Optical fibre coupler and a fabrication method for the same

Abstract: In order to provide an optical fiber coupler in which the mode coupling state between the constituent optical fibers (12) is a multiple mode coupling stage, the optical fiber coupler is formed by fusing together adjacent lateral portions of at least two constituent optical fibers (12) which extend alongside one another and then elongating the fused section to form a fused-elongated region (13), an "SZ" twist perturbation being introduced to the fused elongated region (13).

Description: Background of the Invention The present invention concerns couplers for optical fibers. For coupling optical signals and for causing optical signals to branch over one or more optical fibers, one such coupler known in the art is referred to as the fused-elongated type coupler. In Fig. 1, one example of a conventional fused-elongated type optical fiber coupler is shown. The optical fiber coupler 1 shown in the drawing is formed by aligning a section of each of two component optical fibers 2, mutually thermally fusing the aligned sections thereby forming a fused section, and then elongating or drawing out the heated fused section to form a fused-elongated region 3. In the above described fused-elongated region 3, the diameter of each component optical fiber 2 is reduced as is the core of the component optical fibers 2. To the extent that diameters of the cores of the optical fibers are reduced, a proportionately greater fraction of the energy of the light propagated therein leaks through the cladding which surrounds the core. Also, to the extent that the component optical fibers 2 are drawn out and thereby elongated, the distance between the cores of adjacent optical fibers 2 is reduced, and due to this fact, the coupling between the propagated modes of the individual fibers becomes extremely great. In this way, the optical signal incident on one fiber 2 becomes split and is thus emitted at ports of both optical fibers 2. In the same way, separate optical signals incident on both component optical fibers 2 become combined. With the conventional optical fiber coupler 1 described above, it is known that light incident on one optical fiber becomes proportioned over one or both optical fibers (referred to as coupling ratio) based on the wavelength of the incident light. For example, light incident on port A (or port B) of the optical fiber coupler 1 shown in Fig. 1 may be emitted by port C or port D, depending on the wavelength of the incident light. As illustrated by the graph of Fig. 2 which shows the percentage of light incident on port A of optical fiber coupler 1 that is emitted by port D as a function of the wavelength of the light incident on port A, the relationship between coupling ratio and wavelength is a sinusoidal function. Optical fiber couplers do exist in which the fluctuation of coupling ratio as a function of wavelength is relatively flat. These optical fiber couplers having weak wavelength dependance for coupling ratio are known as wide wavelength range optical fiber couplers, an example of which is shown in Fig. 3. The conventional wide wavelength range optical fiber coupler 4 shown in Fig. 3 includes an optical fiber 5, a portion of which has undergone a preliminary elongation, and a conventional optical fiber 2. To form the optical fiber coupler 4, the initially elongated region of optical fiber 5 and a section of optical fiber 2 are aligned side by side and mutually thermally fused to form a fused section. The fused section thus formed is then elongated or drawn out to form a fused-elongated region 6. By virtue of the above mentioned preliminary elongation process, it is possible to create a propagation constant difference between the component optical fibers 2, 5. By so doing, it is possible to increase the mode coupling between the component optical fibers 2, 5 across the fused-elongated region 6, and thereby attain a desired coupling ratio. With such an optical fiber coupler 4, over a fairly wide range of wavelengths, a relatively level coupling ratio can be obtained, as is shown in Fig. 4. As with Fig. 2 for optical fiber coupler 1, Fig. 4 shows for optical fiber coupler 4, the percentage of light incident on port A that is emitted by port D as a function of the wavelength of the light incident on port A. However, as Fig. 4 shows, for the conventional wide wavelength range optical fiber coupler 4, while relatively level compared with that of conventional optical fiber coupler 1, the coupling ratio as a function of wavelength does demonstrate a hump and thus, the conventional wide wavelength range optical fiber coupler 4 does not achieve truly flat, wavelength independent characteristics. The reason for this is thought to be that for the