CN102135641B - Active optical fiber with photon darkening resistance and preparation method thereof - Google Patents

Abstract

The invention provides an active optical fiber with photon darkening resistance. Silicon dioxide is used as matrix for a fiber core of the active optical fiber which comprises at least one type of active ions and a codoping agent, wherein the active ions are rare earth ions with the atomic number of 57-71; and the codoping agent contains aluminum, yttrium and cerium ions. The invention also provides a preparation method of the active optical fiber with the photon darkening resistance, comprising the following steps of: firstly carrying out polishing treatment on a pure quartz reaction tube; depositing an envelope layer and a core layer, and then soaking in a hydrochloric acid-alcohol mixed solution of the active ions and the cerium and yttrium ions; drying after soaking; vitrifying the pure quartz reaction tube to prepare an optical fiber preformed rod; and finally drawing the prepared optical fiber preformed rod to prepare the active optical fiber. The active optical fiber is greatly enhanced in photon darkening resistant property; and in addition, an optical fiber laser prepared by using the active optical fiber has the slope efficiency kept more than 80 percent and has higher stability and longer service life.

Description

A kind of anti-photon darkening active optical fiber and its preparation method

technical field

The invention relates to an active optical fiber and a preparation method thereof, in particular to an active optical fiber capable of improving anti-photon darkening performance and a preparation method thereof.

Background technique

High-power fiber laser is an all-solid-state laser device that uses rare earth-doped double-clad fiber as the working medium and semiconductor laser as the pump source. Since Snitzer et al. proposed a double-clad laser fiber in 1988, fiber lasers and amplifiers based on cladding pumping technology have developed rapidly. Especially in recent years, with the development of optical fiber preparation technology and semiconductor lasers, fiber lasers have developed rapidly. In 2005, the United States, the United Kingdom, Germany and Japan respectively achieved a single-fiber output power exceeding the kW level. In 2009, the single-fiber output of IPG photonics in the United States reached the order of 10,000 watts, and the beam quality was near the diffraction limit. Because fiber laser has good beam quality, high efficiency and compact structure, it has important applications in high-precision laser welding, cutting and other industrial applications, laser medical treatment and national defense and military fields. However, as the power of fiber lasers increases, the photon darkening effect in the rare earth-doped fiber core becomes one of the main factors that limit the life and stability of lasers.

The photon darkening effect is manifested as the output power of the fiber laser gradually decreases with time after the pump light pumps the rare earth-doped fiber. The permanent light absorption loss induced by the photon darkening effect is extremely significant in the visible light band, and the loss continues to increase with the increase of the pumping time. After a long time, the absorption is saturated and the loss tends to be stable. Moreover, the absorption tail of the visible light band extends to the near-infrared band, causing the transmission loss of the fiber at the pump wavelength and the laser operating wavelength to increase with time, resulting in a decrease in the slope efficiency of the fiber laser. This phenomenon mostly occurs in active fibers based on silica, and has been observed in active fibers such as ytterbium-doped fibers, thulium-doped fibers, praseodymium-doped fibers, and europium-doped fibers. Among them, ytterbium-doped silica matrix fibers The photon darkening effect has been particularly extensively studied.

However, the mechanism of photon darkening effect is still not conclusive. Typically, the underlying physical process of aging in doped fibers can be attributed to the formation of color centers in the silica matrix or other photoinduced structural changes, such as polymer coating loss or radiation damage from high-energy particles. Since photon darkening is the additional loss of the fiber core caused by signal light and pump light, it is easy to distinguish it from the power reduction induced by effects such as polymer coating loss or high-energy particle radiation damage. The current basic consensus is the formation of color centers Eventually lead to the generation of photon darkening effect. For the explanation of the formation mechanism of the color center, there are mainly two views: one view is that the oxygen deficiency center is the precursor of the formation of the color center (see S.Yoo, C.Basu, A.J.Boyland, et.al., Photodarkening in Yb -doped aluminosilicate fibers induced by 488nm irradiation. Optics Letters, 2007.32 (12)), the other is that charge transfer causes changes in the valence state of rare earth ions and forms holes around the oxygen coordination bond, thereby inducing the generation of color centers (see M. Engholm, L. Norin, et.al., Strong UV absorption and visible luminescence in ytterbium-doped aluminum silicate glass under UVexcitation. Optics Letters, 2007.32(22)).

