Theory of Reduction of Residual Amplitude Modulation in Mid-Infrared Wavelength Modulation Spectroscopy by Injection Locking of Quantum Cascade Lasers

A simplified rate equation model is used to investigate suppression of residual amplitude modulation in injection-locked quantum cascade lasers with the master laser modulated by its drive current. Quasi-static and dynamic expressions for intensity modulation suppression and frequency modulation reproduction by the slave laser are presented. The suppression peaks at a specific value of the injection ratio for a given detuning and linewidth enhancement factor. The frequency modulation reproduction is less than 100% and decreases rapidly at frequencies higher than 300 MHz. The intensity modulation suppression remains constant during this frequency range. The effects of injection ratio, detuning, coupling efficiency and linewidth enhancement factor are discussed.