CN118472760A - Annular pumping distributed side pumping optical fiber and optical fiber laser - Google Patents

Abstract

The present invention discloses a ring-pumped distributed side-pumped optical fiber and an optical fiber laser, which relate to the technical field of laser signal transmission. The ring-pumped distributed side-pumped optical fiber comprises an outer cladding, a pump fiber and a signal fiber; the pump fiber and the signal fiber are both located in the outer cladding; there are several pump fibers, when there is only one pump fiber, the pump fiber is a ring-core optical fiber with a depressed central refractive index; when there are multiple pump fibers, at least one pump fiber is a ring-core optical fiber with a depressed central refractive index. The present invention also provides an optical fiber laser, which uses ring light pumping to concentrate energy on the outside of the pump fiber, improve the overall coupling rate from the pump fiber to the signal fiber, and increase the coupling coefficients _k1 and _k2 .

Description

Ring-pumped distributed side-pumped fiber and fiber laser

Technical Field

The present invention relates to the technical field of laser signal transmission, and in particular to a ring-pumped distributed side-pumped optical fiber and an optical fiber laser.

Background Art

With the rapid development of modern science and technology, fiber lasers play an increasingly important role in many fields due to their unique advantages. With the continuous advancement of fiber technology, pumping technology and laser materials, the performance of fiber lasers has been significantly improved. From the initial low-power, single-mode output to the current high-power, multi-mode output, fiber lasers have achieved a qualitative leap. In terms of high-power laser output, long-distance transmission and precision processing, fiber lasers have demonstrated unparalleled potential and application value. With the advancement of science and technology and the growth of demand for high-power, high-beam quality applications, traditional end-pumped fiber lasers have gradually revealed certain deficiencies in thermal management and nonlinear control. To solve this problem, distributed side-coupled cladding-pumped (hereinafter referred to as distributed side pumping, abbreviated as DSCCP, also known as GTwave, composite functional fiber, etc. in different occasions) technology came into being. Its unique pumping method and excellent performance have injected new vitality into the development of fiber laser technology.

DSCCP technology achieves full coupling of pump light and signal light by introducing pump light on the side of the optical fiber. This pumping method effectively solves the problems existing in end-face pumping. It adopts the preparation method of multiple optical fibers bundled and drawn to expand the area where the pump light is coupled into the gain medium to the entire section of the optical fiber, effectively avoiding the local heating problem that is difficult to solve in the end-face pump combiner, and at the same time has better performance in nonlinear control and heat distribution. DSCCP fiber lasers provide more ideas in power expansion level and structural design flexibility. In recent years, it has attracted much attention at home and abroad, and the output power has also achieved a series of milestone leaps, which is no less than that of traditional fiber lasers. The unique structure of its optical fiber is different from the traditional method in the manufacturing process. The multi-fiber bundle drawing scheme needs to overcome many difficulties when it is prepared on the existing optical fiber drawing equipment, including the precision control of the lateral size of the optical fiber, the relative position of the preform rod, and the uniformity of the material, which is difficult to easily meet the high-power design requirements.

The conventional double-clad gain fiber currently used is composed of a rare earth ion doped silica core, a pure silica inner cladding and a low refractive index outer cladding. DSCCP is different from it. Its cross section consists of a signal fiber including a core and an inner cladding and a number of multimode pump fibers (the number is N and N≥1, when N=0, it degenerates to a traditional double-clad pump fiber). For the sake of convenience, the simplest (1+1) type DSCCP fiber is used as an example in this article, but without loss of generality, all the invention contents are applicable to other types of (N+1) type DSCCP fibers.

