JPH05136745A - Optical transmission system - Google Patents

Abstract

PURPOSE:To form the system with high quality by forming a trunk line system port of an optical branching device with a low loss asymmetrical optical branching device and forming a branch line system port of the optical branching device with a high loss asymmetrical optical branching device to set the share of the loss in response to the light receiving level of the branch system. CONSTITUTION:In the optical transmission system having optical tree structure where optical branching devices are arranged at an optional position of an optical transmission line 61, the optical branching devices are made up of asymmetrical optical branching devices 8-1 to 8-n-1 of a low loss for the trunk line system port and a high loss for the branch system port, and the share of the loss of the optical branching devices is set in response to a light receiving level of the branch system. Then an optical analog video signal sent from the optical transmitter 5 is sent through a trunk line system optical transmitter 61 and branched to a branch system transmission line 62 by the asymmetrical optical branching devices 8-1 to 8-n-1 arranged at an optional location in the trunk line system transmission line 61 and received by optical receivers 7-1 to 7-n. In this case, the low loss asymmetrical optical branching devices 8-1 to 8-n-1 are branched for the trunk line system optical transmitter 61 and the high loss asymmetrical optical branching devices 8-1 to 8-n-1 are branched for the branch line system optical transmitter 62.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tree-structured optical transmission system using an asymmetrical optical branching device having different branching ratios for a trunk port and a branching port.

[0002]

2. Description of the Related Art FIG. 2 is a diagram showing a configuration example of a conventional coaxial CATV system. In the figure, 1 is a video transmitter, 2 is a coaxial cable, 3 is a main branch amplifier, and 4 is a distribution amplifier. .. In this coaxial CATV system, a coaxial cable 2 is used as a trunk line, a plurality of trunk line branch amplifiers 3 having a function as a loss compensation amplifier are arranged at arbitrary positions, and a plurality of branch lines are branched from these trunk line branch amplifiers 3. , A so-called tree structure in which a distribution amplifier 4 is connected to each branch line.

Also, in the CATV system using the optical transmission technology, the point-to-point (Po
int-to-Point) service is provided.

FIG. 3 is a diagram showing a configuration example of a conventional optical transmission system to which the point-to-point format is applied. In the figure, 4 is a distribution amplifier, 5 is an optical transmitter, 6 is an optical fiber cable,
Reference numeral 61 is an optical fiber, and 7 is an optical receiver. In this optical transmission system, each optical fiber 61 of the optical fiber cable 6 is
The optical transmission device 5 is arranged on one end side of the optical fiber, and an arbitrary optical fiber 61 is taken out from the optical fiber cable 6 serving as a trunk line to form a branch line at an arbitrary position as a branch point, and the optical receiving device 7 is provided for each branch line. Is arranged.

In such a configuration, there are advantages such as (1) high quality and long distance transmission possible, and (2) economical provision with respect to provision to an area (High-Density) where the subscriber density is high.

[0006]

However, the conventional coaxial CATV system is not shown in FIG.
Since the tree structure is used as shown in (1), the image quality is deteriorated due to the increase in distortion due to the coaxial amplifier serial multi-stage connection based on the following equation. Therefore, the number of amplifier stages is limited, and the transmission distance and the number of accommodated subscribers are limited.

Amount of distortion increase (n: number of amplifiers connected in series) 10 log (n) (secondary beat) (1) 20 log (n) (third beat) (2) Area where subscriber density is low Regarding the provision to (Low-Density), as shown in Fig. 3, when it is attempted to cover the fiber backbone with a tree configuration in a point-to-point format, the number of optical transmitters and the number of optical fiber cables are large, The cost is high and the configuration is complicated. Further, in order to compensate for this, each of the following two configurations also has a drawback. (i) In a tree configuration like a coaxial system, inserting a symmetric optical branching device increases branching loss, making long-distance transmission impossible. (ii) If a repeater (optical amplifier or optical / electrical / optical converter (so-called OS / OR back-to-back)) is inserted in the main line system, quality deterioration due to increase in distortion due to multistage connection cannot be avoided. Furthermore, (iii) the optical amplifier is expensive at present, so the system cost becomes high. (iv) Since the OS / OR depends on the transmission method, it is necessary to replace the OS / OR when changing the method (for example, FM method → AM method).

The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical transmission system capable of constructing a system of higher quality than a coaxial system with the same economic efficiency as the coaxial system. ..

