Influence of Fiber Splicing on Fiber Laser Power

Optical fiber is a cylindrical dielectric waveguide consisting of three parts: core, cladding and coating. Generally, the core diameter of single-mode and multi-mode fiber is 5-15 μm and 40-100 μm, and the cladding diameter is about 125-600 μm. The treated fiber end face is ideally a smooth plane. However, in practice, the processing of the fiber end face often cannot reach the ideal state, such as unsatisfactory polishing, scratches, surface or edge damage, etc., which will complicate the end face situation.

For the coupling of the fiber and other components in the laser and the fusion between the fibers, it is required that the end of the fiber must have a smooth and flat surface, otherwise the loss will increase. In this paper, the causes of fiber loss are classified and introduced. The influence of the quality of the fiber end face on the output power of the fiber laser is verified through experiments. The processing process of the fiber end face is studied, the cutting and grinding methods of the fiber end face are analyzed, and the specific fiber fusion process is proposed. requirements, which provides a reference for the development of similar lasers.

Fiber loss types

Fiber Intrinsic Loss

The intrinsic loss of the optical fiber is the intrinsic loss of the optical fiber. It is mainly due to the defects of the fiber machine matrix material quartz glass itself and the inclusion of metal transition impurities and OH-, which causes the light to scatter, absorb and disperse during the transmission process. Generally, it can be divided into scattering loss and absorption loss. and dispersion loss. Among them, the scattering loss is caused by the fluctuation of atomic density in the material, the non-uniform density caused by the condensation process, and the non-uniform concentration caused by the density fluctuation. The absorption loss is due to the core containing metal transition impurities and OH-absorbing light, especially the inherent absorption of glass in the infrared and ultraviolet spectral regions.

Fiber dispersion can be divided into three categories according to the causes, namely material dispersion, waveguide dispersion and intermodal dispersion. Among them, single-mode fiber transmits in the fundamental mode, so there is no intermodal 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 factors of single-mode fiber is about 0.014dB, and when the mode field diameter is mismatched by 20%, a connection loss of 0.2dB will be generated. The normalized frequency of multimode fiber is V>2.404, and there are multiple waveguide modes transmission. The larger the V value, the more modes. In addition to material dispersion and waveguide dispersion, there is also intermodal dispersion. Generally, intermodal dispersion dominates. The so-called intermodal dispersion refers to the dispersion caused by the different phase constants β of different modes of the fiber at the same frequency, so the group velocity is different.

In addition, 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.

Fiber Additional Loss

The additional loss of fiber is generally composed of radiation loss and application loss. The radiation loss is caused by the fiber drawing process, fiber diameter, fluctuations in ellipticity, the expansion and contraction of the temperature change of the plastic layer, and the low temperature shrinkage of the coating resulting in the microbending of the fiber; the application loss is due to the tension, bending, and extrusion of the fiber. Losses due to macrobending and microbending.

Fiber end face treatment fusion splicing

Fiber end face treatment

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

Stripping of Fiber Coatings

Removing the fiber coating is the first step in fiber end face treatment. Stripping can be done with both wire strippers and a blade. When stripping with a wire stripper, pinch the optical fiber with the thumb and index finger of the left hand, and the exposed length is about 5cm. The remaining fiber is naturally bent between the ring finger and the little finger to increase the strength and prevent slipping. The wire stripper should be perpendicular to the optical fiber. , incline upwards at a certain angle, then lightly clamp the fiber with the jaws, then force the right hand to push it out along the axis of the fiber, the whole process should be natural and smooth, and strive for a success; Soak the 3~5cm long optical fiber end in concentrated sulfuric acid for 1~2 minutes, and wipe it with alcohol cotton. Pinch the fiber tightly with the left hand, hold the fiber flat to prevent slipping, and use the blade with the right hand to follow the direction of the fiber to the end, at a certain angle to the fiber, and strip the polymer material of the surface coating layer in sequence. The solvent method has the disadvantage of serious corrosion of the optical fiber by immersing it for a long time, and it is easier and cleaner than directly scraping with a wire stripper or a blade, and it is not easy to damage the side part of the optical fiber cladding.

