Optical Amplifiers — EDFA, Raman, and Wideband C+L

Long-haul DWDM only works because the signal can be regenerated in the optical domain every 80–120 km without converting back to electrical form. This chapter covers the two amplifier technologies that make this possible — EDFA and Raman — their hybrid combination for very long spans, the parallel architecture used to extend amplification into the L-band, and the noise-figure mathematics that determines how many amplifiers can be cascaded before OSNR collapses.

Optical Amplifiers — EDFA and Raman

EDFA — Erbium-Doped Fibre Amplifier

An EDFA contains a short coil (10-30 metres) of fibre doped with erbium (Er3+) ions [12, 24]. A pump laser at 980 nm or 1480 nm excites these ions to a higher energy state, creating population inversion. When C-band signal photons (1530-1565 nm) enter the excited fibre, they trigger stimulated emission: each signal photon causes an erbium ion to release an identical photon — same wavelength, same phase, same direction. Every wavelength within the erbium gain window is amplified simultaneously and transparently.

The unavoidable cost is ASE (Amplified Spontaneous Emission). Some erbium ions decay spontaneously rather than being stimulated by a signal, generating broadband noise across the entire C-band. This ASE noise accumulates with every amplifier in the chain and ultimately limits the achievable OSNR [21].

flowchart LR
    PUMP["Pump laser<br/>980 or 1480 nm"] -->|"Excites Er3+ ions"| EDF["Erbium-doped<br/>fibre coil<br/>10-30 m"]
    SIG_IN["Signal in<br/>C-band lambdas"] --> EDF
    EDF -->|"Stimulated<br/>emission"| SIG_OUT["Amplified<br/>signal out<br/>+ ASE noise"]
    style PUMP fill:#D85A30,stroke:#993C1D,color:#fff
    style EDF fill:#1D9E75,stroke:#0F6E56,color:#fff
    style SIG_IN fill:#378ADD,stroke:#185FA5,color:#fff
    style SIG_OUT fill:#7F77DD,stroke:#534AB7,color:#fff

EDFA: pump laser excites erbium ions, signal photons trigger stimulated emission, all wavelengths amplified simultaneously.

Placement roles:

Booster — immediately after the transmitter, maximises launch power into the fibre.
Inline amplifier (ILA) — every 80-120 km to compensate span loss. The most common deployment.
Pre-amplifier — before the receiver, lifts weak signals above the detection threshold.

Warning

EDFA gain must match span loss [12, 13]. A 30 dB gain EDFA injects roughly 10× more ASE noise power than a 20 dB gain EDFA (because P_ASE is proportional to G − 1). Over a multi-span chain, excess gain at each site compounds into several dB of unnecessary OSNR degradation — potentially forcing a modulation downgrade that halves per-channel capacity. Modern systems use automatic gain control (AGC) to lock each amplifier’s gain to its span loss within 0.5 dB.

Design parameters:

Noise figure: typically 4.5-6 dB; 5.5 dB is the standard planning value [12, 24].
Gain flattening filter (GFF) inside the EDFA compensates for non-uniform gain across the C-band.
Transient control (AGC) prevents surviving-channel power overshoot when wavelengths are added or removed.

The two plots below quantify the gain-vs-noise tradeoff that motivates AGC. ASE noise rises linearly with gain on the dB scale (each extra 10 dB of gain multiplies noise power by 10×), and end-of-chain OSNR drops in lockstep:

xychart-beta
    title "ASE Noise Power vs EDFA Gain (NF = 5.5 dB)"
    x-axis "EDFA Gain (dB)" [10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30]
    y-axis "ASE Noise per amp (dBm)" -32 --> -18
    line "ASE noise" [-31, -29.5, -28.0, -26.5, -25.2, -24.0, -22.8, -21.8, -20.8, -19.9, -19.0]

xychart-beta
    title "End-of-chain OSNR vs EDFA Gain (10 spans, P_ch = 0 dBm)"
    x-axis "EDFA Gain (dB)" [10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30]
    y-axis "OSNR after 10 spans (dB)" 20 --> 32
    line "OSNR" [31.0, 29.5, 28.0, 26.5, 25.2, 24.0, 22.8, 21.8, 20.8, 19.9, 19.0]

A worked comparison makes the cost of misconfiguration concrete. With a span loss of 17 dB, every dB of gain beyond 17 is excess — it amplifies noise without adding signal:

Parameter	Gain = 17 dB (matched)	Gain = 25 dB (excess)	Difference
ASE per amplifier	-25.5 dBm (2.8 µW)	-19.0 dBm (12.6 µW)	4.5× more noise
Total ASE (10 amps)	-15.5 dBm (28 µW)	-9.0 dBm (126 µW)	4.5× more noise
OSNR at receiver	25.5 dB	21.0 dB	-4.5 dB penalty
Supported modulation	DP-16QAM (needs ~22 dB)	Marginal — may force DP-8QAM	Half the capacity

Warning

Excess gain is the most common misconfiguration in amplified DWDM lines. A 4.5 dB OSNR penalty can force a modulation downgrade from DP-16QAM to DP-8QAM, halving per-channel capacity. AGC must lock each amplifier’s gain to its measured span loss within 0.5 dB — verify on every commissioning turn-up.

Gain Spectrum and Gain Flattening Filter

An EDFA’s gain spectrum is governed by the energy-level structure of erbium ions in silica glass [24]. The raw (unfiltered) gain curve exhibits two dominant features: a sharp peak near 1530-1535 nm — the strongest erbium emission line, where gain can exceed the band edges by 6-8 dB — and a gradual roll-off toward 1565 nm as the erbium emission cross-section decreases. Without correction, a 96-channel DWDM system would see massive inter-channel power divergence after even a single amplifier.

xychart-beta
    title "EDFA Gain vs Wavelength (C-band, no GFF)"
    x-axis "Wavelength (nm)" [1525, 1530, 1535, 1538, 1542, 1545, 1548, 1550, 1553, 1556, 1558, 1560, 1563, 1565, 1568]
    y-axis "Relative Gain (dB)" 15 --> 30
    line "Raw EDFA gain" [18, 22, 28, 26, 23, 22, 21.5, 21, 20.8, 20.5, 20.2, 20, 19.5, 19, 18]
    line "After GFF" [21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21]

The Gain Flattening Filter (GFF) is a passive optical filter inside the EDFA that attenuates peak wavelengths more aggressively than trough wavelengths, producing flat gain (typically ±0.5 dB) across the entire C-band [12, 24]. Every modern commercial EDFA includes a GFF as a standard internal component.

