JPH1168671A - Optical transmission method and its system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the transmission quality of an optical transmission signal by configuring a system of a light source and an optical amplifier so as not to deteriorate a distortion level. SOLUTION: In an optical transmission method where an intensity-modulated optical signal from a light source 14 is amplified by an optical amplifier 15 and transmitted, since deterioration in a signal distortion level is prevented independently of the number of cascade stages when a wavelength chirping amount is 100 MHz/ch or below, a light source whose wavelength chirping amount ranges from 10 MHz/ch to 100 MHz/ch is used and outputs an optical signal with respect to a prescribed intensity modulation level.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission method and apparatus for amplifying and transmitting an optical signal from a light source whose intensity has been modulated by an optical amplifier.

[0002]

2. Related Art Conventionally, this type of optical transmission is used, for example, in an optical CATV system, and an electric signal composed of information such as video and audio generated from a signal generator is modulated by a modulator. A plurality of the modulated signals to be modulated are multiplexed by a mixer, and the multiplexed signal is subjected to intensity modulation of a light source composed of a laser diode. When the intensity modulation is performed in this manner, the optical signal output from the light source with the fluctuation of the supplied current fluctuates in the wavelength direction and spreads as shown in FIG. Hereinafter, “wavelength chirping”) has occurred. After the intensity-modulated optical signal is amplified by an optical amplifier, the optical signal is distributed to several optical signals by an optical distributor and propagated to an optical transmission line including an optical fiber. After being received by the light receiver, it was demodulated by the demodulator.

[0003] The transmission quality of an optical signal transmitted in this manner is evaluated based on noise level versus signal level characteristics and distortion level versus signal level characteristics. It has been experimentally found that this distortion level deteriorates the waveform of a signal having wavelength chirping when the signal propagates through an optical fiber or an optical amplifier. In particular, since the gain of an optical amplifier has wavelength dependence, an optical signal propagates through the optical amplifier, causing distortion degradation characteristics with respect to the amount of wavelength chirping.

[0004]

However, in a conventional system, a light source is generally selected without considering the above-described wavelength chirping amount. Therefore, when a light source having a relatively large wavelength chirping amount is used, Has a problem that the distortion level is deteriorated in proportion to the number of cascade stages of the optical amplifier.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an optical transmission method and an optical transmission method for improving the transmission quality of a transmitted optical signal by configuring a system of a light source and an optical amplifier so that a distortion level does not deteriorate. It is intended to provide a device.

[0006]

In order to achieve the above object, according to the present invention, regardless of the number of cascade stages of an optical amplifier,
For a constant intensity modulation level at the modulator, 100MH _Z / c amount wavelength chirping, for example, from 10 MHz _Z / ch
The optical signal is output using the light source of h. That is, the amount of wavelength chirping 100 MHz _Z / ch Hereinafter, regardless of cascade stages, it is possible to prevent deterioration in the distortion level of the signal has been found by experimentation.

Further, when the optical amplifier is multistage cascaded, for example, when the number of cascade stages is three, the wavelength chirping amount is, for example, 10 MHz / _Z for a certain intensity modulation level.
If the output optical signal with 350MH _Z / ch source from ch, to mitigate the deterioration in the distortion level of the signal, it was found to obtain the desired use quality.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical transmission method and apparatus according to the present invention will be described with reference to FIGS. FIG.
1 is a configuration diagram showing an embodiment of a configuration of an optical transmission system using an optical transmission method according to the present invention. In the figure, an optical transmission device includes a plurality of signal generators 11a to 11d that generate electric signals having different frequencies having information such as video and audio.
Modulators 12a to 12d for modulating signals from the signal generators 11a to 11d, a mixer 13 for multiplexing the plurality of modulated signals, and an optical signal which is intensity-modulated by the multiplexed signal. A light source 14 for outputting
It comprises an optical amplifier 15 for amplifying the optical signal and an optical distributor 16 composed of an optical coupler for distributing the amplified optical signal to several optical signals. Photodetector 18a via fibers 17a to 17d
-18d, then demodulators 19a-19d
Demodulated by

In such an apparatus, the present inventors have:
For example, it has been confirmed by an experiment that when a light source having a relatively large amount of wavelength chirping is used, the distortion level deteriorates in proportion to the number of cascade stages of the optical amplifier. FIG.
FIG. 4 is a characteristic diagram showing a distortion characteristic with respect to a chirp width obtained by the above experiment. In the above experiment, for example, for a given intensity modulation level, a CSO (Composite Second Order beacon) in the case where optical amplifiers are cascaded in 0 to 3 stages.
t: difference between the signal level of the main signal and the signal level of the interference wave). In this experiment, CTB (Composite Triple Beat: 3
(The difference between the signal levels of the two signals), the deterioration of the distortion level was larger in the case of the CSO. Set.

Also, in this experiment, the signal generator
, A signal of 40 channels is generated, the modulation degree of the modulator is about 5% / ch, and the wavelength of the light source is 1560 nm.
And Further, an optical amplifier having a gain with respect to wavelength as shown in FIG. 3 is used. Therefore, in the optical amplifier, the gain for a wavelength of 1560 nm was about 22 dB.

