WO2024169179A1

WO2024169179A1 - Optical fiber, pumping laser emitting device and optical fiber amplifier

Info

Publication number: WO2024169179A1
Application number: PCT/CN2023/120416
Authority: WO
Inventors: 刘沛; 关先朝; 张楷
Original assignee: 华为技术有限公司
Priority date: 2023-02-14
Filing date: 2023-09-21
Publication date: 2024-08-22
Also published as: CN118502015A

Abstract

An optical fiber, a pumping laser emitting device and an optical fiber amplifier, relating to the field of optical devices, and solving the problem of low conversion efficiency of pump light sources. The optical fiber comprises a fiber core, and, sequentially wrapped from inside to outside, a first cladding, a second cladding and a third cladding, the refractive index of the fiber core, the refractive index of the first cladding, the refractive index of the second cladding and the refractive index of the third cladding being sequentially reduced. More energy of a multimode pumping source is coupled into the first cladding having a smaller size, improving the absorption efficiency of the fiber core to the multimode pumping source, thus improving the conversion efficiency of the optical fiber.

Description

Optical fiber, pump laser emitting device and optical fiber amplifier

This application claims the priority of the Chinese patent application filed with the State Intellectual Property Office on February 14, 2023, with application number 202310149817.4 and application name “Optical Fiber, Pump Laser Emitting Device and Fiber Amplifier”, the entire contents of which are incorporated by reference in this application.

Technical Field

The present application relates to the field of optical devices, and in particular to an optical fiber, a pump laser emitting device and an optical fiber amplifier.

Background Art

In optical communication networks, optical signals will attenuate during transmission. In optical transmission lines, optical signal amplifiers are usually required to amplify the power of optical signals. For example, doped optical fiber amplifiers can be used to amplify optical signals. For example, erbium-doped optical fiber amplifiers (EDFA) are used.

There are many factors that affect the amplification efficiency of the erbium-doped fiber amplifier for optical signals, such as the coupling efficiency between the doped fiber and the pump light source, and the conversion efficiency of the doped fiber to the pump light source.

Summary of the invention

The embodiments of the present application provide an optical fiber, a pump laser emitting device and an optical fiber amplifier to improve the conversion efficiency of the pump light source.

In order to achieve the above purpose, this application adopts the following technical solutions:

In the first aspect, an optical fiber is provided, which is used to connect to a multimode pump light source; the optical fiber includes a core, a first cladding, a second cladding and a third cladding. The first cladding is coated outside the core; the second cladding is coated outside the first cladding; the third cladding is coated outside the second cladding; wherein the refractive index of the core, the refractive index of the first cladding, the refractive index of the second cladding and the refractive index of the third cladding decrease in sequence. Thus, part of the energy of the multimode pump light source is coupled into the first cladding, and part of the energy is coupled into the second cladding. Since the second cladding is coated outside the first cladding, the absorption efficiency of the energy entering the first cladding by the core is improved. The outer diameter of the first cladding provided in the embodiment of the present application is smaller than that of the second cladding, and the refractive index of the first cladding is greater than that of the second cladding, which can increase the core efficiency and improve the conversion efficiency of the optical fiber.

In conjunction with the first aspect, in some achievable ways, the numerical aperture of the fiber core is less than or equal to 0.15. Thus, the smaller the numerical aperture of the fiber core, the higher the fiber core efficiency. In addition, the optical fiber provided in the embodiment of the present application is applicable to fiber cores with different numerical apertures.

In conjunction with the first aspect, in some achievable manners, the numerical aperture of the first cladding is greater than or equal to 0.15. Thus, the larger the numerical aperture of the first cladding, the greater the coupling efficiency of the energy of the multimode pump light source coupled to the first cladding. The greater the coupling efficiency, the greater the conversion efficiency of the optical fiber.

In combination with the first aspect, in some achievable manners, the numerical aperture of the second cladding is greater than or equal to 0.15. Thus, the larger the numerical aperture of the second cladding, the greater the coupling efficiency of the energy of the multimode pump light source coupled to the second cladding. The greater the coupling efficiency, the greater the conversion efficiency of the optical fiber.

In conjunction with the first aspect, in some achievable ways, the fiber core is doped with at least one of phosphorus, germanium, neodymium, ytterbium, erbium, bismuth, thulium and holmium. Thus, optical fibers doped with different elements in the fiber core can improve the conversion rate. The doped elements can be selected according to the doping process, etc.

In combination with the first aspect, in some achievable manners, the first cladding is doped with at least one of germanium and fluorine. Thus, the refractive index of the first cladding doped with germanium and fluorine is relatively large, and the energy of the multimode pump light source is coupled to the first cladding and then propagated into the fiber core, and the fiber core efficiency is relatively high.

In combination with the first aspect, in some achievable manners, the second cladding is doped with fluorine. Thus, the refractive index of the second cladding doped with fluorine is relatively small, which is beneficial to improving the coupling efficiency of the second cladding.

In combination with the first aspect, in some achievable manners, the outer diameter of the first cladding is less than or equal to 60 μm. Thus, the refractive index of the first cladding is greater than the refractive index of the second cladding, and the outer diameter of the first cladding is smaller, which can increase the core efficiency and improve the conversion efficiency of the optical fiber.

In combination with the first aspect, in some achievable manners, the outer diameter of the second cladding is less than or equal to 130 μm. Thus, the larger the outer diameter of the second cladding, the higher the coupling efficiency. Since the first cladding can improve the core efficiency, the conversion efficiency of the optical fiber is improved.

In combination with the first aspect, in some achievable methods, a grating is engraved on the fiber core. Thus, there is no need to set the grating on other optical components, which can reduce the cost of raw materials. In addition, the grating is etched on the fiber core, and there is no fusion point between the multimode pump light source and the optical fiber, which will not The conversion efficiency and reliability of the pump laser emitting device are reduced due to fusion loss and fiber mode field jump.

In conjunction with the first aspect, in some achievable manners, the shape of the first cladding is a non-centrosymmetric shape, thereby improving the core efficiency and thus improving the conversion efficiency of the optical fiber.

