KR0183913B1 - Erbium doped fiber amplifier having flat gain and low noise-figure - Google Patents

Abstract

The present invention relates to an erbium-doped optical fiber amplifier having a flat high gain and low noise figure, comprising: a first isolator for blocking the reverse propagation of incident signal light; A pump LD for applying pumping light; A first WDM for splitting and combining the wavelengths of the signal light and the pumped light; A first EDF amplifying the incident light signal into the pumping light of the pump LD and having a negative gain slope; A second WDM for splitting the amplified signal light into a pump light and a signal light; A filter for attenuating signal light of an output signal of the second WDM and having a negative signal attenuation slope; Midway isolator to prevent the signal light passing through the filter in the reverse direction, and to prevent the inflow of the ASE occurs in the next stage; A third WDM for inputting the signal light passed through the Midway isolator, and splitting and coupling the wavelength of the pumping light passed through the second WDM; A second EDF that amplifies the incident light signal, has a positive gain slope, and is made of a different kind of material from the first EDF; And a second isolator for blocking the reverse direction of the signal.

According to the present invention, while having a very high gain against the change in the intensity of the incident light and the pump power, it is flat and has gain characteristics and low noise figure.

Description

Erbium-doped fiber amplifier with flat high gain and low noise figure

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an optical fiber amplifier, and more particularly, to an erbium-doped optical fiber amplifier having a high gain in a wide transmission wavelength region with a change in the intensity of an incident light signal and a pump power, and flattening the gain according to the wavelength and having a low noise figure .

In general, in the repeater for long-distance optical communication, the conventional optical communication relay method is a method of converting the weakened optical signal into an electrical signal to amplify and then converting it into an optical signal to communicate. In such a relay method, there are many problems such as excessively enlarged and increased noise of the relay amplifier system. An optical amplifier that amplifies the optical signal itself is required as a repeater to compensate for this problem and to efficiently perform the optical amplification.

As such optical amplification repeaters, Erbium Doped Fiber Amplifiers (hereinafter referred to as EDFAs) are receiving a lot of attention as next-generation optical communication optical repeaters. The EDFA is used to amplify the optical signal periodically to prevent attenuation of the optical signal due to the long distance transmission when a large amount of data is transmitted over a long fiber.

EDFAs can be classified according to their use, for use as pre-amplifiers, in-line amplifiers, post-amplifiers, and CATV power boosters. In addition, the EDFA has a wide amplification band in the 1.5 μm region showing the lowest loss of the optical fiber when erbium (Er 3+), which is a rare earth element, is easily spliced with the communication optical fiber, and has high gain characteristics, Low noise and low polarization dependence.

EDFA is used in a Wavelength Division Multiplexing (WDM) system in which information carried on different wavelengths of light is bundled and communicated on a single line. However, in order for EDFA to be used for wavelength division multiplexing (WDM), the spectral gain of EDFA must be high, and flat gain characteristics are required over a wide wavelength range.

FIG. 1 is a block diagram illustrating a general EDFA configuration, and includes two optical isolators 100 and 130 for preventing reverse flow of signals to prevent oscillation of light waves other than signals, and passive light for combining pump light and signal light into a single optical fiber. A wavelength division multiplexer (WDM) 120, which is a device, excites erbium on a part of an optical fiber core to excite erbium doped fiber (EDF, 120) and erbium, which are amplification mediums that amplify by inductive radiation principle. It is composed of a pumping laser diode 140, which is an active optical device that provides the required energy. In the EDFA having such a configuration, the pumping laser diode 140 is connected to the EDF, and the pumping light of the pumping laser diode excites Er3 +, a rare earth element doped in the core of the optical fiber, so that spontaneous emission occurs. do. The center wavelength of the pumping laser diode 5 is 980 nm, and the pumping light output is incident on the erbium-doped optical fiber 120 through the wavelength division multiplexer 110 together with the weak optical signal through the isolator 100 on the input side. . The wavelength division multiplexer 110 combines an optical signal having a wavelength of 1530 nm to 1560 nm and a pumping light having a wavelength of 980 nm to enter the EDF 120. In the EDF 120, optical signals having a wavelength of 1530 nm to 1560 nm are amplified by pumping light having a wavelength of 980 nm. In addition, the isolators 100 and 130 prevent light deterioration of amplification efficiency of the signal light because the light traveling in the reverse direction among the output light by the ASE (amplified spontaneous emission) of the EDF is reflected by other photons.