In DSCCP pumping, the coupled absorption gain amplification process can be divided into two steps. The first step is the side coupling process of the pump laser. The pump light is first injected through the end face of the pump fiber, and then transmitted in the pump fiber. At the same time, it is coupled between the pump fiber and the signal fiber inner cladding in the form of an evanescent wave through the tightly fitting fiber side. This process determines that the pump light enters the signal fiber from the pump fiber. The second step is the absorption amplification process. The pump light coupled into the inner cladding of the signal fiber is absorbed and converted by the doped core in its central area, and the signal light in the core is gain-amplified. The second step is similar to the gain amplification in traditional double-clad gain fibers.

The side coupling process in the first step reflects the characteristics of side coupling, which is closely related to factors such as fiber size, refractive index, cross-sectional shape, and bonding method. In order to quantitatively analyze and evaluate the side coupling ability of pump fiber and signal fiber, coupling coefficients _k1 and _k2 are introduced. _k1 represents the strength of pump energy coupling from pump fiber to signal fiber, and the strength of reverse energy coupling is evaluated by parameter _k2 . The pump power that can be injected into the optical fiber maintains a positive correlation with the fiber diameter d, numerical aperture NA, and pump laser brightness. Therefore, in order to increase the pump power that can be injected into the optical fiber to achieve higher power output, under the condition that NA is basically difficult to increase, increasing the pump brightness and fiber diameter is the most commonly used method.

Under the condition of the existing semiconductor pump source (Laser Diode, LD) brightness (typical high brightness 0.23W/(μm ² sr)), the theoretically injectable pump power of a conventional 400μm fiber is less than 20kW. Considering the brightness drop in the step-by-step beam combining, the actual injectable power is even less than 10kW. In order to increase the injected pump power, the fiber diameter needs to be increased. However, in the DSCCP scheme, only increasing the fiber diameter will cause the coupling coefficient to decrease in a quasi-exponential manner when other conditions remain unchanged. When the pump fiber and signal fiber diameters are simultaneously increased from a small size (100μm) to a medium size (250μm), the coupling coefficients _k1 and _k2 decrease by about 2/3. In order to meet certain coupling requirements, the fiber needs to be significantly extended, but too long a fiber will cause serious problems such as stimulated Raman scattering, stimulated Brillouin scattering and reabsorption. Therefore, how to improve the low coupling coefficients _k1 and _k2 of large-size fibers in DSCCP has become a key issue in achieving high-power DSCCP laser output.

Summary of the invention

The object of the present invention is to provide a ring-pumped distributed side-pumped optical fiber and an optical fiber laser to solve the problems existing in the above-mentioned prior art, increase the coupling coefficients _k1 and _k2 , and improve the coupling effect.

To achieve the above object, the present invention provides the following solutions:

The present invention provides a ring-pumped distributed side-pumped optical fiber, comprising: an outer cladding, a pump fiber and a signal fiber; the pump fiber and the signal fiber are both located in the outer cladding; a plurality of pump fibers are provided, and when only one pump fiber is provided, the pump fiber is a ring-core optical fiber with a depressed central refractive index; when a plurality of pump fibers are provided, at least one pump fiber is a ring-core optical fiber with a depressed central refractive index.

Preferably, the ring core optical fiber comprises an inner core and an outer ring core which are nested from inside to outside; and the refractive index of the inner core is lower than that of the outer ring core.

Preferably, the ring-core optical fiber comprises an inner core, a middle ring core and an outer ring core which are nested in sequence from the inside to the outside; and the refractive indexes of the middle ring core, the inner core and the outer ring core gradually increase.

Preferably, the middle ring core is an air core or a solid core.

Preferably, the refractive index distribution of the ring-core optical fiber is gradient.

Preferably, the refractive index distribution of the ring-core optical fiber is step-type.

Preferably, when a plurality of pump fibers are provided, some of the pump fibers are common optical fibers.

The present invention also provides a fiber laser, comprising a ring-pumped light source, a laser signal light source and a ring-pumped distributed side-pumped optical fiber; the ring-pumped distributed side-pumped optical fiber comprises an outer cladding, a pump fiber and a signal fiber; the ring-pumped light source is connected to the signal fiber and transmits a laser signal therein; the ring-pumped light source is connected to the pump fiber and transmits a ring-pumped light therein.

