WO2021129182A1 - 光纤放大器 - Google Patents

光纤放大器 Download PDF

Info

Publication number
WO2021129182A1
WO2021129182A1 PCT/CN2020/126743 CN2020126743W WO2021129182A1 WO 2021129182 A1 WO2021129182 A1 WO 2021129182A1 CN 2020126743 W CN2020126743 W CN 2020126743W WO 2021129182 A1 WO2021129182 A1 WO 2021129182A1
Authority
WO
WIPO (PCT)
Prior art keywords
mode
few
core
doped
fiber
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/CN2020/126743
Other languages
English (en)
French (fr)
Inventor
张�林
刘亚平
杨志群
郭强
周锐
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Huawei Technologies Co Ltd
Original Assignee
Huawei Technologies Co Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Huawei Technologies Co Ltd filed Critical Huawei Technologies Co Ltd
Priority to EP20907255.2A priority Critical patent/EP4054028B1/en
Publication of WO2021129182A1 publication Critical patent/WO2021129182A1/zh
Priority to US17/851,307 priority patent/US20220329035A1/en
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

Links

Images

Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H01S3/06716—Fibre compositions or doping with active elements
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/08018—Mode suppression
    • H01S3/0804—Transverse or lateral modes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0064—Anti-reflection devices, e.g. optical isolaters
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H01S3/06729—Peculiar transverse fibre profile
    • H01S3/06733—Fibre having more than one cladding
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/0675—Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094003—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre
    • H01S3/094007—Cladding pumping, i.e. pump light propagating in a clad surrounding the active core
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
    • H01S3/10023—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by functional association of additional optical elements, e.g. filters, gratings, reflectors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/1301—Stabilisation of laser output parameters, e.g. frequency or amplitude in optical amplifiers
    • H01S3/1302—Stabilisation of laser output parameters, e.g. frequency or amplitude in optical amplifiers by all-optical means, e.g. gain-clamping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/1601—Solid materials characterised by an active (lasing) ion
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/1601—Solid materials characterised by an active (lasing) ion
    • H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/1691—Solid materials characterised by additives / sensitisers / promoters as further dopants
    • H01S3/1695—Solid materials characterised by additives / sensitisers / promoters as further dopants germanium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S2301/00—Functional characteristics
    • H01S2301/04—Gain spectral shaping, flattening
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H01S3/06729—Peculiar transverse fibre profile
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06708—Constructional details of the fibre, e.g. compositions, cross-section, shape or tapering
    • H01S3/06729—Peculiar transverse fibre profile
    • H01S3/06737—Fibre having multiple non-coaxial cores, e.g. multiple active cores or separate cores for pump and gain
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06754—Fibre amplifiers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/09—Processes or apparatus for excitation, e.g. pumping
    • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
    • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
    • H01S3/094003—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre
    • H01S3/094015—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light the pumped medium being a fibre with pump light recycling, i.e. with reinjection of the unused pump light back into the fiber, e.g. by reflectors or circulators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/17—Solid materials amorphous, e.g. glass
    • H01S3/176—Solid materials amorphous, e.g. glass silica or silicate glass

