15 Figure A.3 – OA output power transient response of a input power increase and b input power decrease .... INTERNATIONAL ELECTROTECHNICAL COMMISSION____________ OPTICAL AMPLIFIERS – TE
Terms and definitions
For the purposes of this document, the following terms and definitions apply
3.1.1 input signal optical signal that is input to the OA
3.1.2 input power excursion relative input power difference in dB before, during and after the input power stimulus event that causes an OA transient power excursion
The input power rise time refers to the duration required for the optical signal to increase from 10% to 90% of the total difference between its initial and final levels during a power excursion event.
Note 1 to entry: see Figure A.2
The input power fall time refers to the duration required for the optical signal to decrease from 10% to 90% of the total difference between its initial and final levels during a power drop event.
Note 1 to entry: see Figure A.2
3.1.5 slew rate maximum rate of change of the input optical signal during a power excursion event
The transient power response refers to the maximum or minimum deviation, measured in dB, between the target power of the optical amplifier (OA) and the observed power fluctuation caused by a change in the input channel's power.
When the output power of an amplified channel strays from its intended level, the control electronics in the operational amplifier (OA) should work to correct the power discrepancy This adjustment aims to restore the OA's output power to its original target level, ensuring consistent performance.
3.1.7 transient power settling time amount of time taken to restore the power of the OA to a stable power level close to the target power level
This parameter is measured from the moment the stimulus event causes a power fluctuation until the OA power response stabilizes and meets specifications.
3.1.8 transient power overcompensation response maximum deviation in dB between the amplifier’s target output power and the power resulting from the control electronics instability
Transient power overcompensation response happens following a power excursion, as the amplifier's control electronics work to restore power to its target level This control process is iterative, and initially, the electronics may overcompensate before finally achieving the desired target power level.
Note 2 to entry: The transient power overcompensation response parameter is generally of lesser magnitude than the transient power response and has the opposite sign
3.1.9 steady state power offset difference in dB between the final and initial output power of the OA, prior to the power excursion stimulus event
Following a power excursion, the steady state power level typically differs from the operating average (OA) power prior to the input power stimulus To address this offset, the transient controller utilizes feedback mechanisms.
Abbreviations
ASEP amplified spontaneous emission power
DWDM dense wavelength division multiplexing
EDFA Erbium-doped fibre amplifier
OSNR optical signal-to-noise ratio
SAR signal-to-ASE ratio
Test set-up
Figure 1 shows a generic set-up to characterise the transient response properties of output power controlled single channel OAs
Figure 1 – Power transient test set-up
Characteristics of test equipment
The test equipment listed below is needed, with the required characteristics a) Laser source for supplying the OA input signal with the following characteristics:
The ability to support the specific range of signal wavelengths for testing the optical amplifier (OA) is crucial This can be achieved using a tunable laser or a collection of distributed feedback (DFB) lasers.
To ensure effective testing of the optical amplifier (OA), it is essential to achieve an average output power that exceeds the maximum specified input power of the OA, accounting for any losses from the test equipment between the laser source and the OA Additionally, a polarization scrambler may be utilized to either randomize the incoming polarization state of the laser or to maintain a specific state of polarization (SOP), though its use is optional Furthermore, a variable optical attenuator (VOA) with a sufficient dynamic range is necessary to accommodate the range of signal levels required for testing the OA.
If the laser source's output power can be adjusted within the necessary dynamic range, a Variable Optical Attenuator (VOA) is unnecessary Additionally, an optical modulator is required to alter the Optical Amplifier (OA) input signal to achieve the specified power variation.
– Extinction ratio at rewrite without putting number higher than the maximum drop level for which the OA under test is to be tested
To effectively test the operational amplifier (OA), it is crucial to ensure that the switching time is sufficiently fast to accommodate the highest slew rate Additionally, the use of a channel pass-band filter, while optional, can enhance the ability to distinguish specific signal wavelengths It is also important to support the range of signal wavelengths relevant to the OA under test, which can be achieved through a tunable filter or a selection of discrete filters.
