JPH03210840A - Optical fiber signal transmission system - Google Patents

Abstract

PURPOSE:To eliminate a distortion component caused in a modulator- demodulator circuit almost completely by providing a 2nd optical receiver of the same constitution as an optical receiver of a reception unit in parallel with a transmission unit, feeding back an output of the 2nd optical receiver to an input of the optical transmitter negatively and adding it to the optical transmit ter. CONSTITUTION:A voice signal inputted to a voice signal sum circuit 1 is synthe sized with an output from a voice signal feedback amplifier 5 to correct the distortion caused by an optical fiber signal transmission system. A 1st optical receiver 1 used for a transmission unit 10 and a 1st PLL receiver 8 are of the same internal structure as a 2nd optical receiver 31 and a 2nd PLL receiver 32 used in the reception unit. An output of the optical transmitter 3 is sent to 1st and 2nd optical fibers 8, 20 through an optical branch device 4.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an improvement in an electrical signal transmission system using an optical fiber, and particularly to a distortion correction method using negative feedback of an optical signal.

(Prior Art) Conventionally, systems for transmitting various signals such as audio signals using optical fibers are publicly known, and include analog transmission methods and pulse code modulation transmission methods (i.e., PCM transmission methods).
This is realized by

Typically, such a system is configured with an optical signal transmitting section, a cable, and an optical receiving section as main constituent circuits.

The optical signal transmitter inputs a signal such as an audio signal or digital data, performs appropriate processing on the signal, performs electro-optical conversion, and outputs it as an optical signal.

For example, it is extremely common to input an audio signal, generate a carrier with a frequency higher than twice the frequency of the audio signal, and add the audio signal to this carrier as a modulating wave to perform frequency modulation.

The carrier is in the form of a pulse signal, and the pulse frequency is frequency-modulated with a modulation signal. The frequency-modulated pulse modulation signal is passed through a waveform shaping circuit to a light emitting diode (LED) or a laser diode (L).
D) and other light emitting devices. Through such processing, a frequency modulated optical pulse signal can be obtained.

The optical pulse signal is sent to the optical receiver through a CI type or plastic type optical fiber, and the optical pulse signal sent through the optical fiber is input to the optical receiver. In the optical receiver, the optical pulse signal is converted into an electric pulse signal by an optical signal detection element such as a PIN diode. The electric pulse signal is then regenerated by demodulation, which is the reverse process of the pulse modulator built into the optical transmitter.

For example, the pulse signal may be of a phase-locked loop (PLL) type.
) is reproduced using a demodulation circuit.

The voltage controlled oscillation circuit of the PLL demodulation circuit oscillates at a frequency equal to the center frequency of the FM pulse signal, so the F
This is based on the principle that if the frequency deviation due to M is extracted as a signal, a demodulated output can be obtained.

In such an optical fiber signal transmission system, the distortion generated inside the optical transmitter directly appears in the output of the optical receiver, so the distortion rate is relatively large.

In order to improve this distortion rate, negative feedback may be applied inside the transmitter.

(Problems to be Solved by the Invention) As described above, in the conventional technology, it is possible to improve the distortion factor by applying negative feedback inside the optical transmitter. However, this method has the disadvantage that only the distortion generated on the transmitting side can be corrected.

On the other hand, although it is theoretically possible to return the output of the optical receiver to the input of the optical transmitter via a cable, it is not possible in reality. This is because the optical transmitting section and the optical receiving section are connected by an optical cable, and the length of the optical cable is generally long, and can reach more than 1 km.

Further, due to the nature of optical cables, it is impossible to transmit electrical signals as they are, so an electrical signal line for negative feedback must be provided.

Furthermore, when such a configuration is adopted, the amount of phase rotation of the signal due to the negative feedback loop generally increases, and in some cases,
This is because parasitic oscillation may occur.

