WO2009055888A1

WO2009055888A1 - Optical fiber having a liquid core and cladding, method for simultaneously filling the same, and method for reducing the number of guided modes9

Info

Publication number: WO2009055888A1
Application number: PCT/BR2008/000323
Authority: WO
Inventors: Cristiano Monteiro De Barros Cordeiro; Christiano José Santiago DE MATOS; Eliane Moura Dos Santos; Daniel Dos Santos Ferreira; Carlos Henrique De Brito Cruz
Original assignee: Universidade Estadual De Campinas-Unicamp; Instituto Presbiteriano Mackenzie
Priority date: 2007-10-29
Filing date: 2008-10-29
Publication date: 2009-05-07
Also published as: BRPI0705636B1; BRPI0705636A2

Abstract

The present invention relates to a waveguide, or also called simply an optical fiber, having a liquid core and cladding and which guides light in the liquid core by means of total internal reflection using, as a basis, any hollow-core microstructured fiber with a cladding having a high air content, including those commercially-available. The present invention also discloses a method for simultaneously filling the cladding and the core with liquids, or other different materials such as sol-gel or polymers. Additionally, the present invention relates to a method for reducing the number of guided modes in any fiber provided with a liquid in its core and which is based on a microstructured optical fiber.

Description

Specification of Patent of Invention for

"OPTICAL FIBER HAVING A LIQUID CORE AND

CLADDING, METHOD FOR SIMULTANEOUSLY

FILLING THE SAME, AND METHOD FOR REDUCING THE NUMBER OF GUIDED MODES"

TECHNICAL FIELD

The present invention relates to a wave guide, or optical fiber, made from a microstructured fiber having a hollow core and a cladding with a high air content. The present invention still relates to a method for simultaneously filling the holes of the cladding and the core with previously selected materials. Additionally, the present invention relates to a method for reducing the number of guided modes in any liquid-core optical fiber which is based on a microstructured optical fiber. BACKGROUND OF THE INVENTION

The transmission of light through the optical fiber occurs from the incidence of a light beam onto one of its ends which, due to the optical characteristics of the fiber, travels across the same by means of successive reflections. An optical fiber comprises at least two layers, a core and a cladding. The light transmission occurs in the fiber core due to the refraction index difference between the core and the cladding.

The core always has a higher refraction index than the cladding. This fact, coupled with the incidence angle of the light beam, allows the total internal reflection phenomenon to occur.

With respect to the number of propagating modes, optical fibers may be essentially divided in two types: monomode, which propagates only one mode; or multimode, which propagates more than one mode.

Over the years, improvements on optical fibers have been made and new fibers provided with special characteristics have become available, among which are the microstructured optical fibers.

These fibers, also known as Photonic Crystal Fibres (PCF), are provided with a plurality of holes at the cross- sections of their cladding, which run parallely and across the fiber axis. The core, which is located on the fiber core, may be solid or hollow. The diameter of the cladding holes, as well as the mean distance between said holes, is of the same order of the wavelength of the propagated light and, hence, the microstructure may strongly influence the optical characteristics of the fiber.

These new degrees of freedom related with the microstructured allow for a wide range of possibilities in controlling the optical characteristics of the fiber, such as chromatic dispersion and modal area, allowing essential studies and inaccessible applications with standard optical fibers to be carried out. One of the new applications for the microstructured optical fiber concerns the use of the cladding and/or core holes for inserting liquids or gasses into the interior of the fiber in order to carry out sensing, spectroscopy, non-linear optics experiments, and the like. Due to the long interaction length, the transversal confinement of light, and the possibility of inducing a large superposition between the light and the liquid or gas inserted into the cladding holes, measurements may exhibit high sensitivity. In particular, hollow-core microstructured optical fibers may be used in order to obtain a waveguide in which a liquid is inserted only into the core, thus promoting a virtually complete interaction between the guided light and the liquid medium of interest. The low refraction index of the cladding, obtained due to the high air content in the microstructure, ensures the total internal reflection of the light propagating through the liquid medium will be achieved.

