WO2005096525A1

WO2005096525A1 - Optical signal transmission system and catv transmission system

Info

Publication number: WO2005096525A1
Application number: PCT/JP2005/006574
Authority: WO
Inventors: Hideyuki Omura; Satoshi Takahashi; Yuichi Ohashi
Original assignee: Fuji Photo Film Co., Ltd.
Priority date: 2004-03-30
Filing date: 2005-03-29
Publication date: 2005-10-13

Abstract

The signals modulated by a carrier having certain frequency are subject to intensity modulation in a multi mode resonant type laser diode (16) to generate optical modulation signals. The optical modulation signals are sent to a light receiving module (13) through a multi mode optical fiber (23) as an optical transmission line (12). A light receiving device (23) in the light receiving module (13) photoelectrically converts the optical modulation signals into the carrier modulation signals. The optical fiber (23) is a graded index type plastic optical fiber in which in which the refractive index in the core part gradually changes as the function of the distance from the center to the power of 2.4 to 2.9.

Description

DESCRIPTION

OPTICAL SIGNAL TRANSMISSION SYSTEM AND CATV TRANSMISSION SYSTEM

Technical Field The present invention relates to an optical signal transmission system for transmitting optical signal thiat is modulated by at least one carrier having a certain frequency, and a CATV transmission system to which the optical signal transmission system is applied.

Background Art In a conventional optical transmission system for transmitting analog signals, a single mode optical fiber is used as the transmission line, as described in Japanese Patent Laid-Open Publication No.63-218909. FIG.8 shows a conventional optical signal transmission system 70 having the single mode optical fiber as the transmission line. In the optical signal transmission system 70, base band signals of image, sound , data and so forth, are modulated by a carrier (sine curve) ha^lving a certain frequency to generate modulated signals (carrier modulation signals) . The carrier modulation signals are s ent to a laser module 71 through an amplifier 75. The laser modile 71 generates optical signals (optical modulation signals) in which the amplitude thereof is modulated based on the level of the carrier modulation signals. The optical modulation signa-ls are sent via an optical transmission line 72 to a light receiving module 73 in which the optical modulation signal is converte-d into the carrier modulation signal. The laser module 71 comprises a laser diode 76, a σoncϋenser lens group 77 and. a pigtail 78. The pigtail 78 has an isolator 79, an optical fiber 80 and an optical connector 81. The carrier modulation signals are converted into the optical modulation signals that are focused on a fiber incidence plane 80a of the optical fiber 80 through the condenser lens group 77. Thus, sufficient amount of the optical modulation signals enter the optical fiber 80. The optical modulation signals are transmitted in the optical transmission line 72 that is comprised of an optical connector 82 connected to the optical connector 81 of the pigtail 78, and an optical fiber 83. The optical modulation signals through the optical transmission line 72 are output from the fiber exit plane 83a. The output optical modulation signals are focused on a light receiving surface of a light receiving device 87 through a condenser lens 86. The carrier modulation signals, which are obtained in the light receiving device 87, are output via an amplifier 88. In the conventional optical signal transmission system, the single mode optical fiber (SMF) is used as the optical fiber 80, 83 for the purpose of reducing modal noise. Thus, the laser diode 76 is a distributed feedback laser diode (DFB-LD) that can realize single-mode oscillation. The optical modulation signals, reflected on the optical connector 81, 82 toward the laser diode 76, cause to increase noise. In order to prevent such noise, the optical connectors 81, 82 need to be physical connection (PC) type with low reflection loss, and it is required to sufficiently polish the edges of the optical fiber of the optical connector 81, 82. In addition, for the purpose of preventing reflected light at the light receiving device 87 toward the optical transmission line 72, the light receiving device 87 is inclined to the exit surface 83a of the optical fiber 83. Moreover, the optical isolator 79 is provided to decrease reflected light. Due to recent spread of the Internet, it is required to increase the data transmission speed in the transmission infrastructure for the in-house use, and the optical fiber is suitable for fast data transmission infrastructure. In addition, due to the rapid spread of FTTH (Fiber to the Home) system, the diffusion of the optical access has rapidly increased. Thus, it is desirable to increase the data transmission speed of the in-house data transmission infrastructure. In the optical access as described in JP-A 63-218909, data transmission is carzried out by transmitting the optical signals the intensity of which is modulated by the carrier modulation signals. The DFB-LD used as the light emission device in the system described in JP-A 63-218909, however, has a cavity in the active layer of the device, so the. manufacture process with high precision is required. In addition, for the purpose of reducing the reflection loss of the optical modulation signals, the edge surfaces of the optical connectors 81, 82 need to be precisely polished. As a result, long polishing period is required. Since the core diameter of the SMF is extremely small, it is necessary to achieve high-precision alignment in coupling the laser diode 76 and the fiber entrance plane 80a, and in coupling the fiber exit plane 83a and the light receiving device 87. Even a slight shift in the optical axes of the coupled elements causes leakage of the optical modulation signals out of the optical fiber, and thus the transmission quality becomes deteriorated. In order to prevent such problem, the optical connector needs to have the feature to align the optical axis with high precision. As a result , since the elements in the conventional optical transmission system become expensive, the conventional optical transmission system is not appropriate for in-house purpose. Moreover, since alignment of the optical axes with high precision causes a long alignment time, the conventional optical transmission system is not suitable for the connection work in a house. Due to the fragility, the glass optical fiber is broken due to external stress . Breakage of the glass optical fiber will cause the problems such as disconnection of the data transmission, and badly affect a human body by the piece of the broken optical fiber. For the purpose of reducing the transmission loss of the glass optical fiber, the . wavelength of the optical modulation signals needs to be in the infrared range such as 1310 nm and 1550 nm. Thus, when the optical connector is dislocated while the optical modulation signals are transmitted, there is a possibility that the optical modulation signals enter human's eyes. Instead of the single mode glass optical fiber, JP-A No. 2004-266663 describes the analog optical transmission system in which the multi moαe optical fiber is used as the transmission line. Although JP-A No.2004-266663 describes the analog optical transmission by use of the multi mode optical fiber, this reference merely shows that a pulse signal can be transmitted through the multi mode optical fiber with small distortion . Thus , it is not clear whether or not the system in JP-A No. 2004-266663 is appropriate in transmitting continuously varied analog signals. An object of the present invention is to provide an optical signal transmission system and a CATV transmission system that can convert the carrier modulation signals into optical modulation signals, and that can decrease the possibility of ingress noise caused by the metal cable in the slave station. Another object of the present invention is to provide an optical signal transmission system and a CATV transmission system that can provide easy workability at a low cost . Further object of the present invention is to provide an optical signal transmission system and a CATV transmission system that can decrease the possibility to affect a human body.^"