However, the EDFA has a high gain only for a specific wavelength and has a large gain deviation depending on the wavelength. In particular, there is a problem that the gain spectrum greatly depends on the operating gain range of the EDFA. Thus, when the EDFA is mounted on a real system, it is not only possible to send signals of multiple wavelengths to a single optical line at the same time, but also to reduce the efficiency of optical transmission because the gain decreases depending on the wavelength when the intensity of the pump light decreases after a long time of use. . That is, the wavelength range of the signal light transmittable within the gain change of less than 0.5dB is only about 3nm.

In order to make up for this drawback, efforts have recently been made to planarize the gain of the EDFA. One of them is the pre-filtering EDFA that flattens the gain by using a shear filtering method (hereinafter referred to as pre-filtering) in which a Mach-Zehnder filter is inserted in front of the EDF. FIG. 2 is a block diagram illustrating the structure of the pre-filtering EDFA, and is inserted into the front end of the first isolator 200 and the EDF 240 to block unnecessary backward propagation of the incident light signal. The gender filter 210, the pump laser diode 220 for applying pumping light of 980 nm to amplify the incident signal light, the incident light passing through the mark-gender filter 210 and the wavelength of the pumping light to the EDF 240 A wavelength-division multiplexer 230 that combines wavelength division for transmission, and an erbium-doped optical fiber 240 having a positive gain slope in a wavelength region for obtaining a flat gain characteristic as an amplification medium for amplifying incident signal light. And a second isolator 250 which blocks the reverse direction of the signal and eliminates the gain reduction phenomenon caused by the reflected wave.

On the other hand, referring to the operation principle of the pre-filtering EDFA, the mark-gender filter 210 varies the intensity of the signal light entering the EDF 240 according to the wavelength by varying the attenuation of the input signal with the same power according to the wavelength. Since the EDF 240 has a gain spectrum of 1542 nm and 1552 nm, the short wavelength side has a small gain and the long wavelength side has a large gain, so that the signal light entering the EDF 240 has attenuation at the short wavelength side using the mark-gender filter 210. The smaller, longer wavelengths increase the attenuation, resulting in the opposite of the gain spectrum of the EDF 240. As a result, the attenuation spectrum of the mark-gender filter 210 and the gain spectrum of the EDF 240 complement each other to flatten the gain of the EDFA.

In the case of the pre-filtering EDFA, when the gain is changed to 20.5 dB while the pump power is changed, the gain difference according to the wavelength is within about ± 0.4 dB, and the gain difference is about ± 0.4 dB even when the input signal strength is changed. Within.

However, in the case of the pre-filtering EDFA, the wavelength band that can be transmitted to a single optical path is relatively wide as 10 nm, but the noise figure varies greatly depending on the wavelength, and the value is very high as 8 dB at 1552 nm wavelength. When used as a pre-amplifier installed directly in front of the optical receiver, the EDFA is not suitable because the noise figure is too large. The gain is flattened to within 0.5dB regardless of the wavelength, but the noise figure varies by about 3dB from 5dB to 8dB depending on the wavelength.

