JP2009253654A - Optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-speed visible transmission system which enables two-way communication. <P>SOLUTION: An optical communication system 100 uses an optical fiber 107 having a glass first core 203 with a Graded index type refractive index, a glass second core 203 with a constant refractive index which is smaller than that of the first core 203, and a plastic cladding 201 covering the first and second cores. A signal obtained by modulating infrared laser light with a wavelength of 850 nm or longer is propagated to the first core 203, and a signal obtained by modulating light from a first visible light-emitting diode with a wavelength of 780 nm or shorter to the second core 202. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical communication system used in the fields of optical communication and optical interconnection.

  Optical transmission systems using plastic optical fibers (POF) are widely used for home audio signal transmission, in-car multimedia signal transmission, and data transmission between devices in factories. . Optical communication has advantages such as higher noise resistance compared to telecommunication, and weight reduction by using an electric wire as a fiber. In such a communication system, an acrylic resin fiber is mainly used, and a red light emitting diode (LED: Light Emitting Diode) having the least absorption has been mainly used.

  In recent years, with the spread of home digital devices such as personal computers, digital broadcasting televisions and recorders, etc., there are increasing opportunities for high-speed digital transmission even in homes. Also, in transportation facilities such as automobiles, the number of control devices for safety and environmental measures is increasing, and high-speed transmission means are required. However, even if the light emitting diode is a high-speed type such as a resonant cavity type light emitting diode, it is difficult to operate at a high speed exceeding 1 Gbps, and POF has a large loss and is not suitable for high-speed transmission.

  An optical communication system using a glass fiber core transmission path and a semiconductor laser is suitable for such high-speed optical transmission. Such an optical communication system is already widely applied as a commercial communication system mainly because of its long distance and large capacity. Among them, an optical communication system using a surface emitting laser and a multimode fiber is suitable for communication over a relatively short distance (100 m or less). This method is used for commercial short-range datacom applications that already require high-speed communication. For such short-range communication, a vertical cavity surface emitting laser (VCSEL) having a small size and low power consumption is suitable. Already, high-speed operation of about 10 Gbps is possible.

  By the way, since digital information is not deteriorated even if it is copied, from the viewpoint of copyright protection, encryption processing for prohibiting copying is necessary in image transmission. For secure encryption, in order to establish a trust relationship between the transmission and reception interfaces, it is necessary to use bidirectional communication. Such an interface has already been widely used for the connection between a television and a recorder, but many of them use electric wiring for easy bidirectional communication. However, with the expansion of the transmission band accompanying higher definition, optical wiring is required. An example of such two-way communication for digital moving image transmission is disclosed in Patent Document 1.

  For moving picture transmission, not only image data but also control signals such as sound and aspect ratio, hot plug detection for detecting connection, supply of controller power on the receiving side, and the like are required. Each signal differs in transmission speed, transmission quality, and the like. In an interface using an electric cable, image data is transmitted after being separated into three primary colors (RGB), and a clock is also transmitted separately. On the other hand, some of them using optical wiring are transmitted in parallel with each other, or as described in Patent Document 1, only image data is serialized and transmitted.

  In this case, there are problems such as requiring a plurality of different fibers and an increase in the connector portion. In addition to image transmission, for example, in-vehicle use, it is possible to use a low-speed control signal and a high-speed multimedia signal at a time. The required reliability differs between these multimedia signals and control signals.

  In a multimedia signal composed of image data and audio data, it is not necessary to use an error correction code, and there is no need for certainty of data transmission. This is because slight errors in the multimedia signal cannot be recognized by humans. On the other hand, in the case of a control signal or the like, even one bit does not allow an error. Therefore, error correction using a Reed-Solomon code, retransmission processing when data has not arrived, and the like are required. In such a system that serializes signals having different degrees of reliability and transmits them at a time, even a signal that does not require reliability takes a reliable transmission form. Therefore, the transmission system becomes expensive and the power consumption increases.

  By the way, infrared light is often used in data communication using high-speed laser-modulated light. This is because loss due to glass is low, and a high-speed and highly reliable semiconductor element can be easily manufactured. On the other hand, visual recognition using visible light is desirable because it can intuitively determine the presence or absence of a signal. In particular, an optical transmission line such as an optical fiber is more easily damaged than an electric transmission line, and it cannot be determined whether a signal is normally transmitted only by being connected. Further, there are many advantages such as improvement in workability when the optical system is visible when the optical system is incorporated in the apparatus. Moreover, visible light emitting diodes are also used in communication systems using current POF. For this reason, it is desirable that even a system that speeds it up can be visually recognized in order to maintain compatibility.

