Measurements, on CATV DFB strained QW lasers, of first-order, second-order optical modulation depth (OMD) and, for the first time, of DC component decrease with modulation ΔDC are fitted within a nonlinear model giving a precise determination of gain compression factor ε. This direct determination method at intermediate modulation frequencies is simpler and gives higher precision of ε than existing measurement methods such as relaxation resonance peak characteristics or emission wavelength shift