Optical fiber amplifier

Abstract

The invention discloses an optical fiber amplifier, and relates to the technical field of optical fiber lasers, the optical fiber amplifier comprises a semiconductor laser, a pumping coupler and a plurality of active optical fibers, the input end of the pumping coupler is connected with the semiconductor laser, and the active optical fibers are sequentially connected to form a transmission path. The transmission path comprises a first end and a second end, the first end is connected with a tail fiber at the output end of the pumping coupler, and the numerical apertures of fiber cores of the multiple active optical fibers in the transmission path are sequentially increased in the direction from the first end to the second end; according to the technical scheme, the mode number in the fiber core can be effectively controlled, the laser beam quality and the mode instability threshold value are effectively improved, high-power and high-beam-quality laser output is achieved, pump light is more evenly distributed in the whole amplifier active optical fiber, the temperature of the active optical fiber is effectively reduced, and the service life of the amplifier is prolonged. The risk of burning out the welding point is reduced, and the long-time working stability of the laser is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber lasers, and in particular to an optical fiber amplifier. Background Art

[0002] With the rapid development of high-power fiber materials and high-brightness semiconductor lasers, the output power of fiber lasers has been greatly improved. Traditional high-power fiber lasers mostly use laser synthesis technology, based on fiber power combiners, to synthesize multiple low-power lasers into one beam. However, this type of laser has a complex structure, large size, and poor beam quality. Fiber amplifiers based on master oscillator power amplifier technology (MOPA) have high technical content, good beam quality, and are easy to integrate. With the further increase in power, the shortcomings of traditional MOPA structure amplifiers are gradually exposed. 1. The mode instability threshold is low. When the output power exceeds this threshold, it will cause abnormal power increase or even a sharp drop in power. Ultimately, the output power of high-quality laser beams remains at the kilowatt level; second, based on the quasi-static mode instability phenomenon, as the power increases, the number of modes increases, and the proportion of high-order modes increases, resulting in a larger beam quality and a worse light spot; third, the nonlinearity is strong, and a strong Raman laser component appears, resulting in a decrease in the quantum efficiency of some lasers, causing the mode instability threshold to decrease; fourth, the temperature of the active fiber fusion point of the laser is high and it is easy to burn out; fifth, the temperature of the active fiber is high, and the long-term stability of the laser is poor. After working for a long time, the laser power drops. This scheme proposes a fiber amplifier to solve the above-mentioned problem of poor stability and easy burning of traditional amplifiers. Summary of the invention

[0003] The main purpose of the present invention is to provide an optical fiber amplifier, aiming to solve the above-mentioned problem of poor stability and easy burnout of traditional amplifiers.

[0004] To achieve the above object, the optical fiber amplifier proposed in the present invention includes a semiconductor laser, and the optical fiber amplifier also includes:

[0005] a pump coupler, an input end of which is connected to the semiconductor laser; and

[0006] A plurality of active optical fibers, the plurality of active optical fibers are connected in sequence to form a transmission path, the transmission path comprising a first end and a second end, the first end being connected to an output end pigtail of the pump coupler;

[0007] Among them, the core numerical apertures of the plurality of active optical fibers in the transmission path increase sequentially from the first end to the second end.

[0008] In one embodiment, the core numerical aperture values ​​of the plurality of active optical fibers are in a range of 0.03 to 0.07, and the core numerical aperture value increment between two adjacent active optical fibers from the first end to the second end is in a range of 0.001 to 0.02.

[0009] In one embodiment, the absorption coefficients of the plurality of active optical fibers in the transmission path to the pump light increase sequentially from the first end to the second end.

[0010] In one embodiment, the absorption coefficients of the plurality of active optical fibers to the pump light are all between 0.2 dB / m and 0.07 dB / m, and the absorption coefficient increment of the pump light between two adjacent active optical fibers from the first end to the second end is between 0.01 dB / m and 0.1 dB / m.

[0011] In one embodiment, the core diameters of the plurality of active optical fibers are the same; and,

[0012] The cladding diameters of the plurality of active optical fibers are all the same; and / or,

[0013] The plurality of active optical fibers are arranged in at least two.

[0014] In one embodiment, two adjacent active optical fibers are connected by fusion splicing to form a fusion splice.

[0015] In one embodiment, at the fusion joint, the cores and claddings of the two active optical fibers are aligned and connected, and the outer side of the fusion joint is coated with low-refractive-index glue.

