Measurement method of mode field diameter of (1) single-mode optical fiber

The mode field diameter is a measure of the spatial distribution of the mode field intensity of the fundamental mode (LP0 1) of a single-mode fiber, which depends on the characteristics of the fiber.

Mode field diameter (MFD) can be measured by far-field intensity distribution Pm(θ), complementary aperture power transfer function α (θ) and near-field intensity distribution f2(r). The definition of mode field diameter is closely related to the measurement method.

There are three methods to measure the mode field diameter of single-mode fiber.

● Direct far-field scanning method

Direct far-field scanning method is a benchmark method to measure the mode field diameter of single-mode fiber. According to Peterman's far-field definition, the mode field diameter of single-mode fiber is calculated by measuring the far-field radiation pattern of fiber.

● Far field variable aperture method

Far-field variable aperture method is an alternative method to measure the mode field diameter of single-mode fiber. It calculates the mode field diameter of single-mode fiber by measuring the optical power through two-dimensional far-field diagrams with different aperture sizes, and the mathematical basis for calculating the mode field diameter is Bertman's far-field definition.

Low near-field scanning method

Near-field scanning method is an alternative test method (ATM) to measure the mode field diameter of single-mode fiber. It calculates the mode field diameter of single-mode fiber by measuring the radial near-field diagram of the fiber, and the mathematical basis for calculating the mode field diameter is Bertman's far-field definition.

The general method for measuring the mode field diameter of commercial instruments is the far-field variable aperture method (VAFF).

The instrument used in the test is the optical fiber mode field diameter and attenuation spectrum measuring instrument. The test steps are as follows:

● Prepare a 2m(0.2m) optical fiber sample, peel off the coating from both ends, put it into an optical fiber fixture, and cut a flat end face with a special optical fiber cutter.

● Connect the measured optical fiber 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 end face of the fiber is not clear enough, the position and focal length need to be adjusted.

● Keep the injection condition of the test fiber unchanged at the output end of the light source, make a small ring with a radius of 30mm, filter out the influence of LP 1 1 mode, and test the mode field diameter.

The measured data of fiber mode field diameter are obtained through analysis.

(2) Test method of cut-off wavelength of single-mode fiber and cut-off wavelength of cabled single-mode fiber.

The test method for measuring the cutoff wavelength of single-mode fiber is transmission power method.

When the modes in the optical fiber are uniformly excited, when the ratio of the total optical power including the injected high-order modes to the fundamental mode optical power decreases to a specific value (0. 1dB) with the wavelength, the larger wavelength is the cutoff wavelength. According to the definition of cut-off wavelength, the transmission power method compares the transmission power of the measured optical fiber (or optical cable) with the reference transmission power with the wavelength under certain conditions, and obtains the cut-off wavelength value of the optical fiber (or optical cable).

Transmission power method is usually used to measure the mode field diameter of commercial instruments.

The instrument used in the test is the optical fiber mode field diameter and attenuation spectrum measuring instrument. The test steps are as follows:

① During sample preparation, 2m(0.2m) fiber samples are used for the cut-off wavelength test of single-mode fiber, and 22m single-mode fiber is used for the cut-off wavelength test of cabled single-mode fiber.

(2) Peel off the coating on both ends of the test optical fiber, put it into an optical fiber fixture, and cut the flat end face with a special optical fiber cutter.

(3) Connect the tested optical fiber to the input and output end 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 end face of the fiber 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 in the test fiber.

⑤ Make a small ring with a radius of 30mm at the injection end of the test fiber to filter out the influence of LP 1 1 mode and test the transmission power at this time.

⑥ Compare the two transmission power test curves, and get the cutoff wavelength value of optical fiber (or optical cable) through data analysis and processing.

Optical fiber transmission characteristic parameter test

Test method of (1) attenuation

Attenuation is a measure of the reduction of optical power in optical fiber, which depends on the nature and length of optical fiber and is influenced by measurement conditions. The main test methods of attenuation are as follows:

Low truncation method

Truncation method is a benchmark test method (RTM) for measuring optical fiber attenuation characteristics. Without changing the injection conditions, the optical power passing through two sections of the optical fiber can be measured, thus the attenuation of the optical fiber can be directly obtained.

Low insertion loss method

Insertion loss method is an alternative test method (ATM) for measuring attenuation characteristics of optical fiber, which is similar to truncation method in principle, but the optical power at the injection end of optical fiber is the outgoing optical power at the output end of injection system. The measured optical fiber attenuation includes the attenuation of the test equipment, and the measurement results must be corrected by the additional connector loss and the reference optical fiber segment loss respectively.

Low backscattering method

Backscattering method is an alternative test method (ATM) to measure the attenuation characteristics of optical fiber, which measures the backscattered light power from different points in the optical fiber to the beginning of the optical fiber. This is a single-ended measurement method.

The attenuation testing methods of general commercial instruments include truncation method and backscattering method.

The instrument used in the truncation test is the optical fiber mode field diameter and attenuation spectrum measuring instrument. The test steps are as follows:

① Prepare an optical fiber sample not shorter than 1km or longer (generally, the length of an optical fiber disc is 25km), peel off the coating at both ends, put it into an optical fiber fixture, and cut a flat end face with a special optical fiber cutter.

(2) Connect the outer optical fiber of the test optical fiber disc to the transmitting end of the instrument through a special fixture, and connect the inner optical fiber of the test optical fiber disc to the receiving end of the instrument through a special fixture to check the focusing state of the optical receiving end. If the curve is not in the center of the screen or the end face of the fiber is not clear enough, the position and focal length need to be adjusted.

③ Make a small ring with a radius of 30mm at the fiber injection end to filter out the influence of LP 1 1 mode and test the transmission power at this time.

