In order to meet the requirements of application in DWDM systems, EDFA has its special requirements in performance and function. The optical signal-to-noise ratio (OSNR) is the most important index after optical signal transmission.
In addition to amplifying the input optical signal, the erbium-doped fiber amplifier can also generate ASE noise, which can be equivalent to an ideal amplifier with a gain of G and superimposed with a noise source with a power of Pase (ASE noise power). For a cascaded system of N amplifiers, each amplifier stage amplifies the ASE noise of the previous stage by G times and superimposes the ASE noise generated by itself at the same time. Noise accumulates step by step, so the optical signal-to-noise ratio (OSNR) will degrade step by step.
OSNR= PSIG/PASE (PSIG optical signal power) It can be seen that the optical signal-to-noise ratio can be improved by increasing the optical power of each channel, that is increasing the total output power of Erbium-doped fiber amplifier, or by reducing the spontaneous emission noise (ASE noise) to improve the optical signal-to-noise ratio.For an N-segment optical fiber system with N+1 optical amplifiers cascades, assuming that each stage of the amplifier is the same, and each segment of the optical fiber loss is equal to the gain of the amplifier, then the spontaneous emission noise of the terminal Pase=F(g-1) HVB0 (N+1), it can be seen that Pase increases linearly with the stage of the amplifier.The loss of the optical fiber increases exponentially with the increase of the length of the optical fiber. According to the hypothesis, the gain of the amplifier just compensates the loss of the optical fiber, and the following conclusions can be drawn:When PASE is constant, decreasing the relay length (i.e., decreasing the gain of erbium-doped fiber amplifier) and increasing the number of relays (i.e., increasing the number of cascades of erbium-doped fiber amplifier) can reduce the farther distance of transmission than increasing the relay length.In other words, when the transmission distance is the same, it is better to decrease the relay length and increase the number of relays than to increase the length of relays.When the number of relays is reduced, the PASE is smaller, that is the optical signal-to-noise ratio (OSNR) of the receiver is increased.
In addition, in the system design, besides choosing the relay distance, choosing the Erbium-doped fiber amplifier with low noise index is also an effective method to improve the optical signal-to-noise ratio.In DWDM system, the capacity of the system is increased to accommodate more channels.There are two methods: one is to reduce the channel interval; the other is to expand the operating wavelength range of the erbium-doped fiber amplifier. While expanding the operating wavelength range, the gain level in the operating wavelength range must be maintained, because the erbium-doped fiber amplifier is used in cascade.Each level of small unflatness, after the multi-level cascade, will become very uneven, resulting in a great difference in the power of each channel at the line terminal.
The gain feature of the erbium-doped fiber amplifier is that the gain of the small input optical signal is large, and the gain of the large input optical signal is small. In DWDM system, if the gain of the line fiber amplifier is not controlled, when there are few input channels (small input optical signal), the gain will be larger than when there are many input channels (large input optical signal). When there are few input channels, the output power of each channel is large. In extreme cases, there is only one input channel. Then it may cause the output optical signal to be large enough to produce nonlinear distortion in optical fiber transmission. Therefore, the line amplifier must have gain locking to ensure that the gain is the same in both large and small input signals.That is the output optical power of each channel remains constant as the input channel is increased or decreased.