1 Introduction
Fiber optic cable is the medium for optical signal transmission. It is the basis of optical communication. In this field, the State refers to the International Electrotechnical Commission IEC 793-1-2:1995 "Fiber Optic Part 1: General Specification Part 2: Test Methods for Dimensional Parameters", IEC 793-1-4:1995 "Fiber Optics Part 1: General Specification No.4 Chapter: Test Methods for Transmission Characteristics and Optical Characteristics†and ITU ITU-TG 650:1997 “Definition and test methods for relevant parameters of single-mode optical fibersâ€, ITU-TG 651:1993 Characteristics of 50/125 μm multimode graded-index optical fiber cables And other relevant international standards have formulated the national standard for fiber optic cable GB/T15972.2-1998 "General Rules for Optical Fibers Part 2: Test Methods for Dimensional Parameters" and GB/T15972.4-1998 "General Rules for Optical Fiber Part 4: Transmission Characteristics". And optical characteristics test methods, related to the basic test parameters and test methods of optical fiber.
These standards specify specific performance specifications for fiber optic cables. The characteristic parameters of the optical fiber are divided into geometric characteristic parameters (fiber length, core diameter, cladding diameter, core non-circularity, cladding out-of-roundness, core/cladding concentricity error, etc.) and optical characteristic parameters (mode field diameter) The cut-off wavelength of the single-mode fiber, the cut-off wavelength of the cabled single-mode fiber, the refractive index distribution, the numerical aperture of the multimode fiber, and the transmission characteristic parameters (attenuation, wavelength dispersion, etc.).
2nd, the fiber parameter test method
The test method for the optical fiber parameters is performed with reference to the relevant test methods in the national standard. The following describes some test methods for the basic parameters of the optical fiber. In the characteristic parameters of the optical fiber, the geometrical characteristics parameters relate to the test methods for the diameter of the cladding of the optical fiber, the degree of out-of-roundness of the cladding, and the error of the core/cladding concentricity; the optical characteristic parameters relate to the mode field diameter and the cutoff of the single-mode optical fiber. The wavelength and the cut-off wavelength of the single-mode fiber for cable are described in the relevant test method; the transmission parameters are related to the test methods for the attenuation and wavelength dispersion of the fiber.
2.1, optical fiber geometric parameters test
Optical fiber refractive index profile, cladding diameter, cladding non-circularity, core/cladding concentricity error test method.
The test methods for measuring the diameter of the cladding, the roundness of the cladding, and the error of the core/cladding concentricity are the refractive near field method, the lateral interference method, and the near field light distribution method (measurement of the cross-sectional geometry).
There are three methods for measuring the refractive index profile, cladding diameter, cladding roundness, and core/cladding concentricity error of an optical fiber.
â—Refractive near field method
The refractive near field method is a reference test method (RTM) for measuring the refractive index profile of multimode optical fibers and single mode optical fibers, a reference test method for measuring the dimensional parameters of a multimode optical fiber, and an alternative test method (ATM) for measuring a single mode optical fiber size parameter.
The refractive near field measurement is a direct and accurate measurement. It can directly measure the refractive index change of the cross-section of the optical fiber (core and cladding), has a high resolution, and can be calibrated to give the absolute value of the refractive index. From the refractive index profile, the geometric parameters of the multimode fiber and the single-mode fiber and the maximum theoretical numerical aperture of the multimode fiber can be determined.
â— Lateral interference
Lateral interferometry is an alternative test method (ATM) for the determination of refractive index profiles and dimensional parameters. The transverse interference method employs an interference microscope to illuminate the specimen perpendicular to the axis of the optical fiber sample to generate interference fringes, and the refractive index profile is obtained through video detection and computer processing.
Near-field light distribution
This method is an alternative test method (ATM) for multimode fiber geometries and a reference test method (RTM) for single-mode fiber geometries (except for the mode field diameter). By analyzing the distribution of the near-field light at the output end of the fiber under test, the geometric dimensions of the cross-section of the fiber are determined.
Gray scale method and near field scanning method can be used. The gray scale method uses a video system to implement two-dimensional (xy) near-field scanning, and the near-field scanning method only performs one-dimensional near-field scanning. Due to the effect of core non-circularity, the core diameters obtained by the near-field scanning method and the gray-scale method may differ. Core misalignment can be determined by multi-axis scanning.
