Stair type multi-mode fiber and optical transmission system
The stepped multimode fiber design addresses the issue of large inter-modal loss differences by using a germanium-doped inner core, a pure silica outer core, and a fluorine-doped cladding, achieving reduced inter-modal delay, loss, and enhanced channel capacity.
Patent Information
- Application Number
- JP2023193541
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2023-11-14
- Publication Date
- 2025-05-26
AI Technical Summary
Existing stepped pure silica core multimode optical fibers face challenges with large inter-modal loss differences, which hinder the simultaneous reduction of inter-modal delay difference and propagation loss.
A stepped multimode fiber design featuring an inner core made of germanium-doped silica, an outer core made of pure silica glass, and a cladding with a lower refractive index than the outer core, optimized to reduce the difference in loss coefficients between modes to less than 0.005 dB/km.
This design effectively reduces inter-modal delay difference, propagation loss, and inter-modal loss difference, enhancing channel capacity and compatibility with conventional fiber technologies while minimizing manufacturing costs.
Smart Images
Figure 2025080419000001_ABST
Abstract
Description
Technical Field
[0001] The present disclosure relates to a stepped multimode fiber and an optical transmission system.
Background Art
[0002] For extending the length of a multimode optical fiber (FMF) effective for improving spatial multiplexing, it is important to reduce the intermodal delay difference (DMD) and propagation loss. Non-Patent Document 1 discloses a stepped pure silica core FMF aimed at enhancing the affinity with pure silica glass by making the refractive index distribution a simple step structure and simultaneously reducing DMD and propagation loss.
Prior Art Documents
Non-Patent Documents
[0003]
Non-Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0004] However, according to the stepped pure silica core FMF disclosed in Non-Patent Document 1, there is a problem that the intermodal loss difference (DMA) becomes large.
[0005] The present disclosure has been made in view of the above problems. An object thereof is to provide a stepped multimode fiber and an optical transmission system capable of simultaneously reducing the inter-mode delay difference (DMD) and the propagation loss, and further reducing the inter-mode loss difference (DMA).
Means for Solving the Problems
[0006] In order to solve the above-described problems, a stepped multimode fiber according to an aspect of the present disclosure includes an inner core, an outer core made of silica glass having a refractive index lower than that of the inner core and surrounding the inner core, and a cladding having a refractive index lower than that of the outer core and surrounding all the outer cores. Here, two or more modes propagate at the operating wavelength, and the difference in loss coefficient between the modes is less than 0.005 dB / km.
[0007] Moreover, an optical transmission system according to an aspect of the present disclosure includes the stepped multimode fiber of the present disclosure, a transmitter that generates signal light, a mode multiplexer that converts the signal light into an optical wave mode that propagates through the stepped multimode fiber, an optical coupler that is disposed on one end side of the stepped multimode fiber and inputs input light including the optical wave mode into the inner core and the outer core, an optical extraction unit that is disposed on the other end side of the stepped multimode fiber and extracts output light from the inner core and the outer core, a mode separator that separates the optical wave mode from the output light and extracts the signal light, and a receiver that receives the signal light from the mode separator.
Advantages of the Invention
[0008] According to the present disclosure, it is possible to provide a stepped multimode fiber and an optical transmission system capable of simultaneously reducing the inter-mode delay difference (DMD) and the propagation loss, and further reducing the inter-mode loss difference (DMA).
Brief Description of the Drawings
[0009]
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
[0010] Next, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, the same components are denoted by the same reference numerals and redundant description is omitted.
[0011] [1. Configuration of Stepped Multi-Mode Fiber] FIG. 1 is a cross-sectional view showing the structure of a stepped multi-mode fiber according to an embodiment of the present disclosure. Hereinafter, the stepped multi-mode fiber according to the present embodiment is denoted as optical fiber FB.
[0012] In FIG. 1, a cross-sectional view in a cross-section orthogonal 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 light wave modes propagate, and a cladding 30.
[0013] For example, the inner core 10 is made of quartz glass doped with germanium. The quartz glass doped with germanium has a higher refractive index than the quartz glass before doping. The substance added to increase the refractive index is not limited to germanium (Ge), and any substance having the effect of increasing the refractive index may be used. In addition, the cross-sectional shape of the inner core 10 is circular.
[0014] 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 to surround each inner core 10. For example, the central axes of the inner core 10 and the outer core 20 are common, and the diameter of the outer core 20 is larger than the diameter of the inner core 10. Also, the outer core 20 is made of quartz glass. In particular, the outer core 20 may be made of pure quartz glass. By forming the outer core 20 of quartz glass, the difference in loss coefficient between modes can be made less than 0.005 dB / km.
