Broadband low-differential-mode-gain few-mode thulium-doped optical fiber amplifier and design method thereof
By introducing parabolic thulium ion doping and graded refractive index design into few-mode thulium-doped fiber, the gain imbalance problem of thulium-doped fiber amplifier in the 2-micron band was solved, achieving broadband low differential mode gain and high gain flatness, simplifying the manufacturing process and reducing system complexity.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-30
- Publication Date
- 2026-03-31
AI Technical Summary
Existing thulium-doped fiber amplifiers have difficulty controlling gain flatness and differential mode gain in the 2-micron band, leading to problems such as limited communication bandwidth, mode gain imbalance, and high system complexity.
A few-mode thulium-doped fiber amplifier with a parabolic thulium ion doping distribution and graded refractive index design achieves gain equalization and stable mode transmission by optimizing the refractive index difference between the annular core and the outer cladding, combined with a fundamental mode pump source.
It achieves low differential mode gain and high gain flatness in the 1720-2020nm band, simplifies the manufacturing process, reduces system complexity, and supports stable transmission of multimode fiber communication.
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Figure CN121769622A_ABST
Abstract
Description
Technical Field
[0001] This specification relates to the field of optical fiber communication technology, and in particular to a broadband low differential mode gain few-mode thulium-doped fiber amplifier and its design method. Background Technology
[0002] The explosive growth of bandwidth-hungry applications such as virtual reality (VR), augmented reality (AR), the Internet of Things (IoT), cloud services, big data processing, and ultra-high-definition video has placed unprecedented capacity pressure on optical communication networks. Traditional dense wavelength division multiplexing (DWDM) systems based on the C-band (1530–1565 nm) and L-band (1565–1625 nm) are approaching their spectral resource limits, and the industry urgently needs to extend to longer wavelengths to obtain new transmission windows. Although researchers have attempted to push the gain boundary of erbium-doped fiber amplifiers above 1625 nm by optimizing erbium ion concentration, co-dopersive agents, or pumping schemes, the gain efficiency and noise performance deteriorate sharply beyond this wavelength due to the intrinsic emission characteristics of erbium ions, and further expansion faces insurmountable physical bottlenecks.
[0003] Furthermore, recent breakthroughs in hollow-core fiber technology have injected new vitality into the 2-micron band. Hollow-core fibers confine the light field primarily within the air core, significantly reducing material nonlinearity and dispersion effects, making the 2-micron band an ideal communication window for achieving ultra-low loss, ultra-low latency, ultra-low dispersion, and ultra-high capacity transmission. Simultaneously, gas molecules such as CO2, HCl, and H2O exhibit strong absorption peaks in the 1700–2025 nm range, making the 2-micron band a "fingerprint region" for high-sensitivity spectral detection. Thulium-doped fibers can amplify wavelengths that precisely cover this band, providing broadband, tunable, and high-power light sources and amplifiers for gas sensing, medical diagnostics, and industrial monitoring. More importantly, driven by the low-loss window of hollow-core fibers, the 2-micron communication band is moving from the laboratory to field trials. Thulium-doped fiber amplifiers, as their core gain components, possess both communication and sensing functions and are expected to play a crucial role in future multifunctional integrated networks.
[0004] However, current research on thulium-doped fiber amplifiers mainly focuses on signal amplification of single spatial modes. As an important branch of spatial division multiplexing (SDM) technology, mode division multiplexing (MDM) can significantly improve system capacity by transmitting multiple spatial modes in parallel within the same fiber, and is considered one of the key paths to break through the transmission limits of existing single-mode systems. Unfortunately, research on key components for MDM systems—few-mode thulium-doped fiber amplifiers—is still in its early stages. The core challenges facing its development are mainly concentrated in the following two aspects: 1. Difficulty in controlling gain flatness over ultra-wide bandwidth. The emission cross-section of thulium ions covers an extremely wide band around 2 μm, and the cross-section value fluctuates dramatically with wavelength. Traditional uniform doping combined with step refractive index schemes often exhibit significant fluctuations in gain slope within the communication bandwidth, with a large peak-to-valley difference. Such a large gain difference forces the system to reserve additional power margin and introduce complex post-equalization filters or Raman aids, resulting in additional insertion loss, polarization-dependent loss, and mode-dependent loss. Therefore, how to achieve good inherent gain flatness at the "source"—that is, in the fiber waveguide and doping design stage—is the primary bottleneck for the practical application of thulium-doped fiber amplifiers. 2. High differential mode gain. Differences in field distribution among different spatial modes lead to inconsistent overlap factors with the gain region, causing gain imbalance between modes and severely degrading the signal-to-noise ratio and transmission distance at the receiver. Therefore, ensuring gain balance across all signal modes in a multimode fiber amplifier across the entire frequency band is another pressing technical challenge. Summary of the Invention
[0005] To address the aforementioned shortcomings in the existing technology, this invention provides a broadband low differential mode gain few-mode thulium-doped fiber amplifier and its design method, which solves the problem of how to simplify the manufacturing process and improve the reliability of the finished product.
