JPH06252487A - Optical fiber amplifier - Google Patents

Abstract

PURPOSE:To obtain an optical fiber amplifier having a small noise figure, by providing a light attenuating means which attenuates the intensity of the signal light amplified by an erbium-doped fiber. CONSTITUTION:In an optical fiber amplifier, an incidence signal light from an input connector 1 is passed through a first isolator 2 to be entered in an EDF 3, and here, it is amplified by an incidence excitation light outputted from an LD 9 which is entered in the EDF 3 via a WDM 4. The intensity of the excitation light outputted from the LD 9 is so set as to be nearly the maximum output light intensity of the LD. The signal light amplified largely in the EDF 3 is passed through the WDM 4 and a second isolator 5, and is inputted to an optical attenuator 6, and here, is attenuated to a required output signal light intensity. The value of the attenuation quantity of the optical attenuator 6 is so set that the intensity of the signal light which is amplified by the excitation light having the preset intensity can be lowered to the required output signal light intensity. The signal light attenuated to the required light intensity is outputted from an output connector 8.

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 used in an optical communication system, and more particularly to an optical fiber amplifier having an improved S / N ratio in amplifying an optical signal.

[0002]

2. Description of the Related Art An optical fiber amplifier monitors the light intensity of an output signal and feeds back the result to a light intensity control means to keep the light intensity of the output signal constant.

As shown in FIG. 4, a conventional optical fiber amplifier has an input optical connector 1 for receiving an optical signal and an A
A first optical isolator 2 that passes only the light traveling from the point toward the point B, an erbium-doped fiber (EDF) 3 that receives the pump light and amplifies the signal light, and a point D opposite to the point C Between them, only the signal light passes, and at the CE point that intersects the CD point, only the pumping light passes at the CE point (W
DM) 4, a second isolator 5 for passing only light traveling in the direction from the point D to the opposite point F, and an optical coupler 7 for branching an optical signal incident from the point G in the directions of the H point and the I point.
And an output connector 8 for outputting signal light whose light intensity is kept constant.

The optical fiber amplifier further includes a photodiode (PD) 12 for outputting a current proportional to the intensity of light passing through the point I side of the optical coupler 7 as a mechanism for controlling the intensity of signal light. , The detection circuit 13 that outputs a voltage according to the output current of the photodiode 12, and the output voltage of the detection circuit 13 and the voltage value corresponding to the set value of the output light intensity are compared, and the voltage proportional to the difference is output. Control circuit 11, a drive circuit 10 for supplying a laser diode drive current in an amount corresponding to the output voltage of the control circuit 11, and a pump light laser diode (LD) 9 driven by the drive current of the drive circuit 10. The pump light output from the laser diode 9 for pump light is incident on the EDF 3 through the WDM 4, and E
Excite DF3.

The EDF 3 is composed of an optical fiber mixed with Er ions, and by injection of pumping light of 1.48 μm,
The signal light of 1.55 μm is amplified.

The WDM 4 has a small loss of signal light and a large loss of pumping light between the point C on the EDF 3 side and the point D on the opposite side. On the other hand, at the CE point intersecting with the CD point, the loss is large with respect to the signal light and the loss is small with respect to the pump light. Therefore, only the signal light passes between CDs, and only the excitation light passes between CEs.

The optical coupler 7 has a small loss between GH points,
In comparison, the loss between GI points is large. Therefore,
Most of the light input from point G is output from point H, and only part of the light passes to point I.

The excitation light laser diode 9 driven by the drive circuit 10 outputs light having a wavelength of 1.48 μm.

In this optical fiber amplifier, the signal light incident from the input connector 1 passes through the first isolator 2 and enters the EDF 3 to be amplified. The amplified signal light is
The signal is output from the output connector 8 through the WDM 4, the second isolator 5, and the optical coupler 7. Here, the isolator 2,
Reference numeral 5 is inserted to prevent oscillation that occurs when reflected light returns to the EDF 3 again.

The output light intensity of the optical fiber amplifier is shown in FIG.
As shown in (3), it depends on the intensity of the excitation light output from the LD 9. Therefore, a part of the output signal light is branched by the optical coupler 7, the fluctuation of the output light intensity is monitored by the PD 12 and the detection circuit 13, and the excitation light intensity is controlled.
The output light intensity can be kept constant.

As described above, in the conventional optical fiber amplifier, the pump light intensity is set to the pump light intensity corresponding to the required center value of the optical output, and the APC (for maintaining the output signal light intensity constant). In the Auto Power Control operation, the output signal light intensity can be made constant by monitoring the fluctuation of the output signal light intensity and controlling the excitation light intensity.

The optical fiber amplifier shown in FIG.
Since the pumping light output from 9 is input to the EDF 3 in the direction opposite to the traveling direction of the signal light, it is called “backward pumping type”.
3 in front of WDM4 (WDM4 and second isolator 5
Between the WDM4 and the "forward pumping type", or one WDM4 before and after the EDF3 and connected to each WDM.
A "bidirectional pumping" type amplifying device that pumps EDF3 before and after EDF by D9 is known.

[0013]

In the optical fiber amplifier, the noise figure in amplification, that is, the amount of deterioration of the signal-to-noise ratio, decreases as the pump light intensity increases, as shown in FIG. Therefore, the noise figure can be reduced as the intensity of the pumping light used for amplifying the signal light is increased.

However, in the conventional optical fiber amplifier,
Since the required output signal light intensity is set by adjusting the pump light intensity, when the required output light intensity is low, the pump light intensity must be set low, and therefore the noise figure Could not lower the value of.

