rus
Issue #6/2024
S. B. Bychkov, А. О. Pogonyshev, S. V. Tikhomirov, V. R. Sumkin
Methods for Measuring Return Loss in Fiber Optic Lines and Сomponents
Methods for Measuring Return Loss in Fiber Optic Lines and Сomponents
DOI: 10.22184/1993-7296.FRos.2024.18.6.470.484
This article discusses 3 methods for measuring of optic return loss (ORL) in fiber optic systems being used in modern measuring instruments: continuous wave method (CW), time domain reflectometry (OTDR) and frequency domain reflectometry (OFDR). The authors perform a comparative analysis of these methods, and consider the advantages and limitations of these methods.
This article discusses 3 methods for measuring of optic return loss (ORL) in fiber optic systems being used in modern measuring instruments: continuous wave method (CW), time domain reflectometry (OTDR) and frequency domain reflectometry (OFDR). The authors perform a comparative analysis of these methods, and consider the advantages and limitations of these methods.
Теги: measurements metrological support ofdr optical return loss orl otdr reflectometry измерение метрологическое обеспечение оптические обратные потери рефлектометрия
Methods for Measuring Return Loss in Fiber Optic Lines and Components
S. B. Bychkov1, А. О. Pogonyshev1, S. V. Tikhomirov1, V. R. Sumkin2
Federal State Budgetary Institution “All-Russian Scientific Research Institute of Optical and Physical Measurements” (FGBU “VNIIOFI”), Moscow
LLC “Research and Development Enterprise “Measuring Communication Technology” (“Izmeritelnaya Tekhnika Svyazi”) (NPP “ITS”), Saint-Petersburg
This article discusses 3 methods for measuring of optic return loss (ORL) in fiber optic systems being used in modern measuring instruments: continuous wave method (CW), time domain reflectometry (OTDR) and frequency domain reflectometry (OFDR). The authors perform a comparative analysis of these methods, and consider the advantages and limitations of these methods.
Key words: measurements, optical return loss, ORL, reflectometry, OTDR, OFDR, metrological support
Article received: 08.02.2024
Article accepted: 26.07.2024
Introduction
One of the important characteristics of fiber-optic lines and fiber-optic devices is the level of return loss (ORL, reflection loss) they create – a value equal to the ratio of the power of the backscattering signal created in the device to the power of the optical signal at its input, expressed in decibels (GOST R 54417-2011). Backscattering signals can cause instability and noise in laser emitters used in fiber-optic communication systems, which can lead to failures in telecommunications equipment or even failure of laser emitters. In this regard, the levels of return loss created by the elements and devices of the fiber optic line are usually standardized, and return loss meters are widely used in measuring equipment of fiber-optic communication.
The article discusses the methods of measuring return loss used in such measuring devices, and their comparative analysis is carried out. Currently, the following 3 methods of measuring the return loss value in optical fiber are most common:
Let’s look at these methods in more detail below.
CW measuring method
The CW method is the most obvious way to measure the value of return loss in fiber-optic lines. A diagram explaining the principle of operation of the CW meter is shown in Fig. 1.
The laser (LE) generates continuous-wave optical radiation having a steady, known power of RL, [dBm]. Through branch 1 of the SP splitter, this radiation enters its output 3. The fiber optic line under test or the fiber device under test is connected to it. In this case, the SP introduces losses of А13, [dB], into the signal, and the power of RL-dut < Rl enters branch 3. When radiation passes through the fiber-optic line, part of it is reflected back due to scattering on inhomogeneities of the transmission medium (Rayleigh scattering) and on mirror surfaces (Fresnel reflection). A backscattering signal is received at the input 3 of the SP and, suffering losses of A32, [dB], it enters the input of an optical wattmeter OPM which allows measuring the average power of Pof of this signal. Thus, knowing the average power of optical radiation at Pl laser yield, losses A13 and A32, according to the readings of the OPM Pof, [dBm], it is possible to calculate the value of the integral return losses RL, [dB], in the tract according to the formula (1).
RL = Pof − PL + A13 + A32 + 2 · α, [dB], (1)
where α, [dB] is the loss value at the optical connector connecting the studied fiber-optic line and the output branch of the SP (it is considered steady and is approximately 0.15 dB).
The losses measured in this way are due to all optical components included in the fiber-optic line. In order to determine the return loss from a given section of the line, for example, from the junction of optical connectors, the so-called mandrel is used. A mandrel is a rod of a given diameter on which an optical fiber is wound. The diameter of the rod is such that the condition of the total internal reflection is violated in the optical fiber wound on it, and radiation exits the fiber without creating return loss, and at 5–7 turns of winding, the return loss from the section of the line following the mandrel decreases by more than 70 dB. The diameter of the mandrel rod is determined by the operating wavelength and the type of optical fiber. For example, for Class G.652 optical fiber it is 8 mm when measuring at a wavelength of 1310 nm and 10 mm when measuring at a wavelength of 1550 nm. Thus, for example, in the scheme shown in Fig. 1, to measure the value of the return loss from the optical connector C connecting the two sections of the studied fiber-optic line, it is necessary to remove the return loss from the fiber located after the connector C using the mandrel M, then measure the value of the integral return loss RLsumm, [dB], from the C connector and the optical fiber before it. After that, measure the value of RLbefore, [dB] of the return loss from the section of the path before C, winding the fiber on the mandrel between the meter and the connector C as close as possible to the connector. The required value of the return loss of RLP can be calculated using the formula (2).
RLp = 10 log 10 10RLsumm / 10 − 10RLbefore / 10, [дБ]. (2)
The described CW method for measuring return loss in optical fiber is quite simple to implement. The unit of return loss measured in this way in the Russian metrological support system is traced to the State special measurement standard of units of length and time of signal propagation in a fiber optic, average power, attenuation and wavelength for fiber-optic information transmission systems (GET 170-2024) through units of average power and attenuation of optical radiation. The dynamic range of measurement of return loss by the CW method is determined by:
The measuring range of the average power of the OPM optical wattmeter,
Signal loss in the SP splitter,
The “zero” level of the return loss signal resulting from the reflection of LE radiation from the output connector 3 (see Fig.) of the meter and the SP splitter, as well as due to the presence of crosstalk between lines 1 and 2 of the SP splitter (crosstalk, isolation).
