JPH1093534A - Optical transmission method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the optical transmission method in which the multiplicity of optical signals arranged in a transmission wavelength band is increased and the number of stations sending a transmission signal is increased by relaxing or eliminating a spread of a beat signal due to chirping toward a frequency direction so as to make a signal wavelength interval narrow. SOLUTION: In this method, two optical signals whose wavelength differs obtained by modulating a light from a light source 1 with an external modulator 3 are sent through one optical transmission line 5 and the two optical signal waves or over are received by a light receiving device 6. The two optical signal waves or over take a wavelength interval where a center frequency of the beat signal generated depending on the wavelength difference is selected to be a multiple of four of a maximum frequency in the transmission band.

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 for multiplexing and transmitting optical signals having different wavelengths through a single optical transmission line. The present invention relates to an optical transmission method which is used when a single optical transmission line is transmitted to a central office through a single optical transmission line to monitor and manage multipoints.

[0002]

2. Description of the Related Art An optical transmission method shown in FIG. 5 is known as a method for transmitting a plurality of optical signals through an optical transmission line comprising one optical fiber. In the optical transmission method of FIG. 5, the signal from the modulator 11 of each terminal is directly modulated by the light source 12 (E / O converter: LD, for example), and the modulated light is combined by the optical coupler 13. The multiplexed optical signal is transmitted to one optical fiber 14 and transmitted to the central office. The multiplexed optical signal is received by the optical receiver 15 of the central office, detected, distributed to two or more signals, and demodulated by the demodulator 16 as data and video signals. In FIG. 5, reference numeral 17 denotes a television monitor.

In the transmission of a plurality of signals having different wavelengths, a beat signal is generated in the optical receiver due to mutual interference between two or more optical signals having different wavelengths. The frequency band in which the beat signal is generated corresponds to the wavelength difference between two or more optical signals transmitted through one optical fiber. For example, when two waves of λ ₀ and λ ₁ are transmitted as shown in FIG. 6A and no modulation is performed, a beat signal appears as shown in FIG. 6B. Here, when the modulation signal is input to the E / O converter, chirping occurs in which the beat signal spreads in the frequency direction as shown in FIG. Therefore, when the above-mentioned beat signal is generated in the same frequency band as the transmission signal, the quality of the transmission signal is significantly deteriorated. In order to prevent the transmission signal from deteriorating due to the beat signal, it is necessary to sufficiently separate the wavelength intervals. At the same time, a technique for controlling the wavelength of the light source to be constant is required.

[0004]

In a method of directly modulating an E / O converter, there is a phenomenon (chirping) in which the frequency of a light source, for example, a DFB laser vibrates in the frequency direction. When the optical signal is modulated by a direct modulation method, the beat signal spreads in the frequency direction, so that when the transmission signal is in the same frequency band as the beat signal, the transmission signal deteriorates. That is, when the direct modulation is performed as shown in FIG. 6C, the frequency band of the beat signal is expanded by chirping, as compared with the beat signal when the light source is not directly modulated as shown in FIG. 6B. For this reason, when defining the wavelength interval, the chirping of the beat signal, which is a characteristic of the light source, has an effect, and the narrowing of the wavelength interval is restricted. Since the wavelength resources used for optical transmission are finite, increasing the number of transmission stations has been limited.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problem, and reduces or eliminates the spread of a beat signal in the frequency direction due to chirping, and narrows the wavelength interval of the signal to reduce the number of multiplexed optical signals that can be arranged in the transmission wavelength band. It is an object of the present invention to provide an optical transmission method capable of increasing the number of stations transmitting transmission signals.

[0006]

The present invention has the following means to solve the above problems.

According to a first aspect of the present invention, there is provided an optical transmission method comprising: transmitting two or more optical signals having different wavelengths obtained by modulating light from a light source by an external modulator through a single optical transmission line; An optical transmission method for receiving an optical signal at a photodetector, wherein the two or more optical signals have a wavelength at which the center frequency of a beat signal generated according to the wavelength difference is at least four times the maximum frequency of the transmission band. It is characterized by an interval.

