Method of forming thin film on fiber end surface

Abstract: A method of forming a thin film on an end surface of a fiber which comprises a core of an organic crystal, and a clad made of glass, wherein said method comprises: (a) cutting the fiber under reduced pressure; and (b) forming a thin film on the cut surface of the fiber simultaneously with the cutting, or immediately after the cutting.

Description: The present invention relates to a method of forming a thin film on an end surface of a fiber, such as an optical fiber or a fiber wavelength-conversion element, wherein the fiber consists of a core of an organic crystal and a cladding made of glass. Conventionally, optical waveguides or wavelength-conversion elements, are made up of e.g. a fiber, such as an optical fiber, or a fiber wavelength-conversion element. These fibers are generally made from a core of an organic crystal and a cladding made of glass. However, conventional fibers which have a core made of an organic crystal, have some problems. These arise because, depending on the kind of the organic crystal, the organic crystal core is sublimed from the end surface of the fiber. This reduces the coupling efficiency of laser light with the fiber. In addition, when the molecules which make up the organic crystal core contain hydrogen bonds, the fibers display a poor time-aging stability. This occurs, for example, when the exposed core at the fiber end surfaces contacts water in the atmosphere. The water enters the core and thereby reduces the light transmission properties of the fiber. As one method of solving the foregoing problems, US-A- 4,830,447 discloses a method wherein a fluorine containing polymer is dissolved in a solvent and the resulting solution is applied onto a fiber end surface using a spin coater. This forms a thin film on the fiber end surface. However, there has been a problem with this method because the organic crystal core of the fiber is dissolved in the solvent during the spin coating. This results in a reduction in the coupling efficiency of laser light with a fiber treated in this manner. When a thin film is formed on a fiber end surface using a dry process, such as a vacuum evaporation method, a different problem occurs, namely sublimation of the organic crystal core. This problem occurs because the time taken to evacuate an apparatus is so long that the organic crystal core is sublimed before a thin film can be formed on the end surface. This can occur even when the organic crystal does not sublime at ordinary temperature and ordinary pressure. The object of the present invention is therefore to provide a method of forming a thin film on a fiber end surface, wherein the thin film can be formed without dissolving, subliming, or otherwise changing the quality of an organic crystal core, and thus to allow the production of a fiber which is superior in time-aging stability. According to the present invention this object is attained with a method of forming a thin film(2) on an end surface(13) of a fibre(1) comprising a core(11) of a material selected from an organic crystal, silicone resin, styrene resin and acryl resin and a cladding(12) made of glass, characterized in that (a) the fibre(1) is cut in a vacuum chamber(3) under reduced pressure; and (b) a thin film(2) is formed on the cut surface(13) of said fibre(1) simultaneously with or not later than 20 seconds after the cutting by a vapour phase film forming method in said vacuum chamber(3). FIGURE 1 is a simplified sectional view showing an embodiment of the method of the present invention. FIGURE 2 is a simplified sectional view showing another embodiment of the present method. FIGURE 3 is a graph which shows the time-aging of the amount of light transmitted from a HeNe laser in the fiber wavelength-conversion elements obtained from Examples 1, 3 and 6 and Comparative Example 1 respectively. FIGURE 4 is a graph which shows the time-aging of the amount of light transmitted from a HeNe laser in the fiber wavelength-conversion element obtained in Comparative Example 2. A thin film is formed on a cut surface of a fiber under reduced pressure. As a result, there is no possibility that the organic crystal constituting the core of the fiber will be dissolved or changed in quality. In addition, cutting the fiber under reduced pressure results in the thin film being formed on the cut surface of the fiber simultaneously with or not later than 20 s after the cutting. Further, in fibers where a thin film is formed on the end surface using the present method, the core of organic crystal is prevented from contact with the atmosphere by the thin film. The method according to the present invention will be described more specifically