阅读说明：本技术 一种基于级联拉曼放大辅助的宽波段增益谱优化方法及装置 (Broadband gain spectrum optimization method and device based on cascade Raman amplification assistance ) 是由 忻向军 常天海 李良川 张琦 刘博� 田凤 王光全 姚海鹏 林卓 田清华 高然 于 2021-11-05 设计创作，主要内容包括：本发明提供了一种基于级联拉曼放大辅助的宽波段增益谱优化方法及装置,该方法包括：接收多路信号光,对接收的多路信号光进行波分复用操作；利用多个波长选择开关从波分复用操作后的信号中分别选择出多个波段的信号光,该多个波段包括C波段至U波段之间的多个波段；将选择出的多个波段的信号光中第一组波段的信号光在第一波长的泵浦光的激发下经过掺铒光纤放大器EDFA进行放大,并将第二组波段的信号光在第二波长的泵浦光的激发下经过掺铥光纤放大器TDFA进行放大；将经EDFA或TDFA放大的多波段信号光分别经级联的多个后向RFA进行放大；通过解波分复用分离出所述多路信号光。本发明具有宽带宽、高增益的特点,且使得宽波段增益谱更加平坦。(The invention provides a broadband gain spectrum optimization method and a broadband gain spectrum optimization device based on cascade Raman amplification assistance, wherein the method comprises the following steps: receiving multiple paths of signal light, and performing wavelength division multiplexing operation on the received multiple paths of signal light; selecting signal light of a plurality of wavelength bands from the signals after the wavelength division multiplexing operation by using a plurality of wavelength selection switches, respectively, wherein the plurality of wavelength bands comprise a plurality of wavelength bands from a C band to a U band; amplifying signal light of a first group of wave bands in the selected signal light of the plurality of wave bands by an erbium-doped fiber amplifier (EDFA) under the excitation of pump light of a first wavelength, and amplifying signal light of a second group of wave bands by a thulium-doped fiber amplifier (TDFA) under the excitation of pump light of a second wavelength; amplifying multiband signal light amplified by the EDFA or the TDFA through a plurality of cascaded backward RFAs respectively; and separating the multi-path signal light by wavelength division multiplexing. The broadband gain spectrum filter has the characteristics of wide bandwidth and high gain, and the broadband gain spectrum is flatter.)

1. A broadband gain spectrum optimization method based on cascade Raman amplification assistance is characterized by comprising the following steps:

receiving multiple paths of signal light, and performing wavelength division multiplexing operation on the received multiple paths of signal light;

selecting signal light of a plurality of wavelength bands from the signals after the wavelength division multiplexing operation by using a plurality of wavelength selection switches, wherein the plurality of wavelength bands comprise a plurality of wavelength bands from a C band to a U band;

amplifying signal light of a first group of wave bands in the selected signal light of the plurality of wave bands by an erbium-doped fiber amplifier (EDFA) under the excitation of pump light of a first wavelength, and amplifying signal light of a second group of wave bands in the selected signal light of the plurality of wave bands by a thulium-doped fiber amplifier (TDFA) under the excitation of pump light of a second wavelength;

amplifying multiband signal light amplified by the EDFA or the TDFA through a cascade Raman fiber amplifier RFA comprising a plurality of sections of cascade Raman fiber amplification units respectively;

and separating the multi-path signal light by wavelength division multiplexing.

2. The method according to claim 1, wherein the selecting the signal lights of the plurality of wavelength bands from the signals after the wavelength division multiplexing operation by using the plurality of wavelength selective switches respectively comprises the steps of:

under the condition that the received multi-channel signal light is full-waveband signal light, separating the signal light of a plurality of wavebands through all preset wavelength selective switches corresponding to the full-waveband signal light;

when the received multiplexed signal light is not the full-band signal light, the signal light of the plurality of wavelength bands is separated via a specific plurality of wavelength selective switches corresponding to the multiplexed signal light to be amplified among the all wavelength selective switches.

3. The method of claim 1, wherein the amplifying the multiband signal light amplified by the EDFA or TDFA through a cascaded raman fiber amplifier RFA comprising a multistage cascaded raman fiber amplification unit, respectively, comprises:

amplifying each waveband signal light in the multiband signal light amplified by the EDFA or the TDFA in each cascade Raman fiber amplification unit of the cascade Raman fiber amplifier RFA, wherein the pump light wavelength adopted by each cascade Raman fiber amplification unit is different.

