JPH05343784A - Light amplifier - Google Patents

Abstract

PURPOSE:To provide a constant signal output operation with a low noise by keeping the output light power of a light amplifier constant by using a light attenuator. CONSTITUTION:Light amplifying means for amplifying input light is equipped with an EDF 1, two exciting LD 2, two wave dividing portions 3, two exciting LD drive circuits 10, two light isolators 4 and a wavelength selection filter 5. The EDF 1 as an amplifying medium is excited by the exciting LD 2 with electric power sufficient for providing a region having good noise characteristics. Part of output light is converted into electric signals by PIN-PD 13, said signals are amplified by a monitor circuit 14 and compared to a predetermined set point by a light attenuator control circuit 15, and a variable light attenuator 11 installed in the output stage is controlled through a light attenuator drive circuit 16 in such a manner that their difference becomes zero, thereby making the output light constant. If light signal power is large, then there is no deterioration of S/N, so that a constant output operation with good noise characteristics is possible.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is applied to an optical amplifier,
In particular, it relates to an optical amplifier having means for keeping the output optical signal power (hereinafter, simply referred to as output optical power) constant.

[0002]

2. Description of the Related Art An optical amplifier amplifies and outputs an optical signal as it is, and can be applied to loss compensation of an optical transmission line. In such a case, the optical amplifier may be required to suppress the input fluctuation or the loss fluctuation of the optical components used inside the amplifier to keep the output optical power constant.

In order to keep the output optical power of the optical amplifier constant,
Conventionally, the output optical power is controlled to be constant by increasing or decreasing the gain of the amplification medium itself. There are two methods for this, one that utilizes the output saturation characteristic of the gain medium and one that actively changes the gain of the gain medium.

In the former method, no special control circuit is provided,
The configuration of the amplifier is simple because the pump power may be constant.
FIG. 3 shows an erbium-doped fiber optical amplifier (EDFA: Erbium Doped) that performs optical amplification in the 1.55 μm band.
5 shows an example of output optical power-gain characteristics of a Fiber Amplifier). If the output saturation region in the figure is used, it is possible to suppress the output optical power fluctuation.

On the other hand, the latter method requires active control of the gain of the amplification medium itself, which can be performed relatively easily by adjusting the pumping power to the amplification medium.

FIG. 4 shows a block diagram of a conventional optical amplifier having a constant output control means of an EDFA, and FIG. 5 shows the relationship between the gain of an erbium-doped fiber (EDF) as an amplification medium and the pumping light power. ..

In FIG. 4, reference numeral 1 denotes an ED which is an amplification medium.
F, 2 is a pumping LD (laser diode) that supplies the power required for amplification by light, 3 is a multiplexer, 4 is an optical isolator for preventing oscillation, 5 is a wavelength selective optical filter for extracting the signal light wavelength range, 6 is an optical branching unit that cuts out a part of the output light for monitoring, 7 is a PIN-PD, 8 is a monitoring circuit that extracts necessary information from the converted electric signal, and 9 is a pump LD 2 that receives the monitoring information. An exciting LD control circuit 10 for controlling is an exciting LD drive circuit for driving the exciting LD 2.

As shown in FIG. 5, the EDF can obtain the required gain by increasing or decreasing the pumping light power. Therefore, as shown in FIG. 4, a part (1/100 in the figure) of the output light is converted into an electric signal by the PIN-PD (photodiode) 7, amplified, and then compared with a predetermined set value. If the drive current of the pumping LD 2 is controlled so that the difference becomes zero, the output optical power can be kept constant.

[0009]

However, in the former method utilizing the output saturation characteristic of the amplifying medium, the saturation characteristic generally does not occur rapidly at a certain output value but gradually occurs, so that fluctuation suppression is possible. However, it is not possible to achieve perfect output constant. Further, in the output saturation region, the population inversion parameter n _SP largely deviates from the ideal state (n _SP = 1), so that the noise figure NF (= 2n _SP : EDFA of the S / N ratio of the input / output signal of the amplifier is In this case, the noise characteristics as an amplifier cannot be said to be good. Further, this system has a problem that the power fluctuation of the sum of the optical signal and the optical noise can be suppressed, but the optical signal power cannot be controlled to be constant.

