Optical transmission system and optical / electrical composite cable

Abstract

An optical transmission system and an optical / electrical composite cable that can realize high-quality, large-capacity communication with a simple configuration. [Solution] The optical transmission system comprises an optical signal transmitter that outputs an optical signal, an optical signal receiver that receives the optical signal, and an optical transmission line that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal, wherein when a Gaussian beam emitted from a single-mode optical fiber is input with a positional deviation, the ratio of the maximum to minimum beam diameter of the output beam is 0.7 or greater.

Description

[Technical Field]

[0001] This invention relates to an optical transmission system and an optoelectronic composite cable. [Background technology]

[0002] In recent years, short-distance communication of approximately 100m has been required for indoor communication applications such as data centers. Furthermore, multi-level modulation schemes such as pulse-amplitude modulation (PAM) are being considered for high-capacity communication.

[0003] When employing a multi-level modulation scheme, the difference in amplitude levels between symbols is small, so even lower noise characteristics are required to reduce the error rate and improve communication quality.

[0004] One known error correction method for reducing error rates is called Forward Error Correction (FEC). FEC is implemented using a processor such as a DSP (Digital Signal Processor). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2000-151516 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Using error correction methods such as FEC to reduce the error rate requires adding error correction codes, which increases the amount of data, leading to transmission delays and decreased encoding efficiency. Furthermore, the addition of processors complicates the configuration, and increased heat generation and power consumption due to processor load become problematic.

[0007] The present invention has been made in view of the above, and aims to provide an optical transmission system and an optoelectronic composite cable that can realize high-quality and high-capacity communication with a simple configuration. [Means for solving the problem]

[0008] To solve the above-mentioned problems and achieve the objective, one aspect of the present invention is an optical transmission system comprising: an optical signal transmitter that transmits an optical signal output from a light source; an optical signal receiver that receives the optical signal; and an optical transmission path that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal, wherein the product of the scattering loss and length for the optical signal is 6 dB or less, and when a Gaussian beam emitted from a single-mode optical fiber is input by central excitation, the beam diameter is expanded to three times or more and the output is provided. The first optical transmission path is located immediately after the light source and is further optically connected to the second optical transmission path.

[0009] One aspect of the present invention is an optical transmission system comprising: an optical signal transmitter that transmits an optical signal output from a light source; an optical signal receiver that receives the optical signal; and an optical transmission path that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal, wherein the product of the scattering loss to the optical signal and the length is 6 dB or less, and when a Gaussian beam emitted from a single-mode optical fiber is input by central excitation, the beam diameter is expanded by three times or more and the first optical transmission path is positioned immediately after the light source and is further optically connected to a second optical transmission path that is longer than the first optical transmission path.

[0010] One aspect of the present invention is an optical transmission system comprising: an optical signal transmitter that outputs an optical signal; an optical signal receiver that receives the optical signal; and an optical transmission path that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal, wherein the optical transmission path is a first optical transmission path in which the product of the scattering loss to the optical signal and the length is 6 dB or less, and which expands the beam diameter to three times or more when a Gaussian beam emitted from a single-mode optical fiber is input by central excitation; and a second optical transmission path that is optically connected to the first optical transmission path, is longer than the first optical transmission path, has a transmission loss to the optical signal of 100 dB / km or less, and expands the beam diameter to less than three times when a Gaussian beam emitted from a single-mode optical fiber is input by central excitation.

[0011] One aspect of the present invention comprises an optical signal transmitter that outputs an optical signal, an optical signal receiver that receives the optical signal, and an optical transmission path that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal, wherein the optical transmission path has a product of scattering loss and length of 6 dB or less for the optical signal, and when a Gaussian beam emitted from a single-mode optical fiber is input by central excitation, the beam diameter is expanded to three times or more for output, the modulation method of the optical signal is a multi-level modulation method, and the optical transmission path is error-free without using an error correction method. -12 This optical transmission system achieves the following error rates.

[0012] One aspect of the present invention is an optical transmission system comprising: an optical signal transmitter that outputs an optical signal; an optical signal receiver that receives the optical signal; and an optical transmission path that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal, wherein the optical transmission path has a ratio of the minimum value to the maximum value of the beam diameter of the output beam of 0.7 or more when a Gaussian beam emitted from a single-mode optical fiber is input with a positional shift.

[0013] One aspect of the present invention is an optical transmission system including an optical signal transmitter that transmits an optical signal output from a light source, an optical signal receiver that receives the optical signal, and an optical transmission line that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal. The optical transmission line is formed by connecting two or more optical transmission lines. The optical transmission lines are connected to each other through an air layer at at least one of the connection points. Among the optical transmission lines to be connected, at least the optical transmission line disposed immediately after the light source outputs the beam diameter enlarged by three times or more when a Gaussian beam emitted from a single-mode optical fiber is input with central excitation, and when a Gaussian beam emitted from a single-mode optical fiber is input with a shift in position, the ratio of the minimum value to the maximum value of the beam diameter of the output beam is 0.7 or more, and the product of the scattering loss and the length with respect to the optical signal is 6 dB or less.

[0014] One aspect of the present invention is an optical transmission system including an optical signal transmitter that outputs an optical signal, an optical signal receiver that receives the optical signal, and an optical transmission line that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal. The optical transmission line is formed by connecting two or more optical transmission lines. The optical transmission lines are connected to each other through an air layer at at least one of the connection points. Each optical transmission line outputs the beam diameter enlarged by three times or more when a Gaussian beam emitted from a single-mode optical fiber is input with central excitation, and when a Gaussian beam emitted from a single-mode optical fiber is input with a shift in position, the ratio of the minimum value to the maximum value of the beam diameter of the output beam is 0.7 or more, and the product of the scattering loss and the length with respect to the optical signal is 6 dB or less.

[0015] One aspect of the present invention is an optical and electrical composite cable including the optical transmission system.

[0016] One aspect of the present invention is an optical transmission line used in an optical signal transmitter that outputs an optical signal with a baud rate of 10 Gbaud or more, wherein the product of the scattering loss and the length with respect to the optical signal is 6 dB or less, and the beam diameter is expanded by 3 times or more and output when a Gaussian beam emitted from a single-mode optical fiber is input with central excitation.

Effect of the Invention

[0017] According to the present invention, there is an effect that communication of high quality and large capacity can be realized with a simple configuration.

Brief Description of the Drawings

[0018] [Figure 1] FIG. 1 is a schematic configuration diagram of an optical transmission system according to Embodiment 1. [Figure 2] FIG. 2 is a diagram for explaining a method of measuring the beam diameter. [Figure 3] FIG. 3 is a diagram showing measurement results of the beam diameters of the input light and the output light when a glass optical fiber is used as the second optical transmission line in Reference Example 1. [Figure 4] FIG. 4 is a diagram showing measurement results of the beam diameters of the input light and the output light when a plastic optical fiber is used as the first optical transmission line. [Figure 5] FIG. 5 is a diagram showing the relationship between the ratio of the beam diameter r1 of the output light to the beam diameter r0 of the input light and the length when a plastic optical fiber is used as the first optical transmission line. [Figure 6] FIG. 6 is a diagram showing the relationship between the modulation voltage and the common logarithm of the bit error rate (BER). [Figure 7] FIG. 7 is a schematic configuration diagram of an optical transmission system according to Embodiment 2. [Figure 8] FIG. 8 is a diagram showing measurement results of the beam diameters of the input light and the output light when a plastic optical fiber is used as the optical transmission line. [Figure 9]Figure 9 shows the relationship between the ratio of the output light beam diameter r1 to the input light beam diameter r0 and the length when a plastic optical fiber is used as the optical transmission path. [Figure 10] Figure 10 shows the constellation map for Comparative Example 4. [Figure 11] Figure 11 shows the constellation map of Example 10. [Figure 12] Figure 12 shows the error vector amplitudes over 30 measurements for Example 10 and Comparative Example 4. [Figure 13] Figure 13 illustrates the setting of the x and z axes in an optical transmission path. [Figure 14] Figure 14 shows the relationship between Δx and the beam diameter of the output light in Reference Example 1, Comparative Example 5, and Example 8. [Figure 15] Figure 15 shows the relationship between Δz and the beam diameter of the output light in Reference Example 1, Comparative Example 5, and Example 8. [Figure 16] Figure 16 shows the relationship between Δx and the output beam diameter in Reference Example 1, Comparative Example 5, and Example 8. [Figure 17] Figure 17 shows the relationship between Δz and the output beam diameter in Reference Example 1, Comparative Example 5, and Example 8. [Figure 18] Figure 18 is a diagram showing the normalized output beam diameter of Figure 16. [Figure 19] Figure 19 is a diagram showing the normalized output beam diameter of Figure 17. [Figure 20A] Figure 20A shows the relationship between Δx and the error rate in Reference Example 1. [Figure 20B] Figure 20B shows the relationship between Δx and the error rate in Comparative Example 5. [Figure 20C] Figure 20C shows the relationship between Δx and the error rate in Example 8. [Figure 21A] Figure 21A shows the relationship between Δz and the error rate in Reference Example 1. [Figure 21B]Figure 21B shows the relationship between Δz and the error rate in Comparative Example 5. [Figure 21C] Figure 21C shows the relationship between Δz and the error rate in Example 8. [Figure 22A] Figure 22A shows the relationship between Δx and the coupling loss in Reference Example 1. [Figure 22B] Figure 22B shows the relationship between Δx and the coupling loss in Comparative Example 5. [Figure 22C] Figure 22C shows the relationship between Δx and coupling loss in Example 8. [Figure 23A] Figure 23A shows the relationship between Δz and coupling loss in Reference Example 1. [Figure 23B] Figure 23B shows the relationship between Δz and coupling loss in Comparative Example 5. [Figure 23C] Figure 23C shows the relationship between Δz and coupling loss in Example 8. [Figure 24] Figure 24 shows the conditions for achieving error-free transmission. [Figure 25] Figure 25 is a schematic diagram of an experimental system used to investigate the effect of inter-fiber voids. [Figure 26A] Figure 26A shows the relationship between the modulation voltage and the error rate in Reference Example 1. [Figure 26B] Figure 26B shows the relationship between the modulation voltage and the error rate in Comparative Example 5. [Figure 26C] Figure 26C shows the relationship between the modulation voltage and the error rate in Example 8. [Modes for carrying out the invention]

[0019] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the present invention is not limited to these embodiments.

[0020] (Embodiment 1) [Configuration of the optical transmission system] Figure 1 is a schematic diagram of an optical transmission system according to Embodiment 1. The optical transmission system 100 includes optical signal transceivers 10 and 20, second optical transmission lines 31 and 32, and signal processing circuits 40 and 50. The optical signal transceivers 10 and 20 are examples of both an optical signal transmitter and an optical signal receiver. The optical signal transceivers 10 and 20 can be configured, for example, as modules or as On-Board Optics (OBO).

