JPH05241209A - Optical amplifier control system - Google Patents

Abstract

PURPOSE:To constitute the above system in such a manner that automatic photooutput control (APC) is executed while the internally generated noises are kept low at wide input levels. CONSTITUTION:Stimulating light to put an Er-doped fiber 25 in an amplified state is kept maintained at the specified value sufficient for optical amplification by a stimulating light control circuit 31. The control of the light output level of the optical amplifier is executed by controlling the light attenuation quantity of a variable light attenuator 32 in accordance with the electric signal from a photodetector 29 for monitoring output light which receives a part of the output light from the variable light attenuator 32 and photoelectrically converts this light so as to suppress the fluctuation in the output light by a signal light output control circuit 33. Since the Er-doped fiber 25 is sufficiently excited, the naturally released light coefft. proportional to the noise electric power generated within the optical amplifier is kept always in the low state. The APC is thus realized with the low noise characteristic at the wide input levels.

Description

Detailed Description of the Invention

[0001]

[Industrial application] In recent years, research and development of optical communication systems have been vigorously pursued, and the importance of booster amplifiers, repeaters, and preamplifiers using optical amplification technology has become clear. The present invention relates to a control system for these optical amplifiers, and in particular, a variable optical attenuator is provided on the output side of an optical amplification medium, and the variable optical attenuator is controlled while maintaining the pump light intensity at a constant value sufficient for optical amplification. The present invention relates to an optical amplifier control system for controlling the optical output of an optical amplifier and the gain of the optical amplifier.

[0002]

2. Description of the Related Art In an optical communication system, when an optical amplifier is applied as a booster amplifier, a repeater or a preamplifier, control of the optical amplifier is indispensable. A conventional optical amplifier using an erbium (Erbium, hereinafter referred to as Er) doped fiber as an optical amplification medium is shown in FIG.

As shown in FIG. 15, a part of the input signal light (for example, wavelength 1.55 μm) is branched by the optical coupler 21.
The remaining input signal light is input to the multiplexer 22, and the control circuit 2
The laser light is multiplexed with the pumping light (wavelength: 1.48 μm, for example) output from the pumping light source 24 such as a semiconductor laser diode controlled by No. 3, and then input into the Er-doped fiber 25. Here, the pumping light source 24 is provided to give energy to Er ions in the Er-doped fiber 25 to excite electrons and put the Er-doped fiber 25 in an amplification state.

The signal light input to the Er-doped fiber 25 is amplified in the Er-doped fiber 25 and then output to the pumping light blocking filter 26. Then, of the light input to the pumping light blocking filter 26, the pumping light component is blocked from progressing, and only the amplified signal light and spontaneous emission light are input to the optical coupler 27, and the optical coupler 27 outputs one of them. After the part is branched, the rest is output to the outside.

In the above, each optical coupler 21, 27
A part of the input signal light and the output light branched by the light are respectively received by the input light monitor light receiver 28 and the output light monitor light receiver 29 such as a photodiode, converted into electric signals, and output to the control circuit 23. To be done.

Then, the control circuit 23 controls the automatic light output (hereinafter referred to as APC), which is an important control as an optical amplifier, to keep the output light intensity constant, and the output light intensity (gain) with respect to the input light intensity is made constant. An automatic gain control (hereinafter, referred to as AGC) for keeping is performed.

Here, APC is performed by always controlling the electric signal input from the output light monitoring photodetector 29 to a constant value, and AGC is performed by the input light monitoring photodetector 28.
It is performed by always controlling the ratio of the electric signal input from the output light monitoring photodetector 29 to the electric signal input from the pump to a constant value.
4 has been performed by changing the drive current, that is, by controlling the fluctuation of the excitation light intensity.

[0008]

By the way, in the above-described conventional optical amplifier shown in FIG. 15, the APC and AGC are performed by variably controlling the pumping light intensity. Control has been performed so that the intensity of the excitation light becomes smaller as the intensity becomes larger.

This is because if the excitation light intensity is kept constant,
This is because the output light intensity becomes larger than the desired value when the input signal light intensity to the optical amplifier is high, and as a countermeasure against this, the configuration may be simplified without the need for extra optical parts. In order to reduce the amplification rate, control is performed so as to reduce the pump light intensity.

Here, when the signal light intensity input to the optical amplifier (medium) is increased while the pump light intensity is kept constant at an arbitrary value, the signal light intensity passing through the optical amplification medium is increased. Because of the large size, all the electrons in the upper rank, which are excited in the optical amplification medium, drop to the lower rank, and the amplification weakens, that is, a saturation phenomenon occurs. At this time, the larger the pump light intensity is, the less saturated the input signal light intensity becomes. However, as described above, in the conventional optical amplifier, since the output light intensity becomes larger than the desired value when the input signal light intensity is large, the pump light intensity is controlled to be small. Although the saturation did not occur (or the saturation was low), the saturation occurs in the optical amplification medium.

When the input signal light is amplified in the saturated state, the intensity of noise generated from the inside of the amplifier is larger than that in the case where the same amplification degree (gain) is obtained in the state where the saturated state is not generated. Is the one that is not detailed here but is clarified.