hereafter. FIGURE 1 is a simplified sectional drawing which shows a process of forming a thin film 2 on a cut surface 13 of a fiber 1. In this case, an incision is formed at a predetermined position so as to facilitate cutting at this position of the fiber 1. The fiber 1 has a core 11 made of organic crystal and a cladding 12 made of glass. Fiber 1 is disposed in a vacuum chamber 3 in a vertical orientation so that the end portion 14 which is to be removed when fiber 1 is cut is located on the bottom end of the fiber. The other end portion 15, on which the thin film 2 is to be formed, is fixed by a fixing means 32. Next, the lower end portion 14 of fiber 1 is attached to a rod-like member 33 which is rotatably supported in the vacuum chamber 3. The pressure in the vacuum chamber 3 is reduced by a vacuum pump 34 and a thin film material 20 is scattered from an evaporation source 35. As shown in FIGURE 1, the rod-like member 33 is rotated in the direction of an arrow by a rotating member 36 provided in the vacuum chamber 3. This causes the fiber 1 to be bent at the incision and results in the fiber 1 being cut. Since the thin film material 20 is evaporated in the vacuum chamber 3, the thin film material 20 adheres onto the cut surface 13 of the fiber 1 simultaneously with the cutting of the fiber. This forms the thin film 2. When the core 11 is made of a crystal of an organic material which is susceptible to sublimation, a fiber 1 having an incision formed at a predetermined position is disposed obliquely in a vacuum chamber 3 so that the end portion 14 to be removed when the fiber is cut is at the bottom end of the fiber. The other end portion 15 is fixed by a fixing means 32, as shown in FIGURE 2. The pressure in the vacuum chamber 3 is reduced by a vacuum pump 34. A movable member 37 which is vertically movable is provided in the vacuum chamber 3 and is moved in the direction of the arrow so as to make the end portion 14 rotate. This causes the fiber 1 to be bent so as to be cut at a predetermined position 10 where the incision was formed. During this process, a thin film material 20 is scattered from an evaporation source 35 so as to adhere onto the cut surface 13 of the fiber 1 to thereby form a thin film 2. This operation is preferred as it prevents sublimation of the organic crystal core 11. Sublimation is avoided because the thin film 2 can be more rapidly formed when the fiber 1 is disposed obliquely. Thus, there is no shade portion for the scattering of the thin film material 20 onto the cut surface 13 of the fiber 1 in order to form thin film 2. In addition, fiber 1 can be cooled during the process, so that sublimation of the organic crystal core 11 can be more effectively prevented. When the organic crystal core material has a low sublimation rate, the film forming operation may be performed in the following manner. The fiber is cut and then the film is formed on the cut end. This application of the thin film takes place immediately after the cutting of the fiber. When this process is used, formation of the thin film on the cut surface 20 is started no later than 20 s after the fiber is cut. Suitable methods for applying the thin film material 20 to the cut surface 13 of the fiber 1 to form the thin film 2 includes vapor-phase thin film forming methods. These methods include e.g. a sputtering method, a plasma CVD method, a cluster ion beam method (ICB method) and an electron cyclotron resonance method (ECR method). A preferred method is one which uses ions or plasma to form the thin film 2. This forms a film which adheres well to the cut surface 13. These preferred methods include the sputtering method, the CVD method, the ICB method, and the ECR method. Further, since the highest melting point of the organic crystal core is about 300°C, it is more preferred to use the ICB method or the ECR method. In these methods, a thin film can be formed approximately at room temperature and the resulting thin film adheres well to the cut end. The material for forming the thin film 2 is required to be transparent in the visible and near-infrared radiation regions, and it is preferred that this material is dielectric. Suitable examples of the thin film forming material include inorganic materials including SiO₂, MgF₂, Al₂O₃, Si₄N₃, LiNbO₃, ZnO and organic materials including polyethylene, polymethyl methacrylate, polyacrylic acid. A preferred embodiment is one which uses an inorganic material which is low in permeability for a core material. It is possible to make the thin film 2 a reflection reduction film. This is achieved by accurate control of the thickness of the thin film 2. The term "reflection reduction film" means that the film satisfies the