Disclosure of Invention The above objects are achieved by transmitting optical modulation signals by use of a light emission device of a multi mode oscillation type, and a sending such optical modulation signals through a multi mode type optical fiber. The light emission device generates the optical modulation signals based on the signals modulated by a carrier. The optical modulation signals through the multi mode optical fiber is converted again into electric signals at an optical receiver having a light receiving element . The optical fibers , the optical fiber and the light emission device, and the optical fiber and the light receiving device are connected via an optical connector. Both end of the optical fiber as the signal transmission line may be connected to a pair of pigtail optical fibers that are comprised of the optical fiber and the light transmitter or the light receiver. As for the optical connector, spatial coupling type is preferable. The light emission device is preferably a laser device of visible rang . In a preferable embodiment , the optical fiber is formed from a plastic. The plastic optical fiber is preferably a graded index type in which the refractive index in the core part gradually changes as the function of the distance from the center to the power of 2.4 to 2.9. The diameter of the core part of the optical fiber is preferably 100 μm to 1000 μ-m. In the CATV system according to the present invention, a subscriber's house as a slave station has the optical signal transmission system mentioned above. In the CATV system, the optical modulation signals from a master station are sent to the respective slave stations . According to the present invention, since the optical modulation signals generated by use of a multi-mode light emission device are transferred via a molti mode type plastic optical fiber having refractive index distribution, it is possible to reduce the cost of the constituent in the system and the burden of the fiber alignment process. Thereby, the optical modulation signals can be effectively transmitted without experience devices, so it is possible to provide the. system at a low cost. The plastic optical fiber with relatively large diameter has large bending resistance and ductility, so the plastic optical fiber is hardly broken. Thus, it is possible to prevent disconnection of the communication. Since the optical fiber is formed from plastic material, even a broken piece will not affect human body. The core part of the plastic optical fiber preferably has the refractive index profile in which the refractive index decreases from the center to the outer surface. In the following function N(r) of the refractive index in relation to the distance from the center of the core part, the refractive index distribution coefficient g is preferably 2.4 to 2.9. N(r) = nl{l - r^g x Δ}¹² (0 ≤ r ≤ 1) = nl(l - Δ) (r > 1) Wherein, r indicates the distance from the center that is normalized by the diameter of the optical fiber, nl indicates the maximum value of the refractive index in the optical fiber with respect to the radial direction, n2 indicates the minimum value of the refractive index in the radial direction, and Δ indicates the value of (nl-n2)/nl. By setting the refractive index distribution coefficient from 2.4 to 2.9, the transmission speeds of the optical modulation signals of propagation modes become substantially the same. As a result, a distortion of the optical modulation signals at the receiving section becomes minimized, and thus it is possible to prevent deterioration of the transmitted signals. Moreover, the plastic optical fiber with a large diameter of 100 μm to 1000 μm makes it possible to achieve sufficient transmission of the optical modulation signals , even if the optical fibers are connected with relatively low precision. The refractive index distribution coefficient g is preferably around 2.0 in general, although the optimum value depends on the material as the optical fiber. In the event of an acrylic material as the optical fiber, the transmitted optical modulation signals are badly distorted if the refractive index is outside of the range between 2.4 to 2.9. The plastic optical fiber having such refractive index distribution coefficient can be formed by the method described in Japanese Patent No. 3332922. Compared with the infrared light generally used for the glass optical fiber, the light emission device for the plastic optical fiber has the red wavelength region, such as 650 nm and 780 nm, although the wavelength of 850 nm can be used. Thus, even if the transmission light is leaked from the optical fiber, the user can easily recognize the leakage of the transmission light. As a result , the user can easily avoid watching the leaked laser beam directly. The incident light from the light emission device is preferably transmitted in the optical fiber at the underfilled launch condition that is in a lower mode condition than the normal mode launch condition. Transmission in the underfilled launch condition can reduce the distortion ratio of the analog signals and thus noise reduction and separation of the signals from the carrier wave can be realized effectively. It is possible to obtain the underfilled launch condition by reducing the core size of the multi mode optical fiber, the spot size of the light emission device relative to the numerical aperture, and the numerical aperture. In the CATV system, there are some noise sources such as the subscriber's house, radio wave and so forth. These noises are added in the upstream direction (the direction from the slave station toward the master station) , and then amplified in a relay station. Thus, the conventional system has so-called ingress noise problem in which too much noise is generated in the upstream direction, compared with the downstream direction (the direction from the master station toward the slave station) . This ingress noise is because of deterioration in the shielding property of a coaxial cable, looseness of the connector of the coaxial cable, and so forth. Since the optical signal transmission system is provided with the slave station of the CATV system, it is possible to build a safety and reliable in-house equipment for data transmission at a low cost. Such CATV transmission system including the master station can solve the above ingress noise problem. Brief Description of Drawings FIG. 1 is a schematic block diagram of an optical signal transmission system according to the present invention; FIG. 2 is a schematic block diagram of a system to generate optical modulation signals; FIG. 3 is a graph showing a refractive index profile of an optical fiber used in the optical signal transmission system; FIG. 4 is a schematic diagram of a system to measure the transmission property of the optical fiber of FIG. 3; FIG. 5 is a graph showing the signal/noise ratio of the optical fiber of FIG. 3 in relation to the modulation degree; FIG. 6 is a schematic block diagram of an example of an optical transmission system in a subscriber's home; FIG. 7 is a schematic block diagram of an example of a CATV system; and FIG.8 is a schematic block diagram of a conventional optical transmission system.

