A three-beam fiber laser external cavity spectrum synthesis method and system for reducing spectrum interval

Abstract

The application discloses a three-beam fiber laser external cavity spectrum synthesis method with reduced spectrum interval, which comprises the following steps: firstly, two doped fiber arrays are used to form a resonant cavity among a front cavity mirror, a transform lens, a blazed grating and an output coupling mirror, and a laser oscillation method is used to synthesize a beam containing wavelengths of lambda 1 and lambda 2; secondly, a reflector Bragg grating with a spectrum selectivity less than delta lambda = lambda 2 - lambda 1 is connected in series in the two-beam laser external cavity spectrum synthesis system; finally, a third doped fiber array is used to form an external cavity laser system among a front cavity mirror, the reflector Bragg grating and an output coupling mirror, and a wavelength of lambda 0 of a beam output by the array is incident on the reflector Bragg grating at a Bragg angle, and a diffracted beam of the reflector Bragg grating is coaxially transmitted with the synthesized beam containing the wavelengths of lambda 1 and lambda 2, so that the three-beam laser external cavity spectrum synthesis with a reduced spectrum interval is realized. The application further discloses a three-beam fiber laser external cavity spectrum synthesis system with a reduced spectrum interval.

Description

Technical Field

[0001] This invention relates to the field of fiber laser spectral synthesis, and in particular to a method and system for spectral synthesis of three-beam fiber laser external cavity lasers with reduced spectral spacing. Background Technology

[0002] Laser combining is a technique that combines multiple laser beams into a single, brighter, and more powerful laser beam. It includes both coherent and incoherent combining methods. Spectral combining is a type of incoherent combining scheme, and its principle utilizes wavelength division multiplexing (WDM) technology from optical communication. Spectral combining has become an important approach to achieving high-power, high-brightness lasers, and can be implemented in two modes: with and without an external cavity.

[0003] For volume Bragg grating spectral synthesis without an external cavity, its scalability is limited because only two laser beams can be synthesized from one grating; while for spectral synthesis of multilayer electrolyte gratings, its structure is extremely complex due to the use of master oscillator power amplifier (MOPA).

[0004] Compared to spectral synthesis without an external cavity, spectral synthesis with an external cavity has three major advantages: system robustness, scalability, and quality stability of the synthesized beam. However, external cavity spectral synthesis also has significant drawbacks: due to the mode selection effect of the external cavity, the spectral spacing of the synthesized beam is too large, thus limiting the number of laser array elements that can be synthesized.

[0005] The following section details the reasons why external cavity spectral synthesis leads to excessively large spectral spacing:

[0006] Existing fiber laser external cavity spectral synthesis systems typically include a front cavity mirror, doped fiber, conversion lens, blazed grating, and output coupling mirror. For example... Figure 1 As shown.

[0007] For this synthesis system, the spectral spacing between any two laser beams during spectral synthesis can be approximated as:

[0008] Δλ≈Xdcosα / mF

[0009] In the formula: X is the spacing between the two doped fibers (when synthesizing spectra, the fibers are usually arranged closely in a row, so X is about the diameter of the fiber), d is the distance between the grooves of the grating, α is the incident angle of the incident laser relative to the normal of the grating surface, m is the diffraction order, and F is the focal length of the transforming lens.

[0010] This model shows that, given a fixed fiber element spacing, the spectral spacing of the synthesized beam is directly proportional to the grating slot spacing and inversely proportional to the focal length of the transforming lens. If the grating slot spacing is too small, it is easily damaged when the irradiation power is too high; if the lens focal length is too large, feedback crosstalk will reduce the quality of the synthesized beam. Therefore, d should not be too small, and F should not be too large, which inevitably leads to a relatively large spectral spacing of the synthesized beam.

[0011] for Figure 1 The external cavity spectral synthesis system shown, with a blazed grating line density of 400 lines / mm, an Er / Yb co-doped fiber diameter of 300 μm, and an F of 100 mm, was used to perform a three-channel laser spectral synthesis experiment. The resulting synthesized beam spectral spacing is as follows: Figure 2 As shown in the figure, λ c The center wavelength of the composite beam. Figure 2 It can be seen that the wavelength spacing of the synthesized beam reaches 7.9 nm. If the fluorescence spectrum range of the doped fiber is 35 nm, then the system can only synthesize a maximum of 5 laser beams.