Definitions

  • the present invention relates to the field of optical communication, in particular to an optical fiber amplifier.
  • SDM Spatial Division Multiplexing
  • FMF Few Mode Fiber
  • MCF Multi-core Fiber
  • FIG. 1 it is a scheme for realizing simultaneous amplification of different spatial channels for few-mode optical transmission. It is realized by using multiple single-mode Erbium-Doped Fiber Amplifier (EDFA). Specifically, the mode multiplexed signal input from the few-mode fiber is first demultiplexed into multiple single-mode signals, and then a single-mode EDFA is used to amplify them respectively. The amplified multiple signals are multiplexed into the output few-mode fiber.
  • This method of amplifying a few-mode signal requires three steps of "demultiplexing", "amplifying" and “multiplexing". On the one hand, it increases the complexity and cost of the system, and on the other hand, it introduces additional insertion loss and inter-mode crosstalk. As the transmission distance increases, inter-mode crosstalk continues to accumulate, and the signal-to-noise ratio of each mode continues to deteriorate, making it difficult to support long-distance transmission.
  • EDFA Erbium-Doped Fiber Amplifier
  • FM-EDFA Mode Erbium-Doped Fiber Amplifier
  • DMG mode gain difference
  • NF amplification noise coefficient between different modes
  • the embodiment of the present invention provides an optical fiber amplifier, which realizes strong coupling and co-amplification between signal modes, thereby reducing the differential gain between mode groups.
  • an embodiment of the present invention provides a fiber amplifier, including: a pump laser, a signal pump combiner, and a few-mode doped fiber; wherein: the pump laser is used to output pump light; the signal pump The beam combiner is used to couple the input few-mode signal light and pump light into the few-mode doped fiber; the refractive index of the core of the few-mode doped fiber is gradually distributed along the radial direction of the cross section, and the core is carved along the axial direction. There are periodic gratings, and the period of the grating meets the phase matching condition. The use of periodic gratings of few-mode doped fibers enables strong coupling and co-amplification between optical signal modes, thereby reducing the differential gain between mode groups.
  • the grating period that satisfies the phase matching condition specifically includes: the product of the periods of multiple gratings and the effective refractive index difference between adjacent mode groups of the few-mode signal light is equal to the wavelength of the few-mode signal light;
  • the adjacent mode groups of the mode signal light are: the multiple mode groups contained in the signal light are arranged in sequence according to the corresponding effective refractive index, and the two adjacent mode groups are arranged in sequence.
  • the refractive index of the core of the few-mode doped fiber is gradually distributed along the radial direction of the cross section so that the effective refractive index difference between adjacent mode groups is equal, and the periods of multiple gratings are equal.
  • the core region of the few-mode doped fiber is doped with rare earth element ions.
  • the amplification gain of signals of different wavelengths is realized.
  • the core region is doped with rare earth element ions including uniform doping and layered doping.
  • the core of the few-mode doped fiber is made of silicon dioxide doped with germanium dioxide and the cladding is made of silicon dioxide; or the core is made of silicon dioxide and the cladding is made of fluorine-doped Compound of silicon dioxide. In order to achieve the same refractive index difference between the mode groups.
  • the core periphery of the few-mode doped fiber further includes an inner cladding layer, a trench layer, and an outer cladding layer, and the refractive index of the groove layer is smaller than the refractive index of the inner cladding layer and the outer cladding layer.
  • the few-mode doped fiber has multiple air holes around the core.
  • the air hole is more effective in limiting the light field.
  • the few-mode doped fiber includes multiple cores, and the core pumping method is adopted, and the pump light enters each core. Thereby, the optical fields of the pump light and the signal light are overlapped effectively, and the gain is improved and the gain of each fiber core is balanced.
  • the few-mode doped fiber includes a plurality of cores and adopts a cladding pumping method.
  • the cladding includes a pumping area, and the pump light enters the pumping area.
  • the cladding pumping method has a simple structure and can also achieve the gain balance of each fiber core.
  • the fiber amplifier also includes a beam splitter for separating the signal light and pump light output by the few-mode doped fiber, and input the separated pump light into the signal pump combiner. In this way, the pump light is fully utilized, and the gain of the signal light in each mode is further improved.
  • the fiber amplifier further includes a first isolator and a second isolator.
  • the first isolator is connected to the input end of the signal pumping combiner, and the second isolator is connected to the output of the few-mode doped fiber. ⁇ End connection. This prevents backscattered light from entering the optical fiber link and improves the quality of optical signal transmission.
  • the above technical solution provided by the embodiment of the present invention uses a few-mode doped fiber with periodic gratings in the amplifier to achieve strong coupling and co-amplification between optical signal modes, thereby reducing the difference in differential gain and noise figure between mode groups , which is conducive to long-distance transmission of optical signals.
  • FIG. 1 is a schematic diagram of the structure of an optical fiber amplifier in the prior art
  • FIG. 2 is a schematic structural diagram of a space division multiplexing optical fiber amplifier provided by an embodiment of the present invention
  • FIG. 3a is a schematic diagram of the structure of a few-mode doped fiber in an optical fiber amplifier according to an embodiment of the present invention
  • 3b is a schematic diagram of a cross-sectional refractive index distribution of a few-mode doped optical fiber according to an embodiment of the present invention
  • 4a is a schematic diagram of the structure of a few-mode doped fiber in another optical fiber amplifier provided by an embodiment of the present invention
  • 4b is a schematic diagram of the cross-sectional refractive index distribution of another few-mode doped optical fiber according to an embodiment of the present invention.
  • FIG. 5 is a schematic diagram of a cross-sectional structure of a doped fiber core provided by an embodiment of the present invention.
  • FIG. 6 is a schematic diagram of an erbium-doped fiber model provided by an embodiment of the present invention.
  • FIG. 7 is a graph showing the variation of DMG with the coupling strength and the number of coupling points under uniform doping according to an embodiment of the present invention.
  • FIG. 8 is a contour diagram of DMG with coupling strength and number of coupling points under uniform doping according to an embodiment of the present invention
  • FIG. 9 is a two-dimensional diagram of the relationship between DMG and layered doping parameters b and c according to an embodiment of the present invention.
  • FIG. 10 is a contour map of DMG with coupling strength and coupling points under layered doping according to an embodiment of the present invention
  • 11a is a schematic structural diagram of a cross-section of a groove-assisted optical fiber according to an embodiment of the present invention.
  • 11b is a schematic diagram of a cross-sectional refractive index distribution of a trench assisted optical fiber provided by an embodiment of the present invention along the radial direction;
  • FIG. 12 is a schematic diagram of a cross-sectional structure of an air hole assisted optical fiber according to an embodiment of the present invention.
  • FIG. 13 is a schematic structural diagram of a cross-section of a 3-core optical fiber provided by an embodiment of the present invention.
  • Fig. 14 is a schematic structural diagram of a cross-section of a 7-core optical fiber provided by an embodiment of the present invention.
  • the embodiment of the present invention provides a strong coupling space division multiplexing fiber amplifier, as shown in FIG. 2, including: two optical isolators 201, a pump laser 202, a signal pump combiner 203, and a few-mode doped fiber 204.
  • two optical isolators 201 are respectively at the input end and output end of the fiber amplifier to prevent backscattered light from entering the fiber link and affecting the quality of optical signal transmission;
  • the pump laser 202 is used to output pump light;
  • the combiner 203 is used to couple the few-mode signal light and pump light at the input end into the output end;
  • the few-mode doped fiber 204 is used to achieve strong coupling and co-amplification between signal modes, thereby reducing the differential gain between the mode groups.
  • the signal light entering the fiber amplifier contains M mode groups, and M is an integer greater than or equal to 2.
  • a mode group refers to a group composed of multiple modes with the same propagation constant in an optical fiber, and the speeds of multiple modes in a mode group are equal.
  • the input signal light includes 4 module groups LP01, LP11, LP21, and LP02, and each module group carries different information.
  • the signal light passes through the optical isolator 201, it is coupled into the few-mode doped fiber 204 via the signal pump combiner 203 and the pump light at the same time.
  • the core region is doped with a certain concentration of rare earth element ions.
  • the signal light amplified by the few-mode doped fiber 204 is output through the optical isolator.
  • the gain spectrum range of the few-mode doped fiber is determined by the type of doped ions in the core region. For example, doped with ytterbium ion Yb3+, the corresponding gain band range is 970nm-1200nm.
  • rare earth element ions and the corresponding gain spectrum range include: praseodymium ion Pr3+ (amplified near 1300nm band), neodymium ion Nd3+ (amplified near 1300nm band), Erbium ion Er3+ (amplified 1520nm-1625nm band), bismuth ion Bi3+ (amplified 1300nm- 1380nm band), thulium ion Tm3+ (amplified 1450nm-1520nm band, 1900nm-2000nm band).
  • the few-mode fiber amplifier may also include a circulating pump structure for collecting residual pump light energy and recycling it, thereby greatly improving the pump light utilization rate and increasing the signal light amplification gain.
  • 205 is a signal pump beam splitter.
  • the few-mode signal light amplified by the few-mode doped fiber 204 and the residual pump light enter the component 205, and the signal light and pump light are separated in the signal pump beam splitter, and the amplified signal light enters the next transmission chain
  • the remaining pump light and the pump light emitted by the pump laser 202 are input into the signal pump combiner 203 together to perform a new round of amplification. In this way, the pump light is fully utilized, and the gain of the signal light in each mode is further improved.
  • the core of the few-mode doped optical fiber in the optical fiber amplifier is engraved with a grating.
  • FIG. 3a it is a schematic diagram of a few-mode doped fiber in an optical fiber amplifier provided by an embodiment of the present invention, with a core in the middle and a cladding on the outside of the core.
  • a plurality of periodic gratings are distributed distributedly. The phase matching conditions that the grating period must meet are described below.
  • FIG. 3b is a schematic diagram of the cross-sectional refractive index distribution of a few-mode doped fiber, where ⁇ is the refractive index difference between the core center and the cladding material (generally ⁇ [0.001,0.030]), and R is the core radius (generally R ⁇ [ 2,35]um), ⁇ is the graded refractive index ( ⁇ [1, ⁇ ]). It is assumed that the optical fiber supports M mode group transmission, and different mode groups have different effective refractive indexes.
  • the effective refractive index difference between adjacent mode groups is set to ⁇ n.
  • ⁇ n1 is the effective refractive index difference between the first mode group and the second mode group
  • ⁇ n2 is the effective refractive index difference between the second mode group and the third mode group.
  • the refractive index difference, ⁇ nm is the effective refractive index difference between the m-th mode group and the m+1 mode group.
  • ⁇ nm is the effective refractive index difference between the m-th mode group and the m+1 mode group.
  • m is 1 to (M-1).
  • a plurality of periodic gratings are distributedly written along the axial direction of the optical fiber, so that the refractive index distribution of the optical fiber changes periodically.
  • at least M-1 periodic gratings are engraved on the fiber, where ⁇ 1 is the first grating period, and ⁇ 2 is the second grating period. Segment grating period, ⁇ m is the m-th grating period.
  • the period of the grating needs to meet the phase matching condition, that is, the product of the effective refractive index difference with the adjacent mode group is equal to the wavelength of the few-mode signal light.
  • the adjacent mode groups are amplified together, so that the amplification gain difference between the mode groups is greatly reduced.
  • the coupling between adjacent mode groups including: LP01 and LP11, LP11 and LP21, and LP21 and LP02, will be greatly enhanced.
  • the method of writing periodic gratings on the aforementioned few-mode doped fiber is as follows: a carbon dioxide (CO2) laser is used to write periodic gratings point by point on the few-mode doped fiber.
  • CO2 laser is used to write periodic gratings point by point on the few-mode doped fiber.
  • the laser light emitted by the CO2 laser is focused on the core of the few-mode doped fiber through a microscope objective lens, and the contact position of the laser and the fiber introduces a refractive index change due to the thermal effect.
  • the few-mode doped fiber is fixed on a high-precision three-dimensional mobile platform.
  • the three-dimensional mobile platform controls the fiber to move at a certain stepping speed and exposes point by point until the entire grating is written. Determine the fiber stepping speed, laser energy and exposure time according to the grating writing period and duty cycle.
  • the mode of strong mode coupling is generally applicable to the few-mode fiber with a graded refractive index.
  • M the number of mode groups M is large, there will be M-1 adjacent mode groups effective refractive index difference.
  • the corresponding M-1 long periodic gratings need to be written on the fiber, which improves the grating Requirements, increased manufacturing costs.
  • FIG. 4a it is a schematic diagram of another few-mode doped fiber in the fiber amplifier provided by an embodiment of the present invention.
  • r is a certain radial position in the core
  • n(r) is the refractive index at the radial position r
  • n1 is the refractive index of the core center