– 1dB pass-band of at least ±20 GHz centred around the signal wavelength
The OA 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 around the signal wavelength Additionally, an opto-electronic (O/E) converter is required to detect the filtered output of the OA, adhering to specific characteristics.
– A sufficiently wide optical and electrical bandwidth to support the fastest slew rate for which the OA is to be tested
– A linear response within a ±5 dB range of all signal levels for which the OA under test is to be tested
Laser source VOA Optical modulator
To effectively test the operational amplifier (OA), an oscilloscope is required to measure and capture the transient response of the optically filtered output, ensuring it has a wide electrical bandwidth to accommodate the fastest slew rate Additionally, a function generator is necessary to produce input power transient waveforms for the optical modulator, with pulse widths and slew rates sufficiently short and high to support the OA's maximum testing requirements.
The OA will function under standard operating conditions, and to prevent unwanted reflections that may lead to laser oscillations, it is essential to use optical isolators during testing This approach will help reduce signal instability.
Test preparation
In the experimental setup depicted in Figure 1, an optical signal power is generated from a laser source and can be randomized using an optional polarization scrambler The signal's power is then adjusted to the desired levels with a variable optical attenuator (VOA) before passing through an optical modulator, which is driven by a function generator to create specific input power waveforms This modulated signal is injected into the amplifier under test, with a channel pass-band filter employed to isolate the relevant wavelength The output is monitored using an optical-to-electrical (O/E) converter and an oscilloscope, capturing waveforms for further analysis and processing.
Before measuring the transient response, it is essential to record the input power waveform trace Set up the configuration shown in Figure 1, ensuring that the optical fiber amplifier (OFA) under test is not included Connect the input optical connector from the optical modulator to the channel pass filter.
To stimulate a power excursion at the input of the operational amplifier (OA) under test, the source laser power is set to a typical level The function generator waveform is designed to modulate the input power to the OA, reflecting relevant power excursions and slew rates for the defined test conditions For instance, in the case of an optical receiver, the input power can be increased by 7 dB within 50 microseconds, followed by a sustained hold at this level to simulate a step transient power response, as illustrated in Figure A.1(1) For alternative transient control measurements, the signal generator waveform is adjusted accordingly, and the variable optical attenuator (VOA) is calibrated to match.
Test conditions
Sequential transient control measurements can be conducted based on the specified operating conditions of the optical amplifier Table 1 illustrates various power excursion scenarios, and these measurements are generally carried out across a wide range of input power levels.
Table 1 – Examples of transient control measurement test conditions
Scenario Power excursion Slew rate
Input power step transient increase/reduction 3 dB, 7 dB 500 às, 200 às, 50 às
Input power pulse transient 3 dB, 7 dB 500 às, 200 às, 50 às
Input power lightning bolt transient ±3 dB, ±7 dB 500 às, 200 às, 50 às
Transient parameters can be calculated by processing amplifier output power transient waveforms shown in Figure 2, using the following criteria
– Transient power overcompensation response (dB) = G – A
– Steady state power offset (dB) = E – A
– Transient power response time (μs) = D – C a) Channel input power increase b) Channel input power decrease
Figure 2 – OA output power transient response of a) input power increase
Test settings
The test setting conditions must include the arrangement of the test setup, details such as the make and model of each piece of test equipment, and the specific setup conditions for each device, including the operating speed of the polarization scrambler and the resolution bandwidth of the optical spectrum analyzer (OSA) Additionally, it is essential to document the mounting method of the test sample and the ambient conditions during testing.
IEC f) Input optical wavelength λ in
Test data
The test data to be recorded includes the input optical power (P), output optical power (P out), and the signal-to-ASE ratio (SAR) at operating conditions before and after excursions Additionally, the OFA laser pump power and OA reported input and output power should be noted before and after input excursions, along with the OA reported internal temperature where applicable It is also essential to document the measurement accuracy of each test equipment, the temperature of the test sample, transient power response, transient power overcompensation, steady state power offset, and transient power response time.