[0012] Therefore, there is an urgent need to provide a method to reduce the spectral spacing and achieve external cavity spectral synthesis of more fiber laser beams within a limited range of doped fiber fluorescence spectra. Summary of the Invention

[0013] The technical problem to be solved by the present invention is to invent a method for reducing the spectral spacing of three-beam fiber laser external cavity spectrum synthesis, and to provide a synthesis system for implementing the method to reduce the spectral spacing.

[0014] To solve the above-mentioned technical problems, one technical solution adopted by the present invention is: to provide a method for external cavity spectral synthesis of three-beam fiber lasers with reduced spectral spacing, comprising the following steps:

[0015] First, using two doped fiber array elements, a beam containing wavelengths λ1 and λ2 is synthesized by laser oscillation in a resonant cavity formed between the front cavity mirror, the transformation lens, the blazed grating, and the output coupling mirror.

[0016] Secondly, in the two-way laser external cavity spectrum synthesis system, a reflector Bragg grating with a spectral selectivity less than Δλ is connected in series, where Δλ = λ2 - λ1, to ensure that the synthesized beam containing λ1 and λ2 can be completely transmitted.

[0017] Finally, an external cavity laser system is formed between the front cavity mirror, the reflector Bragg grating, and the output coupling mirror of the third doped fiber array element. The beam with wavelength λ0 and λ1 < λ0 < λ2 output by this array element is incident on the reflector Bragg grating at the Bragg angle, forcing its diffracted beam to be coaxially transmitted with the combined beam containing λ1 and λ2. This achieves external cavity spectral synthesis of three laser beams with reduced spectral spacing.

[0018] In a preferred embodiment of the present invention, in the two-channel laser external cavity spectrum synthesis system, the output end of the doped fiber array element and the blazed grating are respectively placed at one focal length on both sides of the transformation lens.

[0019] In a preferred embodiment of the present invention, in the two-channel laser external cavity spectral synthesis system based on a blazed grating, the spectral spacing Δλ between the two selected laser array elements in the external cavity is calculated using the following formula:

[0020] Δλ≈Xdcosα / mF

[0021] In the formula, X is the spacing between the two fiber array elements, d is the distance between the grooves of the blazed grating, α is the incident angle of the incident beam relative to the normal of the blazed grating surface, m is the diffraction order of the blazed grating, and F is the focal length of the transform lens.

[0022] In a preferred embodiment of the present invention, the reflector Bragg grating should satisfy the following two conditions:

[0023] (1) To ensure that the feedback beam can return to the doped fiber, the normal direction of the grating is perpendicular to the bulk surface;

[0024] (2) To ensure that the composite beam containing wavelengths λ1 and λ2 can be transmitted to the maximum extent, and the beam with wavelength λ0 can be diffracted to the maximum extent, the refractive index, thickness, refractive index modulation, and spatial frequency of the grating material must satisfy the following conditions:

[0025] Optimal transmission conditions:

[0026] Optimal diffraction conditions:

[0027] In the formula: t and δ represent the grating thickness and grating refractive index modulation, respectively; f is the spatial frequency; n is the refractive index of the grating material; and sinθ is the frequency. m =λ0f / 2n.

[0028] To facilitate the implementation of the above method, this invention also provides a three-beam fiber laser external cavity spectral synthesis system with reduced spectral spacing, comprising a first pump source, a first coupling system, a first front cavity mirror, and a first doped fiber array element; a second pump source, a second coupling system, a second front cavity mirror, and a second doped fiber array element; a third pump source, a third coupling system, a third front cavity mirror, and a third doped fiber array element; a transformation lens, a collimating lens, a blazed grating, a reflector Bragg grating, and an output coupling mirror;

[0029] The excitation output from the first pump source and the second pump source is coupled into the first doped fiber array element and the second doped fiber array element through the first coupling system and the second coupling system, respectively. The beams emitted from the first doped fiber array element and the second doped fiber array element are transmitted through the transformation lens and then output through the blazed grating diffraction and transmitted to the output coupling mirror. The two laser external cavity spectra with wavelengths of λ1 and λ2 are synthesized by laser oscillation.

