Apparatus and Method for Preparing Fluoride Optical Fiber Perform

Abstract

The present invention discloses a preparation device for a high-quality fluoride optical fiber preform and a preparation method for the fluoride optical fiber preform. In this device, a low pressure is provided by a heating furnace (with a lifting function), the glass liquid is injected into the mold from bottom to top, and a certain micro-negative pressure is formed in the cavity by pumping, which promotes the bubbles in the glass liquid to float up, and a "gas flotation" effect can be formed. The glass liquid changes from liquid state to solid state through different temperature zones, and the prepared preform has the characteristics of axial uniformity and no bubbles. The preparation method is simple and fast, and the whole process of preform forming is controlled by a program, avoiding the influence of errors caused by manual operation, and is expected to be used for drawing low-loss fluoride optical fibers.

Description

Technical Field

[0001] The present invention relates to a device and method for preparing a fluoride glass optical fiber preform, and is particularly suitable for preparing a uniform and bubble-free optical fiber preform. Technical Background

[0002] Fluoride glass optical fibers have extremely low theoretical losses (0.01 dB / km @ 2.55 μm) and a broad transmission spectrum (0.3 μm - 8 μm), and have great advantages in transmitting the mid-infrared band. At the same time, they also have a relatively high damage threshold (typical value: 10.2 MW / cm 2 , chalcogenide glass: 8.8×10 -3 MW / cm 2 ), making them the most promising materials for applications in military fields such as infrared countermeasure and infrared guidance. The preparation of fluoride optical fiber preforms is one of the most important links in the overall process of fluoride optical fiber preparation. Parameters such as the NA, core-cladding size, and transmission mode of the optical fiber are all determined by the preform structure. The traditional methods for preparing optical fiber preforms and their advantages and disadvantages are shown in Table 1. It can be seen that the casting method and the rotary casting method are prone to generating bubbles in the core layer glass, and the double crucible method and the tube-rod method are prone to generating bubbles and crystallization at the core-cladding interface, and these two methods are complex to operate. Although the suction casting method is not troubled by the bubble problem, the core layer of the prepared preform is conical, which is not conducive to the uniform drawing of the optical fiber over a long distance. Bubbles are one of the main factors causing losses in optical fibers, especially the bubbles in the core layer glass. According to the principle of total reflection transmission, most of the transmission paths of the optical fiber are in the core layer of the optical fiber. Therefore, reducing the bubbles in the core layer glass can effectively reduce the transmission loss of the optical fiber and avoid damage to the optical fiber caused by heat generation due to scattering during high-power transmission of the optical fiber. There are two reasons why the traditional preform preparation methods cannot solve the core layer bubble problem: 1. Microbubbles cannot be completely removed during the glass melting process; 2. Bubbles generated during the glass transfer or casting process cannot be discharged in time. The current process cannot simultaneously meet the requirements for preparing a bubble-free and core-cladding uniform preform. There is an urgent need for a new type of equipment and preparation process to prepare a bubble-free and axially uniform fluoride optical fiber preform. Therefore, we have designed a device and process technology for preparing fluoride optical fiber preforms, which can obtain a bubble-free and axially uniform optical fiber preform.

[0003] Table 1. Traditional Methods for Preparing Optical Fiber Preforms and Their Advantages and Disadvantages

[0004] Summary of the Invention

[0005] The purpose of the present invention is to provide a device and process for preparing fluoride optical fiber preforms. Compared with the traditional devices and processes, this device can prepare bubble-free and axially uniform fluoride optical fiber preforms, providing conditions for drawing low-loss fluoride optical fibers.

[0006] The technical solution of the present invention is as follows:

[0007] A preparation device for a fluoride optical fiber preform, comprising a solid-state forming die, a liquid-state forming die, a distributed temperature gradient heating device, a metal cavity, a glass liquid drainage die, a platinum crucible and a heating furnace;

[0008] The metal cavity is provided with air inlet and air extraction valves at the top of the cavity. The cavity contains a solid-state forming die, a liquid-state forming die and a distributed temperature gradient heating device. The solid-state forming die has a through structure in the middle, its top is communicated with the internal environment of the cavity, and its bottom is connected to the liquid-state forming die through a card slot. The inner diameter of the solid-state forming die is not less than the outer diameter size of the preform. The liquid-state forming die is a through cylindrical structure, divided into an inner column and an outer column. The diameter of the inner column determines the core layer size, and the diameter of the outer column determines the cladding layer size. The bottom end of the die is connected to the heating furnace opening, and a distributed temperature gradient heating device is sleeved outside the die.

