Stair-type pure quartz core number mode fiber and optical transmission system

Abstract

To provide a stair-type pure quartz core number mode fiber and an optical transmission system capable of simultaneously reducing an inter-mode delay difference (DMD) and a propagation loss.SOLUTION: This stair-type pure quartz core number mode fiber comprises: an inner core made of a quartz glass; an outer core lower in refractive index than the inner core and surrounding the inner core; and a clad lower in refractive index than the outer core and surrounding the entire outer core. The effective cutoff wavelength of LP21 or LP02 is 1530 nm or lower, and the mode field diameter of a wavelength 1550 nm is 10-13 μm. A flexure loss at the wavelength of 1625 μm, a flexure diameter of 30 mm, and a bending number of times of 100 is 2 dB or lower, a Rayleigh scattering loss is 0.150 dB / km or lower, and the absolute value of an inter-mode delay difference is 1 ns / km or lower.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to stepped pure silica core few-mode fibers and optical transmission systems. [Background technology]

[0002] Space division multiplexing technology is being actively researched to realize future large-capacity optical networks. Among space division multiplexing technologies, few-mode fiber (FMF) transmission, which propagates multiple modes in one core, can increase spatial multiplexing, and is expected to lead to a dramatic increase in the transmission capacity per optical fiber.

[0003] In FMF transmission, the larger the differential mode delay (DMD) is, the greater the signal processing load for signal restoration becomes. Non-Patent Documents 1 and 2 disclose that in FMF that employs a graded index (GI) type in which the refractive index changes radially in the radial direction, propagation loss equivalent to that of existing single mode fiber (SMF) occurs.

[0004] Non-Patent Document 3 discloses that the propagation loss can be reduced by using a pure silica-core fiber (PSCF) whose core is made of pure silica glass. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] P. Sillard et al., “Low-differential-mode-group-delay 9-LP-mode fiber,” J. Lightw. Technol., vol. 34, no. 2, pp. 425-430 (2016). [Non-Patent Document 2] T. Sakamoto, et al. "Differential mode delay managed transmission line for WDM-MIMO system using multi-step index fiber." J. Lightw. Technol. 30.17 : 2783-2787, (2012). [Non-Patent Document 3] T. Kato et,al., ``Ultra low nonlinearity low loss pure silica core fiber'', OECC, vol. 2, pp. 1575-1576 (1999). Summary of the Invention [Problem to be solved by the invention]

[0006] However, when trying to apply the technology of Non-Patent Document 3 to the FMF disclosed in Non-Patent Documents 1 and 2, it is not easy to control the refractive index distribution in the PSCF. Therefore, there is a problem in that it is difficult to manufacture an FMF that can simultaneously reduce the DMD and the propagation loss.

[0007] The present disclosure has been made in view of the above problems, and has an object to provide a stepped pure silica core few-mode fiber and an optical transmission system capable of simultaneously reducing differential mode delay (DMD) and propagation loss. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, a stepped pure silica core few-mode fiber according to one embodiment of the present disclosure includes an inner core made of silica glass, an outer core having a lower refractive index than the inner core and surrounding the inner core, and a cladding having a lower refractive index than the outer core and surrounding the entire outer core. 21 Or LP 02The effective cutoff wavelength is 1530 nm or less, and the mode field diameter at a wavelength of 1550 nm is 10 to 13 μm. At a wavelength of 1625 μm, a bending diameter of 30 mm, and 100 bending times, the bending loss is 2 dB or less, the Rayleigh scattering loss is 0.150 dB / km or less, and the absolute value of the differential modal delay is 1 ns / km or less.

[0009] Moreover, an optical transmission system according to one aspect of the present disclosure includes the stepped pure quartz-core few-mode fiber of the present disclosure, a transmitter that generates signal light, a mode multiplexer that converts the signal light into a light wave mode that propagates through the stepped pure quartz-core few-mode fiber, an optical coupling unit that is arranged on one end of the stepped pure quartz-core few-mode fiber and inputs input light including a light wave mode to the inner core and the outer core, a light extraction unit that is arranged on the other end of the stepped pure quartz-core few-mode fiber and extracts output light from the inner core and the outer core, a mode separator that separates the light wave mode from the output light to extract signal light, and a receiver that receives the signal light from the mode separator. Effect of the Invention

