AU2020101195A4

AU2020101195A4 - An ultra-wideband high gain multi-core fiber light source

Info

Publication number: AU2020101195A4
Application number: AU2020101195A
Authority: AU
Inventors: Yu Cheng; Donghui Wang; Libo Yuan
Original assignee: Guilin University of Electronic Technology
Current assignee: Guilin University of Electronic Technology
Priority date: 2020-06-30
Filing date: 2020-06-30
Publication date: 2020-08-06
Anticipated expiration: 2028-06-30

Abstract

The invention discloses an ultra-wideband high gain multi-core optical fiber light source. It uses multiple multi-mode pump lasers to generate a broad-spectrum spontaneous emission spectrum in a variety of doped multi-core double-clad fibers, then couples after combining and tapering, and finally outputs the entire high brightness and ultra-wideband spectrum through a single fiber. The invention realizes integrated parallel amplification through multi-core and multi-doped optical fiber, and finally obtains a low-dispersion optical fiber light source with high brightness, ultra-wideband and flat spectrum line. 2/3 4 7 FIG.3 pm -60 -40 0 20 40 60 -60 -40 -20 0 20 40 60 PM FIG.4

Description

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DESCRIPTION TITLE OF INVENTION

An ultra-wideband high gain multi-core fiber light source

TECHNICAL FIELD

[0001] The present invention relates to the field of broadband communication transmission fiber

amplifiers and broadband light sources for sensing applications.

BACKGROUND ART

[0002] The transmission capacity of optical fiber communication has been continuously

improved. Wavelength division multiplexing is one of the main ways to expand the

communication capacity. But applied to the communication system, due to the spectral

characteristics of erbium-doped fiber, can only amplify the C-band and part of the L-band,

resulting in the current communication system only uses the C-band, L-band and 1310 nm single

wavelength.

[0003] Optical fiber sensing has developed rapidly in recent years, and various sensors from the

grating type to the interferometric type have played a huge role in the field of security and

intelligent systems. However, optical fiber sensing is inseparable from the light source,

especially in the construction and research of new types of sensors, ultra-wideband high- brightness fiber light sources.

[0004] In scientific research, spectroscopy measurement is an important method, for example,

absorption spectrum test is an important research method in physics. An important tool in

spectroscopy measurement is the light source. Existing light sources such as tungsten halogen

lamps, SLED light sources, mercury lamps, high-pressure sodium lamps, supercontinuum light

sources, etc.

[0005] As above light sources have very broad spectral lines, but they are expensive, such as

supercontinuum light sources. Some are cheap, but the output brightness is low, the light

intensity is weak, the light is divergent, and it is difficult to couple into the optical fiber, such as a

high-pressure sodium lamp. Fiber light source has the advantages of good output stability and

moderate price. However, the width and brightness of current light sources are insufficient. As a

new luminescent medium, bismuth-doped fiber has a very broad fluorescence emission

spectrum, covering the range from 1100 nm to 1600 nm. However, bismuth-doped fiber is very

susceptible to concentration quenching, resulting in a serious shortage of output brightness.

[0006] Patent CN101719621A discloses a high-power multi-band multi-core fiber laser, which is

characterized by loading a fiber reflection grating on both ends of a multi-core multi-doped fiber

or coating a high reflectivity film on the end surface of the fiber to form a fiber resonator, which

increases the pumping efficiency of the pump light. The patent does not adjust the flatness of the

output spectrum.

[0007] Patent CN101771233A discloses a high-power multi-band multi-layer rare-earth ion

doped annular core laser, which is characterized in that the optical fiber is composed of multiple

layers of different doped materials and realizes the excitation of different rare-earth ions in

different layers. In addition, the fiber grating or the fiber end reflection film for the pump light is loaded at both ends of the fiber to achieve the function of improving the pump efficiency, but it does not adjust the flatness of the output spectrum, and the structure is only a multilayer multi doped fiber. The problem of beam combining between different doped layers is not considered.

[0008] Patent CN104035166A discloses a high-power laser beam combiner based on multi-core fiber. This patent realizes the beam combining function of multiple laser signals. Compared with this patent, there is no doping of multiple materials. The realized function is to combine multiple beams in a multi-core fiber to obtain a high-power output beam. There is no corresponding adjustment and processing for the output spectrum.

