Instruments for metrological support of optical time domain reflectometers and fiber optic sensors
Development, testing and operation of FOCL are not possible without measurements of their characteristics and use of the appropriate measuring instruments, in particular, optical time domain reflectometers. Monitoring systems based on fiber optic communication lines equipped with fiber optic sensors have measuring functions proceeding from their intended purpose.
When developing and operating the fiber optic communication lines based on FOS, the necessity to measure such parameters as length of optical cable, power of optical radiation, attenuation, spectral characteristics of optical radiation, time characteristics of modulated optical radiation occurs. Besides, during the graduation and check of FOS it is necessary to perform measurements of the parameters recorded by sensor using the relevant high-accuracy measuring devices and standards.
In order to reach the required accuracy of measurements of listed characteristics it is required to ensure the traceability of used measuring instruments with respect to state standards of the relevant units. Traceability is the property of unit standard or measuring instrument, which consists in documented establishment of their connection with the state primary standard of the relevant unit by comparison of unit standards, check and calibration of measuring instruments .
Procedure of transfer of physical units from standards to measuring instruments with respect of FOTS parameters, in particular, to fiber optic time domain reflectometers, is established by GOST 8.585-2013 . The fragment of graphic part of this measurement chart, which determines the transfer of measurement units to optical time domain reflectometers, is given in Fig. 1.
The complexes of instruments, which include optical generators, reels of optic fiber and auxiliary equipment, are used in the capacity of industrial standard of the units of length, time of signal propagation in light guide and distributed attenuation. Optical generators form the repeating optical pulses with standardized time intervals between them, which correspond to the set length of optical cable. These pulses are supplied in the capacity of reference signal at the entry of reflectometer in order to determine its error during the measurements of light guide length. In order to verify the error of optical time domain reflectometer during the attenuation measurements the standardization of amplitude of pulses, which come from the optical generator, is used.
Working standards of the measurement units of average power and attenuation are made on the basis of high-accuracy meters of optical power and stabilized radiation sources at the fixed wavelengths. When verifying the measuring instruments (MI) of average power and attenuation the comparison of checked MI with standard meter of optical power is performed.
Working standards of the measurement unity of wavelength are made on the basis of radiation sources and cuvettes with gases, in which the transmission spectrum of optical radiation contains a number of absorption peaks with standardized wavelength. When verifying the optical spectrum analyzers, the measurements of wavelengths of absorption peaks by verified MI and comparison of wavelength measurement results with certified values are performed.
The main regulatory documents, which regulate the methods of verification of measuring instruments of FOTS characteristics, are:
• GOST R 8.720-2010. "State System for Ensuring the Uniformity of Measurements. Optical power meters, sources of optical radiation, meters of inverse losses and optical small testers in FOTS. Methods of verification";
• Recommendation in metrology R 50.2.069-2009. "State System for Ensuring the Uniformity of Measurements. Optical spectrum analyzers in FOTS. Methods of verification";
• Recommendation in metrology R 50.2.070-2009. "State System for Ensuring the Uniformity of Measurements. Attenuators in FOTS. Methods of verification";
• Recommendation in metrology R 50.2.071_2009. "State System for Ensuring the Uniformity of Measurements. Optical time domain reflectometers. Methods of verification".
For the purpose of metrological support of the specialized FOCL, at the Federal State Unitary Enterprise "VNIIFTRI" the complex of equipment intended for the control of optical radiation characteristics is developed; this complex consists of measuring instruments RESM–VS and REDV certified in the capacity of working standards, measuring instruments with respect to time characteristics of optical radiation (oscillograph with broad-band optic-electronic transformer), optical spectrum analyzer. In order to enhance the accuracy of delay between the optical pulses during the calibration of optical time domain reflectometers by the length scale, in these instruments complexing of optical generator with digital memory oscillograph, which performs the measurements of time intervals with sub-nanosecond accuracy. This instrument complex allows solving the main tasks in control of FOCL characteristics and instruments for measurements of their parameters, including the optical time domain reflectometers. The main metrological characteristics of the instrument complex for the control of optical radiation characteristics are given in table. This instrument complex allows testing for the purposes of approval of type, verification and calibration of MI of FOTS characteristics: optical time domain reflectometers, meters of optical radiation power, optical testers, sources of optical radiation, optical attenuators, optical spectrum analyzers, analyzers of digital communication lines; also it allows performing high-accuracy measurements of FOCL characteristics: average power and attenuation of optical radiation, light guide length, spectral and time characteristics of optical radiation.
Metrological support of FOS and measuring systems based on them covers greater number of tasks in comparison with FOTS, which concerns not only characteristics of optical radiation transmitting the signals inside the system but also physical parameters measured by sensors, which characterize the state of object controlled by the system (temperature, pressure, vibration etc.). And provision of required accuracy and reliability of measurements of these parameters, which are input in relation to the system, is the primary task, for the solution of which it is necessary to use unit standards of the parameters measured by the system.
Procedure of experimental determination of metrological characteristics of FOS is explained by the generalized chart given in Fig. 2. The following main elements are shown in chart: FOS with the instruments for signal record, equipment for reproduction of actuating quantities (for example, heat chamber, vibration bed etc.) and reference instrumentation (reference sensor with recording instruments). When calibrating FOS the quantity reproduced by equipment (temperature, pressure, acceleration etc.) influences simultaneously on FOS and reference sensor located in the chamber (at the test bench), signals from sensors get onto the relevant recording instruments where the measured values of the parameter influencing of tested FOS and its actual values, which are determined by reference sensor. It allows determining the values of sensor conversion coefficients and evaluating their error.
In order to carry out the tests for the purpose of type approval and calibration of different groups of fiber optic sensors and systems based on them, the capabilities of FSUE "VNIIFTRI" with respect to high-accuracy measurements of temperature, pressure, movements, parameters of vibration and equipment with the relevant standards, in particular: state primary temperature standard within the range from 0.3 to 273.16 K, state secondary pressure standard within the range from 0.02 to 1600 MPa, working temperature standards, working standard of relative humidity of gases and temperature within the ranges from 5 to 95% and from 5 to 60 °C, working standards of length, speed and acceleration in case of oscillating motion of solid body and standards of other physical parameters.
Use of the standard base existing in country for the purposes of metrological support of monitoring systems based on FOS is necessary in order to meet the requirements of regulatory documents in ensuring the measurement uniformity. Besides, use of high-accuracy MI and standards is reasonable in case of development and studies of prospective FOS with improved metrological characteristics.