[0009]

To achieve the above object, in the present invention, in an optical transmission system having an optical tree structure in which an optical branching device is arranged at an arbitrary position of an optical transmission line, The trunk system port was composed of a low loss and the branch system port was composed of a high loss asymmetric optical branching device, and the loss distribution of the optical branching device was set according to the light receiving level of the branching system.

[0010]

According to the present invention, for example, an optical analog video signal sent from an optical transmitter is transmitted in one trunk optical transmission line, and is asymmetrical arranged at an arbitrary position in the trunk transmission line. Type optical branching device for branching to a branching transmission line and received by an optical receiving device. At this time, in the asymmetrical optical branching device, the branching is performed so that the trunk optical transmission line has a low loss and the branching optical transmission line has a high loss. As a result, long-distance transmission is possible, and since no repeater is arranged in the trunk line system, distortion characteristics do not deteriorate.

[0011]

First Embodiment FIG. 1 is a configuration diagram showing a first embodiment of an optical transmission system to which the method of the present invention is applied, and the same components as those in FIGS. 2 and 3 showing a conventional example are represented by the same reference numerals. .. That is, 1 is a video transmission device, 5 is an optical transmission device, and 6
1 is a trunk optical fiber, 62 is a branch optical fiber, 7-
1 to 7-n are optical receiving devices, and 8-1 to 8-n-1 are asymmetrical optical branching devices. In this system, one optical fiber 6
1 is a trunk line system of a transmission line, and a plurality of 1 × 2 asymmetric type optical branching devices 8-1 to 8-8 are provided at arbitrary points of this trunk line optical fiber 61.
-N-1 is placed.

The asymmetrical optical branching devices 8-1 to 8-n-1 are, for example, 1 × 2 directional couplers having an asymmetrical branching ratio as shown in FIG. 4, and are composed of an input port 81 and a trunk line port. -G 82
Is connected to the trunk optical fiber 61, and the branch port 83 is connected to the branch optical fiber 62. These optical splitters 8
The branching ratios of -1 to 8-n-1 are set so that the trunk port 82 has a low loss and the branch port 83 has a high loss.
The loss distribution is adjusted to match the light receiving level of the branch system. FIG. 5 shows an example of this level diagram.

Specifically, the line loss between the trunk asymmetric optical branching device is L2n-1, the line loss between the branching asymmetric optical branching device and the optical receiver is L2n, and the trunk line of the asymmetrical optical branching device. Assuming that the system port loss is T2n-1 and the branch system port loss is T2n, L1 + L2 + T2 ≈L1 + T1 + L3 + L4 + T4 ≈L1 + T1 + L3 + T3 + L5 + L6 + T6 ≈L1 + T1 + L3 +1 + L + To be configured. The asymmetrical optical branching devices 8-1 to 8-n-1 adopted here can be manufactured with a branching ratio of 0 to 100% (see "Quartz-based optical waveguides and applied optical circuit components". IEICE OCS 89-10
See S).

In such a configuration, the optical analog video signal sent from the optical transmitter 5 is transmitted through one trunk optical fiber 61, and is placed in an arbitrary position of the trunk optical fiber 61. Each of the optical receivers 7 is branched by the optical branching devices 8-1 to 8-n-1 into a branching optical fiber 62.
-1 to 7-n.

At this time, each of the asymmetrical optical branching devices 8-1 to 8-8
At -n-1, the trunk optical fiber 61 has low loss,
Since the optical fiber 62 is branched so as to have a high loss, long-distance transmission is possible. In addition, since the repeater is not provided in the main line system, the distortion characteristic does not deteriorate.

Next, the total transmission distance of the trunk line when an asymmetric type optical branching device is used and when a symmetric type 3 dB coupler is used will be considered.

