A method and system for external cavity spectrum synthesis of N-channel to 2N-1-channel fiber lasers

Abstract

This invention discloses an external cavity spectrum synthesis method and system for expanding N-channel fiber lasers to 2N-1-channel, relating to the field of fiber laser spectrum synthesis. Based on the existing external cavity spectrum synthesis scheme for N-channel fiber laser array elements limited by blazed grids, this invention achieves external cavity spectrum synthesis of 2N-1 fiber lasers by inserting a laser beam with a different wavelength between each two adjacent wavelengths of the synthesized laser, thus solving the problem of the limited number of laser array elements in current external cavity spectrum synthesis.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of fiber laser spectrum synthesis, in particular to an external cavity spectrum synthesis method and system for expanding N-way to 2N-1 way fiber laser. BACKGROUND

[0002] Laser synthesis is a technology of splicing multiple lasers into a laser with higher brightness and power, including coherent and incoherent synthesis. Due to the continuous improvement of grating performance, spectrum synthesis in incoherent synthesis has become an important way to realize high-power and high-brightness laser.

[0003] Spectrum synthesis includes two synthesis methods with and without external cavity. Compared with spectrum synthesis without external cavity, spectrum synthesis with external cavity has advantages such as robustness, easy scalability, stability of synthesized beam quality, etc. However, spectrum synthesis with external cavity also has obvious defects: within the limited fluorescence spectrum range of doped fiber, due to passive mode selection of the external cavity, the spectral spacing between adjacent wavelengths of the synthesized laser is large, thereby limiting the number of array elements of the synthesized laser.

[0004] Assuming the focal length of the transform lens is f, the interval between the array elements is X, the grating constant is d, and the fluorescence spectrum bandwidth is Δλ max , then when 2W+1 array elements (W is the number of array elements arranged axially upward and downward by the system) are synthesized, calculation shows that the maximum value of W is:

[0005]

[0006] In the formula, represents the integer. If the fluorescence spectrum width of the doped fiber is 40 nm, the line density of the grating is 1200 line / mm, the focal length of the transform lens is 100 mm, and the interval X between the array elements (the minimum interval is the fiber diameter) is 450 μm, then the external cavity spectrum synthesis system can at most realize spectrum synthesis of 11 fiber lasers. Further, according to the approximate formula Δλ≈Xd / F of the spectral spacing between adjacent wavelengths, it can be calculated that the spectral spacing between adjacent wavelengths is about 3.75 nm.

[0007] The above calculation shows that the key factor limiting the number of array elements of the external cavity spectrum synthesis laser is the spectral spacing between adjacent wavelengths of the synthesized beam. According to the formula Δλ≈Xd / F, the spectral spacing of the synthesized beam is proportional to the grating groove spacing and inversely proportional to the focal length of the transform lens when the array element spacing is determined. Considering that the grating is easily damaged when the grating groove spacing is too small, and that the feedback crosstalk is caused when the focal length of the transform lens is too large, thereby reducing the beam quality, etc. Therefore, d should not be too small and F should not be too large, which inevitably makes the spectral spacing of the synthesized beam larger, thereby limiting the number of array elements of the synthesized laser.

[0008] To address the limitation on the number of laser array elements in external cavity spectral synthesis, there is an urgent need to invent a method and system for achieving multi-channel fiber laser external cavity spectral synthesis within a limited fluorescence spectrum range by reducing the spectral spacing. Summary of the Invention

[0009] The purpose of this invention is to provide a method and system for external cavity spectrum synthesis of fiber lasers that expands from N channels to 2N-1 channels, that is, to provide a method and system that breaks through the current limit on the number of external cavity spectrum synthesis elements and solves the problem of the limited number of laser array elements in current external cavity spectrum synthesis.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] Based on the existing scheme of external cavity spectrum synthesis of N-channel fiber laser array elements with blazed grating, a method is used to achieve external cavity spectrum synthesis of 2N-1 fiber lasers by inserting a laser with a different wavelength between each two adjacent wavelengths of the synthesized laser.

