like semi-point light sources. However, optical fibers can also serve as linear or semi-linear light sources, if special efforts are undertaken to stimulate the core-transmitted light leakage through the fiber side surface. A number of methods can be used to provide such side-glowing effect, including multiple microbending of the fiber axis 1, mixing of scattering additives into the fiber core or cladding materials 2,3, etc.

   Simple model describing the optical fiber side-emission phenomenon is presented in this paper, as well as experimental results obtained by testing the side-glowing optical fiber samples with pure silica core covered by plastic clad with scattering additives 2. This type of lightguides undoubtedly has good potential in several clinical application areas 4, 5 which will be discussed in more details later in this paper.

 

2. THE MODEL REPRESENTATION

 

   Side-scattering in any type of glowing optical fiber is much stronger than absorption and other losses, therefore only core-transmission and side-scattering can be regarded as two dominant light transport channels. Consequently, the basic assumptions for a simple first-approximation model can be formulated as follows:

   1. Side scattering is the only considerable mechanism causing the losses of the fiber core-transmitted radiation.

  2. All initially scattered photons are penetrating the fiber cylindrical side surface without losses, and are subsequently emitted isotropically into the surrounding space.

   Representing the glowing fiber as a sequence of identical short fragments of length Dx, intensity of the core-transmitted radiation for each fragment can be expressed as

 

 ITX = I0  exp (- k Dx)                (1),

 

where I0 is the input radiation intensity, and k is the side-scattering efficiency coefficient. The side-glowing intensity emitted from a short fiber fragment at any direction per steradian in this case is

 

IS  = (4 p)-1 I0  [1 - exp (- k Dx)]       (2).

 

Following (2), the side-glowing intensity as a function of the actual fiber length x after conversions can be expressed as

IS (x) = A exp (- k x)                             (3),

 

where the constant A, if we propose Dx to be the fiber length unit (Dx = 1), is

 

A = (4 p)-1 I0 (exp k’ - 1)                       (4),

 

where k’ is side-scattering efficiency per unit length.

   The comparison  of (1) and (3) leads  to general conclusion that for simple side-glowing optical fibers both the core-transmitted and side-emitted radiation intensities are decaying monoexponentially with fiber length, and both of them have the same rate constant k. More detailed comments on this topic are given in 6.

 

3. THE SIDE-GLOWING UNIFORMITY ASPECTS

 

   The inevitable intensity decay (4) causes some practical problems, since clinical and other real applications require uniform side-glowing intensity, or at least very limited variations of this intensity along particular fiber length. The model can help to consider quantitatively several design versions focused to increase the uniformity of the side-glowing intensity.

   The simplest version how to get side-glowing is to couple one end of the glowing fiber with appropriate light source, e. g. laser. As for the desired side-glowing uniformity within a specific fiber length, it might be given by user as certain percentage interval; in this case the optimum k-value can be easily adjusted using the expression (3). For example, let us suppose that the required uniformity interval for a 10 m long glowing distance lies within 20 % of the initial side-glowing intensity. To find the optimum k-value, one should just solve the equation for the lowest intensity limit (80%): exp (-10 k) = 0.8, which gives  k = - (ln 0.8) : 10 = 0.023 m-1.

   This design version, however, is not highly efficient from the point of input radiation conversion into the side-glowing: in relatively uniformly-glowing fibers most of the introduced light energy passes through the fiber distal ends instead of being scattered via its side surface. As a rule, the higher is side-glowing uniformity of a single-light-source-coupled fiber, the lower is light conversion efficiency.

   Another design approach - attachment of light  sources at both fiber ends - may look  somewhat more complicated  technically, but  this  version provides more uniform and more intense side-glowing if compared with the above regarded

case. In frame of the presented model, side emissions initiated by both sources are to be summarized. Let us assume that the intensities of radiation coupled into the fiber core at each end of the fiber are equal (say Io), and the length of the given fiber fragment is L. Then the total side-emission intensity at fixed spatial direction as a fiber length function is

 

IS2 (x) = A{ exp(- k x) + exp[- k (L - x)]}     (5),

 

where A is given by (4). The calculated IS2 (x) distribution (5) along a 100 m fiber fragment with fixed k-value (0.01 m-1) is presented on Fig. 1,a.