As a method of such a system, a method of simultaneously transmitting laser-modulated light and light-emitting diode-modulated light is conceivable (see, for example, Patent Documents 2 and 3). In the systems described in Patent Documents 2 and 3, a light emitting diode and a semiconductor laser having the same red wavelength are used. A double core is used, and a laser light source is used on the inner side and a light emitting diode is used on the outer side for transmission. This system has the advantage that compatibility with POF can be maintained.
JP 2005-65034 A JP 2006-64766 A JP 2001-166172 A

  However, the systems described in Patent Documents 2 and 3 have many problems in achieving a transmission rate exceeding 1 Gbps. POF has the smallest loss in the red region, and the loss becomes extremely large at other wavelengths. Therefore, a red light source is used. In this case, since the laser light and the light emitting diode have the same wavelength, crosstalk of signals of the respective wavelengths becomes a problem.

  Further, the red laser light has a large damage to the retina, and it is necessary to reduce the light output in order to make the eye safe. Even if red is used, since the loss due to POF is large, the light output that can reach the receiving side becomes extremely weak. If the optical output is weak, the crosstalk becomes weaker and the SN ratio deteriorates, so high-speed communication cannot be performed stably. Even if a glass fiber is used, the visible light region has a large loss and further has a large chromatic dispersion. Therefore, there is a problem that the spectral width of a laser light source that is often used in a multimode oscillation state must be narrowed.

  The present invention has been made in view of the above, and an object of the present invention is to provide a high-speed transmission system that is bidirectional communication and has visibility.

An optical communication system according to the present invention includes:
A first core made of glass with a refractive index of graded index type;
A second core made of glass having a constant refractive index and having a refractive index lower than that of the first core;
An optical communication system using an optical fiber comprising: a plastic clad surrounding the first and second cores;
A signal obtained by modulating infrared laser light having a wavelength of 850 nm or more is propagated to the first core, and a signal obtained by modulating light from the first visible light emitting diode having a wavelength of 780 nm or less is propagated to the second core. It is characterized by this.

  According to the present invention, it is possible to provide a high-speed transmission system that is bidirectional communication and has visibility.

  In the communication system according to the present invention, a high-speed signal and a low-speed signal are transmitted by a single fiber. Costs can be reduced by combining multiple fibers that are necessary for each.

  A visible light emitting diode is used for the low-speed signal, and an infrared laser is used for the light flux signal. By using a visible light emitting diode, it is possible to visually recognize whether a signal is being output, and it is compatible with a current system using a red light emitting diode and POF. Since it can be visually recognized, it is possible to immediately determine whether a signal is output, and it is easy to determine abnormality or failure. In addition, it is possible to instantaneously determine whether or not a signal is output when assembling such an optical communication system, for example, a car or the like, without any special device, and an improvement in productivity can be expected. It can also be expected to improve the efficiency of repair work at the time of failure or damage.

  For high-speed optical transmission, it is necessary to use a semiconductor laser instead of a light emitting diode due to the limitation of the modulation speed. Here, 1 Gbps is assumed as a high-speed signal. In particular, an infrared semiconductor laser is suitable for high-speed operation, and is excellent in reliability due to achievements in optical communication and the like. In addition, the longer the wavelength, the less damage it will have when entering the human eye directly. Furthermore, considering that high-speed optical transmission requires higher output than low-speed applications, and that the conversion efficiency of photoelectricity deteriorates when the wavelength is shorter, it is not visible but infrared. It is desirable to use a laser.

  As the semiconductor laser, a surface emitting laser that can operate with low power consumption and is suitable for high-speed modulation is preferable. The wavelength is preferably 850 to 1600 nm, which has little influence on human eyes and is suitable for manufacturing a surface emitting laser. If the wavelength is shorter than this wavelength, the composition of the DBR forming the resonator is limited, it becomes difficult to obtain a high reflectance, and the efficiency and band of the surface-emitting laser are lowered. On the other hand, if it exceeds 1600 nm, it is difficult to produce both the light emitting element and the light receiving element. Further, 850 to 1310 nm is more desirable. Within this wavelength band, a high performance surface emitting laser can be easily fabricated on a GaAs substrate.