following equation: nd = ( 1 4 + m 2 ) λ (in the equation, m represents an integer not smaller than 0; n represents the refractive index of the film; d represents the film thickness; and λ represents a wavelength). If the thin film 2 satisfies the following equation, then the refractive index step at the thin film 2 is zero. n = ns (In this equation, n represents the refractive index of the film and ns represents the refractive index of the single crystal of the organic nonlinear optical material which is the core). Further, a second, high-refractive index layer, for example, made of MgF₂ may be formed on the thin film 2 as a reflection reduction film. Various kinds of glass may be used to form the clad 12. Examples of suitable glass include, "PYREX®" glass (registered trademark of CORNING Inc.) and lead glass. A variety of organic materials may be used to form the core 11. These materials include macro-molecular materials such as silicone resin, styrene resin, acryl resin, and crystals such as anthracene, naphthalene, p-nitroaniline. The method of the present invention is particularly effective in forming fiber wavelength-conversion elements where the core is made of an organic nonlinear optical crystal exhibiting a second order nonlinear optical effect and the cladding is made of glass, where the second harmonic of incident light is generated by Cerenkov radiation. Cerenkov radiation is generated when the refractive indexes of the cladding and core satisfy the following condition: ncladding(2ω) > ncore(ω) > ncladding(ω) (ncladding(2ω) represents the refractive index of the cladding in the second harmonic waves, ncore(ω) represents the effective refractive index of the core in the fundamental wave, and ncladding(ω) represents the refractive index of the cladding in the fundamental wave). The organic nonlinear optical crystal material is not particularly limited, and the method according to the present invention is effective with a variety of materials. Examples of suitable nonlinear optical crystal materials include: 3-nitro-5-(N,N-dimethylamino)-acetanilide; 3-(N,N-dimethylamino)-aniline; N-(4′-methoxybenzoyl)-4-cyanoaniline; N-methyl-N-(4-cyanophenyl) aminoacetonitrile: N-(4-cyanophenyl) aminoacetonitrile; 4-nitrobenziriden-2,3-dimethylaniline; 4-nitrobenziriden-2,4-dimethylaniline; 4-nitrobenziriden-2,5-dimethylaniline; 4-nitrobenziriden-4,4-dimethylaniline; 4-nitro-benziriden-3,5-dimethylaniline; 4-nitrobenziriden-2,4-dimethoxyaniline; 4-nitrobenziriden-3,4,5-trimethoxyaniline; 2-nitrobenziriden-3, 4,5-trimethoxyaniline; 3-nitrobenziriden-2,3-dimethylaniline; 3-nitrobenziriden-2,5-dimethylaniline; 3-nitrobenziriden-3,5-dimethylaniline; 2-methyl-4-nitroaniline (MNA); 4-(N,N-dimethylamino)-3-acetamidenitrobenzene (DAN); 4,5-dimethyl-1, 3-dithiol-2-iridencyanoacetate; 1,3-dithiol-2-iridencyanoacetate; N-(4-nitrophenyl)-(S)-prolinol (NPP); N-(5-nitro-2-pyridyl)-(S)-prolinol (NPP); N-(5-nitro-2-pyridyl)-(S)-phenylalalinole (NPPA); 9-methylcarbazole-3-carboxaldehyde. Further, the method according to the present invention can be effectively applied to fibers where the core is made of an organic nonlinear optical crystal which exhibits a third order nonlinear optical effect. Suitable core materials of this type include organic coloring materials such as p-nitroaniline; 4-(dimethylamino)-4′-nitrostilbene; 4′-(N,N′-dimethylamino) benziriden-4-nitroaniline. When the present invention is used, the organic crystal core of the element is not dissolved or changed in quality because a thin film is formed on a cut surface of a fiber under reduced pressure. Further, the thin film can be formed on the fiber end surface without dissolving, quality-changing, or sublimation of the organic crystal core because the fiber is cut in the pressure-reduced state and either at the same time or not later than 20 s after the cutting, the thin film is formed on the cut surface of the fiber. In addition, by forming the thin film on the fiber end surface, the organic crystal core can be prevented from contacting the atmosphere. This allows the production of a fiber which has superior time-aging stability. The following examples illustrate the operation of the present invention. EXAMPLE 1 A capillary tube of lead glass (SF4) having an inner diameter of 2 µm and an outer diameter of 500 µm was inserted into an ampule tube together with 4-(N,N-dimethyl)-amino-3-acetamidenitrobenzene (DAN), and vacuum-sealed. This tube was heated to 170°C in a cylindrical heating furnace so that the DAN was fused. Then, the tube was drawn out of the heating region of the heating furnace at a rate of 3 - 5 mm/h so that a single crystal of DAN was grown in the capillary tube. This produced a fiber