JPS6245733B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a data bus collision detection method that can easily and reliably detect collisions of optical signals on a data bus in an optical network.

[Technical background of the invention]

Recently, optical networks that use light as a carrier for information communication between stations have been attracting attention. 1st
The figure is a schematic configuration diagram of an optical network showing one example.
_-1 , _2-2 to 2 _-o are connected via an optical fiber 3. Each station _2-1 , _2-2 ~ 2 _-o
The light source 4 for transmitting the optical signal is configured using a semiconductor light emitting device such as a semiconductor laser device,
In addition, the photodetector 5 that receives the optical signal is set to, for example, PIN-
It is constructed using a photodiode or an avalanche photodiode. Therefore, the optical signal emitted from the light source 4 of a certain station 2 _-i is
It is transmitted through the optical star network data bus 1 and supplied to each station 2, and
A part of it is returned to station 2 _-i , which is the source of the optical signal. This optical signal is detected by a photodetector 5 to monitor the communication state.

By the way, when each station 2 randomly transmits an optical signal in this optical star network, these optical signals collide on the optical star network data bus 1. Therefore, in order to communicate between the stations 2, it is necessary to monitor and avoid collisions of the data (signals). Conventionally, as shown in Fig. 2 a to c, when two stations simultaneously transmit optical signals as shown in Fig. 2 a and b, respectively,
Since the colliding signals on the optical star network data bus 1 become as shown in FIG. The presence or absence of a collision was determined by determining the signal pulse width by making discrimination based on the level V _th .

[Problems with conventional technology]

However, the waveform of an optical signal used for communication is not necessarily sharp, but generally has a rounded waveform. For this reason, it has been extremely difficult to accurately detect the pulse width. Furthermore, it has been extremely difficult to accurately define the pulse width itself of the transmitted optical signal.

In addition, the semiconductor laser element used as the light source 4 is driven by applying a bias below the normal oscillation threshold in order to operate at high speed, and then applying a pulse voltage in accordance with the transmission signal. For this reason, the light source 4, as shown in FIG. 3a,
Light is generated by adding the pulse signal light P _sig to the spontaneous emission light P _sp due to the above bias. The above-mentioned spontaneous emission light P _sp is constantly output from each station 2, so the level of the optical signal detected by the photodetector 5 is as shown in FIG. 3b, the DC level P _sp This results in a very large overlap. This DC level P _sp is nothing but the superposition of spontaneously emitted light from each station 2 . For this reason, the photodetector 5 generates large shot noise due to the spontaneously emitted light, causing problems such as deterioration of the S/N ratio and a limitation on the number of stations that can participate in the network. Furthermore, such a DC level also makes it extremely difficult to detect the pulse width described above, and as a result, collisions cannot be easily and reliably detected on the data bus of the optical network.

[Purpose of the invention]

The present invention was made in consideration of these circumstances, and its purpose is to provide a highly practical data bus that can easily and reliably detect communication collisions on a data bus in an optical network. The object of the present invention is to provide a collision detection method.

[Summary of the invention]

In an optical network configured by interconnecting stations using the semiconductor light emitting device of the present invention as a light source, each station stops driving the semiconductor light emitting device and stops transmitting light when not transmitting an optical signal. When transmitting an optical signal, the semiconductor light emitting element is initially driven to emit and output DC light at a predetermined level, and then an optical signal data string such as an optical pulse signal is transmitted. . Each station is characterized in that communication collisions are detected by detecting fluctuations in the DC level of the optical detection signal on the data bus.

〔Effect of the invention〕

Therefore, according to the present invention, since each station detects the DC level fluctuation of the detected optical signal due to the DC light of a predetermined level transmitted prior to transmitting the optical signal, it is possible for a plurality of stations to start communication almost simultaneously. In such a case, it becomes possible to easily and reliably detect a collision from the fluctuation in the DC level of the light. Further, since detection only needs to be performed regarding DC level fluctuations, problems such as those encountered when detecting the conventional optical signal pulse width are not caused. Also, in normal optical communications, there is no problem such as the DC light from each station being superimposed on the optical signal, so it is possible to improve the S/N ratio and expand the scale of the optical network. Become. In addition, the semiconductor light emitting device that is the light source emits a predetermined level of DC light prior to optical communication and then outputs an optical signal, so it does not cause problems that depend on the operating characteristics of the semiconductor light emitting device, and can operate at a sufficiently high speed. It is possible to aim for

[Embodiments of the invention]

An embodiment of the method of the present invention will be described below.

The light source 4 of the station constituting the optical star network is composed of, for example, a semiconductor laser element, and the photodetector 5 is composed of an avalanche photodiode. Conventionally, the semiconductor laser element that is the light source 4 is in a spontaneous emission state with a constant bias current flowing through it in order to speed up its operation, and when transmitting a signal, a pulse voltage is is applied to exhibit a laser oscillation effect, and a predetermined laser pulse light is output as signal light. On the other hand, in the station according to the present invention, the semiconductor laser element that is the light source 4 is not driven when not transmitting a signal, that is, when it is in a standby state, that is, when no bias current is passed through it. Set to completely off state.
Therefore, at this time, not only signal light but also spontaneous emission light is not outputted at all, resulting in a stopped state.

