JPH0522226A - Optical communication equipment - Google Patents

Abstract

PURPOSE:To relax the effect of a relevant distortion component caused from a frequency modulation carrier in the distortion signal component caused by nonlinearity of a light source in the optical communication equipment in which a frequency division multiplex signal including the frequency modulation carrier is converted into an optical signal and sent. CONSTITUTION:A spread signal generating circuit 2 applies a spread signal used for distributing uniformly a power spectrum distribution of a distortion signal component from a frequency modulation carrier outputted from a voltage controlled oscillator 3 without being concentrated on a specific frequency to a synthesis circuit 1 receiving a transmission signal, the signals are synthesized in the circuit 1 as a modulation input to the voltage controlled oscillator 3 being a frequency modulator, the result is outputted as a frequency modulation carrier and it is synthesized with other modulation carriers at a synthesis circuit 4 and the output of the circuit 4 is converted into an optical signal at a light source 5. In this case, the effect of the distortion signal component generated from the frequency modulation carrier is relaxed by insertion of the spread signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication device for transmitting a channel image signal or the like.

[0002]

2. Description of the Related Art Conventionally, an apparatus of this type has been shown in FIG. This figure shows Olshansky et al., “Subcarrier Multiplexed”
Lightwave Systems for Bro
adban Distribution "IEEE
JOURNAL OF LIGHTWAVE TE
CHNOLOGY, VOL. 7, NO. 9 133
The structure of the transmission side of the FM or FSK subcarrier multiplexed optical image transmission device of page 0 is shown. In FIG. 2, reference numeral 3 is a voltage controlled oscillator (modulator) for inputting an analog image signal or a digitized image signal and generating a frequency modulation (FM) or frequency shift keying (FSK) carrier signal. 4 is a multiplexing circuit for synthesizing a plurality of modulated carrier waves, and 5 is a laser diode for converting a frequency division multiplexed signal into an optical signal. Reference numeral 6 is a single mode optical fiber which is a transmission medium. As a transmission medium, it is also possible to propagate space other than an optical fiber. FIG. 3 shows the frequency spectrum of a frequency division multiplexed signal of a modulated carrier.

Next, the operation will be described. The analog or digitized video signal is input to the voltage controlled oscillator 3. Since the oscillation frequency of the voltage controlled oscillator 3 changes according to the input voltage, a frequency modulation signal can be obtained. Next, this signal is combined with other frequency-modulated carriers (f _{1 to} f _N ) obtained in the same manner and having different oscillation center frequencies in the multiplexing circuit 4 to obtain a frequency division multiplexed signal.
A transmission optical signal is obtained by directly driving the laser diode of the light source 5 with this signal. The transmitted optical signal is transmitted to the receiving side by the optical fiber 6 which is a transmission medium.

[0004]

The transmission side of the conventional optical transmission device for frequency division multiplexed signals including frequency modulated carrier waves is configured as described above, and when the drive current of the light source 5 is converted into an optical signal. Distorted signal components are generated in the output optical signal due to the generated non-linear characteristics, and if they fall into the transmission band of each modulated carrier, they become interference signals, which is a serious problem in transmission characteristics. The spectrum of the distortion signal component caused by the non-linearity of the light source is similar to the spectrum of the fundamental carrier. Next, referring to FIG. 4 and FIG.
Will be described in detail.

FIG. 4 shows the relationship between one wave and a distorted signal component among a plurality of frequency multiplexed carriers. Actually, the second-order and third-order distortion signal components are problematic in terms of level. Since the spectrum form of the distortion signal component caused by the non-linearity of the light source is similar to the spectrum form of the fundamental carrier, the level difference between the two becomes a D / U ratio, and the distortion signal component is as small as possible, that is, D / U. It is desirable in view of transmission characteristics that the U ratio is as large as possible, and the main signal is, for example, VSB-for transmitting an image.
60 dB or more for AM modulation, 3 for FM modulation of image
0 dB or more is required. FIG. 4 shows the case where the distortion signal component is a non-modulated bright line spectrum component, and the condition is the severest.

FIG. 5 shows a spectrum of a distortion signal component generated from an FM modulation signal when the modulation signal is a digital binary signal (FSK), and FIG. 6 is a case where the modulation signal is a normal NTSC format analog image signal (amplitude component). Indicates the largest horizontal synchronizing signal). In general, when the modulation index of an FM modulated signal is large to some extent, the total signal power becomes constant, and the power is redistributed into a carrier wave and its sidebands depending on the condition of the modulated signal.

