Cladding fiber grating and fiber laser

Abstract

The invention belongs to the technical field of fiber lasers, and discloses a cladding fiber grating which comprises a cladding area and a fiber core area, and the cladding area is inscribed to form a cladding grating of a periodic structure. The cladding fiber bragg grating is adopted, residual pump light which is not absorbed in the fiber laser is reflected back to the gain fiber to be absorbed again, the length of the gain fiber in the fiber laser can be shortened to 30%-50% of the length of the gain fiber in a traditional scheme, and the cost of the fiber laser is reduced; meanwhile, the gain optical fiber with the short length can effectively reduce accumulation of the nonlinear effect, and laser output with higher power can be achieved.

Description

Technical Field

[0001] This invention belongs to the field of fiber laser technology and relates to a cladding fiber grating and a fiber laser. Background Technology

[0002] Fiber lasers possess advantages such as high output power, good beam quality, and high electro-optical conversion efficiency, making them widely used in industrial processing, laser weapons, and optoelectronic warfare. In traditional fiber lasers, the pump light is transmitted through the cladding of the gain fiber, where rare-earth ions in the fiber core absorb the pump light and amplify the signal light. However, limited by pump absorption efficiency, the gain fiber requires a relatively long length (typically tens of meters) to fully absorb the pump energy, leading to increased nonlinear effects such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), which restricts the improvement of output power. Simultaneously, the system size and cost also increase; long gain fibers result in a bulky laser structure and higher material costs.

[0003] In existing technologies, signal light is reflected by writing Bragg gratings (FBGs) into the fiber core, but this cannot solve the problem of pump light residue. The only way to remove it is by etching the wool cladding fiber, but this still cannot solve the problem of nonlinear effect accumulation, which limits the development of high-power lasers.

[0004] The nonlinear effects of fiber lasers are usually positively correlated with the fiber length. Reducing the fiber length of fiber lasers is an effective way to reduce the accumulation of nonlinear effects and achieve high power output. Summary of the Invention

[0005] (a) Purpose of the invention Current fiber lasers are limited by nonlinear effects, making it difficult to achieve higher power output. This invention provides a cladding fiber grating structure and a fiber laser. By using a cladding fiber grating, the absorption of pump light by the laser can be improved, thereby shortening the length of the laser gain fiber, reducing the accumulation of laser nonlinearity, and achieving higher power output.

[0006] (II) Technical Solution To solve the above-mentioned technical problems, the present invention provides a cladding fiber grating, which has a cladding region and a core region, wherein the cladding region is inscribed to form a periodic structure of the cladding grating.

[0007] Furthermore, a core grating with a periodic structure is inscribed in the core region.

[0008] Furthermore, the cladding fiber grating is formed in the cladding region using photomask lithography, femtosecond laser direct writing, or CO2 laser thermal shock methods to create a periodic structure.

[0009] Furthermore, the cladding fiber grating is made of silica fiber, fluoride fiber, or chalcogenide fiber.

[0010] Furthermore, the period Λ of the cladding fiber grating is matched with the pump center wavelength λ_p, and the period Λ = λ_p / (2 * n_eff), where λ_p is the pump center wavelength and n_eff is the effective refractive index of the fiber cladding.

[0011] This invention also provides a single-cavity fiber laser using a cladding fiber grating, comprising: pump light, a pump combiner, a high-reflectivity cavity mirror, a gain fiber, a low-reflectivity cavity mirror, and a cladding fiber grating; the pump light provides pump excitation for the single-cavity fiber laser and is composed of a semiconductor laser or a fiber laser; the pump light is coupled into the single-cavity fiber laser through the pump combiner, the gain fiber absorbs the pump light and converts it into a signal laser, which oscillates and outputs in the resonant cavity composed of the high-reflectivity cavity mirror and the low-reflectivity cavity mirror; the residual pump light that is not completely absorbed and the signal laser emitted through the low-reflectivity cavity mirror are emitted together to the cladding fiber grating, the cladding fiber grating is highly transparent to the signal laser but highly reflective to the pump light, the signal laser continues to propagate forward, and the residual pump light that is not absorbed is reflected back into the resonant cavity by the cladding fiber grating and absorbed again by the gain fiber, thereby increasing the laser output power.

[0012] The present invention also provides a master oscillating fiber amplifier using a cladding fiber grating, comprising: pump light, pump combiner, gain fiber, cladding fiber grating, and seed laser; the pump light is coupled into the master oscillating fiber amplifier through the pump combiner, and the gain fiber absorbs the pump light and converts it into signal laser, thereby shortening the length of the gain fiber and amplifying the output of the signal laser; the cladding fiber grating is highly transparent to the signal laser but highly reflective to the pump light, so the signal laser emitted by the seed laser or the amplified signal laser passes through the cladding fiber grating and can continue to propagate forward, while the pump light is reflected back by the cladding fiber grating.