In order to solve the above problems, a midway filter method that achieves both the flattening of the gain and the low noise figure in the wavelength range of 1542 nm to 1552 nm by using a Mach-Zehnder filter between the same EDF. There is EDFA. 3 is a block diagram showing the configuration of the EDFA of the intermediate filter type, the first isolator 300 for blocking the reverse progress of the incident signal light, the pump laser diode for applying the pumping light to amplify the incident signal light ( 310, amplifying the incident signal light applied from the first wavelength division multiplexer 320 and the first wavelength division multiplexer 320 to combine the wavelengths of the signal light and the pumping light into wavelengths of the pump laser diode 310. And a first erbium-doped optical fiber 330 having a positive gain slope in a wavelength region for obtaining a flat gain characteristic, a second wavelength division multiplexer 340 for splitting the amplified signal light into a pump light and a signal light, and a second wavelength division. Among the signals of the multiplexer 340, a signal gain is input, rather than a positive gain slope of the first erbium-doped optical fiber in a wavelength region in which a flat gain is to be obtained. A mark-gender filter 350 having a large negative signal attenuation slope, and a wavelength dividing and combining wavelength of the signal light passing through the filter 350 and the pumping light passing through the second wavelength division multiplexer 340. Amplifying the incident light signal with the pump light applied from the three wavelength division multiplexer 360 and the third wavelength division multiplexer 360, and having a positive gain slope in the wavelength region to obtain a flat gain characteristic, the first erbium doped The second erbium-doped optical fiber 370 made of a material of the same type as the optical fiber and the second isolator 380 which blocks the reverse propagation of the signal and eliminates the gain reduction phenomenon caused by the reflected wave.

Looking at the EDFA operation of the intermediate filter method, the signal light of the same intensity that is irrespective of the wavelength is passed through the first EDF 330 and the intensity of the amplified signal light is different depending on the wavelength. Here, the amplified signal light is filtered by the mark-gender filter 350 through the second WDM 340 and then amplified by the second EDF 370 after the third WDM 360 so that the overall gain and noise figure are the intensity of the input signal light. B, it is flattened in a given wavelength region (1542nm-1552nm) irrespective of the intensity of the pump light. The first WDM 320 combines with the signal light to excite the first EDF 330, and the remaining pump light splits with the signal light in the second WDM 340, avoiding the mark-gender filter 350, and recombining with the signal light in the third WDM 360. To excite the second EDF 370. That is, the attenuation spectrum of the mark-gender filter 350 and the gain spectra of the two EDFs 330 and 370 complement each other, thereby flattening the gain of the EDFA.

FIG. 4 shows that the center wavelength of the pump laser diode is 980 nm, the current of the pump laser diode is 160 mA (the pump power is 67 mW) using the intermediate filter type EDFA, and the erbium concentration is lowered by the low aluminum-containing alumino-low silicate EDF. As a diagram showing the measurement results of gain and noise figure according to signal input power variation at three input signal wavelengths of 1542.3 nm, 1546.8 nm, and 1552.4 nm for an EDF of 260 ppm, the intensity of the pump light is measured for the three wavelengths. The measured measurements were flattened for gain within ± 0.3dB and noise figure within ± 0.5dB.

FIG. 5 is a diagram illustrating a measurement result of gain and noise index according to a change in pump laser diode current under the same conditions as in FIG. 4. FIG. 5 is a result obtained by changing the intensity of signal light for three wavelengths of 1542 nm, 1546 nm, and 1552 nm. The over noise figure is flattened within ± 0.3dB.

However, the EDFA of the intermediate filter type uses the EDF having a positive slope between 1542 nm and 1552 nm in the front and rear of the mark-gender filter. increase the ratio. In this case, the gain of EDFA can be flattened, but the overall gain is lowered and the noise figure is somewhat higher. If the EDFA's gain is low, it will be costly in terms of increasing the total quantity of EDFA when used as a repeater in long distance communication. In other words, if one of the EDFAs in the middle of several EDFAs fails to function at the end of their lifespan, they have a significant effect on the overall communication. Therefore, the gap between the EDFAs must be narrowed and more EDFAs must be installed. In a given wavelength range (1542nm-1552nm), small signal gain is up to 18 ± 0.3dB with a pump power of 67mW, and a noise figure (NF) of 5.0 ± 0.5dB, regardless of wavelength. The EDFA with no gain flattening is too low considering that the gain in this wavelength range is 25-30 dB. In addition, when EDF with positive gain spectrum in the wavelength range from 1542nm to 1552nm is used before and after the mark-gender filter, the gain spectrum is higher on the long wavelength side and the gain spectrum is lower on the short wavelength side. Becomes larger, and to reduce this, the absorption rate of the mark-gender filter must be further increased. This leads to the problem that the gain of EDFA is inevitably lowered.