In combination with the first aspect, in some achievable manners, the shape of the second cladding is a non-centrosymmetric shape, thereby improving the core efficiency and thus improving the conversion efficiency of the optical fiber.

In a second aspect, a pump laser emitting device is provided, which is used to emit a pump beam of a preset wavelength, and the pump laser emitting device comprises: a multimode pump light source, an optical coupling module, and any one of the optical fibers provided in the first aspect, wherein the optical fiber is connected to the multimode pump light source through the optical coupling module; the multimode pump light source is used to emit a multimode pump laser, and the optical coupling module is used to couple part of the energy of the multimode pump laser to the first cladding, and part of the energy to the second cladding; the optical fiber is used to convert the pump source into a pump beam of the preset wavelength. Therefore, due to the high conversion efficiency of any one of the optical fibers provided in the first aspect, the output power of the pump laser emitting device is high.

In conjunction with the second aspect, in some achievable manners, the energy of the multimode pump laser coupled to the first cladding is greater than the energy of the multimode pump laser coupled to the second cladding. As a result, the energy coupled to the first cladding is higher, and the higher energy enters the first cladding and then propagates into the fiber core, further improving the fiber core efficiency. The conversion efficiency of the optical fiber is significantly improved. The output power of the pump laser emitting device is relatively high.

In a third aspect, a fiber amplifier is provided, which includes: a combining component, a gain fiber, and any one of the pump laser emitting devices provided in the second aspect; the pump laser emitting device is used to emit a pump beam; the combining component is used to combine the received signal beam and the pump beam, and couple the combined beam into the gain fiber; the gain fiber is used to amplify the signal beam under the excitation of the pump beam. Therefore, any one of the pump laser emitting devices provided in the second aspect has a higher output power and a higher energy utilization rate. The amplification performance of the fiber amplifier is significantly improved, and at the same time, the cost of the fiber amplifier is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the structure of a fiber amplifier provided in an embodiment of the present application.

FIG. 2 a is a schematic diagram of the structure of a pump laser emitting device provided in an embodiment of the present application.

FIG2b is a schematic diagram of the energy levels of ytterbium ions.

Figure 2c is the distribution diagram of the absorption cross section and radiation cross section of ytterbium ions in various bands.

FIG. 3 a is a schematic structural diagram of a pump laser emitting device in the related art.

FIG. 3 b is a schematic structural diagram of another pump laser emitting device in the related art.

FIG. 3 c is a schematic structural diagram of another pump laser emitting device in the related art.

FIG. 4 is a schematic diagram of the structure of an optical fiber provided in an embodiment of the present application.

FIG5 is a schematic diagram of the internal structure of an optical fiber provided in an embodiment of the present application.

FIG. 6 a is a schematic diagram of a multi-mode pump light source emitting multi-mode pump laser light.

FIG. 6 b is a schematic diagram of the structure of a multi-mode pump laser and an optical fiber.

FIG6c is a graph showing the relationship between cladding size and core efficiency.

FIG. 7 is a schematic diagram of the structure of another optical fiber provided in an embodiment of the present application.

FIG8 a is another schematic diagram of the structure of the pump laser emitting device provided in an embodiment of the present application.

FIG8 b is another schematic diagram of the structure of the pump laser emitting device provided in an embodiment of the present application.

FIG8 c is another schematic diagram of the connection between the second grating and the cladding pump stripper provided in an embodiment of the present application.

In the figure: 10-fiber amplifier; 11-input end; 12-output end; 20-combining component; 30-gain fiber; 100-pump laser emitting device; 110-multimode pump light source; 200-optical fiber; 01-pump laser emitting device; 02-multimode pump light source; 03-ytterbium-doped fiber; 04-first wavelength selector; 05-second wavelength selector; 031-cladding; 032-core; 033-protective tube; 210-first cladding; 220-second cladding; 230-third cladding; 240-core; 102-second grating; 101-first grating; 103-cladding pump stripper; 104-optical diaphragm; 105-mode field adapter; 106-optical coupling module.

DETAILED DESCRIPTION

In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below in conjunction with the accompanying drawings.

In the following, the terms "first", "second", etc. are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first", "second", etc. may explicitly or implicitly include one or more of the feature. In the description of this application, unless otherwise specified, "plurality" means two or more.

In addition, in the present application, directional terms such as "upper" and "lower" are defined relative to the orientation of the components in the drawings. It should be understood that these directional terms are relative concepts. They are used for relative description and clarification, and they can change accordingly according to the changes in the orientation of the components in the drawings.

In the present application, unless otherwise clearly specified and limited, the term "connection" should be understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a direct connection or an indirect connection through an intermediate medium.

FIG. 1 is a schematic diagram of the structure of an optical fiber amplifier 10 provided in an embodiment of the present application. In FIG. 1 , the optical fiber amplifier 10 includes a pump laser emitting device 100 , a combining component 20 , and a gain optical fiber 30 .

The pump laser emitting device 100 is connected to the gain optical fiber 30 via a multiplexing component 20 .

The pump laser emitting device 100 is used to output a pump beam, and the combiner 20 is used to combine the received signal beam and the pump beam, and couple the combined beam into the gain fiber 30. The gain fiber 30 is used to amplify the signal beam under the excitation of the pump beam.

The gain is the power gain of the amplifier, expressed as the common logarithm of the ratio of the output power to the input power, and the unit is decibel (dB). The higher the gain of the optical fiber amplifier 10, the better the performance of the amplifier.

It is understandable that, in some embodiments, the optical fiber amplifier 10 may further include other components, such as an input end (input) 11 and an output end (output) 12 .

The input end 11 is connected to the multiplexing component 20 , and the output end 12 is connected to the gain optical fiber 30 .

The embodiment of the present application does not limit the type of the optical fiber amplifier 10. For example, it can be an erbium-doped optical fiber amplifier (EDFA), an ytterbium-doped fiber amplifier (YDFA), a neodymium-doped fiber amplifier (NDFA), etc.