An object of the present invention is to provide a second optical receiver having the same configuration as the optical receiver in parallel on the transmitting side, and return the output of the second optical receiver to the input of the optical transmitter by negative feedback. It is an object of the present invention to provide an optical fiber type signal transmission system configured to eliminate the above-mentioned drawbacks by adding an optical fiber to the optical transmitter and to simultaneously correct the distortion caused in the optical transmitter and the optical receiver.

(Means for Solving the Problems) An optical fiber signal transmission system according to the present invention includes an optical signal transmitter, an optical shunt, a first optical fiber, a second optical fiber, and a first optical receiver. 1, a second optical receiver, and a summation circuit.

The optical signal transmitter is for processing input electrical signals, converting them into electrical and optical signals, and transmitting the electrical signals in the form of optical signals.

The optical shunt divides the optical signal output from the optical signal transmitter and sends it to the first and second paths.

The first optical fiber is for transmitting the first optical signal obtained from the first path of the optical shunt to the first optical receiver.

The second optical fiber is for transmitting the second optical signal obtained from the second path of the optical shunt to the second optical receiver.

The first optical receiving means receives the first optical signal sent from the optical signal transmitter through the first path from the first optical fiber, converts it into a corresponding electrical signal, processes it, and inputs the signal. This is for outputting a signal similar to the signal.

The second optical receiving means has an internal configuration similar to that of the first optical receiving means, and receives the second optical signal that has arrived from the second optical fiber through the second optical path. It converts into an electrical signal, processes it, and outputs a signal similar to the input signal.

The sum circuit means adds the output signal of the first optical receiving means with opposite polarity to the electrical signal input to the optical signal transmitting section, and returns the signal by the optical signal transmitting section, the optical shunt, and the first optical receiving means. It is for configuring a loop.

(Example) The present invention will be described in more detail below with reference to the drawings and the like.

FIG. 1 shows a PL for transmitting video signals according to the present invention.
1 is a block diagram showing an overview of an embodiment of an optical fiber signal transmission system using L. FIG.

In Fig. 1, 1 is an audio signal summation circuit, 2 is a PLL type V
C0, 3 is an optical transmitter, 4 is an optical shunt, 5 is an audio signal feedback amplifier, and 6 is a first PLL type receiver.

7 is a first optical receiver, 8 is a first optical fiber, 10 is a transmitting unit, 20 is a second optical fiber, 30 is a receiving unit, 31 is a second optical receiver, 32 is a second PLL It is a type receiver.

The optical fiber signal transmission system according to the present invention includes a transmitting unit 10, a second optical fiber 20, and a receiving unit 30. The transmission unit 1-10 includes an audio signal summation circuit 1, a PLL type VC02, an optical transmitter 3,
an optical shunt 4, a first optical fiber 8, an optical receiver 7,
It consists of a PLL type receiver 6 and an audio signal feedback amplifier 5.

The receiving unit (second receiving means) 30 includes a second optical receiver 31 and a second PLL type receiver 32. The optical transmitter includes a PLL type VCO 2 and an optical transmitter 3. The first optical receiving means includes a first optical receiver 7 and a first PLL type receiver 6. The sum circuit means is composed of an audio signal sum circuit 1 and an audio signal feedback amplifier 5.

FIG. 2 is a circuit block diagram showing an embodiment of the summation circuit means of the optical transmitting unit shown in FIG. 1.

In FIG. 2, 101 is an LPF for isolating the audio signal input side and the inside of the audio signal summation circuit 1, and 102 and 103 are for mixing the input audio signal and the feedback audio signal, respectively. The resistors 104 and 105 are amplifiers.

Further, 501 is an amplifier provided inside the audio signal feedback amplifier 5. Amplifiers 104, 105 and 501
are equipped with feedback resistors 106-108, 502 and 503 to keep the distortion factor extremely low.

The audio signal input to the audio signal summation circuit 1 is processed to correct distortion caused by the optical fiber signal transmission system.
It is combined with the output from the audio signal feedback amplifier 5.