The measurement method for the refractive index distribution of general commercial instruments is the refractive near field method.
The instrument used in the test was a fiber geometry and refractive index profile meter. The test procedure is as follows:
1 When preparing the sample, note that the end face of the sample is clean, smooth and perpendicular to the fiber axis.
2 When measuring the cladding, the angle of inclination of the end face should be less than 1°. Control the end face damage so that it has minimal effect on measurement accuracy.
3 Take care to avoid small bends in the fiber.
4 The coated fiber was stripped off and the flat end face was cut with a special fiber cleaver. The sample was placed in a fiber sample box. The index box was filled with an index matching liquid having a refractive index slightly higher than that of the fiber cladding.
5 Place the fiber sample holder vertically between the light source and the photodetector of the fiber-optic refraction index meter and perform the xy-direction scan test.
6 The test data of fiber refractive index profile, cladding diameter, out-of-roundness of the cladding, and core/cladding concentricity error were obtained through analysis.
2.2, optical fiber optical parameters test
(1) Test method for mode field diameter of single mode optical fiber
The mode field diameter is a measure of the spatial distribution of the mode field strength of the single-mode fiber fundamental mode (LP01) and it depends on the characteristics of the fiber.
The mode field diameter (MFD) can be measured in the far field using the far field intensity distribution Pm(θ), the complementary aperture power transfer function α(θ), and the near field intensity distribution f2(r) in the near field. 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 a single mode fiber:
â—Direct far-field scanning
The direct far-field scanning method is a reference test method (RTM) for measuring the mode field diameter of a single mode fiber. It directly defines the field-mode diameter of a single-mode fiber by measuring the far-field radiation pattern of the fiber in accordance with Petermann's 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 a single mode fiber. It calculates the mode field diameter of the single-mode fiber by measuring the optical power through two-dimensional far-field plots of different sizes of apertures. The mathematical basis for calculating the mode-field diameter is the definition of the Patterman far-field.
Near-field scanning
The near field scanning method is an alternative test method (ATM) for measuring the mode field diameter of a single mode fiber. It calculates the mode field diameter of the single-mode fiber by measuring the radial near-field diagram of the fiber. The mathematical basis for the calculation of the mode field diameter is the definition of the Patterman far-field.
The method for measuring the diameter of a typical commercial instrument model field is the far field variable aperture method (VAFF).
The instrument used in the test is the fiber mode field diameter and attenuation spectrum meter. The test procedure is as follows:
◠Prepare a 2m (±0.2m) fiber sample, remove the coating at both ends, and place it in an optical fixture. Cut the flat end face with a special fiber cleaver.
â— Connect the fiber under test to the input and output of the meter and check the focus 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, you need to adjust the position and focus.
â— Maintain the injection conditions of the test fiber at the output end of the light source, use a small ring with a radius of 30 mm, filter out the effects of the LP11 mode, and test the mode field diameter.
Through the analysis of the fiber model field diameter test data.
(2) Test method for single-mode fiber cut-off wavelength and cabled single-mode fiber cut-off wavelength
The test method for measuring the cutoff wavelength of a single mode fiber and the cutoff wavelength of a cabled single mode fiber is a transmission power method.
When the mode in the fiber is generally uniformly excited, the ratio of the total optical power including the injected higher mode to the fundamental mode optical power decreases as the wavelength decreases to the specified value (0.1 dB). It is the cutoff wavelength. According to the definition of cut-off wavelength, the transmission power method compares the transmission power of the tested optical fiber (or optical cable) with the change of the reference transmission power with the wavelength under certain conditions to obtain the cutoff wavelength of the optical fiber (or optical cable).
The common commercial instrument model field diameter test method is the transmission power law.
The instrument used in the test was a fiber mode field diameter and an attenuation spectrometer. The test procedure is as follows:
1 In the sample preparation, the cut-off wavelength of the single-mode fiber was tested using a 2m (±0.2m) fiber sample, and the cut-off wavelength of the single-mode fiber was tested using a 22m cabled single-mode fiber.
2 Strip the coating at both ends of the test fiber and place it in the fiber clamp. Cut the flat end face with a special fiber cleaver.
3 Connect the fiber to be measured to the input and output of the meter and check the focus 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, you need to adjust the position and focus.
4 First test the reference transmission power in the case that the test fiber does not ring.
5 Then test the fiber at the injection end with a small ring with a radius of 30mm, filter out the effects of LP11 mode, and test the transmission power at this time.