Overview of power transient events in single channel EDFA
Background
The input signal to a terminal Optical Fiber Amplifier (OFA) is typically a single-channel erbium-doped fiber amplifier (EDFA) that exhibits a wide dynamic range due to power fluctuations across the network These fluctuations result from rapid power variations caused by transient overshoot and undershoot from the preceding chain of imperfect EDFAs Despite operating in constant gain mode with advanced gain transient suppression (usually within ±1 dB), these gain transients are influenced by add/drop events in the network As the signal passes through multiple EDFAs, the steepness and magnitude of these transients accumulate, leading to significant power variations at the terminal EDFA's input Consequently, the characteristics of this single-channel power transient are directly linked to the output power shape of the preceding inline EDFAs.
Characteristic input power behaviour
The input power behavior of a single channel terminal Optical Amplifier (OFA) is illustrated in Figure A.1, highlighting the impact of add/drop events in the preceding amplifier chain This figure emphasizes the time-dependent changes in input power, showcasing example timings The transient power responses, including step, pulse, and lightning bolt patterns, along with the power offset response, are crucial for carriers and network equipment manufacturers (NEM), as the terminal OA is directly followed by a channel receiver Proper design is essential for optimal performance.
OA will have small values for these transient parameters
NOTE As an example of receivers, these are example numbers
Figure A.1 – Example OA input power transient cases for a receiver application
Specific measurement parameters of the input power changes are detailed in Figure A.2 with reference to the lightning bolt scenario
T(fall) = 50às a) Input power increase b) Input power decrease
Figure A.2 – Input power measurement parameters for a) input power increase and b) input power decrease
To effectively manage input power transient excursions, it is crucial to operate a single channel optical amplifier (OA) next to a receiver in automatic power control (APC) mode, known as output power transient controlled operation While moderate transient power fluctuations can be handled based on the receiver's dynamic range and the bandwidth of its automatic gain controller (AGC), excessive optical power can lead to data misreadings, resulting in unwanted bit errors, or even cause permanent damage to the receiver.
Parameters for characterizing transient behaviour
The parameters generally used to characterize the transient behaviour of a power controlled
The operational amplifier (OA) behavior during channel step increases and reductions is illustrated in Figure A.3 Specifically, Figure A.3a) depicts the time-dependent output power response of the OA when there is a rapid increase in input power Conversely, Figure A.3b) shows the transient power behavior when the input power is rapidly decreased.
Key transient parameters include transient power overshoot/undershoot, transient power response settling time, and steady-state power offset In a power-controlled amplifier, a decrease in input power leads to an output power undershoot, while an increase in input power causes an output power overshoot This behavior differs from that of a gain-controlled amplifier.
Input p ow er to O FA , li nea r a u.
Input p ow er to O FA , li nea r a u.
A 10% change in input power leads to a 90% change in system response Specifically, a reduction in input power causes a gain overshoot, while an increase in input power results in a gain undershoot This behavior is observed when channel input power is either increased or decreased.
Figure A.3 – OA output power transient response of a) input power increase and b) input power decrease
Background on power transient phenomena in a single channel EDFA
Amplifier chains in optical networks
Optical networks utilize a series of optical amplifiers to address fiber loss and losses from components like dispersion compensation and channel add/drop As these networks evolve into mesh structures, channels may traverse multiple optical paths, leading to unexpected power variations due to the compounded effects of channel add-drops and the transient control of in-line optical amplifiers The ability of the receiver to withstand these optical power fluctuations is crucial for the proper operation of the optical network.
In modern 10 Gb/s systems, the last line amplifier in the WDM link typically serves as a preamplifier, amplifying the entire dense wavelength division multiplexing (DWDM) comb However, there is a growing demand for individual channel amplifiers to enhance the optical channel before it reaches the receiver This single-channel optical fiber amplifier (OFA) is essential for meeting the stringent optical signal-to-noise ratio (OSNR) requirements of advanced modulation formats and compensating for losses from specialized optical components, such as optical discriminators, polarization demultiplexers, and tunable filters The output power of the single-channel OFA consists of both signal power and amplified spontaneous emission (ASE) noise, which may not be filtered or attenuated by downstream components, particularly in colourless receivers that are broadband and not wavelength-specific.