First optical receiver 7 used in transmitting unit 10
and the first PLL receiver 6 are of the same type with the same internal structure as the second optical receiver 31 and the second PLL receiver 32 used in the receiving unit, respectively.

The PLL type VCO2 uses a PLL type VCO that operates at +5 volt DC, and outputs the audio input signal to the corresponding PF.
Convert to M signal. The output of PLL type VC02 is TTL
level and is sent to an optical transmitter 3 (easily available on the market).

The output of the optical transmitter 3 is sent through an optical shunt 4 to first and second optical fibers 8.20.

The optical splitter 4 is for sending the optical power from the optical transmission "a3" to both the second optical fiber 20 and the first optical receiver 7 in the transmission unit 10. The optical power input to the first transmitter unit IO is
is sent to the optical receiver 7.

A second optical receiver 31 in the receiving unit 30 converts optical pulses into electrical pulses, and the electrical pulses are further demodulated by a second PLL receiver 32 . The electrical pulse output from the optical receiver 31 is converted into an audio signal having a value equal to the output of the first PLL receiver 6 in the transmitting unit 10 by demodulation.

Optoelectric feedback loops were established to reduce overall distortion in fiber optic signal transmission systems.

Therefore, the loop includes both electrical and optical circuits.

If the optical pulse signal transmitted from the optical transmitter 3 to the optical receiver 7.31 is distorted, the sound appearing at the output of the first PLL receiver 6 and the output of the second PLL receiver 32 This will cause distortion in the signal. However, since the same type of circuitry used in the receiving unit 30 is also used in the feedback loop within the transmitting unit 10, the same modes of distortion that occur in the transmitting unit 10 are present in the receiving unit 30. This is removed by feedback.

In the transmitting unit 10, distortion of the audio signal is mainly caused by PL.
This occurs in the L type VCO 2 and the first PLL type receiver 6.

If the second optical receiver 31 and the second PLL type receiving circuit 32 are not attached to the receiving unit node 30, distortion of the audio signal will mainly occur in the PLL type VC02. A feedback loop is not formed (if an audio signal is input and its amplitude is large, the distortion of the audio signal occurring in the PLL VCO 2 will increase.The output of the PLL VCO 2 will establish feedback using a normal electrical signal). Since the signal is fed back to the audio signal summation circuit 1 for processing the signal, the distortion of the signal output from the PLL type VCO 2 is reduced.

However, even if the distortion of the transmitting unit 10 is corrected, distortion will occur in the output of the second PLL circuit 32 of the receiving unit 730 if the amplitude of the input audio signal is large. This type of distortion can occur unless a negative feedback loop is established within the PLL receiver 32 of the receiving unit 30.
It will not be corrected. If a negative feedback loop is established within the PLL receiver 32, the signal voltage at the received output will decrease and an additional amplifier will be required to increase the output signal voltage. However, this amplifier
This increases manufacturing costs and complicates the circuit configuration of the receiving unit 30. Therefore, it goes without saying that it is necessary to avoid distortion variations inherent in these internal amplifications.

The second optical receiver 31 and PLL receiver 32 installed in the receiving unit 30 between the input and output increases the manufacturing cost, but it is necessary to install the second optical receiver 31 and the PLL receiver 32 in the receiving unit 30 between input and output.
As shown, by cascading two or more receivers to a transmitter, the overall system cost remains the same.

In FIG. 3, 10 is a transmitter Unison), 201 to 20
4 is an optical fiber, and 301 to 304 are receiver units.

In the system configuration, an optical receiver and a PLL receiver having the same internal configuration as used in the receiving unit 30 in FIG. 1 are used in each of the receiving units 301 to 304.

If the same audio signal waveform that occurs at the output of audio signal feedback amplifier 5 is seen at the outputs of receivers 301, 302, 303 and 304, the distortion occurring in the fiber optic signal transmission system of FIG. 3 can be eliminated. can. In such a system, both the audio signal summation circuit 1 and the audio signal feedback amplifier 5 inside the transmitting unit 10 are designed to have an extremely low distortion value.