In order to reduce the photon darkening effect and improve the stability of fiber lasers, many universities and research institutions have carried out related experiments and proposed ways to suppress photon darkening.

US-A-2000/6154598 discloses a method of preventing light-induced loss. Pavle Gavrilovic et al. increased the lifetime of fiber lasers by deliberately introducing appropriate amounts of other rare earths to suppress the upconversion process.

US-2009/0011233-A1 proposes to improve the anti-photon darkening performance of optical active glass and optical fiber by co-doping phosphorus.

US-2009/0231683-A1 proposes a treatment method to suppress photon darkening effect in ytterbium-doped optical fiber. The treatment method is to irradiate the ytterbium-doped fiber with one or more of gamma rays, X-rays or electron beams. The irradiation energy is greater than the light energy transmitted in the fiber when the fiber is in laser oscillation, so that the fiber forms light-induced defects. Subsequently, hydrogen-carrying treatment was carried out on the optical fiber to obtain an ytterbium-doped optical fiber with anti-photon darkening effect.

M. Engholm et al. found that the doping of cerium in ytterbium-doped optical fiber can improve the photon darkening performance. They pointed out that conventional ytterbium-doped silica fiber produced 40% unsaturated loss at 978nm after 48 hours of operation, while cerium-ytterbium co-doped fiber with the same ytterbium doping concentration produced 7-9% saturation loss after 8 hours of operation (see M. Engholm, P. Jelger, et al., Improved photodarkening resistance in ytterbium-doped fiber lasers by cerium codoping. Optics Letters, 2009.34(8): p.1285-1287.). Although the doping of cerium reduces the photon darkening effect to a certain extent, it has an adverse effect on the emission cross section and fluorescence lifetime of ytterbium rare earth ions, which reduces the slope efficiency of optical lasers.

S.Yoo et al. proposed a ytterbium-doped nanocrystal fiber for reducing photon darkening. The core components of this optical fiber are silica, ytterbium oxide, yttrium oxide, aluminum oxide, phosphorus oxide, lithium oxide and barium oxide, and its performance is 10 to 20 times higher than that of aluminosilicate optical fibers ( See S.Yoo, M.P.Kalita, et al., Ytterbium-doped Y2O3nanoparticle silica optical fibers for high power fiber lasers with suppressed photodarkening. Optics Communications, 2010.). However, the doping of phosphorus and other substances will reduce the mechanical properties of the optical fiber and cause a decrease in the damage threshold of the optical fiber. Moreover, the amount of yttrium ion introduced in this document is relatively high, and a certain degree of phase separation is formed in the optical fiber.

In order to further solve the problem of limited output power and stability of the fiber laser caused by the photon darkening effect in the active fiber, this patent proposes a new active fiber and its preparation method. Through the selection of the new co-dopant components of cerium, yttrium and aluminum in the fiber core, and the determination of the appropriate co-dopant concentration and ratio, the microenvironment of the rare earth ions in the fiber core is adjusted to reduce the photon darkening effect. Combined with the design of the area and concentration distribution of active ions in the core structure, the overlap factor of the fiber signal light, pump light and active area is reduced, so as to further improve the anti-photon darkening performance of the fiber.

Contents of the invention

The object of the present invention is to provide a high-performance optical fiber resistant to photon darkening effect and a preparation method thereof.

The invention provides an anti-photon darkening active optical fiber, the fiber core of which is based on silica and contains at least one active ion and a co-dopant, wherein the active ion is a rare earth with an atomic number of 57-71 ions, characterized in that the co-dopants are aluminum, yttrium and cerium ions.

Further, the molar percentage of active ions in the fiber core is 500ppm-15000ppm, the molar ratio of active ions to yttrium ions is 1:0.05-1:10, and the molar ratio of active ions to cerium ions is 1:0.25-1 :8, the molar ratio of rare earth ions to aluminum ions is 1:3 to 1:10.

Further, the concentration distribution of the active ions in the fiber core is uniform core doping, ring step doping, ring gradient doping or circular lattice doping.

