Improve Sensitivity for a Self-Mixing Laser Diode Sensor by Applying a Pre-feedback

When a part of light emitted by a laser is back-reflected or back-scattered from an external target and re-enters the laser cavity, the laser output power will be modulated. This is called Self-Mixing Effect (SME), which is a universal phenomenon occurring in lasers regardless of type. For SME based laser sensing system, a laser diode is usually employed to make a compact sensing system due to its small size. The laser diode in this case is also called Self-Mixing Laser Diode (SMLD). In some practical cases, a target surface has a very low reflectivity and thus not able to provide enough high feedback light to the laser diode. In this case, the observed sensing signal from the SMLD sensor is blurred and sensing sensitivity is degraded. To improve the sensing ability, we propose to apply a pre-feedback to the SMLD sensor. Investigation from both simulation and experiments are conducted to verify the proposed design. The results show that a proper pre-feedback can greatly enhance the sensing performance for a SMLD sensor.