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Influence on the steady-state oscillation spectrum of a diode laser for feedback of light interacting coherently and incoherently with the field established in the laser cavity.

An analytic model of the effects of optical feedback on the steady-state oscillation spectrum of a diode laser is presented and compared to experimental measurements. To model the laser with feedback, the electric fields at the reflecting surfaces of a three-mirror resonator are solved for and the feedback light is allowed to interact both coherently and incoherently with the field established in the cavity. It is found that the coherently and incoherently interacting feedback light influence the output of the laser in very different and distinct fashions; thus to interpret accurately experimental data, both forms of feedback light must be considered. Incoherent feedback light causes the laser to operate more multimode, and for small amounts (≲0.2% of the laser output) of light returned incoherently, the incoherent feedback increases the above-threshold output power by an amount which is independent of the pumping rate. Coherently interacting feedback light causes the laser to operate more single mode and increases the above-threshold output power by an amount which is a linear function of the pumping rate. For feedback of incoherent light above the ≃0.2% level, the far-above threshold output of the laser is increased by an amount which is an increasing function of the pumping rate. However, since the incoherent feedback increases the output very rapidly with pumping rate at threshold and then decays quickly with pumping to the above-threshold value, the contribution of the incoherent light to the output is easily distinguished from the coherent contribution for all levels of feedback. The model indicates that the distribution of energy among the modes is influenced much more by incoherent feedback while the total power is influenced to a greater extent by the coherent feedback than the incoherent feedback. A good agreement between the model predictions and experimental results is found.