JP6611259B2 - Optical fiber manufacturing method and optical fiber manufacturing apparatus - Google Patents

Description

  The present disclosure relates to a method and apparatus for manufacturing a number mode fiber in which a plurality of propagation modes propagate.

  A number mode optical fiber using a plurality of propagation modes has been proposed as a technique for expanding the transmission capacity. In particular, mode multiplex transmission using a plurality of propagation modes is attracting attention as a new large-capacity transmission system because the transmission capacity can be improved several times the number of modes.

  In transmission using this number mode optical fiber, crosstalk between modes occurs in the transmission path, and therefore, a MIMO (Multiple-Input Multiple-Output) equalizer is used at the receiving end as the compensation means. However, when a loss difference between modes (Mode dependent Loss: hereinafter referred to as MDL) exists, even if a MIMO equalizer is used, the performance degradation of the transmission system becomes a problem (see, for example, Non-Patent Document 1). . However, if the group delay difference (Differential Mode Delay: DMD) between modes is large at the receiving end, the load of digital processing (DSP) related to MIMO increases, and in order to realize long-distance transmission, the DSP Reduction of a load becomes a problem (for example, refer nonpatent literature 2). In order to alleviate the influence of MDL and DMD, use of a mode scrambler that causes coupling between modes has been proposed (see, for example, Non-Patent Document 3). In addition, a ring core type fiber has been proposed in order to actively cause coupling between modes in an optical fiber transmission line (see, for example, Non-Patent Document 4). Furthermore, a mode coupling is caused in the middle of a 2 km 6-mode fiber using a mode multiplexer / demultiplexer, and the DMD reduction effect has been experimentally confirmed (for example, see Non-Patent Document 5).

P. J. et al. Winzer, et al. "Mode-dependent loss, gain, and noise in MIMO-SDM systems", in Proc. ECOC 2014, paper Mo. 3.3.2, 2014. S. O. Arik, D.D. Askarov, J. et al. M.M. Kahn, Effect of mode coupling on signal processing complexity in mode-division multiplexing, J.A. Lightwave Technol. 31 (3) (2013) 423-431. Lobato, A.M. Ferreira, F .; Rabe, J .; Kuschnerov, M .; Spinnler, B .; Rankl, B .; , “Mode scramblers and reduced-search maximum-likelihood detection for mode-dependent-loss-implied transmission, in Optical Communication and EcC 2013. 1-3, 22-26 Sept. 2013 N. Fontaine, R.A. Ryf, M.M. Hirano, and T.R. Sasaki, “Experimental investing of cross accumulation in core-core fiber”, in Proc. IEEE Photon. Soc. Summer Top. Meeting Series, 2013, pp. 111-112. Y. Wakayama, D.W. Soma, K.K. Igarashi, H .; Taga, and T.M. Tsuritani, "Intermediate Mode Interchange for Reduction of Differential Mode-Group Delay Weakly-Coupled 6-Mode Fiber Transmission Line", in Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2016), paper M3E. 6). G. Narayanan, H .; M.M. Presby and A.M. M.M. Vengsarkar, “Band-rejection fiber filter using periodic core deformation”, Electrical Letters, 33 (4), 13th February, 1997, pp. 13-26. 280-281.

  However, Non-Patent Document 4 has a problem that mode coupling for obtaining a sufficient DMD reduction amount is insufficient. In addition, Δβ tends to increase in higher order modes, and it is difficult to cause sufficient mode coupling even when the number of modes increases. Accordingly, the present invention provides an optical fiber manufacturing method and an optical fiber manufacturing apparatus for manufacturing an optical fiber capable of causing mode coupling to obtain a sufficient DMD reduction amount even in a higher-order mode in order to solve the above-described problems. The purpose is to do.

  In order to achieve the above object, an optical fiber manufacturing method according to the present invention gives a periodic structure change corresponding to a propagation constant difference between propagation modes to an optical fiber in which a plurality of modes propagates, thereby providing long-period light. A fiber grating was formed.

Specifically, an optical fiber manufacturing method according to the present invention is an optical fiber manufacturing method for forming a long-period optical fiber grating in an optical fiber,
A delivery step of delivering the optical fiber at a constant speed;
Concavity and convexity imparting step for providing concavity and convexity at regular intervals on the fiber coating of the optical fiber,
Applying stress to the optical fiber after the concavity and convexity providing step, and applying a bending to the optical fiber core in accordance with the concavity and convexity provided to the fiber coating in the concavity and convexity providing step,
A bending fixing step for permanently fixing the periodic bending applied to the core in the stress applying step;
A winding step of winding the optical fiber after the bending and fixing step;
Have
The periodic bending period 2π / Ω given to the core in the bending application step is as follows:
When the propagation constant difference between the two modes propagating through the optical fiber is Δβ, and the length of the core to which the periodic bending is given is L,
Δβ−π / L ≦ Ω ≦ Δβ + π / L
It is the range which satisfy | fills.

  Even in the higher-order mode, mode coupling can be caused with high efficiency by applying a structural change to the optical fiber with a pitch width close to the pitch corresponding to the propagation constant difference between the propagation modes. The optical fiber manufacturing method according to the present invention can give such a structural change of pitch width, and the structural change that has been applied only in a few centimeters as shown in Non-Patent Document 6 to a long fiber. Can be imparted, and stronger mode coupling can occur.

  Therefore, the present invention can provide an optical fiber manufacturing method for manufacturing an optical fiber capable of causing mode coupling for obtaining a sufficient DMD reduction amount even in a higher-order mode.

  In the optical fiber manufacturing method according to the present invention, a resin coating may be attached to the fiber coating of the optical fiber at the predetermined interval in the unevenness providing step.

