Requirements for optical amplifiers in optical fiber communication systems
To understand the requirements of optical fiber communication systems for optical amplifiers, we must first master some important parameter indicators. The main parameters of optical amplifiers are gain, bandwidth and noise figure.
Since optical amplifiers can perform optical amplification, what people are most concerned about is how many times it can be amplified. The parameter describing the magnification of the optical amplifier is called the gain G. of the optical amplifier.
The gain G of the amplifier is defined as
G(dB)=10lgPout(m W)/Pin（mW） (3-1)
In the formula, Pout and Pin are the continuous signal power at the output and input of the amplifier, respectively.
In the optical fiber communication system test, the optical power unit is traditionally expressed by d Bm, then the gain of the optical amplifier can be expressed as:
G（dB）= Pout(dBm)-Pin(d Bm) (3-2)
Optical amplifiers can achieve simultaneous amplification of multiple wavelengths in a wavelength division multiplexing system, so what wavelengths can optical amplifiers amplify? If the amplifier can be amplified at all wavelengths and the gain coefficient is the same, then all wavelengths in the low-loss region of the fiber can be used in the wavelength division multiplexing system to increase the number of channels in the WDM system. However, the amplification effect of the actual optical amplifier is always a certain frequency range, or that the working wavelength of the amplifier can be commercially applied has a certain range.
The operating bandwidth of an optical amplifier is defined as the frequency interval where the small signal gain is lower than the peak small signal gain N (dB). Usually take N=3dD, at this time G is reduced to half of G0, so it is also called half-height full-width bandwidth.
3. Gain saturation and saturation output power
As mentioned earlier in the definition of operating bandwidth, "small signal gain". What is the difference between small signal gain and large signal gain? This is related to the principle of the optical amplifier. First, the optical amplifier requires energy to amplify the optical signal. This energy comes from the pump source laser. The optical amplification process is achieved by using a mechanism to convert the energy of the pump light into the energy of the signal light. The pump source laser has a certain power range, so there must be a certain power range for the amplified signal optical power. Assuming that the optical power of the pump source is 100mW and the gain of the optical amplifier is 20dB, when the optical power of the input signal of the optical amplifier is -20dBm, the output power of the optical amplifier is 0 dBm; but when the optical power of the input signal of the optical amplifier is 10 dBm, the optical The amplifier output cannot reach 30 dBm. Because 30 dBm is equivalent to 1000mW of optical power, and the signal light and pump light add up to 110mW, the maximum output power cannot be greater than 110mW, even if the maximum output power is 110mW, the gain is 10.4 dB relative to the input optical power, and Not 20 dB of the original small signal gain. Because the large-signal amplification process consumes a large amount of particles on the high-energy pole, the gain coefficient gradually decreases as the signal input power increases. This phenomenon is called gain saturation of the optical amplifier. When the signal power increases and the amplifier gain is reduced to half of the peak gain (small signal gain) (the gain coefficient drops by 3 dB), the corresponding output power is called the saturated output power of the optical amplifier. The saturation output power of the optical amplifier is an important parameter of the optical amplifier, which is expressed by Pouts.
4. Noise figure
The amplifier will generate noise during the amplification process. The noise of the optical amplifier mainly originates from its amplified spontaneous emission (ASE). In lasers, spontaneous radiation is indispensable for laser oscillation, but in amplifiers it is the main source of noise. It is transmitted and amplified together with signal light in optical fibers, which reduces the signal-to-noise ratio of signal light. Of course, signal light body noise, pump source noise, etc. will also affect the noise performance of the optical amplifier. Because its impact is relatively small, no further discussion will be made here.
Due to the presence of noise in the amplifier, the amplifier will inevitably increase the noise when amplifying the signal, degrade the signal-to-noise ratio (SNR), and limit the electrical regeneration relay distance of the system. The degradation of the signal-to-noise ratio is expressed by the noise coefficient Fn, which is another important indicator of the optical amplifier. It is defined as the difference between the input signal-to-noise ratio of the signal light and the output signal to the output signal-to-noise ratio:
In the formula, (SNR)in and (SNR)out respectively represent the input signal-to-noise ratio and output signal-to-noise ratio of the signal. They are calculated by converting the optical signal into photocurrent at the receiver.
The requirements for optical amplifiers in optical fiber communication systems are as follows:
(1) High gain;
(2) Wide working range and flat gain;
(3) High saturation output optical power;
(4) Low insertion loss;
(5) Low noise figure;
(6) The working characteristics are stable, preferably independent of temperature and polarization state;
(7) Small size, light weight, low power consumption and long working life.
Actual optical amplifiers are not all of the above indicators are excellent. Optical amplifiers with different performances are placed at different positions in the wavelength division multiplexing system. Lasers with high saturation output power are used for the optical transmission end of the fiber optic communication system. Amplifier (BA); the optical amplifier with low noise figure is placed at the optical receiving end of the optical fiber communication system and is called the pre-amplifier (PA).