As a condition of the system, as shown in FIG. 6, the level difference between transmission and reception of the system ... 30 dB received light level ... -30 dBm optical branching interval ... 2 km line loss ... 1.5 dB / km Optical splitter loss (1) Symmetrical optical splitter ・ ・ ・ 3dB (main line /
(2) Asymmetric type optical branching device ... Low loss in trunk system port, high loss in branching system port → Designed according to received light level, and optical receivers 7-1 to 7- immediately after branching When n is installed, the total transmission distance of the trunk line is the conventional system (using a symmetric optical branching device) → 10 km system (using an asymmetrical optical branching device) → 18 km, which means that long distance transmission is possible with this system. It is possible. It is possible to make the total transmission distance of the conventional system the same as that of the present system by installing a repeater, but in this configuration, as described above, the quality deteriorates due to the increase in distortion due to the multistage connection. In addition, when setting an upstream signal in a coaxial system, a bidirectional amplifier is inserted, so deterioration of the upstream signal cannot be avoided.
Because of the configuration, it can be transmitted to the video transmission device 1 without deterioration, and the quality of the upstream signal is high. Further, since the optical branching device is composed of the directional coupler, the signal does not travel in the reverse direction (downward direction).

FIG. 7 is a diagram showing an example of application of the method of the present invention to a dead zone such as in a tunnel 9 in a mobile communication system such as a car phone or a mobile phone. In this example, the optical signal (π / digital) sent from the optical transmitter 5 is as shown in FIG.
Similar to the configuration example of 1, the optical receivers 7-1 to 7-n are branched by the asymmetrical optical branching devices 8-1 to 8-n-1 and receive appropriate optical power.

In a mobile communication system, a leaky coaxial cable has been used in the past, so that the sending electric level was not constant, but by applying the method of the present invention, the electric sending level and quality of the optical receiving device are equal. Has the advantage that

[0020]

[Embodiment 2] FIG. 8 is a block diagram showing a second embodiment of an optical transmission system to which the method of the present invention is applied.

The present embodiment is different from the first embodiment in that in the first embodiment, after branching by an optical branching device, it is directly received by an optical receiving device via a branch optical fiber and is optically / electrically converted. Instead of the configuration, the branch optical fiber 6
The repeater 10 is inserted in the 2 to form a 2-link configuration. In this case, a repeater is not arranged in the main line system, and the one-link configuration remains unchanged.

Such a structure is possible within the allowable range of the distortion characteristics, and in addition to the effect of the first embodiment described above,
Further long-distance transmission of the branch system can be realized.

FIG. 9 shows a secondary beam when the above system is connected.
Shows the comparison between the third and third beats and the required value. In FIG. 9, in the case of 1 link, the video signal 40c
H, 10km, measured value during transmission (ITEJ Technical Report
Vol.13, No.39, pp37-41.Aug.1989), and in the case of 2 links, it is a value calculated by the formulas (1) and (2) described in the section of the prior art.

As shown in FIG. 9, it can be seen that the required value is substantially satisfied even if the branch system is two links, and two links can be made with a slight improvement in characteristics.

[0025]

As described above, according to the present invention,
By adopting a tree structure similar to that of the coaxial CATV system, it is economical to reduce the number of optical transmitters and the number of cores of the optical fiber cable, and it is not necessary to arrange a repeater in the trunk system. High quality transmission can be realized. In addition, it is possible to provide various services because broadband transmission is possible compared to a coaxial cable. In addition, the transmission distance can be increased.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of an optical transmission system to which the method of the present invention is applied.

FIG. 2 is a diagram showing a configuration example of a conventional coaxial CATV system (main line system).

FIG. 3 is a configuration diagram when a conventional optical transmission system is replaced with a coaxial CATV system.

FIG. 4 is a configuration diagram of an asymmetric optical branching device according to the present invention.

5 is a diagram showing an example of a level diagram in the configuration of FIG.

FIG. 6 is an explanatory diagram for calculating a transmission distance with a conventional / present system.

FIG. 7 is a diagram showing an example of application of a mobile communication system to a dead zone (tunnel).

FIG. 8 is a configuration diagram showing a second embodiment of an optical transmission system to which the method of the present invention is applied.

FIG. 9 is a diagram showing comparison of secondary beats and tertiary beats and required values when the system is connected.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Video transmitter, 5 ... Optical transmitter, 61 ... Trunk optical fiber, 62 ... Branch optical fiber, 7 ... Optical receiver, 8 ...
Asymmetric type optical branching device, 9 ... Tunnel, 10 ... Repeater.

Claims (1)

[Claims] 1. An optical transmission system having an optical tree structure in which an optical branching device is arranged at an arbitrary position of an optical transmission line, wherein the optical branching device is asymmetrical with low loss in a trunk line port and high loss in a branching port. Type optical branching device, and the loss distribution of the optical branching device is set according to the light receiving level of the branching system.