Iec 60793 1 49 2006

NORME INTERNATIONALE CEI IEC INTERNATIONAL STANDARD 60793 1 49 Deuxième édition Second edition 2006 06 Fibres optiques – Partie 1 49 Méthodes de mesure et procédures d''''essai – Retard différentiel de m[.]

Source optique

Utiliser une source optique qui injecte des impulsions de courte durée et de largeur spectrale étroite dans la fibre sonde

The optical pulse must have a sufficiently short duration to measure the expected differential propagation time The maximum allowable duration for the optical pulse, characterized by its full width at 25% of the maximum amplitude, will depend on the value of

The Determined Modal Dispersion (DMD) is influenced by the sample length For instance, if the desired normalized DMD limit is 0.20 ps/m for a 500 m sample, the DMD to be measured is 100 ps, necessitating a pulse duration of less than approximately 110 ps Testing at the same DMD limit over a specified length is essential.

Measuring 10,000 meters of fiber requires a DMD of 2,000 ps, and an impulse width of approximately 2,200 ps can be utilized Detailed limits are provided in section 6.1 and may vary based on the spectral width of the source.

The broadening caused by chromatic dispersion due to the source's spectral width must adhere to the limits specified in Annex A The requirement for spectral width can be met by employing either a spectrally narrow source or by utilizing appropriate optical filtering, either at the source or at the detection end.

La longueur d’onde centrale doit être dans les limites de ±10 nm de la longueur d’onde nominale spécifiée

Un laser titane-saphir à mode bloqué constitue un exemple de source utilisable pour cette application.

Stabilité

The devices must be capable of positioning the test specimen's entry and exit ends with sufficient stability and reproducibility to meet the requirements outlined in sections 4.3 and 4.4.

Système d’injection

The fiber probe positioned between the light source and the test sample must propagate only a single mode at the measurement wavelength The mode field diameter of the fiber probe at wavelength λ should be (8.7λ – 2.39) ± 0.5 µm, where λ represents the measurement wavelength in micrometers, and the mode field diameter is determined using specific calculations.

CEI 60793-1-45 Cette équation produit un diamètre de champ de mode de 5 λm à 850 nm et de 9 λm à 1 310 nm, qui correspond aux fibres monomodales disponibles sur le marché

Ensure that the output of the fiber probe is single-mode One method to achieve this is by removing higher-order modes by wrapping the fiber probe around a 25 mm diameter mandrel for three turns.

La tache de sortie de la fibre sonde doit balayer la face d’extrémité de l’échantillon d’essai avec une prộcision de position infộrieure ou ộgale à ±0,5 àm

Le faisceau de sortie de la fibre sonde doit être perpendiculaire à la face d’extrémité de l’échantillon d’essai dans les limites d’une tolérance angulaire inférieure à 1,0 degré

Le système d’injection doit être capable de centrer de manière reproductible la tache de sortie de la fibre sonde dans les limites de ±1,0 àm

LICENSED TO MECON Limited - RANCHI/BANGALORE FOR INTERNAL USE AT THIS LOCATION ONLY, SUPPLIED BY BOOK SUPPLY BUREAU.

Use an optical source that introduces short duration, narrow spectral width pulses into the probe fibre

To accurately measure the intended differential delay time, the optical pulse must have a sufficiently short duration The maximum permissible duration of the optical pulse, defined as the full width at 25% of the maximum amplitude, is contingent upon the value of the differential mode delay (DMD) to be determined and the length of the sample For instance, the desired length-normalized DMD limit will influence this duration.

To achieve a differential mode delay (DMD) of 100 ps over a 500 m fiber sample with a measurement precision of 0.20 ps/m, a pulse duration of less than approximately 110 ps is necessary In contrast, testing the same DMD limit over a longer 10,000 m fiber length necessitates measuring a DMD of 2,000 ps, allowing for the use of a pulse width of around 2,200 ps.

Detailed limits are given in 6.1, and may depend on the source spectral width

Chromatic dispersion-induced broadening due to source spectral width must adhere to the limits specified in Annex A This spectral width requirement can be satisfied by employing a spectrally narrow source or by utilizing suitable optical filtering at the source or detection end.

The centre wavelength shall be within ±10 nm of the nominal specified wavelength

A mode locked titanium-sapphire laser is an example of a source usable for this application

Devices shall be available to position the input and output ends of the test specimen with sufficient stability and reproducibility to meet the conditions of 4.3 and 4.4

The probe fibre connecting the light source to the test sample must support only a single mode at the measurement wavelength The mode field diameter of the probe fibre at the wavelength λ is essential for accurate measurements.

The mode field diameter, calculated using the equation (8.7λ – 2.39) ± 0.5 µm, where λ represents the measurement wavelength in micrometers, is determined according to IEC 60793-1-45 This results in a mode field diameter of 5 µm at 850 nm and 9 µm at 1,310 nm, aligning with commercially available single-mode fibers.

To achieve single-mode output from the probe fibre, one effective technique is to wrap the fibre three times around a 25-mm diameter mandrel, which helps to strip higher order modes.

The output spot of the probe fibre shall be scanned across the endface of the test sample with a positional accuracy less than or equal to ±0,5 àm

The output beam from the probe fibre shall be perpendicular to the endface of the test sample to within an angular tolerance of less than or equal to 1,0 degree

The launch system shall be capable of reproducibly centring the output spot of the probe fibre to within ±1,0 àm

Dans le cas d’un couplage direct avec l’échantillon d’essai, l’espace entre l’extrémité de sortie de la fibre sonde et l’extrộmitộ de l’ộchantillon d’essai ne doit pas ờtre supộrieur à 10 àm

An optical system in free space, composed of lenses or mirrors, can be employed to project the image of the output point of a fiber probe onto the end face of the sample under test When utilizing this type of injection system, it is essential to ensure that the same modes excited in the test fiber closely match those that would have been excited if the beam had been directly coupled to the output of a single-mode fiber probe.