wavelength-conversion element. Next, SiO₂ was used as the thin film material and was placed in an evaporation source in a vacuum evaporator having a vacuum chamber as shown in FIGURE 2. The fiber wavelength-conversion element having an incision at a predetermined position was disposed as shown in FIGURE 2. The pressure in the vacuum chamber was reduced to 1.3x10⁻⁴ Pa (1x10⁻⁶ torr), and the fiber wavelength-conversion element was cut as shown in FIGURE 2 while SiO₂ was evaporated by electron heating. This formed a thin film of SiO₂ with a thickness of 300 nm (3000 Å) on the cut surface at a rate of 10 nm/s (100 Å/s). When the cut surface was observed through an optical microscope, it was confirmed that the end surface of the DAN core remained specular. Further, the time aging of the amount of transmitted light of a HeNe laser was examined with respect to the fiber wavelength-conversion element having the thin film at its fiber end surface. FIGURE 3 shows the results. The amount of light transmission loss was much less in the fiber on which the thin film was formed. In a fiber wavelength-conversion element having a core made of DAN which is not coated, the core contacts the atmosphere, such as air, at the fiber end surface and water enters the core which increases the amount of transmitted light which is lost. EXAMPLE 2 A fiber wavelength-conversion element was formed in the same manner as in Example 1 except that the formation of the thin film was started 10 s after the cutting of the fiber. When the cut surface of the fiber was observed through an optical microscope, it was confirmed that the end surface of DAN constituting the core remained specular. Further, when time aging of the amount of transmitted light of an HeNe laser was examined, the amount of transmitted light which was lost was much less than the untreated case. EXAMPLE 3 MgF₂ was used as the thin film material, and was placed in a heating crucible in an ICB apparatus. The fiber wavelength-conversion element used in Example 1 was placed in the vacuum chamber of the ICB apparatus in the same manner as in Example 1. The MgF₂ was heated to 1050°C, and an ion cluster was generated under conditions where the ionizing current was 29.5 A and the acceleration voltage was 2 kV. The fiber was cut after the shutter was opened, and a thin film of MgF₂ having a thickness of 190 nm (1900 Å), where the refractive index of the film was minimized at a wavelength of 1.06 µm, was formed at a rate of 1.5 nm/s (15 Å/s). The amount of transmitted light of the fiber wavelength-conversion element having the thin film formed thereon was larger, by 1%, than that of a fiber wavelength-conversion element which had no thin film. Further, the MgF₂ thin film was stable for three months or more and did not separate from the core, although a thin MgF₂ film which was formed through vacuum evaporation was separated after one week. When the amount of transmitted light of a HeNe laser was examined with respect to the fiber wavelength-conversion element, the value did not change over 500 h or more, as shown in FIGURE 3. The same effects were also obtained using SiO₂ in place of MgF₂. EXAMPLE 4 The fiber wavelength-conversion element used in Example 1 was disposed in the vacuum chamber of an ECR apparatus in the same manner as in Example 1. The pressure in the vacuum chamber was reduced to 1.3x10⁻⁴ Pa (1x10⁻⁶ torr), and a thin film made of SiN₃ and having a thickness of 98 nm (980 Å) was formed at a rate of 0.3 nm/s (3 Å/s) by using an input wave of an output of 300 W, while introducing N₂ gas and SiH₄ gas into the chamber. The thin film was stable for three months or more, and did not separate. EXAMPLE 5 A fiber wavelength-conversion element having a core diameter of 5 µm and a cladding outer diameter of 300 µm was formed using 2-chloro-5-nitrobenzaldehye (CNA) as the core and using PYREX® glass (registered trademark of CORNING Inc.) as the cladding in the same manner as in Example 1. The fiber was cut and a thin film of SiO₂ was formed on the cut end surface in the same manner as in Example 1, except that the thin film having a thickness of 18 nm (180 Å) was formed while cooling the fiber wavelength-conversion element to -50°C. When the cut end surface was observed through an optical microscope, it was confirmed that the end surface of the CNA core remained specular. EXAMPLE 6 A thin film of MgF₂ was formed on a cut surface of a fiber wavelength-conversion element having a core diameter of 5 µm and a cladding outer diameter of 300 µm in which the core is made of 3,5-dimethyl-1-(4-nitrophenyl)-pyrazole (PRA) and the cladding is made of the above-mentioned PYREX® glass in the same manner as in Example 2 except that the fiber was