JP6059560B2 - Multimode transmission optical amplifier - Google Patents

Description

  The present invention relates to a technique related to an optical amplifier in multimode transmission.

  In recent years, Internet traffic has continued to increase due to the diversification of services, and the transmission capacity has increased dramatically due to the increase in the number of wavelength multiplexing by the increase in the transmission speed and the wavelength division multiplexing (WDM) technology. . Further, in recent years, further expansion of transmission capacity is expected by digital coherent technology which has been actively studied. In digital coherent transmission systems, frequency utilization efficiency has been improved by using multilevel phase modulation signals, but higher signal-to-noise ratios are required. However, in a transmission system using a conventional single mode fiber (SMF), the transmission capacity is expected to saturate at 100 Tbit / sec due to input power limitation due to nonlinear effects in addition to the theoretical limit. Therefore, further increase in capacity has become difficult.

  In order to further increase the transmission capacity in the future, a medium that realizes innovative transmission capacity expansion is required. Therefore, mode multiplex transmission using a multimode fiber (MMF) that can be expected to improve the signal-to-noise ratio and the space utilization efficiency by using a plurality of propagation modes in an optical fiber as a channel is attracting attention. . Up to now, high-order modes propagating in an optical fiber have been a cause of signal degradation. However, active use has been studied in the development of digital signal processing, multiplexing / demultiplexing techniques, and the like (for example, Non-Patent Document 1). , 2).

The expansion of transmission capacity using MMF has been remarkable in recent years, and has reached 26.4 Tbps in the DP-QPSK system using a three-mode fiber (see, for example, Non-Patent Document 3). In addition, studies have been made to increase the distance of multimode transmission, and reports have been made on the amplification of the LP01 mode in the basic mode and the LP02 mode, which is the fourth higher-order mode, using an Er ³⁺ doped optical amplifier. (For example, refer nonpatent literature 4.).

  In order to maintain the transmission quality of all modes in order to increase the distance of multimode transmission, it is essential to adjust the gain of each mode in the optical amplifier. The gain of each mode is determined by the overlap of the number distribution of excitation elements determined by the electric field distribution of the excitation light incident on the amplification optical fiber and the rare earth element addition distribution and the electric field distribution of the signal light. At present, there are two main structures of the rare earth element added region of the amplification optical fiber: a step index type structure in which the rare earth element is added to the entire core and a center doped type structure in which the rare earth element is added only to the center of the core. However, in an amplification optical fiber for multimode transmission that needs to take into account the gain of higher-order modes, a method of adjusting gain for each propagation mode by connecting two different EDFs in series for gain adjustment, A structure in which a rare earth addition distribution is heavily doped at the edge of the fiber core has been proposed (see, for example, Non-Patent Documents 4 and 5).

  The related multimode transmission optical amplifier is a system using a spatial optical element for entering and exiting the amplification optical fiber 11 for signal light and pumping light as shown in FIG. A method for exciting off-axis when entering the optical fiber 11 (for example, Non-Patent Document 6) or a gain for each propagation mode by converting the excitation light into an arbitrary mode using a phase mask and entering the amplification optical fiber 11 A method of adjusting has been proposed (for example, see Non-Patent Document 7).

  However, since the configuration of the optical amplifier for multimode transmission in the above non-patent document is a spatial optical system, it is used as an element such as the lens 15, the mirror 22, and the phase mask 24 for converting the excitation light into a higher order mode. The resulting loss will occur. Further, in the configuration in which each mode is amplified collectively as in Non-Patent Document 5, the amount of crosstalk between modes generated when light emitted into the space is coupled to the optical fiber becomes large. The crosstalk between modes of signal light becomes a very big problem when mode multiplexing transmission is performed.

