I have a doubt about MMF and SMF cables. If MMF can handle more than one signal light at the same time and both MMF and SMF support full-duplex, why isn't MMF faster than SMF cable?
As the name implies a multi mode fibre can carry multiple modes of light. This is not the same thing as multiple signals.
The multiple modes of the same signal only leads to modal dispersion which stretches out the signal in the time domain, due to the different propagation speed of the modes, making it more difficult to detect at the receiver end. This leads to a lower speed both in theory and practice than a single mode fibre which doesn't suffer from modal dispersion.
Both MMF and SMF fibre can carry multiple signals through the use of wavelength division multiplexing (WDM), although this is much more common with SMF as its characteristics allow it to be used over longer distances where it generally becomes more economical to use WDM equipment.