[0030] In the external cavity spectral synthesis system of two laser beams with wavelengths of λ1 and λ2, a reflector Bragg grating is connected in series; the output end of the third doped fiber array element, the reflector Bragg grating, and the output coupling mirror constitute an external cavity laser system.

[0031] The excitation output from the third pump source is coupled into the third doped fiber array element via the third coupling system. The beam emitted from the third doped fiber array element with wavelength λ0 and λ1<λ0<λ2 is collimated by the collimating lens and then incident on the reflector Bragg grating at the Bragg angle, forcing its diffracted beam to propagate coaxially with the composite beam containing λ1 and λ2. Under the feedback of the output coupling mirror, the three laser beams are combined into a composite laser output with reduced spectral spacing.

[0032] In a preferred embodiment of the present invention, the reflector Bragg grating should satisfy the following two conditions:

[0033] (1) To ensure that the feedback beam can return to the doped fiber, the normal direction of the grating is perpendicular to the bulk surface;

[0034] (2) To ensure that the composite beam containing wavelengths λ1 and λ2 can be transmitted to the maximum extent, and the beam with wavelength λ0 can be diffracted to the maximum extent, the refractive index, thickness, refractive index modulation, and spatial frequency of the grating material must satisfy the following conditions:

[0035] Optimal transmission conditions:

[0036] Optimal diffraction conditions:

[0037] In the formula: t and δ represent the grating thickness and grating refractive index modulation, respectively; f is the spatial frequency; n is the refractive index of the grating material; and sinθ is the frequency. m =λ0f / 2n.

[0038] The beneficial effects of this invention are: this invention can reduce the spectral spacing of the synthesized beams. Through this method of reducing the spectral spacing, it is possible to achieve external cavity spectral synthesis of more fiber laser beams within a limited range of doped fiber fluorescence spectra. Attached Figure Description

[0039] Figure 1 This is a diagram of the optical path structure of an existing fiber laser external cavity spectral synthesis system;

[0040] Figure 2 It is based on Figure 1 A schematic diagram of the synthesized spectrum of the three laser external cavity spectra obtained from the experimental measurement of the system shown.

[0041] Figure 3 This is a diagram of the optical path structure of a two-beam fiber laser external cavity spectral synthesis system;

[0042] Figure 4 yes Figure 3 A schematic diagram showing the spectral composition of the combined external cavity spectra of the two laser beams.

[0043] Figure 5 Is Figure 3 A schematic diagram of the optical path of the series reflector Bragg grating in the system shown.

[0044] Figure 6 This is a schematic diagram of the optical path structure of the three-beam fiber laser external cavity spectral synthesis system with reduced spectral spacing described in this invention;

[0045] Figure 7 This is a schematic diagram illustrating the selection requirements for the Bragg grating of the reflector;

[0046] Figure 8 This is a schematic diagram of the three-beam laser synthesis spectrum achieved by reducing the spectral spacing in the external cavity.

[0047] The components in the attached diagram are labeled as follows: 1. First pump source, 2. First coupling system, 3. First front cavity mirror, 4. First doped fiber array element, 5. Second pump source, 6. Second coupling system, 7. Second front cavity mirror, 8. Second doped fiber array element, 9. Third pump source, 10. Third coupling system, 11. Third front cavity mirror, 12. Third doped fiber array element, 13. Transformer lens, 14. Collimating lens, 15. Blazed grating, 16. Reflector Bragg grating, 17. Output coupling mirror, 18. Feedback beam, 19. Grating normal. Detailed Implementation

[0048] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention.