[0015] The cladding 30 has a lower refractive index than the outer core 20 and is arranged to surround all the outer cores 20. For example, the cladding 30 is made of quartz glass doped with fluorine. The quartz glass doped with fluorine has a lower refractive index than the quartz glass before doping. The substance added to decrease the refractive index is not limited to fluorine (F), and any substance having the effect of decreasing the refractive index may be used. For example, it may be boron trioxide (B 2 O 3 ). In addition, the cross-sectional shape of the cladding 30 is circular.
[0016] The diameter of the cladding 30 may be 126 μm or less. By setting the diameter of the cladding to be equivalent to the standard cladding diameter of an existing single-mode fiber (SMF), the affinity with conventional connector and cabling technologies can be increased, and an increase in manufacturing cost can be suppressed.
[0017] The optical fiber FB may include any number of inner cores 10 and outer cores 20 that is 1 or more. That is, the optical fiber FB may be multi-core. The multi-core optical fiber FB will be described later as a modification example.
[0018] The light rays propagating inside the optical fiber FB propagate by repeating total reflection in the inner core 10 and the outer core 20. Here, the inclination of the light rays is not allowed to be an arbitrary value, and only the light rays having a special angle can propagate. Such a form of light rays is called an optical wave mode. The optical wave modes that can propagate in the inner core 10 and the outer core 20 can be obtained, for example, by electromagnetic field analysis.
[0019] For example, among the optical wave modes, there is a linearly polarized mode. The linearly polarized mode is expressed in the form of "LP" ml Here, "m" is the mode order indicating the state of change in the angular direction of the lateral electric field intensity distribution of the light rays propagating in the inner core 10 and the outer core 20, and "l" is the mode order indicating the state of change in the radial direction of the lateral electric field intensity distribution of the light rays propagating in the inner core 10 and the outer core 20.
[0020] In the present embodiment, it is assumed that the inner core 10 and the outer core 20 have at least two or more optical wave modes. For example, the inner core 10 and the outer core 20 have "LP" 01 and "LP" 11 as optical wave modes. The optical wave modes of the inner core 10 and the outer core 20 are not limited to the examples given here.
[0021] FIG. 2 is a diagram showing an example of the refractive index distribution in a stepped multi-mode fiber according to an embodiment of the present disclosure. In FIG. 2, the vertical axis represents the refractive index, and the horizontal axis represents the position in the radial direction. For example, the refractive index distribution in the cross section passing through the central axis of the optical fiber FB in FIG. 1 has a stepped structure as shown in FIG. 2.
[0022] For the following description, the radius of the inner core 10 is "r"1 」 (unit: μm), and let the radius of the outer core 20 be “r 2 」 (unit: μm). Let the relative refractive index difference between the outer core 20 and the inner core 10 be Δ 1 and the relative refractive index difference between the cladding 30 and the inner core 10 be Δ 2 .
[0023] Note that if the absolute refractive index of the inner core 10 is “n 1 ” and the absolute refractive index of the outer core 20 is “n 2 ”, then “Δ 1 = (n 1 2 - n 2 2 ) / 2n 1 2 ”. Further, if the absolute refractive index of the cladding 30 is “n 3 ”, then “Δ 2 = (n 1 2 - n 3 2 ) / 2n 1 2 ”.
[0024] Also, let the radius ratio and the relative refractive index difference ratio between the inner core 10 and the outer core 20 be R a , R Δ respectively. More specifically, define “R a = r 1 / r 2 ” and “R Δ = Δ 1 / Δ 2 ”.
[0025] [2. Requirements for the optical fiber FB] To examine the requirements imposed on the radius ratio “R a ” and the relative refractive index difference ratio “R Δ ” between the inner core 10 and the outer core 20, structural calculations were performed for various optical fibers FB. As a prerequisite, 2LP mode transmission was assumed for the optical fiber FB.
[0026] Figure 3 shows the core structure dependence of the inter-mode loss difference (DMA) in the outer pure silica core and the inner pure silica core. Here, the outer pure silica core is according to this embodiment, where the inner core is made of germania-doped glass and the outer core is made of pure silica glass. Also, the inner pure silica core is shown in Non-Patent Document 1, where the inner core is made of pure silica glass and the outer core is made of fluorine-doped glass. In Figure 3, the curves of "DMA = 0 dB / km", "DMA = -0.0015 dB / km", and "DMA = -0.003 dB / km" shown by solid lines indicate the cases where the DMA in the outer pure silica core takes on each value.
[0027] Note that the Rayleigh scattering loss of the LP 01 mode is denoted as "α R_01 ", and the Rayleigh scattering loss of the LP 11 mode is denoted as "α R_11 ". The DMA (unit: dB / km) is defined by the following equation.
