JP2525469B2 - Jitterless optical transmission method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a digital data transmission method using an optical fiber, and more particularly to a short-distance jitterless optical transmission method.

[Conventional technology]

As an optical transmission method, in addition to standard communication using quartz fiber, a method for easily performing optical transmission over a short distance (about 100 m or less) has been developed. In this case, since the plastic fiber is mainly used as the optical fiber, the transmission characteristic is lower than that of the quartz fiber. In addition, jitter is likely to occur in the transmission of digital data due to the relationship of the wavelength or the like of the light emitter (LD or LED) on the communication side and the circuit of the light receiving unit.

[Problems to be Solved by the Invention]

As a countermeasure against jitter as described above, a double-track method has been proposed in which, in addition to an optical fiber for data transmission, another optical fiber for transmitting a clock for phase control is provided. In this method, as shown in FIG. 2, the optical fiber 3 is used for data transmission from the transmitter 10A to the receiver 10B. Data from the signal processing unit 1 is converted into an optical signal by a light emitting element (laser diode LD or light emitting diode LED) 2, and the optical fiber 3
Then, it is photoelectrically converted by the light receiving element (photodiode) 4, resynchronized by the resynchronization circuit 5, and output as reception data. Data transmission from the transmitter 10A, receiver 10B
All the data reception is performed in synchronization with the reference clock 6a which is the output of the reference clock generator 6 provided in the receiving unit 10B. Therefore, the reference clock 6a is converted into an optical signal by the light emitting element 7, transmitted through the optical fiber 8, and transmitted by the transmitter 10.
It is sent to A and reproduced by the receiving element 9 as a clock 9a. With this clock 9a, the signal processing unit 1 operates to send data, and in the receiving unit 10B, the resynchronization circuit 5 resynchronizes the data with the reference clock 6a. Even if there is a phase delay or fluctuations in the frequency of the light emitting element 2 of the transmitter 10A, the received data output from the resynchronization circuit 5 does not have jitter.

In addition to the above countermeasures, it is conceivable to use a single optical fiber, write once to the memory provided on the receiving side, and then read at the reference clock. At this time, as the transmission data, data including a clock is sent from the transmission unit, the reception unit extracts the clock, and the clock is written in the memory. Although this clock contains jitter, since the reading is performed with the reference clock, received data without jitter can be obtained.

Each of the above two methods has a drawback as a simple optical transmission method. The first method uses two optical cables,
The second method requires a large amount of RAM and has a drawback that a fixed delay occurs.

An object of the present invention is to eliminate the above-mentioned drawbacks and provide a simple jitterless optical transmission method.

[Means for solving the problem]

The optical transmission method of the present invention is provided with optical branching devices at both ends of an optical fiber, enables bidirectional optical transmission, generates a reference clock on the receiving side, and uses the reference clock or a divided clock as a phase control clock. , Sends to the optical fiber, receives the phase control clock at the transmission side, synchronizes with the phase control clock by a phase locked loop (PLL), generates a clock of the same frequency as the reference clock, and the clock at the transmission side. The transmission data is transmitted as an optical signal in accordance with the phase, and the reception side resynchronizes the reception data with the reference clock and outputs the reception data without jitter.

[Action]

The reference clock or its divided clock is sent from the receiving side to the transmitting side as the phase control clock, and the signal is transmitted from the transmitting side by the clock generated in synchronization with this phase control clock. In the data transmission from the transmitting side, jitter occurs when transmitting the optical fiber, but since the data is resynchronized by the reference clock on the receiving side, it is possible to obtain received data without jitter. Since the optical fiber has optical splitters at both ends, both data transmission from the transmission side to the reception side and clock transmission in the opposite direction can be performed by a single wire.

The reference clock itself may be used as the transmitted clock, but it is preferable to use a divided low-frequency clock because of the design of the optical branching device and the phase comparator of the PLL.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic block diagram of the embodiment. The transmission side (hereinafter referred to as the transmission interface 100A) uses an optical fiber to convert digital data from the information processing unit 11 into an optical signal.
The receiving side via 200 (in the following, receiving interface 100B
Called) and the photoelectrically converted received data is sent to a receiving side device (not shown).