conventional wide wavelength range optical fiber coupler 4, even after elongation and reduction of the respective fibers' diameters, the central axes of each component optical fiber 2, 5 in the fused-elongated region 6 lie in the same plane. Thus, the modes which participate in coupling are thought to be limited to the fundamental modes of each core of each component optical fiber 2, 5. Based on light transmission coupling theory, it is believed that for mode coupling between only two modes, that the mode coupling waveform is such that perfectly flat wavelength characteristics cannot be attained. Summary of the Invention In consideration of the above, it is an object of the present invention to provide a wide wavelength range optical fiber coupler that to the greatest extent possible, allows optical coupling which is wavelength independent, and thus demonstrates a constant coupling ratio over a wide range of wavelengths. As the result of various experiments, it has been found that by giving the fibres an "SZ" twist, various irregularities can be formed in the fused-elongated region of an optical fiber coupler, thereby leading to the generation of additional modes in addition to the fundamental modes, while at the same time causing multiple mode coupling in which the above mentioned additional modes participate in the coupling of the fundamental modes, that it is possible to selectively control the coupling strength at determined wavelengths and thereby achieve optical coupling which is largely wavelength independent and thus demonstrates a relatively constant coupling ratio over a wide range of wavelengths. Accordingly, the present invention provides an optical fibre coupler comprising at least two constituent optical fibres that extend alongside one another and that have been twisted together whereby the coupler is formed by fusing together adjacent lateral portions and then elongating the fused section to form a fused-elongated region, characterised in that the fibres have an "SZ" twist which forms perturbations in the fused-elongated region so that the mode coupling state between said at least two constituent optical fibres in said fused-elongated region is a multiple mode coupling state. Also, the present invention provides a method of manufacturing an optical fibre coupler comprising fusing together adjacent lateral portions of at least two constituent optical fibres that extend alongside one another and that have been twisted together and then elongating the fused section to form a fused-elongated region, characterised in that the fibres have an "SZ" twist which forms perturbations in the fused-elongate region so that the mode coupling state between said at least two constituent optical fibres in said fused-elongate region is a multiple mode coupling state. Brief Explanation of the Drawings Fig. 1 is a side view of an example of a conventional optical fiber coupler. Fig. 2 is a graph illustrating the wavelength dependency properties of the coupling ratio for the conventional optical fiber coupler shown in Fig. 1. Fig. 3 is a side view of an example of a conventional wide bandpass optical fiber coupler. Fig. 4 is a graph illustrating the wavelength dependency properties of the coupling ratio for the conventional wide bandpass optical fiber coupler shown in Fig. 3. Fig. 5 is a side view of the optical fiber coupler of a first preferred embodiment of the present invention. Fig. 6 is an explanatory side view for one example of a manufacturing method for the optical fiber shown in Fig. 5. Detailed Description of the Preferred Embodiment In the following section, an embodiment of the present invention will be described with reference to Fig. 5. The wide bandpass optical fiber coupler 11 (hereafter referred to as optical fiber coupler) of the present embodiment as shown in Fig. 5 consists of a fused-elongated region 13 with a twisting deformation formed from a section of each of two strands of a component single mode optical fiber 12 (hereafter referred to as optical fiber). The twisting deformation of the above mentioned fused-elongated region 13 is preferably formed by first imposing a twisting on a section of each component optical fiber 12 prior to forming the fused-elongated region 13, then mutually thermally fusing the twisted sections, and finally drawing out the twisted, thermally fused section. For the method of twisting the component optical fibers 12, the "SZ" twist shown in Fig. 6 has been found to be effective. With this kind of SZ twist, even though a twist is present, each component optical fiber 12 joins and leaves the fused-elongated region 13 on the same side, thus providing more convenient application. Further, with this kind of twisted fused-elongated region 13, very low optical losses and