Preferably, the annular pumped distributed side-pumped optical fiber adopts the annular pumped distributed side-pumped optical fiber described above.

Preferably, the annular pump light source comprises a forward annular light pump source, a reverse annular light pump source, a reverse pump combiner and a forward pump combiner; the forward annular light pump source, the forward pump combiner and part of the pump fibers are connected in sequence; the reverse annular light pump source, the reverse pump combiner and the remaining part of the pump fibers are connected in sequence;

The laser signal light source includes a seed source, a first pre-amplifier, a second pre-amplifier, a mode field adapter and a first cladding power stripper; the seed source, the first pre-amplifier, the second pre-amplifier, the mode field adapter, the first cladding power stripper and the signal fiber are connected in sequence, and the signal fiber, the second cladding power stripper and the collimating end cap are connected in sequence.

Compared with the prior art, the present invention has achieved the following technical effects:

The present invention uses annular optical pumping to concentrate energy on the outside of the pump fiber, improve the overall coupling rate from the pump fiber to the signal fiber, and increase the coupling coefficients _k1 and _k2 .

BRIEF DESCRIPTION OF THE DRAWINGS

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 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 these drawings without paying creative work.

FIG1 is a ring-pumped distributed side-pumped optical fiber composed of a common multimode pump fiber and a signal fiber and its refractive index distribution;

Figure 2 shows the ring-pumped distributed side-pumped fiber and its refractive index distribution composed of a central refractive index depressed pump fiber and a signal fiber;

Figure 3: Ring-pumped distributed side-pumped optical fiber composed of solid core ring pump fiber and signal fiber and its refractive index distribution;

FIG4 is a ring-pumped distributed side-pumped optical fiber composed of an air-core ring pump fiber and a signal fiber and its refractive index distribution;

Figure 5 (N+1) type ring pumped distributed side pumped fiber composed of various pump fibers;

Figure 6 (N+1) type ring pumped distributed side pumped fiber composed of multiple identical pump fibers;

FIG7 is a schematic diagram of the principle of (2+1) type ring pump distributed side pump fiber ring light pumping;

FIG8 is a schematic diagram of the structure of a (2+1) type ring light pumped ring pumped distributed side pumped fiber laser;

In the figure:

101- signal fiber doped core;

102- signal fiber cladding;

103- pump fiber;

104-Low refractive index coating

301-Inner core;

302-middle ring core;

401-air core;

601-forward ring pump light;

602-forward unabsorbed residual pump light;

603-reverse ring pump light;

604-reverse non-absorbed participating pump light;

605-seed signal light;

606-Amplified output signal light.

701-seed source;

702-first pre-amplifier;

703- second pre-amplifier;

704-mode field adapter;

705-first cladding power stripper;

706 - reverse residual pump output terminal;

707-Forward pump combiner;

708-forward ring optical pump source;

709-Ring pumped distributed side pumped fiber;

710 - second residual pump output terminal;

711-reverse pump combiner;

712-reverse ring optical pump source;

713-second cladding power stripper;

714-Alignment end cap.

DETAILED DESCRIPTION

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.

In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

The inventor of the present invention has found that in the traditional DSCCP pumping, in the process of coupling the pump light energy from the pump fiber to the signal fiber, there are differences in the coupling rates of different regions on the transverse cross section of the pump fiber to the signal fiber. The pump light in the central area of the pump fiber is coupled slowly, and there is even the possibility that it cannot be coupled at all. This not only causes the required optical fiber length to be longer, but also makes part of the pump light energy unable to be absorbed, which leads to low gain conversion efficiency of the DSCCP optical fiber and limited increase in optical fiber output power. Based on this, the following solution is provided to solve this problem.