  A light emitting diode having a wavelength of 400 to 780 nm is suitable because it can be visually recognized by the human eye and needs to have a wavelength difference for signal separation from the semiconductor laser. If it is less than 400 nm, it becomes ultraviolet light and cannot be seen, and the loss due to plastic clad or glass increases. If the wavelength difference between the light-emitting diode that transmits a low-speed signal and the semiconductor laser that transmits a high-speed signal is increased, it is easy to integrate elements that realize the wavelength difference. In addition, interference such as signal crosstalk can be suppressed, and stable communication is possible.

  For low-speed signals, bidirectional communication can be applied to control the modulation state of high-speed signals and to control devices with high-speed signal sources. For further safety, a signal from the high-speed signal receiving side to the transmitting side is used as hot plug detection. When hot plug detection is not performed, laser emission on the transmission side can be stopped to make it safer. In addition, power consumption can be reduced by turning off the power of the transmission-side LSI and the reception-side LSI.

  In the double core optical fiber applied to this communication system, the first core that modulates the semiconductor laser and transmits a high-speed signal needs to have a high mode band. In addition, it is necessary to reduce the light receiving area of the light receiving element. Therefore, it is necessary to reduce the core area. Even if it is large, it is necessary to make it 120 μm or less. By setting the thickness to 50 μm, it is possible to manufacture a fiber having a mode band of at least 500 MHz-km or more, and transmission of about 50 m at 10 G is possible. What core diameter fiber to use may be appropriately selected according to the transmission speed. A fiber having a glass core is suitable for such a fiber because loss can be reduced within a wide range from the visible region to the infrared region. If everything including the clad is vitrified, the cost becomes higher than that of POF. Therefore, it is desirable to use a plastic material for the clad portion.

  The fiber that modulates the light emitting diode and propagates the signal has the first core on the inner side, and therefore needs to be larger than the first core. Since the signal transmitted there is low speed, there is no problem in the mode band even if the core diameter is large, but the length that can be taken from the preform, which is the prototype of the multimode fiber, is shortened and the cost is increased. Therefore, although it is larger than the first core, it is necessary to make it not more than 400 μm, and preferably about 200 μm. Further, the first core may be shifted from the center of the double core fiber. By shifting, it is possible to improve the mountability of a light source such as a semiconductor laser that transmits the first core and an integrated light emitting diode.

  The first effect of the present invention is that high-speed transmission and low-speed transmission can be transmitted without complicated signal processing. The second effect is that the presence or absence of a signal can be visually recognized by using a light emitting diode for a low-speed signal, even though the system is capable of high-speed optical transmission. A third effect is that encrypted data can be transmitted and received and hot plug detection can be performed by bidirectional communication of low-speed signals. The fourth effect is that, when an application such as an automobile is repaired from damage due to an accident or the like, the signal damage portion can be easily diagnosed by using a visually recognizable signal.

Embodiment 1
An optical communication system using a double core fiber according to the first embodiment of the present invention will be described below with reference to FIG. The optical communication system 100 is a system that transmits high-speed transmission data 101 and low-speed transmission data 102 simultaneously using a single fiber. The communication system 100 includes a high-speed signal multiplexing circuit 103 that multiplexes high-speed signals, a low-speed signal multiplexing circuit 104 that multiplexes low-speed signals, a semiconductor laser (LD) 105 that modulates high-speed signals, and a low-speed signal modulation. Light-emitting diode (LED) 106, double-core optical fiber 107, semiconductor light-receiving element (PD) 108 that converts a high-speed optical signal into an electric signal, semiconductor light-receiving element (PD) 109 that converts a low-speed optical signal into an electric signal, transmission A high-speed signal separation circuit 110 and a low-speed signal separation circuit 111 for returning the processed signal to the original data. By this system, the high-speed transmission data 101 and the low-speed transmission data 102 are transmitted as high-speed reception data 112 and low-speed reception data 113.