The embodiment of the present application does not limit the structure of the combining component 20. For example, the combining component 20 can be a device such as an optical coupler, an optical multiplexer or a dichroic mirror, and the combining component 20 can combine two light beams of different wavelengths, namely, a signal light beam and a pump light beam, and couple them into the gain optical fiber 30.

The amplification principle of the gain fiber 30 is: when the rare earth particles in the doped fiber amplifier receive energy at a high energy level, stimulated radiation of light will be generated. If the conditions for continued stimulated radiation are met and it is sensed by the input signal light beam, a stronger signal light beam can be output, thereby achieving an amplification effect.

The embodiment of the present application does not limit the structure of the gain fiber 30, which may be an optical fiber doped with rare earth particles. For example, it may be an erbium-doped optical fiber doped with erbium (Er) ions, an ytterbium-doped optical fiber doped with ytterbium (Yb) ions, a praseodymium-doped optical fiber doped with praseodymium (Pr) ions, a thulium-doped optical fiber doped with thulium (Tm) ions, and the like. The gain fiber 30 may also be an optical fiber co-doped with multiple rare earth particles, such as an erbium-ytterbium co-doped optical fiber.

Different gain fibers 30 amplify different wavelengths. For example, the amplification wavelength range of ytterbium (Yb) ions is 970nm-1200nm. The amplification wavelength range of praseodymium (Pr) ions is near 1300nm, and the amplification wavelength range of neodymium (Nd) ions is near 1300nm. The amplification wavelength range of erbium (Er) ions is 1520nm-1625nm, the amplification wavelength range of bismuth (Bi) ions is 1300nm-1380nm, and the amplification wavelength range of thulium (Tm) ions is 1450nm-1520nm.

The performance of the pump laser emitting device 100 is a key factor in determining the cost and performance of the optical fiber amplifier 10. The higher the pump power of the pump laser emitting device 100, the lower the cost of the optical fiber amplifier 10 and the better the performance.

Among them, the pump laser emitting device 100 has various types, for example, single-mode semiconductor laser (single-mode semi-conductor laser, SMLD) and multi-mode semiconductor laser (multi-mode semi-conductor laser, MMLD).

The maximum output laser power of a single-mode semiconductor laser is limited to about 1 watt (W). As the power increases, the cost of a single-mode semiconductor laser also increases exponentially.

Multimode semiconductor lasers have extremely high reliability and robustness, which can reduce unit costs.

FIG2a is a schematic diagram of a structure of a pump laser emitting device 100 provided in an embodiment of the present application. Referring to FIG2a , the pump laser emitting device 100 includes a multimode pump light source 110, an optical coupling module 106, and an optical fiber 200. The multimode pump light source 110 is connected to the optical fiber 200 via the optical coupling module 106.

The multi-mode pump light source 110 emits a multi-mode pump laser, and the optical coupling module 106 is used to transmit the multi-mode pump light source 110 to The multi-mode pump laser is coupled into the optical fiber, and the optical fiber 200 is used to convert the multi-mode pump laser into a pump beam of a preset wavelength and output a pump beam of a preset wavelength.

The embodiment of the present application does not limit the structure of the optical coupling module 106. Exemplarily, the optical coupling module 106 includes a fast axis collimator, a slow axis collimator and a cylindrical mirror, which are connected in sequence. The optical coupling module 106 can adjust the spot size and divergence angle of the multi-mode pump laser entering the optical fiber 200.

The embodiment of the present application does not limit the connection structure between the multi-mode pump light source 110 and the optical fiber 200. For example, the multi-mode pump light source 110 and the optical fiber 200 are connected via a laser coupling unit.

The embodiment of the present application does not limit the wavelength of the pump beam output by the optical fiber 200. The wavelength of the pump beam is related to the gain medium in the optical fiber 200. Exemplarily, the gain medium can be rare earth particles, such as erbium (Er) ions, ytterbium (Yb) ions, praseodymium (Pr) ions, thulium (Tm) ions, and the like.

FIG2 b is a schematic diagram of the energy levels of ytterbium ions. In conjunction with FIG2 b , the principle of pump source conversion by an optical fiber whose gain medium is ytterbium (hereinafter referred to as ytterbium-doped optical fiber) is introduced.

Most of the ytterbium ions in the ytterbium-doped fiber are in the E1 ground state. When a sufficiently strong pump beam is injected into the ytterbium-doped fiber, the ytterbium ions in the E1 ground state can be pumped to the E3 excited state. The energy level lifetime of the ytterbium ions in the E3 excited state is short, and they will quickly and non-radiatively transfer to the E2 metastable state. The energy level lifetime of the ytterbium ions in the E2 metastable state is long, so a population inversion will be formed between the metastable E2 and E1 ground states. When the ytterbium ions in the metastable state radiate, a large number of photons will be generated.

When the ytterbium ions in the metastable state E2 radiate to the E1 ground state, photons with a wavelength of 980nm are generated. When the ytterbium ions in the metastable state E2 radiate to the E4 ground state, photons with a wavelength of 1μm are generated.

The aforementioned population inversion is also called population inversion. The number of particles at each energy level in a substance in thermal equilibrium follows the Boltzmann distribution, that is, the higher the energy level, the fewer the particles. Under certain conditions, such as external pumping or excitation, particles absorb energy and transition to a higher energy level. When the number of atoms at a high energy level is greater than the number of particles at a low energy level, it is called a population inversion.

The conversion efficiency of the optical fiber 200 is positively correlated with the product of the core efficiency and the pump power. In other words, the greater the product of the core efficiency and the pump power, the higher the conversion efficiency of the optical fiber 200.

The conversion efficiency of the optical fiber 200 is limited by various factors. For example, the conversion efficiency of the optical fiber 200 is related to the pump power and the length of the optical fiber 200 .

Please refer to Figure 2b again. Only when the number of particles in the upper energy level (such as the E2 metastable state) exceeds 50% of the total number of particles can the particle number inversion be achieved to produce lasing. Therefore, the ytterbium ion-doped fiber in the 980nm band requires a very strong pump power to reach the working threshold. Therefore, when the pump power is low, the conversion efficiency of the ytterbium-doped fiber will become very low due to the existence of the working threshold.

