  A new method for measuring the linewidth enhancement factor of a laser is
proposed. It is based on frequency-modulated optical injection, combined with
dual-frequency laser operation. The linewidth enhancement factor {\alpha} is
deduced from the experimental data using a theoretical analysis based on a
standard rate equation model. As the intracavity power is kept constant, the
method allows to free the process from the thermal effects that are usually
present in AM/FM techniques. Measurement of {\alpha}=0.28$\pm$0.04 in a
diode-pumped Nd:YAG laser demonstrates that the method is well-suited for
characterizing small values of {\alpha}.

Optics