  Next, each function will be described. The high-speed signal multiplexing circuit 103 and the low-speed signal multiplexing circuit 104 are devices that respectively multiplex a plurality of high-speed signal trains and low-speed signals. The signal multiplexing circuits 103 and 104 transmit not only data but also a clock at the same time. In FIG. 1, the high-speed signal multiplexing circuit 103 transmits data 1 and 2 and the clock, and the low-speed signal multiplexing circuit 104 transmits data 3 and the clock. Here, in order to facilitate signal separation from the data string, the data string may be subjected to 8B10B conversion. This method is a method of generating a 10-bit signal from an 8-bit signal. By this coding, it is possible to eliminate a long signal sequence, so that the clock can be easily restored.

  Next, a high-speed signal is modulated by the semiconductor laser 105 using the multiplexed signal. A surface emitting laser is suitable for the semiconductor laser 105. This is because the surface emitting laser has low power consumption and is suitable for high-speed modulation. The operating speed can be up to about 10 Gbps. Here, a surface emitting laser having an 850 nm band as an oscillation wavelength is used. This wavelength is widely used in short-distance multimode transmission for datacom applications and the like, and is highly available.

  On the other hand, a red light emitting diode 106 having an oscillation wavelength of 650 nm band is used for a low-speed data signal. Even the light emitting diode can cope with a speed of about several tens of Mbps. Laser-modulated light and light-emitting diode-modulated light are coupled to a multimode double-core optical fiber 107, respectively. After transmission by the double core optical fiber 107, the high-speed laser light and the low-speed light emitting diode modulated light are separated. This method will be described later together with the structure of the fiber.

  The separated light is converted into an electric signal by the light receiving element 108 for high speed signal and the light receiving element 109 for low speed signal. As the light receiving element 108 for high-speed signals, a surface incident type light receiving element made of GaAs is used. On the other hand, a light receiving element made of silicon is used as the light receiving element 109 for low-speed signals. Each of the conversion units may use means for amplifying a signal using an amplifier or the like. After conversion to an electrical signal, the signal separation circuits / CDR units 110 and 111 extract a clock from the signal and separate the signal based on the clock to obtain the high-speed reception data 112 and the low-speed reception data 113 based on the clock. be able to.

  The merit of transmitting a high-speed signal and a low-speed signal separately in this way is useful, for example, when the transmission protocol and the topology are flexibly divided, or when the transmission quality is different. For example, the quality of data differs greatly between audio and image data and signals used for device control. Audio data and image data are output as video or audio from a display or a speaker on the receiving side, but there is no problem because a difference in video and audio cannot be identified even if there is a slight error in the signal. On the other hand, if there is an error in device control or the like, it may cause a malfunction, so that reliable data transmission is required.

  In general, multimedia signals such as images and voices are often required to have high speed, and in the case of the present invention, error correction and data retransmission functions are not required for high-speed data signals. On the other hand, reliable data transmission is required for low-speed data signals. Transmission of low-speed signals makes it easier to improve the quality by increasing margins such as jitter, and because it is low-speed, even if an error correction signal etc. is applied, the burden on the control LSI, here the circuit size and power consumption There is little influence. On the other hand, when a low-speed signal and a high-speed signal are multiplexed, it is necessary to guarantee the data quality for all signals, and complex error correction with a high-speed signal is performed by an LSI that forms a multiplexing or separation circuit. There are many demerits such as large power consumption and circuit scale and transmission delay. Therefore, when the quality and data content required for the low-speed signal and the high-speed signal are different, there is a great merit in transmitting and receiving while separating them.

  Next, the double core optical fiber 107 used in the optical communication system 100 will be described with reference to FIG. The multimode double core optical fiber 107 is a hard plastic clad double glass core fiber. The double core optical fiber 107 includes a fluorinated acrylate resin clad 201, a step index glass core 202, and a germanium-added graded index core 203. Each refractive index is as shown in the lower part of FIG. The refractive index is graded index core 203, step index core 202, and plastic clad 201 in descending order. FIG. 2A shows a case where the centers of the outer circumference circles of the graded index core 203 and the step index core 202 match, and FIG. 2B shows a case where they do not match.

  The laser-modulated light can propagate through the graded index core 203 because there is a difference in refractive index between the step index core 202 and the graded index core 203. Since the graded index core 203 and the step index core 202 have a higher refractive index than the hard plastic clad 201, the light emitting diode light can be propagated by confining the light therein. The graded index core 203 has a refractive index higher than that of the step index core 202 by adding an appropriate amount of germanium. The shape of the refractive index of the graded index core 203 is approximately a parabolic function (square) so that the mode band is the highest.