[0021] The optical signal transceiver 10 includes a transmitting / receiving unit 11, a first optical transmission line 12, an internal optical transmission line 13, and connectors 14 and 15. The optical signal transceiver 20 includes a transmitting / receiving unit 21, a first optical transmission line 22, an internal optical transmission line 23, and connectors 24 and 25.

[0022] First, the components of the optical signal transceiver 10 will be described. The transceiver unit 11 includes a light source LD (Laser Diode: LD) 11a, a photodetector PD (Photo-Diode) 11b, a driver IC 11c, and a TIA (Trans-Impedance Amplifier) ​​IC 11d.

[0023] LD11a is, for example, a VCSEL (Vertical Cavity Surface Emitting laser) that oscillates at a wavelength of, for example, 850 nm. LD11a is controlled by driver-IC11c and outputs an optical signal modulated with a predetermined modulation scheme and a predetermined modulation speed to the first optical transmission line 12. The modulation scheme is a digital modulation scheme, such as a binary modulation scheme like NRZ (Non Return to Zero) or a multi-level modulation scheme like PAM4, with a baud rate of, for example, 10 Gbaud or more, or 25 Gbaud or more. In the case of NRZ, the baud rate and bit rate are equal (for example, 25 Gbaud = 25 Gbps), and in the case of PAM4, the bit rate is twice the baud rate (for example, 25 Gbaud = 50 Gbps). In this embodiment, LD11a is a 4-channel array, but it may be any number of arrays with 2 or more channels, or a single unit.

[0024] The first optical transmission path 12 is optically connected to the LD 11a by connector 14 and optically connected to the second optical transmission path 32 by connector 15. That is, the first optical transmission path 12 is arranged either directly with the light source LD 11a or via a lens system. (In Figure 1, the case via a lens is omitted.) Connectors 14 and 15 may be, for example, MT connectors, MPO connectors, or other connectors of any shape. The first optical transmission path 12 will be described in detail later. In this embodiment, the first optical transmission path 12 is a ribbon-like structure with 4 lanes, but it may be any number of lanes, two or more, or even a single lane.

[0025] As a result, the LD11a transmits the optical signal output from the LD11a to the optical signal transceiver 20 via the first optical transmission path 12, the second optical transmission path 32, and the internal optical transmission path 23.

[0026] PD11b receives the optical signal transmitted from the optical signal transceiver 20 via the first optical transmission path 22, the second optical transmission path 31, and the internal optical transmission path 13, and converts the optical signal into a current signal. TIA-IC11d amplifies the current signal from PD11b and converts it into a voltage signal, which is then output to the signal processing circuit 40. In this embodiment, PD11b and TIA-IC11d are 4-channel arrays, but any number of arrays with 2 or more channels may be used, or they may be used individually.

[0027] The signal processing circuit 40 is electrically connected to the optical signal transceiver 10 by soldering or connector connections. The signal processing circuit 40 supplies a modulated signal superimposed on the optical signal to the driver IC 11c and processes the voltage signal from the TIA IC 11d.

[0028] Next, the components of the optical signal transceiver 20 will be described. The transceiver unit 21 includes an LD21a, a PD21b, a driver-IC21c, and a TIA-IC21d.

[0029] LD21a, like LD11a, is, for example, a VCSEL and oscillates a laser at a wavelength of, for example, 850 nm. LD21a is controlled by driver-IC21c and, like LD11a, outputs an optical signal modulated with a predetermined modulation scheme and a predetermined modulation speed to the first optical transmission line 22. In this embodiment, LD21a is a 4-channel array, but it may be an array of any number of channels (2 or more) or a single unit.

[0030] The first optical transmission path 22 is optically connected to the LD21a by connector 24 and optically connected to the second optical transmission path 31 by connector 25. That is, the first optical transmission path 22 is arranged either directly with the light source LD21a or via a lens system. Connectors 24 and 25 may be, for example, MT connectors, MPO connectors, or other connectors of any shape. The first optical transmission path 22 will be described in detail later. In this embodiment, the first optical transmission path 22 is a ribbon-like structure with 4 lanes, but it may be any number of lanes, two or more, or a single lane.

[0031] As a result, the LD21a transmits the optical signal output from the LD21a to the optical signal transceiver 10 via the first optical transmission path 22, the second optical transmission path 31, and the internal optical transmission path 13.

[0032] PD21b receives the optical signal transmitted from the optical signal transceiver 10 via the first optical transmission path 12, the second optical transmission path 32, and the internal optical transmission path 23, and converts the optical signal into a current signal. TIA-IC21d amplifies the current signal from PD21b and converts it into a voltage signal, which is then output to the signal processing circuit 50. In this embodiment, PD21b and TIA-IC21d are 4-channel arrays, but any number of arrays with 2 or more channels may be used, or they may be used individually.

[0033] The signal processing circuit 50 is electrically connected to the optical signal transceiver 20 by soldering or connector connections. The signal processing circuit 50 supplies a modulated signal superimposed on the optical signal to the driver IC 21c and processes the voltage signal from the TIA IC 21d.

[0034] As described above, the optical transmission system 100 is configured to enable bidirectional optical transmission, with the first optical transmission path 22, the second optical transmission path 31, and the internal optical transmission path 13 constituting an optical transmission path 61 for transmitting optical signals. In addition, the first optical transmission path 12, the second optical transmission path 32, and the internal optical transmission path 23 constituting an optical transmission path 62 for transmitting optical signals. The second optical transmission paths 31 and 32 and the optical signal transceivers 10 and 20 may be integrated to form an Active Optical Cable (AOC).

[0035] [Configuration of optical transmission path] Next, the configuration of the first optical transmission lines 12 and 22, the second optical transmission lines 31 and 32, and the internal optical transmission lines 13 and 23 will be described. First, the second optical transmission lines 31 and 32 are multi-mode optical fibers (MMF) made of glass such as silica-based glass or plastic. The transmission loss of the optical signal in the second optical transmission lines 31 and 32 (for example, the transmission loss at a wavelength of 850 nm) is, for example, 100 dB / km or less, or 50 dB / km or less, furthermore, 10 dB / km or less, and furthermore, 3 dB / km or less, the core diameter is, for example, about 50 μm, and the numerical aperture (NA) is, for example, about 0.2. Also, the second optical transmission lines 31 and 32 are longer than any of the first optical transmission lines 12 and 22 and the internal optical transmission lines 13 and 23. The second optical transmission lines 31 and 32 may also be of the graded-index (GI) type.

[0036] Next, the first optical transmission lines 12 and 22 are optical fibers made of glass such as silica-based glass, or optical fibers made of plastic (Plastic Optical Fiber: POF), and are MMF. The first optical transmission lines 12 and 22 may also be GI type. When the length of the first optical transmission line is a few centimeters to tens of centimeters or less, a GI distribution is not necessary as a sufficient transmission bandwidth is secured, and an SI (Step-Index) type distribution is also acceptable. Furthermore, the shape of the first optical transmission line is not particularly limited as long as it is a transmission line with controlled scattering as defined in the present invention, such as an optical waveguide shape or optical fiber shape. That is, the cross-sectional shape of the first optical transmission line may be, for example, circular, rectangular, or any other arbitrary shape.

[0037] The first optical transmission lines 12 and 22 have scattering losses for optical signals (e.g., scattering losses at a wavelength of 850 nm) of, for example, 50 dB / km or more, or 100 dB / km or more, or 200 dB / km or more, or 500 dB / km or more, or even 1000 dB / km or more. In such first optical transmission lines 12 and 22, the optical signal is transmitted while mode-coupled with higher-order modes by forward scattering. As a result, the first optical transmission lines 12 and 22 output the beam diameter of the optical signal input to each of them expanded by more than three times. In contrast, the second optical transmission lines 31 and 32 output the beam diameter of the optical signal input to each of them expanded by less than three times. Note that such first optical transmission lines 12 and 22 output the beam diameter expanded by more than three times when a Gaussian beam emitted from a single-mode optical fiber is input with central excitation (see evaluation in the central excitation state in Figure 2). Furthermore, the second optical transmission lines 31 and 32 output a beam with a diameter of less than three times when a Gaussian beam emitted from a single-mode optical fiber is input via central excitation.

[0038] The inventors conducted an experiment in which they expanded the beam diameter of the optical signal output from LD11a or LD21a using the first optical transmission line 12 or 22, transmitted it through the second optical transmission line 31 or 32, and received it using PD21b or PD11b. Surprisingly, they found that the error rate in optical transmission was lower when optical transmission lines 61 and 62 were used compared to when the first optical transmission lines 12 and 22 were not present.

[0039] In other words, the optical transmission system 100 can achieve high-quality and high-capacity communication with a simple configuration that adds first optical transmission lines 12 and 22, which have an extremely large beam diameter expansion ratio, to second optical transmission lines 31 and 32, which have a small beam diameter expansion ratio as is commonly used.

[0040] However, if the length of the first optical transmission lines 12 and 22 is too long, the loss of optical signals through the first optical transmission lines 12 and 22 will increase. Therefore, it is preferable that the product of the scattering loss to the optical signal and the length of the first optical transmission lines 12 and 22 is 6 dB or less. In this case, the length of the first optical transmission lines 12 and 22 can be 1 to several tens of centimeters. For example, if the optical signal transceivers 10 and 20 are configured as optical transceiver modules, the length can be 1 to several centimeters. Also, for example, if the optical signal transceivers 10 and 20 are configured as OBOs, the length can be several tens of centimeters due to being routed inside the housing.

[0041] In this embodiment, the internal optical transmission lines 13 and 23 are of the same type as, for example, either of the second optical transmission lines 31 and 32. However, the internal optical transmission lines 13 and 23 may also be of the same type as, for example, either of the first optical transmission lines 12 and 22. The length of the internal optical transmission lines 13 and 23 may be approximately the same as, for example, either of the first optical transmission lines 12 and 22.