[0010] According to the present disclosure, it is possible to provide a stepped pure silica core few-mode fiber and an optical transmission system capable of simultaneously reducing differential modal delay (DMD) and propagation loss. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a cross-sectional view showing the structure of a stepped pure silica core few-mode fiber according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 illustrates an example of a refractive index profile in a stepped pure silica core few-mode fiber according to an embodiment of the present disclosure. [Figure 3A] FIG. 1 shows the core structure dependence of DMD, Rayleigh scattering loss, and bending loss. [Figure 3B] FIG. 1 shows the core structure dependence of DMD, Rayleigh scattering loss, and bending loss. [Figure 3C] FIG. 1 shows the core structure dependence of DMD, Rayleigh scattering loss, and bending loss. [Figure 4A] FIG. 13 shows the MFD dependence of the coefficient on the radius ratio in the boundary curve of bending loss. [Figure 4B] FIG. 13 shows the MFD dependence of the coefficient on the radius ratio in the boundary curve of Rayleigh scattering loss. [Figure 4C] FIG. 13 shows the MFD dependence of coefficient on radius ratio in boundary curve of DMD. [Figure 4D] FIG. 13 shows the MFD dependence of coefficient on radius ratio in boundary curve of DMD. [Diagram 5] FIG. 13 shows the MFD dependence of the coefficient on the radius ratio in the boundary curve of bending loss. [Figure 6] FIG. 13 shows the MFD dependence of the coefficient on the radius ratio in the boundary curve of bending loss. [Figure 7] FIG. 13 is a cross-sectional view illustrating a modified example of a stepped pure silica core few-mode fiber according to an embodiment of the present disclosure. [Figure 8] FIG. 1 is a diagram illustrating a configuration of an optical transmission system using a stepped pure silica core few-mode fiber according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Next, an embodiment of the present disclosure will be described in detail with reference to the drawings. In the description, the same components are designated by the same reference numerals and duplicated description will be omitted.

[0013] [1. Configuration of step-type pure silica core few-mode fiber] 1 is a cross-sectional view showing a structure of a stepped pure silica core few-mode fiber according to an embodiment of the present disclosure. Hereinafter, the stepped pure silica core few-mode fiber according to the present embodiment will be referred to as an optical fiber FB.

[0014] 1, a cross-sectional view of a cross section perpendicular to the extending direction (direction of the central axis) of the optical fiber FB is shown. The optical fiber FB includes an inner core 10 and an outer core 20 through which a light wave mode propagates, and a cladding 30.

[0015] The inner core 10 is made of silica glass. In particular, in order to reduce the propagation loss of the light wave mode, the inner core 10 may be made of pure silica glass. For example, the cross-sectional shape of the inner core 10 is circular.

[0016] The outer core 20 has a lower refractive index than the inner core 10 and is arranged to surround the inner core 10. The outer core 20 is arranged for each inner core 10 so as to surround the inner core 10. For example, the inner core 10 and the outer core 20 share a common central axis, and the diameter of the outer core 20 is larger than the diameter of the inner core 10.

[0017] The cladding 30 has a lower refractive index than the outer cores 20, and is disposed so as to surround all of the outer cores 20. For example, the cross-sectional shape of the cladding 30 is circular. The diameter of the cladding 30 may be 126 μm or less. By making the cladding diameter the same as that of an existing single mode fiber (SMF), compatibility with conventional connectors, cabling technologies, etc. can be improved, and the manufacturing cost can be prevented from increasing.

[0018] The outer core 20 may be doped with fluorine in order to make the refractive index of the outer core 20 lower than that of the inner core 10. Similarly, the outer core 20 may be doped with fluorine in order to make the refractive index of the cladding 30 lower than that of the outer core 20. The substance doped to lower the refractive index is not limited to fluorine (F) and may be any substance that has the effect of lowering the refractive index. For example, diboron trioxide (B2O3) may be used.

[0019] The optical fiber FB may include any number of inner cores 10 and outer cores 20, which may be one or more. That is, the optical fiber FB may be a multi-core optical fiber. A multi-core optical fiber FB will be described later as a modified example.

