26 Figure 6 – Distance calibration with a recirculating delay line ....30 Figure 7 – OTDR trace produced by recirculating delay line ....30 Figure 8 – Determining the reference level and
Organization
The calibration laboratory should satisfy requirements of ISO/IEC 17025
There should be a documented measurement procedure for each type of calibration performed, giving step-by-step operating instructions and equipment to be used.
Traceability
The requirements of ISO/IEC 17025 should be met
All calibration standards must be calibrated following a documented program that ensures traceability to national standards laboratories or accredited calibration laboratories It is recommended to have multiple standards at each hierarchical level to verify their performance through comparisons Additionally, any test equipment that significantly impacts calibration results should also be calibrated, with its traceability chain specified upon request The re-calibration periods must be clearly defined and documented.
Preparation
All tests should be conducted at an ambient room temperature of 23 °C ± 3 °C, unless specified otherwise Ensure that the test equipment is allowed a minimum of 2 hours to acclimate to its environment before testing Additionally, the OTDR must undergo a warm-up period as per the manufacturer's guidelines.
Test conditions
The test conditions usually include the following OTDR external conditions: date, temperature, connector-adapter combination and use of a lead-in fibre
Calibrate the OTDR according to the manufacturer's specifications and procedures Whenever possible, select a variety of test conditions and parameters that replicate the actual field operating conditions This selection should aim to enhance the OTDR's accuracy and resolution, utilizing features such as view windows and zoom options as outlined in the manufacturer's guidelines.
The test conditions usually include the following OTDR parameters: averaging time, pulse width, sample spacing, centroidal wavelength Unless otherwise specified, set the OTDR group index to exactly 1,46
NOTE 1 The calibration results only apply to the set of test conditions used in the calibration process
NOTE 2 Because of the potential for hazardous radiation, be sure to establish and maintain conditions of laser safety Refer to IEC 60825-1 and IEC 60825-2.
Documentation
Calibration certificates must include essential data and their associated uncertainties, such as the location offset \$\Delta L_0\$ and its uncertainty \$\pm 2 u_{\Delta L0}\$, along with the distance scale deviation \$\Delta S_L\$ and its uncertainty \$\pm 2 u_{\Delta SL}\$, or the location deviations \$\Delta L_i\$ and their uncertainties \$\pm 2 u_{\Delta Li}\$ Additionally, they should specify the loss deviations \$\Delta A\$ and their uncertainties \$\pm 2 u_{\Delta A}\$, or the loss scale deviation \$\Delta S_A\$ and its uncertainties \$\pm 2 u_{\Delta SA}\$ The instrument configuration, including pulse width, measurement span, wavelength, and averaging time used during calibration, must also be documented Furthermore, all other requirements for calibration certificates as outlined in ISO/IEC 17025 should be adhered to.
General
The objective of distance calibration is to determine deviations (errors) between the measured and actual distances between points on a fibre, and to characterize the uncertainties of these deviations
An Optical Time-Domain Reflectometer (OTDR) determines the location \( L \) of a feature by measuring the round-trip transit time \( T \) of a light pulse traveling to the feature and back The distance \( L \) is calculated using the speed of light in a vacuum, \( c \) (approximately \( 2.998 \times 10^8 \) m/s), and the group index \( N \) of the fiber.
Errors in measuring the length \( L \) can arise from scale inaccuracies, timebase offsets in the OTDR, and difficulties in locating features relative to the timebase Marker placement for measurement can be performed either manually or automatically by the instrument The resulting error typically depends on the method of marker placement and the nature of the feature being measured, such as point losses, large reflections that saturate the receiver, or small reflections that do not.
Larger errors in measuring the length L can arise from uncertainties in the fiber's group index N, which is not covered by this standard Therefore, the calibration procedures outlined focus solely on the OTDR's capability to accurately measure T For the purposes of this standard, a default group index value of N = 1.46 is adopted, with the uncertainty of N assumed to be zero.
Location deviation model
To characterize location deviations in Optical Time Domain Reflectometers (OTDRs), we will assume a model that reflects their typical behavior The reference location of a feature, denoted as \( L_{\text{ref}} \), is measured from the front panel connector of the OTDR, while the displayed location is represented as \( L_{\text{otdr}} \) It is assumed that the displayed location \( L_{\text{otdr}} \), which utilizes OTDR averaging to reduce noise, functionally depends on the reference location \( L_{\text{ref}} \).
The scale factor \( S_L \) should ideally be 1, while the location offset \( \Delta L_0 \) should be 0, and the distance sampling error \( f(L_{\text{ref}}) \) should also be 0 This distance sampling error is a periodic function with a mean of zero, characterized by a period that corresponds to the distance interval between sampled points on the OTDR For instance, when measuring the location of a significant reflection by marking the first digitized point that indicates an increase in signal, and then incrementing the position of the reflection in fine steps, the function \( f(L_{\text{ref}}) \) may resemble a periodic ramp waveform.
Equation (15) addresses the known errors in location measurements; however, an additional type A uncertainty may still be present This uncertainty impacts both the distance measurements and the precision of the parameters that describe the errors, as outlined in the following procedures.
To determine the values of \( S_L \) and \( \Delta L_0 \), one can measure \( L_{otdr} \) for various \( L_{ref} \) values and then apply the least squares method to fit a straight line to the data In this context, \( S_L \) represents the slope, while \( \Delta L_0 \) denotes the intercept of the fitted line.
Equivalently, a line may be fitted to the location deviation function, that is the difference between L otdr and L ref :
L = =Δ × + Δ + Δ (16) where Δ S L is the slope; and Δ L 0 is still the intercept, as illustrated in Figure 2
The distance sampling error, denoted as f(L ref), can be determined by measuring deviations from the linear approximation for various values of L ref The amplitude of the distance sampling error, Δ L sample, is defined as half the amplitude of f(L ref).
In this standard, the amplitude of the distance sampling error, denoted as Δ L sample, is considered a component of the location readout uncertainty type A Consequently, the reported uncertainty result does not account for the repetitive nature of the sampling error, failing to differentiate between the contributions of the sampling error and uncertainty type A.
Figure 2 – Representation of the location deviation Δ L ( L )
The distance calibration results are defined by several key parameters: the distance scale deviation (\$ \Delta S_L \$) and its uncertainty (\$ u \Delta S_L \$); the location offset (\$ \Delta L_0 \$) and its uncertainty (\$ u \Delta L_0 \$); and the location readout uncertainty (\$ u L_{readout} \$), which encompasses the combined uncertainty from distance sampling errors and type A measurement sample uncertainties, expressed as a standard deviation.
To determine the uncertainty in the readout (\$u_{Lreadout}\$), divide the maximum deviations from the least-squares approximation by the square root of 3 It is important to note that this uncertainty is influenced by factors such as distance, the power level displayed, and the settings of the instrument.
NOTE Δ L sample represents the physical sampling error of the instrument This error is accessible for the user as u Lreadout that includes distance calculation and displaying errors.
Using the calibration results
The error in the location of a feature Δ L = L otdr – L ref can be calculated from the calibration results: ref L
L =Δ + Δ Δ (17) with the uncertainty in ΔL given by the following formula, in which the recommended confidence level of 95 % is used:
D where the displayed location L otdr can be used instead of the reference location L ref without serious consequences
Similarly, the error in the distance between two features ΔD and its uncertainty can be calculated from the following formula:
D = Δ L Δ (18) with uncertainty in ΔD given by the following formula:
2 u =± D u + u ± (18a) where the displayed distance D otdr can be used instead of the reference distance D ref
NOTE The 2 in front of u Lreadout 2 is due to combining two uncorrelated uncertainties
When calibrating, it's essential to consider additional uncertainties if the feature type differs from that used in the calibration process Clearly specify the feature type in the calibration results to ensure accuracy and reliability.