[0011] From the experimental results, the present inventors have found that when chirp width is less 100 MHz _Z / ch does not depend on the cascade stages of the optical amplifier, CSO initial value (chirp width at 10 MHz _Z / ch Value), which is the same as the value (75 dB), and that the distortion amount is not deteriorated (see FIG. 2). Therefore,
In this embodiment, the light source 14 has a wavelength chirping amount of 100.
Using a signal of MH _Z / ch or less, a setting is made so that an optical signal is output from the light source 14 that has been intensity-modulated.

[0012] Thus, in this embodiment, regardless of the cascade stages of the optical amplifier, for a constant intensity modulation level, since setting a wavelength chirping amount below 100 MHz _Z / ch, so that the distortion level is not degraded Thus, a system of a light source and an optical amplifier can be configured. In general, the quality of use for the light source and the optical amplifier when the optical amplifier is cascaded in multiple stages, for example, three stages (distortion level of CSO)
Is about 60 dB. Therefore, as is apparent from the experimental results of FIG.
The use of those following 0MH _Z / ch, using quality 60d
B was found to be greater.

[0013] Thus, in this embodiment, the wavelength chirping amount of the light source 14 is used as follows 350MH _Z / ch, it sets the intensity-modulated modulated light source 14 to output the optical signal. Thus, in this embodiment, when the cascade stages of the optical amplifier is a multistage, for a constant intensity modulation level, 350MH _Z / c the wavelength chirping amount
Since it is set to be equal to or less than h, it is possible to alleviate the degradation of the signal distortion level and obtain a desired use quality.

Normally, in an unmodulated state, for example, D
Optical spectrum width of FB-LD (Distributed Feed Back- Laser Diode) are the order of 10 MHz _Z / ch, since the wavelength chirping can not be narrower than that, in the present embodiment, the wavelength chirping amount The lower limit is 1
It shall be set to 0MH _Z / ch. As a method of adjusting the amount of wavelength chirping, for example, a multi-electrode type laser diode (light source) disclosed in Japanese Patent Application Laid-Open No. 7-38188 is used, May be added to the phase control region or the like.

[0015]

As described above, according to the present invention, in an optical transmission method for amplifying and transmitting an optical signal from a light source whose intensity has been modulated by an optical amplifier, the optical signal does not depend on the number of cascade stages of the optical amplifier. relative intensity modulation level, 100MH _Z / ch weight wavelength chirping from 10 MHz _Z / ch
Since the optical signal is output using the light source, a light source and an optical amplifier system can be configured so that the distortion level does not deteriorate,
The transmission quality of the transmitted optical signal can be improved.

Further, if the optical amplifier were multistage cascade, for a constant intensity modulation level, since the amount of wavelength chirping outputs said optical signal by using the light source of the 350MH _Z / ch from 10 MHz _Z / ch, It is possible to alleviate the deterioration of the distortion level and obtain a desired use quality.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of a configuration of an optical transmission device using an optical transmission method according to the present invention.

FIG. 2 is a characteristic diagram showing a distortion (CSO) characteristic with respect to a chirp width.

FIG. 3 is a relationship diagram showing a relationship between gain and wavelength of an optical amplifier.

FIG. 4 is a relationship diagram showing a relationship between a transmitted optical signal level and a wavelength.

[Explanation of symbols]

 11a-11d Signal Generator 12a-12d Modulator 13 Mixer 14 Light Source (Laser Diode) 15 Optical Amplifier 16 Optical Distributor (Optical Coupler) 17a-17d Optical Fiber 18a-18d Light Receiver 19a-19d Demodulator

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/18

Claims (4)

[Claims] 1. An optical transmission method for amplifying an intensity-modulated optical signal from a light source with an optical amplifier and transmitting the amplified signal, wherein a wavelength charge is applied to a constant intensity modulation level regardless of the number of cascade stages of the optical amplifier. ping amount 100MH _Z
An optical transmission method, wherein the optical signal is output using the light source of / ch or less. 2. An optical transmission method for amplifying an intensity-modulated optical signal from a light source with an optical amplifier and transmitting the amplified signal, wherein when the optical amplifiers are cascaded in multiple stages, wavelength chirping is performed for a given intensity modulation level. the amount is 350MH _Z
An optical transmission method, wherein the optical signal is output using the light source of / ch or less. 3. An optical transmission apparatus for amplifying an intensity-modulated optical signal with an optical amplifier and transmitting the amplified signal, wherein the amount of wavelength chirping is constant for a given intensity modulation level regardless of the number of cascade stages of the optical amplifier. 100MH _Z
An optical transmission device comprising a light source of / ch or less. 4. An optical transmission apparatus for amplifying an intensity-modulated optical signal with an optical amplifier and transmitting the amplified signal, wherein when the optical amplifiers are cascaded in multiple stages, the wavelength chirping amount is 350 MHz for a given intensity modulation level. _Z
An optical transmission device comprising a light source of / ch or less.