In the broadcast system shown in FIG. 3, the transmitting unit 10 is expensive, but the receiving unit 30
There is no problem if 1,302.303 and 304 are not expensive.

FIG. 4 is a graph showing the relationship between input and output of the embodiment shown in FIG. 1.5v p-p for input and output signals
Linearity is maintained up to a certain point, and distortion is reduced.

FIG. 5 is a graph showing the frequency response characteristics of the embodiment shown in FIG. 1, in which the frequency around 20 KHz is 13 dB.
Sufficient responsiveness is ensured.

FIGS. 6 and 7 are graphs comparing the distortion rates when using the transmitting unit with the feedback loop shown in FIG. 1 and when using the transmitting unit without feedback. In FIG. 6, according to the system according to the present invention, the voltage can be reduced to approximately 1% regardless of the output voltage. In the conventional example, the distortion rate is large, ranging from 2% to 4.5%, depending on the output signal voltage.

FIG. 7 shows the distortion rate up to a frequency of 12 KHz. According to the present invention, the distortion rate is around 1% up to a signal frequency of around 12 KHz, but in the conventional example, it is around 3% to 5.
．． It has increased to 5%.

(Effects of the Invention) As described above, the present invention provides a second optical receiver having the same configuration as the optical receiver of the receiving unit in parallel with the transmitting unit, and transmits the output of the second optical receiver to the optical receiver. By giving negative feedback to the input of the transmitter and adding it to the optical transmitter, it is possible to stably and comprehensively correct the distortion generated in the optical transmitter and optical receiver as a system, so it is possible to correct the distortion generated in the modulation/demodulation circuit. This has the effect of almost completely eliminating distortion components.

Furthermore, since the present invention uses an optical cable to transmit signals, there is an advantage that parasitic signals caused by direct feedback of electrical signals, such as those caused by phase rotation that occur at high frequencies, are not generated.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of an optical fiber signal transmission system according to the present invention. FIG. 2 is a block diagram showing an embodiment of the audio signal summation circuit and the audio signal feedback amplifier of the optical transmission unit shown in FIG. FIG. 3 is a block diagram showing an example of cascade-connected transmitting/receiving units configured according to the present invention. FIG. 4 is a graph showing an example of the linearity between input and output obtained in FIG. FIG. 5 is a graph showing an example of the frequency response characteristics obtained in FIG. 1. FIG. 6 is a graph showing an example of the change in the distortion rate obtained in FIG. 1 with respect to the input level. FIG. 7 is a graph showing an example of a change in the distortion factor obtained in FIG. 1 with respect to frequency. 1...Audio signal sum circuit 2-PLL type VCO 3...Optical transmitter 4...Optical shunt 5...Audio signal feedback amplifier 6.32...PLL type receiver 7.31 ... Optical receiver 8.20, 201-204 ... Optical fiber 30.30
1-304...Receiving unit 101...LPF 104.105.501...Amplifier 102.103.106-108,502°503...
·Resistor

Claims (1)

[Claims] an optical signal transmitting section for processing input electrical signals, converting them into electrical signals and transmitting them in the form of optical signals; and dividing the optical signals outputted from the optical signal transmitting section and sending them to first and second paths. an optical shunt for transmitting, and a first optical fiber for transmitting the first optical signal obtained from the first path;
a second optical fiber for transmitting a second optical signal obtained from the second path; and a first optical fiber sent from the optical signal transmitter through the first path; Said first
a first receiving signal from the optical fiber, converting it into a corresponding electrical signal, processing it, and outputting a signal similar to the input signal;
has an internal configuration similar to that of the first optical receiver, and receives a second optical signal that has arrived from the second optical fiber through the second optical path. a second optical receiving means for converting and processing the electrical signal into an electrical signal and outputting a signal similar to the input signal; and the first optical receiving means for the electrical signal input to the optical signal transmitter. 1. An optical fiber signal transmission system comprising: sum circuit means for adding an output signal of the means with opposite polarity to form a feedback loop.