For instance, a system of lenses or mirrors can be employed to project the image of a single-mode fiber's output onto the end face of the test sample.

To eliminate light from the cladding of the sample under test, it is often sufficient to use fiber coating If this is inadequate, mode stripping devices should be employed at both ends of the sample When securing the fiber to the mode stripping devices with small weights, it is crucial to prevent any microbending at these points.

Système de détection

Use an optical detection device suitable for the test wavelength The detection apparatus must couple all guided modes of the sample under test to the active area of the detector, ensuring that the detection sensitivity is not significantly mode-dependent Additionally, the detector and any signal pre-amplifier should respond linearly (within ±5%) across the detected power range.

La réponse temporelle du système détecteur, y compris un affaiblisseur optique facultatif, ne doit pas dépendre du mode de manière significative

A specific essay on mode dependence is provided in section 6.1 Otherwise, the detector's response time can be a function of offset, provided it remains stable during measurement, meaning that the pulse duration (\( \Delta T_{\text{PULSE}}(r) \)) must meet the requirement of ±5%.

The guard ring of the detection system must be restricted so that the maximum positive or negative overshoot is less than 5% of the peak amplitude of the detected optical signal, as measured against the reference.

The detected optical signal waveform must be recorded and displayed on a suitable device, such as a high-speed sampling oscilloscope with calibrated time scanning It is essential for the recording system to average the detected waveform for multiple optical pulses.

Using a delay device, such as a digital delay generator, provides a means to trigger detection electronics at the right moment The delay device can either activate the optical source or be activated by it Additionally, the delay device can be integrated within the recording apparatus or exist as an external component.

The combined effect of synchronization instability and noise in the detection system must be sufficiently low so that the difference between successive measurements of optical propagation time for any fixed injection used in the measurement is less than 5% of the measured value of the DMD Averaging the detected waveform for multiple optical pulses can help reduce the effects of synchronization instability and noise When averaging is employed, each waveform must be recorded using at least the same number of averages as used in determining ∆T PULSE in section 6.1 The system must maintain this level of stability throughout the measurement.

When directly connected to the test sample, the distance between the output end of the probe fiber and the end face of the test sample must not exceed 10 µm.

A free space optics system utilizing lenses or mirrors can effectively image the output spot of the probe fiber onto the end face of the test sample It is crucial to ensure that the same modes are excited in the test fiber as would occur if the beam were directly coupled from the output of the single-mode probe fiber This imaging system allows for accurate coupling by projecting the output of a single-mode fiber onto the test sample's end face.

To effectively eliminate cladding light from the test sample, the fiber coating is typically adequate If additional removal is necessary, cladding mode strippers should be employed at both ends of the sample When using small weights to secure the fiber on the cladding mode strippers, it is crucial to prevent microbending at these locations.

To ensure accurate testing, utilize an optical detection apparatus that is compatible with the test wavelength This apparatus must effectively couple all guided modes from the test sample to the detector's active area, ensuring that detection sensitivity remains consistent across modes Additionally, the detector and any associated signal preamplifier should exhibit a linear response within ±5% across the detected power range.

The temporal response of the detector system, including any optional optical attenuator, shall not be significantly mode dependent

A specific test for mode dependence is outlined in section 6.1 Additionally, the detector's temporal response can vary with offset, provided it remains stable throughout the measurement, ensuring that the requirement of ±5% for ∆T PULSE (r) as stated in section 6.1 is met.

The detector system's ringing must be controlled to ensure that the maximum overshoot or undershoot remains below 5% of the peak amplitude of the detected optical signal, as referenced in measurements.

The detected optical signal's waveform must be recorded and displayed using a high-speed sampling oscilloscope with a calibrated time sweep Additionally, the recording system should have the capability to average the detected waveform across multiple optical pulses.

Utilize a delay device, like a digital delay generator, to accurately time the triggering of detection electronics This device can either trigger the optical source or be activated by it, and it can be integrated within the recording instrument or exist as an external component.

To ensure accurate measurements in the detection system, the combined impact of timing jitter and noise must be minimized, keeping the variation in successive optical delay time measurements for any fixed launch below 5% of the measured DMD value.

Equipement de calcul

Cette méthode d’essai nécessite généralement un ordinateur pour stocker les données intermédiaires et calculer les résultats d’essai

Echantillon d'essai

L’éprouvette doit être une fibre multimodale à cœur en verre à gradient d’indice (catégorie

Faces d’extrémités d'éprouvettes

Préparer les faces terminales pour qu’elles soient planes au niveau des extrémités d'entrée et de sortie de l'éprouvette.

Longueur d'éprouvette

La longueur de la fibre doit être mesurée en utilisant une méthode précise bien appropriée telle que celle de la CEI 60793-1-22.

Emballage de l’éprouvette

Placer la fibre en essai de faỗon que la tension soit dissipộe pour rộduire les microcourbures.

Positionnement de l’éprouvette

Positionner l’extrémité d’entrée de l’échantillon d’essai de manière à ce qu’il soit aligné avec l’extrémité de sortie du système d’injection comme décrit en 4.3

Positionner l’extrémité de sortie de l’échantillon d’essai de manière à ce qu’il soit aligné avec le système de détection comme décrit en 4.4

Régler et mesurer la réponse du système

Connect the output of the fiber probe to the detection device This can be achieved by either mounting the fiber probe directly onto the detection device or by using a short length of fiber.

A short fiber, less than 10 meters, can be installed between the injection system and the detection system, or directly coupled from the probe output to the detector using lenses and mirrors When using a short fiber, it can either be of the same type as the test fiber or a different type.

Adjust the optical pulse amplitude to match the lowest expected peak amplitude of the fiber under test during measurement Typically, the lowest peak amplitude of the fiber will occur at the greatest radial offset.

Adjust the detection system's time scale to match the time scale used for acquiring data from the sample under test, ensuring that the complete impulse is captured (refer to section 6.2).