The present invention also provides a preparation method of an anti-photon darkening active optical fiber, comprising the following steps:

(1) Feed sulfur hexafluoride into the pure quartz reaction tube, and corrode and polish the inner wall of the pure quartz reaction tube;

(2) Into the pure quartz reaction tube, a mixed gas of silicon tetrachloride and oxygen is introduced, and the cladding layer is deposited 2 times in a forward deposition mode;

(3) After the cladding is deposited, feed silicon tetrachloride, oxygen and aluminum chloride into the pure quartz reaction tube, and deposit a core layer by forward deposition;

(4) Soak the deposited reaction tube evenly in a hydrochloric acid alcohol mixture containing active ions, cerium and yttrium ions;

(5) Feed chlorine and oxygen into the pure quartz reaction tube, the flow ratio of chlorine and oxygen is 1: 5-1: 10, and the reaction tube is dried;

(6) Under the mixed atmosphere of chlorine, helium and oxygen, the pure quartz reaction tube is vitrified, the reaction temperature is 2000-2200 degrees Celsius, the flow rate of chlorine gas is 5-50 sccm, the flow rate of helium gas is 10-50 sccm, and the flow rate of oxygen gas is 50- 300 sccm;

(7) Determine whether the concentration distribution of active ions is uniform doping of the fiber core, if so, directly enter step (8), otherwise, repeat steps (3) to (6), until the preparation of the doped fiber core is completed, enter step (8);

(8) shrinking the rod under an atmosphere of 2000-2200 degrees Celsius, a chlorine gas flow rate of 5-30 sccm, and an oxygen flow rate of 100-200 sccm to complete the preparation of the optical fiber preform;

(9) Drawing the prepared optical fiber preform to make an active optical fiber.

The beneficial effects of the present invention are:

1. The co-dopants used in the active optical fiber of the present invention have less influence on the damage threshold of the optical fiber, and maintain the high damage threshold characteristics of the quartz matrix active optical fiber, and are more suitable for preparing high-density active optical fibers than co-doped phosphorus active optical fibers. power fiber laser.

2. The characteristics of the active fiber absorption cross section, emission cross section and fluorescence lifetime of the present invention are almost the same as those of the active fiber co-doped with aluminum and ytterbium, and the prepared fiber laser still maintains a slope efficiency of more than 80%, which greatly improves the selection For other fiber core components, the slope efficiency of the fiber laser is reduced.

3. The anti-photon darkening performance of the active optical fiber of the present invention is greatly improved, and the prepared fiber laser has high stability and long service life. The output power of fiber lasers prepared by traditional active fibers will decrease continuously during operation. Currently, there are reports of 20% reduction in output power. After 50 hours of operation of this fiber laser, the slope efficiency decreases by less than 5%.

Description of drawings

Fig. 1 is the cross section and the schematic diagram of the refractive index distribution of the active optical fiber whose inner cladding is hexagonal and resists the photon darkening effect;

Fig. 2 is a schematic diagram of the distribution of doping concentration in the core part of the optical fiber;

Fig. 3 is the slope efficiency diagram of the laser prepared by the optical fiber described in embodiment one;

Fig. 4 is the attenuation diagram of the laser output power prepared by the optical fiber described in embodiment one;

Fig. 5 is a schematic flow chart of preparing an active optical fiber by MCVD and liquid phase doping technology.

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

The active optical fiber of the present invention has a new core structure, the core is based on silica, and contains at least one active ion and a co-dopant, wherein the co-dopant is aluminum, yttrium and cerium three ions. The active ions are rare earth ions with atomic number 57-71.

The molar percentage of active ions in the fiber core is 500ppm to 15000ppm, the molar ratio of active ions to yttrium ions is 1:0.05 to 1:10, and the molar ratio of active ions to cerium ions is 1:0.25 to 1:8, The total molar ratio of rare earth ions to aluminum ions is 1:3 to 1:10.