                                                                                                                                                                                                                                                                                      a)                                                                                          b)

Fig. 1. Side-glowing intensity distribution along the fiber length using two identical light sources at both fiber ends (a),

           and using a 100 % reflecting end mirror (b).

 

   Mounting of an integrated  reflector, e. g. a miniature plane mirror, on the distal end of fiber can also efficiently increase the light output and uniformity. If the distal reflectivity R and the fiber length L are known, the side-emission intensity at each actual distance x from the input end can be calculated (3):

 

ISR (x) = A {exp(- k x) + R exp[- k (2L - x)]}     (6),

 

where A is defined by (4). For illustration, the calculated ISR (x) distribution curves for 50 m long glowing-fiber fragment with 100 % reflective end mirror are presented on Fig. 1,b.

   The above regarded model gives also a general knowledge how to reach ultimate (100 %) side glowing uniformity by means of specific longitudinal distribution of the side-scattering efficiency along the fiber axis 6.

 

4. EXPERIMENTAL DATA

 

   Several parameters of silica core side-glowing optical fibers have been studied experimentally 5, 6. A brief resume of the main results will be given here.

 

   1. The measured side-glowing intensity decrease with fiber length confirmed the expected (3) monoexponential decay. As example, experimental results for two 0.4 mm silica core glowing fiber samples are presented on Fig. 2. Solid lines represent the least square fitting  to the experimental points collected at 10 measurement  trials, with the corresponding k-values; the dashed lines represent the respective k-values obtained using the cut-off method.

 

   2. The observed spatial distribution of side-emitted intensity for silica core glowing fibers was fairly uniform, but not quite isotropic, as assumed in the above regarded model. In fact, the radial distribution of intensity (perpendicularly to the fiber axis) emitted from each small fiber fragment was practically uniform; however, the measured intensity distributions in axial plane confirmed some predominance of forward scattering, which could be expected from the Mie scattering theory. For example, results of measurements taken from 1 mm long side-emitting fragment located 1 m from the glowing fiber input end at 9 fixed orientations in the axial plane (Fig. 3) clearly showed that the intensity emitted forward at 60 deg. angle was roughly double as much as the intensity  emitted perpendicularly to the fiber axis (0 deg.) and backward up to - 60 deg. One should also note that the emission non-uniformity within the - 60...+30 deg. interval did not exceed 10 %. Use of an end reflector can further increase the spatial uniformity of side-glowing intensity 5.

 

   3. The silica core glowing fibers showed quite unexpected sensitivity to external pressure applied perpendicularly to the fiber axis - approximately 5 times higher than that of conventionally designed plastic clad silica fibers; even the load of few kilograms can be reliably detected by decrease of the core-transmitted radiation intensity. More details on this observation one can find in 6.

Fig. 2. Typical side-glowing intensity decay with

           the fiber length (in semilogarithmic scale).

 

Fig. 3.  The measured angular distribution of side-emission

             intensity for 0.4 mm silica core glowing fiber.

 

5. THE CLINICAL APPLICATION AREAS

 

   Several existing and potentially new clinical applications of the side-glowing optical fibers should be pointed out.

 

6.1. Phototherapy.

   Diffusive optical fiber tips  are widely applied for laser radiation delivery in photo-dynamic therapy (PDT) for cancer treatment; usually they are designed to act as semi-point or semi-linear sources 4. The prolonged glowing fiber application would make possible really linear and also planar (flexible surface) irradiation in PDT - either by dense winding of a single glowing fiber around the tumor location (e. g. finger, ear, tongue), or by using planar illuminators of adjusted shape, made of side-glowing fibers as textile 1,7 or as some other planar optical fiber structure 8,9.

   Side-glowing fibers can appear useful also in soft-laser therapy, including photobiostimulation and laser acupuncture. For instance, the fibers could be fixed on the skin surface to irradiate non-invasively appropriate long underskin blood vessels or specific sequences of bioactive points. The flexibility of glowing fibers make them very attractive as delivery tools for laser therapy of relatively large areas in dentistry and otolaryngology.