[0009] The heating furnace has a programmable lifting function and can push the platinum crucible upward as a whole. The platinum crucible is a double-layer cup-shaped structure. The inner cup is used to hold the core layer glass, and the outer cup is used to hold the cladding layer glass. The size of the platinum crucible matches the size of the inlet end of the glass liquid drainage die. The glass liquid drainage die is welded by four through cylindrical dies of different sizes. The glass liquid drainage die is also divided into an inner die and an outer die. The large mouth end is the inlet end, and its size matches the size of the platinum crucible. The small mouth is the outlet end, and its size matches the size of the liquid-state forming die.

[0010] The heating furnace opening is provided with a fixing device for connecting and fixing the glass liquid drainage die and the liquid-state forming die.

[0011] Let the inner wall diameter of the outer die at the inlet end of the glass liquid drainage die be D 1 , the inner wall diameter of the inner die be D 2 , the height of the inlet end of the glass liquid drainage die be H, the outer die diameter at the outlet end of the glass liquid drainage die be d 1 , the inner die diameter be d 2 , and the total length of the outlet end of the glass liquid drainage die, the liquid-state forming die and the solid-state forming die be L. The above parameters satisfy the following conditions according to the principle of equal volume:

[0012]

[0013] A method for preparing a fluoride optical fiber preform by using the above-mentioned preparation device for a fluoride optical fiber preform, characterized in that the method comprises the following steps:

[0014] Step 1. According to the target core / cladding ratio of the optical fiber, determine the diameter of the small end (exit end) and the large end (inlet end) of the glass liquid drainage mold, the diameters of the liquid forming mold and the solid forming mold, fix them to the furnace mouth of the heating furnace, select the corresponding platinum crucible, put the core layer and cladding glass into it, and place the crucible at the designated position in the heating furnace;

[0015] Step 2. Set the heating temperature of the heating furnace to 750°C - 900°C. The distributed temperature gradient heating device presets three temperature gradients of 200°C - 350°C, 500°C - 600°C, and 750°C - 900°C from top to bottom. At the same time, inject dry inert gas into the metal cavity to complete the air replacement. After the temperature field is stable, raise the heating furnace at a speed of 0 - 1 L / min, push the glass liquid in the crucible into the high-temperature area of the glass liquid drainage mold and then into the liquid forming mold, stop the pushing and evacuate the metal cavity to a slightly negative pressure to promote the floating of bubbles in the glass liquid. After 5 - 10 minutes, continue to push at a speed of 0 - 1 L / min, slowly press the glass liquid to the medium-temperature area. At this time, the glass liquid has started to have a certain viscosity. Continue to push and press at a speed of 0 - 1 L / min. Finally, the core layer and cladding glass are simultaneously pushed out from the top outlet of the liquid forming mold. Under the dual influence of negative pressure and gravity, the core layer and cladding glass are closely attached and enter the solid forming mold and are annealed at 200°C - 350°C for 4 - 6 hours;

[0016] Step 3. After the annealing is completed, turn off the heating device, wait for the equipment to cool naturally to room temperature inside, and then take out the finished product.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] 1) The present invention uses programmed pushing to smoothly inject the molten glass liquid into the mold from bottom to top, thereby reducing the generation of turbulence in the molten glass liquid during the injection process, and further reducing the generation of micro-bubbles in the glass liquid. At the same time, a slightly negative pressure is formed in the metal cavity by vacuum pumping, which promotes the upward movement of the micro-bubbles that have not been removed in the glass liquid, forming an effect similar to "air floatation", reducing the number of micro-bubbles in the preform, and promoting the axial uniformity of the preform;

[0019] 2) The obtained fluoride optical fiber preform has a bubble-free core layer and is axially uniform. The preparation method is simple and effective, and the whole preparation process is programmed to avoid errors caused by manual operation, and it is expected to be used for the drawing of low-loss fluoride optical fibers. Brief Description of the Drawings

[0020] Figure 1 It is a schematic diagram of the equipment for the fluoride optical fiber preform;

[0021] Figure 2 It is a model of the liquid forming mold;

[0022] Figure 3It is a model of the furnace mouth fixing device;

[0023] Figure 4 It is a model of the glass drainage die;

[0024] Figure 5 It is a die assembly model;

[0025] Figure 6 It is a physical diagram of the optical fiber preform in the embodiment;

[0026] Figure 7 It is a physical diagram of the optical fiber preform in the comparative example. Specific implementation manners

[0027] The following specific embodiments are used to give an illustrative description of the present invention and help further understand the present invention. However, the specific details of the implementation cases are only for explaining the present invention and do not represent all the technical solutions under the concept of the present invention. Therefore, it should not be construed as a limitation to the overall technical solution of the present invention. Some non-substantive additions and modifications that do not deviate from the concept of the present invention in the view of those skilled in the art, such as simply replacing or substituting technical features with the same or similar technical effects, all fall within the protection scope of the present invention.