[0009] Patent CN203480085U discloses a fiber laser beam combiner, the laser beam combiner only achieves a multi-fiber taper beam combining, does not involve multi-core active doped fiber and ultra-wideband spectral output.

[0010] Patent CN205122987U discloses a multi-core fiber laser, the laser is composed of NX1 fiber pump coupler, multi-core active fiber, and multi-core fiber grating. From the mechanism point of view, the multi-core active fiber of the device does not involve doping of multiple rare earth ions, and the combination of the pump light is realized by the fiber taper. The output fiber is in the form of multi-core, which does not solve the key problem of multi-core fiber combining.

[0011] Patent CN103682961A discloses an ultra-wideband optical fiber light source system and an optical fiber light source implementation method. This patent combines erbium-doped and bismuth-doped active optical fibers to obtain a broadband ultra-fluorescent light source. Because of its two sets of active fiber pump structures, its device integration is insufficient, and it is not designed for spectral flatness.

[0012] Patent CN200320112294.4 discloses an optical fiber ultra-wideband light source, which

includes an optical fiber, a semiconductor laser, an optical isolator, an erbium-doped fiber, a

reflector, and a wavelength division multiplex coupler. Structurally, the patent uses forward and

backward pumping and mirrors to increase pumping efficiency. The flat range of the output

spectrum is 65nm.

[0013] Patent CN201310560042.6 discloses an ultra-wideband light source based on erbium,

thulium, and neodymium co-doping. The optical fiber in this patent is prepared by co-doping

three rare earth elements, erbium, thulium, and neodymium. The radiated ultra-wideband output

light wave has a wavelength range of 1280 nm-1625 nm. It does not have a corresponding design

for spectral flatness, and only has a single fiber core in structure.

[0014] Patent CN201680037135.2 discloses a technical solution that uses supercontinuum

technology to realize a broadband light source. It mainly uses ultra-short pulse laser to generate

ultra-wideband spectrum through nonlinear effect in high-nonlinear microstructure fiber, and

output the ultra-wideband spectrum through fiber optic components. This patent does not involve

the doping technical solution and the multi-core optical fiber technical solution, because the

generation of the light source depends on the nonlinear effect rather than the electronic

transition.

[0015] Patent CN201280006872.8 discloses a technical solution that uses supercontinuum

supercontinuum in high nonlinear optical fiber, and has a loopback optical path in each high

nonlinear optical fiber, and finally outputs ultra-wideband spectrum through multiple optical

paths. This patent also does not involve doping technology solutions and multi-core fiber

technology solutions.

[0016] Broadband high gain light source is a powerful scientific research tool, widely used in the

field of scientific research and sensing. In summary, supercontinuum, SLED, single-core

broadband light source and multi-core laser light source can be applied, but the cost and

technical methods are different. Using multi-core optical fiber to realize an ultra-wideband high

gain light source in a low-cost manner still has many technical problems that have not been

thoroughly studied.

SUMARRY OF INVENTION

[0017] The object of the present invention is to provide an ultra-wideband high gain multi-core

fiber light source.

[0018] The object of the present invention is achieved as follows:

[0019] The ultra-wideband high gain multi-core fiber light source is composed of multiple pump

laser sources, multi-mode fibers combiner, multi-core double-clad rare-earth-doped fiber, tapered

fiber and output fiber et.al.

[0020] The multi-core double-clad rare-earth-doped fiber used in the invention has multiple

cores, and the co-doped oxide in each core is one or more of

Bi203/ZrO2/SbO2/GeO2/Al203/La23/Er23. Among them, the doping concentration of different

cores varies.