Measuring fibre length
One effective method for OTDR distance calibration involves measuring fibers of known length This standard emphasizes determining fiber length through transit time rather than mechanical measurements, aligning with the OTDR's measurement principles Transit time measurements typically offer greater accuracy, especially for longer fibers Consequently, the standard recommends using fiber transit time over mechanical length when precision is crucial.
To measure the transit time of the fiber, T transit, utilize a pulse generator, a triggerable laser source, an optical-to-electrical converter (O/E converter), and a time interval counter It is crucial that the laser source's centroidal wavelength, λ avg, closely matches that of the test OTDR to avoid discrepancies in transit time caused by chromatic dispersion Alternatively, the OTDR can generate optical pulses, ensuring the centroidal wavelengths align Record the transit time by calculating the difference in arrival times with and without the fiber positioned between the laser source and the O/E converter.
When this fibre is used for OTDR distance calibrations, then the reference distance D ref can be calculated by
In this equation, use a group index N which is identical with the OTDR's group index setting
The time measurement principle makes it possible to use D ref as the reference distance
General
Each of the three alternative calibration methods described below is capable of determining all of the necessary calibration results: location offset, distance scale deviation, and their un- certainties.
External source method
Short description and advantage
The external source method uses a calibrated time-delay generator to simulate the time delay in a fibre and an optical source to simulate the reflected or scattered signal from a fibre
This method is ideal for automated laboratory testing managed by computer control, focusing solely on reflective features for simplicity To calibrate the Optical Time Domain Reflectometer (OTDR) for non-reflective features, the pulsed electro-optical (E/O) converter should be substituted with an optical source that accurately simulates the desired feature.
Equipment
The measurement equipment, as illustrated in Figure 3, comprises several key components: an optical coupler, an optical-to-electrical converter, a digital delay generator with pulse capability, an electrical-to-optical converter, and a variable optical attenuator designed to reduce the pulse amplitude just below the clipping level.
Figure 3 – Equipment for calibration of the distance scale –
The coupler directs the OTDR signal to the optical/electrical (O/E) converter, which activates the delay generator After a predetermined time delay, the generator produces an optical pulse that is subsequently routed back to the OTDR.
The E/O converter functions as a pulsed laser that mimics reflection, utilizing constant pulse amplitude and width to effectively calibrate the distance scale for reflective features Additionally, the attenuator allows for the adjustment of pulse amplitude according to the distance of the reflection from the OTDR's front panel, simulating the variations in reflection amplitude due to fiber attenuation.
To allow accurate calibration of the set-up, fibres F1 and F5 should have the same length (see below) F5 is terminated to absorb reflections.
Calibration of the equipment
Before utilizing the "external source" equipment, it is essential to ensure proper calibration Regular calibration of the digital delay generator is assumed To compute the location offset \$\Delta L_0\$ from the measured data, it is also necessary to determine the insertion delay \$T_{\text{delay}}\$ of the apparatus This can be achieved by incorporating a pulse generator and a calibrated time interval counter into the setup, as illustrated in Figure 4.
Figure 4 – Set-up for calibrating the system insertion delay
To calibrate the insertion delay T delay, proceed as follows
To measure delay time accurately, configure the pulse generator to produce a square wave with a repetition period exceeding twice the delay time Utilize the output pulse from the pulse generator as the start pulse for the time interval counter and to externally trigger the delay generator Ensure the digital delay generator is set for external triggering with zero delay aligned to the leading edge of the pulse generator signal, and adjust the trigger levels for both the delay generator and the counter accordingly.
To minimize uncertainty, it is crucial that the electrical cables E3 and E4, as well as the optical fibres F1 and F5, are of equal length The external source generates an optical square wave, which is converted into an electrical pulse to halt the time interval counter It is important to adjust the optical attenuator for optimal triggering of the time interval counter and to document the time interval displayed (from start to stop) as the insertion delay \( T_{\text{delay}} \) Identical cable numbers in Figures 3 and 4 refer to the same physical cables.
Measurement procedure
Choose between automatic or manual techniques for feature location on the OTDR Set the attenuator to achieve the required pulse amplitude(s) and select a pulse width on the digital delay generator, such as 1 μs.
To optimize the delay generator T i settings, select time intervals that ensure samples are spread across a broad distance range with some randomness, facilitating averaging over the OTDR's distance sampling interval The initial time setting should position the pulse near the OTDR's front panel while remaining outside the initial dead zone for accurate measurements If the testing laboratory does not provide an alternative distance sampling scheme, one of the two specified schemes must be implemented In the first scheme, determine the sample spacing D sample based on the appropriate OTDR instrument settings, such as by zooming into the OTDR trace, and then calculate the corresponding delay difference of the delay generator T sample using the formula: c.
= (20) where N is the OTDR's group index setting and c is the speed of light in vacuum
Then calculate a total number of i delay generator settings, grouped in k clusters of n settings each (i = k n), where each cluster uniformly covers one sample spacing Each cluster shall have the form: n n T n T
In the first scheme, the sample settings are defined as \( T_K, T_{K+1}, T_{K+2}, \ldots, T_{K+(-1)} \), with each cluster containing at least four uniform settings The cluster centers are evenly distributed, extending from just beyond the initial dead zone to a considerable distance for calibration The minimum number of clusters, \( k \), is two In the second scheme, clusters are absent, and the sample spacing, \( D_{sample} \), only needs a rough estimate To calculate \( T_{sample} \) from Equation (20), time settings should be uniformly spaced between the initial dead zone and a significant distance, with each interval incorporating a random time addition These random intervals must follow a uniform probability density within the range of \(-T_1\) to \(T_1\), where \(T_1\) is at least 20 times \(T_{sample}\) but less than 10% of the longest time delay for the tests A minimum of 20 different measurement settings is required.
Prior knowledge of the magnitude of type A uncertainty and acceptable measurement tolerances can guide testing laboratories in choosing an appropriate systematic or random distance sampling scheme.
Begin by selecting the initial time setting, T_i, for the series T_1 Document the delay generator's time, T_1, along with the measured location, L_otdr,1, of the event on the OTDR Follow the time settings outlined in section 6.2.4.1, ensuring to consistently record the time T_i and the corresponding measured location L_otdr,i Continue this process until all time settings have been completed.
Calculations and results
Following the concept of Clause 5, use the time settings to calculate i reference locations L ref,i :
N is the group index setting of the OTDR;
T i are the time settings defined in 6.2.3;
T delay is the calibrated insertion delay of the test equipment (see 6.2.2)
Then, use the reference locations and the displayed locations L otdr,i to calculate the set of i location deviations Δ L i : Δ L i = L otdr,i – L ref, i (23)
To calculate the location offset \$\Delta L_0\$ and the distance scale deviation \$\Delta S_L\$, the location deviation data is fitted to a simplified model that temporarily disregards distance sampling errors The model is expressed as \$\Delta L_{i, \text{model}} = \Delta S_L L_{\text{ref}, i} + \Delta L_0\$.
