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] The present invention relates to an optical transmission system and an optical-electrical composite cable. [Background technology]

[0002] In recent years, short-distance communication of approximately 100 m has become a requirement for indoor communication applications such as data centers. Furthermore, multilevel modulation methods such as pulse-amplitude modulation (PAM) are being considered for large-capacity communication.

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

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

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-151516 Summary of the Invention [Problem to be solved by the invention]

[0006] When using a correction method such as FEC to reduce the error rate, it is necessary to add an error correction code, which increases the amount of data, causing problems such as transmission delays and a decrease in coding efficiency.In addition, adding a processor makes the configuration more complicated, and the processor load causes problems such as increased heat generation and power consumption.

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

[0008] In order to solve the above-mentioned problems and achieve the object, 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 line that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal, wherein a product of scattering loss and length for the optical signal is 6 dB or less, and when a Gaussian beam output from a single-mode optical fiber is input with central excitation, the first optical transmission line expands the beam diameter by three times or more and outputs the expanded beam, the first optical transmission line being located immediately after the light source and further optically connected to a second optical transmission line.

[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 line that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal, wherein a first optical transmission line has a product of scattering loss and length for the optical signal of 6 dB or less, and expands the beam diameter by three times or more when a Gaussian beam output from a single-mode optical fiber is input with central excitation, and is positioned immediately after the light source, and is further optically connected to a second optical transmission line that is longer than the first optical transmission line.

[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 line that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal, wherein the optical transmission line has: a first optical transmission line in which the product of a scattering loss and a length for the optical signal is 6 dB or less, and which expands a beam diameter by three times or more when a Gaussian beam output from a single-mode optical fiber is input with central excitation; and a second optical transmission line that is optically connected to the first optical transmission line, is longer than the first optical transmission line, has a transmission loss for the optical signal of 100 dB / km or less, and expands a beam diameter by less than three times when a Gaussian beam output from the single-mode optical fiber is input with central excitation.

[0011] One aspect of the present invention is 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 the optical transmission line has a product of scattering loss and length for the optical signal of 6 dB or less, expands a beam diameter by three times or more when a Gaussian beam output from a single-mode optical fiber is input with central excitation, and outputs the expanded beam, and the modulation method of the optical signal is a multi-level modulation method, and is error-free without using an error correction method. -12 This is an optical transmission system that 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 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 the minimum beam diameter of the output beam is 0.7 or greater.

[0013] 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 line that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal, wherein the optical transmission line is configured by connecting two or more optical transmission lines, and the optical transmission lines are connected to each other via an air layer at at least one of the connection points, and at least one of the connected optical transmission lines that is located immediately after the light source expands the beam diameter by three times or more when a Gaussian beam output from a single-mode optical fiber is input with central excitation, and outputs the expanded beam diameter, and when the Gaussian beam output from the single-mode optical fiber is input with a positional shift, the ratio of the minimum to the maximum beam diameter of the output beam is 0.7 or more, and the product of the scattering loss and length for the optical signal is 6 dB or less.

[0014] 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 line that optically connects the optical signal transmitter and the optical signal receiver and transmits the optical signal, wherein the optical transmission line is configured by connecting two or more optical transmission lines, and the optical transmission lines are connected to each other via an air layer at at least one of the connection points, and each optical transmission line expands the beam diameter by three times or more when a Gaussian beam output from a single-mode optical fiber is input with central excitation, and outputs the expanded beam diameter, and when the Gaussian beam output from the single-mode optical fiber is input with a positional shift, the ratio of the maximum to the minimum beam diameter of the output beam is 0.7 or more, and the product of the scattering loss and length for the optical signal is 6 dB or less.

[0015] One aspect of the present invention is an optical-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 having a baud rate of 10 Gbaud or more, 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 with central excitation, the optical transmission line outputs the beam diameter expanded by three times or more. [Effects of the Invention]

[0017] The present invention has the effect of realizing high-quality, large-capacity communication with a simple configuration. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic configuration diagram of an optical transmission system according to the first embodiment. [Figure 2] FIG. 2 is a diagram for explaining a method for measuring the beam diameter. [Figure 3] FIG. 3 is a diagram showing the measurement results of the beam diameters of input light and output light when a glass optical fiber is used as the second optical transmission line of Reference Example 1. [Figure 4] FIG. 4 is a diagram showing the measurement results of the beam diameters of input light and 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 length and the ratio of the beam diameter r1 of output light to the beam diameter r0 of input light 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 the second embodiment. [Figure 8] FIG. 8 is a diagram showing the measurement results of the beam diameters of input light and output light when a plastic optical fiber is used as the optical transmission line. [Figure 9]FIG. 9 is a diagram showing the relationship between the length and the ratio of the beam diameter r1 of output light to the beam diameter r0 of input light when a plastic optical fiber is used as the optical transmission line. [Figure 10] FIG. 10 is a diagram showing a constellation map of Comparative Example 4. As shown in FIG. [Figure 11] FIG. 11 is a diagram showing a constellation map of the tenth embodiment. [Figure 12] FIG. 12 is a diagram showing the error vector amplitudes in 30 measurements for Example 10 and Comparative Example 4. [Figure 13] FIG. 13 is a diagram for explaining the setting of the x-axis and z-axis in the optical transmission line. [Figure 14] FIG. 14 is a diagram showing the relationship between Δx and the beam diameter of the output light in Reference Example 1, Comparative Example 5, and Example 8. [Figure 15] FIG. 15 is a diagram showing the relationship between Δz and the beam diameter of the output light in Reference Example 1, Comparative Example 5, and Example 8. [Figure 16] FIG. 16 is a diagram showing the relationship between Δx and the diameter of the emitted beam in Reference Example 1, Comparative Example 5, and Example 8. [Figure 17] FIG. 17 is a diagram showing the relationship between Δz and the diameter of the emitted beam in Reference Example 1, Comparative Example 5, and Example 8. [Figure 18] FIG. 18 is a diagram in which the diameter of the emitted beam in FIG. 16 is standardized. [Figure 19] FIG. 19 is a diagram in which the diameter of the emitted beam in FIG. 17 is standardized. [Figure 20A] FIG. 20A is a diagram showing the relationship between Δx and the error rate in Reference Example 1. FIG. [Figure 20B] FIG. 20B is a diagram showing the relationship between Δx and the error rate in Comparative Example 5. In FIG. [Figure 20C] FIG. 20C is a diagram showing the relationship between Δx and the error rate in the eighth embodiment. [Figure 21A] FIG. 21A is a diagram showing the relationship between Δz and the error rate in Reference Example 1. FIG. [Figure 21B]FIG. 21B is a diagram showing the relationship between Δz and the error rate in Comparative Example 5. In FIG. [Figure 21C] FIG. 21C is a diagram showing the relationship between Δz and the error rate in the eighth embodiment. [Figure 22A] FIG. 22A is a diagram showing the relationship between Δx and coupling loss in Reference Example 1. FIG. [Figure 22B] FIG. 22B is a diagram showing the relationship between Δx and coupling loss in Comparative Example 5. In FIG. [Figure 22C] FIG. 22C is a diagram showing the relationship between Δx and coupling loss in Example 8. [Figure 23A] FIG. 23A is a diagram showing the relationship between Δz and coupling loss in Reference Example 1. FIG. [Figure 23B] FIG. 23B is a diagram showing the relationship between Δz and coupling loss in Comparative Example 5. [Figure 23C] FIG. 23C is a diagram showing the relationship between Δz and coupling loss in Example 8. [Figure 24] FIG. 24 is a diagram showing conditions for realizing error-free transmission. [Figure 25] FIG. 25 is a schematic diagram of an experimental system for investigating the effect of inter-fiber gaps. [Figure 26A] FIG. 26A is a diagram showing the relationship between the modulation voltage and the error rate in Reference Example 1. FIG. [Figure 26B] FIG. 26B is a diagram showing the relationship between the modulation voltage and the error rate in Comparative Example 5. In FIG. [Figure 26C] FIG. 26C is a diagram showing the relationship between the modulation voltage and the error rate in the eighth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, the present invention is not limited to these embodiments.