[0020] A light ray propagating through the optical fiber FB propagates by repeated total reflection in the inner core 10 and the outer core 20. Here, the inclination of the light ray is not allowed to be any value, and only light rays having a specific angle can propagate. Such a form of light ray is called a light wave mode. The light wave modes that can propagate through the inner core 10 and the outer core 20 can be obtained, for example, by electromagnetic field analysis.

[0021] For example, the light wave mode includes a linearly polarized mode. ml Here, "m" is a mode order indicating how the intensity distribution of the transverse electric field of a light beam propagating through the inner core 10 and the outer core 20 changes in the angular direction, and "l" is a mode order indicating how the intensity distribution of the transverse electric field of a light beam propagating through the inner core 10 and the outer core 20 changes in the radial direction.

[0022] In this embodiment, the inner core 10 and the outer core 20 have at least one light wave mode. For example, the inner core 10 and the outer core 20 have an "LP 01 " and "LP 11 The light wave modes of the inner core 10 and the outer core 20 are not limited to the examples given here.

[0023] Fig. 2 is a diagram showing an example of a refractive index profile in a stepped pure silica core few-mode fiber according to an embodiment of the present disclosure. In Fig. 2, the vertical axis indicates the refractive index, and the horizontal axis indicates the radial position. For example, the refractive index profile in a cross section passing through the central axis of the optical fiber FB in Fig. 1 has a stepped structure as shown in Fig. 2.

[0024] For the following explanation, the radius of the inner core 10 is defined as "r1" (unit: μm), and the radius of the outer core 20 is defined as "r2" (unit: μm). The relative refractive index difference of the outer core 20 with respect to the inner core 10 is defined as Δ1, and the relative refractive index difference of the cladding 30 with respect to the inner core 10 is defined as Δ2.

[0025] The radius ratio and the relative refractive index difference ratio of the inner core 10 and the outer core 20 are R a , R Δ More specifically, "R a =r1 / r2," "R a =Δ1 / Δ2".

[0026] [2. Requirements for optical fiber FB] The radius ratio of the inner core 10 to the outer core 20, "R a " and the relative refractive index difference ratio "R Δ In order to consider the requirements imposed on the "Fiber-optic Cable," structural calculations were carried out for various optical fiber FBs.

[0027] As a prerequisite, the wavelength bands of optical communication transmitted by the optical fiber FB are assumed to be C-band and L-band, and furthermore, 2LP mode transmission is assumed. 21 Or LP 02 The design range was assumed to be such that the effective cutoff wavelength of the fiber was 1530 nm or less. This is in accordance with the ITU-T G.654 standard (Reference: ITU-T, G.654, “Characteristics of a cut-off shifted single-mode optical fibre and cable”, (11 / 2016).).

[0028] 3A to 3C are diagrams showing the core structure dependence of DMD, Rayleigh scattering loss, and bending loss. a ,R Δ In the combination of the core radius A2 and the relative refractive index difference Δ2, the mode field diameter (MFD) is 11.5 μm and the LP 21 Or LP 02 Cutoff wavelength (λ c ) can be limited by the condition that the wavelength is 1530 nm.

[0029] The ranges of A2 and Δ2 in the design range shown in FIG. 3A are 6.89 μm≦a2≦10.8 μm and 0.332%≦Δ2≦0.600%, respectively.

[0030] Furthermore, the scattering loss coefficient is 0.2 dB (reference: M. Ohashi et al., “Optical Loss Property of Silica-Based Single-Mode Fibers”, JLT, vol. 10, No. 5, pp. 539-543 (1992)). The bending loss was calculated under the conditions of a wavelength of 1625 nm, a bending diameter of 30 mm, and 100 turns (100 bending times), and the bending loss is plotted as a common logarithm.

[0031] Here, bending loss α B The requirements of α B ≦2dB / 100turn (log 10 2≦0.3), which complies with the ITU-G.654.D standard. In FIG. 3A, the bending loss α B The region that satisfies the above requirements is given by the following formula 1.

number

[0032] In addition, the Rayleigh scattering loss α R The requirements of α R ≦0.150 dB / km. This is in accordance with the loss value of existing PSCFs at a wavelength of 1.55 μm. In FIG. 3A, the Rayleigh scattering loss α R The region that satisfies the above requirements is given by the following formula 2.

number

[0033] Furthermore, if the required condition for the differential mode delay (DMD) τ is −1 ns / km≦τ≦1 ns / km, the region in FIG. 3A that satisfies the required condition for the differential mode delay (DMD) τ is expressed by the following Equation 3.

number

[0034] Next, Fig. 3B shows the core structure dependence of DMD, Rayleigh scattering loss, and bending loss. c The design condition is =1530 nm.