The total internal reflection guiding requires a lower cladding index than that of the liquid to be used. Before the advent of microstructured optical fiber, only liquids having a high refraction index (>~ 1.45) could meet this requirement when placed in the interior of glass capillaries. Nevertheless, unusual materials, difficult to integrate with the technology of conventional silica fibers, allowed the use of liquids having lower refraction indexes. An example of this kind of material is Teflon. Alternatively, it was possible to study and/or sense liquids located on the surroundings of the core of a waveguide. In this case, the interaction between light and the liquid medium of interest is not complete, since it occurs only through the evanescent field. Under these conditions, the fluid- radiation interaction is largely reduced, hardly being higher than 1%. Moreover, interface effects may impair many applications.

In order to be able to use hollow-core microstructured optical fiber for sensing liquids, as described above, only the fiber core must be selectively filled, while maintaining the cladding holes filled only with air.

In this regard, two techniques have been developed. Both techniques are based on processes for blocking the cladding holes while only the core is filled with the liquid. One of such techniques is based on a process which comprises several steps, which are: injecting a special polymer into the fiber, curing (drying) said polymer by illuminating with ultraviolet radiation, and subsequently cutting the fiber. This process is repeated several times. The principle developed in said process consists in pressurizing the polymer into the interior of the fiber, such that said polymer fills a larger length of the higher-sized holes as compared with lower-sized holes [Y. Huang, Y. Xu, and A. Yariv, "Fabrication of functional microstructured optical fibers through a selective-filling technique," Appl. Phys. Lett. 85, 5182-5184 (2004)]. Another technique uses a optical fiber fusion splicer in order to close the cladding holes. The principle of this technique lies in that said machine emits a low intensity/duration electric arc, which is applied to one of the optical fiber ends, so that the cladding holes are closed while the central hole, i.e., the core, remains open [K. Nielsen, D. Noordegraaf, T. Sørensen, A. Bjarklev, and T. P. Hansen, "Selective filling of photonic crystal fibres," J. Opt. A: Pure Appl. Opt. 7, L13-L20 (2005)].

The selective filling of the core by means of the techniques set out above, and the use of a commercially-available hollow-core microstructured optical fiber having a high air content in the cladding, allows the guided radiation by means of the total internal reflection to be achieved in almost every existing liquid. The total internal reflection occurs due to the fact that such fibers have more than 90% of the microstructured cladding area filled with air, what causes the effective refraction index in the cladding to be inferior to 1.2.

However, generally this effective index is excessively low, such that liquid-core microstructured fibers have a high contrast in the core-cladding index and, accordingly, allow the propagation of hundreds or thousands of modes.

The number of optical modes guided in an optical fiber may be calculated by the "parameter V" of said fiber, namely: 2πr 2 2

V — y ¥1 core ^~ Yl cladding λ (D

wherein: λ is the wavelength;

R is the radius of the optical fiber core;

Nc_ore is the refraction index of the optical fiber core; and no_ddi_ng is the refraction index of the optical fiber cladding.

When the parameter V of the optical fiber is inferior to 2.405, only one mode can be guided. Approximately, the number of guided modes (N) may be estimated by the equation N~V²/2, which formula is more accurate as higher the value of V.

A simple example may show the approximate number of modes of a commercially-available hollow-core microstructured optical fiber, namely: A typical microstructured fiber may have ⁿciaddi_ng ~ 1.2 and a core radius of 5 microns. If the core is filled with water (refraction index of ~1.3), we have that for a wavelength λ = 633nm, V ~ 25 and, therefore, N ~ 300.

In a multimode optical fiber, each mode acquires, upon crossing the length of the fiber, a different phase, which renders difficult or unfeasible the adoption of measures which are based or dependant on the wave phase. Among such measures, one could mention those which are WsM in interferometry and Kerr non-linear effects. Kerr non-linear effects include: solitonic effects, self-phase modulation and cross-phase modulation. Determining the phase of the light, or of its variation, with some physical or chemical parameter can be simplified if only one mode propagates through the interior of the optical fiber core, or the waveguide. Thus, methods for reducing the number of guided modes in optical fibers in which a liquid fills the core have been object of research by those skilled in the art.