In this embodiment, the reference clock source is the receiving interface.
Install on 100B. Reference numeral 23 is a reference clock generator using a crystal oscillator, which generates, for example, 2 MHz, and this reference clock 23a is divided by a frequency divider 24, and is, for example, 44 KHz. fiber
An optical branching device 20 provided at the end of the optical fiber 200 sends the optical fiber 200 to the optical fiber 200, and as shown by a dotted arrow, a transmission interface.
Transmitted towards 100A.

This optical signal clock is a photoelectric converter such as a photodiode.
15 becomes an electrical signal clock 15a by 15 and a phase comparator 1
6 Input to one terminal. This clock 15a becomes the phase control clock of the transmission interface 100A. The transmission interface 100A includes a clock oscillator 18, a frequency divider 19, a phase comparator 16, and a low-pass filter 17 to form a phase-locked loop (PLL).
And the clock oscillator 18 generates a clock 18a synchronized with the clock 15a.

The clock 18a is a clock having the same frequency as the reference clock 23a of the reception interface 100B, and is a data transmission clock from the transmission interface 100A. That is,
The transmission data from the information processing unit 11 is sent to the optical fiber 200 via the optical branching device 14 as optical signal data by the electro-optical converter 13 in synchronization with the receiving side.

The optical signal data transmitted through the optical fiber 200 in the arrow direction is converted into digital data from the optical disperser 20 and the photoelectric converter 21. Although this data 21a contains jitter due to the transfer characteristics of the optical fiber 200 and the like, since the clock of the resynchronization circuit 22 is the reference clock 23a, it is output as jitter-free data 22a.

〔The invention's effect〕

In a simple optical transmission system that uses a plastic optical fiber for short-distance communication, jitter cannot be avoided when optical transmission is performed from the transmission side to the reception side. In the present invention, the receiving side has the reference clock source, the phase information of this clock is sent to the transmitting side, the transmitting side generates a clock synchronized with the phase information of this clock, and the sending data is sent by this clock. As a result, the transmission and reception of data between the receiving side and the transmitting side are in a synchronous relationship, but phase jitter occurs due to the characteristics of the optical fiber. This jitter can be completely eliminated on the receiving side by resynchronizing the data with the reference clock.

The optical fiber is advantageous in terms of cost because it can transmit the phase information and the optical signal data by a single line instead of the double line as in the conventional example. Further, as a circuit, a large-scale memory or the like is unnecessary, and a PLL circuit, a reference clock generator, etc. are sufficient, and there is an advantage that the circuit scale is small.

[Brief description of drawings]

FIG. 1 is a schematic circuit block diagram of one embodiment of the present invention, and FIG.
The figure is a conventional example. 11 …… Information processing unit, 13,25 …… Electro-optical converter, 14,20 …… Optical splitter, 15,21 …… Photoelectric converter, 16 …… Phase comparator, 17 …… Low-pass filter, 18… … Clock oscillator, 19,24 …… divider, 22 …… resynchronization circuit, 23 …… reference clock generator, 100A …… transmit interface, 100B …… receive interface.

Claims (1)

(57) [Claims] 1. An optical transmission method for transmitting digital data from a transmission side to a reception side via an optical transmission line formed by an optical fiber, wherein an optical branching device is provided at each end of the optical fiber to enable bidirectional optical transmission. The reference clock is generated on the receiving side, the reference clock or the divided clock is sent to the optical fiber as the phase control clock, the transmitting side receives the phase control clock, and the phase is locked by the phase locked loop (PLL). In synchronization with the control clock, while generating a clock of the same frequency as the reference clock, the transmitting side sends the transmission data as an optical signal in accordance with the phase of the clock, and the receiving side resynchronizes the received data with the reference clock, A jitterless optical transmission method characterized by outputting received data without jitter.