Transmission system and transmission method

Abstract

A transmission system according to the present invention comprises an optical splitter, an upstream-side optical fiber, an upstream-side optical transceiver, a plurality of downstream-side optical fibers, and a plurality of downstream-side optical transceivers. The optical splitter has a trunk section core and a plurality of branch section cores branched from the downstream-side end of the trunk section core. The upstream-side optical fiber is connected to the upstream-side end of the trunk section core. The upstream-side optical transceiver is connected to the upstream-side end of the upstream-side optical fiber. The plurality of downstream-side optical fibers are connected to the downstream-side ends of the corresponding plurality of branch section cores. The plurality of downstream-side optical transceivers are connected to the downstream-side ends of the corresponding plurality of downstream-side optical fibers. The trunk section core is a multimode optical waveguide for transmitting an uplink optical signal in a plurality of waveguide modes.

Description

Transmission System and Transmission Method

[0001] The present invention relates to a transmission system and a transmission method.

[0002] A service called FTTH (Fiber to the Home), which provides broadband communication to subscribers via an optical fiber, is being used. As a system for providing FTTH, a PON (Passive Optical Network) system is known.

[0003] FIG. 5 is an overall configuration diagram of a conventional transmission system 80. The PON system, which is the conventional transmission system 80, is composed of one OLT (Optical Line Termina), which is an optical communication device on the communication carrier side, and one or a plurality of ONUs (Optical Network Unit), which are optical communication devices on the subscriber side. The connection between the OLT and the ONU is a 1:n connection by a combination of optical fibers 82 and 84 and an optical splitter 90. The trunk core 92 of the optical splitter 90 is connected to the OLT via the upstream optical fiber 82. The plurality of branch cores 94 of the optical splitter 90 are connected to the corresponding plurality of ONUs via the corresponding plurality of downstream optical fibers 84.

[0004] "Technical Foundation Course "GE-PON Technology"", NTT Technical Journal, 2005.8, p.71-74 Shigeki Inuma, "Optical Access Wiring Method", Institute of Electronics, Information and Communication Engineers "Knowledge Base", Institute of Electronics, Information and Communication Engineers, 2017 / 05 / 23, Group 5, Chapter 2, p.1-4 Tetsuro Yabu, Shin'nosuke Sawa, "Analysis Method of Optical Waveguides", Optics, 1998, Vol. 27, No. 11, pp.632-639

[0005] The upstream optical signal is input from one ONU to the branch core 94 of the optical splitter 90 via the downstream optical fiber 84. FIG. 6 is an explanatory diagram of the propagation state of the upstream optical signal in the conventional optical splitter 90. The waveforms in FIG. 6 schematically show the electric field (envelope) distribution of the upstream optical signal in the x-axis direction.

[0006] Figure 6A shows the optical signal field before merging. Since the branch core 94 is single-mode, only one waveguide mode of the optical signal field is allowed to propagate in the branch core 94. As a result, the optical signal field becomes the fundamental mode with a peak in the center of the branch core 94. Figure 6B shows the merging of the optical signal field from one branch core 94 to the trunk core 92. The optical signal field is distributed so that the power is 1 / 2 each to the two waveguide modes: the fundamental mode (solid line) and the primary mode (dashed line). Figure 6C shows the optical signal field after merging. Since the trunk core 92 is single-mode, only one waveguide mode of the optical signal field is allowed to propagate in the trunk core 92. As a result, the optical signal field consists only of the fundamental mode with a peak in the center of the trunk core 92. The primary mode disappears as a radiated mode (diffusing outside the trunk core 92).

[0007] Thus, in the conventional optical splitter 90, the power of the upstream optical signal is distributed to the fundamental mode and the primary mode, and only the propagation of the fundamental mode is permitted in the trunk core 92. Therefore, branching loss (power loss) occurs in the upstream optical signal. In an upstream optical signal that passes through a 1:2 optical splitter, branching loss occurs, resulting in a power of 1 / 2. Similarly, in an upstream optical signal that passes through a 1:N optical splitter, branching loss occurs, resulting in a power of 1 / N.

[0008] The problem that this invention aims to solve is to provide a transmission system that can suppress power loss of the uplink optical signal.