The concentration distribution of active ions in the fiber core can be uniform core doping, ring step doping, ring gradient doping and circular lattice doping, etc. The uniform doping of the core means that the active ions in the entire area of the core have the same mole percentage; as shown in Figure 2(a), the annular step doping refers to the introduction of active ions in several annular areas in the core, The active ion concentration in each area is the same; as shown in Figure 2 (b), the annular gradient doping refers to the introduction of active ions in the core according to several annular areas, and the active ion concentration in each area is gradual; as As shown in Figure 2(c), circular lattice doping means that the core is drawn by stacking several active rods containing active ions, that is, the active ion doping area inside the core is several circular areas.

The doping methods such as annular step doping, annular gradient doping, and circular lattice doping reduce the concentration of active ions in the central area of the fiber core, thereby reducing the overlap factor of the fiber signal light, pump light, and active area. , reduce the additional loss caused by the photon darkening effect during the pumping process of the pump light source, slow down the rate of photon darkening, and improve the anti-photon darkening performance of the optical fiber.

The active optical fiber described in the first embodiment of the present invention is a double-clad ytterbium-doped silica fiber, as shown in Figure 1, the composition of the fiber core 11 is a silica matrix, active ion ytterbium ions and co-dopant Aluminum, yttrium, cerium ions, the refractive index _n1 is 1.469; the component of the inner cladding 12 is pure quartz, and its refractive index _n2 is 1.457; the component of the outer cladding 13 is a polymer coating with a low refractive index, the refractive index n ₃ is 1.37; the composition of the coating layer 14 is a polymer coating with a high refractive index, and the refractive index n ₄ is 1.49.

The doping structure of the fiber core adopts an annular step doping method, wherein the composition of the inner region of the fiber core 11 (that is, the circular region inside the fiber core 11 in FIG. 1 ) includes: the molar content of the active ion ytterbium ion is about 1500ppm , the co-dopant cerium ion is about 12000ppm, the yttrium ion is about 1500ppm, and the aluminum ion is about 50000ppm; the outer region of the core 11 includes: a silica matrix, the molar content of the active ion ytterbium ion is about 15000ppm, and the co-dopant cerium ion About 5000ppm, about 7500ppm for yttrium ion, and about 65000ppm for aluminum ion.

A pump source with a maximum power of 15W and a wavelength of 915nm is used, and the working wavelength of the laser is 1080nm. By testing the absorbed pump power and the output laser power, the slope efficiency of the fiber laser is obtained. As shown in Figure 3, the test results show the first The double-clad ytterbium-doped silica fiber described in the examples has a slope efficiency of 82%. After a period of time, the fluctuation of output power of fiber laser reflects the influence of photon darkening effect on laser performance. After a period of continuous operation of the fiber laser prepared by the conventional ytterbium-aluminum co-doped fiber, the output power of the laser has a significant attenuation, and the attenuation is relatively large, while the output power of the fiber laser prepared by the sample fiber is small, as shown in Figure 4 As shown, the test results show that the sample fiber has a high slope efficiency, and the anti-photon darkening performance of the sample fiber is greatly improved.

The method for preparing the double-clad ytterbium-doped silica fiber described in this embodiment by using the MCVD manufacturing process and liquid phase doping technology, as shown in Figure 5, includes the following steps:

(1) Feed sulfur hexafluoride into the pure quartz reaction tube 31, and corrode and polish the inner wall of the pure quartz reaction tube 31;

(2) Feed silicon tetrachloride into the pure quartz reaction tube 31 , introduce silicon tetrachloride gas with oxygen, and deposit the cladding layer 32 twice by using the forward deposition method.

(3) After the cladding is deposited, silicon tetrachloride is introduced into the pure quartz reaction tube 31 in the form of silicon tetrachloride gas carried by oxygen. Aluminum chloride is introduced by baking aluminum chloride powder at high temperature. One pass of the core layer 33 is deposited in the forward direction.

(4) Soak the deposited reaction tube with a hydrochloric acid alcohol mixture containing active ions, cerium and yttrium ions for a long time, and rotate the reaction tube during the soaking process to make the soaking even.

(5) After the solution is fully soaked, chlorine gas and oxygen gas are introduced into the pure quartz reaction tube 31 with a flow ratio of 1:5, and the reaction tube is dried.

(6) Vitrify the pure quartz reaction tube 31 under a mixed atmosphere of chlorine, helium and oxygen, the reaction temperature is 2000 degrees Celsius, the flow rate of chlorine gas is 5 sccm, the flow rate of helium gas is 10 sccm, and the flow rate of oxygen gas is 300 sccm.