The embodiment of the present invention provides a ring-pumped distributed side-pumped optical fiber, hereinafter referred to as the pump optical fiber, comprising: an outer cladding, a pump fiber and a signal fiber; the pump fiber and the signal fiber are both located in the outer cladding; a plurality of pump fibers are provided. When only one pump fiber is provided, it is a (1+1) type pump optical fiber, and the pump optical fiber is a ring-core optical fiber with a depressed central refractive index; when a plurality of pump fibers are provided, it is a (N+1) type pump optical fiber, and at least one pump optical fiber is a ring-core optical fiber with a depressed central refractive index. It can be understood that when a plurality of pump fibers are provided, some of the pump fibers may be ordinary optical fibers, or all of them may be ring-core optical fibers. The ordinary optical fiber in the present invention refers to an optical fiber with a consistent refractive index of the optical fiber core.

The pump fiber provided in this embodiment has a ring-core fiber, and the pump fiber therein is used to transmit annular pump light, which can concentrate energy on the outside (outer ring) of the pump fiber, improve the overall coupling rate from the pump fiber to the signal fiber, and increase the coupling coefficients _k1 and _k2 .

The ring core optical fiber in the present invention includes at least the following:

The first type: the ring core optical fiber includes an inner core 301 and an outer ring core which are nested from the inside to the outside; the refractive index of the inner core 301 is lower than that of the outer ring core.

The second type: the ring core optical fiber comprises an inner core 301, a middle ring core 302 and an outer ring core which are nested in sequence from the inside to the outside; the refractive indexes of the middle ring core 302, the inner core 301 and the outer ring core gradually increase.

Based on the second method, the middle ring core 302 can be an air core or a solid core.

In some embodiments, the refractive index profile of the ring-core optical fiber is graded.

In some embodiments, the refractive index profile of the ring-core optical fiber is step-index.

Below we will take the (1+1) type optical fiber as an example to introduce the pump fiber with a specific pump fiber suitable for the annular spot.

Example 1: Ordinary multimode pump fiber

Ring laser can be transmitted in ordinary multimode optical fiber, so the traditional (1+1) type pump fiber as shown in Figure 1 can be used for ring light pumping. It consists of a multimode pump fiber made of pure quartz and a core-doped gain fiber, which are coated with low-refractive index glue and cured. In Figure 1:

The diameter of the signal fiber doped core 101 is d1, and the refractive index is n1;

The diameter d2 of the signal fiber cladding 102 and the refractive index n2;

The pump fiber 103 has a diameter d3 and a refractive index n2;

The low refractive index coating layer 104 has a diameter d4 and a refractive index n3;

In order to ensure the evanescent wave coupling of the pump light, the pump fiber and the signal fiber cladding must be kept close together, and the refractive index at the close-fitting point must be kept consistent. According to the fiber diameter and corresponding refractive index data in Figure 1, it can be seen that n1>n2>n3; d4>d3≈d2>d1.

Example 2: Pump Fiber with a Central Refractive Index Depression

In order to better control the annular light shift, a low refractive index region can be set in the center of the pump fiber, as shown in Figure 2, to suppress the coupling of the annular light mode to the non-annular mode. In Figure 2:

The central refractive index depression region, ie, the inner core 301, has a diameter d5 and a refractive index n4.

The relationships among the parameters are n1>n2>n4>n3, d4>d3≈d2>d1, d3>d5.

Example 3: Solid Core Ring Pump Fiber

Solid core annular optical fiber is suitable for the transmission of annular light spots, and can be controlled by parameters to constrain the transmission of annular light spots in a specific single mode. Compared with Example 2, it not only has a central refractive index depression area, but also adds a layer of depression area with a lower refractive index between the depression area and the peripheral area to better realize the transmission of annular light spots.

In Figure 3:

The central refractive index depression region, i.e., the inner core 301, has a diameter d5 and a refractive index n4;

The diameter of the annular refractive index depression region, namely the middle ring core 302, is d6, and the refractive index is n5.

