JPH0435330A - Optical reception system - Google Patents

Abstract

PURPOSE:To receive a light stably by utilizing that an optical reception level and a time division multiplex schedule are known and varying a gain of a light receiving circuit or a threshold level of an identification circuit every time slot corresponding to each terminal equipment. CONSTITUTION:Digital transmission accommodation each subscriber in a predetermined time slot is presumed on the optical communication system in which time division multiplex communication is implemented by means of an optical communication network where one station equipment and plural termianl equipments are interconnected by optical fibers and star couplers. When an optical reception level of each subscriber and the sequence of an accommodated time slot are known in advance, the optical reception system realizing economically high speed response AGC and ATC which has been difficult in a conventional system is obtained by varying the gain of the light receiving circuit at every slot corresponding to each subscriber based on the information or a threshold level Vth of an identification circuit even when a large optical loss is present on each subscriber so as to obtain a prescribed electric signal.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to the field of optical communication using optical fibers.
The present invention relates to a method for stably receiving light even when there are differences in received light levels in economical communication using star couplers.

[Prior art and problems to be solved by the invention] FIG. 8 shows a conventional two-way communication method. 1 is an optical fiber transmission line, 2 is an optical transmitter, and 3 is an optical receiver. Normally, the loss of the optical fiber transmission line 1 changes depending on environmental conditions such as vibration. Furthermore, the optical loss varied depending on the distance of the optical fiber transmission line 1. By absorbing changes and variations in this loss,
Conventionally, stable reception has been achieved by changing the gain of the optical receiver circuit or the threshold value of the discrimination circuit.

The former is AGC (automatic gain c
ontrol), and the latter is ATC (autos).
atic threshold control). These are techniques that detect the level of received light and change the amplification degree of the light receiving circuit and the threshold value of the identification circuit based on the detected level. Although this technique is widely used, when it is applied to an optical fiber transmission line as shown in FIG. 9, there are the following problems.

First, the transmission path configuration shown in FIG. 9 will be explained. 4 is a star coupler. In this configuration, the optical fiber transmission line l
Connect star coupler (SC) 4 to
#2. ..., #J, ...) optical transmitters (subscribers: users) 2 are accommodated. This configuration is economical because one station device (optical receiver) 3 is shared by a plurality of subscribers. Furthermore, if the distance ill La between the station (on the optical receiver side) and the star coupler 4 is long, the portion of the optical fiber with length La can be shared by multiple subscribers. This configuration allows multiple subscribers to be accommodated economically. However, due to variations in the distance Lb between the star coupler (SC) 4 and each terminal device (optical transmitter m) 2, variations in the loss of each output port of the star coupler 4, and differences in the transmitted light level of the optical transmitter 2, , the loss value will vary significantly (in some cases, the difference in optical level will be more than 10 dB) than in the case of the configuration shown in Figure 8. Since multiple subscribers share the transmission path, each subscriber A transmission path is used in a time-division manner when communicating at a wavelength of G is a guard time and is provided to absorb variations in the distance Lb (Fig. 9) between the star coupler and the subscribers and to prevent the information of each subscriber from overlapping.The transmission path is shared by each subscriber. Therefore, each subscriber's information rate B
(b/s) more than N times (N: number of accommodated subscribers),
It becomes very fast.

For example, when B = 1.5 Mb/s and N = 16, the transmission speed through the optical fiber transmission line is 24 Mb/s or more. If the length of each time slot is 125 μs, in order to receive stable reception within the first few bits of each time slot, A must respond very quickly at 40 to 100 nsec.
GC and ATC are required. Conventional technology detects the level of received light and changes the amplification of the light receiving circuit and the threshold of the identification circuit based on that value, which requires detection time.
Configuring such a high-speed response is difficult.

SUMMARY OF THE INVENTION An object of the present invention is to economically provide a method for stably receiving light even when there are differences in received light levels in optical communication using optical fibers and star couplers.

[Means for Solving the Problems] The present invention provides an optical communication system in which communication is performed by time division multiplexing between one station device and a plurality of terminal devices using an optical communication network connected by optical fibers and star couplers. , for each time slot corresponding to each subscriber, based on the reception level and time division multiplexing schedule for each optical transmission path obtained in advance.
It is characterized by changing the gain of the light receiving circuit or the threshold value of the discrimination circuit.

