JP2550895B2 - Optical direct amplifier - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal direct amplifier for directly amplifying an optical signal as light.

[0002]

2. Description of the Related Art In an optical communication system using an optical fiber, the intensity of an optical signal is attenuated along with the transmission distance due to the transmission loss of the optical fiber. Therefore, it is necessary to place a repeater at every predetermined distance to amplify the optical signal. In a repeater currently put into practical use, an optical signal is once converted into an electric signal, amplified, and then converted again into an optical signal and transmitted. Therefore, the relay device is complicated and expensive.

Therefore, recently, an optical direct amplifier which directly amplifies an optical signal by using a rare earth-doped fiber has attracted attention.

FIG. 7 shows the structure of an optical direct amplifier of the backward pumping type among such conventional optical direct amplifiers. The input optical signal S ₁ enters from the optical signal input end 11 and is guided to the signal light input end of the erbium-doped fiber 12. The signal light output end of the erbium-doped fiber 12 is connected to the optical signal output end 14 via the wavelength division multiplexing coupler 13.
The output optical signal S ₂ is output from here. On the other hand, the pumping light source driving circuit 15 is connected to the pumping light source 16, and the pumping light output from the pumping light source 16 is guided to the signal light output end of the erbium-doped fiber 12 via the wavelength division multiplexing coupler 13. Further, the signal light output end of the erbium-doped fiber 12 is guided to the optical signal output end 14 via the wavelength division multiplexing coupler 13. In this optical direct amplifier, the erbium ions are excited by the excitation light incident from the signal light output end of the erbium-doped fiber 12, and when the optical signal is input to the erbium ion, stimulated emission occurs and the optical signal is directly amplified. It The method in which the pumping light is incident from the signal light output end of the erbium-doped fiber is called the backward pumping method.

FIG. 8 shows the structure of an optical direct amplifier of the forward pumping type. The optical signal input terminal 11 is connected to the erbium-doped fiber 1 via the wavelength division multiplexing coupler 13.
2 is connected to the signal light input end, and the signal light output end of the erbium-doped fiber 12 is guided to the optical signal output end 14. On the other hand, the excitation light source driving circuit 15
And pumping light output from the pumping light source 16 is guided to the signal light input end of the erbium-doped fiber 12 via the wavelength division multiplexing coupler 13. This optical direct amplifier also directly amplifies an optical signal by stimulated emission by the excitation light incident on the erbium-doped fiber 12 as in the backward pumping optical direct amplifier. The method in which the pumping light is incident from the signal light input end of the erbium-doped fiber is called the forward pumping method.

Such an optical direct amplifier has problems such as improvement of amplification gain and stabilization of amplification gain, and various ideas have been made to improve them. First, as a conventional technique for improving the gain, there is a technique of setting the center wavelength of pumping light to 1.530 to 1.540 micrometers and further cooling an erbium-doped fiber as an amplification medium. The conventional optical direct amplifier is disclosed in Japanese Patent Laid-Open No. 3-75732.

Next, as a conventional technique for stabilizing the amplification gain, there is an optical direct amplifier in which automatic gain control is performed by using a feedback loop. Focusing on the fact that the light intensity in the spontaneous emission band near the wavelength band of the optical signal in such an optical direct amplifier changes depending on the gain of the amplifier, the intensity of the light in the spontaneous emission band after amplification A technique for stabilizing the gain by forming a feedback loop for the pumping light source based on the above is disclosed in JP-A-3-62022.

[0008]

Although various techniques have been devised for the problems of improving the amplification gain of the optical direct amplifier and stabilizing the amplification gain as described above, a low frequency signal is used in the optical direct amplifier. There is another problem that the gain for the component is low.

FIG. 9 shows a gain frequency characteristic of a conventional optical direct amplifier. The characteristic represented by the solid line in FIG. 9 represents the gain frequency characteristic of the output optical signal S ₂ after amplification with respect to the input optical signal S ₁ in the light direct amplifier of FIG. As described above, the conventional optical direct amplifier has a low gain in the low frequency region of several hundreds to several kilohertz or less. This is because the rare earth ion in the excited state has a shorter lifetime than the signal cycle of low frequency, and thus signal amplification by stimulated emission cannot be sufficiently performed. For this reason, for example, a monitoring signal for monitoring the communication state could not be transmitted at a frequency lower than the low cutoff frequency of the optical direct amplifier. This will be specifically described with reference to a waveform diagram.