Best Mode for Carrying Out the Invention In FIG. 1 an optical signal transmission system according to the preferable embodiment of the present invention is illustrated. The optical signal transmission system 10 has a laser module 11 for converting carrier modulation signals into optical modulation signals, an optical transmission line 12 for sending the optical modulation signals, and a receiving module 13 for converting the optical modulation signals into the carrier modulation signals. Referring to FIG. 2, the carrier modulation signal is generated in a modulator 14 to modulate base band signals (such as image, sound and data) with a carrier (sine curve) having a certain frequency. When the base band signals are analog signals , the base band signals are modulated by the analog modulation such as AM (Amplitude Modulation) and FM (Frequency Modulation) . In the event of digital signals, the base band signals are modulated by the digital modulation such as PSK (Phase Shift Keying) and QAM (Quadrature Amplitude Modulation). The carrier modulation signals are input to the laser module 11 via an amplifier 15. In FIG. 1, the laser module 11 has a laser diode 16, a condenser lens group 17 and a pigtail 18. The pigtail 18 comprises an optical connector 19 and an optical fiber 20. The carrier modulation signals are converted to the optical modulation signals in the laser diode 16 that emits laser beams of visible band. The condenser lens group 17 focuses the optical modulation signals on a fiber entrance plane 20a of the optical fiber 20, so sufficient amount of the optical modulation signals can enter the optical fiber 20. The optical modulation signals are transmitted through the optical transmission line 12 that is comprised of an optical connector 22 and an optical fiber 23, and output to a fiber exit plane 23a. The optical connector 22 in the optical transmission line 12 is coupled to the optical connector 19 of the pigtail 18. The output optical modulation signals are focused on a light receiving device 27 through a condenser lens 26, and are converted into the carrier modulation signals that are output via an amplifier 28. In the embodiment of FIG. 1, the optical fiber and the module for light emission or light reception are combined to form a pigtail optical fiber, and the pigtail optical fibers for light emission and the light reception are connected via an optical fiber. Instead, the light emission device and the light receiving device may be connected via a single optical fiber. The optical fibers 20, 23 of the pigtail 18 and the optical transmission line 12 are multi mode optical fibers (MMF) having the core diameter of 100 μm to 1000 μm. The multi mode optical fiber with the core diameter of 50 μm or smaller causes remarkable modal noise, so the conventional optical transmission system applies the single mode optical fiber for the purpose of reducing the modal noise. However, the optical fiber having the core diameter of 100 μm or larger can reduce the modal noise because of increase in the number of the transmission modes . As a result, the S/N ratio improves. In the multi mode optical fiber, the transmission speed of the optical modulation signals is different between the transmission modes, so the optical modulation signal tends to be distorted at the light receiving device 27 due to the difference in the transmission time in the optical fiber. Thus, the reproduced carrier modulation signal is also distorted. In order to prevent this problem, the optical fibers 20, 23 have the core part in which the refractive index gradually changes in the radial direction of the optical fiber. In other words, by providing a graded index (GI) structure with the optical fibers 20, 23, it is possible to cancel the difference in the transmission time between the transmission modes. As shown in FIG. 3, the optical fiber 20, 23 applied in the optical signal transmission system 10 has the refractive index profile in which the refractive index changes as the function of the distance from the center to the power of 2.4 to 2.9. Although the optical fiber 20, 23 may be made of glass, plastic or other materials, the optical fiber 20, 23 is required to be made of a material or composition that has relatively low transmission loss in the visible band (for example, the wavelength of around 790 nm and 650 nm) . Since the multi mode optical fiber is used as the optical fiber 20, 23 in this preferable embodiment, emission light from the single mode light emission device is changed into plural transmission modes during the passage in the optical fiber 20, 23. Thus, the laser diode 16 is not necessary a single mode light emission device, so it is possible to apply multi mode type light emission device, such as the Fabry-Perot laser diode (FP-LD) and the Vertical Cavity Surface-Emitting Laser (VCSEL) , as the laser diode 16. Due to unnecessity of a cavity in the active layer, the multi mode type laser diode has advantages compared with the DFB-LD in simple manufacture process and productivity. Accordingly, it is possible to provide the optical transmission system at a low cost, compared with the conventional system having the DFB-LD. The single mode light emission device like the DFB-LD has high coherency, so reflected light affects the resonant mode of the laser diode. In contrast, the laser diode 16 of the multi mode resonant type has low coherency, and thus has high resistance to reflected light. Thus, the PC coupling with low reflection is not necessary as the optical connector 19, 22. Moreover, it is not necessary to polish the edges of the fiber with high precision. Although the conventional system has the isolator, the system in this preferable embodiment does not require the isolator. Furthermore, it is not required to set the light receiving device 27 inclined to the fiber exit plane 23a. Accordingly, the optical transmission system can be provided at a low cost. The core diameter of the optical fibers 20, 23 is equal to or more than 100 μm, which is much larger than the SMF. Even if there is large shift between the axes of the optical fibers 20, 23, the transmission loss is small. Since the optical modulation signals are effectively transmitted even if the optical axes of the optical fibers 20, 23 are shifted, it is convenient to carry out the process to connect the optical fibers in a subscriber's house. The optical modulation signals emitted from the laser diode 16 is in the visible band, so a user of the system can easily recognize the leakage of the optical modulation signals. Thus, even if the optical connectors 19, 22 are disconnected, or if the optical fibers 20, 23 are broken, it is possible to keep the user's eyes from the optical fibers 20, 23. (Example 1 ) By the method described in JP-A No. 2003-337232, the multi mode plastic optical fiber having the refractive index profile is formed. The refractive