Multi-cladding active optical fiber and preparation method thereof

Abstract

The invention discloses a multi-cladding active optical fiber and a preparation method thereof. The glass part of the optical fiber comprises a rare earth ion doped fiber core and a non-circular pumping cladding. The pumping cladding layer comprises at least two low-refractive-index sunken cladding layers, the sunken cladding layer on the innermost side close to the fiber core is an inner sunken cladding layer, and the lower-limit cladding layer on the outermost side is an outer sunken cladding layer; the NA range of the outer side of the inner sunken cladding layer is 0-0.164; and the NA range of the inner side of the outer sunken cladding layer is 0-0.164. According to the multi-cladding active optical fiber, mode interference is enhanced through the outer sunken cladding and the non-circular pumping cladding, more pumping light enters the inner sunken cladding, the inner sunken cladding filters out a high-order mode, the fundamental mode energy proportion is increased, the light beam quality and the Raman rejection ratio are optimized at the same time, and the multi-cladding active optical fiber is suitable for being used as a gain medium of a high-power laser.

Description

Technical Field

[0001] The present invention belongs to the technical field of active optical fibers, and more specifically, relates to a multi-cladding active optical fiber and a preparation method thereof. Background Art

[0002] The Raman ratio of a laser usually refers to the ratio between the signal light power output by the laser and the power of the Raman scattered light (i.e. Stokes light) generated in a fiber laser due to the stimulated Raman scattering (SRS) effect. This ratio is an important parameter for measuring laser performance, especially in high-power fiber lasers, because it is related to the output power of the laser and the reliability of the system.

[0003] The Raman ratio is affected by many factors of the gain fiber in the fiber laser, including the effective mode field area of ​​the fiber, the fiber length, the fiber refractive index profile structure design, etc. In order to improve the performance and reliability of the laser, it is necessary to control and suppress the SRS effect by optimizing the above factors to maintain a high Raman ratio. At high power, it is a technical challenge to improve the beam quality of the pump light and suppress the Raman scattering effect. Active fibers with larger core diameters are often difficult to optimize the Raman suppression ratio and beam quality at the same time. Summary of the invention

[0004] In view of the above defects or improvement needs of the prior art, the present invention provides a cladding active optical fiber and a preparation method thereof, the purpose of which is to respectively perform mode perturbation and mode separation, i.e., mode scrambling and mode filtering, through multiple layers of depressed cladding, thereby improving the fundamental mode energy ratio while improving the core absorption efficiency, thereby optimizing the Raman suppression ratio and beam quality at the same time, thereby solving the technical problem that the existing double-cladding active optical fiber is often difficult to optimize the Raman suppression ratio and beam quality at the same time.

[0005] To achieve the above object, according to one aspect of the present invention, there is provided a multi-clad active optical fiber, the glass portion of which includes a rare earth ion-doped core and a non-circular pump cladding;

[0006] The pump cladding includes at least two layers of low refractive index depressed claddings, wherein the innermost depressed cladding close to the fiber core is the inner depressed cladding, and the outermost lower limit cladding is the outer depressed cladding;

[0007] The NA of the outer side of the inner depressed cladding is in the range of 0-0.164;

[0008] The NA of the inner side of the outer depressed cladding is in the range of 0-0.164.

[0009] Preferably, in the multi-clad active optical fiber, the refractive index of the inner depressed cladding is lower than or equal to the refractive index of the outer depressed cladding.

[0010] Preferably, in the multi-clad active optical fiber, the F-doped content of the inner depressed cladding is 0.3-3 mol%, and the thickness is ≥1 um, preferably 1 um-10 um, and more preferably 2 um-5 um.

[0011] Preferably, in the multi-clad active optical fiber, the F-doped content of the outer depressed cladding is 0.3-3 mol%, and the thickness is ≥1 um, preferably 1 um-10 um, and preferably 2 um-5 um.

