Influence of optical fiber end face processing on Optical Fiber Laser

1. Preface

A cylindrical media waveguide consists of three parts: Core, package layer, and coating layer. Generally, the diameter of the core of a single-mode or multi-mode optical fiber is 5 ~ 15 μm and 40 ~ 100 μm, with a diameter of about 125 ~ 600 μm. The processed optical fiber end face is ideally a smooth plane. However, in reality, the processing of the optical fiber end face often fails to reach the desired state, such as unsatisfactory polishing, scratches, broken surfaces or edges, and so on. For the coupling between the optical fiber and other components in the laser and the fusion between the optical fiber, the end of the optical fiber must have a smooth and smooth surface; otherwise, the loss will increase. This paper introduces the causes of optical fiber loss, verifies the effect of optical fiber end quality on the output power of the optical fiber laser through experiments, and studies the process of processing the optical fiber end face, this paper analyzes the cutting and grinding methods of the optical fiber end face, puts forward specific requirements for the optical fiber fusion process, and provides a reference for the development of similar lasers.

2. optical fiber loss types

2.1 Optical Fiber intrinsic loss

The intrinsic loss of optical fiber is the inherent loss of optical fiber. Because of the defects of the matrix material of the optical fiber, the crystal glass contains transition impurities and OH-, the light will produce scattering, absorption and dispersion during transmission, it can be divided into scattering loss, absorption loss and dispersion loss. The scattering loss is caused by fluctuations in the subdensity of the material, uneven density during condensation, and uneven density fluctuations. The absorption loss is caused by the inclusion of transition impurities and OH-absorbing light in the core, especially the inherent absorption of glass in infrared and ultraviolet spectra. Fiber dispersion can be divided into three types based on the causes, namely material dispersion, Waveguide Dispersion, and mode color dispersion. The single-mode optical fiber is transmitted as the basis mode, so there is no mode color dispersion. Among the intrinsic factors of single-mode optical fiber, the maximum impact on connection loss is the Mode Field diameter. The connection loss caused by the intrinsic factor of the single-mode optical fiber is approximately 0.014dB. When the module diameter is not 20%, the connection loss of DB will be generated [1]. The normalized frequency of multimode optical fiber is V> 2.404. There are multiple waveguide modes for transmission. The larger the V value, the more modes, in addition to material dispersion and Waveguide Dispersion, there is also the mode color dispersion, in general, the dimming dominant position. The so-called mode color dispersion refers to the dispersion caused by the difference of the phase constant β in different modes of the optical fiber at the same frequency, so the group speed is different.

In addition, the geometric parameters of the optical fiber include the core diameter, outer diameter of the package layer, concentricity of the core/package layer, and non-roundness. The optical parameters include the relative refractive index and the maximum theoretical numerical aperture, as long as one or more mismatches, there will be different degrees of intrinsic loss.

2.2 Additional optical fiber loss

The additional losses of optical fibers are generally composed of radiation losses and application losses. Among them, the radiation loss is caused by the optical fiber pulling process, the fiber diameter, the fluctuation of the elliptic degree, the expansion and contraction of the temperature change of the plastic coating layer and the low temperature shrinkage of the coating; application loss is the loss caused by the macro bending and micro bending caused by the tension, bending, and extrusion of the optical fiber.

3. device and Result

As shown in experimental device 1, the pump light is output by the wavelength 980nmLD pigtail, coupled by the wavelength division multiplexing (WDM) into the ring optical fiber resonator, and output the laser after the coupler splitting. The center wavelength of the optical fiber grating is 1546.3nm, the length of the doped third + optical fiber is 3 m, the doping concentration is 400ppm, and the working wavelength range of the isolator is 1535 ~ The insertion loss of each component is 16.9 dB at 1565nm. The relationship between the output power and input power of the above device is shown in curve 2. The maximum output power is mW. However, because all components of the fiber laser are fused together, insertion loss and Fusion Loss have a great impact on the entire system. In the case of good welding quality, the overall optical efficiency can reach 5.3%. In the case of poor Optical Fiber welding, solder joints may experience serious light leakage, and obvious pump light leakage can be seen through the converter, the overall optical efficiency is seriously affected, and the power difference between the two is about 23%. Therefore, how to reduce the welding loss in the cavity is a key factor affecting the output power of the laser.

4. optical fiber end face processing

As a key process in optical fiber technology, optical fiber end face processing is also called end face preparation. It mainly includes three steps: stripping, cleaning and cutting. The end face quality directly affects the pump optical coupling efficiency and laser output power of the fiber laser.

