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We report a novel radial-firing optical fiber tip containing a conical-shaped air-pocket fabricated by deforming a hollow optical fiber using electric arc-discharge process. The hollow optical fiber was fusion spliced with a conventional optical fiber, simultaneously deforming into the intagliated conical-shaped region along the longitudinal fiber-axis of the fiber due to the gradual collapse of the cavity of the hollow optical fiber. Then the distal-end of the hollow optical fiber was sealed by the additional arc-discharge in order to obstruct the inflow of an external bio-substance or liquid to the inner air surface during the surgical operations, resulting in the formation of encased air-pocket in the silica glass fiber. Due to the total internal reflection of the laser beam at the conical-shaped air surface, the laser beam (λ = 632.8 nm) was deflected to the circumferential direction up to 87 degree with respect to the fiber-axis.
© 2015 Optical Society of America
1. Introduction
Optical fiber-based endoscopic laser devices have been widely used for operation of the several minimally invasive medical surgeries such as gastroenterological surgery, prostatectomy, lithotripsy, varicose vein surgery, and herniated disk surgery in modern medicine [1–7 ]. Optical glass fibers play a central role in emerging and developing these kinds of endoscopic medical surgeries, due to their small invasion size, flexibility, and bio-compatible. In the surgical procedure, the endoscopic fiber-optic probe is inserted into deep diseased tissues through the small incision site and an intense light beam is transmitted from the one end to the other end of the optical fiber in order to treat in vivo lesions. Due to this distinctive therapeutic modality, a remote operation on a deep tissue in human body without a large incision is feasible by physicians [8].
From the initial stage of introduction of the medical optical fiber, the configuration of forward-firing optical fiber tip has been mainly exploited in the endoscopic clinical applications [1,9 ]. The flat end-face of optical fiber tip perpendicular to the longitudinal axis of the fiber was introduced by traditional cleaving system and designed to emit the laser beam to forward direction at the distal-end of the fiber. However, by using the frontal-firing scheme, it is difficult for the laser to target the impregnable areas such as urethra, prostrate, fallopian tube, microvasculature and diseased tissues occurred in the side wall of the small tubular structures. Due to the advances in modern medicine, in addition to the frontal-firing optical fiber, a laterally light-emitting fiber has been required for more accurate medical operation recently [10–12 ]. Therefore, the side-firing and the radial-firing optical fibers were introduced as the effective alternative to the forward-firing optical devices for treatment of the diseased tissue which locates at hard-to-access areas.
Side-firing optical fibers are typically used to deflect the laser beam onto an off-axis of the fiber and appropriate for the treatment of the diseased lesion in the one side of tubular tissue. The beveled surface of an optical fiber tip was fabricated by gradual angled polishing of the end-face of the optical fiber using the lapping sheets of different sized abrasives, causing the laser beam to be totally internally reflected in a certain direction of the transverse-axis of the fiber [3,13,14 ]. On the other hand, the radial-firing fibers are useful for the lesions surrounded in radial-direction. Recently, a radial-firing fiber designed to uniformly emitted laser beam to radial direction of the fiber was demonstrated by machining the terraced annulus pattern of the cross-section of fiber-end using a femtosecond laser followed by electric fusion polishing. To form the terraced pattern descending from outer-edge to central axis, however, the multiple circular disk patterns must be engraved by reducing the diameter concurrent with increasing the depth of a circular disk in each ablation procedure [15].
Although the several methods of the medical devices which are capable of deflecting a longitudinal firing beam to the transverse axis have been established over the past few decades [3,10–20 ], the lateral-firing configuration cannot be easily implemented due to complication of the fabrication process. The micro-processing of the rigid silica fiber is time-consuming, which results in cost-ineffectiveness of the production of the fiber tip. Moreover, these types of fiber tips typically are comprised of an angled element on the outer end-surface of the delivery optical fiber, and thus can be easily exposed to the biological tissue or liquid having the refractive index much higher than that of air during the invasion into human body, resulting in firing of treatment beam toward unwanted direction. Therefore, the prior technologies exceedingly need to be encased in a protective glass cap, to hinder the inflow of the external bio-substances to beveled surface and to facilitate the emission of the beam in the desired lateral direction in the practical medical applications [3,21,22 ]. However, this additional encapsulation process can be not only the factor of the increase of the production cost but also the cause of the bulky product.