The specialized literature discloses an alternative to reduce the number of guided modes in microstructured optical fibers in which a liquid fills the core. Said alternative consists of a fiber with cladding provided with a certain air content, such that its effective refraction index has a specific value, allowing the parameter V to be inferior to 2.405 [J. M. Fini, "Microstructure fibres for optical sensing in gases and liquids," Meas. Sci. Technol. 15, 1120-1 128 (2004)]. However, the possibility proposed in the literature requires the use of optical fibers provided with a specific geometry for each kind of application, for each wavelength to be used, and for each refraction index of the liquid to be used for filling the interior of the core. The use of a specific geometry limits the use of the commercially-available hollow-core optical fiber, since said fibers have a high air content in the cladding microstructure, which is necessary in order that said fibers guide by means of fotonic band gap effect.

Yet part of the literature, optical fibers for the transmission of a radiation beam are described in patent documents, such as patent US2005111804. Said document US2005111804 proposes the total or partial filling of all or some of the holes of a micro structured fiber such as to promote a change of the refraction index in the straight section of the fiber. Additionally, document US2005111804 discloses a project of optical sensors with monomode and multimode operation. The technique to be used for selectively filling said holes described in said document makes the use of masks located on the ends of the optical fiber and the use of UV-curable polymers. However, there is no mention in document US2005111804 as to the simultaneous filling of the core and the cladding with two different liquids. Another factor which was not mentioned throughout said document relates to the use of the different fiber ends for inserting liquids with distinct refraction indexes. The patent literature is well-provided with methods for inserting liquids into microstructured fibers, one of which is described in the document WO2004090510, comprising the insertion of liquids into microstructured fibers, principally through capillarity. Document WO2004001465 discloses the insertion of a liquid, which may be water-based, petrochemical- based or even a liquefied gas, into the core of a microstructured fiber in order that the radiation propagates through the core by means of the total internal reflection. The cladding holes are preferably filled with air, however, different materials such as gas, or vacuum, may also be used. The filling of the holes allows monomode propagation due to the flexibility in choosing the core diameter and the fiber design before being manufactured. Said document WO2004001465 does not make mention, in its preferred embodiment, to the filling of the cladding holes with other liquid for that purpose.

In the document US2002122644, a microstructured fiber with monomode operation is described. Said operation is achieved by means of an array of holes around the core, which is filled with a material having a lower refraction index than that of the core. In said document, there is no mention as to a liquid medium, as well as to the technique to be used for inserting the liquids into the respective holes.

The techniques shown hitherto in the literature require specific microstructured fibers in order to operate in monomode. Such conditions change according to the wavelength and the refraction index of the liquid to be used in the core. This means that it is necessary to manufacture a fiber with specific characteristics for each configuration.

Thus, the literature has not described yet a method which allows the use of virtually any hollow-core microstructured fiber and a high air content in the cladding as a basis, such as to make possible the use of commercially-available hollow-core microstructured fibers. As described herein, this is made possible by filling the cladding and core holes with liquids having different refraction indexes, in order to allow a reduction of the number of guided modes in the core, thereby enabling the application of a liquid-core fiber in any application requiring the use of a monomode fiber.

SUMMARY QF THE INVENTION

The present invention relates to the production of a waveguide, simply called an optical fiber, made from a microstructured optical fiber provided with a hollow core and a cladding having a high air content, used as a basis for the production of liquid-core and liquid-cladding fibers using, as a basis, commercially-available hollow-core microstructured fibers. A particular characteristic of the waveguide of the present invention lies in that it is possible to insert a liquid having a certain refraction index into the fiber core and another liquid, or other type of material, having a different refraction index into the cladding holes. A second object of the present invention relates to a method for simultaneously filling the cladding and core holes of said waveguide obtained herein with previously-selected materials, wherein said method can be applied to any hollow-core microstructured fiber provided with a core diameter which is higher than the diameter of the cladding holes. Further, a third object of the present invention relates to a method for reducing the number of guided modes in any liquid-core fiber based on a microstructured optical fiber, including the already commercially- available hollow-core fibers.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows an electron microscope image of the straight section of a hollow-core microstructured fiber.

Figure 2A shows a schematic drawing of the first end of the fiber, in which the cladding holes are properly blocked. Figure 2B shows a schematic drawing of the second end of the fiber, in which the core entrance is properly blocked.

