Optical injection is compared for distributed feedback semiconductor and fibre lasers whose wavelength is around 1550 nm. A spectral description of an injected semiconductor laser is presented for usual injected power (> 30 dBm), by mapping out several phenomena, such as bistable areas, undamped relaxation and chaos synchronization. For weak injection levels (< 30 dBm), the slave acts as a highquality amplifier of spectrally narrow signals. In this case, the slave frequency is pulled by the master one. Theory of weak injection is demonstrated using a generalized Airy's function method. We also present the temporal response of a fibre laser output power for static injection, and observe original dynamics linked to relaxation processes. A new method using optical injection is proposed to measure narrow linewidth or spontaneous emission rate.