In practice, it is the latter factor that turns out to be the limiting factor. If the dynamic range of modern optical wattmeters usually exceeds 80 dB, the losses A13 and A32 in the SP splitter are about 3 dB, then the level of mutual isolation ports are highly dependent on the design. This characteristic is not standardized for all splitters available on the market, usually the manufacturer indicates exactly the level of return loss. The question of the applicability of splitters of different types to the task of measuring ORL was considered during the joint work of FGBU “VNIIOFI” and NPP “ITS”. Tabl. 1 shows the minimum zero level values obtained in a CW return loss meter using three different types of meters: Y-type fused splitters (having 1 input and 2 outputs – FBT 1x2), Y-type planar splitters (PLC 1x2) and X-type fused splitters (having 2 inputs and 2 outputs – FBT 2x2). All splitters had a count-down ratio of 50% – 50%. The unused port of the X-splitter was wound on the mandrel, which ensured the actual absence of back reflections from the end of the optical fiber. Measurements were carried out at a wavelength of 1550 nm.
The high level of “zero” signal obtained when using a fused FBT 1x2 splitter can be explained by the fact that splitters of this type are performed by fusing two optical fibers, then one of the ports is split off to obtain a Y-configuration. In this case, the area of splitting can create large return loss which determines the low coefficient of inter-channel isolation. The technology of planar splitters allows for greater channel isolation due to higher accuracy and repeatability of production processes. The best result was provided by the X-splitter, the “extra” port of which was jammed. This made it possible to expand the measurement range of return loss to the level of –70 dB and it can be argued that by choosing a better splitter, it is possible to expand the range even more. Nevertheless, this indicator can be considered close to the limit for CW meters. During use, the output connector of the meter is “rubbed out”, as a result of which the Fresnel reflection from it increases, which eventually leads to an increase in the zero signal to a level exceeding –70 dB. Another significant disadvantage of the CW method is the need to use a mandrel to measure losses from a given section of the path, which spoils the optical fiber; moreover, this is not always possible.
Nevertheless, the dynamic range of 70 dB is quite sufficient for a number of applications, and the price of CW return loss meters is very competitive. In addition, the value of the return loss measured by the described method is obviously traced to the attenuation unit for the optical fiber. Therefore, this method is traditionally used in reference technology, in particular, in working measuring standards of return loss units in optical fiber (REOP, registration number in the Measuring Equipment Register: 52363-13, 35981-07), manufactured and supplied by FGBU “VNIIOFI” to metrological centers in Russia.
Reflectometric methods
Recently, devices designed to measure distributed return loss – loss from a selected section of the route, a specific connector or component – have become increasingly common. Such devices have proved to be in demand, for example, in the production of optical patch cords, cables and assemblies – to test the quality of connector polishing. This field of application does not allow spoiling the fiber under test by winding it over a small radius. Devices implementing time-domain and phase-domain reflectometric methods (OTDR and OFDR) are used here. Devices using the OTDR-reflectometric method, for example, include the MAP‑200 (VIAVI, France) and Op‑940 (OptoTest, USA). OFDR-reflectometric method is used, for example, in OVA5000 (Luna, USA). These systems can measure both integral return loss and loss from a given section of the fiber optic cable and have wider dynamic ranges than CW-type meters.
OTDR-метод
Fig. 2 shows a generalized block diagram of an OTDR reflectometer. A pulsed laser emitter LE generates probing optical pulses coming through a fiber-optic SP splitter into the fiber-optic line under study. Propagating in it, the pulses are subjected to backscattering, recorded by a high-speed photodetector (PD), the signal from which is amplified using a broadband amplifier (SA) and fed to a high-speed analog-to-digital converter (ADC) [1]. The digital signal processing (DSP) module controls the start-up LE and the processing of ADC signals. The ADC counts are synchronized in time with the LE start-up moments, for each ADC count produced after the time n Δt, [s] (where Δt is the time between ADC samples, n is an integer) after the radiation of an optical pulse, LE corresponds to the backscattering signal of the probing pulse from a specific section of the fiber-optic path. Thus, a reflectogram is formed – the dependence of the relative power of the backscattering signal on the distance between the meter and the scattering site. During the measurement, the radiator (LE) generates a sequence of optical probing pulses, the period of which is adjusted so that the back reflection signal from the most distant point in the fiber-optic path has time to get to the photodetector (PD) before the next probing pulse is formed. For each point of the reflectogram, a set of averaging of the measured value of the backscattering signal is performed, which reduces the influence of noise from the photodetector channel.
The described principle of operation is the same for a large number of OTDRs used to measure the length of the fiber optic line and troubleshooting on the lines, however, if the reflectometers are optimized for long-range operation (from 1 to 100 km), then OTDR reverse loss meters are optimized for operation in the near zone. They are characterized by higher spatial resolution, higher accuracy and a wider range of backscattering signal levels, but a smaller range of operating lengths of optical lines.
The OTDR principle makes it possible to neutralize the factors limiting the dynamic range of CW return loss meters – the influence of crosstalk of the splitter and the output optical connector. For the OTDR system, these interferences may not be taken into account due to the choice of the minimum interval τ of the time delay of the ADC start relative to the moment of generation of the probing pulse. This allows you to expand the dynamic range of the distributed return loss meter to values corresponding to Rayleigh scattering from the optical fiber section (usually from –70 dB to –80 dB per 1 m). However, the requirement to increase the spatial resolution of the OTDR conflicts with the requirement to increase the dynamic range of the meter. Since an increase in spatial resolution is achieved by reducing the duration of the probing optical pulses, the power of the backscattering signal also decreases. The power of the emitters used to generate probing pulses is limited from above both by the technical capabilities of the emitters themselves and by nonlinear effects in the optical fiber (and usually does not exceed several hundred mW). All this complicates the process of measuring the power of the reverse loss signal and forces OTDR manufacturers to use avalanche photodetectors with worse stability characteristics at higher sensitivity characteristics. Another aspect that prevents obtaining high spatial resolution is the speed of the ADC. For example, in order to obtain a resolution of 10 cm, the amplitude of the backscattering signal must be measured every Δt ≈ 1 ns, which requires a conversion frequency of more than 1 GHz and the corresponding speed from the PD photodetector and the SA amplifier.