FIG. 10 shows a waveform of an optical signal in which a sine wave supervisory signal of several kilohertz or less is superimposed on a high bit rate digital signal, and FIG. 11 amplifies the optical signal of FIG. 10 by a conventional optical direct amplifier. In this case, the waveform of the optical signal after amplification is shown. When the optical signal of FIG. 10 is amplified by the conventional optical direct amplifier, the monitoring signal component of the sine wave is extremely attenuated from the amplified optical signal as the waveform shown in FIG. By the way, in an optical communication system, in order to compensate for attenuation of an optical signal due to transmission loss of an optical fiber, an optical direct amplifier is used for amplification at a relay station. Therefore, a low-frequency signal such as this monitor signal that is attenuated after being amplified by the optical direct amplifier cannot be used in the optical communication system. Therefore, in order to transmit the monitoring signal,
Part of the effective frequency region was used, and the frequency band for transmitting the original signal was narrowed. Further, in order to transmit the monitoring signal, a method of wavelength-multiplexing and transmitting light of different wavelengths can be adopted, but there is a problem such as complication of the device.

Therefore, a first object of the present invention is to provide an optical direct amplifier which compensates for a decrease in gain in a frequency band of several hundreds to several kilohertz or less.

Another object of the present invention is to provide an optical direct amplifier capable of amplifying an optical signal without attenuating a superposed signal component superposed by amplitude modulation such as a monitor signal for monitoring a communication state. It is in.

[0013]

According to a first aspect of the present invention, a rare earth-doped fiber that directly amplifies an optical signal by pumping light, and pumping light is injected into the rare earth-doped fiber.
Amplified by a pumping light source and a rare earth-doped fiber
Low-frequency signal component extracting means for extracting only low-frequency signal components in the frequency range in which the gain of the optical signal is reduced by amplification, and the low-frequency signal component extracting means for extracting low-frequency signal components.
After the wave signal component is amplified by the rare earth doped fiber
The gain at is equal to the gain after amplification of other frequency components.
Low-frequency excitation light output from the excitation light source
Excitation that enhances the gain of the wave signal component compared to other signal components.
The light source control means is provided in the optical direct amplifier.

That is, according to the invention of claim 1, the optical signal is
Gain in the process of amplification by the rare earth-doped fiber
For low frequency signals that decrease, the low frequency signal
It should be extracted by the minute extraction means. And
When pumping pumping light from a pumping source into a rare-earth-doped fiber
In addition, only the low frequency signal component is enhanced, and as a result,
All frequency components are output from the rare earth doped fiber
Output from the pump source so that the
The gain of the low-frequency signal component of the pumping light
I'm trying to increase everything.

According to the second aspect of the present invention, the low frequency signal component extracting means is constituted by a low pass filter which passes only a predetermined frequency component lower than several kilohertz.

That is, according to the second aspect of the present invention, only the signal component of several kilohertz or less at which the amplification gain of the rare earth-doped fiber decreases is extracted from the optical signal, and the pumping light is enhanced according to the amplitude of this signal component. . Therefore, the gain can be compensated only for the low frequency component in which the amplification gain is lowered in the rare earth-doped fiber.

[0017]

[0018]

[0019]

[0020]

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to embodiments.

FIG. 1 shows the structure of an optical direct amplifier according to an embodiment of the present invention. In the optical direct amplifier of the present embodiment, an input optical signal S _{1 1} inputted from the optical signal input end 11 and branched by the optical splitter 17, One of the branched optical signal is incident on the pin photodiode 18 It This output is passed through the current-voltage conversion circuit 19, the low-pass filter 21 and the amplifier 22, and the excitation light source drive circuit 1
Input to 5. Output exciting current of the exciting light source drive circuit 15 is supplied to the excitation light source 16, the excitation light S _{1 3} output from the pumping light source 16 is incident on an erbium-doped fiber 12 through a wavelength division multiplexing coupler 13. on the other hand,
The other optical signal split by the optical splitter 17 is also made incident on the erbium-doped fiber 12, amplified by the erbium-doped fiber 12, and then the wavelength division multiplexer 1
The output optical signal S _{1 2} is guided to the optical signal output end 14 via the signal line 3.