4. The method of claim 1,

the signal light of the plurality of wave bands is signal light of four wave bands, the four wave bands are C wave bands, L wave bands, S wave bands and U wave bands, the first group of wave bands comprise C wave bands and L wave bands, and the second group of wave bands comprise S wave bands and U wave bands;

the multi-section cascade Raman fiber amplification unit is a four-section cascade Raman fiber amplification unit;

the length of the optical fiber corresponding to each Raman optical fiber amplification unit is 6-10 km.

5. The method of claim 3, further comprising:

a pump light separation step of separating pump light from the signal light amplified by each optical fiber amplifier; and

spectra are collected and gain extracted from the input and/or output of each fiber amplifier using a spectrum analyzer.

6. An optical fiber amplifier based on cascaded raman amplification assist, the optical fiber amplifier comprising:

the wavelength division multiplexer is used for receiving the multi-channel signal light and carrying out wavelength division multiplexing on the received multi-channel signal light;

a plurality of wavelength selective switches for separating signal light of a plurality of wavelength bands from the signal light after wavelength division multiplexing, respectively, the plurality of wavelength bands including a plurality of wavelength bands between a C band and a U band;

the erbium-doped fiber amplifier EDFA is used for amplifying signal light of a first group of wave bands in the selected signal light of the plurality of wave bands under the excitation of pump light of a first wavelength;

the thulium-doped optical fiber amplifier TDFA is used for amplifying signal light of a second group of wave bands in the selected signal light of the plurality of wave bands under the excitation of pump light of a second wavelength;

the wave splitter is used for separating the signal light with the first wavelength and the second wavelength from the multiband signal light amplified by the EDFA or the TDFA to obtain multiband signal light to be further amplified;

the cascade Raman fiber amplifier RFA comprises a plurality of sections of cascade Raman fiber amplifying units, and is used for amplifying and mixing the multiband signal light to be further amplified by the corresponding cascade Raman fiber amplifying units; and

and the wavelength division multiplexer is used for performing wavelength division multiplexing operation on the multiband optical signals output by the cascaded RFA mixing mode so as to separate the multipath signal light.

7. The fiber amplifier of claim 6,

under the condition that the received multi-channel signal light is full-waveband signal light, the plurality of wavelength selective switches are all preset wavelength selective switches;

in a case where the received multiplexed signal light is not the full-band signal light, the plurality of wavelength selective switches are a plurality of specific wavelength selective switches corresponding to the bands of the multiplexed signal light among the all wavelength selective switches.

8. The optical fiber amplifier according to claim 6, wherein each of the cascaded raman fiber amplification units in the cascaded raman fiber amplifier RFA is configured to amplify signal light in a corresponding wavelength band of the multiband signal light amplified by the EDFA or the TDFA under excitation of pump light with respective wavelength; the pumping light wavelength of each cascade Raman fiber amplification unit is different.

9. The fiber amplifier of claim 6,

the signal light of the plurality of wave bands is signal light of four wave bands, the four wave bands are C wave bands, L wave bands, S wave bands and U wave bands, the first group of wave bands comprise C wave bands and L wave bands, and the second group of wave bands comprise S wave bands and U wave bands;

the multi-section cascade Raman fiber amplification unit is a four-section cascade Raman fiber amplification unit;

the length of the optical fiber corresponding to each Raman optical fiber amplification unit is 6-10 km.

10. The fiber amplifier according to claim 6, wherein the cascaded Raman fiber amplifier RFA comprises: the cascaded optical fibers and the coupler connected to the front ends of the optical fibers in a distributed manner, wherein a wave splitter is further arranged at the front end of the coupler;

the device further comprises: a spectrum analyzer for collecting spectra from the input and/or output of each fiber amplifier and extracting gain.

Technical Field

The invention relates to the technical field of optical fiber communication, in particular to a broadband gain spectrum optimization method and device based on cascade Raman amplification assistance.

Background

In recent years, with the rising of new services such as high-definition video, virtual reality, and 5G, network traffic is increasing rapidly, and therefore, there is an urgent need for capacity expansion of an existing optical fiber communication network and realization of ultra-long distance transmission. The dense wavelength division multiplexing technology can effectively expand the capacity of the existing optical fiber backbone network, but the requirement on the optical fiber transmission bandwidth is increasingly wide in the future, the wavelengths used in the existing communication are C wave bands and L wave bands, and in order to meet the requirement on the future high-capacity optical communication, the communication wavelengths are expanded to S wave bands, U wave bands and the like, so that an optical fiber amplifier with wider flat gain spectrum and lower noise is required to meet the requirement on broadband communication.