  In the optical fiber manufacturing method according to the present invention, after the resin coating having a predetermined thickness is attached to the optical fiber in the unevenness providing step, a part of the resin coating may be removed at the predetermined interval.

  In the optical fiber manufacturing method according to the present invention, in the unevenness providing step, the optical fiber may be pressed against a pulley having the uneven structure of the constant interval on the outer peripheral portion.

  In the optical fiber manufacturing method according to the present invention, in the bending and fixing step, the periodic bending applied to the core is made permanent by applying heat to the optical fiber to solidify a fiber coating of the optical fiber. It may be fixed.

  In the optical fiber manufacturing method according to the present invention, in the bending and fixing step, the periodic bending given to the core may be permanently fixed by further attaching a coating to the optical fiber.

Moreover, the optical fiber manufacturing apparatus according to the present invention includes:
A heating furnace for heating the optical fiber preform for drawing;
A bending applicator that vibrates a drawn optical fiber drawn from the optical fiber preform heated in the heating furnace with ultrasonic waves and applies a periodic bending to the core of the drawn optical fiber;
A coating applicator for covering and curing the drawn optical fiber treated by the bending applicator with a fiber coating;
A winder for winding the drawn optical fiber treated with the coating applicator;
And manufacturing an optical fiber including a long-period optical fiber grating.

  This optical fiber manufacturing apparatus can adjust the pitch of the structural change of an optical fiber with the frequency of an ultrasonic wave. That is, this optical fiber manufacturing apparatus can easily manufacture an optical fiber having a pitch that promotes mode coupling between desired modes by adjusting the frequency of ultrasonic waves. Therefore, the present invention can provide an optical fiber manufacturing apparatus for manufacturing an optical fiber capable of causing mode coupling for obtaining a sufficient DMD reduction amount even in a higher-order mode.

In addition, another optical fiber manufacturing apparatus according to the present invention,
A heating furnace for heating the optical fiber preform for drawing;
A laser is periodically irradiated to the drawn optical fiber drawn from the optical fiber preform heated in the heating furnace, the clad of the drawn optical fiber is excavated at a constant interval, and the clad is excavated A bending applicator that applies heat to the drawn optical fiber, melts the surface of the drawn optical fiber, and applies periodic bending to the core of the drawn optical fiber;
A coating applicator for covering and curing the drawn optical fiber treated by the bending applicator with a fiber coating;
A winder for winding the drawn optical fiber processed by the coating applicator;
And manufacturing an optical fiber including a long-period optical fiber grating.

  In this optical fiber manufacturing apparatus, a part of the clad of the optical fiber is shaved at an arbitrary period, partially melted by applying heat, and the optical surface is smoothed by the surface tension to bend the core. The pitch of the structural change of the optical fiber can be adjusted by the period of cutting the clad. That is, this optical fiber manufacturing apparatus can easily manufacture an optical fiber having a pitch that promotes mode coupling between desired modes by adjusting the laser irradiation period. Therefore, the present invention can provide an optical fiber manufacturing apparatus for manufacturing an optical fiber capable of causing mode coupling for obtaining a sufficient DMD reduction amount even in a higher-order mode.

  The present invention can provide an optical fiber manufacturing method and an optical fiber manufacturing apparatus for manufacturing an optical fiber capable of causing mode coupling for obtaining a sufficient DMD reduction amount even in a higher-order mode.

It is a figure explaining the basic structure of a long period optical fiber grating. It is a figure explaining the optical fiber manufacturing method which concerns on this invention. It is a figure explaining a mode that new coating | cover is periodically adhered to an optical fiber with resin etc. in a longitudinal direction, and it hardens | cures with a coating curing device. It is a figure explaining the optical fiber manufacturing method which concerns on this invention. It is a figure explaining a mode that a laser for excavation, such as CO2 laser, is irradiated to a fiber coat of an optical fiber. It is a figure explaining the optical fiber manufacturing method which concerns on this invention. It is a figure explaining a mode that the new coating was uniformly adhered to the optical fiber, the coating was shaved with a laser for excavation, etc., and the coating was provided with a periodic structure. It is a figure explaining the optical fiber manufacturing method which concerns on this invention. It is a figure explaining an unevenness | corrugation to the coating | cover of an optical fiber by winding up an optical fiber using the pulley with an unevenness | corrugation. It is a figure explaining the optical fiber manufacturing method which concerns on this invention. It is a figure explaining a mode that periodical bending is given to an optical fiber by giving stress and heat to an optical fiber which gave unevenness to covering. It is a figure explaining the optical fiber manufacturing method which concerns on this invention. It is a figure explaining a mode that cyclic bending is given to an optical fiber by attaching a new coating, giving stress to an optical fiber which gave unevenness to a coating. It is a figure explaining the optical fiber manufacturing method which concerns on this invention. It is a figure explaining a mode that a periodic bending is provided to an optical fiber by periodically attaching a new coating to an optical fiber so as to push the optical fiber. It is a figure explaining the optical fiber manufacturing apparatus which concerns on this invention. It is a figure explaining the optical fiber manufacturing apparatus which concerns on this invention. It is a figure explaining the wavelength dependence of the propagation constant difference between LP01-LP11 modes of an optical fiber, and the propagation constant difference between LP11-LP21 modes. It is a figure explaining the wavelength dependence of the coupling pitch between LP01-LP11 modes of an optical fiber, and the coupling pitch between LP11-LP21 modes. It is a figure showing the relationship with the pitch shift | offset | difference of the mode coupling strength (phi) with respect to the structural change length L of an optical fiber. The maximum deviation of the pitch of the optical fiber structure change that can be approximated when the mode coupling strength φ at an ideal pitch is obtained with respect to the structural change length L of the optical fiber It is a figure explaining the optical fiber manufacturing apparatus which concerns on this invention. It is a figure explaining a mode that an unevenness | corrugation is given to the coating | cover of an optical fiber by winding up an optical fiber using the pulley which provided the unevenness | corrugation to a part of pulley. It is a figure explaining the optical fiber manufacturing apparatus which concerns on this invention. It is a figure explaining a mode that an optical fiber is wound up using a plurality of pulleys, and a covering of an optical fiber is made uneven. It is a figure explaining the comparison of SI fiber and GI fiber of the effective refractive index of each mode. It is a table | surface showing the wavelength dependence of the pitch required for mode coupling between LP01-LP11 and between LP11-LP21.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components. The optical fibers of all the embodiments are optical fibers that can be transmitted in multimode at the wavelength of the optical signal to be transmitted.