Measure the waveform of the optical pulse and determine its temporal width at 25% of the peak amplitude This value, referred to as ∆T PULSE, will be used for calculating test results Linear interpolation can be applied between successive time points to enhance the accuracy of ∆T PULSE.

This test method generally requires a computer to store the intermediate data and calculate the test results

The test sample shall be graded-index glass-core (category A1) multimode fibre

Prepare flat endfaces at the input and output ends of the specimen

The length of the fibre shall be measured using a suitably accurate method such as that of

Support the test fibre in a manner that relieves tension and minimizes microbending

Position the input end of the test sample such that it is aligned to the output end of the launch system as described in 4.3

Position the output end of the test sample such that it is aligned with the detection system, as described in 4.4

6.1 Adjust and measure system response

To connect the probe fibre to the detection apparatus, you can either mount the probe directly in the apparatus or use a short fibre (less than 10 meters) that matches the type of the test fibre Alternatively, you can couple the probe output to the detector using a system of lenses and mirrors.

To ensure accurate measurements, adjust the optical pulse amplitude to align with the smallest peak amplitude anticipated from the test fiber, which typically occurs at the largest radial offset.

Adjust the detection system's time scale to align with the data acquisition time scale of the test sample, ensuring complete capture of the entire pulse.

Measure the optical pulse waveform and determine its temporal width at 25% of the peak amplitude This measurement will be utilized to calculate the test results, referred to as the temporal width.

∆T PULSE Linear interpolation may be used between successive time points to calculate

– Des mesures répétées de ∆T PULSE ne doivent pas différer de plus de 5 % de la valeur mesurée du DMD

When using a short fiber length or a system of lenses and mirrors, the values of ∆T PULSE should not differ by more than 5% from the values obtained by directly coupling the probe fiber into the detection device.

To test and verify that the detection device is not significantly affected by the mode, prepare a short-length sample of the same type as the fiber to be tested Measure the value of ∆T PULSE for each radial offset to be used in the measurement, ensuring that this value meets the requirement specified in section 6.1.

Utiliser l’Annexe A pour calculer une valeur de ∆T REF appropriée aux valeurs de ∆T PULSE , de la largeur spectrale de la source et de la dispersion chromatique de la fibre.

Régler le système de détection

Inject light from the probe fiber into the test fiber Adjust the time scale and trigger time of the detection system to ensure that a complete optical pulse is displayed for all relevant probe point offsets, including all leading and trailing edges with an amplitude of at least 1% of the peak amplitude All data from the test fiber must be collected without any additional adjustments to the propagation time and time scale.

To locate the core center of a test fiber, one effective method involves scanning the probe position across the fiber's surface This process includes identifying the two edges of the fiber's core along an arbitrary "x" axis, where the edge is defined as the position at which the total received power reaches approximately 15% of the maximum Once the probe spot is centered along the "x" axis, it is then necessary to scan the probe spot along the orthogonal "y" axis to find the fiber's core edges and center it along the "y" axis as well.

If necessary, repeat the process to achieve the required positional tolerance When the probe spot is centered, the DMD value will be symmetrical between the positive and negative offsets along the "x" or "y" axes Additionally, IEC 61280-1-4 outlines another method for locating the optical center of the fiber (refer to section 5.4 of IEC 61280-1-4).

Mesurer l’échantillon d’essai

Mesurer la réponse de l’échantillon d’essai, U(r,t), pour des décalages radiaux, r, du point de sonde Pour la mesure du DMD, r est compris entre R INNER ≤ r ≤ R OUTER à des intervalles de

R INNER and R OUTER must be specified according to Article 9, Section 3 Depending on the specified values for R INNER and R OUTER, lower intervals of less than 2 am may be required.

Exemple: Si la spộcification demande que R INNER = 0 et R OUTER = 17 àm, le nombre le plus faible de dộcalages radiaux sera ộgal à 10 (0, 2, …, 16, 17) àm ou (0, 1, …, 15,

17) àm satisferait à l'exigence minimale En variante, on pourrait utiliser

For EMBc measurements, scan from the optical center within a radius of 1 µm from the nominal core diameter Additional radial offsets may be applied For EMBc measurements of multimode fibers A1a.2 with a core diameter of 50 µm, measure U(r,t) over the range of 0 ≤ r ≤ 24 µm at intervals of ≤2 µm.

At each radial shift, measure the waveform of the optical pulse and determine the temporal position of the leading and trailing edges at 25% of the maximum amplitude of the resulting waveform (see Appendix B) Linear interpolation can be employed between successive time points to enhance the accuracy of estimating the leading and trailing edges Record the durations of the leading and trailing edges for each radial shift position.

– Repeated measurements of ∆T PULSE shall differ by no more than 5 % of the value of DMD being measured

– If using either a short length of fibre, or a system of lenses and mirrors, the values of

∆T PULSE shall differ by no more than 5 % from the values obtained by coupling the probe fibre directly into the detection apparatus

To ensure that the detector apparatus is not significantly mode dependent, prepare a short-length test sample identical to the fiber being tested Measure the value of ∆T PULSE for each radial offset used in the measurement, ensuring it meets the requirement specified in section 6.1.

Use Annex A to calculate a value of ∆T REF appropriate for the values of ∆T PULSE , source spectral width, and fibre chromatic dispersion

To conduct the experiment, launch light from the probe fibre into the test fibre and adjust the detection system's time scale and trigger delay to display a complete optical pulse for all relevant probe spot offsets This includes capturing all leading and trailing edges with an amplitude of at least 1% of the peak amplitude Ensure that all data from the test fibre is collected without any additional adjustments to the delay and time scale.

To locate the center of the core of a test fiber, one effective method involves scanning the probe spot across the fiber's face Begin by identifying both edges of the fiber core along an arbitrary "x" axis, where the edge is defined as the point at which the total received power reaches approximately 15% of the maximum Center the probe spot along the "x" axis, then proceed to scan along the orthogonal "y" axis to find the fiber core edges and center the probe spot accordingly This process may require iteration to meet the desired positional tolerance When the probe spot is properly centered, the DMD will exhibit symmetry between positive and negative offsets along both axes Additionally, IEC 61280-1-4 outlines an alternative method for determining the optical center of the fiber.