The present invention was created to solve the above-mentioned problems of the prior art, and the gain spectrum according to the wavelength is complemented by different kinds of EDFs located at the front and rear of the filter, and an optical isolator is placed behind the filter to provide the rear end EDF. By preventing the amplified spontaneous emission (ASE) occurring at the front end, it provides an erbium-doped fiber amplifier having a high gain in the approximately 10 nm transmission wavelength range, but also flattening the gain according to the wavelength and having a low noise figure. There is a purpose.

Figure 1 shows a block diagram of the configuration of a typical EDFA.

Figure 2 shows a block diagram of the structure of the pre-filtering EDFA.

3 is a block diagram showing the configuration of the EDFA of the intermediate filter method.

4 shows that the center wavelength of the pump laser diode is 980 nm, the current of the pump laser diode is 160 mA (the pump power is 67 mW) using EDFA of the intermediate filter method, and the aluminum content of the alumino-low-mole silicate EDF is 260 ppm of erbium. For the EDF, it is a graph showing the measurement results of the gain and noise figure according to the change of signal input power at three input signal wavelengths of 1542.3 nm, 1546.8 nm, and 1552.4 nm.

FIG. 5 is a diagram illustrating a measurement result of gain and noise figure according to a change in the pump laser diode current under the same conditions as in FIG. 4.

FIG. 6 is a block diagram illustrating an embodiment of an erbium-doped fiber amplifier having a high gain in a wide transmission wavelength range while flattening the gain according to the wavelength and having a low noise figure according to the present invention.

FIG. 7 shows measurement results of varying the intensity of the signal light from -35 dB to -15 dB for three wavelengths of 1542.2 nm, 1546.9 nm, and 1553 nm.

FIG. 8 shows the results of measuring the gain and noise figure by varying the intensity of the pump light from 13mW to 67mW for three wavelengths of 1542.2 nm, 1546.9 nm and 1553 nm.

FIG. 8 shows the results of measuring the gain and noise index by changing the intensity of the pump light from 13 kHz to 67 kHz for three wavelengths of 1542.2 nm, 1546.9 nm, and 1553 nm.

According to the present invention for achieving the above object, an erbium-doped optical fiber amplifier having a high gain in the approximately 10 nm transmission wavelength region and flattening the gain according to the wavelength and having a low noise figure is applied to the reflected wave by blocking the reverse propagation of the incident signal light. A first isolator for eliminating the gain deterioration phenomenon caused by the control unit; A pump laser diode that applies pumping light having a wavelength different from that of the incident signal light to amplify the incident signal light; A first wavelength division multiplexer for dividing and coupling the wavelength of the signal light passed through the first isolator and the pumping light; A first erbium-doped optical fiber amplifying the incident light signal applied from the first wavelength division multiplexer into the pumping light of the pump laser diode and having a negative gain slope in a wavelength region for obtaining a flat gain characteristic; A second wavelength division multiplexer for wavelength-dividing the signal light amplified by the first erbium-doped optical fiber into a pump light and a signal light; Attenuates the signal by inputting the signal light of the output signal of the second wavelength division multiplexer, and has a negative signal attenuation slope having an absolute value smaller than the gain slope of the first erbium-doped optical fiber in the wavelength region to obtain a flat gain. filter; Midway isolator to prevent the signal light passing through the filter in the reverse direction, and to prevent the inflow of the ASE generated in the next stage; The third wavelength division multiplexer which receives the signal light passed through the ASE isolator as an input, amplifies the signal light through the first erbium doped optical fiber and passes through the second wavelength division multiplexer, and splits and combines the wavelength of the remaining pumping light. ; A second amplification of the incident light signal with a pump light applied from the third wavelength division multiplexer, having a positive gain slope in a wavelength region for obtaining a flat gain characteristic, and a material of a different type from the first erbium-doped optical fiber Erbium-doped optical fiber; And a second isolator for blocking a backward movement of the signal passing through the second erbium-doped optical fiber to eliminate a decrease in amplification efficiency caused by the reflected wave.