4.1 Optical Fiber Coating Layer Stripping

Removing the optical fiber coating layer is the first step in optical fiber end face processing. Two methods can be used to remove a Wire cutter or a blade. When the wire stripping clamp is used, the left thumb and the index finger are pressed to the optical fiber. The exposed length is about 5cm. The remaining fiber is bent between the ring finger and the small thumb to increase strength and prevent slipping, the strip clamp should be perpendicular to the optical fiber, and the top direction should be tilted to a certain angle. Then, the clamp can be used to gently jam the optical fiber, and the right hand will be forced to push it out along the axial direction of the optical fiber. The whole process should be smooth and smooth, striving for; when the blade is used for stripping, First soak in concentrated sulfuric acid for 3 ~ 5cm long optical fiber head 1 ~ 2 minutes, wipe clean with alcohol cotton [2]. The left hand kneading the optical fiber, holding the fiber to Ping, to prevent slipping, the right hand with the blade along the fiber to the end direction, and the optical fiber into a certain angle of inclination, sequential stripping of surface coating layer polymer material, this method is used to overcome the severe corrosion of Optical Fiber immersed by chemical solvent for a long time. Moreover, it is easier to directly scrape and remove more clean and not easy to damage the side of the optical fiber package.

Clean the surface of the 4.2 package Layer

Check whether all the packages of the optical fiber strip are removed. If there is any residue, remove it. If there is a very small amount of coating layers that are not easy to strip, use cotton balls to dip the appropriate amount of alcohol, dip, and erase. Split the skim cotton into small slices with smooth layers, and use a little alcohol to squeeze the two fingers without Overflow), fold the slice into a V shape, hold the peeled optical fiber, and wipe along the axial direction of the optical fiber, strive for a success, using a piece of cotton 2 ~ Three times later, it should be changed in time. Different Parts and layers of cotton should be used each time. This will not only improve the utilization rate of cotton, but also prevent secondary pollution to the surface of the fiber package.

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 cutting tools are the foundation, and strict scientific operation specifications are the guarantee. Commonly used cutting tools include pen cutting knives and desktop Optical Fiber cutting knives. When using a pen cutting tool to cut the optical fiber, the optical fiber is placed on the finger, and the other hand is holding the knife to cut the optical fiber along the vertical fiber axis about 5mm from the position of the end head, and then gently remove the removed end head; when using a desktop Optical Fiber cutter, you must first clean the cutter blade, place the V-type slot of the optical fiber and the positioning pressure plate, and adjust the position of the cutter to ensure its placement is stable. During cutting, the Movement should be stable and natural, so as not to be heavy or urgent. Avoid the production of bad face such as broken fiber, angle, Burr and crack [3].

The cleaning time of the surface should be closely linked with the cutting time, and the interval should not be too long. In particular, the prepared end face should not be placed in the dirty air. When moving, take it easy to minimize, and avoid rubbing with other objects.

5. optical fiber end Grinding

5.1 Grinding Process

The main factors affecting the Face Grinding Quality include the installation and positioning of optical fiber cables, face grinding, inspection and testing. Among them, the grinding and testing part is the most critical for the development of high-quality optical fiber end face. The main factors that directly affect the grinding effect of the optical fiber end face are: the grinding machine runs stably, the grinding sandpaper particles are uniform, the correct use of the grinding pad, and the grinding parameters set the pressure and time) [4].

Currently, the grinding machine can be divided into three types according to its working principle: Gear Drive, belt drive and Rod Drive. Gear Transmission Mode is adopted. Generally, the horsepower is strong and the stability is high. The belt transmission mode is used. 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 over time. It adopts a rod-connected transmission mode, which features high noise, low stability, easy body jitter and low pressure.

In terms of pressurization, there are single-point center pressurization, air pressure and hydraulic pressure. The pressure on the Single Point center is easily influenced by external changes, such as the limited number of parts per disk; the pressure is difficult to control and stability; the hydraulic control is more accurate, the intensity is relatively large, but the price is expensive.

In the whole grinding process, whether it is the speed, pressure, water or grinding fluid of the grinding machine, the effect of the grinding piece is different. Therefore, when selecting the grinding process, you must take all the factors into consideration, adopt the most reasonable grinding solution.

5.2 Grinding Process

The face grinding process goes through four processes: coarse grinding, medium grinding, fine grinding, and polishing. Among them, the size of the diamond sandpaper used for coarse grinding, medium grinding, and fine grinding is 6, 3, 1, and 0.5 [5], respectively. There are a total of eight parameters for the time and pressure of the four processes. Different schemes can be used to obtain different end quality results. Change the eight parameters during the grinding process to obtain the best solution for grinding the optical fiber face.

6. Optical Fiber fusion

When placing the optical fiber in the V-slot of the melting machine, make sure that there is no foreign body at the bottom of the V-slot and that the optical fiber is close to the bottom of the V-slot. When the machine auto-melting splitters start to weld, the optical fiber cables in the vtrough on the left and right sides will be pushed to each other. A transient discharge will be generated during the push process. The function is to clean the dust on the optical fiber end face, then, the optical fiber will be pushed forward until the optical fiber gap is in the original position. At this time, the melting machine will measure the cutting angle and display the enlarged image near the optical fiber end face on the screen, if the image shown appears, redo it. The core/package layer alignment directly affects the welding loss like the end face. The melting machine is aligned simultaneously on the Y axis and the axial and axial axis deviation parameters are displayed on the screen, if the error is within the permitted range, weld starts.