Figure 2c is a distribution diagram of the absorption cross section and radiation cross section of ytterbium ions in various bands. In Figure 2c, the horizontal axis is wavelength, in nanometers (nm). The vertical axis is area, in ^10-27 square meters ( ^10-27 ^m2 ). In Figure 2c, ytterbium-doped fiber will produce strong gain in the 980nm band. Ytterbium ions also have a very high absorption cross section in the 980nm band. When the length of the ytterbium-doped fiber exceeds the critical value, the 980nm laser signal will be quickly absorbed by the ytterbium fiber, resulting in low conversion efficiency.

Due to the above two reasons, only ytterbium-doped optical fibers that meet the requirements of short length and high pump power can have higher conversion efficiency. However, ytterbium-doped optical fibers with short length and high pump power will lead to high residual pump power, which limits the output power and conversion efficiency of the ytterbium-doped optical fibers.

Similarly, neodymium-doped optical fibers operating in the 910 nm band and erbium-doped optical fibers operating in the 1.5 μm band have corresponding problems, which will not be elaborated here.

Generally, increasing the core diameter or reducing the cladding diameter can improve the conversion efficiency of the optical fiber. The following describes the relevant technology for improving the conversion efficiency of the optical fiber in conjunction with Figures 3a, 3b and 3c.

FIG3 a is a schematic structural diagram of a pump laser emitting device 01 of the related art. Referring to FIG3 a , the pump laser emitting device 01 includes a multimode pump light source 02 , an ytterbium-doped optical fiber 03 , a first wavelength selector 04 and a second wavelength selector 05 .

Among them, the multi-mode pump light source 02, the first wavelength selector 04, the ytterbium-doped optical fiber 03, and the second wavelength selector 05 are connected in sequence.

The multimode pump light source 02 is used to emit a multimode pump laser. The first wavelength selector 04 is used to filter the spontaneous emission signal generated after the multimode pump laser is absorbed by the ytterbium-doped optical fiber 03, and provide optical positive feedback for the laser signal filtered and selected by the second wavelength selector 05, so as to realize the laser resonance of the preset wavelength pump beam.

The second wavelength selector 05 filters the multi-wavelength optical signal output by the first wavelength selector 04, and outputs the filtered multi-wavelength optical signal to the ytterbium-doped optical fiber 03 for re-amplification.

The ytterbium-doped optical fiber 03 includes a cladding 031 and a core 032 , wherein the cladding 031 is coated on the outside of the core 032 .

Increasing the diameter of the core 032 can increase the overlap factor of the cladding pump and the core signal, but increasing the diameter of the core 032 will reduce the single-mode characteristics of the core 032 signal.

In the example of FIG. 3 a , the size of the fiber core 032 is 80 μm, and the fiber core 032 is a photonic crystal structure. The photonic crystal can increase the transmission loss of the high-order mode and realize the single-mode characteristic of the fiber core 032 .

The size of the fiber core 032 in Figure 3a is relatively large, and it is difficult to achieve a small bending radius, so it does not have volume advantages and robustness. The pump laser emitting device 01 is only suitable for laboratories, which limits the scope of use of the pump laser emitting device 01.

FIG. 3 b is a schematic structural diagram of another pump laser emitting device 01 in the related art. The difference between FIG. 3 b and FIG. 3 a lies in the different structure of the ytterbium-doped optical fiber 03 .

In Fig. 3b, the core 032 of the ytterbium-doped optical fiber 03 is a tapered core. Exemplarily, the core 032 is adiabatically tapered so that the diameter of the core 032 at the inlet end is 100 μm and the diameter at the outlet end is 10 μm.

In FIG. 3 b , section B is the end face of the core 032 perpendicular to the axial direction of the core 032 , the solid line c1 in section B is the outer contour of the core 032 , and the dotted line c2 is the mode length of the light in section B.

Similarly, the ytterbium-doped optical fiber 03 in FIG. 3 b also has difficulty in achieving a smaller bending radius and does not have volume advantages and robustness.

FIG3 c is a schematic structural diagram of another pump laser emitting device 01 in the related art. The difference between the example in FIG3 c and the example in FIG3 a lies in the different structure of the ytterbium-doped optical fiber 03 .

In FIG3c , the material of the cladding 031 is air, which is beneficial to increase the numerical aperture of the cladding 031 , and the numerical aperture can reach 1, thereby achieving efficient pump coupling of the ytterbium-doped optical fiber 03 and improving the efficiency of the light source.

The Ytterbium-doped optical fiber 03 is provided with a protective tube 033, which is sleeved outside the cladding 031 and filled with argon gas. The argon gas is used to protect the cladding 031 in a dust-free environment, and the argon gas can prevent the cladding 031 from being damaged by dust and the like.

Accordingly, the protection tube 033 needs to be equipped with an argon gas supply system.

In the example of Fig. 3c, the provision of the protection tube 033 makes the volume of the pump laser emitting device 01 larger. When the outer diameter of the cladding 031 is less than 80 μm, the rigidity of the ytterbium-doped optical fiber 03 is poor and it is difficult to meet the engineering requirements.

It can be seen from the examples of FIG. 3a , FIG. 3b and FIG. 3c that the related technology improves the conversion efficiency of the optical fiber, but imposes many restrictions on the use scenarios of the optical fiber.

Therefore, the embodiment of the present application provides a pump laser emitting device 100, as shown in FIG. 2a, in which the cladding size of the optical fiber 200 is relatively small, and the optical fiber efficiency can be improved on the basis of a relatively high coupling efficiency.

FIG4 is a schematic diagram of the structure of an optical fiber 200 provided in an embodiment of the present application, and the paper surface of FIG4 is perpendicular to the axis of the optical fiber 200 .

4 , the optical fiber 200 includes a core 240 , a first cladding 210 , a second cladding 220 , and a third cladding 230 . The first cladding 210 is coated on the outside of the core 240 , the second cladding 220 is coated on the outside of the first cladding 210 , and the third cladding 230 is coated on the outside of the second cladding 220 .