Equation
[0028] Also, in Figure 3, the curves of "DMA = 0.006 dB / km" and "DMA = 0.005 dB / km" shown by dotted lines indicate the cases where the DMA in the inner pure silica core takes on each value.
[0029] Here, for any combination of the radius ratio "R a " and the relative refractive index difference ratio "R Δ ", the core radius r 2 and the relative refractive index difference Δ 2 are optimized such that the mode field diameter (MFD) at a wavelength of 1.55 μm is 12 μm and the cut-off wavelength (λc) in LP 21 or LP 02 is 1.53 μm or less. This complies 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).).
[0030] The core radius r within the design range shown in FIG. 3 2 and the relative refractive index difference Δ 2 are in the ranges of 7.5 μm ≤ r 2 ≤ 12.5 μm and 0.40% ≤ Δ 2 ≤ 0.56%, respectively.
[0031] As shown by the dotted line in FIG. 3, in the case of an inner pure silica core, it is difficult to achieve |DMA| ≤ 0.003 dB / km by optimizing the core structure. On the other hand, in the case of an outer pure silica core, |DMA| ≤ 0.003 dB / km can be achieved by optimizing the core structure.
[0032] In the outer pure silica core, the region satisfying "|DMA| ≤ 0.003 dB / km" is the part shown in gray in FIG. 3. This region is represented by the following equation.
Equation
[0033] Next, in the outer pure silica core, the boundary line satisfying "DMA = -0.003 dB / km" was investigated. FIG. 4 is a diagram showing the dependence of the coefficient regarding the radius ratio on the mode field diameter in the boundary curve of the inter-mode loss difference. The coefficient regarding the radius ratio "R a " of the inner core 10 and the outer core 20 is denoted as x 0 , x 1 , x 2 and is shown as follows.
Equation
[0034] Here, when investigating the dependence of the coefficient on the MFD, assuming the MFD is "W" (unit: μm), the approximate curves of the coefficient are shown as follows, respectively.
Equation
[0035] Therefore, for any "W", the region satisfying "|DMA| ≤ 0.003 dB / km" is represented by the following equation.
Equation
[0036] Next, FIG. 5 is a diagram showing a region where the Rayleigh scattering loss of the outer pure silica core is less than or equal to the Rayleigh scattering loss of the inner pure silica core. Let the Rayleigh scattering loss of the outer pure silica core be "α R_G ", and the Rayleigh scattering loss of the inner pure silica core be "α R_P ". As the radius ratio "R a " decreases, in the outer pure silica core, the low-loss pure silica glass region in the outer core 20 expands and the scattering loss decreases.
[0037] On the other hand, in the inner pure silica core, the influence of scattering by fluorine added to the outer core 20 to lower the refractive index becomes apparent. Therefore, the scattering loss increases.
[0038] In FIG. 5, the region of the combination of the radius ratio "R R_G " and the relative refractive index difference ratio "R R_P " such that "α a ≤ α Δ " is shown in gray. For any combination of the radius ratio "R a " and the relative refractive index difference ratio "R Δ ", the core radius r 2 and the relative refractive index difference Δ 2 are optimized such that the mode field diameter (MFD) at a wavelength of 1.55 μm is 12 μm, and the cut-off wavelength (λc) in LP 21 or LP 02 is 1.53 μm or less.
[0039] In the design range shown in FIG. 5, the ranges of the core radius r 2 and the relative refractive index difference Δ 2 are 6.9 μm ≤ r 2 ≤ 9.1 μm and 0.50% ≤ Δ 2 ≤ 0.62%, respectively.
[0040] "α R_G ≦α R_P " is satisfied in the region represented by the following formula.
Equation
[0041] Next, the boundary line of the region satisfying "α R_G ≦α R_P " was investigated. Fig. 6 is a diagram showing the dependence of the coefficient on the mode field diameter with respect to the radius ratio in the boundary curve of the region in Fig. 5. Let the coefficients related to the radius ratio "R a " of the inner core 10 and the outer core 20 be y 0 , y 1 , y 2 , then it is shown as follows.
Equation
[0042] Here, when investigating the dependence of the coefficient on the MFD, if the MFD is "W" (unit: μm), the approximate curves of the coefficients are shown as follows respectively.
Equation
[0043] Therefore, for any "W", the region satisfying "α R_G ≦α R_P " is represented by the following formula.