Warning

Gain-tilt accumulation is multiplicative across spans. Even ±1 dB of residual ripple per amplifier compounds to ±10 dB over a 10-span chain — enough to push edge channels below the receiver’s OSNR threshold while centre channels saturate. The GFF flatness specification (typically ±0.5 dB) is one of the most critical parameters on an EDFA datasheet.

Gain Shape Dependence on Operating Point

The EDFA gain profile is not static — it changes with the erbium population inversion level, which is controlled by pump power. At high inversion (high gain settings), the 1530 nm peak dominates. At low inversion (low gain), the peak flattens and the long-wavelength roll-off becomes less severe. This behaviour is called gain tilt versus operating point, and it means variable-gain EDFAs must re-optimise their GFF or employ a Variable Optical Attenuator (VOA) to maintain flatness across the operating range.

Channel loading also affects per-channel gain. When additional wavelengths are lit on a fibre, the EDFA’s total output power is shared among more channels. Modern EDFAs operate in constant-gain (AGC) mode — pump power is adjusted automatically to maintain the same per-channel gain regardless of how many channels are present, preventing surviving-channel power excursions during wavelength provisioning.

EDFA Internal Architecture

The functional blocks inside a modern EDFA follow a fixed signal path:

flowchart LR
    subgraph "Inside an EDFA"
        INPUT["Signal in<br/>96 lambdas"] --> ISO1["Isolator"]
        ISO1 --> EDF["Erbium-doped<br/>fibre coil"]
        PUMP["Pump laser<br/>980 nm"] -->|"WDM coupler"| EDF
        EDF --> GFF["Gain flattening<br/>filter (GFF)"]
        GFF --> VOA["Variable optical<br/>attenuator (VOA)"]
        VOA --> ISO2["Isolator"]
        ISO2 --> MON["Tap + monitor<br/>photodiode"]
        MON --> OUTPUT["Signal out<br/>flat gain"]
    end
    style INPUT fill:#378ADD,stroke:#185FA5,color:#fff
    style PUMP fill:#D85A30,stroke:#993C1D,color:#fff
    style EDF fill:#1D9E75,stroke:#0F6E56,color:#fff
    style GFF fill:#BA7517,stroke:#854F0B,color:#fff
    style VOA fill:#7F77DD,stroke:#534AB7,color:#fff
    style OUTPUT fill:#378ADD,stroke:#185FA5,color:#fff

The input and output isolators prevent backward reflections that would cause parasitic lasing. The tap-monitor photodiode feeds the AGC control loop, and the VOA provides fine output-power adjustment to hit the exact target gain.

Raman Amplifier

Raman amplification exploits the transmission fibre itself as the gain medium [22, 25]. A high-power pump laser (~1450 nm for C-band gain) is injected counter-directionally from the far end of the span. Through Stimulated Raman Scattering (SRS), pump photons transfer energy to signal photons — the energy difference corresponds to a vibrational mode of the silica molecules.

The critical advantage is that Raman gain is distributed across the last 20-30 km of fibre rather than concentrated at a single point. The signal never drops as low as it would without Raman, yielding a 2-4 dB OSNR improvement over EDFA alone — a significant margin in long-haul design.

flowchart RL
    SIG["Signal<br/>propagation"] --> FIBER["Transmission fibre<br/>80+ km<br/>Gain happens here"]
    RPUMP["Raman pump<br/>~1450 nm<br/>counter-propagating"] --> FIBER
    FIBER --> OUT["Amplified<br/>signal"]
    style SIG fill:#378ADD,stroke:#185FA5,color:#fff
    style FIBER fill:#1D9E75,stroke:#0F6E56,color:#fff
    style RPUMP fill:#D85A30,stroke:#993C1D,color:#fff
    style OUT fill:#7F77DD,stroke:#534AB7,color:#fff

Raman: pump laser injects energy into the fibre itself. Distributed amplification across the last 20-30 km keeps the signal above the noise floor.

Eye-Safety

Raman pump lasers operate at high power (300-500 mW). Automatic Laser Shutdown (ALS) must be verified during commissioning — an open fibre connector under full Raman pump power is an eye-safety hazard.

Design considerations:

Raman gain is wavelength-dependent (shorter wavelengths gain more), requiring tilt management via the NMS.
Different fibre types (G.652D, G.655, G.654E) produce different Raman gain profiles [7, 8, 9].
Double Rayleigh Backscatter (DRB) limits practical Raman gain to approximately 15-20 dB.
Hybrid Raman + EDFA is the most common production deployment for long spans — see Hybrid Raman + EDFA — Simulation and Engineering below for the full quantitative analysis.

Raman Gain Profile and Multi-Pump Flattening

Raman gain arises from the vibrational modes of the silica lattice rather than from doped ions, producing a fundamentally different spectral shape from EDFAs [22, 25]. Peak gain occurs at approximately 13 THz (~100 nm) below the pump wavelength — for a pump at 1450 nm, the gain peaks near 1550 nm, precisely targeting the C-band. The useful gain bandwidth from a single pump spans roughly 30-40 nm, and the profile is broad but asymmetric: a gradual rise from the pump wavelength to the peak, followed by a sharper drop-off beyond it.

xychart-beta
    title "Raman Gain vs Wavelength (single pump at 1450 nm)"
    x-axis "Wavelength (nm)" [1470, 1480, 1490, 1500, 1510, 1520, 1530, 1540, 1545, 1550, 1555, 1560, 1570, 1580, 1590]
    y-axis "Raman Gain (dB)" 0 --> 12
    line "Single pump gain" [1, 2, 3.5, 5, 7, 9, 10.5, 11.5, 11.8, 12, 11, 9, 6, 3, 1]

The single-pump gain curve is triangular — shorter C-band wavelengths receive more gain than longer ones — producing a pronounced tilt that must be compensated in multi-span systems.