FIG5 is a schematic diagram of the internal structure of an optical fiber 200 provided in an embodiment of the present application, and the paper surface of FIG5 is parallel to the axis of the optical fiber 200 .

The refractive index of the core 240, the refractive index of the first cladding 210, the refractive index of the second cladding 220 and the refractive index of the third cladding 230 decrease in sequence.

6a is a schematic diagram of a multi-mode pump light source 110 emitting a multi-mode pump laser E. The embodiment of the present application does not limit the structure of the multi-mode pump light source 110. For example, the multi-mode pump light source 110 is a multi-mode semiconductor laser (multi-mode laser diode, MMLD).

The embodiments of the present application do not limit the parameters of the multi-mode semiconductor laser. By way of example, the light emitting window width of the multi-mode semiconductor laser is greater than or equal to 30 μm, and the maximum light output power is greater than or equal to 1 W.

FIG6 b is a schematic diagram of the structure of the multi-mode pump laser E and the optical fiber 200 . Please refer to FIG6 b . The multi-mode pump laser E is partially coupled into the first cladding 210 , and partially coupled into the second cladding 220 .

Under the action of the first cladding 210, the second cladding 220 and the third cladding 230 with successively decreasing refractive indexes, part of the energy of the multimode pump laser emitted by the multimode pump light source 110 is coupled to the first cladding 210, part of the energy is coupled to the second cladding 220, and then propagates into the fiber core 240. The fiber core 240 absorbs the pump source and then converts it into a pump beam of a preset wavelength.

The conversion efficiency F of the optical fiber 200 can be expressed by the following formula:
F＝E(D1)×B(D1,NA)+E(D2)×B(D2,NA)

Wherein, E(D1) is the core efficiency under the size of the first cladding 210.

E(D2) is the core efficiency under the size of the second cladding 220.

B(D1, NA) is the coupling efficiency of the first cladding 210 .

B(D2, NA) is the coupling efficiency of the second cladding 220 .

NA (numerical aperture) is the numerical aperture. The greater the difference in refractive index between the cladding and the core, the larger the numerical aperture.

Figure 6c is a graph showing the relationship between cladding size and core efficiency. In Figure 6c, the horizontal axis is the power of the laser entering the cladding, in watts (W), and the vertical axis is the power of the pump light after conversion into the core, in watts (W).

In Fig. 6c, the optical fiber of line N1, the optical fiber of line N2 and the optical fiber of line N3 are identical in structure except for the cladding size. The optical fiber of line N1, the optical fiber of line N2 and the optical fiber of line N3 are all double-clad optical fibers.

In Figure 6c, the fiber data of line N1 are as follows: the core size is 11 μm, the cladding size is 130 μm, and the core efficiency of the fiber of line N1 is 9%.

The fiber data of line N2 is as follows: the core size is 11 μm, the cladding size is 80 μm, and the core efficiency of line N2 fiber is 23%.

The fiber data of line N3 are as follows: the core size is 11 μm, the cladding size is 40 μm, and the core efficiency of line N3 fiber is 66%.

It can be seen from Figure 6c that the larger the outer diameter of the cladding, the smaller the core efficiency.

In the embodiment of the present application, the larger the outer diameter of the first cladding 210 is, the smaller E(D1) is. The same is true for the second cladding 220.

The coupling efficiency is positively correlated with the product of the outer diameter of the cladding and the numerical aperture. The larger the NA, the greater the coupling efficiency. The larger the outer diameter of the cladding, the greater the coupling efficiency.

For example, the larger the outer diameter of the first cladding 210 is, the larger B(D1, NA) is. The same is true for the second cladding 220.

Since the refractive index of the core 240, the refractive index of the first cladding 210, the refractive index of the second cladding 220 and the refractive index of the third cladding 230 decrease in sequence, the coupling efficiency of the first cladding 210 and the second cladding 220 is relatively high. The outer diameter of the first cladding 210 is smaller than the outer diameter of the second cladding 220, and E(D1) is greater than E(D2). The conversion efficiency F of the optical fiber 200 is improved.

The embodiment of the present application does not limit the energy of the multi-mode pump laser coupled into the first cladding 210 and the second cladding 220 .

Exemplarily, part of the energy of the multi-mode pump laser is coupled to the first cladding 210 , and part of the energy of the multi-mode pump laser is coupled to the second cladding 220 .

The energy of the multimode pump laser coupled to the first cladding 210 is greater than the energy of the multimode pump laser coupled to the second cladding 220. A pump source with a larger energy coupled into the first cladding 210 can improve the core efficiency, thereby achieving a higher conversion efficiency.

The amount of energy coupled to the first cladding 210 can be set by adjusting the parameters of the multi-mode pump light source 110 .

The following description will be made by comparing the theoretical limit of amplification efficiency of the optical fiber 200 provided in the embodiment of the present application and an optical fiber with two layers of cladding outside the fiber core (hereinafter referred to as a two-clad optical fiber).

The double-clad optical fiber has an inner cladding and an outer cladding, and the outer cladding is coated outside the inner cladding.

The coupling efficiency of the multimode pump light source to the optical fiber is closely related to the waveguide brightness of the optical fiber, which can be expressed by the product of the core diameter and the NA.

Assume that the outer diameter of the inner cladding of the double-clad optical fiber is 105μm, the NA is 0.22, and the waveguide brightness of the optical fiber is about 23.1. Thus, when the waveguide brightness is greater than 23.1, the energy of the multimode pump light source can be fully coupled into the waveguide, and when the waveguide brightness is less than 23.1, the ratio of the waveguide brightness to 23.1 can be approximated as the energy ratio of the multimode pump light source that can be coupled into the waveguide.

Therefore, the theoretical limit of amplification efficiency of a double-clad ytterbium-doped optical fiber with an outer diameter of the inner cladding of 80 μm, a core diameter of 10 μm, and a numerical aperture NA of the inner cladding of 0.46 when generating a 980 nm band is about 23%.