[0028] Figure 1 It is a schematic diagram of the equipment for the fluoride optical fiber preform, including a metal cavity 4 located in the upper part and a heating furnace 6 located in the lower part.

[0029] The heating furnace 6 has a programmed lifting function, which can raise the platinum crucible 7 placed at the bottom of the heating furnace 6. A glass liquid drainage die 5 is provided at the top of the heating furnace 6, which can extend into the platinum crucible 7, so that the glass liquid in the platinum crucible 7 is pressed into the metal cavity 4;

[0030] A liquid forming die 2 and a solid forming die 1 are sequentially arranged in the metal cavity 4 from bottom to top. Both the liquid forming die 2 and the solid forming die 1 are of a hollow structure, and a distributed temperature gradient heating device 3 is sleeved outside the die for realizing gradient heating from bottom to top; the liquid forming die 2 and the solid forming die 1 are of a multi-piece detachable structure.

[0031] A negative pressure extraction interface is provided on the metal cavity 4, and a water inlet and a water outlet are provided on the side.

[0032] Example 1

[0033] Select a liquid molding die with a diameter of 6 / 12 cm and a length of 20 cm according to the target optical fiber size. Select a platinum crucible with a diameter of 5 / 10 cm and a height of 5 cm, put the core layer and cladding glass into it, and place the crucible at the designated position in the furnace. Assemble the liquid molding die into the card slot at the furnace mouth to complete the fixation of the die. Then insert the solid molding die into the top card slot of the liquid molding die to complete the fixation. Set the heating temperature of the heating furnace to 750 °C, and preset three temperature gradients of 210 °C, 500 °C, and 750 °C from top to bottom for the distributed temperature gradient heating device. At the same time, inject dry inert gas into the metal cavity to complete the air change. After the glass melts, the heating furnace is lifted at a speed of 0.5 L / min to push the glass liquid in the crucible into the glass liquid drainage die. Continue to push to make the glass liquid enter the high-temperature area of the liquid molding die. Stop pushing and evacuate the metal cavity to a slightly negative pressure (-10 Pa) to promote the floating of bubbles in the glass liquid. After 10 minutes, slowly inject the glass liquid into the medium-temperature area at a speed of 0.2 L / min. At this time, the glass liquid has already started to have a certain viscosity. Continue to push at a speed of 0.1 L / min. Finally, the core layer and cladding glass are simultaneously pushed out from the top outlet of the die. Under the dual influence of negative pressure and gravity, the core layer and cladding glass closely fit into the circular column in the finished product annealing area and are annealed at 210 °C for 6 hours. After the annealing is completed, turn off the heating device, wait for the equipment to cool naturally to room temperature inside, and take out the finished product.

[0034] Comparative Example 1

[0035] Melt the fluoride glass liquid under inert gas and prepare the optical fiber preform by using the traditional rotary casting method. The casting method is from top to bottom. The mold insulation temperature and annealing temperature are 210 °C, and the rotation speed is 1500 r / min.

Claims

1. A preparation device for a fluoride optical fiber preform, characterized in that, it includes a metal cavity (4) located in the upper part and a heating furnace (6) located in the lower part; the heating furnace (6) has a programmed lifting function, which can raise the platinum crucible (7) placed at the bottom of the heating furnace (6), and a glass liquid drainage mold (5) is provided at the top of the heating furnace (6), which can extend into the platinum crucible (7), so that the glass liquid in the platinum crucible (7) is pressed into the metal cavity (4); a liquid forming mold (2) and a solid forming mold (1) are sequentially arranged in the metal cavity (4) from bottom to top; both the liquid forming mold (2) and the solid forming mold (1) are of a hollow structure, and a distributed temperature gradient heating device (3) is sleeved outside the mold for realizing gradient heating from bottom to top; the liquid forming mold (2) and the solid forming mold (1) are of a multi-petal detachable structure; a negative pressure extraction interface is provided on the metal cavity (4), and a water inlet and a water outlet are provided on the side.