When the modulation signal of FIG. 5 is a digital binary signal, the electric power is concentrated as a bright line spectrum at two specific frequencies corresponding to a mark and a space, so that the interference interference level becomes a severe condition. When the modulation signal of FIG. 6 is a normal NTSC format analog image signal, since the spectrum distribution of the modulation image signal is mostly concentrated in the frequency lower than the horizontal synchronizing frequency of 15.75 kHz, the modulation distortion component also has a center frequency. The spectrum distribution is concentrated in the vicinity, and the level of the interference becomes the same severe condition.

The present invention has been made in order to solve the problem of the influence of the distorted signal component generated in the light source in the case of converting the frequency-division-multiplexed signal into an optical signal for communication as described above. An object of the present invention is to obtain an optical communication device capable of reducing the influence of interference interference from the frequency modulation signal when the frequency division modulation signal is contained in the division multiplexed signal.

[0009]

An optical communication device according to the present invention is provided with a voltage-controlled oscillator that outputs a frequency-modulated carrier having an oscillation frequency different in proportion to the voltage level of a transmission information signal, and an output of the voltage-controlled oscillator. In an optical transmitter equipped with a multiplexing circuit that synthesizes another frequency-modulated carrier wave with a different center frequency to generate a frequency-division multiplexed signal and use it as a drive signal for a light-emitting element, a spread spectrum that generates a modulated spectrum energy spread signal A signal generating circuit and a synthesizing circuit for superimposing the transmission information signal together with the modulated spectrum energy spread signal to generate a baseband signal and outputting the baseband signal to the voltage controlled oscillator are provided.

[0010]

According to the present invention, the spread signal generating circuit passes through the synthesizing circuit so that the power spectrum distribution of the modulated carrier wave at the time of modulation is not concentrated in a specific partial frequency region as much as possible in the transmission information signal. By superimposing the modulated spectrum energy spread signal that becomes distributed, the influence of the distortion signal component generated from the frequency-modulated carrier wave output from the voltage controlled oscillator is mitigated.

[0011]

EXAMPLES Example 1. An embodiment of the present invention will be described below with reference to FIG.
Will be explained. Reference numeral 1 is a combining circuit for combining a digital or analog format transmission information signal as a modulated input signal and an energy diffusion signal for evenly distributing the power spectrum of the modulated carrier. 2 is a spread signal generating circuit for generating the energy spread signal, 3 is a voltage controlled oscillator (VCO) for generating a frequency modulated signal, 4 is a multiplexing circuit for combining a plurality of modulated carrier waves, and 5 is a frequency division multiplexed signal. A light source for converting into an optical signal, and 6 is an optical fiber as a transmission medium.

As a transmission medium, it is also possible to propagate a space other than an optical fiber. Reference numeral 7 is a light receiving element for converting a received optical signal into an electric signal, 8 is an amplifier for amplifying the received signal to a predetermined level, and 9 is a bandpass filter for separating a desired signal from the frequency division multiplexed signal. Reference numeral 10 is a frequency modulation signal demodulator, and 11 is a spread signal removing circuit.

Next, the operation will be described. The modulated input signal is combined with the energy spread signal in the combining circuit 1. Generally, when the modulation signal is a periodic signal and the frequency deviation is very large compared to the fundamental frequency, the spectrum has a distribution similar to the density distribution of the amplitude of the signal waveform. For example, in the case of a square wave such as the above-described digital binary signal, the energy of the spectrum of the modulated wave is concentrated near the frequency corresponding to the two amplitudes of the waveform and near the frequency corresponding to this amplitude.

FIG. 7 shows a case of a triangular wave which is a typical example of the energy spread signal. With a triangular wave, the intensity becomes almost constant within the band where the frequency shift is given. Therefore, the total modulated signal power is evenly distributed within the band (the integrated value of the power distribution within the band matches the carrier power during non-modulation), and the level at the frequency at each point becomes the lowest.

As shown in FIG. 6 or 7, the spectrum power distribution is concentrated on a specific frequency for a specific modulation input, whereas the spectrum is distributed when the energy spread signal is superimposed. Since the power distribution is evenly distributed, the peak value of the spectrum power distribution, which is a problem with the distortion signal component, can be reduced, and the D / U ratio can be improved.

In this way, the frequency modulation signal is generated by the voltage controlled oscillator 3 by the modulation signal on which the energy diffusion signal is superimposed. Next, this signal is combined with another modulated carrier wave in the multiplexing circuit 4 to generate a frequency division multiplexed signal. This signal is converted into an optical signal by the light source 5 and transmitted by the optical fiber 6. The transmitted optical signal is converted into an original electric signal by the light receiving element 7 and amplified to a predetermined level by the amplifier 8. Next, the amplified frequency division multiplexed signal is 9
After passing through the band pass filter 9 of 1, the frequency modulation signal is separated and demodulated by the demodulator 10. The demodulated signal passes through the spread signal removing circuit 11 to remove the energy spread signal superimposed on the transmitting side and reproduce the originally transmitted modulated input signal.