On the other hand, in the latter method in which the gain of the amplifying medium is actively changed, as shown in FIG. 5, the EDF is generally used in the region where the gain can be changed by the pumping light power.
The noise characteristics of are not good. This is because the amplification medium must be used in the output saturation region in order to change the gain by changing the pumping light power, as shown in FIG. 5, and as shown in the example of FIG. Parameter n _SP becomes large,
This is because the noise figure of the optical amplifier deteriorates as a result.
Therefore, the control system of FIG. 3 does not essentially have the best noise characteristics. In addition to this, there is a problem that noise may change even if the output is constant.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems by providing an optical amplifier which has a good noise characteristic and does not fluctuate and operates at a constant output.

[0012]

SUMMARY OF THE INVENTION The present invention is an optical amplifier having an optical amplifying means for amplifying an input optical signal, and a variable optical attenuating means for controlling the output optical power of the optical amplifier, and the optical amplifier. And an attenuation amount control means for controlling the attenuation amount of the variable optical attenuation means so that the output optical power becomes a constant value.

Further, in the present invention, it is preferable that the optical amplifying means includes an amplifying medium made of an erbium-doped fiber and a pumping light supplying means for supplying pumping light power to the amplifying medium.

[0014]

The output optical power of the optical amplifier is maintained at a constant value by controlling the attenuation amount of the variable optical attenuating device provided at the output part of the optical amplifier by the attenuation amount controlling device.

Thus, for example, the pumping light power of the EDF can be operated in an output unsaturated region having a good noise characteristic, and an optical amplifier having a good noise characteristic and performing a constant output operation without fluctuation can be obtained. It will be possible.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention.

In this embodiment, an EDF 1, two pump LDs 2, two combiners 3, two pump LD drive circuits 10, and two optical isolators 4 are used as an optical amplifying means for amplifying input light. In an optical amplifier provided with a wavelength selection optical filter 5, a variable optical attenuator 11 as a variable optical attenuator having an input connected to the wavelength selection optical filter 5, and an output of the optical amplifier, which are features of the present invention. The variable optical attenuator 11 monitors the optical power so that the output optical power has a predetermined value.
The optical branching unit 12, the PIN-PD 13, the monitor circuit 14, the optical attenuator control circuit 15, and the optical attenuator drive circuit 16 are provided as the attenuation amount control means for controlling the attenuation amount of the.

The EDF 1 which is the amplification medium is excited by the excitation LD 2 with sufficient power so that it is in the region where the noise characteristic is good (see FIG. 4). Under such conditions, part of the output light (1/100 of the output light in this embodiment) as in the case of FIG.
Is converted into an electric signal by the PIN-PD 13, and this is amplified by the monitor circuit 14, and then by the optical attenuator control circuit 15.
If the variable optical attenuator 11 provided in the output stage is controlled via the optical attenuator drive circuit 16 so that the difference becomes zero as compared with a predetermined set value, the output light can be kept constant.

FIG. 2A shows an example of the variable optical attenuator 11, the optical attenuator control circuit 15 and the optical attenuator drive circuit 16, and FIG. 2B shows the relationship of the attenuation rate with respect to the rotation angle of the optical attenuating plate. It is a characteristic view showing.

In FIG. 2A, the optical attenuator 11a and the lenses 11b and 11c constitute a variable optical attenuator 11, the pulse generator 15a constitutes an optical attenuator control circuit 15, and the pulse motor 16a constitutes an optical attenuator. The attenuator drive circuit 16 is configured.

The light attenuating plate 11a is a glass disk formed with a light and shade shielding film so that the light transmittance changes in the circumferential direction. As shown in FIG. 2B, the light attenuating plate 11a attenuates with respect to a rotation angle (degree). The rate (dB) changes almost linearly. Then, according to the control signal from the monitor circuit 14, the pulse generator 15
a generates a predetermined pulse signal, and the pulse motor 16a drives the light attenuating plate 11a accordingly, so that the light attenuating plate 1
1a is controlled to a rotational position to obtain a desired damping rate.