Further, when the pumping light intensity is set to be small, there is a problem that the value of the noise figure is greatly deteriorated when the pumping light intensity is changed in the APC operation for keeping the output light intensity constant. there were.

The present invention solves these conventional problems, and an object of the present invention is to provide an optical fiber amplifier having a low noise figure.

[0017]

Therefore, according to the present invention, an erbium-based amplifier that amplifies signal light by the incident pumping light is used.
A doped fiber, a wavelength multiplexer that combines signal light and pump light, and a laser that outputs pump light of controlled intensity.
An optical fiber amplifier including a diode is provided with an optical attenuator that attenuates the intensity of the amplified signal light.

[0018]

Therefore, the intensity of the signal light can be attenuated to a predetermined value by the optical attenuating means independently of the amplification of the signal light in the erbium-doped fiber. Therefore,
In the erbium-doped fiber, it becomes possible to amplify the signal light by using a high intensity pumping light such that the value of the noise figure becomes small.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, an optical fiber amplifier according to an embodiment of the present invention has an optical attenuator 6 for attenuating an optical signal connected between a second isolator 5 and an optical coupler 7. In this respect, the structure is different from that of the conventional backward pumping type optical fiber amplifier (FIG. 4).

In this optical fiber amplifying apparatus, the signal light incident from the input connector 1 passes through the first isolator 2 and enters the EDF 3, where it is amplified by the pumping light output from the LD 9 incident through the WDM 4.

The pump light intensity of the LD 9 is set near the maximum output light intensity of the LD so that the noise figure becomes a value close to the minimum.

The signal light largely amplified by the EDF 3 is W
The signal passes through the DM 4 and the second isolator 5 and enters the optical attenuator 6, where it is attenuated to the required output signal light intensity. The attenuation amount of the optical attenuator 6 is set to a value that can reduce the intensity of the signal light amplified by the set excitation light to the desired output signal light intensity.

The signal light attenuated to the required light intensity passes through the optical coupler 7 and is output from the output connector 8.

When the output light intensity fluctuates, the PD 12 and the detection circuit 16 which monitor the branched light of the optical coupler 7 are detected.
The fluctuation is detected by the control circuit 11, the drive current of the drive circuit 10 is adjusted by the control circuit 11, and the excitation light intensity output from the LD 9 is controlled. As a result, the amplification of the signal light in the EDF 3 is adjusted and the output signal light intensity from the EDF 3 is kept constant. As described above, the signal light output from the EDF 3 is attenuated to the required constant signal light intensity by the optical attenuator 6 and output through the output connector 8.

As described above, in this optical fiber amplifier, the output attenuator 6 sets the output signal light intensity independently of the amplification of the signal light in the EDF 3. Therefore, in the EDF 3, it is possible to amplify the signal light by using the excitation light of such a high intensity that the noise figure can be reduced.

Further, FIG. 2 shows a forward pumping type optical fiber amplifier to which the present invention is applied. This device comprises an input connector 1, a first isolator 2, a WDM4, an ED
F3, second isolator 5, optical filter 14 for passing signal light and blocking excitation light, optical attenuator 6, optical coupler 7
And output connector 8 are connected in this order, and WDM4
Excitation light is input to the EDF 3 from the LD 9 connected to the.

Further, FIG. 3 shows a bidirectional pumping type optical fiber amplifier to which the present invention is applied. This device includes an input connector 1, a first isolator 2, and a first W
DM42, EDF3, second WDM41, second isolator 5, optical filter 14, optical attenuator 6, optical coupler 7
And output connector 8 are connected in this order, and two W
The LDs 91 and 92 connected to the DMs 41 and 42 enable EDF from both the same direction and the opposite direction with respect to the traveling direction of the signal light.
Excitation light is input to 3.

Also in the optical fiber amplifiers shown in FIGS. 2 and 3, independently of the amplification of the signal light in the EDF 3,
Since the output signal light intensity is set by the optical attenuator 6, the EDF 3 can amplify the signal light by using the excitation light of high intensity that can reduce the noise figure.

[0029]

As is apparent from the above description of the embodiments, the optical fiber amplifier of the present invention can reduce the noise figure.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of an optical fiber amplifier according to the present invention,

FIG. 2 is a block diagram showing another embodiment of the optical fiber amplifier according to the present invention,

FIG. 3 is a block diagram showing another embodiment of the optical fiber amplifier according to the present invention,

FIG. 4 is a block diagram showing a configuration of a conventional optical fiber amplifier.

FIG. 5 is a diagram showing a relationship between pump light intensity and output light intensity of an optical fiber amplifier;

FIG. 6 is a diagram showing the relationship between excitation light intensity and noise figure.

[Explanation of symbols]

 1 Input Connector 2 First Isolator 3 Erbium Doped Fiber (EDF) 4, 41, 42 Wavelength Multiplexer (WDM) 5 Second Isolator 6 Optical Attenuator 7 Optical Coupler 8 Output Connector 9, 91, 92 Laser Diode (LD) 10 Photodiode (PD) 11 Drive circuit 12 Control circuit 13 Detection circuit

Claims (1)

[Claims] 1. An erbium-doped fiber that amplifies signal light by incident pump light, a wavelength multiplexer that multiplexes the signal light and the pump light, and outputs the pump light with a controlled intensity. An optical fiber amplifier including a laser diode, wherein an optical attenuator for attenuating the intensity of the amplified signal light is provided.