IEC 61290 4 2 Edition 1 0 2011 07 INTERNATIONAL STANDARD NORME INTERNATIONALE Optical amplifiers – Test methods – Part 4 2 Gain transient parameters – Broadband source method Amplificateurs optiques –[.]
General
When the input power to an OFA operating in saturation changes sharply, the gain of the amplifier will typically exhibit a transient response before settling back into the required gain
This response is dictated both by the optical characteristics of the active fibre within the OFA, as well as the performance of the automatic gain control (AGC) mechanism
Changes in input power often happen when certain DWDM channels within the designated transmission band are either added or dropped This article provides definitions that characterize the dynamic events resulting in a transient response, including the definitions of rise and fall times.
Channel Time addition end of change 10 %
Input power to EDFA (linear a.u.)
Channel removal start Channel removal end of change10 %
100 % of change of change90 % timeFall
Input power to EDFA (linear a.u.)
Figure 1 – Definitions of rise and fall times for (a) a channel addition event, and (b) a channel removal event
The parameters generally used to characterize the transient gain behaviour of a gain controlled
The Operational Frequency Analysis (OFA) for channel addition and removal is illustrated in Figure 2 Specifically, Figure 2(a) depicts the time-dependent gain of a surviving channel during the removal of other channels, while Figure 2(b) shows the transient gain behavior of an existing channel when additional channels are introduced Key transient parameters include the transient gain.
The IEC 1583/11 standard addresses key parameters such as response time constant, gain offset, transient net gain overshoot, and transient net gain undershoot These transient gain fluctuations are crucial for carriers and network equipment manufacturers (NEMs) because they can amplify through multiple cascaded amplifiers in the network Therefore, it is essential for optical amplifiers to be designed with minimal values for these transient parameters to ensure optimal performance.
Transient gain response time constant (settling time)
Transient gain response time constant (settling time) overshootGain
Figure 2 – OFA transient gain response for (a) a channel removal event, and (b) a channel addition event
Terms and definitions
For the purposes of this document, the following terms, definitions and abbreviations apply
3.2.1 surviving (pre-existing) signal optical signal that remains (exists) after (before) a drop (add) event
3.2.2 saturating signal optical signal that is switched off (on), thus triggering the drop (add) event
3.2.3 drop (add) level (dB) amount in dB by which the input power decreases (increases) due to dropping (adding) of channels
The rise time refers to the duration required for the input power to increase from 10% to 90% of the total difference between the initial and final input power levels during an add event, as illustrated in Figure 1a.
The drop fall time refers to the duration required for the input power to decrease from 10% to 90% of the total difference between the initial and final input power levels during a drop event.
3.2.6 initial gain gain of the surviving (pre-existing) channel before a drop (add) event
3.2.7 final gain steady state gain of the surviving (pre-existing) channel a very long time (i.e once the gain has stabilized) after a drop (add) event
3.2.8 gain offset change in dB of the gain between initial and final state, defined as final gain – initial gain
NOTE Gain offset may be positive or negative for both channel addition and removal events
3.2.9 gain stability specified peak-to-peak gain fluctuations of the OFA under steady state conditions (i.e not in response to a transient event)
3.2.10 transient gain response time constant (settling time) amount of time required to bring the gain of the surviving (pre-existing) channel to the final gain
This parameter measures the duration from the onset of the drop (add) event that triggers the transient gain response to the moment when the surviving (pre-existing) channel gain first falls within the stability band centered around the final gain.
NOTE 2 Hereon this will also be referred to as settling time
3.2.11 transient gain overshoot difference in dB between the maximum surviving (pre-existing) channel gain reached during the
OFA transient response to a drop (add) event, and the lowest of either the initial gain and final gain
NOTE Hereon this will also be referred to as gain overshoot
3.2.12 transient net gain overshoot difference in dB between the maximum surviving (pre-existing) channel gain reached during the
OFA transient response to a drop (add) event, and the highest of either the initial gain and final gain
The transient net gain overshoot is defined as the transient gain overshoot minus the gain offset This measurement reflects the actual transient response, excluding the effects of the amplifier's shift from its initial steady state to the final steady state.
NOTE 2 Hereon this will also be referred to as net gain overshoot
3.2.13 transient gain undershoot difference in dB between the minimum surviving (pre-existing) channel gain reached during the
OFA transient response to a drop (add) event, and the highest of either the initial gain and final gain
NOTE Hereon this will also be referred to as gain undershoot
3.2.14 transient net gain undershoot difference in dB between the minimum surviving (pre-existing) channel gain reached during the
OFA transient response to a drop (add) event, and the lowest of either the initial gain and final gain
The transient net gain undershoot is defined as the transient gain undershoot minus the gain offset, reflecting the true transient response independent of the amplifier's shift from its initial to final steady state conditions.