[0006] To achieve the aforementioned objectives, the present invention employs the following technical solution: a broadband low differential mode gain few-mode thulium-doped fiber amplifier, comprising: A fundamental mode pump source is used for ring core pumping of few-mode thulium-doped optical fibers. Few-mode thulium-doped fiber is used to amplify the input optical signal to obtain a broadband low-differential-mode gain amplification result.
[0007] The beneficial effects of this invention are as follows: This invention provides a broadband low differential mode gain few-mode thulium-doped fiber amplifier, which can fill the gap of lacking an adaptive gain module in the 2μm band mode division multiplexing communication system. It is used to solve the defects of existing thulium-doped fiber amplifiers, such as severe fluctuations in the transmission cross section and poor gain flatness in the broadband range, as well as the mismatch of the overlap factor between each order mode and the thulium ion doping region and pump mode field distribution when transmitting multiple signal modes. These defects result in the limited number of amplification modes supported by existing thulium-doped fiber amplifiers, limited communication bandwidth, unbalanced mode gain, high system power consumption and high digital signal processing complexity. This invention achieves a low differential mode gain and high gain flatness few-mode thulium-doped fiber amplifier covering the 1720nm-2020nm band.
[0008] Furthermore, the few-mode thulium-doped fiber comprises: The structure consists of an inner cladding, an annular core, a trench, and an outer cladding, arranged sequentially from the inside out. The refractive indices of the annular core, inner cladding, outer cladding, and trench decrease sequentially. The thulium ion doping distribution within the annular core is parabolic.
[0009] Furthermore, the thulium ion doping distribution curve in the annular fiber core is an inverted U-shaped parabolic doping, with a peak value set at a specified radius within the annular core, decreasing symmetrically towards both the inner and outer sides, and the rate of decrease being controlled by the doping width parameter.
[0010] Furthermore, the outer cladding layer is a silicon dioxide component, the inner cladding layer is a silicon dioxide component uniformly doped with germanium or phosphorus, the trench is a silicon dioxide component uniformly doped with boron or fluorine, and the annular fiber core is a silicon dioxide component uniformly doped with either germanium or phosphorus, and is a component doped with thulium ions in a parabolic pattern.
[0011] This invention provides a design method for a broadband low differential mode gain few-mode thulium-doped fiber amplifier, comprising: S1: Obtain the inner and outer ring radii of the annular fiber core, the refractive index of the annular fiber core, and the refractive index of the outer cladding. Calculate the ratio of the inner and outer radii of the annular fiber core and the normalized frequency. S2: Based on the normalized frequency and the ratio of the inner and outer radii of the ring fiber core, the ring fiber core and the trench structure are adjusted to reduce the difference in mode field distribution between different modes while supporting multimode transmission, and the adjustment result is obtained. S3: Based on the adjustment results and combined with the parabolic thulium ion doping distribution curve, calculate the overlap integral between the pump mode and the signal mode; S4: Process the overlap integral to obtain the average overlap integral and the maximum overlap difference; S5: Based on the average overlap integral and the maximum overlap difference, the parameters of the thulium ion doping distribution curve are adjusted to obtain the design results of the few-mode thulium-doped fiber amplifier, thus completing the design of the few-mode thulium-doped fiber amplifier.
[0012] This invention provides a design method for a broadband low differential mode gain few-mode thulium-doped fiber amplifier. By limiting the range of the ratio of the inner ring radius to the outer ring radius of the ring core, the size and thickness of the ring core can be adjusted to a suitable range. This supports the transmission of as many linearly polarized modes as possible while suppressing the generation of higher-order radial modes. It avoids the occurrence of higher-order radial modes due to excessively thick ring cores, while also avoiding the degradation of transmission mode quality due to excessively thin ring cores, thus ensuring stable transmission of linearly polarized modes.
[0013] Furthermore, the expression for the normalized frequency is: ; The expression for the radius ratio is: ; in, Represents the normalized frequency. Indicates wavelength. This indicates the outer ring radius of the annular fiber core. This indicates the refractive index of the toroidal fiber core. Indicates the refractive index of the outer cladding. This represents the ratio of the inner and outer ring radii of the toroidal fiber core. This indicates the inner ring radius of the toroidal fiber core.