[0013] Furthermore, in the master oscillator fiber amplifier of the forward pump structure, the unabsorbed forward residual pump light and the signal laser amplified by the fiber amplifier are emitted together to the cladding fiber grating. The signal laser continues to propagate forward, while the unabsorbed forward residual pump light is reflected back into the amplifier by the cladding fiber grating and absorbed again by the gain fiber, thereby shortening the length of the gain fiber and amplifying the output of the signal laser.

[0014] Furthermore, in the master oscillating fiber amplifier of the reverse pump structure, the signal laser emitted by the seed laser is emitted to the cladding fiber grating and continues to propagate forward into the fiber amplifier where it is amplified. The unabsorbed reverse residual pump light is emitted to the cladding fiber grating and is reflected back into the amplifier by the cladding fiber grating, where it is absorbed again by the gain fiber, thereby shortening the length of the gain fiber and amplifying the output of the signal laser.

[0015] Furthermore, in the master oscillator fiber amplifier of the bidirectional pump structure, the signal laser emitted by the seed laser is emitted to the left cladding fiber grating, continues to propagate forward, enters the fiber amplifier and is amplified, and then continues to propagate forward through the right cladding fiber grating; the unabsorbed forward residual pump light is reflected by the right cladding fiber grating, and the unabsorbed reverse residual pump light is reflected by the left cladding fiber grating, both returning to the fiber amplifier and being absorbed again by the gain fiber; thus achieving the shortening of the gain fiber length and the amplification of the signal laser output.

[0016] (III) Beneficial Effects The cladding fiber grating and fiber laser provided by the above technical solution use a cladding fiber grating to reflect the unabsorbed residual pump light in the fiber laser back into the gain fiber for reabsorption. This can shorten the length of the gain fiber in the fiber laser to 30%-50% of that in traditional solutions, reducing the cost of the fiber laser. At the same time, the shorter length of the gain fiber can effectively reduce the accumulation of nonlinear effects, which helps to achieve higher power laser output. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of a cladding fiber Bragg grating; Figure 2 This is a schematic diagram of a fiber optic grating; Figure 3 This is a schematic diagram of a single-cavity fiber laser using a cladding fiber grating according to an embodiment of the present invention; Figure 4 , Figure 5 and Figure 6 This is a schematic diagram of the application of a cladding fiber grating in the master oscillation fiber amplifier according to an embodiment of the present invention; The markings in the diagram are as follows: 1 is the pump light, 2 is the pump combiner, 3 is the high-reflection cavity mirror, 4 is the gain fiber, 5 is the low-reflection cavity mirror, 6 is the cladding fiber grating, and 7 is the seed laser. Detailed Implementation

[0018] To make the objectives, contents, and advantages of the present invention clearer, the specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0019] Example 1 like Figure 1 and Figure 2 As shown, the cladding fiber grating in this embodiment includes a cladding region and a core region, and the cladding region is inscribed to form a periodic structure of the cladding grating.

[0020] In this process, a core grating with a periodic structure is inscribed in the core region.

[0021] The cladding fiber grating in this embodiment differs from the traditional core fiber grating, which typically only has a grating structure in the fiber core.

[0022] In this embodiment, the reflection wavelength is matched: the cladding fiber grating period is designed to reflect the pump wavelength, rather than the signal light wavelength or other wavelengths; the signal light transparency is achieved: the cladding fiber grating has high reflectivity to the residual pump light and high transmittance to the signal light, ensuring low-loss transmission of the signal light; the application location is determined by the fact that the cladding fiber grating is used to reflect the residual pump light, therefore its application location should be at one end of the gain fiber, opposite to the direction of pump light transmission.

[0023] The cladding fiber grating is formed on the fiber cladding using methods including, but not limited to, photolithography, femtosecond laser direct writing, or CO2 laser thermal shock.

[0024] The materials used for cladding fiber Bragg gratings include, but are not limited to, silica optical fibers, fluoride optical fibers, and chalcogenide optical fibers.

[0025] The period Λ of the cladding fiber grating is matched with the pump center wavelength λ_p. The period Λ = λ_p / (2 * n_eff), where λ_p is the pump center wavelength and n_eff is the effective refractive index of the fiber cladding (e.g., Λ = 337nm @ λ_p = 976nm, n_eff = 1.45).

[0026] The pump wavelength reflected by the cladding fiber grating can be a single wavelength or multiple wavelengths, and the pump center wavelength includes, but is not limited to, 915nm, 976nm, 1018nm, etc.

[0027] The pump wavelength bandwidth reflected by the cladding fiber grating can be designed for either narrowband or broadband.