[0042] (Formation of microheterogeneous structures) Next, preferred examples of the first optical transmission lines 12 and 22 will be described in detail. For example, if the optical fiber core has a microscopic heterogeneous structure with a correlation length of several hundred angstroms or larger, it becomes possible to increase forward scattering, which is different from the so-called Rayleigh scattering observed in silica-based glass optical fibers. For example, polymer chains with molecular weights of several hundred thousand have a coil-like structure, and their radius of inertia is around several hundred angstroms. Furthermore, polymer coils can slightly associate with each other to form large heterogeneous structures. In such cases, the correlation distance increases, as can be derived from Debye's scattering theory, leading to forward scattering and contributing to mode coupling. Microscopic heterogeneous structures can also be formed by copolymers. Generally, copolymers have a compositional distribution, and like monomer units associate with each other, making them even more prone to forming heterogeneous structures than homopolymers. While these heterogeneous structures depend on the extrusion manufacturing conditions, polymer molecular weight, and thermal history, if the enthalpy relaxation phenomenon is effectively utilized and an appropriate metastable enthalpy state can be achieved, polymers with specific micro-heterogeneous structures can be mass-produced without problems for use. Such micro-heterogeneous structures do not exist in quartz glass. Besides creating micro-heterogeneous structures in polymers, another effective method for controlling scattering is to add particles to the polymer or glass. For example, when considering a first optical transmission path of a few centimeters or less that fits within an optical signal transceiver, scattering that enables stronger mode coupling is necessary. To achieve this, it is effective to add submicron or micron-order particles with different refractive indices to the core. Candidate particles are not limited to those with a refractive index different from the polymer or glass medium constituting the core, and include, but are not limited to, metal particles such as iron, silicon particles, silica particles, and mineral particles such as calcium carbonate. To enhance forward scattering with these particles, micron-sized particles are preferable to nano-sized particles. Instead of adding particles, the formation of microvoids produces a similar effect and works effectively. In one embodiment of GI-type POF, a refractive index distribution is formed by radially varying the concentration of a low-molecular-weight dopant having a refractive index different from that of the polymer matrix. The size of the dopant is on the order of a few to tens of angstroms, and the intensity of light scattering from a single molecule is negligibly small. However, if the dopant concentration fluctuates slightly on the order of a few hundred to several thousand angstroms, a micro-heterogeneous structure is formed, inducing forward light scattering. This slight fluctuation / association of dopant arises from slight differences in compatibility between the polymer matrix and the dopant. Therefore, by examining the differences in compatibility between the polymer and the dopant, using solubility parameters as a guide, it becomes possible to control the micro-heterogeneous structure caused by dopant fluctuation / association, and thus control mode coupling. Furthermore, by adding not only dopants for refractive index distribution formation but also low-molecular-weight molecules for forming micro-heterogeneous structures, it is possible to control mode coupling caused by forward scattering using a similar principle.

[0043] For example, acrylic polymers exhibit intramolecular and intermolecular interactions due to ester groups present within the molecule. In contrast, fully fluorinated polymers such as dioxolenes lack such ester groups. Therefore, their intramolecular and intermolecular interactions are smaller than those of acrylic polymers. However, both polymers are aggregates of molecular coils with radii of inertia of several hundred angstroms, and their relatively stable micro-heterogeneous structures can be controlled, for example, in extrusion molding.

[0044] (Examples of polymers used in the first optical transmission line) The polymers constituting the core and cladding portions of the first optical transmission lines 12 and 22 can be manufactured by methods known in the art. For example, a mixture of monomers constituting the polymer can be subjected to solution polymerization, bulk polymerization, emulsion polymerization, or suspension polymerization. Among these, bulk polymerization is preferred from the viewpoint of preventing the inclusion of foreign matter and impurities.

[0045] The polymerization temperature is not particularly limited, but for example, around 80 to 150°C is suitable. The reaction time can be appropriately adjusted depending on the amount and type of monomer, the amount of polymerization initiator and chain transfer agent described later, the reaction temperature, etc., and around 20 to 60 hours is suitable. These polymers may be produced simultaneously or continuously when forming the core and / or cladding.

[0046] The polymers constituting the core include, for example, (meth)acrylic acid ester compounds such as ethyl methacrylate, n-propyl methacrylate, n-butyl methacrylate, ethyl acrylate, n-propyl acrylate, and n-butyl acrylate; styrene compounds such as styrene, α-methylstyrene, chlorostyrene, and bromostyrene; vinyl esters such as vinyl acetate, vinyl benzoate, vinyl phenyl acetate, and vinyl chloroacetate; and maleimides such as N-n-butyl maleimide, N-tert-butyl maleimide, N-isopropyl maleimide, and N-cyclohexyl maleimide. Examples include substances in which some of the hydrogen atoms in the CH bonds of these monomers are substituted with chlorine, fluorine, or deuterium. The stretching vibrations between the CH bonds constituting the above polymers cause absorption loss due to their harmonics, for example, at a light source wavelength of 850 nm. However, when the first optical transmission path is several cm to tens of cm or less, this absorption loss may be negligible. In such cases, general polymers such as unhalogenated acrylics and styrenes may be used.

[0047] When producing polymers, it is preferable to use polymerization initiators and / or chain transfer agents. Examples of polymerization initiators include conventional radical initiators. For example, peroxide compounds such as benzoyl peroxide, t-butyl peroxy-2-ethylhexanate, di-t-butyl peroxide, t-butyl peroxyisopropyl carbonate, and n-butyl 4,4,bis(t-butyl peroxy)valate; 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonnitrile), 2,2'-azobis(2-methylpropane), 2,2'-azobis(2-methylbutane), 2,2'-azobis(2-methylpentane), 2,2'-azobis(2,3-dimethylbutane), and 2,2'-azobis Examples of azo compounds include (2-methylhexane), 2,2'-azobis(2,4-dimethylpentane), 2,2'-azobis(2,3,3-trimethylbutane), 2,2'-azobis(2,4,4-trimethylpentane), 3,3'-azobis(3-methylpentane), 3,3'-azobis(3-methylhexane), 3,3'-azobis(3,4-dimethylpentane), 3,3'-azobis(3-ethylpentane), dimethyl-2,2'-azobis(2-methylpropionate), diethyl-2,2'-azobis(2-methylpropionate), and di-t-butyl-2,2'-azobis(2-methylpropionate). These may be used individually or in combination of two or more. The polymerization initiator is best used at a concentration of approximately 0.01 to 2% by weight relative to the total monomer.

[0048] The chain transfer agent is not particularly limited and any known agent can be used. Examples include alkyl mercaptans (n-butyl mercaptan, n-pentyl mercaptan, n-octyl mercaptan, n-lauryl mercaptan, t-dodecyl mercaptan, etc.) and thiophenols (thiophenol, m-bromothiophenol, p-bromothiophenol, m-toluenethiol, p-toluenethiol, etc.). Among these, alkyl mercaptans such as n-butyl mercaptan, n-octyl mercaptan, n-lauryl mercaptan, and t-dodecyl mercaptan are preferably used. In addition, a chain transfer agent in which the hydrogen atoms of the CH bond are substituted with deuterium atoms or fluorine atoms may be used. These may be used alone or in combination of two or more types.

[0049] Chain transfer agents are typically used to adjust the molecular weight to a suitable level for molding and physical properties. The chain transfer constant of a chain transfer agent for each monomer can be determined experimentally, for example, by referring to "Experimental Methods for Polymer Synthesis" (by Takayuki Otsu and Masayoshi Kinoshita, Kagaku Dojin, 1972), as described in the Polymer Handbook, 3rd Edition (edited by J. Brandrup and Ehim M. Ergut, published by John W. Iley & Son). Therefore, it is preferable to adjust the type and amount of the agent appropriately, taking the chain transfer constant into consideration, depending on the type of monomer. For example, an amount of about 0.1 to 4 parts by weight per 100 parts by weight of the total monomer component is used.

[0050] The polymer constituting the core and / or cladding portion is suitable if it has a weight-average molecular weight in the range of 50,000 to 300,000, and preferably in the range of 100,000 to 250,000. This is to ensure appropriate flexibility, transparency, etc. The molecular weights of the core and cladding portions may differ, for example, to adjust viscosity. The weight-average molecular weight refers to the polystyrene equivalent value measured by GPC (gel permeation chromatography).

[0051] The polymers constituting the first optical transmission lines 12 and 22 may contain, as needed, compounding agents such as thermal stabilization aids, processing aids, heat resistance enhancers, antioxidants, and light stabilizers, provided that these do not impair the transparency, heat resistance, and other properties of the optical fiber. These can be used individually or in combination of two or more, and methods for mixing these compounding agents with monomers or polymers include, for example, hot blending, cold blending, and solution mixing.

[0052] (Fluorine-containing polymer used in the first optical transmission line) When using fluorine-containing polymers (including fully fluorinated and partially fluorinated materials) as core materials for the first optical transmission lines 12 and 22, they can be synthesized by the following method. [Synthesis Example A] Synthesis Method for Total Fluorine-Based Materials As all-fluorine materials, products such as TEFRON-AF (DuPont), HyflonAD (Solvay), and CYTOP (AGC) can generally be used. Alternatively, all-fluorine polymers copolymerized with tetrafluoroethylene or the like in the main ring structure of these materials may be used. All-fluorine polymers with a dioxolene skeleton can also be used. Next, a method for synthesizing all-fluorine materials with a dioxolene skeleton will be described.

[0053] <Synthesis of perfluoro-4-methyl-2-methylene-1,3-dioxolane> 2-Carbomethyl-2-trifluoromethyl-4-methyl-1,3-dioxolane was obtained by a dehydration condensation reaction of 2-chloro-1-propanol, 1-chloro-2-propanol, and methyl trifluoropyruvate. Next, perfluoro-4-methyl-2-methylene-1,3-dioxolane was fluorinated. Using 1,1,2-trichlorotrifluoroethane as the solvent, nitrogen gas and fluorine gas were flowed at constant flow rates, and the previously prepared 2-carbomethyl-2-trifluoromethyl-4-methyl-1,3-dioxolane was slowly added to the reaction vessel under a nitrogen / fluorine atmosphere to perform the fluorination treatment and obtain perfluoro-2,4-dimethyl-1,3-dioxolane-2-carboxylic acid. The above distillate was neutralized with an aqueous potassium hydroxide solution to obtain perfluoro-2,4-dimethyl-2-carboxylic acid potassium-1,3-dioxolane. This potassium salt was vacuum-dried, and then the salt was decomposed under an argon atmosphere to obtain perfluoro-4-methyl-2-methylene-1,3-dioxolane. The perfluoro-4-methyl-2-methylene-1,3-dioxolane obtained above and perfluorobenzoyl peroxide were placed in a glass tube, degassed using a freeze / thaw vacuum machine, refilled with argon, and heated for several hours. The contents became solid, and a transparent polymer was obtained. Optical fibers were fabricated using this polymer.

[0054] The viscosity of fluorine-containing polymers (including fully fluorinated and partially fluorinated materials) in the molten state is 10 at melting temperatures of 200°C to 300°C. 3 ~10 5 Poise is preferred. If the melt viscosity is too high, not only is melt spinning difficult, but the diffusion of dopants necessary for the formation of the refractive index distribution is hindered, making it difficult to form the refractive index distribution. On the other hand, if the melt viscosity is too low, practical problems arise. Specifically, when used as an optical transmission material in electronic devices or automobiles, it softens when exposed to high temperatures, and the optical transmission performance deteriorates.