Measure the response of the test sample, U(r,t), for radial offsets, r, of the probe spot For measurement of DMD, r ranges from R INNER ≤ r ≤ R OUTER at intervals of ≤2 àm R INNER and

R OUTER shall be provided in the specification (see item 3 in clause 9) Depending on the values specified for R INNER and R OUTER , intervals less than 2 àm may be required

If the specifications require an inner radius of 0 and an outer radius of 17 àm, the minimum number of radial offsets needed is ten Acceptable offset sequences include (0, 2, …, 16, 17) àm or (0, 1, …, 15, 17) àm Additionally, it is possible to utilize 18 offsets to fulfill the requirements.

For EMBc measurements, scan from the optical centre to within 1 àm of the nominal core radius Additional radial offsets may be used For 50 àm core diameter A1a.2 multimode fibre

EMBc measurements, measure U(r,t) over the range 0 ≤ r ≤ 24 àm at intervals of ≤2 àm

At each radial offset, measure the optical pulse waveform and identify the temporal positions of the leading and trailing edges at 25% of the maximum amplitude Utilize linear interpolation between successive time points to enhance the accuracy of the leading and trailing edge time estimates Document the leading and trailing edge times for every radial offset position.

7 Calculs et interprétation des résultats

The effective modal bandwidth (EMB) minimum of a fiber refers to the minimum bandwidth corresponding to the excitation of emitters under specified injection conditions For instance, the minimum EMB outlined in IEC 60793-2-10 applies to the injection conditions also defined in this standard The minimum EMB is determined by calculating either the differential mode delay (DMD) or the calculated minimum EMB (EMBc) The purpose of these calculations is to ensure that the fiber's EMB exceeds the requirements for any coherent mode power distribution compatible with the defined emitters Compliance of the emitters can be established by adhering to flux requirements, such as those specified in IEC 60793-2-10, measured according to IEC 61280-1-4.

Retard différentiel de mode (DMD)

Trouver T FAST , le minimum des durées de front pour l’excitation entre R INNER et R OUTER à partir des impulsions de sortie enregistrées en 6.3

Trouver T SLOW , le maximum des durées de flanc arrière pour l’excitation entre R INNER et

R OUTER à partir des impulsions de sortie enregistrées en 6.3

En utilisant la valeur de ∆T REF de 6.1, DMD = (T SLOW – T FAST ) – ∆T REF

The lower limit for the DMD using this equation is 0.9(∆T REF) due to practical measurement issues outlined in Appendix B Therefore, if the calculated value for the DMD is less than 0.9(∆T REF), it should be recorded as "less than 0.9(∆T REF)."

The DMD can alternatively be calculated by performing a deconvolution of the reference impulse from the output impulses obtained from the tested fiber To effectively use deconvolution, the algorithm must not introduce significant errors in the impulse shapes encountered during measurement, particularly due to the selection of a high-frequency noise filter.

Une fibre peut être caractérisée par des valeurs DMD multiples avec chaque valeur évaluée pour une plage différente de R INNER et R OUTER Dans ce cas, toutes les valeurs

DMD can be assessed from the output pulses recorded at 6.3, provided that the radial shift requirements of 6.3 are met for both the INNER and OUTER ranges.

Largeur de bande modale efficace minimale calculée

The minimum EMBc is the lowest value of EMBc established for a specific fiber, utilizing the complete set of weights corresponding to a range of mode power distributions, as calculated in sections 7.2.1 to 7.2.4.

The DMD weights correspond to the range of mode power distribution that aligns with the requirements of the optical emitters used in the application, as detailed in the user specification Users can also specify an additional multiplier to align the EMBc with the effective modal bandwidth required for their application A default set of applicable weights, such as those for IEEE 802.3 10GBASE-S and INCITS 364 10GFC, is specified in IEC 60793-2-10 and is included as an example in Annex D of this document Annex C provides a procedure for generating DMD weights from the encircled flux data.

7 Calculations and interpretation of results

The minimum effective modal bandwidth (EMB) of a fiber refers to the lowest bandwidth achievable when excited by transmitters that meet specific launch conditions According to IEC 60793-2-10, the minimum EMB is defined in relation to these specified launch conditions This value is calculated based on certain criteria outlined in the standard.

DMD, or the minimum calculated EMB (EMBc), is crucial for ensuring that the EMB of the fiber surpasses the requirements for any mode power distribution compatible with conforming transmitters The conformance of these transmitters can be defined by encircled flux requirements, as specified in IEC 60793-2-10 and measured according to IEC 61280-1-4.

Find T FAST , the minimum of the leading edge times for excitation between R INNER and

R OUTER from among the output pulses recorded in 6.3

Find T SLOW , the maximum of the trailing edge times for excitation between R INNER and

Using the value of ∆T REF from 6.1, DMD = (T SLOW – T FAST ) – ∆T REF

The minimum reporting limit for DMD, as determined by the equation, is 0.9(∆T REF) due to practical measurement challenges outlined in Annex B Therefore, if the calculated DMD value falls below 0.9(∆T REF), it should be reported as "less than 0.9(∆T REF)."

DMD can be calculated by deconvolving the reference pulse from the pulses collected from the test fiber It is crucial that the deconvolution algorithm minimizes significant errors in the pulse shapes during measurement, particularly those caused by the selection of a high-frequency noise filter.

A fiber can be defined by various DMD values, each assessed over distinct ranges of R INNER and R OUTER All DMD values can be derived from the output pulses recorded in 6.3, as long as the radial offset criteria of 6.3 are satisfied for each range of R INNER and R OUTER.