Specifically, minimize the difference between the model and the data using the least-squares criterion that is, choose Δ S L and Δ L 0 so that the summation
∑ Δ L i − Δ S L − Δ L (25) is minimized Record Δ L 0 and Δ S L obtained from the approximation
The slope of the linear approximation, as illustrated in Figure 2, indicates the distance scale deviation, denoted as \$\Delta S_L\$ Meanwhile, the intercept on the vertical axis signifies the location offset, represented as \$\Delta L_0\$ It is essential to document the calculated values of \$\Delta S_L\$ and \$\Delta L_0\$.
Uncertainties
A general discussion of the distance uncertainties can be found in Clause 5
The list of uncertainties provided may not be exhaustive, and additional factors should be considered based on the specific measurement setup and procedure It is essential to utilize the mathematical framework outlined in Annex D for calculating and reporting these uncertainties.
The least-squares approximation described in section 6.2.5 utilizes the displayed distances between measurement samples to determine the distance scale deviation It is assumed that measurement samples close to L = 0 and the maximum location L = L max significantly impact the distance scale deviation, while samples in the middle range do not affect the slope of the distance error model.
Applying the standard formula for the propagation of errors to Equation (7) yields the distance scale uncertainty u Δ SL in which ≅ D ref was used
D otdr is D ref ≈ L ref (for the long distances discussed here); u is the standard deviation expressing the uncertainty of the distance samples
The slope uncertainty, denoted as \$u /\$, arises from inaccurate distance measurements and is equivalent to the standard deviation of the slope, \$\Delta S_L\$, in the location model described by Equation (16) This includes uncertainties from marker placement and distance sampling errors The least-squares algorithm used to determine \$\Delta S_L\$ can also be applied to calculate \$u \$ Additionally, \$\Delta L_i\$ may be averaged over the relevant sampling interval The uncertainty of the reference distances is represented by \$u D_{ref}\$, while \$u D_{ref}/D_{ref}\$ indicates the slope uncertainty due to the digital delay generator, reflecting the relative timing uncertainty associated with it.
The location offset \$\Delta L_0\$ is determined by the intercept of the least-squares approximation on the vertical axis This intercept is primarily influenced by the initial samples near the location \$L = 0\$ and the precision of the insertion delay \$T_{\text{delay}}\$.
The location offset uncertainty u Δ L0 can be calculated by using the standard formula for the propagation of errors:
The uncertainty of the differences between the measured values (\$Δ L_i\$) and the least-squares approximation near \$L = 0\$ is represented by \$uΔL\$, which accounts for marker placement uncertainty and distance sampling error This uncertainty is equivalent to the standard deviation of the differences (\$Δ L_i – Δ L_{i, model}\$) near \$L = 0\$ If applicable, \$Δ L_i\$ can be averaged over the corresponding sampling interval The least-squares algorithm used to determine \$Δ L_0\$ can also be applied to calculate \$σ_{Δ L}\$ Additionally, \$uT_{delay}\$ represents the uncertainty of the system insertion delay, with the assumption that the initial setting time is minimal or zero, thereby simplifying the delay generator uncertainty to just the insertion delays.
According to Clause 5, identify the maximum difference between the location deviation samples \$\Delta L_i\$ and the least-squares approximation at \$L = 0\$ To calculate the location readout uncertainty \$u_{L_{readout}}\$ (which accounts for the distance sampling error), divide the maximum difference by the square root of 3 Alternatively, \$u_{L_{readout}}\$ can also be calculated using the least-squares algorithm for determining \$\Delta S_L\$ and \$\Delta L_0\$ or by applying the specified formula.
Concatenated fibre method
Short description and advantages
This method uses calibrated fibres with transit times precisely measured at the wavelength of the OTDR under test to calibrate the distance scale
This method utilizes connectorized lengths of fiber, making it cost-effective and ideal for testing in locations where traditional equipment is impractical Although it is considered a manual testing approach due to the need for multiple connections and disconnections of short fiber lengths to adjust feature locations, automation is possible through the use of optical switches.
Equipment
The equipment, as illustrated in Figure 5, comprises a test OTDR along with several key components: fibre A for identifying location offsets, fibre B for measuring distance scale deviations, and a set of incremental fibres to assess distance sampling errors.
One fibre of the set
Figure 5 – Concatenated fibres used for calibration of the distance scale
Fiber optic cables are typically cabled or packaged for protection and equipped with connectors for easy connection and disconnection Alternatively, they can be utilized with a fiber optic switching mechanism that eliminates the need for connectors.
The specifications for these fibers are outlined as follows: Fiber A can either be a simple fiber with an end reflection or one that incorporates internal reflection or splicing The length of the fiber is not critical, provided it positions the feature to be measured on a backscatter trace that remains largely unaffected by the initial reflections near the OTDR port.
Fibre A can be divided into a short jumper fibre and a specified length with connectorized ends, allowing for the insertion of incremental fibres without needing access to the OTDR's front panel The reflection from the near-end connectors is minimized to prevent interference with the OTDR's dead zone at the end of fibre A.
This alternative may be important in automated systems
Fibre A serves as a lead-in fibre for assessing distance scale deviation using Fibre B, which should feature reflective ends, such as those found on its connectors To minimize uncertainty, it is advisable for Fibre B to have a minimum length of 2 km.
To calibrate the fibre, measure its optical transit time T b as described in Clause 5
For accurate distance calibration, it is crucial that the reflections from both ends of fibre B (connectors C2 and C3) are approximately equal Discrepancies in reflections can lead to inaccurate distance measurements, especially if one end saturates the OTDR while the other does not Although the impact of this difference is minimal for long fibres, it is still important to address When the manufacturer's sampling interval is uncertain and expected to be large, incremental fibres should be utilized to adjust the locations of the reflections on fibre B, ensuring they are less than the OTDR's distance sampling interval To achieve this, select fibre lengths that create at least four evenly spaced distance increments within the sampling interval For instance, with a 10 m sampling interval, using fibres of 2.5 m and 5 m can yield increments of 0 m, 2.5 m, 5 m, and 7.5 m More generally, the fibres should produce length increments of \(x\) where \(x = 2, 1, \ldots\).
0 D D n− D (29) where n ≥ 4, and n D x equals the distance sampling interval of the OTDR under the conditions to be tested
In most cases, it is unnecessary to calibrate the transit time of these fibers as outlined in Clause 5; instead, measuring their physical lengths is sufficient The difference between the true group index and the OTDR group index setting is minimal for such short fibers.
Measurement procedures
Random noise typically has a minimal impact on location deviation, unless the displayed power level approaches the instrument's noise limit In such situations, it is advisable to use longer averaging with the OTDR.
Choose the specific feature type in or at the end of fibre A for which you need to determine the location offset, and select the appropriate fibres Additionally, decide whether to use an automatic or manual technique for placing markers on the feature in fibre A and the reflective ends of fibre B.
To visualize the feature at the far end of fibre A, connect it to the front of the OTDR Next, attach fibre B to the far end of fibre A, allowing the OTDR to capture reflections from both ends of fibre B.
To measure the location of the feature in fibre A, use the OTDR and record the first measured location as \$L_{\text{otdr},1}\$ Next, measure the length of fibre B with the OTDR by utilizing the two reflections generated by this fibre, and record this distance as \$D_{\text{otdr},1}\$.
Insert the shortest incremental fibers between the OTDR and the start of fiber A Measure the location \(L_{\text{otdr},2}\) and the distance \(D_{\text{otdr},2}\) If fiber A is a split fiber, as outlined in section 6.3.2, the incremental fibers can be inserted into the split rather than between the OTDR and the beginning.