Production Raman amplifiers therefore deploy 2-4 pump wavelengths simultaneously. Each pump generates its own gain curve peaked ~100 nm above, and the individual profiles sum to a composite that can be made flat to within ±1 dB across the C-band. Pumps at 1425 nm and 1450 nm, for example, produce overlapping gain profiles whose sum is substantially flatter than either alone:

xychart-beta
    title "Multi-pump Raman — flattened C-band gain"
    x-axis "Wavelength (nm)" [1525, 1530, 1535, 1540, 1545, 1550, 1555, 1560, 1565]
    y-axis "Gain (dB)" 0 --> 14
    line "Pump 1 (1425 nm)" [3, 5, 7, 8, 9, 8.5, 7, 5, 3]
    line "Pump 2 (1450 nm)" [1, 2, 3, 4, 5, 6, 7, 7, 6]
    line "Combined" [8, 9, 10, 10, 10, 10, 10, 9.5, 9]

EDFA vs Raman Flatness Control

EDFA gain flatness depends on a fixed passive GFF — a physical filter designed for a specific gain shape. Raman gain flatness is tuned by adjusting pump powers and wavelengths in software or firmware, making it inherently more flexible but also more complex to optimise. This distinction matters during field commissioning: EDFA flatness is factory-set, while Raman flatness requires per-span characterisation.

Rule of Thumb

Raman either extends usable span length by ~20 km or buys ~3 dB of OSNR at the same distance — often enough to step up one modulation level (DP-16QAM instead of DP-8QAM) on a marginal link.

Hybrid Raman + EDFA — Simulation and Engineering

Hybrid Raman + EDFA amplification is the standard architecture for long-haul and subsea DWDM spans [21, 25]. This subsection quantifies the OSNR advantage through a 170 km span simulation comparing three amplification strategies, explains why standalone Raman is not viable beyond ~75 km spans, details the physical co-location of Raman pump and EDFA at a single site, and provides an engineering decision guide based on span length and terrain.

The OSNR Argument — Why Distributed 12 dB Outperforms Lumped 28 dB

Key Insight

OSNR degradation is determined by the point of lowest signal power along the span, not by the total gain applied at the end. Amplifying a noise-degraded signal produces a louder noise-degraded signal — the damage is irreversible.

Raman gain of 10-12 dB is far less than a typical EDFA’s 20-30 dB, yet Raman consistently delivers superior OSNR on long spans. The reason is spatial: Raman gain is distributed across the last 30-35 km of the fibre itself through Stimulated Raman Scattering, preventing the signal from reaching the deep power minimum that occurs with end-of-span EDFA-only amplification.

An EDFA at the far end restores the signal to any desired power level, but it cannot undo the OSNR penalty accumulated while the signal sat near the noise floor. Raman eliminates that penalty at its source.

Metric	EDFA Only (16 dB gain at end)	Raman 12 dB + EDFA 4 dB at end
Signal at weakest point	-15 dBm (32 µW)	-7 dBm (200 µW)
Effective system NF	5.5 dB	~-1 dB (effective)
OSNR contribution per span	26.5 dB	29.5 dB
OSNR improvement	Baseline	+3 dB better

That 3 dB margin is the difference between DP-8QAM and DP-16QAM on a marginal link — literally doubling per-wavelength capacity, or extending reach by 25-30 % at the same modulation format.

Water-Bucket Analogy

Consider carrying water in a leaky bucket across a desert. The EDFA approach lets the bucket drain completely, then provides a large refill at the destination — severe dehydration occurred over the last 30 km and caused permanent damage. The Raman approach has someone walking backwards from the destination, dripping water into the bucket for the final 30 km — less total water added, but the critical low point was avoided entirely. The smaller refill does not matter because the damage never happened.

Why Raman Is Never Used Alone — DRB Revisited

Raman gain is limited to approximately 10-15 dB before Double Rayleigh Backscatter (DRB) creates a delayed copy of the signal that interferes destructively with itself [22, 25]. DRB arises because a small fraction of the amplified signal is Rayleigh-scattered backward, then Rayleigh-scattered forward again — arriving at the receiver as a delayed, incoherent replica that degrades the signal.

For any span exceeding ~75 km (i.e., more than ~15 dB of loss), Raman alone cannot fully compensate the attenuation. The production deployment is therefore always hybrid Raman + EDFA:

Component	Role	Typical Contribution
Raman pump	Distributed gain in the fibre	10-12 dB
EDFA	Discrete gain at the receiving site	Remainder to match total span loss
Combined	Full span-loss compensation	Matches fibre loss exactly

Physical Architecture — Single Equipment Shelf

Both the Raman pump and the EDFA are co-located in the same equipment shelf at the receiving end of the span (Site B). There is one rack, one power feed, one management connection — no additional infrastructure beyond what a standard EDFA-only site requires.

flowchart LR
    SITEA["Site A<br/>Transmitter<br/>+2 dBm"] -->|"170 km fibre<br/>34 dB total loss"| WDM["WDM<br/>coupler"]
    WDM --> EDFA["EDFA<br/>+22 dB gain"]
    RAMAN["Raman pump<br/>~1450 nm<br/>300-500 mW"] -->|"Pump light<br/>counter-propagating"| WDM
    EDFA --> OUT["Output<br/>+2 dBm<br/>next span"]
    subgraph SITEB ["Site B — single equipment shelf"]
        WDM
        RAMAN
        EDFA
    end
    style SITEA fill:#378ADD,stroke:#185FA5,color:#fff
    style RAMAN fill:#D85A30,stroke:#993C1D,color:#fff
    style EDFA fill:#1D9E75,stroke:#0F6E56,color:#fff
    style OUT fill:#7F77DD,stroke:#534AB7,color:#fff

Signal arrives from the fibre into a WDM coupler. The Raman pump (~1450 nm) is injected backward through the same coupler. After distributed Raman gain over the last ~35 km, the partially amplified signal enters the EDFA for discrete amplification to the target output power.

170 km Span Simulation — Three Amplification Strategies

Key Insight

A 170 km span at 0.2 dB/km incurs 34 dB of total loss. Three amplification strategies produce dramatically different minimum signal powers, OSNR outcomes, and infrastructure requirements.