N. Hanzawa et al. , “Demonstration of Mode-Division multiplexing Transmission 10 km Two-mode Fiber with Mode Coupler” OFC2011, paper OWA4 T.A. Sakamoto et al. , “Modal Disposition Compensation Technology for Long-haul Transmission over Few-mode Fiber with SIMO Configuration”, ECOC2011, We. 10. P1.82 E. Ip et al. "88x3x112-Gb / s WDM Transmission over 50 km of Three-Mode Fiber with Inline Few-Mode Fiber Amplifier" EcoC2011, paper Thr. 13. C. 2 M.M. Salsi et al. "A Six-Mode Erbium-Doped Fiber Amplifier" ECOC 2012 paper Thr. 3. A. 6 M.M. Salsi et al. "In-line Few-Mode Optical Amplifier with Erbium Profile Tuned to Support LP01, LP11 and LP21 Mode Groups" ECOC 2012 Tu. 3. F. 1 Y. Yung et al. , “First demonstration of multimode amplifier for spatial division multiplexed systems”, ECOC 2011 paper Th. 13. K. 4 Neng Bai et al. "Multimode fiber amplifier with tunable modular gaining a reconfigurable multimode pump" OPTICS EXPRESS Vol. 19, no. 17, 2011 Shoichi Sudo, "Erbium-doped fiber amplifier", published by Optronics, ISBN 4-900474-80-0

  An object of the present invention is to perform gain adjustment in each mode of signal light in a multimode transmission optical amplifier without using a spatial optical system for incidence and emission of signal light and excitation light to and from an amplification optical fiber. is there.

The optical amplifier for multimode transmission of the present invention is
A transmission optical fiber for transmitting signal light of a plurality of modes;
An amplification optical fiber that is inserted into the transmission optical fiber and that amplifies a plurality of modes of signal light propagating through the transmission optical fiber;
An excitation light source that outputs excitation light for exciting the amplification optical fiber;
A plurality of mode multiplexing optical couplers inserted in cascade in the transmission optical fiber, each of which converts the pumping light from the pumping light source into a predetermined mode and combines it with the signal light.

And in the optical amplifier for multimode transmission of the present invention,
Each of the plurality of mode multiplexing optical couplers ,
A first port to which the signal light is input;
A third port to which the excitation light is input;
A second port that outputs light obtained by combining the signal light input from the first port and a part of the excitation light input from the third port;
A fourth port that outputs light that has not been combined with the signal light out of the excitation light input from the third port, and
The first mode multiplexing optical coupler inserted in a position farthest from the amplification optical fiber among the plurality of mode multiplexing optical couplers, the third port is connected to the pumping light source,
In the second mode multiplexing optical coupler inserted adjacent to the first mode multiplexing optical coupler, the third port is the fourth mode multiplexing optical coupler of the first mode multiplexing optical coupler. Connected to the port,
A band-pass filter for blocking the signal light is connected to an optical fiber connecting the fourth port of the first mode multiplexing optical coupler and the third port of the second mode multiplexing optical coupler. that has been inserted.

  According to the present invention, in a multimode transmission optical amplifier, gain adjustment in each mode of signal light can be performed without using a spatial optical system for incident and output of signal light and pumping light to an amplification optical fiber. it can. Thereby, loss of signal light and excitation light can be prevented and crosstalk between modes can be reduced.

2 shows an exemplary configuration of an optical amplifier for multimode transmission according to the first embodiment. An example of a 4-mode fiber design is shown. An example of the gain dependence of the propagation mode of the signal light due to the difference in the propagation mode of the excitation light is shown. An example of the detail of an optical coupler is shown. An example of the effective refractive indexes of LP01 and LP11 in an optical fiber for amplifying pumping light and signal light and an optical fiber for multimode transmission is shown. An example of the interaction length dependence of the coupling efficiency between signal light and excitation light is shown. An example of the configuration of an optical amplifier for multimode transmission for adjusting the gain of a propagation mode using X optical couplers and X pumping light sources is shown. An example of the configuration of a multimode transmission optical amplifier for adjusting the gain of each propagation mode using Y optical couplers and one pumping light source is shown. An example of the configuration of an optical amplifier for multimode transmission for adjusting the gain of each propagation mode using Y optical couplers, one pumping light source, a power coupler, and an attenuator is shown. 6 shows a first example of the configuration of a multimode transmission optical amplifier in which a mode converter is placed before optical coupler multiplexing according to a second embodiment. 6 shows a first example of the configuration of an optical amplifier for multimode transmission in which a mode converter is placed after optical coupler multiplexing according to the second embodiment. 6 shows a second example of the configuration of an optical amplifier for multimode transmission in which a mode converter is placed before optical coupler multiplexing according to the second embodiment. 6 shows a second example of the configuration of an optical amplifier for multimode transmission in which a mode converter is placed after optical coupler multiplexing according to the second embodiment. An example of an optical amplifier for multimode transmission using a spatial optical system is shown.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

  As described in Non-Patent Document 8, the multimode transmission optical amplifier according to the present invention includes a transmission optical fiber 14, an amplification optical fiber 11, and a pumping light source 21. The transmission optical fiber 14 transmits signal light in a plurality of modes. The amplification optical fiber 11 is for amplifying a plurality of modes of signal light. For example, a rare earth element is added to the amplification optical fiber 11. The excitation light source 21 outputs the excitation light of the amplification optical fiber 11.