(Embodiment 1)
FIG. 1 shows a basic structure of a long-period optical fiber grating formed on an optical fiber by the optical fiber manufacturing method of the present embodiment. In the long-period optical fiber grating of FIG. 1, the core of the optical fiber is periodically bent. By making the core bending interval coincide with 2π / Δβ corresponding to the propagation constant difference Δβ between the propagation modes of the optical fiber, mode coupling can be caused between the modes. The shape is not limited as long as the bending is periodic. As will be described later, mode coupling occurs between modes even when the core bending interval does not completely match 2π / Δβ. That is, in order to generate mode coupling between modes, the bending interval of the core may be set within a predetermined range including 2π / Δβ.

The periodic optical fiber grating as shown in FIG. 1 can be formed by the first optical fiber manufacturing method. The first optical fiber manufacturing method is an optical fiber manufacturing method for forming a long-period optical fiber grating in an optical fiber,
A delivery step of delivering the optical fiber at a constant speed;
Concavity and convexity imparting step for providing concavity and convexity at regular intervals on the fiber coating of the optical fiber,
Applying stress to the optical fiber after the concavity and convexity providing step, applying a bending to the core of the optical fiber according to the concavity and convexity imparted to the fiber coating in the concavity and convexity providing step,
A bending fixing step for permanently fixing the periodic bending applied to the core in the stress applying step;
A winding step of winding the optical fiber after the bending and fixing step;
Have
The periodic bending period 2π / Ω given to the core in the bending application step is as follows:
When the propagation constant difference between the two modes propagating through the optical fiber is Δβ, and the length of the core to which the periodic bending is given is L,
Δβ−π / L ≦ Ω ≦ Δβ + π / L
It is the range which satisfy | fills.

  In the first optical fiber manufacturing method, in order to bend the core, the fiber coating is made uneven in the unevenness applying step, stress is applied in the bending applying step, and heat treatment is applied in the bending fixing step to fix the coating. To keep the core bent. By solidifying the coating, it is possible to maintain a state in which the core is permanently bent even in the absence of stress.

  For example, in the unevenness applying step, a process of attaching a resin coating to the fiber coating of the optical fiber at the predetermined interval, and after attaching a resin coating having a predetermined thickness to the optical fiber, The fiber coating can be made uneven by performing a process of removing the part or a process of pressing the optical fiber against a pulley having the uneven structure of the constant interval on the outer peripheral part. Further, in the bending and fixing step, the periodic bending applied to the core is performed not only in the process of applying heat to the optical fiber to solidify the fiber coating of the optical fiber but also in the process of further coating the optical fiber. Can be permanently fixed.

Also, the periodic optical fiber grating as shown in FIG. 1 can be formed by the second optical fiber manufacturing method. The second optical fiber manufacturing method is a method in which ultrasonic vibration is given at the time of drawing and structural fluctuation is given to the core itself.
Furthermore, the periodic optical fiber grating as shown in FIG. 1 can be formed by a third optical fiber manufacturing method. The third optical fiber manufacturing method is a method in which a core is bent by surface tension by excavating a clad with a CO ₂ laser at the time of drawing and applying heat treatment or the like.
Further, the periodic optical fiber grating as shown in FIG. 1 can be formed by the fourth optical fiber manufacturing method. The fourth optical fiber manufacturing method does not form a periodic bend in the core, but periodically irradiates a laser pulse such as a CO ₂ laser or a UV laser, and changes the refractive index at an interval of 2π / Δβ. Is a way to give
Each optical fiber manufacturing method will be described in the following embodiments.

(Embodiment 2)
FIG. 2 is a diagram illustrating an optical fiber manufacturing apparatus that realizes the first optical fiber manufacturing method. This optical fiber manufacturing apparatus includes an optical fiber delivery device 11, a device 12 for applying a coating such as a resin, a coating curing device 13, and an optical fiber winder 14. The optical fiber delivery device 11 delivers an optical fiber 50 having a fiber coating at an arbitrary speed. The apparatus 12 for depositing the coating deposits resin or the like on the fiber coating of the optical fiber 50 at predetermined intervals. The coating curing device 13 cures the resin coating. The optical fiber winder 14 winds the optical fiber 50 to which the resin coating is attached at predetermined intervals.

  The present optical fiber manufacturing apparatus has a pitch v of the grating formed on the core of the optical fiber 50 at a speed v at which the optical fiber delivery device 11 delivers the optical fiber 50 and a time interval t at which the coating device 12 applies the resin coating. Can be adjusted. That is, the pitch is given by v × t, and v and t may be set to appropriate values in accordance with the pitch 2π / Δβ causing coupling between modes. Note that an adhesive may be periodically added, and a new coating may be attached thereto.