  • ⁇ ⁇ 1+ ⁇ 2
  • ⁇ 1 is the core center and the package
  • the refractive index difference of the layer materials ⁇ 2 is the refractive index difference between the bottom of the trench and the cladding
  • R is the radius of the core
  • ⁇ is the graded refractive index.
  • the refractive index profile of the core is parabolic.
  • the following fiber parameters: n1, ⁇ 1, ⁇ 2, R, and ⁇ are designed to make the effective refractive index difference ⁇ nm of each adjacent mode group equal.
  • a plurality of periodic gratings are distributedly written along the axial direction of the optical fiber, so that the refractive index distribution of the optical fiber changes periodically.
  • the adjacent mode groups are amplified together, so that the amplification gain difference between the mode groups is greatly reduced.
  • the core can be made of silicon dioxide doped with germanium dioxide, and the cladding can be made of silicon dioxide.
  • the doped germanium dioxide can increase the refractive index of the core.
  • the shape of the refractive index profile of the core region is adjusted by designing the concentration distribution of germanium dioxide doped in the core region to keep it consistent with the theoretical design.
  • the core region is uniformly doped or doped with rare earth element ions in layers (sometimes also referred to as ring doping), and then the preform is drawn into an optical fiber.
  • Figure 5 shows a schematic diagram of the cross-sectional structure of the core, and the doped part is shown in gray.
  • (a) represents a uniform doping structure
  • (b) and (c) represent a layered doping structure
  • (b) is a layered doped structure with center doping
  • (c) is a layered doped structure with non-doped center.
  • the number n of layered doping layers can be any natural number, and is usually less than 15.
  • the doping concentration of each doped region can be different, and can be stepped or gradually distributed.
  • the effective refractive index difference ⁇ nm tolerance of adjacent mode groups of the fiber core is generally controlled within ⁇ 5%, and the same period gratings are written on the fiber to achieve strong coupling. That is, the actual value ⁇ nmR of the effective refractive index difference between adjacent mode groups is controlled within the 5% error range of the design value ⁇ nmD: ⁇ nmR ⁇ [95%* ⁇ nmD, 105%* ⁇ nmD].
  • the optical fiber 204 in the optical fiber amplifier is generally short, and the length of the optical fiber 204 is generally selected in the range of 5 meters to 120 meters.
  • the inscribed grating intensity range is ⁇ ng ⁇ [4*10-7, 3*10-2] is more appropriate; for a layered doped fiber, the inscribed grating intensity range is ⁇ ng ⁇ [1 *10-6, 3*10-2] is more appropriate, please refer to the following description for details.
  • the method of writing periodic gratings is as described above.
  • the refractive index characteristics of the fiber make all adjacent mode groups have the same effective refractive index difference, so a grating with a single periodic structure can be written, which reduces the requirements for the grating and reduces the manufacturing cost. .
  • the amplification performance of the strongly coupled few-mode doped fiber can be evaluated through numerical simulation to further guide fiber design.
  • the transmission matrix solution method is a more accurate method for calculating the gain of each mode in the space division multiplexing few-mode fiber amplifier.
  • the calculated diagonal The square of the element modulus is the mode gain of the corresponding mode.
  • Fig. 6 shows a model diagram of a strong coupling space division multiplexing few-mode fiber amplifier according to an embodiment of the present invention.
  • the erbium-doped optical fiber in the amplifier is divided into K-segment independent amplifying parts.
  • the distributed strong coupling can be represented by the random coupling matrix that occurs between every two independent amplifying parts.
  • the transmission matrix M is:
  • Gi represents the gain matrix of the i-th part
  • Mci represents the random coupling matrix between modes.
  • the matrix M contains all the coupling paths between modes, and the singular value decomposition of M is performed:
  • V and U respectively represent the equivalent mode coupling of input and output
  • N is the number of modes.
  • max(g n ) and min(g n ) represent the maximum and minimum mode gains, respectively.
  • the basic parameters of the fiber in the amplifier are selected as follows: the core radius R is 8 microns, the numerical aperture NA is equal to 0.15, the fiber length is 12 meters, and the erbium ion The doping concentration is selected as 100 ppm. Without external mode coupling, that is, when the grating is not written, the maximum mode gain difference DMG between the fiber's 4-mode groups is 9.3dB.
  • the coupling strength is characterized by two parameters: Number of Couping Points (NCP) and Couping Strength (CS).
  • Figure 7 shows the variation curve of DMG with the coupling strength and the number of coupling points under uniform doping conditions.
  • the abscissa in the left a picture represents the coupling strength CS, the unit is dB/coupling point (pt), and the ordinate is DMG, the unit is dB. It can be seen from figure a that when the number of coupling points NCP is fixed at 300, the DMG decreases from 9.6 ⁇ 0.3dB to 2.1 ⁇ 0.3dB as the coupling strength CS increases, which is a decrease of 7.5dB.
  • the reduction in DMG is found to be 8.3dB.
  • the abscissa indicates the number of coupling points, NCP, and the ordinate indicates DMG, in dB. It can be seen from figure b that when the coupling strength is fixed at -15dB/pt and the number of coupling points is increased, the DMG decreases from 7.9 ⁇ 0.6dB to 4.4 ⁇ 0.7dB. As the coupling strength increases, the change in DMG decreases overall, especially for larger coupling points.
  • the length L of the erbium-doped fiber is usually selected from 5m to 120m, combined with the CO2 laser to write the grating intensity threshold parameter, the lower limit of the grating intensity is 4*10-7, and at the same time, it does not damage the fiber's own conduction conditions.
  • the grating intensity The upper limit is 3*10-2. To sum up, for the strong coupling space division multiplexing fiber amplifier proposed in this patent, the value of the grating intensity is ⁇ ng ⁇ [4*10-7, 3*10-2] is more appropriate.
  • the abscissa represents the coupling strength CS, the unit is dB/pt, the ordinate represents the number of coupling points NCP, and the contour lines of different color shades represent the DMG, the unit is dB; it can be seen from Figure 8, for the uniformly doped strong coupling few-mode fiber
  • the amplifier after introducing strong coupling, the DMG can be significantly reduced to 7.6dB, that is, from 9.3dB to 1.7dB. This is the result of the combined action of the erbium-doped fiber after optimizing the horizontal and vertical structure.
  • the basic parameters of the fiber in the amplifier are selected as follows: the core radius R is 8 microns, and the numerical aperture NA is equal to 0.15, the fiber length is 12 meters.
  • the doping concentration and size of each layer can be numerically optimized to obtain a more balanced mode-dependent gain.
  • the radius b of the first doped region (that is, the center doped region) is selected to be 3 ⁇ m, and the doped
  • the DMG is 2.4dB when there is no mode coupling under this layered doping.
  • FIG. 9 shows a two-dimensional diagram of the relationship between DMG and layered doping parameters b and c.
  • the abscissa c represents the distance between the start boundary of the second doped region and the core, in micrometers ( ⁇ m), and the ordinate b represents the first
  • the radius of a doped area, in micrometers ( ⁇ m) indicates DMG, and the unit is dB. It can be seen that the jitter of b and c brought about by the production process to a certain extent has little effect on DMG in the case of no coupling.
  • the coupling strength is characterized by two parameters: Number of Couping Points (NCP) and Couping Strength (CS).
  • NCP Number of Couping Points
  • CS Couping Strength
  • the number of coupling points and the coupling strength are calculated 100 times, and the DMGs are averaged to obtain the contour map of FIG. 10.
  • the abscissa represents the coupling strength CS
  • the unit is dB/pt
  • the ordinate represents the number of coupling points NCP
  • the contour lines of different color shades represent DMG
  • the unit is dB
  • DMG can be significantly reduced from 3.2dB to 0.5dB. This is the result of the joint action of the erbium-doped fiber after optimizing the horizontal and vertical structure.
  • the average DMG value begins to decrease, and this parameter is used as the critical point of the coupling strength required for layered doping.
  • a 40.1cm-long fiber can complete 3.2% mode power coupling, that is, a distributed coupling strength of -15dB.
  • mode coupling of -15dB can be completed under a fiber length of 4cm.
  • the length of the erbium-doped fiber to be selected from 5m to 120m, and the lower limit of the grating intensity is 1*10-6.
  • the grating intensity is ⁇ ng ⁇ [1*10-6, 3*10-2] is more appropriate.
  • layered doping has higher requirements for coupling strength.
  • the appropriate range of layered doped grating intensity ([1*10-6, 3*10-2]) is included in the appropriate range of uniformly doped grating intensity ([ 4*10-7, 3*10-2]).
  • the above-mentioned grating intensity is the result of the required grating intensity calculated under the fixed b and c parameters. If the b and c parameters are changed, when the DMG without coupling is between 2.4 and 9.3dB, the grating intensity is selected The lower limit should also be between 4*10-7 and 1*10-6.
  • NF noise figure
  • gi and PASE_i respectively represent the mode gain and ASE noise power of the i-th mode group. From the above, the difference of the gain gi of each mode is weakened to a certain extent, and the strong coupling also makes the ASE noise power of each mode tend to be consistent. Therefore, the NF difference between modes in the strongly coupled space division multiplexing fiber amplifier proposed in the embodiment of the present invention is small.
  • the peripheral structure of the core in addition to considering the core refractive index and grating structure parameters of the few-mode doped fiber, can also be introduced to further adjust the mode light field distribution, such as trench assistance or air hole assistance.
  • Fig. 11a shows a schematic structural diagram of a cross-section of a groove-assisted optical fiber
  • Fig. 11b shows a schematic diagram of a cross-sectional refractive index distribution of this optical fiber along the radial direction.
  • the outer cladding layer of the core includes an inner cladding layer and an outer cladding layer, and a groove layer is sandwiched between the inner cladding layer and the outer cladding layer.
  • R is the core radius and W1 is the width of the inner cladding layer. Generally, the value is less than or equal to R/2. Tie the module group in the core.
  • W2 is the width of the groove layer, the refractive index of the groove layer is smaller than the inner and outer cladding layers, ⁇ 1 is the refractive index difference between the core center and the cladding material, ⁇ 2 is the refractive index difference between the groove layer and the cladding layer, ⁇ It is a graded refractive index. Considering the fiber manufacturing process, the wider the groove, the larger the ⁇ 2, and the higher the manufacturing cost.
  • the value range of W2 is generally less than or equal to the core radius R. ⁇ 2 is generally not greater than 0.0045.
  • the introduction of the trench equivalently reduces the refractive index of the cladding, which will limit the optical field, so that the various modes are more concentrated inside the core, reduce the coupling with the cladding mode, and reduce the bending loss.
  • Fig. 12 shows a schematic diagram of a cross-sectional structure of an air hole assisted optical fiber, in which a plurality of air holes are added to the periphery of the core.
  • the function of the air hole is similar to that of the aforementioned groove, which is equivalent to reducing the refractive index of the cladding region, and further restricting the optical field, making it more confined to the core part.
  • R is the core radius
  • b is the radius of the surrounding air holes.
  • the radius b of the air holes is not less than 1.5um, so as to prevent the hole radius from being too small, which is equivalent to defective air bubbles.
  • d is the distance between the air hole and the core. Generally, the smaller the distance d, the better.
  • the air hole is more effective in restricting the light field, but its manufacturing process requires higher.
  • the number of cores in the optical fiber can also be increased, and each core can be designed according to the scheme of the foregoing embodiment to realize a multi-core optical fiber amplifier.
  • the design of the multi-core fiber should try to keep the strong coupling between the mode groups in the core as much as possible, and at the same time weaken the coupling between the mode groups between the cores.
  • the gain balance of the mode groups between the cores is also an important consideration.
  • Different pumping methods can be used for different inter-core arrangements and the number of cores, including core pumping methods and cladding pumping methods. Both of these two pumping methods can make the distribution of pump energy in each fiber core basically the same, and achieve the effect of equalizing the gain between the cores.
  • FIG. 13 shows a schematic diagram of the cross-sectional structure of a 3-core fiber.
  • Each core is a few-mode doped core engraved with periodic gratings.
  • the core pumping method is used in each core to pump light. Both the signal light and the signal light are driven into the core part, so as to effectively realize the optical field overlap of the pump light and the signal light, and achieve the increase of gain and the balance of the gain of each core.
  • the optical fiber includes an inner cladding layer and an outer cladding layer, and multiple cores are located in the inner cladding layer.
  • the inner cladding is the pumping area, and the pump light can penetrate into the pumping area.
  • the refractive index of the inner cladding is generally slightly lower than 1.444, but higher than the refractive index of the outer cladding and lower than the refractive index of the core.
  • the shape of the inner cladding can be rectangular, hexagonal, D-shaped, etc., so that the pump light can penetrate the fiber core multiple times, and the utilization efficiency of the pump light in the fiber core is improved.
  • the inner cladding is generally not circular, because the pump light tends to propagate spirally in the circular inner cladding, becoming vortex light, and cannot enter the core area with rare earth ions and signal light.
  • Each core is a few-mode doped core with periodic gratings, and the inner cladding is a hexagonal area, which is also a pump. area.
  • the cladding pumping method has a simple structure and can realize the gain balance of each core at the same time, but the energy conversion efficiency of the pump light is relatively low.