The pump laser diode is preferably a pump laser diode having a wavelength of 980 nm. The first erbium-doped optical fiber is a high aluminum-containing erbium-doped optical fiber having a negative gain slope in a wavelength region having a flat gain characteristic, and the second erbium-doped optical fiber is positive in a wavelength region having a flat gain characteristic. A low aluminum-containing erbium-doped optical fiber having a gain slope of?, Wherein the filter attenuates the signal by inputting the signal light of the output signal of the second wavelength division multiplexer, and the first erbium doped in the wavelength region to obtain a flat gain. Preferably, the filter has a signal attenuation slope that reduces a gain difference between the optical fiber and the second erbium-doped optical fiber.

In addition, an optical fiber amplifier having a high gain and low noise figure, which is flat against changes in the intensity of an incident light signal and a pump power, has another configuration for achieving the above object. A first isolator to eliminate the degradation phenomenon; A pump laser diode that applies pumping light having a wavelength different from that of the incident signal light to amplify the incident signal light; A first wavelength division multiplexer for dividing and coupling the wavelength of the signal light passed through the first isolator and the pumping light; A first erbium-doped optical fiber amplifying the incident light signal applied from the first wavelength division multiplexer into the pumping light of the pump laser diode and having a positive gain slope in a wavelength region to obtain a flat gain characteristic; A second wavelength division multiplexer for wavelength-dividing the signal light amplified by the first erbium-doped optical fiber into a pump light and a signal light; A filter having a positive attenuation slope having an absolute value smaller than a gain slope of the first erbium-doped optical fiber in a wavelength range in which a signal gain among the output signals of the second wavelength division multiplexer is input to obtain a flat gain; Midway isolator to prevent the signal light passing through the filter in the reverse direction, and to prevent the inflow of the ASE generated in the next stage; The third wavelength division multiplexer which receives the signal light passed through the ASE isolator as an input, amplifies the signal light through the first erbium doped optical fiber and passes through the second wavelength division multiplexer, and splits and combines the wavelength of the remaining pumping light. ; A second amplification of the incident light signal with a pump light applied from the third wavelength division multiplexer, having a negative gain slope in a wavelength region for obtaining a flat gain characteristic, and a material of a different type from the first erbium-doped optical fiber Erbium-doped optical fiber; And a second isolator for blocking a backward movement of the signal passing through the second erbium-doped optical fiber to eliminate a decrease in gain caused by the reflected wave.

The first erbium-doped optical fiber is a low aluminum containing erbium-doped optical fiber having a positive gain slope in the wavelength region to have a flat gain characteristic, and the second erbium-doped optical fiber is negative in a wavelength region to have a flat gain characteristic. A high aluminum-containing erbium-doped optical fiber having a gain slope, wherein the filter attenuates the signal by inputting the signal light of the output signal of the second wavelength division multiplexer, and the first erbium-doped optical fiber in the wavelength region to obtain a flat gain. And a filter having a signal attenuation slope for reducing a gain difference between the second erbium-doped optical fiber and the second erbium-doped optical fiber.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 6 is a block diagram illustrating an erbium-doped optical fiber amplifier having a high gain in a wide transmission wavelength region and a flattened gain according to the wavelength, and having a low noise figure, according to the present invention. ), Pump laser diode 610, first wavelength division multiplexer 620, first erbium-doped optical fiber 630, second wavelength division multiplexer 640, filter 650, midway isolator 690, third wavelength The split multiplexer 660, the second erbium-doped optical fiber 670, and the second isolator 680.

The first isolator 600 serves to eliminate the decrease in gain caused by the reflected wave by blocking the reverse direction of the incident signal light, isolating around 40dB. In order to amplify the incident signal light, the pump laser diode 610 applies pumping light having a wavelength different from that of the incident signal light. The pump laser diode 610 has better amplification efficiency and better noise figure characteristics than the pump LD having a wavelength of 1480 nm. It is a pump LD with a pump light wavelength of 980 nm of Seastar (the name of a pump laser diode manufacturer) with a maximum power of 77 mW.