6 Compare the two transmission power test curves and obtain the cutoff wavelength of the optical fiber (or optical cable) through data analysis and processing.
2.3, optical fiber transmission characteristic parameter test
(1) Attenuation test method
Attenuation is a measure of the reduction in optical power in an optical fiber. It depends on the nature and length of the optical fiber and is influenced by the measurement conditions. The main test methods for attenuation are as follows:
Truncation method
The truncation method is a reference test method (RTM) for measuring the attenuation characteristics of optical fibers. When the injection conditions are not changed, the optical power passing through the two cross-sections of the optical fiber is measured, and the optical fiber attenuation is directly obtained.
Insertion loss method
The insertion loss method is an alternative test method (ATM) to measure the attenuation characteristics of an optical fiber and is similar in principle to a truncation method, but the optical power at the fiber injection end is the output optical power at the output end of the injection system. The attenuation of the measured fiber includes the attenuation of the test device. The measurement results must be corrected with the additional connector loss and the reference fiber segment loss.
Backscattering
The backscattering method is an alternative test method (ATM) for measuring the attenuation characteristics of an optical fiber. It measures the backscattered light power that is scattered back to the beginning of the fiber from different points in the fiber. This is a single-ended measurement method.
The test method for general commercial instrument attenuation is truncation and backscattering.
The instrument used in the cut-off test is the fiber mode field diameter and attenuation spectrometer. The test procedure is as follows:
1 Prepare an optical fiber sample no shorter than 1 km or longer (usually one fiber tray length: 25 km), strip the cover layer at both ends, place it in the fiber clamp, and use a special fiber cleaver to cut out the flat end face.
2 Connect the outer fiber of the test fiber tray to the transmitter end of the meter through a special fixture. Connect the inner fiber of the test fiber tray to the receiving end of the meter through a special fixture and check the focus state of the light receiving end. If the curve is not at the center of the screen or If the fiber end face is not clear enough, you need to adjust the position and focal length.
3 A small ring with a radius of 30 mm is punched at the injection end of the fiber to filter out the influence of the LP11 mode and test the transmission power at this time.
4 Keep the injection state of the light source unchanged (a small ring with a radius of 30mm is injected at the injection end of the fiber). Cut the test fiber sample to a sample of 2m. The fiber is connected to the receiving end of the instrument through a special fixture, and the focus state of the light receiving end is checked. If the curve is not in the center of the screen or the end face of the fiber is not clear enough, you need to adjust the position and focus. Test the transmission power at this time.
Comparing the two transmission power test curves, through data analysis and processing, the attenuation spectrum characteristics of the optical fiber at 1310 nm and 1550 nm are obtained.
The instrument used in the backscattering test is an optical time domain reflectometer. The test procedure is as follows:
1 The outer end of the test fiber tray is welded to the fiber optic connector or bare fiber adapter and the optical time domain reflectometer is connected for testing.
2 The optical time-domain reflectometer uses the Least Squares (LSA) method to calculate the attenuation of the fiber. This method can ignore the impact of possible splices or splice loss in the fiber on the fiber link test.
3 If you need to test the attenuation of a fiber link in stages, use the two-point method for testing.
4 In the fiber attenuation test, the linear region in the fiber test curve should be selected to avoid the saturation region at the proximal end of the test curve and the reflection region at the end, and to test the attenuation (dB/km) of the optical fiber between the two points.
5 Change the test wavelength of the optical time domain reflectometer, and test and analyze the attenuation characteristics of the optical fibers at the wavelengths of 1310 nm and 1550 nm, respectively.
In practical tests, fiber attenuation test data can be verified by two methods of truncation and backscattering. For test fiber samples with fiber optic connectors, single-ended, non-destructive tests can only be performed using the backscattering method in order not to damage the installed fiber optic connectors.
(2) Wavelength dispersion test method
Wavelength dispersion is the broadening of light pulses per unit light source spectral width in an optical fiber caused by transmission of light waves of different wavelengths constituting the light source spectrum at different group velocity, expressed in ps/nm. It depends on the characteristics and length of the fiber. The main test methods for wavelength dispersion are as follows:
â— Phase shift method
The phase shift method is a reference test method (RTM) that measures the wavelength dispersion of an optical fiber. It measures the group delay of different wavelength signals by detecting, recording, and processing the phase shifts of sinusoidal modulated signals of different wavelengths in the frequency domain, thereby deriving the wavelength dispersion of the optical fiber.