[0035] The ranges of a2 and Δ2 in the design range shown in Fig. 3B are 5.93μm ≤ a2 ≤ 7.92μm and 0.623% ≤ Δ2 ≤ 0.789%, respectively.

[0036] Here, bending loss α B The requirements of α B ≦2dB / 100turn (log 10 2≦0.3), which complies with the ITU-G.654.D standard. In FIG. 3B, the bending loss α B The region that satisfies the above requirements is given by the following Equation 4.

number

[0037] In addition, the Rayleigh scattering loss α R The requirements of α R ≦0.150 dB / km. This is in accordance with the loss value of existing PSCFs at a wavelength of 1.55 μm. In FIG. 3B, the Rayleigh scattering loss α R The region that satisfies the above requirements is given by the following Equation 5.

number

[0038] Furthermore, if the required condition for the differential mode delay (DMD) τ is −1 ns / km≦τ≦1 ns / km, the region in FIG. 3B that satisfies the required condition for the differential mode delay (DMD) τ is expressed by the following Equation 6.

number

[0039] Furthermore, FIG. 3C is a diagram showing the core structure dependence of DMD, Rayleigh scattering loss, and bending loss. In Fig. 3C, MFD = 13 μm, λ c The design condition is =1530 nm.

[0040] The ranges of a2 and Δ2 in the design range shown in Fig. 3C are 7.72 μm ≤ a2 ≤ 10.1 μm and 0.390% ≤ Δ2 ≤ 0.574%, respectively.

[0041] Here, bending loss α B The requirements of α B ≦2dB / 100turn (log 10 2≦0.3), which complies with the ITU-G.654.D standard. In FIG. 3C, the bending loss α B The region that satisfies the above requirements is given by the following formula 7.

number

[0042] In addition, the Rayleigh scattering loss α R The requirements of α R ≦0.150 dB / km. This is in accordance with the loss value of existing PSCFs at a wavelength of 1.55 μm. In FIG. 3C, the Rayleigh scattering loss α R The region that satisfies the above requirements is given by the following Equation 8.

number

[0043] Furthermore, if the required condition for the differential mode delay (DMD) τ is −1 ns / km≦τ≦1 ns / km, the region in FIG. 3C that satisfies the required condition for the differential mode delay (DMD) τ is expressed by the following Equation 9.

number

[0044] [2-1. Bending loss α B Requirements for Bending loss α B On the boundary curve satisfying =2 dB, R ΔIf the coefficients for a3, a2, a1, and a0 are a3, a2, a1, and a0, then R Δ is shown in the following formula 10.

number

[0045] 4A is a diagram showing the MFD dependence of the coefficient related to the radius ratio in the boundary curve of the bending loss. If the MFD is W [μm], the approximation curve of the coefficient is expressed by the following Equation 11.

number

[0046] Therefore, the bending loss requirement α B The region that satisfies ≦2 dB is shown by the following formula 12.

number

[0047] [2-2. Rayleigh scattering loss α R Requirements for Rayleigh scattering loss α R = 0.150 dB / km, R Δ If the coefficients for b3, b2, b1, and b0 are b Δ is shown in the following Equation 13.

number

[0048] 4B is a diagram showing the MFD dependence of the coefficient related to the radius ratio in the boundary curve of the Rayleigh scattering loss. If the MFD is W [μm], the coefficient approximation curve is expressed by the following Equation 14.

number

[0049] Therefore, for any MFD, the Rayleigh scattering loss requirement αR The region that satisfies ≦0.150 dB / km is shown by the following formula 15.

number

[0050] [2-3. Differential Mode Delay (DMD) τ Requirements] On the boundary curve that satisfies the differential mode delay (DMD) τ = 1 ns / km, R Δ If the coefficients for are c3, c2, c1, and c0, then R Δ is shown in the following Equation 16.