Equation
[0044] By setting the radius ratio "R a " and the difference ratio of the specific refractive index "R Δ " so as to satisfy the above-mentioned formulas 5 and 9, a stepped multi-mode fiber with a mode field diameter of the fundamental mode LP01 at a wavelength of 1550 nm being 9.5 to 15 μm and a difference in loss coefficient between modes being 0.003 dB / km or less can be configured.
[0045] [Modified Example of Step-Type Multi-Mode Fiber] FIG. 7 is a cross-sectional view showing a modified example of a step-type multi-mode fiber according to an embodiment of the present disclosure. In FIG. 7, the optical fiber FB includes four sets of an inner core 10 and an outer core 20, and is multi-core.
[0046] Also in the optical fiber FB shown in FIG. 7, the outer core 20 has a lower refractive index than the inner core 10 and is arranged to surround the inner core 10. And the cladding 30 has a lower refractive index than the outer core 20 and is arranged to surround all the outer cores 20. In FIG. 7, the optical fiber FB includes a plurality of inner cores 10 and the same number of a plurality of outer cores 20 as the inner cores 10.
[0047] The number of the inner cores 10 and the 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 one or more inner cores 10 and outer cores 20. By configuring each of the cores included in the optical fiber FB with the inner core 10 and the outer core 20, it is possible to increase the spatial multiplexing degree.
[0048] [4. Configuration of Optical Transmission System] FIG. 8 is a diagram showing the configuration of an optical transmission system using a step-type multi-mode fiber according to an embodiment of the present disclosure. The optical transmission system 100 includes an optical fiber FB, a transmitter ST, a mode multiplexer MT, an optical coupler FI, an optical extractor FO, a mode separator MR, and a receiver SR according to an embodiment of the present disclosure.
[0049] The transmitter ST generates signal light. The number of the transmitters ST may be any number of one or more.
[0050] The mode multiplexer MT converts the signal light into optical wave modes. The number of the mode multiplexers MT may be any number of one or more.
[0051] The optical coupling unit FI is arranged on one end side of the optical fiber FB and inputs input light including an optical wave mode into the inner core 10 and the outer core 20. The optical coupling unit FI may be connected to the mode multiplexer MT and introduce the output of each mode multiplexer MT into the inner core 10 and the outer core 20 of the optical fiber FB.
[0052] Alternatively, the optical coupling unit FI may be a MUX (Multiplexer). A MUX is a circuit that converts a plurality of input signals into one output signal. Among the plurality of input signals of the MUX, the signal to be selected and used as the output signal is determined by a control signal called a selection signal.
[0053] The optical extraction unit FO is arranged on the other end side of the optical fiber FB and extracts the output light from the inner core 10 and the outer core 20. The optical 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 into each mode multiplexer MT.
[0054] Alternatively, the optical extraction unit FO may be a DEMUX (Demultiplexer). A DEMUX has a function opposite to that of a MUX and is a circuit that distributes one input signal to a plurality of output signals. The DEMUX distributes the input signal to a plurality of outputs according to the selection signal.
[0055] The mode separator MR separates the optical wave mode from the output light and extracts the signal light. The number of mode separators MR may be any number of 1 or more.
[0056] The receiver SR receives the signal light from the mode separator MR. There may be a plurality of receivers SR. The number of receivers SR may be any number of 1 or more.
[0057] [Effects of the Embodiment] As described in detail above, the stepped multimode fiber according to this embodiment includes an inner core, an outer core made of silica glass that has a lower refractive index than the inner core and surrounds the inner core, and a cladding that has a lower refractive index than the outer core and surrounds all the outer cores. Here, two or more modes are propagated at the operating wavelength, and the difference in loss coefficients between the modes is less than 0.005 dB / km.
[0058] Thereby, while simultaneously reducing the inter-modal delay difference (DMD) and the propagation loss, it is further possible to reduce the inter-modal loss difference (DMA). In particular, by forming the outer core of silica glass, the propagation loss of higher-order modes (such as LP 11 modes) where the optical field distribution is likely to spread is reduced, and it becomes possible to suppress DMA. Furthermore, by reducing DMA, the channel capacity of the stepped multimode fiber can be increased.
[0059] Also, in the stepped multimode fiber according to this embodiment, the mode field diameter of the fundamental mode LP 01 at a wavelength of 1550 nm may be 9.5 to 15 μm, and the difference in loss coefficients between the modes may be 0.003 dB / km or less. Thereby, while simultaneously reducing the DMD and the propagation loss, it is further possible to reduce the DMA.
[0060] Furthermore, in the stepped multimode fiber according to this embodiment, the inner core may be made of silica glass doped with germanium. Thereby, by optimizing the core structure, "|DMA| ≤ 0.003 dB / km" can be realized. Also, compared with the outer core made of silica glass, the refractive index of the inner core can surely be increased.