[0012] Preferably, in the multi-clad active optical fiber, the difference in refractive index between the inner depressed cladding and the outer depressed cladding is within 0.00139.

[0013] Preferably, in the multi-clad active optical fiber, the inner depressed cladding and / or the outer depressed cladding are ring-shaped.

[0014] Preferably, in the multi-clad active optical fiber, the distance between the inner depressed cladding and the core is 10um-80um, preferably 30um-60um; the distance between the outer depressed cladding and the inner depressed cladding is 10um-70um, preferably between 10um-50um.

[0015] Preferably, the multi-clad active optical fiber has a core diameter ranging from 10um to 90um, an NA ranging from 0.02 to 0.09, and is doped with P2O5, Yb2O3, Al2O3, F, and Ce2O3. The doping concentrations of P2O2 are 1-8mol%, Yb2O3 are 0.05-1mol%, Al2O3 are 1-5mol%, F are 0.3-3mol%, and Ce2O3 are 0.1-1mol%. There is a pure quartz layer between the core and the inner sunken cladding, and the outer side of the outer lower limit cladding is a pure quartz layer.

[0016] Preferably, the multi-clad active optical fiber comprises a pure silica layer between the inner depressed cladding and the outer depressed cladding; and comprises one or more layers of low-refractive-index intermediate depressed claddings between the inner depressed cladding and the outer depressed cladding.

[0017] According to another aspect of the present invention, there is provided a method for preparing the multi-clad active optical fiber, comprising the following steps:

[0018] (1) Making a core rod and a cladding tube: According to the cross-sectional design of the multi-cladding active optical fiber, a core rod for forming a rare earth ion-doped core is made by an improved MCVD vapor deposition method or a combined solution method, and a cladding tube for forming a pump cladding including at least two layers of low refractive index depressed cladding is made by PCVD deposition, and the outer contour of the cladding tube is machined into a non-circular shape so that the outer diameter of the core rod matches the inner diameter of the cladding tube;

[0019] (2) Sleeve drawing: inserting the core rod into the cladding sleeve to make the sleeve, and drawing the sleeve to obtain the active optical fiber glass part.

[0020] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects compared with the prior art:

[0021] The multi-clad active optical fiber provided by the present invention realizes enhanced mode scrambling through an outer depressed cladding and a non-circular pump cladding, so that more pump light enters the inner depressed cladding, and the inner depressed cladding filters out high-order modes, thereby increasing the energy proportion of the inner fundamental mode, thereby optimizing the beam quality and the Raman suppression ratio at the same time, and is suitable as a gain medium for high-power lasers. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the end face structure of a multi-clad active optical fiber provided in Examples 1 and 2;

[0023] Figure 2 This is a schematic diagram of the end face structure of a multi-clad active optical fiber provided in Example 3.

[0024] In all the drawings, the same reference numerals are used to represent the same elements or structures, wherein: 1 is a rare earth ion doped core, 2 is a non-circular pump cladding, 3 is an inner depressed cladding, 4 is an outer depressed cladding, and 5 is an intermediate depressed cladding. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0026] The multi-clad active optical fiber provided by the present invention has a glass part comprising a rare earth ion doped fiber core and a non-circular pump cladding; the outer side of the glass part is coated with a low refractive index coating.

[0027] The pump cladding includes at least two layers of low refractive index depressed claddings, wherein the innermost depressed cladding close to the fiber core is the inner depressed cladding, and the outermost lower limit cladding is the outer depressed cladding; the interface NA outside the inner depressed cladding is 0-0.164; the interface NA inside the outer depressed cladding is 0-0.164. The pump light source of the fiber laser is transmitted in the pump cladding, which is more commonly multi-mode transmission, usually meaning higher power, larger mode field diameter, and more effective pumping of the fiber core. Single-mode pumping has higher power density and beam quality.

[0028] For active optical fibers, many factors affect the Raman scattering effect. The current method of increasing the Raman suppression ratio by expanding the core diameter or increasing the mode field area may lead to deterioration of beam quality.