index distribution coefficient g is 2.6 (within the range from 2.4 to 2.9). The property of the plastic optical fiber 101 is measured by use of a measurement system 100 illustrated in FIG. 4. Referring to FIG. 4, the modulation signals are input to a VCSEL 104 via a matrix switch 102 and a variable high-frequency attenuator 103, and the optical modulation signals from the VCSEL 104 are transmitted in the plastic optical fiber 101. The optical modulation signals are converted into electrical signals in the photo diode 106 of the light receiving side, and the reproduced signals is sent to a spectrum analyzer 111 via a variable high-frequency attenuator 108, a band-pass filter 109 and an amplifier 110. For the purpose of measuring the distortion of the received signals with high level, the level of the optical modulation signals are controlled. The modulation signals used for the measurement has the modulation degree per channel of 3 %/ch, and thus the total modulation degree is 26 % at 74 channels. The modulation degree may be adjusted by changing the modulation degree per channel. In the measurement , the signal to noise ratio D/U (dBc) is measured in the overfilled launch condition and the underfilled launch condition by changing the condition to input the optical modulation signals . It is to be noted that the resonant modes of the underfilled launch condition is smaller than the normal launch condition, and that most of the incident light enters the optical fiber in the underfilled launch condition. FIG.5 shows the result of the property of the optical fiber 101. In FIG. 5, the horizontal axis indicates the modulation degree, and the vertical axis indicates the signal to noise ratio D/U (dBc). Compared with the underfilled launch condition, the noise in the overfilled launch condition becomes larger, so the optical signals are distorted. Thus , it is preferable to transmit the optical signals in the lower mode resonant condition than the normal mode resonant condition. As for the comparison example, the plastic optical fiber having the refractive index distribution coefficient g of 4.0 is formed. The property of such optical fiber is measured by use of the same measurement system of FIG. 4. The signal to noise ratio (D/U) becomes smaller by about lOdBσ than that of the optical fiber 101 shown in FIG. 5. (Example 2) In FIG. 6, an example of the optical signal transmission system according to the present invention is illustrated. An in-house type optical signal transmission system 30 has an input side media converter 31, optical transmission lines 32, 33, and an output side media converter 34. By way of the FFTH or CATV system, the carrier modulation signals (image signals and sound signals, for example) enters the input side media converter 31. The media converter 31 has an amplifier 35, a laser diode 36, a condenser lens 37 and a pigtail 38. The laser diode 36 is a multi mode oscillation type FP-LD, and emits laser light having the wavelength of about 650 nm. The pigtail 38 comprises an optical connector 39 and an optical fiber 40. The carrier modulation signals in the media converter 38 enter the laser diode 36 via the amplifier 35, and are converted into amplitude modulated optical modulation signals . The optical signals are focused on a fiber entrance plane 40a of the optical fiber 40 through the condenser lens 37. The optical modulation signals in the pigtail 38 are transmitted through the optical transmission lines 32, 33, and output from the fiber exit plane 43a. The optical transmission lines 32, 33 respectively comprise the spatial coupling type optical connector 43 and an optical fiber 43. The output optical modulation signals enter the light receiving device 46 via a condenser lens 45 in the output side media converter 43. The light receiving device 46 converts the optical modulation signals into the carrier modulation signals . The reproduced carrier modulation signals are input in a setup box 48 via an amplifier 47. In the setup box 48, the carrier modulation signals are reproduced into the image signals, for instance. The image signals are sent to a TV 49, for example, to display an image in the TV 49. Since the multi mode oscillation type light emission device is used as the laser diode 36, the operation of the laser diode 36 is less affected by the reflected light even if the optical connector 42 is the spatial coupling type. Thus, the level of the optical modulation signals does not become lower than the level needed for proper transmission. Similarly, the system is less affected by the reflected light generated between the fiber exit plane 43a and the light receiving device 46, so the light receiving device 46 needs not be inclined to the fiber exit plane 43a. The coupling angle between the fiber exit plane 43a and the light receiving device 46 is^' not limited as long as the light receiving device 46 can receive sufficient level of the optical modulation signals to the. receiving dynamic range. Since the optical modulation signals are laser light in the visible band, the user of the system can easily recognize the leakage of the laser light caused by disconnection of the optical connector 42 or breakage of the optical fiber 43. Thus, it is possible to keep user's eyes from the disconnected optical connector 42 and an edge plane of the optical fiber 43. The core diameter of the optical fibers 40 , 43 of the pigtail 38 and the optical transmission line 32, 33 has 300 μm to 1000 μm, and the optical fibers 40, 43 are multi mode plastic optical fiber having the refractive index profile in which the refractive index changes as the function of the distance from the center to the power of 2.4 to 2.9. Since the core diameter is large, the optical modulation signals can be transmitted even if the optical axes from the laser diode 36 to the fiber entrance plane 40a are relatively fluctuated. Thus, it is not necessary to position the structural parts of the media converter 31 with high precision, so the cost of the media converter 31 decreases . Similarly, since the position accuracy of the optical axes from the fiber exit plane of the media converter 35 to the light receiving device 46 becomes relatively low, the cost of the media converter 35 decreases as well. The core part of the optical fiber 40,43 have the refractive index profile in which the refractive index changes as the function of the distance from the center to the power of 2.4 to 2.9, so the transmission speed of the transmission modes are substantially the same. Since the distortion of the optical modulation signals at the light receiving device 46 becomes minimized, it is possible to send the image signals while keeping the image quality. Since the optical fiber 40, 43 are formed from plastic, the plastic optical fiber 40, 43 are not broken by external stress unless the optical fiber 40, 43 are cut by a cutting tool.