Continue inserting successively increasing length combinations of the incremental fibres Measure the location L otdr,i and the distance D otdr,i until i = n and the total length of the incremental fibres is (n – 1) D x
Calculations and results
Compute the distance < D otdr > (the length of fibre B) as the average of the n values of D otdr,i
Then compute the distance scale deviation as
D ref is the reference distance;
N is the group index setting of the OTDR;
T b is the one-way transit time for fibre B, as measured according to Clause 5
Let < L otdr > be the average of all n values of L otdr,i Compute the location offset on the basis of Equation (16):
< L ref > is the average reference location corresponding to the first reflection, to be calculated with the help of the average length of the incremental fibres;
N is the group index setting of the OTDR;
The one-way transit time for fibre A, denoted as \$T_a\$, is measured in accordance with Clause 5 The distance scale deviation, represented by \$\Delta S_L\$, is calculated using Equation (30) If fibre A is sufficiently short, the term \$\Delta S_L\$ can be disregarded.
Uncertainties
Clause 5 provides a comprehensive overview of distance uncertainties, highlighting that the subsequent list may not encompass all possible uncertainties It is important to consider additional factors that could influence measurements based on the specific setup and procedures used.
The mathematical basis given in Annex D should be used to calculate and state the uncertainties
The distance scale uncertainty u Δ SL should be calculated with the following formula which is derived from Equation (30):
= ⎛ Δ D T u u u m/km (32) where u Dotdr is the uncertainty of the displayed length of fibre B, for example, as caused by the marker placement uncertainty and the distance sampling error
The transit time uncertainty of fibre B, denoted as \$\sigma_{T_b}\$, is calculated using the root-sum-squaring method This uncertainty comprises three components: \$u_{T_b,\text{counter}\$}, which accounts for the uncertainty due to the time interval counter; \$u_{T_b,\lambda}\$, which arises from the difference between the wavelength used for transit time measurement and the OTDR wavelength; and \$u_{T_b,\Theta}\$, which is influenced by the temperature coefficient of fibre B, typically valued at 1 cm/(km °C).
The location offset uncertainty u Δ L0 should be calculated from the following formula which is derived from Equation (31), by neglecting the Δ S L and the (n–1) D x/2 terms:
The uncertainty in measuring the location of the feature at the end or inside of fibre A, denoted as \( N_u c_u (33) \), primarily arises from the marker placement It is assumed that any distance sampling error is effectively mitigated by averaging over a single sampling interval.
The transit time uncertainty of fibre A, denoted as \( u_{Ta} \), should be calculated using the root-sum-squaring method This includes \( u_{Ta,counter} \), which represents the uncertainty due to the time interval counter, \( u_{Ta, \lambda} \), which accounts for the difference between the wavelength used for determining transit time and the OTDR wavelength, and \( u_{Ta, \Theta} \), which reflects the uncertainty due to temperature variations.
Calculate the following two sets of data, one for the location deviation and one for the distance error, using the measurement samples given in 6.3.3
To calculate the location readout uncertainty \( u_{L\text{readout}} \), it is advised to take half the difference between the largest and smallest values of the larger set, either L or D, and divide it by the square root of 3.
Recirculating delay line method
Short description and advantage
The recirculating delay line method uses a calibrated loop of fibre, made with a coupler and a reflector, to generate periodic reflections
The technique resembles the concatenated fibre method, utilizing a fibre artifact without the need for electronic devices This artifact produces numerous calibrated distance samples, which can significantly minimize type A uncertainties that impact the deviation of the distance scale.
The measurements of location offset are limited to the reflective features generated by the recirculating delay line.
Equipment
In addition to the test OTDR, the measurement equipment only includes a recirculating delay line manufactured and calibrated according to Annex A, as shown in Figure 6
Lead-in length (transit time T a)
Figure 6 – Distance calibration with a recirculating delay line
The recirculating delay line on the OTDR display features multiple reflective elements, as illustrated in Figure 7 The first element corresponds to the optical pulse traveling directly to the mirror and returning to the OTDR The second element is created by the optical pulse that travels once through the loop before reaching the mirror and returning directly to the OTDR, coinciding with the pulse that travels directly to the mirror and back through the loop The third pulse involves the optical signal traveling through the loop twice, and this pattern continues with additional reflections.
Accordingly, the ideal displayed locations would be etc
(36) where L a is the length of the lead-in fibre and L b is the length of the fibre loop
Di sp la ye d po we r F d B
Figure 7 – OTDR trace produced by recirculating delay line
Incorporating one or more incremental fibres into the measurement setup can be beneficial, although the necessity is diminished due to the averaging effect caused by multiple reflections from the delay line over the distance sampling interval However, since this averaging is uncontrolled, a systematic approach may be more desirable Following the notation from section 6.3.3, setting \( n = 2 \) may suffice, allowing for a single incremental fibre with a length equal to half of the distance sampling interval.
Measurement procedure
The procedure operates under the assumption that no incremental fibers are utilized However, if incremental fibers are incorporated, the number of recorded distance samples increases, allowing for easy adjustments to the notation and calculations Consequently, the method closely resembles that of section 6.3, where the lead-in fiber corresponds to fiber A and the loop length is analogous to fiber B.
Establish the technique (automatic or manual) of placing the markers at the leading edges of the reflections from the recirculating delay line, following the manufacturer's recommendations
Connect the recirculating delay line assembly directly to the OTDR so that the reflective features can be seen on the OTDR
Measure the positions of consecutive reflections from the recirculating delay line using the OTDR, recording them as \$L_{otdr,i}\$, where the index \$i\$ ranges from 0 to \$k\$, indicating the number of passes through the loop While a higher value of \$k\$ is expected to enhance result accuracy, it is constrained by loss and the OTDR's noise floor.
Calculations and results
Using the calibration data of the recirculating delay line T a and T b the series of reference locations is: i = 1:
= + where N is the group index setting of the OTDR
Then use the displayed locations L otdr,i and the reference locations to calculate the series of location deviations Δ L i:
To find the location offset \$\Delta L_0\$ and the distance scale deviation \$\Delta S_L\$, the location deviation data should be fitted to a simplified model that temporarily disregards the uncertainty in location readouts.
Specifically, minimize the difference between the model and the data using the least-squares criterion, that is choose Δ S L and Δ L 0 so that the summation:
∑ ΔL i − ΔS L − ΔL (39) is minimized Record Δ L 0 and Δ S L obtained from the approximation.
Uncertainties
A general discussion of the distance uncertainties is given in Clause 5
The list of uncertainties provided may not be exhaustive, and additional factors should be considered based on the specific measurement setup and procedure It is essential to utilize the mathematical framework outlined in Annex D for calculating and reporting these uncertainties.
The least-squares approximation described in section 6.4.4 utilizes the displayed distances between measurement samples to determine the distance scale deviation It is assumed that the measurement samples close to L = 0 and those near the maximum location L = L max significantly impact the distance scale deviation, while samples in the middle range do not affect the slope of the distance error model.
Applying the standard formula for the propagation of errors to Equation (7) yields the distance scale uncertainty u Δ SL in which < D otdr > ≅ D ref was used
The relationship between the distance measurements is expressed as D otdr is approximately equal to D ref for long distances The standard deviation, denoted as u , reflects the uncertainty in distance samples based on location samples, which corresponds to the standard deviation of the difference between measured and model distances, accounting for marker placement uncertainty and sampling errors The least squares algorithm applied for determining Δ S L can also be utilized to calculate u When using incremental fibers, Δ L i can be averaged over the corresponding sampling interval Additionally, the uncertainty of the reference distances, u Dref, can be derived from the formula u Dref/D ref = u Tb/T b, where u Tb represents the uncertainty in loop transit time, as specified in the calibration certificate of the recirculating delay line.
