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Summary form only given. An injected (slave) laser can become slaved onto another one (master) and frequency locking occurs following the injected intensity and the frequency difference between both lasers. Optical injection can give a frequency reference or transfer to a slave the spectral properties (linewidth) from a master. For lasers tuned on the same central frequency, decreasing the injected power implies a partial linewidth transfer, when the master linewidth is larger than the slave linewidth. In the converse case (master linewidth smaller than slave linewidth), the laser amplifies at the sample rate both the spontaneous emission from the slave and the external injected field. We have in this way characterized optical spectra of signals in the picowatt range. In our experiment, two single mode lasers are used: a tunable source with a linewidth of 80 kHz for the master and a DFB laser for the slave with a linewidth much wider (2-200 MHz) depending on the bias current. Adding to the laser detector a heterodyne technique, signals in the femtowatt range were detected. The aim is to study the amplification of the laser detector.