KR20010055935A - Two-stage optical amplifier using long-band of erbium doped fiber - Google Patents

Abstract

PURPOSE: An L-band EDFA(Erbium Doped Fiber Amplifier) is provided to be capable of securing a large gain by using an EDF length of the second amplification stage longer than that of the first amplification stage. CONSTITUTION: The first amplification stage(100) has the first EDF(102) of a predetermined length and a reverse-direction pump 980nm laser diode(103) in order to lower a noise figure of an input light of a long wavelength band, and it amplifies the input light. The second amplification stage(300) has the second EDF(303) of a predetermined length and a bidirectional pump 980nm laser diode(302) in order to adjust a gain magnitude, and it amplifies the primarily amplified input light. The length of the second EDF is ten to twentieth times as long as that of the first EDF. An isolator(200) is connected between the first and second amplification stages so as to isolate the first and second amplification stages.

Description

TWO-STAGE OPTICAL AMPLIFIER USING LONG-BAND OF ERBIUM DOPED FIBER}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber transmission apparatus, and a low noise, high efficiency Erbium-Doped Fiber (EDF) capable of amplifying an input signal in the wavelength range of 1565 to 1605 nm with light to light in a wavelength division multiplexed optical transmission apparatus. The present invention relates to a long-band optical amplifier having a long wavelength band as a medium.

In general, an optical amplifier (L-band EDFA) using a long wavelength band of erbium-doped optical fiber (hereinafter referred to as 'EDF') has a relatively small EDF emission coefficient in the wavelength range of 1530-1560 nm in the 1565-1605 nm wavelength band. It is manufactured using very long EDF to get proper gain. For example, using an EDF exhibiting the same characteristics uses an EDF that is approximately 10 times longer than the C-band EDFA.

As shown in FIG. 1, the two-stage optical amplifier proposed in US Pat. No. 5,430,572 amplifies using a relatively short length EDF 11 in the first amplification stage 14, and further in the second amplification stage 18. The structure which amplifies using the EDF 15 is used.

The isolator 19 configured between the first and second amplification stages 14 and 18 has an amplified spontaneous emission (hereinafter, referred to as 'ASE') generated by the second amplification stage 18. 14, the filter 20 blocks the ASE of the 1530 nm wavelength band generated in the first amplifier stage 14 so as not to be connected to the second amplifier stage 18.

Reference numerals 21 and 22 denote laser diodes, reference numerals 13 and 16 denote wavelength division multiplexers (WDM), and reference numerals 12 and 17 denote an isolator.

The optical amplifier may have high efficiency and low noise characteristics in the wavelength range of 1540 nm to 1565 nm. However, erbium-doped fiber optical amplifiers (EDFAs) having gains of approximately 15 dB or more in the wavelength band and high efficiency and low noise characteristics are not suitable as L-band EDFAs because the gains are very small in the long wavelength band. Very long EDF should be used for L-band EDFA.

As shown in FIG. 2, the optical amplifier proposed by H. Ono is a hybrid two-stage structured optical amplifier, the first amplifying stage using a 980nm laser diode (LD) 33. Pump 50 and bi-directionally pump the second amplifier stage 60 using a 1480 nm laser diode (LD) 37,40 (Journal of Lightwave Technology, vol. 17, pp. 490-496).

Here, the first amplifier stage 50 is a pump for short-length EDF with a 980nm laser diode 33 for the purpose of lowering the noise figure, the second amplifier stage 60 is a laser diode of 1480nm wavelength (in order to increase the efficiency) 37,40). At this time, the WDM (36,39) used in the first and second amplification stage 60 has an insertion loss of 0.3 decibel (dB), and is caused by the insertion loss due to the front insertion of the WDM 32 of the first amplification stage 50. We tried to reduce the increase in the noise figure.

However, in the short wavelength band (C-band) when the bidirectional pumping using the 1480nm laser diodes (37, 40), there is a limit to reduce the noise figure increase.

The present invention has been made to solve the above problems, the pumping laser of the two-stage amplification stage using a laser diode of 980nm wavelength band, using a long wavelength band of erbium-added optical fiber suitable for lowering the noise figure and increasing efficiency The purpose is to provide an optical amplifier.