NOTE 2 Hereon this will also be referred to as net gain undershoot.
Abbreviated terms
DWDM dense wavelength division multiplexing
EDFA erbium-doped fibre amplifier
Figure 3 shows a generic setup to characterize the transient response properties of OFAs using the broadband source method
Figure 3 – Transient measurement test set-up for broadband source method
The test equipment listed below, with the required characteristics, is needed a) A laser source for supplying the surviving signal, with the following characteristics
The ability to accommodate various signal wavelengths is crucial for testing the Optical Fiber Amplifier (OFA) This can be achieved using a tunable laser or a collection of distributed feedback (DFB) lasers.
To ensure optimal performance of the optical fiber amplifier (OFA) under test, it is essential to maintain an achievable average output power that exceeds the maximum specified input power Additionally, a broadband source is required to provide the saturating signal, which must possess specific characteristics to meet the operational needs of the OFA.
1) At least 95 % of the output power should be contained within the specified transmission band of the OFA under test
2) A variation of not more than 1dB peak-to-peak of the power level across the specified transmission band of the OFA under test
To ensure effective testing of the optical fiber amplifier (OFA), it is crucial to achieve an output power that exceeds the maximum specified input power of the OFA under evaluation Additionally, a variable optical attenuator (VOA1) must be utilized, featuring a dynamic range adequate to accommodate the necessary range of signal levels for the OFA testing process.
If the laser source's output power can be adjusted within the necessary dynamic range, the use of VOA1 may be unnecessary Conversely, VOA2 must possess a dynamic range adequate to accommodate the required saturating signal powers, which are determined by the combined levels of surviving signals and drop levels during the testing of the optical fiber amplifier (OFA).
NOTE 2 If the output power of the broadband source can be varied over the required dynamic range, then
VOA2 may not be needed e) Block filter – A filter designed to block the broadband signal in the vicinity of the surviving signal wavelength, with the following characteristics
The ability to accommodate various signal wavelengths is crucial for testing the Optical Filter Array (OFA) This can be achieved through the use of a tunable filter or a set of discrete filters, ensuring comprehensive evaluation across the desired wavelength range.
2) Uniform insertion loss to within 0,5 dB over the entire specified transmission band of the OFA under test except in a range of ±125 GHz of the surviving signal wavelength
The optical modulator must achieve an attenuation of at least 15 dB over the uniform insertion loss within a ±75 GHz range of the surviving signal wavelength, effectively switching the saturating signal "on" and "off."
Broadband source VOA2 Optical modulator VOA1
Optical coupler OFA under test
1) Extinction ratio at least 5 dB higher than the maximum drop level for which the OFA under test is to be tested
To ensure optimal performance, the switching time must be sufficiently rapid to accommodate the fastest drop time for the optical fiber amplifier (OFA) under evaluation Additionally, the chosen optical coupler must meet the specified requirements for effective signal transmission Furthermore, a pass filter is essential, designed specifically to allow only the surviving signal wavelength to pass through, adhering to the necessary characteristics for accurate signal processing.
The ability to accommodate various signal wavelengths is crucial for testing the Optical Filter Array (OFA) This can be achieved through the use of a tunable filter or a selection of discrete filters, ensuring comprehensive testing capabilities.
2) 1-dB passband of at least ±20 GHz centered around the surviving signal wavelength
The OFA under test must achieve a minimum attenuation level of 20 dB below the minimum insertion loss across the specified transmission band, excluding a ±100 GHz range centered on the surviving signal wavelength Additionally, a detector is required to accurately detect the filtered output of the OFA, adhering to specific characteristics.
1) A sufficiently wide bandwidth to support the fastest drop time for which the OFA is to be tested
The optical fiber amplifier (OFA) under test should exhibit a linear response within a ±5 dB range across all surviving signal levels An oscilloscope is required to measure the transient response of the filtered output, ensuring it has a wide enough bandwidth to accommodate the fastest drop time of the OFA Additionally, a pulse generator is necessary to create the "on"-"off" signal for the optical modulator, with a pulse width short enough to support the fastest drop time for the OFA being tested.
The OFA must function under nominal operating conditions To prevent unwanted reflections that could lead to laser oscillations, it is essential to use optical isolators to bracket the OFA being tested This approach will help reduce signal instability and enhance measurement accuracy.