At present, erbium-doped fiber amplifiers (EDFAs) are widely used in C-band fiber transmission, but gain flatness is limited for bands other than the C-band, and are not conducive to future development of high-speed large-capacity optical communication. In addition, a Thulium Doped Fiber Amplifier (TDFA) is commonly used for S-band fiber transmission. A Raman Fiber Amplifier (RFA) is mainly composed of a Raman gain medium (optical Fiber), a pump light source, a coupler, and the like. The raman fiber amplifier is classified into a forward pumping type raman fiber amplifier, a backward pumping type raman fiber amplifier, and a bidirectional pumping type raman fiber amplifier. The optical signal to noise ratio (OSNR) and the noise coefficient (NF) of the forward pumping mode are superior to those of the backward pumping mode; the gain saturation rate of the forward pumping mode is greater than that of the backward pumping mode, namely the gain saturation during the forward pumping is more serious than that of the backward pumping mode; the saturation power of the backward pumping mode is greater than that of the forward pumping mode; the forward pumping mode has an improved performance of Amplified Spontaneous Emission (ASE) compared to the backward pumping mode.

The RFA can theoretically amplify signals in any band, but the gain of the RFA is lower than that of the erbium-doped fiber amplifier, and the gain spectrum is not flat, so that the RFA needs to be optimized. The more pumps and the larger bandwidth can improve the gain spectrum of RFA, but the increase in pumps not only increases the cost but also increases the complexity of the system.

Therefore, how to design an optical fiber amplifier with wider flat gain spectrum and lower noise to meet the requirement of broadband communication is a problem to be solved urgently.

Disclosure of Invention

Aiming at the problem that an optical fiber amplifier in the prior art is difficult to meet the future high-capacity optical communication requirement, the invention aims to provide a broadband gain spectrum optimization method and device based on cascade Raman amplification assistance.

In one aspect of the present invention, a method for optimizing a broadband gain spectrum based on cascade raman amplification assistance is provided, the method comprising the following steps:

receiving multiple paths of signal light, and performing wavelength division multiplexing operation on the received multiple paths of signal light;

selecting signal light of a plurality of wavelength bands from the signals after the wavelength division multiplexing operation by using a plurality of wavelength selection switches, wherein the plurality of wavelength bands comprise a plurality of wavelength bands from a C band to a U band;

amplifying signal light of a first group of wave bands in the selected signal light of the plurality of wave bands by an erbium-doped fiber amplifier (EDFA) under the excitation of pump light of a first wavelength, and amplifying signal light of a second group of wave bands in the selected signal light of the plurality of wave bands by a thulium-doped fiber amplifier (TDFA) under the excitation of pump light of a second wavelength;

amplifying multiband signal light amplified by the EDFA or the TDFA through a cascade Raman fiber amplifier RFA comprising a plurality of sections of cascade Raman fiber amplification units respectively;

and separating the multi-path signal light by wavelength division multiplexing.

In some embodiments of the present invention, the selecting, by using a plurality of wavelength selective switches, signal lights of a plurality of wavelength bands from the signals after the wavelength division multiplexing operation respectively includes: under the condition that the received multi-channel signal light is full-waveband signal light, separating the signal light of a plurality of wavebands through all preset wavelength selective switches corresponding to the full-waveband signal light; when the received multiplexed signal light is not the full-band signal light, the signal light of the plurality of wavelength bands is separated via a specific plurality of wavelength selective switches corresponding to the multiplexed signal light to be amplified among the all wavelength selective switches.

In some embodiments of the present invention, the amplifying the multiband signal light amplified by the EDFA or TDFA by the cascaded raman fiber amplifier RFA including the cascaded raman fiber amplifying units respectively includes: amplifying each waveband signal light in the multiband signal light amplified by the EDFA or the TDFA in each section of cascade Raman fiber amplification unit of the cascade RFA, wherein the lengths of the pump light adopted by each section of cascade Raman fiber amplification unit are different.

In some embodiments of the present invention, the signal lights in the plurality of bands are signal lights in four bands, the four bands are a C band, an L band, an S band and a U band, the first group of bands includes the C band and the L band, and the second group of bands includes the S band and the U band; the multi-section cascade Raman fiber amplification unit is a four-section cascade Raman fiber amplification unit; the length of the optical fiber corresponding to each Raman optical fiber amplification unit is 6-10 km.

In some embodiments of the invention, the method further comprises: a pump light separation step of separating pump light from the signal light amplified by each optical fiber amplifier; and collecting spectra from the input and/or output of each fiber amplifier with a spectrum analyzer and extracting gain.