It is to be noted that it is amplification of light that the intensity of the input signal light gradually increases while maintaining the same wavelength due to stimulated emission. In this case, the input signal light faithfully reflects the input signal light. Independently of, the spontaneous emission light emitted when the excited upper electrons fall to the lower order,
In the present invention, noise is used. Since the spontaneous emission light also has the same wavelength as the input signal light, it is amplified without being distinguished from the input signal light.

As described above, since the pump light intensity is controlled to decrease when the input signal light intensity to the optical amplifier increases, saturation occurs in the optical amplification medium as a result, and inside the optical amplifier. There was a problem that the generated noise increased.

Therefore, it is necessary to design the optical amplifier variously according to the input level. Specifically, it is necessary to optimize the length of the Er-doped fiber as the optical amplification medium, for example, according to the purpose of use of the optical preamplifier, the optical repeater, the optical power amplifier, or the like. However, in consideration of mass production, it has been desired that optical amplifiers with the same specifications (for example, in the case of Er-doped fiber, have the same fiber length) be applied to all.

The first object of the present invention is to keep the internally generated noise low in a wide input level,
The second objective is to realize an optical amplifier control system in which automatic optical power control (APC) is performed. Secondly, at a wide input level, while maintaining internally generated noise at a low level, automatic gain control ( It is to realize an optical amplifier control system in which AGC is performed.

[0016]

The means of the first aspect of the present invention is as follows. The optical amplifying medium 1 (see the principle block diagram of FIG. 1, the same applies hereinafter) is composed of, for example, an Er-doped fiber, and amplifies the input signal light and outputs it.

The pumping light control means 2 adjusts the drive current of a pumping light source composed of, for example, a semiconductor laser diode, so that the pumping light for keeping the optical amplifying medium 1 in the amplification state is, for example, the maximum value allowed in the system. Is controlled so as to always maintain the value set in advance.

The variable optical attenuator 3 is provided on the output side of the optical amplifying medium 1 and freely attenuates the light amplified by the optical amplifying medium 1 and input, and outputs the attenuated light. The light output control means 4 includes, for example, a light receiver that monitors the output light from the variable light attenuating means 3, and suppresses fluctuation of the output light based on an electric signal received by the light receiver, photoelectrically converted, and input. The optical output of the optical amplifier is controlled to be constant by controlling the optical attenuation of the variable optical attenuator 3 in the direction.

The means of the second aspect of the present invention is as follows. The optical amplification medium 1 (see the principle block diagram of FIG. 2, the same applies hereinafter) is made of, for example, an Er-doped fiber, and amplifies the input signal light and outputs it.

The pumping light control means 2 adjusts the driving current of the pumping light source, which is composed of a semiconductor laser diode, for example, so that the pumping light for keeping the optical amplification medium 1 in the amplification state becomes the maximum value allowed in the system, for example. Control is performed so that the preset value is always maintained.

The variable optical attenuating means 3 is provided on the output side of the optical amplifying medium 1 and freely attenuates the light amplified by the optical amplifying medium 1 and input, and outputs the attenuated light. The gain control means 5 is provided with, for example, a light receiver for monitoring the input light and a light receiver for monitoring the output light from the variable light attenuating means 3, and the light is received by the light receiver for monitoring the input light and is photoelectrically converted and inputted. The gain of the optical amplifier is controlled to be constant by controlling the optical attenuation amount of the variable optical attenuator 3 based on the ratio of the electric signal received by the photodetector that monitors the output light to the signal and photoelectrically converted and input.

[0022]

First, the principle of the present invention will be described. In order to operate an optical amplifier (for example, an Er-doped fiber is used as an optical amplification medium) with low noise, it is important to keep the Er-doped fiber sufficiently pumped at all times. Considering recent improvements in the efficiency of Er-doped fibers and higher output of semiconductor lasers for pumping, a technique capable of supplying a sufficient pumping light intensity to Er-doped fibers has been established.

As will be described in detail below, by sufficiently exciting the optical amplifying medium, the spontaneous emission coefficient, which is proportional to the noise power generated inside the optical amplifier, is always kept low in a wide input level. Therefore, an optical amplifier with low noise can be realized.

That is, the noise power is the spontaneous emission intensity P _ASE (ν) per unit frequency.

[0025]

[Equation 1]

And the bandwidth Δf of the optical filter,

[0027]

[Equation 2]

Is given by Here, n _sp is the spontaneous emission light coefficient, G is the gain of the optical amplifier, h is Planck's constant, and ν is the frequency of the signal light. As is clear from the above equation, the spontaneous emission light coefficient is proportional to the noise power (spontaneous emission light intensity).

Further, the spontaneous emission light coefficient keeps a small value when sufficiently excited, but the natural emission light coefficient increases when saturation occurs. Because the spontaneous emission coefficient n _sp is distributed in the density n _{2 of} Er ions in which electrons are distributed in the upper level, and the electrons are distributed in the ground level (it is better to call the ground level rather than the lower level). This is because the density is defined as follows using the Er ion density n ₁ .