Pulse delay method
The pulse delay method is an alternative test method (ATM) for measuring the wavelength dispersion of an optical fiber. It derives the wavelength dispersion of the optical fiber by directly detecting, recording, and processing the group delays of pulse signals at different wavelengths in the time domain.
Differential phase shift method
The differential phase shift method is an alternative test method (ATM) for measuring the wavelength dispersion of an optical fiber. It measures the wavelength dispersion coefficient at a specific wavelength from the differential group delay between two similar wavelengths in the wavelength range of 1000 nm to 1700 nm.
The wavelength dispersion method of general commercial instrumentation is the phase shift method.
The equipment used in the test was a dispersion meter. The test procedure is as follows:
1 Test fiber samples should be no shorter than 1km. Optical fiber connectors are used at both ends of the optical fiber.
2 In the dispersion test, two standard fiber patch cables shall be used to connect the input and output ends of the dispersion measuring instrument, and the other end of the two fiber patch cords shall be connected via a flange disk. The dispersion measuring instrument shall be self-looped and tested. The reference value.
3 Then test the fiber through the flange into the fiber loop.
4 According to test fiber samples, set fiber type; data fitting method; group refractive index in fiber test; test fiber length;
5 Test the fiber's zero-dispersion wavelength, zero-dispersion slope, and dispersion coefficient. The dispersion characteristics of the optical fiber are obtained by analyzing the test data.
Uncertainty evaluation method in fiber parameter test: The uncertainty evaluation in fiber parameter test is generally performed by referring to the methods mentioned below. Mainly consider the measurement instrument introduced uncertainty and measurement repeatability two factors.
3, common problems in fiber parameter testing
Examples of single-mode fibers B1.1 (that is, nondispersive shifted single-mode fibers), B1.3 (that is, wavelength-band extended nondispersive shifted single-mode fibers) and B4 (that is, non-zero dispersion shifted single-mode fibers) Explain the common problems in fiber parameter testing. The common problem in the fiber parameter test is the problem of the target wavelength of the single-mode fiber being exceeded.
According to domestic fiber optic cable standards, the cutoff wavelengths can be divided into the cable cutoff wavelength λCC, the fiber cutoff wavelength λC and the jumper cable cutoff wavelength λCj. The cutoff wavelength index of the optical fiber cable should meet the corresponding requirements in Table 2. When the cable length is not less than 22m, it shall comply with the λCC regulations in Table 2. When the use length is less than 22m but not less than 2m, it shall meet the requirements of λCj in Table 2. When the use length is less than 2m, it shall comply with the λC regulation in Table 2 to prevent possible transmission. Generated pattern noise.
The indicators of the cutoff wavelengths of B1.1 and B1.3 fibers in Table 1 are the same. In the following, we refer to B1.1 and B1.3 fibers as B1 fibers.
In the commissioning test of optical fiber cable manufacturers in China, 192 cores (including 178 B1 fibers and 14 B4 fibers) were selected as the sampling base for four fiber optic cable products, and 12 B1 fiber samples were randomly sampled. , B4 type fiber optic sample 4, test single-mode fiber cut-off wavelength parameters. In the test results, the cut-off wavelength of the single-mode optical fiber generally exceeds the standard.
The optical fiber test data is shown in Fig. 1. It can be seen from the figure that the cutoff wavelength of the B4 type fiber satisfies the requirements of the specification. The cutoff wavelength of the B1 type optical fiber is greater than the upper limit of the cutoff wavelength indicator except for the two meet the requirements of the indicator.
Figure 1, fiber cutoff wavelength test results
Fiber optic cable is the medium for optical signal transmission. It is the basis of optical communication. In this field, the State refers to the International Electrotechnical Commission IEC 793-1-2:1995 "Fiber Optic Part 1: General Specification Part 2: Test Methods for Dimensional Parameters", IEC 793-1-4:1995 "Fiber Optics Part 1: General Specification No.4 Chapter: Test Methods for Transmission Characteristics and Optical Characteristics†and ITU ITU-TG 650:1997 “Definition and test methods for relevant parameters of single-mode optical fibersâ€, ITU-TG 651:1993 Characteristics of 50/125 μm multimode graded-index optical fiber cables And other relevant international standards have formulated the national standard for fiber optic cable GB/T15972.2-1998 "General Rules for Optical Fibers Part 2: Test Methods for Dimensional Parameters" and GB/T15972.4-1998 "General Rules for Optical Fiber Part 4: Transmission Characteristics". And optical characteristics test methods, related to the basic test parameters and test methods of optical fiber.
