The rapid development of optical fiber communication technology has increased the demand for optical fiber cables. At present, hundreds of millions of kilometers of optical fiber have been laid all over the world. Optical fiber communication is not only used on land, but also forms a submarine optical cable line across the Atlantic and Pacific Oceans, which almost surrounds the entire earth. According to the laying method of optical cable, it is divided into: self-supporting overhead optical cable, pipeline optical cable, armored buried optical cable and submarine optical cable. According to the optical cable structure, it is divided into: bundled optical cable, layered optical cable, tightly hugged optical cable, ribbon optical cable, non-metallic optical cable and branch optical cable. According to the use of optical cables, there are: optical cables for long-distance communication, short-distance outdoor optical cables, hybrid optical cables and optical cables for buildings. The demand for fiber optic cables in China and the world is very large. It is very important to ensure the quality of fiber optic cables. The test of basic parameters of optical fibers is a guarantee for the quality of fiber optic cables.
These standards specify the specific performance indicators of fiber optic cables. The characteristic parameters of the optical fiber are divided into geometrical characteristic parameters (fiber length, core diameter, cladding diameter, core out-of-roundness, cladding out-of-roundness, core/cladding concentricity error, etc.), optical characteristic parameters (mode field diameter , cut-off wavelength of single-mode fiber, cut-off wavelength of cabled single-mode fiber, refractive index distribution, numerical aperture of multi-mode fiber, etc.), transmission characteristic parameters (attenuation, chromatic dispersion, etc.).
Optical fiber parameter test method
1. Test method of optical fiber parameters
The test methods of optical fiber parameters are carried out with reference to the relevant test methods in the national standard. The test methods of some basic parameters of optical fibers are listed below. Among the characteristic parameters of the optical fiber, the geometrical characteristic parameters make relevant descriptions for the measurement methods of the cladding diameter, cladding out-of-roundness, and core/cladding concentricity error of the optical fiber; The test methods for wavelength and cut-off wavelength of cabled single-mode fibers are explained; the transmission characteristic parameters are related to the test methods for optical fiber attenuation and chromatic dispersion.
1.1. Optical fiber geometric characteristic parameter test
Test methods for refractive index distribution, cladding diameter, cladding out-of-roundness, and core/cladding concentricity errors of optical fibers.
The test methods for measuring cladding diameter, cladding out-of-roundness, and core/cladding concentricity errors are refractive near-field, transverse interferometry, and near-field light distribution (cross-sectional geometry determination).
There are three test methods for the refractive index distribution, cladding diameter, cladding non-circularity, and core/cladding concentricity errors of optical fibers.
Refraction near field method
The refractive near-field method is the benchmark test method (RTM) for the determination of the refractive index distribution of multimode and single mode fibers, as well as the benchmark test method for multimode fiber dimensional parameters and an alternative test method (ATM) for single mode fiber dimensional parameters.
Refraction near-field measurement is a direct and precise measurement. It can directly measure the optical fiber (core and cladding) cross-section refractive index changes, with high resolution, can give the absolute value of the refractive index after calibration. The geometric parameters of multimode fiber and single mode fiber and the maximum theoretical numerical aperture of multimode fiber can be determined from the refractive index profile.
Lateral Interferometry
Transverse Interferometry is an Alternative Test Method (ATM) for the determination of refractive index profiles and dimensional parameters. The transverse interferometry uses an interference microscope to illuminate the sample in the direction perpendicular to the axis of the fiber sample to generate interference fringes, and obtain the refractive index profile through video detection and computer processing.
Near Field Light Distribution Method
This method is the Alternative Test Method (ATM) for Multimode Fiber Geometry and the Reference Test Method (RTM) for Single Mode Fiber Geometry (Removal Mode Field Diameter). By analyzing the near-field light distribution on the output end face of the tested fiber, the geometric parameters of the fiber cross section are determined.
Grayscale and near-field scanning methods can be used. The grayscale method uses a video system to achieve two-dimensional (x-y) near-field scanning, while the near-field scanning method only performs one-dimensional near-field scanning. Due to the influence of the out-of-roundness of the core, the core diameter obtained by the near-field scanning method and the gray-scale method may be different. Core out-of-roundness can be determined by multi-axis scanning.
The measurement method of the refractive index distribution of general commercial instruments is the refractive near-field method.
The instrumentation used in the test is a fiber geometry and refractive index profile meter. The test steps are as follows:
①When preparing the sample, it should be noted that the end face of the sample is clean, smooth and perpendicular to the fiber axis.
②When measuring the cladding, the inclination angle of the end face should be less than 1°. End face damage is controlled with minimal impact on measurement accuracy.