[0030]

[Equation 3]

Here, it may be simply considered that n ₂ is the number of electrons in the upper level and n ₁ is the number of electrons in the ground level. Therefore, while n ₂ is always larger than n ₁ in the light amplification state,

[0032]

[Equation 4]

This is because Further, as described above, it is not preferable to lower the pumping light intensity by control in order to keep the output light of the optical amplifier constant, since this leads to deterioration of optical amplification characteristics.

Therefore, if the optical amplification medium is excited as much as possible and the optical output level of the optical amplifier is controlled by the newly provided optical output control section, the APC with low noise at a wide input level can be obtained. Is achieved.

If the gain of the optical amplifier is pumped as much as possible and the gain of the optical amplifier is controlled by the newly provided gain control unit, AGC can be achieved with low noise at a wide input level. It

The present invention has been made based on the above-mentioned principle. In the first mode, the optical amplification medium 1 receives the input signal light while being pumped by the pumping light control means 2 as sufficiently as possible. The optical output level of the optical amplifier is amplified by controlling the optical output of the variable optical attenuator 3 by the optical output controller 4.

In the second mode, the optical amplification medium 1 amplifies the input signal light in a state where it is pumped by the pumping light control means 2 as sufficiently as possible, and the gain of the optical amplifier is adjusted by the gain control means 5. Is performed by controlling the amount of light attenuation of the variable light attenuator 3.

Therefore, it is possible to realize an optical amplifier control system in which APC and AGC are performed with low noise characteristics over a wide input level. Therefore, the same optical amplifier can be used as an optical preamplifier, an optical repeater, and an optical power amplifier according to the purpose.

[0039]

Embodiments of the present invention will be described below with reference to FIGS. FIG. 3 is a diagram showing the configuration of an optical amplifier control system that realizes the APC of the present invention. Note that FIG.
The same members as those of the conventional example shown in FIG.

As shown in FIG. 3, in this embodiment,
A pumping light control circuit 31 for controlling the pumping light for keeping the Er-doped fiber 25 as an optical amplification medium in an amplified state and a pumping light control circuit 31 for controlling the pumping light to be maintained at a set constant value are provided on the output side of the Er-doped fiber 25 A variable optical attenuator 32 and a signal light output control circuit 33 for controlling the amount of attenuation of the variable optical attenuator 32 are included.

The pumping light control circuit 31 adjusts the drive current of the pumping light source 24 such as the semiconductor laser diode LD in consideration of each component and the system life, so that E
The pumping light intensity for pumping the r-doped fiber 25 is
The system is controlled so as to always maintain the preset value at the maximum value allowed by the system.

The variable optical attenuator 32 freely attenuates the amplified signal light and spontaneous emission light that have passed through the pumping light blocking filter 26 and are input. The signal light output control circuit 33 suppresses the fluctuation of the output light based on the electric signal input from the output light monitor light receiver 29 that receives and photoelectrically converts a part of the output light branched by the optical coupler 27. The output of the optical amplifier is controlled to be constant by controlling the attenuation amount of the variable optical attenuator 32 in the direction.

FIG. 4 is a block diagram of the signal light output control circuit 33 of FIG. (a) is a schematic circuit example, (b) is an explanatory diagram of an output G of the circuit shown in (a), and (c) is a conceptual explanatory diagram of the operation of the circuit shown in (a).

As shown in FIG. 4 (a), the signal light output control circuit 41 includes a resistor R ₁ and a differential amplifier 42 which are provided on the ground side of the output light monitoring photodetector 29. And
The reference voltage V _ref is input to the positive input terminal of the differential amplifier 42, and its negative input terminal and the photodetector 29 for output light monitoring.
And the terminal t ₁ located between the resistor R ₁ and the resistor R ₁ .

Therefore, it is branched by the optical coupler 27 (see FIG. 3).
A part of the output light thus generated is output by the output light monitor light receiver 29.
Photoelectrically converted into photocurrent and resistance R₁Voltage drop caused by
Voltage V depending on_inInput to the negative input terminal of the differential amplifier 42
To do. Therefore, the output G of the differential amplifier 42 is the reference voltage V
_refAnd the voltage V corresponding to the output light received_inDifference with
It is a proportional value). The output G of the differential amplifier 42 is controlled.
Voltage, V_cont[G (V _ref-V_in) = V_cont] As
To use.

[0046] For example, for some reason, if increased output light intensity from the optical amplifier, also increases the voltage V _in generated is converted is received photoelectrically by the photodetector 29. in this case,
As is clear from the relationship shown in FIG. 4 (b), the differential amplifier 4
The output G of 2 decreases. That is, the control voltage V _cont also decreases as shown in FIG. The signal light output control circuit 41 responds to the decrease of the control voltage V _cont by the variable optical attenuator 32.
Control (see FIG. 3) is performed to reduce the transmittance (increase the amount of attenuation). Conversely, when the output light intensity decreases,
The voltage V _in decreases and the output G of the differential amplifier 42 increases, as can be seen from FIG. 4 (b). Therefore, as shown in FIG. 4 (c), since the control voltage V _cont also increases, control is performed to increase the transmittance of the variable optical attenuator 32 (decrease the attenuation amount).

FIG. 5 is a block diagram of another example of the signal light output control circuit 33 of FIG. 3, where (a) is a schematic circuit example and (b) is
7A is an explanatory diagram of an output G of the circuit shown in FIG. 7A, and FIG. 7C is a conceptual explanatory diagram of the operation of the circuit shown in FIG.