Landscapes

  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Plasma & Fusion (AREA)
  • Optics & Photonics (AREA)
  • Lasers (AREA)

Abstract

一种光纤放大器，包括：泵浦激光器（202），信号泵浦合束器（203），以及少模掺杂光纤（204）；其中，泵浦激光器（202）用于输出泵浦光；信号泵浦合束器（203）用于将输入的少模信号光与泵浦光耦合入少模掺杂光纤（204）中；少模掺杂光纤（204）的纤芯的折射率沿横截面半径方向呈渐变分布，纤芯沿轴向刻有周期性光栅，光栅的周期满足相位匹配条件。光纤放大器实现光信号模式间的强耦合与共放大，从而减小模群间的差分增益。

Description

光纤放大器
本申请要求于2019年12月28日提交中国国家知识产权局、申请号为201911383861.1、申请名称为“光纤放大器”的中国专利申请的优先权，以及于2020年3月2日提交中国国家知识产权局、申请号为202010135229.1、申请名称为“光纤放大器”的中国专利申请的优先权，其全部内容通过引用结合在本申请中。
技术领域
本发明涉及光通信领域，尤其涉及一种光纤放大器。
背景技术
随着通信容量需求的迅速增长，出现了基于少模光纤(Few Mode Fiber，FMF)和多芯光纤(Multi-core Fiber，MCF)的空分复用(Spatial Division Multiplexing，SDM)技术，使用SDM技术的长距离传输系统中，需要SDM光纤放大器实现不同空间信道的同时放大。
如图1所示，为一种实现少模光传输不同空间信道同时放大的方案。利用多个单模掺铒光纤放大器(Erbium-Doped Fiber Amplifier，EDFA)实现。具体的，先将少模光纤输入的模式复用信号解复用成多个单模信号，然后分别用单模EDFA放大。放大后的多路信号再复用进输出的少模光纤。这种少模信号放大方法，需要经过“解复用”、“放大”、“复用”三步，一方面增加了系统的复杂度和成本，另一方面引入额外的插损及模间串扰，随着传输距离的增加，模间串扰不断积累，每个模式上信号的信噪比不断恶化，很难支持长距传输。
另外，目前还有少模掺铒光纤放大器(Few Mode Erbium-Doped Fiber Amplifier，FM-EDFA)等。相较于上述多通道单模EDFA，FM-EDFA成本较低，但其各模式间的增益不均衡，模式增益差(Differential Mode Gain，DMG)较大，且不同模式间的放大噪声系数(Noise Figure，NF)差异较大，造成各模式信号的传输性能不一致，不利于长距离传输。
发明内容
本发明实施例提供一种光纤放大器，实现信号模式间的强耦合与共放大，从而减小模式群间的差分增益。
第一方面，本发明实施例提供了一种光纤放大器，包括：泵浦激光器，信号泵浦合束器，以及少模掺杂光纤；其中：泵浦激光器用于输出泵浦光；信号泵浦合束器用于将输入的少模信号光与泵浦光耦合入少模掺杂光纤中；少模掺杂光纤的纤芯的折射率沿横截面半径方向呈渐变分布，纤芯沿轴向刻有周期性光栅，光栅的周期满足相位匹配条件。少模掺杂光纤的周期性光栅的使用，使得光信号模式间强耦合与共放大，从而减小模式群间的差分增益。
在一个可能的设计中，光栅的周期满足相位匹配条件具体包括：多个光栅的周期分别与少模信号光的相邻模群间的有效折射率差的乘积等于少模信号光的波长；少模信号光的相邻模群为：信号光包含的多个模群按照对应的有效折射率依次排列后两个相邻的模群。这种模式强耦合的方式对折射率渐变的少模光纤普遍适用，增大了应用的广泛性。
在又一个可能的设计中，少模掺杂光纤的纤芯的折射率沿横截面半径方向渐变分布使得相邻模群间的有效折射率差相等，多个光栅的周期相等。这种特性使得所有相邻模群具有相同有效折射率差，因而可采用刻写具有单一周期结构的光栅，这样降低了光栅的要求，减小了制造成本。
在又一个可能的设计中，少模掺杂光纤的纤芯区域掺杂有稀土元素离子。实现了不同波长信号的放大增益。
在又一个可能的设计中，纤芯区域掺杂有稀土元素离子包括均匀掺杂和分层掺杂。
在又一个可能的设计中，少模掺杂光纤的纤芯采用掺有二氧化锗的二氧化硅材料，包层采用二氧化硅材料；或者纤芯采用二氧化硅材料，包层采用掺氟化物的二氧化硅。从而实现模群间折射率差相等。
在又一个可能的设计中，少模掺杂光纤的纤芯外围还包括内包层、沟槽层、外包层，沟槽层的折射率小于内包层和外包层的折射率。这种方式等效降低了包层折射率，会起到限制光场的作用，使得各个模式更加聚集于纤芯内部，降低与包层模之间的耦合，减小弯曲损耗。
在又一个可能的设计中，少模掺杂光纤的纤芯外围多个空气孔。空气孔在限制光场方面效果更优。
在又一个可能的设计中，少模掺杂光纤包括多个纤芯，采用纤芯泵浦方式，泵浦光打入每个纤芯。从而有效的实现泵浦光与信号光的光场重叠，达到增益的提高与各纤芯增益的均衡。
在又一个可能的设计中，少模掺杂光纤包括多个纤芯，采用包层泵浦方式，包层中包括泵浦区域，泵浦光打入泵浦区域。包层泵浦方式结构简单，同时也能实现各纤芯的增益均衡，
在又一个可能的设计中，光纤放大器还包括分束器，用于分离少模掺杂光纤输出的信号光和泵浦光，并把分离出的泵浦光输入进信号泵浦合束器。这样实现了对泵浦光充分的利用，进一步提高各模式信号光的增益。
在又一个可能的设计中，光纤放大器还包括第一隔离器和第二隔离器，第一隔离器与信号泵浦合束器的输入端连接，第二隔离器与少模掺杂光纤的输出端连接。这样防止反向散射光进入光纤链路，提高了光信号传输质量。
本发明实施例提供的上述技术方案，通过放大器中使用刻有周期性光栅的少模掺杂光纤，使得光信号模式间实现强耦合与共放大，从而减小模式群间的差分增益和噪声系数差异，有利于光信号的长途传输。
附图说明
图1为现有技术的一种光纤放大器结构示意图；
图2为本发明实施例提供的一种空分复用光纤放大器的结构示意图；
图3a为本发明实施例提供的一种光纤放大器内的少模掺杂光纤结构示意图；
图3b为本发明实施例提供的一种少模掺杂光纤的截面折射率分布示意图；
图4a为本发明实施例提供的另一种光纤放大器内的少模掺杂光纤结构示意图；
图4b为本发明实施例提供的另一种少模掺杂光纤的截面折射率分布示意图；
图5为本发明实施例提供的掺杂纤芯的横截面结构示意图；
图6为本发明实施例提供的掺铒光纤模型示意图；
图7为本发明实施例提供的均匀掺杂下DMG随耦合强度和耦合点数变化曲线图；
图8为本发明实施例提供的均匀掺杂下DMG随耦合强度和耦合点数等高线图；
图9为本发明实施例提供的DMG随分层掺杂参数b、c的变化关系二维图；
图10为本发明实施例提供的分层掺杂下DMG随耦合强度和耦合点数等高线图；
图11a为本发明实施例提供的一种沟槽辅助型的光纤的横截面的结构示意图；
图11b为本发明实施例提供的沟槽辅助型光纤沿径向的截面折射率分布示意图；
图12为本发明实施例提供的一种空气孔辅助型的光纤的横截面结构示意图；
图13为本发明实施例提供的3芯光纤的横截面的结构示意图；
图14为本发明实施例提供的7芯光纤的横截面的结构示意图。
具体实施方式
为使本发明的目的、技术方案和优点更加清楚，下面将结合附图对本发明实施方式作进一步地详细描述。
本发明实施例提供一种强耦合空分复用光纤放大器，如图2所示，包括：两个光隔离器201，泵浦激光器202，信号泵浦合束器203，以及少模掺杂光纤204。其中，两个光隔离器201分别在光纤放大器的输入端和输出端，用于防止反向散射光进入光纤链路影响光信号传输质量；泵浦激光器202用于输出泵浦光；信号泵浦合束器203用于将输入端的少模信号光与泵浦光耦合进入输出端；少模掺杂光纤204用于实现信号模式间的强耦合与共放大，从而减小模群间的差分增益。
图2中，进入光纤放大器的信号光包含M个模群，M为大于等于2的整数。模群是指在光纤中传播常数一致的多个模式组成的群，一个模群中的多个模式的速率相等。例如输入的信号光包括4个模群LP01、LP11、LP21、LP02，每个模群上承载不同的信息。信号光通过光隔离器201后，经信号泵浦合束器203与泵浦光同时耦合进少模掺杂光纤204。在少模掺杂光纤204中，纤芯区域掺杂了一定浓度的稀土元素离子。离子在泵浦光的作用下由低能级跃迁到高能级上，处于激发态，而高能级粒子在入射信号光作用下回到基态时发射与信号光频率相同的光子，使信号光得以放大，且信号光中每个模群获得相同的增益。少模掺杂光纤204放大后的信号光再经光隔离器输出。
少模掺杂光纤的增益谱范围由纤芯区域掺杂离子的类型决定。例如，掺杂镱离子Yb3+， 对应的增益波段范围为970nm-1200nm。其他稀土元素离子和对应的增益谱范围包括：镨离子Pr3+(放大1300nm附近波段)，钕离子Nd3+(放大1300nm附近波段)，铒离子Er3+(放大1520nm-1625nm波段)，铋离子Bi3+(放大1300nm-1380nm波段)，铥离子Tm3+(放大1450nm-1520nm波段、1900nm-2000nm波段)。