③ Pay attention to avoid small bending of the fiber.
④ Strip off the coating of the tested fiber, cut out a flat end face with a special fiber cutter, and put it into the fiber sample box. The sample box is filled with a refractive index matching liquid with a refractive index slightly higher than that of the fiber cladding.
⑤Put the optical fiber sample box vertically between the light source and the photodetector of the optical fiber refractive index distribution measuring instrument, and perform the scanning test in the x-y direction.
⑥ The test data of the optical fiber refractive index distribution, cladding diameter, cladding out-of-roundness, and core/cladding concentricity error are obtained through analysis.
1.2. Optical fiber optical characteristic parameter test
(1) Test method for mode field diameter of single-mode fiber
The mode field diameter is a measure of the spatial distribution of the mode field intensity of the fundamental mode (LP01) of a single-mode fiber, and it depends on the characteristics of the fiber.
The mode field diameter (MFD) can be determined in the far field using the far field intensity distribution Pm(θ), the complementary aperture power transfer function α(θ) and in the near field using the near field intensity distribution f2(r). The definition of the mode field diameter is strictly related to the measurement method.
There are three methods for measuring the mode field diameter of single-mode fibers.
direct far-field scanning
The direct far-field scanning method is the benchmark test method (RTM) for measuring the mode field diameter of single-mode fibers. It directly calculates the mode field diameter of a single-mode fiber by measuring the far-field radiation pattern of the fiber according to the Petermann far-field definition.
Far-field variable aperture method
The far-field variable aperture method is an alternative test method (ATM) for measuring the mode field diameter of single-mode fibers. It calculates the mode field diameter of a single-mode fiber by measuring the two-dimensional far-field images of optical power passing through apertures of different sizes. The mathematical basis for calculating the mode field diameter is the Bertman far-field definition.
near field scanning
The near-field scanning method is an alternative test method (ATM) for measuring the mode field diameter of single-mode fibers. It calculates the mode field diameter of a single-mode fiber by measuring the radial near-field image of the fiber. The mathematical basis for calculating the mode field diameter is the Bertman far-field definition.
The general commercial instrument mode field diameter measurement method is the variable aperture far field method (VAFF).
The instrument used in the test is a fiber mode field diameter and attenuation spectrum meter. The test steps are as follows:
Prepare a 2m (±0.2m) fiber sample, strip off the coating at both ends, place it in the fiber holder, and cut out a flat end face with a special fiber cleaver.
Connect the fiber under test to the input and output ends of the measuring instrument, and check the focusing state of the light receiving end. If the curve is not in the center of the screen or the fiber end face is not clear enough, you need to adjust the position and focus.
At the output end of the light source, keep the injection conditions of the test fiber unchanged, and make a small ring with a radius of 30mm to filter out the influence of the LP11 mode, and test the mode field diameter.
The test data of the fiber mode field diameter are obtained through analysis.
(2) Test method for cut-off wavelength of single-mode fiber and cut-off wavelength of cabled single-mode fiber
The test method for measuring the cut-off wavelength of single-mode fiber and the cut-off wavelength of cabled single-mode fiber is the transmission power method.
When the modes in the fiber are substantially uniformly excited, the larger wavelength corresponding to the ratio of the total optical power including the injected higher-order mode to the fundamental mode optical power decreases to a specified value (0.1dB) with wavelength is the cutoff wavelength. According to the definition of cut-off wavelength, the transmission power method compares the transmission power through the tested optical fiber (or optical cable) with the change of the reference transmission power with wavelength under certain conditions, and obtains the cut-off wavelength value of the optical fiber (or optical cable).
The general commercial instrument mode field diameter test method is the transmission power method.
The instrument used in the test is a fiber mode field diameter and attenuation spectrum meter. The test steps are as follows:
①During sample preparation, the test of the cut-off wavelength of single-mode fiber uses a 2m (±0.2m) fiber sample, and the test of the cut-off wavelength of a cabled single-mode fiber uses a cabled single-mode fiber of 22m.
② Strip off the coating on both ends of the test fiber, place it in the fiber fixture, and cut out a flat end face with a special fiber cleaver.
③Connect the fiber under test to the input and output ends of the measuring instrument, and check the focusing state of the light receiving end. If the curve is not in the center of the screen or the fiber end face is not clear enough, the position and focal length need to be adjusted.
④ First, test the reference transmission power without making a small loop on the test fiber.
⑤ Then put a small ring with a radius of 30mm on the injection end of the test fiber to filter out the influence of the LP11 mode, and test the transmission power at this time.
⑥Compare the two transmission power test curves, and obtain the cut-off wavelength value of the optical fiber (or optical cable) through data analysis and processing.
