High-efficiency visible light super-continuum spectrum generation method and device

Abstract

The invention discloses a high-efficiency visible light super-continuum spectrum generation method and device based on a green light pumping gradient refractive index multimode fiber, and belongs to the technical field of fiber laser. According to the method, a pulse green laser is adopted as a pumping source, the pulse green laser is coupled into a parameter-optimized multimode optical fiber with the gradient refractive index, and the gradient refractive index is obtained by exciting the synergistic effect of nonlinear effects such as geometric parameter instability and light beam self-cleaning which are unique in the optical fiber. And the high-flatness super-continuum spectrum covering the whole visible light wave band is directly and efficiently generated. The defects that a traditional photonic crystal fiber scheme is low in conversion efficiency and poor in flatness in the visible light wave band are overcome, the range and shape of an output spectrum can be accurately regulated and controlled by flexibly designing the fiber core radius, the numerical aperture and the length of the fiber, and meanwhile it is guaranteed that an output light beam is close to the diffraction limit through the light beam self-cleaning effect. According to the scheme, the system is simple in structure and high in robustness, and a novel, reliable and practical way is provided for achieving a high-performance visible light super-continuum spectrum light source.

Description

Technical Field

[0001] This invention relates to the fields of fiber optics and laser technology, and in particular to a method and apparatus for generating a high-efficiency, high-flatness supercontinuum (SC) in the visible light band. Background Technology

[0002] Supercontinuum light sources, especially those covering the visible light band (approximately 400-700 nm), have important applications in biomedical imaging, hyperspectral imaging, spectral analysis, and white light illumination. Currently, the mainstream approach for generating supercontinuum (SC) light is to use a femtosecond or picosecond fiber laser in the 1064 nm band to pump a photonic crystal fiber (PCF). However, this approach has significant drawbacks: the resulting SC spectrum exhibits severely uneven energy distribution, with a large amount of energy concentrated in the near-infrared band, while the visible light band, especially the blue-green region, shows a very low energy proportion and poor flatness. This is mainly because the 1064 nm pump wavelength is far from the visible light band, and the dispersion control of PCF in the visible light band is difficult, resulting in low efficiency in transferring energy to shorter wavelengths.

[0003] To improve output in the visible light band, researchers have proposed various technical improvement schemes, but each has its own limitations: The first approach involves using near-infrared "short wavelength" pumping to pump multi-core PCFs. As disclosed in prior art documents CN105896252B and CN205882383U, this involves using a pulsed laser in the 1000–1025 nm band to pump a specially designed multi-core photonic crystal fiber. This approach, by using a pump wavelength shorter than 1064 nm and a multi-core structure, improves visible light generation efficiency and total output power to some extent. However, its pump wavelength remains in the near-infrared region, and the energy conversion to visible light (especially 400–500 nm blue-green light) still requires a complex nonlinear process, resulting in limited efficiency improvement. Furthermore, its SC spectrum still covers a large amount of near-infrared components and is not a pure or enhanced visible light spectrum. In addition, the design and fabrication process of multi-core PCFs is complex, and controlling the splicing loss between the cores is difficult.

[0004] The second approach involves obtaining green light through nonlinear frequency doubling, followed by pumping the PCF. As disclosed in prior art documents CN204012175U and WO2015172700A1, this system first generates fundamental frequency light using a 1 μm laser, then doubles it to 532 nm green light via a complex spatial optical path or all-fiber frequency doubler, and finally uses this green light to pump the PCF. This approach directly places the pump light in the visible light band, which is beneficial for generating visible SC. However, this approach has inherent drawbacks: the frequency doubling process introduces additional system complexity and insertion loss, leading to a decrease in overall optical-to-optical conversion efficiency; simultaneously, the frequency doubling crystal has a power tolerance limit and is easily damaged, limiting the development of high-power systems; furthermore, its broadening mechanism still heavily relies on the dispersion characteristics of the PCF, requiring precise design of the PCF's zero-dispersion point close to the pump wavelength, which presents a significant technological challenge. As the prior art documents indicate, when the pump wavelength is too far from the zero-dispersion point, the spectral broadening effect deteriorates significantly.

