by wider regions of higher bandgap. The carriers in the active region are generally divided into two populations that occupy a completely different set of energy levels: carriers in the wells and carriers out of the wells. Carriers out of the wells are ordinary pseudo-particles obeying the rules of carrier motion in crystals, and in particular they cannot have any energy in the forbidden band of the semiconductor forming the active region. Inside the quantum well, quantum confinement of the carriers creates energy sub-bands inside the forbidden energy band. The central energy of the sub-bands depends on the well thickness. This is easily understood remembering that in the simple case of a one-dimensional infinite energy well the energy levels of a particle are equal to where n is the quantum number and L is the width of the well. Due to these characteristics the wavelength of the light emitted by a quantum well laser is determined by the width of the active region rather than just the bandgap of the material from which it is constructed. This means that much shorter wavelengths can be obtained from quantum well lasers than from conventional laser diodes using a particular semicon- ductor material. The efficiency of a quantum well laser is also greater than a conventional laser diode due to the stepwise form of its density of states function. Beside the use of a quantum well, in order to shape the energy levels inside the active region, the so-called strain technique can be incorporated. This consists in introducing in the quantum well zone a tensile or compressive strain, whose effect is to change the conduction sub-bands due to the quantum well. One of the main effects of a controlled strain is to reduce the threshold current of the laser due to the increased separation between energy bands that renders recombination more difficult [12]. 5.2.1.4 Source Fabry-Perot Lasers Probably FP semiconductor lasers used as signal sources are the lasers produced in larger volumes on the market for a huge number of applicatio