[0012] A method for external cavity spectrum synthesis of N-channel fiber lasers to 2N-1-channel lasers, the specific steps of which are as follows:

[0013] S1. Describe an existing extreme N-beam fiber laser external cavity spectral synthesis scheme. N doped fiber elements are arranged in a resonant cavity formed between the front cavity mirror, transformation lens, blazed grating, and output coupling mirror. Wavelengths λ1, λ2, ..., λ are selected in a resonant manner. N The spectra are combined into a beam containing wavelengths λ1, λ2, ..., λ N In the synthesized beam of N wavelengths, the spectral spacing Δλ (Δλ = λ) between selected adjacent wavelengths is... n -λ n-1 The distance between two fiber array elements (n = 2, ..., N) can be obtained by the formula Δλ≈Xd / F, where X is the distance between the two fiber array elements, d is the distance between the grooves of the blazed grating, i.e., the grating constant, and F is the focal length of the transform lens.

[0014] S2. In the above-mentioned extreme N-beam fiber laser external cavity spectral synthesis system, a block such as... is first connected in series. Figure 3 The reflector Bragg grating 1 shown has a diffraction wavelength of λ′1=(λ2+λ1) / 2 and a spectral selectivity less than Δλ;

[0015] S3. A laser system with an external cavity and based on a reflector Bragg grating is formed between the front cavity mirror, the reflector Bragg grating, and the output coupling mirror of the inserted doped fiber array element 1. The output beam λ′1 of the inserted doped fiber array element 1 is collimated by a collimating lens and then incident on the reflector Bragg grating at a Bragg angle, resulting in the same Bragg diffraction output. Simultaneously, the beam containing wavelengths λ1, λ2, ..., λ is forced to... NThe beam also passes through the volume Bragg grating at the same Bragg angle, thus the original N-channel fiber laser external cavity spectrum synthesis becomes N+1-channel fiber laser spectrum synthesis.

[0016] S4. Following the methods in steps S2 and S3 above, connect the reflector Bragg gratings 2, ..., N-1 in series, and according to... Figure 4 The method shown constructs N-2 external cavity laser systems based on reflector Bragg gratings between the inserted doped fiber array elements 2,...,N-1, the front cavity mirror, the reflector Bragg grating, and the output coupling mirror, thereby achieving 2N-1-channel fiber laser external cavity spectrum synthesis.

[0017] To ensure that a beam with wavelength λ′1 can undergo laser oscillation and can interact with wavelengths λ1, λ2, ..., λ N The beams are transmitted coaxially to become a single beam. The reflector Bragg grating 1 must satisfy the following conditions: (1) the normal direction of the grating must be perpendicular to the surface of the body; (2) it contains wavelengths λ1, λ2, ..., λ N The incident angle of the beam and the diffraction angle of the λ′1 diffracted beam must be equal; (3) For a diffracted beam with wavelength λ′1, its Bragg angle is θ. m =arcsinλ′1f1 / 2n, where f1 is the spatial frequency of the volume Bragg grating 1, and n is the refractive index of the grating material.

[0018] To ensure the successful implementation of the above synthesis scheme, all N-1 reflector Bragg gratings must satisfy the following conditions: (1) the normal direction of the grating is perpendicular to the bulk surface. This condition is to ensure that the beam fed back by the output coupling mirror can smoothly return to the corresponding doped fiber; (2) the Bragg angles of all N-1 reflector Bragg gratings must be equal, such as... Figure 6 As shown.

[0019] If the same material is used to make the grating, the Bragg angle of the grating must satisfy:

[0020] θ m =arcsinλ′1f1 / 2n=arcsinλ′2f2 / 2n…=arcsinλ′ N-1 f N-1 / 2n,

[0021] In the formula, n is the refractive index of the grating material, f1, f2, ... f N-1 Let λ'1, λ'2, ..., λ'N-1 be the spatial frequencies of the inserted reflector Bragg gratings 1, 2, ..., N-1, and let λ'1 = (λ2 + λ1) / 2; λ'2 = (λ3 + λ2) / 2; ...; λ'N-1 be the wavelengths of the inserted reflectors. N-1 =(λ N +λ N-1 ) / 2.

[0022] Assuming the inserted reflectors have the same thickness, to ensure optimal synthesis efficiency, the wavelengths must be λ′1, λ′2, ..., λ′. N-1 The beam can diffract to the maximum extent, and includes wavelengths λ1, λ2, ..., λ N The beam can be transmitted with maximum efficiency, therefore...