[0005] The third approach involves direct pumping with an ultraviolet / violet semiconductor laser. As disclosed in prior art document CN113054520B, a long section of the power-carrying fiber is directly pumped using a violet semiconductor laser diode in the 400-450 nm band, generating visible light SC based on the cascaded stimulated Raman scattering effect. This approach is simple and low-cost. However, its spectral generation mechanism dictates that its output spectrum consists of discrete, fixed-spaced Stokes lines. Although the gaps between the lines can be filled through a random laser feedback mechanism, the continuity and flatness of its spectrum are inherently difficult to compare with SC based on nonlinear broadening. Furthermore, its spectral range and shape are limited by the Raman frequency shift, making flexible control difficult.

[0006] In summary, existing technologies, in pursuing high-power, high-efficiency visible light SC output, all face a core contradiction: either the pump wavelength is suitable but the system is complex, power-limited, and heavily reliant on precise dispersion control via a PCF; or the system is simple but suffers from poor spectral quality (flatness, continuity) or the pump wavelength is not optimal. Therefore, there is an urgent need for a new solution that can simultaneously address pump source efficiency, nonlinear medium robustness, and output spectral performance.

[0007] In recent years, graded-index multimode fiber (MMF) has attracted attention due to its unique nonlinear effects, such as geometric instability and beam self-cleaning. The SC generation process in MMF has low dispersion dependence, providing a new approach for achieving high-efficiency SC output in specific wavelength bands. However, how to utilize the characteristics of MMF, especially through precise design of its structural parameters and pump conditions, to achieve high-efficiency, high-flatness SC output covering the entire visible light band remains a pressing technical problem to be solved. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention proposes a high-efficiency visible light supercontinuum generation method and apparatus based on green-pumped graded-index multimode fiber. This method achieves SC output with high conversion efficiency, high flatness, and high beam quality in the visible light band by selecting a green light pump source of a specific wavelength and matching precisely designed graded-index multimode fiber parameters.

[0009] To achieve this objective, the present invention adopts the following technical solution: A high-efficiency visible light supercontinuum generation method based on green light-pumped graded-index multimode fiber includes the following steps: S1: providing a pulsed green laser as a pump source; S2: providing a graded-index multimode fiber as a nonlinear broadening medium; S3: inserting the pump laser from step S1 into the graded-index multimode fiber from step S2 via spatial coupling; S4: the pump laser propagates in the graded-index multimode fiber, generating and outputting a high-efficiency, high-flatness supercontinuum covering the visible light band through geometric parametric instabilities and beam self-cleaning effects.

[0010] Furthermore, in step S1, the pulse width of the pulsed green laser is 0.1 ps to 1000 ps, ​​and the single pulse energy is 1 μJ to 10 mJ.

[0011] Furthermore, in step S1, the laser beam quality factor M² output by the pump source is less than 1.3. The high beam quality ensures that the pump light coupled into the optical fiber has a high power density, thus laying the foundation for effectively exciting the supercontinuum.

[0012] Furthermore, in step S2, an input cap is fused to the input end of the graded refractive index multimode fiber, and the incident end face of the input cap is coated with a visible antireflection film; an output cap is fused to the output end of the graded refractive index multimode fiber, and the exit end face of the output cap is an angled surface or coated with an antireflection film to prevent backlighting.

[0013] Furthermore, in step S2, the core radius of the graded-index multimode fiber is 15 μm to 35 μm, and the numerical aperture is 0.08 to 0.2.

[0014] Furthermore, in step S2, the length of the graded-index multimode fiber is 20 m to 60 m.

[0015] Furthermore, in step S3, by selecting a pump source with excellent beam quality and optimizing spatial coupling, it can be ensured that the pump light coupled into the graded-index multimode fiber has sufficient fundamental mode components. Simulation and experimental verification have shown that when the fundamental mode energy accounts for no less than 0.1% of the total energy at the initial coupling moment, favorable conditions are provided for the subsequent occurrence of an effective beam self-cleaning effect.

[0016] An apparatus for implementing the above method includes: a pulsed green light pump source for generating a pulsed laser with a center wavelength of 532 nm; a graded-index multimode fiber, the input end of which is spatially coupled to the output end of the pulsed green light pump source for receiving pump light and generating a supercontinuum; wherein the parameters of the graded-index multimode fiber are optimized to ensure that the generated supercontinuum has high conversion efficiency and high flatness in the visible light band.

