Hello Lama,
1) the Q-factor is a measure of the quality of the received signal and it is related to the statistical parameters of probabilities density functions (pdf) of the received bits 0 and 1. It is described in the datasheet of the BER Analyzer visualizer in OptiSystem as shown in the attached image. You may access the datasheet by right click on the BER Analyzer in OptiSystem then choose component properties and Help. The acceptable value of Q-factor depends on the data rate. Typically, a Q-factor of 10 is acceptable for 10GBps NRZ system.
2. Users of single wavelength transmission systems typically choose the laser at 1550nm because of its minimum attenuation in the fiber. It was also use because of the DSF in late 2000. However, these days DSF is not widely used.
Regards,
Ahmad