[0028] Example 2 like Figure 3 As shown, this embodiment provides a single-cavity fiber laser using a cladding fiber grating, including: pump light 1, pump combiner 2, high-reflection cavity mirror 3, gain fiber 4, low-reflection cavity mirror 5, and cladding fiber grating 6.

[0029] Pump light 1 provides pump excitation for a single-cavity fiber laser and is typically composed of a semiconductor laser or a fiber laser.

[0030] Pump light 1 is coupled into a single-cavity fiber laser through pump combiner 2. Gain fiber 4 absorbs pump light 1 and converts it into signal laser, which then oscillates and outputs in a resonant cavity composed of high-reflection cavity mirror 3 and low-reflection cavity mirror 5.

[0031] The residual pump light that is not completely absorbed and the signal laser emitted through the low-reflection cavity mirror 5 are emitted together to the cladding fiber grating 6. The cladding fiber grating 6 is highly transparent to the signal laser but highly reflective to the pump light. The signal laser can continue to propagate forward, while the residual pump light that is not absorbed is reflected back into the resonant cavity by the cladding fiber grating 6 and absorbed again by the gain fiber, thereby increasing the laser output power.

[0032] To better understand the role of cladding fiber gratings in reducing the length of gain fiber, let's take a certain ytterbium-doped fiber as an example. Its absorption coefficient is about 1.5 dB / m, the gain fiber length is 10 m, and the provided gain coefficient is 15 dB, with about 97% of the pump light being absorbed.

[0033] When using a cladding fiber Bragg grating, only half the length of the original gain fiber, i.e., 5m, is needed. When the pump light passes through the 5m gain fiber for the first time, the gain factor provided is 7.5dB, and about 82% of the pump light is absorbed, leaving 18% of the pump light as residual pump light. After reflection by the cladding fiber Bragg grating, the unabsorbed residual pump light passes through the 5m gain fiber again, and about (1-82%)*82%=15% of the pump light is absorbed.

[0034] The total absorption after passing through the gain fiber twice is 82% + 15% = 97%, which is consistent with the absorption effect of passing through the 10m gain fiber once. This means that using cladding fiber can effectively reduce the length of the gain fiber, thereby reducing nonlinear effects in the fiber laser and achieving greater laser output power.

[0035] Example 3 like Figure 4 or Figure 5 or Figure 6 As shown, this embodiment provides a master oscillating fiber amplifier using a cladding fiber grating, comprising: pump light 1, pump combiner 2, gain fiber 4, cladding fiber grating 6, and seed laser 7.

[0036] Figure 4 It is a master oscillator fiber amplifier with a forward-pumped structure.

[0037] Figure 5 It is a master oscillator fiber amplifier with a reverse-pumped structure.

[0038] Figure 6 It is a master oscillator fiber amplifier with a bidirectional pump structure.

[0039] The seed laser 7 provides the signal laser to the main oscillating fiber amplifier.

[0040] Pump light 1 provides pump excitation for a single-cavity fiber laser and is typically composed of a semiconductor laser or a fiber laser.

[0041] Pump light 1 is coupled into the master oscillating fiber amplifier through pump combiner 2. Gain fiber 4 absorbs pump light 1 and converts it into signal laser, thereby shortening the length of the gain fiber and amplifying the output of the signal laser.

[0042] The cladding fiber grating 6 has high transmittance for the signal laser but high reflectivity for the pump light. The signal laser emitted by the seed laser 7, or the amplified signal laser, can easily pass through the cladding fiber grating 6 and continue to propagate forward. The pump light, however, will be reflected back by the cladding fiber grating.

[0043] like Figure 4 As shown, in the master oscillating fiber amplifier of the forward pump structure, the unabsorbed forward residual pump light and the signal laser amplified by the fiber amplifier are emitted together to the cladding fiber grating 6. The signal laser can continue to propagate forward, while the unabsorbed forward residual pump light is reflected back into the amplifier by the cladding fiber grating 6 and absorbed again by the gain fiber, thereby shortening the length of the gain fiber and amplifying the output of the signal laser.

[0044] Similarly, such as Figure 5 As shown, in the master oscillating fiber amplifier with a reverse pump structure, the signal laser emitted by the seed laser 7 is emitted to the cladding fiber grating 6 and can continue to be transmitted forward and amplified in the fiber amplifier. The unabsorbed reverse residual pump light is emitted to the cladding fiber grating 6 and is reflected back into the amplifier by the cladding fiber grating 6, where it is absorbed again by the gain fiber, thereby shortening the length of the gain fiber and amplifying the output of the signal laser.