  The diameter of the graded index core 203 is set to 50 μm in order to obtain a wide mode band when a high-speed signal of 10 G or more is required. In the case of a speed of about 1G, it is desirable to set the thickness to about 120 μm in consideration of ease of mounting. Since the step index core 202 needs to be larger than the graded index core 203, it is at least about 200 μm, which is the same as a commercially available hard plastic clad fiber. If there is a problem in mounting with an optical element that controls high-speed transmission, there is no problem even if it is expanded to about 400 μm. If the width is too large, the length taken from the processing source of the fiber, which is called a single preform, is shortened. Therefore, it is desirable to keep it at about 400 μm at most from the viewpoint of economy. In addition, as shown in FIG.2 (b), the graded index core 203 may be formed not in the center of the step index glass core 202 but in the position shifted | deviated outside.

  Next, the coupling between the double core optical fiber 107 and each optical element will be described. It is necessary to mount so that the light of the semiconductor laser 105 for high-speed transmission enters directly into the center of the graded index core 203. This is because the transmission characteristics may be deteriorated due to the coupling deviation, and the coupling efficiency is deteriorated because the NA of the fiber cannot be increased.

  On the other hand, the light of the light emitting diode 106 propagating through the step index core 202 does not cause a problem in terms of transmission characteristics and coupling characteristics even if it is incident with a deviation. Therefore, the light emitting diode 106 may be mounted on the end of the step index core 202 at the center of the graded index core 203 with the semiconductor laser 105.

  On the other hand, on the receiving side, a small and small-diameter GaAs light-receiving element 108 may be placed in the center of the large-diameter light-receiving element 109 made of silicon. The small light receiving element 108 is processed as small as possible and is flip-chip mounted on the silicon light receiving element 109. Since the high-speed modulated light propagates through the graded index core 203, the mode field diameter is small and can be condensed. The condensed light is received by a small GaAs light receiving element 108 and converted into an electric signal. The light receiving element 108 is provided with a filter layer so as not to convert the light of the light emitting diode into an electric signal. This is because the forbidden band width of the semiconductor composition is an intermediate composition between 850 nm and 650 nm. It can be absorbed by the filter layer. Further, by making the light receiving element 108 sufficiently smaller than the diameter of the step index core 202, the light propagating through the step index core 202 is made incident on the silicon light receiving element 109, and the modulation signal of the light emitting diode is electrically generated. The signal can be restored.

  Next, a method for manufacturing the double core optical fiber 107 will be described. Although it is a fiber having this double clad core, it is structurally equivalent to a normal graded index fiber with a hard plastic clad. The graded index fiber can be produced by drawing and spinning an optical fiber preform produced by the PCVD method using a drawing furnace. The prepared graded index fiber is coated with a fluorinated acrylate resin clad, and the outer side is coated with a protective coating.

Embodiment 2
Next, as a second embodiment, an optical communication system in which a low-speed signal is bidirectional will be described with reference to FIG. The same components as those in the first embodiment are denoted by the same reference numerals, and description thereof will be omitted as appropriate.

  The optical communication system 100 is a system that simultaneously transmits high-speed transmission data 101, low-speed transmission data 102a, and low-speed transmission data 102b in the reverse direction through a single fiber. This communication system includes a high-speed signal multiplexing circuit 103 that multiplexes a high-speed signal, a low-speed signal multiplexing circuit 104 that multiplexes a low-speed signal, a semiconductor laser (LD) 105 that modulates a high-speed signal, and a low-speed signal. Light-emitting diodes (LEDs) 106a and 106b, a double-core optical fiber 107, a semiconductor light-receiving element (PD) 108 that converts a high-speed optical signal into an electric signal, a semiconductor light-receiving element (PD) 109a that converts a low-speed optical signal into an electric signal, 109b includes a high-speed signal separation circuit 110 and a low-speed signal separation circuit 111 that return the transmitted signal to the original data.

  As in the first embodiment, this system transmits high-speed transmission data 101 and low-speed transmission data 102a as high-speed reception data 112 and low-speed reception data 113a. Furthermore, in the second embodiment, the low-speed transmission data 102b is transmitted in the reverse direction as the low-speed reception data 113b.