[0014] Furthermore, the expression for the thulium ion doping distribution curve is as follows: ; in, The curve represents the thulium ion doping distribution, r represents the radius of the optical fiber, h represents the peak position of the doping curve, and c represents the doping width of the doping curve.
[0015] Furthermore, the expression for the overlap integral of the pump mode and the signal mode is: ; in, This represents the overlap integral of the pump mode and the signal mode. This indicates the inner ring radius of the toroidal fiber core. This indicates the outer ring radius of the annular fiber core. This represents the signal and the pump mode of order i, representing the mode field strength. Indicates radius, Indicates angle, express, The curve represents the distribution of thulium ion doping, where i and j represent the order of the corresponding signal and pump mode, respectively, s represents the signal, and p represents the pump.
[0016] Furthermore, the expression for the average overlap integral is: ; The expression for the maximum overlap difference is: ; in, Represents the average overlap integral. express, express, Indicates the maximum overlap difference. This represents the function that takes the maximum value. This represents the overlap integral of the pump mode and the signal mode. This represents the function that takes the minimum value. Attached Figure Description
[0017] This specification will be further described by way of exemplary embodiments, which will be described in detail with reference to the accompanying drawings. These embodiments are not limiting; in these embodiments, the same reference numerals denote the same structures, wherein: Figure 1 This is an exemplary schematic diagram of a broadband low differential mode gain few-mode thulium-doped fiber amplifier according to some embodiments of this specification. Figure 2 This is an exemplary flowchart illustrating a design method for a broadband low differential mode gain few-mode thulium-doped fiber amplifier according to some embodiments of this specification. Figure 3 This is an exemplary schematic diagram of the cross-section of a few-mode thulium-doped fiber according to some embodiments of this specification; Figure 4 This is an exemplary schematic diagram showing the normalized field strength distribution of each linear polarization mode in a few-mode thulium-doped fiber according to some embodiments of this specification. Figure 5 These are exemplary schematic diagrams illustrating different normalized parabolic doping profiles of few-mode thulium-doped optical fibers according to some embodiments of this specification. Figure 6 This is an exemplary schematic diagram showing the relationship between the thulium ion doping line shape and the mode overlap integral of a few-mode thulium-doped fiber according to some embodiments of this specification; Figure 7 This is an exemplary schematic diagram showing the gain curves of different modes versus wavelength in a few-mode thulium-doped fiber according to some embodiments of this specification; Figure 8 This is an exemplary schematic diagram showing the curves of differential mode gain versus wavelength in a few-mode thulium-doped fiber according to some embodiments of this specification.
[0018] The components are: 1. Inner cladding; 2. Annular core; 3. Groove; 4. Outer cladding. Detailed Implementation
[0019] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.
[0020] Example 1 Figure 1 This is an exemplary schematic diagram of a broadband low differential mode gain few-mode thulium-doped fiber amplifier according to some embodiments of this specification. Figure 1 As shown, a broadband low differential mode gain few-mode thulium-doped fiber amplifier includes: a fundamental mode pump source for ring core pumping of the few-mode thulium-doped fiber; and a few-mode thulium-doped fiber for amplifying the input optical signal to obtain a broadband low differential mode gain amplification result.
[0021] The signal mode field, pump mode field, and thulium ion doping distribution collectively determine the gain equalization performance of a few-mode fiber amplifier. While optimizing the signal optical field—such as finely designing the refractive index profile to compress the energy distribution of each mode into the doped region—can partially reduce the overlap difference between modes, the effect is limited, and excessively complex waveguide structures significantly increase manufacturing difficulty. Using higher-order modes or multimode hybrid pumping can reduce differential mode gain by controlling the pump optical field; however, additional mode switching increases complexity and limits further expansion of the signal modes. In contrast, designing the thulium ion doping distribution using parabolic or other smooth curves not only aligns with the thulium ion diffusion behavior during fiber drawing using improved chemical vapor deposition, reducing process sensitivity, but also precisely compensates for the overlap mismatch between each mode and the gain region, significantly suppressing differential mode gain without relying on complex pump control strategies. This provides a simple, scalable, and easily engineering-implemented gain equalization approach for broadband few-mode thulium-doped fiber amplifiers.
[0022] The fundamental mode pump source is a pump source used for annular fiber core pumping.
[0023] In some embodiments, the few-mode thulium-doped fiber is pumped using a ring core 2 pump and a fundamental mode pump source. By using a ring core 2 pump, the pumping method is simple, reducing the complexity of practical applications; at the same time, using an easily available fundamental mode pump source can save on the cost of practical applications.
[0024] Few-mode thulium-doped fiber is used as an optical fiber amplifier to compensate for losses in ultra-long-distance mode-multiplexed optical fiber communication transmission links.