As shown in FIG. 5A, the optical output control circuit 5
1 differs from the circuit 41 shown in FIG. 4 (a) in that the reference voltage V _ref is input to the negative input terminal of the differential amplifier 52, its positive input terminal, the output light monitoring photodetector 29 and the resistor R ₁ are input. And a terminal t ₁ located between and.

Therefore, a part of the output light branched by the optical coupler 27 (see FIG. 3) is photoelectrically converted into a photocurrent by the output light monitoring photodetector 29, and the voltage V due to the voltage drop caused by the resistor R ₁ is generated. _in is input to the positive input terminal of the differential amplifier 52. Therefore, the output G of the differential amplifier 52 becomes a difference (a value proportional to the difference) between the voltage V _in corresponding to the received output light and the reference voltage V _ref . The output G of the differential amplifier 52 is
It is used as the control voltage V _cont [G (V _in −V _ref ) = V _cont ].

[0050] For example, the factor from what, if increasing the output light intensity from the optical amplifier, also increases the voltage V _in generated is converted is received photoelectrically by the photodetector 29. in this case,
As is clear from the relationship shown in FIG. 5 (b), the differential amplifier 5
The output G of 2 increases. That is, the control voltage V _cont also increases as shown in FIG.

The signal light output control circuit 51 performs control such that the transmittance of the variable optical attenuator 32 (see FIG. 3) is lowered (the amount of attenuation is increased) in response to the increase of the control voltage V _cont . .. On the contrary, when the output light intensity decreases, the voltage V
_{As in} decreases, as can be seen from Fig. 5 (b), the differential amplifier 5
The output G of 2 decreases. Therefore, as shown in FIG. 5C, the control voltage V _cont also decreases, and the variable optical attenuator 32
The control is performed to increase the transmittance (reduce the amount of attenuation).

In this way, the signal light output control circuit 33 (4
1, 51) is the control voltage V _cont due to the fluctuation of the output light intensity.
If a change occurs in the control voltage V _cont , control is performed so as to increase or decrease the transmittance (attenuation amount) of the variable optical attenuator 32 in a direction of canceling the increase or decrease of the output light intensity according to the increase or decrease of the control voltage V _cont. To do.

Since this embodiment is configured as described above, the input signal light (for example, the wavelength of 1.55 μm) is input to the multiplexer 2
2 is input to the pump light source 24 and is output from the pump light source 24 that is controlled by the pump light control circuit 31.
m) and is input to the Er-doped fiber 25. Here, the excitation light is output while being maintained at the maximum value that is a preset value that is allowed by the system.

The signal light input to the Er-doped fiber 25 is amplified in the Er-doped fiber 25 and output to the pump light blocking filter 26. Excitation light blocking filter 2
The excitation light component of the light input to 6 is prevented from traveling, and only the amplified signal light and spontaneous emission light are coupled to the optical coupler 27.
Is input, a part of it is branched, and the rest is output to the outside.

A part of the output light branched by the optical coupler 27 is received by the output light monitoring light receiver 29 such as a photodiode, converted into an electric signal, and output to the signal light output control circuit 33.

From the signal light output control circuit 33, as described with reference to FIGS. 4 and 5, the control voltage which is the difference between the voltage V _in corresponding to the received output light and the reference voltage V _ref. V _cont is output.

If the output light fluctuates due to some load fluctuation, the signal light output control circuit 33 changes the variable light so as to suppress the fluctuation of the output light as described with reference to FIGS. 4 and 5. Since the attenuation amount of the attenuator 32 is increased / decreased, the optical output is always maintained at a constant value and APC is realized.

In this case, since the Er-doped fiber 25 is sufficiently pumped by the pumping light control circuit 31, the spontaneous emission light coefficient proportional to the noise power generated inside the optical amplifier is always low in a wide input level. Can be maintained. Therefore, APC with low noise at wide input level
Is realized.

FIG. 6 is a block diagram of an optical amplifier control system for realizing the AGC of the present invention. The same members as those in the conventional example shown in FIG. 15 and the embodiment shown in FIG. 3 are designated by the same reference numerals, and the duplicate description will be omitted.

As shown in FIG. 6, in the present embodiment,
A pumping light control circuit 31 that controls the pumping light for keeping the Er-doped fiber 25 as an optical amplification medium in an amplified state so as to maintain the pumping light at a set constant value, and a variable provided on the output side of the Er-doped fiber 25. An optical attenuator 32, a gain control circuit 34 for controlling the amount of attenuation of the variable optical attenuator 32,
It is characterized by including.

Here, the pumping light control circuit 31 and the variable optical attenuator 32 use the same members as those of the configuration example shown in FIG. The gain control circuit 34 receives the electric signal from the input light monitor light receiver 28 for receiving and photoelectrically converting the input signal light branched by the optical coupler 21, and the output light branched by the optical coupler 27. An electric signal from the photodetector 29 for photoelectrically converting output light is input.

Then, the gain control circuit 34 tunes the variable light in the direction of suppressing the fluctuation of the output light based on the ratio of the electric signal from the output light monitoring light receiver 29 to the electric signal from the input light monitoring light receiver 28. By controlling the amount of attenuation of the attenuator 32, the gain of the optical amplifier is controlled to be constant.