On the other hand, when the station wishes to transmit an optical signal, first, the semiconductor laser element is initially driven for a predetermined initial period. This initial drive is performed by passing a bias current below and in the vicinity of the laser oscillation threshold of the semiconductor laser element to the semiconductor laser element. By this initial drive, the semiconductor laser element emits spontaneous emission light at a predetermined DC level for a certain period of time using the bias current. After sending out this spontaneous emission light, the semiconductor laser element exhibits a laser oscillation effect by applying a pulse voltage, that is, a pulse voltage sufficiently exceeding the laser oscillation threshold, in accordance with the transmitted data, and the high-level laser light becomes a signal light. is output as Incidentally, the laser oscillation drive of this semiconductor laser element is performed with the above-described bias current flowing, so that the output light takes on the respective levels of laser light and spontaneous emission light depending on the transmitted data. In other words, the optical signal corresponding to the data is the signal level of the laser beam superimposed on the direct current level of the spontaneous emission light.

Now, collision of optical signals transmitted from the station on the data bus is detected in the following manner. That is, the avalanche photodiode constituting the photodetector 5 of each station receives and detects an optical signal sent to the data bus, and detects an electrical signal, such as a voltage signal, corresponding to the level of the optical signal. The DC level of the light-receiving signal detected by this avalanche photodiode is detected by a predetermined level discriminator, and the presence or absence of level fluctuation is determined. When the DC level of the light-receiving signal exceeds a certain set threshold, the station detects that a data collision has occurred on the data bus.

That is, let us now assume that a certain station outputs spontaneous emission light from time _{t 1} as shown in FIG. At this time, another station attempts to communicate from time t ₂ , which is slightly delayed from time t ₁ , and outputs spontaneous emission light by initial drive as shown in FIG. 4b. The DC level of the optical signal varies as shown in FIG. 4c. In other words, the spontaneously emitted light from the two stations due to the initial drive overlaps, which causes the DC level of the optical signal detected by the avalanche photodiode to fluctuate. Note that even when one station is transmitting a data optical signal, there is a DC level of that optical signal, so even if spontaneous emission light is output from another station at this time, the detected DC level will be the same. changes. However, in this case, the data bus has a DC level of zero (0) when all the stations are in the standby state, and a predetermined DC level is obtained by the DC spontaneous emission light by the communication operation of a certain station. , it is possible to detect that communication is already occurring via the data bus from this level fluctuation alone. If there is further fluctuation in the DC level from this state, it is possible to detect this as a data collision.

As described above, according to this method, collision is determined by detecting fluctuations in the DC level of the optical signal on the data bus, so collision detection is extremely simple and reliable. Furthermore, when each station is in a standby state, its light source 4 is completely turned off and no light is emitted, so the S/N on the data bus and, by extension, the S/N of the photodetector 5 can be made sufficiently high. can. This also has the effect of increasing the transmission distance of the optical network and further increasing the number of stations that can participate in the network. Furthermore, the semiconductor laser element that is the light source 4 is driven with a bias current for a certain period of time before transmitting the signal light, and is driven to oscillate after passing through the spontaneous emission state, so its high-speed operation is fully guaranteed. Therefore, various effects that cannot be expected from conventional optical networks can be achieved.

Note that the present invention is not limited to the above embodiments. For example, the DC level light generated by the station is not necessarily spontaneous emission light of a semiconductor laser element, but may be laser oscillation light of a certain DC level. Furthermore, if the optical signal or the DC light level preceding the optical signal can be raised to a certain degree and the signal S/N of the receiving station can be kept sufficiently high, it is not necessarily necessary to completely turn off the light source of the standby station. However, when considering the life span and operational reliability of the semiconductor light emitting device, it is desirable to set the DC light level to a level below and in the vicinity of the laser oscillation threshold. Furthermore, it is of course possible to use a semiconductor light emitting device other than a semiconductor laser device as a light source. In short, the present invention can be implemented with various modifications without departing from the gist thereof.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the optical star network.
Figures 2 a to c are diagrams for explaining the conventional collision detection method, Figures 3 a and b are diagrams for explaining problems in the conventional optical communication system, and Figures 4 a to c are the present invention method. FIG. 2 is a diagram for explaining an optical signal communication form and collision detection thereof. 1... Optical star coupler, 2... Station, 3... Optical fiber, 4... Light source, 5... Photo detector.

Claims (1)

[Scope of Claims] 1. In an optical network in which stations using semiconductor light emitting devices as light sources are interconnected, each station stops driving the semiconductor light emitting device when not transmitting an optical signal, and transmits the optical signal. Sometimes, the semiconductor light emitting device is driven to transmit DC light at a predetermined level and then an optical signal data string is transmitted, and each station detects fluctuations in the DC level of the optical signal transmitted to the data bus of the optical network. A data bus collision detection method characterized by detecting a data collision on the data bus. 2. A patent claim in which the semiconductor light emitting device is composed of a semiconductor laser device, and the direct current light at a predetermined level transmitted before the optical signal data string is spontaneous emission direct current light corresponding to a drive current lower than the laser oscillation of the semiconductor laser device. The data bus collision detection method according to item 1.