少模光纤放大器中还可以包括循环泵浦结构，用于收集残余泵浦光能量，并对其循环利用，从而大大提高泵浦光利用率，提高信号光放大增益。如图2所示，205为信号泵浦分束器。经过少模掺杂光纤204放大的少模信号光与残余的泵浦光进入部件205，在信号泵浦分束器内完成信号光与泵浦光的分离，放大的信号光进入下一段传输链路中，残余的泵浦光与泵浦激光器202发出的泵浦光一同输入信号泵浦合束器203，进行新一轮的放大。这样实现了对泵浦光充分的利用，进一步提高各模式信号光的增益。
为了增强光纤中各模式的空间信道耦合，本发明实施例中，光纤放大器内的少模掺杂光纤的纤芯刻有光栅。如图3a所示，为本发明实施例提供的光纤放大器内的一种少模掺杂光纤示意图，中间为纤芯，纤芯的外面为包层。沿着少模掺杂光纤的轴向方向，分布式刻写多个周期性光栅。光栅周期需满足的相位匹配条件说明如下。
少模掺杂光纤的纤芯折射率沿径向渐变。图3b为少模掺杂光纤的截面折射率分布示意图，其中Δ为纤芯中心与包层材料的折射率差(一般Δ∈[0.001,0.030])，R为纤芯半径(一般R∈[2,35]um)，α为渐变折射率指数(α∈[1,∞])。设光纤支持M个模群传输，不同的模群有不同的有效折射率。将不同模群的有效折射率排列，最大的排在前面，为第一模群，其余的依有效折射率递减顺序排列。相邻模群间的有效折射率差设为Δn，例如，Δn1为第一模群与第二模群之间的有效折射率差，Δn2为第二模群与第三模群之间的有效折射率差，Δnm为第m模群与m+1模群之间的有效折射率差。一般Δnm∈[1*10-5,8*10-3]，m为1到(M-1)。
与上述光纤折射率的特性相匹配，沿着光纤的轴向方向，分布式刻写多个周期性光栅，使光纤折射率分布呈周期性变化。如图3a所示，与上述M-1个相邻模群间的有效折射率差对应，光纤上至少刻有M-1段周期性光栅，其中Λ1为第一段光栅周期，Λ2为第二段光栅周期，Λm为第m段光栅周期。光栅的周期需满足相位匹配条件，即与相邻模群间的有效折射率差的乘积等于少模信号光的波长。也就是说，光栅周期Λm为：Λm＝λ/Δnm，其中λ为光波波长。这样特定相邻模群间会产生最强耦合，放大少模信号光时，相邻模群一起放大，从而使得模群间的放大增益差大幅减小。
例如，对于4个模群的情况，在光纤上刻写光栅后，相邻模群间，包括：LP01和LP11，LP11和LP21，LP21和LP02之间的耦合程度将大大增强。
上述少模掺杂光纤上刻写周期性光栅的方式如下：采用二氧化碳(CO2)激光在少模掺杂光纤上逐点刻写周期性光栅。CO2激光器发出的激光经显微物镜聚焦于少模掺杂光纤的纤芯，激光与光纤接触的位置由于热效应引入折射率变化。少模掺杂光纤被固定在高精度的三维移动平台上，通过三维移动平台控制光纤按一定的步进速度移动，逐点曝光，直至完成整个光栅的刻写。根据光栅刻写周期和占空比确定光纤步进速度和激光能量及曝光时间。
图3a和3b对应的实施例，模式强耦合的方式对折射率渐变的少模光纤普遍适用。在模群数目M较大的情况下，会有M-1个相邻模群有效折射率差，相应的，需在光纤上刻写对应的M-1个长周期性光栅，提高了对光栅的要求，增加了制造成本。
如图4a所示，为本发明实施例提供的光纤放大器内的另一种少模掺杂光纤示意图。在这个实施例中，设计使得少模掺杂光纤的纤芯中，相邻模群有效折射率差相等，则根据前述公式Λm＝λ/Δnm，各段光栅的周期也相同。
图4b所示为本实施例的少模掺杂光纤的截面折射率分布示意图，其中纤芯的折射率渐变分布曲线满足：n(r)2＝n12(1-2Δ(r/R)α)。其中，r为纤芯中某一径向位置，n(r)为该径向位置r处的折射率，n1为纤芯中心的折射率，Δ＝Δ1+Δ2，Δ1为纤芯中心与包层材料的折射率差，Δ2为沟槽底部与包层的折射率差，R为纤芯的半径，α为渐变折射率指数，α＝2时纤芯的折射率分布曲线呈抛物线型。针对多个模群在纤芯区域的光场分布，通过设计以下光纤参数：n1，Δ1、Δ2，R和α等，使得各相邻模群的有效折射率差Δnm相等。
与上述光纤折射率的特性相匹配，沿着光纤的轴向方向，分布式刻写多个周期性光栅，使光纤折射率分布呈周期性变化。如图4a所示，由于M-1个相邻模群间的有效折射率差Δnm相等，在光纤上刻写周期性光栅时，光栅周期Λ只要满足相位匹配条件：Λ＝λ/Δnm，其中λ为光波波长。这样相邻模群间会产生最强耦合，放大少模信号光时，相邻模群一起放大，从而使得模群间的放大增益差大幅减小。
要实现模群间折射率差相等，纤芯可以采用掺有二氧化锗的二氧化硅材料，而包层采用二氧化硅材料。其中掺入的二氧化锗可提高纤芯的折射率。在光纤制备过程中，通过设计纤芯区域中掺入的二氧化锗的浓度分布来调节纤芯区域折射率剖面的形状，使其与理论设计保持一致。进一步，在纤芯区域均匀掺杂或者分层掺杂(有时也可称为环形掺杂)稀土元素离子，然后将预制棒拉制成光纤。
图5所示为纤芯的横截面结构示意图，掺杂的部分用灰色表示。其中，(a)表示均匀掺杂结构，(b)和(c)表示分层掺杂结构。(b)为中心掺杂的分层掺杂结构，(c)为中心非掺杂的分层掺杂结构。分层掺杂的层数n可以为任意自然数，通常小于15。同时，每层掺杂区域的掺杂浓度可有所不同，可阶跃分布或者渐变分布。
经工程实践可知，一般将纤芯的相邻模群有效折射率差Δnm容限控制在±5％范围内，配合在光纤上刻写相同周期的光栅，可实现强耦合。即相邻模群有效折射率差的实际值ΔnmR控制在设计值ΔnmD的5％误差范围内：ΔnmR∈[95％*ΔnmD,105％*ΔnmD]。实际刻写的光栅周期为：Λm＝λ/ΔnmR。
光纤放大器内光纤204通常较短，一般光纤204的长度选取范围介于5米至120米之间。相应的，对于均匀掺杂光纤，刻写的光栅强度范围为Δng∈[4*10-7，3*10-2]更合适；对于分层掺杂光纤，刻写的光栅强度范围为Δng∈[1*10-6，3*10-2]更合适，具体可参见后面的描述。
刻写周期性光栅的方式如前所述。图4a和4b所示实施例中，光纤折射率特性使得所有相邻模群具有相同有效折射率差，因而可采用刻写具有单一周期结构的光栅，这样降低了光栅的要求，减小了制造成本。
对上述实施例的少模掺杂光纤以及放大器，可以通过数值仿真对强耦合少模掺杂光纤的放大性能进行评估，以进一步指导光纤设计。例如，传输矩阵求解法是一种较为精确的计算空分复用少模光纤放大器中各模式增益的方法，通过求解信号光通过放大器的传输矩阵，并对其进行奇异值分解，计算所得对角元素模值的平方即为对应模式的模式增益。
图6所示为本发明实施例的强耦合空分复用少模光纤放大器模型图。放大器内的掺铒光 纤被分成K段独立放大部分，分布式强耦合可由发生在每两段独立放大部分之间的随机耦合矩阵表示，则传输矩阵M为：
M＝G K...G i+1M ciG i...G 2M c1G 1       (1)
其中Gi表示第i部分的增益矩阵，Mci表示模间随机耦合矩阵。矩阵M包含了所有模间耦合路径，对M进行奇异值分解：
M＝V·S·U *           (2)
Figure PCTCN2020126743-appb-000001
其中V、U分别表示输入端和输出端的等效模式耦合，N为模式数。所得目标矢量g＝(g 1,g 2…g N)的元素是M·M *特征值的对数，即N个模式的增益。因此，模式间增益差DMG可被写为：
DMG＝max(g n)-min(g n)        (4)
其中max(g n)、min(g n)分别表示最大和最小模式增益。
下面先讨论纤芯均匀掺杂的情况。假设输入的信号光包括4个模群LP01、LP11、LP21、LP02，放大器中的光纤的基础参数选取为：纤芯半径R为8微米，数值孔径NA等于0.15，光纤长度为12米，铒离子掺杂浓度选取100ppm。无外加模式耦合，即未刻写光栅时，该光纤4模群间最大模式增益差DMG为9.3dB。
引入强耦合，即刻写光栅，可以大幅改进DMG。其耦合强度的大小以耦合点数(Number of Couping Points，NCP)及单点耦合强度(Couping Strength,CS)两个参量共同表征。图7为均匀掺杂条件下，DMG随耦合强度和耦合点数变化曲线。左边a图中横坐标表示耦合强度CS，单位dB/耦合点(pt)，纵坐标为DMG，单位dB。由a图可见，当耦合点数NCP固定为300时，DMG随耦合强度CS的增大从9.6±0.3dB减小到2.1±0.3dB，减小了7.5dB。类似地，当耦合点数为600时，发现DMG的减小量为8.3dB。右边b图中横坐标表示耦合点数NCP，纵坐标表示DMG，单位dB。由b图可见，当耦合强度固定在-15dB/pt，增大耦合点数时，DMG从7.9±0.6dB降低到4.4±0.7dB。随着耦合强度的增加，DMG的变化总体上减小，特别是对于较大的耦合点数。由于耦合本身带来的随机性，从极弱到极强耦合，虽然DMG的数值整体在下降，但其标准差的浮动先增大再减小；这种浮动可能造成DMG的数值在相对较强耦合时反而更高，如图7(a)中CS＝-25dB/pt处所示。值得注意的是，从后面的描述中也可以看出，上述这种现象在纤芯分层掺杂的情况下也会发生。因此为了得到低DMG且性能稳定的光放大器，一般需比较大的耦合强度。