In another aspect of the present invention, there is also provided an optical fiber amplifier based on a cascaded raman amplification assist, the apparatus including:

the wavelength division multiplexer is used for receiving the multi-channel signal light and carrying out wavelength division multiplexing on the received multi-channel signal light;

a plurality of wavelength selective switches for separating signal light of a plurality of wavelength bands from the signal light after wavelength division multiplexing, respectively, the plurality of wavelength bands including a plurality of wavelength bands between a C band and a U band;

the erbium-doped fiber amplifier EDFA is used for amplifying signal light of a first group of wave bands in the selected signal light of the plurality of wave bands under the excitation of pump light of a first wavelength;

the thulium-doped optical fiber amplifier TDFA is used for amplifying signal light of a second group of wave bands in the selected signal light of the plurality of wave bands under the excitation of pump light of a second wavelength;

the wave splitter is used for separating the signal light with the first wavelength and the second wavelength from the multiband signal light amplified by the EDFA or the TDFA to obtain multiband signal light to be further amplified;

the cascade Raman fiber amplifier RFA comprises a plurality of sections of cascade Raman fiber amplifying units, and is used for amplifying and mixing the multiband signal light to be further amplified by the corresponding cascade Raman fiber amplifying units; and

and the wavelength division multiplexer is used for performing wavelength division multiplexing operation on the multiband optical signals output by the cascaded RFA mixing mode so as to separate the multipath signal light.

In some embodiments of the present invention, in a case where the received multiple signal lights are all-band signal lights, the plurality of wavelength selective switches are all preset wavelength selective switches; in a case where the received multiplexed signal light is not the full-band signal light, the plurality of wavelength selective switches are a plurality of specific wavelength selective switches corresponding to the bands of the multiplexed signal light among the all wavelength selective switches.

In some embodiments of the present invention, each of the cascaded raman fiber amplification units in the cascaded RFA is configured to amplify signal light in a corresponding waveband of multiband signal light amplified by the EDFA or TDFA under excitation of pump light with respective wavelength; the pumping light wavelength of each cascade Raman fiber amplification unit is different.

In some embodiments of the invention, the cascaded raman fiber amplifier RFA comprises: the cascaded optical fibers and the coupler connected to the front ends of the optical fibers in a distributed manner, wherein a wave splitter is further arranged at the front end of the coupler; the device further comprises: a spectrum analyzer for collecting spectra from the input and/or output of each fiber amplifier and extracting gain.

The cascade Raman amplification-assisted broadband gain spectrum optimization method and device provided by the invention can realize gain flattening output of multi-path wavelength division multiplexing signals, have the characteristics of wide bandwidth, high gain and the like, and enable the broadband gain spectrum to be flatter, so that a system has better error rate performance and can meet the development of future optical communication.

It will be appreciated by those skilled in the art that the objects and advantages that can be achieved with the present invention are not limited to the specific details set forth above, and that these and other objects that can be achieved with the present invention will be more clearly understood from the detailed description that follows.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.

Fig. 1 is a schematic structural diagram of a multiband hybrid fiber amplifier based on cascaded raman amplification assistance according to an embodiment of the present invention.

Fig. 2 is a schematic flow chart of a method for optimizing a broadband gain spectrum by raman cascade assistance according to an embodiment of the present invention.

Fig. 3A to 3C are gain spectrograms of the C band after different amplifications according to the embodiment of the invention.

Fig. 4A to 4C are gain spectrograms of the L-band after different amplifications according to the embodiment of the invention.

Fig. 5A to 5C are gain spectrograms of the S-band after different amplifications according to the embodiment of the invention.

Fig. 6 is a hybrid gain spectrum of broadband signal light after auxiliary amplification by a cascaded raman fiber amplifier in an embodiment of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the following embodiments and accompanying drawings. The exemplary embodiments and descriptions of the present invention are provided to explain the present invention, but not to limit the present invention.

It should be noted that, in order to avoid obscuring the present invention with unnecessary details, only the structures and/or processing steps closely related to the scheme according to the present invention are shown in the drawings, and other details not so relevant to the present invention are omitted.

It should be emphasized that the term "comprises/comprising/comprises/having" when used herein, is taken to specify the presence of stated features, elements, steps or components, but does not preclude the presence or addition of one or more other features, elements, steps or components.

In order to solve the problems in the prior art, the invention provides a multi-band hybrid fiber amplifier based on Raman cascade assistance, which combines a doped fiber amplifier (such as rare earth doped fiber amplifiers like erbium-doped fiber amplifiers (EDFAs) and thulium-doped fiber amplifiers (TDFAs)) and a Raman Fiber Amplifier (RFA), and correspondingly provides a broadband gain spectrum optimization method based on Raman cascade assistance. The invention realizes the combination of the rare earth-doped fiber amplifier and the RFA amplifier, thereby giving consideration to the high gain of the rare earth-doped fiber amplifier EDFA/TDFA and the on-line amplification of the RFA, and better improving the flat gain bandwidth.