  In the second embodiment, since there are two low-speed data signals, a red light emitting diode 106a having an oscillation wavelength of 650 nm band and a blue light emitting diode 106b having a wavelength of 470 nm are used. By changing the color, the role of signal transmission and reception can be clarified. Each of the laser modulated light and the two light emitting diode modulated lights is coupled to a multimode double core optical fiber 107. The double core optical fiber 107 is the same as that of the first embodiment.

  The transmitted light is separated and converted into an electric signal by the light receiving element 108 for high speed signals and the light receiving elements 109a and 109b for low speed signals. As in the first embodiment, a GaAs surface-incidence light-receiving element is used as the light-receiving element 108 for high-speed signals. On the other hand, a light receiving element made of silicon is used as the light receiving element 109a for low-speed signals. After conversion to an electrical signal, the signal separation circuits / CDR units 110 and 111 extract a clock from the signal and separate the signal based on the clock to obtain the high-speed reception data 112 and the low-speed reception data 113a. . Here, since the low-speed transmission data 102b transmitted from the data reception side is synchronized with the clock of the signal transmitted from the transmission side, there is no need to extract the clock from the low-speed reception data 113b received on the transmission side. .

  In addition to the contents described in the first embodiment, the effect of transmitting a high-speed signal and a low-speed signal separately and transmitting a signal from the receiving side is as follows. First, hot plug detection is mentioned. When there is no optical signal returned from the receiving side, the transmitting side can stop laser transmission and realize a complete eye-safe. Furthermore, it is possible to contribute to power consumption by turning off the power of an LSI with high power consumption such as a high-speed signal multiplexing circuit. Secondly, it is possible to control the transmission side device and the transmission circuit. Third, communication safety can be ensured. By exchanging information between low-speed signals for transmission and reception, it is possible to confirm the trust relationship of the other party of transmission and reception, and to exchange keys for encryption. Therefore, it is possible to safely transmit the contents of high-speed data communication.

  As an example according to the second embodiment, a digital image transmission system will be described as an example with reference to FIG. This digital image system is compliant with the HDMI format used for image transmission between a digital broadcast recorder and a high-definition television.

  The communication system 100 includes a high-speed signal multiplexing circuit 103 that multiplexes high-speed transmission data 101 in which image pixel data and audio signals are multiplexed, data such as image aspect ratio, device control signals, and encryption key signals. The low-speed signal multiplexing circuit 104a that multiplexes the low-speed transmission data 102a and the like, the low-speed signal multiplexing circuit 104b that multiplexes the low-speed transmission data 102b transmitted from the reception side, and the semiconductor laser (LD) that modulates the high-speed signal 105, light-emitting diodes (LEDs) 106a and 106b that modulate low-speed signals, double-core optical fiber 107, semiconductor light-receiving element (PD) 108 that converts high-speed optical signals into electrical signals, and low-speed optical signals that are converted into electrical signals Semiconductor light receiving elements (PD) 109a and 109b, high-speed signals for returning the transmitted signals to the original data Releasing circuit 110 and the low-speed signal separation circuit 111a, it comprises a 111b. Furthermore, 8B10B conversion circuits 401 and 402 for performing 8B10B conversion on the high-speed transmission data 101 and the low-speed transmission data 102a, and CDR circuits 403 and 404 for recovering the received clock data are provided.

  With this system, DDC (Display Data Channel) which is serial data of high-speed transmission data 101 (data 1 to 3 and clock in FIG. 4) such as image and audio data and image format data such as aspect ratio and encryption key data. Signals, CEC (Consumer Electric Control) signals used for device control, and low-speed transmission data 102a including a DDC clock are transmitted as high-speed reception data 112 and low-speed reception data 113a, respectively. Further, the low-speed transmission data 102b such as the data signal for hot plug detection, the control signal / encryption key data (hot plug detection and DDC data output in FIG. 4) is transmitted in the reverse direction as the low-speed reception data 113b.