number

[0051] 4C is a diagram showing the MFD dependency of the coefficient related to the radius ratio in the boundary curve of the DMD. If the MFD is W [μm], the approximate curve of the coefficient is expressed by the following Equation 17.

number

[0052] On the boundary curve that satisfies the differential mode delay (DMD) τ = -1ns / km, R Δ If the coefficients for are d3, d2, d1, and d0, then R Δ is shown in the following Equation 18.

number

[0053] 4D is a diagram showing the MFD dependency of the coefficient related to the radius ratio in the boundary curve of the DMD. If the MFD is W [μm], the approximate curve of the coefficient is expressed by the following Equation 19.

number

[0054] Therefore, the region that satisfies the DMD requirement −1≦τ≦1 for any MFD is given by the following Equation 20.

number

[0055] [2-4. Bending loss α B Requirements (Other Example 1)] Next, the bending loss α B The requirements of α B ≦0.5dB / 100turn (log 10 (0.5)≦-0.3). This is in accordance with the ITU-G.654.B standard, which assumes long-distance undersea transmission. Bending loss α B = 0.5 dB, R Δ If the coefficients for e3, e2, e1, and e0 are e3, e2, e1, and e0, then R Δ is shown in the following Equation 21.

number

[0056] 5 is a diagram showing the MFD dependence of the coefficient related to the radius ratio in the boundary curve of the bending loss. If the MFD is W [μm], the approximation curve of the coefficient is expressed by the following Equation 22.

number

[0057] Therefore, the bending loss requirement α B The region where ≦0.5 dB is satisfied is shown by the following formula 23.

number

[0058] [2-5. Bending loss α B Requirements (Other Example 2)] Next, if the upper limit of the MFD is set to 12 μm, the bending loss α B The requirements of α B≦0.1dB / 100turn (log 10 (0.1)≦-1). This is in accordance with the ITU-G.654.E standard, which assumes medium- to long-distance terrestrial transmission. Bending loss α B On the boundary curve that satisfies =0.1 dB, R Δ If the coefficients for f3, f2, f1, and f0 are f3, f2, f1, and f0, then R Δ is shown in the following Equation 24.

number

[0059] 6 is a diagram showing the MFD dependence of the coefficient related to the radius ratio in the boundary curve of the bending loss. If the MFD is W [μm], the approximation curve of the coefficient is expressed by the following Equation 25.

number

[0060] Therefore, the bending loss requirement α B The region where ≦0.1 dB is satisfied is shown by the following formula 26.

number

[0061] [3. Modifications of step-type pure silica core few-mode fiber] 7 is a cross-sectional view showing a modified example of a stepped pure silica core few-mode fiber according to an embodiment of the present disclosure. In FIG. 7, the optical fiber FB has four pairs of inner cores 10 and outer cores 20, and is made multi-core.

[0062] 7, the inner core 10 is also made of silica glass, and the outer core 20 has a lower refractive index than the inner core 10 and is arranged to surround the inner core 10. The cladding 30 has a lower refractive index than the outer core 20 and is arranged to surround all of the outer cores 20. In FIG. 7, the optical fiber FB includes multiple inner cores 10 and multiple outer cores 20 in the same number as the inner cores 10.

[0063] The number of inner cores 10 and outer cores 20 included in the optical fiber FB is not limited to the example shown in Fig. 7. The optical fiber FB may include any number of inner cores 10 and outer cores 20, which is one or more. By configuring each of the cores included in the optical fiber FB with an inner core 10 and an outer core 20, it is possible to increase spatial multiplexing.

[0064] [4. Optical transmission system configuration] 8 is a diagram showing the configuration of an optical transmission system using a stepped pure silica core few-mode fiber according to an embodiment of the present disclosure. The optical transmission system 100 includes an optical fiber FB according to an embodiment of the present disclosure, a transmitter ST, a mode multiplexer MT, an optical coupler FI, a light extractor FO, a mode separator MR, and a receiver SR.

[0065] The transmitter ST generates a signal light. The number of the transmitters ST may be any number equal to or greater than one.

[0066] The mode multiplexer MT converts the signal light into a lightwave mode. The number of mode multiplexers MT may be one or more.