In an embodiment of the invention, the broadband gain spectrum optimization method combines a doped fiber amplifier (such as EDFA/TDFA) and a backward pumping Raman fiber amplifier by using a Raman cascade-assisted multiband hybrid fiber amplifier, wherein the Raman fiber amplifier adopts a cascade mode to assist in broadband gain spectrum optimization. The Raman cascade-assisted multiband hybrid fiber amplifier can realize amplification from an S wave band to a U wave band and gain flattening, a rare earth-doped fiber amplifier (EDFA/TDFA) can enable the gain of signal amplification to be higher, a cascaded Raman Fiber Amplifier (RFA) can enable a gain spectrum to be flatter, the hybrid fiber amplifier can realize gain flattening output of multiplex wavelength division multiplexing signals, and the hybrid fiber amplifier has the characteristics of wide bandwidth, high gain and the like and is suitable for the development requirements of future optical communication.

Fig. 1 is a schematic structural diagram of a multiband hybrid fiber amplifier based on a cascaded raman amplification assist according to an embodiment of the present invention. Fig. 2 is a schematic flow chart of a method for optimizing a broadband gain spectrum by raman cascade assistance according to an embodiment of the present invention. Now, with reference to fig. 1 and fig. 2, a process of implementing multiband amplification of an S-U band and optimizing a gain spectrum thereof by using a raman cascade-assisted hybrid fiber amplifier to make the gain spectrum thereof more flat will be described.

As shown in fig. 1, the optical fiber amplifier based on the cascaded raman amplification assistance according to the embodiment of the present invention includes: wavelength division multiplexer (or wavelength division multiplexing coupler), multiple wavelength selection switches, EDFA, TDFA, first splitter and multiple backward pumping RFAs cascaded in multiple stages.

The wavelength division multiplexer is used for receiving the multiple paths of signal light, performing wavelength division multiplexing coupling on the received multiple paths of signal light, coupling the broadband signals together and outputting a beam of broadband signal light;

the plurality of wavelength selective switches are used for selecting signal light of a plurality of wavelength bands from the signal light after wavelength division multiplexing. In the embodiment of the present invention, the plurality of wavelength bands may include a plurality of wavelength bands from C band to U band, for example, C band, L band, S band, and U band, that is, the wavelength division multiplexed signal light further passes through the plurality of wavelength selection switches to divide the wide-band signal light into signal light of S band, C band, L band, and U band. Here, the C band, the L band, the S band, and the U band are merely examples, and the present invention is not limited thereto and may be applied to more or less bands.

The EDFA is used for enabling the signal light of a first group of wave bands in the selected signal light of the plurality of wave bands to be at a first wavelengthAmplifying the pump light under the excitation of the pump light; the first set of bands may include, for example, a C-band and an L-band.

TDFA is used for making the signal light of the second group of bands in the selected multiple bands at the second wavelengthAmplifying the pump light under the excitation of the pump light; the second set of bands includes, for example, the S-band and the U-band.

Pump light passing through C wave band and L wave bandIs independently amplified by an erbium-doped fiber amplifier (EDFA) under excitation, and passes through pump light in S wave band and U wave bandAfter the laser is amplified independently by a Thulium Doped Fiber Amplifier (TDFA), the performance imbalance among the bands is caused by the gain fluctuation in the doped fiber amplifier, so that the secondary amplification and gain flattening are further performed by the RFA in the embodiment of the present invention. The inventor finds that the raman amplifier does not meet the requirements of the system on the amplifier well if only one section of optical fiber is used, so in the embodiment of the invention, the gain is smoothed by using the cascade RFA comprising a plurality of sections of cascade raman optical fiber amplifying units. As an example, cascaded RFAs may employ a four-fiber cascade to achieve a flat overall gain.

Before the multiband signal light amplified by the EDFA or the TDFA is input into the cascade RFA, the first wave splitter can separate the signal light with the first wavelength and the second wavelength from the multiband signal light amplified by the EDFA or the TDFA, that is, the pump light of the EDFA and the TDFA is removed, so as to obtain the multiband signal light to be further amplified.

The cascade RFA is used for amplifying multiband signal light to be further amplified in each corresponding cascade Raman optical fiber amplification unit respectively and outputting the multiband signal light in a mixed mode; that is, preferably, the number of bands of the multiband signal light may correspond to the number of raman fiber amplification units of the tandem raman fiber amplifier RFA.