[0025] The broadband low differential gain amplification result is an optical signal gain result with gain equalization effect, characterized by high gain and low differential gain.
[0026] In some embodiments, such as Figure 3 As shown, the few-mode thulium-doped fiber includes: an inner cladding 1, an annular core 2, a trench 3, and an outer cladding 4, which are wrapped sequentially from the inside to the outside; wherein, the refractive index of the annular core 2, the inner cladding 1, the outer cladding 4, and the trench 3 decreases sequentially; the thulium ion doping distribution in the annular core 2 is parabolic.
[0027] In some embodiments, the annular fiber core 2 is doped with germanium or phosphorus to increase the refractive index of the material, thereby creating a refractive index difference with the inner cladding layer 1 and the outer cladding layer 4. This confines the optical field distribution of different signal modes within the same region of the annular fiber core 2. To further reduce mode field differences, a refractive index groove 3 is introduced between the annular fiber core 2 and the outer cladding layer 4 to strengthen the confinement of the optical field, making the field distribution of different modes tend to be consistent. The annular fiber core 2 is doped with thulium ions, and its concentration is distributed parabolically along the radial direction. This can accurately compensate for the overlap difference between each linear polarization mode and the gain region. Combined with the annular fiber core fundamental mode pump, the stimulated emission of thulium ions is used to simultaneously amplify multiple signal modes, significantly reducing the differential mode gain and ensuring the signal-to-noise ratio at the receiver.
[0028] In some embodiments, the thulium ion doping distribution curve in the annular core 2 is an inverted U-shaped parabolic doping, with a peak value set at a specified radius inside the core, decreasing symmetrically towards both the inner and outer sides, and the rate of decrease being controlled by the doping width parameter.
[0029] In some embodiments, the few-mode thulium-doped fiber has several selectable structural parameters, including: the inner ring radius of the ring core. The outer ring radius of the annular fiber core The width w of the trench, and the refractive index of the inner cladding. The refractive index of the annular fiber core The refractive index of the groove The refractive index of the outer cladding layer The peak position h and doping width c of thulium ions in the annular fiber core are specified. By setting the refractive index of the inner cladding 1, the refractive index of the annular fiber core 2, and the refractive index of the trench 3, a large refractive index difference between the annular fiber core 2 and the surrounding area is achieved. This helps to better confine the optical field distribution of each mode within the same area of the annular fiber core 2, thereby ensuring stable modes and a small differential mode gain. It can also suppress the generation of higher-order radial modes and ensure that the optical field distribution of each mode is confined to the same area, supporting the transmission of as many linearly polarized modes as possible while suppressing higher-order radial modes. In addition to the refractive index parameter, the thickness of each structure of the optical fiber also affects the mode distribution of the optical fiber.
[0030] In some embodiments, the relative refractive index difference between the fiber core and the cladding is 0.01, and the ratio of the inner ring radius to the outer ring radius of the annular core ranges from 0.45 to 0.75.
[0031] In some embodiments, the peak position h of thulium ion doping in the annular fiber core ranges from 9 to 10 μm, and the doping width c ranges from 2.5 to 3.5 μm.
[0032] In some embodiments, the outer cladding layer 4 is a silicon dioxide component, the inner cladding layer 1 is a silicon dioxide component uniformly doped with germanium or phosphorus, the trench 3 is a silicon dioxide component uniformly doped with boron or fluorine, and the annular fiber core 2 is a silicon dioxide component uniformly doped with either germanium or phosphorus, and is a component doped with thulium ions in a parabolic pattern.
[0033] In some embodiments, the amplification wavelength range of the few-mode thulium-doped fiber amplifier covers the 1720nm-2020nm band.
[0034] In some embodiments, the annular core 2 is uniformly doped with materials such as germanium or phosphorus, making its refractive index significantly higher than that of the inner cladding 1 and the outer cladding 4, thereby forming a sufficiently large refractive index step. This step can reduce the refractive index of the LP layer. 01 LP 11 LP 21 Signal modes of different orders are simultaneously confined within the same annular core 2 region to prevent mode energy leakage outward. To further reduce the field distribution differences between modes, this embodiment introduces an additional annular refractive index trench structure 3 between the core 2 and the cladding 4. Its refractive index is lower than that of the cladding body, forming a "depressed" optical potential well. This forces more energy from each mode to converge back into the annular core 2, thereby making the peak position, full width at half maximum (FWHM), and edge attenuation of the fundamental mode and each higher-order mode more consistent, significantly reducing the overlap integral difference between different modes. Inside the annular core 2, the thulium ion doping concentration is distributed radially according to a parabolic function. This curve distribution naturally matches the thermal diffusion behavior during fiber drawing, reducing process sensitivity; on the other hand, it can suppress the fundamental mode gain and accurately compensate for the gain imbalance caused by the field distribution differences of different higher-order linear polarization modes, making the effective overlap factor of each mode in the doped region more consistent. During amplification, a fundamental mode core pump is used, and the stimulated emission process of thulium ions is used to provide gain for multiple signal modes, significantly reducing the differential mode gain and ensuring that the receiver obtains a consistent high signal-to-noise ratio, laying a reliable amplification foundation for 2-micron band mode division multiplexing transmission.