[0049] Please see Figure 6 The embodiments of the present invention include:

[0050] A method for reducing the spectral spacing of three-beam fiber laser external cavity spectra includes the following steps:

[0051] In such Figure 3Based on the existing two-beam fiber laser external cavity spectral synthesis scheme shown, a third-beam laser external cavity spectral synthesis with a reduced spectral spacing is achieved by inserting a laser beam of a different wavelength between the two wavelengths of the synthesized laser. The implementation steps include:

[0052] First, two doped fiber elements are combined into a single beam containing wavelengths λ1 and λ2 within a resonant cavity formed by a front cavity mirror, a transforming lens, a blazed grating, and an output coupling mirror. This wavelength is determined by the angle of incidence of the output beam from the element after transformation by the transforming lens and onto the grating, as well as by the output coupling mirror of the resonant cavity. The spectral spacing of this selected wavelength is Δλ (Δλ = λ2 - λ1). This spectral spacing can be approximated using the formula Δλ = λ2 - λ1 ≈ Xdcosα / mF. In this formula, X is the spacing between the two fiber elements, d is the distance between the grooves of the blazed grating, α is the angle of incidence of the incident beam relative to the normal to the blazed grating surface, m is the diffraction order of the blazed grating, and F is the focal length of the transforming lens.

[0053] Secondly, in the aforementioned two-channel laser external cavity spectral synthesis system, a reflective Bragg grating with a spectral selectivity less than Δλ is connected in series, such as... Figure 5 As shown, since the spectral selectivity of the grating is between Δλ and Δλ, according to the transmission characteristics of the reflector Bragg grating, the composite beam containing λ1 and λ2 can be completely transmitted.

[0054] Finally, based on the diffraction characteristics of the reflector Bragg grating, the front cavity mirror, reflector Bragg grating, and output coupling mirror of the third doped fiber element can form a laser system with an external cavity based on the reflector Bragg grating. When the beam emitted from this fiber element (wavelength λ0, and λ1 < λ0 < λ2) is incident on the reflector Bragg grating at the Bragg angle and diffracted out, it can be coaxially transmitted with the synthesized beam containing wavelengths λ1 and λ2 and transmitted through the reflector Bragg grating, thereby realizing the external cavity spectral synthesis of three laser beams with reduced spectral spacing.

[0055] To ensure that a beam with wavelength λ0 can undergo laser oscillation and be spectrally combined with a beam containing wavelengths λ1 and λ2 with maximum efficiency, the reflector Bragg grating must satisfy the following three conditions:

[0056] Laser oscillation conditions: The normal direction of the grating is perpendicular to the volume surface of the grating;

[0057] Optimal transmission conditions:

[0058] Optimal diffraction conditions:

[0059] In the formula: λ0 is the wavelength of the diffracted light, t and δ are the grating thickness and grating refractive index modulation, respectively, f is the grating spatial frequency, n is the refractive index of the grating material, and sinθ is the wavelength of the diffracted light. m =λ0f / 2n.

[0060] See Figure 6 The present invention also provides a three-beam fiber laser external cavity spectral synthesis system with reduced spectral spacing, comprising a first pump source 1, a first coupling system 2, a first front cavity mirror 3, and a first doped fiber array element 4; a second pump source 5, a second coupling system 6, a second front cavity mirror 7, and a second doped fiber array element 8; a third pump source 9, a third coupling system 10, a third front cavity mirror 11, and a third doped fiber array element 12; a transformation lens 13, a collimating lens 14, a blazed grating 15, a reflector Bragg grating 16, and an output coupling mirror 17;

[0061] The excitation output from the first pump source 1 and the second pump source 5 is coupled into the first doped fiber array element 4 and the second doped fiber array element 8 through the first coupling system 2 and the second coupling system 6, respectively. The beams emitted from the first doped fiber array element 4 and the second doped fiber array element 8 are transmitted through the transformation lens 13 and then diffracted by the blazed grating 15 and transmitted to the output coupling mirror 17. The two laser external cavity spectra are synthesized by laser oscillation with wavelengths of λ1 and λ2, respectively.

[0062] In the external cavity spectral synthesis system of two laser beams with wavelengths of λ1 and λ2, a reflector Bragg grating 16 is connected in series; the output end of the third doped fiber array element 12, the reflector Bragg grating 16, and the output coupling mirror 17 constitute an external cavity laser system.