FIG. 10 shows a waveform of an optical signal in which a sine wave signal having a low frequency of several kilohertz or less is superimposed on a digital signal having a high bit rate. The operation of this embodiment will be described by taking the case of amplifying this optical signal as an example. In FIG. 1, an optical signal input from an optical signal input terminal 11 is branched by an optical branching device 17, one of the branched input optical signals is converted into a current signal by a pin photodiode 18, and then converted into a current voltage. The circuit 19 converts the voltage signal into a low-pass filter 21. The frequency characteristic of this low pass filter is provided so as to compensate for the decrease in gain at low frequencies of the erbium-doped fiber.

FIG. 2 shows the normalized gain frequency characteristics of the optical direct amplifier which serves as a reference for defining the characteristics of the low pass filter. The characteristic represented by the solid line in FIG. 2 represents the gain frequency characteristic of the output optical signal S _{1 2} after amplification with respect to the input optical signal S _{1 1} when the intensity of the pumping light is constant. The characteristic represented by the broken line in FIG.
Pumping light S necessary for compensating for the decrease in gain in the low range
₁ is a frequency characteristic of the _third intensity, represent the relationship between the intensity of the excitation light S _{1 3} with respect to the input optical signal S _{1 1.} That is, in order to compensate the amplification gain that decreases as the frequency of the input optical signal decreases, the intensity of the pumping light is increased as the signal frequency decreases.

In order to obtain such a characteristic of the pumping light intensity with respect to the signal frequency characteristic, the pumping light intensity is a pumping current vs. pumping light intensity characteristic showing the intensity of the pumping light with respect to the amount of the pumping current injected into the pumping light source. Is converted into the frequency characteristic of the excitation current given to the excitation light source, and the same frequency characteristic as the frequency characteristic of this excitation current is given to the low-pass filter 21. In this embodiment, the low-pass filter having such a frequency characteristic is composed of a low-pass filter including resistors and capacitors and an operational amplifier.

FIG. 3 shows the output signal waveform of the low pass filter 21 when the optical signal of FIG. 10 is input. The digital signal component is removed and only the low frequency sine wave signal component is extracted.

The output of the low pass filter 21 is amplified by the amplifier 22 and then input to the pumping light source driving circuit 15. Here, an excitation current having a current amount increased according to the amplitude of the signal output from the amplifier 22 is generated in the predetermined DC current, and is output to the excitation light source 16. Such an excitation light source drive circuit is realized by an adder circuit using an operational amplifier or an amplitude modulation circuit that modulates the base potential of a transistor with the output of the amplifier 22. FIG. 4 shows changes in the intensity of the excitation light output from the excitation light source. In this way, the intensity of the pumping light is modulated according to the low frequency signal component of the input optical signal. On the other hand, the other input optical signal split by the optical splitter 17 is input to the erbium-doped fiber 12, amplified by a gain according to the intensity of the pumping light, and output from the optical signal output end 14 via the wavelength division multiplexing coupler 13. Is output.

FIG. 5 shows the waveform of the output optical signal when the optical signal of FIG. 10 is amplified by the optical direct amplifier of this embodiment. As described above, the low-frequency sine wave signal component attenuated in the conventional optical direct amplifier is apparently reproduced by enhancing the intensity of the excitation light with the low-frequency sine wave signal component.

Modification

FIG. 6 shows the structure of an optical direct amplifier according to a modification of the present invention. In this optical direct amplifier, the same parts as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be appropriately omitted. In this modification, the pumping light is supplied by the forward pumping method. That is, the input optical signal and the pumping light supplied from the pumping light source 16 are the wavelength division multiplexing coupler 1
After being multiplexed at 3, it is input to the erbium-doped fiber 12. Also in this modification, as in the previous embodiment, the pumping light source 16 supplies pumping light whose light intensity is enhanced according to the amplitude of the low-frequency signal component of the input optical signal.
Therefore, the amplification gain of the erbium-doped fiber 12 also increases accordingly, and the decrease in gain in the low frequency range is apparently compensated.