The wavelength division multiplexer is used for performing wavelength division multiplexing operation on the multi-band optical signals which are mixed and output by the multi-stage cascaded backward pumping RFA so as to separate the multi-band signal light.

More specifically, in the embodiment of the present invention, in the multi-path signal lightAfter wavelength division multiplexing (n signal lights) by a wavelength division multiplexer, a plurality of wavelengths are selectedThe selection switches WSS 1-WSS 4 respectively select the signal light of a plurality of wave bands, for example, the wavelength selection switch WSS1 is used for selecting the signal light of a wave band S wave band; selecting the signal light of the waveband C by using a wavelength selection switch WSS 2; selecting the signal light of the wave band L wave band by using a wavelength selection switch WSS 3; selecting the signal light of the wave band U wave band by using a wavelength selection switch WSS 4; then the signal light of each wave band passes through the pump lightOrIs amplified by EDFA/TDFA respectively, as shown in fig. 1, C-band signal light passes through pump lightAfter excitation, the signal light is amplified by EDFA, and the L-band signal light passes through pump lightAfter excitation, the light is amplified by EDFA, and S-band signal light passes through pump lightTDFA is used for amplifying after excitation, and signals in U wave band pass through pump lightAmplifying by TDFA after excitation; then, the signal light amplified by the EDFA/TDFA is secondarily amplified by the cascaded RFA, and finally, each path of signal light and the pump light are separated by wavelength division multiplexing. If the signal light to be amplified is not the full-band signal light, the signal light of a specific band is selected by a part of all the preset wavelength selective switches WSS, and then the pump light is excitedOrAmplifying the EDFA/TDFA, and selecting pump light according to the band of the signal light () RFA amplification is carried out; if the full wave band needs to be amplified, all preset WSSs work to separate the signal light of each wave band, and after the signal light of each wave band is independently amplified by EDFA/TDFA, the signal light of each wave band is pumped by pump light (TDFA/TDFA)) Amplification is performed by different Raman fiber amplification units in the cascade RFA respectively. In the embodiment of the invention, the cascaded RFA comprises four cascaded optical fibers and a coupler connected to the front ends of the four optical fibers in a branch manner, and a wave splitter is further arranged at the front end of the coupler and used for removing useless pump light. As shown in FIG. 1, TDFA amplified S-band signal light passes through pump lightExciting the C-band signal light amplified by the first RFA and EDFA to pass through the pump lightExciting the L-band signal light amplified by the second RFA and EDFA to pass through the pump lightExciting the pump light to the U-band signal amplified by the third RFA and TDFAExcitation is amplified by the fourth RFA. After being amplified by the doped fiber amplifier, the optical signals of all the wave bands can have very high gain, but the gain flattening bandwidth of the doped fiber amplifier is too small, so that the signal light is secondarily amplified and gain flattened by the cascaded backward pumping Raman fiber amplifiers. Passing through different wavesThe wave separator of the segment separates and removes the pumping light in the optical signal amplified in each stage. The magnitude of the signal light gain depends on the pump light power and the pump light: () And frequency shift between the signal light, in the cascaded backward pumping Raman fiber amplifier, the four sections of optical fibers are adopted to use different pumping light, so that the gain flatness of the output signal light can be more optimized, and the optical signal is amplified by adopting a mode of cascading the four sections of optical fibers and injecting pumping light with different wavelengths. In the embodiment of the invention, the Raman amplifier selects a backward pumping Raman fiber amplifier to amplify the signal light and flatten the gain spectrum.

Multiple signal lights with different wavelengths (shown in FIG. 1 as wavelengths of lightN signal lights) are wavelength division multiplexed, signal lights of S-band, C-band, L-band and U-band are first selected by a wavelength selection switch, and the signal lights of each band are independently amplified by a doped fiber amplifier (EDFA/TDFA), for example, the optical signals of S-band and U-band pass through TDFA, and the optical signals of C-band and L-band pass through EDFA. After being amplified by each doped fiber amplifier, the optical signals of each waveband can have very high gain, but the gain flattening bandwidth of the optical signals is too small, so that the embodiment of the invention further performs secondary amplification and gain flattening on the signal light through the cascaded raman fiber amplifiers. The magnitude of the signal light gain depends on the pump light power and the frequency shift between the pump light and the signal light, and in the cascaded raman optical fiber amplifier of the embodiment of the present invention, the four optical fibers are used for different pump lights, so that the gain flatness of the output signal light can be more optimized, and therefore, the four optical fibers are cascaded and pump lights with different wavelengths are injected to amplify the optical signal. The Raman fiber amplifier selects a backward pumping Raman fiber amplifier to amplify the signal light and flatten the gain spectrum.