[0067] The optical coupling unit FI is disposed on one end side of the optical fiber FB, and inputs input light including a lightwave mode to the inner core 10 and the outer core 20. The optical coupling unit FI may be connected to a mode multiplexer MT, and introduce the output of each mode multiplexer MT to the inner core 10 and the outer core 20 of the optical fiber FB. Alternatively, the optical coupling unit FI may be a MUX (Multiplexer). A MUX is a circuit that converts multiple input signals into one output signal. A MUX selects only one of multiple input signals to be the output signal, and the selected signal is determined by a control signal called a selection signal.

[0068] The light extraction unit FO is disposed on the other end side of the optical fiber FB and extracts output light from the inner core 10 and the outer core 20. The light extraction unit FO may be connected to the mode separator MR and introduce the outputs of the inner core 10 and the outer core 20 of the optical fiber FB to each mode multiplexer MT. Alternatively, the light extraction unit FO may be a DEMUX (Demultiplexer). A DEMUX has the opposite function to a MUX and is a circuit that distributes one input signal to multiple output signals. A DEMUX distributes an input signal to multiple outputs according to a selection signal.

[0069] The mode separator MR separates the lightwave mode from the output light to extract the signal light. The number of mode separators MR may be any number equal to or greater than one.

[0070] The receiver SR receives the signal light from the mode separator MR. The number of receivers SR may be multiple. The number of receivers SR may be any number equal to or greater than one.

[0071] [Effects of the embodiment] As described above in detail, the stepped pure silica core few-mode fiber according to this embodiment includes an inner core made of silica glass, an outer core having a lower refractive index than the inner core and surrounding the inner core, and a cladding having a lower refractive index than the outer core and surrounding the entire outer core. 21 Or LP 02 The effective cutoff wavelength is 1530 nm or less, and the mode field diameter at a wavelength of 1550 nm is 10 to 13 μm. At a wavelength of 1625 μm, a bending diameter of 30 mm, and 100 bending times, the bending loss is 2 dB or less, the Rayleigh scattering loss is 0.150 dB / km or less, and the absolute value of the differential modal delay is 1 ns / km or less.

[0072] Here, the radius ratio and the relative refractive index difference ratio of the inner core to the outer core are R a , R Δ and the mode field diameter is W,

number

[0073] This makes it possible to provide a stepped pure silica core few-mode fiber that can simultaneously reduce differential mode delay (DMD) and propagation loss. In particular, it can reduce bending loss to 2 dB or less and comply with the ITU-G.654.D standard.

[0074] In addition, because the DMD is smaller, the load on the signal processing side can be reduced. Because the signal processing of the devices connected to both ends of the optical fiber is smaller, the cost of the transmission equipment can also be reduced. In addition, because the propagation loss is smaller, the signal can propagate farther. As a result, the cost of the transmission equipment, especially the device that transmits the signal, can be reduced.

[0075] In addition, the stepped pure silica core few-mode fiber according to this embodiment has the following features:

number

[0076] In addition, the stepped pure silica core few-mode fiber according to this embodiment has the following features:

number

[0077] Furthermore, the stepped pure silica core few-mode fiber according to this embodiment has the following features:

number

[0078] In addition, the stepped pure silica core few-mode fiber according to this embodiment has the following features:

number

[0079] Furthermore, the stepped pure silica core few-mode fiber according to the present embodiment may have a cladding diameter of 126 μm or less. This allows the stepped pure silica core few-mode fiber to have a standard cladding diameter equivalent to that of an existing single mode fiber (SMF). As a result, it is possible to improve compatibility with conventional connectors and cabling technologies, and to suppress the increase in manufacturing costs.

[0080] In addition, the stepped pure silica core few-mode fiber according to this embodiment may have multiple inner cores and multiple outer cores, the same number as the inner cores, and the cladding may surround the multiple outer cores. This can increase the spatial multiplexing and improve the transmission capacity per optical fiber. At the same time, the DMD and propagation loss can be reduced.