  Next, each function will be described. The high-speed signal multiplexing circuit 103 further multiplexes 3 channels of TMDS (data 1 to 3 in FIG. 4) in which the audio signal is multiplexed in advance with the image signal (RGB signal or YUV signal). The multiplexing of the image signal and the audio signal is performed by an image processing system LSI outside the system. In the system of the present invention, the TMDS signals for 3 channels multiplexed in advance are handled from being multiplexed. The low-speed signal multiplexing circuit 104a multiplexes the aspect ratio and encryption key data DDC signal and the device control CEC signal. In the signal multiplexing circuits 103 and 104a, the data string is multiplexed on the basis of a clock (clock and DDC clock in FIG. 4). In this communication system, a clock signal is embedded in a data signal and transmitted, and the clock is restored from the data signal received on the receiving side. In the case of image transmission or the like, the pixel data is likely to be followed by the same data string. Therefore, the data string is converted by the 8B10B conversion circuits 401 and 402 in order to easily extract the clock from the signal and bring the mark probability close to 0.5. To do. This method is a method of generating a 10-bit signal from an 8-bit signal. By this coding, it is possible to eliminate a long signal sequence, and it is easy to restore the clock on the receiving side.

  Next, a high-speed signal is modulated by the semiconductor laser 105 using the multiplexed signal. A surface emitting laser is suitable for this semiconductor laser. This is because the surface emitting laser has low power consumption and is suitable for high-speed modulation. The operating speed can be up to 10 Gbps or higher. It is possible to transmit a video corresponding to so-called full high-definition of about 1920 × 1080, 60 fps of the HDMI 1.3 standard. Here, a surface emitting laser having an 850 nm band as an oscillation wavelength is used. This wavelength is widely used in short-distance multimode transmission for high-speed datacom applications, and is highly available.

  On the other hand, for the low-speed data signals 102a and 102b, a red light emitting diode with an oscillation wavelength of 650 nm band and a blue light emitting diode with 470 nm are used. Even the light emitting diode can cope with a speed of about several tens of Mbps. By changing the color, the role of signal transmission and reception can be clarified. Laser-modulated light and light-emitting diode-modulated light are coupled to the multimode double-core optical fiber 107, respectively.

  The transmitted light is separated and converted into an electrical signal by the light receiving element 108 for high speed signals and the light receiving element 109a for low speed signals. As in the first embodiment, a GaAs surface-incidence light-receiving element is used as the light-receiving element 108 for high-speed signals. On the other hand, a light receiving element made of silicon is used as the light receiving element 109a for low-speed signals. Each conversion unit amplifies the signal using a transimpedance amplifier. After conversion to an electrical signal, clock and data are extracted by clock data recovery (CDR) circuits 403 and 404. In the signal separation circuit / 8B10B conversion circuits 110 and 111a based on the clock signal and the data signal, the signal is separated and decrypted, and the high-speed received data 112 such as image data and audio data, the aspect ratio, the control signal / encrypted key data, etc. Low-speed received data 113a can be obtained. Here, since the low-speed transmission data 102b transmitted from the data reception side is synchronized with the clock of the signal sent from the transmission side, there is no need to extract the clock from the low-speed reception data 113b received on the transmission side. .

  In this way, by transmitting the image and audio signals separately from the control signals and the like and sending the signals also from the receiving side, it is possible to detect hot plugs that are essential in the HDMI device. At this time, on the signal transmission side, when there is no optical signal from the reception side, it is possible to stop laser transmission and make it completely eye-safe. Furthermore, not only the laser but also the power consumption of an LSI with high power consumption such as a high-speed signal multiplexing circuit can be turned off, thereby contributing to power consumption. In addition, it is possible to control a recorder from a high-definition television. Furthermore, secure communication can be ensured. Since high-definition video digital information does not deteriorate even if it is copied, it is necessary to perform copy control from the viewpoint of copyright management. Since transmission / reception is possible, the mutual trust relationship between the devices can be confirmed, a key for encryption can be exchanged, and the image data can be transmitted safely.

  Next, in constructing this optical communication system, the coupling between each optical element and the double core optical fiber 107 will be described. The semiconductor laser 105 for high-speed transmission needs to be mounted so that the light directly enters the center of the graded index core 203. This is because the transmission characteristics may be deteriorated due to the coupling deviation, and the coupling efficiency is deteriorated because the NA of the fiber cannot be increased.

  On the other hand, the light from the light emitting diode 106a propagating through the step index core 202 does not cause a problem in terms of transmission characteristics and coupling characteristics even if it is incident with a deviation. For this reason, the light emitting diode 106 a may be mounted on the end of the step index core 202 at the center of the graded index core 203 with the semiconductor laser 105.