[0009] A transmission system according to one aspect of the present invention comprises an optical splitter, an upstream optical fiber, an upstream optical transceiver, a plurality of downstream optical fibers, and a plurality of downstream optical transceivers. The optical splitter comprises a trunk core and a plurality of branch cores branched from the downstream end of the trunk core. The upstream optical fiber is connected to the upstream end of the trunk core. The upstream optical transceiver is connected to the upstream end of the upstream optical fiber. The plurality of downstream optical fibers are connected to the downstream ends of the corresponding plurality of branch cores. The plurality of downstream optical transceivers are connected to the downstream ends of the corresponding plurality of downstream optical fibers. The trunk core is a multimode optical waveguide that transmits uplink optical signals in multiple waveguide modes.

[0010] One aspect of the present invention relates to a transmission system comprising an optical splitter, an upstream optical fiber, an upstream optical transceiver, a plurality of downstream optical fibers, and a plurality of downstream optical transceivers. The optical splitter comprises a trunk core and a plurality of branch cores branched from the downstream end of the trunk core. The upstream optical fiber is connected to the upstream end of the trunk core. The upstream optical transceiver is connected to the upstream end of the upstream optical fiber. The plurality of downstream optical fibers are connected to the corresponding downstream ends of the plurality of branch cores. The plurality of downstream optical transceivers are connected to the corresponding downstream ends of the plurality of downstream optical fibers. The trunk core includes a step of transmitting an uplink optical signal in a plurality of waveguide modes.

[0011] According to the present invention, power loss of the upstream optical signal can be suppressed.

[0012] Overall configuration diagram of the transmission system of the embodiment. Perspective view of the optical splitter. Diagram illustrating the propagation state of the downlink optical signal in the optical splitter. Diagram illustrating the propagation state of the uplink optical signal in the optical splitter. Overall configuration diagram of the transmission system of the prior art. Diagram illustrating the propagation state of the uplink optical signal in the prior art optical splitter.

[0013] Figure 1 is an overall configuration diagram of the transmission system 1 of the embodiment. As mentioned above, FTTH (Fiber to the Home) is a service that provides broadband communication to subscribers via optical fiber. Various systems exist for providing FTTH, but among them, the PON (Passive Optical Network) system is known to be widely used.

[0014] Figure 1 is an overall configuration diagram of the transmission system 1 of the embodiment. The transmission system 1 of the embodiment is a PON system. The transmission system 1 includes an OLT (Optical Line Terminal; upstream optical transceiver), an OLT (Optical Line Terminal; downstream optical transceiver), an optical splitter 10, an upstream optical fiber 2, and a downstream optical fiber 4. The OLT is an optical communication device on the telecommunications carrier side. There is one OLT. The ONU is an optical communication device on the subscriber side. There is one or more ONUs. In the example of Figure 1, there are two ONUs.

[0015] The outer surface of the optical splitter 10 is provided with trunk input / output terminals 12e and branch input / output terminals 14e. There is one trunk input / output terminal 12e, which is connected to the OLT via the upstream optical fiber 2. There are N branch input / output terminals 14e (where N is an integer of 2 or more), which are connected to the ONU via the downstream optical fiber 4. In the example in Figure 2, there are two branch input / output terminals 14e.

[0016] In this application, the X, Y, and Z axes of the Cartesian coordinate system are defined as follows: The Z axis is parallel to the direction in which the trunk input / output terminal 12e and the branch input / output terminal 14e are separated. The X axis is parallel to the direction in which the two branch input / output terminals 14e are separated. The Y axis is parallel to the direction perpendicular to the Z and X axes.

[0017] Figure 2 is a perspective view of the optical splitter 10. The optical waveguide of the optical splitter 10 is formed by cores 12, 14 and cladding 15. The cores 12 and 14 have a trunk core 12 and a branch core 14. The trunk core 12 and the branch core 14 are formed in the central part of the optical splitter 10 in the Y-axis direction. The trunk input / output terminal 12e is provided at the upstream end of the trunk core 12. The branch input / output terminal 14e is provided at the downstream end of the branch core 14.

[0018] There is one trunk core 12. The trunk core 12 extends parallel to the Z-axis direction from the trunk input / output terminal 12e to the center of the optical splitter 10 in the Z-axis direction. The trunk input / output terminal 12e and the trunk core 12 are formed in the center of the optical splitter 10 in the X-axis direction.

[0019] There are N branch cores 14. In the example in Figure 2, there are two branch cores 14. The branch cores 14 extend from the branch input / output terminal 14e to the center of the optical splitter 10 in the Z-axis and X-axis directions. The branch cores 14 are inclined with respect to the Z-axis direction.