[0045] Similarly, such as Figure 6 As shown, in the master oscillator fiber amplifier with a bidirectional pump structure, the signal laser emitted by the seed laser 7 exits to the left cladding fiber grating 6, where it can continue to propagate forward and enter the fiber amplifier for amplification. It then continues to propagate forward through the right cladding fiber grating 6. Unabsorbed forward residual pump light is reflected by the right cladding fiber grating 6, and unabsorbed reverse residual pump light is reflected by the left cladding fiber grating 6, both returning to the fiber amplifier and being absorbed again by the gain fiber; thus shortening the length of the gain fiber and amplifying the signal laser output.

[0046] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A cladding fiber grating, characterized in that, It includes a cladding region and a core region, with the cladding region inscribed to form a periodic cladding grating.

2. The cladding fiber grating as described in claim 1, characterized in that, The core region is inscribed with a core grating to form a periodic structure.

3. The cladding fiber grating as described in claim 2, characterized in that, The cladding fiber grating is formed in a periodic structure in the cladding region by means of photomask lithography, femtosecond laser direct writing or CO2 laser thermal shock.

4. The cladding fiber grating as described in claim 3, characterized in that, The cladding fiber grating is made of silica fiber, fluoride fiber, or chalcogenide fiber.

5. The cladding fiber grating as described in claim 4, characterized in that, The period Λ of the cladding fiber grating is matched with the pump center wavelength λ_p, and the period Λ = λ_p / (2 * n_eff), where λ_p is the pump center wavelength and n_eff is the effective refractive index of the fiber cladding.

6. A single-cavity fiber laser employing the cladding fiber grating of claim 5, characterized in that, include: Pump light, pump combiner, high-reflection cavity mirror, gain fiber, low-reflection cavity mirror, and cladding fiber grating; The pump light provides pump excitation for the single-cavity fiber laser and is composed of semiconductor laser or fiber laser. The pump light is coupled into the single-cavity fiber laser through a pump combiner. The gain fiber absorbs the pump light and converts it into signal laser, which oscillates and outputs in the resonant cavity composed of high-reflection cavity mirror and low-reflection cavity mirror. The residual pump light that is not completely absorbed and the signal laser emitted through the low-reflection cavity mirror are emitted together to the cladding fiber grating. The cladding fiber grating is highly transparent to the signal laser but highly reflective to the pump light. The signal laser continues to propagate forward, while the residual pump light that is not absorbed is reflected back into the resonant cavity by the cladding fiber grating and is absorbed again by the gain fiber, thereby increasing the laser output power.

7. A master oscillator fiber amplifier using the cladding fiber grating of claim 5, characterized in that, include: Pump light, pump combiner, gain fiber, cladding fiber grating, and seed laser; the pump light is coupled into the master oscillating fiber amplifier through the pump combiner, and the gain fiber absorbs the pump light and converts it into a signal laser, thereby shortening the length of the gain fiber and amplifying the output of the signal laser; the cladding fiber grating is highly transparent to the signal laser but highly reflective to the pump light, so the signal laser emitted by the seed laser or the amplified signal laser passes through the cladding fiber grating and can continue to propagate forward, while the pump light is reflected back by the cladding fiber grating.

8. The master oscillator fiber amplifier with cladding fiber grating as described in claim 7, characterized in that, In the master oscillator fiber amplifier with a forward pump structure, the unabsorbed forward residual pump light and the signal laser amplified by the fiber amplifier are emitted together to the cladding fiber grating. The signal laser continues to propagate forward, while the unabsorbed forward residual pump light is reflected back into the amplifier by the cladding fiber grating and absorbed again by the gain fiber, thereby shortening the length of the gain fiber and amplifying the output of the signal laser.

9. The master oscillator fiber amplifier with cladding fiber grating as described in claim 7, characterized in that, In the master oscillating fiber amplifier with a reverse pump structure, the signal laser emitted by the seed laser is emitted to the cladding fiber grating and continues to propagate forward into the fiber amplifier where it is amplified. The unabsorbed reverse residual pump light is emitted to the cladding fiber grating and is reflected back into the amplifier, where it is absorbed again by the gain fiber, thus shortening the length of the gain fiber and amplifying the output of the signal laser.

10. The master oscillator fiber amplifier with cladding fiber grating as described in claim 7, characterized in that, In the master oscillator fiber amplifier with a bidirectional pump structure, the signal laser emitted by the seed laser is emitted to the left cladding fiber grating, continues to propagate forward, enters the fiber amplifier and is amplified, and then continues to propagate forward through the right cladding fiber grating. The unabsorbed forward residual pump light is reflected by the right cladding fiber grating, and the unabsorbed reverse residual pump light is reflected by the left cladding fiber grating, both returning to the fiber amplifier and being absorbed again by the gain fiber. This achieves the shortening of the gain fiber length and the amplification of the signal laser output.

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