[0020] (Embodiment 1) [Configuration of optical transmission system] 1 is a schematic configuration diagram of an optical transmission system according to a first embodiment. 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 optical signal transmitters and examples of optical signal receivers. The optical signal transceivers 10 and 20 are configured, for example, as modules or as On-Board Optics (OBO).

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

[0022] First, we will explain the components of the optical signal transceiver 10. The transceiver 11 includes an LD (Laser Diode: LD) 11a, which is a light source, a PD (Photo-Diode) 11b, which is a light receiving element, a driver IC 11c, and a TIA (Trans-Impedance Amplifier) ​​IC 11d.

[0023] The LD 11a is, for example, a VCSEL (Vertical Cavity Surface Emitting laser) and oscillates at a wavelength of, for example, 850 nm. The LD 11a is controlled by a driver-IC 11c and outputs an optical signal modulated by a predetermined modulation method at a predetermined modulation speed to the first optical transmission line 12. The modulation method is a digital modulation method, for example, a binary modulation method such as NRZ (Non Return to Zero) or a multi-level modulation method such as PAM4, and the baud rate is, for example, 10 Gbaud or more or 25 Gbaud or more. In the case of NRZ, the baud rate and the 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, the LD 11a is a four-channel array, but it may be an array of any number of channels or more, or it may be a single LD.

[0024] The first optical transmission line 12 is optically connected to the LD 11a via a connector 14, and is optically connected to the second optical transmission line 32 via a connector 15. That is, the first optical transmission line 12 is arranged directly with the LD 11a, which is the light source, or via a lens system. (The case where a lens is used is omitted in FIG. 1.) The connectors 14 and 15 may be, for example, an MT connector, an MPO connector, or a connector of any other shape. The first optical transmission line 12 will be described in detail later. In this embodiment, the first optical transmission line 12 has four lanes like a ribbon, but it may have any number of lanes, such as two or more lanes, or may be a single lane.

[0025] As a result, the LD 11 a transmits the optical signal output from the LD 11 a to the first optical transmission line 12 , the second optical transmission line 32 , and the internal optical transmission line 23 , and transmits the optical signal to the optical signal transceiver 20 .

[0026] The PD 11b receives an 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. The TIA-IC 11d amplifies the current signal from the PD 11b, converts it into a voltage signal, and outputs it to the signal processing circuit 40. In this embodiment, the PD 11b and the TIA-IC 11d are four-channel arrays, but they may be arrays of any number of channels greater than two, or may be single units.

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

[0028] Next, we will explain the components of the optical signal transceiver 20. The transceiver 21 includes an LD 21a, a PD 21b, a driver IC 21c, and a TIA IC 21d.

[0029] Like the LD 11a, the LD 21a is, for example, a VCSEL, and oscillates at a wavelength of, for example, 850 nm. The LD 21a is controlled by the driver-IC 21c, and like the LD 11a, outputs an optical signal modulated by a predetermined modulation method at a predetermined modulation speed to the first optical transmission line 22. In this embodiment, the LD 21a is a four-channel array, but it may be an array of any number of channels greater than two, or may be a single unit.

[0030] The first optical transmission line 22 is optically connected to the LD 21a via a connector 24, and is optically connected to the second optical transmission line 31 via a connector 25. That is, the first optical transmission line 22 is arranged directly with the LD 21a, which is the light source, or via a lens system. The connectors 24 and 25 may be, for example, an MT connector, an MPO connector, or a connector of any other shape. The first optical transmission line 22 will be described in detail later. In this embodiment, the first optical transmission line 22 has four lanes like a ribbon, but it may have any number of lanes, two or more lanes, or may be a single lane.

[0031] As a result, the LD 21 a transmits the optical signal output from the LD 21 a through the first optical transmission line 22 , the second optical transmission line 31 and the internal optical transmission line 13 to the optical signal transceiver 10 .

[0032] The PD 21b receives an 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. The TIA-IC 21d amplifies the current signal from the PD 21b, converts it into a voltage signal, and outputs it to the signal processing circuit 50. In this embodiment, the PD 21b and the TIA-IC 21d are four-channel arrays, but they may be arrays of any number of channels greater than two, or may be single units.

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

[0034] As described above, the optical transmission system 100 is configured to enable bidirectional optical transmission, and the first optical transmission path 22, the second optical transmission path 31, and the internal optical transmission path 13 form an optical transmission path 61 that transmits an optical signal. Furthermore, the first optical transmission path 12, the second optical transmission path 32, and the internal optical transmission path 23 form an optical transmission path 62 that transmits an optical signal. Note that the second optical transmission paths 31, 32 and the optical signal transceivers 10, 20 may be integrated to form an AOC (Active Optical Cable).

[0035] [Configuration of optical transmission path] Next, the configurations 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 glass or plastic. The transmission loss of the optical signal of the second optical transmission lines 31 and 32 (for example, transmission loss at a wavelength of 850 nm) is, for example, 100 dB / km or less, or 50 dB / km or less, further 10 dB / km or less, or even 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. Furthermore, 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 be of a 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 MMFs. The first optical transmission lines 12 and 22 may be GI type. When the first optical transmission lines are several centimeters to several tens of centimeters or less, a sufficient transmission bandwidth is ensured, so that a GI distribution is not necessary and an SI (Step-Index) type distribution may be used. Furthermore, as long as the first optical transmission lines are scattering-controlled transmission lines as defined in the present invention, their shape is not particularly limited and may be an optical waveguide shape, an optical fiber shape, or the like. That is, the cross-sectional shape of the first optical transmission lines may be, for example, circular, rectangular, or any other shape.

[0037] The first optical transmission lines 12 and 22 have a scattering loss for the optical signal (e.g., scattering loss at a wavelength of 850 nm) of, for example, 50 dB / km or more, 100 dB / km or more, 200 dB / km or more, 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 undergoing mode coupling with higher-order modes due to forward scattering. As a result, the first optical transmission lines 12 and 22 output the beam diameter of the optical signal input thereto after expanding it by three times or more. In contrast, the second optical transmission lines 31 and 32 output the beam diameter of the optical signal input thereto after expanding it by less than three times. When a Gaussian beam output from a single-mode optical fiber is input thereto under central excitation, the first optical transmission lines 12 and 22 output the beam diameter after expanding it by three times or more (see the evaluation under central excitation in FIG. 2 ). Furthermore, when a Gaussian beam emitted from a single-mode optical fiber is input to the second optical transmission lines 31 and 32 with central excitation, the second optical transmission lines 31 and 32 expand the beam diameter to less than three times and output the expanded beam.

[0038] The inventors conducted an experiment in which the beam diameter of an optical signal output from the LD 11a or LD 21a was expanded by the first optical transmission line 12 or 22, and then the optical signal was transmitted through the second optical transmission line 31 or 32 and received by the PD 21b or PD 11b. Surprisingly, the inventors found that the error rate in the optical transmission was lower than when the optical transmission lines 61 and 62 did not have the first optical transmission lines 12 and 22.