1 is a view showing a short wavelength band two-stage optical amplifier according to the prior art,

2 is a view showing a long-wavelength hybrid hybrid optical amplifier according to the prior art,

3 is a view showing a two-stage optical amplifier using a long wavelength band of the erbium-added optical fiber according to the first embodiment of the present invention,

4 is a view showing a two-stage optical amplifier using a long wavelength band of the erbium-added optical fiber according to a second embodiment of the present invention,

5 is a view showing a two-stage optical amplifier using a long wavelength band of the erbium-added optical fiber according to a third embodiment of the present invention,

6 is a view showing a two-stage optical amplifier using a long wavelength band of the erbium-added optical fiber according to a fourth embodiment of the present invention,

7 is a view showing a two-stage optical amplifier using a long wavelength band of the erbium-added optical fiber according to a fifth embodiment of the present invention,

8 is a view showing a two-stage optical amplifier using a long wavelength band of the erbium-added optical fiber according to a sixth embodiment of the present invention,

9 is a view showing a two-stage optical amplifier using a long wavelength band of the erbium-added optical fiber according to a seventh embodiment of the present invention,

10 is a view showing a two-stage optical amplifier using a long wavelength band of the erbium-added optical fiber according to an eighth embodiment of the present invention,

11 is a view showing a two-stage optical amplifier using a long wavelength band of the erbium-added optical fiber according to the ninth embodiment of the present invention,

12 is a diagram showing a gain and a noise figure measured in an optical amplifier using a long wavelength band of the erbium-added optical fiber according to the first embodiment of the present invention.

* Description of the symbols for the main parts of the drawings *

100: first amplifier stage

300: second amplifier stage

101,200,400: Isolator

102,303: EDF

103,302,305: 980nm LD

104,301,304: WDM

In order to achieve the above object, the long wavelength band optical amplifier of the present invention includes a first erbium-doped optical fiber having a predetermined length and one reverse pump 980 nm laser diode to reduce the noise index of the input light of the long wavelength band. A first amplifying stage for light-amplifying, a bidirectional pump 980nm laser diode and a second erbium-added optical fiber having a length of 10 to 20 times the length of the first erbium-added optical fiber to increase efficiency, the first optically amplified light 2 And an isolator coupled between the second amplifying stage and the first amplifying stage and the second amplifying stage to isolate the first and second amplifying stages, wherein the first amplifying stage includes light of the long wavelength band. The first erbium-doped optical fiber being input, the reverse laser diode outputting reverse pump light in a 980 nm wavelength band, and the input And an optical coupler for coupling the output of the pump light in a wavelength division scheme, wherein the second amplification stage includes a forward 980 nm laser diode that outputs pump light in a forward direction, a reverse 980 nm laser diode that outputs pump light in a reverse direction, and the primary amplified light. A first wavelength division optical coupler for coupling the pump light of a forward 980 nm laser diode, the second erbium-added optical fiber for second amplifying the primary amplified light in an inverted state having a high energy level by the bidirectional pump light, and the first amplification And a second wavelength division optical coupler for coupling the light and the reverse pump light.

Hereinafter, the preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the technical idea of the present invention. .

3 is a view showing a long wavelength band optical amplifier according to a first embodiment of the present invention, and includes two stages (Erbium-Doped Fibers (EDFs) including a very long Erbium-Doped Fiber (EDF) for optical amplification in the long wavelength band. Two-stage) L-band EDFA is constructed.

That is, the present invention is a two-stage long wavelength band EDFA including a long erbium-added fiber (hereinafter referred to as 'EDF') to amplify the optical in the long wavelength band, and the first amplification stage 100 using the short EDF 102 to lower the noise figure. ) Is pumped backward using one 980nm laser diode (hereinafter referred to as 'LD') 103, and the amplifier amplified bidirectionally pumped with the second amplification stage 300 using two 980nm LDs 302 and 305 to increase efficiency. .

As shown in FIG. 3, a light having a wavelength range of 1565 nm to 1605 nm is incident to the first amplification stage 100 to amplify the first optical amplification, and a second isolator to which the primary amplified light of the first amplification stage 100 is incident. And a second amplification stage 300 for bi-directionally amplifying the output light of the 200 and the second isolator 200.

Here, the first amplification stage 100 outputs pump light in a reverse direction with a first isolator 101 which receives light in the wavelength range of 1565 nm to 1605 nm and prevents the reverse direction of the output light. A first 980 nm LD 103, a first EDF 102 for first amplifying the input light in an inverted state having a high energy level by the pump light, and the first amplified light and the pump light. It consists of a first WDM 104 to combine.