The location offset \$\Delta L_0\$ is determined by the intercept of the least-squares approximation on the vertical axis This intercept is primarily influenced by the initial samples near the location \$L = 0\$ and the precision of the transit time \$T_a\$.
The location offset uncertainty u Δ L0 can be calculated by applying the standard formula for the propagation of errors to Equation (38):
The uncertainty of the differences between the measured values (\$Δ L_i\$) and the least squares approximation near \$L = 0\$ is represented as \$u_Δ L\$, which encompasses both marker placement uncertainty and distance sampling error This uncertainty is equivalent to the standard deviation of the differences between the measured and model values (\$Δ L_i – Δ L_{i, model}\$) near \$L = 0\$ The least squares algorithm used to determine \$Δ L_0\$ can also be applied to calculate \$σ_{Δ L}\$ If incremental fibers are utilized, \$Δ L_i\$ can be averaged over the corresponding sampling interval Additionally, \$u_{Ta}\$ denotes the documented uncertainty of the delay time for the lead-in fiber of the recirculating delay line, while \$u_{Ta, Θ}\$ represents the uncertainty in delay time due to the temperature coefficient of the fiber, typically valued at 1 cm/(km °C).
The method for assessing location readout uncertainty is illustrated in Figure 2 Due to the recirculating delay line, the data generated may be insufficient to demonstrate the repetitive characteristics of the measurement samples It is advisable to identify the maximum differences between the location deviations, denoted as Δ L i (L ref), and the least-squares approximation Subsequently, this difference should be divided by the square root of 3 to calculate the location readout uncertainty, u Lreadout, which encompasses the distance sampling error.
General
The goal of loss calibration is to identify the loss deviation ΔA for power levels F in the OTDR backscatter regime while assessing measurement uncertainties ΔA depends on the displayed power level F and encompasses both the inaccuracies in the displayed loss and the non-linearity of the OTDR power scale.
The loss scale deviation, denoted as Δ S A, serves as an alternative method to present calibration results This approach is particularly beneficial for earlier generations of Optical Time Domain Reflectometers (OTDR), where errors may depend on attenuation levels Users should exercise caution when applying this parameter for attenuations below 1 dB, as the associated uncertainty may exceed that indicated by the formula.
Calibration can be performed using one of four methods: fibre standard, external source, splice simulator, or power reduction The subsequent sections detail the principles of loss calibration.
Determination of the displayed power level F
For each measured loss, determine the displayed power level or an equivalent parameter that can be used to reproduce the vertical position of a measurement sample This level is termed F
Use the OTDR's clipping level at the front panel connector as the default reference point for determining F, with F ref = 0 dB All values of F should be stated in relation to this reference point; for instance, if the displayed power level is x dB below the clipping level, then F = –x dB The clipping level can be identified by introducing a sufficiently large reflection into a fiber length or by using a strong optical pulse from an external source This level is selected because it is the most reproducible on most OTDR power scales.
Di sp la y po we r F dB
–x dB Reference level = clipping level
Figure 8 – Determining the reference level and the displayed power level
Alternative solutions for measuring F include expressing its value in dB relative to a fixed level, provided the OTDR can display power in dB Another option is to use the starting level of the backscatter trace from a specific type of fiber at a designated pulse width as the reference level However, it's important to note that the reproducibility of this reference level can be impacted by the variability in the connection to the OTDR port.
Selection of an appropriate reference loss A ref
The loss calibration principle involves using a reference loss, denoted as A ref, with the OTDR to measure the displayed loss, A otdr,i, in relation to the displayed power level, F i As illustrated in Figure 9, the F i values represent the high power ends of A otdr,i.
From a theoretical point of view, an infinitesimally small value of A ref would be desirable
In practical applications, using a small value of \$A_{ref}\$ can lead to increased measurement uncertainty caused by OTDR noise, while larger values may hide important details It is essential to document the specific \$A_{ref}\$ value used during calibration The suggested range for \$A_{ref}\$ is between 0.5 dB and 2 dB.
Di sp la yed power F d B
Figure 9 – Measurement of the OTDR loss samples
Reference loss in fibre optic components can be either actual or simulated, and it is crucial to recognize that these components often display wavelength-dependent losses To ensure accurate measurements, it is essential to determine the reference loss at the centroidal wavelength of the Optical Time Domain Reflectometer (OTDR) Additionally, minimizing polarization dependence in the reference loss is important for reliable performance.
Development of a test plan
Loss samples are influenced by power levels, distance, and the historical signal shape of the fibre's OTDR signature before the measured feature Additionally, the detector and electronics can be impacted by recovery from the laser's initial firing and by scattering or reflections within the fibre It's important to note that calibration is specific to the distance and signal conditions under which it is conducted.
This standard does not mandate specific signal history conditions It defines an OTDR display region A, which approximates the area where users typically take measurements, based on four key quantities These include the extrapolated start of the backscatter trace for the specific pulse width (F₀), the lowest and highest attenuation as specified in Table 1, and 3 dB margins on either side.
Table 1 – Attenuation coefficients defining region A
Fibre attenuation coefficients Wavelength nm Lowest ( α min ) dB/km
On the same basis, attenuation coefficient values for other wavelengths may be chosen to represent typical single-mode fibres An analytical description of region A is given by
F max should not exceed an upper limit of 1 dB below the clipping level, unless otherwise specified by the OTDR manufacturer The loss calibration points F should lie inside region A
Calibration data for regions B and C can be submitted voluntarily Region B is relevant for fibre paths that contain components with significant loss, while Region C applies to fibre paths featuring components with substantial reflection.
Backscatter traces defined by highest and lowest fibre attenuation
Di sp la y po we r F dB
Figure 10 – Region A, the recommended region for loss measurement samples
Develop a test plan for each method that includes sample placements, combining location and displayed power level Aim for a vertical sample spacing between 0.5 dB and 1 dB, ensuring it does not exceed the reference loss \( A_{ref} \) The attenuation range should be carefully considered.
To achieve optimal results, it is essential to select samples within region A that are evenly distributed and at a noise level of F 0 Additionally, overlapping measurement samples—those taken at the same displayed power level but from different locations—are beneficial, as illustrated in Figure 11.
Di sp la yed power F d B
Figure 11 – Possible placement of sample points within region A
Polarization dependence
Polarization dependence loss (PDL) testing can be performed by introducing a polarized external signal with a variable polarization state and a constant power level to the OTDR port, as specified in IEC 61300-3-2 This can be achieved by integrating a PDL test into the external source method described in section 8.3, where one attenuator is replaced with a polarization controller The polarization controller must be capable of producing all states of polarization while maintaining the output power and degree of polarization nearly constant.
Figure 12 – External source method for testing the polarization dependence of the OTDR
Another possibility for polarization dependence testing is using the reflected signal from, for example, a cleaved fibre end See Figure 13
Figure 13 – Reflection method for testing the polarization dependence of the OTDR
The loss of the polarization controller can vary based on its settings, particularly its rotation dependence, while the coupler in the external source method may also show polarization-dependent loss It is crucial that these dependencies are less than the polarization dependence of the OTDR, as both factors contribute to measurement uncertainty To assess this uncertainty, it is advisable to test the setup by configuring the E/O converter for continuous-wave operation and substituting the OTDR with a polarization-insensitive optical power meter.