Corresponding to the optical fiber amplifier (hybrid optical fiber amplifier) based on the cascaded raman amplification assistance, as shown in fig. 2, the method for optimizing the broadband gain spectrum based on the cascaded raman amplification assistance of the present invention comprises the following steps:

step S110, receiving the multiple signal lights, and performing wavelength division multiplexing coupling operation on the received multiple signal lights.

Step S120, selecting signal lights in a plurality of wavelength bands from the signals after the wavelength division multiplexing operation by using a plurality of wavelength selective switches, wherein the plurality of wavelength bands include a plurality of wavelength bands from C band to U band.

More specifically, in the case where the received multiple signal lights are full-band signal lights, the signal lights of the multiple bands are separated via all preset wavelength selective switches corresponding to the full-band signal lights; when the received multiplexed signal light is not the full-band signal light, the signal light of the plurality of wavelength bands is separated via a specific plurality of wavelength selective switches corresponding to the multiplexed signal light to be amplified among the all wavelength selective switches.

Step S130, the signal light of the first group of bands in the selected signal light of multiple bands is amplified by the erbium-doped fiber amplifier EDFA under the excitation of the pump light of the first wavelength, and the signal light of the second group of bands in the selected signal light of multiple bands is amplified by the thulium-doped fiber amplifier TDFA under the excitation of the pump light of the second wavelength.

Step S140, the multiband signal light amplified by the EDFA or TDFA is amplified by the cascaded raman fiber amplifier RFA including the cascaded raman fiber amplification units.

More specifically, signal light of each waveband in multi-waveband signal light amplified by the EDFA or the TDFA is amplified in each cascade raman fiber amplification unit of the cascade RFA, and the wavelength of pump light adopted by each cascade raman fiber amplification unit is different.

Step S150, separating the multiple signal lights by demultiplexing.

In an exemplary embodiment, the method further comprises: a pump light separation step of separating pump light from the signal light amplified by each optical fiber amplifier; and collecting spectra from the input and/or output of each fiber amplifier with a spectrum analyzer and extracting gain.

When the pump power is 100mW and the wavelength is 980nm, the rate equation of the EDFA is as follows:

（1）

in equation (1):N ₂ (r,z,t)is a metastable particle concentration andN ₁ (r,z,t)in order to be the ground state particle concentration,rindicating the reaction rate of the metastable species,za direction axis indicating a direction of the transmission,trepresents a time variable;R _pa 、R _pe 、W _sa 、W _se 、W _ka andW _ke the absorption rate of the pump light, the emission rate of the pump light, the absorption rate of the signal light, the emission rate of the signal light, and the absorption rate and the emission rate of the kth ASW light source, respectively; k is the total number of ASW light sources; a21 represents the fluorescence lifetime of the bait ions.

（2）

In the formula (2), the first and second groups,G _EDFA in order to obtain the gain of the EDFA,P _soutis the output signal optical power at the end of the fiber,P _sinis the input signal optical power.

Assuming that the pump light and the signal light propagate in the forward direction of the doped fiber, neglecting the fiber loss, the TDFA transmission equation can be expressed as:

（3）

（4）

in the formulas (3) and (4),which is indicative of the power of the signal light,which is indicative of the power of the pump light,is the stimulated absorption sectional area of the thulium-doped optical fiber amplifier to the S wave band (such as 1479nm wave band),is the stimulated emission cross-sectional area,is the stimulated emission cross-sectional area of the different energy levels,is the stimulated absorption cross-sectional area of different energy levels,andfor coincidence integration between signal light and pump light and doping profile, N₀，N₁，N₃Respectively ion beam concentration.

For the cascaded RFA of the present invention, if the fiber length is too small, the raman gain is insufficient because the raman gain increases with the fiber length; if the signal attenuation is larger when the length of the optical fiber is too long, a high-power pumping signal is needed, which is not beneficial to the manufacture of RFA, and the Raman gain is reduced along with the length of the optical fiber because the attenuation is increased. In the embodiment of the invention, the length of the four optical fibers is selected to be about 6-10 km. In four-stage cascaded Raman amplifier, different pump light wavelengths excite wide bandAfter signal light of different wave bands in the system is amplified by the doped fiber amplifier and the cascaded Raman fiber amplifier, the finally superposed mixed gain spectrum can be flatter, and the broadband gain spectrum is optimized. As an example, the lengths of the four sections of optical fibers in the four-section cascaded backward pumping Raman optical fiber amplifier are respectively L₁=8km、L₂=8km、L₃=8km、L₄=8km, the lengths of the four optical fibers are merely examples, and the present invention is not limited thereto, and may be other values.