   Also non-laser applications of side-glowing fibers in phototherapy seem to be promising. For instance, the output efficiency of the blue light local delivery system for treating of infant Hyperbilirubinemia by means of so-called fiber “biliblankets” 1 can be substantially improved by using silica core side-glowing fibers instead of the conventional ones. This was confirmed by testing of the experimental glowing-fiber biliblanket sample manufactured at ANDA OPTEC, Ltd.

   Silica core glowing fibers, thanks to their good transmittance and side-emittance of the ultraviolet radiation, can find new clinical applications also for small- and large-area local UV irradiation. As example, “psoriblankets” of similar design as “biliblankets” could be fabricated of side-glowing silica fibers in future for more convenient local UVA phototherapy of Psoriasis, including also permanent local underwear irradiation. Such design concept might be further extended to special underwear tanning textiles with woven-in  glowing fibers providing dosed delivery of UVA and UVB radiation to skin - e. g., to compensate the sunlight deficit during the dark seasons for those living in polar regions.

 

6.2. Laser Doppler flowmetry.

   Silica core side glowing optical fibers, especially those completed with end mirror 5, have shown ability not only to scatter out the core-transmitted radiation, but also to collect diffuse light from the outside surrounding space into the fiber core; it can be then easily transmitted to appropriate photodetector. This feature appears very useful from the point of single-fiber laser Doppler flowmetry technique 10, since simultaneous laser irradiation and scattered light collection becomes available not only on the top of delivery fiber, but also along its axis.

 

6.3. Laser-tissue dosimetry.

   Ability to collect the diffuse radiation from large areas of tissues makes the side-glowing fibers with end mirrors attractive also for dosimetry of laser radiation exposed to tissues during surgical, therapeutical, or other clinical manipulations. The dosimetry by means of side-scattering fibers eventually would be competitive to presently used point-detection approaches, including that using conventional optical fiber with fluorescent dye-doped tip 11.

 

6.4. Linear and planar “cold light” illumination.

   Besides the specialized clinical procedures, side-glowing fibers can find efficient applications also for illumination of biological objects which are sensitive to thermal injuries and therefore must be protected against heat radiation emitted from conventional light sources, e. g. halogen lamps. Optical fiber ends are successfully used for a long time in many clinics as semipoint “cold light” sources for illumination of hard-to-reach areas of patients (e. g., endoscopic illumination), as well as for illumination of biosamples in microscopy. Glowing fiber based prolonged linear and planar surface light emitters would substantially increase the quality and open new application areas of this specific kind of clinical illumination.

 

6.5. Optical sensing of pressure.

   The observed high sensitivity of side-glowing fibers to externally applied mechanical pressure 6 can also find some clinical applications thanks to the small size, flexibility and electrical safety of the fibers. One potential application area would be recording of the human biting force distribution and its changes during the patient care. Other one would be clinical surveillance, e. g. undercarpet optical pressure sensors in hospital rooms and corridors to check remotely activities of the patients. Glowing fiber based optical pressure sensor designs for fitness control and artificial limb monitoring would be considered in future, too.

 

6. CONCLUSIONS

 

   The recently developed side-glowing optical fibers represent a new class of linear light sources; in spite of relatively few available data on their main features and parameters, there seems to be a good potential for clinical applications in several areas of therapy, diagnostics, patient monitoring and biosensing, as well as for local “cold light” illumination. This stimulates further physical modeling and experimental studies of model samples to improve the side-glowing uniformity along the fiber axis, input radiation conversion efficiency and other parameters which are needed for reliable and safe clinical applications in future.

 

7. ACKNOWLEDGMENTS

 

   The financial support from Latvian Ministry of Education and Science (Grant No. 95-61) is highly appreciated.

 

 

8. REFERENCES

 

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9. Y. Koizumi, “Surface light-emitting ornamental device using optical fibers”, Patent No. WO 90/00699, 1988.

10. E. G. Salerud, P. A. Oberg, “Single-fibre laser Doppler flowmetry. A method for deep tissue perfusion measurements”, Med. Biol. Eng. Comput., 25, 329-334 (1987).

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