Heterodyne detection with an injection laser I. principle of operation and conversion efficiency

If a light signal is passed into the resonant cavity of an actively oscillating injection laser, an electrical signal at the difference between the laser and light signal frequencies flows through the wire supplying the DC bias to the laser. A small-signal analysis of heterodyne detection with such a laser oscillator that is based solely on rate equations and introduces the externally applied signal as a source term into the photon rate equation is discussed. Since neither Maxwell's equations nor the wave equation are involved, this analysis is deemed simpler and more transparent than that reported earlier. Gain saturation of the active medium of the laser, which is responsible for broadening of the relaxation resonance, is taken into account.