7.2 Minimum calculated effective modal bandwidth

The minimum EMBc represents the lowest value of EMBc for a specific fiber, calculated using the complete set of weightings that correspond to various mode power distributions, as outlined in sections 7.2.1 to 7.2.4.

The DMD weightings reflect the mode power distributions that align with the launch condition specifications of the optical transmitters used in the application These weightings are defined by the user's detailed specifications, which may also include an additional multiplier to adjust the effective modal bandwidth (EMBc) to meet the theoretical requirements of the application A default set of weightings is available, such as those applicable to IEEE 802.3 10GBASE.

The S and INCITS 364 10GFC specifications are outlined in IEC 60793-2-10 and are exemplified in Annex D of the document Additionally, Annex C details a method for deriving DMD weights from encircled flux data.

The following calculations pertain to the use of weighting functions derived from near-field flux data of laser sources, which are characteristic of various applications For a given fiber, applying multiple weighting functions will yield several EMBc values, with the minimum representing the lowest EMBc for that fiber.

When DMD data is collected at shifts separated by 2 to m, the values U(r,t) at shifts from 1 to m can be interpolated within these calculations.

Calculer une réponse temporelle de sortie qui en résulte, P o (t) en utilisant les informations d’impulsion de sortie de fibre et une fonction de pondération

U is the sampled output pulse measured at each radial offset r as a function of the measured time t Each output pulse is raw (not amplitude-normalized) after appropriate subtraction of baseline noise.

The weighting function DMD corresponds to the transmitter used in the application For details on calculating W(r), refer to Appendix C, and for examples of W(r) values under specific injection conditions, see Appendix D.

7.2.2 Calculer la fonction de transfert

To perform a deconvolution of the reference temporal response, R(t), from the resulting output response, P_o(t), similar to the bandwidth measurements outlined in IEC 60793-1-41, one can derive the frequency response of the fiber, H_Fib(f), also known as the fiber transfer function.

P o (t) est l’impulsion de sortie résultant de 7.2.1;

R(t) est l’impulsion de référence résultant de 6.1;

FT est la fonction de transformée de Fourier

NOTE Ces calculs produisent un ensemble de nombres complexes

7.2.3 Largeur de bande modale efficace calculée (EMBc)

Normalisation de la longueur

Normalizing the value of DMD or EMBc to a unit length, such as ps/m or MHz⋅km, can be beneficial When normalization is applied to a unit length, it is essential to document the formula related to the length dependency.

Consigner les informations suivantes pour chaque essai

– formule de normalisation de la longueur, si utilisée;

– longueur d’onde source (nominale ou réelle);

– décalages radiaux minimaux et maximaux, R INNER , R OUTER ;

– résultat d’essai: DMD (R INNER , R OUTER ) et/ou EMBc minimale.

Optical source

Use an optical source that introduces short duration, narrow spectral width pulses into the probe fibre

To accurately measure the differential delay time, the optical pulse must have a sufficiently short duration The maximum permissible duration of the optical pulse, defined as the full width at 25% of the maximum amplitude, is influenced by both the value of the differential mode delay (DMD) to be determined and the length of the sample For instance, the desired length-normalized DMD limit will dictate these parameters.

To measure a differential mode delay (DMD) of 100 ps over a 500 m fiber sample with a precision of 0.20 ps/m, a pulse duration of less than approximately 110 ps is necessary In contrast, testing the same DMD limit over a 10,000 m length of fiber necessitates measuring a DMD of 2,000 ps, allowing for the use of a pulse width of around 2,200 ps.

Detailed limits are given in 6.1, and may depend on the source spectral width

Chromatic dispersion-induced broadening due to source spectral width must adhere to the limits specified in Annex A This spectral width requirement can be satisfied by employing a spectrally narrow source or by implementing suitable optical filtering at the source or detection end.

The centre wavelength shall be within ±10 nm of the nominal specified wavelength

A mode locked titanium-sapphire laser is an example of a source usable for this application.

Stability

Devices shall be available to position the input and output ends of the test specimen with sufficient stability and reproducibility to meet the conditions of 4.3 and 4.4.

Launch system

The probe fibre connecting the light source to the test sample must support only a single mode at the measurement wavelength At this wavelength, the mode field diameter of the probe fibre is defined as λ.

The mode field diameter, calculated using the equation (8.7λ – 2.39) ± 0.5 µm, varies with the measurement wavelength λ in micrometers According to IEC 60793-1-45, this results in a mode field diameter of 5 µm at 850 nm and 9 µm at 1,310 nm, aligning with commercially available single-mode fibers.

To achieve single-mode output from the probe fibre, one effective technique is to wrap the fibre three times around a 25-mm diameter mandrel, which helps to eliminate higher order modes.

The output spot of the probe fibre shall be scanned across the endface of the test sample with a positional accuracy less than or equal to ±0,5 àm

The output beam from the probe fibre shall be perpendicular to the endface of the test sample to within an angular tolerance of less than or equal to 1,0 degree

The launch system shall be capable of reproducibly centring the output spot of the probe fibre to within ±1,0 àm

Dans le cas d’un couplage direct avec l’échantillon d’essai, l’espace entre l’extrémité de sortie de la fibre sonde et l’extrộmitộ de l’ộchantillon d’essai ne doit pas ờtre supộrieur à 10 àm

An optical system in free space, composed of lenses or mirrors, can be employed to project the image of the output point of a fiber probe onto the end face of the sample under test When utilizing this type of injection system, it is essential to ensure that the same modes excited in the test fiber closely match those that would have been excited if the beam had been directly coupled from the single-mode fiber probe output.

For instance, a system of lenses or mirrors can be employed to project the image of a single-mode fiber's output onto the end face of the test sample.

To eliminate light from the cladding of the sample under test, a fiber coating is often sufficient If not, mode field extractors should be used at both ends of the sample When securing the fiber to the mode field extractors with small weights, it is crucial to avoid any microbending at these points.