Typical optical amplifier design
An optically amplified receiver typically includes a channel selector, an optical fiber amplifier (OFA), a photon detector, a limiting amplifier, and an electrical low pass filter Pre-amplifier OFAs are crucial for enhancing the sensitivity of the receiver's photon detector However, they generate noise due to the spontaneous de-excitation of excited erbium ions, which leads to the emission of incoherent photons This background noise, known as amplified spontaneous emission (ASE), is the primary noise source in pre-amplifier EDFAs.
Optical power transients are rapid fluctuations in network power levels, occurring due to changes in channel loading, passive loss variations, or network protection switching To maintain quality of service in dynamic networking environments, optical amplifiers must effectively compensate for these power variations For example, during network reconfiguration, a sudden decrease in DWDM channels can lead to increased amplifier gain, resulting in channel power overshoot that negatively impacts network service providers If these power fluctuations are not managed, they can accumulate through multiple optical amplifiers, ultimately affecting a single channel amplifier upstream of a receiver This amplification can cause significant transient overshoot or undershoot, potentially exceeding the receiver's dynamic range and increasing the bit error ratio (BER), which degrades service quality and may even damage the receiver due to excessive optical power.
Optical repeaters in transmission systems usually utilize Line OFA in constant gain mode This mode amplifies and replicates channel power transients from the input to the output, which can negatively impact the performance of a single channel amplifier in an amplified receiver.
To ensure optimal performance and prevent transients, it is crucial for any single channel Optical Fiber Amplifier (OFA) near a receiver to operate in constant output power mode, known as power transient controlled operation While moderate power fluctuations can be managed based on the receiver's dynamic range and the bandwidth of its automatic gain controller (AGC), excessive power excursions pose significant risks These include exceeding the receiver's maximum optical power rating, which can lead to catastrophic optical damage, or surpassing the maximum operating power rating, resulting in eye opening penalties and potential outages Conversely, power levels dropping below the minimum operating rating can also cause similar issues, including eye opening penalties and outages Additionally, rapid oscillations between excessive and insufficient power levels can further exacerbate the risk of outages.
An Optical Fiber Amplifier (OFA) not only amplifies optical channels that carry data but also generates and transmits Amplified Spontaneous Emission (ASE) noise The optical data signal is usually centered on specific wavelengths defined by the International Telecommunications Union (ITU), while ASE is produced over a much broader wavelength range, typically around 40 nm, which falls within the OFA's gain bandwidth The amount of ASE generated is influenced by factors such as the optical signal channel gain, the population inversion, and the temperature of the erbium-doped fiber (EDF) Additionally, the ASE level can fluctuate due to the passive losses of other optical components within the OFA, as these losses impact the gain needed in the EDF to achieve the desired overall gain.
A measure of the amount of ASE relative to the signal power entering a single channel receiver is defined as the signal to ASE ratio (SAR) This is calculated as:
SAR is the signal to ASE ratio in dB;
SigP is the signal power exiting the OFA in dBm;
ASEP is the total ASE power exiting the OFA in dBm
The Signal-to-Amp Noise Ratio (SAR) of an amplifier should ideally be positive, indicating that the signal power surpasses the sum of the Amplified Spontaneous Emission (ASE) power from the Optical Fiber Amplifier (OFA) Achieving higher SAR levels is crucial for minimizing bit error rates in signal detection, leading to the design of single-channel pre-amplifier OFAs that prioritize maximizing SAR Additionally, operational conditions significantly influence the amount of ASE, thereby affecting the SAR value during operation.
The Optical Fiber Amplifier (OFA) operates as a single channel amplifier, resulting in a gain shape that may not be gain flattened across different wavelengths Even with optimal gain flattening, the gain of the Erbium-Doped Fiber (EDF) fluctuates with the input channel wavelength, causing significant variations in Amplified Spontaneous Emission (ASE) under different operational conditions The OFA lacks inherent signal wavelength discrimination, as its control photodetectors (PDs) cannot differentiate between signal channel power and ASE, making them relatively insensitive to wavelength changes This unpredictability in ASE, influenced by variations in OFA input power or channel wavelength, leads to transient power gain offset errors, ultimately resulting in suboptimal output channel power at the receiver and increasing detection errors within the optical network.