  However, in the multimode transmission optical amplifier, not only the pumping light and the signal light are multiplexed, but also one or a plurality of pumping lights are incident on the amplification optical fiber 11 in order to obtain a desired gain in the signal light. Is required.

  Accordingly, the present invention includes the mode multiplexing optical coupler 12 or the mode converter 34. The mode multiplexing optical coupler 12 combines the excitation light with the signal light and converts the excitation light into an arbitrary mode. The mode multiplexing optical coupler 12 may be a fiber type or an optical waveguide type. The mode converter 34 converts the excitation light before or after being incident on the transmission optical fiber 14 into an arbitrary mode. As a result, the multimode transmission optical amplifier of the present invention can adjust the gain in each mode of the signal light without using a spatial optical system for the incidence and emission of the signal light and the excitation light to and from the amplification optical fiber. it can. Thereby, loss of signal light and excitation light can be prevented and crosstalk between modes can be reduced.

(Embodiment 1)
The following is a configuration in which the multiplexing unit is a fiber or a PLC to achieve low loss, low crosstalk, and simplification of the number of constituent elements as compared with a spatial optical amplifier.

  FIG. 1 is a schematic diagram of an optical amplifier for multimode transmission according to this embodiment. In this multimode transmission optical amplifier, the signal light having the N mode propagated through the multimode transmission optical fiber 14 is incident on the multimode transmission optical amplifier. The pumping light is coupled to the same or different desired propagation mode by design by the mode multiplexing optical coupler 12 and is incident on the amplification optical fiber 11. The incident signal light is stimulated and emitted from the rare earth element in an inversion distribution state by the excitation light, and is emitted after optical amplification. If necessary, an isolator 13 is inserted to prevent oscillation in the multimode transmission amplifier.

  From this, the basic theory about amplification is explained for the adjustment of the gain of each mode, and then the difference in the gain of each mode in the signal light is shown by simulation based on the difference in the pumping light incident on the amplification optical fiber 11 based thereon. . Here, the rare earth element to be added is erbium (Er) and will be described below.

When the rate equation when Er is considered as a three-level system is solved and the propagation equation of EDF is obtained, it can be expressed as equation (1).

Here, P _{s, i} is the intensity of the i-mode signal light and spontaneous emission light, z is the light propagation direction in the EDF, and ν is the light wavelength. The i-mode signal light gain coefficient γ _{e, i} (z, ν) and the absorption coefficient γ _{a, i} (z, ν) are the normalized electric field distribution ψ _{s, i} (ν, r, θ) and signal of each mode. Using the emission cross sections σ _{es, i} ^S and absorption cross sections σ _{as, i} ^S for light and excitation light, they can be expressed as the following formulas (2) and (3), respectively.

N ₂ (r, θ, z) and N ₁ (r, θ, z) are Er ³⁺ excitation quasi when one point of the fiber cross section at the position z is represented by a radius r and a declination angle θ. Represents the ion density of the level and ground level. By solving the propagation equation of equation (1) in each propagation mode, the gain of the signal light due to the difference in the mode of the excitation light can be found.

  As the excitation light source 21 of the Er-doped optical fiber (EDF), the 980 nm band and the 1480 nm band are generally used. The same tendency is seen without changing the shape of the mode greatly due to the difference in the light source. In this simulation, the excitation power source 21 of 980 nm has an input power of 200 mW, an Er concentration to be added is 300 ppm, and the length of the EDF used is 20 m. The signal light is 1550 nm and the input power is -10 dBm.