[0023] Optimal diffraction conditions:

[0024] Optimal transmission conditions:

[0025]

[0026] In the formula, λ′1, λ′2,…λ′ N-1 The wavelengths of the diffracted light are δ1,…δ N-1 These represent the refractive index modulations of the corresponding gratings, f1, ..., f N-1 , where are the spatial frequencies of the corresponding gratings, and t is the thickness of the grating.

[0027] An external cavity spectral synthesis system for extended N-channel to 2N-1-channel fiber lasers, based on the above method, comprises: one transformation lens, one blazed grating, one output coupling mirror, N-1 reflector Bragg gratings, N-1 collimating lenses, 2N-1 front cavity mirrors, and 2N-1 doped fiber array elements, such as... Figure 7 As shown.

[0028] The beneficial effects obtained by this invention are:

[0029] This invention provides a method and system for external cavity spectrum synthesis of N-channel fiber lasers to 2N-1-channel, which breaks through the current limit on the number of external cavity spectrum synthesis elements. The technical solution adopted is based on the existing external cavity spectrum synthesis scheme of N-channel fiber laser array elements limited by blazed grid. By inserting a laser beam with a different wavelength between each two adjacent wavelengths of the synthesized laser, the external cavity spectrum synthesis of 2N-1 fiber laser beams is achieved, which solves the problem of the limited number of laser array elements in the current external cavity spectrum synthesis. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of an existing N-channel fiber laser external cavity spectral synthesis system;

[0031] Figure 2 for Figure 1 A schematic diagram of the spectrum obtained by external cavity spectrum synthesis of the system shown;

[0032] Figure 3 A schematic diagram of a reflective Bragg grating connected in series in an existing external cavity spectral synthesis system;

[0033] Figure 4 This is a schematic diagram showing the expansion of the original N-channel fiber laser external cavity spectral synthesis to N+1-channel fiber laser spectral synthesis.

[0034] Figure 5 A schematic diagram of the spectrum obtained by external cavity spectral synthesis after inserting reflector Bragg grating 1;

[0035] Figure 6 This is a schematic diagram showing the parameter requirements for the inserted reflector Bragg grating 1;

[0036] Figure 7 A schematic diagram showing the expansion of the original N-channel fiber laser external cavity spectrum synthesis to 2N-1-channel fiber laser spectrum synthesis;

[0037] Figure 8 This is a schematic diagram of the external cavity spectrum synthesis spectrum for the limit N-beam laser extension to 2N-1-beam laser. Detailed Implementation

[0038] The following description of the embodiments will provide a more detailed explanation of the specific implementation of the present invention, in order to help those skilled in the art to have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention.

[0039] The technical solution adopted in this invention is as follows: Based on the existing external cavity spectrum synthesis scheme of N-channel fiber laser array elements limited by blazed grid, a method is used to achieve external cavity spectrum synthesis of 2N-1 fiber lasers by inserting a laser beam of a different wavelength between each two adjacent wavelengths of the synthesized laser. The specific steps are as follows:

[0040] S1. Describe existing limiting N-beam fiber laser external cavity spectrum synthesis schemes, such as... Figure 1 As shown in the figure, N-channel doped fiber array elements resonate within a resonant cavity formed by the front cavity mirror, the transformation lens, the blazed grating, and the output coupling mirror (the space between the fiber output end, the transformation lens, the blazed grating, and the output coupling mirror is called the external cavity). Wavelengths λ1, λ2, ..., λ are selected in a resonant manner. N The spectra are combined into a beam containing wavelengths λ1, λ2, ..., λ N The light beam, the spectrum of its composite light beam is as follows Figure 2 As shown. In this synthesized beam, the spectral spacing Δλ (Δλ = λ) between selected adjacent wavelengths. n -λ n-1 The distance between the two fiber elements (n = 2, ..., N) can be calculated using the formula Δλ ≈ Xd / F. In this formula, X is the spacing between the two fiber elements, d is the distance between the grooves of the blazed grating (i.e., the grating constant), and F is the focal length of the transform lens.

[0041] S2, in the above limit N fiber laser external cavity spectrum synthesis system, first in series a diffraction wavelength λ'1=(λ2+λ1) / 2, and the spectrum selectivity is less than Δλ of the reflection body Bragg grating 1, as shown in Figure 3 . Because the spectrum selectivity of the grating is between Δλ, therefore the synthesis beam containing λ1, λ2, …, λ N Can be completely transmitted.