[0035] In some embodiments, the few-mode thulium-doped fiber amplifier, by employing an optimized parabolic thulium ion doping distribution, simultaneously achieves high gain and low differential mode gain in the 1720–2020 nm band, and can be directly deployed as an in-line amplifier in 2-micron band mode-division multiplexing transmission links. The parabolic doping curve naturally matches the fiber drawing and thermal diffusion process, significantly reducing process sensitivity; the amplification process relies only on fundamental mode pumping, eliminating the need for higher-order or mixed-mode pumping, thus simplifying the system architecture, reducing costs, and preserving signal mode expansion space. This invention solves the problems of limited amplification modes, bandwidth limitation, and gain imbalance caused by large transmit cross-section fluctuations, insufficient gain flatness, and mode-doping region overlap mismatch in traditional thulium-doped fibers, providing a high-performance and easily manufactured gain means for 2-micron band mode-division multiplexing communication.
[0036] In some embodiments, the few-mode thulium-doped fiber amplifier, through a design combining inner cladding, trench assistance, and graded refractive index, effectively confines the optical field to the annular core region, significantly reducing mode field differences between different modes. By introducing parabolic thulium ion doping into the annular core, excessively high gain of the fundamental mode can be suppressed, while enhancing the gain of higher-order modes. This balances the overlap integral differences between different modes and the thulium ion-doped region, achieving lower differential mode gain without relying on complex pump control. The graded refractive index distribution design considers process feasibility and closely matches the concentration profile formed by natural diffusion during manufacturing, helping to ensure consistency between design specifications and actual performance, simplifying the manufacturing process, and improving product reliability.
[0037] In some embodiments of this specification, a broadband low differential mode gain few-mode thulium-doped fiber amplifier is provided. This amplifier can fill the gap in the lack of a suitable gain module in 2μm band mode division multiplexing communication systems. It addresses the shortcomings of existing thulium-doped fiber amplifiers, such as severe fluctuations in the transmit cross-section and poor gain flatness in the broadband range, as well as the mismatch between the overlap factor of each mode and the thulium ion doping region and pump mode field distribution when transmitting multiple signal modes. These shortcomings result in a small number of amplification modes supported by existing thulium-doped fiber amplifiers, limited communication bandwidth, unbalanced mode gain, high system power consumption and digital signal processing complexity. This amplifier achieves a low differential mode gain and high gain flatness few-mode thulium-doped fiber amplifier covering the 1720nm-2020nm band.
[0038] Example 2 Figure 2 This is an exemplary flowchart illustrating a design method for a broadband low-differential-mode-gain few-mode thulium-doped fiber amplifier according to some embodiments of this specification. Figure 2 As shown, the process includes the following steps. In some embodiments, the process may be executed by a processor.
[0039] S1: Obtain the inner and outer ring radii of the annular fiber core, the refractive index of the annular fiber core, and the refractive index of the outer cladding. Calculate the ratio of the inner and outer radii of the annular fiber core and the normalized frequency.
[0040] Normalized frequency is an important parameter for the cutoff of linearly polarized modes.
[0041] In some embodiments, the processor can be based on the normalized frequency of the few-mode thulium-doped fiber. and the inner ring radius of the annular fiber core 2 With outer ring radius ratio To determine the modes and number supported by few-mode thulium-doped optical fibers.
[0042] In some embodiments, the expression for the normalized frequency is: ; The expression for the radius ratio is: ; in, Represents the normalized frequency. Indicates wavelength. This indicates the outer ring radius of the annular fiber core. This indicates the refractive index of the toroidal fiber core. Indicates the refractive index of the outer cladding. This represents the ratio of the inner and outer ring radii of the toroidal fiber core. This indicates the inner ring radius of the toroidal fiber core.
[0043] In some embodiments, when few-mode thulium-doped fiber is used, the wavelength... The range is determined by the normalized frequency. The calculation formula shows that the outer ring radius of the annular fiber core 2 The greater the refractive index difference between the annular core 2 and the outer cladding 4, the higher the normalized frequency. The higher the value, the more modes that few-mode thulium-doped fibers can support; at normalized frequencies... Given a fixed value, the ratio of the inner ring radius to the outer ring radius of the annular fiber core 2 ,ratio The smaller the size, the more modes that few-mode thulium-doped fiber can support. At the same time, considering the suppression of higher-order radial modes, and taking into account the existing fiber conditions, the size range of the few-mode thulium-doped fiber structure is determined to ensure that the expected effect is achieved.