[0055] The number-average molecular weight of the fluorine-containing polymer is preferably 10,000 to 5,000,000, and more preferably 50,000 to 1,000,000. If the molecular weight is too low, it may impair heat resistance, and if it is too high, it becomes difficult to form an optical transmission material with a refractive index distribution, which is undesirable.

[0056] (Partially chlorinated polymer used in the first optical transmission line) When using partially chlorinated materials as core materials for the first optical transmission lines 12 and 22, they can be synthesized by the same method as the synthesis method for all-fluorine materials, which is a general method of preparation described above.

[0057] [Synthesis Example B] Synthesis of partially chlorinated materials (see Patent No. 5419815) Next, we will briefly describe the method for preparing the partially chlorinated material. Trichloroethyl methacrylate, which has been purified by distillation, cyclohexyl maleimide, which has been purified by sublimation, and diphenyl sulfide, which is used as a refractive index-imparting dopant, were each accurately weighed and placed in a glass container. Furthermore, a predetermined amount of ditertically butyl peroxide as a polymerization initiator and n-lauryl mercaptan as a chain transfer agent were added relative to the total weight. After thoroughly mixing this solution, it was filtered into a glass polymerization container by passing it through a membrane filter with a small pore size. Next, dissolved air was removed from the glass polymerization tube containing this solution by freeze-degassing while introducing argon gas. This glass polymerization tube was placed in an oven, and the temperature of the polymerization container was raised while introducing argon gas to polymerize the monomers, and the polymerization reaction was completed by further raising the temperature. This glass tube was opened to obtain a solidified transparent polymerization rod.

[0058] (Dopants for creating refractive index distributions) When the solubility parameter of the dopant is equal to that of the polymer, indicating good compatibility, the dopant is uniformly distributed within the polymer matrix. On the other hand, as the difference in solubility parameters between the dopant and the polymer increases, the tendency for dopants to aggregate increases, forming a heterogeneous refractive index structure due to the dopant concentration distribution. In addition to general knowledge of solubility parameters, it is also possible to form a microscopic concentration distribution of dopant by adding local interactions between the dopant and the polymer (for example, secondary electronic polarization corresponding to specific functional groups). For dopants used in all-fluorine core materials, materials with a higher refractive index than all-fluorine polymers are usually used. That is, the material dopant is a material that substantially does not have CH bonds for the same reasons as all-fluorine polymers, and it is more preferable that its refractive index is 0.05 or greater than that of the all-fluorine polymer. A higher refractive index means that a smaller amount of dopant is needed to form the desired refractive index distribution, resulting in a smaller decrease in the glass transition temperature and, consequently, an increase in the heat resistance of the optical fiber; therefore, a value of 0.1 or greater is particularly preferable.

[0059] Preferred dopants include low-molecular-weight compounds, oligomers, and polymers containing aromatic rings such as benzene rings, halogen atoms such as chlorine, bromine, and iodine, and bonding groups such as ether bonds. However, in the case of polymers, larger molecular weights reduce compatibility with the total fluorine polymer, resulting in increased light scattering loss; therefore, polymers with excessively large molecular weights are undesirable. Conversely, compounds with very small molecular weights are also undesirable because they lower the glass transition temperature in mixtures with fluorine-containing polymers, causing a decrease in the heat resistance temperature of the optical fiber. Therefore, the number-average molecular weight of the dopant should be 3 × 10⁻⁶. 2 ~2×10 3 Preferably, 3 × 10 2 ~1 × 10 3 This is preferable.

[0060] Specific dopant compounds include oligomers of chlorotrifluoroethylene (5-8

[0061] In addition to halogen-containing aliphatic compounds such as the oligomers mentioned above, halogenated aromatic hydrocarbons and halogen-containing polycyclic compounds that do not contain hydrogen atoms bonded to carbon atoms can also be used. In particular, fluorinated aromatic hydrocarbons and fluorine-containing polycyclic compounds that contain only fluorine atoms as halogen atoms (or a relatively small number of chlorine atoms relative to fluorine atoms) are preferred in terms of compatibility with fluorine-containing polymers. Furthermore, it is more preferable that these halogenated aromatic hydrocarbons and halogen-containing polycyclic compounds do not have polar functional groups such as carbonyl groups and cyano groups.

[0062] Examples of such halogenated aromatic hydrocarbons include compounds represented by the formula Φr-Zb [Φr is a b-valent fluorinated aromatic ring residue in which all hydrogen atoms are replaced by fluorine atoms, Z is a halogen atom other than fluorine, -Rf, -CO-Rf, -O-Rf, or -CN. Here, Rf is a perfluoroalkyl group, a polyfluoroperhaloalkyl group, or a monovalent Φr. b is 0 or an integer of 1 or more.]. The aromatic rings include benzene rings and naphthalene rings. The number of carbon atoms in the perfluoroalkyl group or polyfluoroperhaloalkyl group that is Rf is preferably 5 or less. As halogen atoms other than fluorine, chlorine atoms and bromine atoms are preferred. Specific compounds include, for example, 1,3-dibromotetrafluorobenzene, 1,4-dibromotetrafluorobenzene, 2-bromotetrafluorobenzotrifluoride, clopentafluorobenzene, bromopentafluorobenzene, iodopentafluorobenzene, decafluorobenzophenone, perfluoroacetophenone, perfluorobiphenyl, chloroheptafluoronaphthalene, and bromoheptafluoronaphthalene. Particularly preferred dopants as examples of fluorine-containing polycyclic compounds are chlorotrifluoroethylene oligomer, perfluoro(triphenyltriazine), perfluoroterphenyl, perfluoroquatrophenyl, perfluoro(triphenylbenzene), and perfluoroanthracene, due to their good compatibility with all-fluorine polymers, especially fluorine-containing polymers having a ring structure in the main chain, and their good heat resistance. Due to their good compatibility, substances to be mixed with fluorine-containing polymers, especially fluorine-containing polymers having a ring structure in the main chain, can be easily mixed by heating and melting at 200-300°C. Furthermore, the two substances can be uniformly mixed by dissolving them in a fluorine-containing solvent, mixing them, and then removing the solvent.

[0063] Examples of dopants used in partially chlorinated or partially fluorinated core materials include low molecular weight compounds or compounds in which hydrogen atoms present in these compounds are replaced with deuterium atoms. Examples of low molecular weight compounds with high refractive indices include diphenyl sulfone (DPSO) and diphenyl sulfone derivatives (e.g., chlorinated diphenyl sulfones such as 4,4'-dichlorodiphenyl sulfone and 3,3',4,4'-tetrachlorodiphenyl sulfone), sulfur compounds such as diphenyl sulfide (DPS), diphenyl sulfoxide, dibenzothiophene, and dithiane derivatives; phosphoric acid compounds such as triphenyl phosphate (TPP) and tricresyl phosphate; benzyl benzoate; benzyl n-butyl phthalate; diphenyl phthalate; biphenyl; and diphenylmethane. Examples of low molecular weight compounds with low refractive indices include tris-2-ethylhexyl phosphate (TOP). These may be used individually or in combination of two or more.

[0064] (Method for manufacturing the first optical transmission line) To facilitate the creation of microscopic heterogeneous structures, the temperature and drawing speed during optical fiber spinning may be controlled. Common methods for producing optical fibers using fluorine-containing polymers include the preform method and the melt extrusion method. The preform method involves creating rod-shaped plastic molded bodies called core and cladding rods. These core rods are placed in the center, and the cladding rods, which have a hollow section, are integrated to cover the outer circumference of the core, creating a rod-shaped object called a preform. This preform is then placed in a general spinning apparatus, and the outer circumference of the preform is uniformly heated and melted using a cylindrical heater or similar device. The tip is then drawn and stretched at a constant speed to form a fiber, which is then cooled and wound to obtain an optical fiber.

[0065] On the other hand, the melt extrusion method involves filling a general melt extrusion apparatus with a polymer that has been pre-mixed with a predetermined amount of dopant as the core polymer and a polymer that does not contain dopant as the cladding polymer. The two extruders then combine the molten polymers and co-extrude them, causing both polymers to be discharged from a nozzle to obtain optical fibers. Generally, an extruder with a screw may be used, but melt extrusion may also be performed under pressure such as nitrogen gas. A coating layer can also be provided as needed.

[0066] A heat treatment process following co-extrusion of the molten core polymer and molten cladding polymer can also form a micro-heterogeneous structure. For example, if rapid cooling is performed after co-extrusion, the polymer will vitrify while retaining a large volume before enthalpy relaxation occurs. On the other hand, if sufficient heat treatment is performed near the glass transition temperature, the volume will decrease slightly due to enthalpy relaxation. If this enthalpy relaxation occurs in the micro-region, a so-called micro-heterogeneous structure is formed. Furthermore, if a stretching process is added after co-extrusion, the molecules of the molten-extruded fiber undergo orientation, and orientational birefringence occurs depending on the degree of orientation. This orientational birefringence occurs not only in the fiber axis direction but also consequently in the radial direction and in specific directions. This birefringent structure also promotes mode coupling.

[0067] Methods known in the art can be used to manufacture the optical fiber of the present invention. For example, interfacial gel polymerization, rotational polymerization, melt extrusion dopant diffusion, composite melt spinning, and rod-in-tube methods can be used to form a cladding layer of one or more layers on the outer circumference of a core of one or more layers. Alternatively, a preform may be formed in advance and then stretched, drawn, etc.

[0068] Specifically, one method involves creating a hollow cladding portion and then creating a core portion within the hollow part of this cladding portion. In this case, monomers constituting the core portion are introduced into the hollow part of the cladding portion, and the polymer is polymerized while rotating the cladding portion to form a core portion having a higher refractive index than the cladding portion. This operation may be performed only once to form a single-layer core portion, or it may be repeated to form a core portion consisting of multiple layers.

[0069] The polymerization container used can be a cylindrical tube made of glass, plastic, or metal, possessing sufficient mechanical strength to withstand external forces such as centrifugal force from rotation and heat resistance during thermal polymerization. The rotation speed of the polymerization container during polymerization is typically around 500 to 3000 rpm. Generally, it is preferable to filter the monomer to remove any dust particles before introducing it into the polymerization container.

[0070] Furthermore, to create a GI-type refractive index distribution in optical fibers, examples include interfacial gel polymerization, as described in WO93 / 008488, in which the monomer composition ratio is kept constant, a dopant is added, and the monomers are polymerized in bulk at the polymer interface, thereby imparting a dopant concentration distribution through this reaction; or rotational gel polymerization, in which the reaction mechanism of interfacial gel polymerization is carried out by rotational polymerization; and rotational polymerization, in which the composition ratio of monomers with different refractive indices is gradually changed, that is, the polymerization rate of the previous layer is controlled (the polymerization rate is kept low), and the next layer with a higher refractive index is polymerized, so that the refractive index distribution gradually increases from the interface with the cladding to the center.