a coupling ratio which is relatively wavelength independent can be easily achieved. Here, the example of a fabrication method for the optical fiber coupler 11 as shown in Fig. 6 will be explained. First of all, a portion of each of two component optical fibers 12 are aligned side by side, after which the SZ twist shown in the drawing is imparted on the aligned portion of the component optical fibers 12, thereby forming a twisted region 14. While each end of the twisted region 14 is held in a clamp (not shown in the drawing), the twisted region 14 is fused using a hydrogen-oxygen burner 15, after which the above mentioned clamps are moved in opposite directions, thereby stretching and elongating the fused region and forming the fused-elongated region 13. In this way, the optical fiber coupler 11 shown in Fig. 5 can be fabricated. With this kind of optical fiber coupler 11, by imparting a twisting deformation in the fused-elongated region 13, in addition to the fundamental modes of each component optical fiber 12, additional modes are caused to be formed. Through the participation of these additional modes with the coupling of the fundamental modes, the mode coupling state becomes a multiple mode coupling state, whereby the strength of coupling at predetermined wavelengths can be controlled, and by which means it is possible to create an optical fiber coupler 11 of which the coupling ratio is largely wavelength independent and hence constant over a wide range of wavelengths. [First Experimental Example] Using two quartz single mode optical fibers having a core diameter of 9.5 µm, an external cladding diameter of 125 µm, a core-cladding refractive index difference of 0.33 %, and a primary coating layer of 250 µm outer diameter, the primary coating layer was removed from each fiber over a length of 40 mm, thereby exposing the cladding. Next, the exposed sections from the two optical fibers were aligned side by side so that the exposed portions were in contact. An SZ twist was then imposed one time over the section of contacting exposed cores, with both ends of the exposed sections being fixed in a respective clamp. Next, the exposed sections between the two clamps were heated with a hydrogen-oxygen burner, whereby a central section of the exposed cores of 20 mm length was mutually fused, while at the same time, the clamps were drawn apart, thereby forming a fused-elongated region having a length of 15 mm and an external diameter of 70 µm which is the same as the optical fiber coupler 11 shown in Fig. 5. For the optical fiber coupler thus obtained, the wavelength dependency characteristics of the coupling ratio (the coupling ratio from port A to port C) were measured. The optical fiber coupler 11 produced by the above described method demonstrated a variation in the strength of the coupling ratio with wavelength that was barely detectable, and thus possessed exceedingly flat wavelength dependency characteristics. The various examples of the present invention presented in the above preferred embodiments are merely examples and are in no way to be construed as limiting the present invention. It is possible, for example, to employ three or more optical fibers in the optical fiber coupler of the present invention with acceptable results. It should be understood that the optical fiber coupler of the present invention includes all forms encompassed by the appended claims.

Claims: An optical fibre coupler (11) comprising at least two constituent optical fibres (12) that extend alongside one another and that have been twisted together whereby the coupler is formed by fusing together adjacent lateral portions and then elongating the fused section to form a fused-elongated region (13), characterised in that the fibres have an "SZ" twist which forms perturbations in the fused-elongated region (13) so that the mode coupling state between said at least two constituent optical fibres (12) in said fused-elongated region (13) is a multiple mode coupling state. A method of manufacturing an optical fibre coupler comprising fusing together adjacent lateral portions of at least two constituent optical fibres (12) that extend alongside one another and that have been twisted together and then elongating the fused section to form a fused-elongated region (13), characterised in that the fibres have an "SZ" twist which forms perturbations in the fused-elongate region (13) so that the mode coupling state between said at least two constituent optical fibres (12) in said fused-elongate region (13) is a multiple mode coupling state.

Inventors: KAWAKAMI NOBORU, SUZUKI FUMIO, YAMAUCHI RYOZO, KAWAKAMI, NOBORU, SUZUKI, FUMIO, YAMAUCHI, RYOZO
Applicants: FUJIKURA LTD, FUJIKURA LTD.
Application year: 1990
Main classification: G02B6
IPC codes: G02B6