These standards specify specific performance specifications for fiber optic cables. The characteristic parameters of the optical fiber are divided into geometric characteristic parameters (fiber length, core diameter, cladding diameter, core non-circularity, cladding out-of-roundness, core/cladding concentricity error, etc.) and optical characteristic parameters (mode field diameter) The cut-off wavelength of the single-mode fiber, the cut-off wavelength of the cabled single-mode fiber, the refractive index distribution, the numerical aperture of the multimode fiber, and the transmission characteristic parameters (attenuation, wavelength dispersion, etc.).
2nd, the fiber parameter test method
The test method for the optical fiber parameters is performed with reference to the relevant test methods in the national standard. The following describes some test methods for the basic parameters of the optical fiber. In the characteristic parameters of the optical fiber, the geometrical characteristics parameters relate to the test methods for the diameter of the cladding of the optical fiber, the degree of out-of-roundness of the cladding, and the error of the core/cladding concentricity; the optical characteristic parameters relate to the mode field diameter and the cutoff of the single-mode optical fiber. The wavelength and the cut-off wavelength of the single-mode fiber for cable are described in the relevant test method; the transmission parameters are related to the test methods for the attenuation and wavelength dispersion of the fiber.
2.1, optical fiber geometric parameters test
Optical fiber refractive index profile, cladding diameter, cladding non-circularity, core/cladding concentricity error test method.
The test methods for measuring the diameter of the cladding, the roundness of the cladding, and the error of the core/cladding concentricity are the refractive near field method, the lateral interference method, and the near field light distribution method (measurement of the cross-sectional geometry).
There are three methods for measuring the refractive index profile, cladding diameter, cladding roundness, and core/cladding concentricity error of an optical fiber.
â—Refractive near field method
The refractive near field method is a reference test method (RTM) for measuring the refractive index profile of multimode optical fibers and single mode optical fibers, a reference test method for measuring the dimensional parameters of a multimode optical fiber, and an alternative test method (ATM) for measuring a single mode optical fiber size parameter.
The refractive near field measurement is a direct and accurate measurement. It can directly measure the refractive index change of the cross-section of the optical fiber (core and cladding), has a high resolution, and can be calibrated to give the absolute value of the refractive index. From the refractive index profile, the geometric parameters of the multimode fiber and the single-mode fiber and the maximum theoretical numerical aperture of the multimode fiber can be determined.
â— Lateral interference
Lateral interferometry is an alternative test method (ATM) for the determination of refractive index profiles and dimensional parameters. The transverse interference method employs an interference microscope to illuminate the specimen perpendicular to the axis of the optical fiber sample to generate interference fringes, and the refractive index profile is obtained through video detection and computer processing.
Near-field light distribution
This method is an alternative test method (ATM) for multimode fiber geometries and a reference test method (RTM) for single-mode fiber geometries (except for the mode field diameter). By analyzing the distribution of the near-field light at the output end of the fiber under test, the geometric dimensions of the cross-section of the fiber are determined.
Gray scale method and near field scanning method can be used. The gray scale method uses a video system to implement two-dimensional (xy) near-field scanning, and the near-field scanning method only performs one-dimensional near-field scanning. Due to the effect of core non-circularity, the core diameters obtained by the near-field scanning method and the gray-scale method may differ. Core misalignment can be determined by multi-axis scanning.
The measurement method for the refractive index distribution of general commercial instruments is the refractive near field method.
The instrument used in the test was a fiber geometry and refractive index profile meter. The test procedure is as follows:
1 When preparing the sample, note that the end face of the sample is clean, smooth and perpendicular to the fiber axis.
2 When measuring the cladding, the angle of inclination of the end face should be less than 1°. Control the end face damage so that it has minimal effect on measurement accuracy.
3 Take care to avoid small bends in the fiber.
4 The coated fiber was stripped off and the flat end face was cut with a special fiber cleaver. The sample was placed in a fiber sample box. The index box was filled with an index matching liquid having a refractive index slightly higher than that of the fiber cladding.