The issue of determining the dynamic range of OTDR is discussed in detail in [1] unlike classical reflectometers, OTDR return loss meters are designed to operate both at low, maximum detectable values of the return loss signal power and at higher values (for example, for Op‑940 manufactured by Optotest, USA – from –10 dB). The lower limit of the sensitivity of the OTDR meter can be calculated from the equivalent noise power density of the photodetector signal, NEP W / √—Hz. For photodetectors based on p-i-n photodiodes and avalanche photodiodes, the value of this characteristic is determined by the formula (3).
∆I
NEPpd =− , (3)
S ∙ G
where ΔI is the spectral density of the photodiode noise current, A / √—Hz, S is its spectral sensitivity at the operating wavelength, [A/W], G is the avalanche multiplication coefficient (usually from 1 to 100). The typical NEP value of high-speed photodiodes is about 10–15–10–13 W / √—Hz. Thus, the noise power in the 1 GHz band can be on the order of –75...–55 [dBm].
If the power of the probing pulse is 100 mW (20 dBm), then its ratio to the noise power will be from 95 dB to 75 dB, respectively, which allows, taking into account the requirements for measurement accuracy, to determine the lower limit of the measured return loss. Thus, it can be concluded that the value of the return loss of –80 dB with a spatial resolution of OTDR of 10 cm is quite feasible. However, at the same time, an optical radiation pulse with a power of 100 mW entering the photodiode of the PD should oversaturate the photodiode and is unacceptable, which explains the fact that the upper limit of measuring the return loss value in OTDR meters is usually below –10 dB.
In Russia, for metrological support of OTDR, there is a large fleet of working measuring standards for the average power of optical radiation, attenuation and propagation time of signals in the fiber, and the units are traced to the State Special Measuring Standard GET 170-2024.
OFDR method
Another reflectometric method for measuring distributed return loss is the method using optical frequency-domain reflectometry (OFDR). Similar to OTDR, OFDR is free from disadvantages of the CW method in the form of the influence of crosstalk of the splitter and the optical output connector. Besides, the OFDR method is free from some disadvantages of the OTDR method, such as the contradiction described above between spatial resolution and dynamic range when measuring distributed return loss, however, it has limitations on the length of the studied fiber-optic lines (for commercial samples of the devices under consideration, this value is limited to 2000 meters).
OFDR is an interferometric measurement method that uses a highly coherent laser optical radiation source with a continuously tunable wavelength. The interference pattern is analyzed using the Fourier transform, which makes it possible to obtain the dependence of the intensity of reflected and scattered optical radiation on the distance to the point of introduction of probing optical radiation [2]. The block diagram of the device implementing the OFDR method for measuring distributed return loss in fiber-optic systems is shown in Fig. 3.
The TLE laser source unit is a highly coherent tunable laser that provides spectral rearrangement of optical radiation according to a linear law of time. The recording device consists of a main interferometer with polarization diversity of the signal, an auxiliary interferometer, photodetectors PD1 – PD3 and a data processing and collection device. Finally, the fiber optical coil FOC is a part of an auxiliary interferometer designed to control the optical frequency of radiation during wavelength tuning of the laser.
Optical radiation from a tunable laser using a fiber optic splitter SP1 is divided into two parts, with 90% of the power going to the main interferometer, and 10% to the auxiliary one. In the main interferometer, half of the optical radiation power from the SP2 fiber-optic splitter goes to the fiber-optic circulator and then to the line under study, the differential and absolute propagation delays of optical radiation in which are subject to preliminary determination and measurement. Optical radiation backscattered or reflected from inhomogeneities in the studied line is combined with radiation from the second (reference) branch of the main interferometer on the fiber-optic splitter SP4 and enters the polarization divider. Here, the radiation is divided into two orthogonal polarization s- and p- components and is detected by two photodetectors (PD1, PD2), which are connected via an amplifier to an ADC/DSP data processing and collection device. The data acquisition device is a high-speed three-channel analog-to-digital converter and a computing device designed for mathematical processing of the collected data.
If we consider a simplified model of the measurement process, we can say that at the site of the photodetector of the measuring interferometer, the interference of the signal of the reference channel of the interferometer and the signals of back reflection or scattering (return loss) from each point of the line under test occurs. That is, each inhomogeneity in the fiber-optic line is considered as an individual reflector. With linear adjustment of the laser wavelength, the power of the total signal of the reference channel and the return loss signals from each inhomogeneity will vary according to a sinusoidal law. The frequency of the sine wave will depend only on the rate of adjustment of the laser wavelength and the difference in the lengths of the optical paths in the branches of the interferometer, and the amplitude will depend on the intensity of the signal of distributed return loss. Thus, the signal of the photodetectors PD1 and PD2 makes it possible to restore the dependence of the return loss signal power on the length of the fiber-optic line. The polarization diversity of the signal ensures the independence of the measured signal from changes in the polarization state of radiation caused by scattering or reflection from inhomogeneities in the studied fiber-optic line [3]. Signals polarization components of the radiation Es, Ep, coming to the photo-detectors of the main interferometer, are described by the relations (4) and (5).
Es(ω) =∑i 2rτi gs(τ)T˙s EиT˙s Eоп cos ω(t)τi + ϕτi , (4)
Ep(ω) =∑i 2rτi gp(τ)T˙p EиT˙p Eоп cos ω(t)τi + ϕτi , (5)
where rτi, ϕτi are the amplitude and phase of the complex reflectivity of the inhomogeneity inside the studied fiber-optic line; gs(τ), gp(τ) are the gain coefficients of the measuring channels; T˙s , T˙p are operators describing the polarization separation of the signal; Eи, Eоп are the amplitudes of the electric field vectors of the measuring and reference branches of the main interferometer, respectively; ω(t) is the instantaneous radiation frequency of the tunable laser; τi is the difference in time delays between the measuring and reference branches of the main interferometer.
Equations (4), (5) describe the relationship of return loss signals in the studied fiber-optic line and the observed interference signal in the frequency domain. If the signals Es(ω), Ep(ω) are converted (using the direct Fourier transform) into signals Es(τi), Ep(τi) in the time domain, then these inhomogeneities can be described as a function of the intensity of reflected and scattered optical radiation from the time delay of optical radiation relative to the point of its introduction [2]. As a result, the final signal E(τi) will take the form shown in the equation (6).