[0044] In some embodiments, the processor can set the relative refractive index difference between the fiber core and the cladding to 10. -2 -10 -3The ratio of the inner ring radius to the outer ring radius of the annular fiber core 2 ranges from 0.45 to 0.75. By limiting the range of this ratio, the size and thickness of the annular fiber core 2 can be adjusted to a suitable range. This allows for the transmission of as many linearly polarized modes as possible while suppressing the generation of higher-order radial modes. It avoids the occurrence of higher-order radial modes due to excessive thickness of the annular fiber core 2, while also preventing the transmission mode quality from being too poor due to excessive thinness, thus ensuring stable transmission of linearly polarized modes.
[0045] In some embodiments, the processor can further adjust the distribution of each signal mode in the annular fiber core 2 region by adding trench auxiliary structures and increasing the refractive index of the inner cladding, thereby reducing the differential mode gain to a certain extent.
[0046] S2: Based on the normalized frequency and the ratio of the inner and outer radii of the ring fiber core, the ring fiber core and the trench structure are adjusted to reduce the difference in mode field distribution between different modes while supporting multimode transmission, and the adjustment result is obtained.
[0047] The adjustment results are the result of adjusting the parameters of the annular fiber core and the groove structure.
[0048] In some embodiments, the processor can construct an inverted U-shaped parabolic thulium ion doping distribution in the ring core 2 of a few-mode thulium-doped fiber based on the thulium ion doping distribution curve: the radial coordinate h is set as the peak doping concentration, and the doping concentration decreases symmetrically in a parabolic manner from the peak position to both the inner and outer sides, with the deceleration rate precisely controlled by the doping width parameter c.
[0049] S3: Based on the adjustment results and combined with the parabolic thulium ion doping distribution curve, calculate the overlap integral between the pump mode and the signal mode.
[0050] The thulium ion doping distribution curve is the normalized thulium ion doping distribution curve.
[0051] In some embodiments, the expression for the thulium ion doping distribution curve is: ; in, The curve represents the thulium ion doping distribution, r represents the radius of the optical fiber, h represents the peak position of the doping curve, and c represents the doping width of the doping curve.
[0052] Selecting an appropriate doping curve can suppress the gain difference between modes while ensuring that each signal mode obtains sufficient gain.
[0053] The overlap integral of the pump mode and the signal mode is due to the integral that the signal gain is determined by the common distribution of the pump mode and the signal mode within the doped region.
[0054] In some embodiments, the expression for the overlap integral of the pump mode and the signal mode is: ; in, This represents the overlap integral of the pump mode and the signal mode. This indicates the inner ring radius of the toroidal fiber core. This indicates the outer ring radius of the annular fiber core. This represents the signal and the pump mode of order i, representing the mode field strength. Indicates radius, Indicates angle, express, The curve represents the distribution of thulium ion doping, where i and j represent the order of the corresponding signal and pump mode, respectively, s represents the signal, and p represents the pump.
[0055] S4: Process the overlap integral to obtain the average overlap integral and the maximum overlap difference.
[0056] The average overlap integral is a parameter that reflects the overall gain level.
[0057] Maximum overlap difference is a parameter of gain imbalance between quantization modes.
[0058] In some embodiments, the expression for the average overlap integral is: ; The expression for the maximum overlap difference is: ; in, Represents the average overlap integral. express, express, Indicates the maximum overlap difference. This represents the function that takes the maximum value. This represents the overlap integral of the pump mode and the signal mode. This represents the function that takes the minimum value.
[0059] S5: Based on the average overlap integral and the maximum overlap difference, the parameters of the thulium ion doping distribution curve are adjusted to obtain the design results of the few-mode thulium-doped fiber amplifier, thus completing the design of the few-mode thulium-doped fiber amplifier.
[0060] The design results of the few-mode thulium-doped fiber amplifier are the design results of a few-mode thulium-doped fiber amplifier that balances high gain and mode equalization.
[0061] In some embodiments, the optimization objective of the processor is to make Maximize and By minimizing these parameters, the optimal doping curve parameters can be determined, thereby achieving both high gain and mode equalization over a wide bandwidth.
[0062] By adopting the optimized structural parameters of the few-mode thulium-doped fiber described above, the few-mode thulium-doped fiber amplifier in this embodiment is designed for the 1720–2020nm band and can support stable transmission and amplification of 14 linearly polarized modes.