[0016] In one embodiment, the optical fiber amplifier further comprises a cladding light stripper and an output optical cable;

[0017] The cladding light stripper is connected to the second end, and one end of the output optical cable is connected to the other end of the cladding light stripper.

[0018] In one embodiment, the first end is connected to the output end pigtail of the pump coupler by fusion splicing; and / or,

[0019] The second end is connected to the cladding light stripper by welding.

[0020] In one embodiment, the optical fiber amplifier further comprises a winding portion, and the winding portion is used to wind the transmission path formed by the plurality of active optical fibers;

[0021] Wherein, the winding portion is configured as a circular winding disk or a racetrack-shaped winding disk, and the arc diameter of the winding portion ranges from 0.01 m to 0.5 m.

[0022] The present invention adopts low numerical aperture optical fiber, which can effectively control the number of modes in the fiber core, effectively improve the laser beam quality and mode instability threshold, and realize high-power and high-beam-quality laser output. Low absorption coefficient active optical fiber can further reduce the accumulation of optical fiber heat and improve the thermally induced mode instability threshold. The present invention can effectively reduce the total length of the amplifier active optical fiber, reduce the accumulation of nonlinearity, and further improve the laser mode instability threshold. The present invention makes the pump light more evenly distributed throughout the amplifier active optical fiber, effectively reduces the temperature of the active optical fiber, reduces the risk of burning out the fusion point, and improves the long-term working stability of the laser. Through this method, a 10,000-watt amplifier was successfully developed, and the beam quality was greatly improved. The temperature of the active optical fiber in the amplifier is reduced by nearly 50% compared with traditional amplifiers, reducing the risk of burning out the active optical fiber fusion point, and effectively improving the stability of the amplifier. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0024] Figure 1 A schematic diagram of the structure of the optical fiber amplifier provided by the present invention comprising two active optical fibers;

[0025] Figure 2 for Figure 1 A structural diagram of another embodiment in which the optical fiber amplifier includes a plurality of active optical fibers;

[0026] Figure 3 for Figure 1 A schematic diagram of the structure of a fiber amplifier including multiple active optical fibers and adopting a reverse pumping structure.

[0027] Description of Figure Numbers:

[0028] 1. Pump coupler; 2. Fusion section; 3. Active optical fiber; 4. Cladding light stripper; 5. Output optical cable; 6. Semiconductor laser.

[0029] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative position relationship, movement status, etc. between the components in a certain specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0032] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of technical features indicated. Therefore, the features limited to "first" and "second" may explicitly or implicitly include at least one of the features. In addition, if "and / or" or "and / or" appears in the full text, its meaning includes three parallel solutions. Taking "A and / or B" as an example, it includes solution A, solution B, or solutions that satisfy both A and B. In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the ability of ordinary technicians in this field to implement. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0033] With the rapid development of high-power fiber materials and high-brightness semiconductor lasers, the output power of fiber lasers has been greatly improved. Traditional high-power fiber lasers mostly use laser synthesis technology, based on fiber power combiners, to synthesize multiple low-power lasers into one beam. However, this type of laser has a complex structure, large size, and poor beam quality. Fiber amplifiers based on master oscillator power amplifier technology (MOPA) have high technical content, good beam quality, and are easy to integrate. With the further increase in power, the shortcomings of traditional MOPA structure amplifiers are gradually exposed. 1. The mode instability threshold is low. When the output power exceeds this threshold, it will cause abnormal power increase or even a sharp drop in power. Ultimately, the output power of high-beam-quality lasers remains at the kilowatt level; second, based on the quasi-static mode instability phenomenon, as the power increases, the number of modes increases, and the proportion of high-order modes increases, resulting in a larger beam quality and a worse light spot; third, the nonlinearity is strong, and a strong Raman laser component appears, which causes the quantum efficiency of some lasers to decrease, resulting in a lower threshold of mode instability; fourth, the temperature of the active fiber fusion point of the laser is high and it is easy to burn out; fifth, the temperature of the active fiber is high, and the long-term stability of the laser is poor. After working for a long time, the laser power drops.

[0034] The present invention proposes an optical fiber amplifier for solving the above problems.