Cleaning of cladding surfaces

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 be stripped, use a cotton ball to dip an appropriate amount of alcohol, dipping it and wiping it off. Tear the absorbent cotton into small fan-shaped pieces with a flat surface, dip a little alcohol (it is advisable to squeeze two fingers together without overflowing), fold it into a V shape, clamp the stripped optical fiber, and wipe it along the axial direction of the optical fiber. Cotton should be replaced in time after 2 to 3 times of use, and different parts and layers of cotton should be used each time, which can not only improve the utilization rate of cotton, but also prevent secondary pollution to the surface of the fiber cladding.

Fiber end face cutting

Cutting is the most critical step in the preparation of optical fiber endfaces. Precision and high-quality cutters are the foundation, and strict and scientific operating specifications are the guarantee. Commonly used cleavers include pen cleavers and desktop fiber cleavers. When using a pen-type cleaver to cleave the fiber, place the fiber on your finger, and hold the knife about 5mm away from the end to cleave the fiber in the direction perpendicular to the fiber axis, and then gently remove the cut end; use a desktop fiber When operating the cleaver, first clean the cleaver blade, the V-groove for placing the optical fiber and the positioning platen, and adjust the position of the cleaver to make it stable. When cutting, the action should be smooth and natural, not heavy or urgent, to avoid the occurrence of bad end faces such as broken fibers, bevels, burrs and cracks.

The cleaning and cutting time of the surface should be closely connected, and the interval should not be too long. In particular, the prepared end face should not be placed in the dirty air. When moving, handle it with care to prevent it from rubbing against other objects.

Fiber end face grinding

grinding process

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

The grinding machines currently used can generally be divided into three categories: gear drive, belt drive and rod drive according to their operating principles. The gear transmission method is used, which generally has strong horsepower and high stability; the belt transmission method is generally used, and the horsepower is generally small, and its speed is easy to change under high pressure. In addition, the belt is prone to problems after aging; method, the noise is larger, the stability is lower, the body is easy to shake and the pressure is low.

In terms of pressurization, there are single-point center pressurization, air pressure and hydraulic pressure. Single-point central pressure is easily affected by external changes, such as the limited number of pieces per plate; the air pressure is difficult to control the stability; and the hydraulic control is more precise, the force is relatively large, but the price is expensive.

In the whole grinding process, whether it is the speed of the grinding machine, the pressure, the water or the grinding liquid, the effect of the grinding disc will be different. grinding plan.

Grinding process

The end face grinding process goes through 4 processes: rough grinding, medium grinding, fine grinding and polishing. Among them, the particle sizes of the emery paper used in coarse grinding, medium grinding and fine grinding are different, which are 6, 3, 1 and 0.5 respectively. The time and pressure of the 4 processes have a total of 8 parameters, and different schemes can be used to obtain the end face quality. different results.

Fiber splicing

When placing the fiber into the V-groove of the fusion splicer, make sure that there is no foreign matter at the bottom of the V-groove and that the fiber is close to the bottom of the V-groove. When the automatic fusion splicing machine starts to splicing, the optical fibers in the V-shaped grooves on the left and right sides are first pushed toward each other. During the advancing process, a short-term discharge will be generated. At the original position, the fusion splicer measures the cutting angle and displays the enlarged image near the end face of the fiber on the screen. The alignment of the core/cladding directly affects the splicing loss just like the end face fabrication, and the splicing machine is on the X axis. Align in the Y-axis direction at the same time, and display the axial and axis deviation parameters on the screen. If the error is within the allowable range, start welding.

Observe the welding results. After welding, the machine will automatically evaluate and display the current welding loss. Since it is an estimated value, the end face must be remade if the drum is displayed above 0.3dB. After the fusion splicing, the shape of the optical fiber fusion must be further observed. The machine settings must be adjusted, and the optical fiber end face must be re-fabricated before fusion splicing. The specific implementation is shown in Table 1.

During the welding process, the V-groove, electrodes, objective lens and welding chamber of the welding machine should also be cleaned in time, and the defects such as bubbles, too thin, too thick, virtual melting, and separation should be observed at any time. OTDR can be used to track the monitoring results and analyze them in time. For the reasons for the above-mentioned undesirable phenomena, corresponding improvement measures shall be taken. If the phenomenon of virtual fusion occurs many times, check whether the materials and models of the two optical fibers to be spliced ​​match, whether the cutter and the fusion splicer are polluted by dust, and check the oxidation status of the electrodes. If there is no problem, the fusion current should be appropriately increased.