The relationships among the parameters are n1>n2>n4>n5>n3, d4>d3≈d2>d1, d3>d6>d5.

Example 4: Air-core Ring Pump Fiber

Compared with Example 3, the air core annular pump fiber replaces the annular depression of the refractive index with air, and the lower refractive index can be better used for the transmission of some high-order annular beams. The meanings of the numbers in Figure 4 are as follows:

401: Air core (refractive index n6=1, diameter is the annular area between d5 and d6).

The relationships among the parameters are n1>n2>n4>n3>n6=1, d4>d3≈d2>d1, d3>d6>d5.

The above four different pump fiber types enable the corresponding pump fibers to transmit annular beams. When multiple interconnected pump fibers are combined with one signal fiber, an (N+1) type pump fiber design can be realized, as shown in Figure 5. Taking the solid core annular pump fiber in Example 3 as an example, corresponding (2+1), (3+1) and (6+1) type pump fibers can be designed. In addition, different types of pump fibers can be used according to target requirements or design parameter requirements, as shown in Figures 5 and 6.

It should be emphasized here that the above example only gives the simplest step refractive index design for the refractive index control scheme of the recessed area, but it is not limited to this. It is also possible to use a gradient refractive index for the recessed area. As long as the result of a refractive index recess is achieved, it can be classified into this category.

According to the above-mentioned design ideas of ring pump light and optical fiber, the design of high-power fiber laser is introduced by taking the (2+1) type central refractive index depression pump fiber (Example 2) as an example, as shown in Figure 6:

601 is the forward ring pump light (high power);

602 is the forward unabsorbed residual pump light (very low power);

603 is a reverse ring pump light (high power);

604 is the reverse non-absorbed participating pump light (extremely low power);

605 is a seed signal light (low power);

606 is the output signal light (high power) after amplification.

As shown in Figure 7, high-power annular pump light is injected into the central refractive index depressed pump fiber of the DSCPP fiber from the end face in the forward and reverse directions (Example 2), enters the signal fiber cladding through side coupling in the middle of the DSCPP fiber where it fits the signal fiber, and is then absorbed by the fiber core, thereby amplifying the seed signal light to form a high-power and high-brightness signal light output. The extremely low-power forward and reverse residual pump light that is not absorbed is output from the other side of the pump fiber.

An embodiment of the present invention also provides a fiber laser, as shown in FIG8 , comprising a ring-shaped pump light source, a laser signal light source and a pump fiber; the pump fiber comprises an outer cladding, a pump fiber and a signal fiber; the ring-shaped pump light source is connected to the signal fiber and transmits a laser signal therein; the ring-shaped pump light source is connected to the pump fiber and transmits a ring-shaped pump light therein.

The pump fiber may be a common pump fiber as described in Example 1, or may be a pump fiber as described in Examples 2 to 4.

When the pump optical fiber as described above is used, it has all the advantages described in the above embodiments, which will not be described in detail here.

In some embodiments, the ring pump light source includes a forward ring light pump source 708, a reverse ring light pump source 712, a reverse pump combiner 711, and a forward pump combiner 707; the forward ring light pump source 708, the forward pump combiner 707, and a portion of the pump fibers are sequentially connected; the reverse ring light pump source 712, the reverse pump combiner 711, and the remaining portion of the pump fibers are sequentially connected;

The laser signal light source includes a seed source 701, a first pre-amplifier 702, a second pre-amplifier 703, a mode field adapter 704 and a first cladding power stripper 705; the seed source 701, the first pre-amplifier 702, the second pre-amplifier 703, the mode field adapter 704, the first cladding power stripper 705 and the signal fiber are connected in sequence, and the signal fiber and the second cladding power stripper 713 and the collimating end cap 714 are connected in sequence.