[0035] See also Figure 1 to Figure 2 In one embodiment of the present invention, the optical fiber amplifier includes a semiconductor laser 6, and a pump coupler 1 is connected to one end of the semiconductor laser 6. The seed signal light is coupled into the transmission path formed by multiple active optical fibers 3 through the pump coupler 1. The pump light is absorbed by the multiple active optical fibers 3, and the free electrons of the doped medium transition in energy level to achieve ion number inversion. After the electrons transition from high energy level to low energy level, the energy generated is absorbed by the signal light, thereby being amplified. When the pump light just enters the active optical fiber 3, the pump light power in the cladding is relatively high. At this time, the temperature of this section of active optical fiber 3 is relatively high, which makes the amplifier very prone to mode instability, resulting in poor laser beam quality or low output power. If the power is further increased, the laser output power will drop sharply.

[0036] Through theoretical calculation and experimental verification, it is found that the laser mode instability threshold and beam quality are directly related to the core numerical aperture of the active optical fiber 3. Because the core diameter and core numerical aperture determine the number of modes in the core, and the smaller the core numerical aperture, the fewer the number of modes that support transmission in the core. When the number of modes in the optical fiber core is small, it is easier to obtain a high beam quality laser by properly controlling the high-order mode loss, and the higher the mode stability threshold. When all the amplifier stages use low core numerical aperture active optical fibers 3, they will face low amplification efficiency and it will be difficult to obtain a high-power laser. Taking the above problems into consideration, in this embodiment, the core numerical apertures of the multiple active optical fibers 3 are differentiated, and the core numerical apertures of the multiple active optical fibers 3 in the transmission path increase successively from the first end to the second end. In this way, the active optical fiber 3 with a smaller core numerical aperture is connected to the output end tail fiber of the pump coupler 1, so that the pump light in the cladding of the active optical fiber 3 is stronger. By using a low core numerical aperture optical fiber, the number of modes in the core can be controlled, and the mode instability threshold of the amplifier stage can be increased to achieve high-power and high-beam quality laser output.

[0037] In addition, experiments have found that by reducing the absorption coefficient of the active optical fiber 3, the mode instability threshold can also be greatly increased. Cause analysis: When the active optical fiber 3 absorbs a large amount of pump light, the temperature of the optical fiber will increase significantly, and the signal light will also be strongly amplified. When the power density in the core is large, the core numerical aperture of the optical fiber core will increase, and the thermal effect will cause a thermally induced refractive index grating to be generated in the core, causing energy transfer between different modes, resulting in mode instability. The amount of pump light absorbed by the active optical fiber 3 depends on the power of the pump light in the cladding and the absorption coefficient of the active optical fiber 3 to the pump light. Therefore, in order to reduce the temperature of the active optical fiber 3, the high-power amplifier can use an active optical fiber 3 with a lower absorption coefficient. The entire amplifier uses the active optical fiber 3 with a lower absorption coefficient, so it is necessary to lengthen the length of the active optical fiber 3 to absorb the pump light, which will introduce a stronger nonlinear effect. Therefore, in this solution, not only the core numerical apertures of the plurality of active optical fibers 3 in the transmission path increase in sequence from the first end to the second end as described in the above embodiment, but also the absorption coefficients of the plurality of active optical fibers 3 in the transmission path to the pump light increase in sequence in the above direction. The use of low absorption coefficient active optical fibers 3 can further reduce the accumulation of optical fiber heat and increase the threshold of thermal mode instability. When the pump light just enters the active optical fiber 3, the pump light power in the cladding is relatively high. Here, the active optical fiber with low core numerical aperture and low absorption coefficient is used to control the number of core modes and to increase the threshold of mode instability by controlling the optical fiber temperature. When the pump light is absorbed by the active optical fiber 3, the pump power is reduced, and then enters another active optical fiber 3 adjacent to it. The corresponding active optical fiber 3 has a higher core numerical aperture and a higher absorption coefficient than the previous active optical fiber, and absorbs the pump light step by step through the progressively increased core numerical aperture and absorption coefficient. In this way, the mode deterioration can be more effectively controlled, and the absorption of the pump light by the multiple active optical fibers 3 can be homogenized to reduce the temperature on the multiple active optical fibers 3, and the overall length of the entire transmission path can be more effectively controlled. In this way, the nonlinear effect caused by the long length can be suppressed to a certain extent, and the energy loss in the Raman band can be further reduced.