[0020] The downstream end of the trunk core 12 and the upstream end of the branch core 14 are connected near the center of the optical splitter 10. Multiple branch cores 14 branch off from the downstream end of the trunk core 12. As a result, the optical splitter 10 has a branched (Y-shaped) configuration. The cladding 15 surrounds the cores 12 and 14. The refractive index of light is set higher for the cores 12 and 14 than for the cladding 15.

[0021] Basic optical splitter structures include branched (Y-shaped) and directional coupler types. In the branched (Y-shaped) type, as shown in the embodiment, the ratio of the trunk input / output terminal 12e to the branch input / output terminal 14e is 1:2. In the directional coupler type, two optical waveguides are arranged in parallel. By combining optical splitters of these basic structures in multiple stages, branching ratios such as 1:4, 1:8, and 1:32 can be achieved.

[0022] As shown in Figure 1, the upstream optical fiber 2 is connected to the trunk input / output terminal 12e of the optical splitter 10. An OLT is connected to the upstream end of the upstream optical fiber 2. The upstream optical fiber 2 connects the OLT to the trunk input / output terminal 12e. The downstream optical fiber 4 is connected to the branch input / output terminal 14e of the optical splitter 10. An ONU is connected to the downstream end of the downstream optical fiber 4. The downstream optical fiber 4 connects the branch input / output terminal 14e to the ONU. The waveguide modes of the optical splitter 10, the upstream optical fiber 2, and the downstream optical fiber 4 will be described in detail later.

[0023] In PON systems, standards such as GE-PON, G-PON, 10G-EPON, and XG-PON employ TDM (Time Division Multiplexing) for downlink signals (OLT transmission, ONU reception) and TDMA (Time Division Multiple Access) for uplink signals (ONU transmission, OLT reception) as their communication methods.

[0024] First, let's explain TDM, the downlink signal transmission method. The OLT multiplexes data destined for multiple ONUs into a single optical signal so that they do not overlap in time, and transmits it as a downlink optical signal. Each ONU receives the same downlink optical signal that has been branched and distributed by the optical splitter. The downlink optical signal received by each ONU also contains data destined for other ONUs. Therefore, each ONU extracts only the data destined for itself and discards the data destined for other ONUs.

[0025] Next, we will explain TDMA, an uplink signal transmission method. Each ONU superimposes data destined for the OLT onto an optical signal and transmits it as an uplink signal. The uplink optical signal transmitted by each ONU emits light when it superimposes data destined for the OLT and does not emit light when it does not. If the transmission timing of the uplink optical signals from each ONU is disordered, these uplink optical signals may collide at the optical splitter 10, making communication impossible. Therefore, the transmission timing of the uplink optical signals in each ONU is controlled. As a result, uplink optical signals transmitted from different ONUs can pass through the optical splitter 10 at different timings without colliding and be received at the OLT (see Non-Patent Literature 1).

[0026] Furthermore, standards such as GE-PON, G-PON, 10G-EPON, and XG-PON employ a single-fiber bidirectional transmission method in which the uplink and downlink optical signals transmitted and received between the OLT and ONU are propagated bidirectionally through the same optical fiber. In these standards, the uplink and downlink optical signals are set to have different wavelengths. Therefore, it is possible to combine and decompress the uplink and downlink optical signals using wavelength filters inside the OLT and ONU.

[0027] Referring to Figure 1, the waveguide modes of the optical waveguide, which consists of an optical splitter 10, an upstream optical fiber 2, and a downstream optical fiber 4, will be explained. An optical waveguide has waveguide modes (spatial modes). A waveguide mode is the pattern of the electric field distribution of an optical signal propagating through an optical waveguide. The waveform shown in C of Figure 4 schematically shows the electric field (envelope) distribution of the optical signal in the x-axis direction. Waveguide modes include the fundamental mode and the first-order mode. The fundamental mode has an electric field peak in the center of the optical waveguide in the x-axis direction, as shown by the solid line. The first-order mode has electric field peaks with opposite positive and negative values ​​on either side of the center of the optical waveguide in the x-axis direction, as shown by the dashed line.

[0028] Optical waveguides exist in two types, single-mode and multi-mode, depending on the number of waveguide modes. A single-mode waveguide has only one waveguide mode, which is the fundamental mode (zero-order mode). A multi-mode waveguide has multiple waveguide modes, including the fundamental mode and other modes such as the first-order mode.