Figure 3 shows a schematic drawing of the process for simultaneously inserting two liquids having different refraction indexes: a fist liquid into the interior of the core and a second liquid into the cladding holes.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a waveguide, or optical fiber, having a liquid cladding and core, made from a hollow-core microstructured optical fiber. Said waveguide is provided with a hollow core and a cladding with a high air content and is suitably surrounded by a further cladding of silica, or any other vitreous or polymeric material, which provides a mechanical strength to the whole structure. A particular characteristic of the waveguide of the present invention lies in that a first liquid having a certain refraction index fills its core and a second liquid, having a different refraction index, fills the cladding holes. However, the present invention is not limited to the use of a second liquid for filling the cladding holes, thus said holes may be filled with any different type of material, such as sol-gel or even a polymer.

For ease of understanding, the waveguide of the present invention will be called an "optical fiber" throughout this specification.

An optical fiber provided with a core in which its interior is filled by a liquid allows the realization of sensing, spectroscopy and non-linear optics experiments in liquids, with all of the advantages inherent to optical fibers, such as: a long interaction length, possibility of remote measurements, low weight and cost etc, as well as the fact that only a small volume of liquid is used, generally in the order of nano liters. As an example of this fact, an optical fiber having 1 meter in length and a hole with a radius of 5 microns requires just 78 nanoliters in order to be filled.

In the optical fiber of the present invention the light is guided through the liquid core such as to allow a full superposition/interaction between the liquid and radiation.

In order to better understand the present invention and its technical characteristics, it will be described herein as a preferred embodiment, which consists of an optical fiber with a core and cladding holes provided with liquids having different refraction indexes, without being limited to this embodiment.

The optical fiber developed herein, with a core and cladding holes provided with liquids of different refraction indexes, allows the control of the refraction index difference between the core and the cladding, and thereby allows the fiber numeric aperture and the number of guided modes to be controlled, including the reduction of the number of modes. The optical fiber developed herein has the cladding holes of a first end blocked, in such a way that the cladding holes are not filled by the liquid that is to fill the core and that must be completely unobstructed.

In the second end of the optical fiber, the central hole, i.e., the core, must be blocked, such that it is not filled by the liquid that is to fill the cladding holes, which must be unobstructed for free filling.

The blocking of the optical fiber ends is preferably carried out by an optical fiber fusion splicer and by a polymer to be solidified by means of ultraviolet radiation.

However, the present embodiment is not limited to these two techniques for blocking the optical fiber ends, and other techniques for blocking the first end may also be applied to said fiber, as well as the alternative use of local temperature increase to solidify the polymer. A second object of the present embodiment consists of a method for simultaneously inserting a liquid in the interior of the core and another liquid in the cladding holes of the microstructured fiber used as a basis. The material that will fill the core and the cladding holes, herein preferably liquid, must be selected previously.

Said method may be applied to any hollow-core microstructured fiber provided with a core diameter which is higher than the diameter of the cladding holes. The methodology used in said method herein object of the present embodiment allows the control of the refraction index difference between the liquid which fills the core and the liquid which fills the cladding holes, and hence, the control of the parameter V of the optical fiber to be performed. Further, a third object of the present invention relates to a method for reducing the number of guided modes in any liquid-core fiber based on a microstructured optical fiber, best known as photonic crystal fibers, including the already commercially-available hollow-core fibers. The reduction of the number of modes allows a monomode fiber to be obtained without requiring the production of a fiber specifically designed and manufactured for each different application.

Said method is based on the control of the refraction index of the fiber cladding, such as to allow a reduction of the parameter V of the fiber. The control of the refraction index is made performed by inserting a liquid into the fiber cladding holes, whose refraction index may be selected according to the specific application. Specific values for the refraction index, comprising a suitable range, can be easily obtained by means of a mixture of liquids having different refraction indexes.

The reduction of the index difference allows the light to propagate in only one mode through the interior of the optical fiber core, allowing the use of said fiber in cases where the information to be measured or characterized is encoded in the phase of the guided light.

As can be inferred from Figures 1 to 3, where like references numbers designate like correspondent parts of the object claimed herein, as well as said methods which comprise the present invention, as follows:

The obtainment of the optical fiber (OF) of the present invention consists in blocking the ends of said fiber, such that the liquid selected for filling the core (C) does not occasionally fill the cladding holes (O), as well as the liquid selected for filling the cladding holes (O) does not occasionally fill the core (C) of the optical fiber (OF).