由图7可见，在耦合强度大于-20dB/pt，耦合点数大于200的条件下，DMG的数值开始降低，即光纤放大器的DMG特性开始有所改善。结合长周期光栅所带来的强耦合效应，在光栅强度Δng＝1*10-6时，22.5厘米(cm)长的光纤即可完成1％的模式功率耦合，即-20dB的分布耦合强度。耦合强度反比于光栅长度，因而在光栅强度Δng＝4*10-6时，6cm的光纤长度即可完成-20dB的模式耦合，即可实现上述单点耦合-20dB/pt，点数200的耦合强度。此外，考虑掺铒光纤长度L通常选取范围介于5m至120m之间，结合CO2激光器刻写光栅强度阈值参数，光栅强度下限取值4*10-7，同时为不破坏光纤自身传导条件，光栅强度上限取值3*10-2。综上所述，对于本专利所提出的强耦合空分复用光纤放大器，光栅强度取值为Δng∈[4*10-7，3*10-2]更合适。
由于耦合点数及耦合强度对DMG影响显著，因此，为了光纤放大器性能的提升，需要对这两个重要参数进行扫描和优化。对于均匀掺杂情形，DMG随耦合点数及耦合强度变化的一个示例如图8所示。值得注意的是，即使耦合点数和耦合强度是固定的，DMG仍然会由于单点耦合矩阵的随机性和耦合点的位置而变化。因此，图8的示例中，在每个耦合点数和耦合强度处计算100次，将其DMG平均，得到图8的等高线图。图8中，横坐标表示耦合强度CS，单位dB/pt，纵坐标表示耦合点数NCP，不同颜色深浅的等高线表示DMG，单位dB；由图8可见，对于均匀掺杂强耦合少模光纤放大器，引入强耦合后，DMG可以显著降低到7.6dB，即从9.3dB降低到1.7dB。这是掺铒光纤优化横向和纵向结构后的共同作用结果。
对于纤芯分层掺杂的情况，假设输入的信号光包括4个模群LP01、LP11、LP21、LP02，放大器中的光纤的基础参数选取为：纤芯半径R为8微米，数值孔径NA等于0.15，光纤长度为12米。分层掺杂情形下，每层的掺杂浓度和尺寸可以通过数值优化来获得比较均衡的模式依赖增益。以图5中(b)所示的光纤中心掺杂的分层掺杂结构为例，假设掺杂两层，选取第一掺杂区域(也就是中心掺杂区域)的半径b为3μm，掺杂浓度为65ppm，3μm以外不掺杂，不掺杂宽度i1为1.8μm，第二掺杂区域边界从c＝b+i1＝4.8μm开始，一直到8μm的纤芯边界，掺杂浓度为115ppm。该分层掺杂下无模式耦合时DMG为2.4dB。以上所述分层掺杂光纤参数是通过无耦合情形下掺杂区域参数的优化所得。图9所示为DMG随分层掺杂参数b、c的变化关系二维图，横坐标c表示第二掺杂区域起始边界距离纤芯距离，单位微米(μm)，纵坐标b表示第一掺杂区域半径，单位微米(μm)，颜色深浅的不同表示DMG，单位dB。可看出，由于生产工艺所带来的b、c一定程度上的抖动于无耦合情形下DMG的影响不大。
引入强耦合，即刻写光栅，可以大幅改进DMG。其耦合强度的大小以耦合点数(Number of Couping Points，NCP)及单点耦合强度(Couping Strength,CS)两个参量共同表征。同样，为了得到低DMG且性能稳定的光放大器，一般需比较大的耦合强度。由于耦合点数及耦合强度对DMG影响显著，因此，为了光纤放大器性能的提升，需要对这两个重要参数进行扫描和优化。对于分层掺杂情形，DMG随耦合点数NCP及单点耦合强度CS变化的一个示例如图10所示。同样，即使耦合点数和耦合强度是固定的，DMG仍然会由于单点耦合矩阵的随机性和耦合点的位置而变化。因此，图10的示例中，在每个耦合点数和耦合强度处计算100次，将其DMG平均，得到图10的等高线图。图10中，横坐标表示耦合强度CS，单位dB/pt，纵坐标表示耦合点数NCP，不同颜色深浅的等高线表示DMG，单位dB；可见，对于分层掺杂强耦合少模光纤放大器，引入强耦合后，DMG可以由3.2dB显著降低至0.5dB。这是掺铒光纤优化横向和纵向结构后的共同作用结果。
由分层掺杂等高线图看出，在CS大于-15dB/pt，NCP大于300的条件下，平均DMG的数值开始降低，以此参数作为分层掺杂所需耦合强度的临界点。在光栅强度Δng＝1*10-6时，40.1cm长的光纤即可完成3.2％的模式功率耦合，即-15dB的分布耦合强度。在光栅强度Δng＝1*10-5时，可在4cm的光纤长度下完成-15dB的模式耦合。同样考虑掺铒光纤长度选取范围5m至120m，光栅强度的下限取值为1*10-6。光栅强度为Δng∈[1*10-6，3*10-2]更合适。
一般分层掺杂对耦合强度要求更高，分层掺杂的光栅强度的合适范围([1*10-6，3*10-2])包含于均匀掺杂的光栅强度的合适范围([4*10-7，3*10-2])内。需要注意的是，上述光栅强度是在固定b、c参数下计算的所需光栅强度结果，如若更改b、c参数，无耦合情况下的DMG介于2.4～9.3dB之间时，光栅强度选取的下限应该也会在4*10-7～1*10-6之间。
此外，噪声系数(NF)也是衡量空分复用光纤放大器的重要指标，在传输系统中，模式间较小的噪声系数差异更有利于系统传输性能的提升。本发明实施例所提出的强耦合空分复用光纤放大器能够一定程度上使之降低。NF可由如下公式表示：
Figure PCTCN2020126743-appb-000002
其中gi、PASE_i分别表示第i个模群的模式增益和ASE噪声功率。由上，各模式增益gi的差异性在一定程度上有所减弱，同时强耦合也使得各模式的ASE噪声功率趋于一致。因此，本发明实施例所提出的强耦合空分复用光纤放大器中模式间NF差异较小。
本发明实施例中，除考虑少模掺杂光纤的纤芯折射率和光栅结构参数外，还可以引入纤芯外围结构来进一步调节模式光场分布，如沟槽辅助或空气孔辅助等。
如图11a所示为沟槽辅助型的光纤的横截面的结构示意图，图11b所示为这种光纤沿径向的截面折射率分布示意图。其中纤芯的外围包层包括内包层和外包层，内包层和外包层之间夹一个沟槽层，其中R为纤芯半径，W1为内包层宽度，一般取值小于等于R/2，以将模群束缚在纤芯内。W2为沟槽层的宽度，沟槽层的折射率比内包层和外包层小，Δ1为纤芯中心与包层材料的折射率差，Δ2为沟槽层与包层的折射率差，α为渐变折射率指数。考虑光纤制作工艺，沟槽越宽，Δ2越大，制造成本越高，W2的取值范围是一般小于等于纤芯半径R。Δ2一般不大于0.0045。
沟槽的引入等效降低了包层折射率，会起到限制光场的作用，使得各个模式更加聚集于纤芯内部，降低与包层模之间的耦合，减小弯曲损耗。
图12所示为空气孔辅助型的光纤的横截面结构示意图，其在纤芯外围增加多个空气孔。空气孔的作用与前述沟槽类似，等效于降低包层区域的折射率，进一步限制光场，使其更加束缚于纤芯部分。图中R为纤芯半径，b为周围空气孔的半径，一般空气孔的半径b不小于1.5um，从而避免孔半径过小等效于缺陷气泡。d为空气孔与纤芯之间的距离。一般距离d越小越好。相比于沟槽结构，空气孔在限制光场方面效果更优，但其制作工艺要求较高。
基于上述图2和图3a及3b的实施例，还可以在光纤中增加纤芯的数量，每个纤芯均可按照前述实施例的方案设计，实现多芯光纤放大器。该多芯光纤的设计需尽量保持纤芯内的模群间强耦合，同时弱化纤芯之间的模群间耦合。此外，纤芯之间的模群的增益均衡也是一个重要考虑因素，对于不同的芯间排布以及纤芯数目可采用不同的泵浦方式，包括纤芯泵浦方式和包层泵浦方式。这两种泵浦方式均可以使泵浦能量在各个纤芯中的分布基本一致，达到芯间增益均衡的效果。
当多芯光纤的纤芯数目较少时，可采用纤芯泵浦方式。如图13所示为3芯光纤的横截面的结构示意图，每个纤芯都是刻有周期性光栅的少模掺杂纤芯，各个纤芯内均采用纤芯泵浦方式，泵浦光和信号光均打入纤芯部分，从而有效的实现泵浦光与信号光的光场重叠，达到增益的提高与各纤芯增益的均衡。
当多芯光纤的纤芯数目较多时，可采用包层泵浦方式。光纤包括内包层和外包层，多个纤芯位于内包层中。内包层为泵浦区域，泵浦光可打入泵浦区域。内包层折射率一般略低于 1.444，但高于外包层折射率，低于纤芯折射率。内包层形状可选择矩形、六边形、D形等结构，使得泵浦光可以多次穿透光纤纤芯，提高泵浦光在纤芯中的利用效率。强泵浦光耦合进入内包层，在稀土元素离子掺杂的纤芯区域，泵浦光均匀分布。内包层一般不为圆形，因为泵浦光容易在圆形内包层中呈现螺旋式传播，成为涡旋光，无法进入有稀土离子和信号光的纤芯区域。
图14所示为本发明实施例的7芯光纤的横截面的结构示意图，每个纤芯都是刻有周期性光栅的少模掺杂纤芯，内包层为六边形区域，也是泵浦区域。相比于纤芯泵浦方式，包层泵浦方式结构简单，同时也能实现各纤芯的增益均衡，但泵浦光的能量转换效率相对较低。
尽管在此结合各实施例对本发明进行了描述，然而，在实施所要求保护的本发明过程中，本领域技术人员通过查看所述附图、公开内容、以及所附权利要求书，可理解并实现所述公开实施例的其他变化。在权利要求中，“包括”(comprising)一词不排除其他组成部分或步骤，“一”或“一个”不排除多个的情况。
尽管结合具体特征及其实施例对本发明进行了描述，显而易见的，可对其进行各种修改和组合。相应地，本说明书和附图仅仅是所附权利要求所界定的本发明的示例性说明，且视为已覆盖本发明范围内的任意和所有修改、变化、组合或等同物。显然，本领域的技术人员可以对本发明进行各种改动和变型而不脱离本发明的范围。这样，倘若本发明的这些修改和变型属于本发明权利要求及其等同技术的范围之内，则本发明也意图包含这些改动和变型在内。