Use an optical detection device suitable for the test wavelength The detection apparatus must couple all guided modes of the sample under test to the active area of the detector, ensuring that the detection sensitivity is not significantly mode-dependent Additionally, the detector and any signal pre-amplifier should respond linearly (within ±5%) across the detected power range.

La réponse temporelle du système détecteur, y compris un affaiblisseur optique facultatif, ne doit pas dépendre du mode de manière significative

A specific essay on mode dependence is provided in section 6.1 Otherwise, the detector's response time can be a function of offset, provided it remains stable during measurement, meaning that the pulse duration (\( \Delta T_{\text{PULSE}}(r) \)) must meet the requirement of ±5%.

The guard ring of the detection system must be restricted so that the maximum positive or negative overshoot is less than 5% of the peak amplitude of the detected optical signal as measured against the reference.

The waveform of the detected optical signal must be recorded and displayed on a suitable device, such as a high-speed sampling oscilloscope with calibrated time scanning It is essential for the recording system to average the detected waveform for multiple optical pulses.

Using a delay device, such as a digital delay generator, allows for the precise triggering of detection electronics at the right moment This delay device can either activate the optical source or be activated by it Additionally, the delay device can be integrated within the recording apparatus or exist as an external component.

The combined effect of synchronization instability and noise in the detection system must be sufficiently low so that the difference between successive measurements of optical propagation time for any fixed injection used in the measurement is less than 5% of the measured value of the DMD Averaging the detected waveform for multiple optical pulses can help mitigate the effects of synchronization instability and noise When averaging is employed, each waveform must be recorded using at least the same number of averages as used in determining ∆T PULSE in section 6.1 The system must maintain this level of stability throughout the measurement process.

When directly connected to the test sample, the distance between the output end of the probe fiber and the end face of the test sample must not exceed 10 µm.

A free space optics system utilizing lenses or mirrors can effectively image the output spot of a probe fiber onto the end face of a test sample It is crucial to ensure that the same modes are excited in the test fiber as would occur if the beam were directly coupled from the output of the single-mode probe fiber This imaging system allows for accurate coupling by projecting the output of a single-mode fiber onto the test sample's end face.

To effectively eliminate cladding light from the test sample, the fiber coating is typically adequate If additional removal is necessary, cladding mode strippers should be employed at both ends of the sample When using small weights to secure the fiber on the cladding mode strippers, it is crucial to prevent microbending at these locations.

Detection system

To ensure accurate testing, utilize an optical detection apparatus that is compatible with the test wavelength This apparatus must effectively couple all guided modes from the test sample to the detector's active area, ensuring that detection sensitivity remains largely independent of mode variations Additionally, the detector and any associated signal preamplifier should exhibit a linear response within ±5% across the detected power range.

The temporal response of the detector system, including any optional optical attenuator, shall not be significantly mode dependent

A specific test for mode dependence is outlined in section 6.1 Additionally, the temporal response of the detector can vary with offset, provided it remains stable throughout the measurement, ensuring that the requirement of ±5% for ∆T PULSE (r) as stated in section 6.1 is met.

The detector system's ringing must be controlled to ensure that the maximum overshoot or undershoot remains below 5% of the peak amplitude of the detected optical signal, as referenced in measurements.

The detected optical signal's waveform must be recorded and displayed using a high-speed sampling oscilloscope with a calibrated time sweep Additionally, the recording system should have the capability to average the detected waveform across multiple optical pulses.

Utilize a delay device, like a digital delay generator, to accurately time the triggering of detection electronics This device can either activate the optical source or be activated by it, and it can be integrated within the recording instrument or exist as an external component.

To ensure accurate measurements in the detection system, the combined impact of timing jitter and noise must be minimized Specifically, the variation in optical delay times between consecutive measurements for any fixed launch should remain below 5% of the measured value of DMD.

Averaging multiple detected waveforms from optical pulses can effectively minimize timing jitter and noise When employing averaging, it is essential to record each waveform with a minimum number of averages corresponding to the determination of ∆T PULSE as specified in section 6.1 The system must ensure consistent stability throughout the measurement process.

Cette méthode d’essai nécessite généralement un ordinateur pour stocker les données intermédiaires et calculer les résultats d’essai

L’éprouvette doit être une fibre multimodale à cœur en verre à gradient d’indice (catégorie

Préparer les faces terminales pour qu’elles soient planes au niveau des extrémités d'entrée et de sortie de l'éprouvette

La longueur de la fibre doit être mesurée en utilisant une méthode précise bien appropriée telle que celle de la CEI 60793-1-22

Placer la fibre en essai de faỗon que la tension soit dissipộe pour rộduire les microcourbures

Positionner l’extrémité d’entrée de l’échantillon d’essai de manière à ce qu’il soit aligné avec l’extrémité de sortie du système d’injection comme décrit en 4.3

Positionner l’extrémité de sortie de l’échantillon d’essai de manière à ce qu’il soit aligné avec le système de détection comme décrit en 4.4

6.1 Régler et mesurer la réponse du système

Connect the output of the fiber probe to the detection device This can be achieved by either mounting the fiber probe directly onto the detection device or by using a short length of fiber.

A fiber optic cable shorter than 10 meters can be installed between the injection system and the detection system, or it can connect the probe output directly to the detector using a system of lenses and mirrors When utilizing a short fiber, it can either be of the same type as the test fiber or a different type altogether.

Adjust the optical pulse amplitude to match the lowest expected peak amplitude of the fiber under test during measurement Typically, the lowest peak amplitude of the fiber will occur at the greatest radial offset.

Adjust the detection system's time scale to match the time scale used for acquiring data from the sample under test, ensuring that the complete impulse is captured (refer to section 6.2).

Measure the waveform of the optical pulse and determine its temporal width at 25% of the peak amplitude This value, referred to as ∆T PULSE, will be used for calculating test results Linear interpolation can be applied between successive time points to enhance the accuracy of ∆T PULSE.

Computational equipment

This test method generally requires a computer to store the intermediate data and calculate the test results

Test sample

The test sample shall be graded-index glass-core (category A1) multimode fibre.