  FIG. 6 is a diagram illustrating the stress applicator 15 that performs the bending process and the heat applicator 16 that performs the bending process. FIG. 7 is a diagram illustrating a stress applicator 15 that performs a bending application process and a coating applicator 17 that performs a bending fixation process. The stress applicator 15 applies stress to the optical fiber 50 in which the resin coating is attached to the fiber coating at a cycle of 2π / Δβ, thereby bending the core with the cycle. Then, while maintaining this state, the optical fiber 50 is heat-treated with the heat applicator 16 of FIG. 6 or the optical fiber 50 is covered with a new coating by the coating applicator 17 of FIG. To maintain.

  This optical fiber manufacturing apparatus can form a grating shape on the core of the optical fiber 50 by attaching a resin coating to the fiber coating of the optical fiber 50 at an arbitrary period and applying stress from the outside. And this optical fiber manufacturing apparatus can make it a desired period 2π / Δβ by the delivery speed of the optical fiber 50 and the time interval for attaching the resin coating, and the number of resin coatings to be attached to the optical fiber 50 (resin The length of the grating can be easily adjusted by the coating adhesion work time).

(Embodiment 3)
FIG. 3 is a diagram illustrating an optical fiber manufacturing apparatus that realizes the first optical fiber manufacturing method. This optical fiber manufacturing apparatus includes an excavation laser 18 such as a CO ₂ laser as an alternative to the apparatus 12 for applying the coating of the optical fiber manufacturing apparatus of FIG. The excavation laser 18 excavates a part of the fiber coating of the optical fiber with laser light.

  This optical fiber manufacturing apparatus adjusts the pitch of the grating formed on the core of the optical fiber 50 by the speed v at which the optical fiber delivery device 11 delivers the optical fiber 50 and the time interval t at which the excavation laser 18 irradiates the laser light. can do. That is, the pitch is given by v × t, and v and t may be set to appropriate values in accordance with the pitch 2π / Δβ causing coupling between modes.

  As described with reference to FIGS. 6 and 7, the core can be bent periodically by applying stress to the optical fiber 50 in this state, and the core can be permanently bent by covering with heat treatment or a new coating. To maintain.

  This optical fiber manufacturing apparatus can form a grating shape in the core of the optical fiber 50 by excavating the fiber coating of the optical fiber 50 with an arbitrary period of laser light and applying stress from the outside. The optical fiber manufacturing apparatus can achieve a desired period of 2π / Δβ based on the delivery speed of the optical fiber 50 and the laser light irradiation interval, and the length of the grating can be determined by the number of times of laser light irradiation (excavation work time). It can be adjusted easily.

(Embodiment 4)
FIG. 4 is a diagram for explaining an optical fiber manufacturing apparatus that realizes the first optical fiber manufacturing method. This optical fiber manufacturing apparatus further includes an apparatus 19 for attaching a resin coating to the optical fiber manufacturing apparatus of FIG. In the present optical fiber manufacturing apparatus, a resin coating having a certain film thickness is formed on the fiber coating of the optical fiber 50 by the device 19 for attaching the resin coating before irradiating the excavation laser 18 with the laser beam. The operation of the excavation laser 18 is the same as that described in FIG.

  The present optical fiber manufacturing apparatus excavates the resin coating uniformly formed by the apparatus 19 for applying the resin coating with the excavation laser, and does not excavate the fiber coating of the optical fiber 50. For this reason, even if it is the optical fiber 50 with thin fiber coating, a grating shape can be formed in a core and the reliability of the optical fiber 50 can be improved.

(Embodiment 5)
FIG. 5 is a diagram illustrating an optical fiber manufacturing apparatus that realizes the first optical fiber manufacturing method. The optical fiber manufacturing apparatus includes an optical fiber delivery device 11, a pulley 21, and an optical fiber winder 14. The pulley 21 has a grating plate 22 having a concavo-convex structure at regular intervals wound around the outer periphery. The optical fiber 50 is delivered from the optical fiber delivery device 11, and after being in contact with the pulley 21, the optical fiber 50 is taken up by the optical fiber winder 14.

  The grating plate 22 has irregularities with a period of length 2π / Δβ corresponding to the propagation delay difference Δβ between the propagation modes of the optical fiber. The uneven structure may be any shape as long as it is periodic, such as rectangular, trapezoidal, triangular, sinusoidal, arcuate. The fiber coating of the optical fiber 50 can be made uneven by passing through the pulley 21 having the grating plate 22.

  As described with reference to FIGS. 6 and 7, the core can be bent periodically by applying stress to the optical fiber 50 in this state, and the core can be permanently bent by covering with heat treatment or a new coating. To maintain.

  This optical fiber manufacturing apparatus can form a grating shape in the core of the optical fiber 50 by applying unevenness to the fiber coating of the optical fiber 50 with the pulley 21 having the grating plate 22 and applying stress from the outside. The optical fiber manufacturing apparatus can change the pitch of the core to 2π / Δβ by replacing the pulley 21 with the grating plate 22 having a desired period 2π / Δβ, and further, the grating can be obtained with the optical fiber delivery time. The length can also be easily adjusted.

  The grating plate 22 is not necessarily arranged on the entire outer periphery of the pulley 21. A part of the outer periphery of the pulley 21 may not have a periodic structure. In this case, an intermittent uneven shape is formed on the fiber coating of the optical fiber 50.

(Embodiment 6)
FIG. 8 is a diagram illustrating an optical fiber manufacturing apparatus that realizes the first optical fiber manufacturing method. This optical fiber manufacturing apparatus is similar to the optical fiber manufacturing apparatus of FIG. In this optical fiber manufacturing apparatus, the apparatus 12 for applying the coating of the optical fiber manufacturing apparatus in FIG. 2 is replaced with an apparatus 12a for applying the coating. The apparatus 12a for applying the coating pushes the optical fiber 50 with a period of 2π / Δβ to bend it, and in this state, attaches the resin coating. The coating curing device 13 cures the resin coating in that state. Note that an adhesive may be periodically added at a period of 2π / Δβ while pushing and bending the optical fiber, and a resin coating may be attached thereto.