Approach 1 — EDFA Only at the Far End

Launch power is +2 dBm. The signal decays continuously for 170 km, arriving at +2 - 34 = -32 dBm (0.6 µW). A single EDFA provides 34 dB of gain to restore the signal. The problem: the signal spent the last 50 km below -20 dBm, deep in the noise floor. An EDFA producing 34 dB of gain also injects massive ASE noise (P_ASE is proportional to G - 1).

xychart-beta
    title "Approach 1: EDFA only at end — signal power vs distance"
    x-axis "Distance (km)" [0, 20, 40, 60, 80, 100, 120, 135, 150, 170]
    y-axis "Power (dBm)" -35 --> 5
    line "Signal" [2, -2, -6, -10, -14, -18, -22, -25, -28, -32]

Approach 2 — Mid-Span EDFA at km 85

The first 85 km drops the signal to +2 - 17 = -15 dBm. A mid-span EDFA restores to +2 dBm. The second 85 km drops the signal to -15 dBm again. An end EDFA restores to +2 dBm. The minimum power is only -15 dBm — 17 dB better than Approach 1.

The cost: a powered shelter at km 85 requiring land lease, power supply (often solar + batteries in remote areas), environmental protection, security, and ongoing maintenance access. On desert routes this can cost hundreds of thousands of dollars.

xychart-beta
    title "Approach 2: Mid-span EDFA at km 85 — signal power vs distance"
    x-axis "Distance (km)" [0, 20, 40, 60, 85, 85, 100, 120, 140, 170]
    y-axis "Power (dBm)" -20 --> 5
    line "Signal" [2, -2, -6, -10, -15, 2, -1, -5, -9, -15]

Approach 3 — Raman + EDFA at the Far End

The signal decays identically to Approach 1 for the first 135 km, reaching -25 dBm (3 µW). At km 135, the Raman pump’s gain zone begins. Over the last 35 km, Raman provides ~12 dB of distributed gain while fibre loss continues at 0.2 dB/km, yielding a net +5 dB improvement. The signal arrives at -20 dBm (10 µW).

The EDFA then provides 22 dB of gain (34 - 12 = 22), restoring the signal to +2 dBm. The minimum power of -25 dBm is 7 dB better than Approach 1, with no mid-span infrastructure required.

xychart-beta
    title "Approach 3: Raman + EDFA at end — signal power vs distance"
    x-axis "Distance (km)" [0, 20, 40, 60, 80, 100, 120, 135, 145, 155, 165, 170]
    y-axis "Power (dBm)" -30 --> 5
    line "Signal" [2, -2, -6, -10, -14, -18, -22, -25, -23, -22, -21, -20]

All Three Approaches Overlaid

xychart-beta
    title "170 km span — signal power comparison"
    x-axis "Distance (km)" [0, 20, 40, 60, 85, 100, 120, 135, 150, 170]
    y-axis "Power (dBm)" -35 --> 5
    line "EDFA only" [2, -2, -6, -10, -15, -18, -22, -25, -28, -32]
    line "Mid-span EDFA" [2, -2, -6, -10, 2, -1, -5, -9, -12, -15]
    line "Raman+EDFA" [2, -2, -6, -10, -15, -18, -22, -25, -23, -20]

The three curves diverge after km 85. EDFA-only continues its monotonic decay. Mid-span EDFA resets at km 85, creating a sawtooth. Raman + EDFA bends upward at km 135 as distributed gain fights fibre loss. The vertical gap between curves at km 170 represents the OSNR difference.

Quantitative Comparison

Approach	Equipment Sites	Minimum Signal	OSNR per Span	Practical Notes
EDFA only at end	1	-32 dBm (0.6 µW)	~19 dB — marginal	May force DP-QPSK (reduced capacity)
Mid-span EDFA at km 85	2	-15 dBm (32 µW)	~26 dB — excellent	Requires mid-span powered shelter
Raman + EDFA at end	1	-25 dBm (3 µW)	~23 dB — good	No mid-span site, moderate cost

When Each Approach Wins

Decision Guide

The choice between amplification strategies is driven by span length and mid-span accessibility, not by which technology produces more gain.

EDFA-only at the far end is viable for spans under ~100 km where total loss stays below 20 dB. Signal minimum is manageable, EDFA gain is moderate, and ASE accumulation is acceptable. This is the standard for most terrestrial backbone spans.

Mid-span EDFA delivers the best OSNR but at the highest cost. It is appropriate when the route passes through accessible areas where building a shelter is practical (urban or semi-urban corridors), or on very long spans (over 150 km) where even Raman cannot compensate enough loss.

Raman + EDFA at the far end is the sweet spot for long spans (100-170 km) through remote areas where mid-span construction is prohibitively expensive:

Deployment Context	Why Hybrid Raman + EDFA
Desert routes (e.g., Saudi Arabia, Australia, Sahara crossings)	No power or physical access for hundreds of km
Subsea cables	Every repeater pod on the ocean floor is hybrid Raman + EDFA; mid-span access is physically impossible; spans are 60-80 km but over 6000+ km of cable, every dB of OSNR matters
Mountain crossings	Remote terrain where shelters are extremely expensive to build and maintain

EDFA Gain-Reduction Benefit in Hybrid Deployments

Key Insight

In a hybrid setup, the EDFA operates at significantly lower gain because Raman has already contributed 10-12 dB. Lower gain means dramatically less ASE noise injection.

A secondary but substantial advantage of the hybrid architecture: the EDFA runs at lower gain. For the 170 km simulation, the EDFA produces 22 dB instead of 34 dB. Since ASE noise power is proportional to (G - 1):

Parameter	EDFA-Only (34 dB gain)	Hybrid (22 dB EDFA gain)	Ratio
Linear gain	2512	158	—
P_ASE proportional to (G - 1)	2511	157	16× less noise

The EDFA in the hybrid setup injects 16 times less ASE noise than the EDFA-only approach. Combined with the higher minimum signal power from distributed Raman gain, the total OSNR benefit is substantial — the two effects reinforce each other.

Warning

This gain-reduction benefit applies to the individual amplifier’s ASE contribution. Over a multi-span chain, the cumulative effect is even more pronounced: lower ASE per stage means the noise floor rises more slowly, enabling longer total reach or higher modulation formats at the same distance.

C+L Wideband Amplification

Why a Single EDFA Cannot Cover Both Bands

As C-band capacity fills, operators turn to L-band (1565-1625 nm) to approximately double channel count [21, 24]. Erbium’s emission spectrum exhibits a natural dip between the C and L windows, and achieving gain across L-band requires a much longer erbium fibre at lower population inversion than C-band. No single EDFA design can efficiently span both bands, so production C+L systems employ a parallel architecture with separate C-band and L-band amplifiers.