[0042] S3, in the inserted doped fiber array 1 before the cavity mirror, reflection body Bragg grating and output coupling mirror three, form a band external cavity, based on the reflection body Bragg grating laser system. Let the inserted doped fiber array 1 output beam λ'1 collimated after collimating lens, with Bragg angle incident body Bragg grating, and with the same Bragg diffraction output. At the same time, forced to contain wavelength λ1, λ2, …, λ N The beam also transmits out of the body Bragg grating with the same Bragg angle. Thus, the original N fiber laser external cavity spectrum synthesis becomes N+1 fiber laser spectrum synthesis, as shown in Figure 4 . At this time the external cavity spectrum synthesis spectrum as shown in Figure 5 .

[0043] To ensure that the wavelength of λ'1 beam can occur laser oscillation, and can be transmitted with the beam containing wavelength λ1, λ2, …, λ N Coaxial to become a beam, the reflection body Bragg grating 1 needs to meet the following conditions: (1) the normal direction of the grating must be perpendicular to the body surface; (2) the incident angle of the beam containing wavelength λ1, λ2, …, λ N The size of the diffraction angle of λ'1 diffraction beam needs to be equal, as shown in Figure 6 ; (3) for the wavelength of λ'1 for the diffraction light beam, the Bragg angle is θ m = arcsin λ'1f1 / 2n, in the formula, f1 is the spatial frequency of the body Bragg grating 1, n is the refractive index of the grating material.

[0044] S4, according to the above steps S2 and step S3 method, in the external cavity spectrum synthesis system shown in Figure 3 , in series again reflection body Bragg grating 2, …, N-1, and according to the method shown in Figure 4 , in the inserted doped fiber array 2, …, N-1 before the cavity mirror, reflection body Bragg grating and output coupling mirror, respectively constitute N-2 external cavity, based on the reflection body Bragg grating laser system, so as to realize 2N-1 fiber laser external cavity spectrum synthesis, as shown in Figure 7 , the spectrum of its synthesis beam as shown in Figure 8 .

[0045] To ensure that the above synthesis scheme can be successfully implemented, all N-1 reflector Bragg gratings must meet the following conditions: (1) the normal direction of the grating is perpendicular to the surface of the body. This condition is to ensure that the beam fed back by the output coupling mirror can return smoothly to the corresponding doped fiber; (2) the Bragg angles of all N-1 reflector Bragg gratings must be equal.

[0046] If the same material is used to make the grating, the Bragg angle of the grating must satisfy:

[0047] θ m =arcsinλ′1f1 / 2n=arcsinλ′2f2 / 2n…=arcsinλ′ N-1 f N-1 / 2n,

[0048] In the formula, n is the refractive index of the grating material, f1, f2, ... f N-1 Let λ'1, λ'2, ..., λ'N-1 be the spatial frequencies of the inserted reflector Bragg gratings 1, 2, ..., N-1, and let λ'1 = (λ2 + λ1) / 2; λ'2 = (λ3 + λ2) / 2; ...; λ'N-1 be the wavelengths of the inserted reflectors. N-1 =(λ N +λ N-1 ) / 2.

[0049] Assuming the inserted reflectors have the same thickness, to ensure optimal synthesis efficiency, the wavelengths must be λ′1, λ′2, ..., λ′. N-1 The beam can diffract to the maximum extent, and includes wavelengths λ1, λ2, ..., λ N The beam can be transmitted with maximum efficiency, therefore...

[0050] Optimal diffraction conditions:

[0051] Optimal transmission conditions:

[0052]

[0053] In the formula, λ′1, λ′2,…λ′ N-1 The wavelengths of the diffracted light are δ1,…δ N-1 These represent the refractive index modulations of the corresponding gratings, f1, ..., f N-1 , where are the spatial frequencies of the corresponding gratings, and t is the thickness of the grating.

[0054] The above embodiments are merely illustrative of the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solutions based on the technical concept proposed in this invention shall fall within the scope of protection of this invention. Technologies not covered in this invention can be implemented using existing technologies.