Taking the optical fiber 200 provided in the embodiment of the present application as an example, it is assumed that: the gain medium in the core 240 is ytterbium, the diameter of the core 240 is 10 μm, the outer diameter of the first cladding 210 is 40 μm, the outer diameter of the second cladding 220 is 80 μm, the numerical aperture NA of the first cladding 210 is 0.22, and the numerical aperture NA of the second cladding 220 is 0.46.

Assuming that the energy coupled into the first cladding 210 is about 38%, 38% of the laser energy is in the first cladding with an outer diameter of 40μm, and its core efficiency is about 66%. The remaining 62% of the laser energy is in the second cladding with an outer diameter of 80μm, and its core efficiency is 23%. According to the aforementioned calculation formula of conversion efficiency F, the total efficiency is 38%*66%+62%*23%=39%, so the theoretical limit of the overall amplification efficiency is about 39%.

Obviously, the optical fiber 200 provided in the embodiment of the present application has a 50% improvement in the theoretical limit of the amplification efficiency of the double-clad optical fiber. In addition, the outer diameter of the second cladding 220 of the optical fiber 200 provided in the embodiment of the present application is smaller than the outer diameter of the inner cladding of the double-clad optical fiber.

The embodiment of the present application does not limit the numerical aperture of the fiber core 240. Exemplarily, the numerical aperture of the fiber core 240 is less than or equal to 0.15. For example, the numerical aperture of the fiber core 240 can be 0.15, 01.4, 0.13, 0.12, 0.11, 0.10, 0.09, etc.

The embodiment of the present application does not limit the diameter of the fiber core 240. For example, the diameter of the fiber core 240 may be less than or equal to 20 μm. For example, the diameter of the fiber core 240 is 10 μm, 12 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, etc. The smaller the diameter of 240, the better the single-mode characteristics of the core signal.

It is understandable that, in some embodiments, the diameter of the fiber core 240 may not be within the above range. For example, in an embodiment where the fiber core 240 adopts a photonic crystal structure, the diameter of the fiber core 240 may be 40 μm, 50 μm, etc.

The embodiment of the present application does not limit the material of the fiber core 240. Exemplarily, the material of the fiber core 240 is silicon dioxide.

The embodiment of the present application does not limit the doping elements in the fiber core 240. It can be set according to the refractive index of the fiber core 240. For example, it can be at least one of phosphorus, germanium, neodymium, ytterbium, erbium, bismuth, thulium and holmium. In this way, the optical fiber 200 doped with different elements can improve the conversion efficiency.

The embodiment of the present application does not limit the outer diameter of the first cladding 210, and the outer diameter of the first cladding 210 is greater than the diameter of the core 240. Exemplarily, the outer diameter of the first cladding 210 is less than or equal to 60 μm. For example, the outer diameter of the first cladding 210 is 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 48 μm, 50 μm, 52 μm, 55 μm, 58 μm, 60 μm, etc. The first cladding 210 has a smaller outer diameter, which can increase the core efficiency and improve the conversion efficiency of the optical fiber 200.

Further, the energy of the multimode pump laser coupled to the first cladding 210 is greater than the energy of the multimode pump laser coupled to the second cladding 220. The smaller the outer diameter of the first cladding 210, the higher the core efficiency.

In some other embodiments, the outer diameter of the first cladding 210 may not be within the above range. For example, the outer diameter of the first cladding 210 may be 65 μm, 70 μm, etc.

The outer diameter of the first cladding 210 refers to the maximum dimension of the first cladding 210 along the radial direction of the core 240 .

The embodiment of the present application does not limit the shape of the first cladding 210. Exemplarily, the shape of the first cladding 210 is a non-centrosymmetric shape. In other words, the first cladding 210 is not symmetric about the center of the core 240. For example, the cross section of the first cladding 210 is a polygon such as a hexagon, a pentagon, or an octagon. The aforementioned polygon is a non-regular polygon. The aforementioned cross section is perpendicular to the axis of the core 240, or the shape of the first cladding 210 is an irregular shape. The first cladding 210 is a non-centrosymmetric structure. Increasing the energy coupled into the first cladding 210 can increase the core efficiency.

The embodiment of the present application does not limit the numerical aperture of the first cladding 210. For example, the numerical aperture of the first cladding 210 is greater than or equal to 0.15. For example, the numerical aperture of the first cladding 210 is 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.25, etc. The larger the numerical aperture of the first cladding 210, the greater the coupling efficiency. The greater the coupling efficiency, the greater the conversion efficiency of the optical fiber 200.

The embodiment of the present application does not limit the material of the first cladding layer 210. Exemplarily, the material of the first cladding layer 210 is silicon dioxide.

The embodiment of the present application does not limit the doping element of the first cladding 210. For example, the doping element of the first cladding 210 is at least one of germanium and fluorine. The doping amount can be designed according to the numerical aperture. By designing the type of doping element and the doping amount, the refractive index of the first cladding 210 can be changed, thereby changing the numerical aperture of the first cladding 210, and further improving the coupling efficiency of the first cladding 210.

The embodiment of the present application does not limit the outer diameter of the second cladding 220, and the outer diameter of the second cladding 220 is greater than the outer diameter of the first cladding 210. Exemplarily, the outer diameter of the second cladding 220 is less than or equal to 130 μm. For example, the outer diameter of the second cladding 220 is 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, 100 μm, 105 μm, 110 μm, 115 μm, 120 μm, 125 μm, 130 μm, etc. The larger the outer diameter of the second cladding 220, the higher the coupling efficiency. Since the first cladding 210 can improve the core efficiency, the conversion efficiency of the optical fiber 200 is improved.

Similarly, the outer diameter of the second cladding 220 refers to the maximum dimension of the second cladding 220 along the radial direction of the core 240 .

The embodiment of the present application does not limit the shape of the second cladding 220. For example, the shape of the second cladding 220 is a non-centrosymmetric shape. For example, the cross section of the second cladding 220 is a non-regular polygon. The aforementioned cross section is perpendicular to the axis of the core 240, or the second cladding 220 is an irregular shape. The second cladding 220 is a non-centrosymmetric structure, and increasing the energy coupled into the second cladding 220 can increase the efficiency of the core 240.

