JPH08204267A - Optical fiber amplifier - Google Patents

Abstract

PURPOSE: To provide an optical fiber amplifier which is highly stabilized regardless of the wavelength of excitation light. CONSTITUTION: In an optical fiber amplifier which multiplexes waveforms of excited light and signal light, enters them into an optical fiber doped with rare earth elements, and amplifies signal light by stimulated emission with the excitation light in the optical fiber, a modulated signal generating circuit 2 which modulates the excitation light outputted from an excitation light generating circuit 1 is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier, and more particularly to an optical fiber amplifier which can obtain a stable gain even when a light having a wavelength of 0.98 micron is used for pumping an amplifying fiber.

Fifteen years have passed since optical communication was put into practical use. Initially, a repeater inserted into an optical transmission line was based on a system in which received light was once electrically converted to perform waveform shaping, timing reproduction, and waveform reproduction, and then was again optically converted and sent out to the transmission line. However, recently, an optical amplifier capable of directly amplifying light as it is has been developed.
In particular, an erbium-doped fiber amplifier that amplifies by passing a signal light through an erbium-doped fiber that has been pumped with pumping output light (hereinafter abbreviated as EDFA. When referring to an erbium-doped fiber, abbreviated as EDF)
However, it is most expected because of its high gain efficiency with respect to pumping power, and some have been put to practical use. Usually, in the optical fiber amplifier which has been partially put into practical use, light having a wavelength of 1.48 μm is used as pumping light and light having a wavelength of 1.55 μm is used as signal light.

The EDFA using light having a wavelength of 1.48 μm as the pumping light has a sufficiently wide EDF band with respect to the pumping wavelength, so that there is almost no gain fluctuation with respect to the fluctuation of the pumping wavelength, and 1.48 μm. An optical isolator for micron has been put into practical use, and the reflected return light can be sufficiently suppressed.
Since there is no mode hopping and fluctuations in pumping light output and pumping light wavelength are not seen, gain stability is good.

However, in an EDFA using pumping light with a wavelength of 1.48 μm, the energy level corresponding to the pumping light is close to the energy level corresponding to the signal light (wavelength 1.55 μm). The corresponding level disturbance affects the signal light, and the noise figure is slightly worse than when using pumping light with a wavelength of 0.98 microns.

Therefore, there are expectations for the practical application of an optical fiber amplifier using pumping light having a wavelength of 0.98 microns.

[0006]

2. Description of the Related Art FIG. 7 is a diagram showing a conventional gain stabilizing method for an optical fiber amplifier. In FIG. 7, 11 is an EDFA composed of an erbium-doped fiber and a pump laser diode, 12 is an optical coupler, 13 is an automatic power control circuit, and 7 is an optical isolator. In the configuration of FIG. 7, a part of the output light of the EDFA is branched by a coupler, the branched light is electrically converted, and the laser diode constituting the EDFA is configured by the difference between the average value level of the electric signal and the reference voltage. The gain of the EDFA is kept constant by controlling the power of the output light.

[0007]

However, the wavelength is 0.98.
The following problems occur in the EDFA using the micron excitation light. That is, the EDF for the wavelength of the excitation light in the 0.98 micron band
The gain variation of A is large. The 0.98 micron optical isolator is in the material development stage, and good loss, size, etc. have not been obtained. Therefore, fluctuations in wavelength and level due to mode hopping due to reflected return light occur. .

The above unstable factors can be solved by the configuration of FIG. However, as for the instability factor of, since mode hopping is a high-speed phenomenon, the EDF is a low-pass filter for pumping light even though the loop control of FIG. 7 must be performed at high speed. It works, and its cutoff frequency is as low as several hundred Hz. Therefore,
The control cannot be performed at high speed enough to suppress the effect of mode hopping. Therefore, there is a problem in that the level on the output side of the EDFA is unstable and the noise figure fluctuates, so that the system cannot maintain stable reception sensitivity.

The present invention addresses such problems and addresses the EDF.
It is an object of the present invention to provide an optical fiber amplifier that operates stably even when light having a wavelength of 0.98 μm is used as the pumping light.

[0010]

FIG. 1 illustrates the principle of the present invention. In FIG. 1, 1 is an excitation light generation circuit, 2 is a modulation signal generation circuit, 7 is an optical isolator, 8 is an optical coupler, and 9 is an EDF.

1 is characterized in that the pumping light is modulated by an electric signal having a frequency higher than the cutoff frequency of the EDF output from the modulation signal generating circuit. FIG. 2 is the second principle of the present invention.

In FIG. 2, reference numeral 1 is an excitation light generating circuit, and 3
Is an opto-electric conversion circuit, 4 is an automatic power control circuit, 5 is a modulation amplitude constant control circuit, 7 is an optical isolator, 8 is an optical coupler, and 9 is an EDF. In the configuration of FIG. 2, the modulation signal supply function is provided in the modulation amplitude constant control circuit.

As an example of the configuration of FIG. 2, in the opto-electric conversion circuit, the back light of the pumping laser diode is electrically converted to generate the DC component voltage and the AC component voltage of the electric signal, and the pumping light is generated by the DC component voltage. There is a configuration in which a generation circuit is fed back, a modulation signal is taken out from the AC component voltage, and is applied to an excitation light generation circuit through a modulation amplitude constant control circuit.