[0071] Furthermore, a method may be used to form the core and cladding parts using two or more melt extruders, two or more multilayer dies, and multilayer spinning nozzles. In other words, the polymers constituting the core and cladding parts are heated and melted, and injected from individual channels into the multilayer dies and multilayer spinning nozzles. The core is extruded using these dies and nozzles, and at the same time, one or more concentric cladding parts are extruded around its outer circumference and welded together to form a fiber or preform.

[0072] Furthermore, examples include a melt extrusion dopant diffusion method in which a core and cladding are formed using two or more melt extruders, two or more multilayer dies, and a multilayer spinning nozzle, and then the dopant is diffused toward the periphery or center in a subsequently provided heat treatment zone to impart a dopant concentration distribution; and a method in which polymers with different amounts of dopant are introduced into two or more melt extruders to extrude the core and / or cladding in a multilayer structure.

[0073] When applying an SI-type refractive index distribution, it is suitable to perform rotational polymerization or the like while keeping the monomer composition ratio and / or the amount of dopant added constant from start to finish. When applying a multi-step refractive index distribution, it is preferable to control the polymerization rate of the previous layer (increase the polymerization rate) during rotational polymerization or the like, and polymerize the subsequent layer which will have a higher refractive index.

[0074] (Examples 1-3, Reference Example 1) In Examples 1-3, GI-POF fabricated by the melt extrusion method described above was used as the first optical transmission line. The characteristics of the optical transmission lines in Examples 1-3 and the characteristics of the second optical transmission line of Reference Example 1, which is made of silica-based optical fiber and has a transmission loss of 2.3 dB / km at a wavelength of 850 nm, a core diameter of approximately 50 μm, and an NA of approximately 0.2, were measured.

[0075] Figure 2 illustrates the method for measuring the beam diameter. The beam diameter is obtained from the measurement of the near-field pattern (NFP). Specifically, light 203 (mode field diameter 4.9 μm, Gaussian beam) emitted from the pigtail (APC polished) of a polarization-maintaining single-mode optical fiber 202 of a single-frequency DBR laser 201 with a central wavelength of 850 nm was input to an optical transmission line 205 (first or second optical transmission line) using a lens 204 via a half-mirror. At this time, microscopic observation using a CCD camera 206 was used to ensure that the light was input to the core center of the optical transmission line 205 via the lens 204, and evaluation was performed under central excitation conditions. The NFP of the light 207 output from the end face opposite to the input end face of the optical transmission line 205 was measured using an NFP measuring device 208 (NFP1006 manufactured by Precise Gauges), and the beam diameter of the light output from the optical transmission line 205 was determined. Furthermore, the beam diameter of the light input to the optical transmission line 205 was determined by measuring the light output from the lens 204 in the measurement system shown in Figure 2.

[0076] Figure 3 shows the measurement results of the beam diameters of the input and output light in the second optical transmission path of Reference Example 1. Note that Figure 3 shows an image of the measurement as NFP. The input optical signal is indicated as "Input," and the output optical signal as "Output." The lengths of the second optical transmission path in Reference Example 1 were 10m, 30m, and 100m. The white bars in the figure represent a 10μm scale. The beam diameter (D4σ width) was calculated from the measured NFP using the second moment method. As can be seen from Figure 3, the input light beam diameter was 4.9μm, but in the second optical transmission path of Reference Example 1, the beam diameter hardly expanded even after 100m of transmission, with an expansion ratio of less than 3x.

[0077] Figure 4 shows the measurement results of the beam diameter of the output light in the first optical transmission line of Examples 1 to 3. Figure 4 shows the image measured as NFP. Note that the beam diameter of the incident light is the same as "Input" in Figure 3, so it is omitted from the illustration. The characteristics of the first optical transmission line are as follows.

[0078] The first optical transmission line in Example 1 had an OTDR loss of 460 dB / km, measured at a wavelength of 850 nm using an OTDR (Optical Time Domain Reflectometer). This loss is thought to be mostly due to scattering loss. The core diameter was approximately 40 μm to 45 μm.

[0079] The first optical transmission line in Example 2 had an OTDR loss of 870 dB / km. This loss is thought to be mostly due to scattering loss. The core diameter was approximately 42.8 μm.

[0080] The first optical transmission line in Example 3 had an OTDR loss of 2190 dB / km. This loss is thought to be mostly due to scattering loss. The core diameter was approximately 45.1 μm.

[0081] Furthermore, for each of the first optical transmission lines in Examples 1 to 3, the lengths were set to 0.15m, 0.30m, 0.50m, 1.0m, 2.0m, and 3.0m.

[0082] Figures 4(a), (b), and (c) show NFP images of the first optical transmission lines in Examples 1, 2, and 3, respectively. As can be seen from Figure 4, in all cases the beam diameter had already expanded at a length of 0.15 m in the first optical transmission line, and expanded significantly at 3.0 m.

[0083] Figure 5 shows the relationship between the ratio r1 / r0 (the ratio of the beam diameter r1 of the output light to the beam diameter r0 of the input light) and the length (Fiber Length) in the first optical transmission lines of Examples 1 to 3. As can be seen from Figure 5, in all of the first optical transmission lines of Examples 1 to 3, the beam diameter of the input optical signal is expanded by more than three times before output. That is, in all of the first optical transmission lines of Examples 1 to 3, r1 / r0 is 3 or more, and in Example 3 in particular, r1 / r0 was 6 or more for all lengths. This suggests that mode coupling occurs more significantly in optical transmission lines with greater scattering loss among the first optical transmission lines.

[0084] (Examples 4-7, Comparative Examples 1 and 2) Next, using the first optical transmission line of Example 3 (OTDR loss: 2190 dB / km), an optical transmission system with a configuration similar to the optical transmission system 100 of Example 1 was constructed, and the error rate was measured.

[0085] In this case, as the optical transmission system for Comparative Example 1, the 100m long second optical transmission path shown in Figure 3 for Reference Example 1 was designated as the second optical transmission path 32, the first optical transmission path 12 and the internal optical transmission path 23 were not connected, the optical signal transceiver 10 was used as the optical transmitter, and the optical signal transceiver 20 was used as the optical receiver to construct the optical transmission system.

[0086] Furthermore, as the optical transmission system of Example 4, in the configuration of the optical transmission system 100, the second optical transmission path with a length of 100m shown in Figure 3 was designated as the second optical transmission path 32, the first optical transmission path 12 was set to 0.15m of the first optical transmission path of Example 3, the internal optical transmission path 23 of the equipment was not connected, the optical signal transceiver 10 was used as the optical transmitter, and the optical signal transceiver 20 was used as the optical receiver to construct the optical transmission system. However, the first optical transmission path 12 and the second optical transmission path 32 were connected via an air gap (void) with a gap length of 50μm, and measurements were performed under conditions where axial misalignment in the radial and angular directions could be ignored.

[0087] Furthermore, as the optical transmission system of Example 5, the optical transmission system was constructed by using the configuration of the optical transmission system of Example 4, but with the first optical transmission path 12 being 0.30 m, the same as the first optical transmission path of Example 3.

[0088] Furthermore, as the optical transmission system of Example 6, the optical transmission system was constructed by using the configuration of the optical transmission system of Example 5, but with the first optical transmission path 12 being 0.50 m, the same as the first optical transmission path of Example 3.

[0089] Furthermore, as the optical transmission system of Example 7, the optical transmission system was constructed by using the configuration of the optical transmission system of Example 6, but with the first optical transmission path 12 being the 1.0m first optical transmission path of Example 3.

[0090] Also, as the optical transmission system of Comparative Example 2, in the configuration of the optical transmission system of Example 7, the first optical transmission path 12 was set to 3.0 m of the first optical transmission path of Example 3 to construct an optical transmission system.

[0091] Also, the LD is a VCSEL, with a wavelength of 850 nm, a modulation format of NRZ (Non Return to Zero) PRBS (Pseudo Random Bit Sequence), a bit rate of 10 Gbps, a pattern length of 2 31 -1, a bias current of 5 mA, the modulation voltage (Modulation Voltage) was changed at an interval of 0.02 V from 0.10 V to 0.40 V, and the measurement time was 10 minutes.

[0092] FIG. 6 is a diagram showing the relationship between the modulation voltage and the common logarithm of the bit error rate (BER). In FIG. 6, when log 10 (BER) is -12 or less, the data points are shown as -12.

[0093] As shown in FIG. 6, in Examples 4 to 7, it was confirmed that error-free transmission was achieved at a modulation voltage lower than 0.20 V at which error-free transmission was achieved in Comparative Example 1 with a BER of 10 -12 or less. On the other hand, in the case of Comparative Example 2, the modulation voltage at which error-free transmission could be achieved was 0.26 V, which was higher than that in the case of Comparative Example 1. The reason is considered to be that in Comparative Example 2, the product of the scattering loss and the length for an optical signal with a wavelength of 850 nm was 6.57 dB, which is greater than 6 dB, so the loss of the optical signal was too large and the quality deteriorated instead.

[0094] The refractive index distribution shape within the core of the first optical transmission line is generally represented by the refractive index distribution coefficient g in the commonly used power law approximation (Bell Syst. Tech. J., vol.52, no.9, pp.1563-1578, (1973)). The g value that minimizes modal dispersion is usually around 2, although this also depends on the material dispersion. However, the first optical transmission line may be less than a few centimeters in length, in which case modal dispersion can be almost ignored. In fact, even though the refractive index distribution coefficient g of the first optical transmission line in Example 3 used in Examples 4-7 was around 7.5, it showed a dramatic reduction in BER as shown in Figure 6. That is, even with a large g value that deviates significantly from the ideal distribution g=2, high-quality transmission can be achieved if the beam diameter (r1 / r0) can be sufficiently expanded. The first optical transmission line may also have an SI-type refractive index distribution with an even larger g value for the refractive index distribution shape within the core. To efficiently increase the beam diameter (r1 / r0) over a short distance, a large g value is acceptable, as it causes the refractive index distribution at the center of the core to become nearly flat.

[0095] Furthermore, in the optical transmission systems of Examples 4 to 7, error-free transmission is possible even with a modulation voltage of 0.16V, so it can be expected that error-free transmission will be achieved even if the modulation method is PAM4 or higher multi-level modulation.

[0096] As explained above, the optical transmission system 100 according to Embodiment 1 has the effect of realizing high-quality and high-capacity communication with a simple configuration. This means that signal degradation is suppressed more when the optical transmission path also has a first optical transmission path than when it has only a second optical transmission path.