Further, in an optical amplifier, spontaneous emission light noise is generally added to the signal light. Therefore, if control is performed so that the level of the output light is kept constant, the optical power of the signal light decreases by the amount of the spontaneous emission light noise. It may happen. In such a case, and when the signal light to be amplified contains a constant modulation frequency component, an electrical filter that allows only the modulation frequency component to pass through the monitor circuit 14 after being converted into an electrical signal by the PIN-PD 13 This modulation frequency component is cut out by means of this, and after detecting and smoothing this, it is compared with a predetermined set value, and the variable optical attenuator 11 provided in the output stage is controlled so that the difference becomes zero. It is possible to keep the power of the signal light component of the output light constant without being affected by the noise component from the above. This is also effective when the input light contains a noise component.

Since this embodiment has such a structure, the EDF 1 can be used in the region shown in FIG. 4 where the noise characteristic is excellent. Although the optical signal is attenuated by the optical attenuator at the output stage, the accompanying optical noise is also attenuated in proportion,
If the optical signal power is large, it can be considered that the S / N ratio does not deteriorate. Therefore, a constant output operation with good noise characteristics is possible.

Although the variable optical attenuator is provided at the output stage of the optical amplifier in this embodiment, the output light of the optical amplifier can be controlled by the same operation even if it is provided at the input stage.

[0026]

As described above, since the present invention has means for keeping the output optical power of the optical amplifier constant by using the optical attenuator, it is possible to perform constant signal output operation with low noise. There is an effect.

Further, in the present invention, since it is not necessary to control the pumping power, the output control can be achieved only by connecting the external circuit to the output terminal without modifying the conventional optical amplifier. The effect that the output control means can be easily added to the amplifier is also obtained.

Therefore, the present invention is applied to a long-distance optical transmission system.
R optical repeater [k. Nakagawa, et. al. "T
run and Distribution Net
work Application of Erbiu
m-Doped Fiber Amplifier "Jo
urnal of Lightwave Techno
logy, vol. 9, No. 2, 1991]], it becomes possible to increase the transmission distance (regeneration relay interval) limited by the accumulation of noise, and the effect is great.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing an example of the variable optical attenuator, an optical attenuator control circuit, an optical attenuator drive circuit, and characteristics of the variable optical attenuator.

FIG. 3 is a diagram showing an example of output optical power-gain characteristics of an EDFA.

FIG. 4 is a block diagram showing a conventional example.

FIG. 5 is a diagram showing an example of EDF gain-pump light power characteristics.

FIG. 6 is a diagram showing an example of output optical power-gain characteristics when the pumping optical power of the EDF is changed.

[Explanation of symbols]

 1 EDF 2 Excitation LD 3 Combiner 4 Optical Isolator 5 Wavelength Selective Optical Filter 6 Optical Branch 7 PIN-PD 8 Monitoring Circuit 9 Excitation LD Control Circuit 10 Excitation LD Drive Circuit 11 Variable Optical Attenuator 11a Optical Attenuation Plate 11b, 11c Lens 12 Optical branching unit 13 PIN-PD 14 Monitor circuit 15 Optical attenuator control circuit 15a Pulse generator 16 Optical attenuator drive circuit 16a Pulse motor

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masatoshi Saruwatari 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. An optical amplifier provided with an optical amplifying means for amplifying an input optical signal, wherein a variable optical attenuating means for controlling an output optical signal power of the optical amplifier and an output optical signal power of the optical amplifier are monitored. And an attenuation amount control means for controlling the attenuation amount of the variable optical attenuation means so that the output optical signal power has a constant value. 2. The optical amplifier according to claim 1, wherein the optical amplification means includes an amplification medium made of an erbium-doped fiber, and pumping light supply means for supplying pumping light power to the amplification medium.