KR980013063A - Optical fiber amplifiers with low noise figure and flat gain - Google Patents

Abstract

The present invention relates to an optical fiber amplifier having a low noise figure and a flat gain with respect to changes in the intensity of an incident light signal and a pump power, the optical fiber amplifier comprising: a first isolator for blocking reverse travel of incident signal light; A pump laser diode for applying light for pumping; A first wavelength division multiplexer for wavelength-division-combining the wavelengths of the signal light and the pump light; A filter having a negative signal attenuation slope having an absolute value larger than a positive gain slope of the first erbium doped optical fiber having a positive slope by amplifying an incident light signal with light for pumping the pump laser diode; A third wavelength division multiplexer for wavelength-divisionally combining the wavelengths of the signal light and the pumping light; A second erbium doped optical fiber amplifying the signal light and having a positive gain slope; And a second isolator for blocking the reverse propagation of the signal that has passed through the second erbium doped optical fiber.

According to the present invention, it has a low noise figure and a flat gain for the intensity of the incident light signal and the change of the pump power.

Description

Optical fiber amplifiers with low noise figure and flat gain

The present invention relates to an optical fiber amplifier, and more particularly, to an erbium doped optical fiber amplifier having a low noise figure and flattening a gain according to a wavelength in a transmission wavelength region of approximately 10 nm.

Optical communication is information transmission using light, optical fiber is used as a transmission line, pulses of light are installed, a reproduction repeater is installed in the middle of transmission line to discriminate optical pulses, amplified with a clean waveform for each repeater, to be. In the conventional optical communication relay system, a weak optical signal is converted into an electrical signal, amplified, and then converted into an optical signal for communication. In such a relay system, there are many problems such that the relay amplifier system becomes excessively large and the noise increases. An optical amplifier for amplifying the optical signal itself is required as a repeater for compensating for such problems and performing optical amplification efficiently.

An Erbium Doped Fiber Amplifier (hereinafter referred to as EDFA) has been widely spotlighted as an optical repeater for the next generation optical communication as the above-described optical amplification repeater. The EDFA is used to periodically amplify an optical signal to prevent attenuation of the optical signal due to long-distance transmission, when a large amount of data is transmitted over a single optical fiber over a long distance.

The general structure of the EDFA can be distinguished by the wavelength and position of a pump diode laser diode, and currently commonly used pump wavelengths are 980 nm and 1480 nm. In addition, the EDFA may be a pre-amplifier, an in-line amplifier, a post-amplifier, and a CATV power booster depending on the application.

In addition, the EDFA has a broad amplification band in the 1.5 탆 range showing the lowest loss of the optical fiber when erbium (Er3 +), which is a recurrent earth element, is added, and is easily spliced to a communication optical fiber and has a high gain characteristic, Low noise and low polarization. In addition, the EDFA is used in a WDM (Wavelength Division Multiplexing) system in which information contained in lights of different wavelengths are grouped and communicated by a single line. However, in order for the EDFA to be used for wavelength division multiplexing (WDM), the spectrum gain of the EDFA must be high and a flat gain characteristic over a wide wavelength band is required.

FIG. 1 is a block diagram of a general EDFA. The optical isolator 100 includes two optical isolators 100 and 130 for preventing backflow of a signal and preventing oscillation of a light wave other than a signal, A wavelength division multiplexer (WDM) 120, a passive optical element, an erbium doped fiber (EDF) 120 and an erbium doped fiber (Erbium doped fiber) 120, And a laser diode 140 for pumping, which is an active optical device that provides the energy required for excitation.

In the EDFA having such a structure, the pumping laser diode 140 is connected to the EDF, and the pumping light of the pumping laser diode excites the erbium-doped Er3 + doped in the core of the optical fiber to cause spontaneous emission . 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 with a 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. The isolators 100 and 130 prevent distortion of the input signal due to the light traveling in the opposite direction and the reflected light reflected by other light intensities out of the output light by the amplified spontaneous emission (ASE) of the EDF.

However, the EDFA has a high gain only for a specific wavelength and has a large gain deviation depending on a 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 an actual system, it is impossible to simultaneously transmit signals of several wavelengths to one optical line, and when the intensity of the pump light after the long use is reduced, the gain reduction depending on wavelength is varied, none. That is, the wavelength range of the signal light that can be transmitted within 0.5 dB of the gain change is only about 3 nm in the EDFA.