[0029] Generally, in multimode optical waveguides, waveform degradation of the optical signal occurs due to the difference in the propagation speed of light between the multiple waveguide modes. In contrast, single-mode optical waveguides have only one fundamental mode, so waveform degradation of the optical signal does not occur. Therefore, single-mode optical waveguides are suitable for long-distance and high-bitrate optical transmission. The waveguide mode of an optical waveguide is determined by the refractive index of the cores 12, 14 and cladding 15, the shape such as the width of the cores 12, 14, and the wavelength of the optical signal (see Non-Patent Literature 3).

[0030] In this embodiment, the branch core 14 of the optical splitter 10 is set to single mode for both the downlink and uplink optical signals. In contrast, the trunk core 12 is set to single mode for the downlink optical signal and multimode for the uplink optical signal. Generally, if the wavelength of the optical signal propagating through the optical waveguide is longer than the cutoff wavelength of the optical waveguide, the optical waveguide becomes single-mode. On the other hand, if the wavelength of the optical signal is shorter than the cutoff wavelength, the optical waveguide becomes multimode. Therefore, the wavelength of the uplink optical signal is set to be shorter than the wavelength of the downlink optical signal, and the cutoff wavelength of the trunk core 12 is set between the wavelength of the uplink optical signal and the wavelength of the downlink optical signal.

[0031] The downstream optical fiber 4 is configured for single-mode optical signals for both downstream and upstream optical signals. In contrast, the upstream optical fiber 2 is configured for single-mode optical signals and for multi-mode optical signals for upstream signals. Generally, the cutoff wavelength of an optical waveguide is determined by the refractive index of the core and cladding, as well as the shape of the core, such as its width.

[0032] The propagation of the downlink optical signal in the transmission system 1 of this embodiment will now be described. In Figure 1, the downlink optical signal is input from the OLT to the upstream optical fiber 2. The upstream optical fiber 2 is set to single mode for the downlink optical signal. Therefore, the downlink optical signal is propagated in basic mode through the upstream optical fiber 2. The downlink optical signal is input from the upstream optical fiber 2 to the trunk core 12 of the optical splitter 10.

[0033] Figure 3 is an explanatory diagram of the propagation state of the downlink optical signal in the optical splitter 10. Figure 3A is an electric field distribution diagram of the downlink optical signal in the trunk core 12. The trunk core 12 is set to single mode for the downlink optical signal. Therefore, the downlink optical signal propagates through the trunk core 12 in basic mode. Figure 3B is an electric field distribution diagram of the downlink optical signal at the branching point from the trunk core 12 to the two branch cores 14. The electric field of the downlink optical signal is distributed to the two branch cores 14 so that the power is divided in half. Figure 3C is an electric field distribution diagram of the downlink optical signal in the branch cores 14. The two branch cores 14 are each set to single mode. Therefore, the downlink optical signal propagates through the branch cores 14 in basic mode.

[0034] In Figure 1, the downlink optical signal is output from the branch core 14 of the optical splitter 10 to the downstream optical fiber 4. The downstream optical fiber 4 is set to single mode. Therefore, the downlink optical signal propagates through the downstream optical fiber 4 in basic mode. The downlink optical signal is input to the ONU from the downstream optical fiber 4. Thus, in the transmission system 1 of this embodiment, the entire section is single mode for the downlink optical signal. Therefore, the downlink optical signal propagates through the entire section of the transmission system 1 in basic mode.

[0035] The downlink optical signal is distributed at the branching point from the trunk core 12 to the two branch cores 14, with each receiving half of the power. The power of the downlink optical signal output from one branch core 14 is half of the power of the downlink optical signal input to the trunk core 12. In other words, a branching loss of half occurs in the downlink optical signal.

[0036] As mentioned above, the PON system employs TDM as the transmission method for downlink optical signals. TDM is based on a one-to-many communication model in which the downlink optical signal transmitted by the OLT is distributed and received by all ONUs. In one-to-many communication, the power of the downlink optical signal is distributed to multiple ONUs. Therefore, it is a natural consequence that branching losses occur in the power of the downlink optical signal.

[0037] The propagation of the upstream optical signal in the transmission system 1 of this embodiment will now be described. The upstream optical signal is input from one ONU to the downstream optical fiber 4. The downstream optical fiber 4 is set to single mode. Therefore, the upstream optical signal is propagated in basic mode through the downstream optical fiber 4. The upstream optical signal is input from the downstream optical fiber 4 to the branch core 14 of the optical splitter 10.