The first wavelength division multiplexer 620 combines the wavelength of the signal light passed through the first isolator with the wavelength of the pumping light and combines the wavelength. The fuse type has an insertion loss of about 0.3 dB and an isolation of 18 dB or more. The first erbium-doped optical fiber 630 is a high aluminum-containing erbium-doped optical fiber having an erbium doping concentration of 260 ppm, and has a negative gain slope in a wavelength region for obtaining a flat gain characteristic, and from the first wavelength division multiplexer 620. The incident light signal is amplified by the pumping light of the pump laser diode 610. The second wavelength division multiplexer 640 divides the signal light amplified through the first erbium-doped optical fiber 630 into a pump light and a signal light.

The filter 650 is positioned between the second wavelength division multiplexer 640 and the third wavelength division multiplexer 650, and attenuates the signal by using the signal light among the output signals of the second wavelength division multiplexer 640. And a mark-gender filter having a negative signal attenuation slope having an absolute value smaller than a gain slope of the first erbium-doped optical fiber 630 in a wavelength region for obtaining a flat gain, wherein the first erbium has a flat gain characteristic. It serves to reduce the gain difference between the doped optical fiber 630 and the second erbium doped optical fiber 670.

The midway isolator 690 prevents the signal light passing through the filter 650 from traveling in the reverse direction, and prevents the ASE generated at the next stage from entering the first EDF 630 at the front stage, thereby improving amplification efficiency. Do it.

The third wavelength division multiplexer 660 amplifies the signal light passed through the Midway isolator 690 and the signal light through the first erbium-doped optical fiber 630 passed through the second wavelength division multiplexer 640. The pumping light is input and combined by wavelength division. The second erbium-doped optical fiber 670 amplifies the incident light signal with the pump light applied from the third wavelength division multiplexer 660 and has a positive gain slope in the wavelength region to obtain flat gain characteristics. It is a low aluminum containing erbium-doped optical fiber which is a different kind of material from the erbium-doped optical fiber 630. The second isolator 680 eliminates the gain reduction caused by the reflected wave by blocking the backward movement of the signal passing through the second erbium-doped optical fiber 670.

Meanwhile, the operation of the present invention will be described. When the signal light of the same intensity coming in irrespective of the wavelength passes through the first isolator 600 which prevents the reverse travel and is input to the first WDM 620 together with the pump light of the pump LD 610, the pump light is 1542 nm to 1552 nm. Amplified through a first EDF 630 having a negative gain spectrum therebetween. The signal light amplified by the first EDF 630 and the pump light remaining after exciting the first EDF 630 are divided in the second WDM 640. The pump light output from the second WDM 640 is combined with the signal light directly through the third WDM 660 without being attenuated by the mark-gender filter 650 to excite the second EDF 670 to amplify the signal light. The signal light amplified by the second EDF 670 is prevented from being reversed by the second isolator 680.

That is, since the first EDF 630 has a negative gain slope, the gain on the short wavelength side is higher than the gain of the long wavelength signal light. In addition, since the second EDF 670 has a positive gain spectrum in the given wavelength range, that is, between 1542 nm and 1552 nm, when a signal light having the same intensity but different wavelength is introduced, the second EDF 670 amplifies the signal at the longer wavelength side higher than the short wavelength. Complementary to the first EDF 630 to reduce the gain difference.

However, the gain spectra of the two EDFs 630 and 670 are not exactly symmetrical, so they do not completely compensate for the difference in wavelength. Therefore, by using a tunable mark-gender filter 650 between the EDFs 630 and 670, the signal light output from the second WDM 640 is attenuated to an appropriate size to obtain a gain difference according to a wavelength. As a result, the gain is flattened with high gain. At this time, when the midway traveling backward from the second EDF 670 at the rear end flows into the first EDF 630 at the front end, the amplification efficiency of the signal light is reduced, and thus the Midway isolator behind the filter 650 is prevented. (690) was inserted to increase the amplification efficiency. The mark-gender filter 650 is placed behind the pump LD 610 to have a low noise figure.

FIG. 7 shows measurement results of varying the intensity of the signal light from -35 dB to -15 dB for three wavelengths of 1542.2 nm, 1546.9 nm, and 1553 nm. The gain difference is flattened to within ± 0.2 dB and the noise figure is within ± 0.3 dB. can see. When the signal light intensity was -35dB and the pump power was 67mW, the small signal gain was 25.5 ± 0.2dB, the noise figure was 3.7 ± 0.2dB, and the gain was very high, and the noise figure was very low. FIG. 8 shows the results of measuring gain and noise figure by varying the intensity of the pump light from 13mW to 67mW for three wavelengths of 1542.2 nm, 1546.9 nm, and 1553 nm. The gain and noise figure are flattened within ± 0.2 dB. can see.