[0039] In other words, the optical transmission system 100 can achieve high-quality, high-capacity communication with a simple configuration in which the first optical transmission paths 12 and 22, which have an extremely large beam diameter expansion rate, are added to the second optical transmission paths 31 and 32, which are normally used and have a small beam diameter expansion rate.

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

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

[0042] (Formation of micro-heterogeneous structures) Next, we will describe in detail a preferred example of the first optical transmission line 12, 22. For example, if the core of the optical fiber has a microscopic non-uniform structure with a correlation length of about several hundred angstroms or even longer, it will be 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 coiled structure with a radius of gyration of several hundred angstroms. Furthermore, polymer coils can sometimes associate slightly with each other to form large heterogeneous structures. In such cases, the correlation length increases, as predicted by Debye scattering theory, leading to further forward scattering and mode coupling. Microheterogeneous structures can also be formed in copolymers. Copolymers generally have a compositional distribution and are more likely to form heterogeneous structures than homopolymers, such as through the association of homogeneous monomer units. While these heterogeneous structures depend on the extrusion processing conditions, polymer molecular weight, and thermal history, effective utilization of enthalpy relaxation to achieve the appropriate metastable enthalpy state allows for the mass production of polymers with specific microheterogeneous structures without any problems. Silica glass does not exhibit such microheterogeneous structures. In addition to creating a micro-inhomogeneous structure in the polymer, adding particles to the polymer or glass is also an effective way to control scattering. For example, when considering a first optical transmission line of less than a few centimeters that fits within an optical signal transceiver, scattering that enables stronger mode coupling is required. To achieve this, adding submicron or micron-order particles with a different refractive index to the core is effective. Particle candidates are not limited to any material with a refractive index different from that of the polymer or glass medium that makes up the core. Examples 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 using these particles, larger micron-sized particles are preferable to nano-sized particles. Instead of adding particles, the formation of microvoids can have a similar effect and is also effective. In one embodiment of a GI POF, a refractive index profile is formed by radially varying the concentration of a small-molecule dopant with a refractive index different from that of the polymer matrix. The size of the dopant is on the order of several to several tens of angstroms, and the intensity of light scattering from a single molecule is negligibly small. However, slight fluctuations in the dopant concentration on the order of several hundred to several thousand angstroms result in the formation of a micro-inhomogeneous structure, inducing forward light scattering. This slight fluctuation / association of the dopant is caused by slight differences in the compatibility between the polymer matrix and the dopant. Therefore, by examining the differences in compatibility between the polymer and the dopant using the solubility parameter as a guide, it is possible to control the micro-inhomogeneous structure through the dopant fluctuation / association, thereby controlling mode coupling. Furthermore, by adding not only a dopant for refractive index profile formation but also a small molecule for forming a micro-inhomogeneous structure, mode coupling due to forward scattering can be controlled based on the same principle.

[0043] For example, acrylic polymers have intramolecular and intermolecular interactions due to the ester groups present in the molecules. In contrast, perfluorinated polymers such as dioxolenes do not have such ester groups. Therefore, the intramolecular and intermolecular interactions are weaker than those of acrylic polymers. However, both polymers are aggregates of molecular coils with radii of gyration of several hundred angstroms, and a relatively stable microheterogeneous structure can be controlled, for example, during extrusion molding.

[0044] (Examples of polymers used in the first optical transmission line) The polymers constituting the core and clad of the first optical transmission lines 12 and 22 can be produced by methods known in the art. For example, methods include subjecting a mixture of monomers constituting the polymer to solution polymerization, bulk polymerization, emulsion polymerization, suspension polymerization, etc. 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, and is suitably about 80 to 150° C. The reaction time can be adjusted appropriately depending on the amount and type of monomer, the amount of a polymerization initiator and a chain transfer agent (to be described later), the reaction temperature, etc., and is suitably about 20 to 60 hours. These polymers may be produced simultaneously or continuously with the molding of the core and / or clad.

[0046] Examples of the polymer constituting the core include (meth)acrylic 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-butylmaleimide, N-tert-butylmaleimide, N-isopropylmaleimide, and N-cyclohexylmaleimide, as well as substances in which some of the hydrogen atoms in the C-H bonds of these monomers have been substituted with chlorine, fluorine, or deuterium. The stretching vibration between the C-H bonds constituting the polymer causes absorption loss due to its overtones at a light source wavelength of 850 nm, for example. However, when the first optical transmission line is a few centimeters to a few tens of centimeters or less, the absorption loss may be negligible. In such cases, general polymers such as non-halogenated acrylics and styrenes may be used.

[0047] When producing a polymer, it is preferable to use a polymerization initiator and / or a chain transfer agent. Examples of the polymerization initiator include ordinary radical initiators. For example, peroxide compounds such as benzoyl peroxide, t-butylperoxy-2-ethylhexanate, di-t-butyl peroxide, t-butylperoxyisopropyl carbonate, and n-butyl 4,4,bis(t-butylperoxy)valerate; 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis(2-methylpropane), 2,2'-azobis(2-methylbutane), 2,2'-azobis(2-methylpentane), 2,2'-azobis(2,3-dimethylbutane); Examples of suitable azo compounds include 2,2'-azobis(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 alone or in combination. The polymerization initiator is preferably used in an amount of approximately 0.01 to 2% by weight based on the total monomers.

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

[0049] Chain transfer agents are usually used to adjust the molecular weight to an appropriate 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" (co-authored by Otsu Takayuki and Kinoshita Masayoshi, Kagaku Dojin, published in 1972) in Polymer Handbook, Third Edition (edited by J. Brandrup and Ehim M. Erugut, published by John Weiley & Son). Therefore, it is preferable to appropriately adjust the type and amount of the chain transfer agent depending on the type of monomer, etc., taking into account the chain transfer constant. For example, the amount can be about 0.1 to 4 parts by weight per 100 parts by weight of the total monomer components.

[0050] The polymer constituting the core and / or clad suitably has a weight-average molecular weight in the range of about 50,000 to 300,000, preferably about 100,000 to 250,000, in order to ensure appropriate flexibility, transparency, etc. The core and clad may have different molecular weights, for example, to adjust viscosity, etc. The weight-average molecular weight refers to a polystyrene-equivalent value measured by, for example, GPC (gel permeation chromatography).

[0051] The polymer constituting the first optical transmission lines 12, 22 may be blended with compounding agents, such as heat stabilization aids, processing aids, heat resistance improvers, antioxidants, light stabilizers, etc., as needed, within the scope of not impairing the performance of the optical fiber, such as transparency and heat resistance. These may be used alone or in combination of two or more, and methods for mixing these compounds with the monomer or polymer include, for example, hot blending, cold blending, and solution mixing.

[0052] (Fluorine-containing polymer used in the first optical transmission line) When a fluorine-containing polymer (including fully fluorinated and partially fluorinated materials) is used as the core material of the first optical transmission lines 12 and 22, it can be synthesized by the following method. [Synthesis Example A] Method for synthesizing perfluorinated materials Typical perfluorinated materials that can be used are those with the product names TEFRON-AF (DuPont), Hyflon AD (Solvay), and CYTOP (AGC). Perfluorinated polymers in which tetrafluoroethylene or the like is copolymerized into the main ring structure of these materials may also be used. Perfluorinated polymers with a dioxolene skeleton can also be used. Next, we will describe a method for synthesizing perfluorinated materials with a dioxolene skeleton.