[0097] In particular, the optical transmission system 100 does not use an error correction method. -12 Because the following error rates can be achieved, problems such as increased complexity in configuration due to the addition of processors such as DSPs when using error correction methods, transmission delays, deterioration of encoding efficiency, and increased heat generation and power consumption due to processor load are avoided.

[0098] (Embodiment 2) Figure 7 is a schematic diagram of the optical transmission system according to Embodiment 2. The optical transmission system 100A has a configuration in which connectors 15 and 25 are removed from the optical transmission system 100 shown in Figure 1, optical transmission path 61 is replaced with optical transmission path 61A, and optical transmission path 62 is replaced with optical transmission path 62A.

[0099] Optical transmission lines 61A and 62A are MMFs made of glass such as silica-based glass or plastic. Optical transmission lines 61A and 62A may also be GI type. Optical transmission lines 61A and 62A have a product of scattering loss for an optical signal (e.g., wavelength 850 nm) and length of 6 dB or less, and output a Gaussian beam that is emitted from a single-mode optical fiber and input by central excitation, with the beam diameter expanded by more than three times. The transmission loss for the optical signal (e.g., transmission loss at a wavelength of 850 nm) is, for example, 50 dB / km or more, or 100 dB / km or more, the core diameter is, for example, about 50 μm, and the NA is, for example, about 0.2. Optical transmission lines 61A and 62A have a maximum length (transmission distance) of, for example, 100 m.

[0100] As with optical transmission system 100A, even when using optical transmission lines 61A and 62A, high-quality and high-capacity communication can be achieved with a simple configuration, similar to optical transmission system 100.

[0101] For example, in an optical transmission system 100A, the modulation method of the optical signal is a multi-level modulation method such as PAM4, and error-free without using an error correction method 10 -12 The following error rates can be achieved.

[0102] (Example 8) As Example 8, a GI-POF fabricated by the melt extrusion method described above was used as the optical transmission path. The optical transmission path in Example 8 had a transmission loss of 120 dB / km at a wavelength of 850 nm. This loss is thought to be mostly due to scattering loss. The core diameter was approximately 50 μm, and the NA was approximately 0.185. The characteristics of the optical transmission path in Example 8 were then measured. Specifically, the beam diameters of the light input to and output from the optical transmission path in Example 8 were measured using the measurement method shown in Figure 2.

[0103] Figure 8 shows the measurement results of the beam diameter of the output light in the optical transmission line of Example 8. Figure 8 shows an image measured as NFP. The lengths were 1m, 5m, 10m, 30m, 50m, and 100m. As can be seen from Figure 8, the beam diameter gradually increased as the length of the optical transmission line increased.

[0104] Figure 9 shows the relationship between the ratio r1 / r0 (the ratio of the beam diameter r1 of the output light to the beam diameter r0 of the input light) and the length (Fiber Length) in the optical transmission path of Example 8. As can be seen from Figure 9, the optical transmission path of Example 8 outputs the input optical signal with a beam diameter expanded by more than three times. That is, in the optical transmission path of Example 8, r1 / r0 was 3 or more, and specifically, when the length was 1 m or more, r1 / r0 was 4 or more.

[0105] (Example 9, Comparative Example 3) Next, an optical transmission system of Example 9, with a configuration similar to the optical transmission system 100A according to Example 2, was constructed using the optical transmission path of Example 8 with a length of 10 m, and the error rate was measured. However, the optical transmission path 62A, consisting of the optical transmission path of Example 8 with a length of 10 m, was connected to PD11b, and a single-channel loopback measurement was performed.

[0106] In this case, the LD is a VCSEL, with a wavelength of 850 nm, a modulation scheme of PAM4 PRBS, and a pattern length of 2 31The value was set to -1, the baud rate to 26.5625 Gbaud, the bit rate to 53.125 Gbps, the modulation voltage to 0.6 V (differential signal, peak-to-peak value), and the measurement time to 30 minutes.

[0107] As a result, over a 30-minute period, the BER was 1.0 × 10⁻⁶ -13 The following was achieved, resulting in error-free transmission.

[0108] Here, as Comparative Example 3, an optical transmission system was constructed by replacing the 10m long optical transmission path of Example 8 with a 2m long glass optical fiber optical transmission path, in the configuration of the optical transmission system of Example 9. This 2m long optical transmission path is optically connected to LD11a and PD11b by mini-MT connectors. The error rate was then measured under the same conditions as the optical transmission system of Example 9. However, the measurement time for each measurement was set to 3 minutes, and the mini-MT connector was inserted and removed after each measurement, resulting in a total of 5 BER measurements. As a result, the BER was 10 in each measurement. -6 ~10 -12 The values ​​fluctuated significantly within this range. This confirmed that transmission quality depends on the interconnection status of the light source (VCSEL), photodetector (PD), and optical transmission path, and that error correction by FEC is necessary to achieve high-quality error-free transmission.

[0109] In optical links, transmission quality deteriorates due to various types of noise (such as reflected light noise and modal noise) that depend on the connection status of the optical fiber. In particular, reflected light noise is considered to be a major cause of transmission quality deterioration due to its significant increase in noise levels.

[0110] In typical silica-based optical fibers, the effects of mode coupling within the fiber are negligible, so the reflected light maintains high coherence, efficiently coupling to the laser resonator, which can cause significant noise and degrade transmission quality (bit error rate).

[0111] In a GI-type POF, mode coupling randomizes the spatial distribution of the field pattern, coherence, and polarization of the reflected light, reducing the optical coherence. This reduces the correlation between the reflected light and the modes within the laser resonator, decreases the self-coupling rate of the reflected light to the laser resonator, and reduces reflected light noise. Therefore, by using a GI-type POF, transmission quality degradation caused by reflected light is suppressed, and error-free transmission can be preferably achieved without using FEC.

[0112] However, even with silica-based optical fibers, error-free transmission can be achieved without using FEC by appropriately generating mode coupling by having scatterers (such as fine particles) within the core.

[0113] (Analog modulation method) In the above embodiments, examples, and comparative examples, the modulation method is a digital modulation method, but the present invention can also be applied to optical transmission systems using an analog modulation method. Radio over Fiber (RoF) is an example of an optical transmission system using an analog modulation method.

[0114] RoF (Resonance of Fiber) is a technology that transmits wireless signals using optical fibers, and its use is advancing in the fields of wireless communication and broadcasting. For example, in wireless communication, RoF is used to transmit wireless signals between base stations and antennas. With 5G and the future Beyond 5G and 6G, a much larger number of antennas will be required to improve transfer speeds and eliminate dead zones, so the importance of RoF technology is expected to increase even further in the future.

[0115] In RoF transmission, the light source is directly modulated by a wireless signal, and the wireless signal waveform is converted directly into an optical signal waveform and transmitted through an optical fiber. This is equivalent to analog modulation of the optical signal, so even slight noise generated in the optical transmission path can degrade the transmission quality in RoF transmission. In other words, reducing the noise level of the optical transmission system is a crucial challenge in establishing high-quality RoF transmission technology.

[0116] An analog modulation optical transmission system according to an embodiment of the present invention can have the same configuration as the optical transmission system 100 according to Embodiment 1 shown in Figure 1, or the optical transmission system 100A according to Embodiment 2 shown in Figure 7. However, in the analog modulation system, an RF amplifier or the like is used instead of a TIA-IC. Furthermore, for broadcast applications, the analog modulation optical transmission system may be configured as a unidirectional system for distribution only, rather than a bidirectional configuration like optical transmission systems 100 and 100A.

[0117] (Example 10, Comparative Example 4) As an analog modulation optical transmission system for Example 10, similar to Example 6, an optical transmission system was constructed using the 100m long second optical transmission path from Reference Example 1 shown in Figure 3 and the 0.50m long first optical transmission path from Example 3 as optical transmission paths. In addition, as an analog modulation optical transmission system for Comparative Example 4, an optical transmission system was constructed using only the 100m long second optical transmission path from Reference Example 1 as the optical transmission path. RoF transmission experiments were then conducted in the optical transmission systems of Example 10 and Comparative Example 4.

[0118] The LD was a VCSEL with a wavelength of 850 nm and a bias current of 5 mA. The transmitted signal was a radio signal using orthogonal frequency division multiplexing (OFDM), with 64-level quadrature amplitude modulation (64QAM), a center frequency of 880 MHz, a bandwidth of 20 MHz, and an input signal strength of -10 dBm.

[0119] Figure 10 shows the constellation map of Comparative Example 4. In this case, the error vector amplitude (EVM) was 9.93%. In contrast, Figure 11 shows the constellation map of Example 10. In this case, the error vector amplitude (EVM) was 3.97%. As can be seen from Figures 10 and 11, in Example 10, the variation in signal points in the constellation map was greatly reduced compared to Comparative Example 4, and the EVM of the transmitted signal was reduced. This is thought to be due to the significant reduction in noise generated in the optical transmission path by mode coupling inside the first optical transmission path.

[0120] Figure 12 shows the error vector amplitude over 30 measurements for Example 10 and Comparative Example 4. In Comparative Example 4, the EVM was approximately 9%, but in Example 10, the EVM was reduced to approximately 3%. These results demonstrate that the optical transmission system according to the embodiment of the present invention is useful not only for digital data transmission but also for RoF transmission.

[0121] (Embodiment 3) Incidentally, when a light source such as an LD in an optical signal transmitter is optically connected to an optical fiber in an optical transmission path via a lens or the like, the light source may become unstable due to reflected light from the optical fiber, increasing noise and degrading transmission quality.

[0122] It is known that the effect of reflected light from an optical fiber can be reduced by slightly misaligning the optical fiber and the light source, or by defocusing the light source. However, positional misalignments such as axial misalignment and defocusing increase the coupling loss between the light source and the optical fiber, which degrades the signal-to-noise ratio of the transmitted signal.

[0123] As a result of diligent research, the inventors have found that by utilizing mode coupling in the optical fiber, the dependence of the beam diameter of light emitted from the optical fiber on the excitation conditions can be reduced, thereby mitigating the effect of reflected light, which depends on the misalignment between the optical fiber and the light source. Furthermore, they have found that this significantly relaxes the optical fiber alignment requirements necessary for achieving high-quality optical transmission. Embodiment 3 is described below as an example of this embodiment.

[0124] The optical transmission system according to Embodiment 3 has the same configuration as the optical transmission system according to Embodiment 2 shown in Figure 7. Here, in the optical transmission system according to Embodiment 3, when a Gaussian beam emitted from a single-mode optical fiber is input with a misalignment, the ratio of the minimum value to the maximum value of the output Gaussian beam diameter is 0.7 or more. This significantly relaxes the optical fiber alignment requirements for achieving high-quality optical transmission. As a result, high-precision alignment of optical elements (light source, lens, optical fiber, etc.) becomes unnecessary, making it possible to manufacture the optical transmission system at low cost and easily. Preferably, the optical transmission line has a product of scattering loss with respect to the optical signal and length of 6 dB or less.