FIG. 7 is a schematic configuration diagram of the gain control circuit 34 shown in FIG. As shown in FIG. 7, the gain control circuit 3
4, a resistor R ₂ is provided on the ground side of the output light monitoring photodetector 29, and the terminal t ₂ located between the photodetector 29 and the resistor R ₂ and the positive input terminal of the differential amplifier 71 are logarithmic. Amplifier 7
It is connected through 3. A resistor R ₃ is provided on the ground side of the input light monitoring photodetector 28, and a terminal t ₃ located between the photodetector 28 and the resistor R ₃ and the negative input terminal of the differential amplifier 71 are logarithmic amplifiers. It is connected via 74.

The output terminal of the differential amplifier 71 is connected to the positive input terminal of the differential amplifier 72, and the differential amplifier 7
The reference voltage V _ref is input to the negative input terminal of 2.
Therefore, the output light branched by the optical coupler 27 (see FIG. 6) is photoelectrically converted into a photocurrent by the output light monitoring photodetector 29, and the voltage V due to the voltage drop caused by the resistor R ₂ is generated.
₂ is output from the logarithmic amplifier 73 as its logarithmic value logV ₂ . Further, the input light branched by the optical coupler 21 (see FIG. 6) is photoelectrically converted into a photocurrent by the input light monitoring photodetector 28, and the voltage V ₃ due to the voltage drop caused by the resistor R ₃ is output from the logarithmic amplifier 74. It is output as the logarithmic value logV ₃ . Further, from the differential amplifier 71, the difference (value proportional to) (logV ₂ −logV ₃ = logV ₂ / V)
₃ ) = V _in is output. Then, the difference (a value proportional to) between the voltage V _in corresponding to the ratio of the output light to the input light and the reference voltage V _ref is output from the differential amplifier 72. The output G of the differential amplifier 72 is controlled by the control voltage V _cont [G (V
_in −V _ref ) = V _cont ].

For example, due to some factor, the optical coupler 27
If the output light intensity from the output light fluctuates, the output G of the differential amplifier 72, which is the difference between the voltage V _in corresponding to the ratio of the output light to the input light and the reference voltage V _ref , changes. Thus, by utilizing the output G as a control voltage V _cont, when the control voltage V _cont is increased, it causes the variable optical attenuator 32 (increased attenuation) reducing the transmittance (see Fig. 6). On the contrary, the control voltage V _cont
When is decreased, the transmittance of the variable optical attenuator 32 is increased (the amount of attenuation is decreased).

Since this embodiment is configured as described above, part of the input signal light (for example, wavelength 1.55 μm) is branched by the optical coupler 21, and the remaining input signal light is combined by the multiplexer 22.
Pumping light (for example, a wavelength of 1.48 μm) that is input from the pumping light control circuit 31 and is output from the pumping light source 24.
And is input to the Er-doped fiber 25. Here, the excitation light is output while being maintained at the maximum value that is a preset value that is allowed by the system.

The signal light input to the Er-doped fiber 25 is amplified in the Er-doped fiber 25 and output to the pumping light blocking filter 26. Of the light input to the pumping light blocking filter 26, the pumping light component is blocked from progressing, and only the amplified signal light and spontaneous emission light are included in the optical coupler 2.
7 is input to the optical coupler 27, a part of which is branched by the optical coupler 27, and the rest is output to the outside.

A part of the input signal light and the output light branched by the respective optical couplers 21 and 27 are respectively received by the input light monitor light receiver 28 and the output light monitor light receiver 29 and converted into electric signals. And output to the gain control circuit 34.

From the gain control circuit 34, as shown in FIG.
As explained with reference to the ratio of output light to input light
Corresponding voltage V_inAnd reference voltage V_refControl voltage which is the difference between
Pressure V _contIs output.

When the output light fluctuates due to some load fluctuation, the gain control circuit 34 controls the attenuation amount of the variable optical attenuator 32 so as to suppress the fluctuation of the output light, as will be described with reference to FIG. , The gain is always maintained at a constant value, and AGC is realized.

In this case, since the Er-doped fiber 25 is sufficiently pumped by the pumping light control circuit 31, the spontaneous emission light coefficient proportional to the noise power generated inside the optical amplifier is always low in a wide input level. Can be maintained. Therefore, in a wide input level, AGC with low noise
Is realized.

FIG. 8 shows an example of constructing a variable optical attenuator using a Faraday rotator. As shown in FIG. 8, the Faraday rotator 81 is sandwiched between the polarizer 82 and the analyzer 83, and the variable optical attenuator works without polarization dependency.

After the input light from the optical fiber on the input side (not shown) is collimated by a lens, it is separated into two orthogonal polarization components by a polarizer 82 (for example, a rutile polarization separation plate), and a Faraday rotator 81 is formed. Pass through. The polarization planes of the two orthogonal polarization component lights rotate according to the strength of the magnetic field H generated by the coil 81a while passing through the Faraday rotator 81. After that, the two polarized component lights are combined by an analyzer 83 (for example, a rutile polarization separation plate) and coupled to an optical fiber (not shown) on the output side.