In order to overcome such disadvantages, efforts have been actively made to flatten the gain of the EDFA. One of them is the EDFA, in which the gain is flattened by using a pre-filtering method in which a Mach-Zehnder filter is inserted in front of the EDF.

2 shows a block diagram of the structure of the pre-filtering EDFA and includes a first isolator 200, a mark-gender filter 210, a pump laser diode 220, a wavelength division multiplexer 230, an erbium doped optical fiber 240 and a second isolator 250.

The first isolator 200 blocks unnecessary reverse propagation of an incident light signal and eliminates interference due to reflected waves. The Mach-Zehnder filter 210 is inserted in front of the EDF 240 The signal light is filtered according to the wavelength and the signal of the first isolator 200 is inputted to reduce the signal attenuation in the short wavelength side of the wavelength range where the flat gain is to be obtained and the signal attenuation in the long wavelength side is increased. The pump laser diode 220 applies a pumping light having a wavelength different from that of the incident light signal to amplify the incident light signal, and is a pump LD having a pump wavelength of 980 nm.

The wavelength division multiplexer (WDM) 230 wavelength-divides and combines the incident light and the wavelength of the pumping light to pass through the mark-gender filter 210 to the EDF 240, and transmits the erbium doped optical fiber (EDF) 240 Is an amplification medium for amplifying the incident light signal applied from the wavelength division multiplexer (WDM) 230 with light for pumping the pump laser diode (LD) 220, and is a positive amplification medium in a wavelength range in which a flat gain characteristic is to be obtained Low-mannosilicate, erbium-doped optical fiber with a positive gain slope. The second isolator 250 blocks the reverse propagation of the signal that has passed through the erbium doped optical fiber 240, thereby eliminating the interference due to the reflected wave.

Meanwhile, the operation of the pre-filtering EDFA will be described. In the mark-gender filtering 210, the intensity of a signal input to the EDF 240 varies depending on the wavelength by making the input signal having the same power different in attenuation according to the wavelength And the EDF 440 has a gain of a short wavelength side and a gain of a long wavelength side in the 1542 nm and 1552 nm regions of the gain spectrum, the signal light entering the EDF 240 is attenuated by the mark- And the longer wavelength side increases the attenuation so as to be opposite to the gain spectrum of the EDF 240 as a result. As a result, the attenuation spectrum of the mark-gender filter 210 and the gain spectrum of the EDF 240 are complementary to each other to flatten the gain of the EDFA. The gain spectra of the pre-filtering EDFA 240 are complementary to each other Thereby flattening the gain of the EDFA. In the case of the pre-filtering EDFA, when the gain is changed to 20.5 dB while changing the pump power, the gain difference according to the wavelength is within ± 0.4 dB as shown in FIG. 4, The gain difference is within +/- 0.4 dB as shown in Fig. That is, FIG. 3 shows that the center wavelength of the pump laser diode is 980 nm, the current of the pump laser diode is 160 mA (pump power is 67 mW), and the EDF having the erbium concentration of 260 ppm and the length of 17 m by the low- 1542.3 nm, 1546.8 nm, and 1552.4 nm, respectively, and the results of measurement of gain and noise figure according to signal input power change at three input signal wavelengths. And FIG. 4 is a graph showing a measurement result of a gain and a noise figure according to a change of the pump laser diode current under the condition of FIG.

However, in the case of the pre-filtering EDFA, the wavelength band that can be transmitted to one optical line is relatively wide as 10 nm, but the noise figure is significantly different according to the wavelength and is 8 dB at the wavelength of 1552 nm. When used as a pre-amplifier installed just before 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 from 5dB to 8dB by 3dB depending on the wavelength, so that multiple wavelength signals can not be transmitted on one optical line.

The present invention is created to compensate for the disadvantages of an unflattening EDFA having a large gain variation according to the wavelength and a pre-filtering EDFA having a large difference in noise figure according to wavelengths. Zehnder filter to simultaneously flatten the gain and noise figure from 1542 nm to 1552 nm. SUMMARY OF THE INVENTION The present invention is directed to an optical fiber amplifier having a low noise figure and a flat gain.