[0063] In some embodiments, such as Figure 4 The diagram shows the normalized field intensity distribution of each linearly polarized mode in the few-mode thulium-doped fiber of this embodiment. The area between the two dashed lines represents the annular fiber core region. The horizontal axis represents the radial position (micrometers) of the few-mode thulium-doped fiber, and the vertical axis represents the normalized optical field intensity. Because the optical field intensity distribution differs between different modes, the overlap integral between the thulium ion region and the optical field intensity distribution is different, which is the source of differential mode gain. Therefore, it is necessary to select a suitable doping curve to achieve gain equalization in the fiber amplifier.
[0064] In some embodiments, such as Figure 5 The figure shows the parabolic normalized doping distribution in the few-mode thulium-doped fiber as a function of parameters in this embodiment. The horizontal axis represents the radial position (micrometers) of the few-mode thulium-doped fiber, and the vertical axis represents the normalized thulium ion doping concentration. By adjusting the peak position h and the doping width c, the shape of the doping curve can be precisely controlled, achieving optimized design for gain balance across modes.
[0065] In some embodiments, such as Figure 6 The figure shows the scanning results of the average overlap integral ops_avg and the maximum overlap difference ops_d in the few-mode thulium-doped fiber amplifier of this embodiment, as a function of doping parameters h and c. The horizontal axis represents the peak position h (micrometers), the vertical axis represents the doping width c (micrometers), the grayscale level represents the magnitude of the average overlap integral ops_avg, and the contour lines represent the value of the maximum overlap difference ops_d. Within the region of h ≈ 9.5 micrometers and c ≈ 3 micrometers, ops_d approaches its minimum value, while ops_avg remains relatively large. This can be directly determined as the optimal doping scheme, achieving low differential mode gain amplification without additional pump mode control.
[0066] In some embodiments, such as Figure 7 The figure shows the curves of different mode gains versus wavelength in the few-mode thulium-doped fiber amplifier of this embodiment; the horizontal axis represents wavelength (nm) and the vertical axis represents gain (dB). Under the conditions of pump power of 4W and signal power of -10dBm (0.1mW), the gain is greater than 20dB in the 1720-1990nm range (bandwidth 270nm) and greater than 14dB in the 1720-2020nm range (bandwidth 300nm), demonstrating high mode gain and wide coverage bandwidth, meeting the requirements of practical applications.
[0067] In some embodiments, such as Figure 8The figure shows the curves of the maximum differential mode gain (DMG) between different modes in the few-mode thulium-doped fiber of this embodiment as a function of wavelength; the horizontal axis represents wavelength (nm) and the vertical axis represents differential mode gain (dB). As can be seen from the figure, the differential mode gain fluctuates with wavelength, but its value is relatively small, below 1.42dB in the 300nm range of 1720-2020nm.
[0068] In some embodiments, a design method for a broadband low differential mode gain few-mode thulium-doped fiber amplifier is applied to design a broadband low differential mode gain few-mode thulium-doped fiber amplifier, comprising: a fundamental mode pump source for ring core pumping of the few-mode thulium-doped fiber; and a few-mode thulium-doped fiber for amplifying the input optical signal to obtain a broadband low differential mode gain amplification result.
[0069] This invention provides an optical fiber amplifier, comprising a few-mode thulium-doped fiber provided by any of the above embodiments, wherein the few-mode thulium-doped fiber amplifies the input optical signal and increases the gain of the input optical signal; the optical fiber amplifier further comprises a fundamental mode pump source, which is used to pump the few-mode thulium-doped fiber in a ring core 2.
[0070] In some embodiments, the fundamental mode pump source includes a fundamental mode fiber, which comprises, from the inside out, a fundamental mode inner cladding, a fundamental mode annular core, a trench auxiliary structure, and a fundamental mode outer cladding. That is, the structure of the fundamental mode fiber is similar to and has the same dimensions as the few-mode thulium-doped fiber of the present invention, except that the fundamental mode annular core is not doped with thulium ions. The fundamental mode, LP, is generated through the fundamental mode fiber. 01 The mold is used to pump the annular fiber core 2.
[0071] In some embodiments of this specification, a design method for a broadband low differential mode gain few-mode thulium-doped fiber amplifier is provided. By limiting the range of the ratio of the inner ring radius to the outer ring radius of the ring core 2, the size and thickness of the ring core 2 can be adjusted to a suitable range. This supports the transmission of as many linearly polarized modes as possible while suppressing the generation of higher-order radial modes, avoiding the occurrence of higher-order radial modes due to excessive thickness of the ring core 2, and avoiding the occurrence of extremely poor transmission mode quality due to excessively thin thickness of the ring core 2, thus ensuring stable transmission of linearly polarized modes.