[0125] As the optical transmission path for the optical transmission system according to Embodiment 3, for example, the GI-POF of Example 8 can be used. Therefore, the results of experiments using the optical transmission path of Reference Example 1, the optical transmission path of Example 8, and the optical transmission path of Comparative Example 5, which is a commercially available GI-POF, will be described below. The optical transmission path of Comparative Example 5 had an OTDR loss of approximately 60 dB / km. This loss is thought to be mostly due to scattering loss. The core diameter was approximately 50 μm.

[0126] First, for the optical transmission lines of Reference Example 1, Comparative Example 5, and Example 8, each with a length of 10 m, the relationship between the positional displacement and the beam diameter was measured using the measurement system shown in Figure 2. Here, as shown in Figure 13, the optical axis direction of the emitted light 203 (or lens 204) was taken as the z-axis, and the radial direction as the x-axis. The optical transmission line 205 was then shifted in the z-axis or x-axis direction around the focal point of the emitted light 203 from lens 204, and the emitted light 203 was incident on it. In this case, a displacement of Δx=0 indicates that the optical axis of the emitted light 203 and the optical axis of the optical transmission line 205 coincide. A displacement of Δz=0 indicates that the focal point of the emitted light 203 coincides with the input end face of the optical transmission line 205. Then, the NFP of the light output from the end face opposite to the input end face of the optical transmission line 205 was measured using an NFP measuring device, and the beam diameter (D4σ width) of the light output from the optical transmission line 205 was determined from the NFP using the second moment method. Furthermore, the beam diameter of the light input to the optical transmission line 205 was determined by measuring the light output from the lens 204 in the measurement system shown in Figure 2.

[0127] Figure 14 shows the relationship between Δx and the beam diameter of the output light in Reference Example 1, Comparative Example 5, and Example 8. Figure 14 shows an image measured as NFP. The white bars in the figure represent a scale of 10 μm in length. As can be seen from Figure 14, in Reference Example 1 and Comparative Example 5, a tendency was observed for the beam diameter of the output beam to expand with increasing displacement Δx. This is attributed to the excitation of higher-order modes as the beam incidence position moves from the core center to the outer periphery of the optical transmission path. On the other hand, in Example 8, the size of the output beam remained almost constant regardless of the displacement Δx. This is thought to be because higher-order modes are excited regardless of the excitation conditions due to strong mode coupling in the optical transmission path.

[0128] Figure 15 shows the relationship between Δz and the beam diameter of the output light in Reference Example 1, Comparative Example 5, and Example 8. Similar to the case of Δx, in Reference Example 1 and Comparative Example 5, the beam diameter of the output beam tended to expand as the displacement amount Δz increased. On the other hand, in Example 8, the size of the output beam remained almost constant regardless of the displacement amount Δz.

[0129] Figure 16 shows the relationship between Δx and the output beam diameter in Reference Example 1, Comparative Example 5, and Example 8. Figure 17 shows the relationship between Δz and the output beam diameter in Reference Example 1, Comparative Example 5, and Example 8. As can be seen from Figures 16 and 17, in Reference Example 1 and Comparative Example 5, the output beam diameter was smallest near Δx=0 and Δz=0, and there was a tendency for the output beam diameter to increase with increasing displacement. On the other hand, in Example 8, the output beam diameter increased significantly near Δx=0 and Δz=0, and the dependence of the output beam diameter on the displacement amount decreased significantly.

[0130] Figure 18 shows the normalized output beam diameter from Figure 16. Figure 19 shows the normalized output beam diameter from Figure 17. The output beam diameter was normalized by dividing the output beam diameter at each displacement by the maximum output beam diameter. That is, the vertical axis in Figures 18 and 19 is (output beam diameter) / (maximum output beam diameter).

[0131] As can be seen from Figure 18, the ratio of (minimum output beam diameter) to (maximum output beam diameter) for positional misalignment in the x-axis direction is 0.27 for Reference Example 1, 0.41 for Comparative Example 5, and 0.79 for Example 8. Also, as can be seen from Figure 19, the ratio of (minimum output beam diameter) to (maximum output beam diameter) for positional misalignment in the z-axis direction is 0.41 for Reference Example 1, 0.63 for Comparative Example 5, and 0.86 for Example 8.

[0132] Next, an optical transmission system similar to the optical transmission system 100A according to Embodiment 2 was constructed using the optical transmission lines of Reference Example 1, Comparative Example 5, and Example 8, each with a length of 10 m. The optical signal transceiver 10 was used as the optical transmitter, and the optical signal transceiver 20 was used as the optical receiver. The error rate was then measured by varying the amount of deviation in the x-axis direction and the amount of deviation in the z-axis direction.

[0133] In this case, the LD is a VCSEL, with a wavelength of 850 nm, a modulation scheme of NRZ PRBS, a bit rate of 10 Gbps, and a pattern length of 2 31The setting was -1, the bias current was set to 5mA, the modulation voltage to 0.12V, and the measurement time to 5 minutes.

[0134] Figures 20A, 20B, and 20C show the relationship between Δx and the error rate in Reference Example 1, Comparative Example 5, and Example 8, respectively. Figures 21A, 21B, and 21C show the relationship between Δz and the error rate in Reference Example 1, Comparative Example 5, and Example 8, respectively.

[0135] As can be seen from Figures 20A-20C and 21A-21C, in Reference Example 1 and Comparative Example 5, the BER deteriorates and has peaks around Δx=0 and Δz=0, and the peak value is 10 in Reference Example 1. -4 In comparative example 5, the degree was 10. -8 The results were as follows. Furthermore, a tendency was observed for the BER to decrease with increasing displacement. This is attributed to the fact that the effect of reflected light decreases with increasing displacement. On the other hand, when the displacement became too large (exceeding approximately ±20 μm for Δx and ±1000 μm for Δz), the effect of increased coupling loss became dominant, and the BER began to increase.

[0136] Based on the above, in the case of Reference Example 1 and Comparative Example 5, the ratio of (minimum output beam diameter) to (maximum output beam diameter) is less than 0.7. However, in order to minimize coupling loss and suppress the effects of reflected light, more precise alignment between the light source and the optical transmission path is required.

[0137] On the other hand, in Example 8, no degradation of BER was observed near Δx=0 and Δz=0, and the BER was 10 under all conditions of -14.5μm≦Δx≦+18μm and -850μm≦Δz≦+730μm. -12 The following error-free transmission was achieved. Therefore, in Example 8, even though (minimum output beam diameter) / (maximum output beam diameter) is 0.7 or greater, error-free transmission can be achieved without precise alignment.

[0138] Next, for the optical transmission paths in Reference Example 1, Comparative Example 5, and Example 8, the coupling loss was measured by varying the amount of displacement in the x-axis direction and the amount of displacement in the z-axis direction.

[0139] Figures 22A, 22C, and 22C show the relationship between Δx and coupling loss in Reference Example 1, Comparative Example 5, and Example 8, respectively. Figures 23A, 23B, and 23C show the relationship between Δz and coupling loss in Reference Example 1, Comparative Example 5, and Example 8, respectively. In all cases of Reference Example 1, Comparative Example 5, and Example 8, a tendency for coupling loss to increase with increasing displacement was observed.

[0140] Figure 24 shows the conditions for achieving error-free transmission using the optical transmission path of Example 8, obtained from the results in Figures 20C, 21C, 22C, and 23C. When using the optical transmission path of Example 8, error-free transmission was achieved in the range -14.5 μm ≤ Δx ≤ +18 μm. The coupling loss at Δx = -14.5 μm was 1.08 dB, and the coupling loss at Δx = +18 μm was 2.66 dB. Furthermore, when using the optical transmission path of Example 8, error-free transmission was achieved in the range -850 μm ≤ Δz ≤ +730 μm. The coupling loss at Δz = -850 μm was 7.78 dB, and the coupling loss at Δx = +730 μm was 6.69 dB. From these results, it can be concluded that, according to the optical transmission path of Example 8, under optical coupling conditions where the coupling loss between the optical signal transmitter (light source) and the optical transmission path is 1 dB or less, regardless of the amount of misalignment, 10 -12 The following error rates can be achieved. Furthermore, for example, in the optical transmission lines of Reference Example 1 and Comparative Example 5, even under optical coupling conditions where the coupling loss between the optical signal transmitter (light source) and the optical transmission line is 1 dB or less, the worst-case error rate is 10 -8 It is larger. In contrast, according to the optical transmission path of Example 8, under optical coupling conditions where the coupling loss between the optical signal transmitter (light source) and the optical transmission path is 1 dB or less, the worst value of the error rate is 10 -8 The following applies:

[0141] Incidentally, in actual optical transmission systems, optical signals are often transmitted by connecting two or more optical fibers to form an optical transmission path. When connecting optical fibers, if there is an air gap (air layer) between the fibers, it increases reflection loss and reflected light noise. Therefore, suppressing reflection at the fiber connection point is considered important for high-quality optical transmission.

[0142] Various methods have been proposed to suppress reflection at fiber optic connections, such as fusion splicing and connections using refractive index matching agents, but PC (Physical Contact) connections are the most widely used. In PC connections, reflection is prevented by pressing the fiber end faces (connector end faces), which have been polished into a convex spherical shape, against each other to create a tight seal.

[0143] However, connecting to a PC requires not only precise polishing of the connector end faces but also strong mating force during connection. This can lead to problems such as reduced workability and increased costs due to the need for more robust connectors. Furthermore, since PC connections involve physical contact between the fiber end faces, if mating is performed with foreign matter present on the fiber end faces, the foreign matter may be pressed against the fiber end faces, potentially damaging them.

[0144] Therefore, if we can establish an optical transmission technology that enables high-quality signal transmission regardless of the presence or absence of reflection at the fiber connection point, conventional mating techniques used to suppress reflection at fiber connection points, such as those used in PC connections, will become unnecessary, and optical fiber connections will be made extremely simple.

[0145] Figure 25 is a schematic diagram of an experimental system for investigating the effect of inter-fiber gaps. The inventors used the experimental system shown in Figure 25 to evaluate the effect of reflections occurring at optical fiber connections on transmission quality. In the experimental system shown in Figure 25, connector C12 of a 1m long optical fiber F1 equipped with connectors C11 and C12, and connector C21 of a 10m long optical fiber F2 equipped with connectors C21 and C22, were connected at an optical fiber connection part C to form an optical transmission path. Then, an optical signal 302 from a light source 301 was focused on the end face of connector C11 by lens 303 and transmitted through the optical transmission path. Then, an optical signal 304 emitted from connector C22 was focused by lens system 305 and received by photodetector 306, which is a photodiode, and the BER was measured. At this time, the BER was compared and evaluated in the case where a 50 μm air gap (air layer) was present in the optical fiber connection part C and in the case where there was no air gap (i.e., when optical fibers F1 and F2 (connectors C12 and C21) were pressed together). At this time, the two optical fibers F1 and F2 were precisely aligned and measurements were taken after eliminating axial misalignment in the radial and angular directions. The optical signal was a 10 Gbps NRZ signal, and the pattern length of the PRBS was 2 31 The BER was set to -1, and the measurement time was set to 5 minutes. Light source 301 was a VCSEL with a wavelength of 850 nm, the bias current was set to 5 mA, and the excitation condition was center excitation.