The seed source 701, the first pre-amplifier 702, the second pre-amplifier 703, and the mode field adapter 704 can be replaced by modules that generate corresponding seed light, such as a resonant cavity, etc. The forward ring light pump source 708, the forward pump combiner 707, and the reverse ring light pump source 712, and the reverse pump combiner 711 in the figure can be used separately.

Among some alternatives:

The present invention can replace the annular light spot (annular pump light) from a vector light beam and a vortex light beam to any other type of light beam with an annular energy spot intensity.

The present invention can change the bidirectional pumping mode into a forward or reverse pumping mode.

The present invention can inject the annular light spot (annular pump light) into the pump optical fiber through other spatial optical devices rather than the all-optical fiber form.

The present invention can change the cross section of the pump fiber into other shapes besides circular.

The laser signal light source in the present invention may include multiple stages of pre-amplifiers, not limited to two stages, and may not even require a pre-amplifier.

The present invention uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the method and core ideas of the present invention. At the same time, for those skilled in the art, according to the ideas of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims (10)

1. A ring-pumped distributed side-pumped optical fiber, characterized in that it comprises: an outer cladding, a pump fiber and a signal fiber; the pump fiber and the signal fiber are both located in the outer cladding; a plurality of pump fibers are provided, and when only one pump fiber is provided, the pump fiber is a ring-core optical fiber with a central refractive index depression; when a plurality of pump fibers are provided, at least one pump fiber is a ring-core optical fiber with a central refractive index depression. 2. The ring-pumped distributed side-pumped optical fiber according to claim 1 is characterized in that: the ring-core optical fiber comprises an inner core and an outer ring core nested from inside to outside; the refractive index of the inner core is lower than that of the outer ring core, and the inner core forms a central refractive index depression area. 3. The ring-pumped distributed side-pumped optical fiber according to claim 1 is characterized in that: the ring-core optical fiber comprises an inner core, a middle ring core and an outer ring core which are nested in sequence from the inside to the outside; and the refractive index of the middle ring core, the inner core and the outer ring core gradually increases. 4 . The ring-pumped distributed side-pumped optical fiber according to claim 3 , wherein the middle ring core is an air core or a solid core. 5 . The ring-pumped distributed side-pumped optical fiber according to claim 1 , wherein the refractive index distribution of the ring-core optical fiber is a gradient type. 6 . The ring-pumped distributed side-pumped optical fiber according to claim 1 , wherein the refractive index distribution of the ring-core optical fiber is step-type. 7. The annularly pumped distributed side-pumped optical fiber according to claim 1, characterized in that when a plurality of pump fibers are provided, some of the pump fibers are ordinary optical fibers. 8. A fiber laser, characterized in that it includes a ring pump light source, a laser signal light source and a ring pump distributed side pump fiber; the ring pump distributed side pump fiber includes an outer cladding, a pump fiber and a signal fiber; the ring pump light source is connected to the signal fiber and transmits a laser signal therein; the ring pump light source is connected to the pump fiber and transmits a ring pump light therein. 9. The optical fiber laser according to claim 8, characterized in that: The annular pumped distributed side-pumped optical fiber adopts the annular pumped distributed side-pumped optical fiber according to any one of claims 1 to 7. 10. The optical fiber laser according to claim 8, characterized in that: The annular pump light source comprises a forward annular light pump source, a reverse annular light pump source, a reverse pump combiner and a forward pump combiner; the forward annular light pump source, the forward pump combiner and part of the pump fibers are connected in sequence; the reverse annular light pump source, the reverse pump combiner and the remaining part of the pump fibers are connected in sequence; The laser signal light source includes a seed source, a first pre-amplifier, a second pre-amplifier, a mode field adapter and a first cladding power stripper; the seed source, the first pre-amplifier, the second pre-amplifier, the mode field adapter, the first cladding power stripper and the signal fiber are connected in sequence, and the signal fiber, the second cladding power stripper and the collimating end cap are connected in sequence.