[0021] The traditional erbium-bismuth-ytterbium co-doped silica fiber is laser pumped at 830

nm, and the small signal gain bandwidth is very wide, reaching 490 nm (1100nm-1590nm),but the gain is very small, and the spectral line intensity is only around -60 dB /nm, that equals around 1 nW/nm. The spectral line intensity of the SLED light source can reach about 0.1 mW/nm, and the spectral line intensity of the supercontinuum light source can reach 5 mW/nm. The general test result of the line flatness of the supercontinuum is about 8dB, but the flatness of the SLED's spectrum is generally around 3 dB. In the field of scientific research and sensing, we hope to obtain a gain-flat light source. The gain of the traditional erbium-bismuth-ytterbium co doped fiber is small, the main reason is the quenching phenomenon caused by the concentration distribution of bismuth doping. Moreover, with the increase of bismuth doping concentration, the loss of the fiber itself is also very large, resulting in lower spectral gain and more unevenness spectral lines. Therefore, we design multiple cores respectively doped with one or more of Bi203/ZrO2/SbO2/GeO2/Al203/La2O3/Er2O3 to form integrated parallel amplification in order to increase the gain, the ultimate purpose is to make the output spectrum gain and flatness are better.

[0022] The main reason we use Bi203/ZrO2/SbO2/GeO2/Al203/La2O3/Er2O3 as the doping

element is that it is impossible for a single element emission center to realize a fluorescence spectrum with a width of several hundred nanometers. Here, we use bismuth and erbium as the basis for multiple spontaneous radiation sources in optical fibers. At the same time, the crystal field induces the splitting of the energy level of the erbium element to produce a new small energy level structure. The broad-spectrum emission formed under the combined action of the emission centers of various polymorphisms and materials is the basis of our ultra-wideband light

[0023] In the doped multi-core fiber, the design of the doping element concentration of different cores is different. At the same time, we use multiple pump combination optimization techniques to optimally control the center wavelength of each core output spectrum for red shift or blue shift, in this way, in addition to the greater gain of the multi-core double-clad rare-earth-doped fiber light source, the spectral line is also flatter. In the invention patent, the pump sources we selected include 976nm pump, 830nm pump, and 1480nm pump and so on.

[0024] The number of cores of the multi-core double-clad rare-earth-doped fiber of the present

invention is 7 to 37, and the crosstalk coefficient between the cores is less than -45dB/100m.

[0025] Since the cores in the multi-core optical fiber share a cladding, there must be a crosstalk

phenomenon. For a broad-spectrum light source, in order to prevent crosstalk from other cores

from forming into stimulated radiation in adjacent cores, it is necessary to design the structure of

the optical fiber to reduce crosstalk. To solve this problem, we propose a model based on

supermode theory to analyze the relationship between the transmission constant of the fiber core

and fiber crosstalk.

[0026] 1A; = e(f EpxHjdS)(ff EjXHydS) 1 (f EpxHydS) Re(ff EjxH dS)

[0027] Aj represents the amplitude of thej-th supermode.

[0028] Pcore-n (z) ff dScore-n(j=1 A;Eje-iz+ipi) 2 (2)

[0029] Pcorenrepresents the transmitted optical power of the Nth core, and the area of the Nth

core is Score-n. represents the transfer constant of the j-th supermode, and rpj represents the initial

phase of thej-th supermode.

[0030] XTmax = 10log max(Pn(z)) (3) max(Pmwh))

[003 1] XT is the crosstalk value, which is defined as the ratio of the signal power of the Mth core to the Nth core. There is a specific relationship between the transmission constant of the fiber core and fiber crosstalk. Therefore, we first use the supermode theory to analyze the transmission constant of the optical fiber at the minimum crosstalk, and then design the composition of the fiber structure and rare earth doping so that the crosstalk coefficient between the cores is less than -45 dB/100 m. The condition to be satisfied is that the distance between two adjacent cores must not be less than 30[m.

[0032] In the fabrication of multi-core rare-earth-doped optical fiber, first of all rare-earth-doped optical fiber is prepared by MCVD process, after completion, the preform refractive index and rare earth ion concentration are tested. The outer diameter of the preform is polished to achieve the designed core distance. After stacking multiple rare-earth-doped preforms, they are put into a pure quartz sleeve (refractive index: 1.4571), and the gap is filled with quartz glass filaments. The prepared preform is installed on the drawing tower for drawing, the coating material is changed to a low-refractive index coating (refractive index is less than 1.40), and a double-clad rare-earth-doped multi-core optical fiber is prepared by a photo-curing process.