(4) Keep the injection state of the light source unchanged (make a small ring with a radius of 30mm at the injection end of the optical fiber), cut the test optical fiber sample into 2m samples, connect the optical fiber to the receiving end of the instrument through a special fixture, and check the focusing state of the optical receiving end. If the curve is not in the center of the screen or the end face of the fiber is not clear enough, the position and focal length need to be adjusted. Test the transmission power at this time.

Comparing the two transmission power test curves, through data analysis and processing, the attenuation spectral characteristics of optical fiber in 13 10nm and 1550nm bands are obtained.

The instrument used in backscattering test is optical time domain reflectometer. The test steps are as follows:

① Connect the outer end of the test optical fiber disc to the optical time domain reflectometer through fused optical fiber connector or bare fiber optic adapter for testing.

② In the test, the optical time domain reflectometer uses the least square method (LSA) to calculate the attenuation of the optical fiber, which can ignore the influence of the possible welding or joint loss in the optical fiber on the optical fiber link test.

③ If it is necessary to test the attenuation of optical fiber links in sections, two-point method can be used.

④ In the optical fiber attenuation test, we should choose the linear area in the optical fiber test curve, avoid the saturated area near the test curve and the reflection area at the end, and test the optical fiber attenuation (dB/km) between two points.

⑤ The optical fiber attenuation characteristics at 13 10nm and 1550nm were tested and analyzed by changing the testing wavelength of the optical time domain reflectometer.

In the actual test, the test data of optical fiber attenuation can be verified by truncation method and backscattering method. For the test optical fiber samples with optical fiber connectors, in order not to damage the installed optical fiber connectors, the backscattering method can only be used for single-ended nondestructive testing.

Testing method of wavelength dispersion

Wavelength dispersion refers to the optical pulse broadening of unit light source spectral width in optical fiber when light waves with different wavelengths are transmitted at different group speeds, which is expressed by ps/nm. This depends on the characteristics and length of the fiber. The main testing methods of wavelength dispersion are as follows:

● Phase shift method

Phase shift method is a benchmark test method for measuring wavelength dispersion of optical fiber. By detecting, recording and processing the phase shift of sinusoidal modulation signals with different wavelengths in frequency domain, it measures the group delay of signals with different wavelengths, thus deducing the wavelength dispersion of optical fibers.

Low pulse delay method

Pulse delay method is an alternative test method for measuring wavelength dispersion of optical fiber. By directly detecting, recording and processing the group delay of pulse signals with different wavelengths in time domain, the wavelength dispersion of optical fiber is deduced.

● Differential phase shift method

Differential phase shift method is an alternative test method for measuring wavelength dispersion of optical fiber. It measures the wavelength dispersion coefficient at a specific wavelength by the differential group delay between two similar wavelengths in the wavelength range of 1000 nm to 1700 nm.

The testing method of wavelength dispersion of commercial instruments is phase shift method.

The equipment used in the test is a dispersion measuring instrument. The test steps are as follows:

① The optical fiber sample to be tested shall be no less than 1km. Fabricate optical fiber connectors at both ends of the optical fiber.

② During dispersion test, two standard optical fiber jumpers should be used to connect the input end and the output end of the dispersion meter respectively, and the other ends of the two optical fiber jumpers should be connected through flanges, so that the dispersion meter can cycle by itself and test the reference value at this time.

③ Connect the test optical fiber to the optical fiber loop through the flange.

(4) setting the optical fiber type according to the test optical fiber sample; Data fitting method; Group refractive index in optical fiber testing: testing the length of optical fiber; ; Test wavelength range; Wavelength interval, etc.

⑤ Test the zero dispersion wavelength, zero dispersion slope and dispersion coefficient of the fiber. By analyzing and processing the test data, the dispersion characteristics of optical fiber are obtained.

Evaluation method of optical fiber parameter test uncertainty: The evaluation of optical fiber parameter test uncertainty is generally carried out with reference to the following methods. This paper mainly considers the uncertainty and measurement repeatability introduced by measuring instruments.

Common problems in optical fiber parameter testing

Taking single-mode fibers B 1. 1 (i.e. non-dispersion shifted single-mode fiber), B 1.3 (i.e. non-dispersion shifted single-mode fiber with extended wavelength band) and B4 (i.e. non-zero dispersion shifted single-mode fiber) as examples, the common problems in fiber parameter testing are explained. The common problem in fiber parameter testing is that the cutoff wavelength index of single-mode fiber exceeds the standard.

According to the domestic optical fiber and cable standards, the cutoff wavelength can be divided into the cutoff wavelength λCC of optical fiber and cable, the cutoff wavelength λC of optical fiber and cable and the cutoff wavelength λCj of jumper cable, and the cutoff wavelength index of optical fiber and cable shall meet the corresponding provisions in Table 2. When the service length of optical cable is not less than 22m, it shall comply with the provisions of λCC in Table 2, λCj in Table 2 when the service length is less than 22m but not less than 2m, and λC in Table 2 when the service length is less than 2m, so as to prevent possible mode noise during transmission.

In the debugging and testing of optical cable products of domestic optical fiber and cable manufacturers, two samples of B 192 core (including B 1 optical fiber 178 core and B4 fiber 14 core) and four samples of B4 fiber were randomly selected. In the test results, the cutoff wavelength of single-mode fiber generally exceeds the standard.

In the debugging and testing of optical cable products of domestic optical fiber and cable manufacturers, two samples of B 192 core (including B 1 optical fiber 178 core and B4 fiber 14 core) and four samples of B4 fiber were randomly selected. In the test results, the cutoff wavelength of single-mode fiber generally exceeds the standard.