Thus, a first end (A) of the optical fiber (OF) was subjected to an optical fiber fusion splicer to block the cladding holes (O) in said end (A). An electric arc applied onto said end (A) of the optical fiber (OF) blocked the cladding holes (C), so that said cladding holes (O) of the fiber were not filled with the liquid that is to be applied to the fiber to fill the core (C) by the end (A).

Next, a second end (B) of the optical fiber (OF) is inserted into a container, which contains a special polymer in the liquid form. The selected polymer to be used in the present embodiment must be provided with low viscosity and become solid by means of UV-curing or local temperature rise. Preferably, said polymer must exhibit a good adhesion to glass. For the present embodiment, the optical adhesive NOA73 was preferably used as a polymeric material.

Said polymer is introduced into the center of the optical fiber (OF), i.e., into the core (C) by means of the suction technique, which is applied to the first end (A) such as to promote the blocking of the core (C) in the end (B).

The blocking of the core (C) at the end (B) occurs due to the suction effect which will act only on the central hole, the core (C), located on the second end (B), since the cladding holes (O) of the first end (A) are properly blocked by the electric arc applied by the fusion splicer. The suction is applied to the end (A) until the filling of a small area of the core (C) has occurred, after which it is stopped. The area of the core (C) to be filled with the solidified polymer by means of suction is of about 5-10 mm. The second end (B) of the optical fiber (OF) which was immersed into the polymer in liquid phase to block the central hole, which provides access to the core (C), is subjected to UV-radiation illumination such as to promote the solidification of the polymer and, consequently, to obstruct the core (C) at the end (B). From Figure 1, it is possible to see the cladding holes (O) and the central core entrance hole (C), since said Figure is an electron microscopy photograph showing of one of the ends of an optical fiber (OF), which was used to better illustrate the technical subject of the present invention. Following the previously-described steps, an optical fiber (OF) ready to be tilled with liquids having different refraction indexes is obtained, such that each of the liquids is introduced by means of each of the ends (A and B), simultaneously. The filling of the core (C) and the cladding holes (O) of the optical fiber (OF) occurs by means of the simultaneous filling method subject of the present embodiment herein. Said method comprises the steps of:

- pressurizing a first liquid into the interior of the core (C) of the optical fiber (OF); and

- pressurizing a second liquid into the interior of the cladding holes (OC) of the optical fiber (OF).

A first liquid having a higher refraction index

(Ll) is pressurized into the interior of the optical fiber (OF) by the first end (A) such that said liquid having a higher refraction index (Ll) fills only the central hole of said fiber, i.e., the core (C), as shown in Figure 2a.

Simultaneously, a second liquid having a lower refraction index (L2) is pressurized into the interior of the cladding holes (O) of the optical fiber (OF) by the second end (B). Thus, only the cladding holes (O) will be filled with the liquid having a lower refraction index (L2), as shown in Figure 2b.

Due to the positive pressure applied externally to both ends (A and B) of the fiber, both liquids will displace in the interior of the cladding holes (O) and in the interior of the core

(C) of the optical fiber (OF). Assuming an optimum scenario, without the occurrence of any unforeseen accident, the displacement of liquids will occur until the internal pressure of the optical fiber (OF) equals the external pressure.

The internal pressure of the optical fiber (OF) results from the air which is trapped in the interior of the optical fiber (OF), as shown in Figure 3.

A typical pressure value applied to the external side of the optical fiber (OF) is on the order of 5 ami. However, the pressure value is not limited to 5 arm and higher pressure values can be applied.

From Figure 3, it can be seen the area (x) not filled by the liquid having a higher refraction index (Ll) in the central hole, the core, and the area (x') not filled by the liquid having a lower refraction index (L2) of the optical fiber (OF) cladding.

The method for reducing modes based on the control of the refraction index by means of the filling of the cladding holes (O), as well as the filling of the core (C) of the optical fiber (OF) obtained herein with their respective liquids is carried out as follows:

After filling the cladding (O) and core (C) holes of the optical fiber (OF), cross-sections (T) transversal to the optical fiber (OF) are made in order to remove fiber portions which are not filled in the area (x) of the core and in the area (x') of the cladding holes, as shown in Figure 3.