Meanwhile, another embodiment of the present invention will be described. Another embodiment of the present invention is similar in component to that of the embodiment, but the filter 650 and the two EDFs 630 and 670 are different from those used in the embodiment. That is, the first EDF 630 is a low aluminum containing EDF having a positive gain slope, and the second EDF 670 is a high aluminum containing EDF having a negative gain slope. The filter 650 adjusts the gain slope to reduce the gain difference due to the difference between the gain slopes of the first EDF 630 and the second EDF 670. Even in this configuration, the first EDF 630 and the second EDF 670 complement each other so as to have a flattened gain in a gain plane, and the fine gain difference is reduced by the filter 650, resulting in a pump. Even if the LD and the input power are changed, the gain is very high at a transmission wavelength of approximately 10 nm, but the gain characteristics are very flat.

The effects of the present invention are described above. Using EDFs with the same gain spectrum at the front and back leads to higher gain wavelengths and lower gain wavelengths, resulting in a larger gain deviation over the wavelength of the entire EDFA. The absolute gain of EDFA is lowered because the absorption ratio of the mark-gender filter must be increased to prevent this phenomenon.

In the present invention, the gain spectra of the two EDFs are opposite to each other, and the gain and noise figure of the EDFA are simultaneously flattened by using a filter and an isolator between the two EDFs. Compared with the prior art, the absolute value of the EDFA gain is higher and the noise figure is lower, resulting in improved EDFA performance.

Since the EDF with complementary gain spectrum is used, the extinction ratio of the mark-gender filter can be reduced and high gain can be obtained. If the complementary characteristics of the two EDFs are improved in the future, the filter does not need to be used. It is expected that the gain of EDFA can be flattened over a wider wavelength range.

Midway isolators block only the reverse ASE, so using a filter that can pass a given wavelength (1542nm-1552nm) and block the forward ASE is expected to yield higher gains.

In addition, if the characteristics of the EDF are well selected, the flattened EDFA may be configured to be wider than the flattened 10 nm wavelength range in the present invention. When using the same type of EDF, the gain is higher and the noise figure is lower. Therefore, it is useful as a pre-amplifier as well as long distance communication.

It should be noted that, in the above embodiments, the absolute value of the gain slope of the first EDF 630 and the second EDF 670 may be slightly larger. And the difference is adjusted through the filter 650, resulting in a flat gain and placing the isolator behind the filter to prevent backward ASE in the back end, so many of the things not mentioned by the above embodiments are not mentioned. Still other embodiments are possible for those skilled in the art to which the present invention pertains.

Claims (7)