The input signal power to the optical fiber amplifier (OFA) under test consists of a discrete wavelength that represents the surviving signal, along with a broadband source that simulates a saturating signal The power levels of both sources can be adjusted accordingly.
To simulate the add and drop events for testing, the VOA must achieve the required power ratio at the input to the OFA A block filter is used to ensure that the radiation near the surviving signal wavelength remains significantly below the signal level Additionally, the broadband source is activated and deactivated by a pulse generator controlling the optical modulator, effectively simulating the desired events.
The output of the Optical Fiber Amplifier (OFA) is filtered to detect only the surviving signal power, ensuring that radiation from the broadband source near the surviving signal wavelength is excluded By using a pulse generator as a trigger, the oscilloscope is set up to display the transient response of the surviving signal's power following both drop and add events.
Measurement of the various transient response parameters from the oscilloscope display is described in detail in IEC 61290-4-1:–
To conduct a single transient measurement, first, adjust the laser source's wavelength along with the block and pass filters to match the signal wavelength being tested Next, configure the optical fiber amplifier (OFA) to the necessary operating gain for the measurement, which can be determined using its internal calibrated gain measurement function.
OFA, or directly according to one of the following standards: IEC 61290-1-1, IEC 61290-1-2,
Généralités
When the input power for a saturated fiber amplifier (AFO) changes abruptly, the amplifier's gain typically exhibits a transient response before stabilizing at the desired level This response is influenced by the optical characteristics of the active fiber in the AFO and the effectiveness of the automatic gain control (AGC) mechanism.
Automatic Gain Control (AGC) is essential for managing variations in input power, which often occur when channels in the specified DWDM transmission band are added or removed Definitions are established to describe the dynamic events that lead to transient responses, with specific representations for rise and fall times.
Début de l’ajout d’un canal
Temps Fin de l’ajout d’un canal
90 % variation de Puissance d’entrée pour l’EDFA (linéaire a.u.)
Début de suppression d’un canal
Fin de suppression d’un canal
Puissance d’entrée pour l’EDFA (linéaire a.u.)
Figure 1 – Définitions des temps de montée et de descente pour (a) un événement d’ajout de canal, et (b) un événement de suppression de canal
The parameters used to characterize the transient gain behavior of a gain-controlled AFO in the context of channel addition or removal are illustrated in Figure 2 Specifically, Figure 2(a) shows the time-dependent gain of one of the retained channels when others are removed, while Figure 2(b) depicts the transient gain behavior of a pre-existing channel when channels are added The key transient parameters include the transient gain response time constant (adaptation time), gain offset, net positive transient gain overshoot, and net negative transient gain overshoot Both positive and negative overshoots of the transient gain are particularly critical for carriers and network equipment manufacturers.
NEM 2 highlights that the speed and amplitude of gain fluctuations combine as the optical signal traverses an increasing number of cascaded amplifiers Properly designed optical amplifiers exhibit very low values for these transient parameters.
Constante de temps de réponse du gain transitoire (temps d’établissement)
Dépassement négatif du gain net
Dépassement négatif du gain net
Stabilité du gain initialGain Gain final
Constante de temps de réponse du gain transitoire (temps d’établissement)
Figure 2 – Réponse du gain transitoire d’un AFO pour un événement (a) de suppression de canal, et un événement (b) d’ajout de canal
Termes et définitions
Les termes, définitions et abréviations suivants s’appliquent pour les besoins du présent document
3.2.1 signal (préexistant) conservé signal optique qui demeure (existe) après (avant) un événement de suppression (d’ajout)
3.2.2 signal de saturation signal optique désactivé (activé), déclenchant ainsi l’événement de suppression (d’ajout)
3.2.3 niveau de suppression (d’ajout) (dB) quantité en dB de laquelle la puissance d’entrée diminue (augmente) du fait de la suppression
The rise time for the addition refers to the duration required for the input power to increase by 10% to 90% of the total difference between the initial and final input power levels during an addition event.
The descent time for suppression refers to the duration required for the input power to decrease from 90% to 10% of the total difference between the initial and final input power levels during a suppression event (see Figure 1b).