[0146] In this experimental setup, transmission quality may be degraded not only by reflections occurring at the optical fiber connection point C, but also by reflected backlight from the optical fiber F2 end face on the receiver 306 side. Therefore, in this measurement, in order to focus on the effect of reflections occurring at the optical fiber connection point C, the optical fiber F2 end face on the receiver 306 side (the end face of connector C22) was subjected to a 12-degree oblique polishing process. As a result, the light reflected from the end face of connector C22 is radiated outside the core of optical fiber F2, eliminating the effect of reflected backlight from the end face of connector C22. The other optical fiber end faces (end faces of connectors C11, C12, and C21) were subjected to convex spherical polishing.

[0147] Figures 26A, 26B, and 26C show the relationship between modulation voltage and error rate (worst-case value) when optical fibers F1 and F2 are the optical transmission lines of Reference Example 1, Comparative Example 5, and Example 8, respectively. As shown in Figures 26A, 26B, and 26C, in all of Reference Example 1, Comparative Example 5, and Example 8, if there is no air gap in the optical fiber connection C, the BER is 10 regardless of the modulation voltage. -12 The following error-free transmission was achieved.

[0148] However, if there is an air gap in the optical fiber connection C, the worst-case BER value in Reference Example 1 is up to 10 -6 The degradation was so severe that in Comparative Example 5, the worst-case BER value reached a maximum of 10. -8 The signal quality deteriorated to this extent. This is thought to be because the presence of an air gap in the optical fiber connection C caused reflected light to be generated, increasing the noise.

[0149] On the other hand, in Example 8, a slight degradation of BER was observed when the modulation voltage was 0.1V, but when the modulation voltage was 0.12V or higher, error-free transmission (BER ≤ 10) was observed under all measured conditions. -12 ) was achieved. The above results show that by using the optical transmission path (POF) of Example 8, low-noise signal transmission is possible regardless of whether or not there is reflection at the optical fiber connection part C, eliminating the need for conventional mating techniques to suppress reflection at the optical fiber connection part C such as PC connections, and enabling high-quality optical transmission even with extremely simple optical fiber connections.

[0150] In the case where optical fibers F1 and F2 are the optical transmission lines of Example 8, this is an example of a case where an optical transmission line is composed of two or more optical transmission lines connected together, and where each optical transmission line is connected to the others via an air layer. Among the connected optical transmission lines, at least the optical transmission line located immediately after the light source is an example of an optical transmission line that, when a Gaussian beam emitted from a single-mode optical fiber is input by central excitation, expands the beam diameter to three times or more, and when a Gaussian beam emitted from a single-mode optical fiber is input with a positional shift, the ratio of the minimum value to the maximum value of the output beam diameter is 0.7 or more, and the product of the scattering loss to the optical signal and the length is 6 dB or less. In the case where there are multiple connection points between each optical transmission line, it is sufficient that at least one of those connection points the optical transmission lines are connected to each other via an air layer. Alternatively, when there are multiple connection points between each optical transmission line, it is also acceptable for all of those connection points to be connected to each other via an air layer.

[0151] Furthermore, if optical fibers F1 and F2 are the optical transmission lines of Example 8, this is an example of a case where an optical transmission line is constructed by connecting two or more optical transmission lines, and where each optical transmission line is connected to the others via an air layer. This is an example of an optical transmission line where, when a Gaussian beam emitted from a single-mode optical fiber is input by central excitation, the beam diameter is expanded to three times or more and output, and when a Gaussian beam emitted from a single-mode optical fiber is input with a positional shift, the ratio of the minimum value to the maximum value of the output beam diameter is 0.7 or more, and the product of the scattering loss to the optical signal and the length is 6 dB or less. Note that if there are multiple connection points between each optical transmission line, it is sufficient that at least one of those connection points the optical transmission lines are connected to each other via an air layer. Also, if there are multiple connection points between each optical transmission line, it is acceptable that all of those connection points be connected to each other via an air layer.

[0152] In the above embodiment, the first optical transmission path, the second optical transmission path, and the optical transmission path are all optical fibers, but the first optical transmission path, the second optical transmission path, and the optical transmission path are not limited to these, and may also be optical waveguides or a single-piece molded multi-optical transmission sheet as disclosed in International Publication No. 2019 / 177068.

[0153] Furthermore, the second optical transmission path is not limited to being longer than the first optical transmission path. In other words, the length of the second optical transmission path may be less than or equal to the length of the first optical transmission path.

[0154] As described above, the optical transmission path disclosed herein can also be configured as an optoelectronic composite cable, which is a composite cable that combines with metal cables for transmitting power and low-speed signals. It can also be used to accelerate signals from existing standards (e.g., HDMI®, USB, etc.) by combining them with an optical system, or as a transmission path between new devices. The shape of the optoelectronic composite connector can be backward compatible with existing standards or can be newly designed.

[0155] Furthermore, the optical transmission system disclosed herein can also be configured as a Wavelength Division Multiplexing (WDM) system that uses multiple LDs of different wavelengths, does not increase the number of optical transmission paths, and can achieve a speed increase by a multiple of the number of wavelengths used. In this case, the emitted light (optical signals) from LDs of different wavelengths (wavelengths: λ1 to λn) are coupled to the first optical transmission path in Embodiment 1 or the optical transmission path in Embodiment 2 using a multiplexer and transmitted. The optical signals transmitted through the optical transmission path are separated into optical signals of each wavelength by a demultiplexer and received by a photodiode. A set of LDs of different wavelengths, a multiplexer, a demultiplexer, and a photodiode can also be used to correspond to multiple optical transmission paths.

[0156] Furthermore, the present invention is not limited by the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Moreover, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible. [Explanation of symbols]

[0157] 10, 20: Optical signal transceivers 11, 21: Transmitter / Receiver 11a, 21a:LD 11b, 21b: PD 11c, 21c: Driver IC 11d, 21d: TIA-IC 12, 22: First optical transmission line 13, 23: Optical transmission path within equipment 14, 15, 24, 25, C11, C12, C21, C22: Connectors 31, 32: Second optical transmission line 40, 50: Signal processing circuits 61, 62, 61A, 62A: Optical transmission lines 100, 100A: Optical transmission system 201: DBR laser 202: Polarization-maintaining single-mode optical fiber 203: Emitted light 204, 303: Lens 205: Optical transmission path 206: CCD camera 207: light 208:NFP measurement device 301 :Light source 302, 304: Optical signals 305: Lens system 306:Receiver C: Optical fiber connection section F1, F2: Optical fiber

Claims

1. An optical signal transmitter for transmitting an optical signal output from a light source having a laser diode; an optical signal receiver for receiving the optical signal; an optical transmission line that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal; Equipped with the optical transmission line has a scattering loss of 50 dB / km or more for light with a wavelength of 850 nm, transmits the optical signal in a multimode manner, and a product of the scattering loss and the length for light with a wavelength of 850 nm is 6 dB or less; when a Gaussian beam output from a single-mode optical fiber is input by central excitation, the beam diameter is expanded by a factor of three or more due to a micro non-uniform structure and output, and mode-couples with a higher-order mode due to forward scattering; When a Gaussian beam emitted from a single mode optical fiber is input with a positional deviation, the ratio of the maximum beam diameter to the minimum beam diameter of the output beam is 0.7 or more. Optical transmission system.

2. The modulation method of the optical signal is a digital modulation method that is a binary modulation method or a multi-level modulation method, and under optical coupling conditions in which the coupling loss between the optical signal transmitter and the optical transmission line is 1 dB or less, the worst value of the error rate is 10 -8 is less than or equal to 2. The optical transmission system according to claim 1.

3. The modulation method of the optical signal is a digital modulation method that is a binary modulation method or a multi-level modulation method, and under optical coupling conditions in which the coupling loss between the optical signal transmitter and the optical transmission line is 1 dB or less, the optical signal transmitter can be modulated by 10 -12 Achieve the following error rate:

2. The optical transmission system according to claim 1.

4. An optical signal transmitter for transmitting an optical signal output from a light source having a laser diode; an optical signal receiver for receiving the optical signal; an optical transmission line that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal; Equipped with the optical transmission line is configured by connecting two or more optical transmission lines, The optical transmission lines are connected to each other via an air layer at least at one of the connection points; Among the optical transmission lines to be connected, at least the optical transmission line arranged immediately after the light source has a scattering loss of 50 dB / km or more for light with a wavelength of 850 nm, transmits the optical signal in multimode, and when a Gaussian beam output from a single-mode optical fiber is input by central excitation, the beam diameter is expanded by three times or more due to a micro non-uniform structure and output, and is mode-coupled with a higher-order mode by forward scattering, and the product of the scattering loss and length for light with a wavelength of 850 nm is 6 dB or less. Optical transmission system.

5. An optical signal transmitter for transmitting an optical signal output from a light source having a laser diode; an optical signal receiver for receiving the optical signal; an optical transmission line that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal; Equipped with the optical transmission line is configured by connecting two or more optical transmission lines, The optical transmission lines are connected to each other via an air layer at least at one of the connection points; Each optical transmission line has a scattering loss of 50 dB / km or more for light with a wavelength of 850 nm, transmits the optical signal in multimode, and when a Gaussian beam output from a single-mode optical fiber is input with central excitation, the beam diameter is expanded by three times or more due to the micro non-uniform structure and output, and mode-coupled with a higher mode by forward scattering, and the product of the scattering loss and length for light with a wavelength of 850 nm is 6 dB or less. Optical transmission system.

6. A digital modulation method in which the modulation method of the optical signal is a binary modulation method or a multi-level modulation method, and the error rate is 10 -8 is less than or equal to 6. An optical transmission system according to claim 4 or 5.

7. The modulation method of the optical signal is a digital modulation method that is a binary modulation method or a multi-level modulation method, and when the optical signal is center-excited, -12 Achieve the following error rate:

6. An optical transmission system according to claim 4 or 5.

8. The optical transmission system according to any one of claims 1 to 7, which is a wavelength division multiplexing (WDM) system.

9. An optical / electrical composite cable comprising the optical transmission system according to any one of claims 1 to 8.

Images