Approaches to address detection errors
Several methods exist to tackle the issue of amplified spontaneous emission (ASE) in dense wavelength division multiplexing (DWDM) optical fiber amplifiers (OFAs) A common approach involves calibrating to estimate ASE levels under various operational conditions, but this process is often time-consuming and expensive due to the influence of factors like signal gain, temperature, and channel wavelength While inserting a fixed wavelength discriminating filter can effectively filter out ASE for a specific channel, this method is inflexible and impractical, as it necessitates a unique OFA for each channel wavelength Additionally, fixed filters are unsuitable for systems with multiple channels, such as those using tunable laser sources Although ASE flattening filters (AFFs) can enhance the signal-to-noise ratio by reducing ASE wavelength dependence, they require optimization for a single gain and temperature, thus still necessitating ASE calibration and increasing costs The use of tunable optical filters offers a more flexible solution for ASE suppression, but their large size, high cost, and need for additional controls make them less appealing for cost-sensitive applications.
Due to the constraints of fixed filters and the expense and bulk of tuneable filters, many single-channel Optical Fiber Amplifiers (OFA) do not utilize gain flattening, Amplified Spontaneous Emission (ASE) flattening, or tuneable filters Consequently, the OFA controller relies solely on the total output power at the output photodetector (PD) as a control signal parameter, since the output PD measures the combined total output power from both the amplified signal channel and the ASE power.
The power output of a single channel erbium-doped optical fiber amplifier (OFA) is influenced by four key factors: input optical power, input optical channel wavelength, optical pump power, and the population inversion level The inversion level indicates the proportion of erbium atoms ready to energize the input optical signal, leading to optical gain Generally, an increase in optical pump power raises the inversion level, while an increase in input optical power lowers it Consequently, to sustain output power, the optical pump power must be increased when channel power rises at the OFA input, and conversely, it must be decreased swiftly when channel power falls.
The output power of an Optical Fiber Amplifier (OFA) is adjustable by modifying the pump laser output power through pump current adjustments This control scheme includes measuring the input and output power of the OFA using signal taps and monitor photodiodes An error signal is computed from these monitor signals, and a high-speed proportional integral-derivative (PID) controller, which may utilize both feedforward and feedback control, is employed to regulate the pump power effectively.
Steady state power offset error arises from any discrepancies in post-transient output power, influenced by the post-transient ASE level, channel wavelength, and temperature The power transient settling time, which indicates how long it takes for the OFA to stabilize at the correct output power, is affected by the pump controller's response time and the pumping rate into the EDF This rate is contingent upon monitoring response, controller bandwidth, algorithm latency, and the Er recovery and saturation time constants The overall output power transient settling time is a cumulative measure of these factors and varies with output power, channel wavelength, and EDF temperature Typically, amplifiers with higher output power exhibit quicker transient response times, while increasing EDF temperature and decreasing channel wavelength can further enhance this response time.
The response of an Optical Fiber Amplifier (OFA) to input transients is influenced by two key time constants associated with the Erbium-Doped Fiber (EDF) The first is the Er recovery time constant, which measures the duration required for pump power to induce a change in the population inversion within the EDF The second time constant also plays a crucial role in this dynamic process.
The Er saturation time constant is linked to the decay time of the EDF population inversion, with both time constants decreasing as population inversion increases In operational OFAs, the saturation rate can significantly exceed the Er recovery rate Additionally, both recovery and saturation time constants are influenced by wavelength and temperature, with longer channel wavelengths and cooler EDF temperatures leading to the longest saturation time constant.
A single channel Output Feedback Amplifier (OFA) designed to mitigate input power transients must utilize a controller that functions in constant output power mode, incorporating a power transient suppression algorithm with its own time constant Upon detection of an input power transient, the controller adjusts the pump power to maintain the output power at the desired level.