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

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

FIG. 3 is a graph showing the relationship between the erbium concentration and the erbium concentration in the pre-filtering EDFA, in which the center wavelength of the pump laser diode is 980 nm, the current of the pump laser diode is 160 nmA (pump power is 67 mW) Is a graph showing the measurement results of the gain and the noise figure according to the signal input power change at three input signal wavelengths of 1542.3 nm, 1546.8 nm and 1552.4 nm for an EDF having a length of 17 m.

FIG. 4 is a graph showing a measurement result of a gain and a noise figure according to a change of a pump laser diode current under the same condition as FIG.

FIG. 5 is a block diagram illustrating an optical fiber amplifier having a low noise figure and a flat gain with respect to variations in the intensity of an incident light signal and pump power according to the present invention.

FIG. 6 is a graph showing the relationship between the center wavelength of the pump laser diode and the pump laser diode using the EDFA according to the present invention. The center wavelength of the pump laser diode is 980 nm, the current of the pump laser diode is 160 mA (pump power is 67 mW) For the EDF, which is 1542.3 nm, 1546.8 nm, and 1552.4 nm, the gain and noise figure according to the signal input power change at three input signal wavelengths.

7 is a graph showing the measurement results of the gain and the noise figure according to the change of the pump laser diode current under the condition of FIG.

According to an aspect of the present invention, there is provided an optical fiber amplifier having a low noise figure and a flat gain with respect to changes in the intensity of an incident light signal and pump power, A first isolator; A pump laser diode for applying 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 wavelength-division-multiplexing the signal light having passed through the isolator and the wavelength of the pumping light; A first erbium doped optical fiber amplifying the incident light signal applied from the first wavelength division multiplexer with light for pumping the pump laser diode and having a positive gain slope in a wavelength range in which flat gain characteristics are to be obtained; A second wavelength division multiplexer for wavelength division of the signal light amplified through the first erbium doped optical fiber into a pump light and a signal light; A filter having a negative signal attenuation slope having an absolute value larger than a positive gain slope of the first erbium doped optical fiber in a wavelength region where signal light of the second wavelength division multiplexer is to be input to obtain a flat gain; A third wavelength division multiplexer for wavelength-division multiplexing the signal light having passed through the filter and the wavelength of pumping light that has passed through the second WDM fiber and amplifies the signal light through the first Er-doped optical fiber; Doped optical fiber, a second wavelength division multiplexer for amplifying the incident light signal by the pump light applied from the third wavelength division multiplexer, and having a positive gain slope in a wavelength range in which a flat gain characteristic is to be obtained, Erbium doped fiber; And a second isolator for intercepting the reverse propagation of the signal having passed through the second erbium doped optical fiber to eliminate an interference phenomenon caused by reflected waves.

The first and second erbium doped optical fibers have a free spectral range (FSR) of 32 nm. The first erbium doped optical fiber and the second erbium doped optical fiber have a wavelength Lt; RTI ID = 0.0 > alumino-low-mannosilicate < / RTI > erbium doped optical fiber having a positive gain slope in the region.

In order to achieve the above object, an optical fiber amplifier having a low noise figure and a flat gain with respect to variation of a granular optical signal and a pump power is capable of preventing the reverse propagation of an incident signal light, Giving a first isolator; A pump laser diode for applying 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 wavelength-division-multiplexing the signal light having passed through the isolator and the wavelength of the pumping light; A first erbium doped optical fiber amplifying the incident light signal applied from the first wavelength division multiplexer with light for pumping the pump laser diode and having a negative gain slope in a wavelength range in which a flat gain characteristic is to be obtained; A second wavelength division multiplexer for wavelength division of the signal light amplified through the first erbium doped optical fiber into a pump light and a signal light; A filter having a positive signal attenuation slope having an absolute value larger than a negative gain slope of the first erbium doped optical fiber in a wavelength region where signal light of the signal of the second wavelength division multiplexer is to be input to obtain a flat gain; A second wavelength division multiplexer for wavelength-division-multiplexing wavelengths of the signal light having passed through the filter and the pumping light passing through the second wavelength division multiplexer; Doped optical fiber, and a third wavelength division multiplexer for amplifying the incident light signal by the pump light applied from the third wavelength division multiplexer and having a negative gain slope in a wavelength range where flat gain characteristics are to be obtained, Erbium doped fiber; And a second isolator for blocking reverse propagation of the signal that has passed through the erbium-doped optical fiber, thereby eliminating interference due to reflected waves.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to the accompanying drawings. In the present invention, two optical isolators are disposed at the front and rear of the EDF, a mark-gender filter is used between the two EDFs, and two WDMs are further disposed to pump the front EDF into the 980 nm pump LD.