3.2.6 gain initial gain du canal conservé (préexistant) avant un événement de suppression (d’ajout)
3.2.7 gain final gain en état stable du canal conservé (préexistant) très longtemps (c’est-à-dire une fois le gain stabilisé) après un événement de suppression (d’ajout)
3.2.8 décalage de gain variation en dB du gain entre l’état initial et l’état final, défini comme le gain final – gain initial
NOTE Le décalage de gain peut être positif ou négatif tant pour l’événement d’ajout que pour l’événement de suppression de canaux
3.2.9 stabilité du gain fluctuations spécifiées du gain crête-à-crête de l’AFO dans des conditions d’état stable (c’est- à-dire qu’il ne s’agit pas d’une réponse à un événement transitoire)
3.2.10 constante de temps de réponse du gain transitoire (temps d’établissement) durée nécessaire pour amener le gain du canal conservé (préexistant) au gain final
This parameter measures the time from the start of the deletion (or addition) event that triggered the transient gain response, until the moment the retained channel gain (pre-existing) first enters the stability band centered around the final gain.
NOTE 2 Par la suite, on le désigne également sous le terme de temps d’établissement
The positive overshoot of the transient gain is defined as the difference, expressed in dB, between the maximum gain of the retained channel (pre-existing) reached during the transient response of the AFO to a suppression (or addition) event, and the lowest value of both the initial and final gains.
NOTE Par la suite, on le désigne également sous le terme de dépassement positif de gain
The positive net overshoot of the transient gain is defined as the difference, expressed in dB, between the maximum gain of the retained channel (pre-existing) reached during the transient response of the AFO to a suppression (or addition) event, and the highest value of both the initial and final gains.
The net positive overshoot of the transient gain is defined as the positive overshoot of the transient gain minus the gain offset This metric reflects the actual transient response that is not associated with the amplifier's offset as it transitions from the initial steady state to the final steady state.
NOTE 2 Par la suite, on le désigne également sous le terme de dépassement positif net du gain
The negative overshoot of the transient gain is defined as the difference, expressed in dB, between the minimum gain of the retained channel (pre-existing) reached during the transient response of the AFO to a suppression (or addition) event, and the highest value between the initial gain and the final gain.
NOTE Par la suite, on le désigne également sous le terme dépassement négatif de gain
The net negative overshoot of the transient gain is defined as the difference in decibels (dB) between the minimum gain of the retained channel (pre-existing) during the transient response of the AFO to a suppression (or addition) event, and the lower value between the initial gain and the final gain.
The net negative overshoot of the transient gain is defined as the negative overshoot of the transient gain minus the gain offset This metric represents the actual transient response that is independent of the amplifier's offset, transitioning from the initial steady-state condition to the final steady-state condition.
NOTE Par la suite, on le désigne également sous le terme de dépassement négatif net de gain.
Abréviations
AGC commande automatique de gain (automatic gain control)
DFB contre-réaction répartie (distributed feedback)
DWDM multiplexage par répartition en longueur d’onde dense (dense wavelength division multiplexing)
EDFA amplificateur à fibre dopée à l'erbium (erbium-doped fibre amplifier)
NEM fabricant d’équipements de réseaux (network equipment manufacturer)
SHB consommation du trou spectral (spectral-hole-burning)
VOA affaiblisseur optique variable (variable optical attenuator)
WDM multiplexage par répartition en longueur d’onde (wavelength division multiplexing)
La Figure 3 présente un montage générique pour caractériser les propriétés des réponses transitoires des AFO au moyen de la méthode par source large bande
Figure 3 – Montage d’essai de mesure transitoire pour la méthode par source large bande
Le matériel d'essai énuméré ci-dessous, avec les caractéristiques requises, est nécessaire: a) Une source laser en vue fournir le signal conservé, comportant les caractéristiques suivantes
The ability to withstand the range of signal wavelengths for which the AFO must be tested is crucial To achieve this, a tunable laser may be utilized, or alternatively, a distributed feedback laser array can be employed.
AFO testing requires an average output power that exceeds the specified maximum input power Additionally, a wideband source is needed to provide the saturation signal, which must include specific characteristics.
1) Il convient qu’au moins 95 % de la puissance de sortie soient contenus dans la bande de transmission spécifiée de l’AFO en essai
2) Une variation inférieure ou égale à 1dB crête-à-crête du niveau de puissance dans la bande de transmission spécifiée de l’AFO en essai
The output power must be achievable such that the input to the tested Optical Amplifier (AFO) exceeds its specified maximum input power Additionally, the Variable Optical Attenuator (VOA1) should have a dynamic range sufficient to accommodate the required range of signal levels that the AFO must endure during testing.
If the output power of the laser source can vary within the required dynamic range, then VOA1 may not be necessary VOA2 is designed with a dynamic range sufficient to accommodate the required power levels of saturation signals, which are determined by the sum of the retained signal levels and the suppression level that the tested AFO must endure during the evaluation.