To evaluate uncertainty using the reflection method, set the OTDR to continuous-wave mode or use a continuous-wave (polarized) laser source if necessary Connect the fiber end to a polarization-insensitive optical power meter and adjust the polarization controller to generate a wide range of polarization states covering the entire Poincaré sphere Measure the peak-to-peak change in the displayed power level due to polarization state variations, denoted as \$\Delta A_{pdl,set-up}\$, which includes the power meter's polarization dependence Finally, document the results as an uncertainty for the PDL measurement setup, represented as \$u_{pdl,set-up}\$.
3 2 set-up pdl, -up set pdl, u A otdr
To evaluate the polarization dependence of an OTDR, position the optical pulse at the center of the screen and ensure that the displayed power levels are well below the OTDR's clipping level by appropriately adjusting the attenuator Next, modify the polarization controller to generate a wide range of polarization states, and monitor the changes in the displayed power level as the polarization state varies Finally, document the fluctuations in pulse amplitude, denoted as ±Δ A_{pdl,otdr}.
Include the PDL measurement result into the documentation If PDL testing was not carried out, then this is to be specifically stated in the documentation.
Calculation of the calibration results
From the measured values A otdr,i , calculate the loss deviation Δ A i: ref i i A A
The loss uncertainties u Δ A are discussed in conjunction with the calibration methods.
Using the calibration results
The error in the loss value, denoted as ΔY_i, measured with the OTDR ΔA_i, along with its uncertainty, can be calculated using the calibration results This calculation employs a formula that incorporates a recommended confidence level of 95%.
Notice that this error applies, in a rigorous sense, only to measured losses at the displayed power level and displayed location for which Δ A i was recorded
General
Loss calibration methods involve generating a reference loss outside the OTDR and testing the OTDR's response to this loss When the reference loss is based on two signals with the same polarization state, the calibration results remain unaffected by the OTDR's polarization dependence However, if the reference loss arises from signals with different polarization states, it is recommended to use a polarization controller between the OTDR and the reference loss device to mitigate the impact of polarization dependence Ultimately, ensuring that loss calibration results are independent of the OTDR's polarization dependence is crucial for accurate measurements.
Fibre standard method
Short description and advantage
The fibre standard method involves calibrating the OTDR power scale using an optical fibre standard with a precisely determined loss, as detailed in Annex B This reference loss is essential for determining A otdr,i and ensures that the calibration method accurately simulates the conditions for measuring fibre attenuation with an OTDR It is especially effective for calibrating region A, as illustrated in Figure 10.
Equipment
The measurement equipment for the test OTDR comprises several essential components: a fibre standard as outlined in Annex B, a set of type B single-mode lead-in fibres in accordance with IEC 60793-2-50, a variable attenuator, and optionally, a polarization controller.
The attenuator and lead-in fibres are designed to position the fibre standard at various locations within region A of the OTDR display For instance, by combining four lead-in fibres with attenuation values of 2 dB, 4 dB, 8 dB, and 16 dB, the displayed power level can be adjusted within a range of 30 dB in 2 dB increments.
To achieve optimal performance, fibers must utilize low-reflectance connectors, as the attenuation values account for standard connector losses For the recommended finer sample spacing of 0.5 dB, the attenuator should be adjusted in increments of 0.5 dB, ranging from 0 dB to 1.5 dB.
It should be noted that the attenuator is not necessary if the recommended steps of 0,5 dB are generated with a larger number of lead-in fibres
The purpose of the optional polarization controller is to reduce the possible influence of the OTDR polarization dependence (see Figure 14)
Set of lead-in fibres
Figure 14 – Loss calibration with a fibre standard
Both the attenuator and the polarization controller shall have low reflections, because reflections can influence the attenuation measurement results
8.2.2.2 Calibration of the reference loss
To measure the total length \( D_2 \) of the fibre standard, follow the OTDR manufacturer's instructions Select a section of the fibre standard, \( D_1 \), that is outside the attenuation dead zone caused by connectors Ensure that the starting point of this section minimizes the difference \( \Delta F_{\text{max}} \) between the actual backscatter trace and its linear extrapolation, which may require a lead-in fibre Finally, measure the length of the selected section \( D_1 \).
Di sp la yed power F d B
Small reflection caused by connector in front of fibre standard
Earliest start of section D 1 Start of section D 2 Δ F max
Figure 15 – Placing the beginning of section D 1 outside the attenuation dead zone
For optimal performance, a length D1 corresponding to a reference loss of approximately 0.5 dB is recommended It is crucial to avoid back reflections from the far end of the fiber standard, as they can affect the preceding backscatter trace Thanks to the longitudinal uniformity of the fiber standard and the directional independence of its backscatter loss, the reference loss A_ref at the marker locations is determined with high accuracy.
The calibrated loss of the fiber standard at the centroidal wavelength of the OTDR, denoted as A std (λ avg), is represented in decibels as A λ dB (46) It is essential that A ref remains constant during subsequent measurements, or alternatively, the distance between marker locations must be fixed Additionally, the calculation eliminates the influence of the group index.
Measurement procedure
To achieve a vertical sample spacing of approximately 0.5 dB and ensure all measurement samples fall within region A of the OTDR display, it is essential to develop a comprehensive test plan for fibre/attenuator settings Overlapping measurement samples, which occur at the same displayed power level but at different locations, are beneficial for accurate results Additionally, to address any inaccessible dynamic range due to the insertion loss of the attenuator and connectors, directly connecting the shortest lead-in fibre to the OTDR and adjusting the insertion loss by bending the lead-in fibre is recommended.
To minimize uncertainty in measurements, it is advisable to average the loss values \$A_{otdr,i}\$ around the markers or over the entire length \$D_1\$ rather than relying on single power levels \$F_i\$ Longer averaging times for OTDR measurements are particularly beneficial at lower displayed power levels to reduce type A uncertainty, and all applied averaging times should be documented Additionally, it is essential to record all displayed power levels \$F_i\$ and their corresponding locations \$L_i\$.
The measurement results are likely affected by the polarization dependence of the OTDR (PDL), as the signals from the start and end of section D1 may exhibit different polarization states To mitigate the impact of PDL, a polarization controller can be utilized, as illustrated in Figure 14 By measuring multiple samples of \( A_{\text{otdr},i} \) at various positions of the polarization controller, these samples can be averaged to produce a single \( A_{\text{otdr},i} \), effectively reducing the influence of PDL.
Calculations and results
Calculate the loss deviation samples Δ A i using Equation (47): ref i i A A
Alternatively report the loss scale deviation using Equation (48): ref i ref otdr, i
Uncertainties
Note that the following list of uncertainties may not be complete Additional contributions may have to be taken into account, depending on the measurement set-up and procedure
The guidelines of the mathematical basis given in Annex D should be used to calculate and state the uncertainties
The standard deviation characterizing the loss uncertainty u Δ A of an individual Δ A can be calculated from Equation (47), using the standard formula for the propagation of errors
The uncertainty u Aref of the reference loss, that is of the fibre standard, and u Aotdr of the OTDR response shall be determined
The uncertainty u Aref should be accumulated, by root-sum-squaring, from the following contributions: u A,std the loss uncertainty, in decibels, of the fibre standard as documented according to
Annex B discusses the distance loss uncertainty in decibels resulting from not utilizing the full length of the fiber standard This uncertainty must be calculated by root-sum-squaring the uncertainty associated with determining the length relation \( D_1 / D_2 \) (refer to Equation (46)) and the uncertainty stemming from the uniformity of longitudinal attenuation (see Annex B).