[0053] <Synthesis of perfluoro-4-methyl-2-methylene-1,3-dioxolane> Purified 2-carbomethyl-2-trifluoromethyl-4-methyl-1,3-dioxolane was obtained by dehydration condensation 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 each flowed at a constant flow rate. Under a nitrogen / fluorine atmosphere, the previously prepared 2-carbomethyl-2-trifluoromethyl-4-methyl-1,3-dioxolane was slowly added to the reaction vessel for fluorination, yielding perfluoro-2,4-dimethyl-1,3-dioxolane-2-carboxylic acid. The distillate was neutralized with aqueous potassium hydroxide to yield potassium perfluoro-2,4-dimethyl-2-carboxylate-1,3-dioxolane. This potassium salt was vacuum dried and further decomposed under an argon atmosphere to obtain perfluoro-4-methyl-2-methylene-1,3-dioxolane. The perfluoro-4-methyl-2-methylene-1,3-dioxolane and perfluorobenzoyl peroxide obtained above were placed in a glass tube, which was degassed using a freeze / thaw vacuum machine, then refilled with argon and heated for several hours. The contents became solid, and a transparent polymer was obtained. This polymer was used to fabricate optical fiber.

[0054] The viscosity of fluorine-containing polymers (including fully fluorinated and partially fluorinated materials) in the molten state is 10 at a melting temperature of 200 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 the dopant necessary for forming the refractive index distribution is also difficult to occur, making it difficult to form the refractive index distribution. On the other hand, if the melt viscosity is too low, practical problems arise. That is, when used as an optical transmission medium in electronic devices, automobiles, etc., the material softens when exposed to high temperatures, resulting in a decrease in light transmission performance.

[0055] The number average molecular weight of the fluorine-containing polymer is preferably 10,000 to 5,000,000, more preferably 50,000 to 1,000,000. If the molecular weight is too small, heat resistance may be impaired, while if it is too large, it becomes difficult to form an optical transmission element having a refractive index distribution, which is not preferable.

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

[0057] [Synthesis Example B] Synthesis of partially chlorinated materials (see Patent No. 5419815) The preparation method for the partially chlorinated material will be briefly described below. Trichloroethyl methacrylate, purified by distillation, cyclohexylmaleimide, and diphenyl sulfide, a refractive index-imparting dopant, were each precisely weighed and placed in a glass container. Furthermore, a predetermined amount of di-tert-butyl peroxide as a polymerization initiator and normal-lauryl mercaptan as a chain transfer agent were added based on the total weight. After thorough mixing, the solution was filtered through a fine-pore membrane filter into a glass polymerization vessel. Next, argon gas was introduced into the glass polymerization tube containing the solution, and dissolved air was removed by freeze degassing. The glass polymerization tube was then placed in an oven, and the temperature of the polymerization vessel was increased while argon gas was introduced to polymerize the monomers. The polymerization reaction was then completed by further increasing the temperature. The glass tube was then opened, yielding a solidified, transparent polymerized rod.

[0058] (Dopants for forming refractive index distribution) When the solubility parameter of a dopant is equal to that of the polymer and the compatibility is good, 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, dopants tend to aggregate, resulting in the formation of a refractive index heterogeneity due to the dopant concentration distribution. In addition to general solubility parameter knowledge, local interactions between the dopant and the polymer (e.g., secondary electronic polarization corresponding to specific functional groups) can also be used to create a microscopic dopant concentration distribution. Dopants for perfluorinated core materials typically have a higher refractive index than perfluorinated polymers. That is, for similar reasons as perfluorinated polymerization, the dopant is essentially free of C—H bonds, and a refractive index 0.05 or greater than that of perfluorinated polymers is preferred. A higher refractive index requires a smaller dopant content to achieve the desired refractive index profile, resulting in a smaller decrease in the glass transition temperature and, consequently, improved heat resistance of the optical fiber. Therefore, a refractive index 0.1 or greater is particularly preferred.

[0059] As the dopant, 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 are preferred. However, in the case of polymers, as the molecular weight increases, compatibility with perfluoropolymers decreases, resulting in increased light scattering loss, so those with too high a molecular weight are not preferred. Conversely, in the case of compounds with low molecular weights, the glass transition temperature in a mixture with a fluorine-containing polymer decreases, causing a decrease in the heat resistance temperature of the optical fiber, so too low a molecular weight is not preferred. Therefore, the number average molecular weight of the dopant should be 3×10 2 ~2×10 3 is preferred, 3 × 10 2 ~1×10 3 is more preferred.

[0060] Specific dopant compounds include oligomers that are pentamer to octamer of chlorotrifluoroethylene, oligomers that are pentamer to octamer of dichlorodifluoroethylene, as described in JP-A-8-5848, and dimer to pentamer oligomers obtained by polymerizing a monomer that gives an oligomer with a high refractive index (for example, a monomer having a chlorine atom) among the monomers that form the perfluoropolymer.

[0061] In addition to halogen-containing aliphatic compounds such as the above-mentioned oligomers, 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 fluorine atoms and a relatively small number of chlorine 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 contain 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 substituted with fluorine atoms, Z is a halogen atom other than fluorine, -Rf, -CO-Rf, -O-Rf, or -CN, where Rf is a perfluoroalkyl group, a polyfluoroperhaloalkyl group, or a monovalent Φr, and b is an integer of 0 or greater). Examples of aromatic rings include benzene rings and naphthalene rings. The number of carbon atoms in the perfluoroalkyl group or polyfluoroperhaloalkyl group represented by Rf is preferably 5 or less. The halogen atom other than fluorine is preferably a chlorine atom or a bromine atom. Specific examples of such compounds include 1,3-dibromotetrafluorobenzene, 1,4-dibromotetrafluorobenzene, 2-bromotetrafluorobenzotrifluoride, chloropentafluorobenzene, bromopentafluorobenzene, iodopentafluorobenzene, decafluorobenzophenone, perfluoroacetophenone, perfluorobiphenyl, chloroheptafluoronaphthalene, and bromoheptafluoronaphthalene. Particularly preferred examples of fluorine-containing polycyclic compounds as dopants include chlorotrifluoroethylene oligomer, perfluoro(triphenyltriazine), perfluoroterphenyl, perfluoroquatrophenyl, perfluoro(triphenylbenzene), and perfluoroanthracene, because of their good compatibility with perfluorinated polymers, particularly fluorinated polymers having a ring structure in the main chain, and good heat resistance. Because of their good compatibility, the fluorinated polymers, particularly fluorinated polymers having a ring structure in the main chain, can be easily mixed with substances to be mixed by heating and melting at 200 to 300°C. Alternatively, the two can be mixed uniformly by dissolving them in a fluorine-containing solvent, mixing them, and then removing the solvent.

[0063] Dopants used in partially chlorinated or partially fluorinated core materials include low-molecular-weight compounds and compounds in which hydrogen atoms in these compounds are replaced with deuterium atoms. Examples of low-molecular-weight compounds with high refractive indices include sulfur compounds such as 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), diphenyl sulfide (DPS), diphenyl sulfoxide, dibenzothiophene, and dithiane derivatives; phosphate compounds such as triphenyl phosphate (TPP) and tricresyl phosphate; benzyl benzoate; benzyl n-butyl phthalate; diphenyl phthalate; biphenyl; and diphenylmethane. Examples of low-refractive-index low-molecular-weight compounds include tris-2-ethylhexyl phosphate (TOP). These compounds may be used alone or in combination.

[0064] (Method for manufacturing the first optical transmission line) To facilitate the creation of a microscopic non-uniform structure, the temperature and drawing speed during spinning of the optical fiber may be controlled. The preform method and melt extrusion method are well-known common methods for producing optical fiber using fluorine-containing polymers. In the preform method, a rod-shaped plastic molded body called a rod, consisting of a core and a cladding, is first prepared. This core rod is placed at the center, and the cladding rod, which has a hollow portion and is integrated so as to cover the outer periphery of the core, produces a rod-shaped object called a preform. This preform is set in a common spinning device, and the outer periphery of the preform is uniformly heated and melted using a cylindrical heater or the like. The tip portion is drawn and drawn at a constant speed to form a fiber, which is then cooled and wound up to obtain the optical fiber.

[0065] On the other hand, the melt extrusion method involves filling a general melt extrusion device with a core polymer that is premixed with a predetermined amount of dopant and a cladding polymer that does not contain a dopant, and then co-extruding the molten polymers through two extruders to extrude both polymers from a nozzle to obtain optical fiber. Generally, an extruder with a screw can be used, but melt extrusion under pressure such as nitrogen gas can also be used. A coating layer can also be provided as needed.

[0066] A heat treatment process after coextrusion of the molten core polymer and molten cladding polymer can also create microheterogeneous structures. For example, if the coextrusion is rapidly cooled, the polymer will retain a large volume and become vitrified before enthalpy relaxation occurs. On the other hand, if a sufficient heat treatment process is performed near the glass transition temperature, the volume will decrease slightly due to enthalpy relaxation. If this enthalpy relaxation occurs in the microregion, a so-called microheterogeneous structure will form. Furthermore, if a drawing process is performed after coextrusion, the molecules of the melt-extruded fiber will be oriented, and orientation birefringence will occur depending on the degree of orientation. This orientation birefringence will result in birefringence not only in the fiber axial direction but also in the radial direction and in a specific direction. This birefringent structure also promotes mode coupling.

[0067] The optical fiber of the present invention can be manufactured by any method known in the art. For example, to form one or more cladding layers around one or more core layers, interfacial gel polymerization, rotational polymerization, melt extrusion dopant diffusion, composite melt spinning, rod-in-tube, etc. can be used. Alternatively, a preform can be formed in advance and then drawn, drawn, etc.

[0068] Specifically, a hollow cladding is fabricated, and a core is fabricated in the hollow cladding. In this case, a monomer constituting the core is introduced into the hollow cladding, and the polymer is polymerized while rotating the cladding, forming a core having a higher refractive index than the cladding. This process may be performed only once to form a single-layer core, or may be repeated to form a core consisting of multiple layers.

[0069] The polymerization vessel used is a cylindrical tube made of glass, plastic, or metal, and can be one that has the mechanical strength to withstand external forces such as centrifugal force due to rotation and the heat resistance during thermal polymerization. The rotation speed of the polymerization vessel during polymerization is, for example, about 500 to 3,000 rpm. It is usually preferable to filter the monomer through a filter to remove dust contained in the monomer before introducing it into the polymerization vessel.

[0070] To impart a GI type refractive index profile to an optical fiber, for example, as described in WO93 / 008488, interfacial gel polymerization is used, in which a monomer composition ratio is kept constant, a dopant is added, and the monomer is polymerized in bulk at the interface of the polymer, resulting in a dopant concentration profile through the reaction; rotational gel polymerization is used to simulate the reaction mechanism of interfacial gel polymerization; and rotational polymerization is used to gradually change the composition ratio of monomers with different refractive indexes, i.e., to control the polymerization rate of the previous layer (to lower the polymerization rate) and polymerize the next layer, which will have a higher refractive index, so that the refractive index profile gradually increases from the interface with the cladding to the center.

[0071] Furthermore, a method of forming the core and clad portions may be used, using two or more melt extruders and two or more multi-layer dies and a multi-layer spinning nozzle. That is, the polymers constituting the core and clad portions are heated and melted, respectively, and then injected into the multi-layer die and multi-layer spinning nozzle through separate channels. The core portion is extruded using this die and nozzle, and at the same time, one or more concentric clad layers are extruded around the core portion and fused together to form a fiber or preform.

[0072] Other examples include a melt extrusion dopant diffusion method in which a core and clad portion 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 subsequent heat treatment zone to impart a dopant concentration distribution; and a method in which polymers containing different amounts of dopant are introduced into two or more melt extruders, and the core and / or clad portion are extruded to form a multilayer structure.

[0073] When an SI type refractive index distribution is to be imparted, it is suitable to carry out rotational polymerization etc. while keeping the monomer composition ratio and / or the amount of dopant added constant from start to finish. When a multi-step type refractive index distribution is to be imparted, it is preferable to control (increase) the polymerization rate of the previous layer in rotational polymerization etc. and polymerize the next layer with a higher refractive index.

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

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

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

[0077] Fig. 4 is a diagram showing the measurement results of the beam diameter of the light output in the first optical transmission path in Examples 1 to 3. Fig. 4 shows an image measured as NFP. Note that the beam diameter of the incident light is the same as "Input" in Fig. 3, so it is not shown in the figure. The characteristics of the first optical transmission path are as follows.

[0078] The first optical transmission line of Example 1 had an OTDR loss of 460 dB / km measured with an OTDR (Optical Time Domain Reflectometer) at a wavelength of 850 nm. 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 of 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 of 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, the lengths of the first optical transmission lines in all of Examples 1 to 3 were set to 0.15 m, 0.30 m, 0.50 m, 1.0 m, 2.0 m, and 3.0 m.

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

[0083] 5 is a diagram showing the relationship between the ratio r1 / r0 of the beam diameter r1 of output light to the beam diameter r0 of input light in the first optical transmission lines of Examples 1 to 3 and the length (fiber length). As can be seen from FIG. 5, all of the first optical transmission lines of Examples 1 to 3 output the beam diameter of the input optical signal expanded by three times or more. That is, in all of the first optical transmission lines of Examples 1 to 3, r1 / r0 was 3 or more, and in Example 3 in particular, r1 / r0 was 6 or more at all lengths. This is thought to mean that the greater the scattering loss in the first optical transmission line, the more significant the mode coupling.

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

[0085] Here, as the optical transmission system of Comparative Example 1, the second optical transmission line of Reference Example 1 shown in Figure 3, which was 100 m long, was used as the second optical transmission line 32, the first optical transmission line 12 and the internal optical transmission line 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, and an optical transmission system was constructed.

[0086] As an optical transmission system of Example 4, an optical transmission system was constructed in which, in the configuration of the optical transmission system 100, the second optical transmission line with a length of 100 m shown in Fig. 3 was used as the second optical transmission line 32, the first optical transmission line 12 was set to 0.15 m of the first optical transmission line of Example 3, the intra-device optical transmission line 23 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. However, the first optical transmission line 12 and the second optical transmission line 32 were connected via an air layer (gap) with a gap length of 50 µm, and measurements were performed under conditions where axial misalignment in the radial and angular directions was negligible.

[0087] Furthermore, as an optical transmission system of Example 5, an optical transmission system was constructed in which the first optical transmission line 12 in the configuration of the optical transmission system of Example 4 was changed to 0.30 m, which is the length of the first optical transmission line in Example 3.

[0088] As an optical transmission system of Example 6, an optical transmission system was constructed in which the first optical transmission line 12 in the configuration of the optical transmission system of Example 5 was changed to 0.50 m, which is the length of the first optical transmission line in Example 3.

[0089] Moreover, as an optical transmission system of Example 7, an optical transmission system was constructed in which the first optical transmission line 12 in the configuration of the optical transmission system of Example 6 was changed to 1.0 m of the first optical transmission line of Example 3.

[0090] As an optical transmission system of Comparative Example 2, an optical transmission system was constructed in which the first optical transmission line 12 in the configuration of the optical transmission system of Example 7 was changed to 3.0 m, which is the same as the first optical transmission line of Example 3.

[0091] The LD is a VCSEL, the wavelength is 850 nm, the modulation method is NRZ (Non Return to Zero) PRBS (Pseudo Random Bit Sequence), the bit rate is 10 Gbps, and the pattern length is 2 31 The voltage was set to -1, the bias current was set to 5 mA, the modulation voltage was changed from 0.10 V to 0.40 V in 0.02 V increments, and the measurement time was set to 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). 10 If the BER is less than or equal to -12, the data point is marked as -12.

[0093] As shown in FIG. 6, in Examples 4 to 7, the BER was 10 -12 It was confirmed that error-free transmission was achieved at a modulation voltage lower than 0.20 V at which error-free transmission below 100 kHz was achieved. 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 Comparative Example 1. The reason for this is thought to be that in Comparative Example 2, the product of the scattering loss and length for an optical signal with a wavelength of 850 nm was 6.57 dB, which was larger than 6 dB, and therefore the loss of the optical signal was too large, resulting in a deterioration in quality.

[0094] The refractive index profile within the core of the first optical transmission line is expressed by the refractive index profile 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 it depends on the material dispersion. However, the first optical transmission line may be several centimeters or less in length, in which case modal dispersion can be almost negligible. In fact, as shown in Figure 6, the refractive index profile coefficient g of the first optical transmission line used in Examples 4 to 7 was around 7.5, yet it exhibited a dramatic reduction in BER. In other words, even if the g value is significantly larger than the ideal profile g = 2, high-quality transmission can be achieved as long as the beam diameter (r1 / r0) can be sufficiently expanded. The first optical transmission line may have an SI-type refractive index profile, which has an even larger g value for the refractive index profile within the core. In order to efficiently expand the beam diameter (r1 / r0) over a short distance, a large g value may be used so that the refractive index profile at the center of the core approaches flat.

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

[0096] As described above, the optical transmission system 100 according to the first embodiment has an advantage of being able to realize high-quality, high-capacity communication with a simple configuration. This means that signal degradation is suppressed more effectively when the optical transmission line also includes the first optical transmission line than when the optical transmission line includes only the second optical transmission line.

[0097] In particular, the optical transmission system 100 can transmit 10 -12 Since the following error rate can be achieved, problems such as the complexity of the configuration due to the addition of processors such as DSPs when using error correction methods, transmission delays, deterioration of coding efficiency, and increased heat and power consumption due to processor load do not occur.

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

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

[0100] Like the optical transmission system 100, the optical transmission system 100A can achieve high-quality, large-capacity communication with a simple configuration using the optical transmission lines 61A and 62A.

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

[0102] Example 8 In Example 8, a GI-POF fabricated by the melt extrusion method described above was used as an optical transmission line. The optical transmission line of 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 line of Example 8 were then measured. Specifically, the beam diameters of the light input to and output from the optical transmission line of Example 8 were measured using the measurement method shown in FIG. 2.

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

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

[0105] (Example 9, Comparative Example 3) Next, an optical transmission system of Example 9 having a configuration similar to that of the optical transmission system 100A according to Embodiment 2 was constructed using the optical transmission line of Example 8 with a length of 10 m, and error rates were measured. However, an optical transmission line 62A consisting of the 10 m long optical transmission line of Example 8 was connected to PD 11b, and loopback measurement of one channel was performed.

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

[0107] As a result, over 30 minutes, the BER was 1.0 x 10 -13 and error-free transmission was achieved.

[0108] Here, as an optical transmission system of Comparative Example 3, an optical transmission system was constructed by replacing the 10 m long optical transmission line of Example 8 in the configuration of the optical transmission system of Example 9 with a 2 m long glass optical fiber optical transmission line. This 2 m long optical transmission line was optically connected to LD 11a and PD 11b by a mini-MT connector. Then, the error rate was measured under the same conditions as the optical transmission system of Example 9. However, each measurement took 3 minutes, and the mini-MT connector was inserted and removed for each measurement, so a total of 5 BER measurements were performed. As a result, the BER was 10 -6 ~10 -12 This indicates that the transmission quality depends on the interconnection state between the light source (VCSEL), photodetector (PD), and optical transmission line, and that error correction by FEC is necessary for high-quality error-free transmission.

[0109] In optical links, transmission quality is degraded by various noises (reflected optical noise, modal noise, etc.) that depend on the connection state of the optical fiber. In particular, reflected optical noise causes a significant increase in the noise level and is thought to be the main cause of transmission quality degradation.

[0110] In ordinary silica-based optical fibers, the effects of mode coupling within the optical fiber can be ignored, so the reflected light maintains high coherence and efficiently couples to the laser resonator, causing large noise and possibly degrading transmission quality (bit error rate).

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

[0112] However, even with silica-based optical fibers, if the core contains scatterers (such as particles) and mode coupling is appropriately generated, error-free transmission can be achieved without using FEC.

[0113] (analog modulation method) In the above-described embodiment, example, and comparative example, the modulation method is a digital modulation method, but the present invention can also be applied to optical transmission systems using analog modulation method. Radio over Fiber (RoF) is a technology related to optical transmission systems using analog modulation method.

[0114] RoF is a technology that transmits wireless signals using optical fiber, and is being increasingly used 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. 5G and future Beyond 5G and 6G will require the installation of more antennas than before to improve transfer speeds and eliminate radio dead zones, so the importance of RoF technology is expected to increase even more in the future.

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

[0116] An optical transmission system using an analog modulation method according to an embodiment of the present invention can have a configuration similar to that of the optical transmission system 100 according to the first embodiment shown in Fig. 1 or the optical transmission system 100A according to the second embodiment shown in Fig. 7. However, in the case of the analog modulation method, an RF amplifier or the like is used instead of the TIA-IC. Furthermore, in the case of broadcasting applications, the optical transmission system using the analog modulation method can be configured as a unidirectional system for distribution only, rather than as a bidirectional system like the optical transmission systems 100 and 100A.

[0117] (Example 10, Comparative Example 4) As an analog modulation optical transmission system of Example 10, an optical transmission system was constructed using the 100 m long second optical transmission line of Reference Example 1 and the 0.50 m long first optical transmission line of Example 3 as optical transmission lines, as in Example 6. Furthermore, as an analog modulation optical transmission system of Comparative Example 4, an optical transmission system was constructed using only the 100 m long second optical transmission line of Reference Example 1 as an optical transmission line. Then, RoF transmission experiments were carried out 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 transmission signal was an orthogonal frequency division multiplexing (OFDM) radio signal, with a modulation method of 64-quadrature amplitude modulation (64QAM), a center frequency of 880 MHz, a bandwidth of 20 MHz, and an input signal strength of -10 dBm.

[0119] FIG. 10 is a diagram showing a constellation map of Comparative Example 4. The error vector magnitude (EVM) in this case was 9.93%. In contrast, FIG. 11 is a diagram showing a constellation map of Example 10. The error vector magnitude (EVM) in this case was 3.97%. As can be seen from FIGS. 10 and 11, in Example 10, the variation of signal points in the constellation map was significantly reduced compared to Comparative Example 4, and the EVM of the transmission signal was reduced. This is thought to be due to the fact that mode coupling within the first optical transmission line significantly reduces the noise generated in the optical transmission line.