In the absence of a pump power controller, the OFA power transient response exhibits a constant gain when an inverse step input power transient occurs, leading to a simultaneous reduction in output power Following the initial drop in input power at time \( t_0 \), the gain increases due to changes in amplifier saturation conditions, resulting in a rise in output power Eventually, at time \( t_S \), the output power stabilizes at a new level that corresponds to the reduced input power and gain Despite the higher post-transient gain, the output power remains lower than before the transient, resulting in a steady-state power offset error This power offset's magnitude is influenced by the input power levels before and after the change, as well as factors such as temperature, ASE level, and the amplifier's saturation state.
Figure B.1 – Transient response to a) input power drop (inverse step transient) with transient control, b) deactivated (constant pump power), and c) activated (power control)
The OFA transient response with the controller activated demonstrates the pump controller's ability to adjust to changes in output power, aiming to maintain a consistent total output power target When the input power transient step reduces the instantaneous input power at time \( t_0 \), the OFA's instantaneous gain remains constant, leading to a drop in output power, known as the output power transient undershoot The OFA controller detects this drop and adjusts the pump power to enhance the inversion level of the EDF During the Er recovery time (\( t_R \)), the inversion level remains relatively stable, causing a gradual increase in gain After \( t_R \), the controller effectively influences the output power, resulting in a quicker rise in gain compared to an uncontrolled scenario Consequently, the post-transient output power approaches the pre-transient level with a reduced steady-state power offset error and a faster settling time (\( t_C \)) However, if the control is poorly damped, there may be an overcompensation of output power before stabilizing at the correct value.
When analyzing the OFA transient response without pump power control, a step increase in input power leads to a constant gain in the uncontrolled OFA Consequently, the output power rises in direct correlation with the increase in input power.
Settling time (t S ) a) Input power b) Uncontrolled output power c) Controlled output power
Input P ow er O ut put P ow er Power offset t 0
Following an input power transient, the gain in an IEC system decreases due to increased gain saturation Eventually, the output power stabilizes at a new level, corresponding to the adjusted input power Despite the lower post-transient gain, the output power is higher than before the transient, resulting in a gain offset error This gain offset error's magnitude is influenced by the input power conditions and the associated gain saturation level.
Figure B.2 – Transient response to a) input power rise (step transient) with transient control, b) deactivated (constant pump power), and c) activated (power control)
In the activated OFA transient response, the pump controller adjusts to maintain a consistent total output power target despite changes in output power, as the output SigP and ASEP are indistinguishable When a step transient occurs at time \( t_0 \), the OFA's instantaneous gain remains constant, leading to an increase in output power, known as the output power transient overshoot The OFA controller then reduces the output power, and after the saturation recovery time \( t_R \), the output power decreases rapidly due to diminished inversion of the EDF Consequently, both gain and pump power decline more swiftly than in an uncontrolled scenario, allowing the post-transient output power to approach the pre-transient level with a smaller steady-state power offset error, settling in a time \( t_C \) that is quicker than the uncontrolled case However, if the control is poorly damped, it may result in overcompensation of the output power before stabilizing at the correct value.
Settling time (t S ) a) Input power c) Controlled output power b) Uncontrolled output power
Input P ow er O ut put P ow er
(informative ) Slew rate effect on transient gain response
When measuring transient performance, it is crucial to consider the speed of input power transients as channel powers fluctuate The power transient control of Optical Fiber Amplifiers (OFA) is achieved by monitoring input and output levels and adjusting the pump laser current Factors such as optical design, monitor and controller bandwidth, and control algorithms significantly influence transient response Additionally, the slew rate of the input power transient impacts OFA performance, with variations in the slew rate arising from the speed of transient events within the network or the switch speed of the testing apparatus.
Slow variations in input power to the Optical Fiber Amplifier (OFA) allow the power control system to effectively adjust pump power, thereby reducing transient overshoot or undershoot This capability minimizes the transient response of the OFA.