[Function] The present invention obtains in advance the unique reception level and time division multiplexing schedule of each optical transmission path determined by each terminal device and the optical fiber connected thereto, and based on these, each terminal device By changing the gain of the light receiving circuit or the threshold value of the discriminating circuit for each time slot corresponding to , a constant signal can be obtained even if there is a large optical loss value for each terminal device.

[Example code] The present invention will be explained in detail below.

FIG. 1 shows the first principle of the present invention. First, the operating principle will be explained. The invention assumes a digital transmission that accommodates each subscriber in a defined time slot. If this fact is utilized, the received light level will fluctuate significantly, but the fluctuation will be based on the optical reception level obtained in advance (this optical reception level is determined from the output level of each terminal device and the preceding transmission path loss value). It can be predicted based on the time division multiplex schedule.

The difference between this predicted value and the actual optical reception level is due to changes in temperature and the like, and is generally small. That is, each subscriber has a unique optical loss, and the optical peak level when the subscriber is communicating is unique for each subscriber.

Therefore, if the optical reception level of each subscriber and the order of the accommodated time slots are known in advance, the gain of the light receiving circuit can be changed based on this information for each time slot corresponding to each subscriber. Therefore, it is possible to obtain a constant electrical signal even if there is a large optical loss value for each subscriber. With the configuration of the present invention, stable reception is possible even when the level of received light changes significantly.

FIG. 2 shows a second principle diagram of the present invention. The operating principle in this case will be explained. Based on the optical reception level and time division multiplexing schedule obtained in advance, the threshold value vth of the identification circuit is changed for each time slot corresponding to each subscriber. This configuration allows stable reception even when the level of received light changes significantly.

Note that it is also possible to use both the first principle and the second principle at the same time.

As explained above, the present invention utilizes the fact that the optical reception level and time division multiplexing schedule are known in advance to economically realize high-speed response 800 and ATC, which has been difficult in the past. It provides a method.

FIG. 3 shows a circuit configuration as an implementation means based on the first principle. 5 is a photoelectric conversion circuit, which is a photodiode (PD)
This is a circuit that converts signals into electrical signals using a photoelectric conversion element such as an avalanche photodiode (PD). 6 is an amplifier, 7
8 is a feedback resistor group of the amplifier 6, 8 is a high speed switch (SW), and 9 is a control circuit for controlling the SW.

The feedback resistor group 7 of the amplifier 6 is set according to the optical reception level obtained in advance. The individual resistances of the resistor group 7 correspond to the loss values of each subscriber. Settings can be done electrically automatically or manually by changing individual variable resistors.

The control circuit 9 controls SW for each time slot corresponding to each subscriber based on a time division multiplexing schedule obtained in advance.
8 to one of the feedback resistor groups 7 is electrically controlled. As a result, the amplification degree of the amplifier 6 changes rapidly. Therefore, stable reception can be achieved at high speed even with large fluctuations in the received light level.

Equivalent to Fig. 3, but with SW8 and feedper.

ROM (read only) instead of resistor group 7
It is possible to configure the feedback of the amplifier using a read-only memory (read-only memory) and a D/A converter. That is, the voltage value to be fed back to the amplifier for each time slot is stored in the ROM according to the optical reception level obtained in advance, and the voltage value is read out sequentially, and the analog value voltage is generated by the D/A converter. This constitutes the feedback of the amplifier.

FIG. 4 shows another circuit configuration based on the first principle. 1o
is a variable resistor, and 11 is a control circuit. The control circuit 11 is
Based on the time division multiplexing schedule and optical reception level obtained in advance, the variable resistor 1o is changed for each time slot corresponding to each subscriber.

FIG. 5 shows a circuit configuration as an implementation means based on the second principle. 12 is an amplifier, 13 is a comparator, 14 is a threshold generation circuit, and 15 is a control circuit.

Amplifier 12 is a linear amplifier or a nonlinear amplifier with a fixed amplification factor. In the case of nonlinear amplification, the output signal is not proportional to the input signal, but has a function in which the amplification degree is large when the signal is weak and the amplification degree is small when the signal is strong. The comparator 13 has a function of comparing a threshold value and an input value to identify the presence or absence of a signal. The threshold generation circuit 14 is a circuit that supplies a threshold value to the comparator, and is a DC voltage generator with a variable voltage value. As a configuration of the threshold generation circuit 14, RO
It is also possible to use M and D/A converters.