_____________
E(τi) = √ Es(τi)2 + Ep(τi)2 . (6)
The received signal E(τi) is nothing more than a function of the distribution of back-reflected (or scattered) optical power from the position of the corresponding inhomogeneities in the studied fiber-optic line. For precision measurements of distributed return loss using the method under consideration, the E(τi) value is calibrated using a reference – a fiber-optic element with a known reflection coefficient and, accordingly, return loss. As a rule, a gold-coated fiber-optic mirror is used as this fiber-optic element, the reflection coefficient of which is known with a high degree of accuracy, which makes it possible to measure distributed return loss by the OFDR method with an error of no more than 0.1 dB.
The dynamic measurement range is limited mainly by the intrinsic noise of the tunable laser used. The use of tunable lasers with a low relative noise level (RIN) of the order of –160 dB / Hz makes it possible to obtain a dynamic measurement range of return loss of the order of 80 dB. It should also be noted that the OFDR method has a well better spatial resolution compared to the OTDR method. The spatial resolution of the OFDR reflectometer ΔL, m, is determined by the laser wavelength and can be estimated using the ratio (7).
λ2
ΔL =− , (7)
2 ∙ n ∙ Δλ
where λ is the central wavelength of the tunable laser, [m] (Fig. 3); Δλ is the spectral tuning range of the tunable laser, [m]; n – the refractive index of the studied fiber-optic line.
Thus, in accordance with the ratio (7), the high spatial resolution of measurements of distributed return loss of the OFDR type does not contradict the high dynamic range of measurements of the return loss value, unlike OTDR meters. Thus, in the OBR 4600 meters (Luna, USA), the OFDR method makes it possible to measure distributed return loss with a spatial resolution of about 20 microns with a length of the studied line up to 70 meters and 1 mm with a corresponding length up to 2000 meters, which is the best value of the spatial resolution characteristic relative to other methods considered. The disadvantages of the described method include the fact that the wavelength range of the fiber-optic path is limited by the coherence length of the laser. Also, as the path length increases, the bandwidth of the interference signal increases at the same laser tuning speed. This requires the use of higher-performance ADCs operating at a sampling rate of hundreds of MHz, and significantly increases the requirements for computing equipment. Another disadvantage is the high cost of tunable lasers with a long coherence length. However, the successes of modern laser technology lead to a tendency for its decline.
The currently existing reference base in the Russian Federation does not provide metrological support for the length scales of OFDR reflectometers with the accuracy claimed by manufacturers, however, work is underway in this direction [4]. For full-fledged metrological support of such devices, the development of an appropriate working standard is required.
Comparison of the fields of application of the described methods
Tabl. 2 provides information on the technical and metrological characteristics of the measuring devices mentioned in the article that implement OTDR, OFDR and CW measurements of return loss in optical fiber. It should be noted that for most devices, the data are taken out of the specifications from the manufacturers, i. e. these devices have not been tested for type approval in the territory of the Russian Federation. However, these characteristics make it possible to confirm the conclusions about the limitations and advantages of the described measurement methods. The symbol “*” next to the name of the measuring device indicates that the device was entered into the state register of measuring instruments (as a type or as a single sample).
Conclusions
The development of fiber-optic communication systems increases the requirements for measuring equipment necessary for their construction and operation. The continuous improvement of the characteristics of lasers, photodetectors and information and measuring equipment, as well as the desire of manufacturers of measuring devices to meet the increasing needs of fiber-optic communication, lead both to the improvement and rethinking of existing measurement methods, and to the emergence of new measurement methods, as well as to their implementation in mass-produced measuring devices. The considered methods for measuring return losses in optical fiber were formed from the simpler CW method to the high-tech OFDR refletometry method, however, at present it is impossible to say about the complete predominance of any one of them, they have their own specifics and fields of application.
CW return loss meters occupy a niche of portable devices with limited functionality, but competitive cost, and reference meters with small measurement errors, but designed for rare use. They are used to measure integral return loss and have great limitations when measuring the return loss from a given section of the path. If the error of the CW-working measuring instruments is large enough and is usually about 0.5 dB, then the high accuracy of the reference meters is achieved by good polishing of the optical connectors and the use of special reference gauges. The return loss measurement range by CW meters is usually from 0 to –65…-70 dB. Portable CW return loss meters are often included in optical testers designed to measure insertion losses and average optical radiation power in fiber and are used in the installation and maintenance of fiber-optic communication lines.
OTDR return loss meters are, in fact, an adaptation of OTDR for operation in the near-field and in a wider range of backscattering signals. Such devices allow measuring the value of the return loss from a given section of the path, and are used, for example, in the production of optical components and patch cords. Error rate of OTDR meters, on average, are slightly higher than the error of CW meters, but it is provided in a wider measurement range (up to –80 dB). The spatial resolution ranges from ones to tens centimeters.
OFDRs are more often rather large multifunctional measuring systems (optical vector analyzers) that, in addition to return loss, can measure insertion loss, dispersion characteristics, polarization extinction and polarization-dependent loss in fiber-optic lines. But the main feature of these devices is the spatial resolution comparable to the wavelength of optical radiation (on the order of tens of microns) at distances of the order of hundreds of meters. The OFDR return loss measurement range is from 0 to –80 dB, and the measurement error is from 0.1 to 0.2 dB. The described characteristics make OFDR an ideal solution for short-range measurements, including for integrated photonics applications. The resolution of the submillimeter range specific to OFDR is fundamentally unattainable for OTDR meters, however, currently OTDRs can have a compact implementation, lower cost and allow measurements at a set of different discrete wavelengths, whereas OFDRs operate in a not very wide spectral range of a tunable laser.
REFERENCES
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AUTHORS
Bychkov S. B., junior research scientist, Federal State Budgetary Institution “All-Russian Scientific Research Institute of Optical and Physical Measurements” (FGBU “VNIIOFI”), Moscow, Russia.
ORCID: 0009-0000-5118-6368
Pogonyshev A. O., junior research scientist, VNIIOFI, Moscow, Russia.
Web of Science Researcher ID: HJB‑3251-2022
Scopus Author ID: 57204943217
Tikhomirov S. V., Dr. of Science (Eng.), postgraduate teacher, VNIIOFI, Moscow, Russia.