Claims (14)

  1. 一种光纤放大器，其特征在于，包括：泵浦激光器，信号泵浦合束器，以及少模掺杂光纤；其中：
    所述泵浦激光器用于输出泵浦光；
    所述信号泵浦合束器用于将输入的少模信号光与所述泵浦光耦合入所述少模掺杂光纤中；
    所述少模掺杂光纤的纤芯的折射率沿横截面半径方向呈渐变分布，所述纤芯沿轴向刻有周期性光栅，光栅的周期满足相位匹配条件。
  2. 如权利要求1所述的光纤放大器，其特征在于，所述光栅的周期满足相位匹配条件具体包括：多个光栅的周期分别与所述少模信号光的相邻模群间的有效折射率差的乘积等于少模信号光的波长；所述少模信号光的相邻模群为：所述信号光包含的多个模群按照对应的有效折射率依次排列后两个相邻的模群。
  3. 如权利要求2所述的光纤放大器，其特征在于，所述少模掺杂光纤的纤芯的折射率沿横截面半径方向渐变分布使得相邻模群间的有效折射率差相等，所述多个光栅的周期相等。
  4. 如权利要求1-3任一项所述的光纤放大器，其特征在于，所述少模掺杂光纤的纤芯区域掺杂有稀土元素离子。
  5. 如权利要求4所述的光纤放大器，其特征在于，所述纤芯区域掺杂有稀土元素离子包括均匀掺杂和分层掺杂。
  6. 如权利要求5所述的光纤放大器，其特征在于，所述刻写的光栅的强度范围为：[4*10-7，3*10-2]。
  7. 如权利要求6所述的光纤放大器，其特征在于，所述纤芯区域分层掺杂有稀土元素离子，所述刻写的光栅的强度范围为：[1*10-6，3*10-2]。
  8. 如权利要求1-7任一项所述的光纤放大器，其特征在于，所述少模掺杂光纤的纤芯采用掺有二氧化锗的二氧化硅材料，包层采用二氧化硅材料。
  9. 如权利要求1-8任一项所述的光纤放大器，其特征在于，所述少模掺杂光纤的纤芯外围还包括内包层、沟槽层、外包层，所述沟槽层的折射率小于内包层和外包层的折射率。
  10. 如权利要求1-8任一项所述的光纤放大器，其特征在于，所述少模掺杂光纤的纤芯外围多个空气孔。
  11. 如权利要求1-8任一项所述的光纤放大器，其特征在于，所述少模掺杂光纤包括多个纤芯，采用纤芯泵浦方式，泵浦光打入每个纤芯。
  12. 如权利要求1-6任一项所述的光纤放大器，其特征在于，所述少模掺杂光纤包括多个纤芯，采用包层泵浦方式，包层中包括泵浦区域，泵浦光打入所述泵浦区域。
  13. 如权利要求1-12所述的光纤放大器，其特征在于，所述光纤放大器还包括分束器，用于分离所述少模掺杂光纤输出的信号光和泵浦光，并把分离出的泵浦光输入进所述信号泵浦合束器。
  14. 如权利要求1-12所述的光纤放大器，其特征在于，所述光纤放大器还包括第一隔离器和第二隔离器，所述第一隔离器与所述信号泵浦合束器的输入端连接，第二隔离器与所述少模掺杂光纤的输出端连接。
PCT/CN2020/126743 2019-12-28 2020-11-05 光纤放大器 Ceased WO2021129182A1 (zh)

Priority Applications (2)

Application Number Priority Date Filing Date Title
EP20907255.2A EP4054028B1 (en) 2019-12-28 2020-11-05 Fiber amplifier
US17/851,307 US20220329035A1 (en) 2019-12-28 2022-06-28 Fiber amplifier

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
CN201911383861 2019-12-28
CN201911383861.1 2019-12-28
CN202010135229.1A CN111211471B (zh) 2019-12-28 2020-03-02 光纤放大器
CN202010135229.1 2020-03-02

Related Child Applications (1)

Application Number Title Priority Date Filing Date
US17/851,307 Continuation US20220329035A1 (en) 2019-12-28 2022-06-28 Fiber amplifier

Publications (1)

Publication Number Publication Date
WO2021129182A1 true WO2021129182A1 (zh) 2021-07-01

Family

ID=70788636

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/CN2020/126743 Ceased WO2021129182A1 (zh) 2019-12-28 2020-11-05 光纤放大器

Country Status (4)

Country Link
US (1) US20220329035A1 (zh)
EP (1) EP4054028B1 (zh)
CN (1) CN111211471B (zh)
WO (1) WO2021129182A1 (zh)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN113866884A (zh) * 2021-10-12 2021-12-31 桂林电子科技大学 一种基于光子灯笼的涡旋光放大装置
CN114594544A (zh) * 2022-01-25 2022-06-07 华南师范大学 一种分布式共掺微结构光纤
CN116387949A (zh) * 2023-04-18 2023-07-04 北京交通大学 一种含有增益均衡六模掺铒光纤的光纤放大器