Specimen endfaces

Prepare flat endfaces at the input and output ends of the specimen.

Specimen length

The length of the fibre shall be measured using a suitably accurate method such as that of

Specimen packaging

Support the test fibre in a manner that relieves tension and minimizes microbending.

Specimen positioning

Position the input end of the test sample such that it is aligned to the output end of the launch system as described in 4.3

Position the output end of the test sample such that it is aligned with the detection system, as described in 4.4

Adjust and measure system response

To connect the probe fibre to the detection apparatus, you can either mount the probe directly within the apparatus, use a short fibre (less than 10 meters) between the launch and detection systems, or couple the probe output to the detector using lenses and mirrors If opting for a short fibre, ensure it matches the type of the test fibre.

To ensure accurate measurements, adjust the optical pulse amplitude to align with the minimum peak amplitude anticipated from the test fiber, which typically occurs at the maximum radial offset.

Adjust the detection system's time scale to align with the data acquisition time scale of the test sample, ensuring complete capture of the entire pulse.

Measure the optical pulse waveform and determine its temporal width at 25% of the peak amplitude This measurement will be utilized to calculate the test results, referred to as the temporal width.

∆T PULSE Linear interpolation may be used between successive time points to calculate

– Des mesures répétées de ∆T PULSE ne doivent pas différer de plus de 5 % de la valeur mesurée du DMD

When using a short fiber length or a system of lenses and mirrors, the values of ∆T PULSE should not differ by more than 5% from those obtained by directly coupling the probe fiber into the detection device.

To test and verify that the detection device is not significantly affected by the mode, prepare a short-length sample of the same type as the fiber to be tested Measure the value of ∆T PULSE for each radial offset to be used in the measurement, ensuring that this value meets the requirement of 6.1.

Utiliser l’Annexe A pour calculer une valeur de ∆T REF appropriée aux valeurs de ∆T PULSE , de la largeur spectrale de la source et de la dispersion chromatique de la fibre

6.2 Régler le système de détection

Inject light from the probe fiber into the test fiber Adjust the time scale and trigger time of the detection system to ensure that a complete optical pulse is displayed for all relevant probe point offsets, including all leading and trailing edges with an amplitude of at least 1% of the peak amplitude All data from the test fiber must be collected without any additional adjustments to the propagation time and time scale.

To locate the core center of the fiber under test, one effective method involves scanning the probe position across the fiber's surface This process includes identifying the two edges of the fiber's core along an arbitrary "x" axis, where the edge is defined as the position at which the total received power reaches approximately 15% of the maximum Once the probe spot is centered along the "x" axis, it is then necessary to scan the probe spot along the orthogonal "y" axis to determine the core edges and achieve centering along the "y" axis as well.

If necessary, repeat the process to achieve the required positional tolerance When the probe spot is centered, the DMD value will be symmetrical between the positive and negative offsets along the "x" or "y" axes Additionally, IEC 61280-1-4 outlines another method for determining the optical center of the fiber (refer to section 5.4 of IEC 61280-1-4).

Mesurer la réponse de l’échantillon d’essai, U(r,t), pour des décalages radiaux, r, du point de sonde Pour la mesure du DMD, r est compris entre R INNER ≤ r ≤ R OUTER à des intervalles de

R INNER and R OUTER must be specified according to Article 9, Section 3 Depending on the specified values for R INNER and R OUTER, lower intervals of less than 2 am may be required.

Exemple: Si la spộcification demande que R INNER = 0 et R OUTER = 17 àm, le nombre le plus faible de dộcalages radiaux sera ộgal à 10 (0, 2, …, 16, 17) àm ou (0, 1, …, 15,

17) àm satisferait à l'exigence minimale En variante, on pourrait utiliser

For EMBc measurements, scan from the optical center within a radius of 1 µm from the nominal core radius Additional radial offsets may be applied For EMBc measurements of multimode fibers A1a.2 with a core diameter of 50 µm, measure U(r,t) over the range of 0 ≤ r ≤ 24 µm at intervals of ≤2 µm.

At each radial shift, measure the waveform of the optical pulse and determine the temporal position of the leading and trailing edges at 25% of the maximum amplitude of the resulting waveform (see Appendix B) Linear interpolation can be employed between successive time points to enhance the accuracy of estimating the leading and trailing edges Record the durations of the leading and trailing edges for each radial shift position.

– Repeated measurements of ∆T PULSE shall differ by no more than 5 % of the value of DMD being measured

– If using either a short length of fibre, or a system of lenses and mirrors, the values of

∆T PULSE shall differ by no more than 5 % from the values obtained by coupling the probe fibre directly into the detection apparatus

To ensure that the detector apparatus is not significantly mode dependent, prepare a short-length test sample identical to the fiber being tested Measure the value of ∆T PULSE for each radial offset used in the measurement, ensuring it meets the requirement specified in section 6.1.

Use Annex A to calculate a value of ∆T REF appropriate for the values of ∆T PULSE , source spectral width, and fibre chromatic dispersion.

Adjust detection system

To conduct the experiment, launch light from the probe fibre into the test fibre and adjust the detection system's time scale and trigger delay to display a complete optical pulse for all relevant probe spot offsets This includes capturing all leading and trailing edges with an amplitude of at least 1% of the peak amplitude Ensure that all data from the test fibre is collected without any additional adjustments to the delay and time scale.

To locate the center of the core of a test fiber, one effective method involves scanning the probe spot across the fiber's face Begin by identifying both edges of the fiber core along an arbitrary "x" axis, where the edge is defined as the position at which the total received power reaches approximately 15% of the maximum Center the probe spot along the "x" axis, then proceed to scan along the orthogonal "y" axis to find the fiber core edges and center the probe spot accordingly This process may require iteration to meet the desired positional tolerance When the probe spot is properly centered, the DMD will exhibit symmetry between positive and negative offsets along both axes Additionally, IEC 61280-1-4 outlines an alternative method for determining the optical center of the fiber.