The mechanical strength of the optical fiber is reduced due to the removal of the coating on the joint part when the optical fiber is connected. Therefore, to strengthen the protection of the joint part, the optical fiber heat shrink protection tube (heat shrink tube) can be used to protect the optical fiber joint part. The heat shrinkable tube should be penetrated before peeling, and it is strictly prohibited to penetrate after the end face is prepared. Move the heat-shrinkable tube pre-inserted on one end of the optical fiber to the fiber connector, so that the fusion point is in the middle of the heat-shrinkable tube, gently straighten the fiber connector, put it in the heater to heat, the vinyl acetate inner tube melts, and the polyethylene tube shrinks Tightly sleeve it on the connected optical fiber, because there is a stainless steel rod in the tube, it not only increases the tensile strength (withstand tensile force is 1000 ~ 2300g), but also avoids the shrinkage of the polyethylene tube. bend.

coil fiber

Fiber coiling is a technology. The scientific fiber coiling method can make the fiber layout reasonable, the additional loss is small, can withstand the test of time and harsh environment, and can avoid fiber breakage caused by extrusion. There are many ways to coil the fiber. You can start from the fiber on one side, fix the heat-shrinkable tube, and then process the remaining fiber on the other side. This method can flexibly choose the location of the heat-shrinkable tube according to the length of the remaining fiber on one side, which is convenient and fast. It can avoid the phenomenon of sharp bends and small circles; it is also possible to place the heat shrinkable sleeves in the fixing grooves one by one, and then deal with the remaining fibers on both sides. This method is commonly used when the reserved disk space is small and the fiber is not easy to coil and fix; when the individual fiber is too long or too short, it can be coiled separately at the end; when it has special optical devices, it can be coiled separately If it is co-coated with ordinary optical fibers, it should be placed lightly on the ordinary optical fibers, and a buffer liner should be added between the two to prevent fiber breakage caused by extrusion, and the pigtails of special optical devices should not be too long. According to the actual situation, various shapes such as circle, ellipse and "∝" can be used to coil the fiber. According to the length of the remaining fiber and the size of the reserved disk space, it can be coiled naturally according to the situation. Effectively reduce the additional loss caused by the coiled fiber.

Measurement of Fiber Splice Loss

The measurement of optical fiber splice point loss is an important indicator to measure the quality of optical fiber splice. The optical loss of optical fiber splice can be determined by measuring methods such as optical time domain reflectometry (OTDR) or loss evaluation scheme of splice splice.

The principle of OTDR is that since the mode field diameter of the fiber affects its backscattering, the fibers on both sides of the splice may produce different backscattering, thereby obscuring the true loss of the splice. If the loss of the splice is measured from both directions, and the two results are averaged, the artifact of a unidirectional OTDR measurement can be eliminated. Strengthening the monitoring of the OTDR is of great significance to ensure the quality of optical fiber fusion and reduce the additional loss caused by the coiled fiber and the loss that may be caused by the packaging to the optical fiber. In the whole splicing work, the 4 monitoring procedures of the OTDR must be strictly implemented: real-time tracking and monitoring of each optical fiber during the splicing process to check the quality of each splicing point; Determine the additional loss caused by the coiled fiber; test all the fibers before packaging to find out whether there is any leakage and whether the fiber and the connector are squeezed; after packaging, perform a final inspection on all the fibers to check whether the packaging is lossy to the fiber .

In addition, some fusion splicers use a cross-section alignment system for imaging the fiber and measuring geometric parameters. By viewing the fiber from two perpendicular directions, the computer processes and analyzes the image to determine cladding shift, core distortion, fiber OD variation and Other key parameters, use these parameters to evaluate the loss of the joint. Splice losses that depend on splice and loss estimation algorithms can vary significantly from the true value.

Summarize

In this paper, the causes of various fiber losses are introduced by classification, the influence of the quality of the fiber end face on the output power of the fiber laser is verified by experiments, the processing process of the fiber end face is studied, and the cutting and grinding methods of the fiber end face are analyzed. Specific requirements and references are provided.