In this study, we demonstrated a radial-firing optical fiber probe containing a conical-shaped air-pocket. The present novel technique is simply implemented by electric arc-discharge procedure using commercially available fusion splicer. An intagliated conical shaped fiber region resulted from the gradual collapse of the cavity of the hollow optical fiber was formed by fusion splicing a hollow optical fiber with a conventional optical fiber. Then the fiber-end of the hollow optical fiber was discharged again to form an encased conical-shaped air-pocket.
2. Experiments
Figure 1 shows the fabrication process of the proposed radial-firing optical fiber containing an intagliated conical-shaped air-pocket. Firstly, a hollow silica glass optical fiber (HOF, inner/outer diameters; 70/125 μm) was fusion spliced with a conventional optical fiber (COF, core/cladding diameters; 8.2/125 μm) by discharging an electric-arc at the interface between the COF and HOF [Fig. 1(a)]. The cross-section images of the HOF and the COF used for the experiment are shown in the figure. As the result, the intagliated conical shaped region can be formed in the HOF. Due to the gradual power distribution of arc discharge between the electro-rods [23], the hole of HOF is collapsed completely at the central arc region (splice point), whereas the hole in beside region was contracted gradually along the fiber- (longitudinal) axis, resulting in a conical cavity that inner diameter increases with distance along the fiber-axis as shown in Fig. 1(b). Afterward, the HOF section at the ~120 μm away from the vertex of the conical surface was induced the additional electric arc in order to entrap the air region inside the optical fiber tip [Fig. 1(c)]. Figure 2 shows the side views of a COF with the flat end surface and a radial-firing fiber tip containing a conical-shaped air-pocket. Note that the photo-images were obtained in an air environment. As shown in Fig. 2(c), we can clearly see that the angled surface relative to the longitudinal axis was formed inside the glass fiber tip. Only the electric arc-discharge technique using a conventional fusion splicer (Fitel S176) was conducted without high-tech equipment in the whole process.
Fig. 1 Fabrication process of the radial-firing optical fiber tip containing a conical-shaped air-pocket and the cross-section images of the hollow optical fiber (HOF) and the conventional optical fiber (COF).
Fig. 2 Side view of (a) the fabricated radial-firing fiber [Fig. 1(c)], and the magnified images of end region of (b) the conventional optical fiber (COF) with flat end surface and (c) the radial-firing optical fiber tip.
In order to investigate the dependence of emission angle of the laser beam at the fiber tip upon the internal surface angle (φ) of the conical-shaped air-pocket, the three radial-firing optical fibers having the different internal angles were fabricated and are shown in Fig. 3 . The internal angle (φ) with the fiber-axis of the surface angle can be simply controlled by adjusting the power of the electric arc-discharge. The precise inner air region and the angles relative to the longitudinal fiber-axis were observed by using an optical microscope (Olympus BX51) before placing a drop of index-matching oil (noil = 1.457) on the tip to restrain the occurrence of refraction due to the cylindrical surface of the optical fiber. The angles relative to fiber-axis of the air-pockets (Sample 1, 2, and 3) were measured to be ~44, 47, and 60 degree (°), respectively [Fig. 3]. Note that the surface angle above the critical angle (θc = 90 ° ‒ φ) of 43 ° with respect to the normal to the surface (glass/air boundary) is necessary for the total internal reflection (TIR), when the light emerges from glass to air.
Fig. 3 Microscope images of the encased air-pockets of the radial-firing fibers, (a); Sample 1, (b); Sample 2, (c); Sample 3.
Figure 4 shows the experimental setup for measuring the angular distribution of the laser light deflected from the fiber tip (emission profile). The optical fiber tip was fixed along longitudinal axis by a fiber mounter and a He-Ne laser beam (Melles Griot 25-LHR-171 (λ = 632.8 nm)) was focused into the fiber core of the lead-in COF end-face. To measure the optical power of the laser beam at the different angular positions, an optical iris and a focusing lens in alignment with the collecting fiber along the longitudinal fiber-axis (alignment system) were simultaneously rotated around the point of the fiber tip as shown in the figure. The optical iris (hole diameter; 0.5 mm) was placed at 2.5 cm away from the fiber tip to extract the firing laser from the precise angular region, and then the optical power (λ = 632.8 nm) of the focused beam passed through a collecting multi-mode optical fiber (core/cladding diameters; 62.5/125 μm) was measured by an optical power meter.
Fig. 4 Experimental setup to measure the angular emission profile of the laser beam irradiated from the radial-firing fiber tip.