[0038] Figure 4 is an explanatory diagram of the propagation state of the uplink optical signal in the optical splitter 10. Figure 4A is an electric field distribution diagram of the uplink optical signal in the branch core 14. The branch core 14 is set to single mode. Therefore, the uplink optical signal propagates through the branch core 14 in basic mode. Figure 4B is an electric field distribution diagram of the uplink optical signal at the confluence of multiple branch cores 14 to the trunk core 12. The electric field of the uplink optical signal is distributed to two waveguide modes, the basic mode (solid line) and the primary mode (dashed line), with the power being divided equally between them. Figure 4C is an electric field distribution diagram of the uplink optical signal in the trunk core 12. The trunk core 12 is set to multimode for the uplink optical signal. Therefore, the uplink optical signal propagates through the trunk core 12 in basic mode and primary mode.

[0039] In Figure 1, the upstream optical signal is output from the trunk core 12 of the optical splitter 10 to the upstream optical fiber 2. The upstream optical fiber 2 is set to multimode for the upstream optical signal. Therefore, the upstream optical signal propagates through the upstream optical fiber 2 in both basic and primary modes. The upstream optical signal is input from the upstream optical fiber 2 to the OLT.

[0040] Figure 6 is an explanatory diagram of the propagation state of an uplink optical signal in a conventional optical splitter 90. At the confluence of multiple branch cores 94 to the trunk core 92, the electric field of the uplink optical signal is distributed to two waveguide modes, the fundamental mode (solid line) and the primary mode (dashed line), with each receiving half of the power. In the conventional optical splitter 90, the trunk core 92 is set to single mode for both downlink and uplink optical signals. The uplink optical signal propagates through the trunk core 92 only in fundamental mode. At this time, the primary mode diffuses outside the trunk core 92 as a radiated mode and disappears.

[0041] The power of the uplink optical signal output from the trunk core 92 is halved compared to the power of the uplink optical signal input to one branch core 94. Therefore, in conventional optical splitters 90, branching loss occurs in the uplink optical signal as well. In an uplink optical signal that passes through a 1:2 optical splitter 90, branching loss occurs, resulting in a power reduction to 1 / 2. Similarly, in an uplink optical signal that passes through a 1:N optical splitter, branching loss occurs, resulting in a power reduction to 1 / N.

[0042] As mentioned above, the PON system employs TDMA as the transmission method for uplink optical signals. TDMA is a method based on 1:1 optical communication, where an uplink optical signal transmitted from one ONU is received by one OLT. In 1:1 optical communication, the power of the uplink optical signal is transmitted to one OLT, so ideally, no branching loss occurs.

[0043] In the embodiment shown in Figure 4, the trunk core 12 of the optical splitter 10 is set to multimode for the uplink optical signal. The uplink optical signal is propagated through the trunk core 92 in basic mode and primary mode. Furthermore, in the embodiment shown in Figure 1, the downstream optical fiber 4 is also set to multimode for the uplink optical signal. The uplink optical signal is propagated through the downstream optical fiber 4 in basic mode and primary mode. As a result, the transmission system 1 of the embodiment can suppress power loss (branching loss) of the uplink optical signal.

[0044] Accordingly, it is possible to achieve relaxation of the optical reception sensitivity of the OLT, reduction of the power consumption of the OLT optical reception unit and cost reduction of the OLT optical reception unit, and relaxation of the optical transmission output of the ONU, reduction of the power consumption of the ONU optical transmission unit and cost reduction of the ONU optical transmission unit.

[0045] Since a single-mode optical fiber has only the fundamental mode as a guided mode, waveform degradation of an optical signal due to mode dispersion does not occur. Mode dispersion is waveform degradation of an optical signal that occurs because the propagation speeds of light are different among a plurality of guided modes. Therefore, a single-mode optical fiber is suitable for long-distance or high-bit-rate optical transmission. On the other hand, waveform degradation of an optical signal due to mode dispersion is a concern in long-distance transmission using a multimode optical fiber. However, digital signal transmission at about 400 m at 10 Gbps is possible for short distances.