The optical transmission method according to a second aspect of the present invention is characterized in that the external modulator is a modulator utilizing Mach-Zehnder interference.

According to the optical transmission method of the present invention, the optical signal of two or more waves is modulated by the external modulator, and the center frequency of the beat signal generated according to the wavelength difference is at least four times the maximum frequency of the transmission band. Since the wavelength spacing of the signal is reduced or eliminated, the spread of the beat signal in the frequency direction due to chirping is reduced, and the multiplexing number of optical signals that can be arranged in the transmission wavelength band is increased by reducing the wavelength spacing of the signal. Can be increased.

That is, the beat signal is an optical signal of two or more waves.
(Wavelength λ₀, Wavelength λ₁) Is generated according to the wavelength difference. Departure
The frequency of the generated beat signal is based on the logic expressed by the following equation.
Is determined. E₀(T) = A₀sin (ω₀t) Detected by the receiver
Wavelength λ₀Power E₁(T) = A₁sin (ω₁t) Detected by the receiver
Wavelength λ₁The power of に よ り E₀(T) + E₁(T)｝^Two= A₀ ^Twosin^Two(Ω
₀t) + A₁ ^Twosin^Two(Ω ₁t) + A₀A₁｛Cos (ω
₀−ω₁) T-cos (ω₀+ Ω₁)｝ E: electric field of light, A: constant, ω: wavelength of light, t: time, and the difference between both wavelengths (ω₀−ω₁) Is in Fig. 6 (b)
It appears as a beat signal.

For example, if the optical signal λ in the optical transmission band is 1.55
In the μm band, a beat signal is generated with a center frequency of 125 GHz at a wavelength difference of light of 1 nm. At this time, the sum of the two wavelengths (ω ₀ + ω ₁ ) also becomes a beat signal, but its generation frequency is as high as 10 ¹⁴ GHz, which is out of the optical transmission band, and poses no problem. In the conventional method of directly modulating the E / O converter, the beat signal spreads in the frequency direction due to chirping as shown in FIG. 7, so that it is necessary to define the wavelength interval at a position where the maximum frequency of the transmission band does not deteriorate. Was. On the other hand, in the modulation by the external modulator according to the optical transmission method of the present invention, the spread in the frequency direction as shown in FIG. 7 is not observed, but the peak corresponding to the fundamental wave of the transmission frequency and the second harmonic are supported. Peaks and peaks corresponding to the third harmonic are generated. However, a wavelength interval may be defined so that no peak corresponding to a higher-order frequency appears any more, and the maximum frequency of the transmission band does not approach the beat center frequency more than the peak corresponding to the third harmonic of the beat signal.

For example, specifically, the transmission band of the external modulator is about 3 GHz, and assuming a modulation degree of about 90%, the maximum frequency of the transmission band of 3 GHz and the third order of the beat signal are obtained in the transmission method by the external modulation. As a wavelength interval at which the peak corresponding to the harmonic does not become the same frequency and does not enter the direction of the center frequency of the beat signal, the center frequency of the beat signal is four times the maximum frequency 3 GHz of the transmission band, that is, 12 GHz or more. The interval is 0.1 nm or more. When the maximum frequency of the transmission band is a low frequency, for example, when there is a boundary of required quality at a frequency 4 GHz away from the center frequency of the beat signal, the maximum frequency of the transmission band needs to be arranged at least 4 GHz away from the center frequency of the beat signal. is there. Now, if the maximum frequency of the transmission band is 1 GHz, the center frequency of the beat signal is 5 GHz or more and the wavelength interval of two or more waves is 0.04 nm or more in order to transmit the signal with required quality.