Moreover, the cut piece of the optical fiber 40, 43 will not affect the human body. Thus, the system according to the present invention is appropriate for handing the optical fiber in a house . (Example 3) Referring to FIG.7 , the CATV system comprises a CATV center 50, optical nodes 51, optical transmission lines 552, coaxial cables 53, multiple direction amplifier 54 and safety devices 55. The safety device 55 is provided with respective subscriber's house 56-58, and connected to the in-house type optical signal transmission system 30 of FIG. 6. In the conventional CATV system, the coaxial cable is used as the transmission line in the subscriber's house. Because of noises mixed into the transmission line caused by deterioration in the shielding property of the coaxial cable or looseness of the connector. Such noise causes the problem of transferring to the CATV center 50 that is connected to many subscribers' houses. The noise level transferred to the CATV center 50 becomes large, so the ingress noise problem to deteriorate the signal quality happens . In order to prevent this, it is preferable to apply the optical signal transmission system in the house, but the conventional system i.s not appropriate in terms of the cost and safety. In contrast, applying the optical signal transmission system 30 of FIG.6 can solve the ingress noise problem ancl provide the in-house equipment with safety and reliability at a low cost. Since the optical transmission system can be provided -with the slave station at a low cost, it is advantageous in promoting the optical transmission in the slave station (subscriber side) of the CATV system in which HFC (Hybrid Fiber and Coaxial) is promoted in the master station.