Here, in a certain magnetic field H, adjustment is made so that the coupling efficiency to the optical fiber on the output side is maximized at a specific (angle of polarization) rotation angle. In this case, the rotation angle of the plane of polarization can be changed by changing the strength of the magnetic field H. Therefore, if the magnetic field H created by the coil 81a is changed by a control circuit (not shown), the coupling efficiency with respect to the optical fiber on the output side can be improved. Change. That is, it is possible to attenuate incident light at a desired ratio and output it to the optical fiber on the output side without polarization dependency.

FIG. 9 shows an example of the structure of the voltage-current conversion circuit 91 for forming the drive current for driving the Faraday rotator shown in FIG. 8, where (a) is a schematic circuit example and (b) is its operation. FIG.

In the signal light output control circuit 33 of FIG. 3 and the gain control circuit 34 of FIG. 6, the variable optical attenuator 32 is controlled by the control voltage V _cont . Here, since the Faraday rotator 81 is controlled by the electric current,
The voltage-current conversion circuit 91 described above is required.

As shown in FIG. 9A, a resistor R ₄ and a coil 81a of the Faraday rotator 81 and a voltage source 93 are provided between the source (S) and the drain (D) of the field effect transistor (FET) 92. Connect a series circuit. Then, the control voltage V _cont is applied between the gate (G) and the source (S). A control current I _cont flows through the coil 81a,
The increase / decrease changes the magnetic field H generated by the coil 81a of the Faraday rotator 81, and the amount of attenuation changes.

As shown in FIG. 9B, when the output light intensity increases, the control voltage V _cont increases and the control voltage I _cont flowing to the coil 81a decreases. Therefore, the magnetic field H of the Faraday rotator 81 decreases and the rotation angle of the polarization plane of the polarized component light passing through the Faraday rotator 81 decreases. Therefore, the coupling efficiency with the optical fiber on the output side (not shown) decreases, and the amount of attenuation increases.

On the contrary, when the output light intensity decreases, the control voltage V
_cont decreases and the control current I _cont flowing to the coil 81a increases. Therefore, the magnetic field H of the Faraday rotator 81 increases, and the rotation angle of the polarization plane of the polarized component light increases. Therefore, the coupling efficiency with the optical fiber on the output side is increased and the amount of attenuation is reduced.

FIG. 10 shows an example of constructing a variable optical attenuator using a semiconductor modulator. (a) is a conceptual block diagram of a variable optical attenuator, (b) is a configuration example for eliminating polarization dependence, (c) is an example of a semiconductor modulator, and (d) is a transmittance characteristic diagram.

As shown in FIG. 10A, a desired amount of attenuation is obtained by applying a control voltage to the semiconductor modulator 101 having no polarization dependence in the direction perpendicular to the direction of the input light and increasing or decreasing the control voltage. Output light is obtained.

Even when the semiconductor modulator has polarization dependency, as shown in FIG. 10B, the two semiconductor modulators 102 and 103 are arranged such that the control voltage application directions are orthogonal to each other. By arranging so that each semiconductor modulator 10
The polarization dependences of 2 and 103 are canceled out, and a variable optical attenuator having no polarization dependence can be configured.

The semiconductor modulator is, for example, Electronics Lett.
ers 15th September 1988 Vol, 24No, 19pp. 1194/1195
As shown in FIG. 10, the structure shown in FIG. 10 (c) is known.

As shown in FIG. 10C, the semiconductor modulator has a Si-InP layer provided on a substrate (n-InP) and an insulating film (SiO ₂ ) formed on the Si-InP layer. Is provided with a polyimide layer. And
As a part appears on the left side of the semiconductor modulator,
A waveguide (GaInAsP) is formed in the central portion of the Si-InP layer, a cladding layer (p-InP) is formed on the waveguide (GaInAsP), and a cap layer (p ⁺ -GaInAsP) is formed on the cladding layer (p-InP). Is provided. Further, electrodes are formed on the cap layer (p ⁺ -GaInAsP) and on the periphery of the polyimide layer, and on the lower surface of the substrate (n-InP).

The semiconductor modulator has the effect of shifting the absorption edge to the long wavelength side when a voltage is applied to the semiconductor. Franz-Keldys
It uses the h effect. That is, when a voltage is applied to the semiconductor, the energy band structure is inclined and the band gap becomes narrower. Therefore, when light having a wavelength near the absorption edge is passed through the semiconductor and a voltage is applied, the light is absorbed. In the case of a semiconductor modulator, the amplitude of input light is modulated.

The characteristic of the transmittance with respect to the applied control voltage is as shown in FIG. 10 (d), and the transmittance linearly decreases (the attenuation increases) as the control voltage increases.
To do. That is, a part of the input light is absorbed and the light is attenuated.

FIG. 11 shows an example of constructing a variable optical attenuator using liquid crystal. As in the case of the Faraday rotator 81, the liquid crystal 111 is sandwiched between the polarizer 112 and the analyzer 113, and the variable optical attenuator operates without polarization dependency.