[0017]

As described above, according to the present invention, when a frequency division multiplexed signal of a plurality of carrier waves modulated by various modulation systems is converted into an optical signal and transmitted, the light source for converting an electric signal into an optical signal is used. There is an effect that the influence of the interference interference generated from the frequency-modulated carrier wave among the distortion signal components generated from the nonlinearity can be significantly improved.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical transmission system of a frequency division multiplexed signal including a frequency modulation signal according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a transmission side of an optical transmission system of a frequency division multiplexed signal including a conventional frequency modulation signal.

FIG. 3 is an array diagram showing a frequency array of a conventional frequency division multiplexed signal.

FIG. 4 is a level characteristic diagram showing a relationship between one of the frequency-division-multiplexed carrier waves and a distortion signal component caused by non-linearity of a light source.

FIG. 5 is a spectrum characteristic diagram showing a modulation spectrum of a distortion component generated from a carrier frequency-modulated by a binary digital signal.

FIG. 6 is a spectrum characteristic diagram showing a modulation spectrum of a distortion component generated from a carrier frequency-modulated by an NTSC video signal (horizontal synchronizing signal).

FIG. 7 is a spectrum characteristic diagram showing a modulation spectrum of a carrier frequency-modulated by a triangular wave that is a typical example of an energy spread signal.

[Explanation of symbols]

 1 Synthesis Circuit 2 Spreading Signal Generation Circuit 3 Voltage Controlled Oscillator 4 Multiplexing Circuit 5 Light Source 6 Optical Fiber 7 Photoreceptor 8 Amplifier 9 Bandpass Filter 10 Frequency Modulation Demodulator 11 Spreading Signal Remover

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[Procedure amendment]

[Submission date] September 13, 1991

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 1

[Correction method] Change

[Correction content]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0001

[Correction method] Change

[Correction content]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication device for transmitting, for example, multi- channel image signals.

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] 0003

[Correction method] Change

[Correction content]

Next, the operation will be described. The analog or digitized video signal is input to the voltage controlled oscillator 3. Since the oscillation frequency of the voltage controlled oscillator 3 changes according to the input voltage, a frequency modulation signal can be obtained. Next, this signal is combined with another frequency-modulated carrier wave ( f _{2 to} f _N ) obtained in the same manner and having a different oscillation center frequency in the multiplexing circuit 4 to obtain a frequency division multiplexed signal.
A transmission optical signal is obtained by directly driving the laser diode of the light source 5 with this signal. The transmitted optical signal is transmitted to the receiving side by the optical fiber 6 which is a transmission medium.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0009

[Correction method] Change

[Correction content]

[0009]

An optical communication device according to the present invention is provided with a voltage-controlled oscillator that outputs a frequency-modulated carrier having an oscillation frequency different in proportion to the voltage level of a transmission information signal, and an output of the voltage-controlled oscillator. to synthesize other modulation carrier waves different center frequency, and generates a frequency division multiplexed signal, in the optical transmitting apparatus and a multiplexing circuit for the drive signal of the light-emitting element to generate a modulated spectrum energy spread signal spread A signal generating circuit and a synthesizing circuit for superimposing the transmission information signal together with the modulated spectrum energy spread signal to generate a baseband signal and outputting the baseband signal to the voltage controlled oscillator are provided.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Correction content]

In this way, the frequency modulation signal is generated by the voltage controlled oscillator 3 by the modulation signal on which the energy diffusion signal is superimposed. Next, this signal is combined with another modulated carrier wave in the multiplexing circuit 4 to generate a frequency division multiplexed signal. This signal is converted into an optical signal by the light source 5 and transmitted by the optical fiber 6. The transmitted optical signal is converted into an original electric signal by the light receiving element 7 and amplified to a predetermined level by the amplifier 8. Next, the amplified frequency division multiplexed signal is 9
Passes through the band-pass filter, the frequency-modulated signal is separated and demodulated by the demodulator 10. The demodulated signal passes through the spread signal removing circuit 11 to remove the energy spread signal superimposed on the transmitting side and reproduce the originally transmitted modulated input signal.

Claims (1)

Claim: What is claimed is: 1. A voltage-controlled oscillator that outputs a frequency-modulated carrier having an oscillation frequency different in proportion to the voltage level of a transmission information signal, and another voltage-controlled oscillator whose center frequency is different from the output of the voltage-controlled oscillator. A spread signal generation circuit for generating a modulated spectrum energy spread signal in an optical transmitter provided with a multiplexing circuit for combining a frequency modulated carrier wave to generate a frequency division multiplexed signal and using it as a drive signal for a light emitting element, and the modulated signal. An optical communication device comprising: a spectrum energy spread signal and a combining circuit that superimposes the transmission information signal to generate a baseband signal and outputs the baseband signal to the voltage controlled oscillator.