Families Citing this family (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111211471B (zh) * 2019-12-28 2021-08-20 华为技术有限公司 光纤放大器
CN111653659B (zh) * 2020-07-16 2021-05-18 湖北大学 一种降低发光二极管发射光发散角的装置及其制作方法
CN114063208A (zh) * 2020-07-30 2022-02-18 华为技术有限公司 一种光纤及光缆
CN115867838B (zh) * 2020-09-09 2025-12-09 恩梯梯株式会社 长周期光纤光栅以及光传输系统
CN112612076B (zh) * 2020-12-23 2022-09-16 武汉邮电科学研究院有限公司 一种少模多芯微结构光纤及少模光纤放大器
CN115524847A (zh) * 2021-06-24 2022-12-27 上海禾赛科技有限公司 激光器性能的表征方法及装置、光源设计方法和系统
CN113866124B (zh) * 2021-09-26 2024-06-21 岭南师范学院 一种spr差分强度调制传感器
CN116009140A (zh) * 2021-10-22 2023-04-25 中国移动通信有限公司研究院 一种少模光纤
CN114124226B (zh) * 2022-01-26 2022-04-15 南京信息工程大学 基于混合泵浦的密集多芯光纤增益均衡放大系统和方法
US20240243539A1 (en) * 2023-01-17 2024-07-18 Ii-Vi Delaware, Inc. Short fiber length multi-core fiber (mcf) erbium-doped fiber amplifiers (edfas)
CN115986537B (zh) * 2023-03-14 2023-08-22 武汉长进光子技术股份有限公司 一种基于高斯掺杂环芯少模光纤的纤芯泵浦光纤放大器
CN116520481A (zh) * 2023-04-25 2023-08-01 鹏城实验室 掺杂光纤、光信号传输系统、模式相关损耗补偿方法
CN116742454A (zh) * 2023-06-20 2023-09-12 暨南大学 一种模式均衡的多模掺铒光纤放大器模块

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102544999A (zh) * 2012-02-01 2012-07-04 中国科学技术大学 基于少模光纤光栅的全光纤轴对称偏振光束激光器及产生方法
US20120219026A1 (en) * 2008-08-21 2012-08-30 Nlight Photonics Corporation Uv-green converting fiber laser using active tapers
CN104185804A (zh) * 2012-01-12 2014-12-03 康宁股份有限公司 用于掺铒放大器的少模光纤以及使用该光纤的放大器
CN105826798A (zh) * 2015-01-26 2016-08-03 恩耐公司 高功率单模纤维源
CN108899751A (zh) * 2018-08-20 2018-11-27 桂林电子科技大学 支持六线偏振模信号光放大的edfa及其模式增益均衡方法
CN109417263A (zh) * 2016-07-04 2019-03-01 株式会社藤仓 光纤以及激光装置
CN111211471A (zh) * 2019-12-28 2020-05-29 华为技术有限公司 光纤放大器

Family Cites Families (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5740292A (en) * 1996-09-12 1998-04-14 Lucent Technologies Inc. Mode coupling optical waveguide grating
JPH10275953A (ja) * 1996-12-31 1998-10-13 Daewoo Telecommun Ltd 光繊維増幅器
US7064889B2 (en) * 2002-05-17 2006-06-20 The Board Of Trustees Of The Leland Stanford Junior University Double-clad fiber lasers and amplifiers having long-period fiber gratings
CN101622560A (zh) * 2007-03-05 2010-01-06 株式会社藤仓 光子带隙光纤
WO2013023193A1 (en) * 2011-08-10 2013-02-14 Ofs Fitel, Llc Few moded fiber device employing mode conversion
US9197030B2 (en) * 2012-07-31 2015-11-24 Corning Incorporated Few mode rare earth doped optical fibers for optical amplifiers, and amplifiers using such fibers
CN103474867A (zh) * 2013-08-13 2013-12-25 江苏天元激光科技有限公司 大模场面积高功率光纤激光装置
JP6452242B2 (ja) * 2015-03-09 2019-01-16 Kddi株式会社 光伝送システム、光増幅器及びその励起光制御方法
CN105022114B (zh) * 2015-07-27 2018-06-26 江苏大学 一种级联型长周期光纤光栅的滤模器
CN105490160B (zh) * 2016-02-06 2019-03-01 上海理工大学 全光式锁定重复频率的激光装置及应用该装置的方法
CN106707408A (zh) * 2017-03-28 2017-05-24 中山大学 一种基于环形结构纤芯的折射率渐变型光纤
CN108181723B (zh) * 2018-01-30 2019-09-03 电子科技大学 一种基于强耦合多芯光纤随机激光的无散斑成像光源
US10277354B1 (en) * 2018-02-15 2019-04-30 Futurewei Technologies, Inc. Efficient mode coupling using ladder fibers
CN109521518B (zh) * 2018-12-14 2020-06-02 南京师范大学 基于直流调制的多通道光纤光栅滤波器及其制作方法

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20120219026A1 (en) * 2008-08-21 2012-08-30 Nlight Photonics Corporation Uv-green converting fiber laser using active tapers
CN104185804A (zh) * 2012-01-12 2014-12-03 康宁股份有限公司 用于掺铒放大器的少模光纤以及使用该光纤的放大器
CN102544999A (zh) * 2012-02-01 2012-07-04 中国科学技术大学 基于少模光纤光栅的全光纤轴对称偏振光束激光器及产生方法
CN105826798A (zh) * 2015-01-26 2016-08-03 恩耐公司 高功率单模纤维源
CN109417263A (zh) * 2016-07-04 2019-03-01 株式会社藤仓 光纤以及激光装置
CN108899751A (zh) * 2018-08-20 2018-11-27 桂林电子科技大学 支持六线偏振模信号光放大的edfa及其模式增益均衡方法
CN111211471A (zh) * 2019-12-28 2020-05-29 华为技术有限公司 光纤放大器

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See also references of EP4054028A4

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN113866884A (zh) * 2021-10-12 2021-12-31 桂林电子科技大学 一种基于光子灯笼的涡旋光放大装置
CN114594544A (zh) * 2022-01-25 2022-06-07 华南师范大学 一种分布式共掺微结构光纤
CN114594544B (zh) * 2022-01-25 2023-01-31 华南师范大学 一种分布式共掺微结构光纤
CN116387949A (zh) * 2023-04-18 2023-07-04 北京交通大学 一种含有增益均衡六模掺铒光纤的光纤放大器

Also Published As

Publication number Publication date
CN111211471B (zh) 2021-08-20
EP4054028B1 (en) 2026-04-22
CN111211471A (zh) 2020-05-29
EP4054028A4 (en) 2023-01-11
US20220329035A1 (en) 2022-10-13
EP4054028A1 (en) 2022-09-07

Similar Documents

Publication Publication Date Title
WO2021129182A1 (zh) 光纤放大器
US10197727B2 (en) Large core holey fibers
JP4511831B2 (ja) 高開口数の光ファイバー、その製造方法並びにその使用法
Kang et al. Design of four-mode erbium doped fiber amplifier with low differential modal gain for modal division multiplexed transmissions
US20150333473A1 (en) High Power Raman-Based Fiber Laser System and Method of Operating the Same
US20230123319A1 (en) Optical amplifying fiber, optical fiber amplifier, and optical communication system
US9448359B2 (en) Single mode propagation in microstructured optical fibers
CN106772778A (zh) 热致超大模场光纤
JP2020161600A (ja) マルチコア光増幅ファイバ、マルチコア光ファイバ増幅器および光通信システム
JP7444677B2 (ja) マルチコア光増幅ファイバ、マルチコア光ファイバ増幅器および光通信システム
JP2018198287A (ja) 増幅用ファイバ
Gaur et al. Dual-core few mode EDFA for amplification of 20 modes
CN114859460B (zh) 一种多芯增益光纤、光纤放大器和光纤振荡器
US9225142B2 (en) Fiber amplifier with multi section core
CN118050846B (zh) 一种具有抑制模式不稳定效果的部分掺杂光纤设计方法
CN115986537B (zh) 一种基于高斯掺杂环芯少模光纤的纤芯泵浦光纤放大器
US20250105578A1 (en) Optical fiber for amplification, optical amplifier, and method for controlling optical amplifier
CN115047561B (zh) 一种铒镱共掺双包层双环形的少模增益光纤
JP2023119383A (ja) 光増幅ファイバ、光ファイバ増幅器および光通信システム
CN113589425A (zh) 一种多芯微结构光纤
Takasaka et al. Absorption ratio dependence of gain characteristics on cladding pumped multicore EDFAs
CN221828346U (zh) 一种五模掺铒光纤放大器
US20260045754A1 (en) Optical fiber for amplification and cladding-pumped optical fiber amplifier
WO2013002787A1 (en) High power raman-based fiber laser system and method of operating the same
DK202631098A1 (en) Multi-core optical amplyfying fiber and optical amplifier

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 20907255

Country of ref document: EP

Kind code of ref document: A1

ENP Entry into the national phase

Ref document number: 2020907255

Country of ref document: EP

Effective date: 20220531

NENP Non-entry into the national phase

Ref country code: DE

WWG Wipo information: grant in national office

Ref document number: 2020907255

Country of ref document: EP