[0029] The present invention arranges an inner depressed cladding and an outer depressed cladding in the pump cladding, respectively. The inner depressed cladding separates the high-order mode and the fundamental mode, filters out the high-order mode of the pump light, and increases the power ratio occupied by the fundamental mode. The outer depressed cladding is combined with the non-circular profile of the pump cladding to disturb the propagation mode of the pump light, so that more light energy enters the inner side of the inner depressed cladding. The inner depressed cladding limits the pump light to be transmitted inside the inner side, thereby increasing the fiber core absorption and reducing the application length of the optical fiber. By combining the inner and outer depressed claddings with the non-circular pump cladding profile, the beam quality and the Raman suppression ratio are optimized at the same time.

[0030] In a preferred embodiment, the refractive index of the inner depressed cladding is lower than or equal to the refractive index of the outer depressed cladding, and the outer side is shallowly doped with fluorine, while the inner side is deeply doped with fluorine to further separate the high-order mode and the fundamental mode, thereby increasing the proportion of the fundamental mode.

[0031] Specifically, the fluorine content of the inner depressed cladding is 0.3-3 mol%, and the thickness is 1um-10um, preferably 2um-5um; the fluorine content of the outer depressed cladding is 0.3-3 mol%, and the thickness is 1um-10um, preferably 2um-5um. The refractive index difference between the inner depressed cladding and the outer depressed cladding is within 0.00139. Usually the inner depressed cladding and / or the outer depressed cladding are annular, which is convenient for manufacturing. The core-to-cladding ratio of the inner depressed cladding relative to the core is between 4.7-6.9, the distance between the inner depressed cladding and the core is 10um-80um, preferably 30um-60um, and the distance between the outer depressed cladding and the inner depressed cladding is 10um-70um, preferably between 10-50um, so that the outer depressed cladding and the inner depressed cladding are effectively separated to realize their respective functions. A pure quartz layer is located between the fiber core and the inner depressed cladding, and a pure quartz layer is located outside the outer lower limit cladding.

[0032] Between the inner depressed cladding and the outer depressed cladding, there can be a pure quartz layer commonly used as an active optical fiber pump cladding, or one or more layers of low-refractive-index intermediate depressed cladding can be added. In order to improve the effect of mode scrambling, the refractive index range and thickness are the same as those of the inner depressed cladding and the outer depressed cladding.

[0033] The core diameter ranges from 10um to 90um, generally from 20um to 50um, and most commonly from 20um to 30um. The NA ranges from 0.02 to 0.09. The doping includes P2O5, Yb2O3, Al2O3, F, and Ce2O3. The doping concentrations are 1-8mol% for P2O2, 0.05-1mol% for Yb2O3, 1-5mol% for Al2O3, 0.3-3mol% for F, and 0.1-1mol% for Ce2O3. There is a pure quartz layer between the core and the inner sunken cladding, and a pure quartz layer outside the outer lower limit cladding.

[0034] The outer contour of the pump cladding may be a D-shape, a quincunx shape, or a regular polygon with 4 to 10 sides, and a regular octagon is common.

[0035] The method for preparing a multi-clad active optical fiber provided by the present invention comprises the following steps:

[0036] (1) Making a core rod and a cladding tube: According to the cross-sectional design of the multi-cladding active optical fiber, a core rod for forming a rare earth ion-doped core is made by MCVD deposition, and a cladding tube for forming a pump cladding including at least two layers of low refractive index depressed cladding is made by PCVD deposition, and the outer contour of the cladding tube is machined into a non-circular shape so that the outer diameter of the core rod matches the inner diameter of the cladding tube;

[0037] (2) Sleeve drawing: inserting the core rod into the cladding sleeve to make the sleeve, and drawing the sleeve to obtain the active optical fiber glass part.