5 Place the fiber sample holder vertically between the light source and the photodetector of the fiber-optic refraction index meter and perform the xy-direction scan test.
6 The test data of fiber refractive index profile, cladding diameter, out-of-roundness of the cladding, and core/cladding concentricity error were obtained through analysis.
2.2, optical fiber optical parameters test
(1) Test method for mode field diameter of single mode optical fiber
The mode field diameter is a measure of the spatial distribution of the mode field strength of the single-mode fiber fundamental mode (LP01) and it depends on the characteristics of the fiber.
The mode field diameter (MFD) can be measured in the far field using the far field intensity distribution Pm(θ), the complementary aperture power transfer function α(θ), and the near field intensity distribution f2(r) in the near field. 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 a single mode fiber:
â—Direct far-field scanning
The direct far-field scanning method is a reference test method (RTM) for measuring the mode field diameter of a single mode fiber. It directly defines the field-mode diameter of a single-mode fiber by measuring the far-field radiation pattern of the fiber in accordance with Petermann's 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 a single mode fiber. It calculates the mode field diameter of the single-mode fiber by measuring the optical power through two-dimensional far-field plots of different sizes of apertures. The mathematical basis for calculating the mode-field diameter is the definition of the Patterman far-field.
Near-field scanning
The near field scanning method is an alternative test method (ATM) for measuring the mode field diameter of a single mode fiber. It calculates the mode field diameter of the single-mode fiber by measuring the radial near-field diagram of the fiber. The mathematical basis for the calculation of the mode field diameter is the definition of the Patterman far-field.
The method for measuring the diameter of a typical commercial instrument model field is the far field variable aperture method (VAFF).
The instrument used in the test is the fiber mode field diameter and attenuation spectrum meter. The test procedure is as follows:
◠Prepare a 2m (±0.2m) fiber sample, remove the coating at both ends, and place it in an optical fixture. Cut the flat end face with a special fiber cleaver.
â— Connect the fiber under test to the input and output of the meter and check the focus 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, you need to adjust the position and focus.
â— Maintain the injection conditions of the test fiber at the output end of the light source, use a small ring with a radius of 30 mm, filter out the effects of the LP11 mode, and test the mode field diameter.
Through the analysis of the fiber model field diameter test data.
(2) Test method for single-mode fiber cut-off wavelength and cabled single-mode fiber cut-off wavelength
The test method for measuring the cutoff wavelength of a single mode fiber and the cutoff wavelength of a cabled single mode fiber is a transmission power method.
When the mode in the fiber is generally uniformly excited, the ratio of the total optical power including the injected higher mode to the fundamental mode optical power decreases as the wavelength decreases to the specified value (0.1 dB). It is the cutoff wavelength. According to the definition of cut-off wavelength, the transmission power method compares the transmission power of the tested optical fiber (or optical cable) with the change of the reference transmission power with the wavelength under certain conditions to obtain the cutoff wavelength of the optical fiber (or optical cable).
The common commercial instrument model field diameter test method is the transmission power law.
The instrument used in the test was a fiber mode field diameter and an attenuation spectrometer. The test procedure is as follows:
1 In the sample preparation, the cut-off wavelength of the single-mode fiber was tested using a 2m (±0.2m) fiber sample, and the cut-off wavelength of the single-mode fiber was tested using a 22m cabled single-mode fiber.
2 Strip the coating at both ends of the test fiber and place it in the fiber clamp. Cut the flat end face with a special fiber cleaver.
3 Connect the fiber to be measured to the input and output of the meter and check the focus 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, you need to adjust the position and focus.
4 First test the reference transmission power in the case that the test fiber does not ring.
5 Then test the fiber at the injection end with a small ring with a radius of 30mm, filter out the effects of LP11 mode, and test the transmission power at this time.
6 Compare the two transmission power test curves and obtain the cutoff wavelength of the optical fiber (or optical cable) through data analysis and processing.
2.3, optical fiber transmission characteristic parameter test
(1) Attenuation test method
Attenuation is a measure of the reduction in optical power in an optical fiber. It depends on the nature and length of the optical fiber and is influenced by the measurement conditions. The main test methods for attenuation are as follows:
Truncation method
The truncation method is a reference test method (RTM) for measuring the attenuation characteristics of optical fibers. When the injection conditions are not changed, the optical power passing through the two cross-sections of the optical fiber is measured, and the optical fiber attenuation is directly obtained.