DF which is a light source for light transmission which is usually used
The wavelength variable width of the B laser is specified by the temperature and is 0.1 n at 1 ° C.
m, and the temperature variable width is usually ± 5 ° C., so that the actual wavelength variable ability is 1 nm. Here, when the number of transmission terminal stations is set to ten and the wavelengths are arranged at equal intervals, the wavelength interval becomes 0.1 nm or less. That is, the wavelength interval is set to 0.04 nm or more.
If the thickness is set to 1 nm or less, transmission becomes possible without deteriorating the transmission quality. As described above, when an external modulator is used, chirping is basically eliminated, the wavelength interval is narrowed,
There is no need to perform wavelength selection of the light source.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an optical transmission method according to the present invention will be described in more detail with reference to FIGS. FIGS. 1A, 1B, and 1C are explanatory views showing an embodiment of an optical transmission method according to the present invention, wherein outputs from light sources 1 (E / O converters: for example, DFB lasers) of a plurality of terminal stations are provided. The modulated light is driven by a transmission signal from the external modulation signal source 2 to drive, for example, an external modulator 3 including a modulator utilizing Mach-Zehnder interference, and the optical signal is intensity-modulated. The optical signals are multiplexed by the coupler 4 and sent to one optical transmission line (optical fiber) 5 to transmit the multiplexed optical signal to the central office. The multiplexed optical signal is received and detected by the optical receiver 6 of the central office, then distributed into two or more signals, demodulated by the demodulator 7 as data and video signals, and received by, for example, the monitor television 8. is there.

1A is a bus type, FIG. 1B is a star type,
(C) shows a bus star type system configuration, and the optical transmission method of the present invention can be applied to any system configuration. In each system configuration of the optical transmission method shown in FIG. A beat signal is generated. If a beat signal is generated in the frequency band of the modulated signal sent from the modulator 3, the transmission quality is significantly deteriorated. The frequency at which the beat signal is generated is determined according to the wavelength difference of the optical signal multiplexed on the optical transmission line 5, and the center frequency of the beat signal is generated at a position of 125 GHz for a wavelength interval of 1 nm.

As described above, when light is modulated by external modulation, peaks corresponding to the fundamental wave, the second harmonic, and the third harmonic appear as shown in FIG. At this time, if the maximum frequency of the transmission signal becomes the same frequency as the peak corresponding to the third harmonic of the beat signal, the transmission quality deteriorates. FIG.
As shown in the figure, the beat signal has peaks corresponding to the beat center frequency and the fundamental wave, the second harmonic, and the third harmonic of the transmission signal in the range up to the modulation factor of 90%. The relative intensity noise at this peak is significantly degraded. Therefore, by setting the maximum frequency of the transmission signal at a position farther from the center frequency of the beat signal than the peak corresponding to the third harmonic of the beat signal, transmission can be performed without deteriorating the transmission quality.

In the optical transmission method of the present invention, the wavelength interval is defined so as to be at least four times the maximum frequency of the transmission band with respect to the center frequency of the beat signal. For example, if the maximum frequency of the transmission band is 3 GHz, the center frequency of the beat signal is 12
GHz or more, and at this time, the wavelength interval is 0.1 nm or more. In each system configuration shown in FIG. 1, when there is a boundary of required quality at a frequency 4 GHz away from the center frequency of the beat signal as shown in FIG. 4, the maximum frequency of the transmission band is separated from the center frequency of the beat signal by 4 GHz or more. Deploy. In this case, in order to satisfy the required quality and transmit, for example, if the maximum frequency of the transmission band is 1 GHz, the center frequency of the beat needs to be 5 GHz or more. In a 1.55 μm band light source, the wavelength interval is 1 nm and the beat center frequency is 125 GHz. Therefore, the wavelength interval of two or more waves is specified to be 0.04 nm or more.

As described above, when an external modulator including a modulator utilizing Mach-Zehnder interference is used, chirping is eliminated in principle, the wavelength interval is narrowed, and the necessity of wavelength selection of a light source is eliminated. The wavelength tunable width of a DFB laser, which is a light source for optical transmission that is usually used, is defined by temperature and changes by 0.1 nm at 1 ° C. Since the temperature tunable width is usually ± 5 ° C., the actual wavelength tunable is 1 nm If the number of transmission terminal stations is 10 and the wavelengths are arranged at equal intervals, the wavelength interval is 0.1 nm or less.