[0038] The double-clad optical fiber is prepared by combining MCVD and PCVD, wherein the core rod is prepared by improving the MCVD vapor deposition method combined with the chelate vapor method or the solution method, and the design of the special cladding structure relies on the PCVD preparation. The special cladding structure is designed, wherein the cladding is designed with one or more layers of gully structure, and the gully is a structure including but not limited to F-doping, which is realized by the PCVD process.

[0039] The following are examples:

[0040] The embodiment of the present invention adopts a regular octagonal active optical fiber, represented by a standard optical fiber of 250 specifications, and the structure is as follows: Figure 1 As shown, the glass part includes, from the inside to the outside: a rare earth ion doped core, an octagonal pump cladding, wherein the octagonal cladding has an inner depressed cladding and an outer depressed cladding. The optical fiber structure and refractive index parameters are as follows:

[0041]

[0042] The pump cladding diameter refers to the distance between the opposite sides of the octagonal cladding; the depressed cladding is F-doped quartz glass, and the refractive index is adjusted by adjusting the fluorine doping amount. The pump cladding except the depressed cladding is made of pure quartz material.

[0043] The method for preparing the multi-clad active optical fiber in the embodiment comprises the following steps:

[0044] (1) Making a core rod and a cladding sleeve: According to the cross-sectional design of the multi-cladding active optical fiber, a core rod for forming a rare earth ion-doped core is made by using an improved MCVD chemical vapor deposition method combined with a solution doping technology, specifically:

[0045] First, polish to remove impurities inside and outside the liner, deposit a loose SiO2-P2O5-F layer at 1200℃-1600℃, remove the deposition tube and pour it into the prepared solution for soaking. The solution contains doping such as YbCl3 and AlCl3. Soak for 40 minutes, take out the deposition tube and reconnect it to the lathe, and pass a mixed gas of high-purity Cl2 and O2 at 1000℃ to dry and dehydrate the preform. Then heat the loose core layer until it is transparent. Adjust the number of deposition layers and other parameters according to design requirements. The thickness of the deposition core layer is 3.6mm.

[0046] The cladding casing is made by PCVD deposition; specifically:

[0047] Pure quartz part: according to the size design, a pure quartz layer is deposited between the inner sunken cladding and the outer sunken cladding, or a pure quartz part is deposited in the innermost layer of the cladding casing, and the deposition thickness is adjusted according to the size requirements;

[0048] Inner and outer sag blanket: According to the design delta and size range, inner sag F-doped layer and outer sag F-doped layer are deposited on the PCVD liner, and the CSA of the deposited inner sag F-doped layer is 16.5mm. 2 The CSA of the deposited outer depression F-doped layer is 26.28 mm 2 , the refractive indexes were 1.4560 and 1.4566 respectively. After deposition, the refractive index profile was tested to confirm the thickness and delta;

[0049] Machining the outer contour of the cladding sleeve into an octagon so that the outer diameter of the core rod matches the inner diameter of the cladding sleeve;

[0050] (2) Sleeve drawing: inserting the core rod into the cladding sleeve to make the sleeve, and drawing the sleeve to obtain the active optical fiber glass part.

[0051] A 100W Q-switched 30 / 250 laser was used to test the Raman suppression ratio and beam quality. The fiber length used was 5.2m and the coil diameter was 9cm. The beam quality was measured using M 2The Raman suppression ratio is measured by the value measurement method, and the test is carried out at full pump power; while the Raman suppression ratio is tested by the spectrometer, and the signal light intensity is subtracted from the Raman peak intensity at full pump power for calculation. 2 It can reach 1.1, Raman suppression ratio 30dB, and the optical fiber length is shorter when the optical efficiency is 75%. Compared with the active optical fiber provided by comparative example 1 and comparative example 2 with a single-layer depressed cladding of the same total thickness, the Raman suppression ratio and the beam quality are optimized at the same time.