FIG. 3 shows the third principle of the present invention. FIG.
In the figure, 1 is an excitation light generation circuit, 3 is an opto-electric conversion circuit, 4 is an automatic power control circuit, 6 is a constant modulation degree control circuit, 7 is an optical isolator, 8 is an optical coupler, and 9 is an EDF. In the configuration of FIG. 3, the modulation signal supply function is provided in the constant modulation degree control circuit.

As an example of the configuration of FIG. 3, in the opto-electric conversion circuit, the back light of the pump laser diode is electrically converted to generate the DC component voltage and the AC component voltage of the electric signal, and the pump light is excited by the DC component voltage. There is a configuration in which the generation circuit is fed back, the modulation factor is detected from the AC component voltage, and the excitation light generation circuit is fed back through the constant modulation factor control circuit.

[0016]

In the structure shown in FIG. 1, when the pumping laser diode is modulated at a certain frequency by an electric signal, the pumping laser diode oscillates in multiple modes. For this reason, the energy of the pumping light is the sum of the energies of many modes, and even if wavelength fluctuation occurs in a certain mode, the total change is small, so the gain fluctuation is suppressed. Further, since there are many modes, the coherence is lowered, the influence of mode hopping due to the returning light is reduced, and the noise figure is stabilized.

The modulation frequency is the ED for the pumping light.
Since it is higher than the cutoff frequency of FA, the level of the excitation light does not appear to be modulated when viewed from the EDFA side, and does not affect the amplification degree.

In the configuration of FIG. 2, in addition to realizing the same action as that of the configuration of FIG. 1, the action of stabilizing the pumping light level by applying automatic power control to the pumping laser diode and detecting The function of stabilizing the level of the modulated signal output from the modulated signal generation circuit is realized, and more stable characteristics can be obtained.

In the configuration of FIG. 3, in addition to realizing the same action as that of the configuration of FIG. 1, the action of stabilizing the pumping light level by applying automatic power control to the pumping laser diode and detecting The degree of modulation achieves the effect of stabilizing the level of the modulation signal output from the modulation signal generation circuit, and more stable characteristics can be obtained.

[0020]

EXAMPLE FIG. 4 shows an example of the present invention. In FIG. 4, 11 is a pump laser diode, 12 is a transistor for driving the pump laser diode, 13 is a drive current determination circuit, and 14 is a choke coil, which constitutes a pump light generation circuit. Reference numeral 21 is an oscillator, and 22 is a blocking capacitor, which constitutes a modulation signal generation circuit.

In the configuration of FIG. 4, the pump laser diode oscillates at an average level corresponding to the output voltage of the drive current determining circuit, and the modulation signal is applied thereto, so that the envelope curve is centered on the average level. At the same time as the level changes similar to the waveform of the modulation signal, the spectrum of the output light is the sum of the original oscillation spectrum and the spectrum of the modulation signal.
It becomes the subtracted spectrum. The increase in the longitudinal mode associated with the modulation increases the effect of reducing the coherence.

FIG. 5 shows a second embodiment of the present invention. In FIG. 5, 11 is a pump laser diode, 12 is a transistor that drives the pump laser diode, and 14
Is a choke coil and constitutes an excitation light generation circuit. 31
Is a photodiode for electrically converting the back light of the excitation laser diode, 32 is a resistor, 33 is a DC component detection circuit, and 34 is a peak value detection circuit, which constitutes an opto-electric conversion circuit. Reference numeral 41 is a first differential amplifier for extracting the difference between the DC component voltage and the first reference voltage, which constitutes an automatic power control circuit. Further, 51 is an oscillator, 52 is a blocking capacitor, 53 is an automatic gain control amplifier, 54 is a second differential amplifier for extracting a difference between a DC component and a peak value, that is, an AC component voltage, and 55 is a voltage of the AC component. And a third differential amplifier that takes out the difference between the second reference voltage and the second reference voltage constitutes a modulation amplitude constant control circuit. In the configuration of FIG. 5, the back light of the pump laser diode is electrically converted by the photodiode, and the DC component voltage and the AC component voltage are detected from the electrical signal. The detected DC component voltage is guided to the first differential amplifier, converted into a differential voltage with respect to the first reference voltage, and the current of the drive transistor is controlled by the differential voltage. on the other hand,
The second differential amplifier detects the voltage corresponding to the amplitude of the modulation signal by taking the difference between the DC component voltage and the peak value voltage, and detects the modulation signal by the voltage of the difference between the voltage and the second reference voltage. The gain of the automatic gain control amplifier that amplifies
Stabilizes the level of the modulation signal.

Thus, in addition to the function of the structure of FIG. 4, the structure of FIG. 5 stabilizes the level of the pumping light by automatic power control and stabilizes the level of the modulation signal by feeding back the modulation signal. Will also have, and will make the characteristics of the optical fiber amplifier more stable.