  In mode multiplexing transmission, all modes are required to realize a desired bending loss. The bending loss recommended in ITU-T G652 is 0.5 dB / 100 turn or less at a bending radius of 30 mm. The amplification optical fiber 11 used in this simulation has a core radius (hereinafter abbreviated as “core diameter”) and is assumed to be a step type in which the relative refractive index difference between the core and the cladding is Δ. FIG. 2 shows a calculation range of 1530 to 1565 nm, a bending loss of 0.1 dB / 100 turn or less, and a propagation range of LP01 mode, LP11 mode, LP21 mode, and LP02 mode. A region indicated by diagonal lines is a region where the LP02 mode satisfies a desired bending loss and does not propagate the LP31 mode.

ａ＝７．５μｍ、Δ＝０．６５％とした際にＣ帯の波長域で基本モードであるＬＰ０１モードから第三高次モードであるＬＰ０２モードが伝搬する構造である。また９８０ｎｍの励起光においては増幅用光ファイバ１１中ではＬＰ０１、ＬＰ０２、ＬＰ０３、ＬＰ１１、ＬＰ１２、ＬＰ２１、ＬＰ２２、ＬＰ３１、ＬＰ４１、ＬＰ５１の１０モードが存在し得る。 Here, the relative refractive index difference Δ is expressed by the following equation (4).

When a = 7.5 μm and Δ = 0.65%, the LP02 mode, which is the third higher order mode, propagates from the LP01 mode, which is the fundamental mode, in the wavelength band of the C band. In the 980 nm excitation light, the amplification optical fiber 11 may have 10 modes of LP01, LP02, LP03, LP11, LP12, LP21, LP22, LP31, LP41, and LP51.

  FIG. 3 shows a difference in gain in each mode LP01, LP11, LP21, and LP02 of signal light due to a difference in mode in pumping light (980 nm) incident on the amplification optical fiber 11. FIG. 3 shows the gain ratio (dB) based on the LP01 mode. Thus, the gain for each mode of signal light differs depending on the propagation mode of pumping light.

  In the present invention, in order to enter a desired pumping light mode into the amplification optical fiber 11, it is necessary to design the mode multiplexing optical coupler 12 in order to couple the pumping light to the transmission optical fiber 14. The design method of the mode multiplexing optical coupler 12 will be described below.

  FIG. 4 shows details of the optical coupler for mode multiplexing shown in FIG. The incidence of the excitation light in the mode multiplexing optical coupler 12 is assumed to be a fiber type or a waveguide type. In the production of the fiber type, the distance between the fiber cores is reduced by using a method such as melt drawing or side polishing of the optical fiber 25 for transmitting the transmission optical fiber 14 for propagating multimode signal light and the optical fiber 25 for receiving the excitation light. Alternatively, the coupling portion 121 is an optical waveguide type, and is manufactured by setting the waveguide width and the waveguide interval DC.

  For simplicity, an example is shown in which the transmission optical fiber 14 is a two-mode fiber that propagates only the fundamental mode and the first higher-order mode in the C band, and the optical fiber 25 is a fiber that operates in a single mode at 1550 nm. In this example, the 1550 nm signal light LP01 and LP11 modes incident on the transmission optical fiber 14 are emitted from the port 2 without being coupled to the port 4, and the excitation light having a wavelength of 980 nm incident from the port 3 as the LP01 mode is mode-multiplexed. The coupling portion 121 of the optical coupler 12 is required to couple to the port 2 as an LP11 mode and emit. In order to couple the excitation light incident from the port 3 to the port 2 as a desired mode, the effective refractive index of the transmission optical fiber 14 and the effective refractive index of the desired propagation mode of the optical fiber 25 are required to be equal. In the signal light, it is desirable that the effective refractive index of the first higher-order mode propagating through the amplification optical fiber 11 is different from the effective refractive index of the fundamental mode of the optical fiber 25. Each effective refractive index is determined by the core diameter a of the optical fiber to be guided and the relative refractive index difference Δ.

  FIG. 5 shows the core diameter dependence of the effective refractive index when Δ of both the LP11 mode in the transmission optical fiber 14 and the LP01 mode in the optical fiber 25 at 980 nm is 0.4%. FIG. 5 shows that good coupling efficiency can be obtained when the core diameter of the transmission optical fiber 14 is 7 μm and the core diameter of the optical fiber 25 is 3.9 μm. However, since the effective refractive index of the LP11 mode in the optical fiber 11 for amplifying signal light and the effective refractive index in the optical fiber 25 are close, there is a risk of coupling. Therefore, it is aimed to obtain an arbitrary characteristic by adjusting the interaction length LC between the transmission optical fiber 14 and the optical fiber 25.