[0033] In the multi-core double-clad rare-earth-doped fiber mentioned in the present invention, multiple multi-mode pumps are pumped to the multi-core double-clad rare-earth-doped fiber through the common cladding of the fan-in beam combiner to achieve integrated parallel amplification

[0034] The multiple pump sources include a 976nm pump, an 830nm pump, a 1480nm pump, etc. A fan-in beam combiner is used to combine multiple wavelength laser energy into a single fiber. This optical fiber is fused with a double-clad rare-earth-doped multi-core optical fiber. Apply a low-refractive index coating after welding. Because the multimode pump laser is always transmitted in the quartz core and total reflection occurs at the interface of the low refractive index coating, the fiber fused with the double-clad rare-earth-doped multicore fiber can be pure silica (also known as a coreless fiber) without any doping. At this time, energy is reflected multiple times through the cladding interface and absorbed by multiple cores. Because the core spacing of this design is small and dense, the energy coupling cross section of the fiber core and the pump is very large, so the pumping efficiency is greater than the absorption coefficient of the common single-core double-clad fiber. This is beneficial to the broadening of the rare earth emission spectrum. Ordinary double-clad fiber usually needs to process the quartz cladding into an asymmetric shape in order to increase the pump absorption, but the present invention can achieve strong absorption of the pump because of the dense core arrangement design, so there is no need to use mechanical polishing of quartz cladding. The design of the present invention also reduces the difficulty and cost of production.

[0035] In addition, in the present invention, it is mentioned that the fan-in beam combiner fiber may also be a coreless fiber, but the outer layer must be coated with a low refractive index paint.

[0036] The cross-sectional structure and the core distance designed by the present invention need to consider the relationship between the mode field diameter and crosstalk. Because the invention utilizes the concept of integrated parallel amplification. The rare-earth-doped in different cores is different. Crosstalk may cause signals from one core to enter another core to form stimulated radiation, which will cause extremely uneven spectrum.

[0037] A plurality of fiber cores are subjected to mode conversion through a specially designed tapered fiber, and a plurality of individual LPO1 modes are converted into a near-Gaussian beam. The structure of the multi-core optical fiber may be different from the double-clad rare-earth doped multi-core optical fiber. The specific difference is that the mode field diameter at the input end of the tapered fiber is larger than that of the double-clad rare-earth-doped multi-core fiber, and there is crosstalk. At short distances, such as within lcm, the light spot at the input end of the tapered fiber is independent, and the optical energy of each core is Gaussian distributed. The light spot at the output end of the tapered fiber is a large light spot with the highest energy density in the center, and the energy density gradually decreases along the direction of increasing radius and only a small peak of local energy density exists around r=15 mm. This indicates that there is a certain high-order mode at the output end of the tapered fiber, so it is appropriate for us to use a multimode fiber as the coupling output end.

[0038] In the multi-core double-clad rare-earth-doped fiber mentioned in the present invention,

the independent spectra of multiple cores are subjected to mode conversion through a specially

designed tapered fiber to output a near-Gaussian beam.

[0039] According to the simulation result, this near-Gaussian beam forms a stable multi-modal

transmission. When the wavelength range is in the range of 1000 nm to 1650 nm, tapered fibers

can form stable multi-mode transmission. Therefore, the output fiber can be spliced with a

multimode fiber and tapered fiber, and the splice loss is not greater than 0.5 dB.

[0040] The multi-core double-clad rare-earth-doped fiber mentioned in the present invention, the

tapered fiber is fused with the multimode fiber after being cut at a suitable taper surface. This

device has the characteristics of low dispersion loss without dispersion.

[0041] After the appropriate cone cutting of the tapered fiber is fused with a multimode fiber, the

output spectrum will keep the near-Gaussian beam transmitted in the fiber. Most other light

sources are highly divergent light sources, and their brightness (energy density per unit area) is

small. The light source of the present invention is a low divergence light source, and integrated

parallel amplification is achieved through multi-core doped fiber, and finally obtained high

brightness and flat broadband light source.

BRIEF DESCRIPTION OF DRAWINGS

[0042] FIG. 1 is a schematic diagram of a double-clad rare-earth-doped 19-core fiber.