[0038] Among them, in order to ensure the stability of signal transmission between multiple sections of the active optical fiber 3, the relevant parameters between two adjacent sections of the optical fiber should meet the characteristics of gradual progression and small interval difference, so in this embodiment, the increment of the core numerical aperture and absorption coefficient of the multiple active optical fibers 3 used in the above embodiments are all subject to certain restrictions. Specifically, the core numerical aperture of the multiple active optical fibers 3 is in the range of 0.03-0.07, the core numerical aperture of the multiple active optical fibers 3 increases successively from the first end to the second end, and the increment of the core numerical aperture between two adjacent active optical fibers 3 is controlled to be 0.001-0.02. The specific parameter value setting should be adjusted under the condition of satisfying the above relationship value, but theoretically speaking, the increment value is controlled in a relatively stable state, which is conducive to the improvement of the stability of the entire transmission path. In addition, the absorption coefficient value range of the multiple active optical fibers 3 used for pump light is set in the range of 0.2-2dB / m, and along with the above-mentioned transmission direction, the absorption coefficient of the multiple active optical fibers 3 is gradually increased, and the specific increase value range between each two adjacent active optical fibers 3 is 0.01-0.1dB / m. It can be imagined that, similar to the core numerical aperture of the active optical fiber 3, when the incremental amount of the absorption coefficient is kept in a relatively stable increase state, it is more conducive to the stability of the entire transmission path. The specific value can be calculated and set according to actual needs on the premise of satisfying the above-mentioned increasing relationship, and no excessive restrictions are made here. In actual application, through the above structure, a 10,000-watt amplifier is developed, and the beam quality is greatly improved. The temperature of the active optical fiber 3 in the amplifier is reduced by nearly 50% compared with the traditional amplifier, reducing the risk of burning the fusion part 2 of the active optical fiber 3, and effectively improving the stability of the amplifier.

[0039] According to the above, we know that the actual number of active optical fibers 3 is at least two to meet the above parameter gradient setting requirements. In order to ensure the transmission efficiency and low loss requirements of the entire transmission path, the core diameters of the multiple active optical fibers 3 are the same, and the cladding diameters of the multiple active optical fibers 3 are the same.

[0040] In addition, the active optical fibers 3 are generally connected by fusion splicing, and the connection position of the fusion splice 2 is the fault connection point of the two active optical fibers 3. The processing of the fusion splice 2 affects the transmission efficiency and effect of the entire transmission path to a certain extent. In this embodiment, in order to ensure the transmission effect between the two adjacent active optical fibers 3 and ensure the low-loss transmission of the pump light in the cladding structure, when the fusion splicing is performed, the cores and claddings of the two active optical fibers 3 are aligned and connected respectively, and a layer of low-refractive-index glue is coated on the outside of the fusion splice 2, thereby ensuring the low-loss transmission of the pump light between different active optical fibers 3.

[0041] The optical fiber amplifier also includes an output structure at its tail, and the output structure includes a cladding light stripper 4 and an output optical cable 5. When installed and connected, the cladding light stripper 4 is connected to the second end, and one end of the output optical cable 5 is connected to the other end of the cladding light stripper 4. The second end is connected to the cladding light stripper 4 by fusion, and the first end is connected to the output end fiber pigtail of the pump coupler 1 by fusion.

[0042] It can be imagined that the position of the fusion joint 2 formed between the above two structural parts must still meet the above-mentioned requirement that when performing fusion, the cladding and the core of the two optical fiber structures must be connected to each other as much as possible, and the outer layer structure must be coated with low-refractive index glue to ensure the transmission performance at the fusion joint position.

[0043] According to the above-mentioned embodiment of this scheme, the entire transmission path will be composed of a plurality of active optical fibers 3, which have a certain transmission length. In the actual optical fiber amplifier, a corresponding winding part will be set to wind the excessively long light path. Generally, the winding part will be set as a circular winding part or a runway-type winding part. The runway-type winding part is similar to the path of the runway on the playground, and includes two arc-shaped winding parts. In order to ensure that the transmission performance of the optical fiber after winding is not affected, generally speaking, the arc diameter on the winding part will be as large as possible if the overall installation space allows. However, the overall size specification of the optical fiber amplifier is limited. In order to ensure the compactness of the entire optical fiber amplifier, in this embodiment, the arc diameter range of the winding part is between 0.01m and 0.5m. The specific diameter size can be determined according to the actual diameter size of the active optical fiber 3 and its specific extension length.