Sumkin V. R., general director “Research and Production Enterprise “Measuring Communications Equipment” (“NPP”ITS”), Saint-Petersburg, Russia.
CONFLICT OF INTEREST
The authors state that they have no conflict of interest. All the authors took part in writing the article and supplemented the manuscript in part of their work.
CONTRIBUTION OF THE MEMBERS
OF THE AUTHOR’S TEAM
The article is based on the work of all members of the author’s team.
S. B. Bychkov1, А. О. Pogonyshev1, S. V. Tikhomirov1, V. R. Sumkin2
Federal State Budgetary Institution “All-Russian Scientific Research Institute of Optical and Physical Measurements” (FGBU “VNIIOFI”), Moscow
LLC “Research and Development Enterprise “Measuring Communication Technology” (“Izmeritelnaya Tekhnika Svyazi”) (NPP “ITS”), Saint-Petersburg
This article discusses 3 methods for measuring of optic return loss (ORL) in fiber optic systems being used in modern measuring instruments: continuous wave method (CW), time domain reflectometry (OTDR) and frequency domain reflectometry (OFDR). The authors perform a comparative analysis of these methods, and consider the advantages and limitations of these methods.
Key words: measurements, optical return loss, ORL, reflectometry, OTDR, OFDR, metrological support
Article received: 08.02.2024
Article accepted: 26.07.2024
Introduction
One of the important characteristics of fiber-optic lines and fiber-optic devices is the level of return loss (ORL, reflection loss) they create – a value equal to the ratio of the power of the backscattering signal created in the device to the power of the optical signal at its input, expressed in decibels (GOST R 54417-2011). Backscattering signals can cause instability and noise in laser emitters used in fiber-optic communication systems, which can lead to failures in telecommunications equipment or even failure of laser emitters. In this regard, the levels of return loss created by the elements and devices of the fiber optic line are usually standardized, and return loss meters are widely used in measuring equipment of fiber-optic communication.
The article discusses the methods of measuring return loss used in such measuring devices, and their comparative analysis is carried out. Currently, the following 3 methods of measuring the return loss value in optical fiber are most common:
- CW-method based on measuring the average power of backscatter from the fiber-optic line under study with a steady probing optical signal;
- The method of optical time-domain reflectometry (OTDR) based on measuring the change in the power of the backscatter signal with a pulse-modulated probing signal;
- The method of optical frequency-domain reflectometry (OFDR) based on the analysis of an interference pattern linearly tunable along the wavelength of a laser used as a source of a probing signal and a backscatter signal.
Let’s look at these methods in more detail below.
CW measuring method
The CW method is the most obvious way to measure the value of return loss in fiber-optic lines. A diagram explaining the principle of operation of the CW meter is shown in Fig. 1.
The laser (LE) generates continuous-wave optical radiation having a steady, known power of RL, [dBm]. Through branch 1 of the SP splitter, this radiation enters its output 3. The fiber optic line under test or the fiber device under test is connected to it. In this case, the SP introduces losses of А13, [dB], into the signal, and the power of RL-dut < Rl enters branch 3. When radiation passes through the fiber-optic line, part of it is reflected back due to scattering on inhomogeneities of the transmission medium (Rayleigh scattering) and on mirror surfaces (Fresnel reflection). A backscattering signal is received at the input 3 of the SP and, suffering losses of A32, [dB], it enters the input of an optical wattmeter OPM which allows measuring the average power of Pof of this signal. Thus, knowing the average power of optical radiation at Pl laser yield, losses A13 and A32, according to the readings of the OPM Pof, [dBm], it is possible to calculate the value of the integral return losses RL, [dB], in the tract according to the formula (1).
RL = Pof − PL + A13 + A32 + 2 · α, [dB], (1)
where α, [dB] is the loss value at the optical connector connecting the studied fiber-optic line and the output branch of the SP (it is considered steady and is approximately 0.15 dB).
The losses measured in this way are due to all optical components included in the fiber-optic line. In order to determine the return loss from a given section of the line, for example, from the junction of optical connectors, the so-called mandrel is used. A mandrel is a rod of a given diameter on which an optical fiber is wound. The diameter of the rod is such that the condition of the total internal reflection is violated in the optical fiber wound on it, and radiation exits the fiber without creating return loss, and at 5–7 turns of winding, the return loss from the section of the line following the mandrel decreases by more than 70 dB. The diameter of the mandrel rod is determined by the operating wavelength and the type of optical fiber. For example, for Class G.652 optical fiber it is 8 mm when measuring at a wavelength of 1310 nm and 10 mm when measuring at a wavelength of 1550 nm. Thus, for example, in the scheme shown in Fig. 1, to measure the value of the return loss from the optical connector C connecting the two sections of the studied fiber-optic line, it is necessary to remove the return loss from the fiber located after the connector C using the mandrel M, then measure the value of the integral return loss RLsumm, [dB], from the C connector and the optical fiber before it. After that, measure the value of RLbefore, [dB] of the return loss from the section of the path before C, winding the fiber on the mandrel between the meter and the connector C as close as possible to the connector. The required value of the return loss of RLP can be calculated using the formula (2).
RLp = 10 log 10 10RLsumm / 10 − 10RLbefore / 10, [дБ]. (2)
The described CW method for measuring return loss in optical fiber is quite simple to implement. The unit of return loss measured in this way in the Russian metrological support system is traced to the State special measurement standard of units of length and time of signal propagation in a fiber optic, average power, attenuation and wavelength for fiber-optic information transmission systems (GET 170-2024) through units of average power and attenuation of optical radiation. The dynamic range of measurement of return loss by the CW method is determined by:
The measuring range of the average power of the OPM optical wattmeter,
Signal loss in the SP splitter,
The “zero” level of the return loss signal resulting from the reflection of LE radiation from the output connector 3 (see Fig.) of the meter and the SP splitter, as well as due to the presence of crosstalk between lines 1 and 2 of the SP splitter (crosstalk, isolation).