The liquid crystal 111 is used in the TN mode, for example.
That is, when the electric field is not applied, the liquid crystal molecule basic axis is 90 degrees between the substrates.
Have the effect of twisting and rotating polarized light by 90 degrees,
This is a so-called twisted nematic system in which an electric field is applied to this to convert from twisted orientation to orientation in the direction of the electric field to eliminate the rotation of polarized light.

The input light from the optical fiber 113 on the input side is split into two orthogonal polarization components by the polarizer 112. Each polarization component light passes through the liquid crystal 111, so that each polarization plane is rotated according to the intensity of the applied control voltage. After that, the two polarized component lights are combined by the analyzer 113 and coupled to the optical fiber 115 on the output side.

Here, adjustment is made so that the coupling efficiency to the optical fiber on the output side is maximized at a specific rotation angle (polarization plane) at a certain control voltage. In this case, the rotation angle of the plane of polarization can be changed depending on the magnitude of the control voltage. Therefore, if the control voltage applied by the control circuit (not shown) is changed, the coupling efficiency with respect to the optical fiber 115 on the output side changes. That is, it is possible to output the incident light to the output-side optical fiber 115 at a desired ratio without polarization dependency.

FIG. 12 shows an example of constructing a variable optical attenuator by changing the coupling position in the optical coupling system. In the light coupling system 121, the input light from the input side optical fiber 122 is collimated by the front lens 123, passes through the rear lens 124, and is input to the output side optical fiber 125.

In the above, the output side optical fiber 125
The efficiency of coupling to the fiber 125 on the output side is maximized when the end face of the is at the focal position of the rear lens 124. Then, the end surface of the optical fiber 125 on the output side is attached to the rear lens 12
The coupling efficiency decreases with increasing distance from the focal point of No. 4.

That is, by providing a means for moving the output side fiber 125 in the optical axis direction indicated by the arrow by a control signal from a control circuit (not shown), and changing the position with respect to the rear lens 124, the coupling efficiency is changed to a desired value. Can be obtained.

Instead of moving the output side fiber 125, the input side fiber 122 may be moved, or the front lens 123 or the rear lens 124 may be moved.

FIG. 13 shows an example of constructing a variable optical attenuator using a disc-shaped optical attenuator. Disc-shaped optical attenuator 131
Has a continuously different attenuation amount in the circumferential direction. FIG.
As shown in FIG. 3, the light input from the input side optical fiber 132 is collimated by the front lens 133, passes through the disc-shaped optical attenuator 131, and is guided to the output side optical fiber 135 by the rear lens 134. A means for rotating the disc-shaped optical attenuator 131 as shown by an arrow is provided by a control signal from a control circuit (not shown). The desired amount of attenuation is obtained by rotating the disc in accordance with the control signal.

FIG. 14 shows an example of constructing a variable optical attenuator by utilizing the filter characteristic of an optical filter. (a) is a configuration diagram thereof, and (b) is a principle diagram of variable attenuation. For example, as the optical filter, a dielectric multilayer film 141 well known as an optical bandpass filter is used. The dielectric multilayer film 141 generally has a center wavelength λ _c depending on the incident angle of input light.
Changes.

As shown in FIG. 14 (a), the light input from the optical fiber 142 on the input side is collimated by the front lens 143, passes through the dielectric multilayer film 141, and passes through the rear lens 1.
It is guided to the optical fiber 145 on the output side by 44. A means for rotating the dielectric multilayer film 141 as indicated by an arrow by a control signal from a control circuit (not shown) is provided.

As shown in FIG. 14B, the dielectric multilayer film 1
By rotating 41, the transmittance curve moves from the dotted line to the solid line, and the center wavelength λ _{c of the} filter shifts, for example, to the left of the wavelength λ _{sig of the} signal light, and the transmittance decreases (the attenuation amount decreases). To increase. That is, when the dielectric multilayer film 141 rotates according to the control signal, the attenuation amount with respect to the signal light wavelength changes. The optical filter is not limited to the dielectric multilayer film.

The variable optical attenuator having any of the configurations shown in FIGS. 8 and 10 to 14 can be applied to the optical amplifier control system realizing the APC and the optical amplifier control system realizing the AGC of the present invention. Is. In this case, since there is no polarization dependence, there is no signal distortion, and the sufficient amount of counter-attenuation can be obtained, so that the reflected light does not return to the optical amplification medium, and the operation of the optical amplifier does not become unstable.

In the present invention, APC and A
The GC is not realized by changing the pumping light intensity as in the conventional case, but the pumping light is held constant at the maximum value allowed by the system and the optical amplification medium is pumped sufficiently.

Therefore, the spontaneous emission coefficient, which is proportional to the noise power generated inside the optical amplifier, can always be kept low in a wide input level, and a low-noise optical amplifier control system can be realized. Therefore, the same optical amplifier can be used as an optical preamplifier, an optical repeater, and an optical power amplifier according to the purpose.

The embodiment shown in FIGS. 3 and 6 is as follows.
The forward pumping configuration is such that the input signal light is pumped from the same direction as it is incident on the optical amplification medium, but the backward pumping configuration is also possible, where the input signal light is pumped from the opposite direction.

Further, the optical amplification medium has been described by taking the Er-doped fiber as an example, but the present invention is not limited to this, and neodymium (Neod) is used.
The medium may be a medium doped with a rare earth element such as ymium, Nd) or praseodymium (Pr).