[0044] This solution also includes another embodiment scheme, such as Figure 3 As shown, a high-power and high-beam-quality optical fiber amplifier using multiple active optical fibers 3 as described above but using reverse pumping, the structure of which is specifically referred to Figure 3. On the basis of the above embodiments, a backward signal / pump coupler 1 is used to change the forward pumping mode to a backward pumping mode. The input fiber of the backward signal / pump coupler 1 is connected and fused with the active optical fiber 3, and the fusion joint 2 is coated with low-refractive index glue. The pump fiber of the backward signal / pump coupler 1 is connected to the output pigtail of the semiconductor laser 6. The signal output fiber of the backward signal / pump coupler 1 is connected to the cladding light stripper 4 and the output optical cable 5 in sequence, and the laser is output through the output optical cable 5. Because the pump light is reversely input into the active optical fiber 3, multiple active optical fibers 3 are fused in sequence, and the fusion joint 2 is coated with low-refractive index glue to ensure low-loss transmission of the cladding pump light. When the pump light power in the cladding of the active optical fiber 3 is high, an active optical fiber 3 with a low core numerical aperture and a low absorption coefficient is used; when the pump power in the cladding of the active optical fiber 3 is low, an active optical fiber 3 with a low core numerical aperture and a high absorption coefficient is used. As the pump light is transmitted, the numerical apertures of the cores of the plurality of active optical fibers 3 gradually increase from right to left, and the absorption coefficients also gradually increase from right to left.

[0045] The above reverse pumping structure also adopts the core technical solution in this solution. In the specific implementation process, it also has the beneficial technical effects in the above embodiments, which will not be described one by one here.

[0046] The above description is only an exemplary embodiment of the present invention, and does not limit the patent scope of the present invention. All equivalent structural changes made by using the contents of the present invention specification and drawings under the technical concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. An optical fiber amplifier, comprising a semiconductor laser, characterized in that: The optical fiber amplifier further comprises: a pump coupler, an input end of which is connected to the semiconductor laser; and A plurality of active optical fibers, the plurality of active optical fibers are connected in sequence to form a transmission path, the transmission path comprising a first end and a second end, the first end being connected to an output end pigtail of the pump coupler; Among them, the core numerical apertures of the plurality of active optical fibers in the transmission path increase sequentially from the first end to the second end.

2. The optical fiber amplifier according to claim 1, wherein: The core numerical aperture values ​​of the plurality of active optical fibers are all in the range of 0.03 to 0.07, and the core numerical aperture value increment between two adjacent active optical fibers in the direction from the first end to the second end is in the range of 0.001 to 0.

02.

3. The optical fiber amplifier according to claim 1, wherein: From the first end to the second end, the absorption coefficients of the plurality of active optical fibers in the transmission path to the pump light increase sequentially.

4. The optical fiber amplifier according to claim 3, characterized in that: The absorption coefficients of the active optical fibers to pump light are all between 0.2dB / m and 0.07dB / m, and the absorption coefficient increment of the pump light between two adjacent active optical fibers from the first end to the second end is between 0.01dB / m and 0.1dB / m.

5. The optical fiber amplifier according to claim 1, wherein: The core diameters of the plurality of active optical fibers are all the same; and, The cladding diameters of the plurality of active optical fibers are all the same; and / or, The plurality of active optical fibers are arranged in at least two.

6. The optical fiber amplifier according to claim 5, characterized in that: Two adjacent active optical fibers are connected to each other by fusion splicing to form a fusion splice.

7. The optical fiber amplifier according to claim 6, wherein: At the fusion joint, the cores and claddings of the two active optical fibers are aligned and connected respectively, and the outer side of the fusion joint is coated with low-refractive index glue.

8. The optical fiber amplifier according to claim 1, wherein: The optical fiber amplifier also includes a cladding light stripper and an output optical cable; The cladding light stripper is connected to the second end, and one end of the output optical cable is connected to the other end of the cladding light stripper.

9. The optical fiber amplifier according to claim 8, characterized in that: The first end is connected to the output end pigtail of the pump coupler by fusion splicing; and / or, The second end is connected to the cladding light stripper by welding.

10. The optical fiber amplifier according to claim 2, wherein: The optical fiber amplifier further comprises a winding portion, and the winding portion is used to wind the transmission path formed by the plurality of active optical fibers; Wherein, the winding portion is configured as a circular winding disk or a racetrack-shaped winding disk, and the arc diameter of the winding portion ranges from 0.01 m to 0.5 m.

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