[0046] As described in detail above, the transmission system 1 of the embodiment includes an optical splitter 10, an upstream optical fiber 2, an OLT, a plurality of downstream optical fibers 4, and a plurality of ONUs. The optical splitter 10 includes a trunk core 12 and a plurality of branch cores 14 branched from the downstream end of the trunk core 12. The upstream optical fiber 2 is connected to the upstream end of the trunk core 12. The OLT is connected to the upstream end of the upstream optical fiber 2. The plurality of downstream optical fibers 4 are connected to the downstream ends of the corresponding plurality of branch cores 14. The plurality of ONUs are connected to the downstream ends of the corresponding plurality of downstream optical fibers 4. The trunk core 12 is a multimode optical waveguide that transmits an upstream optical signal in a plurality of guided modes.

[0047] The upstream optical signal is distributed such that the power is divided equally into two guided modes, the fundamental mode and the first-order mode, at the junction from the branch core 14 to the trunk core 12. The trunk core 12 is set to multimode with respect to the upstream optical signal. The trunk core 12 propagates the upstream optical signal in the fundamental mode and the first-order mode. Thereby, the power loss of the upstream optical signal is suppressed.

[0048] The upstream optical fiber 2 is a multimode optical waveguide that transmits an upstream optical signal in a plurality of guided modes. The trunk core 12 transmits the upstream optical signal in the fundamental mode and the first-order mode. Thereby, the power loss of the upstream optical signal is suppressed.

[0049] The cutoff wavelengths of the trunk core 12 and the upstream optical fiber 2 are longer than the wavelength of the upstream optical signal and shorter than the wavelength of the downstream optical signal. The trunk core 12 and the upstream optical fiber 2 are multimode with respect to the upstream optical signal and single mode with respect to the downstream optical signal. Thereby, waveform degradation of the downstream optical signal is suppressed.

[0050] The transmission method of the embodiment assumes a transmission system 1 having an optical splitter 10, an upstream optical fiber 2, an OLT, a plurality of downstream optical fibers 4, and a plurality of ONUs. The optical splitter 10 has a trunk core 12 and a plurality of branch cores 14 branched from the downstream end of the trunk core 12. The upstream optical fiber 2 is connected to the upstream end of the trunk core 12. The OLT is connected to the upstream end of the upstream optical fiber 2. The plurality of downstream optical fibers 4 are connected to the downstream ends of the corresponding plurality of branch cores 14. The plurality of ONUs are connected to the downstream ends of the corresponding plurality of downstream optical fibers 4. The trunk core 12 has a step of transmitting the upstream optical signal in a plurality of guided modes. The trunk core 12 transmits the upstream optical signal in the fundamental mode and the first-order mode. Thereby, the power loss of the upstream optical signal is suppressed.

[0051] As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and designs and the like within the scope not departing from the gist of the present invention are also included.

[0052] OLT... Upstream optical transceiver ONU... Downstream optical transceiver 1... Transmission system 2... Upstream optical fiber 4... Downstream optical fiber 10... Optical splitter 12... Trunk core 14... Branch core

Claims

1. A transmission system comprising: an optical splitter having a trunk core and a plurality of branch cores branched from the downstream end of the trunk core; an upstream optical fiber connected to the upstream end of the trunk core; an upstream optical transceiver connected to the upstream end of the upstream optical fiber; a plurality of downstream optical fibers connected to the corresponding downstream ends of the plurality of branch cores; and a plurality of downstream optical transceivers connected to the corresponding downstream ends of the plurality of downstream optical fibers, wherein the trunk core is a multimode optical waveguide that transmits uplink optical signals in a plurality of waveguide modes.

2. The transmission system according to claim 1, wherein the upstream optical fiber is a multimode optical waveguide that transmits upstream optical signals in multiple waveguide modes.

3. The transmission system according to claim 2, wherein the cutoff wavelength of the trunk core and the upstream optical fiber is longer than the wavelength of the upstream optical signal and shorter than the wavelength of the downstream optical signal.

4. A transmission system comprising: an optical splitter having a trunk core and a plurality of branch cores branched from the downstream end of the trunk core; an upstream optical fiber connected to the upstream end of the trunk core; an upstream optical transceiver connected to the upstream end of the upstream optical fiber; a plurality of downstream optical fibers connected to the corresponding downstream ends of the plurality of branch cores; and a plurality of downstream optical transceivers connected to the corresponding downstream ends of the plurality of downstream optical fibers, wherein the trunk core transmits an uplink optical signal in a plurality of waveguide modes.

Images