An optical fiber amplifier having a high gain and a low noise figure that is flat with a change in the intensity of an incident light signal and a pump power, the optical fiber amplifier comprising: a first isolator which blocks a backward movement of the incident signal light and eliminates a gain reduction phenomenon due to reflected waves; A pump laser diode that applies pumping light having a wavelength different from that of the incident signal light to amplify the incident signal light; A first wavelength division multiplexer for dividing and coupling the wavelength of the signal light passed through the first isolator and the pumping light; A first erbium-doped optical fiber amplifying the incident light signal applied from the first wavelength division multiplexer into the pumping light of the pump laser diode and having a negative gain slope in a wavelength region for obtaining a flat gain characteristic; A second wavelength division multiplexer for wavelength-dividing the signal light amplified by the first erbium-doped optical fiber into a pump light and a signal light; Attenuates the signal by inputting the signal light of the output signal of the second wavelength division multiplexer, and has a negative signal attenuation slope having an absolute value smaller than the gain slope of the first erbium-doped optical fiber in the wavelength region to obtain a flat gain. filter; Midway isolator to prevent the signal light passing through the filter in the reverse direction, and to prevent the inflow of the ASE generated in the next stage; The third wavelength division multiplexer which receives the signal light passed through the ASE isolator as an input, amplifies the signal light through the first erbium doped optical fiber and passes through the second wavelength division multiplexer, and splits and combines the wavelength of the remaining pumping light. ; A second amplification of the incident light signal with a pump light applied from the third wavelength division multiplexer, having a positive gain slope in a wavelength region for obtaining a flat gain characteristic, and a material of a different type from the first erbium-doped optical fiber Erbium-doped optical fiber; And a second isolator for blocking a reverse propagation of the signal passing through the second erbium-doped optical fiber to eliminate gain reduction caused by reflected waves. The erbium-doped fiber amplifier of claim 1, wherein the filter is a mark-gender filter for filtering signal light according to a wavelength. The erbium-doped optical fiber amplifier of claim 1, wherein the pump laser diode is a pump laser diode having a wavelength of 980 nm. The erbium-doped optical fiber of claim 1, wherein the first erbium-doped optical fiber is a high aluminum containing erbium-doped optical fiber having a negative gain slope in a wavelength region to have a flat gain characteristic, and the second erbium-doped optical fiber has a flat gain characteristic. Low-aluminum-containing erbium-doped optical fiber having a positive gain slope in the wavelength region, and the filter attenuates the signal by inputting the signal light of the output signal of the second wavelength division multiplexer and obtains a flat gain in the wavelength region. An erbium doped fiber amplifier having a flat high gain and low noise figure, characterized in that the filter having a signal attenuation slope to reduce the gain difference between the first erbium-doped optical fiber and the second erbium-doped optical fiber. An optical fiber amplifier having a high gain and a low noise figure that is flat with a change in the intensity of an incident light signal and a pump power, the optical fiber amplifier comprising: a first isolator which blocks a backward movement of the incident signal light and eliminates a gain reduction phenomenon due to reflected waves; A pump laser diode that applies pumping light having a wavelength different from that of the incident signal light to amplify the incident signal light; A first wavelength division multiplexer for dividing and coupling the wavelength of the signal light passed through the first isolator and the pumping light; A first erbium-doped optical fiber amplifying the incident light signal applied from the first wavelength division multiplexer into the pumping light of the pump laser diode and having a positive gain slope in a wavelength region to obtain a flat gain characteristic; A second wavelength division multiplexer for wavelength-dividing the signal light amplified by the first erbium-doped optical fiber into a pump light and a signal light; A filter having a positive attenuation slope having an absolute value smaller than a gain slope of the first erbium-doped optical fiber in a wavelength range in which a signal gain among the output signals of the second wavelength division multiplexer is input to obtain a flat gain; Midway isolator to prevent the signal light passing through the filter in the reverse direction, and to prevent the inflow of the ASE generated in the next stage; The third wavelength division multiplexer which receives the signal light passed through the ASE isolator as an input, amplifies the signal light through the first erbium doped optical fiber and passes through the second wavelength division multiplexer, and splits and combines the wavelength of the remaining pumping light. ; A second amplification of the incident light signal with a pump light applied from the third wavelength division multiplexer, having a negative gain slope in a wavelength region for obtaining a flat gain characteristic, and a material of a different type from the first erbium-doped optical fiber Erbium-doped optical fiber; And a second isolator for blocking a reverse propagation of the signal passing through the second erbium-doped optical fiber to eliminate gain reduction caused by reflected waves. The erbium-doped fiber amplifier of claim 1, wherein the filter is a mark-gender filter for filtering signal light according to a wavelength. The erbium-doped optical fiber of claim 5, wherein the first erbium-doped optical fiber is a low aluminum-containing erbium-doped optical fiber having a positive gain slope in a wavelength region to have a flat gain characteristic, and the second erbium-doped optical fiber has a flat gain characteristic. A high aluminum-containing erbium doped optical fiber having a negative gain slope in a wavelength range, wherein the filter attenuates a signal by inputting signal light of an output signal of the second wavelength division multiplexer and obtains a flat gain in a wavelength range. An erbium doped fiber amplifier having a flat high gain and low noise figure, characterized in that the filter having a signal attenuation slope to reduce the gain difference between the first erbium-doped optical fiber and the second erbium-doped optical fiber.