Linewidth enhancement factor measurement based on FM-modulated optical injection: application to rare-earth doped active medium

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}.