[0017] Furthermore, the input end of the graded-index multimode fiber is fused with an input end cap, the incident end face of which is coated with a visible antireflection film to optimize the incident efficiency of the pump light; the output end of the graded-index multimode fiber is fused with an output end cap, the exit end face of which is a beveled surface or coated with an antireflection film to suppress backlight. Furthermore, the core radius of the graded-index multimode fiber is between 15 μm and 35 μm, the numerical aperture is between 0.08 and 0.2, and the length is between 20 m and 60 m.

[0018] Beneficial technical effects of the present invention: High efficiency and high flatness: By using green light to pump graded-index multimode fiber, new frequency components are efficiently generated directly on both sides of the pump light using the geometric parametric instability effect. By optimizing fiber parameters, the spectrum is precisely confined and filled with the entire visible light band, avoiding the ineffective energy dissipation to the near-infrared in traditional schemes, and achieving high conversion efficiency and high flatness output in the visible light band.

[0019] High beam quality: The inherent self-cleaning effect of the beam in graded-index multimode fiber can automatically concentrate energy to the fundamental mode while generating the supercontinuum, making the output beam close to a Gaussian distribution, which significantly improves the quality of the output beam and facilitates subsequent applications.

[0020] Flexible design and robustness: The present invention does not rely on the zero dispersion point of the optical fiber. By flexibly adjusting the core radius, numerical aperture and length of the graded refractive index multimode fiber, the range and shape of the output spectrum can be precisely controlled, making it more adaptable.

[0021] System simplification: This solution can directly use commercially available pulsed green lasers, and the graded-index fiber itself is easy to couple with the spatial optical path, reducing system complexity and cost. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of the device described in an embodiment of the present invention.

[0023] The diagram shows: 1. Pump source, 2. Spatial optical coupling component, 3. Input cap, 4. Gradient-index multimode fiber, and 5. Output cap.

[0024] Figure 2 This is a schematic diagram illustrating the position of the sideband wavelength of geometric parametric instability under the core radius of multimode optical fibers with different graded refractive indices in an embodiment of the present invention.

[0025] Figure 3 This is a schematic diagram showing the position of the sideband wavelength of geometric parametric instability under numerical aperture of multimode optical fibers with different graded refractive indices in an embodiment of the present invention.

[0026] Figure 4 This is a diagram showing the refractive index distribution of graded-index multimode fiber and step-index fiber in an embodiment of the present invention.

[0027] Figure 5 This is a schematic diagram illustrating the evolution of the energy of the fundamental mode (FM), higher-order mode group (HOMS), and intermediate state (IMS) with transmission distance during the beam self-cleaning process in an embodiment of the present invention.

[0028] Figure 6 This is a comparison diagram showing the light field distribution before and after the beam self-cleaning effect in an embodiment of the present invention; wherein Figure 6 (a) is a diagram showing the light field distribution of the multimode pump light spot at the input end; Figure 6 (b) is the light field distribution diagram of the near-fundamental Gaussian distribution spot at the output end.

[0029] Figure 7 This is the supercontinuum spectrum output in the embodiments of the present invention.

[0030] Figure 8 This is a three-dimensional schematic diagram in an embodiment of the present invention, used to determine the range of optimized parameters for graded refractive index multimode fiber used in high-efficiency visible light supercontinuum generation. It shows the relationship between the redshift sideband wavelength and the core radius and numerical aperture. Detailed Implementation

[0031] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0032] like Figure 1 As shown, the device of the present invention mainly includes a pump source 1, a spatial optical coupling component 2, an input cap 3, a graded refractive index multimode fiber 4, and an output cap 5.

[0033] The pump source 1 is a pulsed green laser used to generate the required pump light.

[0034] The spatial optical coupling component 2 is used to collimate and focus the pump light and couple it into the input cap 3. In this embodiment, the spatial optical coupling component 2 is a set of lenses.

[0035] The incident end face of the input cap 3 is coated with a visible anti-reflection film to optimize the incident efficiency of the pump light.

[0036] The output end cap 5 has an angled surface for emission to suppress reflected light.

[0037] Pump source 1 can be a picosecond pulse laser obtained by frequency doubling of a 1064 nm fiber laser, with an output wavelength of 532 nm, a pulse width of 50 ps, ​​a single pulse energy of 10 μJ, and a beam quality M² of approximately 1.2.