In addition, the second isolator 200 receives the first amplified light and prevents the reverse progress of the second amplified light of the second amplifying stage 300.

The second amplification stage 300 includes a second 980 nm LD 302 for inputting the pump light in the forward direction, the first amplified light incident thereto, a third 980 nm LD 305 for outputting the pump light in the reverse direction, and the primary signal. A second WDM for coupling the amplified light and the pump light of the second 980 nm LD 302, and a second EDF 303 for second amplifying the primary amplified light in an inverted state having a high energy level by the bidirectional pump light. And a third WDM 304 for coupling the secondary amplified light and the pump light. And a third isolator 400 which prevents backward travel of the combined secondary amplified light.

Wherein the L-band EDFA of the two-stage long wavelength band has a low noise figure and high efficiency, the length of the first EDF 102 of the first amplifying stage 100 is equal to that of the second amplifying stage 300. 2 Make it 5-10% of the length of EDF (303). This is a case where the EDFs of the respective amplification stages 100 and 300 exhibit the same characteristics. If the EDFs having different characteristics are used, the lengths inversely proportional to the absorption maximum values of the respective EDFs are used.

The operation of the long wavelength band optical amplifier according to the first embodiment of the present invention configured as described above will be described.

First, light in the wavelength range of 1565 nm to 1605 nm enters the input terminal of the first isolator 101 and is output to the output terminal. The first isolator 101 suppresses the reverse movement of the light from the output terminal of the first isolator 101 to the input terminal. do.

Then, the light output from the output terminal of the first isolator 101 passes through the first EDF 102, where the length of the first EDF 102 is equal to the length of the second EDF 303 configured in the second amplifying stage 300. Short length of 5-10%.

Meanwhile, the pump light output from the first 980 nm LD 103 exits to the input terminal connected to the first EDF 102 through the pump input terminal of the first WDM 104 (A), and finally enters the first EDF 102. As a result, the energy level of Erbium (Er) ions in the first EDF 102 is increased to be inverted. In the inverted state, the light of the long wavelength band input to the first EDF 102 through the output terminal of the first isolator 101 is first amplified through an induced emission process. This first amplified light is input to the first WDM 104 and then output to the output stage.

Subsequently, the first amplified light passes through the input terminal of the second isolator 200 to the output terminal and is input to the second amplifier stage 300. At this time, the second isolator 200 prevents the reverse direction of the output primary amplified light.

Subsequently, the forward pump light output from the second 980 nm LD 302 exits to the output end of the second WDM 301 through the pump input end of the second WDM 301 ('B'), and the pump light passes through the second EDF 303. Input to invert the erbium ions. In this case, the second EDF 303 varies depending on the gain to be obtained, but uses a length about 10 times longer than that of the C-band EDFA.

The reverse pump light output from the third 980 nm LD 305 is output to the input terminal of the third WDM 304 through the pump input terminal of the third WDM 304 ('C') and the second EDF 303 is pumped backward. do.

Subsequently, the first amplified light that has passed through the output terminal of the second isolator 200 is input through the input terminal of the second WDM 301, coupled to the pump light in a wavelength division scheme, and output to the output terminal to enter the second EDF 303. .

As described above, the energy level of the erbium (Er) ions in the second EDF 303 is increased by the bidirectional pump light so as to be inverted. In the inverted state, the first amplified light passing through the output terminal of the second isolator 200 is combined with the forward pump light in a wavelength division method, and then input to the second EDF 303 to undergo an induction emission process to generate the first amplified light. Amplify the car.

The second amplified light is coupled to the pump light output from the second EDF 303 in a wavelength division scheme and then input to the input terminal of the third isolator 400.

As described above, the optical amplifier of the long wavelength band according to the first embodiment of the present invention uses the length of the EDF 303 of the second amplification stage 100 to have a gain having an appropriate size in the long wavelength band. The EDF 102 of 100 uses a short length of 5 to 10% of the length of the EDF 303 of the second amplifying stage 300. In addition, a high inversion is made by using the 980 nm LD 103, and the noise index is low, and the gain index of the second amplifier 300 has a sufficient gain to reduce the overall value. The ASE noise generated in the first amplifier stage 100 serves to amplify the input light of the long wavelength band in the second amplifier stage 300.