The uncertainty u Aotdr should be accumulated, by root-sum-squaring, from the following contributions: u A, λ the loss uncertainty, in decibels, introduced by the uncertainty of the OTDR centroidal wavelength u λ
At wavelengths around 1,550 nm, the uncertainty in fiber loss is minimal due to the slight wavelength dependence However, at 1,300 nm, where loss is mainly attributed to Rayleigh scattering, the uncertainty becomes more significant, represented as ref λ λ.
The loss uncertainty in decibels caused by the attenuation dead zone following the connector(s) is denoted as \$u_{A,\text{deadzone}}\$ The loss uncertainty of the optical attenuator, represented as \$u_{A,\text{attenuator}}\$, arises from the potential polarization of the backscatter signal, which may lead to polarization-dependent loss Additionally, \$u_{A,\text{pdl}}\$ refers to the loss uncertainty due to the OTDR's polarization dependence, as the backscatter signal can exhibit varying polarization states; this uncertainty can be mitigated as discussed in section 8.2.3.2 Lastly, \$u_{A,\text{type A}}\$ represents the type A loss uncertainty, measured in decibels, which must be experimentally determined and is influenced by the displayed power level and the OTDR averaging time.
Additional contributions may have to be taken into account, depending on the measurement set-up and procedure.
External source method (see Figure 16)
Short description and advantage
This technique employs a calibrated delay generator to replicate time delays in fiber optics, alongside an optical source that produces a measurable change in optical pulse amplitude The setup resembles the external source method used for distance calibration, with the added capability of controlling optical amplitude By integrating variable delay and pulse amplitude, this method enables comprehensive characterization of the OTDR power scale and its distance-dependent behavior.
The method is ideal for fully automated laboratory testing under computer control, allowing for the calibration of all regions A, B, and C as shown in Figure 10 For clarity, the following discussion focuses solely on a pulsed signal source that generates points in region A, assuming no backscatter light in the signal history To adapt the OTDR calibration for different conditions, the described pulsed light source should be substituted with an optical source that accurately simulates the required signal.
Equipment
The measurement equipment for the test OTDR comprises several essential components: an optical fibre coupler for system interconnection, an optical to electrical (O/E) converter, and a delay generator featuring variable delay, pulse width, and amplitude Additionally, it includes an electrical to optical (E/O) converter with a centroidal wavelength matching that of the OTDR, capable of generating continuous-wave optical power for reference loss calibration The setup also consists of a variable optical attenuator (attenuator 1), a second optical attenuator (attenuator 2) with high repeatability for precise attenuation adjustments (recommended between 1 dB and 4 dB), optical fibre jumper cables for connectivity, and an optical power meter.
Figure 16 – Loss calibration with the external source method
The integration of variable attenuator A1 and a variable delay generator enables precise positioning of the optical pulse peak within the two-dimensional space of the OTDR display Additionally, attenuator A2 facilitates the toggling of a fixed attenuation step, allowing for controlled changes in the optical pulse amplitude It is important to consider the reference loss in this context.
A ref equals half the attenuation step, due to the OTDR's vertical scaling factor of 0,5
The optional polarization controller aims to minimize the impact of polarization dependence in the attenuators and power meter during reference loss measurements It should be utilized for both calibrating reference loss and conducting actual measurements by averaging multiple polarization states, as outlined in section 8.2.3.2.
Calibration of the reference loss
Select a suitable value for the reference loss A ref A value of 0,5 dB to 2 dB is recommended – this equals an attenuation step of 1 dB to 4 dB
To ensure accurate measurements, set the E/O converter to continuous wave (CW) operation and connect the optical power meter to the OTDR end of fiber F1 Adjust the optical power meter's wavelength correction to match that of the E/O converter Measure the attenuation step of attenuator 2 multiple times to determine the reference attenuation (A ref) and its uncertainty (type A u A,step) Be aware that optical interference from reflections in the measurement setup and the narrow spectral width of the source may introduce unwanted variability in the results.
Calculate the reference loss as the average attenuation step divided by 2.
Measurement procedure
To achieve evenly distributed measurement samples within region A, select suitable combinations of pulse amplitudes and delay settings, ensuring that the samples are spaced between 0.5 dB and 1 dB apart (refer to Figure 10) It is beneficial to have overlapping measurement samples, which are samples taken at the same displayed power level but from different locations Additionally, measurement samples outside of region A may be included voluntarily.
To ensure a flat top on the displayed pulse, select an E/O pulse width of approximately 1 µs Choose the OTDR pulse width to be shorter than the return path to avoid overlapping with the E/O pulse Adjust attenuators 1 and 2 and the delay generator accordingly Alternatively, consider varying the pulse width for each new location, ensuring that the pulse consistently starts at L = 0, and determine the attenuation step at the end of the pulse.
The method for utilizing an Optical Time Domain Reflectometer (OTDR) to assess power level variations due to the adjustment of attenuator 2 involves averaging multiple data points near the measurement site Additionally, it is essential to define the approach for measuring the displayed power level, which is specified in Clause 7 as the higher of the two power levels being analyzed.
Ensure all equipment is powered on, allowing the E/O converter adequate warm-up time to stabilize its pulse amplitude Toggle attenuator 2 several times to confirm the stability of the displayed power level and the attenuation step with the OTDR.
Determine the 0 dB reference level as described in Clause 7
For each of the preselected combinations of attenuator 1 settings and delay generator settings proceed as follows
To minimize uncertainty in measurements, it is essential to establish the measurement location at the center of the pulse width, denoted as L i Additionally, it is crucial to maintain a consistent value for L i throughout the entire set of measurements at this specific location.
Di sp la yed powe r F dB
Initial dead zone (may be used to determine clipping level)
Figure 17 – Location and measurements for external source method otdr
To toggle the attenuator 2 between its predefined settings, record the displayed power level \( F_i \) (the upper level of the two) and note the change in displayed power level \( A_{otdr,i} \) as illustrated in Figure 17.
To ensure accuracy, it is recommended to take multiple measurements of A otdr,i and calculate the average Additionally, obtaining several samples at each location with varying displayed power levels can be beneficial by adjusting the pulse power using attenuator 1 Afterward, set the delay generator to the subsequent location.
Calculations and results
For each measurement sample, calculate the loss deviation using Equation (44): ref i i A A
Record the displayed power level \( F_i \) and the corresponding location \( L_i \) for each loss deviation value It is recommended to graph the loss deviation values against the displayed power level \( F_i \).
Alternatively report the loss scale deviation using Equation (52): ref i ref otdr, i
Uncertainties
The list of uncertainties provided may not be exhaustive, and additional factors should be considered based on the specific measurement setup and procedure It is essential to utilize the mathematical framework outlined in Annex D for calculating and reporting these uncertainties.
The standard deviation characterizing the loss uncertainty can be calculated from Equation
(51) using the standard formula for the propagation of errors:
The uncertainty of the reference loss, denoted as \( u_{A_{\text{ref}}} \), should be calculated by accumulating contributions through root-sum-squaring This includes \( u_{A,\pm} \), the uncertainty in decibels from calibrating the reference loss with the power meter, which arises from factors such as the power meter's non-linearity, polarization dependence, inhomogeneity, and noise Additionally, this uncertainty must also account for \( u_{A,\text{refl}} \), which reflects the uncertainty due to variations in the reflection characteristics of the setup.
