Influence of end treatment and fusion of the hotte

2022-08-18
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Abstract: This paper introduces the causes of various fiber losses, verifies the influence of fiber end quality on fiber laser output power through experiments, studies the fiber end treatment process, analyzes the cutting and grinding methods of fiber end, and puts forward specific requirements for the fiber fusion process, It provides a reference for the development of similar lasers

key words: fiber laser doped er3+ fiber fiber grinding fiber end face processing fiber fusion

1, preface

fiber is a cylindrical dielectric waveguide, which is composed of fiber core, cladding and coating. Generally, the core diameter of single-mode and multi-mode fiber is 5 ~ 15 respectively μ M and 40 ~ 100 μ m. The cladding diameter is about 125~600 μ m。 The end face of the treated optical fiber is ideally a smooth plane. However, in practice, the processing of optical fiber end faces often cannot reach the ideal state, such as poor polishing, scratches, broken surface or edge damage, etc., which will complicate the end face situation. For the coupling between the optical fiber and other components in the laser and the welding between the optical fibers, and turning on the plotter switch, it is required that the end of the optical fiber must have a smooth and flat surface, otherwise the loss will be increased. This paper introduces the causes of fiber loss, verifies the influence of fiber end quality on the output power of fiber laser through experiments, studies the fiber end treatment process, analyzes the cutting and grinding methods of fiber end, and puts forward specific requirements for the fiber fusion process, which provides a reference for the development of similar lasers

2. Type of optical fiber loss

2.1 optical fiber intrinsic loss

optical fiber intrinsic loss is the inherent loss of optical fiber, which is mainly due to the defects of quartz glass, the matrix material of optical fiber machine, and the inclusion of metal transition impurities and oh-, so that light has scattering, absorption and dispersion in the transmission process. Generally, it can be divided into scattering loss, absorption loss and dispersion loss. Among them, the scattering loss is caused by the fluctuation of atomic density in the material, which causes uneven density in the condensation process and uneven concentration caused by density fluctuation. Absorption loss is due to the fact that the fiber core contains metal transition impurities and oh absorption light, especially the inherent absorption of glass in the infrared and ultraviolet spectral region. Fiber dispersion can be divided into three categories according to the causes, namely material dispersion, waveguide dispersion and inter mode dispersion. Among them, single-mode fiber is transmitted in fundamental mode, so there is no inter mode dispersion. Among the intrinsic factors of single-mode fiber, the mode field diameter has the greatest influence on the connection loss. The connection loss caused by the intrinsic factor of single-mode fiber is about 0.014db. When the mode field diameter mismatch is 20%, it will produce a connection loss of 0.2db. The normalized frequency of multimode fiber is v>2.404, and there are multiple waveguide modes for transmission. The larger the value of V, the more modes. In addition to material dispersion and waveguide dispersion, there is also intermodal dispersion, which is generally dominant. The so-called inter mode dispersion refers to the phase constants of different modes of optical fibers at the same frequency β Dispersion caused by different group velocities

in addition, optical fiber geometric parameters such as fiber core diameter, cladding outer diameter, core/cladding concentricity, out of roundness, optical parameters such as relative refractive index, maximum theoretical numerical aperture, etc., as long as one or more mismatches, will produce different degrees of intrinsic loss

2.2 additional loss of optical fiber

the additional loss of optical fiber is generally composed of radiation loss and application loss. Among them, the radiation loss is caused by the optical fiber micro bending caused by the optical fiber drawing process, the fluctuation of optical fiber diameter and ovality, the expansion and contraction of the temperature change of the plastic coating and the low-temperature shrinkage of the coating; Application loss is the loss caused by macro bending and micro bending caused by tension, bending and extrusion of optical fiber

3. Experimental device and results

er3+ doped fiber ring cavity laser experimental device, the pump light is output from the 980nmld pigtail, coupled into the ring fiber resonator by wavelength division multiplexer (WDM), and then output the laser after splitting by coupler. Among them, the central wavelength of fiber grating is 1546.3nm, the length of er3+ doped fiber is 3M, the doping concentration is 400ppm, the working wavelength range of isolator is 1535~1565nm, and the insertion loss of each element is 0.4db. The relationship curve between output power and input power of the above device is shown in Figure 2, and the maximum output power can reach 16.9mw. However, because the components of the fiber laser are fused together, the insertion loss and fusion loss have a great impact on the whole system. Under the condition of good welding quality, the overall optical efficiency can reach 5.3%. Under the condition of poor optical fiber welding, the solder joint has serious light leakage. Obvious pump light leakage can be seen with the conversion sheet, which seriously affects the overall optical efficiency, and the power difference between the two is about 23%. Therefore, how to reduce the welding loss in the cavity is the key factor affecting the output power of the laser

4. Optical fiber end treatment

optical fiber end treatment, also known as end preparation, is a key process in optical fiber technology, mainly including stripping, cleaning and cutting. The end quality directly affects the pump light coupling efficiency and laser output power of fiber lasers

4.1 stripping of optical fiber coating

removing optical fiber coating is the first step of optical fiber end face treatment. Wire strippers and blades can be used for stripping. When stripping with wire stripper, the left thumb and index finger pinch the optical fiber tightly, and the exposed length is about 5cm. The remaining fiber is naturally bent between the ring finger and the little finger to increase strength and prevent slipping. The wire stripper should be vertical to the optical fiber, and tilt a certain angle in the upward direction. Then use the jaw to gently clamp the optical fiber, and then use the right hand to force it, and push it out horizontally along the axial direction of the optical fiber. The whole process should be natural and smooth, and strive for a success; When the blade is used for stripping, first soak the 3-5cm long optical fiber end with concentrated sulfuric acid for 1-2 minutes, and wipe it with alcohol cotton. Pinch the optical fiber with your left hand and hold it flat to prevent slipping. Use a blade with your right hand to peel off the surface coating polymer material in sequence along the direction of the optical fiber to the end at a certain angle with the optical fiber. This method overcomes the shortcomings of serious corrosion of the optical fiber soaked in chemical solvent for a long time, and it is easier and cleaner than scraping directly with a stripper or blade, and is not easy to damage the side part of the optical fiber cladding