As can be seen from equation (1), z = L in the optical fiber₁+L₂+L₃+L₄The respective signal optical powers at are:

（5）

in the formula (5), the first and second groups,for the effective interaction length of the 1 st segment of fiber,for the effective interaction length of the 2 nd fiber,for the effective interaction length of the 3 rd length of fiber,is the effective interaction length of the 4 th optical fiber;g _1i is the 1 st pump light and the secondiA raman gain coefficient between signal lights of the channels,g _2i is the 2 nd pump light and the 2 ndiA raman gain coefficient between signal lights of the channels,g _3i is the No. 3 pump light and the No. 3iA raman gain coefficient between signal lights of the channels,g _4i is the 4 th pump light and the secondiA raman gain coefficient between signal lights of the channels;P _p1 (0) for the 1 st pump lightThe power of the electric motor is controlled by the power controller,P _p2 (0) for the 2 nd pump light power,P _p3 (0) for the 3 rd pump light power,P _p4 (0) the 4 th pump light power;is the signal optical attenuation coefficient in the 1 st channel,is the signal optical attenuation coefficient in the 2 nd channel,is the signal optical attenuation coefficient in the 3 rd channel,is the signal light attenuation coefficient in the 4 th channel, M is the polarization maintaining coefficient, A_eIs the effective cut-off area of the fiber.

In the embodiment of the invention, after EDFA/TDFA and cascade RFA are mixed and amplified, the gain of the multiband mixed optical fiber amplifier combining the EDFA/TDFA and the cascade RFA is expressed as follows:

G_hybrid= G_Raman·G_EDFA/TDFA （6）

in equation (6): g_RamanIs the switching gain of the Raman amplifier, G_EDFA/TDFAIs the gain of the EDFA/TDFA.

A Gain Spectrum (EDFA Gain Spectrum) of the C-band signal after being amplified by the EDFA alone is shown in fig. 3A, a Gain Spectrum (RFA Gain Spectrum) of the C-band signal after being amplified by the RFA alone is shown in fig. 3B, and a Gain Spectrum (Mixed Gain Spectrum) of the C-band signal after being amplified by the EDFA and RFA Mixed amplifier is shown in fig. 3C, and it can be seen that the maximum and minimum difference of the Gain spectrums after being amplified by the Mixed amplifier is only about 4dB, which indicates that the Gain Spectrum of the C-band can be more flat when the C-band is jointly amplified. A Gain Spectrum (EDFA Gain Spectrum) of the L-band signal after being separately amplified by the EDFA is shown in fig. 4A, a Gain Spectrum (RFA Gain Spectrum) of the L-band signal after being separately amplified by the RFA is shown in fig. 4B, and a Gain Spectrum (Mixed Gain Spectrum) of the L-band signal after being amplified by the EDFA and RFA Mixed amplifier is shown in fig. 4C, and it can be seen that the maximum and minimum difference of the Gain spectrums after being amplified by the Mixed amplifier is only about 5dB, which indicates that the Gain Spectrum of the L-band can be more flat by the combined amplification of the L-band. A Gain Spectrum (TDFA Gain Spectrum) of the S-band signal after being amplified by TDFA alone is shown in fig. 5A, a Gain Spectrum (RFA Gain Spectrum) of the S-band signal after being amplified by RFA alone is shown in fig. 5B, and a Gain Spectrum (Mixed Gain Spectrum) of the S-band signal after being amplified by a hybrid amplifier of TDFA and RFA is shown in fig. 5C.

Fig. 6 shows a hybrid gain spectrum optimization graph of broadband signal light after being assisted by a cascaded raman amplifier. As shown in fig. 6, it can be seen that the method for optimizing the broadband gain spectrum based on cascaded raman amplification can make the broadband gain spectrum flatter, so that the system has better error rate performance.

The cascade Raman amplification-assistance-based broadband gain spectrum optimization method and device provided by the invention can realize gain flattening output of multi-path wavelength division multiplexing signals, have the characteristics of wide bandwidth, high gain and the like, and enable the broadband gain spectrum to be flatter, so that the system has better error rate performance and can meet the development of future optical communication.

It should be noted that the exemplary embodiments of the present invention describe some methods or systems based on a series of steps or devices. However, the present invention is not limited to the order of the above-described steps, that is, the steps may be performed in the order mentioned in the embodiments, may be performed in an order different from the order in the embodiments, or may be performed simultaneously.

Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments in the present invention.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and various modifications and changes may be made to the embodiment of the present invention by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.