CN103728691A - Gain fiber with step and gauss composite ion doping concentration distribution - Google Patents

Abstract

The invention relates to a Gaussian composite type doping ion concentration distribution gain fiber which belongs to the technical field of fiber laser. In the prior art, the proportion of the fundamental mode light power in the gain light output is not high. The step-Gaussian composite type doping ion concentration distribution gain fiber of the present invention is a kind of large-core-diameter multimode fiber, has a double-clad structure, and is doped with rare earth ions in the fiber core doping radius _R'1 , and is characterized in that, The distribution of the dopant ion concentration N is divided into two areas, the circular area from 0 to R ₀ within the range of the core radius R ₁ is the step area, R ₀ is the radius of the step area, R ₀ <R' ₁ , in the step The dopant ion concentration N distribution in the jump region is step-type, and the dopant ion concentration N is the maximum value N _max ; the R ₀ ~ R' ₁ ring area within the range of the core radius R ₁ is a Gaussian region, and in the Gaussian region The distribution of doping ion concentration N is Gaussian, and the doping ion concentration N is determined by the following formula:

In the formula: r is the fiber radius of the gain fiber.

Description

Stepped Gaussian composite doped ion concentration distribution gain fiber

technical field

The invention relates to a step-Gauss compound type doping ion concentration distribution gain fiber, which belongs to the technical field of fiber laser.

Background technique

Gain fibers are used as wavelength converters, optical amplifiers, fiber lasers, etc., and have a double-clad structure. The fibers are core 1, inner cladding 2, outer cladding 3, and protective layer 4 from the inside to the outside, as shown in Figure 1. The core 1 and the outer cladding 3 are circular, the radius of the core 1 is the core radius R ₁ , and the inner cladding 2 generally adopts a special-shaped structure, and its cross-sectional shape is elliptical, rectangular, quincunx, D-shaped, and hexagonal, etc. , commonly used rectangle, such as a square, at this time, the radius R ₂ of the inner cladding 2 refers to the radius of the inscribed circle of the square, and the rectangular inner cladding 2 can increase the laser conversion efficiency to 50%. The refractive indices of the core 1, the inner cladding 2, and the outer cladding 3 are n ₁ , n ₂ , and n ₃ in sequence, and n ₁ >n ₂ >n ₃ . Rare earth ions are doped within the doping radius R'1 of the fiber core ₁ , and the doping radius _R'1 is smaller than the fiber core radius _R1 . The pumping source is two or four high-power semiconductor lasers. The pumping light enters the inner cladding 2 from both ends of the double-clad fiber, is totally reflected by the interface between the inner cladding 2 and the outer cladding 3, and passes through the fiber core 1. Doped ions provide pump energy to obtain gain light, and a single fiber has achieved 1000W single-mode continuous output of gain light.

The gain fiber used as a power device is usually designed as a large-core multimode fiber in order to meet the requirements of high-power working conditions. Due to mode competition, the gain light contains high-order modes. The concentration distribution of dopant ions in the fiber core 1 is usually a step type, as shown in Figure 2, that is, within the doping radius _R'1 , the dopant ion concentration N everywhere is the same dopant ion concentration The maximum value N _max . Therefore, in the process of optical gain, the basic mode and high-order mode are gained and output at the same time, and the quality of the gain light is not high; at the same time, the pump energy is wasted. In order to solve this problem, a parabolic doping ion concentration distribution scheme has appeared in the prior art, as shown in Figure 3, within the doping radius R' ₁ , the doping ion concentration N increases with the fiber radius r from the center O At the beginning of the increase, the dopant ion concentration N _max decreases along a parabola. When the fiber radius r reaches the doping radius _R'1 , the dopant ion concentration N decreases to zero. According to this scheme, the higher-order mode gain in the peripheral region of the mode field is suppressed. However, under the premise of the same dopant ion concentration maximum value N _max , the fundamental mode gain in the central region of the mode field is also reduced compared with the step type. Moreover, the unilateral parabola has no inflection point and is a complete convex curve. The dopant ion concentration N does not decrease rapidly as the fiber radius r increases from the center O, and the suppression effect on high-order mode gain is not optimal.

Contents of the invention

In order to improve the fundamental mode gain of the large-core multimode gain fiber, suppress the high-order mode gain, and improve the quality of the gain light, we invented a step-Gaussian composite doped ion concentration distribution gain fiber.