In addition, unlike the conventional method, power consumption is reduced by using only the 980 nm LDs 302 and 305 in the second amplifier stage 300, and the second amplifier stage 300 is bidirectionally pumped, thereby obtaining sufficient output.

4 is a view showing an optical amplifier of a long wavelength band according to a second embodiment of the present invention, in which the first isolator 101 of the first amplifier stage 100 is removed from the arrangement of FIG. There is an advantage in preventing the increase of the noise figure.

FIG. 5 is a view showing an optical amplifier of a long wavelength band according to a third embodiment of the present invention, in order to increase efficiency by removing the isolator 400 connected to the output terminal of the second amplifier 300 in the arrangement of FIG. 3. .

FIG. 6 is a view showing an optical amplifier of a long wavelength band according to a fourth embodiment of the present invention. The 2x2 optical coupler 900 and the long wavelength band which share two reverse pumps LD 901 and 902 and output the same by dividing by a constant ratio are shown in FIG. A first amplifying stage 500 for sharing the first optical amplification of the input light by sharing one of the first EDF 502 having a predetermined length and one of the two reverse pumps LD 901 and 902 so as to lower the noise index of the input light of the first light; The primary optical amplification using a second EDF 703 having a length of 10 to 20 times the length of the first EDF 502 and having one of the two reverse pumps LD 901, 902 and a forward pump LD 702. A second isolator 600 connected between the second amplification stage 700 and the first amplification stage 500 and the second amplification stage 700 to isolate the first and second amplification stages 500 and 700. It consists of.

Here, the first amplification stage 500 uses the reverse pump light of one of the outputs of the first isolator 501 and the 2 × 2 optical coupler 900 to isolate the long wavelength band input light from the reflection. A first EDF 502 for amplifying, a first WDM 503 for coupling one reverse pumped light of the primary amplified light and the output of the 2x2 optical coupler 900, wherein the second amplifying stage A forward LD 702 for outputting pump light in a forward direction, a second WDM 701 for coupling the forward light with the output light of the second isolator 600, and an output of the forward pump light and the 2x2 optical coupler 900 A second EDF 703 for second amplifying the first amplified light using one reverse pump light, and a second for combining one reverse pump light among the outputs of the second amplified light and the 2 × 2 optical combiner 900. It consists of three WDM 704.

A third isolator 800 for isolating the output light of the third WDM 704 is connected to the output terminal of the second amplifier stage 700. The laser diodes used in each amplifier stage are 980 nm LD and the first EDF 502. ) Has a length of 5-10% of the length of the second EDF 703.

By sharing the reverse pump LD 901 of the first amplifier stage 500 and the reverse pump LD 902 of the second amplifier stage 700 through the 2x2 optical coupler 900, even if one LD fails, the other It is efficient because it can be reverse pumped using LD.

FIG. 7 is a view showing an optical amplifier of a long wavelength band according to a fifth embodiment of the present invention. The reverse deflection pump LD 901 and the second amplifier 700 of the first amplifier stage 500 are provided through a 2x2 optical coupler 900. Forward pump LD 902 is shared.

FIG. 8 is a view showing an optical amplifier of a long wavelength band according to a sixth embodiment of the present invention, in which the isolator 501 is not included in the input portion of the first amplifier 500 in the arrangement of FIG. The optocoupler 900 shares the reverse pump LD 901 of the first amplifier stage 500 and the reverse pump LD 902 of the second amplifier stage 700.

FIG. 9 is a view illustrating an optical amplifier in a long wavelength band according to a seventh embodiment of the present invention. In the arrangement of FIG. 7, the isolator does not have an input portion of the first amplification stage 500. As shown in FIG. Through 900, the reverse pump LD 901 of the first amplifier stage 500 and the forward pump LD 902 of the second amplifier stage 700 are shared.

FIG. 10 is a view illustrating an optical amplifier in a long wavelength band according to an eighth embodiment of the present invention, in which the isolator 800 is not connected to the output terminal of the second amplifier 700 in the arrangement of FIG. The optocoupler 900 shares the reverse pump LD 901 of the first amplifier stage 500 and the reverse pump LD 902 of the second amplifier stage 700.

FIG. 11 is a view showing an optical amplifier of a long wavelength band according to a ninth embodiment of the present invention, in which there is no isolator 800 connected to an output terminal of the second amplifier 700 in the arrangement of FIG. The reverse pump LD 901 of the first amplifier stage 500 and the forward pump LD 902 of the second amplifier stage 700 are shared by the 2x2 optical coupler 900.