[0063] The excitation output from the third pump source 10 is coupled into the third doped fiber array element 12 via the third coupling system 9. The beam emitted from the third doped fiber array element 12 with wavelength λ0 and λ1<λ0<λ2 is collimated by the collimating lens 14 and then incident on the reflector Bragg grating 16 at the Bragg angle, forcing its diffracted beam to be coaxially transmitted with the composite beam containing λ1 and λ2. Under the feedback action of the output coupling mirror 17, the three laser beams are combined into a composite laser output with reduced spectral spacing.

[0064] The implementation process of a three-beam laser external cavity spectral synthesis system for reducing spectral spacing, as provided in the present invention, will be described in detail below with reference to the accompanying drawings.

[0065] 1. Figure 3This paper presents a method for combining two laser beams using an external cavity comprised of a transforming lens 13, a blazed grating 15, and an output coupling mirror 17, based on an existing two-doped fiber array. In the figure, the coupling system (composed of two collimating lenses) efficiently couples the pump energy into the doped fiber. Due to the different arrangements of the doped fiber array elements, the wavelength of the selected beam in the external cavity will inevitably differ. Furthermore, factors such as the focal length of the transforming lens and the grating markings will inevitably result in a relatively large spectral spacing in this combining scheme. Figure 4 A spectral graph of the synthesis scheme is presented.

[0066] 2. To reduce the spectral spacing in external cavity spectrum synthesis, the solution provided in this invention is: Figure 3 A reflector Bragg grating 16 with spectral selectivity less than Δλ is connected in series in the external cavity, as shown. Figure 5 As shown. Because the grating's spectral selectivity is less than Δλ, the beam containing wavelengths λ1 and λ2 can be completely transmitted.

[0067] 3. The output beam of the third doped fiber element 12 is incident on the emitter Bragg grating 16 at a Bragg angle, forcing its diffracted beam to propagate coaxially with the beam containing wavelengths λ1 and λ2, as shown below. Figure 6 As shown. To generate laser oscillation, a laser resonant system must be formed between the front cavity mirror of the third doped fiber array element 12, the reflector Bragg grating 16, and the output coupling mirror 17.

[0068] 4. To ensure that the output beam (wavelength λ0) of the third doped fiber element 12 can be coaxially transmitted with the beam containing wavelengths λ1 and λ2, and that laser oscillation can be formed between the third front cavity mirror 11 and the output coupling mirror 17 of the third doped fiber element 12, the reflector Bragg grating 16 must satisfy two conditions: the diffraction angle of the beam diffracted with wavelength λ0 is equal to the transmission angle of the beam containing wavelengths λ1 and λ2; and the beam with wavelength λ0 fed back by the output coupling mirror 17 can return to the third doped fiber element 12. This condition is achieved by ensuring that the direction of the grating normal 19 is perpendicular to the bulk surface, such as... Figure 7 As shown, 18 is the feedback beam.

[0069] 5. The reflector Bragg grating 16 satisfies Figure 7 In the scenario shown, an external cavity is used, and laser oscillation is employed to obtain a composite spectrum of three laser beams with reduced spectral spacing. The composite spectrum is as follows: Figure 8 As shown.

[0070] 6. To ensure maximum combining efficiency of the synthesized beam, the thickness, refractive index modulation, spatial frequency, and refractive index of the grating material of the reflector Bragg grating 16 should also meet the following conditions:

[0071] Optimal transmission conditions:

[0072] Optimal diffraction conditions:

[0073] In the formula: θ m Let λ be the Bragg angle, λ0 be the wavelength of the diffracted light, t and δ be the grating thickness and grating refractive index modulation, respectively, f be the grating spatial frequency, n be the refractive index of the grating material, and sinθ be the wavelength of the diffracted light. m =λ0f / 2n.