4.2 cleaning of cladding surface

observe whether the cladding of the stripped part of the optical fiber is completely removed. If there is any residue, it must be removed. If there is a very small amount of coating that is not easy to peel off, dip a cotton ball with an appropriate amount of alcohol, dip it and wipe it at the same time. Tear the absorbent cotton into fan-shaped small pieces with flat layer, dip it with a little alcohol (it is appropriate to pinch it with two fingers without overflow), fold it into a V shape, clamp the stripped optical fiber, wipe it along the axial direction of the optical fiber, and strive to succeed at one time. A piece of cotton should be replaced in time after being used for 2-3 times, and different parts and layers of cotton should be used each time, so as to improve the utilization rate of cotton and prevent secondary pollution to the surface of the optical fiber cladding

4.3 optical fiber end face cutting

cutting is the most critical step in the preparation of optical fiber end faces. Precision and high-quality cutters are the basis, and strict and scientific operation specifications are the guarantee. Commonly used cutting knives include pen type cutting knives and table type fiber cutting knives. When cutting the optical fiber with a pen cutter, place the optical fiber on your finger, cut the optical fiber along the vertical direction of the optical fiber axis at a position about 5mm away from the end with another hand-held cutter, and then gently remove the cut end; When using a table type optical fiber cutter for operation, first clean the cutter blade, place the V-shaped groove of the optical fiber and the positioning pressing plate, and adjust the position of the cutter to make it placed smoothly. When cutting, the action should be stable and natural, not heavy, not urgent, and avoid fiber breakage, bevel, burr, crack and other bad end faces

the cleaning and cutting time of the surface should be closely connected, and the interval should not be too long, especially the prepared end face should not be placed in the dirty air. Handle with care when moving to prevent collision with other objects

5, optical fiber end grinding

5.1 grinding process

the main factors affecting the quality of end grinding are the installation and positioning of optical fiber, end grinding, inspection and testing. Among them, grinding and testing are the most critical to the development of high-quality optical fiber end faces. The main factors that directly affect the grinding effect of the optical fiber end face are: the stable operation of the grinding machine, the uniform particles of the grinding sandpaper, the correct use of the grinding plate, and the setting of the grinding parameters (pressure and time)

currently used grinders can generally be divided into gear drive, belt drive and rod drive according to their operation principle. Gear transmission mode is adopted, which generally has strong horsepower and high stability; Belt transmission mode is adopted. Generally, the horsepower is small, and its speed is easy to change under high pressure. In addition, the belt is prone to problems after aging with time; The multi pole transmission mode is adopted, which has high noise, low stability, easy shaking of the machine body and low pressure

in terms of pressurization, there are single point central pressurization, air pressure and hydraulic pressure. Single point central pressurization is vulnerable to external influences, such as the limited number of pieces per plate; It is difficult to control the stability of air pressure; The hydraulic control is more accurate and relatively strong, but the price is expensive

in the whole grinding process, no matter the speed, pressure, water or grinding fluid of the grinder, the effect of the grinding plate will be different. Therefore, when selecting the grinding treatment, we must consider all factors and adopt the most reasonable grinding scheme

5.2 grinding process

the end face grinding process goes through four processes: rough grinding, medium grinding, fine grinding and polishing. The particle sizes of emery paper used in rough grinding, medium grinding and fine grinding are 6, 3, 1 and 0.5 respectively. The time and pressure of the four processes have a total of 8 parameters. With different schemes, different results of end face quality can be obtained

6. Optical fiber welding

when putting the optical fiber into the V-groove of the welding machine, make sure that there is no foreign matter at the bottom of the V-groove and the optical fiber is close to the bottom of the V-groove. Machine automatic welding when the machine starts welding, first push the optical fibers in the V-grooves on the left and right sides towards each other, and a short discharge will be generated during the process. Its function is to clean the dust on the end face of the optical fiber, and then continue to push the optical fiber until the optical fiber gap is at the originally set position. At this time, the welding machine measures the cutting angle, and displays the enlarged image near the end face of the optical fiber on the screen. The core/cladding alignment directly affects the welding loss as the end face fabrication. The welding opportunity is aligned at the same time in the x-axis and Y-axis direction, and the axial and axial deviation parameters are displayed on the screen. If the error is within the allowable range, the welding begins

observe the welding results. After welding, the machine will automatically evaluate and display the current welding loss. Because it is an estimated value, if the drum is displayed above 0.3dB, the end face must be remade. After welding, further observe the welding shape of the optical fiber. If there is a situation as shown in Figure 5, the machine setting must be adjusted, and the optical fiber end face must be remade before welding. The specific implementation method is shown in Table 1

the welding collet is firmly clamped at 25mm from the outer end of the flexible adherend. During the process, the V-groove, electrode, objective lens and welding chamber of the welding machine should also be cleaned in time to observe whether there are bubbles, too fine, too thick, virtual fusion, separation and other adverse phenomena in the welding at any time. OTDR can be used to track the monitoring results, analyze the causes of the above adverse phenomena in time, and take corresponding improvement measures. If virtual fusion occurs for many times, check whether the materials and models of the two fused optical fibers match, and whether the cutting knife and welding machine are polluted by dust, and

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