Parallel C+L EDFA Architecture

flowchart LR
    INPUT["C+L signal<br/>in"] --> SPLIT["C/L band<br/>splitter"]
    SPLIT -->|"C-band<br/>1530-1565"| CEDFA["C-band<br/>EDFA<br/>short EDF<br/>high inversion"]
    SPLIT -->|"L-band<br/>1565-1625"| LEDFA["L-band<br/>EDFA<br/>long EDF<br/>low inversion"]
    CEDFA --> COMBINE["C/L band<br/>combiner"]
    LEDFA --> COMBINE
    COMBINE --> OUTPUT["C+L signal<br/>out"]
    style INPUT fill:#378ADD,stroke:#185FA5,color:#fff
    style SPLIT fill:#BA7517,stroke:#854F0B,color:#fff
    style CEDFA fill:#7F77DD,stroke:#534AB7,color:#fff
    style LEDFA fill:#D85A30,stroke:#993C1D,color:#fff
    style COMBINE fill:#BA7517,stroke:#854F0B,color:#fff
    style OUTPUT fill:#378ADD,stroke:#185FA5,color:#fff

The C/L band splitter and combiner are passive dichroic filters that separate or merge the two bands at ~1567-1568 nm with approximately 0.5 dB insertion loss. Each EDFA is independently optimised for its band:

Parameter	C-band EDFA	L-band EDFA
EDF length	10-15 m	50-100 m
Pump wavelength	980 nm (primary)	1480 nm (primary)
Population inversion	High (~70%)	Low (~40%)
Gain per metre	High	Low (compensated by fibre length)
Noise figure	4.5-5.5 dB	5.0-6.5 dB
Gain flatness (with GFF)	±0.5 dB	±1.0 dB
Power consumption	Lower	Higher (longer fibre, more pump power)
Typical gain (C+L with SRS compensation)	20-22 dB	16-18 dB

Note

L-band EDFAs have both worse noise figure and lower gain setting (to avoid over-amplifying SRS-boosted channels). Combined, L-band channels carry approximately 2-3 dB less OSNR than C-band at the same span count. This may force a modulation downgrade on marginal links — a span supporting DP-16QAM on C-band might only sustain DP-8QAM on L-band. Link budgets must be calculated separately for each band.

C+L Gain Profile — A Step, Not a Slope

Each band’s EDFA is independently flattened by its own GFF, so the combined C+L gain profile exhibits a discrete step at the band boundary rather than a continuous slope. The C-band EDFA typically operates at 20-22 dB gain; the L-band EDFA at 16-18 dB. The 4-5 dB offset compensates for bulk SRS power transfer from C-band to L-band during fibre propagation [22] — it is not a deficiency of the L-band amplifier but a deliberate design choice.

xychart-beta
    title "C+L EDFA gain — SRS-compensated operating point"
    x-axis "Wavelength (nm)" [1530, 1535, 1540, 1545, 1550, 1555, 1560, 1565, 1570, 1575, 1580, 1585, 1590, 1595, 1600, 1610, 1620]
    y-axis "Gain (dB)" 14 --> 24
    line "C-band EDFA (higher)" [22, 22, 22, 22, 22, 22, 22, 22, 0, 0, 0, 0, 0, 0, 0, 0, 0]
    line "L-band EDFA (lower)" [0, 0, 0, 0, 0, 0, 0, 0, 17, 17, 17, 17, 17, 17, 17, 16.5, 16]

Common expectation	Actual behaviour	Reason
One continuous gain curve sloping from C to L	Two separate flat curves with a step between them	C and L are physically separate EDFAs with independent GFFs
Smooth slope compensates SRS	Step offset between band gains compensates bulk SRS	Each EDFA is independently flattened
EDFA handles all SRS compensation	Three layers work together: EDFA offset + pre-tilt + DGE	SRS tilt is nonlinear and varies with channel loading

SRS Compensation — Three Layers

Stimulated Raman Scattering transfers approximately 3-4 dB of power from C-band channels to L-band channels per 80 km span [22]. Compensation is distributed across three mechanisms:

Layer 1 — EDFA Gain Offset Between Bands

The C-band EDFA is set to a higher gain than the L-band EDFA (e.g. 22 dB vs 17 dB). This 5 dB offset accounts for C-band channels arriving weaker and L-band channels arriving stronger after SRS. Within each band, gain remains flat — the offset is a step, not a tilt.

Layer 2 — Pre-Tilt at Launch

Transponders or per-channel VOAs at the transmit side launch C-band channels 2-3 dB higher than L-band channels. After SRS redistributes power during propagation, channels arrive roughly equalised at the far-end amplifier.

Layer 3 — Dynamic Gain Equalisation (DGE)

At ROADM nodes, a Wavelength Selective Switch measures every channel’s power individually and adjusts each within ±0.5 dB. This per-channel fine-tuning catches residual tilt that the first two layers cannot perfectly correct — particularly important because SRS tilt is not perfectly linear across the band and varies with channel loading.

flowchart LR
    TX["Transmitter<br/>Pre-tilt:<br/>C launched higher<br/>L launched lower"] -->|"80 km fibre<br/>SRS: C loses ~3 dB<br/>L gains ~3 dB"| AMP["C/L EDFAs<br/>C-EDFA: 22 dB<br/>L-EDFA: 17 dB"]
    AMP --> DGE["DGE / WSS<br/>Per-channel<br/>fine adjustment<br/>±0.5 dB"]
    DGE --> NEXT["To next span<br/>channels equalised"]
    style TX fill:#378ADD,stroke:#185FA5,color:#fff
    style AMP fill:#1D9E75,stroke:#0F6E56,color:#fff
    style DGE fill:#7F77DD,stroke:#534AB7,color:#fff
    style NEXT fill:#BA7517,stroke:#854F0B,color:#fff

Wideband Raman for C+L

Raman amplification handles C+L more naturally than EDFA because gain bandwidth is determined by pump count rather than doped-fibre chemistry [25]. A 4-pump Raman architecture covers the full 1525-1620 nm range without requiring a band splitter or combiner — the gain occurs in the transmission fibre itself, which naturally carries both bands:

Pump Wavelength	Gain Peak	Band Coverage
~1425 nm	~1525 nm	Short C-band
~1450 nm	~1550 nm	Mid C-band
~1470 nm	~1570 nm	Long C / short L
~1495 nm	~1595 nm	Mid L-band

Independent adjustment of the four pump powers shapes the composite gain to be flat across the entire C+L range. This flexibility is a key reason hybrid Raman + EDFA is preferred for C+L systems: Raman provides broadband pre-amplification across both bands in a single fibre, then split C/L EDFAs supply the remaining discrete gain.

