For the needs of transmission and practical applications, sometimes the optical fiber is drawn into a tapered shape. The fusion tapered fiber coupling device is one of the important basic devices in the optical fiber communication system and can be used for various proportions. Power splitter (splitter)/combiner (combiner), wavelength division multiplexer (WDM), fiber optic total mirror and other key components of various optical measuring instruments. Therefore, the coupling effect between tapered fibers is studied, The application of tapered fibers is very important. Analyze the influence of various parameters of the tapered fiber itself and the relative position between the two tapered fibers on the energy coupling between the fibers, so as to promote the better application of tapered fibers in optical fiber communications.
Optical fiber provides a large-capacity information transmission medium for wired communication, and optical fiber communication has become an indispensable part of today's information society. There is no doubt that the reliability of optical fiber communication is an important indicator for judging the performance of optical networks, and the reliability of optical fiber communication mainly depends on the distortion of optical signals when they propagate in optical fibers. Among them, the polarization characteristics of optical signals are an important factor. . In addition, the polarization characteristics of optical signals in optical fibers are also widely used in the fields of imaging and measurement. Therefore, studying the polarization characteristics of tapered fibers has certain practical value.
Structure of tapered optical fiber
Common processing methods of tapered optical fiber include chemical etching, grinding and fusion drawing method, among which fusion drawing method is the most widely used method. The tapered fiber is made by the fusion drawing method. The characteristic of the optical fiber taper made by the fusion drawing method is that the diameter of the cladding and core of the optical fiber becomes gradually thinner along the axial direction of the optical fiber. Video microscope image of optical fiber. Where l is the length of the optical taper, α is the taper of the optical taper, a is the radius of the thick end of the optical fiber taper, and b is the tip radius. The relationship between the geometric parameters of the tapered fiber can be obtained by simple calculation. It can be seen that when other parameters have been set, the larger the tip diameter, the larger α; the shorter the tapered transition zone of the fiber, that is, the smaller the value of l, the larger the relative cone angle α, and the sharper the cone change.
As long as the light source and the tapered fiber are at a proper distance, the fiber coupling efficiency can reach an ideal level.
Coupling efficiency of tapered fiber
According to the mode coupling theory, the coupling of LD to fiber is essentially the mode field matching between the two. If a flat-end optical fiber is used for direct coupling, because of the mode field mismatch, the coupling efficiency is very low, about 10%, while a tapered fiber is used for coupling, which can greatly improve the coupling efficiency. This paper uses optical design simulation software to study the tapered fiber, and strive to find the relationship between the geometry of the tapered fiber and the coupling efficiency, so as to optimize the shape of the tapered fiber and improve the coupling efficiency.The principle of the tapered fiber simulation research is ray tracing, and the coupling efficiency of the tapered fiber is calculated by tracing the propagation path of different light rays entering and exiting the tapered fiber. The light source used in the simulation is a He-Ne laser with strong directivity and good stability, a wavelength of 0.6328μm, and a fiber parameter of 50/125μm.
Fabrication of tapered optical fiber[2]
There are two main methods for making optical fiber cones: fusion drawing method and etching method. The characteristics of optical fiber cones made by fusion drawing method are The diameters of the cladding and core of the optical fiber are gradually tapered along the axis of the optical fiber. It can generally be considered that the ratio of the diameter of the cladding to the core remains constant throughout the tapered area. The probe obtained by the etching method has the diameter of the cladding along the The z-direction gradually decreases, while the core diameter is basically the same. Only when it is close to the tip of the cone, the core diameter gradually decreases. The tapered fiber used in the experiment is a common single-mode silica fiber, which is self-made by thermal drawing The device is drawn by the fusion drawing method. The fusion drawing method uses a CO2 laser to melt the optical fiber and apply a pulling force at both ends. , The section naturally forms a smooth plane. This method of making optical fiber tapers is easy to control, with good repeatability, and the surface after taping is smooth. It is an ideal production method.
Coupling between tapered fibers Characteristics
The coupling characteristic between tapered fibers refers to the ratio of the light energy in the signal fiber coupled to the coupling fiber. Since the two tapered fibers are both single-mode step weakly conductive fibers, on the optical scale The distance is sufficiently far apart to meet the conditions of local mode coupling, which can be analyzed by the local mode coupling theory.
The light source adopts a helium-neon laser with good monochromaticity and stability, and is focused by a microscope objective. The laser shines on the coupler and enters the fiber. The fiber is about 40m long. The optical signal establishes a steady-state mode distribution during the transmission process, and the output power is relatively stable. The square dashed frame in the device is a cassette, silicon photodiode and the end of the coupling fiber The end is placed in it. The rectangular dashed frame in the picture is a special readable microscope with two movable stages, one on the left and one on the left. In the field of view of the microscope is the joint of the two tapered optical fibers. The movable stage of the microscope can adjust the relative position of the two tapered fibers. The specific data of the relative position can be read through the micrometer eyepiece of the microscope. The experiment is carried out in a dark room, and the specific experimental steps are as follows: 1) Before the experiment, turn on the helium-neon laser to preheat for a few minutes to stabilize the output power. The micro current amplifier is preheated for about half an hour, and on the basis of calibration and zero adjustment, adjust to the appropriate range. 2) Put the two fibers as shown in the figure Place the position shown, the plane end of the signal fiber is connected to the coupler, and the plane end of the coupling fiber is glued in the square cassette, and the end face of the fiber is about 1mm away from the photosensitive surface of the silicon photodiode. The tapered ends of the two fibers are placed on the readout microscope. On the movable platform on the left and right. 3) Turn on the helium-neon laser, adjust the relative position of the two tapered ends, the light emitted from the end of the coupling fiber is on the silicon photodiode, and the photocurrent is generated in the circuit. 4) Fix the overlap length l and measure The current emitted from the end of the coupling fiber changes with the vertical distance. 5) The vertical distance d is fixed, and the change of the output current with the overlap length is also measured. 6) The tapered end of the fiber is placed flat under the reading microscope, and the projection of the cone is Think of it as an isosceles triangle, read the length of the right side (that is, the length of the cone) and the length of the base, and use the tangent The formula calculates the cone angles of the two cones separately.
Classification of optical fibers
There are many ways to classify optical fibers, mainly based on operating wavelength, refractive index distribution, transmission mode, manufacturing material, etc. Classification .
1. Divided according to the working wavelength: the frequency of ultraviolet fiber, visible fiber, and the commonly used fiber light wave are in the visible light range. Near-infrared optical fiber, infrared optical fiber negative.
2. Divided according to different manufacturing materials: all-plastic fiber, glass fiber, quartz fiber, composite fiber, infrared fiber, etc. The optical fiber used in optical fiber communication is mainly made of quartz material. For the tapered fiber to be studied in the thesis, the quartz fiber is also the object, which is mainly drawn from ordinary quartz fiber.
3. According to the refractive index distribution of the fiber cross section, it can be divided into: step fiber and graded fiber. Step-type fiber means that the core refractive index and the cladding refractive index remain constant along the radial direction. At the boundary between the core and the cladding, the refractive index changes stepwise. Graded fiber refers to the fact that the refractive index of the core layer gradually decreases along the radial direction, while the refractive index in the cladding layer remains unchanged.
4. Divided according to the mode of transmission in the core: single-mode fiber and multi-mode fiber. Mode is a form of field structure distribution of electromagnetic field. Different modes have different field structures. An optical fiber that transmits only one mode in an optical fiber is called a single-mode fiber, and an optical fiber that can transmit multiple modes at the same time is called a multimode fiber.