The embodiment of the present application does not limit the numerical aperture of the second cladding 220. For example, the numerical aperture of the second cladding 220 is greater than or equal to 0.15. For example, the numerical aperture of the second cladding 220 is 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.25, etc. The larger the numerical aperture of the second cladding 220, the greater the coupling efficiency. The greater the coupling efficiency, the greater the conversion efficiency of the optical fiber 200.

As mentioned above, the numerical aperture of the second cladding 220 is related to the refractive index of the second cladding 220 , and the refractive index of the second cladding 220 is related to the type and amount of doping elements in the second cladding 220 .

The embodiment of the present application does not limit the material of the second cladding layer 220. Exemplarily, the material of the second cladding layer 220 is silicon dioxide.

For example, the element doped in the second cladding layer 220 is fluorine. Thus, the refractive index of the second cladding layer 220 is relatively small, and the doping process of doping with fluorine element is relatively simple, and the preparation cost is low.

The embodiment of the present application does not limit the outer diameter of the third cladding 230. Exemplarily, the outer diameter of the third cladding 230 is greater than the outer diameter of the second cladding 220. For example, the outer diameter of the third cladding 230 is 100 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, etc.

The embodiment of the present application does not limit the material of the third cladding layer 230 . For example, a resin polymer is coated on the surface of the second cladding layer 220 to form the third cladding layer 230 .

The embodiment of the present application does not limit the type of the resin polymer of the third cladding layer 230 , for example, it can be polyacrylate, polyimide resin, etc.

The embodiment of the present application does not limit the refractive index of the third cladding layer 230. For example, the refractive index of the third cladding layer 230 is less than or equal to 1.4. For example, the refractive index of the third cladding layer 230 is 1.4, 1.3, 1.2, 1.1, etc. This is beneficial to improve the NA value of the second cladding layer 220.

FIG7 is a schematic diagram of the structure of another optical fiber 200 provided in an embodiment of the present application. Please refer to FIG7 . In the example shown in FIG7 , the optical fiber 200 further includes a first grating 101 and a second grating 102 . The first grating 101 and the second grating 102 are both etched on the fiber core 240 .

The first grating 101 and the second grating 102 are respectively located at opposite ends of the fiber core 240. In this way, there is no need to set the first grating 101 and the second grating 102 on other optical elements, which can reduce the cost of the pump laser emitting device 100. In addition, there is no fusion point between the multi-mode pump light source 110 and the optical fiber 200, and the conversion efficiency and reliability of the pump laser emitting device 100 will not decrease due to fusion loss and fiber mode field jump.

Exemplarily, the first grating 101 is located at an end of the fiber core 240 close to the multi-mode pump light source 110 . The second grating 102 is located at an end of the fiber core 240 far from the multi-mode pump light source 110 .

The first grating 101 is used to reflect the spontaneous radiation and stimulated radiation generated by the fiber core 240 after being excited by the multimode pump light source 110, provide the required optical positive feedback for the selected laser wavelength, and prevent the spontaneous radiation and stimulated radiation laser from affecting the multimode pump light source 110.

The second grating 102 is used to select a specific laser wavelength emitted by the multi-mode pump light source 110 and provide the required optical positive feedback for the selected laser wavelength.

The embodiment of the present application does not limit the structure of the first grating 101. Exemplarily, the first grating 101 is a Bragg grating. For example, the first grating 101 can be a high reflective fiber Bragg grating (HR-FBG).

The embodiment of the present application does not limit the structure of the second grating 102. Exemplarily, the second grating 102 is a Bragg grating. For example, the second grating 102 can be a low reflective fiber Bragg grating (LR-FBG).

It is understood that in some embodiments of the present application, the first grating 101 may not be disposed on the optical fiber 200. For example, the first grating 101 is disposed on a passive fiber core, or the first grating 101 is disposed on a multimode pump light source 110, and so on.

Accordingly, in some embodiments of the present application, the second grating 102 may not be disposed on the optical fiber 200. For example, the second grating 102 is disposed on a passive fiber core and so on.

In other words, neither the first grating 101 nor the second grating 102 may be disposed on the optical fiber 200 , or one of the first grating 101 and the second grating 102 may be selectively disposed on the optical fiber 200 .

Please refer to FIG. 2 a again. In FIG. 2 a , the first grating 101 and the second grating 102 are both disposed on the core 240 of the optical fiber 200 .

In FIG. 2a , the pump laser emitting device 100 may further include a cladding pump stripper (CPS) 103 . The cladding pump stripper 103 is connected to an end of the optical fiber 200 away from the multimode pump light source 110 .

The cladding pump stripper 103 is used to strip residual pump sources, for example, to strip pump wavelengths other than the preset wavelengths.

In the embodiment of the present application, the cladding pump stripper 103 and the optical fiber 200 may be directly connected or indirectly connected.

In FIG2a , the pump laser emitting device 100 can also include a mode field adapter (MFA) 105, wherein the mode field adapter 105 is connected to the cladding pump stripper 103. The mode field adapter 105 is used to reduce the loss during the fusion splicing of optical fibers with different mode field diameters and numerical apertures, so that the fundamental mode signal can obtain the maximum transmittance at the melting point.

FIG8a is another schematic diagram of the structure of the pump laser emitting device 100 provided in an embodiment of the present application. Please refer to FIG8a. The first grating 101 and the second grating 102 are not provided in the optical fiber 200 in FIG8a. Please refer to FIG2a for the remaining structures.

In FIG8a, the first grating 101 is connected to an end of the optical fiber 200 close to the multimode pump light source 110. Cladding pump stripper 103 The second grating 102 is connected to the optical fiber 200 .

Exemplarily, the first grating 101 can be arranged on a passive triple-clad optical fiber. The second grating 102 can be arranged on a passive triple-clad optical fiber. The aforementioned passive triple-clad optical fiber is an optical fiber with three claddings arranged sequentially on the core, and there is no doping element in the core. The process of etching the first grating 101 or the second grating 102 on the passive triple-clad optical fiber is simpler, reducing the process cost of the first grating 101 and the second grating 102.