FIG. 12 is a diagram illustrating a gain and a noise figure measured according to the first embodiment of the present invention, wherein the first amplification stage 100 uses a short length EDF 102 as a 980 nm LD 103. The inversion is increased to lower the noise figure, and the ASE is generated by the optical energy of the signal incident on the amplifier passing through the long length EDF 303 pumped from the second amplifier stage 300 to the 980 nm LD 302, 305. Is transferred.

Referring to FIG. 12, an input light having a wavelength range of 1570 to 1600 nm is incident with -10 dBm intensity, and an EDF 5.4 having an maximum absorption value of 13 dB / m in the first EDF 102 of the first amplifying stage 100. Using m and connecting the EDF 70m to the second EDF 303 of the second amplifier stage 300, the pump strength of the first 980nm LD (103) of the first amplifier stage 100 is 100mW, the second amplifier stage 300 When pumped to the forward second 980nm LD 302 at 100mW and the reverse third 980nm LD 305 at 70mW, the gain of 25dB and the change within 1dB in the 30nm wavelength band of 1570-1600nm are shown. It shows a noise figure of 3.9 dB.

Although the technical idea of the present invention has been described in detail according to the above preferred embodiment, it should be noted that the above-described embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

As described above, the long-wavelength optical amplifier of the present invention has a large gain by using the length of the second amplifier EDF longer than the length of the first amplifier EDF, and makes a high inversion state by using the 980nm LD to create a noise figure. Is low, and the noise index of the second amplifying stage has a sufficient gain to lessen the overall value.

In addition, the ASE noise generated in the first amplifier stage amplifies the input light of the long wavelength band in the second amplifier stage can implement a high efficiency optical amplifier.

In addition, by using only 980nm LD, power consumption can be reduced, and the second amplifier stage can be bi-directionally pumped, so that sufficient output efficiency can be obtained and the noise figure of the entire optical amplifier can be improved.

Claims (22)