  FIG. 6 shows differences in coupling efficiency from the port 1 to the port 4 in the signal light LP11 mode and the coupling efficiency from the port 3 to the port 2 in the pumping light LP01 mode, depending on the interaction length. As shown in FIG. 6, when the interaction length is 8950 μm, the coupling efficiency from the port 1 to the port 2 of the signal light is 0.93 in the C band, and the coupling efficiency from the port 3 to the port 2 of the pumping light is 0.97. I can do it. Further, the desired mode multiplexing optical coupler 12 can be obtained by designing the higher-order modes in the same manner. In addition, in order to obtain a desired gain in each mode of signal light, it is effective to pump the amplification optical fiber 11 with pumping light as a plurality of propagation modes.

  Accordingly, an example of a configuration in which a plurality of modes are incident on the amplification optical fiber 11 by connecting a plurality of mode multiplexing optical couplers 12 in cascade will be described below. The plurality of mode multiplexing optical couplers 12 used here are configured to couple the pumping light incident on the port 3 to the port 2 as different propagation modes. For example, the mode multiplexing optical coupler 12-1 couples the LP01 mode input to port 3 to the port 2 as the LP11 mode, and the mode multiplexing optical coupler 12-X converts the LP01 mode input to the port 3 to LP21. Connect to port 2 as mode. The propagation mode coupled in each mode multiplexing optical coupler 12 is determined by adjusting the core diameter (waveguide width) of the fiber (optical waveguide) and the relative refractive index difference.

  As shown in FIG. 7, the pumping light is coupled to the port 2 as a desired propagation mode, and the coupled propagation modes are all connected by connecting a plurality of different mode multiplexing optical couplers 12 in cascade. Can be incident. In this configuration, one pump light source 21 is attached to each of X (X is an integer of 1 or more) mode multiplexing optical couplers 12, and each of the X mode multiplexing optical couplers 12 propagates pumping light differently. It is configured to couple to port 2 as a mode.

  FIG. 8 shows a mode multiplexing optical coupler 12 in which Y (Y is an integer greater than 1) pumping lights are incident from the port 3 and coupled to the port 2 and have different propagation modes, and Y mode multiplexing optical couplers. 12 includes one pumping light source 21, and these mode multiplexing optical couplers 12 are cascaded in the same manner as in FIG. 7 to make pumping light into a plurality of propagation modes and enter the optical fiber 11 for amplification. be able to. In this configuration, the coupling efficiency from the port 3 to the port 2 of the mode multiplexing optical coupler 12 is an arbitrary value less than 100%, and the excitation light that is not coupled from the port 4 of each mode multiplexing optical coupler 12 Is output. The pumping light output from the port 4 enters from the port 3 of the adjacent mode multiplexing optical coupler 12 and is coupled again to the port 2 with an arbitrary coupling efficiency. By configuring the connected Y mode multiplexing optical couplers 12 to have the same configuration as described above, excitation light can be incident as a plurality of modes. In the configuration shown in FIG. 8, since the number of pumping light sources 21 can be reduced, the configuration of the multimode transmission optical amplifier can be simplified and the degree of freedom in designing the mode multiplexing optical coupler 12 can be improved. be able to.

  Here, in the configuration of FIG. 8, it is preferable to insert a band pass filter 31 that blocks signal light between the port 4 and the port 3 of each mode multiplexing optical coupler 12. As a result, the signal light that slightly leaks from the port 1 to the port 4 can be cut, so that crosstalk between modes can be suppressed.

  In FIG. 9, Y (Y is an integer greater than 1) mode multiplexing optical couplers 12 have one pumping light source 21, and these mode multiplexing optical couplers 12 are cascaded as in FIG. By connecting, it can enter into the optical fiber 11 for amplification as excitation light of multiple modes. The pumping light source 21 is first incident on a Y-branchable power coupler 33, and the pumping light branched by the power coupler 33 is incident on the port 3 of the Y mode multiplexing optical couplers 12. As a result, each pump light is coupled to port 2 of Y mode multiplexing optical couplers 12 as respective modes. In order to adjust the intensity of each propagation mode of the excitation light incident on the amplification optical fiber 11, an optical attenuator 32 for attenuating the excitation light after branching by the power coupler 33 is installed. With such a configuration, it is possible to adjust the pumping light intensity of each propagation mode of the pumping light incident on the amplification optical fiber 11, and the configuration of the multimode transmission optical amplifier is simplified by using the pumping light source 21 in common. Become.