cooled to -20°C. When the cut surface was observed through an optical microscope, it was confirmed that the end surface of the PRA core remained specular. When the amount of transmitted light of an HeNe laser was examined with respect to the fiber wavelength-conversion element, the value did not change for 500 h or more as shown in FIGURE 3. EXAMPLE 7 The P fiber wavelength-conversion element used in Example 6 was placed in a commercially available plasma polymerization apparatus in which the substrate temperature was kept at 0°C, and the pressure in the chamber was reduced to 0.13 Pa (10⁻³ torr). Then, in the chamber, a hexatrifluorobenzene monomer which is used as the thin film material and N₂ were added at pressures 33.2 Pa (0.25 torr) and 13.3 Pa (0.1 torr) respectively. A plasma was generated using a discharge frequency of 13.56 MHz, a discharge output of 25 W, and an electrode distance of 30 mm. After stabilization of the plasma, the fiber was cut in the chamber and a thin film having a thickness of 0.5 µm was formed on the cut surface. When the cut surface was observed through an optical microscope, it was confirmed that the end surface the PRA core remained specular. Further, when the amount of transmitted light of a HeNe laser was examined with respect to the fiber wavelength-conversion element, the value did not change for 500 h or more. EXAMPLE 8 A thin film was formed on a cut surface of a fiber wavelength-conversion element in the same manner as in Example 7 except that tetramethylsilane was used in place of hexafluorobenzene and the 25 pressure of the tetramethylsilane in the chamber was 26.6 Pa (0.2 torr), O₂ was used in place of N₂ at a pressure of 53.2 Pa (0.4 torr), and the discharge output was 50 W. When the cut surface was observed through an optical microscope, it was confirmed that the end surface of the PRA core remained specular. The film formed by this method was harder than that formed in Example 7, and was similar in physical characteristics to an inorganic material film. When the amount of transmitted light of a HeNe laser was examined with respect to the fiber wavelength-conversion element, the value did not change for 1000 h or more. COMPARATIVE EXAMPLE 1 A fiber wavelength-conversion element was obtained in the same manner as in Example 1 except that no thin film was formed on the end surface. When the end surface of the fiber wavelength-conversion element was observed through an optical microscope, sublimation of the crystal constituting the core was not observed. The amount of transmitted light of a HeNe laser was examined with respect to the fiber wavelength-conversion element. The amount of transmitted light decreased over time as shown in FIGURE 3. It was observed that the rate of reduction of the quantity of transmission light increased as the humidity rose. Further, when the fiber wavelength-conversion element was cut, and then inserted into a vacuum chamber, and a thin film was formed thereon in the same manner as in Example 1, it was found that the crystal core was slightly sublimed and the amount of transmitted light of a HeNe laser was reduced to about 85% as much as that before the formation of the thin film. COMPARATIVE EXAMPLE 2 A fiber wavelength-conversion element was obtained in the same manner as in Example 5 except that no thin film was formed on the end surface. When the end surface of the fiber wavelength-conversion element was observed through an optical microscope, it was observed that the crystal constituting the core was sublimed to a depth of a few µm. Further, when the amount of transmitted light of a HeNe laser was examined with respect to the fiber wavelength-conversion element, the amount of transmitted light was reduced over elapsed time and the value became half as much as that in the beginning, as shown in FIGURE 4. The fiber was cut and then placed in a vacuum chamber of 0.13 Pa (1x10⁻³ torr) for 10 min. On examination after this treatment, the crystal core had sublimed by about 500 µm. From Examples 1 through 8 and Comparative Examples 1 and 2, it was observed that the method of the present invention produces a thin film on a fiber end surface with no sublimation of the organic crystal core. Further, as seen from FIGURES 3 and 4, reduction of the amount of transmitted light over time can be prevented or remarkably improved by formation of a thin film on a fiber end surface using the present method. Further, from Example 8, it was found that when a thin film is formed on an end surface of a fiber using a silane compound or a silicone monomer, a harder film, similar to an inorganic material was obtained. The reduction of the amount of transmitted light over time was particularly improved.