In the two stage optical amplifier, A first amplifier stage for first amplifying the input light with a first erbium-doped optical fiber having a predetermined length and a reverse pump 980 nm laser diode to lower the noise figure of the input light of the long wavelength band; A second amplification stage including a second erbium-doped fiber having a length of 10 to 20 times the length of the first erbium-doped fiber and a bidirectional pump 980 nm laser diode to amplify the first optically amplified light to adjust the gain size; ; And An isolator connected between the first amplifier stage and the second amplifier stage to isolate the first and second amplifier stages. Long wavelength band optical amplifier, characterized in that configured to include. The method of claim 1, The first amplification stage, The first erbium-doped optical fiber to which light in the long wavelength band is input; The laser diode outputting reverse pump light at a wavelength of 980 nm; And A wavelength division optical coupler for coupling the input light and the output of the pump light in a wavelength division method Long wavelength band optical amplifier, characterized in that configured to include. The method of claim 1, The second amplification stage, A first laser diode outputting forward pump light in the 980 nm wavelength band; A second laser diode outputting reverse pump light at a wavelength of 980 nm; A first wavelength division optical coupler for coupling the first amplified light and the pump light of the first laser diode; The second erbium-doped optical fiber for second amplifying the primary amplified light in an inverted state having a high energy level by the bidirectional pump light; And A second wavelength split optical coupler coupling the first amplified light and the reverse pump light; Long wavelength band optical amplifier, characterized in that configured to include. The method of claim 1, And said first amplifier stage comprises an isolator for isolating said input light. The method of claim 1, And said second amplifier stage comprises an isolator for isolating said secondary amplified light. The method of claim 1, The second erbium-doped optical fiber has a small signal gain of 15 dB to 30 dB in the wavelength range of 1565 nm to 1605 nm. In the two-stage optical amplifier using the erbium-added optical fiber, A first isolator to isolate the input light of the long wavelength band; A first laser diode outputting reverse pump light at a wavelength of 980 nm; A first erbium-doped optical fiber for first amplifying the input light in an inverted state by the reverse pump light; A first wavelength division optical coupler for coupling the output of the first amplified light and the reverse pump light in a wavelength division scheme; A second isolator for isolating the output light of the wavelength split optical coupler; A second laser diode outputting forward pump light in the 980 nm wavelength band; A third laser diode outputting reverse pump light in the 980 nm wavelength band; A second wavelength division optical coupler coupling the first amplified light and the forward pump light; The second erbium-doped optical fiber for second amplifying the primary amplified light in an inverted state having a high energy level by the bidirectional pump light; A third wavelength split optical coupler coupling the first amplified light and the reverse pump light; A third isolator to isolate the output light of the third wavelength split optical coupler Long wavelength band optical amplifier, characterized in that configured to include. The method of claim 7, wherein The first isolator has a long wavelength band optical amplifier, characterized in that the light of a long wavelength band including a wavelength range of 1565nm to 1605nm is incident. The method of claim 7, wherein And the first erbium-doped optical fiber uses 5% to 10% of the length of the second erbium-doped optical fiber. The method of claim 7, wherein And removing the first isolator to lower the noise figure of the first amplifier stage. The method of claim 7, wherein And removing the third isolator to adjust the gain size. In the two-stage optical amplifier using the erbium-added optical fiber, A 2 × 2 optical coupler which shares two reverse pump laser diodes and divides them by a constant ratio; A first amplifying stage for first optically amplifying the input light by using a first erbium-added optical fiber having a predetermined length and one of the two reverse pump laser diodes to lower the noise figure of the input light of the long wavelength band; A second erbium-doped optical fiber having a length of 10 to 20 times the length of the first erbium-doped optical fiber, one of the two reverse pump laser diodes, and a forward pump laser diode are used to secondary amplify the primary optically amplified light. A second amplification stage; And An isolator connected between the first amplifier stage and the second amplifier stage to isolate the first and second amplifier stages. Long wavelength band optical amplifier, characterized in that configured to include. The method of claim 12, The first amplification stage, The first erbium-doped optical fiber to which light in the wavelength range of 1565 nm to 1605 nm is input; And A wavelength division optical coupler for coupling one of the output pump light of the 2 × 2 optical coupler output light output in the 980 nm wavelength band and the input light in a wavelength division scheme; Long wavelength optical amplifier, characterized in that configured to include The method of claim 12, The second amplification stage, A first laser diode outputting forward pump light in the 980 nm wavelength band; A first wavelength division optical coupler coupling the output light of the second isolator and the forward pump light; The second erbium-added optical fiber for second amplifying by using the reverse pump light and the forward pump light of one of the 2 × 2 optical coupler output light output in the 980 nm wavelength band; And A second wavelength split optical coupler coupling the secondary amplified light and the one reverse pump light; Long wavelength band optical amplifier, characterized in that configured to include. The method of claim 12, And removing the first isolator to lower the noise figure of the first amplifier stage. The method of claim 12, And removing the third isolator to adjust the gain size of the second amplifier stage. In the two-stage optical amplifier using the erbium-added optical fiber, A 2x2 optical coupler which shares one reverse pump first laser diode and one forward pump second laser diode and divides the output by a predetermined ratio; A first isolator that isolates the long wavelength input light from reflection; A first erbium-doped optical fiber for first amplifying the input light by using reverse pump light in the output of the 2 × 2 optical coupler; A first wavelength division optical coupler for coupling one reverse pump light of the primary amplified light and the output of the 2x2 optical coupler; A second isolator for isolating output light of the first wavelength division optical coupler; A second wavelength split optical coupler for coupling the output light of the second isolator and the forward pump light among the outputs of the 2x2 optical coupler; A reverse third laser diode outputting pump light in a reverse direction; A second erbium-added optical fiber for secondly amplifying the output light of the second wavelength split optical coupler using the forward pump light and the reverse pump light among the outputs of the 2x2 optical coupler; A third wavelength division optical coupler coupling the reverse pump light and the second amplified light; And A third isolator that isolates the output light of the third wavelength split optical coupler from reflection Long wavelength band optical amplifier, characterized in that configured to include. The method of claim 17, And the first erbium-doped optical fiber is 5% to 10% of the length of the second erbium-doped optical fiber. The method of claim 17, And the first, second and third laser diodes oscillate in a wavelength range of 980 nm. The method of claim 17, The first and second erbium-doped optical fiber is a long-wavelength optical amplifier, characterized in that for amplifying the input light in the wavelength range of 1565nm to 1605nm. The method of claim 17, And removing the first isolator to lower the noise figure of the first amplifier stage. The method of claim 17, And removing the third isolator to adjust the gain size of the second amplifier stage.