3. Results and discussion
Figure 5 shows the images of laser emission at the fabricated radial-firing optical fiber tips with the different conical-shaped air-pockets and the conventional optical fiber (COF) with the flat end surface. Whereas the laser from the COF lighted on the frontal screen perpendicular to longitudinal-axis of the fiber [Fig. 5(d)], the tremendous difference in direction of the beam divergence was observed in the proposed radial-firing fibers as shown in Figs. 5 (a) and (b). The laser beam from the radial-firing fiber tips (Sample 1 and 2) was deflected to the circumferential direction and projected onto the side screen parallel to the fiber-axis. On the other hand, in the case of the fiber tip (Sample 3) with the conical surface angle (φ) of ~60 °, the laser beam was transmitted toward the frontal direction.
Fig. 5 Emission images of the laser beam (λ = 632.8) at the radial-firing optical fiber tips, (a); Sample 1, (b); Sample 2, (c); Sample 3, (d); COF.
Figure 6 shows the measured angular emission profile of the laser beam from the radial-firing fiber tip. The laser beams deflected from the fiber Sample 1 and 2 were emitted with the angle of ~ ± 71 and 87 °, respectively. Based on the above results, we can deduce that the radially firing beam was resulted from the TIR of the laser at the glass/air surface in the fibers. Because the incident laser beam from the fiber core strikes the conical-shaped surface at the angle (θ) larger than the critical angle (θc = 90 ° ‒ φ) of 43 ° with respect to normal to the interface. Furthermore, the fluent radial emission (2nd mode) was found in the emission profiles of the radial-firing fibers (α = ~ ± 40 °) as shown in the figure. In the previous report, the detailed explanations of frontal-direction modes of the laser beam through an intagliaged conical surface of optical fiber was depicted by several methods [24]. However, we assumed that the fluent radial emission in the case of the present optical fiber tip was originated from the refraction of the fractional laser beam instead of being reflected through the air surface owing to the propagation angle (αo (> 0 °) = αNA) of the laser beam passing through the glass fiber. If the laser beam traveling within the angle of 0 < αNA ≤ ~5.4 °, which is the maximum acceptance angle (αmax = ~5.4 °) is transmitted inside the fiber core (COF numerical aperture; NA = n⋅sinαmax = 0.14) and emerges the conical surface (φ; ~44 and 47 °), the incidence angle (θ = 90° ‒ αNA ‒ φ < θc) with respect to the normal to the surface can be below the critical angle. The TIR condition of the laser beam emerging with the beveled angle (αo = αNA ≤ αmax) with respect to the fiber-axis is not valid. Thus the laser beam circularly divided at the vertex of conical surface can be transmitted through the air surface region, resulting in the fluent radial-firing laser (2nd mode) in comparison with the abrupt radial firing of the laser (1st mode) as shown in Fig. 6. While the majority of the laser beam (αo = 0 °) was totally internally reflected by the angled air surface, the optical power of the fractional radial-firing 2nd mode of the laser beam was much smaller (~10 times) than that of the major radial firing mode (Fig. 6).
Fig. 6 Angular emission profile of the radial-firing optical fibers (Sample 1, 2) and forward-firing optical fiber (Sample3) containing a conical-shaped air pocket, Inset: Emission profile of the COF.
By using the Snell’s law (ni⋅sinθi = nj⋅sinθj), the emission angle (theoretical values; lines) of the deflected laser beam at the fiber tip of the Samples 1 and 2 was estimated and compared with the measured data (circles) as shown in Fig. 7 . We considered that the laser beam was totally internally reflected at the glass/air-pocket boundary and then refracted out at the angled outer surface of the fiber tip (φos = ~20 °) as shown in Fig. 3. When the reflected laser beam at the conical surface enters the outer surface of the fiber with the angle of θi, the emission angle (α) with the fiber-axis was estimated by using the following relation, α = 90 ° ‒ θj + φos, where θj is the angle of output laser beam with respect to the normal to the outer surface (φos) of the fiber tip [Fig. 3(d)]. Note that the refractive indices of silica glass and air are nsilica = 1.457 @ 632.8 nm and nair = 1, respectively. The divergence angle of the laser beam increased with the increase of the conical angle, until the condition of the total internal reflection was thwarted (below θc) at the conical surface. On the basis of the aspects, the emission angle can be expected to be widely controllable by tailoring the internal surface angle of conical-shaped air-pocket.
Fig. 7 Comparison of the theoretical (lines) and the experimental (circles) emission angle of the laser beam at the optical fiber tips, S1; Sample 1, S2; Sample 2, S3; Sample 3.