NOTE 2 Si la puissance de sortie de la source large bande peut être soumise à des variations sur la plage dynamique requise, alors VOA2 peut ne pas être nécessaire
Source large bande VOA2 Modulateur optique VOA1
IEC 1586/11 e) Filtre coupe-bande – Filtre conỗu pour bloquer le signal large bande à proximitộ de la longueur d’onde du signal conservé, avec les caractéristiques suivantes
The ability to withstand the range of wavelengths for which the AFO must be tested is crucial To achieve this, a tunable laser or a series of discrete filters could be utilized.
2) Perte d’insertion uniforme à 0,5 dB sur toute la bande de transmission spécifiée de l’AFO en essai, hormis dans une plage de ±125 GHz de la longueur d’onde du signal conservé
The optical modulator is designed to activate and deactivate the saturation signal, achieving a uniform insertion loss reduction of at least 15 dB across a wavelength range of ±75 GHz.
1) Taux d’extinction d’au moins 5 dB supérieur au niveau de suppression maximal pour lequel l’AFO en essai doit être soumis à l’essai
Sufficient switching time is required to support the fastest suppression time for which the tested AFO must undergo evaluation An optical coupler must be selected to meet the specified requirements Additionally, a bandpass filter is designed to allow only the wavelength of the retained signal to pass, featuring specific characteristics.
The ability to withstand the range of wavelengths of the signal being tested is crucial for the AFO under examination To achieve this, a tunable laser or a series of discrete filters may be utilized.
2) Bande passante à 1 dB d’au moins ±20 GHz centrée autour de la longueur d’onde du signal conservé
The attenuation level must be at least 20 dB below the minimum insertion loss across the entire specified transmission band of the tested Optical Frequency Oscillator (AFO), except within a range of ±100 GHz centered around the wavelength of the retained signal Additionally, a detector is required to identify the filtered output of the tested AFO, adhering to specific characteristics.
1) Une largeur de bande suffisamment importante pour supporter le temps de suppression le plus rapide pour lequel l’AFO doit être soumis à l’essai
A linear response within a range of ±5 dB is required for all signal levels to which the Device Under Test (DUT) will be subjected An oscilloscope is necessary to measure the transient response of the DUT's filtered output, ensuring a bandwidth sufficient to visualize the fastest suppression time the DUT must achieve Additionally, a pulse generator is needed to create the "on"-"off" signal for the optical modulator, with pulse widths short enough to support the DUT's fastest suppression time.
The AFO must operate under nominal conditions If the AFO is likely to cause laser oscillations due to stray reflections, it is advisable to use optical isolators at both the input and output during testing This will help reduce signal instability and measurement inaccuracies.
In the described setup, the input signal power for the tested AFO is a combination of a discrete wavelength representing the retained signal and a broadband source representing a saturation signal The power of each source can be adjusted using the appropriate VOA to achieve the desired power ratio at the AFO's input, which is essential for simulating addition and deletion events during testing The broadband source is filtered through a band-stop filter to ensure that the radiation near the retained signal's wavelength remains significantly lower than the retained signal level Additionally, the broadband source is activated and deactivated via a pulse generator controlling the optical modulator, effectively simulating the addition and deletion events.
The output of the AFO during testing is filtered to ensure that only the power of the retained signal is detected by the detector and the oscilloscope, considering that radiation from the nearby broadband source at the wavelength of the retained signal has already been filtered by the band-stop filter By using the pulse generator as a trigger, the oscilloscope can be configured to display the transient response of the retained signal's power following suppression and addition events The measurement of various parameters of the transient response from the oscilloscope display is specified in IEC 61290-4-1.
To perform a unique transient measurement, follow these steps: a) Adjust the wavelength of the laser source, the bandpass filter, and the bandpass filter according to the length of the signal to be tested b) Set the gain of the AFO under test to the required operating gain for the measurement The gain can be measured using either an internal calibrated gain measurement function of the AFO or directly according to one of the following standards: IEC 61290-1-1, IEC 61290-1-2.
Adjust the power levels of the laser source and the broadband source using Variable Optical Attenuators (VOAs) and ensuring the modulator is in the "on" position, to meet the specified conditions at the input of the Device Under Test (DUT) as outlined in CEI 61290-1-3.
1) La puissance d’entrée totale est égale à la puissance d’entrée de fonctionnement requise pour la mesure
2) La différence entre la puissance d’entrée totale et la puissance de la source laser est le niveau de suppression