JP6348861B2 - Optical transmission apparatus and optical transmission method - Google Patents

Description

  The present invention relates to an optical transmission apparatus and an optical transmission method for transmitting high power light using an optical fiber.

  With the progress of high power lasers, industrial processing applications such as welding using laser light are widely used. In particular, in recent years, high-power fiber lasers with an output of 10 kW have been developed and are expected to be used for medical and industrial purposes. In such a high-power fiber laser, for example, as shown in Non-Patent Document 1, the output power limitation due to nonlinearity is relaxed by expanding the core area in a short optical fiber of several meters or less. In laser processing, the beam quality of the emitted light greatly affects the processing efficiency. Since the beam quality strongly depends on the mode state of the emitted light, an optical fiber capable of single mode transmission is used in the fiber laser.

  Furthermore, as shown in Non-Patent Document 2, an optical fiber is connected to the emission end of the high-power laser, and this technique is also applied to remote welding. In this case, the beam quality at the output end is affected by the excitation state of the higher mode in the connected optical fiber. Therefore, a multi-mode optical fiber having a large core area can transmit high power light such as several kW for several tens of meters or more, but the beam quality at the exit end is low. Further, by using a single-mode optical fiber, it is possible to obtain emitted light with high beam quality, but there is a trade-off between single-mode transmission and core area expansion, and the transmittable power is limited. In Non-Patent Document 3, the nonlinear effect is suppressed by using a photonic band gap structure, and the core area is expanded while maintaining the single mode transmission condition.

Himeno, “Basics and Features of High-Power Lasers”, Fujikura Technical Report, vol. 1, pp. 1-6, January 2014 Yamazaki et al., “8-core long cable for 10 kW laser transmission”, Mitsubishi Electric Industrial Times, No. 105, pp. 24-27, October 2008 Himeno, “Fiber laser and advanced optical technology”, Fujikura Technical Report, vol. 2, pp. 33-37, February 2012 Arao et al., “Fan-out parts for lens-coupled multi-core fibers”, IEICE Technical Report, OFT 2013-67, pp. 67-70, February 2014

  As described above, in the conventional single mode optical fiber, there is a trade-off relationship between the single mode transmission condition and the expansion of the core area, and there is a problem that the transmittable power and distance are limited. In addition, the optical fiber using the photonic band gap structure has a very large variation in characteristics due to structural deviations during manufacturing, and the transmission loss is very large at 20 dB / km, so that it is not suitable for high-power optical transmission to a remote location. There was a problem.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical transmission apparatus and an optical transmission method that can alleviate restrictions on transmittable power and distance and reduce transmission loss.

  In the optical transmission device according to the present invention, the light to be transmitted is dispersed to each core of the multi-core optical fiber, and the emitted light from each core of the multi-core optical fiber is condensed at a predetermined position at the light receiving end.

Specifically, the optical transmission device according to the present invention is:
A multi-core optical fiber having a plurality of core regions in one optical fiber;
A beam splitter that focuses light emitted from the core region of the multi-core optical fiber at a predetermined position;
A light collecting unit arranged at the predetermined position and outputting the light collected by the beam splitter as light of one mode field;
Is provided.

  The optical transmission method according to the present invention branches light from a light source and couples it to each core of a multicore optical fiber having a plurality of core regions in one optical fiber, and the core of the multicore optical fiber. Light emitted from the region is condensed at a predetermined position and output as light of one mode field.

  The optical transmission device and the optical transmission method according to the present invention can increase the transmittable optical power by the number of cores while maintaining the single mode condition by dispersing light to each core of the multi-core optical fiber. . Furthermore, the multi-core optical fiber has less characteristic variation and transmission loss than the optical fiber using the photonic band gap structure. For this reason, high-power optical transmission to a remote is also possible.

  Therefore, the present invention can provide an optical transmission apparatus and an optical transmission method that can alleviate restrictions on transmittable power and distance and reduce transmission loss.

であることを特徴とする。
上記条件を満たすとき、マルチコア光ファイバの全コアの出射光は集光部へ入射される。従って、光の伝送損失を低減することができる。 The optical transmission apparatus according to the present invention includes a longest distance Λ among distances between a fiber center of the multi-core optical fiber and a center of each core region, a distance L between the beam splitter and the condensing unit, and the collection point. The numerical aperture NA of the optical part is

It is characterized by being.
When the above conditions are satisfied, the light emitted from all the cores of the multi-core optical fiber is incident on the condensing unit. Therefore, light transmission loss can be reduced.

  The optical transmission device according to the present invention is characterized in that the core region of the multi-core optical fiber is arranged concentrically or concentrically and in the center of the fiber. Since the shape of the beam splitter can be simplified, the cost of the apparatus can be reduced.

  The multi-core optical fiber of the optical transmission device according to the present invention is characterized in that a refractive index distribution between a clad and a core is a W type or a trench type. The multi-core optical fiber of the optical transmission device according to the present invention may have a hole assist structure. The trade-off regarding the expansion of the effective area of the core and the guarantee of single mode operation can be eased.

  The condensing unit of the optical transmission device according to the present invention combines light of different wavelengths transmitted for each core region of the multi-core optical fiber, condenses the combined light at a predetermined position, It outputs as light of one mode field. Since this optical transmission apparatus has a different wavelength for each core of the multi-core optical fiber, interference between the cores during multi-core optical fiber transmission is extremely small, and a stable optical output can be obtained.

  INDUSTRIAL APPLICABILITY The present invention can provide an optical transmission apparatus and an optical transmission method that can alleviate restrictions on transmittable power and distance and have low transmission loss.

It is a figure explaining the structure of the optical transmission apparatus which concerns on this invention. It is a figure explaining the structure of the optical transmission apparatus which concerns on this invention. It is a figure explaining the structure of the optical transmission apparatus which concerns on this invention. It is a characteristic view showing the relationship between NA (relative refractive index difference of a core and a clad) of a condensing part of an optical transmission device concerning the present invention, and the distance between a beam splitter and a condensing part. It is sectional drawing of the multi-core optical fiber of the optical transmission apparatus which concerns on this invention. It is sectional drawing of a tape type multicore fiber. It is a characteristic view showing the relationship between the effective area of an optical fiber, and the power which can be transmitted. It is a characteristic view showing the design example of the core structure of an optical fiber. It is a characteristic view showing the relationship between the core structure of an optical fiber, and an effective area. It is a figure explaining the relationship between the clad diameter of the multi-core optical fiber of the optical transmission apparatus which concerns on this invention, and the power which can be transmitted. It is sectional drawing explaining the core structure of an optical fiber. It is sectional drawing explaining the core structure of an optical fiber. It is sectional drawing explaining the core structure of an optical fiber. It is sectional drawing explaining the core structure of an optical fiber. It is a figure explaining the structure which couple | bonds light to each core area | region of a multi-core optical fiber from a laser. It is a figure explaining the structure which couple | bonds light to each core area | region of a multi-core optical fiber from a laser. It is a figure explaining the structure of the optical transmission apparatus which concerns on this invention.

  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.

FIG. 1 is a diagram for explaining the structure of the optical transmission apparatus according to the present embodiment. This optical transmission device
A multi-core optical fiber 10 having a plurality of core regions 11 in a single optical fiber;
A beam splitter 20 for condensing light emitted from the core region 11 of the multi-core optical fiber 10 at a predetermined position;
A light collecting unit 30 arranged at the predetermined position and outputting the light collected by the beam splitter 20 as light of one mode field;
Is provided.
The beam splitter 20 is a prism, for example. The condensing unit 30 is, for example, a collimator.

  The light wave emitted from each core region 11 of the multi-core optical fiber 10 is incident on the central position of the light collecting unit 30 by the beam splitter 20. Here, the light wave incident on the condensing unit 30 can be incident on the condensing unit 30 when the incident angle θ of the light wave is equal to or smaller than the NA in the condensing unit 30, and the light wave incident beyond the NA leaks.

であること好ましい。数１の関係を満たすとき、マルチコア光ファイバ１０のコア領域１１からの出射光を全て集光部３０へ入射できる。 Therefore, the largest Λ _max among the distances Λ between the fiber center of the multi-core optical fiber 10 and the center of each core region 11, the distance L between the beam splitter 20 and the light collecting unit 30, and the light collecting unit 30. NA

Preferably it is. When satisfy | filling the relationship of Formula 1, all the emitted light from the core area | region 11 of the multi-core optical fiber 10 can enter into the condensing part 30. FIG.

  FIGS. 15 and 16 are diagrams illustrating a structure for coupling light from the laser 1 to each core region 11 of the multi-core optical fiber 10. In the structure of FIG. 15, the light output from the laser 1 is branched into the number of cores of the multi-core optical fiber 10 by the beam splitter 2 and coupled to each core region 11. In the structure of FIG. 16, the same number of lasers 1 and fan-in devices 3 as the number of cores are used, and individual laser outputs are incident on each core 11 of a single multi-core optical fiber 10 via the fan-in device 3.

  FIG. 2 is a diagram for explaining an optical transmission device in which the condensing unit 30 couples the emitted light to the incident end of a single core optical fiber 40 capable of propagating only a single propagation mode at the wavelength. . Condensing efficiency can be improved by disposing the condensing unit 30 such as a collimator in front of the single core optical fiber 40. Further, since the single core optical fiber 40 propagates only in a single mode, the emitted light is emitted as one mode field at the exit end of the single core optical fiber 40.

  That is, the present optical transmission apparatus divides the light to be transmitted into a plurality of core regions 11 of the multi-core optical fiber 10 and reduces the optical power in each core, thereby enabling high-power optical transmission such as 10 kW. . Furthermore, since this optical transmission apparatus can collect the light from all the cores of the multi-core optical fiber 10 in the condensing part 30, and can radiate | emit the light of a single mode field from the single core optical fiber 40, its beam quality is high. Light can be emitted.

  FIG. 3 is a diagram illustrating an optical transmission device when a single core optical fiber 30 a is used as the configuration of the light collecting unit 30. That is, the optical transmission device in FIG. 3 is of a type in which the light from the beam splitter 20 is directly incident on the single core optical fiber 30a. In this case, it is sufficient that the single core optical fiber 30a has a length of several millimeters to several centimeters or more, and single mode propagation is guaranteed in the used wavelength band. When a single core optical fiber is used as the condensing unit 30, even if there is a slight misalignment of the incident angle of the light wave, a component that is coupled only to a single mode and does not deteriorate quality does not occur. Irradiation is obtained, which is preferable.

  Here, the shape of the beam splitter 20 changes depending on the arrangement of the core region 11 of the multi-core optical fiber 10, and the more the core is multi-layered, the more complicated the shape of the beam splitter 20 becomes, and the more difficult the manufacturing becomes. Therefore, if the core is arranged concentrically with respect to the center of the multi-core optical fiber 10, the beam splitter 20 can have a simple structure such as a polygonal pyramid, which is preferable. At this time, a core may exist at the center of the multi-core optical fiber 10.

FIG. 4 shows the minimum distance L _min between the beam splitter 20 and the condensing unit 30 required for the inter-core distance Λ of the multi-core optical fiber 10 for the optical transmission apparatus of FIG. The horizontal axis is the core / cladding relative refractive index difference in the single-core optical fiber 30a. It can be seen that L _min increases as the distance Λ increases and the relative refractive index difference of the single core optical fiber 30a decreases. If the distance between the cores of the multi-core optical fiber 10 is the same, the beam splitter 20 can have a simple structure, which is preferable from the viewpoint of beam splitter processing.

  On the other hand, the single core optical fiber 30a needs to have a relatively large effective area in order to efficiently connect to the multicore optical fiber 10. In order to increase the effective area, it is essential to reduce the relative refractive index difference. For example, a material having a very small NA such as a relative refractive index difference of 0.1% or less is required (NA and relative refractive index difference are proportional to each other). To do.)

FIG. 5 is a cross-sectional view of the multi-core optical fiber 10, and FIG. 6 is a cross-sectional view of the tape-type multi-core fiber. The tape type multi-core fiber has a coating region on the outer periphery of the clad of each fiber, and the distance between fibers (inter-core distance) is an outer diameter including the coating. A conventional single-core optical fiber has a coating outer diameter of at least 250 μm, and the one that guides high-power light ranges from 500 to 1000 μm. Therefore, the inter-core distance Λ of the tape-type multicore fiber is 250 μm or more. When a tape-type multi-core fiber is used for optical transmission, if the relative refractive index difference of the single core optical fiber 30a of the condensing unit 30 is 0.01%, L _{min of} about 1 cm is required from FIG. From the curve of Λ = 250 μm.)

When the light emitted from the optical fiber is collected by a spatial system, the focal length is about several mm. For example, as shown in Non-Patent Document 4, when the distance from the emitting end to the light collecting portion is as long as several millimeters or more, the coupling loss increases. Furthermore, loss increases easily due to disturbances such as vibration. For this reason, when light is dispersed and transmitted through a tape-type multicore fiber as shown in FIG. 6, since L _min = 1 cm, a large coupling loss occurs.

On the other hand, since each core shares a single clad, the multi-core optical fiber 10 can efficiently accommodate a plurality of cores and can be arranged with a distance between cores of about several tens of μm. For example, when the distance between the cores is 50 μm, L _min can be designed in a very short region of 1.6 mm or less. Therefore, the above-described coupling loss can be reduced. Since the coupling loss is reduced, the optical power can be reduced, and measures against burning of the fiber end face due to water cooling or the like can be reduced.

  FIG. 7 is a diagram showing the upper limit value of the optical power that can be propagated per core. In high-power optical transmission, stimulated Raman scattering (SRS) light generated in the fiber becomes a limiting factor, and optical power that exceeds the SRS generation threshold cannot propagate. As shown in the figure, in order to propagate higher optical power, it is necessary to enlarge the effective area or limit the fiber length.

FIG. 8 shows the conditions of the core structure of an optical fiber that enables single mode transmission at a wavelength of 1060 nm. The horizontal axis is the core diameter, and the vertical axis is the relative refractive index difference between the core and the clad. The solid line is a condition for the cutoff wavelength, and means that the optical fiber operates in the single mode or the fundamental mode LP ₀₁ and the first higher-order mode LP ₁₁ in the region below the solid line. A broken line is a condition for bending loss, and means that when a predetermined bending radius (R = 140 mm, R = 500 mm), the loss can be suppressed to 0.01 dB or less per 100 m of fiber length in a region beyond the broken line.

For example, when the optical fiber is operated in a single mode at a wavelength of 1060 nm and the bending loss is 0.01 dB or less per 100 m of the fiber length at the bending radius R = 140 mm, the core diameter needs to be 19 μm or less (see region D). ). Further, the LP ₀₁ mode and the LP ₁₁ mode use the fact that coupling between modes is less likely to occur, and excite only the LP ₀₁ mode in a region where both can propagate to perform effective single mode transmission, thereby providing a condition for the core structure. Can be relaxed. For example, the core diameter can be expanded to 33 μm with respect to the bending loss condition (see region D ′).

Even LP ₁₁ mode is slightly excited by axial displacement and the mode field mismatch of the entrance end of the single-core optical fiber 30a, since LP ₁₁ mode generated in the incident end to leaks in a single core optical fiber 30a, the single The quality of the beam emitted from the one-core optical fiber 30a is not deteriorated.

FIG. 9 is a diagram illustrating the relationship between the core diameter and the effective cross-sectional area. The horizontal axis is the core diameter, and the vertical axis is the effective cross-sectional area. A solid line represents a case where the optical fiber guarantees single mode operation, and a broken line represents a case where the optical fiber can propagate in two modes and performs effective single mode transmission. In the case of single mode operation, the core diameter needs to be 18 μm or less, so the effective area is about 300 μm ² or less.

If this is applied to FIG. 7, transmission of 0.5 kW per core is possible with a fiber length of 100 m. When performing effective single mode transmission, the core diameter can be expanded to 33 μm, so the effective area can be increased to about 600 μm ² , and transmission of 1 kW per core is possible at 100 m. By using the multi-core optical fiber 10 using these core structures, it is possible to increase the power that can be transmitted by the number of cores.

  FIG. 10 is a diagram illustrating an example of the relationship between the optical power that can be transmitted by the multi-core optical fiber 10 and the cladding diameter. In this example, the wavelength was 1060 nm, the length of the multi-core optical fiber 10 was 100 m, the core diameter and the relative refractive index difference were 30 μm and 0.06%, respectively, and the core interval was 50 μm. This core structure is an area in which two modes can be transmitted from FIG. 8, and an effective single mode transmission is assumed.

  The required cladding diameter of the multi-core optical fiber 10 is set to be 2.5 times or more the mode field diameter from the center of the outermost core region from the viewpoint of suppressing an increase in transmission loss in cores other than the center. . A solid line and a broken line in the drawing respectively represent a case where the core is arranged only concentrically and a case where the core is arranged concentrically in the center.

  The following can be understood from FIG. When this core structure is arranged only concentrically, it is possible to arrange a plurality of cores with a clad diameter of 110 μm or more, and when the clad diameter is 270 μm or more, 13 cores can be arranged and high power of 10 kW Light can be transmitted 100 meters. When the core structure is arranged concentrically with the center, seven or more cores can be arranged with a cladding diameter of 160 μm or more, and high power light of 5 kW or more can be transmitted.

  The core structure, inter-core pitch, and fiber length shown in FIG. 10 are examples. By using the multi-core optical fiber 10 having another core structure, inter-core pitch, and fiber length, it becomes possible to transmit optical power significantly higher than that of a single-core optical fiber over a long distance.