[0052] Example 3

[0053] The multi-clad active optical fiber provided in this embodiment has a structure as follows Figure 2 As shown, the difference between it and Example 1 is that it has a three-layer lower limit cladding structure, that is, a fluorine-doped quartz layer is deposited between the inner and outer depressed claddings as the middle depressed cladding, and its CSA is 23.4 mm 2 , refractive index is 1.4563, outer diameter is 175, inner diameter is 170; this scheme M 2 It can reach 1.1, Raman suppression ratio 31dB, and the optical fiber length is 5m when the optical efficiency reaches 75%. It also has good beam quality and Raman suppression ratio, especially it can improve the mode scrambling effect to a certain extent on the basis of double layer, improve absorption, and when the optical efficiency reaches 75%, the optical fiber length is shorter.

[0054] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A multi-clad active optical fiber, characterized in that: Its glass part includes a rare earth ion doped core and a non-circular pump cladding; The pump cladding includes at least two layers of low refractive index depressed claddings, wherein the innermost depressed cladding close to the fiber core is the inner depressed cladding, and the outermost lower limit cladding is the outer depressed cladding; The NA of the outer side of the inner depressed cladding is in the range of 0-0.164; The NA of the inner side of the outer depressed cladding is in the range of 0-0.

164.

2. The multi-clad active optical fiber according to claim 1, characterized in that: The refractive index of the inner depressed cladding is lower than or equal to the refractive index of the outer depressed cladding.

3. The multi-clad active optical fiber according to claim 1 or 2, characterized in that: The F-doped content of the inner sunken cladding is 0.3-3 mol%, and the thickness is ≥1 um, preferably 1 um-10 um, and more preferably 2 um-5 um.

4. The multi-clad active optical fiber according to claim 1 or 2, characterized in that: The F-doped content of the outer sunken cladding is 0.3-3 mol%, and the thickness is ≥1 um, preferably 1 um-10 um, and preferably 2 um-5 um.

5. The multi-clad active optical fiber according to claim 1 or 2, characterized in that: The refractive index difference between the inner depressed cladding and the outer depressed cladding is within 0.00139.

6. The multi-clad active optical fiber according to any one of claims 1 to 5, characterized in that: The inner depressed cladding and / or the outer depressed cladding are annular in shape.

7. The multi-clad active optical fiber according to claim 1, characterized in that: The distance between the inner depressed cladding and the core is 10um-80um, preferably 30um-60um; the distance between the outer depressed cladding and the inner depressed cladding is 10um-70um, preferably between 10um-50um.

8. The multi-clad active optical fiber according to claim 1, characterized in that: The core diameter ranges from 10um to 90um, the NA ranges from 0.02 to 0.09, and the doping includes P2O5, Yb2O3, Al2O3, F, and Ce2O3. The doping concentrations of P2O2 are 1-8mol%, Yb2O3 are 0.05-1mol%, Al2O3 are 1-5mol%, F are 0.3-3mol%, and Ce2O3 are 0.1-1mol%. There is a pure quartz layer between the core and the inner sunken cladding, and the outer side of the outer lower limit cladding is a pure quartz layer.

9. The multi-clad active optical fiber according to claim 1, characterized in that: A pure quartz layer is included between the inner depressed cladding and the outer depressed cladding; and one or more layers of low-refractive-index intermediate depressed cladding are included between the inner depressed cladding and the outer depressed cladding.

10. The method for preparing a multi-clad active optical fiber according to any one of claims 1 to 9, characterized in that: The following steps are involved: (1) Making a core rod and a cladding tube: According to the cross-sectional design of the multi-cladding active optical fiber, a core rod for forming a rare earth ion-doped core is made by MCVD deposition, and a cladding tube for forming a pump cladding including at least two layers of low-refractive-index depressed cladding is made by PCVD deposition, and the outer contour of the cladding tube is machined into a non-circular shape so that the outer diameter of the core rod matches the inner diameter of the cladding tube; (2) Sleeve drawing: inserting the core rod into the cladding sleeve to make the sleeve, and drawing the sleeve to obtain the active optical fiber glass part.

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