Claims

1. An external cavity spectral synthesis method for expanding N-ary to 2N-1-ary fiber laser, characterized in that, On the basis of the existing fiber laser array external cavity spectrum synthesis scheme N On the basis of the existing fiber laser array external cavity spectrum synthesis scheme N The specific steps of the external cavity spectrum synthesis of one fiber laser include: S1. Given the existing limits N Fiber laser external cavity spectrum synthesis scheme, N The doped fiber array elements select the wavelength in a resonant manner within a resonant cavity formed by the front cavity mirror, the transformation lens, the blazed grating, and the output coupling mirror. The spectrum is synthesized into a beam containing of N A beam of wavelengths, wherein the spectral spacing between selected adjacent wavelengths in the synthesized beam is . ,in It can be done according to the formula. We obtain the equation, where... X The distance between two fiber array elements. d The distance between the grooves of the blazed grating, i.e., the grating constant. F To change the focal length of the lens; S2, in the above limit N In the beam fiber laser external cavity spectrum synthesis system, first in series a diffraction wavelength of , and the spectrum selection is less than Reflection body Bragg grating 1; S3, between the front cavity mirror, the volume Bragg grating and the output coupling mirror of the inserted doped fiber array 1, a volume cavity based on the volume Bragg grating is formed, and a laser system based on the volume Bragg grating is formed, so that the inserted doped fiber array 1 outputs a light beam After collimation by the collimating lens, the Bragg angle is used to enter the volume Bragg grating, and the same Bragg diffraction output is used, and the light beam containing wavelengths is also transmitted out of the volume Bragg grating at the same Bragg angle, so that the original N road fiber laser external cavity spectrum synthesis becomes N +1 road fiber laser spectrum synthesis; S4. The method according to steps S2 and S3 above, further comprising a Fabry-Perot cavity in series with the volume Bragg grating , the doped fiber array is inserted between the front mirror, the volume Bragg grating and the output coupler N , respectively, to form N two out-coupled, volume Bragg grating based laser systems, thereby realizing two 2. The method of claim 1, wherein the external cavity spectrum synthesis method is extended from N to 2N-1 paths. The N+1 fiber laser spectrum synthesis described in step S3 above is to ensure that the wavelength is The beam can generate laser oscillations and can interact with wavelengths containing... The beams are transmitted coaxially to become a single beam. The reflector Bragg grating 1 must meet the following conditions: (1) the normal direction of the grating must be perpendicular to the surface of the body; (2) it contains wavelengths The incident angle of the beam and The diffraction angles of the diffracted beams must be equal; (3) for wavelengths of The beam is a diffracted beam, and its Bragg angle is . In the formula, f The spatial frequency of reflector Bragg grating 1 is... n denoted as the refractive index of the grating material.

3. The method of claim 1, wherein the external cavity spectrum synthesis method is extended from N to 2N-1 paths. To ensure the successful implementation of the synthesis scheme, all N -1 block reflector Bragg grating must meet: (1) the grating normal direction is perpendicular to the surface, which is to ensure that the light beam feedback by the output coupling mirror can smoothly return to the corresponding doped fiber; (2) all N -1 block reflector Bragg grating Bragg angle must be equal.

4. The method of claim 1, wherein the external cavity spectrum synthesis method is extended from N to 2N-1 paths. If the gratings are made of the same material, the grating Bragg angle must satisfy: ， wherein n is the refractive index of the grating material; are the spatial frequencies of the inserted reflector Bragg gratings 1, and the inserted wavelength .

5. The method of claim 4, wherein the external cavity spectrum synthesis method is extended from N to 2N-1 paths. Assuming the thickness of the inserted reflection Bragg grating is the same, to ensure that the combined system has the best combination efficiency, it is required that the light beams with wavelengths of are diffracted to the maximum extent, and the light beams with wavelengths of are transmitted with the maximum efficiency, therefore, Optimum diffraction conditions: , Optimum transmission condition: wherein is the wavelength of the diffracted light, are the refractive index modulations of the corresponding gratings, are the spatial frequencies of the corresponding gratings, t is the thickness of the grating.

6. A system for external cavity supercontinuum generation using the method of claim 1-5, wherein, The system comprises: 1 transform lens, 1 blazed grating, 1 output coupling mirror, N -1 reflective volume Bragg grating, N -1 collimating lens, 2 N -1 front cavity mirror, 2 N -1 doped fiber array element.

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