SRS Power Tilt — The C+L Engineering Problem

SRS in the transmission fibre causes shorter wavelengths to transfer energy to longer wavelengths — the same physical effect exploited by Raman amplifiers, but here it is uncontrolled and unwanted [22]. In a loaded C+L system, the effect is dramatic:

xychart-beta
    title "SRS Power Tilt — shorter wavelengths lose power to longer wavelengths"
    x-axis "Wavelength (nm)" [1530, 1540, 1550, 1560, 1570, 1580, 1590, 1600, 1610, 1620]
    y-axis "Relative Power Change (dB)" -4 --> 4
    line "SRS tilt after 80 km" [-3.5, -2.8, -2.0, -1.2, -0.3, 0.5, 1.3, 2.0, 2.8, 3.5]

The shortest C-band channels (1530 nm) lose 3-4 dB of power per span, while the longest L-band channels (1620 nm) gain 3-4 dB. The total tilt across C+L can reach 6-8 dB per 80 km span. Left uncorrected over 10 spans, this compounds into a massive power imbalance that renders edge channels unusable.

In a C-band-only system, SRS tilt is modest — approximately 1-2 dB across the 35 nm C-band, easily handled by the EDFA’s GFF and per-channel power adjustments. Doubling the spectral width to ~90 nm for C+L roughly triples the SRS interaction bandwidth and doubles the channel count, pushing per-span tilt to 6-8 dB — far beyond static GFF compensation.

Parameter	C-band Only	C+L Band	Impact
Spectral width	~35 nm	~90 nm	2.5× more SRS interaction
SRS tilt per span	~1-2 dB	~6-8 dB	Requires active per-channel management
Number of channels	~96	~192	More channels = more total power = more SRS
Pre-tilt requirement	Minimal	2-4 dB across C+L	Must be calculated per link
Amplifier complexity	Single C-band EDFA	Split C + L EDFAs	Double the amplifier count and cost
OSNR penalty	Negligible	0.5-1.5 dB for L-band	L-band channels have slightly worse OSNR
Nonlinear threshold	Higher per channel	Must reduce per-channel power	Total fibre power constrained

SRS is not the only nonlinear concern. Self-Phase Modulation (SPM), Cross-Phase Modulation (XPM), and Four-Wave Mixing (FWM) all worsen as total fibre power increases [22, 30]. With 192 channels in C+L, total launch power can reach +22 to +24 dBm — approaching the fibre’s nonlinear regime (~+18 to +21 dBm composite for typical SMF, channel-count and spacing dependent; +23 dBm is on the high end for standard G.652D).

The design must balance per-channel power high enough for adequate receiver OSNR against total power low enough to avoid nonlinear distortion. Typical per-channel launch power in C+L systems is -3 to +1 dBm, chosen so the sum of all channels remains below the nonlinear threshold. Modern planning tools (Ciena MCP / OnePlanner, Nokia WaveSuite) solve this optimisation automatically.

Practical C+L Upgrade Example

A Tier-1 SP upgrading an existing 88-channel C-band backbone (88 × 400G = 35.2 Tbps) to C+L faces the following engineering sequence:

L-band EDFAs are added at every amplifier site in parallel with existing C-band EDFAs, connected through C/L band splitters and combiners.
SRS pre-tilt is calculated by the NMS for each span based on measured fibre parameters.
Per-channel launch power is reduced from 0 dBm (C-only) to approximately -2 dBm (C+L) to keep total fibre power below the nonlinear threshold.
The 2 dB per-channel power reduction plus L-band EDFA’s ~1 dB worse noise figure costs L-band channels approximately 3 dB of OSNR. Links running DP-16QAM on C-band may require DP-8QAM on L-band.
Existing C-band transponders continue operating unchanged. New transponders are added for L-band channels.
ROADM WSS modules are upgraded to C+L variants if the existing hardware does not cover L-band.

The result: capacity approximately doubles from 35 Tbps to ~65 Tbps on the same fibre pair, at the cost of more complex amplification and reduced per-channel performance on L-band.

Noise Figure — Deep Dive

NF Definition

Every optical amplifier degrades the signal-to-noise ratio [12, 24]. Noise Figure quantifies this degradation:

NF = SNR_input / SNR_output    (linear ratio)
NF (dB) = 10 × log10(SNR_in / SNR_out)

An EDFA with NF = 5.5 dB degrades the SNR by a factor of 3.55. The quantum-mechanical minimum NF for any phase-insensitive optical amplifier is 3 dB (factor of 2) — this limit is fundamental because stimulated emission (useful gain) is always accompanied by spontaneous emission (unavoidable noise) [21].

NF and ASE — The Exact Formula

NF determines the ASE noise power injected by each amplifier:

P_ASE = 2 × NF_linear × h × v × dv × (G - 1)

Variable	Value	Description
NF_linear	10^(NF_dB / 10)	e.g. 10^(5.5/10) = 3.55
h	6.626 × 10⁻³⁴ J·s	Planck’s constant
v	193.4 × 10¹² Hz	C-band centre frequency
dv	12.5 × 10⁹ Hz	0.1 nm reference bandwidth (ITU)
G	10^(Gain_dB / 10)	e.g. 10^(20/10) = 100

The factor of 2 accounts for ASE in both polarisation modes. This formula connects directly to the OSNR approximation in Essential Optical Engineering Formulas above, where the constant “58” derives from 10 × log10(2 × h × v × dv) evaluated in dBm.

Worked Example — NF = 5.5 dB, Gain = 20 dB

Step	Calculation	Result
Convert NF to linear	10^(5.5/10)	3.55
Convert gain to linear	10^(20/10)	100
Calculate P_ASE	2 × 3.55 × 6.626e-34 × 193.4e12 × 12.5e9 × 99	1.125 µW = -29.5 dBm
OSNR after 1 amplifier	10 × log10(1000 µW / 1.125 µW)	29.5 dB
OSNR after 10 amplifiers	10 × log10(1000 / 11.25)	19.5 dB

Key Insight

Each amplifier in a cascade adds equal ASE noise power. After N identical amplifiers, OSNR degrades by exactly 10 × log10(N) — hence the 10 dB penalty going from 1 to 10 amplifiers (29.5 dB to 19.5 dB in the example above).