FIG. 6 shows a third embodiment of the present invention. In FIG. 6, 11 is a pump laser diode, 12 is a transistor for driving the pump laser diode, and 14
Is a choke coil and constitutes an excitation light generation circuit. 31
Is a photodiode for electrically converting the back light of the excitation laser diode, 32 is a resistor, 33 is a DC component detection circuit, and 34 is a peak value detection circuit, which constitutes an opto-electric conversion circuit. Reference numeral 41 is a first differential amplifier for extracting the difference between the DC component voltage and the first reference voltage, which constitutes an automatic power control circuit. Further, 61 is an oscillator, 62 is a blocking capacitor, 63 is an automatic gain control amplifier, 64 is a second differential amplifier for extracting the difference between the DC component and the peak value, 65 is a division circuit, 66 is the output of the division circuit and The fourth differential amplifier for extracting the difference from the third reference voltage constitutes a constant modulation degree control circuit.

In the configuration of FIG. 6, the back light of the pump laser diode is electrically converted by the photodiode, and the DC component voltage and the AC component voltage are detected from the electrical signal.
The detected DC component voltage is guided to the first differential amplifier, converted into a differential voltage with respect to the first reference voltage, and the current of the drive transistor is controlled by the differential voltage. On the other hand, the second differential amplifier detects the voltage corresponding to the amplitude of the modulation signal by taking the difference between the DC component voltage and the peak value voltage, and the ratio between the voltage and the DC component voltage, that is, the modulation, in the division circuit. The voltage of the automatic gain control amplifier that generates the voltage corresponding to the frequency and takes out the voltage of the difference between the generated voltage and the third reference voltage to set the level of the modulation signal, and controls the level of the modulation signal. Stabilize. The division circuit is commercialized as an IC and is commercially available.

As a result, in addition to the function of the structure of FIG. 4, the structure of FIG. 6 stabilizes the level of the pumping light by the automatic power control and stabilizes the level of the modulation signal by feeding back the modulation degree signal. Will also be provided, and the characteristics of the optical fiber amplifier will be made more stable.

In the above description, the EDFA using the excitation light having the wavelength of 0.98 μm has been described as an example, but it goes without saying that the present invention can be applied to the case where the excitation light having another wavelength is used. Further, the technique of the present invention is applied to EDF.
The present invention is not limited to A, and can naturally be applied to an optical fiber amplifier doped with another element.

[0028]

As described above, according to the present invention, it is possible to stabilize the gain and noise figure of the EDFA even when pumping light having a wavelength of 0.98 μm is used.
As described above, the development of the optical isolator for 0.98 micron has just started, and sufficient characteristics have not been obtained yet. Therefore, the effect of the present invention is great.

Further, with respect to the EDFA, the technique can be applied not only when pumping light having a wavelength other than 0.98 μm is used but also to an optical fiber amplifier other than the EDFA. wide.

[Brief description of drawings]

FIG. 1 is a principle of the present invention.

FIG. 2 is the second principle of the present invention.

FIG. 3 is a third principle of the present invention.

FIG. 4 is an embodiment of the present invention.

FIG. 5 is a second embodiment of the present invention.

FIG. 6 is a third embodiment of the present invention.

FIG. 7 is a conventional gain stabilizing method for an optical fiber amplifier.

[Explanation of symbols]

 1 Pumping Light Generation Circuit 2 Modulation Signal Generation Circuit 7 Optical Isolator 8 Coupler 9 Erbium Doped Fiber

Claims (3)

[Claims] 1. An optical fiber amplifier for wavelength-multiplexing pumping light and signal light, making the light incident on an optical fiber doped with a rare earth element, and amplifying the signal light by stimulated emission by the pumping light in the rare earth element doped optical fiber. 2. An optical fiber amplifier, comprising: a modulation signal generation circuit (2) for modulating the excitation light output from the excitation light generation circuit (1) at a frequency equal to or higher than the cutoff frequency of the rare earth-doped optical fiber. 2. The optical fiber amplifier according to claim 1, wherein an optical-electrical conversion circuit that electrically converts the back light of the pump laser diode and outputs a DC component voltage and an AC component voltage of the electric signal, An automatic power control circuit for applying a voltage difference between the DC component voltage and the first reference voltage to the driving transistor of the pump laser diode, and a modulation signal to generate a modulation signal, which corresponds to the modulation amplitude at the output of the photoelectric conversion circuit. An optical fiber amplifier, comprising: a modulation amplitude constant control circuit that generates a voltage that is a difference between the AC component voltage and a second reference voltage. 3. The optical fiber amplifier according to claim 1, wherein an optical-electrical conversion circuit that electrically converts the back light of the pump laser diode and outputs a DC component voltage and an AC component voltage of the electric signal, An automatic power control circuit for applying a difference voltage between the DC component voltage and the first reference voltage to the drive transistor of the pump laser diode, and a modulation signal for generating the modulation signal and outputting the AC-component voltage to the opto-electric conversion circuit. A constant modulation degree control circuit for extracting a voltage corresponding to a modulation degree that is a ratio to the DC component voltage and generating a voltage of a difference between the voltage corresponding to the modulation degree and a third reference voltage. Optical fiber amplifier.