[0061] Also, in the stepped multimode fiber according to this embodiment, the cladding may be made of silica glass doped with fluorine. Thereby, compared with the outer core made of silica glass, the refractive index of the cladding can surely be decreased.
[0062] Furthermore, the stepped multi-mode fiber according to the present embodiment may have a cladding diameter of 126 μm or less. As a result, the stepped multi-mode fiber has a standard cladding diameter equivalent to that of an existing single-mode fiber (SMF). Consequently, the compatibility with conventional connectors, cabling technologies, etc. can be enhanced, and an increase in manufacturing costs can be suppressed.
[0063] Also, the stepped multi-mode fiber according to the present embodiment may include a plurality of inner cores and a plurality of outer cores equal in number to the inner cores, and the cladding may surround the plurality of outer cores. Thereby, the spatial multiplexing degree can be increased, and the transmission capacity per optical fiber can be improved. While simultaneously reducing DMD and propagation loss, DMA can be further reduced.
[0064] Furthermore, the optical transmission system according to the present embodiment includes the stepped multi-mode fiber of the present disclosure, a transmitter that generates signal light, a mode multiplexer that converts the signal light into an optical wave mode propagating through the stepped multi-mode fiber, an optical coupling unit that is disposed on one end side of the stepped multi-mode fiber and inputs input light including the optical wave mode into the inner core and the outer core, an optical extraction unit that is disposed on the other end side of the stepped multi-mode fiber and extracts output light from the inner core and the outer core, a mode separator that separates the optical wave mode from the output light and extracts the signal light, and a receiver that receives the signal light from the mode separator.
[0065] Thereby, it is possible to provide an optical transmission system capable of simultaneously reducing DMD and propagation loss and further reducing DMA. Also, since DMD becomes smaller, the load on the signal processing side can be reduced. Since the signal processing of the devices connected to both ends of the optical fiber becomes smaller, the cost of the transmission device can also be reduced. Also, since the propagation loss becomes smaller, the signal can propagate farther. As a result, the cost of the transmission device, particularly the device side that emits the signal, can be suppressed.
[0066] The above has described the content of the present disclosure in accordance with the embodiments. However, it is obvious to those skilled in the art that the present disclosure is not limited to these descriptions, and various modifications and improvements are possible. It should not be understood that the discussions and drawings forming a part of this disclosure limit the present disclosure. Various alternative embodiments, examples, and operation techniques will be apparent to those skilled in the art from this disclosure.
[0067] The present disclosure of course includes various embodiments and the like not described herein. Therefore, the technical scope of the present disclosure is defined only by the matters specifying the invention according to the legitimate claims derived from the above description.
Explanation of Reference Numerals
[0068] 10 Inner core 20 Outer core 30 Cladding 100 Optical transmission system FB Optical fiber (step-index multimode fiber) FI Optical coupling section FO Optical extraction section MR Mode separator MT Mode multiplexer SR Receiver ST Transmitter
Claims
1. An inner core, an outer core made of silica glass, which has a lower refractive index than the inner core and surrounds the inner core, a cladding that has a lower refractive index than the outer core and surrounds all of the outer cores, and a stepped multi-mode fiber that propagates two or more modes at the operating wavelength and has a difference in loss coefficients between modes of less than 0.005 dB / km.
2. The mode field diameter of the fundamental mode LP at a wavelength of 1550 nm ０１ is 9.5 to 15 μm, and the difference in loss coefficients between modes is 0.003 dB / km or less. The stepped multimode fiber according to claim 1.
3. The stepped multi-mode fiber according to claim 1, wherein the inner core is made of silica glass doped with germanium.
4. The stepped multi-mode fiber according to claim 1, wherein the cladding is made of silica glass doped with fluorine.
5. The stepped multi-mode fiber according to claim 1, wherein the diameter of the cladding is 126 μm or less.
6. A stepped multi-mode fiber comprising a plurality of the inner cores and a plurality of the outer cores equal in number to the inner cores, wherein the cladding surrounds the plurality of the outer cores.
7. A stepped multi-mode fiber according to any one of claims 1 to 6, a transmitter that generates signal light, a mode multiplexer that converts the signal light into an optical wave mode that propagates through the stepped multi-mode fiber, an optical coupler disposed on one end side of the stepped multi-mode fiber, for inputting input light including the optical wave mode into the inner core and the outer core, an optical extraction unit disposed on the other end side of the stepped multi-mode fiber, for extracting output light from the inner core and the outer core, a mode separator that separates the optical wave mode from the output light and extracts the signal light, and a receiver that receives the signal light from the mode separator. An optical transmission system comprising the above components.