Industrial Applicability The present invention is applicable in transmitting optical signals that are modulated on the basis of a carrier having certain frequency.

Claims

CLAIMS 1. An optical signal transmission system for transmitting optical modulation signals that are modulated based on a carrier, the system comprising: an optical signal transmitter having a molti mode oscillation type light emission device for emitting the optical modulation signals ; a graded index type multi mode optical fiber having a core part for transmitting the optical modulation signals , the core part having a refractive index profile in which the refractive index gradually decreases in relation to the distance from the center of the core part; and an optical signal receiver having a light receiving device for receiving the optical modulation signals through the optical fiber and converting the optical modulation signals into electric signals .

2. The system according to claim 1 , wherein the optical fiber is formed from a plastic material.

3. The system according to claim 1, wherein the refractive index distribution coefficient of the plastic optical fiber is 2.4 to 2.9.

4. The system according to claim 1, wherein the core part of the optical fiber has the diameter of 100 μm to 1000 μm.

5. The system according to claim 1, wherein the optical modulation signal emitted from the light emission device is laser light having the wavelength in the visible band.

6. The system according to claim 1 , further comprising an optical connector of spatial coupling type; wherein the optical signal transmitter, the optical fiber and the optical signal receiver are mutually connected via the optical connectors .

7. An optical signal transmission system for in-house use having the optical signal transmission system according to claim 1, wherein the optical signals, the intensity of which is modulated in accordance with the carrier, are transmit ed in the house.

8. An optical CATV transmission system for transmitting optical modulation signals, the intensity of which is modulated based on a signal modulated by a carrier, from a master station or a node to a slave station having the optical signal transmission system of claim 1.