  This optical fiber manufacturing apparatus can form a grating shape in the core of the optical fiber 50 without the stress applicator 15, the heat applicator 16, and the coating applicator 17 shown in FIGS.

(Embodiment 7)
FIG. 9 is a diagram for explaining an optical fiber manufacturing apparatus that realizes the second optical fiber manufacturing method. This optical fiber manufacturing equipment
A heating furnace 31 that heats the optical fiber preform 60 for drawing;
A bending applicator 32 that vibrates the drawn optical fiber 61 drawn from the optical fiber preform 60 heated in the heating furnace 31 with ultrasonic waves, and applies periodic bending to the core of the drawn optical fiber 61;
A coating applicator 33 for covering and hardening the drawn optical fiber 61 processed by the bending applicator 32 with a fiber coating;
A winder 34 for winding the drawn optical fiber 61 processed by the coating applicator 33;
And manufacturing an optical fiber including a long-period optical fiber grating.

  The heating furnace 31 heats and melts the optical fiber preform 60. The bending applicator 32 is, for example, an ultrasonic exciter, and applies an acoustic wave to the thinned optical fiber 61 to apply bending to the core. The coating applicator 33 includes a coating application unit and a coating curing device. The coating application unit applies a coating to the drawing optical fiber 61, and the coating curing device cures the applied coating. The winder 34 includes a capstan and a bobbin. The capstan applies tension to the drawn optical fiber 61 from the optical fiber preform 60 to control the diameter of the drawn optical fiber. The bobbin winds the drawing optical fiber 61.

  When the acoustic wave is sufficiently faster than the drawing speed, the acoustic wave emitted from the ultrasonic vibrator is directly transmitted to the drawing optical fiber 61. The bending period applied to the core of the drawing optical fiber 61 increases as the frequency of the acoustic wave decreases, and conversely, the bending period decreases as the frequency increases. Thus, the bending period given to the core of the drawing optical fiber 61 is determined by the frequency of the acoustic wave. Therefore, the optical fiber manufacturing apparatus adjusts the frequency of the acoustic wave radiated from the ultrasonic vibrator and uses a bending of length 2π / Δβ corresponding to the propagation constant difference Δβ between modes propagating to the optical fiber as a core. Can be granted.

(Embodiment 8)
FIG. 10 is a diagram illustrating an optical fiber manufacturing apparatus that realizes the third optical fiber manufacturing method. This optical fiber manufacturing equipment
A heating furnace 31 that heats the optical fiber preform 60 for drawing;
A laser was periodically irradiated to the drawn optical fiber 61 drawn from the optical fiber preform 60 heated in the heating furnace 31, and the clad of the drawn optical fiber 61 was excavated at regular intervals, and the clad was excavated. A bending applicator 32 that applies heat to the drawing optical fiber 61, melts the surface of the drawing optical fiber 61, and applies periodic bending to the core of the drawing optical fiber 61;
A coating applicator 33 for covering and hardening the drawn optical fiber 61 processed by the bending applicator 32 with a fiber coating;
A winder 34 for winding the drawn optical fiber 61 processed by the coating applicator 33;
And manufacturing an optical fiber including a long-period optical fiber grating.

This optical fiber manufacturing apparatus is different from the optical fiber manufacturing apparatus of FIG. 9 in the configuration of the bending applicator 32. The bending applicator 32 of this optical fiber manufacturing apparatus includes an excavation laser and a heat applicator. The excavation laser irradiates the drawing optical fiber 61 with laser light at a period of length 2π / Δβ corresponding to the propagation constant difference Δβ between modes propagating through the optical fiber, and cuts a part of the cladding. The heat applicator partially melts by applying heat to the drawn optical fiber 61, and bends the core by smoothing the surface of the drawn optical fiber 61 by the surface tension at that time. At this time, the optical fiber manufacturing apparatus adjusts the drawing speed and the pulse irradiation interval from the excavation laser in order to set the bending of the core to a period of 2π / Δβ. When the drawing speed is Vs [m / s], the period 2π / Δβ [m] is Λ, and the pulse interval of the laser light is Tp [s], Equation 1 is established. This optical fiber manufacturing apparatus determines Vs and Tp so as to satisfy Formula 1.

(Embodiment 9)
The number of propagation modes of the optical fiber is N (N is an integer of 2 or more). In order to reduce DMD, there is a method of reducing DMD by combining a mode having the largest DMD of the optical fiber and a mode having the next largest. Furthermore, a larger DMD reduction effect can be obtained by combining other modes. For this purpose, it is necessary to impart structural variations of a plurality of pitches to the optical fiber as necessary. Similarly, when a wavelength is multiplexed and transmitted, a higher DMD reduction effect can be expected by giving a structure having a plurality of pitches to the optical fiber.

  For example, in the case of a step index fiber having a core radius of 12 μm and a relative refractive index difference Δ of 0.4% relative to the core cladding, as shown in FIG. 11, the propagation constant difference Δβ between LP01 and LP11 and the propagation between LP11 and LP21. The constant difference Δβ is different. Each has wavelength dependency. Therefore, as shown in FIG. 12, the period of structural fluctuation (pitch on the vertical axis) of the optical fiber in which the coupling between the modes occurs differs depending on the set of modes causing the coupling and also varies depending on the wavelength.

  In other words, it is necessary to impart to the optical fiber a structural change in pitch according to the modes to be coupled and the wavelength of light to be transmitted. An example of a step index fiber having a core radius of 12 μm and Δ0.4% is shown in FIG. For example, when only mode coupling between LP01 and LP11 is generated, a pitch in the range of 30 μm from 1620 to 1650 μm is formed in the optical fiber as a structural variation in the C band. Further, in order to cover the C to L bands, a pitch in the range of 1580 to 1650 μm is formed as the structural variation, and in order to cover the O to L bands, a pitch in the range of 1580 to 1900 μm is formed in the optical fiber. Further, in order to cause mode coupling between the LP11 and LP21 modes, as shown in FIG. 18, a structural variation with a finer pitch than the structural variation causing mode coupling between LP01 and LP11 is formed in the optical fiber.

ここで、モードｌとモードｍの伝搬定数をそれぞれβｌとβｍとおき（つまりΔβ＝βｌ−βｍ）、理想的な構造変化のピッチを２π／（βｌ−βｍ）とした場合に、光ファイバに与えた構造変化の周期を２π／Ωとおき、構造変化のトータル長をＬとする。 (Embodiment 10)
The period of the structural variation imparted to the optical fiber does not necessarily have to be a length corresponding to 2π / Δβ using the propagation constant difference Δβ between modes propagating through the optical fiber. Depending on the coupling conditions, the period of structural variation has a predetermined range. In general, the mode coupling strength φ is expressed by Equation 2.

Here, when the propagation constants of mode l and mode m are respectively βl and βm (that is, Δβ = βl−βm), and the ideal structure change pitch is 2π / (βl−βm), an optical fiber is used. The period of the given structural change is set to 2π / Ω, and the total length of the structural change is L.

When the propagation constant of the LP01 mode is βl and the propagation constant of the LP11 mode is βm at a wavelength of 1550 nm with a step index fiber having a core radius of 12 μm and Δ0.4%, the strength of coupling between the LP01 mode and the LP11 mode φ is expressed as shown in FIG. The deviation between the structure period 2π / Ω given to the optical fiber and the pitch corresponding to Δβ between the LP01-LP11 modes is expressed by Equation 3.

When Δ _pitch is 0, the pitch width corresponding to Δβ between the propagation modes matches the period of structural change given to the optical fiber, so the LPG length (the length at which the structural change occurs) L is increased. The stronger the mode coupling strength φ is. The ideal pitch width 2π / Δβ is 1633 μm, but assuming that there is a deviation of 10 μm in the pitch width, the mode coupling strength φ approximates a plot of Δ _pitch = 0 when the LPG length L of the optical fiber is short. However, as the LPG length L increases, the value of the mode coupling strength φ decreases. Similarly, when there is a deviation of Δ _pitch = 20 μm and a deviation of Δ _pitch = 30 μm, the plot approximation of Δ _pitch = 0 can be performed only when the LPG length L is small.

From FIG. 13, as the LPG length L is shorter, a value closer to the mode coupling strength φ when the pitch is 2π / Δβ is obtained even if the pitch is deviated from 2π / Δβ. That is, the LPG length L needs to be shortened as the pitch deviation from 2π / Δβ increases. From FIG. 13, the allowable pitch range in which the mode coupling strength φ can be regarded as substantially equal to the pitch 2π / Δβ is expressed by Equation 4.

  FIG. 14 shows the result of obtaining the structural change length L of the optical fiber and the allowable pitch deviation 2π / Ω from this relational expression. The shorter the L, the larger the allowable deviation. From this, when L is short, mode coupling can be generated in a wide wavelength band even if the pitch interval of the structural change is constant. For example, since the allowable deviation is 100 μm at a structural change length of 1.41 cm, it can be said that mode coupling occurs within a desired pitch width of ± 100 μm.

  As shown in FIG. 18, the band to be converted differs depending on the wavelength, but by defining the coupling change length L that covers the band, it is possible to cover a wide band with a small number of pitch widths.

(Embodiment 11)
If the length L of the structural change applied to the optical fiber is short, the band to be converted is widened. Therefore, it is preferable to cause the structural change intermittently rather than to cause a uniform structural change in the optical fiber transmission line. For example, in the method of FIGS. 2 to 4 in which unevenness is imparted to the coating, if the irregularity is given to a certain length and then a section where the predetermined length irregularity is not given is provided, the interval for applying the coating, the cutting interval, etc. are adjusted. Good. The same applies to the method of FIGS. 8 and 9 in which the structural change is directly applied to the core and the method of FIG. 10 in which the cladding is uneven.

  For example, in the method of FIG. 5, a grating plate 22 in which a section with unevenness on the outer periphery of the pulley 21 and a section without unevenness are alternately arranged as shown in FIG. 15, or a plurality of pulleys 21 are provided as shown in FIG. The structure of the short LPG length L is intermittently applied to the optical fiber by adjusting the position of the pulley and the length of the unevenness of the grating plate 22 so that there is a portion where the unevenness portion of the grating plate 22 does not hit, Mode coupling can be generated to cover the transmission band.

  In the case where a plurality of pitch widths are required to cover the transmission band, or when there are two or more propagation modes and coupling between the plurality of modes is desired, the grating plate 22 shown in FIG. Or a grating plate 22 having a different pitch width for each pulley in FIG. Similarly, in the methods shown in FIGS. 2 to 5 and FIGS. 8 to 10, the resin addition, the CO 2 laser irradiation interval, and the excitation interval by the ultrasonic vibrator may be adjusted so as to change the pitch interval.

Embodiment 12
When a graded index (GI) fiber is used as the transmission optical fiber, it is possible to generate inter-mode coupling of a plurality of modes in one kind of core bending period. FIG. 17 shows the effective refractive index neff in each propagation mode. The effective refractive index neff is proportional to the propagation constant β as shown in Equation 5.