(7) Steps (3) to (6) are repeated once to complete the preparation of the doped fiber core.

(8) Shrunk the rod under an atmosphere of 2000 degrees Celsius, a chlorine gas flow rate of 5 sccm, and an oxygen flow rate of 100 sccm to complete the preparation of the optical fiber preform.

(9) Drawing the optical fiber preform into a double-clad optical fiber by using a drawing tower. That is, after the optical fiber preform is prepared, the preform is processed into the required geometric shape through precision machining. Subsequently, the optical fiber preform is drawn into an optical fiber using a drawing tower. A low-refractive-index polymer coating is used for primary coating, and a high-refractive-index polymer coating is used for secondary coating to obtain a double-clad optical fiber. The refractive index of the low refractive index polymer coating is 1.37, and the refractive index of the high refractive index polymer coating is 1.49.

The active optical fiber described in the second embodiment of the present invention is a double-clad ytterbium-doped silica optical fiber, and the core 11 components are silica matrix, active ion ytterbium ions and co-dopants aluminum, yttrium, and cerium ions , the refractive index n ₁ is about 1.459; the inner cladding 12 components are pure quartz, and its refractive index n ₂ is 1.457; the outer cladding 13 components are low refractive index polymer coatings, and the refractive index n ₃ is 1.37; the coating layer The 14-component is a high-refractive-index polymer coating with a refractive index _n4 of 1.49.

The doping structure of the fiber core adopts a uniform doping method, that is, the active ion doping concentration of the fiber core 11 is evenly distributed. The fiber core components are: silica matrix, the molar content of the active ion ytterbium ion is about 500ppm, the co-dopant cerium ion is about 500ppm, the yttrium ion is about 4000ppm, and the aluminum ion is about 10000ppm.

The preparation method of this double-clad ytterbium-doped silica optical fiber, steps (1)-(4) are the same as the preparation method in the first embodiment, and step (5) feeds chlorine gas and oxygen into the pure silica reaction tube, and the flow ratio is 1 : 10, drying the reaction tube.

(6) Vitrify the pure quartz reaction tube in a mixed atmosphere of chlorine, helium and oxygen, the reaction temperature is 2200 degrees Celsius, the flow rate of chlorine gas is 50 sccm, the flow rate of helium gas is 50 sccm, and the flow rate of oxygen gas is 50 sccm, and the core doping part is completed preparation.

(7) Shrunk the rod under an atmosphere of 2200 degrees Celsius, chlorine gas flow rate of 30 sccm, and oxygen flow rate of 200 sccm to complete the preparation of the optical fiber preform.

(8) Drawing the optical fiber preform into a double-clad optical fiber by using a drawing tower.

The active optical fiber described in the third embodiment of the present invention is a double-clad ytterbium-doped photonic crystal fiber, and the core 11 components are silica matrix, active ion ytterbium ions and co-dopants aluminum, yttrium, and cerium Ion, the capillary duty cycle of the inner air hole layer is 18%, and the capillary duty cycle of the outer air hole layer is 89%. Among them, the doping structure of the fiber core adopts a circular lattice doping method, as shown in Figure 2(c), the fiber core component is: silica matrix, the molar content of the active ion ytterbium ion is about 3000ppm, co-doped The cerium ion is about 1500ppm, the yttrium ion is about 30000ppm, and the aluminum ion is about 13500ppm.

The preparation method of this double-clad ytterbium-doped photonic crystal fiber, steps (1)-(4) are the same as the preparation method in the first embodiment, and step (5) feeds chlorine gas and oxygen into the pure quartz reaction tube, the flow ratio The ratio is 1:7, and the reaction tube is dried.

(6) Under the mixed atmosphere of chlorine, helium and oxygen, the pure quartz reaction tube is vitrified, the reaction temperature is 2100 degrees Celsius, the flow rate of chlorine gas is 25 sccm, the flow rate of helium gas is 25 sccm, and the flow rate of oxygen gas is 200 sccm.

(7) Steps (3) to (6) are repeated to complete the preparation of the doped fiber core.