Measure the test sample

Measure the response of the test sample, U(r,t), for radial offsets, r, of the probe spot For measurement of DMD, r ranges from R INNER ≤ r ≤ R OUTER at intervals of ≤2 àm R INNER and

R OUTER shall be provided in the specification (see item 3 in clause 9) Depending on the values specified for R INNER and R OUTER , intervals less than 2 àm may be required

If the specifications require an inner radius of 0 and an outer radius of 17 àm, the minimum number of radial offsets needed is ten This can be achieved with offsets of either (0, 2, …, 16, 17) àm or (0, 1, …, 15, 17) àm Additionally, it is possible to utilize 18 offsets to meet the requirements.

For EMBc measurements, scan from the optical centre to within 1 àm of the nominal core radius Additional radial offsets may be used For 50 àm core diameter A1a.2 multimode fibre

EMBc measurements, measure U(r,t) over the range 0 ≤ r ≤ 24 àm at intervals of ≤2 àm

At each radial offset, measure the optical pulse waveform and identify the temporal positions of the leading and trailing edges at 25% of the maximum amplitude Utilize linear interpolation between successive time points to enhance the accuracy of the leading and trailing edge time estimates Document the leading and trailing edge times for every radial offset position.

7 Calculs et interprétation des résultats

The effective modal bandwidth (EMB) minimum of a fiber refers to the minimum bandwidth corresponding to the excitation of emitters under specified injection conditions For instance, the minimum EMB outlined in IEC 60793-2-10 applies to the injection conditions also defined in this standard The minimum EMB is determined by calculating either the differential mode delay (DMD) or the calculated minimum EMB (EMBc) The purpose of these calculations is to ensure that the fiber's EMB exceeds the requirements for any coherent mode power distribution compatible with the defined emitters Compliance of the emitters can be established by adhering to flux requirements, such as those specified in IEC 60793-2-10 and measured by IEC 61280-1-4.

7.1 Retard différentiel de mode (DMD)

Trouver T FAST , le minimum des durées de front pour l’excitation entre R INNER et R OUTER à partir des impulsions de sortie enregistrées en 6.3

Trouver T SLOW , le maximum des durées de flanc arrière pour l’excitation entre R INNER et

R OUTER à partir des impulsions de sortie enregistrées en 6.3

En utilisant la valeur de ∆T REF de 6.1, DMD = (T SLOW – T FAST ) – ∆T REF

The lower limit for the DMD using this equation is 0.9(∆T REF) due to the practical measurement issues outlined in Appendix B Therefore, if the calculated value for the DMD is less than 0.9(∆T REF), record the result as "less than 0.9(∆T REF)."

The DMD can alternatively be calculated by performing a deconvolution of the reference impulse from the output impulses obtained from the tested fiber To effectively use deconvolution, the algorithm must not introduce significant errors in the impulse shapes encountered during measurement, particularly due to the selection of a high-frequency noise filter.

Une fibre peut être caractérisée par des valeurs DMD multiples avec chaque valeur évaluée pour une plage différente de R INNER et R OUTER Dans ce cas, toutes les valeurs

DMD can be assessed from the output pulses recorded at 6.3, provided that the radial offset requirements of 6.3 are met for each range of R INNER and R OUTER.

7.2 Largeur de bande modale efficace minimale calculée

The minimum EMBc is the lowest value of EMBc established for a specific fiber, utilizing the complete set of weights corresponding to a range of mode power distributions, as calculated in sections 7.2.1 to 7.2.4.

The DMD weights correspond to the range of mode power distribution consistent with the requirements of the encircled flux of optical emitters used in the application, as detailed in the user specification Users can also specify an additional multiplier to align the EMBc with the effective modal bandwidth required by the application A default set of applicable weights, such as those for IEEE 802.3 10GBASE-S and INCITS 364 10GFC, is specified in IEC 60793-2-10 and is also included as an example in Annex D of this document Annex C provides a procedure for generating DMD weights from encircled flux data.

7 Calculations and interpretation of results

The minimum effective modal bandwidth (EMB) of a fiber refers to the least bandwidth associated with excitation from transmitters that meet specific launch conditions According to IEC 60793-2-10, the minimum EMB is defined in relation to these specified launch conditions This value is calculated based on certain criteria outlined in the standard.

DMD, or the minimum calculated EMB (EMBc), is crucial for ensuring that the EMB of the fiber surpasses the necessary requirements for any mode power distribution that aligns with conforming transmitters The conformance of these transmitters can be defined by encircled flux requirements, as specified in IEC 60793-2-10, and measured according to IEC 61280-1-4.

Differential mode delay (DMD)

Find T FAST , the minimum of the leading edge times for excitation between R INNER and

Find T SLOW , the maximum of the trailing edge times for excitation between R INNER and

Using the value of ∆T REF from 6.1, DMD = (T SLOW – T FAST ) – ∆T REF

The minimum reporting limit for DMD, as determined by the equation, is 0.9(∆T REF) due to practical measurement challenges outlined in Annex B Therefore, if the calculated DMD value falls below 0.9(∆T REF), it should be reported as "less than 0.9(∆T REF)."

DMD can be calculated by deconvolving the reference pulse from the pulses collected from the test fiber It is crucial that the deconvolution algorithm minimizes significant errors in the pulse shapes during measurement, particularly those caused by the selection of a high-frequency noise filter.

A fiber can be defined by various DMD values, each assessed for distinct ranges of R INNER and R OUTER All DMD values can be derived from the output pulses recorded in section 6.3, as long as the radial offset criteria of 6.3 are satisfied for each range of R INNER and R OUTER.

Tiêu đề	Differential Mode Delay
Trường học	International Electrotechnical Commission (IEC)
Chuyên ngành	Optical Fibres
Thể loại	International Standard
Năm xuất bản	2006

Định dạng
Số trang	66
Dung lượng	683,16 KB