  FIG. 11 and FIG. 12 are diagrams for explaining a structural example of the refractive index distribution of the core of the multi-core optical fiber 10. 4 to 10, the multi-core optical fiber having a simple step-type refractive index distribution core has been described. However, it has a W-type core as shown in FIG. 11 or a trench-type refractive index core as shown in FIG. A multi-core optical fiber can also be employed. By using a multi-core optical fiber having a core with these refractive index profiles, the optical transmission device can expand the effective area while performing single mode transmission, and can transmit light with high power and good beam quality. .

  FIGS. 13 and 14 are diagrams illustrating an example of the structure of the multi-core optical fiber 10. FIG. 13 illustrates a hole assist structure using both the refractive index difference and the holes. FIG. 14 illustrates a photonic crystal fiber structure in which multiple holes are provided in a single material. By adopting a hole-assist structure or photonic crystal fiber structure as a multi-core optical fiber, the optical transmission device can expand the effective area while performing single-mode transmission, and transmit light with high power and good beam quality. can do.

FIG. 17 is a diagram illustrating the structure of the optical transmission apparatus according to this embodiment. This optical transmission device
Transmitting light waves of different wavelengths to each core region 11 of the multi-core optical fiber 10,
A wavelength multiplexer 50 that combines the light waves emitted from the multi-core optical fiber 10 and collected by the beam splitter 20 is provided, and the combined light is collected at a predetermined position and output as light of one mode field. A light collecting unit 30 is provided.

  In this configuration, light waves (λ 1, λ 2) having different wavelengths are incident on each core 11 of the multi-core optical fiber 10. The light emitted from each core 11 is condensed on the wavelength multiplexer 50 via the beam splitter 20 and combined into one light wave. The combined light combined by the wavelength multiplexer 50 is coupled to the condensing unit 30. The condensing unit 30 condenses the combined light and couples it to the single core optical fiber 40 as one mode field. Although the condensing unit 30 may be unnecessary depending on the configuration, the efficiency of coupling each light wave to the single core optical fiber 40 is improved by arranging the condensing unit 30. The optical transmission apparatus of FIG. 17 has a different wavelength for each core of the multi-core optical fiber 10, so that interference between the cores during multi-core optical fiber transmission is extremely small, and a stable optical output is obtained.

  The present invention can be used for industrial processing using high power light.

1: Laser 2: Beam splitter 3: Fan-in device 10: Multi-core optical fiber 11: Core region 20: Beam splitter 30: Condenser 30a: Single core optical fiber 40: Single core optical fiber 41: Core region 50: Wavelength multiplexer

Claims (7)

A multi-core optical fiber having a plurality of core regions capable of transmitting light incident on a single optical fiber while maintaining a single mode condition ;
A beam splitter that focuses light emitted from the core region of the multi-core optical fiber at a predetermined position;
A light collecting unit arranged at the predetermined position and outputting the light collected by the beam splitter as light of one mode field;
An optical transmission device comprising :
An optical transmission device characterized in that by condensing light transmitted in a single mode in the core region, the optical power that can be transmitted in a single mode is increased by the number of the core region . Among the distances between the fiber center of the multi-core optical fiber and the center of each core region, the longest interval Λ, the distance L between the beam splitter and the condensing unit, and the numerical aperture NA of the condensing unit are:

The optical transmission device according to claim 1, wherein:   3. The optical transmission device according to claim 1, wherein the core region of the multi-core optical fiber is arranged concentrically or concentrically and in the center of the fiber.   4. The optical transmission device according to claim 1, wherein the multi-core optical fiber has a refractive index distribution between a clad and a core of a W type or a trench type. 5.   The optical transmission device according to claim 1, wherein the multi-core optical fiber has a hole assist structure. The condensing part is
The light having different wavelengths transmitted for each of the core regions of the multi-core optical fiber is multiplexed, and the combined light is condensed at a predetermined position and output as light of one mode field. The optical transmission device according to claim 1. Branching light from a light source and coupling light incident on a single optical fiber to each core of a multi-core optical fiber having a plurality of core regions capable of transmitting while maintaining a single mode condition ;
Condensing the light emitted from the core region of the multi-core optical fiber at a predetermined position;
An optical transmission method for outputting light as one mode field ,
An optical transmission method characterized in that by condensing light transmitted in a single mode in the core region, the optical power that can be transmitted in a single mode is increased by the number of the core region .

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