FIG. 5 is a block diagram illustrating an optical fiber amplifier having a low noise figure and a flat gain with respect to changes in intensity and pump power of an incident light signal according to an embodiment of the present invention. The first isolator 500, A pump laser diode 510, a first wavelength division multiplexer 520, a first erbium doped fiber 530, a second wavelength division multiplexer 540, a mark-gander filter 550, a third wavelength division multiplexer 560, A second erbium doped optical fiber 570 and a second isolator 580.

The first isolator 500 blocks unnecessary reverse propagation of incident signal light to eliminate interference due to reflected waves, and performs isolation of about 40 dB. The pump laser diode 510 applies pump light having a wavelength different from that of the incident signal light to amplify the incident signal light. The pump laser diode 510 has a higher amplification efficiency than the pump LD having a pump wavelength of 1480 nm and a noise figure characteristic It is a pump LD with a pump wavelength of 980nm which is excellent and has a maximum power of 77mW by Seastar (name of a pump laser diode manufacturer).

The first wavelength division multiplexer 520 couples the signal light passing through the isolator and the wavelength of the pumping light by wavelength division. The first wavelength division multiplexer 520 is a fuse type with an insertion loss of about 0.3 dB and an isolation of 18 dB or more.

The first erbium doped optical fiber 530 is a low aluminum-containing alumino-low-mannosilicate erbium doped optical fiber having an erbium doping concentration of 260 ppm. The first erbium doped optical fiber 530 is a low- 510), and has a positive gain slope in a wavelength range where flat gain characteristics are to be obtained.

The second wavelength division multiplexer 540 wavelength-divides the amplified signal light into the pump light and the signal light through the first erbium doped optical fiber 530. The mark-gender filter 550 is located between the second wavelength division multiplexer 540 and the third wavelength division multiplexer 560 and receives the signal light of the signals of the second wavelength division multiplexer 540, Doped optical fiber 520 has a negative signal attenuation slope having an absolute value larger than a positive gain slope of the first erbium doped optical fiber 520 and has a free spectral range (FSR) of 32 nm in a wavelength range where gain is to be obtained.

The third wavelength division multiplexer 560 amplifies the signal light through the first erbium doped optical fiber 530 that has passed through the mark-gender filter 550 and has passed through the second WDM multiplexer 540, And the wavelength of the pumping light is wavelength-divisionally combined.

The second erbium doped optical fiber 570 amplifies the incident light signal by the pump light applied from the third wavelength division multiplexer 560 and has a positive gain slope in a wavelength range in which a flat gain characteristic is to be obtained, It is a kind of material such as erbium doped optical fiber. The second isolator 580 blocks the reverse propagation of the signal that has passed through the second erbium doped optical fiber 570, thereby eliminating interference due to reflected waves.

The operation of the present invention will be described. The signal intensity of the same intensity coming in regardless of the wavelength is amplified while passing through the first EDF 530, and the intensity of the amplified signal varies depending on the wavelength. The amplified signal light is filtered by the mark-gender filter 550 through the second WDM 540 and then amplified by the second EDF 570 through the third WDM 560. As a result, the overall gain and the noise figure are determined by the intensity And is flattened in a given wavelength range (1542nm - 1552nm) regardless of the intensity of the pump light. The pump light that is combined with the signal light in the first WDM 520 to excite the first EDF 530 is separated from the signal light in the second WDM 540 to bypass the mark-gender filter 550 and is recombined with the signal light in the third WDM 560 And excites the second EDF 570. That is, the attenuation spectrum of the mark-gender filter 410 and the gain spectrum of the two EDFs 530 and 570 are complementary to each other, thereby flattening the gain of the EDFA.