The length of non-filled areas, or even the displacement of liquids, may be estimated from the following equation: x=x'=L/P wherein: L is the value of the optical fiber (OF) length; e

P is the value of external pressure, in atmospheres (atm), applied to both ends (A and B) of the optical fiber (OF).

Thus, for an optical fiber (OF) having about 30 cm in length and subjected to an external pressure of about 5 atm in each of its ends, the displacements of the liquid having a higher refraction index (Ll) in the core and the liquid having a lower refraction index (L2) are of approximately 24cm; the portions of the core and cladding which will not be filled with their respective liquids will be of approximately 6cm. The central portion of the optical fiber (OF), of about 18cm (=30cm-6cm-6cm), will be completely filled, including the cladding and core holes. Accordingly, the embodiments described above are intended to better explain the modes known for practicing the invention and to allow those skilled in the art to use the invention in such embodiments, or others, with any modifications required by the specific applications or purposes of the present invention. It is intended that the present invention cover all modifications and variations thereof which are included within the scope described in the specification and the appended claims.

Claims

1. An optical fiber having a liquid core and cladding using, as a basis, a microstructured fiber provided with a hollow core and a cladding having a high air content suitably surrounded by a further cladding of silica, or other vitreous or polymeric material, in order to give a mechanical strength to the whole structure characterized in that a first liquid having a certain refraction index fills the core (C) and a second liquid having a different refraction index fills the cladding holes (O).

2. Microstructured optical fiber according to claim 1, characterized in that the cladding holes (O) are alternatively filled with any other type of material, selected from sol-gel and a polymer.

3. Microstructured optical fiber according to claim 1 , characterized in that the light is guided through the core (C) filled in its interior with a liquid having a certain refraction index to allow a total superposition/interaction between the liquid and the radiation.

4. Microstructured optical fiber according to claim 1, characterized in that the cladding holes (O) of a first end (A) of the fiber are firstly blocked by means of an electric arc generated by the fiber fusion splicer and the central hole of the second end (B) of the fiber is blocked by means of a polymer introduced into the core (C) by means of suction applied to the first end (A) and solidified by means of ultraviolet radiation.

5. Microstructured optical fiber according to claim 4, characterized in that the selected polymer is alternatively solidified by means of a local temperature rise.

6. Microstructured optical fiber according to claim 4, characterized in that other processes for blocking the cladding holes (O) may be utilized.

7. Microstructured optical fiber according to claims 1 and 4, characterized in that it is ready for being filled with liquids having different refraction indexes.

8. Method for simultaneously filling the core and the cladding holes of an optical fiber as claimed in 1 to 7, characterized by comprising the steps of: - pressurizing a first liquid having a higher refraction index (Ll) to fill the hollow core (C); and

- pressurizing a second liquid having a lower refraction index (L2) to fill the cladding holes (O).

9. Method for filling according to claim 8, characterized in that the displacement of liquids occurs until the internal pressure of the optical fiber (OF) equals the external pressure.

10. Method for filling according to claim 9, characterized in that a typical pressure value applied onto the external side of the optical fiber (OF) is preferably on the order of 5atm.

11. Method for reducing the number of guided modes in any optical fiber provided with a liquid core based on a microstructured optical fiber characterized by being based on the control of the refraction index of the fiber cladding in order to allow a reduction on the parameter V of the fiber.

12. Method for reducing the number of guided modes, according to claim 11, characterized in that the control of the refraction index is made by inserting a liquid into the cladding holes (O) of the fiber, the refraction index of which may be selected according to the specific application.

13. Method for reducing the number of guided modes according to claim 11 , characterized in that the reduction of the number of modes allows the light to propagate in only one mode through the interior of the core (C) of the waveguide.

14. Method for reducing according to claim 11, characterized in that after filling the cladding (O) and core (C) holes of the optical fiber (OF), cross-sections (T) transversal to the optical fiber (OF) are made in order to remove the fiber portions which are not filled in the area (X) of the core and in the area (x') of the cladding holes, and in that the liquid displacement is estimated from equation x=x'=L/P.