Exemplarily, the aforementioned passive triple-clad optical fiber is different from the optical fiber 200 in that no element is doped in the fiber core.

The embodiment of the present application does not limit the positional relationship between the first grating 101 and the optical coupling module 106. In some embodiments, the first grating 101 is located between the optical coupling module 106 and the multi-mode pump light source 110. In some embodiments, the optical coupling module 106 is located between the first grating 101 and the multi-mode pump light source 110.

FIG8 b is another structural schematic diagram of the pump laser emitting device 100 provided in an embodiment of the present application. In FIG8 b , the pump laser emitting device 100 is not provided with a first grating, and an optical film 104 is used to replace the effect of the first grating 101 shown in FIG8 a .

In FIG. 8 b , the optical film 104 is located between the optical fiber 200 and the multi-mode pump light source 110 .

The optical film 104 has no requirements on the photosensitivity of the optical fiber 200, which can reduce the requirements on the photosensitivity of the optical fiber 200 and reduce the difficulty of designing and manufacturing the optical fiber 200. In addition, the optical film 104 can filter out the reverse amplified spontaneous emission (ASE) of the optical fiber 200, reducing the impact of the reverse amplified spontaneous emission on the reliability of the multi-mode pump light source 110. In addition, the optical film 104 can further lock the wavelength of the multi-mode pump light source 110, thereby increasing the stability of the light source emitted by the pump laser emitting device 100.

In the embodiment where the second grating 102 is not disposed on the optical fiber 200, the positional relationship between the second grating 102 and the cladding pump stripper 103 is not limited. For example, the second grating 102 is located between the optical fiber 200 and the cladding pump stripper 103. Alternatively, the cladding pump stripper 103 is located between the optical fiber 200 and the second grating 102.

In FIG. 8 a and FIG. 8 b , the second grating 102 is located between the optical fiber 200 and the cladding pump stripper 103 .

FIG8c is another connection diagram of the second grating 102 and the cladding pump stripper 103 provided in an embodiment of the present application. In FIG8c, the cladding pump stripper 103 is located between the optical fiber 200 and the second grating 102. In this way, the positional relationship between the cladding pump stripper 103 and the second grating 102 can be set according to requirements.

Please refer to FIG. 2 a , FIG. 8 a , FIG. 8 b and FIG. 8 c again, wherein the pump laser emitting device 100 includes a multi-mode pump light source 110 .

In other embodiments of the present application, the pump laser emitting device 100 may include two, three or more multi-mode pump light sources 110 . The multi-mode pump lasers of the multiple multi-mode pump light sources 110 are all coupled into the optical fiber 200 .

The first cladding 210 of the optical fiber 200 provided in the embodiment of the present application is relatively small in size, and part of the energy is coupled into the first cladding 210, which improves the absorption efficiency of the core 240 for the multi-mode pump laser, thereby improving the conversion efficiency of the optical fiber 200. The working threshold of the pump laser emitting device 100 is lowered, and the output power and signal-to-noise ratio of the pump laser emitting device 100 are improved.

The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

An optical fiber, characterized in that the optical fiber is used to connect to a multimode pump light source; the optical fiber comprises:

Fiber core;

A first cladding, wherein the first cladding is coated outside the fiber core;

a second cladding layer, wherein the second cladding layer is coated outside the first cladding layer;

a third cladding layer, the third cladding layer covering the second cladding layer;

The refractive index of the core, the refractive index of the first cladding, the refractive index of the second cladding and the refractive index of the third cladding decrease in sequence.
The optical fiber according to claim 1, characterized in that the numerical aperture of the core is less than or equal to 0.15.
The optical fiber according to claim 1 or 2, characterized in that the numerical aperture of the first cladding is greater than or equal to 0.15.
The optical fiber according to any one of claims 1 to 3 is characterized in that the value of the aperture of the second cladding number is greater than or equal to 0.15.
The optical fiber according to any one of claims 1 to 4, characterized in that the core is doped with at least one of phosphorus, germanium, neodymium, ytterbium, erbium, bismuth, thulium and holmium.
The optical fiber according to any one of claims 1 to 5, characterized in that the first cladding is doped with at least one of germanium and fluorine.
The optical fiber according to any one of claims 1 to 6, characterized in that the second cladding is doped with fluorine.
The optical fiber according to any one of claims 1 to 7, characterized in that the outer diameter of the first cladding is less than or equal to 60 μm.
The optical fiber according to any one of claims 1 to 8, characterized in that the outer diameter of the second cladding is less than or equal to 130 μm.
The optical fiber according to any one of claims 1 to 9, characterized in that a grating is engraved on the core.
The optical fiber according to any one of claims 1 to 10, characterized in that the shape of the first cladding is non-centrosymmetric.
The optical fiber according to any one of claims 1 to 11, characterized in that the shape of the second cladding is non-centrosymmetric.
A pump laser emitting device, characterized in that the pump laser emitting device is used to emit a pump light beam of a preset wavelength, the pump laser emitting device comprises: a multimode pump light source, an optical coupling module and the optical fiber according to any one of claims 1 to 12, the optical fiber and the multimode pump light source are connected through the optical coupling module;

The multimode pump light source is used to emit a multimode pump laser, and the optical coupling module is used to couple part of the energy of the multimode pump laser to the first cladding, and part of the energy to the second cladding;

The optical fiber is used to convert the multi-mode pump laser into a pump light beam of the preset wavelength.
The pump laser emitting device according to claim 13, characterized in that the energy of the multi-mode pump laser coupled to the first cladding is greater than the energy of the multi-mode pump laser coupled to the second cladding.
An optical fiber amplifier, characterized in that the optical fiber amplifier comprises: a wave combining component, a gain optical fiber and the pump laser emitting device according to claim 13 or 14; the pump laser emitting device is connected to the wave combining component, and the gain optical fiber is connected to the wave combining component; the pump laser emitting device is used to emit a pump light beam;

The combining component is used to combine the received signal light beam and the pump light beam, and couple the combined light beam into the gain optical fiber;

The gain optical fiber is used to amplify the signal light beam under the excitation of the pump light beam.