[0081] Furthermore, the optical transmission system according to the present embodiment includes the stepped pure silica few-core mode fiber of the present disclosure, a transmitter that generates signal light, a mode multiplexer that converts the signal light into a light wave mode that propagates through the stepped pure silica few-core mode fiber, an optical coupling unit that is arranged at one end of the stepped pure silica few-core mode fiber and inputs input light including a light wave mode to the inner core and the outer core, a light extraction unit that is arranged at the other end of the stepped pure silica few-core mode fiber and extracts output light from the inner core and the outer core, a mode separator that separates the light wave mode from the output light to extract signal light, and a receiver that receives the signal light from the mode separator.

[0082] This makes it possible to provide an optical transmission system that can simultaneously reduce the DMD and propagation loss. In addition, because the DMD is smaller, the load on the signal processing side can be reduced. The cost of the transmission device can also be reduced because the signal processing of the devices connected to both ends of the optical fiber is reduced. In addition, because the propagation loss is smaller, the signal can propagate further. As a result, the cost of the transmission device, especially the device that transmits the signal, can be reduced.

[0083] Although the contents of the present disclosure have been described above along with the embodiments, the present disclosure is not limited to these descriptions, and various modifications and improvements are possible, which are obvious to those skilled in the art. The descriptions and drawings forming a part of this disclosure should not be understood as limiting the present disclosure. Various alternative embodiments, examples, and operating techniques will be apparent to those skilled in the art from this disclosure.

[0084] Of course, the present disclosure includes various embodiments not described herein. Therefore, the technical scope of the present disclosure is defined only by the invention-specifying matters related to the scope of the claims appropriate from the above description. [Explanation of symbols]

[0085] 10 Inner Core 20 Outer Core 30 Clad 100 Optical Transmission System FB optical fiber (step-type pure silica core few-mode fiber) FI optical coupling section FO light extraction section MR mode separator MT mode multiplexer SR Receiver ST Transmitter

Claims

1. an inner core made of quartz glass; an outer core having a refractive index lower than that of the inner core and surrounding the inner core; a cladding having a refractive index lower than that of the outer core and surrounding the entire outer core; Equipped with LP ２１ Or LP ０２ The effective cutoff wavelength is 1530 nm or less, The mode field diameter at a wavelength of 1550 nm is 10 to 13 μm, The bending loss at a wavelength of 1625 μm, a bending diameter of 30 mm, and 100 bending times is 2 dB or less, Rayleigh scattering loss is 0.150 dB / km or less; The absolute value of the differential mode delay is 1 ns / km or less; The radius ratio and the relative refractive index difference ratio of the inner core to the outer core are respectively R ａ , R Δ and the mode field diameter is W, [0010] This pure silica core multimode fiber satisfies the above requirements. 【Request 2】 【Number 2】 2. The stepped pure silica core few-mode fiber according to claim 1, wherein the Rayleigh scattering loss is 0.150 dB / km or less by satisfying the above. 【Request 3】 【Number 3】 2. The stepped pure silica core few-mode fiber according to claim 1, wherein the absolute value of the differential mode delay is 1 ns / km or less by satisfying the above. 【Request 4】 【Number 4】 2. The stepped pure silica core few-mode fiber according to claim 1, wherein the bending loss is 0.5 dB or less by satisfying the above. 【Request 5】 【Number 5】 2. The stepped pure silica core few-mode fiber according to claim 1, wherein the bending loss is 0.1 dB or less when the mode field diameter is 10 to 12 μm by satisfying the above.

6. 2. The stepped pure silica core few-mode fiber according to claim 1, wherein the cladding has a diameter of 126 μm or less.

7. A plurality of the inner cores and a plurality of the outer cores, the number of which is equal to the number of the inner cores, The stepped pure silica core few-mode fiber of claim 1 , wherein the cladding surrounds a plurality of the outer cores.

8. A stepped pure silica core few-mode fiber according to any one of claims 1 to 7; A transmitter for generating a signal light; a mode multiplexer that converts the signal light into a light wave mode that propagates through the stepped pure silica core few-mode fiber; an optical coupling section disposed on one end side of the stepped pure silica core few-mode fiber, for inputting input light including the light wave mode into the inner core and the outer core; a light extraction section disposed on the other end side of the stepped pure silica core few-mode fiber and configured to extract output light from the inner core and the outer core; a mode separator that separates the lightwave mode from the output light to extract the signal light; a receiver for receiving the signal light from the mode separator; An optical transmission system comprising:

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