  Since the propagation constant is proportional to the effective refractive index, the propagation constant difference Δβ between modes related to mode coupling is proportional to the effective refractive index Δneff. For example, the effective refractive index of the SI fiber shown in FIG. Subsequently, mode numbers 2 and 3 have the next largest effective refractive index, and these have the same effective refractive index. Modes having the same effective refractive index as the modes 2 and 3 are called mode groups. In addition, modes 4, 5, 7, 8, etc. are the mode group in the SI fiber.

  In the case of the SI fiber, since the effective refractive index difference is different between the mode groups, it is necessary to give the optical fiber a structural change with a pitch width corresponding to different 2π / Δβ when coupling between a plurality of sets of modes is caused. . On the other hand, in the GI fiber, even when the effective refractive index difference between the mode groups is equal to cause coupling between a plurality of sets of modes, a single 2π / Δβ is possible. In other words, the GI fiber can easily couple between a plurality of modes when the number of modes is expanded.

(Embodiment 13)
An optical fiber having a long-period optical fiber grating manufactured by the optical fiber manufacturing method described in the above embodiment is used in a low DMD transmission line using a low DMD fiber, a ring core fiber that easily generates coupling, a DMD compensation transmission line, and the like. It is also possible to do. In this case, it is possible to expect a further effect of reducing the difference in propagation time between modes due to mode coupling by the long period grating, and it is possible to provide an optical transmission system capable of transmission with a very small signal processing load.

[Appendix]
The following describes the optical fiber manufacturing method and the optical fiber manufacturing apparatus of the present embodiment.

(Task)
Provided a manufacturing method and a manufacturing apparatus capable of manufacturing a long-period optical fiber grating capable of efficiently promoting coupling between propagation modes for a mode multiplexing optical transmission system using a number mode optical fiber in which a plurality of propagation modes propagate. The purpose is to do.

(Problem means)
In order to achieve the above object, the period of the structural change applied to the optical fiber is set to a pitch width corresponding to the propagation constant difference between the propagation modes. In addition, a mode change with a narrow pitch width corresponding to the difference in the propagation constant is given to the mode having a large difference in the higher-order propagation constant.

(1): The first optical fiber manufacturing method is
A delivery process for delivering an optical fiber at a constant speed;
Concavity and convexity imparting step for providing concavity and convexity at regular intervals on the fiber coating of the optical fiber,
Applying stress to the optical fiber after the concavity and convexity providing step, and applying a bending to the optical fiber core in accordance with the concavity and convexity provided to the fiber coating in the concavity and convexity providing step,
A bending fixing step for permanently fixing the periodic bending applied to the core in the stress applying step;
A winding step of winding the optical fiber after the bending and fixing step;
A manufacturing method for forming a long-period optical fiber grating on the optical fiber.

(2): The optical fiber manufacturing method according to the above (1), wherein, in the unevenness applying step, a resin coating is attached to the fiber coating of the optical fiber at the predetermined interval.

(3) In the above (1), the resin coating having a predetermined thickness is attached to the optical fiber in the irregularity applying step, and then a part of the resin coating is removed at the predetermined interval. Optical fiber manufacturing method.

(4): The optical fiber manufacturing method according to (1), wherein the optical fiber is pressed against a pulley having an uneven structure with a constant interval on an outer peripheral portion in the unevenness providing step.

(5): In the bending and fixing step, the periodic bending applied to the core is permanently fixed by applying heat to the optical fiber to solidify a fiber coating of the optical fiber. The optical fiber manufacturing method according to any one of (1) to (4) above.

(6): In the bending and fixing step, the periodic bending given to the core is permanently fixed by further attaching a coating to the optical fiber. ) Any one of the optical fiber manufacturing methods.

(7): The second optical fiber manufacturing method is:
A heating step of heating the optical fiber preform to draw,
A bending step of applying a bending to the core of the drawn optical fiber by exciting the drawn optical fiber drawn from the optical fiber preform in the heating step with ultrasonic waves;
A coating applying step of covering and hardening the drawn optical fiber after the bending applying step with a fiber coating;
A winding step of winding the drawn optical fiber after the coating applying step;
And an optical fiber having a long-period optical fiber grating.

(8): The third optical fiber manufacturing method is
A heating step of heating the optical fiber preform to draw,
The drawn optical fiber drawn from the optical fiber preform in the heating step is periodically irradiated with a laser, the clad of the drawn optical fiber is excavated at regular intervals, and the drawn light with the clad excavated is excavated. Applying a bend to applying heat to the fiber, melting the surface of the drawn optical fiber, and periodically bending the core of the drawn optical fiber;
A coating applying step of covering and hardening the drawn optical fiber after the bending applying step with a fiber coating;
A winding step of winding the drawn optical fiber after the coating applying step;
And an optical fiber having a long-period optical fiber grating.

(9): The periodic bending period 2π / Ω given to the core in the bending step is as follows:
When the propagation constant difference between the two modes propagating through the optical fiber is Δβ, and the length of the core to which the periodic bending is given is L,
Δβ−π / L ≦ Ω ≦ Δβ + π / L
The optical fiber manufacturing method according to any one of (1) to (8) above, wherein

(10): The first optical fiber manufacturing apparatus is
A delivery device for delivering an optical fiber at a constant speed;
An irregularity imparting device for imparting irregularities at regular intervals to the fiber coating of the optical fiber;
A bending applicator that applies stress to the optical fiber, and that imparts periodic bending to the core of the optical fiber in accordance with the unevenness applied to the fiber coating by the unevenness applicator;
A bending fixture for permanently fixing the periodic bending imparted to the core by the stress applicator;
A winder that winds up the optical fiber processed by the bending fixture;
A long-period optical fiber grating is formed on the optical fiber.