In practice, it is the latter factor that turns out to be the limiting factor. If the dynamic range of modern optical wattmeters usually exceeds 80 dB, the losses A13 and A32 in the SP splitter are about 3 dB, then the level of mutual isolation ports are highly dependent on the design. This characteristic is not standardized for all splitters available on the market, usually the manufacturer indicates exactly the level of return loss. The question of the applicability of splitters of different types to the task of measuring ORL was considered during the joint work of FGBU “VNIIOFI” and NPP “ITS”. Tabl. 1 shows the minimum zero level values obtained in a CW return loss meter using three different types of meters: Y-type fused splitters (having 1 input and 2 outputs – FBT 1x2), Y-type planar splitters (PLC 1x2) and X-type fused splitters (having 2 inputs and 2 outputs – FBT 2x2). All splitters had a count-down ratio of 50% – 50%. The unused port of the X-splitter was wound on the mandrel, which ensured the actual absence of back reflections from the end of the optical fiber. Measurements were carried out at a wavelength of 1550 nm.
The high level of “zero” signal obtained when using a fused FBT 1x2 splitter can be explained by the fact that splitters of this type are performed by fusing two optical fibers, then one of the ports is split off to obtain a Y-configuration. In this case, the area of splitting can create large return loss which determines the low coefficient of inter-channel isolation. The technology of planar splitters allows for greater channel isolation due to higher accuracy and repeatability of production processes. The best result was provided by the X-splitter, the “extra” port of which was jammed. This made it possible to expand the measurement range of return loss to the level of –70 dB and it can be argued that by choosing a better splitter, it is possible to expand the range even more. Nevertheless, this indicator can be considered close to the limit for CW meters. During use, the output connector of the meter is “rubbed out”, as a result of which the Fresnel reflection from it increases, which eventually leads to an increase in the zero signal to a level exceeding –70 dB. Another significant disadvantage of the CW method is the need to use a mandrel to measure losses from a given section of the path, which spoils the optical fiber; moreover, this is not always possible.
Nevertheless, the dynamic range of 70 dB is quite sufficient for a number of applications, and the price of CW return loss meters is very competitive. In addition, the value of the return loss measured by the described method is obviously traced to the attenuation unit for the optical fiber. Therefore, this method is traditionally used in reference technology, in particular, in working measuring standards of return loss units in optical fiber (REOP, registration number in the Measuring Equipment Register: 52363-13, 35981-07), manufactured and supplied by FGBU “VNIIOFI” to metrological centers in Russia.
Reflectometric methods
Recently, devices designed to measure distributed return loss – loss from a selected section of the route, a specific connector or component – have become increasingly common. Such devices have proved to be in demand, for example, in the production of optical patch cords, cables and assemblies – to test the quality of connector polishing. This field of application does not allow spoiling the fiber under test by winding it over a small radius. Devices implementing time-domain and phase-domain reflectometric methods (OTDR and OFDR) are used here. Devices using the OTDR-reflectometric method, for example, include the MAP‑200 (VIAVI, France) and Op‑940 (OptoTest, USA). OFDR-reflectometric method is used, for example, in OVA5000 (Luna, USA). These systems can measure both integral return loss and loss from a given section of the fiber optic cable and have wider dynamic ranges than CW-type meters.
OTDR-метод
Fig. 2 shows a generalized block diagram of an OTDR reflectometer. A pulsed laser emitter LE generates probing optical pulses coming through a fiber-optic SP splitter into the fiber-optic line under study. Propagating in it, the pulses are subjected to backscattering, recorded by a high-speed photodetector (PD), the signal from which is amplified using a broadband amplifier (SA) and fed to a high-speed analog-to-digital converter (ADC) [1]. The digital signal processing (DSP) module controls the start-up LE and the processing of ADC signals. The ADC counts are synchronized in time with the LE start-up moments, for each ADC count produced after the time n Δt, [s] (where Δt is the time between ADC samples, n is an integer) after the radiation of an optical pulse, LE corresponds to the backscattering signal of the probing pulse from a specific section of the fiber-optic path. Thus, a reflectogram is formed – the dependence of the relative power of the backscattering signal on the distance between the meter and the scattering site. During the measurement, the radiator (LE) generates a sequence of optical probing pulses, the period of which is adjusted so that the back reflection signal from the most distant point in the fiber-optic path has time to get to the photodetector (PD) before the next probing pulse is formed. For each point of the reflectogram, a set of averaging of the measured value of the backscattering signal is performed, which reduces the influence of noise from the photodetector channel.
The described principle of operation is the same for a large number of OTDRs used to measure the length of the fiber optic line and troubleshooting on the lines, however, if the reflectometers are optimized for long-range operation (from 1 to 100 km), then OTDR reverse loss meters are optimized for operation in the near zone. They are characterized by higher spatial resolution, higher accuracy and a wider range of backscattering signal levels, but a smaller range of operating lengths of optical lines.
The OTDR principle makes it possible to neutralize the factors limiting the dynamic range of CW return loss meters – the influence of crosstalk of the splitter and the output optical connector. For the OTDR system, these interferences may not be taken into account due to the choice of the minimum interval τ of the time delay of the ADC start relative to the moment of generation of the probing pulse. This allows you to expand the dynamic range of the distributed return loss meter to values corresponding to Rayleigh scattering from the optical fiber section (usually from –70 dB to –80 dB per 1 m). However, the requirement to increase the spatial resolution of the OTDR conflicts with the requirement to increase the dynamic range of the meter. Since an increase in spatial resolution is achieved by reducing the duration of the probing optical pulses, the power of the backscattering signal also decreases. The power of the emitters used to generate probing pulses is limited from above both by the technical capabilities of the emitters themselves and by nonlinear effects in the optical fiber (and usually does not exceed several hundred mW). All this complicates the process of measuring the power of the reverse loss signal and forces OTDR manufacturers to use avalanche photodetectors with worse stability characteristics at higher sensitivity characteristics. Another aspect that prevents obtaining high spatial resolution is the speed of the ADC. For example, in order to obtain a resolution of 10 cm, the amplitude of the backscattering signal must be measured every Δt ≈ 1 ns, which requires a conversion frequency of more than 1 GHz and the corresponding speed from the PD photodetector and the SA amplifier.
The issue of determining the dynamic range of OTDR is discussed in detail in [1] unlike classical reflectometers, OTDR return loss meters are designed to operate both at low, maximum detectable values of the return loss signal power and at higher values (for example, for Op‑940 manufactured by Optotest, USA – from –10 dB). The lower limit of the sensitivity of the OTDR meter can be calculated from the equivalent noise power density of the photodetector signal, NEP W / √—Hz. For photodetectors based on p-i-n photodiodes and avalanche photodiodes, the value of this characteristic is determined by the formula (3).