(8) Shrunk the rod at 2100 degrees Celsius under an atmosphere with a chlorine gas flow rate of 15 sccm and an oxygen flow rate of 150 sccm to complete the preparation of the optical fiber preform.

(9) The optical fiber preform is drawn by a secondary drawing method to make an active photonic crystal optical fiber. That is, after the optical fiber preform is prepared, the optical fiber preform is drawn into a 1mm thin rod with a drawing tower, and then several thin rods are piled up and drawn into a 0.85mm thin rod as the fiber core. 5 layers of capillary tubes with a diameter of 0.85mm and 18% duty ratio are used as the inner air hole layer; quartz glass tubes are used as the quartz inner cladding layer; a layer of capillary tubes with a duty ratio of 89% is arranged outside the quartz inner cladding layer as the outer layer. The air hole layer, and the jacketed quartz glass tube is used as the quartz outer cladding. After drawing, a photonic crystal fiber with a circular lattice doping structure is obtained after the second coating.

The present invention is not limited to the above-mentioned specific embodiments, and those skilled in the art can adopt various other specific embodiments to implement the present invention according to the disclosed content of the present invention. Changes or modified designs all fall within the protection scope of the present invention.

Claims (4)

1. An anti-photon darkening active optical fiber, the core of which is based on silica, contains at least one active ion, and also includes a co-dopant, wherein the active ion is atomic number 57-71 Rare earth ions, characterized in that the co-dopants are aluminum, yttrium and cerium ions; The molar percentage of active ions in the fiber core is 500ppm to 15000ppm, the molar ratio of active ions to yttrium ions is 1:0.05 to 1:10, and the molar ratio of active ions to cerium ions is 1:0.25 to 1:8. The molar ratio of active ions to aluminum ions is 1:3 to 1:10; The concentration distribution of the active ions in the fiber core is uniform core doping, ring step doping, ring gradient doping or circular lattice doping. 2. A preparation method of an anti-photon darkening active optical fiber, comprising the following steps: (1) Feed sulfur hexafluoride into the pure quartz reaction tube, and corrode and polish the inner wall of the pure quartz reaction tube; (2) Into the pure quartz reaction tube, a mixed gas of silicon tetrachloride and oxygen is introduced, and the cladding layer is deposited 2 times in a forward deposition mode; (3) After the cladding is deposited, feed silicon tetrachloride, oxygen and aluminum chloride into the pure quartz reaction tube, and deposit a core layer by forward deposition; (4) Soak the deposited reaction tube evenly in a hydrochloric acid alcohol mixture containing active ions, cerium and yttrium ions; wherein the active ions are rare earth ions with atomic numbers 57 to 71, and the molar percentage of active ions in the core is 500ppm~15000ppm, the molar ratio of active ions to yttrium ions is 1:0.05~1:10, the molar ratio of active ions to cerium ions is 1:0.25~1:8, the molar ratio of active ions to aluminum ions 1:3～1:10; (5) Feed chlorine and oxygen into the pure quartz reaction tube, the flow ratio of chlorine and oxygen is 1: 5-1: 10, and the reaction tube is dried; (6) Under the mixed atmosphere of chlorine, helium and oxygen, the pure quartz reaction tube is vitrified, the reaction temperature is 2000-2200 degrees Celsius, the flow rate of chlorine gas is 5-50 sccm, the flow rate of helium gas is 10-50 sccm, and the flow rate of oxygen gas is 50- 300 sccm; (7) Determine whether the concentration distribution of active ions is uniform doping of the fiber core, if so, directly enter step (8), otherwise, repeat steps (3) to (6), until the preparation of the doped fiber core is completed, enter step (8); (8) shrinking the rod under an atmosphere of 2000-2200 degrees Celsius, a chlorine gas flow rate of 5-30 sccm, and an oxygen flow rate of 100-200 sccm to complete the preparation of the optical fiber preform; (9) Drawing the prepared optical fiber preform to make an active optical fiber. 3. The preparation method according to claim 2, characterized in that, in step (9), the optical fiber preform is drawn into an active photonic crystal optical fiber by a secondary drawing method. 4. The preparation method according to claim 2, characterized in that in step (9), a drawing tower is used to draw the optical fiber preform into a double-clad optical fiber.

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