[0104]

As described above, according to the first aspect of the present invention, the optical output level of the optical amplifier is controlled by the separately provided optical output control section after the optical amplification medium is sufficiently excited. Therefore, automatic optical output control (APC) is realized with low noise at a wide input level.

Further, according to the second aspect of the present invention, since the optical amplifying medium is sufficiently pumped and the gain control of the optical amplifier is performed by the separately provided gain control section, it is wide. Low noise and automatic gain control (AGC) at input level
Is realized.

Therefore, the same optical amplifier can be used as an optical preamplifier, an optical repeater, and an optical power amplifier according to the purpose.

[Brief description of drawings]

FIG. 1 is a principle block diagram of a first aspect of the present invention.

FIG. 2 is a principle block diagram of a second aspect of the present invention.

FIG. 3 is a diagram showing the configuration of an optical amplifier control system that realizes the APC of the present invention.

4 is a configuration diagram of the signal light output control circuit of FIG.
(a) is a schematic circuit example, (b) is explanatory drawing of the output of the circuit of (a), (c) is a conceptual explanatory drawing of operation of the circuit of (a).

5A and 5B are configuration diagrams of another example of the signal light output control circuit of FIG. 3, where FIG. 5A is a schematic circuit example, and FIG. 5B is an output G of the circuit of FIG.
And (c) is a conceptual explanatory view of the operation of the circuit of (a).

FIG. 6 is a diagram showing a configuration of an optical amplifier control system that realizes the AGC of the present invention.

7 is a schematic configuration diagram of the gain control circuit shown in FIG.

FIG. 8 is a diagram showing an example of configuring a variable optical attenuator using a Faraday rotator.

9 is a diagram showing a configuration example of a voltage-current conversion circuit for forming a drive current for driving the Faraday rotator of FIG. 8, (a) is a schematic circuit example, and (b) is (a). FIG. 7 is an explanatory diagram of an operation of the circuit of FIG.

10A and 10B are diagrams showing an example of configuring a variable optical attenuator using a semiconductor modulator, in which FIG. 10A is a conceptual block diagram, FIG. 10B is a configuration example for eliminating polarization dependence, and FIG. Is an example of a semiconductor modulator, and (d) is a transmittance characteristic diagram.

FIG. 11 is a diagram showing an example of configuring a variable optical attenuator using liquid crystal.

FIG. 12 is a diagram showing an example in which a variable optical attenuator is configured by changing a coupling position in a light coupling system.

FIG. 13 is a diagram showing an example of configuring a variable optical attenuator using a disc-shaped attenuator.

FIG. 14 is a diagram showing an example of configuring a variable optical attenuator by utilizing the filter characteristic of an optical filter.

FIG. 15 is a diagram showing a configuration example of a conventional optical amplifier.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical amplification medium 2 Excitation light control means 3 Variable light attenuation means 4 Optical output control means 5 Gain control means 81 Faraday rotator 101 Semiconductor modulator 111 Liquid crystal 121 Optical coupling system 131 Disc-shaped optical attenuator 141 Optical filter

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H04B 10/16

Claims (8)

[Claims] 1. An optical amplification medium (1) for amplifying input light, and pumping light for keeping the optical amplification medium (1) in an amplification state,
Excitation light control means for maintaining a constant value sufficient for optical amplification (2)
A variable optical attenuating means (3) provided on the output side of the optical amplifying medium (1) and capable of attenuating and outputting the light from the optical amplifying medium (1); Optical output control means (4) for controlling the optical output of the optical amplifier to a constant value by controlling the optical attenuation of the variable optical attenuation means (3) based on the output light from (3).
An optical amplifier control system comprising: 2. An optical amplification medium (1) for amplifying input light, and pumping light for keeping the optical amplification medium (1) in an amplification state,
Excitation light control means for maintaining a constant value sufficient for optical amplification (2)
A variable optical attenuator (3) provided on the output side of the optical amplification medium (1) and capable of attenuating and outputting light from the optical amplification medium (1); Gain control means (5) for controlling the gain of the optical amplifier to a constant value by controlling the optical attenuation amount of the variable optical attenuating means (3) based on the ratio of the output light from the variable optical attenuating means (3); An optical amplifier control system comprising: 3. The optical amplifier control system according to claim 1, wherein the variable optical attenuator (3) is formed by using a Faraday rotator (81). 4. The optical amplifier control system according to claim 1, wherein the variable optical attenuating means (3) is formed by using a semiconductor modulator (101). 5. The variable light attenuation means (3) comprises a liquid crystal (1).
The optical amplifier control system according to claim 1 or 2, wherein the optical amplifier control system is formed by using (11). 6. The light according to claim 1, wherein the variable light attenuating means (3) is a device for changing a coupling position in a light coupling system (121) to change a coupling efficiency. Amplifier control system. 7. The optical amplifier control system according to claim 1, wherein the variable optical attenuator (3) is formed by using a disc-shaped optical attenuator (131). 8. The optical amplifier control system according to claim 1, wherein the variable optical attenuator (3) is a device that utilizes the filter characteristics of the optical filter (141).