6 shows that the center wavelength of the pump laser diode is 980 nm, the current of the pump laser diode is 160 mA, and the pump power is 67 mW) using the EDFA according to the present invention. The low-aluminum-containing alumino-low-mannosilicate EDF has an erbium concentration of 260 ppm Is a graph showing the measurement results of the gain and the noise figure according to the signal input power change at three input signal wavelengths of 1542.3 nm, 1546.8 nm and 1552.4 nm for an EDF having a length of 17 m.

FIG. 4 is a graph showing a measurement result of a gain and a noise figure according to a change of a pump laser diode current under the same condition as FIG.

FIG. 5 is a block diagram illustrating an optical fiber amplifier having a low noise figure and a flat gain with respect to variations in the intensity of an incident light signal and pump power according to the present invention.

FIG. 6 is a graph showing the relationship between the center wavelength of the pump laser diode and the pump laser diode using the EDFA according to the present invention. The center wavelength of the pump laser diode is 980 nm, the current of the pump laser diode is 160 mA (pump power is 67 mW) For the EDF, which is 1542.3 nm, 1546.8 nm, and 1552.4 nm, the gain and noise figure according to the signal input power change at three input signal wavelengths. 7 is a graph showing the measurement results of the gain and the noise figure according to the change of the pump laser diode current under the condition of FIG.

As a result of adjusting the lengths of the front and rear EDFs 530 and 570 in order to optimize the gain and noise figure of the EDFA, the first EDF 530 was set to 4.7 meters and the second EDF 570 was set to 17 meters. If the length or type of the EDF is changed, the absorption rate and the tuning range of the mark-gender filter 410 should be adjusted according to the gain spectrum of a given EDF.

As shown in FIG. 6, the intensity of the pump light for exciting the EDF is maximum 67 mW, and the gain and the noise figure are 18.5 dB and 5.5 dB, respectively. As shown in FIG. 7, the gain and the noise figure were flattened within ± 0.3 dB as a result of varying the intensity of the signal for three wavelengths of 1542 nm, 1546 nm and 1552 nm. Also, as shown in FIG. 6, when the intensity of the pump light was varied with respect to these three wavelengths, the gain was flattened within ± 0.3 dB and the noise figure within ± 0.5 dB.

Hereinafter, another embodiment of the present invention will be described. In other embodiments of the present invention, the components are the same except for the difference in characteristics of the mark-gander filter 550 and the EDF 530 and 570 in comparison with the above embodiment. In other words, the mark-gender filter 550 receives the signal of the first isolator 500 and outputs a signal to the EDFs 530 and 570 in each of the two EDFs 530 and 570 in a wavelength region where a flat gain is to be obtained, And has a positive signal attenuation slope having a larger absolute value than the gain slope. The two EDFs 530 and 570 also have a 'negative gain slope' in the wavelength region where flat gain characteristics are to be obtained and a low aluminum Containing alumino-low-mannosilicate erbium doped optical fiber. Even in such a configuration, the attenuation spectrum of the mark-gender filter 550 and the gain spectrum of the EDFs 530 and 570 are complementary to each other, thereby flattening the gain of the EDFA.

In order to transmit multiple wavelength signals to one optical line, the gain factor as well as the noise figure should be flattened in a given wavelength range. According to the present invention, the gain is flattened to 0.3 dB or less, and the noise figure is flattened to 0.5 dB or less irrespective of the intensity of the signal light or the intensity of the pump light for the signal light of 1542 nm to 1552 nm. In a long-distance optical communication, several EDFAs are used as a repeater. In EDFAs of the related art, since the intensity of the pump light is decreased, the difference in gain of the EDFAs according to the wavelengths can be greatly different, and only two or three wavelength signals can be transmitted. However, when multiple EDFAs are used, each EDFA gain is slightly different depending on the wavelength, and cascaded signal light in the entire EDFA can not transmit perfectly one wavelength. However, since the gain difference is constant regardless of the intensity of the pump light and the intensity of the signal light, the EDFA of the present invention can transmit multiple signals in a given wavelength range (1542 nm - 1552 nm) like the initial signal light, .