As described above, in the beat signal, in addition to the peak of the beat signal, the boundary of required quality defined by the noise characteristics of the transmission signal is 4G from the center frequency of the beat signal.
Hz. Therefore, the required quality of the transmission signal is not satisfied unless the frequency is at least 4 GHz away from the center frequency of the beat signal. The boundary of the required quality due to the peak of the beat signal and the boundary of the required quality due to the noise characteristic overlap at 1.33 GHz. Below this frequency, the wavelength interval satisfying the required quality of the transmission signal is defined by the boundary of the noise characteristic of the beat signal. In the case of a frequency of 1.33 GHz or less, transmission becomes possible by setting the wavelength interval to 0.4 nm or more.

In the above embodiment, a case has been described where an external modulator uses a modulator utilizing Mach-Zehnder interference. However, the modulator is not limited to a modulator utilizing Mach-Zehnder interference, and Well-known external modulators other than modulators using Mach-Zehnder interference, such as electroabsorption modulators, which are other types of external modulators other than modulators using Zender interference, can also be used. Further, in the above-described embodiment, the transmission signal has been described for a video, but it goes without saying that the transmission signal can transmit data, audio, and the like in addition to the video.

[0021]

As described above, according to the optical transmission method of the first aspect of the present invention, the optical signal of two or more waves is modulated by the external modulator, and the beat signal generated according to the wavelength difference is modulated. Since the center frequency is at least four times the wavelength interval of the maximum frequency of the transmission band, the spread of the beat signal in the frequency direction due to chirping is reduced or eliminated, and the signal wavelength interval is reduced by narrowing the signal wavelength interval. It is possible to increase the number of multiplexable optical signals and increase the number of stations transmitting transmission signals.

According to the optical transmission method of the second aspect of the present invention,
The use of a modulator utilizing Mach-Zehnder interference without chirping in principle prevents the beat signal from deteriorating the required quality of the transmission signal in the communication band.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an embodiment of an optical transmission method according to the present invention.

FIG. 2 is an explanatory diagram showing a relationship between a beat signal and a transmission band when modulated by an external modulator.

FIG. 3 is an explanatory diagram showing a relationship of a relative intensity noise that changes with respect to a beat frequency.

FIG. 4 is an explanatory diagram showing a relationship between a beat signal and required quality.

FIG. 5 is an explanatory diagram showing an example of a conventional optical transmission method.

FIG. 6 is an explanatory diagram showing a relationship between a wavelength interval of an optical signal and a beat signal in a conventional optical transmission method.

FIG. 7 is an explanatory diagram showing a relationship between a beat signal and a transmission band when an optical signal is directly modulated.

[Explanation of symbols]

 Reference Signs List 1 light source 2 external modulation signal source 3 external modulator 4 optical coupler 5 optical transmission line 6 light receiver 7 demodulator 8 monitor TV

Continued on the front page (72) Inventor Nozomu Matsuo 2-6-1 Marunouchi, Chiyoda-ku, Tokyo Inside Furukawa Electric Co., Ltd. (72) Inventor Yukihisa Shinoda 1-3-1 Uchisaiwaicho, Chiyoda-ku, Tokyo Tokyo Inside Electric Power Company

Claims (2)

[Claims] 1. A light for transmitting two or more optical signals having different wavelengths obtained by modulating light from a light source by an external modulator through one optical transmission line, and receiving the two or more optical signals by a light receiver. A transmission method, wherein the two or more optical signals have a wavelength interval such that the center frequency of the beat signal generated according to the wavelength difference is at least four times the maximum frequency of the transmission band. Optical transmission method. 2. The optical transmission method according to claim 1, wherein the external modulator is a modulator using Mach-Zehnder interference.