[0120] 12 is a diagram showing the error vector magnitudes obtained by 30 measurements for Example 10 and Comparative Example 4. In Comparative Example 4, the EVM was about 9%, but in Example 10, the EVM was reduced to about 3%. The above 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) However, when a light source such as an LD in an optical signal transmitter is optically connected to an optical fiber of an optical transmission path via a lens or the like, the light source may become unstable due to return light from the optical fiber, which may increase noise and impair transmission quality.

[0122] It is known that the influence of light returning from an optical fiber can be reduced by slightly misaligning the optical fiber and the light source or by defocusing the light source. However, misalignment such as misalignment and defocusing increases the coupling loss between the light source and the optical fiber, which causes a deterioration in the signal-to-noise ratio of the transmitted signal.

[0123] As a result of extensive research, the inventors have found that by utilizing mode coupling in an optical fiber to reduce the dependency of the beam diameter of light emitted from the optical fiber on the excitation conditions, it is possible to reduce the influence of returned light that depends on the amount of misalignment between the optical fiber and the light source. This has also led to the discovery that the requirements for aligning optical fibers to achieve high-quality optical transmission can be significantly relaxed. Below, a third embodiment will be described as an example of this.

[0124] The optical transmission system according to the third embodiment has the same configuration as the optical transmission system according to the second embodiment shown in FIG. 7. Here, in the optical transmission line of the optical transmission system according to the third embodiment, when a Gaussian beam output from a single-mode optical fiber is input with a positional deviation, the ratio of the minimum to the maximum beam diameter of the output Gaussian beam is 0.7 or more. This significantly eases the requirement for aligning the optical fiber to achieve high-quality optical transmission. As a result, high-precision alignment of optical elements (light source, lens, optical fiber, etc.) is no longer necessary, making it possible to easily fabricate the optical transmission system at low cost. It is preferable that the product of the scattering loss and length of the optical transmission line for the optical signal is 6 dB or less.

[0125] The optical transmission line of the optical transmission system according to the third embodiment can be, for example, the GI-POF of Example 8. Hereinafter, the results of an experiment using the optical transmission line of Reference Example 1, the optical transmission line of Example 8, and the optical transmission line of Comparative Example 5, which is a commercially available GI-POF, will be described. The optical transmission line of Comparative Example 5 had an OTDR loss of approximately 60 dB / km. Most of this loss is thought to be due to scattering loss. The core diameter was approximately 50 μm.

[0126] First, the relationship between positional deviation and beam diameter was measured for the 10-m-long optical transmission lines of Reference Example 1, Comparative Example 5, and Example 8 using the measurement system shown in FIG. 2. Here, as shown in FIG. 13, the z-axis represents the optical axis direction of the emitted light 203 (or lens 204), and the x-axis represents the radial direction. The optical transmission line 205 was shifted in the z-axis direction or the x-axis direction around the focusing position of the emitted light 203 from the lens 204, and the emitted light 203 was then incident. In this case, a deviation amount Δx=0 indicates that the optical axis of the emitted light 203 and the optical axis of the optical transmission line 205 coincide with each other. A deviation amount Δz=0 indicates that the focusing position of the emitted light 203 coincides with the input end face of the optical transmission line 205. The NFP of the light output from the end face opposite the input end face of the optical transmission line 205 was measured using an NFP measurement device, and the beam diameter (D4σ width) of the light output from the optical transmission line 205 was calculated from the NFP using the second-order moment method. 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 of FIG.

[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, the beam diameter of the output beam tended to expand as the offset Δx increased. This is likely due to the excitation of higher-order modes as the beam incidence position moves from the core center to the core periphery of the optical transmission line. On the other hand, in Example 8, the size of the output beam remained almost constant regardless of the offset Δx. This is thought to be due to the strong mode coupling in the optical transmission line, which excites higher-order modes regardless of the excitation conditions.

[0128] 15 is a diagram showing the relationship between Δz and the beam diameter of the output light in Reference Example 1, Comparative Example 5, and Example 8. As in the case of Δx, in the cases of Reference Example 1 and Comparative Example 5, the beam diameter of the output beam tends to expand as the deviation amount Δz increases. On the other hand, in the case of Example 8, the size of the output beam was almost constant regardless of the deviation amount Δz.

[0129] FIG. 16 is a diagram showing the relationship between Δx and the emitted beam diameter in Reference Example 1, Comparative Example 5, and Example 8. FIG. 17 is a diagram showing the relationship between Δz and the emitted beam diameter in Reference Example 1, Comparative Example 5, and Example 8. As can be seen from FIGS. 16 and 17, in the cases of Reference Example 1 and Comparative Example 5, the emitted beam diameter was smallest near Δx=0 and Δz=0, and the emitted beam diameter tended to increase as the amount of deviation increased. On the other hand, in the case of Example 8, the emitted beam diameter significantly increased near Δx=0 and Δz=0, and the dependency of the emitted beam diameter on the amount of deviation significantly decreased.

[0130] Fig. 18 is a diagram showing the normalized output beam diameter of Fig. 16. Fig. 19 is a diagram showing the normalized output beam diameter of Fig. 17. The output beam diameter was normalized by dividing the output beam diameter at each deviation amount by the maximum output beam diameter. In other words, the vertical axis of Figs. 18 and 19 is (output beam diameter) / (maximum output beam diameter).

[0131] 18, the ratio (minimum value of output beam diameter) / (maximum value of output beam diameter) for positional displacement 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 Fig. 19, the ratio (minimum value of output beam diameter) / (maximum value of output beam diameter) for positional displacement 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 the second embodiment was constructed using the optical transmission paths of Reference Example 1, Comparative Example 5, and Example 8, each 10 m long, and the optical signal transceiver 10 was used as the optical transmitter and the optical signal transceiver 20 was used as the optical receiver. Then, the amount of deviation in the x-axis direction and the amount of deviation in the z-axis direction were variously changed, and the error rate was measured.

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

[0134] 20A, 20B, and 20C are diagrams showing the relationship between Δx and the error rate in Reference Example 1, Comparative Example 5, and Example 8, respectively. FIGS. 21A, 21B, and 21C are diagrams showing 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 FIGS. 20A to 20C and 21A to 21C, in the case of Reference Example 1 and Comparative Example 5, the BER deteriorates and has peaks near Δx=0 and Δz=0. -4 In Comparative Example 5, the -8 The BER tended to decrease as the misalignment increased. This was due to the fact that the influence of optical feedback decreased as the misalignment increased. On the other hand, when the misalignment became too large (exceeding approximately ±20 μm for Δx and ±1000 μm for Δz), the influence of increased coupling loss became dominant, and the BER began to increase.

[0136] From the above, in the case of Reference Example 1 and Comparative Example 5, although (minimum value of output beam diameter) / (maximum value of output beam diameter) is less than 0.7, more precise alignment of the light source and the optical transmission path is required to minimize the coupling loss and suppress the influence of returned light.

[0137] On the other hand, in the case of Example 8, no deterioration of the BER was observed near Δx=0 and Δz=0, and the BER was 10 -12 The following error-free transmission was achieved: From the above, in the case of Example 8, although (minimum value of output beam diameter) / (maximum value of output beam diameter) is 0.7 or more, error-free transmission can be achieved without precise alignment.

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

[0139] 22A, 22C, and 22C are diagrams showing the relationship between Δx and coupling loss in Reference Example 1, Comparative Example 5, and Example 8, respectively. Figures 23A, 23B, and 23C are diagrams showing the relationship between Δz and coupling loss in Reference Example 1, Comparative Example 5, and Example 8, respectively. In all of Reference Example 1, Comparative Example 5, and Example 8, a tendency for coupling loss to increase as the amount of misalignment increases was observed.