Also, since the noise figure is as low as 5dB, it is also suitable as a pre-amplifier that amplifies at the front of the receiver. It is expected that EDFA can be designed to flatten the gain and noise figure of wider wavelength band according to the type of Mark-Gender filter and EDF.

Claims (6)

1. An optical fiber amplifier having a low noise figure and a flat gain with respect to variation of an intensity of an incident light signal and pump power, comprising: a first isolator for blocking reverse propagation of an incident signal light to eliminate an interference phenomenon by reflected waves; A pump laser diode for applying 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 wavelength-division-multiplexing the signal light having passed through the isolator and the wavelength of the pumping light; A first erbium doped optical fiber amplifying the incident light signal applied from the first wavelength division multiplexer with light for pumping the pump laser diode and having a positive gain slope in a wavelength range in which flat gain characteristics are to be obtained; And a second wavelength division multiplexer for wavelength-splitting the signal light amplified through the first erbium doped optical fiber into a pump light and a signal light; A filter having a negative signal attenuation slope having an absolute value larger than a positive gain slope of the first erbium doped optical fiber in a wavelength region where signal light of the second wavelength division multiplexer is to be input to obtain a flat gain; A third wavelength division multiplexer for wavelength-division multiplexing the signal light having passed through the filter and the wavelength of pumping light that has passed through the second wavelength division multiplexer, amplifies the signal light through the first erbium doped optical fiber 530, and wavelengths of the remaining pumping light; Doped optical fiber, a second wavelength division multiplexer for amplifying the incident light signal by the pump light applied from the third wavelength division multiplexer, and having a positive gain slope in a wavelength range in which a flat gain characteristic is to be obtained, Erbium doped fiber; And a second isolator for blocking reverse propagation of a signal that has passed through the second erbium doped optical fiber and eliminating interference due to reflected waves, wherein the second isolator has a low noise figure and a flat gain characteristic. The optical fiber amplifier according to claim 1, wherein the filter is a mark-gender filter that has a free spectral range (FSR) of 32 nm and filters the signal light to a wavelength, and has a low noise figure and a flat gain. The optical fiber of claim 1, wherein the first erbium doped optical fiber and the second erbium doped optical fiber are low-aluminum-containing alumino-low-mannosilicate erbium doped optical fibers having a positive gain slope in a wavelength range intended to have flat gain characteristics Optical fiber amplifier with low noise figure and flat gain. The optical fiber amplifier according to claim 1, wherein the pump laser diode is a pump laser diode having a pump wave length of 980 nm. 1. An optical fiber amplifier having a low noise figure and a flat gain with respect to variation of an intensity of an incident light signal and pump power, comprising: a first isolator for blocking reverse propagation of an incident signal light to eliminate an interference phenomenon by reflected waves; A pump laser diode for applying 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 wavelength-division-multiplexing the signal light having passed through the isolator and the wavelength of the pumping light; A first erbium doped optical fiber amplifying the incident light signal applied from the first wavelength division multiplexer with light for pumping the pump laser diode and having a negative gain slope in a wavelength range in which a flat gain characteristic is to be obtained; And a second wavelength division multiplexer for wavelength-splitting the signal light amplified through the first erbium doped optical fiber into a pump light and a signal light; A filter having a positive signal attenuation slope having an absolute value larger than a negative gain slope of the first erbium doped optical fiber in a wavelength region where signal light of the signal of the second wavelength division multiplexer is to be input to obtain a flat gain; A second wavelength division multiplexer for wavelength-division-multiplexing the wavelength of the pumping light that has passed through the filter and the second wavelength division multiplexer; A second wavelength division multiplexer for amplifying the incident light signal with the pump light applied thereto and having a negative gain slope in a wavelength range in which a flat gain characteristic is to be obtained and a second erbium doped with the same kind of material as the first erbium doped optical fiber, Doped optical fiber; And a second isolator for blocking reverse propagation of a signal transmitted through the erbium doped optical fiber to eliminate interference due to reflected waves. 7. The optical fiber of claim 5, wherein the first erbium doped optical fiber and the second erbium doped optical fiber are low-aluminum-containing alumino-low-mannosilicate erbium doped optical fibers having a negative gain slope in a wavelength range intended to have flat gain characteristics Optical fiber amplifier with low noise figure and flat gain. ※ Note: It is disclosed by the contents of the first application.