KR100289038B1 - Gain-equalized multi-channel optical amplifier using semiconductor optical amplifier and erbium-doped fiber - Google Patents

Abstract

PURPOSE: A gain-equalized multi-channel optical amplifier is provided to equalize a gain easily and extend a gain bandwidth using a semiconductor optical amplifier and an erbium-doped fiber at the same time. CONSTITUTION: The first optical amplifier part(5) controls gain variation according to input light strength and density-inversion variations using an erbium-doped fiber for the first amplification. The second optical amplifier part(6) secondly amplifies lights amplified through the first optical amplifier part(5) and equalizes a gain, using a semiconductor optical amplifier. The gain of an input signal of the light amplified by the first optical amplifier part(5) has a gain characteristic curve which is lowered as a wavelength is prolonged. The gain of an input signal of the light amplified by the second optical amplifier part(6) has a gain characteristic curve which is increased as a wavelength is prolonged.

Description

Gain-Equalized Multichannel Optical Amplifier Using Semiconductor Optical Amplifier and Rare Earth Element-Added Fiber

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical amplifiers, and more particularly to gain-equalized multichannel optical amplifiers for use in wavelength division multiple optical transmission systems.

The optical amplifier amplifies the signal weakened by the loss of the optical path and signal branch in the optical transmission system to enable long-distance transmission. In the early optical amplifier, there was a semiconductor optical amplifier developed using the principle of a semiconductor laser. As the development of semiconductor manufacturing technology, which is a key technology for the manufacturing method of semiconductors, has been delayed, the basic research has been stopped. Has rapidly reached its current commercial level, and with the proliferation of fiber-optic amplifiers, optical communication systems have enabled long-distance communications of thousands of kilometers.

On the other hand, when the optical transmission system sends different information to multiple wavelengths for large capacity, the research on the wavelength division multiplexing which can increase the capacity of the system is carried out, and the multi-channel optical amplifier having the wide gain wavelength band characteristics. A need arises for optical amplifiers used in such multichannel optical transmission systems that the gain between transmission channels, i.e., transmission wavelength, must be uniform. However, in the case of erbium-doped optical amplifiers, which are widely used, the amplification rate varies depending on the wavelength due to the fundamental gain characteristics of erbium ions in the optical fiber, that is, the gain characteristics of the erbium-added optical amplifiers for long-distance transmission in a multichannel optical transmission system. As the nonuniformity is accumulated through several amplifications and degrades the final light intensity and signal-to-noise ratio characteristics between the signal channels, a method using an optical filter opposite to the gain characteristic to improve the gain nonuniformity of the erbium-doped optical amplifier, In order to compensate for the difference between the optical channels at the receiving end, a method of adjusting the light intensity between each channel at the transmitting end and a method of controlling gain equalization by adjusting the intensity of the excitation light at each of the two optical amplifiers are used.

However, the optical filter has not yet implemented a device with a perfect state, and when controlling the light intensity between the transmission channel has a significant problem in terms of flexibility of the system, and gain equalization using two optical fiber amplifiers In this case, the complexity of optimization and the expansion of gain bandwidth are difficult.

Fig. 1 shows the structure of a rare earth element-added optical fiber amplifier composed of two stages, which is most commonly used for gain equalization as a conventional technique.

The optical fiber amplifier of FIG. 1 includes a first stage optical amplifier 1 using a rare earth element-added optical fiber and a second stage optical fiber amplifier 2 using a rare earth element-added optical fiber. In the optical fiber amplifier, when the first stage optical fiber amplifier is used as the erbium-doped optical fiber, the slope of the circled portion in the gain characteristic curve 3 of the optical signal as shown in Fig. 2 and the second stage optical fiber amplifier are erbium When used as an additive optical fiber, when the portions indicated by circles in the gain characteristic curve 4 of the optical signal as shown in Fig. 2 are superimposed, the gain characteristic slope is opposite, so that the gain of the final output is uniform in this region.

In order to obtain the above characteristics, different density reversal conditions are used. Since the inclination and gain bandwidth of the gain characteristics are not freely adjusted due to the basic characteristics of the rare earth element-added optical fiber, the gain equalization condition is difficult and the gain bandwidth is increased. If it is difficult and has a difference in gain characteristics according to the wavelength, it is a factor that lowers the reception sensitivity of the receiver when applied to the wavelength division multiplexing optical transmission system. Although the erbium-added optical amplifier having the structure shown in FIG. 1 is used, not only is it difficult to optimize but also has difficulty in securing a wide gain bandwidth.

On the other hand, in the case of semiconductor optical amplification devices, the technology for reducing the significant residual reflectance of optical amplification devices has been remarkably improved in recent years, and its characteristics have been remarkably improved. Since the wide and gain peaks can be adjusted by changing the injection current, efforts to use them as multi-channel optical amplifiers continue.

However, the optical amplification device has a gain fluctuation due to an increase in the influence of residual reflectance in a high gain region, and has not yet secured a flattened gain bandwidth enough to be used in a system. When the amplifier and the semiconductor optical amplifying device are used at the same time, it is possible to combine the high gain characteristics of the optical fiber amplifier and the flexibility of adjusting the gain characteristics of the semiconductor optical amplifier, thereby obtaining a multi-channel optical amplifier having a wide gain band and high gain characteristics. Can be.

As mentioned above, in the conventional wavelength division multiplexing transmission system, when the gain difference between wavelengths is generated, the performance of the system is degraded. Therefore, the conventional method used to compensate for the gain difference between the wavelengths is difficult to obtain the equalization of the gain. The disadvantage is narrow bandwidth.