Insertion loss method
The insertion loss method is an alternative test method (ATM) to measure the attenuation characteristics of an optical fiber and is similar in principle to a truncation method, but the optical power at the fiber injection end is the output optical power at the output end of the injection system. The attenuation of the measured fiber includes the attenuation of the test device. The measurement results must be corrected with the additional connector loss and the reference fiber segment loss.
Backscattering
The backscattering method is an alternative test method (ATM) for measuring the attenuation characteristics of an optical fiber. It measures the backscattered light power that is scattered back to the beginning of the fiber from different points in the fiber. This is a single-ended measurement method.
The test method for general commercial instrument attenuation is truncation and backscattering.
The instrument used in the cut-off test is the fiber mode field diameter and attenuation spectrometer. The test procedure is as follows:
1 Prepare an optical fiber sample no shorter than 1 km or longer (usually one fiber tray length: 25 km), strip the cover layer at both ends, place it in the fiber clamp, and use a special fiber cleaver to cut out the flat end face.
2 Connect the outer fiber of the test fiber tray to the transmitter end of the meter through a special fixture. Connect the inner fiber of the test fiber tray to the receiving end of the meter through a special fixture and check the focus state of the light receiving end. If the curve is not at the center of the screen or If the fiber end face is not clear enough, you need to adjust the position and focal length.
3 A small ring with a radius of 30 mm is punched at the injection end of the fiber to filter out the influence of the LP11 mode and test the transmission power at this time.
4 Keep the injection state of the light source unchanged (a small ring with a radius of 30mm is injected at the injection end of the fiber). Cut the test fiber sample to a sample of 2m. The fiber is connected to the receiving end of the instrument through a special fixture, and the focus state of the light receiving end is checked. If the curve is not in the center of the screen or the end face of the fiber is not clear enough, you need to adjust the position and focus. Test the transmission power at this time.
Comparing the two transmission power test curves, through data analysis and processing, the attenuation spectrum characteristics of the optical fiber at 1310 nm and 1550 nm are obtained.
The instrument used in the backscattering test is an optical time domain reflectometer. The test procedure is as follows:
1 The outer end of the test fiber tray is welded to the fiber optic connector or bare fiber adapter and the optical time domain reflectometer is connected for testing.
2 The optical time-domain reflectometer uses the Least Squares (LSA) method to calculate the attenuation of the fiber. This method can ignore the impact of possible splices or splice loss in the fiber on the fiber link test.
3 If you need to test the attenuation of a fiber link in stages, use the two-point method for testing.
4 In the fiber attenuation test, the linear region in the fiber test curve should be selected to avoid the saturation region at the proximal end of the test curve and the reflection region at the end, and to test the attenuation (dB/km) of the optical fiber between the two points.
5 Change the test wavelength of the optical time domain reflectometer, and test and analyze the attenuation characteristics of the optical fibers at the wavelengths of 1310 nm and 1550 nm, respectively.
In practical tests, fiber attenuation test data can be verified by two methods of truncation and backscattering. For test fiber samples with fiber optic connectors, single-ended, non-destructive tests can only be performed using the backscattering method in order not to damage the installed fiber optic connectors.
(2) Wavelength dispersion test method
Wavelength dispersion is the broadening of light pulses per unit light source spectral width in an optical fiber caused by transmission of light waves of different wavelengths constituting the light source spectrum at different group velocity, expressed in ps/nm. It depends on the characteristics and length of the fiber. The main test methods for wavelength dispersion are as follows:
â— Phase shift method
The phase shift method is a reference test method (RTM) that measures the wavelength dispersion of an optical fiber. It measures the group delay of different wavelength signals by detecting, recording, and processing the phase shifts of sinusoidal modulated signals of different wavelengths in the frequency domain, thereby deriving the wavelength dispersion of the optical fiber.
Pulse delay method
The pulse delay method is an alternative test method (ATM) for measuring the wavelength dispersion of an optical fiber. It derives the wavelength dispersion of the optical fiber by directly detecting, recording, and processing the group delays of pulse signals at different wavelengths in the time domain.
Differential phase shift method
The differential phase shift method is an alternative test method (ATM) for measuring the wavelength dispersion of an optical fiber. It measures the wavelength dispersion coefficient at a specific wavelength from the differential group delay between two similar wavelengths in the wavelength range of 1000 nm to 1700 nm.