The step-Gaussian composite type doped ion concentration distribution gain fiber of the present invention is a large-core multimode fiber with a double-clad structure, and is doped with rare earth ions within the fiber core 1 doping radius R' ₁ , and is characterized in that , as shown in Figure 4, the distribution of the dopant ion concentration N is divided into two regions, the circular region from 0 to R ₀ within the range of the core radius R ₁ is the step region, R ₀ is the radius of the step region, and R ₀ <R' ₁ , the dopant ion concentration N distribution in the step region is step type, and the dopant ion concentration N is the maximum value N _max ; the R ₀ ~ R' ₁ ring region within the core radius R ₁ is In the Gaussian region, the dopant ion concentration N distribution in the Gaussian region is Gaussian, and the dopant ion concentration N is determined by the following formula:

$\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="R"\; filenames="HDA0000447297230000011.png"\; state="\{\{state\}\}">R</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; ,</span>$

In the formula: r is the fiber radius of the gain fiber.

The effect of the present invention is that since the value of dopant ion concentration N in the step region of the core 1 is the maximum value N _max everywhere, the fundamental mode gain has a high gain level of the existing step gain fiber. Moreover, in the Gaussian region, the dopant ion concentration N increases with the fiber radius r starting from the center O, and the dopant ion concentration N decreases from the maximum value N _max along the Gaussian curve. When the fiber radius r reaches the doping radius R' When ₁ , the doping ion concentration N decreases to zero, while the Gaussian curve has an inflection point compared with the parabola. With the increase of the fiber radius r, the doping ion concentration N decreases faster, and the higher-order modes are more effectively suppressed. The ratio of the fundamental mode of the optical power in the gain light is increased, which effectively avoids the pulse broadening and nonlinear effects caused by mode competition, homogenizes the distribution of the optical power in the core 1, and improves the output beam quality and output power.

Description of drawings

Figure 1 is a schematic cross-sectional view of a double-clad fiber structure. Fig. 2 is a distribution diagram of the existing step-type dopant ion concentration. Fig. 3 is a prior art parabolic dopant ion concentration distribution diagram. Fig. 4 is a step-Gaussian complex type doping ion concentration distribution diagram of the gain fiber doping ion concentration distribution diagram of the present invention, which is also used as a summary drawing.

Detailed ways

The step-Gaussian composite doped ion concentration distribution gain fiber of the present invention is a large-core multimode fiber with a double-clad structure, and rare earth ions are doped in the fiber core 1 doping radius _R'1 , as shown in Figure 4 As shown, the distribution of doping ion concentration N is divided into two regions, the circular region from 0 to R ₀ within the range of core radius R ₁ is the step region, R ₀ is the radius of the step region, R ₀ <R' ₁ , and the specific relationship between the step region radius R ₀ and the doping radius R' ₁ is: R ₀ =0.5～0.7R' ₁ , the distribution of doping ion concentration N in the step region is a step type, and the doping ion concentration N is the maximum value N _max ; the ring region R ₀ ~ R' ₁ within the range of the core radius R ₁ is a Gaussian region, and the dopant ion concentration N distribution in the Gaussian region is Gaussian, and the dopant ion concentration N is given by the following formula Decide:

$\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="R"\; filenames="HDA0000447297230000011.png"\; state="\{\{state\}\}">R</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; ,</span>$

In the formula: r is the fiber radius of the gain fiber.

The dopant ion is Yb ³⁺ , and the output wavelength is 1.064 μm. R ₀ =0.7R' ₁ , R' ₁ =15 μm. The refractive index profile of the gain fiber is a step type. Through numerical calculation, it is found that under the premise that the relative gain coefficient of the fundamental mode is the largest, the relative suppression coefficient of the high-order mode reaches the maximum, such as 0.2088, while under the same conditions, the relative suppression coefficient of the high-order mode of the parabolic doping concentration distribution gain fiber is only 0.1418.

Claims (2)

1. the compound doping ion concentration distribution of a step Gauss gain fibre, for the large core diameter multimode optical fiber of one, has double clad structure, fibre core (1) doping radius R ' ₁interior doping with rare-earth ions, is characterized in that, its distribution of doping ion concentration N is divided into two regions, fiber core radius R ₁0～R in scope ₀border circular areas is step district, R ₀for step district radius, R ₀<R' ₁, the ion concentration of adulterating in step district N is distributed as step change type, and doping ion concentration N is maximal value N _max; Fiber core radius R ₁r in scope ₀～R' ₁circle ring area is Gauss district, and the ion concentration of adulterating in Gauss district N is distributed as Gaussian, and doping ion concentration N is determined by following formula:

$N\left(r\right)=N_{\max }\&CenterDot;\exp [-\frac{1}{2}{\left(\frac{r-R_0}{R_1^{\&prime;}-R_0}\right)}^2],$

In formula: the fiber radius that r is described gain fibre.

2. the compound doping ion concentration distribution of step Gauss according to claim 1 gain fibre, is characterized in that, step district radius R ₀with doping radius R ' ₁physical relationship be: R ₀=0.5～0.7R' ₁.

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