[0038] like Figure 2 As shown, this experiment demonstrates the effect of the core radius of a graded-index multimode fiber on the sideband wavelength of geometric parametric instability (GPI) under a fixed numerical aperture. Experimental results show that as the core radius decreases, significant blue and red shifts occur in the sidebands, leading to spectral broadening. Therefore, selecting a smaller core radius (e.g., in the 15-35 μm range) can effectively promote spectral broadening. However, an excessively small core radius causes the sidebands to move too far away from the pump wavelength, resulting in energy escaping into the visible light band and consequently reducing visible light conversion efficiency.

[0039] like Figure 3 As shown, this experiment demonstrates the effect of the numerical aperture of a graded-index multimode fiber on the sideband wavelengths of geometrically parametric instabilities (GPIs) with a fixed core radius. Experimental results show that increasing the numerical aperture leads to an increase in the inter-sideband spacing, which in turn causes spectral broadening. Therefore, by selecting a larger numerical aperture (e.g., in the range of 0.08–0.2), the nonlinear effect can be enhanced. However, similar to the core radius, an excessively large numerical aperture can also cause the sidebands to extend beyond the visible light range, which is detrimental to energy concentration in the visible light band.

[0040] Based on the above Figure 2 and Figure 3 By analyzing the relationship between the sideband positions of geometric parametric instabilities and fiber parameters, we selected the following optimal parameters for graded-index multimode fiber 4: core radius 25 μm, numerical aperture 0.1, and length 40 meters, after balancing spectral broadening capability and visible light conversion efficiency. This fiber is a highly nonlinear graded-index multimode fiber, with its core doped with a high concentration of germanium, resulting in a nonlinear refractive index coefficient significantly higher than that of standard silica fiber. This unique structure combining graded refractive index and high nonlinearity provides the physical basis for efficiently stimulating geometric parametric instabilities and beam self-cleaning effects.

[0041] The mathematical expression describing the refractive index distribution of graded-index multimode fiber 4 is as follows: (1) in, The radial coordinates on the cross-section of the optical fiber are defined as originating from the center of the fiber core. =0) Outward radial distance; For radial coordinates The refractive index at that point; It is the center position of the fused silica fiber core diameter ( The refractive index of (=0), This refers to the refractive index of the fiber cladding. For relative refractive index difference, , Represents the fiber core radius. It is called the refractive index, and its value is greater than zero. Different values ​​represent different refractive index distributions. Formula (1) can express different fiber refractive index distributions. Figure 4 The refractive index distributions of graded-index multimode fibers and step-index fibers were compared. For example... Figure 4 As shown, the parabolic refractive index distribution of graded-index multimode fiber provides it with a periodic self-imaging effect, which is the physical basis for generating key nonlinear effects such as geometric parametric instability and beam self-cleaning; while the uniform core of step-index fiber cannot provide such periodic refractive index guidance, and therefore it is difficult to efficiently generate the nonlinear process described in this invention.

[0042] Pump light is efficiently injected into the core of graded-index multimode fiber 4 via spatial optical coupling component 2. During propagation within the fiber, the pump light first undergoes a beam self-cleaning effect over an extremely short distance (e.g., 0.3 meters). Figure 5 This demonstrates that the pulse undergoes self-cleaning at the transmission front end, the fundamental mode energy rapidly increases, and the beam quality is optimized.

[0043] The shape changes of the output light spot are as follows: Figure 6 As shown. Figure 6 (a) shows the multimode pump light spot after spatial coupling at the input end, with a clear outline indicating that multiple transverse modes were excited; in stark contrast, Figure 6 (b) The output beam spot transforms into a circular beam spot with uniform intensity distribution and regular shape. The relatively blurred state of this output beam spot is a direct reflection of its extremely high spatial coherence and high energy concentration in the fundamental mode, which is consistent with the far-field distribution characteristics of a near-diffraction-limited fundamental mode Gaussian beam. Thus, the experiment experimentally verifies the successful occurrence of the beam self-cleaning effect.

[0044] Meanwhile, in the spectral domain, a series of spectral sidebands are symmetrically generated on both sides of the pump light based on the geometric parametric instability effect. Through optimized fiber parameters, these nonlinear effects in the time and frequency domains work synergistically, ultimately enabling these sidebands to effectively cover and fill the entire visible light band, producing a highly flat supercontinuum.

[0045] After 40 meters of nonlinear transmission, the final output spectrum is as follows: Figure 7 As shown, the spectrum has a 10 dB bandwidth of 309.5 nm, covering a range from 470.5 nm to 780 nm, completely covering the visible light band and extending into part of the near-infrared. Its visible light portion has a high power ratio and good flatness, achieving high-efficiency and high-flatness visible light supercontinuum output.