Claims
1. A broadband low differential mode gain few-mode thulium-doped fiber amplifier, characterized in that, include: A fundamental mode pump source is used for ring core pumping of few-mode thulium-doped optical fibers. Few-mode thulium-doped fiber is used to amplify the input optical signal to obtain a broadband low-differential-mode gain amplification result.
2. The broadband low differential mode gain few-mode thulium-doped fiber amplifier according to claim 1, characterized in that, The few-mode thulium-doped fiber comprises: The inner cladding (1), the annular core (2), the groove (3) and the outer cladding (4) are wrapped from the inside to the outside. The refractive indices of the annular core (2), the inner cladding (1), the outer cladding (4) and the groove (3) decrease sequentially. The thulium ion doping distribution in the annular core (2) is parabolic.
3. The broadband low differential mode gain few-mode thulium-doped fiber amplifier according to claim 2, characterized in that, The thulium ion doping distribution curve in the annular fiber core (2) is an inverted U-shaped parabolic doping, with a peak value set at a specified radius inside the annular core, decreasing symmetrically towards both the inner and outer sides, and the rate of decrease is controlled by the doping width parameter.
4. The broadband low differential mode gain few-mode thulium-doped fiber amplifier according to claim 2, characterized in that, The outer cladding layer (4) is a silicon dioxide component, the inner cladding layer (1) is a silicon dioxide component uniformly doped with germanium or phosphorus, the trench (3) is a silicon dioxide component uniformly doped with boron or fluorine, and the annular fiber core (2) is a silicon dioxide component uniformly doped with either germanium or phosphorus, and is a thulium ion component doped in a parabolic pattern.
5. A design method for a broadband low differential mode gain few-mode thulium-doped fiber amplifier, applied to the broadband low differential mode gain few-mode thulium-doped fiber amplifier as described in any one of claims 1 to 4, characterized in that, include: S1: Obtain the inner and outer ring radii of the annular fiber core, the refractive index of the annular fiber core, and the refractive index of the outer cladding. Calculate the ratio of the inner and outer radii of the annular fiber core and the normalized frequency. S2: Based on the normalized frequency and the ratio of the inner and outer radii of the ring fiber core, the ring fiber core and the trench structure are adjusted to reduce the difference in mode field distribution between different modes while supporting multimode transmission, and the adjustment result is obtained. S3: Based on the adjustment results and combined with the parabolic thulium ion doping distribution curve, calculate the overlap integral between the pump mode and the signal mode; S4: Process the overlap integral to obtain the average overlap integral and the maximum overlap difference; S5: Based on the average overlap integral and the maximum overlap difference, the parameters of the thulium ion doping distribution curve are adjusted to obtain the design results of the few-mode thulium-doped fiber amplifier, thus completing the design of the few-mode thulium-doped fiber amplifier.
6. The design method of the broadband low differential mode gain few-mode thulium-doped fiber amplifier according to claim 5, characterized in that, The expression for the normalized frequency is: ； The expression for the radius ratio is: ； in, Represents the normalized frequency. Indicates wavelength. This indicates the outer ring radius of the annular fiber core. This indicates the refractive index of the toroidal fiber core. Indicates the refractive index of the outer cladding. This represents the ratio of the inner and outer ring radii of the toroidal fiber core. This indicates the inner ring radius of the toroidal fiber core.
7. The design method of the broadband low differential mode gain few-mode thulium-doped fiber amplifier according to claim 5, characterized in that, The expression for the thulium ion doping distribution curve is as follows: ； in, The curve represents the thulium ion doping distribution, r represents the radius of the optical fiber, h represents the peak position of the doping curve, and c represents the doping width of the doping curve.
8. The design method of the broadband low differential mode gain few-mode thulium-doped fiber amplifier according to claim 5, characterized in that, The expression for the overlap integral of the pump mode and the signal mode is: ； in, This represents the overlap integral between the pump mode and the signal mode. This indicates the inner ring radius of the toroidal fiber core. This indicates the outer ring radius of the annular fiber core. This represents the normalized field strength distribution of the i-th signal mode. Indicates radius, Indicates azimuth. This represents the normalized field strength distribution of the j-th pump mode. The curve represents the distribution of thulium ion doping, where i and j represent the order of the corresponding signal and pump mode, respectively, s represents the signal, and p represents the pump.
9. The design method of the broadband low differential mode gain few-mode thulium-doped fiber amplifier according to claim 5, characterized in that, The expression for the average overlap integral is: ； The expression for the maximum overlap difference is: ； in, Represents the average overlap integral. This represents the function of taking the average value. This represents the overlap integral between the pump mode and the signal mode. Indicates the maximum overlap difference. This represents the function that takes the maximum value. This represents the function that takes the minimum value.