In the case of the fiber tip with below the critical angle (Sample 3), it was found that the laser beam was illuminated on the distributed area larger than that of the COF on the frontal screen which was 4 cm (L78) far from the fiber tip. The diameter of the circular lighting area (D) and the maximum emission angle (α) were measured to be ~2.5 cm and ~16.5 °, respectively. We considered that the laser beam was not totally internally reflected at the conical surface but refracted through the three curved boundary, as illustrated in Fig. 8 . For detailed analysis of refraction in the fiber tip of the Sample 3 having more than three refractions, the ABCD matrix method was used. The matrix equation representing the beam passed through the four translation and the three refraction region is given by [25,26 ],
withwhere ni and nj are the refractive indices of the left (ith) and right (jth) sides of the media between the certain boundary, respectively, Rij is the radius of curvature at ij boundary, Mij is the ray matrix presenting the beam parameters, Lij is the distance from ith to jth regions, and γ and α are the position and the emission angle of the ray relative to the optical axis, respectively.Fig. 8 Schematic diagram of the optical fiber with air-pocket (Sample 3) and the parameters for the ABCD matrix calculation.
By assuming that the majority of the laser beam from the fiber core propagates straight along the fiber-axis (αo = 0 °) in the silica region (L12) and the conical region (M23) is regarded as the curved surface (R23 = 19.7 μm), in the case of the Sample 3, the size of the circular beam, D ( = 2γ), on the screen and the emission angle of the laser, α, were estimated. The parameters used in the matrix equation of the Sample 3 were L12 = 186.1 μm, L34 = 86.8 μm, L56 = 35.9 μm, L78 = 40.0 mm, R23 = 19.7 μm, R45 = 53.8 μm, and R67 = 77.0 μm. After the calculation with the equation, the results (D = 2.4 cm and α = 17.3 °) were found to well match with the experimental results (D = ~2.5 cm and α = ~16.5 °).
In order to confirm the feasibility to utilize the proposed radial-firing optical fiber tip for bio-medical applications, the capability of the radial-firing of the tip inside the bio-compatible medium was investigated. A saline (NaCl = 0.9%) was used as a biological fluid in this experiment. Note that the reflective index of biological tissue is slightly larger than that of the water (nbio > nwater ( = 1.331 @ λ = 632.8 nm)) which is major component of the living tissue [27]. Figure 9 shows the emission images of the laser beam at the optical fiber tip (Sample 2) inside the solution. Even though optical fiber tip was in contact with the saline solution (nsaline = ~1.333) having the high enough refractive index, which can be frustrated the TIR condition of the laser beam at the surface, the fiber tip emitted the laser beam to radial direction of the optical fiber and circularly lighted the red laser beam on the cylindrical wall of the opaque bottle (polypropylene), as can be easily observed in the figure. This result implies that the TIR of the laser beam at the conical inner surface was retained due to the encased air-pocket which obstructs the inflow of an external bio-substance. Therefore, without the secondary process of encapsulation of the fiber tip, the proposed radial-firing fiber tip appears to be sufficient to use for practical bio-medical applications.
Fig. 9 Emission image (Sample 2) of the radial-firing inside a bottle of saline, (left) side view and (right) top view.
4. Conclusion
We demonstrated a novel radial-firing optical fiber tip containing a conical-shaped air-pocket using electric arc-discharge process. A hollow optical fiber was fusion spliced with a conventional optical fiber, simultaneously deforming into an intagliated conical-shaped region along the longitudinal fiber-axis of the optical fiber by discharging electric-arc. Then the end-face of the hollow optical fiber was discharged again to form an encased air-pocket inside the silica fiber. Due to the conical-shaped air region, the laser beam from the delivery optical fiber was totally internally reflected at the conical surface and deflected toward the radial direction of the optical fiber. It was found that the uniformly radially firing laser beam was emitted with the side angle up to ± 87 degree. Therefore, we expect that the developed radial-firing optical fiber can be applied for practical medial applications such as laser surgeries and photodynamic therapy (PDT) to treat the diseased tissue occurred in the side wall of the micro-tubular structures.
Acknowledgments
This work was supported in part by Basic Science Research Program through the National Research Foundation of Korea, Ministry of Education, under Grant 2013R1A1A2063250, the Korea government under Grants 2011-0031840 and 2010-0020794, the New Growth Engine industry Project of Ministry of Trade, Industry, and Energy, the Brain Korea-21 Plus Information Technology Project, and by the Ultrashort Quantum Beam Facility Program under a grant provided by the Gwangju Institute of Science and Technology, South Korea.
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