[0046] Systematic studies have shown that reducing the core radius or increasing the numerical aperture of graded-index multimode fibers leads to an increase in the sideband frequency shift due to geometrical instability, thereby broadening the spectrum. However, excessive adjustment can cause the sidebands to extend beyond the visible light range, reducing visible light efficiency. Increasing the fiber length helps improve spectral flatness and width, but a trade-off must be made with visible light efficiency.

[0047] This optimization criterion is in Figure 8 The three-dimensional relationship is intuitively illustrated in the figure. This figure clearly reveals the variation of the redshift sideband wavelength with the core radius and numerical aperture: that is, decreasing or increasing these two parameters directly leads to the redshift sideband shifting towards longer wavelengths. To ensure that the redshift sideband broadens sufficiently without excessively entering the near-infrared band to maintain high visible light efficiency, its wavelength needs to be constrained to near the upper limit of the visible light band (approximately 700 nm). Based on this principle, through… Figure 8 The analysis of the relationships shown can determine the optimal combination of core radius and numerical aperture parameters that simultaneously satisfies the requirements for spectral width and visible light efficiency. The dotted areas in the figure represent the suggested intervals.

[0048] Theoretical analysis and experiments show that when the fundamental mode energy ratio of the pump light at the initial coupling moment is not less than 0.1, good spectral broadening and beam self-cleaning effects can be guaranteed simultaneously. In this embodiment, this condition is met by selecting a pump source with an M² of approximately 1.2.

[0049] In summary, this invention provides a practical and innovative solution for generating high-performance visible light supercontinuum.

Claims

1. A method for high efficiency visible supercontinuum generation, characterized by, The application relates to a high-efficiency visible light supercontinuum spectrum generation method. The method comprises the following steps: using a green laser as a pump source; coupling laser generated by the pump source into a graded-index multimode fiber; and using the geometric parameter instability and the beam self-cleaning effect in the graded-index multimode fiber to generate and output a high-flatness supercontinuum spectrum covering the visible light band.

2. The high-efficiency visible light supercontinuum spectrum generation method according to claim 1, wherein: an input end cap is fused to the input end of the graded-index multimode fiber, and the incident end face of the input end cap is coated with a visible light anti-reflection film; and an output end cap is fused to the output end of the graded-index multimode fiber, and the emergent end face of the output end cap is a bevel face or is coated with an anti-reflection film to prevent back light.

3. The high-efficiency visible light supercontinuum spectrum generation method according to claim 1, wherein: the pulse width of the pulse laser is between 0.1 ps and 1000 ps, and the single pulse energy is between 1 muJ and 10 mJ.

4. The high-efficiency visible light supercontinuum spectrum generation method according to claim 1, wherein: the core radius of the graded-index multimode fiber is between 15 mu m and 35 mu m.

5. The high-efficiency visible light supercontinuum spectrum generation method according to claim 1, wherein: the numerical aperture of the graded-index multimode fiber is between 0.08 and 0.

2.

6. The high-efficiency visible light supercontinuum spectrum generation method according to claim 1, wherein: the length of the graded-index multimode fiber is between 20 m and 60 m.

7. The high-efficiency visible light supercontinuum spectrum generation method according to claim 1, wherein: characterized in that the laser beam quality factor M<2> output by the pump source is less than 1.

3.

8. An apparatus for implementing the high-efficiency visible light supercontinuum spectrum generation method according to any one of claims 1-7, comprising: a pump source for generating green pulse laser; and a graded-index multimode fiber, the input end of which receives the output light of the pump source through spatial coupling.

9. The apparatus for implementing the high-efficiency visible light supercontinuum spectrum generation method according to claim 8, wherein: an input end cap is fused to the input end of the graded-index multimode fiber, and the incident end face of the input end cap is coated with a visible light anti-reflection film to optimize the incident efficiency of the pump light; and an output end cap is fused to the output end of the graded-index multimode fiber, and the emergent end face of the output end cap is a bevel face or is coated with an anti-reflection film to suppress back light.

10. The apparatus for implementing the high-efficiency visible light supercontinuum spectrum generation method according to claim 8, wherein: the core radius of the graded-index multimode fiber is between 15 mu m and 35 mu m, the numerical aperture is between 0.08 and 0.2, and the length is between 20 m and 60 m.

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