(Embodiment 2)
Although this configuration is based on FIG. 1 of the first embodiment, this configuration shows an example in which the mode converter 34 is used instead of the gain adjustment by the design of the mode coupling optical coupler 12. In this configuration, since only one mode multiplexing optical coupler 12 is required, loss can be further reduced, and the configuration of the optical amplifier for multimode transmission can be simplified.

  As the mode converter 34, a long period fiber Bragg grating (LPG) is used here. In LPG, it is possible to convert excitation light propagating by refractive index fluctuation according to the effective refractive index difference between two modes at a specific wavelength λ into different propagation modes. Here, mode conversion is realized only at the pumping light wavelength by using narrow band LPG. However, this configuration cannot be used when the wavelength of the signal light overlaps the LPG band.

式（５）のｎ_１とｎ_２はそれぞれ波長λにおける変換する前のモードの実効屈折率と変換後のモードの実効屈折率を示している。この実効屈折率ｎ_１とｎ_２は光ファイバの構造パラメータ、波長、モードの次数により決定する。 The grating interval Λ for conversion can be expressed by the following equation (5).

In Equation (5), n ₁ and n ₂ indicate the effective refractive index of the mode before conversion and the effective refractive index of the mode after conversion, respectively, at the wavelength λ. The effective refractive indexes n ₁ and n ₂ are determined by the structural parameters, wavelength, and mode order of the optical fiber.

  FIG. 10 shows an example in which excitation light is converted into a desired mode before being coupled from port 3 to port 2. The optical coupler 17 used in this configuration is preferably designed so that the coupling efficiency of the mode converted by the LPG 34 is high. FIG. 11 shows an example in which the excitation light is multiplexed from port 3 to port 2 and then converted to a desired mode. In this configuration, there are no restrictions on the modes to be coupled, and those with high coupling efficiency are desirable.

  In both of the configurations of FIGS. 10 and 11, when a plurality of modes of pumping light are required to obtain a necessary gain, the number of mode converters 34 of the desired mode is added as shown in FIGS. What is necessary is just to arrange.

  The present invention is an optical amplifier for multimode transmission, and realizes gain adjustment for each propagation mode and extension of transmission distance in transmission using a plurality of modes.

11: optical fiber for amplification 12: optical coupler for mode multiplexing 121: coupling unit 13: isolator 14: optical fiber for N-mode transmission 15: lens 16: dichroic mirror 17: optical coupler 21: excitation light source 22: mirror 23: half Mirror 24: Phase mask 25: Optical fiber 31: Band pass filter 32: Optical attenuator 33: Power coupler 34: LPG

Claims (1)

A transmission optical fiber for transmitting signal light of a plurality of modes;
An amplification optical fiber that is inserted into the transmission optical fiber and that amplifies a plurality of modes of signal light propagating through the transmission optical fiber;
An excitation light source that outputs excitation light for exciting the amplification optical fiber;
A plurality of mode multiplexing optical couplers inserted in cascade in the transmission optical fiber, each converting the pumping light from the pumping light source into a predetermined mode and multiplexing the signal light;
The predetermined modes to be converted by the plurality of mode multiplexing optical couplers are different from each other,
Each of the plurality of mode multiplexing optical couplers,
A first port to which the signal light is input;
A third port to which the excitation light is input;
A second port that outputs light obtained by combining the signal light input from the first port and a part of the excitation light input from the third port;
A fourth port that outputs light that has not been combined with the signal light out of the excitation light input from the third port, and
The first mode multiplexing optical coupler inserted in a position farthest from the amplification optical fiber among the plurality of mode multiplexing optical couplers, the third port is connected to the pumping light source,
In the second mode multiplexing optical coupler inserted adjacent to the first mode multiplexing optical coupler, the third port is the fourth mode multiplexing optical coupler of the first mode multiplexing optical coupler. Connected to the port,
A band-pass filter for blocking the signal light is connected to an optical fiber connecting the fourth port of the first mode multiplexing optical coupler and the third port of the second mode multiplexing optical coupler. An optical amplifier for multimode transmission characterized by being inserted.

Images