  The semiconductor laser 105 and the light emitting diode 106a are mounted on a silicon light receiving element 109b. Since the heat sink made of silicon absorbs visible light, a low-speed signal sent from the receiving side can be returned to an electric signal, and since it has high thermal conductivity, it can also serve as a heat sink. In order to allow sufficient light to enter the light receiving element 109b, for example, as shown in FIG. 2B, the graded index core 203 is formed by being shifted from the center of the step index core 202, or the light emitting diode 106a. Shift to the other side to implement.

  On the other hand, on the signal receiving side, a small and small-diameter GaAs light-receiving element 108 and a light-emitting diode 106b are arranged in the center of the large-diameter light-receiving element 109a made of silicon. The small light receiving element 108 is processed as small as possible and flip-chip mounted on the silicon light receiving element 109a. Since the high-speed modulated light propagates through the graded index core 203, the mode field diameter is small and can be condensed. The condensed light is received by a small GaAs light receiving element 108 and converted into an electric signal. The light receiving element 108 is provided with a filter layer so that light from the light emitting diode is not converted into an electric signal. This can be absorbed by the filter layer when the forbidden band width of the semiconductor composition is an intermediate composition between 850 nm and 650 nm. Further, by making the light receiving element 108 sufficiently smaller than the diameter of the step index core 202, the light propagating through the step index core 203 is incident on the silicon light receiving element 109a, and the modulation signal of the light emitting diode is electrically converted. The signal can be restored. The light emitting diode 106b is mounted away from the center of the silicon light receiving element 109a so as not to prevent light absorption into the silicon light receiving element 109a.

1 is a configuration diagram of an optical communication system according to a first embodiment of the present invention. It is sectional drawing of the optical fiber used with the optical communication system of this invention. It is a block diagram of the optical communication system which concerns on the 2nd Embodiment of this invention. 1 is a configuration diagram of an optical communication system according to a first embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Optical communication system 101 High speed transmission data 102, 102a, 102b Low speed transmission data 103 High speed signal multiplexing circuit 104, 104a, 104b Low speed signal multiplexing circuit 105 Semiconductor laser (LD)
106, 106a, 106b Light emitting diode (LED)
107 Double-core optical fiber 108 Light-receiving element (PD)
109, 109a, 109b Light receiving element (PD)
110, 110a, 110b High-speed signal separation circuit 111, 111a, 111b Low-speed signal separation circuit 112, 112a, 112b High-speed reception data 113, 113a, 113b Low-speed reception data 201 Fluoroacrylate resin clad 202 Step index core 203 Graded index core 401, 402 8B10B conversion circuit 403, 404 CDR circuit

Claims (7)

A first core made of glass with a refractive index of graded index type;
A second core made of glass having a constant refractive index and having a refractive index lower than that of the first core;
An optical communication system using an optical fiber comprising: a plastic clad surrounding the first and second cores;
A signal obtained by modulating infrared laser light having a wavelength of 850 nm or more is propagated to the first core, and a signal obtained by modulating light from the first visible light emitting diode having a wavelength of 780 nm or less is propagated to the second core. An optical communication system.   2. The optical communication system according to claim 1, wherein a diameter of the first core portion is 120 μm or less, and a diameter of the second core is 400 μm or less.   The center of a circle constituting the outer periphery of the first core and the center of a circle constituting the outer periphery of the second core do not coincide with each other in the cross section of the optical fiber. The optical communication system according to 2.   The optical communication system according to claim 1, wherein a surface emitting laser is used to generate the infrared laser light.   A signal obtained by modulating light from a second visible light emitting diode having a wavelength different from that of light from the first visible light emitting diode is propagated in the opposite direction to the second core to perform bidirectional communication. 5. The optical communication system according to claim 4.   A mechanism is provided for stopping emission of the laser light when a signal indicating that the signal has been received does not reach from the reception side of the signal modulated with the infrared laser light to the transmission side. 5. An optical communication system according to 5.   7. The optical communication system according to claim 5, wherein two light emitting diodes among a red light emitting diode, a blue light emitting diode, and a green light emitting diode are used as the first and second visible light emitting diodes. .

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