Accordingly, an object of the present invention for solving the problems of the prior art is to provide a gain equalized optical amplifier that can extend the gain bandwidth and ease of gain equalization by using a semiconductor optical amplifier and a rare earth element-added optical fiber amplifier simultaneously.

The gain-equalized multichannel optical amplifier of the present invention for achieving the above object is a method for controlling the gain change according to the change of the input light intensity and the density inversion by using an optical fiber to which a rare earth element that performs primary optical amplification is added. And a second stage optical amplifier configured to perform secondary optical amplification and gain equalization of the light amplified through the first stage optical amplifier and the first stage optical amplifier using a semiconductor optical amplifier.

1 is a gain-equalized multi-channel optical amplifier composed of two stages using only a rare earth element-added optical fiber,

2 is a graph showing an optical gain spectrum;

3 is a gain-equalized multichannel optical amplifier using a rare earth element-added optical fiber and a semiconductor optical amplifier according to the present invention;

4 is a graph showing the light gain spectrum of FIG.

5 is a graph showing the measured light gain spectrum when the erbium-doped optical fiber is high density inverted;

6 is a graph showing the measured light gain spectrum when the erbium-doped optical fiber is low density inverted;

7 is a graph showing an example of an optical gain spectrum measured according to an injection current of a semiconductor optical amplifier.

<Explanation of symbols for main parts of drawing>

1: First stage optical amplification part using rare earth element-added optical fiber

2: second stage optical amplification part using rare earth element-added optical fiber

3: Gain characteristic curve according to the wavelength of the first stage optical amplification section when the rare earth element-added optical fiber is used as the erbium-doped optical fiber

4: Gain characteristic curve according to the wavelength of the second stage optical amplification part when the rare earth element-added optical fiber is used as the erbium-doped optical fiber

5: first stage optical amplification part using rare earth element-added optical fiber

6: second stage optical amplifier using semiconductor optical amplifier

7: Gain characteristic curve according to the wavelength of the first stage optical amplification section when the rare earth element-added optical fiber is used as the erbium-doped optical fiber

8: Gain characteristic curve according to the wavelength of the second stage optical amplifier using semiconductor optical amplifier

Hereinafter, with reference to the accompanying drawings will be described an embodiment of the present invention;

Figure 3 shows a gain-equalized multi-channel optical amplifier according to the present invention, comprising a first stage optical amplifier 5 using a rare earth element-added optical fiber and a semiconductor optical amplifier for secondary optical amplification and gain equalization. It consists of a two stage optical amplifier 6.

The gain-equalized optical amplifier of the present invention has a gain characteristic in which the gain of the input signal of the light amplified through the first stage optical amplifier 5 using the rare earth element-added optical fiber is lowered as the wavelength is longer. The gain of the input signal passing through the two-stage semiconductor optical amplification section 6 is shown by the curve 7 and the gain characteristic curve 8 increases as the wavelength becomes longer, as shown in FIG. By using the gain characteristic of, gain equalization is possible. That is, unlike the case of FIG. 1, when using a semiconductor optical amplifier, the gain peak value and the gain characteristic slope can be freely converted by changing the injection current, so that the gain equalization is easy and the gain bandwidth is increased. In this case, the semiconductor optical amplifier uses a semiconductor optical amplifier to control the change in the gain of the first stage optical amplifier according to the change in the input light intensity.

In addition, if the arrangement in FIG. 2 is reversed, gain equalization is possible. When the long wavelength region of the gain peak is used in the semiconductor optical amplification device of the first stage, the gain characteristics of the fiber amplifier with the rare earth element added in the second stage are used. Gain equalization is also possible, and gain equalization can be achieved by changing the gain characteristics by adjusting the injection current of the semiconductor optical amplifier of the second stage optical amplifier.

FIG. 5 is an example of an optical gain spectrum measured when density inversion is strongly performed in an erbium-doped fiber amplifier, and has a negative gain characteristic slope in a wavelength range used, and FIG. 6 is measured when density inversion is weak in an erbium-doped fiber amplifier. As an example of the obtained optical gain spectrum, it has a positive gain characteristic slope in the wavelength range of use, and FIG. 7 is an optical gain spectrum measured while varying the injection current of the semiconductor optical amplification device from 50 mA to 200 mA. As shown in, as the injection current increases, the gain peak value of the semiconductor optical amplifier moves to short wavelength, the slope of the gain characteristic changes steeply, and the short wavelength side has a positive gain slope around the gain peak in the gain characteristic curve. The side has a negative gain slope.

As described above, the gain-equalized multi-channel optical amplifier of the present invention can be easily used for gain equalization and increase gain bandwidth by using the optical gain of the semiconductor optical amplifier in combination with the optical fiber amplifier. Unlike the additive fiber amplifier, the position of the gain peak can be adjusted according to the change of the injection current, and the gain slope can be freely adjusted according to the wavelength. Therefore, when combined with the rare earth element-added fiber amplifier, the gain characteristic can be adjusted. Compared with the case of only the rare earth element-added optical fiber amplifier, it has an easy effect.

Claims (1)

In the multi-channel optical amplifier for equalizing the gain of the optical amplifier in the multi-channel optical transmission system, A first stage optical amplification unit for controlling a gain change according to a change in input light intensity and density inversion using an optical fiber to which a rare earth element having primary optical amplification is added; And And a second stage optical amplification unit configured to perform secondary optical amplification and gain equalization using the semiconductor optical amplification device for the light amplified through the first stage optical amplification unit. Channel optical amplifier.

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