The return loss measured by the power meter differs from that of the coupler/OTDR combination, potentially affecting the reference loss magnitude Additionally, the loss uncertainty, expressed in decibels, can be influenced by factors such as external source instability, optical interference, and the coupler's polarization dependence, which may vary with the state of polarization.
The total uncertainty of the OTDR measurement samples, denoted as \$u_{A,otdr}\$, is calculated by root-sum-squaring various contributions These include \$u_{A,\lambda}\$, the uncertainty in decibels due to differences between the centroidal wavelengths of the OTDR and the optical source used for calibration; \$u_{A,pdl}\$, the uncertainty in decibels arising from changes in polarization state during the attenuation step; and \$u_{A,type A}\$, the uncertainty in decibels linked to variability in attenuation step measurements, which may result from limited readout resolution and power levels nearing the noise limit This overall uncertainty can be assessed alongside \$u_{A,step}\$.
NOTE The uncertainties due to polarization dependence can be reduced or eliminated by the optional polarization controller.
Splice simulator method
Short description and advantage
This technique employs a splice simulator to accurately calibrate the OTDR power scale, featuring a fixed reference loss of about 1.5 dB It effectively simulates the conditions encountered when measuring splice attenuation with an OTDR Notably, this calibration method does not require any electronic equipment and is particularly effective for calibrating regions A and B as illustrated in Figure 10.
Equipment
The measurement equipment for the test OTDR comprises several essential components: a splice simulator as outlined in Annex C, a set of lead-in fibres conforming to type B of IEC 60793-2-50, a variable attenuator if required, and optionally, a polarization controller.
The test set-up is shown in Figure 18
(optional) Set of lead-in fibres
Figure 18 – Set-up for loss calibration with splice simulator
The calibrated reference loss of approximately 1.5 dB is observed at the end of fibre F1, as stated in Annex C The lead-in fibres are designed to produce measurement data within region A, as detailed in Clause 7 and illustrated in Figure 19.
The loss produced by the splice simulator is the difference between a) the sum of the two backscatter signals from fibres F1 and F2, and b) the backscatter signal from fibre F2
The signals a) and b) typically exhibit distinct polarization states To mitigate the polarization dependence of the OTDR, it is beneficial to employ a polarization controller that generates various polarization states for both signals This approach enables the averaging of the effects caused by polarization variations.
It is important that the attenuator and the polarization controller have low reflections, because they can influence the calibration result
NOTE The smaller circle represents the OTDR response to the reference loss
Figure 19 – OTDR display with splice simulator
Procedure
To achieve a vertical sample spacing of 0.5 dB to 1 dB on the OTDR display, it is essential to select the right combinations of lead-in fibre and attenuator settings Ensuring that all measurement samples fall within region A of the display is crucial Overlapping measurement samples, which occur at the same displayed power level but at different locations, are beneficial Additionally, utilizing a variable attenuator can help increase the number of distinct displayed power levels.
Connect the splice simulator to the OTDR as shown in Figure 18
8.4.3.2 Taking the splice loss measurements
As illustrated in Figure 20, the splice loss A otdr is defined as the vertical distance between the two lines representing the attenuation coefficients
To measure splice loss, follow the two-point method using two cursors or apply the least squares approximation (LSA method) While alternative techniques like the five cursors method or automatic measurement are available, it is essential to clearly specify the method employed.
Figure 20 – Measurement of the splice loss
When utilizing the optional polarization controller, it is essential to repeat the procedure multiple times for various positions of the controller By averaging all n samples, a single value, A otdr,i, is obtained, which minimizes the influence of Polarization Dependent Loss (PDL).
For each combination of lead-in fibers and attenuator settings, first, define point X1b near the splice, positioned to the left on a straight, clean backscatter trace Next, establish point X1a to the left of X1b, ensuring it is at least 500 m away A straight, clean backscatter trace should connect these two points Finally, record the distance between the two points as Ds1.
A distance of 500 meters was selected to ensure that the backscatter loss between the two points is at least 0.1 dB This choice is crucial depending on the method employed, whether it is the two-point method or the LSA method.
1) if using the two-point method, record the two power levels, F 1a and F 1b , and compute the displayed attenuation coefficient of the fibre as:
Note that the attenuation coefficient given by the OTDR may be used;
2) if using the LSA method, compute the best-fit line and determine the slope α 1 and the intercept with the vertical axis F 10 The fit should be performed on all points between
To analyze the data, first, record the values of α 1 and F 10 at points X 1a and X 1b Next, establish point X 2a to the right of the splice, ensuring it is positioned after any signal drop effects on a straight, clean backscatter trace Subsequently, define point X 2b at least 500 m to the right of X 2a, ensuring a straight, clean backscatter trace connects both points Finally, measure and document the distance between these two points, denoted as D s2, using either the two-point method or the LSA method.
1) if using the two-point method, record the two power levels, F 2a and F 2b , and compute the displayed attenuation coefficient of the fibre as
Note that the attenuation coefficient given by the OTDR may be used;
2) if using the LSA method, compute the best-fit line and determine the slope α 2 and the intercept with the vertical axis F 20 The fit shall be performed on all points between X 2a and X 2b Record α 2 and F 20 ; g) define the splice location L s,i as the turning point of the backscatter trace; h) compute the distance D 1 between point X 1b and the splice location Compute the distance
D 2 between the splice location and point X 2a
Calculations and results
a) According to the method used – the two-point method or the LSA method:
1) if using the two-point method, compute the splice loss A otdr,i and the correspondent power level F i (the subscript i indicates the i th splice loss) as:
2) if using the LSA method, compute the splice loss A otdr,i and the correspondent power level F i , as:
F −α otdr, b) from the loss values A otdr , calculate the loss deviations Δ A : ref i i A A
Record the loss deviations, the displayed power levels and the splice locations State the method used for splice loss analysis (for example two-point method or least-squares approximation)
Alternatively report the loss scale deviation using Equation (59): ref i ref otdr, i
Uncertainties
The list of uncertainties provided may not be exhaustive, and additional factors should be considered based on the specific measurement setup and procedure It is essential to utilize the mathematical framework outlined in Annex D for calculating and reporting these uncertainties.
The standard deviation characterizing the loss uncertainty u ΔA of an individual ΔA can be calculated from Equation (58), using the standard formula for propagation of uncertainties:
The uncertainty \$u_{Aref}\$ can be obtained from the calibration certificate of the splice simulator The total uncertainty \$\sigma_{Aotdr}\$ should be calculated using root-sum-squaring from several contributions: \$u_{A,\lambda}\$, the splice loss uncertainty in decibels due to the difference between the centroidal wavelengths of the splice simulator and the OTDR; \$u_{A,pdl}\$, the splice loss uncertainty from the OTDR's polarization dependence, which can be minimized with an optional polarization controller; and \$u_{A,pos}\$, the splice loss uncertainty introduced by the splice location uncertainty \$\sigma_L\$, which can be approximated as half of the OTDR pulse width in meters The relationship between these uncertainties is defined by the equation \$u_{A,pos} = |\alpha_1 - \alpha_2| \sigma_L\$ Additionally, \$u_{A,slope}\$ represents the splice loss uncertainty from the slope measurement uncertainty \$\sigma_s\$.
The relation between these uncertainties is given by: