Special instructions for erbium-doped fiber amplifier(EDFA Optical Amplifier)
You have purchased an erbium-doped fiber amplifier (EDFA Optical Amplifier) with a specification of 30dB gain and a saturation output power of+20dBm.
Connect 0dBm input light and read an output of+27dBm. You can calculate that 30-3=27, and the gain is not a problem.
But what if you input -20dBm? The nominal gain of 30dB means that the output should be+10dBm, but the actual measurement is only+7dBm – a full 3dB less. This is not a quality issue. 30dB is the small signal gain, while ASE noise and noise figure consume your gain. In fact, the measured gain often falls below the nominal gain, which clarifies that this phenomenon is not a quality issue of the equipment, but rather determined by the working mechanism of the amplifier.
Small signal gain ≠ actual gain:
1. Core contradiction: The gain indicated on the specification sheet (e.g. 30dB) is a small signal gain, which is the ideal measurement value when the input signal power is very low (e.g. -20~-30dBm) and at the rated pump power. This does not equal the actual gain when the input signal power is high in practical applications.
2. The main reason for the decrease in gain:
2.1 Gain saturation: As the input signal power increases, the EDFA Optical Amplifier enters the saturation region, causing the gain to decrease from its maximum value.
2.2 Diversion of ASE noise: Amplified spontaneous emission (ASE) noise competes with signal light and consumes limited pump power. The stronger the ASE noise, the lower the effective gain used to amplify signal light. This is one of the fundamental reasons why the measured gain is lower than the nominal value.
2.3 Quantitative relationship: The higher the input signal power, the greater the compression of the actual gain (G_actual) compared to the small signal gain (G_small). The compression amount mainly comes from saturation compression (Δ G_sat) and gain from ASE noise consumption (Δ G_ASE). For example, when the input power is 0dBm, it is common for the measured gain to be more than 3dB lower than the nominal value.
3. Engineering practice suggestions:
3.1 Link budget: Small signal gain should not be directly used for calculation, but a more realistic link budget formula should be adopted:
P_out ≈ P_in+G_small-NF-3dB (safety margin)
Among them, NF is the noise figure (typical value 4-6dB).
3.2 Reverse calibration: If the measured output power does not match the formula budget, the formula can be used to calculate the actual system noise figure (NF) in reverse, thus enabling more accurate link design and calibration.
Conclusion: When evaluating and using EDFA Optical Amplifier, engineers need to pay attention to their input signal power and understand the characteristic of gain compression under high signal conditions. When designing the link, the budget should be based on the actual input power and engineering formulas that include noise factor and safety margin, rather than simply relying on the nominal value of small signal gain on the specification sheet. After receiving the EDFA Optical Amplifier, first ask what the input power is, and then use the link budget formula to calculate the expected output. Don’t use small signal gain for full power budgeting.
Post time: Apr-27-2026