NF by Technology

NF (dB)	Linear Factor	SNR Degradation	Typical Technology
3.0	2.0×	Quantum limit	Theoretical minimum — unachievable in practice
4.0	2.5×	Near-quantum-limited	Research laboratory prototypes
4.5	2.8×	Excellent	Premium low-noise EDFA, 980 nm pump
5.0	3.2×	Very good	High-quality commercial EDFA
5.5	3.5×	Standard planning value	Typical commercial EDFA
6.0	4.0×	Acceptable	Budget EDFA or L-band EDFA
6.5	4.5×	Mediocre	Older or budget L-band EDFA
7.0	5.0×	Poor	End-of-life equipment

The 1 dB NF Rule

Every 1 dB improvement in noise figure translates directly to 1 dB better OSNR at the receiver — always, regardless of span count, gain setting, or channel count. Upgrading from NF 6.0 to NF 4.5 across all amplifier sites yields 1.5 dB of OSNR margin. That 1.5 dB might enable DP-16QAM where only DP-8QAM was previously feasible (doubling per-channel capacity) or extend reach by approximately 15-20 km (equivalent to eliminating one amplifier site).

Friis Formula — Why the First Amplifier Dominates

In a cascade of amplifiers, the overall noise figure is dominated by the first stage [21]:

NF_total = NF1 + (NF2 - 1)/G1 + (NF3 - 1)/(G1 × G2) + ...

Because G1 is large (e.g. 100× for 20 dB gain), the second and subsequent terms become negligible. The first amplifier’s NF sets the noise floor for the entire chain. This is why service providers deploy a premium low-noise pre-amplifier (NF ~4.5 dB) at the receiver end where the signal is weakest — even when inline amplifiers are cheaper models with NF 5.5-6 dB.

Warning

A common cost-saving mistake is deploying budget amplifiers everywhere, including the pre-amplifier position. The Friis formula shows that saving 1 dB of NF at the pre-amplifier is worth far more than saving 1 dB at any inline site. Premium pre-amplifiers pay for themselves in reach and capacity.

Raman Effective NF Below 3 dB

A standalone EDFA cannot achieve NF below 3 dB — this is a quantum-mechanical limit. However, hybrid Raman + EDFA configurations routinely achieve an effective combined NF of 0 to -2 dB [25]. This does not violate physics.

The mechanism: Raman provides distributed gain in the fibre before the signal reaches the EDFA. The signal arrives at the EDFA 10-15 dB stronger than it would without Raman pre-amplification. The EDFA injects the same absolute ASE noise power, but relative to the much stronger incoming signal, the effective SNR degradation is smaller. When the combined system is characterised as a single “black box” amplifier, its effective NF falls below the 3 dB quantum limit — because the measurement convention assumes the signal enters at its un-amplified level, not at the Raman-boosted level.

This is the fundamental physical reason hybrid Raman + EDFA improves OSNR by 2-4 dB over EDFA-only designs, and it explains why every long-haul and subsea system employs distributed Raman amplification.

Reading NF on Amplifier Datasheets

When evaluating EDFA specifications, four curves and trade-offs require attention:

Datasheet Characteristic	What to Check	Typical Variation
NF vs gain	NF at the actual operating gain, not just the headline value	0.5-1 dB across the gain range
NF vs wavelength	NF at band edges vs centre (lowest near 1540-1550 nm)	±0.5 dB across C-band
NF at low input power	NF may increase when per-channel input approaches the noise floor	Check at actual per-channel input power
Single-stage vs dual-stage	Dual-stage (with mid-stage GFF/DCM/VOA access) has slightly higher NF	0.5-1 dB penalty, offset by better gain-flatness control

Amplifier Selection Guide

Scenario	Recommendation
Standard terrestrial spans under 100 km	EDFA only — sufficient and simplest to operate
Long spans (100-150 km) or OSNR-sensitive routes	Hybrid Raman + EDFA
Subsea cable systems	Always Raman + EDFA — every fraction of a dB matters
Metro DCI under 80 km	Often unamplified — 400ZR+ pluggable optics handle the loss budget
L-band or S-band expansion	Raman required — standard EDFAs cover C-band only; dedicated L-band EDFAs exist but Raman offers more flexibility

The decision-tree below provides the engineering rationale behind those rows, walking from “what bands do you need” through the appropriate amplifier architecture and SRS-management requirements:

flowchart TD
    START["What band(s)<br/>do you need?"] -->|"C-band only"| CONLY{"Span<br/>length?"}
    CONLY -->|"Under 100 km"| EDFA_ONLY["EDFA only<br/>simplest, cheapest"]
    CONLY -->|"100-150 km"| HYBRID["Hybrid Raman + EDFA<br/>better OSNR"]
    CONLY -->|"Over 150 km<br/>(subsea)"| HYBRID_MAX["Raman + EDFA<br/>+ extended fibre types<br/>(G.654E)"]
    START -->|"C+L band"| CL{"Amplifier<br/>architecture"}
    CL --> SPLIT_EDFA["Split C/L EDFAs<br/>+ band splitter/combiner<br/>at every amp site"]
    CL --> WIDEBAND_RAMAN["Wideband Raman (4 pumps)<br/>+ split C/L EDFAs<br/>for maximum reach"]
    SPLIT_EDFA --> SRS_MGMT["SRS management required:<br/>pre-tilt + DGE + power optimisation"]
    WIDEBAND_RAMAN --> SRS_MGMT
    style EDFA_ONLY fill:#1D9E75,stroke:#0F6E56,color:#fff
    style HYBRID fill:#BA7517,stroke:#854F0B,color:#fff
    style HYBRID_MAX fill:#D85A30,stroke:#993C1D,color:#fff
    style SPLIT_EDFA fill:#7F77DD,stroke:#534AB7,color:#fff
    style WIDEBAND_RAMAN fill:#7F77DD,stroke:#534AB7,color:#fff
    style SRS_MGMT fill:#E24B4A,stroke:#A32D2D,color:#fff

Rule of Thumb

Below 100 km with C-band only: EDFA is sufficient and simplest to operate. Above 100 km or when deploying C+L: hybrid Raman + EDFA is the standard choice. Subsea systems always use Raman — every fraction of a dB matters when amplifier sites are separated by hundreds of kilometres.