The control circuit 15 is a control circuit for changing the threshold value for each time slot corresponding to each subscriber based on the optical reception level and time division multiplexing schedule obtained in advance. This configuration does not detect the level of received light and change the threshold value based on the detected value, but instead always generates a periodically determined threshold value, so it is possible to increase the speed.

There are the following two methods for obtaining the reception level (or optical transmission line loss value) and time division multiplexing schedule for each optical transmission line obtained in advance.

(1) Before all subscribers sharing a star coupler start service, the optical transmission path loss value for each subscriber is assumed by calculation or actual measurement, and the transmitted light level of the optical transmitter is known. , it is possible to know the reception level for each optical transmission path.

Furthermore, it is possible to know the time division multiplexing schedule in advance by specifying the order and time length of allocating time slots to each subscriber.

(2) If some subscribers who share the star coupler have already started service, when new subscriber #J starts service, the reception level and time division are determined using the method shown in Figure 6. You can learn about multiple schedules. FIG. 6 will be explained. It is assumed that subscribers #1 and #2 are already using the service. Using an unused time slot, from the terminal device (optical transmitter) 2 of #J to the station device (optical receiver) 3/
<-Sends a strike signal. This signal is extracted on the optical receiver side and the reception level is determined. The time division multiplex schedule is finalized by reserving this unused time slot for #J.

FIG. 10 shows a case where the positional relationship between the transmitter and receiver is reversed. In this case, the optical level sent from the station equipment (optical transmitter) 2 to each terminal equipment (optical receiver; #1 to #n) 3 varies, the branching ratio at SC4 also varies, and furthermore, the optical fiber transmission Due to variations in the length of the path 1 and other variations as mentioned above, the light level reaching the terminal device 3 varies as shown in the lower graph of FIG. #J is incorrect (in order to receive it, it is necessary to set AGCSATC according to #J. Therefore, as in Fig. 6, it is necessary to recognize the time slot of #J to be received and set it according to #J. A
Set GC and A'TC. In this case as well, the station equipment (optical transmitter) It is possible to transmit a burst signal from 2 and obtain the reception level and time division multiplexing schedule using the same procedure as shown in FIG.

[Effects of the Invention] The present invention utilizes the fact that the optical reception level and time division multiplexing schedule are known in advance to economically realize AGC and ATC that respond quickly to large changes in the reception level. It is possible. As a result, it is possible to stably realize a communication system in which a transmission path is shared using a star coupler, etc., and an economical optical communication system can be realized.

[Brief explanation of drawings]

FIG. 1 is a diagram for explaining changes in the optical reception level according to the first principle of the present invention, and FIG. 2 is a diagram for explaining the setting state of the threshold value according to the second principle of the present invention. FIG. 3 is a diagram illustrating a circuit configuration that is an implementation means based on the first principle of the present invention, and FIG. 4 is a diagram explaining another circuit configuration that is an implementation means based on the first principle of the present invention. Figures 6 and 7 illustrate circuit configurations that are implementation means based on the second principle of the present invention. Figures 6 and 7 show that some subscribers sharing the star coupler have already FIG. 8 is a diagram explaining how to know the reception level and time division multiplexing schedule when a new subscriber starts a new service. FIG. 8 shows the conventional two-way communication method. FIG. 9 is a diagram explaining a method of communicating using a star coupler, and FIG. 10 is a diagram showing the time position relationship of each subscriber's signal when communicating using FIG. 9. Fig. 1 1... Optical fiber transmission line, 2... Terminal equipment, 3... Station equipment, 4... Star coupler 5...・Photoelectric conversion circuit, 6...
Amplifier, 7...Feed amplifier 9. resistance group, 8.
...High speed switch, 9...Control circuit, 1
0...Variable resistance, 11...Control circuit,
12...Amplifier, 13...Comparator, 1
4... Threshold generation circuit, 15... Control circuit. Figure 2 Figure 5 Figure 3 Figure 4 Figure 7 Figure 8

Claims (1)

[Claims] In an optical communication system that performs time-division multiplexing communication between one station device and multiple terminal devices using an optical communication network connected by optical fibers and star couplers, the received level and reception level for each optical transmission path obtained in advance are Based on the time division multiplex schedule,
An optical reception system characterized by changing the gain of a light receiving circuit or the threshold value of an identification circuit for each time slot corresponding to each subscriber.