The wavelength dispersion method of general commercial instrumentation is the phase shift method.
The equipment used in the test was a dispersion meter. The test procedure is as follows:
1 Test fiber samples should be no shorter than 1km. Optical fiber connectors are used at both ends of the optical fiber.
2 In the dispersion test, two standard fiber patch cables shall be used to connect the input and output ends of the dispersion measuring instrument, and the other end of the two fiber patch cords shall be connected via a flange disk. The dispersion measuring instrument shall be self-looped and tested. The reference value.
3 Then test the fiber through the flange into the fiber loop.
4 According to test fiber samples, set fiber type; data fitting method; group refractive index in fiber test; test fiber length;
5 Test the fiber's zero-dispersion wavelength, zero-dispersion slope, and dispersion coefficient. The dispersion characteristics of the optical fiber are obtained by analyzing the test data.
Uncertainty evaluation method in fiber parameter test: The uncertainty evaluation in fiber parameter test is generally performed by referring to the methods mentioned below. Mainly consider the measurement instrument introduced uncertainty and measurement repeatability two factors.
3, common problems in fiber parameter testing
Examples of single-mode fibers B1.1 (that is, nondispersive shifted single-mode fibers), B1.3 (that is, wavelength-band extended nondispersive shifted single-mode fibers) and B4 (that is, non-zero dispersion shifted single-mode fibers) Explain the common problems in fiber parameter testing. The common problem in the fiber parameter test is the problem of the target wavelength of the single-mode fiber being exceeded.
According to domestic fiber optic cable standards, the cutoff wavelengths can be divided into the cable cutoff wavelength λCC, the fiber cutoff wavelength λC and the jumper cable cutoff wavelength λCj. The cutoff wavelength index of the optical fiber cable should meet the corresponding requirements in Table 2. When the cable length is not less than 22m, it shall comply with the λCC regulations in Table 2. When the use length is less than 22m but not less than 2m, it shall meet the requirements of λCj in Table 2. When the use length is less than 2m, it shall comply with the λC regulation in Table 2 to prevent possible transmission. Generated pattern noise.
The indicators of the cutoff wavelengths of B1.1 and B1.3 fibers in Table 1 are the same. In the following, we refer to B1.1 and B1.3 fibers as B1 fibers.
Table 1
In the commissioning test of optical fiber cable manufacturers in China, 192 cores (including 178 B1 fibers and 14 B4 fibers) were selected as the sampling base for four fiber optic cable products, and 12 B1 fiber samples were randomly sampled. , B4 type fiber optic sample 4, test single-mode fiber cut-off wavelength parameters. In the test results, the cut-off wavelength of the single-mode optical fiber generally exceeds the standard.
The optical fiber test data is shown in Fig. 1. It can be seen from the figure that the cutoff wavelength of the B4 type fiber satisfies the requirements of the specification. The cutoff wavelength of the B1 type optical fiber is greater than the upper limit of the cutoff wavelength indicator except for the two meet the requirements of the indicator.
Figure 1, fiber cutoff wavelength test results
Figure 1, fiber cutoff wavelength test results
Because the fiber cut-off wavelength is too large, it exceeds the system's expected operating wavelength. The effects of mode noise and dispersion power at the time of optical transmission in the system cannot be avoided. The cut-off wavelength of the optical fiber exceeds the standard, so that the optical fiber cannot be in the single-mode (base mode LP01) working state, causing interference to the transmission of optical signals in the system and affecting the transmission quality. Therefore, the range of the cut-off wavelength of the optical fiber cable should be strictly controlled, the production process of the optical fiber core should be improved, and the cable-laying process of the optical fiber cable should be monitored.
4 Conclusion
The fiber parameter test is an important technical indicator in the test of optical fibers and optical fibers, and it is crucial to the quality of optical fiber cables. This article summarizes the testing methods of optical fiber parameters, the evaluation criteria of uncertainty, and the problems in fiber optic cable testing. Summarizes the application and improvement of test methods in actual testing, as well as possible problems and solutions.
4 Conclusion
The fiber parameter test is an important technical indicator in the test of optical fibers and optical fibers, and it is crucial to the quality of optical fiber cables. This article summarizes the testing methods of optical fiber parameters, the evaluation criteria of uncertainty, and the problems in fiber optic cable testing. Summarizes the application and improvement of test methods in actual testing, as well as possible problems and solutions.