[0043] FIG. 2 is a schematic diagram of a double-clad rare-earth-doped 37-core fiber.

[0044] FIG. 3 is an ultra-wideband multi-core fiber light source solution. 1 is the pump laser. 2 is the pump input port. 3 is a double-clad fan-in beam combiner. 4 is the coreless part of the fan-in beam combiner after fusing tapered, and the outer layer is coated with a low refractive index coating. 5 is the quartz part of the double-clad rare-earth-doped multi-core optical fiber. 6 is a low refractive index coating. 7 is a tapered fiber. 8 is the outputfiber.

[0045] FIG. 4 is the spot diagram of the output end of a double-clad rare-earth-doped multi-core fiber (the core interval is 30 m, the number is 7), the energy of multiple cores is localized in multiple cores and there is no obvious crosstalk.

[0046] FIG. 5 is a spot diagram of the output end of the taperedfiber, the energy of multiple cores is coupled into an approximate Gaussian beam.

[0047] FIG. 6 is an energy distribution diagram of the output end of the tapered optical fiber, indicating that the fiber after tapering outputs a stable energy field approximately Gaussian distribution.

DESCRIPTION OF EMBODIMENTS

[0048] Embodiments described in further detail below:

[0049] A 19-core ultra-wideband high gain fiber light source solution. It contains 19 cores

fabricated by MCVD, each core is doped with one or more of Bi203/Er2O3 and other oxides

ZrO2/SbO2/GeO2/Al203/La2. Three wavelength (976 nm, 830 nm, 1480 nm) multimode pumps

are coupled into a double-clad rare-earth-doped 19-core fiber through a double-clad fan-in beam

combiner. The spectrum of multiple cores is converted by a specially designed tapered fiber, and

finally coupled out through a multi-mode fiber.

[0050] A 37-core ultra-wideband high gain fiber light source solution. It contains 37 cores

ZrO2/SbO2/GeO2/Al203/La2. 7 multi-mode pumps with 3 wavelengths are coupled into a

double-clad rare-earth-doped 37-core fiber through a double-clad fan-in beam combiner, and the

spectrum of multiple cores is subjected to mode conversion through a specially designed tapered

fiber. The output is coupled through a multimode fiber. Among them, the number of Bi-doped

cores is 6 times that of Er-doped cores, mainly because the single gain of bismuth-doped fiber is

small, and the integrated parallel amplification method can obtain a high-gain broadband

emission spectrum.

[0051] Although the design parameters in the above embodiments have been preferred, the above

embodiments also describe the present invention in detail, but those skilled in the art can

understand that: Various changes, modifications, substitutions, and variations can be made to

these embodiments without departing from the principle and spirit of the present invention. The

scope of the present invention is limited by the claims and their equivalents.

Claims

1. An ultra-wideband high gain multi-core fiber light source, characterized in that it consists of multiple pump laser sources, multi-mode fibers combiner, multi-core double-clad rare-earth doped fiber, tapered fiber and output fiber et.al.

2. The multi-core double-clad rare-earth-doped fiber according to claim 1, characterized in that: (1) It has multiple cores, and the co-doped oxide in each core is one or more of Bi203/ZrO2/SbO2/GeO2/Al203/La2O3/Er2O3, and the doping concentrations of different

cores varies. (2) The number of optical fiber cores is 7 to 37, and the crosstalk coefficient between the optical cores is less than -45 dB/100 m.

3. The multi-core double-clad rare-earth-doped fiber according to claim 1, characterized in that: (1) Multiple multimode pumps are pumped to the multi-core double-clad rare-earth doped fiber through the common cladding of the fan-in combiner to achieve integrated parallel amplification. (2) The fiber of fan-in combiner can be a coreless fiber, but the outer layer must be coated with a low refractive index coating.

4. The multi-core double-clad rare-earth-doped fiber as claimed in claim 1, the independent spectra of multiple cores are subjected to mode conversion through a specially designed tapered fiber to output a near-Gaussian beam.

5. The multi-core double-clad rare-earth-doped fiber according to claim 1, the tapered fiber

is fused with the multimode fiber after being cut at a suitable cone surface, and the device has

colorless low coupling loss characteristics.