Although the erbium-doped fiber is used as the amplification medium of the optical direct amplifier in the embodiment, it is also possible to use other rare-earth-doped fiber. Although a low pass filter is used in this embodiment,
A bandpass filter may be used according to the wavelength of the low frequency signal component to be transmitted. In this case, since the pumping light is enhanced corresponding to only the low frequency signal component to be transmitted, unnecessary low frequency component can be removed.

[0032]

According to the invention of claim 1, wherein, as described in the foregoing, to extract the signal components of the low-frequency included in the input optical signal, the gain is the same as the gain of the signal components of other frequency
So that the pumping light of this low frequency signal component
Injecting into rare-earth-doped fiber with enhancement over its constituents
I chose Therefore, the gain of the low frequency component originally decreases.
A low-frequency signal such as a monitor signal on a rare earth-doped fiber
Can be input with signals of other frequencies,
Contributes to simplification of communication system and wide band of transmitted signal
Will be done.

Further, according to the second aspect of the present invention, a low-frequency signal component extracting means, since the low-pass filter having a pass band of less than a few kilohertz, for example the frequency band
Can send the monitoring signal well,
Simplification of the communication system without the need for a dedicated signal transmission path
Can be achieved.

[0034]

[0035]

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a configuration of an optical direct amplifier of a backward pumping type in an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a frequency characteristic of normalized gain of an optical direct amplifier and a frequency characteristic of pumping light intensity in an example.

FIG. 3 is a waveform diagram showing an output voltage waveform of the low pass filter in the present embodiment.

FIG. 4 is a waveform diagram showing the intensity of the enhanced excitation light in this example.

5 is a waveform diagram showing a waveform of an output optical signal when the optical signal of FIG. 10 is input to the optical direct amplifier according to the present embodiment.

FIG. 6 is a schematic configuration diagram showing a configuration of an optical direct amplifier in a modified example of the present invention.

FIG. 7 is a schematic configuration diagram showing the configuration of a backward pumping optical direct amplifier.

FIG. 8 is a schematic configuration diagram showing a configuration of a forward pumping type optical direct amplifier.

FIG. 9 is a characteristic diagram 0 showing an example of a normalized gain frequency characteristic of a conventional optical direct amplifier.

FIG. 10 is a waveform diagram showing an example of a waveform of an optical signal in which a sine wave signal is superimposed on a digital signal.

11 is a waveform diagram showing the waveform of an output optical signal when the optical signal of FIG. 10 is amplified by a conventional optical direct amplifier.

[Explanation of symbols]

11 Optical Signal Input Terminal 12 Erbium Doped Fiber 13 Wavelength Division Multiplexing Coupler 14 Optical Signal Output Terminal 15 Excitation Light Source Driving Circuit 16 Excitation Light Source 17 Optical Divider 18 Pin Photodiode 19 Current-Voltage Conversion Circuit 21 Low-Pass Filter 22 Amplifier S ₁ , S _{1 1} input optical signal S ₂ , S _{1 2} output optical signal S _{1 3} pumping light

Claims (2)

(57) [Claims] 1. A rare earth-doped fiber for directly amplifying an optical signal by pumping light, and a pumping light source for injecting pumping light into the rare earth-doped fiber.
And the optical signal amplified by the rare earth-doped fiber
And a low-frequency signal component extracting means for extracting only a low-frequency signal component in a frequency range in which the gain is reduced by amplification, and a low-frequency signal component extracted by the low-frequency signal component extracting means.
After being amplified by the rare earth-doped fiber of the No. component
The gain at is equal to the gain after amplification of other frequency components.
Of the excitation light output from the excitation light source so that
Enhance the gain of the low frequency signal component compared to other signal components
A pumping light source control means for controlling the optical direct amplifier. 2. The optical direct amplifier according to claim 1, wherein the low-frequency signal component extracting means is a low-pass filter that passes only a predetermined frequency component lower than several kilohertz.