Claims: A method of forming a thin film(2) on an end surface(13) of a fibre(1) comprising a core(11) of a material selected from an organic crystal, silicone resin, styrene resin and acryl resin and a cladding (12) made of glass, characterized in that (a) the fibre(1) is cut in a vacuum chamber (3) under reduced pressure; and (b) a thin film(2) is formed on the cut surface(13) of said fibre(1) simultaneously with or not later than 20 seconds after the cutting by a vapour phase film forming method in said vacuum chamber (3). The method according to claim 1, wherein the thin film (2) is formed using a method selected from a vapor deposition method, a sputtering method, a plasma CVD method, a cluster ion beam method, and an electron cyclotron resonance method. The method according to claim 2, wherein the thin film is formed using a method selected from a cluster ion beam method and an electron cyclotron resonance method. The method according to claim 1, wherein the thin film (2) is transparent to visible and near-infrared radiation. The method according to claim 4, wherein the thin film (2) is a dielectric material. The method according to claim 1, wherein the thin film (2) is formed of a film-forming material, selected from organic material and an inorganic material. The method according to claim 6, wherein the thin film (2) is formed of a material selected from SiO₂, MgF₂, Al₂O₃, Si₄N₃, LiNbO₃ and ZnO. The method according to claim 6, wherein the thin film is formed of a material selected from polyethylene, polmethyl methacrylate, polyacrylic acid, silane, silicone monomer and hexatrifluoro benzene monomer. The method according to claim 1, wherein the glass cladding (12) is selected from PYREX glass and lead glass. The method according to claim 1, wherein the organic crystal is selected from organic-nonlinear optical crystal exhibiting a second order non-linear optical effect, and organic non-linear optical crystal exhibiting a third order non-linear optical effect. The method according to claim 10, wherein the organic crystal core (11) is an organic nonlinear optical crystal exhibiting a second order non-linear optical effect selected from 3-nitro-5-(N,N-dimethylamino)-acetanilide; 3-(N,N-dimethylamino)-aniline; N-(4′-methoxybenzoyl)-4-cyanoaniline, N-methyl-N-(4-cyanophenyl)aminoaeetonitrile, N-(4-cyanophenyl) aminoacetonitrile, 4-nitrobenziriden-2,3-dimethylaniline, 4-nitrobenziriden-2,4-dimethylaniline, 4-nitrobenziriden-2,5-dimethylaniline, 4-nitrobenziriden-4,4-dimethylaniline, 4-nitrobenziriden-3,5-dimethylaniline, 4-nitrobenziriden-2,4-dimethoxyaniline, 4-nitrobenziriden-3,4,5-trimethoxyaniline, 2-nitrobenziriden-3,4,5-trimethoxyaniline, 3-nitrobenziriden-2,3-dimethylaniline, 3-nitrobenziriden-2,5-dimethylaniline, 3-nitrobenziriden-3,5-dimethylaniline, 2-methyl-4-nitroaniline (MNA), 4-(N,N-dimethylamino)-3-acetamidenitrobenzene (DAN), 4,5-dimethyl-1,3-dithiol-2-iridencyanoacetate, 1,3-dithiol-2-iridencyanoacetate, N-(4-nitrophenyl)-(S)-prolinol (NPP), N-(5-nitro-2-pyridyl)-(S)-prolinol (NPP), N-(5-nitro-2-pyridyl)-(S)-phenylalalinole (NPPA) and 9-methylcarbazole-3-carboxaldehyde. The method according to claim 10, wherein the organic crystal core (11) is an organic nonlinear optical crystal exhibiting a third order non-linear optical effect selected from p-nitroaniline, 4-(dimethylamino) -4′-nitrostilbene, and 4′-(N,N′-dimethylamino) benziriden-4-nitroaniline.

Inventors: HATTORI YASUHIRO C O OSAKA WOR, MIZOGUCHI AKIRA C O OSAKA WORK, UEMIYA TAKAFUMI C O OSAKA WORK, UENISHI NAOTO C O OSAKA WORKS, HATTORI, YASUHIRO, C/O OSAKA WORKS, MIZOGUCHI, AKIRA, C/O OSAKA WORKS, UEMIYA, TAKAFUMI, C/O OSAKA WORKS, UENISHI, NAOTO, C/O OSAKA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: G02B6
IPC codes: G02B6, G02F1