(11): The optical fiber manufacturing apparatus according to (10), wherein the unevenness imparting device attaches a resin coating to the fiber coating of the optical fiber at the predetermined interval.

(12): The unevenness imparting device removes a part of the resin coating at the predetermined interval after a resin coating having a predetermined thickness is attached to the optical fiber. Optical fiber manufacturing equipment.

(13):
The apparatus for producing an optical fiber according to (10), wherein the unevenness imparting device presses the optical fiber against a pulley having the uneven structure of the constant interval on an outer peripheral portion.

(14): The bending fixture permanently fixes the periodic bending applied to the core by applying heat to the optical fiber to solidify a fiber coating of the optical fiber. The optical fiber manufacturing apparatus according to any one of (10) to (13).

(15): The bending fixture permanently fixes the periodic bending applied to the core by further attaching a coating to the optical fiber. The optical fiber manufacturing apparatus according to any one of the above.

(16): The second optical fiber manufacturing apparatus is
A heating furnace for heating the optical fiber preform for drawing;
A bending applicator that vibrates a drawn optical fiber drawn from the optical fiber preform heated in the heating furnace with ultrasonic waves and applies a periodic bending to the core of the drawn optical fiber;
A coating applicator for covering and curing the drawn optical fiber treated by the bending applicator with a fiber coating;
A winder for winding the drawn optical fiber treated with the coating applicator;
And manufacturing an optical fiber including a long-period optical fiber grating.

(17): The third optical fiber manufacturing apparatus is
A heating furnace for heating the optical fiber preform for drawing;
A laser is periodically irradiated to the drawn optical fiber drawn from the optical fiber preform heated in the heating furnace, the clad of the drawn optical fiber is excavated at a constant interval, and the clad is excavated A bending applicator that applies heat to the drawn optical fiber, melts the surface of the drawn optical fiber, and applies periodic bending to the core of the drawn optical fiber;
A coating applicator for covering and curing the drawn optical fiber treated by the bending applicator with a fiber coating;
A winder for winding the drawn optical fiber processed by the coating applicator;
And manufacturing an optical fiber including a long-period optical fiber grating.

(18) The bending applicator is:
The periodic bending period 2π / Ω given to the core is:
When the propagation constant difference between the two modes propagating through the optical fiber is Δβ, and the length of the core to which the periodic bending is given is L,
Δβ−π / L ≦ Ω ≦ Δβ + π / L
The optical fiber manufacturing apparatus according to any one of (10) to (17), wherein the optical fiber manufacturing apparatus is in a range satisfying

(effect)
INDUSTRIAL APPLICABILITY The present invention can provide a manufacturing method for manufacturing an optical fiber in which efficient mode coupling occurs by applying a periodic structural change (long-period optical fiber grating) of Δβ / 2π to the optical fiber. In particular, by providing a periodic structural change in accordance with the propagation constant difference between propagation modes, it is possible to provide a manufacturing method capable of reliably coupling between modes of any propagation constant difference. The present invention can realize a large capacity and a long distance of optical fiber transmission by using a higher-order mode in the fiber.

  The refractive index profile of the optical fiber forming the long-period optical fiber grating may be selected as long as it can propagate a plurality of modes, such as a step index type, a graded index type, a multistep type, and a ring type.

11: Optical fiber delivery device 12, 12a: Coating device 13: Coating curing device 14: Optical fiber winder 15: Stress applicator 16: Heat applicator 17: Coating applicator 18: Laser for excavation 19: Resin Coating device 21: pulley 22: grating plate 31: heating furnace 32: bending applicator 33: coating applicator 34: winder 50: optical fiber 60: optical fiber preform 61: drawn optical fiber

Claims (3)

An optical fiber manufacturing method for forming a long-period optical fiber grating in an optical fiber,
A delivery step of delivering the optical fiber at a constant speed;
Concavity and convexity imparting step for providing concavity and convexity at regular intervals on the fiber coating of the optical fiber,
And the uneven stresses imparted to the optical fiber after the application step, applying step bending provide periodic bending corresponding to the irregularities given to the fiber coated with the irregularity applying step to the core of the optical fiber,
A bending fixing step to permanently fix the periodic bending given to the core by the bending applying step,
A winding step of winding the optical fiber after the bending and fixing step;
Have
When the propagation constant difference between the two modes propagating through the optical fiber is Δβ, the periodic bending period 2π / Ω given to the core in the bending step is 2π / Δβ.
The concavo-convex applying step, the optical fiber manufacturing method characterized that you deposit a resin coated with the predetermined intervals in the fiber coating of the optical fiber. An optical fiber manufacturing method for forming a long-period optical fiber grating in an optical fiber,
A delivery step of delivering the optical fiber at a constant speed;
Concavity and convexity imparting step for providing concavity and convexity at regular intervals on the fiber coating of the optical fiber,
And the uneven stresses imparted to the optical fiber after the application step, applying step bending provide periodic bending corresponding to the irregularities given to the fiber coated with the irregularity applying step to the core of the optical fiber,
A bending fixing step to permanently fix the periodic bending given to the core by the bending applying step,
A winding step of winding the optical fiber after the bending and fixing step;
Have
When the propagation constant difference between the two modes propagating through the optical fiber is Δβ, the periodic bending period 2π / Ω given to the core in the bending step is 2π / Δβ.
The concavo-convex applying step, after depositing the resin coating of the optical fiber to a predetermined thickness, the optical fiber manufacturing method characterized that you remove a portion of the resin coated at the predetermined intervals. 3. The optical fiber manufacturing according to claim 1, wherein in the bending fixing step, the periodic bending applied to the core is permanently fixed by further attaching a coating to the optical fiber. Method.

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