∆I
NEPpd =− , (3)
S ∙ G
where ΔI is the spectral density of the photodiode noise current, A / √—Hz, S is its spectral sensitivity at the operating wavelength, [A/W], G is the avalanche multiplication coefficient (usually from 1 to 100). The typical NEP value of high-speed photodiodes is about 10–15–10–13 W / √—Hz. Thus, the noise power in the 1 GHz band can be on the order of –75...–55 [dBm].
If the power of the probing pulse is 100 mW (20 dBm), then its ratio to the noise power will be from 95 dB to 75 dB, respectively, which allows, taking into account the requirements for measurement accuracy, to determine the lower limit of the measured return loss. Thus, it can be concluded that the value of the return loss of –80 dB with a spatial resolution of OTDR of 10 cm is quite feasible. However, at the same time, an optical radiation pulse with a power of 100 mW entering the photodiode of the PD should oversaturate the photodiode and is unacceptable, which explains the fact that the upper limit of measuring the return loss value in OTDR meters is usually below –10 dB.
In Russia, for metrological support of OTDR, there is a large fleet of working measuring standards for the average power of optical radiation, attenuation and propagation time of signals in the fiber, and the units are traced to the State Special Measuring Standard GET 170-2024.
OFDR method
Another reflectometric method for measuring distributed return loss is the method using optical frequency-domain reflectometry (OFDR). Similar to OTDR, OFDR is free from disadvantages of the CW method in the form of the influence of crosstalk of the splitter and the optical output connector. Besides, the OFDR method is free from some disadvantages of the OTDR method, such as the contradiction described above between spatial resolution and dynamic range when measuring distributed return loss, however, it has limitations on the length of the studied fiber-optic lines (for commercial samples of the devices under consideration, this value is limited to 2000 meters).
OFDR is an interferometric measurement method that uses a highly coherent laser optical radiation source with a continuously tunable wavelength. The interference pattern is analyzed using the Fourier transform, which makes it possible to obtain the dependence of the intensity of reflected and scattered optical radiation on the distance to the point of introduction of probing optical radiation [2]. The block diagram of the device implementing the OFDR method for measuring distributed return loss in fiber-optic systems is shown in Fig. 3.
The TLE laser source unit is a highly coherent tunable laser that provides spectral rearrangement of optical radiation according to a linear law of time. The recording device consists of a main interferometer with polarization diversity of the signal, an auxiliary interferometer, photodetectors PD1 – PD3 and a data processing and collection device. Finally, the fiber optical coil FOC is a part of an auxiliary interferometer designed to control the optical frequency of radiation during wavelength tuning of the laser.
Optical radiation from a tunable laser using a fiber optic splitter SP1 is divided into two parts, with 90% of the power going to the main interferometer, and 10% to the auxiliary one. In the main interferometer, half of the optical radiation power from the SP2 fiber-optic splitter goes to the fiber-optic circulator and then to the line under study, the differential and absolute propagation delays of optical radiation in which are subject to preliminary determination and measurement. Optical radiation backscattered or reflected from inhomogeneities in the studied line is combined with radiation from the second (reference) branch of the main interferometer on the fiber-optic splitter SP4 and enters the polarization divider. Here, the radiation is divided into two orthogonal polarization s- and p- components and is detected by two photodetectors (PD1, PD2), which are connected via an amplifier to an ADC/DSP data processing and collection device. The data acquisition device is a high-speed three-channel analog-to-digital converter and a computing device designed for mathematical processing of the collected data.
If we consider a simplified model of the measurement process, we can say that at the site of the photodetector of the measuring interferometer, the interference of the signal of the reference channel of the interferometer and the signals of back reflection or scattering (return loss) from each point of the line under test occurs. That is, each inhomogeneity in the fiber-optic line is considered as an individual reflector. With linear adjustment of the laser wavelength, the power of the total signal of the reference channel and the return loss signals from each inhomogeneity will vary according to a sinusoidal law. The frequency of the sine wave will depend only on the rate of adjustment of the laser wavelength and the difference in the lengths of the optical paths in the branches of the interferometer, and the amplitude will depend on the intensity of the signal of distributed return loss. Thus, the signal of the photodetectors PD1 and PD2 makes it possible to restore the dependence of the return loss signal power on the length of the fiber-optic line. The polarization diversity of the signal ensures the independence of the measured signal from changes in the polarization state of radiation caused by scattering or reflection from inhomogeneities in the studied fiber-optic line [3]. Signals polarization components of the radiation Es, Ep, coming to the photo-detectors of the main interferometer, are described by the relations (4) and (5).
Es(ω) =∑i 2rτi gs(τ)T˙s EиT˙s Eоп cos ω(t)τi + ϕτi , (4)
Ep(ω) =∑i 2rτi gp(τ)T˙p EиT˙p Eоп cos ω(t)τi + ϕτi , (5)
where rτi, ϕτi are the amplitude and phase of the complex reflectivity of the inhomogeneity inside the studied fiber-optic line; gs(τ), gp(τ) are the gain coefficients of the measuring channels; T˙s , T˙p are operators describing the polarization separation of the signal; Eи, Eоп are the amplitudes of the electric field vectors of the measuring and reference branches of the main interferometer, respectively; ω(t) is the instantaneous radiation frequency of the tunable laser; τi is the difference in time delays between the measuring and reference branches of the main interferometer.
Equations (4), (5) describe the relationship of return loss signals in the studied fiber-optic line and the observed interference signal in the frequency domain. If the signals Es(ω), Ep(ω) are converted (using the direct Fourier transform) into signals Es(τi), Ep(τi) in the time domain, then these inhomogeneities can be described as a function of the intensity of reflected and scattered optical radiation from the time delay of optical radiation relative to the point of its introduction [2]. As a result, the final signal E(τi) will take the form shown in the equation (6).
_____________
E(τi) = √ Es(τi)2 + Ep(τi)2 . (6)
The received signal E(τi) is nothing more than a function of the distribution of back-reflected (or scattered) optical power from the position of the corresponding inhomogeneities in the studied fiber-optic line. For precision measurements of distributed return loss using the method under consideration, the E(τi) value is calibrated using a reference – a fiber-optic element with a known reflection coefficient and, accordingly, return loss. As a rule, a gold-coated fiber-optic mirror is used as this fiber-optic element, the reflection coefficient of which is known with a high degree of accuracy, which makes it possible to measure distributed return loss by the OFDR method with an error of no more than 0.1 dB.