[0140] FIG. 24 is a diagram showing the conditions for realizing error-free transmission using the optical transmission line of Example 8, obtained from the results of FIGS. 20C, 21C, 22C, and 23C. When the optical transmission line of Example 8 was used, error-free transmission was achieved in the range of -14.5 μm≦Δx≦+18 μm. Furthermore, 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 the optical transmission line of Example 8 was used, error-free transmission was achieved in the range of -850 μm≦Δz≦+730 μm. Furthermore, the coupling loss at Δz=-850 μm was 7.78 dB, and the coupling loss at Δx=+730 μm was 6.69 dB. From the above results, it can be seen that, with the optical transmission line of Example 8, 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, error-free transmission can be achieved in the range of 10 μm≦Δz≦+730 μm, regardless of the amount of misalignment. -12 Furthermore, for example, in the optical transmission lines of Reference Example 1 and Comparative Example 5, even under optical coupling conditions in which 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 On the other hand, in the optical transmission line of Example 8, under the optical coupling condition where the coupling loss between the optical signal transmitter (light source) and the optical transmission line is 1 dB or less, the worst value of the error rate is 10 -8 The following is the result.

[0141] In actual optical transmission systems, optical signals are often transmitted over an optical transmission path formed by connecting two or more optical fibers. When connecting optical fibers, any gaps (air layers) between the fibers can increase reflection loss and reflected back optical noise. Therefore, suppressing reflections at fiber connections is considered important for high-quality optical transmission.

[0142] Various methods have been proposed to suppress reflection at fiber connections, such as fusion splicing and connections using refractive index matching agents, but the most widely used is the physical contact (PC) connection, which prevents reflection by pressing the fiber end faces (connector end faces) polished to a convex spherical shape together to form a tight seal.

[0143] However, PC connections not only require precise polishing of the connector end faces, but also require 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, because PC connections require physical contact between the fiber end faces, if foreign matter is present on the fiber end faces when mating, the foreign matter may be pressed against the fiber end faces, causing damage to the end faces.

[0144] Therefore, if optical transmission technology can be established that enables high-quality signal transmission regardless of the presence or absence of reflection at the fiber connection, conventional mating techniques for suppressing reflection at fiber connections, such as PC connections, will become unnecessary, and optical fiber connections will become extremely simple.

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

[0146] In this experimental system, in addition to reflections occurring at optical fiber connection C, there is also the possibility that transmission quality may be degraded by reflected back light occurring at the light-emitting end face of optical fiber F2 on the optical receiver 306 side. Therefore, in this measurement, in order to focus on the effects of reflections occurring at optical fiber connection C, the light-emitting end face (end face of connector C22) of optical fiber F2 on the optical receiver 306 side was polished at a 12-degree angle. This allows light reflected at the end face of connector C22 to be emitted outside the core of optical fiber F2, eliminating the effects of reflected back light occurring at the end face of connector C22. The other optical fiber end faces (end faces of connectors C11, C12, and C21) were polished to a convex spherical shape.

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

[0148] However, when there is an air gap at the optical fiber connection part C, the worst BER value in Reference Example 1 is 10 -6 In Comparative Example 5, the worst BER value was 10 -8 This is thought to be due to the presence of a gap at optical fiber connection C, which caused reflected light to be reflected back, resulting in an increase in noise.

[0149] On the other hand, in Example 8, when the modulation voltage was 0.1 V, a slight deterioration in BER was observed, but when the modulation voltage was 0.12 V or higher, error-free transmission (BER≦10 -12 ) was achieved. The above results show that by using the optical transmission line (POF) of Example 8, low-noise signal transmission is possible regardless of the presence or absence of reflection at the optical fiber connection part C, and the conventional fitting technology for suppressing reflection at the optical fiber connection part C such as PC connection is no longer necessary, and high-quality optical transmission can be achieved even with an extremely simple optical fiber connection.

[0150] Note that when the optical fibers F1 and F2 are the optical transmission lines of Example 8, this is an example of a case where the optical transmission line is formed by connecting two or more optical transmission lines, and is an example of a case where the optical transmission lines are connected 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 output from a single-mode optical fiber is input with central excitation, expands the beam diameter by three times or more and outputs it, and when a Gaussian beam output from the single-mode optical fiber is input with a positional shift, the ratio of the maximum to the minimum beam diameter of the output beam is 0.7 or more, and the product of the scattering loss and length for the optical signal is 6 dB or less. Note that, when there are multiple connection points between the optical transmission lines, it is sufficient that the optical transmission lines are connected via an air layer at at least one of these connection points. Furthermore, when there are multiple connection points between the optical transmission lines, the optical transmission lines may be connected via an air layer at all of these connection points.

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

[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 this and may be optical waveguides or lumped multi-optical transmission sheets such as those disclosed in International Publication No. 2019 / 177068.

[0153] Furthermore, the second optical transmission line is not limited to being longer than the first optical transmission line, that is, the length of the second optical transmission line may be equal to or shorter than the length of the first optical transmission line.

[0154] As described above, the optical transmission path disclosed herein can also be combined with a metal cable that transmits power or low-speed signals to form an optical-electrical composite cable. It can also be used to increase the speed of signals of existing standards (e.g., HDMI (registered trademark), USB, etc.) combined with an optical system, or as a transmission path between new devices. The shape of the optical-electrical composite connector can be made backward compatible with existing standards or can be made new.

[0155] The optical transmission system of the present disclosure can also be configured as a wavelength division multiplexing (WDM) system that uses multiple LDs of different wavelengths and can increase the speed by a multiple of the number of wavelengths used without increasing the number of optical transmission lines. In this case, light (optical signals) emitted from LDs of different wavelengths (wavelengths: λ1 to λn) is coupled to the first optical transmission line in embodiment 1 or the optical transmission line in embodiment 2 using a multiplexer, and transmitted. The optical signal transmitted through the optical transmission line is demultiplexed by a demultiplexer into optical signals of each wavelength, and then received by a PD. A set of LDs of different wavelengths, a multiplexer, a demultiplexer, and a PD can also be used to support multiple optical transmission lines.

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

[0157] 10, 20: Optical signal transmitter / receiver 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 inside the device 14, 15, 24, 25, C11, C12, C21, C22: Connectors 31, 32: Second optical transmission line 40, 50: Signal processing circuit 61, 62, 61A, 62A: Optical transmission line 100, 100A: Optical transmission system 201: DBR laser 202: Polarization-maintaining single-mode optical fiber 203: Emitted light 204, 303: Lens 205: Optical transmission line 206: CCD camera 207: light 208:NFP measurement device 301 :Light source 302, 304: Optical signal 305: Lens system 306:Receiver C: Optical fiber connection F1, F2: Optical fiber

Claims

1. An optical signal transmitter that transmits 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 multimode, and the product of the scattering loss and length for light with a wavelength of 850 nm is 6 dB or less, 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 a micro-inhomogeneous 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 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 case error rate is 10 -8 is 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 is 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 paths 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 with central excitation, expands the beam diameter by three times or more due to a micro-nonuniform structure and outputs it, undergoes mode coupling with a higher-order mode due to forward scattering, and has a product of the scattering loss and length for light with a wavelength of 850 nm of 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 paths 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-inhomogeneous structure and output, and mode coupling occurs with a higher-order mode due to 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. 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 the error rate is 10 -8 is 6. The 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, 10 -12 Achieve the following error rate 6. The optical transmission system according to claim 4 or 5.

8. 8. The optical transmission system according to claim 1, 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.

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