2. The preparation device for a fluoride optical fiber preform according to claim 1, characterized in that, the liquid forming mold (2) is of a middle-through cylindrical structure, which is divided into an inner mold and an outer mold. The inner mold is used for core layer glass forming, and the outer mold is used for cladding glass forming.

3. The preparation device for a fluoride optical fiber preform according to claim 1, characterized in that, the distributed temperature gradient heating device (3) is composed of a heating resistor, two independent cooling water circulation systems and a thermocouple.

4. The preparation device for a fluoride optical fiber preform according to claim 1, characterized in that, the platinum crucible (7) is of a double-layer cup-shaped structure, the inner cup is used for accommodating core layer glass, and the outer cup is used for accommodating cladding glass.

5. The preparation device for a fluoride optical fiber preform according to claim 1, characterized in that, the glass liquid drainage mold (5) is welded by middle-through cylindrical molds of four sizes. The glass liquid drainage mold (5) is also divided into an inner mold and an outer mold. The large-mouth end is the inlet end, and its size matches that of the platinum crucible (7). The small-mouth end is the outlet end, and its size matches that of the liquid forming mold (2).

6. The preparation device for a fluoride optical fiber preform according to claim 1, characterized in that, a fixing device is provided at the furnace mouth of the heating furnace (6) for connecting and fixing the glass liquid drainage mold (5) and the liquid forming mold (2).

7. The preparation device for a fluoride optical fiber preform according to claim 1, characterized in that, The inner wall diameter of the outer mold at the inlet end of the glass liquid drainage mold (5) is D 1 , and the inner wall diameter of the inner mold is D 2 . The height of the inlet end of the glass liquid drainage mold (5) is H. The outer mold diameter at the outlet end of the glass liquid drainage mold (5) is d 1 , and the inner mold diameter is d 2 . The total length L of the outlet end of the glass liquid drainage mold (5), the liquid forming mold (2), and the solid forming mold (1). The above parameters satisfy the following conditions according to the principle of equal volume:

8. The preparation device for a fluoride optical fiber preform according to any one of claims 1-7, characterized in that, the inner surfaces of the glass liquid drainage mold (5), the liquid forming mold (2) and the solid forming mold (1) are gold-plated.

9. The preparation device for a fluoride optical fiber preform according to any one of claims 1-7, characterized in that, the distributed temperature gradient heating device (3) preset three temperature gradients of 200°C - 350°C, 500°C - 600°C and 750°C - 900°C from top to bottom.

10. A method for preparing a fluoride optical fiber preform using the preparation device of the fluoride optical fiber preform according to any one of claims 1-7, characterized in that, the method comprises the following steps: Step 1. According to the target fiber core / cladding ratio, determine the diameter of the small-end outlet and the large-end inlet of the glass liquid drainage die (5), the diameters of the liquid forming die (2) and the solid forming die (1), and fix them to the furnace mouth of the heating furnace. Select the corresponding platinum crucible (7), put the core layer and cladding glass into it, and place the crucible at a designated position in the heating furnace (6); Step 2. Set the heating temperature of the heating furnace (6) to 750°C to 900°C. The distributed temperature gradient heating device (3) preset three temperature gradients of 200°C - 350°C, 500°C to 600°C, and 750°C to 900°C from top to bottom, respectively forming a low-temperature zone, a medium-temperature zone, and a high-temperature zone. Inject dry inert gas into the metal cavity (4) to complete air replacement. After the temperature field is stable, the heating furnace is lifted at a speed of 0 to 1 L / min, and the glass liquid in the crucible (7) is pushed into the glass liquid drainage die (5) and enters the high-temperature area of the liquid forming die (2). Stop pushing and evacuate the metal cavity to a slightly negative pressure to promote the floating of bubbles in the glass liquid. After 5-10 minutes, continue to push at a speed of 0 to 1 L / min, slowly press the glass liquid to the medium-temperature zone. At this time, the glass liquid has already started to have a certain viscosity. Continue to press at a speed of 0 to 1 L / min. Finally, the core layer and cladding glass are simultaneously pushed out from the top outlet of the liquid forming die (2). Under the dual influence of negative pressure and gravity, the core layer and cladding glass are closely attached and enter the solid forming die (1) and are annealed at 200°C - 350°C for 4-6 hours; Step 3. After the annealing is completed, turn off the heating device, wait for the inside of the equipment to cool naturally to room temperature, and take out the finished product.

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