[0074] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A three-beam fiber laser external cavity spectrum synthesis method for reducing spectral spacing, characterized in that, The method comprises the following steps: First, two doped fiber arrays are used to combine two beams of wavelengths , into one beam in a resonant cavity formed by a front mirror, a transform lens, a blazed grating, and an output coupler. Secondly, in the two-way laser external cavity spectrum synthesis system, a reflection Bragg grating with a spectrum selectivity less than is connected in series, wherein , so as to ensure that the synthesized light beam containing , can be completely transmitted. Finally, between the output end of the third doped fiber array element, the reflector Bragg grating and the output coupling mirror, an external cavity laser system is formed, so that the light beam output by the array element, with a wavelength of and , is incident on the reflector Bragg grating at a Bragg angle, and the diffracted light beam is coaxially transmitted with the combined light beam containing , , so that the three laser beams realize external cavity spectrum synthesis with reduced spectral spacing. The reflection Bragg grating should satisfy the following two conditions: (1) To ensure that the feedback beam can return to the doped fiber, the normal direction of the grating is perpendicular to the surface of the body; (2) To ensure that the synthetic light beam containing wavelengths of , is maximally transmitted, and the light beam with wavelength of is maximally diffracted, the material refractive index, thickness, refractive index modulation, and spatial frequency of the grating satisfy the following conditions: Optimum transmission conditions: Optimum diffraction conditions: wherein: t , are the grating thickness and grating refractive index modulation, respectively, is the spatial frequency, n is the refractive index of the grating material, .

2. The reduced-linewidth three-beam fiber laser external-cavity spectrum combining method of claim 1, wherein, In the two-laser external cavity spectrum synthesis system, the output end of the doped fiber array element and the blazed grating are respectively placed at one focal length on both sides of the conversion lens.

3. The reduced-linewidth three-beam fiber laser external-cavity spectrum combining method of claim 1, wherein, In a two-laser external cavity spectrum synthesis system based on blazed grating, the wavelength of two selected laser array elements in the external cavity, and the spectral interval of the two selected laser array elements are calculated by the following formula: wherein X is the pitch between two fiber array elements, d is the distance between the grooves of the blazed grating, is the angle of incidence of the incident beam with respect to the surface normal of the blazed grating, m is the diffraction order of the blazed grating, F is the focal length of the transform lens.

4. A three-beam fiber laser external cavity spectrum combining system for reducing spectral spacing, characterized in that, It comprises a first pump source, a first coupling system, a first front cavity mirror, a first doped fiber array element; a second pump source, a second coupling system, a second front cavity mirror, a second doped fiber array element; a third pump source, a third coupling system, a third front cavity mirror, a third doped fiber array element; a conversion lens, a collimating lens, a blazed grating, a reflection Bragg grating, and an output coupling mirror; The excitation outputted by the first pump source and the second pump source is coupled into the first doped fiber array and the second doped fiber array through the first coupling system and the second coupling system respectively, and the light beams outputted by the first doped fiber array and the second doped fiber array are transmitted through the transform lens and then diffracted by the blazed grating to be transmitted to the output coupling mirror, so that the wavelength of two beams of laser external cavity spectrum synthesis is realized through laser oscillation 、 . In the wavelength of the two laser external cavity spectrum synthesis system respectively , A block of reflection Bragg grating is connected in series; an external cavity laser system is formed among the output end of the third doped fiber array element, the reflection Bragg grating and the output coupling mirror. The excitation output by the third pump source is coupled into the third doped fiber array through a third coupling system, and the wavelength of the light beam emitted by the third doped fiber array is and The light beam is collimated by a collimating lens, and then is incident on the reflection Bragg grating at a Bragg angle, and forces the diffracted light beam to coaxially transmit with a synthetic light beam containing 、 Under the feedback of the output coupling mirror, the three laser beams are combined into one synthetic laser output with a reduced spectral interval. The reflection Bragg grating should satisfy the following two conditions: (1) To ensure that the feedback beam can return to the doped fiber, the normal direction of the grating is perpendicular to the surface of the body; (2) To ensure that the synthetic light beam containing wavelengths of , can be maximally transmitted, and the light beam with wavelength of can be maximally diffracted, the refractive index, thickness, refractive index modulation, and spatial frequency of the grating material satisfy the following conditions: Optimum transmission conditions: Optimum diffraction conditions: wherein: t , are the grating thickness and grating refractive index modulation, respectively, is the spatial frequency, n is the refractive index of the grating material, .

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