References

Standards (ITU-T)

ITU-T G.694.1 — Spectral grids for WDM applications: DWDM frequency grid (10/2020). https://www.itu.int/rec/T-REC-G.694.1
ITU-T G.694.2 — Spectral grids for WDM applications: CWDM wavelength grid (12/2003). https://www.itu.int/rec/T-REC-G.694.2
ITU-T G.709/Y.1331 — Interfaces for the Optical Transport Network (06/2020). https://www.itu.int/rec/T-REC-G.709
ITU-T G.798 — Characteristics of optical transport network hierarchy equipment functional blocks (12/2017). https://www.itu.int/rec/T-REC-G.798
ITU-T G.872 — Architecture of optical transport networks (10/2017). https://www.itu.int/rec/T-REC-G.872
ITU-T G.873.1 — Optical Transport Network — Linear protection (03/2022). https://www.itu.int/rec/T-REC-G.873.1
ITU-T G.652 — Characteristics of a single-mode optical fibre and cable (11/2016). https://www.itu.int/rec/T-REC-G.652
ITU-T G.654 — Characteristics of cut-off shifted single-mode optical fibre and cable (03/2020). https://www.itu.int/rec/T-REC-G.654
ITU-T G.655 — Characteristics of non-zero dispersion-shifted single-mode optical fibre and cable (11/2009). https://www.itu.int/rec/T-REC-G.655
ITU-T G.657 — Characteristics of a bending-loss insensitive single-mode optical fibre and cable (11/2016). https://www.itu.int/rec/T-REC-G.657
ITU-T G.707/Y.1322 — Network node interface for the synchronous digital hierarchy (SDH) (01/2007). https://www.itu.int/rec/T-REC-G.707
ITU-T G.661 — Definitions and test methods for the relevant generic parameters of optical amplifier devices and subsystems (07/2018). https://www.itu.int/rec/T-REC-G.661
ITU-T G.662 — Generic characteristics of optical amplifier devices and subsystems. https://www.itu.int/rec/T-REC-G.662
ITU-T G.975 — Forward error correction for submarine systems (10/2000). https://www.itu.int/rec/T-REC-G.975
ITU-T G.975.1 — Forward error correction for high bit-rate DWDM submarine systems (02/2004). https://www.itu.int/rec/T-REC-G.975.1
ITU-T G.698.2 — Amplified multichannel dense wavelength division multiplexing applications with single channel optical interfaces. https://www.itu.int/rec/T-REC-G.698.2

Standards (IEEE / OIF / MSA)

IEEE 802.3 — Ethernet; specifically 802.3bs (400 GbE), 802.3ck, 802.3df (800 GbE). https://standards.ieee.org/standard/802_3-2022.html
OIF 400ZR Implementation Agreement (OIF-400ZR-01.0). https://www.oiforum.com/documents/
OpenZR+ MSA specification. https://www.openzrplus.org/
OIF Common Management Interface Specification (CMIS). https://www.oiforum.com/documents/

Books

G. P. Agrawal, Fiber-Optic Communication Systems, 5th ed., Wiley, 2021.
G. P. Agrawal, Nonlinear Fiber Optics, 6th ed., Academic Press, 2019.
I. P. Kaminow, T. Li, A. E. Willner (Eds.), Optical Fiber Telecommunications VI-A & VI-B, Academic Press, 2013.
P. C. Becker, N. A. Olsson, J. R. Simpson, Erbium-Doped Fiber Amplifiers — Fundamentals and Technology, Academic Press, 1999.
C. Headley & G. P. Agrawal (Eds.), Raman Amplification in Fiber Optical Communication Systems, Academic Press, 2005.
V. Alwayn, Optical Network Design and Implementation, Cisco Press, 2004.
R. Ramaswami, K. N. Sivarajan, G. H. Sasaki, Optical Networks: A Practical Perspective, 3rd ed., Morgan Kaufmann, 2009.

Papers

K. Roberts et al., “Beyond 100 Gb/s: Capacity, Flexibility, and Network Optimization,” J. Lightwave Technol. 35 (2017).
J. Cho et al., “Probabilistic Constellation Shaping for Optical Fiber Communications,” J. Lightwave Technol. 37, 1590 (2019).
A. D. Ellis et al., “Performance limits in optical communications due to fiber nonlinearity,” Adv. Opt. Photon. 9, 429 (2017).

Transport Network Guide

Explorer

Ch 03 — Optical Amplifiers — EDFA, Raman, and Wideband C+L

Optical Amplifiers — EDFA, Raman, and Wideband C+L

Optical Amplifiers — EDFA and Raman

EDFA — Erbium-Doped Fibre Amplifier

Gain Spectrum and Gain Flattening Filter

Gain Shape Dependence on Operating Point

EDFA Internal Architecture

Raman Amplifier

Raman Gain Profile and Multi-Pump Flattening

Hybrid Raman + EDFA — Simulation and Engineering

The OSNR Argument — Why Distributed 12 dB Outperforms Lumped 28 dB

Why Raman Is Never Used Alone — DRB Revisited

Physical Architecture — Single Equipment Shelf

170 km Span Simulation — Three Amplification Strategies

Approach 1 — EDFA Only at the Far End

Approach 2 — Mid-Span EDFA at km 85

Approach 3 — Raman + EDFA at the Far End

All Three Approaches Overlaid

Quantitative Comparison

When Each Approach Wins

EDFA Gain-Reduction Benefit in Hybrid Deployments

C+L Wideband Amplification

Why a Single EDFA Cannot Cover Both Bands

Parallel C+L EDFA Architecture

C+L Gain Profile — A Step, Not a Slope

SRS Compensation — Three Layers

Layer 1 — EDFA Gain Offset Between Bands

Layer 2 — Pre-Tilt at Launch

Layer 3 — Dynamic Gain Equalisation (DGE)

Wideband Raman for C+L

SRS Power Tilt — The C+L Engineering Problem

Practical C+L Upgrade Example

Noise Figure — Deep Dive

NF Definition

NF and ASE — The Exact Formula

Worked Example — NF = 5.5 dB, Gain = 20 dB

NF by Technology

The 1 dB NF Rule

Friis Formula — Why the First Amplifier Dominates

Raman Effective NF Below 3 dB

Reading NF on Amplifier Datasheets

Amplifier Selection Guide

See Also

References

Standards (ITU-T)

Standards (IEEE / OIF / MSA)

Books

Papers

Graph View

Table of Contents

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