The dynamic measurement range is limited mainly by the intrinsic noise of the tunable laser used. The use of tunable lasers with a low relative noise level (RIN) of the order of –160 dB / Hz makes it possible to obtain a dynamic measurement range of return loss of the order of 80 dB. It should also be noted that the OFDR method has a well better spatial resolution compared to the OTDR method. The spatial resolution of the OFDR reflectometer ΔL, m, is determined by the laser wavelength and can be estimated using the ratio (7).
λ2
ΔL =− , (7)
2 ∙ n ∙ Δλ
where λ is the central wavelength of the tunable laser, [m] (Fig. 3); Δλ is the spectral tuning range of the tunable laser, [m]; n – the refractive index of the studied fiber-optic line.
Thus, in accordance with the ratio (7), the high spatial resolution of measurements of distributed return loss of the OFDR type does not contradict the high dynamic range of measurements of the return loss value, unlike OTDR meters. Thus, in the OBR 4600 meters (Luna, USA), the OFDR method makes it possible to measure distributed return loss with a spatial resolution of about 20 microns with a length of the studied line up to 70 meters and 1 mm with a corresponding length up to 2000 meters, which is the best value of the spatial resolution characteristic relative to other methods considered. The disadvantages of the described method include the fact that the wavelength range of the fiber-optic path is limited by the coherence length of the laser. Also, as the path length increases, the bandwidth of the interference signal increases at the same laser tuning speed. This requires the use of higher-performance ADCs operating at a sampling rate of hundreds of MHz, and significantly increases the requirements for computing equipment. Another disadvantage is the high cost of tunable lasers with a long coherence length. However, the successes of modern laser technology lead to a tendency for its decline.
The currently existing reference base in the Russian Federation does not provide metrological support for the length scales of OFDR reflectometers with the accuracy claimed by manufacturers, however, work is underway in this direction [4]. For full-fledged metrological support of such devices, the development of an appropriate working standard is required.
Comparison of the fields of application of the described methods
Tabl. 2 provides information on the technical and metrological characteristics of the measuring devices mentioned in the article that implement OTDR, OFDR and CW measurements of return loss in optical fiber. It should be noted that for most devices, the data are taken out of the specifications from the manufacturers, i. e. these devices have not been tested for type approval in the territory of the Russian Federation. However, these characteristics make it possible to confirm the conclusions about the limitations and advantages of the described measurement methods. The symbol “*” next to the name of the measuring device indicates that the device was entered into the state register of measuring instruments (as a type or as a single sample).
Conclusions
The development of fiber-optic communication systems increases the requirements for measuring equipment necessary for their construction and operation. The continuous improvement of the characteristics of lasers, photodetectors and information and measuring equipment, as well as the desire of manufacturers of measuring devices to meet the increasing needs of fiber-optic communication, lead both to the improvement and rethinking of existing measurement methods, and to the emergence of new measurement methods, as well as to their implementation in mass-produced measuring devices. The considered methods for measuring return losses in optical fiber were formed from the simpler CW method to the high-tech OFDR refletometry method, however, at present it is impossible to say about the complete predominance of any one of them, they have their own specifics and fields of application.
CW return loss meters occupy a niche of portable devices with limited functionality, but competitive cost, and reference meters with small measurement errors, but designed for rare use. They are used to measure integral return loss and have great limitations when measuring the return loss from a given section of the path. If the error of the CW-working measuring instruments is large enough and is usually about 0.5 dB, then the high accuracy of the reference meters is achieved by good polishing of the optical connectors and the use of special reference gauges. The return loss measurement range by CW meters is usually from 0 to –65…-70 dB. Portable CW return loss meters are often included in optical testers designed to measure insertion losses and average optical radiation power in fiber and are used in the installation and maintenance of fiber-optic communication lines.
OTDR return loss meters are, in fact, an adaptation of OTDR for operation in the near-field and in a wider range of backscattering signals. Such devices allow measuring the value of the return loss from a given section of the path, and are used, for example, in the production of optical components and patch cords. Error rate of OTDR meters, on average, are slightly higher than the error of CW meters, but it is provided in a wider measurement range (up to –80 dB). The spatial resolution ranges from ones to tens centimeters.
OFDRs are more often rather large multifunctional measuring systems (optical vector analyzers) that, in addition to return loss, can measure insertion loss, dispersion characteristics, polarization extinction and polarization-dependent loss in fiber-optic lines. But the main feature of these devices is the spatial resolution comparable to the wavelength of optical radiation (on the order of tens of microns) at distances of the order of hundreds of meters. The OFDR return loss measurement range is from 0 to –80 dB, and the measurement error is from 0.1 to 0.2 dB. The described characteristics make OFDR an ideal solution for short-range measurements, including for integrated photonics applications. The resolution of the submillimeter range specific to OFDR is fundamentally unattainable for OTDR meters, however, currently OTDRs can have a compact implementation, lower cost and allow measurements at a set of different discrete wavelengths, whereas OFDRs operate in a not very wide spectral range of a tunable laser.
REFERENCES
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AUTHORS
Bychkov S. B., junior research scientist, Federal State Budgetary Institution “All-Russian Scientific Research Institute of Optical and Physical Measurements” (FGBU “VNIIOFI”), Moscow, Russia.
ORCID: 0009-0000-5118-6368
Pogonyshev A. O., junior research scientist, VNIIOFI, Moscow, Russia.
Web of Science Researcher ID: HJB‑3251-2022
Scopus Author ID: 57204943217
Tikhomirov S. V., Dr. of Science (Eng.), postgraduate teacher, VNIIOFI, Moscow, Russia.
Sumkin V. R., general director “Research and Production Enterprise “Measuring Communications Equipment” (“NPP”ITS”), Saint-Petersburg, Russia.
CONFLICT OF INTEREST
The authors state that they have no conflict of interest. All the authors took part in writing the article and supplemented the manuscript in part of their work.
CONTRIBUTION OF THE MEMBERS
OF THE AUTHOR’S TEAM
The article is based on the work of all members of the author’s team.
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