After observing the weld result, the machine will automatically evaluate it and display the current weld loss. Because it is an estimated value, the end face must be re-created when the drum is displayed above dB. After the fusion, we need to further observe the optical fiber fusion shape. If there are 5 situations, we must adjust the machine settings and re-create the optical fiber end face for fusion. The specific implementation method is shown in table 1.

During the welding process, the V-shaped Trough, electrode, objective lens and welding chamber of the melting machine should also be cleaned in time, and there should be no bubbles, meticulous, rough, virtual fusion, separation and other adverse phenomena in the welding machine at any time, OTDR can be used to track the monitoring results, analyze the causes of the above adverse phenomena in a timely manner, and take corresponding improvement measures. If virtual fusion occurs multiple times, check whether the materials and models of the two optical fiber cables are matched, whether the cutter and melting machine are contaminated by dust, and check whether the electrode is oxidized. If no problem exists, the welding current should be appropriately increased.

Because the coating layer of the joint is removed from the optical fiber connection, the mechanical strength of the coating layer is reduced. Therefore, the joint part must be reinforced and protected. The Optical Fiber Thermal Contraction protection tube can be used to protect the optical fiber joint. The heat shrinkable tube should be passed in before stripping. It is strictly prohibited to wear it after the end surface preparation. Move the heat shrinkable tube with a certain end of the pre-passed optical fiber to the optical fiber joint so that the weld point is located in the middle of the heat shrinkable tube, gently pull the Optical Fiber Joint, put it into the heater for heating, and melt the vinyl acetate tube, after the polyethylene tube shrinks, It is tightly mounted on the connected optical fiber. Because there is a stainless steel rod in the tube, not only increasing the tensile strength to 1000 ~ 2300 GB), and avoid the slight bending of the connecting part due to the contraction of the polyethylene pipe.

7. Disk Fiber

As a technology, the scientific method can make the layout of the optical fiber reasonable, the additional loss is small, can withstand the test of time and bad environment, and avoid the broken fiber caused by extrusion. There are many ways to resize the disk. You can start from one side of the optical fiber disk, fix the heat shrinkable tube, and then process the remaining side of the fiber. This method can be used to flexibly choose the position of the Heat Shrinkable Tube Based on the length of the remaining one side, it is convenient and fast, which can avoid sharp bending and small circles. You can also place the heat shrinkable sleeve tubes one by one in a fixed slot and then process the remaining fibers on both sides. This method is conducive to protecting the optical fiber contact, this method is often used to avoid the possible damage caused by the disk and the disk space reserved by the optical fiber is small, and the optical fiber is not easy to be flushed or fixed. When an individual optical fiber is too long or too short, it can be placed in the end for independent winding; when it comes with special optical devices, it can be independently wound. If it is shared with a common optical fiber, it should be placed on the ordinary optical fiber, A buffer pad is added between the two to prevent fiber disconnection caused by extrusion, And the pigtails of special optical devices cannot be too long. According to the actual situation, you can use a variety of graphics such as circle, elliptical, and "small" disks, according to the length of the remaining fiber and the size of the reserved disk space, take advantage of the natural winding, do not pull hard, use the reserved disk space as much as possible to effectively reduce the additional losses caused by the disk fiber.

8. measurement of optical fiber fusion point loss

The measurement of the loss of Optical Fiber splices is an important indicator for measuring the quality of Optical Fiber splices. The loss of Optical Fiber splices can be determined by measuring methods such as OTDR or the loss evaluation scheme of the splices.

The principle of OTDR is: Because the diameter of the optical fiber field affects the backward scattering, the optical fiber on both sides of the joint may produce different backward scattering, thus shielding the actual loss of the joint. If the loss of the joint is measured in two directions and the average value of the two results is obtained, the human factor error measured by one-way OTDR can be eliminated. It is of great significance to strengthen OTDR monitoring to ensure the optical fiber fusion quality and reduce the loss caused by additional losses and encapsulation caused by the disk fiber. In the entire connection, the OTDR monitoring program must be strictly implemented: Each optical fiber is monitored and tracked in real time during the welding process to check the quality of each fusion point. After each fiber tray, inspect the optical fiber to determine the additional losses caused by the disk fiber; inspect all optical fiber before encapsulation to check whether there is any leakage test and whether there is any squeeze on the optical fiber and the connector; after encapsulation, the final inspection of all optical fibers is performed to check whether the Encapsulation has any loss on the optical fibers [6].

In addition, some splices use a cross-section Arrangement System for optical fiber imaging and measurement of Geometric Parameters by observing the optical fiber from two vertical directions, the computer processes and analyzes the image to determine the package layer offset, core distortion, outer diameter variation, and other key parameters. These parameters are used to evaluate the joint loss. The connection loss calculated based on the joint and loss evaluation algorithms may be significantly different from the actual values.

9. Summary

In summary, the process of establishing a set of optical fiber end face processing and fusion is shown in Step 6. This paper introduces the causes of various optical fiber losses, verifies the effect of optical fiber end quality on the output power of the optical fiber laser, and studies the process of processing the optical fiber end face, this paper analyzes the cutting and grinding methods of the optical fiber end face, puts forward specific requirements for the optical fiber fusion process, and provides a reference for the development of similar lasers.